Neurotrophin-3 and brain-derived neurotrophic factor activate multiple signal transduction events but are not survival factors for hippocampal pyramidal neurons.

Expression of the neurotrophin-3 (NT-3) receptor (TrkC) and the effects of NT-3 on signal transduction were investigated in highly enriched populations of embryonic rat hippocampal pyramidal neurons grown in bilaminar cultures. PCR analysis revealed that the predominant trkC isoform is K1, which lacks an insert in the kinase domain. Polyclonal TrkC-specific antibodies stained > 90% of the neurons and revealed a single approximately 145-kDa protein in immunoblots of extracts from adult hippocampus and pyramidal neuron cultures. Addition of NT-3 (50 ng/ml) to these cultures induced the tyrosine phosphorylation of TrkC but not TrkB, as determined by anti-phosphotyrosine staining of immunoprecipitates; thus, all the effects of NT-3 are mediated through TrkC. NT-3 also increased the tyrosine phosphorylation of 42-, 44-, 49-, 55-, 95-, and 145-kDa proteins; the pattern induced by brain-derived neurotrophic factor (BDNF) was similar but not identical to that induced by NT-3, suggesting that subtle differences may exist in signaling by TrkB and TrkC receptors. Immunoprecipitation of p21ras from 32P-prelabeled cells showed that NT-3 increased the level of the GTP-bound form of the protein threefold over the control within 5 min. Mitogen-activated protein (MAP) kinase activity was maximally elevated by NT-3 within 2 min and then returned slowly toward baseline over the next 60 min. Tyrosine phosphorylation of phospholipase C-gamma increased rapidly after NT-3, suggesting that this enzyme becomes activated. Consistent with this, the neurotrophin rapidly increased protein kinase C activity as well as intracellular Ca2+ levels. The effects of both NT-3 and BDNF on Ca2+ levels were attenuated in Ca(2+)-free medium, suggesting that both neurotrophins increase Ca2+ flux across the plasma membrane as well as release from internal stores. NT-3 also increased c-Fos expression in > 80% of the cells; the effect peaked at 30 min and declined to baseline by 120 min. Despite the activation of ras-MAP kinase and phosphoinositide signaling pathways, neither NT-3 nor BDNF alone or in combination could sustain hippocampal pyramidal neurons deprived of glial support. We conclude that in this system NT-3 and BDNF do not appear to be acting as classical "neurotrophic" factors and that activation of the MAP kinase pathway is insufficient for the promotion of neuronal survival.

BDNF-activated signal transduction in rat cortical glial cells.

Cortical glial cells in culture were found to be responsive to the neurotrophin brain-derived neurotrophic factor (BDNF), as evidenced by activation of multiple signal transduction processes. BDNF produced an increase in mitogen-activated protein (MAP) kinase tyrosine phosphorylation, MAP kinase activity, intracellular calcium concentration and c-fos expression in the glial cells. Only a subset of the glial cells responded to BDNF, as reflected in single-cell analysis of calcium transients and c-fos expression. BDNF had no detectable effect on glial mitotic 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 receptors with a cytoplasmic tyrosine kinase domain (gp145trkB). Pretreatment of glial cultures with K252a, which at low concentrations specifically inhibits the trk tyrosine kinases, abolished BDNF effects on MAP kinase stimulation, suggesting that BDNF was acting through gp145trkB. However, subsequent studies showed that gp145trkB was expressed at extremely low levels in the cultures: gp145trkB mRNA transcripts could only be detected using the reverse transcription-polymerase chain reaction, and gp145trkB protein was not detected by either immunoblotting or immunocytochemistry. On the other hand, the glia expressed significantly higher levels of gp95trkB mRNA and protein, which represent truncated forms of trkB receptors lacking the tyrosine kinase domain. The results of these studies demonstrate that a subset of cultured CNS glia respond to BDNF with the activation of conventional signal transduction processes. The mechanism of BDNF-initiated signal transduction in glial cells most likely involves a relatively small number of gp145trkB receptors, but involvement of the more abundant truncated gp95trkB receptors cannot be excluded.

Signal transduction events mediated by the BDNF receptor gp 145trkB in primary hippocampal pyramidal cell culture.

The trkB gene encodes a tyrosine kinase receptor, gp145trkB, for brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NT-4). To understand the role of gp145trkB in the nervous system, we have investigated its expression in embryonic rat hippocampal pyramidal cell cultures and examined the effects of BDNF on signal transduction in the primary neurons. The expression of trkB transcripts was established by PCR analysis and in situ hybridization. In addition to gp145trkB, the pyramidal neuronal cultures expressed transcripts specific for the NT-3 receptor gp145trkC, but not for the high-affinity NGF receptor gp140trk or for p75LNGFR, a low-affinity receptor for all known members of the NGF family of neurotrophins including the gp145trkB ligands, BDNF and NT-4. The presence of gp145trkB receptors in the primary neuronal cultures was confirmed by immunocytochemical analysis in which > 90% of the cells stained with affinity-purified polyclonal antibodies to gp145trkB. Immunoblots using this antibody revealed a single approximately 140 kDa protein in both adult hippocampus and pyramidal cultures. Addition of recombinant BDNF to these cultures induced the tyrosine phosphorylation of gp145trkB, as determined by antiphosphotyrosine staining of gp145trkB immunoprecipitates. Moreover, BDNF treatment activated the microtubule-associated protein (MAP) kinases, as determined by an increase in MAP2 phosphorylation in vitro. Both the 41 and 44 kDa forms of MAP kinase were activated by BDNF. BDNF also increased c-fos expression in over 90% of the cells. These results indicate that gp145trkB does not require p75LNGFR to form a functional receptor for BDNF in hippocampal pyramidal neurons.