In vitro and in vivo behaviour of NDF-expanded monkey Schwann cells

Schwann cells: Rescuers of central demyelination

The presence of peripheral myelinating cells in the central nervous system (CNS) has gained the neurobiologist attention over the years. Despite the confirmed presence of Schwann cells in the CNS in pathological conditions, and the long list of their beneficial effects on central remyelination, the cues that impede or allow Schwann cells to successfully conquer and remyelinate central axons remain partially undiscovered. A better knowledge of these factors stands out as crucial to foresee a rational therapeutic approach for the use of Schwann cells in CNS repair. Here, we review the diverse origins of Schwann cells into the CNS, both peripheral and central, as well as the CNS components that inhibit Schwann survival and migration into the central parenchyma. Namely, we analyze the astrocyte- and the myelin-derived components that restrict Schwann cells into the CNS. Finally, we highlight the unveiled mode of invasion of these peripheral cells through the central environment, using blood vessels as scaffolds to pave their ways toward demyelinated lesions. In short, this review presents the so far uncovered knowledge of this complex CNS-peripheral nervous system (PNS) relationship.

Remyelination of Demyelinated CNS Axons by Transplanted Human Schwann Cells: The Deleterious Effect of Contaminating Fibroblasts

Areas of demyelination can be remyelinated by transplanting myelin-forming cells. Schwann cells are the naturally remyelinating cells of the peripheral nervous system and have a number of features that may make them attractive for cell implantation therapies in multiple sclerosis, in which spontaneous but limited Schwann cell remyelination has been well documented. Schwann cells can be expanded in vitro, potentially affording the opportunity of autologous transplantation; and they might also be spared the demyelinating process in multiple sclerosis. Although rat, cat, and monkey Schwann cells have been transplanted into rodent demyelinating lesions, the behavior of transplanted human Schwann cells has not been evaluated. In this study we examined the consequences of injecting human Schwann cells into areas of acute demyelination in the spinal cords of adult rats. We found that transplants containing significant fibroblast contamination resulted in deposition of large amounts of collagen and extensive axonal degeneration. However, Schwann cell preparations that had been purified by positive immunoselection using antibodies to human low-affinity nerve growth factor receptor containing less than 10% fibroblasts were associated with remyelination. This result indicates that fibroblast contamination of human Schwann cells represents a greater problem than would have been appreciated from previous studies.

The activation of the WNT signaling pathway is a Hallmark in neurofibromatosis type 1 tumorigenesis

PURPOSE

The hallmark of neurofibromatosis type 1 (NF1) is the onset of dermal or plexiform neurofibromas, mainly composed of Schwann cells. Plexiform neurofibromas can transform into malignant peripheral nerve sheath tumors (MPNST) that are resistant to therapies.

EXPERIMENTAL DESIGN

The aim of this study was to identify an additional pathway in the NF1 tumorigenesis. We focused our work on Wnt signaling that is highly implicated in cancer, mainly in regulating the proliferation of cancer stem cells. We quantified mRNAs of 89 Wnt pathway genes in 57 NF1-associated tumors including dermal and plexiform neurofibromas and MPNSTs. Expression of two major stem cell marker genes and five major epithelial-mesenchymal transition marker genes was also assessed. The expression of significantly deregulated Wnt genes was then studied in normal human Schwann cells, fibroblasts, endothelial cells, and mast cells and in seven MPNST cell lines.

RESULTS

The expression of nine Wnt genes was significantly deregulated in plexiform neurofibromas in comparison with dermal neurofibromas. Twenty Wnt genes showed altered expression in MPNST biopsies and cell lines. Immunohistochemical studies confirmed the Wnt pathway activation in NF1-associated MPNSTs. We then confirmed that the knockdown of NF1 in Schwann cells but not in epithelial cells provoked the activation of Wnt pathway by functional transfection assays. Furthermore, we showed that the protein expression of active β-catenin was increased in NF1-silenced cell lines. Wnt pathway activation was strongly associated to both cancer stem cell reservoir and Schwann-mesenchymal transition.

CONCLUSION

We highlighted the implication of Wnt pathway in NF1-associated tumorigenesis.

