Abstract 17079: MYBPC3 Truncation Mutations Cause Contractile Dysregulation in iPSC-Derived Cardiomyocytes

The mechanism of MYBPC3 (encoding cardiac myosin binding protein C, MyBP-C) truncation mutations, the most common genetic cause of hypertrophic cardiomyopathy, has been incompletely resolved. We hypothesized that truncating MYBPC3 mutations cause myofibrillar protein assembly defects and/or contractile dysfunction. Methods and Results: Control iPSCs were CRISPR/Cas9-edited to create cell lines with homozygous (homCT) and heterozygous (hetCT) C-terminal MYBPC3 truncation mutations and heterozygous MYBPC3 promoter knock-out (hetPROM). HetCT was further edited to add an N-terminal flag tag (FhetCT). Purified cardiomyocytes were assayed at day ~25. RNAseq showed a 50% reduction in MYBPC3 mRNA in hetCT (p<0.01). Full ablation of MyBP-C was demonstrated in homCT but MyBP-C content was not reduced in hetCT or hetPROM by quantitative mass spectrometry. Immunofluorescence and co-IP of FhetCT showed no truncated MyBP-C. Sarcomere assembly, quantified by aligned myofilament number after replating onto 7:1 rectangular micropatterns, did not differ between lines; homCT (23±3), hetCT (23±3), hetPROM (22±3 myofilaments), control (22±3), p=NS. Quantitative mass spectrometry demonstrated that the stoichiometry of other major thick and thin filament proteins was not altered (all p=NS). Maximum force was reduced in homCT (866±366 uN, p<0.001) and hetCT (1213±545 uN, p=0.03) vs controls (1509±441 uN), and not significantly different for hetPROM (1238±360 uN, p=0.07), when measured in single micropatterned iPSC-CMs on 8.7 kPa hydrogels by traction force microscopy. Time to peak contraction was shorter in homCT (0.27 s, p=0.02) vs control (0.42 sec), but not in hetCT (0.46 s) and hetPROM (0.49 s). Contractile deceleration time was reduced only in homCT (0.12 s vs 0.25 s, p=0.002). Conclusions: Heterozygous MYBPC3 truncation mutations result in haploinsufficent mRNA with steady state compensation of MYBP3 protein and no MYBPC3 truncated peptide. Even complete ablation of MyBP-C loss does not alter the overall stoichiometry of other sarcomeric thick and thin filament proteins. MyBP-C ablation and heterozygous truncating mutations impair maximal contractile force, strongly implicating contractile dysregulation as the primary mechanism of MYBPC3-HCM.

Myosin MgADP release rate decreases at longer sarcomere length to prolong myosin attachment time in skinned rat myocardium

Cardiac contractility increases as sarcomere length increases, suggesting that intrinsic molecular mechanisms underlie the Frank-Starling relationship to confer increased cardiac output with greater ventricular filling. The capacity of myosin to bind with actin and generate force in a muscle cell is Ca2+ regulated by thin-filament proteins and spatially regulated by sarcomere length as thick-to-thin filament overlap varies. One mechanism underlying greater cardiac contractility as sarcomere length increases could involve longer myosin attachment time ( t on) due to slowed myosin kinetics at longer sarcomere length. To test this idea, we used stochastic length-perturbation analysis in skinned rat papillary muscle strips to measure t on as [MgATP] varied (0.05–5 mM) at 1.9 and 2.2 μm sarcomere lengths. From this t on-MgATP relationship, we calculated cross-bridge MgADP release rate and MgATP binding rates. As MgATP increased, t on decreased for both sarcomere lengths, but t on was roughly 70% longer for 2.2 vs. 1.9 μm sarcomere length at maximally activated conditions. These t on differences were driven by a slower MgADP release rate at 2.2 μm sarcomere length (41 ± 3 vs. 74 ± 7 s−1), since MgATP binding rate was not different between the two sarcomere lengths. At submaximal activation levels near the pCa50 value of the tension-pCa relationship for each sarcomere length, length-dependent increases in t on were roughly 15% longer for 2.2 vs. 1.9 μm sarcomere length. These changes in cross-bridge kinetics could amplify cooperative cross-bridge contributions to force production and thin-filament activation at longer sarcomere length and suggest that length-dependent changes in myosin MgADP release rate may contribute to the Frank-Starling relationship in the heart.

Ubiquitylation by Trim32 causes coupled loss of desmin, Z-bands, and thin filaments in muscle atrophy

During muscle atrophy, myofibrillar proteins are degraded in an ordered process in which MuRF1 catalyzes ubiquitylation of thick filament components (Cohen et al. 2009. J. Cell Biol. http://dx.doi.org/10.1083/jcb.200901052). Here, we show that another ubiquitin ligase, Trim32, ubiquitylates thin filament (actin, tropomyosin, troponins) and Z-band (α-actinin) components and promotes their degradation. Down-regulation of Trim32 during fasting reduced fiber atrophy and the rapid loss of thin filaments. Desmin filaments were proposed to maintain the integrity of thin filaments. Accordingly, we find that the rapid destruction of thin filament proteins upon fasting was accompanied by increased phosphorylation of desmin filaments, which promoted desmin ubiquitylation by Trim32 and degradation. Reducing Trim32 levels prevented the loss of both desmin and thin filament proteins. Furthermore, overexpression of an inhibitor of desmin polymerization induced disassembly of desmin filaments and destruction of thin filament components. Thus, during fasting, desmin phosphorylation increases and enhances Trim32-mediated degradation of the desmin cytoskeleton, which appears to facilitate the breakdown of Z-bands and thin filaments.