Stac proteins associate with the critical domain for excitation-contraction coupling in the II-III loop of CaV1.1

Intrinsic contractile dysfunction due to impaired sarcoplasmic reticulum Ca2+ release in compensatory hypertrophied muscle fibers following synergist ablation

Synergist ablation (SA) is an experimental procedure for the induction of hypertrophy. However, SA causes a decrease in specific force (i.e., force per cross-sectional area), likely due to excessive muscle use. Here, we investigated the mechanisms behind the SA-induced intrinsic contractile dysfunction, especially focusing on the excitation-contraction (EC) coupling. Male Wistar rats had unilateral surgical ablation of gastrocnemius and soleus muscles to induce compensatory hypertrophy in the plantaris muscles. Two weeks after SA, plantaris muscle was dissected from each animal and used for later analyses. SA significantly increased the mean fiber cross-sectional area (+18%). On the other hand, the ratio of depolarization-induced force to the maximum Ca2+-activated specific force, an indicator of sarcoplasmic reticulum (SR) Ca2+ release, was markedly reduced in mechanically skinned fibers from the SA group (-51%). These functional defects were accompanied by an extensive fragmentation of the SR Ca2+ release channel, the ryanodine receptor 1 (RyR1), and a decrease in the amount of other triad proteins (i.e., DHPR, STAC3, and junctophilin1). SA treatment also caused activation of calpain-1 and increased the amount of NADPH oxidase 2, endoplasmic reticulum (ER) stress proteins (i.e., Grp78, Grp94, PDI, and Ero1), and lipid peroxidation [i.e., 4-hydroxynonenal (4-HNE)] in SA-treated muscles. Our findings show that SA causes skeletal muscle weakness due to impaired EC coupling. This is likely to be induced by Ca2+-dependent degradation of triad proteins, which may result from Ca2+ leak from fragmented RyR1 triggered by increased oxidative stress.NEW & NOTEWORTHY Synergist ablation (SA) has widely been used to understand the mechanisms behind skeletal muscle hypertrophy. However, compensatory hypertrophied muscles display intrinsic contractile dysfunction, i.e., a hallmark of overuse. Here, we demonstrate that SA-induced compensatory hypertrophy is accompanied by muscle weakness due to impaired sarcoplasmic reticulum Ca2+ release. This dysfunction may be caused by the degradation of triad proteins due to the reciprocal amplification of reactive oxygen species and Ca2+ signaling at the junctional space microdomain.

CaV1.1 Calcium Channel Signaling Complexes in Excitation-Contraction Coupling: Insights from Channelopathies

In skeletal muscle, excitation-contraction (EC) coupling relies on the mechanical coupling between two ion channels: the L-type voltage-gated calcium channel (CaV1.1), located in the sarcolemma and functioning as the voltage sensor of EC coupling, and the ryanodine receptor 1 (RyR1), located on the sarcoplasmic reticulum serving as the calcium release channel. To this day, the molecular mechanism by which these two ion channels are linked remains elusive. However, recently, skeletal muscle EC coupling could be reconstituted in heterologous cells, revealing that only four proteins are essential for this process: CaV1.1, RyR1, and the cytosolic proteins CaVβ1a and STAC3. Due to the crucial role of these proteins in skeletal muscle EC coupling, any mutation that affects any one of these proteins can have devastating consequences, resulting in congenital myopathies and other pathologies.Here, we summarize the current knowledge concerning these four essential proteins and discuss the pathophysiology of the CaV1.1, RyR1, and STAC3-related skeletal muscle diseases with an emphasis on the molecular mechanisms. Being part of the same signalosome, mutations in different proteins often result in congenital myopathies with similar symptoms or even in the same disease.

Dissociation of SH3 and cysteine rich domain 3 and junctophilin 1 from dihydropyridine receptor in dystrophin-deficient muscles

