Relative levels of the proprotein and cleavage-activated form of circulating human anti-Müllerian hormone are sexually dimorphic and variable during the life cycle

Characterization of the molecular mechanisms that govern anti-Müllerian hormone synthesis and activity

The roles of anti-Müllerian hormone (AMH) continue to expand, from its discovery as a critical factor in sex determination, through its identification as a regulator of ovarian folliculogenesis, its use in fertility clinics as a measure of ovarian reserve, and its emerging role in hypothalamic-pituitary function. In light of these actions, AMH is considered an attractive therapeutic target to address diverse reproductive needs, including fertility preservation. Here, we set out to characterize the molecular mechanisms that govern AMH synthesis and activity. First, we enhanced the processing of the AMH precursor to >90% by introducing more efficient proprotein convertase cleavage sites (RKKR or ISSRKKRSVSS [SCUT]). Importantly, enhanced processing corresponded with a dramatic increase in secreted AMH activity. Next, based on species differences across the AMH type II receptor-binding interface, we generated a series of human AMH variants and assessed bioactivity. AMHSCUT potency (EC50 4 ng/mL) was increased 5- or 10-fold by incorporating Gln484 Met/Leu535 Thr (EC50 0.8 ng/mL) or Gln484 Met/Gly533 Ser (EC50 0.4 ng/mL) mutations, respectively. Furthermore, the Gln484 Met/Leu535 Thr double mutant displayed enhanced efficacy, relative to AMHSCUT . Finally, we identified residues within the wrist pre-helix of AMH (Trp494 , Gln496 , Ser497 , and Asp498 ) that likely mediate type I receptor binding. Mutagenesis of these residues generated gain- (Trp494 Phe or Gln496 Leu) or loss- (Ser497 Ala) of function AMH variants. Surprisingly, combining activating type I and type II receptor mutations only led to modest additive increases in AMH potency/efficacy. Our study is the first to characterize AMH residues involved in type I receptor binding and suggests a step-wise receptor-complex assembly mechanism, in which enhancement in the affinity of the ligand for either receptor can increase AMH activity beyond the natural level.

Proteolytic activation of anti-Müllerian hormone is suppressed in adolescent girls

PURPOSE

The ratio of the anti-Müllerian hormone (AMH) precursor (proAMH) to active AMH (AMH_N,C) is higher in childhood than in adulthood but has never been quantified during adolescence. The ratio of proAMH to total AMH (AMH prohormone index, API) was examined during the puberty in healthy girls. The API was also compared between girls with and without polycystic ovary syndrome (PCOS) to determine if there were differences that could assist in PCOS diagnosis during adolescence.

METHODS

Total AMH and proAMH were measured by immunoassay in a single-centre, cross-sectional observational study; 61 controls and 29 girls with PCOS were included in the study (age range 8-21 years). The API was calculated as proAMH as a percentage of total AMH. Differences in API between control and PCOS subjects and across age-groups were examined by Welch's ANOVA. The relationship between API and a range of metabolic parameters was examined by Pearson correlation.

RESULTS

The API in healthy females increased between the ages of 10~15 years and declined from 15~20 years (p < 0.001). The API was negatively correlated with body mass index in the control (p = 0.04) and PCOS groups (p = 0.007). The API was associated with factors related to adiposity and lipid metabolism. The API was not significantly different in control girls and girls with PCOS.

CONCLUSIONS

Higher API during adolescence suggests that proteolytic activation of proAMH is suppressed during this life stage. API was not different between control girls and girls with PCOS indicating that it is not useful in diagnosis of PCOS during adolescence.

Molecular Mechanisms of AMH Signaling

Anti-Müllerian Hormone (AMH) is a secreted glycoprotein hormone with critical roles in reproductive development and regulation. Its chemical and mechanistic similarities to members of the Transforming Growth Factor β (TGF-β) family have led to its placement within this signaling family. As a member of the TGF-β family, AMH exists as a noncovalent complex of a large N-terminal prodomain and smaller C-terminal mature signaling domain. To produce a signal, the mature domain will bind to the extracellular domains of two type I and two type II receptors which results in an intracellular SMAD signal. Interestingly, as will be discussed in this review, AMH possesses several unique characteristics which set it apart from other ligands within the TGF-β family. In particular, AMH has a dedicated type II receptor, Anti-Müllerian Hormone Receptor Type II (AMHR2), making this interaction intriguing mechanistically as well as therapeutically. Further, the prodomain of AMH has remained largely uncharacterized, despite being the largest prodomain within the family. Recent advancements in the field have provided valuable insight into the molecular mechanisms of AMH signaling, however there are still many areas of AMH signaling not understood. Herein, we will discuss what is known about the biochemistry of AMH and AMHR2, focusing on recent advances in understanding the unique characteristics of AMH signaling and the molecular mechanisms of receptor engagement.

