A synthetic peptide corresponding to the third cytoplasmic loop (residues 533 to 555) of the testicular follicle-stimulating hormone receptor affects signal transduction in rat testis membranes and in intact cultured rat Sertoli cells

Structure-Function Relationships of the Follicle-Stimulating Hormone Receptor

The follicle-stimulating hormone receptor (FSHR) plays a crucial role in reproduction. This structurally complex receptor is a member of the G-protein coupled receptor (GPCR) superfamily of membrane receptors. As with the other structurally similar glycoprotein hormone receptors (the thyroid-stimulating hormone and luteinizing hormone-chorionic gonadotropin hormone receptors), the FSHR is characterized by an extensive extracellular domain, where binding to FSH occurs, linked to the signal specificity subdomain or hinge region. This region is involved in ligand-stimulated receptor activation whereas the seven transmembrane domain is associated with receptor activation and transmission of the activation process to the intracellular loops comprised of amino acid sequences, which predicate coupling to effectors, interaction with adapter proteins, and triggering of downstream intracellular signaling. In this review, we describe the most important structural features of the FSHR intimately involved in regulation of FSHR function, including trafficking, dimerization, and oligomerization, ligand binding, agonist-stimulated activation, and signal transduction.

[Novel achievements in development and application of GPCR-peptides]

One of the approaches to creating the regulators of G-protein-coupled receptors (GPCR) is the development of peptides that structurally correspond to the functionally important regions of the intracellular extracellular loops of the receptors. GPCR-peptides can selectively regulate the functional activity of homologous receptor and affect the hormonal signal transduction via the receptor. Among the peptides corresponding to the intracellular regions of GPCR, their derivatives modified with hydrophobic radicals exhibit the highest activity and selectivity of action in vitro and in vivo. Ample evidence demonstrates that lipophilic GPCR-peptides may be used to treat diseases and various abnormalities that depend on the functional activity of receptors homologous to them. In turn, the peptides corresponding to the extracellular regions of GPCR can be used as functional probes for studying the specific interaction between the receptors and their ligands, as well as for studying the etiology and pathogenesis of autoimmune diseases caused by the production of antibodies to GPCR antigenic determinants that are localized in the receptor extracellular loops. The present review focuses on the recent achievements in development and application of GPCR-peptides and on the prospects for their further use in medicine and fundamental biology.

[Prospects for use of peptides and their derivatives, structurally corresponding to the G protein-coupled receptors, in medicine]

The regulation of signaling pathways involved in the control of many physiological functions is carried out via the heterotrimeric G protein-coupled receptors (GPCR). The search of effective and selective regulators of GPCR and intracellular signaling cascades coupled with them is one of the important problems of modern fundamental and clinical medicine. Recently data suggest that synthetic peptides and their derivatives, structurally corresponding to the intracellular and transmembrane regions of GPCR, can interact with high efficiency and selectivity with homologous receptors and influence, thus, the functional activity of intracellular signaling cascades and fundamental cellular processes controlled by them. GPCR-peptides are active in both in vitro and in vivo. They regulate hematopoiesis, angiogenesis and cell proliferation, inhibit tumor growth and metastasis, and prevent the inflammatory diseases and septic shock. These data show greatest prospects in the development of the new generations of drugs based on GPCR-derived peptides, capable of regulating the important functions of the organism.

Role of the Extracellular and Intracellular Loops of Follicle-Stimulating Hormone Receptor in Its Function

