Differentiation signals induce APOBEC3A expression via GRHL3 in squamous epithelia and squamous cell carcinoma

Two APOBEC (apolipoprotein-B mRNA editing enzyme catalytic polypeptide-like) DNA cytosine deaminase enzymes (APOBEC3A and APOBEC3B) generate somatic mutations in cancer, driving tumour development and drug resistance. Here we used single cell RNA sequencing to study APOBEC3A and APOBEC3B expression in healthy and malignant mucosal epithelia, validating key observations with immunohistochemistry, spatial transcriptomics and functional experiments. Whereas APOBEC3B is expressed in keratinocytes entering mitosis, we show that APOBEC3A expression is confined largely to terminally differentiating cells and requires Grainyhead-like transcription factor 3 (GRHL3). Thus, in normal tissue, neither deaminase appears to be expressed at high levels during DNA replication, the cell cycle stage associated with APOBEC-mediated mutagenesis. In contrast, we show that in squamous cell carcinoma tissues, there is expansion of GRHL3 expression and activity to a subset of cells undergoing DNA replication and concomitant extension of APOBEC3A expression to proliferating cells. These findings indicate a mechanism for acquisition of APOBEC3A mutagenic activity in tumours.

Structural basis of promiscuous substrate transport by Organic Cation Transporter 1

Organic Cation Transporter 1 (OCT1) plays a crucial role in hepatic metabolism by mediating the uptake of a range of metabolites and drugs. Genetic variations can alter the efficacy and safety of compounds transported by OCT1, such as those used for cardiovascular, oncological, and psychological indications. Despite its importance in drug pharmacokinetics, the substrate selectivity and underlying structural mechanisms of OCT1 remain poorly understood. Here, we present cryo-EM structures of full-length human OCT1 in the inward-open conformation, both ligand-free and drug-bound, indicating the basis for its broad substrate recognition. Comparison of our structures with those of outward-open OCTs provides molecular insight into the alternating access mechanism of OCTs. We observe that hydrophobic gates stabilize the inward-facing conformation, whereas charge neutralization in the binding pocket facilitates the release of cationic substrates. These findings provide a framework for understanding the structural basis of the promiscuity of drug binding and substrate translocation in OCT1.

Timeline to dysphagia resolution after endoscopic intervention of an interarytenoid defect based on Video Fluoroscopic Swallow Study dysphagia severity

INTRODUCTION

We previously reported that endoscopic repair of a Type 1 Laryngeal Cleft (LC1) or Deep Interarytenoid Groove (DIG) improves swallowing function postoperatively. However, caregivers often ask about the timeline to resolution of the need for thickening. This study re-examines this cohort to answer this important caregiver-centered question.

METHODS

We reassessed a 3-year retrospective, single-center dataset of children with dysphagia found to have a LC-1 or DIG on endoscopic exam. The primary outcome was rate of complete resolution of dysphagia at 2, 6, and 12 months after endoscopic intervention. A sub-group analysis was made based on severity of dysphagia prior to intervention and by type of endoscopic repair.

RESULTS

Thirty-nine patients with mean age 1.35 years that had a LC-1 or DIG met criteria for inclusion. Rate of complete dysphagia resolution increased over time. Those with mild dysphagia (flow-reducing nipple and/or IDDSI consistency 1 or 2) had brisker resolution than those with moderate dysphagia (IDDSI consistency 3 or 4) at 2 months (67% vs 5%, p < 0.01) and at 6 months (80% vs 18%, p < 0.01) after endoscopic repair. There was no difference in dysphagia resolution between patients grouped by type of endoscopic repair.

CONCLUSION

Addressing an interarytenoid defect in patients will not result in immediate, complete dysphagia resolution in most patients. However, patients that only require a flow-reducing nipple and/or thickening to an IDDSI consistency 1 or 2 have brisker resolution of the need for thickening than those that require an IDSSI consistency 3 or 4 prior to intervention. These results inform pre-operative discussions of the timeline to resolution based upon severity of dysphagia and help manage caregiver expectations.

Type 2 diabetes influences intraepithelial corneal nerve parameters and corneal stromal-epithelial nerve penetration sites

INTRODUCTION

The quantification of intraepithelial corneal basal nerve parameters by in vivo confocal microscopy represents a promising modality to identify the earliest manifestations of diabetic peripheral neuropathy. However, its diagnostic accuracy is hampered by its dependence on neuron length, with minimal consideration for other parameters, including the origin of these nerves, the corneal stromal-epithelial nerve penetration sites. This study sought to utilize high-resolution images of murine corneal nerves to analyze comprehensively the morphological changes associated with type 2 diabetes progression.

MATERIALS AND METHODS

βIII-Tubulin immunostained corneas from prediabetic and type 2 diabetic mice and their respective controls were imaged by scanning confocal microscopy and analyzed automatically for nerve parameters. Additionally, the number and distribution of penetration sites was manually ascertained and the average length of the axons exiting them was computed.

RESULTS

The earliest detectable changes included a significant increase in nerve density (6.06 ± 0.41% vs 8.98 ± 1.99%, P = 0.03) and branching (2867.8 ± 271.3/mm² vs 4912.1 ± 1475.3/mm² , P = 0.03), and in the number of penetration sites (258.80 ± 20.87 vs 422.60 ± 63.76, P = 0.0002) at 8 weeks of age. At 16 weeks, corneal innervation decreased, most notably in the periphery. The number of penetration sites remained significantly elevated relative to controls throughout the monitoring period. Similarly, prediabetic mice exhibited an increased number of penetration sites (242.2 ± 13.55 vs 305.6 ± 30.96, P = 0.003) without significant changes to the nerves.

CONCLUSIONS

Our data suggest that diabetic peripheral neuropathy may be preceded by a phase of neuron growth rather than regression, and that the peripheral cornea is more sensitive than the center for detecting changes in innervation.

Primary cilia, A-kinase anchoring proteins and constitutive activity at the orphan G protein-coupled receptor GPR161: A tale about a tail

Primary cilia are non-motile antennae-like structures responsible for sensing environmental changes in most mammalian cells. Ciliary signalling is largely mediated by the Sonic Hedgehog (Shh) pathway, which acts as a master regulator of ciliary protein transit and is essential for normal embryonic development. One particularly important player in primary cilia is the orphan G protein-coupled receptor, GPR161. In this review, we introduce GPR161 in the context of Shh signalling and describe the unique features on its C-terminus such as PKA phosphorylation sites and an A-kinase anchoring protein motif, which may influence the function of the receptor, cAMP compartmentalisation and/or trafficking within primary cilia. We discuss the recent putative pairing of GPR161 and spexin-1, highlighting the additional steps needed before GPR161 could be considered 'deorphanised'. Finally, we speculate that the marked constitutive activity and unconventional regulation of GPR161 may indicate that the receptor may not require an endogenous ligand.

