Mechanisms regulating neurotrophin responsiveness during development and injury

The neurotrophins are a conserved family of secreted growth factors which regulate the survival, differentiation and maintenance of cells of the nervous system and we sought to understand the mechanisms regulating neurotrophin responsiveness during development and injury. Using neuroblastoma differentiation as a model, we found that developmental responsiveness to NGF depended on activation of both the RET and TRKA signaling pathways. RET receptor activation leads to an increase in the expression of the RET receptor complex. Sustained activation of the RET receptor pathway causes growth cessation and arrest in the G0-G1 phase of the cell cycle. Furthermore, GDNF synergizes with CNTF to enhance TRKA receptor expression thus facilitating terminal differentiation in the presence of NGF. Neuroblastoma cell lines differentiated with the combination of GDNF, CNTF and NGF cease proliferation and begin to express normal neuronal marker genes while at the same time shutting off expression of the N-myc gene. The cells become trophic factor dependent, relying only on TRKA signaling for maintenance of the differentiated phenotype. Thus, collaborative interactions between the RET and TRKA signaling pathways reactivate the normal differentiation process in malignant neuroblastoma cells.; Expression of the p75 neurotrophin receptor (p75NTR) is induced in response to a diverse array of different diseases and injuries affecting the nervous system and evidence suggests that in these contexts, p75NTR mediated signals may determine whether the affected cells live or die. In order to identify the mechanisms underlying injury induced p75NTR expression, we first developed an in vitro model of osmotic cell swelling which recapitulates many aspects of nerve tissue injury including the dramatic induction of p75NTR expression. Transcriptional activation of p75NTR occurred in a wide variety of cell types and was dependent on the activities of PLC, PKC, calcium, and NOS. Intriguingly, we found that other stimuli which increase cell size including lactic acidosis, migration, and transformation lead to enhanced p75NTR expression, suggesting that p75NTR expression is regulated by cell volume. In addition, we demonstrate that p75NTR receptor density is directly proportional to osmotically induced increases in cell volume, perhaps suggesting that p75NTR functions as a regulator of cell volume homeostasis.