Journal of General Physiology

Mutations in {beta}-cardiac myosin are a common cause of inherited cardiomyopathies. The dilated cardiomyopathy E525K mutation alters both thick filament regulation and motor activity, yet the structural basis for these effects has remained unclear. Here, we combined cryo-EM and MD simulations to determine how E525K affects the conformational landscape of cardiac myosin in both its autoinhibited interacting-heads motif (IHM) and isolated myosin head states. E525K-mutated myosin exclusively adopted a single S2 conformation in IHM, in contrast to the conformational heterogeneity observed in wild-type myosin. Structural and electrostatic analyses revealed that the mutation increased positive charge density at the blocked-head (BH) - S2 (subfragment-2) interface, strengthening interactions with negatively charged residues within S2. 3D variability analysis and MD simulations of the E525K IHM demonstrated reduced S2 mobility, consistent with strengthened BH - S2 interactions, and decreased conformational flexibility, providing a structural mechanism for stabilization of the IHM state. To investigate the isolated myosin head, we determined cryo-EM structures of wild-type and E525K subfragment-1 (S1). The mutation induced local conformational rearrangements in the activation-loop and Loop 3 region, the SH3-like domain, and the essential light chain of S1, revealing structural changes that support the enhanced motor activity previously reported for isolated E525K myosin motors. These findings show that E525K stabilizes the autoinhibited IHM through electrostatic stabilization of the BH - S2 interface while inducing structural changes consistent with activation of the isolated motor domain, providing structural insights into how a single DCM mutation differentially regulates cardiac myosin S1 and IHM structure and function.

Voltage-gated Kv7 (KCNQ, M-type) potassium channels regulate cellular excitability through interactions with phosphatidylinositol 4,5-bisphosphate (PIP2;) and calmodulin (CaM). The distal B helix of Kv7.2-5 channels contains a conserved protein kinase C (PKC) phosphorylation site, suggesting that phosphorylation may regulate channel activity by altering CaM- and PIP2;-dependent mechanisms. Here, we investigated the effects of phosphorylation of Thr552 within the Kv7.4 B helix using electrophysiology, biophysical assays, NMR spectroscopy, and molecular dynamics simulations. Phosphomimetic substitution of Thr552 produced a loss-of-function phenotype characterized by reduced current density and a depolarizing shift in voltage-dependent activation without altering channel surface expression. Biophysical studies demonstrated that phosphorylation produced only modest effects on overall CaM association. However, NMR spectroscopy revealed phosphorylation-dependent changes in apoCaM interactions, and molecular dynamics simulations identified remodeling of the local interaction network surrounding the distal B-helix polybasic (KRK motif) region. In contrast, phosphorylation markedly reduced PIP2; binding to both the isolated B helix and CaM-associated Kv7.4 regulatory complexes. Mutation of the distal B-helix KRK motif similarly impaired PIP2; binding, identifying this region as an important determinant of lipid recognition. Together, these findings support a model in which phosphorylation of Thr552 suppresses Kv7.4 activity primarily by reducing PIP2; interactions while reorganizing, rather than disrupting, CaM binding. These results identify Thr552 as a critical regulatory site linking phosphorylation-dependent signaling to Kv7.4 channel gating.

Store-operated calcium entry (SOCE) is a ubiquitous calcium (Ca2+) signalling mechanism that replenishes endoplasmic and sarcoplasmic reticulum Ca2+ stores. We previously identified STIM1L, a skeletal muscle-enriched STIM1 splice variant that enables rapid SOCE activation in human myotubes. To determine its physiological role in vivo, we generated a constitutive knockout mouse model that selectively abolishes STIM1L expression while preserving the canonical STIM1 isoform. Stim1l-KO mice developed normally and exhibited no overt abnormalities, but both sexes showed reduced voluntary wheel-running activity, suggesting neuromuscular perturbation. Skeletal muscle deprived of STIM1L did not show any histological alterations or changes in fibre- type and size. Likewise, force-frequency relationships and fatigue resistance measured ex vivo in isolated extensor digitorum longus and soleus muscles were indistinguishable from control muscles. This lack of difference may be explained by the increased expression of STIM1 in several skeletal muscles and suggests that changes in the voluntary running activity of Stim1l-KO mice may arise from STIM1L loss in non-muscular tissues, including the central nervous system. Notwithstanding, primary myotubes derived from Stim1l-KO mice displayed a marked delay in SOCE activation kinetics, whereas SOCE amplitude remained comparable to controls. This was consistent with a compensatory STIM1 upregulation maintaining overall Ca2+ entry capacity. Interestingly, electrically evoked Ca2+ transients in intact Stim1l-KO flexor digitorum brevis fibres exhibited significantly faster decay kinetics than controls. This revealed another role of STIM1L in Ca2+ handling in skeletal muscle that appears independent of its contribution to SOCE activation. Together, these findings establish STIM1L as a regulator of SOCE activation kinetics and indicate an additional role in myofibre Ca2+ clearance, while suggesting that compensatory upregulation of canonical STIM1 may preserve muscle structure and contractile function in its absence in mice.

