Serine proteinase inhibitors in human skeletal muscle: Expression of β‐amyloid protein precursor and α1‐antichymotrypsin in vivo and during myogenesis in vitro

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Abstract
The balance of serine proteases and inhibitors in nerve and muscle is altered during programmed‐ and injury‐induced remodeling. A serpin, α1‐antichymotrypsin (α1‐ACT), and Kunitz‐inhibitor containing forms of the β‐amyloid precursor protein (βAPP) may be important components of this balance. In the present study, we analyzed their expression in primary cultures of human myogenic (satellite) cells that mimic myogenic differentiation using Western blotting and immunocytochemistry. In vitro results were compared to in vivo results from normal adult human skeletal muscle biopsies. Using an anti‐α1‐ACT polyclonal antibody, we detected a 62 kDa immunoreactive band both in cultured human myogenic cells (mononucleated myoblasts as well as multi‐nucleated myotubes) and in extracts of human muscle biopsies. With a polyclonal anti‐βAPP antibody we found two bands (105 and 120 kDa) in myoblasts and myotubes in culture. However, the same antibody recognized only a single band at 92 kDa in biopsies. By immunocytochemistry, both α1‐ACT and βAPP were indistinctly present on localized to the surface of myoblasts in culture. In contrast, these inhibitors were dense on myotube surfaces, where they often formed distinct aggregates and frequently co‐localized. In permeabilized muscle cells, α1‐ACT and βAPP appeared to be localized to the perikarya of both myoblasts and myotubes. Confirming previous results, both α1‐ACT and βAPP were present at the neuromuscular junction in human muscle sections. These developmental changes found during in vitro myogenesis for α1‐ACT and βAPP, both serine protease inhibitors, reinforce the hypothesis that regulation of the serine proteases and serine protease inhibitors plays an important role in neuromuscular differentiation. © 1995 Wiley‐Liss Inc.