Limiting multiple sclerosis related axonopathy by blocking Nogo receptor and CRMP-2 phosphorylation

p75NTR and TROY: Uncharted Roles of Nogo Receptor Complex in Experimental Autoimmune Encephalomyelitis

Multiple sclerosis (MS) and its animal model, experimental autoimmune encephalomyelitis (EAE), have been on the forefront of drug discovery for most of the myelin inhibitory molecules implicated in axonal regenerative process. Nogo-A along with its putative receptor NgR and co-receptor LINGO-1 has paved the way for the production of pharmaceutical agents such as monoclonal antibodies, which are already put into handful of clinical trials. On the other side, little progress has been made towards clarifying the role of neurotrophin receptor p75 (p75NTR) and TROY in disease progression, other key players of the Nogo receptor complex. Previous work of our lab has shown that their exact location and type of expression is harmonized in a phase-dependent manner. Here, in this review, we outline their façade in normal and diseased central nervous system (CNS) and suggest a role for p75NTR in chronic axonal regeneration whereas TROY in acute inflammation of EAE intercourse.

Nogo receptor expression in microglia/macrophages during experimental autoimmune encephalomyelitis progression

Myelin-associated inhibitory factors within the central nervous system (CNS) are considered to be one of the main obstacles for axonal regeneration following disease or injury. The nogo receptor 1 (NgR1) has been well documented to play a key role in limiting axonal regrowth in the injured and diseased mammalian CNS. However, the role of nogo receptor in immune cell activation during CNS inflammation is yet to be mechanistically elucidated. Microglia/macrophages are immune cells that are regarded as pathogenic contributors to inflammatory demyelinating lesions in multiple sclerosis (MS). In this study, the animal model of MS, experimental autoimmune encephalomyelitis (EAE) was induced in ngr1^+/+ and ngr1^–/– female mice following injection with the myelin oligodendrocyte glycoprotein (MOG_35–55) peptide. A fate-map analysis of microglia/macrophages was performed throughout spinal cord sections of EAE-induced mice at clinical scores of 0, 1, 2 and 3, respectively (increasing locomotor disability) from both genotypes, using the CD11b and Iba1 cell markers. Western immunoblotting using lysates from isolated spinal cord microglia/macrophages, along with immunohistochemistry and flow cytometric analysis, was performed to demonstrate the expression of nogo receptor and its two homologs during EAE progression. Myelin protein engulfment during EAE progression in ngr1^+/+ and ngr1^–/– mice was demonstrated by western immunblotting of lysates from isolated spinal cord microglia/macrophages, detecting levels of Nogo-A and MOG. The numbers of M1 and M2 microglia/macrophage phenotypes present in the spinal cords of EAE-induced ngr1^+/+ and ngr1^–/– mice, were assessed by flow cytometric analysis using CD38 and Erg-2 markers. A significant difference in microglia/macrophage numbers between ngr1^+/+ and ngr1^–/– mice was identified during the progression of the clinical symptoms of EAE, in the white versus gray matter regions of the spinal cord. This difference was unrelated to the expression of NgR on these macrophage/microglial cells. We have identified that as EAE progresses, the phagocytic activity of microglia/macrophages with myelin debris, in ngr1^–/– mice, was enhanced. Moreover, we show a modulation from a predominant M1-pathogenic to the M2-neurotrophic cell phenotype in the ngr1^–/– mice during EAE progression. These findings suggest that CNS-specific macrophages and microglia of ngr1^–/– mice may exhibit an enhanced capacity to clear inhibitory molecules that are sequestered in inflammatory lesions.

Amyloid-beta-dependent phosphorylation of collapsin response mediator protein-2 dissociates kinesin in Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by accumulation of amyloid plaques and neurofibrillary tangles. Prior to the development of these characteristic pathological hallmarks of AD, anterograde axonal transport is impaired. However, the key proteins that initiate these intracellular impairments remain elusive. The collapsin response mediator protein-2 (CRMP-2) plays an integral role in kinesin-1-dependent axonal transport and there is evidence that phosphorylation of CRMP-2 releases kinesin-1. Here, we tested the hypothesis that amyloid-beta (Aβ)-dependent phosphorylation of CRMP-2 disrupts its association with the kinesin-1 (an anterograde axonal motor transport protein) in AD. We found that brain sections and lysates from AD patients demonstrated elevated phosphorylation of CRMP-2 at the T555 site. Additionally, in the transgenic Tg2576 mouse model of familial AD (FAD) that exhibits Aβ accumulation in the brain with age, we found substantial co-localization of pT555CRMP-2 and dystrophic neurites. In SH-SY5Y differentiated neuronal cultures, Aβ-dependent phosphorylation of CRMP-2 at the T555 site was also elevated and this reduced the CRMP-2 association with kinesin-1. The overexpression of an unphosphorylatable form of CRMP-2 in neurons promoted the re-establishment of CRMP-2-kinesin association and axon elongation. These data suggest that Aβ-dependent phosphorylation of CRMP-2 at the T555 site may directly impair anterograde axonal transport protein function, leading to neuronal defects.

Neuroprotective and neurotrophic effects of Lanthionine Ketimine Ester

Lanthionine ketimine ethyl ester (LKE) is a synthetic derivative of the naturally occurring amino acid lanthionine ketimine. We previously showed that LKE reduced clinical signs in a mouse model of multiple sclerosis (MS) associated with reductions in axonal damage; however, whether LKE has direct beneficial actions on mammalian neuronal cells was not examined. In the current study, we tested the effects of LKE in SH-SY5Y human neuronal cells and in primary mouse cerebellar granule neurons. In both cell types, LKE dose-dependently reduced the cell death that occurred spontaneously followed a change in media. LKE also reduced cell death due to glutamate excitoxicity, accompanied by a reduction in production of reactive oxygen species. LKE induced neuritogenesis in both undifferentiated SH-SY5Y cells and in primary neuron, increasing process numbers and lengths. These results demonstrate that direct neuroprotective and neurotrophic effects of LKE likely contribute to its beneficial actions in vivo.

