Specialized cytonemes induce self-organization of stem cells

Significance

Many questions about how stem cells communicate with neighboring cells and self-organize to initiate tissue formation remain unanswered. We uncovered mechanisms employed by embryonic stem cells (ESCs) and trophoblast stem cells (TSCs) to coform embryo-like structures. We describe ESC-generated cytonemes that react to self-renewal–promoting Wnt ligands secreted by TSCs. We identified glutamatergic activity upon formation of ESC–TSC interaction. This cellular connection is required for the transmission of Wnt signals to ESCs for Wnt/β-catenin pathway activation, a process that regulates morphogenesis. Given that many stem cell types express glutamate receptors and rely on niche-secreted Wnt ligands for self-renewal, we propose that Wnt and glutamatergic signaling crosstalk may prove prevalent in various mammalian tissues to regulate stem cell–niche interactions.

Abstract

Spatial cellular organization is fundamental for embryogenesis. Remarkably, coculturing embryonic stem cells (ESCs) and trophoblast stem cells (TSCs) recapitulates this process, forming embryo-like structures. However, mechanisms driving ESC–TSC interaction remain elusive. We describe specialized ESC-generated cytonemes that react to TSC-secreted Wnts. Cytoneme formation and length are controlled by actin, intracellular calcium stores, and components of the Wnt pathway. ESC cytonemes select self-renewal–promoting Wnts via crosstalk between Wnt receptors, activation of ionotropic glutamate receptors (iGluRs), and localized calcium transients. This crosstalk orchestrates Wnt signaling, ESC polarization, ESC–TSC pairing, and consequently synthetic embryogenesis. Our results uncover ESC–TSC contact–mediated signaling, reminiscent of the glutamatergic neuronal synapse, inducing spatial self-organization and embryonic cell specification.