The RyR1 P3528S Substitution Alters Mouse Skeletal Muscle Contractile Properties and RyR1 Ion Channel Gating

The recessive Ryanodine Receptor Type 1 (RyR1) P3527S mutation causes mild muscle weakness in patients and increased resting cytoplasmic [Ca2+] in transformed lymphoblastoid cells. In the present study, we explored the cellular/molecular effects of this mutation in a mouse model of the mutation (RyR1 P3528S). The results were obtained from 73 wild type (WT/WT), 82 heterozygous (WT/MUT) and 66 homozygous (MUT/MUT) mice with different numbers of observations in individual data sets depending on the experimental protocol. The results showed that WT/MUT and MUT/MUT mouse strength was less than that of WT/WT mice, but there was no difference between genotypes in appearance, weight, mobility or longevity. The force frequency response of extensor digitorum longus (EDL) and soleus (SOL) muscles from WT/MUT and MUT/MUT mice was shifter to higher frequencies. The specific force of EDL muscles was reduced and Ca2+ activation of skinned fibres shifted to a lower [Ca2+], with an increase in type I fibres in EDL muscles and in mixed type I/II fibres in SOL muscles. The relative activity of RyR1 channels exposed to 1 µM cytoplasmic Ca2+ was greater in WT/MUT and MUT/MUT mice than in WT/WT mice. We suggest the altered RyR1 activity due to the P2328S substitution could increase resting [Ca2+] in muscle fibres, leading to changes in fibre type and contractile properties.

Biophysical reviews top five: voltage-dependent charge movement in nerve and muscle

The discovery of gating currents and asymmetric charge movement in the early 1970s represented a remarkable leap forward in our understanding of the biophysical basis of voltage-dependent events that underlie electrical signalling that is vital for nerve and muscle function. Gating currents and charge movement reflect a fundamental process in which charged amino acid residues in an ion channel protein move in response to a change in the membrane electrical field and therefore activate the specific voltage-dependent response of that protein. The detection of gating currents and asymmetric charge movement over the past 50 years has been pivotal in unraveling the multiple molecular and intra-molecular processes which lead to action potentials in excitable tissues and excitation-contraction (EC) coupling in skeletal muscle. The recording of gating currents and asymmetric charge movement remains an essential component of investigations into the basic molecular mechanisms of neuronal conduction and muscle contraction.

FKBP12 binds to the cardiac ryanodine receptor with negative cooperativity: implications for heart muscle physiology in health and disease

Cardiac ryanodine receptors (RyR2) release the Ca²⁺ from intracellular stores that is essential for cardiac myocyte contraction. The ion channel opening is tightly regulated by intracellular factors, including the FK506 binding proteins, FKBP12 and FKBP12.6. The impact of these proteins on RyR2 activity and cardiac contraction is debated, with often apparently contradictory experimental results, particularly for FKBP12. The isoform that regulates RyR2 has generally been considered to be FKBP12.6, despite the fact that FKBP12 is the major isoform associated with RyR2 in some species and is bound in similar proportions to FKBP12.6 in others, including sheep and humans. Here, we show time- and concentration-dependent effects of adding FKBP12 to RyR2 channels that were partly depleted of FKBP12/12.6 during isolation. The added FKBP12 displaced most remaining endogenous FKBP12/12.6. The results suggest that FKBP12 activates RyR2 with high affinity and inhibits RyR2 with lower affinity, consistent with a model of negative cooperativity in FKBP12 binding to each of the four subunits in the RyR tetramer. The easy dissociation of some FKBP12/12.6 could dynamically alter RyR2 activity in response to changes in in vivo regulatory factors, indicating a significant role for FKBP12/12.6 in Ca²⁺ signalling and cardiac function in healthy and diseased hearts. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

Feedback contributions to excitation-contraction coupling in native functioning striated muscle

