The conserved sites for the FK506-binding proteins in ryanodine receptors and inositol 1,4,5-trisphosphate receptors are structurally and functionally different

Direct Gating of the TRPM2 Channel by cADPR via Specific Interactions with the ADPR Binding Pocket

cADPR is a well-recognized signaling molecule by modulating the RyRs, but considerable debate exists regarding whether cADPR can bind to and gate the TRPM2 channel, which mediates oxidative stress signaling in diverse physiological and pathological processes. Here, we show that purified cADPR evoked TRPM2 channel currents in both whole-cell and cell-free single-channel recordings and specific binding of cADPR to the purified NUDT9-H domain of TRPM2 by surface plasmon resonance. Furthermore, by combining computational modeling with electrophysiological recordings, we show that the TRPM2 channels carrying point mutations at H1346, T1347, L1379, S1391, E1409, and L1484 possess distinct sensitivity profiles for ADPR and cADPR. These results clearly indicate cADPR is a bona fide activator at the TRPM2 channel and clearly delineate the structural basis for cADPR binding, which not only lead to a better understanding in the gating mechanism of TRPM2 channel but also shed light on a cADPR-induced RyRs-independent Ca²⁺ signaling mechanism.

FK506 regulates Ca2+ release evoked by inositol 1,4,5-trisphosphate independently of FK-binding protein in endothelial cells

Background and Purpose

FK506 and rapamycin are modulators of FK‐binding proteins (FKBP) that are used to suppress immune function after organ and hematopoietic stem cell transplantations. The drugs share the unwanted side‐effect of evoking hypertension that is associated with reduced endothelial function and nitric oxide production. The underlying mechanisms are not understood. FKBP may regulate IP₃ receptors (IP₃R) and ryanodine receptors (RyR) to alter Ca²⁺ signalling in endothelial cells.

Experimental Approach

We investigated the effects of FK506 and rapamycin on Ca²⁺ release via IP₃R and RyR in hundreds of endothelial cells, using the indicator Cal‐520, in intact mesenteric arteries from male Sprague‐Dawley rats. IP₃Rs were activated by acetylcholine or localised photo‐uncaging of IP₃, and RyR by caffeine.

Key Results

While FKBPs were present, FKBP modulation with rapamycin did not alter IP₃‐evoked Ca²⁺ release. Conversely, FK506, which modulates FKBP and blocks calcineurin, increased IP₃‐evoked Ca²⁺ release. Inhibition of calcineurin (okadiac acid or cypermethrin) also increased IP₃‐evoked Ca²⁺ release and blocked FK506 effects. When calcineurin was inhibited, FK506 reduced IP₃‐evoked Ca²⁺ release. These findings suggest that IP₃‐evoked Ca²⁺ release is not modulated by FKBP, but by FK506‐mediated calcineurin inhibition. The RyR modulators caffeine and ryanodine failed to alter Ca²⁺ signalling suggesting that RyR is not functional in native endothelium.

Conclusion and Implications

The hypertensive effects of the immunosuppressant drugs FK506 and rapamycin, while mediated by endothelial cells, do not appear to be exerted at the documented cellular targets of Ca²⁺ release and altered FKBP binding to IP₃ and RyR.

Reciprocal sensitivity of diffuse large B-cell lymphoma cells to Bcl-2 inhibitors BIRD-2 versus venetoclax

Bcl-2 is often upregulated in cancers to neutralize the BH3-only protein Bim at the mitochondria. BH3 mimetics (e.g. ABT-199 (venetoclax)) kill cancer cells by targeting Bcl-2's hydrophobic cleft and disrupting Bcl-2/Bim complexes. Some cancers with elevated Bcl-2 display poor responses towards BH3 mimetics, suggesting an additional function for anti-apoptotic Bcl-2 in these cancers. Indeed, Bcl-2 via its BH4 domain prevents cytotoxic Ca²⁺ release from the endoplasmic reticulum (ER) by directly inhibiting the inositol 1,4,5-trisphosphate receptor (IP₃R). The cell-permeable Bcl-2/IP₃R disruptor-2 (BIRD-2) peptide can kill these Bcl-2-dependent cancers by targeting Bcl-2's BH4 domain, unleashing pro-apoptotic Ca²⁺-release events. We compared eight "primed to death" diffuse large B-cell lymphoma cell lines (DLBCL) for their apoptotic sensitivity towards BIRD-2 and venetoclax. By determining their IC₅₀ using cytometric cell-death analysis, we discovered a reciprocal sensitivity towards venetoclax versus BIRD-2. Using immunoblotting, we quantified the expression levels of IP₃R2 and Bim in DLBCL cell lysates, revealing that BIRD-2 sensitivity correlated with IP₃R2 levels but not with Bim levels. Moreover, the requirement of intracellular Ca²⁺ for BIRD-2- versus venetoclax-induced cell death was different. Indeed, BAPTA-AM suppressed BIRD-2-induced cell death, but promoted venetoclax-induced cell death in DLBCL cells. Finally, compared to single-agent treatments, combining BIRD-2 with venetoclax synergistically enhanced cell-death induction, correlating with a Ca²⁺-dependent upregulation of Bim after BIRD-2 treatment. Our findings suggest that some cancer cells require Bcl-2 proteins at the mitochondria, preventing Bax activation via its hydrophobic cleft, while others require Bcl-2 proteins at the ER, preventing cytotoxic Ca²⁺-signaling events via its BH4 domain.

From Stores to Sinks: Structural Mechanisms of Cytosolic Calcium Regulation

All eukaryotic cells have adapted the use of the calcium ion (Ca²⁺) as a universal signaling element through the evolution of a toolkit of Ca²⁺ sensor, buffer and effector proteins. Among these toolkit components, integral and peripheral proteins decorate biomembranes and coordinate the movement of Ca²⁺ between compartments, sense these concentration changes and elicit physiological signals. These changes in compartmentalized Ca²⁺ levels are not mutually exclusive as signals propagate between compartments. For example, agonist induced surface receptor stimulation can lead to transient increases in cytosolic Ca²⁺ sourced from endoplasmic reticulum (ER) stores; the decrease in ER luminal Ca²⁺ can subsequently signal the opening surface channels which permit the movement of Ca²⁺ from the extracellular space to the cytosol. Remarkably, the minuscule compartments of mitochondria can function as significant cytosolic Ca²⁺ sinks by taking up Ca²⁺ in a coordinated manner. In non-excitable cells, inositol 1,4,5 trisphosphate receptors (IP₃Rs) on the ER respond to surface receptor stimulation; stromal interaction molecules (STIMs) sense the ER luminal Ca²⁺ depletion and activate surface Orai1 channels; surface Orai1 channels selectively permit the movement of Ca²⁺ from the extracellular space to the cytosol; uptake of Ca²⁺ into the matrix through the mitochondrial Ca²⁺ uniporter (MCU) further shapes the cytosolic Ca²⁺ levels. Recent structural elucidations of these key Ca²⁺ toolkit components have improved our understanding of how they function to orchestrate precise cytosolic Ca²⁺ levels for specific physiological responses. This chapter reviews the atomic-resolution structures of IP₃R, STIM1, Orai1 and MCU elucidated by X-ray crystallography, electron microscopy and NMR and discusses the mechanisms underlying their biological functions in their respective compartments within the cell.

Bcl-2 and FKBP12 bind to IP3 and ryanodine receptors at overlapping sites: the complexity of protein-protein interactions for channel regulation

The 12- and 12.6-kDa FK506-binding proteins, FKBP12 (12-kDa FK506-binding protein) and FKBP12.6 (12.6-kDa FK506-binding protein), have been implicated in the binding to and the regulation of ryanodine receptors (RyRs) and inositol 1,4,5-trisphosphate receptors (IP3Rs), both tetrameric intracellular Ca2+-release channels. Whereas the amino acid sequences responsible for FKBP12 binding to RyRs are conserved in IP3Rs, FKBP12 binding to IP3Rs has been questioned and could not be observed in various experimental models. Nevertheless, conservation of these residues in the different IP3R isoforms and during evolution suggested that they could harbour an important regulatory site critical for IP3R-channel function. Recently, it has become clear that in IP3Rs, this site was targeted by B-cell lymphoma 2 (Bcl-2) via its Bcl-2 homology (BH)4 domain, thereby dampening IP3R-mediated Ca2+ flux and preventing pro-apoptotic Ca2+ signalling. Furthermore, vice versa, the presence of the corresponding site in RyRs implied that Bcl-2 proteins could associate with and regulate RyR channels. Recently, the existence of endogenous RyR-Bcl-2 complexes has been identified in primary hippocampal neurons. Like for IP3Rs, binding of Bcl-2 to RyRs also involved its BH4 domain and suppressed RyR-mediated Ca2+ release. We therefore propose that the originally identified FKBP12-binding site in IP3Rs is a region critical for controlling IP3R-mediated Ca2+ flux by recruiting Bcl-2 rather than FKBP12. Although we hypothesize that anti-apoptotic Bcl-2 proteins, but not FKBP12, are the main physiological inhibitors of IP3Rs, we cannot exclude that Bcl-2 could help engaging FKBP12 (or other FKBP isoforms) to the IP3R, potentially via calcineurin.

