Nuclear localization of a1A-adrenergic receptors is required for signaling in cardiac myocytes: an “inside-out” a1-AR signaling pathway

Signalling of Adrenoceptors: Canonical Pathways and New Paradigms

The concept of G protein-coupled receptors initially arose from studies of the β-adrenoceptor, adenylyl cyclase, and cAMP signalling pathway. Since then both canonical G protein-coupled receptor signalling pathways and emerging paradigms in receptor signalling have been defined by experiments focused on adrenoceptors. Here, we discuss the evidence for G protein coupling specificity of the nine adrenoceptor subtypes. We summarise the ability of each of the adrenoceptors to activate proximal signalling mediators including cAMP, calcium, mitogen-activated protein kinases, and protein kinase C pathways. Finally, we highlight the importance of precise spatial and temporal control of adrenoceptor signalling that is controlled by the localisation of receptors at intracellular membranes and in larger protein complexes.

Recent advances on the role of monoamine oxidases in cardiac pathophysiology

Numerous physiological and pathological roles have been attributed to the formation of mitochondrial reactive oxygen species (ROS). However, the individual contribution of different mitochondrial processes independently of bioenergetics remains elusive and clinical treatments unavailable. A notable exception to this complexity is found in the case of monoamine oxidases (MAOs). Unlike other ROS-producing enzymes, especially within mitochondria, MAOs possess a distinct combination of defined molecular structure, substrate specificity, and clinically accessible inhibitors. Another significant aspect of MAO activity is the simultaneous generation of hydrogen peroxide alongside highly reactive aldehydes and ammonia. These three products synergistically impair mitochondrial function at various levels, ultimately jeopardizing cellular metabolic integrity and viability. This pathological condition arises from exacerbated MAO activity, observed in many cardiovascular diseases, thus justifying the exploration of MAO inhibitors as effective cardioprotective strategy. In this context, we not only summarize the deleterious roles of MAOs in cardiac pathologies and the positive effects resulting from genetic or pharmacological MAO inhibition, but also discuss recent findings that expand our understanding on the role of MAO in gene expression and cardiac development.

Nuclear Calcium in Cardiac (Patho)Physiology: Small Compartment, Big Impact

The nucleus of a cardiomyocyte has been increasingly recognized as a morphologically distinct and partially independent calcium (Ca²⁺) signaling microdomain, with its own Ca²⁺-regulatory mechanisms and important effects on cardiac gene expression. In this review, we (1) provide a comprehensive overview of the current state of research on the dynamics and regulation of nuclear Ca²⁺ signaling in cardiomyocytes, (2) address the role of nuclear Ca²⁺ in the development and progression of cardiac pathologies, such as heart failure and atrial fibrillation, and (3) discuss novel aspects of experimental methods to investigate nuclear Ca²⁺ handling and its downstream effects in the heart. Finally, we highlight current challenges and limitations and recommend future directions for addressing key open questions.

Norepinephrine activates β1 -adrenergic receptors at the inner nuclear membrane in astrocytes

Norepinephrine exerts powerful influences on the metabolic, neuroprotective and immunoregulatory functions of astrocytes. Until recently, all effects of norepinephrine were believed to be mediated by receptors localized exclusively to the plasma membrane. However, recent studies in cardiomyocytes have identified adrenergic receptors localized to intracellular membranes, including Golgi and inner nuclear membranes, and have shown that norepinephrine can access these receptors via transporter-mediated uptake. We recently identified a high-capacity norepinephrine transporter, organic cation transporter 3 (OCT3), densely localized to outer nuclear membranes in astrocytes, suggesting that adrenergic signaling may also occur at the inner nuclear membrane in these cells. Here, we used immunofluorescence and western blot to show that β1 -adrenergic receptors are localized to astrocyte inner nuclear membranes; that key adrenergic signaling partners are present in astrocyte nuclei; and that OCT3 and other catecholamine transporters are localized to astrocyte plasma and nuclear membranes. To test the functionality of nuclear membrane β1 -adrenergic receptors, we monitored real-time protein kinase A (PKA) activity in astrocyte nuclei using a fluorescent biosensor. Treatment of astrocytes with norepinephrine induced rapid increases in PKA activity in the nuclear compartment. Pretreatment of astrocytes with inhibitors of catecholamine uptake blocked rapid norepinephrine-induced increases in nuclear PKA activity. These studies, the first to document functional adrenergic receptors at the nuclear membrane in any central nervous system cell, reveal a novel mechanism by which norepinephrine may directly influence nuclear processes. This mechanism may contribute to previously described neuroprotective, metabolic and immunoregulatory actions of norepinephrine.

