Small GTPases RhoA and RhoC coordinately regulate cardiomyocyte size in the mouse heart

Rho family GTPases regulate cardiomyocyte growth and cytoskeletal organization, yet the overlapping and individual function of the two primary isoforms RhoA and RhoC, remain largely unknown in the heart. Here, we examined cardiac structure and function at baseline and in response to mechanical and neurohumoral stress in mice with cardiomyocyte-specific deletion of Rhoa , global loss of Rhoc , or combined deficiency of both isoforms. Deletion of either Rhoa or Rhoc increased myocardial expression of the other isoform, showing reciprocal compensation. At baseline, Rhoc deficiency produced elongated, narrower cardiomyocytes, whereas additional loss of Rhoa restored cell width but length remained longer. Combined Rhoa/Rhoc deficiency also disrupted intercalated disc protein expression and organization and caused age-dependent cardiac hypertrophy with mild pulmonary congestion despite preserved systolic function. Following pressure overload, loss of either isoform increased hypertrophy and fibrosis compared with control groups, while combining Rhoa/Rhoc deficiency significantly worsened cardiac remodeling and systolic dysfunction. Neurohumoral stimulation with angiotensin II and phenylephrine similarly revealed cooperative control of hypertrophic growth in Rhoa/Rhoc double deleted cardiomyocytes versus either single deletion. These findings identify RhoA and RhoC as partially compensatory effectors of cardiomyocyte growth and ventricular remodeling with stress stimulation, although their individual contributions to hypertrophic growth, structural integrity, and cardiac function depend on both the nature and duration of cardiac stress. KEY POINTS SUMMARY Cardiac hypertrophy is regulated by small GTPase proteins including RhoA, but the role of the closely related protein RhoC in the heart remains poorly defined. Using mice with genetic deletion of Rhoa and Rhoc , we show that they compensate for one another, and that loss of Rhoc alone alters the size and shape of cardiomyocytes even without added stress. Loss of Rhoc , but not Rhoa , reduced heart function within two weeks of a sudden increase in workload, whereas Rhoa was important with prolonged workload. Combined loss of Rhoa and Rhoc in cardiomyocytes produces significantly greater hypertrophy to neurohumoral signals compared with deletion of either gene alone. These findings suggest that RhoA and RhoC play both overlapping and non-redundant roles in the heart’s response to stress, with implications for understanding and treating cardiac hypertrophy and failure.