Structural insights into the activation of human calcium-sensing receptor

Preparation and characterization of LGR5 LOOP region-specific nanobodies

Leucine-rich repeat-containing G-protein-coupled receptor 5 (LGR5), also known as G-protein-coupled receptor 49 (GPR49), is a class A G-protein-coupled receptor (GPCR) that plays a pivotal role in embryonic development and functions as a marker for adult stem cells in various tissues and organs. LGR5 possesses a large extracellular domain (ecto-domain) enriched with leucine-rich repeats (LRR), primarily responsible for binding to ligands such as R-spondins. The C-terminal LRR extracellular LOOP region of LGR5 refers to the loop structure connecting the C-terminus of LGR5 to the first transmembrane helix. As the LOOP region is located extracellularly, it is readily accessible to exogenous molecules such as antibodies, nanobodies, or small-molecule drugs. In this study, we successfully expressed and purified the LGR5 LOOP region protein in a prokaryotic expression system. The purified protein was subsequently used as an antigen to immunize camels, leading to the generation of nanobodies. These nanobodies are composed solely of the variable domain of the heavy-chain antibody (VHH), with a molecular weight of approximately 15 kDa. Using the purified LGR5 LOOP region protein as an antigen, we isolated nanobodies that specifically bind to it. Subsequent assays demonstrated that the selected nanobody, NB 4C4 and NB 3E8, specifically targeted the LGR5 LOOP region, exhibited an inhibitory effect on β-catenin-mediated Wnt signaling to a certain extent. This study provides insights for the development of LGR5-targeted diagnostic reagents and antibody-based therapeutic strategies.

High-affinity nanobodies targeting IL-12B for the detection of fluorescence resonance energy transfer

AIMS

IL-12B, a subunit of the IL-23 family of cytokines, plays a crucial role in various diseases such as viral infections, autoimmune disorders, and tumors. This study aimed to identify high-affinity nanobodies that bind to distinct epitopes of IL-12B to and assess their potential for therapeutic and diagnostic applications, particularly through fluorescence resonance energy transfer(FRET) to evaluate their ability to target IL-12B.

METHODS

IL-12B protein was expressed in eukaryotic cells and used to immunize camels to induce an immune response. Camel-derived anti-IL-12B nanobodies were isolated and screened via phage display to identify those with high specificity and affinity for IL-12B. Binding affinity and epitope interactions were further analyzed using high-performance liquid chromatography (HPLC) and ForteBio Octet assays. A FRET-based assay was developed to evaluate protein interactions for precise therapeutic targeting.

RESULTS

Several high-affinity nanobodies targeting IL-12B were successfully generated. These nanobodies exhibited strong binding to various epitopes of IL-12B. Screening by HPLC and ForteBio Octet confirmed their high specificity and affinity, while fluorescence analysis demonstrated efficient energy transfer between thenanobodies, indicating successful interactions.

CONCLUSIONS

This study identified high-affinity nanobodies against IL-12B and used FRET to characterize their interactions. These nanobodies show promise for therapeutic potential targeting IL-12B-related diseases, including viral infections, autoimmune disorders, and cancer. However, further clinical studies are needed to fully explore their potential for diagnostic and therapeutic applications.

Epitope-directed selection of GPCR nanobody ligands with evolvable function

Antibodies have the potential to target G protein-coupled receptors (GPCRs) with high receptor, cellular, and tissue selectivity; however, few antibody ligands for GPCRs exist. Here, we describe a generalizable selection method to enrich for GPCR ligands from a synthetic camelid antibody fragment (nanobody) library. Our strategy yielded multiple nanobody ligands for the angiotensin II type I receptor (AT1R), a prototypical GPCR and important drug target. We found that nanobodies readily act as allosteric modulators, encoding selectivity for both the receptor and chemical features of GPCR ligands. We then used structure-guided design to convert two nanobodies from allosteric ligands to competitive AT1R inhibitors through simple mutations. This work demonstrates that nanobodies can encode multiple pharmacological behaviors and have great potential as evolvable scaffolds for the development of next-generation GPCR therapeutics.

The calcium-sensing receptor: a comprehensive review on its role in calcium homeostasis and therapeutic implications

The calcium-sensing receptor (CaSR), a key member of the family C G protein-coupled receptors (GPCRs), plays a crucial role in regulating calcium homeostasis and parathyroid hormone (PTH) secretion. It responds to various physiological ligands, including calcium ions and amino acids, activating multiple signaling pathways through interactions with different G proteins and β-arrestin. This review focuses on the structural features of CaSR, emphasizing recent advances in understanding its activation mechanisms via agonists and allosteric modulators. CaSR holds significant therapeutic potential, particularly in treating calcitropic disorders such as hyperparathyroidism and hypoparathyroidism. Current pharmacological agents, including calcimimetics such as cinacalcet and etelcalcetide, have proven effective in managing secondary hyperparathyroidism (SHPT); however, they are associated with side effects such as hypocalcemia. Emerging investigational drugs, including palopegteriparatide and other small molecules, show promise in addressing various calcium-related conditions. Despite challenges that have led to the discontinuation of some drug developments, ongoing research is focused on refining CaSR-targeted therapies to improve efficacy, reduce adverse effects, and enhance patient outcomes.

