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

Antibody fragments targeting the extracellular domain of follicular stimulating hormone receptor for contraception in male dogs and cats

The increased LH levels resulting from the absence of negative feedback after castration has been linked to long-term health issues. A need exists for an alternative contraceptive agent that functions without interfering the LH pathways. This study aimed to develop antibody fragments against the follicular-stimulating hormone receptor (anti-FSHr) using phage-display technology and evaluate its effects on Sertoli cell functions. Phage clones against the extracellular domain of dog and cat FSHr selected from an antibody fragment phagemid library were analyzed for binding kinetics by surface plasmon resonance. Sertoli cells were isolated from testes of adult animals (five dogs and five cats). Efficacy test was performed by treating Sertoli cell cultures (SCCs) with anti-FSHr antibody fragments compared with untreated in triplicates. Expressions of androgen binding protein (ABP), inhibin subunit beta B (IHBB) and vascular endothelial growth factor A (VEGFA) mRNA in SCCs were quantified by RT-qPCR. The results demonstrated that the molecular weight of the purified dog and cat anti-FSHr antibody fragment was 25 kDa and 15 kDa, respectively. Based on protein molecular weight, the antibody fragment of dogs and cats was therefore, so-called single-chain variable fragments (scFv) and nanobody (nb), respectively. The binding affinity with dissociation constant (KD) was 2.32 × 10-7 M and 2.83 × 10-9 M for dog and cat anti-FSHr antibody fragments, respectively. The cross-binding kinetic interactions between the dog anti-FSHr scFv and the cat ECD of FSHr could not be fitted to the curves to determine the binding kinetics. However, the cross-binding affinity KD between the cat anti-FSHr nb and the dog ECD FSHr was 1.75 × 10-4 M. The mRNA expression of ABP, IHBB and VEGFA in SCCs was less (P < 0.05) in both dogs (12.26, 4.07 and 5.11 folds, respectively) and cats (39.53, 14.07 and 20.29 folds, respectively) treated with anti-FSHr antibody fragments, indicating the Sertoli cell functions were suppressed. In conclusion, this study demonstrated the establishment of species-specific antibody fragments against FSHr in SCCs for dogs and cats. The fragment proteins illustrate potential to be developed as non-surgical contraceptive agent targeting FSHr in companion animals.

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.

Impaired Testicular Function without Altering Testosterone Concentration Using an Anti-Follicular-Stimulating Hormone Receptor (Anti-FSHr) Single-Chain Variable Fragment (scFv) in Long-Tailed Macaques (Macaca fascicularis)

FSHr antibodies have been shown to inhibit the differentiation of spermatogonia to primary spermatocytes, resulting in infertility without a pathological effect on reproductive organs. The aim of this study was to develop single-chain variable fragments (scFvs) against the follicular-stimulating hormone receptor (anti-FSHr) using phage-display technology and to evaluate the effects of intratesticular administration of the anti-FSHr scFv on testicular function and testosterone production. A phage clone against the extracellular domain of FSHr selected from a scFv phagemid library was analyzed for binding kinetics by surface plasmon resonance. Using ultrasound guidance, three adult macaques (M. fascicularis) were administered with 1 mL of 0.4 mg/mL anti-FSHr scFv (treatment) and 1 mL sterile phosphate buffer solution (control) into the left and right rete testis, respectively. Testicular appearance and volume, ejaculate quality, and serum testosterone levels were recorded on day 0 (before injection) and on days 7, 28, and 56 (after injection). Testicular tissue biopsies were performed on day 7 and day 56 to quantify the mRNA expressions of androgen binding protein (ABP), inhibin subunit beta B (IHBB), and vascular endothelial growth factor A (VEGFA). The results demonstrated that the anti-FSHr scFv molecule was calculated as 27 kDa with a dissociation constant (K_D) of 1.03 µM. The volume of the anti-FSHr scFv-injected testicle was reduced on days 28 and 56 compared with day 0 (p < 0.05). Total sperm number was reduced from day 0 (36.4 × 10⁶ cells) to day 56 (1.6 × 10⁶ cells) (p < 0.05). The percentage of sperm motility decreased from day 0 (81.7 ± 1.0%) to day 7 (23.3 ± 1.9%), day 28 (41.7 ± 53.4%), and day 56 (8.3 ± 1.9%) (p < 0.05). Sperm viability on day 0 was 86.8 ± 0.5%, which reduced to 64.2 ± 1.5%, 67.1 ± 2.2%, and 9.3 ± 1.1% on days 7, 28, and 56, respectively (p < 0.05). The expression of ABP and VEGFA on days 7 (14.2- and 3.2-fold) and 56 (5.6- and 5.5-fold) was less in the scFv-treated testicle compared with the controls (p < 0.05). On day 56, the expression of IHBB was less (p < 0.05) in the treated testis (1.3-fold) compared with the controls. Serum testosterone levels were unchanged throughout the study period (p > 0.05). This study characterized the anti-FSHr scFv and demonstrated that treatment with anti-FSHr ameliorates testicular function without altering testosterone levels, offering a potential alternative contraceptive for the long-tailed macaques.

