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Mutations in the cardiac splicing factor RBM20 lead to malignant dilated cardiomyopathy (DCM). To understand the mechanism of RBM20-associated DCM, we engineered isogenic iPSCs with DCM-associated missense mutations in RBM20 as well as RBM20 knockout (KO) iPSCs. iPSC-derived engineered heart tissues made from these cell lines recapitulate contractile dysfunction of RBM20-associated DCM and reveal greater dysfunction with missense mutations than KO. Analysis of RBM20 RNA binding by eCLIP reveals a gain-of-function preference of mutant RBM20 for 3′ UTR sequences that are shared with amyotrophic lateral sclerosis (ALS) and processing-body associated RNA binding proteins (FUS, DDX6). Deep RNA sequencing reveals that the RBM20 R636S mutant has unique gene, splicing, polyadenylation and circular RNA defects that differ from RBM20 KO. Super-resolution microscopy verifies that mutant RBM20 maintains very limited nuclear localization potential; rather, the mutant protein associates with cytoplasmic processing bodies (DDX6) under basal conditions, and with stress granules (G3BP1) following acute stress. Taken together, our results highlight a pathogenic mechanism in cardiac disease through splicing-dependent and -independent pathways.

 

Platelets are currently stored at room temperature before transfusion to maximize circulation time. This approach has numerous downsides, including limited storage duration, bacterial growth risk, and increased costs. Cold storage could alleviate these problems. However, the functional consequences of cold exposure for platelets are poorly understood. In the present study, we compared the function of cold-stored platelets (CSP) and room temperature-stored platelets (RSP) in vitro, in vivo, and post-transfusion. CSP formed larger aggregates under in vitro shear while generating similar contractile forces compared to RSP. We found significantly reduced GPVI levels after cold exposure of 5-7 days. After transfusion in humans, CSP were mostly equivalent to RSP yet aggregated significantly less to the GPVI agonist collagen. In a mouse model of platelet transfusion, we found a significantly lower response to the GPVI-dependent agonist convulxin and significantly lower GPVI levels on the surface of transfused platelets after cold storage. In summary, our data support an immediate but short-lived benefit of CSP and highlight the need for thorough investigations of this product. (NCT03787927) 

Abstract Mitochondrial trifunctional protein deficiency, due to mutations in hydratase subunit A (HADHA), results in sudden infant death syndrome with no cure. To reveal the disease etiology, we generated stem cell-derived cardiomyocytes from HADHA-deficient hiPSCs and accelerated their maturation via an engineered microRNA maturation cocktail that upregulated the epigenetic regulator, HOPX .  Here we report, matured HADHA mutant cardiomyocytes treated with an endogenous mixture of fatty acids manifest the disease phenotype: defective calcium dynamics and repolarization kinetics which results in a pro-arrhythmic state. Single cell RNA-seq reveals a cardiomyocyte developmental intermediate, based on metabolic gene expression. This intermediate gives rise to mature-like cardiomyocytes in control cells but, mutant cells transition to a pathological state with reduced fatty acid beta-oxidation, reduced mitochondrial proton gradient, disrupted cristae structure and defective cardiolipin remodeling. This study reveals that HADHA (tri-functional protein alpha), a monolysocardiolipin acyltransferase-like enzyme, is required for fatty acid beta-oxidation and cardiolipin remodeling, essential for functional mitochondria in human cardiomyocytes. 

Background: The giant sarcomere protein titin is important in both heart health and disease. Mutations in the gene encoding for titin ( TTN ) are the leading known cause of familial dilated cardiomyopathy. The uneven distribution of these mutations within TTN motivated us to seek a more complete understanding of this gene and the isoforms it encodes in cardiomyocyte (CM) sarcomere formation and function. Methods: To investigate the function of titin in human CMs, we used CRISPR/Cas9 to generate homozygous truncations in the Z disk (TTN-Z −/− ) and A-band (TTN-A −/− ) regions of the TTN gene in human induced pluripotent stem cells. The resulting CMs were characterized with immunostaining, engineered heart tissue mechanical measurements, and single-cell force and calcium measurements. Results: After differentiation, we were surprised to find that despite the more upstream mutation, TTN-Z −/− -CMs had sarcomeres and visibly contracted, whereas TTN-A −/− -CMs did not. We hypothesized that sarcomere formation was caused by the expression of a recently discovered isoform of titin, Cronos, which initiates downstream of the truncation in TTN-Z −/− -CMs. Using a custom Cronos antibody, we demonstrate that this isoform is expressed and integrated into myofibrils in human CMs. TTN-Z −/− -CMs exclusively express Cronos titin, but these cells produce lower contractile force and have perturbed myofibril bundling compared with controls expressing both full-length and Cronos titin. Cronos titin is highly expressed in human fetal cardiac tissue, and when knocked out in human induced pluripotent stem cell derived CMs, these cells exhibit reduced contractile force and myofibrillar disarray despite the presence of full-length titin. Conclusions: We demonstrate that Cronos titin is expressed in developing human CMs and is able to support partial sarcomere formation in the absence of full-length titin. Furthermore, Cronos titin is necessary for proper sarcomere function in human induced pluripotent stem cell derived CMs. Additional investigation is necessary to understand the molecular mechanisms of this novel isoform and how it contributes to human cardiac disease.