β-Arrestin-Biased Angiotensin II Receptor Agonists for COVID-19

New insights into sperm physiology regulation: Enlightenment from G-protein-coupled receptors

BACKGROUND

G-protein-coupled receptors are critical in many physiological and pathological processes in various organs. Serving as the control panel for sensing extracellular stimuli, G-protein-coupled receptors recognise various ligands, including light, temperature, odours, pheromones, hormones, neurotransmitters, chemokines, etc. Most recently, G-protein-coupled receptors residing in spermatozoa have been found to be indispensable for sperm function.

OBJECTIVE

Here, we have summarised cutting-edge findings on the functional mechanisms of G-protein-coupled receptors that are known to be associated with sperm functions and the activation of their downstream effectors, providing new insights into the roles of G-protein-coupled receptors in sperm physiology.

RESULTS

Emerging studies hint that alterations in G-protein-coupled receptors could affect sperm function, implicating their role in fertility, but solid evidence needs to be continuing excavated with various means. Several members of the G-protein-coupled receptor superfamily, including olfactory receptors, opsins, orphan G-protein-coupled receptors, CXC chemokine receptor 4, CC chemokine receptor 5 and CC chemokine receptor 6 as well as their downstream effector β-arrestins, etc., were suggested to be essential for sperm motility, capacitation, thermotaxis, chemotaxis, Ca2+ influx through CatSper channel and fertilisation capacity.

CONCLUSION

The present review provides a comprehensive overview of studies describing G-protein-coupled receptors and their potential action in sperm function. We also present a critical discussion of these issues, and a possible framework for future investigations on the diverse ligands, biological functions and cell signalling of G-protein-coupled receptors in spermatozoa. Here, the G-protein-coupled receptors and their related G proteins that specifically were identified in spermatozoa were summarised, and provided references valuable for further illumination, despite the evidence that is not overwhelming in most cases.

Development of a Non-Peptide Angiotensin II Type 1 Receptor Ligand by Structural Modification of Olmesartan as a Biased Agonist

As a biased agonist, peptide angiotensin II (Ang II) type 1 (AT₁) receptor ligand antagonizes Ang II-stimulated G protein signaling but stimulates several kinase pathways. Here, we developed a non-peptide AT₁ receptor compound as a biased ligand. We synthesized three non-peptide AT₁ receptor ligands (R239470, R781253, and R794847) as candidates of biased ligands. Extracellular signal-regulated kinase (ERK) 1/2 activation and inositol phosphate (IP) production were measured using a cell system that overexpressed AT₁ receptors (wild-type, L112A, Q257A, Y292A, and N295A receptors). We also examined the modes of receptor-ligand binding using a competition binding study. The K_d values of R239470, R781253, and R794847 for the AT₁ wild-type receptor were 0.8, 21, and 48 nM, respectively, as assessed in a competition binding study. Those of R239470, R781253, and R794847 for the L112A receptor were 37, 23, and 31 nM, respectively. R781253 and R794847 decreased and increased IP production, respectively, whereas R239470 did not change IP production. R781253 and R794847, but not R239470, activated ERK1/2. In conclusion, R239470, R781253, and R794847 act as a neutral antagonist, an inverse agonist, and an agonist with regard to IP production, respectively. On the other hand, R781253 and R794847, but not R239470, are agonists toward ERK1/2 activation. Thus, we developed a non-peptide AT₁ receptor compound as a biased ligand.

The effect of TRV027 on coagulation in COVID-19: A pilot randomized, placebo-controlled trial

COVID-19 causes significant thrombosis and coagulopathy, with elevated D-dimer a predictor of adverse outcome. The precise mechanism of this coagulopathy remains unclear; one hypothesis is that loss of angiotensin-converting enzyme 2 activity during viral endocytosis leads to pro-inflammatory angiotensin-II accumulation, loss of angiotensin-1-7 and subsequent vascular endothelial activation. We undertook a double-blind randomized, placebo-controlled experimental medicine study to assess the effect of TRV027, a synthetic angiotensin-1-7 analogue on D-dimer in 30 patients admitted to hospital with COVID-19. The study showed a similar rate of adverse events in TRV027 and control groups. There was a numerical decrease in D-dimer in the TRV027 group and increase in D-dimer in the placebo group; however, this did not reach statistical significance (P = .15). A Bayesian analysis demonstrated that there was a 92% probability that this change represented a true drug effect.

