# Endothelial oncogenic KRAS mutation drives the dynamics of microglia and macrophages in brain arteriovenous malformation

## TL;DR

This study shows that mutant KRAS in brain blood vessels activates microglia and macrophages, leading to dangerous bleeding in brain arteriovenous malformations.

## Contribution

The study reveals a novel mechanism linking endothelial KRAS mutations to immune cell activation and hemorrhage in bAVMs.

## Key findings

- KRASG12V in endothelial cells triggers microglia activation and macrophage infiltration in bAVMs.
- Activated microglia/macrophages correlate with blood-brain barrier leakage and hemorrhagic conversion.
- Depletion or inhibition of microglia/macrophages reduces intracerebral hemorrhage in bAVMs.

## Abstract

Mutation of KRAS in endothelial cells (KRAS-ECs) leads to intracerebral hemorrhage (ICH) in brain arteriovenous malformation (bAVM), resulting in severe disabilities or even death. However, it is unclear what causes this hemorrhagic conversion of bAVMs. Here, using a locally established, clinically relevant sporadic bAVM mouse model, created by overexpressing mutant KRAS (KRASG12V) in brain ECs, we demonstrate that KRAS-ECs act as trigger for activation of microglia (MG) and infiltration of macrophages (Mϕ). Using a 3-dimensional immunostaining approach with cleared human and mouse bAVM tissues, we demonstrate an abundance of MG/Mϕ around the bAVM nidus. The presence of MG/Mϕ was correlated to the blood-brain barrier leakage in bAVM areas. Time-lapsed intravital imaging in Cx3cr1-gfp;Ccr2-rfp reporter mice demonstrated the dynamic activation of MG and infiltration of Mϕ toward mutant KRASG12V–modified dysplastic vessels. Importantly, a time-course analysis showed that these activated MG and infiltrated Mϕ are present around the bAVMs prior to hemorrhagic conversion, and controlled depletion of MG/Mϕ reduced ICH incidence in bAVMs. Inhibition of MG/Mϕ with long-term minocycline treatment attenuated the incidence of ICHs around bAVMs. Our study indicates that MG/Mϕ are involved in destabilization of KRASG12V-induced bAVM, leading to hemorrhagic conversion/ICH. Thus, modulation of MG/Mϕ may reduce ICH risk in patients with bAVM.

The study focuses on the mechanism of intracerebral hemorrhage by understanding the role of microglia and macrophages in mutant KRAS-induced brain arteriovenous malformation.