Changing hypothalamopituitary function: its role in aging of the female reproductive system

Changes in female reproductive function occur relatively early during the life span in many mammalian species. Therefore, this physiological system is an excellent model system in which to study the effects of age on specific endocrine relationships since changes occur prior to the occurrence of multiple pathologies associated with later stages of aging. Data from several laboratories suggest that changes in hypothalamic, pituitary and ovarian function may contribute to age-related deterioration of fertility in females. We will focus our attention on the role of hypothalamic changes in the cascade of events that eventually lead to acyclicity and infertility. Data suggest that changes in the diurnal rhythmicity of catecholaminergic neurotransmitters and their receptors occur during middle age. These changes may regulate the pattern of release of GnRH since alterations in the pulsatile pattern of LH secretion also become detectable at this age. Some age-related changes in hypothalamic and pituitary function are not irreversible or absolutely determined. Instead it appears that the ovarian steroidal milieu modulates the rate of aging of several aspects of hypothalamohypophysial function. In summary, changes in hypothalamic and pituitary function appear to contribute to the aging of the female reproductive system.     

Prolongation and cessation of estrous cycles in aging C57BL/6J mice are differentially regulated events

The relative contributions of ovarian failure and hypothalamic-pituitary dysfunction to the prolongation and cessation of estrous cycles were assessed by measuring the ability of acutely ovariectomized (OVX) middle-aged (12 mo) mice to cycle after receiving grafts (under the renal capsule) of ovaries from young (2 mo) mice. The potentially disruptive effect of the acyclic state on the cycling response to grafted, young ovaries was avoided restricting grafting to middle-aged hosts that were still cycling. The effect of chronic exposure to ovarian secretions before the cessation of cyclicity on age-related hypothalamic-pituitary dysfunction was also assessed. The cycling ability of long-term OVX middle-aged mice (i.e., OVX at 3 mo) bearing grafts of young ovaries was compared to that of age-matched acutely OVX controls. Grafted young ovaries extended the cycling lifespan of acutely OVX middle-aged hosts by 60%. The length of this extended cycling lifespan, however, was only 80% of that achieved by young hosts bearing grafts of young ovaries. Young ovaries in middle-aged mice markedly lowered the incidence of long cycles (greater than 5 days), shifting the modal cycle length to 5 days. However, young ovaries in middle-aged mice failed to increase the incidence of 4-day cycles, the modal cycle of young controls. Middle-aged ovaries grafted into young hosts lengthened their cycles and shortened their cycling lifespan to middle-aged values. Long-term ovariectomy failed to increase the cycling lifespan of middle-aged hosts bearing grafts of young ovaries beyond that achieved in acutely OVX mice. Long-term ovariectomy did shorten the modal cycle length of middle-aged mice to 4 days, although the duration of 4-day cycling was only one-third (2 mo) that of young controls. These results indicate that the relative contributions of ovarian and neuroendocrine factors to three major events of reproductive aging vary with each event. Whereas the hypothalamic-pituitary unit appears to play an important role in the initial shift from 4- to 5-day cycles, the aging ovary plays the major role in the subsequent shift to longer cycles and in the ultimate cessation of cyclicity. Although chronic exposure to ovarian secretions during the period of cyclicity does not play a major role in the cessation of cyclicity, it appears to contribute to the hypothalamic-pituitary changes responsible for the initial shift from 4- to 5-day cycles.     

Cognitive changes across the menopause transition: A longitudinal evaluation of the impact of age and ovarian status on spatial memory

Cognitive changes that occur during mid-life and beyond are linked to both aging and the menopause transition. Studies in women suggest that the age at menopause onset can impact cognitive status later in life; yet, little is known about memory changes that occur during the transitional period to the postmenopausal state. The 4-vinylcyclohexene diepoxide (VCD) model simulates transitional menopause in rodents by depleting the immature ovarian follicle reserve and allowing animals to retain their follicle-deplete ovarian tissue, resulting in a profile similar to the majority of perimenopausal women. Here, Vehicle or VCD treatment was administered to ovary-intact adult and middle-aged Fischer-344 rats to assess the trajectory of cognitive change across time with normal aging and aging with transitional menopause via VCD-induced follicular depletion, as well as to evaluate whether age at the onset of follicular depletion plays a role in cognitive outcomes. Animals experiencing the onset of menopause at a younger age exhibited impaired spatial memory early in the transition to a follicle-deplete state. Additionally, at the mid- and post- follicular depletion time points, VCD-induced follicular depletion amplified an age effect on memory. Overall, these findings suggest that age at the onset of menopause is a critical parameter to consider when evaluating learning and memory across the transition to reproductive senescence. From a translational perspective, this study illustrates how age at menopause onset might impact cognition in menopausal women, and provides insight into time points to explore for the window of opportunity for hormone therapy during the menopause transition period. Hormone therapy during this critical juncture might be especially efficacious at attenuating age- and menopause- related cognitive decline, producing healthy brain aging profiles in women who retain their ovaries throughout their lifespan.     

