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'.

Short-Term Very High Carbohydrate Diet and Gut-Training Have Minor Effects on Gastrointestinal Status and Performance in Highly Trained Endurance Athletes

We implemented a multi-pronged strategy (MAX) involving chronic (2 weeks high carbohydrate [CHO] diet + gut-training) and acute (CHO loading + 90 g·h−1 CHO during exercise) strategies to promote endogenous and exogenous CHO availability, compared with strategies reflecting lower ranges of current guidelines (CON) in two groups of athletes. Nineteen elite male race walkers (MAX: 9; CON:10) undertook a 26 km race-walking session before and after the respective interventions to investigate gastrointestinal function (absorption capacity), integrity (epithelial injury), and symptoms (GIS). We observed considerable individual variability in responses, resulting in a statistically significant (p < 0.001) yet likely clinically insignificant increase (Δ 736 pg·mL−1) in I-FABP after exercise across all trials, with no significant differences in breath H2 across exercise (p = 0.970). MAX was associated with increased GIS in the second half of the exercise, especially in upper GIS (p < 0.01). Eighteen highly trained male and female distance runners (MAX: 10; CON: 8) then completed a 35 km run (28 km steady-state + 7 km time-trial) supported by either a slightly modified MAX or CON strategy. Inter-individual variability was observed, without major differences in epithelial cell intestinal fatty acid binding protein (I-FABP) or GIS, due to exercise, trial, or group, despite the 3-fold increase in exercise CHO intake in MAX post-intervention. The tight-junction (claudin-3) response decreased in both groups from pre- to post-intervention. Groups achieved a similar performance improvement from pre- to post-intervention (CON = 39 s [95 CI 15−63 s]; MAX = 36 s [13−59 s]; p = 0.002). Although this suggests that further increases in CHO availability above current guidelines do not confer additional advantages, limitations in our study execution (e.g., confounding loss of BM in several individuals despite a live-in training camp environment and significant increases in aerobic capacity due to intensified training) may have masked small differences. Therefore, athletes should meet the minimum CHO guidelines for training and competition goals, noting that, with practice, increased CHO intake can be tolerated, and may contribute to performance outcomes.

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.

Calmodulin inhibition of human RyR2 channels requires phosphorylation of RyR2-S2808 or RyR2-S2814

Calmodulin (CaM) is a Ca-binding protein that binds to, and can directly inhibit cardiac ryanodine receptor calcium release channels (RyR2). Animal studies have shown that RyR2 hyperphosphorylation reduces CaM binding to RyR2 in failing hearts, but data are lacking on how CaM regulates human RyR2 and how this regulation is affected by RyR2 phosphorylation. Physiological concentrations of CaM (100 nM) inhibited the diastolic activity of RyR2 isolated from failing human hearts by ~50% but had no effect on RyR2 from healthy human hearts. Using FRET between donor-FKBP12.6 and acceptor-CaM bound to RyR2, we determined that CaM binds to RyR2 from healthy human heart with a K_d = 121 ± 14 nM. Ex-vivo phosphorylation/dephosphorylation experiments suggested that the divergent CaM regulation of healthy and failing human RyR2 was caused by differences in RyR2 phosphorylation by protein kinase A and Ca-CaM-dependent kinase II. Ca²⁺-spark measurements in murine cardiomyocytes harbouring RyR2 phosphomimetic or phosphoablated mutants at S2814 and S2808 suggest that phosphorylation of residues corresponding to either human RyR2-S2808 or S2814 is both necessary and sufficient for RyR2 regulation by CaM. Our results challenge the current concept that CaM universally functions as a canonical inhibitor of RyR2 across species. Rather, CaM's biological action on human RyR2 appears to be more nuanced, with inhibitory activity only on phosphorylated RyR2 channels, which occurs during exercise or in patients with heart failure.

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.

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.

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.

Regions of ryanodine receptors that influence activation by the dihydropyridine receptor β1a subunit

Background

Although excitation-contraction (EC) coupling in skeletal muscle relies on physical activation of the skeletal ryanodine receptor (RyR1) Ca²⁺ release channel by dihydropyridine receptors (DHPRs), the activation pathway between the DHPR and RyR1 remains unknown. However, the pathway includes the DHPR β_1a subunit which is integral to EC coupling and activates RyR1. In this manuscript, we explore the isoform specificity of β_1a activation of RyRs and the β_1a binding site on RyR1.

Methods

We used lipid bilayers to measure single channel currents and whole cell patch clamp to measure L-type Ca²⁺ currents and Ca²⁺ transients in myotubes.

Results

We demonstrate that both skeletal RyR1 and cardiac RyR2 channels in phospholipid bilayers are activated by 10–100 nM of the β_1a subunit. Activation of RyR2 by 10 nM β_1a was less than that of RyR1, suggesting a reduced affinity of RyR2 for β_1a. A reduction in activation was also observed when 10 nM β_1a was added to the alternatively spliced (ASI(−)) isoform of RyR1, which lacks ASI residues (A3481-Q3485). It is notable that the equivalent region of RyR2 also lacks four of five ASI residues, suggesting that the absence of these residues may contribute to the reduced 10 nM β_1a activation observed for both RyR2 and ASI(−)RyR1 compared to ASI(+)RyR1. We also investigated the influence of a polybasic motif (PBM) of RyR1 (K3495KKRRDGR3502) that is located immediately downstream from the ASI residues and has been implicated in EC coupling. We confirmed that neutralizing the basic residues in the PBM (RyR1 K-Q) results in an ~50 % reduction in Ca²⁺ transient amplitude following expression in RyR1-null (dyspedic) myotubes and that the PBM is also required for β_1a subunit activation of RyR1 channels in lipid bilayers. These results suggest that the removal of β_1a subunit interaction with the PBM in RyR1 could contribute directly to ~50 % of the Ca²⁺ release generated during skeletal EC coupling.