Polyamines are cationic metabolites that support critical cellular processes including transcription, translation, and cell proliferation. Although polyamine synthesis and transport are emerging therapeutic targets in human disease, the polyamine transport system remains poorly understood. ATP13A2-4 were recently identified as endolysosomal polyamine exporters, transporting putrescine (PUT), spermidine (SPD), and spermine (SPM) from the vesicular lumen to the cytosol. Despite interest in pharmacologically targeting these transporters, existing assays of transporter function rely on indirect or endpoint measures that cannot resolve transport as it occurs. Here, we report the first voltage-clamp recordings of polyamine transporter activity, directly detecting ATP13A2- and ATP13A3-mediated charge movement across cell membranes. When expressed in Xenopus oocytes, these transporters localized to the plasma membrane, enabling electrophysiological measurements. Polyamines induced inward currents dependent on transporter catalytic activity, and their dose-dependence correlated with ATPase assays. ATP13A2 currents were largest in response to SPM, whereas ATP13A3 currents were largest with PUT, consistent with their established substrate preferences. Polyamine-induced currents were blocked by the polyamine transport inhibitor AMXT-1501. These findings establish ATP13A2 and ATP13A3 as electrogenic polyamine pumps and demonstrate the utility of two-electrode voltage clamp as a label-free, transporter-resolved assay to complement other approaches in polyamine transporter-targeted drug discovery.

KCNQ2 (Kv7.2) subunits contribute to the channels that carry the neuronal M-current, and variants in KCNQ2 cause epilepsies that range from self-limited neonatal seizures to developmental and epileptic encephalopathy (DEE). Structures of KCNQ2 bound to PIP2 and cannabidiol show a widened activation gate, yet it has remained unclear which point of the activated pore limits K+ flux and how DEE variants at that point act. We simulated the KCNQ2 pore domain (PDB 8J01) in a POPC bilayer under an applied field of about 565 mV, restraining only the terminal residues of S5 and S6 so that G310 and S314 were free. The activated pore conducted K+ at 4.3 pS (95% CI 2.5-6.9; 17 net filter crossings in 1120 ns over four replicas), while the closed-state pore (PDB 7CR3) conducted none in 560 ns. Conduction proceeded by knock-on through a filter that was doubly occupied in 84-92% of frames. Along S6 the narrowest position of the activated pore was not the canonical gate residue S314 but the glycine of the Pro-Ala-Gly motif, G310: activation widens S314 by 0.37 nm but G310 by only 0.07 nm, and restraining G310 to its closed-state width in one replica abolished gate crossings. The DEE variant G310S reduced conduction about six-fold (G310S/wild-type ratio 0.16, 95% CI 0.02-0.66; p = 0.003). Its serine hydroxyls project into the constriction and halve the water column in the G310 plane (8.7 to 4.7 waters), an effect reproduced in CHARMM36m (9.9 to 4.3). The first coordination shell of a K+ ion at the constriction was, however, conserved (6.3 vs 6.5 oxygens), because the serine hydroxyls replace the waters they displace. The hydroxyls did not retain the ion: contacts lasted tens of picoseconds, and ions were instead turned back at the serine ring, fewer reaching the gate plane (45 vs 79 excursions per microsecond) and fewer of those crossing it (5% vs 23%). In mixed tetramers carrying one or two variant subunits the effect on the water column was additive, each subunit removing about a quarter of it (f = 0.22 and 0.58 of the full effect). These results identify G310 as the flux-limiting constriction of the activated KCNQ2 pore in our simulations and point to a permeation defect as one component of G310S loss of function.

