Induction of myelin genes during peripheral nerve remyelination requires a continuous signal from the ingrowing axon

Regulatory role of cytochrome P450scc and pregnenolone in myelination by rat Schwann cells

To investigate the production of steroid hormones by Schwann cells and to examine the regulation of steroid hormone production during myelination, cultures of rat Schwann cells were differentiated into their myelinating phenotype in the absence of neurons with dibutyryl cAMP (db-cAMP). During this process, the expression of P450scc (involved in steroid biosynthesis) was elevated at both the mRNA and protein levels as evident in RT-PCR, Western blots, and immunostaining. Labeling of the cells with [14C] acetate revealed enhanced production of pregnenolone during differentiation into the myelinating phenotype. Disruption of P450scc's activity with an inhibitor diminished the extent of differentiation into the myelinating phenotype as levels of mRNA and protein expression of myelin protein zero (P0) declined. However, the effect was reversed with the addition of pregnenolone. Furthermore, when the differentiating cultures were treated with pregnenolone, mRNA expression of P0 was upregulated, suggesting the stimulation of the differentiation process. Together, these results provide evidence for Schwann cells as a major producer of steroid hormones and pregnenolone production by P450scc as an important regulatory step during myelination.

Desert hedgehog-patched 2 expression in peripheral nerves during Wallerian degeneration and regeneration

Hedgehog proteins are important in the development of the nervous system. As Desert hedgehog (Dhh) is involved in the development of peripheral nerves and is expressed in adult nerves, it may play a role in the maintenance of adult nerves and degeneration and regeneration after injury. We firstly investigated the Dhh-receptors, which are expressed in mouse adult nerves. The Dhh receptor patched(ptc)2 was detected in adult sciatic nerves using RT-PCR, however, ptc1 was undetectable under the same experimental condition. Using RT-PCR in purified cultures of mouse Schwann cells and fibroblasts, we found ptc2 mRNA in Schwann cells, and at much lower levels, in fibroblasts. By immunohistochemistry, Ptc2 protein was seen on unmyelinated nerve fibers. Then we induced crush injury to the sciatic nerves of wild-type (WT) and dhh-null mice and the distal stumps of injured nerves were analyzed morphologically at different time points and expression of dhh and related receptors was also measured by RT-PCR in WT mice. In dhh-null mice, degeneration of myelinated fibers was more severe than in WT mice. Furthermore, in regenerated nerves of dhh-null mice, minifascicular formation was even more extensive than in dhh-null intact nerves. Both dhh and ptc2 mRNA levels were down-regulated during the degenerative phase postinjury in WT mice, while levels rose again during the phase of nerve regeneration. These results suggest that the Dhh-Ptc2 signaling pathway may be involved in the maintenance of adult nerves and may be one of the factors that directly or indirectly determines the response of peripheral nerves to injury.

Expression of P0 glycoprotein in CNS glia: Effects of overexpression in N20.1 cells

To examine effects of expression of the PNS myelin P0 glycoprotein in glial cells of CNS lineage, we transfected murine N20.1 glial cells with a rat P0 cDNA. A stably transfected cell line expressing high levels of P0 message showed P0 immunostaining, along with changes in morphology. Polymerase chain reaction (PCR) identified the predicted rat P0 sequence in the transfected N20.1 cells and further revealed low levels of mouse P0 message in the nontransfected cells and in primary mouse astrocytes. This is the first evidence of endogenous expression of message for P0 glycoprotein in CNS glia. Quantitative RT-PCR confirmed the expression of rat P0 mRNA in the transfected N20.1 cells, at levels about 400 times greater than murine P0 in nontransfected cells. A 27-kD band was detected in the transfected cells by Western blot with P0 antibody, but not in mock-transfected or nontransfected N20.1 cells. Immunocytochemistry following permeabilization showed intracellular vesicular localization of P0 in the cytoplasm and perinuclear rings in transfected cells, with a similar pattern but much lower levels in nontransfected cells. Faint surface staining for P0 protein without permeabilization was seen only on the transfected cells. A few transfected cells with membrane sheets stained more intensely for surface P0. Quantitative RT-PCR was used to determine if P0 overexpression altered expression of other myelin-related genes compared with glial fibrillary acidic protein (GFAP); the ratios of myelin basic protein (MBP)/GFAP and proteolipid protein (PLP)/GFAP were increased 2- to 3-fold in the P0-transfected cells. We conclude that P0 overexpression alters N20.1 gene expression and cell morphology, and shifts the cells from astroglial to oligodendroglial phenotype.

