The role of neurotrophins in axonal growth, guidance, and regeneration.

Neurotrophins are proteins that regulate neuronal survival, axonal growth, synaptic plasticity and neurotransmission. They are members of the neurotrophic factors family and include factors such as the nerve growth factor (NGF), the brain derived neurotrophic factor (BDNF), the neurotrophin-3 (NT-3), and the neurotrophin-4/5 (NT-4/5). These molecules bind to two types of receptors: i) tyrosine kinase receptors (TrkA, TrkB, TrkC) and ii) a common neurotrophin receptor (p75NTR). The two receptor types can either suppress or enhance each other's actions. Neurotrophins have a multifunctional role both in the central and peripheral nervous system. They have been suggested as axonal guidance molecules during the growth and regeneration of nerves. It has also been proven that they stimulate axonal growth by mediating the polymerization and accumulation of F-actin in growth cones and axon shafts. Neurotrophins, as other neurotrophic factors, have been shown that they reduce neuronal injury by exposure to excitotoxins, glucose deprivation, or ischemia. Furthermore, the nerve regeneration promoting effect of these growth factors is well documented for many different models of central or peripheral nervous system injury. Several studies have shown that exogenous administration of these factors has protective properties for injured neurons and stimulates axonal regeneration. Based on these properties, these molecules may be used as therapeutic agents for treating degenerative diseases and traumatic injuries of both the central and peripheral nervous system.