L. Lefaucheur et al., TRANSITORY EXPRESSION OF ALPHA-CARDIAC MYOSIN HEAVY-CHAIN IN A SUBPOPULATION OF SECONDARY GENERATION MUSCLE-FIBERS IN THE PIG, Developmental dynamics, 210(2), 1997, pp. 106-116
Unlike the random distribution of fiber types seen in skeletal muscles
of most mammals, pig muscle exhibits a rosette pattern consisting of
islets of slow fibers surrounded by concentric circles of type IIA and
IIB fibers. Within each islet of slow fibers, one of the central fibe
rs is a primary myofiber, whereas all others are secondary fibers. The
present study demonstrates that a subpopulation of the slow secondary
fibers transiently expresses alpha-myosin heavy chain (MHC). Two cDNA
libraries were made from longissimus dorsi skeletal muscle of 14-day-
old piglet and adult pig atrium; the latter muscle is mainly composed
of alpha-MHC. Screening of the libraries with a human anti-alpha-MHC m
Ab (F8812F8) demonstrated the presence of positive MHC clones in both
libraries; the nucleotide sequence of the 3'-untranslated region (3'-U
TR) was identical in both libraries. As this MHC 3'-UTR had 75% homolo
gy with the human alpha-MHC, it was identified as pig alpha-MHC. Using
specific cRNA probes and mAbs against pig alpha-cardiac and beta/slow
/type I MHC, we studied the expression of these MHCs in developing pig
semitendinosus muscle by combining in situ hybridization and immunocy
tochemistry on serial sections at 90 days of gestation, and at 1, 6, 3
5 days and 6 months of age. The results showed that a subpopulation of
secondary fibers that directly abut primary fibers, transiently produ
ced alpha-MHC, both at the levels of the protein and its transcript. S
ubsequently, these fibres expressed beta-MHC. At 1 day, immunocytochem
istry showed that 16% of the secondary fibers expressed alpha-MHC, amo
ng which 20% did not yet express beta-MHC. At 6 days, alpha- and beta-
MHCs were mostly present in the same fibers, i.e., 23% of the secondar
y fibers. Thereafter, the proportion of secondary fibers reacting with
alpha-MHC mAb decreased to 10% at 5 weeks and 0% at 6 months, whereas
beta-MHC was still accumulating in about 38% of the secondary fibers.
During the period studied, the distribution of alpha- and beta-MHC tr
anscripts closely matched that of the corresponding proteins. Expressi
on of alpha-MHC was not detected in primary type I muscle fibers and s
low type I secondary fibers at the periphery of the rosettes of slow f
ibers. This study is the first unequivocal demonstration of a transito
ry expression of alpha-MHC in a subpopulation of secondary fibers in a
limb skeletal muscle during mammalian development. (C) 1997 Wiley-Lis
s, Inc.