Nogo-A antibodies enhance axonal repair and remyelination in neuro-inflammatory and demyelinating pathology

Two hallmarks of chronic multiple sclerosis lesions are the absence of significant spontaneous remyelination and primary as well as secondary neurodegeneration. Both characteristics may be influenced by the presence of inhibitory factors preventing myelin and neuronal repair. We investigated the potential of antibodies against Nogo-A, a well-known inhibitory protein for neuronal growth and plasticity, to enhance neuronal regeneration and remyelination in two animal models of multiple sclerosis. We induced a targeted experimental autoimmune encephalomyelitis (EAE) lesion in the dorsal funiculus of the cervical spinal cord of adult rats resulting in a large drop of skilled forelimb motor functions. We subsequently observed improved recovery of forelimb function after anti-Nogo-A treatment. Anterograde tracing of the corticospinal tract revealed enhanced axonal sprouting and arborisation within the spinal cord gray matter preferentially targeting pre-motor and motor spinal cord laminae on lesion level and above in the anti-Nogo-A-treated animals. An important additional effect of Nogo-A-neutralization was enhanced remyelination observed after lysolecithin-induced demyelination of spinal tracts. Whereas remyelinated fiber numbers in the lesion site were increased several fold, no effect of Nogo-A-inhibition was observed on oligodendrocyte precursor proliferation, migration, or differentiation. Enhancing remyelination and promoting axonal regeneration and plasticity represent important unmet medical needs in multiple sclerosis. Anti-Nogo-A antibodies hold promise as a potential new therapy for multiple sclerosis, in particular during the chronic phase of the disease when neurodegeneration and remyelination failure determine disability evolution.

Nogo receptor 1 regulates Caspr distribution at axo-glial units in the central nervous system

Axo-glial units are highly organised microstructures propagating saltatory conduction and are disrupted during multiple sclerosis (MS). Nogo receptor 1 (NgR1) has been suggested to govern axonal damage during the progression of disease in the MS-like mouse model, experimental autoimmune encephalomyelitis (EAE). Here we have identified that adult ngr1 -/- mice, previously used in EAE and spinal cord injury experiments, display elongated paranodes, and nodes of Ranvier. Unstructured paranodal regions in ngr1 -/- mice are matched with more distributed expression pattern of Caspr. Compound action potentials of optic nerves and spinal cords from naïve ngr1 -/- mice are delayed and reduced. Molecular interaction studies revealed enhanced Caspr cleavage. Our data suggest that NgR1 may regulate axo-myelin ultrastructure through Caspr-mediated adhesion, regulating the electrophysiological signature of myelinated axons of central nervous system (CNS).

Blockade of chondroitin sulfate proteoglycans-induced axonal growth inhibition by LOTUS

Chondroitin sulfate proteoglycans (CSPGs) are axon growth inhibitors in the glial scar, and restrict axon regeneration following damage to the adult mammalian central nervous system. CSPGs have recently been identified as functional ligands for Nogo receptor-1 (NgR1), which is the common receptor for Nogo proteins, myelin-associated glycoprotein (MAG), oligodendrocyte myelin glycoprotein (OMgp) and B lymphocyte stimulator (BLyS). We have previously reported that through its binding to NgR1, lateral olfactory tract usher substance (LOTUS) suppresses Nogo, MAG, OMgp, and BLyS-induced axon growth inhibition. However, it remains unknown whether LOTUS also exerts this suppressive action on CSPG-induced axon growth inhibition. LOTUS overexpression rescued CSPG-induced growth cone collapse and neurite outgrowth inhibition in cultured dorsal root ganglion neurons, which only weakly express endogenous LOTUS. In cultured olfactory bulb neurons, which endogenously express LOTUS, the growth cone was insensitive to CSPG-induced collapse, but was sensitive to collapse induced by CSPGs in lotus-deficient mice. Our data demonstrate that LOTUS suppresses CSPG-induced axon growth inhibition, suggesting that LOTUS may represent a promising therapeutic agent for promoting axon regeneration.

Nogo-A in the visual system development and in ocular diseases

Nogo-A is a potent myelin-associated inhibitor for neuronal growth and plasticity in the central nervous system (CNS). Its effects are mediated by the activation of specific receptors that intracellularly control cytoskeleton rearrangements, protein synthesis and gene expression. Moreover, Nogo-A has been involved in the development of the visual system and in a variety of neurodegenerative diseases and injury processes that can alter its function. For example, Nogo-A was shown to influence optic nerve myelinogenesis, the formation and maturation of retinal axon projections, and retinal angiogenesis. In adult animals, the inactivation of Nogo-A exerted remarkable effects on visual plasticity. Relieving Nogo-A-induced inhibition increased axonal sprouting after optic nerve lesion and axonal rewiring in the visual cortex of intact adult mice. This review aims at presenting our current knowledge on the role of Nogo-A in the visual system and to discuss how its therapeutic targeting may promote visual improvement in ophthalmic diseases.