Ca2+ mishandling and mitochondrial dysfunction: a converging road to prediabetic and diabetic cardiomyopathy

Diabetic cardiomyopathy is defined as the myocardial dysfunction that suffers patients with diabetes mellitus (DM) in the absence of hypertension and structural heart diseases such as valvular or coronary artery dysfunctions. Since the impact of DM on cardiac function is rather silent and slow, early stages of diabetic cardiomyopathy, known as prediabetes, are poorly recognized, and, on many occasions, cardiac illness is diagnosed only after a severe degree of dysfunction was reached. Therefore, exploration and recognition of the initial pathophysiological mechanisms that lead to cardiac dysfunction in diabetic cardiomyopathy are of vital importance for an on-time diagnosis and treatment of the malady. Among the complex and intricate mechanisms involved in diabetic cardiomyopathy, Ca²⁺ mishandling and mitochondrial dysfunction have been described as pivotal early processes. In the present review, we will focus on these two processes and the molecular pathway that relates these two alterations to the earlier stages and the development of diabetic cardiomyopathy.

Chronological and biological aging of the human left ventricular myocardium: Analysis of microRNAs contribution

Aging is the main risk factor for cardiovascular diseases. In humans, cardiac aging remains poorly characterized. Most studies are based on chronological age (CA) and disregard biological age (BA), the actual physiological age (result of the aging rate on the organ structure and function), thus yielding potentially imperfect outcomes. Deciphering the molecular basis of ventricular aging, especially by BA, could lead to major progresses in cardiac research. We aim to describe the transcriptome dynamics of the aging left ventricle (LV) in humans according to both CA and BA and characterize the contribution of microRNAs, key transcriptional regulators. BA is measured using two CA-associated transcriptional markers: CDKN2A expression, a cell senescence marker, and apparent age (AppAge), a highly complex transcriptional index. Bioinformatics analysis of 132 LV samples shows that CDKN2A expression and AppAge represent transcriptomic changes better than CA. Both BA markers are biologically validated in relation to an aging phenotype associated with heart dysfunction, the amount of cardiac fibrosis. BA-based analyses uncover depleted cardiac-specific processes, among other relevant functions, that are undetected by CA. Twenty BA-related microRNAs are identified, and two of them highly heart-enriched that are present in plasma. We describe a microRNA-gene regulatory network related to cardiac processes that are partially validated in vitro and in LV samples from living donors. We prove the higher sensitivity of BA over CA to explain transcriptomic changes in the aging myocardium and report novel molecular insights into human LV biological aging. Our results can find application in future therapeutic and biomarker research.

Attenuation of inward rectifier potassium current contributes to the α1-adrenergic receptor-induced proarrhythmicity in the caval vein myocardium

AIM

This study is aimed at investigation of electrophysiological effects of α1-adrenoreceptor (α1-AR) stimulation in the rat superior vena cava (SVC) myocardium, which is one of the sources of proarrhythmic activity.

METHODS

α1-ARs agonists (phenylephrine-PHE or norepinephrine in presence of atenolol-NE + ATL) were applied to SVC and atrial tissue preparations or isolated cardiomyocytes, which were examined using optical mapping, glass microelectrodes or whole-cell patch clamp. α1-ARs distribution was evaluated using immunofluorescence. Kir2.X mRNA and protein level were estimated using RT-PCR and Western blotting.

RESULTS

PHE or NE + ATL application caused a significant suppression of the conduction velocity (CV) of excitation and inexcitability in SVC, an increase in the duration of electrically evoked action potentials (APs), a decrease in the maximum upstroke velocity (dV/dt_max ) and depolarization of the resting membrane potential (RMP) in SVC to a greater extent than in atria. The effects induced by α1-ARs activation in SVC were attenuated by protein kinase C inhibition (PKC). The whole-cell patch clamp revealed PHE-induced suppression of outward component of I_K1 inward rectifier current in isolated SVC, but not atrial myocytes. These effects can be mediated by α1A subtype of α-ARs found in abundance in rat SVC. The basal I_K1 level in SVC was much lower than in atria as a result of the weaker expression of Kir2.2 channels.

CONCLUSION

Therefore, the reduced density of I_K1 in rat SVC cardiomyocytes and sensitivity of this current to α1A-AR stimulation via PKC-dependent pathways might lead to proarrhythmic conduction in SVC myocardium by inducing RMP depolarization, AP prolongation, CV and dV/dt_max decrease.

Catecholestradiol Activation of Adrenergic Receptors Induces Endometrial Cell Survival via p38 MAPK Signaling

CONTEXT

Enhanced levels of catecholestradiols, 2-hydroxyestradiol (2-OHE2) or 4-hydroxyestradiol (4-OHE2), are reported in endometriosis. During gestation, catecholestradiol activation of adrenergic receptors (AR) elevates estrogen receptor (ER)-independent proliferation of uterine arterial endothelial cells.

OBJECTIVE

To investigate β-AR-mediated catecholestradiol effects on human endometrial stromal cell (HESC) and epithelial cell survival in endometriosis.

