Na + /K + ATPase-Ca v 1.2 nanodomain differentially regulates intracellular [Na + ], [Ca 2+ ] and local adrenergic signaling in cardiac myocytes

Background

The intracellular Na + concentration ([Na + ] i ) is a crucial but understudied regulator of cardiac myocyte function. The Na + /K + ATPase (NKA) controls the steady-state [Na + ] i and thereby determines the set-point for intracellular Ca 2+ . Here, we investigate the nanoscopic organization and local adrenergic regulation of the NKA macromolecular complex and how it differentially regulates the intracellular Na + and Ca 2+ homeostases in atrial and ventricular myocytes.

Methods

Multicolor STORM super-resolution microscopy, Western Blot analyses, and in vivo examination of adrenergic regulation are employed to examine the organization and function of Na + nanodomains in cardiac myocytes. Quantitative fluorescence microscopy at high spatiotemporal resolution is used in conjunction with cellular electrophysiology to investigate intracellular Na + homeostasis in atrial and ventricular myocytes.

Results

The NKAα1 (NKAα1) and the L-type Ca 2+ -channel (Ca v 1.2) form a nanodomain with a center-to center distance of ∼65 nm in both ventricular and atrial myocytes. NKAα1 protein expression levels are ∼3 fold higher in atria compared to ventricle. 100% higher atrial I NKA , produced by large NKA "superclusters", underlies the substantially lower Na + concentration in atrial myocytes compared to the benchmark values set in ventricular myocytes. The NKA's regulatory protein phospholemman (PLM) has similar expression levels across atria and ventricle resulting in a much lower PLM/NKAα1 ratio for atrial compared to ventricular tissue. In addition, a huge PLM phosphorylation reserve in atrial tissue produces a high ß-adrenergic sensitivity of I NKA in atrial myocytes. ß-adrenergic regulation of I NKA is locally mediated in the NKAα1-Ca v 1.2 nanodomain via A-kinase anchoring proteins.

Conclusions

NKAα1, Ca v 1.2 and their accessory proteins form a structural and regulatory nanodomain at the cardiac dyad. The tissue-specific composition and local adrenergic regulation of this "signaling cloud" is a main regulator of the distinct global intracellular Na + and Ca 2+ concentrations in atrial and ventricular myocytes.

Detyrosinated microtubule arrays drive myofibrillar malformations in mdx muscle fibers

Altered myofibrillar structure is a consequence of dystrophic pathology that impairs skeletal muscle contractile function and increases susceptibility to contraction injury. In murine Duchenne muscular dystrophy (mdx), myofibrillar alterations are abundant in advanced pathology (>4 months), an age where we formerly established densified microtubule (MT) arrays enriched in detyrosinated (deTyr) tubulin as negative disease modifiers impacting cell mechanics and mechanotransduction. Given the essential role of deTyr-enriched MT arrays in myofibrillar growth, maintenance, and repair, we examined the increased abundance of these arrays as a potential mechanism for these myofibrillar alterations. Here we find an increase in deTyr-tubulin as an early event in dystrophic pathology (4 weeks) with no evidence myofibrillar alterations. At 16 weeks, we show deTyr-enriched MT arrays significantly densified and co-localized to areas of myofibrillar malformation. Profiling the enzyme complexes responsible for deTyr-tubulin, we identify vasohibin 2 (VASH2) and small vasohibin binding protein (SVBP) significantly elevated in the mdx muscle at 4 weeks. Using the genetic increase in VASH2/SVBP expression in 4 weeks wild-type mice we find densified deTyr-enriched MT arrays that co-segregate with myofibrillar malformations similar to those in the 16 weeks mdx. Given that no changes in sarcomere organization were identified in fibers expressing sfGFP as a control, we conclude that disease-dependent densification of deTyr-enriched MT arrays underscores the altered myofibrillar structure in dystrophic skeletal muscle fibers.

