Abnormalities expressed in long-term cultures of dorsal root ganglia from the dystrophic mouse

Axonal damage and demyelination in long-term dorsal root ganglia cultures from a rat model of Charcot-Marie-Tooth type 1A disease

Clinical progression in hereditary and acquired demyelinating disorders of both the central and peripheral nervous system is mainly due to a time-dependent axonal impairment. We established 90-day dorsal root ganglia (DRG) cultures from a rat model of Charcot-Marie-Tooth type 1A (CMT1A) neuropathy to evaluate the structure of myelin and axons, and the expression of myelin-related proteins and cytoskeletal components, by morphological and molecular techniques. Both wild-type and CMT1A cultures were rich in myelinated fibres. Affected cultures showed dysmyelinated internodes and focal myelin swellings. Furthermore, uncompacted myelin and smaller axons with increased neurofilament (NF) density were found by electron microscopy, and Western blots showed higher levels of nonphosphorylated NF. Confocal microscopy demonstrated an abnormal distribution of the myelin-associated glycoprotein which, instead of being expressed at the noncompact myelin level, showed focal accumulation along the internodes while other myelin proteins were normally distributed. These findings suggest that CMT1A DRG cultures, similarly to the animal model and human disease, undergo axonal atrophy over a period of time. This model may be utilized to study the molecular changes underlying demyelination and secondary axonal impairment. As axonal damage may occur after just 3 months and tissue cultures represent a strictly controlled environment, this model may be ideal for testing neuroprotective therapies.

Rho kinase regulates schwann cell myelination and formation of associated axonal domains

The myelin sheath forms by the spiral wrapping of a glial membrane around an axon. The mechanisms involved are poorly understood but are likely to involve coordinated changes in the glial cell cytoskeleton. Because of its key role as a regulator of the cytoskeleton, we investigated the role of Rho kinase (ROCK), a major downstream effector of Rho, in Schwann cell morphology, differentiation, and myelination. Pharmacologic inhibition of ROCK activity results in loss of microvilli and stress fibers in Schwann cell cultures and strikingly aberrant myelination in Schwann cell-neuron cocultures; there was no effect on Schwann cell proliferation or differentiation. Treated Schwann cells branch aberrantly and form multiple, small, independent myelin segments along the length of axons, each with associated nodes and paranodes. This organization partially resembles myelin formed by oligodendrocytes rather than the single long myelin sheath characteristic of Schwann cells. ROCK regulates myosin light chain phosphorylation, which is robustly, but transiently, activated at the onset of myelination. These results support a key role of Rho through its effector ROCK in coordinating the movement of the glial membrane around the axon at the onset of myelination via regulation of myosin phosphorylation and actomyosin assembly. They also indicate that the molecular machinery that promotes the wrapping of the glial membrane sheath around the axon is distributed along the entire length of the internode.

PMP22 transgenic dorsal root ganglia cultures show myelin abnormalities similar to those of human CMT1A

Charcot-Marie-Tooth 1A (CMT1A) neuropathy is caused by duplication of the peripheral myelin protein 22 (PMP22) gene, leading to protein overexpression. Although this protein has a role in regulating Schwann cell growth and peripheral myelin compaction, how altered concentrations of PMP22 impair myelination is unknown. We established dorsal root ganglia (DRG) cultures from a transgenic rat overexpressing PMP22 (PMP22tg) to study the behavior of PMP22tg Schwann cells in early stages of development and myelination. We used reverse transcriptase-polymerase chain reaction and light and electron microscopy to study PMP22 expression and myelin formation. Myelin ultrastructure was evaluated in sural nerves from CMT1A patients to compare experimental and human findings. PMP22tg DRG cultures contained a greater number of internodes devoid of myelin, in the absence of remyelination, and increased periodicity of myelin lamellae compared with normal cultures. Widening of myelin lamellae was also observed in CMT1A biopsy specimens. Our results suggest that both functions of PMP22, in regulating Schwann cell differentiation and contributing to peripheral myelin compaction, are affected by its overexpression. The presence of similar myelin abnormalities in PMP22tg cultures and human nerves emphasizes the importance of developing in vitro models of hereditary neuropathies to study their underlying pathomechanisms.

