Unveiling the intricate role of S100A1 in regulating RyR1 activity: A commentary on "Structural insights into the regulation of RyR1 by S100A1".

S100A1, a calcium-binding protein, plays a crucial role in regulating Ca2+ signaling pathways in skeletal and cardiac myocytes via interactions with the ryanodine receptor (RyR) to affect Ca2+ release and contractile performance. Biophysical studies strongly suggest that S100A1 interacts with RyRs but have been inconclusive about both the nature of this interaction and its competition with another important calcium-binding protein, calmodulin (CaM). Thus, high-resolution cryo-EM studies of RyRs in the presence of S100A1, with or without additional CaM, were needed. The elegant work by Weninger et al. demonstrates the interaction between S100A1 and RyR1 through various experiments and confirms that S100A1 activates RyR1 at sub-micromolar Ca2+ concentrations, increasing the open probability of RyR1 channels.

Scalable Hypothalamic Arcuate Neuron Differentiation from Human Pluripotent Stem Cells Suitable for Modeling Metabolic and Reproductive Disorders.

The hypothalamus, composed of several nuclei, is essential for maintaining our body's homeostasis. The arcuate nucleus (ARC), located in the mediobasal hypothalamus, contains neuronal populations with eminent roles in energy and glucose homeostasis as well as reproduction. These neuronal populations are of great interest for translational research. To fulfill this promise, we used a robotic cell culture platform to provide a scalable and chemically defined approach for differentiating human pluripotent stem cells (hPSCs) into pro-opiomelanocortin (POMC), somatostatin (SST), tyrosine hydroxylase (TH) and gonadotropin-releasing hormone (GnRH) neuronal subpopulations with an ARC-like signature. This robust approach is reproducible across several distinct hPSC lines and exhibits a stepwise induction of key ventral diencephalon and ARC markers in transcriptomic profiling experiments. This is further corroborated by direct comparison to human fetal hypothalamus, and the enriched expression of genes implicated in obesity and type 2 diabetes (T2D). Genome-wide chromatin accessibility profiling by ATAC-seq identified accessible regulatory regions that can be utilized to predict candidate enhancers related to metabolic disorders and hypothalamic development. In depth molecular, cellular, and functional experiments unveiled the responsiveness of the hPSC-derived hypothalamic neurons to hormonal stimuli, such as insulin, neuropeptides including kisspeptin, and incretin mimetic drugs such as Exendin-4, highlighting their potential utility as physiologically relevant cellular models for disease studies. In addition, differential glucose and insulin treatments uncovered adaptability within the generated ARC neurons in the dynamic regulation of POMC and insulin receptors. In summary, the establishment of this model represents a novel, chemically defined, and scalable platform for manufacturing large numbers of hypothalamic arcuate neurons and serves as a valuable resource for modeling metabolic and reproductive disorders.

N-terminal region is responsible for mHv1 channel activity in MDSCs.

Voltage-gated proton channels (Hv1) are important regulators of the immunosuppressive function of myeloid-derived suppressor cells (MDSCs) in mice and have been proposed as a potential therapeutic target to alleviate dysregulated immunosuppression in tumors. However, till date, there is a lack of evidence regarding the functioning of the Hvcn1 and reports on mHv1 isoform diversity in mice and MDSCs. A computational prediction has suggested that the Hvcn1 gene may express up to six transcript variants, three of which are translated into distinct N-terminal isoforms of mHv1: mHv1.1 (269 aa), mHv1.2 (269 + 42 aa), and mHv1.3 (269 + 4 aa). To validate this prediction, we used RT-PCR on total RNA extracted from MDSCs, and the presence of all six predicted mRNA variances was confirmed. Subsequently, the open-reading frames (ORFs) encoding for mHv1 isoforms were cloned and expressed in Xenopus laevis oocytes for proton current recording using a macro-patch voltage clamp. Our findings reveal that all three isoforms are mammalian mHv1 channels, with distinct differences in their activation properties. Specifically, the longest isoform, mHv1.2, displays a right-shifted conductance-voltage (GV) curve and slower opening kinetics, compared to the mid-length isoform, mHv1.3, and the shortest canonical isoform, mHv1.1. While mHv1.3 exhibits a V0.5 similar to that of mHv1.1, mHv1.3 demonstrates significantly slower activation kinetics than mHv1.1. These results suggest that isoform gating efficiency is inversely related to the length of the N-terminal end. To further explore this, we created the truncated mHv1.2 ΔN20 construct by removing the first 20 amino acids from the N-terminus of mHv1.2. This construct displayed intermediate activation properties, with a V0.5 value lying intermediate of mHv1.1 and mHv1.2, and activation kinetics that were faster than that of mHv1.2 but slower than that of mHv1.1. Overall, these findings indicate that alternative splicing of the N-terminal exon in mRNA transcripts encoding mHv1 isoforms is a regulatory mechanism for mHv1 function within MDSCs. While MDSCs have the capability to translate multiple Hv1 isoforms with varying gating properties, the Hvcn1 gene promotes the dominant expression of mHv1.1, which exhibits the most efficient gating among all mHv1 isoforms.