Skeletal and cardiac muscle excitation-contraction coupling commences with Na_v1.4/Na_v1.5-mediated, surface and transverse (T-) tubular, action potential generation. This initiates feedforward, allosteric or Ca²⁺-mediated, T-sarcoplasmic reticular (SR) junctional, voltage sensor-Cav1.1/Cav1.2 and ryanodine receptor-RyR1/RyR2 interaction. We review recent structural, physiological and translational studies on possible feedback actions of the resulting SR Ca²⁺ release on Na_v1.4/Na_v1.5 function in native muscle. Finite-element modelling predicted potentially regulatory T-SR junctional [Ca²⁺]_TSR domains. Na_v1.4/Na_v1.5, III-IV linker and C-terminal domain structures included Ca²⁺ and/or calmodulin-binding sites whose mutations corresponded to specific clinical conditions. Loose-patch-clamped native murine skeletal muscle fibres and cardiomyocytes showed reduced Na⁺ currents (I_Na) following SR Ca²⁺ release induced by the Epac and direct RyR1/RyR2 activators, 8-(4-chlorophenylthio)adenosine-3',5'-cyclic monophosphate and caffeine, abrogated by the RyR inhibitor dantrolene. Conversely, dantrolene and the Ca²⁺-ATPase inhibitor cyclopiazonic acid increased I_Na. Experimental, catecholaminergic polymorphic ventricular tachycardic RyR2-P2328S and metabolically deficient Pgc1β^-/- cardiomyocytes also showed reduced I_Na accompanying [Ca²⁺]_i abnormalities rescued by dantrolene- and flecainide-mediated RyR block. Finally, hydroxychloroquine challenge implicated action potential (AP) prolongation in slowing AP conduction through modifying Ca²⁺ transients. The corresponding tissue/organ preparations each showed pro-arrhythmic, slowed AP upstrokes and conduction velocities. We finally extend discussion of possible Ca²⁺-mediated effects to further, Ca²⁺, K⁺ and Cl^-, channel types. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

How does flecainide impact RyR2 channel function?

Flecainide, a cardiac class 1C blocker of the surface membrane sodium channel (NaV1.5), has also been reported to reduce cardiac ryanodine receptor (RyR2)-mediated sarcoplasmic reticulum (SR) Ca2+ release. It has been introduced as a clinical antiarrhythmic agent for catecholaminergic polymorphic ventricular tachycardia (CPVT), a condition most commonly associated with gain-of-function RyR2 mutations. Current debate concerns both cellular mechanisms of its antiarrhythmic action and molecular mechanisms of its RyR2 actions. At the cellular level, it targets NaV1.5, RyR2, Na+/Ca2+ exchange (NCX), and additional proteins involved in excitation-contraction (EC) coupling and potentially contribute to the CPVT phenotype. This Viewpoint primarily addresses the various direct molecular actions of flecainide on isolated RyR2 channels in artificial lipid bilayers. Such studies demonstrate different, multifarious, flecainide binding sites on RyR2, with voltage-dependent binding in the channel pore or voltage-independent binding at distant peripheral sites. In contrast to its single NaV1.5 pore binding site, flecainide may bind to at least four separate inhibitory sites on RyR2 and one activation site. None of these binding sites have been specifically located in the linear RyR2 sequence or high-resolution structure. Furthermore, it is not clear which of the inhibitory sites contribute to flecainide's reduction of spontaneous Ca2+ release in cellular studies. A confounding observation is that flecainide binding to voltage-dependent inhibition sites reduces cation fluxes in a direction opposite to physiological Ca2+ flow from SR lumen to cytosol. This may suggest that, rather than directly blocking Ca2+ efflux, flecainide can reduce Ca2+ efflux by blocking counter currents through the pore which otherwise limit SR membrane potential change during systolic Ca2+ efflux. In summary, the antiarrhythmic effects of flecainide in CPVT seem to involve multiple components of EC coupling and multiple actions on RyR2. Their clarification may identify novel specific drug targets and facilitate flecainide's clinical utilization in CPVT.

