Isolation and characterization of axolemma-enriched fractions from discrete areas of bovine CNS

Oligodendrocyte progenitor cells proliferate and survive in an immature state following treatment with an axolemma-enriched fraction

The ability of an AEF (axolemma-enriched fraction) to influence the proliferation, survival and differentiation of OPC (oligodendrocyte progenitor cells) was evaluated. Following addition of AEF to cultured OPC, the AEF associated with the outer surface of OPC so that subsequent metabolic events were likely mediated by direct AEF-OPC contact. Addition of AEF to the cultured OPC resulted in a dose- and time-dependent increase in proliferation that was partially dependent on Akt (protein kinase B) and MAPK (mitogen-activated protein kinase) activation. The major mitogen in an AEF-SE (soluble 2.0 M NaCl extract of the AEF) was identified as aFGF (acidic fibroblast growth factor) and accounted for 50% of the mitogenicity. The remaining 50% of the mitogenicity had properties consistent with bFGF (basic fibroblast growth factor) but was not unequivocally identified. Under conditions that limit the survival of OPC in culture, AEF treatment prolonged the survival of the OPC. Antigenic and morphological examination of the AEF-treated OPC indicated that the AEF treatment helped the OPC survive in a more immature state. The potential downstream metabolic pathways potentially activated in OPC by AEF and the consequences of these activated pathways are discussed. The results of these studies are consistent with the view that direct contact of axons with OPC stimulates their proliferation and survival while preventing their differentiation.

Differential localization of neuregulin-1 type III in the central and peripheral nervous system

In the developing PNS, axonal neuregulin-1 (NRG1) type III is the key determinant for myelination. However, the specific role for NRG1 (III) in the CNS has not been established. To address this issue, isotype-specific antibodies were generated, characterized, and used for the immunofluorescent localization of NRG1 (III) in the developing and adult CNS of rat. In contrast to adult peripheral nerve, which showed robust axonal staining, no immunoreactivity was observed in CNS myelinated tracts during the period of active myelination or in the adult CNS. Surprisingly, NRG1 (III) was prominently expressed on dendrites and soma in both the developing and adult CNS. These findings were corroborated through the subcellular fractionation of adult rat brain combined with an immunoblotting analysis. The immunolocalization of NRG1 (III) suggests that it plays a novel role in the myelination fate of CNS axons possibly through undetermined roles in neuronal maturation, or dendritic development and activation.

Are there enhanced MBP autoantibodies in autism?

Autoantibodies to central nervous system antigens, such as myelin basic protein (MBP), may play a role in autism. We measured autoantibody titers to MBP in children with autism, both classic onset and regressive onset forms, controls (healthy age- and gender-matched) and individuals with Tourette syndrome via enzyme-linked immunosorbent assays. We found a significant difference in autoantibody titers to MBP, not accounted for by age or medication, between Tourette and classic autism (both significantly lower) when compared to regressive autism, but not when compared to controls. Autoantibody responses against MBP are unlikely to play a pathogenic role in autism.

Polyreactive myelin oligodendrocyte glycoprotein antibodies: Implications for systemic autoimmunity in progressive experimental autoimmune encephalomyelitis

Two myelin oligodendrocyte glycoprotein (MOG92-106) monoclonal antibodies (mAbs) were produced from an A.SW mouse with progressive experimental autoimmune encephalomyelitis. Polyreactivity/specificity of the mAbs was demonstrated by ELISA. Functionality and a potential role in pathogenesis of systemic autoimmunity were demonstrated in vitro in a lymphocytotoxicity assay and in vivo upon injection into naïve mice. Injection of MOG mAb producing hybridomas into naïve mice resulted in immunoglobulin deposition in kidneys and liver. This model will be useful in determining whether transitional forms between CNS (organ)-specific and systemic autoimmune diseases exist, and whether progressive multiple sclerosis has features of a systemic autoimmune disease.

