Abstract 1703: Role of Obscurin in Cardiomyocyte Hypertrophy: Z-Disk and A-Band Patterning during Premyofibril to Myofibril Transition
Obscurin is a giant muscle protein that is localized in the M-line areas of A-bands and in Z-disks of myofibrils. We assessed the hypothesis that during cardiac hypertrophic growth obscurin is involved in assembly of sarcomeres and in lateral fusion of newly formed premyofibrillar bundles at the time of their differentiation into myofibrils. Given the fact that this protein is present in several sarcomeric regions, we examined the dynamics of its incorporation in nascent myofibrils with special reference to Z-disk and A-band assembly in cultures of rat cardiac muscle cells during hypertrophic response induced by phenylephrine. High resolution microscopy detected early presence of obscurin at the level of the middle regions of the A-bands of newly formed sarcomeres. Obscurin incorporation in the A-bands occurred nearly simultaneously with assembly of myosin filaments. We found the presence of obscurin associated with the nascent A-bands both in the open I-A-band interfaces, and between the complete Z- to Z-band outlines of differentiating sarcomeres. This process intensely progressed before the development of morphologically mature M-lines. Our ultrastructural studies confirmed the fact that assembly and elongation of myosin filaments into nascent sarcomeres preceded the formation of M-line, and that this process can occur within open I-A-band interface before the addition of the next Z-disk. Incorporation of obscurin at the level of Z-disks occurred at the stage of lateral fusion of their dot-like structural precursors into the transverse striated pattern during premyofibril-to-myofibril transition and rapidly followed its inclusion into the mid-regions of the A-bands. These results allow us to conclude that 1) obscurin incorporation is directly related to the process of assembly of Z-disks and A-bands and is one of the earliest events in M-line morphogenesis, 2) involvement of obscurin in different topographical regions of nascent myofibrils follows distinct temporal pattern. These conclusions are supported by our recent findings concerning the effect of downregulation of obscurin on the progression of myofibril assembly. Thus, obscurin is an indispensable structural protein directly involved in the process of myocardial hypertrophy.