To understand brain disorders, consider astrocytes

It’s been slow. “We spent a lot of time, probably the first few years, really just doing immunopanning and culturing astrocytes,” recalls Caldwell. One challenge was making sure that the medium contained very little protein — proteins that would interfere with their measurements. The scientists also needed to make sure that growing astrocytes in a petri dish didn’t change the way they behave in the brain.

Once they determined that the cultured cells behaved normally and maintained their ability to direct neuron development, the scientists studied the proteins they made and the genes they expressed. They then compared these cells to normal cells. In all three disease models, they found that 88 proteins and about 11 genes were up-regulated — meaning their numbers or expression increased.

Both Caldwell and Allen were surprised how often the two were out of sync. While one might think that an increase in gene expression is related to an increase in the protein associated with it, this is not the case. Across the three diseases, there was not much overlap between the most overexpressed genes and the most overproduced proteins. “I think it really stands out, especially for different diseases, you really have to focus on proteins,” Allen said, rather than just gene expression.

Baldwin, who was not involved in the study, agrees — noting that this lack of overlap is a “striking” result. “What sequencing doesn’t capture what proteomics can capture is all the regulation that happens when a protein is produced,” she said. Sequencing can tell you which gene transcripts are available, but “not necessarily tell you which gene transcripts are being converted into proteins, or at what rate they are being converted into proteins,” she added.

Allen’s team focused on a few specific proteins that peaked in all three disease models. One, called Igfbp2, inhibits the gene pathway for insulin-like growth factor (IGF), a hormone that normally helps brain development. “The idea is that astrocytes are producing too much of this inhibitor,” Allen said. So the lab tried to suppress it. They gave live mice with Rett syndrome an antibody that blocks Igfbp2, and they found that their neurons grew more normally.

Another protein that is overproduced in all three animal models is called Bmp6. It is thought to regulate the maturation of astrocytes. Again, the team tested what happened when they rejected the protein. First, they put mouse neurons in a petri dish and then added proteins secreted by Fragile X mouse astrocytes. Neurons cannot grow many neurite tendrils. But when the scientists tried again, this time with exudates from Fragile X astrocytes treated with a Bmp6 inhibitor, the tendrils grew. Knocking down the production of the Bmp6 protein appears to result in more normal neuronal development.

It turns out that the two proteins may be related — the presence of Bmp6 may also be the presence of Igfbp2, Allen said, “which led to some of these defects.”

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