Alzheimer's disease is one of the most feared diagnoses among older adults, particularly for those who know they carry a high-risk gene that magnifies their odds. But even many people with this gene-APOE ε4-dodge the disease and stay mentally sharp into their 80s.
As Columbia researchers discovered two years ago, it isn't just luck preventing some of these APOE4 carriers from developing Alzheimer's. The protection from the disease comes from rare mutation in a gene for fibronectin (FN1). APOE ε4 carriers accumulate unusually high levels of fibronectin in the blood-brain barrier, the researchers found, while the protective variant, located in the fibronectin gene, limits that buildup and the damage it causes.
The finding raised the exciting possibility of developing a drug that could block this damaging process, and the researchers' latest study, published in Nature Aging, has uncovered the biological evidence that the idea may be attainable."We knew that changes in fibronectin could protect against Alzheimer's, but we didn't know why excess fibronectin was harmful in the first place. This study gives us that detailed mechanism-and with it, ideas for how to reproduce that natural protection."
"This new study takes us from a genetic clue to a disease mechanism," says Caghan Kizil, the study's corresponding author and professor of neurological sciences in the Department of Neurology and the Taub Institute for Research on Alzheimer's Disease and the Aging Brain at Columbia University Vagelos College of Physicians and Surgeons.
"We knew that changes in fibronectin could protect against Alzheimer's, but we didn't know why excess fibronectin was harmful in the first place. This study gives us that detailed mechanism-and with it, ideas for how to reproduce that natural protection."
APOE ε4 damages blood-brain barrier
The blood-brainbarrier, a tightly packed layer of cells around the brain's blood vessels, is instrumental in removing toxic substances from the brain.
To learn how fibronectin affects the blood-brain barrier, the researchers utilized a panoply of new and established methods, including studies of human brain tissue and cerebrospinal fluid with genetic and epigenetic analyses, single-cell sequencing, spatial imaging, human stem-cell models, three-dimensional vascular cultures, and experiments in zebrafish and mice.
The results from all these systems converged onto a single verdict: APOE ε4 increased fibronectin and promoted its accumulation around brain blood vessels. In mice carrying human APOE ε4, brain fibronectin levels were nearly twice those found in mice carrying the normal human APOE version-APOE ε3-and coincided with evidence of barrier leakage.
The most decisive experiment showed that fibronectin is not simply associated with vascular damage, it causes it: By increasing human fibronectin specifically in astrocytes, the cells that maintain the barrier, the researchers saw that the fibronectin increase was sufficient by itself to make the barrier leak.
"The animal experiments allowed us to move beyond association," says co-first author Prabesh Bhattarai, an associate research scientist in the Kizil lab. "When we increased fibronectin in astroglia, the barrier became permeable. When we reduced fibronectin, barrier function improved."
The magenta color reveals the damaging fibronectin around the brain's blood vessels in APOE ε4 carriers. Photo provided by Caghan Kizil / Columbia University Vagelos College of Physicians and Surgeons.
The researchers also identified how fibronectin causes damage. Excess fibronectin sends abnormal signals into cells through integrins-proteins that sense the cells' surroundings-and an enzyme called focal adhesion kinase. This disrupts a chain of growth factors called VEGF, HBEGF, and IGF1. These factors allow astrocytes and blood-vessel cells to communicate and work together to maintain the barrier.
"Single-cell analysis allowed us to examine this pathway in the different cells that maintain the brain's blood vessels," says co-first author Elanur Yilmaz, an associate research scientist in the Kizil lab. "We observed the same biological pattern across experimental models and human Alzheimer's tissue, making the mechanism especially compelling."
The same destructive processes likely occur in Alzheimer's patients with the APOE ε4 gene. Analyses of human brain tissue and cerebrospinal fluid showed that high fibronectin levels were associated with inflamed astrocytes, while genetic and gene-regulation data linked FN1 to vascular disease and changes in VEGFA, a growth factor needed to maintain healthy brain blood vessels.
"The convergence of these independent human datasets strengthens the case that fibronectin is part of the APOE ε4-related vascular disease process," says Badri Vardarajan, associate professor of neurological science in the Department of Neurology, Gertrude H. Sergievsky Center, and Taub Institute, who collaborated on this discovery of the FN1 mutation.
A framework for drug development
APOE ε4 is the strongest common genetic risk factor for Alzheimer's disease, and roughly one in five people carries at least one copy.
The findings suggest several possible treatment strategies for people with APOE ε4: prevent excess fibronectin from accumulating, block the harmful signals it triggers, or restore the growth signals needed to keep the blood-brain barrier healthy.
The researchers are currently exploring these possibilities in laboratory experiments. Future treatments will need to target pathological fibronectin without interfering with its normal roles in tissue structure and repair. Nevertheless, defining the mechanism provides a foundation for developing more selective interventions and identifying the best time to use them.
"By showing how fibronectin contributes to early vascular damage, this study gives us new therapeutic targets and potential ways to measure whether treatments are working," says Richard Mayeux, chair of the Department of Neurology and a collaborator on the project. "Clinically, it connects a major genetic risk factor for Alzheimer's to an early disease process that may be possible to modify."
