[Neurotrophins. I: Molecular features].

Neurotrophic factors are proteins necessary for neuronal development and survival. The first isolated and characterized neurotrophin was nerve growth factor (NGF). Other structurally related neurotrophins are brain-derived neurotrophic factor (BDNF) and the neurotrophins NT-3, NT-4/5 and NT-6. The neurotrophins bind to specific tyrosine-kinase (trk) receptors, which are transmembrane proteins with intrinsic phosphorylating activity. Three trk receptor subtypes are known at present, trk A, trk B and trk C, which preferentially bind NGF, BDNF and NT-3 respectively. All of the neurotrophins bind to another low-affinity receptor called p75. The neurotrophins are generally synthesized in target cells and, after receptor binding, are transported retrogradely from the nerve terminal to the nucleus where they regulate gene expression. The role of neurotrophins in development has been further characterized by means of gene disruption studies that show profound phenotype alterations in mice lacking any of the trk receptors.