Wellness Among Interventional Radiologists: Results From a Multidimensional Survey

RATIONALE AND OBJECTIVES

To evaluate wellness among interventional radiologists using a multidimensional survey.

MATERIALS AND METHODS

An anonymous 53-item survey, including 36 Perceived Wellness Survey (PWS) prompts, was created in Qualtrics (an online survey tool) to assess wellness among interventional radiology attendings and residents. The survey was open from June to September, 2022, 2 years into the COVID-19 Pandemic. The survey was distributed via Society of Interventional Radiology Forums, social media (Twitter, LinkedIn, and Facebook), and personal correspondence. PWS scores were categorized into Wellness Composite and subscores (physical, emotional, intellectual, psychological, social, and spiritual).

RESULTS

367 surveys were completed. 300 (81.7%) respondents were male and 67 (18.3%) were female. Respondents included attending physicians (297; 81.0%) and residents (70; 19.0%). Practice settings included academic (174; 47.4%), private (114; 31.0%), private-academic hybrid (62; 16.9%), and others (17; 4.7%). Mean Wellness Composite at academic centers (14.74 ± 3.16; range: 5.42-23.50) was significantly lower than at private (15.22 ± 3.37; range: 6.13-23.51) and hybrid (16.14 ± 2.47; range: 10.20-23.50) practices (p = 0.01). Respondents < 40 years old had significantly lower emotional wellness subscores compared to those ≥ 40 years old (4.34 ± 0.86 vs. 4.60 ± 0.87; p = 0.006). There were no significant differences between geographic regions in the United States, however, International respondents had significantly lower social and intellectual wellness (INT) subscores. Physical wellness subscore (4.00 ± 0.9) was significantly lower than the other subscores (4.59 ± 0.81) (p < .001). Overall mean Wellness Composite was 15.11 ± 3.13 (range: 5.42-23.51).

CONCLUSION

Overall self-reported wellness was lower among interventional radiologists practicing at academic centers. Interventional Radiologists < 40 years old and residents had lower emotional wellness, while international respondents had lower social and INT. Overall wellness scores were lower than prior PWS studies.

Common and differential mechanisms of gonadotropin receptors

The gonadotropin receptors are G-protein-coupled receptors with unique structural and functional features, consisting of two halves. The N-terminal extracellular half (exodomain) binds the hormones, whereas the C-terminal membrane-associated half (endodomain) is responsible for receptor activation. In this review, the novel ternary interactions, contact points and mutual modulations among the exodomain, endodomain and hormone for hormone binding and signal generation are described based on the latest observations. This discussion is contrary to the yiew that the exodomain and endodomain are independent, at least functionally, and provides new insights into the receptor mechanisms for the gonadotropins and other G-protein-coupled receptors.

The role of the hinge region of the luteinizing hormone receptor in hormone interaction and signal generation

Luteinizing hormone receptor, a G protein-coupled receptor, consists of two halves, the N-terminal extracellular hormone binding domain (exodomain) and the C-terminal membrane-associated, signal-generating domain (endodomain). The exodomain has seven to nine Leu-rich repeats, which are generally thought to form a 1/3 donut-like structure and interact with human choriogonadotropin (hCG). The resulting hCG-exodomain complex adjusts the structure and its association with the endodomain, which results in signal generation in the endodomain. It is unclear whether the rigid 1/3 donut structure could provide the agility and versatility of this dynamic action. In addition, there is no clue as to where the endodomain contact point (the signal modulator) in the exodomain is. To address these issues, the exodomain was examined by Ala scan and multiple substitutions, while receptor peptides were used for photoaffinity labeling and affinity cross-linking. Our results show that the C-flanking sequence (hinge region), Thr(250)-Gln(268), of the Leu-rich repeats (LRRs) specifically interacts with hCG, preferentially hCGalpha. This interaction is inhibited by exoloop 2 of the endodomain but not by exoloops 1 and 3, suggesting an intimate relationship between Thr(250)-Gln(268), exoloop 2, and hCG. Taken together, our observations in this article suggest a new paradigm that the LRRs contact the front of hCG, while both flanking regions of the LRRs interact with the sides of hCG. This would trap hCG in the 1/3 donut structure of the LRRs and enhance the binding affinity. In addition, mutations of conserved Ser(255) in the sequence can constitutively activate the receptor. This provides a clue for the signal modulator in the exodomain. In contrast, a phenyl or phenolic group is necessary at conserved Tyr(253) for targeting the receptor to the surface.

Hormone interactions to Leu-rich repeats in the gonadotropin receptors. III. Photoaffinity labeling of human chorionic gonadotropin with receptor Leu-rich repeat 4 peptide

Human chorionic gonadotropin (hCG) binds to the extracellular N-terminal domain, exodomain, of its receptor, and the resulting hCG-exodomain complex is thought to modulate the membrane associated domain, endodomain, of the receptor to generate hormone signal. The bulk of the exodomain is speculated to assume a crescent structure consisting of eight to nine Leu-rich repeats (LRRs), which may provide the hormone contact sites. Unfortunately, little experimental evidence is available for the precise hormone contact points in the exodomain and the endodomain. The two preceding articles (Song, Y., Ji, I., Beauchamp, J., Isaacs, N., and Ji, T. (2001) J. Biol. Chem. 276, 3426-3435; Song, Y., Ji, I., Beauchamp, J., Isaacs, N., and Ji, T. (2001) J. Biol. Chem. 276, 3436-3442) show that putative LRR2 and LRR4 are crucial for hormone binding. In particular, the N-terminal region of LRR4 assumes the hydrophobic core of the LRR4 loop, whereas the C-terminal region is crucial for signal generation. However, it is unclear whether LRR4 interacts hCG and the endodomain and how it might be involved in signal generation. In this article, our affinity labeling results present the first evidence that the N-terminal region of LRR4 interacts with hCG, preferentially the hCGalpha subunit and that the hCG/LRR4 complex interacts with exoloop 2 of the endodomain. This interaction offers a mechanism to generate hormone signal.

Hormone interactions to Leu-rich repeats in the gonadotropin receptors. II. Analysis of Leu-rich repeat 4 of human luteinizing hormone/chorionic gonadotropin receptor

The luteinizing hormone receptor (LHR) consists of an approximately 350-amino acid-long N-terminal extracellular exodomain and a membrane-associated endodomain of similar size. Human chorionic gonadotropin (hCG) binds to the exodomain, and then hCG/exodomain complex is thought to make a secondary contact with the endodomain and generate hormone signals. The sequence alignment of the exodomain shows imperfectly matching eight to nine Leu-rich repeats (LRRs). In the preceding article (Song, Y., Ji, I., Beauchamp, J., Isaacs, N., and Ji, T. (2001) J. Biol. Chem. 276, 3426-3435), we have shown that LRR2 and LRR4 are crucial for hormone binding. In this work, we have examined the residues of LRR4, in particular Leu(103) and Ile(105) in the putative beta strand. Our data show that Leu(103) and Ile(105) are involved in the specific, hydrophobic interaction of the LRR4 loop, likely to form the hydrophobic core. This loop is crucial for the structural integrity of all of the LRRs. In contrast, the downstream sequence consisting of Asn(107), Thr(108), Gly(109), and Ile(110) of LRR4 is crucial for cAMP induction but not for hormone binding, folding, and surface expression. This implicates, for the first time, its involvement in the interaction with the endodomain and signal generation. The evidence for the interaction is presented in the following article.

