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White matter disorders of the CNS such as MS, lead to failure of nerve conduction and long-lasting neurological disabilities affecting a variety of sensory and motor systems including vision. While most disease-modifying therapies target the immune and inflammatory response, the promotion of remyelination has become a new therapeutic avenue, to prevent neuronal degeneration and promote recovery. Most of these strategies are developed in short-lived rodent models of demyelination, which spontaneously repair and do not reflect the size, organization, and biology of the human CNS. Thus, well-defined non-human primate models are required to efficiently advance therapeutic approaches for patients. Here, we followed the consequence of long-term toxin-induced demyelination of the macaque optic nerve on remyelination and axon preservation, as well as its impact on visual functions. Findings from oculo-motor behavior, ophthalmic examination, electrophysiology, and retinal imaging indicate visual impairment involving the optic nerve and retina. These visual dysfunctions fully correlated at the anatomical level, with sustained optic nerve demyelination, axonal degeneration, and alterations of the inner retinal layers. This non-human primate model of chronic optic nerve demyelination associated with axonal degeneration and visual dysfunction, recapitulates several key features of MS lesions and should be instrumental in providing the missing link to translate emerging repair pro-myelinating/neuroprotective therapies to the clinic for myelin disorders such as MS.Significance StatementPromotion of remyelination has become a new therapeutic avenue, to prevent neuronal degeneration and promote recovery in white matter diseases such as MS. To date most of these strategies are developed in short-lived rodent models of demyelination, which spontaneously repair. Well-defined non-human primate models closer to man would allow to efficiently advance therapeutic approaches. Here we present a non-human primate model of optic nerve demyelination that recapitulates several features of MS lesions. The model leads to failed remyelination, associated with progressive axonal degeneration and visual dysfunction, thus providing the missing link to translate emerging pre-clinical therapies to the clinic for myelin disorders such as MS.

The presence of putative stem/progenitor cells has been suggested in adult peripheral nervous system (PNS) tissue, including the dorsal root ganglion (DRG). To date, their identification and fate in pathophysiological conditions have not been addressed. Combining multiple in vitro and in vivo approaches, we identified the presence of stem cells in the adult DRG satellite glial population, and progenitors were present in the DRGs and sciatic nerve. Cell-specific transgenic mouse lines highlighted the proliferative potential of DRG stem cells and progenitors in vitro. DRG stem cells had gliogenic and neurogenic potentials, whereas progenitors were essentially gliogenic. Lineage tracing showed that, under physiological conditions, adult DRG stem cells maintained DRG homeostasis by supplying satellite glia. Under pathological conditions, adult DRG stem cells replaced DRG neurons lost to injury in addition of renewing the satellite glial pool. These novel findings open new avenues for development of therapeutic strategies targeting DRG stem cells for PNS disorders.

International audience; Slit1 is a secreted axon guidance molecule, also involved in adult neurogenesis. In physiological conditions, Slit1 loss promotes ectopic dispersal of SVZ-derived neural precursors (SVZ-NPCs) into periventricular structures such as the corpus callosum. Demyelination of the corpus callosum triggers SVZ-NPC migration to ectopic locations and their recruitment by the lesion, suggesting a possible role for Slit1 in SVZ-NPCs ectopic dispersal regulation in pathological conditions. Here, we have investigated the function of Slit1 protein in the recruitment of SVZ-NPCs after CNS demyelination. We find that the dynamics of oligodendrogenesis and temporal profile of developmental myelination in Slit1 -/- mice are similar to Slit1 +/- controls. SVZ micro-dissection and RT-PCR from wild-type mice, show that Slits and Robos are physiologically regulated at the transcriptional level in response to corpus callosum demyelination suggesting their role in the process of SVZ-NPC ectopic migration in demyelinating conditions. Moreover, we find that the number of SVZ-NPCs recruited by the lesion increases in Sli1-/- mice compared to Slit1 +/- mice, leading to higher numbers of Olig2+ cells within the lesion. Time-lapse video-microscopy of immuno-purified NPCs shows that Slit1-deficient cells migrate faster and make more frequent directional changes than control NPCs, supporting a cell-autonomous mechanism of action of Slit1 in NPC migration. In conclusion, while Slit1 does not affect the normal developmental process of oligodendrogenesis and myelination, it regulates adult SVZ-NPC ectopic migration in response to demyelination, and consequently oligodendrocyte renewal within the lesion.

