Carlo Manno

Paradoxical buffering of calcium by calsequestrin demonstrated for the calcium store of skeletal muscle.

Contractile activation in striated muscles requires a Ca2+ reservoir of large capacity inside the sarcoplasmic reticulum (SR), presumably the protein calsequestrin. The buffering power of calsequestrin in vitro has a paradoxical dependence on [Ca2+] that should be valuable for function. Here, we demonstrate that this dependence is present in living cells. Ca2+ signals elicited by membrane depolarization under voltage clamp were compared in single skeletal fibers of wild-type (WT) and double (d) Casq-null mice, which lack both calsequestrin isoforms. In nulls, Ca2+ release started normally, but the store depleted much more rapidly than in the WT. This deficit was reflected in the evolution of SR evacuability, E, which is directly proportional to SR Ca2+ permeability and inversely to its Ca2+ buffering power, B. In WT mice E starts low and increases progressively as the SR is depleted. In dCasq-nulls, E started high and decreased upon Ca2+ depletion. An elevated E in nulls is consistent with the decrease in B expected upon deletion of calsequestrin. The different value and time course of E in cells without calsequestrin indicate that the normal evolution of E reflects loss of B upon SR Ca2+ depletion. Decrement of B upon SR depletion was supported further. When SR calcium was reduced by exposure to low extracellular [Ca2+], release kinetics in the WT became similar to that in the dCasq-null. E became much higher, similar to that of null cells. These results indicate that calsequestrin not only stores Ca2+, but also varies its affinity in ways that progressively increase the ability of the store to deliver Ca2+ as it becomes depleted, a novel feedback mechanism of potentially valuable functional implications. The study revealed a surprisingly modest loss of Ca2+ storage capacity in null cells, which may reflect concurrent changes, rather than detract from the physiological importance of calsequestrin.

Synthetic localized calcium transients directly probe signalling mechanisms in skeletal muscle

•  The signal for skeletal muscle contraction is a rapid increase in cytosolic Ca2+ concentration, which requires the coordinated opening of ryanodine receptor (RyR) channels in the sarcoplasmic reticulum. •  Channel opening is controlled by voltage‐sensing dihydropyridine receptors (DHPRs) of plasma membrane and T tubules. Whether or not their signal is amplified by Ca2+‐induced Ca2+ release (CICR) is controversial. •  We used two‐photon lysis of an advanced Ca2+ cage to produce local Ca2+ concentration transients that were large, fast, reproducible and quantifiable, while monitoring the cellular response with a dual confocal laser scanner. •  Single frog muscle cells in physiological solutions responded to transients greater than 0.28 μm with propagated CICR waves. •  Mouse cells did not respond to stimuli up to 8 μm, unless channel opening drugs were present. •  We conclude that CICR contributes to physiological Ca2+ release in frog but not mouse muscle. •  Mice and presumably other mammals do have a capability for CICR that is normally inhibited. It could be manifested under special circumstances, including diseases.

The couplonopathies: A comparative approach to a class of diseases of skeletal and cardiac muscle.

A novel category of diseases of striated muscle is proposed, the couplonopathies, as those that affect components of the couplon and thereby alter its operation. Couplons are the functional units of intracellular calcium release in excitation–contraction coupling. They comprise dihydropyridine receptors, ryanodine receptors (Ca2+ release channels), and a growing list of ancillary proteins whose alteration may lead to disease. Within a generally similar plan, the couplons of skeletal and cardiac muscle show, in a few places, marked structural divergence associated with critical differences in the mechanisms whereby they fulfill their signaling role. Most important among these are the presence of a mechanical or allosteric communication between voltage sensors and Ca2+ release channels, exclusive to the skeletal couplon, and the smaller capacity of the Ca stores in cardiac muscle, which results in greater swings of store concentration during physiological function. Consideration of these structural and functional differences affords insights into the pathogenesis of several couplonopathies. The exclusive mechanical connection of the skeletal couplon explains differences in pathogenesis between malignant hyperthermia (MH) and catecholaminergic polymorphic ventricular tachycardia (CPVT), conditions most commonly caused by mutations in homologous regions of the skeletal and cardiac Ca2+ release channels. Based on mechanistic considerations applicable to both couplons, we identify the plasmalemma as a site of secondary modifications, typically an increase in store-operated calcium entry, that are relevant in MH pathogenesis. Similar considerations help explain the different consequences that mutations in triadin and calsequestrin have in these two tissues. As more information is gathered on the composition of cardiac and skeletal couplons, this comparative and mechanistic approach to couplonopathies should be useful to understand pathogenesis, clarify diagnosis, and propose tissue-specific drug development.