Nogo-A Antibodies for Progressive Multiple Sclerosis

Most of the current therapies, as well as many of the clinical trials, for multiple sclerosis (MS) target the inflammatory autoimmune processes, but less than 20% of all clinical trials investigate potential therapies for the chronic progressive disease stage of MS. The latter is responsible for the steadily increasing disability in many patients, and there is an urgent need for novel therapies that protect nervous system tissue and enhance axonal growth and/or remyelination. As outlined in this review, solid pre-clinical data suggest neutralization of the neurite outgrowth inhibitor Nogo-A as a potential new way to achieve both axonal and myelin repair. Several phase I clinical studies with anti-Nogo-A antibodies have been conducted in different disease paradigms including MS and spinal cord injury. Data from spinal cord injury and amyotrophic lateral sclerosis (ALS) trials accredit a good safety profile of high doses of anti-Nogo-A antibodies administered intravenously or intrathecally. An antibody against a Nogo receptor subunit, leucine rich repeat and immunoglobulin-like domain-containing protein 1 (LINGO-1), was recently shown to improve outcome in patients with acute optic neuritis in a phase II study. Nogo-A-suppressing antibodies could be novel drug candidates for the relapsing as well as the progressive MS disease stage. In this review, we summarize the available pre-clinical and clinical evidence on Nogo-A and elucidate the potential of Nogo-A-antibodies as a therapy for progressive MS.

Collapsin response mediator protein-2 plays a major protective role in acute axonal degeneration

Axonal degeneration is a key pathological feature in many neurological diseases. It often leads to persistent deficits due to the inability of axons to regenerate in the central nervous system. Therefore therapeutic approaches should optimally both attenuate axonal degeneration and foster axonal regeneration. Compelling evidence suggests that collapsin response mediator protein-2 (CRMP2) might be a molecular target fulfilling these requirements. In this mini-review, we give a compact overview of the known functions of CRMP2 and its molecular interactors in neurite outgrowth and in neurodegenerative conditions. Moreover, we discuss in detail our recent findings on the role of CRMP2 in acute axonal degeneration in the optic nerve. We found that the calcium influx induced by the lesion activates the protease calpain which cleaves CRMP2, leading to impairment of axonal transport. Both calpain inhibition and CRMP2 overexpression effectively protected the proximal axons against acute axonal degeneration. Taken together, CRMP2 is further characterized as a central molecular player in acute axonal degeneration and thus evolves as a promising therapeutic target to both counteract axonal degeneration and foster axonal regeneration in neurodegenerative and neurotraumatic diseases.

Increased phosphorylation of collapsin response mediator protein-2 at Thr514 correlates with β-amyloid burden and synaptic deficits in Lewy body dementias

Collapsin response mediator protein-2 (CRMP2) regulates axonal growth cone extension, and increased CRMP2 phosphorylation may lead to axonal degeneration. Axonal and synaptic pathology is an important feature of Lewy body dementias (LBD), but the state of CRMP2 phosphorylation (pCRMP2) as well as its correlations with markers of neurodegeneration have not been studied in these dementias. Hence, we measured CRMP2 phosphorylation at Thr509, Thr514 and Ser522, as well as markers of β-amyloid (Aβ), tau-phosphorylation, α-synuclein and synaptic function in the postmortem neocortex of a longitudinally assessed cohort of LBD patients characterized by low (Parkinson’s disease dementia, PDD) and high (dementia with Lewy bodies, DLB) burden of Alzheimer type pathology. We found specific increases of pCRMP2 at Thr514 in DLB, but not PDD. The increased CRMP2 phosphorylation correlated with fibrillogenic Aβ as well as with losses of markers for axon regeneration (β-III-tubulin) and synaptic integrity (synaptophysin) in LBD. In contrast, pCRMP2 alterations did not correlate with tau-phosphorylation or α-synuclein, and also appear unrelated to immunoreactivities of putative upstream kinases glycogen synthase kinase 3β and cyclin-dependent kinase 5, as well as to protein phosphatase 2A. In conclusion, increased pCRMP2 may underlie the axonal pathology of DLB, and may be a novel therapeutic target. However, antecedent signaling events as well as the nature of pCRMP2 association with Aβ and other neuropathologic markers require further study.

CRMPs Function in Neurons and Glial Cells: Potential Therapeutic Targets for Neurodegenerative Diseases and CNS Injury

Neurodegeneration in the adult mammalian central nervous system (CNS) is fundamentally accelerated by its intrinsic neuronal mechanisms, including its poor regenerative capacity and potent extrinsic inhibitory factors. Thus, the treatment of neurodegenerative diseases faces many obstacles. The degenerative processes, consisting of axonal/dendritic structural disruption, abnormal axonal transport, release of extracellular factors, and inflammation, are often controlled by the cytoskeleton. From this perspective, regulators of the cytoskeleton could potentially be a therapeutic target for neurodegenerative diseases and CNS injury. Collapsin response mediator proteins (CRMPs) are known to regulate the assembly of cytoskeletal proteins in neurons, as well as control axonal growth and neural circuit formation. Recent studies have provided some novel insights into the roles of CRMPs in several inhibitory signaling pathways of neurodegeneration, in addition to its functions in neurological disorders and CNS repair. Here, we summarize the roles of CRMPs in axon regeneration and its emerging functions in non-neuronal cells, especially in inflammatory responses. We also discuss the direct and indirect targeting of CRMPs as a novel therapeutic strategy for neurological diseases.

Sub-Chronic Neuropathological and Biochemical Changes in Mouse Visual System after Repetitive Mild Traumatic Brain Injury

