Transient loss of voltage control of Ca2+ release in the presence of maurocalcine in skeletal muscle

Probenecid affects muscle Ca2+ homeostasis and contraction independently from pannexin channel block

Tight control of skeletal muscle contractile activation is secured by the excitation-contraction (EC) coupling protein complex, a molecular machinery allowing the plasma membrane voltage to control the activity of the ryanodine receptor Ca2+ release channel in the sarcoplasmic reticulum (SR) membrane. This machinery has been shown to be intimately linked to the plasma membrane protein pannexin-1 (Panx1). We investigated whether the prescription drug probenecid, a widely used Panx1 blocker, affects Ca2+ signaling, EC coupling, and muscle force. The effect of probenecid was tested on membrane current, resting Ca2+, and SR Ca2+ release in isolated mouse muscle fibers, using a combination of whole-cell voltage-clamp and Ca2+ imaging, and on electrically triggered contraction of isolated muscles. Probenecid (1 mM) induces SR Ca2+ leak at rest and reduces peak voltage-activated SR Ca2+ release and contractile force by 40%. Carbenoxolone, another Panx1 blocker, also reduces Ca2+ release, but neither a Panx1 channel inhibitory peptide nor a purinergic antagonist affected Ca2+ release, suggesting that probenecid and carbenoxolone do not act through inhibition of Panx1-mediated ATP release and consequently altered purinergic signaling. Probenecid may act by altering Panx1 interaction with the EC coupling machinery, yet the implication of another molecular target cannot be excluded. Since probenecid has been used both in the clinic and as a masking agent for doping in sports, these results should encourage evaluation of possible effects on muscle function in treated individuals. In addition, they also raise the question of whether probenecid-induced altered Ca2+ homeostasis may be shared by other tissues.

Superfast excitation-contraction coupling in adult zebrafish skeletal muscle fibers

The zebrafish has emerged as a very relevant animal model for probing the pathophysiology of human skeletal muscle disorders. This vertebrate animal model displays a startle response characterized by high-frequency swimming activity powered by contraction of fast skeletal muscle fibers excited at extremely high frequencies, critical for escaping predators and capturing prey. Such intense muscle performance requires extremely fast properties of the contractile machinery but also of excitation-contraction coupling, the process by which an action potential spreading along the sarcolemma induces a change in configuration of the dihydropyridine receptors, resulting in intramembrane charge movements, which in turn triggers the release of Ca2+ from the sarcoplasmic reticulum. However, thus far, the fastest Ca2+ transients evoked by vertebrate muscle fibers has been described in muscles used to produce sounds, such as those in the toadfish swim bladder, but not in muscles used for locomotion. By performing intracellular Ca2+ measurements under voltage control in isolated fast skeletal muscle fibers from adult zebrafish and mouse, we demonstrate that fish fast muscle fibers display superfast kinetics of action potentials, intramembrane charge movements, and action potential-evoked Ca2+ transient, allowing fusion and fused sustained Ca2+ transients at frequencies of excitation much higher than in mouse fast skeletal muscle fibers and comparable to those recorded in muscles producing sounds. The present study is the first demonstration of superfast kinetics of excitation-contraction coupling in skeletal muscle allowing superfast locomotor behaviors in a vertebrate.

Ca2+-induced sarcoplasmic reticulum Ca2+ release in myotubularin-deficient muscle fibers

Skeletal muscle deficiency in the 3-phosphoinositide (PtdInsP) phosphatase myotubularin (MTM1) causes myotubular myopathy which is associated with severe depression of voltage-activated sarcoplasmic reticulum Ca²⁺ release through ryanodine receptors. In the present study we aimed at further understanding how Ca²⁺ release is altered in MTM1-deficient muscle fibers, at rest and during activation. While in wild-type muscle fibers, SR Ca²⁺ release exhibits fast stereotyped kinetics of activation and decay throughout the voltage range of activation, Ca²⁺ release in MTM1-deficient muscle fibers exhibits slow and unconventional kinetics at intermediate voltages, suggestive of partial loss of the normal control of ryanodine receptor Ca²⁺ channel activity. In addition, the diseased muscle fibers at rest exhibit spontaneous elementary Ca²⁺ release events at a frequency 30 times greater than that of control fibers. Eighty percent of the events have spatiotemporal properties of archetypal Ca²⁺ sparks while the rest take either the form of lower amplitude, longer duration Ca²⁺ release events or of a combination thereof. The events occur at preferred locations in the fibers, indicating spatially uneven distribution of the parameters determining spontaneous ryanodine receptor 1 opening. Spatially large Ca²⁺ release sources were obviously involved in some of these events, suggesting that opening of ryanodine receptors in one cluster can activate opening of ryanodine receptors in a neighboring one. Overall results demonstrate that opening of Ca²⁺-activated ryanodine receptors is promoted both at rest and during excitation-contraction coupling in MTM1-deficient muscle fibers. Because access to this activation mode is denied to ryanodine receptors in healthy skeletal muscle, this may play an important role in the associated disease situation.