The disruption of excitation-contraction (EC) coupling and subsequent reduction in Ca²⁺ release from the sarcoplasmic reticulum (SR) have been shown to account for muscle weakness seen in patients with Duchenne muscular dystrophy (DMD). Here, we examined the mechanisms underlying EC uncoupling in skeletal muscles from mdx52 and DMD-null/NSG mice, animal models for DMD, focusing on the SH3 and cysteine rich domain 3 (STAC3) and junctophilin 1 (JP1), which link the dihydropyridine receptor (DHPR) in the transverse tubule and the ryanodine receptor 1 in the SR. The isometric plantarflexion torque normalized to muscle weight of whole plantar flexor muscles was depressed in mdx52 and DMD-null/NSG mice compared to their control mice. This was accompanied by increased autolysis of calpain-1, decreased levels of STAC3 and JP1 content, and dissociation of STAC3 and JP1 from DHPR-α_1s in gastrocnemius muscles. Moreover, in vitro mechanistic experiments demonstrated that STAC3 and JP1 underwent Ca²⁺-dependent proteolysis which was less pronounced in dystrophin-deficient muscles where calpastatin, the endogenous calpain inhibitor, was upregulated. Eccentric contractions further enhanced autolysis of calpain-1 and proteolysis of STAC3 and JP1 that were associated with severe torque depression in gastrocnemius muscles from DMD-null/NSG mice. These data suggest that Ca²⁺-dependent proteolysis of STAC3 and JP1 may be an essential factor causing muscle weakness due to EC coupling failure in dystrophin-deficient muscles.

From α1s splicing to γ1 function: A new twist in subunit modulation of the skeletal muscle L-type Ca2+ channel

Melzer discusses a recent JGP study showing that alternative splicing of the skeletal muscle L-type calcium channel impacts on a modulatory effect of its γ subunit.

Structural Insight Into Ryanodine Receptor Channelopathies

The ryanodine receptors (RyRs) are large cation-selective ligand-gated channels that are expressed in the sarcoplasmic reticulum (SR) membrane. They mediate the controlled release of Ca²⁺ from SR and play an important role in many cellular processes. The mutations in RyRs are associated with several skeletal muscle and cardiac conditions, including malignant hyperthermia (MH), central core disease (CCD), catecholaminergic polymorphic ventricular tachycardia (CPVT), and arrhythmogenic right ventricular dysplasia (ARVD). Recent breakthroughs in structural biology including cryo-electron microscopy (EM) and X-ray crystallography allowed the determination of a number of near-atomic structures of RyRs, including wildtype and mutant structures as well as the structures in complex with different modulating molecules. This allows us to comprehend the physiological gating and regulatory mechanisms of RyRs and the underlying pathological mechanisms of the disease-causing mutations. In this review, based on the insights gained from the available high-resolution structures of RyRs, we address several questions: 1) what are the gating mechanisms of different RyR isoforms; 2) how RyRs are regulated by multiple channel modulators, including ions, small molecules, and regulatory proteins; 3) how do disease-causing mutations affect the structure and function of RyRs; 4) how can these structural information aid in the diagnosis of the related diseases and the development of pharmacological therapies.

Molecular interactions of STAC proteins with skeletal muscle dihydropyridine receptor and excitation‐contraction coupling

Excitation‐contraction coupling (ECC) is the physiological process in which an electrical signal originating from the central nervous system is converted into muscle contraction. In skeletal muscle tissue, the key step in the molecular mechanism of ECC initiated by the muscle action potential is the cooperation between two Ca²⁺ channels, dihydropyridine receptor (DHPR; voltage‐dependent L‐type calcium channel) and ryanodine receptor 1 (RyR1). These two channels were originally postulated to communicate with each other via direct mechanical interactions; however, the molecular details of this cooperation have remained ambiguous. Recently, it has been proposed that one or more supporting proteins are in fact required for communication of DHPR with RyR1 during the ECC process. One such protein that is increasingly believed to play a role in this interaction is the SH3 and cysteine‐rich domain‐containing protein 3 (STAC3), which has been proposed to bind a cytosolic portion of the DHPR α_1S subunit known as the II–III loop. In this work, we present direct evidence for an interaction between a small peptide sequence of the II–III loop and several residues within the SH3 domains of STAC3 as well as the neuronal isoform STAC2. Differences in this interaction between STAC3 and STAC2 suggest that STAC3 possesses distinct biophysical features that are potentially important for its physiological interactions with the II–III loop. Therefore, this work demonstrates an isoform‐specific interaction between STAC3 and the II–III loop of DHPR and provides novel insights into a putative molecular mechanism behind this association in the skeletal muscle ECC process.