DESIGN

β-AR immunostaining of eutopic and ectopic endometria (n = 9). Assays for cell viability, 5-bromo-2'-deoxyuridine proliferation, apoptosis, quantitative PCR, and estrogenicity (alkaline phosphatase activity), as well as siRNA β-AR silencing and immunoblot analyses of cultured HESCs or Ishikawa cells treated with control or 2-OHE2 or 4-OHE2 ±β-AR antagonist or ±p38 MAPK inhibitor.

SETTING

University research institution.

PATIENTS

Women with or without endometriosis.

INTERVENTIONS

None.

MAIN OUTCOME MEASURES

β-AR expression in eutopic vs ectopic endometria and regulation of HESC survival by 2-OHE2 and 4-OHE2.

RESULTS

Eutopic and ectopic endometrial stromal and epithelial cells displayed β2-AR immunoreactivity with increased staining in the functionalis vs basalis layer (P < 0.05). Both 2-OHE2 and 4-OHE2 enhanced HESC and Ishikawa cell survival (P < 0.05), an effect abrogated by β-AR antagonist propranolol, but not ER antagonist ICI182,780. 2-OHE2 or 4-OHE2 failed to induce cell survival and estrogenic activity in ADRB2-silenced HESCs and in Ishikawa cells, respectively. Although 2-OHE2 inhibited apoptosis and BAX mRNA expression, 4-OHE2 induced proliferation and decreased apoptosis (P < 0.05). Both catecholestradiols elevated phospho-p38 MAPK levels (P < 0.05), which was blocked by propranolol, and p38 MAPK inhibitor reversed catecholestradiol-enhanced HESC survival.

CONCLUSIONS

Catecholestradiols increase endometrial cell survival by an ER-independent β-AR-mediated p38 MAPK activation, suggesting that agents blocking β-AR (e.g., propranolol) or inhibiting 2-OHE2- or 4-OHE2-generating enzymes (i.e., CYP1A1/B1) could treat endometriosis.

Cardiac α1A-adrenergic receptors: emerging protective roles in cardiovascular diseases

α1-Adrenergic receptors (ARs) are catecholamine-activated G protein-coupled receptors (GPCRs) that are expressed in mouse and human myocardium and vasculature, and play essential roles in the regulation of cardiovascular physiology. Though α1-ARs are less abundant in the heart than β1-ARs, activation of cardiac α1-ARs results in important biologic processes such as hypertrophy, positive inotropy, ischemic preconditioning, and protection from cell death. Data from the Antihypertensive and Lipid-Lowering Treatment to Prevent Heart Attack Trial (ALLHAT) indicate that nonselectively blocking α1-ARs is associated with a twofold increase in adverse cardiac events, including heart failure and angina, suggesting that α1-AR activation might also be cardioprotective in humans. Mounting evidence implicates the α1A-AR subtype in these adaptive effects, including prevention and reversal of heart failure in animal models by α1A agonists. In this review, we summarize recent advances in our understanding of cardiac α1A-ARs.

Location Bias as Emerging Paradigm in GPCR Biology and Drug Discovery

GPCRs are the largest receptor family that are involved in virtually all biological processes. Pharmacologically, they are highly druggable targets, as they cover more than 40% of all drugs in the market. Our knowledge of biased signaling provided insight into pharmacology vastly improving drug design to avoid unwanted effects and achieve higher efficacy and selectivity. However, yet another feature of GPCR biology is left largely unexplored, location bias. Recent developments in this field show promising avenues for evolution of new class of pharmaceuticals with greater potential for higher level of precision medicine. Further consideration and understanding of this phenomenon with deep biochemical and molecular insights would pave the road to success. In this review, we critically analyze this perspective and discuss new avenues of investigation.

Angiotensin Receptors Heterodimerization and Trafficking: How Much Do They Influence Their Biological Function?

G-protein–coupled receptors (GPCRs) are targets for around one third of currently approved and clinical prescribed drugs and represent the largest and most structurally diverse family of transmembrane signaling proteins, with almost 1000 members identified in the human genome. Upon agonist stimulation, GPCRs are internalized and trafficked inside the cell: they may be targeted to different organelles, recycled back to the plasma membrane or be degraded. Once inside the cell, the receptors may initiate other signaling pathways leading to different biological responses. GPCRs’ biological function may also be influenced by interaction with other receptors. Thus, the ultimate cellular response may depend not only on the activation of the receptor from the cell membrane, but also from receptor trafficking and/or the interaction with other receptors. This review is focused on angiotensin receptors and how their biological function is influenced by trafficking and interaction with others receptors.

