Loss of Endogenously Cycling Adult Cardiomyocytes Worsens Myocardial Function

• Published in: Circulation research Vol. 128; no. 2; pp. 155 - 168
• Main Authors: Bradley, Leigh A, Young, Alexander, Li, Hongbin, Billcheck, Helen O, Wolf, Matthew J
• Format: Journal Article
• Language: English
• Published: United States 22.01.2021
• Subjects: Animals; Cell Cycle; Cell Proliferation; Diphtheria Toxin - genetics; Diphtheria Toxin - metabolism; Disease Models, Animal; Female; HEK293 Cells; Humans; Integrases - genetics; Integrases - metabolism; Ki-67 Antigen - genetics; Luminescent Proteins - genetics; Luminescent Proteins - metabolism; Male; Mice; Mice, Inbred C57BL; Mice, Transgenic; Myocardial Infarction - genetics; Myocardial Infarction - metabolism; Myocardial Infarction - pathology; Myocardial Infarction - physiopathology; Myocardial Reperfusion Injury - genetics; Myocardial Reperfusion Injury - metabolism; Myocardial Reperfusion Injury - pathology; Myocardial Reperfusion Injury - physiopathology; Myocytes, Cardiac - metabolism; Myocytes, Cardiac - pathology; Myosin Heavy Chains - genetics; Myosin Heavy Chains - metabolism; Peptide Fragments - genetics; Peptide Fragments - metabolism; Promoter Regions, Genetic; Time Factors; Ventricular Function, Left; Ventricular Remodeling; myocytes, cardiac; mice, transgenic; polyploidy; cell cycle; myocardial infarction
• ISSN: 0009-7330; 1524-4571
• DOI: 10.1161/CIRCRESAHA.120.318277

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. We created and validated a new transgenic mouse, (alpha myosin heavy chain) (denoted [cardiomyocyte-specific αMHC-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 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. A tandem transgene was designed, validated in cultured cells, and used to make transgenic mice. The transgene integrated between and and did not disrupt expression of the surrounding genes. Compared with controls, ) mice treated with Tamoxifen expressed tdTomato+ in cardiomyocytes with rare Bromodeoxyuridine+, eGFP+ cardiomyocytes, consistent with reentry of the cell cycle. We then pretreated 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 ≈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 (diphtheria) mice caused progressive worsening left ventricular chamber size and function after I/R MI, compared with controls. Although scarce, endogenously cycling adult cardiomyocytes contribute to myocardial function after injury, suggesting that these cells may be physiologically relevant.