Cardiac contraction and relaxation are regulated by distinct subcellular cAMP pools

Study on the underlying mechanism of Huachansu Capsule induced cardiotoxicity of normal rat by integrating transcriptomics, metabolomics and network toxicology

ETHNOPHARMACOLOGICAL RELEVANCE

Huachansu Capsule (HCSc) is a simple enteric-coated capsule refined from the skin of the dried toad, a traditional medicinal herb. It has been used clinically for many years to treat a variety of malignant tumors with remarkable efficacy. To date, a number of main components of HCSc have been reported to be cardiotoxic, but the specific mechanism of cardiotoxicity is still unknown.

AIM OF THE STUDY

The aim of this study was to elucidate the possible cardiotoxic symptoms caused by high-doses of HCSc and to further reveal the complex mechanisms by which it causes cardiotoxicity.

MATERIALS AND METHODS

UPLC-Q-Exactive Orbitrap MS and network toxicology were used to identify and predict the potential toxic components, related signaling pathways. Then, we used acute and sub-acute toxicity experiments to reveal the apparent phenomenon of HCSc-induced cardiotoxicity. Finally, we combined transcriptomics and metabolomics to elucidate the potential mechanism of action, and verified the putative mechanism by molecular docking, RT-qPCR, and Western blot.

RESULTS

We found 8 toad bufadienolides components may be induced cardiac toxicity HCSc main toxic components. Through toxicity experiments, we found that high dose of HCSc could increase a variety of blood routine indexes, five cardiac enzymes, heart failure indexes (BNP), troponin (cTnI and cTnT), heart rate and the degree of heart tissue damage, while low-dose of HCSc had no such changes. In addition, by molecular docking, found that 8 kinds of main toxic components and cAMP, AMPK, IL1β, mTOR all can be a very good combination, especially in the cAMP. Meanwhile, RT-qPCR and Western blot results showed that HCSc could induce cardiotoxicity by regulating a variety of heart-related differential genes and activating the cAMP signaling pathway.

CONCLUSIONS

In this study, network toxicology, transcriptomics and metabolomics were used to elucidate the complex mechanism of possible cardiotoxicity induced by high-dose HCSc. Animal experiments, molecular docking, Western blot and RT-qPCR experiments were also used to verify the above mechanism. These findings will inform further mechanistic studies and provide theoretical support for its safe clinical application.

Inhibition of the upregulated phosphodiesterase 4D isoforms improves SERCA2a function in diabetic cardiomyopathy

BACKGROUND AND PURPOSE

Sarcoplasmic reticulum Ca2+-ATPase (SERCA2a) is impaired in heart failure. Phosphodiesterases (PDEs) are implicated in the modulation of local cAMP signals and protein kinase A (PKA) activity essential for cardiac function. We characterise PDE isoforms that underlie decreased activities of SERCA2a and reduced cardiac contractile function in diabetic cardiomyopathy.

EXPERIMENTAL APPROACH

Wild type mice were fed with either normal chow or a high-fat diet (HFD). Cardiomyocytes were isolated for excitation-contraction coupling (ECC), fluorescence resonant energy transfer PKA biosensor and proximity ligation assays.

KEY RESULTS

The upregulated PDE4D3 and PDE4D9 isoforms in HFD cardiomyocytes specifically bound to SERCA2a but not ryanodine receptor 2 (RyR2) on the sarcoplasmic reticulum (SR). The increased association of PDE4D isoforms with SERCA2a in HFD cardiomyocytes led to reduced local PKA activities and phosphorylation of phospholamban (PLB) but minimally effected the PKA activities and phosphorylation of RyR2. These changes correlate with slower calcium decay tau in the SR and attenuation of ECC in HFD cardiomyocytes. Selective inhibition of PDE4D3 or PDE4D9 restored PKA activities and phosphorylation of PLB at the SERCA2a complex, recovered calcium decay tau, and increased ECC in HFD cardiomyocytes. Therapies with PDE4 inhibitor roflumilast, PDE4D inhibitor BPN14770 or genetical deletion of PDE4D restored PKA phosphorylation of PLB and cardiac contractile function.