Further development of reconstructive and cell tissue-engineering technology for treatment of complete peripheral nerve injury in rats

In this work we evaluated the efficacy of biodegradable composite co-polymer guiding neurotube, based on tissue-engineering technology, for the treatment of complete peripheral nerve injury where the nerve defect is significant. The right sciatic nerve of 12 three-month-old rats was completely transected and peripheral nerve segment was removed. A 2.2-cm biodegradable co-polymer neurotube containing viscous gel (NVR-N-Gel) with survival factors, neuroprotective agents and Schwann cells was placed between the proximal and the distal parts of the transected nerve for reconnection a 2-cm nerve defect. The proximal and distal parts of the nerve were fixed into the neurotube using 10-0 sutures. Ultrasound observation showed growth of the axons into the composite neurotube 2 months after the surgery. Electrophysiological study indicated compound muscle action potentials in nine out of 12 rats, 2-4 months after peripheral nerve reconstructive surgery. The postoperative follow-up (up to 4 months) on the operated rats that underwent peripheral nerve reconstruction using composite co-polymer neurotube, showed beginning of re-establishment of active foot movements. The tube was dissolved and nerve showed complete reconnection. Histological observation of the nerve showed growth of myelinated axons into the site where a 2-cm nerve defect replaced by composite co-polymer neurotube and into the distal part of the nerve.

IN CONCLUSION

(1) an innovative composite neurotube for reconstruction of significant loss of peripheral nerve segment is described; (2) a viscous gel, containing survival factors, neuroprotective agents and Schwann cells served as a regenerative environment for repair. Further investigations of this reconstructive procedure are being conducted.

Exogenous schwann cells migrate, remyelinate and promote clinical recovery in experimental auto-immune encephalomyelitis

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 intra-thecal 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.

Cell therapy for multiple sclerosis

The spontaneous recovery observed in the early stages of multiple sclerosis (MS) is substituted with a later progressive course and failure of endogenous processes of repair and remyelination. Although this is the basic rationale for cell therapy, it is not clear yet to what degree the MS brain is amenable for repair and whether cell therapy has an advantage in comparison to other strategies to enhance endogenous remyelination. Central to the promise of stem cell therapy is the therapeutic plasticity, by which neural precursors can replace damaged oligodendrocytes and myelin, and also effectively attenuate the autoimmune process in a local, nonsystemic manner to protect brain cells from further injury, as well as facilitate the intrinsic capacity of the brain for recovery. These fundamental immunomodulatory and neurotrophic properties are shared by stem cells of different sources. By using different routes of delivery, cells may target both affected white matter tracts and the perivascular niche where the trafficking of immune cells occur. It is unclear yet whether the therapeutic properties of transplanted cells are maintained with the duration of time. The application of neural stem cell therapy (derived from fetal brain or from human embryonic stem cells) will be realized once their purification, mass generation, and safety are guaranteed. However, previous clinical experience with bone marrow stromal (mesenchymal) stem cells and the relative easy expansion of autologous cells have opened the way to their experimental application in MS. An initial clinical trial has established the probable safety of their intravenous and intrathecal delivery. Short-term follow-up observed immunomodulatory effects and clinical benefit justifying further clinical trials.

Secretion of EGF-like domain of heregulinβ promotes axonal growth and functional recovery of injured sciatic nerve

Neuregulin 1 (NRG1) and epidermal growth factor receptor (ErbB) signaling pathways control Schwann cells during axonal regeneration in an injured peripheral nervous system. We investigated whether a persistent supply of recombinant NRG1 to the injury site could improve axonal growth and recovery of sensory and motor functions in rats during nerve regeneration. We generated a recombinant adenovirus expressing a secreted form of EGF-like domain from Heregulinβ (sHRGβE-Ad). This virus, sHRGβE-Ad allowed for the secretion of 30-50 ng of small sHRGβE peptides per 10(7-8) virus particle outside cells and was able to phosphorylate ErbB receptors. Transduction of the concentrated sHRGβE-Ad into an axotomy model of sciatic nerve damage caused an effective promotion of nerve regeneration, as shown by histological features of the axons and Schwann cells, as well as increased expression of neurofilaments, GAP43 and S100 in the distal stump of the injury site. This result is consistent with longer axon lengths and thicker calibers observed in the sHRGβE-Ad treated animals. Furthermore, sensory and motor functions were significantly improved in sHRGβE-Ad treated animals when evaluated by a behavioral test. These results suggest a therapeutic potential for sHRGβE-Ad in treatment of peripheral nerve injury.

