Phosphoinositide signalling and cardiac arrhythmias

Structure-based virtual screening of FDA-approved drugs to discover potential inhibitors of phosphoinositide kinase, PIKfyve

The phosphoinositide kinase, PIKfyve is a lipid kinase that plays a vital role in membrane trafficking, endosomal transport, retroviral budding, and toll-like receptor signaling. Thus, it has emerged as a potential therapeutic target for several diseases, including, cancer, viral infections, and autoimmune diseases. However, a limited number of PIKfyve inhibitors have been reported so far. Herein, we report a structure-based virtual screening-driven identification of new PIKfyve inhibitors from a library of FDA-approved small molecule drugs. Labetalol, capsaicin and ibrutinib occupy the ATP pocket of PIKfyve with dock scores of -10.3, -10.6 and -12.24 kcal/mol, and MMGBSA binding energy of -57.3, -53.7 and -66.4 kcal/mol, respectively. These drugs inhibit PIKfyve with IC50 values of 0.292, 0.965 and 0.678 µM, respectively, in an in vitro ADP-Glo kinase assay. Among the top hits from SBVS, labetalol as well as capsaicin display a stable interaction with the critical amino acid, LEU 119 of the hinge region during the 100 ns MD simulation. The results obtained herein warrant the exploration of these new inhibitors in preclinical disease models.

Atomistic mechanisms of the regulation of small-conductance Ca2+-activated K+ channel (SK2) by PIP2

Small-conductance Ca2+-activated K+ channels (SK, KCa2) are gated solely by intracellular microdomain Ca2+. The channel has emerged as a therapeutic target for cardiac arrhythmias. Calmodulin (CaM) interacts with the CaM binding domain (CaMBD) of the SK channels, serving as the obligatory Ca2+ sensor to gate the channels. In heterologous expression systems, phosphatidylinositol 4,5-bisphosphate (PIP2) coordinates with CaM in regulating SK channels. However, the roles and mechanisms of PIP2 in regulating SK channels in cardiomyocytes remain unknown. Here, optogenetics, magnetic nanoparticles, combined with Rosetta structural modeling, and molecular dynamics (MD) simulations revealed the atomistic mechanisms of how PIP2 works in concert with Ca2+-CaM in the SK channel activation. Our computational study affords evidence for the critical role of the amino acid residue R395 in the S6 transmembrane segment, which is localized in propinquity to the intracellular hydrophobic gate. This residue forms a salt bridge with residue E398 in the S6 transmembrane segment from the adjacent subunit. Both R395 and E398 are conserved in all known isoforms of SK channels. Our findings suggest that the binding of PIP2 to R395 residue disrupts the R395:E398 salt bridge, increasing the flexibility of the transmembrane segment S6 and the activation of the channel. Importantly, our findings serve as a platform for testing of structural-based drug designs for therapeutic inhibitors and activators of the SK channel family. The study is timely since inhibitors of SK channels are currently in clinical trials to treat atrial arrhythmias.

Calpain Regulation and Dysregulation-Its Effects on the Intercalated Disk

The intercalated disk is a cardiac specific structure composed of three main protein complexes-adherens junctions, desmosomes, and gap junctions-that work in concert to provide mechanical stability and electrical synchronization to the heart. Each substructure is regulated through a variety of mechanisms including proteolysis. Calpain proteases, a class of cysteine proteases dependent on calcium for activation, have recently emerged as important regulators of individual intercalated disk components. In this review, we will examine how calcium homeostasis regulates normal calpain function. We will also explore how calpains modulate gap junctions, desmosomes, and adherens junctions activity by targeting specific proteins, and describe the molecular mechanisms of how calpain dysregulation leads to structural and signaling defects within the heart. We will then examine how changes in calpain activity affects cardiomyocytes, and how such changes underlie various heart diseases.

Activation of Phospholipase C β by Gβγ and Gαq Involves C-Terminal Rearrangement to Release Autoinhibition

Phospholipase C (PLC) enzymes hydrolyze phosphoinositide lipids to inositol phosphates and diacylglycerol. Direct activation of PLCβ by Gα_q and/or Gβγ subunits mediates signaling by Gq and some Gi coupled G-protein-coupled receptors (GPCRs), respectively. PLCβ isoforms contain a unique C-terminal extension, consisting of proximal and distal C-terminal domains (CTDs) separated by a flexible linker. The structure of PLCβ3 bound to Gα_q is known, however, for both Gα_q and Gβγ; the mechanism for PLCβ activation on membranes is unknown. We examined PLCβ2 dynamics on membranes using hydrogen-deuterium exchange mass spectrometry (HDX-MS). Gβγ caused a robust increase in dynamics of the distal C-terminal domain (CTD). Gα_q showed decreased deuterium incorporation at the Gα_q binding site on PLCβ. In vitro Gβγ-dependent activation of PLC is inhibited by the distal CTD. The results suggest that disruption of autoinhibitory interactions with the CTD leads to increased PLCβ hydrolase activity.

