Review

Axon guidance proteins in neurological disorders Eljo Y Van Battum*, Sara Brignani*, R Jeroen Pasterkamp

Many neurological disorders are characterised by structural changes in neuronal connections, ranging from presymptomatic synaptic changes to the loss or rewiring of entire axon bundles. The molecular mechanisms that underlie this perturbed connectivity are poorly understood, but recent studies suggest a role for axon guidance proteins. Axon guidance proteins guide growing axons during development and control structural plasticity of synaptic connections in adults. Changes in expression or function of these proteins might induce pathological changes in neural circuits that predispose to, or cause, neurological diseases. For some neurological disorders, such as midline crossing disorders, investigators have identified causative mutations in genes for axon guidance. However, for most other disorders, evidence is correlative and further studies are needed to confirm the pathological role of defects in proteins for axon guidance. Importantly, further insight into how dysregulation of axon guidance proteins causes disease will help the development of therapeutic strategies for neurological disorders.

Introduction Pathological changes in neural circuitry are a unifying hallmark of many neurological diseases. In some diseases, structural connectivity defects are a result of the disease process, whereas in other disorders they have an early and causal role. The molecular mechanisms underlying perturbed structural connectivity are often poorly understood. Accumulating evidence from both human genetic and animal studies implicates axon guidance proteins in the rearrangement and loss of neuronal connections during neurological disease. Axon guidance proteins were originally identified as instructive cues to guide embryonic axons. These cues act as attractants or repellents for growing axons and guide them towards or away from specific regions in the developing brain or embryo.1,2 The concerted action of several different axon guidance proteins enables growing axons to find their targets. However, axon guidance proteins have many roles beyond embryonic axon guidance. These include functions in developmental processes, such as neuron cell body migration, but also roles in the adult nervous system—eg, in the control of synaptic plasticity. Human and animal studies have associated axon guidance proteins with neuronal connectivity changes in three classes of neurological disorders. First, mutations in genes encoding axon guidance proteins and receptors can cause congenital axon guidance disorders characterised by aberrant pathfinding of embryonic axons and neurons.3,4 Second, changes in axon guidance (receptor) genes and changed expression or function of these cues could contribute to the aetiology of multifactorial disorders such as autism, in which neuronal morphology and structural connectivity are affected during development.5–7 Third, subtle defects in axon guidance proteins could contribute to the development of late-onset brain disorders such as neurodegenerative diseases. These defects might increase disease susceptibility or might act as causative factors.8,9 In this Review, we highlight recent advances in understanding of how axon guidance proteins contribute to neurological diseases. We focus on the role of canonical axon guidance proteins and discuss examples in which defects in these

proteins mediate pathological changes in neuronal morphology and connectivity. Data from human studies are discussed, together with experiments in cellular and animal models, to explain disease mechanisms and to highlight opportunities for the development of therapeutic strategies. Defects in axon guidance proteins can also affect the cardiovascular and immune systems and thereby indirectly contribute to neurological disease. For example, in multiple sclerosis, axon guidance proteins mediate an aberrant immune response towards neuronal tissue. These non-neuronal effects are beyond the scope of our Review and are discussed elsewhere.10,11

Lancet Neurol 2015 Published Online March 11, 2015 http://dx.doi.org/10.1016/ S1474-4422(14)70257-1 *Authors contributed equally Department of Translational Neuroscience, Brain Center Rudolf Magnus, University Medical Center Utrecht, Utrecht, Netherlands (E Y Van Battum MSc, S Brignani MSc, R J Pasterkamp PhD) Correspondence to: Dr R Jeroen Pasterkamp, Department of Translational Neuroscience, Brain Center Rudolf Magnus, University Medical Center Utrecht, Utrecht 3584 CG, Netherlands [email protected]

Axon guidance proteins Work in the past two decades has led to the discovery of a class of proteins that help to establish functional neural circuits through guidance of axons to their specific targets during development. These so-called axon guidance proteins are expressed along the trajectories of growing axons and are detected by a sensory structure at the axon tip—the growth cone (figure 1). Specialised receptor proteins at the growthcone-cell surface detect axon guidance proteins and trigger intracellular signalling cascades that cause axon steering through induction of changes in the growth-cone cytoskeleton. Axon guidance proteins can act as attractants or repellents—ie, either directing axons towards a specific structure or diverting them away from specific regions. Furthermore, they exist as both membrane-associated cues acting at short ranges or as secreted agents with longer range effects. Researchers have identified five families of canonical guidance proteins: Semaphorins, Netrins, Slits, Repulsive guidance molecules, and Ephrins (figure 1C).1,2 Semaphorins are a large family of proteins that includes both secreted (Semaphorin class three) and membraneassociated attractants and repellents (Semaphorins classes four to seven). In mammals, Semaphorins have been subdivided into five classes on the basis of sequence and structural similarities. The major growth-cone receptors for Semaphorins are members of the Plexin and Neuropilin protein families. Semaphorins and their receptors regulate many different aspects of neural circuit development, in both the central and peripheral nervous systems, including

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topographically organised axon connections between the eye and the brain.16 In addition to these canonical axon guidance proteins, additional protein families previously recognised for other functions have been implicated in axon guidance (eg, cell adhension molecules and morphogens).17–19 Many axon guidance proteins are pleiotropic and have diverse functions unrelated to axon guidance. This includes roles in other aspects of neural circuit development (eg, in axon fasciculation, pruning, and synaptogenesis), but also effects on non-neuronal cells in other organ systems (eg, in angiogenesis and bone formation).13,15,16,20

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Figure 1: Axonal guidance and canonical axon guidance proteins (A) During embryonic development, axons are guided along predetermined routes by proteins in the extracellular environment. The growth cone, a sensory structure at the axon tip, enables axons to react to axon guidance proteins. Receptor proteins at the growth cone cell surface bind to the guidance cues and induce axon steering. Axon guidance cues can have repulsive (blue, –) or attractive (pink, +) effects on growing axons and exist as secreted proteins that form gradients in the extracellular matrix or as membrane-bound proteins with short-range effects. (B) Similar to axons, migrating cells in the developing nervous system are guided to their targets by repulsive and attractive axon guidance proteins. (C) Five families of canonical axon guidance proteins have been identified (top) that bind and signal through specific growth cone receptors (bottom). RGM=repulsive guidance molecules.

axon growth and guidance, axon bundling, and pruning.12 Two closely related secreted Netrin proteins have been identified in mammals, which signal through DCC receptors and UNC-5 receptors to induce attractive and repulsive effects, respectively. One of the best characterised effects of Netrins is their ability to attract axons to the midline of the nervous system during the formation of brain commissures.13 The Slit proteins (Slit1, Slit2, and Slit3) are secreted repellents that use proteins from the roundabout (Robo) family as receptors. Similar to Netrins, Slits have an important role during midline guidance. Once an axon has crossed the midline, repulsion by Slits prevents recrossing.14 Repulsive guidance molecules (RGMA-C) are glycosylphosphatidylinositol (GPI)-anchored repellents that signal via the Neogenin receptor. Although Repulsive guidance molecules remain poorly characterised, evidence is emerging that these cues mediate axon guidance events in different brain regions.15 EphrinA and EphrinB proteins act as short-range attractants or repellents. The EphrinA proteins (A1–A5) are tethered to the membrane via GPI anchors and interact with class A (A1–A8) Eph receptors; the EphrinB proteins (B1–B3) are transmembrane proteins that interact with class B (B1–B6) Eph receptors. Ephrins and Eph receptors are known for the development of 2

Principles of commissural axon guidance Commissural axon projections connect the left and right sides of the brain and the spinal cord to integrate information for coordinated sensory, motor, and cognitive functions. There are six major brain commissures (the anterior commissure, posterior commissure, corpus callosum, hippocampal commissure, habenular commissure, and the decussation of the superior cerebellar peduncle) and one spinal cord commissure (the anterior white commissure). In addition, several other axon bundles cross the midline, including the optic nerve and auditory pathway. Commissural axon guidance has been best studied for embryonic spinal commissural axons crossing the floorplate and for axons from pyramidal neurons in the cerebral cortex that form the corpus callosum (figure 2).21–23 Semaphorins and their Neuropilin receptors, Ephrins and Eph receptors, Netrins and DCC, and Slits and Robo receptors control midline guidance in different model organisms. Mutations in DCC or ROBO lead to midline guidance defects in humans. For a review of human genetic disorders of axon guidance caused by mutations in non-canonical guidance proteins we direct the reader to other articles.23,24

Congenital mirror movements Congenital mirror movements are pathological involuntary movements that mirror intentional movements on the contralateral side. Congenital mirror movements occur mostly in the forearms, hands, and fingers, and they can be a symptom of other brain disorders, such as Joubert’s syndrome. Non-syndromic congenital mirror movements is a very rare disorder that runs in an autosomal-dominant inheritance pattern in a few families worldwide, but can also present as a sporadic disease. Although congenital mirror movements can occur in healthy young children, they typically disappear by age 7 years. Imaging studies in patients have identified abnormal development of the corpus callosum and incomplete development, including abnormal decussation, of the corticospinal tracts, as anatomical defects underlying congenital mirror movements.3,4 Mutations in the Netrin-1 receptor DCC were recently reported to underlie non-syndromic congenital mirror