Hormone interactions to Leu-rich repeats in the gonadotropin receptors. I. Analysis of Leu-rich repeats of human luteinizing hormone/chorionic gonadotropin receptor and follicle-stimulating hormone receptor

The luteinizing hormone receptor (LHR) and follicle-stimulating hormone receptor (FSHR) have an approximately 350-amino acid-long, N-terminal extracellular exodomain. This exodomain binds hormone with high affinity and specificity and contains eight to nine putative Leu-rich repeat (LRR) sequences. LRRs are known to assume the horseshoe structure in ribonuclease inhibitors, and the inner lining of the horseshoe consists of the beta-stranded Leu/Ile-X-Leu/Ile motif. In the case of ribonuclease inhibitors, these beta strands interact with ribonuclease. However, it is unclear whether the putative LRRs of LHR and FSHR play any role in the structure and function. In this work, the beta-stranded Leu/Ile residues in all LRRs of the human LHR and FSHR were Ala-scanned and characterized. In addition, the 23 residues around LRR2 of LHR were Ala-scanned. The results show that beta-stranded Leu and Ile residues in all LRRs are important but not equally. These Leu/Ile-X-Leu/Ile motifs appear to form the hydrophobic core of the LRR loop, crucial for the LRR structure. Interestingly, the hot spots are primarily in the upstream and downstream LRRs of the LHR exodomain, whereas important LRRs spread throughout the FSHR exodomain. This may explain the distinct hormone specificity despite the structural similarity of the two receptors.

Differentiation of granulosa cell line: follicle-stimulating hormone induces formation of lamellipodia and filopodia via the adenylyl cyclase/cyclic adenosine monophosphate signal

FSH plays a crucial role in granulosa cell differentiation and follicular development during the ovulation cycle. The early events of granulosa cell differentiation in cell culture involve changes in the cell morphology and cell-to-cell interactions. To determine the cause and signaling mechanism for these changes, we examined an undifferentiated rat ovarian granulosa cell line that grows in a defined serum-free medium, expresses the FSH receptor, terminally differentiates when exposed to FSH, and undergoes apoptosis upon FSH withdrawal. FSH bound the FSH receptor on rat ovarian granulosa cells, and the liganded receptor activated adenylyl cyclase (AC) to produce cAMP but did not mobilize Ca2+. In addition, we observed massive reorganization of the actin cytoskeleton within 3 h of FSH treatment. This involves formation of lamellipodia and filopodia and spreading of multilayer cell aggregates to monolayers. This actin reorganization and cell transformation could also be induced by the AC activator, forskolin, in the absence of FSH. Furthermore, AC inhibitors blocked the FSH-dependent actin reorganization and transformation. On the other hand, phospholipase C inhibitors did not block the FSH-induced changes. Taken together, our observations indicate that the AC/cAMP signal is necessary and sufficient for FSH-dependent granulosa cell differentiation, including massive reorganization of the actin cytoskeleton and changes in the cell morphology and cell-to-cell interactions. There is no evidence that the phospholipase C signal and Ca2+ mobilization are involved in this process.

The alpha-subunit of human choriogonadotropin interacts with the exodomain of the luteinizing hormone/choriogonadotropin receptor

The LH/CG receptor, a G protein-coupled receptor, consists of two parts, the N-terminal extracellular segment (exodomain) and the membrane-associated C-terminal segment (endodomain). hCG initially binds the exodomain of the receptor and then, the hormone/exodomain complex is thought to make the secondary contact with the endodomain of the receptor and generate a hormone signal. However, little direct evidence is available about which hormone subunits (alpha or beta) interact with which domains of the receptor. To determine whether the alpha-subunit contacts the exodomain of its receptor, hCG containing [125I]alpha and truncated exodomain lacking the endodomain were prepared. They were chemically cross-linked, and the resulting cross-linked complexes were solubilized and electrophoresed. The results indicate that the alpha-subunit of hCG was directly and specifically cross-linked to the exodomain. To verify the cross-linked exodomain by the independent method, the Flag epitope was inserted between the signal sequence and the mature exodomain. hCG containing [125I]alpha was cross-linked to the Flag exodomain, and the resulting cross-linked hCG/Flag exodomain complexes were immunoprecipitated with anti-Flag antibody. The results show that the material cross-linked to hCG containing [125I]alpha is indeed the exodomain. In conclusion, our results show the direct interaction of the alpha-subunit with the exodomain and, therefore, its crucial role in the hormone-receptor interaction in addition to its involvement in signal generation.

High affinity hormone binding to the extracellular N-terminal exodomain of the follicle-stimulating hormone receptor is critically modulated by exoloop 3

The human follicle-stimulating hormone receptor (FSH-R) consists of two distinct domains of >330 amino acids, the N-terminal extracellular exodomain and membrane-associated endodomain. The exodomain alone binds hormone with high affinity, whereas the endodomain is the site of receptor activation. Coordination of these two domains is essential for successful hormone action but little is known about their functional and structural relationship. In this communication, we report that exoloop 3 of FSH-R constrains follicle-stimulating hormone binding to the exodomain. When the FSH-R exodomain was prepared by truncating its endodomain, the hormone binding affinity of the exodomain was slightly improved, compared with the wild type receptor. The binding affinity was further improved by >3-fold when the exodomain was attached to the membrane-associated domain of CD8. These results suggest that the FSH-R endodomain attenuates hormone binding at the exodomain. As a first step to test this hypothesis, the 11 amino acids except Ala589 of exoloop 3 were individually substituted with Ala. Ala substitution for Leu583 or Ile584 improved the hormone binding affinity by 4-6-fold while totally abolishing cAMP induction, indicating an inverse relationship. The Ala substitution for Lys580 or Pro582 had a similar trend but to a lesser extent. This significant improvement in the binding affinity suggests that the four residues at the N-terminal region of exoloop 3 interact with the exodomain and constrain the hormone binding in the wild type receptor. This effect is specific since substitutions for other than the 4 residues did not improve the hormone binding affinity. Computer modeling shows that the 4 residues can be positioned on one side of exoloop 3. This result and the apparent inverse relationship of hormone binding and cAMP induction suggest that these two essential functions may work against each other. Therefore, hormone binding might be compromised to preserve cAMP inducibility while maintaining a reasonably high, but below maximum, binding affinity.

