D4cpv-calsequestrin: a sensitive ratiometric biosensor accurately targeted to the calcium store of skeletal muscle

Genetic Manipulation of CB1 Cannabinoid Receptors Reveals a Role in Maintaining Proper Skeletal Muscle Morphology and Function in Mice

The endocannabinoid system (ECS) refers to a widespread signaling system and its alteration is implicated in a growing number of human diseases. Cannabinoid receptors (CBRs) are highly expressed in the central nervous system and many peripheral tissues. Evidence suggests that CB1Rs are expressed in human and murine skeletal muscle mainly in the cell membrane, but a subpopulation is present also in the mitochondria. However, very little is known about the latter population. To date, the connection between the function of CB1Rs and the regulation of intracellular Ca²⁺ signaling has not been investigated yet. Tamoxifen-inducible skeletal muscle-specific conditional CB1 knock-down (skmCB1-KD, hereafter referred to as Cre^+/-) mice were used in this study for functional and morphological analysis. After confirming CB1R down-regulation on the mRNA and protein level, we performed in vitro muscle force measurements and found that peak twitch, tetanus, and fatigue were decreased significantly in Cre^+/- mice. Resting intracellular calcium concentration, voltage dependence of the calcium transients as well as the activity dependent mitochondrial calcium uptake were essentially unaltered by Cnr1 gene manipulation. Nevertheless, we found striking differences in the ultrastructural architecture of the mitochondrial network of muscle tissue from the Cre^+/- mice. Our results suggest a role of CB1Rs in maintaining physiological muscle function and morphology. Targeting ECS could be a potential tool in certain diseases, including muscular dystrophies where increased endocannabinoid levels have already been described.

Constitutive assembly of Ca2+ entry units in soleus muscle from calsequestrin knockout mice

Calcium (Ca2+) entry units (CEUs) are junctions within the I band of the sarcomere between stacks of sarcoplasmic reticulum (SR) cisternae and extensions of the transverse (T)-tubule. CEUs contain STIM1 and Orai1 proteins, the molecular machinery of store-operated Ca2+ entry (SOCE). In extensor digitorum longus (EDL) fibers of wild-type (WT) mice, CEUs transiently assemble during acute exercise and disassemble several hours thereafter. By contrast, calsequestrin-1 (CASQ1) ablation induces a compensatory constitutive assembly of CEUs in EDL fibers, resulting in enhanced constitutive and maximum SOCE that counteracts SR Ca2+ depletion during repetitive activity. However, whether CEUs form in slow-twitch fibers, which express both the skeletal CASQ1 and the cardiac CASQ2 isoforms, is unknown. Herein, we compared the structure and function of soleus muscles from WT and knockout mice that lack either CASQ1 (CASQ1-null) or both CASQs (dCASQ-null). Ultrastructural analyses showed that SR/T-tubule junctions at the I band, virtually identical to CEUs in EDL muscle, were present and more frequent in CASQ1-null than WT mice, with dCASQ-null exhibiting the highest incidence. The greater incidence of CEUs in soleus from dCASQ-null mice correlated with increased specific force production during repetitive, high-frequency stimulation, which depended on Ca2+ entry. Consistent with this, Orai1 expression was significantly increased in soleus of CASQ1-null mice, but even more in dCASQ-null mice, compared with WT. Together, these results strengthen the concept that CEU assembly strongly depends on CASQ expression and provides an alternative source of Ca2+ needed to refill SR Ca2+ stores to maintain specific force production during sustained muscle activity.

A mathematical model to quantify RYR Ca2+ leak and associated heat production in resting human skeletal muscle fibers

Cycling of Ca2+ between the sarcoplasmic reticulum (SR) and myoplasm is an important component of skeletal muscle resting metabolism. As part of this cycle, Ca2+ leaks from the SR into the myoplasm and is pumped back into the SR using ATP, which leads to the consumption of O2 and generation of heat. Ca2+ may leak through release channels or ryanodine receptors (RYRs). RYR Ca2+ leak can be monitored in a skinned fiber preparation in which leaked Ca2+ is pumped into the t-system and measured with a fluorescent dye. However, accurate quantification faces a number of hurdles. To overcome them, we developed a mathematical model of Ca2+ movement in these preparations. The model incorporated Ca2+ pumps that move Ca2+ from the myoplasm to the SR and from the junctional space (JS) to the t-system, Ca2+ buffering by EGTA in the JS and myoplasm and by buffers in the SR, and Ca2+ leaks from the SR into the JS and myoplasm and from the t-system into the myoplasm. The model accurately simulated Ca2+ uptake into the t-system, the relationship between myoplasmic [Ca2+] and steady-state t-system [Ca2+], and the effect of blocking RYR Ca2+ leak on t-system Ca2+ uptake. The magnitude of the leak through the RYRs would contribute ∼5% of the resting heat production of human muscle. In normal resting fibers, RYR Ca2+ leak makes a small contribution to resting metabolism. RYR-focused pathologies have the potential to increase RYR Ca2+ leak and the RYR leak component of resting metabolism.