Alterations in phospholipidomic profile in the brain of mouse model of depression induced by chronic unpredictable stress

Depression is a worldwide disability disease associated with high morbidity and has increased dramatically in the last few years. The differential diagnosis and the definition of an individualized therapy for depression are hampered by the absence of specific biomarkers. The aim of this study was to evaluate the phospholipidomic profile of the brain and myocardium in a mouse model of depression induced by chronic unpredictable stress (CUS). The lipidomic profile was evaluated by thin layer and liquid chromatography and mass spectrometry and lipid oxidation was estimated by FOX II assay. Antioxidant enzyme activity and the oxidized/reduced glutathione (GSH/GSSG) ratio were also evaluated. Results showed that chronic stress affects primarily the lipid profile of the brain, inducing an increase in lipid hydroperoxides, which was not detected in the myocardium. A significant decrease in phosphatidylinositol (PI) and in cardiolipin (CL) relative contents and also oxidation of CL and a significant increase of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) were observed in the brain of mice after unpredictable chronic stress conditions. In the myocardium only an increase in PC content was observed. Nevertheless, both organs present a decreased GSH/GSSG ratio when compared to control groups, corroborating the occurrence of oxidative stress. The enzyme activities of catalase (CAT) and superoxide dismutase (SOD) were found to be decreased in the myocardium and increased in the brain, while glutathione reductase (GR) was decreased in the brain. Our results indicate that in a mouse model for studying depression induced by CUS, the modification of the expression of oxidative stress-related enzymes did not prevent lipid oxidation in organs, particularly in the brain. These observations suggest that depression has an impact on the brain lipidome and that further studies are needed to better understand lipids role in depression and to evaluate their potential as future biomarkers.

Structural insights into phospholipase C-β function

Phospholipase C (PLC) enzymes convert phosphatidylinositol-4,5-bisphosphate into the second messengers diacylglycerol and inositol-1,4,5-triphosphate. The production of these molecules promotes the release of intracellular calcium and activation of protein kinase C, which results in profound cellular changes. The PLCβ subfamily is of particular interest given its prominent role in cardiovascular and neuronal signaling and its regulation by G protein-coupled receptors, as PLCβ is the canonical downstream target of the heterotrimeric G protein Gαq. However, this is not the only mechanism regulating PLCβ activity. Extensive structural and biochemical evidence has revealed regulatory roles for autoinhibitory elements within PLCβ, Gβγ, small molecular weight G proteins, and the lipid membrane itself. Such complex regulation highlights the central role that this enzyme plays in cell signaling. A better understanding of the molecular mechanisms underlying the control of its activity will greatly facilitate the search for selective small molecule modulators of PLCβ.

Full-length Gα(q)-phospholipase C-β3 structure reveals interfaces of the C-terminal coiled-coil domain

Phospholipase C-β (PLCβ) is directly activated by Gα_q, but the molecular basis for how its distal C-terminal domain (CTD) contributes to maximal activity is poorly understood. Herein we present both the crystal structure and cryo-EM 3D reconstructions of human full-length PLCβ3 in complex with murine Gα_q. The distal CTD forms an extended, monomeric helical bundle consisting of three anti-parallel segments with structural similarity to membrane-binding bin–amphiphysin–Rvs (BAR) domains. Sequence conservation of the distal CTD identifies putative membrane and protein interaction sites, the latter of which bind the N-terminal helix of Gα_q in both the crystal structure and cryo-EM reconstructions. Functional analysis suggests the distal CTD plays roles in membrane targeting and in optimizing the orientation of the catalytic core at the membrane for maximal rates of lipid hydrolysis.

No contribution of IP3-R(2) to disease phenotype in models of dilated cardiomyopathy or pressure overload hypertrophy

BACKGROUND

We investigated the contribution of inositol(1,4,5)-trisphosphate (Ins(1,4,5)P3 [IP3]) receptors (IP3-R) to disease progression in mouse models of dilated cardiomyopathy (DCM) and pressure overload hypertrophy. Mice expressing mammalian sterile 20-like kinase and dominant-negative phosphatidylinositol-3-kinase in heart (Mst1×dn-PI3K-2Tg; DCM-2Tg) develop severe DCM and conduction block, associated with increased expression of type 2 IP3-R (IP3-R(2)) and heightened generation of Ins(1,4,5)P3. Similar increases in Ins(1,4,5)P3 and IP3-R(2) are caused by transverse aortic constriction.

