The therapeutic potential of GLP-1 receptor biased agonism

The impact of GLP-1 signalling on the energy metabolism of pancreatic islet β-cells and extrapancreatic tissues

Glucagon-like peptide-1 signalling impacts glucose homeostasis and appetite thereby indirectly affecting substrate availability at the whole-body level. The incretin canonically produces an insulinotropic effect, thereby lowering blood glucose levels by promoting the uptake and inhibiting the production of the sugar by peripheral tissues. Likewise, GLP-1 signalling within the central nervous system reduces the appetite and food intake, whereas its gastric effect delays the absorption of nutrients, thus improving glycaemic control and reducing the risk of postprandial hyperglycaemia. We review the molecular aspects of the GLP-1 signalling, focusing on its impact on intracellular energy metabolism. Whilst the incretin exerts its effects predominantly via a Gs receptor, which decodes the incretin signal into the elevation of intracellular cAMP levels, the downstream signalling cascades within the cell, acting on fast and slow timescales, resulting in an enhancement or an attenuation of glucose catabolism, respectively.

Glucagon-like peptide 1 receptor activation: anti-inflammatory effects in the brain

The glucagon-like peptide 1 is a pleiotropic hormone that has potent insulinotropic effects and is key in treating metabolic diseases such as diabetes and obesity. Glucagon-like peptide 1 exerts its effects by activating a membrane receptor identified in many tissues, including different brain regions. Glucagon-like peptide 1 activates several signaling pathways related to neuroprotection, like the support of cell growth/survival, enhancement promotion of synapse formation, autophagy, and inhibition of the secretion of proinflammatory cytokines, microglial activation, and apoptosis during neural morphogenesis. The glial cells, including astrocytes and microglia, maintain metabolic homeostasis and defense against pathogens in the central nervous system. After brain insult, microglia are the first cells to respond, followed by reactive astrocytosis. These activated cells produce proinflammatory mediators like cytokines or chemokines to react to the insult. Furthermore, under these circumstances, microglia can become chronically inflammatory by losing their homeostatic molecular signature and, consequently, their functions during many diseases. Several processes promote the development of neurological disorders and influence their pathological evolution: like the formation of protein aggregates, the accumulation of abnormally modified cellular constituents, the formation and release by injured neurons or synapses of molecules that can dampen neural function, and, of critical importance, the dysregulation of inflammatory control mechanisms. The glucagon-like peptide 1 receptor agonist emerges as a critical tool in treating brain-related inflammatory pathologies, restoring brain cell homeostasis under inflammatory conditions, modulating microglia activity, and decreasing the inflammatory response. This review summarizes recent advances linked to the anti-inflammatory properties of glucagon-like peptide 1 receptor activation in the brain related to multiple sclerosis, Alzheimer's disease, Parkinson's disease, vascular dementia, or chronic migraine.

Targeting GLP-1 receptors to reduce nicotine use disorder: Preclinical and clinical evidence

Nicotine use disorder (NUD) remains a leading cause of preventable death in the U.S. Unfortunately, current FDA-approved pharmacotherapies for smoking cessation have limited efficacy and are associated with high rates of relapse. One major barrier to long-term smoking abstinence is body weight gain during withdrawal. Nicotine withdrawal-induced body weight gain can also lead to development of chronic disease states like obesity and type II diabetes mellitus. Therefore, it is critical to identify novel pharmacotherapies for NUD that decrease relapse and nicotine withdrawal symptoms including body weight gain. Recent studies demonstrate that glucagon-like peptide-1 receptor (GLP-1R) agonists attenuate voluntary nicotine taking and seeking and prevent withdrawal-induced hyperphagia and body weight gain. Emerging evidence also suggests that GLP-1R agonists improve cognitive deficits, as well as depressive- and anxiety-like behaviors, which contribute to smoking relapse during withdrawal. While further studies are necessary to fully characterize the effects of GLP-1R agonists on NUD and understand the mechanisms by which GLP-1R agonists decrease nicotine withdrawal-mediated behaviors, the current literature supports GLP-1R-based approaches to treating NUD.