Proteomics reveals potential non-neuronal cholinergic receptor-effectors in endothelial cells

AIM

The non-neuronal acetylcholine system (NNAS) in endothelial cells participates in modulating endothelial function, vascular tone, angiogenesis and inflammation, thus plays a critical role in cardiovascular diseases. In this study, we used a proteomic approach to study potential downstream receptor-effectors of NNAS that were involved in regulating cellular function in endothelial cells.

METHODS

Human umbilical vein endothelial cells were incubated in the presence of acetylcholine, oxotremorine, pilocarpine or nicotine at the concentration of 10 μmol/L for 12 h, and the expressed proteins in the cells were separated and identified with two-dimensional electrophoresis (2-DE) and LC-MS. The protein spots with the largest changes were identified by LC-MS. Biowork software was used for database search of the peptide mass fingerprints.

RESULTS

Over 1200 polypeptides were reproducibly detected in 2-DE with a pH range of 3-10. Acetylcholine, oxotremorine, pilocarpine and nicotine treatment caused 16, 9, 8 and 9 protein spots, respectively, expressed differentially. Four protein spots were identified as destrin, FK506 binding protein 1A (FKBP1A), macrophage migration inhibitory factor (MIF) and profilin-1. Western blotting analyses showed that treatment of the cells with cholinergic agonists significantly decreased the expression of destrin, FKBP1A and MIF, and increased the expression of profilin-1.

CONCLUSION

A set of proteins differentially expressed in endothelial cells in response to cholinergic agonists may have important implications for the downstream biological effects of NNAS.

Bcl-2 binds to and inhibits ryanodine receptors

The anti-apoptotic B-cell lymphoma-2 (Bcl-2) protein not only counteracts apoptosis at the mitochondria by scaffolding pro-apoptotic Bcl-2-family members, but also acts at the endoplasmic reticulum, thereby controlling intracellular Ca2+ dynamics. Bcl-2 inhibits Ca2+ release by targeting the inositol 1,4,5-trisphosphate receptor (IP3R). Sequence analysis revealed that the Bcl-2-binding site on the IP3R displays strong homology with a conserved sequence present in all three ryanodine-receptor (RyR) isoforms. We now report that, Bcl-2 co-immunoprecipitated with RyRs in ectopic expression systems and in native rat hippocampi, indicating the existence of endogenous RyR/Bcl-2 complexes. Purified RyR domains containing the putative Bcl-2-binding site bound full-length Bcl-2 in pull-down experiments and interacted with Bcl-2's BH4 domain in surface-plasmon-resonance experiments, suggesting a direct interaction. Exogenous expression of full-length Bcl-2 or electroporation loading of Bcl-2's BH4-domain dampened RyR-mediated Ca2+ release in HEK293 cell models. Finally, introducing the BH4-domain peptide into hippocampal neurons via a patch pipette decreased RyR-mediated Ca2+ release. In conclusion, this study identifies Bcl-2 as a novel inhibitor of RyR-based intracellular Ca2+-release channels.

ATP interacts with the CPVT mutation-associated central domain of the cardiac ryanodine receptor

BACKGROUND

This study was designed to determine whether the cardiac ryanodine receptor (RyR2) central domain, a region associated with catecholamine polymorphic ventricular tachycardia (CPVT) mutations, interacts with the RyR2 regulators, ATP and the FK506-binding protein 12.6 (FKBP12.6).

METHODS

Wild-type (WT) RyR2 central domain constructs (G(2236)to G(2491)) and those containing the CPVT mutations P2328S and N2386I, were expressed as recombinant proteins. Folding and stability of the proteins were examined by circular dichroism (CD) spectroscopy and guanidine hydrochloride chemical denaturation.

RESULTS

The far-UV CD spectra showed a soluble stably-folded protein with WT and mutant proteins exhibiting a similar secondary structure. Chemical denaturation analysis also confirmed a stable protein for both WT and mutant constructs with similar two-state unfolding. ATP and caffeine binding was measured by fluorescence spectroscopy. Both ATP and caffeine bound with an EC50 of ~200-400μM, and the affinity was the same for WT and mutant constructs. Sequence alignment with other ATP binding proteins indicated the RyR2 central domain contains the signature of an ATP binding pocket. Interaction of the central domain with FKBP12.6 was tested by glutaraldehyde cross-linking and no association was found.

CONCLUSIONS

The RyR2 central domain, expressed as a 'correctly' folded recombinant protein, bound ATP in accord with bioinformatics evidence of conserved ATP binding sequence motifs. An interaction with FKBP12.6 was not evident. CPVT mutations did not disrupt the secondary structure nor binding to ATP.

GENERAL SIGNIFICANCE

Part of the RyR2 central domain CPVT mutation cluster, can be expressed independently with retention of ATP binding.

Disparities in the association of the ryanodine receptor and the FK506-binding proteins in mammalian heart

The FK506-binding proteins (FKBP12 and FKBP12.6; also known as FKBP1A and FKBP1B, respectively) are accessory subunits of the ryanodine receptor (RyR) Ca(2+) release channel. Aberrant RyR2-FKBP12.6 interactions have been proposed to be the underlying cause of channel dysfunction in acquired and inherited cardiac disease. However, the stoichiometry of the RyR2 association with FKBP12 or FKBP12.6 in mammalian heart is currently unknown. Here, we describe detailed quantitative analysis of cardiac stoichiometry between RyR2 and FKBP12 or FKBP12.6 using immunoblotting and [(3)H]ryanodine-binding assays, revealing striking disparities between four mammalian species. In mouse and pig heart, RyR2 is found complexed with both FKBP12 and FKBP12.6, although the former is the most abundant isoform. In rat heart, RyR2 is predominantly associated with FKBP12.6, whereas in rabbit it is associated with FKBP12 only. Co-immunoprecipitation experiments demonstrate RyR2-specific interaction with both FKBP isoforms in native cardiac tissue. Assuming four FKBP-binding sites per RyR2 tetramer, only a small proportion of available sites are occupied by endogenous FKBP12.6. FKBP interactions with RyR2 are very strong and resistant to drug (FK506, rapamycin and cyclic ADPribose) and redox (H(2)O(2) and diamide) treatment. By contrast, the RyR1-FKBP12 association in skeletal muscle is readily disrupted under oxidative conditions. This is the first study to directly assess association of endogenous FKBP12 and FKBP12.6 with RyR2 in native cardiac tissue. Our results challenge the widespread perception that RyR2 associates exclusively with FKBP12.6 to near saturation, with important implications for the role of the FK506-binding proteins in RyR2 pathophysiology and cardiac disease.

Ryanodine receptors

Excitation-contraction coupling involves the faithful conversion of electrical stimuli to mechanical shortening in striated muscle cells, enabled by the ubiquitous second messenger, calcium. Crucial to this process are ryanodine receptors (RyRs), the sentinels of massive intracellular calcium stores contained within the sarcoplasmic reticulum. In response to sarcolemmal depolarization, RyRs release calcium into the cytosol, facilitating mobilization of the myofilaments and enabling cell contraction. In order for the cells to relax, calcium must be rapidly resequestered or extruded from the cytosol. The sustainability of this cycle is crucially dependent upon precise regulation of RyRs by numerous cytosolic metabolites and by proteins within the lumen of the sarcoplasmic reticulum and those directly associated with the receptors in a macromolecular complex. In addition to providing the majority of the calcium necessary for contraction of cardiac and skeletal muscle, RyRs act as molecular switchboards that integrate a multitude of cytosolic signals such as dynamic and steady calcium fluctuations, β-adrenergic stimulation (phosphorylation), nitrosylation and metabolic states, and transduce these signals to the channel pore to release appropriate amounts of calcium. Indeed, dysregulation of calcium release via RyRs is associated with life-threatening diseases in both skeletal and cardiac muscle. In this paper, we briefly review some of the most outstanding structural and functional attributes of RyRs and their mechanism of regulation. Further, we address pathogenic RyR dysfunction implicated in cardiovascular disease and skeletal myopathies.

