Structure-Based Design of Highly Selective and Potent G Protein-Coupled Receptor Kinase 2 Inhibitors Based on Paroxetine

GRK2 Kinases in the Primary Cilium Initiate SMOOTHENED-PKA Signaling in the Hedgehog Cascade

During Hedgehog (Hh) signal transduction in development and disease, the atypical G protein-coupled receptor (GPCR) SMOOTHENED (SMO) communicates with GLI transcription factors by binding the protein kinase A catalytic subunit (PKA-C) and physically blocking its enzymatic activity. Here we show that GPCR kinase 2 (GRK2) orchestrates this process during endogenous Hh pathway activation in the vertebrate primary cilium. Upon SMO activation, GRK2 rapidly relocalizes from the ciliary base to the shaft, triggering SMO phosphorylation and PKA-C interaction. Reconstitution studies reveal that GRK2 phosphorylation enables active SMO to bind PKA-C directly. Lastly, the SMO-GRK2-PKA pathway underlies Hh signal transduction in a range of cellular and in vivo models. Thus, GRK2 phosphorylation of ciliary SMO, and the ensuing PKA-C binding and inactivation, are critical initiating events for the intracellular steps in Hh signaling. More broadly, our study suggests an expanded role for GRKs in enabling direct GPCR interactions with diverse intracellular effectors.

A Simple and Scalable Zebrafish Model of Sonic Hedgehog Medulloblastoma

Medulloblastoma (MB) is the most common malignant brain tumor in children and is stratified into three major subgroups. The Sonic hedgehog (SHH) subgroup represents ~30% of all MB cases and has significant survival disparity depending upon TP53 status. Here, we describe the first zebrafish model of SHH MB using CRISPR to mutate ptch1, the primary genetic driver in human SHH MB. These tumors rapidly arise adjacent to the valvula cerebelli and resemble human SHH MB by histology and comparative genomics. In addition, ptch1-deficient MB tumors with loss of tp53 have aggressive tumor histology and significantly worse survival outcomes, comparable to human patients. The simplicity and scalability of the ptch1 MB model makes it highly amenable to CRISPR-based genome editing screens to identify genes required for SHH MB tumor formation in vivo, and here we identify the grk3 kinase as one such target.

Cardioprotective Effects of the GRK2 Inhibitor Paroxetine on Isoproterenol-Induced Cardiac Remodeling by Modulating NF-κB Mediated Prohypertrophic and Profibrotic Gene Expression

Pathological cardiac remodeling is associated with cardiovascular disease and can lead to heart failure. Nuclear factor-kappa B (NF-κB) is upregulated in the hypertrophic heart. Moreover, the expression of the G-protein-coupled receptor kinase 2 (GRK2) is increased and linked to the progression of heart failure. The inhibitory effects of paroxetine on GRK2 have been established. However, its protective effect on IκBα/NFκB signaling has not been elucidated. This study investigated the cardioprotective effect of paroxetine in an animal model of cardiac hypertrophy (CH), focusing on its effect on GRK2-mediated NF-κB-regulated expression of prohypertrophic and profibrotic genes. Wistar albino rats were administered normal saline, paroxetine, or fluoxetine, followed by isoproterenol to induce CH. The cardioprotective effects of the treatments were determined by assessing cardiac injury, inflammatory biomarker levels, histopathological changes, and hypertrophic and fibrotic genes in cardiomyocytes. Paroxetine pre-treatment significantly decreased the HW/BW ratio (p < 0.001), and the expression of prohypertrophic and profibrotic genes Troponin-I (p < 0.001), BNP (p < 0.01), ANP (p < 0.001), hydroxyproline (p < 0.05), TGF-β1 (p < 0.05), and αSMA (p < 0.01) as well as inflammatory markers. It also markedly decreased pIκBα, NFκB(p105) subunit expression (p < 0.05) and phosphorylation. The findings suggest that paroxetine prevents pathological cardiac remodeling by inhibiting the GRK2-mediated IκBα/NF-κB signaling pathway.

