Lipid droplet synthesis is associated with angiogenesis in mouse ovarian follicles†

Lipid droplets (LDs) are endoplasmic reticulum (ER)-derived organelles comprising a core of neutral lipids surrounded by a phospholipid monolayer. Lipid droplets play important roles in lipid metabolism and energy homeostasis. Mammalian ovaries have been hypothesized to use neutral lipids stored in LDs to produce the hormones and nutrients necessary for rapid follicular development; however, our understanding of LD synthesis remains incomplete. In this study, we generated transgenic reporter mice that express mCherry fused to HPos, a minimal peptide that localizes specifically to nascent LDs synthesized at the ER. With this tool for visualizing initial LD synthesis in ovaries, we found that LDs are synthesized continuously in theca cells but rarely in inner granulosa cells (Gc) during early follicular development. Administration of exogenous gonadotropin enhances LD synthesis in the Gc, suggesting that LD synthesis is hormonally regulated. In contrast, we observed copious LD synthesis in the corpus luteum, and excessive LDs accumulation in atretic follicles. Furthermore, we demonstrated that LD synthesis is synchronized with angiogenesis around the follicle and that suppressing angiogenesis caused defective LD biosynthesis in developing follicles. Overall, our study is the first to demonstrate a spatiotemporally regulated interplay between LD synthesis and neovascularization during mammalian follicular development.