Partial characterization of circulating inhibin-B and pro-alphaC during development in the male rhesus monkey

Common markers of testicular Sertoli cells

INTRODUCTION

Sertoli cells play central roles in the development of testis formation in fetuses and the initiation and maintenance of spermatogenesis in puberty and adulthood, and disorders of Sertoli cell proliferation and/or functional maturation can cause male reproductive disorders at various life stages. It's well documented that various genes are either overexpressed or absent in Sertoli cells during the conversion of an immature, proliferating Sertoli cell to a mature, non-proliferating Sertoli cell, which are considered as Sertoli cell stage-specific markers. Thus, it is paramount to choose an appropriate Sertoli cell marker that will be used not only to identify the developmental, proliferative, and maturation of Sertoli cell status in the testis during the fetal period, prepuberty, puberty, or in the adult, but also to diagnose the mechanisms underlying spermatogenic dysfunction.

AREAS COVERED

In this review, we principally enumerated 5 categories of testicular Sertoli cell markers - including immature Sertoli cell markers, mature Sertoli cell markers, immature/mature Sertoli cell markers, Sertoli cell functional markers, and others.

EXPERT OPINION

By delineating the characteristics and applications of more than 20 Sertoli cell markers, this review provided novel Sertoli cell markers for the more accurate diagnosis and mechanistic evaluation of male reproductive disorders.

Pubertal Status and Gonadal Functions in Obese Boys with Fatty Liver

BACKGROUND

In adult studies, obese subjects with nonalcoholic fatty liver disease (NAFLD) have been shown to have poor sperm quality, and lower testosterone and luteinizing hormone levels. The aim of this study was to investigate the pubertal status and gonadal functions in obese boys with NAFLD.

MATERIALS AND METHODS

The study included 119 obese and 78 nonobese age-matched adolescents. The obese boys were separated into two groups based on the presence (NAFLD group) or absence of liver steatosis with high transaminases (non-NAFLD group). The levels of serum AMH (anti-Mullerian hormone), inhibin B, gonadotropins, total testosterone, lipids, high-sensitivity C-reactive protein, fasting glucose, insulin levels, and aortic intima media thickness were measured in all subjects.

RESULTS

Of the total 197 children, 174 had reached puberty. There were no significant differences between the groups in respect of testicular sizes and the prevalence of pubertal status among the groups (84.3% of NAFLD vs. 70.6% of non-NAFLD vs. 98.7% of control subjects). No significant differences were found in respect of gonadotropins and AMH levels. Total testosterone levels in the NAFLD group were significantly lower than those of the non-NAFLD obese group (P < 0.001) and the control group (P < 0.001). Inhibin B levels were also significantly lower in all (NAFLD and non-NAFLD) obese groups compared to the control group (P = 0.008).

CONCLUSIONS

The results of the study demonstrated that diminished testosterone and inhibin B levels occur in pubertal obese boys with NAFLD. No significant differences were detected according to pubertal status, AMH levels, and testicular volumes in the age-matched groups.

Inhibin at 90: from discovery to clinical application, a historical review

When it was initially discovered in 1923, inhibin was characterized as a hypophysiotropic hormone that acts on pituitary cells to regulate pituitary hormone secretion. Ninety years later, what we know about inhibin stretches far beyond its well-established capacity to inhibit activin signaling and suppress pituitary FSH production. Inhibin is one of the major reproductive hormones involved in the regulation of folliculogenesis and steroidogenesis. Although the physiological role of inhibin as an activin antagonist in other organ systems is not as well defined as it is in the pituitary-gonadal axis, inhibin also modulates biological processes in other organs through paracrine, autocrine, and/or endocrine mechanisms. Inhibin and components of its signaling pathway are expressed in many organs. Diagnostically, inhibin is used for prenatal screening of Down syndrome as part of the quadruple test and as a biochemical marker in the assessment of ovarian reserve. In this review, we provide a comprehensive summary of our current understanding of the biological role of inhibin, its relationship with activin, its signaling mechanisms, and its potential value as a diagnostic marker for reproductive function and pregnancy-associated conditions.

Serum inhibin-B and follicle stimulating hormone as predictors of the presence of sperm in testicular fine needle aspirate in men with azoospermia

OBJECTIVE

Inhibin-B (Inh-B) is produced by Sertoli cells and controls Follicle Stimulating Hormone (FSH) secretion through a negative feedback mechanism. The primary aim of this study was to compare Iotanh-B with FSH as predictors of the recovery of sperm in testicular fine needle aspirate in men with azoospermia.

