The effect of diet and cardiovascular risk on ovarian aging in cynomolgus monkeys (Macaca fascicularis)

Unraveling female reproductive senescence to enhance healthy longevity

In a society where women often want successful careers and equal opportunities to men, the early nature of ovarian aging often forces women to make difficult life choices between career and family development. Fertility in women begins to decline after the age of 37 years and it is rare for pregnancies to occur after 45. This reproductive decline in women is inevitable and culminates in menopause, which is a major driver of age-related diseases. In a world where biomedical advances are leading to modifiable biological outcomes, it is time to focus on mitigating female reproductive senescence to maintain fertility and preserve age-related hormonal functions, with the goal of providing increased life choices and enhancing healthspan. To date, reproductive longevity research remains an understudied field. More needs to be done to unravel the biology of the ovarian follicles, which are the functional units of reproductive lifespan and are comprised of cell types including the oocyte (female gamete) and a group of specialized supporting somatic cells. Biological attempts to maintain the quality and quantity of follicles in animal models through manipulating pathways involved in aging can potentially prolong female reproductive lifespan and healthspan. Here, we summarize the molecular events driving ovarian aging and menopause and the interventional strategies to offset these events. Developing solutions to female reproductive senescence will open doors to discover ways to enhance true healthy longevity for both men and women.

Ovarian aging: mechanisms and intervention strategies

Ovarian reserve is essential for fertility and influences healthy aging in women. Advanced maternal age correlates with the progressive loss of both the quantity and quality of oocytes. The molecular mechanisms and various contributing factors underlying ovarian aging have been uncovered. In this review, we highlight some of critical factors that impact oocyte quantity and quality during aging. Germ cell and follicle reserve at birth determines reproductive lifespan and timing the menopause in female mammals. Accelerated diminishing ovarian reserve leads to premature ovarian aging or insufficiency. Poor oocyte quality with increasing age could result from chromosomal cohesion deterioration and misaligned chromosomes, telomere shortening, DNA damage and associated genetic mutations, oxidative stress, mitochondrial dysfunction and epigenetic alteration. We also discuss the intervention strategies to delay ovarian aging. Both the efficacy of senotherapies by antioxidants against reproductive aging and mitochondrial therapy are discussed. Functional oocytes and ovarioids could be rejuvenated from pluripotent stem cells or somatic cells. We propose directions for future interventions. As couples increasingly begin delaying parenthood in life worldwide, understanding the molecular mechanisms during female reproductive aging and potential intervention strategies could benefit women in making earlier choices about their reproductive health.

Induction of meiosis by embryonic gonadal somatic cells differentiated from pluripotent stem cells

Background

Depletion of oocytes leads to ovarian aging-associated infertility, endocrine disruption and related diseases. Excitingly, unlimited oocytes can be generated by differentiation of primordial germ cell like cells (PGCLCs) from pluripotent stem cells. Nevertheless, development of oocytes and follicles from PGCLCs relies on developmentally matched gonadal somatic cells, only available from E12.5 embryos in mice. It is therefore imperative to achieve an in vitro source of E12.5 gonadal somatic cells.

Methods

We explored to identify small molecules, which can induce female embryonic stem cells (ESCs) into gonadal somatic cell like cells.

Results

Using RNA-sequencing, we identified signaling pathways highly upregulated in E12.5_gonadal somatic cells (E12.5_GSCs). Through searching for the activators of these pathways, we identified small-molecule compounds Vitamin C (Vc) and AM580 in combination (V580) for inducing differentiation of female embryonic stem cells (ESCs) into E12.5_GSC-like cells (E12.5_GSCLCs). After V580 treatment for 6 days and sorted by a surface marker CD63, the cell population yielded a transcriptome profile similar to that of E12.5_GSCs, which promoted meiosis progression and folliculogenesis of primordial germ cells. This approach will contribute to the study of germ cell and follicle development and oocyte production and have implications in potentially treating female infertility.

Conclusion

ESCs can be induced into embryonic gonadal somatic cell like cells by small molecules.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02672-4.