Identification of genes potentially involved in the increased risk of malignancy in NF1-microdeleted patients

Patients with NF1 microdeletion develop more neurofibromas at a younger age, and have an increased risk of malignant peripheral nerve sheath tumors (MPNSTs). We postulated that the increased risk of malignancy could be due to inactivation, in addition to NF1, of a second tumor suppressor gene located in the typical 1.4-Mb microdeletion found in most of the microdeleted patients. We investigated the expression of NF1, the other 16 protein-coding genes and the 2 microRNAs located in the 1.4-Mb microdeletion by means of real-time quantitative reverse-transcription polymerase chain reaction (RT-PCR) in a large series of human dermal and plexiform neurofibromas and MPNSTs. Five genes were significantly upregulated: OMG and SUZ12 in plexiform neurofibromas and ATAD5, EVI2A and C17orf79 in MPNSTs. More interestingly, two genes were significantly downregulated (RNF135 and CENTA2) in tumor Schwann cells from MPNST biopsies and in MPNST cell lines. This study points to the involvement of several genes (particularly RNF135 and CENTA2) in the increased risk of malignancy observed in NF1-microdeleted patients.

Differential expression of CCN1/CYR61, CCN3/NOV, CCN4/WISP1, and CCN5/WISP2 in neurofibromatosis type 1 tumorigenesis

The hallmark of neurofibromatosis type 1 is the development of dermal and plexiform neurofibromas. Neurofibromatosis type 1 patients with plexiform neurofibromas are at risk of developing malignant peripheral nerve sheath tumors. We applied a 22,000-oligonucleotide microarray transcriptomic approach to a series of plexiform neurofibromas in comparison with dermal neurofibromas, and results were confirmed with real-time quantitative reverse transcription-polymerase chain reaction. Thirteen genes were upregulated and 10 were downregulated in plexiform neurofibromas. The upregulated genes mainly encode molecules involved in cell adhesion, extracellular matrix, fibrogenesis, and angiogenesis. Several CCN gene family members were dysregulated in neurofibromatosis type 1 tumorigenesis; the angiogenic gene CCN1/CYR61 was specifically upregulated in the plexiform neurofibromas; CCN4/WISP1 was upregulated, and CCN3/NOV and CCN5/WISP2 were downregulated in paired comparisons of plexiform neurofibroma and malignant peripheral nerve sheath tumor from the same patients. CCN1 and CCN3 proteins were detected by immunohistochemistry in neurofibromatosis type 1-associated tumors. Upregulation of S100A8, S100A9, and CD36 was also observed and suggests a role of this pathway in inflammation-associated genesis of plexiform neurofibromas. In summary, a limited number of pathways are potentially involved in plexiform neurofibroma development. Some of the genes identified, particularly CCN1, might be useful diagnostic or prognostic markers or form the basis for novel therapeutic strategies.

Ectopic expression of polysialylated neural cell adhesion molecule in adult macaque Schwann cells promotes their migration and remyelination potential in the central nervous system

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.

Prospects of cell therapy for disorders of myelin

Recent advances in stem cell biology have raised expectations that both diseases of, and injuries to, the central nervous system may be ameliorated by cell transplantation. In particular, cell therapy has been studied for inducing efficient remyelination in disorders of myelin, including both the largely pediatric disorders of myelin formation and maintenance and the acquired demyelinations of both children and adults. Potential cell-based treatments of two major groups of disorders include both delivery of myelinogenic replacements and mobilization of residual oligodendrocyte progenitor cells as a means of stimulating endogenous repair; the choice of modality is then predicated upon the disease target. In this review we consider the potential application of cell-based therapeutic strategies to disorders of myelin, highlighting the promises as well as the problems and potential perils of this treatment approach.