Control of Gαq signaling dynamics and GPCR cross-talk by GRKs

Numerous processes contribute to the regulation of G protein-coupled receptors (GPCRs), but relatively little is known about rapid mechanisms that control signaling on the seconds time scale or regulate cross-talk between receptors. Here, we reveal that the ability of some GPCR kinases (GRKs) to bind Gα_q both drives acute signaling desensitization and regulates functional interactions between GPCRs. GRK2/3-mediated acute desensitization occurs within seconds, is rapidly reversible, and can occur upon local, subcellular activation. This rapid desensitization is kinase independent, insensitive to pharmacological inhibition, and generalizable across receptor families and effectors. We also find that the ability of GRK2 to bind G proteins also enables it to regulate the extent and timing of Gα_q-dependent signaling cross-talk between GPCRs. Last, we find that G protein/GRK2 interactions enable a novel form of GPCR trafficking cross-talk. Together, this work reveals potent forms of Gα_q-dependent GPCR regulation with wide-ranging pharmacological and physiological implications.

A bead-based GPCR phosphorylation immunoassay for high-throughput ligand profiling and GRK inhibitor screening

Analysis of agonist-driven phosphorylation of G protein-coupled receptors (GPCRs) can provide valuable insights into the receptor activation state and ligand pharmacology. However, to date, assessment of GPCR phosphorylation using high-throughput applications has been challenging. We have developed and validated a bead-based immunoassay for the quantitative assessment of agonist-induced GPCR phosphorylation that can be performed entirely in multiwell cell culture plates. The assay involves immunoprecipitation of affinity-tagged receptors using magnetic beads followed by protein detection using phosphorylation state-specific and phosphorylation state-independent anti-GPCR antibodies. As proof of concept, five prototypical GPCRs (MOP, C5a1, D1, SST2, CB2) were treated with different agonizts and antagonists, and concentration-response curves were generated. We then extended our approach to establish selective cellular GPCR kinase (GRK) inhibitor assays, which led to the rapid identification of a selective GRK5/6 inhibitor (LDC8988) and a highly potent pan-GRK inhibitor (LDC9728). In conclusion, this versatile GPCR phosphorylation assay can be used extensively for ligand profiling and inhibitor screening.

A G protein-coupled receptors (GPCRs) phosphorylation assay for cell culture plates can be used for ligand profiling and inhibitor screening, as evidenced by the identification of two GRK inhibitor compounds.

Sex/Gender- and Age-Related Differences in β-Adrenergic Receptor Signaling in Cardiovascular Diseases

Sex differences in cardiovascular disease (CVD) are often recognized from experimental and clinical studies examining the prevalence, manifestations, and response to therapies. Compared to age-matched men, women tend to have reduced CV risk and a better prognosis in the premenopausal period. However, with menopause, this risk increases exponentially, surpassing that of men. Although several mechanisms have been provided, including sex hormones, an emerging role in these sex differences has been suggested for β-adrenergic receptor (β-AR) signaling. Importantly, β-ARs are the most important G protein-coupled receptors (GPCRs), expressed in almost all the cell types of the CV system, and involved in physiological and pathophysiological processes. Consistent with their role, for decades, βARs have been considered the first targets for rational drug design to fight CVDs. Of note, β-ARs are seemingly associated with different CV outcomes in females compared with males. In addition, even if there is a critical inverse correlation between β-AR responsiveness and aging, it has been reported that gender is crucially involved in this age-related effect. This review will discuss how β-ARs impact the CV risk and response to anti-CVD therapies, also concerning sex and age. Further, we will explore how estrogens impact β-AR signaling in women.

Case Report: A Novel GNB1 Mutation Causes Global Developmental Delay With Intellectual Disability and Behavioral Disorders

Diseases of neurodevelopment mostly exhibit neurological and psychiatric symptoms that go from very mild to extremely severe. While the etiology of most cases of neurodevelopmental disease is still unknown, the discovery of underlying genetic causes is rapidly increasing, with hundreds of genes being currently implicated as disease-causing. Here, we report a clinical case of a patient with a previously undiagnosed syndrome comprising severe global developmental delay, intellectual disability, and behavioral disorders (such as attention-deficit/hyperactivity disorder, autism spectrum disorder and recurrent bouts of aggressive behavior). After genetic testing, a pathogenic variant was detected in the GNB1 gene, which codes for the G-protein subunit β1. The detected variant (c.217G>A, p.A73T) has not been previously reported in any of the 58 published cases of GNB1 encephalopathy. However, it localizes to the mutational hotspot in exons 6 and 7 in which 88% of all missense mutations occur. An in silico model predicts that this mutation is likely to disrupt the WD40 domain of the GNB1 protein, which is required for its interaction with other G-proteins and, consequently, for downstream signal transduction. In conclusion, we reported an additional GNB1 encephalopathy patient, bearing a novel mutation, taking another step toward a better understanding of its clinical presentation and prospective development of treatments for the disease.

