O-205 Poor mitochondrial metabolism impairs meiosis and contributes to reduced oocyte maturation rates in patients with advanced maternal age
Authors: Pietroforte S , Martins M , Ibañez E , Popovic M , Sanchez T , Vassena R , Sakkas D , Zambelli F 
Published: June 2023

Abstract

Study question
Is there a relationship between poor mitochondrial metabolism and impaired meiotic progression of germinal vesicle (GVs) oocytes in women of advanced maternal age (AMA)?

Summary answer
Poor mitochondrial metabolism impairs meiotic progression in human oocytes, contributing to a lower oocyte maturation capacity in women of AMA.

What is known already
High aneuploidy rates and impaired metabolism are characteristic features of AMA oocytes. Studies in mouse suggest that successful meiotic maturation and metabolic competency may be functionally linked, however this relationship has not been clearly established in human. Metabolic dynamics can be visualized by indirect measurements through mitochondrial staining and quantified more directly using Fluorescence Lifetime Imaging Microscopy (FLIM). This live-imaging approach can generate metabolic timelapse profiles of oocytes throughout meiosis. We performed an extensive characterization of oocyte metabolism in maturing GVs, obtained from both young and AMA patients, to establish the importance of mitochondrial metabolism during meiosis.

Study design, size, duration
A total of 401 GVs from young (≤30 years, n = 264) and AMA (&gt;37 years, n = 137) women were included in the study. GVs were matured in vitro in G2-plus medium for 30 hours. Maturation was determined by the presence of an extruded polar body. Oocytes were used for FLIM (n = 175) and immunofluorescence (IF) analysis (n = 226). Further, mitochondrial metabolism loss of function studies (n = 64) were performed by treating young GVs with 1μM Trifluoromethoxy-carbonylcyanide-phenylhydrazone (FCCP) for 30 minutes.

Participants/materials, setting, methods
The proteins Dihydrolipoamide-S-Acetyltransferase (D-LAT) and Translocase-of-outer mitochondrial-membrane (TOMM20) were analysed in young and AMA oocytes by IF (Arbitrary-Unit, AU) to assess mitochondrial activity and localization, respectively. Fluorescence mean intensities were quantified with ImageJ and compared by t-test; maturation rates were compared by Chi-squared test. FLIM comprehensive metabolism (NADH; FAD+) were taken at GV stage. Different FLIM parameters (fluorescence intensity, fraction bound, short/long lifetime) were evaluated individually and combined into the Redox ratio (NADH intensity/FAD+ intensity).

Main results and the role of chance
Mitochondrial staining showed a common pattern in young and AMA GV oocytes, with a uniform localization of mitochondria in the ooplasm (TOMM20) and a subcortical localization of active organelles (D-LAT). These patterns were confirmed by in-live FLIM analysis. The total mitochondria abundance was comparable between AMA and young GV oocytes (intensity of 61674±24322 AU in young, 32186±33414 AU in AMA, p = 0.195), however, active mitochondria were diminished in AMA compared to young GVs both by IF and FLIM (intensity of 78614±58534 AU in young, 12517±10187 AU in AMA, p = 0.003; Redox ratio in young 2e+00±0.15, in AMA 1e+00 ± 0.16, p = 2.969e-05). Notably, young oocytes matured at a significantly higher rate (86.3%; 63/73) than AMA oocytes (62.3%; 38/61; p = 0.002). Moreover, FLIM imaging revealed that GVs with a higher metabolism were more likely to complete meiosis to MII (Redox ratio 2e+00±0.17 in GVs matured to MII, 1e+00±0.18 in non-matured, p = 1.054e-06). Following treatment with FCCP, young GVs showed a similar phenotype to AMA oocytes, with a significant decrease in mitochondrial activity (intensity of 78614±58534 IU in untreated and 11554±16131 IU in treated GVs, p = 0.019), and an associated drop-in maturation rate. Ultimately, only 39.5% (17/43) of the young treated GVs were able to accomplish meiotic maturation.

Limitations, reasons for caution
Maturation rates were assessed by the presence of an extruded PB and variations in spindle assembly timings may have been overlooked. The quantification of mitochondrial activity in loss of function studies was assessed only by IF staining.

Wider implications of the findings
Our findings demonstrate the presence of a functional link between oocyte metabolism and impaired meiosis, which may contribute to the decline of oocyte quality with age. Given that live-metabolic imaging is able to discern GVs that progress through meiosis, this approach may represent a valuable non-invasive tool in oocyte selection.

Trial registration number
not applicable