Homeobox gene expression in adult dorsal root ganglia during sciatic nerve regeneration: is regeneration a recapitulation of development?

After damage of the sciatic nerve, a regeneration process is initiated. Neurons in the dorsal root ganglion regrow their axons and functional connections. The molecular mechanisms of this neuronal regenerative process have remained elusive, but a relationship with developmental processes has been conceived. This chapter discusses the applicability of the developmental hypothesis of regeneration to the dorsal root ganglion; this hypothesis states that regeneration of dorsal root ganglion neurons is a recapitulation of development. We present data on changes in gene expression upon sciatic nerve damage, and the expression and function of homeobox genes. This class of transcription factors plays a role in neuronal development. Based on these data, it is concluded that the hypothesis does not hold for dorsal root ganglion neurons, and that regeneration-specific mechanisms exist. Cytokines and the associated Jak/STAT (janus kinase/signal transducer and activator of transcription) signal transduction pathway emerge as constituents of a regeneration-specific mechanism. This mechanism may be the basis of pharmacological strategies to stimulate regeneration.

Expression of protein zero is increased in lesioned axon pathways in the central nervous system of adult zebrafish

The immunoglobulin superfamily molecule protein zero (P0) is important for myelin formation and may also play a role in adult axon regeneration, since it promotes neurite outgrowth in vitro. Moreover, it is expressed in the regenerating central nervous system (CNS) of fish, but not in the nonregenerating CNS of mammals. We identified a P0 homolog in zebrafish. Cell type-specific expression of P0 begins in the ventromedial hindbrain and the optic chiasm at 3-5 days of development. Later (at 4 weeks) expression has spread throughout the optic system and spinal cord. This is consistent with a role for P0 in CNS myelination during development. In the adult CNS, glial cells constitutively express P0 mRNA. After an optic nerve crush, expression is increased within 2 days in the entire optic pathway. Expression peaks at 1 to 2 months and remains elevated for at least 6 months postlesion. After enucleation, P0 mRNA expression is also upregulated but fails to reach the high levels observed in crush-lesioned animals at 4 weeks postlesion. Spinal cord transection leads to increased expression of P0 mRNA in the spinal cord caudal to the lesion site. The glial upregulation of P0 mRNA expression after a lesion of the adult zebrafish CNS suggests roles for P0 in promoting axon regeneration and remyelination after injury.

P0 and myelin basic protein-like immunoreactivities following ligation of the sciatic nerve in the rat

In this work we analyzed variations in the expression of MBPs and P0 in ligated sciatic nerves of young and adult rats at 3, 7, and 14 days postligation (PL), by immunohistochemistry and SDS-PAGE of isolated myelin. A protein redistribution was seen in the distal stump of ligated nerves with the appearance of immunoreactive clusters. Using the KS400 image analyzer, immunostained area values were obtained from the different nerves dissected. In adult rats, there was an increase of the immunostained area for MBP from 3 to 7 days PL, coincident with a reorganization of the marker in clusters, followed by a marked decrease at 14 days. P0 immunolabeling gave similar results without, however, a decrease of the immunostained area at the longer survival time tested. Young animals showed an acceleration in the process of protein redistribution and digestion within ligated nerves, which followed a similar pattern as that of adult animals. Analysis by electrophoresis showed a marked decrease in P0 and MBP at 7 days PL in young rats and 14 days PL in adult rats. The functional significance of protein clustering within myelin in injured nerves deserves further analysis.

