Reproductive capacity in women starts to decline beyond their mid-30s and pregnancies in older women result in higher rates of miscarriage with aneuploidy. Age-related decline in fertility is strongly attributed to ovarian aging, diminished ovarian reserves, and decreased developmental competence of oocytes. In this review, we discuss the underlying mechanisms of age-related decline in oocyte quality, focusing on oxidative stress (OS) in oocytes. The primary cause is the accumulation of spontaneous damage to the mitochondria arising from increased reactive oxygen species (ROS) in oocytes, generated by the mitochondria themselves during daily biological metabolism. Mitochondrial dysfunction reduces ATP synthesis and influences the meiotic spindle assembly responsible for chromosomal segregation. Moreover, reproductively aged oocytes produce a decline in the fidelity of the protective mechanisms against ROS, namely the ROS-scavenging metabolism, repair of ROS-damaged DNA, and the proteasome and autophagy system for ROS-damaged proteins. Accordingly, increased ROS and increased vulnerability of oocytes to ROS lead to spindle instability, chromosomal abnormalities, telomere shortening, and reduced developmental competence of aged oocytes.

Impact of Oxidative Stress on Age-Associated Decline in Oocyte Developmental Competence.

As women delay childbearing because of demographic and socioeconomic trends, reproductive aging and ensuing ovarian dysfunction become increasingly more prevalent causes of infertility. Age-related decline in fertility is characterized by both quantitative and qualitative deterioration of the ovarian reserve. Importantly, disorders of aging are frequently associated with mitochondrial dysfunction, as are impaired oogenesis and embryogenesis. Ongoing research explores the role of mitochondrial dysfunction in ovarian aging, and potential ways to exploit mitochondrial mechanisms to slow down or reverse age-related changes in female gonads.

Mitochondrial Dysfunction and Ovarian Aging.

Mitochondria in Ovarian Aging and Reproductive Longevity.

How, what and why.

Mitochondria: Their relevance during oocyte ageing

Mitochondria are dynamic organelles that continually adapt their structure through fusion and fission in response to changes in their bioenergetic environment. Targeted deletion of mitochondrial fusion protein mitofusin1 (MFN1) in oocytes resulted in female infertility associated with failure to achieve oocyte maturation. Oocyte-granulosa cell communication was impaired, and cadherins and connexins were downregulated, resulting in follicle developmental arrest at the secondary follicle stage. Deletion of MFN1 in oocytes resulted in mitochondrial dysfunction and altered mitochondrial dynamics, as well as accumulation of ceramide, which contributed to increased apoptosis and a reproductive phenotype that was partially rescued by treatment with ceramide synthesis inhibitor myriocin. Absence of MFN1 and resulting apoptotic cell loss also caused depletion of ovarian follicular reserve, and a phenotype consistent with accelerated female reproductive aging.

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Mitofusin 1 is required for female fertility and to maintain ovarian follicular reserve

Mitochondria change their shape through fusion and fission in order to adapt to their metabolic milieu. Mitofusin-2 (MFN2) is a key regulatory protein in this process, mediating mitochondrial fusion and interaction with endoplasmic reticulum. Targeted deletion of Mfn2 in oocytes resulted in mitochondrial dysfunction and female subfertility associated with impaired oocyte maturation and follicle development. Oocytes lacking MFN2 showed shortened telomeres and increased apoptosis, resulting in compromised oocyte quality and accelerated follicular depletion, consistent with a reproductive aging phenotype.

/pdf/mitofusin-2-plays-a-role-in-oocyte-and-follicle-development-2w8x69c5d9.pdf

Mitofusin 2 plays a role in oocyte and follicle development, and is required to maintain ovarian follicular reserve during reproductive aging.

MFN1 is required for follicle development, oocyte maturation, and female fertility

Microtubule acetylation is found in populations of stable, long-lived microtubules, occurring on the conserved lysine 40 (K40) residue of α-tubulin by α-tubulin acetyltransferases (αTATs). α-tubulin K40 acetylation has been shown to stabilize microtubules via enhancing microtubule resilience against mechanical stress. Here we show that a previously uncharacterized αTAT, Drosophila CG17003/leaky (lky), is required for α-tubulin K40 acetylation in early germ cells in Drosophila ovary. We found that loss of lky resulted in a progressive egg chamber fusion phenotype accompanied with mislocalization of germline-specific Vasa protein in somatic follicle cells. The same phenotype was observed upon replacement of endogenous α-tubulin84B with non-acetylatable α-tubulin84BK40A, suggesting α-tubulin K40 acetylation is responsible for the phenotype. Chemical disturbance of microtubules by Colcemid treatment resulted in a mislocalization of Vasa in follicle cells within a short period of time (~30 min), suggesting that the observed mislocalization is likely caused by direct leakage of cellular contents between germline and follicle cells. Taken together, this study provides a new function of α-tubulin acetylation in maintaining the cellular identity possibly by preventing the leakage of tissue-specific gene products between juxtaposing distinct cell types.

Drosophila CG17003/leaky (lky) is required for microtubule acetylation in early germ cells in Drosophila ovary

Drosophila male germline stem cells (GSCs) reside at the tip of the testis and surround a cluster of niche cells. It has been believed that the niche-derived Decapentaplegic (Dpp) ligand has a role in maintaining stem cells in close proximity but has no role in the differentiating cells spaced one-cell layer away. However, the range of Dpp diffusion has never been tested. Here, using genetically encoded nanobodies called Morphotrap, we physically block Dpp diffusion without interfering with niche-stem cell signaling. When Dpp diffusion is perturbed, differentiating germ cells frequently de-differentiated, suggesting that Dpp ensures differentiation of GSC daughter cells, opposing to its role in maintenance of GSC in the niche. Our work provides the evidence that a single niche ligand induces distinct cellular responses inside versus outside the niche, which may be a common mechanism to regulate tissue homeostasis. One sentence summary A soluble BMP ligand diffuses from the niche and has dual, and opposite roles on stem cells and differentiating daughter cells.

/pdf/diffusing-fraction-of-niche-bmp-ligand-safeguards-stem-cell-1fi8e015.pdf

Muhammed Burak Bener

Papers

Mitofusin 1 is required for female fertility and to maintain ovarian follicular reserve

Mitofusin 2 plays a role in oocyte and follicle development, and is required to maintain ovarian follicular reserve during reproductive aging.

MFN1 is required for follicle development, oocyte maturation, and female fertility

Drosophila CG17003/leaky (lky) is required for microtubule acetylation in early germ cells in Drosophila ovary

Diffusing fraction of niche BMP ligand safeguards stem-cell differentiation