Blocking of β-adrenergic receptors during the subfertile period inhibits spontaneous ovarian cyst formation in rats

As aging proceeds, fertility problems arise, and the success rate of in vitro fertilization declines. During reproductive aging, rat ovaries present spontaneous formation of cysts, followed by a concomitant increase in sympathetic nerve activity, causing infertility and cessation of ovarian function. β2-Adrenergic receptors, which are activated by noradrenaline (NA), modify follicular development and steroid secretions; thus, increased nerve activity has been associated with the development and maintenance of cystic structures. The purpose of this work was to block the effect of this sympathetic activity through in vivo administration of propranolol (a β-adrenergic receptor antagonist) to determine whether it delays cyst formation and cessation of the ovarian function in rats that had reached the subfertile period. Propranolol was administrated daily to 8- and 10-month-old rats for 2 months. Estrous cycling activity was monitored by vaginal smear, serum concentration of the steroidal hormones was determined by enzyme-immune assay and morphological analysis of the ovaries was performed using 6 μm tissue slices stained with hematoxylin-eosin. Propranolol increased the number of healthy follicles, the ovulation rate, and levels of serum sexual steroids (androstenedione, testosterone, and estradiol) and recovered estrous cycling activity. It also decreased the number of follicular cysts. These results suggest that the blockade of β-adrenergic receptors recovered ovarian function during reproductive aging. It is suggested that propranolol induces a time-dependent extension of the subfertile window, and it could be used to increase the success rate of fertility programs in aging woman.     

Neuroendocrinology and ovarian aging

The ovarian aging, a dynamic process that precedes the clinical manifestations of menopause, can be assessed using ovarian reserve biomarkers. It is well-known that reproduction during the later years of reproductive life has known limitations that challenge the success of assisted reproduction. Therefore, a review of the neuroendocrine modifications during this critical period of reproductive life may help to elucidate the ovarian aging process and its impact on reproduction. In this review, we aim to further the discussion of neuroendocrine changes taking place during the ovarian aging process that may impact reproductive function.     

Rotifers in aging research: use of rotifers to test various theories of aging

Four theories of aging are discussed to examine how effectively they might explain the aging process in rotifers. One of the early theories, the rate of living theory of aging can perhaps be discounted. Although the theory predicts that increased biological energy expenditure, in the form of increased activity or reproduction, would lead to a shorter lifespan, these predictions are not born out by experimental evidence. At the whole animal level, a case can be made for a theory of programmed aging, where the end of reproduction signals the end of the lifespan. Support for this view comes from the observation that lifespan is positively correlated with reproductive parameters, that treatments that extend lifespan usually act to extend the reproductive period, and that the end of reproduction is associated with high mortality and senescent biochemical changes. Two molecular theories of aging are also discussed; the free radical theory of aging and the calcium theory of aging. These theories point to the fact that molecular damage accumulates and that calcium influx increases in the course of aging. When free radical buildup or calcium homeostasis is reduced, lifespan is extended. A molecular explanation of aging does not necessarily exclude the idea of programmed aging. It is probable that an eventual understanding of the aging process will rest on both a physiological and molecular basis.     

Genetic and pharmacological factors that influence reproductive aging in nematodes

Age-related degenerative changes in the reproductive system are an important aspect of aging, because reproductive success is the major determinant of evolutionary fitness. Caenorhabditis elegans is a prominent organism for studies of somatic aging, since many factors that extend adult lifespan have been identified. However, mechanisms that control reproductive aging in nematodes or other animals are not well characterized. To use C. elegans to measure reproductive aging, we analyzed mated hermaphrodites that do not become sperm depleted and monitored the duration and level of progeny production. Mated hermaphrodites display a decline of progeny production that culminates in reproductive cessation before the end of the lifespan, demonstrating that hermaphrodites undergo reproductive aging. To identify factors that influence reproductive aging, we analyzed genetic, environmental, and pharmacological factors that extend lifespan. Dietary restriction and reduced insulin/insulin-like growth factor signaling delayed reproductive aging, indicating that nutritional status and a signaling pathway that responds to environmental stress influence reproductive aging. Cold temperature delayed reproductive aging. The anticonvulsant medicine ethosuximide, which affects neural activity, delayed reproductive aging, indicating that neural activity can influence reproductive aging. Some of these factors decrease early progeny production, but there is no consistent relationship between early progeny production and reproductive aging in strains with an extended lifespan. To directly examine the effects of early progeny production on reproductive aging, we used sperm availability to modulate the level of early reproduction. Early progeny production neither accelerated nor delayed reproductive aging, indicating that reproductive aging is not controlled by use-dependent mechanisms. The implications of these findings for evolutionary theories of aging are discussed.     