Gene expression analyses of TAS1R taste receptors relevant to the treatment of cardiometabolic disease

The sweet taste receptor (STR) is a G protein-coupled receptor (GPCR) responsible for mediating cellular responses to sweet stimuli. Early evidence suggests that elements of the STR signaling system are present beyond the tongue in metabolically active tissues, where it may act as an extraoral glucose sensor. This study aimed to delineate expression of the STR in extraoral tissues using publicly available RNA-sequencing repositories. Gene expression data was mined for all genes implicated in the structure and function of the STR, and control genes including highly expressed metabolic genes in relevant tissues, other GPCRs and effector G proteins with physiological roles in metabolism, and other GPCRs with expression exclusively outside the metabolic tissues. Since the physiological role of the STR in extraoral tissues is likely related to glucose sensing, expression was then examined in diseases related to glucose-sensing impairment such as type 2 diabetes. An aggregate co-expression network was then generated to precisely determine co-expression patterns among the STR genes in these tissues. We found that STR gene expression was negligible in human pancreatic and adipose tissues, and low in intestinal tissue. Genes encoding the STR did not show significant co-expression or connectivity with other functional genes in these tissues. In addition, STR expression was higher in mouse pancreatic and adipose tissues, and equivalent to human in intestinal tissue. Our results suggest that STR expression in mice is not representative of expression in humans, and the receptor is unlikely to be a promising extraoral target in human cardiometabolic disease.

Don't Turn Off the Tap! The Importance of Discovery Science to the Australian Cardiovascular Sector and Improving Clinical Outcomes Into the Future

Despite significant advances in interventional and therapeutic approaches, cardiovascular disease (CVD) remains the leading cause of death and mortality. To lower this health burden, cardiovascular discovery scientists need to play an integral part in the solution. Successful clinical translation is achieved when built upon a strong foundational understanding of the disease mechanisms involved. Changes in the Australian funding landscape, to place greater emphasis on translation, however, have increased job insecurity for discovery science researchers and especially early-mid career researchers. To highlight the importance of discovery science in cardiovascular research, this review compiles six science stories in which fundamental discoveries, often involving Australian researchers, has led to or is advancing to clinical translation. These stories demonstrate the importance of the role of discovery scientists and the need for their work to be prioritised now and in the future. Australia needs to keep discovery scientists supported and fully engaged within the broader cardiovascular research ecosystem so they can help realise the next game-changing therapy or diagnostic approach that diminishes the burden of CVD on society.

Orphan GPR146: an alternative therapeutic pathway to achieve cholesterol homeostasis?

Atherosclerosis predisposes to myriad cardiovascular complications, including myocardial infarction and stroke. Statins have revolutionised cholesterol management but they do not work for all patients, particularly those with familial hypercholesterolaemia (FH). Genome-wide association studies have linked SNPs at orphan G protein-coupled receptor 146 (GPR146) to human atherosclerosis but how GPR146 influences serum cholesterol homeostasis was only recently described. Gpr146 deletion in mice reduces serum cholesterol and atherosclerotic plaque burden, confirming GPR146 as a potential therapeutic target for managing circulating cholesterol. Critically, this effect was independent of the low-density lipoprotein receptor. While still an orphan, the activation of GPR146 by serum suggests identification of its endogenous ligand is tantalisingly close. Herein, we discuss the evidence for GPR146 inhibition as a treatment for atherosclerosis.

Metabolic Profiling of Mice with Deletion of the Orphan G Protein-Coupled Receptor, GPR37L1

Understanding the neurogenic causes of obesity may reveal novel drug targets to counter the obesity crisis and associated sequelae. Here, we investigate whether the deletion of GPR37L1, an astrocyte-specific orphan G protein-coupled receptor, affects whole-body energy homeostasis in mice. We subjected male Gpr37l1−/− mice and littermate wildtype (Gpr37l1+/+, C57BL/6J background) controls to either 12 weeks of high-fat diet (HFD) or chow feeding, or to 1 year of chow diet, with body composition quantified by EchoMRI, glucose handling by glucose tolerance test and metabolic rate by indirect calorimetry. Following an HFD, Gpr37l1−/− mice had similar glucose handling, body weight and fat mass compared with wildtype controls. Interestingly, we observed a significantly elevated respiratory exchange ratio in HFD- and chow-fed Gpr37l1−/− mice during daylight hours. After 1 year of chow feeding, we again saw no differences in glucose and insulin tolerance or body weight between genotypes, nor in energy expenditure or respiratory exchange ratio. However, there was significantly lower fat mass accumulation, and higher ambulatory activity in the Gpr37l1−/− mice during night hours. Overall, these results indicate that while GPR37L1 may play a minor role in whole-body metabolism, it is not a viable clinical target for the treatment of obesity.

Involvement of GPR37L1 in murine blood pressure regulation and human cardiac disease pathophysiology

Multiple mouse lines lacking the orphan G protein-coupled receptor, GPR37L1, have elicited disparate cardiovascular phenotypes. The first Gpr37l1 knockout mice study to be published reported a marked elevation in systolic blood pressure (SBP; ∼60 mmHg), revealing a potential therapeutic opportunity. The phenotype differed from our own independently generated knockout line, where male mice exhibited equivalent baseline blood pressure to wild type. Here, we attempted to reproduce the first study by characterizing the cardiovascular phenotype of both the original knockout and transgenic lines alongside a C57BL/6J control line, using the same method of blood pressure measurement. The present study supports the findings from our independently developed Gpr37l1 knockout line, finding that SBP and diastolic blood pressure (DBP) are not different in the original Gpr37l1 knockout male mice (SBP: 130.9 ± 5.3 mmHg; DBP: 90.7 ± 3.0 mmHg) compared with C57BL/6J mice (SBP: 123.1 ± 4.1 mmHg; DBP: 87.0 ± 2.7 mmHg). Instead, we attribute the apparent hypertension of the knockout line originally described to comparison with a seemingly hypotensive transgenic line (SBP 103.7 ± 5.0 mmHg; DBP 71.9 ± 3.7 mmHg). Additionally, we quantified myocardial GPR37L1 transcript in humans, which was suggested to be downregulated in cardiovascular disease. We found that GPR37L1 has very low native transcript levels in human myocardium and that expression is not different in tissue samples from patients with heart failure compared with sex-matched healthy control tissue. These findings indicate that cardiac GPR37L1 expression is unlikely to contribute to the pathophysiology of human heart failure.NEW & NOTEWORTHY This study characterizes systolic blood pressure (SBP) in a Gpr37l1 knockout mouse line, which was previously reported to have ∼60 mmHg higher SBP compared with a transgenic line. We observed only a ∼27 mmHg SBP difference between the lines. However, when compared with C57BL/6J mice, knockout mice showed no difference in SBP. We also investigated GPR37L1 mRNA abundance in human hearts and observed no difference between healthy and failing heart samples.

