Mol Neurobiol (2014) 49:1017–1030 DOI 10.1007/s12035-013-8576-6

Cysteine Cathepsins in Neurological Disorders Anja Pišlar & Janko Kos

Received: 28 July 2013 / Accepted: 21 October 2013 / Published online: 15 November 2013 # Springer Science+Business Media New York 2013

Abstract Increased proteolytic activity is a hallmark of several pathological processes, including neurodegeneration. Increased expression and activity of cathepsins, lysosomal cysteine proteases, during degeneration of the central nervous system is frequently reported. Recent studies reveal that a disturbed balance of their enzymatic activities is the first insult in brain aging and age-related diseases. Leakage of cathepsins from lysosomes, due to their membrane permeability, and activation of pro-apoptotic factors additionally contribute to neurodegeneration. Furthermore, in inflammation-induced neurodegeneration the cathepsins expressed in activated microglia play a pivotal role in neuronal death. The proteolytic activity of cysteine cathepsins is controlled by endogenous protein inhibitors—the cystatins—which evidently fail to perform their function in neurodegenerative processes. Exogenous synthetic inhibitors, which may augment their inhibitory potential, are considered as possible therapeutic tools for the treatment of neurological disorders. Keywords Cathepsins . Lysosomal system . Neurodegeneration . Neuronal death . Inflammation . Microglia Abbreviations 6-OHDA AD

6-hydroxydopamine Alzheimer’s disease

A. Pišlar (*) : J. Kos (*) Department of Pharmaceutical Biology, Faculty of Pharmacy, University of Ljubljana, Aškerčeva 7, 1000 Ljubljana, Slovenia e-mail: [email protected] e-mail: [email protected] J. Kos Department of Biotechnology, Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia

ALS APP Aβ CNS EPM1 HCHWA-I HD LPS MHC NCL NPC NPY PD

Amiotrophic lateral sclerosis Amyloid precursor protein Amyloid beta Central nervous system Progressive myoclonic epilepsy type 1 Hereditary cerebral haemorrhage with amyloidosis of Icelandic-type Huntington’s disease Lipopolysaccharide Major histocompatibility complex Neuronal ceroid lipofuscinosis Niemann–Pick disease type C Neuropeptide Y Parkinson’s disease

Introduction There is no cure for devastating neurodegenerative disorders such as Alzheimer’s (AD), Parkinson’s (PD) and Huntington’s diseases (HD) or amyotrophic lateral sclerosis (ALS) that cause long-term suffering to thousands of patients and ultimately their death. These disorders are characterized by pathological changes in disease-specific areas of the brain. In each disease, the pathological processes lead to dysfunction and degeneration in distinct subsets of neurons [1]. Accumulating evidence suggests that the lysosomal proteolytic system plays important roles in the development, plasticity, and neurodegeneration in the brain [2–5]. Abnormalities of lysosomal cathepsins have been found in certain neurodegenerative diseases [6]. This review focuses on the roles of cysteine cathepsins in neurological disorders.

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Cysteine Cathepsins The maintenance of a healthy organism relies largely on controlled biosynthesis, maturation, function, and terminal breakdown of proteins [7]. Proteolytic enzymes contribute to these processes by irreversible cleavage of peptide bonds in a polypeptide chain by a nucleophilic attack on the carbonyl carbon. The proteases are either exopeptidases, cleaving one or a few amino acids at the N or C terminus, or endopeptidases cleaving internal peptide bonds [8, 9]. Endopeptidases are classified, according to their catalytic mechanism, into aspartic, serine, threonine, metallo and cysteine endopeptidases (see MEROPS database [10]). The latter, cysteine proteases, constitute the largest cathepsin family, with 11 proteases in humans referred to as clan CA, members of the C1 family. The human family of cysteine cathepsins comprises cathepsins B, C (also known as cathepsin J and dipeptidyl-peptidase 1), F, H, K (also known as cathepsin O2), L, O, S, W, V (also known as cathepsin L2), and X (also known as cathepsin Z and cathepsin P), which share a conserved active site formed by cysteine, histidine and asparagine residues [11]. Most cysteine cathepsins are endopeptidases, although cathepsins B and H may also function as a dipeptidyl carboxypeptidase and an aminopeptidase, respectively. Cathepsin C is an aminopeptidase and cathepsin X is a carboxy mono- or di-peptidase [12]. Cysteine cathepsins are synthesized as inactive precursors, which are normally activated in the acidic environment of lysosomes. For this reason, they were initially considered as intracellular enzymes, responsible for the non-specific, bulk proteolysis in the acidic environment of the endosomal/lysosomal compartments, where they degrade intracellular and extracellular proteins [13, 14]. Despite this view, important and specific functions of cathepsins have been discovered that occur extracellularly and in other locations inside cells, such as secretory vesicles [15], the cytosol [16], and the nucleus [17]. They are also involved in proteolytic processing of specific substrates [13, 18]. Thus, cathepsins contribute to protein [14], neuropeptide and hormone processing [19–21], major histocompatibility complex (MHC) classII-mediated antigen presentation [22], bone remodelling [23, 24], apoptosis [25] and to keratinocyte differentiation [26]. Besides their normal physiological roles, cysteine cathepsins are involved in several pathologies, such as tumour development and progression [27], inflammation [28], psoriasis [29], muscular dystrophy [30], atherosclerosis [31], rheumatoid arthritis [24], osteoporosis and other bone disorders [24, 32], acute pancreatitis [33] and neurodegeneration [6].