Conclusions

We conclude that the β_1a subunit likely binds to a region that is largely conserved in RyR1 and RyR2 and that this region is influenced by the presence of the ASI residues and the PBM in RyR1.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-015-0049-3) contains supplementary material, which is available to authorized users.

Differences in the regulation of RyR2 from human, sheep, and rat by Ca²⁺ and Mg²⁺ in the cytoplasm and in the lumen of the sarcoplasmic reticulum

Cardiac ryanodine receptors (RyR2) from humans, rats, and sheep show differential sensitivity to calcium and magnesium, with regulation of human RyR2 resembling that of sheep more than that of rat.

Regulation of the cardiac ryanodine receptor (RyR2) by intracellular Ca²⁺ and Mg²⁺ plays a key role in determining cardiac contraction and rhythmicity, but their role in regulating the human RyR2 remains poorly defined. The Ca²⁺- and Mg²⁺-dependent regulation of human RyR2 was recorded in artificial lipid bilayers in the presence of 2 mM ATP and compared with that in two commonly used animal models for RyR2 function (rat and sheep). Human RyR2 displayed cytoplasmic Ca²⁺ activation (K_a = 4 µM) and inhibition by cytoplasmic Mg²⁺ (K_i = 10 µM at 100 nM Ca²⁺) that was similar to RyR2 from rat and sheep obtained under the same experimental conditions. However, in the presence of 0.1 mM Ca²⁺, RyR2s from human were 3.5-fold less sensitive to cytoplasmic Mg²⁺ inhibition than those from sheep and rat. The K_a values for luminal Ca²⁺ activation were similar in the three species (35 µM for human, 12 µM for sheep, and 10 µM for rat). From the relationship between open probability and luminal [Ca²⁺], the peak open probability for the human RyR2 was approximately the same as that for sheep, and both were ∼10-fold greater than that for rat RyR2. Human RyR2 also showed the same sensitivity to luminal Mg²⁺ as that from sheep, whereas rat RyR2 was 10-fold more sensitive. In all species, modulation of RyR2 gating by luminal Ca²⁺ and Mg²⁺ only occurred when cytoplasmic [Ca²⁺] was <3 µM. The activation response of RyR2 to luminal and cytoplasmic Ca²⁺ was strongly dependent on the Mg²⁺ concentration. Addition of physiological levels (1 mM) of Mg²⁺ raised the K_a for cytoplasmic Ca²⁺ to 30 µM (human and sheep) or 90 µM (rat) and raised the K_a for luminal Ca²⁺ to ∼1 mM in all species. This is the first report of the regulation by Ca²⁺ and Mg²⁺ of native RyR2 receptor activity from healthy human hearts.

C-terminal residues of skeletal muscle calsequestrin are essential for calcium binding and for skeletal ryanodine receptor inhibition

Background

Skeletal muscle function depends on calcium signaling proteins in the sarcoplasmic reticulum (SR), including the calcium-binding protein calsequestrin (CSQ), the ryanodine receptor (RyR) calcium release channel, and skeletal triadin 95 kDa (trisk95) and junctin, proteins that bind to calsequestrin type 1 (CSQ1) and ryanodine receptor type 1 (RyR1). CSQ1 inhibits RyR1 and communicates store calcium load to RyR1 channels via trisk95 and/or junctin.

Methods

In this manuscript, we test predictions that CSQ1’s acidic C-terminus contains binding sites for trisk95 and junctin, the major calcium binding domain, and that it determines CSQ1’s ability to regulate RyR1 activity.

Results

Progressive alanine substitution of C-terminal acidic residues of CSQ1 caused a parallel reduction in the calcium binding capacity but did not significantly alter CSQ1’s association with trisk95/junctin or influence its inhibition of RyR1 activity. Deletion of the final seven residues in the C-terminus significantly hampered calcium binding, significantly reduced CSQ’s association with trisk95/junctin and decreased its inhibition of RyR1. Deletion of the full C-terminus further reduced calcium binding to CSQ1 altered its association with trisk95 and junctin and abolished its inhibition of RyR1.

Conclusions

The correlation between the number of residues mutated/deleted and binding of calcium, trisk95, and junctin suggests that binding of each depends on diffuse ionic interactions with several C-terminal residues and that these interactions may be required for CSQ1 to maintain normal muscle function.

