FK-binding protein is associated with the ryanodine receptor of skeletal muscle in vertebrate animals

Complex Actions of FKBP12 on RyR1 Ion Channel Activity Consistent with Negative Co-Operativity in FKBP12 Binding to the RyR1 Tetramer

The association of the 12 KDa FK506 binding protein (FKBP12) with ryanodine receptor type 1 (RyR1) in skeletal muscle is thought to suppress RyR1 channel opening and contribute to healthy muscle function. The strongest evidence for this role is increased RyR1 channel activity following FKBP12 dissociation. However, the corollary that channel activity will decrease when FKBP12 is added back to FKBP12-depleted RyR1 is not well established, and when reported, the time- and concentration-dependence of inhibition vary over orders of magnitude. Here, we address this problem with an investigation of the molecular mechanisms of the FKBP12 regulation of RyR1. Muscle processing to obtain sarcoplasmic reticulum (SR) vesicle preparations enriched in RyR1 resulted in substantial FKBP12 dissociation from RyR1, indicating low-affinity binding. Conversely, high-affinity binding was indicated by some FKBP12 remaining bound to RyR1 after solubilization. We report, for the first time, an increase in the activity of FKBP12-depleted channels after the addition of exogenous FKBP12 (5 nM to 5 µM), followed by a reduction in activity consistent with inhibition after 20-30 min exposure to higher [FKBP12]s. Both the increase and later decline in activity were time- and concentration-dependent. The results suggest a high-affinity activation when FKBP12 binding sites on the RyR1 tetramer are partially occupied by FKBP12 and lower affinity inhibition as more RyR1 monomers become occupied. These novel results imply negative cooperativity in FKBP12 binding to RyR1 and a dynamic role for FKBP12/RyR1 interactions in intact muscle fibers.

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'.

Mitochondrial Disruption by Amyloid Beta 42 Identified by Proteomics and Pathway Mapping

Alzheimer’s disease (AD) is marked by chronic neurodegeneration associated with the occurrence of plaques containing amyloid β (Aβ) proteins in various parts of the human brain. An increase in several Aβ fragments is well documented in patients with AD and anti-amyloid targeting is an emerging area of therapy. Soluble Aβ can bind to various cell surface and intracellular molecules with the pathogenic Aβ₄₂ fragment leading to neurotoxicity. Here we examined the effect of Aβ₄₂ on network adaptations in the proteome of nerve growth factor (NGF) differentiated PC12 cells using liquid-chromatography electrospray ionization mass spectrometry (LC-ESI MS/MS) proteomics. Whole-cell peptide mass fingerprinting was coupled to bioinformatic gene set enrichment analysis (GSEA) in order to identify differentially represented proteins and related gene ontology (GO) pathways within Aβ₄₂ treated cells. Our results underscore a role for Aβ₄₂ in disrupting proteome responses for signaling, bioenergetics, and morphology in mitochondria. These findings highlight the specific components of the mitochondrial response during Aβ₄₂ neurotoxicity and suggest several new biomarkers for detection and surveillance of amyloid disease.

Ryanodine receptor dysfunction in human disorders

Regulation of intracellular calcium (Ca²⁺) is critical in all cell types. The ryanodine receptor (RyR), an intracellular Ca²⁺ release channel located on the sarco/endoplasmic reticulum (SR/ER), releases Ca²⁺ from intracellular stores to activate critical functions including muscle contraction and neurotransmitter release. Dysfunctional RyR-mediated Ca²⁺ handling has been implicated in the pathogenesis of inherited and non-inherited conditions including heart failure, cardiac arrhythmias, skeletal myopathies, diabetes, and neurodegenerative diseases. Here we have reviewed the evidence linking human disorders to RyR dysfunction and describe novel approaches to RyR-targeted therapeutics.

Ryanodine receptor and FK506 binding protein 1 in the Atlantic killifish (Fundulus heteroclitus): A phylogenetic and population-based comparison

Non-dioxin-like polychlorinated biphenyls (NDL PCBs) activate ryanodine receptors (RyR), microsomal Ca²⁺ channels of broad significance. Teleost fish may be important models for NDL PCB neurotoxicity, and we used sequencing databases to characterize teleost RyR and FK506 binding protein 12 or 12.6kDa (genes FKBP1A; FKBP1B), which promote NDL PCB-triggered Ca²⁺ dysregulation. Particular focus was placed on describing genes in the Atlantic killifish (Fundulus heteroclitus) genome and searching available RNA-sequencing datasets for single nucleotide variants (SNV) between PCB tolerant killifish from New Bedford Harbor (NBH) versus sensitive killifish from Scorton Creek (SC), MA. Consistent with the teleost whole genome duplication (tWGD), killifish have six RyR genes, corresponding to a and b paralogs of mammalian RyR1, 2 and 3. The presence of six RyR genes was consistent in all teleosts investigated including zebrafish. Killifish have four FKBP1; one FKBP1b and three FKBP1a named FKBP1aa, FKBP1ab, likely from the tWGD and a single gene duplicate FKBP1a3 suggested to have arisen in Atherinomorphae. The RyR and FKBP1 genes displayed tissue and developmental stage-specific mRNA expression, and the previously uncharacterized RyR3, herein named RyR3b, and all FKBP1 genes were prominent in brain. We identified a SNV in RyR3b encoding missense mutation E1458D. In NBH killifish, 57% were heterozygous and 28% were homozygous for this SNV, whereas almost all SC killifish (94%) lacked the variant (n≥39 per population). The outlined sequence differences between mammalian and teleost RyR and FKBP1 together with outlined population differences in SNV frequency may contribute to our understanding of NDL PCB neurotoxicity.