Peripheral nerve fibroblasts as a source of IL-6, TNFalpha and IL-1 and their modulation by IFNgamma

Interleukin-6 (IL-6), tumor necrosis factor alpha (TNFalpha), and interleukin-1 (IL-1) are immunomodulatory cytokines produced by Schwann cells of the peripheral nervous system (PNS). Their upregulation has been associated with autoimmune inflammatory diseases of the PNS such as Guillain-Barré Syndrome (GBS) and Chronic Inflammatory Demyelinating Neuropathy (CIDP). We now report that PNS fibroblasts and a PNS fibroblast cell line - MA-1 express mRNA for IL-6, TNFalpha and IL-I and that the MA-1 cell line secretes these molecules. Flow cytometry and fluorescent activated cell sorting defined that 76% of MA-1 fibroblasts were Thy1.1+ and 24% were Thy1.1-. Each subset expressed major histocompatibility class (MHC) I molecules and intercellular adhesion molecule-1 (ICAM-1). IFNgamma stimulation induced the expression of MHC II molecules in Thy1.1+, but not Thy1.1(-) cells. All MA-1 cells expressed mRNA for IL-6, TNFalpha, and IL-1 plus or minus IFNgamma stimulation. IFNgamma stimulation significantly reduced the production of IL-6 but increased TNFalpha production. Direct in situ reverse-transcriptase polymerase chain reaction (RT-PCR) showed that IL-1 mRNA staining increased significantly following IFNgamma stimulation. These results provide evidence for the first time that not only Schwann cells, but also peripheral nerve fibroblasts are a source of immunomodulatory cytokines within the PNS and may contribute to inflammatory processes in PNS disease.

Schwann cell extracellular matrix protein production is modulated by Mycobacterium leprae and macrophage secretory products

Extracellular matrix (ECM) protein deposition is an important feature of leprous nerves, where Schwann cells (SCs) and macrophages are the main hosts for Mycobacterium leprae. Since, SCs are involved in the synthesis of ECM proteins and its production is regulated by macrophage secretory factors, the present study aimed to determine in vitro, the effect of M. leprae infection and macrophage secretory products on secretion of ECM proteins by SCs in two strains of mice, Swiss White (SW) and C57BL/6, that are known to differ in their nerve pathology and macrophage functions in response to infection. Following six days of M. leprae infection, SCs from SW mice responded with increased secretion of 14C-leucine radiolabelled proteins and a concomitant increase in laminin and collagens type I, III and IV, as determined by enzyme-linked immunosorbent assay. In contrast infected C57BL/6 SCs responded with decreased secretion of total proteins and fibronectin. Exposure of SCs to macrophage conditioned medium resulted in decreased ECM protein secretion in both strains of mice. This decrease was a function of protein breakdown by macrophage derived proteases and also active regulation by macrophage secreted cytokines. A similar effect of M. leprae and macrophage secretory products on SC metabolism in leprous nerves would have major ramifications on damage and repair activities. In addition ECM proteins would also influence the composition of the infiltrating cell population in lepromatous and tuberculoid nerves.

Fibroblasts are required for Schwann cell basal lamina deposition and ensheathment of unmyelinated sympathetic neurites in culture