An activating calcium-sensing receptor variant with biased signaling reveals a critical residue for Gα11 coupling

Autosomal dominant hypocalcemia (ADH) is due to enhanced calcium-dependent signaling caused by heterozygous gain-of-function (GOF) variants in the CASR gene (ADH1) or in the GNA11 gene, encoding Gα11 (ADH2). Both ADH1 and ADH2 are associated with hypocalcemia and normal or inappropriately low levels of circulating PTH. ADH1 patients typically manifest hypercalciuria, while ADH2 is associated with short stature in approximately 42% of cases. We evaluated a 10-yr-old boy with hypoparathyroidism and short stature. Biochemical analyses revealed hypocalcemia, hyperphosphatemia, and inconsistent hypercalciuria. Genetic analyses revealed a de novo heterozygous p.Leu723Arg variant in CASR. We characterized the expression of recombinant WT and Leu723Arg calcium-sensing receptor (CaSR) proteins in HEK293 cells and assessed G protein activation in vitro by CaSR using bioluminescence resonance energy transfer. Transient expression studies showed the Leu723Arg variant was normally expressed but resulted in a significantly lower EC50 for extracellular calcium activation of G11 but not other G proteins (ie, Gi, Gq, Gs). The Leu723Arg substitution has a novel GOF phenotype that leads to biased CaSR activation of G11 signaling, suggesting that residue 723 specifies activation of G11 but not other G proteins. Similar studies of a previously described CaSR variant associated with hypoparathyroidism and short stature, Leu616Val, showed no changes in any G protein pathways, indicating it is likely to be a benign variant. Given the preferential activation of G11 by the Leu723Arg CaSR variant, we propose that the patient's short stature shares a similar basis to that in patients with ADH2 due to GOF variants in GNA11.

CASRdb: A Publicly Accessible Comprehensive Database for Disease-Associated Calcium-Sensing Receptor Variants

CONTEXT

Genetic testing of the calcium-sensing receptor (CASR) gene is crucial for confirming diagnoses of familial hypocalciuric hypercalcemia type I (FHH1) and autosomal dominant hypocalcemia type I (ADH1). Therefore, we created a publicly accessible comprehensive database of the disease-causing variants of the CASR gene.

EVIDENCE ACQUISITION

We used 2 sources for variant reports: (1) we conducted a systematic review in the Embase and PubMed databases from inception to March 2023, using search strategies associated with CASR. We identified all articles reporting CASR variants associated with disorders of calcium metabolism. (2) Additionally, data associated with pathogenic (P) or likely pathogenic (LP) variants in the ClinVar and LOVD databases were retrieved. Benign or likely benign variants were excluded. Variants of uncertain significance (VUS) were included only if they were reported in the literature. We generated a library of CASR variants associated with phenotypes, which has been made available on a website.

EVIDENCE SYNTHESIS

We identified a total of 498 variants, of which 121 (24.3%) were associated with ADH1 and 377 (75.7%) with FHH1. Most included variants were identified from the literature (117 activating and 352 inactivating variants), and the majority of these were not documented in ClinVar/LOVD (73/117, 62.4% activating variants; 207/352, 58.8% inactivating variants).

CONCLUSION

We developed CASRdb, a database that compiles information on all CASR variants associated with disorders of calcium metabolism from existing literature and genomic databases. Our database stands out due to the substantially higher number of disease-associated variants it contains, highlighting its comprehensive nature. The website is available at http://casrdb.mgh.harvard.edu.

Structural insights into the LGR4-RSPO2-ZNRF3 complexes regulating WNT/β-catenin signaling

WNT/β-catenin signaling plays key roles in development and cancer1,2. ZNRF3/RNF43 modulates Frizzleds through ubiquitination, dampening WNT/β-catenin signaling. Conversely, RSPO1-4 binding to LGR4-6 and ZNRF3/RNF43 enhances WNT/β-catenin signaling3-5. Here, we elucidate the overall landscape of architectures in multiple LGR4, RSPO2, and ZNRF3 assemblies, showcasing varying stoichiometries and arrangements. These structures reveal that LGR4 and RSPO2 capture distinct states of ZNRF3. The intrinsic heterogeneity of the LGR4-RSPO2-ZNRF3 assembly is influenced by LGR4 content. Particularly, in the assembly complex with a 2:2:2 ratio, two LGR4 protomers induce and stabilize the inactive state of ZNRF3, characterized by a wide inward-open conformation of two transmembrane helices (TM helices). This specific assembly promotes a stable complex, facilitating LGR4-induced endocytosis of ZNRF3. In contrast, the active dimeric ZNRF3, bound by a single LGR4, adopts a coiled-coil TM helices conformation and dimerization of RING domains. Our findings unveil how LGR4 content mediates diverse assemblies, leading to conformational rearrangements in ZNRF3 to regulate WNT/β-catenin signaling, and provide a structural foundation for drug development targeting Wnt-driven cancers.