Impaired excitation-contraction coupling in muscle fibres from the dynamin2R465W mouse model of centronuclear myopathy

KEY POINTS

Dynamin 2 is a ubiquitously expressed protein involved in membrane trafficking processes. Mutations in the gene encoding dynamin 2 are responsible for a congenital myopathy associated with centrally located nuclei in the muscle fibres. Using muscle fibres from a mouse model of the most common mutation responsible for this disease in humans, we tested whether altered Ca²⁺ signalling and excitation-contraction coupling contribute to muscle weakness. The plasma membrane network that carries the electrical excitation is moderately perturbed in the diseased muscle fibres. The excitation-activated Ca²⁺ input fluxes across both the plasma membrane and the membrane of the sarcoplasmic reticulum are defective in the diseased fibres, which probably contributes to muscle weakness in patients.

ABSTRACT

Mutations in the gene encoding dynamin 2 (DNM2) are responsible for autosomal dominant centronuclear myopathy (AD-CNM). We studied the functional properties of Ca²⁺ signalling and excitation-contraction (EC) coupling in muscle fibres isolated from a knock-in (KI) mouse model of the disease, using confocal imaging and the voltage clamp technique. The transverse-tubule network organization appeared to be unaltered in the diseased fibres, although its density was reduced by ∼10% compared to that in control fibres. The density of Ca²⁺ current through CaV1.1 channels and the rate of voltage-activated sarcoplasmic reticulum Ca²⁺ release were reduced by ∼60% and 30%, respectively, in KI vs. control fibres. In addition, Ca²⁺ release in the KI fibres reached its peak value 10-50 ms later than in control ones. Activation of Ca²⁺ transients along the longitudinal axis of the fibres was more heterogeneous in the KI than in the control fibres, with the difference being exacerbated at intermediate membrane voltages. KI fibres exhibited spontaneous Ca²⁺ release events that were almost absent from control fibres. Overall, the results of the present study demonstrate that Ca²⁺ signalling and EC coupling exhibit a number of dysfunctions likely contributing to muscle weakness in DNM2-related AD-CNM.

Phosphatidylinositol 3-kinase inhibition restores Ca2+ release defects and prolongs survival in myotubularin-deficient mice

Mutations in the gene encoding the phosphoinositide 3-phosphatase myotubularin (MTM1) are responsible for a pediatric disease of skeletal muscle named myotubular myopathy (XLMTM). Muscle fibers from MTM1-deficient mice present defects in excitation-contraction (EC) coupling likely responsible for the disease-associated fatal muscle weakness. However, the mechanism leading to EC coupling failure remains unclear. During normal skeletal muscle EC coupling, transverse (t) tubule depolarization triggers sarcoplasmic reticulum (SR) Ca²⁺ release through ryanodine receptor channels gated by conformational coupling with the t-tubule voltage-sensing dihydropyridine receptors. We report that MTM1 deficiency is associated with a 60% depression of global SR Ca²⁺ release over the full range of voltage sensitivity of EC coupling. SR Ca²⁺ release in the diseased fibers is also slower than in normal fibers, or delayed following voltage activation, consistent with the contribution of Ca²⁺-gated ryanodine receptors to EC coupling. In addition, we found that SR Ca²⁺ release is spatially heterogeneous within myotubularin-deficient muscle fibers, with focally defective areas recapitulating the global alterations. Importantly, we found that pharmacological inhibition of phosphatidylinositol 3-kinase (PtdIns 3-kinase) activity rescues the Ca²⁺ release defects in isolated muscle fibers and increases the lifespan and mobility of XLMTM mice, providing proof of concept for the use of PtdIns 3-kinase inhibitors in myotubular myopathy and suggesting that unbalanced PtdIns 3-kinase activity plays a critical role in the pathological process.

