MIT researchers have discovered that two common genetic mutations that cause Rett syndrome each set off a molecular chain of events that compromises the structural integrity of developing brain blood vessels, making them leaky. The study traces the problem to overexpression of a particular microRNA (miRNA-126-3p), and shows that tamping down the miRNA's levels helps to rescue the vascular defect.
Rett syndrome is a severe developmental disorder affecting both the brain and body. It is caused by various mutations in the widely expressed MECP2 gene, but the first symptoms don't become apparent until affected children (mostly girls) reach 2-3 years of age. Because that's a critical time in development for the brain's blood vessels, neuroscientists in The Picower Institute for Learning and Memory at MIT embarked on a study to model how two common but distinct MeCP2 mutations may affect vascular development and contribute to the disease's profound neurological pathology.
To conduct the research published in Molecular Psychiatry, lead author and Research Scientist Tatsuya Osaki and senior author Mriganka Sur developed advanced human tissue cultures to model vessel development, with and without the MeCP2 mutations. The cultures not only enabled them to model and closely observe how the mutations affected the vessels, but also allowed them to molecularly dissect the problems they observed and then to test an intervention that helped.
"A role for microRNAs in Rett syndrome has been shown, but now demonstrating that miRNA-126-3p is actually downstream of MeCP2 and directly implicated in the endothelial cell dysfunction is an important piece of the Rett syndrome puzzle," said Sur, Newton Professor of Neuroscience in The Picower Institute and MIT's Department of Brain and Cognitive Sciences.
Building vessels and spotting leaks
Building on years of tissue engineering experience, including time as a postdoc in the lab of co-author and MIT Mechanical Engineering and Biological Engineering Professor Roger D. Kamm, Osaki built "3-dimentional microvascular networks" using human induced pluripotent stem cells (iPS cells) donated by patients with Rett syndrome. The donated cells were induced to become stem cells and then endothelial cells (the backbone of blood vessels. Embedded in a gel and mixed with fibroblast cells, the endothelial cells self-assembled into networks of tubes, which Osaki then hooked up to microfluidics to provide circulation.
One set of the cultures harbored the mutation R306C. Osaki created a control microvasculature that was genetically identical except that it did not have the mutation. Another set of the cultures had the R168X mutation. And again, Osaki paired that with control culture that was identical except for the mutation using CRISPR.
The research team chose these two mutations because they are each relatively common but affect the MeCP2 gene differently, Sur said. The finding that each of these distinct Rett-causing mutations ultimately led to upregulating miRNA-126-3p and undermining blood vessel integrity suggests that vascular problems are indeed a central feature of the disease.
"There is something common across these mutations," Sur said.
In particular, lab tests showed that the vessels harboring either mutation showed reduced expression of a protein called ZO-1, which is critical for ensuring that the junctions among endothelial cells in blood vessels form a tight seal (like the grout in a tile floor). ZO-1 also didn't localize to those junctions as well. Sure enough, further tests showed that the Rett-mutation vessel cultures were relatively leaky compared to the controls.
Similar deficiencies were evident in another cell culture the team created in which they added astrocyte cells to even more closely simulate the blood-brain-barrier (BBB), which tightly regulates what can go in or out of blood vessels and into the brain. BBB problems are widely suspected of contributing to neurodegenerative diseases such as Alzheimer's , Huntington's , and ALS and frontotemporal dementia .
To gain some insight into how the vascular problems might undermine neural function in Rett syndrome, the researchers exposed neurons to medium from their Rett vasculature cultures. Those nerve cells showed reduced electrical activity, a possible sign that secretions from the Rett endothelial cells disrupted the neurons.
Catching a culprit
Generally speaking, the role of MeCP2 is to repress the expression of other genes. The scientists' expectation, therefore, was that when MeCP2 is compromised by mutations the result would be overexpression of many genes. Yet ZO-1 was downregulated. Something had to account for that and miRNAs were a suspect, Osaki said, because they function as regulators of gene expression.
"That's why we hypothesized that we should have some mediator between the MeCP2 mutation and ZO-1 downregulation and the BBB permeability increase," Osaki said. "We focused on the microRNAs."
Indeed, by profiling miRNAs in the Rett cultures and the controls, the scientists found that miRNA-126-3p was overexpressed. And by sequencing RNA, the team identified more molecular pathways needed to support vascular integrity that were dysregulated in the Rett cultures.
While the sequencing and profile associated miRNA-126-3p upregulation with the altered molecular chain of events, Osaki and Sur sought more definitive proof. To obtain it, they treated the Rett-mutation cultures with an "antisense" – a molecule that reduces miRNA-126-3p levels. Doing that resulted in an increase in ZO-1 expression and a partial restoration of endothelial cell barrier function—meaning less leakiness—in the vessel cultures. Knocking down the miRNA's expression also restored the molecular pathways the scientists were tracking to more healthy states.
It turns out that there is a drug that inhibits miR-126 called miRisten that is undergoing clinical testing for Leukemia. Osaki and Sur say they are planning on administering it to mice modeling Rett syndrome to see if it helps them.
In addition to Osaki, Sur and Kamm, the paper's co-authors are Zhengpeng Wan, Koji Haratani, Ylliah Jin, Marco Campisi, and David Barbie.
Funding for the study came from sources including the National Institutes of Health, a MURI grant, The Freedom Together Foundation and the Simons Center for the Social Brain.