Bias translation: The final frontier?

Biased signalling is a natural result of GPCR allosteric function and should be expected from any and all synthetic and natural agonists. Therefore, it may be encountered in all agonist discovery projects and must be considered as a beneficial (or possible detrimental) feature of new candidate molecules. While bias is detected easily, the synoptic nature of GPCR signalling makes translation of simple in vitro bias to complex in vivo systems problematic. The practical outcome of this is a difficulty in predicting the therapeutic value of biased signalling due to the failure of translation of identified biased signalling to in vivo agonism. This is discussed in this review as well as some new ways forward to improve this translation process and better exploit this powerful pharmacologic mechanism.

Targeting the incretin system in obesity and type 2 diabetes mellitus

Obesity and type 2 diabetes mellitus (T2DM) are widespread, non-communicable diseases that are responsible for considerable levels of morbidity and mortality globally, primarily in the form of cardiovascular disease (CVD). Changes to lifestyle and behaviour have insufficient long-term efficacy in most patients with these diseases; metabolic surgery, although effective, is not practically deliverable on the scale that is required. Over the past two decades, therapies based on incretin hormones, spearheaded by glucagon-like peptide 1 (GLP1) receptor agonists (GLP1RAs), have become the treatment of choice for obesity and T2DM, and clinical evidence now suggests that these agents have benefits for CVD. We review the latest advances in incretin-based pharmacotherapy. These include 'GLP1 plus' agents, which combine the known advantages of GLP1RAs with the activity of additional hormones, such as glucose-dependent insulinotropic peptide, glucagon and amylin, to achieve desired therapeutic goals. Second-generation non-peptidic oral GLP1RAs promise to extend the benefits of GLP1 therapy to those who do not want, or cannot have, subcutaneous injection therapy. We conclude with a discussion of the knowledge gaps that must be addressed before incretin-based therapies can be properly deployed for maximum benefit in the treatment of obesity and T2DM.

Duodenal enteroendocrine cells and GIP as treatment targets for obesity and type 2 diabetes

The duodenum is an important source of endocrine and paracrine signals controlling digestion and nutrient disposition, notably including the main incretin hormone glucose-dependent insulinotropic polypeptide (GIP). Bariatric procedures that prevent nutrients from contact with the duodenal mucosa are particularly effective interventions to reduce body weight and improve glycaemic control in obesity and type 2 diabetes. These procedures take advantage of increased nutrient delivery to more distal regions of the intestine which enhances secretion of the other incretin hormone glucagon-like peptide-1 (GLP-1). Preclinical experiments have shown that either an increase or a decrease in the secretion or action of GIP can decrease body weight and blood glucose in obesity and non-insulin dependent hyperglycaemia, but clinical studies involving administration of GIP have been inconclusive. However, a synthetic dual agonist peptide (tirzepatide) that exerts agonism at receptors for GIP and GLP-1 has produced marked weight-lowering and glucose-lowering effects in people with obesity and type 2 diabetes. This appears to result from chronic biased agonism in which the novel conformation of the peptide triggers enhanced signalling by the GLP-1 receptor through reduced internalisation while reducing signalling by the GIP receptor directly or via functional antagonism through increased internalisation and degradation.

G protein-coupled receptors driven intestinal glucagon-like peptide-1 reprogramming for obesity: Hope or hype?

