Neurotrophic factors and neuromuscular disease: I. General comments, the neurotrophin family, and neuropoietic cytokines

Neurotrophic factors are growth factors or cytokines that are inducible polypeptides and permit intercellular communication. An explosion of information about neurotrophic factors is setting the stage for significant advances in neural disease therapy in the next century. The effects of these trophic factors are overlapping and pleiotropic, acting on many cell types and tissues to control proliferation and differentiation of developing neurons and to exert a variety of functions on mature neurons. Studies of receptors unique to several neurotrophic factor families have revealed exquisite mechanisms of signal transduction. Preclinical trials in neuromuscular disease were promising, but results from initial clinical trials have been disappointing; new and better designed clinical trials are under way. Laboratory investigators also are exploring techniques to deliver factors directly to the central nervous system by means of viral vectors or to exert neurotrophic signals on the nervous system using novel small molecules that stimulate neurotrophic factor or neuroimmunophilin receptors. Combination therapies, refined delivery techniques, and treatment timing may be the key for successful treatment with neurotrophic factors. In this two-part review, we discuss the neurobiology of neurotrophic factors, the characteristics of the major neurotrophic factors, and their therapeutic potential in neuromuscular disease.