Aims Duchenne muscular dystrophy (DMD) is a severe striated muscle disease due to the absence of dystrophin. Dystrophin deficiency results in dysfunctional sodium channels and conduction abnormalities in hearts of mdx mice. Disease progression in the mdx mouse only modestly reflects that of DMD patients, possibly due to utrophin up-regulation. Here, we investigated mice deficient in both dystrophin and utrophin [double knockout (DKO)] to assess the role of utrophin in the regulation of the cardiac sodium channel (Nav1.5) in mdx mice. Methods and results Co-immunoprecipitation studies in HEK293 cells showed that utrophin interacts with Nav1.5 via syntrophin proteins, an interaction abolished by deletion of the PDZ (PSD-95, Dlg, and Zona occludens) domain-binding motif of Nav1.5. We also provide evidence for such interaction in mouse heart using Nav1.5 C-terminus fusion proteins. In hearts of DKO mice, Nav1.5 protein levels were decreased by 25 ± 8%, together with a 42 ± 12% reduction of syntrophins compared with mdx, where utrophin was up-regulated by 52 ± 9% compared with C57BL/10 control mice. Sodium current was found to be reduced by 41 ± 5% in DKO cardiomyocytes compared with mdx, representing a loss of 63 ± 3% when compared with C57BL/10 wild-type control mice. Decreased Nav1.5 protein and current in DKO were reflected in a significant slowing of 27 ± 6% of maximal upstroke velocity of the cardiac action potential compared with mdx. Conclusion Utrophin plays a central role in the regulation of Nav1.5 in mdx mice. These findings provide support for therapeutic strategies aimed at overexpressing utrophin in the hopes of reducing cardiac pathology in DMD patient
