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S100A1, a small homodimeric EF-hand Ca2+-binding protein (~21 kDa), plays an important regulatory role in Ca2+ signaling pathways involved in various biological functions including Ca2+ cycling and contractile performance in skeletal and cardiac myocytes. One key target of the S100A1 interactome is the ryanodine receptor (RyR), a huge homotetrameric Ca2+ 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 Ca2+. Ca2+-free apo-S100A1 binds beneath the bridging solenoid (BSol) and forms contacts with the junctional solenoid and the shell-core linker of RyR1. Upon Ca2+-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, Ca2+-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 Ca2+ 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. [ABSTRACT FROM AUTHOR]

Patients with heart failure (HF) have augmented vascular tone, which increases cardiac workload, impairing ventricular output and promoting further myocardial dysfunction. The molecular mechanisms underlying the maladaptive vascular responses observed in HF are not fully understood. Vascular smooth muscle cells (VSMCs) control vasoconstriction via a [Ca.sup.2+]-dependent process, in which the type 1 inositol 1,4,5-trisphosphate receptor (IP3R1) on the sarcoplasmic reticulum (SR) plays a major role. To dissect the mechanistic contribution of intracellular [Ca.sup.2+] release to the increased vascular tone observed in HF, we analyzed the remodeling of IP3R1 in aortic tissues from patients with HF and from controls. VSMC IP3R1 channels from patients with HF and HF mice were hyperphosphorylated by both serine and tyrosine kinases. VSMCs isolated from [IP3R1.sup.VSMC-/-] mice exhibited blunted [Ca.sup.2+] responses to angiotensin II (ATII) and norepinephrine compared with control VSMCs. [IP3R1.sup.VSMC-/-] mice displayed significantly reduced responses to ATII, both in vivo and ex vivo. HF [IP3R1.sup.VSMC-/-] mice developed significantly less afterload compared with HF [IP3R1.sup.fl/fl] mice and exhibited significantly attenuated progression toward decompensated HF and reduced interstitial fibrosis. [Ca.sup.2+]-dependent phosphorylation of the MLC by MLCK activated VSMC contraction. MLC phosphorylation was markedly increased in VSMCs from patients with HF and HF mice but reduced in VSMCs from HF [IP3R1.sup.VSMC-/-] mice and HF WT mice treated with ML-7. Taken together, our data indicate that VSMC IP3R1 is a major effector of increased vascular tone, which contributes to increased cardiac afterload and decompensation in HF., Introduction Heart failure (HF) is a complex clinical syndrome that remains a leading cause of mortality despite decades of intense efforts to develop novel therapeutics (1, 2). Beginning early in [...]

Cancer-associated muscle weakness is a poorly understood phenomenon, and there is no effective treatment. Here we find that seven different mouse models of human osteolytic bone metastases--representing breast, lung and prostate cancers, as well as multiple myeloma--exhibited impaired muscle function, implicating a role for the tumor-bone microenvironment in cancer-associated muscle weakness. We found that transforming growth factor (TGF)-[beta], released from the bone surface as a result of metastasis-induced bone destruction, upregulated NADPH oxidase 4 (Nox4), resulting in elevated oxidization of skeletal muscle proteins, including the ryanodine receptor and calcium (Ca[sup.2+]) release channel (RyR1). The oxidized RyR1 channels leaked Ca[sup.2+], resulting in lower intracellular signaling, which is required for proper muscle contraction. We found that inhibiting RyR1 leakage, TGF-[beta] signaling, TGF-[beta] release from bone or Nox4 activity improved muscle function in mice with MDA-MB-231 bone metastases. Humans with breast- or lung cancer-associated bone metastases also had oxidized skeletal muscle RyR1 that is not seen in normal muscle. Similarly, skeletal muscle weakness, increased Nox4 binding to RyR1 and oxidation of RyR1 were present in a mouse model of Camurati-Engelmann disease, a nonmalignant metabolic bone disorder associated with increased TGF-[beta] activity. Thus, pathological TGF-[beta] release from bone contributes to muscle weakness by decreasing Ca[sup.2+]-induced muscle force production., Author(s): David L Waning [1]; Khalid S Mohammad [1]; Steven Reiken [2]; Wenjun Xie [2]; Daniel C Andersson [2]; Sutha John [1]; Antonella Chiechi [1]; Laura E Wright [1]; Alisa [...]

The type 2 ryanodine receptor (RyR2) is a [Ca.sup.2] release channel on the endoplasmic reticulum (ER) of several types of cells, including cardiomyocytes and pancreatic [beta] cells. In cardiomyocytes, RyR2-dependent [Ca.sup.2] release is critical for excitation-contraction coupling; however, a functional role for RyR2 in [beta] cell insulin secretion and diabetes mellitus remains controversial. Here, we took advantage of rare RyR2 mutations that were identified in patients with a genetic form of exercise-induced sudden death (catecholaminergic polymorphic ventricular tachycardia [CPVT]). As these mutations result in a 'leaky' RyR2 channel, we exploited them to assess RyR2 channel function in [beta] cell dynamics. We discovered that CPVT patients with mutant leaky RyR2 present with glucose intolerance, which was heretofore unappreciated. In mice, transgenic expression of CPVT-associated RyR2 resulted in impaired glucose homeostasis, and an in-depth evaluation of pancreatic islets and [beta] cells from these animals revealed intracellular [Ca.sup.2] leak via oxidized and nitrosylated RyR2 channels, activated ER stress response, mitochondrial dysfunction, and decreased fuel-stimulated insulin release. Additionally, we verified the effects of the pharmacological inhibition of intracellular [Ca.sup.2] leak in CPVT-associated RyR2-expressing mice, in human islets from diabetic patients, and in an established murine model of type 2 diabetes mellitus. Taken together, our data indicate that RyR2 channels play a crucial role in the regulation of insulin secretion and glucose homeostasis., Introduction The molecular mechanisms underlying insulin secretion and glucose metabolism have not been fully elucidated (1-6). Numerous studies have demonstrated that calcium ([Ca.sup.2+]) plays a pivotal role in insulin secretion [...]

Ryanodine receptors (RyRs) mediate the rapid release of calcium ([Ca.sup.2+]) from intracellular stores into the cytosol, which is essential for numerous cellular functions including excitation-contraction coupling in muscle. Lack of sufficient structural detail has impeded understanding of RyR gating and regulation. Here we report the closed-state structure of the 2.3-megadalton complex of the rabbit skeletal muscle type 1 RyR (RyR1), solved by single-particle electron cryomicroscopy at an overall resolution of 4.8 Å. We fitted a polyalanine-level model to all 3,757 ordered residues in each protomer, defining the transmembrane pore in unprecedented detail and placing all cytosolic domains as tertiary folds. The cytosolic assembly is built on an extended α-solenoid scaffold connecting key regulatory domains to the pore. The RyR1 pore architecture places it in the six-transmembrane ion channel superfamily. A unique domain inserted between the second and third transmembrane helices interacts intimately with paired EF-hands originating from the α-solenoid scaffold, suggesting a mechanism for channel gating by [Ca.sup.2+]., Ryanodine receptors (RyRs) are intracellular [Ca.sup.2+] release channels on the sarcoplasmic and endoplasmic reticula required for fundamental cellular functions in most tissues, including skeletal and cardiac muscle excitation-contraction coupling, synaptic [...]