The superfast extraocular myosin (MYH13) is localized to the innervation zone in both the global and orbital layers of rabbit extraocular muscle
2002
SUMMARY Extraocular muscles (EOMs) are the most molecularly heterogeneous and
physiologically diverse mammalian striated muscles. They express the entire
array of striated muscle myosins, including a specialized myosin heavy chain
MYH13, which is restricted to extraocular and laryngeal muscles. EOMs also
exhibit a breadth of contractile activity, from superfast saccades to slow
tracking and convergence movements. These movements are accomplished by the
action of six ultrastructurally defined fiber types that differ from the type
IIa, IIb, IIx and I fibers found in other skeletal muscles. Attempts to
associate different eye movements with either the expression of different
myosins or the activity of particular EOM fiber types are complicated by the
molecular heterogeneity of several of the fiber types, and by electromyography
studies showing that the majority of extraocular motor units participate in
both fast and slow eye movements. To better understand the role of MYH13 in
ocular motility, we generated MYH13-sequence-specific antibodies and used
SDS-PAGE to quantify the regional distribution of myosin in EOM and to
characterize its heterogeneity in single fibers. These studies demonstrate
that MYH13 is preferentially expressed in the majority of orbital and global
fibers in the central innervation zone of rabbit EOM. Many individual fibers
express MYH13 with the fast IIb myosin and varying amounts of IIx myosin. The
differential localization of MYH13, coupled with specialization of the
sarcoplasmic reticulum and thin filament systems, probably explains how
activation of the endplate band region enables the majority of EOM fibers to
contribute to superfast contractions.
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