In cellulo phosphorylation induces pharmacological reprogramming of maurocalcin, a cell-penetrating venom peptide

The venom peptide maurocalcin (MCa) is atypical among toxins because of its ability to rapidly translocate into cells and potently activate the intracellular calcium channel type 1 ryanodine receptor (RyR1). Therefore, MCa is potentially subjected to posttranslational modifications within recipient cells. Here, we report that MCa Thr(26) belongs to a consensus PKA phosphorylation site and can be phosphorylated by PKA both in vitro and after cell penetration in cellulo. Unexpectedly, phosphorylation converts MCa from positive to negative RyR1 allosteric modulator. Thr(26) phosphorylation leads to charge neutralization of Arg(24), a residue crucial for MCa agonist activity. The functional effect of Thr(26) phosphorylation is partially mimicked by aspartyl mutation. This represents the first case, to our knowledge, of both ex situ posttranslational modification and pharmacological reprogramming of a small natural cystine-rich peptide by target cells. So far, phosphorylated MCa is the first specific negative allosteric modulator of RyR1, to our knowledge, and represents a lead compound for further development of phosphatase-resistant analogs.

Depression of voltage-activated Ca2+ release in skeletal muscle by activation of a voltage-sensing phosphatase

Transverse tubule PIP₂ modulates Ca²⁺ release from the SR during EC coupling.

Phosphoinositides act as signaling molecules in numerous cellular transduction processes, and phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P₂) regulates the function of several types of plasma membrane ion channels. We investigated the potential role of PtdIns(4,5)P₂ in Ca²⁺ homeostasis and excitation–contraction (E-C) coupling of mouse muscle fibers using in vivo expression of the voltage-sensing phosphatases (VSPs) Ciona intestinalis VSP (Ci-VSP) or Danio rerio VSP (Dr-VSP). Confocal images of enhanced green fluorescent protein–tagged Dr-VSP revealed a banded pattern consistent with VSP localization within the transverse tubule membrane. Rhod-2 Ca²⁺ transients generated by 0.5-s-long voltage-clamp depolarizing pulses sufficient to elicit Ca²⁺ release from the sarcoplasmic reticulum (SR) but below the range at which VSPs are activated were unaffected by the presence of the VSPs. However, in Ci-VSP–expressing fibers challenged by 5-s-long depolarizing pulses, the Ca²⁺ level late in the pulse (3 s after initiation) was significantly lower at 120 mV than at 20 mV. Furthermore, Ci-VSP–expressing fibers showed a reversible depression of Ca²⁺ release during trains, with the peak Ca²⁺ transient being reduced by ∼30% after the application of 10 200-ms-long pulses to 100 mV. A similar depression was observed in Dr-VSP–expressing fibers. Cav1.1 Ca²⁺ channel–mediated current was unaffected by Ci-VSP activation. In fibers expressing Ci-VSP and a pleckstrin homology domain fused with monomeric red fluorescent protein (PLCδ₁PH-mRFP), depolarizing pulses elicited transient changes in mRFP fluorescence consistent with release of transverse tubule–bound PLCδ₁PH domain into the cytosol; the voltage sensitivity of these changes was consistent with that of Ci-VSP activation, and recovery occurred with a time constant in the 10-s range. Our results indicate that the PtdIns(4,5)P₂ level is tightly maintained in the transverse tubule membrane of the muscle fibers, and that VSP-induced depletion of PtdIns(4,5)P₂ impairs voltage-activated Ca²⁺ release from the SR. Because Ca²⁺ release is thought to be independent from InsP₃ signaling, the effect likely results from an interaction between PtdIns(4,5)P₂ and a protein partner of the E-C coupling machinery.

Phosphoinositide substrates of myotubularin affect voltage-activated Ca²⁺ release in skeletal muscle

Skeletal muscle excitation–contraction (E–C) coupling is altered in several models of phosphatidylinositol phosphate (PtdInsP) phosphatase deficiency and ryanodine receptor activity measured in vitro was reported to be affected by certain PtdInsPs, thus prompting investigation of the physiological role of PtdInsPs in E–C coupling. We measured intracellular Ca2+ transients in voltage-clamped mouse muscle fibres microinjected with a solution containing a PtdInsP substrate (PtdIns(3,5)P2 or PtdIns(3)P) or product (PtdIns(5)P or PtdIns) of the myotubularin phosphatase MTM1. No significant change was observed in the presence of either PtdIns(5)P or PtdIns but peak SR Ca2+ release was depressed by ~30% and 50% in fibres injected with PtdIns(3,5)P2 and PtdIns(3)P, respectively, with no concurrent alteration in the membrane current signals associated with the DHPR function as well as in the voltage dependence of Ca2+ release inactivation. In permeabilized muscle fibres, the frequency of spontaneous Ca2+ release events was depressed in the presence of the three tested phosphorylated forms of PtdInsP with PtdIns(3,5)P2 being the most effective, leading to an almost complete disappearance of Ca2+ release events. Results support the possibility that pathological accumulation of MTM1 substrates may acutely depress ryanodine receptor-mediated Ca2+ release. Overexpression of a mCherry-tagged form of MTM1 in muscle fibres revealed a striated pattern consistent with the triadic area. Ca2+ release remained although unaffected by MTM1 overexpression and was also unaffected by the PtdIns-3-kinase inhibitor LY2940002, suggesting that the 3-phosphorylated PtdIns lipids active on voltage-activated Ca2+ release are inherently maintained at a low level, inefficient on Ca2+ release in normal conditions.