CONCLUSION AND IMPLICATIONS

The current study identifies upregulation of specific PDE4D isoforms that selectively inhibit SERCA2a function in HFD-induced cardiomyopathy, indicating that this remodelling can be targeted to restore cardiac contractility in diabetic cardiomyopathy.

Identifying sex similarities and differences in structure and function of the sinoatrial node in the mouse heart

Background

The sinoatrial node (SN) generates the heart rate (HR). Its spontaneous activity is regulated by a complex interplay between the modulation by the autonomic nervous system (ANS) and intrinsic factors including ion channels in SN cells. However, the systemic and intrinsic regulatory mechanisms are still poorly understood. This study aimed to elucidate the sex-specific differences in heart morphology and SN function, particularly focusing on basal HR, expression and function of hyperpolarization-activated HCN4 and HCN1 channels and mRNA abundance of ion channels and mRNA abundance of ion channels contributing to diastolic depolarization (DD) and spontaneous action potentials (APs).

Methods

Body weight, heart weight and tibia length of 2- to 3-month-old male and female mice were measured. Conscious in-vivo HR of male and female mice was recorded via electrocardiography (ECG). Unconscious ex-vivo HR, stroke volume (SV) and ejection fraction (EF) were recorded via echocardiography. Ex-vivo HR was measured via Langendorff apparatus. Volume of atria, ventricles and whole hearts were measured from the ex-vivo hearts by microcomputed tomography (micro-CT). Immunohistochemistry targeting HCN4 and HCN1 was conducted in the SN and RA tissues from both male and female hearts. The funny current (I f) of SN cells in 1 nM and following wash-on of 1 μM isoproterenol (ISO) were recorded via whole cell patch clamp. The APs of SN tissue were recorded via sharp microelectrode and optical mapping of membrane voltage. The relative abundance of mRNAs was measured in male and female mice by qPCR.

Results

Heart weight to tibia length ratio and heart volume of females were significantly smaller than males. Unconscious in-vivo HR in male mice was higher than that in females. Conscious in-vivo HR, ex-vivo HR, SV, and EF showed no notable difference between male and female mice. Immunohistochemistry revealed HCN4, HCN1, and the sum of HCN4 and HCN1, expression in the SN was notably elevated compared with the RA in both male and females, but there was no sex difference in these channels expression. There were also no significant sex differences in the V 0.5 of I f in SN cells in the presence of 1 nM ISO, however wash-on 1 μM ISO in the same cells induced a significantly increased shift of V 0.5 to more positive voltages in males than in females. The expression of mRNA coding for adrenergic receptor beta-1 (Adrb1) and cholinergic receptors muscarinic 2 (chrm2) in male mice was higher compared with that in female mice. Early diastolic depolarization (EDD) rate in APs from peripheral SN (pSN) from male mice were higher than these in female mice. Mice of both sexes showed equivalent frequency of SN APs and spatial localization of the leading site in control, and similar significant response to ISO 100 nM superfusion.

Conclusion

Males display faster in-vivo HR, but not ex-vivo HR, than females associated with increased expression of Adrb1 in male versus female. This suggests a possible difference in the β-adrenergic modulation in males and females, possibly related to the greater ISO response of I f observed in cells from males. The role of hormonal influences or differential expression of other ion channels may explain these sex-specific variations in HR dynamics. Further investigations are necessary to pinpoint the precise molecular substrates responsible for these differences.

Protein kinase A and local signaling in cancer

Protein kinase A (PKA) is a basophilic kinase implicated in the modulation of many cell-signaling and physiological processes. PKA also contributes to cancer-relevant events such as growth factor action, cell cycle control, cell migration and tumor metabolism. Germline and somatic mutations in PKA, gene amplifications, and chromosome rearrangements that encode kinase fusions, are linked to a growing number of malignant neoplasms. Mislocalization of PKA by exclusion from A-Kinase Anchoring Protein (AKAP) signaling islands further underlies cancer progression. This article highlights the influence of AKAP signaling and local kinase action in selected hallmarks of cancer. We also feature the utility of kinase inhibitor drugs as frontline and future anti-cancer therapies.