'Globesity' is a foremost challenge to the healthcare system. The limited efficacy and adverse effects of available oral pharmacotherapies pose a significant obstacle in the fight against obesity. The biology of the leading incretin hormone glucagon-like-peptide-1 (GLP-1) has been highly captivated during the last decade owing to its multisystemic pleiotropic clinical outcomes beyond inherent glucoregulatory action. That fostered a pharmaceutical interest in synthetic GLP-1 analogues to tackle type-2 diabetes (T2D), obesity and related complications. Besides, mechanistic insights on metabolic surgeries allude to an incretin-based hormonal combination strategy for weight loss that emerged as a forerunner for the discovery of injectable 'unimolecular poly-incretin-agonist' therapies. Physiologically, intestinal enteroendocrine L-cells (EECs) are the prominent endogenous source of GLP-1 peptide. Despite comprehending the potential of various G protein-coupled receptors (GPCRs) to stimulate endogenous GLP-1 secretion, decades of translational GPCR research have failed to yield regulatory-approved endogenous GLP-1 secretagogue oral therapy. Lately, a dual/poly-GPCR agonism strategy has emerged as an alternative approach to the traditional mono-GPCR concept. This review aims to gain a comprehensive understanding by revisiting the pharmacology of a few potential GPCR-based complementary avenues that have drawn attention to the design of orally active poly-GPCR agonist therapy. The merits, challenges and recent developments that may aid future poly-GPCR drug discovery are critically discussed. Subsequently, we project the mechanism-based therapeutic potential and limitations of oral poly-GPCR agonism strategy to augment intestinal GLP-1 for weight loss. We further extend our discussion to compare the poly-GPCR agonism approach over invasive surgical and injectable GLP-1-based regimens currently in clinical practice for obesity.

What is the pipeline for future medications for obesity?

Obesity is a chronic disease associated with increased risk of obesity-related complications and mortality. Our better understanding of the weight regulation mechanisms and the role of gut-brain axis on appetite has led to the development of safe and effective entero-pancreatic hormone-based treatments for obesity such as glucagon-like peptide-1 (GLP-1) receptor agonists (RA). Semaglutide 2.4 mg once weekly, a subcutaneously administered GLP-1 RA approved for obesity treatment in 2021, results in 15-17% mean weight loss (WL) with evidence of cardioprotection. Oral GLP-1 RA are also under development and early data shows similar WL efficacy to semaglutide 2.4 mg. Looking to the next generation of obesity treatments, combinations of GLP-1 with other entero-pancreatic hormones with complementary actions and/or synergistic potential (such as glucose-dependent insulinotropic polypeptide (GIP), glucagon, and amylin) are under investigation to enhance the WL and cardiometabolic benefits of GLP-1 RA. Tirzepatide, a dual GLP-1/GIP receptor agonist has been approved for glycaemic control in type 2 diabetes as well as for obesity management leading in up to 22.5% WL in phase 3 obesity trials. Other combinations of entero-pancreatic hormones including cagrisema (GLP-1/amylin RA) and the triple agonist retatrutide (GLP-1/GIP/glucagon RA) have also progressed to phase 3 trials as obesity treatments and early data suggests that may lead to even greater WL than tirzepatide. Additionally, agents with different mechanisms of action to entero-pancreatic hormones (e.g. bimagrumab) may improve the body composition during WL and are in early phase clinical trials. We are in a new era for obesity pharmacotherapy where combinations of entero-pancreatic hormones approach the WL achieved with bariatric surgery. In this review, we present the efficacy and safety data for the pipeline of obesity pharmacotherapies with a focus on entero-pancreatic hormone-based treatments and we consider the clinical implications and challenges that the new era in obesity management may bring.

Applications of oxetanes in drug discovery and medicinal chemistry

The compact and versatile oxetane motifs have gained significant attention in drug discovery and medicinal chemistry campaigns. This review presents an overview of the diverse applications of oxetanes in clinical and preclinical drug candidates targeting various human diseases, including cancer, viral infections, autoimmune disorders, neurodegenerative conditions, metabolic disorders, and others. Special attention is given to biologically active oxetane-containing compounds and their disease-related targets, such as kinases, epigenetic and non-epigenetic enzymes, and receptors. The review also details the effect of the oxetane motif on important properties, including aqueous solubility, lipophilicity, pKa, P-glycoprotein (P-gp) efflux, metabolic stability, conformational preferences, toxicity profiles (e.g., cytochrome P450 (CYP) suppression and human ether-a-go-go related gene (hERG) inhibition), pharmacokinetic (PK) properties, potency, and target selectivity. We anticipate that this work will provide valuable insights that can drive future discoveries of novel bioactive oxetane-containing small molecules, enabling their effective application in combating a wide range of human diseases.

