Cardiomyocyte Proliferation and Maturation: Two Sides of the Same Coin for Heart Regeneration

Abstract

In the past few decades, cardiac regeneration has been the central target for restoring the injured heart. In mammals, cardiomyocytes are terminally differentiated and rarely divide during adulthood. Embryonic and fetal cardiomyocytes undergo robust proliferation to form mature heart chambers in order to accommodate the increased workload of a systemic circulation. In contrast, postnatal cardiomyocytes stop dividing and initiate hypertrophic growth by increasing the size of the cardiomyocyte when exposed to increased workload. Extracellular and intracellular signaling pathways control embryonic cardiomyocyte proliferation and postnatal cardiac hypertrophy. Harnessing these pathways could be the future focus for stimulating endogenous cardiac regeneration in response to various pathological stressors. Meanwhile, patient-specific cardiomyocytes derived from autologous induced pluripotent stem cells (iPSCs) could become the major exogenous sources for replenishing the damaged myocardium. Human iPSC-derived cardiomyocytes (iPSC-CMs) are relatively immature and have the potential to increase the population of cells that advance to physiological hypertrophy in the presence of extracellular stimuli. In this review, we discuss how cardiac proliferation and maturation are regulated during embryonic development and postnatal growth, and explore how patient iPSC-CMs could serve as the future seed cells for cardiac cell replacement therapy.

Keywords: cardiac regeneration; cardiac stem cell therapy; cardiomyocyte hypertrophy; cardiomyocyte maturation; cardiomyocyte proliferation; induced pluriopotent stem cells.

FIGURE 1

Extracellular signaling and intracellular pathways drive embryonic cardiac proliferation and postnatal cardiac hypertrophy. Signaling molecules listed on the left are positive and negative regulators for embryonic cardiac proliferation whereas those on the right are essential for physiological hypertrophy in postnatal cardiomyocytes. These pathways and molecules could be prospectively manipulated to stimulate cardiomyocyte proliferation and growth for therapeutic interventions of cardiovascular disease.

FIGURE 2

Robust proliferation of human iPSC-CMs is promoted by WNT signaling activation. (A) Typical morphology of human iPSC-CMs stained with antibodies against cardiac troponin T (TNNT2, green) and α-actinin (red). Nuclei were counterstained with DAPI (blue). (B) Zoom-in sarcomere structure of human iPSC-CMs with intercalated TNNT2 (green) and α-actinin (red). (C,D) Robust proliferation of human iPSC-CMs from Day 1 (D1,C) to D7 (D) in the presence of 2 μM of a WNT activator (CHIR99021). (E,F) Dramatic increase of dividing CMs is propelled by CHIR99021. Cells were stained with Ki67 (green) and TNNT2 (red). Nuclei were counterstained with DAPI (blue). Double positive cells (indicated by white arrows) are dividing cardiomyocytes. The percentage of Ki67⁺ TNNT2⁺ cells is increased in CHIR99021-treated iPSC-CMs (F) compared to the controls (E). Scale bars: 10 μm (B); 100 μm (A,E,F); 200 μm (C,D).