The amino-terminal region of the luteinizing hormone/choriogonadotropin receptor contacts both subunits of human choriogonadotropin. I. Mutational analysis

The luteinizing hormone/choriogonadotropin receptor is a seven-transmembrane receptor. Unlike most seven-transmembrane receptors, it is composed of two halves of equal size, the N-terminal extracellular exodomain and the C-terminal membrane-associated endodomain. The exodomain is exclusively responsible for high affinity hormone binding, whereas receptor activation occurs only in the endodomain. This mutually exclusive physical separation of the two functional domains sets the lutropin receptor and its subfamily of receptors apart from all other seven-transmembrane receptors. The mechanisms of hormone binding and receptor activation also appear to be different from those of other receptors in that binding occurs in at least two steps. However, the precise hormone contact sites in the exodomain are unknown. To determine the hormone/receptor contact sites, we have examined the receptor using progressive truncation from the C terminus, Ala scanning, immunofluorescence microscopy, and antibody binding. Progressive truncation from the C terminus of the receptor indicates several discrete regions that impact hormone binding. These regions are around the boundaries of exons 1-2, 4-5, 6-7, and 9-10. Ala scanning of the Asp17-Arg26 region near the exon 1-2 junction uncovered three alternating residues (Leu20, Cys22, and Gly24) crucial for hormone binding. Ala substitution for any one of these residues abolished hormone binding, although the resulting mutant receptors were successfully expressed on the cell surface. In contrast, Ala substitution for their flanking and intervening residues did not impair hormone binding. These results and the data in the accompanying article (Phang, T., Kundu, G., Hong, S., Ji, I., and Ji, T. (1998) J. Biol. Chem. 273, 13841-13847) indicate that this region directly contacts the hormone and suggest a novel mode of embracing the hormone.

The amino-terminal region of the luteinizing hormone/choriogonadotropin receptor contacts both subunits of human choriogonadotropin. II. Photoaffinity labeling

The luteinizing hormone/choriogonadotropin receptor, a seven-transmembrane receptor, is composed of two equal halves, the N-terminal extracellular exodomain and the C-terminal membrane-associated endodomain. Unlike most seven-transmembrane receptors, the exodomain alone is responsible for high affinity hormone binding, whereas signal is generated in the endodomain. These physical separations of hormone-binding and receptor activation sites are attributed to unique mechanisms for hormone binding and receptor activation of this receptor and its subfamily members. However, the precise hormone contact sites in the exodomain are unclear. In the preceding article (Hong, S., Phang, T., Ji, I., and Ji, T. H. (1998) J. Biol. Chem. 273, 13835-13840), a region immediately downstream of the N terminus of the exodomain was shown to be crucial for hormone binding. To test if the region interacts with the hormone, human choriogonadotropin (hCG) was photoaffinity-labeled with a peptide mimic corresponding to Gly18-Tyr36 of the receptor. This peptide mimic specifically photoaffinity-labeled both the alpha- and beta-subunits of hCG. Interestingly, hCGalpha was preferentially labeled. On the other hand, denatured hCG was not labeled, and a mutant analog of the peptide failed to label hCG. Furthermore, the affinity labeling was UV-dependent and saturable, indicating the specificity of the photoaffinity labeling. Our results indicate that the region of the exodomain interacts with hCG and that the contact points are near both subunits of hCG. Particularly, the alternate residues (Leu20, Cys22, and Gly24) are crucial for hCG binding. In addition, the results underscore the fact that there is a crucial hormone contact site outside of the popularly believed primary hormone-binding site that is composed of Leu-rich repeats and is located in the middle of the exodomain. Our observations are crucial for understanding the molecular mechanism through which the initial high affinity hormone binding leads to receptor activation in the endodomain.

Gene, interaction, signal generation, signal divergence and signal transduction of the LH/CG receptor

Trophoblastic neoplasms and choriocarcinoma cells express high levels of the hCG receptor. The hCG receptor is encoded by a single gene in chromosome 2p21-p16, spanning over -70 kb with 11 exons and 10 introns. Multiple mRNA species are produced from the gene utilizing two proximal promoters and several Sp-1 elements as well as proximal and distal suppressors. In fact, regulatory proteins which bind to one of these suppressors are expressed less in choriocarcinoma cell lines than in placenta. The LH/CG receptor is comprised of two structurally and functionally distinct domains, extracellular N-terminal exodomain and membrane embedded endodomain. These two domains can separately be expressed and processed, including folding. The exodomain alone has the high affinity hormone binding site but is not capable of generating hormonal signal. In contrast, the endodomain alone has the site for receptor activation. These two domains contact each other in holo-receptor and split receptor. This interaction, particularly through exoloops 2 and 3, constrains the high affinity hormone binding at the exodomain. Conversely, the exodomain could be involved in receptor activation. Therefore, these two domains are not entirely independent although they can be independently synthesized and processed. The existing evidence indicate that hCG and the receptor undergo multiple stages of interactions leading to receptor activation. Initial high affinity binding of hCG to the exodomain results into conformational adjustments of the hCG/exodomain complex. This leads to the secondary, low affinity contact of the hCG/exodomain complex with the endodomain. This secondary contact is responsible for generating signals. They are transduced through TM to the cytoplasmic portion (cytoloops and the C-terminal tail) of the receptor and then, transferred to cytoplasmic signaling molecules, such as G protein. Mutations in the exodomain and endodomain (N-extension, exoloops, TM, cytoloops, and cytoplasmic tail) have the potential to interfere with receptor activation at different steps, signal generation, transduction and transfer. Binding of hCG to the LH/CG receptor are known to induce two signals, one for adenylyl cyclase/ cAMP and the other for phospholipase C/inositol phosphate/diacylglycerol. The cAMP signal and IP signal diverge at the surface of the receptor. These independent signals are separately transduced through the transmembrane domains to the cytoplasmic part of the receptor, indicating the existence of the distinct transducers for each of the signals. Furthermore, it is likely that the divergent signals are separately transferred to cytoplasmic signal molecules such as G protein. In addition, each of the cAMP signal and IP signal consists of at least three separate subsignals: affinity signal, maximal production (efficacy) signal and basal level signal. In heterodimeric hCG, there are distinct parts responsible for high affinity receptor binding and receptor activation. Particularly, the C-terminal reduces of the alpha subunit play a crucial role in receptor activation. This alpha subunit is shared with other glycoprotein hormones, follicle stimulating hormone and thyroid stimulating hormone. Interesting, the alpha C-terminal residues play distinct roles in all three hormones, despite its common nature.

Modulation of high affinity hormone binding. Human choriogonadotropin binding to the exodomain of the receptor is influenced by exoloop 2 of the receptor

The lutropin/choriogonadotropin receptor is a seven-transmembrane receptor and consists of two major domains of similar size, an extracellular exodomain and a membrane-associated endodomain which includes 3 exoloops. The uniquely large exodomain is responsible for high affinity hormone binding whereas receptor activation occurs at the endodomain. However, little is known about the relationship between the exodomain and endodomain. It was reported that hormone binding to the exodomain was improved when the endodomain was truncated. This result suggests that hormone binding to the exodomain was influenced by the endodomain. To test this hypothesis, amino acids of exoloop 2 were examined by Ala substitutions. The binding affinity was enhanced by some Ala substitutions but attenuated by others. These results indicate that exoloop 2 influences the hormone binding to the exodomain. Particularly, the high affinity hormone binding at the exodomain is constrained by a group of amino acids, Ser484, Asn485, Lys488, Ser490, and Ser499. Computer modeling suggests these residues may be positioned on one side of exoloop 2. It also influences the affinity for cAMP induction and the maximal cAMP production in distinct ways, in addition to its influence on the hormone binding affinity. The distinct ways of influencing these functions are sometimes in conflict and compromised to attain the maximal affinity for cAMP induction. As a result, the exodomain attains the maximal affinity for hormone binding when the endodomain is truncated and cAMP induction is disengaged.

