How our brains become smarter disease fighters

Overview: CRISPR gene editing created the G795A amino acid that was introduced into microglia derived from human stem cells. Researchers were able to transplant the donor’s microglial immune cells into humanized rodent models while administering an FDA-approved cancer drug called pexidartinib. The inclusion of the amino acid allows the donated microglia to thrive and resist the drug, while the host microglia died. The findings open the door to new methods of using microglia to treat a range of neurodegenerative disorders.

Source: U.C. Irvine

Fighting Alzheimer’s disease and other neurodegenerative diseases by injecting healthy new immune cells into the brain has taken a leap towards reality. Neuroscientists at the University of California, Irvine and the University of Pennsylvania have found a way to safely thwart the brain’s resistance to them, overcoming a major hurdle in the search.

Their discovery about brain cells called microglia heralds countless possibilities for treating and even preventing neurodegenerative disorders. The team’s newspaper appears in the Journal of Experimental Medicine.

When microglia are healthy, they serve as the central nervous system’s primary disease fighters. “However, there is overwhelming evidence that they can become dysfunctional in many neurological conditions,” said Mathew Blurton-Jones, UCI professor of neurobiology and behavior and co-lead author of the study.

“Until recently, scientists have focused on the mechanisms that cause microglial dysfunction and tried to find drugs to alter their activity. But with this study, we found a way to use microglia themselves to treat those diseases.”

Frederick “Chris” Bennett, assistant professor of psychiatry at Penn and co-lead author, added: “There’s an obstacle, because once our own microglia develop where they should be in our brain, they give not on that space. They block the ability to deliver new cells that would take their place. If you want to put in donor microglia, you have to deplete the host microglia to free up space.”

Bennett and his lab collaborated with Blurton-Jones and his lab on the project.

Microglia depend on signaling by a protein on their surface called CSF1R for their survival. The FDA-approved cancer drug pexidartinib blocks those signaling and kills them. This process appears to provide a way to free up space in the brain to insert healthy donor microglia.

However, there is a dilemma: unless pexidartinib is stopped before the donor microglia are added, they will also be eliminated. But once the drug is finished, the host’s microglia regenerate too quickly to be effectively introduced into the donor cells.

This dilemma presents a challenge for treating people with certain rare and serious neurological disorders. One is Krabbe disease, in which the body cells cannot digest certain fats that are very abundant in the brain. Currently, clinicians are using bone marrow transplant and chemotherapy to try to introduce new immune cells, similar to microglia, into the brain. But this approach can be toxic and must be done before Krabbe symptoms manifest.

“Our team believed that if we could overcome the brain’s resistance to accept new microglia, we could successfully transplant them into patients using a safer, more effective process to treat a wide range of diseases. suits,” said co-first author Sonia Lombroso, a Penn Ph.D. student and member of the Bennett Lab. “We decided to investigate whether we could make the donor microglia resistant to the drug that eliminates their host counterparts.”

This shows the outline of a head
When microglia are healthy, they serve as the central nervous system’s primary disease fighters. The image is in the public domain

The researchers used CRISPR gene-editing technology to create one amino acid mutation, known as G795A, which they introduced into donor microglia produced from human stem cells or a mouse microglial cell line. They then injected the donor microglia into humanized rodent models while administering pexidartinib, with exciting results.

“We found that this one small mutation caused the donor’s microglia to resist the drug and thrive, while the host’s microglia continued to die,” said co-first author Jean Paul Chadarevian, a UCI Ph.D. student who is a member of the Blurton-Jones Lab. “This finding could lead to many options for developing new microglia-based treatments. Pexidartinib is already approved for clinical use and appears to be relatively well tolerated by patients.”

Approaches can range from fighting disease by replacing dysfunctional microglia with healthy ones to designing microglia that can recognize impending threats and attack them with therapeutic proteins before they do any damage.

The UCI-Penn team believes that treatments based on this kind of microglial method could be developed within a decade. Their next studies include studying in rodent models how the approach could be used to attack the brain plaques associated with Alzheimer’s and counter Krabbe and other similar diseases.

financing: Support for the project was provided by the National Institutes of Health, National Science Foundation, The Paul Allen Frontiers Group, Klingenstein-Simons Fellowship Award in Neuroscience, and the Susan Scott Foundation.

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About this neuroscience research news

Writer: Ethan Perez
Source: U.C. Irvine
Contact: Ethan Perez–UC Irvine
Image: The image is in the public domain

Original research: Closed access.
“Development of an inhibitor-resistant human CSF1R variant for microglia replacement” by Mathew Blurton-Jones et al. Journal of Experimental Medicine


Engineering an inhibitor-resistant human CSF1R variant for microglia replacement

Hematopoietic stem cell transplantation (HSCT) can replace endogenous microglia with macrophages derived from the circulation, but has a high mortality rate. To reduce the risks of HSCT and expand the potential for microglia replacement, we developed an inhibitor-resistant CSF1R that enables robust microglia replacement.

A glycine to alanine substitution at position 795 of human CSF1R (G795A) confers resistance to several CSF1R inhibitors, including PLX3397 and PLX5622. Biochemical and cell-based assays show no discernible gain or loss of function. Macrophage-expressing G795A but not wild-type CSF1R efficiently implant the brains of PLX3397-treated mice and persist after cessation of inhibitor treatment.

To measure translational potential, we engineered human-induced pluripotent stem cell-derived microglia (iMG) using CRISPR to express G795A. Xenotransplantation studies show that G795A-iMG shows nearly identical gene expression to wild-type iMG, responds to inflammatory stimuli and gradually expands in the presence of PLX3397, replacing endogenous microglia to completely occupy the brain.

In summary, we have developed a human CSF1R variant that allows for non-toxic, cell-type and tissue-specific replacement of microglia.

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