Dynamic changes occur in the DNA gut virome of female cynomolgus macaques during aging

Aging is a critical factor affecting physical health and disease in mammals. Emerging evidence indicates that aging may affect the gut bacteriome in cynomolgus macaques, but little is known about whether or how the gut virome changes with age. Here, we compared the DNA gut viral composition of 16 female cynomolgus monkeys (Macaca fascicularis) at three life stages (young, adult, and old) using the shotgun metagenome sequencing method. We found that the DNA gut virome from these monkeys differed substantially among the three groups. The gut viruses were dominated by bacteriophages, the most abundant of which was the Caudovirales order (i.e., Siphoviridae, Myoviridae, and Podoviridae families). Additionally, the co‐occurrence analysis revealed that the age‐related bacteriophages were correlated in an extensive and complex manner with the main intestinal bacteria (i.e., Firmicutes, Bacteroidetes, Proteobacteria, and Actinobacteria phyla). Furthermore, the age‐related DNA gut viral functions were enriched for genetic information processing, nucleotide, and folate metabolism. Our gut virome analysis provides new insight into how aging influences the gut virome of non‐human primates.

Single-Cell Transcriptomic Atlas of Primate Ovarian Aging

Molecular mechanisms of ovarian aging and female age-related fertility decline remain unclear. We surveyed the single-cell transcriptomic landscape of ovaries from young and aged non-human primates (NHPs) and identified seven ovarian cell types with distinct gene-expression signatures, including oocyte and six types of ovarian somatic cells. In-depth dissection of gene-expression dynamics of oocytes revealed four subtypes at sequential and stepwise developmental stages. Further analysis of cell-type-specific aging-associated transcriptional changes uncovered the disturbance of antioxidant signaling specific to early-stage oocytes and granulosa cells, indicative of oxidative damage as a crucial factor in ovarian functional decline with age. Additionally, inactivated antioxidative pathways, increased reactive oxygen species, and apoptosis were observed in granulosa cells from aged women. This study provides a comprehensive understanding of the cell-type-specific mechanisms underlying primate ovarian aging at single-cell resolution, revealing new diagnostic biomarkers and potential therapeutic targets for age-related human ovarian disorders.

Mediterranean versus Western Diet Effects on Caloric Intake, Obesity, Metabolism, and Hepatosteatosis in Nonhuman Primates

OBJECTIVE

This study aimed to determine the effects of humanlike Western and Mediterranean diets on caloric intake, obesity, metabolism, and hepatosteatosis in an established nonhuman primate model of obesity, cardiometabolic syndrome, type 2 diabetes, and atherosclerosis.

METHODS

A 38-month, randomized, preclinical, nonhuman primate primary prevention trial of 38 socially housed, middle-aged adult females was conducted. The monkeys were characterized during a 7-month baseline phase while consuming chow and then randomized to either Western or Mediterranean diets; the groups were balanced on baseline characteristics. Western and Mediterranean diets were formulated to closely reflect human diets, matched on macronutrient content, with protein and fat derived largely from animal sources in the Western diet and plant sources in the Mediterranean diet. Food consumption, activity levels, energy expenditure, body composition, carbohydrate metabolism, and hepatosteatosis were measured during baseline and treatment phases.

RESULTS

The Western diet increased caloric intake for the first 6 months and body fat, activity, energy expenditure, insulin resistance, and hepatosteatosis after 2.5 years, whereas the Mediterranean diet reduced triglyceride levels.

CONCLUSIONS

This is the first report of differential caloric intake and obesity with long-term consumption of a Western versus Mediterranean diet under controlled experimental conditions and the first experimental evidence that a Mediterranean diet protects against hepatosteatosis compared with a Western diet.

Dietary Protein Intake and Early Menopause in the Nurses' Health Study II

Early menopause, which is the cessation of ovarian function before age 45 years, affects 5%-10% of Western women and is associated with an increased risk of adverse health outcomes. Literature suggests that high levels of vegetable protein intake may prolong female reproductive function. We evaluated the association of long-term intake of vegetable protein, animal protein, and specific protein-rich foods with incidence of early natural menopause in the Nurses' Health Study II cohort. Women included in analyses (n = 85,682) were premenopausal at baseline (1991) and followed until 2011 for onset of natural menopause. Protein intake was assessed via food frequency questionnaire. In Cox proportional hazard models that were adjusted for age, smoking, body mass index, and other factors, women in the highest quintile of cumulatively averaged vegetable protein intake (median, 6.5% of calories) had a significant 16% lower risk of early menopause compared with women in the lowest quintile (3.9% of calories; 95% confidence interval: 0.73, 0.98; P for trend = 0.02). Intake of specific foods, including pasta, dark bread, and cold cereal, was also associated with lower risk (P < 0.05). Conversely, animal protein intake was unrelated to risk. High consumption of vegetable protein, equivalent to 3-4 servings per day of protein-rich foods, is associated with lower incidence of early menopause in US women.