Identification and comparative analysis of the peptidyl-prolyl cis/trans isomerase repertoires of H. sapiens, D. melanogaster, C. elegans, S. cerevisiae and Sz. pombe

The peptidyl-prolyl cis/trans isomerase (PPIase) class of proteins comprises three member families that are found throughout nature and are present in all the major compartments of the cell. Their numbers appear to be linked to the number of genes in their respective genomes, although we have found the human repertoire to be smaller than expected due to a reduced cyclophilin repertoire. We show here that whilst the members of the cyclophilin family (which are predominantly found in the nucleus and cytoplasm) and the parvulin family (which are predominantly nuclear) are largely conserved between different repertoires, the FKBPs (which are predominantly found in the cytoplasm and endoplasmic reticulum) are not. It therefore appears that the cyclophilins and parvulins have evolved to perform conserved functions, while the FKBPs have evolved to fill ever-changing niches within the constantly evolving organisms. Many orthologous subgroups within the different PPIase families appear to have evolved from a distinct common ancestor, whereas others, such as the mitochondrial cyclophilins, appear to have evolved independently of one another. We have also identified a novel parvulin within Drosophila melanogaster that is unique to the fruit fly, indicating a recent evolutionary emergence. Interestingly, the fission yeast repertoire, which contains no unique cyclophilins and parvulins, shares no PPIases solely with the budding yeast but it does share a majority with the higher eukaryotes in this study, unlike the budding yeast. It therefore appears that, in comparison with Schizosaccharomyces pombe, Saccharomyces cerevisiae is a poor representation of the higher eukaryotes for the study of PPIases.

Regulation by FK506 and rapamycin of Ca2+ release from the sarcoplasmic reticulum in vascular smooth muscle: the role of FK506 binding proteins and mTOR

BACKGROUND AND PURPOSE

The sarcoplasmic reticulum (SR), regulates the cytoplasmic Ca(2+) concentration ([Ca(2+)](cyto)) in vascular smooth muscle. Release from the SR is controlled by two intracellular receptor/channel complexes, the ryanodine receptor (RyR) and the inositol 1,4,5-trisphosphate receptor (IP(3)R). These receptors may be regulated by the accessory FK506-binding protein (FKBP) either directly, by binding to the channel, or indirectly via FKBP modulation of two targets, the phosphatase, calcineurin or the kinase, mammalian target of rapamycin (mTOR).

EXPERIMENTAL APPROACH

Single portal vein myocytes were voltage-clamped in whole cell configuration and [Ca(2+)](cyto) measured using fluo-3. IP(3)Rs were activated by photolysis of caged IP(3) and RyRs activated by hydrostatic application of caffeine.

KEY RESULTS

FK506 which displaces FKBP from each receptor (to inhibit calcineurin) increased the [Ca(2+)](cyto) rise evoked by activation of either RyR or IP(3)R. Rapamycin which displaces FKBP (to inhibit mTOR) also increased the amplitude of the caffeine-evoked, but reduced the IP(3)-evoked [Ca(2+)](cyto) rise. None of the phosphatase inhibitors, cypermethrin, okadaic acid or calcineurin inhibitory peptide, altered either caffeine- or IP(3)-evoked [Ca(2+)](cyto) release; calcineurin did not contribute to FK506-mediated potentiation of RyR- or IP(3)R-mediated Ca(2+) release. The mTOR inhibitor LY294002, like rapamycin, decreased IP(3)-evoked Ca(2+) release.

CONCLUSIONS AND IMPLICATIONS

Ca(2+) release in portal vein myocytes, via RyR, was modulated directly by FKBP binding to the channel; neither calcineurin nor mTOR contributed to this regulation. However, IP(3)R-mediated Ca(2+) release, while also modulated directly by FKBP may be additionally regulated by mTOR. Rapamycin inhibition of IP(3)-mediated Ca(2+) release may be explained by mTOR inhibition.

Ryanodine receptor-mediated arrhythmias and sudden cardiac death

The cardiac ryanodine receptor-Ca²⁺ release channel (RyR2) is an essential sarcoplasmic reticulum (SR) transmembrane protein that plays a central role in excitation–contraction coupling (ECC) in cardiomyocytes. Aberrant spontaneous, diastolic Ca²⁺ leak from the SR due to dysfunctional RyR2 contributes to the formation of delayed after-depolarisations, which are thought to underlie the fatal arrhythmia that occurs in both heart failure (HF) and in catecholaminergic polymorphic ventricular tachycardia (CPVT). CPVT is an inherited disorder associated with mutations in either the RyR2 or a SR luminal protein, calsequestrin. RyR2 shows normal function at rest in CPVT but the RyR2 dysfunction is unmasked by physical exercise or emotional stress, suggesting abnormal RyR2 activation as an underlying mechanism. Several potential mechanisms have been advanced to explain the dysfunctional RyR2 observed in HF and CPVT, including enhanced RyR2 phosphorylation status, altered RyR2 regulation at luminal/cytoplasmic sites and perturbed RyR2 intra/inter-molecular interactions. This review considers RyR2 dysfunction in the context of the structural and functional modulation of the channel, and potential therapeutic strategies to stabilise RyR2 function in cardiac pathology.

FK506-binding protein (FKBP12) regulates ryanodine receptor-evoked Ca2+ release in colonic but not aortic smooth muscle

In smooth muscle, the ryanodine receptor (RyR) mediates Ca(2+) release from the sarcoplasmic reticulum (SR) Ca(2+) store. Release may be regulated by the RyR accessory FK506-binding protein (FKBP12) either directly, as a result of FKBP12 binding to RyR, or indirectly via modulation of the activity of the phosphatase calcineurin or kinase mTOR. Here we report that RyR-mediated Ca(2+) release is modulated by FKBP12 in colonic but not aortic myocytes. Neither calcineurin nor mTOR are required for FKBP12 modulation of Ca(2+) release in colonic myocytes to occur. In colonic myocytes, co-immunoprecipitation techniques established that FKBP12 and calcineurin each associated with the RyR2 receptor isoform (the main isoform in this tissue). Single colonic myocytes were voltage clamped in the whole cell configuration and cytoplasmic Ca(2+) concentration ([Ca(2+)](c)) increases evoked by the RyR activator caffeine. Under these conditions FK506, which displaces FKBP12 (to inhibit calcineurin) and rapamycin, which displaces FKBP12 (to inhibit mTOR), each increased the [Ca(2+)](c) rise evoked by caffeine. Notwithstanding, neither mTOR nor calcineurin are required to potentiate caffeine-evoked Ca(2+) increases evoked by each drug. Thus, the mTOR and phosphatidylinositol 3-kinase inhibitor, LY294002, which directly inhibits mTOR without removing FKBP12 from RyR, did not alter caffeine-evoked [Ca(2+)](c) transients. Nor did inhibition of calcineurin by cypermethrin, okadaic acid or calcineurin inhibitory peptide block the FK506-induced increase in RyR-mediated Ca(2+) release. In aorta, although RyR3 (the main isoform), FKBP12 and calcineurin were each present, RyR-mediated Ca(2+) release was unaffected by either FK506, rapamycin or the calcineurin inhibitors cypermethrin and okadaic acid in single voltage clamped aortic myocytes. Presumably failure of FKBP12 to associate with RyR3 resulted in the immunosuppressant drugs (FK506 and rapamycin) being unable to alter the activity of RyR. The effects of these drugs are therefore, apparently dependent on an association of FKBP12 with RyR. Together, removal of FKBP12 from RyR augmented Ca(2+) release via the channel in colonic myocytes. Neither calcineurin nor mTOR are required for the FK506- or rapamycin-induced potentiation of RyR Ca(2+) release to occur. The results indicate that FKBP12 directly inhibits RyR channel activity in colonic myocytes but not in aorta.

Modulation of the Ryanodine Receptor and Intracellular Calcium

Ryanodine receptors (RyRs)/Ca2+ release channels, on the endoplasmic and sarcoplasmic reticulum of most cell types, are required for intracellular Ca2+ release involved in diverse cellular functions, including muscle contraction and neurotransmitter release. The large cytoplasmic domain of the RyR serves as a scaffold for proteins that bind to and modulate the channel's function and that comprise a macromolecular signaling complex. These proteins include calstabins [FK506-binding proteins (FKBPs)], calmodulin (CaM), phosphodiesterase, kinases, phosphatases, and their cognate targeting proteins. This review focuses on recent progress in the understanding of RyR regulation and disease mechanisms that are associated with channel dysfunction.

Inositol trisphosphate receptor Ca2+ release channels

The inositol 1,4,5-trisphosphate (InsP3) receptors (InsP3Rs) are a family of Ca2+ release channels localized predominately in the endoplasmic reticulum of all cell types. They function to release Ca2+ into the cytoplasm in response to InsP3 produced by diverse stimuli, generating complex local and global Ca2+ signals that regulate numerous cell physiological processes ranging from gene transcription to secretion to learning and memory. The InsP3R is a calcium-selective cation channel whose gating is regulated not only by InsP3, but by other ligands as well, in particular cytoplasmic Ca2+. Over the last decade, detailed quantitative studies of InsP3R channel function and its regulation by ligands and interacting proteins have provided new insights into a remarkable richness of channel regulation and of the structural aspects that underlie signal transduction and permeation. Here, we focus on these developments and review and synthesize the literature regarding the structure and single-channel properties of the InsP3R.

