Background How the sarcomeric complex is continuously turned-over in long-living cardiomyocytes is unclear. According to the prevailing model of sarcomere maintenance, sarcomeres are maintained by cytoplasmic soluble protein pools with free recycling between pools and sarcomeres. Methods We imaged and quantified the turnover of expressed and endogenous sarcomeric proteins, including the giant protein titin, in cardiomyocytes in culture and in vivo, at the single cell and at the single sarcomere level using pulse-chase labeling of Halo-tagged proteins with covalent ligands. Results We disprove the prevailing ‘protein pool’ model and instead show an ordered mechanism in which only newly translated proteins enter the sarcomeric complex while older ones are removed and degraded. We also show that degradation is independent of protein age, and that proteolytic extraction is a rate limiting step in the turnover. We show that replacement of sarcomeric proteins occurs at a similar rate within cells and across the heart and is slower in adult cells. Conclusions Our findings establish a ‘unidirectional replacement’ model for cardiac sarcomeres subunit replacement and identify their turnover principles.

Background: Heart failure is a major health problem and progress in this field relies on better understanding of the mechanisms and development of novel therapeutics using animal models. The rat may be preferable to the mouse as a cardiovascular disease model due to its closer physiology to humans and due to its large size that facilitates surgical and monitoring procedures. However, unlike the mouse, genetic manipulation of the rat genome is challenging. Methods: Here we developed a simple, refined, and robust cardiac-specific rat transgenic model based on an adeno-associated virus (AAV) 9 containing a cardiac troponin T promoter. This model uses a single intraperitoneal injection of AAV and does not require special expertise or equipment. Results: We characterize the AAV dose required to achieve a high cardiac specific level of expression of a transgene in the rat heart using a single intraperitoneal injection to neonates. We show that at this AAV dose GFP expression does not result in hypertrophy, a change in cardiac function or other evidence for toxicity. Conclusions: The model shown here allows easy and fast transgenic based disease modeling of cardiovascular disease in the rat heart, and can also potentially be expanded to deliver Cas9 and gRNAs or to deliver small hairpin (sh)RNAs to also achieve gene knockouts and knockdown in the rat heart.

Cardiac fibroblasts play key roles in both health and disease. Their regulatory elements, transcription factors (TFs), and mechanisms of expression control have not been fully elucidated. We used a differential open chromatin approach, coupled with active enhancer mark, transcriptomic, and computational TFs binding analysis to map cell-type-specific active enhancers in cardiac fibroblasts and cardiomyocytes, and outline the TFs families that control them. This approach was validated by its ability to uncover the known cardiomyocyte TF biology in an unbiased manner, and was then applied to cardiac fibroblasts. We identified Tead, Sox9, Smad, Tcf, Meis, Rbpj, and Runx1 as the main cardiac fibroblasts TF families. Our analysis shows that in both cell types, distal enhancers, containing concentrated combinatorial clusters of multiple tissue expressed TFs recognition motifs, are combinatorically clustered around tissue specific genes. This model for tissue specific gene expression in the heart supports the general "billboard" model for enhancer organization.