The Builders of the Junction: Roles of Junctophilin1 and Junctophilin2 in the Assembly of the Sarcoplasmic Reticulum–Plasma Membrane Junctions in Striated Muscle

Contraction of striated muscle is triggered by a massive release of calcium from the sarcoplasmic reticulum (SR) into the cytoplasm. This intracellular calcium release is initiated by membrane depolarization, which is sensed by voltage-gated calcium channels Ca_V1.1 (in skeletal muscle) and Ca_V1.2 (in cardiac muscle) in the plasma membrane (PM), which in turn activate the calcium-releasing channel ryanodine receptor (RyR) embedded in the SR membrane. This cross-communication between channels in the PM and in the SR happens at specialized regions, the SR-PM junctions, where these two compartments come in close proximity. Junctophilin1 and Junctophilin2 are responsible for the formation and stabilization of SR-PM junctions in striated muscle and actively participate in the recruitment of the two essential players in intracellular calcium release, Ca_V and RyR. This short review focuses on the roles of junctophilins1 and 2 in the formation and organization of SR-PM junctions in skeletal and cardiac muscle and on the functional consequences of the absence or malfunction of these proteins in striated muscle in light of recently published data and recent advancements in protein structure prediction.

Preconditioning contractions prevent prolonged force depression and Ca2+-dependent proteolysis of STAC3 after damaging eccentric contractions

Preconditioning contractions (PCs) have been shown to markedly improve recovery from eccentric contractions (ECCs)-induced force depression. We here examined the mechanism behind the effects of PCs with focusing on the SH3 and cysteine-rich domain 3 (STAC3) that is essential for coupling membrane depolarization to Ca²⁺ release from the sarcoplasmic reticulum. Rat medial gastrocnemius (MG) muscles were excised immediately (REC0), 1 day (REC1), and 4 days (REC4) after exposure to 100 repeated damaging ECCs in vivo. PCs with 10 repeated nondamaging ECCs were applied 2 days before the damaging ECCs. Damaging ECCs induced in vivo isometric torque depression at 50 and 100 Hz stimulation frequencies, which was accompanied by a significant decrease in the amount of full-length STAC3, an activation of calpain 1, and an increased number of Evans Blue dye-positive fibers in MG muscles at REC1 and REC4. Interestingly, PCs attenuated all these deleterious alterations induced by damaging ECCs. Moreover, mechanistic experiments performed on normal muscle samples exposed to various concentration of Ca²⁺ showed a Ca²⁺-dependent proteolysis of STAC3, which was prevented by calpain inhibitor MDL-28170. In conclusion, PCs may improve recovery from force depression after damaging ECCs, in part by inhibiting the loss of STAC3 due to the increased permeability of cell membrane and subsequent activation of calpain 1.NEW & NOTEWORTHY The SH3 and cysteine-rich domain 3 (STAC3) is a skeletal muscle-specific protein that couples membrane depolarization to sarcoplasmic reticulum Ca²⁺ release. No studies, however, examined the role of STAC3 in protective effects of preconditioning contractions (PCs) against damaging eccentric contractions (ECCs). Here, we demonstrate that PCs may improve recovery from damaging ECCs-induced force depression, in part by an inhibition of Ca²⁺-dependent proteolysis of STAC3 due to increased membrane permeability and subsequent calpain 1 activation.

Loss of dysferlin or myoferlin results in differential defects in excitation-contraction coupling in mouse skeletal muscle

Muscular dystrophies are disorders characterized by progressive muscle loss and weakness that are both genotypically and phenotypically heterogenous. Progression of muscle disease arises from impaired regeneration, plasma membrane instability, defective membrane repair, and calcium mishandling. The ferlin protein family, including dysferlin and myoferlin, are calcium-binding, membrane-associated proteins that regulate membrane fusion, trafficking, and tubule formation. Mice lacking dysferlin (Dysf), myoferlin (Myof), and both dysferlin and myoferlin (Fer) on an isogenic inbred 129 background were previously demonstrated that loss of both dysferlin and myoferlin resulted in more severe muscle disease than loss of either gene alone. Furthermore, Fer mice had disordered triad organization with visibly malformed transverse tubules and sarcoplasmic reticulum, suggesting distinct roles of dysferlin and myoferlin. To assess the physiological role of disorganized triads, we now assessed excitation contraction (EC) coupling in these models. We identified differential abnormalities in EC coupling and ryanodine receptor disruption in flexor digitorum brevis myofibers isolated from ferlin mutant mice. We found that loss of dysferlin alone preserved sensitivity for EC coupling and was associated with larger ryanodine receptor clusters compared to wildtype myofibers. Loss of myoferlin alone or together with a loss of dysferlin reduced sensitivity for EC coupling, and produced disorganized and smaller ryanodine receptor cluster size compared to wildtype myofibers. These data reveal impaired EC coupling in Myof and Fer myofibers and slightly potentiated EC coupling in Dysf myofibers. Despite high homology, dysferlin and myoferlin have differential roles in regulating sarcotubular formation and maintenance resulting in unique impairments in calcium handling properties.