Stimulation of the four isoforms of receptor tyrosine kinase ErbB4, but not ErbB1, confers cardiomyocyte hypertrophy

Epidermal growth factor (EGF) receptors (ErbB1-ErbB4) promote cardiac development and growth, although the specific EGF ligands and receptor isoforms involved in growth/repair versus pathology remain undefined. We challenged ventricular cardiomyocytes with EGF-like ligands and observed that selective activation of ErbB4 (the receptor for neuregulin 1 [NRG1]), but not ErbB1 (the receptor for EGF, EGFR), stimulated hypertrophy. This lack of direct ErbB1-mediated hypertrophy occurred despite robust activation of extracellular-regulated kinase 1/2 (ERK) and protein kinase B. Hypertrophic responses to NRG1 were unaffected by the tyrosine kinase inhibitor (AG1478) at concentrations that are selective for ErbB1 over ErbB4. NRG1-induced cardiomyocyte enlargement was suppressed by small interfering RNA (siRNA) knockdown of ErbB4 and ErbB2, whereas ERK phosphorylation was only suppressed by ErbB4 siRNA. Four ErbB4 isoforms exist (JM-a/JM-b and CYT-1/CYT-2), generated by alternative splicing, and their expression declines postnatally and following cardiac hypertrophy. Silencing of all four isoforms in cardiomyocytes, using an ErbB4 siRNA, abrogated NRG1-induced hypertrophic promoter/reporter activity, which was rescued by coexpression of knockdown-resistant versions of the ErbB4 isoforms. Thus, ErbB4 confers cardiomyocyte hypertrophy to NRG1, and all four ErbB4 isoforms possess the capacity to mediate this effect.

Deletion of Orphan G Protein-Coupled Receptor GPR37L1 in Mice Alters Cardiovascular Homeostasis in a Sex-Specific Manner

GPR37L1 is a family A orphan G protein-coupled receptor (GPCR) with a putative role in blood pressure regulation and cardioprotection. In mice, genetic ablation of Gpr37l1 causes sex-dependent effects; female mice lacking Gpr37l1 (GPR37L1^-/-) have a modest but significant elevation in blood pressure, while male GPR37L1^-/- mice are more susceptible to cardiovascular dysfunction following angiotensin II-induced hypertension. Given that this receptor is highly expressed in the brain, we hypothesize that the cardiovascular phenotype of GPR37L1^-/- mice is due to changes in autonomic regulation of blood pressure and heart rate. To investigate this, radiotelemetry was employed to characterize baseline cardiovascular variables in GPR37L1^-/- mice of both sexes compared to wildtype controls, followed by power spectral analysis to quantify short-term fluctuations in blood pressure and heart rate attributable to alterations in autonomic homeostatic mechanisms. Additionally, pharmacological ganglionic blockade was performed to determine vasomotor tone, and environmental stress tests were used to assess whether cardiovascular reactivity was altered in GPR37L1^-/- mice. We observed that mean arterial pressure was significantly lower in female GPR37L1^-/- mice compared to wildtype counterparts, but was unchanged in male GPR37L1^-/- mice. GPR37L1^-/- genotype had a statistically significant positive chronotropic effect on heart rate across both sexes when analyzed by two-way ANOVA. Power spectral analysis of these data revealed a reduction in power in the heart rate spectrum between 0.5 and 3 Hz in female GPR37L1^-/- mice during the diurnal active period, which indicates that GPR37L1^-/- mice may have impaired cardiac vagal drive. GPR37L1^-/- mice of both sexes also exhibited attenuated depressor responses to ganglionic blockade with pentolinium, indicating that GPR37L1 is involved in maintaining sympathetic vasomotor tone. Interestingly, when these mice were subjected to aversive and appetitive behavioral stressors, the female GPR37L1^-/- mice exhibited an attenuation of cardiovascular reactivity to aversive, but not appetitive, environmental stimuli. Together, these results suggest that loss of GPR37L1 affects autonomic maintenance of blood pressure, giving rise to sex-specific cardiovascular changes in GPR37L1^-/- mice.

Pressure overload by suprarenal aortic constriction in mice leads to left ventricular hypertrophy without c-Kit expression in cardiomyocytes

Animal models of pressure overload are valuable for understanding hypertensive heart disease. We characterised a surgical model of pressure overload-induced hypertrophy in C57BL/6J mice produced by suprarenal aortic constriction (SAC). Compared to sham controls, at one week post-SAC systolic blood pressure was significantly elevated and left ventricular (LV) hypertrophy was evident by a 50% increase in the LV weight-to-tibia length ratio due to cardiomyocyte hypertrophy. As a result, LV end-diastolic wall thickness-to-chamber radius (h/R) ratio increased, consistent with the development of concentric hypertrophy. LV wall thickening was not sufficient to normalise LV wall stress, which also increased, resulting in LV systolic dysfunction with reductions in ejection fraction and fractional shortening, but no evidence of heart failure. Pathological LV remodelling was evident by the re-expression of fetal genes and coronary artery perivascular fibrosis, with ischaemia indicated by enhanced cardiomyocyte Hif1a expression. The expression of stem cell factor receptor, c-Kit, was low basally in cardiomyocytes and did not change following the development of robust hypertrophy, suggesting there is no role for cardiomyocyte c-Kit signalling in pathological LV remodelling following pressure overload.

