Crystal structure of LGR4-Rspo1 complex: insights into the divergent mechanisms of ligand recognition by leucine-rich repeat G-protein-coupled receptors (LGRs)

Evolutionary origin and distribution of leucine-rich repeat-containing G protein-coupled receptors in insects

Leucine-rich repeat-containing G protein-coupled receptors (LGRs) are crucial for animal growth and development. They were categorized into four types (A, B, C1, and C2) based on their sequence and domain structures. Despite the widespread distribution of LGRs across bilaterians, a thorough investigation of their distribution and evolutionary history remains elusive. Recent studies insect LGRs, especially the emergence of type C2 LGRs in various hemimetabolous insects, had prompted our study to address these problems. Initially, we traced the origins of LGRs by exploiting data from 99 species spanning 11 metazoan phyla, and discovered that type A and B LGRs originated from sponges, while type C LGRs originated from cnidarians. Subsequently, through comprehensive genomic and transcriptomic analyses across 565 species across 25 orders of insects, we found that both type A and C1 LGRs divided into two gene clusters. These clusters can be traced back to basal Insecta and an early ancestor of the Arthropoda, respectively. Furthermore, the absence of type B LGRs in wingless insects suggests a role in wing development, while the absence of type C2 LGRs in holometabolous insects hints at novel functions unrelated to insect metamorphosis. According to the origin of LGRs and the investigation of LGRs in insects, we speculated that type A and B LGRs appeared first among four types of LGRs, type A evolved into type C LGRs later, and type A and C1 LGRs independently duplicated during the evolutionary process. This study provides a more comprehensive view of the evolution of LGR genes than previously available, and sheds light on the evolutionary history and significance of LGRs in insect biology.

RSPO2 as Wnt signaling enabler: Important roles in cancer development and therapeutic opportunities

R-spondins are secretory proteins localized in the endoplasmic reticulum and Golgi bodies and are processed through the secretory pathway. Among the R-spondin family, RSPO2 has emanated as a novel regulator of Wnt signaling, which has now been acknowledged in numerous in vitro and in vivo studies. Cancer is an abnormal growth of cells that proliferates and spreads uncontrollably due to the accumulation of genetic and epigenetic factors that constitutively activate Wnt signaling in various types of cancer. Colorectal cancer (CRC) begins when cells in the colon and rectum follow an indefinite pattern of division due to aberrant Wnt activation as one of the key hallmarks. Decades-long progress in research on R-spondins has demonstrated their oncogenic function in distinct cancer types, particularly CRC. As a critical regulator of the Wnt pathway, it modulates several phenotypes of cells, such as cell proliferation, invasion, migration, and cancer stem cell properties. Recently, RSPO mutations, gene rearrangements, fusions, copy number alterations, and altered gene expression have also been identified in a variety of cancers, including CRC. In this review, we addressed the recent updates regarding the recurrently altered R-spondins with special emphasis on the RSPO2 gene and its involvement in potentiating Wnt signaling in CRC. In addition to the compelling physiological and biological roles in cellular fate and regulation, we propose that RSPO2 would be valuable as a potential biomarker for prognostic, diagnostic, and therapeutic use in CRC.

Anti-Pan-Rspo Chimeric Protein-Conjugated Albumin Nanoparticle Provides Promising Opportunities in Cancer Targeted Therapy

Rspos (R-spondins) belong to a family of secreted proteins that causes various cancers via interacting the corresponding receptors. However, targeted therapeutic approaches against Rspos are largely lacking. In this study, a chimeric protein Rspo-targeting anticancer chimeric protein (RTAC) is originally designed, engineered, and characterized. RTAC shows satisfactory anticancer effects through inhibition of pan-Rspo-mediated Wnt/β-catenin signaling activation both in vitro and in vivo. Furthermore, a conceptually novel antitumor strategy distinct from traditional drug delivery systems that release drugs inside tumor cells is proposed. A special "firewall" nano-system is designed to enrich on tumor cell surface and cover the plasma membrane, rather than undergoing endocytosis, to block oncogenic Rspos from binding to receptors. Cyclic RGD (Arg-Gly-Asp) peptide-linked globular cluster serum albumin nanoparticles (SANP) are integrated as a vehicle for conjugating RTAC (SANP-RTAC/RGD) for tumor tissue targeting. These nanoparticles can adhere to the tumor cell surface and enable RTAC to locally capture free Rspos with high spatial efficiency and selectivity to antagonize cancer progression. Therefore, this approach offers a new nanomedical anticancer route and obtains the "dual-targeting" capability for effective tumor clearance and low potential toxicity. This study presents a proof-of-concept for anti-pan-Rspo therapy and a nanoparticle-integrated paradigm for targeted cancer treatment.

