MYOBLAST FUSION PROMOTES THE APPEARANCE OF ACTIVE PROTEASE NEXIN-I ONHUMAN MUSCLE-CELL SURFACES

Protease nexin I (PM) is a 43- to 50-kDa glycoprotein capable of inhib iting a number of serine proteases and belongs to the serpin superfami ly. PNI is identical to glia-derived nexin, a neurite outgrowth promot er by virtue of its thrombin-inhibiting activity. Of particular releva nce to neuromuscular biology and pathology, PNI was the first serpin s hown to be highly localized to the neuromuscular junction and it maps to precisely the same locus as autosomal recessive amyotrophic lateral sclerosis (ALSJ) at chromosome 2q33-35. In the present report, we now show that in cultures of human skeletal muscle, PNI protein is expres sed only after myoblast fusion into multinuclear myotubes and is local ized in patches on their surfaces. We performed complex formation expe riments with labeled thrombin, another target protease for PNI, with i ntact human muscle cells in culture. We detected specific SDS-stable P NI/thrombin complexes in myotube extracts only, indicating that active PM was bound to their surfaces. We studied the gene expression of PNI mRNA using a 300-bp cDNA synthesized from the published sequence of h uman PM. Confirming the protein data, upregulation of PM appears in my otubes using Northern blot analysis. The current results reinforce the hypothesis that the regulation of the balance of serine proteases and serpins, such as PNI, is involved in muscle differentiation. They als o prompt us to explore PNI abnormalities in several neuromuscular dise ases, including ALSJ. (C) 1996 Academic Press, Inc.