Modulation Of Receptor Function Research Articles

Melanocortin-4 Receptor PLC Activation Is Modulated by an Interaction with the Monocarboxylate Transporter 8.

The melanocortin-4 receptor (MC4R) is a key player in the hypothalamic leptin-melanocortin pathway that regulates satiety and hunger. MC4R belongs to the G protein-coupled receptors (GPCRs), which are known to form heterodimers with other membrane proteins, potentially modulating receptor function or characteristics. Like MC4R, thyroid hormones (TH) are also essential for energy homeostasis control. TH transport across membranes is facilitated by the monocarboxylate transporter 8 (MCT8), which is also known to form heterodimers with GPCRs. Based on the finding in single-cell RNA-sequencing data that both proteins are simultaneously expressed in hypothalamic neurons, we investigated a putative interplay between MC4R and MCT8. We developed a novel staining protocol utilizing a fluorophore-labeled MC4R ligand and demonstrated a co-localization of MC4R and MCT8 in human brain tissue. Using in vitro assays such as BRET, IP1, and cAMP determination, we found that MCT8 modulates MC4R-mediated phospholipase C activation but not cAMP formation via a direct interaction, an effect that does not require a functional MCT8 as it was not altered by a specific MCT8 inhibitor. This suggests an extended functional spectrum of MCT8 as a GPCR signaling modulator and argues for the investigation of further GPCR-protein interactions with hitherto underrepresented physiological functions.

IUPHAR review: Recent progress in the development of Mu opioid receptor modulators to treat opioid use disorders

Opioid Use Disorder (OUD) can be described as intense preoccupation with using or obtaining opioids despite the negative consequences associated with their use. As the number of OUD cases in the U.S. increase, so do the number of opioid-related overdose deaths. In 2022, opioid-related overdose became the No. 1 cause of death for individuals in the U.S. between the ages of 25 and 64 years of age. Because of the introduction of highly potent synthetic opioids (e.g. fentanyl) to the illicit drug market, there is an urgent need for therapeutics that successfully reduce the number of overdoses and can help OUD patients maintain sobriety. Most abused opioids stimulate the mu-opioid receptor (MOR) and activation of this receptor can lead to positive (e.g., euphoria) consequences. However, the negative side effects of MOR stimulation can be fatal (e.g., sedation, respiratory depression). Therefore, the MOR is an attractive target for developing medications to treat OUD. Current FDA drugs include MOR agonists that aid in detoxification and relapse prevention, and MOR antagonists that also serve as maintenance therapies or reverse overdose. These medications are limited by their abuse potential, adverse effects, or pharmacological profiles which leaves ample room for research into designing new chemical entities with optimal physiological effects. These includes, orthosteric ligands that target the primary binding site of the MOR, allosteric ligands that positively, negatively, or “silently” modulate receptor function, and lastly, bitopic ligands target both the orthosteric and allosteric sites simultaneously.

Transient Ca2+ entry by plasmalogen-mediated activation of receptor potential cation channel promotes AMPK activity.

Ethanolamine-containing alkenyl ether glycerophospholipids, plasmalogens, are major cell membrane components of mammalian cells that activate membrane protein receptors such as ion transporters and G-protein coupled receptors. However, the mechanism by which plasmalogens modulate receptor function is unknown. Here, we found that exogenously added plasmalogens activate transient receptor potential cation channel subfamily C member 4 (TRPC4) to increase Ca2+ influx, followed by calcium/calmodulin-dependent protein kinase 2-mediated phosphorylation of AMP-activated protein kinase (AMPK). Upon topical application of plasmalogens to the skin of mice, AMPK activation was observed in TRPC4-expressing hair bulbs and hair follicles. Here, TRPC4 was co-localized with the leucine-rich repeat containing G protein-coupled receptor 5, a marker of hair-follicle stem cells, leading to hair growth. Collectively, this study indicates that plasmalogens could function as gate openers for TRPC4, followed by activating AMPK, which likely accelerates hair growth in mice.