Background: KCNH2 encodes Kv11.1 channel proteins that conduct the rapidly activating delayed-rectifier K+ current (IKr), which is critical for cardiac repolarization. KCNH2 encodes two functional isoforms, Kv11.1a and Kv11.1b, via alternative transcription start sites. Kv11.1a is the principal determinant of cardiac IKr and ventricular repolarization. The circadian clock, a transcriptional-translational feedback loop that cycles with a period of ~24 hours and drives the circadian expression of many genes, including Kcnh2 in the mouse heart. Because daily body temperature rhythms provide a systemic signal that synchronizes cardiac circadian clocks, we tested whether physiological temperature cycles drive the circadian promoter activity of the cloned human KCNH2 (hKCNH2) promoter. Hypothesis: hKCNH2 is a direct transcriptional target of the circadian clock, with temperature driving its promoter activity through BMAL1:CLOCK acting at a conserved tandem E-box. Methods: We cloned the conserved proximal promoter of KCNH2 (-1631 bp upstream of Kv11.1a exon 1) to generate hKCNH2 promoter luciferase reporter constructs. Constructs were transfected into C2C12 myotubes and synchronized by serum shock (static 37{degrees}C) or temperature cycling (36.5-38.5{degrees}C). Bioluminescence was recorded and assessed for period, phase, and amplitude. BMAL1:CLOCK dependence was tested via dominant-negative CLOCK{Delta}19 co-expression. Results: Temperature cycling did not exhibit the rapid damping characteristic of serum-shock-synchronized oscillations, consistent with continuous entrainment by an external zeitgeber rather than a free-running oscillator. Deletion analysis identified a conserved tandem E-box required for oscillation under both serum shock and temperature cycling, and for BMAL1:CLOCK-dependent transactivation (1.75 {+/-} 0.21 vs. 0.86 {+/-} 0.06 RLU, p = 0.0038). CLOCK{Delta}19 reduced hKCNH2 promoter amplitude under temperature cycling without altering period. Conclusion: The circadian clock regulates KCNH2 promoter activity through daily temperature rhythms.

Background: Loss- and gain-of-function variants in SCN5A, encoding the cardiac sodium channel Nav1.5, are major causes of inherited cardiac channelopathies. Their pathophysiology has almost exclusively been attributed to altered cardiomyocyte excitability and cardiac conduction. However, growing evidence indicates that autonomic dysfunction contributes to these disorders, raising the possibility that Nav1.5 dysfunction may also affect the intrinsic cardiac nervous system. Objectives: We investigated whether Nav1.5 is functionally expressed in murine intrinsic cardiac neurons and evaluated its contribution to neuronal excitability. Methods: Scn5a expression was investigated by digital PCR and immunocytochemistry in isolated murine intracardiac neurons. Whole-cell patch-clamp recordings were used to characterize sodium currents and action potential properties in wild-type and Scn5a+/- mice. Results: Scn5a transcripts and Nav1.5 protein were detected in intrinsic cardiac neurons. Voltage-clamp recordings identified a tetrodotoxin-resistant sodium current accounting for approximately 8% of the total sodium current and displaying pharmacological and biophysical properties consistent with Nav1.5. Activation and inactivation analyses indicated that Nav1.5 is the predominant sodium conductance available at most polarized membrane potentials. Accordingly, Scn5a haploinsufficiency selectively altered action potential threshold, maximal upstroke velocity, overshoot and duration in neurons resting below -60 mV, whereas more depolarized neurons were essentially unaffected. No structural or neurochemical remodeling of intrinsic cardiac ganglia was detected. Conclusion: Nav1.5 is a functional determinant of intrinsic cardiac neuronal excitability. These findings identify the intrinsic cardiac nervous system as a new cellular target of SCN5A dysfunction and suggest that inherited cardiac sodium channelopathies could involve alterations in both myocardial and intrinsic neuronal electrophysiology.