Repetitive mild traumatic brain injury (r-mTBI) results in neuropathological and biochemical consequences in the human visual system. Using a recently developed mouse model of r-mTBI, with control mice receiving repetitive anesthesia alone (r-sham) we assessed the effects on the retina and optic nerve using histology, immunohistochemistry, proteomic and lipidomic analyses at 3 weeks post injury. Retina tissue was used to determine retinal ganglion cell (RGC) number, while optic nerve tissue was examined for cellularity, myelin content, protein and lipid changes. Increased cellularity and areas of demyelination were clearly detectable in optic nerves in r-mTBI, but not in r-sham. These changes were accompanied by a ~25% decrease in the total number of Brn3a-positive RGCs. Proteomic analysis of the optic nerves demonstrated various changes consistent with a negative effect of r-mTBI on major cellular processes like depolymerization of microtubules, disassembly of filaments and loss of neurons, manifested by decrease of several proteins, including neurofilaments (NEFH, NEFM, NEFL), tubulin (TUBB2A, TUBA4A), microtubule-associated proteins (MAP1A, MAP1B), collagen (COL6A1, COL6A3) and increased expression of other proteins, including heat shock proteins (HSP90B1, HSPB1), APOE and cathepsin D. Lipidomic analysis showed quantitative changes in a number of phospholipid species, including a significant increase in the total amount of lysophosphatidylcholine (LPC), including the molecular species 16:0, a known demyelinating agent. The overall amount of some ether phospholipids, like ether LPC, ether phosphatidylcholine and ether lysophosphatidylethanolamine were also increased, while the majority of individual molecular species of ester phospholipids, like phosphatidylcholine and phosphatidylethanolamine, were decreased. Results from the biochemical analysis correlate well with changes detected by histological and immunohistochemical methods and indicate the involvement of several important molecular pathways. This will allow future identification of therapeutic targets for improving the visual consequences of r-mTBI.

The cytoskeleton as a novel therapeutic target for old neurodegenerative disorders

Cytoskeleton defects, including alterations in microtubule stability, in axonal transport as well as in actin dynamics, have been characterized in several unrelated neurodegenerative conditions. These observations suggest that defects of cytoskeleton organization may be a common feature contributing to neurodegeneration. In line with this hypothesis, drugs targeting the cytoskeleton are currently being tested in animal models and in human clinical trials, showing promising effects. Drugs that modulate microtubule stability, inhibitors of posttranslational modifications of cytoskeletal components, specifically compounds affecting the levels of tubulin acetylation, and compounds targeting signaling molecules which regulate cytoskeleton dynamics, constitute the mostly addressed therapeutic interventions aiming at preventing cytoskeleton damage in neurodegenerative disorders. In this review, we will discuss in a critical perspective the current knowledge on cytoskeleton damage pathways as well as therapeutic strategies designed to revert cytoskeleton-related defects mainly focusing on the following neurodegenerative disorders: Alzheimer's Disease, Parkinson's Disease, Huntington's Disease, Amyotrophic Lateral Sclerosis and Charcot-Marie-Tooth Disease.

Inhibition of CRMP2 phosphorylation repairs CNS by regulating neurotrophic and inhibitory responses

Central nervous system (CNS) regeneration is restricted by both the lack of neurotrophic responses and the presence of inhibitory factors. As of yet, a common mediator of these two pathways has not been identified. Microtubule dynamics is responsible for several key processes after CNS injuries: intracellular trafficking of receptors for neurotrophic factors, axonal retraction by inhibitory factors, and secondary tissue damages by inflammation and scarring. Kinases regulating microtubule organization, such as Cdk5 or GSK3β, may play pivotal roles during CNS recovery, but the molecular mechanisms remain to be elucidated. Collapsin response mediator protein 2 (CRMP2) stabilizes cytoskeletal polymerization, while CRMP2 phosphorylation by Cdk5 and GSK3β loses its affinity for cytoskeleton proteins, leading to the inhibition of axonal growth. Here, we characterized CRMP2 phosphorylation as the first crucial factor regulating neurotrophic and inhibitory responses after spinal cord injury (SCI). We found that pharmacological inhibition of GSK3β enhanced brain-derived neurotrophic factor (BDNF)-induced axonal growth response in cultured dorsal root ganglion (DRG) neurons. DRG neurons from CRMP2 knock-in (Crmp2KI/KI) mice, where CRMP2 phosphorylation was eliminated, showed elevated sensitivity to BDNF as well. Additionally, cultured Crmp2KI/KI neurons exhibited suppressed axonal growth inhibition by chondroitin sulfate proteoglycan (CSPG). These data suggest a couple of new molecular insights: the BDNF/GSK3β/CRMP2 and CSPG/GSK3β/CRMP2 pathways. Next, we tested the significance of CRMP2 phosphorylation after CNS injury in vivo. The phosphorylation level of CRMP2 was enhanced in the injured spinal cord. Crmp2KI/KI mice exhibited prominent recovery of locomotive and nociceptive functions after SCI, which correlated with the enhanced axonal growth of the motor and sensory neurons. Neuroprotective effects against SCI, such as microtubule stabilization, reduced inflammation, and suppressed scarring were also observed by inhibiting CRMP2 phosphorylation. Therefore, inhibition of CRMP2 phosphorylation demonstrates the unique potential to repair SCI by both enhancing sensitivity to BDNF and reducing inhibitory responses.

Axonal degeneration in multiple sclerosis: defining therapeutic targets by identifying the causes of pathology

Current therapeutics in multiple sclerosis (MS) target the putative inflammation and immune attack on CNS myelin. Despite their effectiveness in blunting the relapse rate in MS patients, such therapeutics do not prevent MS disease progression. Importantly, specific clinical dilemma arises through inability to predict MS progression and thereby therapeutically target axonal injury during MS, limiting permanent disability. The current review identifies immune and neurobiological principles that govern the sequelae of axonal degeneration during MS disease progression. Defining the specific disease arbiters, inflammatory and autoimmune, oligodendrocyte dystrophy and degenerative myelin, we discuss a basis for a molecular mechanism in axons that may be targeted therapeutically, in spatial and temporal manner to limit axonal degeneration and thereby halt progression of MS.