Angiotensin and Endothelin Receptor Structures With Implications for Signaling Regulation and Pharmacological Targeting

In conjunction with the endothelin (ET) type A (ETAR) and type B (ETBR) receptors, angiotensin (AT) type 1 (AT1R) and type 2 (AT2R) receptors, are peptide-binding class A G-protein-coupled receptors (GPCRs) acting in a physiologically overlapping context. Angiotensin receptors (ATRs) are involved in regulating cell proliferation, as well as cardiovascular, renal, neurological, and endothelial functions. They are important therapeutic targets for several diseases or pathological conditions, such as hypertrophy, vascular inflammation, atherosclerosis, angiogenesis, and cancer. Endothelin receptors (ETRs) are expressed primarily in blood vessels, but also in the central nervous system or epithelial cells. They regulate blood pressure and cardiovascular homeostasis. Pathogenic conditions associated with ETR dysfunctions include cancer and pulmonary hypertension. While both receptor groups are activated by their respective peptide agonists, pathogenic autoantibodies (auto-Abs) can also activate the AT1R and ETAR accompanied by respective clinical conditions. To date, the exact mechanisms and differences in binding and receptor-activation mediated by auto-Abs as opposed to endogenous ligands are not well understood. Further, several questions regarding signaling regulation in these receptors remain open. In the last decade, several receptor structures in the apo- and ligand-bound states were determined with protein X-ray crystallography using conventional synchrotrons or X-ray Free-Electron Lasers (XFEL). These inactive and active complexes provide detailed information on ligand binding, signal induction or inhibition, as well as signal transduction, which is fundamental for understanding properties of different activity states. They are also supportive in the development of pharmacological strategies against dysfunctions at the receptors or in the associated signaling axis. Here, we summarize current structural information for the AT1R, AT2R, and ETBR to provide an improved molecular understanding.

Molecular Effects of Auto-Antibodies on Angiotensin II Type 1 Receptor Signaling and Cell Proliferation

The angiotensin II (Ang II) type 1 receptor (AT1R) is involved in the regulation of blood pressure (through vasoconstriction) and water and ion homeostasis (mediated by interaction with the endogenous agonist). AT1R can also be activated by auto-antibodies (AT1R-Abs), which are associated with manifold diseases, such as obliterative vasculopathy, preeclampsia and systemic sclerosis. Knowledge of the molecular mechanisms related to AT1R-Abs binding and associated signaling cascade (dys-)regulation remains fragmentary. The goal of this study was, therefore, to investigate details of the effects of AT1R-Abs on G-protein signaling and subsequent cell proliferation, as well as the putative contribution of the three extracellular receptor loops (ELs) to Abs-AT1R signaling. AT1R-Abs induced nuclear factor of activated T-cells (NFAT) signaling, which reflects Gq/11 and Gi activation. The impact on cell proliferation was tested in different cell systems, as well as activation-triggered receptor internalization. Blockwise alanine substitutions were designed to potentially investigate the role of ELs in AT1R-Abs-mediated effects. First, we demonstrate that Ang II-mediated internalization of AT1R is impeded by binding of AT1R-Abs. Secondly, exclusive AT1R-Abs-induced Gq/11 activation is most significant for NFAT stimulation and mediates cell proliferation. Interestingly, our studies also reveal that ligand-independent, baseline AT1R activation of Gi signaling has, in turn, a negative effect on cell proliferation. Indeed, inhibition of Gi basal activity potentiates proliferation triggered by AT1R-Abs. Finally, although AT1R containing EL1 and EL3 blockwise alanine mutations were not expressed on the human embryonic kidney293T (HEK293T) cell surface, we at least confirmed that parts of EL2 are involved in interactions between AT1R and Abs. This current study thus provides extended insights into the molecular action of AT1R-Abs and associated mechanisms of interrelated pathogenesis.

