Upregulation of the hyperpolarization-activated current increases pacemaker activity of the sinoatrial node and heart rate during pregnancy in mice.

BACKGROUND: Pregnancy is associated with a faster heart rate (HR), which is a risk factor for arrhythmias. However, the underlying mechanisms for this increased HR are poorly understood. Therefore, this study was performed to gain mechanistic insight into the pregnancy-induced increase in HR. METHODS AND RESULTS: Using surface ECG we observed that pregnant (P) mice have faster HR (531±14 beats per minute [bpm]) compared with nonpregnant (NP) mice (470±27 bpm; P<0.03). Results obtained with Langendorff-perfused hearts showed that this difference persisted in the absence of autonomic nervous innervation (NP, 327±16 bpm; P, 385±18 bpm; P<0.02). Spontaneous action potentials of sinoatrial node cells from pregnant mice exhibited higher automaticity (NP, 292±13 bpm; P, 330±12 bpm; P=0.047) and steeper diastolic depolarization (NP, 0.20±0.03 V/s; P, 0.40±0.06 V/s; P=0.004). Pregnancy increased the density of the hyperpolarization-activated current (If) (at -90mV: NP, -15.2±1.0 pA/pF; P, -28.6±2.9 pA/pF; P=0.0002) in sinoatrial node cells. Voltage dependence of the If activation curve and the intracellular cAMP levels were unchanged in sinoatrial node cells of pregnant mice. However, there was a significant increase in HCN2 channel protein expression with no change in HCN4 expression. Maximal depolarizing shift of the If activation curve induced by isoproterenol was attenuated in pregnancy. This reduced response to isoproterenol may be attributable to the lower cAMP sensitivity of HCN2 isoform compared with that of HCN4. CONCLUSIONS: This study shows that an increase in If current density contributes to the acceleration of sinoatrial node automaticity and explains, in part, the higher HR observed in pregnancy.

GABA-B(1) receptors are coupled to the ERK1/2 MAP kinase pathway in the absence of GABA-B(2) subunits.

In the current model of gamma-aminobutyric acid (GABA) B receptor function, there is a requirement for GABA-B(1/2) heterodimerisation for targetting to the cell surface. However, different lines of evidence suggest that the GABA-B(1) subunit can form a functional receptor in the absence of GABA-B(2). We observed coupling of endogenous GABA-B(1) receptors in the DI-TNC1 glial cell line to the ERK pathway in response to baclofen even though these cells do not express GABA-B(2). GABA-B(1A) receptors were also able to mediate a rapid, transient, and dose-dependent activation of the ERK1/2 MAP kinase pathway when transfected alone into HEK 293 cells. The response was abolished by G(i/o) and MEK inhibition, potentiated by inhibitors of phospholipase C and protein kinase C and did not involve PI-3-kinase activity. Finally, using bioluminescence resonance energy transfer and co-immunoprecipitation, we show the existence of homodimeric GABA-B(1A) receptors in transfected HEK293 cells. Altogether, our observations show that GABA-B(1A) receptors are able to activate the ERK1/2 pathway despite the absence of surface targetting partner GABA-B(2) in both HEK 293 cells and the DI-TNC1 cell line.

Signalling complexes associated with adenylyl cyclase II are assembled during their biosynthesis.

We have previously demonstrated that adenylyl cyclase II (ACII) interacts with beta2-adrenergic receptors and heterotrimeric G proteins as part of a pre-assembled signalling complex. In this study, we further show that AC interacts with these proteins before it is targetted to the cell surface. Using a combination of approaches including bioluminescence resonance energy transfer (BRET) in concert with subcellular fractionation, we show that ACII and beta2AR initially interact in the ER. Further, dominant-negative Rab1 and Sar1 GTPases which block anterograde trafficking out of the ER have no effect on either ACII/receptor or ACII/Gbetagamma protein interactions. However, DN Rab1 and Sar1 constructs (but not DN Rabs 2, 6, 8 or 11) prevent the inclusion of Galpha subunits in ACII signalling complexes suggesting it assembles into the complex at a slightly later stage. Thus, like Kir3.1 inwardly rectifying potassium channels, signalosomes containing ACII are formed during their biosynthesis and not in response to agonist at the cell surface.

