The Nematode Leucine-Rich Repeat-Containing, G Protein-Coupled Receptor (LGR) Protein Homologous to Vertebrate Gonadotropin and Thyrotropin Receptors is Constitutively Activated in Mammalian Cells

Abstract

The receptors for LH, FSH, and TSH belong to the large G protein-coupled, seven-transmembrane protein family and are unique in having a large N-terminal extracellular (ecto-) domain containing leucine-rich repeats important for interactions with the large glycoprotein hormone ligands. Recent studies indicated the evolution of an expanding family of homologous leucine-rich repeat-containing, G protein-coupled receptors (LGRs), including the three known glycoprotein hormone receptors; mammalian LGR4 and LGR5; and LGRs in sea anemone, fly, and snail. We isolated nematode LGR cDNA and characterized its gene from the Caenorhabditis elegans genome. This receptor cDNA encodes 929 amino acids consisting of a signal peptide for membrane insertion, an ectodomain with nine leucine-rich repeats, a seven-TM region, and a long C-terminal tail. The nematode LGR has five potential N-linked glycosylation sites in its ectodomain and multiple consensus phosphorylation sites for protein kinase A and C in the cytoplasmic loop and C tail. The nematode receptor gene has 13 exons; its TM region and C tail, unlike mammalian glycoprotein hormone receptors, are encoded by multiple exons. Sequence alignments showed that the TM region of the nematode receptor has 30% identity and 50% similarity to the same region in mammalian glycoprotein hormone receptors. Although human 293T cells expressing the nematode LGR protein do not respond to human glycoprotein hormones, these cells exhibited major increases in basal cAMP production in the absence of ligand stimulation, reaching levels comparable to those in cells expressing a constitutively activated mutant human LH receptor found in patients with familial male-limited precocious puberty. Analysis of cAMP production mediated by chimeric receptors further indicated that the ectodomain and TM region of the nematode LGR and human LH receptor are interchangeable and the TM region of the nematode LGR is responsible for constitutive receptor activation. Thus, the identification and characterization of the nematode receptor provides the basis for understanding the evolutionary relationship of diverse LGRs and for future analysis of mechanisms underlying the activation of glycoprotein hormone receptors and related LGRs.

Adipokinetic hormone signaling through the gonadotropin-releasing hormone receptor modulates egg-laying in Caenorhabditis elegans

“…Although expression of Ce-GnRHR in the gonads favors a direct effect of Ce-AKH-GnRH on egg laying, Ce-AKH-GnRH may have an indirect effect by modulating the release of gonadotropins, analogous to the hypothalamic-pituitary-ovary axis in vertebrates. A leucine-rich GPCR (LGR), called FSHR-1 (follicle-stimulating hormone receptor 1), with sequence similarity to vertebrate pituitary gonadotropin receptors (32) and estrogen binding proteins (33), have been reported in C. elegans. The FSHR has a role in the control of germ-cell differentiation, proliferation, and worm survival (34).…”

Evolution of GPCR: Change and continuity

“…Within rhodopsin-like receptors, a phylogenetically old subfamily can be identified that according to its prominent feature is referred to as leucine-rich repeat-containing GPCR. Members of this group can already be identified in molluscs, nematodes and cnidaria [57][58][59] and include glycoprotein and relaxin receptors. They share a long N terminus that contains leucine-rich repeats flanked by cysteine-rich sequences [60].…”

Lgr4 Promotes Glioma Cell Proliferation through Activation of Wnt Signaling

“…GPR48, also known as leucine-rich repeat (LRR)-containing G protein-coupled receptor 4 (LGR4), is a member of the G protein coupled receptor (GPCR) family of proteins (Kudo et al, 2000;Loh et al, 2001). The GPCR family containing seven transmembrane domains function as the receptors for various classes of ligands, such as growth factors, chemokines and peptide hormones, and have become major targets for pharmaceutical development (Lagerström et al, 2008;Lappano et al, 2011;Stevens et al, 2013).…”