Disorganization of myelinogenesis in tissue culture by anti-CNS antiserum

Negative regulation of oligodendrocyte differentiation by galactosphingolipids

Galactocerebroside and sulfatide, major galactosphingolipid components of oligodendrocyte plasma membranes and myelin, are first expressed at a critical point, when progenitors cease to proliferate and commence terminal differentiation. We showed previously that an antibody to galactocerebroside/sulfatide arrested terminal differentiation, suggesting a role for these galactolipids in oligodendrocyte differentiation. We have now investigated the differentiation of oligodendrocytes (1) in response to other anti-galactolipid antibodies, showing that anti-sulfatide O4 but not anti-galactocerebroside O1 blocks terminal differentiation, perhaps by mimicking an endogenous ligand, and (2) in a transgenic mouse unable to synthesize these lipids because of mutation of the gene for ceramide galactosyltransferase, a key enzyme for galactosphingolipid synthesis. We find that galactosyltransferase mRNA expression begins at the late progenitor [pro-oligodendroblast (Pro-OL)] stage of the lineage and that the late progenitor marker pro-oligodendroblast antigen is not synthesized in the absence of galactosyltransferase. The principal outcome of the elimination of these galactolipids is a two- to threefold enhancement in the number of terminally differentiated oligodendrocytes both in culture and in vivo. Because the general pattern of differentiation and the level of progenitor proliferation and survival appear to be unaltered in the mutant cultures, we conclude that the increased number of oligodendrocytes is caused by an increased rate and probability of differentiation. In agreement with these two experimental approaches, we present a model in which galactosphingolipids (in particular galactocerebroside and/or sulfatide) act as sensors and/or transmitters of environmental information, interacting with endogenous ligands to function as negative regulators of oligodendrocyte differentiation, monitoring the timely progress of Pro-OLs into terminally differentiating, myelin-producing oligodendrocytes.

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.

Spontaneous and induced remyelination in multiple sclerosis and the Theiler's virus model of central nervous system demyelination

Remyelination in the central nervous system, originally thought to occur rarely, if ever, is now an established phenomena in multiple sclerosis patients. However, the extent of myelin repair is incomplete and limited. Experimental models of central nervous system demyelination provide an opportunity to study the cellular and molecular events involved in remyelination. These models may provide some clue to why remyelination in multiple sclerosis is incomplete as well as suggest potential methods to stimulate central nervous system repair. In this review we examine the morphological aspects of central nervous system remyelination and discuss both spontaneous and induced remyelination in multiple sclerosis and experimental models of central nervous system demyelination. We give special emphasis to the Theiler's virus model of central nervous system demyelination and its usefulness to identify therapeutic agents to promote remyelination. The role of immunoglobulins in promoting remyelination in both the Theiler's model system and in multiple sclerosis is discussed. Finally, we examine the potential physiological role of demyelination and remyelination and its relationship with clinical manifestations of central nervous system disease.

Regulation of gene expression in mature oligodendrocytes by the specialized myelin-like membrane environment: antibody perturbation in culture with the monoclonal antibody R-mAb

We have previously shown that the growth of oligodendrocyte progenitors in the presence of a monoclonal antibody (R-mAb) reacting with a cell surface component reversibly blocks their further differentiation at a specific, late progenitor stage of the lineage. This block is characterized by a nearly complete elimination of the onset of terminal differentiation at the level of RNA expression. In the present study, mature oligodendrocytes already expressing markers of terminal differentiation were exposed to R-mAb. This resulted in a retraction of cell processes and the formation of round, swollen cells, and a dose-dependent, antibody-specific partial reduction (30-50%) in the steady state levels of markers of terminal differentiation. Upon removing the perturbing antibody, all markers returned to control levels within 2 days. This inhibition was due to modulations of the levels of the specific mRNAs and proteins, not to cell loss. Total protein and levels of a marker of astrocytic differentiation were not affected by the treatment. Monoclonal antibody O1 did not cause the effects observed with R-mAb. We conclude that the response of terminally differentiating oligodendrocytes to the effects of R-mAb is different from that of oligodendrocyte late progenitors. Whereas the latter appears to operate through perturbation of the onset of gene expression (mRNA transcription and/or stability), the partial down-regulation of previously activated myelinogenic gene expression appears to be due to the loss of a normal, myelin-like, membrane environment needed for the stability of myelin mRNA and protein components.

