{Beta}-blocker drugs mediate calcium signaling in native central nervous system neurons by {beta}-arrestin-biased agonism

Carvedilol Selectively Stimulates βArrestin2-Dependent SERCA2a Activity in Cardiomyocytes to Augment Contractility

Heart failure (HF) carries the highest mortality in the western world and β-blockers [β-adrenergic receptor (AR) antagonists] are part of the cornerstone pharmacotherapy for post-myocardial infarction (MI) chronic HF. Cardiac β1AR-activated βarrestin2, a G protein-coupled receptor (GPCR) adapter protein, promotes Sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA)2a SUMO (small ubiquitin-like modifier)-ylation and activity, thereby directly increasing cardiac contractility. Given that certain β-blockers, such as carvedilol and metoprolol, can activate βarrestins and/or SERCA2a in the heart, we investigated the effects of these two agents on cardiac βarrestin2-dependent SERCA2a SUMOylation and activity. We found that carvedilol, but not metoprolol, acutely induces βarrestin2 interaction with SERCA2a in H9c2 cardiomyocytes and in neonatal rat ventricular myocytes (NRVMs), resulting in enhanced SERCA2a SUMOylation. However, this translates into enhanced SERCA2a activity only in the presence of the β2AR-selective inverse agonist ICI 118,551 (ICI), indicating an opposing effect of carvedilol-occupied β2AR subtype on carvedilol-occupied β1AR-stimulated, βarrestin2-dependent SERCA2a activation. In addition, the amplitude of fractional shortening of NRVMs, transfected to overexpress βarrestin2, is acutely enhanced by carvedilol, again in the presence of ICI only. In contrast, metoprolol was without effect on NRVMs’ shortening amplitude irrespective of ICI co-treatment. Importantly, the pro-contractile effect of carvedilol was also observed in human induced pluripotent stem cell (hIPSC)-derived cardiac myocytes (CMs) overexpressing βarrestin2, and, in fact, it was present even without concomitant ICI treatment of human CMs. Metoprolol with or without concomitant ICI did not affect contractility of human CMs, either. In conclusion, carvedilol, but not metoprolol, stimulates βarrestin2-mediated SERCA2a SUMOylation and activity through the β1AR in cardiac myocytes, translating into direct positive inotropy. However, this unique βarrestin2-dependent pro-contractile effect of carvedilol may be opposed or masked by carvedilol-bound β2AR subtype signaling.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Agonist Effects of Propranolol on Non-Tumor Human Breast Cells

The β-blocker propranolol (PROP) has been proposed as a repurposed treatment for breast cancer. The similarity of action between β-agonists and antagonists found on breast cells encouraged us to compare PROP and isoproterenol (ISO, agonist) signaling pathways on a human breast cell line. Cell proliferation was measured by cell counting and DNA-synthesis. Cell adhesion was measured counting the cells that remained adhered to the plastic after different treatments. Changes in actin cytoskeleton were observed by fluorescence staining and Western Blot. ISO and PROP caused a diminution of cell proliferation and an increase of cell adhesion, reverted by the pure β-antagonist ICI-118551. ISO and PROP induced a reorganization of actin cytoskeleton increasing F-actin, p-COFILIN and p-LIMK. While ISO elicited a marked enhancement of cAMP concentrations and an increase of vasodilator-stimulated phosphoprotein (VASP) and cAMP response element-binding protein (CREB) phosphorylation, PROP did not. Clathrin-mediated endocytosis inhibition or β-arrestin1 dominant-negative mutant abrogated PROP-induced cell adhesion and COFILIN phosphorylation. The fact that PROP has been proposed as an adjuvant drug for breast cancer makes it necessary to determine the specific action of PROP in breast models. These results provide an explanation for the discrepancies observed between experimental results and clinical evidence.

