360 ISSN 1990-519X, Cell and Tissue Biology, 2019, Vol. 13, No. 5, pp. 360–365. © Pleiades Publishing, Ltd., 2019. Russian Text © The Author(s), 2019, published in Tsitologiya, 2019, Vol. 61, No. 4, pp. 319–325. Redistribution of Sarcomeric Myosin and α-Actinin in Cardiomyocytes in Culture upon the Rearrangement of their Contractile Apparatus N. B. Bildyug a, * and S. Yu. Khaitlina a a Institute of Cytology, Russian Academy of Sciences, St. Petersburg, 194064 Russia *e-mail: nbildyug@gmail.com Received January 10, 2018; revised January 28, 2019; accepted January 28, 2019 Abstract—Cardiomyocytes in culture undergo reversible rearrangement of their contractile apparatus with conversion of typical myofibrils into structures resembling stress fibers of nonmuscle cells. Such rearrange- ment is accompanied by the replacement of cardiac actin, the main protein of myofibrils, with its smooth muscle isoform. This study shows that along with the replacement of actin isoform the key structural sarco- meric proteins are released from actin structures and stored in cell cytoplasm as inclusions not bound with actin. The data obtained are indicative of the incompatibility of smooth muscle actin with sarcomeric iso- forms of these proteins and myofibrillar organization in general. Keywords: cardiomyocytes in culture, contractile apparatus, myosin, α-actinin, actin DOI: 10.1134/S1990519X1905002X INTRODUCTION Cardiomyocytes (CMs) are heart muscle cells responsible for heart contraction due to a highly orga- nized contractile apparatus represented by myofibrils, which are composed of sarcomeres, these being struc- tural and functional units. Fibrillar actin forms thin filaments of myofibrils, whereas myosin forms thick filaments and provides motor function. Another important structural protein of myofibrils is α-actinin, the main protein of Z-disks. It forms cross bridges between actin filaments and thus binds individual sar- comeres. The myofibrillar apparatus of CMs is considered to be a generally stable system. However, during cardio- genesis, myofibrils are formed from dynamic cytoskel- etal structures, since the precursors of CMs are non- muscle cells. In addition, myofibrillar apparatus of CMs can be reorganized during pathological processes in the heart. Although the processes of CM differentiation, as well as pathological changes, are well described, the mechanisms underlying the rearrangements of their contractile apparatus remain unclear. Investigation of these mechanisms may contribute to understanding how the dynamics of the contractile system in CMs is regulated in vivo and allow its reorganization to be controlled in the future. CMs in primary culture are a good model to inves- tigate alterations of the contractile system in vitro. These cells undergo a reversible rearrangement of their contractile apparatus with conversion of typical myo- fibrils into nonstriated structures resembling stress fibers of nonmuscle cells (Nag and Cheng, 1981; Bor- isov et al., 1989, Bildyug and Pinaev, 2013, Bildyug et al., 2016). The data from the literature and our previous results show that the rearrangement of contractile apparatus in CMs in culture is accompanied by switching of actin isoforms with a transient expression of nonsarcomeric smooth muscle α-actin (van Bilsen and Chien, 1993; Schaub et al., 1997; Bildyug et al., 2016), which is normally restricted to smooth muscle cells and myofibroblasts and is also expressed in CMs during embryogenesis (Vandekerckhove et al., 1986; Ruzicka and Schwartz, 1988; Handel et al., 1991; van Bilsen and Chien, 1993) and pathological changes (Winegrad et al., 1990; Clément et al., 1999). Along with actin, myofibrils contain many actin-interacting proteins. Our study was aimed at investigating the dis- tribution of two key structural proteins of myofibrils, myosin and α-actinin, in CMs at the stages corre- sponding to the initial organization of their contractile apparatus and its total rearrangement with the disap- pearance of myofibrils and the replacement of cardiac actin with its smooth muscle isoform. Abbreviations: CM—cardiomyocyte, PBS—phosphate buffered saline.