Review: The evolution of peptidergic signaling in Cnidaria and Placozoa, including a comparison with Bilateria

Bilateria have bilateral symmetry and are subdivided into Deuterostomia (animals like vertebrates) and Protostomia (animals like insects and mollusks). Neuropeptides occur in both Proto- and Deuterostomia and they are frequently structurally related across these two lineages. For example, peptides belonging to the oxytocin/vasopressin family exist in both clades. The same is true for the G protein-coupled receptors (GPCRs) of these peptides. These observations suggest that these neuropeptides and their GPCRs were already present in the common ancestor of Proto- and Deuterostomia, which lived about 700 million years ago (MYA). Furthermore, neuropeptides and their GPCRs occur in two early-branching phyla that diverged before the emergence of Bilateria: Cnidaria (animals like corals and sea anemones), and Placozoa (small disk-like animals, feeding on algae). The sequences of these neuropeptides and their GPCRs, however, are not closely related to those from Bilateria. In addition, cnidarian neuropeptides and their receptors are not closely related to those from Placozoa. We propose that the divergence times between Cnidaria, Placozoa, and Bilateria might be too long for recognizing sequence identities. Leucine-rich repeats-containing GPCRs (LGRs) are a special class of GPCRs that are characterized by a long N-terminus containing 10-20 leucine-rich domains, which are used for ligand binding. Among the ligands for LGRs are dimeric glycoprotein hormones, and insulin-like peptides, such as relaxin. LGRs have been found not only in Proto- and Deuterostomia, but also in early emerging phyla, such as Cnidaria and Placozoa. Humans have eight LGRs. In our current review, we have revisited the annotations of LGRs from the sea anemone Nematostella vectensis and the placozoan Trichoplax adhaerens. We identified 13 sea anemone LGRs and no less than 46 LGRs from T. adhaerens. All eight human LGRs appear to have orthologues in sea anemones and placozoans. LGRs and their ligands, therefore, have a long evolutionary history, going back to the common ancestor of Cnidaria and Placozoa.

LGR4, a G Protein-Coupled Receptor With a Systemic Role: From Development to Metabolic Regulation

Leucine-rich repeat-containing G protein-coupled receptor 4 (LGR4/GPR48), a member of the GPCR (G protein-coupled receptors) superfamily, subfamily B, is a common intestinal crypt stem cell marker. It binds R-spondins/Norrin as classical ligands and plays a crucial role in Wnt signaling potentiation. Interaction between LGR4 and R-spondins initiates many Wnt-driven developmental processes, e.g., kidney, eye, or reproductive tract formation, as well as intestinal crypt (Paneth) stem cell pool maintenance. Besides the well-described role of LGR4 in development, several novel functions of this receptor have recently been discovered. In this context, LGR4 was indicated to participate in TGFβ and NFκB signaling regulation in hematopoietic precursors and intestinal cells, respectively, and found to be a new, alternative receptor for RANKL (Receptor Activator of NF kappa B Ligand) in bone cells. LGR4 inhibits the process of osteoclast differentiation, by antagonizing the interaction between RANK (Receptor Activator of NF kappa B) and its ligand-RANKL. It is also known to trigger anti-inflammatory responses in different tissues (liver, intestine, cardiac cells, and skin), serve as a sensor of the circadian clock in the liver, regulate adipogenesis and energy expenditure in adipose tissue and skeletal muscles, respectively. The extracellular domain of LGR4 (LGR4-ECD) has emerged as a potential new therapeutic for osteoporosis and cancer. LGR4 integrates different signaling pathways and regulates various cellular processes vital for maintaining whole-body homeostasis. Yet, the role of LGR4 in many cell types (e.g. pancreatic beta cells) and diseases (e.g., diabetes) remains to be elucidated. Considering the broad spectrum of LGR4 actions, this review aims to discuss both canonical and novel roles of LGR4, with emphasis on emerging research directions focused on this receptor.