From Receptor Selectivity to Functional Selectivity: The Rise of Biased Agonism in 5-HT1A Receptor Drug Discovery

Despite extensive efforts to design serotonin 5-HT1A receptor compounds, there are currently no clinically available selective agonists to explore the therapeutic potential of activating this receptor. Commonly used drugs targeting 5-HT1A receptors, such as buspirone or other azapirone compounds, possess only limited selectivity over cross-reacting sites, act as partial agonists for 5-HT1A receptor activation, and are metabolically labile, generating active metabolites. In addition, drug discovery has been hampered by the multiplicity of 5-HT1A receptor subpopulations, expressed in different brain regions, that are coupled to distinct molecular signaling mechanisms and mediate a wide variety of physiological responses, both desired and undesired. In this context, advances in 5-HT1A receptor drug discovery have attracted attention of novel 'biased agonists' that are selective, efficacious and preferentially target the brain regions that mediate therapeutic activity without triggering side effects. The prototypical first-in-class compound NLX-101 (a.k.a. F15599; 3-chloro-4-fluorophenyl-[4-fluoro-4-[[(5-methylpyrimidin-2-ylmethyl)amino]methyl]piperidin- 1-yl]methanone), preferentially activates 5-HT1A receptors in cortical regions and exhibits potent, rapidacting and sustained antidepressant-like and procognitive properties in animal models. Here the background has been reviewed that led to the discovery of the class of 1-(1-benzoylpiperidin-4- yl)methanamine derivatives, including NLX-101, as well as recent advances in discovery of novel 5-HT1A receptor biased agonists, notably aryloxyethyl derivatives of 1‑(1-benzoylpiperidin-4yl)methanamine which show promising pharmacological activity both in vitro and in vivo. Overall, the results suggest that opportunities exist for innovative drug discovery of selective 5-HT1A receptor biased agonists that may open new avenues for the treatment of CNS disorders involving dysfunction of serotonergic neurotransmission.

Therapeutic Targets for Treatment of Heart Failure: Focus on GRKs and β-Arrestins Affecting βAR Signaling

Heart failure (HF) is a heart disease that is classified into two main types: HF with reduced ejection fraction (HFrEF) and HF with preserved ejection fraction (HFpEF). Both types of HF lead to significant risk of mortality and morbidity. Pharmacological treatment with β-adrenergic receptor (βAR) antagonists (also called β-blockers) has been shown to reduce the overall hospitalization and mortality rates and improve the clinical outcomes in HF patients with HFrEF but not HFpEF. Although, the survival rate of patients suffering from HF continues to drop, the management of HF still faces several limitations and discrepancies highlighting the need to develop new treatment strategies. Overstimulation of the sympathetic nervous system is an adaptive neurohormonal response to acute myocardial injury and heart damage, whereas prolonged exposure to catecholamines causes defects in βAR regulation, including a reduction in the amount of βARs and an increase in βAR desensitization due to the upregulation of G protein-coupled receptor kinases (GRKs) in the heart, contributing in turn to the progression of HF. Several studies show that myocardial GRK2 activity and expression are raised in the failing heart. Furthermore, β-arrestins play a pivotal role in βAR desensitization and, interestingly, can mediate their own signal transduction without any G protein-dependent pathway involved. In this review, we provide new insight into the role of GRKs and β-arrestins on how they affect βAR signaling regarding the molecular and cellular pathophysiology of HF. Additionally, we discuss the therapeutic potential of targeting GRKs and β-arrestins for the treatment of HF.

β-adrenergic signaling broadly contributes to LTP induction

Long-lasting forms of long-term potentiation (LTP) represent one of the major cellular mechanisms underlying learning and memory. One of the fundamental questions in the field of LTP is why different molecules are critical for long-lasting forms of LTP induced by diverse experimental protocols. Further complexity stems from spatial aspects of signaling networks, such that some molecules function in the dendrite and some are critical in the spine. We investigated whether the diverse experimental evidence can be unified by creating a spatial, mechanistic model of multiple signaling pathways in hippocampal CA1 neurons. Our results show that the combination of activity of several key kinases can predict the occurrence of long-lasting forms of LTP for multiple experimental protocols. Specifically Ca²⁺/calmodulin activated kinase II, protein kinase A and exchange protein activated by cAMP (Epac) together predict the occurrence of LTP in response to strong stimulation (multiple trains of 100 Hz) or weak stimulation augmented by isoproterenol. Furthermore, our analysis suggests that activation of the β-adrenergic receptor either via canonical (G_s-coupled) or non-canonical (G_i-coupled) pathways underpins most forms of long-lasting LTP. Simulations make the experimentally testable prediction that a complete antagonist of the β-adrenergic receptor will likely block long-lasting LTP in response to strong stimulation. Collectively these results suggest that converging molecular mechanisms allow CA1 neurons to flexibly utilize signaling mechanisms best tuned to temporal pattern of synaptic input to achieve long-lasting LTP and memory storage.