PEP-1-FK506BP12 inhibits matrix metalloproteinase expression in human articular chondrocytes and in a mouse carrageenan-induced arthritis model

The 12 kDa FK506-binding protein (FK506BP12), an immunosuppressor, modulates T cell activation via calcineurin inhibition. In this study, we investigated the ability of PEP-1-FK506BP12, consisting of FK506BP12 fused to the protein transduction domain PEP-1 peptide, to suppress catabolic responses in primary human chondrocytes and in a mouse carrageenan-induced paw arthritis model. Western blotting and immunofluorescence analysis showed that PEP-1-FK506BP12 efficiently penetrated chondrocytes and cartilage explants. In interleukin-1β (IL-1β)-treated chondrocytes, PEP-1-FK506BP12 significantly suppressed the expression of catabolic enzymes, including matrix metalloproteinases (MMPs)-1, -3, and -13 in addition to cyclooxygenase-2, at both the mRNA and protein levels, whereas FK506BP12 alone did not. In addition, PEP-1-FK506BP12 decreased IL-1β-induced phosphorylation of the mitogen-activated protein kinase (MAPK) complex (p38, JNK, and ERK) and the inhibitor kappa B alpha. In the mouse model of carrageenan-induced paw arthritis, PEP-1-FK506BP12 suppressed both carrageenan-induced MMP-13 production and paw inflammation. PEP-1-FK506BP12 may have therapeutic potential in the alleviation of OA progression. [BMB Reports 2015; 48(7): 407-412]

Expression and function of ryanodine receptor related pathways in PCB tolerant Atlantic killifish (Fundulus heteroclitus) from New Bedford Harbor, MA, USA

Atlantic killifish (Fundulus heteroclitus) thrive in New Bedford Harbor (NBH), MA, highly contaminated with polychlorinated biphenyls (PCBs). Resident killifish have evolved tolerance to dioxin-like (DL) PCBs, whose toxic effects through the aryl hydrocarbon receptor (AhR) are well studied. In NBH, non-dioxin like PCBs (NDL PCBs), which lack activity toward the AhR, vastly exceed levels of DL congeners yet how killifish counter NDL toxic effects has not been explored. In mammals and fish, NDL PCBs are potent activators of ryanodine receptors (RyR), Ca(2+) release channels necessary for a vast array of physiological processes. In the current study we compared the expression and function of RyR related pathways in NBH killifish with killifish from the reference site at Scorton Creek (SC, MA). Relative to the SC fish, adults from NBH displayed increased levels of skeletal muscle RyR1 protein, and increased levels of FK506-binding protein 12 kDa (FKBP12) an accessory protein essential for NDL PCB-triggered changes in RyR channel function. In accordance with increased RyR1 levels, NBH killifish displayed increased maximal ligand binding, increased maximal response to Ca(2+) activation and increased maximal response to activation by the NDL PCB congener PCB 95. Compared to SC, NBH embryos and larvae had increased levels of mtor and ryr2 transcripts at multiple stages of development, and generations, while levels of serca2 were decreased at 9 days post-fertilization in the F1 and F2 generations. These findings suggest that there are compensatory and heritable changes in RyR mediated Ca(2+) signaling proteins or potential signaling partners in NBH killifish.

Structure-activity relationship of non-coplanar polychlorinated biphenyls toward skeletal muscle ryanodine receptors in rainbow trout (Oncorhynchus mykiss)

Research addressing the health impacts of polychlorinated biphenyls (PCBs) has primarily focused on the effects of coplanar, or dioxin-like (DL), congeners, which is especially true for research assessing impacts in fish species. Ortho substituted non-coplanar, termed non-dioxin-like (NDL), PCBs have received less attention. In mammals, NDL PCBs enhance the activity of ryanodine receptors (RyR), calcium release channels necessary for engaging excitation-contraction (EC) coupling in striated muscle. We utilized in vitro receptor binding analysis to determine whether NDL PCB congeners detected in aquatic environments alter the activity of RyR isoform 1 (RyR1) found in the skeletal muscle of rainbow trout. Congeners 52, 95, 136, and149 were the most efficacious leading to an increase in receptor activity that was approximately 250% greater than that found under solvent control conditions. Other environmentally relevant congeners, namely PCB 153, 151 and 101, which all contain two or more chlorines in the ortho-position, enhanced receptor activity by greater than 160% of baseline. The mono-ortho congeners or the non-ortho PCB 77 had negligible impact on the RyR1. When combined, in binary or environmentally relevant mixtures, congeners shown to enhance receptor activity appeared to display additivity and when the active PCB 95 was present with the non-active congener PCB 77 the impact on receptor activity was reduced from 250% to 230%. The important role of the RyR and the demonstrated additive nature of NDL congeners toward altering channel function calls for further investigation into the ecological implications of altered RyR function in fish with high PCB burdens.

