Further studies on the mitogenic response of cultured Schwann cells to rat CNS axolemma-enriched fractions

Quantitation of changes in P0 mRNA by polymerase chain reaction in primary cultured Schwann cells stimulated by axolemma-enriched fraction

A requirement for large numbers of primary culture cells has frequently restricted investigations of gene expression in glial cells. We have developed a non-radioactive method based on reverse transcription-polymerase chain reaction (RT-PCR) to accurately assess small changes in the expression of the myelin specific gene P0 in Schwann cells. Using axolemma-enriched fraction (AEF) as an inducing agent, we demonstrate that RT-PCR can be used to detect 4-8-fold increases in P0 mRNA levels occurring in a time and dose dependent manner, utilizing only 250000 cells per assay. Initial experiments used an in vitro transcribed RNA for P0 constructed with a 300 bp deletion for quantitation by competitive RT-PCR. Relative quantitation by co-amplification of the housekeeping gene glyceraldehyde-phosphate dehydrogenase was established and provided similar results. Product evaluation was enhanced 50-100-fold by the incorporation of primers labelled with biotin at the 5' end, allowing for the sensitive detection of PCR product by enhanced chemiluminescence and autoradiography. This technique provides sensitivity to detect and evaluate picogram amounts of DNA. Our results validate the assay for P0 gene expression and indicate that the technique should facilitate the study of multiple genes of interest in glial cell systems.

Glucocorticoids enhance the potency of Schwann cell mitogens

Previous studies have documented that cultured Schwann cells require serum-containing medium to respond maximally to mitogens. We now report that Schwann cells are able to proliferate to a mitogenic response in a serum-free defined medium termed oligodendrocyte defined media (ODM). Glucocorticoids are the essential component of ODM which allow Schwann cell proliferation in the serum-free medium. Charcoal treatment of the fetal calf serum decreases the mitogenic potency of the axolemma-enriched fraction (AEF) by 50%. The addition of 2 microM hydrocortisone to charcoal-treated fetal calf serum restores 75% of the lost mitogenicity. These observations are consistent with the view that glucocorticoids present in fetal calf serum are potent co-mitogens essential for AEF-induced Schwann cell proliferation. The synthetic glucocorticoid, dexamethasone, is a more potent co-mitogen than hydrocortisone, with a maximal effect at concentrations less than 10 nM. In contrast, other steroids including aldosterone, progesterone, testosterone, and 17 beta-estradiol have no effect on enhancing the mitogenic response of Schwann cells to the AEF. The glucocorticoid antagonists RU 486 and dehydroepiandrosterone (DHEA), but not the antiestrogenic compound tamoxifen, block AEF-induced Schwann cell proliferation. These results suggest that glucocorticoid-induced Schwann cell proliferation is mediated through a glucocorticoid receptor (GR) mechanism. We detected immunoreactivity to the GR in the cytoplasm, but not in the nuclei of Schwann cells grown in ODM lacking dexamethasone. The addition of 100 nM dexamethasone to these cultures resulted in immunoreactivity in the nucleus. This data suggests that glucocorticoids working through the GR are potent co-mitogens for Schwann cell proliferation.

Distribution of fibroblast growth factor in cultured dorsal root ganglion neurons and Schwann cells. I. Localization during maturation in vitro

The distribution of acidic and basic fibroblast growth factor in co-cultures of dorsal root ganglion neurons and Schwann cells was examined as a function of time in culture. After two days in vitro, the cytoplasm of the neuronal cell bodies demonstrated both acidic and basic FGF immunoreactivity, whereas the cytoplasm of the neurites was not immunoreactive. Schwann cells, in contrast, exhibited both acidic and basic fibroblast growth factor cytoplasmic immunoreactivity. After two days in culture, immunoreactivity was not detected on the plasma membrane surface of either the neurons or the Schwann cells. By 10 days in vitro, fibroblast growth factor immunoreactivity was observed in the cytoplasm of the most proximal portion of some, but not all, neurites but was unchanged in Schwann cells. At 20 days in vitro, immunoreactivity was still restricted to the intracellular compartment of both Schwann cells and neurons. Acidic fibroblast growth factor was primarily localized to the cytoplasm of Schwann cells, neuron cell bodies and along the entire length of the neurites. In contrast, basic fibroblast growth factor was predominantly localized to the nuclei of Schwann cells and small to medium size neurons. In many cases, the nucleolar region demonstrated the most intense basic fibroblast growth factor. The cytoplasm of the neurites was also immunoreactive for basic fibroblast growth factor. At 30 days in vitro the intracellular distribution of fibroblast growth factor immunoreactivity was similar to that observed at 20 days. However, both acidic and basic fibroblast growth factor were detected on the surface of the neurites. In contrast, no fibroblast growth factor immunoreactivity was detected at the Schwann cell surface at any time point examined. The distribution of fibroblast growth factor in Schwann cells cultured by themselves was similar to that of Schwann cells co-cultured with neurons after 20 days in vitro. Both Schwann cells and dorsal root ganglia exhibited increased fibroblast growth factor immunoreactivity with increased time in culture and an increased expression of basic fibroblast growth factor in the nucleus. Of particular interest was the appearance of fibroblast growth factor on the surface of neurites after 30 days in vitro where it could function to modulate neuron-glial cell interactions.