METHODS AND RESULTS

To evaluate the contribution of IP3-R(2) to disease progression, the DCM-2Tg mice were further crossed with mice in which the type 2 IP3-R (IP3-R(2)-/-) had been deleted (DCM-2Tg×IP3-R(2)-/-) and transverse aortic constriction was performed on IP3-R(2)-/- mice. Hearts from DCM-2Tg mice and DCM-2Tg×IP3-R(2)-/- were similar in terms of chamber dilatation, atrial enlargement, and ventricular wall thinning. Electrophysiological changes were also similar in the DCM-2Tg mice, with and without IP3-R(2). Deletion of IP3-R(2) did not alter the progression of heart failure, because DCM-2Tg mice with and without IP3-R(2) had similarly reduced contractility, increased lung congestion, and atrial thrombus, and both strains died between 10 and 12 weeks of age. Loss of IP3-R(2) did not alter the progression of hypertrophy after transverse aortic constriction.

CONCLUSIONS

We conclude that IP3-R(2) do not contribute to the progression of DCM or pressure overload hypertrophy, despite increased expression and heightened generation of the ligand, Ins(1,4,5)P3.

Inositol 1,4,5-trisphosphate receptors and pacemaker rhythms

Intracellular Ca(2+) plays an important role in the control of the heart rate through the interaction between Ca(2+) release by ryanodine receptors in the sarcoplasmic reticulum (SR) and the extrusion of Ca(2+) by the sodium-calcium exchanger which generates an inward current. A second type of SR Ca(2+) release channel, the inositol 1,4,5-trisphosphate receptor (IP(3)R), can release Ca(2+) from SR stores in many cell types, including cardiac myocytes. However, it is still uncertain whether IP(3)Rs play any functional role in regulating the heart rate. Accumulated evidence shows that IP(3) and IP(3)R are involved in rhythm control in non-cardiac pacemaker tissues and in the embryonic heart. In this review we focus on intracellular Ca(2+) oscillations generated by Ca(2+) release from IP(3)R that initiates membrane depolarization and provides a common mechanism producing spontaneous activity in a range of cells with pacemaker function. Emerging new evidence also suggests that IP(3)/IP(3)Rs play a functional role in normal and diseased hearts and in cardiac rhythm control. Several membrane currents, including a store-operated Ca(2+) current, might be activated by Ca(2+) release from IP(3)Rs. IP(3)/IP(3)R may thus add another dimension to the complex regulation of heart rate.

Phosphoinositides and cardiovascular diseases

Phosphoinositides (PIs), a family of phosphorylated derivatives of the membrane lipid phosphatidylinositol, are established regulators of multiple cellular functions. An increasing amount of evidence has highlighted potential links between PI-mediated signaling pathways and the etiology of many human diseases, including cardiovascular pathologies. This chapter will provide a detailed overview of the peculiar functions of the major cardiovascular PIs in the pathogenesis of atherosclerosis, heart failure, and arrhythmias.

An autoinhibitory helix in the C-terminal region of phospholipase C-β mediates Gαq activation

Phospholipase C-β (PLCβ) is a key regulator of intracellular calcium levels whose activity is controlled by heptahelical receptors that couple to G_q. We have determined atomic structures of two invertebrate homologs of PLCβ (PLC21) from cephalopod retina and identified a helix from the C-terminal regulatory region that interacts with a conserved surface of the catalytic core of the enzyme. Mutations designed to disrupt the analogous interaction in human PLCβ3 dramatically increase basal activity and diminish stimulation by Gα_q. Gα_q binding requires displacement of the autoinhibitory helix from the catalytic core, thus providing an allosteric mechanism for activation of PLCβ.

Multiple aberrations in shared inflammatory and oxidative & nitrosative stress (IO&NS) pathways explain the co-association of depression and cardiovascular disorder (CVD), and the increased risk for CVD and due mortality in depressed patients