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: structure, expression, molecular details, and function in calcium release

Ryanodine receptors (RyRs) are located in the sarcoplasmic/endoplasmic reticulum membrane and are responsible for the release of Ca(2+) from intracellular stores during excitation-contraction coupling in both cardiac and skeletal muscle. RyRs are the largest known ion channels (> 2MDa) and exist as three mammalian isoforms (RyR 1-3), all of which are homotetrameric proteins that interact with and are regulated by phosphorylation, redox modifications, and a variety of small proteins and ions. Most RyR channel modulators interact with the large cytoplasmic domain whereas the carboxy-terminal portion of the protein forms the ion-conducting pore. Mutations in RyR2 are associated with human disorders such as catecholaminergic polymorphic ventricular tachycardia whereas mutations in RyR1 underlie diseases such as central core disease and malignant hyperthermia. This chapter examines the current concepts of the structure, function and regulation of RyRs and assesses the current state of understanding of their roles in associated disorders.

Mitochondrial Ca2+ transport and permeability transition in zebrafish (Danio rerio)

We have studied mitochondrial Ca(2+) transport and the permeability transition (PT) in the teleost zebrafish (Danio rerio), a key model system for human diseases. Permeabilized zebrafish embryo cells displayed a mitochondrial energy-dependent Ca(2+) uptake system that, like the Ca(2+) uniporter of mammals, was inhibited by ruthenium red. Zebrafish mitochondria underwent a Ca(2+)-dependent PT that displayed Pi-dependent desensitization by cyclosporin A, and responded appropriately to key modulators of the mammalian PT pore (voltage, pH, ubiquinone 0, dithiol oxidants and cross linkers, ligands of the adenine nucleotide translocator, arachidonic acid). Opening of the pore was documented in intact cells, where it led to death that could largely be prevented by cyclosporin A. Our results represent a necessary step toward the use of zebrafish for the screening and validation of PTP inhibitors of potential use in human diseases, as recently shown for collagen VI muscular dystrophy [Telfer et al., 2010].

Mapping the ryanodine receptor FK506-binding protein subunit using fluorescence resonance energy transfer

The 12-kDa FK506-binding proteins (FKBP12 and FKBP12.6) are regulatory subunits of ryanodine receptor (RyR) Ca(2+) release channels. To investigate the structural basis of FKBP interactions with the RyR1 and RyR2 isoforms, we used site-directed fluorescent labeling of FKBP12.6, ligand binding measurements, and fluorescence resonance energy transfer (FRET). Single-cysteine substitutions were introduced at five positions distributed over the surface of FKBP12.6. Fluorescent labeling at position 14, 32, 49, or 85 did not affect high affinity binding to the RyR1. By comparison, fluorescent labeling at position 41 reduced the affinity of FKBP12.6 binding by 10-fold. Each of the five fluorescent FKBPs retained the ability to inhibit [(3)H]ryanodine binding to the RyR1, although the maximal extent of inhibition was reduced by half when the label was attached at position 32. The orientation of FKBP12.6 bound to the RyR1 and RyR2 was examined by measuring FRET from the different labeling positions on FKBP12.6 to an acceptor attached within the RyR calmodulin subunit. FRET was dependent on the position of fluorophore attachment on FKBP12.6; however, for any given position, the distance separating donors and acceptors bound to RyR1 versus RyR2 did not differ significantly. Our results show that FKBP12.6 binds to RyR1 and RyR2 in the same orientation and suggest new insights into the discrete structural domains responsible for channel binding and inhibition. FRET mapping of RyR-bound FKBP12.6 is consistent with the predictions of a previous cryoelectron microscopy study and strongly supports the proposed structural model.

