Cardiac-Specific Overexpression of Cyclin-Dependent Kinase 2 Increases Smaller Mononuclear Cardiomyocytes

Abstract

Cyclin-dependent kinase 2 (cdk2) plays a critical role in the G1- to S-phase checkpoint of the cell cycle. Adult cardiomyocytes are believed to withdraw from the cell cycle. To determine whether forced overexpression of cdk2 results in altered cell-cycle regulation in the adult heart, we generated transgenic mice specifically overexpressing cdk2 in hearts. Transgenic hearts expressed high levels of both cdk2 mRNA and catalytically active cdk2 proteins. Cdk2 overexpression significantly increased the levels of cdk4 and cyclins A, D3, and E. There was an increase in both DNA synthesis and proliferating cell nuclear antigen levels in the adult transgenic hearts. The ratio of heart weight to body weight in cdk2 transgenic mice was significantly increased in neonatal day 2 but not in adults compared with that of wild-type mice. Analysis of dispersed individual adult cardiomyocytes showed a 5.6-fold increase in the proportion of smaller mononuclear cardiomyocytes in the transgenic mice. Echocardiography revealed that transgenic heart was functionally normal. However, adult transgenic ventricles expressed beta-myosin heavy chain and atrial natriuretic factor. Surgically induced pressure overload caused an exaggerated maladaptive hypertrophic response in transgenic mice but did not change the proportion of mononuclear cardiomyocytes. The data suggest that overexpression of cdk2 promotes smaller, less-differentiated mononuclear cardiomyocytes in adult hearts that respond in an exaggerated manner to pressure overload.

The α1A/C- and α1B-adrenergic receptors are required for physiological cardiac hypertrophy in the double-knockout mouse

“…By weaning, myocyte DNA synthesis is largely complete, and subsequent growth of the heart is due to myocyte hypertrophy (1,21,22). Taken together, the findings suggest a model in which increased cardiac sympathetic innervation after weaning, increased norepinephrine release during daily life, and subsequent activation of α 1 -ARs play an important role in the physiological myocyte hypertrophy of normal postnatal development.…”

“…During the first 2 weeks of normal postnatal development, mouse myocytes undergo a final round of DNA synthesis and become binucleate by nuclear division without cell division (21). A delay in terminal differentiation can increase the number of smaller, mononuclear myocytes (22) and was a potential explanation for the reduced myocyte hypertrophy observed in the male ABKO mice. However, the majority of isolated adult male myocytes were binucleate in both genotypes (ABKO, 91% ± 0.2%; WT, 89% ± 0.7%; n = 2 hearts per genotype, 100-120 myocytes per heart), suggesting normal terminal differentiation.…”

View from the heart: cardiac fibroblasts in development, scarring and regeneration

“…Remarkable progress has been achieved on this front in the past decade by forced expression of general cell cycle regulators in adult cardiomyocytes, including cyclin A2 (Chaudhry et al, 2004;Shapiro et al, 2014;Woo et al, 2006), cyclin D1/2/3 (Hassink et al, 2008;Pasumarthi et al, 2005;Soonpaa et al, 1997) and cyclin-dependent kinase 2 (Cdk2) (Liao et al, 2001). In all cases, reactivation of the cell cycle regulator was achieved in rodent or porcine hearts using transgenic or viral approaches.…”

“…In all cases, reactivation of the cell cycle regulator was achieved in rodent or porcine hearts using transgenic or viral approaches. Cardiomyocyte-specific Cdk2 transgene overexpression is among the first strategies to show increased DNA synthesis and proliferation index in adult murine hearts (Liao et al, 2001). However, challenge to transgenic hearts using pressure overload caused maladaptive hypertrophy in transgenic animals, suggesting that this approach might not be pro-regenerative.…”

Adenoviral delivery of human CDC5 promotes G2/M progression and cell division in neonatal ventricular cardiomyocytes

“…18,19 However, these cells have been shown to be either abnormally small 18 or aneuploid 19 suggesting that components of the Cdk2-CLND1 complex alone are insufficient to drive the normal transcriptional program required for cardiomyocyte cell division. Similarly, others have demonstrated that the forced expression of G1/S transcriptional regulators, such as E2F1 or E1A, can drive cardiomyocyte cell cycle re-entry and DNA synthesis, 6,7,9 however, these transcriptionally re-programmed cells fail to complete mitosis.…”