Distribution of proteolipid protein and myelin basic protein in cultured mouse oligodendrocytes: primary vs. secondary cultures

Induction of oligodendrocyte differentiation and in vitro myelination by inhibition of rho-associated kinase

In inflammatory demyelinating diseases such as multiple sclerosis (MS), myelin degradation results in loss of axonal function and eventual axonal degeneration. Differentiation of resident oligodendrocyte precursor cells (OPCs) leading to remyelination of denuded axons occurs regularly in early stages of MS but halts as the pathology transitions into progressive MS. Pharmacological potentiation of endogenous OPC maturation and remyelination is now recognized as a promising therapeutic approach for MS. In this study, we analyzed the effects of modulating the Rho-A/Rho-associated kinase (ROCK) signaling pathway, by the use of selective inhibitors of ROCK, on the transformation of OPCs into mature, myelinating oligodendrocytes. Here we demonstrate, with the use of cellular cultures from rodent and human origin, that ROCK inhibition in OPCs results in a significant generation of branches and cell processes in early differentiation stages, followed by accelerated production of myelin protein as an indication of advanced maturation. Furthermore, inhibition of ROCK enhanced myelin formation in cocultures of human OPCs and neurons and remyelination in rat cerebellar tissue explants previously demyelinated with lysolecithin. Our findings indicate that by direct inhibition of this signaling molecule, the OPC differentiation program is activated resulting in morphological and functional cell maturation, myelin formation, and regeneration. Altogether, we show evidence of modulation of the Rho-A/ROCK signaling pathway as a viable target for the induction of remyelination in demyelinating pathologies.

Epidermal growth factor promotes oligodendrocyte process formation and regrowth after injury

Oligodendrocytes (OLs) form myelin within the central nervous system and are targets in numerous demyelinating diseases and injuries. OLs grown in culture maintain the developmental timetable which occurs in vivo and mature into cells with a relatively normal phenotype. In this study, cultured cells are used to test whether EGF can modulate process formation in OLs both before and after transection injury. EGF had no effect on the formation of new processes by OLs at any stage of development. To test the effect of EGF on process outgrowth after injury, mature OLs were selected and injured by laser transection of a single process, then imaged at 24-h intervals for 120 h. EGF promoted the recovery and regrowth of injured processes and also significantly increased outgrowth in uninjured processes. As well, it increased the number of new sprouts formed by OLs after injury. Results suggest that the effects of EGF on process outgrowth are a consequence of EGF interaction with a signaling pathway that is specifically activated within injured OLs. The potent effect of EGF on OL process formation after an injury suggests that modulation of the signaling pathways involved might provide a mechanism to promote remyelination.

Insulin-like growth factor I protects oligodendrocytes from tumor necrosis factor-alpha-induced injury

Tumor necrosis factor-alpha (TNF-alpha) has been causally implicated in several demyelinating disorders, including multiple sclerosis. Because insulin-like growth factor I (IGF-I) is a potent stimulator of myelination, we investigated whether it can protect oligodendrocytes and myelination from TNF-alpha-induced damage using mouse glial cultures as a model. Compared with controls, TNF-alpha decreased oligodendrocyte number by approximately 40% and doubled the number of apoptotic oligodendrocytes and their precursors. Addition of Boc-aspartyl(Ome)-fluoromethyl ketone (BAF), an inhibitor of interleukin-1beta converting enzyme (ICE)/caspase proteases, blocked TNF-alpha-induced reductions in oligodendrocytes, indicating that the TNF-alpha-induced reduction in oligodendrocytes is, at least in part, due to apoptosis, and that ICE/caspases are one of TNF-alpha action mediators. Simultaneous addition of IGF-I to TNF-alpha-treated cultures negated these TNF-alpha effects nearly completely. Furthermore, IGF-I promoted oligodendrocyte precursor proliferation and/or differentiation in TNF-alpha-treated cultures. To analyze TNF-alpha and IGF-I actions on oligodendrocyte function, we measured the abundance of messenger RNAs (mRNAs) for two major myelin-specific proteins, myelin basic protein (MBP) and proteolipid protein (PLP). While TNF-alpha decreased MBP and PLP mRNA abundance by 5- to 6-fold, IGF-I abrogated TNF-alpha-induced reductions in a dose- and time-dependent manner. The changes in MBP and PLP mRNA abundance could not be completely explained by the changes in oligodendrocyte number, indicating that myelin protein gene expression is regulated by both TNF-alpha and IGF-I. These data support the hypothesis that TNF-alpha can mediate oligodendrocyte and myelin damage, and indicate that IGF-I protects oligodendrocytes from TNF-alpha insults by blocking TNF-alpha-induced apoptosis, and by promoting oligodendrocyte and precursor proliferation/differentiation and myelin protein gene expression.