In vitro studies of pigment epithelium-derived factor in human Schwann cells after treatment with axolemma-enriched fraction

Pigment epithelium-derived factor (PEDF) is a multifunctional protein with known anti-angiogenic and trophic properties, capable of promoting the survival and growth of Schwann cells (SC). Normal rat SCs and ganglioneuroma-derived human SCs secrete PEDF. The ability of normal SC to secrete a number of trophic factors is controlled by axonal contact. Normal human Schwann cells (HSC) and malignant peripheral nerve sheath tumors (MPNST) cell lines synthesize and secrete PEDF as determined by reverse transcription PCR analysis for PEDF mRNA, immunocytochemistry, and Western blot analysis for PEDF protein. Two MPNST cell lines secreted higher levels of PEDF than did HSC. A 90.3% decrease in PEDF mRNA and a 29.3% decrease in secreted PEDF were observed after treatment of HSC with axolemma-enriched fraction (AEF, 100 microg/ml), a neuronal membrane fraction of the axonal plasma membrane used with cultured SC to mimic axonal contact in vitro. PEDF levels remained unchanged, however, in MPNST-derived SC conditioned media under the same treatment paradigm. These results suggest that MPNST SC lose the ability to downregulate PEDF upon axonal contact, which is characteristic of HSC. The elevated PEDF levels expressed by MPNST cell lines may serve to promote their proliferation and survival.

Localization of neuregulin isoforms and erbB receptors in myelinating glial cells

Neuregulins (NRGs) are growth factors present in neurons and glial cells of the central and peripheral nervous systems and play a role in the survival, proliferation, and differentiation of these cells. We now report the localization of the two major isoforms of NRG (alpha and beta) and their receptors (erbB) in cultured Schwann cells and oligodendrocytes isolated from neonatal rat pups. Immunocytochemistry and Western blots for NRG and erbB receptors in defined subcellular fractions were utilized to assess cellular localization. Less differentiated oligodendrocytes contain both NRG isoforms in the cell bodies but not the processes, while only NRG-1beta was found in the nucleus. In contrast, more differentiated oligodendrocytes contained neither isoform in the nucleus while both isoforms were colocalized in the cytoplasm and cell processes. In Schwann cells, both NRG-1beta and NRG-1alpha were colocalized in the cytoplasm and processes. The Schwann cell nucleus had weak immunoreactivity for both NRG-1 isoforms, although NRG-1beta was predominant. ErbB2 and erbB3 receptors, which transduce the NRG-1 signal in Schwann cells, were found throughout the cytoplasm and in the processes and were also localized in the cell nucleus. The nuclear localization of NRG-1 isoforms and/or erbB receptors in both cell types was confirmed by Western blotting of nuclear and cytoplasmic extracts. Stimulation of Schwann cells with mitotic agents increased NRG-1beta expression in the nucleus and dramatically suppressed NRG-1alpha expression throughout the cell. The functional implications of this differential localization in myelinating cells are discussed.

The neuronal lipid membrane permeability was markedly increased by bupivacaine and mildly affected by lidocaine and ropivacaine

We investigated the local anesthetic action on ionic membrane conductance (membrane conductance) and selectivity in membranes formed with neuronal phospholipids in the absence and presence of cholesterol. In membranes without cholesterol, 1 mM bupivacaine and ropivacaine increased the membrane conductance approximately 4.5-fold; and 5 mM lidocaine, ropivacaine and bupivacaine increased the membrane conductance by 2.7-, 2.8- and 22.2-fold, respectively. In the presence of cholesterol, 5 mM ropivacaine had no effect, lidocaine decreased the membrane conductance by 2-fold, and bupivacaine increased the membrane conductance by 17-fold. Local anesthetics did not affect the ion selectivity in membranes without cholesterol, but they all decreased the Na(+) selectivity in membranes with cholesterol. Cholesterol reduced the lidocaine- and ropivacaine-induced membrane conductance increase by eliminating or reversing the Na(+) conductance increase and by lowering the Cl(-) conductance increase. In the absence of cholesterol, 5 mM bupivacaine increased both Na(+) conductance (38-fold) and Cl(-) conductance (19-fold), while in the presence of cholesterol it only increased Cl(-) conductance (26-fold). Of the local anesthetics studied, ropivacaine was the least membrane toxic while bupivacaine was the most toxic.

Widespread expression of netrin-1 by neurons and oligodendrocytes in the adult mammalian spinal cord

