Reproductive failure in mice expressing transgenic follicle-stimulating hormone is not caused by loss of oocyte quality

Gonadotropin elevation is ootoxic to ovulatory oocytes and inhibits oocyte maturation, and activin decoy receptor ActRIIB:Fc therapeutically restores maturation

BACKGROUND

Elevated FSH often occurs in women of advanced maternal age (AMA, age ≥ 35) and in infertility patients undergoing controlled ovarian stimulation (COS). There is controversy on whether high endogenous FSH contributes to infertility and whether high exogenous FSH adversely impacts patient pregnancy rates.

METHODS

The senescence-accelerated mouse-prone-8 (SAMP8) model of female reproductive aging was employed to assess the separate impacts of age and high FSH activity on the percentages (%) of viable and mature ovulated oocytes recovered after gonadotropin treatment. Young and midlife mice were treated with the FSH analog equine chorionic gonadotropin (eCG) to model both endogenous FSH elevation and exogenous FSH elevation. Previously we showed the activin inhibitor ActRIIB:Fc increases oocyte quality by preventing chromosome and spindle misalignments. Therefore, ActRIIB:Fc treatment was performed in an effort to increase % oocyte viability and % oocyte maturation.

RESULTS

The high FSH activity of eCG is ootoxic to ovulatory oocytes, with greater decreases in % viable oocytes in midlife than young mice. High FSH activity of eCG potently inhibits oocyte maturation, decreasing the % of mature oocytes to similar degrees in young and midlife mice. ActRIIB:Fc treatment does not prevent eCG ootoxicity, but it restores most oocyte maturation impeded by eCG.

CONCLUSIONS

FSH ootoxicity to ovulatory oocytes and FSH maturation inhibition pose a paradox given the well-known pro-growth and pro-maturation activities of FSH in the earlier stages of oocyte growth. We propose the FOOT Hypothesis ("FSH OoToxicity Hypothesis), that FSH ootoxicity to ovulatory oocytes comprises a new driver of infertility and low pregnancy success rates in DOR women attempting spontaneous pregnancy and in COS/IUI patients, especially AMA women. We speculate that endogenous FSH elevation also contributes to reduced fecundity in these DOR and COS/IUI patients. Restoration of oocyte maturation by ActRIB:Fc suggests that activin suppresses oocyte maturation in vivo. This contrasts with prior studies showing activin A promotes oocyte maturation in vitro. Improved oocyte maturation with agents that decrease endogenous activin activity with high specificity may have therapeutic benefit for COS/IVF patients, COS/IUI patients, and DOR patients attempting spontaneous pregnancies.

Maternal age and gonadotrophin elevation cooperatively decrease viable ovulated oocytes and increase ootoxicity, chromosome-, and spindle-misalignments: '2-Hit' and 'FSH-OoToxicity' mechanisms as new reproductive aging hypotheses

While there is consensus that advanced maternal age (AMA) reduces oocyte yield and quality, the notion that high FSH reduces oocyte quality and causes aneuploidy remains controversial, perhaps due to difficulties controlling the confounding variables of age and FSH levels. Here, contributions of age and gonadotrophin elevation were separately controlled using a mouse model of human female reproductive aging. Ovulated oocytes were collected from young and midlife mice after 0-, 2.6-, or 17-day treatment with the FSH analog equine chorionic gonadotrophin (eCG), to model both exogenous FSH elevation within a single treatment cycle (as in controlled ovarian stimulation (COS)), and chronic endogenous FSH elevation during multiple cycles (as in diminished ovarian reserve). After 17-day eCG, fewer total oocytes/mouse are ovulated in midlife than young mice, and a precipitous decline in viable oocytes/mouse is observed in midlife but not young mice throughout eCG treatment. eCG is potently ootoxic to ovulatory oocytes and strongly induces chromosome- and spindle-misalignments within 2.6 days of eCG in midlife, but only after 17 days in young mice. These data indicate that AMA increases susceptibility to multiple adverse effects of elevated FSH activity in ovulated oocytes, including declines in total and viable oocytes/mouse, and induction of ootoxicity and aneuploidy. Two hypotheses are proposed for underlying causes of infertility in women. The FSH OOToxicity Hypothesis ('FOOT Hypothesis') posits that high FSH is ootoxic to ovulatory oocytes and that FSH ootoxicity is a root cause of low pregnancy success rates in naturally cycling women with high FSH and IUI patients undergoing COS. The '2-Hit Hypothesis' posits that AMA increases susceptibility to FSH-induced ootoxicity and aneuploidy.