Immune promotion of central nervous system remyelination

Remyelination by oligodendrocytes is the normal response to injury of the central nervous system following experimental demyelination by toxins and viruses in rodents. By contrast, in immune-mediated myelin disorders such as human MS, Theiler's virus-induced demyelination or EAE, remyelination is incomplete. We have considered two hypotheses to explain why myelin repair is incomplete in these disorders. Hypothesis I is that myelin repair is the normal consequence of primary myelin injury but there are immune factors which prevent its full expression. To test hypothesis I, we depleted T cells in Theiler's virus infected mice with cyclophosphamide or with monoclonal antibodies to CD4, CD8, or immune response gene products (Ia). Enhanced remyelination and proliferation of glial cells was observed in mice depleted of CD4+ or CD8+ T cells. Hypothesis II is that there are immune factors within some demyelinated lesions which, when present, promote new myelin synthesis. We envision these factors to be present in those lesions showing remyelination but absent in those lesions that remain demyelinated. To test hypothesis II, we generated polyclonal immunoglobulins directed against normal CNS antigens. Transfer of immunoglobulins from mice immunized repeatedly with spinal cord homogenate resulted in 4-5-fold enhancement of remyelination in Theiler's virus infected mice. We have also generated a series of monoclonal antibodies directed against normal autoantigens which also promote CNS remyelination. These experiments support the concept that full CNS remyelination is possible in human demyelinating diseases such as MS. Manipulation of the immune response either by inhibiting the function of T cells or by treatment with immunoglobulins (possibly normal autoantibodies) appears to promote remyelination. These experiments provide hope for patients with fixed neurological deficits for whom there are currently no available therapies.

Novel oligodendrocyte transmembrane signaling systems. Investigations utilizing antibodies as ligands

Antibodies are increasingly being used as tools to study the function of cell surface markers. Several types of responses may occur upon the selective binding of an antibody to an epitope on a receptor. Antibody binding may trigger signals that are normally transduced by endogenous ligands. Moreover, antibody binding may activate normal signals in a manner that disrupts a sequence of events that coordinates either differentiation, mitogenesis, or morphogenesis. Alternately, it is possible that binding elicits either a modified signal or no signal. This article focuses on the cascade of events that occur following specific antibody binding to myelin markers expressed by cultured murine oligodendrocytes. Binding of specific antibodies to the oligodendrocyte membrane surface markers myelin/oligodendrocyte glycoprotein (MOG), myelin/oligodendrocyte specific protein (MOSP), galactocerebroside (GalC), and sulfatide on cultured murine oligodendrocytes results in different effects with regard to phospholipid turnover, Ca2+ influxes, and antibody:marker distribution. The consequence of each antibody-elicited cascade of events appears to be the regulation of the cytoskeleton within the oligodendroglial membrane sheets. The antibody binding studies described in this article demonstrate that these myelin surface markers are capable of transducing signals. Since endogenous ligands for these myelin markers have yet to be identified, it is not known if these signals are normally transduced or are a modification of normally transduced signals.

Antibody-dependent cellular cytotoxicity in antimyelin antibody-induced oligodendrocyte damage in vitro

Treatment of dissociated murine brain cell cultures with an antibody recognizing galactocerebroside (GalC) led to degeneration of oligodendrocytes with loss of their cell processes. F(ab')2 fragments prepared from this antibody showed no effect. The anti-GalC antibody--but not its F(ab')2 fragments b2 was able to stimulate macrophages in these cultures as seen in a chemiluminescence assay. Therefore, antibodies bound to oligodendrocytes stimulated nearby macrophages by interacting with their Fc receptors. The oligodendroglial damage coincided with the release of toxic compounds by the stimulated macrophages, since treatment of the cultures with the anti-GalC antibody and a variety of other macrophage stimulating agents led to secretion of reactive oxygen species and--in some experiments--tumor necrosis factor, both known to be toxic for oligodendrocytes. These in vitro experiments show evidence that antibody-dependent cellular cytotoxicity may be an important mechanism of tissue destruction in inflammatory demyelinating diseases.

