Biomedical subjects

M R Sairam

Publications and source records attributed to M R Sairam.

At least 19 recordsLinked to original sources

PubMed2002

Chronic estrogen deficiency leads to molecular aberrations related to neurodegenerative changes in follitropin receptor knockout female mice.

The follitropin receptor knockout (FORKO) mouse undergoes ovarian failure, thereby providing an animal model to investigate the consequences of the depletion of circulating estrogen in females. The estrogen deficiency causes marked defects in the female reproductive system, obesity, and skeletal abnormalities. In light of estrogen's known pleiotropic effects in the nervous system, our study examined the effects of genetically induced estrogen-testosterone imbalance on this system in female FORKO mice. Circulating concentrations of 17-beta-estradiol (E2) in FORKO mice are significantly decreased (FORKO -/-: 1.13+/-0.34 pg/ml; wild-type +/+: 17.6+/-3.5 pg/ml, P<0.0001, n=32-41); in contrast, testosterone levels are increased (-/-: 37.7+/-2.3 pg/ml; wild-type +/+: 3.9+/-1.7 pg/ml, P<0.005, n=25-33). The focus was on the activities of key enzymes in the central cholinergic and peripheral nervous systems, on dorsal root ganglia (DRGs) capacity for neurite outgrowth, and on the phosphorylation state of structural neurofilament (NF) proteins. Choline acetyltransferase activity was decreased in several central cholinergic structures (striatum 50+/-3%, hippocampus 24+/-2%, cortex 12+/-3%) and in DRGs (11+/-6%). Moreover, we observed aberrations in the enzymatic activities of mitogen-activated protein kinases (extracellular-regulated kinase and c-Jun N-terminal kinase) in the hippocampus, DRGs, and sciatic nerves. Hippocampal and sensory ganglia samples from FORKO mice contained hyper-phosphorylated NFs. Finally, explanted ganglia of FORKO mice displayed decreased neurite outgrowth (20-50%) under non-treated conditions and when treated with E2 (10 nM). Our results demonstrate that genetic depletion of circulating estrogen leads to biochemical and morphological changes in central and peripheral neurons, and underlie the importance of estrogen in the normal development and functioning of the nervous system. In particular, the findings suggest that an early and persisting absence of the steroid leads to neurodegenerative changes and identify several key enzymes that may contribute to the process. This model provides a system to explore the consequences of circulating estrogen deprivation and other hormonal imbalances in the nervous system.

Acetylcholine↗

PubMed2001

Characterization of regulatory elements of ovine follicle-stimulating hormone (FSH) receptor gene: the role of E-box in the regulation of ovine FSHreceptor expression.

Expression and activation of follicle-stimulating hormone receptor (FSHR) in the granulosa and Sertoli cells are required for normal development of the ovarian follicles and germ cells. However, little is known regarding the mechanisms by which FSHR expression is regulated. We fused an ovine FSHR promoter to a luciferase gene to understand the promoter regulation in two gonadal cell lines. Deletion studies revealed that the strongest promoter was at -200 to +163 relative to the transcription start site. One of cis-elements protected from DNase I digestion was mapped to between +32 and +54 of the 174-base pair (bp) minimal promoter. Electrophoretic mobility shift assay with a 26-bp probe (+32 to +57) and nuclear extracts from Sertoli (15P1) and granulosa (JC-410) cell lines demonstrated a sequence-specific DNA-protein complex. Southwestern analysis detected a 43-kDa protein bound to the 26-bp probe. Gel supershift with upstream stimulatory factor 1 and 2 (USF-1/2) antibodies revealed that the DNA-protein complex contained these two transcription factors. Mutation within the E-box of the promoter abolished the sequence-specific binding and the minimal promoter activity but also greatly reduced the transcription of the proximal promoters by 49%-70%. These data suggest that the USF-1/2 binding to the promoter is required for the expression of the ovine FSHR in the gonadal cells.

5' Untranslated Regions↗

PubMed2001

Delay in sexual maturity of the follicle-stimulating hormone receptor knockout male mouse.