Endomembrane GPCR signaling: 15 years on, the quest continues

G-protein-coupled receptors (GPCRs) are the largest family of cell receptors. They mediate the effects of a multitude of endogenous and exogenous cues, are deeply involved in human physiology and disease, and are major pharmacological targets. Whereas GPCRs were long thought to signal exclusively at the plasma membrane, research over the past 15 years has revealed that they also signal via classical G-protein-mediated pathways on membranes of intracellular organelles such as endosomes and the Golgi complex. This review provides an overview of recent advances and emerging concepts related to endomembrane GPCR signaling, as well as ongoing research aimed at a better understanding of its mechanisms, physiological relevance, and potential therapeutic applications.

Visualization of endogenous G proteins on endosomes and other organelles

Classical G-protein-coupled receptor (GPCR) signaling takes place in response to extracellular stimuli and involves receptors and heterotrimeric G proteins located at the plasma membrane. It has recently been established that GPCR signaling can also take place from intracellular membrane compartments, including endosomes that contain internalized receptors and ligands. While the mechanisms of GPCR endocytosis are well understood, it is not clear how well internalized receptors are supplied with G proteins. To address this gap, we use gene editing, confocal microscopy, and bioluminescence resonance energy transfer to study the distribution and trafficking of endogenous G proteins. We show here that constitutive endocytosis is sufficient to supply newly internalized endocytic vesicles with 20-30% of the G protein density found at the plasma membrane. We find that G proteins are present on early, late, and recycling endosomes, are abundant on lysosomes, but are virtually undetectable on the endoplasmic reticulum, mitochondria, and the medial-trans Golgi apparatus. Receptor activation does not change heterotrimer abundance on endosomes. Our findings provide a subcellular map of endogenous G protein distribution, suggest that G proteins may be partially excluded from nascent endocytic vesicles, and are likely to have implications for GPCR signaling from endosomes and other intracellular compartments.

Compartmentalization in cardiomyocytes modulates creatine kinase and adenylate kinase activities

Intracellular molecules are transported by motor proteins or move by diffusion resulting from random molecular motion. Cardiomyocytes are packed with structures that are crucial for function, but also confine the diffusional spaces, providing cells with a means to control diffusion. They form compartments in which local concentrations are different from the overall, average concentrations. For example, calcium and cyclic AMP are highly compartmentalized, allowing these versatile second messengers to send different signals depending on their location. In energetic compartmentalization, the ratios of AMP and ADP to ATP are different from the average ratios. This is important for the performance of ATPases fuelling cardiac excitation-contraction coupling and mechanical work. A recent study suggested that compartmentalization modulates the activity of creatine kinase and adenylate kinase in situ. This could have implications for energetic signaling through, for example, AMP-activated kinase. It highlights the importance of taking compartmentalization into account in our interpretation of cellular physiology and developing methods to assess local concentrations of AMP and ADP to enhance our understanding of compartmentalization in different cell types.

cAMP-PKA signaling pathway and anxiety: Where do we go next?