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Figure 2: Midline crossing of embryonic spinal commissural and callosal axons (A) The development of axon projections from spinal commissural neurons in the embryonic mouse dorsal spinal cord requires many different axon guidance proteins. Here, two classes of axon guidance proteins are shown: Netrin-1 and Slits. Netrin-1 attracts mouse spinal commissural axons expressing DCC towards the floorplate. As these axons approach the floorplate, Robo3.1 blocks Robo1 on the growth cone, and thereby blocks axon repulsion mediated by Slit proteins expressed at the floorplate, allowing axons to cross the midline. After midline crossing, Robo3.2 is expressed instead of Robo3.1. As a result, Robo1, Robo2, and Robo3.2 detect the Slit proteins, which repel the spinal commissural axons away from the floorplate. Netrin-1/DCC attraction is silenced by Robo1 to prevent recrossing of the spinal commissural axons. (Robo1* indicates inactivation by Robo3.1; DCC* indicates DCC silencing). (B) Callosal axons in the embryonic mouse brain originate from layer V pyramidal neurons in the cerebral cortex and project through the corpus callosum to connect both hemispheres. At the midline, callosal axons interact with axon guidance proteins that are expressed on the medial zipper glia and guidepost cells that bridge the two hemispheres. Neuronal guidepost cells located in the developing corpus callosum express Sema3C, which attracts callosal axons that express the Neuropilin-1 (Nrp1) receptor to the midline. Netrin-1–DCC signalling also attracts callosal axons to the midline. Axons are repelled and funnelled through the developing corpus callosum by glial guidepost cells in the indusium griseum and glial wedge that express Slits 1–3 and Draxin, and their receptors (Robo 1 and 2, and DCC, respectively). SCA=spinal commissural axons. FP=ventral floor plate. CA=callosal axons. IG=indusium griseum. CC=corpus callosum. GW=glial wedge. MZG=medial zipper glia.

movements in three unrelated families (table). Many of these mutations are predicted to produce truncated DCC proteins that do not have binding sites for Netrin-1 or do not have the intracellular domains needed for signalling.25,26 However, whether these truncated proteins are produced at all and whether they reach the plasma membrane still needs to be established. The ability of Netrin-1 and DCC to attract axons to the nervous system midline, together with the abnormal development of commissures in mice without DCC,72 support the hypothesis that in patients with congenital mirror movements who carry DCC mutations commissural axons do not reach and cross the midline because of reduced midline attraction. The finding that several other families and sporadic patients with congenital mirror movements do not have DCC mutations indicates that this is a genetically heterogeneous disorder.25,26,73 Axon guidance proteins and receptors with roles in midline guidance, other than DCC and ROBO3, such as Netrin-1, Slits, and Semaphorins, are potential candidate proteins that should be investigated. Further work is also needed to identify additional mutations in patients with

congenital mirror movements and to identify the molecular mechanisms through which DCC mutations cause these disorders.

Horizontal gaze palsy with progressive scoliosis Horizontal gaze palsy with progressive scoliosis is a rare disorder that presents with congenital absence of horizontal eye movement and scoliosis that develops during the first decade of life. It is an autosomal-recessive disorder that runs in around ten families worldwide and is caused by nonsense, frame-shift, splice-site, or missense mutations in ROBO3 (table).27 Electrophysiological and imaging studies show the absence of decussating axons in the pons and medulla. Uncrossed oculomotor and abducens intranuclear tracts are thought to account for the horizontal gaze.4 The cause of the scoliosis is not known but could be the result of asynchronous breathing.3,4 The heterogeneous nature of the mutations and their presence throughout the ROBO3 gene indicates a loss of ROBO3 function in patients. Two ROBO3 splice variants exist: ROBO3.1 and ROBO3.2. The ROBO3 mutations reported in horizontal

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Study type

Disease mechanism linked to defect in axon guidance gene*

GWAS; affected-only linkage analysis in 11 patients of two unrelated families;25 linkage analysis in 13 patients from three unrelated families26

Abnormal development and crossing of corpus callosum and corticospinal tracts

Genetic disorders of midline crossing Congenital mirror movements DCC

Horizontal gaze palsy with progressive scoliosis ROBO3

Linkage analysis in 25 patients from eight unrelated families;27 DNA sequence Abnormal decussation of axons in the pons and medulla analysis of two patients from two unrelated familes;28 DNA sequence analysis of seven patients from five unrelated families;29 linkage analysis in ten patients from four unrelated families30

Dyslexia ROBO1

Case report;31 polymorphism screening in 19 patients from one family;31 functional magneto-encephalography study;32 association study in cohort of 1177 individuals of 538 families;33 association study in 804 patients of 493 families (1727 total sample);34 and association study in 1416 individuals from 421 families and association study in 749 individuals from 101 families35

A specific haplotype31,32 decreases Robo1 protein expression, which leads to crossing defects in the auditory pathway

SEMA4F

Association study in 180 twins and their parents36

Possibly defects in visual pathways

SEMA3A

Comparative genomic hybridisation array in 386 unrelated patients;37 case report, 1 patient selected from 48 patients with no mutations in known responsible genes;38 Sanger sequencing, SNP analysis in 66 unrelated patients39

Failure of GnRH neurons to migrate to the hypothalamus during development

SEMA7A

Sanger sequencing, SNP analysis in 66 unrelated patients39

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Genetic disorders of neuron migration Kallmann’s syndrome

Hirschsprung’s disease SEMA3A

SNP analysis in 119 patients and 93 controls;40 mutational screening in 200 patients Defects in enteric neural precursor migration and 200 controls41

SEMA3D

Mutational screening in 200 patients and 200 controls41

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NRP2

GWAS in 169 Chinese Han family trios42

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ROBO3; ROBO4

GWAS, a trio association study using DNA from 252 families recruited to AGRE43

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SEMA4D; ROBO2; SEMA5A; SEMA6A; RGMA; UNC5D; PLXNC1; NRP2

Family-based autism GWAS of 597 families and a second existing autism GWAS of 696 families from AGRE44

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PLXNA4; ROBO2

Expression study, samples from anterior cingulate cortex of eight patients and 13 controls45

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SEMA5A

Expression study (mRNA from Epstein–Barr virus transformed B lymphocytes from six unrelated patients and six controls)46

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EPHA7; PLXNB2; SEMA3A; SEMA5A; SEMA3D; SEMA7A

Genome-wide copy number variation in 286 patients coming from four distinct populations of patients with autism47

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SEMA3A

Expression study (entorhinal cortex) in rats with electrically induced epileptogenesis48

Mossy fibre sprouting into molecular layer of the dentate gyrus

EFNA; EPHA

Functional study (kindling rat model)49

Application of Ephrin A and EphA agonists and antagonists modulates mossy fibre sprouting and seizure development

EFNA4; EPHA10

Expression study (hippocampus) in mouse model of temporal lobe epilepsy by intraperitoneal administration of pilocarpine50

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SEMA3C; SEMA3F

Expression study (hippocampus) in 30 rats with kainic-acid-induced status epilepticus51

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SLIT2

Expression study in temporal neocortex of 35 epileptic patients and 15 controls, and 54 rats (control or in which epilepsy was induced by lithium-pilocarpine administration)52

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EPHA3

Genome-wide copy number variation in 117 Turkish patients with ALS and 109 matched healthy controls53

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EPHA4

Expression study in human blood (120 patients and 58 controls);54 DNA sequencing of a patient with sporadic ALS;54 functional study in animal models: mutant SOD1 zebrafish, hSOD-G93A rats and hSOD1-G93A mice54

EPHA4 mutation is significantly protective for ALS, and leads to a long survival; genetic as well as pharmacological inhibition of Epha4 signalling rescues the mutant SOD1 phenotype in zebrafish and increases survival in models of ALS

Autism spectrum disorders and epilepsy Autism spectrum disorders

Epilepsy

Neurodegenerative diseases ALS

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Study type

Disease mechanism linked to defect in axon guidance gene*

SEMA3A

Expression study (neuromuscular junction of hSOD-G93A mouse model55)

Axon retraction or inhibition of axonal sprouting at neuromuscular junction

SEMA6A

GWAS on 250 patients with sporadic ALS and 250 controls from Chinese Han population56

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DCC; EPHB1

GWAS on 2 groups (443 sibling pairs discordant for Parkinson’s disease; 332 matched case-unrelated control pairs) of USA or European origin57

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SEMA5A

GWAS on two groups (443 sibling pairs discordant for Parkinson’s disease; 332 matched case-unrelated control pairs) of USA or European origin;58 metaanalysis of 12 studies in different populations (3539 cases and 3250 controls)59

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UNC5C

GWAS on two groups (443 sibling pairs discordant for Parkinson’s disease; 332 matched case-unrelated control pairs) of USA or European origin60

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DCC; EPHA4; EPHA7; EPHB1; EPHB2; PLNXA2; ROBO3; SEMA5A; SLIT3

Highest ranked SNPs in genomic pathway analyses (meta-analysis of multiple studies)60,61

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RGMA; ROBO2; SEMA5A

Expression study (substantia nigra of four patients with Parkinson’s disease vs controls)62