There is evidence that there is a bidirectional relationship between major depression and cardiovascular disorder (CVD): depressed patients are a population at risk for increased cardiac morbidity and mortality, and depression is more frequent in patients who suffer from CVD. There is also evidence that inflammatory and oxidative and nitrosative stress (IO&NS) pathways underpin the common pathophysiology of both CVD and major depression. Activation of these pathways may increase risk for both disorders and contribute to shared risk. The shared IO&NS pathways that may contribute to CVD and depression comprise the following: increased levels of pro-inflammatory cytokines, like interleukin-1β (IL-1β), IL-2, IL-6, IL-8, IL-12, tumor necrosis factor-α, and interferon-γ; T cell activation; increased acute phase proteins, like C-reactive protein, haptoglobin, fibrinogen and α1-antitrypsin; complement factors; increased LPS load through bacterial translocation and subsequent gut-derived inflammation; induction of indoleamine 2,3-dioxygenase with increased levels of tryptophan catabolites; decreased levels of antioxidants, like coenzyme Q10, zinc, vitamin E, glutathione and glutathione peroxidase; increased O&NS characterized by oxidative damage to low density lipoprotein (LDL) and phospholipid inositol, increased malondialdehyde, and damage to DNA and mitochondria; increased nitrosative stress; and decreased ω3 polyunsaturated fatty acids (PUFAs). The complex interplay between the abovementioned IO&NS pathways in depression results in pro-atherogenic effects and should be regarded as a risk factor to future clinical CVD and due mortality. We suggest that major depression should be added as a risk factor to the Charlson "comorbidity" index. It is advised that patients with (sub)chronic or recurrent major depression should routinely be assessed by serology tests to predict if they have an increased risk to cardiovascular disorders.

Cardioprotection by omega-3 fatty acids: involvement of PKCs?

It has been known since the 1970s that an increased consumption of n-3 long chain polyunsaturated fatty acids such as eicosapentaenoic acid and docosahexaenoic acid has cardioprotective effects. Epidemiological studies have reported that this effect is due to the prevention of the arrhythmias responsible for sudden cardiac death. Mechanistically, different hypotheses have been put forward to give an explanation. Among them, there are a direct effect of the polyunsaturated fatty acids on ion channels and/or a modification of the regulation of ion channels by protein kinase C's.

Transient receptor potential channelopathies

In the past years, several hereditary diseases caused by defects in transient receptor potential channels (TRP) genes have been described. This review summarizes our current knowledge about TRP channelopathies and their possible pathomechanisms. Based on available genetic indications, we will also describe several putative pathological conditions in which (mal)function of TRP channels could be anticipated.

Targeted G-protein inhibition as a novel approach to decrease vagal atrial fibrillation by selective parasympathetic attenuation

AIMS

The parasympathetic nervous system is thought to play a key role in atrial fibrillation (AF). Since parasympathetic signalling is primarily mediated by the heterotrimeric G-protein, Galpha(i)betagamma, we hypothesized that targeted inhibition of Galpha(i) interactions in the posterior left atrium (PLA) would modify the substrate for vagal AF.

METHODS AND RESULTS

Cell-penetrating(cp)-Galpha(i)1/2 and cp-Galpha(i)3 C-terminal peptides were assessed for their ability to attenuate cholinergic-parasympathetic signalling in isolated feline atrial myocytes and in canine left atrium (LA). Confocal fluorescence microscopy indicated that cp-Galpha(i)1/2 and/or cp-Galpha(i)3 peptides moderated carbachol attenuation of cellular Ca(2+) transients in isolated atrial myocytes. High-density epicardial mapping of dog PLA, left atrial pulmonary veins (PVs), and left atrial appendage (LAA) indicated that the delivery of cp-Galpha(i)1/2 peptide or cp-Galpha(i)3 peptide into the PLA prolonged effective refractory periods at baseline and during vagal stimulation in the PLA and to varying extents also in the LAA and PV regions. After delivery of cp-Galpha(i) peptides into the PLA, AF inducibility during vagal stimulation was significantly diminished.

CONCLUSION

These results demonstrate the feasibility of using specific G(i)-protein inhibition to achieve selective parasympathetic denervation in the PLA, with a resulting change in vagal responsiveness across the entire LA.

Caveolae, ion channels and cardiac arrhythmias

Caveolae are specialized membrane microdomains enriched in cholesterol and sphingolipids which are present in multiple cell types including cardiomyocytes. Along with the essential scaffolding protein caveolin-3, a number of different ion channels and transporters have been localized to caveolae in cardiac myocytes including L-type Ca2+ channels (Ca(v)1.2), Na+ channels (Na(v)1.5), pacemaker channels (HCN4), Na+/Ca2+ exchanger (NCX1) and others. Closely associated with these channels are specific macromolecular signaling complexes that provide highly localized regulation of the channels. Mutations in the caveolin-3 gene (CAV3) have been linked with the congenital long QT syndrome (LQT9), and mutations in caveolar-localized ion channels may contribute to other inherited arrhythmias. Changes in the caveolar microdomain in acquired heart disease may also lead to dysregulation and dysfunction of ion channels, altering the risk of arrhythmias in conditions such as heart failure. This review highlights the existing evidence identifying and characterizing ion channels localized to caveolae in cardiomyocytes and their role in arrhythmogenesis.