BDNF-ACTIVATED SIGNAL-TRANSDUCTION IN RAT CORTICAL GLIAL-CELLS

Cortical glial cells in culture were found to be responsive to the neu rotrophin brain-derived neurotrophic factor (BDNF), as evidenced by ac tivation of multiple signal transduction processes. BDNF produced an i ncrease in mitogen-activated protein (MAP) kinase tyrosine phosphoryla tion, MAP kinase activity, intracellular calcium concentration and c-f os expression in the glial cells. Only a subset of the glial cells res ponded to BDNF, as reflected in single-cell analysis of calcium transi ents and c-fos expression. BDNF had no detectable effect on glial mito tic activity, as measured by DNA synthesis. In parallel studies, nerve growth factor and neurotrophin-3 had no effect on signalling in these cultures. BDNF has previously been demonstrated to act via trkB recep tors with a cytoplasmic tyrosine kinase domain (gp145(trkB)). Pretreat ment of glial cultures with K252a, which at low concentrations specifi cally inhibits the trk tyrosine kinases, abolished BDNF effects on MAP kinase stimulation, suggesting that BDNF was acting through gp145(trk B). However, subsequent studies showed that gp145(trkB) was expressed at extremely low levels in the cultures: gp145(trkB) mRNA transcripts could only be detected using the reverse transcription-polymerase chai n reaction, and gp145(trkB) protein was not detected by either immunob lotting or immunocytochemistry. On the other hand, the glia expressed significantly higher levels of gp95(trkB) mRNA and protein, which repr esent truncated forms of trkB receptors lacking the tyrosine kinase do main. The results of these studies demonstrate that a subset of cultur ed CNS glia respond to BDNF with the activation of conventional signal transduction processes. The mechanism of BDNF-initiated signal transd uction in glial cells most likely involves a relatively small number o f gp145(trkB) receptors, but involvement of the more abundant truncate d gp95(trkB) receptors cannot be excluded.