Myelin-specific domain on the plasmalemma of oligodendroglia: differential expression in the rat and hypomyelinating mouse mutants jimpy and quaking

The microtubule-associated protein tumor overexpressed gene/cytoskeleton-associated protein 5 is necessary for myelin basic protein expression in oligodendrocytes

Tumor overexpressed gene (TOG) protein, encoded by cytoskeleton-associated protein CKAP5, is a microtubule-associated protein that binds to heterogeneous nuclear ribonucleoprotein (hnRNP) A2. hnRNP A2 is an RNA trafficking factor that associates with myelin basic protein (MBP) mRNA. In oligodendrocytes, TOG, hnRNP A2, and MBP mRNA colocalize in granules that assemble in the perikaryon and are transported to the peripheral network of processes that extends from it. MBP accumulates preferentially in the membrane of the medial and distal portions of these cellular processes. MBP expression was reduced when TOG level was lowered by short-hairpin (sh) RNA. The reduction in TOG did not affect overall cell morphology or the assembly, transport, localization, or number of MBP mRNA-containing granules. Reduced levels of TOG did not affect another oligodendrocyte-specific component, myelin oligodendrocyte glycoprotein, which is expressed at the same time as MBP but translated from mRNA localized in the cell body. Expression in a neural cell line of a green fluorescent protein (GFP)-MBP fusion protein derived from a construct containing GFP and the full-length cDNA for the rat 14 kDa MBP was reduced when TOG level was lowered by shRNA treatment. Expression of GFP, derived from GFP mRNA containing the hnRNP A2 binding element of MBP mRNA, was similarly reduced in cells with low TOG levels. These data indicate that TOG is necessary for efficient translation of MBP mRNA and suggest that this role is mediated by its interaction with hnRNP A2.

Transport of proteolipid protein to the plasma membrane does not depend on glycosphingolipid cotransport in oligodendrocyte cultures

The possibility that transport of proteolipid protein (PLP) from its site of synthesis to the plasma membrane is dependent on cotransport with (sulfo)galacto-cerebrosides was investigated in primary cultured oligodendrocytes and Chinese hamster ovary (CHO) cells expressing PLP. Sulfation was inhibited by growing oligodendrocytes in the presence of a competitive inhibitor of this process, sodium chlorate. Under these circumstances, sulfatide synthesis was inhibited by 85%. Nevertheless, PLP was still delivered to the plasma membrane in quantitative amounts. Furthermore, when PLP was expressed in CHO cells, which normally synthesize very low amounts of galactosyl ceramide (GalCer) and no sulfatide, PLP was transported to the plasma membrane. Moreover, in CHO cells coexpressing PLP and ceramide galactosyl transferase, PLP cell surface labeling was unaltered. Noting that it has been demonstrated that proteins destined for the apical surface of epithelial cells colocalize with glycolipid-enriched microdomains, we isolated detergent-insoluble membrane complexes from cultured oligodendrocytes. We found, however, that most of the PLP is present in the detergent-soluble fraction and, furthermore, that PLP could not be chased into or out of the insoluble fraction. Taken together, these data make it very likely that in oligodendrocytes PLP transport takes place irrespective of the presence of glycosphingolipids GalCer and sulfatide.

In vitro death of jimpy oligodendrocytes: correlation with onset of DM-20/PLP expression and resistance to oligodendrogliotrophic factors