Emerging Roles of Anti-Müllerian Hormone in Hypothalamic-Pituitary Function

Since its initial discovery in the 1940s, research into the physiological actions of anti-Müllerian hormone (AMH), from its eponymous role in male developmental biology to its routine clinical use in female reproductive health, has undergone a paradigm shifting change. With several exciting studies recently reporting hitherto unforeseen AMH actions at all levels in the hypogonadal-pituitary-gonadal axis, the importance of this hormone for both hypothalamic and pituitary reproductive control is finding increasing support and significance. In this review, we will briefly summarize what is known about the traditional roles and biology of AMH and how this could be integrated with new findings of AMH actions at the level of the hypothalamic-pituitary axis. We also synthesize the important findings from these new studies and discuss their potential impact and significance to our understanding of one of the most common reproductive disorders currently affecting women, polycystic ovary syndrome.

Towards international standardization of immunoassays for Müllerian inhibiting substance/anti-Müllerian hormone

Research question

Is formulated and lyophilized, recombinant human Müllerian inhibiting substance, also known as anti-Müllerian hormone (AMH), suitable for the preparation of a WHO international standard to calibrate AMH immunoassays?

Design

The AMH content of a trial preparation, coded SS-581, was determined by five laboratories using seven immunoassay methods. Participants were requested to report the content of the preparation in terms of their method calibrators through the measurement of a minimum of five concentrations in the linear part of the dose-response curve. Participants were also asked to measure, concomitantly, a panel of six serum samples containing AMH at concentrations of 0.1–13.0 ng/ml.

Results

Across all assays, including two automated assays in development, the geometric mean content was 361.76 ng/ampoule with a geometric coefficient of variation (GCV%) of 39.95%. When measured by immunoassays that were commercially available at the time of the study, the mean content was 423.08 ng/ampoule, with a GCV% of 26.67%. The inter-method geometric means of five serum samples with an AMH concentration >0.3 ng/ml and measured concomitantly with dilutions of SS-581 varied with a range of GCV% of 14.90–22.35%, which may reflect the use of serum sample value transfer to calibrate current immunoassays, some of which use non-human AMH calibrators. The AMH in trial preparation SS-581 was shown to be biologically active in the Müllerian duct regression assay.

Conclusions

A reference material prepared using human recombinant AMH is a promising candidate for the preparation of an international standard for AMH for immunoassays calibrated to recombinant human AMH.

New Biomarkers to Evaluate Hyperandrogenemic Women and Hypogonadal Men

Androgens can have variable effects on men and women. Women may be evaluated for androgen excess for several reasons. Typically, young premenopausal women present with clinical symptoms of hirsutism, alopecia, irregular menses, and/or infertility. The most common cause of these symptoms is polycystic ovary syndrome. After menopause, even though ovaries stop producing estrogen, they continue to produce androgen, and women can have new onset of hirsutism and alopecia. Laboratory evaluation involves measurement of the major ovarian and adrenal androgens. In women, age, phase of the menstrual cycle, menopausal status, obesity, metabolic health, and sex hormone-binding proteins significantly affect total-androgen levels and complicate interpretation. This review will summarize the clinically relevant evaluation of hyperandrogenemia at different life stages in women and highlight pitfalls associated with interpretation of commonly used hormone measurements. Hypogonadism in men is a clinical syndrome characterized by low testosterone and/or low sperm count. Symptoms of hypogonadism include decreased libido, erectile dysfunction, decreased vitality, decreased muscle mass, increased adiposity, depressed mood, osteopenia, and osteoporosis. Hypogonadism is a common disorder in aging men. Hypogonadism is observed rarely in young boys and adolescent men. Based on the defects in testes, hypothalamus, and/or pituitary glands, hypogonadism can be broadly classified as primary, secondary, and mixed hypogonadism. Diagnosis of hypogonadism in men is based on symptoms and laboratory measurement. Biomarkers in use/development for hypogonadism are classified as hormonal, Leydig and Sertoli cell function, semen, genetic/RNA, metabolic, microbiome, and muscle mass-related. These biomarkers are useful for diagnosis of hypogonadism, determination of the type of hypogonadism, identification of the underlying causes, and therapeutic assessment. Measurement of serum testosterone is usually the most important single diagnostic test for male hypogonadism. Patients with primary hypogonadism have low testosterone and increased luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Patients with secondary hypogonadism have low testosterone and low or inappropriately normal LH and FSH. This review provides an overview of hypogonadism in men and a detailed discussion of biomarkers currently in use and in development for diagnosis thereof.