P-type ATPases maintain cellular metal homeostasis by coupling ATP hydrolysis to the active translocation of ions across biological membranes. Within this superfamily, P1B -type ATPases play a central role in the transport and detoxification of transition metals such as Cu+ and Ag+. Despite the availability of several crystallographic structures for bacterial ATPases, molecular determinants governing ion selectivity, coordination dynamics, and transport efficiency are not yet fully resolved, particularly for plant homologs, whose conformational landscapes have not been explored in detail at atomistic resolution. In this study, we combine molecular dynamics (MD) simulations with enhanced sampling through metadynamics to characterize the conformational dynamics and ion-binding mechanisms of representative heavy-metal P-type ATPases. By comparing bacterial and plant homologs in phosphorylated and dephosphorylated states, we examine how phosphorylation is associated with changes in structural plasticity, channel accessibility and ion coordination environments. The simulations indicate phosphorylation-dependent rearrangements of the cytosolic domains and transmembrane helices, which may alter access to the metal-binding pathway. Enhanced sampling identifies distinct conformational basins and possible transition pathways along functionally relevant collective variables, allowing characterization of the free-energy landscape associated with ion displacement within the transmembrane region. Together, these results provide an atomistic perspective on the relationship between metal coordination and protein conformational dynamics. Overall, our findings support a model in which phosphorylation state, structural dynamics, and local coordination environments collectively contribute to the differential behavior of metal ions in P1B -type ATPases.

Skeletal muscles generate force and shortening through a structural powerstroke in actin-bound myosin motors linked to ATP hydrolysis, but they also act as brakes to resist stretching by an external load. The molecular structural basis of muscle braking has remained obscure. Here we show that muscle braking is driven both by structural changes in myosin motors and by distortion of the lattice of thick and thin filaments including their M-band and Z-disk connections. A strained motor conformation is established early during stretch, but force continues to increase as thin filaments are displaced from their symmetrical lattice positions, the lateral shear between thick filaments increases, and the lattice volume decreases. Moreover, whilst the motor conformation rapidly recovers after the stretch, the distortion of the filament lattice is largely maintained, storing the work done on the muscle during stretch and accounting for the residual force enhancement after stretch.

The adrenergic response is a fundamental mechanism that regulates heart rate (chronotropy), cardiac contractility (inotropy) and relaxation (lusitropy). Adrenergic stress is also a recognized trigger of arrhythmia in disease. Yet, our understanding of the underlying molecular basis remains incomplete. Protein kinase A (PKA) and the calcium/calmodulin-dependent kinase II (CaMKII) phosphorylate multiple targets proposed to participate in the adrenergic response, including the GTP-binding protein Rad, phospholamban (PLB) and ryanodine receptor 2 (RyR2). Here we demonstrate that phosphorylation of both Rad and PLB is necessary for inotropy and lusitropy. We show that changes in cardiac contractility and relaxation are primarily dependent on intracellular calcium handling. Finally, we report that Rad and PLB control stress-induced arrhythmogenesis, despite the phosphorylation of other pro-arrhythmic targets. We have identified the essential molecular components of the adrenergic response, resolving a long-standing debate in cardiac excitation-contraction coupling and refining current models of sympathetic regulation in health and disease.

Background: Human induced pluripotent stem cell-derived cardiomyocytes are valuable for cardiac disease modelling and drug testing but remain limited by immature structural and electrophysiological properties in conventional two-dimensional cultures. Three-dimensional cardiac models, including cardioids, provide a more physiological environment through multicellular organization and enable tissue-level functional analysis. Their electrophysiological properties and maturation, however, remain incompletely characterized. Methods: We performed an integrated electrophysiological characterization of human cardioids over the first three weeks of maturation using sharp microelectrode recordings, calcium transient measurements, whole-cell patch clamp recordings, immunofluorescence, transcriptomic analysis, and pharmacological interventions. Results: During maturation, cardioids exhibited progressive electrophysiological maturation, characterized by more physiological resting membrane potentials (-54.0{+/-}3.1mV to -68.1{+/-}1.2mV), increased action potential amplitude (73.5{+/-}4.8mV to 101.7{+/-}2.1mV) and upstroke velocity (44.3{+/-}5mV/ms to 86.6{+/-}8.4mV/ms), and shorter action potential duration (178.9{+/-}8.7ms to 144.0{+/-}4.8ms at 1Hz stimulation). These changes were associated with progressively increased INa density and enhanced SCN5A mRNA and Nav1.5 protein expression, consistent with progressive maturation of cardiac excitability. IKr current and KCNH2/hERG expression were already present at early maturation stages and remained stable, as evidenced by preserved sensitivity to dofetilide-blockade. Pharmacological inhibition of barium- and HMR-sensitive currents altered resting membrane and/or action potential characteristics in intact cardioids, whereas in isolated cardiomyocytes classical IK1 remained undetectable and IKs was small, highlighting a certain degree of persisting immaturity of the model. Conclusions: These findings demonstrate that human cardioids undergo progressive electrophysiological maturation and provide complementary tissue- and cellular-level insights into cardiac function, establishing their suitability for investigating INa and IKr-related cardiac pathologies.

