Experimental Alzheimer's molecule improves cognition in mice by tamping down on inflammation

A molecule developed by researchers at the Massachusetts Institute of Technology (MIT) appears to improve cognitive function in mouse models of Alzheimer’s disease, setting the stage for a new drug that could one day complement existing therapies.

In an article published Aug. 29 in the Journal of Experimental Medicine, a team led by researchers from MIT described how deactivating a protein called PU.1 could reduce inflammation in the brain’s immune cells, a likely contributor to the etiology of Alzheimer’s disease. They also showed that by suppressing PU.1 activity in microglia, their molecule, dubbed A11, could improve Alzheimer’s biomarkers and symptoms in mice.

“This preclinical study demonstrates that A11 reduces inflammation in human microglia-like cells as well as in multiple mouse models of Alzheimer’s disease and significantly improves cognition in the mice,” co-corresponding author Li-Huei Tsai, Ph.D., said in a press release. “We believe A11 therefore merits further development and testing.”

PU.1 is what’s known as a transcription factor, a type of protein that controls the rate at which DNA is converted, or transcribed, into RNA. It’s involved in the differentiation of myeloid cells in the bone marrow as well as in the function and development of microglia, the brain’s resident immune cells.

It’s here that the link to Alzheimer’s arises. In 2015, Tsai’s lab published a study showing that microglia are highly activated as the disease progresses. The same study also found that genetic variants associated with Alzheimer’s disease are linked to immune function—and that PU.1 regulates transcription of many of them. Furthermore, the same genetic region that encodes PU.1 was itself associated with Alzheimer’s.  

The new study builds on the findings by validating PU.1 as a therapeutic target. To do that, the researchers started by analyzing gene expression patterns in microglia from brain samples donated by people who died with Alzheimer’s and comparing them with controls. This showed that the disease causes changes to microglial gene expression as well as that gene variants linked to the condition had higher numbers of PU.1 binding motifs, or sequences where the transcription factor is meant to latch on.

The same results were seen in mouse models with Alzheimer’s. Moreover, the team showed that genetically suppressing PU.1 binding in the same type of model reduced inflammation and neurodegeneration.

Given that PU.1 is also involved in other important processes, the researchers needed to find a way to suppress its activity exclusively in the microglia. To that end, they screened more than 58,000 small molecules from libraries of both FDA-approved and novel drugs in cell cultures, looking for ones that reduced both inflammation and expression of Alzheimer’s-associated genes regulated by PU.1. Several screening rounds later, A11 emerged the winner.

Next, the scientists tested various doses of A11 on microglia-like cells grown from the stem cells of patients with Alzheimer’s, exposing them to immune molecules that would typically trigger inflammation. In the treated cells, they found that doing so reduced signs of inflammation from multiple angles: The cells had lower expression of inflammatory molecules, retained their shape compared to inflamed cells and had the same gene expression patterns as non-activated microglia.

Further probing revealed a mechanism. The researchers found that A11 wasn’t actually changing PU.1 levels but was instead counteracting it. The molecule did so by recruiting a complex of proteins to the PU.1 binding motif sites, which tamped down on the expression of genes PU.1 had previously activated.

“A11 regulated genes putatively by recruiting a repressive complex … to PU.1 motifs, thus representing a novel mechanism and class of molecules,” the researchers wrote in the paper.

With a mechanism established, the researchers turned to testing A11 in mice. After seeing that the molecule was able to cross the blood-brain barrier and could effectively be cleared from healthy animals, they gave it to male and female mice of three different Alzheimer’s models, each representing a different component of the condition. One model had severe neurodegeneration; another had excessive amounts of the tau protein in their brains, known as tauopathy; and the third had large amounts of amyloid deposits.

The pathologies of all three of the mouse models improved compared to untreated controls. There was lower inflammation and fewer neurons lost in the mice with neurodegeneration. The mice with tauopathy had less abnormal tau protein in the memory-forming region of their brains. And amyloid buildup was reduced in the amyloid model.

Finally, the researchers tested whether the molecule could actually improve cognitive function. They dosed two separate mouse models of severe neurodegeneration and tauopathy with A11 and placed them in various mazes designed to test short-term memory and long-term learning. In both instances, the treated mice outperformed their untreated counterparts.

While there is much more to learn about A11, the scientists behind the research think it could eventually be used alone or as a complement to recently approved and forthcoming Alzheimer’s drugs like Biogen and Eisai’s lecanemab, trade name Leqembi, and Eli Lilly's donanemab. Both target amyloid pathologies.