Reactive astrocytes: the brain's firefighters that sometimes start fires
When the brain is injured, its most abundant support cells transform. New research explains how astrocytes amplify inflammation through the TNF–STAT3 axis and the complement protein C3 — and what that means for Alzheimer's disease.
Ask a textbook what astrocytes do and you will get a peaceful picture: star-shaped support cells that feed neurons, recycle neurotransmitters, and keep the brain’s chemistry stable. That picture is missing a plot twist. When the brain is injured or diseased, astrocytes transform — and the transformed versions, called reactive astrocytes, can be either healers or arsonists.
Two faces of the same cell
Reactive astrocytes are not one thing. Some reactive states seal wounds and calm inflammation. Others do the opposite: they secrete immune signals, recruit more inflammation, and can even attack synapses. Which program an astrocyte switches into depends heavily on the inflammatory signals it receives.
Two names come up again and again in this story. The first is TNF (tumor necrosis factor), a classic inflammatory messenger. The second is STAT3, a molecular switch that TNF flips inside the astrocyte. Our work in a human stem-cell model of the blood-brain barrier showed that this TNF–STAT3 axis is how reactive astrocytes broadcast inflammation: astrocytes on one side of the barrier translate inflammatory signals into effects on the other side. That paper now has over 250 citations, a sign of how much interest this relay mechanism has attracted.
The C3 connection
The second name is C3, the central protein of the complement cascade — an ancient part of the immune system. In a healthy brain, complement helps prune synapses during development. In a diseased brain, C3-positive reactive astrocytes are increasingly seen as a hallmark of harmful inflammation, from Alzheimer’s disease to multiple sclerosis.
The exciting question is whether we can separate the two faces: silence the harmful programs while keeping the healing ones. That requires knowing exactly which cells are in which state — and being able to intervene only in the harmful ones.
From watching to steering
This is where our two research threads meet. We use human iPSC-derived astrocytes and CRISPR screens to discover which genes push astrocytes into inflammatory states. Then we design RNA-editing sensors — tools that recruit the cell’s own ADAR enzyme — to retune those genes inside the inflammatory cells themselves, without touching DNA.
The long-term goal: turn down the fires, keep the firefighters.
Curious how RNA editing works under the hood? Start with the invisible proofreader.