Netrins are a family of secreted proteins that function as chemotropic axon guidance cues during neural development. Here we demonstrate that netrin-1 continues to be expressed in the adult rat spinal cord at a level similar to that in the embryonic CNS. In contrast, netrin-3, which is also expressed in the embryonic spinal cord, was not detected in the adult. In situ hybridization analysis demonstrated that cells in the white matter and the gray matter of the adult spinal cord express netrin-1. Colocalization studies using the neuronal marker NeuN revealed that netrin-1 is expressed by multiple classes of spinal interneurons and motoneurons. Markers identifying glial cell types indicated that netrin-1 is expressed by most, if not all, oligodendrocytes but not by astrocytes. During neural development, netrin-1 has been proposed to function as a diffusible long-range cue for growing axons. We show that in the adult spinal cord the majority of netrin-1 protein is not freely soluble but is associated with membranes or the extracellular matrix. Fractionation of adult spinal cord white matter demonstrated that netrin-1 was absent from fractions enriched for compact myelin but was enriched in fractions containing periaxonal myelin and axolemma, indicating that netrin-1 protein may be localized to the periaxonal space. These findings suggest that in addition to its role as a long-range guidance cue for developing axons, netrin may have a short-range function associated with the cell surface that contributes to the maintenance of appropriate neuronal and axon-oligodendroglial interactions in the mature nervous system.

Phosphorylation of CREB in axon-induced Schwann cell proliferation

Axonal contact regulates Schwann cell (SC) proliferation during development. However, the intracellular signal transduction pathways involved in the axon-induced proliferation of SC have not been described. We have previously shown that SC proliferation induced by axolemma-enriched fractions (AEF) is accompanied by increased expression of cyclic AMP-responsive element binding protein, CREB. We now report the AEF and dorsal root ganglion neuritic-induced signal transduction pathway(s) which regulate the phosphorylation of CREB that correlate with the SC proliferative response. The phosphorylated form of CREB was significantly increased after 16 hr of axonal stimulation, continued to increase for 48 hr, and subsequently decreased as monitored by immunocytochemistry and Western blot analysis. Treatment with protein kinase A (PKA) inhibitor, H89, completely abolished both the CREB activation and SC proliferation. In contrast, treatment with protein kinase C (PKC) inhibitor (bisindolylmaleimide) inhibited AEF-induced SC proliferation, but did not immediately affect CREB phosphorylation. These data are consistent with the view that PKA and PKC pathways are essential for AEF-induced SC proliferation. Since PKC can influence SC proliferation without initially affecting CREB phosphorylation, PKC may regulate SC proliferation at pathways distal to the immediate CREB activation.

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.

Role of axonal components during myelination

Myelination is a multistep ordered process whereby Schwann cells in the peripheral nervous system (PNS) and oligodendrocytes in the central nervous system (CNS), produce and extend membranous processes that envelop axons. Mechanisms that regulate this complex process are not well understood. Advances in deciphering the regulatory components of myelination have been carried out primarily in the PNS and although the mechanisms for triggering and directing myelination are not known, it is well established that myelination does not occur in the absence of axons or axon/neuron-derived factors. This appears to be true both in PNS and CNS. Progress in understanding CNS myelinogenesis has been relatively slow because of the unavailability of a suitable culture system, which, in turn, is partly due to complexity in the cellular organization of the CNS. Though the myelin composition differs between PNS and CNS, the regulation of myelination seems to parallel rather than differ between these two systems. This article reviews the regulatory role of axonal components during myelination. The first half consists of an overview of in vitro and in vivo studies carried out in the nervous system. The second half discusses the use of a cerebellar slice culture system and generation of anti-axolemma monoclonal antibodies to investigate the role of axonal membrane components that participate in myelination. It also describes the characterization of an axonal protein involved in myelination.

Antibodies to the axolemma-enriched fraction in the cerebrospinal fluid and serum of patients with multiple sclerosis and other neurological diseases

Antibodies to an axolemma-enriched fraction (AEF) antigen have been detected in the cerebrospinal fluid (CSF) and serum of patients with Multiple Sclerosis (MS) using an enzyme-linked immunosorbent assay (ELISA). A marginal elevation (P < 0.08) of anti-AEF IgG was found in MS CSF when compared with OND samples. When CSF was diluted to a standardized IgG concentration, the anti-AEF IgG level in MS CSF was significantly elevated (P=0.007) when compared to OND CSF. MS serum was also found to contain a significantly higher level (P < 0.001) of anti-AEF IgG when compared to OND serum using the ELISA technique.

