β-Arrestin-1 is required for adaptive β-cell mass expansion during obesity

Multiomics reveal key inflammatory drivers of severe obesity: IL4R, LILRA5, and OSM

Polygenic severe obesity (body mass index [BMI] ≥40 kg/m2) has increased, especially in Hispanic/Latino populations, yet we know little about the underlying mechanistic pathways. We analyzed whole-blood multiomics data to identify genes differentially regulated in severe obesity in Mexican Americans from the Cameron County Hispanic Cohort. Our RNA sequencing analysis identified 124 genes significantly differentially expressed between severe obesity cases (BMI ≥40 kg/m2) and controls (BMI <25 kg/m2); 33% replicated in an independent sample from the same population. Our integrative approach identified inflammatory genes, including IL4R, ZNF438, and LILRA5. Several genes displayed transcriptomic effects on severe obesity in subcutaneous adipose tissue. We further showed that the genetic regulation of these genes is associated with several traits in a large biobank, including bone fractures, obstructive sleep apnea, and hyperaldosteronism, illuminating potential risk mechanisms. Our findings furnish a molecular architecture of the severe obesity phenotype across multiple molecular domains.

Regulation of GLP-1 and Glucagon Receptor Function by β-Arrestins in Metabolically Important Cell Types

Glucagon-like peptide-1 (GLP-1) and glucagon (GCG) are polypeptides derived from a common precursor (preproglucagon) that modulates the activity of numerous cell types involved in regulating glucose and energy homeostasis. GLP-1 and GCG exert their biological functions via binding to specific G protein-coupled receptors (GLP-1Rs and GCGRs). Ligand-activated GLP-1Rs and GCGRs preferentially activate the heterotrimeric G protein Gs, resulting in increased cytosolic cAMP levels. However, activation of the two receptors also leads to the recruitment of β-arrestin-1 and -2 (βarr1 and βarr2, respectively) to the intracellular surface of the receptor proteins. The binding of β-arrestins to the activated receptors contributes to the termination of receptor-stimulated G protein coupling. In addition, receptor-β-arrestin complexes can act as signaling nodes in their own right by modulating the activity of many intracellular signaling pathways. In this Review, we will discuss the roles of βarr1 and βarr2 in regulating key metabolic functions mediated by activated GLP-1Rs and GCGRs. During the past decade, GLP-1R agonists have emerged as highly efficacious antidiabetic and antiobesity drugs. Moreover, dual agonists that stimulate both GLP-1Rs and GCGRs are predicted to offer additional therapeutic benefits as compared to GLP-1R agonist monotherapy. We will summarize and try to synthesize a series of studies suggesting that the development of G protein-biased GLP-1R and/or GCGR agonists, which do not lead to the recruitment of β-arrestins, may lead to even more efficacious therapeutic agents.

LGR4 is essential for maintaining β-cell homeostasis through suppression of RANK

OBJECTIVE

Loss of functional β-cell mass is a major cause of diabetes. Thus, identifying regulators of β-cell health is crucial for treating this disease. The Leucine-rich repeat-containing G-protein-coupled receptor (GPCR) 4 (LGR4) is expressed in β-cells and is the fourth most abundant GPCR in human islets. Although LGR4 has regenerative, anti-inflammatory, and anti-apoptotic effects in other tissues, its functional significance in β-cells remains unknown. We have previously identified Receptor Activator of Nuclear Factor Kappa B (NFκB) (RANK) as a negative regulator of β-cell health. In this study, we assessed the regulation of Lgr4 in islets, and the role of LGR4 and LGR4/RANK stoichiometry in β-cell health under basal and stress-induced conditions, in vitro and in vivo.

METHODS

We evaluated Lgr4 expression in mouse and human islets in response to acute (proinflammatory cytokines), or chronic (high fat fed mice, db/db mice, and aging) stress. To determine the role of LGR4 we employed in vitro Lgr4 loss and gain of function in primary rodent and human β-cells and examined its mechanism of action in the rodent INS1 cell line. Using Lgr4fl/fl and Lgr4fl/fl/Rankfl/fl × Ins1-Cre mice we generated β-cell-specific conditional knockout (cko) mice to test the role of LGR4 and its interaction with RANK in vivo under basal and stress-induced conditions.