The ability to purify and recombine populations of peripheral neurons, Schwann cells and fibroblasts in tissue culture has enabled us to examine the contribution of fibroblasts to Schwann cell basal lamina assembly and ensheathment of unmyelinated rat superior cervical ganglion neurites in vitro. Purified perinatal superior cervical ganglion neurons were grown in culture either with Schwann cells or with Schwann cells plus fibroblasts derived from either superior cervical ganglion capsule or cranial periosteum. The cultures were maintained for 2-8 weeks on a collagen substratum in a medium known to promote Schwann cell differentiation (myelin, basal lamina formation) in the presence of dorsal root ganglion neurons. The extent of Schwann cell differentiation (ensheathment, basal lamina formation) in the presence of superior cervical ganglion neurons was evaluated in this study using electron microscopy. In superior cervical ganglion neuron plus Schwann cell cultures (without fibroblasts), Schwann cells achieved only a moderate degree of ensheathment; also, Schwann cell basal lamina was discontinuous and extracellular collagen fibrils were sparse. Although only discontinuous basal lamina was demonstrable by electron microscopy in these cultures, surprisingly, Schwann cell/neurite fascicles were uniformly immunostained for laminin, type IV collagen, and heparan sulfate proteoglycan. The addition of fibroblasts to superior cervical ganglion neuron plus Schwann cell cultures increased the deposition of basal lamina around the Schwann cell/neurite units, the number of collagen fibrils, and the extent of neurite ensheathment. We propose that the presence of basal lamina increases the Schwann cell's ability to ensheathe superior cervical ganglion neurites, possibly through an augmentation of specific extracellular matrix components or by increasing in some way the capacity of these components to become organized into basal lamina. We conclude that, unlike dorsal root ganglion neurons, superior cervical ganglion neurons are unable to stimulate full Schwann cell extracellular matrix expression with the result that these Schwann cells require the extraneuronal influence of fibroblasts to deposit basal lamina and attain their mature phenotype in culture.

Movements of the Schwann cell nucleus implicate progression of the inner (axon-related) Schwann cell process during myelination

Although it has been known for several decades that peripheral myelin is formed from an extended, spiraled, and compacted sheet of Schwann cell (SC) plasma membrane, the mechanism by which this unique spiraling is accomplished remains unknown. We have studied the movements of SC nuclei before, during, and subsequent to myelin formation (over periods of 24-72 h) to determine if this nuclear motion (noted in earlier reports) would provide useful insights into the mechanism of myelinogenesis. We used rodent sensory neuron and SC cultures in which initiation of myelinogenesis is relatively synchronized and bright field conditions that allowed resolution of the axon, compact myelin, and position of the SC nucleus. Observed areas were subsequently examined by electron microscopy (EM); eight myelinating SCs with known nuclear movement history were subjected to detailed EM analysis. We observed that, prefatory to myelination, SCs extended along the length of larger axons, apparently competing with adjacent SCs for axonal surface contact. This lengthening preceded the deposition of compact myelin. SC nuclear circumnavigation of the axon was found to attend early myelin sheath formation. This movement was rarely greater than 0.25 turns per 3 h; on the average, more nuclear motion was seen in relation to internodes that formed during observation (0.8 +/- 0.1 turns/24 h) than in relation to those that had begun to form before observation (0.3 +/- 0.1 turns/24 h). Nuclear circumnavigation generally proceeded in one direction, could be in similar or opposite direction in neighboring myelinating SCs on the same axon, and was not proportional to the number of major dense lines within the myelin sheath. A critical finding was that, in all eight cases examined, the overall direction of nuclear movement was the same as that of the inner end of the spiraling SC process, and thus opposite the direction of the outer end of the spiral. We conclude that the correspondence of the direction of nuclear rotation and inner end of the spiraling cytoplasmic lip implicates active progression of the inner lip over the axonal surface to form the membranous spiral of myelin, the nuclear motion resulting from towing by the advancing adaxonal lip. This interpretation fits with finding basal lamina and macular adhering junctions associated with the external lip of SC cytoplasm; these attributes would imply anchorage rather than movement of this region of the SC.

