# Spontaneous Overactivation of Xenopus Frog Eggs Triggers Necrotic Cell Death

## TL;DR

This study shows that spontaneous overactivation in Xenopus frog eggs leads to necrotic cell death, similar to stress-induced cases, and may be caused by mechanical stress during oviposition.

## Contribution

The study identifies the cytological and biochemical events of spontaneous egg overactivation and links it to necrotic cell death and mechanical stress during oviposition.

## Key findings

- Spontaneous overactivation in Xenopus eggs involves rapid cortical contraction, ATP depletion, and cyclin B2 degradation.
- Overactivated eggs leak ATP and ADP, indicating plasma membrane rupture and necrotic cell death.
- Egg overactivation can occur in the frog's genital tract, suggesting mechanical stress as a potential trigger during oviposition.

## Abstract

The excessive activation of frog eggs, referred to as overactivation, can be initiated by strong oxidative stress, leading to expedited calcium-dependent non-apoptotic cell death. Overactivation also occurs spontaneously, albeit at a low frequency, in natural populations of spawned frog eggs. Currently, the cytological and biochemical events of the spontaneous process have not been characterized. In the present study, we demonstrate that the spontaneous overactivation of Xenopus frog eggs, similarly to oxidative stress- and mechanical stress-induced overactivation, is characterized by the fast and irreversible contraction of the egg’s cortical layer, an increase in egg size, the depletion of intracellular ATP, a drastic increase in the intracellular ADP/ATP ratio, and the degradation of M phase-specific cyclin B2. These events manifest in eggs in the absence of caspase activation within one hour of triggering overactivation. Importantly, substantial amounts of ATP and ADP leak from the overactivated eggs, indicating that plasma membrane integrity is compromised in these cells. The rupture of the plasma membrane and acute depletion of intracellular ATP explicitly define necrotic cell death. Finally, we report that egg overactivation can occur in the frog’s genital tract. Our data suggest that mechanical stress may be a key factor promoting egg overactivation during oviposition in frogs.