A new cytoplasmic interaction between junctin and ryanodine receptor Ca2+ release channels

Junctin, a non-catalytic splice variant encoded by the aspartate-β-hydroxylase (Asph) gene, is inserted into the membrane of the sarcoplasmic reticulum (SR) Ca²⁺ store where it modifies Ca²⁺ signalling in the heart and skeletal muscle through its regulation of ryanodine receptor (RyR) Ca²⁺ release channels. Junctin is required for normal muscle function as its knockout leads to abnormal Ca²⁺ signalling, muscle dysfunction and cardiac arrhythmia. However, the nature of the molecular interaction between junctin and RyRs is largely unknown and was assumed to occur only in the SR lumen. We find that there is substantial binding of RyRs to full junctin, and the junctin luminal and, unexpectedly, cytoplasmic domains. Binding of these different junctin domains had distinct effects on RyR1 and RyR2 activity: full junctin in the luminal solution increased RyR channel activity by ∼threefold, the C-terminal luminal interaction inhibited RyR channel activity by ∼50%, and the N-terminal cytoplasmic binding produced an ∼fivefold increase in RyR activity. The cytoplasmic interaction between junctin and RyR is required for luminal binding to replicate the influence of full junctin on RyR1 and RyR2 activity. The C-terminal domain of junctin binds to residues including the S1–S2 linker of RyR1 and N-terminal domain of junctin binds between RyR1 residues 1078 and 2156.

Adverse effects of doxorubicin and its metabolic product on cardiac RyR2 and SERCA2A

The use of anthracycline chemotherapeutic drugs is restricted owing to potentially fatal cardiotoxic side effects. It has been hypothesized that anthracycline metabolites have a primary role in this cardiac dysfunction; however, information on the molecular interactions of these compounds in the heart is scarce. Here we provide novel evidence that doxorubicin and its metabolite, doxorubicinol, bind to the cardiac ryanodine receptor (RyR2) and to the sarco/endoplasmic reticulum Ca(2+) ATPase (SERCA2A) and deleteriously alter their activity. Both drugs (0.01 μM-2.5 μM) activated single RyR2 channels, and this was reversed by drug washout. Both drugs caused a secondary inhibition of RyR2 activity that was not reversed by drug washout. Preincubation with the reducing agent dithiothreitol (DTT, 1 mM) prevented drug-induced inhibition of channel activity. Doxorubicin and doxorubicinol reduced the abundance of thiol groups on RyR2, further indicating that oxidation reactions may be involved in the actions of the compounds. Ca(2+) uptake into sarcoplasmic reticulum vesicles by SERCA2A was inhibited by doxorubicinol, but not doxorubicin. Unexpectedly, in the presence of DTT, doxorubicinol enhanced the rate of Ca(2+) uptake by SERCA2A. Together the evidence provided here shows that doxorubicin and doxorubicinol interact with RyR2 and SERCA2A in similar ways, but that the metabolite acts with greater efficacy than the parent compound. Both compounds modify RyR2 and SERCA2A activity by binding to the proteins and also act via thiol oxidation to disrupt SR Ca(2+) handling. These actions would have severe consequences on cardiomyocyte function and contribute to clinical symptoms of acute anthracycline cardiotoxicity.

Cardiac ryanodine receptor activation by a high Ca²⁺ store load is reversed in a reducing cytoplasmic redox environment

Here, we report the impact of redox potential on isolated cardiac ryanodine receptor (RyR2) channel activity and its response to physiological changes in luminal [Ca²⁺]. Basal leak from the sarcoplasmic reticulum is required for normal Ca²⁺ handling, but excess diastolic Ca²⁺ leak attributed to oxidative stress is thought to lower the threshold of RyR2 for spontaneous sarcoplasmic reticulum Ca²⁺ release, thus inducing arrhythmia in pathological situations. Therefore, we examined the RyR2 response to luminal [Ca²⁺] under reducing or oxidising cytoplasmic redox conditions. Unexpectedly, as luminal [Ca²⁺] increased from 0.1 to 1.5 mM, RyR2 activity declined when pretreated with cytoplasmic 1 mM DTT or buffered with GSH∶GSSG to a normal reduced cytoplasmic redox potential (−220 mV). Conversely, with 20 µM cytoplasmic 4,4′-DTDP or buffering of the redox potential to an oxidising value (−180 mV), RyR2 activity increased with increasing luminal [Ca²⁺]. The luminal redox potential was constant at −180 mV in each case. These responses to luminal [Ca²⁺] were maintained with cytoplasmic 2 mM Na₂ATP or 5 mM MgATP (1 mM free Mg²⁺). Overall, the results suggest that the redox potential in the RyR2 junctional microdomain is normally more oxidised than that of the bulk cytoplasm.