Identification of the regulatory region of the peripheral myelin protein 22 (PMP22) gene that directs temporal and spatial expression in development and regeneration of peripheral nerves

Minor changes in PMP22 gene dosage have profound effects on the development and maintenance of peripheral nerves. This is evident from the genetic disease mechanisms in Charcot-Marie-Tooth disease type 1A (CMT1A) and hereditary neuropathy with liability to pressure palsies (HNPP) as well as transgenic animals with altered PMP22 gene dosage. Thus, regulation of PMP22 is a crucial aspect in understanding the function of this protein in health and disease. In this study, we have generated transgenic mice containing 10 kb of the 5'-flanking region of the PMP22 gene, including the two previously identified alternative promoters, fused to a lacZ reporter gene. We show that this part of the PMP22 gene contains the necessary information to mirror the endogenous expression pattern in peripheral nerves during development and regeneration and in mouse models of demyelination due to genetic lesions. Transgene expression is strongly regulated during myelination, demyelination, and remyelination in Schwann cells, demonstrating the crucial influence of neuron-Schwann cell interactions in the regulation of PMP22. In addition, the region of the PMP22 gene present on this transgene confers also neuronal expression in sensory and motor neurons. These results provide the crucial basis for further dissection of the elements that direct the temporal and spatial regulation of the PMP22 gene and to elucidate the molecular basis of the master program regulating peripheral nerve myelination.

Retroviral inhibition of cAMP-dependent protein kinase inhibits myelination but not Schwann cell mitosis stimulated by interaction with neurons

Schwann cells are the myelinating glia of the peripheral nervous system. Neuron-Schwann cell contact profoundly affects several aspects of Schwann cell phenotype, including stimulation of mitosis and myelin formation. Many reports suggest that neuronal contact exerts this influence on Schwann cells by elevating Schwann cell cAMP and activating cAMP-dependent protein kinase A (PKA). To elucidate the importance of Schwann cell PKA in neuronal stimulation of Schwann cell mitosis and myelination, the gene encoding the PKA inhibitory protein RIalphaAB or PKIEGFP was delivered to Schwann cells using retroviral vectors. PKA inhibitory retroviral vectors effectively blocked forskolin-stimulated Schwann cell mitosis and morphological change, demonstrating the ability of the vectors to inhibit PKA in infected Schwann cells. Treatment of dorsal root ganglia neuron-Schwann cell cocultures with H-89 (10 microm) or KT5720 (1-10 microm), chemical inhibitors selective for PKA, significantly inhibited neuronal stimulation of Schwann cell mitosis. In contrast, retrovirus-mediated inhibition of Schwann cell PKA had no effect on the ability of neurons to stimulate Schwann cell mitosis. However, markedly fewer myelin segments were formed by Schwann cells expressing PKA inhibitory proteins compared with controls. These results suggest that activation of Schwann cell PKA is required for myelin formation but not for Schwann cell mitosis stimulated by interaction with neurons.

Mononuclear cell proliferation and hyperplasia during Wallerian degeneration in the visual system of the goldfish in the presence or absence of regenerating optic axons

Patterns of proliferation and changes in non-neuronal cell number in the visual system of the goldfish have been quantitatively examined during optic axon regeneration after an optic nerve crush (ONC). In addition, in order to examine the effect of the regenerating axons on cellular responses in the visual pathways, we did a similar analysis of animals with the right eye removed (ER). Finally, we used double labeling protocols to demonstrate that the proliferating cells that we were counting were mostly phagocytic cells of the mononuclear lineage. In animals with an ONC, we observed an early burst of proliferation that peaked between 7 and 14 days after surgery in all parts of the visual system. In the optic tract, there was also a secondary rise that peaked at 21 days. Levels of proliferation returned to normal by 32 days postoperative in the tract and tectum, while they remained somewhat elevated in the optic nerve for at least 93 days. The total number of non-neuronal cells in the visual paths also rose to peak values between 7 and 14 days after ONC surgery. In the optic tract and tectum, the values fell rapidly after this time, while in the optic nerve, there was a secondary peak at 32 days after which values remained elevated for the duration of the experiment. As compared to animals with an ONC, enucleation resulted in elevated proliferation and hyperplasia at early postoperative intervals. However, because these differences occurred when axons had not yet regenerated into the affected structures, these data do not provide strong evidence for a direct effect of regenerating optic axons on the early cellular responses during Wallerian degeneration in the goldfish. In addition, in the tectum, there was an early increment in cell number that was not associated with elevated levels of proliferation. We believe that this increment represents immigration of resident microglia from other regions of the brain.