Actin cross-linkers are essential modulators of the actin cytoskeleton, enabling structural diversity and dynamic remodeling. Among them, calcium/calmodulin-dependent protein kinase II (CaMKII) is unique in serving a dual role as a kinase and as a multivalent structural binder of actin in dendritic spines, where it plays a major role in supporting dendritic spine structure via its {beta} subunit. To understand the basis of the structural function of CaMKII, we quantified the morphology and mechanics of actin bundles formed by CaMKII and directly compared them with bundles assembled by the canonical bivalent cross-linkers -actinin and fascin. Using fluorescence microscopy, we measured contour length, straightness ratio, and persistence length across different cross-linker concentrations. Fascin generated progressively shorter, straighter, and stiffer bundles with increasing concentration, whereas -actinin bundles remained largely unchanged. In contrast, increasing CaMKII concentration reduced both bundle straightness and persistence length, revealing a unique concentration-dependent increase in bundle flexibility. To investigate the structural origins of these mechanical behaviors, we complemented the experiments with coarse-grained simulations of cross-linked actin bundles. Simulations revealed that CaMKII-generated bundles retained a substantially larger fraction of their curvature in their time-averaged configuration than bundles formed by -actinin or fascin, indicating that bundle mechanics are strongly influenced by organizational features beyond thermal bending fluctuations alone. CaMKII bundles displayed behavior consistent with additional structural complexity arising from their multivalent architecture and flexible linker domains. Together, our experimental and computational results identify CaMKII as a structurally and mechanically distinct actin cross-linker. We propose that the combination of multivalency and linker flexibility enables CaMKII to function as a molecular structural pivot, promoting flexible, adaptable actin assemblies that are mechanically compatible with the dynamic remodeling required for dendritic spine plasticity.

Longitudinal muscle growth occurs through the addition of sarcomeres in series, a process termed serial sarcomerogenesis. To investigate contraction mode dependence of sarcomerogenesis in Sprague-Dawley rats (n=36, 14 to18 weeks, ~468g), we compared the effects of isometric, eccentric, and concentric training (4 to 6 weeks; 3 times/week) on morphological and mechanical adaptations of the plantar flexors. Isometric training was performed at a joint angle corresponding to the plateau nearing the descending limb of the force length relationship, eccentric contractions began at a shorter muscle length and lengthened to this same joint position, and concentric contractions started at this lengthened position but then actively shortened. Owing to the rapid regulation of serial sarcomere number the 4-and 6-week timepoints were collapsed. While force output increased across all training modes (+16%; p<0.05), longitudinal muscle growth was regulated by contraction type. Both isometric and eccentric training were highly effective at increasing serial sarcomere number when optimal loading conditions (high forces at longer lengths) are provided (+7-9%; p<0.05). Concentric training, even when initiated at long lengths with high forces was not an effective stimulus for longitudinal remodelling (+2%; p>0.05). Therefore, these findings identify the mechanical loading environment associated with isometric and eccentric training as a key driver of serial sarcomerogenesis.

Vocal communication depends on the biomechanical integrity of the vocal folds, yet how impairment alters vocal behavior remains poorly understood. We used the California mouse (Peromyscus californicus), a highly vocal species that produces songs through airflow-induced vocal fold vibration, to experimentally examine how defined vocal fold injury alters vocal production. Longitudinal recordings were combined with histological assessment to track changes in vocal function after impairment. Injury produced acute edema of the lamina propria and thyroarytenoid muscle. Despite stable levels of vocal activity, the vocal phenotype shifted characteristically following injury. Thus, vocal behavior can persist despite substantial alteration of the biomechanical tissues underlying sound production, demonstrating that maintenance of behavioral output does not necessarily indicate preservation of the physiological system that generates it. More broadly, these findings establish an experimental link between vocal fold injury and vocal phenotype and provide a framework for understanding how tissue damage and remodeling can reshape acoustic communication even when vocal behavior is maintained. Because repeated vocalization can impose substantial mechanical demands on vocal folds, this framework may have broad relevance for understanding how vocal use, injury, and tissue remodeling interact to shape vocal communication.