DESIGN

In 51 men with azoospermia basal values of Luteinizing Hormone (LH), FSH, prolactin and testosterone as well as Inh-B values before and 24 h and 48 h after the administration of 300 IU recombinant human FSH were determined. Testicular Fine Needle Aspiration (FNA) was also carried out. Thirty-one young healthy men were also enrolled in the study as controls.

RESULTS

There was significant difference between men with azoospermia and controls with regard to the basal Inh-B levels [median (interquartile range) 37.2 (36) vs. 103.0 (90) pg/mL, respectively, p=0.003] but not to the stimulated Inh-B levels [40.5 (41) vs. 73.0 (44) pg/mL, p=0.113 at 24 h and 34.3 (34) vs. 82.0 (50) pg/mL, p=0.098 at 48 h)]. The Area Under Curve in Receiver Operating Characteristic curves were similar for Inh-B and FSH (0.610 vs. 0.716, respectively, p=0.151) as far as prediction of sperm retrieval is concerned.

CONCLUSIONS

Basal serum Inh-B values are significantly lower in men with azoospermia compared to controls. However, Inh-B is not superior to FSH in predicting the presence of sperm in testicular fine needle aspirate.

Activating mutations in the luteinizing hormone receptor gene: a human model of non-follicle-stimulating hormone-dependent inhibin production and germ cell maturation

CONTEXT

Familial male-limited precocious puberty is a dominant autosomal genetic disease caused by activating LH receptor gene mutations, clinically expressed only in males. In preliminary studies, in addition to the expected testosterone increase, we found high inhibin B levels before the age of normal puberty.

OBJECTIVES

The objective of the study was to assess the cellular origin of serum inhibin thanks to testis section immunostaining.

MAIN OUTCOME MEASURES

Serum testosterone, gonadotropin, inhibin B, pan-alphaC-inhibin, and anti-Mullerian hormone levels were measured. Immunostaining was performed using specific anti-alpha- and anti-beta-subunit antibodies.

SUBJECTS AND METHODS

Five boys from three families (mutation M398T or I542L) were investigated at onset (2-6 yr), on ketoconazole treatment, and at adolescence. Testis biopsies were performed in three subjects before the disease was fully characterized.

RESULTS

The high testosterone levels were suppressed by ketoconazole. Anti-Mullerian hormone levels were inversely related to testosterone: low at diagnosis, elevated after testosterone suppression. Despite FSH suppression, inhibin B and pan-alphaC-inhibin levels were high from clinical onset to adolescence. Biopsy specimens showed normal Sertoli cell complement and germ cell maturation until the spermatocyte II stage. Sertoli and Leydig cells displayed positive inhibin alpha-subunit immunostaining. Only Leydig cells and spermatogonia stained positively for the inhibin betaB-subunit.

CONCLUSIONS

Familial male-limited precocious puberty is a unique model of inhibin B secretion, demonstrating that Leydig cells can produce significant amounts of the dimeric molecule. Our results also suggest that the pubertal FSH rise is not required for full expression of the two inhibin B genes and for the initiation of germ cell maturation.

Prepubertal expansion of dark and pale type A spermatogonia in the rhesus monkey (Macaca mulatta) results from proliferation during infantile and juvenile development in a relatively gonadotropin independent manner

The purpose of the present study was to determine whether dark and pale type A spermatogonia (Ad and Ap, respectively) are mitotically active during prepubertal development and whether proliferation of these germ cells during this protracted phase of primate development occurs predominantly during infancy before gonadotropin secretion is arrested. Four neonate (1-2 days of age), four infant (4-5 mo of age), and four juvenile (14-17 mo of age) rhesus monkeys (Macaca mulatta) were castrated 2 h after receiving an i.v. bolus of 5-bromo2'-deoxy-uridine (BrdU, 33 mg/kg body weight). Tissue was fixed in Bouin solution, and 5-microm paraffin sections were cut. Using periodic acid-Schiff reagent/Gill hematoxylin staining, the number per testis of Ad and Ap spermatogonia were determined. BrdU S-phase-labeled nuclei were identified using immunofluorescence. Conservative criteria were employed for classifying cell types, and this resulted in a fraction of A spermatogonia remaining unclassified. Ad, Ap, and the unclassified A spermatogonia each showed an approximately 4-fold increase over the 5-mo period from birth to infancy, and a similar increase was observed over the 10-mo period between infancy and the juvenile stage of development. Both Ad and Ap (and unclassified A spermatogonia) exhibited robust and similar S-phase labeling at the three stages of development. We conclude that the prepubertal expansion of Ad and Ap spermatogonia is achieved by mitotic proliferation that is relatively gonadotropin independent. This conclusion raises the question of the nature of the signal that arrests the cell cycle of Ad in adult testis.