LGR5 enhances the osteoblastic differentiation of MC3T3-E1 cells through the Wnt/β-catenin pathway

Leucine-rich repeat-containing G-protein coupled receptor 5 (LGR5) is a Wnt-associated gene that contributes to cell proliferation and self-renewal in various organs. LGR5 is expressed in Ewing sarcoma, and LGR5-overexpressing mesenchymal stem cells promote fracture healing. However, the effects of LGR5 on osteoblastic differentiation remain unclear. The aim of the present study was to explore the function of LGR5 in osteoblastic differentiation. LGR5 was overexpressed or knocked down in the MC3T3-E1 pre-osteoblastic cell line via lentiviral transfection and its function in osteoblastic differentiation was investigated. The mRNA expression levels of the osteoblast differentiation markers alkaline phosphatase (ALP), osteocalcin and collagen type I a1 were determined, and ALP and Alizarin red staining were performed. In addition, the effects of LGR5 modulation on β-catenin and the expression of target genes in the Wnt pathway were investigated. The results revealed that the overexpression of LGR5 promoted osteoblastic differentiation. This was associated with enhancement of the stability of β-catenin and its levels in the cell nucleus, which enabled it to activate Wnt signaling. By contrast, the inhibition of LGR5 decreased the osteogenic capacity of MC3T3-E1 cells. These results indicate that LGR5 is a positive regulator of osteoblastic differentiation, whose effects are mediated through the Wnt/β-catenin signaling pathway. This suggests suggesting that the regulation of LGR5/Wnt/β-catenin signaling has potential as a therapy for osteoporosis.

Unlike LGR4, LGR5 potentiates Wnt-β-catenin signaling without sequestering E3 ligases

LGR4 and LGR5 encode two homologous receptors with critical, yet distinct, roles in organ development and adult stem cell survival. Both receptors are coexpressed in intestinal crypt stem cells, bind to R-spondins (RSPOs) with high affinity, and potentiate Wnt-β-catenin signaling, presumably by the same mechanism: forming RSPO-bridged complexes with the E3 ligases RNF43 and ZNRF3 to inhibit ubiquitylation of Wnt receptors. However, direct evidence for RSPO-bound, full-length LGR5 interacting with these E3 ligases in whole cells has not been reported, and only LGR4 is essential for the self-renewal of intestinal stem cells. Here, we examined the mechanisms of action of LGR4 and LGR5 in parallel using coimmunoprecipitation, proximity ligation, competition binding, and time-resolved FRET assays in whole cells. Full-length LGR4 formed a tight complex with ZNRF3 and RNF43 even without RSPO, whereas LGR5 did not interact with either E3 ligase with or without RSPO. Domain-swapping experiments with LGR4 and LGR5 revealed that the seven-transmembrane domain of LGR4 conferred interaction with the E3 ligases. Native LGR4 and LGR5 existed as dimers on the cell surface, and LGR5 interacted with both FZD and LRP6 of the Wnt signalosome to enhance LRP6 phosphorylation and potentiate Wnt-β-catenin signaling. These findings provide a molecular basis for the weaker activity of LGR5 in the potentiation of Wnt signaling that may underlie the distinct roles of LGR4 and LGR5 in organ development, as well as the self-renewal and fitness of adult stem cells.

Ablation of LGR4 signaling enhances radiation sensitivity of prostate cancer cells

AIM

Our previous study has shown that leucine-rich repeat containing GPCR-4 (LGR4, or GPR48) LGR4 plays a role in cell migration, invasion, proliferation and apoptosis of prostate cancer (PCa). In this study, we aimed to explore whether LGR4 would affect radiation response in PCa.