Calcium Release Channels: Structure and Function of IP3 Receptors and Ryanodine Receptors

Ca²⁺-release channels are giant membrane proteins that control the release of Ca²⁺ from the endoplasmic and sarcoplasmic reticulum. The two members, ryanodine receptors (RyRs) and inositol-1,4,5-trisphosphate Receptors (IP₃Rs), are evolutionarily related and are both activated by cytosolic Ca²⁺. They share a common architecture, but RyRs have evolved additional modules in the cytosolic region. Their massive size allows for the regulation by tens of proteins and small molecules, which can affect the opening and closing of the channels. In addition to Ca²⁺, other major triggers include IP₃ for the IP₃Rs, and depolarization of the plasma membrane for a particular RyR subtype. Their size has made them popular targets for study via electron microscopic methods, with current structures culminating near 3Å. The available structures have provided many new mechanistic insights int the binding of auxiliary proteins and small molecules, how these can regulate channel opening, and the mechanisms of disease-associated mutations. They also help scrutinize previously proposed binding sites, as some of these are now incompatible with the structures. Many questions remain around the structural effects of post-translational modifications, additional binding partners, and the higher-order complexes these channels can make in situ. This review summarizes our current knowledge about the structures of Ca²⁺-release channels and how this informs on their function.

Structure and function of STAC proteins: Calcium channel modulators and critical components of muscle excitation-contraction coupling

In skeletal muscle tissue, an intriguing mechanical coupling exists between two ion channels from different membranes: the L-type voltage-gated calcium channel (Ca_V1.1), located in the plasma membrane, and ryanodine receptor 1 (RyR1) located in the sarcoplasmic reticulum membrane. Excitable cells rely on Ca_vs to initiate Ca²⁺ entry in response to action potentials. RyRs can amplify this signal by releasing Ca²⁺ from internal stores. Although this process can be mediated through Ca²⁺ as a messenger, an overwhelming amount of evidence suggests that RyR1 has recruited Ca_V1.1 directly as its voltage sensor. The exact mechanisms that underlie this coupling have been enigmatic, but a recent wave of reports have illuminated the coupling protein STAC3 as a critical player. Without STAC3, the mechanical coupling between Ca_v1.1 and RyR1 is lost, and muscles fail to contract. Various sequence variants of this protein have been linked to congenital myopathy. Other STAC isoforms are expressed in the brain and may serve as regulators of L-type Ca_Vs. Despite the short length of STACs, several points of contacts have been proposed between them and Ca_Vs. However, it is currently unclear whether STAC3 also forms direct interactions with RyR1, and whether this modulates RyR1 function. In this review, we discuss the 3D architecture of STAC proteins, the biochemical evidence for their interactions, the relevance of these connections for functional modulation, and their involvement in myopathy.

Insights into the molecular mechanisms underlying the inhibition of acid-sensing ion channel 3 gating by stomatin

Stomatin (STOM) is a monotopic integral membrane protein found in all classes of life that has been shown to regulate members of the acid-sensing ion channel (ASIC) family. However, the mechanism by which STOM alters ASIC function is not known. Using chimeric channels, we combined patch-clamp electrophysiology and FRET to search for regions of ASIC3 critical for binding to and regulation by STOM. With this approach, we found that regulation requires two distinct sites on ASIC3: the distal C-terminus and the first transmembrane domain (TM1). The C-terminal site is critical for formation of the STOM–ASIC3 complex, while TM1 is required only for the regulatory effect. We then looked at the mechanism of STOM-dependent regulation of ASIC3 and found that STOM does not alter surface expression of ASIC3 or shift the pH dependence of channel activation. However, a point mutation (Q269G) that prevents channel desensitization also prevents STOM regulation, suggesting that STOM may alter ASIC3 currents by stabilizing the desensitized state of the channel. Based on these findings, we propose a model whereby STOM is anchored to the channel via a site on the distal C-terminus and stabilizes the desensitized state of the channel via an interaction with TM1.

Targeted Therapies for Skeletal Muscle Ion Channelopathies: Systematic Review and Steps Towards Precision Medicine

BACKGROUND

Skeletal muscle ion channelopathies include non-dystrophic myotonias (NDM), periodic paralyses (PP), congenital myasthenic syndrome, and recently identified congenital myopathies. The treatment of these diseases is mainly symptomatic, aimed at reducing muscle excitability in NDM or modifying triggers of attacks in PP.

OBJECTIVE

This systematic review collected the evidences regarding effects of pharmacological treatment on muscle ion channelopathies, focusing on the possible link between treatments and genetic background.

