The goal of this study is to understand how a particular mutation in obscurin proteins leads to congenital heart disease.

The intercalated disc of cardiac muscle embodies a highly-ordered, multifunctional network, essential for the synchronous contraction of the heart. Over 200 known proteins localize to the intercalated disc. The challenge now lies in their characterization as it relates to the coupling of neighboring cells and whole heart function. Using molecular, biochemical and imaging techniques, we characterized for the first time two small obscurin isoforms, obscurin-40 and obscurin-80, which are enriched at distinct locations of the intercalated disc. Both proteins bind specifically and directly to select phospholipids via their pleckstrin homology (PH) domain. Overexpression of either isoform or the PH-domain in cardiomyocytes results in decreased cell adhesion and size via reduced activation of the PI3K/AKT/mTOR pathway that is intimately linked to cardiac hypertrophy. In addition, obscurin-80 and obscurin-40 are significantly reduced in acute (myocardial infarction) and chronic (pressure overload) murine cardiac-stress models underscoring their key role in maintaining cardiac homeostasis. Our novel findings implicate small obscurins in the maintenance of cardiomyocyte size and coupling, and the development of heart failure by antagonizing the PI3K/AKT/mTOR pathway.

Myosin Binding Protein-C slow (sMyBP-C) is expressed in skeletal muscles where it plays structural and regulatory roles. The functions of sMyBP-C are modulated through alternative splicing and phosphorylation. Herein, we examined the phosphorylation profile of sMyBP-C in mouse slow-twitch soleus muscle isolated from fatigued or non-fatigued young (2-4-months old) and old (~14-months old) wild type and mdx mice. Our findings are two-fold. First, we identified the phosphorylation events present in individual sMyBP-C variants at different states. Secondly, we quantified the relative abundance of each phosphorylation event, and of sMyBP-C phospho-species as a function of age and dystrophy, in the presence or absence of fatigue. Our results revealed both constitutive and differential phosphorylation of sMyBP-C. Moreover, we noted a 10–40% and a 25–35% reduction in the phosphorylation levels of select sites in old wild type and young or old mdx soleus muscles, respectively. On the contrary, we observed a 5–10% and a 20–25% increase in the phosphorylation levels of specific sites in young fatigued wild type and mdx soleus muscles, respectively. Overall, our studies showed that the phosphorylation pattern of sMyBP-C is differentially regulated following reversible (i.e. fatigue) and non-reversible (i.e. age and disease) (patho)physiological stressors.

This Article contains errors in Figure 2, where the blots for (−) GST-sMyBP-C NH 2 aa1-285 , (+) GST-fMyBP-C NH 2 aa1-249 , (−) GST-fMyBP-C NH 2 aa1-249 , (+) GST-protein and (−) GST-protein (the right half of the panel) were duplicated between all panels. Additionally, blots for (+) GST-sMyBP-C NH 2 Δ 21-59 aa1-285 in panel 2c and (+) GST-sMyBP-C NH 2 aa1-285 in panel 2e were duplicated. The authors re-scanned the images for the blots presented in this figure. Data in panel 2a, samples (+) GST-sMyBP-C NH 2 Δ 21-59 aa1-285 , (+) GST-sMyBP-C NH 2 Δ 34-59 aa1-285 , (+) GST-sMyBP-C NH 2 aa1-285 , (+) GST-fMyBP-C NH 2 aa1-249, panel 2b, samples (+) GST-sMyBP-C NH 2 Δ 21-59 aa1-285 , (+) GST-sMyBP-C NH 2 Δ 34-59 aa1-285 , and (+) GST-sMyBP-C NH 2 aa1-285 , and panel 2d sample (+) GST-sMyBP-C NH 2 Δ 34-59 aa1-285 remain unchanged. All other images were replaced with re-scanned blots. All samples in the corrected figure are shown with the wider field of view. The correct Figure 2 appears below as Figure 1 and the Supplementary Information file has now been replaced. In the revised version of the Supplement the unprocessed images of full-length blots for all blots presented in Figure 2 are now included as Supplementary Figures 2-6.

Recent experiments [1] show that both striation, an indication of the structural registry in muscle fibers, as well as the contractile strains produced by beating cardiac muscle cells, can be optimized by substrate stiffness. We show theoretically how the substrate rigidity dependence of the registry data can be mapped onto that of the strain measurements. We determine [2] the elasticity-mediated structural registry including both the elastic interactions of neighboring, actively contractile acto-myosin units as well as the noise inherent in biological systems. By assuming that structurally registered myofibrils also tend to beat in phase, we explain the observed dependence of both structural striation and beating strain measurements of heart muscle cells on substrate stiffness in a unified manner. The agreement of our ideas with experiment suggests that the correlated beating of heart cells may be limited by the structural registry of the myofibrils which in turn is regulated by their elastic environment. New theoretical predictions focus on the role of the substrate and the intercellular distance in the synchronization of the beating of cellular assemblies.[1] S.