Function of Cysteine Cathepsins in Brain Cells Cysteine cathepsins exhibit different expression patterns, levels and specificities, all of which contribute to their

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different functions. Some, like cathepsins B, H, L and C, are present ubiquitously in tissues, whereas others (cathepsins S, V, X, O, K, F and W) are expressed by specific cell types [34, 35]. Cathepsins B, L, H and F are widely distributed throughout the central nervous system (CNS), while others are considered as being cell or tissue-type specific [36–40]. Specific expression has been observed for cathepsins C and S in cells of the mononuclear phagocytic system, particularly in microglial cells [41, 42] and for cathepsin V in brain cortex and hippocampus [43]. Cathepsin X, however, has been found in almost all neuronal cells in the brain, with a preference for microglial cells and astrocytes [40]. Cathepsin B is probably the best studied cathepsin found in the CNS. It is distributed abundantly throughout the different areas of the brain, being expressed preferentially in neocortical and hippocampal neurons [36, 38]. Cathepsin B is the most abundant lysosomal protease of the papain family, being required for the housekeeping function of lysosomes in protein turnover. It is capable of digesting cell proteins, chromatin, complex carbohydrates and lipids, and is related to the physiological turnover of cell proteins in neurons [44]. It is also capable of cleaving the myristoylated alanine-rich C kinase substrate [45], a major cellular substrate of protein kinase C. In neuronal chromaffin cells, cathepsin B in secretory vesicles has been recently identified as a β-secretase for producing neurotoxic amyloid-β (Aβ) peptides [46–48]. Furthermore, cathepsin B was identified as the main enzyme responsible for endosomal proteolysis of internalized epidermal growth factor receptor complexes [49] and insulin-like growth factor [50]. Nevertheless, Ryan et al. have shown that, in response to the inflammatory molecule lipopolysaccharide (LPS), murine microglia also secrete cathepsin B [51]. Moreover, secreted cathepsin B is a major causative factor of microglia-induced neuronal apoptosis [52]. This observation is consistent with evidence that lysosomal proteases are involved in neuronal apoptosis [12]. Cathepsin B has been implicated in the activation of the proinflammatory caspases 1 and 11 and consequently induces nuclear apoptosis [53, 54]. Furthermore, cathepsin B, like cathepsins H, K, L and S, can cleave the Bcl-2 family member Bid, predominantly at residue Arg65 [55, 56], which may lead to the release of cytochrome c from the mitochondria and subsequent caspase activation [57]. Additionally, under certain conditions such as hypoxia, cathepsin B has been suggested to participate in the cleavage of caspase-3 substrates in brain cells [58]. Cathepsin L also participates in the production of the active secretory vesicle peptides required for cell–cell communication in the nervous and endocrine systems [4, 20, 59]. Cathepsin L is present in secretory vesicles of neuroendocrine cells where it produces brain peptide neurotransmitters that include neuropeptide Y (NPY), dynorphins and cholecystokinin [20, 60]. Notably, cathepsin L has been shown to convert proenkephalin to the active enkephalin opioid peptide