Differences in density and distribution of surface glycoconjugates between normal and dystrophic mouse Schwann cells detected by statistical analyses of lectin-ferritin binding

Cultures of Schwann cells and neurons from dorsal root ganglia of normal (C57bl/6J +/+) and dystrophic (C57bl/6J dy2j/dy2j) mice were labeled with wheat germ agglutinin (WGA) and Ricinus communis agglutinin (RCA-I) conjugated to ferritin. Statistical methods were used to compare the regional densities and distribution characteristics of lectin binding in these two types of Schwann cells, which differ in their capacities to ensheath and myelinate axons in vivo and in cultures. Regional variations in lectin binding densities and distributions were observed in both types of Schwann cells. WGA-ferritin was bound at lower densities in dystrophic mouse Schwann cells than in corresponding regions of normal cells. In both normal and dystrophic cells, WGA-ferritin was distributed at greater densities on the free surfaces of Schwann cells than on the substrate-associated surfaces. WGA-ferritin was clustered in all regions of both normal and dystrophic mouse cells. RCA-ferritin densities did not differ significantly between corresponding regions of normal and dystrophic mouse Schwann cells. However, in normal mouse Schwann cells, the density of RCA-ferritin was significantly greater in the thinner, peripheral processes of Schwann cells than in thicker perinuclear regions of the cells. Differences in the degree of RCA-ferritin clustering were also detected between normal and dystrophic Schwann cells. These results indicate that regional differences in the density and distributions of cell surface glycoconjugates occur in Schwann cells of normal and dystrophic mice.

Adhesive interactions between normal and dystrophic human skin fibroblasts

The adhesive properties of skin fibroblasts from patients with Duchenne muscular dystrophy (DMD) were studied by analysing cell aggregate formation in suspensions consisting of normal and DMD fibroblasts. By the use of aggregation kinetics and fluorescent labelling, the genotypic composition of aggregates in mixed-cell suspensions could be visualised. The distribution of normal and DMD cells within these aggregates could then be compared to theoretical binomial distributions which assume no difference in the specific adhesiveness between the two genotypes. Analysis of the 3- and 4-cell aggregates which were produced by co-aggregating normal and DMD cells demonstrate that there is no qualitative (specific) difference in the adhesiveness between normal and DMD fibroblasts. However, quantitative changes in the cell-cell adhesion of DMD fibroblasts may be present, and this is supported by the relatively small proportion of intermediate size heterotypic aggregates which were formed in mixed-genotype cell suspensions. In such mixtures, fewer aggregates consisting of 5 or more cells were formed compared to fibroblast suspensions derived from pairs of normal individuals. Furthermore, cell suspensions from pairs of DMD patients produced even less greater than or equal to 5-cell aggregates than were found in the mixed-genotype experiments. These findings are considered in relation to previous reports of abnormal cell adhesiveness and other adhesion-related mechanisms in DMD cells.

Schwann cells fail to differentiate when co-cultured in contact with PC12 neurites

Schwann cells derived from mouse or rat dorsal root ganglia (DRG) were co-cultured with either DRG neurons or nerve growth factor (NGF)-responsive PC12 pheochromocytoma cells for up to 7 weeks. When Schwann cells were grown in the presence of DRG neurites, they displayed normal ensheathing behavior and produced basal laminae and small diameter collagen fibrils within 5-19 days in vitro. However, when Schwann cells were co-cultured in direct contact with PC12 cells and without DRG neurons, they largely failed to ensheath PC12 neurites, and failed to assemble either basal lamina or small diameter collagen fibrils at any point during 7 weeks. Schwann cell proliferation continued in the presence of PC12 neurites, indicating that PC12 cells produced a mitogenic activity for Schwann cells functionally similar to previously described neurite-associated activities. These results demonstrate that Schwann cell contact with PC12 cells does not elicit the final morphogenetic events in Schwann cells (ensheathment, basal lamina formation and collagen fibril assembly) that normally occur when Schwann cells are co-cultured in contact with DRG neurons.

Myelin proteins and collagen in the spinal roots and sciatic nerves of muscular dystrophic mice

Proteins of lumbosacral spinal roots and sciatic nerves of adult dystrophic mice (Bar Harbor 129REJ dydy) were analyzed by SDS gel electrophoresis to determine if identifiable proteins were affected. All peripheral nerve myelin proteins (P0 glycoprotein, P1 and Pr basic proteins, X protein and a high-molecular-weight protein) were decreased in the roots but not in the sciatic nerves. Central nervous system myelin proteins were not increased in either roots or sciatic nerves. Although dystrophic spinal roots and sciatic nerves contained less collagen, Type I, III and V collagens were present.