Roles of transmembrane prolines and proline-induced kinks of the lutropin/choriogonadotropin receptor

The lutropin/choriogonadotropin receptor is a seven-helix transmembrane (TM) receptor. A unique feature of TM helices is the content of Pro, which generally is absent in alpha helices of globular proteins. Because Pro disrupts helices and introduces a approximately 26 degrees kink, it has been speculated that Pro plays a crucial role in the structure of TM helices, exoloops, and cytoloops of TM receptors. To examine the roles of the five TM Pros of the lutropin/choriogonadotropin receptor, these residues were individually substituted. Mutant receptors were examined for surface expression, hormone binding, and cAMP induction. Surface expression was monitored after introducing the flag epitope into the receptors. Flag epitopes slightly affected cAMP induction but not hormone binding or surface expression of receptors as monitored by immunofluorescence microscopy and 125I-anti-flag antibody. The results indicate that Pro479 in TM 4 and Pro598 in TM 7 play important yet contrasting roles. Pro479 is crucial for hormone binding at the cell surface but not after solubilization of the receptor. This is more likely due to the Pro side chain than the Pro-induced kink. Pro598 is important for surface expression. The kinks of Pro463 of TM 4, Pro562 of TM 6, or Pro591 of TM 7 are not important because the substitution of Phe for these residues did not significantly impact surface expression, hormone binding, and cAMP induction.

Interaction, signal generation, signal divergence, and signal transduction of LH/CG and the receptor

The LH/CG receptor is comprised of two structurally and functionally distinct domains, extracellular N-terminal exodomain and membrane-embedded endodomain. These two domains can separately be expressed and processed, including folding. The exodomain alone has the high-affinity hormone binding site but is not capable of generating hormonal signal. In contrast, the endodomain alone has the site for receptor activation. These two domains contact each other in holo-receptor and split receptor. This interaction, particularly through exoloops 2 and 3, constrains the high-affinity hormone binding at the exodomain. Conversely, the exodomain could be involved in receptor activation. Therefore, these two domains are not entirely independent, although they can be independently synthesized and processed. The existing evidence indicates that hCG and the receptor undergo multiple stages of interactions leading to receptor activation. Initial high-affinity binding of hCG to the exodomain results in conformational adjustments of the hCG/exodomain complex. This leads to the secondary, low-affinity contact of the hCG/exodomain complex with the endodomain. This secondary contact is responsible for generating signals. They are transduced through transmembrane domains (TM) to the cytoplasmic portion (cytoloops and the C-terminal tail) of the receptor and then transferred to cytoplasmic signaling molecules such as G protein. Mutations in the exodomain and endodomain (N-extension, exoloops, TM, cytoloops, and cytoplasmic tail) have the potential to interfere with receptor activation at different steps: signal generation, transduction, and transfer. Binding of hCG to the LH/CG receptor is known to induce two signals, one for adenylyl cyclase/ cAMP and the other for phospholipase C/inositol phosphate/diacylglycerol. The cAMP signal and IP signal diverge at the surface of the receptor. These independent signals are separately transduced through the transmembrane domains to the cytoplasmic part of the receptor, indicating the existence of the distinct transducers for each of the signals. Furthermore, it is likely that the divergent signals are separately transferred to cytoplasmic signal molecules such as G protein. In addition, each cAMP signal and IP signal consists of at least three separate subsignals: affinity signal, maximal production (efficacy) signal, and basal level signal. In heterodimeric hCG there are distinct parts responsible for high-affinity receptor binding and receptor activation. Particularly, the C-terminal residues of the alpha subunit play a crucial role in receptor activation. This alpha subunit is shared with other glycoprotein hormones, follicle-stimulating hormone, and thyroid-stimulating hormone. Interestingly, the alpha C-terminal residues play distinct roles in all three hormones, despite its common nature.

Molecular mechanism of LH/CG receptor activation

It is known that the N-terminal half of the LH/CG receptor is responsible for high hCG binding whereas the C-terminal half is capable of receptor activation. Our results suggest that initial hCG binding at the high affinity site in the N-half receptor induces conformational adjustments. This leads to low affinity secondary contacts of the complex of hCG/the N-half receptor with the C-half receptor. This low affinity secondary contact is responsible for activating the receptor. This is based on the following observations. The C-terminal tail of hCG alpha is known to be involved in activation of the LH/CG receptor. In addition to hCG, we examined the C-terminal three residues (His90-Lys91-Ser92) of the common alpha subunit of FSH and TSH. The results show their differential roles in the three hormones. Ser92 is important for binding and cAMP induction of TSH but not for hCG and FSH. Lys91 is important for binding and cAMP induction of hCG, and cAMP induction but not binding of FSH. It is not important for binding or cAMP induction of TSH. His90 is important for all three hormones. When all three residues were truncated, FSH and TSH lose their affinity for binding and cAMP induction, whereas hCG is still capable of binding but not cAMP induction. Therefore, the three amino acids contribute differently in receptor binding and cAMP induction of hCG, FSH and TSH. Our data also indicate that the evolution of the alpha subunit has been constrained in order not to impair any of the hormones. This suggests that each hormone can be independently engineered to improve the potency. To chemically identify the contact site of the alpha C-tail of hCG in the LH/CG receptor, a decamer peptide corresponding to the alpha subunit sequence from His83 to Ser92 (peptide alpha 81-92) was derivatized with UV sensitive reagent, ABG and radio-iodinated. The resulting ABG-125I-peptide alpha 83-92 was capable of binding and activating the LH/CG receptor. Furthermore, it specifically photoaffinity-labeled the LH/CG receptor. In addition, the amino group of alpha Lys91 of peptide alpha 83-92 is crosslinked to a carboxyl group of the receptor, an indication of close association. Reciprocal mutagenesis of alpha Lys91 and Asp397 in exoloop 1 of the LH/CG receptor suggests the complementary of this pair in receptor activation but not the high affinity interaction of hCG and the receptor. In addition, Lys583 of exoloop 3 is also crucial for receptor activation. To test the conformational adjustment, ABG was attached to hCG alpha and reassociated with untreated beta to produce ABG-125I-alpha/beta. The extent of inter-subunit crosslinking of ABG-125I-alpha/beta bound to the receptor was two to three fold less than unbound ABG-125I-alpha/beta. This result indicates structural change at the subunit interface in response to hCG binding to the receptor.

The luteinizing hormone/chorionic gonadotropin receptor has distinct transmembrane conductors for cAMP and inositol phosphate signals

The luteinizing hormone/chorionic gonadotropin receptor is a member of the seven-transmembrane receptor family. It is coupled, presumably via Gs and Gq, to two signal pathways involving adenylyl cyclase/cAMP and phospholipase C/inositol phosphate (IP). Little is known about the events prior to G-protein coupling: for example, whether these signals are generated from a single or multiple independent origins and mechanisms, when and where they diverge, and how they are transduced. We report novel observations that the cAMP signal and the IP signal originate and diverge upstream of G-protein coupling. The generation of these two signals independently involves Lys583 in exoloop 3 of the rat receptor. For this study, Lys583 of the receptor was substituted with a panel of amino acids, and mutant receptors were assayed for hormone binding and induction of cAMP, inositol monophosphate, inositol bisphosphate, and inositol trisphosphate. No substitutions for Lys583 were permissible for cAMP induction, despite successful surface expression and hormone binding. In contrast, several substitutions were permissible for IP induction. Our results suggest two distinct transmembrane signal conductors for cAMP and inositol phosphate signals and imply particular models of receptor activation not previously suggested.