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neurotransmitter within the secretory vesicles of neuroendocrine chromaffin cells [15]. Cathepsin L is localized in bovine chromaffin cells of the sympathetic nervous system, where it co-localizes primarily with NPY and enkephalin [15, 59] and in secretory vesicles of pituitary cells, in which it co-localizes with β-endorphin and α-melanocyte-stimulating factor [21]. Both cathepsins B and L are involved in recycling processes during axon outgrowth and synapse formation in the developing postnatal central nervous system [61]. Likewise, cathepsins B and L cathepsins H and V have been associated with secretory vesicles. Cathepsin H was identified as an aminopeptidase in secretory vesicles of the adrenal medulla of mice brains, cleaving peptide intermediates with N-terminal basic residue extensions generated by cathepsin L. In particular, cathepsin H generates (Met)enkephalin by sequential removal of basic residues from KR-(Met)enkephalin and KK-(Met)enkephalin [62]. In contrast, in secretory vesicles in the human brain, cathepsin H acts as an endopeptidase, metabolizing neuropeptides and bradykinin [63]. Cathepsin V is present in secretory vesicles of human brain cortex and hippocampus, where enkephalin and NPY are produced, as well as being associated with neuropeptides within the cells. A unique function of human cathepsin V has therefore been proposed—the production of the enkephalin and NPY neuropeptides required for neurotransmission in health and in neurological diseases [43]. In contrast to other cathepsins, cathepsin X was discovered only recently [64]. It is not widely expressed in cells and tissues [34], but is restricted to the cells of the immune system [65] and the central nervous system [40]. Specifically, in the CNS, cathepsin X shows the highest expression in glial cells, microglia and astrocytes, although it has also been localized in neurons and ependymal cells of the mouse CNS. In neuronal cells, cathepsin X sequentially cleaves C-terminal amino acids of γ-enolase, abolishing its neurotrophic activity [66]. Additionally, the involvement of cathepsin X in the generation of plasmin has been demonstrated. In this way, the neuronal differentiation and the length of neurites, especially the early phase of neurite outgrowth, can be regulated [66]. Moreover, neuronal survival and apoptosis are regulated by cathepsin X activity [67]. Cathepsin S, expressed predominantly in cells of mononuclear phagocytic origin [68], has also been discovered in cells of the CNS. It is expressed abundantly in all regions of the brain, with preferential localization in microglia cells [38, 69]. Cathepsin S is released by microglia and macrophages on stimulation with inflammatory mediators [42, 68, 70]. It retains its activity after prolonged incubation at neutral pH, at which all other cathepsins become irreversibly inactivated [13, 71]. In this regard, cathepsin S is able to degrade a number of components of the extracellular matrix in both acidic and non-acidic extracellular environments, including molecules of the extracellular matrix found in the CNS, such

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as neurocan, phosphacan, and basement membrane heparin sulphate proteoglycan [68, 72–74]. Cathepsin S also plays a role in the migration of microglia in vitro, in this way, protecting facial motoneurons against axotomy-induced injury [75]. Cathepsin F is expressed throughout the CNS, although its physiological role has not been established [39]. It does, however, display a potent activity in lipoprotein degradation [76, 77]. Although the actual substrate of cathepsin F in neurons is not known, the lipoprotein component of the lipofuscin complex has been proposed [39]. Cathepsin C is expressed constitutively in various tissues but, in CNS tissues, it is hardly detectable. However, under certain pathological conditions, activated microglia is strongly immunopositive for cathepsin C. This suggests that cathepsin C may participate in inflammatory processes accompanying pathogenesis in the CNS [41].

Role of Cysteine Cathepsins in Neurodegenerative Processes There is increasing evidence that disturbance of the normal balance and extralysosomal localization of cathepsins contribute to the processes of neurodegeneration in AD, tauopathies, PD, HD and lysosomal storage diseases [2, 36, 78–84] as overviewed in Table 1. Dysfunction of the endosomal/ lysosomal system in neurons is closely associated with activation of microglia, which could initiate an inflammatory response to provoke neurodegeneration. Activated microglia also releases certain cathepsins that induce neuronal death through degradation of extracellular matrix proteins [70] (Fig. 1). During neurodegeneration, cathepsins contribute to neuronal injury induced by excitotoxins, through degradation of axonal and myelin proteins, by converting protein precursor into active peptide neurotransmitters and by amplifying apoptotic signalling [3, 4]. Furthermore, a central role in the neuronal cell death mechanism has been proposed for cathepsins [12, 85]. In terms of which cysteine cathepsins are specifically involved in neurodegeneration, cathepsins B and L have been investigated most intensively. They have been reported to induce age-related changes that are closely related to neuronal degeneration [84]. On the other hand, they have been shown to be essential for maturation and integrity of the postnatal CNS in which both proteases compensate each other in vivo [86]. Much less is known about the association with neurodegeneration of other members of the cysteine cathepsin family. Levels of cysteine cathepsins H, S and X have been associated with inflammatory neurological diseases [87, 88] and cathepsin X has been implicated in processes during normal aging as well as in neurodegenerative processes in AD and ALS [40, 89]. Information on the remaining family members is limited.

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Table 1 Overview of cysteine cathepsin functions in neurological disorders CNS diseases

Cathepsins

Main findings

References

Alzheimer’s disease

B, H, L, S and X B and L

Enhanced levels of cathepsins were found in cells surrounding senile plaques in AD brain; cathepsin B, L and S possess β-secretase activity in APP processing. Mice lacking B and L develop an accumulation of ultrastructurally and biochemically unique lysosomal bodies in large cortical neurons and massive apoptosis of selected neurons. Increased level/activity and altered subcellular distribution of cathepsin B were found in the cerebellum of NPC1 mouse brain. Accumulation of eosinophilic granules and lipofuscin in neurons is increased in association with decreased cathepsin F expression. Increased levels of cathepsins were found in response to neurotoxin 6-OHDA, where cathepsin L was found to promote 6-OHDA neuronal apoptosis. Increased levels of cathepsin B were found in the degenerating motor neurons. Cathepsin X was found upregulated in numerous glial cells of degenerating brain regions in a mouse model of ALS. Increased expression of cathepsin B reduced protein level of mutant huntingtin; increased levels of cathepsin H were found in HD brain; cathepsin X mediates N-terminal proteolysis and toxicity of mutant huntungtin.