Regulation of fibronectin alternative splicing during peripheral nerve repair

Wallerian degeneration following peripheral nerve injury is associated with increased production of fibronectin and other extracellular matrix molecules that are thought to enhance repair. We have shown previously that alternative splicing of the mRNA for fibronectin also changes following sciatic nerve lesions so as to reexpress forms of mRNA seen during embryogenesis. In the present study, we have examined the role of the regenerating axons in the regulation of this splicing. We have compared the patterns of fibronectin mRNA splicing seen in sciatic nerve development with that seen in cut nerves (that do not regenerate), crushed nerves (that regenerate successfully), and Schwann cells cultured in forskolin so as to mimic axonal signals. By using a reverse transcriptase polymerase chain reaction assay to examine all three regions of fibronectin mRNA splicing in a quantitative manner, we found that embryonic patterns of fibronectin mRNA splicing appear rapidly following injury and are not then altered by reestablishment of axons in the nerve. In addition, we found that forskolin has no effect on fibronectin mRNA splicing in cultured cells. We conclude that axonal signals do not regulate the pattern of fibronectin alternative splicing in peripheral nerve repair.

Promyelinating Schwann cells express Tst-1/SCIP/Oct-6

Tst-1/SCIP/Oct-6, a POU domain transcription factor, is transiently expressed by developing Schwann cells and is required for their normal development into a myelinating phenotype. In tst-1/scip/oct-6-null sciatic nerves, Schwann cells are transiently arrested at the "promyelinating" stage, when they have a one-to-one relationship with an axon but before they have elaborated a myelin sheath. To determine when Schwann cells express Tst-1/SCIP/Oct-6, we examined beta-galactosidase (beta-gal) expression in heterozygous tst-1/scip/oct-6 mice, in which one copy of the tst-1/scip/oct-6 gene has been replaced with the LacZ gene. beta-Gal expression from the LacZ gene seems to parallel Tst-1/SCIP/Oct-6 expression from the endogenous tst-1/scip/oct-6 gene in developing and regenerating sciatic nerves. Furthermore, electron microscopic examination of 5bromo-4-chloro-3-indolyl-beta-D-galactopyranoside- (X-gal) and halogenated indolyl-beta-D-galactoside- (Bluo-gal) stained nerves showed that promyelinating Schwann cells express the highest levels of beta-gal, both in developing and in regenerating nerves. Thus, the expression of beta-gal, a surrogate marker of Tst-1/SCIP/Oct-6, peaks at the same stage of Schwann cell development at which development is arrested in tst-1/scip/oct-6-null mice, indicating that Tst-1/SCIP/Oct-6 has a critical role in promyelinating Schwann cells.

Transient expression of the neurofilament proteins NF-L and NF-M by Schwann cells is regulated by axonal contact

Expression of the genes that encode neurofilament proteins is considered to be confined normally to neurons. However, in demyelinating peripheral nerves Schwann cells upregulate the mRNA for the medium-sized neurofilament protein (NF-M), and cultured Schwann cells of the myelin-forming phenotype can also synthesize and incorporate NF-M protein into their intermediate filament (IF) cytoskeleton. The purpose of this study was to establish how axonal contact might influence glial neurofilament gene expression and regulate the synthesis of neurofilament proteins. We show that the gene encoding NF-M is expressed at early stages of differentiation in myelin-forming Schwann cells in vivo; nevertheless, little NF-M protein can be detected in these cells. The transient induction of NF-M mRNA is also apparent in dedifferentiating Schwann cells during Wallerian degeneration. In these Schwann cells the mRNAs for NF-M and NF-L (the smallest polypeptide), but not NF-H (the largest neurofilament subunit), are coordinately expressed. In contrast to differentiating myelin-forming Schwann cells, the cells of degenerating nerves express both NF-M and NF-L polypeptides. Restoration of axonal contact in the growing nerve stimulates the recapitulation of Schwann cell differentiation including the elevation of NF-M and NF-L mRNA expression. These results demonstrate that the transient induction of neurofilament mRNAs in Schwann cells is a feature of both differentiation and dedifferentiation. However translation of these mRNAs is confined to Schwann cells deprived of axonal contact either by nerve injury or by culture in the absence of axons. These findings suggest that the expression of the NF-M and NF-L polypeptides is an important characteristic of those Schwann cells that will contribute to the repair of damaged peripheral nerves.

