Introduction: miR-208a, a small non-coding RNA expressed only in the heart, has a profound influence on cardiac gene expression. It has been previously demonstrated that genetic deletion of miR-208a does not affect viability or induce gross morphological heart defects, suggesting it is not required for normal cardiac growth and function (van Rooij et al., 2007; Callis et al., 2009). However, recent data from our lab indicate that miR-208a knock-out (KO) mice (developed in the laboratory of Da-Zhi Wang and subsequently maintained by inbreeding) display gross morphological cardiac abnormalities and dramatically reduced survival. Therefore, this investigation sought out to determine the mechanism(s) responsible for the maladaptive growth.
Methods: Wild-type (WT) and miR-208a KO littermate mice (3-5 mon old) were used for this investigation. Cardiac function was assessed via echocardiography, cardiac fibrosis by collagen fiber staining, and gene expression by real-time PCR and Western blotting. We also measured basal Ca2+ transients and unloaded cell shortening in isolated mouse ventricular myocytes.
Results: We observed KO mice have significantly lower survival rates, developed marked cardiac hypertrophy, fibrosis and have significantly reduced cardiac function. Compared to WT controls, cardiomyocytes from KO mice exhibited a significantly lower percent of cell shortening, slower rates of relaxation and contraction, lower amplitude, and slower kinetics (tau) of Ca2+ transients. Additionally, there was a reduced phosphorylation of phospholamban (Ser16) in miR-208 KO mice indicating a lower Ca2+ affinity of the SR Ca2+-ATPase.
Conclusions: These data provide evidence that the observed reduction in cardiac function in miR-208a KO mice is likely due, in part, to alterations in cellular Ca2+ fluxes. To our knowledge, this is the first study to demonstrate genetic deletion of miR-208a induces a maladaptive phenotype at baseline. Our findings indicate the importance of considering variables such as genetic/environmental constraints when perturbing the expression of the miR-208a in the heart.