Cyclic adenosine monophosphate (cAMP) is an intracellular second messenger that is derived from the conversion of adenosine triphosphate catalysed by adenylyl cyclase (AC). Protein kinase A (PKA), the main effector of cAMP, is a dimeric protein kinase consisting of two catalytic subunits and two regulatory subunits. When cAMP binds to the regulatory subunits of PKA, it leads to the dissociation and activation of PKA, which allows the catalytic subunit of PKA to phosphorylate target proteins, thereby regulating various physiological functions and metabolic processes in cellular function. Recent researches also implicate the involvement of cAMP-PKA signaling in the pathologenesis of anxiety disorder. However, there are still debates on the prevention and treatment of anxiety disorders from this signaling pathway. To review the function of cAMP-PKA signaling in anxiety disorder, we searched the publications with the keywords including "cAMP", "PKA" and "Anxiety" from Pubmed, Embase, Web of Science and CNKI databases. The results showed that the number of publications on cAMP-PKA pathway in anxiety disorder tended to increase. Bioinformatics results displayed a close association between the cAMP-PKA pathway and the occurrence of anxiety. Mechanistically, cAMP-PKA signaling could influence brain-derived neurotrophic factor and neuropeptide Y and participate in the regulation of anxiety. cAMP-PKA signaling could also oppose the dysfunctions of gamma-aminobutyric acid (GABA), intestinal flora, hypothalamic-pituitary-adrenal axis, neuroinflammation, and signaling proteins (MAPK and AMPK) in anxiety. In addition, chemical agents with the ability to activate cAMP-PKA signaling demonstrated therapy potential against anxiety disorders. This review emphasizes the central roles of cAMP-PKA signaling in anxiety and the targets of the cAMP-PKA pathway would be potential candidates for treatment of anxiety. Nevertheless, more laboratory investigations to improve the therapeutic effect and reduce the adverse effect, and continuous clinical research will warrant the drug development.

Subcellular activation of β-adrenergic receptors using a spatially restricted antagonist

Gprotein-coupled receptors (GPCRs) regulate several physiological and pathological processes and represent the target of approximately 30% of Food and Drug Administration-approved drugs. GPCR-mediated signaling was thought to occur exclusively at the plasma membrane. However, recent studies have unveiled their presence and function at subcellular membrane compartments. There is a growing interest in studying compartmentalized signaling of GPCRs. This requires development of tools to separate GPCR signaling at the plasma membrane from the ones initiated at intracellular compartments. We leveraged the structural and pharmacological information available for β-adrenergic receptors (βARs) and focused on β1AR as exemplary GPCR that functions at subcellular compartments, and rationally designed spatially restricted antagonists. We generated a cell-impermeable βAR antagonist by conjugating a suitable pharmacophore to a sulfonate-containing fluorophore. This cell-impermeable antagonist only inhibited β1AR on the plasma membrane. In contrast, a cell-permeable βAR antagonist containing a nonsulfonated fluorophore efficiently inhibited both the plasma membrane and Golgi pools of β1ARs. Furthermore, the cell-impermeable antagonist selectively inhibited the phosphorylation of PKA downstream effectors near the plasma membrane, which regulate sarcoplasmic reticulum (SR) Ca2+ release in adult cardiomyocytes, while the β1AR Golgi pool remained active. Our tools offer promising avenues for investigating compartmentalized βAR signaling in various contexts, potentially advancing our understanding of βAR-mediated cellular responses in health and disease. They also offer a general strategy to study compartmentalized signaling for other GPCRs in various biological systems.

An Integrated Optogenetic and Bioelectronic Platform for Regulating Cardiomyocyte Function

Bioelectronic medicine is emerging as a powerful approach for restoring lost endogenous functions and addressing life-altering maladies such as cardiac disorders. Systems that incorporate both modulation of cellular function and recording capabilities can enhance the utility of these approaches and their customization to the needs of each patient. Here we report an integrated optogenetic and bioelectronic platform for stable and long-term stimulation and monitoring of cardiomyocyte function in vitro. Optical inputs are achieved through the expression of a photoactivatable adenylyl cyclase, that when irradiated with blue light causes a dose-dependent and time-limited increase in the secondary messenger cyclic adenosine monophosphate with subsequent rise in autonomous cardiomyocyte beating rate. Bioelectronic readouts are obtained through a multi-electrode array that measures real-time electrophysiological responses at 32 spatially-distinct locations. Irradiation at 27 µW mm-2 results in a 14% elevation of the beating rate within 20-25 min, which remains stable for at least 2 h. The beating rate can be cycled through "on" and "off" light states, and its magnitude is a monotonic function of irradiation intensity. The integrated platform can be extended to stretchable and flexible substrates, and can open new avenues in bioelectronic medicine, including closed-loop systems for cardiac regulation and intervention, for example, in the context of arrythmias.