Surface behavior of axolemma monolayers: physico-chemical characterization and use as supported planar membranes for cultured Schwann cells

The axolemma membrane forms a stable and reproducible monomolecular layer at the air-aqueous interface. The major lipids and proteins are present in this monolayer in molar ratios similar to the original membrane. Acetylcholinesterase and Na-K-ATPase activities are preserved in the monolayer to levels of 64% and 25%, respectively. The total lipid fraction forms a homogeneously mixed phase. The presence of proteins in the monolayer introduces surface inhomogeneties. Among other features, this is revealed by the presence of two values of lateral pressure at which the monolayer shows partial or total collapse: a broad partial collapse at surface pressures between 13 to 30 mN/m and a sharp collapse point at 46 mN/m. The average molecular areas, the broad collapse point, and the variation of the surface potential per molecule suggest the relocation of protein components at surface pressures between 13 to 30 mN/m. The behavior is consistent with the extrusion and exposure of proteins toward the aqueous medium that depends on the lateral pressure. Schwann cells grown on coverslips coated with axolemma monolayers at 13 mN/m (beginning of the broad collapse) and 34 mN/m (above the broad collapse) recognize the difference in the surface organization of axolemma caused by the lateral pressure which affects their proliferation, morphology, and spatial pattern of organization. Our results show for the first time that response of Schwann cells depends on the intermolecular organization of the axolemma surface with which they interact. These results suggest that the local expression of putative surface molecules of axolemma that may mediate membrane recognition and the signalling of morphological and proliferative changes can be modulated by long range supramolecular properties.

Isolation and characterization of neonatal Schwann cells from cryopreserved rat sciatic nerves

Much of our knowledge about the development and maintenance of the peripheral nervous system has been learned through studying the interaction of neurons, or their isolated membranes, with Schwann cells (SC), in tissue culture. Numerous approaches have been employed to obtain an adequate quantity of SC, but all have been limited by either the uncertainty of obtaining a sufficient amount of starting material, the time and expertise required to isolate the SC, or by the limited number of SC that can be generated. We have developed a procedure to isolate SC from cryopreserved sciatic nerves. This procedure allows for sciatic nerves to be pooled until adequate numbers of nerves are obtained, yet still produces cells that retain the functional abilities of SC isolated from fresh nerves. Sciatic nerves were isolated from 2 day old rat pups, placed in either DME media and used fresh or placed in a freezing solution containing DME media (25%), DMSO (25%), fetal calf serum (50%), frozen at -70 degrees C and stored in liquid nitrogen. The frozen nerves were rapidly thawed to 37 degrees C and single cells were prepared from both fresh and frozen nerves using enzymatic and mechanical disruption as previously described (Brockes et al., Brain Res 165: 105-118, 1979). Comparable cell yields were obtained for SC isolated from both frozen and fresh nerves. Immunohistochemical staining of both fresh and frozen SC produced similar staining patterns with antibodies to GFAP, laminin, CNPase, S100, MBP, and P0 protein. Addition of axolemmal enriched membrane fractions to both the frozen and fresh SC gave a similar dose response curve of 3H-thymidine incorporation, with SC from frozen sciatic nerves responding even better than fresh sciatic nerves at higher doses (50 micrograms and 100 micrograms of protein/ml). As demonstrated by the cell yield, immunohistochemical staining and responses to axolemmal mitogens, this procedure produces SC from frozen sciatic nerves with similar characteristics to those isolated from fresh nerves. This procedure will allow the production and utilization of a large number of SC, which will be critical in further studies on the development and maintenance of the peripheral nervous system.

Isolated growth cones stimulate proliferation of cultured Schwann cells

A growth cone-enriched fraction was prepared from 3-4 day rat cerebra. Examination of the growth cone fraction by electron microscopy revealed numerous structures circular in appearance that contain a number of features common to neuronal growth cones in vivo. The isolated growth cones stimulated a dose-dependent incorporation of [3H]-thymidine into cultured Schwann cells in a manner similar to that observed with an axolemma-enriched fraction prepared from adult rat brainstem. The mitogenic activities of both the growth cone fraction and axolemma-enriched fraction were decreased 50% and 20%, respectively, by treatment with heparitinase I. The mitogen for Schwann cells present in the isolated growth cones appears to be similar to that found in axolemma-enriched fractions prepared from adult rats.