De novo Fc-based receptor dimerizers differentially modulate PlexinB1 function

Signaling by single-pass transmembrane receptors often involves a formation of ligand-induced receptor dimers with particular conformation, and bivalent receptor binders can modulate receptor functions by inducing different receptor dimer conformations, although such agents are difficult to design. Here, we describe the generation of both antagonistic and agonistic receptor dimerizers toward PlexinB1 (PlxnB1), a receptor for semaphorin 4D (Sema4D), by grafting two different PlxnB1-binding peptides onto the human immunoglobulin G1 (IgG1) Fc protein. The function-modulating activity of a peptide Fc was strongly dependent on the type of the peptide as well as the grafting site, with the best variants showing activity at an nM concentration range. Structural analysis of each peptide-PlxnB1 complex revealed that the agonistic Fc dimerizes PlxnB1 in a face-to-face fashion similar to that induced by Sema4D, whereas antagonistic Fc would induce signaling-incompetent PlxnB1 dimer conformation, enforcing the idea that plexin activation is primarily controlled by the receptor orientation within the dimer.

Role of Neto1 extracellular domain in modulation of kainate receptors

Kainate receptors play fundamental roles in regulating synaptic transmission and plasticity in central nervous system and are regulated by their cognate auxiliary subunits Neuropilin and tolloid-like proteins 1 and 2 (Neto). While electrophysiology-based insights into functions of Neto proteins are known, biophysical and biochemical studies into Neto proteins have been largely missing till-date. Our biochemical, biophysical, and functional characterization of the purified extracellular domain (ECD) of Neto1 shows that Neto1-ECD exists as monomers in solution and has a micromolar affinity for GluK2 receptors in apo state or closed state. Remarkably, the affinity was ~2.8 fold lower for receptors trapped in the desensitized state, highlighting the conformation-dependent interaction of Neto proteins with kainate receptors. SAXS analysis of Neto1-ECD reveals that their dimensions are long enough to span the entire extracellular domain of kainate receptors. The shape and conformation of Neto1-ECD seems to be altered by calcium ions pointing towards its possible role in modulating Neto1 functions. Functional assays using GluK2 receptors and GluK2/GluA2 chimeric receptors reveal a differential role of Neto1 domains in modulating receptor functions. Although the desensitization rate was not affected by the Neto1-ECD, the recovery rates from the desensitized state are altered.

Structural Arrangement Produced by Concanavalin A Binding to Homomeric GluK2 Receptors.

Kainate receptors are members of the ionotropic glutamate receptor family. They form cation-specific transmembrane channels upon binding glutamate that desensitize in the continued presence of agonists. Concanavalin A (Con-A), a lectin, stabilizes the active open-channel state of the kainate receptor and reduces the extent of desensitization. In this study, we used single-molecule fluorescence resonance energy transfer (smFRET) to investigate the conformational changes underlying kainate receptor modulation by Con-A. These studies showed that Con-A binding to GluK2 homomeric kainate receptors resulted in closer proximity of the subunits at the dimer–dimer interface at the amino-terminal domain as well as between the subunits at the dimer interface at the agonist-binding domain. Additionally, the modulation of receptor functions by monovalent ions, which bind to the dimer interface at the agonist-binding domain, was not observed in the presence of Con-A. Based on these results, we conclude that Con-A modulation of kainate receptor function is mediated by a shift in the conformation of the kainate receptor toward a tightly packed extracellular domain.

Modulation of Host Immune System by Staphylococcal Superantigen-like (SSL) Proteins

Staphylococcus aureus is a common pathogen causing a wide range of infectious diseases in humans and animals. This bacterium secretes a variety of exoproteins, including toxins known as superantigens, such as toxic shock syndrome toxin-1 (TSST-1) and enterotoxins. Staphylococcal superantigen-like (SSL) proteins are a family of exoproteins showing structural similarities with superantigens but no superantigenic activity. This family is composed of 14 members (SSL1-SSL14), and recent studies have revealed that these members exhibit various immunomodulatory activities: e.g., inhibition of antibody and complement functions, impairment of leukocyte trafficking, modulation of receptor functions, inappropriate activation of immunocytes, and inhibition of blood coagulation. These activities have been proposed to contribute to immune evasion of the bacteria. The interactions between SSL proteins and their target molecules in the host immune system and the pathophysiological roles of SSL proteins in the bacterial infections are reviewed in this article.