Reproductive Aging Drives Protein Accumulation in the Uterus and Limits Lifespan in C. elegans

Aging in Caenorhabditis elegans is characterized by widespread physiological and molecular changes, but the mechanisms that determine the rate at which these changes occur are not well understood. In this study, we identify a novel link between reproductive aging and somatic aging in C. elegans. By measuring global age-related changes in the proteome, we identify a previously uncharacterized group of secreted proteins in the adult uterus that dramatically increase in abundance with age. This accumulation is blunted in animals with an extended reproductive period and accelerated in sterile animals lacking a germline. Uterine proteins are not removed in old post-reproductive animals or in young vulvaless worms, indicating that egg-laying is necessary for their rapid removal in wild-type young animals. Together, these results suggest that age-induced infertility contributes to extracellular protein accumulation in the uterus with age. Finally, we show that knocking down multiple age-increased proteins simultaneously extends lifespan. These results provide a mechanistic example of how the cessation of reproduction contributes to detrimental changes in the soma, and demonstrate how the timing of reproductive decline can influence the rate of aging.     

Gene Pathways That Delay Caenorhabditis elegans Reproductive Senescence

Reproductive senescence is a hallmark of aging. The molecular mechanisms regulating reproductive senescence and its association with the aging of somatic cells remain poorly understood. From a full genome RNA interference (RNAi) screen, we identified 32 Caenorhabditis elegans gene inactivations that delay reproductive senescence and extend reproductive lifespan. We found that many of these gene inactivations interact with insulin/IGF-1 and/or TGF-β endocrine signaling pathways to regulate reproductive senescence, except nhx-2 and sgk-1 that modulate sodium reabsorption. Of these 32 gene inactivations, we also found that 19 increase reproductive lifespan through their effects on oocyte activities, 8 of them coordinate oocyte and sperm functions to extend reproductive lifespan, and 5 of them can induce sperm humoral response to promote reproductive longevity. Furthermore, we examined the effects of these reproductive aging regulators on somatic aging. We found that 5 of these gene inactivations prolong organismal lifespan, and 20 of them increase healthy life expectancy of an organism without altering total life span. These studies provide a systemic view on the genetic regulation of reproductive senescence and its intersection with organism longevity. The majority of these newly identified genes are conserved, and may provide new insights into age-associated reproductive senescence during human aging.     

The effects of superovulation and reproductive aging on the epigenome of the oocyte and embryo

A societal preference of delaying maternal age at first childbirth has increased reliance on assisted reproductive technologies/therapies (ART) to conceive a child. Oocytes that have undergone physiologic aging (≥35 years for humans) are now commonly used for ART, yet evidence is building that suboptimal reproductive environments associated with aging negatively affect oocyte competence and embryo development—although the mechanisms underlying these relationship are not yet well understood. Epigenetic programming of the oocyte occurs during its growth within a follicle, so the ovarian stimulation protocols that administer exogenous hormones, as part of the first step for all ART procedures, may prevent the gamete from establishing an appropriate epigenetic state. Therefore, understanding how oocyte. Therefore, understanding how hormone stimulation and oocyte physiologic age independently and synergistically physiologic age independently and synergistically affect the epigenetic programming of these gametes, and how this may affect their developmental competence, are crucial to improved ART outcomes. Here, we review studies that measured the developmental outcomes affected by superovulation and aging, focusing on how the epigenome (i.e., global and imprinted DNA methylation, histone modifications, and epigenetic modifiers) of gametes and embryos acquired from females undergoing physiologic aging and exogenous ovarian stimulation is affected.     