Attenuating persistent sodium current-induced atrial myopathy and fibrillation by preventing mitochondrial oxidative stress

Mechanistically driven therapies for atrial fibrillation (AF), the most common cardiac arrhythmia, are urgently needed, the development of which requires improved understanding of the cellular signaling pathways that facilitate the structural and electrophysiological remodeling that occurs in the atria. Similar to humans, increased persistent Na+ current leads to the development of an atrial myopathy and spontaneous and long-lasting episodes of AF in mice. How increased persistent Na+ current causes both structural and electrophysiological remodeling in the atria is unknown. We crossbred mice expressing human F1759A-NaV1.5 channels with mice expressing human mitochondrial catalase (mCAT). Increased expression of mCAT attenuated mitochondrial and cellular reactive oxygen species (ROS) and the structural remodeling that was induced by persistent F1759A-Na+ current. Despite the heterogeneously prolonged atrial action potential, which was unaffected by the reduction in ROS, the incidences of spontaneous AF, pacing-induced after-depolarizations, and AF were substantially reduced. Expression of mCAT markedly reduced persistent Na+ current-induced ryanodine receptor oxidation and dysfunction. In summary, increased persistent Na+ current in atrial cardiomyocytes, which is observed in patients with AF, induced atrial enlargement, fibrosis, mitochondrial dysmorphology, early after-depolarizations, and AF, all of which can be attenuated by resolving mitochondrial oxidative stress.

Sarcomeric deficits underlie MYBPC1-associated myopathy with myogenic tremor

Myosin binding protein-C slow (sMyBP-C) comprises a subfamily of cytoskeletal proteins encoded by MYBPC1 that is expressed in skeletal muscles where it contributes to myosin thick filament stabilization and actomyosin cross-bridge regulation. Recently, our group described the causal association of dominant missense pathogenic variants in MYBPC1 with an early-onset myopathy characterized by generalized muscle weakness, hypotonia, dysmorphia, skeletal deformities, and myogenic tremor, occurring in the absence of neuropathy. To mechanistically interrogate the etiologies of this MYBPC1-associated myopathy in vivo, we generated a knock-in mouse model carrying the E248K pathogenic variant. Using a battery of phenotypic, behavioral, and physiological measurements spanning neonatal to young adult life, we found that heterozygous E248K mice faithfully recapitulated the onset and progression of generalized myopathy, tremor occurrence, and skeletal deformities seen in human carriers. Moreover, using a combination of biochemical, ultrastructural, and contractile assessments at the level of the tissue, cell, and myofilaments, we show that the loss-of-function phenotype observed in mutant muscles is primarily driven by disordered and misaligned sarcomeres containing fragmented and out-of-register internal membranes that result in reduced force production and tremor initiation. Collectively, our findings provide mechanistic insights underscoring the E248K-disease pathogenesis and offer a relevant preclinical model for therapeutic discovery.

Calcium Signaling Silencing in Atrial Fibrillation: Implications for Atrial Sodium Homeostasis

Atrial fibrillation (AF) is the most common type of cardiac arrhythmia, affecting more than 33 million people worldwide. Despite important advances in therapy, AF's incidence remains high, and treatment often results in recurrence of the arrhythmia. A better understanding of the cellular and molecular changes that (1) trigger AF and (2) occur after the onset of AF will help to identify novel therapeutic targets. Over the past 20 years, a large body of research has shown that intracellular Ca2+ handling is dramatically altered in AF. While some of these changes are arrhythmogenic, other changes counteract cellular arrhythmogenic mechanisms (Calcium Signaling Silencing). The intracellular Na+ concentration ([Na+])i is a key regulator of intracellular Ca2+ handling in cardiac myocytes. Despite its importance in the regulation of intracellular Ca2+ handling, little is known about [Na+]i, its regulation, and how it might be changed in AF. Previous work suggests that there might be increases in the late component of the atrial Na+ current (INa,L) in AF, suggesting that [Na+]i levels might be high in AF. Indeed, a pharmacological blockade of INa,L has been suggested as a treatment for AF. Here, we review calcium signaling silencing and changes in intracellular Na+ homeostasis during AF. We summarize the proposed arrhythmogenic mechanisms associated with increases in INa,L during AF and discuss the evidence from clinical trials that have tested the pharmacological INa,L blocker ranolazine in the treatment of AF.