METHODS

We searched databases for randomized clinical trials (RCT) and other human studies reporting pharmacological treatments. Preclinical studies were considered to gain further information regarding mutation-dependent drug effects. All steps were performed by two independent investigators, while two others critically reviewed the entire process.

RESULTS

For NMD, RCT showed therapeutic benefits of mexiletine and lamotrigine, while other human studies suggest some efficacy of various sodium channel blockers and of the carbonic anhydrase inhibitor (CAI) acetazolamide. Preclinical studies suggest that mutations may alter sensitivity of the channel to sodium channel blockers in vitro, which has been translated to humans in some cases. For hyperkalemic and hypokalemic PP, RCT showed efficacy of the CAI dichlorphenamide in preventing paralysis. However, hypokalemic PP patients carrying sodium channel mutations may have fewer benefits from CAI compared to those carrying calcium channel mutations. Few data are available for treatment of congenital myopathies.

CONCLUSIONS

These studies provided limited information about the response to treatments of individual mutations or groups of mutations. A major effort is needed to perform human studies for designing a mutation-driven precision medicine in muscle ion channelopathies.

Physiological and Pathological Relevance of Selective and Nonselective Ca2+ Channels in Skeletal and Cardiac Muscle

Contraction of the striated muscle is fundamental for human existence. The action of voluntary skeletal muscle enables activities such as breathing, establishing body posture, and diverse body movements. Additionally, highly precise motion empowers communication, artistic expression, and other activities that define everyday human life. The involuntary contraction of striated muscle is the core function of the heart and is essential for blood flow. Several ion channels are important in the transduction of action potentials to cytosolic Ca2+ signals that enable muscle contraction; however, other ion channels are involved in the progression of muscle pathologies that can impair normal life or threaten it. This chapter describes types of selective and nonselective Ca2+ permeable ion channels expressed in the striated muscle, their participation in different aspects of muscle excitation and contraction, and their relevance to the progression of some pathological states.

Skeletal muscle CaV1.1 channelopathies

Ca_V1.1 is specifically expressed in skeletal muscle where it functions as voltage sensor of skeletal muscle excitation-contraction (EC) coupling independently of its functions as L-type calcium channel. Consequently, all known Ca_V1.1-related diseases are muscle diseases and the molecular and cellular disease mechanisms relate to the dual functions of Ca_V1.1 in this tissue. To date, four types of muscle diseases are known that can be linked to mutations in the CACNA1S gene or to splicing defects. These are hypo- and normokalemic periodic paralysis, malignant hyperthermia susceptibility, Ca_V1.1-related myopathies, and myotonic dystrophy type 1. In addition, the Ca_V1.1 function in EC coupling is perturbed in Native American myopathy, arising from mutations in the Ca_V1.1-associated protein STAC3. Here, we first address general considerations concerning the possible roles of Ca_V1.1 in disease and then discuss the state of the art regarding the pathophysiology of the Ca_V1.1-related skeletal muscle diseases with an emphasis on molecular disease mechanisms.

Dihydropyridine Receptor Congenital Myopathy In A Consangineous Turkish Family

Dihydropyridine receptor congenital myopathy is a recently described congenital myopathy caused by dominant or recessive mutations in the CACNA1S gene. To date, only 11 cases from 7 families were described in a single report. Here, we describe a consanguineous family with three affected children, presenting congenital hypotonia, contractures, ophthalmoplegia and respiratory insufficiency, with a novel homozygous mutation in the CACNA1S gene. They also showed cognitive delay, pes equinovarus deformity and neurogenic changes that have not been associated with this myopathy in the previous reports. This report expands the phenotypic spectrum of dihydropyridine receptor congenital myopathy and underscores the importance of whole exome sequencing in early onset neuromuscular disorders.

Excitation-contraction coupling in skeletal muscle: recent progress and unanswered questions

Excitation-contraction coupling (ECC) is a physiological process that links excitation of muscles by the nervous system to their mechanical contraction. In skeletal muscle, ECC is initiated with an action potential, generated by the somatic nervous system, which causes a depolarisation of the muscle fibre membrane (sarcolemma). This leads to a rapid change in the transmembrane potential, which is detected by the voltage-gated Ca²⁺ channel dihydropyridine receptor (DHPR) embedded in the sarcolemma. DHPR transmits the contractile signal to another Ca²⁺ channel, ryanodine receptor (RyR1), embedded in the membrane of the sarcoplasmic reticulum (SR), which releases a large amount of Ca²⁺ ions from the SR that initiate muscle contraction. Despite the fundamental role of ECC in skeletal muscle function of all vertebrate species, the molecular mechanism underpinning the communication between the two key proteins involved in the process (DHPR and RyR1) is still largely unknown. The goal of this work is to review the recent progress in our understanding of ECC in skeletal muscle from the point of view of the structure and interactions of proteins involved in the process, and to highlight the unanswered questions in the field.