Long-term potentiation of the strength of synaptic connections is a mechanism of learning and memory storage. One of the most confusing aspects of hippocampal synaptic potentiation is that numerous experiments have revealed the requirement for a plethora of signaling molecules. Furthermore the degree to which molecules activated by the stress response modify hippocampal synaptic potentiation and memory is still unclear. We used a computational model to demonstrate that this molecular diversity can be explained by considering a combination of several key molecules. We also show that activation of β-adrenergic receptors by the stress response appears to be involved in most forms of synaptic potentiation, though in some cases unconventional mechanisms are utilized. This suggests that novel treatments for stress-related disorders may have more success if they target unconventional mechanisms activated by β-adrenergic receptors.

The Multiple Waves of Cannabinoid 1 Receptor Signaling

The cannabinoid 1 receptor (CB1R) is one of the most abundant G protein-coupled receptors (GPCRs) in the central nervous system, with key roles during neurotransmitter release and synaptic plasticity. Upon ligand activation, CB1Rs may signal in three different spatiotemporal waves. The first wave, which is transient (<10 minutes) and initiated by heterotrimeric G proteins, is followed by a second wave (>5 minutes) that is mediated by β-arrestins. The third and final wave occurs at intracellular compartments and could be elicited by G proteins or β-arrestins. This complexity presents multiple challenges, including the correct classification of receptor ligands, the identification of the signaling pathways regulated by each wave, and the underlying molecular mechanisms and physiologic impacts of these waves. Simultaneously, it provides new opportunities to harness the therapeutic potential of the cannabinoid system and other GPCRs. Over the last several years, we have significantly expanded our understanding of the mechanisms and pathways downstream from the CB1R. The identification of receptor mutations that can bias signaling to specific pathways and the use of siRNA technology have been key tools to identifying which signaling cascades are controlled by G proteins or β-arrestins. Here, we review our current knowledge on CB1R signaling, with particular emphasis on the mechanisms and cascades mediated by β-arrestins downstream from the CB1R.

Achievement of a target dose of bisoprolol may not be a preferred option for attenuating pressure overload-induced cardiac hypertrophy and fibrosis

Bisoprolol is a drug that acts via the mechanism of specifically and selectively inhibiting the β1-adrenoreceptor in cardiac myocytes, and provides a pure reduction of heart rate without changing other cardiac parameters. It has long been clinically used to treat cerebrovascular and cardiovascular illnesses. However, there is little information available on whether the role of bisoprolol in the attenuation of ventricular remodeling is dependent upon the achievement of a target dose, and whether it must be used as a preferred option. The aim of the present study was to clarify the underlying benefits of bisoprolol in the attenuation of pressure overload-induced cardiac hypertrophy and fibrosis at different doses. C57BL/6J male mice, aged 6-8 weeks, were treated with saline or one of three different doses of bisoprolol (Biso: 2.5, 5 or 10 mg/kg/day) for 8 weeks from day 1 after aortic banding (AB). A number of mice underwent sham surgery and were treated with saline or bisoprolol. The mice were randomly assigned into the sham (n=24) and AB (n=62) groups. The results revealed that bisoprolol had a protective role against the cardiac hypertrophy, fibrosis and dysfunction caused by AB. This was determined on the basis of heart/body and lung/body weight ratios and heart weight/tibia length ratios, as well as echocardiographic and hemodynamic parameters, histological analysis, and the gene expression levels of hypertrophic and fibrotic markers. The present study revealed that administration of bisoprolol for a long time period may enhance its role in the prevention of cardiac hypertrophy and fibrosis induced by AB, whereas no statistically significant difference was observed between the middle- and high-doses. These observations indicated that the function of bisoprolol in protecting against cardiac hypertrophy, fibrosis and dysfunction is time-dependent. Furthermore, it is proposed that a middle dose of bisoprolol may be a better option for patients with cardiovascular illnesses, particularly those undertaking coronary artery bypass graft and cardiac pacemaker surgeries. These promising results require further clinical investigation.