An intracellular VHH targeting the Luteinizing Hormone receptor modulates G protein-dependent signaling and steroidogenesis

Luteinizing hormone (LH) is essential for reproduction, controlling ovulation and steroidogenesis. Its receptor (LHR) recruits various transducers leading to the activation of a complex signaling network. We recently identified iPRC1, the first variable fragment from heavy-chain-only antibody (VHH) interacting with intracellular loop 3 (ICL3) of the follicle-stimulating hormone receptor (FSHR). Because of the high sequence similarity of the human FSHR and LHR (LHCGR), here we examined the ability of the iPRC1 intra-VHH to modulate LHCGR activity. In this study, we demonstrated that iPRC1 binds LHCGR, to a greater extent when the receptor was stimulated by the hormone. In addition, it decreased LH-induced cAMP production, cAMP-responsive element-dependent transcription, progesterone and testosterone production. These impairments are not due to Gs nor β-arrestin recruitment to the LHCGR. Consequently, iPRC1 is the first intra-VHH to bind and modulate LHCGR biological activity, including steroidogenesis. It should help further understand signaling mechanisms elicited at this receptor and their outcomes on reproduction.

Visualization of endogenous G proteins on endosomes and other organelles

Classical G protein-coupled receptor (GPCR) signaling takes place in response to extracellular stimuli and involves receptors and heterotrimeric G proteins located at the plasma membrane. It has recently been established that GPCR signaling can also take place from intracellular membrane compartments, including endosomes that contain internalized receptors and ligands. While the mechanisms of GPCR endocytosis are well understood, it is not clear how well internalized receptors are supplied with G proteins. To address this gap we use gene editing, confocal microscopy, and bioluminescence resonance energy transfer to study the distribution and trafficking of endogenous G proteins. We show here that constitutive endocytosis is sufficient to supply newly internalized endocytic vesicles with 20-30% of the G protein density found at the plasma membrane. We find that G proteins are present on early, late, and recycling endosomes, are abundant on lysosomes, but are virtually undetectable on the endoplasmic reticulum, mitochondria, and the medial Golgi apparatus. Receptor activation does not change heterotrimer abundance on endosomes. Our findings provide a subcellular map of endogenous G protein distribution, suggest that G proteins may be partially excluded from nascent endocytic vesicles, and are likely to have implications for GPCR signaling from endosomes and other intracellular compartments.

G Protein-Coupled Receptors: A Century of Research and Discovery

GPCRs (G protein-coupled receptors), also known as 7 transmembrane domain receptors, are the largest receptor family in the human genome, with ≈800 members. GPCRs regulate nearly every aspect of human physiology and disease, thus serving as important drug targets in cardiovascular disease. Sharing a conserved structure comprised of 7 transmembrane α-helices, GPCRs couple to heterotrimeric G-proteins, GPCR kinases, and β-arrestins, promoting downstream signaling through second messengers and other intracellular signaling pathways. GPCR drug development has led to important cardiovascular therapies, such as antagonists of β-adrenergic and angiotensin II receptors for heart failure and hypertension, and agonists of the glucagon-like peptide-1 receptor for reducing adverse cardiovascular events and other emerging indications. There continues to be a major interest in GPCR drug development in cardiovascular and cardiometabolic disease, driven by advances in GPCR mechanistic studies and structure-based drug design. This review recounts the rich history of GPCR research, including the current state of clinically used GPCR drugs, and highlights newly discovered aspects of GPCR biology and promising directions for future investigation. As additional mechanisms for regulating GPCR signaling are uncovered, new strategies for targeting these ubiquitous receptors hold tremendous promise for the field of cardiovascular medicine.

Differential Downregulation of β1-Adrenergic Receptor Signaling in the Heart

BACKGROUND

Chronic sympathetic stimulation drives desensitization and downregulation of β1 adrenergic receptor (β1AR) in heart failure. We aim to explore the differential downregulation subcellular pools of β1AR signaling in the heart.

