Search

Heart failure, the leading cause of mortality and morbidity in the developed world, is characterized by cardiac ryanodine receptor 2 channels that are hyperphosphorylated, oxidized, and depleted of the stabilizing subunit calstabin-2. This results in a diastolic sarcoplasmic reticulum Ca 2+ leak that impairs cardiac contractility and triggers arrhythmias. Genetic mutations in ryanodine receptor 2 can also cause Ca 2+ leak, leading to arrhythmias and sudden cardiac death. Here, we solved the cryogenic electron microscopy structures of ryanodine receptor 2 variants linked either to heart failure or inherited sudden cardiac death. All are in the primed state, part way between closed and open. Binding of Rycal drugs to ryanodine receptor 2 channels reverts the primed state back towards the closed state, decreasing Ca 2+ leak, improving cardiac function, and preventing arrhythmias. We propose a structural-physiological mechanism whereby the ryanodine receptor 2 channel primed state underlies the arrhythmias in heart failure and arrhythmogenic disorders., (© 2024. The Author(s).)

Coronary heart disease (CHD) is a prevalent cardiac disease that causes over 370,000 deaths annually in the USA. In CHD, occlusion of a coronary artery causes ischemia of the cardiac muscle, which results in myocardial infarction (MI). Junctophilin-2 (JPH2) is a membrane protein that ensures efficient calcium handling and proper excitation-contraction coupling. Studies have identified loss of JPH2 due to calpain-mediated proteolysis as a key pathogenic event in ischemia-induced heart failure (HF). Our findings show that calpain-2-mediated JPH2 cleavage yields increased levels of a C-terminal cleaved peptide (JPH2-CTP) in patients with ischemic cardiomyopathy and mice with experimental MI. We created a novel knock-in mouse model by removing residues 479-SPAGTPPQ-486 to prevent calpain-2-mediated cleavage at this site. Functional and molecular assessment of cardiac function post-MI in cleavage site deletion (CSD) mice showed preserved cardiac contractility and reduced dilation, reduced JPH2-CTP levels, attenuated adverse remodeling, improved T-tubular structure, and normalized SR Ca 2+ -handling. Adenovirus mediated calpain-2 knockdown in mice exhibited similar findings. Pulldown of CTP followed by proteomic analysis revealed valosin-containing protein (VCP) and BAG family molecular chaperone regulator 3 (BAG3) as novel binding partners of JPH2. Together, our findings suggest that blocking calpain-2-mediated JPH2 cleavage may be a promising new strategy for delaying the development of HF following MI., Competing Interests: Declaration of competing interest XHTW is a consultant for Pfizer and Rocket Pharmaceuticals, and a founding partner and board member of Elex Biotech Inc., a start-up company that developed drug molecules that target ryanodine receptors to treat cardiac arrhythmia disorders. LZ is the co-founder and consultant for Pelagos Pharma Inc., a start-up company that develop drug molecules that target nuclear receptors to treat heart failure and neurodegenerative disorders. Other authors have no potential conflicts to disclose. AI-assisted technology is not used in the preparation of this work (except checking grammar and spelling)., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

S100A1, a small homodimeric EF-hand Ca 2+ -binding protein (~21 kDa), plays an important regulatory role in Ca 2+ signaling pathways involved in various biological functions including Ca 2+ cycling and contractile performance in skeletal and cardiac myocytes. One key target of the S100A1 interactome is the ryanodine receptor (RyR), a huge homotetrameric Ca 2+ release channel (~2.3 MDa) of the sarcoplasmic reticulum. Here, we report cryoelectron microscopy structures of S100A1 bound to RyR1, the skeletal muscle isoform, in absence and presence of Ca 2+ . Ca 2+ -free apo-S100A1 binds beneath the bridging solenoid (BSol) and forms contacts with the junctional solenoid and the shell-core linker of RyR1. Upon Ca 2+ -binding, S100A1 undergoes a conformational change resulting in the exposure of the hydrophobic pocket known to serve as a major interaction site of S100A1. Through interactions of the hydrophobic pocket with RyR1, Ca 2+ -bound S100A1 intrudes deeper into the RyR1 structure beneath BSol than the apo-form and induces sideways motions of the C-terminal BSol region toward the adjacent RyR1 protomer resulting in tighter interprotomer contacts. Interestingly, the second hydrophobic pocket of the S100A1-dimer is largely exposed at the hydrophilic surface making it prone to interactions with the local environment, suggesting that S100A1 could be involved in forming larger heterocomplexes of RyRs with other protein partners. Since S100A1 interactions stabilizing BSol are implicated in the regulation of RyR-mediated Ca 2+ release, the characterization of the S100A1 binding site conserved between RyR isoforms may provide the structural basis for the development of therapeutic strategies regarding treatments of RyR-related disorders., Competing Interests: Competing interests statement:Columbia University and A.R.M. own stock inARMGO, a company developing compounds targeting RyR and have patents on Rycals.The remaining authors declare no competing interests.