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.

Redox sensitivity of the ryanodine receptor interaction with FK506-binding protein

The ryanodine receptor (RyR) calcium release channel functions as a redox sensor that is sensitive to channel modulators. The FK506-binding protein (FKBP) is an important regulator of channel activity, and disruption of the RyR2-FKBP12.6 association has been implicated in cardiac disease. In the present study, we investigated whether the RyR-FKBP association is redox-regulated. Using co-immunoprecipitation assays of solubilized native RyR2 from cardiac muscle sarcoplasmic reticulum (SR) with recombinant [(35)S]FKBP12.6, we found that the sulfydryl-oxidizing agents, H(2)O(2) and diamide, result in diminished RyR2-FKBP12.6 binding. Co-sedimentation experiments of cardiac SR vesicles with [(35)S]FKBP12.6 also demonstrated that oxidizing reagents decreased FKBP binding. Matching results were obtained with skeletal muscle SR. Notably, H(2)O(2) and diamide differentially affected the RyR2-FKBP12.6 interaction, decreasing binding to approximately 75 and approximately 50% of control, respectively. In addition, the effect of H(2)O(2) was negligible when the channel was in its closed state or when applied after FKBP binding had occurred, whereas diamide was always effective. A cysteine-null mutant FKBP12.6 retained redox-sensitive interaction with RyR2, suggesting that the effect of the redox reagents is exclusively via sites on the ryanodine receptor. K201 (or JTV519), a drug that has been proposed to prevent FKBP12.6 dissociation from the RyR2 channel complex, did not restore normal FKBP binding under oxidizing conditions. Our results indicate that the redox state of the RyR is intimately connected with FKBP binding affinity.

Ryanodine receptors and ventricular arrhythmias: emerging trends in mutations, mechanisms and therapies

It has been six years since the first reported link between mutations in the cardiac ryanodine receptor Ca(2+) release channel (RyR2) and catecholaminergic polymorphic ventricular tachycardia (CPVT), a malignant stress-induced arrhythmia. In this time, rapid advances have been made in identifying new mutations, and in understanding how these mutations disrupt normal channel function to cause VT that frequently degenerates into ventricular fibrillation (VF) and sudden death. Functional characterisation of these RyR2 Ca(2+) channelopathies suggests that mutations alter the ability of RyR2 to sense its intracellular environment, and that channel modulation via covalent modification, Ca(2+)- and Mg(2+)-dependent regulation and structural feedback mechanisms are catastrophically disturbed. This review reconciles the current status of RyR2 mutation-linked etiopathology, the significance of mutational clustering within the RyR2 polypeptide and the mechanisms underlying channel dysfunction. We will also review new data that explores the link between abnormal Ca(2+) release and the resultant cardiac electrical instability in VT and VF, and how these recent developments impact on novel anti-arrhythmic therapies. Finally, we evaluate the concept that mechanistic differences between CPVT and other arrhythmogenic disorders may preclude a common therapeutic strategy to normalise RyR2 function in cardiac disease.

Ca2+ microdomains in smooth muscle

In smooth muscle, Ca(2+) controls diverse activities including cell division, contraction and cell death. Of particular significance in enabling Ca(2+) to perform these multiple functions is the cell's ability to localize Ca(2+) signals to certain regions by creating high local concentrations of Ca(2+) (microdomains), which differ from the cytoplasmic average. Microdomains arise from Ca(2+) influx across the plasma membrane or release from the sarcoplasmic reticulum (SR) Ca(2+) store. A single Ca(2+) channel can create a microdomain of several micromolar near (approximately 200 nm) the channel. This concentration declines quickly with peak rates of several thousand micromolar per second when influx ends. The high [Ca(2+)] and the rapid rates of decline target Ca(2+) signals to effectors in the microdomain with rapid kinetics and enable the selective activation of cellular processes. Several elements within the cell combine to enable microdomains to develop. These include the brief open time of ion channels, localization of Ca(2+) by buffering, the clustering of ion channels to certain regions of the cell and the presence of membrane barriers, which restrict the free diffusion of Ca(2+). In this review, the generation of microdomains arising from Ca(2+) influx across the plasma membrane and the release of the ion from the SR Ca(2+) store will be discussed and the contribution of mitochondria and the Golgi apparatus as well as endogenous modulators (e.g. cADPR and channel binding proteins) will be considered.

The aggregation of alpha‐synuclein is stimulated by FK506 binding proteins as shown by fluorescence correlation spectroscopy

Aggregation of alpha-synuclein (alpha-SYN) plays a key role in Parkinson's disease (PD). We have used fluorescence correlation spectroscopy (FCS) to study alpha-SYN aggregation in vitro and discovered that this process is clearly accelerated by addition of FK506 binding proteins (FKBPs). This effect was observed both with E. coli SlyD FKBP and with human FKBP12 and was counteracted by FK506, a specific inhibitor of FKBP. The alpha-SYN aggregates formed in the presence of FKBP12 showed fibrillar morphology. The rotamase activity of FKBP apparently accelerates the folding and subsequent aggregation of alpha-SYN. Since FK506 and other non-immunosuppressive FKBP inhibitors are known to display neuroregenerative and neuroprotective properties in disease models, the observed inhibition of rotamase activity and alpha-SYN aggregation, may explain their mode of action. Our results open perspectives for the treatment of PD with immunophilin ligands that inhibit a specific member of the FKBP family.

Pyridine nucleotides and calcium signalling in arterial smooth muscle: from cell physiology to pharmacology

It is generally accepted that the mobilisation of intracellular Ca2+ stores plays a pivotal role in the regulation of arterial smooth muscle function, paradoxically during both contraction and relaxation. However, the spatiotemporal pattern of different Ca2+ signals that elicit such responses may also contribute to the regulation of, for example, differential gene expression. These findings, among others, demonstrate the importance of discrete spatiotemporal Ca2+ signalling patterns and the mechanisms that underpin them. Of fundamental importance in this respect is the realisation that different Ca2+ storing organelles may be selected by the discrete or coordinated actions of multiple Ca2+ mobilising messengers. When considering such messengers, it is generally accepted that sarcoplasmic reticulum (SR) stores may be mobilised by the ubiquitous messenger inositol 1,4,5 trisphosphate. However, relatively little attention has been paid to the role of Ca2+ mobilising pyridine nucleotides in arterial smooth muscle, namely, cyclic adenosine diphosphate-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP). This review will therefore focus on these novel mechanisms of calcium signalling and their likely therapeutic potential.

In smooth muscle, FK506-binding protein modulates IP3 receptor-evoked Ca2+ release by mTOR and calcineurin

Ca2+ release from the sarcoplasmic reticulum (SR) by the IP3 receptors (IP3Rs) crucially regulates diverse cell signalling processes from reproduction to apoptosis. Release from the IP3R may be modulated by endogenous proteins associated with the receptor, such as the 12 kDa FK506-binding protein (FKBP12), either directly or indirectly by inhibition of the phosphatase calcineurin. Here, we report that, in addition to calcineurin, FKPBs modulate release through the mammalian target of rapamycin (mTOR), a kinase that potentiates Ca2+ release from the IP3R in smooth muscle. The presence of FKBP12 was confirmed in colonic myocytes and co-immunoprecipitated with the IP3R. In aortic smooth muscle, however, although present, FKBP12 did not co-immunoprecipitate with IP3R. In voltage-clamped single colonic myocytes rapamycin, which together with FKBP12 inhibits mTOR (but not calcineurin), decreased the rise in cytosolic Ca2+ concentration ([Ca2+]c) evoked by IP3R activation (by photolysis of caged IP3), without decreasing the SR luminal Ca2+ concentration ([Ca2+]l) as did the mTOR inhibitors RAD001 and LY294002. However, FK506, which with FKBP12 inhibits calcineurin (but not mTOR), potentiated the IP3-evoked [Ca2+]c increase. This potentiation was due to the inhibition of calcineurin; it was mimicked by the phosphatase inhibitors cypermethrin and okadaic acid. The latter two inhibitors also prevented the FK506-evoked increase as did a calcineurin inhibitory peptide (CiP). In aortic smooth muscle, where FKBP12 was not associated with IP3R, the IP3-mediated Ca2+ release was unaffected by FK506 or rapamycin. Together, these results suggest that FKBP12 has little direct effect on IP3-mediated Ca2+ release, even though it is associated with IP3R in colonic myocytes. However, FKBP12 might indirectly modulate Ca2+ release through two effector proteins: (1) mTOR, which potentiates and (2) calcineurin, which inhibits Ca2+ release from IP3R in smooth muscle.

