Scientists have successfully treated ovarian aging in mice with compounds that target fibrosis and promote mitochondrial health [1].
Why do ovaries age faster?
Ovarian aging is fascinating because one bodily system completely shuts down while the rest of the body is still in good health. Therefore, studying ovarian aging could provide us with valuable insights into aging in general.
More importantly, female reproductive aging is a huge problem that must be solved. The decline in fertility begins long before menopause, when women are in their 30s, while exhaustion of oocytes only occurs in their late 40s. In today’s world, this is the time window when many women would like to have children, but creeping ovarian aging makes it hard and, for some, impossible.
A possible culprit: fibrosis
It is still not entirely certain why fertility begin to decline while there are still plenty of healthy oocytes in a woman’s body. One theory blames fibrosis, a phenomenon tightly linked to aging. Ovarian follicles are embedded within stromal tissue and surrounded by the extracellular matrix (ECM). Fibrosis affects the ECM in many tissues, such as lung and liver, making it stiffer.
Fibrosis is believed to develop in response to accumulating tissue stress, damage, and inflammation [2]. Responding to pro-inflammatory signals, macrophages stimulate collagen production by resident fibroblasts, which leads to fibrosis, a process akin to wound healing.
Recent research has found that ovaries of postmenopausal women and animal models of reproductive aging show increased collagen deposition [3]. In this study, the researchers set out to investigate whether the ECM, stiffened by age-related fibrosis, constrains follicle growth, barring the release of otherwise healthy oocytes.
Obesity, which also increases fibrosis, negatively affects fertility, with symptoms resembling those of ovarian aging. The scientists showed that obese and reproductively aged mice from the same colony exhibit remarkably similar levels of fibrosis. The ovaries of young controls contained many corpora lutea, bodies that appear in the ovary after the oocyte is released. However, both in aging and obese mice, ovaries contained unruptured follicles, in which oocytes were produced but could not be released.
Just two approved anti-fibrosis drugs, pirfenidone and nintedanib, exist. Both are used to treat advanced pulmonary disease and work by inhibiting pro-inflammatory signaling, which slows fibrosis progression. Aged or obese mice were treated with either pirfenidone or nintendanib for two weeks. As expected, untreated aged mice produced no oocytes, and untreated obese mice produced very few, but more than half of the mice on pirfenidone did ovulate. The oocytes were viable and developed into normal blastocysts following in vitro fertilization. Analysis showed that pirfenidone but not nintendanib significantly reduces fibrosis.
The researchers found that fibrosis was already widespread in the ovaries of 12-month-old mice (corresponding to about 35 human years), which is considerably earlier than previously thought. In those younger mice, pirfenidone was also able to restore the dwindling ovarian function. However, it did not influence ovulation in even younger mice – probably because those animals were not burdened by fibrosis to begin with.
Mitochondrial health as an upstream cause
While fibrosis can be directly caused by cellular stress and inflammation, upstream of those factors lies mitochondrial dysfunction [4], one of the hallmarks of aging. Treating aged and obese mice with the molecule BGP-15, which stimulates mitochondrial activity, resulted in effects similar to those of pirfenidone, increasing the number of oocytes and reducing ovarian fibrosis. BGP-15 did not affect ovarian function in young mice.
Studying ovarian stromal cells in vitro, the researchers confirmed that cells from reproductively old females had impaired energy metabolism as measured by mitochondrial respiration, glycolysis, and fatty acid oxidation. BGP-15 treatment partially reversed those changes.
In an additional experiment, the researchers treated mice with two more molecules that are known to improve mitochondrial function: metformin and MitoQ. Metformin is already used against the loss of ovulation associated with insulin resistance. Both metformin and MitoQ reduced ovarian fibrosis in aged and obese mice but could only significantly improve ovulation in obese animals.
Interestingly, treatment with BGP-15 did not reduce the expression of collagen-producing genes, which are upregulated in both old and obese mice, but it increased the levels of matrix metalloproteinase 13 (MMP13), an enzyme that cleaves collagen. This probably means that the effects of BGP-15 involve removing excessive collagen rather than inhibiting its production. BGP-15 also decreased some markers of inflammation and oxidative stress.
Finally, the researchers tried giving young healthy mice rotenone, a molecule that impairs mitochondrial activity. This treatment caused symptoms of ovarian dysfunction similar to those observed in obese and aging mice, including elevated fibrosis.
Conclusion
This study shows that ovarian dysfunction is linked to fibrosis and can be successfully treated by targeting inflammation and/or mitochondrial dysfunction. If confirmed in humans, these results can signal a breakthrough in treating accelerated ovarian aging and possibly other types of aging.
Literature
[1] Umehara, T., Winstanley, Y. E., Andreas, E., Morimoto, A., Williams, E. J., Smith, K. M., … & Robker, R. L. (2022). Female reproductive life span is extended by targeted removal of fibrotic collagen from the mouse ovary. Science Advances, 8(24), eabn4564.
[2] Henderson, N. C., Rieder, F., & Wynn, T. A. (2020). Fibrosis: from mechanisms to medicines. Nature, 587(7835), 555-566.
[3] Amargant, F., Manuel, S. L., Tu, Q., Parkes, W. S., Rivas, F., Zhou, L. T., … & Duncan, F. E. (2020). Ovarian stiffness increases with age in the mammalian ovary and depends on collagen and hyaluronan matrices. Aging Cell, 19(11), e13259.
[4] Li, X., Zhang, W., Cao, Q., Wang, Z., Zhao, M., Xu, L., & Zhuang, Q. (2020). Mitochondrial dysfunction in fibrotic diseases. Cell death discovery, 6(1), 1-14.