The axon-glia unit in white matter stroke: mechanisms of damage and recovery

Approximately one quarter of all strokes in humans occur in white matter, and the progressive nature of white matter lesions often results in severe physical and mental disability. Unlike cortical grey matter stroke, the pathology of white matter stroke revolves around disrupted connectivity and injured axons and glial cells, rather than neuronal cell bodies. Consequently, the mechanisms behind ischemic damage to white matter elements, the regenerative responses of glial cells and their signaling pathways, all differ significantly from those in grey matter. Development of effective therapies for white matter stroke would require an enhanced understanding of the complex cellular and molecular interactions within the white matter, leading to the identification of new therapeutic targets. This review will address the unique properties of the axon-glia unit during white matter stroke, describe the challenging process of promoting effective white matter repair, and discuss recently-identified signaling pathways which may hold potential targets for repair in this disease. This article is part of a Special Issue entitled SI: Cell Interactions In Stroke.

Non-canonical actions of Nogo-A and its receptors

Nogo-A is a myelin associated protein and one of the most potent neurite growth inhibitors in the central nervous system. Interference with Nogo-A signaling has thus been investigated as therapeutic target to promote functional recovery in CNS injuries. Still, the finding that Nogo-A presents a fairly ubiquitous expression in many types of neurons in different brain regions, in the eye and even in the inner ear suggests for further functions besides the neurite growth repression. Indeed, a growing number of studies identified a variety of functions including regulation of neuronal stem cells, modulation of microglial activity, inhibition of angiogenesis and interference with memory formation. Aim of the present commentary is to draw attention on these less well-known and sometimes controversial roles of Nogo-A. Furthermore, we are addressing the role of Nogo-A in neuropathological conditions such as ischemic stroke, schizophrenia and neurodegenerative diseases.

CRMPs: critical molecules for neurite morphogenesis and neuropsychiatric diseases

Neuronal polarity and spatial rearrangement of neuronal processes are central to the development of all mature nervous systems. Recent studies have highlighted the dynamic expression of Collapsin-Response-Mediator Proteins (CRMPs) in neuronal dendritic/axonal compartments, described their interaction with cytoskeleton proteins, identified their ability to activate L- and N-type voltage-gated calcium channels (VGCCs) and delineated their crucial role as signaling molecules essential for neuron differentiation and neural network development and maintenance. In addition, evidence obtained from genome-wide/genetic linkage/proteomic/translational approaches revealed that CRMP expression is altered in human pathologies including mental (schizophrenia and mood disorders) and neurological (Alzheimer's, prion encephalopathy, epilepsy and others) disorders. Changes in CRMPs levels have been observed after psychotropic treatments, and disrupting CRMP2 binding to calcium channels blocked neuropathic pain. These observations, altogether with those obtained from genetically modified mice targeting individual CRMPs and RNA interference approaches, pave the way for considering CRMPs as potential early disease markers and modulation of their activity as therapeutic strategy for disorders associated with neurite abnormalities.

New Insights into the Roles of Nogo-A in CNS Biology and Diseases

Nogos have become a hot topic for its well-known number Nogo-A's big role in clinical matters. It has been recognized that the expression of Nogo-A and the receptor NgR1 inhibit the neuron's growth after CNS injuries or the onset of the MS. The piling evidence supports the notion that the Nogo-A is also involved in the synaptic plasticity, which was shown to negatively regulate the strength of synaptic transmission. The occurrence of significant schizophrenia-like behavioral phenotypes in Nogo-A KO rats also added strong proof to this conclusion. This review mainly focuses on the structure of Nogo-A and its corresponding receptor-NgR1, its intra- and extra-cellular signaling, together with its major physiological functions such as regulation of migration and distribution and its related diseases like stroke, AD, ALS and so on.

Lanthionine ketimine ester provides benefit in a mouse model of multiple sclerosis

Lanthionine ketimine (LK) is a natural sulfur amino acid metabolite which binds to collapsin response mediator protein-2 (CRMP2), an abundant brain protein that interacts with multiple partners to regulate microtubule dynamics, neurite growth and retraction, axonal transport, and neurotransmitter release. LK ethyl-ester (LKE) is a cell-permeable synthetic derivative that promotes neurogenesis, suppresses nitric oxide production from microglia, and reduces neurotoxicity of microglia-conditioned medium. These properties led us to test the effects of LKE in experimental autoimmune encephalomyelitis (EAE), a commonly used mouse model of multiple sclerosis. Female C57Bl/6 mice were immunized with myelin oligodendrocyte glycoprotein peptide 35-55 to develop a chronic disease. LKE was provided in the chow at 100 ppm, ad libitum beginning when the mice reached moderate clinical signs. Over the following 4 weeks the LKE-treated mice showed a significant reduction in clinical signs compared to vehicle-treated mice. LKE dose dependently reduced IFNγ production from splenic T cells, but had no effect on IL-17 production suggesting protective effects were mediated within the CNS. Electron microscopy revealed that, compared to sham mice, EAE mice had significant neurodegeneration in both the optic nerve and spinal cord, which was reduced in the LKE-treated mice. In contrast only minimal disruption of myelin was observed at this time point. In the optic nerve, measurements of axon caliber and myelin thickness showed little changes between sham and EAE mice, however, treatment with LKE increased the percentage of axons with thicker myelin and with larger axon calibers. In the spinal cord, only smaller effects of LKE on myelin thickness were observed. The effects of LKE were associated with a reduced relative level of phosphorylated CRMP2 to CRMP2. Together, these results demonstrate that LKE reduces neurodegeneration in a chronic EAE model of MS, which could have translation potential for treatment of progressive forms of MS.

Central nervous system regenerative failure: role of oligodendrocytes, astrocytes, and microglia

Animal studies are now showing the exciting potential to achieve significant functional recovery following central nervous system (CNS) injury by manipulating both the inefficient intracellular growth machinery in neurons, as well as the extracellular barriers, which further limit their regenerative potential. In this review, we have focused on the three major glial cell types: oligodendrocytes, astrocytes, and microglia/macrophages, in addition to some of their precursors, which form major extrinsic barriers to regrowth in the injured CNS. Although axotomized neurons in the CNS have, at best, a limited capacity to regenerate or sprout, there is accumulating evidence that even in the adult and, especially after boosting their growth motor, neurons possess the capacity for considerable circuit reorganization and even lengthy regeneration when these glial obstacles to neuronal regrowth are modified, eliminated, or overcome.