Ca2+ release in muscle fibers expressing R4892W and G4896V type 1 ryanodine receptor disease mutants

The large and rapidly increasing number of potentially pathological mutants in the type 1 ryanodine receptor (RyR1) prompts the need to characterize their effects on voltage-activated sarcoplasmic reticulum (SR) Ca²⁺ release in skeletal muscle. Here we evaluated the function of the R4892W and G4896V RyR1 mutants, both associated with central core disease (CCD) in humans, in myotubes and in adult muscle fibers. For both mutants expressed in RyR1-null (dyspedic) myotubes, voltage-gated Ca²⁺ release was absent following homotypic expression and only partially restored following heterotypic expression with wild-type (WT) RyR1. In muscle fibers from adult WT mice, both mutants were expressed in restricted regions of the fibers with a pattern consistent with triadic localization. Voltage-clamp-activated confocal Ca²⁺ signals showed that fiber regions endowed with G4896V-RyR1s exhibited an ∼30% reduction in the peak rate of SR Ca²⁺ release, with no significant change in SR Ca²⁺ content. Immunostaining revealed no associated change in the expression of either α1S subunit (Cav1.1) of the dihydropyridine receptor (DHPR) or type 1 sarco(endo)plasmic reticulum Ca²⁺ ATPase (SERCA1), indicating that the reduced Ca²⁺ release resulted from defective RyR1 function. Interestingly, in spite of robust localized junctional expression, the R4892W mutant did not affect SR Ca²⁺ release in adult muscle fibers, consistent with a low functional penetrance of this particular CCD-associated mutant.

Sarcoplasmic reticulum Ca2+ permeation explored from the lumen side in mdx muscle fibers under voltage control

Under resting conditions, external Ca²⁺ is known to enter skeletal muscle cells, whereas Ca²⁺ stored in the sarcoplasmic reticulum (SR) leaks into the cytosol. The nature of the pathways involved in the sarcolemmal Ca²⁺ entry and in the SR Ca²⁺ leak is still a matter of debate, but several lines of evidence suggest that these Ca²⁺ fluxes are up-regulated in Duchenne muscular dystrophy. We investigated here SR calcium permeation at resting potential and in response to depolarization in voltage-controlled skeletal muscle fibers from control and mdx mice, the mouse model of Duchenne muscular dystrophy. Using the cytosolic Ca²⁺ dye Fura2, we first demonstrated that the rate of Ca²⁺ increase in response to cyclopiazonic acid (CPA)–induced inhibition of SR Ca²⁺-ATPases at resting potential was significantly higher in mdx fibers, which suggests an elevated SR Ca²⁺ leak. However, removal of external Ca²⁺ reduced the rate of CPA-induced Ca²⁺ increase in mdx and increased it in control fibers, which indicates an up-regulation of sarcolemmal Ca²⁺ influx in mdx fibers. Fibers were then loaded with the low-affinity Ca²⁺ dye Fluo5N-AM to measure intraluminal SR Ca²⁺ changes. Trains of action potentials, chloro-m-cresol, and depolarization pulses evoked transient Fluo5N fluorescence decreases, and recovery of voltage-induced Fluo5N fluorescence changes were inhibited by CPA, demonstrating that Fluo5N actually reports intraluminal SR Ca²⁺ changes. Voltage dependence and magnitude of depolarization-induced SR Ca²⁺ depletion were found to be unchanged in mdx fibers, but the rate of the recovery phase that followed depletion was found to be faster, indicating a higher SR Ca²⁺ reuptake activity in mdx fibers. Overall, CPA-induced SR Ca²⁺ leak at −80 mV was found to be significantly higher in mdx fibers and was potentiated by removal of external Ca²⁺ in control fibers. The elevated passive SR Ca²⁺ leak may contribute to alteration of Ca²⁺ homeostasis in mdx muscle.