Early Vascular Damage in Young Women with DM-1 and Its Relation to Anti-Müllerian Hormone: A Cross-Sectional Study

Vascular function is suggested to be associated with ovarian reserve, but the relationship with microvascular function has never been studied. In this cross-sectional pilot study, the relationship of microvascular damage markers with AMH was studied in premenopausal women. Twenty-two regularly cycling women with type 1 diabetes (DM-1) and a reference group of 20 healthy regularly cycling women were included, from whom blood was drawn in the early follicular phase of the menstrual cycle. The main outcome was the correlation between circulating progenitor cells (CPCs), markers for early vascular damage, and AMH, a marker for ovarian reserve. Secondary endpoints for early vascular impairment were circulating angiogenic cells and additional biomarkers. Median AMH levels were 2.2 µg/L [1.2-3.5] in the DM-1 group and 2.1 µg/L [0.85-3.8] in the reference group. CPCs were significantly decreased in women with DM-1; 1204 ± 537 CD34+/CD45dim cells were counted in the DM-1 group, compared to 2264 ± 1124 in the reference group. CPCs and other markers of early vascular damage were not correlated with AMH levels in a multivariable analysis. These results underscore previous findings of early vascular damage in DM-1 and suggest that there may not be a relationship between vascular function and ovarian reserve. Trial Registration. This trial is registered with Clinicaltrials.gov NCT01665716.

Contributions of Nonhuman Primates to Research on Aging

Aging is the biological process of declining physiologic function associated with increasing mortality rate during advancing age. Humans and higher nonhuman primates exhibit unusually longer average life spans as compared with mammals of similar body mass. Furthermore, the population of humans worldwide is growing older as a result of improvements in public health, social services, and health care systems. Comparative studies among a wide range of organisms that include nonhuman primates contribute greatly to our understanding about the basic mechanisms of aging. Based on their genetic and physiologic relatedness to humans, nonhuman primates are especially important for better understanding processes of aging unique to primates, as well as for testing intervention strategies to improve healthy aging and to treat diseases and disabilities in older people. Rhesus and cynomolgus macaques are the predominant monkeys used in studies on aging, but research with lower nonhuman primate species is increasing. One of the priority topics of research about aging in nonhuman primates involves neurologic changes associated with cognitive decline and neurodegenerative diseases. Additional areas of research include osteoporosis, reproductive decline, caloric restriction, and their mimetics, as well as immune senescence and chronic inflammation that affect vaccine efficacy and resistance to infections and cancer. The purpose of this review is to highlight the findings from nonhuman primate research that contribute to our understanding about aging and health span in humans.

Effect of soy isoflavones on thyroid hormones in intact and ovariectomized cynomolgus monkeys (Macaca fascicularis)

OBJECTIVE

Soy isoflavones are commonly used to alleviate menopause-related symptoms. Postmenopausal women are at increased risk for hypothyroidism, and there are concerns that isoflavones may be detrimental to thyroid health. The aim of this study was to examine the effects of soy protein and isoflavones on thyroid function and the relationship between thyroid function and ovarian function.

METHODS

Adult female cynomolgus monkeys (Macaca fascicularis) were randomized to consume two diets differing only in protein source: casein-lactalbumin (n = 44) or soy protein with isoflavones (n = 41). After 34 months, all animals were ovariectomized via laparotomy. Half of the monkeys from each diet treatment group continued to consume their preovariectomy treatment phase diet (either isolated soy protein [n = 19] or casein-lactalbumin [n = 21]) for an additional 34 months. The remaining animals did not continue their diets and thus were not considered further. Circulating progesterone, triiodothyronine, thyroxine, and thyroid-stimulating hormone were measured at baseline. Thyroid hormones were remeasured during each treatment phase.

RESULTS

Dietary soy increased triiodothyronine in preovariectomized monkeys and prevented a decline in thyroxine after surgical menopause (both P's < 0.05). Mean progesterone concentrations were positively correlated with triiodothyronine at baseline in preovariectomized monkeys (P < 0.01).

CONCLUSIONS

Progesterone levels and triiodothyronine are positively correlated in macaques. Dietary soy increases triiodothyronine in preovariectomized monkeys and prevents a decline in thyroxine after surgical menopause. The outcomes observed in this study suggest that soy protein and isoflavone consumption does not adversely affect-and may even preserve-thyroid function in postmenopausal women.