In the highly organized and complex process of mammalian spermatogenesis, the development of an undifferentiated diploid germ cell into a fully differentiated and mature spermatozoon is orchestrated in a time frame unique for each species including man. If the various hormonal signals including environmental cues that play a critical part in initiating these events are not properly executed, various deficiencies including delay in sexual maturity or puberty are likely. In this study we have followed testicular development and spermatogenesis in the FSH receptor knockout (FORKO) mice from Day 7 onward by using histology and quantitative DNA flow cytometry. The drastic reduction in testicular weight and shrinkage of seminiferous tubules that occurred at this early age persisted into the adult stage in the FORKOs, suggesting inhibition of the initial developmental processes. The round spermatids that were clearly abundant on Day 21 in the wild-type and heterozygous males were few and present only in some tubules of the FORKOs. There were no elongated spermatids in FORKO males on Day 35. The sperm produced by Day 49 FORKOs were already aberrant, a feature that persisted into adulthood in these animals. As all these changes occurred in a background of normal circulating testosterone levels, we may conclude that the delay in testicular development is a consequence of the loss of FSH-receptor signaling. The delay in sexual maturity of FORKOs was accompanied by reduction in fertility as evidenced by mating studies. Based on these data we suggest that the FORKO mouse might be a useful experimental model to define the molecular mechanisms that underlie the delay in puberty.

Animals↗

PubMed2001

Role of CACC-box in the regulation of ovine follicle-stimulating hormone receptor expression.

Tissue-specific and stage-specific expression of follicle-stimulating hormone receptor (FSH-R) in granulosa and Sertoli cells is required for normal development of ovarian follicles and germ cells. However, little is known of the transcription factors that regulate the FSH-R gene and its promoter. Using an ovine FSH-R promoter as a model system, we have identified a second DNase I footprinting 2 (FP2) region from -46 to -67 of the strongest ovine FSH-R promoter (-200 to +163) relative to the transcription start site. Electrophoretic mobility shift assay with a 22-base pair DNA probe (-46 to -67) and nuclear extracts from Sertoli (15P1) and granulosa (JC-410) cell lines demonstrated a sequence-specific DNA-protein complex. Further Southwestern and UV cross-linking analyses detected three predominant proteins of molecular weights 87, 60, and 50 kDa present in both Sertoli and granulosa cells bound to a 32P-labeled DNA probe as a complex. Gel competition experiments with DNA probes containing known Krupple-like factor binding sites revealed that the testis-specific zinc finger protein, ZNF202-like factor, Ras-responsive element binding protein-like factor, or both, may be among the potential candidate regulators. Mutation within the CACC box of the promoter abolished Krupple-like factor binding and significantly diminished promoter activity in both gonadal cells. These data suggest that Krupple-like transcription factors may play a role in the regulation of ovine FSH-R expression.

Animals↗

PubMed2001

Intercellular communication between Sertoli cells and Leydig cells in the absence of follicle-stimulating hormone-receptor signaling.

Effective interactions among the various compartments of the testis are necessary to sustain efficiency of the spermatogenic process. To study the intercellular communication between the Sertoli and Leydig cells in the complete absence of FSH receptor signaling, we have examined several indices of Leydig cell function in FSH receptor knockout (FORKO) mice. The serum testosterone levels were reduced in the 3- to 4-mo-old adult FORKO males compared to wild-type mice despite no significant alteration in circulating LH levels. Treatment with ovine LH resulted in a dose-dependent increase in serum testosterone levels in all three genotypes (+/+, +/-, and -/-). However, the response in FORKO males was significantly reduced. Similarly, the total intratesticular testosterone per testis was also lower, but the intratesticular testosterone per milligram of testis was significantly elevated in the FORKO males. Western blot analysis revealed an apparent higher expression of the enzyme 3beta-hydroxysteroid dehydrogenase (3beta-HSD) as well as LH-receptor density in the testis of FORKO males. Immunohistochemistry also showed an increase in the intensity of 3beta-HSD staining in the testicular sections of FORKO males. Although LH receptor binding increased per unit weight in FORKO mice, the total LH binding remained the same in all genotypes. Taken together, the results of the present study suggest that, in the absence of FSH receptor signaling, the testicular milieu is altered to affect Leydig cell response to LH such that circulating testosterone is reduced in the adult mutant. Studies are currently under way to understand the mechanisms underlying this phenomenon.

3-Hydroxysteroid Dehydrogenases↗

PubMed2001

Hormone-induced receptor gene splicing: enhanced expression of the growth factor type I follicle-stimulating hormone receptor motif in the developing mouse ovary as a new paradigm in growth regulation.