Mechanisms of Biased β-Arrestin-Mediated Signaling Downstream from the Cannabinoid 1 Receptor

Activation of G protein-coupled receptors results in multiple waves of signaling that are mediated by heterotrimeric G proteins and the scaffolding proteins β-arrestin 1/2. Ligands can elicit full or subsets of cellular responses, a concept defined as ligand bias or functional selectivity. However, our current understanding of β-arrestin-mediated signaling is still very limited. Here we provide a comprehensive view of β-arrestin-mediated signaling from the cannabinoid 1 receptor (CB1R). By using a signaling biased receptor, we define the cascades, specific receptor kinases, and molecular mechanism underlying β-arrestin-mediated signaling: We identify the interaction kinetics of CB1R and β-arrestin 1 during their endocytic trafficking as directly proportional to its efficacy. Finally, we demonstrate that signaling results in the control of genes clustered around prosurvival and proapoptotic functions among others. Together, these studies constitute a comprehensive description of β-arrestin-mediated signaling from CB1Rs and suggest modulation of receptor endocytic trafficking as a therapeutic approach to control β-arrestin-mediated signaling.

Imaging GPCRs trafficking and signaling with total internal reflection fluorescence microscopy in cultured neurons

Total internal reflection fluorescence (TIRF) microscopy allows probing the cellular events occurring close and at the plasma membrane. Over the last decade, we have seen a significant increase in the number of publications applying TIRF microscopy to unravel some of the fundamental biological questions regarding G protein-coupled receptors (GPCRs) function such as the mechanisms controlling receptor trafficking, quaternary structure, and signaling among others. Most of the published work has been performed in heterologous systems such as HEK293 and CHO cells, where the imaging surface available is higher and smoother when compared with the narrow processes or the smaller cell bodies of neurons. However, some publications have expanded our understanding of these events to primary cell cultures, mostly rat hippocampal and striatal neuronal cultures. Results from these cells provide a bona fide model of the complex events controlling GPCR function in living cells. We believe more work needs to be performed in primary cultures and eventually in intact tissue to complement the knowledge obtained from heterologous cell models. Here, we described a step-by-step protocol to investigate the surface trafficking and signaling from GPCRs in rat hippocampal and striatal primary cultures.

AKAP-dependent modulation of BCAM/Lu adhesion on normal and sickle cell disease RBCs revealed by force nanoscopy

Human normal and sickle red blood cells (RBCs) adhere with high affinity to the alpha5 chain of laminin (LAMA5) via the basal cell adhesion molecule/Lutheran (BCAM/Lu) receptor, which is implicated in vasoocclusive episodes in sickle cell disease and activated through the cyclic adenosine monophosphate (cAMP) signaling pathway. However, the effect of the cAMP pathway on the expression of active BCAM/Lu receptors at the single-molecule level is unknown. We established an in vitro technique, based on atomic force microscopy, which enables detection of single BCAM/Lu proteins on the RBC surface and measures the unbinding force between BCAM/Lu and LAMA5. We showed that the expression of active BCAM/Lu receptors is higher in homozygous sickle RBCs (SS-RBCs) than normal RBCs and that it is critically dependent on the cAMP signaling pathway on both normal and SS-RBCs. Of importance, we illustrated that A-kinase anchoring proteins are crucial for BCAM/Lu receptor activation. Furthermore, we found that SS-RBCs from hydroxyurea-treated patients show a lower expression of active BCAM/Lu receptors, a lower unbinding force to LAMA5, and insignificant stimulation by epinephrine as compared to SS-RBCs from untreated patients. To our knowledge, these findings may lead to novel antiadhesive targets for vasoocclusive episodes in sickle cell disease.