Protein kinase C-δ interacts with and phosphorylates ARD1

Protein kinase C-δ (PKCδ) is a diacylglycerol-dependent, calcium-independent novel PKC isoform that is engaged in various cell signaling pathways, such as cell proliferation, apoptosis, inflammation, and oxidative stress. In this study, we searched for proteins that bind PKCδ using a yeast two-hybrid assay and identified murine arrest-defective 1 (mARD1) as a binding partner. The interaction between PKCδ and mARD1 was confirmed by glutathione S-transferase pull-down and co-immunoprecipitation assays. Furthermore, recombinant PKCδ phosphorylated full-length mARD1 protein. The NetPhos online prediction tool suggested PKCδ phosphorylates Ser⁸⁰ , Ser¹⁰⁸ , and Ser¹¹⁴ residues of mARD1 with the highest probability. Based on these results, we synthesized peptides containing these sites and examined their phosphorylations using recombinant PKCδ. Autoradiography confirmed these sites were efficiently phosphorylated. Consequent mass spectrometry and peptide sequencing in combination with MALDI-TOF MS/MS confirmed that Ser⁸⁰ and Ser¹⁰⁸ were major phosphorylation sites. The alanine mutations of Ser⁸⁰ and Ser¹⁰⁸ abolished the phosphorylation of mARD1 by PKCδ in 293T cells supporting these observations. In addition, kinase assays using various PKC isotypes showed that Ser⁸⁰ of ARD1 was phosphorylated by PKCβI and PKCζ isotypes with the highest selectivity, while Ser¹⁰⁸ and/or Ser¹¹⁴ were phosphorylated by PKCγ with activities comparable to that of the PKCδ isoform. Overall, these results suggest the possibility that PKCδ transduces signals by regulating phosphorylation of ARD1.

Coupling to Gq Signaling Is Required for Cardioprotection by an Alpha-1A-Adrenergic Receptor Agonist

RATIONALE

Gq signaling in cardiac myocytes is classically considered toxic. Targeting Gq directly to test this is problematic, because cardiac myocytes have many Gq-coupled receptors.

OBJECTIVE

Test whether Gq coupling is required for the cardioprotective effects of an alpha-1A-AR (adrenergic receptor) agonist.

METHODS AND RESULTS

In recombinant cells, a mouse alpha-1A-AR with a 6-residue substitution in the third intracellular loop does not couple to Gq signaling. Here we studied a knockin mouse with this alpha-1A-AR mutation. Heart alpha-1A receptor levels and antagonist affinity in the knockin were identical to wild-type. In wild-type cardiac myocytes, the selective alpha-1A agonist A61603-stimulated phosphoinositide-phospholipase C and myocyte contraction. In myocytes with the alpha-1A knockin, both A61603 effects were absent, indicating that Gq coupling was absent. Surprisingly, A61603 activation of cardioprotective ERK (extracellular signal-regulated kinase) was markedly impaired in the KI mutant myocytes, and A61603 did not protect mutant myocytes from doxorubicin toxicity in vitro. Similarly, mice with the α1A KI mutation had increased mortality after transverse aortic constriction, and A61603 did not rescue cardiac function in mice with the Gq coupling-defective alpha-1A receptor.

CONCLUSIONS

Gq coupling is required for cardioprotection by an alpha-1A-AR agonist. Gq signaling can be adaptive.

Alpha-1 Adrenergic Receptor Agonist Phenylephrine Inhibits Sepsis-Induced Cardiomyocyte Apoptosis and Cardiac Dysfunction via Activating ERK1/2 Signal Pathway

It was demonstrated that α1 adrenergic receptor (α1-AR) activation by phenylephrine (PE) attenuated cardiac dysfunction in lipopolysaccharide (LPS)-challenged mice. However, it is unclear whether PE suppresses sepsis-induced cardiomyocyte apoptosis. Here, we investigated the effects of PE on cardiomyocyte apoptosis in LPS-treated adult rat ventricular myocytes (ARVMs) and septic rats induced by cecal ligation and puncture. Cardiomyocyte apoptosis and caspase activity were detected by TUNEL and spectrophotometrical assay, respectively. Bax, Bcl-2 and cytochrome c (Cyt c) levels as well as IκBα, ERK1/2, p38 MAPK, JNK and cardiac troponin I (cTnI) phosphorylation were analyzed by Western blotting, and TNF-α concentration was analyzed by ELISA. PE inhibited LPS-induced caspase-3 activation in ARVMs, which was reversed by prazosin (a membrane permeable α1-AR antagonist), but not by CGP12177A (a membrane impermeable α1-AR antagonist). PE upregulated phosphorylated ERK1/2 and Bcl-2 contents, decreased TNF-α and Bax levels, Cyt c release, caspase-8/-9 activities as well as IκBα, p38MAPK and JNK phosphorylation in LPS-treated ARVMs, all of which were abolished by prazosin. Treatment with U0126 (a specific ERK1/2 inhibitor) reversed the effects of PE on IκBα, p38MAPK and JNK phosphorylation as well as caspase-3/-8/-9 activation in LPS-treated ARVMs. In septic rats, PE not only inhibited myocardial apoptosis as well as IκBα, p38MAPK, and JNK phosphorylation, but also upregulated myocardial phosphorylated ERK1/2. Furthermore, PE inhibited myocardial cTnI phosphorylation and improved cardiac function in septic rats. Taken together, our data suggest that α1-AR activation by PE inhibits sepsis-induced cardiomyocyte apoptosis and cardiac dysfunction via activating ERK1/2 signal pathway.