Cardiomyocyte contraction is essential for the pumping action of the heart and deteriorates after myocardial damage, either as a consequence of irreversible cardiomyocyte injury or stiffening of the extracellular matrix, a process described as fibrosis. Cell geometry and substrate stiffness not only influence sarcomere architecture and contractility, they also determine how much work the cardiomyocytes can transfer to their environment. Human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes provide a model system to examine these effects in a controlled manner, enabling us to mimic key features of the cardiac environment in vitro. Here we show that geometric confinement and substrate stiffness influence different aspects of cardiomyocyte force generation. By combining traction force microscopy and live-cell imaging of hiPSC-derived cardiomyocytes on soft and patterned substrates, we find that geometric confinement leads to longer resting sarcomeres and more pronounced sarcomere shortening on substrates with both physiological (10 kPa) and fibrotic (30 kPa) stiffness. Interestingly, the substrate stiffness itself has little effect on resting sarcomere length, but influences the peak contractile stress. On 10 kPa substrates, confined cells generate higher peak contractile stresses than unconfined cells, whereas this difference is not observed at 30 kPa. Confinement also results in faster mechanical relaxation at 10 kPa, with no detectable difference at 30 kPa. The combination of defined cell geometry and physiological stiffness leads to longer resting sarcomere lengths, more pronounced sarcomere shortening, higher peak contractile stress, and faster relaxation, features associated with a more mature cardiomyocyte phenotype. These findings show that the mechanical consequences of cell geometry depend on substrate stiffness and, therefore, both aspects have to be considered when employing hiPSC-derived cardiomyocytes as a model system in mechanobiology research, disease modeling and drug testing.

Mitochondrial reactive oxygen species (ROS) are implicated in cardiac dysfunction, but complex dynamic interactions between ROS, intracellular calcium, and electrophysiology make it difficult to resolve mechanisms experimentally. We developed the Zukowski excitation-contraction-mitochondrial-ROS (ECM-ROS) model, a computational model that couples human ventricular electrophysiology with mitochondrial calcium handling, energetics, ROS production, and ROS dependent modulation of the ryanodine receptor, SERCA, L-type calcium current, and late sodium current. Model predictions were evaluated against independent experimental measurements of ROS induced changes in action potential duration and intracellular calcium. We then used the model to characterize ROS calcium feedback across physiological pacing rates and applied it to doxorubicin induced oxidative stress. Increasing cytosolic calcium produced nonlinear amplification of mitochondrial activity and cytosolic ROS, with greater amplification at faster pacing rates. Moderate ROS elevation initially enhanced calcium release, whereas greater oxidative stress depleted sarcoplasmic reticulum calcium stores. During simulated doxorubicin exposure, faster pacing lowered the level of oxidative stress required to produce calcium dysregulation and action potential prolongation. Mitochondrial ROS and calcium accumulation preceded detectable electrophysiological remodeling. In populations incorporating variability in ion channel expression, neither doxorubicin induced oxidative stress nor subtle pharmacological IKr block alone produced early afterdepolarizations. However, early afterdepolarizations were predicted to emerge with combined perturbations. These findings establish a computational framework for investigating bidirectional coupling among mitochondrial function, ROS, calcium handling, and human cardiac electrophysiology. The model generates testable predictions regarding the early detection of oxidative cardiac injury and the physiological conditions that increase susceptibility to arrhythmia.

Adherent cells must continuously adapt to rapid environmental fluctuations to preserve mechanical integrity. However, conventional theoretical frameworks predominantly rely on quasi-static assumptions, thereby limiting their ability to capture the transient dynamics of cellular responses to high-rate perturbations, such as acute osmotic shocks. To address this limitation, we developed a biophysical model incorporating a Hill-type law that couples active cytoskeletal mechanics to cell-edge velocity, explaining the counterintuitive experimental observation that traction forces transiently decrease during rapid hypotonic swelling despite an increase in cell size. We further construct a kinematic-geometric phase diagram that connects classical quasi-static theories with our dynamic model. Moreover, we show that traction-force dynamics are sensitive to the loading rate and amplitude of osmotic shock. In addition, stiffness-associated simulations and supporting perturbation evidence suggest a cellular stiffness-associated recovery trend in which stiffer cells exhibit faster volume recovery driven by stronger recoil of hydrostatic pressure.