Effects of crush injury on the abnormalities in the spinal roots and peripheral nerves of dystrophic mice

Lumbosacral spinal roots and peroneal nerves in dystrophic and control mice were crushed and allowed to regenerate. Six weeks after crush injury, the dystrophic roots no longer showed the typical groups of unensheathed axons that characterize the uncrushed roots. Thus, the location of this ensheathment defect in the spinal roots cannot be the exclusive mechanism responsible for its development. Crush injury and regeneration also tended to correct a second abnormality in the peripheral nervous system of dystrophic mice: the discontinuities in the Schwann cell basal laminas. Because the regenerated nerves contained increased amounts of collagen, the results of this study support the evidence from tissue culture experiments that the extracellular matrix may be involved in the pathogenesis of these disorders. However, the outcome of the present in vivo experiments indicates that genetically normal fibroblasts are not required for this change to occur.

Biosynthesis of type IV collagen by cultured rat Schwann cells

We have obtained evidence that rat Schwann cells synthesize and secrete type IV procollagen. Metabolic labeling of primary cultures of Schwann cells plus neurons and analysis by SDS PAGE revealed the presence of a closely spaced pair of polypeptides in the medium of these cultures that (a) were susceptible to digestion by purified bacterial collagenase, (b) co-migrated with type IV procollagen secreted by rat parietal endoderm cells, and (c) were specifically immunoprecipitated by antibodies against mouse type IV collagen. Limited pepsin digestion of metabolically labeled medium or cell layers produced a pepsin-resistant fragment characteristic of pro-alpha 1(IV) chains. Removal of neuronal cell bodies from the cultures immediately before labeling did not reduce the amount of type IV procollagen detected in the medium. This indicated that Schwann cells, not neurons, were responsible for synthesis of type IV procollagen. We believe type IV procollagen is a major constituent of the Schwann-cell extracellular matrix based upon (a) its presence in a detergent-insoluble matrix preparation, (b) its presence in the cell layer of the cultures in a state in which it can be removed by brief treatment with bacterial collagenase or trypsin, and (c) positive immunofluorescence of Schwann cell-neuron cultures with anti-type-IV collagen antibodies. Secretion of type IV procollagen was substantially reduced when Schwann cells were maintained in the absence of neurons. This observation may account for the previously reported finding that Schwann cells assemble a basal lamina only when co-cultured with neurons (Bunge, M. B., A. K. Williams, and P. M. Wood, 1982, Dev. Biol., 92:449).

Potassium channel distribution in spinal root axons of dystrophic mice

We have used 4-aminopyridine (4AP), a potassium channel blocker, to assess the presence and distribution of potassium channels in the congenitally abnormally myelinated spinal root axons of dystrophic mice. 1 mM-4AP slightly depressed the amplitude but had no effect on the half-width of the monophasic action potential of normal A fibres, indicating the absence of a significant concentration of potassium channels at normal mouse nodes of Ranvier. By progressively increasing stimulus intensity it was possible to elicit three more or less discrete components of the compound action potential from dystrophic mouse spinal roots, presumably corresponding to myelinated fibres, large diameter bare axons and, in the case of dorsal roots, C fibres. The amplitude and duration of all three components were increased on exposure to 4AP, indicating the presence of potassium channels in all types of dystrophic mouse spinal root axons. Conduction in single fibres was studied using longitudinal current analysis. Both saltatory and continuous conduction were observed corresponding to the myelinated and bare portions of dystrophic mouse spinal root axons. Three types of 'nodal' membrane could be inferred from the membrane current recordings from myelinated dystrophic mouse axons: (1) pure sodium channel membrane, (2) membrane containing both sodium and potassium channels, and (3) membrane containing predominantly, if not exclusively, potassium channels. The large early outward currents at the latter two types of nodes suggested that these nodes were wider than normal. Recordings of continuous conduction indicated that potassium channels were also distributed irregularly along bare portions of the dystrophic mouse axons. These abnormalities of ion channel distribution are interpreted as reflecting failure of normal axon-Schwann cell communication in the dystrophic mouse spinal roots.