Schwann cell proliferation in vitro is under negative autocrine control

In healthy adult peripheral nerve, Schwann cells are believed to be generally quiescent. Similarly, cultures of isolated rat sciatic nerve Schwann cells hardly proliferate in serum-supplemented medium. The possibility that Schwann cells negatively regulate their own proliferation was supported by the demonstration that conditioned media from Schwann cell cultures inhibited the proliferation of mitogen-stimulated test cultures. The inhibition could be complete, was dose dependent, and was exhibited when the test Schwann cells were under the influence of different types of mitogens such as cholera toxin, laminin, and living neurons. The inhibition of proliferation was completely reversible and a rapid doubling of cell number resulted when treatment with conditioned medium was withdrawn from mitogen-stimulated Schwann cells. Conditioned medium from cholera toxin-stimulated and immortalized Schwann cell cultures contained less antiproliferative activity than that found in medium from quiescent Schwann cell cultures. However, media conditioned by two actively proliferating rat Schwannoma cell lines were rich sources of antiproliferative activity for Schwann cells. Unlike the mitogen-stimulated Schwann cells, whose proliferation could be inhibited completely, the immortalized and transformed Schwann cell types were nearly unresponsive to the antiproliferative activity. The antiproliferative activity in Schwann and Schwannoma cell conditioned media was submitted to gel filtration and SDS-PAGE. The activity exists in at least two distinct forms: (a) a high molecular weight complex with an apparent molecular mass greater than 1,000 kD, and (b) a lower molecular weight form having a molecular mass of 55 kD. The active 55-kD form could be derived from the high molecular weight form by gel filtration performed under dissociating conditions. The 55-kD form was further purified to electrophoretic homogeneity. These results suggest that Schwann cells produce an autocrine factor, which we designate as a "neural antiproliferative protein," which completely inhibits the in vitro proliferation of Schwann cells but not that of immortalized Schwann cells or Schwannoma lines.

Evidence for the colocalization of the axonal mitogen for Schwann cells and oligodendrocytes

Axons that normally will encounter either CNS or PNS glia have been shown to contain a powerful mitogen for both Schwann cells and oligodendrocytes. The normally nonmyelinated, nonglial ensheathed cerebellar granule cells have been shown to possess a proliferative signal for Schwann cells, suggesting that a glial mitogen is common to all axons. To determine if a glial mitogen capable of stimulating both Schwann cells and oligodendrocytes is colocalized on all types of axons we have (1) cocultured granule cells with oligodendrocytes, (2) incubated oligodendrocytes with granule cell membranes, and (3) evaluated the ability of heparin extracts of granule cell membranes, splenic nerve microsomes, and axolemma-enriched fractions isolated from rat and bovine CNS to stimulate mitosis of cultured oligodendrocytes. Neither the intact granule cells nor the granule cell membrane fraction stimulated cultured oligodendrocytes to divide. However, heparin extracts of the granule cell membranes were significantly mitogenic to the cultured oligodendrocytes. Heparin extracts of splenic nerve microsomes were more mitogenic than the comparable extract obtained from bovine CNS axolemma-enriched fractions. These results suggest that the neuronal mitogen for oligodendroglia is colocalized with the neuronal mitogen for Schwann cells.

Proliferation and differentiation of a transfected Schwann cell line is altered by an artificial basement membrane

Rapidly dividing transfected Schwann cells were grown on Matrigel, a reconstituted basement membrane gel. Matrigel decreased the proliferation of the cells by 75% when compared to sister cultures that were grown on an untreated plastic substrate. Some transfected cells plated onto a Matrigel substrate formed colonies similar to that observed when the cells were plated on a plastic substrate. Additionally, many cells on Matrigel assembled themselves into fascicles projecting away from the colonies. These fascicles were composed of transfected Schwann cells that had assumed a bipolar appearance reminiscent of quiescent secondary Schwann cells in culture. Transfected cells grown on Matrigel contained approximately 10-fold less glial fibrillary acidic protein when compared to sister cultures grown on an untreated plastic substrate. By indirect immunofluorescence laminin immunoreactivity appeared as globules within the cytoplasm of the cells which were cultured on a plastic substrate. However, cells that were grown on the Matrigel substrate appear to organize laminin in a linear array around themselves. These results demonstrate that the presence of an artificial basement membrane alters the morphology, rate of proliferation, and state of differentiation of a transfected Schwann cell line.

