Dynamic Alterations to α-Actinin Accompanying Sarcomere Disassembly and Reassembly during Cardiomyocyte Mitosis

Abstract

Although mammals are thought to lose their capacity to regenerate heart muscle shortly after birth, embryonic and neonatal cardiomyocytes in mammals are hyperplastic. During proliferation these cells need to selectively disassemble their myofibrils for successful cytokinesis. The mechanism of sarcomere disassembly is, however, not understood. To study this, we performed a series of immunofluorescence studies of multiple sarcomeric proteins in proliferating neonatal rat ventricular myocytes and correlated these observations with biochemical changes at different cell cycle stages. During myocyte mitosis, α-actinin and titin were disassembled as early as prometaphase. α-actinin (representing the sarcomeric Z-disk) disassembly precedes that of titin (M-line), suggesting that titin disassembly occurs secondary to the collapse of the Z-disk. Sarcomere disassembly was concurrent with the dissolution of the nuclear envelope. Inhibitors of several intracellular proteases could not block the disassembly of α-actinin or titin. There was a dramatic increase in both cytosolic (soluble) and sarcomeric α-actinin during mitosis, and cytosolic α-actinin exhibited decreased phosphorylation compared to sarcomeric α-actinin. Inhibition of cyclin-dependent kinase 1 (CDK1) induced the quick reassembly of the sarcomere. Sarcomere dis- and re-assembly in cardiomyocyte mitosis is CDK1-dependent and features dynamic differential post-translational modifications of sarcomeric and cytosolic α-actinin.

In vivo analysis of cardiomyocyte proliferation during trabeculation

“…A distinctive feature of cardiomyocytes is the presence of contractile filaments, called myofibrils, which are built by a repetitive arrangement of sarcomeres (Guan et al, 1999). It is thought that the limited proliferative capacity of adult mammalian cardiomyocytes is partially due to the presence of voluminous and stable mature myofibrils (Ahuja et al, 2004;Bersell et al, 2009;Fan et al, 2015). Consistent with this hypothesis, the regenerative capacity of adult zebrafish hearts upon injury is thought to be achieved through cardiomyocyte dedifferentiation with partial sarcomere disassembly followed by cardiomyocyte proliferation (Jopling et al, 2010;Kikuchi et al, 2010).…”

“…As mentioned earlier, a very distinctive feature of cardiomyocytes is the presence of contractile elements: the sarcomeres. Studies of mammalian cardiomyocyte cultures have shown that sarcomere disassembly precedes cardiomyocyte division (Ahuja et al, 2004;Engel et al, 2006;Fan et al, 2015). We wanted to address sarcomere behavior during in vivo cardiomyocyte division in the zebrafish heart.…”

Matrix metalloproteinase-2 in oncostatin M-induced sarcomere degeneration in cardiomyocytes

“…Although sarcomeric proteins are downregulated in OSM-induced dedifferentiation (20), a role of the sarcomeric protease MMP-2 implicates a proteolytic process in sarcomere degeneration. We also extended the evidence for OSM-induced sarcomere degeneration and cardiomyocyte dedifferentiation from postmitotic adult mouse cardiomyocytes (15) to rat neonatal cardiomyocytes capable of a limited degree of mitotic activity (9). Whether or not sarcomere degeneration is a sign of cardiomyocyte dedifferentiation (32), NRVM may be a useful tool to investigate this issue.…”

“…We previously showed that MMPs are not required for sarcomere disassembly during neonatal cardiomyocyte mitosis (9). The mechanism driving sarcomere disassembly in mitotic cardiomyocytes is thus likely distinct from that in OSMinduced sarcomere degeneration.…”

Cardiac Regeneration

“…However, the merits of this strategy are unknown and depend upon better understanding the “dedifferentiation” process. Although sarcomere disassembly has been suggested as a prerequisite for completion of CM mitosis 158, 180 , it is unclear whether the “dedifferentiation” observed during regeneration is a true reversal of differentiation state or simply a change of cytoskeletal architecture of CMs undergoing mitosis. Second, whether dedifferentiation will necessarily enhance proliferation is also unclear.…”