Quantitative studies of the abnormal axon-Schwann cell relationship in the peripheral motor and sensory nerves of the dystrophic mouse

The nature and extent of abnormal axon-Schwann cell relationships in peripheral portions of dystrophic motor and sensory nerves were quantitatively evaluated between 1 and 9 months of age using teased fibres and electron micrographs. The results show that in the dystrophic (dy/dy) common peroneal (CPN) and tibial nerves (TN), and less in the dy/dy sural nerve (SN): (1) the number of Schwann cell nuclei associated with myelinated axons is increased with respect to normal; (2) the average internodal length is correspondingly reduced; (3) the average dystrophic internode elongates roughly in parallel with the average normal internode, and with the dystrophic limb; the longitudinal growth of the dystrophic limb is normal; (4) the variation of internodal length is greater than normal; it does not increase with age; (5) the incidence of the nodes of Ranvier which are wider than the normal 3 micrometers limit does not increase with age; and (6) the number of myelinated axons is reduced in the dy/dy CPN and TN but not in the dy/dy SN; it shows no change with age. These data indicate that: (1) in the dy/dy peripheral nerves (PNS) the abnormal axon-Schwann cell relationships and the reduced number of myelinated axons have been established prior to 1 month of age, thereafter progressive degenerative processes do not appear to take place, and (2) the dy/dy sensory nerves are less affected than the motor ones.

Factors affecting schwann cell basal lamina formation in cultures of dorsal root ganglia from mice with muscular dystrophy

Pure populations of sensory neurons (N), Schwann cells (S) and fibroblasts (Fb) were established in culture from normal and dystrophic (dy) mice in order to investigate the cellular origin(s) of the peripheral nervous system abnormalities present in murine muscular dystrophy. These cell types were placed together in various combinations and their subsequent interactions were monitored with the light and electron microscope. The formation of the basal lamina (BL) which in normal tissue, completely surrounds the external aspect of the Schwann cell (when in contact with axons) was documented by morphometric analysis of electron micrographs. Defects in Schwann cell BL formation, observed throughout the PNS of the dy mouse in vivo, were used as a marker for the expression of the dystrophic abnormality in culture. Initially mature cultures of dy tissues containing only S and N (SN) without Fb were examined and found to contain an incomplete BL that surrounded only 82.8 +/- 12.2% of the externally directed plasmalemma of axon-related Schwann cells. The following recombination cultures were established: (1) normal S were placed on dystrophic N; (2) dystrophic S were placed on dystrophic N; (3) dystrophic S were placed on normal N; and (4) normal Fb were added to a dystrophic SN culture. After a 5-week period, the BL formed by normal S in direct contact with dystrophic N was thick and continuous (97.7 +/- 2.2 coverage). On the other hand, in culture situations (without Fb) containing dystrophic S in contact with either dystrophic or normal neurites, the BL coverage was considerably less (58.5 +/- 14.8% and 55.4 +/- 13.2%, respectively). The addition of normal Fb obtained from sciatic nerve explants to dystrophic SN cultures in time resulted in the formation of a morphologically complete BL (98.9 +/- 1.4% coverage). We conclude that neuronal signal(s) are adequate to induce complete BL formation by Schwann cells in the dystrophic tissue but that dystrophic Schwann cells are incapable of forming a complete BL. Furthermore, this deficiency of dy Schwann cells is apparently corrected by the presence of normal Fb by an unknown mechanism.