Background and ObjectivesTo test whether low concentrations of teriflunomide (TF) could promote remyelination, we investigate the effect of TF on oligodendrocyte in culture and on remyelination in vivo in 2 demyelinating models.MethodsThe effect of TF on oligodendrocyte precursor cell (OPC) proliferation and differentiation was assessed in vitro in glial cultures derived from neonatal mice and confirmed on fluorescence-activated cell sorting–sorted adult OPCs. The levels of the 8,9-unsaturated sterols lanosterol and zymosterol were quantified in TF- and sham-treated cultures. In vivo, TF was administered orally, and remyelination was assessed both in myelin basic protein–GFP-nitroreductase (Mbp:GFP-NTR) transgenic Xenopus laevis demyelinated by metronidazole and in adult mice demyelinated by lysolecithin.ResultsIn cultures, low concentrations of TF down to 10 nM decreased OPC proliferation and increased their differentiation, an effect that was also detected on adult OPCs. Oligodendrocyte differentiation induced by TF was abrogated by the oxidosqualene cyclase inhibitor Ro 48-8071 and was mediated by the accumulation of zymosterol. In the demyelinated tadpole, TF enhanced the regeneration of mature oligodendrocytes up to 2.5-fold. In the mouse demyelinated spinal cord, TF promoted the differentiation of newly generated oligodendrocytes by a factor of 1.7-fold and significantly increased remyelination.DiscussionTF enhances zymosterol accumulation in oligodendrocytes and CNS myelin repair, a beneficial off-target effect that should be investigated in patients with multiple sclerosis.

Schwann cells (SC) enter the central nervous system (CNS) in pathophysiological conditions. However, how SC invade the CNS to remyelinate central axons remains undetermined. We studied SC migratory behavior ex vivo and in vivo after exogenous transplantation in the demyelinated spinal cord. The data highlight for the first time that SC migrate preferentially along blood vessels in perivascular extracellular matrix (ECM), avoiding CNS myelin. We demonstrate in vitro and in vivo that this migration route occurs by virtue of a dual mode of action of Eph/ephrin signaling. Indeed, EphrinB3, enriched in myelin, interacts with SC Eph receptors, to drive SC away from CNS myelin, and triggers their preferential adhesion to ECM components, such as fibronectin via integrinβ1 interactions. This complex interplay enhances SC migration along the blood vessel network and together with lesion-induced vascular remodeling facilitates their timely invasion of the lesion site. These novel findings elucidate the mechanism by which SC invade and contribute to spinal cord repair. Electronic supplementary material The online version of this article (10.1007/s00401-019-02011-1) contains supplementary material, which is available to authorized users.

Induced pluripotent stem cell–derived (iPS-derived) neural precursor cells may represent the ideal autologous cell source for cell-based therapy to promote remyelination and neuroprotection in myelin diseases. So far, the therapeutic potential of reprogrammed cells has been evaluated in neonatal demyelinating models. However, the repair efficacy and safety of these cells has not been well addressed in the demyelinated adult CNS, which has decreased cell plasticity and scarring. Moreover, it is not clear if these induced pluripotent–derived cells have the same reparative capacity as physiologically committed CNS-derived precursors. Here, we performed a side-by-side comparison of CNS-derived and skin-derived neural precursors in culture and following engraftment in murine models of adult spinal cord demyelination. Grafted induced neural precursors exhibited a high capacity for survival, safe integration, migration, and timely differentiation into mature bona fide oligodendrocytes. Moreover, grafted skin–derived neural precursors generated compact myelin around host axons and restored nodes of Ranvier and conduction velocity as efficiently as CNS-derived precursors while outcompeting endogenous cells. Together, these results provide important insights into the biology of reprogrammed cells in adult demyelinating conditions and support use of these cells for regenerative biomedicine of myelin diseases that affect the adult CNS.

