# Rac1 Temporarily Suppresses Fertilization Envelope Formation Immediately After 1-Methyladenine Stimulation

## TL;DR

This study reveals that the Rac1 signaling pathway temporarily prevents the formation of a fertilization envelope in starfish oocytes after hormone stimulation.

## Contribution

The study identifies a transient 'no FE phase' and links it to the activation of the GEF/Rac1 signaling cascade following 1-MA stimulation.

## Key findings

- Co-treatment with 1-MA and calcium ionophore A23187 prevents fertilization envelope formation.
- The GEF/Rac1 signaling cascade is activated during the transient 'no FE phase'.
- Constitutively active Rac1 inhibits fertilization envelope formation even without 1-MA.

## Abstract

In starfish oocytes, the hormone 1-methyladenine (1-MA) induces germinal vesicle breakdown (GVBD) through a signaling cascade involving PI3K, SGK, Cdc25, and Cdk1/cyclin via G-proteinβγ subunit. Following GVBD, fertilization triggers an intracellular calcium increase, leading to the formation of the fertilization envelope (FE) via cortical granule exocytosis. While transient calcium elevations are known to occur after 1-MA stimulation even without fertilization, the inability of these calcium elevations to induce cortical granule exocytosis and FE formation remained unexplained. In this study, we found that co-treatment with 1-MA and calcium ionophore A23187 prevents FE formation, revealing a transient period termed the “no FE phase” persisting for several minutes. After no FE phase, the oocytes regain full competence to form the FE. Furthermore, we identified that the GEF/Rac1 signaling cascade is activated during the no FE phase. Notably, constitutively active Rac1 expressed in oocytes reproduces this inhibition even in the absence of 1-MA stimulation. These findings suggest that the GEF/Rac1 cascade, triggered by 1-MA, initiates the no FE phase and plays a critical role in coordinating the progression of subsequent fertilization events.