ß-Adrenergic stimulation increases RyR2 activity via intracellular Ca2+ and Mg2+ regulation

Here we investigate how ß-adrenergic stimulation of the heart alters regulation of ryanodine receptors (RyRs) by intracellular Ca²⁺ and Mg²⁺ and the role of these changes in SR Ca²⁺ release. RyRs were isolated from rat hearts, perfused in a Langendorff apparatus for 5 min and subject to 1 min perfusion with 1 µM isoproterenol or without (control) and snap frozen in liquid N₂ to capture their phosphorylation state. Western Blots show that RyR2 phosphorylation was increased by isoproterenol, confirming that RyR2 were subject to normal ß-adrenergic signaling. Under basal conditions, S2808 and S2814 had phosphorylation levels of 69% and 15%, respectively. These levels were increased to 83% and 60%, respectively, after 60 s of ß-adrenergic stimulation consistent with other reports that ß-adrenergic stimulation of the heart can phosphorylate RyRs at specific residues including S2808 and S2814 causing an increase in RyR activity. At cytoplasmic [Ca²⁺] <1 µM, ß-adrenergic stimulation increased luminal Ca²⁺ activation of single RyR channels, decreased luminal Mg²⁺ inhibition and decreased inhibition of RyRs by mM cytoplasmic Mg²⁺. At cytoplasmic [Ca²⁺] >1 µM, ß-adrenergic stimulation only decreased cytoplasmic Mg²⁺ and Ca²⁺ inhibition of RyRs. The K_a and maximum levels of cytoplasmic Ca²⁺ activation site were not affected by ß-adrenergic stimulation.

Our RyR2 gating model was fitted to the single channel data. It predicted that in diastole, ß-adrenergic stimulation is mediated by 1) increasing the activating potency of Ca²⁺ binding to the luminal Ca²⁺ site and decreasing its affinity for luminal Mg²⁺ and 2) decreasing affinity of the low-affinity Ca²⁺/Mg²⁺ cytoplasmic inhibition site. However in systole, ß-adrenergic stimulation is mediated mainly by the latter.

Proteins within the intracellular calcium store determine cardiac RyR channel activity and cardiac output

SUMMARY

The contractile function of the heart requires the release of Ca(2+) from intracellular Ca(2+) stores in the sarcoplasmic reticulum (SR) of cardiac muscle cells. The efficacy of Ca(2+) release depends on the amount of Ca(2+) loaded into the Ca(2+) store and the way in which this 'Ca(2+) load' influences the activity of the cardiac ryanodine receptor Ca(2+) release channel (RyR2). The effects of the Ca(2+) load on Ca(2+) release through RyR2 are facilitated by: (i) the sensitivity of RyR2 itself to luminal Ca(2+) concentrations; and (ii) interactions between the cardiac Ca(2+) -binding protein calsequestrin (CSQ) 2 and RyR2, transmitted through the 'anchoring' proteins junctin and/or triadin. Mutations in RyR2 are linked to catecholaminergic polymorphic ventricular tachycardia (CPVT) and sudden cardiac death. The tachycardia is associated with changes in the sensitivity of RyR2 to luminal Ca(2+) . Triadin-, junctin- or CSQ-null animals survive, but their longevity and ability to tolerate stress is compromised. These studies reveal the importance of the proteins in normal muscle function, but do not reveal the molecular nature of their functional interactions, which must be defined before changes in the proteins leading to CPVT and heart disease can be understood. Herein, we discuss known interactions between the RyR, triadin, junctin and CSQ with emphasis on the cardiac isoforms of the proteins. Where there is little known about the cardiac isoforms, we discuss evidence from skeletal isoforms.

Effects of brief stress exposure during early postnatal development in Balb/CByJ mice: II. Altered cortical morphology

Early life experience can significantly determine later mental health status and cognitive function. Neonatal stress, in particular, has been linked to the etiology of mental health disorders as divergent as mood disorder, schizophrenia, and autism. Our study uses a Balb/CByJ mouse model to test the hypothesis, that neonatal stress will alter development and subsequent environmental modulation of neocortex. Using a split litter design, we generated stressed mice (STR) and within litter controls (LMC) along with age-matched, untreated animals (AMC), to serve as across litter controls. Short, daily exposure to a psychosocial/physical stressor, during the first week of life, resulted by adulthood in significant changes in neocortical thickness and architecture, which were further modulated by exposure to behavioral testing. Surprisingly, cortical size in LMC mice was also affected. These observations were compared to the effects of environmental enrichment in the same mouse strain. Our data indicate that LMC and STR males share with environmentally enriched males, an increase in thickness in infra-granular cortical layers, while STR also display a stress selective decrease in supragranular layers, in response to behavioral training as adults.

The ryanodine receptor: a pivotal Ca2+ regulatory protein and potential therapeutic drug target

The ryanodine receptor (RyR) calcium release channel is an essential intracellular ion channel that is central to Ca(2+) signaling and contraction in the heart and skeletal muscle. The rapid release of Ca(2+) from the internal sarcoplasmic reticulum Ca(2+) stores through the RyR during excitation-contraction coupling is facilitated by the unique arrangement of the surface and sarcoplasmic reticulum membrane systems. Debilitating and sometimes fatal skeletal and cardiomyopathies result from changes in RyR activity that disrupt normal Ca(2+) signaling. Such changes can be caused by point mutations in many different regions of the RyR protein or acquired as a result of stress associated with exercise, heart failure, age or drugs. In general, both inherited and acquired changes include an increase in RyR channel activity. Because of its central function, the RyR is a potential therapeutic target for the inherited disorders and many of the acquired disorders. The RyR is currently used as a therapeutic target in malignant hyperthermia where dantrolene is effective and to relieve ventricular arrhythmia, with the use of JTV519 and flecainide. These drugs show that the RyR is a valid therapeutic target, but have side effects that prevent their chronic use. Thus there is an urgent need for the development of skeletal and cardiac specific drugs to treat these diverse muscle disorders. In this review, we discuss the mutations that cause skeletal myopathies and cardiac arrhythmias and how these mutations pinpoint residues within the RyR protein that are functionally significant and might be developed as targets for therapeutic drugs.