Human GLP1R variants affecting GLP1R cell surface expression are associated with impaired glucose control and increased adiposity

The glucagon-like peptide 1 receptor (GLP1R) is a major drug target with several agonists being prescribed in individuals with type 2 diabetes and obesity1,2. The impact of genetic variability of GLP1R on receptor function and its association with metabolic traits are unclear with conflicting reports. Here, we show an unexpected diversity of phenotypes ranging from defective cell surface expression to complete or pathway-specific gain of function (GoF) and loss of function (LoF), after performing a functional profiling of 60 GLP1R variants across four signalling pathways. The defective insulin secretion of GLP1R LoF variants is rescued by allosteric GLP1R ligands or high concentrations of exendin-4/semaglutide in INS-1 823/3 cells. Genetic association studies in 200,000 participants from the UK Biobank show that impaired GLP1R cell surface expression contributes to poor glucose control and increased adiposity with increased glycated haemoglobin A1c and body mass index. This study defines impaired GLP1R cell surface expression as a risk factor for traits associated with type 2 diabetes and obesity and provides potential treatment options for GLP1R LoF variant carriers.

Structural basis of hydroxycarboxylic acid receptor signaling mechanisms through ligand binding

Hydroxycarboxylic acid receptors (HCA) are expressed in various tissues and immune cells. HCA2 and its agonist are thus important targets for treating inflammatory and metabolic disorders. Only limited information is available, however, on the active-state binding of HCAs with agonists. Here, we present cryo-EM structures of human HCA2-Gi and HCA3-Gi signaling complexes binding with multiple compounds bound. Agonists were revealed to form a salt bridge with arginine, which is conserved in the HCA family, to activate these receptors. Extracellular regions of the receptors form a lid-like structure that covers the ligand-binding pocket. Although transmembrane (TM) 6 in HCAs undergoes dynamic conformational changes, ligands do not directly interact with amino acids in TM6, suggesting that indirect signaling induces a slight shift in TM6 to activate Gi proteins. Structural analyses of agonist-bound HCA2 and HCA3 together with mutagenesis and molecular dynamics simulation provide molecular insights into HCA ligand recognition and activation mechanisms.

Interaction of the AKT and β-catenin signalling pathways and the influence of photobiomodulation on cellular signalling proteins in diabetic wound healing

The induction of a cells destiny is a tightly controlled process that is regulated through communication between the matrix and cell signalling proteins. Cell signalling activates distinctive subsections of target genes, and different signalling pathways may be used repeatedly in different settings. A range of different signalling pathways are activated during the wound healing process, and dysregulated cellular signalling may lead to reduced cell function and the development of chronic wounds. Diabetic wounds are chronic and are characterised by the inability of skin cells to act in response to reparative inducements. Serine/threonine kinase, protein kinase B or AKT (PKB/AKT), is a central connection in cell signalling induced by growth factors, cytokines and other cellular inducements, and is one of the critical pathways that regulate cellular proliferation, survival, and quiescence. AKT interacts with a variety of other pathway proteins including glycogen synthase kinase 3 beta (GSK3β) and β-catenin. Novel methodologies based on comprehensive knowledge of activated signalling pathways and their interaction during normal or chronic wound healing can facilitate quicker and efficient diabetic wound healing. In this review, we focus on interaction of the AKT and β-catenin signalling pathways and the influence of photobiomodulation on cellular signalling proteins in diabetic wound healing.