Pregnancy and oestrogen regulate sinoatrial node calcium homeostasis and accelerate pacemaking.

Aims: During pregnancy, there is a significant increase in heart rate (HR) potentially associated with an increased risk of arrhythmias or exacerbation of pre-existing cardiac conditions endangering both mother and foetus. Calcium homeostasis plays an important role in regulating automaticity of the sinoatrial node (SAN); however, its contribution to the accelerated HR during pregnancy remains unknown. Methods and results: Using murine SAN cells, we showed that pregnancy increased L-type Ca2+ current (ICaL) and CaV1.3 mRNA expression, whereas T-type Ca2+ current (ICaT) and its underlying channel were unchanged. Analysis of SAN intra-cellular Ca2+ oscillations showed that the rate of spontaneous Ca2+ transients was significantly higher in pregnant mice along with a higher mRNA expression of ryanodine receptor. Assessment of supra-ventricular arrhythmias using programmed electrical stimulation protocols on anaesthetized mice revealed higher susceptibility in pregnancy. Of note, the modifications associated with pregnancy were reversible following delivery. Furthermore, chronic administration of 17ß-estradiol (E2) to nodal-like human-induced pluripotent stem cell-derived cardiomyocytes (N-hiPSC-CM), control mice, oestrogen-receptor-ß knockout (ERKOß) but not ERKOα mice, accelerated cardiac automaticity, recapitulating the pregnancy phenotype in both mouse and human SAN cell models. Conclusion: Together, these results indicate that pregnancy considerably alters intra-cellular Ca2+ homeostasis sustaining faster HR during pregnancy. Importantly, these changes were dependent on an oestrogen receptor α (ERα) mechanism that resulted in increased ICaL and spontaneous Ca2+ release from the sarcoplasmic reticulum, highlighting a novel role for oestrogen in regulating HR.

Effects of resveratrol (trans-3,5,4'-trihydroxystilbene) treatment on cardiac remodeling following myocardial infarction.

Resveratrol (RES; trans-3,5,4'-trihydroxystilbene) has been shown to improve health and slow the progression of disease in various models. Several cardioprotective mechanisms have been identified including antioxidant, anti-inflammatory, and antifibrotic actions. Each of these actions is thought to have the ability to attenuate the pathophysiology underlying the deleterious cardiac structural remodeling that results from acute myocardial infarction (MI). Therefore, we evaluated the effect of resveratrol treatment on the progression of cardiac remodeling after MI. Four groups of rats (sham, n = 6; sham + RES, n = 21; MI, n = 26; MI + RES, n = 24) were treated for 13 weeks, starting 7 days before ligation of the left anterior descending coronary artery. Serial transthoracic echocardiography revealed that resveratrol had no effect on MI-induced left-ventricular and left-atrial dilatation or reduction in left-ventricular fractional shortening. Consistent with these findings, resveratrol did not improve the deterioration of hemodynamic function or reduce infarct size at 12 weeks post-MI. Resveratrol-treated animals did, however, show preserved cardiac contractile reserve in response to dobutamine administration. Radioligand binding revealed that MI reduced beta-adrenergic receptor density. Resveratrol administration increased beta-adrenoceptor density, so that resveratrol-treated MI rats had beta-adrenoceptor densities similar to normal rats. Real-time reverse transcription-polymerase chain reaction revealed that MI-induced changes in sarcoplasmic reticulum Ca2+-ATPase 2 and transforming growth factor beta-1 expression were unaltered by resveratrol, whereas MI-induced increases in atrial natriuretic factor (ANF) and connective tissue growth factor (CTGF) expression were attenuated. Resveratrol treatment does not improve cardiac remodeling and global hemodynamic function post-MI but does preserve contractile reserve and attenuate ANF and CTGF up-regulation.