Gating of RYR2 channels from the arrhythmic RYR2-P2328S mouse heart and some unexpected actions of flecainide

The P2328S mutation in mice is associated with arrhythmia and spontaneous diastolic calcium release in atrial and ventricular myocytes and there is a corresponding leftward shift in the Ca2+-activation curve for mutant RYR2 channels from homozygous mouse hearts (Salvage et al. 2019. J Cell Sci. https://doi.org/10.1242/jcs.229039). P2328 is located in helical domain 1 (HD1) of RYR2. Local structural changes likely result when structurally active proline residues are replaced by structurally inert serine residues. We speculate that local structural changes in HD1 lead to sequential intradomain and interdomain stearic changes through the protein to the distant channel gate, which favor the open pore conformation. The drug flecainide prevents arrhythmia in humans and mouse models of CPVT by blocking NaV1.5 and RYR2 channels. Conventionally, flecainide blocks RYR2 channels in a voltage-dependent manner. We did not observe voltage-dependent pore block. This was possibly because, in contrast to previous studies, the only channel modulators that we used to produce end-diastolic control channel activity were 1 µM cytoplasmic Ca2+ and 1 mM luminal Ca2+. We observed previously unreported, voltage-independent increases in WT and P2328S channel activity at low flecainide concentrations, followed by a decline in activity at higher concentrations. The increase in activity dominated the effect of flecainide on P2328S channels. These effects suggested high-affinity flecainide binding to an activation site and lower-affinity binding to an inhibition site, both distant from the channel pore (Salvage et al. 2021. Cells. https://doi.org/10.3390/cells10082101). Unlike channel block by flecainide, the drug under our conditions stabilized intrinsic sub-conductance activity at +40 mV and -40 mV. Since flecainide effectively reduces CPVT arrythmia clinically and in animal models, we conclude that voltage-independent inhibition and voltage-dependent channel block prevail under cellular conditions. However, channel activation is important to note as it may be unmasked in other circumstances such as acquired cardiac disorders, mutations, or additional drug applications.

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

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

Molecular Changes in the Cardiac RyR2 With Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT)

The cardiac ryanodine receptor Ca²⁺ release channel (RyR2) is inserted into the membrane of intracellular sarcoplasmic reticulum (SR) myocyte Ca²⁺ stores, where it releases the Ca²⁺ essential for contraction. Mutations in proteins involved in Ca²⁺ signaling can lead to catecholaminergic polymorphic ventricular tachycardia (CPVT). The most common cellular phenotype in CPVT is higher than normal cytoplasmic Ca²⁺ concentrations during diastole due to Ca²⁺ leak from the SR through mutant RyR2. Arrhythmias are triggered when the surface membrane sodium calcium exchanger (NCX) lowers cytoplasmic Ca²⁺ by importing 3 Na⁺ ions to extrude one Ca²⁺ ion. The Na⁺ influx leads to delayed after depolarizations (DADs) which trigger arrhythmia when reaching action potential threshold. Present therapies use drugs developed for different purposes that serendipitously reduce RyR2 Ca²⁺ leak, but can adversely effect systolic Ca²⁺ release and other target processes. Ideal drugs would specifically reverse the effect of individual mutations, without altering normal channel function. Such drugs will depend on the location of the mutation in the 4967-residue monomer and the effect of the mutation on local structure, and downstream effects on structures along the conformational pathway to the pore. Such atomic resolution information is only now becoming available. This perspective provides a summary of known or predicted structural changes associated with a handful of CPVT mutations. Known molecular changes associated with RyR opening are discussed, as well one study where minute molecular changes with a particular mutation have been tracked from the N-terminal mutation site to gating residues in the channel pore.

Flecainide Paradoxically Activates Cardiac Ryanodine Receptor Channels under Low Activity Conditions: A Potential Pro-Arrhythmic Action

Cardiac ryanodine receptor (RyR2) mutations are implicated in the potentially fatal catecholaminergic polymorphic ventricular tachycardia (CPVT) and in atrial fibrillation. CPVT has been successfully treated with flecainide monotherapy, with occasional notable exceptions. Reported actions of flecainide on cardiac sodium currents from mice carrying the pro-arrhythmic homozygotic RyR2-P2328S mutation prompted our explorations of the effects of flecainide on their RyR2 channels. Lipid bilayer electrophysiology techniques demonstrated a novel, paradoxical increase in RyR2 activity. Preceding flecainide exposure, channels were mildly activated by 1 mM luminal Ca²⁺ and 1 µM cytoplasmic Ca²⁺, with open probabilities (P_o) of 0.03 ± 0.01 (wild type, WT) or 0.096 ± 0.024 (P2328S). Open probability (P_o) increased within 0.5 to 3 min of exposure to 0.5 to 5.0 µM cytoplasmic flecainide, then declined with higher concentrations of flecainide. There were no such increases in a subset of high P_o channels with P_o ≥ 0.08, although P_o then declined with ≥5 µM (WT) or ≥50 µM flecainide (P2328S). On average, channels with P_o < 0.08 were significantly activated by 0.5 to 10 µM of flecainide (WT) or 0.5 to 50 µM of flecainide (P2328S). These results suggest that flecainide can bind to separate activation and inhibition sites on RyR2, with activation dominating in lower activity channels and inhibition dominating in more active channels.

