L-Proline Supplementation Drives Self-Renewing Mouse Embryonic Stem Cells to a Partially Primed Pluripotent State: The Early Primitive Ectoderm-Like Cell

Signalling pathway crosstalk stimulated by L-proline drives mouse embryonic stem cells to primitive-ectoderm-like cells

The amino acid L-proline exhibits growth factor-like properties during development - from improving blastocyst development to driving neurogenesis in vitro. Addition of 400 μM L-proline to self-renewal medium drives naïve mouse embryonic stem cells (ESCs) to early primitive ectoderm-like (EPL) cells - a transcriptionally distinct primed or partially primed pluripotent state. EPL cells retain expression of pluripotency genes, upregulate primitive ectoderm markers, undergo a morphological change and have increased cell number. These changes are facilitated by a complex signalling network hinging on the Mapk, Fgfr, Pi3k and mTor pathways. Here, we use a factorial experimental design coupled with statistical modelling to understand which signalling pathways are involved in the transition between ESCs and EPL cells, and how they underpin changes in morphology, cell number, apoptosis, proliferation and gene expression. This approach reveals pathways which work antagonistically or synergistically. Most properties were affected by more than one inhibitor, and each inhibitor blocked specific aspects of the naïve-to-primed transition. These mechanisms underpin progression of stem cells across the in vitro pluripotency continuum and serve as a model for pre-, peri- and post-implantation embryogenesis.

Capturing Transitional Pluripotency through Proline Metabolism

In this paper, we summarize the current knowledge of the role of proline metabolism in the control of the identity of Embryonic Stem Cells (ESCs). An imbalance in proline metabolism shifts mouse ESCs toward a stable naïve-to-primed intermediate state of pluripotency. Proline-induced cells (PiCs), also named primitive ectoderm-like cells (EPLs), are phenotypically metastable, a trait linked to a rapid and reversible relocalization of E-cadherin from the plasma membrane to intracellular membrane compartments. The ESC-to-PiC transition relies on the activation of Erk and Tgfβ/Activin signaling pathways and is associated with extensive remodeling of the transcriptome, metabolome and epigenome. PiCs maintain several properties of naïve pluripotency (teratoma formation, blastocyst colonization and 3D gastruloid development) and acquire a few traits of primed cells (flat-shaped colony morphology, aerobic glycolysis metabolism and competence for primordial germ cell fate). Overall, the molecular and phenotypic features of PiCs resemble those of an early-primed state of pluripotency, providing a robust model to study the role of metabolic perturbations in pluripotency and cell fate decisions.