[2, 36, 40, 78, 92, 95] [61, 86]

Lysosomal storage disease Niemann–Pick disease type C Neuronal ceroid lipofuscinosis Parkinson’s disease Amiotrophic lateral sclerosis

B F B, L and X B and X

Huntington’s disease B, H and X

[113] [39] [128, 130] [40, 165]

[82, 166, 167]

Brain aging is associated with progressive decline of the cognitive and memory functions and accompanied by alterations in the expression and activity of lysosomal proteases. Alterations in the concentrations and localization of cysteine cathepsins in CNS have been reported in normally aged brain [40, 83]. In rat brain, cathepsin B activity increases from 2 to 28 months in the neostriatum, while cathepsin L activity decreases in all areas of the brain [37]. Mice lacking cathepsins B and L revealed pathological features such as atrophy in the cerebral and cerebellar regions of the brain, suggesting their necessity for neuronal development [86]. Further, suppression of cathepsins B and L by exposing cultured hippocampal slices to a selective cathepsin inhibitor provoked

changes similar to those occurring during brain aging, such as an increased number of lysosomes and the formation of neurites [90]. Increased number of lysosomes has been observed in neurons of aged rats and humans, and also in venerable neurons in AD [79, 80, 91]. As shown by Wendt et al., cathepsin X is widely expressed in a developing mouse brain and age-dependently upregulated in amount and in activity, with a dense accumulation in glial cells [40]. Further, the cathepsin S protein level also increases in microglial cells of aged mouse brain [69]. Upregulation of the cysteine cathepsins has also been demonstrated in pathologically altered areas within the brain, such as the neurofibrillary tangles and senile plaques characteristic of AD [2, 36, 92]. Cathepsin B has been localized in neurofibrillary tangles and in degenerated neurites within senile

Fig. 1 Cysteine cathepsins involvement in neurodegeneration and inflammation-induced neurodegeneration. Lysosomal cysteine cathepsins can be altered by many toxic insults leading to enhanced cathepsin expression and proteolytic activity, which contribute to neuronal injury

during neurodegeneration. Activated microglia mediate neuroinflammation by secreting inflammatory cytokines thereby inducing neuronal death. In addition to cytokines, activated microglia secretes certain cathepsins thus upscaling inflammation-induced neurodegeneration

Cysteine Cathepsins and Age-Related Disorders

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Fig. 2 Cathepsin Ximmunopositive cells, surrounding amyloid plaque in a mouse model of AD. a Representative transmission image of senile plaque (arrow) of consecutive brain section obtained from 19-month-old transgenic Tg2576 mice. b Representative images of consecutive brain sections, immunostained for cathepsin X (red fluorescence) and counterstained with thioflavine-S (green fluorescence), demonstrating a strong cathepsin X overexpression on or very close to senile plaques (arrows). c,d Representative images of double immunofluorescence staining for cathepsin X (red fluorescence) with microglial marker OX-6 (c, green fluorescence) and neuronal marker NeuN (d, green fluorescence), where expression of cathepsin X by activated microglia is restricted to close proximity to the plaque (c, arrows), whereas cathepsin X-positive neuronal cells are seen in the wider area of amyloid plaque (d, arrows). Scale bars 20 μm

plaques in brains of patients with AD and dementia. In contrast to the control brain—in which cathepsin B was found only in some degenerated neurons within the cell body—the enzyme was widespread throughout the cells in the pathological brain, and also in neurites and dendrites [93]. Besides cathepsin B, enhanced levels of cathepsins L and H were found in the majority of astroglia and microglia, both within and outside senile plaques [93]. In neuritic plaques in brains of patients with AD, enhanced levels of cathepsins L and S, but not of cathepsin B, have been observed [94]. In a human AD brain tissue, increased immunoreactivity for cathepsin S has been shown in neocortical and hippocampal neurons and in glial cells. Immunostaining of cathepsin S in particular was observed in astrocytes in the periphery of amyloid plaques [92]. Likewise, the appearance of cathepsin X immunopositive cells in glial cells in a transgenic mouse model of AD and in AD patients was observed [40]. Cathepsin X protein and activity were strongly upregulated in these cells, particularly in microglial cells around senile plaques observed in a transgenic mouse model of AD (Fig. 2) [89]. In aged brain and in AD, cysteine cathepsins play a role in Aβ peptide generation from amyloid precursor protein (APP). Three cathepsins, B, L and S, were identified as possessing βsecretase activity in APP cleavage [95]. Recent studies have