Biotherapies in multiple sclerosis: a step toward remyelination and neuroprotection?

Multiple sclerosis (MS) is a complex disease of the central nervous system (CNS), characterized by CNS-restricted inflammation with subsequent demyelination and neurodegeneration. Current disease-modifying therapies efficiently reduce relapse rate and new lesions appearance, but still fail to impact the progressive course of the disease. There is a great need for the avenue of new therapies aimed at promoting myelin repair or reducing neurodegeneration that should result in the prevention of neurological disability in this chronic disease. This review will focus on the potentials and limitations of biotherapies that are currently developed for the promotion of CNS repair in MS, either monoclonal antibodies targeting axonal growth and remyelination, or cell therapies aimed at replacing the depleted myelinating cells within the CNS. As other researches aimed at promoting neuroprotection or remyelination are following a classical pharmacological approach, they will not be described in this review, which will focus on antibody-based therapies and cell therapies.

Nogo receptor inhibition enhances functional recovery following lysolecithin-induced demyelination in mouse optic chiasm

BACKGROUND

Inhibitory factors have been implicated in the failure of remyelination in demyelinating diseases. Myelin associated inhibitors act through a common receptor called Nogo receptor (NgR) that plays critical inhibitory roles in CNS plasticity. Here we investigated the effects of abrogating NgR inhibition in a non-immune model of focal demyelination in adult mouse optic chiasm.

METHODOLOGY/PRINCIPAL FINDINGS

A focal area of demyelination was induced in adult mouse optic chiasm by microinjection of lysolecithin. To knock down NgR levels, siRNAs against NgR were intracerebroventricularly administered via a permanent cannula over 14 days, Functional changes were monitored by electrophysiological recording of latency of visual evoked potentials (VEPs). Histological analysis was carried out 3, 7 and 14 days post demyelination lesion. To assess the effect of NgR inhibition on precursor cell repopulation, BrdU was administered to the animals prior to the demyelination induction. Inhibition of NgR significantly restored VEPs responses following optic chiasm demyelination. These findings were confirmed histologically by myelin specific staining. siNgR application resulted in a smaller lesion size compared to control. NgR inhibition significantly increased the numbers of BrdU+/Olig2+ progenitor cells in the lesioned area and in the neurogenic zone of the third ventricle. These progenitor cells (Olig2+ or GFAP+) migrated away from this area as a function of time.

CONCLUSIONS/SIGNIFICANCE

Our results show that inhibition of NgR facilitate myelin repair in the demyelinated chiasm, with enhanced recruitment of proliferating cells to the lesion site. Thus, antagonizing NgR function could have therapeutic potential for demyelinating disorders such as Multiple Sclerosis.

Axonal degeneration in multiple sclerosis: can we predict and prevent permanent disability?

Axonal degeneration is a major determinant of permanent neurological impairment during multiple sclerosis (MS). Due to the variable course of clinical disease and the heterogeneity of MS lesions, the mechanisms governing axonal degeneration may differ between disease stages. While the etiology of MS remains elusive, there now exist potential prognostic biomarkers that can predict the conversion to clinically definite MS. Specialized imaging techniques identifying axonal injury and drop-out are becoming established in clinical practice as a predictive measure of MS progression, such as optical coherence tomography (OCT) or diffusion tensor imaging (DTI). However, these imaging techniques are still being debated as predictive biomarkers since controversy surrounds their lesion-specific association with expanded disability status scale (EDSS). A more promising diagnostic measure of axonal degeneration has been argued for the detection of reduced N-acetyl aspartate (NAA) and Creatine ratios via magnetic resonance spectroscopic (MRS) imaging, but again fail with its specificity for predicting actual axonal degeneration. Greater accuracy of predictive biomarkers is therefore warranted and may include CSF neurofilament light chain (NF-L) and neurofilament heavy chain (NF-H) levels, for progressive MS. Furthermore, defining the molecular mechanisms that occur during the neurodegenerative changes in the various subgroups of MS may in fact prove vital for the future development of efficacious neuroprotective therapies. The clinical translation of a combined Na⁺ and Ca²⁺ channel blocker may lead to the establishment of a bona fide neuroprotective agent for the treatment of progressive MS. However, more specific therapeutic targets to limit axonal damage in MS need investigation and may include such integral axonal proteins such as the collapsin response mediator protein-2 (CRMP-2), a molecule which upon post-translational modification may propagate axonal degeneration in MS. In this review, we discuss the current clinical determinants of axonal damage in MS and consider the cellular and molecular mechanisms that may initiate these neurodegenerative changes. In particular we highlight the therapeutic candidates that may formulate novel therapeutic strategies to limit axonal degeneration and EDSS during progressive MS.

Interleukin-18 alters protein expressions of neurodegenerative diseases-linked proteins in human SH-SY5Y neuron-like cells

