Identification and characterization of differentiation-dependent Schwann cell surface antigens by novel monoclonal antibodies: introduction of a marker common to the non-myelin-forming phenotype

Contribution of Schwann Cells to Remyelination in a Naturally Occurring Canine Model of CNS Neuroinflammation

Gliogenesis under pathophysiological conditions is of particular clinical relevance since it may provide evidence for regeneration promoting cells recruitable for therapeutic purposes. There is evidence that neurotrophin receptor p75 (p75^NTR)-expressing cells emerge in the lesioned CNS. However, the phenotype and identity of these cells, and signals triggering their in situ generation under normal conditions and certain pathological situations has remained enigmatic. In the present study, we used a spontaneous, idiopathic and inflammatory CNS condition in dogs with prominent lympho-histiocytic infiltration as a model to study the phenotype of Schwann cells and their relation to Schwann cell remyelination within the CNS. Furthermore, the phenotype of p75^NTR-expressing cells within the injured CNS was compared to their counter-part in control sciatic nerve and after peripheral nerve injury. In addition, organotypic slice cultures were used to further elucidate the origin of p75^NTR-positive cells. In cerebral and cerebellar white and grey matter lesions as well as in the brain stem, p75^NTR-positive cells co-expressed the transcription factor Sox2, but not GAP-43, GFAP, Egr2/Krox20, periaxin and PDGFR-α. Interestingly, and contrary to the findings in control sciatic nerves, p75^NTR-expressing cells only co-localized with Sox2 in degenerative neuropathy, thus suggesting that such cells might represent dedifferentiated Schwann cells both in the injured CNS and PNS. Moreover, effective Schwann cell remyelination represented by periaxin- and P0-positive mature myelinating Schwann cells, was strikingly associated with the presence of p75^NTR/Sox2-expressing Schwann cells. Intriguingly, the emergence of dedifferentiated Schwann cells was not affected by astrocytes, and a macrophage-dominated inflammatory response provided an adequate environment for Schwann cells plasticity within the injured CNS. Furthermore, axonal damage was reduced in brain stem areas with p75^NTR/Sox2-positive cells. This study provides novel insights into the involvement of Schwann cells in CNS remyelination under natural occurring CNS inflammation. Targeting p75^NTR/Sox2-expressing Schwann cells to enhance their differentiation into competent remyelinating cells appears to be a promising therapeutic approach for inflammatory/demyelinating CNS diseases.

Schwann cell-free adult canine olfactory ensheathing cell preparations from olfactory bulb and mucosa display differential migratory and neurite growth-promoting properties in vitro

Background

Transplantation of olfactory ensheathing cells (OEC) and Schwann cells (SC) is a promising therapeutic strategy to promote axonal growth and remyelination after spinal cord injury. Previous studies mainly focused on the rat model though results from primate and porcine models differed from those in the rat model. Interestingly, canine OECs show primate-like in vitro characteristics, such as absence of early senescence and abundance of stable p75^NTR expression indicating that this species represents a valuable translational species for further studies. So far, few investigations have tested different glial cell types within the same study under identical conditions. This makes it very difficult to evaluate contradictory or confirmatory findings reported in various studies. Moreover, potential contamination of OEC preparations with Schwann cells was difficult to exclude. Thus, it remains rather controversial whether the different glial types display distinct cellular properties.

Results

Here, we established cultures of Schwann cell-free OECs from olfactory bulb (OB-OECs) and mucosa (OM-OECs) and compared them in assays to Schwann cells. These glial cultures were obtained from a canine large animal model and used for monitoring migration, phagocytosis and the effects on in vitro neurite growth. OB-OECs and Schwann cells migrated faster than OM-OECs in a scratch wound assay. Glial cell migration was not modulated by cGMP and cAMP signaling, but activating protein kinase C enhanced motility. All three glial cell types displayed phagocytic activity in a microbead assay. In co-cultures with of human model (NT2) neurons neurite growth was maximal on OB-OECs.

Conclusions

These data provide evidence that OB- and OM-OECs display distinct migratory behavior and interaction with neurites. OB-OECs migrate faster and enhance neurite growth of human model neurons better than Schwann cells, suggesting distinct and inherent properties of these closely-related cell types. Future studies will have to address whether, and how, these cellular properties correlate with the in vivo behavior after transplantation.

