Intracellular proteases and sarcomere disassembly in neonatal cardiomyocytes

Abstract

Mammals lose the capacity to regenerate their hearts shortly after birth. Although it is known that neonatal cardiomyocytes need to undergo sarcomere disassemble for cytokinesis, the mechanism of this is unclear. We studied this in neonatal rat ventricular myocytes (NRVM). We performed immunofluorescence studies of multiple sarcomeric proteins in proliferating NRVM using confocal microscopy and time lapse imaging. We also developed a fluorescence activated cell sorting (FACS)‐based method to separate NRVM in different cell cycle stages. During NRVM division α‐actinin, troponin I and titin were completely disassembled. Both Z‐disk and M‐band ends of titin disassemble simultaneously. Interestingly, sarcomere disassembly begins promptly at late prophase, at the same time when the nuclear envelope breaks down. Reassembly of the myofibril starts when the nuclear envelope re‐forms. Western blot from cells separated into G0G1, S and G2M phases revealed that α‐actinin, troponin I and myosin binding protein C were most prominently degraded in the G2M phase. Live cell imaging of NRVM expressing Halo‐ tagged matrix metalloproteinase‐2 revealed a strong signal inside nuclei which is released into the cytosol when the nuclear envelope breaks down. This study suggests that a protease regulated in a precisely timed manner may be involved in the disassembly of the sarcomere during mitosis.