Cloning of a novel inhibin alpha cDNA from rhesus monkey testis

BACKGROUND

Inhibins are dimeric gonadal protein hormones that negatively regulate pituitary FSH synthesis and secretion. Inhibin B is produced by testicular Sertoli cells and is the primary circulating form of inhibin in most adult male mammals. Inhibin B is comprised of the inhibin alpha subunit disulfide-linked to the inhibin/activin betaB subunit. Here we describe the cloning of the cDNAs encoding these subunits from adult rhesus monkey testis RNA.

METHODS

The subunit cDNAs were cloned by a combination of reverse transcriptase polymerase chain reaction (RT-PCR) and 5' rapid amplification of cDNA ends (RACE) RT-PCR from adult rhesus monkey testis RNA.

RESULTS

Both the inhibin alpha and betaB subunit nucleotide and predicted protein sequences are highly conserved with other mammalian species, particularly with humans. During the course of these investigations, a novel inhibin alpha mRNA isoform was also identified. This form, referred to as rhesus monkey inhibin alpha-variant 2, appears to derive from both alternative transcription initiation as well as alternative splicing. rmInhibin alpha-variant 2 is comprised of a novel 5' exon (exon 0), which is spliced in-frame with exon 2 of the conventional inhibin alpha isoforms (variant 1). Exon 1 is skipped in its entirety such that the pro-alpha and part of the alpha N regions are not included in the predicted protein. rmInhibin alpha-variant 2 is of relatively low abundance and its biological function has not yet been ascertained.

CONCLUSION

The data show that the predicted inhibin B protein is very similar between monkeys and humans. Therefore, studies in monkeys using recombinant human inhibins are likely to reflect actions of the homologous ligands. In addition, we have observed the first inhibin alpha subunit mRNA variant. It is possible that variants will be observed in other species as well and this may lead to novel insights into inhibin action.

Molecular weight forms of inhibin a and inhibin B in the bovine testis change with age

To investigate alterations in the molecular weight forms of inhibin in bull testis from the infantile (4-5 wk of age) to postpubertal (49-56 wk of age) periods, testicular homogenates were obtained from animals of various ages and fractionated by a combination of immunoaffinity chromatography and SDS-PAGE. Subsequently, the fractions eluted from the SDS gels were assayed for total inhibin, inhibin A, and inhibin B by fluoroimmunoassay or immunofluorometric assays (IFMAs) and for inhibin bioactivity by an in vitro bioassay. The molecular mass patterns of inhibin A and inhibin B in the testis, as determined by the dimer-specific IFMAs, showed the presence of a peak of approximate 47 kDa until 21-26 wk of age. However, the peak disappeared after 31-32 wk of age. As bulls aged, especially after 31-32 wk of age, inhibin A and inhibin B levels increased in the molecular mass region of 27-34 kDa. Total inhibin showed two peaks, of between 20 and 26 kDa and at approximately 47 kDa, until 21-26 wk of age and a single peak between 20 and 30 kDa after 31-32 wk of age. The eluted fractions corresponding to 29, 31, or 47 kDa gave a dose-response curve that was parallel to the curve generated with 32-kDa inhibin A or 29-kDa inhibin B standard in the IFMA for inhibin A or inhibin B. The fractions corresponding to 29 and 31 kDa suppressed basal release of FSH from rat pituitary cells, but the 47-kDa fraction had a lower FSH-suppressing activity. In the testes of older bulls, immunoblot analysis revealed the presence of a 29-kDa band cross-reacting with inhibin alpha and inhibin betaB antibodies and of a 31-kDa band cross-reacting with inhibin alpha and inhibin betaA antibodies. The 47-kDa band was recognized by the alpha, betaA, and betaB antibodies. Immunohistochemisty of the testis at each age showed that inhibin alpha subunits were found exclusively in Sertoli cells, but the intensity of immunostaining diminished in older bulls, in parallel with the decrease in the testicular concentrations of total inhibin. We conclude that 1) bovine Sertoli cells produce both inhibin A and inhibin B, 2) inhibin production in Sertoli cells during the prepubertal period is characterized by the 47 kDa inhibin-related material that contains precursor forms of inhibin A and inhibin B, and 3) the proportion of the mature forms of inhibin A and inhibin B increases as bulls age, although total inhibin production in Setroli cells decreases.

