Astrocytes: the brain's quiet housekeepers that start failing as you age
They are not neurons and they rarely get the attention, but astrocytes are everywhere in the brain and they do almost everything: nutrition, waste removal, synapse regulation. As you get older, they begin to fail one by one, and those failures add up.
Astrocytes are one of the largest cell populations in the central nervous system. For a long time they were considered passive, a kind of connective tissue of the brain. That picture has changed fundamentally. Astrocytes regulate the blood-brain barrier, clear used neurotransmitters, feed neurons, and modulate synaptic connections. Without properly functioning astrocytes, the brain simply does not work normally.
A detailed review on Fight Aging lays out what goes wrong with astrocytes as they age, and it is a multifaceted story. Astrocytes can fall into two problematic states as they get older. The first is reactivity: cells become activated in response to damage or inflammation, producing substances that are protective in the short term but harmful over time. The second state is senescence: cells stop dividing, do not die, but chronically secrete pro-inflammatory substances, the notorious SASP.
Why the difference between reactive and senescent matters
The distinction is not merely academic. Reactive astrocytes can, in principle, return to normal functioning once the trigger disappears. Senescent astrocytes cannot, they remain in a chronically damaged state and effectively contaminate their surroundings with inflammatory signals. As the brain ages, the balance shifts further and further toward senescence, and the brain's microenvironment deteriorates in a lasting way.
That has direct consequences for neurons. Synapses are maintained less efficiently, the blood-brain barrier becomes leaky, and waste products accumulate. This is linked to the elevated risk of neurodegenerative diseases such as Alzheimer's and Parkinson's in older people, although cause and effect are difficult to untangle in these complex interactions.
Senolytics as a solution, but which cells do you target?
Scientific interest has turned to senolytics: agents that specifically clear out senescent cells. In animal models, senolytics have reduced brain inflammation and slowed cognitive decline. The challenge is selectivity, you want to target senescent astrocytes without harming healthy cells or useful reactive astrocytes. That requires a more precise characterisation of exactly what sets a senescent astrocyte apart from its healthier relatives. That characterisation is well under way, but not yet complete. The biology of brain ageing is rarely as straightforward as we would like.