The acquisition of FSH receptor(s) during follicular growth and their coupling to signaling pathways are key events in follicular development and dominance. However, little is known about the precise nature of the FSH receptor(s) involved in the growth-promoting phases of hormone action. To investigate the hormonal regulation of a newly discovered, alternatively spliced, growth factor type 1 receptor (designated FSH-R3) for the hormone, we examined expression in the adult mouse and the effect of PMSG treatment in the immature mouse ovary. Using RT-PCR and primers based on the established sheep ovarian transcript, a part of the FSH-R3 message was amplified only in wild-type (+/+), but not in the FSH-R knockout (-/-), mouse ovary. Semiquantitative RT-PCR using 3'-end primers specific for FSH-R1 (G(s)-coupled) and FSH-R3 indicated expression levels of the latter to be higher when follicular growth was induced by PMSG. Using FSH-R3-specific peptide IgG, FSH-R3 protein was detected by Western blotting in extracts of adult mouse ovary and was localized in granulosa cell membrane of mature follicles. In the immature mouse, levels of FSH-R3 protein that increased after PMSG administration in a time-dependent manner were also localized only on granulosa cell membranes of large follicles. The results reveal for the first time the expression of a different growth-promoting receptor for FSH in the developing and cycling mouse ovary. These observations introduce a new paradigm in the control of ovarian function.

Alternative Splicing↗

PubMed2001

Ovarian pathology and high incidence of sex cord tumors in follitropin receptor knockout (FORKO) mice.

In this investigation we describe our observations of the status of the aging ovary in mice with disruption of the receptor for FSH. Knockout mice at 3-5 months of age are acyclic and sterile, with very small, underdeveloped ovaries. Thus, they exhibit hypergonadotropic-hypogonadism with high levels of circulating FSH similar to the postmenopausal state in women. By 12 months more than 92% of these animals developed various kinds of ovarian pathology, including neoplasms of sex cord-stromal type as well as cysts. Interestingly, the majority of tumors were located in the right ovary, with the contralateral ovary remaining unaffected but atrophic. The ovary from heterozygotes also showed pathology after 15 months. None of the age-matched wild-type mice that remained fertile developed any sign of ovarian tumors. Circulating LH and FSH levels were increased in follitropin receptor knockout mice and remained severalfold higher in tumor-bearing animals. The histological appearances of ovarian tumors were similar to the pathology observed in some types of sex cord-stromal neoplasms in women. The tumor burden caused weight loss and cachexia in follitropin receptor knockout mice. Based on these characteristics as well as the high incidence of ovarian pathology in the aging mutant, we propose that the loss of the FSH receptor signaling mechanisms predispose the ovary to molecular and structural changes leading to tumor formation. Hence, in the intact and fertile animal, FSH receptor signaling offers a protective mechanism that is lost upon reproductive senescence (menopause in women). Further studies are warranted in this genetic model to explore the molecular changes underlying the development of ovarian neoplasia.

Animals↗

PubMed2000

Activation of extracellular-regulated kinase pathways in ovarian granulosa cells by the novel growth factor type 1 follicle-stimulating hormone receptor. Role in hormone signaling and cell proliferation.

Follicle-stimulating hormone (FSH) regulated growth and function of the ovarian follicle was previously thought to be mediated solely through activation of G(s)-coupled receptors. In this study, we show for the first time that this function is predominantly mediated through the alternatively spliced and novel growth factor type 1 receptor (oFSH-R3) that is also present in the ovary. Immortalized granulosa cells lacking endogenous FSH receptors, when transfected with either oFSH-R3 cDNA (JC-R3) or the G(s)-coupled oFSH-R1 (JC-R1), expressed the corresponding glycosylated receptor. In JC-R3 or JC-R1 cells labeled with bromodeoxyuridine or [(3)H]thymidine, FSH stimulated the cells to progress through S-phase and divide. The growth promoting effect of recombinant FSH in JC-R3 cells was preceded by the rapid activation of ERK1 and ERK2. This effect was hormone-specific and transient. In JC-R3 cells inhibitors like calphostin C, PD98059, Ag 18, or calcium chelators EGTA or 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid/AM inhibited both mitogen-activated protein kinase activation and bromodeoxyuridine incorporation. FSH induced phosphorylation of the FSH-R3 receptor was blocked by pretreating cells with calphostin C. There was no cAMP induction by FSH in JC-R3 cells. The cAMP independent growth promoting effect of FSH is mediated by activation of Ca(2+) and mitogen-activated protein kinase-dependent pathways. Thus, alternative splicing of a G-protein coupled receptor creates the expression of a novel receptor motif that can mediate a widely recognized function of the glycoprotein hormone.

