Transforming growth factor-β signaling in motor neuron diseases.

Transforming growth factor β (TGF-β), a pleiotropic cytokine, regulates a diverse range of cellular responses, such as proliferation, differentiation, migration, and apoptosis. The TGF-β1, -β2, and -β3 isoforms are expressed by neurons and glial cells, and their receptors are expressed throughout the central nervous system. Several lines of evidence demonstrate that TGF-β signaling protects neurons from glutamate-mediated excitotoxicity, a putative mechanism underlying the pathogenesis of various neurodegenerative disorders such as amyotrophic lateral sclerosis (ALS). Recent studies indicate that the TGF-β-Smad2/3 pathway restores motor function in a mouse model of ALS, and that disruption of TGF-β signaling due to the transcriptional dysregulation of its receptor is associated with polyglutamine-induced motor neuron damage in spinal and bulbar muscular atrophy. Moreover, the TGF-β-Smad2/3 pathway regulates the function of glial cells, although the implication of this regulation in neurodegeneration remains elusive. Conversely, myostatin, a member of the TGF-β superfamily, has gained attention as a potential therapeutic target for neuromuscular disorders because genetic deletion of this factor results in increased muscle volume. Signal transduction by BMP, a member of the TGF-β super family, regulates the function and growth of the neuromuscular junction, while the disruption of this signaling has been reported in animal models of hereditary spastic paraplegia. These findings support the hypothesis that the disruption of TGF-β signaling is an important molecular event in the pathogenesis of motor neuron diseases, and that the modification of this signaling pathway represents a new therapeutic strategy against these devastating disorders.