Sequential myofibrillar breakdown accompanies mitotic division of mammalian cardiomyocytes

Abstract

The contractile tissue of the heart is composed of individual cardiomyocytes. During mammalian embryonic development, heart growth is achieved by cell division while at the same time the heart is already exerting its essential pumping activity. There is still some debate whether the proliferative activity is carried out by a less differentiated, stem cell-like type of cardiomyocytes or whether embryonic cardiomyocytes are able to perform both of these completely different dynamic tasks, contraction and cell division. Our analysis of triple-stained specimen of cultured embryonic cardiomyocytes and of whole mount preparations of embryonic mouse hearts by confocal microscopy revealed that differentiated cardiomyocytes are indeed able to proliferate. However, to go through cell division, a disassembly of the contractile elements, the myofibrils, has to take place. This disassembly occurs in two steps with Z-disk and thin (actin)-filament-associated proteins getting disassembled before disassembly of the M-bands and the thick (myosin) filaments happens. After cytokinesis reassembly of the myofibrillar proteins to their mature cross-striated pattern can be seen. Another interesting observation was that the cell-cell contacts remain seemingly intact during division, probably reflecting the requirement of intact integration sites of the individual cells in the contractile tissue. Our results suggest that embryonic cardiomyocytes have developed an interesting strategy to deal with their major cytoskeletal elements, the myofibrils, during mitosis. The complex disassembly-reassembly process might also provide a mechanistic explanation, why cardiomyocytes cede to divide postnatally.

An improved protocol for the isolation and cultivation of embryonic mouse myocytes

“…2, 4). Our observations are consistent with Ahuja et al (2004), demonstrating that myofibrils need to be disassembled during proliferation.…”

“…Nonessential amino acids are not often reported as required additive for the culture of cardiomyocyte cells (Burt 1982;Sreejit et al 2008). Though Sreejit et al (2008) and Song et al (2000) report using 1% gelatin and Ahuja et al (2004) reported using fibronectin as a pre-coat for myocyte culture dishes, such pre-coat steps are not often reported (Burt 1982;Nuss and Marban 1994;Fu et al 2005;Rosenblatt-Velin et al 2005;Nickson et al 2007). We, however, determined that a pre-coating with laminin was absolutely necessary for effective embryonic myocyte adhesion.…”

Isolation and Culture of Neonatal Mouse Cardiomyocytes

“…Even with the addition of pre-plating steps to reduce the population of cardiac fibroblasts and endothelial cells, the remaining fibroblasts will undergo cell proliferation and significantly change the cultured cell-population over time. 23,32,33 . Usage of AraC should be judged , such as phenylephrine (0.1 mM, preferentially for neonatal rat cardiomyocytes; Sigma P6126), or isoproterenol (1 μM, preferentially for neonatal mouse cardiomyocytes; Sigma I6501) greatly increases sarcomerogenesis, spreading of cardiomyocytes on the plate as well as spontaneous beating of cells.…”

Krp1 (Sarcosin) promotes lateral fusion of myofibril assembly intermediates in cultured mouse cardiomyocytes

“…During embryonic development, cardiomyocytes undergo cycles of myofibril assembly and disassembly that accompany each round of cell division [43]. During mitosis, Z-disks and actin filaments appear to disassemble before myosin filaments and myomesin ( figure 10, step 7).…”