Mucin-Type O-GalNAc Glycosylation in Health and Disease.

Mucin-type GalNAc O-glycosylation is one of the most abundant and unique post-translational modifications. The combination of proteome-wide mapping of GalNAc O-glycosylation sites and genetic studies with knockout animals and genome-wide analyses in humans have been instrumental in our understanding of GalNAc O-glycosylation. Combined, such studies have revealed well-defined functions of O-glycans at single sites in proteins, including the regulation of pro-protein processing and proteolytic cleavage, as well as modulation of receptor functions and ligand binding. In addition to isolated O-glycans, multiple clustered O-glycans have an important function in mammalian biology by providing structural support and stability of mucins essential for protecting our inner epithelial surfaces, especially in the airways and gastrointestinal tract. Here the many O-glycans also provide binding sites for both endogenous and pathogen-derived carbohydrate-binding proteins regulating critical developmental programs and helping maintain epithelial homeostasis with commensal organisms. Finally, O-glycan changes have been identified in several diseases, most notably in cancer and inflammation, where the disease-specific changes can be used for glycan-targeted therapies. This chapter will review the biosynthesis, the biology, and the translational perspectives of GalNAc O-glycans.

Signaling via Src Family Kinases Is Required for Normal Internalization of the Receptor c-Kit

AbstractStem cell factor (SCF) exerts its biological effects by binding to a specific receptor, the tyrosine kinase c-Kit, which is expressed on the cell surface. Although normal cellular trafficking of growth factor receptors may play a critical role in the modulation of receptor function, the mechanisms that regulate the distribution of c-Kit on the cell surface and the internalization of c-Kit have not been fully defined. We investigated whether signal transduction via Src family kinases is required for normal c-Kit trafficking. Treatment of the SCF-responsive human hematopoietic cell line MO7e with the inhibitor of Src family kinases PP1 blocked SCF-induced capping of c-Kit and internalization of c-Kit. c-Kit was able to associate with clathrin in the presence of PP1, suggesting that entry of c-Kit into clathrin-coated pits occurs independently of Src family kinases. SCF-induced internalization of c-Kit was also diminished in the D33-3 lymphoid cell line in which expression of Lyn kinase was disrupted by homologous recombination. These results indicate that Src family kinases play a role in ligand-induced trafficking of c-Kit.

Structure of human GABAB receptor in an inactive state.

Human GABAB G protein-coupled receptor (GPCR), a member of the class C family, mediates inhibitory neurotransmission and is implicated in epilepsy, pain, and addiction1. A unique GPCR known to require heterodimerization for function2–6, its two subunits, GABAB1 and GABAB2, are structurally homologous but perform distinct and complementary functions. GABAB1 recognizes orthosteric ligand7,8, while GABAB2 couples with G protein9–14. Each subunit is characterized by an extracellular Venus flytrap (VFT) module, a descending peptide linker, a seven-helix transmembrane (TM) domain, and a cytoplasmic tail15. Whereas the VFT heterodimer structure has been resolved16, the structure of the full-length receptor and its transmembrane signaling mechanism remain unknown. Here we present a near full-length structure of the GABAB receptor, captured in an inactive state via cryo-electron microscopy (EM). Our structure reveals multiple ligands pre-associated with the receptor, including two large endogenous phospholipids embedded within the TM domains to maintain receptor integrity and modulate receptor function. We also identify a novel heterodimer interface between TM helices 5 and 3 of both subunits, which serves as a signature of the inactive conformation. A unique ′intersubunit latch′ within this TM interface maintains the inactive state, and its disruption leads to constitutive receptor activity.

