Unraveling a Force-Generating Allosteric Pathway of Actomyosin Communication Associated with ADP and Pi Release

Omecamtiv Mecarbil in Systolic Heart Failure: Clinical Efficacy and Future Directions of a Novel Myosin-Activating Inotropic Agent

Heart failure with reduced ejection fraction (HFrEF) remains a global health challenge, associated with high morbidity, mortality, and rising healthcare costs. Despite advances in guideline-directed therapy, many patients, especially those with severely reduced ejection fraction, remain symptomatic. Traditional inotropes, including β-agonists and phosphodiesterase inhibitors, are limited in chronic HFrEF due to risks of arrhythmias, calcium overload, and increased myocardial oxygen demand. Omecamtiv mecarbil (OM) is a novel cardiac myosin activator that enhances systolic contraction by increasing the efficiency of actin-myosin interactions. It prolongs systolic ejection time and improves stroke volume without elevating intracellular calcium or energy consumption. Although theoretical concerns exist regarding impaired diastolic filling, clinical trials have not confirmed such adverse effects in most patients. This narrative review discusses OM's pharmacologic profile, clinical trial data, and its potential as an adjunct therapy in advanced HFrEF. While not yet guideline-recommended, OM may benefit patients who remain symptomatic despite optimal treatment.

An allosteric inhibitor of RhoGAP class-IX myosins suppresses the metastatic features of cancer cells

Aberrant Ras homologous (Rho) GTPase signalling is a major driver of cancer metastasis, and GTPase-activating proteins (GAPs), the negative regulators of RhoGTPases, are considered promising targets for suppressing metastasis, yet drug discovery efforts have remained elusive. Here, we report the identification and characterization of adhibin, a synthetic allosteric inhibitor of RhoGAP class-IX myosins that abrogates ATPase and motor function, suppressing RhoGTPase-mediated modes of cancer cell metastasis. In human and murine adenocarcinoma and melanoma cell models, including three-dimensional spheroid cultures, we reveal anti-migratory and anti-adhesive properties of adhibin that originate from local disturbances in RhoA/ROCK-regulated signalling, affecting actin-dynamics and actomyosin-based cell-contractility. Adhibin blocks membrane protrusion formation, disturbs remodelling of cell-matrix adhesions, affects contractile ring formation, and disrupts epithelial junction stability; processes severely impairing single/collective cell migration and cytokinesis. Combined with the non-toxic, non-pathological signatures of adhibin validated in organoids, mouse and Drosophila models, this mechanism of action provides the basis for developing anti-metastatic cancer therapies.

Switch-2 determines Mg2+ADP-release kinetics and fine-tunes the duty ratio of Dictyostelium class-1 myosins

Though myosins share a structurally conserved motor domain, single amino acid variations of active site elements, including the P-loop, switch-1 and switch-2, which act as nucleotide sensors, can substantially determine the kinetic signature of a myosin, i.e., to either perform fast movement or enable long-range transport and tension generation. Switch-2 essentially contributes to the ATP hydrolysis reaction and determines product release. With few exceptions, class-1 myosin harbor a tyrosine in the switch-2 consensus sequence DIYGFE, at a position where class-2 myosins and a selection of myosins from other classes have a substitution. Here, we addressed the role of the tyrosine in switch-2 of class-1 myosins as potential determinant of the duty ratio. We generated constitutively active motor domain constructs of two class-1 myosins from the social amoeba Dictyostelium discoideum, namely, Myo1E, a high duty ratio myosin and Myo1B, a low duty ratio myosin. In Myo1E we introduced mutation Y388F and in Myo1B mutation F387Y. The detailed functional characterization by steady-state and transient kinetic experiments, combined with in vitro motility and landing assays revealed an almost reciprocal relationship of a number of critical kinetic parameters and equilibrium constants between wild-type and mutants that dictate the lifetime of the strongly actin-attached states of myosin. The Y-to-F mutation increased the duty ratio of Moy1B by almost one order of magnitude, while the introduction of the phenylalanine in switch-2 of Myo1E transformed the myosin into a low duty ratio motor. These data together with structural considerations propose a role of switch-2 in fine-tuning ADP release through a mechanism, where the class-specific tyrosine together with surrounding residues contributes to the coordination of Mg2+ and ADP. Our results highlight the importance of conserved switch-2 residues in class-1 myosins for efficient chemo-mechanical coupling, revealing that switch-2 is important to adjust the duty ratio of the amoeboid class-1 myosins for performing movement, transport or gating functions.