MATERIALS AND METHODS

LGR4 expression was silenced by shRNA transfection. qRT-PCR was employed to determine mRNA expression of LGR4 and DNA damage repair genes. Western blot was used to evaluate protein expression of LGR4, RSPO1-4, androgen receptor (AR), cyclic AMP response-element binding protein (CREB1), γH2A.X, and H2A.X. Cell proliferation was detected by CCK-8 assay and apoptosis was assayed by flow cytometry. Additionally, a xenograft model was also established to validate the role of LGR4 in PCa cells after radiation.

KEY FINDINGS

LGR4 expression was enhanced in PCa cells by radiation treatment in dose- and time-dependent means. RSPO1-4 were also upregulated post-radiation. Furthermore, LGR4 knockdown exacerbated apoptosis, reduced cell viabilities and strengthened nuclear γH2A.X staining in AR positive PCa cells but not in AR negative cells in the presence of radiation. Likewise, LGR4 ablation diminished AR and CREB1 expression induced by radiation. In contrast, RSPO1 stimulation augmented cell viabilities, promoted AR and CREB1 expression, and upregulated DNA repair gene expression, which could be reversed by enzalutamide, except for AR expression. Additionally, LGR4 knockdown further suppressed tumor growth and AR/CREB1 expression but enhanced γH2A.X expression in xenografts.

SIGNIFICANCE

In all, our study suggested that LGR4 might serve as an important regulator of radiation sensitivity in PCa.

LGR5 constitutively activates NF-κB signaling to regulate the growth of intestinal crypts

Mammalian LGR5 and LGR4, markers of adult stem cells, are involved in many physiological functions by enhancing WNT signaling. However, whether LGR5 and LGR4 are coupled to other intracellular signaling pathways to regulate stem cell function remains unknown. Here, we show that LGR5 and LGR4 can constitutively activate NF-κB signaling in a ligand-independent manner, which is dependent on their C-termini, but independent of receptor endocytosis. Moreover, the C-termini of LGR5/4 interact with TROY, which is required for activating NF-κB signaling. In small intestinal crypt organoids, overexpression of a C-terminal deletion mutant of LGR5 inhibits the growth and bud formation of organoids, whereas overexpression of the R-spondin-binding mutant of LGR5 that is defective for WNT signaling can still promote organoid growth. Our study reveals that NF-κB signaling, regulated by LGR5 and LGR4, plays an important role in the survival of colon cancer cells and the growth of intestinal crypts. Our findings also suggest that LGR5/4-induced NF-κB signaling and WNT signaling may co-regulate the growth of LGR5⁺ adult stem cells and intestinal crypts.

LGR6 is a potential diagnostic and prognostic marker for esophageal squamous cell carcinoma

BACKGROUND

Leucine-rich repeat-coupled receptor 6 (LGR6) is a marker of the skin, nails, and other types of adult tissue stem cells and has been widely found to be related to the development and progression of a variety of cancer types. The clinical significance and biological function of LGR6 in esophageal squamous cell carcinoma (ESCC) have not been determined.

METHODS

The expression of LGR6 at the transcriptional level was analyzed by searching the TCGA and UCSC data sets. Immunohistochemistry, WB, and q-PCR were used to detect the expression of LGR6 in ESCC and adjacent normal tissues. LGR6 PPI networks and KEGG pathways were used to analyze the potential biological functions of LGR6.

RESULTS

The expression of LGR6 in ESCC tissues was significantly higher than that in normal tissues and was negatively correlated with the differentiation degree of ESCC and the prognosis of the patients but not closely correlated with the TNM stage of ESCC. PPI networks showed that LGR6 had a close interaction with RSPO1, RSPO2, RSPO3, and RSPO4. KEGG pathway analysis showed that LGR6 activated the Wnt/β-catenin signaling pathway by binding with RSPO ligands to promote the progression of ESCC.

CONCLUSION

LGR6 can serve as a potential diagnostic and prognostic marker for ESCC.