Targeted inhibition of GRK2 kinase domain by CP-25 to reverse fibroblast-like synoviocytes dysfunction and improve collagen-induced arthritis in rats

Rheumatoid arthritis (RA) is an autoimmune disease and is mainly characterized by abnormal proliferation of fibroblast-like synoviocytes (FLS). The up-regulated cellular membrane expression of G protein coupled receptor kinase 2 (GRK2) of FLS plays a critical role in RA progression, the increase of GRK2 translocation activity promotes dysfunctional prostaglandin E4 receptor (EP4) signaling and FLS abnormal proliferation. Recently, although our group found that paeoniflorin-6ʹ-O-benzene sulfonate (CP-25), a novel compound, could reverse FLS dysfunction via GRK2, little is known as to how GRK2 translocation activity is suppressed. Our findings revealed that GRK2 expression up-regulated and EP4 expression down-regulated in synovial tissues of RA patients and collagen-induced arthritis (CIA) rats, and prostaglandin E2 (PGE2) level increased in arthritis. CP-25 could down-regulate GRK2 expression, up-regulate EP4 expression, and improve synovitis of CIA rats. CP-25 and GRK2 inhibitors (paroxetine or GSK180736A) inhibited the abnormal proliferation of FLS in RA patients and CIA rats by down-regulating GRK2 translocation to EP4 receptor. The results of microscale thermophoresis (MST), cellular thermal shift assay, and inhibition of kinase activity assay indicated that CP-25 could directly target GRK2, increase the protein stability of GRK2 in cells, and inhibit GRK2 kinase activity. The docking of CP-25 and GRK2 suggested that the kinase domain of GRK2 might be an important active pocket for CP-25. G201, K220, K230, A321, and D335 in kinase domain of GRK2 might form hydrogen bonds with CP-25. Site-directed mutagenesis and co-immunoprecipitation assay further revealed that CP-25 down-regulated the interaction of GRK2 and EP4 via controlling the key amino acid residue of Ala321 of GRK2. Our data demonstrate that FLS proliferation is regulated by GRK2 translocation to EP4. Targeted inhibition of GRK2 kinase domain by CP-25 improves FLS function and represents an innovative drug for the treatment of RA by targeting GRK2.

Adding another GRK to the fire of heart failure

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Nuisance compounds in cellular assays

Compounds that exhibit assay interference or undesirable mechanisms of bioactivity ("nuisance compounds") are routinely encountered in cellular assays, including phenotypic and high-content screening assays. Much is known regarding compound-dependent assay interferences in cell-free assays. However, despite the essential role of cellular assays in chemical biology and drug discovery, there is considerably less known about nuisance compounds in more complex cell-based assays. In our view, a major obstacle to realizing the full potential of chemical biology will not just be difficult-to-drug targets or even the sheer number of targets, but rather nuisance compounds, due to their ability to waste significant resources and erode scientific trust. In this review, we summarize our collective academic, government, and industry experiences regarding cellular nuisance compounds. We describe assay design strategies to mitigate the impact of nuisance compounds and suggest best practices to efficiently address these compounds in complex biological settings.

Targeting G protein-coupled receptor kinases (GRKs) to G protein-coupled receptors

G protein-coupled receptors (GPCRs) interact with three protein families following agonist binding: heterotrimeric G proteins, G protein-coupled receptor kinases (GRKs) and arrestins. GRK-mediated phosphorylation of GPCRs promotes arrestin binding to uncouple the receptor from G protein, a process called desensitization, and for many GPCRs, arrestin binding also promotes receptor endocytosis and intracellular signaling. Thus, GRKs play a central role in modulating GPCR signaling and localization. Here we review recent advances in this field which include additional insight into how GRKs target GPCRs and bias signaling, and the development of specific inhibitors to dissect GRK function in model systems.