Regulation of mitochondrial metabolism in murine skeletal muscle by the medium-chain fatty acid receptor Gpr84

Fatty acid receptors have been recognized as important players in glycaemic control. This study is the first to describe a role for the medium-chain fatty acid (MCFA) receptor G-protein-coupled receptor (Gpr) 84 in skeletal muscle mitochondrial function and insulin secretion. We are able to show that Gpr84 is highly expressed in skeletal muscle and adipose tissue. Mice with global deletion of Gpr84 [Gpr84 knockout (KO)] exhibit a mild impairment in glucose tolerance when fed a MCFA-enriched diet. Studies in mice and pancreatic islets suggest that glucose intolerance is accompanied by a defect in insulin secretion. MCFA-fed KO mice also exhibit a significant impairment in the intrinsic respiratory capacity of their skeletal muscle mitochondria, but at the same time also exhibit a substantial increase in mitochondrial content. Changes in canonical pathways of mitochondrial biogenesis and turnover are unable to explain these mitochondrial differences. Our results show that Gpr84 plays a crucial role in regulating mitochondrial function and quality control.-Montgomery, M. K., Osborne, B., Brandon, A. E., O'Reilly, L., Fiveash, C. E., Brown, S. H. J., Wilkins, B. P., Samsudeen, A., Yu, J., Devanapalli, B., Hertzog, A., Tolun, A. A., Kavanagh, T., Cooper, A. A., Mitchell, T. W., Biden, T. J., Smith, N. J., Cooney, G. J., Turner, N. Regulation of mitochondrial metabolism in murine skeletal muscle by the medium-chain fatty acid receptor Gpr84.

The APOBEC3 genes and their role in cancer: insights from human papillomavirus

The interaction between human papillomaviruses (HPV) and the apolipoprotein-B mRNA-editing catalytic polypeptide-like (APOBEC)3 (A3) genes has garnered increasing attention in recent years, with considerable efforts focused on understanding their apparent roles in both viral editing and in HPV-driven carcinogenesis. Here, we review these developments and highlight several outstanding questions in the field. We consider whether editing of the virus and mutagenesis of the host are linked or whether both are essentially separate events, coincidentally mediated by a common or distinct A3 enzymes. We discuss the viral mechanisms and cellular signalling pathways implicated in A3 induction in virally infected cells and examine which of the A3 enzymes might play the major role in HPV-associated carcinogenesis and in the development of therapeutic resistance. We consider the parallels between A3 induction in HPV-infected cells and what might be causing aberrant A3 activity in HPV-independent cancers such as those arising in the bladder, lung and breast. Finally, we discuss the implications of ongoing A3 activity in tumours under treatment and the therapeutic opportunities that this may present.

Cryo-EM reveals distinct conformations of E. coli ATP synthase on exposure to ATP

ATP synthase produces the majority of cellular energy in most cells. We have previously reported cryo-EM maps of autoinhibited E. coli ATP synthase imaged without addition of nucleotide (Sobti et al. 2016), indicating that the subunit ε engages the α, β and γ subunits to lock the enzyme and prevent functional rotation. Here we present multiple cryo-EM reconstructions of the enzyme frozen after the addition of MgATP to identify the changes that occur when this ε inhibition is removed. The maps generated show that, after exposure to MgATP, E. coli ATP synthase adopts a different conformation with a catalytic subunit changing conformation substantially and the ε C-terminal domain transitioning via an intermediate 'half-up' state to a condensed 'down' state. This work provides direct evidence for unique conformational states that occur in E. coli ATP synthase when ATP binding prevents the ε C-terminal domain from entering the inhibitory 'up' state.

GPCRs in context: sexual dimorphism in the cardiovascular system

Cardiovascular disease (CVD) remains the largest cause of mortality worldwide, and there is a clear gender gap in disease occurrence, with men being predisposed to earlier onset of CVD, including atherosclerosis and hypertension, relative to women. Oestrogen may be a driving factor for female-specific cardioprotection, though androgens and sex chromosomes are also likely to contribute to sexual dimorphism in the cardiovascular system (CVS). Many GPCR-mediated processes are involved in cardiovascular homeostasis, and some exhibit clear sex divergence. Here, we focus on the G protein-coupled oestrogen receptor, endothelin receptors ET_A and ET_B and the eicosanoid G protein-coupled receptors (GPCRs), discussing the evidence and potential mechanisms leading to gender dimorphic responses in the vasculature. The use of animal models and pharmacological tools has been essential to understanding the role of these receptors in the CVS and will be key to further delineating their sex-specific effects. Ultimately, this may illuminate wider sex differences in cardiovascular pathology and physiology.

LINKED ARTICLES

This article is part of a themed section on Molecular Pharmacology of GPCRs. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.21/issuetoc.

Temporal parameters of post-stress prophylactic glucose treatment in rats

Acute trauma can lead to life-long changes in susceptibility to psychiatric disease, such as post-traumatic stress disorder (PTSD). Rats given free access to a concentrated glucose solution for 24 h beginning immediately after trauma failed to show stress-related pathology in the learned helplessness model of PTSD and comorbid major depression. We assessed effective dosing and temporal constraints of the glucose intervention in three experiments. We exposed 120 male Sprague-Dawley rats to 100, 1 mA, 3-15 s, inescapable and unpredictable electric tail shocks (over a 110-min period) or simple restraint in the learned helplessness procedure. Rats in each stress condition had access to a 40% glucose solution or water. We measured fluid consumption under 18-h free access conditions, or limited access (1, 3, 6, 18 h) beginning immediately after trauma, or 3-h access with delayed availability of the glucose solution (0, 1, 3, 6 h). We hypothesized that longer and earlier access following acute stress would improve shuttle-escape performance. Rats exposed to traumatic shock and given 18-h access to glucose failed to show exaggerated fearfulness and showed normal reactivity to foot shock during testing as compared to their water-treated counterparts. At least 3 h of immediate post-stress access to glucose were necessary to see these improvements in test performance. Moreover, delaying access to glucose for more than 3 h post-trauma yielded no beneficial effects. These data clearly identify limits on the post-stress glucose intervention. In conclusion, glucose should be administered almost immediately and at the highest dose after trauma.

The G protein-coupled receptor N-terminus and receptor signalling: N-tering a new era

G protein-coupled receptors (GPCRs) are a vast family of membrane-traversing proteins, essential to the ability of eukaryotic life to detect, and mount an intracellular response to, a diverse range of extracellular stimuli. GPCRs have evolved with archetypal features including an extracellular N-terminus and intracellular C-terminus that flank a transmembrane structure of seven sequential helices joined by intracellular and extracellular loops. These structural domains contribute to the ability of a GPCR to be correctly synthesised and inserted into the cell membrane, to interact with its cognate ligand(s) and to couple with signal-transducing heterotrimeric G proteins, allowing the activated receptor to selectively modulate a number of signalling cascades. Whilst well known for its importance in receptor translation and trafficking, the GPCR N-terminus is underexplored as a participant in receptor signalling. This review aims to discuss and integrate recent advances in knowledge of the vital roles of the GPCR N-terminus in receptor signalling.