Effect of neu differentiation factor isoforms on neonatal oligodendrocyte function

Previous studies have suggested that neu differentiation factor (NDF), a member of the neuregulin (NRG) family of growth factors, may regulate the development of PNS and CNS glial cells. There is limited information concerning the potential role of NDF on the development of neonatal (immature) oligodendrocytes (OLG) into adult OLG. We now report the effect of the two major isoform families of NDF (NDF alpha and NDF beta) on the development of cultured rat neonatal OLG. Immunocytochemical and western blot analyses of neonatal OLG using anti-erb-B antibodies revealed that these immature OLG express all four members of NRG (erb-B) receptors. Treatment of neonatal OLG with varying concentrations of either NDF alpha or NDF beta did not have a mitogenic effect on cultured neonatal OLG. Pretreatment of immature OLG with either of the NDF isoforms also did not influence the subsequent mitogenicity of other known OLG mitogens. However, treatment of neonatal OLG with either isoform of NDF influenced the survival of these cells by protecting the cells from apoptosis. Additionally, treatment of neonatal OLG with either NDF alpha or NDF beta resulted in more extensive process formation compared to control, non-treated OLG.

Immunolocalization of cytoplasmic and myelin mcalpain in transfected Schwann cells: II. Effect of withdrawal of growth factors

We have examined the reversal of the regulatory effect of growth factors on calpain/calpastatin activity in transfected Schwann cells (tSc) after their subsequent withdrawal. Removal of nerve growth factor (NGF) or cyclic adenosine monophosphate (cAMP) from tSc resulted in a smaller loss of mu calpain (37%) and mcalpain (36.5 %) activity compared to treated cells from which the growth factors were not withdrawn. The mu calpain activity increased approximately 12% following withdrawal of acidic fibroblast growth factor (aFGF) and basic fibroblast growth factor (bFGF) at 24 hr, while the increased mcalpain activity was more than 30-40% compared with that of cells that were continuously treated. The activity of both isoforms returned to their normal levels (untreated) at 48-72 hr following withdrawal of various growth factors, including NGF, cAMP, aFGF, bFGF, platelet-derived growth factor aa (PDGFaa), and PDGFbb. The inhibitory activity of calpastatin was greater than control following withdrawal of NGF, cAMP, PDGFaa, or PDGFbb at 24 hr and this inhibitory activity was less with treatment by aFGF and bFGF. The control activity was restored at 48 hr following withdrawal of these factors. The intensity of the cytoplasmic calpain immunoreactivity was significantly decreased in the nuclear and non-nuclear regions of the cytoplasm, respectively, following withdrawal of cAMP at 144 hr. Removal of bFGF from the medium resulted in an increase of cytoplasmic calpain immunoreactivity in the nuclear regions and cytoplasm, while there was dramatic loss of myelin calpain immunoreactivity from both the nuclear region and cytoplasm. The changes in calpain activity and immunoreactivity in tSc following withdrawal of growth factors suggest that release of calpain from membrane to cytosol may be regulated by these factors.

Modulation of Schwann cell Po glycoprotein and galactocerebroside by the surface organization of axolemma

The nature of the axon signal for the induction of proliferation and differentiation of peripheral glial cells is still unknown. Besides the existence of interactions among surface molecules the cellular responses can also be regulated by physicochemical parameters of the membrane. We have previously reported that planar axolemma monolayers coated on glass cover-slips at different defined surface molecular packing affected the Schwann cell (SC) morphology and their proliferative response (Calderon et al.: J Neurosci Res 34:206-218, 1993). In this paper we report that relative to SC cultured on uncoated coverslips, the level of expression of both glycoprotein Po and galactocerebroside (GC) (as revealed by immunofluorescence) was increased 2-4 times in SC cultured on axolemma monolayers with either high or low molecular packing. However, the cellular distribution of these antigens was profoundly influenced by the molecular packing density of the axolemma monolayer. SC cultured on an axolemma monolayer at high molecular packing showed preferential expression of Po at the SC surface whereas GC was concentrated intracellularly. On the other hand, SC grown on an axolemma monolayer at low molecular density GC showed preferential expression at the cell surface whereas Po was concentrated intracellularly.

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. II. Redistribution after neural injury