Functional Site-Directed Fluorometry in Native Cells to Study Skeletal Muscle Excitability.

Functional site-directed fluorometry has been the technique of choice to investigate the structure-function relationship of numerous membrane proteins, including voltage-gated ion channels. This approach has been used primarily in heterologous expression systems to simultaneously measure membrane currents, the electrical manifestation of the channels' activity, and fluorescence measurements, reporting local domain rearrangements. Functional site-directed fluorometry combines electrophysiology, molecular biology, chemistry, and fluorescence into a single wide-ranging technique that permits the study of real-time structural rearrangements and function through fluorescence and electrophysiology, respectively. Typically, this approach requires an engineered voltage-gated membrane channel that contains a cysteine that can be tested by a thiol-reactive fluorescent dye. Until recently, the thiol-reactive chemistry used for the site-directed fluorescent labeling of proteins was carried out exclusively in Xenopus oocytes and cell lines, restricting the scope of the approach to primary non-excitable cells. This report describes the applicability of functional site-directed fluorometry in adult skeletal muscle cells to study the early steps of excitation-contraction coupling, the process by which muscle fiber electrical depolarization is linked to the activation of muscle contraction. The present protocol describes the methodologies to design and transfect cysteine-engineered voltage-gated Ca2+ channels (CaV1.1) into muscle fibers of the flexor digitorum brevis of adult mice using in vivo electroporation and the subsequent steps required for functional site-directed fluorometry measurements. This approach can be adapted to study other ion channels and proteins. The use of functional site-directed fluorometry of mammalian muscle is particularly relevant to studying basic mechanisms of excitability.

Voltage sensor current, SR Ca2+ release, and Ca2+ channel current during trains of action potential-like depolarizations of skeletal muscle fibers.

In skeletal muscle, CaV 1.1 serves as the voltage sensor for both excitation-contraction coupling (ECC) and L-type Ca2+ channel activation. We have recently adapted the technique of action potential (AP) voltage clamp (APVC) to monitor the current generated by the movement of intramembrane voltage sensors (IQ ) during single imposed transverse tubular AP-like depolarization waveforms (IQAP ). We now extend this procedure to monitoring IQAP , and Ca2+ currents during trains of tubular AP-like waveforms in adult murine skeletal muscle fibers, and compare them with the trajectories of APs and AP-induced Ca2+ release measured in other fibers using field stimulation and optical probes. The AP waveform remains relatively constant during brief trains (<1 sec) for propagating APs in non-V clamped fibers. Trains of 10 AP-like depolarizations at 10 Hz (900 ms), 50 Hz (180 ms), or 100 Hz (90 ms) did not alter IQAP amplitude or kinetics, consistent with previous findings in isolated muscle fibers where negligible charge immobilization occurred during 100 ms step depolarizations. Using field stimulation, Ca2+ release did exhibit a considerable decline from pulse to pulse during the train, also consistent with previous findings, indicating that the decline of Ca2+ release during a short train of APs is not correlated to modification of charge movement. Ca2+ currents during single or 10 Hz trains of AP-like depolarizations were hardly detectable, were minimal during 50 Hz trains, and became more evident during 100 Hz trains in some fibers. Our results verify predictions on the behavior of the ECC machinery in response to AP-like depolarizations and provide a direct demonstration that Ca2+ currents elicited by single AP-like waveforms are negligible, but can become more prominent in some fibers during short high-frequency train stimulation that elicits maximal isometric force.