Follicle-stimulating hormone receptor (FSHR) is a leucine-rich repeat containing class A G-protein coupled receptor belonging to the subfamily of glycoprotein hormone receptors (GPHRs), which includes luteinizing hormone/choriogonadotropin receptor (LH/CGR) and thyroid-stimulating hormone receptor. Its cognate ligand, follicle-stimulating hormone binds to, and activates FSHR expressed on the surface of granulosa cells of the ovary, in females, and Sertoli cells of the testis, in males, to bring about folliculogenesis and spermatogenesis, respectively. FSHR contains a large extracellular domain (ECD) consisting of leucine-rich repeats at the N-terminal end and a hinge region at the C-terminus that connects the ECD to the membrane spanning transmembrane domain (TMD). The TMD consists of seven α-helices that are connected to each other by means of three extracellular loops (ELs) and three intracellular loops (ILs) and ends in a short-cytoplasmic tail. It is well established that the ECD is the primary hormone binding domain, whereas the TMD is the signal transducing domain. However, several studies on the ELs and ILs employing site directed mutagenesis, generation of chimeric receptors and in vitro characterization of naturally occurring mutations have proven their indispensable role in FSHR function. Their role in every phase of the life cycle of the receptor like post translational modifications, cell surface trafficking, hormone binding, activation of downstream signaling, receptor phosphorylation, hormone-receptor internalization, and recycling of hormone-receptor complex have been documented. Mutations in the loops causing dysregulation of these processes lead to pathophysiological conditions. In other GPHRs as well, the loops have been convincingly shown to contribute to various aspects of receptor function. This review article attempts to summarize the extensive contributions of FSHR loops and C-terminal tail to its function.

Signal protein-derived peptides as functional probes and regulators of intracellular signaling

The functionally important regions of signal proteins participating in their specific interaction and responsible for transduction of hormonal signal into cell are rather short in length, having, as a rule, 8 to 20 amino acid residues. Synthetic peptides corresponding to these regions are able to mimic the activated form of full-size signal protein and to trigger signaling cascades in the absence of hormonal stimulus. They modulate protein-protein interaction and influence the activity of signal proteins followed by changes in their regulatory and catalytic sites. The present review is devoted to the achievements and perspectives of the study of signal protein-derived peptides and to their application as selective and effective regulators of hormonal signaling systems in vitro and in vivo. Attention is focused on the structure, biological activity, and molecular mechanisms of action of peptides, derivatives of the receptors, G protein α subunits, and the enzymes generating second messengers.

Multiple facets of follicle-stimulating hormone receptor function

Follicle-stimulating hormone (FSH) is a glycoprotein hormone produced by the anterior pituitary gland. This gonadotropin plays an essential role in reproduction. Its receptor (FSHR) belongs to the superfamily of G protein-coupled receptors (GPCR), specifically the family of rhodopsin-like receptors. Agonist binding to the FSHR triggers the rapid activation of multiple signaling cascades, mainly the cAMP-adenylyl cyclase-protein kinase A cascade, that impact diverse biological effects of FSH in the gonads. As in other G protein-coupled receptors, the several cytoplasmic domains of the FSHR are involved in signal transduction and termination of the FSH signal. Here we summarize some recent information on the signaling cascades activated by FSH as well as on the role of the intracytoplasmic domains of the FSHR in coupling to membrane and cytosolic proteins linked to key biological functions regulated by the FSH-FSHR system.

Role of the intracellular domains of the human FSH receptor in G(alphaS) protein coupling and receptor expression