Ligand-specific endocytic dwell times control functional selectivity of the cannabinoid receptor 1

G protein-coupled receptors (GPCRs) are the major transducers of external stimuli and key therapeutic targets in many pathological conditions. When activated by different ligands, one receptor can elicit multiple signalling cascades that are mediated by G proteins or β-arrestin, a process defined as functional selectivity or ligand bias. However, the dynamic mechanisms underlying β-arrestin signalling remain unknown. Here by studying the cannabinoid receptor 1 (CB1R), we identify ligand-specific endocytic dwell times, that is, the time during which receptors are clustered into clathrin pits together with β-arrestins before endocytosis, as the mechanism controlling β-arrestin signalling. Agonists inducing short endocytic dwell times produce little or no β-arrestin signalling, whereas those eliciting prolonged dwell times induce robust signalling. Remarkably, extending CB1R dwell times by preventing endocytosis substantially increased β-arrestin signalling. These studies reveal how receptor activation translates into β-arrestin signalling and identify a mechanism to control this pathway.

G-protein coupled receptors can signal through G-proteins or through β-arrestin, however mechanisms determining pathway selection remain unclear. Here the authors show that the duration of cannabinoid receptor clustering in clathrin coated pits prior to endocytosis determines the strength of β-arrestin signalling.

Quantification of ligand bias for clinically relevant β2-adrenergic receptor ligands: implications for drug taxonomy

The concepts of functional selectivity and ligand bias are becoming increasingly appreciated in modern drug discovery programs, necessitating more informed approaches to compound classification and, ultimately, therapeutic candidate selection. Using the β2-adrenergic receptor as a model, we present a proof of concept study that assessed the bias of 19 β-adrenergic ligands, including many clinically used compounds, across four pathways [cAMP production, extracellular signal-regulated kinase 1/2 (ERK1/2) activation, calcium mobilization, and receptor endocytosis] in the same cell background (human embryonic kidney 293S cells). Efficacy-based clustering placed the ligands into five distinct groups with respect to signaling signatures. In some cases, apparent functional selectivity originated from off-target effects on other endogenously expressed adrenergic receptors, highlighting the importance of thoroughly assessing selectivity of the responses before concluding receptor-specific ligand-biased signaling. Eliminating the nonselective compounds did not change the clustering of the 10 remaining compounds. Some ligands exhibited large differences in potency for the different pathways, suggesting that the nature of the receptor-effector complexes influences the relative affinity of the compounds for specific receptor conformations. Calculation of relative effectiveness (within pathway) and bias factors (between pathways) for each of the compounds, using an operational model of agonism, revealed a global signaling signature for all of the compounds relative to isoproterenol. Most compounds were biased toward ERK1/2 activation over the other pathways, consistent with the notion that many proximal effectors converge on this pathway. Overall, we demonstrate a higher level of ligand texture than previously anticipated, opening perspectives for the establishment of pluridimensional correlations between signaling profiles, drug classification, therapeutic efficacy, and safety.

Insights into the structural biology of G-protein coupled receptors impacts drug design for central nervous system neurodegenerative processes

In the last few years, there have been important new insights into the structural biology of G-protein coupled receptors. It is now known that allosteric binding sites are involved in the affinity and selectivity of ligands for G-protein coupled receptors, and that signaling by these receptors involves both G-protein dependent and independent pathways. The present review outlines the physiological and pharmacological implications of this perspective for the design of new drugs to treat disorders of the central nervous system. Specifically, new possibilities are explored in relation to allosteric and orthosteric binding sites on dopamine receptors for the treatment of Parkinson's disease, and on muscarinic receptors for Alzheimer's disease. Future research can seek to identify ligands that can bind to more than one site on the same receptor, or simultaneously bind to two receptors and form a dimer. For example, the design of bivalent drugs that can reach homo/hetero-dimers of D2 dopamine receptor holds promise as a relevant therapeutic strategy for Parkinson's disease. Regarding the treatment of Alzheimer's disease, the design of dualsteric ligands for mono-oligomeric rinic receptors could increase therapeutic effectiveness by generating potent compounds that could activate more than one signaling pathway.