Photoaffinity labeling of the lutropin receptor with synthetic peptide for carboxyl terminus of the human choriogonadotropin alpha subunit

Human choriogonadotropin (hCG) consists of an alpha subunit and a beta subunit. The existing evidence from various studies using truncation, substitution, synthetic hormone peptides, and hCG crystals suggests that the C-terminal region of the alpha subunit contacts the luteinizing hormone/choriogonoadotropin (LH/CG) receptor and is involved in receptor activation. Despite a deluge of the speculation and the important role of the alpha C-terminal region, direct evidence for its interaction with the receptor has been elusive. Because of the significant biological activity, it is imperative to prove the interaction of the alpha C-terminal region. For this purpose, decamer peptides corresponding to the alpha subunit sequence from His83 to Ser92 (alpha 83-92) were derivatized with the N-hydroxysuccinimide ester of 4-azidobenzoylglycine (ABG) and radioiodinated. The resulting ABG-125I-alpha 83-92 was capable of binding and activating the LH/CG receptor. Furthermore, UV-sensitive ABG-125I-alpha 83-92 exclusively photoaffinity-labeled an approximately of 86-kDa molecule. This labeled molecule was shown to be the LH/CG receptor by various methods including immunoprecipitation by anti-LH/CG receptor antiserum. In addition, evidence is presented that the amino group of alpha Lys91 of alpha 83-92 is in such close proximity to a carboxyl group of the receptor that this pair is cross-linked to form an amide, a zero length cross-link. This low affinity contact of alpha 83-92 and the receptor is sufficient for receptor activation and is crucial for the full understanding of the mechanistics of the receptor activation steps.

Exoloop 3 of the luteinizing hormone/choriogonadotropin receptor. Lys583 is essential and irreplaceable for human choriogonadotropin (hCG)-dependent receptor activation but not for high affinity hCG binding

The luteinizing hormone/choriogonadotropin (CG) receptor belongs to a subfamily of glycoprotein hormone receptors within the seven-transmembrane receptor family. It is comprised of an extracellular N-terminal half of 341 amino acids and a membrane-associated C-terminal half of 303 amino acids. The N-terminal half is capable of high affinity hormone binding whereas the C-terminal half is capable of low affinity hormone binding and receptor activation. However, the precise location of the receptor activation site is currently unknown. We present evidence for the first time that Lys583 of exoloop 3 is crucial and irreplaceable for receptor activation to induce cAMP synthesis. Exoloop 3 is comprised of 11 amino acids and flanked by two Lys residues, Lys573 and Lys583, that are located at the boundaries with the transmembrane columns 6 and 7, respectively. All substitutions including Arg for Lys583 did not affect the high affinity human CG binding, but they resulted in the complete loss of cAMP synthesis induced by human CG. Ala substitutions of the other amino acids in exoloop 3 did not make such a dramatic impact on cAMP induction. The Ala scan revealed two distinct groups of amino acids in terms of their importance in cAMP induction, one group being more important than the other. Interestingly, these two groups of amino acids are arranged in an alternate sequence. This result suggests a specific structure similar to a beta-like structure for exoloop 3.

Receptor binding dependent structural changes in human choriogonadotropin: photochemical inter-subunit crosslinking

Activation of surface receptors is thought to occur in multiple transient steps with conformational adjustments of hormones and receptors beginning from the initial hormone-receptor contact. In this study, we have established a sensitive photochemical crosslinking method to detect structural change of hCG upon receptor binding. hCG consists of an α subunit and a β subunit. Free α subunit was derivatized with photosensitive reagents and reassociated with unmodified β subunit. Reassociated hCG αβ dimer was capable of high affinity receptor binding and activation. The reagents attached to the α subunit were capable of crosslinking the α subunit to the β subunit. However, the extent of inter-subunit cross-linking in solution was two-three fold greater than inter-subunit crosslinking after hCG bound to the receptor. This difference indicates a novel structural change at the subunit interface in response to hCG binding to the receptor. Although highly unlikely, other microenvironmental factors might have interfered with the crosslinking efficiency without impacting the structure of hCG. This study lays the ground work to precisely define the location and nature of the change. Such information will be crucial for the understanding of the molecular mechanism of the hormone-receptor interaction and receptor activation.

Role of the carboxy-terminal residues of the alpha-subunit in the expression and bioactivity of human thyroid-stimulating hormone

The glycoprotein hormones TSH, CG, LH, and FSH are heterodimers consisting of a hormone-specific beta-subunit and a common alpha-subunit. The aim of the present study was to investigate the role of the carboxy terminus of the common alpha-subunit (amino acids Tyr89-His90-Lys91-Ser92), which has been shown to be important for human (h) CG and hFSH, for the activity of hTSH. Successive truncations of the alpha-carboxy terminus by site-directed mutagenesis revealed a stepwise reduction of bioactivity occurring at residues alpha Ser92 and alpha His90 to 64% and 13%, respectively. This contrasts with previous findings for hCG and hFSH, where loss of bioactivity occurred in a single step with the deletion of alpha Lys91 but alpha Ser92 was not important. The decreased bioactivities of the hTSH alpha-truncation mutants were reflected by concomitant reductions of cAMP production, thyroid hormone synthesis and cell growth and were accompanied by a loss of receptor binding. Substitution of residues alpha Lys91 or alpha His90 with either a hydrophobic or a bulkier residues resulted in a reduction of receptor binding and signal transduction, indicating that the alpha-carboxy terminus of hTSH may interact with the TSH receptor in a tight contact area. Conversely, substitution of alpha His90 with smaller residues enhanced bioactivity. In addition, the integrity of the alpha-carboxy terminus was essential for hTSH expression. Thus, we showed common and different roles of the alpha-carboxy-terminal residues for the glycoprotein hormones. The unique role of alpha Ser92 in hTSH activity explains the evolutionary constraint to preserve the alpha-carboxy-terminal Ser92 in all glycoprotein hormones.

Differential roles of exoloop 1 of the human follicle-stimulating hormone receptor in hormone binding and receptor activation

The follicle-stimulating hormone (FSH) receptor is a member of the glycoprotein hormone receptor subfamily of the seven-transmembrane receptor superfamily. These receptors have an extracellular N-terminal half of approximately 350 amino acids and a membrane-associated C-terminal half of approximately 350 amino acids. The N-terminal halves have the high affinity hormone binding site. On the other hand, the C-terminal half of the lutropin/choriogonadotropin receptor has the receptor activation site. However, little is known about the activation site and mechanism of the FSH receptor, although the existing evidence indicates crucial differences in the activation of the FSH receptor and the lutropin/choriogonadotropin receptor. As a first step to resolve this issue, we examined the upstream juxtamembrane five amino acids, Asp405-Ile406-His407-Thr408-Lys409, of the exoloop 1. Ala scan and multi-substitutions show that the five amino acid sequence is important for both hormone binding and receptor activation to induce cAMP synthesis, despite its short length. Specifically, His407 is important for high affinity hormone binding, whereas Asp405, Thr408, and Lys409 are crucial for receptor activation. The data suggest that the five amino acids may form a turn of helix that is an extension of the transmembrane helix 2. In this helical arrangement, Asp405, Thr408, and Lys409 are grouped to form a hydrophilic face of the helix, suggesting a correlation between this arrangement and receptor activation. In addition, the diverse and differential roles of the five amino acids indicate that high affinity hormone binding and receptor activation are discernible functions. These novel observations will be helpful for understanding the activation mechanism of the FSH receptor.