RESULTS

Lgr4 expression in rodent and human islets was reduced by multiple stressors. In vitro, Lgr4 knockdown decreased proliferation and survival in rodent β-cells, while overexpression protected against cytokine-induced cell death in rodent and human β-cells. Mechanistically, LGR4 protects β-cells by suppressing RANK- Tumor necrosis factor receptor associated factor 6 (TRAF6) interaction and subsequent activation of NFκB. Lgr4cko mice exhibit normal glucose homeostasis but increased β-cell death in both sexes and decreased β-cell proliferation and maturation only in females. Male Lgr4cko mice under stress displayed reduced β-cell proliferation and a further increase in β-cell death. The impaired β-cell phenotype in Lgr4cko mice was rescued in Lgr4/Rank double ko (dko) mice. Upon aging, both male and female Lgr4cko mice displayed impaired β-cell homeostasis, however, only female mice became glucose intolerant with decreased plasma insulin.

CONCLUSIONS

These data demonstrate a novel role for LGR4 as a positive regulator of β-cell health under basal and stress-induced conditions, through suppressing the negative effects of RANK.

Advances in the molecular understanding of GPCR-arrestin complexes

Arrestins are essential proteins for the regulation of G protein-coupled receptors (GPCRs). They mediate GPCR desensitization after the activated receptor has been phosphorylated by G protein receptor kinases (GRKs). In addition, GPCR-arrestin interactions may trigger signaling pathways that are distinct and independent from G proteins. The non-visual GPCRs encompass hundreds of receptors with varying phosphorylation patterns and amino acid sequences, which are regulated by only two human non-visual arrestin isoforms. This review describes recent findings on GPCR-arrestin complexes, obtained by structural techniques, biophysical, biochemical, and cellular assays. The solved structures of complete GPCR-arrestin complexes are of limited resolution ranging from 3.2 to 4.7 Å and reveal a high variability in the relative receptor-arrestin orientation. In contrast, biophysical and functional data indicate that arrestin recruitment, activation and GPCR-arrestin complex stability depend on the receptor phosphosite sequence patterns and density. At present, there is still a manifest lack of high-resolution structural and dynamical information on the interactions of native GPCRs with both GRKs and arrestins, which could provide a detailed molecular understanding of the genesis of receptor phosphorylation patterns and the specificity GPCR-arrestin interactions. Such insights seem crucial for progress in the rational design of advanced, arrestin-specific therapeutics.

SRSF2 is essential for maintaining pancreatic beta-cell identity and regulating glucose homeostasis in mice

Diabetes is characterized by decreased beta-cell mass and islet dysfunction. The splicing factor SRSF2 plays a crucial role in cell survival, yet its impact on pancreatic beta cell survival and glucose homeostasis remains unclear. We observed that the deletion of Srsf2 specifically in beta cells led to time-dependent deterioration in glucose tolerance, impaired insulin secretion, decreased islet mass, an increased number of alpha cells, and the onset of diabetes by the age of 10 months in mice. Single-cell RNA sequencing (scRNA-seq) analyses revealed that, despite an increase in populations of unfolded protein response (UPR)-activated and undifferentiated beta cells within the SRSF2_KO group, there was a notable decrease in the expression of UPR-related and endoplasmic reticulum (ER)-related genes, accompanied by a loss of beta-cell identity. This suggests that beta cells have transitioned from an adaptive phase to a maladaptive phase in islets of 10-month-old SRSF2_KO mice. Further results demonstrated that deletion of SRSF2 caused decreased proliferation in beta cells within 3-month-old islets and Min6 cells. These findings underscore the essential role of SRSF2 in controlling beta-cell proliferation and preserving beta-cell function in mice.

LGR4 is essential for maintaining β-cell homeostasis through suppression of RANK

Pancreatic β-cell stress contributes to diabetes progression. This study demonstrates that Leucine-rich repeat-containing G-protein-coupled-receptor-4 (LGR4) is critical for maintaining β-cell health and is modulated by stressors. In vitro , Lgr4 knockdown decreases proliferation and survival in rodent β-cells, while overexpression protects against cytokine-induced cell death in rodent and human β-cells. Mechanistically, LGR4 suppresses Receptor Activator of Nuclear Factor Kappa B (NFκB) (RANK) and its subsequent activation of NFκB to protect β-cells. β-cell-specific Lgr4 -conditional knockout (cko) mice exhibit normal glucose homeostasis but increased β-cell death in both sexes and decreased proliferation only in females. Male Lgr4 cko mice under stress display reduced β-cell proliferation and a further increase in β-cell death. Upon aging, both male and female Lgr4 cko mice display impaired β-cell homeostasis, however, only female mice are glucose intolerant with decreased plasma insulin. We show that LGR4 is required for maintaining β-cell health under basal and stress-induced conditions, through suppression of RANK.

Teaser

LGR4 receptor is critical for maintaining β-cell health under basal and stressed conditions, through suppression of RANK.