Severe hypomyelination in the jimpy (jp) mouse mutation results from premature death of most oligodendrocytes (OCs). We have applied an immunopanning technique to successfully purify oligodendroblasts (OBs) directly from neonatal jp brainstem in order to determine if their death during differentiation into OCs is preventable in culture by diffusible oligodendrogliotrophic factors. No significant differences in the yield (0.9-1.1 x 10(5) cells/brainstem) or viability (approximately 90%) of OB populations from jp and wild-type (wt) littermates were observed, indicating that cell death occurs at a later stage in the mutant lineage. When cultured in a basally defined, insulin-containing medium, wt and jp OBs died 1-2 days later as their differentiation into GalC+ OCs began. Survival was not enhanced by known trophic factors (ciliary neurotrophic factor, leukemia inhibitory factor, neurotrophin-3) for differentiating rat OCs. In medium conditioned by neonatally derived rat or wt mouse astrocytes, however, wt OBs survived terminal OC differentiation, expressing first GalC, then DM-20/PLP on their surface 1-2 days later, before elaborating myelin-like membrane. By contrast, jp OBs in sister cultures survived differentiation initially as well as their normal counterparts did but rapidly died thereafter, beginning at the time when PLP/DM-20 immunoreactivity became detectable on premature wt GalC+ OCs. Additionally under these conditions, there survived a minor population (<5%) of jp cells, including mature OCs, which expressed stunted membranes and DM-20/PLP immunoreactivity in their cytoplasm, and undifferentiated progenitors. This model supports the concept that OC death in jp is effected by an intrinsic program, one mechanistically related to jp PLP/DM-20 gene expression and refractory to trophic cues in the environment.

Conservation of topology, but not conformation, of the proteolipid proteins of the myelin sheath

The proteolipid protein gene products DM-20 and PLP are adhesive intrinsic membrane proteins that make up >/=50% of the protein in myelin and serve to stabilize compact myelin sheaths at the extracellular surfaces of apposed membrane lamellae. To identify which domains of DM-20 and PLP are positioned topologically in the extracellular space to participate in adhesion, we engineered N-glycosylation consensus sites into the hydrophilic segments and determined the extent of glycosylation. In addition, we assessed the presence of two translocation stop-transfer signals and, finally, mapped the extracellular and cytoplasmic dispositions of four antibody epitopes. We find that the topologies of DM-20 and PLP are identical, with both proteins possessing four transmembrane domains and N and C termini exposed to the cytoplasm. Consistent with this notion, DM-20 and PLP contain within their N- and C-terminal halves independent stop-transfer signals for insertion into the bilayer of the rough endoplasmic reticulum during de novo synthesis. Surprisingly, the conformation (as opposed to topology) of DM-20 and PLP may differ, which has been inferred from the divergent effects that many missense mutations have on the intracellular trafficking of these two isoforms. The 35 amino acid cytoplasmic peptide in PLP, which distinguishes this protein from DM-20, imparts a sensitivity to mutations in extracellular domains. This peptide may normally function during myelinogenesis to detect conformational changes originating across the bilayer from extracellular PLP interactions in trans and trigger intracellular events such as membrane compaction in the cytoplasmic compartment.

Orientation of the myelin proteolipid protein C-terminus in oligodendroglial membranes

The topology of the integral membrane proteolipid protein (PLP) has important structural and functional implications for central nervous system myelin. To determine the orientation of the carboxyl-terminal portion of PLP, cultured mouse oligodendrocytes were probed with polyclonal antibodies raised against a synthetic terminal peptide corresponding to PLP residues 264-276 and with ten separate monoclonal antibodies that react with this region. Cells were examined by double-label indirect immunofluorescence for the presence of the PLP C-terminus and either oligodendrocyte-specific surface or intracellular antigens. To detect surface antigens, both living and paraformaldehyde-fixed cells were incubated with primary antibodies and then stained with fluorochrome-conjugated second antibodies. Antigens located within the cytoplasmic space were identified after fixation and permeabilization of cells. Live-labeled oligodendrocytes were stained brightly for myelin-oligodendrocyte glycoprotein, galactocerebroside, and other surface markers but did not stain for the PLP C-terminus or the intracellular proteins myelin basic protein and beta-tubulin. Fixation alone was sufficient for partial permeabilization of oligodendrocytes to antibodies and resulted in limited staining of the PLP C-terminus and intracellular proteins. The permeabilized oligodendrocytes stained intensely for the PLP C-terminus, myelin basic protein, and beta-tubulin. Finally, trypsinization of living oligodendrocytes eliminated surface myelin-oligodendrocyte glycoprotein staining but did not change the immunostaining properties of the PLP C-terminus. These results provide evidence that the carboxyl-terminus of PLP is located at the cytoplasmic face of oligodendroglial membranes.