The localization of fibroblast growth factor was examined in both immature (< 20 days in vitro) and mature (> 30 days in vitro) dorsal root ganglion neuron-glial cell co-cultures as a function of time after in vitro crush injury of the neurites. In the 20 day cultures, neuritic membrane vesicles were seen adhering to Schwann cells following neurite injury. Fibroblast growth factor was not detected on the surface of these membrane vesicles when they were associated with either the degenerating neurites or the surface of Schwann cells. However, the cytoplasm of the Schwann cells demonstrated fibroblast growth factor immunoreactivity at all times. In contrast, injury to neurites after 30 days in vitro resulted in demonstrable fibroblast growth factor immunoreactivity on the surfaces of the neuritic membrane vesicles both before and after their association with the Schwann cells. Furthermore, there was a change in the pattern of fibroblast growth factor immunoreactivity on the surface of Schwann cells after injury: initially the staining was patchy but with increasing time it became more uniform and more intense. A similar pattern of staining was noted on the surface of oligodendrocytes co-cultured with dorsal root ganglion neurons. However, astrocytes which were co-cultured with dorsal root ganglion neurons did not show any fibroblast growth factor immunoreactivity. Also, after injury at 30 days in vitro, the neuronal cell bodies began to express fibroblast growth factor immunoreactivity on their extracellular surfaces and the regenerating neurites exhibited fibroblast growth factor immunoreactive material on the surface of their plasma membranes. This redistribution of fibroblast growth factor via degenerating neuritic membrane vesicles to the plasma membrane of Schwann cells may be involved in neuronal signalling to glial cells after neuronal injury.

Increased P0 glycoprotein gene expression in primary and transfected rat Schwann cells after treatment with axolemma-enriched fraction

To elucidate the role of axonal plasma membrane factors in the differentiation of Schwann cells, we investigated the effect of an axolemma-enriched fraction (AEF) isolated from myelinated CNS tissue on the expression of P0 glycoprotein, the major glycoprotein in peripheral myelin, in primary rat Schwann cells (PSC) isolated from sciatic nerve, as well as in a transfected rat Schwann cell line (TSC). AEF increased PO-mRNA levels in PSC and TSC in a concentration-dependent manner, producing a maximal induction of nearly twofold after 48 hr of treatment. A similar induction of P0 mRNA was elicited in TSC by the cAMP-activating agents 8-bromo-cAMP and forskolin, which have been shown to induce myelin proteins in PSC. In addition to inducing P0 mRNA, AEF and forskolin also increased the amount of P0 protein in TSC, as indicated by increased P0-immunoreactive staining. However, in TSC, axolemma caused no increase in expression of CAT linked to a P0 promoter while forskolin caused a marked increase in the expression from the P0 promoter. These results suggest that AEF, in contrast to forskolin, does not regulate P0-mRNA expression at the level of transcriptional activity. These in vitro systems may be useful for the study of axolemmal factors that induce Schwann cell differentiation.

Isolation and characterization of axolemma-enriched fractions from rabbit and bovine peripheral nerve

Axolemma-enriched fractions were isolated from bovine spinal accessory nerves, bovine intradural dorsal roots, and rabbit sciatic nerve by differential centrifugation and separation on a linear 10-40% sucrose (w/w) gradient. The fractions were enriched 4 to 10 fold in acetylcholinesterase, a biochemical marker for axolemma. Axolemma-enriched fractions isolated from uniformly well-myelinated fibers (bovine spinal accessory nerve) contained lower CNPase activity and higher acetylcholinesterase activity than comparable fractions isolated from variably myelinated fibers (rabbit sciatic nerve and bovine intradural roots). Separation by polyacrylamide electrophoresis showed that the molecular weight distribution of all peripheral nerve axolemma-enriched fractions was similar and ranged from 20 to over 150 kilodaltons. All axolemma-enriched fractions appeared to contain a small but variable amount of myelin-specific proteins. Based on biochemical properties, peripheral nerves containing uniformly well-myelinated fibers yield an axolemma-enriched fraction which is least contaminated with myelin-related membranes.

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.

Identification of a 58-kDa antigen with increased immunoreactivity in the cerebella of multiple sclerosis patients

An increase in immunoreactivity associated with a 58-kDa antigen was found in a majority of MS cerebellar homogenates examined by Western blot analysis using antisera obtained by selective immunization of rabbits with autopsy cerebella. Two-dimensional immunoblotting demonstrated that the majority of the increased immunoreactivity observed in MS cerebella was associated with the highest apparent pI of three immunoreactive species at 58 kDa. Immuno-crossreaction with rat cerebellar homogenates demonstrated that the 58-kDa antigen was developmentally regulated, showing the greatest immunoreactivity at embryonic day 15. The 58-kDa cerebellar antigen may represent a membrane protein which is re-expressed as part of the onset of MS.

