Abstract 1105 Lipid Desaturation Regulates the Balance between Self-renewal and Differentiation in Blastocyst-derived Stem Cells

Stem cells are defined by their ability to self-renew and to differentiate, both shown in multiple studies to be regulated by metabolic processes. To decipher cellular metabolic signatures of self-renewal in the early embryo, we compared early differentiating embryonic stem cells (ESCs) and their extra-embryonic counterparts - trophoblast T(SCs) to their self-renewing counterparts. A metabolomics analysis pointed to the desaturation of fatty acyl chains as a metabolic signature of differentiating blastocyst SCs via the upregulation of delta-6 desaturase (D6D; FADS2) and delta-5 desaturase (D5D; FADS1), key enzymes in the biosynthesis of polyunsaturated fatty acids (PUFAs). The inhibition of D6D or D5D retained stemness in ESCs and TSCs, and attenuated ER stress-related apoptosis. D6D inhibition upregulated stearoyl-CoA desaturase-1 (Scd1), essential to maintain endoplasmic reticulum (ER) homeostasis in ESCs and increased their viability. In TSCs, however, D6D inhibition downregulated Scd1, while increasing their viability. Addition of oleic acid – the product of Scd1, essential to ESCs, to TSCs culture medium, was detrimental to TSCs (in contrast to its essentiality to ESCs). Scd1 showed higher mRNA expression and high levels of monounsaturated fatty acyl chain products in TSCs in comparison to ESCs. Interestingly, Scd1 protein was highly expressed as a 63 kDa variant, rather than the expected 37 kDa. This phenomenon was previously described, and was attributed to the formation of dimers. Our results point to embryonic cells with differential expression and regulation of Scd1, and suggest that cellular concentrations of monounsaturated lipids regulate the self-renewal of cell populations in the early embryo, whereas PUFAs drive differentiation. Taken together, our data point to lipid desaturation as a metabolic regulator of the balance between differentiation and self-renewal of the blastocyst stem cells, driving differentiation, and regulating ER homeostasis and cell viability.