Peptide mimetic compounds can activate or inhibit cardiac and skeletal ryanodine receptors

AIMS

Our aim was to characterise the actions of novel BIT compounds with structures based on peptides and toxins that bind to significant regulatory sites on ryanodine receptor (RyR) Ca²⁺ release channels. RyRs, located in sarcoplasmic reticulum (SR) Ca²⁺ store membranes of striated muscle, are essential for muscle contraction. Although severe sometimes-deadly myopathies occur when the channels become hyperactive following genetic or acquired changes, specific inhibitors of RyRs are rare.

MAIN METHODS

The effect of BIT compounds was determined by spectrophotometric analysis of Ca²⁺ release from isolated SR vesicles, analysis of single RyR channel activity in artificial lipid bilayers and contraction of intact and skinned skeletal muscle fibres.

KEY FINDINGS

The inhibitory compounds reduced: (a) Ca²⁺ release from SR vesicles with IC50s of 1.1-2.5 μM, competing with activation by parent peptides and toxins; (b) single RyR ion channel activity with IC50s of 0.5-1.5 μM; (c) skinned fibre contraction. In contrast, activating BIT compounds increased Ca²⁺ release with an IC50 of 5.0 μM and channel activity with AC50s of 2 to 12 nM and enhanced skinned fibre contraction. Sub-conductance activity dominated channel activity with both inhibitors and activators. Effects of all compounds on skeletal and cardiac RyRs were similar and reversible. Competition experiments suggest that the BIT compounds bind to the regulatory helical domains of the RyRs that impact on channel gating mechanisms through long-range allosteric interactions.

SIGNIFICANCE

The BIT compounds are strong modulators of RyR activity and provide structural templates for novel research tools and drugs to combat muscle disease.

Ion channel gating in cardiac ryanodine receptors from the arrhythmic RyR2-P2328S mouse

Mutations in the cardiac ryanodine receptor Ca²⁺ release channel (RyR2) can cause deadly ventricular arrhythmias and atrial fibrillation (AF). The RyR2-P2328S mutation produces catecholaminergic polymorphic ventricular tachycardia (CPVT) and AF in hearts from homozygous RyR2^{P2328S/P2328S} (denoted RyR2^S/S) mice. We have now examined P2328S RyR2 channels from RyR2^S/S hearts. The activity of wild-type (WT) and P2328S RyR2 channels was similar at a cytoplasmic [Ca²⁺] of 1 mM, but P2328S RyR2 was significantly more active than WT at a cytoplasmic [Ca²⁺] of 1 µM. This was associated with a >10-fold shift in the half maximal activation concentration (AC₅₀) for Ca²⁺ activation, from ∼3.5 µM Ca²⁺ in WT RyR2 to ∼320 nM in P2328S channels and an unexpected >1000-fold shift in the half maximal inhibitory concentration (IC₅₀) for inactivation from ∼50 mM in WT channels to ≤7 μM in P2328S channels, which is into systolic [Ca²⁺] levels. Unexpectedly, the shift in Ca²⁺ activation was not associated with changes in sub-conductance activity, S2806 or S2814 phosphorylation or the level of FKBP12 (also known as FKBP1A) bound to the channels. The changes in channel activity seen with the P2328S mutation correlate with altered Ca²⁺ homeostasis in myocytes from RyR2^S/S mice and the CPVT and AF phenotypes.This article has an associated First Person interview with the first author of the paper.