The Role of G Protein-Coupled Receptors and Receptor Kinases in Pancreatic β-Cell Function and Diabetes

Type 2 diabetes (T2D) mellitus has emerged as a major global health concern that has accelerated in recent years due to poor diet and lifestyle. Afflicted individuals have high blood glucose levels that stem from the inability of the pancreas to make enough insulin to meet demand. Although medication can help to maintain normal blood glucose levels in individuals with chronic disease, many of these medicines are outdated, have severe side effects, and often become less efficacious over time, necessitating the need for insulin therapy. G protein-coupled receptors (GPCRs) regulate many physiologic processes, including blood glucose levels. In pancreatic β cells, GPCRs regulate β-cell growth, apoptosis, and insulin secretion, which are all critical in maintaining sufficient β-cell mass and insulin output to ensure euglycemia. In recent years, new insights into the signaling of incretin receptors and other GPCRs have underscored the potential of these receptors as desirable targets in the treatment of diabetes. The signaling of these receptors is modulated by GPCR kinases (GRKs) that phosphorylate agonist-activated GPCRs, marking the receptor for arrestin binding and internalization. Interestingly, genome-wide association studies using diabetic patient cohorts link the GRKs and arrestins with T2D. Moreover, recent reports show that GRKs and arrestins expressed in the β cell serve a critical role in the regulation of β-cell function, including β-cell growth and insulin secretion in both GPCR-dependent and -independent pathways. In this review, we describe recent insights into GPCR signaling and the importance of GRK function in modulating β-cell physiology. SIGNIFICANCE STATEMENT: Pancreatic β cells contain a diverse array of G protein-coupled receptors (GPCRs) that have been shown to improve β-cell function and survival, yet only a handful have been successfully targeted in the treatment of diabetes. This review discusses recent advances in our understanding of β-cell GPCR pharmacology and regulation by GPCR kinases while also highlighting the necessity of investigating islet-enriched GPCRs that have largely been unexplored to unveil novel treatment strategies.

β-Arrestins: Structure, Function, Physiology, and Pharmacological Perspectives

The two β-arrestins, β-arrestin-1 and -2 (systematic names: arrestin-2 and -3, respectively), are multifunctional intracellular proteins that regulate the activity of a very large number of cellular signaling pathways and physiologic functions. The two proteins were discovered for their ability to disrupt signaling via G protein-coupled receptors (GPCRs) via binding to the activated receptors. However, it is now well recognized that both β-arrestins can also act as direct modulators of numerous cellular processes via either GPCR-dependent or -independent mechanisms. Recent structural, biophysical, and biochemical studies have provided novel insights into how β-arrestins bind to activated GPCRs and downstream effector proteins. Studies with β-arrestin mutant mice have identified numerous physiologic and pathophysiological processes regulated by β-arrestin-1 and/or -2. Following a short summary of recent structural studies, this review primarily focuses on β-arrestin-regulated physiologic functions, with particular focus on the central nervous system and the roles of β-arrestins in carcinogenesis and key metabolic processes including the maintenance of glucose and energy homeostasis. This review also highlights potential therapeutic implications of these studies and discusses strategies that could prove useful for targeting specific β-arrestin-regulated signaling pathways for therapeutic purposes. SIGNIFICANCE STATEMENT: The two β-arrestins, structurally closely related intracellular proteins that are evolutionarily highly conserved, have emerged as multifunctional proteins able to regulate a vast array of cellular and physiological functions. The outcome of studies with β-arrestin mutant mice and cultured cells, complemented by novel insights into β-arrestin structure and function, should pave the way for the development of novel classes of therapeutically useful drugs capable of regulating specific β-arrestin functions.

TRPM7 kinase is required for insulin production and compensatory islet responses during obesity

Most overweight individuals do not develop diabetes due to compensatory islet responses to restore glucose homeostasis. Therefore, regulatory pathways that promote β cell compensation are potential targets for treatment of diabetes. The transient receptor potential cation channel subfamily M member 7 protein (TRPM7), harboring a cation channel and a serine/threonine kinase, has been implicated in controlling cell growth and proliferation. Here, we report that selective deletion of Trpm7 in β cells disrupted insulin secretion and led to progressive glucose intolerance. We indicate that the diminished insulinotropic response in β cell-specific Trpm7-knockout mice was caused by decreased insulin production because of impaired enzymatic activity of this protein. Accordingly, high-fat-fed mice with a genetic loss of TRPM7 kinase activity displayed a marked glucose intolerance accompanied by hyperglycemia. These detrimental glucoregulatory effects were engendered by reduced compensatory β cell responses because of mitigated protein kinase B (AKT)/ERK signaling. Collectively, our data identify TRPM7 kinase as a potentially novel regulator of insulin synthesis, β cell dynamics, and glucose homeostasis under obesogenic diet.

