CONSTITUTIVE PHOSPHORYLATION OF TRKC RECEPTORS IN CULTURED CEREBELLARGRANULE NEURONS MIGHT BE RESPONSIBLE FOR THE INABILITY OF NT-3 TO INCREASE NEURONAL SURVIVAL AND TO ACTIVATE P21 RAS

The neurotrophins brain derived neurotrophic factor (BDNF) and neurotr ophin-3 (NT-3) are both expressed in developing cerebellum in addition to their tyrosine kinase receptors, TrkB and TrkC. In contrast to BDN F, NT-3 has only a negligible or a transient survival activity on cult ured cerebellar granule neurons. The granule neurons however, express both TrkC and Trk B receptors which suggests a basic difference in sig naling between BDNF and NT-3 in these neurons. Here we have studied wh ether this difference can be attributed to the presence of alternative TrkC receptor variants on the granule neurons and which signaling pat hway is specifically activated by BDNF but not by NT-3 in these neuron s. Using RT-PCR it was shown that the cerebellar granule neurons expre ss the full length TrkC receptor, in addition to variant receptors con taining small inserts in the receptor tyrosine kinase domain. There wa s no dramatic change in the relative amounts of different TrkC recepto rs during development. However, we found the TrkC receptor constitutiv ely phosphorylated even in the absence of added ligand suggesting an i nteraction of TrkC with endogenously produced NT-3. In addition, NT-3 was able to phosphorylate the BDNF receptor, TrkB but only at higher c oncentration (50 ng/ml). There were also distinct differences in the a ctivation of intracellular molecules by BDNF and NT-3. Thus, p21 Ras a nd PLC gamma were activated by BDNF but not by NT-3 whereas both BDNF and NT-3 increased calcium and c-fos mRNA in the granule neurons. Thes e results show that differential activation of specific intracellular pathways such as that of p21 Ras determines the specific effects of BD NF and NT-3 on granule neuron survival. In addition, since calcium is increased by NT-3 in the cerebellar granule neurons, this neurotrophin might have some unknown important effects on these neurons.