Evolutionary history of calcium-sensing receptors unveils hyper/hypocalcemia-causing mutations

Despite advancements in understanding the structure and functions of the Calcium Sensing Receptor (CaSR), gaps persist in our knowledge of the specific functions of its residues. In this study, we used phylogeny-based techniques to identify functionally equivalent orthologs of CaSR, predict residue significance, and compute specificity-determining position (SDP) scores to understand its evolutionary basis. The analysis revealed exceptional conservation of the CaSR subfamily, emphasizing the critical role of residues with high SDP scores in receptor activation and pathogenicity. To further enhance the findings, gradient-boosting trees were applied to differentiate between gain- and loss-of-function mutations responsible for hypocalcemia and hypercalcemia. Lastly, we investigated the importance of these mutations in the context of receptor activation dynamics. In summary, through comprehensive exploration of the evolutionary history of the CaSR subfamily, coupled with innovative phylogenetic methodologies, we identified activating and inactivating residues, providing valuable insights into the regulation of calcium homeostasis and its connections to associated disorders.

Cryo-EM structure of human class C orphan GPCR GPR179 involved in visual processing

GPR179, an orphan class C GPCR, is expressed at the dendritic tips of ON-bipolar cells in the retina. It plays a pivotal role in the initial synaptic transmission of visual signals from photoreceptors, and its deficiency is known to be the cause of complete congenital stationary night blindness. Here, we present the cryo-electron microscopy structure of human GPR179. Notably, the transmembrane domain (TMD) of GPR179 forms a homodimer through the TM1/7 interface with a single inter-protomer disulfide bond, adopting a noncanonical dimerization mode. Furthermore, the TMD dimer exhibits architecture well-suited for the highly curved membrane of the dendritic tip and distinct from the flat membrane arrangement observed in other class C GPCR dimers. Our structure reveals unique structural features of GPR179 TMD, setting it apart from other class C GPCRs. These findings provide a foundation for understanding signal transduction through GPR179 in visual processing and offers insights into the underlying causes of ocular diseases.

The Impact of Nanobodies on G Protein-Coupled Receptor Structural Biology and Their Potential as Therapeutic Agents

The family of human G protein-coupled receptors (GPCRs) comprises about 800 different members, with about 35% of current pharmaceutical drugs targeting GPCRs. However, GPCR structural biology, necessary for structure-guided drug design, has lagged behind that of other membrane proteins, and it was not until the year 2000 when the first crystal structure of a GPCR (rhodopsin) was solved. Starting in 2007, the determination of additional GPCR structures was facilitated by protein engineering, new crystallization techniques, complexation with antibody fragments, and other strategies. More recently, the use of camelid heavy-chain-only antibody fragments (nanobodies) as crystallographic chaperones has revolutionized the field of GPCR structural biology, aiding in the determination of more than 340 GPCR structures to date. In most cases, the GPCR structures solved as complexes with nanobodies (Nbs) have revealed the binding mode of cognate or non-natural ligands; in a few cases, the same Nb has acted as an orthosteric or allosteric modulator of GPCR signaling. In this review, we summarize the multiple ingenious strategies that have been conceived and implemented in the last decade to capitalize on the discovery of nanobodies to study GPCRs from a structural perspective. SIGNIFICANCE STATEMENT: G protein-coupled receptors (GPCRs) are major pharmacological targets, and the determination of their structures at high resolution has been essential for structure-guided drug design and for insights about their functions. Single-domain antibodies (nanobodies) have greatly facilitated the structural determination of GPCRs by forming complexes directly with the receptors or indirectly through protein partners.