The variable lymphocyte receptor as an antibody alternative

Variable lymphocyte receptors (VLRs) are leucine-rich repeat proteins in jawless vertebrates that function similarly to Ig antibodies. However, VLRs possess a distinct crescent-shaped structure and modularity that results in a concave binding interface that contrasts significantly with Ig antibodies. Antigen binding interactions result in specific, high affinity VLR binding interactions with both proteins and glycans. The natural sourcing of VLRs allows for immunization strategies, while the modularity enables a whole host of protein engineering approaches including consensus scaffolds, designed libraries and directed evolution with display technologies. VLR technologies have been recently deployed for applications in cell-specific targeting, drug delivery, tumor diagnostics and even protein stabilization. It is anticipated that the VLR field will continue to emerge to provide unique solutions for targeting glycans, evolutionarily conserved proteins and cellular specificity.

Differential activities and mechanisms of the four R-spondins in potentiating Wnt/β-catenin signaling

The four R-spondins (RSPO1-4) strongly potentiate Wnt signaling and play critical roles in normal development, adult stem cell survival, and cancer development and aggressiveness. All four RSPOs have been suggested to potentiate Wnt signaling by binding to three related receptors, i.e. leucine-rich repeat-containing, G protein-coupled receptors 4, 5, and 6 (LGR4/5/6), and then inducing the clearance of two E3 ubiquitin ligases (RNF43 and ZNRF3) that otherwise would ubiquitinate Wnt receptors for degradation. Here, we show that RSPO1-4 have differential dependence on LGRs in potentiating Wnt/β-catenin signaling and that RSPO2 can enhance this pathway without any LGR. LGR4 knockout (LGR4KO) in HEK293 cells completely abrogated the Wnt/β-catenin signaling response to RSPO1 and RSPO4 and strongly impaired the response to RSPO3. RSPO2, however, retained robust activity albeit with decreased potency. Complete rescue of RSPO1-4 activity in LGR4KO cells required the seven-transmembrane domain of LGR4. Furthermore, an RSPO2 mutant with normal binding affinity to ZNRF3 but no or little binding to LGR4 or LGR5 still potentiated Wnt/β-catenin signaling in vitro, supported the growth of intestinal organoids ex vivo, and stimulated intestinal crypt growth in vivo Mechanistically, RSPO2 could increase Wnt receptor levels in the absence of any LGR without affecting ZNRF3 endocytosis and stability. These findings suggest that RSPO1-4 use distinct mechanisms in regulating Wnt and other signaling pathways, which have important implications for understanding the pleiotropic functions of RSPOs and LGRs in both normal and cancer development.

R-spondin3-LGR4 signaling protects hepatocytes against DMOG-induced hypoxia/reoxygenation injury through activating β-catenin

BACKGROUND & AIMS

Leucine-rich repeat G-protein-coupled receptor 4 (LGR4) and its ligands R-spondin1-4 (Rspos) have been vastly investigated in embryonic development. The biological functions of Rspos-LGR4 system in liver remains largely unknown. Here, we explored whether it protects hepatocytes against hypoxia/reoxygenation (H/R) induced damage.

METHODS

H/R injury was induced by dimethyloxalylglycine (DMOG) in AML12 cells and the effects of Rspo3 on cell proliferation and apoptosis were assessed. Specific shRNAs were used to interfere LGR4 or β-catenin.

RESULTS

DMOG caused hepatocytes damage evidenced by increase in HIF-1α, cell death and apoptosis genes p27 and Bax, with concurrent decrease of cell proliferation genes PCNA and CyclinD1. Of all the Rspos, Rspo3 is predominantly expressed in AML12 hepatocytes. Importantly, Rspo3 demonstrated an alteration in a manner similar to proliferation-related genes during H/R injury. Rspo3 pretreatment rendered hepatocytes less vulnerable to DMOG induced H/R injury. Ablation of LGR4 using shRNA attenuated the protective effects of Rspo3. Wnt3a also protected AML12 cells from damages caused by H/R, showing enhanced proliferation activity. Notably, knockdown of β-catenin in hepatocytes completely abolished the effect of Rspo3 pretreatment on the expression levels of PCNA and CyclinD1.

CONCLUSION

Rspo3-LGR4 axis protects hepatocytes from H/R injury via activating β-catenin.