Expression of the trembler mouse mutation in organotypic cultures of dorsal root ganglia

Organotypic dorsal root ganglion (DRG) cultures were established from all the embryos of two trembler (Tr/+) female mice mated to normal (+/+) males to determine if the trembler mutation would be expressed in nerve tissue culture. Dorsal root ganglia from normal mice maintained in our culture system exhibit substantial myelination after 6 weeks of growth. This normal pattern was observed in approximately one half of the cultures in the present series. The remaining half of the explants had marked PNS myelin abnormalities readily detectable at the light microscopic level in living cultures; furthermore, the ultrastructural appearance of these Tr/+ cultures was similar to that of adult trembler sciatic nerve. An analysis of unmyelinated nerve fibers in Tr/+ cultures revealed that the number of neurites resident within each non-myelinating Tr/+ Schwann cell was significantly less than the number observed in +/+ cultures. There are distinctive PNS myelin abnormalities which: (1) develop in DRG cultures established from embryos at risk for the trembler mutation; (2) are highly reliable and readily detectable markers for the trembler genotype; and (3) are similar to the trembler PNS defects detectable in vivo.

Basal lamina deficiency in Schwann cells induced by beta-aminopropionitrile (BAPN) in rat dorsal root ganglion culture

In cultures of rat dorsal root ganglion (DRG) treated with beta-aminopropionitrile (BAPN), the following abnormalities were observed. Abnormalities in the size and the shape of collagen fibers; longer periodicity of collagen fibers in BAPN (70 +/- 5 nm) than in control (66 +/- 6 nm) (P less than 0.025); lack of basal lamina of both unmyelinated and myelinated Schwann cells; and abundant amorphous materials in the interstitial area between fibroblasts or peripheral cells and Schwann cells associated with neurites. Detachment of the explant from the collagen substratum also occurred and showed dose dependency. In contrast to the alterations of connective tissue, the neurites were well preserved in this experimental regimen.

Factors influencing the release of proteins by cultured Schwann cells

Cultured rat schwann cells grown in association with sensory neurons when labeled with [(3)H]leucinem, [(3)H]glucosamine, or [(35)S]methionine release labeled polypeptides into the culture medium. Analysis by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) of the culture medium reveals a reproducible pattern of more than 20 polypeptides with molecular weights ranging from 15,000 to more than 250,000. Five major polypeptides (apparent molecular weights 225,000, 210,000, 90,000, 66,000, 50,000, and 40,000) account for approximately 40 percent of the leucine or methionine radioactivity in medium polypeptide. Schwann cells grown in a serum-free defined medium, in which schwann cells do not relate normally to axons, release approximately four times less labeled medium polypeptides tha cultures grown in medium supplemented with serum and chick embryo extract. In addition, there is a qualitative difference in the pattern of medium polypeptides resolved by SDS-PAGE, so that a single polypeptide (mol wt 40,000) accounts for nearly all of the label in medium polypeptides. Switching of cultures grown in defined medium to supplemented medium for 2 d results in a fourfold increase in the amount of labeled polypeptides appearing in the culture medium, and a return to the normal pattern of medium polypeptides appearing in the culture medium, and a return to the normal pattern of medium polypeptides as resolved by SDS-PAGE. This change in the pattern of polypeptides release by schwann cells is accompanied by changes in the association between schwann cells and axons. An early step in the establishment of normal axon-schwann cell relations appears to be an inward migration of schwann cells into axonal bundles and spreading of schwann cells along neurites. These changes are evident within 48 h after medium shift. Our results thus suggest that the release of proteins by schwann cells may be important for the development of normal axonal ensheathment.

Ongoing block of Schwann cell differentiation and deployment in dystrophic mouse spinal roots

In the spinal roots of dystrophic mice, there are bundles of unensheathed axons and two populations of axon-associated cells: the typical Schwann cells of myelinated fibers and 'uncommitted' cells at the margin of the bundles. Because these 'uncommitted' cells continue to divide in adult animals but fail to ensheath the axons they appose, they can be labelled with tritiated thymidine. In the present experiments, we show that these cells may differentiate into typical Schwann cells of myelinated or unmyelinated fibers when spinal roots from [3H]thymidine-labelled dystrophic mice are grafted into the sciatic nerves of non-dystrophic animals. Thus, this study demonstrates that the 'uncommitted' cells of dystrophic spinal roots are undifferentiated Schwann cells whose differentiation in the intact spinal roots is continuously prevented by some unknown mechanism.