The Meta-Position of Phe4 in Leu-Enkephalin Regulates Potency, Selectivity, Functional Activity, and Signaling Bias at the Delta and Mu Opioid Receptors.

As tool compounds to study cardiac ischemia, the endogenous δ-opioid receptors (δOR) agonist Leu5-enkephalin and the more metabolically stable synthetic peptide (d-Ala2, d-Leu5)-enkephalin are frequently employed. However, both peptides have similar pharmacological profiles that restrict detailed investigation of the cellular mechanism of the δOR’s protective role during ischemic events. Thus, a need remains for δOR peptides with improved selectivity and unique signaling properties for investigating the specific roles for δOR signaling in cardiac ischemia. To this end, we explored substitution at the Phe4 position of Leu5-enkephalin for its ability to modulate receptor function and selectivity. Peptides were assessed for their affinity to bind to δORs and µ-opioid receptors (µORs) and potency to inhibit cAMP signaling and to recruit β-arrestin 2. Additionally, peptide stability was measured in rat plasma. Substitution of the meta-position of Phe4 of Leu5-enkephalin provided high-affinity ligands with varying levels of selectivity and bias at both the δOR and µOR and improved peptide stability, while substitution with picoline derivatives produced lower-affinity ligands with G protein biases at both receptors. Overall, these favorable substitutions at the meta-position of Phe4 may be combined with other modifications to Leu5-enkephalin to deliver improved agonists with finely tuned potency, selectivity, bias and drug-like properties.

A Review on Mammary Tumors in Rabbits: Translation of Pathology into Medical Care.

Simple SummaryIn recent years mammary cancer has been increasingly recognized in pet rabbits. In addition to uterine carcinomas—the most common tumor of female rabbits—mammary cancer can also markedly reduce the life expectancy of pet rabbits. The aim of this review is to raise awareness for these tumors and to report recent progress in related research. Their detailed characterization will likely improve medical care for affected rabbits. Moreover, study results will contribute to comparative pathology and may reveal if the rabbit is a suitable model for certain types of breast cancer in humans. Available information suggests that most invasive cancer cases develop through stepwise progression from non-invasive forms. Thus, early recognition will likely improve a complete cancer cure. So far, the only treatment option is surgical excision and prognostic factors are unknown. Recent investigations have identified tumor features with likely prognostic value. They have also revealed differences and similarities to mammary tumors in other species and breast cancer in women. Despite these initial data, continued research is necessary to gain more insights into the development of these tumors and their molecular features.The aim of this review is to raise awareness for mammary tumors in rabbits and to report progress in related research. Currently, a standardized tumor classification for rabbits is not available, prognostic factors are unknown and the only treatment option is surgical excision. Studies showed that affected rabbits have a wide age range and are nearly exclusively female or female spayed. Most mammary tumors are carcinomas. These may occur together with non-neoplastic or benign mammary lesions. Frequent microscopic findings are lipid droplets in tumor cells, secretory activity and microscopic heterogeneity. Since carcinomas are often negative for estrogen and progesterone receptors (ER-α/PR), modulation of receptor function will unlikely be beneficial for most rabbits. ER-α and PR status may have prognostic significance, since ER-α- or PR-negative tumors have significantly higher mitotic rates than ER-α- or PR-positive tumors. The frequent secretory activity of rabbit mammary tumors may suggest an influence of prolactin on tumorigenesis. Available data contribute to comparative pathology and are the basis for future molecular studies into the identification of additional prognostic factors and novel therapeutic options. They will also reveal the suitability of the rabbit as a model for certain types of breast cancer in women.

Bispecific Aptamer Induced Artificial Protein-Pairing: A Strategy for Selective Inhibition of Receptor Function.