Three-dimensional visualization of FKBP12.6 binding to an open conformation of cardiac ryanodine receptor

The cardiac isoform of the ryanodine receptor (RyR2) from dog binds predominantly a 12.6-kDa isoform of the FK506-binding protein (FKBP12.6), whereas RyR2 from other species binds both FKBP12.6 and the closely related isoform FKBP12. The role played by FKBP12.6 in modulating calcium release by RyR2 is unclear at present. We have used cryoelectron microscopy and three-dimensional (3D) reconstruction techniques to determine the binding position of FKBP12.6 on the surface of canine RyR2. Buffer conditions that should favor the "open" state of RyR2 were used. Quantitative comparison of 3D reconstructions of RyR2 in the presence and absence of FKBP12.6 reveals that FKBP12.6 binds along the sides of the square-shaped cytoplasmic region of the receptor, adjacent to domain 9, which forms part of the four clamp (corner-forming) structures. The location of the FKBP12.6 binding site on "open" RyR2 appears similar, but slightly displaced (by 1-2 nm) from that found previously for FKBP12 binding to the skeletal muscle ryanodine receptor that was in the buffer that favors the "closed" state. The conformation of RyR2 containing bound FKBP12.6 differs considerably from that depleted of FKBP12.6, particularly in the transmembrane region and in the clamp structures. The x-ray structure of FKBP12.6 was docked into the region of the 3D reconstruction that is attributable to bound FKBP12.6, to show the relative orientations of amino acid residues (Gln-31, Asn-32, Phe-59) that have been implicated as being critical in interactions with RyR2. A thorough understanding of the structural basis of RyR2-FKBP12.6 interaction should aid in understanding the roles that have been proposed for FKBP12.6 in heart failure and in certain forms of sudden cardiac death.

Intracellular calcium release and cardiac disease

Intracellular calcium release channels are present on sarcoplasmic and endoplasmic reticuli (SR, ER) of all cell types. There are two classes of these channels: ryanodine receptors (RyR) and inositol 1,4,5-trisphosphate receptors (IP3R). RyRs are required for excitation-contraction (EC) coupling in striated (cardiac and skeletal) muscles. RyRs are made up of macromolecular signaling complexes that contain large cytoplasmic domains, which serve as scaffolds for proteins that regulate the function of the channel. These regulatory proteins include calstabin1/calstabin2 (FKBP12/FKBP12.6), a 12/12.6 kDa subunit that stabilizes the closed state of the channel and prevents aberrant calcium leak from the SR. Kinases and phosphatases are targeted to RyR2 channels and modulate RyR2 function in response to extracellular signals. In the classic fight or flight stress response, phosphorylation of RyR channels by protein kinase A reduces the affinity for calstabin and activates the channels leading to increased SR calcium release. In heart failure, a cardiac insult causes a mismatch between blood supply and metabolic demands of organs. The chronically activated fight or flight response leads to leaky channels, altered calcium signaling, and contractile dysfunction and cardiac arrhythmias.

Involvement of ryanodine receptors in pacemaker Ca2+ oscillation in murine gastric ICC

Using a cell cluster preparation from the stomach smooth muscle tissue of mice, we measured intracellular Ca(2+) oscillations in interstitial cells of Cajal (ICCs) in the presence of nifedipine. Pacemaker [Ca(2+)](i) activity in ICCs was significantly suppressed by caffeine application and restored after washout. Application of either ryanodine or FK-506 terminated the pacemaker [Ca(2+)](i) activity irreversibly. Immunostaining of smooth muscle tissue showed that c-Kit-immunopositive cells (that form network-like structure cells in the myenteric plexus, equivalent to ICCs) clearly express ryanodine receptors (RyR). RT-PCR revealed that ICCs (identified with c-Kit-immunoreactivity) predominantly express type 3 RyR (RyR3). Furthermore, the FK-binding proteins 12 and 12.6, both of which would interact with RyR3, were detected. In conclusion, we provide first evidence for the essential contribution of RyR to generating pacemaker activity in gastric motility. Similar mechanisms might account for spontaneous rhythmicity seen in smooth muscle tissues distributed in the autonomic nervous system.

Regulation of ryanodine receptors by FK506 binding proteins

Ryanodine receptors (RyRs) are the major sarcoplasmic reticulum calcium-release channels required for excitation-contraction coupling in skeletal and cardiac muscle. Mutations in RyRs have been linked to several human diseases. Mutations in the cardiac isoform of RyR2 are associated with catecholaminergic polymorphic ventricular arrhythmias (CPVT), and arrhythmogenic right ventricular dysplasia type 2 (ARVD2), whereas mutations in the skeletal muscle isoform (RyR1) are linked to malignant hyperthermia (MH) and central core disease (CCD). RyRs are modulated by several other proteins, including the FK506 binding proteins (FKBPs), FKBP12 and FKBP12.6. These immunophilins appear to stabilize a closed state of the channel and are important for cooperative interactions among the subunits of RyRs. This review discusses the regulation of RyRs by FKBPs and the possibility that defective modulation of RyR2 by FKBP12.6 could play a role in heart failure, CPVT, and ARVD2. Also discussed are the consequences of FKBP12 depletion to skeletal muscle and the possibility of FKBP12 involvement in certain forms of MH or CCD.

Requirement of ryanodine receptors for pacemaker Ca2+ activity in ICC and HEK293 cells

Intracellular Ca(2+) ([Ca(2+)](i)) oscillations seen in interstitial cells of Cajal (ICCs) are considered to be the primary pacemaker activity in the gut. Here, we show evidence that periodic Ca(2+) release from intracellular Ca(2+) stores produces [Ca(2+)](i) oscillations in ICCs, using cell cluster preparations isolated from mouse ileum. The pacemaker [Ca(2+)](i) oscillations in ICCs are preserved in the presence of dihydropyridine Ca(2+) antagonists, which suppress Ca(2+) activity in smooth muscle cells. However, applications of drugs affecting either ryanodine receptors or inositol 1,4,5-trisphosphate receptors terminated [Ca(2+)](i) oscillations at relatively low concentrations. RT-PCR analyses revealed a predominant expression of type 3 RyR (RyR3) in isolated c-Kit-immunopositive cells (ICCs). Furthermore, we demonstrate that pacemaker-like global [Ca(2+)](i) oscillation activity is endowed by introducing RyR3 into HEK293 cells, which originally express only IP(3)Rs. The reconstituted [Ca(2+)](i) oscillations in HEK293 cells possess essentially the same pharmacological characteristics as seen in ICCs. The results support the functional role of RyR3 in ICCs.

Emerging role of cyclic ADP-ribose (cADPR) in smooth muscle

Cyclic adenosine diphosphate ribose (cADPR) is a naturally occurring cyclic nucleotide and represents a novel class of endogenous Ca(2+) messengers implicated in the regulation of the gating properties of ryanodine receptors (RyRs). This action of cADPR occurs independently from the inositol-1,4,5-trisphosphate (IP(3)) receptor. The regulation of intracellular Ca(2+) release is a fundamental element of cellular Ca(2+) homeostasis since a number of smooth muscle functions (tone, proliferation, apoptosis, and gene expression) are modulated by intracellular Ca(2+) concentration ([Ca(2+)](i)). There has been a surge in the efforts aimed at understanding the mechanisms of cADPR-mediated Ca(2+) mobilization and its impact on smooth muscle function. This review summarizes the proposed roles of cADPR in the regulation of smooth muscle tone.

Interaction of FKBP12.6 with the cardiac ryanodine receptor C-terminal domain

The ryanodine receptor-calcium release channel complex (RyR) plays a pivotal role in excitation-contraction coupling in skeletal and cardiac muscle. RyR channel activity is modulated by interaction with FK506-binding protein (FKBP), and disruption of the RyR-FKBP association has been implicated in cardiomyopathy, cardiac hypertrophy, and heart failure. Evidence for an interaction between RyR and FKBP is well documented, both in skeletal muscle (RyR1-FKBP12) and in cardiac muscle (RyR2-FKBP12.6), however definition of the FKBP-binding site remains elusive. Early reports proposed interaction of a short RyR central domain with FKBP12/12.6, however this site has been questioned, and recently an alternative FKBP12.6 interaction site has been identified within the N-terminal half of RyR2. In this study, we report evidence for the human RyR2 C-terminal domain as a novel FKBP12.6-binding site. Using competition binding assays, we find that short C-terminal RyR2 fragments can displace bound FKBP12.6 from the native RyR2, although they are unable to exclusively support interaction with FKBP12.6. However, expression of a large RyR2 C-terminal construct in mammalian cells encompassing the pore-forming transmembrane domains exhibits rapamycin-sensitive binding specifically to FKBP12.6 but not to FKBP12. We also obtained some evidence for involvement of the RyR2 N-terminal, but not the central domain, in FKBP12.6 interaction. Our studies suggest that a novel interaction site for FKBP12.6 may be present at the RyR2 C terminus, proximal to the channel pore, a sterically appropriate location that would enable this protein to play a central role in the modulation of this critical ion channel.