Characterization of ovarian aging and reproductive senescence in vervet monkeys (Chlorocebus aethiops sabaeus)

Female vervet monkeys (Chlorocebus aethiops sabaeus) are used as an experimental model for chronic diseases relevant to women's health. However, reproductive senescence (menopause) has not yet been characterized for vervet monkeys. Here we describe the histologic, hormonal, and menstrual markers of reproductive senescence in vervet monkeys from the Wake Forest Vervet Research Colony. Ovaries from monkeys (age, 0 to 27 y) were serially sectioned (5 μm), stained, and photographed. In every 100th section, the numbers of primordial, primary, and secondary follicles were determined, and triplicate measurements were used to calculate mean numbers of follicles per ovary. Antimüllerian hormone (AMH), follicle stimulating hormone, and menstrual cycle length were measured in additional monkeys. Primordial follicles and AMH decreased significantly with age, and significant correlations between numbers of primordial and primary follicles and between numbers of primary and secondary follicles were noted. Histologic evaluation revealed that ovaries from 4 aged monkeys (older than 23 y) were senescent. One aged monkey transitioned to menopause, experiencing cycle irregularity over 4 y, eventual cessation of menses, and plasma AMH below the level of detection. Finally, with increasing age, the percentage of female vervets with offspring declined significantly. The present study provides insight into ovarian aging and reproductive senescence in vervet monkeys. Results highlight the importance of considering this nonhuman primate as a model to investigate the relationships between ovarian aging and chronic disease risk.

Effects of a Western-type diet on plasma lipids and other cardiometabolic risk factors in African green monkeys (Chlorocebus aethiops sabaeus)

Our goal was to assess a nonhuman primate diet that mimicked the Western-type diet of humans with regard to palatability and the diet's effects on plasma lipid concentrations and other cardiometabolic risk factors. We evaluated male (n = 8) and female (n = 11) African green monkeys (vervets; Chlorocebus aethiops sabaeus) that initially were fed a standard diet. Each cohort then was divided into 2 groups, which received either standard chow or the Western diet. Food consumption and fecal quality were measured weekly. Body weight, waist circumference, and body-mass index were measured every 2 wk. CBC and clinical chemistry analyses were performed at baseline and 4 wk after the diet change. Plasma lipid concentrations, total cholesterol, HDL cholesterol, LDL cholesterol, triglycerides, glucose, insulin, and fructosamine were measured at baseline and at 4, 8, and 12 wk after the diet change. Isoflavones were measured in the male monkeys at 6 wk after diet change, and lipid particle size was measured in the female monkeys at the 12-wk point. Green monkeys readily ate the Western diet and maintained baseline body weight and morphometric measures, with no adverse effects on fecal quality or clinical measures. Total plasma cholesterol was higher in monkeys fed the Western diet compared with standard chow. Isoflavones were higher in male monkeys fed standard chow compared with the Western diet, but lipid particle size did not differ by diet in female monkeys. Our data indicate that the Western diet led to changes in various biomedical risk factors of green monkeys to become similar to those of humans in the United States.

Effect of ovarian aging on androgen biosynthesis in a cynomolgus macaque model

OBJECTIVE

The role of androgens in chronic disease pathogenesis, cognitive function and libido during menopause is of increasing interest. The aim of this study was to characterize the distribution and expression of androgenic proteins in the macaque ovary and to investigate the relationship between serum androgen concentrations, follicle number, and the persistence of androgenesis in the aging macaque ovary.

METHODS

The subjects were 26 adult female cynomolgus macaques. Ovaries were immunostained for cytochrome P450 17α-hydroxylase/17-20 lyase (P450c17), 3β-hydroxysteroid dehydrogenase (3βHSD), and cytochrome b5 (cytb5). Based on primordial follicle counts, animals were divided into tertiles (low (≤200), intermediate (226-1232), and high (2372-4356)) to evaluate differences in androgen staining and changes in serum androgen concentrations following ovariectomy.

RESULTS

Positive immunostaining for P450c17 and cytb5 within the theca interna layer of growing follicles persisted in advanced atretic follicles and secondary interstitial cells (residual stromal cells). Ovaries with low follicle numbers had less staining for all androgenic proteins compared to ovaries with higher numbers of growing follicles. Immunostaining for cytb5 was the most reliable marker for persistent androgenesis in ovaries with minimal primordial follicle numbers (<100) and residual stromal cells. Following ovariectomy, a significant decrease in testosterone (-27.7%, -30.8%, -27.5%; p < 0.01) and androstenedione (-33.4%, -35.7%, -46.0%; p < 0.01) was observed in monkeys with low, intermediate, and high primordial follicle counts, respectively.