Identifying ligands at orphan GPCRs: current status using structure-based approaches

GPCRs are the most successful pharmaceutical targets in history. Nevertheless, the pharmacology of many GPCRs remains inaccessible as their endogenous or exogenous modulators have not been discovered. Tools that explore the physiological functions and pharmacological potential of these 'orphan' GPCRs, whether they are endogenous and/or surrogate ligands, are therefore of paramount importance. Rates of receptor deorphanization determined by traditional reverse pharmacology methods have slowed, indicating a need for the development of more sophisticated and efficient ligand screening approaches. Here, we discuss the use of structure-based ligand discovery approaches to identify small molecule modulators for exploring the function of orphan GPCRs. These studies have been buoyed by the growing number of GPCR crystal structures solved in the past decade, providing a broad range of template structures for homology modelling of orphans. This review discusses the methods used to establish the appropriate signalling assays to test orphan receptor activity and provides current examples of structure-based methods used to identify ligands of orphan GPCRs. Linked Articles This article is part of a themed section on Molecular Pharmacology of G Protein-Coupled Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.20/issuetoc.

Metalloprotease cleavage of the N terminus of the orphan G protein-coupled receptor GPR37L1 reduces its constitutive activity

Little is known about the pharmacology or physiology of GPR37L1, a G protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptor that is abundant in the cerebellum. Mice deficient in this receptor exhibit precocious cerebellar development and hypertension. We showed that GPR37L1 coupled to the G protein Gα(s) when heterologously expressed in cultured cells in the absence of any added ligand, whereas a mutant receptor that lacked the amino terminus was inactive. Conversely, inhibition of ADAMs (a disintegrin and metalloproteases) enhanced receptor activity, indicating that the presence of the amino terminus is necessary for GPR37L1 signaling. Metalloprotease-dependent processing of GPR37L1 was evident in rodent cerebellum, where we detected predominantly the cleaved, inactive form. However, comparison of the accumulation of cAMP (adenosine 3',5'-monophosphate) in response to phosphodiesterase inhibition in cerebellar slice preparations from wild-type and GPR37L1-null mice showed that some constitutive signaling remained in the wild-type mice. In reporter assays of Gα(s) or Gα(i) signaling, the synthetic, prosaposin-derived peptide prosaptide (TX14A) did not increase GPR37L1 activity. Our data indicate that GPR37L1 may be a constitutively active receptor, or perhaps its ligand is present under the conditions that we used for analysis, and that the activity of this receptor is instead controlled by signals that regulate metalloprotease activity in the tissue.

Ablation of cardiac myosin binding protein-C disrupts the super-relaxed state of myosin in murine cardiomyocytes

Cardiac myosin binding protein-C (cMyBP-C) is a structural and regulatory component of cardiac thick filaments. It is observed in electron micrographs as seven to nine transverse stripes in the central portion of each half of the A band. Its C-terminus binds tightly to the myosin rod and contributes to thick filament structure, while the N-terminus can bind both myosin S2 and actin, influencing their structure and function. Mutations in the MYBPC3 gene (encoding cMyBP-C) are commonly associated with hypertrophic cardiomyopathy (HCM). In cardiac cells there exists a population of myosin heads in the super-relaxed (SRX) state, which are bound to the thick filament core with a highly inhibited ATPase activity. This report examines the role cMyBP-C plays in regulating the population of the SRX state of cardiac myosin by using an assay that measures single ATP turnover of myosin. We report a significant decrease in the proportion of myosin heads in the SRX state in homozygous cMyBP-C knockout mice, however heterozygous cMyBP-C knockout mice do not significantly differ from the wild type. A smaller, non-significant decrease is observed when thoracic aortic constriction is used to induce cardiac hypertrophy in mutation negative mice. These results support the proposal that cMyBP-C stabilises the thick filament and that the loss of cMyBP-C results in an untethering of myosin heads. This results in an increased myosin ATP turnover, further consolidating the relationship between thick filament structure and the myosin ATPase.

Role of miRNAs and alternative mRNA 3'-end cleavage and polyadenylation of their mRNA targets in cardiomyocyte hypertrophy

miRNAs play critical roles in heart disease. In addition to differential miRNA expression, miRNA-mediated control is also affected by variable miRNA processing or alternative 3'-end cleavage and polyadenylation (APA) of their mRNA targets. To what extent these phenomena play a role in the heart remains unclear. We sought to explore miRNA processing and mRNA APA in cardiomyocytes, and whether these change during cardiac hypertrophy. Thoracic aortic constriction (TAC) was performed to induce hypertrophy in C57BL/6J mice. RNA extracted from cardiomyocytes of sham-treated, pre-hypertrophic (2 days post-TAC), and hypertrophic (7 days post-TAC) mice was subjected to small RNA- and poly(A)-test sequencing (PAT-Seq). Differential expression analysis matched expectations; nevertheless we identified ~400 mRNAs and hundreds of noncoding RNA loci as altered with hypertrophy for the first time. Although multiple processing variants were observed for many miRNAs, there was little change in their relative proportions during hypertrophy. PAT-Seq mapped ~48,000 mRNA 3'-ends, identifying novel 3' untranslated regions (3'UTRs) for over 7000 genes. Importantly, hypertrophy was associated with marked changes in APA with a net shift from distal to more proximal mRNA 3'-ends, which is predicted to decrease overall miRNA repression strength. We independently validated several examples of 3'UTR proportion change and showed that alternative 3'UTRs associate with differences in mRNA translation. Our work suggests that APA contributes to altered gene expression with the development of cardiomyocyte hypertrophy and provides a rich resource for a systems-level understanding of miRNA-mediated regulation in physiological and pathological states of the heart.

A Molecular Mechanism for Sequential Activation of a G Protein-Coupled Receptor

Ligands targeting G protein-coupled receptors (GPCRs) are currently classified as either orthosteric, allosteric, or dualsteric/bitopic. Here, we introduce a new pharmacological concept for GPCR functional modulation: sequential receptor activation. A hallmark feature of this is a stepwise ligand binding mode with transient activation of a first receptor site followed by sustained activation of a second topographically distinct site. We identify 4-CMTB (2-(4-chlorophenyl)-3-methyl-N-(thiazol-2-yl)butanamide), previously classified as a pure allosteric agonist of the free fatty acid receptor 2, as the first sequential activator and corroborate its two-step activation in living cells by tracking integrated responses with innovative label-free biosensors that visualize multiple signaling inputs in real time. We validate this unique pharmacology with traditional cellular readouts, including mutational and pharmacological perturbations along with computational methods, and propose a kinetic model applicable to the analysis of sequential receptor activation. We envision this form of dynamic agonism as a common principle of nature to spatiotemporally encode cellular information.