STAC proteins: The missing link in skeletal muscle EC coupling and new regulators of calcium channel function

Excitation-contraction coupling is the signaling process by which action potentials control calcium release and consequently the force of muscle contraction. Until recently, three triad proteins were known to be essential for skeletal muscle EC coupling: the voltage-gated calcium channel CaV1.1 acting as voltage sensor, the SR calcium release channel RyR1 representing the only relevant calcium source, and the auxiliary CaV β1a subunit. Whether CaV1.1 and RyR1 are directly coupled or whether their interaction is mediated by another triad protein is still unknown. The recent identification of the adaptor protein STAC3 as fourth essential component of skeletal muscle EC coupling prompted vigorous research to reveal its role in this signaling process. Accumulating evidence supports its possible involvement in linking CaV1.1 and RyR1 in skeletal muscle EC coupling, but also indicates a second, much broader role of STAC proteins in the regulation of calcium/calmodulin-dependent feedback regulation of L-type calcium channels.

Crystal structures of Ca2+-calmodulin bound to NaV C-terminal regions suggest role for EF-hand domain in binding and inactivation

Voltage-gated sodium (Na_V) and calcium channels (Ca_V) form targets for calmodulin (CaM), which affects channel inactivation properties. A major interaction site for CaM resides in the C-terminal (CT) region, consisting of an IQ domain downstream of an EF-hand domain. We present a crystal structure of fully Ca²⁺-occupied CaM, bound to the CT of Na_V1.5. The structure shows that the C-terminal lobe binds to a site ∼90° rotated relative to a previous site reported for an apoCaM complex with the Na_V1.5 CT and for ternary complexes containing fibroblast growth factor homologous factors (FHF). We show that the binding of FHFs forces the EF-hand domain in a conformation that does not allow binding of the Ca²⁺-occupied C-lobe of CaM. These observations highlight the central role of the EF-hand domain in modulating the binding mode of CaM. The binding sites for Ca²⁺-free and Ca²⁺-occupied CaM contain targets for mutations linked to long-QT syndrome, a type of inherited arrhythmia. The related Na_V1.4 channel has been shown to undergo Ca²⁺-dependent inactivation (CDI) akin to Ca_Vs. We present a crystal structure of Ca²⁺/CaM bound to the Na_V1.4 IQ domain, which shows a binding mode that would clash with the EF-hand domain. We postulate the relative reorientation of the EF-hand domain and the IQ domain as a possible conformational switch that underlies CDI.

The binding interactions that maintain excitation-contraction coupling junctions in skeletal muscle

Calcium for contraction of skeletal muscles is released via tetrameric ryanodine receptor (RYR1) channels of the sarcoplasmic reticulum (SR), which are assembled in ordered arrays called couplons at junctions where the SR abuts T tubules or plasmalemma. Voltage-gated Ca2+ (CaV1.1) channels, found in tubules or plasmalemma, form symmetric complexes called CaV tetrads that associate with and activate underlying RYR tetramers during membrane depolarization by conveying a conformational change. Intriguingly, CaV tetrads regularly skip every other RYR tetramer within the array; therefore, the RYRs underlying tetrads (named V), but not the voltage sensor-lacking (C) RYRs, should be activated by depolarization. Here we hypothesize that the checkerboard association is maintained solely by reversible binary interactions between CaVs and RYRs and test this hypothesis using a quantitative model of the energies that govern CaV1.1-RYR1 binding, which are assumed to depend on number and location of bound CaVs. A Monte Carlo simulation generates large statistical samples and distributions of state variables that can be compared with quantitative features in freeze-fracture images of couplons from various sources. This analysis reveals two necessary model features: (1) the energy of a tetramer must have wells at low and high occupation by CaVs, so that CaVs positively cooperate in binding RYR (an allosteric effect), and (2) a large energy penalty results when two CaVs bind simultaneously to adjacent RYR protomers in adjacent tetramers (a steric clash). Under the hypothesis, V and C channels will eventually reverse roles. Role reversal justifies the presence of sensor-lacking C channels, as a structural and functional reserve for control of muscle contraction.