shown that cathepsin B in secretory vesicles participates as a β-secretase in producing the neurotoxic Aβ peptide. Cathepsin B shows a clear preference for cleaving wild-type β-secretase substrate, whereas for the Swedish mutant βsecretase substrate, it showed essentially no activity [96]. Inhibition of cathepsin B by the cysteine protease inhibitor E64d and the related inhibitor CA074Me, that preferentially inhibits intracellular cathepsin B, results in reduction of brain Aβ peptides and significant improvement in memory in a mouse model of AD. Inhibition by E64d had no effect in AD mice expressing the Swedish mutant β-secretase site of APP [96, 97]. Knocking out the cathepsin B gene results in reduction of brain Aβ and the C-terminal β-secretase fragments in mice expressing APP with the wild-type β-secretase site, but not in mice expressing the Swedish mutant site of APP [60, 98]. Recently, Kindy et al. showed improvement of memory deficits after cathepsin B gene knockout in an AD mouse model expressing the wild type β-secretase site of AβPP that is present in most AD patients [48]. In contrast, Mueller-Steiner et al. reported that, through proteolytic cleavage, cathepsin B actually reduces the levels of Aβ peptides, especially the aggregation-prone species of full-length peptide Aβ1-42, [99]. In addition, the amount of amyloid plaques in aged AD model mice was decreased by lentivirus-mediated

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expression of cathepsin B. It has also been reported that lysosomal protease inhibitors induce symptoms of AD, such as axonal dystrophy [100]. The role of lysosomal activation in AD pathology is still unclear. It has been proposed that the release of cathepsins from ruptured lysosomes contributes to the pathology of many neurodegenerative diseases [101]. On the other hand, protein accumulation events trigger lysosomal activation to slow progressive neurodegeneration [100, 102]. However, other cathepsins have also been identified that, like cathepsin B, display the β-secretase activity associated with the pathogenesis of AD. Cathepsins S and L also cleave the wild-type β-secretase site [95]. Transfection of human kidney cells with cathepsin S increased the secretion of Aβ, and E-64d reduced its secretion by cathepsin S transfected cells [103]. In addition, Liuzzo et al. demonstrated that cathepsin S is able to degrade monomers and dimers of the Aβ peptide in vitro [104]. It is known that Aβ peptides are taken up predominantly by microglia and that the peptides are accumulated and degraded in endosomal/lysosomal systems of microglia [70]. Thus, microglial cathepsin S may assist in the extracellular clearance of intracellularly formed Aβ or soluble Aβ, and modulate the levels of the peptide at the very initial stages of peptide aggregation, which in turn might have an effect on Aβ neurotoxicity [105].

Cysteine Cathepsins and Lysosomal Storage Disorders Lysosomal instability is a distinct feature of brain aging that results in gradual changes and increased risk of protein accumulation and an aggregated protein stress response. These occurrences contribute to the neurodegeneration in lysosomal storage disease, leading to abnormal brain development, dynamic changes in synapses, and associated cognitive deterioration [106, 107]. The primary defect underlying lysosomal storage disease is a severe loss of function of acidic endosomal/lysosomal proteins [108]. Purely lysosomeinitiated cell death may be achieved by the endocytosis of oxidizable substrates that selectively injure lysosomal membranes, cause leakage of cathepsins and induce apoptosis [108, 109]. Lysosomal storage defects frequently result in neurological defects, underscoring the critical importance of lysosomal function in the CNS [110]. Mice lacking cathepsins B and L display a pronounced lysosomal storage disease that leads to extensive neuron death in the CNS, and to the development of pronounced brain atrophy due to massive apoptosis of selected neurons in the cerebral cortex and the cerebellar Purkinje and granule cell layers [86]. However, prior to neuronal cell death, neurons lacking cathepsins B and L develop a lysosomal storage disorder similar to the human neuronal ceroid lipofuscinosis (NCL), suggesting that cathepsins B and L are essential for the maturation and integrity of postnatal CNS [86, 111].

