Molecular basis for gating of cardiac ryanodine receptor explains the mechanisms for gain- and loss-of function mutations

Propagation of conformational instability in FK506-binding protein FKBP12

FKBP12 is the archetype of the FK506 binding domains that define the family of FKBP proteins which participate in the regulation of various distinct physiological signaling processes. As the drugs FK506 and rapamycin inhibit many of these FKBP proteins, there is need to develop therapeutics which exhibit selectivity within this family. The long β4-β5 loop of the FKBP domain is known to regulate transcriptional activity for the steroid hormone receptors and appears to participate in regulating calcium channel activity for the cardiac and skeletal muscle ryanodine receptors. The β4-β5 loop of FKBP12 has been shown to undergo extensive conformational dynamics, and here we report hydrogen exchange measurements for a series of mutational variants in that loop which indicate deviations from a two-state kinetics for those dynamics. In addition to a previously characterized local transition near the tip of this loop, evidence is presented for a second site of conformational dynamics in the stem of this loop. These mutation-dependent hydrogen exchange effects extend beyond the β4-β5 loop, primarily by disrupting the hydrogen bond between the Gly 58 amide and the Tyr 80 carbonyl oxygen which links the two halves of the structural rim that surrounds the active site cleft. Mutationally-induced opening of the cleft between Gly 58 and Tyr 80 not only modulates the global stability of the protein, it promotes a conformational transition in the distant β2-β3a hairpin that modulates the binding affinity for a FKBP51-selective inhibitor previously designed to exploit a localized conformational transition at the homologous site.

An inherited life-threatening arrhythmia model established by screening randomly mutagenized mice

Inherited arrhythmia syndromes (IASs) can cause life-threatening arrhythmias and are responsible for a significant proportion of sudden cardiac deaths (SCDs). Despite progress in the development of devices to prevent SCDs, the precise molecular mechanisms that induce detrimental arrhythmias remain to be fully investigated, and more effective therapies are desirable. In the present study, we screened a large-scale randomly mutagenized mouse library by electrocardiography to establish a disease model of IASs and consequently found one pedigree that exhibited spontaneous ventricular arrhythmias (VAs) followed by SCD within 1 y after birth. Genetic analysis successfully revealed a missense mutation (p.I4093V) of the ryanodine receptor 2 gene to be a cause of the arrhythmia. We found an age-related increase in arrhythmia frequency accompanied by cardiomegaly and decreased ventricular contractility in the Ryr2I4093V/+ mice. Ca2+ signaling analysis and a ryanodine binding assay indicated that the mutant ryanodine receptor 2 had a gain-of-function phenotype and enhanced Ca2+ sensitivity. Using this model, we detected the significant suppression of VA following flecainide or dantrolene treatment. Collectively, we established an inherited life-threatening arrhythmia mouse model from an electrocardiogram-based screen of randomly mutagenized mice. The present IAS model may prove feasible for use in investigating the mechanisms of SCD and assessing therapies.

A potent and selective cis-amide inhibitor of ryanodine receptor 2 as a candidate for cardiac arrhythmia treatment

Ryanodine receptor 2 (RyR2) is a Ca2+ release channel mainly located on the sarcoplasmic reticulum (SR) membrane of heart muscle cells and regulates the concentration of Ca2+ in the cytosol. RyR2 overactivation causes potentially lethal cardiac arrhythmias, but no specific inhibitor is yet available. Herein we developed the first highly potent and selective RyR2 inhibitor, TMDJ-035, containing 3,5-difluoro substituents on the A ring and a 4-fluoro substituent on the B ring, based on a comprehensive structure-activity relationship (SAR) study of tetrazole compound 1. The SAR study also showed that the amide conformation is critical for inhibitory potency. Single-crystal X-ray diffraction analysis and variable-temperature 1H NMR revealed that TMDJ-035 strongly favors cis-amide configuration, while the inactive analogue TMDJ-011 with a secondary amide takes trans-amide configuration. Examination of the selectivity among RyRs indicated that TMDJ-035 displayed high selectivity for RyR2. TMDJ-035 suppressed abnormal Ca2+ waves and transients in isolated cardiomyocytes from RyR2-mutated mice. It appears to be a promising candidate drug for treating cardiac arrhythmias due to RyR2 overactivation, as well as a tool for studying the mechanism and dynamics of RyR2 channel gating.