Mechanochemical consequences of myopathy-linked mutations in Tpm2.2 on striated muscle contractility

Tropomyosin (Tpm) is an actin-binding protein central to muscle contraction regulation. The Tpm sequence consists of periodic repeats corresponding to seven actin-binding sites, further divided in two functionally distinct halves. To clarify the importance of the first and second halves of the actin-binding periods in regulating the interaction of myosin with actin, we introduced hypercontractile mutations D20H, E181K located in the N-terminal halves of periods 1 and 5 and hypocontractile mutations E41K, N202K located in the C-terminal halves of periods 1 and 5 of the skeletal muscle Tpm isoform Tpm2.2. Wild-type and mutant Tpms displayed similar actin-binding properties, however, as revealed by FRET experiments, the hypercontractile mutations affected the binding geometry and orientation of Tpm2.2 on actin, causing a stimulation of myosin motor performance. Contrary, the hypocontractile mutations led to an inhibition of both, actin activation of the myosin ATPase and motor activity, that was more pronounced than with wild-type Tpm2.2. Single ATP turnover kinetic experiments indicate that the introduced mutations have opposite effects on product release kinetics. While the hypercontractile Tpm2.2 mutants accelerated product release, the hypocontractile mutants decelerated product release from myosin, thus having either an activating or inhibitory influence on myosin motor performance, which agrees with the muscle disease phenotypes caused by these mutations.

Interaction of Vanadium Complexes with Proteins: Revisiting the Reported Structures in the Protein Data Bank (PDB) since 2015

The structural determination and characterization of molecules, namely proteins and enzymes, is crucial to gaining a better understanding of their role in different chemical and biological processes. The continuous technical developments in the experimental and computational resources of X-ray diffraction (XRD) and, more recently, cryogenic Electron Microscopy (cryo-EM) led to an enormous growth in the number of structures deposited in the Protein Data Bank (PDB). Bioinorganic chemistry arose as a relevant discipline in biology and therapeutics, with a massive number of studies reporting the effects of metal complexes on biological systems, with vanadium complexes being one of the relevant systems addressed. In this review, we focus on the interactions of vanadium compounds (VCs) with proteins. Several types of binding are established between VCs and proteins/enzymes. Considering that the V-species that bind may differ from those initially added, the mentioned structural techniques are pivotal to clarifying the nature and variety of interactions of VCs with proteins and to proposing the mechanisms involved either in enzymatic inhibition or catalysis. As such, we provide an account of the available structural information of VCs bound to proteins obtained by both XRD and/or cryo-EM, mainly exploring the more recent structures, particularly those containing organic-based vanadium complexes.

Background and roles: myosin in autoimmune diseases

The myosin superfamily is a group of molecular motors. Autoimmune diseases are characterized by dysregulation or deficiency of the immune tolerance mechanism, resulting in an immune response to the human body itself. The link between myosin and autoimmune diseases is much more complex than scientists had hoped. Myosin itself immunization can induce experimental autoimmune diseases of animals, and myosins were abnormally expressed in a number of autoimmune diseases. Additionally, myosin takes part in the pathological process of multiple sclerosis, Alzheimer's disease, Parkinson's disease, autoimmune myocarditis, myositis, hemopathy, inclusion body diseases, etc. However, research on myosin and its involvement in the occurrence and development of diseases is still in its infancy, and the underlying pathological mechanisms are not well understood. We can reasonably predict that myosin might play a role in new treatments of autoimmune diseases.

Myosin's powerstroke occurs prior to the release of phosphate from the active site

Myosins are a family of motor proteins responsible for various forms of cellular motility, including muscle contraction and vesicular transport. The most fundamental aspect of myosin is its ability to transduce the chemical energy from the hydrolysis of ATP into mechanical work, in the form of force and/or motion. A key unanswered question of the transduction process is the timing of the force-generating powerstroke relative to the release of phosphate (P_i ) from the active site. We examined the ability of single-headed myosin Va to generate a powerstroke in a single molecule laser trap assay while maintaining P_i in its active site, by either elevating P_i in solution or by introducing a mutation in myosin's active site (S217A) to slow P_i -release from the active site. Upon binding to the actin filament, WT myosin generated a powerstoke rapidly (≥500 s^-1 ) and without a detectable delay, both in the absence and presence of 30 mM P_i . The elevated levels of P_i did, however, affect event lifetime, eliminating the longest 25% of binding events, confirming that P_i rebound to myosin's active site and accelerated detachment. The S217A construct also generated a powerstroke similar in size and rate upon binding to actin despite the slower P_i release rate. These findings provide direct evidence that myosin Va generates a powerstroke with P_i still in its active site. Therefore, the findings are most consistent with a model in which the powerstroke occurs prior to the release of P_i from the active site.