Ganglioside synthesis and transport in regenerating sensory neurons of the rat sciatic nerve

The sciatic nerves of rats were crushed with fine forceps and allowed to survive for 3 or 7 days, at which time the 5th lumbar dorsal root ganglion was injected with [3H]glucosamine. Animals were killed 18 h later and the nerves proximal and distal to the crush site were cut into 3 mm segments. Gangliosides were purified from these segments, and radioactivity was separately measured in gangliosides, neutral glycolipids and glycoproteins. For all 3 fractions, radioactivity was distributed similarly between the crush site and point of maximum axonal elongation. A second smaller peak of ganglioside radioactivity was seen to span a few segments immediately distal to the point of maximum axonal elongation. We propose two possible explanations for this: (1) it represents ganglioside synthesis by Schwann cells (from blood-borne [3H]glucosamine) as part of the mitogenic response of these cells to the reappearance of axons; or (2) recently synthesized, transported gangliosides are released from the growth cone and taken up by adjacent mitogenic Schwann cells.

Cerebellar granule cells contain a membrane mitogen for cultured Schwann cells

Proliferation of Schwann cells is one of the first events that occurs after contact with a growing axon. To further define the distribution and properties of this axonal mitogen, we have (a) cocultured cerebellar granule cells, which lack glial ensheathment in vivo with Schwann cells; and (b) exposed Schwann cell cultures to isolated granule cell membranes. Schwann cells cocultured with granule cells had a 30-fold increase in the labeling index over Schwann cells cultured alone, suggesting that the mitogen is located on the granule cell surface. Inhibition of granule cell proteoglycan synthesis caused a decrease in the granule cells' ability to stimulate Schwann cell proliferation. Membranes isolated from cerebellar granule cells when added to Schwann cell cultures caused a 45-fold stimulation in [3H]thymidine incorporation. The granule cell mitogenic signal was heat and trypsin sensitive and did not require lysosomal processing by Schwann cells to elicit its proliferative effect. The ability of granule cells and their isolated membranes to stimulate Schwann cell proliferation suggests that the mitogenic signal for Schwann cells is a ubiquitous factor present on all axons regardless of their ultimate state of glial ensheathment.

Schwann cell recruitment from intact nerve fibers

Two proximal branches of the rat facial nerve were transected and anastomosed end-to-end within a silicone tube, each of them being exposed to a massive invasion of ascending regenerating axons. The proximal nerves contained extremely large bundles of regenerated fibers, often associated with preexistent "parent fibers." The bundles showed many signs of rash and disordered cell proliferation and myelination. These included multiple Schwann cells surrounded by a common basement membrane, occurrence of different phases of myelination and even myelination of two axons by one Schwann cell. There was no evidence of mitogenic signals for fibrocytes. This model may be used for studying the mitogenic effect of axons on Schwann cells. It also suggests that so-called "groups of regenerating fibers" in neuropathy are caused by Schwann cell recruitment.

Mitogenic effect of axolemma-enriched fraction on cultured oligodendrocytes

An in vitro system has been devised to study the mitogenic effect of axolemma on cultured oligodendrocytes. Addition of axolemma-enriched fraction to cultured oligodendrocytes results in a dose-dependent mitotic response with an 11-fold stimulation at a membrane concentration of 200 micrograms/ml. The interaction between oligodendrocytes and axolemma is specific, as myelin-enriched fraction, astrocyte membrane, and red blood cell membrane showed little or no effect on the oligodendroglial proliferation under similar conditions. In addition, cultured astrocytes were tested with the same axolemma membrane, and no mitotic stimulation was observed. The mitogenicity of AEF membrane on cultured oligodendrocytes is sensitive to heat and trypsin treatment, suggesting that the axolemma mitogen may be a protein.

1-oleoyl-2-acetylglycerol and A23187 potentiate axolemma- and myelin-induced Schwann cell proliferation