Personal recollections on the discovery of the ryanodine receptors of muscle

The intracellular Ca(2+) release channels are indispensable molecular machinery in practically all eukaryotic cells of multicellular animals. They serve a key role in cell signaling by way of Ca(2+) as a second messenger. In response to a signaling event, the channels release Ca(2+) from intracellular stores. The resulting rise in cytoplasmic Ca(2+) concentration triggers the cell to carry out its specialized role, after which the intracellular Ca(2+) concentration must be reduced so that the signaling event can again be repeated. There are two types of intracellular Ca(2+) release channels, i.e., the ryanodine receptors and the inositol triphosphate receptors. My focus in this minireview is to present a personal account, from the vantage point our laboratory, of the discovery, isolation, and characterization of the ryanodine receptors from mammalian muscle. There are three isoforms: ryanodine receptor 1 (RyR1), first isolated from rabbit fast twitch skeletal muscle; ryanodine receptor 2 (RyR2), first isolated from dog heart; and ryanodine receptor 3 (RyR3), first isolated from bovine diaphragm muscle. The ryanodine receptors are the largest channel structures known. The RyR isoforms are very similar albeit with important differences. Natural mutations in humans in these receptors have already been associated with a number of muscle diseases.

Ryanodine receptor interaction with the SNARE-associated protein snapin

The ryanodine receptor (RyR) is a widely expressed intracellular calcium (Ca(2+))-release channel regulating processes such as muscle contraction and neurotransmission. Snapin, a ubiquitously expressed SNARE-associated protein, has been implicated in neurotransmission. Here, we report the identification of snapin as a novel RyR2-interacting protein. Snapin binds to a 170-residue predicted ryanodine receptor cytosolic loop (RyR2 residues 4596-4765), containing a hydrophobic segment required for snapin interaction. Ryanodine receptor binding of snapin is not isoform specific and is conserved in homologous RyR1 and RyR3 fragments. Consistent with peptide fragment studies, snapin interacts with the native ryanodine receptor from skeletal muscle, heart and brain. The snapin-RyR1 association appears to sensitise the channel to Ca(2+) activation in [(3)H]ryanodine-binding studies. Deletion analysis indicates that the ryanodine receptor interacts with the snapin C-terminus, the same region as the SNAP25-binding site. Competition experiments with native ryanodine receptor and SNAP25 suggest that these two proteins share an overlapping binding site on snapin. Thus, regulation of the association between ryanodine receptor and snapin might constitute part of the elusive molecular mechanism by which ryanodine-sensitive Ca(2+) stores modulate neurosecretion.

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.

Structural Characterization of the RyR1–FKBP12 Interaction

The 12 kDa FK506-binding protein (FKBP12) constitutively binds to the calcium release channel RyR1. Removal of FKBP12 using FK506 or rapamycin causes an increased open probability and an increase in the frequency of sub-conductance states in RyR1. Using cryo-electron microscopy and single-particle image processing, we have determined the 3D difference map of FKBP12 associated with RyR1 at 16 A resolution that can be fitted with the atomic model of FKBP12 in a unique orientation. This has allowed us to better define the surfaces of close apposition between FKBP12 and RyR1. Our results shed light on the role of several FKBP12 residues that had been found critical for the specificity of the RyR1-FKBP12 interaction. As predicted from previous immunoprecipitation studies, our results suggest that Gln3 participates directly in this interaction. The orientation of RyR1-bound FKBP12, with part of its FK506 binding site facing towards RyR1, allows us to propose how FK506 is involved in the dissociation of FKBP12 from RyR1.

Depletion of FKBP does not affect the interaction between isolated ryanodine receptors

The ryanodine receptors/calcium release channels (RyRs) usually form two dimensional regular lattices in the endoplasmic/sarcoplasmic reticulum membranes. The native RyR is associated with many auxiliary proteins, including FKBP. It has been indicated that FKBP may play a role in the intermolecular interaction and coupled gating of neighboring RyRs. However, a more recent study shows that FKBP12 is not involved in the physical linkage between neighboring RyR1s. In the present work, the effect of FKBP12 on the interaction between RyR1s isolated from rabbit skeletal muscle was investigated in an aqueous medium with photon correlation spectroscopy. We found that the depletion of FKBP12 did not affect the oligomerization of RyR1s in the medium containing different [KCl] or under different channel functional states. No evidence is obtained for the involvement of FKBP12 in the intermolecular interaction between RyR1s.

Physical Coupling between Ryanodine Receptor–Calcium Release Channels

Ryanodine receptor-calcium release channels play a pivotal role in the calcium signaling that mediates muscle excitation-contraction coupling. Their membrane organization into regular patterns, functional gating studies and theoretical analysis of receptor clustering have led to models that invoke allosteric interaction between individual channel oligomers as a critical mechanism for control of calcium release. Here we show that in reconstituted "checkerboard-like" lattices that mimic in situ membrane channel arrays, each oligomer is interlocked physically with four adjacent oligomers via a specific domain-domain interaction. Direct physical coupling between ryanodine receptors provides structural evidence for an inter-oligomer allosteric mechanism in channel regulation. Therefore, in addition to established cytosolic and luminal regulation of function, these observations indicate that channel-channel communication through physical coupling provides a novel mode of regulation of intracellular calcium release channels.

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.

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.