Drug Discovery Opportunities at the Endothelin B Receptor-Related Orphan G Protein-Coupled Receptors, GPR37 and GPR37L1

Orphan G protein-coupled receptors (GPCRs) represent a largely untapped resource for the treatment of a variety of diseases, despite sophisticated advances in drug discovery. Two promising orphan GPCRs are the endothelin B receptor-like proteins, GPR37 [ET(B)R-LP, Pael-R] and GPR37L1 [ET(B)R-LP-2]. Originally identified through searches for homologs of endothelin and bombesin receptors, neither GPR37 nor GPR37L1 were found to bind endothelins or related peptides. Instead, GPR37 was proposed to be activated by head activator (HA) and both GPR37 and GPR37L1 have been linked to the neuropeptides prosaposin and prosaptide, although these pairings are yet to be universally acknowledged. Both orphan GPCRs are widely expressed in the brain, where GPR37 has received the most attention for its link to Parkinson’s disease and parkinsonism, while GPR37L1 deletion leads to precocious cerebellar development and hypertension. In this review, the existing pharmacology and physiology of GPR37 and GPR37L1 is discussed and the potential therapeutic benefits of targeting these receptors are explored.

NVP-QBE170: an inhaled blocker of the epithelial sodium channel with a reduced potential to induce hyperkalaemia

BACKGROUND AND PURPOSE

Inhaled amiloride, a blocker of the epithelial sodium channel (ENaC), enhances mucociliary clearance (MCC) in cystic fibrosis (CF) patients. However, the dose of amiloride is limited by the mechanism-based side effect of hyperkalaemia resulting from renal ENaC blockade. Inhaled ENaC blockers with a reduced potential to induce hyperkalaemia provide a therapeutic strategy to improve mucosal hydration and MCC in the lungs of CF patients. The present study describes the preclinical profile of a novel ENaC blocker, NVP-QBE170, designed for inhaled delivery, with a reduced potential to induce hyperkalaemia.

EXPERIMENTAL APPROACH

The in vitro potency and duration of action of NVP-QBE170 were compared with amiloride and a newer ENaC blocker, P552-02, in primary human bronchial epithelial cells (HBECs) by short-circuit current. In vivo efficacy and safety were assessed in guinea pig (tracheal potential difference/hyperkalaemia), rat (hyperkalaemia) and sheep (MCC).

KEY RESULTS

In vitro, NVP-QBE170 potently inhibited ENaC function in HBEC and showed a longer duration of action to comparator molecules. In vivo, intratracheal (i.t.) instillation of NVP-QBE170 attenuated ENaC activity in the guinea pig airways with greater potency and duration of action than that of amiloride without inducing hyperkalaemia in either guinea pig or rat. Dry powder inhalation of NVP-QBE170 by conscious sheep increased MCC and was better than inhaled hypertonic saline in terms of efficacy and duration of action.

CONCLUSIONS AND IMPLICATIONS

NVP-QBE170 highlights the potential for inhaled ENaC blockers to exhibit efficacy in the airways with a reduced risk of hyperkalaemia, relative to existing compounds.

Rapid Knockout and Reporter Mouse Line Generation and Breeding Colony Establishment Using EUCOMM Conditional-Ready Embryonic Stem Cells: A Case Study

As little as a decade ago, generation of a single knockout mouse line was an expensive and time-consuming undertaking available to relatively few researchers. The International Knockout Mouse Consortium, established in 2007, has revolutionized the use of such models by creating an open-access repository of embryonic stem (ES) cells that, through sequential breeding with first FLP1 recombinase and then Cre recombinase transgenic mice, facilitates germline global or conditional deletion of almost every gene in the mouse genome. In this Case Study, we describe our experience using the repository to create mouse lines for a variety of experimental purposes. Specifically, we discuss the process of obtaining germline transmission of two European Conditional Mouse Mutagenesis Program (EUCOMM) "knockout-first" gene targeted constructs and the advantages and pitfalls of using this system. We then outline our breeding strategy and the outcomes of our efforts to generate global and conditional knockouts and reporter mice for the genes of interest. Line maintenance, removal of recombinase transgenes, and cryopreservation are also considered. Our approach led to the generation of heterozygous knockout mice within 6 months of commencing breeding to the founder mice. By describing our experiences with the EUCOMM ES cells and subsequent breeding steps, we hope to assist other researchers with the application of this valuable approach to generating versatile knockout mouse lines.

RhoA/ROCK signaling and pleiotropic α1A-adrenergic receptor regulation of cardiac contractility

Aims

To determine the mechanisms by which the α_1A-adrenergic receptor (AR) regulates cardiac contractility.

Background

We reported previously that transgenic mice with cardiac-restricted α_1A-AR overexpression (α_1A-TG) exhibit enhanced contractility but not hypertrophy, despite evidence implicating this Gα_q/11-coupled receptor in hypertrophy.

Methods

Contractility, calcium (Ca²⁺) kinetics and sensitivity, and contractile proteins were examined in cardiomyocytes, isolated hearts and skinned fibers from α_1A-TG mice (170-fold overexpression) and their non-TG littermates (NTL) before and after α_1A-AR agonist stimulation and blockade, angiotensin II (AngII), and Rho kinase (ROCK) inhibition.

Results

Hypercontractility without hypertrophy with α_1A-AR overexpression is shown to result from increased intracellular Ca²⁺ release in response to agonist, augmenting the systolic amplitude of the intracellular Ca²⁺ concentration [Ca²⁺]_i transient without changing resting [Ca²⁺]_i. In the absence of agonist, however, α_1A-AR overexpression reduced contractility despite unchanged [Ca²⁺]_i. This hypocontractility is not due to heterologous desensitization: the contractile response to AngII, acting via its Gα_q/11-coupled receptor, was unaltered. Rather, the hypocontractility is a pleiotropic signaling effect of the α_1A-AR in the absence of agonist, inhibiting RhoA/ROCK activity, resulting in hypophosphorylation of both myosin phosphatase targeting subunit 1 (MYPT1) and cardiac myosin light chain 2 (cMLC2), reducing the Ca²⁺ sensitivity of the contractile machinery: all these effects were rapidly reversed by selective α_1A-AR blockade. Critically, ROCK inhibition in normal hearts of NTLs without α_1A-AR overexpression caused hypophosphorylation of both MYPT1 and cMLC2, and rapidly reduced basal contractility.

