Characterization of TrkB receptor-mediated signaling pathways in rat cerebellar granule neurons: involvement of protein kinase C in neuronal survival.

TrkB belongs to the Trk family of tyrosine kinase receptors and mediates the response to brain-derived neurotrophic factor (BDNF) and neurotrophin-4/5 (NT-4/5). Here, we report that both truncated and full-length forms of TrkB receptors are expressed in developing cerebellar granule neurons. BDNF and NT-4/5 increased the survival of cultured cerebellar granule neurons. BDNF and NT-4/5 also induced an autophosphorylation of TrkB receptors and subsequently resulted in a phosphorylation and binding of phospholipase C-gamma (PLC-gamma) and SH2-containing sequence to the autophosphorylated TrkB receptors. Both contain src homology 2 (SH2) regions. In keeping with a signaling function of PLC-gamma, BDNF increased the phosphatidylinositol (PI) turnover and elevated intracellular calcium levels. To investigate the involvement of protein kinase C (PKC) in the survival of granular neurons, we show here activation of PKC after BDNF or TPA treatment and blocking of the observed survival-promoting effects of BDNF and TPA with calphostin C, a specific PKC inhibitor. In addition, BDNF activated c-ras in a concentration-dependent manner. These results suggest that two different pathways, the c-ras and the PLC-gamma pathway, are activated by TrkB receptors in primary neurons and that PKC activation is involved in the survival promoting effect of BDNF.