Injury stimulates outgrowth and motility of oligodendrocytes grown in vitro

Oligodendrocytes which form myelin within the CNS develop from small, highly motile cells that are largely bipolar into mature cells which extend many processes and which produce myelin membranes around multiple axons. The production of myelin sheaths is thought to anchor mature oligodendrocytes (OLs), limiting their motility. When the brain sustains an injury, OLs do not make a significant effort to remyelinate, a fact attributed to both their lack of proliferation and their inability to migrate or extend processes into areas of injury. To test the motility and growth potential of mature OLs, we have designed an in vitro system in which individual cells can undergo long-term observation. Additionally, cells can be mechanically injured by transection of processes using a low-power laser beam. Both control and injured OLs undergo several types of structural change, including extension and retraction of processes and membranes, as well as changes in process caliber. Some OLs exhibit a high degree of motility, moving several hundred micrometers within days. Rather than interfering with the cells' ability to undergo structural change, injury actually stimulated outgrowth of new processes and motility. Neither injury nor addition of basic fibroblast growth factor (bFGF) increased the rate of OL division. However, bFGF paradoxically caused an increase in uptake of the DNA synthesis marker bromodeoxyuridine and had negative effects on OL survival. The unexpected findings that OLs with a mature phenotype are motile and undergo constant structural modification in vitro and that injury induces certain behaviors suggest that myelin-forming OLs in the brain may be capable of a high degree of plasticity under certain conditions.

Complexity analysis of oligodendroglial processes expressing myelin-associated glycoprotein

Oligodendroglia synthesize myelin in the mammalian central nervous system. Mature oligodendroglia have been identified in culture by two criteria; the expression of molecules characteristic of myelin, such as galactocerebroside (galC) and myelin-associated glycoprotein (MAG), and the elaboration of complex processes. Myelin gene expression can be documented by the binding of specific antibodies and antisera to the myelin-specific molecules; process complexity can be described by the fractal dimension, D. In this study, anti-MAG antisera was used to document MAG expression in the processes of oligodendroglia. Eighty percent of the galC+ oligodendroglia bound anti-MAG antiserum. With time in culture, MAG immunoreactivity seemed to extend from the cell soma into the oligodendroglial processes. To quantify this observation, fractal dimensions were calculated using either galC or MAG immunoreactivity to visualize oligodendroglial processes. A fractal dimension of 1.5 was calculated for O1+ processes by day 4 of culture; this value for D remained constant over the course of 1 month in culture. The fractal dimension calculated for MAG+ processes increased from 1.2 to 1.5 over the course of 28 days in culture. This change in fractal dimension confirms our visual impression that galC-containing processes acquire MAG slowly over the course of several weeks in culture.

Orientation of myelin proteolipid protein in the oligodendrocyte cell membrane

The orientation of proteins within a cell membrane can often be difficult to determine. A number of models have been proposed for the orientation of the myelin protein, proteolipid protein (PLP), each of which includes exposed domains on the intracellular and extracellular membrane faces. Immunolabeling experiments have localized the C-terminus and the region spanning amino acids 103-116 to the cytoplasmic face of the membrane, but no well characterized antibodies have been available that label extracellular PLP domains. In this report, we describe the generation and characterization of mouse monoclonal antibodies (mAb) against putative extramembrane domains. Three of the mAb, specific for PLP peptides 40-59, 178-191, or 215-232, immunostain live oligodendrocytes, indicating that these regions of the molecule are exposed on the external surface of the cell. In addition, we have used these mAb to study the time-course of incorporation of PLP into the oligodendrocyte membrane. These studies increase our knowledge of the orientation of PLP in the lipid bilayer and are relevant for understanding myelin function.