Toward defining the regenerative potential of olfactory mucosa: establishment of Schwann cell-free adult canine olfactory ensheathing cell preparations suitable for transplantation

Olfactory mucosa (OM)-derived olfactory ensheathing cells (OECs) are attractive candidates for autologous cell transplantation-based therapy of nervous system injury. However, defining the regenerative capacity of OM-derived OECs is impeded by the fact that cell cultures used for transplantation may contain significant amounts of contaminating trigeminal nerve Schwann cells that escape identification by sharing in vitro expression of OEC markers. The aim of the present study, therefore, was to quantify contaminating Schwann cells in OEC preparations and to develop a protocol for their specific depletion. Based on the observation that freshly dissociated, but not cultured, OECs and Schwann cells display differential expression of HNK-1 and p75(NTR), magnet-activated cell sorting (MACS) was used to deplete myelinating (HNK-1-positive) and nonmyelinating (p75(NTR)-positive) Schwann cells from primary cell suspensions containing HNK-1-/p75(NTR)-negative OECs. Upregulation of p75(NTR) expression in OECs during culturing allowed their subsequent MACS-based separation from fibroblasts. Immunofluorescence analysis of freshly dissociated OM prior to MACS depletion revealed that 21% of the total and 56% of all CNPase-positive cells, representing both OECs and Schwann cells, expressed the Schwann cell antigens HNK-1 or p75(NTR), indicating that freshly dissociated OM prior to culturing contained as many Schwann cells as OECs, while olfactory bulb (OB) primary cell suspensions revealed lower levels of Schwann cell contamination. Interestingly, neurite growth of neonatal rat dorsal root ganglion (DRG) neurons cocultured with OM-OECs, OB-OECs, and fibular nerve (FN) Schwann cells used as control was significantly higher in the presence of OECs than of Schwann cells. The first report on identification and specific depletion of Schwann cells from OEC preparations provides a solid basis for future efforts to fully define the regenerative potential of nasal mucosa OECs.

Recent progress in histochemistry and cell biology

Studies published in Histochemistry and Cell Biology in the year 2011 represent once more a manifest of established and newly sophisticated techniques being exploited to put tissue- and cell type-specific molecules into a functional context. The review is therefore the Histochemistry and Cell Biology's yearly intention to provide interested readers appropriate summaries of investigations touching the areas of tissue biology, developmental biology, the biology of the immune system, stem cell research, the biology of subcellular compartments, in order to put the message of such studies into natural scientific-/human- and also pathological-relevant correlations.

Increased p75 neurotrophin receptor expression in the canine distemper virus model of multiple sclerosis identifies aldynoglial Schwann cells that emerge in response to axonal damage

Gliogenesis under pathophysiological conditions is of particular clinical relevance since it may provide regeneration-promoting cells recruitable for therapeutic purposes. There is accumulating evidence that aldynoglial cells with Schwann cell-like growth-promoting properties emerge in the lesioned CNS. However, the characterization of these cells and the signals triggering their in situ generation have remained enigmatic. In the present study, we used the p75 neurotrophin receptor (p75(NTR) ) as a marker for Schwann cells to study gliogenesis in the well-defined canine distemper virus (CDV)-induced demyelination model. White matter lesions of CDV-infected dogs contained bi- to multipolar, p75(NTR) -expressing cells that neither expressed MBP, GFAP, BS-1, or P0 identifying oligodendroglia, astrocytes, microglia, and myelinating Schwann cells nor CDV antigen. Interestingly, p75(NTR) -expression became apparent prior to the onset of demyelination in parallel to the expression of β-amyloid precursor protein (β-APP), nonphosphorylated neurofilament (n-NF), BS-1, and CD3, and peaked in subacute lesions with inflammation. To study the role of infiltrating immune cells during differentiation of Schwann cell-like glia, organotypic slice cultures from the normal olfactory bulb were established. Despite the absence of infiltrating lymphocytes and macrophages, a massive appearance of p75(NTR) -positive Schwann-like cells and BS-1-positive microglia was noticed at 10 days in vitro. It is concluded that axonal damage as an early signal triggers the differentiation of tissue-resident precursor cells into p75(NTR) -expressing aldynoglial Schwann cells that retain an immature pre-myelin state. Further studies have to address the role of microglia during this process and the regenerative potential of aldynoglial cells in CDV infection and other demyelinating diseases.