Pharmacological modulation of sarcoplasmic reticulum function in smooth muscle

The sarco/endoplasmic reticulum (SR/ER) is the primary storage and release site of intracellular calcium (Ca2+) in many excitable cells. The SR is a tubular network, which in smooth muscle (SM) cells distributes close to cellular periphery (superficial SR) and in deeper aspects of the cell (deep SR). Recent attention has focused on the regulation of cell function by the superficial SR, which can act as a buffer and also as a regulator of membrane channels and transporters. Ca2+ is released from the SR via two types of ionic channels [ryanodine- and inositol 1,4,5-trisphosphate-gated], whereas accumulation from thecytoplasm occurs exclusively by an energy-dependent sarco-endoplasmic reticulum Ca2+-ATPase pump (SERCA). Within the SR, Ca2+ is bound to various storage proteins. Emerging evidence also suggests that the perinuclear portion of the SR may play an important role in nuclear transcription. In this review, we detail the pharmacology of agents that alter the functions of Ca2+ release channels and of SERCA. We describe their use and selectivity and indicate the concentrations used in investigating various SM preparations. Important aspects of cell regulation and excitation-contractile activity coupling in SM have been uncovered through the use of such activators and inhibitors of processes that determine SR function. Likewise, they were instrumental in the recent finding of an interaction of the SR with other cellular organelles such as mitochondria. Thus, an appreciation of the pharmacology and selectivity of agents that interfere with SR function in SM has greatly assisted in unveiling the multifaceted nature of the SR.

Role of FKBP12.6 in hypoxia- and norepinephrine-induced Ca2+ release and contraction in pulmonary artery myocytes

The cellular and molecular processes underlying the regulation of ryanodine receptor (RyR) Ca(2+) release in smooth muscle cells (SMCs) are incompletely understood. Here we show that FKBP12.6 proteins are expressed in pulmonary artery (PA) smooth muscle and associated with type-2 RyRs (RyR2), but not RyR1, RyR3, or IP(3) receptors (IP(3)Rs) in PA sarcoplasmic reticulum. Application of FK506, which binds to FKBPs and dissociates these proteins from RyRs, induced an increase in [Ca(2+)](i) and Ca(2+)-activated Cl(-) and K(+) currents in freshly isolated PASMCs, whereas cyclosporin, an agent known to inhibit calcineurin but not to interact with FKBPs, failed to induce an increase in [Ca(2+)](i). FK506-induced [Ca(2+)](i) increase was completely blocked by the RyR antagonist ruthenium red and ryanodine, but not the IP(3)R antagonist heparin. Hypoxic Ca(2+) response and hypoxic vasoconstriction were significantly enhanced in FKBP12.6 knockout mouse PASMCs. FK506 or rapamycin pretreatment also enhanced hypoxic increase [Ca(2+)](i), but did not alter caffeine-induced Ca(2+) release (SR Ca(2+) content) in PASMCs. Norepinephrine-induced Ca(2+) release and force generation were also markedly enhanced in PASMCs from FKBP12.6 null mice. These findings suggest that FKBP12.6 plays an important role in hypoxia- and neurotransmitter-induced Ca(2+) and contractile responses by regulating the activity of RyRs in PASMCs.

Association of Immunophilins with Mammalian TRPC Channels

Drosophila photoreceptor channels TRP and TRPL are held in a large signalplex by the scaffolding protein, INAD. Immunophilin FKBP59, another member of the signalplex, binds to both INAD and TRPL. Mutation P702Q or P709Q in the highly conserved TRPL sequence (701)LPPPFNVLP(709), eliminates TRPL interaction with FKBP59. The first leucylprolyl (LP) dipeptide in this region is conserved in mammalian TRPC channel proteins. However, the second LP is changed to isoleucylprolyl (IP) in TRPC1, -C4, and -C5, and valylprolyl (VP) in TRPC3, -C6, and -C7. The purpose of the present study was to determine if mammalian FKBP12 or FKBP52 interact with TRPC channel proteins. Using TRPC-specific antibodies, immunoprecipitations from Sf9 cells individually co-expressing each of the TRPC proteins along with the immunophilins showed that TRPC3, -C6, and -C7 interact with FKBP12, whereas TRPC1, -C4, and -C5 interact with FKBP52. The binding of FKBP12 and FKBP52 was specific and could be displaced by the immunosuppressant drug FK506, at concentrations of 0.5 and 10 microm, respectively. To evaluate TRPC-immunophilin interactions in vivo, immunoprecipitations were performed using membrane lysates of rat cerebral cortex. FKBP12 co-immunoprecipitated with TRPC3, -C6, and -C7 from rat brain, whereas FKBP52 was found to associate with TRPC1, -C4, and -C5. The association of immunophilins with the TRPC channels in rat brain lysates could be displaced by FK506. Receptor-mediated activation of TRPC6, stably expressed in HEK cells, was significantly inhibited by FK506, which also disrupted interaction between TRPC6 and the endogenous immunophilin found in HEK cells. Pro to Gln mutations in the first LP dipeptide in the putative FKBP binding domain eliminated FKBP12 and FKBP52 interaction with TRPC3 and -C6, and TRPC1 and -C4, respectively. However, mutual swap of VP and IP in TRPC3 and TRPC5 did not alter the association or the selectivity of the channels for their respective immunophilin binding partner. These results suggest that immunophilins are TRPC channel accessory proteins that play an important role in the mechanism of channel activation following receptor stimulation.

Inositol 1,4,5-trisphosphate receptors as signal integrators

The inositol 1,4,5 trisphosphate (IP3) receptor (IP3R) is a Ca2+ release channel that responds to the second messenger IP3. Exquisite modulation of intracellular Ca2+ release via IP3Rs is achieved by the ability of IP3R to integrate signals from numerous small molecules and proteins including nucleotides, kinases, and phosphatases, as well as nonenzyme proteins. Because the ion conduction pore composes only approximately 5% of the IP3R, the great bulk of this large protein contains recognition sites for these substances. Through these regulatory mechanisms, IP3R modulates diverse cellular functions, which include, but are not limited to, contraction/excitation, secretion, gene expression, and cellular growth. We review the unique properties of the IP3R that facilitate cell-type and stimulus-dependent control of function, with special emphasis on protein-binding partners.

N-terminal region of FKBP12 is essential for binding to the skeletal ryanodine receptor

It is known that the two types of FK506-binding proteins FKBP12 and FKBP12.6 are tightly associated with the skeletal (RyR1) and cardiac ryanodine receptors (RyR2), respectively, and their interactions are important for channel functions of the RyR. In the case of cardiac muscle, three amino acid residues (Gln-31, Asn-32, and Phe-59) of FKBP12.6 could be essential for the selective binding to RyR2 (Xin, H. B., Rogers, K., Qi, Y., Kanematsu, T., and Fleischer, S. (1999) J. Biol. Chem. 274, 15315-15319). In this study to identify amino acid residues of FKBP12 that are important for the selective binding to RyR1, we mutated 9 amino acid residues of FKBP12 that differ from the counterparts of FKBP12.6 (Q3E, R18A, E31Q, D32N, M49R, R57A, W59F, H94A, and K105A), and we examined binding properties of these mutants to RyR1 by in vitro binding assay by using glutathione S-transferase-fused proteins of the mutants and Triton X-100-solubilized, FKBP12-depleted rabbit skeletal sarcoplasmic reticulum vesicles. Among the nine mutants tested, only Q3E and R18A lost their selective binding ability to RyR1. Furthermore, co-immunoprecipitation of RyR1 with 33 various mutants for the 9 positions produced by introducing different size, charge, and hydrophobicity revealed that an integration of the hydrogen bonds by the irreplaceable Gln-3 and the hydrophobic interactions by the residues Arg-18 and Met-49 could be a possible mechanism for the binding of FKBP12 to RyR1. Therefore, these results suggest that the N-terminal regions of FKBP12 (Gln-3 and Arg-18) and Met-49 are essential and unique for binding of FKBP12 to RyR1 in skeletal muscle.