Recent advances in understanding the ryanodine receptor calcium release channels and their role in calcium signalling

The ryanodine receptor calcium release channel is central to cytoplasmic Ca ²⁺ signalling in skeletal muscle, the heart, and many other tissues, including the central nervous system, lymphocytes, stomach, kidney, adrenal glands, ovaries, testes, thymus, and lungs. The ion channel protein is massive (more than 2.2 MDa) and has a structure that has defied detailed determination until recent developments in cryo-electron microscopy revealed much of its structure at near-atomic resolution. The availability of this high-resolution structure has provided the most significant advances in understanding the function of the ion channel in the past 30 years. We can now visualise the molecular environment of individual amino acid residues that form binding sites for essential modulators of ion channel function and determine its role in Ca ²⁺ signalling. Importantly, the structure has revealed the structural environment of the many deletions and point mutations that disrupt Ca ²⁺ signalling in skeletal and cardiac myopathies and neuropathies. The implications are of vital importance to our understanding of the molecular basis of the ion channel’s function and for the design of therapies to counteract the effects of ryanodine receptor-associated disorders.

Calcium Channelopathies: Structural Insights into Disorders of the Muscle Excitation–Contraction Complex

Ion channels are membrane proteins responsible for the passage of ions down their electrochemical gradients and across biological membranes. In this, they generate and shape action potentials and provide secondary messengers for various signaling pathways. They are often part of larger complexes containing auxiliary subunits and regulatory proteins. Channelopathies arise from mutations in the genes encoding ion channels or their associated proteins. Recent advances in cryo-electron microscopy have resulted in an explosion of ion channel structures in multiple states, generating a wealth of new information on channelopathies. Disease-associated mutations fall into different categories, interfering with ion permeation, protein folding, voltage sensing, ligand and protein binding, and allosteric modulation of channel gating. Prime examples of these are Ca2+-selective channels expressed in myocytes, for which multiple structures in distinct conformational states have recently been uncovered. We discuss the latest insights into these calcium channelopathies from a structural viewpoint.

STAC3 variants cause a congenital myopathy with distinctive dysmorphic features and malignant hyperthermia susceptibility

SH3 and cysteine-rich domain-containing protein 3 (STAC3) is an essential component of the skeletal muscle excitation-contraction coupling (ECC) machinery, though its role and function are not yet completely understood. Here, we report 18 patients carrying a homozygous p.(Trp284Ser) STAC3 variant in addition to a patient compound heterozygous for the p.(Trp284Ser) and a novel splice site change (c.997-1G > T). Clinical severity ranged from prenatal onset with severe features at birth, to a milder and slowly progressive congenital myopathy phenotype. A malignant hyperthermia (MH)-like reaction had occurred in several patients. The functional analysis demonstrated impaired ECC. In particular, KCl-induced membrane depolarization resulted in significantly reduced sarcoplasmic reticulum Ca²⁺ release. Co-immunoprecipitation of STAC3 with Ca_V 1.1 in patients and control muscle samples showed that the protein interaction between STAC3 and Ca_V 1.1 was not significantly affected by the STAC3 variants. This study demonstrates that STAC3 gene analysis should be included in the diagnostic work up of patients of any ethnicity presenting with congenital myopathy, in particular if a history of MH-like episodes is reported. While the precise pathomechanism remains to be elucidated, our functional characterization of STAC3 variants revealed that defective ECC is not a result of Ca_V 1.1 sarcolemma mislocalization or impaired STAC3-Ca_V 1.1 interaction.

Allosteric regulators selectively prevent Ca2+-feedback of CaV and NaV channels

Calmodulin (CaM) serves as a pervasive regulatory subunit of Ca_V1, Ca_V2, and Na_V1 channels, exploiting a functionally conserved carboxy-tail element to afford dynamic Ca²⁺-feedback of cellular excitability in neurons and cardiomyocytes. Yet this modularity counters functional adaptability, as global changes in ambient CaM indiscriminately alter its targets. Here, we demonstrate that two structurally unrelated proteins, SH3 and cysteine-rich domain (stac) and fibroblast growth factor homologous factors (fhf) selectively diminish Ca²⁺/CaM-regulation of Ca_V1 and Na_V1 families, respectively. The two proteins operate on allosteric sites within upstream portions of respective channel carboxy-tails, distinct from the CaM-binding interface. Generalizing this mechanism, insertion of a short RxxK binding motif into Ca_V1.3 carboxy-tail confers synthetic switching of CaM regulation by Mona SH3 domain. Overall, our findings identify a general class of auxiliary proteins that modify Ca²⁺/CaM signaling to individual targets allowing spatial and temporal orchestration of feedback, and outline strategies for engineering Ca²⁺/CaM signaling to individual targets.