Remyelination of CNS axons by Schwann cells (SCs) is not efficient, in part due to the poor migration of SCs into the adult CNS. Although it is known that migrating SCs avoid white matter tracts, the molecular mechanisms underlying this exclusion have never been elucidated. We now demonstrate that myelin-associated glycoprotein (MAG), a well known inhibitor of neurite outgrowth, inhibits rat SC migration and induces their death via γ-secretase-dependent regulated intramembrane proteolysis of the p75 neurotrophin receptor (also known as p75 cleavage). Blocking p75 cleavage using inhibitor X (Inh X), a compound that inhibits γ-secretase activity before exposing to MAG or CNS myelin improves SC migration and survival in vitro Furthermore, mouse SCs pretreated with Inh X migrate extensively in the demyelinated mouse spinal cord and remyelinate axons. These results suggest a novel role for MAG/myelin in poor SC-myelin interaction and identify p75 cleavage as a mechanism that can be therapeutically targeted to enhance SC-mediated axon remyelination in the adult CNS.SIGNIFICANCE STATEMENT Numerous studies have used Schwann cells, the myelin-making cells of the peripheral nervous system to remyelinate adult CNS axons. Indeed, these transplanted cells successfully remyelinate axons, but unfortunately they do not migrate far and so remyelinate only a few axons in the vicinity of the transplant site. It is believed that if Schwann cells could be induced to migrate further and survive better, they may represent a valid therapy for remyelination. We show that myelin-associated glycoprotein or CNS myelin, in general, inhibit rodent Schwann cell migration and induce their death via cleavage of the neurotrophin receptor p75. Blockade of p75 cleavage using a specific inhibitor significantly improves migration and survival of the transplanted Schwann cells in vivo.

Rapid and efficient protocols to generate oligodendrocytes (OL) from human induced pluripotent stem cells (iPSC) are currently lacking, but may be a key technology to understand the biology of myelin diseases and to develop treatments for such disorders. Here, we demonstrate that the induction of three transcription factors (SOX10, OLIG2, NKX6.2) in iPSC-derived neural progenitor cells is sufficient to rapidly generate O4+ OL with an efficiency of up to 70% in 28 d and a global gene-expression profile comparable to primary human OL. We further demonstrate that iPSC-derived OL disperse and myelinate the CNS of Mbpshi/shiRag-/- mice during development and after demyelination, are suitable for in vitro myelination assays, disease modeling, and screening of pharmacological compounds potentially promoting oligodendroglial differentiation. Thus, the strategy presented here to generate OL from iPSC may facilitate the studying of human myelin diseases and the development of high-throughput screening platforms for drug discovery.

The basic helix-loop-helix transcription factor Olig2 is a key determinant for the specification of neural precursor cells into oligodendrocyte progenitor cells. However, the functional role of Olig2 in oligodendrocyte migration and differentiation remains elusive both during developmental myelination and under demyelinating conditions of the adult central nervous system. To decipher Olig2 functions, we generated transgenic mice (TetOlig2:Sox10(rtTA/+)) overexpressing Olig2 in Sox10(+) oligodendroglial cells in a doxycycline inducible manner. We show that Olig2 overexpression increases the generation of differentiated oligodendrocytes, leading to precocious myelination of the central nervous system. Unexpectedly, we found that gain of Olig2 function in oligodendrocyte progenitor cells enhances their migration rate. To determine whether Olig2 overexpression in adult oligodendrocyte progenitor cells promotes oligodendrocyte regeneration for myelin repair, we induced lysophosphatidylcholine demyelination in the corpus callosum of TetOlig2:Sox10(rtTA/+) and control mice. We found that Olig2 overexpression enhanced oligodendrocyte progenitor cell differentiation and remyelination. To assess the relevance of these findings in demyelinating diseases, we also examined OLIG2 expression in multiple sclerosis lesions. We demonstrate that OLIG2 displays a differential expression pattern in multiple sclerosis lesions that correlates with lesion activity. Strikingly, OLIG2 was predominantly detected in NOGO-A(+) (now known as RTN4-A) maturing oligodendrocytes, which prevailed in active lesion borders, rather than chronic silent and shadow plaques. Taken together, our data provide proof of principle indicating that OLIG2 overexpression in oligodendrocyte progenitor cells might be a possible therapeutic mechanism for enhancing myelin repair.

