Neurotrophic factors are a family of proteins that are responsible for the growth and survival of developing neurons and the maintenance of mature neurons. Recent research has shown that neurotrophic factors promote the initial growth and development of neurons in the central nervous system and peripheral nervous system and that they are capable of regrowing damaged neurons in test tubes and animal models. Neurotrophic factors comprise a broad family, each family has its own distinct signaling family though the cellular responses elicited often do overlap. Neurotrophic factors are essential for keeping neurons alive and properly connected. During development, these factors play a critical role in nourishing the neurons in the spinal cord that connect to the muscle cells to prevent the death of the nerve cell. In addition, neurotrophic factors regulate growth of neurons, associated metabolic functions such as protein synthesis, and the ability of the neuron to make the neurotransmitters that carry chemical signals which allow the neuron to communicate with other neurons or with other targets.
The first neurotrophic factor family discovered was the neurotrophins which consist of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4). Each of the four mammalian neurotrophins has been shown to activate one or more of the three members of the tropomyosin-related kinase (Trk) family of receptor tyrosine kinases (TrkA, TrkB and TrkC). In addition, each neurotrophin activates p75 neurotrophin receptor (p75NTR), a member of the tumour necrosis factor receptor superfamily. Members of numerous other proteins also regulate neural survival, development and function through activation of receptor tyrosine kinases, most notably the glial cell-derived neurotrophic factor (GDNF) family. There are at least four members of this family including GDNF, neurturin, artemin, and persephin.
Neurotrophic factors control the survival and development of neurons. Currently, neurotrophic factors are being intensely studied for use in bioartificial nerve conduits because they also play important roles in regulating axon growth, dendrite cell growth and pruning and the expression of proteins, such as ion channels, transmitter biosynthetic enzymes and neuropeptide transmitters that are essential for normal neuronal function. The neurotrophic factors may or may not be immobilized to the scaffold structure, though immobilization is preferred because it allows for the creation of permanent, controllable gradients. In some cases, such as neural drug delivery systems, they are loosely immobilized such that they can be selectively released at specified times and in specified amounts.