Neurotrophins (NGF, BDNF, NT-3, NT-4/5) play pivotal roles in survival, differentiation, and plasticity of neurons in the peripheral and central nervous system. They are produced, and secreted in minute amounts, by a variety of tissues. They signal through two types of receptors: TRK tyrosine kinase receptors (TRKA, TRKB, TRKC), which specifically interact with the different neurotrophins, and p75NTR, which interacts with all neurotrophins. TRK receptors are reported in a variety of tissues in addition to neurons. p75NTRs are also widespread.
Neurotrophins and their receptors are synthesized as several different splice variants, which differ in terms of their biological activities. The nerve growth factor (NGF) was the first growth factor to be identified and has served as a model for studying the mechanisms of action of neurotrophins and growth factors. The mechanisms by which NGF generates diverse cellular responses have been studied extensively in the rat pheochromocytoma cell line PC12. When exposed to NGF, PC12 cells exit the cell cycle and differentiate into sympathetic neuron-like cells. Current data show that signalling by the other neurotrophins is similar to NGF signalling.