The role of weight control in the management of type 2 diabetes mellitus: Perspectives on semaglutide

Glucagon-like peptide-1 receptor agonists (GLP-1 RAs) are widely used to address multiple aspects of type 2 diabetes mellitus (T2DM) management, including glycaemic control, weight loss, and cardiovascular risk reduction. Semaglutide, a well-established GLP-1 RA approved for T2DM treatment and weight management, demonstrates marked efficacy in achieving these clinically important goals. The American Diabetes Association (ADA) and the European Association for the Study of Diabetes (EASD) consensus report emphasizes the importance of a holistic approach to T2DM treatment, with weight control as a key component for improving patient outcomes. Notably, semaglutide is mentioned in the consensus report as having 'very high' efficacy for both glucose lowering and weight loss in T2DM treatment. Nevertheless, as has been observed with other weight-lowering drugs, weight loss observed with semaglutide appears less profound in individuals with T2DM than in those with obesity without T2DM, a phenomenon requiring further investigation. The semaglutide safety and tolerability profiles are well established, and it is approved in some countries to reduce cardiovascular risk in certain populations with T2DM. Thus, semaglutide offers a well-established therapeutic option that aligns well with guideline recommendations for T2DM management, emphasizing the high importance of weight control and amelioration of other cardiometabolic risk factors.

G Protein-Coupled Receptors and the Rise of Type 2 Diabetes in Children

The human genome counts hundreds of GPCRs specialized to sense thousands of different extracellular cues, including light, odorants and nutrients in addition to hormones. Primordial GPCRs were likely glucose transporters that became sensors to monitor the abundance of nutrients and direct the cell to switch from aerobic metabolism to fermentation. Human β cells express multiple GPCRs that contribute to regulate glucose homeostasis, cooperating with many others expressed by a variety of cell types and tissues. These GPCRs are intensely studied as pharmacological targets to treat type 2 diabetes in adults. The dramatic rise of type 2 diabetes incidence in pediatric age is likely correlated to the rapidly evolving lifestyle of children and adolescents of the new century. Current pharmacological treatments are based on therapies designed for adults, while youth and puberty are characterized by a different hormonal balance related to glucose metabolism. This review focuses on GPCRs functional traits that are relevant for β cells function, with an emphasis on aspects that could help to differentiate new treatments specifically addressed to young type 2 diabetes patients.

Divergent acute versus prolonged pharmacological GLP-1R responses in adult β cell-specific β-arrestin 2 knockout mice

The glucagon-like peptide-1 receptor (GLP-1R) is a major type 2 diabetes therapeutic target. Stimulated GLP-1Rs are rapidly desensitized by β-arrestins, scaffolding proteins that not only terminate G protein interactions but also act as independent signaling mediators. Here, we have assessed in vivo glycemic responses to the pharmacological GLP-1R agonist exendin-4 in adult β cell-specific β-arrestin 2 knockout (KO) mice. KOs displayed a sex-dimorphic phenotype consisting of weaker acute responses that improved 6 hours after agonist injection. Similar effects were observed for semaglutide and tirzepatide but not with biased agonist exendin-phe1. Acute cyclic adenosine 5'-monophosphate increases were impaired, but desensitization reduced in KO islets. The former defect was attributed to enhanced β-arrestin 1 and phosphodiesterase 4 activities, while reduced desensitization co-occurred with impaired GLP-1R recycling and lysosomal targeting, increased trans-Golgi network signaling, and reduced GLP-1R ubiquitination. This study has unveiled fundamental aspects of GLP-1R response regulation with direct application to the rational design of GLP-1R-targeting therapeutics.

GLP-1R Signaling and Functional Molecules in Incretin Therapy

Glucagon-like peptide-1 receptor (GLP-1R) is a critical therapeutic target for type 2 diabetes mellitus (T2DM). The GLP-1R cellular signaling mechanism relevant to insulin secretion and blood glucose regulation has been extensively studied. Numerous drugs targeting GLP-1R have entered clinical treatment. However, novel functional molecules with reduced side effects and enhanced therapeutic efficacy are still in high demand. In this review, we summarize the basis of GLP-1R cellular signaling, and how it is involved in the treatment of T2DM. We review the functional molecules of incretin therapy in various stages of clinical trials. We also outline the current strategies and emerging techniques that are furthering the development of novel therapeutic drugs for T2DM and other metabolic diseases.

