Plasminogen activators and their inhibitors in the neuromuscular system: II. Serpins and serpin: Protease complex receptors increase during in vitro myogenesis

In the course of studies on the regulation of plasminogen activator‐mediated extracellular matrix degradation in muscle we found the presence of a factor, a cellular inhibitor of serine proteases having features similar to the serpin protease nexin I (PNI). This factor was present in the medium and at maximum concentration following fusion of skeletal muscle cells in culture. The ability of the PNI homologue in mouse muscle to inhibit ECM degradation by urokinase in myo‐blast medium was compared to that of human PNI purified from human fibro‐blasls. Stable (to SDS) 1:1 molar ratio complex formation between PNI and proteases, the proposed means by which these enzymes are regulated and removed, was also detected. Cell surface receptors for protease:PNI complexes, the specific binding sites for inactive complex internalization, were found on multinucleated myotubes, while little or no receptor activity was detected on myoblasts. These data suggest that developmental regulation of (a) increased PNI proteolytic inhibitory activity expression and (b) the appearance of protease:inhibitor complex receptors on muscle cell surfaces during myogenesis may constitute important regulatory features of muscle suface proteolytic activity. They complement previous studies of proteoglycan metabolism in muscle, which itself contains molecules capable of regulating the activity of myotube surface proteases.