Identification and characterization of a B-cell determinant within the amphipathic domain (residues 178-238) of the myelin proteolipid protein

Pooled polyclonal rabbit anti-rat myelin and mouse anti-human proteolipid protein (PLP) antisera were screened against a panel of PLP synthetic peptides spanning residues 178-238 of the protein. Cross-reactivity against one determinant defined by PLP(200-219) was particularly prominent in both the anti-myelin and anti-PLP antisera and was chosen for further study. Competitive inhibition studies, utilizing a panel of overlapping synthetic peptides, demonstrated that the C-terminal portion of PLP(200-219), specifically residues comprising PLP(200-217), was important for antibody recognition of this region. Immunohistochemical analyses with an affinity-purified rabbit anti-PLP(200-219) antiserum demonstrated antibody cross-reactivity with PLP in both paraffin- and gelatin-embedded brain sections and immunocytochemical staining of mouse oligodendrocyte-enriched cultures demonstrated antibody binding with native PLP in situ. Staining of living non-permeabilized cells localized binding to the extracellular face of the myelin membrane. Collectively, these data argue for the presence of an immunodominant B-cell determinant defined by PLP residues 200-219. Furthermore, the structural conformation of this determinant in native PLP can be mimicked by the synthetic peptide, resulting in the generation of an antibody reagent that has considerable utility for immunohistochemical and immunocytochemical investigations of PLP expression and localization within the central nervous system myelin membrane.

Characterization and development of glial and other non-neuronal cells in chick Edinger Westphal cultures

Cultures of dissociated Edinger Westphal nuclei, dissected from embryonic chick brainstems, were screened immunohistochemically for a variety of non-neuronal cell markers. In young cultures, small clusters of cells were stained by the oligodendrocyte-specific antibodies 04 and 01. In older cultures, larger groups of cells were 04 and 01 positive, sheets of myelin-like membrane were elaborated, and immunoreactivity for proteolipid protein appeared. This sequence resembles that observed in well-characterized rodent brain cultures and suggests that oligodendrocytes in chick Edinger Westphal cultures differentiate in a pattern similar to rodent oligodendrocytes in culture. Variable numbers of cells were immunoreactive for glial fibrillary acidic protein. Many vimentin positive cells were observed, some of which morphologically resembled flat astrocytes. Together with the widespread presence of vimentin, large patches of fibronectin-like immunoreactivity suggested the presence of fibroblasts and/or endothelial cells. An anti-thymocyte polyclonal antibody stained a subset of cobblestone-shaped cells, possibly endothelial cells, in both Edinger Westphal cultures and control cultures of skin fibroblasts. Staining for smooth muscle myosin was detected in several patches of cells, tentatively identifying them as pericytes or smooth muscle cells. In conclusion, Edinger Westphal cultures contain a diverse and varying population of non-neuronal cells loosely organized in large, overlapping islands of cell types and including oligodendrocytes, astrocytes, possibly fibroblasts, endothelial cells, pericytes and/or smooth muscle cells.