Disparate bone anabolic cues activate bone formation by regulating the rapid lysosomal degradation of sclerostin protein

The downregulation of sclerostin in osteocytes mediates bone formation in response to mechanical cues and parathyroid hormone (PTH). To date, the regulation of sclerostin has been attributed exclusively to the transcriptional downregulation of the Sost gene hours after stimulation. Using mouse models and rodent cell lines, we describe the rapid, minute-scale post-translational degradation of sclerostin protein by the lysosome following mechanical load and PTH. We present a model, integrating both new and established mechanically and hormonally activated effectors into the regulated degradation of sclerostin by lysosomes. Using a mouse forelimb mechanical loading model, we find transient inhibition of lysosomal degradation or the upstream mechano-signaling pathway controlling sclerostin abundance impairs subsequent load-induced bone formation by preventing sclerostin degradation. We also link dysfunctional lysosomes to aberrant sclerostin regulation using human Gaucher disease iPSCs. These results reveal how bone anabolic cues post-translationally regulate sclerostin abundance in osteocytes to regulate bone formation.

Tubulin acetylation increases cytoskeletal stiffness to regulate mechanotransduction in striated muscle

Microtubules tune cytoskeletal stiffness, which affects cytoskeletal mechanics and mechanotransduction of striated muscle. While recent evidence suggests that microtubules enriched in detyrosinated α-tubulin regulate these processes in healthy muscle and increase them in disease, the possible contribution from several other α-tubulin modifications has not been investigated. Here, we used genetic and pharmacologic strategies in isolated cardiomyocytes and skeletal myofibers to increase the level of acetylated α-tubulin without altering the level of detyrosinated α-tubulin. We show that microtubules enriched in acetylated α-tubulin increase cytoskeletal stiffness and viscoelastic resistance. These changes slow rates of contraction and relaxation during unloaded contraction and increased activation of NADPH oxidase 2 (Nox2) by mechanotransduction. Together, these findings add to growing evidence that microtubules contribute to the mechanobiology of striated muscle in health and disease.

Dynamic Measurement and Imaging of Capillaries, Arterioles, and Pericytes in Mouse Heart

Coronary arterial tone along with the opening or closing of the capillaries largely determine the blood flow to cardiomyocytes at constant perfusion pressure. However, it is difficult to monitor the dynamic changes of the coronary arterioles and the capillaries in the whole heart, primarily due to its motion and non-stop beating. Here we describe a method that enables monitoring of arterial perfusion rate, pressure and the diameter changes of the arterioles and capillaries in mouse right ventricular papillary muscles. The mouse septal artery is cannulated and perfused at a constant flow or pressure with the other dynamically measured. After perfusion with a fluorescently labeled lectin (e.g., Alexa Fluor-488 or -633 labeled Wheat-Germ Agglutinin, WGA), the arterioles and capillaries (and other vessels) in right ventricle papillary muscle and septum could be readily imaged. The vessel-diameter changes could then be measured in the presence or absence of heart contractions. When genetically encoded fluorescent proteins were expressed, specific features could be monitored. For examples, pericytes were visualized in mouse hearts that expressed NG2-DsRed. This method has provided a useful platform to study the physiological functions of capillary pericytes in heart. It is also suitable for studying the effect of reagents on the blood flow in heart by measuring the vascular/capillary diameter and the arterial luminal pressure simultaneously. This preparation, combined with a state-of-the-art optic imaging system, allows one to study the blood flow and its control at cellular and molecular level in the heart under near-physiological conditions.

TRPV4 calcium influx controls sclerostin protein loss independent of purinergic calcium oscillations

Skeletal remodeling is driven in part by the osteocyte's ability to respond to its mechanical environment by regulating the abundance of sclerostin, a negative regulator of bone mass. We have recently shown that the osteocyte responds to fluid shear stress via the microtubule network-dependent activation of NADPH oxidase 2 (NOX2)-generated reactive oxygen species and subsequent opening of TRPV4 cation channels, leading to calcium influx, activation of CaMKII, and rapid sclerostin protein downregulation. In addition to the initial calcium influx, purinergic receptor signaling and calcium oscillations occur in response to mechanical load and prior to rapid sclerostin protein loss. However, the independent contributions of TRPV4-mediated calcium influx and purinergic calcium oscillations to the rapid sclerostin protein downregulation remain unclear. Here, we showed that NOX2 and TRPV4-dependent calcium influx is required for calcium oscillations, and that TRPV4 activation is both necessary and sufficient for sclerostin degradation. In contrast, calcium oscillations are neither necessary nor sufficient to acutely decrease sclerostin protein abundance. However, blocking oscillations with apyrase prevented fluid shear stress induced changes in osterix (Sp7), osteoprotegerin (Tnfrsf11b), and sclerostin (Sost) gene expression. In total, these data provide key mechanistic insights into the way bone cells translate mechanical cues to target a key effector of bone formation, sclerostin.