Animals↗

PubMed2000

Effect of deprival of LH on Leydig cell proliferation: involvement of PCNA, cyclin D3 and IGF-1.

The levels of proliferating cell nuclear antigen (PCNA) and cyclin D3 which are known markers of cellular proliferation were monitored by immunoblotting in progenitor Leydig cells (PLC), immature Leydig cells (ILC) and adult Leydig cells (ALC) isolated from 21, 35 and 90 day old rats, respectively which represent the Leydig cells at different stages of development. The levels of PCNA and cyclin D3 were highest in PLC, intermediate in ILC and lowest in ALC. Following administration of an antiserum to LH to deprive endogenous LH in 21 day old rats, a significant decrease in the levels of PCNA and Cyclin D3 were observed suggesting the involvement of Lutenizing hormone (LH) in PLC proliferation. In support of this observation, Bromodeoxyuridine (BrdU) incorporation was highest in PLC when compared with ILC and ALC, and administration of LH antiserum to 21 day old rats led to a total absence of BrdU incorporation by the isolated PLC. Also, there was a decrease in the level of IGF-1 and IGF-1 receptor mRNA levels by 55 and 35%, respectively as assessed by semi-quantitative RT-PCR. In addition, the PLC isolated from rats deprived of endogenous LH incorporated much less BrdU following addition of IGF-1. These results, which are obtained using an in vivo model system establish that LH has a very important role in Leydig cell proliferation in immature rats.

Animals↗

PubMed2000

Follicle-stimulating hormone mediated calcium signaling by the alternatively spliced growth factor type I receptor.

Ovarian granulosa cell and testicular Sertoli cell functions are regulated by the tropic action of the pituitary follicle-stimulating hormone (FSH), which may exert pleiotropic effects using a variety of signaling pathways. The effects of FSH on the mobilization of Ca(2+) into granulosa and Sertoli cells have been widely studied, but whether all the effects of the hormone are mediated by the single G-protein-coupled (G(s)) receptor with the seven-transmembrane structure (R1) has remained an enigma. With the object of resolving this mystery, we have compared the hormonal responses of HEK 293 cells transfected with three different cloned FSH receptor cDNAs of testis/ovary, designated R1 (G(s)), R2 (similar to R1 but having a shorter carboxyl terminus), and R3, a novel FSH receptor exhibiting a growth factor type I receptor motif. The latter two that use the same DNA segment for alternative splicing of the single large 80- to 100-kilobase gene create different structural motifs and carboxyl termini. Of the three receptors, only the FSH-R3 type induced a significant rise in intracellular free calcium concentration ([Ca(2+)](i)), as measured by single cell fluorescence digital imaging with the Ca(2+) sensitive dye fura-2AM. FSH induced a rapid [Ca(2+)](i) response that was concentration dependent. The response was hormone-specific, as neither its individual alpha/beta subunits nor the related glycoprotein hormone LH were effective. To determine whether the [Ca(2+)](i) response was due to Ca(2+) influx or to intracellular Ca(2+) mobilization, cells were exposed to Ca(2+)-free buffer and to the Ca(2+)-channel blocker diltiazem (10(-5) M). FSH-Induced [Ca(2+)](i) responses were inhibited in Ca(2+)-free buffer and abrogated in the presence of diltiazem. These novel data demonstrate that FSH can increase [Ca(2+)](i) through L-type voltage-dependent Ca(2+) channels via the growth factor type 1 receptor. Our findings support the concept that different receptor motifs act to integrate intracellular signaling events.

Alternative Splicing↗

PubMed2000

Qualitative and quantitative decline in spermatogenesis of the follicle-stimulating hormone receptor knockout (FORKO) mouse.