Allosteric modulation of gonadotropin receptors

Gonadotropins regulate reproductive functions by binding to G protein-coupled receptors (FSHR and LHCGR) expressed in the gonads. They activate multiple, cell-specific signalling pathways, consisting of ligand-dependent intracellular events. Signalling cascades may be modulated by synthetic compounds which bind allosteric sites of FSHR and LHCGR or by membrane receptor interactions. Despite the hormone binding to the orthosteric site, allosteric ligands, and receptor heteromerizations may reshape intracellular signalling pattern. These molecules act as positive, negative, or neutral allosteric modulators, as well as non-competitive or inverse agonist ligands, providing a set of new compounds of a different nature and with unique pharmacological characteristics. Gonadotropin receptor allosteric modulation is gathering increasing interest from the scientific community and may be potentially exploited for clinical purposes. This review summarizes the current knowledge on gonadotropin receptor allosteric modulation and their potential, clinical use.

Recent advances in understanding gonadotropin signaling

Gonadotropins are glycoprotein sex hormones regulating development and reproduction and bind to specific G protein–coupled receptors expressed in the gonads. Their effects on multiple signaling cascades and intracellular events have recently been characterized using novel technological and scientific tools. The impact of allosteric modulators on gonadotropin signaling, the role of sugars linked to the hormone backbone, the detection of endosomal compartments supporting signaling modules, and the dissection of different effects mediated by these molecules are areas that have advanced significantly in the last decade. The classic view providing the exclusive activation of the cAMP/protein kinase A (PKA) and the steroidogenic pathway by these hormones has been expanded with the addition of novel signaling cascades as determined by high-resolution imaging techniques. These new findings provided new potential therapeutic applications. Despite these improvements, unanswered issues of gonadotropin physiology, such as the intrinsic pro-apoptotic potential to these hormones, the existence of receptors assembled as heteromers, and their expression in extragonadal tissues, remain to be studied. Elucidating these issues is a challenge for future research.

Biased Signaling and Allosteric Modulation at the FSHR

Knowledge on G protein-coupled receptor (GPCRs) structure and mechanism of activation has profoundly evolved over the past years. The way drugs targeting this family of receptors are discovered and used has also changed. Ligands appear to bind a growing number of GPCRs in a competitive or allosteric manner to elicit balanced signaling or biased signaling (i.e., differential efficacy in activating or inhibiting selective signaling pathway(s) compared to the reference ligand). These novel concepts and developments transform our understanding of the follicle-stimulating hormone (FSH) receptor (FSHR) biology and the way it could be pharmacologically modulated in the future. The FSHR is expressed in somatic cells of the gonads and plays a major role in reproduction. When compared to classical GPCRs, the FSHR exhibits intrinsic peculiarities, such as a very large NH2-terminal extracellular domain that binds a naturally heterogeneous, large heterodimeric glycoprotein, namely FSH. Once activated, the FSHR couples to Gαs and, in some instances, to other Gα subunits. G protein-coupled receptor kinases and β-arrestins are also recruited to this receptor and account for its desensitization, trafficking, and intracellular signaling. Different classes of pharmacological tools capable of biasing FSHR signaling have been reported and open promising prospects both in basic research and for therapeutic applications. Here we provide an updated review of the most salient peculiarities of FSHR signaling and its selective modulation.