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

•  The rise in cytosolic calcium ion concentration that triggers muscle contraction requires release of a large amount of calcium from the cellular store, sarcoplasmic reticulum (SR), where it is stored bound, largely to the protein calsequestrin. •  Binding of calcium by calsequestrin is a complex process, believed to involve changes in protein conformation and aggregation. We want to know to what extent these properties, observed in vitro, apply inside cells. •  We measured the calcium buffering power of the SR, defined as the ratio of change in total SR calcium by change in free [Ca2+]SR, in muscle cells of wild type or calsequestrin‐lacking mice, using two different methods to monitor [Ca2+]SR and deriving changes in total SR calcium content from simultaneous measurements of cytosolic [Ca2+]. •  The average buffering power during a large, depleting calcium release event was 157 in the wild type and 40 in the calsequestrin‐null mice, suggesting that three‐quarters of the calcium released normally comes from the calsequestrin‐bound pool. •  The Ca2+ buffering ability of the SR is different from that of the equivalent concentration of calsequestrin in aqueous solution, the SR exhibiting greater affinity and cooperativity. We conclude that calsequestrin adopts different properties inside cells. •  SR buffering power depends on the SR Ca2+ load and on the rate of its changes, a dependence that could be, at least in part, explained by the unique Ca2+ binding properties of calsequestrin. •  This study reveals Ca2+ buffering as a highly dynamic process, marking it as both a vulnerable link in diseases that involve loss of control of Ca2+ release, and a candidate for further study and intervention.

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

•  Malignant Hyperthermia (MH) affects the Ca2+ movements that control muscle contraction. We measured Ca2+ movements in skeletal muscle of “Y522S” mice, with a tyrosine‐to‐serine mutation in the RyR channel that causes MH in mice and humans. •  In YS cells, [Ca2+] inside the Ca2+ store (sarcoplasmic reticulum, SR) was 45% of that in the wild type (WT), but the SR membrane permeability increased 2‐fold, resulting in Ca2+ release of initially normal value. •  During Ca2+release, cytosolic [Ca2+] and SR Ca2+ buffering power evolved differently in YS and WT. These variables became similar in WT exposed to BAPTA, an inhibitor of Ca2+‐dependent inactivation (CDI) of the RyR, suggesting that tyrosine 522 is involved in CDI. •  Similar paradoxical observations in YS and WT cells with reduced content of the SR protein calsequestrin, revealed the importance of balance between SR Ca permeability (increased in YS) and storage capability (decreased when calsequestrin is low).

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

Significance We show that calsequestrin, the main Ca2+ storing protein of muscle, is polymerized inside the sarcoplasmic reticulum (SR) and its mobility increases greatly upon SR depletion, indicating depolymerization. Deep depletion causes massive calsequestrin migration and radical SR remodeling, often accompanied by a surge in intra-SR free Ca2+. The changes in calsequestrin polymerization observed in aqueous solutions therefore also occur in vivo. These changes help explain some uniquely advantageous properties of the SR as a source of calcium for contractile activation. The results support untested hypotheses about additional calsequestrin roles in the control of channel gating and facilitation of calcium flux. They also provide insights on the consequences of calsequestrin mutations for functional competence and structural stability of skeletal and cardiac 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.