New insights into the structure and function of class B1 GPCRs

G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors. Class B1 GPCRs constitute a subfamily of 15 receptors that characteristically contain large extracellular domains (ECDs) and respond to long polypeptide hormones. Class B1 GPCRs are critical regulators of homeostasis, and as such, many are important drug targets. While most transmembrane proteins, including GPCRs, are recalcitrant to crystallization, recent advances in electron cryo-microscopy (cryo-EM) have facilitated a rapid expansion of the structural understanding of membrane proteins. As a testament to this success, structures for all the class B1 receptors bound to G proteins have been determined by cryo-EM in the past five years. Further advances in cryo-EM have uncovered dynamics of these receptors, ligands, and signalling partners. Here, we examine the recent structural underpinnings of the class B1 GPCRs with an emphasis on structure-function relationships.

Biased Agonism and Polymorphic Variation at the GLP-1 Receptor: Implications for the Development of Personalised Therapeutics

Glucagon-like peptide-1 receptor (GLP-1R) is a well-studied incretin hormone receptor and target of several therapeutic drugs for type 2 diabetes (T2D), obesity and, more recently, cardiovascular disease. Some signalling pathways downstream of GLP-1R may be responsible for drug adverse effects such as nausea, while others mediate therapeutic outcomes of incretin-based T2D therapeutics. Understanding the interplay between different factors that alter signalling, trafficking, and receptor activity, including biased agonism, single nucleotide polymorphisms and structural modifications is key to develop the next-generation of personalised GLP-1R agonists. However, these interactions remain poorly described, especially for novel therapeutics such as dual and tri-agonists that target more than one incretin receptor. Comparison of GLP-1R structures in complex with G proteins and different peptide and non-peptide agonists has revealed novel insights into important agonist-residue interactions and networks crucial for receptor activation, recruitment of G proteins and engagement of specific signalling pathways. Here, we review the latest knowledge on GLP-1R structure and activation, providing structural evidence for biased agonism and delineating important networks associated with this phenomenon. We survey current biased agonists and multi-agonists at different stages of development, highlighting possible challenges in their translational potential. Lastly, we discuss findings related to non-synonymous genomic variants of GLP1R and the functional importance of specific residues involved in GLP-1R function. We propose that studies of GLP-1R polymorphisms, and specifically their effect on receptor dynamics and pharmacology in response to biased agonists, could have a significant impact in delineating precision medicine approaches and development of novel therapeutics.

Modifying a Hydroxyl Patch in Glucagon-like Peptide 1 Produces Biased Agonists with Unique Signaling Profiles

Glucagon-like peptide-1 (GLP-1) lowers blood glucose by inducing insulin but also has other poorly understood properties. Here, we show that hydroxy amino acids (Thr11, Ser14, Ser17, Ser18) in GLP-1(7-36) act in concert to direct cell signaling. Mutating any single residue to alanine removes one hydroxyl group, thereby reducing receptor affinity and cAMP 10-fold, with Ala11 or Ala14 also reducing β-arrestin-2 10-fold, while Ala17 or Ala18 also increases ERK1/2 phosphorylation 5-fold. Multiple alanine mutations more profoundly bias signaling, differentially silencing or restoring one or more signaling properties. Mutating three serines silences only ERK1/2, the first example of such bias. Mutating all four residues silences β-arrestin-2, ERK1/2, and Ca²⁺ maintains the ligand and receptor at the membrane but still potently stimulates cAMP and insulin secretion in cells and mice. These novel findings indicate that hydrogen bonding cooperatively controls cell signaling and highlight an important regulatory hydroxyl patch in hormones that activate class B G protein-coupled receptors.

