Dantrolene rescues aberrant N-terminus intersubunit interactions in mutant pro-arrhythmic cardiac ryanodine receptors

Molecular Aspects Implicated in Dantrolene Selectivity with Respect to Ryanodine Receptor Isoforms

Dantrolene is an intra-cellularly acting skeletal muscle relaxant used for the treatment of the rare genetic disorder, malignant hyperthermia (MH). In most cases, MH susceptibility is caused by dysfunction of the skeletal ryanodine receptor (RyR1) harboring one of nearly 230 single-point MH mutations. The therapeutic effect of dantrolene is the result of a direct inhibitory action on the RyR1 channel, thus suppressing aberrant Ca2+ release from the sarcoplasmic reticulum. Despite the almost identical dantrolene-binding sequence exits in all three mammalian RyR isoforms, dantrolene appears to be an isoform-selective inhibitor. Whereas RyR1 and RyR3 channels are competent to bind dantrolene, the RyR2 channel, predominantly expressed in the heart, is unresponsive. However, a large body of evidence suggests that the RyR2 channel becomes sensitive to dantrolene-mediated inhibition under certain pathological conditions. Although a consistent picture of the dantrolene effect emerges from in vivo studies, in vitro results are often contradictory. Hence, our goal in this perspective is to provide the best possible clues to the molecular mechanism of dantrolene’s action on RyR isoforms by identifying and discussing potential sources of conflicting results, mainly coming from cell-free experiments. Moreover, we propose that, specifically in the case of the RyR2 channel, its phosphorylation could be implicated in acquiring the channel responsiveness to dantrolene inhibition, interpreting functional findings in the structural context.

Defective ryanodine receptor N-terminus inter-subunit interaction is a common mechanism in neuromuscular and cardiac disorders

The ryanodine receptor (RyR) is a homotetrameric channel mediating sarcoplasmic reticulum Ca2+ release required for skeletal and cardiac muscle contraction. Mutations in RyR1 and RyR2 lead to life-threatening malignant hyperthermia episodes and ventricular tachycardia, respectively. In this brief report, we use chemical cross-linking to demonstrate that pathogenic RyR1 R163C and RyR2 R169Q mutations reduce N-terminus domain (NTD) tetramerization. Introduction of positively-charged residues (Q168R, M399R) in the NTD-NTD inter-subunit interface normalizes RyR2-R169Q NTD tetramerization. These results indicate that perturbation of NTD-NTD inter-subunit interactions is an underlying molecular mechanism in both RyR1 and RyR2 pathophysiology. Importantly, our data provide proof of concept that stabilization of this critical RyR1/2 structure-function parameter offers clear therapeutic potential.

Z16b, a natural compound from Ganoderma cochlear is a novel RyR2 stabilizer preventing catecholaminergic polymorphic ventricular tachycardia

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited, lethal ventricular arrhythmia triggered by catecholamines. Mutations in genes that encode cardiac ryanodine receptor (RyR2) and proteins that regulate RyR2 activity cause enhanced diastolic Ca2+ release (leak) through the RyR2 channels, resulting in CPVT. Current therapies for CPVT are limited. We found that Z16b, a meroterpenoid isolated from Ganoderma cochlear, inhibited Ca2+ spark frequency (CaSF) in R2474S/ + cardiomyocytes in a dose-dependent manner, with an IC50 of 3.2 μM. Z16b also dose-dependently suppressed abnormal post-pacing Ca2+ release events. Intraperitoneal injection (i.p.) of epinephrine and caffeine stimulated sustained ventricular tachycardia in all R2474S/+ mice, while pretreatment with Z16b (0.5 mg/kg, i.p.) prevented ventricular arrhythmia in 9 of 10 mice, and Z16b administration immediately after the onset of VT abolished sVT in 9 of 12 mice. Of translational significance, Z16b significantly inhibited CaSF and abnormal Ca2+ release events in human CPVT iPS-CMs. Mechanistically, Z16b interacts with RyR2, enhancing the "zipping" state of the N-terminal and central domains of RyR2. A molecular docking simulation and point mutation and pulldown assays identified Z16b forms hydrogen bonds with Arg626, His1670, and Gln2126 in RyR2 as a triangle shape that anchors the NTD and CD interaction and thus stabilizes RyR2 in a tight "zipping" conformation. Our findings support that Z16b is a novel RyR2 stabilizer that can prevent CPVT. It may also serve as a lead compound with a new scaffold for the design of safer and more efficient drugs for treating CPVT.