S100beta and fibroblast growth factor-2 are present in cultured Schwann cells and may exert paracrine actions on the peripheral nerve injury

PURPOSE

The neurotrophic factor fibroblast growth factor-2 (FGF-2, bFGF) and Ca++ binding protein S100beta are expressed by the Schwann cells of the peripheral nerves and by the satellite cells of the dorsal root ganglia (DRG). Recent studies have pointed out the importance of the molecules in the paracrine mechanisms related to neuronal maintenance and plasticity of lesioned motor and sensory peripheral neurons. Moreover, cultured Schwann cells have been employed experimentally in the treatment of central nervous system lesions, in special the spinal cord injury, a procedure that triggers an enhanced sensorymotor function. Those cells have been proposed to repair long gap nerve injury.

METHODS

Here we used double labeling immunohistochemistry and Western blot to better characterize in vitro and in vivo the presence of the proteins in the Schwann cells and in the satellite cells of the DRG as well as their regulation in those cells after a crush of the rat sciatic nerve.

RESULTS

FGF-2 and S100beta are present in the Schwann cells of the sciatic nerve and in the satellite cells of the DRG. S100beta positive satellite cells showed increased size of the axotomized DRG and possessed elevated amount of FGF-2 immunoreactivity. Reactive satellite cells with increased FGF-2 labeling formed a ring-like structure surrounding DRG neuronal cell bodies.Reactive S100beta positive Schwann cells of proximal stump of axotomized sciatic nerve also expressed higher amounts of FGF-2.

CONCLUSION

Reactive peripheral glial cells synthesizing FGF-2 and S100beta may be important in wound repair and restorative events in the lesioned peripheral nerves.

Transfection of adult canine Schwann cells and olfactory ensheathing cells at early and late passage with human TERT differentially affects growth factor responsiveness and in vitro growth

Adult canine Schwann cells and olfactory ensheathing cells (OECs) are closely related cell types that are considered attractive candidates for translational studies of neural repair. To establish a reliable cell source by comparing the in vitro properties of immortalized Schwann cells and OECs for transplantation purposes, we transfected both cell types with human telomerase reverse transcriptase (hTERT). Ectopic hTERT expression has been shown to induce immortalization of various cell types without substantial alterations of their phenotypes. Schwann cells and OECs were isolated from adult dogs, transfected with hTERT at early (P4) and late passage (P26), characterized regarding in vitro proliferation, antigenic expression and senescence-associated genes in the presence and absence of fibroblast growth factor-2 (FGF-2). Ectopic hTERT expression in late passage glia treated with but not without FGF-2 prevented the decline in proliferation observed in non-transfected cells. Immortalization did not alter p75(NTR) and GFAP but O4 and A2B5 expression. Contrary to this, early passage hTERT transfection significantly reduced proliferation independent of FGF-2 and lowered expression of O4 and GFAP in both cell types. Transfection did not alter mRNA expression of senescence-associated genes such as p53 and p16. No substantial differences were found between Schwann cells and OECs underscoring the close relationship of both cell types. Taken together, we established a stable source of adult canine Schwann cells and OECs and demonstrated that the effects of hTERT expression on in vitro growth and growth factor responsiveness depend on the replicative age.

Cellular analysis of S100Beta and fibroblast growth factor-2 in the dorsal root ganglia and sciatic nerve of rodents. focus on paracrine actions of activated satellite cells after axotomy

The role of satellite cells, a type of peripheral glia, in the paracrine mechanisms related to neuronal maintenance and plasticity in the dorsal root ganglia (DRG) needs to be further investigated. This study employed immunohistochemistry and image analysis to investigate basic fibroblast growth factor (bFGF, FGF-2) and S100Beta immunoreactivities in the DRG and sciatic nerve of the rat and mouse. Well-characterized antibodies against bovine (residues 1-24) and rat (residues 1-23) FGF-2 were employed. Furthermore, the state of satellite cell reaction and changes in the FGF-2/S100Beta immunoreactivity were analyzed after axotomy of rat sciatic nerve. Scattered neurons and the majority of the satellite cells of the rat DRG and also Schwann cells of the rat sciatic nerve stained for S100Beta. In the mouse, strong S100Beta was encountered in the majority of sensory neurons and Schwann cells. Moderate FGF-2 (residues 1-24) immunoreactivity was found in scattered small size neurons of the rat DRG. A strong FGF-2 (residues 1-23) immunoreactivity was achieved in the satellite cells of rat DRG. Both FGF-2 antisera showed strong labeling in the mouse DRG sensory neurons. Activated satellite cells of the axotomized DRG possessed increased amount of FGF-2 and S100Beta immunoreactivity as demonstrated by quantitative image analysis. The proximal stump of the lesioned rat sciatic nerve showed increased FGF-2 (residues 1-24 and 1-23) in the Schwann cells, myelin sheaths, and neuronal fibers, without changes in the level of S100Beta immunoreactivity. Results suggested a possible interaction between FGF-2 and S100Beta in activated satellite cells of the DRG, which might trigger paracrine actions in the axotomized sensory neurons.