Osteocalcin binds to a GPRC6A Venus fly trap allosteric site to positively modulate GPRC6A signaling

GPRC6A, a member of the Family C G-protein coupled receptors, regulates energy metabolism and sex hormone production and is activated by diverse ligands, including cations, L-amino acids, the osteocalcin (Ocn) peptide and the steroid hormone testosterone. We sought a structural framework for the ability of multiple distinct classes of ligands to active GPRC6A. We created a structural model of GPRC6A using Alphafold2. Using this model we explored a putative orthosteric ligand binding site in the bilobed Venus fly trap (VFT) domain of GPRC6A and two positive allosteric modulator (PAM) sites, one in the VFT and the other in the 7 transmembrane (7TM) domain. We provide evidence that Ocn peptides act as a PAM for GPRC6A by binding to a site in the VFT that is distinct from the orthosteric site for calcium and L-amino acids. In agreement with this prediction, alternatively spliced GPRC6A isoforms 2 and 3, which lack regions of the VFT, and mutations in the computationally predicted Ocn binding site, K352E and H355P, prevent Ocn activation of GPRC6A. These observations explain how dissimilar ligands activate GPRC6A and set the stage to develop novel molecules to activate and inhibit this previously poorly understood receptor.

Molecular regulation of calcium-sensing receptor (CaSR)-mediated signaling

Calcium-sensing receptor (CaSR), a family C G-protein-coupled receptor, plays a crucial role in regulating calcium homeostasis by sensing small concentration changes of extracellular Ca2+, Mg2+, amino acids (e.g., L-Trp and L-Phe), small peptides, anions (e.g., HCO3 - and PO4 3-), and pH. CaSR-mediated intracellular Ca2+ signaling regulates a diverse set of cellular processes including gene transcription, cell proliferation, differentiation, apoptosis, muscle contraction, and neuronal transmission. Dysfunction of CaSR with mutations results in diseases such as autosomal dominant hypocalcemia, familial hypocalciuric hypercalcemia, and neonatal severe hyperparathyroidism. CaSR also influences calciotropic disorders, such as osteoporosis, and noncalciotropic disorders, such as cancer, Alzheimer's disease, and pulmonary arterial hypertension. This study first reviews recent advances in biochemical and structural determination of the framework of CaSR and its interaction sites with natural ligands, as well as exogenous positive allosteric modulators and negative allosteric modulators. The establishment of the first CaSR protein-protein interactome network revealed 94 novel players involved in protein processing in endoplasmic reticulum, trafficking, cell surface expression, endocytosis, degradation, and signaling pathways. The roles of these proteins in Ca2+-dependent cellular physiological processes and in CaSR-dependent cellular signaling provide new insights into the molecular basis of diseases caused by CaSR mutations and dysregulated CaSR activity caused by its protein interactors and facilitate the design of therapeutic agents that target CaSR and other family C G-protein-coupled receptors.

The path to the G protein-coupled receptor structural landscape: Major milestones and future directions

G protein-coupled receptors (GPCRs) play a crucial role in cell function by transducing signals from the extracellular environment to the inside of the cell. They mediate the effects of various stimuli, including hormones, neurotransmitters, ions, photons, food tastants and odorants, and are renowned drug targets. Advancements in structural biology techniques, including X-ray crystallography and cryo-electron microscopy (cryo-EM), have driven the elucidation of an increasing number of GPCR structures. These structures reveal novel features that shed light on receptor activation, dimerization and oligomerization, dichotomy between orthosteric and allosteric modulation, and the intricate interactions underlying signal transduction, providing insights into diverse ligand-binding modes and signalling pathways. However, a substantial portion of the GPCR repertoire and their activation states remain structurally unexplored. Future efforts should prioritize capturing the full structural diversity of GPCRs across multiple dimensions. To do so, the integration of structural biology with biophysical and computational techniques will be essential. We describe in this review the progress of nuclear magnetic resonance (NMR) to examine GPCR plasticity and conformational dynamics, of atomic force microscopy (AFM) to explore the spatial-temporal dynamics and kinetic aspects of GPCRs, and the recent breakthroughs in artificial intelligence for protein structure prediction to characterize the structures of the entire GPCRome. In summary, the journey through GPCR structural biology provided in this review illustrates how far we have come in decoding these essential proteins architecture and function. Looking ahead, integrating cutting-edge biophysics and computational tools offers a path to navigating the GPCR structural landscape, ultimately advancing GPCR-based applications.

Glutathione-Based Photoaffinity Probe Identifies Caffeine as a Positive Allosteric Modulator of the Calcium-Sensing Receptor

The calcium-sensing receptor (CaSR), abundantly expressed in the parathyroid gland and kidney, plays a central role in calcium homeostasis. In addition, CaSR exerts multimodal roles, including inflammation, muscle contraction, and bone remodeling, in other organs and tissues. The diverse functions of CaSR are mediated by many endogenous and exogenous ligands, including calcium, amino acids, glutathione, cinacalcet, and etelcalcetide, that have distinct binding sites in CaSR. However, strategies to evaluate ligand interactions with CaSR remain limited. Here, we developed a glutathione-based photoaffinity probe, DAZ-G, that analyzes ligand binding to CaSR. We showed that DAZ-G binds to the amino acid binding site in CaSR and acts as a positive allosteric modulator of CaSR. Oxidized and reduced glutathione and phenylalanine effectively compete with DAZ-G conjugation to CaSR, while calcium, cinacalcet, and etelcalcetide have cooperative effects. An unexpected finding was that caffeine effectively competes with DAZ-G's conjugation to CaSR and acts as a positive allosteric modulator of CaSR. The effective concentration of caffeine for CaSR activation (<10 μM) is easily attainable in plasma by ordinary caffeine consumption. Our report demonstrates the utility of a new chemical probe for CaSR and discovers a new protein target of caffeine, suggesting that caffeine consumption can modulate the diverse functions of CaSR.