Contribution of skeletal muscle-specific microRNA-133b to insulin resistance in heart failure.

Insulin resistance (IR) is one of the hallmarks of heart failure (HF). Abnormalities in skeletal muscle (SM) metabolism have been identified in patients with HF. However, the underlying mechanisms of IR development in SM in HF are poorly understood. Herein, we hypothesize that HF upregulates miR-133b in SM and in turn alters glucose metabolism and the propensity toward IR. Mitochondria isolated from SM of mice with HF induced by transverse aortic constriction (TAC) showed lower respiration and downregulation of muscle-specific components of the tricarboxylic acid (TCA) cycle, AMP deaminase 1 (AMPD1), and fumarate compared with those from control animals. RNA-Seq and subsequent qPCR validation confirmed upregulation of SM-specific microRNA (miRNA), miR-133b, in TAC versus sham animals. miR-133b overexpression alone resulted in significantly lower mitochondrial respiration, cellular glucose uptake, and glycolysis along with lower ATP production and cellular energy reserve compared with the scramble (Scr) in C2C12 cells. miR-133b binds to the 3'-untranslated region (UTR) of KLF15, the transcription factor for the insulin-sensitive glucose transporter, GLUT4. Overexpression of miR-133b lowers GLUT4 and lowers pAkt in presence of insulin in C2C12 cells. Finally, lowering miR-133b in primary skeletal myocytes isolated from TAC mice using antagomir-133b reversed the changes in KLF15, GLUT4, and AMPD1 compared with the scramble-transfected myocytes. Taken together, these data demonstrate a role for SM miR-133b in altered glucose metabolism in HF and suggest the therapeutic potential in HF to improve glucose uptake and glycolysis by restoring GLUT4 abundance. The data uncover a novel mechanism for IR and ultimately SM metabolic abnormalities in patients with HF.NEW & NOTEWORTHY Heart failure is associated with systemic insulin resistance and abnormalities in glucose metabolism but the underlying mechanisms are poorly understood. In the skeletal muscle, the major peripheral site of glucose utilization, we observe an increase in miR-133b in heart failure mice, which reduces the insulin-sensitive glucose transporter (GLUT4), glucose uptake, and metabolism in C2C12 and in myocytes. The antagomir for miR-133b restores GLUT4 protein and markers of metabolism in skeletal myocytes from heart failure mice demonstrating that miR-133b is an exciting target for systemic insulin resistance in heart failure and an important player in the cross talk between the heart and the periphery in the heart failure syndrome.

Advances in CaV1.1 gating: New insights into permeation and voltage-sensing mechanisms.

The CaV1.1 voltage-gated Ca2+ channel carries L-type Ca2+ current and is the voltage-sensor for excitation-contraction (EC) coupling in skeletal muscle. Significant breakthroughs in the EC coupling field have often been close on the heels of technological advancement. In particular, CaV1.1 was the first voltage-gated Ca2+ channel to be cloned, the first ion channel to have its gating current measured and the first ion channel to have an effectively null animal model. Though these innovations have provided invaluable information regarding how CaV1.1 detects changes in membrane potential and transmits intra- and inter-molecular signals which cause opening of the channel pore and support Ca2+ release from the sarcoplasmic reticulum remain elusive. Here, we review current perspectives on this topic including the recent application of functional site-directed fluorometry.

Trapping Charge Mechanism in Hv1 Channels (CiHv1).