Next generation GLP-1/GIP/glucagon triple agonists normalize body weight in obese mice

Objective

Pharmacological strategies that engage multiple mechanisms-of-action have demonstrated synergistic benefits for metabolic disease in preclinical models. One approach, concurrent activation of the glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic peptide (GIP), and glucagon (Gcg) receptors (i.e. triagonism), combines the anorectic and insulinotropic activities of GLP-1 and GIP with the energy expenditure effect of glucagon. While the efficacy of triagonism in preclinical models is known, the relative contribution of GcgR activation remains unassessed. This work aims to addresses that central question.

Methods

Herein, we detail the design of unimolecular peptide triagonists with an empirically optimized receptor potency ratio. These optimized peptide triagonists employ a protraction strategy permitting once-weekly human dosing. Additionally, we assess the effects of these peptides on weight-reduction, food intake, glucose control, and energy expenditure in an established DIO mouse model compared to clinically relevant GLP-1R agonists (e.g. semaglutide) and dual GLP-1R/GIPR agonists (e.g. tirzepatide).

Results

Optimized triagonists normalize body weight in DIO mice and enhance energy expenditure in a manner superior to that of GLP-1R mono-agonists and GLP-1R/GIPR co-agonists.

Conclusions

These pre-clinical data suggest unimolecular poly-pharmacology as an effective means to target multiple mechanisms contributing to obesity and further implicate GcgR activation as the differentiating factor between incretin receptor mono- or dual-agonists and triagonists.

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Details the design of unimolecular peptide triagonists for GLP-1R/GIPR/GCGR.

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Optimal weight-loss is achieved when receptor potency ratio is weighted toward GCGR vs GLP-1R or GIPR.

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These agonists are protracted for once-weekly human dosing.

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Optimized triagonists normalizes body weight & enhance energy expenditure in mice.

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Efficacy of optimized triagonists is superior to GLP-1R & GLP-1R/GIPR agonists.

New Horizons in the Treatment of Age-Associated Obesity, Sarcopenia and Osteoporosis

The rapid increase in both the lifespan and proportion of older adults in developed countries is accompanied by the dramatic growth of age-associated chronic diseases, including obesity, sarcopenia, and osteoporosis. Hence, prevention and treatment of age-associated chronic diseases has become increasingly urgent. The key to achieving this goal is a better understanding of the mechanisms underlying their pathophysiology, some aspects of which, despite extensive investigation, are still not fully understood. Aging, obesity, sarcopenia, and osteoporosis are characterized by the creation of a systemic, chronic, low-grade inflammation (SCLGI). The common mechanisms that govern the development of these chronic conditions include a failed resolution of inflammation. Physiologically, the process of inflammation resolution is provided mainly by specialized pro-resolving mediators (SPMs) acting via cognate G protein-coupled receptors (GPCRs). Noteworthy, SPM levels and the expression of their receptors are significantly reduced in aging and the associated chronic disorders. In preclinical studies, supplementation of SPMs or their stable, small-molecule SPM mimetics and receptor agonists reveals clear beneficial effects in inflammation-related obesity and sarcopenic and osteoporotic conditions, suggesting a translational potential. Age-associated chronic disorders are also characterized by gut dysbiosis and the accumulation of senescent cells in the adipose tissue, skeletal muscle, and bones. Based on these findings, we propose SCLGI resolution as a novel strategy for the prevention/treatment of age-associated obesity, sarcopenia, and osteoporosis. Our approach entails the enhancement of inflammation resolution by SPM mimetics and receptor agonists in concert with probiotics/prebiotics and compounds that eliminate senescent cells and their pro-inflammatory activity.