Activation of RyR2 by class I kinase inhibitors

BACKGROUND AND PURPOSE

Kinase inhibitors are a common treatment for cancer. Class I kinase inhibitors that target the ATP-binding pocket are particularly prevalent. Many of these compounds are cardiotoxic and can cause arrhythmias. Spontaneous release of Ca²⁺ via cardiac ryanodine receptors (RyR2), through a process termed store overload-induced Ca²⁺ release (SOICR), is a common mechanism underlying arrhythmia. We explored whether class I kinase inhibitors could modify the activity of RyR2 and trigger SOICR to determine if this contributes to the cardiotoxic nature of these compounds.

EXPERIMENTAL APPROACH

The impact of class I and II kinase inhibitors on SOICR was studied in HEK293 cells and ventricular myocytes using single-cell Ca²⁺ imaging. A specific effect on RyR2 was confirmed using single channel recordings. Ventricular myocytes were also used to determine if drug-induced changes in SOICR could be reversed using anti-SOICR agents.

KEY RESULTS

Class I kinase inhibitors increased the propensity of SOICR. Single channel recording showed that this was due to a specific effect on RyR2. Class II kinase inhibitors decreased the activity of RyR2 at the single channel level but had little effect on SOICR. The promotion of SOICR mediated by class I kinase inhibitors could be reversed using the anti-SOICR agent VK-II-86.

CONCLUSIONS AND IMPLICATIONS

Part of the cardiotoxicity of class I kinase inhibitors can be assigned to their effect on RyR2 and increase in SOICR. Compounds with anti-SOICR activity may represent an improved treatment option for patients.

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

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

Ryanodine receptor Ca2+ release channel post-translational modification: Central player in cardiac and skeletal muscle disease

Calcium release from internal stores is a quintessential event in excitation-contraction coupling in cardiac and skeletal muscle. The ryanodine receptor Ca²⁺ release channel is embedded in the internal sarcoplasmic reticulum Ca²⁺ store, which releases Ca²⁺ into the cytoplasm, enabling contraction. Ryanodine receptors form the hub of a macromolecular complex extending from the extracellular space to the sarcoplasmic reticulum lumen. Ryanodine receptor activity is influenced by the integrated effects of associated co-proteins, ions, and post-translational phosphor and redox modifications. In healthy muscle, ryanodine receptors are phosphorylated and redox modified to basal levels, to support cellular function. A pathological increase in the degree of both post-translational modifications disturbs intracellular Ca²⁺ signalling, and is implicated in various cardiac and skeletal disorders. This review summarises our current understanding of the mechanisms linking ryanodine receptor post-translational modification to heart failure and skeletal myopathy and highlights the challenges and controversies within the field.

Multiple targets for flecainide action: implications for cardiac arrhythmogenesis

Flecainide suppresses cardiac tachyarrhythmias including paroxysmal atrial fibrillation, supraventricular tachycardia and arrhythmic long QT syndromes (LQTS), as well as the Ca2+ -mediated, catecholaminergic polymorphic ventricular tachycardia (CPVT). However, flecainide can also exert pro-arrhythmic effects most notably following myocardial infarction and when used to diagnose Brugada syndrome (BrS). These divergent actions result from its physiological and pharmacological actions at multiple, interacting levels of cellular organization. These were studied in murine genetic models with modified Nav channel or intracellular ryanodine receptor (RyR2)-Ca2+ channel function. Flecainide accesses its transmembrane Nav 1.5 channel binding site during activated, open, states producing a use-dependent antagonism. Closing either activation or inactivation gates traps flecainide within the pore. An early peak INa related to activation of Nav channels followed by rapid de-activation, drives action potential (AP) upstrokes and their propagation. This is diminished in pro-arrhythmic conditions reflecting loss of function of Nav 1.5 channels, such as BrS, accordingly exacerbated by flecainide challenge. Contrastingly, pro-arrhythmic effects attributed to prolonged AP recovery by abnormal late INaL following gain-of-function modifications of Nav 1.5 channels in LQTS3 are reduced by flecainide. Anti-arrhythmic effects of flecainide that reduce triggering in CPVT models mediated by sarcoplasmic reticular Ca2+ release could arise from its primary actions on Nav channels indirectly decreasing [Ca2+ ]i through a reduced [Na+ ]i and/or direct open-state RyR2-Ca2+ channel antagonism. The consequent [Ca2+ ]i alterations could also modify AP propagation velocity and therefore arrhythmic substrate through its actions on Nav 1.5 channel function. This is consistent with the paradoxical differences between flecainide actions upon Na+ currents, AP conduction and arrhythmogenesis under circumstances of normal and increased RyR2 function.