Small efficient cell-penetrating peptides derived from scorpion toxin maurocalcine

Maurocalcine is the first demonstrated example of an animal toxin peptide with efficient cell penetration properties. Although it is a highly competitive cell-penetrating peptide (CPP), its relatively large size of 33 amino acids and the presence of three internal disulfide bridges may hamper its development for in vitro and in vivo applications. Here, we demonstrate that several efficient CPPs can be derived from maurocalcine by replacing Cys residues by isosteric 2-aminobutyric acid residues and sequence truncation down to peptides of up to 9 residues in length. A surprising finding is that all of the truncated maurocalcine analogues possessed cell penetration properties, indicating that the maurocalcine is a highly specialized CPP. Careful examination of the cell penetration properties of the truncated analogues indicates that several maurocalcine-derived peptides should be of great interest for cell delivery applications where peptide size matters.

Junctophilin 1 and 2 proteins interact with the L-type Ca2+ channel dihydropyridine receptors (DHPRs) in skeletal muscle

Junctophilins (JPs) anchor the endo/sarcoplasmic reticulum to the plasma membrane, thus contributing to the assembly of junctional membrane complexes in striated muscles and neurons. Recent studies have shown that JPs may be also involved in regulating Ca2+ homeostasis. Here, we report that in skeletal muscle, JP1 and JP2 are part of a complex that, in addition to ryanodine receptor 1 (RyR1), includes caveolin 3 and the dihydropyridine receptor (DHPR). The interaction between JPs and DHPR was mediated by a region encompassing amino acids 230-369 and amino acids 216-399 in JP1 and JP2, respectively. Immunofluorescence studies revealed that the pattern of DHPR and RyR signals in C2C12 cells knocked down for JP1 and JP2 was rather diffused and characterized by smaller puncta in contrast to that observed in control cells. Functional experiments revealed that down-regulation of JPs in differentiated C2C12 cells resulted in a reduction of intramembrane charge movement and the L-type Ca2+ current accompanied by a reduced number of DHPRs at the plasma membrane, whereas there was no substantial alteration in Ca2+ release from the sterol regulatory element-binding protein. Altogether, these results suggest that JP1 and JP2 can facilitate the assembly of DHPR with other proteins of the excitation-contraction coupling machinery.

Defects in Ca2+ release associated with local expression of pathological ryanodine receptors in mouse muscle fibres

Mutations of the gene encoding the type 1 ryanodine receptor (RyR1) are associated with skeletal muscle disorders including malignant hyperthermia susceptibility (MHS) and central core disease (CCD). We used in vivo expression of EGFP-RyR1 constructs in fully differentiated mouse muscle fibres to characterize the function of several RyR1 mutants. Wild-type and Y523S, R615C, R2163H and I4897T mutants of RyR1 were separately expressed and found to be present within restricted regions of fibres with a pattern consistent with triadic localization. Confocal measurements of voltage-clamp-activated myoplasmic Ca(2+) transients demonstrated alterations of sarcoplasmic reticulum (SR) Ca(2+) release spatially correlated with the presence of exogenous RyR1s. The Y523S, R615C and R2163H RyR1 MHS-related mutants were associated with enhanced peak Ca(2+) release for low and moderate levels of depolarization, whereas the I4897T CCD mutant produced a chronic reduction of peak SR Ca(2+) release. For example, peak Ca(2+) release in response to a depolarization to -20 mV in regions of fibres expressing Y523S and I4897T was 2.0 ± 0.3 (n = 9) and 0.46 ± 0.1 (n = 5) times the corresponding value in adjacent, non-expressing regions of the same fibre, respectively. Interestingly no significant change in the estimated total amount of Ca(2+) released at the end of large depolarizing pulses was observed for any of the mutant RyR1 channels. Overall, results are consistent with an 'inherent' increase in RyR1 sensitivity to activation by the voltage sensor for the MHS-related RyR1 mutants and a partial failure of voltage-gated release for the CCD-related I4897T mutant, that occur with no sign of change in SR Ca(2+) content. Furthermore, the results indicate that RyR1 channel density is tightly regulated even under the present conditions of forced exogenous expression.