The human (h) follicle-stimulating hormone receptor (FSHR) belongs to the superfamily of G protein-coupled receptors (GPCRs). This receptor consists of 695 amino acid residues and is preferentially coupled to the G(s) protein. This receptor is highly conserved among species (overall homology, 85%), with a 25-69% homology drop when compared to the human LH and TSH receptors. Although studies in prototypical rhodopsin/beta-adrenergic receptors suggest that multiple domains in the intracellular loops (iL) and the carboxyl-terminus (Ctail) of these receptors contribute to G protein coupling and receptor expression, there is a paucity of structure/function data on the role of these domains in FSHR function. Employing point mutations we have found that several residues present in the iL2 of the hFSHR are important for both coupling the receptor to the G(s) protein and maintaining the receptor molecule in an inactive conformation. In fact, HEK-293 cells expressing several hFSHR mutants with substitutions at R(450) (central to the highly conserved ERW triplet motif) and T(453) (a potential target for phosphorylation) failed to mediate ligand-provoked G(s) protein activation but not agonist binding, whereas substitutions at the hydrophobic L(460) (a conserved residue present in all glycoprotein hormone receptors) conferred elevated basal cAMP to the transfected cells. Thus, this particular loop apparently acts as a conformational switch for allowing the receptor to adopt an active conformation upon agonist stimulation. Residues in both ends of the iL3 are important for signal transduction in a number of GPCRs, including the FSHR. We have recently explored the importance of the reversed BBXXB motif (BXXBB; where B represents a basic residue and X a non-basic residue) present in the juxtamembrane region of the hFSHR iL3. A hFSHR mutant with all basic amino acids present in the iL3 BXXBB motif replaced by alanine failed to bind agonist and activate effector, and was expressed as an immature < or =62kDa form of the receptor. Individual substitutions of basic residues resulted in mutants that bound agonist normally but failed to activate effector when replaced at R(552) or R(556). Triple mutations in the same motif located in the NH(2)-end of the Ctail resulted in a complete inability of the receptor to bind agonist and activate effector, whereas individual substitutions resulted in decreased or virtually abolished agonist binding and cAMP accumulation, with both functions correlating with the detected levels of mature (80kDa) forms of the receptor. Thus, the BXXBB motif at the iL3 of the FSHR is essential for coupling the activated receptor to the G(s) protein, whereas the same motif in the Ctail is apparently more important for membrane expression. The role of cysteine residues present in the Ctail of the FSHR is an enigma since there are no conserved cysteines amongst LHR, FSHR and TSHR. C(629) and C(655) are conserved in the gonadotropin receptors but not in the TSHR. Alanine replacement of C(627) had no effect on hFSHR expression and function, whereas the same mutation at C(629) altered membrane expression and signal transduction. Serine or threonine substitutions of C(655) did not modify any of the parameters analyzed. In the hFSHR, C(629) may be a target for palmitoylation, and apparently it is the only cysteine residue in the Ctail domain that might play an important role in receptor function.

Functional significance of the BBXXB motif reversed present in the cytoplasmic domains of the human follicle-stimulating hormone receptor

The minimal structural motif, BBXXB (where B represents a basic amino acid residue and X a non-basic residue), located in particular regions of the intracellular domains of cell surface membrane receptors is involved in the G protein-activating activity of a number of G protein-coupled receptors. The human FSH receptor (hFSHR) exhibits a reversed BBXXB motif (BXXBB) in the juxtamembrane region of the third intracellular loop (IL3) and the carboxyl terminus (Ctail) of the receptor; however the importance of this sequence on receptor function remains unclear. In the present study, we analyzed the effects of mutations in this structural motif on hFSHR expression, receptor-mediated effector activation and agonist-provoked receptor internalization. Human embryonic kidney 293 cells were transiently transfected with plasmids containing the cDNA of the wild-type (Wt) hFSHR or several hFSHR mutants in which basic amino acids of the minimal structural motif at the IL3 and Ctail were replaced with alanine (i.e. AXXAA, AXXBB, BXXAB and BXXBA mutants). Alanine substitution of the three basic residues present in the IL3-BXXBB (IL3-AXXAA mutant) yielded a < or =60 kDa possibly under-glycosylated form of the FSHR, whereas the same substitutions in the Ctail resulted in the immature >62 kDa form of the receptor; both AXXAA hFSHR mutants completely failed to bind agonist and activate effector. Individual substitutions resulted in different cAMP responses to agonist stimulation: the IL3-AXXBB and IL3-BXXBA mutant hFSHRs failed to evoke Gs protein activation, whereas agonist-stimulated cAMP production was completely normal when the IL3-BXXAB mutant was expressed. All three IL3 mutants bound [125I]-labelled FSH in a similar fashion to the Wt hFSHR. Ligand-binding, cell surface membrane receptor expression and agonist-provoked effector activation were significantly affected by the individual substitutions at the Ctail-BXXBB motif: the Ctail-AXXBB variant exhibited reduced (approximately 50%) maximal cAMP response and ability to bind ligand, whereas both ligand binding and effector activation was severely reduced or abolished by expression of the Ctail-BXXBA and -BXXAB hFSHR mutants; the expression levels of the 80 kDa form of the receptor correlated with the magnitude of ligand-provoked cAMP production and binding capability of the mutant receptors. Upon stimulation by agonist, all mutants with detectable ligand-binding activity internalized following the pattern exhibited by the Wt hFSHR species. These results indicate that the BXXBB motif at the IL3 of the hFSHR is essential for coupling the activated receptor to the Gs protein, whereas the same motif in the Ctail is apparently more important for membrane expression.