Chronic Sympathetic Hyperactivity Triggers Electrophysiological Remodeling and Disrupts Excitation-Contraction Coupling in Heart

The sympathetic nervous system is essential for maintenance of cardiac function via activation of post-junctional adrenergic receptors. Prolonged adrenergic receptor activation, however, has deleterious long-term effects leading to hypertrophy and the development of heart failure. Here we investigate the effect of chronic adrenergic receptors activation on excitation-contraction coupling (ECC) in ventricular cardiomyocytes from a previously characterized mouse model of chronic sympathetic hyperactivity, which are genetically deficient in the adrenoceptor α2A and α2C genes (ARDKO). When compared to wild-type (WT) cardiomyocytes, ARDKO displayed reduced fractional shortening (~33%) and slower relaxation (~20%). Furthermore, ARDKO cells exhibited several electrophysiological changes such as action potential (AP) prolongation (~50%), reduced L-type calcium channel (LCC) current (~33%), reduced outward potassium (K+) currents (~30%), and increased sodium/calcium exchanger (NCX) activity (~52%). Consistent with reduced contractility and calcium (Ca2+) currents, the cytosolic Ca2+ ([Ca2+]i) transient from ARDKO animals was smaller and decayed slower. Importantly, no changes were observed in membrane resting potential, AP amplitude, or the inward K+ current. Finally, we modified our existing cardiac ECC computational model to account for changes in the ARDKO heart. Simulations suggest that cellular changes in the ARDKO heart resulted in variable and dyssynchronous Ca2+-induced Ca2+ release therefore altering [Ca2+]i transient dynamics and reducing force generation. In conclusion, chronic sympathetic hyperactivity impairs ECC by changing the density of several ionic currents (and thus AP repolarization) causing altered Ca2+ dynamics and contractile activity. This demonstrates the important role of ECC remodeling in the cardiac dysfunction secondary to chronic sympathetic activity.

Diminazene aceturate (DIZE) has cellular and in vivo antiarrhythmic effects

Diminazene aceturate (DIZE) is an anti-protozoan compound that has been previously reported to increase the activity of the angiotensin-converting enzyme 2 (ACE2) and thus increase Angiotensin-(1-7) production, leading to cardioprotection against post-myocardial infarction dysfunction and structural remodelling. Moreover, DIZE is able to ameliorate morpho-functional changes after myocardial infarction by enhancing ACE2 activity, thus increasing Angiotensin-(1-7) production (a benefic peptide of the renin-angiotensin system). However, despite the improvement in cardiac function/structure, little is known about DIZE effects on arrhythmia suppression, contraction/excitable aspects of the heart and importantly its mechanisms of action. Thus, our aim was to test the acute effect of DIZE cardioprotection at the specific level of potential antiarrhythmic effects and modulation in excitation-contraction coupling. For this, we performed in vitro and in vivo techniques for arrhythmia induction followed by an acute administration of DIZE. For the first time, we described that DIZE can reduce arrhythmias which is explained by modulation of cardiomyocyte contraction and excitability. Such effects were independent of Mas receptor and nitric oxide release. Development of a new DIZE-based approach to ameliorate myocardial contractile and electrophysiological dysfunction requires further investigation; however, DIZE may provide the basis for a future beneficial therapy to post-myocardial infarction patients.