LINKED ARTICLES

This article is part of a themed section on Spotlight on Small Molecules in Cardiovascular Diseases. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.8/issuetoc.

Functional and structural characterization of a novel malignant hyperthermia-susceptible variant of DHPR-β1a subunit (CACNB1)

Malignant hyperthermia (MH) susceptibility has been recently linked to a novel variant of β_1a subunit of the dihydropyridine receptor (DHPR), a channel essential for Ca²⁺ regulation in skeletal muscle. Here we evaluate the effect of the mutant variant V156A on the structure/function of DHPR β_1a subunit and assess its role on Ca²⁺ metabolism of cultured myotubes. Using differential scanning fluorimetry, we show that mutation V156A causes a significant reduction in thermal stability of the Src homology 3/guanylate kinase core domain of β_1a subunit. Expression of the variant subunit in β₁-null mouse myotubes resulted in increased sensitivity to caffeine stimulation. Whole cell patch-clamp analysis of β_1a-V156A-expressing myotubes revealed a -2 mV shift in voltage dependence of channel activation, but no changes in Ca²⁺ conductance, current kinetics, or sarcoplasmic reticulum Ca²⁺ load were observed. Measurement of resting free Ca²⁺ and Na⁺ concentrations shows that both cations were significantly elevated in β_1a-V156A-expressing myotubes and that these changes were linked to increased rates of plasmalemmal Ca²⁺ entry through Na⁺/Ca²⁺ exchanger and/or transient receptor potential canonical channels. Overall, our data show that mutant variant V156A results in instability of protein subdomains of β_1a subunit leading to a phenotype of Ca²⁺ dysregulation that partly resembles that of other MH-linked mutations of DHPR α_1S subunit. These data prove that homozygous expression of variant β_1a-V156A has the potential to be a pathological variant, although it may require other gene defects to cause a full MH phenotype.

Core skeletal muscle ryanodine receptor calcium release complex

The core skeletal muscle ryanodine receptor (RyR1) calcium release complex extends through three compartments of the muscle fibre, linking the extracellular environment through the cytoplasmic junctional gap to the lumen of the internal sarcoplasmic reticulum (SR) calcium store. The protein complex is essential for skeletal excitation-contraction (EC)-coupling and skeletal muscle function. Its importance is highlighted by perinatal death if any one of the EC-coupling components are missing and by myopathies associated with mutation of any of the proteins. The proteins essential for EC-coupling include the DHPR α_1S subunit in the transverse tubule membrane, the DHPR β_1a subunit in the cytosol and the RyR1 ion channel in the SR membrane. The other core proteins are triadin and junctin and calsequestrin, associated mainly with SR. These SR proteins are not essential for survival but exert structural and functional influences that modify the gain of EC-coupling and maintain normal muscle function. This review summarises our current knowledge of the individual protein/protein interactions within the core complex and their overall contribution to EC-coupling. We highlight significant areas that provide a continuing challenge for the field. Additional important components of the Ca²⁺ release complex, such as FKBP12, calmodulin, S100A1 and Stac3 are identified and reviewed elsewhere.

Association of FK506 binding proteins with RyR channels - effect of CLIC2 binding on sub-conductance opening and FKBP binding

Ryanodine receptor (RyR) Ca²⁺ channels are central to striated muscle function and influence signalling in neurons and other cell types. Beneficially low RyR activity and maximum conductance opening may be stabilised when RyRs bind to FK506 binding proteins (FKBPs) and destabilised by FKBP dissociation, with submaximal opening during RyR hyperactivity associated with myopathies and neurological disorders. However, the correlation with submaximal opening is debated and quantitative evidence is lacking. Here, we have measured altered FKBP binding to RyRs and submaximal activity with addition of wild-type (WT) CLIC2, an inhibitory RyR ligand, or its H101Q mutant that hyperactivates RyRs, which probably causes cardiac and intellectual abnormalities. The proportion of sub-conductance opening increases with WT and H101Q CLIC2 and is correlated with reduced FKBP-RyR association. The sub-conductance opening reduces RyR currents in the presence of WT CLIC2. In contrast, sub-conductance openings contribute to excess RyR 'leak' with H101Q CLIC2. There are significant FKBP and RyR isoform-specific actions of CLIC2, rapamycin and FK506 on FKBP-RyR association. The results show that FKBPs do influence RyR gating and would contribute to excess Ca²⁺ release in this CLIC2 RyR channelopathy.