PDX1 is the cornerstone of pancreatic β-cell functions and identity

Diabetes has been a worldwide healthcare problem for many years. Current methods of treating diabetes are still largely directed at symptoms, aiming to control the manifestations of the pathology. This creates an overall need to find alternative measures that can impact on the causes of the disease, reverse diabetes, or make it more manageable. Understanding the role of key players in the pathogenesis of diabetes and the related β-cell functions is of great importance in combating diabetes. PDX1 is a master regulator in pancreas organogenesis, the maturation and identity preservation of β-cells, and of their role in normal insulin function. Mutations in the PDX1 gene are correlated with many pancreatic dysfunctions, including pancreatic agenesis (homozygous mutation) and MODY4 (heterozygous mutation), while in other types of diabetes, PDX1 expression is reduced. Therefore, alternative approaches to treat diabetes largely depend on knowledge of PDX1 regulation, its interaction with other transcription factors, and its role in obtaining β-cells through differentiation and transdifferentiation protocols. In this article, we review the basic functions of PDX1 and its regulation by genetic and epigenetic factors. Lastly, we summarize different variations of the differentiation protocols used to obtain β-cells from alternative cell sources, using PDX1 alone or in combination with various transcription factors and modified culture conditions. This review shows the unique position of PDX1 as a potential target in the genetic and cellular treatment of diabetes.

Dietary carbohydrates: Pathogenesis and potential therapeutic targets to obesity-associated metabolic syndrome

Metabolic syndrome (MetS) is a common feature in obesity, comprising a cluster of abnormalities including abdominal fat accumulation, hyperglycemia, hyperinsulinemia, dyslipidemia, and hypertension, leading to diabetes and cardiovascular diseases (CVD). Intake of carbohydrates (CHO), particularly a sugary diet that rapidly increases blood glucose, triglycerides, and blood pressure levels is the predominant determining factor of MetS. Complex CHO, on the other hand, are a stable source of energy taking a longer time to digest. In particular, resistant starch (RS) or soluble fiber is an excellent source of prebiotics, which alter the gut microbial composition, which in turn improves metabolic control. Altering maternal CHO intake during pregnancy may result in the child developing MetS. Furthermore, lifestyle factors such as physical inactivity in combination with dietary habits may synergistically influence gene expression by modulating genetic and epigenetic regulators transforming childhood obesity into adolescent metabolic disorders. This review summarizes the common pathophysiology of MetS in connection with the nature of CHO, intrauterine nutrition, genetic predisposition, lifestyle factors, and advanced treatment approaches; it also emphasizes how dietary CHO may act as a key element in the pathogenesis and future therapeutic targets of obesity and MetS.

Dual pancreatic adrenergic and dopaminergic signaling as a therapeutic target of bromocriptine

Bromocriptine is approved as a diabetes therapy, yet its therapeutic mechanisms remain unclear. Though bromocriptine's actions have been mainly attributed to the stimulation of brain dopamine D2 receptors (D2R), bromocriptine also targets the pancreas. Here, we employ bromocriptine as a tool to elucidate the roles of catecholamine signaling in regulating pancreatic hormone secretion. In β-cells, bromocriptine acts on D2R and α2A-adrenergic receptor (α2A-AR) to reduce glucose-stimulated insulin secretion (GSIS). Moreover, in α-cells, bromocriptine acts via D2R to reduce glucagon secretion. α2A-AR activation by bromocriptine recruits an ensemble of G proteins with no β-arrestin2 recruitment. In contrast, D2R recruits G proteins and β-arrestin2 upon bromocriptine stimulation, demonstrating receptor-specific signaling. Docking studies reveal distinct bromocriptine binding to α2A-AR versus D2R, providing a structural basis for bromocriptine's dual actions on β-cell α2A-AR and D2R. Together, joint dopaminergic and adrenergic receptor actions on α-cell and β-cell hormone release provide a new therapeutic mechanism to improve dysglycemia.

Double life: How GRK2 and β-arrestin signaling participate in diseases

G-protein coupled receptor (GPCR) kinases (GRKs) and β-arrestins play key roles in GPCR and non-GPCR cellular responses. In fact, GRKs and arrestins are involved in a plethora of pathways vital for physiological maintenance of inter- and intracellular communication. Here we review decades of research literature spanning from the discovery, identification of key structural elements, and findings supporting the diverse roles of these proteins in GPCR-mediated pathways. We then describe how GRK2 and β-arrestins partake in non-GPCR signaling and briefly summarize their involvement in various pathologies. We conclude by presenting gaps in knowledge and our prospective on the promising pharmacological potential in targeting these proteins and/or downstream signaling. Future research is warranted and paramount for untangling these novel and promising roles for GRK2 and arrestins in metabolism and disease progression.