The β(1a) subunit of the skeletal DHPR binds to skeletal RyR1 and activates the channel via its 35-residue C-terminal tail

Although it has been suggested that the C-terminal tail of the β(1a) subunit of the skeletal dihyropyridine receptor (DHPR) may contribute to voltage-activated Ca(2+) release in skeletal muscle by interacting with the skeletal ryanodine receptor (RyR1), a direct functional interaction between the two proteins has not been demonstrated previously. Such an interaction is reported here. A peptide with the sequence of the C-terminal 35 residues of β(1a) bound to RyR1 in affinity chromatography. The full-length β(1a) subunit and the C-terminal peptide increased [(3)H]ryanodine binding and RyR1 channel activity with an AC(50) of 450-600 pM under optimal conditions. The effect of the peptide was dependent on cytoplasmic Ca(2+), ATP, and Mg(2+) concentrations. There was no effect of the peptide when channel activity was very low as a result of Mg(2+) inhibition or addition of 100 nM Ca(2+) (without ATP). Maximum increases were seen with 1-10 μM Ca(2+), in the absence of Mg(2+) inhibition. A control peptide with the C-terminal 35 residues in a scrambled sequence did not bind to RyR1 or alter [(3)H]ryanodine binding or channel activity. This high-affinity in vitro functional interaction between the C-terminal 35 residues of the DHPR β(1a) subunit and RyR1 may support an in vivo function of β(1a) during voltage-activated Ca(2+) release.

Quantification of calsequestrin 2 (CSQ2) in sheep cardiac muscle and Ca2+-binding protein changes in CSQ2 knockout mice

Calsequestrin 2 (CSQ2) is generally regarded as the primary Ca2+-buffering molecule present inside the sarcoplasmic reticulum (SR) in cardiac cells, but findings from CSQ2 knockout experiments raise major questions about its role and necessity. This study determined the absolute amount of CSQ2 present in cardiac ventricular muscle to gauge its likely influence on SR free Ca2+ concentration ([Ca2+]) and maximal Ca2+ capacity. Ventricular tissue from hearts of freshly killed sheep was examined by SDS-PAGE without any fractionation, and CSQ2 was detected by Western blotting; this method avoided the >90% loss of CSQ2 occurring with usual fractionation procedures. Band intensities were compared against those for purified CSQ2 run on the same blots. Fidelity of quantification was verified by demonstrating that CSQ2 added to homogenates was detected with equal efficacy as purified CSQ2 alone. Ventricular tissue from sheep (n=8) contained 24±2 μmol CSQ2/kg wet wt. Total Ca2+ content of the ventricular tissue, measured by atomic absorption spectroscopy, was 430±20 μmol/kg (with SR Ca2+ likely<250 μmol/kg) and displayed a linear correlation with CSQ2 content, with gradient of ∼10 Ca2+ per CSQ2. The large amount of CSQ2 bestows the SR with a high theoretical maximal Ca2+-binding capacity (∼1 mmol Ca2+/kg ventricular tissue, assuming a maximum of ∼40 Ca2+ per CSQ2) and would keep free [Ca2+] within the SR relatively low, energetically favoring Ca2+ uptake and reducing SR leak. In mice with CSQ2 ablated, histidine-rich Ca2+-binding protein was upregulated ∼35% in ventricular tissue, possibly in compensation.

Ca(2+) signaling in striated muscle: the elusive roles of triadin, junctin, and calsequestrin

This review focuses on molecular interactions between calsequestrin, triadin, junctin and the ryanodine receptor in the lumen of the sarcoplasmic reticulum. These interactions modulate changes in Ca(2+) release in response to changes in the Ca(2+) load within the sarcoplasmic reticulum store in striated muscle and are of fundamental importance to Ca(2+) homeostasis, since massive adaptive changes occur when expression of the proteins is manipulated, while mutations in calsequestrin lead to functional changes which can be fatal. We find that calsequestrin plays a different role in the heart and skeletal muscle, enhancing Ca(2+) release in the heart, but depressing Ca(2+) release in skeletal muscle. We also find that triadin and junctin exert independent influences on the ryanodine receptor in skeletal muscle where triadin alone modifies excitation-contraction coupling, while junctin alone supports functional interactions between calsequestrin and the ryanodine receptor.

Unique isoform-specific properties of calsequestrin in the heart and skeletal muscle

Calcium signaling in myocytes is dependent on the cardiac ryanodine receptor (RyR2) calcium release channel and the calcium buffering protein in the sarcoplasmic reticulum, cardiac calsequestrin (CSQ2). The overall properties of CSQ2 and its regulation of RyR2 have not been explored in detail or directly compared with skeletal CSQ1 and its regulation of the skeletal RyR1, with physiological ionic strength and Ca(2+) concentrations. We find that there are major differences between the two isoforms under these physiological conditions. Ca(2+) binding to CSQ2 is 50% lower than to CSQ1. Only approximately 30% of CSQ2 is bound to cardiac junctional face membrane (JFM), compared with approximately 70% of CSQ1 and the ratio of CSQ2 to RyR2 is only 50% of the CSQ1/RyR1 ratio. Chemical crosslinking shows that CSQ2 is mostly monomer/dimer, while CSQ1 is mostly polymerized. In single channel lipid bilayer experiments, CSQ2 monomers and/or dimers increase the open probability of both RyR1 and RyR2 channels, while CSQ1 polymers decrease the activity of RyR1. We speculate that CSQ2 facilitates high rates of Ca(2+) release through RyR2 during systole, while CSQ1 curtails RyR1 opening in response to a single action potential to maintain Ca(2+) and allow repeated Ca(2+) release and graded activation with increased stimulation frequency.