Reversible inhibition of oligodendrocyte progenitor differentiation by a monoclonal antibody against surface galactolipids

We have hypothesized that oligodendrocyte (OL) surface glycolipids, specifically galactocerebroside and sulfatide, play a role in the regulation of OL development by acting as sensors/transmitters of environment information. In support of this hypothesis we report here a reversible inhibition of OL progenitor cell differentiation by a monoclonal antibody [Ranscht mAb (R-mAb); Ranscht, B., Clapshaw, P. A. & Seifert, W. (1982) Proc. Natl. Acad. Sci. USA 79, 2709-2713] that reacts with these glycolipids. When isolated OL progenitors or mixed primary cultures are grown in the presence of the antibody, myelinogenic development is blocked in a dose-dependent manner at concentrations as low as 2 micrograms of IgG per ml. The inhibited cells express the OL progenitor markers O4 and vimentin but are negative for galactosylcerebroside, sulfatide, 2',3'-cyclic nucleotide 3'-phosphohydrolase, myelin basic protein, and myelin basic protein RNA expression. In contrast, the levels of total cellular protein and the expression of astrocytic glial fibrillary acidic protein in mixed cultures are not affected. Antibody-blocked cells have a distinctive morphology in which long, sparsely branched processes emanate from round cell bodies. Upon removing the perturbing antibody, the cells rapidly resume differentiation. Reverted mixed primary cultures, in which OL progenitors of several sequential developmental stages are present at the time of plating, differentiate more rapidly than control cultures, suggesting that the antibody-induced block results in a synchronization of developmental progression along the OL lineage by accumulating cells at the inhibition point. However, the normal temporal sequence of marker expression is maintained. Control studies with several other antibodies recognizing OL cell surface antigens, including HNK-1, neural cellular adhesion molecule (N-CAM), 1A9, anticholesterol, and O1, did not inhibit development. Since the inhibition occurs in highly enriched populations of OL progenitors, the inhibition does not involve cell-cell interactions between OLs and other cell types but concerns interactions of OLs with themselves, soluble factors, or OL extracellular matrix molecules and adhesion factors that provide essential environmental signals required for normal myelinogenic development.

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.

Glial conditioned medium enables jimpy oligodendrocytes to express properties of normal oligodendrocytes: production of myelin antigens and membranes

Cultures of cerebral cells from 2-day-old jimpy (jp) and normal animals were immunocytochemically stained with antibodies to galactocerebroside, 2', 3'-cyclic nucleotide 3'-phosphohydrolase and proteolipid protein (PLP). As the normal cultures mature, oligodendrocytes express these markers in cell bodies, in processes and in expansive membrane sheets produced along the lengths of cell processes. In contrast, oligodendrocytes from jp cultures do not produce large membrane sheets and their total numbers are reduced. Only rarely do jp oligodendrocytes exhibit immunostaining for PLP. Thus, jp oligodendrocytes grown in vitro mimic deficiencies of jp CNS in situ. In order to examine the effect of environment and cell-cell interactions on expression of the jp mutation we grew jp cerebral cells in the presence of medium conditioned for short periods by normal cerebral cells. Under these conditions jp oligodendroglia appeared nearly normal by immunostaining criteria. Their numbers were increased; they were able to produce and maintain membrane sheets; and some cells expressed PLP. These results show that jp oligodendrocytes have the capacity to express certain normal phenotypic parameters, and when given an appropriate environment they do so. The effects of normal conditioned medium may be due to a secreted factor, possibly produced by the astrocytes, which constitute the vast majority of cells in the conditioning cultures. The dramatic effect of normal glial conditioned medium on jp oligodendrocytes suggests that the steps leading to hypomyelination in jp are exceedingly complex and may involve glial-glial interactions.