The Two β-Arrestins Regulate Distinct Metabolic Processes: Studies with Novel Mutant Mouse Models

The two β-arrestins (β-arrestin-1 and -2; alternative names: arrestin-2 and -3, respectively) are well known for their ability to inhibit signaling via G protein-coupled receptors. However, β-arrestins can also act as signaling molecules in their own right. Although the two proteins share a high degree of sequence and structural homology, early studies with cultured cells indicated that β-arrestin-1 and -2 are not functionally redundant. Recently, the in vivo metabolic roles of the two β-arrestins have been studied using mutant mice selectively lacking either β-arrestin-1 or -2 in cell types that are of particular relevance for regulating glucose and energy homeostasis. These studies demonstrated that the β-arrestin-1 and -2 mutant mice displayed distinct metabolic phenotypes in vivo, providing further evidence for the functional heterogeneity of these two highly versatile signaling proteins.

In vivo metabolic effects after acute activation of skeletal muscle Gs signaling

Objective

The goal of this study was to determine the glucometabolic effects of acute activation of G_s signaling in skeletal muscle (SKM) in vivo and its contribution to whole-body glucose homeostasis.

Methods

To address this question, we studied mice that express a G_s-coupled designer G protein-coupled receptor (Gs-DREADD or GsD) selectively in skeletal muscle. We also identified two G_s-coupled GPCRs that are endogenously expressed by SKM at relatively high levels (β₂-adrenergic receptor and CRF₂ receptor) and studied the acute metabolic effects of activating these receptors in vivo by highly selective agonists (clenbuterol and urocortin 2 (UCN2), respectively).

Results

Acute stimulation of GsD signaling in SKM impaired glucose tolerance in lean and obese mice by decreasing glucose uptake selectively into SKM. The acute metabolic effects following agonist activation of β₂-adrenergic and, potentially, CRF₂ receptors appear primarily mediated by altered insulin release. Clenbuterol injection improved glucose tolerance by increasing insulin secretion in lean mice. In SKM, clenbuterol stimulated glycogen breakdown. UCN2 injection resulted in decreased glucose tolerance associated with lower plasma insulin levels. The acute metabolic effects of UCN2 were not mediated by SKM G_s signaling.

Conclusions

Selective activation of G_s signaling in SKM causes an acute increase in blood glucose levels. However, acute in vivo stimulation of endogenous G_s-coupled receptors enriched in SKM has only a limited impact on whole-body glucose homeostasis, most likely due to the fact that these receptors are also expressed by pancreatic islets where they modulate insulin release.

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A novel mouse model allowed us to study the in vivo metabolic effects of acute activation of G_s signaling in skeletal muscle (SKM).

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Acute stimulation of this pathway resulted in impaired glucose tolerance in lean and obese mice due to decreased glucose uptake by SKM.

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Acute treatment of mice with selective β₂-adrenergic and CRF₂ receptor agonists (both receptors couple to G_s and are enriched in SKM) resulted in complex in vivo metabolic outcomes, primarily due to altered insulin release.

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Our study provides an excellent example of how different tissue expression patterns of receptors can affect the acute effects of GPCR agonists on whole-body glucose homeostasis

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Our findings also highlight the importance of studying both acute and chronic effects of GPCR agonist treatment to properly assess translationally relevant metabolic outcomes.

β-Arrestins as Important Regulators of Glucose and Energy Homeostasis

β-Arrestin-1 and -2 (also known as arrestin-2 and -3, respectively) are ubiquitously expressed cytoplasmic proteins that dampen signaling through G protein-coupled receptors. However, β-arrestins can also act as signaling molecules in their own right. To investigate the potential metabolic roles of the two β-arrestins in modulating glucose and energy homeostasis, recent studies analyzed mutant mice that lacked or overexpressed β-arrestin-1 and/or -2 in distinct, metabolically important cell types. Metabolic analysis of these mutant mice clearly demonstrated that both β-arrestins play key roles in regulating the function of most of these cell types, resulting in striking changes in whole-body glucose and/or energy homeostasis. These studies also revealed that β-arrestin-1 and -2, though structurally closely related, clearly differ in their metabolic roles under physiological and pathophysiological conditions. These new findings should guide the development of novel drugs for the treatment of various metabolic disorders, including type 2 diabetes and obesity. Expected final online publication date for the Annual Review of Physiology, Volume 84 is February 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.