RNA-seq analysis of Lgr6+ stem cells and identification of an Lgr6 isoform

We studied Lgr6⁺ stem cells in experimental UV carcinogenesis in hairless mice. For further characterization through RNA-seq, these stem cells were isolated by FACS from transgenic hairless mice bearing an EGFP-Ires-CreERT2 reporter cassette inserted into exon 1 of the Lgr6 gene (purity confirmed by human ERT2 expression). Between Lgr6/EGFP⁺ and Lgr6/EGFP^- basal cells (Tg/wt), 682 RNAs were differentially expressed, indicating stemness and expression of cancer-related pathways in Lgr6/EGFP⁺ cells. We discovered that suspected "Lgr6 null" mice (Tg/Tg) expressed RNA of an Lgr6 isoform (delta-Lgr6, lacking 74 N-terminal aa) which could be functional and explain the lack of a phenotype.

Reactivity to Novel Autoantigens in Patients with Coexisting Central Nervous System Demyelinating Disease and Autoimmune Thyroid Disease

Several lines of evidence suggest a definite and unique link between CNS demyelinating diseases and autoimmune thyroid disease (AITD). The aim of the current study was to systematically compare the clinical and laboratory features of patients with coexistent AITD and CNS demyelinating disease with those of patients with just CNS demyelinating disease. Forty-four patients with coexisting CNS demyelinating disease and AITD were identified and their clinical and radiological features were recorded. Blood and DNA were collected and tested for HLA type and for the response of T cells and antibodies to a variety of antigens. Patients with multiple sclerosis (MS) without AITD and healthy individuals were included as controls. Patients with coexisting AITD and CNS demyelinating disease were almost exclusively female (43/44) and had prominent spinal cord involvement as the main neurological finding. The HLA molecules carried by individuals with CNS demyelinating disease and AITD differed from both other MS patients and healthy individuals. Furthermore, patients with both CNS disease and AITD showed less T cell reactivity than patients with MS alone to myelin proteolipid protein, but, compared to other groups, showed elevated levels of T cell reactivity to the calcitonin gene-related peptide, which is present in both the CNS and the thyroid, and elevated levels of T cell and antibody to the leucine-rich repeat-containing G-protein coupled receptor 4 (LGR4), a molecule that is expressed in the brainstem and spinal cord, and which is a homolog of the thyroid-stimulating hormone receptor. We suggest that reactivity of autoreactive immune cells in these patients against antigens present in both the thyroid and the spinal cord is a potential mechanism underlying the pattern of lesion development in the CNS in patients with coexisting AITD and MS and might indicate a novel mechanism of disease pathogenesis in these patients.

Generation and characterization of monoclonal antibodies against human LGR6

Leucine-rich repeat-containing G protein-coupled receptor 6 (LGR6) is a seven-pass transmembrane protein known to be a marker of stem cells in several organs. To deepen our understanding of the cell biology of LGR6-positive cells, including stem cells, we generated monoclonal antibodies (mAbs) against human LGR6. DNA immunization followed by whole-cell immunization with LGR6-expressing transfectants was performed to obtain mAbs that recognized the native form of LGR6. Hybridomas were screened by flow cytometry using LGR6-transfected cells. Because the molecules of LGR4, LGR5, and LGR6 are 50% homologous at the amino acid level, specificity of the mAbs was confirmed by transfectants expressing LGR4, LGR5, or LGR6. Three LGR6-specific mAbs were generated. Two of the three mAbs (designated 43A6 and 43D10) recognized the large N-terminal extracellular domain of LGR6, and competitively blocked the binding of R-spondin 1, which is known to be the ligand for LGR6. The other mAb, 43A25, recognized the seven-pass transmembrane domain of LGR6, and was able to be used for immunoblot analysis. In addition, mAbs 43A6 and 43D10 detected endogenous expression of LGR6 in cancer cell lines. We expect that our mAbs will contribute to widening our understanding of LGR6-positive cells in humans.

GLY67ARG substitution in RSPO4 disrupts the WNT signaling pathway due to an abnormal binding pattern with LGRs leading to anonychia

R-Spondins regulate the WNT/β-catenin signaling pathway by interacting with leucine rich-repeat containing G-protein coupled receptors (LGR4–6).