Axon regeneration through scars and into sites of chronic spinal cord injury

Cellular and extracellular inhibitors are thought to restrict axon growth after chronic spinal cord injury (SCI), confronting the axon with a combination of chronic astrocytosis and extracellular matrix-associated inhibitors that collectively constitute the chronic "scar." To examine whether the chronically injured environment is strongly inhibitory to axonal regeneration, we grafted permissive autologous bone marrow stromal cells (MSCs) into mid-cervical SCI sites of adult rats, 6 weeks post-injury without resection of the "chronic scar." Additional subjects received MSCs genetically modified to express neurotrophin-3 (NT-3), providing a further local stimulus to axon growth. Anatomical analysis 3 months post-injury revealed extensive astrocytosis surrounding the lesion site, together with dense deposition of the inhibitory extracellular matrix molecule NG2. Despite this inhibitory environment, axons penetrated the lesion site through the chronic scar. Robust axonal regeneration occurred into chronic lesion cavities expressing NT-3. Notably, chronically regenerating axons preferentially associated with Schwann cell surfaces expressing both inhibitory NG2 substrates and the permissive substrates L1 and NCAM in the lesion site. Collectively, these findings indicate that inhibitory factors deposited at sites of chronic SCI do not create impenetrable boundaries and that inhibition can be balanced by local and diffusible signals to generate robust axonal growth even without resecting chronic scar tissue.

Purification and in vitro characterization of adult canine olfactory ensheathing cells

Olfactory ensheathing cells (OECs) are known to promote neural repair under experimental conditions. The experimental focus has so far been almost entirely on rodent OECs (rOECs), and hence whether human OECs (humOECs) display similar properties is unclear. Studies on larger mammals as an "intermediate" model may be helpful for translating the experimental evidence gathered so far into novel therapeutic strategies. In the present study, we purified adult canine OECs (caOECs) from the olfactory bulb and analyzed their in vitro properties with respect to antigen expression, proliferation, and differentiation. Secondary caOECs shared the expression of marker molecules and the reactivity toward growth factors, with rOECs and humOECs. CaOECs were positively immunostained for the low affinity neurotrophin receptor p75, GFAP, and O4 and proliferated in response to fibroblast growth factor-2 and heregulin-1beta. No decline in proliferation was noted at higher passages (>8). The effects of forskolin, which neither increased proliferation nor stimulated the expression of O4, were clearly different from those on rOECs. Moreover, caOECs displayed their typical spindle-shaped morphology only upon growth factor/forskolin addition, whereas mitotically quiescent caOECs had a flattened morphology. Thus, caOECs can readily be purified from adult canine olfactory bulb and expanded by using established OEC mitogens. The behavior of caOECs toward forskolin suggests that caOECs and humOECs share a number of properties amd implies the presence of common intracellular signalling pathways. CaOECs therefore represent a suitable model system relevant for humOECs in neural repair studies.

Phagocytosis of O4+ axonal fragments in vitro by p75? neonatal rat olfactory ensheathing cells

Olfactory ensheathing cells (OECs) have gained wide interest because of their unique regeneration-promoting capacity. However, despite their frequent use in regeneration studies, the characterization of the cells has remained fragmentary. In the present study, we analyzed freshly dissociated neonatal rat OECs at the light and electron microscopic level and studied their fate in vitro using a novel two-step labeling protocol based on antibody internalization. We report the identification and characterization of two distinct OEC populations in situ and in primary cell suspensions that differed in number, p75 NGF receptor expression, and O4 immunoreactivity. The major OEC population in primary cells suspensions did not express p75 but stained positive for the glycolipid O4 (p75-/O4+). During culturing, these cells upregulated p75 expression and lost O4 immunoreactivity. Conversely, the minor OEC population consisted of p75+/O4- OECs that maintained p75 expression in vitro. Interestingly, ultrastructural analysis revealed not only that O4 immunoreactivity of p75- OECs was, in fact, due to O4+ axonal fragments adhering to the cell surface but also that p75- OECs rapidly phagocytosed these fragments in vitro. Taken together, the identification of two distinct OEC populations in the neonatal olfactory bulb that converge into single p75+ phenotype in vitro is reported. The observation that upregulation of p75 receptor expression in vitro was only apparent in those OECs closely associated with O4+ axonal processes may suggest that axonal signalling in vivo negatively regulates p75 receptor expression. The strong phagocytic activity of OECs in vitro may reflect one important aspect of their physiological function.