Cell surface receptors play a critical role in modulating intracellular signal transduction, making them important drug targets. However, it remains challenging to develop a selective and efficient strategy for regulating receptor function. Herein, we develop a strategy, called bispecific aptamer induced artificial protein-pairing, to selectively regulate receptor function. In this strategy, bispecific aptamer probes act as molecular mediators to bind to both a target receptor protein and a paired protein, which brings the two proteins into close proximity on the living cell membrane. Importantly, the paired proteins work not only as a cancer biomarker for enhancing cell selectivity but also as a blocking assistant to inhibit target receptor function via strong steric hindrance effect. Compared with single-aptamer-mediated regulation, the proposed bispecific aptamer probes afford substantial improvement in selective and efficient regulation of receptor function and downstream signaling pathways. This work offers a versatile methodology to design molecular mediators that can modulate receptor function, thereby providing a new way for developing novel therapeutic drugs.

Ligand-Induced Allosteric Effects Governing SR Signaling

Steroid receptors (SRs) are a class of ligand-regulated transcription factors that regulate gene expression in response to the binding of steroid hormones. Ligand binding drives conformational changes within the SR ligand binding domain that alters the receptors' affinity for coregulator proteins that in turn modulate chromatin state and either promote or block the recruitment of transcriptional machinery to a gene. Structural characterizations of SRs have provided insight into how these conformational rearrangements modulate receptor function, including signaling between the ligand binding pocket and the site of coregulator binding. Here, we review some of the proposed structural mechanisms put forward to explain the ability of ligands to modulate SR function. We also provide a discussion on computational methods that have contributed to the elucidation of SR allosteric regulation. Finally, we consider broader discussions of allostery within the SR family, such as receptor-induced reverse allostery and allosteric binding sites located outside of the canonical ligand interaction site.

Abstract 2504: Sialylation of EGFR by the glycosyltransferase ST6Gal-I results in receptor activation and resistance to gefitinib mediated apoptosis

Abstract The Epidermal Growth Factor Receptor (EGFR) is one of the most commonly altered receptors in cancer. In the present study, we report that sialylation of EGFR by the tumor-associated sialyltransferase ST6Gal-I increases receptor activation resulting in resistance to the EGFR tyrosine kinase inhibitor gefitinib. ST6Gal-I adds an α2-6 sialic acid (a bulky, negatively charged sugar) to select cell surface receptors, thereby modulating receptor function. Previously our group has shown that ST6Gal-I activity confers hallmark cancer stem cell characteristics such as invasion, chemoresistance, and resistance to microenvironmental stressors such as growth factor deprivation and hypoxia. Kinomics data collected for this study reveal that in OV4 ovarian cancer cells, where ST6Gal-I has been overexpressed using a lentiviral construct, ST6Gal-I overexpressing cells have an increase in overall tyrosine kinase activity in comparison to cells without ST6Gal-I expression, whereas the overall serine/threonine kinase activity is largely unaffected by ST6Gal-I expression. Specifically, the receptor tyrosine kinase, EGFR, is one of the most significantly altered kinases in ST6Gal-I expressing cells, with ST6Gal-I expressing cells having an increase in overall activity in comparison to non-expressing cells. Based on these findings, we further interrogated the role of ST6Gal-I on EGFR function. To this end, we first confirmed the sialylation status of EGFR in cells with either forced ST6Gal-I overexpression or shRNA knockdown. Using OV4 and Skov3 cells, ovarian cancer cells, as well as BxPC3 pancreatic cancer cells, we conclude that not only is EGFR a substrate of ST6Gal-I, but that sialylation of EGFR by ST6Gal-I results in basal activation of the receptor, independent of EGF ligand treatment. Further, upon stimulation with EGF, receptor activation is increased in ST6Gal-I expressing versus non-expressing cells. To determine the functional significance of EGFR sialylation by ST6Gal-I, we treated cells with differential ST6Gal-I expression with gefitinib, an EGFR tyrosine kinase inhibitor widely used as an anti-cancer therapy. These studies revealed that ST6Gal-I expressing cells were protected from gefitinib mediated apoptosis. Finally, given that EGFR activation has been shown to promote epithelial to mesenchymal transition (EMT), we evaluated ST6Gal-I expressing cells treated with EGF for the EMT marker Slug. We report that while ST6Gal-I expressing cells inherently have elevated Slug expression versus cells without ST6Gal-I, treatment with EGF greatly enhances Slug expression in ST6Gal-I expressing versus non-expressing cells. Collectively, these results reveal a novel functional role for sialylated EGFR and suggest that the sialylation of EGFR may facilitate EMT as well as provide cells a mechanism to overcome chemoresistance. Citation Format: Colleen Britain, Andrew Holdbrooks, Susan Bellis. Sialylation of EGFR by the glycosyltransferase ST6Gal-I results in receptor activation and resistance to gefitinib mediated apoptosis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2504.

