Cell Cycle-Mediated Cardiac Regeneration in the Mouse Heart

Abstract

Purpose of review: Many forms of heart disease result in the essentially irreversible loss of cardiomyocytes. The ability to promote cardiomyocyte renewal may be a promising approach to reverse injury in diseased hearts. The purpose of this review is to describe the impact of cardiomyocyte cell cycle activation on cardiac function and structure in several different models of myocardial disease.

Recent findings: Transgenic mice expressing cyclin D2 (D2 mice) exhibit sustained cardiomyocyte renewal in the adult heart. Earlier studies demonstrated that D2 mice exhibited progressive myocardial regeneration in experimental models of myocardial infarction, and that cardiac function was normalized to values seen in sham-operated litter mates by 180 days post-injury. D2 mice also exhibited markedly improved atrial structure in a genetic model of atrial fibrosis. More recent studies revealed that D2 mice were remarkably resistant to heart failure induced by chronic elevated afterload as compared with their wild type (WT siblings), with a 6-fold increase in median survival as well as retention of relatively normal cardiac function. Finally, D2 mice exhibited a progressive recovery in cardiac function to normal levels and a concomitant reduction in adverse myocardial remodeling in an anthracycline cardiotoxicity model. The studies reviewed here make a strong case for the potential utility of inducing cardiomyocyte renewal as a means to treat injured hearts. Several challenges which must be met to develop a viable therapeutic intervention based on these observations are discussed.

Keywords: Cardiac regeneration; Cardiomyocyte renewal; Cell cycle regulation.

Figure 1.

Targeted expression of cyclin D2 results in cardiomyocyte cell cycle activity which can promote structural and functional recovery following myocardial injury. A. Schematic diagram of the MHC-cycD2 transgene (upper panel), and documentation of transgene expression (lower left panel, anti-cyclin D2 immune histology, brown signal), endogenous CDK4 expression (lower middle panel, anti-CDK4 immune histology, brown signal) and cardiomyocyte S-phase activity (lower right panel, tritiated thymidine incorporation in cardiomyocyte nuclei, black punctate signal over blue nuclear signal). (Reproduced from: Pasumarthi KB, Nakajima H, Nakajima HO, Soonpaa MH, Field LJ: Circ Res. 2005;96(1):110–8, with permission from Wolters Kluwer Health, Inc.) [14] B. Histologic analysis of WT and D2 hearts at 7 and 180 days post-infarction, sections sampled at 1 mm intervals from apex to base and stained with Azan (magenta indicates viable myocardium, grey indicates scar). (Reproduced from: Hassink RJ, Pasumarthi KB, Nakajima H, Rubart M, Soonpaa MH, de la Riviere AB et al.: Cardiovasc Res. 2008;78(1):18–25., with permission from Oxford University Press) [15] C. Cardiac function (as quantitated by dP/dt_max/EDV) in WT (open bars) and D2 (solid bars) at 7, 60 and 180 days post-infarction; data is normalized to the percent activity observed in sham-operated littermates. (Reproduced from: Hassink RJ, Pasumarthi KB, Nakajima H, Rubart M, Soonpaa MH, de la Riviere AB et al.: Cardiovasc Res. 2008;78(1):18–25., with permission from Oxford University Press) [15] D. Histologic analysis of right atria from 12 week old TGF (left panel) and TGF / D2 double transgenic (right panel) mice; sections were stained with Sirius Red and Fast Green (green indicates viable myocardium, red indicates collagen deposition). (Reproduced from: Nakajima H, Nakajima HO, Dembowsky K, Pasumarthi KB, Field LJ: Circ Res. 2006;98(1):141–8, with permission from Wolters Kluwer Health, Inc.) [17].

Figure 2.

Cyclin D2 expression blocks progression to heart failure in a chronic pressure overload model. A. Survival of WT (blue trace) and D2 (red trace) mice following TAC surgery (black trace shows survival of sham operated animals). B. Fractional shortening (FS) of WT and D2 mice 10 weeks after sham or TAC surgery as measured by echocardiography. C. Detection of p-H3 immune reactivity in cardiomyocyte nuclei (left panel, black punctate signal over blue nuclear signal) and quantitation of levels in D2 mice 10 weeks after sham or TAC surgery. D. Cardiomyocyte number determined from dispersed cell preparations of WT and D2 hearts 10 weeks after sham or TAC surgery. E. Activated caspase 3-positive cardiomyocytes per mm² tissue in WT and D2 mice at 3 weeks after sham or TAC surgery. F. Histologic analysis of WT (left panel) and D2 (right panel) mice 10 weeks after TAC; sections were stained with Sirius Red and Fast Green (green indicates viable myocardium, red indicates collagen deposition). (Reproduced from: Toischer K, Zhu W, Hunlich M, Mohamed BA, Khadjeh S, Reuter SP et al.: J Clin Invest. 2017;127(12):4285–4296, with permission from American Society for Clinical Investigation) [18].

Figure 3.

Cyclin D2 expression restores cardiac function in a juvenile anthracycline cardiotoxicity model. A. Cardiac function in saline-treated WT (black trace), saline-treated D2 (magenta), DOX-treated WT (red trace) and DOX-treated D2 (blue trace) mice at various time points in the study; vertical arrows indicate the time of saline or DOX injections. B. Quantitation of tritiated thymidine incorporation and activated caspase 3 immune reactivity in DOX-treated WT and D2 mice at the acute and late stages; results are plotted as events per mm² to allow direct comparison of the results. C. Histologic analysis of acute and late stage WT and D2 hearts 13 weeks after DOX treatment; sections were stained with Sirius Red and Fast Green (green indicates viable myocardium, red indicates collagen deposition). (Reproduced from: Zhu W , Reuter S, Field LJ.: Cardiovasc Res. 2019;115(5):960–5, with permission from Oxford University Press) [19].