# Nuclear RhoA Activation Regulates Nucleus Size and DNA Content via Nuclear Activation of ROCK and pErk

## TL;DR

This study shows that RhoA in the nucleus controls DNA content and nucleus size through ROCK and Erk activation, separate from its cytoplasmic role.

## Contribution

The paper introduces a method to distinguish nuclear and cytoplasmic RhoA functions and identifies a novel nuclear signaling pathway.

## Key findings

- Nuclear RhoA activation regulates DNA amount and nuclear size, not cytoplasmic RhoA.
- ROCK and Erk are sequentially activated in the nucleus by RhoA to control DNA and nucleus size.
- GDP-bound RhoA, not activated RhoA, moves to the nucleus, showing distinct signaling compartments.

## Abstract

RhoA is a major regulator of the actin cytoskeleton. Its function in the nucleus, however, is unclear. Fusing wildtype, fast cycling, constitutively active, and dominant negative forms of RhoA with tags promoting nuclear or cytoplasmic location and allowing specific detection, we established a platform to distinguish the functions of nuclear and cytoplasmic RhoA. Our data show that nuclear but not cytoplasmic activation of RhoA regulates DNA amount and nuclear size. This is mediated by sequential nuclear activation of the RhoA effector ROCK and Erk, a major cell cycle regulating kinase. The inhibition of ROCK or Erk activation in untransfected cells reduced DNA amounts to a similar extent, suggesting that endogenous activation levels of nuclear RhoA-ROCK-Erk signaling are sufficient for regulation. We reveal, furthermore, that GDP-bound, but not activated RhoA, translocates to the nucleus, indicating relatively separated cytoplasmic and nuclear RhoA signaling. Moreover, even the massive nuclear activation of RhoA does not cause an obvious increase in nuclear F-actin, indicating that RhoA activation is not critical for nuclear F-actin formation.