Abstract 4092: Glycosyltransferase ST6Gal-I promotes pancreatic ductal adenocarcinoma progression and metastasis

Abstract Sialyltransferase ST6Gal-I adds α2-6 sialic acids to select N-glycosylated cell surface receptors, thereby modulating receptor function and intracellular signaling. ST6Gal-I is upregulated in multiple human cancers, including >70% of pancreatic ductal adenocarcinoma (PDAC) samples. ST6Gal-I confers cancer stem cell (CSC) properties evidenced by tumorspheroid growth, chemoresistance and tumor initiating potential. As an example, we previously reported that ST6Gal-I imparts gemcitabine resistance by dampening DNA damage. One possible mechanism by which ST6Gal-I promotes a CSC phenotype is by increasing the expression of the master stem cell transcription factor, Sox-9. Forced overexpression or knockdown of ST6Gal-I directly regulates Sox9 expression in a wide array of established cancer lines. Sox9 is known to be a potent driver of acinar to ductal metaplasia (ADM), a key PDAC initiating event. To interrogate the role of ST6Gal-I and Sox9 in ADM, ST6Gal-I was overexpressed in the nontransformed acinar cell line, 266-6. ST6Gal-I overexpression was found to induce expression of Sox9 and other ductal markers. Furthermore, ST6Gal-I overexpressing cells spontaneously formed foci in adherent culture, a hallmark of malignant transformation. Contrarily, ST6Gal-I knockdown in 266-6 cells decreased cell viability. Post ADM, PDAC progresses through the formation of PanINs. This process of progressive lesion formation leading to frank PDAC can be replicated in murine models of pancreatic cancer utilizing pancreas specific oncogenic K-Ras (KC). To investigate the role of ST6Gal-I pertaining to PDAC in vivo we generated ST6Gal-I knock-in (KI) mice with oncogenic K-Ras (KC-ST6Gal-I KI). The KC-ST6Gal-I KI mice presented with significantly decreased overall survival (median = 4.3 months) compared to the KC mice (median = 14 months). This was coupled with a marked increase in liver and lung metastasis in KC-ST6Gal-I KI mice compared to KC mice. To better understand the molecular events underlying ST6Gal-I's PDAC-promoting function, we generated organoids from KC, KC-ST6Gal-I KI, ST6Gal-I KI and WT mice. Preliminary data indicate that there is increased Sox-9 expression in ST6Gal-I KI organoids compared to WT controls. As well, the number of organoids formed in culture is increased in ST6Gal-I KI mice compared to the control. Since organoids are derived from the resident stem cells, this data further indicates increased stemness with enhanced ST6Gal-I expression. In ongoing studies we are defining the signaling mechanisms underpinning a ST6Gal-I/Sox9 axis in PDAC initiation events and other aspects of CSC behavior. These collective studies highlight a novel role for tumor glycans in PDAC pathogenesis, and implicate ST6Gal-I as a promising therapeutic target. Citation Format: Asmi Chakraborty, Robert B. Jones, Christopher A. Klug, Lesley E. Smythies, Susan L. Bellis. Glycosyltransferase ST6Gal-I promotes pancreatic ductal adenocarcinoma progression and metastasis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4092.

Closely related, yet unique: Distinct homo- and heterodimerization patterns of G protein coupled chemokine receptors and their fine-tuning by cholesterol.

