Single chain human chorionic gonadotropin, hCGalphabeta: effects of mutations in the alpha subunit on structure and bioactivity

Hormonal and Allosteric Regulation of the Luteinizing Hormone/Chorionic Gonadotropin Receptor

Luteinizing hormone (LH) and human chorionic gonadotropin (CG), like follicle-stimulating hormone, are the most important regulators of the reproductive system. They exert their effect on the cell through the LH/CG receptor (LHCGR), which belongs to the family of G protein-coupled receptors. Binding to gonadotropin induces the interaction of LHCGR with various types of heterotrimeric G proteins (Gs, Gq/11, Gi) and β-arrestins, which leads to stimulation (Gs) or inhibition (Gi) of cyclic adenosine monophosphate-dependent cascades, activation of the phospholipase pathway (Gq/11), and also to the formation of signalosomes that mediate the stimulation of mitogen-activated protein kinases (β-arrestins). The efficiency and selectivity of activation of intracellular cascades by different gonadotropins varies, which is due to differences in their interaction with the ligand-binding site of LHCGR. Gonadotropin signaling largely depends on the status of N- and O-glycosylation of LH and CG, on the formation of homo- and heterodimeric receptor complexes, on the cell-specific microenvironment of LHCGR and the presence of autoantibodies to it, and allosteric mechanisms are important in the implementation of these influences, which is due to the multiplicity of allosteric sites in different loci of the LHCGR. The development of low-molecular-weight allosteric regulators of LHCGR with different profiles of pharmacological activity, which can be used in medicine for the correction of reproductive disorders and in assisted reproductive technologies, is promising. These and other issues regarding the hormonal and allosteric regulation of LHCGR are summarized and discussed in this review.

Dissecting the structural and functional features of the Luteinizing hormone receptor using receptor specific single chain fragment variables

The Luteinizing hormone receptor (LHR) has a large extracellular domain (amino acid residues, a.a.1-355) and a transmembrane domain (TMD; a.a. 356-699), essential for hormone binding and signaling, respectively. The LHR hinge region (a.a. 256-355) connects the two domains and acts as an activating switch for the receptor by an unknown mechanism. LHR hinge-specific Single chain fragment variables (ScFv) stimulated cAMP production by the stable and transiently transfected cell lines expressing LHR in a hormone-independent manner and the C-terminal region of LHR hinge (a.a. 313-349) was identified as the probable epitope for one agonistic ScFv. This epitope attained a helical conformation upon agonistic ScFv binding and the activity of the ScFv was dependent on Y331 sulfation. ScFv was also able to activate TMD mutants, D578Y and A593P, reemphasizing the role of TM helix VI in LHR activation.

New Frontier in Glycoprotein Hormones and Their Receptors Structure-Function

Last two decades of structure-function studies performed in numerous laboratories provided substantial progress in understanding basic science, physiological, pathophysiological, pharmacological, and comparative aspects of glycoprotein hormones (GPHs) and their cognate receptors. Multiple concepts and models developed based on experimental data in the past stood the test of time and have been, at least in part, confirmed and/or remained compatible with the new structures resolved at the atomic level. Major advances in understanding of the ligand-receptor relationships are heralding the dawn of a new era for GPHs and their receptors, although many basic questions still remain unanswered. This article examines retrospectively several basic science aspects of GPH super-agonists and related "biosuperiors" in a broader context of the advances in the ligand-receptor structure-function relationships and new mechanistic models generated based on the structure elucidation. Due to selective focus of my comments and perspectives in certain parts, the reader is directed to the most relevant publications and reviews in the field for more comprehensive analyses.

Membrane tethered bursicon constructs as heterodimeric modulators of the Drosophila G protein-coupled receptor rickets

The study of complex heterodimeric peptide ligands has been hampered by a paucity of pharmacological tools. To facilitate such investigations, we have explored the utility of membrane tethered ligands (MTLs). Feasibility of this recombinant approach was explored with a focus on Drosophila bursicon, a heterodimeric cystine-knot protein that activates the G protein-coupled receptor rickets (rk). Rk/bursicon signaling is an evolutionarily conserved pathway in insects required for wing expansion, cuticle hardening, and melanization during development. We initially engineered two distinct MTL constructs, each composed of a type II transmembrane domain, a peptide linker, and a C terminal extracellular ligand that corresponded to either the α or β bursicon subunit. Coexpression of the two complementary bursicon MTLs triggered rk-mediated signaling in vitro. We were then able to generate functionally active bursicon MTLs in which the two subunits were fused into a single heterodimeric peptide, oriented as either α-β or β-α. Carboxy-terminal deletion of 32 amino acids in the β-α MTL construct resulted in loss of agonist activity. Coexpression of this construct with rk inhibited receptor-mediated signaling by soluble bursicon. We have thus generated membrane-anchored bursicon constructs that can activate or inhibit rk signaling. These probes can be used in future studies to explore the tissue and/or developmental stage-dependent effects of bursicon in the genetically tractable Drosophila model organism. In addition, our success in generating functionally diverse bursicon MTLs offers promise that such technology can be broadly applied to other complex ligands, including the family of mammalian cystine-knot proteins.