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EPHA4

Expression study in post-mortem hippocampal tissue from three patients and three controls;63 expression study in hippocampus from Alzheimer’s disease mouse model overexpressing human APP with familial swe/ind mutations;63 functional study in APPswe/PSEN1ΔE9 transgenic mice64

Binding of Aβ to EPHA4 induces synaptic dysfunction or synaptic loss

NETRIN-1

Functional study in APP mutant mice in a mixed background of C57BL6, 129SvEv, and 129 Ola65

Netrin-1 binds APP and regulates cleavage of Aβ from APP

EPHB2

Functional study in hAPP line J20 mouse model66

Increasing EphB2 levels or function could be beneficial in Alzheimer’s disease

CRMP2; SEMA3A; PLXNA1; PLXNA2

Multiprotein complex from the hippocampus of patients with Alzheimer’s disease67,68

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EPHA4; EPHA5; EPHA6; ROBO2; SEMA3C; SEMA3E; SEMA4A; SEMA6B; SLIT1; SLIT3; UNC5A

Expression study (posterior cingulate area of seven patients with sporadic early onset Alzheimer’s disease, seven patients with familial monogenic PSEN1 Alzheimer’s disease, and seven controls)69

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EPHA1

GWAS in 6992 patients and 13 472 controls (meta-analysis)70,71

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(Continued from previous page)

Parkinson’s disease

Alzheimer’s disease

For more information see appendix. GnRH=gonadotropin-releasing hormone. GWAS=genome-wide association study. Aβ=amyloid β. APP=amyloid precursor protein. AGRE=Autism Genetic Resource Exchange. ALS=amyotrophic lateral sclerosis. swe/ind=Swedish/Indiana. *Disease mechanisms proposed on basis of combined data from human genetics, imaging, expression, and animal studies.

Table: Canonical axon guidance proteins in neurological disease

gaze palsy with progressive scoliosis affect both splice variants, and midline crossing defects are probably caused by the inability of axons to block their axonal Slit responses, preventing axons from crossing the midline. Recently, conditional knockout mice have been generated to selectively ablate Robo3 expression in mouse hindbrain neurons.74 These mice will be instrumental to establish the causal mechanisms of horizontal gaze palsy with progressive scoliosis.

Dyslexia Dyslexia has a prevalence of about 5% of school-age children. Twin and family studies show a strong genetic basis for dyslexia, and several genomic loci harbour candidate dyslexia susceptibility genes. Several of these genes encode for axon guidance proteins or receptors, including SEMA4F and ROBO1 (table).75 In a Finnish family, a rare, weakly expressing haplotype of ROBO1 was reported to cosegregate with dyslexia in a dominant manner.31 Dyslexia associates with various auditory disorders, and electrophysiological assessment of the Finnish

family revealed crossing defects in the auditory pathway in family members with dyslexia.32 This, together with the observation that Robo1 knockout mice show various midline crossing defects,76 suggests that normal crossing of auditory pathways in human beings might need ROBO1 expression. However, no differences were shown in ROBO1 expression between healthy individuals versus dyslexic individuals and other studies could not replicate the association between ROBO1 and dyslexia in human beings (table).31,77 Therefore, further studies are needed to assess whether or not dysregulation of ROBO1 contributes to the pathogenesis of dyslexia.

See Online for appendix

Genetic disorders of neuron migration Axon guidance proteins in cell migration Axon guidance proteins not only guide growing axons but also guide migrating cells (figure 1B). Newly differentiated neurons and glial cells use axon guidance proteins as directional cues in the developing nervous system.78 Members of all five families of canonical axon guidance proteins have been implicated in neural

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migration, and changes in their expression or function are linked to disorders characterised by aberrant cell migration.

Kallmann’s syndrome Kallmann’s syndrome affects about one in 8000 men and one in 40 000 women, and is characterised by congenital anosmia and hypogonadotropic hypogonadism. In patients with Kallmann’s syndrome, neurons that produce gonadotropin-releasing hormone (GnRH) do not reach the A GnRH neuron migration

AO AOB OB GnRH neuron OB OE OB SEMA3A mutation Sema7A Sema777A

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Sema3A VNO

hypothalamus, resulting in an absence of sexual development during puberty. Mutations in several different genes have been linked to Kallmann’s syndrome; many of these genes are implicated in axon growth and guidance.79 During embryonic development, GnRH neurons use olfactory axons as a scaffold to reach the hypothalamus (figure 3A). It is proposed that in a subset of patients with Kallmann’s syndrome, defects in olfactory axon guidance cause misrouting of GnRH neurons. Several loss-offunction mutations in SEMA3A and some rare mutations in SEMA7A have been noted in studies of patients with Kallmann’s syndrome (table).37,38 Knockout mice provide valuable insight into the possible disease process in patients with Kallmann’s syndrome who carry these mutations. Knockout mice for Sema3A or functional Sema3A receptors have a Kallmann’s syndrome-like phenotype, with olfactory axon guidance defects, aberrant GnRH neuron migration, and reduced fertility.37,80 In these mice, olfactory axon misrouting directs GnRH neurons to regions of the brain other than the hypothalamus (figure 3A).37,80,81 By contrast, Sema7A is not needed for the guidance of olfactory axons but does act as a molecular signal on olfactory axons to aid GnRH neuron migration. In the absence of Sema7A, GnRH neurons detach from olfactory axons and fail to migrate into the brain (figure 3A).82 Although further work is needed to establish how mutations in SEMA3A and SEMA7A linked to

SEMA7A mutation

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Figure 3: Axon guidance proteins and human genetic disorders of neuron migration (A) Head of a mouse embryo. The secreted semaphorin Sema3A (blue gradient) is expressed along the trajectory of olfactory axons from the olfactory epithelium, which project to the OB and the VNO, which project to the AOB. The membranebound semaphorin Sema7A is expressed on the olfactory axons. GnRH neurons (in green) in the developing olfactory system migrate along olfactory axons to the OB and then along central VNO to the hypothalamus. Mutations in SEMA3A are proposed to lead to defects in olfactory axon guidance. As a result, GnRH neurons stall in the nasal compartment or follow misrouted olfactory axons into abnormal regions of the brain. Sema7A is required on olfactory axons to promote the migration of GnRH neurons. SEMA7A mutations can cause GnRH neurons to detach from olfactory axons and accumulate in the nasal compartment. (B) During the development of the mouse enteric nervous system, enteric neural precursor cells (green) are derived from either the vagal neural crest or the sacral neural crest. Enteric neural precursor cells derived from vagal neural crest cells exit the hindbrain area and migrate to the developing intestinal tract. Once at the intestinal tract, these cells express the RET receptor, which promotes proliferation, migration, and differentiation. The gut mesoderm expresses GDNF, which is a substrate of RET coreceptor GFRα1. RET signalling drives the innervation of the intestinal tract by vagal neural crest-derived cells in a rostral to caudal direction. Sema3A is expressed in the distal gut (blue). Initially, repulsion of enteric neural precursor cells derived from S-NCC by Sema3A prevents entry of these cells into the intestinal tract. After a delay until later development, Sema3A expression is reduced, allowing the innervation of the gut by the sacral neural crest-derived cells in a caudal to rostral direction. Mutations in RET cause Hirschsprung’s disease. RET deficiency results in decreased innervation of the gut by both populations of enteric neural precursor cells, leading to partial enteric agangliosis. A subset of patients with Hirschsprung’s disease have SEMA3A mutations and display enhanced expression of SEMA3A (blue) in the gut. This prevents or delays the entry of sacral neural crest-derived enteric neural precursor cells into the intestinal tract. OE=olfactory epithelium. OB=olfactory bulb. VNO=vomeronasal axons. AOB=accessory olfactory bulb. GnRH=Gonadotropin-releasing hormone. GDNF=glial derived neurotrophic factor. V-NCC= vagal neural crest cells. S-NCC=sacral neural crest cells.

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Kallmann’s syndrome affect the function of these proteins, these results support an important role for Semaphorins in the pathogenesis of Kallmann’s syndrome. So far, disease-causing mutations have been identified in roughly 30% of patients with Kallmann’s syndrome, but more genetic studies are needed. Several other axon guidance proteins have been implicated in GnRH neuron migration—eg, Slit2 and Sema4D—and are candidate proteins for Kallmann’s syndrome.83,84

Hirschsprung’s disease Hirschsprung’s disease is characterised by a congenital absence of ganglia containing enteric neurons along variable lengths of the distal intestinal tract. This causes tonic contraction, intestinal obstruction, and bowel distention.85 Agangliosis is attributed to a defect in the time-dependent migration of neural-crest-cell-derived enteric neural precursors to the intestinal tract. Hirschsprung’s disease is rare and affects about 0·01% of the European population.86 Most cases of Hirschsprung’s disease are caused by heterogeneous mutations in RET, a proto-oncogene that causes total intestinal agangliosis when mutated in mice (figure 3B).87 SEMA3A and SEMA3D mutations have been identified in some families with Hirschsprung’s disease and a subset of patients show increased SEMA3A expression in affected colon tissue (table).41,88 Normally, SEMA3A is expressed in the mesenchyme of the distal intestine and acts as a repulsive signal for migrating enteric neural precursor cells from the sacral neural crest. In Sema3Aknockout mice, enteric neural precursor cells from the sacral neural crest enter the distal gut prematurely.89 Based on these findings, researchers have proposed that enhanced expression of SEMA3A in patients with Hirschsprung’s disease inhibits the migration of sacral neural crest enteric neural precursor cells into the developing distal intestine (figure 3B). Several patients with SEMA3A mutations have cosegregating RET mutations and it has been suggested that both mutations work synergistically.89 Both populations of enteric neural precursor cells might be affected in patients with the two mutations, compared with single mutation carriers, leading to a more severe disease phenotype.