Remyelination of the central nervous system: a valuable contribution from the periphery

The loss of myelin, a major element involved in the saltatory conduction of the electrical impulse of the nervous system, is a major target of current research. Serious long-term disabilities are observed in patients with demyelinating disease of the central nervous system, such as multiple sclerosis. New therapeutic strategies aimed at overcoming myelin damage and axonal loss focus on the repair potential of myelin-forming cells. This review examines the use of peripheral myelin-forming cells, the Schwann cells, to promote myelin repair.

Baculovirus-mediated gene delivery into Mammalian cells does not alter their transcriptional and differentiating potential but is accompanied by early viral gene expression

Gene delivery to neural cells is central to the development of transplantation therapies for neurological diseases. In this study, we used a baculovirus derived from the domesticated silk moth, Bombyx mori, as vector for transducing a human cell line (HEK293) and primary cultures of rat Schwann cells. Under optimal conditions of infection with a recombinant baculovirus containing the reporter green fluorescent protein gene under mammalian promoter control, the infected cells express the transgene with high efficiency. Toxicity assays and transcriptome analyses suggest that baculovirus infection is not cytotoxic and does not induce differential transcriptional responses in HEK293 cells. Infected Schwann cells retain their characteristic morphological and molecular phenotype as determined by immunocytochemistry for the marker proteins S-100, glial fibrillary acidic protein, and p75 nerve growth factor receptor. Moreover, baculovirus-infected Schwann cells are capable of differentiating in vitro and express the P0 myelination marker. However, transcripts for several immediate-early viral genes also accumulate in readily detectable levels in the transduced cells. This transcriptional activity raises concerns regarding the long-term safety of baculovirus vectors for gene therapy applications. Potential approaches for overcoming the identified problem are discussed.

Neuregulin growth factors and their ErbB receptors form a potential signaling network for schwannoma tumorigenesis

Sporadic and neurofibromatosis type 2-associated schwannomas contain a glial growth factor (GGF)-like activity that has been hypothesized to promote neoplastic Schwann cell mitogenesis. It is not known whether this GGF-like activity is neuregulin-1 (NRG-1), an epidermal growth factor (EGF)-related molecule that regulates the proliferation, survival, and differentiation of developing Schwann cells, the related factor NRG-2, or another NRG/EGF ligand. We report that neoplastic Schwann cells within schwannomas overexpress multiple alpha and beta transmembrane precursors from the class II and class III NRG-1 subfamilies. NRG-2 alpha and beta transcripts are similarly overexpressed in some tumors. Of the other 8 known NRG/EGF ligands, only heparin-binding EGF, epiregulin, and TGFalpha are detectable in schwannomas. Neoplastic Schwann cells almost uniformly express erbB2 and erbB3, 2 membrane receptor tyrosine kinases mediating NRG-1 and NRG-2 action. Expression of the NRG receptor erbB4 and EGF receptor is also evident in schwannomas, but is more limited, occurring in only a subset of these tumors. ErbB2, the preferred dimerization partner for all erbB kinases, is constitutively phosphorylated in schwannomas. These observations suggest that autocrine, paracrine, and/or juxtacrine NRG-1/NRG-2 signaling promotes schwannoma pathogenesis and that this signaling pathway may be an important therapeutic target in schwannomas.

Remyelination of dorsal column axons by endogenous Schwann cells restores the normal pattern of Nav1.6 and Kv1.2 at nodes of Ranvier

Demyelination of CNS axons occurs in a number of pathological conditions, including multiple sclerosis and contusion-type spinal cord injury. The demyelination can be repaired by remyelination in both humans and rodents, and even within the CNS remyelination can be achieved by endogenous and/or exogenous Schwann cells, the myelinating cells of the PNS. Remyelinated axons can often conduct impulses securely, but the organization of ion channels at long-term remyelinated nodes is not known. In the present study, the expression of voltage-gated sodium (Na(v)) and potassium (K(v)) channels along central axons remyelinated by endogenous Schwann cells has been studied in lesions induced more than 1 year previously by the intraspinal injection of ethidium bromide (EB). The expression of the channels at long-term nodes formed by Schwann cell remyelination has been compared with that present in nascent nodes formed in the adult at 18 and 23 days post-EB injection. Immunohistochemical studies revealed that long-term nodes formed by Schwann cell remyelination exhibit a clustering of Na(v)1.6 sodium channels within the nodal membrane, with the Shaker-type potassium channel K(v)1.2 segregated within the juxtaparanodal region, similar to the arrangement at normal mature CNS nodes. Na(v)1.2 was not detected at nodes formed by Schwann cells at any stage of their development. Moreover, Na(v)1.6, but not Na(v)1.2, was clustered at nascent nodes formed by remyelinating Schwann cells 18 and 23 days following EB injection. These observations show that endogenous Schwann cells can establish and maintain nodes of Ranvier on central axons for over one year, and that the nodes exhibit an apparently normal distribution of sodium and potassium channels, with Na(v)1.6 the predominant subtype of sodium channel present at such nodes at all stages of their development.