Quantification of the calcium signaling deficit in muscles devoid of triadin

Triadin, a protein of the sarcoplasmic reticulum (SR) of striated muscles, anchors the calcium-storing protein calsequestrin to calcium release RyR channels at the junction with t-tubules, and modulates these channels by conformational effects. Triadin ablation induces structural SR changes and alters the expression of other proteins. Here we quantify alterations of calcium signaling in single skeletal myofibers of constitutive triadin-null mice. We find higher resting cytosolic and lower SR-luminal [Ca²⁺], 40% lower calsequestrin expression, and more Ca_V1.1, RyR1 and SERCA1. Despite the increased Ca_V1.1, the mobile intramembrane charge was reduced by ~20% in Triadin-null fibers. The initial peak of calcium release flux by pulse depolarization was minimally altered in the null fibers (revealing an increase in peak calcium permeability). The “hump” phase that followed, attributable to calcium detaching from calsequestrin, was 25% lower, a smaller change than expected from the reduced calsequestrin content and calcium saturation. The exponential decay rate of calcium transients was 25% higher, consistent with the higher SERCA1 content. Recovery of calcium flux after a depleting depolarization was faster in triadin-null myofibers, consistent with the increased uptake rate and lower SR calsequestrin content. In sum, the triadin knockout determines an increased RyR1 channel openness, which depletes the SR, a substantial loss of calsequestrin and gains in other couplon proteins. Powerful functional compensations ensue: activation of SOCE that increases [Ca²⁺]_cyto; increased SERCA1 activity, which limits the decrease in [Ca²⁺]_SR and a restoration of SR calcium storage of unknown substrate. Together, they effectively limit the functional loss in skeletal muscles.

Pre-assembled Ca2+ entry units and constitutively active Ca2+ entry in skeletal muscle of calsequestrin-1 knockout mice

Store-operated Ca²⁺ entry (SOCE) is a ubiquitous Ca²⁺ influx mechanism triggered by depletion of Ca²⁺ stores from the endoplasmic/sarcoplasmic reticulum (ER/SR). We recently reported that acute exercise in WT mice drives the formation of Ca²⁺ entry units (CEUs), intracellular junctions that contain STIM1 and Orai1, the two key proteins mediating SOCE. The presence of CEUs correlates with increased constitutive- and store-operated Ca²⁺ entry, as well as sustained Ca²⁺ release and force generation during repetitive stimulation. Skeletal muscle from mice lacking calsequestrin-1 (CASQ1-null), the primary Ca²⁺-binding protein in the lumen of SR terminal cisternae, exhibits significantly reduced total Ca²⁺ store content and marked SR Ca²⁺ depletion during high-frequency stimulation. Here, we report that CEUs are constitutively assembled in extensor digitorum longus (EDL) and flexor digitorum brevis (FDB) muscles of sedentary CASQ1-null mice. The higher density of CEUs in EDL (39.6 ± 2.1/100 µm² versus 2.0 ± 0.3/100 µm²) and FDB (16.7 ± 1.0/100 µm² versus 2.7 ± 0.5/100 µm²) muscles of CASQ1-null compared with WT mice correlated with enhanced constitutive- and store-operated Ca²⁺ entry and increased expression of STIM1, Orai1, and SERCA. The higher ability to recover Ca²⁺ ions via SOCE in CASQ1-null muscle served to promote enhanced maintenance of peak Ca²⁺ transient amplitude, increased dependence of luminal SR Ca²⁺ replenishment on BTP-2-sensitive SOCE, and increased maintenance of contractile force during repetitive, high-frequency stimulation. Together, these data suggest that muscles from CASQ1-null mice compensate for the lack of CASQ1 and reduction in total releasable SR Ca²⁺ content by assembling CEUs to promote constitutive and store-operated Ca²⁺ entry.

Altered Ca2+ Handling and Oxidative Stress Underlie Mitochondrial Damage and Skeletal Muscle Dysfunction in Aging and Disease

