Investigating the trafficking, storage and release of lipids in pluripotent cells during mouse peri-implantation development

Abstract

Embryonic development from pre-to-post-implantation involves critical transformations of the pluripotent lineage. Although the associated metabolic remodeling has often been regarded as survival adaptations to the implantation environment, emerging roles of metabolites played in cell state transition start to be recognized. Mammalian pre-implantation embryos are rich in lipids, which are not only essential structural building blocks, but also energy-yielding sub- strates and signaling molecules. Yet the mechanistic roles of the stored lipids in developmental progression are neglected. In mouse blastocysts, lipids appeared to be built up in the form of enlarged lipid droplets (LDs), which is an adaptive property shared by lipid-rich adult tissues, for sustaining storage while allowing timely release of lipid derivatives in response to physio- logical demands. Here we explore the mechanism of LD enlargement and thus the functional roles of associated lipid storage in the developing pluripotent lineage. By using embryonic stem cells (ESCs) as in vitro models, we uncovered a conserved role of adipocyte LD-residing factor, CIDEA, to promote LD-LD fusion in pre-implantation pluripo- tent cells. Early differentiation upon implantation downregulates the factor, accompanied by the loss of the lipid storage. Loss-of-function analysis reveals CIDEA is not only important for metabolic activity, but also unexpectedly involved in peri-implantation-associated epithelization and subsequent lumenogenesis. The role of CIDEA in morphogenesis is directly linked to the timely mobilization of lipid storage built up in enlarged LDs. Mechanistically, CIDEA-mediated LD-LD fusion increases lipid storage capacity by protecting LDs from immediate degradation, but allowing subsequent lysosomal hydrolysis of the organelles, known as lipophagy. Inhibiting lipophagy leads to similar morphogenesis defect as CIDEA loss-of-function, highlighting that the functionality of lipid storage in the event lies in its subsequent usage. Further analyzing the profiles of LD-stored lipids (triacylglycerol; TG and cholesteryl ester; CE) reveals that only CE is selectively utilized with the mobilization of LDs. Functionally, only the inhibition of CE synthesis, but not TG synthesis, disrupts morphogenesis. Cholesterol esterification in differentiating pluripotent cells is associated with the continual build-up of free cholesterol, which accumulates in lysosomes. Lysosomal cholesterol was recently identified as metabolite input for mTORC1 activation (Castellano et al., 2017). We find that disrupting the cellular cholesterol balance of differentiating ESCs inactivates mTORC1, while direct attenua- tion of mTORC1 activity also disrupts proper morphogenesis. Our results propose a potential link between LD dynamics and mTORC1 activity through cholesterol metabolism. Collectively, this study uncovers a novel role of LDs in orchestrating tissue remodeling and highlights the underappreciated significance of lipid metabolism in early embryonic development.