Nucleoligands-repurposing G Protein-coupled Receptor Ligands to Modulate Nuclear-localized G Protein-coupled Receptors in the Cardiovascular System

There is significant evidence that internal pools of G protein-coupled receptors (GPCRs) exist and may be affected by both endogenous signaling molecules and hydrophobic pharmaceutical ligands, once assumed to only affect cell surface versions of these receptors. Here, we discuss evidence that the biology of nuclear GPCRs in particular is complex, rich, and highly interactive with GPCR signaling from the cell surface. Caging existing GPCR ligands may be an excellent means of further stratifying the phenotypic effects of known pharmacophores such as β-adrenergic, angiotensin II, and type B endothelin receptor ligands in the cardiovascular system. We describe some synthetic strategies we have used to design ligands to go from in cellulo to in vivo experiments. We also consider how surface and intracellular GPCR signaling might be integrated and ways to dissect this. If they could be selectively targeted, nuclear GPCRs and their associated nucleoligands would represent a completely novel area for exploration by Pharma.

Organic cation transporter 3: A cellular mechanism underlying rapid, non-genomic glucocorticoid regulation of monoaminergic neurotransmission, physiology, and behavior

Contribution to Special Issue on Fast effects of steroids. Corticosteroid hormones act at intracellular glucocorticoid receptors (GR) and mineralocorticoid receptors (MR) to alter gene expression, leading to diverse physiological and behavioral responses. In addition to these classical genomic effects, corticosteroid hormones also exert rapid actions on physiology and behavior through a variety of non-genomic mechanisms, some of which involve GR or MR, and others of which are independent of these receptors. One such GR-independent mechanism involves corticosteroid-induced inhibition of monoamine transport mediated by "uptake₂" transporters, including organic cation transporter 3 (OCT3), a low-affinity, high-capacity transporter for norepinephrine, epinephrine, dopamine, serotonin and histamine. Corticosterone directly and acutely inhibits OCT3-mediated transport. This review describes the studies that initially characterized uptake₂ processes in peripheral tissues, and outlines studies that demonstrated OCT3 expression and corticosterone-sensitive monoamine transport in the brain. Evidence is presented supporting the hypothesis that corticosterone can exert rapid, GR-independent actions on neuronal physiology and behavior by inhibiting OCT3-mediated monoamine clearance. Implications of this mechanism for glucocorticoid-monoamine interactions in the context-dependent regulation of behavior are discussed.

Subcellular compartmentalization of proximal Gαq-receptor signaling produces unique hypertrophic phenotypes in adult cardiac myocytes

G protein-coupled receptors that signal through Gα_q (G_q receptors), such as α₁-adrenergic receptors (α₁-ARs) or angiotensin receptors, share a common proximal signaling pathway that activates phospholipase Cβ1 (PLCβ1), which cleaves phosphatidylinositol 4,5-bisphosphate (PIP₂) to produce inositol 1,4,5-trisphosphate (IP₃) and diacylglycerol. Despite these common proximal signaling mechanisms, G_q receptors produce distinct physiological responses, yet the mechanistic basis for this remains unclear. In the heart, G_q receptors are thought to induce myocyte hypertrophy through a mechanism termed excitation-transcription coupling, which provides a mechanistic basis for compartmentalization of calcium required for contraction versus IP₃-dependent intranuclear calcium required for hypertrophy. Here, we identified subcellular compartmentalization of G_q-receptor signaling as a mechanistic basis for unique G_q receptor-induced hypertrophic phenotypes in cardiac myocytes. We show that α₁-ARs co-localize with PLCβ1 and PIP₂ at the nuclear membrane. Further, nuclear α₁-ARs induced intranuclear PLCβ1 activity, leading to histone deacetylase 5 (HDAC5) export and a robust transcriptional response (i.e. significant up- or down-regulation of 806 genes). Conversely, we found that angiotensin receptors localize to the sarcolemma and induce sarcolemmal PLCβ1 activity, but fail to promote HDAC5 nuclear export, while producing a transcriptional response that is mostly a subset of α₁-AR-induced transcription. In summary, these results link G_q-receptor compartmentalization in cardiac myocytes to unique hypertrophic transcription. They suggest a new model of excitation-transcription coupling in adult cardiac myocytes that accounts for differential G_q-receptor localization and better explains distinct physiological functions of G_q receptors.