Dynamics of the mitochondrial permeability transition pore: Transient and permanent opening events

A gentle optical examination of the mitochondrial permeability transition pore (mPTP) opening events was carried out in isolated quiescent ventricular myocytes by tracking the inner membrane potential (ΔΨM) using TMRM (tetramethylrhodamine methyl ester). Zeiss Airyscan 880 ″super-resolution" or "high-resolution" imaging was done with very low levels of illumination (0.009% laser power). In cellular areas imaged every 9 s (ROI or regions of interest), transient depolarizations of variable amplitudes occurred at increasing rates for the first 30 min. The time to first depolarization events was 8.4 min (±1.1 SEM n = 21 cells). At longer times, essentially permanent and irreversible depolarizations occurred at an increasing fraction of all events. In other cellular areas surrounding the ROI, mitochondria were rarely illuminated (once per 5 min) and virtually no permanent depolarization events occurred for over 1 h of imaging. These findings suggest that photon stress due to the imaging itself plays an important role in the generation of both the transient mPTP opening events as well as the permanent mPTP opening events. Consistent with the evidence that photon "stress" in mitochondria loaded with virtually any photon absorbing substance, generates reactive oxygen species (ROS) [1-5], we show that cyclosporine-A (CsA, 10 μM) and the antioxidant n-acetyl cysteine (NAC, 10 mM), reduced the number of events by 80% and 93% respectively. Furthermore, CsA and NAC treatment led to the virtual disappearance of permanent depolarization events. Nevertheless, all transient depolarization events in any condition (control, CsA and NAC) appeared to repolarize with a similar half-time of 30 ± 6 s (n = 478) at 37 °C. Further experiments showed quantitatively similar results in cerebral vascular smooth muscle cells, using a different confocal system, and different photon absorbing reagent (TMRE; tetramethylrhodamine ethyl ester). In these experiments, using modest power (1% laser power) transient depolarization events were seen in only 8 out of 23 cells while with higher power (8%), all cells showed transient events, which align with the level of photon stress being the driver of the effect. Together, our findings suggest that photon-induced ROS is sufficient to cause depolarization events of individual mitochondria in quiescent cells; without electrical or mechanical activity to stimulates mitochondrial metabolism, and without raising the mitochondrial matrix Ca2+. In a broad context, these findings neither support nor deny the relevance or occurrence of ΔΨM depolarization events in specific putatively physiologic mitochondrial behaviors such as MitoFlashes [6,7] or MitoWinks [8]. Instead, our findings raise a caution with regards to the physiological and pathophysiological functions attributed to singular ΔΨM depolarization events when those functions are investigated using photon absorbing substances. Nevertheless, using photon stress as a tool ("Optical Stress-Probe"), we can extract information on the activation, reversibility, permanency and kinetics of mitochondrial depolarization. These data may provide new information on mPTP, help identify the mPTP protein complex, and establish the physiological function of the mPTP protein complex and their links to MitoFlashes and MitoWinks.

Quantitative tests reveal that microtubules tune the healthy heart but underlie arrhythmias in pathology

KEY POINTS

Our group previously discovered and characterized the microtubule mechanotransduction pathway linking diastolic stretch to NADPH oxidase 2-derived reactive oxygen species signals that regulate calcium sparks and calcium influx pathways. Here we used focused experimental tests to constrain and expand our existing computational models of calcium signalling in heart. Mechanistic and quantitative modelling revealed new insights in disease including: changes in microtubule network density and properties, elevated NOX2 expression, altered calcium release dynamics, how NADPH oxidase 2 is activated by and responds to stretch, and finally the degree to which normalizing mechano-activated reactive oxygen species signals can prevent calcium-dependent arrhythmias.

ABSTRACT

Microtubule (MT) mechanotransduction links diastolic stretch to generation of NADPH oxidase 2 (NOX2)-dependent reactive oxygen species (ROS), signals we term X-ROS. While stretch-elicited X-ROS primes intracellular calcium (Ca²⁺ ) channels for synchronized activation in the healthy heart, the dysregulated excess in this pathway underscores asynchronous Ca²⁺ release and arrhythmia. Here, we expanded our existing computational models of Ca²⁺ signalling in heart to include MT-dependent mechanotransduction through X-ROS. Informed by new focused experimental tests to properly constrain our model, we quantify the role of X-ROS on excitation-contraction coupling in healthy and pathological conditions. This approach allowed for a mechanistic investigation that revealed new insights into X-ROS signalling in disease including changes in MT network density and post-translational modifications (PTMs), elevated NOX2 expression, altered Ca²⁺ release dynamics (i.e. Ca²⁺ sparks and Ca²⁺ waves), how NOX2 is activated by and responds to stretch, and finally the degree to which normalizing X-ROS can prevent Ca²⁺ -dependent arrhythmias.