The 12 kDa FK506-binding protein, FKBP12, modulates the Ca(2+)-flux properties of the type-3 ryanodine receptor

We have characterised the functional regulation of the type-3 ryanodine receptor by the 12 kDa FK506-binding protein. Wild-type type-3 ryanodine receptor and mutant type-3 ryanodine receptor in which the critical valine at position 2322 in the central 12 kDa FK506-binding protein binding site was substituted by aspartate, were stably expressed in human embryonic kidney cells. In contrast to the wild-type receptor, the mutant receptor was strongly impaired in binding to immobilised glutathione S-transferase 12 kDa FK506-binding protein. Caffeine-induced 45Ca(2+)-efflux was markedly increased in cells expressing mutant type-3 ryanodine receptor whereas the maximal-releasable Ca2+ was not affected. Confocal Ca2+ imaging provided clear evidence for a much higher sensitivity of the mutant receptor, which showed global Ca2+ release at about 20-fold lower caffeine concentrations than the wild-type receptor. Spontaneous Ca2+ sparks were observed in both wild-type- and mutant-expressing cells but the number of sparking cells was about 1.5-fold higher in the mutant group, suggesting that the degree of FK506 binding controls the stability of the closed state of ryanodine receptor channels. Furthermore, overexpression of 12 kDa FK506-binding protein decreased the number of sparking cells in the wild-type-expressing cells whereas it did not affect the number of sparking cells in cells expressing the mutant receptor. Concerning spark properties, the amplitude and duration of Ca2+ sparks mediated by mutant channels were significantly reduced in comparison to wild-type channels. This suggests that functional coupling between different mutant type-3 ryanodine receptor channels in a cluster is impaired. Our findings show for the first time that the central binding site for the 12 kDa FK506-binding protein of type-3 ryanodine receptor, encompassing the critical valine proline motif, plays a crucial role in the modulation of the Ca2+ release properties of the type-3 ryanodine receptor channel, including the regulation of both global Ca2+ responses and spontaneous Ca2+ sparks.

Calcineurin and intracellular Ca2+-release channels: regulation or association?

The Ca(2+)- and calmodulin-dependent phosphatase calcineurin was reported to interact with the inositol 1,4,5-trisphosphate receptor (IP(3)R) and the ryanodine receptor (RyR) and to modulate their phosphorylation status and activity. However, controversial data on the molecular mechanisms involved and on the functional relevance of calcineurin for these channel-complexes have been described. Hence, we will focus on the functional importance of calcineurin for IP(3)R and RyR function and on the different mechanisms by which Ca(2+)-dependent dephosphorylation can affect the gating of those intracellular Ca(2+)-release channels. Since many studies made use of immunosuppressive drugs that are inhibiting calcineurin activity, we will also have to take the different side effects of these drugs into account for the proper interpretation of the effects of calcineurin on intracellular Ca(2+)-release channels. In addition, it became recently known that various other phosphatases and kinases can associate with these channels, thereby forming macromolecular complexes. The relevance of these enzymes for IP(3)R and RyR functioning will be reviewed since in some cases they could interfere with the effects ascribed to calcineurin. Finally, we will discuss the downstream effects of calcineurin on the regulation of the expression levels of intracellular Ca(2+)-release channels as well as the relation between IP(3)R- and RyR-mediated Ca(2+) release and calcineurin-dependent gene expression.

Reduction of calcium release from the endoplasmic reticulum could only provide partial neuroprotection against beta-amyloid peptide toxicity

Beta-amyloid (Abeta) peptide has been suggested to play important roles in the pathogenesis of Alzheimer's disease (AD). Abeta peptide neurotoxicity was shown to induce disturbance of cellular calcium homeostasis. However, whether modulation of calcium release from the endoplasmic reticulum (ER) can protect neurons from Abeta toxicity is not clearly defined. In the present study, Abeta peptide-triggered ER calcium release in primary cortical neurons in culture is modulated by Xestospongin C, 2-aminoethoxydiphenyl borate or FK506. Our results showed that reduction of ER calcium release can partially attenuate Abeta peptide neurotoxicity evaluated by LDH release, caspase-3 activity and quantification of apoptotic cells. While stress signals associated with perturbations of ER functions such as up-regulation of GRP78 was significantly attenuated, other signaling machinery such as activation of caspase-7 transmitting death signals from ER to other organelles could not be altered. We further provide evidence that molecular signaling in mitochondria play also a significant role in determining neuronal apoptosis because Abeta peptide-triggered activation of caspase-9 was not significantly reduced by attenuating ER calcium release. Our results suggest that neuroprotective strategies aiming at reducing Abeta toxicity should include molecular targets linked to ER perturbations associated with ER calcium release as well as mitochondrial stress.

Cyclic ADP-ribose increases Ca2+ removal in smooth muscle

Ca2+ release via ryanodine receptors (RyRs) is vital in cell signalling and regulates diverse activities such as gene expression and excitation-contraction coupling. Cyclic ADP ribose (cADPR), a proposed modulator of RyR activity, releases Ca2+ from the intracellular store in sea urchin eggs but its mechanism of action in other cell types is controversial. In this study, caged cADPR was used to examine the effect of cADPR on Ca2+ signalling in single voltage-clamped smooth muscle cells that have RyR but lack FKBP12.6, a proposed target for cADPR. Although cADPR released Ca2+ in sea urchin eggs (a positive control), it failed to alter global or subsarcolemma [Ca2+]c, to cause Ca2+-induced Ca2+ release or to enhance caffeine responses in colonic myocytes. By contrast, caffeine (an accepted modulator of RyR) was effective in these respects. The lack of cADPR activity on Ca2+ release was unaffected by the introduction of recombinant FKBP12.6 into the myocytes. Indeed in western blots, using brain membrane preparations as a source of FKBP12.6, cADPR did not bind to FKBPs, although FK506 was effective. However, cADPR increased and its antagonist 8-bromo-cADPR slowed the rate of Ca2+ removal from the cytoplasm. The evidence indicates that cADPR modulates [Ca2+]c but not via RyR; the mechanism may involve the sarcolemma Ca2+ pump.

FKBP12 binding to RyR1 modulates excitation-contraction coupling in mouse skeletal myotubes

The skeletal muscle sarcoplasmic reticulum (SR) Ca2+ release channel or ryanodine receptor (RyR1) binds four molecules of FKBP12, and the interaction of FKBP12 with RyR1 regulates both unitary and coupled gating of the channel. We have characterized the physiologic effects of previously identified mutations in RyR1 that disrupt FKBP12 binding (V2461G and V2461I) on excitation-contraction (EC) coupling and intracellular Ca2+ homeostasis following their expression in skeletal myotubes derived from RyR1-knockout (dyspedic) mice. Wild-type RyR1-, V246I-, and V2461G-expressing myotubes exhibited similar resting Ca2+ levels and maximal responses to caffeine (10 mm) and cyclopiazonic acid (30 microm). However, maximal voltage-gated Ca2+ release in V2461G-expressing myotubes was reduced by approximately 50% compared with that attributable to wild-type RyR1 (deltaF/Fmax = 1.6 +/- 0.2 and 3.1 +/- 0.4, respectively). Dyspedic myotubes expressing the V2461I mutant protein, that binds FKBP12.6 but not FKBP12, exhibited a comparable reduction in voltage-gated SR Ca2+ release (deltaF/Fmax = 1.0 +/- 0.1). However, voltage-gated Ca2+ release in V2461I-expressing myotubes was restored to a normal level (deltaF/Fmax = 2.9 +/- 0.6) following co-expression of FKBP12.6. None of the mutations that disrupted FKBP binding to RyR1 significantly affected RyR1-mediated enhancement of L-type Ca2+ channel activity (retrograde coupling). These data demonstrate that FKBP12 binding to RyR1 enhances the gain of skeletal muscle EC coupling.

Three-dimensional localization of divergent region 3 of the ryanodine receptor to the clamp-shaped structures adjacent to the FKBP binding sites

Of the three divergent regions of ryanodine receptors (RyRs), divergent region 3 (DR3) is the best studied and is believed to be involved in excitation-contraction coupling as well as in channel regulation by Ca(2+) and Mg(2+). To gain insight into the structural basis of DR3 function, we have determined the location of DR3 in the three-dimensional structure of RyR2. We inserted green fluorescent protein (GFP) into the middle of the DR3 region after Thr-1874 in the sequence. HEK293 cells expressing this GFP-RyR2 fusion protein, RyR2(T1874-GFP,) were readily detected by their green fluorescence, indicating proper folding of the inserted GFP. RyR2(T1874-GFP) was further characterized functionally by assays of Ca(2+) release and [(3)H]ryanodine binding. These analyses revealed that RyR2(T1874-GFP) functions as a caffeine- and ryanodine-sensitive Ca(2+) release channel and displays Ca(2+) dependence and [(3)H]ryanodine binding properties similar to those of the wild type RyR2. RyR2(T1874-GFP) was purified from cell lysates in a single step by affinity chromatography using GST-FKBP12.6 as the affinity ligand. The three-dimensional structure of the purified RyR2(T1874-GFP) was then reconstructed using cryoelectron microscopy and single particle image analysis. Comparison of the three-dimensional reconstructions of wild type RyR2 and RyR2(T1874-GFP) revealed the location of the inserted GFP, and hence the DR3 region, in one of the characteristic domains of RyR, domain 9, in the clamp-shaped structure adjacent to the FKBP12 and FKBP12.6 binding sites. COOH-terminal truncation analysis demonstrated that a region between 1815 and 1855 near DR3 is essential for GST-FKBP12.6 binding. These results provide a structural basis for the role of the DR3 region in excitation-contraction coupling and in channel regulation.