Autism spectrum disorders Neurodevelopmental disorders such as autism spectrum disorders and attention-deficit hyperactivity disorder have an onset during early childhood and are caused by largely uncharacterised defects in neural development. Genetic evidence that implicates defective axon guidance protein function in neurodevelopmental disease is still sparse, but at the moment is strongest for autism spectrum disorders. Patients with autism spectrum disorders show impaired social and communicative skills and repetitive behaviours. Genetic risk contributes to the disorders, but the underlying genetic alterations remain unknown in

most cases.90 However, some human genetic, imaging, and neuropathological studies have identified connectivity as a converging neural substrate. It is generally believed that autism spectrum disorders are caused by a decrease in connectivity between specific brain regions (hypoconnectivity) together with an increase in connectivity within regions (hyperconnectivity). The hyperconnectivity phenotype includes an increase in the number of neurons, dendritic branches, and spines, whereas reduced connectivity between brain regions might arise from defective axon growth and guidance (figure 4A).5–7 These phenotypes could arise from defects in axon guidance proteins, and human genetic and tissue profiling studies have begun to identify axon guidance genes as susceptibility factors to autism spectrum disorders (table). In addition, expression of axon guidance proteins and receptors is decreased in post-mortem brain tissue or blood in patients with autism (table). However, functional evidence to link these findings to autism spectrum disorders is largely absent.

Epilepsy Genetic risk factors have an important role in epilepsy, but evidence to support a causal role for genetic defects in axon guidance genes is lacking. Rather, the strongest support for a role for axon guidance proteins in epilepsy is from animal studies. Seizure-induced changes in expression of axon guidance proteins have been shown in experimental models of epilepsy (table)50–52,91 and could cause the rewiring of neuronal circuits during later stages of epilepsy, contributing to its progressive and chronic nature. In the epileptic hippocampus, seizures lead to a progressive remodelling of the axons of dentate gyrus granule cells, known as mossy fibres. Normally mossy fibres extend towards the CA3 region and form synapses on the dendrites of CA3 pyramidal neurons (figure 4B). However, as a consequence of recurrent seizures, dentate gyrus granule cells can generate new mossy fibre collaterals, which aberrantly innervate the molecular layer and synapse with their own dendrites (figure 4B). This change leads to a recurrent excitatory circuit that causes the spontaneous recurrence of focal seizures and propagation of an epileptic state.92 Semaphorins have been linked to this rewiring event in rodent models of epilepsy.48,51 Sema3A is a repellent for mossy fibres and is normally delivered from the axons of neurons in the entorhinal cortex to the outer part of the molecular layer. In a rat model for temporal lobe epilepsy, Sema3A is downregulated in neurons in the entorhinal cortex, reducing the repulsive nature of the molecular layer. This reduction might allow ingrowth of mossy fibre collaterals and the formation of aberrant synapses in a recurrent excitatory circuit (figure 4B).48 Whether analogous expression changes occur in temporal lobe epilepsy in human beings and whether in-vivo manipulation of Sema3A function can alter mossy fibre sprouting await further study. The EphrinA5–EphA5 ligand–receptor

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system might also contribute to functional mossy fibre rewiring, because infusion of EphrinA and EphA antagonists modulates the extent of mossy fibre sprouting and seizure activity, although the mechanism is unknown.49 Furthermore, the effect of regulation of axon guidance proteins in network remodelling in epilepsy might not be limited to mossy fibre sprouting; several other regions, such as CA1 and CA3, of the rodent and human hippocampus display seizure-induced changes in expression of axon guidance proteins (table). Epilepsy is not only characterised by marked changes in neuronal connectivity but also by the abnormal migration and location of hippocampal neurons.93 Given the role of axon

guidance proteins in cell migration, it is tempting to speculate that dysregulation of axon guidance proteins could contribute to epilepsy-associated migration defects.

Neurodegenerative diseases Neuronal connectivity and neurodegenerative diseases Accumulating evidence indicates that changes in neuronal connectivity are not only a result of the neurodegenerative disease process but could also have a causative role in disease susceptibility, onset, or progression. For example, in many neurodegenerative diseases morphological and functional changes in axons and synapses precede symptom onset.94,95 Dendritic spine

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Figure 4: Axon guidance proteins in autism spectrum disorders and epilepsy (A) Autism spectrum disorders are characterised by a reduction in axonal connections between specific brain regions and an increase in connectivity within brain regions, including an increase in the number of neurons, dendritic branches, and spines. (B) Adult mouse hippocampus. DG granule cells (green) in the hippocampus normally project mossy fibre axons towards CA3, making synaptic contacts (yellow) with the dendrites of pyramidal cells (blue). Neurons in the entorhinal cortex innervate the outer part of the molecular layer and secrete Sema3A, which repels mossy fibres in the molecular layer through interaction with Neuropilin 1 (Nrp1), and possibly Plexin A4. In experimental models of temporal lobe epilepsy, Sema3A expression is downregulated in EC neurons and the molecular layer. This could allow newly formed mossy fibre collaterals (red), generated as a result of cell death or seizure activity, to invade the molecular layer and form aberrant synapses on DG granule cells. A reduction in the expression of unknown axon guidance proteins in CA3 is also proposed to contribute to the formation of aberrant mossy fibre synapses in this region of the hippocampus. EC=entorhinal cortex. DG=dentate gyrus.

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Genome-wide association, microarray profiling, and proteomics studies link axon guidance proteins to neurodegenerative diseases (table). Given their role in maintenance of axonal and synaptic connections in human adults,96,97 changes in axon guidance proteins could possibly trigger defects in neuronal networks, such as synaptic changes, which lead to neuronal dysfunction and loss in patients with neurodegenerative disease.

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Amyotrophic lateral sclerosis Although progress has been made to determine the genetic basis of amyotrophic lateral sclerosis, the mechanism of how genetic changes cause the disease is incompletely understood.98 In amyotrophic lateral sclerosis, the most distant cellular sites of the motor neuron (ie, distal axons and synapses) are affected first during early stages of the disease.94 It is therefore thought that subtle changes in neuronal connections might be causally related to the disease process, perhaps in conjunction with other causal or susceptibility factors. Several pieces of experimental evidence support the idea that dysregulation of axon guidance proteins might trigger the denervation of neuromuscular junctions, the synapses connecting motor neuron axons to muscle fibres (table). SOD1G93A mice, a commonly used model for amyotrophic lateral sclerosis, have a marked increase in the expression of Sema3A in terminal Schwann cells, which surround neuromuscular junctions (figure 5A). This increase is greatest for terminal Schwann cells surrounding motor nerve terminals (axons innervating fast-fatigable or type IIb/IIx muscle fibres), which have a low intrinsic ability for compensatory nerve sprouting after injury and that are lost first in SOD1G93A animal model.55 Researchers have therefore proposed that increased expression of Sema3A inhibits motor axon sprouting or triggers de-adhesion or repulsion of motor axons at the neuromuscular junctions, leading to muscle denervation and motor neuron degeneration. This model of denervation induced by axon guidance proteins is also supported by work on EPHA4.54 Gene knockdown and pharmacological inhibition of EphA4 rescue the axon outgrowth and branching defects observed in mutant SOD1 transgenic zebrafish. Similarly, genetic deletion of one EphA4 allele in SOD1G93A mice reduces muscle denervation and motor neuron loss, and significantly prolongs survival. Knockdown of EphA4 also rescues axonopathies induced by expression of TDP43, another protein that can cause amyotrophic lateral sclerosis, and by knockdown of SMN1 (survival motor neuron 1), the causative gene for spinal muscular atrophy. Thus, EPHA4 might be a generic therapeutic target in several motor neuron diseases. In patients with amyotrophic lateral sclerosis, EPHA4 expression is inversely correlated with age at disease onset and survival; loss-of-function mutations in EPHA4 are associated with longer survival.54 Motor neurons that are most vulnerable to degeneration (innervating fast-

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Figure 5: Axon guidance proteins in neurodegenerative disease (A) Axons of motor neurons form specialised synapses, called neuromuscular junctions, on muscle fibres. These synapses are surrounded by terminal Schwann cells (left panel). In the right panel, G93A-hSOD1 mice, a mouse model for amyotrophic lateral sclerosis, have a marked increase in the expression of the motor axon repellent Sema3A in terminal Schwann cells. In addition, expression of the neurite growth inhibitor Nogo-A is increased in skeletal muscle and concentrations of EphA4, a receptor for the repulsive ephrinA protein family, are increased in experimental models of and in patients with amyotrophic lateral sclerosis. These molecular events are proposed to participate in the establishment of a repulsive environment at the neuromuscular junction, causing axon retraction or impaired axonal sprouting. (B) Collapsin response mediator protein (CRMP)2 is a cytosolic protein that binds Tubulin dimers, and promotes microtubule polymerisation and consequently axon elongation. Following binding of Sema3A to its Nrp1/PlexinA receptor, CRMP2 is recruited to the PlexinA receptor and becomes sequentially phosphorylated at Ser522 and Thr509 by Cdk5 and GSK-3β kinases, respectively. The phosphorylated form of CRMP2 shows a reduced association with Tubulin. Therefore, microtubule polymerisation is inhibited upon Sema3A exposure, inducing growth cone collapse and axon repulsion. In Alzheimer’s disease, CRMP2 is hyperphosphorylated. This might induce enhanced repulsive signaling by Sema3A and retraction of synaptic contacts and neuron loss. Aβ phosphorylates CRMP2 in a RhoA GTPase-dependent manner. Netrin-1 binds APP and might inhibit the cleavage of APP into Aβ. Reduced expression of Netrin-1 leads to increased Aβ levels and an exacerbation of Alzheimer’s-disease-like phenotypes in mouse models. Aβ=amyloid β. APP=Amyloid precursor protein.

