Using cloned DNA probes specific for two isoforms of cardiac myosin light chains (MLCs), nonphosphorylatable MLC1 and phosphorylatable, regulatory MLC2, we have observed that the MLC1 messenger RNA of ventricular type does not appear in detectable amounts in atrial cells of either normotensive Wistar-Kyoto rat strain (WKY) or spontaneously hypertensive rat strain (SHR). The messenger RNA of regulatory isoform of ventricular MLC2, on the other hand, is found in threefold excess in atria of SHR relative to that of age-matched WKY. The increased level of MLC2 messenger RNA is present even in 6-week-old SHR atria where there is no established overloading of the heart. Thus, it appears that the increased expression of the regulatory MLC2 gene in SHR atrial cells is a predetermined event, which, most likely, participates in functional adaptation of the myocardium in response to pressure overload and subsequent hypertrophy.
The emergence of abundant-class mRNAs specific for contractile muscle proteins and their distribution between polysomal and free mRNP fractions were studied in skeletal muscle excised from chicken embryos during the transition from myoblasts (day 9) to myotubes (day 18). Muscle-specific cDNA was selectively prepared by hybridizing cDNA to template RNA (polysomal poly(A)+mRNA) from day-14 embryos followed by isolation ofthe abundant class, which represents %20% oftotal mRNA. The specificity of the cDNA probe for this class was confirmed by the differential degree of hybridization to cytoplasmic RNA from cultured myotube and myoblast cells and by its inability to hybridize with mRNA from nonmuscle cells such as liver. Except for muscle from day-9 embryos, the concentrations of the abundant-class muscle-specific mRNAs were higher in polysomes than in free mRNP fractions. Furthermore, the levels ofthese mRNAs in polysomes increased 12-fold from day 9 (myoblast) to day 14 (intermediate) with a further 3.6-fold increase from day 14 to day 18 (myotube). In contrast to this 45-fold net increase in the polysomal level of these mRNAs from day 9 to day 18, the levels in the free mRNP fraction showed only a 3-fold decrease during this period. Because the amount of mRNA lost from the mRNP fraction is much less than the net increase in the polysome fraction, mRNP does not serve as a reservoir of untranslated muscle-specific mRNA for transfer to polysomes. Consequently, the emergence of muscle-specific polysomal mRNA for contractile proteins during myogenesis in ovo appears to be regulated primarily by transcriptional control.In embryonic muscle, the mononucleated myoblast cells replicate for two or three cycles to become postmitotic mononucleated myoblasts, which then fuse and form multinucleated myotubes. After fusion there is a several-hundred-fold increase in synthesis of muscle-specific contractile (myofibrillar) proteins. At issue is whether this change in pattern ofprotein synthesis during myogenesis is regulated at the transcriptional or translational level. Much of the evidence relates to muscle that differentiates in tissue culture. In vitro translation shows that the mRNAs coding for the myofibrillar proteins in cultures of quail breast muscle are present in negligible amounts in the dividing myoblasts and accumulate coordinately after fusion, suggesting transcriptional regulation (1). A similar conclusion was based on studies of the abundance of mRNA for myosin heavy chain in myoblasts and myotubes in chicken embryonic muscle culture that use either translation of mRNA (2) or in situ hybridization with a cDNA probe (3). Other studies show that the transition from the myoblast to myotube stage in primary cultures ofchicken embryonic muscle (4) and rat skeletal muscle (5) is associated with the appearance of a new abundant-class population of mRNA consisting of at least six different mRNA sequences-the increased synthesis of myofibrillar proteins at or after fusion being coincident with the appearance ofthis...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.