Elevated prenatal anti-Müllerian hormone reprograms the fetus and induces polycystic ovary syndrome in adulthood

Polycystic ovary syndrome (PCOS) is the main cause of female infertility worldwide and corresponds with a high degree of comorbidities and economic burden. How PCOS is passed on from one generation to the next is not clear, but it may be a developmental condition. Most women with PCOS exhibit higher levels of circulating luteinizing hormone, suggestive of heightened gonadotropin-releasing hormone (GnRH) release, and Anti-Müllerian Hormone (AMH) as compared to healthy women. Excess AMH in utero may affect the development of the female fetus. However, as AMH levels drop during pregnancy in women with normal fertility it was unclear if their levels were also elevated in pregnant women with PCOS. Here, we measured AMH in a cohort of pregnant women with PCOS and control women and found that AMH is significantly more elevated in the former group versus the latter. To determine if the elevation of AMH during pregnancy in women with PCOS is a bystander effect or a driver of the condition in the offspring, we modelled our clinical findings by treating pregnant mice with AMH and followed the neuroendocrine phenotype of their female progeny postnatally. This treatment resulted in maternal neuroendocrine-driven testosterone excess and diminished placental metabolism of testosterone to estradiol, resulting in a masculinization of the exposed female fetus and a PCOS-like reproductive and neuroendocrine phenotype in adulthood. We found that the affected females had persistently hyperactivated GnRH neurons and that GnRH antagonist treatment in the adult female offspring restored their neuroendocrine phenotype to a normal state. These findings highlight a critical role for excess prenatal AMH exposure and subsequent aberrant GnRH receptor signaling in the neuroendocrine dysfunctions of PCOS, while offering a new potential therapeutic avenue to treat the condition during adulthood.

New biomarkers for diagnosis and management of polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is the most common endocrine disorder affecting young women. Even though the definition of PCOS has changed over the years, all diagnostic criteria include two or more of the following: oligomenorrhea/oligoovulation/anovulation, androgen excess and polycystic ovaries (PCO). Traditional method of assessing the ovarian morphology has been transvaginal pelvic ultrasound. Recent studies support that serum anti-Mullerian hormone (AMH) levels correlate with the number of ovarian follicles and cysts. Hence, measurement of AMH is adequate to make the diagnosis. Traditionally, hyperandrogenemia has been assessed by measuring total-testosterone. The literature stresses the importance of sex hormone binding globulin (SHBG) measurements and bioavailable-testosterone and free-testosterone calculations, because insulin resistance decreases SHBG, lowers total-testosterone, and leads to under-estimation of bioavailable- and free-testosterone. Since 50-60% of PCOS patients have metabolic syndrome, assessment of metabolic risk is also necessary. It is important to diagnose insulin resistance before development of glucose intolerance and diabetes. This requires measurements of not only plasma glucose but also insulin concentrations. Determination of HgBA1 can be informative as well. This review aims to present an accurate and cost-effective approach to diagnosis and management of PCOS.