Lys91 and His90 of the alpha-subunit are crucial for receptor binding and hormone action of follicle-stimulating hormone (FSH) and play hormone-specific roles in FSH and human chorionic gonadotropin

Glycoprotein hormones, FSH, LH, CG, and TSH, consist of a common alpha-subunit and a hormone-specific beta-subunit. Both subunits are thought to interact with the hormone receptors. Although several C-terminal residues of hCG alpha are known to contact the LH/CG receptor, little is known about the roles of individual C-terminal residues of FSH alpha. In this report, substitutions of various amino acids for the penultimate Lys91 and the upstream His90 of the alpha-subunit demonstrate that these two residues of FSH alpha are important for high affinity receptor binding and hormone action to induce cAMP production. In contrast, the same residues of hCG alpha are more important for cAMP induction than for high affinity receptor binding. Some substitutions significantly improved receptor binding of FSH and hCG, whereas others were detrimental. Some had the same effect on both hormones, and others impacted differently. Particularly, the substitution of Val for alpha Lys91 resulted in an improved receptor binding of and a loss of cAMP induction by FSH and hCG. On the other hand, the substitution of Arg or Pro for alpha His90 abolished receptor binding of FSH, but not of hCG. These results allowed us to generate an antagonist to FSH. Our results indicate that alpha His90 and alpha Lys91 play roles in receptor binding and cAMP induction of FSH and hCG in strikingly different ways. They will be useful to elucidate the underlying mechanisms for the interaction of FSH and hCG with their complementary receptors as well as for receptor activation.

Cloning and expression of a receptor for an insecticidal toxin of Bacillus thuringiensis

Environmentally friendly toxins of Bacillus thuringiensis are effective in controlling agriculturally and biomedically harmful insects. However, little is known about the insect receptor molecules that bind these toxins and the mechanism of insecticidal activity. We report here for the first time the cloning and expression of a cDNA that encodes a receptor (BT-R1) of the tobacco hornworm Manduca sexta for an insecticidal toxin of B. thuringiensis. The receptor is a 210-kDa membrane glycoprotein that specifically binds the cryIA(b) toxin of B. thuringiensis subsp. berliner and leads to death of the hornworm. BT-R1 shares sequence similarity with the cadherin superfamily of proteins.

Activation of membrane receptors

Many extracellular messengers interact with discriminate receptors on the cell surface. Some of bound ligands activate receptors whereas others fail to do so. Only activated receptors are capable of generating and transferring signal through the membrane. Recent advances in our understanding of agonist-induced and constitutive receptor activation suggest several molecular mechanisms for receptor activation, signal generation and transmembrane signal transfer.

Characterization of different sizes of rat luteinizing hormone/chorionic gonadotropin receptor messenger ribonucleic acids

The LH/CG receptor is uniquely expressed in the gonads of both sexes at specific stages of development. In ovaries, its expression marks particular steps of the ovulation cycle. An enigmatic aspect of expression of the LH/CG receptor is the dramatically diverse transcript sizes [from 7 to < 1 kilobase (kb)] and development-dependent expression of different sizes of mRNAs. It has been thought that mRNAs larger than 2.1 kb encode full-length receptors, whereas those smaller than 2.1 kb encode truncated receptor, because the full-length coding sequence is 2.1 kb. As a first step in elucidation of these diverse mRNAs and corresponding proteins, we have produced a series of cDNA clones and determined their DNA sequences and deduced the amino acid sequences of the resulting proteins. Our data demonstrate that variant mRNAs are produced by alternate splicing and polyadenylation, and they encode significantly shorter truncated receptor peptides. Surprisingly, many of these variant mRNAs are larger than 2.1 kb, and some are 4.2 kb. Some of them are polyadenylated in introns 3, 4, and 10. These alternate mRNAs were successfully expressed in 293 cells to produce receptor peptides 81, 116, and 294 amino acids in length compared to the wild-type receptor, which consists of 674 amino acids. Although these receptor peptides are not secreted, they are capable of binding the hormone, indicating the presence of a hormone contact site(s) in the short peptide fragments, particularly the N-terminal 81-amino acid segment. The data presented here will be helpful for understanding the functions of different sizes of mRNAs and also be valuable in studies designed to investigate whether individual cells express a specific message or multiple messages and how different classes of LH/CG receptor mRNAs are selectively expressed dependent on differentiation and development of the gonads.

Receptor activation of and signal generation by the lutropin/choriogonadotropin receptor. Cooperation of Asp397 of the receptor and alpha Lys91 of the hormone

We have developed a novel method of reciprocal substitution mutation to identify pairing of amino acids within a receptor and its ligand. Using this method, we demonstrate for the first time that a pair of counterionic amino acids, one from the lutropin/choriogonadotropin receptor and the other from the ligand (human choriogonadotropin), cooperate and perhaps interact with each other to activate the receptor and to generate hormonal signal. In this study, Asp397 of the receptor was converted to Lys while Lys91 of the alpha subunit of human choriogonadotropin was substituted with Asp, thus maintaining the counterionic nature of this pair of amino acids. Mutation at each of these positions does not affect the hormone-receptor interaction but results in the significant or complete loss of the bioactivity of both the receptor and the hormone. However, when the impotent mutant receptor and mutant hormone were paired to interact together, they induced cAMP synthesis, resulting in a potent receptor-hormone couple. Substitutions with other amino acids that eliminated the counterionic nature failed to induce cAMP synthesis. Our results shed light on the molecular mechanisms of receptor activation and will serve as a model for other G-protein-coupled peptide receptors, particularly glycoprotein hormone receptors.

Receptor activation is distinct from hormone binding in intact lutropin-choriogonadotropin receptors and Asp397 is important for receptor activation

Hormone binding to receptors on the cell surface triggers a sequence of events (receptor activation and signal generation) leading to activation of effectors in the cytoplasm. Receptor activation and signal generation are difficult to study as both are intimately associated with hormone binding. The lutropin-choriogonadotropin (LH/CG) receptor offers a unique model to differentiate and examine receptor activation and signal generation from hormone binding. It belongs to a subfamily of glycoprotein hormone receptors within the G-protein-coupled receptor family. This receptor subfamily has several structural features different from the structures of other G-protein-coupled receptors. These receptors consist of a large extracellular N-terminal half and membrane-associated C-terminal half of similar size. The truncated N-terminal half alone is capable of high affinity hormone binding, whereas the truncated C-terminal half alone is capable of low affinity hormone binding and cAMP induction. However, this distinction between the high affinity hormone binding and low affinity hormone binding associated with cAMP induction has not been established in intact receptors. As a step to identify a structural element which is responsible for receptor activation and signal generation, we have identified an extracellular Asp of the C-terminal half of the LH/CG receptor which is unique and common to the glycoprotein hormone receptors. Evidence is presented for the first time that Asp397 is important for induction of cAMP synthesis but not essential for hormone binding. Since extracellular Asp397 cannot interact with G-protein in the cytoplasm, the inability of the mutant LH/CG receptors with an Asp397 substitution to induce cAMP synthesis is likely to be caused by a defect in the intermediate steps (receptor activation and signal generation) between hormone binding and activation of G-protein. Therefore, our results not only demonstrate that receptor activation and signal generation are distinct from high affinity hormone binding in intact LH/CG receptors, but they also identify an amino acid important for the processes.