Skeletal muscle contraction relies on both high-fidelity calcium (Ca²⁺) signals and robust capacity for adenosine triphosphate (ATP) generation. Ca²⁺ release units (CRUs) are highly organized junctions between the terminal cisternae of the sarcoplasmic reticulum (SR) and the transverse tubule (T-tubule). CRUs provide the structural framework for rapid elevations in myoplasmic Ca²⁺ during excitation-contraction (EC) coupling, the process whereby depolarization of the T-tubule membrane triggers SR Ca²⁺ release through ryanodine receptor-1 (RyR1) channels. Under conditions of local or global depletion of SR Ca²⁺ stores, store-operated Ca²⁺ entry (SOCE) provides an additional source of Ca²⁺ that originates from the extracellular space. In addition to Ca²⁺, skeletal muscle also requires ATP to both produce force and to replenish SR Ca²⁺ stores. Mitochondria are the principal intracellular organelles responsible for ATP production via aerobic respiration. This review provides a broad overview of the literature supporting a role for impaired Ca²⁺ handling, dysfunctional Ca²⁺-dependent production of reactive oxygen/nitrogen species (ROS/RNS), and structural/functional alterations in CRUs and mitochondria in the loss of muscle mass, reduction in muscle contractility, and increase in muscle damage in sarcopenia and a wide range of muscle disorders including muscular dystrophy, rhabdomyolysis, central core disease, and disuse atrophy. Understanding the impact of these processes on normal muscle function will provide important insights into potential therapeutic targets designed to prevent or reverse muscle dysfunction during aging and disease.

Rapid subcellular calcium responses and dynamics by calcium sensor G-CatchER. iScience 2021;24:102129. [PMID: 33665552 PMCID: PMC7900224 DOI: 10.1016/j.isci.2021.102129]

The precise spatiotemporal characteristics of subcellular calcium (Ca²⁺) transients are critical for the physiological processes. Here we report a green Ca²⁺ sensor called "G-CatchER⁺" using a protein design to report rapid local ER Ca²⁺ dynamics with significantly improved folding properties. G-CatchER⁺ exhibits a superior Ca²⁺ on rate to G-CEPIA1er and has a Ca²⁺-induced fluorescence lifetimes increase. G-CatchER⁺ also reports agonist/antagonist triggered Ca²⁺ dynamics in several cell types including primary neurons that are orchestrated by IP₃Rs, RyRs, and SERCAs with an ability to differentiate expression. Upon localization to the lumen of the RyR channel (G-CatchER⁺-JP45), we report a rapid local Ca²⁺ release that is likely due to calsequestrin. Transgenic expression of G-CatchER⁺ in Drosophila muscle demonstrates its utility as an in vivo reporter of stimulus-evoked SR local Ca²⁺ dynamics. G-CatchER⁺ will be an invaluable tool to examine local ER/SR Ca²⁺ dynamics and facilitate drug development associated with ER dysfunction.

The Role of Orai1 in Regulating Sarcoplasmic Calcium Release, Mitochondrial Morphology and Function in Myostatin Deficient Skeletal Muscle

In mice a naturally occurring 12-bp deletion in the myostatin gene is considered responsible for the compact phenotype (Mstn^{Cmpt–dl1Abc}, Cmpt) labeled by a tremendous increase in body weight along with signs of muscle weakness, easier fatigability, decreased Orai1 expression and store operated calcium entry (SOCE). Here, on the one hand, Cmpt fibers were reconstructed with venus-Orai1 but this failed to restore SOCE. On the other hand, the endogenous Orai1 was silenced in fibers from wild type C57Bl6 mice which resulted in ∼70% of Orai1 being silenced in whole muscle homogenates as confirmed by Western blot, accompanied by an inhibitory effect on the voltage dependence of SR calcium release that manifested in a slight shift toward more positive potential values. This maneuver completely hampered SOCE. Our observations are consistent with the idea that Orai1 channels are present in distinct pools responsible for either a rapid refilling of the SR terminal cisternae connected to each voltage-activated calcium transient, or a slow SOCE associated with an overall depletion of calcium in the SR lumen. Furthermore, when Cmpt cells were loaded with the mitochondrial membrane potential sensitive dye TMRE, fiber segments with depolarized mitochondria were identified covering on average 26.5 ± 1.5% of the fiber area. These defective areas were located around the neuromuscular junction and displayed significantly smaller calcium transients. The ultrastructural analysis of the Cmpt fibers revealed changes in the mitochondrial morphology. In addition, the mitochondrial calcium uptake during repetitive stimulation was higher in the Cmpt fibers. Our results favor the idea that reduced function and/or expression of SOCE partners (in this study Orai1) and mitochondrial defects could play an important role in muscle weakness and degeneration associated with certain pathologies, perhaps including loss of function of the neuromuscular junction and aging.

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.

Sarcoplasmic reticulum Ca2+ decreases with age and correlates with the decline in muscle function in Drosophila

Sarcopenia, the loss of muscle mass and strength associated with age, has been linked to impairment of the cytosolic Ca2+ peak that triggers muscle contraction, but mechanistic details remain unknown. Here we explore the hypothesis that a reduction in sarcoplasmic reticulum (SR) Ca2+ concentration ([Ca2+]SR) is at the origin of this loss of Ca2+ homeostasis. We engineered Drosophila melanogaster to express the Ca2+ indicator GAP3 targeted to muscle SR, and we developed a new method to calibrate the signal into [Ca2+]SRin vivo [Ca2+]SR fell with age from ∼600 µM to 50 µM in close correlation with muscle function, which declined monotonically when [Ca2+]SR was <400 µM. [Ca2+]SR results from the pump-leak steady state at the SR membrane. However, changes in expression of the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) pump and of the ryanodine receptor leak were too modest to explain the large changes seen in [Ca2+]SR Instead, these changes are compatible with increased leakiness through the ryanodine receptor as the main determinant of the [Ca2+]SR decline in aging muscle. In contrast, there were no changes in endoplasmic reticulum [Ca2+] with age in brain neurons.This article has an associated First Person interview with the first author of the paper.