Phosphorylation of skeletal muscle calsequestrin enhances its Ca2+ binding capacity and promotes its association with junctin

Calcium signaling, intrinsic to skeletal and cardiac muscle function, is critically dependent on the amount of calcium stored within the sarcoplasmic reticulum. Calsequestrin, the main calcium buffer in the sarcoplasmic reticulum, provides a pool of calcium for release through the ryanodine receptor and acts as a luminal calcium sensor for the channel via its interactions with triadin and junctin. We examined the influence of phosphorylation of calsequestrin on its ability to store calcium, to polymerise and to regulate ryanodine receptors by binding to triadin and junctin. Our hypothesis was that these parameters might be altered by phosphorylation of threonine 353, which is located near the calcium and triadin/junctin binding sites. Although phosphorylation increased the calcium binding capacity of calsequestrin nearly 2-fold, it did not alter calsequestrin polymerisation, its binding to triadin or junctin or inhibition of ryanodine receptor activity at 1 mM luminal calcium. Phosphorylation was required for calsequestrin binding to junctin when calcium concentration was low (100 nM), and ryanodine receptors were activated by dephosphorylated calsequestrin when it bound to triadin alone. These novel data shows that phosphorylated calsequestrin is required for high capacity calcium buffering and suggest that ryanodine receptor inhibition by calsequestrin is mediated by junctin.

Calsequestrin content and SERCA determine normal and maximal Ca2+ storage levels in sarcoplasmic reticulum of fast- and slow-twitch fibres of rat

Whilst calsequestrin (CSQ) is widely recognized as the primary Ca2+ buffer in the sarcoplasmic reticulum (SR) in skeletal muscle fibres, its total buffering capacity and importance have come into question. This study quantified the absolute amount of CSQ isoform 1 (CSQ1, the primary isoform) present in rat extensor digitorum longus (EDL) and soleus fibres, and related this to their endogenous and maximal SR Ca2+ content. Using Western blotting, the entire constituents of minute samples of muscle homogenates or segments of individual muscle fibres were compared with known amounts of purified CSQ1. The fidelity of the analysis was proven by examining the relative signal intensity when mixing muscle samples and purified CSQ1. The CSQ1 contents of EDL fibres, almost exclusively type II fibres, and soleus type I fibres [SOL (I)] were, respectively, 36 +/- 2 and 10 +/- 1 micromol (l fibre volume)(-1), quantitatively accounting for the maximal SR Ca2+ content of each. Soleus type II [SOL (II)] fibres (approximately 20% of soleus fibres) had an intermediate amount of CSQ1. Every SOL (I) fibre examined also contained some CSQ isoform 2 (CSQ2), which was absent in every EDL and other type II fibre except for trace amounts in one case. Every EDL and other type II fibre had a high density of SERCA1, the fast-twitch muscle sarco(endo)plasmic reticulum Ca2+-ATPase isoform, whereas there was virtually no SERCA1 in any SOL (I) fibre. Maximal SR Ca2+ content measured in skinned fibres increased with CSQ1 content, and the ratio of endogenous to maximal Ca2+ content was inversely correlated with CSQ1 content. The relative SR Ca2+ content that could be maintained in resting cytoplasmic conditions was found to be much lower in EDL fibres than in SOL (I) fibres (approximately 20 versus >60%). Leakage of Ca2+ from the SR in EDL fibres could be substantially reduced with a SR Ca2+ pump blocker and increased by adding creatine to buffer cytoplasmic [ADP] at a higher level, both results indicating that at least part of the Ca2+ leakage occurred through SERCA. It is concluded that CSQ1 plays an important role in EDL muscle fibres by providing a large total pool of releasable Ca2+ in the SR whilst maintaining free [Ca2+] in the SR at sufficiently low levels that Ca2+ leakage through the high density of SERCA1 pumps does not metabolically compromise muscle function.