Grafts of brain-derived neurotrophic factor and neurotrophin 3-transduced primate Schwann cells lead to functional recovery of the demyelinated mouse spinal cord

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 differentiation in 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.

Efficiency of adeno-associated virus type-2 vectors in non-human primate Schwann cells

Adult macaque Schwann cells were infected using adeno-associated virus type-2-derived vectors expressing the green fluorescent protein reporter gene under the control of the cytomegalovirus, the hybrid cytomegalovirus-betaactin, the myelin basic protein or the tetracycline-inducible promoters. On the basis of green fluorescent protein expression, gene transfer efficiency was compared in resting and dividing conditions following or not following hydroxyurea or etoposide treatment. Hydroxyurea allowed promoter-dependent expression of green fluorescent protein in infected Schwann cells. Etoposide treatment led to a high percentage of green fluorescent protein expressing cells (over 50%) with all promoters tested. When infected cells were grafted into demyelinated nude mice spinal cord, green fluorescent protein expression was only observed with the cytomegalovirus-betaactin and tetracycline-inducible promoters. In addition, adeno-associated virus type-2 infection reduced the grafted cell survival but increased their differentiation.

Superparamagnetic iron oxide-labeled Schwann cells and olfactory ensheathing cells can be traced in vivo by magnetic resonance imaging and retain functional properties after transplantation into the CNS

Schwann cell (SC) and olfactory ensheathing cell (OEC) transplantation has been shown experimentally to promote CNS axonal regeneration and remyelination. To advance this technique into a clinical setting it is important to be able to follow the fates of transplanted cells by noninvasive imaging. Previous studies, using complex modification processes to enable uptake of contrast agents, have shown that cells labeled in vitro with paramagnetic contrast agents transplanted into rodent CNS can be visualized using magnetic resonance imaging (MRI). Here we show that SCs and OECs efficiently internalize dextran-coated superparamagnetic iron oxide (SPIO) from the culture medium by fluid phase pinocytosis. After transplantation into focal areas of demyelination in adult rat spinal cord both transplanted SPIO-labeled SCs and OECs produce a signal reduction using T(2)-weighted MRI in anesthetized rats that persists for up to 4 weeks. Although signal reduction was discernable after transplantation of unlabelled cells, this is nevertheless distinguishable from that produced by transplanted labeled cells. The region of signal reduction in SPIO-labeled cell recipients correlates closely with areas of remyelination. Because the retention of functional integrity by labeled cells is paramount, we also show that SPIO-labeled SCs and OECs are able to myelinate normally after transplantation into focal areas of demyelination. These studies demonstrate the feasibility of noninvasive imaging of transplanted SCs and OECs and represent a significant step toward the clinical application of promising experimental approaches.

Cell-based remyelinating therapies in multiple sclerosis: evidence from experimental studies

PURPOSE OF REVIEW

Spontaneous remyelination occurs in the central nervous system of patients with multiple sclerosis. However, this process is not robust enough to promote a functional and stable recovery of the myelin architecture. The development of cell-based therapies, aimed at promoting multifocal remyelination, is therefore foreseen.

RECENT FINDINGS

Several experimental cell-based strategies aimed at replacing damaged myelin-forming cells have been developed in the last few years. However, most of these therapeutic approaches - although consistently able to form new myelin sheaths at the transplantation site - are unfeasible owing to the mutifocality of the demyelinating process in multiple sclerosis patients and the inability to grow and produce large numbers of differentiated myelin-forming cells in vitro. Stem cell-based therapies that partially overcome these limitations have been proposed recently.

