The identification of muscle protein isoforms has
become an important approach used by investigators in the
field to distinguish muscle fiber types. Further, with the use
of techniques such as protein electrophoresis and immunereactivity
to antibodies it has been demonstrated that a
single muscle fiber may express, simultaneously, more than
one type of myosin heavy chain; for example type I and IIa
or IIa and IIx together. A shift in the expression of myosin
heavy chain along the length-axis of a single fiber can
result from the transcription of different mRNA’s in different
nuclear domains. These so-called hybrid fibers have
been shown to increase with exercise, aging [27], and in
some pathological conditions. Isoforms of many muscle
proteins other than myosin have been identified and associated
with specific muscle fiber types [12]. For example
slow and fast troponin T isoforms are known to exist in
type I and II fibers, respectively [23]. Several myogenic
transcription factors such as MyoD and myogenin play a
significant role in the synthesis of muscle proteins that are
specific for each of the fiber types. This process of fiber
differentiation is influenced also by factors external to
muscle such as the levels of thyroid hormones and the
activity pattern of peripheral nerves.
The identification of muscle protein isoforms hasbecome an important approach used by investigators in thefield to distinguish muscle fiber types. Further, with the useof techniques such as protein electrophoresis and immunereactivityto antibodies it has been demonstrated that asingle muscle fiber may express, simultaneously, more thanone type of myosin heavy chain; for example type I and IIaor IIa and IIx together. A shift in the expression of myosinheavy chain along the length-axis of a single fiber canresult from the transcription of different mRNA’s in differentnuclear domains. These so-called hybrid fibers havebeen shown to increase with exercise, aging [27], and insome pathological conditions. Isoforms of many muscleproteins other than myosin have been identified and associatedwith specific muscle fiber types [12]. For exampleslow and fast troponin T isoforms are known to exist intype I and II fibers, respectively [23]. Several myogenictranscription factors such as MyoD and myogenin play asignificant role in the synthesis of muscle proteins that arespecific for each of the fiber types. This process of fiberdifferentiation is influenced also by factors external tomuscle such as the levels of thyroid hormones and theactivity pattern of peripheral nerves.
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