Loss of Endogenously Cycling Adult Cardiomyocytes Worsens Myocardial Function

Rationale: Endogenously cycling adult cardiomyocytes increase after myocardial infarction (MI) but remain scarce and are generally thought not to contribute to myocardial function. However, this broadly held assumption has not been tested, mainly because of the lack of transgenic reporters that restrict Cre expression to adult cardiomyocytes that reenter the cell cycle. Objective: We created and validated a new transgenic mouse, alpha MHC (alpha myosin heavy chain)-MerDreMer-Ki67p-RoxedCre (denoted alpha DKRC [cardiomyocyte-specific alpha MHC-MerDreMer-Ki67p-RoxedCre]) that restricts Cre expression to cycling adult cardiomyocytes and uniquely integrates spatial and temporal adult cardiomyocyte cycling events based on the DNA specificities of orthologous Dre and Cre recombinases. We then created alpha DKRC::DTA mice that expressed an inducible diphtheria toxin in adult cycling cardiomyocytes and examined the effects of ablating these endogenously cycling cardiomyocytes on myocardial function after ischemic-reperfusion (I/R) MI. Methods and Results: A tandem alpha DKRC transgene was designed, validated in cultured cells, and used to make transgenic mice. The alpha DKRC transgene integrated between MYH6 and MYH7 and did not disrupt expression of the surrounding genes. Compared with controls, alpha DKRC::RLTG (Rox-Lox-tdTomato-eGFP) mice treated with Tamoxifen expressed tdTomato+ in cardiomyocytes with rare Bromodeoxyuridine+, eGFP+ cardiomyocytes, consistent with reentry of the cell cycle. We then pretreated alpha DKRC::RLTG mice with Tamoxifen to activate the reporter before sham or reperfusion (I/R) MI surgeries. Compared with Sham surgery, the I/R MI group had increased single and paired eGFP+ (enhanced green fluorescent protein)+ cardiomyocytes predominantly in the border zones (5.8 +/- 0.5 versus 3.3 +/- 0.3 cardiomyocytes per 10-micron section, N=8-9 mice per group, n=16-24 sections per mouse), indicative of cycled cardiomyocytes. The single to paired eGFP+ cardiomyocyte ratio was approximate to 9 to 1 (5.2 +/- 0.4 single versus 0.6 +/- 0.2 paired cardiomyocytes) in the I/R MI group after MI, suggesting that cycling cardiomyocytes were more likely to undergo polyploidy than replication. The ablation of endogenously cycling adult cardiomyocytes in alpha DKRC::DTA (diphtheria) mice caused progressive worsening left ventricular chamber size and function after I/R MI, compared with controls. Conclusions: Although scarce, endogenously cycling adult cardiomyocytes contribute to myocardial function after injury, suggesting that these cells may be physiologically relevant.