Lipid droplets in mammalian eggs are utilized during embryonic diapause

Abstract

Embryonic diapause (ED) is a temporary arrest of an embryo at the blastocyst stage when it waits for the uterine receptivity signal to implant. ED used by over 100 species may also occur in normally “nondiapausing” mammals when the uterine receptivity signal is blocked or delayed. A large number of lipid droplets (LDs) are stored throughout the preimplantation embryo development, but the amount of lipids varies greatly across different mammalian species. Yet, the role of LDs in the mammalian egg and embryo remains unknown. Here, using a mouse model, we provide evidence that LDs play a crucial role in maintaining ED. By mechanical removal of LDs from zygotes, we demonstrated that delipidated embryos are unable to survive during ED. LDs are not essential for normal prompt implantation, without ED. We further demonstrated that with the progression of ED, the amount of intracellular lipid reduces, and composition changes. This decrease in lipid is caused by a switch from carbohydrate metabolism to lipid catabolism in diapausing blastocysts, which also exhibit increased release of exosomes reflecting elevated embryonic signaling to the mother. We have also shown that presence of LDs in the oocytes of various mammals positively corelates with their species-specific length of diapause. Our results reveal the functional role of LDs in embryonic development. These results can help to develop diagnostic techniques and treatment of recurrent implantation failure and will likely ignite further studies in developmental biology and reproductive medicine fields.

Molecular Regulation of Paused Pluripotency in Early Mammalian Embryos and Stem Cells

“…The lipid content of the mouse oocyte is relatively low compared to other species (4 ng compared to 161 ng in the pig, 63 in cows, and 89 in sheep) (Loewenstein and Cohen, 1964;McEvoy et al, 2000;Sturmey and Leese, 2003). Nonetheless, removal of lipid droplets from mouse zygotes impairs the survival of the diapause blastocysts, indicating active utilization during diapause (Arena et al, 2021). This functional evidence is supported by molecular analysis of different lipid species during diapause, which found depletion of neutral lipids usually enriched in lipid droplets (e.g., triacylglycerol) and increase in processed intermediates such as phosphatidylcholines and fatty acids (Hussein et al, 2020).…”

“…In addition to eliminating estrogen, progesterone supplementation is required to sustain the pregnancy. Mouse diapause can be sustained for up to 36 days and possibly longer in vivo (twice as long as gestation), however, embryo loss occurs over time (Arena et al, 2021). Exit from diapause is triggered by an increase in uterine receptivity that either occurs when lactation ends or can be experimentally induced by injection of estradiol.…”

“…Prolonged diapause, on the other hand, may compromise the fitness of the embryo. In mice, fewer diapause embryos (Arena et al, 2021) and resulting fetuses (Weitlauf and Greenwald, 1968) are recovered with longer duration of diapause. Also, ex vivo survival of embryos in a diapause-like state diminishes over time (Bulut-Karslioglu et al, 2016;Liu et al, 2020).…”

Glucocorticoids, Stress and Delta-9 Tetrahydrocannabinol (THC) during Early Embryonic Development

“…This provides further support for a relationship between cortisol concentrations and lipid content during pre-implantation development. Lipid droplets are important resources for signaling and meeting the energy requirements of the embryo during the preimplantation stages of development across many mammalian species [ 82 ]. A study by Arena et al (2021) provides evidence to suggest that lipid storage in the oocyte and embryo is especially important for successful survival during embryonic diapause (ED).…”

“…A study by Arena et al (2021) provides evidence to suggest that lipid storage in the oocyte and embryo is especially important for successful survival during embryonic diapause (ED). ED is a temporary arrest of the embryo that occurs prior to receiving adequate maternal signaling needed for uterine implantation [ 82 ]. Without adequate energy stored during ED, it was shown that mouse blastocysts fail to survive [ 82 ].…”