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Cathepsins B and L can compensate for each other in vivo, since only cathepsin B−/−L−/− double mutant mice develop neurodegeneration accompanied by pronounced reactive astrocytosis. This is accompanied by an accumulation of ultrastructurally and biochemically unique lysosomal bodies in large cortical neurons, by axonal enlargements, and by elevated levels of proteins localized to neuronal biosynthetic, recycling/endocytotic or lysosomal compartments [61, 86]. Nitatori et al. showed that immunoreactivity for cathepsins B, H and L was increased in the CA1 pyramidal neurons 3 days after ischemic insult [112]. The cathepsin B-immunopositive lysosomes were mostly autolysosomes or autophagic vacuoles corresponding to membrane-bound vacuoles. The latter observations indicate the degradation of proteins in lysosomes of CA1 pyramidal neurons. The process proceeds by forming autolysosomes, and cysteine cathepsins play an essential role in protein turnover in these neuronal cells. Recently, the increased level/activity and altered subcellular distribution of cathepsin B in the cerebellum of Niemann–Pick disease type C (NPC) mouse brain has been associated with the underlying cause of neuronal vulnerability in brains of NPC [113]. The latter is a progressive lysosomal storage disorder characterized by intracellular accumulation and redistribution of cholesterol in a number of tissues, including the brain [110]. Mice lacking cathepsin F accumulated eosinophilic granules in neurons that have features typical of lysosomal lipofuscin. Additionally, large amounts of autofluorescent lipofuscin, characteristic of the lysosomal storage disorder neuronal ceroid lipofuscinosis (NCL), accumulated throughout the CNS in mice with cathepsin F deficiency. Pronounced gliosis, an indicator of neuronal stress and neurodegeneration, was also apparent in mice lacking cathepsin F [39]. These observations show conclusively that cathepsin F is the only cysteine cathepsin whose inactivation alone causes a lysosomal storage defect and progressive neurological features in mice.

Cysteine Cathepsins and Inflammation-Induced Neurodegeneration Inflammation plays a central role in the processes that have been associated with neurodegeneration. The inflammatory response is normally mediated by activated microglia, the resident immune cells of the CNS. These cells respond to neuronal damage and remove the damaged cells by phagocytosis. The activation of microglia is a hallmark of brain pathology [114, 115]. In addition to inflammatory cytokines, activated microglia also secretes cathepsins [70, 116]. Microgliosis can promote sprouting of injured neurons by providing neurotrophic factors [117]. However, in several neuropathologies in which chronic inflammation is present, the inflammatory products derived from activated microglia may also promote neurodegeneration and contribute to

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neuronal loss [118]. In response to LPS, which induces death of the nigral dopaminergic neurons through microglial activation, there is a substantial increase in secretion from microglia of cathepsins S and X [38, 116]. Besides cathepsins S and X, LPS also induces the expression and release of cathepsin B [116] and cathepsin C [119] and induces their enzymatic activity. During LPS-induced inflammation, cathepsin B is translocated from lysosomes to other subcellular compartments in hippocampal neurons, including the cytoplasm, autophagic vacuoles and, sporadically, mitochondria. Also, cathepsin B-dependent autophagy in the hippocampus has been induced during systemic inflammation [120]. It has been demonstrated that secreted cathepsin B is a major causative factor of microglia-induced neuronal apoptosis [52]. Like microglia cells, dendritic cells are also involved in the inflammatory response. Dendritic cells are restricted to meninges and choroid plexuses and are absent from brain parenchyma [121, 122]. Under pathological conditions, these leukocytes infiltrate the brain and contribute to the progression and chronicity of inflammation [88]. Dendritic cells capture and process antigens and display large amounts of MHCpeptide complexes at their surface. In the aged brain, dendritic cells express cathepsins S and X whose localization were restricted to granules or vesicles in the cytoplasm, sparing the nucleus. This implies their involvement in an age-related immune response in the brain [88]. Cathepsin S is also involved in the proteolytic processing of various antigens, including myelin basic protein, a potential autoantigen implicated in the pathogenesis of multiple sclerosis, a chronic inflammatory disease of the CNS [123]. Selective inhibition of cathepsin S ameliorated disease in mice and rats [124]. Progressive inflammation is also a contributing factor to early development of PD [125], a chronic neurodegenerative disorder resulting from progressive loss of dopaminergic neurons in the substantia nigra, striatal dopamine depletion and motor impairments [126, 127]. It has been reported that the lysosomal proteolytic system participates in the apoptosis of neuronal cells induced by 6-hydroxydopamine (6-OHDA), a common neurotoxin used in models of PD. The latter increased the expression of cathepsin B, although inhibition of cathepsin B failed to protect neuronal cells [128]. On the other hand, cathepsin L plays an active role in neuronal injury induced by 6-OHDA, and thus could be involved in the progression of Parkinson’s disease [129, 130]. The similar role in neuronal apoptosis of dopaminergic neurons has been proposed for cathepsin X [67].

Endogenous and Exogenous Inhibitors of Cysteine Cathepsins Many pathophysiological processes in the brain are modulated by a balance between cathepsins and their endogenous