The effects of 1-oleoyl-2-acetylglycerol (OAG) and the calcium ionophore A23187 on the proliferation of Schwann cells stimulated with either a myelin-enriched membrane fraction (MEF) or an axolemma-enriched membrane fraction (AEF) have been examined. Using incorporation of [3H]thymidine as an index of proliferation, 16% of the cells became labeled after incubation with MEF (20 micrograms protein/ml) and AEF (40 micrograms protein/ml) for 72 h. Only 0.5% of the cells became labeled in cultures which were not exposed to the membrane fractions. Addition of OAG (10-500 microM) or A23187 (1.9-190 nM) in the absence of the membrane mitogens had no effect on the proliferative response of quiescent cultures of Schwann cells. When added simultaneously, however, OAG and A23187 were able to induce proliferation of the cells, although the response was only 30% of the response achieved with maximal doses of either AEF or MEF. Both OAG and A23187 were able to potentiate the mitogenicity of AEF or MEF, but only when AEF and MEF were added at submaximal concentrations. When Schwann cells were prelabeled with [3H]glycerol and then stimulated to proliferate with AEF or MEF, the amount of [3H]diacylglycerol was increased two- to threefold above that in control cultures for time periods up to 1 h. These results suggest that the proliferation of Schwann cells induced by either AEF or MEF is partially mediated through the combined effects of diacylglycerol and an increase in intracellular calcium.

The neuronal cell-surface molecule mitogenic for Schwann cells is a heparin-binding protein

The cell surface of embryonic peripheral neurons provides a mitogenic stimulus for Schwann cells. We report (i) the solubilization of this mitogenic activity from rat dorsal root ganglion neurons grown in tissue culture and (ii) the solubilization and partial purification of mitogenic activity from neonatal rat brains. Extracted mitogenic activity is peripheral rather than intrinsic to the membrane, stable after extraction, and active as a mitogen in the absence of serum (the most stringent criterion defining the neuronal mitogen). We have previously provided evidence suggesting that a neuronal cell-surface heparan sulfate proteoglycan is required for expression of the neurons' mitogenic activity. We now show that mitogenic activity can be extracted from the membrane dissociated from proteoglycan as assayed by its ability to bind to immobilized heparin. After dissociation, low concentrations of heparin (1 micrograms/ml) inhibit the ability of the mitogen to stimulate Schwann cell division. Basic fibroblast growth factor (FGF) is weakly mitogenic for Schwann cells, but it is not present in mitogenic brain extracts (based on immunoblotting). Immunodepletion experiments with specific antibodies to FGF indicate that the mitogenic activity extracted from neurons is not a form of this heparin-binding mitogen. Acidic FGF is not mitogenic for Schwann cells and is not present in mitogenic brain extracts. We suggest that these and previous data indicate the neurite mitogen is a proteoglycan-growth factor complex that limits mitogenic activity to the axonal surface, protects mitogen against inactivation by other proteoglycans, and provides for effective presentation of mitogen to the Schwann cell.

Schwann cell proliferation is accompanied by enhanced inositol phospholipid metabolism

Inositol phospholipid metabolism during mitogen-induced Schwann cell proliferation has been examined. Addition of axolemma- and myelin-enriched membrane fractions (AXL and MYE, respectively) to cultured Schwann cells stimulated 32P incorporation into phosphatidylinositol 4-monophosphate [PtdIns(4)P] and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2]. During the first 5 min of incubation with the mitogens, the amount of 32P incorporated into PtdIns(4)P and PtdIns(4,5)P2 was four- to fivefold above control values. The phosphorylation of the inositol phospholipids was dependent on the concentration of membrane mitogens and was maximal within 1 h. Schwann cells that were prelabeled with [3H]glycerol and then stimulated with AXL and MYE displayed a 30-70% increase in the amounts of [3H]PtdIns(4)P and [3H]PtdIns(4,5)P2 and a 60-80% increase in the amount of [3H]phosphatidic acid. A concomitant 20% decrease in the content of [3H]PtdIns was observed after stimulation. These results suggest that the increased metabolism of PtdIns, PtdIns(4)P, and PtdIns(4,5)P2 may be one of the initial molecular events in the transduction of the mitogenic signal across the Schwann cell plasma membrane.

Macrophage-mediated myelin-related mitogenic factor for cultured Schwann cells

Conditioned medium from cultured peritoneal macrophages that have phagocytosed a myelin membrane fraction is mitogenic for cultured Schwann cells. Production of the mitogenic supernatant was time- and dose-dependent with a maximal Schwann cell-proliferative response from supernatants after 48-hr incubation of cultured macrophages with myelin-enriched fraction (200 micrograms of protein per ml). The response was specific for myelin membrane: supernatants derived from macrophages incubated with axolemma, liver microsomes, polystyrene beads, or lipopolysaccharide were not mitogenic. Lysosomal processing of the myelin membrane was necessary for the production of the mitogenic factor, which was shown to be heat labile and trypsin sensitive. There was no species specificity because myelin membranes isolated from the central and peripheral nervous systems of rat, bovine, and human were equally potent in eliciting mitogenic supernatant. However, supernatants derived from central nervous system myelin membranes were two to three times more mitogenic than those obtained from peripheral nervous system fractions of the same species. Previous observations that myelin is mitogenic for cultured Schwann cells may, in part, involve the intermediate processing of myelin by macrophages that are present in Schwann cell cultures. These results suggest that macrophages play a crucial role in Schwann cell proliferation during Wallerian degeneration.