Direct autoradiography after radio-ligand binding to identify mammalian cell clones expressing hormone receptor cDNA

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COOH-terminal amino acids of the alpha subunit play common and different roles in human choriogonadotropin and follitropin

Human choriogonadotropin (hCG) and follitropin (FSH) belong to the glycoprotein hormone family. These hormones are heterodimers and composed of a common alpha subunit and a distinct beta subunit which confers receptor-binding specificities. In addition to this structural similarity, they share a similar signal pathway involving G protein, adenylyl-cyclase and induction of cAMP synthesis. Therefore, a presumptive relationship of these common structure and function has been the subject of extensive past investigations, but a definitive clue has been elusive. As a step to address this important issue, a series of recombinant mutants of hCG and human FSH were generated in which the COOH-terminal amino acids of the alpha subunit were successively removed or substituted. Furthermore, a set of peptides were synthesized with sequences corresponding to different regions of the alpha subunit. Deletion of the alpha COOH-terminal Ser92 had no effect on receptor-binding or cAMP induction by FSH and hCG. Truncation of alpha Lys91-Ser92 or alpha His90-Lys91-Ser92 abolished the ability of both hormones to induce cAMP synthesis. It significantly reduced receptor binding of FSH but not hCG. The different functions of the alpha COOH-terminal region are further noticed with a peptide corresponding to the last 10 amino acids of alpha. It failed to bind to the FSH receptor but was capable of binding to the LH/CG receptor and stimulating cAMP synthesis. These results are the first conclusive evidence that alpha His90-Lys91 play an essential role in cAMP induction of both hormones. In contrast to this common role, they are necessary for FSH binding to the FSH receptor but not for hCG binding to the LH/CG receptor. The hCG alpha COOH-terminal region makes direct contact with the LH/CG receptor, and this low affinity contact is necessary and sufficient to activate the receptor for signal generation. This conclusion is supported by the study using mutant hCGs in which either alpha His90 or Lys91 was substituted.

Analyses of ovine corpora lutea for tumor necrosis factor mRNA and bioactivity during prostaglandin-induced luteolysis

It has been suggested that tumor necrosis factor (TNF) participates in the mechanism of regression of the corpus luteum. We measured luteal expression of TNF alpha mRNA and biological activity during prostaglandin-induced luteolysis in sheep. Initiation of functional luteolysis was marked by a sharp decline in concentrations of progesterone in luteal tissue beginning 4 h after administration of luteolysin. Structural regression of corpora lutea was manifested by a reduction in glandular weight at 16 h. A luteal cytotoxic factor with TNF alpha-like bioactivity was isolated after the decrease in tissue progesterone had occurred, but before evidence of luteal resorption. We were unable to detect temporal alterations in TNF alpha mRNA in luteal samples by classical Northern blot or in situ hybridization analyses. These results imply that luteal TNF alpha is derived primarily as a preformed entity from an extraovarian source, such as infiltrating leukocytes. These results raise the possibility that this cytokine might not be involved in the early stages of luteal regression in the ewe, yet could play a secondary role, perhaps in the subsequent opsonization and removal of degenerating cells.

Conversion of lysine 91 to methionine or glutamic acid in human choriogonadotropin alpha results in the loss of cAMP inducibility

Human choriogonadotropin (hCG) is a heterodimeric glycoprotein hormone. The alpha subunit comprises 92 amino acids, of which 6 are Lys residues (Morgan, F.G., Birken, S., and Canfield, R.E. (1975) J. Biol. Chem. 250, 5247-5258). Our photoaffinity-labeling studies indicate that several of these Lys residues make contact with the lutropin receptor and are covalently cross-linked to the receptor. Lys-91 of the alpha subunit is of interest because deletion of the two alpha C-terminal residues, Lys-91 and Ser-92, results in a significant reduction in the bioactivity of lutropin and thyrotropin (Cheng, K.-W., Glazer, A.N., and Pierce, J.G. (1973) J. Biol. Chem. 248, 7930-7937). To determine the importance of Lys-alpha 91, we substituted it with Arg, Met, or Glu. The resulting mutant alpha cDNA constructs were co-transfected into CHO cells with the wild type hCG beta cDNA construct. Secreted hCG dimers were assayed for binding to receptors on porcine granulosa cells and stimulation of cAMP synthesis in a murine Leydig tumor cell line. The natural hCG, wild type hCG, and all mutant hCGs recognized the receptor, although with somewhat divergent affinities. However, there was a striking difference in the ability of cAMP induction. The natural hCG, wild type hCG, and Lys-91----Arg mutant hCG induced cAMP synthesis, whereas the Lys-91----Met and Lys-91----Glu mutants did not. These results demonstrate that Lys-91 is important for receptor modulation in the stimulation of cAMP synthesis.

Asp383 in the second transmembrane domain of the lutropin receptor is important for high affinity hormone binding and cAMP production

The lutropin (LH), follitropin, and thyrotropin receptors belong to the superfamily of G-protein coupled receptors and have some unique structural features. These glycoprotein hormone receptors comprise a C-terminal half and an N-terminal half of similar size. The C-terminal half is equivalent to the entire structure of other G-protein coupled receptors and has seven transmembrane domains, three cytoplasmic loops, three exoplasmic loops, and a C terminus. In contrast, the hydrophilic N-terminal half is exoplasmic and unique to the glycoprotein hormone receptors. This large N-terminal half of the LH receptor has recently been shown to be capable of binding the hormone. Therefore, these glycoprotein hormone receptors are structurally and functionally different from other G-protein coupled receptors. In an attempt to define the role of the membrane-associated C-terminal half of the LH receptor, we have prepared several mutant receptors in which an Asp or Glu in the seven transmembrane domains has been converted to Asn or Gln, respectively. These include Asp383----Asn in the second transmembrane domain, Glu410----Gln in the third transmembrane domain, and Asp556----Asn in the sixth transmembrane domain. All these mutant receptors were successfully expressed in Cos 7A cells. The Glu410----Gln and Asp556----Asn mutants maintained normal affinities for hormone binding and cAMP production, but the Asp383----Asn mutant showed significantly lower affinities. Although Asp383 of the LH receptor is conserved in all G-protein coupled receptors cloned to date except the substance P receptor, which has Glu in the place of the Asp residue, this is the first observation of the critical role of the Asp in hormone binding and subsequent stimulation of cAMP production.

