Neurotrophin receptors: Old friends with new partners.

Neurotrophins are important regulators of embryonic development and adult function of most populations of neurons in vertebrate nervous systems. This signaling system regulates many diverse activities, including survival, axon outgrowth, and synaptic plasticity. In mammals, neurotrophin action is mediated by four receptors, p75(NTR), TrkA, TrkB, and TrkC. Although early studies viewed these receptors as solitary agents in the cells outer membrane, recent discoveries reveal that the cell outer membrane is a crowded and highly interactive neighborhood. Neurotrophin receptors partner with a diverse array of membrane proteins, dramatically expanding their functional repertoire. This review will focus on some of the most recent discoveries concerning the promiscuous partnering of neurotrophin receptors.