Subcellular localization of phosphoprotein B-50 in regenerating muscle. An immuno-electron microscopic study

Phosphoprotein B-50, also termed growth-associated protein GAP43, is a membrane-bound phosphoprotein expressed in neurons. It is particularly abundant during periods of axonal outgrowth in development and regeneration of the central and peripheral nervous systems. Recently the expression of phosphoprotein B-50 in regenerating muscle fibers was reported and it was assumed that phosphoprotein B-50, in muscles, plays a role in the determination of the growth morphology of regenerating muscle fibers. Thus, phosphoprotein B-50 no longer can be regarded as a neuron-specific molecule. In this paper we study the subcellular localization of phosphoprotein B-50 in regenerating human skeletal muscle fibers by electron immunohistochemistry. Phosphoprotein B-50 immunoreactivity is randomly distributed over the nuclear and perinuclear area of regenerating muscle fibers. Previously, by light-microscopy phosphoprotein B-50 immunoreactivity was demonstrated on the inner face of the sarcolemma in hypotrophic type 1 fibers in congenital fiber type disproportion. It is this distribution of phosphoprotein B-50 in developmentally disordered myocytes in particular which allows an analogy to the corresponding results found for growing axons. But we did not find subsarcolemmal expression of B-50 in regenerating muscle fibers. Probably this indicates that there is a difference in B-50 expression between regeneration of muscle fibers and developmentally retarded/immature myofibers. The presented data suggest that phosphoprotein B-50 is inserted only stage-dependent in the extending sarcolemma of the growing muscle fiber. Analogously to the nervous system, phosphoprotein B-50 may serve a local function involving transmembrane signalling by means of calmodulin binding of phosphoprotein B-50 and/or phosphoprotein B-50 phosphorylation and dephosphorylation.