Heterotrimeric Gq proteins act as a switch for GRK5/6 selectivity underlying β-arrestin transducer bias

Signaling-biased ligands acting on G-protein-coupled receptors (GPCRs) differentially activate heterotrimeric G proteins and β-arrestins. Although a wealth of structural knowledge about signaling bias at the GPCR level exists (preferential engagement of a specific transducer), little is known about the bias at the transducer level (different functions mediated by a single transducer), partly due to a poor understanding of GPCR kinase (GRK)-mediated GPCR phosphorylation. Here, we reveal a unique role of the Gq heterotrimer as a determinant for GRK-subtype selectivity that regulates subsequent β-arrestin conformation and function. Using the angiotensin II (Ang II) type-1 receptor (AT1R), we show that β-arrestin recruitment depends on both GRK2/3 and GRK5/6 upon binding of Ang II, but solely on GRK5/6 upon binding of the β-arrestin-biased ligand TRV027. With pharmacological inhibition or genetic loss of Gq, GRK-subtype selectivity and β-arrestin functionality by Ang II is shifted to those of TRV027. Single-molecule imaging identifies relocation of AT1R and GRK5, but not GRK2, to an immobile phase under the Gq-inactive, AT1R-stimulated conditions. These findings uncover a previously unappreciated Gq-regulated mechanism that encodes GRK-subtype selectivity and imparts distinct phosphorylation-barcodes directing downstream β-arrestin functions.

GPCR kinases (GRKs) phosphorylate active-form G-protein-coupled receptors (GPCRs). Here, the authors reveal that Gq heterotrimer coupled with the angiotensin II type-1 receptor (AT1R) determines the GRK subtypes recruited to the complex in a microdomain, thus defining subsequent AT1R phosphorylation patterns, β-arrestin conformation and functionality.

Viewpoints on the First Transatlantic GPCR Symposium for Early-Career Investigators

In July 2021, we organized a virtual symposium aimed at early-career investigators (ECIs) in G protein-coupled receptor (GPCR) research: the first Transatlantic ECI GPCR Symposium. Here, we discuss the proceedings of this symposium and the unique networking events with GPCR leaders including the Nobel Laureates Dr. Robert Lefkowitz and Dr. Brian Kobilka.

The Release of Nitric Oxide Is Involved in the β-Arrestin1-Induced Antihypertensive Effect in the Rostral Ventrolateral Medulla

β-Arrestin1 is a multifunctional scaffold protein with the ability to interact with diverse signaling molecules independent of G protein-coupled receptors. We previously reported that overexpression of β-arrestin1 in the rostral ventrolateral medulla (RVLM) decreased blood pressure (BP) and renal sympathetic nerve activity (RSNA) in spontaneously hypertensive rats (SHRs). Nitric oxide (NO) is widely reported to be involved in central cardiovascular regulation. The goal of this study was to investigate whether NO signaling contributes to the β-arrestin1-mediated antihypertensive effect in the RVLM. It was found that bilateral injection of adeno-associated virus containing Arrb1 gene (AAV-Arrb1) into the RVLM of SHRs significantly increased NO production and NO synthase (NOS) activity. Microinjection of the non-selective NOS inhibitor N-nitro-L-arginine methyl ester (L-NAME; 10 nmol) into the RVLM prevented the β-arrestin1-induced cardiovascular inhibitory effect. Furthermore, β-arrestin1 overexpression in the RVLM significantly upregulated the expression of phosphorylated neuronal NOS (nNOS) by 3.8-fold and extracellular regulated kinase 1/2 (ERK1/2) by 5.6-fold in SHRs. The β-arrestin1-induced decrease in BP and RSNA was significantly abolished by treatment with ERK1/2 small interfering RNA (ERK1/2 siRNA). Moreover, ERK1/2 siRNA attenuated the β-arrestin1-induced NO production, NOS activity, and nNOS phosphorylation in the RVLM. Taken together, these data demonstrate that the antihypertensive effect of β-arrestin1 in the RVLM is mediated by nNOS-derived NO release, which is associated with ERK1/2 activation.