Heart failure is a serious global health challenge, affecting more than 6.2 million people in the United States and is projected to reach over 8 million by 2030. Independent of etiology, failing hearts share common features, including defective calcium (Ca 2+ ) handling, mitochondrial Ca 2+ overload, and oxidative stress. In cardiomyocytes, Ca 2+ not only regulates excitation-contraction coupling, but also mitochondrial metabolism and oxidative stress signaling, thereby controlling the function and actual destiny of the cell. Understanding the mechanisms of mitochondrial Ca 2+ uptake and the molecular pathways involved in the regulation of increased mitochondrial Ca 2+ influx is an ongoing challenge in order to identify novel therapeutic targets to alleviate the burden of heart failure. In this review, we discuss the mechanisms underlying altered mitochondrial Ca 2+ handling in heart failure and the potential therapeutic strategies.

Ryanodine receptor type 2 (RyR2) mutations have been linked to an inherited form of exercise-induced sudden cardiac death called catecholaminergic polymorphic ventricular tachycardia (CPVT). CPVT results from stress-induced sarcoplasmic reticular Ca 2+ leak via the mutant RyR2 channels during diastole. We present atomic models of human wild-type (WT) RyR2 and the CPVT mutant RyR2-R2474S determined by cryo-electron microscopy with overall resolutions in the range of 2.6 to 3.6 Å, and reaching local resolutions of 2.25 Å, unprecedented for RyR2 channels. Under nonactivating conditions, the RyR2-R2474S channel is in a "primed" state between the closed and open states of WT RyR2, rendering it more sensitive to activation that results in stress-induced Ca 2+ leak. The Rycal drug ARM210 binds to RyR2-R2474S, reverting the primed state toward the closed state. Together, these studies provide a mechanism for CPVT and for the therapeutic actions of ARM210.

Calpains are calcium-activated neutral proteases involved in the regulation of key signaling pathways. Junctophilin-2 (JP2) is a Calpain-specific proteolytic target and essential structural protein inside Ca 2+ release units required for excitation-contraction coupling in cardiomyocytes. While downregulation of JP2 by Calpain cleavage in heart failure has been reported, the precise molecular identity of the Calpain cleavage sites and the (patho-)physiological roles of the JP2 proteolytic products remain controversial. We systematically analyzed the JP2 cleavage fragments as function of Calpain-1 versus Calpain-2 proteolytic activities, revealing that both Calpain isoforms preferentially cleave mouse JP2 at R565, but subsequently at three additional secondary Calpain cleavage sites. Moreover, we identified the Calpain-specific primary cleavage products for the first time in human iPSC-derived cardiomyocytes. Knockout of RyR2 in hiPSC-cardiomyocytes destabilized JP2 resulting in an increase of the Calpain-specific cleavage fragments. The primary N-terminal cleavage product NT 1 accumulated in the nucleus of mouse and human cardiomyocytes in a Ca 2+ -dependent manner, closely associated with euchromatic chromosomal regions, where NT 1 is proposed to function as a cardio-protective transcriptional regulator in heart failure. Taken together, our data suggest that stabilizing NT 1 by preventing secondary cleavage events by Calpain and other proteases could be an important therapeutic target for future studies., (© 2022. The Author(s).)

[Figure: see text].