Functional beta-adrenergic receptor signalling on nuclear membranes in adult rat and mouse ventricular cardiomyocytes.

OBJECTIVE: We sought to determine if different beta-adrenergic receptor (betaAR) subtypes, and their associated signalling machinery, are functionally localized to nuclear membranes. METHODS: Employing enriched nuclear preparations, we assayed the specific presence of betaAR by measuring 125I-cyanopindolol (CYP) binding, Western blotting, confocal microscopy and functional assays. RESULTS: Western blots of rat heart nuclear fractions and confocal immunofluorescent analysis of adult rat and mouse ventricular cardiomyocytes displayed the presence of beta 1AR and beta 3AR but, surprisingly, not the beta 2AR on nuclear membranes. Nuclear localization of downstream signalling partners Gs, Gi and adenylyl cyclases II and V/VI was also demonstrated. The functional relevance of nuclear betaAR was shown by receptor-mediated stimulation of adenylyl cyclase activity by isoproterenol but not the beta 3AR-selective agonist CL 316243 in enriched nuclear preparations. We also examined the effect of subtype-selective ligands on the initiation of RNA synthesis in isolated nuclei. Both isoproterenol and another beta 3AR-selective agonist, BRL 37344, increased RNA synthesis which was inhibited by pertussis toxin (PTX). Neither a beta 1AR-selective agonist, xamoterol, nor a beta 2AR-selective agonist, procaterol, was able to stimulate transcription. However, both CGP 20712A and ICI 118,551 blocked isoproterenol-mediated effects to varying extents. PTX treatment also revealed that nuclear betaAR may be coupled to other signalling pathways in addition to Gi, as stimulation under these conditions reduced initiation of transcription below basal levels. CONCLUSION: These results highlight differential subcellular localization for betaAR subtypes and indicate that betaAR may have specific roles in regulating nuclear function in cardiomyocytes.

Characterization of a Human Induced Pluripotent Stem Cell-Derived Cardiomyocyte Model for the Study of Variant Pathogenicity: Validation of a KCNJ2 Mutation.

BACKGROUND: Long-QT syndrome is a potentially fatal condition for which 30% of patients are without a genetically confirmed diagnosis. Rapid identification of causal mutations is thus a priority to avoid at-risk situations that can lead to fatal cardiac events. Massively parallel sequencing technologies are useful for the identification of sequence variants; however, electrophysiological testing of newly identified variants is crucial to demonstrate causality. Long-QT syndrome could, therefore, benefit from having a standardized platform for functional characterization of candidate variants in the physiological context of human cardiomyocytes. METHODS AND RESULTS: Using a variant in Kir2.1 (Gly52Val) revealed by whole-exome sequencing in a patient presenting with symptoms of long-QT syndrome as a proof of principle, we demonstrated that commercially available human induced pluripotent stem cell-derived cardiomyocytes are a powerful model for screening variants involved in genetic cardiac diseases. Immunohistochemistry experiments and whole-cell current recordings in human embryonic kidney cells expressing the wild-type or the mutant Kir2.1 demonstrated that Kir2.1-52V alters channel cellular trafficking and fails to form a functional channel. Using human induced pluripotent stem cell-derived cardiomyocytes, we not only confirmed these results but also further demonstrated that Kir2.1-52V is associated with a dramatic prolongation of action potential duration with evidence of arrhythmic activity, parameters which could not have been studied using human embryonic kidney cells. CONCLUSIONS: Our study confirms the pathogenicity of Kir2.1-52V in 1 patient with long-QT syndrome and also supports the use of isogenic human induced pluripotent stem cell-derived cardiomyocytes as a physiologically relevant model for the screening of variants of unknown function.

Dopamine receptor-interacting protein 78 acts as a molecular chaperone for Ggamma subunits before assembly with Gbeta.