Paroxetine-Overview of the Molecular Mechanisms of Action

In the 21st century and especially during a pandemic, the diagnosis and treatment of depression is an essential part of the daily practice of many family doctors. It mainly affects patients in the age category 15–44 years, regardless of gender. Anxiety disorders are often diagnosed in children and adolescents. Social phobias can account for up to 13% of these diagnoses. Social anxiety manifests itself in fear of negative social assessment and humiliation, which disrupts the quality of social functioning. Treatment of the above-mentioned disorders is based on psychotherapy and pharmacotherapy. Serious side effects or mortality from antidepressant drug overdose are currently rare. Recent studies indicate that paroxetine (ATC code: N06AB), belonging to the selective serotonin reuptake inhibitors, has promising therapeutic effects and is used off-label in children and adolescents. The purpose of this review is to describe the interaction of paroxetine with several molecular targets in various points of view including the basic chemical and pharmaceutical properties. The central point of the review is focused on the pharmacodynamic analysis based on the molecular mechanism of binding paroxetine to various therapeutic targets.

Generation of Highly Selective, Potent, and Covalent G Protein-Coupled Receptor Kinase 5 Inhibitors

The ability of G protein-coupled receptor (GPCR) kinases (GRKs) to regulate the desensitization of GPCRs has made GRK2 and GRK5 attractive targets for treating diseases such as heart failure and cancer. Previously, our work showed that Cys474, a GRK5 subfamily-specific residue located on a flexible loop adjacent to the active site, can be used as a covalent handle to achieve selective inhibition of GRK5 over GRK2 subfamily members. However, the potency of the most selective inhibitors remained modest. Herein, we describe a successful campaign to adapt an indolinone scaffold with covalent warheads, resulting in a series of 2-haloacetyl-containing compounds that react quickly and exhibit three orders of magnitude selectivity for GRK5 over GRK2 and low nanomolar potency. They however retain a similar selectivity profile across the kinome as the core scaffold, which was based on Sunitinib.

Hit-to-lead optimization and discovery of a potent, and orally bioavailable G protein coupled receptor kinase 2 (GRK2) inhibitor

G-protein coupled receptor kinase 2 (GRK2), which is upregulated in the failing heart, appears to play a critical role in heart failure (HF) progression in part because enhanced GRK2 activity promotes dysfunction of β-adrenergic signaling and myocyte death. An orally bioavailable GRK2 inhibitor could offer unique therapeutic outcomes that cannot be attained by current heart failure treatments that directly target GPCRs or angiotensin-converting enzyme. Herein, we describe the discovery of a potent, selective, and orally bioavailable GRK2 inhibitor, 8h, through high-throughput screening, hit-to-lead optimization, structure-based design, molecular modelling, synthesis, and biological evaluation. In the cellular target engagement assays, 8h enhances isoproterenol-mediated cyclic adenosine 3',5'-monophosphate (cAMP) production in HEK293 cells overexpressing GRK2. Compound 8h was further evaluated in a human stem cell-derived cardiomyocyte (HSC-CM) contractility assay and potentiated isoproterenol-induced beating rate in HSC-CMs.

Docking and 3D-QSAR Studies of Hydrazone and Triazole Derivatives for Selective Inhibition of GRK2 over ROCK2

Introduction:

G protein-coupled receptor kinase 2 (GRK2) is known to be implicated in heart failure, and therefore serves as an important drug target. GRK2 belongs to the protein kinase A, G, and C family and shares high sequence similarity with its closely related protein, the Rhoassociated coiled-coil protein kinase 2 (ROCK2). Therefore, selective inhibition of GRK2 over ROCK2 is considered crucial for heart failure therapy.

Objective:

To understand the structural factors for enhancing the inhibitory activity for GRK2 and selectivity over ROCK2, we analyzed and compared molecular interactions using the same set of ligands against both receptors.

Methods:

We have performed molecular docking and three-dimensional quantitative structure activity relationship (3D-QSAR) studies on a series of hydrazone and triazole derivatives.

Results:

The presence of hydrophobic substituents at the triazole ring, electronegative substituents between the pyridine and triazole ring and hydrophobic substituents near the benzene ring increases the activity of both kinases. Whereas, having non-bulky substituents near the triazole ring, bulky and hydrophobic substations at the benzene ring and electronegative and H-bond acceptor substituents at the triazole ring showed a higher inhibitory preference for GRK2 over ROCK2.

Conclusion:

The outcome of this study may be used in the future development of potent GRK2 inhibitors having ROCK2 selectivity.