A cell's ability to maintain its volume is a prerequisite for its essential physiological functions. Changes in extracellular osmolarity induce the flux of water and solutes across the cell and nuclear membrane and, therefore, alters cell and nuclear volumes. Here, we investigated the equilibrium and dynamic volumetric responses of cells and nuclei in adherent human hepatoma (HepG2) cell monolayers using live three-dimensional imaging. Through image segmentation, 3D reconstruction, and volume rendering, precise cell and nuclear volumes were quantified. We found that during hypoosmotic stress, cells swell and lose osmolytes through active volume regulation, and nuclei undergo greater relative expansion than cells. During hyperosmotic stress, we found that both cells and nuclei shrink rapidly, develop irregular morphologies and undergo passive volume regulation. Furthermore, we explored the osmotic responses in these cells by restoring isosmotic conditions following osmotic stress and found that nuclei returned to volumes greater than their initial isosmotic volume, whereas cytoplasmic volume returned to a value below its initial isosmotic volume following swelling. By fitting coupled transport models water and solute permeability estimates for both cells and nuclei were made from their dynamic responses to different anisosmotic media with and without the presence of permeating solutes dimethyl sulfoxide and ethylene glycol. Together, these quantitative measurements of cell, cytoplasm, and nuclear volumetric responses and associated transport parameters have important implications for understanding cell physiology and the mechanism of osmoregulation.

Transient receptor potential melastatin 3 (TRPM3) is a calcium-permeable ion channel implicated in chronic pain and developmental and epileptic encephalopathies. The voltage sensor-like domain (VSLD) is a major site of pharmacological modulation, but the molecular determinants of ligand recognition and efficacy remain incompletely understood. Here we combine cryo-electron microscopy, molecular dynamics simulations, mutagenesis, and medicinal chemistry to investigate the interactions of TRPM3 with the inhibitors isosakuranetin, maprotiline, and diclofenac. We show that structurally diverse ligands occupy a common VSLD cavity but engage distinct interaction networks. Functional and computational analyses identify ligand-specific residue dependencies and support a halogen bond between diclofenac and D947 that contributes to inhibitor potency. Furthermore, esterification or amidation of the diclofenac carboxylate converts inhibition into potentiation, and mutagenesis and molecular dynamics simulations support partially overlapping VSLD configurations associated with these opposing effects. Together, these findings establish the TRPM3 VSLD as an adaptable allosteric pocket in which ligand-specific interactions shape recognition, potency, and efficacy.

Objective: Mitochondrial oxidative capacity in striated muscle is governed by the PGC-1/ERR transcriptional axis, which controls not only steady-state respiration but the capacity to expand energy production when demand rises. Whether this reserve function requires dedicated downstream effectors, and whether its loss is detectable at rest, remains unclear. We asked whether Perm1, a striated muscle-specific PGC-1/ERR-induced regulator, is required cell-autonomously for cardiac bioenergetic reserve. Methods: We generated cardiomyocyte-specific Perm1 knockout mice using alphaMHC-nCre, complemented by an adult tamoxifen-inducible alphaMHC-MerCreMer model, and assessed cardiac development, structure, and function longitudinally through 12 months of age in both sexes. We evaluated mitochondrial function by high-resolution respirometry, transmission electron microscopy, and molecular profiling. We tested energetic capacity under load by treadmill exercise with tissue ATP quantification and by beta-adrenergic stimulation of cultured neonatal cardiomyocytes. Results: Perm1 deletion did not perturb embryonic cardiac development, and neither constitutive nor adult-onset deletion impaired cardiac structure, systolic function, or diastolic function through 12 months. Despite this preserved resting phenotype, mitochondrial respiration was reduced by 20 - 25% across complex I- and complex I+II-supported conditions, without changes in citrate synthase activity, mtDNA copy number, or OXPHOS subunit abundance. Instead, the MICOS components IMMT (MIC60) and CHCHD3 (MIC19) were selectively reduced, and mitochondrial subpopulations shifted toward fewer subsarcolemmal and more interfibrillar mitochondria, with unchanged total mitochondrial density. Functionally, Perm1-deficient mice of both sexes showed reduced treadmill capacity at 6 and 12 months and failed to maintain cardiac ATP during exercise at a submaximal workload, while skeletal muscle ATP and plasma lactate were unaffected. This deficit was cardiomyocyte-autonomous: beta-adrenergic stimulation increased ATP content in control cardiomyocytes, but Perm1 knockdown reduced this response. Conclusions. Perm1 is a cardiomyocyte-intrinsic determinant of bioenergetic reserve, dispensable for resting cardiac function but required to expand ATP production under increased demand. These findings identify a mode of metabolic regulation that is invisible to resting functional phenotyping.