Pituitary follistatin and activin gene expression, and the testicular regulation of FSH in the adult Rhesus monkey (Macaca mulatta)

In rats, FSHbeta gene expression and FSH secretion are increased and decreased, respectively, by pituitary activin and follistatin. Because little information is available on the paracrine control of FSH secretion in the primate, follistatin and activin/inhibin beta(B) messenger RNA (mRNA) levels were measured in pituitaries of adult male rhesus monkeys 6 weeks after castration or sham surgery (n = 5/group). Follistatin mRNA was determined by quantitative RT-PCR assay using oligonucleotide primers designed to span exons 3-5 of the human follistatin gene. Activin/inhibin beta(B) mRNA levels were measured by ribonuclease protection. Orchidectomy resulted in a 100-fold increase in plasma FSH concentrations and a 60-fold rise in those of LH. In castrated monkeys, levels of mRNA encoding FSHbeta, LHbeta, alpha- subunit, and GnRH receptor (GnRH-R) were increased 21-, 2.1-, 1.7-, and 1.7-fold, respectively (P < 0.01). Levels of pituitary follistatin and activin/inhibin beta(B) mRNAs, however, were similar in castrated and intact animals. These data suggest that the paracrine control of FSH secretion in the male differs substantially in primates and rodents. Specifically, the relatively greater postcastration rise in FSHbeta gene expression and FSH secretion in the adult male monkey may result because in this species pituitary follistatin gene expression does not increase after orchidectomy, as it does in the rat.

Negative feedback regulation of the secretion and actions of gonadotropin-releasing hormone in males

This minireview considers the state of knowledge regarding the interactions of testicular hormones to regulate the secretion and actions of GnRH in males, with special focus on research conducted in rams and male rhesus monkeys. In these two species, LH secretion is under the negative feedback regulation of testicular steroids that act predominantly within the central nervous system to suppress GnRH secretion. The extent to which these actions of testicular steroids result from the direct actions of testosterone or its primary metabolites, estradiol or dihydrotestosterone, is unclear. Because GnRH neurons do not contain steroid receptors, the testicular steroids must influence GnRH neurons via afferent neurons, which are largely undefined. The feedback regulation of FSH is controlled by inhibin acting directly at the pituitary gland. In male rhesus monkeys, the feedback regulation of FSH secretion is accounted for totally by the physiologically relevant form of inhibin, which appears to be inhibin B. In rams, the feedback regulation of FSH secretion involves the actions of inhibin and testosterone and interactions between these hormones, but the physiologically relevant form of inhibin has not been determined. The mechanisms of action for inhibin are not known.

Influence of the degree of stimulation of the pituitary by gonadotropin-releasing hormone on the action of inhibin and testosterone to suppress the secretion of the gonadotropins in rams

This experiment determined if the degree of stimulation of the pituitary gland by GnRH affects the suppressive actions of inhibin and testosterone on gonadotropin secretion in rams. Two groups (n = 5) of castrated adult rams underwent hypothalamopituitary disconnection and were given two i.v. injections of vehicle or 0.64 microg/kg of recombinant human inhibin A (rh-inhibin) 6 h apart when treated with i.m. injections of oil and testosterone propionate every 12 h for at least 7 days. Each treatment was administered when the rams were infused i.v. with 125 ng of GnRH every 4 h (i.e., slow-pulse frequency) and 125 ng of GnRH every hour (i.e., fast-pulse frequency). The FSH concentrations and LH pulse amplitude were lower and the LH concentrations higher during the fast GnRH pulse frequency. The GnRH pulse frequency did not influence the ability of rh-inhibin and testosterone to suppress FSH secretion. Testosterone did not affect LH secretion. Following rh-inhibin treatment, LH pulse amplitude decreased at the slow, but not at the fast, GnRH pulse frequency, and LH concentrations decreased at both GnRH pulse frequencies. We conclude that the degree of stimulation of the pituitary by GnRH does not influence the ability of inhibin or testosterone to suppress FSH secretion in rams. Inhibin may be capable of suppressing LH secretion under conditions of low GnRH.