Molecular, Subcellular, and Arrhythmogenic Mechanisms in Genetic RyR2 Disease

The ryanodine receptor (RyR2) has a critical role in controlling Ca²⁺ release from the sarcoplasmic reticulum (SR) throughout the cardiac cycle. RyR2 protein has multiple functional domains with specific roles, and four of these RyR2 protomers are required to form the quaternary structure that comprises the functional channel. Numerous mutations in the gene encoding RyR2 protein have been identified and many are linked to a wide spectrum of arrhythmic heart disease. Gain of function mutations (GoF) result in a hyperactive channel that causes excessive spontaneous SR Ca²⁺ release. This is the predominant cause of the inherited syndrome catecholaminergic polymorphic ventricular tachycardia (CPVT). Recently, rare hypoactive loss of function (LoF) mutations have been identified that produce atypical effects on cardiac Ca²⁺ handling that has been termed calcium release deficiency syndrome (CRDS). Aberrant Ca²⁺ release resulting from both GoF and LoF mutations can result in arrhythmias through the Na⁺/Ca²⁺ exchange mechanism. This mini-review discusses recent findings regarding the role of RyR2 domains and endogenous regulators that influence RyR2 gating normally and with GoF/LoF mutations. The arrhythmogenic consequences of GoF/LoF mutations will then be discussed at the macromolecular and cellular level.

Cardiac ryanodine receptor N-terminal region biosensors identify novel inhibitors via FRET-based high-throughput screening

The N-terminal region (NTR) of ryanodine receptor (RyR) channels is critical for the regulation of Ca²⁺ release during excitation–contraction (EC) coupling in muscle. The NTR hosts numerous mutations linked to skeletal (RyR1) and cardiac (RyR2) myopathies, highlighting its potential as a therapeutic target. Here, we constructed two biosensors by labeling the mouse RyR2 NTR at domains A, B, and C with FRET pairs. Using fluorescence lifetime (FLT) detection of intramolecular FRET signal, we developed high-throughput screening (HTS) assays with these biosensors to identify small-molecule RyR modulators. We then screened a small validation library and identified several hits. Hits with saturable FRET dose–response profiles and previously unreported effects on RyR were further tested using [³H]ryanodine binding to isolated sarcoplasmic reticulum vesicles to determine effects on intact RyR opening in its natural membrane. We identified three novel inhibitors of both RyR1 and RyR2 and two RyR1-selective inhibitors effective at nanomolar Ca²⁺. Two of these hits activated RyR1 only at micromolar Ca²⁺, highlighting them as potential enhancers of excitation–contraction coupling. To determine whether such hits can inhibit RyR leak in muscle, we further focused on one, an FDA-approved natural antibiotic, fusidic acid (FA). In skinned skeletal myofibers and permeabilized cardiomyocytes, FA inhibited RyR leak with no detrimental effect on skeletal myofiber excitation–contraction coupling. However, in intact cardiomyocytes, FA induced arrhythmogenic Ca²⁺ transients, a cautionary observation for a compound with an otherwise solid safety record. These results indicate that HTS campaigns using the NTR biosensor can identify compounds with therapeutic potential.

"Ryanopathies" and RyR2 dysfunctions: can we further decipher them using in vitro human disease models?