The elusive role of the SPRY2 domain in RyR1

The second of three SPRY domains (SPRY2, S1085 -V1208) located in the skeletal muscle ryanodine receptor (RyR1) is contained within regions of RyR1 that influence EC coupling and bind to imperatoxin A, a toxin probe of RyR1 channel gating. We examined the binding of the F loop (P1107-A1121) in SPRY2 to the ASI/basic region in RyR1 (T3471-G3500, containing both alternatively spliced (ASI) residues and neighboring basic amino acids). We then investigated the possible influence of this interaction on excitation contraction (EC) coupling. A peptide with the F loop sequence and an antibody to the SPRY2 domain each enhanced RyR1 activity at low concentrations and inhibited at higher concentrations. A peptide containing the ASI/basic sequence bound to SPRY2 and binding decreased ~10-fold following mutation or structural disruption of the basic residues. Binding was abolished by mutation of three critical acidic F loop residues. Together these results suggest that the ASI/basic and SPRY2 domains interact in an F loop regulatory module. Although a region that includes the SPRY2 domain influences EC coupling, as does the ASI/basic region, Ca2+ release during ligand- and depolarization-induced RyR1 activation were not altered by mutation of the three critical F loop residues following expression of mutant RyR1 in RyR1-null myotubes. Therefore the electrostatic regulatory interaction between the SPRY2 F loop residues (that bind to imperatoxin A) and the ASI/basic residues of RyR1 does not influence bi-directional DHPR-RyR1 signaling during skeletal EC coupling, possibly because the interaction is interrupted by the influence of factors present in intact muscle cells.

Model calcium sensors for network homeostasis: sensor and readout parameter analysis from a database of model neuronal networks

In activity-dependent homeostatic regulation (ADHR) of neuronal and network properties, the intracellular Ca(2+) concentration is a good candidate for sensing activity levels because it is correlated with the electrical activity of the cell. Previous ADHR models, developed with abstract activity sensors for model pyloric neurons and networks of the crustacean stomatogastric ganglion, showed that functional activity can be maintained by a regulation mechanism that senses activity levels solely from Ca(2+). At the same time, several intracellular pathways have been discovered for Ca(2+)-dependent regulation of ion channels. To generate testable predictions for dynamics of these signaling pathways, we undertook a parameter study of model Ca(2+) sensors across thousands of model pyloric networks. We found that an optimal regulation signal can be generated for 86% of model networks with a sensing mechanism that activates with a time constant of 1 ms and that inactivates within 1 s. The sensor performed robustly around this optimal point and did not need to be specific to the role of the cell. When multiple sensors with different time constants were used, coverage extended to 88% of the networks. Without changing the sensors, it extended to 95% of the networks by letting the sensors affect the readout nonlinearly. Specific to this pyloric network model, the sensor of the follower pyloric constrictor cell was more informative than the pacemaker anterior burster cell for producing a regulatory signal. Conversely, a global signal indicating network activity that was generated by summing the sensors in individual cells was less informative for regulation.

Neurotoxic and cytotoxic effects of venom from different populations of the Egyptian Scorpio maurus palmatus

Neurotoxic and cytotoxic effects of venoms from Scorpio maurus palmatus taken from different populations were assessed for geographic based variability in toxicity, and to evaluate their insecticidal potency. Scorpions were collected from four regions. Three locations were mutually isolated pockets in the arid area of Southern Sinai. The fourth sample was collected from a population inhabiting the semi-arid environment of Western Mediterranean Coastal Desert. The neurotoxic (paralytic) effect of the venom from each population was assayed by its ability to induce permanent disability in adult cockroaches within 3h. Venom was applied using microinjection techniques through an intersegmental membrane. Probit analysis was used to calculate the Paralytic Effective Dose (PED(50), ng/100mg). Levels of glutathione, lipid peroxidation, protein carbonyl content and nitric oxide, as well as the activities of superoxide dismutase, catalase and cholinesterase, were measured to assess the cytotoxicity of the venom. The results show that the injected venom from each population induced obvious spasticity, followed by flaccid paralysis. All the tested biochemical parameters, except glutathione content, revealed significant differences in toxicity in venom taken from the different scorpion populations. We conclude that (i) the venom of this scorpion has significant neurotoxic and cytotoxic effects on insect cells, (ii) its efficacy, as assessed by the PED(50) unit, exhibited variation across its geographic range, and (iii) components in the venom may have the potential for being developed into effective and environmentally friendly bioinsecticides.

Voltage-activated elementary calcium release events in isolated mouse skeletal muscle fibers

The elementary Ca(2+)-release events underlying voltage-activated myoplasmic Ca(2+) transients in mammalian muscle remain elusive. Here, we looked for such events in confocal line-scan (x,t) images of fluo-3 fluorescence taken from isolated adult mouse skeletal muscle fibers held under voltage-clamp conditions. In response to step depolarizations, spatially segregated fluorescence signals could be detected that were riding on a global increase in fluorescence. These discrete signals were separated using digital filtering in the spatial domain; mean values for their spatial half-width and amplitude were 1.99 +/- 0.09 microm and 0.16 +/- 0.005 DeltaF/F(0) (n = 151), respectively. Under control conditions, the duration of the events was limited by the pulse duration. In contrast, in the presence of maurocalcine, a scorpion toxin suspected to disrupt the process of repolarization-induced ryanodine receptor (RyR) closure, events uninterrupted by the end of the pulse were readily detected. Overall results establish these voltage-activated low-amplitude local Ca(2+) signals as inherent components of the physiological Ca(2+)-release process of mammalian muscle and suggest that they result from the opening of either one RyR or a coherently operating group of RyRs, under the control of the plasma membrane polarization.