Thyrotropin in teleost fish

Thyrotropin (TSH), a pituitary glycoprotein hormone that stimulates the thyroid gland, has been cloned and sequenced from over a dozen teleost fish species. Although TSH is established as a primary driver of systemic thyroid status in mammals, its importance in the regulation of fish thyroid function is still uncertain. We review recent studies indicating that TSH structure is highly conserved across species representing six teleost families. These studies have found TSH messenger RNA consistently expressed in teleost pituitary tissue, although ectopic expression, particularly in gonads, has also been observed. They have also provided evidence for negative feedback inhibition of TSH expression by thyroid hormones, as well as stimulation by hypothalamic peptides. Descriptive studies have found increased TSHbeta expression associated with life history events thought to be promoted by thyroid hormones. These results, coupled with the discovery of a G-protein coupled TSH receptor in several teleost species, supports an active and conserved role for TSH in the regulation of teleost thyroid function. The relative importance of central pathways in regulating thyroid hormone provision to targets and the identity of a proposed thyrotropin-inhibiting factor in teleost fish are still unanswered questions whose resolution will be facilitated by development of methods to measure circulating TSH and its secretion from the pituitary gland.

Translational fusion of two beta-subunits of human chorionic gonadotropin results in production of a novel antagonist of the hormone

The strategy of translationally fusing the alpha- and beta-subunits of human chorionic gonadotropin (hCG) into a single-chain molecule has been used to produce novel analogs of hCG. Previously we reported expression of a biologically active single-chain analog hCGalphabeta expressed using Pichia expression system. Using the same expression system, another analog, in which the alpha-subunit was replaced with the second beta-subunit, was expressed (hCGbetabeta) and purified. hCGbetabeta could bind to LH receptor with an affinity three times lower than that of hCG but failed to elicit any response. However, it could inhibit response to the hormone in vitro in a dose-dependent manner. Furthermore, it inhibited response to hCG in vivo indicating the antagonistic nature of the analog. However, it was unable to inhibit human FSH binding or response to human FSH, indicating the specificity of the effect. Characterization of hCGalphabeta and hCGbetabeta using immunological tools showed alterations in the conformation of some of the epitopes, whereas others were unaltered. Unlike hCG, hCGbetabeta interacts with two LH receptor molecules. These studies demonstrate that the presence of the second beta-subunit in the single-chain molecule generated a structure that can be recognized by the receptor. However, due to the absence of alpha-subunit, the molecule is unable to elicit response. The strategy of fusing two beta-subunits of glycoprotein hormones can be used to produce antagonists of these hormones.

Enzymatic removal of asparagine-linked carbohydrate chains from heterodimer human chorionic gonadotrophin and effect on bioactivity

The native form of human chorionic gonadotropin (hCG) is a heterodimer protein with two asparagine (Asn)-linked carbohydrate chains on each subunit. Removal of the Asn-linked carbohydrate chains from hCG has resulted in hCG variants with consistent antagonistic properties on isolated murine cells. Specific and direct enzymatic removal of these carbohydrate chains from native hCG with resultant antagonistic properties has not been reported. An antagonist to the hCG/luteinising hormone (LH) receptor could be used as an anticancer therapy, emergency contraceptive or for therapeutic resolution of ectopic pregnancies. Therefore, our aim was to use enzymes to specifically remove Asn-linked carbohydrate chains from hCG in the heterodimer form and analyse the resultant bioactivity. Native hCG was treated with endoglycosidases, carbohydrate removal was analysed with electrophoresis and the hCG variants were tested for altered bioactivity with human and murine cells. Endoglycosidases were able to cleave most of the Asn-linked carbohydrate chains from the native hCG. The deglycosylated hCG demonstrated a 75% reduction in bioactivity on a murine Leydig cell line and a 65% reduction in bioactivity on human granulosa cells. These results exemplify a simple and efficient method for creating deglycosylated hCG and provide the most direct evidence for the importance of Asn-linked carbohydrate chains in maintaining hCG bioactivity.