Eeyarestatin Compounds Selectively Enhance Sec61-Mediated Ca2+ Leakage from the Endoplasmic Reticulum

Eeyarestatin 1 (ES1) inhibits p97-dependent protein degradation, Sec61-dependent protein translocation into the endoplasmic reticulum (ER), and vesicular transport within the endomembrane system. Here, we show that ES1 impairs Ca²⁺ homeostasis by enhancing the Ca²⁺ leakage from mammalian ER. A comparison of various ES1 analogs suggested that the 5-nitrofuran (5-NF) ring of ES1 is crucial for this effect. Accordingly, the analog ES24, which conserves the 5-NF domain of ES1, selectively inhibited protein translocation into the ER, displayed the highest potency on ER Ca²⁺ leakage of ES1 analogs studied and induced Ca²⁺-dependent cell death. Using small interfering RNA-mediated knockdown of Sec61α, we identified Sec61 complexes as the targets that mediate the gain of Ca²⁺ leakage induced by ES1 and ES24. By interacting with the lateral gate of Sec61α, ES1 and ES24 likely capture Sec61 complexes in a Ca²⁺-permeable, open state, in which Sec61 complexes allow Ca²⁺ leakage but are translocation incompetent.

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ES1, ES2, and ES24 deplete Ca²⁺ in ER

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ESR35 and ES47 do not affect cellular Ca²⁺ homeostasis

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The most potent eeyarestatin, ES24, comprises only the 5-nitrofuran domain

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ES1 and ES24 target Sec61 complexes in ER

Calcium Dynamics Mediated by the Endoplasmic/Sarcoplasmic Reticulum and Related Diseases

The flow of intracellular calcium (Ca2+) is critical for the activation and regulation of important biological events that are required in living organisms. As the major Ca2+ repositories inside the cell, the endoplasmic reticulum (ER) and the sarcoplasmic reticulum (SR) of muscle cells are central in maintaining and amplifying the intracellular Ca2+ signal. The morphology of these organelles, along with the distribution of key calcium-binding proteins (CaBPs), regulatory proteins, pumps, and receptors fundamentally impact the local and global differences in Ca2+ release kinetics. In this review, we will discuss the structural and morphological differences between the ER and SR and how they influence localized Ca2+ release, related diseases, and the need for targeted genetically encoded calcium indicators (GECIs) to study these events.

The GCaMP-R Family of Genetically Encoded Ratiometric Calcium Indicators

We report on GCaMP-Rs, a new family of genetically encoded ratiometric calcium indicators that extend the virtues of the GCaMP proteins to ratiometric measurements. We have engineered a tandem construct of calcium-dependent GCaMP and calcium-independent mCherry fluorescent proteins. The tandem design assures that the two proteins localize in the same cellular compartment(s) and facilitates pixelwise ratiometric measurements; however, Förster resonance energy transfer (FRET) between the fluorophores reduces brightness of the sensor by up to half (depending on the GCaMP variant). To eliminate FRET, we introduced a rigid α-helix, the ER/K helix, between GCaMP and mCherry. Avoiding FRET significantly increases the brightness (notably, even at low calcium concentrations), the signal-to-noise ratio, and the dynamic range.

Calsequestrin depolymerizes when calcium is depleted in the sarcoplasmic reticulum of working muscle

Calsequestrin, the only known protein with cyclical storage and supply of calcium as main role, is proposed to have other functions, which remain unproven. Voluntary movement and the heart beat require this calcium flow to be massive and fast. How does calsequestrin do it? To bind large amounts of calcium in vitro, calsequestrin must polymerize and then depolymerize to release it. Does this rule apply inside the sarcoplasmic reticulum (SR) of a working cell? We answered using fluorescently tagged calsequestrin expressed in muscles of mice. By FRAP and imaging we monitored mobility of calsequestrin as [Ca²⁺] in the SR--measured with a calsequestrin-fused biosensor--was lowered. We found that calsequestrin is polymerized within the SR at rest and that it depolymerized as [Ca²⁺] went down: fully when calcium depletion was maximal (a condition achieved with an SR calcium channel opening drug) and partially when depletion was limited (a condition imposed by fatiguing stimulation, long-lasting depolarization, or low drug concentrations). With fluorescence and electron microscopic imaging we demonstrated massive movements of calsequestrin accompanied by drastic morphological SR changes in fully depleted cells. When cells were partially depleted no remodeling was found. The present results support the proposed role of calsequestrin in termination of calcium release by conformationally inducing closure of SR channels. A channel closing switch operated by calsequestrin depolymerization will limit depletion, thereby preventing full disassembly of the polymeric calsequestrin network and catastrophic structural changes in the SR.

