The transforming growth factor-betas: multifaceted regulators of the development and maintenance of skeletal muscles, motoneurons and Schwann cells.

This review discusses the roles of the transforming growth factor-betas (TGF-betas) as part of a complex network that regulates the development and maintenance of the neuromuscular system. The actions of the TGF-betas often vary depending on which other growth factors are present, making it difficult to extrapolate results from in vitro experiments to the in vivo situation. A new approach has therefore been needed to understand the physiological functions of the TGF-betas. The behaviours (proliferation, fusion, apoptosis) of many of the cells in the neuromuscular system have a complex pattern which varies in space and time. The actions of growth factors in this system can thus be deduced based on how well their pattern of expression correlates with known cellular behaviours. Hypotheses based on this molecular anatomical evidence can then be further tested with genetically modified mice. From this type of evidence, we suggest that: (1) TGF-beta1 is an autocrine regulator of Schwann cells; (2) maternally-derived TGF-beta1 helps to suppress self and maternal immune attack; (3) TGF-beta2 regulates when and where myoblasts fuse to myotubes; (4) motoneuron survival is regulated by multiple sources of TGF-betas, with TGF-beta2 being the more important isoform. The concept of TGF-beta1 as a regulator of secondary myotube formation is not supported by either the location of the TGF-beta1 in developing muscles or by the phenotype of TGF-beta1-/- mice. The review concludes with a discussion of whether all of these of postulated functions can occur independently of each other, within the confines of the neuromuscular system.