Anti-Müllerian hormone signaling is influenced by Follistatin 288, but not 14 other transforming growth factor beta superfamily regulators

The hypothesis that, in contrast to other transforming growth factor-beta (TGFβ) superfamily ligands, the dose-response curve of Anti-Müllerian hormone (AMH) is unmodulated was tested by examining whether known TGFB superfamily modulators affect AMH signaling, using a P19/BRE luciferase reporter assay. AMH_C and AMH_N,C activated the reporter with an EC50 of approximately 0.5 nM. Follistatins (FS) produced concentration-dependent increases in AMH_C - and AMH_N,C -initiated reporter activity, with FS288 being more potent than FS315; however, the maximum bioactivity of AMH was not altered by either follistatin. Thirteen other TGFβ regulators (Chordin, Chordin-like 1, Chordin-like 2, Differential screening-selected gene aberrative in neuroblastoma [DAN], Decorin, Endoglin, Follistatin-like 1, Follistatin-like 3, Follistatin-like 4, Noggin, α2 macroglobulin, TGFβ receptor 3, Von Willebrand factor C domain-containing 2) had little or no effect. Surface plasmon resonance analysis showed no significant association between FS288 and AMH_C , suggesting that FS288 indirectly regulates AMH signaling. Activin A, a direct target of FS288, did not itself induce reporter activity in P19 cells, but did prevent the FS288-induced increase in AMH signaling. Hence, local concentrations of FS288 and Activin A may influence the response of some cell types to AMH.

Relative levels of the proprotein and cleavage-activated form of circulating human anti-Müllerian hormone are sexually dimorphic and variable during the life cycle

Anti‐Müllerian hormone (AMH) is a gonadal hormone, which induces aspects of the male phenotype, and influences ovarian follicular recruitment. AMH is synthesized as a proprotein (proAMH), which is incompletely cleaved to the receptor‐competent AMH_N,C. AMH ELISAs have not distinguished between proAMH and AMH_N,C; consequently, the physiological ranges of circulating proAMH and AMH_N,C are unknown. A novel proAMH ELISA has been used to assay serum proAMH in humans. Total AMH was also measured, enabling the AMH_N,C concentration to be calculated. Stored serum from 131 boys, 80 younger, and 106 older men were examined, with serum from 14 girls and 18 women included for comparison. The mean levels of proAMH and AMH_N,C in pM were respectively: boys (253, 526), men (7.7, 36), elderly men (5.7, 19), girls (3.3, 15), and women (5.2, 27) (boys vs. men, P < 0.001; girls vs. women, P = 0.032). The proportion of proAMH as a percentage of total AMH (API) was approximately twofold higher in boys than men (P < 0.001) with little overlap between the ranges, with girls also exhibiting lesser cleavage of their AMH than women (P < 0.001). The API varied within each population group. In young men, the API did not correlate with circulating levels of the other testicular hormones (testosterone, InhB, and INSL3). In conclusion, the cleavage of circulating AMH varies extensively within the human population, with most individuals having significant levels of proAMH. The physiological and clinical relevance of circulating proAMH needs to be established.

Changes in Circulating ProAMH and Total AMH during Healthy Pregnancy and Post-Partum: A Longitudinal Study

Circulating Anti-Müllerian hormone (AMH) is derived from the gonads, and is a mixture of the prohormone (proAMH), which does not bind to AMH receptors, and receptor-competent AMH. The functions of a hormone are partially defined by the factors that control its levels. Ovarian reserve accounts for 55~75% of the woman-to-woman variation in AMH level, leaving over 25% of the biological variation to be explained. Pregnancy has been reported to decrease circulating AMH levels, but the observations are inconsistent, with the effect of pregnancy on the bioactivity of AMH being unknown. We have therefore undertaken a longitudinal study of circulating proAMH and total AMH during pregnancy. Serum samples were drawn at 6–8 gestational time-points (first trimester to post-partum) from 25 healthy women with prior uneventful pregnancies. The total AMH and proAMH levels were measured at each time-point using ELISA. The level of circulating total AMH progressively decreased during pregnancy, in all women (p<0.001). On average, the percentage decline between the first trimester and 36–39 weeks’ gestation was 61.5%, with a standard deviation of 13.0% (range 30.4–81.2%). The percentage decline in total AMH levels associated with maternal age (R = -0.53, p = 0.024), but not with the women’s first trimester AMH level. The postpartum total AMH levels showed no consistent relationship to the woman’s first trimester values (range 31–273%). This raises the possibility that a fundamental determinant of circulating AMH levels is reset during pregnancy. The ratio of proAMH to total AMH levels exhibited little or no variation during pregnancy, indicating that the control of the cleavage/activation of AMH is distinct from the mechanisms that control the total level of AMH.