Effect of lithium on Schwann cell proliferation stimulated by axolemma- and myelin-enriched fractions

Cultured Schwann cells stimulated with an axolemma- or myelin-enriched fraction incorporated 2.5 to three times as much [3H]thymidine when 10 mM lithium was added to the extracellular medium. The ability of lithium to enhance the mitogenic activity of either fraction was dose dependent. This result was not due to an increase in osmolarity, because addition of 10 mM NaCl had no effect on the amount of labeled thymidine accumulated by Schwann cells treated with either membrane fraction. In an earlier study, the effect of either membrane fraction could be potentiated with active phorbol esters. Lithium significantly enhanced the incorporation of [3H]thymidine into Schwann cells treated with a myelin-enriched fraction and phorbol esters. In contrast, lithium slightly increased the amount of labeled thymidine incorporated into Schwann cells stimulated with an axolemma-enriched fraction and phorbol esters. The mitogenic activity of either membrane fraction was impaired when the calcium channel blockers Mn2+ and nifedipine were added. Addition of lithium stimulated an increase in the amount of [3H]thymidine accumulated by Schwann cells treated with either the axolemma- or myelin-enriched fraction in the presence of either Mn2+ or nifedipine.

Developmental changes in myelin-induced proliferation of cultured Schwann cells

Schwann cell proliferation induced by a myelin-enriched fraction was examined in vitro. Although nearly all the Schwann cells contained material that was recognized by antisera to myelin basic protein after 24 h, only 1% of the cells were synthesizing DNA. 72 h after the addition of the mitogen a maximum of 10% of the cells incorporated [3H]thymidine. If the cultures were treated with the myelin-enriched fraction for 24 h and then washed, the number of proliferating Schwann cells decreased by 75% when compared with those cells that were incubated with the mitogen continuously. When Schwann cells were labeled with [14C]thymidine followed by a pulse of [3H]thymidine 24 h later, every Schwann cell labeled with [3H]thymidine was also labeled with [14C]thymidine. Although almost every Schwann cell can metabolize the myelin membranes within 24 h of exposure, a small population of cell initially utilizes the myelin as a mitogen, and this population continues to divide only if myelin is present in the extracellular media. The percentage of the Schwann cells that initially recognize the myelin-enriched fraction as a mitogen is dependent upon the age of the animal from which the cells were prepared.

Effects of delayed myelination by oligodendrocytes and Schwann cells on the macromolecular structure of axonal membrane in rat spinal cord

The macromolecular structure of axonal membrane from dorsal funiculi of control and irradiated spinal cord of 45-day-old rats was examined with freeze-fracture electron microscopy. In control spinal cords, virtually all myelination is mediated by oligodendrocytes, and the internodal axonal membrane of these fibres displays highly asymmetrical partitioning of intramembranous particles (IMPs). The internodal P-face particle density is approximately 2350IMPs per micron 2, whereas the E-face IMP density is approximately 150 per micron 2. In control dorsal spinal roots, myelination is mediated by Schwann cells, and the ultrastructure of the internodal axolemma of the myelinated fibres is similar to that displayed by myelinated fibres of dorsal funiculi. On the internodal P-face of Schwann cell-myelinated fibres the IMP density is approximately 2350 per micron 2, whereas on the E-face the density is approximately 175 per micron 2. Irradiation of the lumbosacral spinal cord at 3 days of age results in a glial cell-deficient region within the spinal cord such that myelination in irradiated dorsal funiculi is delayed and subsequent myelination is mediated by both oligodendrocytes and Schwann cells. By 45 days of age, dorsal funiculi of irradiated spinal cords are well populated with fibres myelinated by oligodendrocytes and Schwann cells. However, fibres myelinated by oligodendrocytes display very thin myelin sheaths whereas Schwann cell-myelinated fibres exhibit myelin sheaths with normal thicknesses. Internodal membrane of fibres myelinated by Schwann cells and oligodendrocytes exhibit similar macromolecular structure, with approximately 2400 IMPs per micron 2 on P-faces and approximately 150 IMPs per micron 2 on E-faces. Occasional large (greater than 1.5 micron diameter) axons without glial-Schwann cell ensheathment are observed. These axons display a high density of P-face particles (approximately 2000 per micron 2) and a moderate density (approximately 350 per micron 2) of E-face IMPs on their fracture faces. These results demonstrate that CNS fibers exhibit similar axonal membrane ultrastructure irrespective of whether they are myelinated by Schwann cells or oligodendrocytes, or whether myelination is delayed. Moreover, when myelination does not occur, the axolemmal E-face IMP density, which may be related to the density of voltage-sensitive sodium channels, is not reduced.