GPCR signalling from within the cell

Traditionally, signal transduction from GPCRs is thought to emanate from the cell surface where receptor interactions with external stimuli can be transformed into a broad range of cellular responses. However, emergent data show that numerous GPCRs are also associated with various intracellular membranes where they may couple to different signalling systems, display unique desensitization patterns and/or exhibit distinct patterns of subcellular distribution. Although many GPCRs can be activated at the cell surface and subsequently endocytosed and transported to a unique intracellular site, other intracellular GPCRs can be activated in situ either via de novo ligand synthesis, diffusion of permeable ligands or active transport of nonpermeable ligands. Current findings reinforce the notion that intracellular GPCRs play a dynamic role in various biological functions including learning and memory, contractility and angiogenesis. As new intracellular GPCR roles are defined, the need to selectively tailor agonists and/or antagonists to both intracellular and cell surface receptors may lead to the development of more effective therapeutic tools.

LINKED ARTICLES

This article is part of a themed section on Molecular Pharmacology of GPCRs. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.21/issuetoc.

Calcium Signaling and Transcriptional Regulation in Cardiomyocytes

Calcium (Ca 2+ ) is a universal regulator of various cellular functions. In cardiomyocytes, Ca 2+ is the central element of excitation–contraction coupling, but also impacts diverse signaling cascades and influences the regulation of gene expression, referred to as excitation–transcription coupling. Disturbances in cellular Ca 2+ -handling and alterations in Ca 2+ -dependent gene expression patterns are pivotal characteristics of failing cardiomyocytes, with several excitation–transcription coupling pathways shown to be critically involved in structural and functional remodeling processes. Thus, targeting Ca 2+ -dependent transcriptional pathways might offer broad therapeutic potential. In this article, we (1) review cytosolic and nuclear Ca 2+ dynamics in cardiomyocytes with respect to their impact on Ca 2+ -dependent signaling, (2) give an overview on Ca 2+ -dependent transcriptional pathways in cardiomyocytes, and (3) discuss implications of excitation–transcription coupling in the diseased heart.

Sarcomeric protein modification during adrenergic stress enhances cross-bridge kinetics and cardiac output

Molecular adaptations to chronic neurohormonal stress, including sarcomeric protein cleavage and phosphorylation, provide a mechanism to increase ventricular contractility and enhance cardiac output, yet the link between sarcomeric protein modifications and changes in myocardial function remains unclear. To examine the effects of neurohormonal stress on posttranslational modifications of sarcomeric proteins, mice were administered combined α- and β-adrenergic receptor agonists (isoproterenol and phenylephrine, IPE) for 14 days using implantable osmotic pumps. In addition to significant cardiac hypertrophy and increased maximal ventricular pressure, IPE treatment accelerated pressure development and relaxation (74% increase in dP/dt_max and 14% decrease in τ), resulting in a 52% increase in cardiac output compared with saline (SAL)-treated mice. Accelerated pressure development was maintained when accounting for changes in heart rate and preload, suggesting that myocardial adaptations contribute to enhanced ventricular contractility. Ventricular myocardium isolated from IPE-treated mice displayed a significant reduction in troponin I (TnI) and myosin-binding protein C (MyBP-C) expression and a concomitant increase in the phosphorylation levels of the remaining TnI and MyBP-C protein compared with myocardium isolated from saline-treated control mice. Skinned myocardium isolated from IPE-treated mice displayed a significant acceleration in the rate of cross-bridge (XB) detachment (46% increase) and an enhanced magnitude of XB recruitment (43% increase) at submaximal Ca²⁺ activation compared with SAL-treated mice but unaltered myofilament Ca²⁺ sensitivity of force generation. These findings demonstrate that sarcomeric protein modifications during neurohormonal stress are molecular adaptations that enhance in vivo ventricular contractility through accelerated XB kinetics to increase cardiac output.NEW & NOTEWORTHY Posttranslational modifications to sarcomeric regulatory proteins provide a mechanism to modulate cardiac function in response to stress. In this study, we demonstrate that neurohormonal stress produces modifications to myosin-binding protein C and troponin I, including a reduction in protein expression within the sarcomere and increased phosphorylation of the remaining protein, which serve to enhance cross-bridge kinetics and increase cardiac output. These findings highlight the importance of sarcomeric regulatory protein modifications in modulating ventricular function during cardiac stress.