The β-blocker Nebivolol Is a GRK/β-arrestin biased agonist

Nebivolol, a third generation β-adrenoceptor (β-AR) antagonist (β-blocker), causes vasodilation by inducing nitric oxide (NO) production. The mechanism via which nebivolol induces NO production remains unknown, resulting in the genesis of much of the controversy regarding the pharmacological action of nebivolol. Carvedilol is another β-blocker that induces NO production. A prominent pharmacological mechanism of carvedilol is biased agonism that is independent of Gα_s and involves G protein-coupled receptor kinase (GRK)/β-arrestin signaling with downstream activation of the epidermal growth factor receptor (EGFR) and extracellular signal-regulated kinase (ERK). Due to the pharmacological similarities between nebivolol and carvedilol, we hypothesized that nebivolol is also a GRK/β-arrestin biased agonist. We tested this hypothesis utilizing mouse embryonic fibroblasts (MEFs) that solely express β₂-ARs, and HL-1 cardiac myocytes that express β₁- and β₂-ARs and no detectable β₃-ARs. We confirmed previous reports that nebivolol does not significantly alter cAMP levels and thus is not a classical agonist. Moreover, in both cell types, nebivolol induced rapid internalization of β-ARs indicating that nebivolol is also not a classical β-blocker. Furthermore, nebivolol treatment resulted in a time-dependent phosphorylation of ERK that was indistinguishable from carvedilol and similar in duration, but not amplitude, to isoproterenol. Nebivolol-mediated phosphorylation of ERK was sensitive to propranolol (non-selective β-AR-blocker), AG1478 (EGFR inhibitor), indicating that the signaling emanates from β-ARs and involves the EGFR. Furthermore, in MEFs, nebivolol-mediated phosphorylation of ERK was sensitive to pharmacological inhibition of GRK2 as well as siRNA knockdown of β-arrestin 1/2. Additionally, nebivolol induced redistribution of β-arrestin 2 from a diffuse staining pattern into more intense punctate spots. We conclude that nebivolol is a β₂-AR, and likely β₁-AR, GRK/β-arrestin biased agonist, which suggests that some of the unique clinically beneficial effects of nebivolol may be due to biased agonism at β₁- and/or β₂-ARs.

Induction of cardiac fibrosis by β-blocker in G protein-independent and G protein-coupled receptor kinase 5/β-arrestin2-dependent Signaling pathways

G-protein coupled receptors (GPCRs) have long been known as receptors that activate G protein-dependent cellular signaling pathways. In addition to the G protein-dependent pathways, recent reports have revealed that several ligands called "biased ligands" elicit G protein-independent and β-arrestin-dependent signaling through GPCRs (biased agonism). Several β-blockers are known as biased ligands. All β-blockers inhibit the binding of agonists to the β-adrenergic receptors. In addition to β-blocking action, some β-blockers are reported to induce cellular responses through G protein-independent and β-arrestin-dependent signaling pathways. However, the physiological significance induced by the β-arrestin-dependent pathway remains much to be clarified in vivo. Here, we demonstrate that metoprolol, a β(1)-adrenergic receptor-selective blocker, could induce cardiac fibrosis through a G protein-independent and β-arrestin2-dependent pathway. Metoprolol, a β-blocker, increased the expression of fibrotic genes responsible for cardiac fibrosis in cardiomyocytes. Furthermore, metoprolol induced the interaction between β(1)-adrenergic receptor and β-arrestin2, but not β-arrestin1. The interaction between β(1)-adrenergic receptor and β-arrestin2 by metoprolol was impaired in the G protein-coupled receptor kinase 5 (GRK5)-knockdown cells. Metoprolol-induced cardiac fibrosis led to cardiac dysfunction. However, the metoprolol-induced fibrosis and cardiac dysfunction were not evoked in β-arrestin2- or GRK5-knock-out mice. Thus, metoprolol is a biased ligand that selectively activates a G protein-independent and GRK5/β-arrestin2-dependent pathway, and induces cardiac fibrosis. This study demonstrates the physiological importance of biased agonism, and suggests that G protein-independent and β-arrestin-dependent signaling is a reason for the diversity of the effectiveness of β-blockers.