Evidence that axons are mitogenic for oligodendrocytes isolated from adult animals

Recovery from demyelinating events in the adult central nervous system (CNS) may depend in some situations on the generation, by cell division, of new oligodendrocytes (ODCs). Adult ODCs, identified by morphological criteria, have been shown to divide in vivo in response to induced demyelination or to physical trauma, although the factors that elicit this response are unknown. Adult ODCs do not proliferate in vitro under normal conditions (ref. 4 and P.M.W., unpublished observations). Encouraged by observations that axons provide a mitogenic stimulus for Schwann cells and support the proliferation of embryonic ODC progenitors, we have studied the response of adult ODCs, identified by immunocytological criteria, to axons of dorsal root ganglion neurones in culture. We have now found that adult ODCs proliferate in vitro when neurones are present but not when neurones are absent, suggesting that neurons can elicit ODC division under the conditions prevailing in our culture system.

A neuronal cell surface heparan sulfate proteoglycan is required for dorsal root ganglion neuron stimulation of Schwann cell proliferation

Axons of dorsal root ganglion neurons express on their surfaces one or more proteins which are mitogenic for Schwann cells (Salzer, J., R. P. Bunge, and L. Glaser, 1980, J. Cell Biol., 84:767-778). Incubation of co-cultures of dorsal root ganglion neurons and Schwann cells with 4-methylumbelliferyl-beta-D-xyloside, an inhibitor of proteoglycan biosynthesis, decreases the mitogenic response of the Schwann cell by over 95%. The effect of the beta-D-xyloside has been localized to the neurons; pretreatment of neurons but not of Schwann cells with the inhibitor causes a marked reduction of the mitogenic response. In addition, Schwann cells treated with beta-D-xyloside are still mitogenically responsive to soluble Schwann cell mitogens (cholera toxin and glial growth factor). Neurons treated with heparitinase and membrane vesicles prepared from heparitinase-treated neurons show diminished mitogenicity for Schwann cells, while other proteoglycan lyases have no effect. We conclude that a cell surface heparan sulfate proteoglycan is a component of the Schwann cell mitogen present on the surface of dorsal root ganglion neurons.

Differential proliferative responses of cultured Schwann cells to axolemma and myelin-enriched fractions. II. Morphological studies

Axolemma-enriched and myelin-enriched fractions were prepared from bovine CNS white matter and conjugated to fluorescein isothiocyanate (FITC). Both unlabelled and FITC-labelled axolemma and myelin were mitogenic for cultured rat Schwann cells. Treatment of Schwann cells with the FITC-labelled mitogens for up to 24 h resulted in two distinct morphological appearances. FITC-myelin-treated cells were filled with numerous round, fluorescent-labelled intracellular vesicles, while FITC-axolemma-treated cells appeared to be coated with a patchy, ill-defined fluorescence, primarily concentrated around the cell body but extending onto the cell processes. These observations were corroborated under phase microscopy. Electron microscopy revealed multiple, membrane-bound, membrane-containing phagosomes within myelin-treated cells and to a far lesser extent in axolemma-treated cells. The effect on the expression of the myelin-mediated and axolemma-mediated mitogenic signal when Schwann cells were treated with the lysosomal inhibitors, ammonium chloride and chloroquine, was evaluated. The mitogenicity of myelin was reduced 70-80% by these agents whereas the mitogenicity of axolemma was not significantly altered under these conditions. These results suggest that axolemma and myelin stimulate the proliferation of cultured Schwann cells by different mechanisms. Myelin requires endocytosis and lysosomal processing for expression of its mitogenic signal; in contrast, the mitogenicity of axolemma may be transduced at the Schwann cell surface.

Differential expression of 2':3'-cyclic nucleotide 3'-phosphodiesterase in cultured central, peripheral, and extraneural cells

The relative levels of the central nervous system myelin marker enzyme 2':3'-cyclic nucleotide 3'-phosphodiesterase (EC 3.1.4.37, CNPase) were determined in neuroblastoma, astrocyte, oligodendrocyte and Schwann cell cultures and in freshly isolated human lymphocytes and platelets. The highest specific activities were associated with the cells that elaborate myelin membrane in the central and peripheral nervous system, oligodendrocytes and Schwann cells, respectively. Antiserum to bovine CNPase recognized both CNP1 and CNP2 in CNS myelin and human oligodendroglioma. In addition, a 53,000 dalton protein was evident on autoradiographs of immunoblotted PNS myelin and human oligodendroglioma proteins. Cultured rat oligodendrocyte, C6 and mouse NA neuroblastoma CNPase appear to share common determinants with the corresponding normal rat CNS enzyme.