Exploring cells with targeted biosensors

Cellular signaling networks are composed of multiple pathways, often interconnected, that form complex networks with great potential for cross-talk. Signal decoding depends on the nature of the message as well as its amplitude, temporal pattern, and spatial distribution. In addition, the existence of membrane-bound organelles, which are both targets and generators of messages, add further complexity to the system. The availability of sensors that can localize to specific compartments in live cells and monitor their targets with high spatial and temporal resolution is thus crucial for a better understanding of cell pathophysiology. For this reason, over the last four decades, a variety of strategies have been developed, not only to generate novel and more sensitive probes for ions, metabolites, and enzymatic activity, but also to selectively deliver these sensors to specific intracellular compartments. In this review, we summarize the principles that have been used to target organic or protein sensors to different cellular compartments and their application to cellular signaling.

Characterization of the ER-Targeted Low Affinity Ca(2+) Probe D4ER

Calcium ion (Ca²⁺) is a ubiquitous intracellular messenger and changes in its concentration impact on nearly every aspect of cell life. Endoplasmic reticulum (ER) represents the major intracellular Ca²⁺ store and the free Ca²⁺ concentration ([Ca²⁺]) within its lumen ([Ca²⁺]_ER) can reach levels higher than 1 mM. Several genetically-encoded ER-targeted Ca²⁺ sensors have been developed over the last years. However, most of them are non-ratiometric and, thus, their signal is difficult to calibrate in live cells and is affected by shifts in the focal plane and artifactual movements of the sample. On the other hand, existing ratiometric Ca²⁺ probes are plagued by different drawbacks, such as a double dissociation constant (K_d) for Ca²⁺, low dynamic range, and an affinity for the cation that is too high for the levels of [Ca²⁺] in the ER lumen. Here, we report the characterization of a recently generated ER-targeted, Förster resonance energy transfer (FRET)-based, Cameleon probe, named D4ER, characterized by suitable Ca²⁺ affinity and dynamic range for monitoring [Ca²⁺] variations within the ER. As an example, resting [Ca²⁺]_ER have been evaluated in a known paradigm of altered ER Ca²⁺ homeostasis, i.e., in cells expressing a mutated form of the familial Alzheimer’s Disease-linked protein Presenilin 2 (PS2). The lower Ca²⁺ affinity of the D4ER probe, compared to that of the previously generated D1ER, allowed the detection of a conspicuous, more clear-cut, reduction in ER Ca²⁺ content in cells expressing mutated PS2, compared to controls.

Voltage-gated Ca(2+) influx through L-type channels contributes to sarcoplasmic reticulum Ca(2+) loading in skeletal muscle

Muscle contraction is triggered by Ca(2+) ions released from the sarcoplasmic reticulum (SR) in response to depolarization of skeletal muscle fibres. Muscle activation is also associated with a voltage-activated trans-sarcolemmal Ca(2+) influx early identified as a current flowing through L-type Ca(2+) channels. Because removal of external Ca(2+) does not impede fibres from contracting, a negligible role was given to this voltage-activated Ca(2+) entry, although the decline of Ca(2+) release is more pronounced in the absence of Ca(2+) during long-lasting activation. Furthermore, it is not clearly established whether Ca(2+) exclusively flows through L-type channels or in addition through a parallel voltage-activated pathway distinct from L-type channels. Here, by monitoring the quenching of fura-2 fluorescence resulting from Mn(2+) influx in voltage-controlled mouse and zebrafish isolated muscle fibres, we show that the L-type current is the only contributor to Ca(2+) influx during long-lasting depolarizations in skeletal muscle. Calibration of the Mn(2+) quenching signal allowed us to estimate a mean Mn(2+) current of 0.31 ± 0.06 A F(-1) flowing through L-type channels during a train of action potentials. Measurements of SR Ca(2+) changes with fluo-5N in response to depolarization revealed that an elevated voltage-activated Ca(2+) current potentiated SR Ca(2+) loading and addition of external Mn(2+) produced quenching of fluo-5N in the SR, indicating that voltage-activated Ca(2+) /Mn(2+) influx contributes to SR Ca(2+) /Mn(2+) loading.