A New Paroxetine-Based GRK2 Inhibitor Reduces Internalization of the μ-Opioid Receptor

G protein-coupled receptor (GPCR) kinases (GRKs) play a key role in terminating signals initiated by agonist-bound GPCRs. However, chronic stimulation of GPCRs, such as that which occurs during heart failure, leads to the overexpression of GRKs and maladaptive downregulation of GPCRs on the cell surface. We previously reported the discovery of potent and selective families of GRK inhibitors based on either the paroxetine or GSK180736A scaffold. A new inhibitor, CCG258747, which is based on paroxetine, demonstrates increased potency against the GRK2 subfamily and favorable pharmacokinetic parameters in mice. CCG258747 and the closely related compound CCG258208 also showed high selectivity for the GRK2 subfamily in a kinome panel of 104 kinases. We developed a cell-based assay to screen the ability of CCG258747 and 10 other inhibitors with different GRK subfamily selectivities and with either the paroxetine or GSK180736A scaffold to block internalization of the μ-opioid receptor (MOR). CCG258747 showed the best efficacy in blocking MOR internalization among the compounds tested. Furthermore, we show that compounds based on paroxetine had much better cell permeability than those based on GSK180736A, which explains why GSK180736A-based inhibitors, although being potent in vitro, do not always show efficacy in cell-based assays. This study validates the paroxetine scaffold as the most effective for GRK inhibition in living cells, confirming that GRK2 predominantly drives internalization of MOR in the cell lines we tested and underscores the utility of high-resolution cell-based assays for assessment of compound efficacy. SIGNIFICANCE STATEMENT: G protein-coupled receptor kinases (GRKs) are attractive targets for developing therapeutics for heart failure. We have synthesized a new GRK2 subfamily-selective inhibitor, CCG258747, which has nanomolar potency against GRK2 and excellent selectivity over other kinases. A live-cell receptor internalization assay was used to test the ability of GRK2 inhibitors to impart efficacy on a GRK-dependent process in cells. Our data indicate that CCG258747 blocked the internalization of the μ-opioid receptor most efficaciously because it has the ability to cross cell membranes.

Leveraging Signaling Pathways to Treat Heart Failure With Reduced Ejection Fraction

Advances in the treatment of heart failure with reduced ejection fraction due to systolic dysfunction are engaging an ever-expanding compendium of molecular signaling targets. Well established approaches modifying hemodynamics and cell biology by neurohumoral receptor blockade are evolving, exploring the role and impact of modulating intracellular signaling pathways with more direct myocardial effects. Even well-tread avenues are being reconsidered with new insights into the signaling engaged and thus opportunity to treat underlying myocardial disease. This review explores therapies that have proven successful, those that have not, those that are moving into the clinic but whose utility remains to be confirmed, and those that remain in the experimental realm. The emphasis is on signaling pathways that are tractable for therapeutic manipulation. Of the approaches yet to be tested in humans, we chose those with a well-established experimental history, where clinical translation may be around the corner. The breadth of opportunities bodes well for the next generation of heart failure therapeutics.

Structure-Based Design of Selective, Covalent G Protein-Coupled Receptor Kinase 5 Inhibitors

The ability of G protein-coupled receptor (GPCR) kinases (GRKs) to regulate desensitization of GPCRs has made GRK2 and GRK5 attractive targets for treating heart failure and other diseases such as cancer. Although advances have been made toward developing inhibitors that are selective for GRK2, there have been far fewer reports of GRK5 selective compounds. Herein, we describe the development of GRK5 subfamily selective inhibitors, 5 and 16d that covalently interact with a nonconserved cysteine (Cys474) unique to this subfamily. Compounds 5 and 16d feature a highly amenable pyrrolopyrimidine scaffold that affords high nanomolar to low micromolar activity that can be easily modified with Michael acceptors with various reactivities and geometries. Our work thereby establishes a new pathway toward further development of subfamily selective GRK inhibitors and establishes Cys474 as a new and useful covalent handle in GRK5 drug discovery.