Chemokine receptors, a subclass of G protein coupled receptors (GPCRs), play essential roles in the human immune system, they are involved in cancer metastasis as well as in HIV-infection. A plethora of studies show that homo- and heterodimers or even higher order oligomers of the chemokine receptors CXCR4, CCR5, and CCR2 modulate receptor function. In addition, membrane cholesterol affects chemokine receptor activity. However, structural information about homo- and heterodimers formed by chemokine receptors and their interplay with cholesterol is limited. Here, we report homo- and heterodimer configurations of the chemokine receptors CXCR4, CCR5, and CCR2 at atomistic detail, as obtained from thousands of molecular dynamics simulations. The observed homodimerization patterns were similar for the closely related CC chemokine receptors, yet they differed significantly between the CC receptors and CXCR4. Despite their high sequence identity, cholesterol modulated the CC homodimer interfaces in a subtype-specific manner. Chemokine receptor heterodimers display distinct dimerization patterns for CXCR4/CCR5 and CXCR4/CCR2. Furthermore, associations between CXCR4 and CCR5 reveal an increased cholesterol-sensitivity as compared to CXCR4/CCR2 heterodimerization patterns. This work provides a first comprehensive structural overview over the complex interaction network between chemokine receptors and indicates how heterodimerization and the interaction with the membrane environment diversifies the function of closely related GPCRs.

Taste Receptor Polymorphisms and Immune Response: A Review of Receptor Genotypic-Phenotypic Variations and Their Relevance to Chronic Rhinosinusitis.

Bitter (T2R) and sweet taste (T1R) receptors have emerged as regulators of upper airway immune responses. Genetic variation of these taste receptors additionally confers susceptibility to infection and has been implicated in severity of disease in chronic rhinosinusitis (CRS). Ongoing taste receptor research has identified a variety of biologically active compounds that activate T1R and T2R receptors, increasing our understanding of not only additional receptor isoforms and their function but also how receptor function may contribute to the pathophysiology of CRS. This review will discuss the function of taste receptors in mediating airway immunity with a focus on recently described modulators of receptor function and directions for future research into the potential role of genotypic and phenotypic receptor variation as a predictor of airway disease and response to therapy.

Actin cytoskeleton-dependent regulation of corticotropin-releasing factor receptor heteromers.

Stress responses are highly nuanced and variable, but how this diversity is achieved by modulating receptor function is largely unknown. Corticotropin-releasing factor receptors (CRFRs), class B G protein-coupled receptors, are pivotal in mediating stress responses. Here we show that the two known CRFRs interact to form heteromeric complexes in HEK293 cells coexpressing both CRFRs and in vivo in mouse pancreas. Coimmunoprecipitation and mass spectrometry confirmed the presence of both CRF1R and CRF2βR, along with actin in these heteromeric complexes. Inhibition of actin filament polymerization prevented the transport of CRF2βR to the cell surface but had no effect on CRF1R. Transport of CRF1R when coexpressed with CRF2βR became actin dependent. Simultaneous stimulation of cells coexpressing CRF1R+CRF2βR with their respective high-affinity agonists, CRF+urocortin2, resulted in approximately twofold increases in peak Ca2+ responses, whereas stimulation with urocortin1 that binds both receptors with 10-fold higher affinity did not. The ability of CRFRs to form heteromeric complexes in association with regulatory proteins is one mechanism to achieve diverse and nuanced function.

GABAA Receptors and the Diversity in their Structure and Pharmacology.

GABAA receptors (GABAARs) are a class of ligand-gated ion channels with high physiological and therapeutic significance. In the brain, these pentameric receptors occur with diverse subunit composition, which confers highly complex pharmacology to this receptor class. An impressive range of clinically used therapeutics are known to bind to distinct sites found on GABAARs to modulate receptor function. Numerous experimental approaches have been used over the years to elucidate the binding sites of these drugs, but unequivocal identification is challenging due to subtype- and ligand-dependent pharmacology. Here, we review the current structural and pharmacological understanding of GABAARs, besides highlighting recent evidence which has revealed greater complexity than previously anticipated.