Stac Proteins Suppress Ca2+-Dependent Inactivation of Neuronal l-type Ca2+ Channels

Stac protein (named for its SH3- and cysteine-rich domains) was first identified in brain 20 years ago and is currently known to have three isoforms. Stac2, Stac1, and Stac3 transcripts are found at high, modest, and very low levels, respectively, in the cerebellum and forebrain, but their neuronal functions have been little investigated. Here, we tested the effects of Stac proteins on neuronal, high-voltage-activated Ca²⁺ channels. Overexpression of the three Stac isoforms eliminated Ca²⁺-dependent inactivation (CDI) of l-type current in rat neonatal hippocampal neurons (sex unknown), but not CDI of non-l-type current. Using heterologous expression in tsA201 cells (together with β and α₂-δ₁ auxiliary subunits), we found that CDI for Ca_V1.2 and Ca_V1.3 (the predominant, neuronal l-type Ca²⁺ channels) was suppressed by all three Stac isoforms, whereas CDI for the P/Q channel, Ca_V2.1, was not. For Ca_V1.2, the inhibition of CDI by the Stac proteins appeared to involve their direct interaction with the channel's C terminus. Within the Stac proteins, a weakly conserved segment containing ∼100 residues and linking the structurally conserved PKC C1 and SH3_1 domains was sufficient to fully suppress CDI. The presence of CDI for l-type current in control neonatal neurons raised the possibility that endogenous Stac levels are low in these neurons and Western blotting indicated that the expression of Stac2 was substantially increased in adult forebrain and cerebellum compared with neonate. Together, our results indicate that one likely function of neuronal Stac proteins is to tune Ca²⁺ entry via neuronal l-type channels.SIGNIFICANCE STATEMENT Stac protein, first identified 20 years ago in brain, has recently been found to be essential for proper trafficking and function of the skeletal muscle l-type Ca2+ channel and is the site of mutations causing a severe, inherited human myopathy. In neurons, however, functions for Stac protein have remained unexplored. Here, we report that one likely function of neuronal Stac proteins is tuning Ca2+ entry via l-type, but not that via non-l-type, Ca2+ channels. Moreover, there is a large postnatal increase in protein levels of the major neuronal isoform (Stac2) in forebrain and cerebellum, which could provide developmental regulation of l-type channel Ca2+ signaling in these brain regions.

Duplex signaling by CaM and Stac3 enhances CaV1.1 function and provides insights into congenital myopathy

CaV1.1 is essential for skeletal muscle excitation-contraction coupling. Its functional expression is tuned by numerous regulatory proteins, yet underlying modulatory mechanisms remain ambiguous as CaV1.1 fails to function in heterologous systems. In this study, by dissecting channel trafficking versus gating, we evaluated the requirements for functional CaV1.1 in heterologous systems. Although coexpression of the auxiliary β subunit is sufficient for surface-membrane localization, this baseline trafficking is weak, and channels elicit a diminished open probability. The regulatory proteins calmodulin and stac3 independently enhance channel trafficking and gating via their interaction with the CaV1.1 carboxy terminus. Myopathic stac3 mutations weaken channel binding and diminish trafficking. Our findings demonstrate that multiple regulatory proteins orchestrate CaV1.1 function via duplex mechanisms. Our work also furnishes insights into the pathophysiology of stac3-associated congenital myopathy and reveals novel avenues for pharmacological intervention.

STAC3 incorporation into skeletal muscle triads occurs independent of the dihydropyridine receptor

Excitation‐contraction (EC) coupling in skeletal muscles operates through a physical interaction between the dihydropyridine receptor (DHPR), acting as a voltage sensor, and the ryanodine receptor (RyR1), acting as a calcium release channel. Recently, the adaptor protein SH3 and cysteine‐rich containing protein 3 (STAC3) has been identified as a myopathy disease gene and as an additional essential EC coupling component. STAC3 interacts with DHPR sequences including the critical EC coupling domain and has been proposed to function in linking the DHPR and RyR1. However, we and others demonstrated that incorporation of recombinant STAC3 into skeletal muscle triads critically depends only on the DHPR but not the RyR1. On the contrary, here, we provide evidence that endogenous STAC3 incorporates into triads in the absence of the DHPR in myotubes and muscle fibers of dysgenic mice. This finding demonstrates that STAC3 interacts with additional triad proteins and is consistent with its proposed role in directly or indirectly linking the DHPR with the RyR1.

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.