Silicon-Rhodamine Functionalized Evocalcet Probes Potently and Selectively Label Calcium Sensing Receptors In Vitro, In Vivo, and Ex Vivo

The calcium sensing receptor (CaSR) is a ubiquitously expressed G-protein coupled receptor (GPCR) that regulates extracellular calcium signals via the parathyroid glands. CaSR has recently also been implicated in noncalcitropic pathophysiologies like asthma, gut inflammation, and cancer. To date, molecular tools that enable the bioimaging of CaSR in tissues are lacking. Based on in silico analyses of available structure-activity relationship data on CaSR ligands, we designed and prepared silicon-rhodamine (SiR) conjugates of the clinically approved drug evocalcet. The new probes EvoSiR4 and EvoSiR6, with differing linker lengths at the evocalcet carboxyl end, both showed a 6-fold and 3-fold increase in potency toward CaSR (EC50 < 45 nM) compared to evocalcet and the evocalcet-linker conjugate, respectively, in an FLIPR-based cellular functional assay. The specificity of the EvoSiR probes toward CaSR binding and the impact of albumin was evaluated in live cell experiments. Both probes showed strong albumin binding, which facilitated the clearance of nonspecific binding interactions. Accordingly, in zebrafish embryos, EvoSiR4 specifically labeled the high CaSR expressing neuromasts of the lateral line in vivo. EvoSiR4 was also assessed in human parathyroid tissues ex vivo, showing a specific absolute CaSR-associated fluorescence compared to that of parathyroid autofluorescence. In summary, functionalization of evocalcet by SiR led to the preparation of potent and specific fluorescent CaSR probes. EvoSiR4 is a versatile small-molecular probe that can be employed in CaSR-related biomedical analyses where antibodies are not applicable.

Promiscuous G-protein activation by the calcium-sensing receptor

The human calcium-sensing receptor (CaSR) detects fluctuations in the extracellular Ca2+ concentration and maintains Ca2+ homeostasis1,2. It also mediates diverse cellular processes not associated with Ca2+ balance3-5. The functional pleiotropy of CaSR arises in part from its ability to signal through several G-protein subtypes6. We determined structures of CaSR in complex with G proteins from three different subfamilies: Gq, Gi and Gs. We found that the homodimeric CaSR of each complex couples to a single G protein through a common mode. This involves the C-terminal helix of each Gα subunit binding to a shallow pocket that is formed in one CaSR subunit by all three intracellular loops (ICL1-ICL3), an extended transmembrane helix 3 and an ordered C-terminal region. G-protein binding expands the transmembrane dimer interface, which is further stabilized by phospholipid. The restraint imposed by the receptor dimer, in combination with ICL2, enables G-protein activation by facilitating conformational transition of Gα. We identified a single Gα residue that determines Gq and Gs versus Gi selectivity. The length and flexibility of ICL2 allows CaSR to bind all three Gα subtypes, thereby conferring capacity for promiscuous G-protein coupling.

Absence of calcium-sensing receptor basal activity due to inter-subunit disulfide bridges

G protein-coupled receptors naturally oscillate between inactive and active states, often resulting in receptor constitutive activity with important physiological consequences. Among the class C G protein-coupled receptors that typically sense amino-acids and their derivatives, the calcium sensing receptor (CaSR) tightly controls blood calcium levels. Its constitutive activity has not yet been studied. Here, we demonstrate the importance of the inter-subunit disulfide bridges in maintaining the inactive state of CaSR, resulting in undetectable constitutive activity, unlike the other class C receptors. Deletion of these disulfide bridges results in strong constitutive activity that is abolished by mutations preventing amino acid binding. It shows that this inter-subunit disulfide link is necessary to limit the agonist effect of amino acids on CaSR. Furthermore, human genetic mutations deleting these bridges and associated with hypocalcemia result in elevated CaSR constitutive activity. These results highlight the physiological importance of fine tuning the constitutive activity of G protein-coupled receptors.