Structure of the luteinizing hormone receptor gene and multiple exons of the coding sequence

The genomic structure of the LH receptor is important to our understanding of its expression mechanisms, functional domains, relationships with other hormone receptors, and evolution. We have isolated four overlapping cosmid clones and six subgenomic clones of the rat LH receptor gene. They span a total of 95.6 kilobases (kb) and extend from 23 kb upstream of the translation start site to 13 kb down-stream of the stop codon. In addition, part of the human LH receptor gene has been isolated. The coding region of the rat hormone receptor gene spans over 60 kb and consists of 11 exons and 10 introns. Southern blots hybridized with exon 1 and exon 11 probes as well as gene dose analyses demonstrate that a single copy gene encodes the rat LH receptor. Sequence comparison suggests that the porcine and human LH receptor genes have similar, if not identical, exon-intron structures. There is no consensus cAMP-responsive element within 600 basepairs up-stream of the translation start site in spite of the cAMP responsiveness of the LH receptor gene. There are, however, unconventional cAMP-responsive elements in the region: one which is identical, several which are homologous to the activating protein-2-binding elements, CCCCAGGC, and several sequences which are similar to the G-rich cAMP-responsive element found in P450c21, a steroid 21-hydroxylase. The first 10 exons encode the N-terminal half of the molecule, while exon 11 encodes the C-terminal half of the molecule. This last exon is the same in the rat and human genes. The DNA and amino acid sequences of the first 10 exons show significant similarities and reveal repetitive sequence motifs. They have similar sizes which occur in the range of 69 and 183 bases; 8 of them are from 69-81 bases. Despite these remarkable similarities, structural predictions of exons 1-10 show a diversity of structures. The N-terminal half of the LH receptor appears to have a folded structure, with frequent turns and an extensive surface area. Part of the surface is predicted to be covered by amphiphilic helices and beta structures, types of secondary structure frequently found at the interfaces between subunits or between 2 interacting molecules. The introns dividing these exons also share many similarities.(ABSTRACT TRUNCATED AT 400 WORDS)

Exons 1-10 of the rat LH receptor encode a high affinity hormone binding site and exon 11 encodes G-protein modulation and a potential second hormone binding site

We have reported that the rat LH receptor is encoded by 11 exons of a single copy gene. Exons 1-10 encode the N-terminal half and exon 11 the C-terminal half. Since exon splice sites often mark structural transitions of multiexon molecules, we have attempted to define the function of the exons by generating mutant receptors with missing exons. As a first step, we have constructed two LH mutant receptors, one containing exons 1-10 (LH receptor (exon)1-10) and the other containing exon 1 and exon 11 (LH receptor(exon)1&11). These mutant receptors were functionally expressed in Cos 7A cells. The LH mutant receptor(exon)1-10, which lacks the membrane associated C-terminal half of the receptor, showed a high affinity for hCG. Surprisingly, the LH mutant receptor(exon)1&11 recognized hCG with a low affinity and stimulated G-proteins and cAMP production. The results demonstrate that exons 1-10 encode a high affinity hCG binding site and proves an important hypothesis that exon 11 encodes the site for receptor-modulation to activate G-proteins. Furthermore, the results raises an intriguing possibility of a second hormone binding site in the C-terminal half and multistep hormone binding.

Differential interactions of human choriogonadotropin and its antagonistic aglycosylated analog with their receptor

Human choriogonadotropin (hCG) is a heterodimeric hormone consisting of an alpha subunit and a beta subunit. hCG and aglycosylated hCG (aghCG) have similar receptor binding affinities but differ in their ability to activate hormone-responsive adenylate cyclase. aghCG is an effective antagonist. The mechanisms of this antagonism and interactions of antagonistic aghCG with the receptor are not understood. To address this critical question, we have examined the interaction of this hormone analog with the receptor. The hormone receptor on porcine granulosa cells is a glycoprotein of 86 kDa and thas three domains of 24 kDa, 28 kDa, and 34 kDa, which are disulfide-linked. They undergo proteolysis, particularly when bound to the hormones, to produce three polypeptide components. These three receptor components can readily be identified through the use of affinity labeling with the hormones. Affinity labeling with an amino-specific homobifunctional reagent and subsequent cleavage indicate that hCG is cross-linked directly to the 24-kDa receptor component. In contrast, aghCG is cross-linked directly to the 34-kDa component. The peptide map of the cross-linked aghCG-34-kDa receptor component produced by papain treatment is different from the peptide map of the cross-linked complex of hCG-24-kDa component. This difference in receptor binding may be a factor determining the success or failure of signal transduction from the receptor to the effector system, guanine nucleotide-binding regulatory protein, and adenylate cyclase.

Photoaffinity labeling of the gonadotropin receptor with native, asialo, and deglycosylated choriogonadotropin

Human choriogonadotropin (hCG) is a heterodimeric hormone composed of an alpha and a beta subunit. hCG and its asialo (ashCG) and deglycosylated (dghCG) forms vary in their ability to stimulate hormone responsive adenylate cyclase. ashCG is a partial agonist, and dghCG is an antagonist. Photoactivatable moieties were coupled to hCG, ashCG, and dghCG, and the derivatives were radioiodinated. Competitive binding studies indicate that all of the derivatives had a similar affinity for the gonadotropin receptor on porcine granulosa cell membranes. Radiolabeled derivatives were used to photoaffinity label the gonadotropin receptor. Radiolabeled complexes were separated by NaDodSO4/PAGE. All of the derivatives produced similar autoradiographic patterns, except that dghCG produced an additional 48-kDa complex. To investigate the structure of the complexes further, peptide mapping of proteolytic digests was used. All, except for the 48-kDa complex, generated similar peptide maps indicating a relationship between those complexes in which the smaller components are part of the larger. The 48-kDa complex contained both subunits of 40-kDa dghCG. Therefore, this complex is expected to contain an additional component of 8 kDa. The complex was generated whether the hormone-receptor complex was photoaffinity labeled on cells, on isolated membranes, or after solubilizing in detergent. Formation was blocked by excess hCG and did not occur in the absence of UV irradiation. We conclude that the hCG derivatives are able to photoaffinity label the hCG receptor but that the dghCG derivative can photoaffinity label an additional component that was not observed when derivatives of hCG or ashCG were used to label the receptor.

A single amino acid substitution in an ectopic alpha subunit of a human carcinoma choriogonadotropin

Human choriogonadotropin [hCG] has two dissimilar noncovalently associated subunits, designated alpha and beta. An ectopically secreted hCG alpha subunit that fails to associate with the beta subunit and displays an anomalously high molecular weight on molecular sieve chromatography but not on sodium dodecyl sulfate-polyacrylamide gel electrophoresis has been sequenced. A single substitution of Glu56 by Ala56 has been found in the altered subunit. No evidence for conformational differences between normal and ectopic alpha could be found using circular dichroism or intrinsic fluorescence as measures of secondary and tertiary structure, respectively. Hydrophobicity profiles as determined by the method of Kyte and Doolittle (Kyte, J., and Doolittle, R. F. (1982) J. Mol. Biol. 157, 105-132) predicted, however, that the hydrophilic segment, Thr54-Ser55-Glu56-Ser57-Thr58, becomes an extension of the preceding hydrophobic segment when Glu56 is substituted with Ala. This solitary hemoglobin S-like mutation may lead to an altered tertiary structure, self dimerization, or an alteration in glycosylation that could be responsible for the ectopic alpha subunit's failure to associate with the beta subunit.