LGR4 and Its Role in Intestinal Protection and Energy Metabolism

Leucine-rich repeat-containing G protein-coupled receptors were identified by the unique nature of their long leucine-rich repeat extracellular domains. Distinct from classical G protein-coupled receptors which act via G proteins, LGR4 functions mainly through Wnt/β-catenin signaling to regulate cell proliferation, differentiation, and adult stem cell homeostasis. LGR4 is widely expressed in tissues ranging from the reproductive system, urinary system, sensory organs, digestive system, and the central nervous system, indicating LGR4 may have multiple functions in development. Here, we focus on the digestive system by reviewing its effects on crypt cells differentiation and stem cells maintenance, which are important for cell regeneration after injury. Through effects on Wnt/β-catenin signaling and cell proliferation, LGR4 and its endogenous ligands, R-spondins, are involved in colon tumorigenesis. LGR4 also contributes to regulation of energy metabolism, including food intake, energy expenditure, and lipid metabolism, as well as pancreatic β-cell proliferation and insulin secretion. This review summarizes the identification of LGR4, its endogenous ligand, ligand-receptor binding and intracellular signaling. Physiological functions include intestinal development and energy metabolism. The potential effects of LGR4 and its ligand in the treatment of inflammatory bowel disease, chemoradiotherapy-induced gut damage, colorectal cancer, and diabetes are also discussed.

Discovery and Development of Small Molecule Allosteric Modulators of Glycoprotein Hormone Receptors

Glycoprotein hormones, follicle-stimulating hormone (FSH), luteinizing hormone (LH), and thyroid-stimulating hormone (TSH) are heterodimeric proteins with a common α-subunit and hormone-specific β-subunit. These hormones are dominant regulators of reproduction and metabolic processes. Receptors for the glycoprotein hormones belong to the family of G protein-coupled receptors. FSH receptor (FSHR) and LH receptor are primarily expressed in somatic cells in ovary and testis to promote egg and sperm production in women and men, respectively. TSH receptor is expressed in thyroid cells and regulates the secretion of T3 and T4. Glycoprotein hormones bind to the large extracellular domain of the receptor and cause a conformational change in the receptor that leads to activation of more than one intracellular signaling pathway. Several small molecules have been described to activate/inhibit glycoprotein hormone receptors through allosteric sites of the receptor. Small molecule allosteric modulators have the potential to be administered orally to patients, thus improving the convenience of treatment. It has been a challenge to develop a small molecule allosteric agonist for glycoprotein hormones that can mimic the agonistic effects of the large natural ligand to activate similar signaling pathways. However, in the past few years, there have been several promising reports describing distinct chemical series with improved potency in preclinical models. In parallel, proposal of new structural model for FSHR and in silico docking studies of small molecule ligands to glycoprotein hormone receptors provide a giant leap on the understanding of the mechanism of action of the natural ligands and new chemical entities on the receptors. This review will focus on the current status of small molecule allosteric modulators of glycoprotein hormone receptors, their effects on common signaling pathways in cells, their utility for clinical application as demonstrated in preclinical models, and use of these molecules as novel tools to dissect the molecular signaling pathways of these receptors.

In a Class of Their Own - RXFP1 and RXFP2 are Unique Members of the LGR Family

The leucine-rich repeat-containing G protein-coupled receptors (LGRs) family consists of three groups: types A, B, and C and all contain a large extracellular domain (ECD) made up of the structural motif - the leucine-rich repeat (LRR). In the LGRs, the ECD binds the hormone or ligand, usually through the LRRs, that ultimately results in activation and signaling. Structures are available for the ECD of type A and B LGRs, but not the type C LGRs. This review discusses the structural features of LRR proteins, and describes the known structures of the type A and B LGRs and predictions that can be made for the type C LGRs. The mechanism of activation of the LGRs is discussed with a focus on the role of the low-density lipoprotein class A (LDLa) module, a unique feature of the type C LGRs. While the LDLa module is essential for activation of the type C LGRs, the molecular mechanism for this process is unknown. Experimental data for the potential interactions of the type C LGR ligands with the LRR domain, the transmembrane domain, and the LDLa module are summarized.