Sertoli cells express functional receptors for FSH, one of the two pituitary hormones that regulate spermatogenesis in mammals. We recently produced genetic mutant (FORKO) mice that lack FSH receptor, in order to examine the effects on testicular function and fertility. Mutant males exhibited weight loss of testis, epididymis, and seminal vesicle as well as low levels of testosterone. Except for reduced seminiferous tubular diameter, no gross changes were apparent upon histological examination. Analysis of testicular germ cells by flow cytometry revealed a significant increase in the percentage of 2C cells (spermatogonia and non-germ cells) and a significant decrease in the percentage of HC cells (elongated spermatids) of FORKO males. The absolute number of homogenization-resistant elongated spermatids was also significantly reduced in the mutant males. A 2-fold increase in c-kit-positive 2C cells was recorded in the mutant males. Elongated spermatids of FORKO males showed a dramatic increase in propidium iodide binding suggesting reduced nuclear compaction. The increase in size of the sperm head in mutants, as well as susceptibility to dithiothreitol-induced decondensation, suggests the inadequate condensation of sperm chromatin. Sperm chromatin structure assay, a technique that reflects DNA stability, revealed that sperm from FORKO males are susceptible to acid denaturation, indicating the poor quality of sperm. These data allow us to conclude that genetic disruption of FSH receptor signaling in the rodent induces major changes that might contribute to reduced fertility.

Animals↗

PubMed2000

Estrogen deficiency, obesity, and skeletal abnormalities in follicle-stimulating hormone receptor knockout (FORKO) female mice.

Targeted disruption of the receptor for glycoprotein hormone, FSH (FSH-R) causes a gene dose-related endocrine and gametogenic abnormality in female mice. The resulting FSH-R knockout (FORKO) mutants have disordered estrous cycles, ovulatory defects, and atrophic uterus. The heterozygous animals that initially show reduced fertility undergo early reproductive senescence and stop breeding altogether. Lack of FSH-R signaling in females causes severe ovarian underdevelopment producing chronic estrogen deficiency. This was accompanied by increases in serum testosterone levels. Ovarian aromatase gene transcription and translation are unaltered in the mutants. Early loss of estrogen in the null mutants leads to obesity and skeletal abnormalities that intensify with age producing (kyphosis), a hunchback appearance. Both these changes also become apparent in older heterozygous mice coincident with early reproductive senescence. The expression of nuclear estrogen receptor(s) alpha and beta genes and the corresponding proteins in the ovary and uterus of FORKO mice appear to be intact. The loss of ovarian estrogen creates an imbalance in A and B forms of the progesterone receptor in the uterus of both heterozygotes and null mutants. Some of the changes we have documented here in FORKO mice are reminiscent of the ovarian dysfunction and other major symptoms that are usually associated with estrogen deficiency. In null mutants, estradiol-17beta administration promptly induced uterine growth and reversed the accumulation of adipose tissue indicating that estrogen receptors are functional. Thus, the phenotypes evident in these genetically altered FSH-R mutants may provide an experimental system to explore the effects of estrogenic compounds on different targets including the ovary in a nonsurgical setting.

Animals↗

PubMed1999

Structural features and expression of an alternatively spliced growth factor type I receptor for follitropin signaling in the developing ovary.

The pleiotropic actions of pituitary follitropin (FSH), regulate the expression of many cell cycle genes controlling ovarian follicular development and differentiation. In this study we asked the question whether different receptor motifs are created by the alternative splicing of the single large 80-100 Kb receptor gene. A 1.2 Kb transcript identified from a cDNA library of hormone primed (immature) sheep ovaries, codes for a putative protein lacking the seven transmembrane segment. The receptor of 259 amino acids designated FSH-R3 is derived from a transcript comprising the first eight exons of the Gs coupled larger FSH receptor (R1) spliced to another DNA segment. This event produces a different carboxyl terminus at the junction creating a novel receptor motif with a single membrane spanning domain, assigning it to the growth factor type I receptor family. In transfected cells the expressed receptor localizes on the cell surface and specific antibodies directed against the unique C-terminal portion (residues 242-259) of FSH-R3 demonstrate the presence of the receptor protein in solubilized ovarian and testicular membrane preparations. FSH binding to the transfected cells induced [Ca2+]i identifying coupling of the R3 receptor to calcium signaling pathways. Thus, a growth factor type I receptor for FSH may be implicated in the growth promoting actions of FSH in the ovary. This is the first documentation of alternative splicing of a G protein coupled receptor gene creating a different signaling motif for cellular signaling.

Alternative Splicing↗

PubMed1999

Hormonal regulation of natriuretic peptide system during induced ovarian follicular development in the rat.