Myelination and glial ensheathment of Purkinje cells in cerebellar cultures are not inhibited by antibodies to the neural cell adhesion molecule, N-CAM

Mouse cerebellar cultures were exposed to anti-N-CAM antibodies throughout their in vitro development. Some cultures were stripped of myelinating oligodendrocytes and functionally competent astrocytes by treatment with cytosine arabinoside (Ara C), while others were left untreated and were potentially capable of forming myelin around axons and astrocytic sheaths around Purkinje cell somata and dendrites. As expected, the antibodies inhibited axonal fasciculation in the Ara C treated cultures. However, the same antibodies had no discernible effect on formation of myelin or astrocytic sheaths in cultures not treated with Ara C. N-CAM is expressed on the surfaces of neurons, oligodendroglia and astrocytes, and has been proposed as the signal molecule governing both kinds of neuron-glia interactions. The observations of the present study strongly suggest, however, that N-CAM does not have an indispensable role in such interactions.

Glycolipids and transmembrane signaling: antibodies to galactocerebroside cause an influx of calcium in oligodendrocytes

This is the first study to provide evidence that one function for the surface glycolipid galactocerebroside (GalC) is participation in the opening of Ca2+ channels in oligodendroglia in culture. This glycolipid is a unique differentiation marker for myelin-producing cells; antibodies to GalC have been shown to markedly alter oligodendroglial morphology via disruption of microtubules (Dyer, C. A., and J. A. Benjamins. 1988. J. Neurosci. 8:4307-4318). This study demonstrates that extracellular EGTA blocks anti-GalC-induced disassembly of microtubules in oligodendroglial membrane sheets, demonstrating that an influx of extracellular Ca2+ mediates the cytoskeletal changes. The Ca2+ influx was examined directly by loading oligodendroglia with the fluorescent dye Indo-1 in defined medium, and measuring changes in Ca2+ in individual cells with a laser cytometer. Upon addition of anti-GalC IgG, a marked sustained increase in intracellular Ca2+ occurred in 80% of the oligodendroglia observed. EGTA blocked the increase, indicating the increase is due to an influx of extracellular Ca2+, and not due to release from intracellular stores. The effect is specific, since Ca2+ levels remain normal in oligodendroglia treated with nonimmune IgG; astrocytes do not respond to the anti-GalC. The Ca2+ response in oligodendrocytes is dependent on concentration of antibody and GalC on the oligodendroglial membrane surface. The Ca2+ influx is not mediated by voltage-sensitive Ca2+ channels: it is not blocked by cadmium, and depolarization with K+ does not mimic the response. The kinetics of the response suggest that second messenger-mediated opening of Ca2+ channels is involved.

Stimulation of oligodendrocyte differentiation in culture by growth in the presence of a monoclonal antibody to sulfated glycolipid

Perturbation of myelinogenesis by monoclonal antibodies against galactolipids is being used to study the role of these lipids in oligodendrocyte differentiation. We report here a marked stimulatory effect on oligodendrocyte differentiation when mixed primary cultures initiated from 19-21 day fetal rat telencephala are grown in the presence of a monoclonal antibody against sulfogalactolipids. When such cultures were grown in the presence of the IgM antibody 04 [Sommer and Schachner, Dev Biol 83:311-327 1981], the oligodendrocytes formed aggregates connected by fasciculated processes. Immunofluorescence microscopy and biochemical analyses of treated cultures demonstrated 2-3 fold increases in the fraction of 04-positive cells expressing myelin basic protein, and in the levels of myelin basic protein RNA, myelin basic protein, 2',3'-cyclic nucleotide 3'-phosphohydrolase activity, and 35SO4 incorporation into sulfatide. Greater than 90% of the cells positive for myelin basic protein in treated cultures were in aggregates. The specific activities of oligodendrocyte markers were unaffected in control cultures grown with nonspecific myeloma IgM. Since there was no increase in the total number of 04-positive cells in treated cultures, the increases in the specific activities of the myelin protein markers appears to be due to an increase in the fraction of cells expressing these markers. Time course studies demonstrated that both the rate and extent of oligodendrocyte differentiation were enhanced in treated cultures. These data are discussed with regard to possible mechanisms of the stimulation, considering not only potential direct effects of the antibody on the cell physiology, but also possible indirect effects due to antibody-induced aggregation.

