Neurotrophin signalling.

The neurotrophins act through their signalling competent trk tyrosine kinase receptors (trkA, trkB and trkC), and, in addition, they share a common low-affinity receptor, p75. Acting alone, trk kinases can mediate neurotrophin action, including survival, fiber outgrowth, differentiation and proliferation. The p75 receptor modulates trk activity and also couples to an independent signalling mechanism involving the sphingomyelin cycle. The elucidation of pathways that couple trk receptor activation to fiber outgrowth and gene expression has made good progress. New work on signalling in postmitotic neurons is beginning to reveal that similarities and differences in these pathways exist, which depend on the neuronal type or the developmental stage.