All components of the natriuretic peptide (NP) system have been found in the ovary. The purpose of this study was to determine the hormonal regulation of the NP system during follicular growth and ovulation induced by gonadotropins eCG and hCG. Ovarian membrane binding, before and after treatment, revealed the presence of guanylyl cyclase-type receptors exclusively. Equine CG treatment increased Bmax from 225 +/- 50 fmol/mg protein in control animals to 354 +/- 51 fmol/mg protein, and additional hCG treatment increased it further to 492 +/- 130 fmol/mg protein (p < 0.05), without changing receptor affinity. The increased binding was consistent with increased ability of atrial natriuretic peptide (ANP) to activate guanylyl cyclase in the ovarian cells obtained from hormone-treated animals. In confirmation, autoradiography of 125I-tyroCNP and 125I-ANP binding to the rat ovary showed that both guanylyl cyclase GC-A and GC-B receptor subtypes are localized to the granulosa cells of antral follicles. Quantitative analysis of GC-A and GC-B receptors by reverse transcription-polymerase chain reaction showed that the expression level of both receptors started to increase at 2 h and reached maximal levels at 6 h following eCG treatment. Increased levels of GC-B mRNA were also observed 12 h after eCG injection. At 24 and 48 h the receptor levels were below basal. Stimulation of NP receptors by eCG was paralleled by activation of both ovarian ANP and C-type natriuretic peptide (CNP) gene expression. ANP mRNA increased as early as 1 h after eCG injection and remained elevated up to 6 h. CNP mRNA increased at 2 h after eCG injection, peaked (5-fold) at 6 h, and remained elevated 48 h later, a stage at which follicular maturation continues. Incubation of ovaries with ANP significantly decreased eCG-induced estradiol level, indicating the functionality of the ovarian NP system. These results implicate the NP system in the induction and maintenance of fluid balance in the rapidly developing ovarian follicle.

Animals↗

PubMed1998

Impairing follicle-stimulating hormone (FSH) signaling in vivo: targeted disruption of the FSH receptor leads to aberrant gametogenesis and hormonal imbalance.

Pituitary gonadotropins follicle-stimulating hormone (FSH) and luteinizing hormone stimulate the gonads by regulating germ cell proliferation and differentiation. FSH receptors (FSH-Rs) are localized to testicular Sertoli cells and ovarian granulosa cells and are coupled to activation of the adenylyl cyclase and other signaling pathways. Activation of FSH-Rs is considered essential for folliculogenesis in the female and spermatogenesis in the male. We have generated mice lacking FSH-R by homologous recombination. FSH-R-deficient males are fertile but display small testes and partial spermatogenic failure. Thus, although FSH signaling is not essential for initiating spermatogenesis, it appears to be required for adequate viability and motility of the sperms. FSH-R-deficient females display thin uteri and small ovaries and are sterile because of a block in folliculogenesis before antral follicle formation. Although the expression of marker genes is only moderately altered in FSH-R -/- mice, drastic sex-specific changes are observed in the levels of various hormones. The anterior lobe of the pituitary gland in females is enlarged and reveals a larger number of FSH- and thyroid-stimulating hormone (TSH)-positive cells. The phenotype of FSH-R -/- mice is reminiscent of human hypergonadotropic ovarian dysgenesis and infertility.

Animals↗

PubMed1998

Is there a true requirement for follicle stimulating hormone in promoting spermatogenesis and fertility in primates?

Although the role of follicle stimulating hormone (FSH) in regulating ovarian function is well accepted, its need in regulating testicular function of the adult continues to be debated. Sertoli cells of all mammals have FSH receptors and are known to regulate differentiation and transformation of germ cells to spermatozoa. However, there appear to be species and age differences in the way in which FSH regulates spermatogenesis. An attempt has been made in the current paper to examine critically the newer data available in support of and against the concept that FSH is required to regulate spermatogenesis and fertility of the primate. As there is no evidence to indicate that testicular function in monkeys and humans is regulated differently, the information available using the monkey as the experimental surrogate model is discussed in some depth. These are correlated, wherever feasible, to the newer information emerging from clinical studies. It appears from these studies that in the primate (including humans) FSH, besides promoting quantitative spermatogenesis leading to production of a normal number of spermatozoa, has a critical role in regulating spermiogenesis, the process that controls the formation of normal fertilizable mature spermatozoa. While the requirement for FSH in promoting fertility in the male monkey is reasonably well established, in humans the evidence currently available in favour of the concept is still circumstantial and more work needs to be done to establish the hypothesis beyond any doubt.

