Duchenne muscular dystrophy (DMD) is the most common form of the inherited muscular dystrophies. Cardiomyopathy is present in about 90% of patients and heart failure accounts for at least 20% of DMD-associated deaths. Dystrophin-deficient cardiomyopathy recapitulates many of the contractile phenotypes found in the majority of patients with end-stage dilated heart failure stemming from a variety of etiologies. Therefore, treatment targeting dystrophin-deficient cardiomyopathy may also be useful beyond the field of muscular dystrophy. New types of therapy can not be developed without understanding the molecular and cellular processes that contribute to heart failure. We set out to assess how contractility is dynamically regulated in healthy myocardium. We also used animal models of DMD to explore novel therapeutic treatments of cardiomyopathy.
We investigated the beat-to-beat regulation of cardiac contractility in healthy myocardium under near physiological conditions. We developed a random cycle length (CL) approach, during which the twitch contractions of 5 different CLs were randomized around a physiological stimulation baseline. It was shown that the history of at least 3 CLs prior to a contraction influences myocardial contractility. The pattern of CL contribution to a given twitch is different between rat and dog, which have substantial differences in calcium handling. This suggests that investigation of calcium handling on a beat-to-beat basis will provide us with more insights into dynamic regulation of cardiac contractility. Calcium indicator bis-fura-2 was used to acquire calcium transients. Our data indicates that the changes in calcium transients are minor compared to the dramatic changes in contractile force in the cycle lengths protocol. The absolute systolic Ca2+ ion concentration is not the sole determinant in the calcium-force relationship in the isometric twitch of an intact muscle.
We also explored novel therapeutic treatments of cardiomyopathy in DMD. Our data showed that inhibition of the NF-kappaB pathway using a NEMO Binding Domain (NBD) peptide and a virus-based gene delivery to sustain levels of claudin-5 protein in the dystrophin-utrophin double knockout (dko) mouse increased cardiac contractile force. The treatments also improved key governing mechanisms of contractile force, which are typically impaired in patients with heart failure, including force-frequency behavior and the response to beta-adrenergic stimulation. Further investigation is warranted to elucidate the mechanisms leading to the improvements of cardiac contractile function by these novel treatments.
Finally, we developed a lengthening-contraction model in vitro to facilitate the investigation of mechanical stress in myocardium by exerting various stretch-release ramps while the muscle is contracting. We tested our model in age matched (young and adult) dystrophin-deficient mdx and wild type mouse right ventricular trabeculae. The peak isometric active developed tension (Fdev, in mN/mm2) and kinetic parameters such as time to peak tension (TTP, in ms) and time from peak tension to half-relaxation (RT50, in ms) were measured. Our results indicate lengthening-contractions significantly impact contractile behavior, and that dystrophin-deficient myocardium in mdx mice is significantly more susceptible to these damaging lengthening-contractions. This model will be helpful to further understand the link between mechanical stress and molecular remodeling in the progress of cardiac malfunction.