Conclusions

We report for the first time pleiotropic α_1A-AR signaling and the physiological role of RhoA/ROCK signaling in maintaining contractility in the normal heart.

The impact of the two-week wait referral pathway on rectal cancer survival

AIM

The aim of this study was to compare the outcome of patients with rectal cancer referred through the two-week wait (TWW) system with those identified by routine referral pathways (non-TWW).

METHOD

A prospective study was carried out of 125 consecutive patients diagnosed with rectal cancer between January 2000 and December 2005 (6 years) in one district general hospital. Data were recorded prospectively in a local clinicopathological registry. The patients were divided into two groups: group 1 (TWW) and group 2 (routine referral pathway).

RESULTS

Fifty-two (41%) of the 125 patients were diagnosed through the TWW (group 1). There was no significant difference in patient demographics, including baseline tumour characteristics, between the two groups. There was no difference in preoperative or postoperative T stage between the two groups (P = 0.63). There was no significant difference in circumferential margin positivity (five of 52 in group 1 vs four of 73 in group 2; P = 0.52) or local recurrence rates (P = 0.37). The 5-year all-cause mortality was 49% for group 1 and 52% for group 2 (P = 0.3). The overall disease-free survival was similar in the two groups (1521 days for group 1 vs 1591 days for group 1, P = 0.29).

CONCLUSION

Referral under the TWW strategy does not translate into improved survival in rectal cancer.

Low affinity GPCRs for metabolic intermediates: challenges for pharmacologists

The discovery that a number of metabolites and metabolic intermediates can act through G protein-coupled receptors has attracted great interest in the field and has led to new therapeutic targets for diseases such as hypertension, type 2 diabetes, inflammation, and metabolic syndrome. However, the low apparent affinity of these ligands for their cognate receptors poses a number of challenges for pharmacologists interested in investigating receptor structure, function or physiology. Furthermore, the endogenous ligands matched to their receptors have other, well established metabolic roles and thus selectivity is difficult to achieve. This review discusses some of the issues researchers face when working with these receptors and highlights the ways in which a number of these obstacles have been overcome.

Extracellular loop 2 of the free fatty acid receptor 2 mediates allosterism of a phenylacetamide ago-allosteric modulator

Allosteric agonists are powerful tools for exploring the pharmacology of closely related G protein-coupled receptors that have nonselective endogenous ligands, such as the short chain fatty acids at free fatty acid receptors 2 and 3 (FFA2/GPR43 and FFA3/GPR41, respectively). We explored the molecular mechanisms mediating the activity of 4-chloro-α-(1-methylethyl)-N-2-thiazolylbenzeneacetamide (4-CMTB), a recently described phenylacetamide allosteric agonist and allosteric modulator of endogenous ligand function at human FFA2, by combining our previous knowledge of the orthosteric binding site with targeted examination of 4-CMTB structure-activity relationships and mutagenesis and chimeric receptor generation. Here we show that 4-CMTB is a selective agonist for FFA2 that binds to a site distinct from the orthosteric site of the receptor. Ligand structure-activity relationship studies indicated that the N-thiazolyl amide is likely to provide hydrogen bond donor/acceptor interactions with the receptor. Substitution at Leu(173) or the exchange of the entire extracellular loop 2 of FFA2 with that of FFA3 was sufficient to reduce or ablate, respectively, allosteric communication between the endogenous and allosteric agonists. Thus, we conclude that extracellular loop 2 of human FFA2 is required for transduction of cooperative signaling between the orthosteric and an as-yet-undefined allosteric binding site of the FFA2 receptor that is occupied by 4-CMTB.

Allostery at G protein-coupled receptor homo- and heteromers: uncharted pharmacological landscapes

For many years seven transmembrane domain G protein-coupled receptors (GPCRs) were thought to exist and function exclusively as monomeric units. However, evidence both from native cells and heterologous expression systems has demonstrated that GPCRs can both traffic and signal within higher-order complexes. As for other protein-protein interactions, conformational changes in one polypeptide, including those resulting from binding of pharmacological ligands, have the capacity to alter the conformation and therefore the response of the interacting protein(s), a process known as allosterism. For GPCRs, allosterism across homo- or heteromers, whether dimers or higher-order oligomers, represents an additional topographical landscape that must now be considered pharmacologically. Such effects may offer the opportunity for novel therapeutic approaches. Allosterism at GPCR heteromers is particularly exciting in that it offers additional scope to provide receptor subtype selectivity and tissue specificity as well as fine-tuning of receptor signal strength. Herein, we introduce the concept of allosterism at both GPCR homomers and heteromers and discuss the various questions that must be addressed before significant advances can be made in drug discovery at these GPCR complexes.

When simple agonism is not enough: emerging modalities of GPCR ligands

Recent advances in G protein-coupled receptors have challenged traditional definitions of agonism, antagonism, affinity and efficacy. The discovery of agonist functional selectivity and receptor allosterism has meant researchers have an expanded canvas for designing and discovering novel drugs. Here we describe modes of agonism emerging from the discovery of functional selectivity and allosterism. We discuss the concept of ago-allosterism, where ligands can initiate signaling by themselves and influence the actions of another ligand at the same receptor. We introduce the concept of dualsteric ligands that consist of distinct elements which bind to each of the orthosteric and an allosteric domain on a single receptor to enhance subtype selectivity. Finally, the concept that efficacy should be defined by the activity of an endogenous ligand will be challenged by the discovery that some ligands act as 'super-agonists' in specific pathways or at certain receptor mutations.