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fatigable or type IIb/IIx muscle fibers)and have low intrinsic ability for disease-induced neuromuscular reinnervation express higher levels of EphA4 in SOD1G93A transgenic mice. These observations suggest that high concentrations of axonal EphA4 might inhibit motor axon sprouting or cause deadhesion of neuromuscular junctions through interactions with repulsive EphA4 ligands, such as the EphrinA proteins, in the neuromuscular junctions or muscle (figure 5A). The neurite growth inhibitory molecule Nogo-A has been suggested to also have axon growth inhibitory effects at the neuromuscular junction. Patients with amyotrophic lateral sclerosis and SOD1 transgenic mice show an upregulation of Nogo-A in skeletal muscles.99 Although further work is needed to establish how changes in Nogo-A expression contribute to the pathogenesis of amyotrophic lateral sclerosis, investigators have shown that application of anti-Nogo-A antibodies delays disease progression in SOD1G93A transgenic mice.100 These observations have triggered the development of a humanised anti-NOGO-A antibody (ozanezumab), which is now being tested in a phase 2 clinical trial for application in patients with amyotrophic lateral sclerosis (NCT01753076). Collectively, these data support a model in which dysregulation of axon guidance proteins in amyotrophic lateral sclerosis contributes to motor axon retraction or impaired axonal sprouting, leading to muscle denervation and motor neuron death. Important goals for future research are to provide further functional data to support this model and to establish whether or not other axon guidance proteins associated with amyotrophic lateral sclerosis101–103 have similar roles in the disease process.

Parkinson’s disease Gene expression profiling in the substantia nigra pars compacta of patients with Parkinson’s disease or in experimental animal models of Parkinson’s disease shows differences in the expression of axon guidance proteins and receptors (table). In addition, genomic pathway analyses have identified axon guidance pathways in the aetiology of Parkinson’s disease (table). Although axon guidance proteins play a crucial part in the development of dopaminergic circuits,104 whether and how these expression and genetic changes are functionally linked to Parkinson’s disease needs to be established. Understanding of the potential role of these cues in Parkinson’s disease will need further analysis of their role in the dopaminergic system. This information could also help to improve cell replacement strategies in patients with Parkinson’s disease. The transplantation of healthy dopaminergic neurons into the brains of patients is a promising approach to alleviate the progressive symptoms of Parkinson’s disease.105 Transplanted dopaminergic neurons often establish insufficient and inappropriate axon connections leading to side-effects in patients. A better understanding of the 10

axon guidance mechanisms that normally wire the dopaminergic system could help molecular strategies to direct the axons of transplanted dopaminergic neurons to appropriate regions of the brain.

Alzheimer’s disease A recent whole-genome-expression profiling study in patients with familial and sporadic early-onset Alzheimer’s disease showed changes in the expression of several axon guidance proteins (table).69 Although this work implicates the axon guidance pathway in Alzheimer’s disease, early evidence of a pathological role for axon guidance proteins came from the isolation of a protein complex from the hippocampus of patients with Alzheimer’s disease that contained SEMA3A and collapsin response mediator protein (CRMP)2.67 CRMP2 is a microtubule-assembly factor downstream of Sema3A. Sequential phosphorylation of CRMP2 at ser522 and thr509 by the protein kinases Cdk5 and GSK-3β, respectively, reduces its interaction with tubulin and is required for axon repulsion induced by Sema3A (figure 5B). Neurofibrillary tangles contain a hyperphosphorylated form of CRMP2 that has increased phosphorylation of several aminoacids, including ser522 and thr509.106 This hyperphosphorylated state might possibly potentiate Sema3A signalling, leading to increased axon repulsion, retraction of synaptic contacts, and neuron loss. Amyloid β (Aβ) increases the phosphorylation of CRMP2 through a RhoA GTPasedependent mechanism.68 This system hints at cross-talk between Aβ and Sema3A signalling in Alzheimer’s disease. Axon guidance proteins can function both upstream and downstream of Aβ. Netrin-1 can bind the Aβ region of amyloid precursor protein and might negatively regulate cleavage of amyloid precursor protein.65 This is supported by the observation that models of Alzheimer’s disease with deletion of one Netrin-1 allele have significantly increased Aβ concentrations. Conversely, intracerebroventricular injection of Netrin-1 decreases Aβ concentrations in transgenic mice and improves memory function.65 Another interesting link between Netrin-1 and Alzheimer’s disease is the observation that processing of DCC by the protease Presenilin-1 is needed for Netrin-1-mediated axon guidance.107 Mutations in Presenilin-1 are a cause of familial Alzheimer’s disease. Therefore, in addition to contributing to defects in amyloid precursor protein processing, Presenilin-1 mutations could lead to changes in Netrin-1 signalling. In addition to Netrin-1, Aβ also interacts with EphB2 and EphA4. In mouse models of Alzheimer’s disease, Aβ directly binds EphB2, inducing the internalisation and degradation of the receptor. This causes a reduction in the cell surface expression of the NMDA glutamate receptor and leads to deficits in synaptic plasticity and cognitive function.66 Increases in EphB2 concentrations in the hippocampus of mice in models of Alzheimer’s

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disease reverses their cognitive and behavioural deficits associated with Alzheimer’s disease.63,65 Similarly, Aβ binds to and signals via EphA4 to induce synaptic damage. The synaptotoxic effects of Aβ oligomers can be reduced by in-vivo knockdown of EphA4 expression.64 These data indicate that Netrin-1 and Eph receptors could have promise as potential therapeutic targets to modulate the concentration and effects of Aβ in Alzheimer’s disease.

Conclusions and future directions Because of their prominent role in neural circuit development, investigators have long considered changes in the expression and function of axon guidance proteins as potential causative events in neurological diseases with a developmental origin. Indeed, genetic studies have identified specific mutations in axon guidance receptors that cause congenital disorders of axon guidance such as congenital mirror movements and horizontal gaze palsy with progressive scoliosis. So far only few disorders have been reported, and in most cases their molecular and cellular causal mechanisms remain poorly understood. The small number of disorders directly linked to mutations in axon guidance genes could be explained by the proteins’ crucial role in cellular processes such as heart development, angiogenesis, and bone formation. Mutations in some axon guidance genes might simply not be compatible with life. For congenital mirror movements and horizontal gaze palsy with progressive scoliosis, findings in mouse models have clearly established a causal role for mutations in DCC and ROBO3 receptor genes, respectively. This evidence base contrasts with that for most other neurological disorders, for which evidence for the involvement of specific axon guidance proteins is correlative and based on gene expression studies or genome-wide association studies. In these cases functional studies that use molecular approaches and genetic mouse models for disease-specific mutations, are needed to establish causality. However, even in diseases such as congenital mirror movements and horizontal gaze palsy with progressive scoliosis, further molecular and cellular studies are needed to establish the precise effects of the reported DCC and ROBO3 mutations. Often mutations occur throughout the entire protein and probably impair different properties of the protein such as ligand binding or dimerisation. Understanding of the mechanism of action of specific mutations will help to design targeted therapeutic strategies. Advances in genetic, neuroimaging, and electrophysiological techniques are needed to identify and characterise neural circuit malformations caused by defects in axon guidance proteins in undiagnosed neurological patients. This analysis can be guided by work in animal models. Anatomical analyses of mouse models with disease-associated mutations in, or knockout models for, specific axon guidance proteins will not only further understanding of the role of the axon guidance proteins in neurological disease but also present a means for the in-depth identification of axon tract and

Search strategy and selection criteria We searched PubMed with the terms “axon guidance and neurological disease”, by combining the term “axon guidance” with the various diseases covered in this Review (eg, Alzheimer’s disease), and by combining a specific class of axon guidance proteins (eg, semaphorin) with a specific disease. We searched for reports published from Jan 1, 1990 to May 1, 2014. We also manually searched the reference lists of published work to identify further relevant references. Only papers published in English were considered. The final reference list was generated on the basis of originality and relevance to the broad scope of this Review. When many papers addressed the same topic, one or more were cited as representative of the issue.

