Friends and Foes: Astrocytes as Disease Targets

Astrocytes quickly respond to trouble in the brain. Under normal physiological conditions, these abundant reactive glial cells rapidly adapt to environmental changes, which allows them to regulate healthy brain function.¹ However, in disease contexts, astrocytes switch to harmful states that increase nerve cell loss and damage the central nervous system. Although researchers know that neurotoxic reactive astrocytes are pervasive in several neurodegenerative diseases, investigating relevant changes for high throughput therapeutic discovery remains difficult.²

“A big challenge in the field was that anytime you put astrocytes in culture, they were already changing their state,” said neuroscientist Paul Tesar, who studies glial cell function at Case Western Reserve University. Current serum-free techniques help researchers avoid culture-induced changes while examining astrocyte reactivity in vitro, but require time consuming and low yield isolation methods.³ In their latest work published in Nature Neuroscience, Tesar and his team established a large-scale isolation and enrichment protocol to grow astrocytes in physiological states and control disease-relevant reactive transitions in response to specific inflammatory stimuli.⁴ They cultured millions of functional mouse astrocytes and developed a drug screen to identify inhibitors of rogue astrocyte reactivity in culture and mouse models of neurodegeneration.

Led by postdoctoral researcher Benjamin Clayton, Tesar’s team built and validated their new discovery platform with sequencing and cellular technologies, examining cell morphologies, transcriptomes, and chromatin conformations of primary mouse cortical astrocytes grown without serum or activating cytokines. “Ben put a lot of effort into developing the cell culture protocols so that the starting astrocytes were physiological or otherwise normal,” said Tesar. “Understanding that if we could mimic at least certain aspects of that with our culture platform, it might allow us to mechanistically identify why the cells were transitioning to one particular reactive astrocyte state.”

To really clearly show a mechanism, show a pathway, and validate it multiple different ways, I think it's a beautiful piece of work, exactly how discovery biology should be done.
–Shane Liddelow, New York University

After the researchers demonstrated that the astrocytes were growing without baseline reactivity in culture, they transitioned the astrocytes to a pathogenic state by adding microglia-derived cytokines, which are the molecular cues that induce reactivity in neurodegenerative diseases.² Using this system to modulate physiological and pathogenic states, the scientists screened thousands of compounds, searching for drugs that prevented or reversed harmful astrocyte transitions. Their screen identified histone deacetylase 3 (HDAC3) inhibitors that suppressed neurotoxic astrocyte reactivity in culture and in vivo, and protected nerve cells from degeneration in three separate mouse models of neuroinflammation, demyelination, and neural injury.

“They have done not only the screen but built the system and validated the system as well, which is really nice,” said Shane Liddelow, a neuroscientist at New York University who investigates astrocyte reactivity mechanisms in health and disease, and who was not involved in this study. “To really clearly show a mechanism, show a pathway, and validate it multiple different ways, I think it's a beautiful piece of work, exactly how discovery biology should be done.”

According to Liddelow, this robust approach is a big step forward and simultaneously just the tip of the iceberg for astrocyte-targeting therapeutic interventions; how blocking astrocyte reactivity will affect different disease courses in humans remains unknown. Hits from Tesar’s screen, such as HDAC3 inhibition, may help elucidate which diseases could benefit from suppressing reactive astrocytes and when to pump the breaks on reactivity during chronic neurodegenerative disease onset and progression.

“We know that this reactivity is really important and competently able to drive the death of neurons and dysfunction of neurons, in addition to some very simple acute models,” said Liddelow. Scientists are still interrogating the potential benefits of blocking neurotoxic astrocyte reactivity in chronic neurodegenerative diseases such as Alzheimer's disease. “Understanding whether or not these HDAC inhibitors could be an interesting novel therapeutic angle to go after. I think that would be really exciting,” Liddelow added.

Paul Tesar cofounded Convelo Therapeutics and serves on its Board.

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