Delay of CNTF decrease following peripheral nerve injury in C57BL/Wld mice

In peripheral nerves, ciliary neurotrophic factor (CNTF) is localized to a subset of Schwann cells and is decreased in synthesis during Wallerian degeneration. This pattern of expression is similar to that of myelin protein genes. In the present study, C57BL/Wld mice, which exhibit delayed Wallerian degeneration, were used to determine the role of axonal contact on the regulation of CNTF synthesis. Western blot analysis showed that CNTF immunoreactivity in Wld nerves remained almost normal even 10 days after ligation when it was almost undetectable in control mice. Reverse transcriptase polymerase chain reaction (RT-PCR) analysis revealed that 4 days after ligation, concentrations of CNTF mRNA in Wld mice had decreased much less than in control mice, but that at 10 days CNTF mRNA concentrations in Wld and control mice were comparably low. These observations suggest that maintenance of axonal contact in the absence of axonal transport from the cell body delays the decrease of CNTF mRNA normally seen after injury. Also, during Wallerian degeneration in Wld mice, the decrease of CNTF protein is delayed for many days longer than the decrease in CNTF mRNA.

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

In an attempt to identify and characterize novel Schwann cell surface molecules with putative functions during development, maintenance, and regeneration of the peripheral nervous system (PNS), we have produced monoclonal antibodies against viable neonatal rat Schwann cells. Using a sensitive live cell ELISA protocol, three monoclonal antibodies reactive with cultured Schwann cells, designated 27B10, 26F2, and 27C7 were isolated. The 27B10 and 26F2 antibodies specifically labelled forskolin-stimulated secondary Schwann cells in vitro as determined by live cell ELISA implying that the expression of the antigens in situ is regulated by axonal contact. The observation that the antigens seemed to be associated with both Schwann cell phenotypes clearly discriminated them from the well characterized myelin proteins as well as from molecules known to be confined to the non-myelin-forming phenotype. Interestingly, both antigens were found to be concentrated at the nodes of Ranvier. Further studies therefore have to show whether the identified antigens share structural or functional homology with adhesion or channel molecules, which display a similar distribution. Following transection of the adult sciatic nerve, the 26F2 antigen was rapidly down-regulated in the distal nerve stump. The 27C7 antibody reacted with an 80 kDa cell surface molecule common to non-myelin-forming Schwann cells. No differences in expression of the antigen between forskolin-treated and untreated Schwann cells in vitro were found, suggesting that the antigen is expressed independently from axonal contact. Two weeks after nerve transection in the absence of myelinating Schwann cells, the antigen was associated with S-100-positive Schwann cells of the distal nerve stump. The antigen was found to be expressed also by non-neuronal tissues, the level of the protein declined towards the adult stage. Comparison of the 27C7 antigen with previously described marker molecules suggests that we have identified a novel Schwann cell surface antigen of the non-myelin-forming phenotype.

Axonal regeneration through acellular muscle grafts

The management of peripheral nerve injury remains a major clinical problem. Progress in this field will almost certainly depend upon manipulating the pathophysiological processes which are triggered by traumatic injuries. One of the most important determinants of functional outcome after the reconstruction of a transected peripheral nerve is the length of the gap between proximal and distal nerve stumps. Long defects (> 2 cm) must be bridged by a suitable conduit in order to support axonal regrowth. This review examines the cellular and acellular elements which facilitate axonal regrowth and the use of acellular muscle grafts in the repair of injuries in the peripheral nervous system.

Periaxin expression in myelinating Schwann cells: modulation by axon-glial interactions and polarized localization during development

Periaxin is a newly described protein that is expressed exclusively by myelinating Schwann cells. In developing nerves, periaxin is first detected as Schwann cells ensheathe axons, prior to the appearance of the proteins that characterize the myelin sheath. Periaxin is initially concentrated in the adaxonal membrane (apposing the axon) but, during development, as myelin sheaths mature, periaxin becomes predominately localized at the abaxonal Schwann cell membrane (apposing the basal lamina). In permanently axotomized adult nerves, periaxin is lost from the abaxonal and adaxonal membranes, becomes associated with degenerating myelin sheaths and is phagocytosed by macrophages. In crushed nerves, in which axons regenerate and are remyelinated, periaxin is first detected in the adoxonal membrane as Schwann cells ensheathe regenerating axons, but again prior to the appearance of other myelin proteins. Periaxin mRNA and protein levels change in parallel with those of other myelin-related genes after permanent axotomy and crush. These data demonstrate that periaxin is expressed by myelinating Schwann cells in a dynamic, developmentally regulated manner. The shift in localization of periaxin in the Schwann cell after completion of the spiralization phase of myelination suggests that periaxin participates in membrane-protein interactions that are required to stabilize the mature myelin sheath.