International audience; Oligodendrocyte and myelin deficits have been reported in mental/psychiatric diseases. The p21-activated kinase 3 (PAK3), a serine/threonine kinase, whose activity is stimulated by the binding of active Rac and Cdc42 GTPases is affected in these pathologies. Indeed, many mutations of Pak3 gene have been described in non-syndromic intellectual disability diseases. Pak3 is expressed mainly in the brain where its role has been investigated in neurons but not in glial cells. Here, we showed that PAK3 is highly expressed in oligodendrocyte precursors (OPCs) and its expression decreases in mature oligodendrocytes. In the developing white matter of the Pak3 knockout mice, we found defects of oligodendrocyte differentiation in the corpus callosum and to a lesser extent in the anterior commissure, which were compensated at the adult stage. In vitro experiments in OPC cultures, derived from Pak3 knockout and wild type brains, support a developmental and cell-autonomous role for PAK3 in regulating OPC differentiation into mature oligodendrocytes. Moreover, we did not detect any obvious alterations of the proliferation or migration of Pak3 null OPCs compared to wild type. Overall, our data highlight PAK3 as a new regulator of OPC differentiation.

The factors that determine whether remyelination fails or succeeds in multiple sclerosis remain unknown. By grafting lymphocytes from patients into demyelinated lesions in mice, El Behi, Sanson et al. show that lymphocytes differ in their ability to induce remyelination. Unravelling the basis of this heterogeneity reveals prerequisites for efficient myelin repair., One major challenge in multiple sclerosis is to understand the cellular and molecular mechanisms leading to disease severity progression. The recently demonstrated correlation between disease severity and remyelination emphasizes the importance of identifying factors leading to a favourable outcome. Why remyelination fails or succeeds in multiple sclerosis patients remains largely unknown, mainly because remyelination has never been studied within a humanized pathological context that would recapitulate major events in plaque formation such as infiltration of inflammatory cells. Therefore, we developed a new paradigm by grafting healthy donor or multiple sclerosis patient lymphocytes in the demyelinated lesion of nude mice spinal cord. We show that lymphocytes play a major role in remyelination whose efficacy is significantly decreased in mice grafted with multiple sclerosis lymphocytes compared to those grafted with healthy donors lymphocytes. Mechanistically, we demonstrated in vitro that lymphocyte-derived mediators influenced differentiation of oligodendrocyte precursor cells through a crosstalk with microglial cells. Among mice grafted with lymphocytes from different patients, we observed diverse remyelination patterns reproducing for the first time the heterogeneity observed in multiple sclerosis patients. Comparing lymphocyte secretory profile from patients exhibiting high and low remyelination ability, we identified novel molecules involved in oligodendrocyte precursor cell differentiation and validated CCL19 as a target to improve remyelination. Specifically, exogenous CCL19 abolished oligodendrocyte precursor cell differentiation observed in patients with high remyelination pattern. Multiple sclerosis lymphocytes exhibit intrinsic capacities to coordinate myelin repair and further investigation on patients with high remyelination capacities will provide new pro-regenerative strategies.

Pelizaeus-Merzbacher disease (PMD) results from an X-linked misexpression of proteolipid protein 1 (PLP1). This leukodystrophy causes severe hypomyelination with progressive inflammation, leading to neurological dysfunctions and shortened life expectancy. While no cure exists for PMD, experimental cell-based therapy in the dysmyelinated shiverer model suggested that human oligodendrocyte progenitor cells (hOPCs) or human neural precursor cells (hNPCs) are promising candidates to treat myelinopathies. However, the fate and restorative advantages of human NPCs/OPCs in a relevant model of PMD has not yet been addressed. Using a model of Plp1 overexpression, resulting in demyelination with progressive inflammation, we compared side-by-side the therapeutic benefits of intracerebrally grafted hNPCs and hOPCs. Our findings reveal equal integration of the donor cells within presumptive white matter tracks. While the onset of exogenous remyelination was earlier in hOPCs-grafted mice than in hNPC-grafted mice, extended lifespan occurred only in hNPCs-grafted animals. This improved survival was correlated with reduced neuroinflammation (microglial and astrocytosis loads) and microglia polarization toward M2-like phenotype followed by remyelination. Thus modulation of neuroinflammation combined with myelin restoration is crucial to prevent PMD pathology progression and ensure successful rescue of PMD mice. These findings should help to design novel therapeutic strategies combining immunomodulation and stem/progenitor cell-based therapy for disorders associating hypomyelination with inflammation as observed in PMD.