The skeletal muscle ryanodine receptor isoform 1 is found at the intercalated discs in human and mouse hearts

The ryanodine receptors (RyRs) are a class of intracellular calcium release channels of which there are three isoforms. In striated muscle, isoform 1 and isoform 2 are mainly expressed in the terminal cisternae of sarcoplasmic reticulum of skeletal muscle and heart, respectively. Isoform 3 is widely distributed in tissues but in minuscule amounts. These channels release calcium ions from intracellular stores in excitation-contraction coupling for cell signaling. Here, we report the presence of skeletal muscle isoform 1 localized in the intercalated discs (IDs) of human and mouse hearts. By using RyR1 and connexin43 specific antibodies and dual immunofluorescent techniques, both were localized in the proximity of the IDs of human and mouse hearts. We confirmed that RyR1 is localized to the IDs by selective immunoprecipitation of RyR isoform 1 from a subcellular fraction containing IDs from human heart tissue. The functional significance of our observation remains to be elucidated as isoform 1 is involved in depolarization induced calcium release, unlike RyR isoforms 2 and 3 which appear to be involved in calcium induced calcium release.

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.

Ryanodine receptor calcium release channels

The ryanodine receptors (RyRs) are a family of Ca2+ release channels found on intracellular Ca2+ storage/release organelles. The RyR channels are ubiquitously expressed in many types of cells and participate in a variety of important Ca2+ signaling phenomena (neurotransmission, secretion, etc.). In striated muscle, the RyR channels represent the primary pathway for Ca2+ release during the excitation-contraction coupling process. In general, the signals that activate the RyR channels are known (e.g., sarcolemmal Ca2+ influx or depolarization), but the specific mechanisms involved are still being debated. The signals that modulate and/or turn off the RyR channels remain ambiguous and the mechanisms involved unclear. Over the last decade, studies of RyR-mediated Ca2+ release have taken many forms and have steadily advanced our knowledge. This robust field, however, is not without controversial ideas and contradictory results. Controversies surrounding the complex Ca2+ regulation of single RyR channels receive particular attention here. In addition, a large body of information is synthesized into a focused perspective of single RyR channel function. The present status of the single RyR channel field and its likely future directions are also discussed.

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

We compared the interaction of the FK506-binding protein (FKBP) with the type 3 ryanodine receptor (RyR3) and with the type 1 and type 3 inositol 1,4,5-trisphosphate receptor (IP(3)R1 and IP(3)R3), using a quantitative GST-FKBP12 and GST-FKBP12.6 affinity assay. We first characterized and mapped the interaction of the FKBPs with the RyR3. GST-FKBP12 as well as GST-FKBP12.6 were able to bind approximately 30% of the solubilized RyR3. The interaction was completely abolished by FK506, strengthened by the addition of Mg(2+), and weakened in the absence of Ca(2+) but was not affected by the addition of cyclic ADP-ribose. By using proteolytic mapping and site-directed mutagenesis, we pinpointed Val(2322), located in the central modulatory domain of the RyR3, as a critical residue for the interaction of RyR3 with FKBPs. Substitution of Val(2322) for leucine (as in IP(3)R1) or isoleucine (as in RyR2) decreased the binding efficiency and shifted the selectivity to FKBP12.6; substitution of Val(2322) for aspartate completely abolished the FKBP interaction. Importantly, the occurrence of the valylprolyl residue as alpha-helix breaker was an important determinant of FKBP binding. This secondary structure is conserved among the different RyR isoforms but not in the IP(3)R isoforms. A chimeric RyR3/IP(3)R1, containing the core of the FKBP12-binding site of IP(3)R1 in the RyR3 context, retained this secondary structure and was able to interact with FKBPs. In contrast, IP(3)Rs did not interact with the FKBP isoforms. This indicates that the primary sequence in combination with the local structural environment plays an important role in targeting the FKBPs to the intracellular Ca(2+)-release channels. Structural differences in the FKBP-binding site of RyRs and IP(3)Rs may contribute to the occurrence of a stable interaction between RyR isoforms and FKBPs and to the absence of such interaction with IP(3)Rs.

FKBP12 associates tightly with the skeletal muscle type 1 ryanodine receptor, but not with other intracellular calcium release channels

This study compared the relative levels of ryanodine receptor (RyR) isoforms, inositol 1,4,5-trisphosphate receptor (IP(3)R) isoforms, and calcineurin, plus their association with FKBP12 in brain, skeletal and cardiac tissue. FKBP12 demonstrated a very tight, high affinity association with skeletal muscle microsomes, which was displaced by FK506. In contrast, FKBP12 was not tightly associated with brain or cardiac microsomes and did not require FK506 for removal from these organelles. Furthermore, of the proteins solubilised from skeletal muscle, cardiac muscle and brain microsomes, only skeletal muscle RyR1 bound to an FKBP12-glutathione-S-transferase fusion protein, in a high affinity FK506 displaceable manner. These results suggest that RyR1 has distinctive FKBP12 binding properties when compared to RyR2, RyR3, all IP(3)R isoforms and calcineurin.