The majority of voltage-gated ion channels contain a defined voltage-sensing domain and a pore domain composed of highly conserved amino acid residues that confer electrical excitability via electromechanical coupling. In this sense, the voltage-gated proton channel (Hv1) is a unique protein in that voltage-sensing, proton permeation and pH-dependent modulation involve the same structural region. In fact, these processes synergistically work in concert, and it is difficult to separate them. To investigate the process of Hv1 voltage sensor trapping, we follow voltage-sensor movements directly by leveraging mutations that enable the measurement of Hv1 channel gating currents. We uncover that the process of voltage sensor displacement is due to two driving forces. The first reveals that mutations in the selectivity filter (D160) located in the S1 transmembrane interact with the voltage sensor. More hydrophobic amino acids increase the energy barrier for voltage sensor activation. On the other hand, the effect of positive charges near position 264 promotes the formation of salt bridges between the arginines of the voltage sensor domain, achieving a stable conformation over time. Our results suggest that the activation of the Hv1 voltage sensor is governed by electrostatic-hydrophobic interactions, and S4 arginines, N264 and selectivity filter (D160) are essential in the Ciona-Hv1 to understand the trapping of the voltage sensor.

Selective posttranslational inhibition of CaVß1-associated voltage-dependent calcium channels with a functionalized nanobody.

Ca2+ influx through high-voltage-activated calcium channels (HVACCs) controls diverse cellular functions. A critical feature enabling a singular signal, Ca2+ influx, to mediate disparate functions is diversity of HVACC pore-forming α1 and auxiliary CaVß1-CaVß4 subunits. Selective CaVα1 blockers have enabled deciphering their unique physiological roles. By contrast, the capacity to post-translationally inhibit HVACCs based on CaVß isoform is non-existent. Conventional gene knockout/shRNA approaches do not adequately address this deficit owing to subunit reshuffling and partially overlapping functions of CaVß isoforms. Here, we identify a nanobody (nb.E8) that selectively binds CaVß1 SH3 domain and inhibits CaVß1-associated HVACCs by reducing channel surface density, decreasing open probability, and speeding inactivation. Functionalizing nb.E8 with Nedd4L HECT domain yielded Chisel-1 which eliminated current through CaVß1-reconstituted CaV1/CaV2 and native CaV1.1 channels in skeletal muscle, strongly suppressed depolarization-evoked Ca2+ influx and excitation-transcription coupling in hippocampal neurons, but was inert against CaVß2-associated CaV1.2 in cardiomyocytes. The results introduce an original method for probing distinctive functions of ion channel auxiliary subunit isoforms, reveal additional dimensions of CaVß1 signaling in neurons, and describe a genetically-encoded HVACC inhibitor with unique properties.

Voltage sensor movements of CaV1.1 during an action potential in skeletal muscle fibers.

The skeletal muscle L-type Ca2+ channel (CaV1.1) works primarily as a voltage sensor for skeletal muscle action potential (AP)-evoked Ca2+ release. CaV1.1 contains four distinct voltage-sensing domains (VSDs), yet the contribution of each VSD to AP-evoked Ca2+ release remains unknown. To investigate the role of VSDs in excitation-contraction coupling (ECC), we encoded cysteine substitutions on each S4 voltage-sensing segment of CaV1.1, expressed each construct via in vivo gene transfer electroporation, and used in cellulo AP fluorometry to track the movement of each CaV1.1 VSD in skeletal muscle fibers. We first provide electrical measurements of CaV1.1 voltage sensor charge movement in response to an AP waveform. Then we characterize the fluorescently labeled channels' VSD fluorescence signal responses to an AP and compare them with the waveforms of the electrically measured charge movement, the optically measured free myoplasmic Ca2+, and the calculated rate of Ca2+ release from the sarcoplasmic reticulum for an AP, the physiological signal for skeletal muscle fiber activation. A considerable fraction of the fluorescence signal for each VSD occurred after the time of peak Ca2+ release, and even more occurred after the earlier peak of electrically measured charge movement during an AP, and thus could not directly reflect activation of Ca2+ release or charge movement, respectively. However, a sizable fraction of the fluorometric signals for VSDs I, II, and IV, but not VSDIII, overlap the rising phase of charge moved, and even more for Ca2+ release, and thus could be involved in voltage sensor rearrangements or Ca2+ release activation.