Boundary cap cells (BC), which express the transcription factor Krox20, participate in the formation of the boundary between the central nervous system and the peripheral nervous system. To study BC stemness, we developed a method to purify and amplify BC in vitro from Krox20 Cre/+ , R26R YFP/+ mouse embryos. We show that BC progeny are EGF/FGF2-responsive, form spheres, and express neural crest markers. Upon growth factor withdrawal, BC progeny gave rise to multiple neural crest and CNS lineages. Transplanted into the developing murine forebrain, they successfully survived, migrated, and integrated within the host environment. Surprisingly, BC progeny generated exclusively CNS cells, including neurons, astrocytes, and myelin-forming oligodendrocytes. In vitro experiments indicated that a sequential combination of ventralizing morphogens and glial growth factors was necessary to reprogram BC into oligodendrocytes. Thus, BC progeny are endowed with differentiation plasticity beyond the peripheral nervous system. The demonstration that CNS developmental cues can reprogram neural crest-derived stem cells into CNS derivatives suggests that BC could serve as a source of cell type-specific lineages, including oligodendrocytes, for cell-based therapies to treat CNS disorders.

During development, boundary cap cells (BC) and neural crest cell (NCC) derivatives generate Schwann cells (SC) of the spinal roots and a subpopulation of neurons and satellite cells in the dorsal root ganglia. Despite their stem-like properties, their therapeutic potential in the diseased central nervous system (CNS) was never explored. The aim of this work was to explore BC therapeutic potential for CNS remyelination. We derived BC from Krox20Cre × R26RYfp embryos at E12.5, when Krox20 is exclusively expressed by BC. Combining microdissection and cell fate mapping, we show that acutely isolated BC are a unique population closely related but distinct from NCC and SC precursors. Moreover, when grafted in the demyelinated spinal cord, BC progeny expands in the lesion through a combination of time-regulated processes including proliferation and differentiation. Furthermore, when grafted away from the lesion, BC progeny, in contrast to committed SC, show a high migratory potential mediated through enhanced interactions with astrocytes and white matter, and possibly with polysialylated neural cell adhesion molecule expression. In response to demyelinated axons of the CNS, BC progeny generates essentially myelin-forming SC. However, in contact with axons and astrocytes, some of them generate also myelin-forming oligodendrocytes. There are two primary outcomes of this study. First, the high motility of BC and their progeny, in addition to their capacity to remyelinate CNS axons, supports the view that BC are a reservoir of interest to promote CNS remyelination. Second, from a developmental point of view, BC behavior in the demyelinated CNS raises the question of the boundary between central and peripheral myelinating cells.

Experimental studies provided overwhelming proof that transplants of myelin-forming cells achieve efficient remyelination in the CNS. Among cellular candidates, Schwann cells can be used for autologous transplantation to ensure robust remyelination of lesions and to deliver therapeutic factors in the CNS. In the present study, macaque Schwann cells expressing green fluorescent protein (GFP) were infected with human immunodeficiency virus-derived vectors overexpressing brain-derived neurotrophic factor (BDNF) or Neurotrophin 3 (NT-3), two neurotrophins that also modulate glial cell biology. The ability of transgenic Schwann cells to secrete growth factors was assessed by ELISA and showed 35- and 62-fold increases in BDNF and NT-3, respectively, in transduced macaque Schwann cell supernatants. Conditioned media of BDNF- and NT-3-transduced Schwann cells reduced Schwann cell proliferation and favored their differentiationin vitro. Transgenic cells were grafted in demyelinated spinal cords of adult nude mice. Two behavioral assays showed that NT-3- and BDNF-transduced Schwann cells promoted faster and stronger functional recovery than GFP-transduced Schwann cells. Morphological analysis indicated that functional recovery correlated with enhanced proliferation and differentiation of resident oligodendrocyte progenitors and enhanced oligodendrocyte and Schwann cell differentiation. Moreover, NT-3-transduced Schwann cells provided neuroprotection and reduced astrogliosis. These results underline the potential therapeutic benefit of combining neuroprotection and activation of myelin-forming cells to restore altered functions in demyelinating diseases of the CNS.