Heterotrimeric G proteins play a central role in intracellular communication mediated by extracellular signals, and both Galpha and Gbetagamma subunits regulate effectors downstream of activated receptors. The particular constituents of the G protein heterotrimer affect both specificity and efficiency of signal transduction. However, little is known about mechanistic aspects of G protein assembly in the cell that would certainly contribute to formation of heterotrimers of specific composition. It was recently shown that phosducin-like protein (PhLP) modulated both Gbetagamma expression and subsequent signaling by chaperoning nascent Gbeta and facilitating heterodimer formation with Ggamma subunits (Lukov, G. L., Hu, T., McLaughlin, J. N., Hamm, H. E., and Willardson, B. M. (2005) EMBO J. 24, 1965-1975; Humrich, J., Bermel, C., Bunemann, M., Harmark, L., Frost, R., Quitterer, U., and Lohse, M. J. (2005) J. Biol. Chem. 280, 20042-20050). Here we demonstrate using a variety of techniques that DRiP78, an endoplasmic reticulum resident protein known to regulate the trafficking of several seven transmembrane receptors, interacts specifically with the Ggamma subunit but not Gbeta or Galpha subunits. Furthermore, we demonstrate that DRiP78 and the Gbeta subunit can compete for the Ggamma subunit. DRiP78 also protects Ggamma from degradation until a stable partner such as Gbeta is provided. Furthermore, DRiP78 interaction may represent a mechanism for assembly of specific Gbetagamma heterodimers, as selectivity was observed among Ggamma isoforms for interaction with DRiP78 depending on the presence of particular Gbeta subunits. Interestingly, we could detect an interaction between DRiP78 and PhLP, suggesting a role of DRiP78 in the assembly of Gbetagamma by linking Ggamma to PhLP.Gbeta complexes. Our results, therefore, suggest a role of DRiP78 as a chaperone in the assembly of Gbetagamma subunits of the G protein.

Seven transmembrane receptor core signaling complexes are assembled prior to plasma membrane trafficking.

Much is known about beta2-adrenergic receptor trafficking and internalization following prolonged agonist stimulation. However, less is known about outward trafficking of the beta2-adrenergic receptor to the plasma membrane or the role that trafficking might play in the assembly of receptor signaling complexes, important for targeting, specificity, and rapidity of subsequent signaling events. Here, by using a combination of bioluminescence resonance energy transfer, bimolecular fluorescence complementation, and confocal microscopy, we evaluated the steps in the formation of the core receptor-G protein heterotrimer complex. By using dominant negative Rab and Sar GTPase constructs, we demonstrate that receptor dimers and receptor-G betagamma complexes initially associate in the endoplasmic reticulum, whereas G alpha subunits are added to the complex during endoplasmic reticulum-Golgi transit. We also observed that G protein heterotrimers adopt different trafficking itineraries when expressed alone or with stoichiometric co-expression with receptor. Furthermore, deliberate mistargeting of specific components of these complexes leads to diversion of other members from their normal subcellular localization, confirming the role of these early interactions in targeting and formation of specific signaling complexes.

Heterotrimeric G proteins form stable complexes with adenylyl cyclase and Kir3.1 channels in living cells.

Bioluminescence resonance energy transfer (BRET) and co-immunoprecipitation experiments revealed that heterotrimeric G proteins and their effectors were found in stable complexes that persisted during signal transduction. Adenylyl cyclase, Kir3.1 channel subunits and several G-protein subunits (Galpha(s), Galpha(i), Gbeta(1) and Ggamma(2)) were tagged with luciferase (RLuc) or GFP, or the complementary fragments of YFP (specifically Gbeta(1)-YFP(1-158) and Ggamma(2)-YFP(159-238), which heterodimerize to produce fluorescent YFP-Gbeta(1)gamma(2)). BRET was observed between adenylyl-cyclase-RLuc or Kir3.1-RLuc and GFP-Ggamma(2), GFP-Gbeta(1) or YFP-Gbeta(1)gamma(2). Galpha subunits were also stably associated with both effectors regardless of whether or not signal transduction was initiated by a receptor agonist. Although BRET between effectors and Gbetagamma was increased by receptor stimulation, our data indicate that these changes are likely to be conformational in nature. Furthermore, receptor-sensitive G-protein-effector complexes could be detected before being transported to the plasma membrane, providing the first direct evidence for an intracellular site of assembly.

