BRAIN 2015: 138; 3132–3140
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SCIENTIFIC COMMENTARIES
Blocking bad This scientific commentary refers to ‘Targeting the colony stimulating factor 1 receptor alleviates two forms of Charcot–Marie–Tooth disease in mice’, by Klein et al. (doi:10.1093/brain/awv240).
femoral nerve. Unlike mice that genetically lack CSF 1, the CSF 1R antagonist did not result in fewer abnormally myelinated axons, but it still had positive effects on hindlimb grip strength, the amplitude of the compound muscle action potential of the intrinsic foot muscles, and the innervation of an intrinsic foot muscle (flexor digitorum brevis). These beneficial effects were associated with fewer pathological changes in axons and more axonal sprouts. The CSF 1R antagonist had similar effects in Mpz + / mice. It reduced the number of endoneurial macrophages, increased hindlimb grip strength, the amplitude of the compound muscle action potential of the intrinsic foot muscles, and the innervation of the flexor digitorum brevis. In addition, treatment reduced the number of abnormally myelinated axons. The effects in a Pmp22 overexpressing transgenic line, by contrast, were minimal. These findings are important because they show that the immune system contributes to the pathogenesis of some inherited demyelinating neuropathies. Why the CSF 1R inhibitor only helped some forms of CMT1 remains to be determined. Nevertheless, if CSF 1R inhibitors prove to be safe in humans, then one can envision a clinical trial to assess their efficacy in patients with CMT1X and perhaps the rare forms of CMT1B that are caused by loss-of-function mutations (Sanmaneechai et al., 2015). To be effective, it seems likely that the CSF 1R inhibitor will need to penetrate the blood–nerve (to reach the
ß The Author (2015). Published by Oxford University Press on behalf of the Guarantors of Brain. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://brain.oxfordjournals.org/ by guest on October 26, 2015
Axonal loss is chiefly responsible for deterioration of neurological function in chronic peripheral neuropathies. In most kinds of neuropathy, the axonal loss is conceptualized as a dying back of the longest axons, and is progressive over time. The same pattern is observed in inherited demyelinating neuropathies (Krajewski et al., 2000), even though the defective genes are expressed by myelinating Schwann cells; however, the underlying cause of the axonal loss is obscure (Nave, 2010). While the contribution of activated macrophages to demyelination and axonal loss was not anticipated, it nevertheless represents a therapeutic opportunity, as illustrated by Klein and co-workers in this issue of Brain (Klein et al., 2015). A well-known consequence of axonal loss (and demyelination) is a marked increase in the number of macrophages within peripheral nerves. In experimental models, these macrophages are derived from preexisting endoneurial macrophages and from the immigration of exogenous monocytes into the nerve (Ma¨urer et al., 2003). Until recently, their only well described role was the phagocytosis of degenerating axons, particularly their myelin sheaths. Work from Rudolf Martini’s laboratory, however, has revealed a detrimental role of macrophages in some animal
models of inherited demyelinating neuropathies. Their studies of genetically authentic mouse models of Charcot-Marie-Tooth disease 1B (CMT1B; created by a heterozygous loss-of-function mutation in Mpz; Mpz + / ) and CMT1X (Gjb1-null mice) have led to the proposal that macrophages mediate demyelination and axonal pathology. In both models, macrophages are physically associated with abnormally myelinated axons. More tellingly, Mpz + / and Gjb1-null mice with a genetically engineered deficiency in colony stimulating factor 1 (CSF 1) have far fewer macrophages and abnormally myelinated axons (Carenini et al., 2001; Groh et al., 2015). Because CSF 1 is a pro-inflammatory cytokine that activates macrophages, these findings indicate that there is a complicated pathway in peripheral nerve (Groh et al., 2012), involving fibroblasts (which express CSF 1) and macrophages (which express the CSF 1 receptor; CSF 1R), resulting in the activation of macrophages that, in turn, injure myelinating Schwann cells. In this issue of Brain, Martini’s group has made an important addition to this story (Klein et al., 2015). They treated three different mouse models of inherited demyelinating neuropathy (Mpz + / , Gjb1null, and a line of Pmp22 overexpressing mice; modelling CMT1A) with a pharmacological inhibitor of the CSF 1R. As expected, treating Gjb1-null mice for 3 or 6 months with this CSF 1R inhibitor reduced the number of macrophages in the
Scientific Commentaries
endoneurium), and even the blood– brain barrier (to reach the nerve roots). If reaching the nerve roots is not important, then one could avoid depleting the microglia in the CNS, which are also a target of the CSF 1R antagonists (Elmore et al., 2014). Steven S. Scherer, M.D., Ph.D. Perelman School of Medicine at the University of Pennsylvania, USA