Ovarian aging and menopause: current theories, hypotheses, and research models

Aging of the reproductive system has been studied in numerous vertebrate species. Although there are wide variations in reproductive strategies and hormone cycle components, many of the fundamental changes that occur during aging are similar. Evolutionary hypotheses attempt to explain why menopause occurs, whereas cellular hypotheses attempt to explain how it occurs. It is commonly believed that a disruption in the hypothalamic-pituitary-gonadal axis is responsible for the onset of menopause. Data exist to demonstrate that the first signs of menopause occur at the level of the brain or the ovary. Thus, finding an appropriate and representative animal model is especially important for the advancement of menopause research. In primates, there is a gradual decline in the function of the hypothalamic-pituitary-gonadal (HPG) axis ultimately resulting in irregularities in menstrual cycles and increasingly sporadic incidence of ovulation. Rodents also exhibit a progressive deterioration in HPG axis function; however, they also experience a period of constant estrus accompanied by intermittent ovulations, reduced progesterone levels, and elevated circulating estradiol levels. It is remarkable to observe that females of other classes also demonstrate deterioration in HPG axis function and ovarian failure. Comparisons of aging in various taxa provide insight into fundamental biological mechanisms of aging that could underlie reproductive decline.     

Neuroendocrine mechanisms and aging.

Evidence describing altered neuroendocrine function during aging from this and other laboratories is reviewed, with focus on changes in the brain-pituitary-ovarian-adrenal-hepatic and in the brain-pituitary-ovarian systems. Difficulties in interpreting the discordant data on age-related changes in pituitary function are discussed. Among mechanisms of reproductive aging are changes at both the ovarian and hypothalamic level (including reduced catecholamine levels, turnover, and synaptosomal uptake). However, it cannot yet be concluded that impairments of hypothalamic catecholamine metabolism are the primary cause for the loss of regular cycles. Evidence for dopaminergic impairments in the basal ganglions of humans and rodents during normal aging suggests that these changes may be a general phenomenon of aging. Although the origins of the changes are not yet known, neuronal cell loss in the substantia nigra would not seem to be the only cause.     

Radical ovarian resection advances the onset of persistent vaginal cornification but only transiently disrupts hypothalamic-pituitary regulation of cyclicity in C57BL/6J mice

In aging laboratory rodents, neuroendocrine failure to support estrous cyclicity is in part the consequence of exposure to ovarian secretions during adulthood. Moreover, some evidence suggests that those secretions associated with the predominant postcyclic state, persistent vaginal cornification (PVC), are more deleterious than those associated with cyclicity. However, it is not clear whether postcyclic hormonal secretions are intrinsically more deleterious or whether vulnerability to ovarian secretions, regardless of their nature, increases during aging. Using relatively young, age-matched mice, this study was designed to control for age and to determine if the hormonal milieu associated with PVC would be more deleterious to neuroendocrine function than that associated with regular cyclicity. Onset of PVC was advanced about 5 mo by resecting 90-95% of the ovarian tissue from 5-mo-old mice. The resultant PVC was similar in duration, vaginal cytology and ovarian histology to that seen in normally aging mice. At age 13 mo, when mean duration of PVC was 3 mo in resected mice but only 1 mo in sham-operated controls, the ability of mice to support cyclicity upon receipt of ovarian grafts from 4-mo-old donors was tested as an index of neuroendocrine function. The response of resected mice was slightly impaired, but only during the first month after grafting. This transient disruption of neuroendocrine function in mice prematurely exposed to PVC stands in contrast to the irreversible loss of cycling potential in older animals.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pathways coordinating oocyte attrition and abundance during mammalian ovarian reserve establishment

The mammalian ovarian reserve is comprised of a finite pool of primordial follicles, representing the lifetime reproductive capacity of females. In most mammals, the reserve is produced during embryonic and early postnatal development with oocyte numbers peaking during mid‐to‐late gestation, and then experiencing a dramatic decline continuing until shortly after birth. Oocytes remaining after the bulk of this attrition are subsequently surrounded by a layer of somatic pre‐granulosa cells with these units then referred to as “primordial follicles.” The complex and varied cell death mechanisms intrinsic to this process are not only characteristic of, but also essential for, the proper formation of this pool of follicles, and as a result must be immaculately balanced to ensure long‐term fertility and reproductive health. Too few follicles can lead to Primary Ovarian Insufficiency, resulting in fertility loss and other features of aging, such as an overall shorter lifespan. On the other hand, whereas an excess of follicles might extend reproductive lifespan, this might also be the underlying etiology of other ovarian pathologies. The last decade, in particular, has vastly expanded our understanding of oocyte attrition and determinants of ovarian reserve abundance. By continuing to decipher the intricacies underlying the cell death processes and development of the initial primordial follicle pool, we may be in a much better position to understand idiopathic cases of premature follicle depletion and improve ovarian health in reproductive‐age women.     

Profiles of random change during aging contain hidden information about longevity and the aging process.