SUMMARY

Stem cell-based remyelinating therapies can be considered a plausible alternative strategy in immune-mediated demyelinating disorders. However, before any potential applications in patients with multiple sclerosis can be envisaged, it is necessary to confront the following preliminary, and still unsolved, questions: (1) the ideal stem cell source for transplantation; (2) the most appropriate route of stem cell administration; and, last but not least, (3) the best approach for achieving an appropriate, functional and long-lasting integration of transplanted stem cells into the host tissue.

Derivation and characterization of monkey embryonic stem cells

Embryonic stem (ES) cell based therapy carries great potential in the treatment of neurodegenerative diseases. However, before clinical application is realized, the safety, efficacy and feasibility of this therapeutic approach must be established in animal models. The rhesus macaque is physiologically and phylogenetically similar to the human, and therefore, is a clinically relevant animal model for biomedical research, especially that focused on neurodegenerative conditions. Undifferentiated monkey ES cells can be maintained in a pluripotent state for many passages, as characterized by a collective repertoire of markers representing embryonic cell surface molecules, enzymes and transcriptional factors. They can also be differentiated into lineage-specific phenotypes of all three embryonic germ layers by epigenetic protocols. For cell-based therapy, however, the quality of ES cells and their progeny must be ensured during the process of ES cell propagation and differentiation. While only a limited number of primate ES cell lines have been studied, it is likely that substantial inter-line variability exists. This implies that diverse ES cell lines may differ in developmental stages, lineage commitment, karyotypic normalcy, gene expression, or differentiation potential. These variables, inherited genetically and/or induced epigenetically, carry obvious complications to therapeutic applications. Our laboratory has characterized and isolated rhesus monkey ES cell lines from in vitro produced blastocysts. All tested cell lines carry the potential to form pluripotent embryoid bodies and nestin-positive progenitor cells. These ES cell progeny can be differentiated into phenotypes representing the endodermal, mesodermal and ectodermal lineages. This review article describes the derivation of monkey ES cell lines, characterization of the undifferentiated phenotype, and their differentiation into lineage-specific, particularly neural, phenotypes. The promises and limitations of primate ES cell-based therapy are also discussed.

Myelin oligodendrocyte glycoprotein is expressed in the peripheral nervous system of rodents and primates

The myelin oligodendrocyte glycoprotein (MOG) is a minor CNS myelin-specific protein that is an important candidate autoantigen in multiple sclerosis. We now report that MOG mRNA transcripts are present in the peripheral nervous system of rodents and primates at levels approximately ten-fold lower than in brain as demonstrated by real time PCR. A major source of this signal are Schwann cells which are also shown to express MOG protein within their cytoplasm in vitro by immunohistochemistry. Expression of MOG by Schwann cells associated with tissue innervation may account for the widespread distribution of low levels of MOG mRNA transcripts, and potentially may provide a source of antigen that can influence the composition and function of the MOG-specific immune repertoire.

Biology of oligodendrocyte and myelin in the mammalian central nervous system

Oligodendrocytes, the myelin-forming cells of the central nervous system (CNS), and astrocytes constitute macroglia. This review deals with the recent progress related to the origin and differentiation of the oligodendrocytes, their relationships to other neural cells, and functional neuroglial interactions under physiological conditions and in demyelinating diseases. One of the problems in studies of the CNS is to find components, i.e., markers, for the identification of the different cells, in intact tissues or cultures. In recent years, specific biochemical, immunological, and molecular markers have been identified. Many components specific to differentiating oligodendrocytes and to myelin are now available to aid their study. Transgenic mice and spontaneous mutants have led to a better understanding of the targets of specific dys- or demyelinating diseases. The best examples are the studies concerning the effects of the mutations affecting the most abundant protein in the central nervous myelin, the proteolipid protein, which lead to dysmyelinating diseases in animals and human (jimpy mutation and Pelizaeus-Merzbacher disease or spastic paraplegia, respectively). Oligodendrocytes, as astrocytes, are able to respond to changes in the cellular and extracellular environment, possibly in relation to a glial network. There is also a remarkable plasticity of the oligodendrocyte lineage, even in the adult with a certain potentiality for myelin repair after experimental demyelination or human diseases.