Specialized structural and functional roles of residues selectively conserved in subfamilies of the pleckstrin homology domain family

Homologous domains embedded in multidomain proteins of different domain architectures (DA) may exhibit subtle, but important, differences in their structure and function. Here, we consider two multidomain proteins, Arf nucleotide binding site opener (ARNO) and G protein‐coupled receptor kinase 2 (GRK2), which have very different DAs, but both contain pleckstrin homology (PH) domains. We analyzed the roles of residues selectively conserved in these subfamilies of PH domains from ARNO and GRK2 proteins. DA‐specific residues in PH domain are found to contribute to structural and functional specialization of ARNO and GRK2 in terms of (a) specific intra‐ and interprotein interactions; (b) specificity for phospholipids; and (c) participation in conformational excursions, leading to various functional forms. Our approach can also be applied to subfamilies of other protein families to identify subfamily‐specific residues and their specialized roles.

Computational study of paroxetine-like inhibitors reveals new molecular insight to inhibit GRK2 with selectivity over ROCK1

The G-protein coupled receptor kinase 2 (GRK2) regulates the desensitization of beta-adrenergic receptors (β-AR), and its overexpression has been implicated in heart failure. Hence, the inhibition of GRK2 is considered to be an important drug target for the treatment of heart failure. Due to the high sequence similarity of GRK2 with the A, G, and C family (AGC family) of kinases, the inhibition of GRK2 also leads to the inhibition of AGC kinases such as Rho-associated coiled-coil kinase 1 (ROCK1). Therefore, unraveling the mechanisms to selectively inhibit GRK2 poses an important challenge. We have performed molecular docking, three dimensional quantitative structure activity relationship (3D-QSAR), molecular dynamics (MD) simulation, and free energy calculations techniques on a series of 53 paroxetine-like compounds to understand the structural properties desirable for enhancing the inhibitory activity for GRK2 with selectivity over ROCK1. The formation of stable hydrogen bond interactions with the residues Phe202 and Lys220 of GRK2 seems to be important for selective inhibition of GRK2. Electropositive substituents at the piperidine ring and electronegative substituents near the amide linker between the benzene ring and pyrazole ring showed a higher inhibitory preference for GRK2 over ROCK1. This study may be used in designing more potent and selective GRK2 inhibitors for therapeutic intervention of heart failure.

G protein-coupled receptor kinases as therapeutic targets in the heart

G protein-coupled receptors (GPCRs) are critical cellular sensors that mediate numerous physiological processes. In the heart, multiple GPCRs are expressed on various cell types, where they coordinate to regulate cardiac function by modulating critical processes such as contractility and blood flow. Under pathological settings, these receptors undergo aberrant changes in expression levels, localization and capacity to couple to downstream signalling pathways. Conventional therapies for heart failure work by targeting GPCRs, such as β-adrenergic receptor and angiotensin II receptor antagonists. Although these treatments have improved patient survival, heart failure remains one of the leading causes of mortality worldwide. GPCR kinases (GRKs) are responsible for GPCR phosphorylation and, therefore, desensitization and downregulation of GPCRs. In this Review, we discuss the GPCR signalling pathways and the GRKs involved in the pathophysiology of heart disease. Given that increased expression and activity of GRK2 and GRK5 contribute to the loss of contractile reserve in the stressed and failing heart, inhibition of overactive GRKs has been proposed as a novel therapeutic approach to treat heart failure.

Designer Approaches for G Protein-Coupled Receptor Modulation for Cardiovascular Disease

The new horizon for cardiac therapy may lie beneath the surface, with the downstream mediators of G protein–coupled receptor (GPCR) activity. Targeted approaches have shown that receptor activation may be biased toward signaling through G proteins or through GPCR kinases (GRKs) and β-arrestins, with divergent functional outcomes. In addition to these canonical roles, numerous noncanonical activities of GRKs and β-arrestins have been demonstrated to modulate GPCR signaling at all levels of receptor activation and regulation. Further, research continues to identify novel GRK/effector and β-arrestin/effector complexes with distinct impacts on cardiac function in the normal heart and the diseased heart. Coupled with the identification of once orphan receptors and endogenous ligands with beneficial cardiovascular effects, this expands the repertoire of GPCR targets. Together, this research highlights the potential for focused therapeutic activation of beneficial pathways, with simultaneous exclusion or inhibition of detrimental signaling, and represents a new wave of therapeutic development.