Characterization of Two Human Skeletal Calsequestrin Mutants Implicated in Malignant Hyperthermia and Vacuolar Aggregate Myopathy

Calsequestrin 1 is the principal Ca(2+) storage protein of the sarcoplasmic reticulum of skeletal muscle. Its inheritable D244G mutation causes a myopathy with vacuolar aggregates, whereas its M87T "variant" is weakly associated with malignant hyperthermia. We characterized the consequences of these mutations with studies of the human proteins in vitro. Equilibrium dialysis and turbidity measurements showed that D244G and, to a lesser extent, M87T partially lose Ca(2+) binding exhibited by wild type calsequestrin 1 at high Ca(2+) concentrations. D244G aggregates abruptly and abnormally, a property that fully explains the protein inclusions that characterize its phenotype. D244G crystallized in low Ca(2+) concentrations lacks two Ca(2+) ions normally present in wild type that weakens the hydrophobic core of Domain II. D244G crystallized in high Ca(2+) concentrations regains its missing ions and Domain II order but shows a novel dimeric interaction. The M87T mutation causes a major shift of the α-helix bearing the mutated residue, significantly weakening the back-to-back interface essential for tetramerization. D244G exhibited the more severe structural and biophysical property changes, which matches the different pathophysiological impacts of these mutations.

Endoplasmic reticulum stress in insulin resistance and diabetes

The endoplasmic reticulum is the main intracellular Ca(2+) store for Ca(2+) release during cell signaling. There are different strategies to avoid ER Ca(2+) depletion. Release channels utilize first Ca(2+)-bound to proteins and this minimizes the reduction of the free luminal [Ca(2+)]. However, if release channels stay open after exhaustion of Ca(2+)-bound to proteins, then the reduction of the free luminal ER [Ca(2+)] (via STIM proteins) activates Ca(2+) entry at the plasma membrane to restore the ER Ca(2+) load, which will work provided that SERCA pump is active. Nevertheless, there are several noxious conditions that result in decreased activity of the SERCA pump such as oxidative stress, inflammatory cytokines, and saturated fatty acids, among others. These conditions result in a deficient restoration of the ER [Ca(2+)] and lead to the ER stress response that should facilitate recovery of the ER. However, if the stressful condition persists then ER stress ends up triggering cell death and the ensuing degenerative process leads to diverse pathologies; particularly insulin resistance, diabetes and several of the complications associated with diabetes. This scenario suggests that limiting ER stress should decrease the incidence of diabetes and the mobility and mortality associated with this illness.

Altered Ca2+ concentration, permeability and buffering in the myofibre Ca2+ store of a mouse model of malignant hyperthermia

  Malignant hyperthermia (MH) is linked to mutations in the type 1 ryanodine receptor, RyR1, the Ca2+ channel of the sarcoplasmic reticulum (SR) of skeletal muscle. The Y522S MH mutation was studied for its complex presentation, which includes structurally and functionally altered cell 'cores'. Imaging cytosolic and intra-SR [Ca2+] in muscle cells of heterozygous YS mice we determined Ca2+ release flux activated by clamp depolarization, permeability (P) of the SR membrane (ratio of flux and [Ca2+] gradient) and SR Ca2+ buffering power (B). In YS cells resting [Ca2+]SR was 45% of the value in normal littermates (WT). P was more than doubled, so that initial flux was normal. Measuring [Ca2+]SR(t) revealed dynamic changes in B(t). The alterations were similar to those caused by cytosolic BAPTA, which promotes release by hampering Ca2+-dependent inactivation (CDI). The [Ca2+] transients showed abnormal 'breaks', decaying phases after an initial rise, traced to a collapse in flux and P. Similar breaks occurred in WT myofibres with calsequestrin reduced by siRNA; calsequestrin content, however, was normal in YS muscle. Thus, the Y522S mutation causes greater openness of the RyR1, lowers resting [Ca2+]SR and alters SR Ca2+ buffering in a way that copies the functional instability observed upon reduction of calsequestrin content. The similarities with the effects of BAPTA suggest that the mutation, occurring near the cytosolic vestibule of the channel, reduces CDI as one of its primary effects. The unstable SR buffering, mimicked by silencing of calsequestrin, may help precipitate the loss of Ca2+ control that defines a fulminant MH event.

Dynamic measurement of the calcium buffering properties of the sarcoplasmic reticulum in mouse skeletal muscle

The buffering power, B, of the sarcoplasmic reticulum (SR), ratio of the changes in total and free [Ca(2+)], was determined in fast-twitch mouse muscle cells subjected to depleting membrane depolarization. Changes in total SR [Ca(2+)] were measured integrating Ca(2+) release flux, determined with a cytosolic [Ca(2+)] monitor. Free [Ca(2+)](SR) was measured using the cameleon D4cpv-Casq1. In 34 wild-type (WT) cells average B during the depolarization (ON phase) was 157 (SEM 26), implying that of 157 ions released, 156 were bound inside the SR. B was significantly greater when BAPTA, which increases release flux, was present in the cytosol. B was greater early in the pulse - when flux was greatest - than at its end, and greater in the ON than in the OFF. In 29 Casq1-null cells, B was 40 (3.6). The difference suggests that 75% of the releasable calcium is normally bound to calsequestrin. In the nulls the difference in B between ON and OFF was less than in the WT but still significant. This difference and the associated decay in B during the ON were not artifacts of a slow SR monitor, as they were also found in the WT when [Ca(2+)](SR) was tracked with the fast dye fluo-5N. The calcium buffering power, binding capacity and non-linear binding properties of the SR measured here could be accounted for by calsequestrin at the concentration present in mammalian muscle, provided that its properties were substantially different from those found in solution. Its affinity should be higher, or K(D) lower than the conventionally accepted 1 mm; its cooperativity (n in a Hill fit) should be higher and the stoichiometry of binding should be at the higher end of the values derived in solution. The reduction in B during release might reflect changes in calsequestrin conformation upon calcium loss.

