Inositol 1,4,5-trisphosphate receptor - reactive oxygen signaling domain regulates excitation-contraction coupling in atrial myocytes.

The inositol 1,4,5-trisphosphate receptor (InsP3R) is up-regulated in patients with atrial fibrillation (AF) and InsP3-induced Ca2+ release (IICR) is linked to pro-arrhythmic spontaneous Ca2+ release events. Nevertheless, knowledge of the physiological relevance and regulation of InsP3Rs in atrial muscle is still limited. We hypothesize that InsP3R and NADPH oxidase 2 (NOX2) form a functional signaling domain where NOX2 derived reactive oxygen species (ROS) regulate InsP3R agonist affinity and thereby Ca2+ release. To quantitate the contribution of IICR to atrial excitation-contraction coupling (ECC) atrial myocytes (AMs) were isolated from wild type and NOX2 deficient (Nox2-/-) mice and changes in the cytoplasmic Ca2+ concentration ([Ca2+]i; fluo-4/AM, indo-1) or ROS (2',7'-dichlorofluorescein, DCF) were monitored by fluorescence microscopy. Superfusion of AMs with Angiotensin II (AngII: 1 µmol/L) significantly increased diastolic [Ca2+]i (F/F0, Ctrl: 1.00 ± 0.01, AngII: 1.20 ± 0.03; n = 7; p < 0.05), the field stimulation induced Ca2+ transient (CaT) amplitude (ΔF/F0, Ctrl: 2.00 ± 0.17, AngII: 2.39 ± 0.22, n = 7; p < 0.05), and let to the occurrence of spontaneous increases in [Ca2+]i. These changes in [Ca2+]i were suppressed by the InsP3R blocker 2-aminoethoxydiphenyl-borate (2-APB; 1 µmol/L). Concomitantly, AngII induced an increase in ROS production that was sensitive to the NOX2 specific inhibitor gp91ds-tat (1 µmol/L). In NOX2-/- AMs, AngII failed to increase diastolic [Ca2+]i, CaT amplitude, and the frequency of spontaneous Ca2+ increases. Furthermore, the enhancement of CaTs by exposure to membrane permeant InsP3 was abolished by NOX inhibition with apocynin (1 µM). AngII induced IICR in Nox2-/- AMs could be restored by addition of exogenous ROS (tert-butyl hydroperoxide, tBHP: 5 µmol/L). In saponin permeabilized AMs InsP3 (5 µmol/L) induced Ca2+ sparks that increased in frequency in the presence of ROS (InsP3: 9.65 ± 1.44 sparks*s-1*(100µm)-1; InsP3 + tBHP: 10.77 ± 1.5 sparks*s-1*(100µm)-1; n = 5; p < 0.05). The combined effect of InsP3 + tBHP was entirely suppressed by 2-APB and Xestospongine C (XeC). Changes in IICR due to InsP3R glutathionylation induced by diamide could be reversed by the reducing agent dithiothreitol (DTT: 1 mmol/L) and prevented by pretreatment with 2-APB, supporting that the ROS-dependent post-translational modification of the InsP3R plays a role in the regulation of ECC. Our data demonstrate that in AMs the InsP3R is under dual control of agonist induced InsP3 and ROS formation and suggest that InsP3 and NOX2-derived ROS co-regulate atrial IICR and ECC in a defined InsP3R/NOX2 signaling domain.

Muscle calcium stress cleaves junctophilin1, unleashing a gene regulatory program predicted to correct glucose dysregulation.

Calcium ion movements between cellular stores and the cytosol govern muscle contraction, the most energy-consuming function in mammals, which confers skeletal myofibers a pivotal role in glycemia regulation. Chronic myoplasmic calcium elevation ("calcium stress"), found in malignant hyperthermia-susceptible (MHS) patients and multiple myopathies, has been suggested to underlie the progression from hyperglycemia to insulin resistance. What drives such progression remains elusive. We find that muscle cells derived from MHS patients have increased content of an activated fragment of GSK3ß - a specialized kinase that inhibits glycogen synthase, impairing glucose utilization and delineating a path to hyperglycemia. We also find decreased content of junctophilin1, an essential structural protein that colocalizes in the couplon with the voltage-sensing CaV1.1, the calcium channel RyR1 and calpain1, accompanied by an increase in a 44 kDa junctophilin1 fragment (JPh44) that moves into nuclei. We trace these changes to activated proteolysis by calpain1, secondary to increased myoplasmic calcium. We demonstrate that a JPh44-like construct induces transcriptional changes predictive of increased glucose utilization in myoblasts, including less transcription and translation of GSK3ß and decreased transcription of proteins that reduce utilization of glucose. These effects reveal a stress-adaptive response, mediated by the novel regulator of transcription JPh44.

Loss of p21-activated kinase 1 (Pak1) promotes atrial arrhythmic activity.

