Introduction:
Cardiomyocytes carry the pump function of the heart. What molecular and cellular mechanisms control proliferation of cardiomyocytes is an unresolved question with high impact on regenerative medicine.
Hypothesis:
The growth factor neuregulin1 (NRG1) and its tyrosine kinase receptor ErbB4 control cardiomyocyte proliferation during prenatal development. NRG1 and ErbB4 are expressed in the adult heart. We hypothesized that activating NRG1 signaling stimulates cell cycle reentry and division of a subpopulation of differentiated cardiomyocytes.
Methods:
We determined cardiomyocyte cell cycle reentry and division
in vitro
using immunofluorescence microscopy and live cell imaging as read-out assays. We tested the
in vivo
proliferative effect of controlling NRG1 signaling at the level of its receptor using inducible cardiomyocyte-specific ErbB4 knockout mice and ErbB4 transgenic mice.
Results:
NRG1 induced cell cycle reentry of large, striated, and rod-shaped cardiomyocytes that express cardiac contractile proteins, consistent with a differentiated phenotype. Live cell imaging demonstrated in real time that NRG1 induces differentiated cardiomyocytes to undergo karyokinesis (nuclear division) and cytokinesis (cytoplasmic division), followed by separation into two differentiated daughter cardiomyocytes. During cell division, cardiomyocytes disassembled their contractile fibrils in the regions of the mitotic spindle and the cleavage furrow. NRG1 induced karyokinesis in 33% of mononucleated and in 1% of binucleated cardiomyocytes. Mononucleated cardiomyocytes completed cytokinesis, while binucleated cardiomyocytes did not.
In vivo,
postnatal genetic inactivation of the NRG1 receptor ErbB4 reduced cycling of differentiated mononucleated cardiomyocytes (Ctr. 5%
vs.
ko 0%,
P
< 0.01). In contrast, transgenic expression of ErbB4 increased cycling of differentiated mononucleated cardiomyocytes (Ctr. 7.2%
vs.
tg 22.6%,
P
< 0.05).
Conclusions:
ErbB4 is required and NRG1 and ErbB4 are sufficient to induce proliferation of a subpopulation of differentiated cardiomyocytes. NRG1 and the pathway that it regulates may provide new therapeutic targets to enhance mammalian cardiac regeneration.
This research has received full or partial funding support from the American Heart Association, AHA Founders Affiliate (Connecticut, Maine, Massachusetts, New Hampshire, New Jersey, New York, Rhode Island, Vermont).
Visualizing the metazoan proliferation-quiescence decision in vivo