It has been proposed that the adult dorsal root ganglia (DRG) harbor neural stem/progenitor cells (NPCs) derived from the neural crest. However, the thorough characterization of their stemness and differentiation plasticity was not addressed. In this study, we investigated adult DRG-NPC stem cell properties overtime, and their fate when ectopically grafted in the central nervous system. We compared them in vitro and in vivo to the well-characterized adult spinal cord-NPCs derived from the same donors. Using micro-dissection and neurosphere cultures, we demonstrate that adult DRG-NPCs have quasi unlimited self-expansion capacities without compromising their tissue specific molecular signature. Moreover, they differentiate into multiple peripheral lineages in vitro. After transplantation, adult DRG-NPCs generate pericytes in the developing forebrain but remyelinating Schwann cells in response to spinal cord demyelination. In addition, we show that axonal and endothelial/astrocytic factors as well astrocytes regulate the fate of adult DRG-NPCs in culture. Although the adult DRG-NPC multipotency is restricted to the neural crest lineage, their dual responsiveness to developmental and lesion cues highlights their impressive adaptive and repair potentials making them valuable targets for regenerative medicine. Stem Cells 2015;33:2011–2024

Schwann cell (SC) transplantation is currently being discussed as a strategy that may promote functional recovery in patients with multiple sclerosis (MS) and other inflammatory demyelinating diseases of the central nervous system (CNS). However this assumes they will not only survive but also remyelinate demyelinated axons in the chronically inflamed CNS. To address this question we investigated the fate of transplanted SCs in myelin oligodendrocyte glycoprotein (MOG)- induced experimental autoimmune encephalomyelitis (EAE) in the Dark Agouti rat; an animal model that reproduces the complex inflammatory demyelinating immunopathology of MS. We now report that SCs expressing green fluorescent protein (GFP-SCs) allografted after disease onset not only survive but also migrate to remyelinate lesions in the inflamed CNS. GFP-SCs were detected more frequently in the parenchyma after direct injection into the spinal cord, than via intrathecal delivery into the cerebrospinal fluid. In both cases the transplanted cells intermingled with astrocytes in demyelinated lesions, aligned with axons and by twenty one days post transplantation had formed Pzero protein immunoreactive internodes. Strikingly, GFP-SCs transplantation was associated with marked decrease in clinical disease severity in terms of mortality; all GFP-SCs transplanted animals survived whilst 80% of controls died within 40 days of disease. peerReviewed

Recent findings suggested that inducing neural cell adhesion molecule polysialylation in rodents is a promising strategy for promoting tissue repair in the injured central nervous system. Since autologous grafting of Schwann cells is one potential strategy to promote central nervous system remyelination, it is essential to show that such a strategy can be translated to adult primate Schwann cells and is of interest for myelin diseases. Adult macaque Schwann cells were transduced with a lentiviral vector encoding sialyltransferase, an enzyme responsible for neural cell adhesion molecule polysialylation. In vitro, we found that ectopic expression of polysialylate promoted adult macaque Schwann cell migration and improved their integration among astrocytes in vitro without modifying their antigenic properties as either non-myelinating or pro-myelinating. In addition, forced expression of polysialylate in adult macaque Schwann cells decreased their adhesion with sister cells. To investigate the ability of adult macaque Schwann cells to integrate and migrate in vivo, focally induced demyelination was targeted to the spinal cord dorsal funiculus of nude mice, and both control and sialyltransferase expressing Schwann cells overexpressing green fluorescein protein were grafted remotely from the lesion site. Analysis of the spatio-temporal distribution of the grafted Schwann cells performed in toto and in situ, showed that in both groups, Schwann cells migrated towards the lesion site. However, migration of sialyltransferase expressing Schwann cells was more efficient than that of control Schwann cells, leading to their accelerated recruitment by the lesion. Moreover, ectopic expression of polysialylated neural cell adhesion molecule promoted adult macaque Schwann cell interaction with reactive astrocytes when exiting the graft, and their 'chain-like' migration along the dorsal midline. The accelerated migration of sialyltransferase expressing Schwann cells to the lesion site enhanced their ability to compete for myelin repair with endogenous cells, while control Schwann cells were unable to do so. Finally, remyelination by the exogenous sialyltransferase expressing Schwann cells restored the normal distribution of paranodal and nodal elements on the host axons. These greater performances of sialyltransferase expressing Schwann cell correlated with their sustained expression of polysialylated neural cell adhesion molecule at early times when migrating from the graft to the lesion, and its progressive downregulation at later times during remyelination. These results underline the potential therapeutic benefit to genetically modify Schwann cells to overcome their poor migration capacity and promote their repair potential in demyelinating disorders of the central nervous system.