Calcium buffering properties of sarcoplasmic reticulum and calcium-induced Ca(2+) release during the quasi-steady level of release in twitch fibers from frog skeletal muscle

Experiments were performed to characterize the properties of the intrinsic Ca²⁺ buffers in the sarcoplasmic reticulum (SR) of cut fibers from frog twitch muscle. The concentrations of total and free calcium ions within the SR ([Ca_T]_SR and [Ca²⁺]_SR) were measured, respectively, with the EGTA/phenol red method and tetramethylmurexide (a low affinity Ca²⁺ indicator). Results indicate SR Ca²⁺ buffering was consistent with a single cooperative-binding component or a combination of a cooperative-binding component and a linear binding component accounting for 20% or less of the bound Ca²⁺. Under the assumption of a single cooperative-binding component, the most likely resting values of [Ca²⁺]_SR and [Ca_T]_SR are 0.67 and 17.1 mM, respectively, and the dissociation constant, Hill coefficient, and concentration of the Ca-binding sites are 0.78 mM, 3.0, and 44 mM, respectively. This information can be used to calculate a variable proportional to the Ca²⁺ permeability of the SR, namely d[Ca_T]_SR/dt ÷ [Ca²⁺]_SR (denoted release permeability), in experiments in which only [Ca_T]_SR or [Ca²⁺]_SR is measured. In response to a voltage-clamp step to −20 mV at 15°C, the release permeability reaches an early peak followed by a rapid decline to a quasi-steady level that lasts ∼50 ms, followed by a slower decline during which the release permeability decreases by at least threefold. During the quasi-steady level of release, the release amplitude is 3.3-fold greater than expected from voltage activation alone, a result consistent with the recruitment by Ca-induced Ca²⁺ release of 2.3 SR Ca²⁺ release channels neighboring each channel activated by its associated voltage sensor. Release permeability at −60 mV increases as [Ca_T]_SR decreases from its resting physiological level to ∼0.1 of this level. This result argues against a release termination mechanism proposed in mammalian muscle fibers in which a luminal sensor of [Ca²⁺]_SR inhibits release when [Ca_T]_SR declines to a low level.

Synthetic localized calcium transients directly probe signalling mechanisms in skeletal muscle

The contribution of Ca2+-induced Ca2+ release (CICR) to trigger muscle contraction is controversial. It was studied on isolated muscle fibres using synthetic localized increases in Ca2+ concentration, SLICs, generated by two-photon photorelease from nitrodibenzofuran (NDBF)-EGTA just outside the permeabilized plasma membrane. SLICs provided a way to increase cytosolic [Ca2+] rapidly and reversibly, up to 8 μM, levels similar to those reached during physiological activity. They improve over previous paradigms in rate of rise, locality and reproducibility. Use of NDBF-EGTA allowed for the separate modification of resting [Ca2+], trigger [Ca2+] and resting [Mg2+]. In frog muscle, SLICs elicited propagated responses that had the characteristics of CICR. The threshold [Ca2+] for triggering a response was 0.5 μM or less. As this value is much lower than concentrations prevailing near channels during normal activity, the result supports participation of CICR in the physiological control of contraction in amphibian muscle. As SLICs were applied outside cells, the primary stimulus was Ca2+, rather than the radiation or subproducts of photorelease. Therefore the responses qualify as ‘classic' CICR. By contrast, mouse muscle fibres did not respond unless channel-opening drugs were present at substantial concentrations, an observation contrary to the physiological involvement of CICR in mammalian excitation–contraction coupling. In mouse muscle, the propagating wave had a substantially lower release flux, which together with a much higher threshold justified the absence of response when drugs were not present. The differences in flux and threshold may be ascribed to the absence of ryanodine receptor 3 (RyR3) isoforms in adult mammalian muscle.