Structural and biophysical analyses of the skeletal dihydropyridine receptor β subunit β1a reveal critical roles of domain interactions for stability

Excitation-contraction (EC) coupling in skeletal muscle requires a physical interaction between the voltage-gated calcium channel dihydropyridine receptor (DHPR) and the ryanodine receptor Ca²⁺ release channel. Although the exact molecular mechanism that initiates skeletal EC coupling is unresolved, it is clear that both the α₁ and β subunits of DHPR are essential for this process. Here, we employed a series of techniques, including size-exclusion chromatography-multi-angle light scattering, differential scanning fluorimetry, and isothermal calorimetry, to characterize various biophysical properties of the skeletal DHPR β subunit β_1a Removal of the intrinsically disordered N and C termini and the hook region of β_1a prevented oligomerization, allowing for its structural determination by X-ray crystallography. The structure had a topology similar to that of previously determined β isoforms, which consist of SH3 and guanylate kinase domains. However, transition melting temperatures derived from the differential scanning fluorimetry experiments indicated a significant difference in stability of ∼2-3 °C between the β_1a and β_2a constructs, and the addition of the DHPR α_1s I-II loop (α-interaction domain) peptide stabilized both β isoforms by ∼6-8 °C. Similar to other β isoforms, β_1a bound with nanomolar affinity to the α-interaction domain, but binding affinities were influenced by amino acid substitutions in the adjacent SH3 domain. These results suggest that intramolecular interactions between the SH3 and guanylate kinase domains play a role in the stability of β_1a while also providing a conduit for allosteric signaling events.

Ryanodine receptor modification and regulation by intracellular Ca2+ and Mg2+ in healthy and failing human hearts

RATIONALE

Heart failure is a multimodal disorder, of which disrupted Ca²⁺ homeostasis is a hallmark. Central to Ca²⁺ homeostasis is the major cardiac Ca²⁺ release channel - the ryanodine receptor (RyR2) - whose activity is influenced by associated proteins, covalent modification and by Ca²⁺ and Mg²⁺. That RyR2 is remodelled and its function disturbed in heart failure is well recognized, but poorly understood.

OBJECTIVE

To assess Ca²⁺ and Mg²⁺ regulation of RyR2 from left ventricles of healthy, cystic fibrosis and failing hearts, and to correlate these functional changes with RyR2 modifications and remodelling.

METHODS AND RESULTS

The function of RyR2 from left ventricular samples was assessed using lipid bilayer single-channel measurements, whilst RyR2 modification and protein:protein interactions were determined using Western Blots and co-immunoprecipitation. In all failing hearts there was an increase in RyR2 activity at end-diastolic cytoplasmic Ca²⁺ (100nM), a decreased cytoplasmic [Ca²⁺] required for half maximal activation (K_a) and a decrease in inhibition by cytoplasmic Mg²⁺. This was accompanied by significant hyperphosphorylation of RyR2 S²⁸⁰⁸ and S²⁸¹⁴, reduced free thiol content and a reduced interaction with FKBP12.0 and FKBP12.6. Either dephosphorylation of RyR2 using PP1 or thiol reduction using DTT eliminated any significant difference in the activity of RyR2 from healthy and failing hearts. We also report a subgroup of RyR2 in failing hearts that were not responsive to regulation by intracellular Ca²⁺ or Mg²⁺.

CONCLUSION

Despite different aetiologies, disrupted RyR2 Ca²⁺ sensitivity and biochemical modification of the channel are common constituents of failing heart RyR2 and may underlie the pathological disturbances in intracellular Ca²⁺ signalling.