Expression of microtubule-associated proteins MAP2 and tau in cultured rat brain oligodendrocytes

Oligodendrocytes in culture are characterized by large membranous sheets containing an elaborate network of microtubules. Microtubule-associated proteins (MAPs) participate in microtubule stability and the regulation of the cellular architecture. We have investigated the expression of two major groups of MAPs, MAP2 and tau, in cultured rat brain oligodendrocytes. Alternatively spliced isoforms of mRNAs encoding MAP2 and tau were assessed by means of reverse transcription and polymerase chain reaction using a newly designed set of MAP2- and tau-specific primers. The data were compared with data obtained with cultures of rat brain astrocytes and rat cerebral neurons, and adult rat brain. The results show that oligodendrocytes, similarly to neurons, express mainly MAP2c transcripts containing three microtubule-binding repeats. They also contain small amounts of MAP2b mRNA. Six low molecular weight tau isoforms, namely tau 1-6, have been described in the brain (Goedert et al. 1991). The major isoform of tau mRNA in oligodendrocytes was found to be tau 1, which represents a marker typical for immature neurons. Tau 2 and tau 4 isoforms were also detected, albeit at a very low level. Immunoblot analysis of oligodendroglia cell extracts confirmed the presence of tau protein. It migrates as a single polypeptide with an apparent molecular weight of approximately 55 kDa. In addition, oligodendrocytes express MAP2c protein, which migrates as a close double band with an apparent molecular weight around 70 kDa. Indirect immunofluorescence staining indicated that tau and MAP2 immunoreactivity was expressed in oligodendrocytes of immature and mature morphologies in the cell somata and cellular processes. Tau was particularly found in the end of the cellular extensions, and both proteins exhibited a distribution similar to myelin basic protein. Thus, oligodendroglia, like neuronal cells, contain microtubule-associated proteins, mainly MAP2c and the tau 1 isoform, although at a much lower level. The presence of these MAPs in myelin-forming cells further points to the functional significance of the cytoskeleton during oligodendrocyte differentiation, process outgrowth, and myelin formation.