connectivity defects that characterise the diseases. These features can subsequently be used to screen populations of patients and identify those with mutations in similar or related genes. The feasibility of this approach can be inferred from midline guidance disorders such as congenital muscle movements or horizontal gaze palsy with progressive scoliosis for which patients with mutations in DCC and ROBO have similar midline guidance defects to mice with the equivalent genetic defects. The role of axon guidance proteins in multifactorial neurological diseases remains incompletely understood. In some diseases, such as epilepsy, axon guidance proteins might be dysregulated as a result of the disease process and contribute to disease progression, severity, or duration. In other complex diseases, such as autism spectrum disorders, defects in axon guidance genes might have a causative role but the functional contribution of changes in individual proteins is often small. For example, in patients with Parkinson’s disease, consistent replication of the associations between individual single nucleotide polymorphisms in axon guidance genes and disease onset, duration, or severity has been difficult to achieve. By contrast, genomic pathway analyses that take into account several different single nucleotide polymorphisms indicate a clear role for the axon guidance pathway. Therefore, a combination of several small effects, caused by variants in different axon guidance genes, is probably needed to reach a level of biological dysfunction that triggers disease. Many of the observations that link axon guidance proteins to neurological disease (eg, changes in gene expression) are still correlative and should be interpreted with caution. They might simply be changes that are a result of the disease or just markers of the disease and but not contribute to its aetiology. Further cellular and animal studies are needed to test the functional contribution of these variants. Recent progress in stem cell biology that enables the conversion of somatic cells, such as skin fibroblasts, into stem cells (induced pluripotent stem cells [iPSC]) and,

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subsequently, into differentiated neurons, is an invaluable method to further study the disease mechanisms by which defective axon guidance proteins cause neurological disease. For example, recent studies have used iPSC technology to transduce patient-derived fibroblasts into neurons to study synaptic defects associated with autism spectrum disorders.108,109 iPSC-derived neurons from patients with autism spectrum disorders are a promising means to assess the role of axon guidance proteins in the changes in neuronal morphology and connectivity that are found in patients. Furthermore, these cultures will not only help understand the cellular defects mediated by axon guidance protein dysfunction, but can also be used to functionally assess specific mutations at the biochemical and cellular levels. Although still in its infancy, the study of axon guidance proteins in neurological disease is an area of great potential. Although the pathogenic role of axon guidance proteins needs to be firmly established for most neurological disorders, in some disorders axon guidance proteins and their receptors have already been validated as therapeutic targets in experimental disease models. The signalling cues and cellular pathways that function downstream of axon guidance proteins and receptors could be used as targets for treatments. For example, investigators have postulated that defects in local protein synthesis contribute to the cause of neurodevelopmental disorders.110 mRNAs can be targeted to axons and dendrites, which enables rapid changes in the local proteome through local translation.111 Local translation has an important role in axon guidance, and axon guidance proteins such as Sema3A and Netrin-1 are known to regulate local protein synthesis to exert their effects on growing axons.111 Dysregulation of axon guidance proteins might lead to changed protein synthesis and thereby to disease. Another important downstream target of axon guidance proteins is the cytoskeleton. Several genes that encode proteins that regulate cytoskeletal dynamics have been linked to neurological disease. For example, mutations in CHN1, which encodes α2-Chimaerin, perturb axon guidance in the oculomotor system and lead to the eye movement disorder Duane retraction syndrome.112 With use of the CHN1 mutations as a starting point, cell culture and mouse studies have shown that the Semaphorins Sema3A and Sema3C act upstream of α2-Chimaerin and use this signalling cue to control the cytoskeleton and oculomotor axon guidance.112 This work not only identifies Sema3 signalling proteins as therapeutic targets in Duane retraction syndrome but also exemplifies how an intracellular signalling cue can be used to identify other downstream and upstream components of a specific disease pathway. In future studies, it will be interesting to establish whether the genes encoding Sema3s, their receptors, and downstream signalling cues carry mutations in patients with Duane retraction syndrome. 12

Another class of molecules that are promising therapeutic targets for disorders related to axon guidance proteins are microRNAs. For example, pathway analysis of the targets of microRNAs dysregulated in experimental temporal lobe epilepsy identifies the axon guidance pathway as one of the most frequently targeted pathways.113 This not only confirms the importance of axon guidance proteins in epilepsy but also raises the possibility that methods to manipulate microRNAs might be used to counteract epilepsy-associated pathological changes in neuronal connectivity caused by the abnormal expression of axon guidance proteins. Although much remains to be learned about the role and mechanism of action of axon guidance proteins in neurological disorders, prospects are good for the development of novel treatments by use of the axon guidance pathway as a therapeutic target. Contributors EVYB, SB and RJP reviewed the literature and wrote the manuscript. EVYB and SB prepared the figures and table, with input from RJP. Declaration of interests We declare no competing interests. Acknowledgments Our work was supported by the Dutch Epilepsy Fund, the Prinses Beatrix Fund, the International Parkinson Fund, the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme (FP7) 2007–2013 under REA grant agreement number 289581 (NPlast), and the FP7-HEALTH-2013-INNOVATION-1 Collaborative project Epi-miRNA. Our work was partly performed within the framework of CTMM (to RJP), the Center for Translational Molecular Medicine, project EMINENCE (01C-204). References 1 Kolodkin AL, Tessier-Lavigne M. Mechanisms and molecules of neuronal wiring: a primer. Cold Spring Harb Perspect Biol 2011; 3: pii: a001727 2 Pasterkamp RJ, Kolodkin AL. SnapShot: Axon Guidance. Cell 2013; 153: 494–494e1–2. 3 Izzi L, Charron F. Midline axon guidance and human genetic disorders. Clin Genet 2011; 80: 226–34. 4 Nugent AA, Kolpak AL, Engle EC. Human disorders of axon guidance. Curr Opin Neurobiol 2012; 22: 837–43. 5 Geschwind DH, Levitt P. Autism spectrum disorders: developmental disconnection syndromes. Curr Opin Neurobiol 2007; 17: 103–11. 6 Amaral DG, Schumann CM, Nordahl CW. Neuroanatomy of autism. Trends Neurosci 2008; 31: 137–45. 7 McFadden K, Minshew NJ. Evidence for dysregulation of axonal growth and guidance in the etiology of ASD. Front Hum Neurosci 2013; 7: 671. 8 Lin L, Lesnick TG, Maraganore DM, Isacson O. Axon guidance and synaptic maintenance: preclinical markers for neurodegenerative disease and therapeutics. Trends Neurosci 2009; 32: 142–49. 9 Schmidt ERE, Pasterkamp RJ, van den Berg LH. Axon guidance proteins: novel therapeutic targets for ALS? Prog Neurobiol 2009; 88: 286–301. 10 Kubo T, Tokita S, Yamashita T. Repulsive guidance molecule-A and demyelination: implications for multiple sclerosis. J Neuroimmune Pharmacol 2012; 7: 524–28. 11 Kumanogoh A, Kikutani H. Immunological functions of the neuropilins and plexins as receptors for semaphorins. Nat Rev Immunol 2013; 13: 802–14. 12 Pasterkamp RJ. Getting neural circuits into shape with semaphorins. Nat Rev Neurosci 2012; 13: 605–18. 13 Lai Wing Sun K, Correia JP, Kennedy TE. Netrins: versatile extracellular cues with diverse functions. Development 2011; 138: 2153–69.

