UCLA

Human pluripotent stem cells can self-renew indefinitely or be induced to differentiate into the three embryonic germ lineages: endoderm, mesoderm, and ectoderm. Multiple studies show that reconfiguring metabolic pathway activities influences the fate of pluripotent stem cells through metabolite-regulated changes in intracellular signal transduction pathways and the epigenome. Through these and potentially additional mechanisms, the conversion of available nutrients into metabolite fluxes and steady-state levels can tip the balance between pluripotent stem cell self-renewal and differentiation, although which nutrients and metabolites favor specific cell fate decisions are largely unidentified.

Glutamine participates in a wide variety of cellular processes beyond biomass and energy-producing reactions in pluripotent and adult-type stem cells. We showed that endoderm, mesoderm, and ectoderm all consume abundant glutamine supplied in cell culture media. Additionally, exogenous glutamine withdrawal biased spontaneous human pluripotent stem cell differentiation toward ectoderm and away from mesoderm. However, a mechanism for this nutrient-directed cell fate bias was, until recently, unknown.

We uncovered that while all three germ lineages are capable of de novo glutamine synthesis, only ectoderm generates sufficient glutamine to sustain cell viability and differentiation, clarifying lineage fate restrictions observed with glutamine withdrawal. While exogenous glutamine is dispensable for ectoderm, complete glutamine starvation obliterates this lineage, suggesting a critical role for glutamine in ectoderm specification. We uncovered that glutamine fulfillment of mTORC1 activation, but not biosynthetic demands and protein translation, is critical for initiating ectoderm fate commitment. Glutamine acts as a signaling molecule to activate mTORC1 signaling in ectoderm that supersedes lineage-specifying cytokine induction. In contrast, glutamine in mesoderm and endoderm serves as the preferred precursor of -ketoglutarate without a direct role in cell fate signaling. Our findings show that a nutrient acts as a signaling molecule to control cell fate, raising a question about whether the nutrient environment functions directly in cell differentiation during development.

For these reasons, we hypothesize that intracellular glutamine may help coordinate differentiation of the three embryonic germ layers. Differing patterns of nutrient availability occur during early embryonic germ lineage differentiation. Interestingly, transcriptome analysis of a gastrulation-stage human embryo shows unique glutamine enzyme-encoding gene expression patterns may also distinguish germ lineages in vivo.

In summary, these findings reveal critical roles for metabolic regulation and nutrient signaling during early embryonic development.