Schwann cell and oligodendrocyte remyelination in lysolecithin-induced lesions in irradiated rat spinal cord

Localised irradiation of adult rat spinal cord was achieved by implanting for 2 weeks a 192Ir pin alongside vertebral segments in the thoraco-lumbar region of the spinal column. Following removal of the implant, lysolecithin (LPC) was injected directly into the dorsal columns in order to induce demyelination in the most intensely irradiated segments of spinal cord. Eight weeks after LPC injection, remyelination was much less extensive in dorsal columns which absorbed more than 40Gy than in LPC lesions in less intensely irradiated spinal cords or in unirradiated animals. No oligodendrocytes, few astrocyte processes and little myelin debris lay among the demyelinated axons. However, capillary vessels were surrounded by astroglial end-feet so that the glial-limiting membrane remained intact in the demyelinated regions. There were some oligodendrocyte remyelinated fibres around the edges of the demyelinated zones but none among the naked axons. Schwann cells, which probably migrated into the lesions from the proximal segments of the dorsal roots, provided some fibres with myelin sheaths. These remyelinated fibres abutted demyelinated axons without an intervening glial limiting membrane or astrocyte process. Oligodendrocytes may fail to migrate into the demyelinated regions because of the scarcity of astrocyte processes. A possible explanation for the limited Schwann cell remyelination may be that the presence of astroglial end-feet around capillaries deprived Schwann cells of ready access to the demyelinated regions.

Differential proliferative responses of cultured Schwann cells to axolemma- and myelin-enriched fractions. I. Biochemical studies

Cultured rat Schwann cells were treated for 72 h with axolemma- and myelin-enriched fractions prepared from rat brainstem. [3H]Thymidine was added to the cultures 48 h before the termination of the experiment. Although, both fractions produced a dose-dependent uptake of label into Schwann cells, the shape of the dose response curves and rates at which [3H]thymidine was incorporated were different. The axolemma-enriched fraction produced a sigmoid dose response curve with a Hill coefficient of 2.05. The dose response curve for myelin rose sharply and saturated at a level that was approximately 50% of the maximal response observed with axolemma. Schwann cells that had been treated with axolemma exhibited little change in the rate of [3H]thymidine incorporation from 36-72 h after the addition of the membranes. In contrast, Schwann cells accumulated label three times faster during the 48-72-h period following the addition of myelin to the cultures when compared with the rate during the preceding 12-h interval. Furthermore, the mitogenic activity of the myelin-enriched fraction was decreased by the addition of ammonium chloride, a lysosomal inhibitor, whereas the activity of the axolemmal fraction was not impaired.

Cyclic AMP and calcium as potential mediators of stimulation of cultured Schwann cell proliferation by axolemma-enriched and myelin-enriched membrane fractions

The roles of cyclic AMP and calcium in the transduction of the mitogenic effects of central nervous system axolemma and myelin-enriched fractions on cultured Schwann cells were examined. Cyclic AMP levels were not elevated in axolemma or myelin-stimulated Schwann cells, but were increased when stimulated with cholera toxin, an adenyl cyclase activator. The mitogenicity of axolemma and myelin was markedly reduced by 2.5 mM citrate, a calcium chelator, and 10 uM trifluoroperazine, an inhibitor of calmodulin. Treatment of Schwann cells with several tumor-promoting phorbol esters caused significant enhancement of the mitogenicity of the axolemma and myelin preparations. These data suggest that the mitogenic effects of axolemma and myelin are not mediated by cyclic AMP, but may be mediated by calcium ions.

PC12 cells as a source of neurite-derived cell surface mitogen, which stimulates Schwann cell division

Primary cultures of rat dorsal root ganglion Schwann cells were used to assay the efficacy of PC12 cells in stimulating Schwann cell proliferation. Co-cultures of PC12 cells and Schwann cells assayed by [3H]thymidine labeling followed by autoradiography showed proliferation of Schwann cells only where contact occurred between PC12 neurites and Schwann cells. Membranes derived from PC12 cells were shown to have many characteristics similar to membranes derived from sensory neurons; both could mimic whole cells in stimulating Schwann cell division; both were inactivated by mild heat treatment and by trypsinization, and both elevated intracellular cyclic AMP concentrations in Schwann cells 16 h after addition of membranes. We conclude that PC12 cells will be a valuable source for the isolation of the neuronal cell surface component which controls proliferation of Schwann cells during development of the peripheral nervous system.