Nanobodies as negative allosteric modulators for human calcium sensing receptor

Human calcium sensing receptor (CaSR) senses calcium ion concentrations in vivo and is an important class of drug targets. Mutations in the receptor can lead to disorders of calcium homeostasis, including hypercalcemia and hypocalcemia. Here, 127 CaSR-targeted nanobodies were generated from camels, and four nanobodies with inhibitory function were further identified. Among these nanobodies, NB32 can effectively inhibit the mobilization of intracellular calcium ions (Ca2+i) and suppress the G12/13 and ERK1/2 signaling pathways downstream of CaSR. Moreover, it enhanced the inhibitory effect of the calcilytics as a negative allosteric modulator (NAM). We determined the structure of complex and found NB32 bound to LB2 (Ligand-binding 2) domain of CaSR to prevent the interaction of LB2 domains of two protomers to stabilize the inactive state of CaSR.

Structure, function and drug discovery of GPCR signaling

G protein-coupled receptors (GPCRs) are versatile and vital proteins involved in a wide array of physiological processes and responses, such as sensory perception (e.g., vision, taste, and smell), immune response, hormone regulation, and neurotransmission. Their diverse and essential roles in the body make them a significant focus for pharmaceutical research and drug development. Currently, approximately 35% of marketed drugs directly target GPCRs, underscoring their prominence as therapeutic targets. Recent advances in structural biology have substantially deepened our understanding of GPCR activation mechanisms and interactions with G-protein and arrestin signaling pathways. This review offers an in-depth exploration of both traditional and recent methods in GPCR structure analysis. It presents structure-based insights into ligand recognition and receptor activation mechanisms and delves deeper into the mechanisms of canonical and noncanonical signaling pathways downstream of GPCRs. Furthermore, it highlights recent advancements in GPCR-related drug discovery and development. Particular emphasis is placed on GPCR selective drugs, allosteric and biased signaling, polyphamarcology, and antibody drugs. Our goal is to provide researchers with a thorough and updated understanding of GPCR structure determination, signaling pathway investigation, and drug development. This foundation aims to propel forward-thinking therapeutic approaches that target GPCRs, drawing upon the latest insights into GPCR ligand selectivity, activation, and biased signaling mechanisms.

Asymmetric activation of dimeric GABAB and metabotropic glutamate receptors

G protein-coupled receptors (GPCRs) represent the largest family of membrane proteins and are important drug targets. GPCRs are allosteric machines that transduce an extracellular signal to the cell by activating heterotrimeric G proteins. Herein, we summarize the recent advancements in the molecular activation mechanism of the γ-aminobutyric acid type B (GABAB) and metabotropic glutamate (mGlu) receptors, the most important class C GPCRs that modulate synaptic transmission in the brain. Both are mandatory dimers, this quaternary structure being needed for their function The structures of these receptors in different conformations and in complexes with G proteins have revealed their asymmetric activation. This asymmetry is further highlighted by the recent discovery of mGlu heterodimers, where the eight mGlu subunits can form specific and functional heterodimers. Finally, the development of allosteric modulators has revealed new possibilities for regulating the function of these receptors by targeting the transmembrane dimer interface. This family of receptors never ceases to astonish and serve as models to better understand the diversity and asymmetric functioning of GPCRs.NEW & NOTEWORTHY γ-aminobutyric acid type B (GABAB) and metabotropic glutamate (mGlu) receptors form constitutive dimers, which are required for their function. They serve as models to better understand the diversity and activation of G protein-coupled receptors (GPCRs). The structures of these receptors in different conformations and in complexes with G proteins have revealed their asymmetric activation. This asymmetry is further highlighted by the recent discovery of specific and functional mGlu heterodimers. Allosteric modulators can be developed to target the transmembrane interface and modulate the asymmetry.

The impact of cryo-EM on determining allosteric modulator-bound structures of G protein-coupled receptors

G-protein coupled receptors (GPCRs) are important therapeutic targets for the treatment of human disease. Although GPCRs are highly successful drug targets, there are many challenges associated with the discovery and translation of small molecule ligands that target the endogenous ligand-binding site for GPCRs. Allosteric modulators are a class of ligands that target alternative binding sites known as allosteric sites and offer fresh opportunities for the development of new therapeutics. However, only a few allosteric modulators have been approved as drugs. Advances in GPCR structural biology enabled by the cryogenic electron microscopy (cryo-EM) revolution have provided new insights into the molecular mechanism and binding location of small molecule allosteric modulators. This review highlights the latest findings from allosteric modulator-bound structures of Class A, B, and C GPCRs with a focus on small molecule ligands. Emerging methods that will facilitate cryo-EM structures of more difficult ligand-bound GPCR complexes are also discussed. The results of these studies are anticipated to aid future structure-based drug discovery efforts across many different GPCRs.