Muscle-specific GSTM2-2 on the luminal side of the sarcoplasmic reticulum modifies RyR ion channel activity

We show that a glutathione transferase (GST) protein, which is recognised by an antibody against the muscle-specific human GSTM2-2 (hGSTM2-2), is associated with the lumen of the sarcoplasmic reticulum (SR) of cardiac muscle, but not skeletal muscle. We further show that hGSTM2-2 modifies both cardiac and skeletal ryanodine receptor (RyR) activity when it binds to the luminal domain of the RyR channel complex. The properties of hGSTM2-2 were compared with those of the calsequestrin (CSQ), a Ca(2+) binding protein also present in the lumen of the SR which, like GSTM2-2, contains a thioredoxin-fold structure and modifies RyR activity (Wei, L., Varsanyi, M., Dulhunty, A. F., Beard, N. A. (2006). The Biophysical Journal, 91, 1288-1301). The glutathione transferase activity of hGSTM2-2 is strong, while CSQ is essentially inactive. Conversely CSQ is a strong Ca(2+) binder, but hGSTM2-2 is not. The effects of luminal hGSTM2-2 on RyR activity differ from those of CSQ in that hGSTM2-2 activates RyRs by increasing their open probability and conductance and the effects are independent of luminal Ca(2+) concentration. The results suggest that GSTM2-2 can interact with specific luminal sites on the RyR complex and that the interaction is likely to be within the pore of the RyR channel. The differences between the effects of CSQ and hGSTM2-2 suggest that the thioredoxin fold is not a major determinant of the luminal actions of either protein. The results indicate that GSTM2-2 is a novel luminal regulator of the RyR channels in the heart.

Triadin binding to the C-terminal luminal loop of the ryanodine receptor is important for skeletal muscle excitation contraction coupling

Ca²⁺ release from intracellular stores is controlled by complex interactions between multiple proteins. Triadin is a transmembrane glycoprotein of the junctional sarcoplasmic reticulum of striated muscle that interacts with both calsequestrin and the type 1 ryanodine receptor (RyR1) to communicate changes in luminal Ca²⁺ to the release machinery. However, the potential impact of the triadin association with RyR1 in skeletal muscle excitation–contraction coupling remains elusive. Here we show that triadin binding to RyR1 is critically important for rapid Ca²⁺ release during excitation–contraction coupling. To assess the functional impact of the triadin-RyR1 interaction, we expressed RyR1 mutants in which one or more of three negatively charged residues (D4878, D4907, and E4908) in the terminal RyR1 intraluminal loop were mutated to alanines in RyR1-null (dyspedic) myotubes. Coimmunoprecipitation revealed that triadin, but not junctin, binding to RyR1 was abolished in the triple (D4878A/D4907A/E4908A) mutant and one of the double (D4907A/E4908A) mutants, partially reduced in the D4878A/D4907A double mutant, but not affected by either individual (D4878A, D4907A, E4908A) mutations or the D4878A/E4908A double mutation. Functional studies revealed that the rate of voltage- and ligand-gated SR Ca²⁺ release were reduced in proportion to the degree of interruption in triadin binding. Ryanodine binding, single channel recording, and calcium release experiments conducted on WT and triple mutant channels in the absence of triadin demonstrated that the luminal loop mutations do not directly alter RyR1 function. These findings demonstrate that junctin and triadin bind to different sites on RyR1 and that triadin plays an important role in ensuring rapid Ca²⁺ release during excitation–contraction coupling in skeletal muscle.

Agonists and antagonists of the cardiac ryanodine receptor: Potential therapeutic agents?

This review addresses the potential use of the intracellular ryanodine receptor (RyR) Ca(2+) release channel as a therapeutic target in heart disease. Heart disease encompasses a wide range of conditions with the major contributors to mortality and morbidity being ischaemic heart disease and heart failure (HF). In addition there are many rare, but devastating conditions, some of which are either genetically linked to the RyR and its regulatory proteins or involve drug-induced modification of the proteins. The defects in Ca(2+) signalling vary with the nature of the heart disease and the stage in its progress and therefore specific corrections require different modifications of Ca(2+) signalling. Compounds that activate the RyR are potential inotropic agents to increase the Ca(2+) transient and strength of contraction. Compounds that reduce RyR activity are potentially useful in conditions where excess RyR activity initiates arrhythmias, or depletes the Ca(2+) store, as in end stage HF. It has recently been discovered that the cardio-protective action of the drug JTV519 can be attributed partly to its ability to stabilise the interaction between the RyR and the 12.6 kDa binding protein for the commonly used immunosuppressive drug FK506 (FKBP12.6, known as tacrolimus). This has established the credibility of the RyR as a therapeutic target. We explore the possibility that mutations causing the rare RyR-linked arrhythmias will open the door to identification of novel RyR-based therapeutic agents. The use of regulatory binding sites within the RyR complex or on its associated proteins as templates for drug design is discussed.

A variably spliced region in the type 1 ryanodine receptor may participate in an inter-domain interaction

The aim of the present study was to examine residues that are variably spliced in the juvenile and adult isoforms of the skeletal-muscle RyR1 (type 1 ryanodine receptor). The juvenile ASI(-) splice variant is less active than the adult ASI(+) variant and is overexpressed in patients with DM (myotonic dystrophy) [Kimura, Nakamori, Lueck, Pouliquin, Aoike, Fujimura, Dirksen, Takahashi, Dulhunty and Sakoda (2005) Hum. Mol. Genet. 14, 2189-2200]. In the present study, we explore the ASI region using synthetic peptides corresponding to rabbit RyR1 residues Thr3471-Gly3500 either containing [PASI(+)] or lacking [PASI(-)] the ASI residues. Both peptides increased [3H]ryanodine binding to rabbit RyR1s, increased Ca2+ release from sarcoplasmic reti-culum vesicles and increased single RyR1 channel activity. The peptide PASI(-) was more active in each case than PASI(+). [3H]Ryanodine binding to recombinant ASI(+)RyR1 or ASI(-)-RyR1 was enhanced more by PASI(-) than PASI(+), with the greatest increase seen when PASI(-) was added to ASI(-)RyR1. The activation of the RyR channels is consistent with the hypo-thesis that the peptides interrupt an inhibitory inter-domain inter-action and that PASI(-) is more effective at interrupting this interaction than PASI(+). We therefore suggest that the ASI(-) sequence interacts more tightly than the ASI(+) sequence with its binding partner, so that the ASI(-)RyR1 is more strongly inhibited (less active) than the ASI(+)RyR1. Thus the affinity of the binding partners in this inter-domain interaction may deter-mine the activities of the mature and juvenile isoforms of RyR1 and the stronger inhibition in the juvenile isoform may contribute to the myopathy in DM.

