# Seasonal Change in Glucose Metabolism and Steroidogenesis in the Ovaries of Wild Ground Squirrels (Spermophilus dauricus)

## TL;DR

This study explores how glucose metabolism and steroid hormone production in the ovaries of wild ground squirrels change with the seasons, affecting their reproductive cycles.

## Contribution

The study is the first to reveal the tight coupling of ovarian glucose metabolism and steroidogenesis during the breeding season, identifying GSK-3β as a novel regulatory target.

## Key findings

- Ovarian glucose metabolism and steroidogenesis are enhanced during the breeding season, promoting follicular development.
- Glycogen synthase kinase-3β (GSK-3β) is upregulated in the non-breeding season, maintaining metabolic quiescence.
- Transcriptomic and metabolomic analyses confirm seasonal changes in glucose transporters and steroidogenic enzymes.

## Abstract

The wild ground squirrel (Spermophilus dauricus) is a typical seasonal breeder with distinct cyclical changes in ovarian function and follicular development. During the breeding season (BS), its ovaries contain primary follicles, secondary follicles, antral follicles, Graafian follicles, and corpora lutea; in contrast, only a small number of early-stage follicles exist in the non-breeding season (NBS), with no Graafian follicles or corpora lutea. This pronounced seasonal contrast reflects precise temporal regulation of follicular development and ovulatory activity. Folliculogenesis relies on adequate glucose metabolism (providing energy and essential substrates for follicular growth and oocyte maturation) and steroid hormones (regulating follicular maturation and ovulation). Using integrated multi-omics analyses and experimental validation, our study is the first to reveal that enhanced ovarian glucose metabolism is tightly coupled with elevated steroidogenesis during the BS, synergistically promoting follicular development and ovulatory competence. Conversely, glycogen synthase kinase-3β (GSK-3β)—a negative regulator of glucose metabolism—is upregulated in the NBS, acting as a novel regulatory target to maintain ovarian metabolic quiescence. These findings elucidate the coordinated dynamics of glucose metabolism and steroid hormone synthesis across the seasonal reproductive cycle, providing novel insights into the metabolic–endocrine crosstalk underlying seasonal follicular development in this species and the physiological mechanisms of periodic reproduction.

Precise regulation of ovarian glucose metabolism and steroidogenesis is pivotal for the reproductive success of seasonal breeders. This study investigated seasonal variations in ovarian glucose metabolism and its association with follicular development and steroidogenesis in wild ground squirrels (Spermophilus dauricus). Results showed that ovarian weight and volume, and serum levels of luteinizing hormone (LH), follicle-stimulating hormone (FSH), 17β-estradiol, and progesterone were significantly higher in the breeding season (BS) than in the non-breeding season (NBS). Ovarian glycogen content was elevated in the BS, whereas circulating glucose levels decreased remarkably. Histologically, the BS ovaries contained follicles at all developmental stages (primary to Graafian) and corpora lutea, while corpora lutea were absent in the NBS. Integrated transcriptomic and metabolomic analyses revealed upregulated glycogen synthesis, glycolytic pathways, and altered steroidogenic metabolites in the BS. Quantitative real-time PCR (qRT-PCR) confirmed higher expression of glucose transporter 1 (GLUT1), glucose metabolism-related enzymes (G6PD, PFKFB3, PFKM), glycogen synthase (GYS), and steroidogenic factors (StAR, P450scc, P450c17, 3β-HSD, P450arom) in the BS, whereas glycogen synthase kinase-3β (Gsk-3β) was upregulated in the NBS. Immunohistochemistry demonstrated colocalization of steroidogenic and glucose-metabolizing enzymes in granulosa and theca cells, and correlation analyses confirmed positive associations between glucose metabolism, steroidogenesis, and ovarian weight. Collectively, these findings indicate that enhanced ovarian glucose metabolism in the BS is tightly coupled with elevated steroidogenesis, synergistically promoting seasonal follicular development and ovulatory competence in this species.