Screening for Novel Type 2 Ryanodine Receptor Inhibitors by Endoplasmic Reticulum Ca2+ Monitoring

Type 2 ryanodine receptor (RyR2) is a Ca2+ release channel on the endoplasmic (ER)/sarcoplasmic reticulum that plays a central role in the excitation-contraction coupling in the heart. Hyperactivity of RyR2 has been linked to ventricular arrhythmias in patients with catecholaminergic polymorphic ventricular tachycardia and heart failure, where spontaneous Ca2+ release via hyperactivated RyR2 depolarizes diastolic membrane potential to induce triggered activity. In such cases, drugs that suppress RyR2 activity are expected to prevent the arrhythmias, but there is no clinically available RyR2 inhibitors at present. In this study, we searched for RyR2 inhibitors from a well-characterized compound library using a recently developed ER Ca2+-based assay, where the inhibition of RyR2 activity was detected by the increase in ER Ca2+ signals from R-CEPIA1er, a genetically encoded ER Ca2+ indicator, in RyR2-expressing HEK293 cells. By screening 1535 compounds in the library, we identified three compounds (chloroxylenol, methyl orsellinate, and riluzole) that greatly increased the ER Ca2+ signal. All of the three compounds suppressed spontaneous Ca2+ oscillations in RyR2-expressing HEK293 cells and correspondingly reduced the Ca2+-dependent [3H]ryanodine binding activity. In cardiomyocytes from RyR2-mutant mice, the three compounds effectively suppressed abnormal Ca2+ waves without substantial effects on the action-potential-induced Ca2+ transients. These results confirm that ER Ca2+-based screening is useful for identifying modulators of ER Ca2+ release channels and suggest that RyR2 inhibitors have potential to be developed as a new category of antiarrhythmic drugs. SIGNIFICANCE STATEMENT: We successfully identified three compounds having RyR2 inhibitory action from a well-characterized compound library using an endoplasmic reticulum Ca2+-based assay, and demonstrated that these compounds suppressed arrhythmogenic Ca2+ wave generation without substantially affecting physiological action-potential induced Ca2+ transients in cardiomyocytes. This study will facilitate the development of RyR2-specific inhibitors as a potential new class of drugs for life-threatening arrhythmias induced by hyperactivation of RyR2.

Calcium Release Deficiency Syndrome: A New Inherited Arrhythmia Syndrome

Calcium release deficiency syndrome (CRDS) is a newly described form of inherited arrhythmia caused by damaging loss-of-function variants in the cardiac ryanodine receptor (RyR2). Unlike the prototypical RyR2 gain-of-function channelopathy, known as catecholaminergic polymorphic ventricular tachycardia, patients with CRDS are predisposed to sudden death usually in the absence of any electrical abnormalities at rest or during stress electrocardiography. This makes diagnosis incredibly challenging, however, an invasive electrophysiologic test appears to be effective in unmasking the phenotype, called the long-burst, long-pause, short-coupled ventricular extra-stimulus protocol. Optimal therapies for patients with CRDS remain unestablished, although flecainide appears to be a promising candidate drug.