Mechanisms of troponin release into serum in cardiac injury associated with COVID-19 patients

Serum levels of thin filament proteins, cardiac troponin T (cTnT) and cardiac troponin I (cTnI) employing high sensitivity antibodies provide a state-of-the art determination of cardiac myocyte injury in COVID-19 patients. Although there is now sufficient evidence of the value of these determinations in patients infected with SARS-CoV-2, mechanisms of their release have not been considered in depth. We summarize the importance of these mechanisms with emphasis on their relation to prognosis, stratification, and treatment of COVID-19 patients. Apart from frank necrotic cell death, there are other mechanisms of myocyte injury leading to membrane fragility that provoke release of cTnT and cTnI. We discuss a rationale for understanding these mechanisms in COVID-19 patients with co-morbidities associated with myocyte injury such as heart failure, hypertension, arrythmias, diabetes, and inflammation. We describe how understanding these significant aspects of these mechanisms in the promotion of angiotensin signaling by SARS-CoV-2 can affect treatment options in the context of individualized therapies. Moreover, with likely omic data related to serum troponins and with the identification of elevations of serum troponins now more broadly detected employing high sensitivity antibodies, we think it is important to consider molecular mechanisms of elevations in serum troponin as an element in clinical decisions and as a critical aspect of development of new therapies.

Targeting the renin-angiotensin signaling pathway in COVID-19: Unanswered questions, opportunities, and challenges

The role of the renin-angiotensin signaling (RAS) pathway in COVID-19 has received much attention. A central mechanism for COVID-19 pathophysiology has been proposed: imbalance of angiotensin converting enzymes (ACE)1 and ACE2 (ACE2 being the severe acute respiratory syndrome coronavirus 2 [SARS-CoV-2] virus "receptor") that results in tissue injury from angiotensin II (Ang II)-mediated signaling. This mechanism provides a rationale for multiple therapeutic approaches. In parallel, clinical data from retrospective analysis of COVID-19 cohorts has revealed that ACE inhibitors (ACEIs) or angiotensin receptor blockers (ARBs) may be beneficial in COVID-19. These findings have led to the initiation of clinical trials using approved drugs that target the generation (ACEIs) and actions (ARBs) of Ang II. However, treatment of COVID-19 with ACEIs/ARBs poses several challenges. These include choosing appropriate inclusion and exclusion criteria, dose optimization, risk of adverse effects and drug interactions, and verification of target engagement. Other approaches related to the RAS pathway might be considered, for example, inhalational administration of ACEIs/ARBs (to deliver drugs directly to the lungs) and use of compounds with other actions (e.g., activation of ACE2, agonism of MAS1 receptors, β-arrestin-based Angiotensin receptor agonists, and administration of soluble ACE2 or ACE2 peptides). Studies with animal models could test such approaches and assess therapeutic benefit. This Perspective highlights questions whose answers could advance RAS-targeting agents as mechanism-driven ways to blunt tissue injury, morbidity, and mortality of COVID-19.

ACE2 (Angiotensin-Converting Enzyme 2) in Cardiopulmonary Diseases: Ramifications for the Control of SARS-CoV-2

Discovery of ACE2 (angiotensin-converting enzyme 2) revealed that the renin-angiotensin system has 2 counterbalancing arms. ACE2 is a major player in the protective arm, highly expressed in lungs and gut with the ability to mitigate cardiopulmonary diseases such as inflammatory lung disease. ACE2 also exhibits activities involving gut microbiome, nutrition, and as a chaperone stabilizing the neutral amino acid transporter, B⁰AT1, in gut. But the current interest in ACE2 arises because it is the cell surface receptor for the novel coronavirus, severe acute respiratory syndrome coronavirus-2, to infect host cells, similar to severe acute respiratory syndrome coronavirus-2. This suggests that ACE2 be considered harmful, however, because of its important other roles, it is paradoxically a potential therapeutic target for cardiopulmonary diseases, including coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus-2. This review describes the discovery of ACE2, its physiological functions, and its place in the renin-angiotensin system. It illustrates new analyses of the structure of ACE2 that provides better understanding of its actions particularly in lung and gut, shedding of ACE2 by ADAM17 (a disintegrin and metallopeptidase domain 17 protein), and role of TMPRSS2 (transmembrane serine proteases 2) in severe acute respiratory syndrome coronavirus-2 entry into host cells. Cardiopulmonary diseases are associated with decreased ACE2 activity and the mitigation by increasing ACE2 activity along with its therapeutic relevance are addressed. Finally, the potential use of ACE2 as a treatment target in COVID-19, despite its role to allow viral entry into host cells, is suggested.