Nucleoplasmic calcium regulation in rabbit aortic vascular smooth muscle cells

In this study, we investigated whether nucleoplasmic free Ca2+ in aortic vascular smooth muscle cells (VSMCs) might be independently regulated from cytosolic free Ca2+. Understanding mechanisms and pathways responsible for this regulation is especially relevant given the role of a numerous intranuclear Ca2+-sensitive proteins in transcriptional regulation, apoptosis and cell division. The question of an independent regulatory mechanism remains largely unsettled because the previous use of intensitometric fluorophores (e.g., Fluo-3) has been criticized on technical grounds. To circumvent the potential problem of fluorescence artifact, we utilized confocal laser scanning microscopy to image intracellular Ca2+ movements with the ratiometric fluorophore Indo-1. In cultured rabbit VSMCs, we found sarcoplasmic reticulum (SR) Ca2+ ATPase (SERCA) pumps and ryanodine receptor (RyR) Ca2+ channel proteins to be discretely arranged within a perinuclear locus, as determined by fluorescent staining patterns of BODIPY FL thapsigargin and BODIPY FL-X Ry. When intracellular Ca2+ stores were mobilized by addition of thapsigargin (5 microM) and activatory concentrations of ryanodine (1 microM), Indo-1 ratiometric signals were largely restricted to the nucleoplasm. Cytosolic signals, by comparison, were relatively small and even then its spatial distribution was largely perinuclear rather homogeneous. These observations indicate perinuclear RyR and SERCA proteins are intimately involved in regulating VSMC nucleoplasmic Ca2+ concentrations. We also observed a similar pattern of largely nucleoplasmic Ca2+ mobilization upon exposure of cells to the immunosuppressant drug FK506 (tacrolimus), which binds to the RyR-associated immunophillin-binding proteins FKBP12 and FKBP12.6. However, initial FK506-induced nucleoplasmic Ca2+ mobilization was followed by marked reduction of Indo-1 signal intensity close to pretreatment levels. This suggested FK506 exerts both activatory and inhibitory effects upon RyR channels. The latter was reinforced by observed effects of FK506 to only reduce nucleoplasmic Indo-1 signal intensity when added following pretreatment with both activatory and inhibitory concentrations of ryanodine. These latter observations raise the possibility that VSMC nuclei represent an important sink of intracellular Ca2+ and may help explain vasodilatory actions of FK506 observed by others.

Localization of the 12.6-kDa FK506-binding protein (FKBP12.6) binding site to the NH2-terminal domain of the cardiac Ca2+ release channel (ryanodine receptor)

The 12.6-kDa FK506-binding protein (FKBP12.6) interacts with the cardiac ryanodine receptor (RyR2) and modulates its channel function. However, the molecular basis of FKBP12.6-RyR2 interaction is poorly understood. To investigate the significance of the isoleucine-proline (residues 2427-2428) dipeptide epitope, which is thought to form an essential part of the FKBP12.6 binding site in RyR2, we generated single and double mutants, P2428Q, I2427E/P2428A, and P2428A/L2429E, expressed them in HEK293 cells, and assessed their ability to bind GST-FKBP12.6. None of these mutations abolished GST-FKBP12.6 binding, indicating that this isoleucine-proline motif is unlikely to form the core of the FKBP12.6 binding site in RyR2. To systematically define the molecular determinants of FKBP12.6 binding, we constructed a series of internal and NH(2)- and COOH-terminal deletion mutants of RyR2 and examined the effect of these deletions on GST-FKBP12.6 binding. These deletion analyses revealed that the first 305 NH(2)-terminal residues and COOH-terminal residues 1937-4967 are not essential for GST-FKBP12.6 binding, whereas multiple sequences within a large region between residues 305 and 1937 are required for GST-FKBP12.6 interaction. Furthermore, an NH(2)-terminal fragment containing the first 1937 residues is sufficient for GST-FKBP12.6 binding. Co-expression of overlapping NH(2) and COOH-terminal fragments covering the entire sequence of RyR2 produced functional channels but did not restore GST-FKBP12.6 binding. These data suggest that FKBP12.6 binding is likely to be conformationdependent. Binding of FKBP12.6 to the NH(2)-terminal domain may play a role in stabilizing the conformation of this region.

Modulation of type 1 inositol (1,4,5)-trisphosphate receptor function by protein kinase a and protein phosphatase 1alpha

Type 1 inositol (1,4,5)-trisphosphate receptors (InsP3R1s) play a major role in neuronal calcium (Ca2+) signaling. The InsP3R1s are phosphorylated by protein kinase A (PKA), but the functional consequences of InsP3R1 phosphorylation and the mechanisms that control the phosphorylated state of neuronal InsP3R1s are poorly understood. In a yeast two-hybrid screen of rat brain cDNA library with the InsP3R1-specific bait, we isolated the protein phosphatase 1alpha (PP1alpha). In biochemical experiments, we confirmed the specificity of the InsP3R1-PP1alpha association and immunoprecipitated the InsP3R1-PP1 complex from rat brain synaptosomes and from the neostriatal lysate. We also established that the association with PP1 facilitates dephosphorylation of PKA-phosphorylated InsP3R1 by the endogenous neostriatal PP1 and by the recombinant PP1alpaha. We demonstrated that exposure of neostriatal slices to 8-bromo-cAMP, dopamine, calyculin A, or cyclosporine A, but not to 10 nM okadaic acid, promotes the phosphorylation of neostriatal InsP3R1 by PKA in vivo. We discovered that PKA activates and PP1alpha inhibits the activity of recombinant InsP3R1 reconstituted into planar lipid bilayers. We found that phosphorylation of InsP3R1 by PKA induces at least a fourfold increase in the sensitivity of InsP3R1 to activation by InsP3 without shifting the peak of InsP3R1 bell-shaped Ca2+ dependence. Based on these data, we suggest that InsP3R1 may participate in cross talk between cAMP and Ca2+ signaling in the neostriatum and possibly in other regions of the brain.

The role of calmodulin for inositol 1,4,5-trisphosphate receptor function

Intracellular calcium release is a fundamental signaling mechanism in all eukaryotic cells. The ryanodine receptor (RyR) and inositol 1,4,5-trisphosphate receptor (IP(3)R) are intracellular calcium release channels. Both channels can be regulated by calcium and calmodulin (CaM). In this review we will first discuss the role of calcium as an activator and inactivator of the IP(3)R, concluding that calcium is the most important regulator of the IP(3)R. In the second part we will further focus on the role of CaM as modulator of the IP(3)R, using results of the voltage-dependent Ca(2+) channels and the RyR as reference material. Here we conclude that despite the fact that different CaM-binding sites have been characterized, their function for the IP(3)R remains elusive. In the third part we will discuss the possible functional role of CaM in IP(3)-induced Ca(2+) release (IICR) by direct and indirect mechanisms. Special attention will be given to the Ca(2+)-binding proteins (CaBPs) that were shown to activate the IP(3)R in the absence of IP(3).

Molecular genetics of ryanodine receptors Ca2+-release channels

The family of ryanodine receptor (RyR) genes encodes three highly related Ca(2+)-release channels: RyR1, RyR2 and RyR3. RyRs are known as the Ca(2+)-release channels that participate to the mechanism of excitation-contraction coupling in striated muscles, but they are also expressed in many other cell types. Actually, in several cells two or three RyR isoforms can be co-expressed and interactive feedbacks among them may be important for generation of intracellular Ca(2+) signals and regulation of specific cellular functions. Important developments have been obtained in understanding the biochemical complexity underlying the process of Ca(2+) release through RyRs. The 3-D structure of these large molecules has been obtained and some regulatory regions have been mapped within these 3-D reconstructions. Recent studies have clarified the role of protein kinases and phosphatases that, by physically interacting with RyRs, appear to play a role in the regulation of these Ca(2+)-release channels. These and other recent advancements in understanding RyR biology will be the object of this review.