Structural diversity, hypothetical biosynthesis, chemical synthesis, and biological activity of Ganoderma meroterpenoids

Covering: 2018 to 2022Meroterpenoids found in fungal species of the genus Ganoderma and known as Ganoderma meroterpenoids (GMs) are substances composed of a 1,2,4-trisubstituted benzene and a polyunsaturated side chain. These substances have attracted the attention of chemists and pharmacologists due to their diverse structures and significant bioactivity. In this review, we present the structures and possible biosynthesis of representative GMs newly found from 2018 to 2022, as well as chemical synthesis and biological activity of some interesting GMs. We propose for the first time a plausible biosynthetic pathway for GMs, which will certainly motivate further research on the biosynthetic pathway in Ganoderma species, as well as on chemical synthesis of GMs as important bioactive compounds for the purpose of drug development.

Drug development for the treatment of RyR1-related skeletal muscle diseases

Type 1 ryanodine receptor (RyR1) is an intracellular Ca²⁺ release channel on the sarcoplasmic reticulum of skeletal muscle, and it plays a central role in excitation-contraction (E-C) coupling. Mutations in RyR1 are implicated in various muscle diseases including malignant hyperthermia, central core disease, and myopathies. Currently, no specific treatment exists for most of these diseases. Recently, high-throughput screening (HTS) assays have been developed for identifying potential candidates for treating RyR-related muscle diseases. Currently, two different methods, namely a FRET-based assay and an endoplasmic reticulum Ca²⁺-based assay, are available. These assays identified several compounds as novel RyR1 inhibitors. In addition, the development of a reconstituted platform permitted HTS assays for E-C coupling modulators. In this review, we will focus on recent progress in HTS assays and discuss future perspectives of these promising approaches.

Ligand sensitivity of type-1 inositol 1,4,5-trisphosphate receptor is enhanced by the D2594K mutation

Inositol 1,4,5-trisphosphate receptor (IP₃R) and ryanodine receptor (RyR) are homologous cation channels that mediate release of Ca²⁺ from the endoplasmic/sarcoplasmic reticulum (ER/SR) and thereby are involved in many physiological processes. In previous studies, we determined that when the D2594 residue, located at or near the gate of the IP₃R type 1, was replaced by lysine (D2594K), a gain of function was obtained. This mutant phenotype was characterized by increased IP₃ sensitivity. We hypothesized the IP₃R1-D2594 determines the ligand sensitivity of the channel by electrostatically affecting the stability of the closed and open states. To test this possibility, the relationship between the D2594 site and IP₃R1 regulation by IP₃, cytosolic, and luminal Ca²⁺ was determined at the cellular, subcellular, and single-channel levels using fluorescence Ca²⁺ imaging and single-channel reconstitution. We found that in cells, D2594K mutation enhances the IP₃ ligand sensitivity. Single-channel IP₃R1 studies revealed that the conductance of IP₃R1-WT and -D2594K channels is similar. However, IP₃R1-D2594K channels exhibit higher IP₃ sensitivity, with substantially greater efficacy. In addition, like its wild type (WT) counterpart, IP₃R1-D2594K showed a bell-shape cytosolic Ca²⁺-dependency, but D2594K had greater activity at each tested cytosolic free Ca²⁺ concentration. The IP₃R1-D2594K also had altered luminal Ca²⁺ sensitivity. Unlike IP₃R1-WT, D2594K channel activity did not decrease at low luminal Ca²⁺ levels. Taken together, our functional studies indicate that the substitution of a negatively charged residue by a positive one at the channels' pore cytosolic exit affects the channel's gating behavior thereby explaining the enhanced ligand-channel's sensitivity.

Three-dimensional perspective on ryanodine receptor mutations causing skeletal and cardiac muscle-related diseases

Mutations in RyR alter the cell's Ca2+ homeostasis and can cause serious health problems for which few effective therapies are available. Until recently, there was little structural context for the hundreds of mutations linked to muscular disorders reported for this large channel. Growing knowledge of the three-dimensional structure of RyR starts to illustrate the fine control of Ca2+ release. Current efforts directed towards understanding how disease mutations impinge in such processes will be crucial for future design of novel therapies. In this review article we discuss the up-to-date information about mutations according to their role in the 3D structure, and classified them to provide context from a structural perspective.