Myelin glycoprotein P0 is expressed at early stages of chicken and rat embryogenesis

Although previous studies suggest that P0 is expressed only in myelinating Schwann cells, monoclonal antibody 1E8 reacts with P0, yet also stains early Schwann cell precursors and non-myelinating Schwann cells (Bhattacharyya et al.: Neuron 7:831-844, 1991). We therefore characterized the 1E8 epitope and analyzed P0 mRNA expression during development. Immunoblot analyses of P0 fusion proteins and of deglycosylated P0 indicated that the 1E8 epitope is polypeptide. Northern blot and polymerase chain reaction (PCR) analyses revealed that P0 is encoded by a single mRNA that is expressed in chicken embryos as early as E4 and in rat embryos as early as E14. These data indicate that the antigen recognized by 1E8 in early chicken embryos is P0 and that, during development of both chickens and rats, P0 mRNA is expressed long before myelination.

NGFR-mRNA expression in sciatic nerve: a sensitive indicator of early stages of axonopathy

Expression of the low-affinity nerve growth factor receptor (NGFR) in the sciatic nerve (particularly Schwann cells) is high during development but is downregulated upon establishment of the mature axon-Schwann cell relationship. NGFR is re-expressed by Schwann cells if this relationship is altered by degeneration of axons (axotomy) or myelin (tellurium intoxication). To determine the sensitivity of NGFR expression to axonal injury, we have assayed NGFR-mRNA levels in proximal and distal regions of nerves exposed to the axonopathic agents acrylamide and isoniazid, as well as in proximal and distal stumps of axotomized nerves. NGFR-mRNA was elevated in all three models and correlated regionally with sites of axonal perturbation. In distal regions of acrylamide- and isoniazid-intoxicated nerves, NGFR-mRNA was elevated at least 2 days prior to visible signs of axonal degeneration as assayed by morphological techniques utilizing light microscopy. NGFR-mRNA was also elevated in proximal regions of axotomized and acrylamide-intoxicated nerves prior to signs of axonal degeneration. In these models, increased mRNA expression correlated with alterations in the size distribution of axonal cross sections. The common response in all of these situations indicates that NGFR expression, in addition to being a marker for axonal degeneration, is also a sensitive indicator of less profound perturbations in normal axon-Schwann cell interactions, including early stages of axonopathy. We suggest that assay for NGFR-mRNA may be utilized as a rapid and simple method (relative to more labor-intensive morphological methods) to screen for peripheral neurotoxicity.(ABSTRACT TRUNCATED AT 250 WORDS)

Schwann cell ATP-mediated calcium increases in vitro and in situ are dependent on contact with neurons

Schwann cells freshly isolated from the sciatic nerves of neonatal rats respond to exogenously applied ATP with a rapid increase in cytosolic calcium. This increase in [Ca2+]i is mediated by a P2Y-purinergic pathway (Lyons et al.: J. Neurochem. 63:552-560, 1994) and was measured using the calcium indicator dye, fura-2/AM, and a video-enhanced calcium imaging system. The ability to respond to ATP with increases in intracellular calcium is lost over a period of several days in culture; this loss can be prevented or reversed by application of cAMP analogs in a defined medium. We now demonstrate that the direct contact of Schwann cells with neurons also induces and stabilizes this ATP responsiveness. The induction of ATP responsiveness was observed among all Schwann cells contacting neurites, including those forming myelin, and regardless of whether the source of neurons was dorsal root ganglion neurons or superior cervical ganglion neurons. Approximately 85% of Schwann cells responded to ATP over the time studied (72 d in coculture). Addition of axolemma to Schwann cell cultures did not induce ATP responsiveness. We also examined the ATP responsiveness of Schwann cells in situ (excised nerves) using laser-scanning confocal microscopy and the calcium indicator dye, fluo-3/AM. Schwann cells in intact sciatic nerve segments isolated from neonatal and 16-day-old rats exhibited ATP-mediated [Ca2+]i increases. We conclude that neuronal contact is necessary for the expression of the ATP-mediated calcium responses in Schwann cells and that these responses are independent of myelin formation or maintenance.