Regulation of follitropin receptor cell surface residency by the ubiquitin-proteasome pathway

Little is known of the normal physiological processes that govern the cell surface residency of the human follitropin receptor (hFSHR), a G protein-coupled receptor expressed in the ovary and testis. In the hFSHR, the third intracellular (3i) loop is considered to be pivotal in attenuation of ligand activation, particularly internalization. To gain a better understanding of these processes, we used a yeast-based interaction trap to identify cytoplasmic proteins in a human ovarian cDNA library that interacted with the hFSHR 3i loop. Among the cDNA identified, four encoded isoforms of ubiquitin. Immunoprecipitated hFSHR probed with an antiubiquitin antibody revealed that the receptor is ubiquitinated, although not exclusively on the 3i loop. Cell-surface hFSHR levels increased when expressed at nonpermissive temperature in a temperature-sensitive, ubiquitination-defective cell line. Similarly, after treatment with proteasome inhibitors, HEK293 cells stably transfected with an hFSHR expression plasmid showed an increase in follitropin binding. Proteasome inhibitors did not affect the rate of FSH internalization when receptors were saturated before internalization was measured. In contrast, internalization decreased when binding experiments were performed under nonequilibrium conditions. A mutant hFSHR-K555R, which removes the only lysine in the 3i loop available for ubiquitination, was still ubiquitinated, illustrating that, although the third loop enables and interaction with ubiquitin, it is not the sole site of ubiquitination. These observations are consistent with a role for ubiquitination in the regulation of hFSHR cell surface residency. Additionally, it can be inferred that a sequence in the 3i loop is involved in regulating receptor ubiquitination and internalization.

Molecular, structural, and cellular biology of follitropin and follitropin receptor

Follitropin and the follitropin receptor are essential for normal gamete development in males and females. This review discusses the molecular genetics and structural and cellular biology of the follitropin/follitropin receptor system. Emphasis is placed on the human molecules when possible. The structure and regulation of the genes for the follitropin beta subunit and the follitropin receptor is discussed. Control of systemic and cellular protein levels is explained. The structural biology of each protein is described, including protein structure, motifs, and activity relationships. Finally, the follitropin/follitropin receptor signal transduction system is discussed.

Structural determinants in the second intracellular loop of the human follicle-stimulating hormone receptor are involved in G(s) protein activation

In the present study, we analyzed the structural determinants present in the second intracellular loop (IL-2) of the human follicle-stimulating hormone (FSH) receptor (R) involved in G(s) protein-mediated signal transduction. Human embryonic kidney 293 (HEK-293) cells, stably expressing wild-type (Wt) human FSHR (HEK-293((+))), were transiently transfected with plasmids containing cDNAs encoding the entire IL-2 or several IL-2 sequences mutated in R467 (a residue located at the center of the conserved ERW motif in the glycoprotein hormone receptors), T470 (a potential site for phosphorylation by protein kinase-A and -C) or L477 (a residue conserved in all glycoprotein hormone receptors). Expression of the IL-2 Wt in HEK-293((+)) cells reduced the maximum FSH-stimulated cAMP production significantly by approximately 40%; similar results were observed with the R467A and R467K IL-2 mutants. The IL-2(R467H), IL-2(T470A), the triple R467A/T470A/L477A IL-2 mutant and the IL-2 of the oxytocin receptor (G(q/11)-coupled) had no effects on Wt FSHR-mediated intracellular signaling whereas the L477A mutation provoked a higher ( approximately 55%) inhibition of FSH-stimulated cAMP than the free, Wt IL-2. These results suggested a specific role of IL-2 residues in FSHR function. Site directed mutagenesis of the FSHR and the expression of resulting mutants in HEK-293 cells were performed in order to corroborate the effects of these substitutions. Expression of FSHR(R467H), FSHR(R467A) and FSHR(T470A) failed to mediate ligand-provoked G(s) protein activation, whereas the R467K mutant behaved as the Wt receptor. Interestingly, the expression of L477A, L477D and L477P FSHR mutants conferred elevated basal cAMP levels to HEK-293 cells. This study indicates that the IL-2 of the human FSHR possesses amino acid residues that are important for both coupling the receptor to the G(s) protein (R467 and T470) and maintaining the receptor molecule in an inactive conformation (L477). It appears that this particular intracellular domain may act as a conformational switch to produce the activation of G proteins as has been reported for the IL-2 of other G protein-coupled receptors.