Axonal regeneration through regions of chondroitin sulfate proteoglycan deposition after spinal cord injury: a balance of permissiveness and inhibition

Increased expression of certain extracellular matrix (ECM) molecules after CNS injury is believed to restrict axonal regeneration. The chondroitin sulfate proteoglycans (CSPGs) are one such class of ECM molecules that inhibit neurite outgrowth in vitro and are upregulated after CNS injury. We examined growth responses of several classes of axons to this inhibitory environment in the presence of a cellular fibroblast bridge in a spinal cord lesion site and after a growth factor stimulus at the lesion site (fibroblasts genetically modified to secrete NGF). Immunohistochemical analysis showed dense labeling of the CSPGs NG2, brevican, neurocan, versican, and phosphacan at the host-lesion interface after spinal cord injury (SCI). Furthermore, robust expression of NG2, and to a lesser extent versican, was also observed throughout grafts of control and NGF-secreting fibroblasts. Despite this inhibitory milieu, several axonal classes penetrated control fibroblast grafts, including dorsal column sensory, rubrospinal, and nociceptive axons. Axon growth was amplified more in the presence of NGF-secreting grafts. Confocal microscopy demonstrated that axon growth was, paradoxically, preferentially associated with NG2-rich substrates in both graft types. NG2 expression also increased after sciatic nerve injury, wherein axons successfully regenerate. Cellular sources of NG2 in SCI and peripheral nerve lesion sites included Schwann cells and endothelial cells. Notably, these same cellular sources in lesion sites produced the cell adhesion molecules L1 and laminin, and these molecules all colocalized. Thus, axons grow along substrates coexpressing both inhibitory and permissive molecules, suggesting that regeneration is successful when local permissive signals balance and exceed inhibitory signals.

Purification of Schwann cells by selection of p75 low affinity nerve growth factor receptor expressing cells from adult peripheral nerve

The intrinsic capacity of Schwann cells to promote regeneration after limited peripheral nerve lesions has been successfully transferred to extensive peripheral nerve injuries and central nervous system lesions by autologous transplantation strategies. However, both the intrinsic ability of axotomized neurons to regenerate and the permissiveness of the parenchyma surrounding the acute injury site diminish over time. Therefore, the autologous transplantation mode requires a fast and effective method to isolate Schwann cells from peripheral nerve biopsies. Here, we report a method to purify p75 low affinity nerve growth factor receptor (p75LNGFr) expressing Schwann cells from peripheral nerve biopsies in adult rats using magnetic-activated cell separation (MACS). After 1 week of nerve degeneration in culture, nerve fragments were dissociated resulting in mixed cultures containing Schwann cells and fibroblasts. After incubation with specific anti-p75LNGFr antibodies and secondary magnetic bead conjugated antibodies followed by one cycle of MACS, 95% pure Schwann cell cultures were generated as confirmed by flow-cytometry and immunocytochemistry. In contrast to established methods, MACS separation of p75LNGFr expressing cells allows the reliable purification of Schwann cells within 9 days after biopsy employing direct selection of Schwann cells rather than fibroblast depletion assays. Therefore, this method represents an effective and fast means to generate autologous Schwann cells for clinical transplantation strategies aiming for axon repair and remyelination.