Real-time scratch assay reveals mechanisms of early calcium signaling in breast cancer cells in response to wounding

Aggressive cellular phenotypes such as uncontrolled proliferation and increased migration capacity engender cellular transformation, malignancy and metastasis. While genetic mutations are undisputed drivers of cancer initiation and progression, it is increasingly accepted that external factors are also playing a major role. Two recently studied modulators of breast cancer are changes in the cellular mechanical microenvironment and alterations in calcium homeostasis. While many studies investigate these factors separately in breast cancer cells, very few do so in combination. This current work sets a foundation to explore mechano-calcium relationships driving malignant progression in breast cancer. Utilizing real-time imaging of an in vitro scratch assay, we were able to resolve mechanically-sensitive calcium signaling in human breast cancer cells. We observed rapid initiation of intracellular calcium elevations within seconds in cells at the immediate wound edge, followed by a time-dependent increase in calcium in cells at distances up to 500μm from the scratch wound. Calcium signaling to neighboring cells away from the wound edge returned to baseline within seconds. Calcium elevations at the wound edge however, persisted for up to 50 minutes. Rigorous quantification showed that extracellular calcium was necessary for persistent calcium elevation at the wound edge, but intercellular signal propagation was dependent on internal calcium stores. In addition, intercellular signaling required extracellular ATP and activation of P2Y₂ receptors. Through comparison of scratch-induced signaling from multiple cell lines, we report drastic reductions in response from aggressively tumorigenic and metastatic cells. The real-time scratch assay established here provides quantitative data on the molecular mechanisms that support rapid scratch-induced calcium signaling in breast cancer cells. These mechanisms now provide a clear framework for investigating which short-term calcium signals promote long-term changes in cancer cell biology.

Absence of synemin in mice causes structural and functional abnormalities in heart

Cardiomyopathies have been linked to changes in structural proteins, including intermediate filament (IF) proteins located in the cytoskeleton. IFs associate with the contractile machinery and costameres of striated muscle and with intercalated disks in the heart. Synemin is a large IF protein that mediates the association of desmin with Z-disks and stabilizes intercalated disks. It also acts as an A-kinase anchoring protein (AKAP). In murine skeletal muscle, the absence of synemin causes a mild myopathy. Here, we report that the genetic silencing of synemin in mice (synm -/-) causes left ventricular systolic dysfunction at 3months and 12-16months of age, and left ventricular hypertrophy and dilatation at 12-16months of age. Isolated cardiomyocytes showed alterations in calcium handling that indicate defects intrinsic to the heart. Although contractile and costameric proteins remained unchanged in the old synm -/- hearts, we identified alterations in several signaling proteins (PKA-RII, ERK and p70S6K) critical to cardiomyocyte function. Our data suggest that synemin plays an important regulatory role in the heart and that the consequences of its absence are profound.

Microtubules tune mechanotransduction through NOX2 and TRPV4 to decrease sclerostin abundance in osteocytes

The adaptation of the skeleton to its mechanical environment is orchestrated by mechanosensitive osteocytes, largely by regulating the abundance of sclerostin, a secreted inhibitor of bone formation. We defined a microtubule-dependent mechanotransduction pathway that linked fluid shear stress to reactive oxygen species (ROS) and calcium (Ca²⁺) signals that led to a reduction in sclerostin abundance in cultured osteocytes. We demonstrated that microtubules stabilized by detyrosination, a reversible posttranslational modification of polymerized α-tubulin, determined the stiffness of the cytoskeleton, which set the mechanoresponsive range of cultured osteocytes to fluid shear stress. We showed that fluid shear stress through the microtubule network activated NADPH oxidase 2 (NOX2)-generated ROS that target the Ca²⁺ channel TRPV4 to elicit Ca²⁺ influx. Furthermore, tuning the abundance of detyrosinated tubulin affected cytoskeletal stiffness to define the mechanoresponsive range of cultured osteocytes to fluid shear stress. Finally, we demonstrated that NOX2-ROS elicited Ca²⁺ signals that activated the kinase CaMKII to decrease the abundance of sclerostin protein. Together, these discoveries may identify potentially druggable targets for regulating osteocyte mechanotransduction to affect bone quality.