Loss of function of the myotubularin (MTM)-related protein 2 (MTMR2) in Schwann cells causes Charcot–Marie–Tooth disease type 4B1, a severe demyelinating neuropathy, but the consequences of MTMR2 disruption in Schwann cells are unknown. We established the expression profile of MTMR2 by real-time RT-PCR during rat myelination and showed it to be preferentially expressed at the onset of the myelination period. We developed a model in which MTMR2 loss of function was reproduced in primary cultures of Schwann cells by RNA interference. We found that depletion of MTMR2 in Schwann cells decreased their rate of proliferation. Furthermore, when cultivated in serum-free medium, MTMR2 depletion increased the number of Schwann cells that died by a caspase-dependent process. These results support the hypothesis that loss of MTMR2 in patients, by decreasing Schwann cells proliferation and survival, may impair the first stages of myelination of the peripheral nervous system.

The mechanisms limiting myelin repair in human central nervous system (CNS) remain unknown. Models of induced-demyelination in the nonhuman primate CNS may provide the necessary grounds to unravel these mechanisms and to investigate the development of strategies to promote myelin repair. To address this issue, we developed a model of focal demyelination in the adult Macaca fascicularis CNS. Lesions were induced by microinjection of lysolecithin in the optic nerve and the profile of remyelination was compared to that of lysolecithin-induced lesions of the spinal cord. In both structures, the time-course of demyelination as well as the onset of remyelination were found to be similar to that in the rodent CNS. While spinal cord lesions were remyelinated within 6 weeks, optic nerve lesions remained demyelinated for up to 3 months post-injection. The failure of remyelination in the optic nerve correlated with a reduced density of NG2+ oligodendrocyte progenitor cells, the presence of oligodendrocytes that fail to ensheath naked axons in the lesion and the absence of astrocyte recruitment in the lesion compared with spinal cord lesions. Our present data suggest that the reduced oligodendrocyte progenitor population, the improper activation of oligodendrocytes at the onset of remyelination in the optic nerve, and possibly, the involvement of astrocytes contribute to the chronicity of the optic nerve lesion. This model of chronic demyelination in the macaque optic nerve stresses its pertinence to unraveling the mechanisms limiting remyelination in multiple sclerosis.

Experimental transplantation in rodent models of CNS demyelination has led to the idea that Schwann cells may be candidates for cell therapy in human myelin diseases. Here we investigated the ability of Schwann cells autografts to generate myelin in the demyelinated monkey spinal cord. We report that monkey Schwann cells derived from adult peripheral nerve biopsies retain, after growth factor expansion and transduction with a lentiviral vector encoding green fluorescent protein, the ability to differentiate in vitro into promyelinating cells. When transplanted in the demyelinated nude mouse spinal cord, they promoted functional and anatomical repair of the lesions (n = 12). Furthermore, we obtained evidence by immunohistochemistry (n = 2) and electron microscopy (n = 4) that autologous transplantation of expanded monkey Schwann cells in acute lesions of the monkey spinal cord results in the repair of large areas of demyelination; up to 55% of the axons were remyelinated by donor Schwann cells, the remaining ones being remyelinated by oligodendrocytes. Autologous grafts of Schwann cells may thus be of therapeutic value for myelin repair in the adult CNS.

Transmissible spongiform encephalopathies arise as a consequence of infection of the central nervous system (CNS) by prions. Spreading of the infectious agent through the peripheral nervous system (PNS) may represent a crucial step toward CNS neuroinvasion, but the modalities of this process have yet to be clarified. Here we provide further evidence that PNS glial cells are likely targets for infection by prions. Glial cell clones originating from dorsal root ganglia of transgenic mice expressing ovine PrP (tgOv) and simian virus 40 T antigen were found to be readily infectible by sheep scrapie agent. This led us to establish two stable cell lines that exhibited features of Schwann cells. These cells were shown to sustain an efficient and stable replication of sheep prion based on the high level of accumulation of abnormal PrP and infectivity in exposed cultures. We also provide evidence for abnormal PrP deposition in peripheral neuroglial cells from scrapie-infected tgOv mice and sheep. These findings have potential implications in terms of designing new cell systems permissive to prions and of peripheral pathobiology of prion infections.