E-mail: [email protected] doi:10.1093/brain/awv279
Carenini S, Ma¨urer M, Werner A,Blazyca H, Toyka KV, Schmid CD, et al. The role of macrophages in demyelinating peripheral
nervous system of mice heterozygously deficient in P0. J Cell Biol 2001; 152:301–8. Elmore MR, Najafi AR, Koike MA, Dagher NN, Spangenberg EE, Rice RA, et al. (2014). Colony-stimulating factor 1 receptor signaling is necessary for microglia viability, unmasking a microglia progenitor cell in the adult brain. Neuron 2014; 82:380–97. Groh J, Klein I, Hollmann C, Wettmarshausen J, Klein D, Martini R. CSF-1-activated macrophages are targetdirected and essential mediators of Schwann cell dedifferentiation and dysfunction in Cx32-deficient mice. Glia 2015; 977–986. Groh J, Weis J, Zieger H, Stanley ER, Heuer H, Martini R. Colony-stimulating factor-1 mediates macrophage-related neural damage in a model for Charcot-MarieTooth disease type 1X. Brain 2012; 135: 88–104. Klein D, Patzko A, Schreiber D, van Hauwermeiren A, Baier M, Groh J, et al. Targeting the colony-stimulating factor-1
| 3133
receptor alleviates two forms of CharcotMarie-Tooth disease in mice. Brain 2015; 138: 3193–205. Krajewski KM, Lewis RA, Fuerst DR, Turansky C, Hinderer SR, Garbern J, et al. Neurological dysfunction and axonal degeneration in Charcot-MarieTooth disease. Brain 2000: 123: 1516–27. Ma¨urer M, Muller M, Kobsar I, Leonhard C, Martini R, Kiefer R. Origin of pathogenic macrophages and endoneurial fibroblast-like cells in an animal model of inherited neuropathy. Mol Cell Neurosci 2003: 23: 351–9. Nave KA. Myelination and the trophic support of long axons. Nat Rev Neurosci 2010: 11: 275–83. Sanmaneechai O, Feely S, Scherer SS, Herrmann DN, Burns J, Muntoni F, et al. Genotype–phenotype characteristics and baseline natural history of heritable neuropathies caused by mutations in the MPZ gene. Brain 2015, in press.
Cerebral adrenoleukodystrophy: a demyelinating disease that leaves the door wide open This scientific commentary refers to ‘Brain endothelial dysfunction in cerebral adrenoleukodystrophy’ by Musolino et al. (doi: 10.1093/ awv250) Cerebral adrenoleukodystrophy (ALD) can be distinguished from other inherited diseases of white matter (known collectively as ‘leukodystrophies’) by the presence of severe neuroinflammation and blood–brain barrier disruption, evidenced in brain MRI by the leakage of gadolinium-diethylenetriamine pentaacetic acid (Gd-DPTA) at the periphery of demyelinating lesions on T1-weighted sequences. In contrast to multiple sclerosis, blood–brain barrier breakdown occurs downstream of the demyelinating process. Moreover, it is always followed by very rapid progression of demyelinating lesions, likely resulting from
myelin destruction by inflammatory cells. In this issue of Brain, Musolino et al. (2015) unravel for the first time some of the molecular mechanisms that underlie blood–brain barrier breakdown in ALD. Using ALD brain tissue, and human brain microvascular cells (HBMECs) in which the ABCD1 (ALD) gene was silenced via siRNA, they demonstrate an upregulation of intercellular adhesion molecule 1 (ICAM1) and vascular cell adhesion protein 1 (VCAM1) expression that reflected endothelial activation and that facilitated monocyte adhesion. They also show an increase in the expression of transforming growth factor b1 (TGF-b1), and in the number of vessels, probably due to production of vascular endothelial growth factor (VEGF) by astrocytes and/or microglia. Furthermore, they reveal an increase in the expression of matrix metallopeptidase 9 (MMP9), which
contributed to blood–brain barrier disruption, and in turn resulted in leakage of fibrinogen (an exclusively intravascular protein) into the perivascular space, recruitment of monocytes/ macrophages and the activation of microglia around vessels in active demyelinating areas. This correlated with the displacement of two members of the endothelial tight junction complex, claudin 5 (CLDN5) and ZO-1, from the membrane to the cytoplasm of endothelial cells: a clear indication that the blood–brain barrier had been breached, boosting the ingress of proinflammatory lymphocytes and monocytes into the white matter. Importantly, ABCD1 silencing in HBMECs also displaced CLDN5 to the endothelial cell cytoplasm, as in vivo. Studies in HBMECs provided two key additional pieces of information: (i) the silencing of ABCD1 resulted in transcriptional inhibition of CLDN5
Downloaded from http://brain.oxfordjournals.org/ by guest on October 26, 2015
References
BRAIN 2015: 138; 3132–3140
| 3132
SCIENTIFIC COMMENTARIES
Blocking bad This scientific commentary refers to ‘Targeting the colony stimulating factor 1 receptor alleviates two forms of Charcot–Marie–Tooth disease in mice’, by Klein et al. (doi:10.1093/brain/awv240).