KvLQT1 modulates the distribution and biophysical properties of HERG. A novel alpha-subunit interaction between delayed rectifier currents.

Cardiac repolarization is under joint control of the slow (IKs) and rapid (IKr) delayed rectifier currents. Experimental and clinical evidence indicates important functional interactions between these components. We hypothesized that there might be more direct interactions between the KvLQT1 and HERG alpha-subunits of IKs and IKr and tested this notion with a combination of biophysical and biochemical techniques. Co-expression of KvLQT1 with HERG in a mammalian expression system significantly accelerated HERG current deactivation at physiologically relevant potentials by increasing the contribution of the fast component (e.g. upon repolarization from +20 mV to -50 mV: from 20 +/- 3 to 32 +/- 5%, p < 0.05), making HERG current more like native IKr. In addition, HERG current density was approximately doubled (e.g. tail current after a step to +10 mV: 18 +/- 3 versus 39 +/- 7 pA/picofarad, p < 0.01) by co-expression with KvLQT1. KvLQT1 co-expression also increased the membrane immunolocalization of HERG by approximately 2-fold (p < 0.05). HERG and KvLQT1 co-immunolocalized in canine ventricular myocytes and co-immunoprecipitated in cultured Chinese hamster ovary cells as well as in native cardiac tissue, indicating physical interactions between HERG and KvLQT1 proteins in vitro and in vivo. Protein interaction assays also demonstrated binding of KvLQT1 (but not another K+ channel alpha-subunit, Kv3.4) to a C-terminal HERG glutathione S-transferase fusion protein. Co-expression with HERG did not affect the membrane localization or ionic current properties of KvLQT1. This study shows that the alpha-subunit of IKs can interact with and modify the localization and current-carrying properties of the alpha-subunit of IKr, providing potentially novel insights into the molecular function of the delayed rectifier current system.

G protein-coupled receptors form stable complexes with inwardly rectifying potassium channels and adenylyl cyclase.

A large number of studies have demonstrated co-purification or co-immunoprecipitation of receptors with G proteins. We have begun to look for the presence of effector molecules in these receptor complexes. Co-expression of different channel and receptor permutations in COS-7 and HEK 293 cells in combination with co-immunoprecipitation experiments established that the dopamine D(2) and D(4), and beta(2)-adrenergic receptors (beta(2)-AR) form stable complexes with Kir3 channels. The D(4)/Kir3 and D(2) receptor/Kir3 interaction does not occur when the channel and receptor are expressed separately and mixed prior to immunoprecipitation, indicating that the interaction is not an artifact of the experimental protocol and reflects a biosynthetic event. The observed complexes are stable in that they are not disrupted by receptor activation or modulation of G protein alpha subunit function. However, using a peptide that binds Gbetagamma (betaARKct), we show that Gbetagamma is critical for dopamine receptor-Kir3 complex formation, but not for maintenance of the complex. We also provide evidence that Kir3 channels and another effector, adenylyl cyclase, are stably associated with the beta(2)-adrenergic receptor and can be co-immunoprecipitated by anti-receptor antibodies. Using bioluminescence resonance energy transfer, we have shown that in living cells under physiological conditions, beta(2)AR interacts directly with Kir3.1/3.4 and Kir3.1/3.2c heterotetramers as well as with adenylyl cyclase. All of these interactions are stable in the presence of receptor agonists, suggesting that these signaling complexes persist during signal transduction. In addition, we provide evidence that the receptor-effector complexes are also found in vivo. The observation that several G protein-coupled receptors form stable complexes with their effectors suggests that this arrangement might be a general feature of G protein-coupled signal transduction.