BACKGROUND: Atrial fibrillation (AF) is initiated through arrhythmic atrial excitation from outside the sinus node or remodeling of atrial tissue that allows reentry of excitation. Angiotensin II (AngII) has been implicated in the initiation and maintenance of AF through changes in Ca2+ handling and production of reactive oxygen species (ROS). OBJECTIVE: We aimed to determine the role of p21-activated kinase 1 (Pak1), a downstream target in the AngII signaling cascade, in atrial electrophysiology and arrhythmia. METHODS: Wild-type and Pak1-/- mice were used to determine atrial function in vivo on the organ and cellular level by quantification of electrophysiological and Ca2+ handling properties. RESULTS: We demonstrate that reduced Pak1 activity increases the inducibility of atrial arrhythmia in vivo and in vitro. On the cellular level, Pak1-/- atrial myocytes (AMs) exhibit increased basal and AngII (1 µM)-induced ROS production, sensitivity to the NADPH oxidase-2 (NOX2) inhibitors gp91ds-tat and apocynin (1 µM), and enhanced membrane translocation of Ras-related C3 substrate 1 (Rac1) that is part of the multimolecular NOX2 complex. Upon stimulation with AngII, Pak1-/- AMs exhibit an exaggerated increase in the intracellular Calcium concentration ([Ca2+]i) and arrhythmic events that were sensitive to sodium-calcium exchanger (NCX) inhibitors (KB-R7943 and SEA0400; 1 µM) and suppressed in AMs from NOX2-deficient (gp91phox-/-) mice. Pak1 stimulation (FTY720; 200 nM) in wild-type AMs and AMs from a canine model of ventricular tachypacing-induced AF prevented AngII-induced arrhythmic Ca2+ overload by attenuating NCX activity in a NOX2-dependent manner. CONCLUSION: The experimental results support that Pak1 stimulation can attenuate NCX-dependent Ca2+ overload and prevent triggered arrhythmic activity by suppressing NOX2-dependent ROS production.

Pharmacological targeting of the mammalian clock reveals a novel analgesic for osteoarthritis-induced pain.

Environmental disruption of the circadian rhythm is linked with increased pain due to osteoarthritis (OA). We aimed to characterize the role of the clock gene in OA-induced pain more systemically using both genetic and pharmacological approaches. Genetically modified mice, (bmal1f/fNav1.8CreERT mice), generated by deleting the critical clock gene, bmal1, from Nav1.8 sensory neurons, were resistant to the development of mechanical hyperalgesia associated with OA induced by partial medial meniscectomy (PMM) of the knee. In wild-type mice, induction of OA by PMM surgery led to a substantial increase in BMAL1 expression in DRG neurons. Interestingly, pharmacological activation of the REV-ERB (a negative regulator of bmal1 transcription) with SR9009 resulted in reduction of BMAL1 expression, and a significant decrease in mechanical hyperalgesia associated with OA. Cartilage degeneration was also significantly reduced in mice treated with the REV-ERB agonist SR9009. Based on these data, we also assessed the effect of pharmacological activation of REV-ERB using a model of environmental circadian disruption with its associated mechanical hyperalgesia, and noted that SR9009 was an effective analgesic in this model as well. Our data clearly demonstrate that genetic disruption of the molecular clock, via deletion of bmal1 in the sensory neurons of the DRG, decreases pain in a model of OA. Furthermore, pharmacological activation of REV-ERB leading to suppression of BMAL1 expression may be an effective method for treating OA-related pain, as well as to reduce joint damage associated with this disease.

Forkhead, a new cross regulator of metabolism and innate immunity downstream of TOR in Drosophila.

Antimicrobial peptides (AMPs) are conserved cationic peptides which act both as defense molecules of the host immune system and as regulators of the commensal microbiome. Expression of AMPs is induced in response to infection by the Toll and Imd pathway. Under non-infected conditions, the transcription factor dFOXO directly regulates a set of AMP expression at low levels when nutrients are limited. Here we have analyzed whether target of rapamycin (TOR), another major regulator of growth and metabolism, also modulates AMP responses in Drosophila. We found that downregulation of TOR by feeding the drug rapamycin or by overexpressing the negative TOR regulators TSC1/TSC2, resulted in a specific induction of the AMPs Diptericin (Dpt) and Metchnikowin (Mtk). In contrast, overexpression of Rheb, which positively regulates TOR led to a repression of the two AMPs. Genetic and pharmacological experiments indicate that Dpt and Mtk activation is controlled by the transcription factor Forkhead (FKH), the founding member of the FoxO family. Shuttling of FKH from the cytoplasm to the nucleus is induced in the fat body and in the posterior midgut in response to TOR downregulation. The FKH-dependent induction of Dpt and Mtk can be triggered in dFOXO null mutants and in immune-compromised Toll and IMD pathway mutants indicating that FKH acts in parallel to these regulators. Together, we have discovered that FKH is the second conserved member of the FoxO family cross-regulating metabolism and innate immunity. dFOXO and FKH, which are activated upon downregulation of insulin or TOR activities, respectively, act in parallel to induce different sets of AMPs, thereby modulating the immune status of metabolic tissues such as the fat body or the gut in response to the oscillating energy status of the organism.