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protease inhibitors—cystatins, thyropins and some members of the serpin serine protease inhibitors [131, 132]. Thyropins comprise a superfamily of inhibitors homologous to the thyroglobulin type-1 domains [133]. The best characterized human representative is the MHC class Ii-associated p41 invariant chain fragment (Ii), which strongly inhibits cathepsin L and cruzipain [134, 135]. Cystatins are a superfamily of evolutionarily related protein inhibitors of cysteine proteases that are found in plants, fungi and animals, as well as in viruses. Type I cystatins (also designated stefins), are cytosolic and nuclear proteins, in contrast to type II cystatins that are secreted into the extracellular environment [136]. The seven members of the latter group are cystatins C,E/M, D, F, S, SA and SN, together with the male reproductive tract cystatins 8 (CRES, cystatin-related epididymal and spermatogenic), 9 (testatin), 11 and 12 (cystatin T), a bone marrow derived, cystatin-like molecule (CLM, cystatin 13) and secreted phosphoprotein (SPP-24, cystatin 14) [11]. Type III cystatins, the kininogens, are large multifunctional plasma proteins containing three type II cystatin-like domains. Fetuins and latexins are constituted by two tandem cystatin-like domains; however, they do not possess inhibitory activity against cysteine proteases. The tertiary structure of cystatins is conserved and exhibits the typical cystatin fold [137]. In general, cystatins are tight-binding inhibitors of the C1 family of cysteine proteases, whereas type II cystatins also possess a second reactive site for inhibition of the C13 family of cysteine proteases [138]. The main physiological role of cystatins is the regulation of excess cysteine proteinase activity, either in cells and tissues or in body fluids. In general, cystatins act as »emergency« inhibitors, trapping and neutralizing protease activity in either cell cytoplasm or extracellular fluids. However, regulatory roles have also been proposed. Stefin A, for example, controls the normal keratinocyte proliferation and differentiation, plays a role in apoptosis, and protects epithelial and lymphoid tissues from cysteine proteases produced by invading pathogens. Mouse stefin A has also been shown to control ovarian follicular growth and maturation [139]. For stefin B, a specific function was proposed in the regulation of bone resorption by down-regulating intracellular cathepsin K activity [140]. Since cystatins are present in the cytoplasm and extracellular fluids and their targets, cysteine cathepsins are predominantly localized in lysosomes, it is not obvious under what circumstances cystatins meet the targeted proteases. Recent studies provide evidence that extracellular type 2 cystatins can be internalized by neuroblastoma and immune cells, in this way, entering endosomal/lysosomal vesicles [141, 142]. An exception among cystatins is cystatin F, which is translocated towards endosomes/lysosomes within cells [143]. Vesicular localization of cystatins may affect a number of cell functions, including antigen presentation [144] or processing of extracellular matrix proteins in the case of tumour progression [145].

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It has been reported that cystatins regulate proteolytic activity in various diseases that are associated with increased expression of cysteine cathepsins. The best studied is their role in cancer, cardiovascular diseases and atherosclerosis, although there is an increasing number of studies revealing their contribution in neurological disorders as well. The results are controversial, as are those describing the roles of particular cathepsins in these processes. Cysteine protease inhibitors, in particular cystatin C, may impair the β-secretase activity of cathepsin B and release of Aβ peptide in transgenic AD mice [97]. This is in line with the fact that lower cystatin C levels have been associated with higher risk of AD [146]. However, other studies have provided opposing results, suggesting that cathepsin B does not induce cleavage at the β-secretase site in primary neurons but degrades Aβ aggregates [99]. Reduced levels of cystatin C promote cathepsin B-induced Aβ degradation and attenuated Aβ-associated cognitive deficits, behavioural abnormalities, and restore synaptic plasticity in the hippocampus [147]. On the other hand, in brains of APP-transgenic mice, cystatin C should bind Aβ and inhibit its fibril formation [148]. Cystatin C, in its N-terminally truncated form and found in the cerebrospinal fluid, was associated with multiple sclerosis [149]. However; the truncation was identified later as a result of inappropriate sample storage at −20 °C [150]. Cystatin C was found to be increased in nigrostriatal neurons, astrocytes and microglia, following destruction of the nigrostriatal dopaminergic pathway with 6-OHDA. Its role is suggested as being highly neuroprotective [151]. Changes in cystatin C level have been observed in various models of neuronal injuries, including ALS [152].

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CysB (stefin B) gene [154]. The most common change found is the dodecamer repeat expansion in the promoter region [155], which leads to reduced mRNA and protein levels. In stefin B-deficient mice, the lower levels of stefin B were accompanied by increased cathepsin B and D activities in the brain, which were previously shown to play a role in apoptosis [156]. By using double-knockout mice for cathepsins and stefin B, cathepsin B was found as the main contributor to the apoptotic phenotype of stefin B-deficient mice and of humans with EPM1 [157]. Other mechanisms of the stefin B function in EPM1 have also been proposed, such as the regulation of the cell cycle through inhibition of cathepsin L in the nucleus, or the formation of stefin B dimers or oligomers by mutations that are toxic for neuronal cells [158]. HCHWA-I is an autosomal-dominant disorder with, often fatal, early onset cerebral haemorrhage, while dementia may develop in those surviving the initial episode of haemorrhagic stroke [159]. It was found as a rare, fatal amyloid disease in young people in Iceland [160]. The disease is associated with a glutamine for leucine amino acid substitution resulting from an A to T point mutation at codon 68 of the cystatin C gene located on chromosome 20. The protein species, widely deposited as vascular amyloid in the leptomeninges, cerebral