The regulation of intracellular calcium (Ca²⁺) homeostasis is fundamental to maintain normal functions in many cell types. The ryanodine receptor (RyR), the largest intracellular calcium release channel located on the sarco/endoplasmic reticulum (SR/ER), plays a key role in the intracellular Ca²⁺ handling. Abnormal type 2 ryanodine receptor (RyR2) function, associated to mutations (ryanopathies) or pathological remodeling, has been reported, not only in cardiac diseases, but also in neuronal and pancreatic disorders. While animal models and in vitro studies provided valuable contributions to our knowledge on RyR2 dysfunctions, the human cell models derived from patients’ cells offer new hope for improving our understanding of human clinical diseases and enrich the development of great medical advances. We here discuss the current knowledge on RyR2 dysfunctions associated with mutations and post-translational remodeling. We then reviewed the novel human cellular technologies allowing the correlation of patient’s genome with their cellular environment and providing approaches for personalized RyR-targeted therapeutics.

Impaired Binding to Junctophilin-2 and Nanostructural Alteration in CPVT Mutation

[Figure: see text].

Identification of an amino-terminus determinant critical for ryanodine receptor/Ca2+ release channel function

AIMS

The cardiac ryanodine receptor (RyR2), which mediates intracellular Ca2+ release to trigger cardiomyocyte contraction, participates in development of acquired and inherited arrhythmogenic cardiac disease. This study was undertaken to characterize the network of inter- and intra-subunit interactions regulating the activity of the RyR2 homotetramer.

METHODS AND RESULTS

We use mutational investigations combined with biochemical assays to identify the peptide sequence bridging the β8 with β9 strand as the primary determinant mediating RyR2 N-terminus self-association. The negatively charged side chains of two aspartate residues (D179 and D180) within the β8-β9 loop are crucial for the N-terminal inter-subunit interaction. We also show that the RyR2 N-terminus domain interacts with the C-terminal channel pore region in a Ca2+-independent manner. The β8-β9 loop is required for efficient RyR2 subunit oligomerization but it is dispensable for N-terminus interaction with C-terminus. Deletion of the β8-β9 sequence produces unstable tetrameric channels with subdued intracellular Ca2+ mobilization implicating a role for this domain in channel opening. The arrhythmia-linked R176Q mutation within the β8-β9 loop decreases N-terminus tetramerization but does not affect RyR2 subunit tetramerization or the N-terminus interaction with C-terminus. RyR2R176Q is a characteristic hypersensitive channel displaying enhanced intracellular Ca2+ mobilization suggesting an additional role for the β8-β9 domain in channel closing.

CONCLUSION

These results suggest that efficient N-terminus inter-subunit communication mediated by the β8-β9 loop may constitute a primary regulatory mechanism for both RyR2 channel activation and suppression.

Association of cardiac myosin-binding protein-C with the ryanodine receptor channel - putative retrograde regulation?

The cardiac muscle ryanodine receptor-Ca²⁺ release channel (RyR2) constitutes the sarcoplasmic reticulum (SR) Ca²⁺ efflux mechanism that initiates myocyte contraction, while cardiac myosin-binding protein-C (cMyBP-C; also known as MYBPC3) mediates regulation of acto-myosin cross-bridge cycling. In this paper, we provide the first evidence for the presence of direct interaction between these two proteins, forming a RyR2-cMyBP-C complex. The C-terminus of cMyBP-C binds with the RyR2 N-terminus in mammalian cells and the interaction is not mediated by a fibronectin-like domain. Notably, we detected complex formation between both recombinant cMyBP-C and RyR2, as well as between the native proteins in cardiac tissue. Cellular Ca²⁺ dynamics in HEK293 cells is altered upon co-expression of cMyBP-C and RyR2, with lowered frequency of RyR2-mediated spontaneous Ca²⁺ oscillations, suggesting that cMyBP-C exerts a potential inhibitory effect on RyR2-dependent Ca²⁺ release. Discovery of a functional RyR2 association with cMyBP-C provides direct evidence for a putative mechanistic link between cytosolic soluble cMyBP-C and SR-mediated Ca²⁺ release, via RyR2. Importantly, this interaction may have clinical relevance to the observed cMyBP-C and RyR2 dysfunction in cardiac pathologies, such as hypertrophic cardiomyopathy.