A dihydropyridine receptor alpha1s loop region critical for skeletal muscle contraction is intrinsically unstructured and binds to a SPRY domain of the type 1 ryanodine receptor

The II-III loop of the dihydropyridine receptor (DHPR) alpha(1s) subunit is a modulator of the ryanodine receptor (RyR1) Ca(2+) release channel in vitro and is essential for skeletal muscle contraction in vivo. Despite its importance, the structure of this loop has not been reported. We have investigated its structure using a suite of NMR techniques which revealed that the DHPR II-III loop is an intrinsically unstructured protein (IUP) and as such belongs to a burgeoning structural class of functionally important proteins. The loop does not possess a stable tertiary fold: it is highly flexible, with a strong N-terminal helix followed by nascent helical/turn elements and unstructured segments. Its residual structure is loosely globular with the N and C termini in close proximity. The unstructured nature of the II-III loop may allow it to easily modify its interaction with RyR1 following a surface action potential and thus initiate rapid Ca(2+) release and contraction. The in vitro binding partner for the II-III was investigated. The II-III loop interacts with the second of three structurally distinct SPRY domains in RyR1, whose function is unknown. This interaction occurs through two preformed N-terminal alpha-helical regions and a C-terminal hydrophobic element. The A peptide corresponding to the helical N-terminal region is a common probe of RyR function and binds to the same SPRY domain as the full II-III loop. Thus the second SPRY domain is an in vitro binding site for the II-III loop. The possible in vivo role of this region is discussed.

Charged surface area of maurocalcine determines its interaction with the skeletal ryanodine receptor

The 33 amino acid scorpion toxin maurocalcine (MCa) has been shown to modify the gating of the skeletal-type ryanodine receptor (RyR1). Here we explored the effects of MCa and its mutants ([Ala(8)]MCa, [Ala(19)]MCa, [Ala(20)]MCa, [Ala(22)]MCa, [Ala(23)]MCa, and [Ala(24)]MCa) on RyR1 incorporated into artificial lipid bilayers and on elementary calcium release events (ECRE) in rat and frog skeletal muscle fibers. The peptides induced long-lasting subconductance states (LLSS) on RyR1 that lasted for several seconds. However, their average length and frequency were decreased if the mutation was placed farther away in the 3D structure from the critical (24)Arg residue. The effect was strongly dependent on the direction of the current through the channel. If the direction was similar to that followed by calcium during release, the peptides were 8- to 10-fold less effective. In fibers long-lasting calcium release events were observed after the addition of the peptides. The average length of these events correlated well with the duration of LLSS. These data suggest that the effect of the peptide is governed by the large charged surface formed by residues Lys(20), Lys(22), Arg(23), Arg(24), and Lys(8). Our observations also indicate that the results from bilayer experiments mimic the in situ effects of MCa on RyR1.

Design of a disulfide-less, pharmacologically inert, and chemically competent analog of maurocalcine for the efficient transport of impermeant compounds into cells

Maurocalcine is a 33-mer peptide initially isolated from the venom of a Tunisian scorpion. It has proved itself valuable as a pharmacological activator of the ryanodine receptor and has helped the understanding of the molecular basis underlying excitation-contraction coupling in skeletal muscles. Because of its positively charged nature, it is also an innovative vector for the cell penetration of various compounds. We report a novel maurocalcine analog with improved properties: (i) the complete loss of pharmacological activity, (ii) preservation of the potent ability to carry cargo molecules into cells, and (iii) coupling chemistries not affected by the presence of internal cysteine residues of maurocalcine. We did this by replacing the six internal cysteine residues of maurocalcine by isosteric 2-aminobutyric acid residues and by adding an additional N-terminal biotinylated lysine (for a proof of concept analog) or an N-terminal cysteine residue (for a chemically competent coupling analogue). Additional replacement of a glutamate residue by alanyl at position 12 further improves the potency of these analogues. Coupling to several cargo molecules or nanoparticles are presented to illustrate the cell penetration potency and usefulness of these pharmacologically inactive analogs.