Role of FK506-binding protein 12 in development of the chick embryonic heart

A cDNA encoding chicken FK506-binding protein 12 (FKBP12) was isolated and sequenced. The predicted amino acid sequence of the chicken protein shows high homology to those of FKBP12 proteins of other species ranging from human to frog. The possible role of FKBP12 in chick embryonic cardiac development was examined. Northern blot analysis revealed that FKBP12 mRNA is distributed widely in chick embryos, being especially abundant in the heart; the amount of FKBP12 mRNA in the embryonic heart decreased with time. Administration of FK506 to chick embryos at 7 to 9 days resulted in marked cardiac enlargement. FK506 also reduced the expression of myosin, induced a more elongated cell morphology, and impaired network formation in cultured chick embryonic cardiomyocytes. These results suggest that FKBP12 is important in the regulation of contractile function and phenotypic expression in chick cardiomyocytes during embryonic development.

FKBP binding characteristics of cardiac microsomes from diverse vertebrates

FK506 binding protein (FKBP) is a cytosolic receptor for the immunosuppressive drug FK-506. The common isoform, FKBP12, was found to be associated with the calcium release channel (ryanodine receptor 1) of different species of vertebrate skeletal muscle, whereas 12.6, a novel FKBP isoform was found to be associated with canine cardiac ryanodine receptor (ryanodine receptor 2). Until recently, canine cardiac sarcoplasmic reticulum was considered to be the prototype for studying heart RyR2 and its interactions with FKBP. In this study, cardiac microsomes were isolated from diverse vertebrates: human, rabbit, rat, mice, dog, chicken, frog, and fish and were analyzed for their ability to bind or exchange with FKBP isoforms 12 and 12.6. Our studies indicate that RyR2 from seven out of the eight animals contain both FKBP12 and 12.6. Dog is the exception. It can now be concluded that the association of FKBP isoforms with RyR2 is widely conserved in the hearts of different species of vertebrates.

Overexpression of FK506-binding protein FKBP12.6 in cardiomyocytes reduces ryanodine receptor-mediated Ca(2+) leak from the sarcoplasmic reticulum and increases contractility

The FK506-binding protein FKBP12.6 is tightly associated with the cardiac sarcoplasmic reticulum (SR) Ca(2+)-release channel (ryanodine receptor type 2 [RyR2]), but the physiological function of FKBP12.6 is unclear. We used adenovirus (Ad)-mediated gene transfer to overexpress FKBP12.6 in adult rabbit cardiomyocytes. Western immunoblot and reverse transcriptase-polymerase chain reaction analysis revealed specific overexpression of FKBP12.6, with unchanged expression of endogenous FKBP12. FKBP12.6-transfected myocytes displayed a significantly higher (21%) fractional shortening (FS) at 48 hours after transfection compared with Ad-GFP-infected control cells (4.8+/-0.2% FS versus 4+/-0.2% FS, respectively; n=79 each; P:=0.001). SR-Ca(2+) uptake rates were monitored in beta-escin-permeabilized myocytes using Fura-2. Ad-FKBP12.6-infected cells showed a statistically significant higher rate of Ca(2+) uptake of 0.8+/-0.09 nmol/s(-)(1)/10(6) cells (n=8, P:<0.05) compared with 0.52+/-0.1 nmol/s(-)(1)/10(6) cells in sham-infected cells (n=8) at a [Ca(2+)] of 1 micromol/L. In the presence of 5 micromol/L ruthenium red to block Ca(2+) efflux via RyR2, SR-Ca(2+) uptake rates were not significantly different between groups. From these measurements, we calculate that SR-Ca(2+) leak through RyR2 is reduced by 53% in FKBP12.6-overexpressing cells. Caffeine-induced contractures were significantly larger in Ad-FKBP12.6-infected myocytes compared with Ad-GFP-infected control cells, indicating a higher SR-Ca(2+) load. Taken together, these data suggest that FKBP12.6 stabilizes the closed conformation state of RyR2. This may reduce diastolic SR-Ca(2+) leak and consequently increase SR-Ca(2+) release and myocyte shortening.

Immunophilins: switched on protein binding domains?

Peptidylprolyl isomerases (PPIases) are a group of cytosolic enzymes first characterized by their ability to catalyze the cis-trans isomerization of cis-peptidylprolyl bonds. Subsequently, some PPIases were also identified as the initial targets of the immunosuppressant drugs-cyclosporin A (CsA), FK506, and rapamycin-have been called immunophilins. Immunophilins have been found to be both widely distributed and abundantly expressed leading to suggestions that they may play a general role in cellular biochemistry. However, the nature of this role has been difficult to elucidate and is still controversial in vivo. A number of roles for these enzymes have been identified in vitro including the ability to catalyze the refolding of partly denatured proteins and stabilize multiprotein complexes such as Ca(2+) channels, inactive steroid receptor complexes, and receptor protein tyrosine kinases. Generally, these effects appear to depend on the ability of immunophilins to selectively bind to other proteins. This review will examine in detail experimental and structural investigations of the mechanism of PPIase activity for both FKBPs and cyclophilins and suggest a mechanism for these enzymes, which depends on their ability to recognize a specific peptide conformation rather than sequence. Examination of structures of immunophilin-protein complexes will then be used to further suggest that the ability of these enzymes to recognize specific peptide conformations is central to the formation of these complexes and may constitute a general function of immunophilin enzymes. The binding of ligand to immunophilins will also be shown to stabilize specific conformations in surface loops of these proteins that are observed to play a critical role in a number of immunophilin-protein complexes suggesting that the immunophilins may constitute a class of ligand-triggered selective protein binders.