Asymmetric activation of class C GPCRs

Class C G-protein-coupled receptors (GPCRs) comprise a unique GPCR subfamily with large ligand-binding extracellular domains and function as obligate dimers. The recently resolved cryo-EM structures of full-length GABA_B, CaSR, and mGlus have revealed that these receptors are activated in an asymmetric manner, leading to G-protein-coupling on one protomer within the receptor dimer. In this review we discuss the mechanisms of asymmetric activation in class C GPCRs and the unique mode of interaction with the inhibitory Gi protein. Upon activation, the two seven-transmembrane domains (7TMs) of class C GPCRs rearrange to form a conserved asymmetric TM6-TM6 interface. In contrast to class A and B GPCRs, G-protein coupling does not involve the cytoplasmic opening of TM6, but is facilitated through the coordination of intracellular loops. Furthermore, positive and negative allosteric modulators (PAMs and NAMs) adopt distinct conformations to regulate the activity of class C GPCRs. Taken together, these recent findings on the mechanism of asymmetric activation of class C GPCRs highlight a novel mechanism of G protein activation and provide new insights into the design of therapeutics targeting these receptors.

Recent advances in calcium-sensing receptor structures and signaling pathways

The calcium-sensing receptor (CaSR) is a class C GPCR that has a fundamental role in extracellular calcium homeostasis by regulating parathyroid hormone release and urinary calcium excretion. Germline mutations in the receptor cause disorders of calcium homeostasis and studies of the functional effects of these mutations has facilitated understanding of CaSR signaling and how allosteric modulators affect these responses. In the past year, five cryo-EM structures of the near full-length CaSR have been published, demonstrating how agonist-binding transmits changes in the CaSR extracellular domain to the transmembrane region to activate G proteins, and how allosteric modulators affect these structural dynamics. Additionally, several recent studies have identified CaSR interacting proteins that regulate CaSR signaling and trafficking and contribute to understanding how the receptor achieves rapid and diverse physiological responses.

Calcium-Sensing Receptor (CaSR)-Mediated Intracellular Communication in Cardiovascular Diseases

The calcium-sensing receptor (CaSR), a G-protein-coupled receptor (GPCR), is a cell-surface-located receptor that can induce highly diffusible messengers (IP3, Ca²⁺, cAMP) in the cytoplasm to activate various cellular responses. Recently, it has also been suggested that the CaSR mediates the intracellular communications between the endoplasmic reticulum (ER), mitochondria, nucleus, protease/proteasome, and autophagy-lysosome, which are involved in related cardiovascular diseases. The complex intracellular signaling of this receptor challenges it as a valuable therapeutic target. It is, therefore, necessary to understand the mechanisms behind the signaling characteristics of this receptor in intracellular communication. This review provides an overview of the recent research progress on the various regulatory mechanisms of the CaSR in related cardiovascular diseases and the heart-kidney interaction; the associated common causes are also discussed.

Accelerating GPCR Drug Discovery With Conformation-Stabilizing VHHs

The human genome encodes 850 G protein-coupled receptors (GPCRs), half of which are considered potential drug targets. GPCRs transduce extracellular stimuli into a plethora of vital physiological processes. Consequently, GPCRs are an attractive drug target class. This is underlined by the fact that approximately 40% of marketed drugs modulate GPCRs. Intriguingly 60% of non-olfactory GPCRs have no drugs or candidates in clinical development, highlighting the continued potential of GPCRs as drug targets. The discovery of small molecules targeting these GPCRs by conventional high throughput screening (HTS) campaigns is challenging. Although the definition of success varies per company, the success rate of HTS for GPCRs is low compared to other target families (Fujioka and Omori, 2012; Dragovich et al., 2022). Beyond this, GPCR structure determination can be difficult, which often precludes the application of structure-based drug design approaches to arising HTS hits. GPCR structural studies entail the resource-demanding purification of native receptors, which can be challenging as they are inherently unstable when extracted from the lipid matrix. Moreover, GPCRs are flexible molecules that adopt distinct conformations, some of which need to be stabilized if they are to be structurally resolved. The complexity of targeting distinct therapeutically relevant GPCR conformations during the early discovery stages contributes to the high attrition rates for GPCR drug discovery programs. Multiple strategies have been explored in an attempt to stabilize GPCRs in distinct conformations to better understand their pharmacology. This review will focus on the use of camelid-derived immunoglobulin single variable domains (VHHs) that stabilize disease-relevant pharmacological states (termed ConfoBodies by the authors) of GPCRs, as well as GPCR:signal transducer complexes, to accelerate drug discovery. These VHHs are powerful tools for supporting in vitro screening, deconvolution of complex GPCR pharmacology, and structural biology purposes. In order to demonstrate the potential impact of ConfoBodies on translational research, examples are presented of their role in active state screening campaigns and structure-informed rational design to identify de novo chemical space and, subsequently, how such matter can be elaborated into more potent and selective drug candidates with intended pharmacology.