METHODS AND RESULTS

We applied chronic infusion of isoproterenol to induced cardiomyopathy in male C57BL/6J mice. We applied confocal and proximity ligation assay to examine β1AR association with L-type calcium channel, ryanodine receptor 2, and SERCA2a ((Sarco)endoplasmic reticulum calcium ATPase 2a) and Förster resonance energy transfer-based biosensors to probe subcellular β1AR-PKA (protein kinase A) signaling in ventricular myocytes. Chronic infusion of isoproterenol led to reduced β1AR protein levels, receptor association with L-type calcium channel and ryanodine receptor 2 measured by proximity ligation (puncta/cell, 29.65 saline versus 14.17 isoproterenol, P<0.05), and receptor-induced PKA signaling at the plasma membrane (Förster resonance energy transfer, 28.9% saline versus 1.9% isoproterenol, P<0.05) and ryanodine receptor 2 complex (Förster resonance energy transfer, 30.2% saline versus 10.6% isoproterenol, P<0.05). However, the β1AR association with SERCA2a was enhanced (puncta/cell, 51.4 saline versus 87.5 isoproterenol, P<0.05), and the receptor signal was minimally affected. The isoproterenol-infused hearts displayed decreased PDE4D (phosphodiesterase 4D) and PDE3A and increased PDE2A, PDE4A, and PDE4B protein levels. We observed a reduced role of PDE4 and enhanced roles of PDE2 and PDE3 on the β1AR-PKA activity at the ryanodine receptor 2 complexes and myocyte shortening. Despite the enhanced β1AR association with SERCA2a, the endogenous norepinephrine-induced signaling was reduced at the SERCA2a complexes. Inhibiting monoamine oxidase A rescued the norepinephrine-induced PKA signaling at the SERCA2a and myocyte shortening.

CONCLUSIONS

This study reveals distinct mechanisms for the downregulation of subcellular β1AR signaling in the heart under chronic adrenergic stimulation.

The M3 Muscarinic Acetylcholine Receptor Can Signal through Multiple G Protein Families

The M3 muscarinic acetylcholine receptor (M3R) is a G protein-coupled receptor (GPCR) that regulates important physiologic processes, including vascular tone, bronchoconstriction, and insulin secretion. It is expressed on a wide variety of cell types, including pancreatic beta, smooth muscle, neuronal, and immune cells. Agonist binding to the M3R is thought to initiate intracellular signaling events primarily through the heterotrimeric G protein Gq. However, reports differ on the ability of M3R to couple to other G proteins beyond Gq. Using members from the four primary G protein families (Gq, Gi, Gs, and G13) in radioligand binding, GTP turnover experiments, and cellular signaling assays, including live cell G protein dissociation and second messenger assessment of cAMP and inositol trisphosphate, we show that other G protein families, particularly Gi and Gs, can also interact with the human M3R. We further show that these interactions are productive as assessed by amplification of classic second messenger signaling events. Our findings demonstrate that the M3R is more promiscuous with respect to G protein interactions than previously appreciated. SIGNIFICANCE STATEMENT: The study reveals that the human M3 muscarinic acetylcholine receptor (M3R), known for its pivotal roles in diverse physiological processes, not only activates intracellular signaling via Gq as previously known but also functionally interacts with other G protein families such as Gi and Gs, expanding our understanding of its versatility in mediating cellular responses. These findings signify a broader and more complex regulatory network governed by M3R and have implications for therapeutic targeting.

cAMP signaling: a remarkably regional affair

Louis Pasteur once famously said 'in the fields of observation chance favors only the prepared mind'. Much of chance is being in the right place at the right time. This is particularly true in the crowded molecular environment of the cell where being in the right place is often more important than timing. Although Brownian motion argues that enzymes will eventually bump into substrates, this probability is greatly enhanced if both molecules reside in the same subcellular compartment. However, activation of cell signaling enzymes often requires the transmission of chemical signals from extracellular stimuli to intracellular sites of action. This review highlights new developments in our understanding of cAMP generation and the 3D utilization of this second messenger inside cells.