Mitotic potential of adult rat oligodendrocytes in culture

The proliferation of adult oligodendrocytes (OLGs) was examined in response to membrane bound and soluble mitogens. OLGs were isolated according to Vick, et al., J Neurosci Res 25:524-534, 1990, and co-cultured with dorsal root ganglia (DRGs). Less than 5% of the total cells incorporated a 48 hr pulse of 3H thymidine during the first 4 days of co-culture. From day 4 to day 6 there was a dramatic increase in proliferation which reached a plateau at 40% and gradually decreased to 25% from days 10 to 20 of co-culture. Axolemma-enriched fractions (AEF) were weak mitogens for OLGs (less than 5% proliferation after 7 days of stimulation), however, heparin extracts of AEF were five-fold more mitogenic than the AEF from which they were derived. Basic and acidic fibroblast growth factor were effective mitogens for the adult OLGs (labelling indices of 28% and 12%, respectively) provided that the cells were treated for 7 days with the growth factor and that the cells had been in culture for at least 14 days. Other soluble growth factors (IL-2 and PDGF) gave no mitotic response. We conclude that adult OLGs are mitotically responsive to mitogens provided that (1) the adult OLG has been cultured for sufficient time (14 days, acidic or basic fibroblast growth factor); (2) the axonal mitogen is allowed to interact with the OLG for a sufficient time (neuritic mitogen); and (3) the axonal mitogen is presented to the OLG in an activated form (AEF-heparin extract).

Isolation and characterization of periaxolemmal and axolemmal enriched membrane fractions from the rat central nervous system

In this report, we describe the fractionation of crude axolemmal fractions from rat lower brainstem into subfractions enriched in markers for either periaxolemmal myelin or axolemma. These subfractions were isolated on density gradients as bands layering on 0.8M and 1.0M sucrose. Both subfractions consisted of unilamellar vesicles. Relative to myelin purified from the same starting material, the 0.8M subfraction was enriched in MAG, CNPase, carbonic anhydrase and Na+, K+ ATPase but was extremely low in PLP and MBP. In addition, this fraction exhibited a protein profile distinct from myelin. The 1.0M fraction was also highly enriched in Na+, K+ ATPase and had an overall composition similar to the 0.8M subfraction. However, it differed from the 0.8M subfraction by being low in MAG, CNPase, and carbonic anhydrase, but enriched in voltage-dependent Na+ channel, axon-specific fodrin, and MAP-1B. Based on these characteristics we concluded that the 0.8M and 1.0M subfractions were highly enriched in periaxolemmal myelin and axolemmal membrane, respectively. Plasmolipin10 was unique with equally high levels in myelin and in the 0.8M and 1.0M subfractions. Both subfractions were enriched, relative to myelin, in the alpha subunit of the GTP binding protein, Go, and the alpha subunit common to all G proteins, GA/1. Electrophysiology with membrane subfractions fused to lipid bilayers showed that both membranes contained sets of K+ and Cl- channels, which based on channel sizes and open times, are largely distinct from one another.

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.

The phylogenic expression of plasmolipin in the vertebrate nervous system

Plasmolipin is a plasma membrane proteolipid is a major myelin membrane component (Cochary et al., 1990). In this study we report the phylogenic expression of plasmolipin in the vertebrate nervous system. Using Western blot analysis with polyclonal antibodies, we have analyzed membrane fractions, including myelin, from elasmobranchs, teleosts, amphibians, reptiles, birds and mammals. On the basis of immune detection, plasmolipin appears to be restricted to the mammalian nervous system. Comparison of the central and peripheral nervous systems of mammals showed only minor differences in the level of plasmolipin in these two regions. Within mammals, little quantitative differences were observed when rat, human and bovine membrane fractions were compared. The late evolutionary expression of plasmolipin which results in its restriction to mammals makes it unique among the (major) myelin proteins. The potential physiologic significance of these data are discussed.

Glass micro-fibers: a model system for study of early events in myelination

A system was developed to analyze early events in the process of myelination. Primary cultures of rat oligodendrocytes were maintained in the presence of glass micro-fibers which served as artificial axons. A culture chamber was constructed which allowed the close apposition of fibers and cells in a three-dimensional arrangement designed to resemble an in vivo environment. Cells cultured in the presence of glass micro-fibers coated with a glial cell matrix extract were induced to organize into clusters around the fibers. Examination of oligodendrocyte-fiber sandwiches by SEM revealed the presence of a number of cell contacts with the fibers. TEM images showed that, in most cases, fibers were surrounded by the cells and not multiply wrapped. Only occasionally was a loose wrapping of cell membrane observed around the fibers. Cells cultured in the presence of matrix-coated glass micro-fibers showed an increased production of sulfolipids that was at least partially dependent on the presence of the matrix coating. Coating of these "artificial axons" may aid in the identification of signal molecules produced by neurons which enable them to be myelinated.

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.