Many different morphological and physiological changes occur during the yeast replicative lifespan. It has been proposed that change is a cause rather than an effect of aging. It is difficult to ascribe causality to processes that manifest themselves at the level of the entire organism, because of their global nature. Although causal connections can be established for processes that occur at the molecular level, their exact contributions are obscured, because they are immersed in a highly interactive network of processes. A top-down approach that can isolate crucial features of aging processes for further study may be a productive avenue. We have mathematically depicted the complicated and random changes that occur in cellular spatial organization during the lifespan of individual yeast cells. We call them budding profiles. This has allowed us to demonstrate that budding profiles are a highly individual characteristic, and that they are correlated with an individual cell's longevity. Additional information can be extracted from our model, indicating that random budding is associated with longevity. This expectation was confirmed, providing new avenues for exploring causal factors in yeast aging. The methodology described here can be readily applied to other aspects of aging in yeast and in higher organisms.     

Current understanding of ovarian aging

The reproductive system of human female exhibits a much faster rate of aging than other body systems. Ovarian aging is thought to be dominated by a gradual decreasing numbers of follicles, coinciding with diminished quality of oocytes. Menopause is the final step in the process of ovarian aging. This review focuses on the mechanisms underlying the ovarian aging involving a poor complement of follicles at birth and a high rate of attrition each month, as well as the alternated endocrine factors. We also discuss the possible causative factors that contribute to ovarian aging, e.g., genetic factors, accumulation of irreparable damage of microenvironment, pathological effect and other factors. The appropriate and reliable methods to assess ovarian aging, such as quantification of follicles, endocrine measurement and genetic testing have also been discussed. Increased knowledge of the ovarian aging mechanisms may improve the prevention of premature ovarian failure.     

5-Azacytidine-induced demethylation of DNA to senescent level does not block proliferation of human fibroblasts.

IMR-90 human diploid fibroblasts (HDF) lose from 30-50% of their genomic 5-methyldeoxycytidine (5mdC) during the cellular aging process. In contrast, immortal SV40-transformed IMR-90 maintain a constant level of 5mdC in culture. Precrisis SV40-transformed HDF (AG3204) represent a stage in between normal cell aging and immortalization because these cells still have a finite proliferative lifespan, but it is longer than that of normal HDF and ends in cell death rather than in G1-arrest. We find that AG3204 cells continue to lose from 12-33% of their 5mdC after a population has become 99% positive for SV40 T-antigen. Both IMR-90 cells and AG3204 cells have similar levels of 5mdC (average of 2.25%) at the end of lifespan. We investigated whether this level of 5mdC is an absolute block to further proliferation by treating IMR-90 and AG3204 cells with 5-azacytidine (5azaC) to reduce their 5mdC levels below the terminal level normally achieved at end of lifespan. We find that both IMR-90 and AG3204 cells undergo extensive proliferation with subterminal levels of 5mdC and that the lifespans of both cell types are shortened by 5azaC treatment. These studies indicate that random genomic DNA demethylation to a specific level of 5mdC is not a direct cause of finite proliferative lifespan. However, the correlation between accelerated DNA demethylation and accelerated aging still suggests that these two phenomena are related. Two ways to explain this relationship are: (1) DNA demethylation during aging is not random, and/or (2) both DNA demethylation and other independent aging processes cooperate to produce finite lifespan. In both cases, accelerated random DNA demethylation could accelerate aging, but not necessarily in direct relationship to the final genomic level of 5mdC achieved during the normal aging process.     

Drosophila melanogaster as a model system for the evaluation of anti-aging compounds

Understanding the causes of aging is a complex problem due to the multiple factors that influence aging, which include genetics, environment, metabolism and reproduction, among others. These multiple factors create logistical difficulties in the evaluation of anti-aging agents. There is a need for good model systems to evaluate potential anti-aging compounds. The model systems used should represent the complexities of aging in humans, so that the findings may be extrapolated to human studies, but they should also present an opportunity to minimize the variables so that the experimental results can be accurately interpreted. In addition to positively affecting lifespan, the impact of the compound on the physiologic confounders of aging, including fecundity and the health span-the period of life where an organism is generally healthy and free from serious or chronic illness-of the model organism needs to be evaluated. Fecundity is considered a major confounder of aging in fruit flies. It is well established that female flies that are exposed to toxic substances typically reduce their dietary intake and their reproductive output and display an artifactual lifespan extension. As a result, drugs that achieve longevity benefits by reducing fecundity as a result of diminished food intake are probably not useful candidates for eventual treatment of aging in humans and should be eliminated during the screening process.