Thick filament mechanosensing has been proposed as the mechanism by which myosin motors in cardiac muscle become available to bind actin. Accordingly, Caremani et al., using x-ray diffraction from intact rat trabeculae, show that myosin motors fully return to their OFF state during diastole independently of inotropic interventions.

Omecamtiv mecarbil (OM) is a putative positive inotropic tool for treatment of systolic heart dysfunction, based on the finding that in vivo it increases the ejection fraction and in vitro it prolongs the actin-bond life time of the cardiac and slow-skeletal muscle isoforms of myosin. OM action in situ, however, is still poorly understood as the enhanced Ca 2+-sensitivity of the myofilaments is at odds with the reduction of force and rate of force development observed at saturating Ca 2+. Here we show, by combining fast sarcomere-level mechanics and ATPase measurements in single slow demembranated fibres from rabbit soleus, that the depressant effect of OM on the force per attached motor is reversed, without effect on the ATPase rate, by physiological concentrations of inorganic phosphate (Pi) (1-10 mM). This mechanism could underpin an energetically efficient reduction of systolic tension cost in OM-treated patients, whenever [Pi] increases with heartbeat frequency.

The myosin-directed chaperone UNC-45B is essential for sarcomeric organization and muscle function from Caenorhabditis elegans to humans. The pathological impact of UNC-45B in muscle disease remained elusive. We report ten individuals with bi-allelic variants in UNC45B who exhibit childhood-onset progressive muscle weakness. We identified a common UNC45B variant that acts as a complex hypomorph splice variant. Purified UNC-45B mutants showed changes in folding and solubility. In situ localization studies further demonstrated reduced expression of mutant UNC-45B in muscle combined with abnormal localization away from the A-band towards the Z-disk of the sarcomere. The physiological relevance of these observations was investigated in C. elegans by transgenic expression of conserved UNC-45 missense variants, which showed impaired myosin binding for one and defective muscle function for three. Together, our results demonstrate that UNC-45B impairment manifests as a chaperonopathy with progressive muscle pathology, which discovers the previously unknown conserved role of UNC-45B in myofibrillar organization.

We hypothesized GSK-3b's ability to alter calcium sensitivity is directly related to its ability to localize to the myofilament. Using co-immunoprecipitation and immunofluorescence, we found GSK-3b localized to the z-disc when phosphorylated at Y216. Overexpression and pull-down experiments in NRVMs in which WT and mutant (phospho-mimetic and null) GSK-3b adenoviruses confirmed pY216 is required for interaction with the myofilament. As pY216 appears to be essential for myofilament localization, we identified a sole myofilament protein with an SH2 domain, Tensin-1, also identified at the z-disc. Using overexpression studies, we have shown that the binding of these two proteins is inhibited when either pY216 or the SH2 domain is mutated. Together, our data reveal a possible therapeutic target via re-localization of GSK-3b to the myofilament via Y216 phosphorylation after MI.