Discovery of a Cushing's syndrome protein kinase A mutant that biases signaling through type I AKAPs

Adrenal Cushing's syndrome is a disease of cortisol hypersecretion often caused by mutations in protein kinase A catalytic subunit (PKAc). Using a personalized medicine screening platform, we discovered a Cushing's driver mutation, PKAc-W196G, in ~20% of patient samples analyzed. Proximity proteomics and photokinetic imaging reveal that PKAcW196G is unexpectedly distinct from other described Cushing's variants, exhibiting retained association with type I regulatory subunits (RI) and their corresponding A kinase anchoring proteins (AKAPs). Molecular dynamics simulations predict that substitution of tryptophan-196 with glycine creates a 653-cubic angstrom cleft between the catalytic core of PKAcW196G and type II regulatory subunits (RII), but only a 395-cubic angstrom cleft with RI. Endocrine measurements show that overexpression of RIα or redistribution of PKAcW196G via AKAP recruitment counteracts stress hormone overproduction. We conclude that a W196G mutation in the kinase catalytic core skews R subunit selectivity and biases AKAP association to drive Cushing's syndrome.

Cardiac optogenetics: shining light on signaling pathways

In the early 2000s, the field of neuroscience experienced a groundbreaking transformation with the advent of optogenetics. This innovative technique harnesses the properties of naturally occurring and genetically engineered rhodopsins to confer light sensitivity upon target cells. The remarkable spatiotemporal precision offered by optogenetics has provided researchers with unprecedented opportunities to dissect cellular physiology, leading to an entirely new level of investigation. Initially revolutionizing neuroscience, optogenetics quickly piqued the interest of the wider scientific community, and optogenetic applications were expanded to cardiovascular research. Over the past decade, researchers have employed various optical tools to observe, regulate, and steer the membrane potential of excitable cells in the heart. Despite these advancements, achieving control over specific signaling pathways within the heart has remained an elusive goal. Here, we review the optogenetic tools suitable to control cardiac signaling pathways with a focus on GPCR signaling, and delineate potential applications for studying these pathways, both in healthy and diseased hearts. By shedding light on these exciting developments, we hope to contribute to the ongoing progress in basic cardiac research to facilitate the discovery of novel therapeutic possibilities for treating cardiovascular pathologies.

Selective activation of intracellular β1AR using a spatially restricted antagonist

G-protein-coupled receptors (GPCRs) regulate several physiological and pathological processes and represent the target of approximately 30% of FDA-approved drugs. GPCR-mediated signaling was thought to occur exclusively at the plasma membrane. However, recent studies have unveiled their presence and function at subcellular membrane compartments. There is a growing interest in studying compartmentalized signaling of GPCRs. This requires development of novel tools to separate GPCRs signaling at the plasma membrane from the ones initiated at intracellular compartments. We took advantage of the structural and pharmacological information available for β1-adrenergic receptor (β1AR), an exemplary GPCR that functions at subcellular compartments, and rationally designed spatially restricted antagonists. We generated a cell impermeable β1AR antagonist by conjugating a suitable pharmacophore to a sulfonate-containing fluorophore. This cell-impermeable antagonist only inhibited β1AR on the plasma membrane. In contrast, a cell permeable β1AR agonist containing a non-sulfonated fluorophore, efficiently inhibited both the plasma membrane and Golgi pools of β1ARs. Furthermore, the cell impermeable antagonist selectively inhibited the phosphorylation of downstream effectors of PKA proximal to the plasma membrane in adult cardiomyocytes while β1AR intracellular pool remained active. Our tools offer promising avenues for investigating compartmentalized β1AR signaling in various context, potentially advancing our understanding of β1AR-mediated cellular responses in health and disease. They also offer a general strategy to study compartmentalized signaling for other GPCRs in various biological systems.