Cardiomyocyte nuclearity and ploidy: when is double trouble?

Considerable effort has gone into investigating mechanisms that underlie the developmental transition in which mammalian cardiomyocytes (CMs) switch from being able to proliferate during development, to essentially having lost that ability at maturity. This problem is interesting not only for scientific curiosity, but also for its clinical relevance because controlling the ability of mature CMs to replicate would provide a much-needed approach for restoring cardiac function in damaged hearts. In this review, we focus on the propensity of mature mammalian CMs to be multinucleated and polyploid, and the extent to which this may be necessary for normal physiology yet possibly disadvantageous in some circumstances. In this context, we explore whether the concept of the myonuclear domain (MND) in multinucleated skeletal muscle fibers might apply to cardiomyocytes, and whether cardio-MND size might be related to the transition of CMs to become multinuclear. Nuclei in CMs are almost certainly integrators of not only biochemical, but also-because of their central location within the myofibrils-mechanical information, and this multimodal, integrative function in adult CMs-involving molecules that have been extensively studied along with newly identified possibilities-could influence both gene expression as well as replication of the genome and the nuclei themselves.

Do different nuclei in a binucleated cardiomyocyte have different rates of nuclear protein import?

The functional significance of having two nuclei in a cell is unknown. Having two stores of genetic material may be advantageous for cell growth. Nuclear protein import is at a critical juncture in the cell to modify cell growth. This study addressed the potential for differential nuclear protein import in two nuclei of the same cell. Isolated adult rat cardiomyocytes were microinjected with an exogenous fluorescent protein conjugated with nuclear localization amino acid sequences to facilitate nuclear import and its detection. Our results demonstrate the rate of nuclear protein import was not significantly different between the two nuclei in the same cell. These data demonstrate that the two nuclei are functionally similar in a binucleated cardiomyocyte, at least as far as nucleocytoplasmic transport is concerned.