Selective orthosteric free fatty acid receptor 2 (FFA2) agonists: identification of the structural and chemical requirements for selective activation of FFA2 versus FFA3

Free fatty acid receptor 2 (FFA2; GPR43) is a G protein-coupled seven-transmembrane receptor for short-chain fatty acids (SCFAs) that is implicated in inflammatory and metabolic disorders. The SCFA propionate has close to optimal ligand efficiency for FFA2 and can hence be considered as highly potent given its size. Propionate, however, does not discriminate between FFA2 and the closely related receptor FFA3 (GPR41). To identify FFA2-selective ligands and understand the molecular basis for FFA2 selectivity, a targeted library of small carboxylic acids was examined using holistic, label-free dynamic mass redistribution technology for primary screening and the receptor-proximal G protein [³⁵S]guanosine 5′-(3-O-thio)triphosphate activation, inositol phosphate, and cAMP accumulation assays for hit confirmation. Structure-activity relationship analysis allowed formulation of a general rule to predict selectivity for small carboxylic acids at the orthosteric binding site where ligands with substituted sp³-hybridized α-carbons preferentially activate FFA3, whereas ligands with sp²- or sp-hybridized α-carbons prefer FFA2. The orthosteric binding mode was verified by site-directed mutagenesis: replacement of orthosteric site arginine residues by alanine in FFA2 prevented ligand binding, and molecular modeling predicted the detailed mode of binding. Based on this, selective mutation of three residues to their non-conserved counterparts in FFA3 was sufficient to transfer FFA3 selectivity to FFA2. Thus, selective activation of FFA2 via the orthosteric site is achievable with rather small ligands, a finding with significant implications for the rational design of therapeutic compounds selectively targeting the SCFA receptors.

Angiotensin1-9 antagonises pro-hypertrophic signalling in cardiomyocytes via the angiotensin type 2 receptor

The renin–angiotensin system (RAS) regulates blood pressure mainly via the actions of angiotensin (Ang)II, generated via angiotensin converting enzyme (ACE). The ACE homologue ACE2 metabolises AngII to Ang1-7, decreasing AngII and increasing Ang1-7, which counteracts AngII activity via the Mas receptor. However, ACE2 also converts AngI to Ang1-9, a poorly characterised peptide which can be further converted to Ang1-7 via ACE. Ang1-9 stimulates bradykinin release in endothelium and has antihypertrophic actions in the heart, attributed to its being a competitive inhibitor of ACE, leading to decreased AngII, rather than increased Ang1-7. To date no direct receptor-mediated effects of Ang1-9 have been described. To further understand the role of Ang1-9 in RAS function we assessed its action in cardiomyocyte hypertrophy in rat neonatal H9c2 and primary adult rabbit left ventricular cardiomyocytes, compared to Ang1-7. Cardiomyocyte hypertrophy was stimulated with AngII or vasopressin, significantly increasing cell size by approximately 1.2-fold (P < 0.05) as well as stimulating expression of the hypertrophy gene markers atrial natriuretic peptide, brain natriuretic peptide, β-myosin heavy chain and myosin light chain (2- to 5-fold, P < 0.05). Both Ang1-9 and Ang1-7 were able to block hypertrophy induced by either agonist (control, 186.4 μm; AngII, 232.8 μm; AngII+Ang1-7, 198.3 μm; AngII+Ang1-9, 195.9 μm; P < 0.05). The effects of Ang1-9 were not inhibited by captopril, supporting previous evidence that Ang1-9 acts independently of Ang1-7. Next, we investigated receptor signalling via angiotensin type 1 and type 2 receptors (AT1R, AT2R) and Mas. The AT1R antagonist losartan blocked AngII-induced, but not vasopressin-induced, hypertrophy. Losartan did not block the antihypertrophic effects of Ang1-9, or Ang1-7 on vasopressin-stimulated cardiomyocytes. The Mas antagonist A779 efficiently blocked the antihypertrophic effects of Ang1-7, without affecting Ang1-9. Furthermore, Ang1-7 activity was also inhibited in the presence of the bradykinin type 2 receptor antagonist HOE140, without affecting Ang1-9. Moreover, we observed that the AT2R antagonist PD123,319 abolished the antihypertrophic effects of Ang1-9, without affecting Ang1-7, suggesting Ang1-9 signals via the AT2R. Radioligand binding assays demonstrated that Ang1-9 was able to bind the AT2R (pKi = 6.28 ± 0.1). In summary, we ascribe a direct biological role for Ang1-9 acting via the AT2R. This has implications for RAS function and identifying new therapeutic targets in cardiovascular disease.

Deconvolution of complex G protein-coupled receptor signaling in live cells using dynamic mass redistribution measurements

Label-free biosensor technology based on dynamic mass redistribution (DMR) of cellular constituents promises to translate GPCR signaling into complex optical 'fingerprints' in real time in living cells. Here we present a strategy to map cellular mechanisms that define label-free responses, and we compare DMR technology with traditional second-messenger assays that are currently the state of the art in GPCR drug discovery. The holistic nature of DMR measurements enabled us to (i) probe GPCR functionality along all four G-protein signaling pathways, something presently beyond reach of most other assay platforms; (ii) dissect complex GPCR signaling patterns even in primary human cells with unprecedented accuracy; (iii) define heterotrimeric G proteins as triggers for the complex optical fingerprints; and (iv) disclose previously undetected features of GPCR behavior. Our results suggest that DMR technology will have a substantial impact on systems biology and systems pharmacology as well as for the discovery of drugs with novel mechanisms.

Agonism and allosterism: the pharmacology of the free fatty acid receptors FFA2 and FFA3

The free fatty acid receptors FFA2 and FFA3 are recently de-orphanized G protein-coupled receptors that share a group of short-chain free fatty acids as endogenous ligands. The expression of FFA2 and FFA3 by immune cells, in parts of the gastro-intestinal tract and by white adipocytes has suggested their potential as therapeutic targets in conditions including inflammation and obesity. However, although FFA2 and FFA3 display distinct structure-activity relationships for stimulation by short-chain free fatty acids, the overlap between these endogenous agonists and the lack of synthetic small molecule ligands that display selectivity between these two receptors has, until recently, hindered efforts to resolve their individual functions. Recently, chloro-alpha-(1-methylethyl)-N-2-thiazolylbenzeneacetamide has been described as an FFA2 selective ago-allosteric ligand, not only being a direct agonist but also acting as a positive allosteric modulator of the function of short-chain free fatty acids at FFA2. Mutation of a pair of key arginine residues near the top of transmembrane domains V and VII of both FFA2 and FFA3 eliminates the function of short-chain free fatty acids but is without effect on the direct agonist action of chloro-alpha-(1-methylethyl)-N-2-thiazolylbenzeneacetamide at FFA2, confirming the distinct nature of the binding site of the ago-allosteric regulator from the orthosteric binding site for free fatty acids. An understanding of structure-activity relationships for ligands related to chloro-alpha-(1-methylethyl)-N-2-thiazolylbenzeneacetamide is likely to provide greater insight into the mode of action and site of binding of this ligand, but further FFA2 and FFA3 selective ligands, preferably with higher potency/affinity, will be required to fully explore the physiological function of these receptors.