Effects of the immunosuppressant FK506 on intracellular Ca2+ release and Ca2+ accumulation mechanisms

The immunophilin FKBP12 associates with intracellular Ca2+ channels and this interaction can be disrupted by the immunosuppressant FK506. We have investigated the effect of FK506 on Ca2+ release and Ca2+ uptake in permeabilized cell types. Changes in medium free [Ca2+] were detected by the fluorescent Ca2+ indicator fluo-3 in digitonin-permeabilized SH-SY5Y human neuroblastoma cells, DT40 and R23-11 (i.e. triple inositol 1,4,5-trisphosphate (IP3) receptor knockout cells) chicken B lymphocytes and differentiated and undifferentiated BC3H1 skeletal muscle cells. 45Ca2+ fluxes were studied in saponin-permeabilized A7r5 rat smooth muscle cells. Addition of FK506 to permeabilized SH-SY5Y cells led to a sustained elevation of the medium [Ca2+] corresponding to approximately 30 % of the Ca2+ ionophore A23187-induced [Ca2+] rise. This rise in [Ca2+] was not dependent on mitochondrial activity. This FK506-induced [Ca2+] rise was related to the inhibition of the sarcoplasmic/endoplasmic reticulum Ca2+-Mg2+-ATPase (SERCA) Ca2+ pump. Oxalate-facilitated 45Ca2+ uptake in SH-SY5Y microsomes was inhibited by FK506 with an IC50 of 19 microM. The inhibition of the SERCA Ca2+ pump was not specific since several macrocyclic lactone compounds (ivermectin > FK506, ascomycin and rapamycin) were able to inhibit Ca2+ uptake activity. FK506 (10 microM) did not affect IP3-induced Ca2+ release in permeabilized SH-SY5Y and A7r5 cells, but enhanced caffeine-induced Ca2+ release via the ryanodine receptor (RyR) in differentiated BC3H1 cells. In conclusion, FK506 inhibited active Ca2+ uptake by the SERCA Ca2+ pump; in addition, FK506 enhanced intracellular Ca2+ release through the RyR, but it had no direct effect on IP3-induced Ca2+ release.

Three-dimensional structure of ryanodine receptor isoform three in two conformational states as visualized by cryo-electron microscopy

Using cryo-electron microscopy and single particle image processing techniques, we present the first three-dimensional reconstructions of isoform 3 of the ryanodine receptor/calcium release channel (RyR3). Reconstructions were carried out on images obtained from a purified, detergent-solubilized receptor for two different buffer conditions, which were expected to favor open and closed functional states of the channel. As for the heart (RyR2) and skeletal muscle (RyR1) receptor isoforms, RyR3 is a homotetrameric complex comprising two main components, a multidomain cytoplasmic assembly and a smaller ( approximately 20% of the total mass) transmembrane region. Although the isoforms show structural similarities, consistent with the approximately 70% overall sequence identity of the isoforms, detailed comparisons of RyR3 with RyR1 showed one region of highly significant difference between them. This difference indicated additional mass present in RyR1, and it likely corresponds to a region of the RyR1 sequence (residues 1303-1406, known as diversity region 2) that is absent from RyR3. The reconstructions of RyR3 determined under "open" and "closed" conditions were similar to each other in overall architecture. A difference map computed between the two reconstructions reveals subtle changes in conformation at several widely dispersed locations in the receptor, the most prominent of which is a approximately 4 degrees rotation of the transmembrane region with respect to the cytoplasmic assembly.

Molecular cloning of cDNA encoding a drosophila ryanodine receptor and functional studies of the carboxyl-terminal calcium release channel