Voltage clamp methods for the study of membrane currents and SR Ca(2+) release in adult skeletal muscle fibres

Skeletal muscle excitation-contraction (E-C)(1) coupling is a process composed of multiple sequential stages, by which an action potential triggers sarcoplasmic reticulum (SR)(2) Ca(2+) release and subsequent contractile activation. The various steps in the E-C coupling process in skeletal muscle can be studied using different techniques. The simultaneous recordings of sarcolemmal electrical signals and the accompanying elevation in myoplasmic Ca(2+), due to depolarization-initiated SR Ca(2+) release in skeletal muscle fibres, have been useful to obtain a better understanding of muscle function. In studying the origin and mechanism of voltage dependency of E-C coupling a variety of different techniques have been used to control the voltage in adult skeletal fibres. Pioneering work in muscles isolated from amphibians or crustaceans used microelectrodes or 'high resistance gap' techniques to manipulate the voltage in the muscle fibres. The development of the patch clamp technique and its variant, the whole-cell clamp configuration that facilitates the manipulation of the intracellular environment, allowed the use of the voltage clamp techniques in different cell types, including skeletal muscle fibres. The aim of this article is to present an historical perspective of the voltage clamp methods used to study skeletal muscle E-C coupling as well as to describe the current status of using the whole-cell patch clamp technique in studies in which the electrical and Ca(2+) signalling properties of mouse skeletal muscle membranes are being investigated.

Residual sarcoplasmic reticulum Ca2+ concentration after Ca2+ release in skeletal myofibers from young adult and old mice

Contrasting information suggests either almost complete depletion of sarcoplasmic reticulum (SR) Ca(2+) or significant residual Ca(2+) concentration after prolonged depolarization of the skeletal muscle fiber. The primary obstacle to resolving this controversy is the lack of genetically encoded Ca(2+) indicators targeted to the SR that exhibit low-Ca(2+) affinity, a fast biosensor: Ca(2+) off-rate reaction, and can be expressed in myofibers from adult and older adult mammalian species. This work used the recently designed low-affinity Ca(2+) sensor (Kd = 1.66 mM in the myofiber) CatchER (calcium sensor for detecting high concentrations in the ER) targeted to the SR, to investigate whether prolonged skeletal muscle fiber depolarization significantly alters residual SR Ca(2+) with aging. We found CatchER a proper tool to investigate SR Ca(2+) depletion in young adult and older adult mice, consistently tracking SR luminal Ca(2+) release in response to brief and repetitive stimulation. We evoked SR Ca(2+) release in whole-cell voltage-clamped flexor digitorum brevis muscle fibers from young and old FVB mice and tested the maximal SR Ca(2+) release by directly activating the ryanodine receptor (RyR1) with 4-chloro-m-cresol in the same myofibers. Here, we report for the first time that the Ca(2+) remaining in the SR after prolonged depolarization (2 s) in myofibers from aging (~220 μM) was larger than young (~132 μM) mice. These experiments indicate that SR Ca(2+) is far from fully depleted under physiological conditions throughout life, and support the concept of excitation-contraction uncoupling in functional senescent myofibers.

Measurement of RyR permeability reveals a role of calsequestrin in termination of SR Ca(2+) release in skeletal muscle

The mechanisms that terminate Ca²⁺ release from the sarcoplasmic reticulum are not fully understood. D4cpv-Casq1 (Sztretye et al. 2011. J. Gen. Physiol. doi:10.1085/jgp.201010591) was used in mouse skeletal muscle cells under voltage clamp to measure free Ca²⁺ concentration inside the sarcoplasmic reticulum (SR), [Ca²⁺]_SR, simultaneously with that in the cytosol, [Ca²⁺]_c, during the response to long-lasting depolarization of the plasma membrane. The ratio of Ca²⁺ release flux (derived from [Ca²⁺]_c(t)) over the gradient that drives it (essentially equal to [Ca²⁺]_SR) provided directly, for the first time, a dynamic measure of the permeability to Ca²⁺ of the releasing SR membrane. During maximal depolarization, flux rapidly rises to a peak and then decays. Before 0.5 s, [Ca²⁺]_SR stabilized at ∼35% of its resting level; depletion was therefore incomplete. By 0.4 s of depolarization, the measured permeability decayed to ∼10% of maximum, indicating ryanodine receptor channel closure. Inactivation of the t tubule voltage sensor was immeasurably small by this time and thus not a significant factor in channel closure. In cells of mice null for Casq1, permeability did not decrease in the same way, indicating that calsequestrin (Casq) is essential in the mechanism of channel closure and termination of Ca²⁺ release. The absence of this mechanism explains why the total amount of calcium releasable by depolarization is not greatly reduced in Casq-null muscle (Royer et al. 2010. J. Gen. Physiol. doi:10.1085/jgp.201010454). When the fast buffer BAPTA was introduced in the cytosol, release flux became more intense, and the SR emptied earlier. The consequent reduction in permeability accelerated as well, reaching comparable decay at earlier times but comparable levels of depletion. This observation indicates that [Ca²⁺]_SR, sensed by Casq and transmitted to the channels presumably via connecting proteins, is determinant to cause the closure that terminates Ca²⁺ release.