Lipid droplets (LDs) are intracellular organelles, which have long been ignored as passive aggregates of lipids. However, recent research firmly established LDs as dynamic structures, actively regulating energy storage and generating signalling molecules. Concomitant with unfolding the regulatory mechanisms of LDs, including the identification of LD-associated proteins, their functional importance is increasingly being recognised in various pathologies such as metabolic diseases and cancer.

Early embryogenesis is a tightly orchestrated process, often investigated through in vitro models, such as pluripotent stem cells. Interestingly, dynamic LD enlargement and lipid storage was found to operate in in vivo and blastocyst-derived embryonic stem cells (ESCs). The overall objective of this PhD research project is to specifically investigate the biology and roles of LDs in regulating pluripotent stem cell biology and metabolism. Here, we demonstrate an essential role for LD-associated protein Cidea in promoting LD enlargement in ESCs using gain- and loss-of-function approaches. In vivo, we observe that Cidea expression peaks at the morula-blastocyst transition consistent with the formation of enlarged LDs throughout the embryo. Yet these are mobilised in the pluripotent (epiblast) compartment at the onset of implantation, pointing to enhanced, tissue-specific lipid usage during this key developmental step.

Using ESC models, we recognize a possible role for Cidea during the morphogenesis events accompanying epiblast maturation post-implantation. From a metabolic perspective, we find that enforced Cidea expression and LD enlargement correlate with enhanced oxygen consumption and TCA cycle activity with unchanged anaerobic glycolytic capacity. The functional significance of these alterations is further investigated using self-renewing, ESC clonal populations that harbour different LD morphologies, gene expression patterns and metabolic capacities. Integrating RNA-sequencing data, GC-MS and Seahorse-based experiments, we propose that ESCs might transiently enhance their oxidative status to sustain the energy-demanding process of lipid synthesis taking place at the exit of ESC pluripotency.