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Ypsilanti AR, Zagar Y, Chédotal A. Moving away from the midline: new developments for Slit and Robo. Development 2010; 137: 1939–52. Severyn CJ, Shinde U, Rotwein P. Molecular biology, genetics and biochemistry of the repulsive guidance molecule family. Biochem J 2009; 422: 393–403. Klein R. Eph/ephrin signalling during development. Development 2012; 139: 4105–09. Avilés EC, Wilson NH, Stoeckli ET. Sonic hedgehog and Wnt: antagonists in morphogenesis but collaborators in axon guidance. Front Cell Neurosci 2013; 7: 86. Charron F, Tessier-Lavigne M. The Hedgehog, TGF-beta/BMP and Wnt families of morphogens in axon guidance. Adv Exp Med Biol 2007; 621: 116–33. Hollis ER 2nd, Zou Y. Expression of the Wnt signaling system in central nervous system axon guidance and regeneration. Front Mol Neurosci 2012; 5: 5. Jongbloets BC, Pasterkamp RJ. Semaphorin signalling during development. Development 2014; 141: 3292–97. Chédotal A. Further tales of the midline. Curr Opin Neurobiol 2011; 21: 68–75. Nawabi H, Castellani V. Axonal commissures in the central nervous system: how to cross the midline? Cell Mol Life Sci 2011; 68: 2539–53. Edwards TJ, Sherr EH, Barkovich AJ, Richards LJ. Clinical, genetic and imaging findings identify new causes for corpus callosum development syndromes. Brain 2014; 137: 1579–613. Engle EC. Human genetic disorders of axon guidance. Cold Spring Harb Perspect Biol 2010; 2: a001784. Srour M, Rivière J-B, Pham JMT, et al. Mutations in DCC cause congenital mirror movements. Science 2010; 328: 592. Depienne C, Cincotta M, Billot S, et al. A novel DCC mutation and genetic heterogeneity in congenital mirror movements. Neurology 2011; 76: 260–64. Jen JC, Chan W-M, Bosley TM, et al. Mutations in a human ROBO gene disrupt hindbrain axon pathway crossing and morphogenesis. Science 2004; 304: 1509–13. Chan W-M, Traboulsi EI, Arthur B, Friedman N, Andrews C, Engle EC. Horizontal gaze palsy with progressive scoliosis can result from compound heterozygous mutations in ROBO3. J Med Genet 2006; 43: e11. Abu-Amero KK, al Dhalaan H, al Zayed Z, Hellani A, Bosley TM. Five new consanguineous families with horizontal gaze palsy and progressive scoliosis and novel ROBO3 mutations. J Neurol Sci 2009; 276: 22–26. Amouri R, Nehdi H, Bouhlal Y, Kefi M, Larnaout A, Hentati F. Allelic ROBO3 heterogeneity in Tunisian patients with horizontal gaze palsy with progressive scoliosis. J Mol Neurosci 2009; 39: 337–41. Hannula-Jouppi K, Kaminen-Ahola N, Taipale M, et al. The axon guidance receptor gene ROBO1 is a candidate gene for developmental dyslexia. PLoS Genet 2005; 1: e50. Lamminmäki S, Massinen S, Nopola-Hemmi J, Kere J, Hari R. Human ROBO1 regulates interaural interaction in auditory pathways. J Neurosci 2012; 32: 966–71. Bates TC, Luciano M, Medland SE, Montgomery GW, Wright MJ, Martin NG. Genetic variance in a component of the language acquisition device: ROBO1 polymorphisms associated with phonological buffer deficits. Behav Genet 2011; 41: 50–57. Mascheretti S, Riva V, Giorda R, et al. KIAA0319 and ROBO1: evidence on association with reading and pleiotropic effects on language and mathematics abilities in developmental dyslexia. J Hum Genet 2014; 59: 189–97. Tran C, Wigg KG, Zhang K, et al. Association of the ROBO1 gene with reading disabilities in a family-based analysis. Genes Brain Behav 2014; 13: 430–38. Francks C, Fisher SE, Olson RK, et al. Fine mapping of the chromosome 2p12-16 dyslexia susceptibility locus: quantitative association analysis and positional candidate genes SEMA4F and OTX1. Psychiatr Genet 2002; 12: 35–41. Hanchate NK, Giacobini P, Lhuillier P, et al. SEMA3A, a gene involved in axonal pathfinding, is mutated in patients with Kallmann syndrome. PLoS Genet 2012; 8: e1002896. Young J, Metay C, Bouligand J, et al. SEMA3A deletion in a family with Kallmann syndrome validates the role of semaphorin 3A in human puberty and olfactory system development. Hum Reprod 2012; 27: 1460–65.

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Känsäkoski J, Fagerholm R, Laitinen E-M, et al. Mutation screening of SEMA3A and SEMA7A in patients with congenital hypogonadotropic hypogonadism. Pediatr Res 2014; 75: 641–44. DOI:10.1038/pr.2014.23. Wang L-L, Zhang Y, Fan Y, et al. SEMA3A rs7804122 polymorphism is associated with Hirschsprung disease in the Northeastern region of China. Birth Defects Res A Clin Mol Teratol 2012; 94: 91–95. Luzón-Toro B, Fernández RM, Torroglosa A, et al. Mutational spectrum of semaphorin 3A and semaphorin 3D genes in Spanish Hirschsprung patients. PLoS One 2013; 8: e54800. Wu S, Yue W, Jia M, et al. Association of the neuropilin-2 (NRP2) gene polymorphisms with autism in Chinese Han population. Am J Med Genet B Neuropsychiatr Genet 2007; 144B: 492–95. Anitha A, Nakamura K, Yamada K, et al. Genetic analyses of roundabout (ROBO) axon guidance receptors in autism. Am J Med Genet B Neuropsychiatr Genet 2008; 147B: 1019–27. Hussman JP, Chung R-H, Griswold AJ, et al. A noise-reduction GWAS analysis implicates altered regulation of neurite outgrowth and guidance in autism. Mol Autism 2011; 2: 1. Suda S, Iwata K, Shimmura C, et al. Decreased expression of axonguidance receptors in the anterior cingulate cortex in autism. Mol Autism 2011; 2: 14. Melin M, Carlsson B, Anckarsater H, et al. Constitutional downregulation of SEMA5A expression in autism. Neuropsychobiology 2006; 54: 64–69. Sbacchi S, Acquadro F, Calò I, Calì F, Romano V. Functional annotation of genes overlapping copy number variants in autistic patients: focus on axon pathfinding. Curr Genomics 2010; 11: 136–45. Holtmaat AJGD, Gorter JA, De Wit J, et al. Transient downregulation of Sema3A mRNA in a rat model for temporal lobe epilepsy. A novel molecular event potentially contributing to mossy fiber sprouting. Exp Neurol 2003; 182: 142–50. Xu B, Li S, Brown A, Gerlai R, Fahnestock M, Racine RJ. EphA/ ephrin-A interactions regulate epileptogenesis and activitydependent axonal sprouting in adult rats. Mol Cell Neurosci 2003; 24: 984–99. Xia Y, Luo C, Dai S, Yao D. Increased EphA/ephrinA expression in hippocampus of pilocarpine treated mouse. Epilepsy Res 2013; 105: 20–29. Barnes G, Puranam RS, Luo Y, McNamara JO. Temporal specific patterns of semaphorin gene expression in rat brain after kainic acid-induced status epilepticus. Hippocampus 2003; 13: 1–20. Fang M, Liu G-W, Pan Y-M, et al. Abnormal expression and spatiotemporal change of Slit2 in neurons and astrocytes in temporal lobe epileptic foci: a study of epileptic patients and experimental animals. Brain Res 2010; 1324: 14–23. Uyan Ö, Ömür Ö, Ağım ZS, et al. Genome-wide copy number variation in sporadic amyotrophic lateral sclerosis in the Turkish population: deletion of EPHA3 is a possible protective factor. PLoS One 2013; 8: e72381. Van Hoecke A, Schoonaert L, Lemmens R, et al. EPHA4 is a disease modifier of amyotrophic lateral sclerosis in animal models and in humans. Nat Med 2012; 18: 1418–22. De Winter F, Vo T, Stam FJ, et al. The expression of the chemorepellent Semaphorin 3A is selectively induced in terminal Schwann cells of a subset of neuromuscular synapses that display limited anatomical plasticity and enhanced vulnerability in motor neuron disease. Mol Cell Neurosci 2006; 32: 102–17. Xie T, Deng L, Mei P, et al. Genome-wide association study combining pathway analysis for typical sporadic amyotrophic lateral sclerosis in Chinese Han populations. Neurobiol Aging 2014; 35: e9, e23. Kim J-M, Park SK, Yang JJ, et al. SNPs in axon guidance pathway genes and susceptibility for Parkinson’s disease in the Korean population. J Hum Genet 2011; 56: 125–29. Maraganore DM, de Andrade M, Lesnick TG, et al. High-resolution whole-genome association study of Parkinson disease. Am J Hum Genet 2005; 77: 685–93. Yu X, Wang F, Zhang JP. Meta analysis of the association of rs7702187 SNP in SEMA5A gene with risk of Parkinson’s disease. Eur Rev Med Pharmacol Sci 2014; 18: 900–04. Lesnick TG, Papapetropoulos S, Mash DC, et al. A genomic pathway approach to a complex disease: axon guidance and Parkinson disease. PLoS Genet 2007; 3: e98.