Mechanism of sensitivity modulation in the calcium-sensing receptor via electrostatic tuning

Transfer of information across membranes is fundamental to the function of all organisms and is primarily initiated by transmembrane receptors. For many receptors, how ligand sensitivity is fine-tuned and how disease associated mutations modulate receptor conformation to allosterically affect receptor sensitivity are unknown. Here we map the activation of the calcium-sensing receptor (CaSR) - a dimeric class C G protein-coupled receptor (GPCR) and responsible for maintaining extracellular calcium in vertebrates. We show that CaSR undergoes unique conformational rearrangements compared to other class C GPCRs owing to specific structural features. Moreover, by analyzing disease associated mutations, we uncover a large permissiveness in the architecture of the extracellular domain of CaSR, with dynamics- and not specific receptor topology- determining the effect of a mutation. We show a structural hub at the dimer interface allosterically controls CaSR activation via focused electrostatic repulsion. Changes in the surface charge distribution of this hub, which is highly variable between organisms, finely tune CaSR sensitivity. This is potentially a general tuning mechanism for other dimeric receptors.

Extracellular Calcium Receptor as a Target for Glutathione and Its Derivatives

Extracellular glutathione (GSH) and oxidized glutathione (GSSG) can modulate the function of the extracellular calcium sensing receptor (CaSR). The CaSR has a binding pocket in the extracellular domain of CaSR large enough to bind either GSH or GSSG, as well as the naturally occurring oxidized derivative L-cysteine glutathione disulfide (CySSG) and the compound cysteinyl glutathione (CysGSH). Modeling the binding energies (ΔG) of CySSG and CysGSH to CaSR reveals that both cysteine derivatives may have greater affinities for CaSR than either GSH or GSSG. GSH, CySSG, and GSSG are found in circulation in mammals and, among the three, CySSG is more affected by HIV/AIDs and aging than either GSH or GSSG. The beta-carbon linkage of cysteine in CysGSH may model a new class of calcimimetics, exemplified by etelcalcetide. Circulating glutathionergic compounds, particularly CySSG, may mediate calcium-regulatory responses via receptor-binding to CaSR in a variety of organs, including parathyroids, kidneys, and bones. Receptor-mediated actions of glutathionergics may thus complement their roles in redox regulation and detoxification. The glutathionergic binding site(s) on CaSR are suggested to be a target for development of drugs that can be used in treating kidney and other diseases whose mechanisms involve CaSR dysregulation.

Targeting trafficking as a therapeutic avenue for misfolded GPCRs leading to endocrine diseases

G protein-coupled receptors (GPCRs) are plasma membrane proteins associated with an array of functions. Mutations in these receptors lead to a number of genetic diseases, including diseases involving the endocrine system. A particular subset of loss-of-function mutant GPCRs are misfolded receptors unable to traffic to their site of function (i.e. the cell surface plasma membrane). Endocrine disorders in humans caused by GPCR misfolding include, among others, hypo- and hyper-gonadotropic hypogonadism, morbid obesity, familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism, X-linked nephrogenic diabetes insipidus, congenital hypothyroidism, and familial glucocorticoid resistance. Several in vitro and in vivo experimental approaches have been employed to restore function of some misfolded GPCRs linked to endocrine disfunction. The most promising approach is by employing pharmacological chaperones or pharmacoperones, which assist abnormally and incompletely folded proteins to refold correctly and adopt a more stable configuration to pass the scrutiny of the cell's quality control system, thereby correcting misrouting. This review covers the most important aspects that regulate folding and traffic of newly synthesized proteins, as well as the experimental approaches targeted to overcome protein misfolding, with special focus on GPCRs involved in endocrine diseases.

Symmetric activation and modulation of the human calcium-sensing receptor

The human extracellular calcium-sensing (CaS) receptor controls plasma Ca²⁺ levels and contributes to nutrient-dependent maintenance and metabolism of diverse organs. Allosteric modulation of the CaS receptor corrects disorders of calcium homeostasis. Here, we report the cryogenic-electron microscopy reconstructions of a near-full-length CaS receptor in the absence and presence of allosteric modulators. Activation of the homodimeric CaS receptor requires a break in the transmembrane 6 (TM6) helix of each subunit, which facilitates the formation of a TM6-mediated homodimer interface and expansion of homodimer interactions. This transformation in TM6 occurs without a positive allosteric modulator. Two modulators with opposite functional roles bind to overlapping sites within the transmembrane domain through common interactions, acting to stabilize distinct rotamer conformations of key residues on the TM6 helix. The positive modulator reinforces TM6 distortion and maximizes subunit contact to enhance receptor activity, while the negative modulator strengthens an intact TM6 to dampen receptor function. In both active and inactive states, the receptor displays symmetrical transmembrane conformations that are consistent with its homodimeric assembly.