Novel regulators of RyR Ca2+ release channels: insight into molecular changes in genetically-linked myopathies

There are many mutations in the ryanodine receptor (RyR) Ca2+ release channel that are implicated in skeletal muscle disorders and cardiac arrhythmias. More than 80 mutations in the skeletal RyR1 have been identified and linked to malignant hyperthermia, central core disease or multi-minicore disease, while more than 40 mutations in the cardiac RyR2 lead to ventricular arrhythmias and sudden cardiac death in patients with structurally normal hearts. These RyR mutations cause diverse changes in RyR activity which either excessively activate or block the channel in a manner that disrupts Ca2+ signalling in the muscle fibres. In a different myopathy, myotonic dystrophy (DM), a juvenile isoform of the skeletal RyR is preferentially expressed in adults. There are two regions of RyR1 that are variably spiced and developmentally regulated (ASI and ASII). The juvenile isoform (ASI(-)) is less active than the adult isoform (ASI(+)) and its over-expression in adults with DM may contribute to functional changes. Finally, mutations in an important regulator of the RyR, the Ca2+ binding protein calsequestrin (CSQ), have been linked to a disruption of Ca2+ homeostasis in cardiac myocytes that results in arrhythmias. We discuss evidence supporting the hypothesis that mutations in each of these situations alter protein/protein interactions within the RyR complex or between the RyR and its associated proteins. The disruption of these protein-protein interactions can lead either to excess Ca2+ release or reduced Ca2+ release and thus to abnormal Ca2+ homeostasis. Much of the evidence for disruption of protein-protein interactions has been provided by the actions of a group of novel RyR regulators, domain peptides with sequences that correspond to sequences within the RyR and which compete with the endogenous residues for their interaction sites.

The conformation of calsequestrin determines its ability to regulate skeletal ryanodine receptors

Ca2+ efflux from the sarcoplasmic reticulum decreases when store Ca2+ concentration falls, particularly in skinned fibers and isolated vesicles where luminal Ca2+ can be reduced to very low levels. However ryanodine receptor activity in many single channel studies is higher when the luminal free Ca2+ concentration is reduced. We investigated the hypothesis that prolonged exposure to low luminal Ca2+ causes conformational changes in calsequestrin and deregulation of ryanodine receptors, allowing channel activity to increase. Lowering of luminal Ca2+ from 1 mM to 100 microM for several minutes resulted in conformational changes with dissociation of 65-75% of calsequestrin from the junctional face membrane. The calsequestrin remaining associated no longer regulated channels. In the absence of this regulation, ryanodine receptors were more active when luminal Ca2+ was lowered from 1 mM to 100 microM. In contrast, when ryanodine receptors were calsequestrin regulated, lowering luminal Ca2+ either did not alter or decreased activity. Ryanodine receptors are regulated by calsequestrin under physiological conditions where calsequestrin is polymerized. Since depolymerization occurs slowly, calsequestrin can regulate the ryanodine receptor and prevent excess Ca2+ release when the store is transiently depleted, for example, during high frequency activity or early stages of muscle fatigue.

Regulation of ryanodine receptors by calsequestrin: effect of high luminal Ca2+ and phosphorylation

Calsequestrin, the major calcium sequestering protein in the sarcoplasmic reticulum of muscle, forms a quaternary complex with the ryanodine receptor calcium release channel and the intrinsic membrane proteins triadin and junctin. We have investigated the possibility that calsequestrin is a luminal calcium concentration sensor for the ryanodine receptor. We measured the luminal calcium concentration at which calsequestrin dissociates from the ryanodine receptor and the effect of calsequestrin on the response of the ryanodine receptor to changes in luminal calcium. We provide electrophysiological and biochemical evidence that: 1), luminal calcium concentration of >/=4 mM dissociates calsequestrin from junctional face membrane, whereas in the range of 1-3 mM calsequestrin remains attached; 2), the association with calsequestrin inhibits ryanodine receptor activity, but amplifies its response to changes in luminal calcium concentration; and 3), under physiological calcium conditions (1 mM), phosphorylation of calsequestrin does not alter its ability to inhibit native ryanodine receptor activity when the anchoring proteins triadin and junctin are present. These data suggest that the quaternary complex is intact in vivo, and provides further evidence that calsequestrin is involved in the sarcoplasmic reticulum calcium signaling pathway and has a role as a luminal calcium sensor for the ryanodine receptor.