Targeting star-shaped brain cells reduces fragile X seizures in mice: Study
Researchers suppressed the BMP signaling pathway in these astrocytes
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Suppressing the bone morphogenetic protein (BMP) signaling pathway specifically in astrocytes, the star-shaped brain cells that support neurons, may help reduce seizure severity in fragile X syndrome, a mouse study shows.
“Seeing that targeting the BMP pathway in astrocytes alleviated some [fragile X] symptoms makes us optimistic about astrocytes being important for consideration in future therapeutics,” James Deng, the study’s first author who led the work as a graduate student researcher in the Salk Institute for Biological Studies in California, said in an institute news story. “We really hope our work can help accelerate patient impact.”
Added Nicola Allen, PhD, the study’s senior author and a professor at the Salk Institute in California: “This [data set] identifies astrocyte-specific alterations to proteins that allow astrocytes to regulate neurons in a whole-brain context. It’s a great resource for fragile X syndrome researchers, but also for the scientific community beyond any single disorder or condition.”
The study, “Suppression of astrocyte BMP signaling improves molecular signatures and functional deficits in a fragile X syndrome mouse model,” was published in Nature Communications.
BMP signaling overly active in fragile X astrocytes
In fragile X syndrome, mutations in the FMR1 gene lead to low or absent levels of FMRP, a protein that regulates the production of several other proteins. Many of the proteins affected are involved in the formation and maturation of synapses, the close-contact points between neurons that allow them to communicate.
While research on fragile X syndrome has mostly focused on changes to neurons, a growing body of work points to a role for astrocytes, a type of neuron-supporting cell that helps regulate synapse formation and maturation.
“Recent research, including in our lab, has shown that astrocytes have many changed genes and proteins in fragile X syndrome,” Deng said. “Our study accelerates this ongoing work by studying fragile X syndrome astrocytes through multiple angles in a living system, which gives us novel insights into those changes.”
Prior cell culture experiments by Allen’s team showed that BMP signaling, a molecular pathway involved in several neurodevelopmental processes and astrocyte maturation, is overly active in fragile X astrocytes and acts upstream of the protein changes observed in these cells.
Mice were exposed to loud sound, scored on their response
In this study, the team set out to assess the effects of selectively deleting the Smad4 gene, which plays a central role in transmitting BMP signals inside cells, in astrocytes of a mouse model of fragile X.
Audiogenic seizures, or seizures triggered by loud sound, are a standard test in fragile X mouse research and model the sensory hypersensitivity seen in patients. Mice were exposed to loud sound and scored on their response on a scale of zero (no reaction) to 2 (respiratory failure).
As expected, unmodified fragile X mice had more severe seizures than healthy mice in response to a loud sound. But fragile X mice lacking Smad4, and therefore BMP signaling, had significantly fewer severe seizures than unmodified fragile X mice.
To explain this finding, researchers examined synaptic changes in the auditory cortex, the primary brain region responsible for processing sound.
In fragile X mice, the density of inhibitory synapses, which send signals to calm brain activity, was 59% of normal, supporting the higher audiogenic seizure activity. In Smad4-deficient fragile X mice, inhibitory synapse density was restored to normal levels. And removing Smad4 also improved electrical signals that reflect inhibitory synapse activity.
Still, deleting Smad4 did not improve fragile X-related visual deficits or behaviors like hyperactivity or anxiety.
A striking aspect of our fragile X syndrome astrocyte-specific RNA and protein datasets was the low amount of overlap between syndrome-related changes at the RNA versus protein levels. It really illustrates the idea that you have to look at things from multiple different angles and levels to make impactful breakthroughs.
To understand the molecular changes in Smad4-deficient fragile X astrocytes, the researchers assessed changes in gene activity through levels of RNA molecules derived from genes when they are read to produce proteins.
They found that genes encoding proteins secreted by astrocytes were among the most significantly altered in activity, and FMRP target genes tended to show greater activity changes in fragile X astrocytes than genes overall.
After deleting Smad4, many fragile X-associated gene activity changes no longer differed significantly from those in healthy animals. In particular, genes involved in two energy metabolism pathways (oxidative phosphorylation and glycolysis) that showed higher activity in fragile X astrocytes were suppressed by Smad4 deletion.
Researchers also measured protein levels in astrocytes because gene activity changes do not always predict changes in protein levels.
Experiments in fragile X mice showed reduced levels of membrane proteins linked to autism and components of perineuronal nets, mesh-like structures surrounding certain neurons that help stabilize connections. Other reduced proteins were known FMRP targets. Deleting Smad4 normalized many of these changes, including the pathways involved in protein secretion.
The team noted that changes in gene activity and protein levels were poorly correlated, highlighting the value of measuring both independently.
“A striking aspect of our fragile X syndrome astrocyte-specific RNA and protein [data sets] was the low amount of overlap between syndrome-related changes at the RNA versus protein levels,” Allen said. “It really illustrates the idea that you have to look at things from multiple different angles and levels to make impactful breakthroughs.”
Allen added that the tools developed for this study to assess astrocyte-specific protein changes can be applied to other developmental conditions, like Rett syndrome.
