Regulatory factors that determine actual cell size and number are relatively unknown. It has been suggested that tissues regulate their own size by synthesizing "inhibitors" that feedback on individual cells. When the collective concentration of tissue-specific inhibitor reaches a critical level, tissue-specific cells are frozen in size and number. It has also been proposed that cells outside the tissue of interest, clear or non-functionally bind circulating inhibitor. Thus, the proportion of cells secreting "inhibitor" relative to all cells making up the organism determines the availability of any limiting factor. The larger the quantity of tissue secreting inhibitor, the more inhibitor available to the secreting tissue after body-wide clearance.
One tissue that appears to support the theory of growth inhibition is skeletal muscle. Initial interest in myostatin (or GDF-8; growth/differentiation factor 8) as an inhibitor of skeletal muscle was based on the observation that the GDF-8 gene in double-muscled Belgian Blue and Piedmontese cattle was mutated, thus creating a non-functional protein.1,2 In Belgian Blue cattle, the gene for GDF-8 codes for a mature GDF-8 that is less than 10 amino acids (aa) long; in Piedmontese cattle, a critical cysteine is lost, resulting in a misfolded molecule.2 Recent studies in mice have confirmed the initial observations in cattle; a GDF-8 null mouse results in a super-muscular animal with abnormal muscle mass.3-5 Experiments have also shown that a functional GDF-8 inhibits the growth and proliferation of mouse C2C12 skeletal muscle cells.6,7 GDF-8 is a member of the TGF-beta superfamily (TGF-beta SF), demonstrating a characteristic cysteine knot configuration with extremely high aa conservation across species.2,4,8-11 Like all TGF-beta SF members, GDF-8 is synthesized as a pre-proprotein that undergoes intracellular proteolytic cleavage to create a 109 aa, 24-30 kDa mature, disulfide-linked dimer.4,6,10,12,13 Unlike most members of the TGF-beta SF, the mature GDF-8 dimer is secreted as a latent complex, retaining its pro-region in a non-covalent interaction that is analogous to that for TGF-beta 1. This pro-region confers latency on the mature GDF-8 dimer.14-16
Questions still remain, however, as to whether or not GDF-8 is truly a tissue-specific, diffusible "size regulator." First, it is not clear that it actually circulates, and thus a balance between production and clearance is difficult to envision.8,17 Second, cells other than skeletal muscle are reported to express GDF-8. These include fibroblasts,18 adipocytes,4 cardiac muscle cells,19 and possibly vascular smooth muscle cells.19 These results imply that GDF-8 is not a tissue-specific factor regulating its own mass. Third, it is unclear if GDF-8 is differentially expressed by slow or fast-type muscles. If so, then the model for GDF-8 activity may be more complex than initially proposed.15,17,20,21 Finally, correlations related to the temporal expression of GDF-8 in muscle undergoing hypertrophy and atrophy are poor.20,21 This may be due to the storage form that GDF-8 takes after secretion.
In summary, GDF-8 has the potential to negatively influence skeletal muscle growth. The factors that regulate the biological activity of GDF-8, however, have yet to be determined.