Emerging Role of cAMP/AMPK Signaling

The 5′-Adenosine monophosphate (AMP)-activated protein kinase (AMPK) is a natural energy sensor in mammalian cells that plays a key role in cellular and systemic energy homeostasis. At the cellular level, AMPK supports numerous processes required for energy and redox homeostasis, including mitochondrial biogenesis, autophagy, and glucose and lipid metabolism. Thus, understanding the pathways regulating AMPK activity is crucial for developing strategies to treat metabolic disorders. Mounting evidence suggests the presence of a link between cyclic AMP (cAMP) and AMPK signaling. cAMP signaling is known to be activated in circumstances of physiological and metabolic stress due to the release of stress hormones, such as adrenaline and glucagon, which is followed by activation of membrane-bound adenylyl cyclase and elevation of cellular cAMP. Because the majority of physiological stresses are associated with elevated energy consumption, it is not surprising that activation of cAMP signaling may promote AMPK activity. Aside from the physiological role of the cAMP/AMPK axis, numerous reports have suggested its role in several pathologies, including inflammation, ischemia, diabetes, obesity, and aging. Furthermore, novel reports have provided more mechanistic insight into the regulation of the cAMP/AMPK axis. In particular, the role of distinct cAMP microdomains generated by soluble adenylyl cyclase in regulating basal and induced AMPK activity has recently been demonstrated. In the present review, we discuss current advances in the understanding of the regulation of the cAMP/AMPK axis and its role in cellular homeostasis and explore some translational aspects.

Biased and allosteric modulation of bone cell-expressing G protein-coupled receptors as a novel approach to osteoporosis therapy

On the cellular level, osteoporosis (OP) is a result of imbalanced bone remodeling, in which osteoclastic bone resorption outcompetes osteoblastic bone formation. Currently available OP medications include both antiresorptive and bone-forming drugs. However, their long-term use in OP patients, mainly in postmenopausal women, is accompanied by severe side effects. Notably, the fundamental coupling between bone resorption and formation processes underlies the existence of an undesirable secondary outcome that bone anabolic or anti-resorptive drugs also reduce bone formation. This drawback requires the development of anti-OP drugs capable of selectively stimulating osteoblastogenesis and concomitantly reducing osteoclastogenesis. We propose that the application of small synthetic biased and allosteric modulators of bone cell receptors, which belong to the G-protein coupled receptors (GPCR) family, could be the key to resolving the undesired anti-OP drug selectivity. This approach is based on the capacity of these GPCR modulators, unlike the natural ligands, to trigger signaling pathways that promote beneficial effects on bone remodeling while blocking potentially deleterious effects. Under the settings of OP, an optimal anti-OP drug should provide fine-tuned regulation of downstream effects, for example, intermittent cyclic AMP (cAMP) elevation, preservation of Ca²⁺ balance, stimulation of osteoprotegerin (OPG) and estrogen production, suppression of sclerostin secretion, and/or preserved/enhanced canonical β-catenin/Wnt signaling pathway. As such, selective modulation of GPCRs involved in bone remodeling presents a promising approach in OP treatment. This review focuses on the evidence for the validity of our hypothesis.

Effects of Incretin-Related Diabetes Drugs on Bone Formation and Bone Resorption

Patients with type 2 diabetes have an increased risk of fracture compared to the general population. Glucose absorption is accelerated by incretin hormones, which induce insulin secretion from the pancreas. The level of the incretin hormone, glucagon-like peptide-1 (GLP-1), shows an immediate postprandial increase, and the circulating level of intact GLP-1 is reduced rapidly by dipeptidyl peptidase-4 (DPP-4)-mediated inactivation. Therefore, GLP-1 receptor agonists and DPP-4 inhibitors are effective in the treatment of type 2 diabetes. However, these incretin-related diabetic agents have been reported to affect bone metabolism, including bone formation and resorption. These agents enhance the expression of bone markers, and have been applied to improve bone quality and bone density. In addition, they have been reported to suppress chronic inflammation and reduce the levels of inflammatory cytokine expression. Previously, we reported that these incretin-related agents inhibited both the expression of inflammatory cytokines and inflammation-induced bone resorption. This review presents an overview of current knowledge regarding the effects of incretin-related diabetes drugs on osteoblast differentiation and bone formation as well as osteoclast differentiation and bone resorption. The mechanisms by which incretin-related diabetes drugs regulate bone formation and bone resorption are also discussed.