P-370 The age-associated increase in ovarian stiffness compromises follicle development and induces changes in the follicles’ transcriptome, resulting in poor-quality oocytes
Authors: Pietroforte S , Amargant F 
Published: July 2024

Abstract

Study question
Does the age-associated increase in ovarian stiffness trigger the decline in oocyte quantity and quality observed in women of advanced maternal age?

Summary answer
An encapsulated in vitro follicle culture mimicking ovarian biomechanics revealed that age-associated increase in stiffness impacts follicle growth, modulates follicles’ transcriptome and reduces oocyte quality.

What is known already
Female reproductive aging is characterized by a progressive decline in oocyte quality and quantity, resulting in sub-fertility. We recently discovered that with aging the mouse ovary assumes a pro-fibrotic milieu which induces ovarian stiffness. Moreover, human ovaries also become stiffer with aging; however, it is not known whether this age-associated increase in stiffness impacts ovarian function. The present study aims to examine whether stiffness is a novel mechanism that mediates the age-associated progressive decline in oocyte quality and how stiffness affects the transcriptome of growing follicles, resulting in poor-quality oocytes.

Study design, size, duration
We used an in vitro alginate-encapsulated follicle culture to mimic young and old environments in which follicles grow. Alginate hydrogel synthesis replicated the soft (0.5% - 1.79±0.08 kPa) and stiff (2% - 4.56±2.03 kPa) environments of young and aged ovaries, respectively. Mouse secondary follicles were cultured in 0.5 or 2% alginate and used for i) long-term culture (day 0-10) to evaluate follicle development and oocyte quality; ii) short-term culture (hour 0-24) to investigate follicles’ transcriptome.

Participants/materials, setting, methods
During the long-term culture, markers of follicle development were evaluated (n = 31 follicles), including: follicle size, estradiol production (ELISA), cell death (cleaved-caspase3,CC3; immunohistochemistry). At D10, oocyte quality was inspected on isolated oocytes. For the short-term culture experiment, 32 follicles were analyzed by RNAsequencing at 3h (baseline) and 24h. We compared 3h and 24h gene expression (DESeq2,R/Bioconductor) in 0.5% and 2%. Significant differences reported at padj&lt;0.05 and log2FoldChange±2. Two mice strains (CB6F1;CD1) were used in all experiments.

Main results and the role of chance
Follicles cultured in stiff environment showed a significant reduction in follicle size compared to follicles cultured in soft environment (0.5% 226.9±17.4um, 2% 160.8±9.9um; p &lt; 0.0001). These size differences suggest that granulosa cells are not proliferating in a stiff environment. Estradiol production was reduced in follicles cultured at 2% (0.5% 11.3±15.9ng/ml, 2% 0.3±0.5ng/ml, p = 0.296), while CC3 intensity was increased (CC3 staining intensity/area, 0.5% 20.4±5.8a.u., 2% 27.1±4.3a.u., p &lt; 0.05), suggesting that the stiff environment impacts granulosa cell viability. Oocyte quality significantly declined in follicles cultured in 2%, with 68.9±16.8% of the oocytes degenerated, compared to 23.6±9.2% in 0.5% alginate. The use of two different mice strains yielded comparable results. Using RNAsequencing analysis, we found 1033 differentially expressed genes (DEGs) between 3h and 24h of culture in 2% alginate, mostly involved in apoptosis, inflammation and collagen processes, biological pathways typically associated with aging. The 64% of those DEGs were upregulated at 24h. Conversely, the 1295 DEGs (61% upregulated at 24h) detected comparing 3h and 24h in 0.5% alginate were relate to metabolism, cell cycle and development pathways, biological processes associated to the normal behavior of growing follicles. These differences in gene expression suggest an early-response in the transcriptome of follicles cultured in a stiff environment.

Limitations, reasons for caution
The stiffness of the alginate gels was based on whole ovary biomechanics; future investigations are needed to measure each follicle-stage-specific stiffness. The changes in follicles’ transcriptome in the stiff environment were evaluated only in short-term culture. This study utilizes in vitro approaches, in vivo modulation of stiffness is currently undergoing.

Wider implications of the findings
Increased ovarian stiffness is observed in conditions such as aging or ovaries exposed to cancer treatments. Studying how stiffness impacts oocyte quality will allow the development of suitable in vitro systems to support human folliculogenesis and in vivo approaches to modulate biomechanics of the ovary, preserving female fertility.

Trial registration number
not applicable