POSTNATAL-DEVELOPMENT AND PLASTICITY OF SPECIALIZED MUSCLE-FIBER CHARACTERISTICS IN THE HINDLIMB

Recent progress in defining molecular components of pathways controlli ng early stages of myogenesis has been substantial, but regulatory fac tors that govern the striking functional specialization of adult skele tal muscle fibers in vertebrate organisms have not yet been identified . A more detailed understanding of the temporal and spatial patterns b y which specialized fiber characteristics arise may provide clues to t he identity of the relevant regulatory factors. In this study, we used immunohistochemical, in situ hybridization, and Northern blot analyse s to examine the time course and spatial characteristics of expression of myoglobin protein and mRNA during development of the distal hindli mb in the mouse. in adult animals, myoglobin is expressed selectively in oxidative, mitochondria-rich, fatigue-resistant myofibers, and it p rovides a convenient marker for this particular subset of specialized fibers. We observed only minimal expression of myoglobin in the hindli mb prior to the second day after birth, but a rapid and large (50-fold ) induction of this gene in the ensuing neonatal period. Myoglobin exp ression was limited, however, to fibers located centrally within the l imb which coexpress myosin isoforms characteristic of type I, IIA, and IIX fibers. This induction of myoglobin expression within the early p ostnatal period was accompanied by increased expression of nuclear gen es encoding mitochondrial proteins, and exhibited a time course simila r to the upregulaiion of myoglobin and mitochondrial protein expressio n that can be induced in adult muscle fibers by continuous motor nerve stimulation. This comparison suggests that progressive locomotor acti vity of neonatal animals may provide signals which trigger the develop ment of the specialized features of oxidative, fatigue-resistant skele tal muscle fibers. (C) 1996 Wiley-liss, Inc.