Calsequestrin interacts directly with the cardiac ryanodine receptor luminal domain

Cardiac muscle contraction requires sarcoplasmic reticulum (SR) Ca²⁺ release mediated by the quaternary complex comprising the ryanodine receptor 2 (RyR2), calsequestrin 2 (CSQ2), junctin (encoded by ASPH) and triadin. Here, we demonstrate that a direct interaction exists between RyR2 and CSQ2. Topologically, CSQ2 binding occurs at the first luminal loop of RyR2. Co-expression of RyR2 and CSQ2 in a human cell line devoid of the other quaternary complex proteins results in altered Ca²⁺-release dynamics compared to cells expressing RyR2 only. These findings provide a new perspective for understanding the SR luminal Ca²⁺ sensor and its involvement in cardiac physiology and disease.

Genetic and Biochemical Approaches for In Vivo and In Vitro Assessment of Protein Oligomerization: The Ryanodine Receptor Case Study

Oligomerization is often a structural requirement for proteins to accomplish their specific cellular function. For instance, tetramerization of the ryanodine receptor (RyR) is necessary for the formation of a functional Ca²⁺ release channel pore. Here, we describe detailed protocols for the assessment of protein self-association, including yeast two-hybrid (Y2H), co-immunoprecipitation (co-IP) and chemical cross-linking assays. In the Y2H system, protein self-interaction is detected by β-galactosidase assay in yeast co-expressing GAL4 bait and target fusions of the test protein. Protein self-interaction is further assessed by co-IP using HA- and cMyc-tagged fusions of the test protein co-expressed in mammalian HEK293 cells. The precise stoichiometry of the protein homo-oligomer is examined by cross-linking and SDS-PAGE analysis following expression in HEK293 cells. Using these different but complementary techniques, we have consistently observed the self-association of the RyR N-terminal domain and demonstrated its intrinsic ability to form tetramers. These methods can be applied to protein-protein interaction and homo-oligomerization studies of other mammalian integral membrane proteins.

CaMKII-dependent phosphorylation of RyR2 promotes targetable pathological RyR2 conformational shift

Diastolic calcium (Ca) leak via cardiac ryanodine receptors (RyR2) can cause arrhythmias and heart failure (HF). Ca/calmodulin (CaM)-dependent kinase II (CaMKII) is upregulated and more active in HF, promoting RyR2-mediated Ca leak by RyR2-Ser2814 phosphorylation. Here, we tested a mechanistic hypothesis that RyR2 phosphorylation by CaMKII increases Ca leak by promoting a pathological RyR2 conformation with reduced CaM affinity. Acute CaMKII activation in wild-type RyR2, and phosphomimetic RyR2-S2814D (vs. non-phosphorylatable RyR2-S2814A) knock-in mouse myocytes increased SR Ca leak, reduced CaM-RyR2 affinity, and caused a pathological shift in RyR2 conformation (detected via increased access of the RyR2 structural peptide DPc10). This same trio of effects was seen in myocytes from rabbits with pressure/volume-overload induced HF. Excess CaM quieted leak and restored control conformation, consistent with negative allosteric coupling between CaM affinity and DPc10 accessible conformation. Dantrolene (DAN) also restored CaM affinity, reduced DPc10 access, and suppressed RyR2-mediated Ca leak and ventricular tachycardia in RyR2-S2814D mice. We propose that a common pathological RyR2 conformational state (low CaM affinity, high DPc10 access, and elevated leak) may be caused by CaMKII-dependent phosphorylation, oxidation, and HF. Moreover, DAN (or excess CaM) can shift this pathological gating state back to the normal physiological conformation, a potentially important therapeutic approach.

Structural and functional interactions within ryanodine receptor

The ryanodine receptor/Ca2+ release channel plays a pivotal role in skeletal and cardiac muscle excitation-contraction coupling. Defective regulation leads to neuromuscular disorders and arrhythmogenic cardiac disease. This mini-review focuses on channel regulation through structural intra- and inter-subunit interactions and their implications in ryanodine receptor pathophysiology.