CONCLUSIONS

Despite low follicle numbers, the aging macaque ovary retains the necessary proteins for androgenesis within residual stromal cells and contributes to peripheral androgen concentrations.

The development of small primate models for aging research

Nonhuman primate (NHP) aging research has traditionally relied mainly on the rhesus macaque. But the long lifespan, low reproductive rate, and relatively large body size of macaques and related Old World monkeys make them less than ideal models for aging research. Manifold advantages would attend the use of smaller, more rapidly developing, shorter-lived NHP species in aging studies, not the least of which are lower cost and the ability to do shorter research projects. Arbitrarily defining "small" primates as those weighing less than 500 g, we assess small, relatively short-lived species among the prosimians and callitrichids for suitability as models for human aging research. Using the criteria of availability, knowledge about (and ease of) maintenance, the possibility of genetic manipulation (a hallmark of 21st century biology), and similarities to humans in the physiology of age-related changes, we suggest three species--two prosimians (Microcebus murinus and Galago senegalensis) and one New World monkey (Callithrix jacchus)--that deserve scrutiny for development as major NHP models for aging studies. We discuss one other New World monkey group, Cebus spp., that might also be an effective NHP model of aging as these species are longer-lived for their body size than any primate except humans.

Experimental induction of reduced ovarian reserve in a nonhuman primate model (Macaca fascicularis)

Chronic diseases including coronary heart disease and osteoporosis represent a substantial health burden to postmenopausal women, yet the initiation of these conditions and their relationships with reproductive aging remain poorly understood. This situation is due, in part, to the lack of animal models reflecting ovarian and hormonal characteristics of peri- and postmenopausal women. Ovaries of women approaching menopause are nearly depleted of primordial follicles but retain a pool of larger developing follicles and androgen-producing stroma, a condition known as reduced ovarian reserve (ROR). The long-term goal of the research presented here was to create a monkey model of reproductive aging, beginning with ROR and progressing to perimenopause and finally postmenopause. Here we sought to develop a method to reduce primordial follicles in cynomolgus monkeys (Macaca fascicularis) and document hormonal changes associated with follicle reduction or ROR. At 30 d after surgical placement of a biodegradable fiber containing approximately 200 mg of 4-vinlycyclohexene diepoxide (VCD) next to one ovary in each of 8 monkeys, primordial follicles were reduced by approximately 70%, with a corresponding decrease (83%) in antimüllerian hormone (AMH, a serum marker of ovarian follicle numbers). At 4 mo after VCD-treatment of both ovaries in 29 monkeys (approximately 200 mg VCD per ovary), AMH was reduced 56% from baseline, testosterone was unchanged, and follicular phase estradiol was slightly increased. These data indicate that VCD treatment markedly reduced primordial follicles while preserving larger estradiol- and testosterone-producing follicles and ovarian stroma, a condition that mimics ROR in women.

Reproductive aging and risk for chronic disease: Insights from studies of nonhuman primates

Reproductive aging and ovarian senescence have considerable public health relevance because they are associated with increased risk for coronary heart disease (CHD), osteoporosis and other degenerative conditions including cognitive decline and potentially the metabolic syndrome. It has been suggested that the hormonal dysregulation that occurs during the perimenopausal transition may play a role in the initiation of pathobiological changes (e.g., adverse lipid profiles, atherosclerotic plaques) that will increase risk for chronic disease (e.g., CHD) during the postmenopausal years. Moreover, these early changes are suspected to establish a trajectory of disease progression that may be difficult to alter if interventions are not begun until after menopause. Even a slight increase in the rate of disease progression during the pre- or perimenopausal years could have substantial consequences for health and quality of life over the postmenopausal lifespan. Thus, the years leading up to menopause may offer a "critical window" for interventions aimed at reducing the postmenopausal disease burden. The relationship between perimenopausal hormonal dysregulation and the risk for chronic disease is poorly understood due, in large part, to the lack of appropriate animal models of the perimenopausal transition and natural menopause. In this review we assesses studies of nonhuman primates (NHPs) evaluated in various reproductive stages (naturally pre-, peri- and postmenopausal, surgically menopausal) and their contribution to our understanding about risk factors for chronic disease. Finally, because large numbers of naturally perimenopausal and menopausal NHPs are not available for research at present, experimental approaches that have the potential to hasten the onset of the perimenopausal transition will be described.