femoral nerve. Unlike mice that genetically lack CSF 1, the CSF 1R antagonist did not result in fewer abnormally myelinated axons, but it still had positive effects on hindlimb grip strength, the amplitude of the compound muscle action potential of the intrinsic foot muscles, and the innervation of an intrinsic foot muscle (flexor digitorum brevis). These beneficial effects were associated with fewer pathological changes in axons and more axonal sprouts. The CSF 1R antagonist had similar effects in Mpz + / mice. It reduced the number of endoneurial macrophages, increased hindlimb grip strength, the amplitude of the compound muscle action potential of the intrinsic foot muscles, and the innervation of the flexor digitorum brevis. In addition, treatment reduced the number of abnormally myelinated axons. The effects in a Pmp22 overexpressing transgenic line, by contrast, were minimal. These findings are important because they show that the immune system contributes to the pathogenesis of some inherited demyelinating neuropathies. Why the CSF 1R inhibitor only helped some forms of CMT1 remains to be determined. Nevertheless, if CSF 1R inhibitors prove to be safe in humans, then one can envision a clinical trial to assess their efficacy in patients with CMT1X and perhaps the rare forms of CMT1B that are caused by loss-of-function mutations (Sanmaneechai et al., 2015). To be effective, it seems likely that the CSF 1R inhibitor will need to penetrate the blood–nerve (to reach the
ß The Author (2015). Published by Oxford University Press on behalf of the Guarantors of Brain. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://brain.oxfordjournals.org/ by guest on October 26, 2015
Axonal loss is chiefly responsible for deterioration of neurological function in chronic peripheral neuropathies. In most kinds of neuropathy, the axonal loss is conceptualized as a dying back of the longest axons, and is progressive over time. The same pattern is observed in inherited demyelinating neuropathies (Krajewski et al., 2000), even though the defective genes are expressed by myelinating Schwann cells; however, the underlying cause of the axonal loss is obscure (Nave, 2010). While the contribution of activated macrophages to demyelination and axonal loss was not anticipated, it nevertheless represents a therapeutic opportunity, as illustrated by Klein and co-workers in this issue of Brain (Klein et al., 2015). A well-known consequence of axonal loss (and demyelination) is a marked increase in the number of macrophages within peripheral nerves. In experimental models, these macrophages are derived from preexisting endoneurial macrophages and from the immigration of exogenous monocytes into the nerve (Ma¨urer et al., 2003). Until recently, their only well described role was the phagocytosis of degenerating axons, particularly their myelin sheaths. Work from Rudolf Martini’s laboratory, however, has revealed a detrimental role of macrophages in some animal
models of inherited demyelinating neuropathies. Their studies of genetically authentic mouse models of Charcot-Marie-Tooth disease 1B (CMT1B; created by a heterozygous loss-of-function mutation in Mpz; Mpz + / ) and CMT1X (Gjb1-null mice) have led to the proposal that macrophages mediate demyelination and axonal pathology. In both models, macrophages are physically associated with abnormally myelinated axons. More tellingly, Mpz + / and Gjb1-null mice with a genetically engineered deficiency in colony stimulating factor 1 (CSF 1) have far fewer macrophages and abnormally myelinated axons (Carenini et al., 2001; Groh et al., 2015). Because CSF 1 is a pro-inflammatory cytokine that activates macrophages, these findings indicate that there is a complicated pathway in peripheral nerve (Groh et al., 2012), involving fibroblasts (which express CSF 1) and macrophages (which express the CSF 1 receptor; CSF 1R), resulting in the activation of macrophages that, in turn, injure myelinating Schwann cells. In this issue of Brain, Martini’s group has made an important addition to this story (Klein et al., 2015). They treated three different mouse models of inherited demyelinating neuropathy (Mpz + / , Gjb1null, and a line of Pmp22 overexpressing mice; modelling CMT1A) with a pharmacological inhibitor of the CSF 1R. As expected, treating Gjb1-null mice for 3 or 6 months with this CSF 1R inhibitor reduced the number of macrophages in the
Scientific Commentaries
endoneurium), and even the blood– brain barrier (to reach the nerve roots). If reaching the nerve roots is not important, then one could avoid depleting the microglia in the CNS, which are also a target of the CSF 1R antagonists (Elmore et al., 2014). Steven S. Scherer, M.D., Ph.D. Perelman School of Medicine at the University of Pennsylvania, USA