New Insights in Cardiac β-Adrenergic Signaling During Heart Failure and Aging

Heart failure (HF) has become increasingly common within the elderly population, decreasing their survival and overall quality of life. In fact, despite the improvements in treatment, many elderly people suffer from cardiac dysfunction (HF, valvular diseases, arrhythmias or hypertension-induced cardiac hypertrophy) that are much more common in an older fragile heart. Since β-adrenergic receptor (β-AR) signaling is abnormal in failing as well as aged hearts, this pathway is an effective diagnostic and therapeutic target. Both HF and aging are characterized by activation/hyperactivity of various neurohormonal pathways, the most important of which is the sympathetic nervous system (SNS). SNS hyperactivity is initially a compensatory mechanism to stimulate contractility and maintain cardiac output. Unfortunately, this chronic stimulation becomes detrimental and causes decreased cardiac function as well as reduced inotropic reserve due to a decrease in cardiac β-ARs responsiveness. Therapies which (e.g., β-blockers and physical activity) restore β-ARs responsiveness can ameliorate cardiac performance and outcomes during HF, particularly in older patients. In this review, we will discuss physiological β-adrenergic signaling and its alterations in both HF and aging as well as the potential clinical application of targeting β-adrenergic signaling in these disease processes.

New Insights in Cardiac β-Adrenergic Signaling During Heart Failure and Aging

Heart failure (HF) has become increasingly common within the elderly population, decreasing their survival and overall quality of life. In fact, despite the improvements in treatment, many elderly people suffer from cardiac dysfunction (HF, valvular diseases, arrhythmias or hypertension-induced cardiac hypertrophy) that are much more common in an older fragile heart. Since β-adrenergic receptor (β-AR) signaling is abnormal in failing as well as aged hearts, this pathway is an effective diagnostic and therapeutic target. Both HF and aging are characterized by activation/hyperactivity of various neurohormonal pathways, the most important of which is the sympathetic nervous system (SNS). SNS hyperactivity is initially a compensatory mechanism to stimulate contractility and maintain cardiac output. Unfortunately, this chronic stimulation becomes detrimental and causes decreased cardiac function as well as reduced inotropic reserve due to a decrease in cardiac β-ARs responsiveness. Therapies which (e.g., β-blockers and physical activity) restore β-ARs responsiveness can ameliorate cardiac performance and outcomes during HF, particularly in older patients. In this review, we will discuss physiological β-adrenergic signaling and its alterations in both HF and aging as well as the potential clinical application of targeting β-adrenergic signaling in these disease processes.

GRK2 as a therapeutic target for heart failure

INTRODUCTION

G protein-coupled receptor (GPCR) kinase-2 (GRK2) is a regulator of GPCRs, in particular β-adrenergic receptors (ARs), and as demonstrated by decades of investigation, it has a pivotal role in the development and progression of cardiovascular disease, like heart failure (HF). Indeed elevated levels and activity of this kinase are able to promote the dysfunction of both cardiac and adrenal α- and β-ARs and to dysregulate other protective signaling pathway, such as sphingosine 1-phospate and insulin. Moreover, recent discoveries suggest that GRK2 can signal independently from GPCRs, in a 'non-canonical' manner, via interaction with non-GPCR molecule or via its mitochondrial localization. Areas covered: Based on this premise, GRK2 inhibition or its genetic deletion has been tested in several disparate animal models of cardiovascular disease, showing to protect the heart from adverse remodeling and dysfunction. Expert opinion: HF is one of the leading cause of death worldwide with enormous health care costs. For this reason, the identification of new therapeutic targets like GRK2 and strategies such as its inhibition represents a new hope in the fight against HF development and progression. Herein, we will update the readers about the 'state-of-art' of GRK2 inhibition as a potent therapeutic strategy in HF.

Structural Determinants Influencing the Potency and Selectivity of Indazole-Paroxetine Hybrid G Protein-Coupled Receptor Kinase 2 Inhibitors

