Reduced Cardiac Transmembrane Protein 65 Resulted In Dilated Cardiomyopathy And Progressive Cardiac Fibrosis In Vivo

Intercalated discs (ICDs) are unique and functionally indispensible to the heart, but its structural organization remains less understood. Previously, we showed that an ICD-bound transmembrane protein 65 (Tmem65) was required for Connexin 43 (Cx43) localization in cultured mouse neonatal cardiomyocytes, and that reduced Tmem65 was associated with a decrease and internalization of Cx43, and impaired electrical conduction between neighboring cardiomyocytes. Here, we investigated the role of Tmem65 in vivo by injecting CD1 mice with recombinant adeno-associated virus 9 (rAAV9) harboring Tmem65 (or scrambled) shRNA. Quantitative polymerase chain reactions and immunoblots confirmed greater than 90% reduction in Tmem65 expression in mouse ventricles compared to control samples. Immunoblots and immunofluorescence showed reduced and internalized Cx43 in Tmem65 knockdown (KD) hearts compared to controls, respectively. Kaplain-Meier survival plot showed that all Tmem65 KD mice died within 7 weeks (> 50% death 3 weeks post viral injection), whereas no death was seen in control mice. Tmem65 KD mice developed eccentric hypertrophic cardiomyopathy in 3 weeks and dilated cardiomyopathy with severe cardiac fibrosis in 7 weeks as confirmed by H&E and Masson’s Trichrome staining. Echocardiography also confirmed ventricular dilatation and showed a 60% reduction in cardiac output (19.27±1.46 mL/min in control vs. 6.63±0.52 mL/min in Tmem65 KD mice, p<0.01, n = 6 per group) at 7 weeks. Transmission electron microscopy showed an altered ICD pattern and disorganized myofibril structure in Tmem65 KD ventricles while control tissues had smooth ICDs and organized myofibers. Together, these findings suggest a critical role of Tmem65 in maintaining cardiac ICD and myofibers and loss of Tmem65 leads to cardiomyopathy in vivo .