Distortion of the Actin A-Triad Results in Contractile Disinhibition and Cardiomyopathy

Abstract

SUMMARYStriated muscle contraction is regulated by the movement of tropomyosin over the thin filament surface, which blocks or exposes myosin binding sites on actin. Findings suggest that electrostatic contacts, particularly those between K326, K328, and R147 on actin and tropomyosin, establish an energetically favorable F-actin-tropomyosin configuration, with tropomyosin positioned in a location that impedes actomyosin associations and promotes relaxation. Here, we provide data that directly support a vital role for these actin residues, termed the A-triad, in tropomyosin positioning in intact functioning muscle. By examining the effects of an A295S α-cardiac actin hypertrophic cardiomyopathy-causing mutation, over a range of increasingly complex in silico, in vitro, and in vivo Drosophila muscle models, we propose that subtle A-triad-tropomyosin perturbation can destabilize thin filament regulation, which leads to hypercontractility and triggers disease. Our efforts increase understanding of basic thin filament biology and help unravel the mechanistic basis of a complex cardiac disorder.

Prolonged cross-bridge binding triggers muscle dysfunction in a Drosophila model of myosin-based hypertrophic cardiomyopathy

“…Therefore, the proportion of time during the cardiac cycle that was spent generating tension, calculated by determining the ratio of systolic interval to heart period (SI/HP), was significantly greater for the mutant heart tubes relative to controls at all ages studied ( Figure 7C ). Together, these findings illustrate that the R146N / + hearts exhibit reduced cardiac diameters and potentially elevated tone during diastole, as well as prolonged periods of systolic tension generation, which are indicative of restrictive physiology, diastolic dysfunction, and impaired relaxation ( Cammarato et al, 2008 ; Viswanathan et al, 2014 ; Viswanathan et al, 2017 ).…”

Diabetes with heart failure increases methylglyoxal modifications in the sarcomere, which inhibit function

“…On actin, we identified MG-modification of K293, a residue that is very near tropomyosin's position on F-actin in the absence of calcium, which blocks myosin binding. Two studies clearly illustrate how this region is critical for actin-tropomyosin interaction, as the authors mutated neighboring residues on actin (D294V and A297S by the numbering used here) and both disrupted tropomyosin's position on actin and affected calcium sensitivity (41,42). Thus, it is highly likely that MG-modification of K293 would have a similar effect, explaining the observed decrease in calcium sensitivity.…”

Fropofol decreases force development in cardiac muscle

“…Therefore, the drug may elicit effects via binding to actin subdomain 3, which contains numerous residues that favorably interact with tropomyosin. These contact sites help establish tropomyosin's inhibitory position along thin filaments and are critical in maintaining proper Ca 2+ activation (38)(39)(40)(41)(42). In fact, mutations in this region have recently been shown to induce significant effects on muscle function and, specifically, on thin filament Ca 2+ sensitivity (42).…”

“…These contact sites help establish tropomyosin's inhibitory position along thin filaments and are critical in maintaining proper Ca 2+ activation (38)(39)(40)(41)(42). In fact, mutations in this region have recently been shown to induce significant effects on muscle function and, specifically, on thin filament Ca 2+ sensitivity (42). Therefore, fropofol binding may induce a structural change in this region of actin that acts to desensitize the thin filament to Ca 2+ , which can further reduce myosin binding and overall force production.…”