Mutations in Genes Encoding Cystatins are Associated with Neurological Disorders Mutations in genes encoding stefin B and cystatin C were reported to cause two neurological disorders, UnverrichtLundborg disease (progressive myoclonic epilepsy type 1 (EPM1)) and hereditary cerebral haemorrhage with amyloidosis of Icelandic-type (HCHWA-I). EPM1, caused by autosomal-recessive loss-of-function mutations in the gene encoding stefin B, is the most common cause of progressive myoclonus epilepsy worldwide [153]. This is a neurodegenerative disorder with age of onset from 6 to 16 years, characterized by stimulus-sensitive myoclonus and tonic-clonic epileptic seizures. Ataxia, loss of coordination and tremor develop several years after onset of the disease. Individuals with EPM1 are mentally alert but show depression and mild decline in intellectual performance over time. Cerebellar atrophy and motor cortex degeneration are largely responsible for the loss in equilibrium, correlating with the motor symptoms of the disease. The diagnosis of EPM1 is confirmed by identifying disease-causing mutations in the

Fig. 3 Cathepsins regulation in neuronal cells. Cysteine cathepsins are involved in the proteolytic processing and generation of neurotoxic peptides that can accumulate in the cells initiating a variety of neurological disorders. These processes can be blocked by selective cysteine cathepsin inhibitors reducing the harmful effects on neuronal cells and resulting in reduced neuronal death and neurodegeneration

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cortex, basal ganglia, brainstem and cerebellum, is an Nterminal degradation product of the mutated cystatin C protein. The truncation of ten N-terminal amino acids of cystatin C molecule is also a hallmark of the disease. The protease responsible for the truncation has not been identified. It could be a serine proteinase with elastase specificity since, in vitro, elastase is capable of generating cystatin C lacking the first 10 amino acids [161]. The truncated cystatin C form exhibits significantly lower inhibition against cathepsin L.

Potential Therapeutic Use of Inhibitors of Cysteine Cathepsins Due to the harmful action of cysteine cathepsins in pathological processes of neurodegeneration, cathepsin inhibitors constitute a possible tool for therapeutic interventions to inhibit excessive proteolytic activity, as indicated in Fig. 3. Some beneficial effects of cystatins have been demonstrated; however, they are general inhibitors, not selective for particular cathepsins therefore, and off-target side effects are expected if they are used as drugs for treating patients. Instead of cystatins and other endogenous cysteine protease inhibitors therefore, synthetic cathepsin inhibitors have been suggested as agents for treating neurodegenerative disorders. Aβ levels in brain in vivo are significantly reduced by cysteine protease inhibitor E64d and the related inhibitor CA074Me, which specifically inhibits intracellular cathepsin B [4, 162–164]. In vitro neuroprotection by cathepsin L inhibitor was recently demonstrated in a cell model of PD. An irreversible inhibitor of cathepsin L, Z-FY(t-Bu)-DMK, significantly reduces 6-OHDA-induced apoptosis [130]. Likewise, the irreversible epoxysuccinyl inhibitor of cathepsin X was observed to exert a protective effect on 6-OHDA-induced neurodegeneration [67]. New generations of selective and reversible cathepsin inhibitors are expected to significantly improve the protease-targeted therapy of neurodegenerative diseases.

Conclusion It is widely accepted that overexpression, increased enzymatic activity and mis-localization of cysteine cathepsins in cells of the nervous system are associated with pathological processes in neurodegenerative disorders. Until recently, the molecular mechanisms linking the proteolytic activity of the enzymes with neuronal damage were unknown. However, during the last few years, evidence has been growing that protein molecules, which when modified by proteolytic cleavage, trigger the neurodegeneration processes or, alternatively, are no longer able to promote growth and differentiation of neuronal cells. In particular, molecular targets for cathepsin proteolytic activity have been identified in neuronal apoptosis and

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autophagy, as well as in the processing of APP precursors. However, in inflammation-induced neurodegeneration, the molecular targets remain highly intriguing. On the other hand, cystatins, the main endogenous inhibitors of cysteine cathepsins, deserve equal attention regarding their role in neurodegeneration. The mutations in their genes, leading to severe neurological disorders, underline the fact that effective control of proteolytic activity is crucial for the normal functioning of the neuronal system. Although cystatins are not appropriate molecules for therapeutic intervention, small synthetic inhibitors of appropriate specificities could replace them and improve the treatment of patients with neurodegenerative disorders. The design of such synthetic inhibitors that bind cysteine cathepsins reversibly and selectively together with the modes of their delivery to the site of action need to be given increased emphasis. Acknowledgments The authors sincerely acknowledge Prof. Roger Pain for the critical review of the manuscript. This project was supported by a grant from Research Agency of the Republic of Slovenia (grants P40127 and J4-4123 to JK).

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