# Extracellular presentation of syntaxin4 as a potential trigger for region-specific gastrulation

## TL;DR

The paper explores how syntaxin4, a membrane protein, may trigger region-specific gastrulation by influencing cell signaling and behavior during early embryonic development.

## Contribution

The study reveals a novel extracellular role of syntaxin4 in initiating gastrulation through localized signaling pathways.

## Key findings

- Extracellular syntaxin4 deactivates focal adhesion kinase (FAK), impacting AKT/PI3K signaling.
- Increased P-cadherin expression and brachyury induction occur due to extracellular syntaxin4 activity.
- Activation of Rho/ROCK signaling in embryonic stem cells leads to morphological changes.

## Abstract

During early embryogenesis, gastrulation occurs within a specific region of the pluripotent epiblast, where cells undergo significant changes in their context. The induction of these cellular transformations in particular cell populations suggests the involvement of non-diffusible factors that activate signaling pathways in a spatiotemporal manner. Syntaxin4 (Stx4), a type IV membrane protein that functions as an intravesicular fusion mediator, often translocates across membranes to perform a latent extracellular role in locally regulating cellular behaviors. Through the culture of mouse embryonic egg cylinders isolated from E6.0 embryos and embryonic stem cells (ESCs), we demonstrate that the membrane translocation of Stx4 may play a crucial role in this early stage of development. Using membrane-impermeable antagonistic peptides against extracellular Stx4, along with several small-molecule inhibitors and activators, we found that cells with extracellular Stx4 deactivate focal adhesion kinase (FAK), which then impacts AKT/PI3K signaling and results in increased expression of P-cadherin, ultimately inducing the expression of the gastrulation marker brachyury. Activation of this signaling pathway also triggers Rho/ROCK signaling in ESCs, leading to morphological changes. These findings offer important insights into gastrulation by shedding light on the molecular mechanisms that initiate the spatiotemporal changes in the uniform pluripotent cell sheet.