Inhibin Inactivation in Female Mice Leads to Elevated FSH Levels, Ovarian Overstimulation, and Pregnancy Loss

Inhibins are members of the transforming growth factor-β family, composed of a common α-subunit disulfide-linked to 1 of 2 β-subunits (βA in inhibin A or βB in inhibin B). Gonadal-derived inhibin A and B act in an endocrine manner to suppress the synthesis of follicle-stimulating hormone (FSH) by pituitary gonadotrope cells. Roles for inhibins beyond the pituitary, however, have proven difficult to delineate because deletion of the inhibin α-subunit gene (Inha) results in unconstrained expression of activin A and activin B (homodimers of inhibin β-subunits), which contribute to gonadal tumorigenesis and lethal cachectic wasting. Here, we generated mice with a single point mutation (Arg233Ala) in Inha that prevents proteolytic processing and the formation of bioactive inhibin. In vitro, this mutation blocked inhibin maturation and bioactivity, without perturbing activin production. Serum FSH levels were elevated 2- to 3-fold in InhaR233A/R233A mice due to the loss of negative feedback from inhibins, but no pathological increase in circulating activins was observed. While inactivation of inhibin A and B had no discernible effect on male reproduction, female InhaR233A/R233A mice had increased FSH-dependent follicle development and enhanced natural ovulation rates. Nevertheless, inhibin inactivation resulted in significant embryo-fetal resorptions and severe subfertility and was associated with disrupted maternal ovarian function. Intriguingly, heterozygous Inha+/R233A females had significantly enhanced fecundity, relative to wild-type littermates. These studies have revealed novel effects of inhibins in the establishment and maintenance of pregnancy and demonstrated that partial inactivation of inhibin A/B is an attractive approach for enhancing female fertility.

The putative roles of FSH and AMH in the regulation of oocyte developmental competence: from fertility prognosis to mechanisms underlying age-related subfertility

BACKGROUND

Fertility loss during female ageing is associated with increasing basal FSH and decreasing anti-Müllerian hormone (AMH) concentrations, together with compromised oocyte quality, presumably due to increased oxidative stress (OS) and DNA damage, as well as reduced metabolic and meiotic competences. Basal FSH and AMH circulatory concentrations have been broadly utilized as IVF success predictors, regardless of fluctuations in prognostic accuracy; basal FSH and AMH perform better in pre-advanced maternal age (AMA: >35 years) and AMA patients, respectively. The relationships between FSH and AMH intrafollicular levels and IVF outcomes suggest, nevertheless, that both hormones regulate oocyte competence, supporting the hypothesis that changes in FSH/AMH levels cause, at least in part, oocyte quality degradation during ageing. To understand the reasons behind the fluctuations in FSH and AMH prognostic accuracies and to clarify their participation in mechanisms determining oocyte competence and age-related subfertility, a deeper knowledge of the regulation of FSH and AMH intrafollicular signalling during the female reproductive lifespan, and of their effects on the cumulus-oocyte complex, is required.

OBJECTIVE AND RATIONALE

An extensive body of information on the regulation of FSH and AMH intrafollicular availability and signalling, as well as on the control of folliculogenesis and oocyte metabolism, has been accumulated. However, these datasets have been explored within the relatively narrow boundaries of their specific subjects. Given the aforementioned gaps in knowledge and their clinical relevance, herein we integrate clinical and basic data, within a wide biological perspective, aiming to shed light on (i) the reasons for the variability in the accuracy of serum FSH and AMH as fertility markers, and on (ii) the potential roles of these hormones in mechanisms regulating oocyte quality, particularly those associated with ageing.

SEARCH METHODS

The PubMed database encompassing the period between 1960 and 2021 was searched. Principal search terms were FSH, FSH receptor, AMH, oocyte, maternal age, cumulus, transzonal projections (TZPs), actin, OS, redox, reactive oxygen species, mitochondria, DNA damage, DNA repair, aneuploidy, spindle, meiosis, gene expression, transcription, translation, oocyte secreted factors (OSFs), cAMP, cyclic guanosine monophosphate, natriuretic peptide C, growth differentiation factor 9, bone morphogenetic protein 15 and fibroblast growth factor.

OUTCOMES

Our analysis suggests that variations in the accuracy of fertility prognosis reflect a modest association between circulatory AMH levels and oocyte quality as well as increasing basal FSH inter-cycle variability with age. In addition, the basic and clinical data articulated herein support the hypothesis that increased intrafollicular FSH levels, as maternal age advances, may override the physiological protective influences of AMH and OSFs against excessive FSH signalling in cumulus cells. This would result in the disruption of oocyte homeostasis via reduced TZP-mediated transfer of cumulus-derived molecules essential for meiotic competence, gene expression, redox activity and DNA repair.

WIDER IMPLICATIONS

In-depth data analysis, encompassing a wide biological perspective has revealed potential causative mechanisms of age-related subfertility triggered by alterations in FSH/AMH signalling during the female reproductive life. Insights from new mechanistic models arising from this analysis should contribute to advancing our comprehension of oocyte biology in humans and serve as a valuable reference for novel AMA subfertility treatments aimed at improving oocyte quality through the modulation of AMH/FSH action.