Spontaneous and voltage-activated Ca2+ release in adult mouse skeletal muscle fibres expressing the type 3 ryanodine receptor

The physiological properties and role of the type 3 ryanodine receptor (RyR3), a calcium release channel expressed in a wide variety of cell types, remain mysterious. We forced, in vivo, the expression of RyR3 in adult mouse skeletal muscle fibres using a GFP-RyR3 DNA construct. GFP fluorescence was found within spatially restricted regions of muscle fibres where it exhibited a sarcomere-related banded pattern consistent with a localization within or near the junctional sarcoplasmic reticulum membrane. Immunostaining confirmed the presence of RyR3 together with RyR1 within the GFP-positive areas. In approximately 90% of RyR3-positive fibres microinjected with the calcium indicator fluo-3, we detected repetitive spontaneous transient elevations of intracellular Ca2+ that persisted when fibres were voltage-clamped at -80 mV. These Ca2+ transients remained essentially confined to the RyR3 expression region. They ranged from wide local events to propagating Ca2+ waves and were in some cases associated with local contractile activity. When voltage-clamp depolarizations were applied while fluo-3 or rhod-2 fluorescence was measured within the RyR3-expressing region, no voltage-evoked 'spark-like' elementary Ca2+ release event could be detected. Still global voltage-activated Ca2+ release exhibited a prominent early peak within the RyR3-expressing regions. Measurements were also taken from muscles fibres expressing a GFP-RyR1 construct; positive fibres also yielded a local banded pattern of GFP fluorescence but exhibited no spontaneous Ca2+ release. Results demonstrate that RyR3 is a very potent source of voltage-independent Ca2+ release activity. Conversely we find no evidence that it could contribute to the production of discrete voltage-activated Ca2+ release events in differentiated mammalian skeletal muscle.

Critical amino acid residues of maurocalcine involved in pharmacology, lipid interaction and cell penetration

Maurocalcine (MCa) is a 33-amino acid residue peptide that was initially identified in the Tunisian scorpion Scorpio maurus palmatus. This peptide triggers interest for three main reasons. First, it helps unravelling the mechanistic basis of Ca(2+) mobilization from the sarcoplasmic reticulum because of its sequence homology with a calcium channel domain involved in excitation-contraction coupling. Second, it shows potent pharmacological properties because of its ability to activate the ryanodine receptor. Finally, it is of technological value because of its ability to carry cell-impermeable compounds across the plasma membrane. Herein, we characterized the molecular determinants that underlie the pharmacological and cell-penetrating properties of maurocalcine. We identify several key amino acid residues of the peptide that will help the design of cell-penetrating analogues devoid of pharmacological activity and cell toxicity. Close examination of the determinants underlying cell penetration of maurocalcine reveals that basic amino acid residues are required for an interaction with negatively charged lipids of the plasma membrane. Maurocalcine analogues that penetrate better have also stronger interaction with negatively charged lipids. Conversely, less effective analogues present a diminished ability to interact with these lipids. These findings will also help the design of still more potent cell penetrating analogues of maurocalcine.

Maurocalcine interacts with the cardiac ryanodine receptor without inducing channel modification

We have previously shown that MCa (maurocalcine), a toxin from the venom of the scorpion Maurus palmatus, binds to RyR1 (type 1 ryanodine receptor) and induces strong modifications of its gating behaviour. In the present study, we investigated the ability of MCa to bind to and modify the gating process of cardiac RyR2. By performing pull-down experiments we show that MCa interacts directly with RyR2 with an apparent affinity of 150 nM. By expressing different domains of RyR2 in vitro, we show that MCa binds to two domains of RyR2, which are homologous with those previously identified on RyR1. The effect of MCa binding to RyR2 was then evaluated by three different approaches: (i) [(3)H]ryanodine binding experiments, showing a very weak effect of MCa (up to 1 muM), (ii) Ca(2+) release measurements from cardiac sarcoplasmic reticulum vesicles, showing that MCa up to 1 muM is unable to induce Ca(2+) release, and (iii) single-channel recordings, showing that MCa has no effect on the open probability or on the RyR2 channel conductance level. Long-lasting opening events of RyR2 were observed in the presence of MCa only when the ionic current direction was opposite to the physiological direction, i.e. from the cytoplasmic face of RyR2 to its luminal face. Therefore, despite the conserved MCa binding ability of RyR1 and RyR2, functional studies show that, in contrast with what is observed with RyR1, MCa does not affect the gating properties of RyR2. These results highlight a different role of the MCa-binding domains in the gating process of RyR1 and RyR2.