Elastic filaments in situ in cardiac muscle: Deep-etch replica analysis in combination with selective removal of actin and myosin filaments

Takashi Funatsu, Eiji Kono, Hideo Higuchi, Sumiko Kimura, Shin'ichi Ishiwata, Toshitada Yoshioka, Koscak Maruyama, Shoichiro Tsukita

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    Abstract

    To clarify the full picture of the connectin (titin) filament network in situ, we selectively removed actin and myosin filaments from cardiac muscle fibers by gelsolin and potassium acetate treatment, respectively, and observed the residual elastic filament network by deep-etch replica electron microscopy. In the A bands, elastic filaments of uniform diameter (6-7 nm) projecting from the M line ran parallel, and extended into the I bands. At the junction line in the I bands, which may correspond to the N2 line in skeletal muscle, individual elastic filaments branched into two or more thinner strands, which repeatedly joined and branched to reach the Z line. Considering that cardiac muscle lacks nebulin, it is very likely that these elastic filaments were composed predominantly of connectin molecules; indeed, anti-connectin monoclonal antibody specifically stained these elastic filaments. Further, striations of ∼4 nm, characteristic of isolated connectin molecules, were also observed in the elastic filaments. Taking recent analyses of the structure of isolated connectin molecules into consideration, we concluded that individual connectin molecules stretched between the M and Z lines and that each elastic filament consisted of laterally-associated connectin molecules. Close comparison of these images with the replica images of intact and S1-decorated sarcomeres led us to conclude that, in intact sarcomeres, the elastic filaments were laterally associated with myosin and actin filaments in the A and I bands, respectively. Interestingly, it was shown that the elastic property of connectin filaments was not restricted by their lateral association with actin filaments in intact sarcomeres. Finally, we have proposed a new structural model of the cardiac muscle sarcomere that includes connectin filaments.

    Original languageEnglish
    Pages (from-to)711-724
    Number of pages14
    JournalJournal of Cell Biology
    Volume120
    Issue number3
    Publication statusPublished - 1993 Feb

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    ASJC Scopus subject areas

    • Cell Biology

    Cite this

    Funatsu, T., Kono, E., Higuchi, H., Kimura, S., Ishiwata, S., Yoshioka, T., Maruyama, K., & Tsukita, S. (1993). Elastic filaments in situ in cardiac muscle: Deep-etch replica analysis in combination with selective removal of actin and myosin filaments. Journal of Cell Biology, 120(3), 711-724.