© 1997 American Heart Association, Inc.
Articles |
Skeletal Muscle–Specific Myosin Binding Protein-H Is Expressed in Purkinje Fibers of the Cardiac Conduction System
From the Department of Cell Biology and Anatomy, Cornell University Medical College, New York, NY.
Correspondence to Dr Takashi Mikawa, Department of Cell Biology and Anatomy, Cornell University Medical College, 1300 York Ave, New York, NY 10021. E-mail tmikaw{at}mail.med.cornell.edu
Abstract Heart contraction is coordinated by conduction of
electrical excitation through specialized tissues of the cardiac
conduction system. By retroviral single-cell tagging and lineage
analyses in the embryonic chicken heart, we have recently
demonstrated that a subset of cardiac muscle cells terminally
differentiates as cells of the peripheral conduction system
(Purkinje fibers) and that this occurs invariably in perivascular
regions of developing coronary arteries. Cis
regulatory elements that function in transcriptional regulation of
cells in the conducting system have been distinguished from those in
contractile cardiac muscle cells; eg, 5' regulatory sequences of the
desmin gene act as enhancer elements in skeletal muscle and in the
conduction system but not in cardiac muscle. We hypothesize that
Purkinje fiber differentiation involves a switch of the gene expression
program from that characteristic of cardiac muscle to one typical of
skeletal muscle. To test this hypothesis, we examined the expression of
myosin binding protein-H (MyBP-H) in Purkinje fibers of chicken hearts.
This unique myosin binding protein is present in skeletal but not
cardiac myocytes. A site-directed polyclonal antibody (AB105) was
generated against MyBP-H. Immunohistological
analysis of the myocardium mapped the AB105 antigen
predominantly to A bands of myofibrils within Purkinje fibers. Western
blot analysis of whole extracts from the
ventricular wall of adult chicken hearts revealed that the
AB105 epitope was restricted to a single protein of 
86 kD, the same
size as MyBP-H in skeletal muscle. Biochemical properties of the
Purkinje fiber 86-kD protein and RNase protection analyses of
its mRNA indicate that Purkinje fiber 86-kD protein is
indistinguishable from skeletal muscle MyBP-H. The results provide
evidence that skeletal muscle MyBP-H is expressed in a subset of
cardiac muscle cells that differentiate into Purkinje fibers of the
heart.
Key Words: Purkinje fiber • heart conduction system • myosin binding protein-H • myofibril
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