Chronic inflammation and oxidative stress (OS) are present in Alzheimer's disease (AD) brains in addition to neuronal loss, Amyloid-β (Aβ) plaques and hyperphosphorylated tau-protein neurofibrillary tangles (NFTs). Previously we showed that levels of the pro-inflammatory cytokine, interleukin-18 (IL-18), are elevated in post-mortem AD brains. IL-18 can modulate the tau kinases, Cdk5 and GSK3β, as well as Aβ-production. IL-18 levels are also increased in AD risk diseases, including type-2 diabetes and obesity. Here, we explored other IL-18 regulated proteins in neuron-like SH-SY5Y cells. Differentiated SH-SY5Y cells, incubated with IL-18 for 24, 48, or 72 h, were analyzed by two-dimensional gel electrophoresis (2D-DIGE). Specific altered protein spots were chosen and identified with mass spectrometry (MS) and verified by western immunoblotting (WIB). IL-18 had time-dependent effects on the SH-SY5Y proteome, modulating numerous protein levels/modifications. We concentrated on those related to OS (DDAH2, peroxiredoxins 2, 3, and 6, DJ-1, BLVRA), Aβ-degradation (MMP14, TIMP2), Aβ-aggregation (Septin-2), and modifications of axon growth and guidance associated, collapsin response mediator protein 2 (CRMP2). IL-18 significantly increased antioxidative enzymes, indicative of OS, and altered levels of glycolytic α- and γ-enolase and multifunctional 14-3-3γ and -ε, commonly affected in neurodegenerative diseases. MMP14, TIMP2, α-enolase and 14-3-3ε, indirectly involved in Aβ metabolism, as well as Septin-2 showed changes that increase Aβ levels. Increased 14-3-3γ may contribute to GSK3β driven tau hyperphosphorylation and CRMP2 Thr514 and Ser522 phosphorylation with the Thr555-site, a target for Rho kinase, showing time-dependent changes. IL-18 also increased caspase-1 levels and vacuolization of the cells. Although our SH-SY5Y cells were not aged, as neurons in AD, our work suggests that heightened or prolonged IL-18 levels can drive protein changes of known relevance to AD pathogenesis.

Axonal transport rate decreased at the onset of optic neuritis in EAE mice

Optic neuritis is frequently the first symptom of multiple sclerosis (MS), an inflammatory demyelinating neurodegenerative disease. Impaired axonal transport has been considered as an early event of neurodegenerative diseases. However, few studies have assessed the integrity of axonal transport in MS or its animal models. We hypothesize that axonal transport impairment occurs at the onset of optic neuritis in experimental autoimmune encephalomyelitis (EAE) mice. In this study, we employed manganese-enhanced MRI (MEMRI) to assess axonal transport in optic nerves in EAE mice at the onset of optic neuritis. Axonal transport was assessed as (a) optic nerve Mn(2+) accumulation rate (in % signal change/h) by measuring the rate of increased total optic nerve signal enhancement, and (b) Mn(2+) transport rate (in mm/h) by measuring the rate of change in optic nerve length enhanced by Mn(2+). Compared to sham-treated healthy mice, Mn(2+) accumulation rate was significantly decreased by 19% and 38% for EAE mice with moderate and severe optic neuritis, respectively. The axonal transport rate of Mn(2+) was significantly decreased by 43% and 65% for EAE mice with moderate and severe optic neuritis, respectively. The degree of axonal transport deficit correlated with the extent of impaired visual function and diminished microtubule-associated tubulins, as well as the severity of inflammation, demyelination, and axonal injury at the onset of optic neuritis.

Nogo limits neural plasticity and recovery from injury

The expression of Nogo-A and the receptor NgR1 limits the recovery of adult mammals from central nervous system injury. Multiple studies have demonstrated efficacy from targeting this pathway for functional recovery and neural repair after spinal cord trauma, ischemic stroke, optic nerve injury and models of multiple sclerosis. Recent molecular studies have added S1PR2 as a receptor for the amino terminal domain of Nogo-A, and have demonstrated shared components for Nogo-A and CSPG signaling as well as novel Nogo antagonists. It has been recognized that neural repair involves plasticity, sprouting and regeneration. A physiologic role for Nogo-A and NgR1 has been documented in the restriction of experience-dependent plasticity with maturity, and the stability of synaptic, dendritic and axonal anatomy.

CRMP2: functional roles in neural development and therapeutic potential in neurological diseases

Cytoskeletal restructuring is essential for nearly all cellular processes in the developing brain. After cell fate determination, newborn cortical neurons must migrate to their final positions while establishing proper axon-dendrite polarity. Significant progress has recently been made towards understanding the cellular and molecular mechanisms underlying neuronal polarization in vivo. Collapsin response mediator protein 2 (CRMP2) has long been identified as a microtubule-binding protein that regulates neuronal polarity in vitro. Recent studies provide new insights into the roles of CRMP2 in neuronal migration and subsequent neuronal differentiation. Both the expression and activity of CRMP2 are tightly regulated during cortex development. CRMP2 is suggested to be important in the multipolar-bipolar transition in radial migration. The increasing number of known interaction partners indicates that CRMP2 has functions beyond cytoskeletal regulation, including axonal transport, vesicle trafficking, and neurotransmitter release. This review discusses the current knowledge about CRMP2 in the context of neuronal development and highlights a recent emerging theme regarding its potential therapeutic applications.

Nogo-receptor 1 deficiency has no influence on immune cell repertoire or function during experimental autoimmune encephalomyelitis

The potential role of Nogo-66 Receptor 1 (NgR1) on immune cell phenotypes and their activation during neuroinflammatory diseases such as multiple sclerosis (MS) and its animal model, experimental autoimmune encephalomyelitis (EAE), is unclear. To further understand the function of this receptor on haematopoietically-derived cells, phenotypic and functional analyses were performed using NgR1-deficient (ngr1-/-) animals. Flow cytometry-based phenotypic analyses performed on blood, spleen, thymus, lymph nodes, bone marrow and central nervous-system (CNS)-infiltrating blood cells revealed no immunological defects in naïve ngr1-/- animals versus wild-type littermate (WTLM) controls. EAE was induced by either recombinant myelin oligodendrocyte glycoprotein (rMOG), a model in which B cells are considered to contribute pathogenically, or by MOG_35–55 peptide, a B cell-independent model. We have demonstrated that in ngr1-/- mice injected with MOG_35–55, a significant reduction in the severity of EAE correlated with reduced axonal damage present in the spinal cord when compared to their WTLM controls. However, despite a reduction in axonal damage observed in the CNS of ngr1-/- mice at the chronic stage of disease, no clinical differences could be attributed to a specific genotype when rMOG was used as the encephalitogen. Following MOG_35–55-induction of EAE, we could not derive any major changes to the immune cell populations analyzed between ngr1-/- and WTLM mice. Collectively, these data demonstrate that NgR1 has little if any effects on the repertoire of immune cells, their activation and trafficking to the CNS.

