Cellular and Molecular Events During Myofibrillogenesis
The heart is the first organ to differentiate and function in the embryo. Cardiac muscle cells synthesize sarcomeric proteins, which assemble into myofibrils. These early myofibrils are responsible for the first contractions and, later, for the continuous contractions of the growing heart with full capability of undergoing mitosis and cytokinesis. Upon cell division, myofibrils can disassemble into their component filaments and molecules, some of which are used to form the contractile cleavage furrow used in cytokinesis (see Research Interests).
The precise developmental cascade of myfibrillogenesis is poorly understood. As the spreading edges of the cell surface advance, the previously formed premyofibrils are left in place, which would permit other large proteins (e.g., muscle myosin II and titan) to gain access to the minisarcomeres. Premyofibrils are not, as some have suggested, breakdown products of mature myofibrils. The Z-bodies of the premyofibrils appear to fuse laterally with one another to form the Z-bands of mature myofibrils. With increasing time, mature myofibrils and Z-bands emerge in regions where linear arrays of Z-bodies had been. A proposed three stage model of myofibril formation progresses with premyofibrils to nascent myofibrils to mature myofibrils. Premyofibrils, found at the spreading edges of the cardiomyocytes, are composed of minisarcomeres. The sarcomeric equivalent of the Z-band in the premyofibril is a Z-body; both contain the cytoskeletal protein alpha actinin (100 kDA). This protein is found concentrated in stress fibers, Z-bodies and Z-bands of muscles marking the boundaries of the sarcomeric untis in nonmuscle stress fibers and in muscle myofibrils. Our results have demonstrated premyofibrils precede in space and time to mature myofibrils. These beaded Z-bodies are attached to the cell surface and are responsible for the attachment of the short actin filaments to the cell surfaces. Most importantly, premyofibrils contain nonmuscle myosin IIB, which is believed to be responsible for the antipolar arrangement of the actin filaments in these minisarcomeres.
Rhee et al. presented evidence that the premyofibrils are transformed into nascent myofibrils with the capture of muscle thick filaments by the actin filaments and the newly added muscle thtin molecules to the Z-bodies. The fusion of adjacent premyofibrils to form nascent myofibrils occurs at the level of the Z-bodies and is marked by the association of at least two proteins, titin and zeugmatin. We have shown that zeugmatin, which can be detected in the fused regions of the Z-bodies in the nascent myofibrils, is part of the titin molecule embedded in the Z-band. We suggest the possibility that the widely reported ability of cardiomyocytes in hypertrophic hearts to reinitiate the synthesis of fetal sarcomeric proteins may be related to the reinitiation of the embryonic program for myofibrillogenesis, that is the premyofibril model.
During the transition from premyofibril to myofibril, it is postulated that there is an exchange of nonmuscle myosin IIB filaments for muscle myosin II filaments and a growth and fusion of Z-bodies into Z-bands. The Z-bodies appear initially as discrete aggregates of alph actinin along the premyofibrils. As the myofibrils increase in width, the Z-bands appear to be composed of laterally aligned Z-bodies and finally continuous bands of alpha actinin. A second model of myofibrillogenesis proposes that the first fibrils that form at the periphery of spreading cardiomyocytes act as temporary scaffolds along which myofibrils assemble. Finally, there is a third hypothesis that spatially separate complexes of actin filaments and Z-bands, IZI brushes, and groups of myosin thick filaments assemble independently of one another and become spliced together by titin filaments and then inserted at the ends of fully formed myofibrils.
Time-lapse observations of sarcomeric proteins in single cells undergoing myofibrillogenesis allows aspects of these models to be tested. Our laboratory uses GFP (green fluorescent protein) to observe various muscle peoteins in living cells for extended periods of time. We have generated GFP fusion proteins to numberous cytodkeletal proteins and observed them in spreading embryonic chicken cardiomyocytes to follow the complex regulation of myofibrillogenesis.