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J. Cell Biol.,
Volume 141, Number 4, May 18, 1998 943-953
* Department of Microbiology and Immunology, Myosin in adult murine skeletal muscle is
composed primarily of three adult fast myosin heavy
chain (MyHC) isoforms. These isoforms, MyHC-IIa,
-IId, and -IIb, are >93% identical at the amino acid
level and are broadly expressed in numerous muscles, and their genes are tightly linked. Mice with a null mutation in the MyHC-IId gene have phenotypes that include growth inhibition, muscle weakness, histological
abnormalities, kyphosis (spinal curvature), and aberrant kinetics of muscle contraction and relaxation. Despite the lack of MyHC-IId, IId null mice have normal
amounts of myosin in their muscles because of compensation by the MyHC-IIa gene. In each muscle examined
from IId null mice, there was an increase in MyHC-IIa-
containing fibers. MyHC-IIb content was unaffected in
all muscles except the masseter, where its expression was extinguished in the IId null mice. Cross-sectional fiber areas, total muscle cross-sectional area, and total fiber number were affected in ways particular to each
muscle. Developmental expression of adult MyHC
genes remained unchanged in IId null mice. Despite this universal compensation of MyHC-IIa expression,
IId null mice have severe phenotypes. We conclude
that despite the similarity in sequence, MyHC-IIa and
-IId have unique roles in the development and function
of skeletal muscle.
Department of Molecular Genetics, Albert Einstein College of Medicine, New
York 10461; and § Department of Molecular, Department of Kinesiology, University of
Colorado, Boulder, Colorado 80309
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