G protein-coupled receptor kinases (GRKs) phosphorylate activated receptors to promote arrestin binding, decoupling from heterotrimeric G proteins, and internalization. GRK2 and GRK5 are overexpressed in the failing heart and thus have become therapeutic targets. Previously, we discovered two classes of GRK2-selective inhibitors, one stemming from GSK180736A, a Rho-associated coiled-coil containing kinase 1 (ROCK1) inhibitor, the other from paroxetine, a selective serotonin-reuptake inhibitor. These two classes of compounds bind to the GRK2 active site in a similar configuration but contain different hinge-binding "warheads": indazole and benzodioxole, respectively. We surmised from our prior studies that an indazole would be the stronger hinge binder and would impart increased potency when substituted for benzodioxole in paroxetine derivatives. To test this hypothesis, we synthesized a series of hybrid compounds that allowed us to compare the effects of inhibitors that differ only in the identity of the warhead. The indazole-paroxetine analogs were indeed more potent than their respective benzodioxole derivatives but lost selectivity. To investigate how these two warheads dictate selectivity, we determined the crystal structures of three of the indazole hybrid compounds (CCG224061, CCG257284, and CCG258748) in complex with GRK2-Gβγ Comparison of these structures with those of analogous benzodioxole-containing complexes confirmed that the indazole-paroxetine hybrids form stronger interactions with the hinge of the kinase but also stabilize a distinct conformation of the kinase domain of GRK2 compared with previous complexes with paroxetine analogs. This conformation is analogous to one that can be assumed by GRK5, at least partially explaining the loss in selectivity.

Design, Synthesis, and Evaluation of the Highly Selective and Potent G-Protein-Coupled Receptor Kinase 2 (GRK2) Inhibitor for the Potential Treatment of Heart Failure

A novel class of therapeutic drug candidates for heart failure, highly potent and selective GRK2 inhibitors, exhibit potentiation of β-adrenergic signaling in vitro studies. Hydrazone derivative 5 and 1,2,4-triazole derivative 24a were identified as hit compounds by HTS. New scaffold generation and SAR studies of all parts resulted in a 4-methyl-1,2,4-triazole derivative with an N-benzylcarboxamide moiety with highly potent activity toward GRK2 and selectivity over other kinases. In terms of subtype selectivity, these compounds showed enough selectivity against GRK1, 5, 6, and 7 with almost equipotent inhibition to GRK3. Our medicinal chemistry efforts led to the discovery of 115h (GRK2 IC₅₀ = 18 nM), which was obtained the cocrystal structure with human GRK2 and an inhibitor of GRK2 that potentiates β-adrenergic receptor (βAR)-mediated cAMP accumulation and prevents internalization of βARs in β2AR-expressing HEK293 cells treated with isoproterenol. Therefore, 115h appears to be a novel class of therapeutic for heart failure treatment.

Impact of paroxetine on proximal β-adrenergic receptor signaling

β-adrenergic receptors (βAR) regulate numerous functions throughout the body, however G protein-coupled receptor kinase (GRK)-dependent desensitization of βAR has long been recognized as a maladaptive process in the progression of various disease states. Thus, the development of small molecule inhibitors of GRKs for the study of these processes and as potential therapeutics has been at the forefront of recent research efforts. Via structural and biochemical analyses, the selective serotonin reuptake inhibitor (SSRI) paroxetine was identified as a GRK2 inhibitor that enhances βAR-dependent cardiomyocyte and cardiac contractility and reverses cardiac dysfunction and myocardial βAR expression in mouse models of heart failure. Despite these functional outcomes, consistent with diminished βAR desensitization, the proximal βAR signaling mechanisms sensitive to paroxetine have not been reported. In this study, we aimed to determine whether paroxetine prevents classic βAR desensitization-related signaling mechanisms at a molecular level. Therefore, via immunoblotting, radioligand binding, fluorescence resonance energy transfer (FRET) and microscopy assays, we have performed an assessment of the effect of paroxetine on proximal βAR signaling responses. Indeed, paroxetine treatment inhibited ligand-induced β2AR phosphorylation in a concentration-dependent manner. Additionally, for both β1AR and β2AR, paroxetine decreased ligand-induced βarrestin2 recruitment and subsequent receptor internalization. Thus, paroxetine inhibits βAR desensitization mechanisms consistent with GRK2 inhibition and provides a useful pharmacological tool for studying these proximal GPCR signaling responses.

G protein-coupled receptor kinases: Past, present and future

This review is provided in recognition of the extensive contributions of Dr. Robert J. Lefkowitz to the G protein-coupled receptor (GPCR) field and to celebrate his 75th birthday. Since one of the authors trained with Bob in the 80s, we provide a history of work done in the Lefkowitz lab during the 80s that focused on dissecting the mechanisms that regulate GPCR signaling, with a particular emphasis on the GPCR kinases (GRKs). In addition, we highlight structure/function characteristics of GRK interaction with GPCRs as well as a review of two recent reports that provide a molecular model for GRK-GPCR interaction. Finally, we offer our perspective on some future studies that we believe will drive this field.