Deletions in the third intracellular loop of the thyrotropin receptor. A new mechanism for constitutive activation

Gain-of-function mutations of the thyrotropin receptor (TSHR) gene have been invoked as one of the major causes of toxic thyroid adenomas. In a toxic thyroid nodule, we recently identified a 9-amino acid deletion (amino acid positions 613-621) within the third intracellular (i3) loop of the TSHR resulting in constitutive receptor activity. This finding exemplifies a new mechanism of TSHR activation and raises new questions concerning the function of the i3 loop. Because the i3 loop is thought to be critical for receptor/G protein interaction in many receptors, we systematically reexamined the role of the TSHR's i3 loop for G protein coupling. Thus, various deletion mutants were generated and functionally characterized. We identified an optimal deletion length responsible for constitutive activity. If the number of deleted amino acids was reduced, elevated basal cAMP accumulation was found to be concomitantly diminished. Expansion of the deletion dramatically impaired cell surface expression of the receptor. Shifting the deletion toward the N terminus of the i3 loop resulted in unaltered strong constitutive receptor activity. In contrast, translocation of the deletion toward the C terminus led to significantly reduced basal cAMP formation, most probably due to destruction of a conserved cluster of amino acids. In this study, we show for the first time that amino acid deletions within the i3 loop of a G protein-coupled receptor result in constitutive receptor activity. In the TSHR, 75% of the i3 loop generally assumed to play an essential role in G protein coupling can be deleted without rendering the mutant receptor unresponsive to thyrotropin. These findings support a novel model explaining the molecular events accompanying receptor activation by agonist.

Synthetic peptides corresponding to residues 551 to 555 and 650 to 653 of the rat testicular follicle-stimulating hormone (FSH) receptor are sufficient for post-receptor modulation of Sertoli cell responsiveness to FSH stimulation

We have recently demonstrated that synthetic peptides corresponding to the third cytoplasmic (3i) loop (residues 533 to 555) and a region in the carboxy-terminal cytoplasmic tail (residues 645 to 653) of the rat testicular follicle-stimulating hormone receptor (FSHR) affected signal transduction in rat testis membranes and cultured rat Sertoli cells. In order to define more precisely the peptide domains involved, we synthesized truncated peptide amides corresponding to FSHR residues 551-555 (KIAKR) and 650-653 (RKSH), respectively. These two regions were chosen since they contained a minimal structural motif present in G protein activator regions of several other G protein-coupled receptors (i.e., B-X-X-B-B or B-B-X-B, B representing a basic amino acid). Neither peptide inhibited binding of FSH to testis membrane receptors. Each peptide significantly reduced FSH-stimulated estradiol biosynthesis by intact cultured rat Sertoli cells. The same results were obtained with streptolysin O-permeabilized Sertoli cells. No effect was noted on forskolin-induced steroidogenesis, indicating that the peptide effects were not due to interaction with adenylyl cyclase. Each peptide amide, however, induced concentration-dependent increases in guanine nucleotide exchange in rat testis membranes. Our results indicate that interaction of FSH receptor with its associated G protein may involve relatively restricted peptide sequences, and include residues 551-555 (KIAKR) in the third cytoplasmic loop, and residues 650-653 (RKSH) in the carboxy-terminal cytoplasmic tail of the FSH receptor.