Organic cation transporter 3 (OCT3) is localized to intracellular and surface membranes in select glial and neuronal cells within the basolateral amygdaloid complex of both rats and mice

Organic cation transporter 3 (OCT3) is a high-capacity, low-affinity transporter that mediates corticosterone-sensitive uptake of monoamines including norepinephrine, epinephrine, dopamine, histamine and serotonin. OCT3 is expressed widely throughout the amygdaloid complex and other brain regions where monoamines are key regulators of emotional behaviors affected by stress. However, assessing the contribution of OCT3 to the regulation of monoaminergic neurotransmission and monoamine-dependent regulation of behavior requires fundamental information about the subcellular distribution of OCT3 expression. We used immunofluorescence and immuno-electron microscopy to examine the cellular and subcellular distribution of the transporter in the basolateral amygdaloid complex of the rat and mouse brain. OCT3-immunoreactivity was observed in both glial and neuronal perikarya in both rat and mouse amygdala. Electron microscopic immunolabeling revealed plasma membrane-associated OCT3 immunoreactivity on axonal, dendritic, and astrocytic processes adjacent to a variety of synapses, as well as on neuronal somata. In addition to plasma membrane sites, OCT3 immunolabeling was also observed associated with neuronal and glial endomembranes, including Golgi, mitochondrial and nuclear membranes. Particularly prominent labeling of the outer nuclear membrane was observed in neuronal, astrocytic, microglial and endothelial perikarya. The localization of OCT3 to neuronal and glial plasma membranes adjacent to synaptic sites is consistent with an important role for this transporter in regulating the amplitude, duration, and physical spread of released monoamines, while its localization to mitochondrial and outer nuclear membranes suggests previously undescribed roles for the transporter in the intracellular disposition of monoamines.

Nuclear calcium in cardiac myocytes

Calcium (Ca) is a universal second messenger involved in the regulation of various cellular processes, including electrical signaling, contraction, secretion, memory, gene transcription, and cell death. In heart, Ca governs cardiomyocyte contraction, is central in electrophysiological properties, and controls major signaling pathway implicated in gene transcription. How cardiomyocytes decode Ca signal to regulate gene expression without interfering with, or being controlled by, "contractile" Ca that floods the entire cytosol during each heartbeat is still elusive. In this review, we summarize recent findings on nuclear Ca regulation and its downstream signaling in cardiomyocytes. We will address difficulties in reliable quantification of nuclear Ca fluxes and discuss its role in the development and progression of cardiac hypertrophy and heart failure. We also point out key open questions to stimulate future work.

Regulation of phosphatidylinositol-specific phospholipase C at the nuclear envelope in cardiac myocytes

Phosphatidylinositol 4,5-bisphosphate hydrolysis at the plasma membrane by phospholipase C is one of the major hormone regulated intracellular signaling systems. The system generates the diffusible second messenger IP3 and the membrane bound messenger diacylglycerol. Spatial regulation of this system has been thought to be through specific subcellular distributions of the IP3 receptor or PKC. As is becoming increasingly apparent, receptor-stimulated signaling systems are also found at intracellular membranes. As discussed in this issue, G protein-coupled receptors have been identified at the nuclear envelope implying intracellular localization of the signaling systems that respond to G protein-coupled receptors. Here, we discuss the evidence for the existence of PLC signals that regulate nuclear processes, as well as the evidence for nuclear and nuclear envelope localization of PLC signaling components, and their implications for cardiac physiology and disease.

Nuclear compartmentalization of α1-adrenergic receptor signaling in adult cardiac myocytes

Although convention dictates that G protein-coupled receptors localize to and signal at the plasma membrane, accumulating evidence suggests that G protein-coupled receptors localize to and signal at intracellular membranes, most notably the nucleus. In fact, there is now significant evidence indicating that endogenous alpha-1 adrenergic receptors (α1-ARs) localize to and signal at the nuclei in adult cardiac myocytes. Cumulatively, the data suggest that α1-ARs localize to the inner nuclear membrane, activate intranuclear signaling, and regulate physiologic function in adult cardiac myocytes. Although α1-ARs signal through Gαq, unlike other Gq-coupled receptors, α1-ARs mediate important cardioprotective functions including adaptive/physiologic hypertrophy, protection from cell death (survival signaling), positive inotropy, and preconditioning. Also unlike other Gq-coupled receptors, most, if not all, functional α1-ARs localize to the nuclei in adult cardiac myocytes, as opposed to the sarcolemma. Together, α1-AR nuclear localization and cardioprotection might suggest a novel model for compartmentalization of Gq-coupled receptor signaling in which nuclear Gq-coupled receptor signaling is cardioprotective.

Nuclear G protein signaling: new tricks for old dogs

According to the standard model of G protein-coupled receptor (GPCR) signaling, GPCRs are localized to the cell membrane where they respond to extracellular signals. Stimulation of GPCRs leads to the activation of heterotrimeric G proteins and their intracellular signaling pathways. However, this model fails to accommodate GPCRs, G proteins, and their downstream effectors that are found on the nuclear membrane or in the nucleus. Evidence from isolated nuclei indicates the presence of GPCRs on the nuclear membrane that can activate similar G protein-dependent signaling pathways in the nucleus as at the cell surface. These pathways also include activation of cyclic adenosine monophosphate, calcium and nitric oxide synthase signaling in cardiomyocytes. In addition, a number of distinct heterotrimeric and monomeric G proteins have been found in the nucleus of various cell types. This review will focus on understanding the function of nuclear G proteins with a focus on cardiac signaling where applicable.