The C57BL/6 genetic background confers cardioprotection in iron-overloaded mice

BACKGROUND

Chronic transfusion therapy causes a progressive iron overload that damages many organs including the heart. Recent evidence suggests that L-type calcium channels play an important role in iron uptake by cardiomyocytes under conditions of iron overload. Given that beta-adrenergic stimulation significantly enhances L-type calcium current, we hypothesised that beta-adrenergic blocking drugs could reduce the deleterious effects of iron overload on the heart.

METHODS

Iron overload was generated by intraperitoneal injections of iron dextran (1g/kg) administered once a week for 8 weeks in male C57bl/6 mice, while propranolol was administered in drinking water at the dose of 40 mg/kg/day. Cardiac function and ventricular remodelling were evaluated by echocardiography and histological methods.

RESULTS

As compared to placebo, iron injection caused cardiac iron deposition. Surprisingly, despite iron overload, myocardial function and ventricular geometry in the iron-treated mice resulted unchanged as compared to those in the placebo-treated mice. Administration of propranolol increased cardiac performance in iron-overloaded mice. Specifically, as compared to the values in the iron-overloaded group, in iron-overloaded animals treated with propranolol left ventricular fractional shortening increased (from 31.6% to 44.2%, P =0.01) whereas left ventricular end-diastolic diameter decreased (from 4.1 ± 0.1 mm to 3.5 ± 0.1 mm, P =0.03). Propranolol did not alter cardiac systolic function or left ventricular sizes in the placebo group.

CONCLUSIONS

These results demonstrate that C57bl/6 mice are resistant to iron overload-induced myocardial injury and that treatment with propranolol is able to increase cardiac performance in iron-overloaded mice. However, since C57bl/6 mice were resistant to iron-induced injury, it remains to be evaluated further whether propranolol could prevent iron-overload cardiomyopathy.

Divergent agonist selectivity in activating β1- and β2-adrenoceptors for G-protein and arrestin coupling

The functional selectivity of adrenergic ligands for activation of β1- and β2-AR (adrenoceptor) subtypes has been extensively studied in cAMP signalling. Much less is known about ligand selectivity for arrestin-mediated signalling pathways. In the present study we used resonance energy transfer methods to compare the ability of β1- and β2-ARs to form a complex with the G-protein β-subunit or β-arrestin-2 in response to a variety of agonists with various degrees of efficacy. The profiles of β1-/β2-AR selectivity of the ligands for the two receptor-transducer interactions were sharply different. For G-protein coupling, the majority of ligands were more effective in activating the β2-AR, whereas for arrestin coupling the relationship was reversed. These data indicate that the β1-AR interacts more efficiently than β2-AR with arrestin, but less efficiently than β2-AR with G-protein. A group of ligands exhibited β1-AR-selective efficacy in driving the coupling to arrestin. Dobutamine, a member of this group, had 70% of the adrenaline (epinephrine) effect on arrestin via β1-AR, but acted as a competitive antagonist of adrenaline via β2-AR. Thus the structure of such ligands appears to induce an arrestin-interacting form of the receptor only when bound to the β1-AR subtype.

Medicinal chemistry for 2020

Rapid advances in our collective understanding of biomolecular structure and, in concert, of biochemical systems, coupled with developments in computational methods, have massively impacted the field of medicinal chemistry over the past two decades, with even greater changes appearing on the horizon. In this perspective, we endeavor to profile some of the most prominent determinants of change and speculate as to further evolution that may consequently occur during the next decade. The five main angles to be addressed are: protein-protein interactions; peptides and peptidomimetics; molecular diversity and pharmacological space; molecular pharmacodynamics (significance, potential and challenges); and early-stage clinical efficacy and safety. We then consider, in light of these, the future of medicinal chemistry and the educational preparation that will be required for future medicinal chemists.