E-mail: [email protected] doi:10.1093/brain/awv279
Carenini S, Ma¨urer M, Werner A,Blazyca H, Toyka KV, Schmid CD, et al. The role of macrophages in demyelinating peripheral
nervous system of mice heterozygously deficient in P0. J Cell Biol 2001; 152:301–8. Elmore MR, Najafi AR, Koike MA, Dagher NN, Spangenberg EE, Rice RA, et al. (2014). Colony-stimulating factor 1 receptor signaling is necessary for microglia viability, unmasking a microglia progenitor cell in the adult brain. Neuron 2014; 82:380–97. Groh J, Klein I, Hollmann C, Wettmarshausen J, Klein D, Martini R. CSF-1-activated macrophages are targetdirected and essential mediators of Schwann cell dedifferentiation and dysfunction in Cx32-deficient mice. Glia 2015; 977–986. Groh J, Weis J, Zieger H, Stanley ER, Heuer H, Martini R. Colony-stimulating factor-1 mediates macrophage-related neural damage in a model for Charcot-MarieTooth disease type 1X. Brain 2012; 135: 88–104. Klein D, Patzko A, Schreiber D, van Hauwermeiren A, Baier M, Groh J, et al. Targeting the colony-stimulating factor-1
| 3133
receptor alleviates two forms of CharcotMarie-Tooth disease in mice. Brain 2015; 138: 3193–205. Krajewski KM, Lewis RA, Fuerst DR, Turansky C, Hinderer SR, Garbern J, et al. Neurological dysfunction and axonal degeneration in Charcot-MarieTooth disease. Brain 2000: 123: 1516–27. Ma¨urer M, Muller M, Kobsar I, Leonhard C, Martini R, Kiefer R. Origin of pathogenic macrophages and endoneurial fibroblast-like cells in an animal model of inherited neuropathy. Mol Cell Neurosci 2003: 23: 351–9. Nave KA. Myelination and the trophic support of long axons. Nat Rev Neurosci 2010: 11: 275–83. Sanmaneechai O, Feely S, Scherer SS, Herrmann DN, Burns J, Muntoni F, et al. Genotype–phenotype characteristics and baseline natural history of heritable neuropathies caused by mutations in the MPZ gene. Brain 2015, in press.
Cerebral adrenoleukodystrophy: a demyelinating disease that leaves the door wide open This scientific commentary refers to ‘Brain endothelial dysfunction in cerebral adrenoleukodystrophy’ by Musolino et al. (doi: 10.1093/ awv250) Cerebral adrenoleukodystrophy (ALD) can be distinguished from other inherited diseases of white matter (known collectively as ‘leukodystrophies’) by the presence of severe neuroinflammation and blood–brain barrier disruption, evidenced in brain MRI by the leakage of gadolinium-diethylenetriamine pentaacetic acid (Gd-DPTA) at the periphery of demyelinating lesions on T1-weighted sequences. In contrast to multiple sclerosis, blood–brain barrier breakdown occurs downstream of the demyelinating process. Moreover, it is always followed by very rapid progression of demyelinating lesions, likely resulting from
myelin destruction by inflammatory cells. In this issue of Brain, Musolino et al. (2015) unravel for the first time some of the molecular mechanisms that underlie blood–brain barrier breakdown in ALD. Using ALD brain tissue, and human brain microvascular cells (HBMECs) in which the ABCD1 (ALD) gene was silenced via siRNA, they demonstrate an upregulation of intercellular adhesion molecule 1 (ICAM1) and vascular cell adhesion protein 1 (VCAM1) expression that reflected endothelial activation and that facilitated monocyte adhesion. They also show an increase in the expression of transforming growth factor b1 (TGF-b1), and in the number of vessels, probably due to production of vascular endothelial growth factor (VEGF) by astrocytes and/or microglia. Furthermore, they reveal an increase in the expression of matrix metallopeptidase 9 (MMP9), which
contributed to blood–brain barrier disruption, and in turn resulted in leakage of fibrinogen (an exclusively intravascular protein) into the perivascular space, recruitment of monocytes/ macrophages and the activation of microglia around vessels in active demyelinating areas. This correlated with the displacement of two members of the endothelial tight junction complex, claudin 5 (CLDN5) and ZO-1, from the membrane to the cytoplasm of endothelial cells: a clear indication that the blood–brain barrier had been breached, boosting the ingress of proinflammatory lymphocytes and monocytes into the white matter. Importantly, ABCD1 silencing in HBMECs also displaced CLDN5 to the endothelial cell cytoplasm, as in vivo. Studies in HBMECs provided two key additional pieces of information: (i) the silencing of ABCD1 resulted in transcriptional inhibition of CLDN5
Downloaded from http://brain.oxfordjournals.org/ by guest on October 26, 2015
References
BRAIN 2015: 138; 3132–3140