Enhancement of autoimmune disease using recombinant vaccinia virus encoding myelin proteolipid protein

Viral infections have been associated with the initiation and exacerbations often observed with autoimmune disease. Mechanisms by which viruses may play a role in the development of autoreactive immune responses include polyclonal activation of B and T cells, molecular mimicry, viral infection of immune cells, exposure of sequestered antigens, or altered host cell expression (neoantigen or altered self) in virus infected host cells. We have been studying the immune response generated to self proteins in association with viral infection. Here we evaluate the effects of viral infection on the development of an autoimmune disease of the central nervous system, experimental allergic encephalomyelitis. A vaccinia virus construct, VVplp was made containing the coding region for rat myelin proteolipid protein (PLP). Cells infected with VVplp were found to express PLP protein in vitro. Central nervous system disease was not detectable in mice vaccinated with VVplp. However, mice vaccinated with VVplp and later challenged with encephalitogenic peptides derived from PLP were found to have enhanced disease with earlier onset of symptoms when compared to mice treated with a control vaccinia virus construct. This enhancement of disease was found to peak at 10 days post challenge with the encephalitogenic PLP peptide. Clinical disease and an inflammatory response in the central nervous system was evident in mice previously vaccinated with VVplp but not in control vaccinated mice at this time. These results indicate that prior infection with a virus capable of coding for self protein can predispose the host to an accentuated autoimmune response.

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.

Glial cells from adult rat olfactory bulb: immunocytochemical properties of pure cultures of ensheathing cells

Three morphologically and immunohistochemically distinct types of cell were present in primary cultures of adult rat olfactory nerve and glomerular layers of the olfactory bulb. One cell type was multipolar and stained positively for glial fibrillary acidic protein; a second type had fried egg-like morphology and stained with antibodies to epitope ED1; the third cell type had fusiform morphology, reacted with antibodies to vimentin and laminin and was glial fibrillary acidic protein- and ED1-negative. Trypsinization of these primary cultures (3 min, 37 degrees C), detached multipolar and fusiform cells only. When detached cells were set up in secondary culture on a glass substrate, fusiform cells did not attach, resulting in a pure culture of multipolar cells. Multipolar cells were glial fibrillary acidic protein- and myelin basic protein-positive and had the properties of so-called ensheathing cells or Blanes' glia. Immunoreactivity with anti-nerve growth factor receptor and anti-fibronectin allowed us to identify four distinct populations of multipolar ensheathing cells. One population was nerve growth factor receptor-positive, fibronectin-negative. A second was nerve growth factor receptor-negative and fibronectin-positive. A third was positive for both markers and the remaining cells did not stain for either of them. The morphological and immunological characteristics of cultured cells from olfactory nerve and glomerular layers were similar to those of Schwann cells and the similarities could account for the permissivity to axonal growth of the olfactory bulb.

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.

Monoclonal antibodies against myelin proteolipid protein: identification and characterization of two major determinants

This report describes the preparation and characterization of a panel of monoclonal antibodies (mAbs) against the myelin proteolipid protein (PLP). A Lewis rat was immunized with bovine proteolipid apoprotein and 27 mAbs were selected based on their reactivity against bovine PLP on enzyme-linked immunosorbent assays. Eleven mAbs recognized the PLP carboxyl-terminal sequence when tested against a panel of synthetic peptides in a solid-phase assay. A carboxyl-terminal pentapeptide (residues 272-276) was sufficient for antibody binding and the terminal phenylalanine residue was found particularly important. Deletion, modification, or replacement of this residue markedly reduced or obliterated antigen-antibody interaction. Nine mAbs reacted with a second antigenic determinant, residues 209-217, but these could be identified only by competitive immunoassays. This peptide was a more effective inhibitor than the longer peptides 202-217 and 205-221, suggesting that flanking residues may interfere with peptide-antibody interaction. Seven antibodies did not react with any of the synthetic peptides tested and their determinants remain unidentified. Immunoblot analysis showed that the mAbs reacted with both the PLP and the DM-20 isoforms. Twenty-three of the mAbs were of the immunoglobulin G2a or b isotype; the remaining antibodies were immunoglobulin M and all of these were specific for residues 209-217. Cultured murine oligodendrocytes were stained by most of the mAbs tested, but the most intense reactivity was observed with the carboxyl-terminus-specific mAbs. The immunocytochemical analyses demonstrate that the mAbs react with the native PLP in situ and show their potential usefulness for studies of the cell biology of myelin and oligodendrocytes.