CaMKII oxidation is a critical performance/disease trade-off acquired at the dawn of vertebrate evolution.

Antagonistic pleiotropy is a foundational theory that predicts aging-related diseases are the result of evolved genetic traits conferring advantages early in life. Here we examine CaMKII, a pluripotent signaling molecule that contributes to common aging-related diseases, and find that its activation by reactive oxygen species (ROS) was acquired more than half-a-billion years ago along the vertebrate stem lineage. Functional experiments using genetically engineered mice and flies reveal ancestral vertebrates were poised to benefit from the union of ROS and CaMKII, which conferred physiological advantage by allowing ROS to increase intracellular Ca2+ and activate transcriptional programs important for exercise and immunity. Enhanced sensitivity to the adverse effects of ROS in diseases and aging is thus a trade-off for positive traits that facilitated the early and continued evolutionary success of vertebrates.

Mechanoactivation of NOX2-generated ROS elicits persistent TRPM8 Ca2+ signals that are inhibited by oncogenic KRas.

Changes in the mechanical microenvironment and mechanical signals are observed during tumor progression, malignant transformation, and metastasis. In this context, understanding the molecular details of mechanotransduction signaling may provide unique therapeutic targets. Here, we report that normal breast epithelial cells are mechanically sensitive, responding to transient mechanical stimuli through a two-part calcium signaling mechanism. We observed an immediate, robust rise in intracellular calcium (within seconds) followed by a persistent extracellular calcium influx (up to 30 min). This persistent calcium was sustained via microtubule-dependent mechanoactivation of NADPH oxidase 2 (NOX2)-generated reactive oxygen species (ROS), which acted on transient receptor potential cation channel subfamily M member 8 (TRPM8) channels to prolong calcium signaling. In contrast, the introduction of a constitutively active oncogenic KRas mutation inhibited the magnitude of initial calcium signaling and severely blunted persistent calcium influx. The identification that oncogenic KRas suppresses mechanically-induced calcium at the level of ROS provides a mechanism for how KRas could alter cell responses to tumor microenvironment mechanics and may reveal chemotherapeutic targets for cancer. Moreover, we find that expression changes in both NOX2 and TRPM8 mRNA predict poor clinical outcome in estrogen receptor (ER)-negative breast cancer patients, a population with limited available treatment options. The clinical and mechanistic data demonstrating disruption of this mechanically-activated calcium pathway in breast cancer patients and by KRas activation reveal signaling alterations that could influence cancer cell responses to the tumor mechanical microenvironment and impact patient survival.

LRP1 (Low-Density Lipoprotein Receptor-Related Protein 1) Regulates Smooth Muscle Contractility by Modulating Ca2+ Signaling and Expression of Cytoskeleton-Related Proteins.

Objective- Mutations affecting contractile-related proteins in the ECM (extracellular matrix), microfibrils, or vascular smooth muscle cells can predispose the aorta to aneurysms. We reported previously that the LRP1 (low-density lipoprotein receptor-related protein 1) maintains vessel wall integrity, and smLRP1-/- mice exhibited aortic dilatation. The current study focused on defining the mechanisms by which LRP1 regulates vessel wall function and integrity. Approach and Results- Isometric contraction assays demonstrated that vasoreactivity of LRP1-deficient aortic rings was significantly attenuated when stimulated with vasoconstrictors, including phenylephrine, thromboxane receptor agonist U-46619, increased potassium, and L-type Ca2+ channel ligand FPL-64176. Quantitative proteomics revealed proteins involved in actin polymerization and contraction were significantly downregulated in aortas of smLRP1-/- mice. However, studies with calyculin A indicated that although aortic muscle from smLRP1-/- mice can contract in response to calyculin A, a role for LRP1 in regulating the contractile machinery is not revealed. Furthermore, intracellular calcium imaging experiments identified defects in calcium release in response to a RyR (ryanodine receptor) agonist in smLRP1-/- aortic rings and cultured vascular smooth muscle cells. Conclusions- These results identify a critical role for LRP1 in modulating vascular smooth muscle cell contraction by regulating calcium signaling events that potentially protect against aneurysm development.