Ryanodine is a plant alkaloid that was originally used as an insecticide. To study the function and regulation of the ryanodine receptor (RyR) from insect cells, we have cloned the entire cDNA sequence of RyR from the fruit fly Drosophila melanogaster. The primary sequence of the Drosophila RyR contains 5134 amino acids, which shares approximately 45% identity with RyRs from mammalian cells, with a large cytoplasmic domain at the amino-terminal end and a small transmembrane domain at the carboxyl-terminal end. To characterize the Ca(2+) release channel activity of the cloned Drosophila RyR, we expressed both full-length and a deletion mutant of Drosophila RyR lacking amino acids 277-3650 (Drosophila RyR-C) in Chinese hamster ovary cells. For subcellular localization of the expressed Drosophila RyR and Drosophila RyR-C proteins, green fluorescent protein (GFP)-Drosophila RyR and GFP-Drosophila RyR-C fusion constructs were generated. Confocal microscopic imaging identified GFP-Drosophila RyR and GFP-Drosophila RyR-C on the endoplasmic reticulum membranes of transfected cells. Upon reconstitution into the lipid bilayer membrane, Drosophila RyR-C formed a large conductance cation-selective channel, which was sensitive to modulation by ryanodine. Opening of the Drosophila RyR-C channel required the presence of microM concentration of Ca(2+) in the cytosolic solution, but the channel was insensitive to inhibition by Ca(2+) at concentrations as high as 20 mM. Our data are consistent with our previous observation with the mammalian RyR that the conduction pore of the calcium release channel resides within the carboxyl-terminal end of the protein and further demonstrate that structural and functional features are essentially shared by mammalian and insect RyRs.

Pharmacological clues to calmodulin-mediated activation of skeletal ryanodine receptor using [3H]-ryanodine binding

The hypothesis that calmodulin (CaM) may act as a positive modulator of junctional SR Ca2+-release channel/ ryanodine receptor (RyRl) rests largerly on the demonstrated capacity of CaM to interact structurally and functionally with RyRl at pCa > 8 (Tripathy et al., 1995). The goal of the present [3H]-ryanodine binding study was to produce, in isolated terminal cisternae (TC) and in purified junctional face membrane (JFM), CaM-mediated activation of RyRl at less extreme pCa values, i.e. closer to resting myoplasmic pCa, and to analyze more accurately the corresponding changes in binding affinity for ryanodine of the receptor. We were able to monitor these changes at an optimum pCa of 6.5, following pre-activation of native RyRl by mM concentrations of caffeine or microM concentrations of antraquinone compound doxorubicin, and at various doses of these triggers. CaM increased the affinity of ryanodine binding to isolated TC in the presence of 1 mM AMP-PCP as an activator of RyRl; the Kd for ryanodine binding was reduced from 21.8 nM to 13.2 nM by 1microM CaM. Similar effects of CaM were seen when AMP-PCP was replaced by either caffeine or doxorubicin. In order to discount the involvement of SR extrajunctional proteins in this effect, the experiments were repeated on purified JFM. Again, CaM increased the affinity of ryanodine binding; the Kd was reduced from 11.1 nM to 7.0 nM by 1 microM CaM (in the presence of doxorubicin). Pharmacological triggers of CaM-activatory action on native RyRl, like caffeine and doxorubicin, have long been characterized for their ability to activate RyRl by increasing the Ca2+-sensitivity of the receptor. We speculate that the triggering effect of these agents on the CaM-mediated mechanism in vitro might mimick one of the early effects of the activation of RyRl in skeletal muscle, during E-C coupling.

Differential effects of insulin-like growth factor I (IGF-I) and IGF-binding protein-1 on protein metabolism in human skeletal muscle cells

Insulin-like growth factor-binding protein-1 (BP-1) is a multifunctional protein that binds IGF-I in solution and integrins on the cell surface. BP-1 is overexpressed during catabolic illnesses, and the protein accumulates in skeletal muscle. To define a potential physiological role for BP-1 in regulating muscle protein balance, we have examined the effect of IGF-I and BP-1 on protein synthesis and degradation in human skeletal muscle cells. IGF-I-stimulated protein synthesis by 20%, and this was completely inhibited by either phosphorylated or nonphosphorylated BP-1. Half-maximal inhibition of protein synthesis occurred at a molar ratio of BP-1 to IGF-I of 1.5:1. BP-1 failed to form a complex with a truncated form of IGF-I (desIGF-I), and consequently, BP-1 failed to inhibit the ability of desIGF-I to stimulate protein synthesis. IGF-I and BP-1 dose-dependently inhibited protein degradation individually, and both BP-1 phosphovariants failed to block the ability of IGF-I to do the same. Blocking integrin receptor occupancy with the integrin antagonist echistatin blunted the ability of BP-1 to inhibit protein degradation, but had no significant effect on IGF-I-mediated changes in protein synthesis or degradation. The extracellular matrix protein vitronectin also inhibited protein degradation, but vitronectin receptor antibodies failed to block BP-1 action. In contrast, antibodies to the beta1 integrin subunit blocked BP-1-mediated inhibition of protein degradation. Rapamycin inhibited IGF-I-dependent protein synthesis, but not the ability of IGF-I to inhibit proteolysis. In contrast, rapamycin completely blocked the ability of BP-1 to inhibit proteolysis. Our results demonstrate that BP-1 inhibits IGF-I-mediated protein synthesis by binding to IGF-I. BP-1, acting independently of IGF-I, inhibits protein degradation. The IGF-independent response occurs via beta1 integrin binding and stimulation of a rapamycin-sensitive signal transduction pathway.