The role of reticulons in neurodegenerative diseases

Reticulons (RTNs) are a group of membrane-associated proteins mainly responsible for shaping the tubular endoplasmic reticulum network, membrane trafficking, inhibition of axonal growth, and apoptosis. These proteins share a common sequence feature, the reticulon homology domain, which consists of paired hydrophobic stretches that are believed to induce membrane curvature by acting as a wedge in bilayer membranes. RTNs are ubiquitously expressed in all tissues, but each RTN member exhibits a unique expression pattern that prefers certain tissues or even cell types. Recently, accumulated evidence has suggested additional and unexpected roles for RTNs, including those on DNA binding, autophagy, and several inflammatory-related functions. These manifold actions of RTNs account for their ever-growing recognition of their involvement in neurodegenerative diseases like Alzheimer's disease, amyotrophic lateral sclerosis, multiple sclerosis, as well as hereditary spastic paraplegia. This review summarizes the latest discoveries on RTNs in human pathophysiology, and the engagement of these in neurodegeneration, along with the implications of these findings for a better understanding of the molecular events triggered by RTNs and their potential exploitation as next-generation therapeutics.

Multiple sclerosis: does Nogo play a role?

Over the past decade, there has been substantial interest in the role of the integral myelin protein, Nogo-A, from fundamental neurobiological to clinical perspectives. It is now a well-known inhibitor of neurite outgrowth through its cognate receptor, Nogo receptor 1 (NgR1). Nogo-A can only signal through NgR1 upon heteromeric collaboration with p75(NTR), TROY, and LINGO-1 to induce axonal retraction. Both Nogo-A and NgR1 are expressed in multiple sclerosis (MS) lesions, suggesting that Nogo signaling may play a pivotal role in disease progression. There are several approaches targeting Nogo signaling in animal models of MS, and these therapeutic effects are currently in debate. One of the points of contention arises from the localization of the aforementioned signaling molecules, considering that MS and its animal models of disease are governed by inflammatory infiltration of the central nervous system. Furthermore, an impressive list of ligands for NgR1 continues to be compiled, possibly leading to disparities in the results obtained from the various animal models. In this review, we systematically dissect the complexities of Nogo signaling, which may be relevant in the future directions of neuroprotective therapies for MS.

Opening Pandora's jar: a primer on the putative roles of CRMP2 in a panoply of neurodegenerative, sensory and motor neuron, and central disorders

CRMP2, also known as DPYSL2/DRP2, Unc-33, Ulip or TUC2, is a cytosolic phosphoprotein that mediates axon/dendrite specification and axonal growth. Mapping the CRMP2 interactome has revealed previously unappreciated functions subserved by this protein. Together with its canonical roles in neurite growth and retraction and kinesin-dependent axonal transport, it is now known that CRMP2 interacts with numerous binding partners to affect microtubule dynamics; protein endocytosis and vesicular cycling, synaptic assembly, calcium channel regulation and neurotransmitter release. CRMP2 signaling is regulated by post-translational modifications, including glycosylation, oxidation, proteolysis and phosphorylation; the latter being a fulcrum of CRMP2 functions. Here, the putative roles of CRMP2 in a panoply of neurodegenerative, sensory and motor neuron, and central disorders are discussed and evidence is presented for therapeutic strategies targeting CRMP2 functions.

Insight into the role of CRMP2 (collapsin response mediator protein 2) in T lymphocyte migration: the particular context of virus infection

Lymphocyte migration into the central nervous system is a critical step in the physiopathology of a variety of neurological diseases, including multiple sclerosis and virus-induced neuroinflammation. To better understand the molecular mechanisms involved in cells migration, we focused our studies on collapsin response mediator proteins (CRMPs), a group of phosphoproteins that mediate neural cell motility. There is now evidence that collapsin response mediator protein 2 (CRMP2) plays critical roles in the polarization (uropod formation) of T lymphocytes and their subsequent migration. CRMP2 was known to respond to semaphorin, ephrin and neurotrophin signaling in neurons. The link between the chemokine CXCL12, CRMP2 activity and cell migration has been demonstrated in T lymphocytes. These observations and comparisons of the activity of CRMPs in immune and non-immmune cells are summarized here. The ability of a human retrovirus to enhance lymphocyte migration through the modulation of CRMP2 activity is also discussed. In conclusion, viruses have the ability to manipulate the lymphocyte motility machinery, intensifying neural tissue invasion in infected patients.

Neurotrophic factors and the regeneration of adult retinal ganglion cell axons

The adult central nervous system (CNS) has only a limited capacity to regenerate axons after injury. This is due to a number of factors including the presence of extrinsic inhibitory factors that limit plasticity, lack of effective trophic support, and intrinsic changes in neuronal responsiveness. In this review, we describe the expression and role of neurotrophins in retinal ganglion cells (RGCs) during development and adulthood, and the receptors and miscellaneous signaling systems that influence axonal regeneration after injury. The impact of exogenous neurotrophic factors on adult RGCs injured at different sites in the visual pathway is described for several modes of delivery, including recombinant factors, viral vectors, cell transplantation, as well as combinatorial treatments involving other pharmacotherapeutic agents. Indirect, off-target effects of neurotrophic factors on RGC axonal regeneration are also considered. There remain unresolved issues relating to optimal delivery of neurotrophic factors, and we emphasize the need to develop safe, reliable methods for the regulation of exogenous supply of these factors to the injured CNS.