Loss of S100A1 expression leads to Ca2+ release potentiation in mutant mice with disrupted CaM and S100A1 binding to CaMBD2 of RyR1.

Calmodulin (CaM) and S100A1 fine-tune skeletal muscle Ca2+ release via opposite modulation of the ryanodine receptor type 1 (RyR1). Binding to and modulation of RyR1 by CaM and S100A1 occurs predominantly at the region ranging from amino acid residue 3614-3640 of RyR1 (here referred to as CaMBD2). Using synthetic peptides, it has been shown that CaM binds to two additional regions within the RyR1, specifically residues 1975-1999 and 4295-4325 (CaMBD1 and CaMBD3, respectively). Because S100A1 typically binds to similar motifs as CaM, we hypothesized that S100A1 could also bind to CaMBD1 and CaMBD3. Our goals were: (1) to establish whether S100A1 binds to synthetic peptides containing CaMBD1 and CaMBD3 using isothermal calorimetry (ITC), and (2) to identify whether S100A1 and CaM modulate RyR1 Ca2+ release activation via sites other than CaMBD2 in RyR1 in its native cellular context. We developed the mouse model (RyR1D-S100A1KO), which expresses point mutation RyR1-L3625D (RyR1D) that disrupts the modulation of RyR1 by CaM and S100A1 at CaMBD2 and also lacks S100A1 (S100A1KO). ITC assays revealed that S100A1 binds with different affinities to CaMBD1 and CaMBD3. Using high-speed Ca2+ imaging and a model for Ca2+ binding and transport, we show that the RyR1D-S100A1KO muscle fibers exhibit a modest but significant increase in myoplasmic Ca2+ transients and enhanced Ca2+ release flux following field stimulation when compared to fibers from RyR1D mice, which were used as controls to eliminate any effect of binding at CaMBD2, but with preserved S100A1 expression. Our results suggest that S100A1, similar to CaM, binds to CaMBD1 and CaMBD3 within the RyR1, but that CaMBD2 appears to be the primary site of RyR1 regulation by CaM and S100A1.

Voltage sensing mechanism in skeletal muscle excitation-contraction coupling: coming of age or midlife crisis?

The process by which muscle fiber electrical depolarization is linked to activation of muscle contraction is known as excitation-contraction coupling (ECC). Our understanding of ECC has increased enormously since the early scientific descriptions of the phenomenon of electrical activation of muscle contraction by Galvani that date back to the end of the eighteenth century. Major advances in electrical and optical measurements, including muscle fiber voltage clamp to reveal membrane electrical properties, in conjunction with the development of electron microscopy to unveil structural details provided an elegant view of ECC in skeletal muscle during the last century. This surge of knowledge on structural and biophysical aspects of the skeletal muscle was followed by breakthroughs in biochemistry and molecular biology, which allowed for the isolation, purification, and DNA sequencing of the muscle fiber membrane calcium channel/transverse tubule (TT) membrane voltage sensor (Cav1.1) for ECC and of the muscle ryanodine receptor/sarcoplasmic reticulum Ca2+ release channel (RyR1), two essential players of ECC in skeletal muscle. In regard to the process of voltage sensing for controlling calcium release, numerous studies support the concept that the TT Cav1.1 channel is the voltage sensor for ECC, as well as also being a Ca2+ channel in the TT membrane. In this review, we present early and recent findings that support and define the role of Cav1.1 as a voltage sensor for ECC.

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.

Acute Elevated Glucose Promotes Abnormal Action Potential-Induced Ca2+ Transients in Cultured Skeletal Muscle Fibers.