G protein-coupled receptor signaling in cardiac nuclear membranes

G protein-coupled receptors (GPCRs) play key physiological roles and represent a significant target for drug development. However, historically, drugs were developed with the understanding that GPCRs as a therapeutic target exist solely on cell surface membranes. More recently, GPCRs have been detected on intracellular membranes, including the nuclear membrane, and the concept that intracellular GPCRs are functional is become more widely accepted. Nuclear GPCRs couple to effectors and regulate signaling pathways, analogous to their counterparts at the cell surface, but may serve distinct biological roles. Hence, the physiological responses mediated by GPCR ligands, or pharmacological agents, result from the integration of their actions at extracellular and intracellular receptors. The net effect depends on the ability of a given ligand or drug to access intracellular receptors, as dictated by its structure, lipophilic properties, and affinity for nuclear receptors. This review will discuss angiotensin II, endothelin, and β-adrenergic receptors located on the nuclear envelope in cardiac cells in terms of their origin, activation, and role in cardiovascular function and pathology.

Intracellular mGluR5 plays a critical role in neuropathic pain

Spinal mGluR5 is a key mediator of neuroplasticity underlying persistent pain. Although brain mGluR5 is localized on cell surface and intracellular membranes, neither the presence nor physiological role of spinal intracellular mGluR5 is established. Here we show that in spinal dorsal horn neurons >80% of mGluR5 is intracellular, of which ∼60% is located on nuclear membranes, where activation leads to sustained Ca(2+) responses. Nerve injury inducing nociceptive hypersensitivity also increases the expression of nuclear mGluR5 and receptor-mediated phosphorylated-ERK1/2, Arc/Arg3.1 and c-fos. Spinal blockade of intracellular mGluR5 reduces neuropathic pain behaviours and signalling molecules, whereas blockade of cell-surface mGluR5 has little effect. Decreasing intracellular glutamate via blocking EAAT-3, mimics the effects of intracellular mGluR5 antagonism. These findings show a direct link between an intracellular GPCR and behavioural expression in vivo. Blockade of intracellular mGluR5 represents a new strategy for the development of effective therapies for persistent pain.

Caged ligands to study the role of intracellular GPCRs

In addition to cell surface membranes, numerous G protein-coupled receptors (GPCRs) are located on intracellular membranes including the nuclear envelope. Although the role of numerous GPCRs at the cell surface has been well characterized, the physiological function of these same receptors located on intracellular membranes remains to be determined. Here, we employ a novel caged Ang-II analog, cAng-II, to compare the effects of the activation of cell surface versus intracellular angiotensin receptors in intact cardiomyocytes. When added extracellularly to HEK 293 cells, Ang-II and photolysed cAng-II increased ERK1/2 phosphorylation (via AT1R) and cGMP production (AT2R). In contrast unphotolysed cAng-II did not. Cellular uptake of cAng-II was 6-fold greater than that of Ang-II and comparable to the HIV TAT(48-60) peptide. Intracellular photolysis of cAng-II induced an increase in nucleoplasmic Ca(2+) ([Ca(2+)]n) that was greater than that induced by extracellular application of Ang-II. We conclude that cell-permeable ligands that can access intracellular GPCRs may evoke responses distinct from those with access restricted to the same receptor located on the cell surface.

An integrated mechanism of cardiomyocyte nuclear Ca(2+) signaling

In cardiomyocytes, Ca(2+) plays a central role in governing both contraction and signaling events that regulate gene expression. Current evidence indicates that discrimination between these two critical functions is achieved by segregating Ca(2+) within subcellular microdomains: transcription is regulated by Ca(2+) release within nuclear microdomains, and excitation-contraction coupling is regulated by cytosolic Ca(2+). Accordingly, a variety of agonists that control cardiomyocyte gene expression, such as endothelin-1, angiotensin-II or insulin-like growth factor-1, share the feature of triggering nuclear Ca(2+) signals. However, signaling pathways coupling surface receptor activation to nuclear Ca(2+) release, and the phenotypic responses to such signals, differ between agonists. According to earlier hypotheses, the selective control of nuclear Ca(2+) signals by activation of plasma membrane receptors relies on the strategic localization of inositol trisphosphate receptors at the nuclear envelope. There, they mediate Ca(2+) release from perinuclear Ca(2+) stores upon binding of inositol trisphosphate generated in the cytosol, which diffuses into the nucleus. More recently, identification of such receptors at nuclear membranes or perinuclear sarcolemmal invaginations has uncovered novel mechanisms whereby agonists control nuclear Ca(2+) release. In this review, we discuss mechanisms for the selective control of nuclear Ca(2+) signals with special focus on emerging models of agonist receptor activation.