Animals↗

PubMed1997

Molecular cloning, structure, and expression of a testicular follitropin receptor with selective alteration in the carboxy terminus that affects signaling function.

During the molecular cloning of the ovine testicular follicle-stimulating (FSH) receptor that couples to the Gs-type effector systems, we discovered novel cDNA clones that were highly homologous. Some of these clones contained an insert of 1,584 bp, which consisted of a divergent 3' region spliced with a 5' region that was identical to nucleotides 724-1,924, forming part of the 9th and 10th exons, of the coding region of the ovine FSH receptor gene. The prominence of alternately spliced clone, which suggested important functional implications, prompted this detailed investigation. Screening of the library by polymerase chain reaction and Northern analysis of testicular messenger RNA with a specific ribo-probe directed to the divergent 3' region of this transcript suggested existence of a full-length transcript of roughly 2.4 kb size. The cDNA was assembled and characterized for its structure. The predicted full-length sequence consisting of nucleotides -121-1,924 of the ovine FSH receptor and the novel 3' region (nucleotides 1,925-2,307) encoded a protein of 670 amino acids containing the entire extracellular and transmembrane domains of the ovine FSH receptor. However, a frame-shift in the coding sequence at the point of divergence resulted in a shorter (40 residues vs. 65 for ovine FSH receptor) C-terminus with three cysteine residues and a reduced number of potential phosphorylation sites. Two of the cysteine residues were adjacent and are apparently potential double palmitoylation sites compared to the single site present in the Gs coupled ovine FSH receptor. Stable expression of this novel transcript in human embryonic kidney (HEK 293) cells revealed the complete absence of cyclic AMP inducible functions, but presence of specific hormone binding activity on plasma membranes and prominent cell surface immunostaining by antireceptor antiserum. There was no alteration in hormone binding specificity because the structurally analogous luteinizing hormone (LH) did not bind to the receptor. The loss of cyclic AMP stimulation in the transfected cells was completely opposite to the properties of the cells expressing the active wild-type receptor. When cells expressing active receptors were cotransfected with the altered FSH receptor cDNA, hormone action was inhibited, suggesting that it could be functioning as a dominant negative receptor. The existence of this ovine FSH receptor with an altered carboxyl terminus predicts the utilization of an alternative splicing mechanism for regulation of receptor expression, signalling and gonadal function. Our study reveals that the modular structure of the FSH receptor gene generates motifs that allows coupling to different effectors. This could become a common feature for all glycoprotein hormone receptors.

Adenylyl Cyclases↗

PubMed1997

Alternative splicing converts the G-protein coupled follitropin receptor gene into a growth factor type I receptor: implications for pleiotropic actions of the hormone.

Pituitary follitropin (FSH) has pleiotropic actions on gonads, but it is not certain if all these events are mediated by a single receptor. A single gene for the FSH receptor undergoes extensive alternate splicing generating multiple transcripts, and several of these have been cloned and characterized from the sheep testis. In this study we have investigated the expression in HEK (human embryonic kidney) 293 cells of a cloned cDNA encoding the first eight exons of the FSH receptor along with a carboxyterminal extension that contributed a hypothetical single transmembrane domain. This cDNA, which lacked the conventional seven transmembrane motif of the full-length 695 residue wild-type receptor protein, was also efficiently expressed on the cell surface and exhibited high affinity and specificity for FSH binding. LH did not compete for FSH binding indicating that these structures contained all the motifs necessary for specific hormone recognition. Following hormone binding and affinity crosslinking the deduced M(r) of the expressed receptor was compatible with dimer formation. The expression of these altered FSH receptors on the cell surface was confirmed by immunohistochemistry, which revealed punctate labeling in a pattern comparable to that shown by cells transfected by wild-type receptor cDNA. Addition of FSH stimulated 3H-thymidine incorporation in transfected cells in a biphasic manner. By performing RT-PCR we could show that similar altered receptor transcripts were present in both the ovary and testis. Our results reveal for the first time that the seven transmembrane structure of FSH-receptor is not absolutely necessary for cell surface expression and hormone binding provided other compensating motifs are present in the protein structure for membrane insertion. Some of these features are typical of growth factor receptors. Our investigations also demonstrate that alternate splicing of the FSH receptor gene provides a mechanism for creating receptor diversity and suggest that multiple receptors could be involved in regulation of hormone action.

Alternative Splicing↗