Equine maternal aging affects the metabolomic profile of oocytes and follicular cells during different maturation time points

Abstract

Introduction: Oocyte quality and fertility decline with advanced maternal age. During maturation within the ovarian follicle, the oocyte relies on the associated somatic cells, specifically cumulus and granulosa cells, to acquire essential components for developmental capacity.Methods: A nontargeted metabolomics approach was used to investigate the effects of mare age on different cell types within the dominant, follicular-phase follicle at three time points during maturation. Metabolomic analyses from single oocytes and associated cumulus and granulosa cells allowed correlations of metabolite abundance among cell types.Results and Discussion: Overall, many of the age-related changes in metabolite abundance point to Impaired mitochondrial metabolic function and oxidative stress in oocytes and follicular cells. Supporting findings include a higher abundance of glutamic acid and triglycerides and lower abundance of ceramides in oocytes and somatic follicular cells from old than young mares. Lower abundance of alanine in all follicular cell types from old mares, suggests limited anaerobic energy metabolism. The results also indicate impaired transfer of carbohydrate and free fatty acid substrates from cumulus cells to the oocytes of old mares, potentially related to disruption of transzonal projections between the cell types. The identification of age-associated alterations in the abundance of specific metabolites and their correlations among cells contribute to our understanding of follicular dysfunction with maternal aging.

Ovarian aging: energy metabolism of oocytes

“…Specifically, as age increases, the cross-regional transport of substances such as pyruvate and lactate salts from granulosa cells to oocytes decreases, but metabolites such as pyruvate, lactate salts, and glutamine gradually accumulate in the oocytes [7,22]. Glucose [7], glucose-6-phosphate [7], sorbitol [13], mannitol [13], urea cycle intermediates such as aspartate [7], ornithine [7], and arginine [7] increase in oocytes of older mothers, indicating that energy substrates are diverted to the pentose phosphate pathway, hexosamine synthesis pathway, and urea cycle; the TCA cycle cannot process available substrates. Furthermore, the TCA cycle intermediates succinate [7], fumarate [7], citrate [11],…”

“…For example, in the oocytes of horses, although the glucose abundance in the cumulus cells of older horses is higher, the level of pyruvate in the oocytes of older mares is consistently lower than that of young mares during the GV, MI, and MII stages. This suggests impaired transport or production of pyruvate, possibly due to reduced transzonal transport [13,22]. These differences may be attributed to variations in samples and species.…”

“…OXPHOS is highly active in oocytes [39]. With age, ATP generation through oxidative phosphorylation in oocytes decreases, and the function of ETC is impaired [12][13][14]. The expression of the majority of genes encoding subunits of respiratory chain complexes I to V is downregulated [11,23,40].…”

“…However, the activities of fatty acids metabolism-related substances such as carnitine [7], 3-ketoacyl-CoA thiolase (ACAA) [75], and enoyl-CoA hydratase 3-hydroxy acyl-CoA dehydrogenase (EHHADH) [75] in oocytes show an age-dependent decline. N-acyl ethanolamine, as a signaling molecule, promotes fatty acid breakdown and inhibits fatty acid synthesis, but its expression is downregulated in oocytes of aged mares [13]. These factors lead to lipid accumulation and increased abundance of free fatty acids in oocytes, while inhibiting fatty acid oxidation, impacting oocyte quality in multiple ways.…”

“…1). Although cumulus cells produce ATP and provide it to oocytes [7], a decrease in ATP, a decrease in energy production capacity, and a decline in mitochondrial function are all part of the aging of the oocytes [8][9][10][11][12][13][14][15]. Reduced ATP production leads to a decline in oocyte quality, specifically resulting in decreased metabolic activity, which may affect cell cycle regulation, spindle formation during mitosis, chromosome segregation, fertilization, embryo development, and implantation, as discussed in other literature [16][17][18].…”