Production and characterization of high titer antibodies to galactocerebroside

High titer antibodies primarily of the IgG class were produced against galactocerebroside (GalC) by including keyhole limpet hemocyanin (KLH) and supplemental M. tuberculosis in the adjuvant mixture used for immunization of rabbits. Antibody titers were determined by an ELISA in which microtiter wells were coated with liposomes containing lecithin, cholesterol and GalC. The antibodies showed reactivity with GalC and psychosine, but not glucocerebroside, sulfatide, mixed gangliosides or asialo GM1. Specificity was further demonstrated by absorption of antibodies with GalC. Binding was inhibited by galactose, but only at high concentrations. Further, the antibodies did not bind to any brain proteins on immunoblots, indicating lack of reactivity with glycoproteins which might contain a terminal galactose. Antibodies to GalC are directed against different determinants than those reacting with peanut agglutinin since the lectin will not react with GalC, and the antibodies will not react with asialo GM1. The antibodies raised to GalC by this method show specific staining for oligodendroglia in culture. Peanut agglutinin binds intensely to process-bearing GalC+ oligodendroglia, but very poorly to the membrane sheets elaborated by oligodendroglia after longer times in culture. Other process-bearing GalC-, GFAP- cells were also stained with peanut agglutinin; these cells may represent glial precursors.

Monoclonal cell surface antibodies do not produce short-term effects on electrical properties of mouse oligodendrocytes in culture

Eleven monoclonal antibodies (O1-O11) directed against the surface of oligodendrocytes were applied individually or in combination during measurement of membrane potential, input resistance and K+-pump activity in explant cultures of mouse spinal cord. Antibody binding to oligodendrocytes was verified by indirect immunofluorescence. None of the antibodies affected the electrical properties studied. On the basis of these observations, it is possible to identify oligodendrocytes immunocytologically prior to electrophysiological characterization.

Increased proliferation of oligodendrocytes in the hypomyelinated mouse mutant-jimpy

Previous studies of the hypomyelinated mouse mutant jimpy have shown that the number of oligodendrocytes are reduced about 50%. To determine the cause of the cellular reduction, light and electron microscopy were combined with thymidine autoradiographic techniques. The number of neuroglial cells which incorporate radioactive thymidine in the mutants is increased severalfold over control values. Electron microscopic autoradiograms indicate the majority of the labeled cells are oligodendroblasts. However, the total number of glia in the white matter of jimpy and control animals is the same during development and even up to the time of the animal's death. The presence of mitotic cells suggest that the oligodendrocytes undergo division but the abundance of dying cells suggests that they die sometime afterwards. The results of the quantitative autoradiographic studies in combination with our other data strongly suggest that the immediate failure of these cells to form myelin sheaths is due to a shortened life span and/or continued cell proliferation.

A marker for oligodendrocytes and its relation to myelinogenesis: an immunocytochemical study with experimental allergic encephalomyelitis serum and C.N.S. cultures

To investigate a possible marker for oligodendrocytes and its relation to myelinogenesis, experimental allergic encephalomyelitis (EAE) serum has been used to study C.N.S. cultures from the time of explantation to maturity at 26 days in vitro (DIV). Cultures of foetal mouse spinal cord were exposed for 1 h to heated (complement-inactivated), rabbit anti-bovine white matter (WM-EAE) or control serum, fixed and processed by an immunoperoxidase technique for demonstrating bound immunoglobulin (Ig) by light and electron microscopy. From 5 to 26 DIV, cells morphologically identical to oligodendrocytes displayed binding of Ig to the plasmalemma of the cell body and its processes. At 5 DIV, immunoreactive oligodendrocytes had a large nucleus and nucleolus, prominent Golgi apparatus, and microtubules but no filaments. Occasionally a centriole was present, suggesting an early stage of differentiation. In myelinated cultures (from 11-12 DIV onwards), reaction product was present on the oligodendroglial outer plasmalemma apposed to myelin and along the outer loop. Sometimes it extended into the external mesaxon, outer layer of myelin, inner mesaxon and periaxonal space. No other structures were reactive, and oligodendroglia did not bind control Ig. These findings indicate that WM-EAE serum can be used as a marker for oligodendrocytes in cultures from 5 DIV onwards. The findings that oligodendrocytes acquire the antigen(s) prior to myelination and that the antigen(s) is localized on the plasmalemma of the inner and outer loops of actively myelinating oligodendroglial processes suggest that the antigen(s) may have a role in oligodendrocyte maturation and myelinogenesis. The antigen(s) involved is not yet established, but it is probably not myelin basic protein. This marker should prove useful in studies of C.N.S. development and the demyelinating diseases.