A common comorbidity of diabetes is skeletal muscle dysfunction, which leads to compromised physical function. Previous studies of diabetes in skeletal muscle have shown alterations in excitation-contraction coupling (ECC)-the sequential link between action potentials (AP), intracellular Ca2+ release, and the contractile machinery. Yet, little is known about the impact of acute elevated glucose on the temporal properties of AP-induced Ca2+ transients and ionic underlying mechanisms that lead to muscle dysfunction. Here, we used high-speed confocal Ca2+ imaging to investigate the temporal properties of AP-induced Ca2+ transients, an intermediate step of ECC, using an acute in cellulo model of uncontrolled hyperglycemia (25 mM, 48 h.). Control and elevated glucose-exposed muscle fibers cultured for five days displayed four distinct patterns of AP-induced Ca2+ transients (phasic, biphasic, phasic-delayed, and phasic-slow decay); most control muscle fibers show phasic AP-induced Ca2+ transients, while most fibers exposed to elevated D-glucose displayed biphasic Ca2+ transients upon single field stimulation. We hypothesize that these changes in the temporal profile of the AP-induced Ca2+ transients are due to changes in the intrinsic excitable properties of the muscle fibers. We propose that these changes accompany early stages of diabetic myopathy.

The Activation of Protein Kinase A by the Calcium-Binding Protein S100A1 Is Independent of Cyclic AMP.

Biochemical and structural studies demonstrate that S100A1 is involved in a Ca2+-dependent interaction with the type 2α and type 2ß regulatory subunits of protein kinase A (PKA) (RIIα and RIIß) to activate holo-PKA. The interaction was specific for S100A1 because other calcium-binding proteins (i.e., S100B and calmodulin) had no effect. Likewise, a role for S100A1 in PKA-dependent signaling was established because the PKA-dependent subcellular redistribution of HDAC4 was abolished in cells derived from S100A1 knockout mice. Thus, the Ca2+-dependent interaction between S100A1 and the type 2 regulatory subunits represents a novel mechanism that provides a link between Ca2+ and PKA signaling, which is important for the regulation of gene expression in skeletal muscle via HDAC4 cytosolic-nuclear trafficking.

Altered nuclear dynamics in MDX myofibers.

Duchenne muscular dystrophy (DMD) is a genetic disorder in which the absence of dystrophin leads to progressive muscle degeneration and weakness. Although the genetic basis is known, the pathophysiology of dystrophic skeletal muscle remains unclear. We examined nuclear movement in wild-type (WT) and muscular dystrophy mouse model for DMD (MDX) (dystrophin-null) mouse myofibers. We also examined expression of proteins in the linkers of nucleoskeleton and cytoskeleton (LINC) complex, as well as nuclear transcriptional activity via histone H3 acetylation and polyadenylate-binding nuclear protein-1. Because movement of nuclei is not only LINC dependent but also microtubule dependent, we analyzed microtubule density and organization in WT and MDX myofibers, including the application of a unique 3D tool to assess microtubule core structure. Nuclei in MDX myofibers were more mobile than in WT myofibers for both distance traveled and velocity. MDX muscle shows reduced expression and labeling intensity of nesprin-1, a LINC protein that attaches the nucleus to the microtubule and actin cytoskeleton. MDX nuclei also showed altered transcriptional activity. Previous studies established that microtubule structure at the cortex is disrupted in MDX myofibers; our analyses extend these findings by showing that microtubule structure in the core is also disrupted. In addition, we studied malformed MDX myofibers to better understand the role of altered myofiber morphology vs. microtubule architecture in the underlying susceptibility to injury seen in dystrophic muscles. We incorporated morphological and microtubule architectural concepts into a simplified finite element mathematical model of myofiber mechanics, which suggests a greater contribution of myofiber morphology than microtubule structure to muscle biomechanical performance.NEW & NOTEWORTHY Microtubules provide the means for nuclear movement but show altered organization in the muscular dystrophy mouse model (MDX) (dystrophin-null) muscle. Here, MDX myofibers show increased nuclear movement, altered transcriptional activity, and altered linkers of nucleoskeleton and cytoskeleton complex expression compared with healthy myofibers. Microtubule architecture was incorporated in finite element modeling of passive stretch, revealing a role of fiber malformation, commonly found in MDX muscle. The results suggest that alterations in microtubule architecture in MDX muscle affect nuclear movement, which is essential for muscle function.