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Myosin isozyme transitions occurring during the postnatal development of the rat soleus muscle
Insights
Developing rat soleus muscles show myosin isozyme transitions early after birth. A preprogrammed sequence of embryonic, neonatal, and adult fast myosin occurs, with potential for slow myosin accumulation.
Area of Science:
- Muscle physiology
- Developmental biology
- Biochemistry
Background:
- Myosin isozymes are critical for muscle function and adapt during development.
- The soleus muscle's myosin composition changes significantly from birth to adulthood.
Purpose of the Study:
- To investigate myosin isozyme expression in rat soleus muscle during the first six weeks of postnatal development.
- To understand the sequence and cellular basis of myosin isozyme transitions in developing skeletal muscle.
Main Methods:
- Biochemical analysis (electrophoresis) of native myosin.
- Immunological techniques including immunotransfer and immunocytochemistry.
- Use of specific polyclonal antibodies against embryonic, neonatal, and adult fast and slow myosins.
Main Results:
- Adult slow myosin is present in the soleus at 1 week post-birth, alongside embryonic and neonatal myosin.
- Neonatal myosin heavy chain decreases between 2-3 weeks and is absent in adults.
- Immunocytochemistry revealed cellular heterogeneity and distinct myosin isozyme combinations in fibers during development, indicating transitions.
Conclusions:
- Myosin isozyme transitions in the developing rat soleus follow a preprogrammed sequence: embryonic → neonatal → adult fast.
- Fibers initially expressing embryonic and slow myosin eventually express only slow myosin.
- Other fibers transition from embryonic/neonatal to adult fast myosin, with some later accumulating slow myosin, suggesting developmental plasticity.
Abstract:
The myosin isozymes present in the developing rat soleus muscle from 1 week to 6 weeks after birth were investigated using biochemical and immunological methods. Electrophoresis of native myosin reveals that adult slow myosin is present in the soleus as early as 1 week after birth. At this time, embryonic and neonatal myosin can also be demonstrated. Using an immunotransfer technique, the presence of slow myosin heavy chain can be demonstrated at all time points examined whereas neonatal myosin heavy chain diminishes in quantity between 2 and 3 weeks, and is undetectable in the adult soleus. Specific polyclonal antibodies were prepared to embryonic, neonatal, and adult fast and slow myosins. Immunocytochemistry reveals a cellular heterogeneity at all stages examined. Different combinations of myosin isozymes can be found in the soleus fibers depending on the stage of development; these results suggest therefore that myosin isozyme transitions are occurring. Approximately half the fibers contain embryonic and slow myosin at 1 week after birth; these fibers subsequently contain only slow myosin. A second group of fibers contains embryonic and neonatal myosin at 1 week and most of them subsequently accumulate adult fast myosin. A portion of this latter group begins to acquire slow myosin from 4 weeks of age. These data are interpreted to suggest that a preprogrammed sequence of myosin isozymes is embryonic----neonatal----adult fast. At any time during development of an individual fiber, induction of slow myosin accumulation and repression of other types can occur.