Immunohistochemical characterization of axonal sprouting and reactive tissue changes after long-term implantation of a polyimide sieve electrode to the transected adult rat sciatic nerve

The development of artificial microstructures suited for interfacing of peripheral nerves is not only relevant for basic neurophysiological research but also for future prosthetic approaches. Aim of the present study was to provide a detailed analysis of axonal sprouting and reactive tissue changes after implantation of a flexible sieve electrode to the proximal stump of the adult rat sciatic nerve. We report here that massive neurite growth after implantation, steadily increasing over a period of 11 months, was observed. Parallel to this increase was the expression of myelin markers like Po, whereas non-myelin-forming Schwann cells did not change. Compared to five weeks post-implantation. where both Schwann-cell phenotypes were intermingled with each other, non-myelin-forming Schwann cells occupied a peripheral position in each microfascicle after 11 months. After an initial increase, hematogenous macrophages were down-regulated in number but maintained close contact with the implant. However, at no time were signs of its degradation observed. It is concluded that the introduced flexible polyimide electrode is suitable for contacting peripheral nerves since it permits substantial neurite growth and offers excellent long-term stability.

Macrowell cultures identify a subpopulation of neonatal rat dorsal root ganglionic neurons displaying nerve growth factor independent survival

Dorsal root ganglionic (DRG) neurons of the newborn rat in vitro die by apoptosis within 24-48 h unless nerve growth factor (NGF) is added. Using a novel cell culture system (macrowell), we identified a neuronal subpopulation displaying NGF-independent survival in vitro. Neurons were grown on glass coverslips at standard cell density in different volumes of defined medium (standard: 500 microl; macrowell: 10 ml). In standard culture, 40% of neurons survived in the presence of NGF whereas there was no survival under control conditions. In macrowell culture, however, about 15% of neurons survived even in the absence of NGF. Addition of NGF to these cultures increased survival up to 65%. Neurons surviving independent of NGF in macrowell culture were heterogeneous in size and were lacking the low-affinity NGF receptor.

Nerve growth factor-hypersecreting Schwann cell grafts augment and guide spinal cord axonal growth and remyelinate central nervous system axons in a phenotypically appropriate manner that correlates with expression of L1

Schwann cells contribute to efficient axonal regeneration after peripheral nerve injury and, when grafted to the central nervous system (CNS), also support a modest degree of central axonal regeneration. This study examined (1) whether Schwann cells grafted to the CNS exhibit normal patterns of differentiation and association with spinal axons and what signals putatively modulate these interactions, and (2) whether Schwann cells overexpressing neurotrophic factors enhance axonal regeneration. Thus, primary Schwann cells were transduced to hypersecrete human nerve growth factor (NGF) and were grafted to spinal cord injury sites in adult rats. Comparisons were made to nontransfected Schwann cells. From 3 days to 6 months later, grafted Schwann cells exhibited a phenotypic and temporal course of differentiation that matched patterns normally observed after peripheral nerve injury. Schwann cells spontaneously aligned into regular spatial arrays within the cord, appropriately remyelinated coerulospinal axons that regenerated into grafts, and appropriately ensheathed but did not myelinate sensory axons extending into grafts. Coordinate expression of the cell adhesion molecule L1 on Schwann cells and axons correlated with establishment of appropriate patterns of axon-Schwann cell ensheathment. Transduction of Schwann cells to overexpress NGF robustly increased axonal growth but did not otherwise alter the nature of interactions with growing axons. These findings suggest that signals expressed on Schwann cells that modulate peripheral axonal regeneration and myelination are also recognized in the CNS and that the modification of Schwann cells to overexpress growth factors significantly augments their capacity to support extensive axonal growth in models of CNS injury.

The monoclonal antibody 23E9 defines a novel developmentally-regulated Schwann cell surface antigen

The present study describes the identification and partial characterization of a novel Schwann cell surface molecule by means of a monoclonal antibody (23E9). The 23E9 antigen was found in association with Schwann cells of the peripheral nerve but not with sensory neurons and satellite cells of the dorsal root ganglion. The expression of the antigen in the sciatic nerve starts after birth, is high around postnatal day 8 and becomes down-regulated towards the adult stage. This suggests that it may be involved in the induction of myelin formation. On Western blots, the antibody identified two major bands of approximately 27 and 42 kDa. Treatment of cultured Schwann cells with forskolin, an agent known to mimic neuronal contact in vitro, stimulated the up-regulation of the antigen. This implies that the expression of 23E9 is induced and maintained by axon-derived signals in vivo. Comparison of the presented data with the literature suggests that we have identified a novel cell surface molecule not previously characterized in the context of Schwann cell biology. To clarify the molecular identity of the antigen and define its physiological relevance, the antibody will be used in future studies for immunoprecipitation and functional in vitro assays.