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Review

61

62

63

64

65

66

67

68

69

70

71

72

73

74

75 76

77

78 79 80

81 82

83

14

Srinivasan BS, Doostzadeh J, Absalan F, et al. Whole genome survey of coding SNPs reveals a reproducible pathway determinant of Parkinson disease. Hum Mutat 2009; 30: 228–38. Bossers K, Meerhoff G, Balesar R, et al. Analysis of gene expression in Parkinson’s disease: possible involvement of neurotrophic support and axon guidance in dopaminergic cell death. Brain Pathol 2009; 19: 91–107. Simón AM, de Maturana RL, Ricobaraza A, et al. Early changes in hippocampal Eph receptors precede the onset of memory decline in mouse models of Alzheimer’s disease. J Alzheimers Dis 2009; 17: 773–86. Vargas LM, Leal N, Estrada LD, et al. EphA4 activation of c-Abl mediates synaptic loss and LTP blockade caused by amyloid-β oligomers. PLoS One 2014; 9: e92309. Lourenço FC, Galvan V, Fombonne J, et al. Netrin-1 interacts with amyloid precursor protein and regulates amyloid-beta production. Cell Death Differ 2009; 16: 655–63. Cissé M, Halabisky B, Harris J, et al. Reversing EphB2 depletion rescues cognitive functions in Alzheimer model. Nature 2011; 469: 47–52. Good PF, Alapat D, Hsu A, et al. A role for semaphorin 3A signaling in the degeneration of hippocampal neurons during Alzheimer’s disease. J Neurochem 2004; 91: 716–36. Petratos S, Li Q-X, George AJ, et al. The beta-amyloid protein of Alzheimer’s disease increases neuronal CRMP-2 phosphorylation by a Rho-GTP mechanism. Brain 2008; 131: 90–108. Antonell A, Lladó A, Altirriba J, et al. A preliminary study of the whole-genome expression profile of sporadic and monogenic earlyonset Alzheimer’s disease. Neurobiol Aging 2013; 34: 1772–78. Hollingworth P, Harold D, Sims R, et al, for the Alzheimer’s Disease Neuroimaging Initiative, the CHARGE consortium, and the EADI1 consortium. Common variants at ABCA7, MS4A6A/ MS4A4E, EPHA1, CD33 and CD2AP are associated with Alzheimer’s disease. Nat Genet 2011; 43: 429–35. Naj AC, Jun G, Beecham GW, et al. Common variants at MS4A4/ MS4A6E, CD2AP, CD33 and EPHA1 are associated with late-onset Alzheimer’s disease. Nat Genet 2011; 43: 436–41. Fazeli A, Dickinson SL, Hermiston ML, et al. Phenotype of mice lacking functional Deleted in colorectal cancer (Dcc) gene. Nature 1997; 386: 796–804. Djarmati-Westenberger A, Brüggemann N, Espay AJ, Bhatia KP, Klein C. A novel DCC mutation and genetic heterogeneity in congenital mirror movements. Neurology 2011; 77: 1580. Renier N, Schonewille M, Giraudet F, et al. Genetic dissection of the function of hindbrain axonal commissures. PLoS Biol 2010; 8: e1000325. Scerri TS, Schulte-Körne G. Genetics of developmental dyslexia. Eur Child Adolesc Psychiatry 2010; 19: 179–97. Andrews W, Liapi A, Plachez C, et al. Robo1 regulates the development of major axon tracts and interneuron migration in the forebrain. Development 2006; 133: 2243–52. Venkatesh SK, Siddaiah A, Padakannaya P, Ramachandra NB. Lack of association between genetic polymorphisms in ROBO1, MRPL19/C2ORF3 and THEM2 with developmental dyslexia. Gene 2013; 529: 215–19. Marín O, Valiente M, Ge X, Tsai LH. Guiding neuronal cell migrations. Cold Spring Harb Perspect Biol 2010; 2: a001834. Dodé C, Hardelin J-P. Clinical genetics of Kallmann syndrome. Ann Endocrinol 2010; 71: 149–57. Cariboni A, Davidson K, Rakic S, Maggi R, Parnavelas JG, Ruhrberg C. Defective gonadotropin-releasing hormone neuron migration in mice lacking SEMA3A signalling through NRP1 and NRP2: implications for the aetiology of hypogonadotropic hypogonadism. Hum Mol Genet 2011; 20: 336–44. Giacobini P, Prevot V. Semaphorins in the development, homeostasis and disease of hormone systems. Semin Cell Dev Biol 2013; 24: 190–98. Messina A, Ferraris N, Wray S, et al. Dysregulation of Semaphorin7A/β1-integrin signaling leads to defective GnRH-1 cell migration, abnormal gonadal development and altered fertility. Hum Mol Genet 2011; 20: 4759–74. Giacobini P, Messina A, Morello F, et al. Semaphorin 4D regulates gonadotropin hormone-releasing hormone-1 neuronal migration through PlexinB1-Met complex. J Cell Biol 2008; 183: 555–66.

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85

86

87

88

89

90

91 92

93 94

95

96

97 98 99

100

101

102

103

104 105 106

107

Cariboni A, Andrews WD, Memi F, et al. Slit2 and Robo3 modulate the migration of GnRH-secreting neurons. Development 2012; 139: 3326–31. Romeo G, Ronchetto P, Luo Y, et al. Point mutations affecting the tyrosine kinase domain of the RET proto-oncogene in Hirschsprung’s disease. Nature 1994; 367: 377–78. Best KE, Addor M-C, Arriola L, et al. Hirschsprung’s disease prevalence in Europe: A register based study. Birth Defects Res Part A Clin Mol Teratol 2014; 100: 695–702 Schuchardt A, D’Agati V, Larsson-Blomberg L, Costantini F, Pachnis V. Defects in the kidney and enteric nervous system of mice lacking the tyrosine kinase receptor Ret. Nature 1994; 367: 380–83. Wang L-L, Fan Y, Zhou F-H, et al. Semaphorin 3A expression in the colon of Hirschsprung disease. Birth Defects Res A Clin Mol Teratol 2011; 91: 842–47. Anderson RB, Bergner AJ, Taniguchi M, et al. Effects of different regions of the developing gut on the migration of enteric neural crest-derived cells: a role for Sema3A, but not Sema3F. Dev Biol 2007; 305: 287–99. Huguet G, Ey E, Bourgeron T. The genetic landscapes of autism spectrum disorders. Annu Rev Genomics Hum Genet 2013; 14: 191–213. Koyama R1, Ikegaya Y. Mossy fiber sprouting as a potential therapeutic target for epilepsy. Curr Neurovasc Res 2004; 1: 3–10. Babb TL, Pretorius JK, Mello LE, Mathern GW, Levesque MF. Synaptic reorganizations in epileptic human and rat kainate hippocampus may contribute to feedback and feedforward excitation. Epilepsy Res Suppl 1992; 9: 193–202. Houser CR. Granule cell dispersion in the dentate gyrus of humans with temporal lobe epilepsy. Brain Res 1990; 535: 195–204. Murray LM, Talbot K, Gillingwater TH. Review: neuromuscular synaptic vulnerability in motor neurone disease: amyotrophic lateral sclerosis and spinal muscular atrophy. Neuropathol Appl Neurobiol 2010; 36: 133–56. Saxena S, Caroni P. Selective neuronal vulnerability in neurodegenerative diseases: from stressor thresholds to degeneration. Neuron 2011; 71: 35–48. Lai KO, Ip NY. Synapse development and plasticity: roles of ephrin/Eph receptor signaling. Curr Opin Neurobiol 2009; 19: 275–83. Pasterkamp RJ, Giger RJ. Semaphorin function in neural plasticity and disease. Curr Opin Neurobiol 2009; 19: 263–74. Robberecht W, Philips T. The changing scene of amyotrophic lateral sclerosis. Nat Rev Neurosci 2013; 14: 248–64. Dupuis L, Gonzalez de Aguilar J-L, di Scala F, et al. Nogo provides a molecular marker for diagnosis of amyotrophic lateral sclerosis. Neurobiol Dis 2002; 10: 358–65. Bros-Facer V, Krull D, Taylor A, et al. Treatment with an antibody directed against Nogo-A delays disease progression in the SOD1G93A mouse model of Amyotrophic lateral sclerosis. Hum Mol Genet 2014; 23: 4187–200. Jiang Y-M, Yamamoto M, Kobayashi Y, et al. Gene expression profile of spinal motor neurons in sporadic amyotrophic lateral sclerosis. Ann Neurol 2005; 57: 236–51. Lesnick TG, Sorenson EJ, Ahlskog JE, et al. Beyond Parkinson disease: amyotrophic lateral sclerosis and the axon guidance pathway. PLoS One 2008; 3: e1449. Li J, Li T, Zhang X, Tang Y, Yang J, Le W. Human superoxide dismutase 1 overexpression in motor neurons of Caenorhabditis elegans causes axon guidance defect and neurodegeneration. Neurobiol Aging 2014; 35: 837–46. Van den Heuvel DMA, Pasterkamp RJ. Getting connected in the dopamine system. Prog Neurobiol 2008; 85: 75–93. Bjorklund A, Kordower JH. Cell therapy for Parkinson’s disease: what next? Mov Disord 2013; 28: 110–15. Soutar MPM, Thornhill P, Cole AR, Sutherland C. Increased CRMP2 phosphorylation is observed in Alzheimer’s disease; does this tell us anything about disease development? Curr Alzheimer Res 2009; 6: 269–78. Bai G, Chivatakarn O, Bonanomi D, et al. Presenilin-dependent receptor processing is required for axon guidance. Cell 2011; 144: 106–18.

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108 Marchetto MCN, Carromeu C, Acab A, et al. A model for neural development and treatment of Rett syndrome using human induced pluripotent stem cells. Cell 2010; 143: 527–39. 109 Prilutsky D, Palmer NP, Smedemark-Margulies N, Schlaeger TM, Margulies DM, Kohane IS. iPSC-derived neurons as a higherthroughput readout for autism: promises and pitfalls. Trends Mol Med 2014; 20: 91–104. 110 Zukin RS, Richter JD, Bagni C. Signals, synapses, and synthesis: how new proteins control plasticity. Front Neural Circuits 2009; 3: 14. 111 Jung H, Yoon BC, Holt CE. Axonal mRNA localization and local protein synthesis in nervous system assembly, maintenance and repair. Nat Rev Neurosci 2012; 13: 308–24.

112 Ferrario JE, Baskaran P, Clark C, et al. Axon guidance in the developing ocular motor system and Duane retraction syndrome depends on Semaphorin signaling via alpha2-chimaerin. Proc Natl Acad Sci USA 2012; 109: 14669–74. 113 Gorter JA, Iyer A, White I, et al. Hippocampal subregion-specific microRNA expression during epileptogenesis in experimental temporal lobe epilepsy. Neurobiol Dis 2014; 62: 508–20.

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Axon guidance proteins in neurological disorders.

Many neurological disorders are characterised by structural changes in neuronal connections, ranging from presymptomatic synaptic changes to the loss ...
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