Confocal Imaging of Transmembrane Voltage by SEER of di-8-ANEPPS

Imaging, optical mapping, and optical multisite recording of transmembrane potential (Vm) are essential for studying excitable cells and systems. The naphthylstyryl voltage-sensitive dyes, including di-8-ANEPPS, shift both their fluorescence excitation and emission spectra upon changes in Vm. Accordingly, they have been used for monitoring Vm in nonratioing and both emission and excitation ratioing modes. Their changes in fluorescence are usually much less than 10% per 100 mV. Conventional ratioing increases sensitivity to between 3 and 15% per 100 mV. Low sensitivity limits the value of these dyes, especially when imaged with low light systems like confocal scanners. Here we demonstrate the improvement afforded by shifted excitation and emission ratioing (SEER) as applied to imaging membrane potential in flexor digitorum brevis muscle fibers of adult mice. SEER—the ratioing of two images of fluorescence, obtained with different excitation wavelengths in different emission bands—was implemented in two commercial confocal systems. A conventional pinhole scanner, affording optimal setting of emission bands but less than ideal excitation wavelengths, achieved a sensitivity of up to 27% per 100 mV, nearly doubling the value found by conventional ratioing of the same data. A better pair of excitation lights should increase the sensitivity further, to 35% per 100 mV. The maximum acquisition rate with this system was 1 kHz. A fast “slit scanner” increased the effective rate to 8 kHz, but sensitivity was lower. In its high-sensitivity implementation, the technique demonstrated progressive deterioration of action potentials upon fatiguing tetani induced by stimulation patterns at >40 Hz, thereby identifying action potential decay as a contributor to fatigue onset. Using the fast implementation, we could image for the first time an action potential simultaneously at multiple locations along the t-tubule system. These images resolved the radially varying lag associated with propagation at a finite velocity.

A better method to measure total calcium in biological samples yields immediate payoffs

The calcium ion is the most universal and widely studied vector of chemical signals in living cells. Its presence and dynamic distribution in diverse organelles are associated with various functions, which include controlling protein expression and activity in response to functional demands. Calcium

Abnormal calcium signalling and the caffeine–halothane contracture test

Background: The variable clinical presentation of malignant hyperthermia (MH), a disorder of calcium signalling, hinders its diagnosis and management. Diagnosis relies on the caffeine–halothane contracture test, measuring contraction forces upon exposure of muscle to caffeine or halothane (FC and FH, respectively). Patients with above‐threshold FC or FH are diagnosed as MH susceptible. Many patients test positive to halothane only (termed ‘HH’). Our objective was to determine the characteristics of these HH patients, including their clinical symptoms and features of cytosolic Ca2+ signalling related to excitation–contraction coupling in myotubes. Methods: After institutional ethics committee approval, recruited patients undergoing contracture testing at Torontos MH centre were assigned to three groups: HH, doubly positive (HS), and negative patients (HN). A clinical index was assembled from musculoskeletal symptoms and signs. An analogous calcium index summarised four measures in cultured myotubes: resting [Ca2+]cytosol, frequency of spontaneous cytosolic Ca2+ events, Ca2+ waves, and cell‐wide Ca2+ spikes after electrical stimulation. Results: The highest values of both indexes were found in the HH group; the differences in calcium index between HH and the other groups were statistically significant. The principal component analysis confirmed the unique cell‐level features of the HH group, and identified elevated resting [Ca2+]cytosol and spontaneous event frequency as the defining HH characteristics. Conclusions: These findings suggest that HH pathogenesis stems from excess Ca2+ leak through sarcoplasmic reticulum channels. This identifies HH as a separate diagnostic group and opens their condition to treatment based on understanding of pathophysiological mechanisms.

The voltage sensor of excitation–contraction coupling in mammals: Inactivation and interaction with Ca2+

In skeletal muscle, the four-helix voltage-sensing modules (VSMs) of CaV1.1 calcium channels simultaneously gate two Ca2+ pathways: the CaV1.1 pore itself and the RyR1 calcium release channel in the sarcoplasmic reticulum. Here, to gain insight into the mechanism by which VSMs gate RyR1, we quantify intramembrane charge movement associated with VSM activation (sensing current) and gated Ca2+ release flux in single muscle cells of mice and rats. As found for most four-helix VSMs, upon sustained depolarization, rodent VSMs lose the ability to activate Ca2+ release channels opening; their properties change from a functionally capable mode, in which the mobile sensor charge is called charge 1, to an inactivated mode, charge 2, with a voltage dependence shifted toward more negative voltages. We find that charge 2 is promoted and Ca2+ release inactivated when resting, well-polarized muscle cells are exposed to low extracellular [Ca2+] and that the opposite occurs in high [Ca2+]. It follows that murine VSMs are partly inactivated at rest, which establishes the reduced availability of voltage sensing as a pathogenic mechanism in disorders of calcemia. We additionally find that the degree of resting inactivation is significantly different in two mouse strains, which underscores the variability of voltage sensor properties and their vulnerability to environmental conditions. Our studies reveal that the resting and activated states of VSMs are equally favored by extracellular Ca2+. Promotion by an extracellular species of two states of the VSM that differ in the conformation of the activation gate requires the existence of a second gate, inactivation, topologically extracellular and therefore accessible from outside regardless of the activation state.