APOE ε4, Fibronectin and the Blood-Brain Barrier: A New Clue to Alzheimer’s Risk

A vascular mechanism may explain why Alzheimer’s risk varies

Carrying APOE ε4 raises the risk of Alzheimer’s disease, but it doesn’t determine who will develop it. A Columbia University study offers a possible explanation for some of that variation: in certain people, a rare mutation in the FN1 gene appears to limit a protein buildup that can damage the brain’s blood vessels.

The protein, fibronectin, helps support tissue structure. Yet too much of it around brain vessels may disrupt the blood-brain barrier, the system that regulates what passes from the bloodstream into brain tissue. The new work, published in Nature Aging, traces how excess fibronectin may compromise that barrier and identifies biological signals that researchers could investigate as treatment targets.

The study builds on a finding the Columbia team reported two years ago. Some people with APOE ε4 also carry a rare protective mutation in FN1. That observation suggested a genetic defense against Alzheimer’s-related damage; the newer research probes what the defense may be counteracting.

“This new study takes us from a genetic clue to a disease mechanism,” says corresponding author Caghan Kizil, a professor of neurological sciences at Columbia University Vagelos College of Physicians and Surgeons and the Taub Institute for Research on Alzheimer’s Disease and the Aging Brain. The team had evidence that changes in fibronectin could protect against Alzheimer’s, he says, but lacked a detailed account of why excess fibronectin might be harmful.

From protein buildup to barrier damage

The blood-brain barrier is formed by tightly connected cells surrounding brain vessels. It controls the movement of substances into brain tissue and helps maintain conditions that support the removal of potentially harmful waste. If the barrier becomes too permeable, substances that it would normally restrict may enter more readily, potentially contributing to inflammation and other processes associated with neurological disease.

To examine fibronectin’s role, the researchers combined human samples with experiments in several model systems. They studied brain tissue and cerebrospinal fluid, and used genetic and epigenetic analyses to assess DNA variation and gene activity. Single-cell sequencing helped identify activity in individual cells, while spatial imaging showed where changes occurred in tissue. The team also used human stem-cell models, three-dimensional blood vessel cultures, zebrafish and mice.

Across these approaches, the findings pointed to a link between APOE ε4 and increased fibronectin around brain vessels. In mice engineered to carry human APOE ε4, the researchers measured nearly twice as much brain fibronectin as in mice carrying APOE ε3, another common form of the gene. The APOE ε4 mice also showed signs of a leakier blood-brain barrier.

That comparison alone couldn’t establish whether fibronectin was causing the damage or accumulating in response to it. The team tested the distinction by increasing human fibronectin in astrocytes, support cells that help maintain the blood-brain barrier. Raising fibronectin in those cells made the barrier more permeable. Reducing it improved barrier function.

“When we increased fibronectin in astroglia, the barrier became permeable. When we reduced fibronectin, barrier function improved,” says co-first author Prabesh Bhattarai, an associate research scientist in the Kizil lab. The experiments, he says, let the team move beyond observing an association between the protein and barrier damage.

Disrupted signals link fibronectin to vascular dysfunction

The researchers also examined how excess fibronectin might affect the cells responsible for keeping the barrier intact. They found that it can trigger abnormal signaling through integrins, proteins that help cells sense their surroundings, and focal adhesion kinase, an enzyme involved in cellular signaling.

These changes interfere with a network involving the growth factors VEGF, HBEGF and IGF1. The molecules have roles in development and repair, but they also help astrocytes and blood vessel cells coordinate their work. When that communication is disrupted, the barrier may lose integrity.

Single-cell analysis allowed the team to examine these pathways across the different cell types involved in maintaining brain vessels. Co-first author Elanur Yilmaz, an associate research scientist in the Kizil lab, says the same biological pattern appeared across experimental models and human Alzheimer’s tissue. That consistency gives the proposed mechanism added support, though it does not by itself establish that fibronectin drives disease in every person with APOE ε4.

Human samples provide another line of evidence. Analyses of brain tissue and cerebrospinal fluid linked elevated fibronectin with inflammation in astrocytes. Genetic and gene-regulation analyses also connected FN1 with blood vessel disease and changes involving VEGFA, a growth factor involved in maintaining healthy brain vessels.

Badri Vardarajan, an associate professor of neurological science at Columbia who collaborated on the discovery of the FN1 mutation, says the results from independent human datasets strengthen the case that fibronectin is part of APOE ε4-related vascular disease. The human evidence supports a connection; the controlled experiments provide the clearest evidence in the study that changing fibronectin can alter barrier function.

Potential targets, with a significant biological constraint

About one in five people carries at least one copy of APOE ε4, the strongest common genetic risk factor for Alzheimer’s disease. The variant raises the likelihood of developing the disease but doesn’t make it inevitable. Understanding why some carriers remain unaffected could help researchers identify ways to protect the brain before substantial damage develops.

The study points to several experimental strategies: limiting fibronectin buildup around brain vessels, blocking the abnormal signals associated with excess fibronectin, or restoring growth-factor communication between astrocytes and vessel cells. Researchers are investigating these possibilities in laboratory experiments, but the study does not establish that any approach is safe or effective as a treatment for people.

One constraint is that fibronectin has essential functions throughout the body. It supports tissue structure and helps with wound healing, so broadly suppressing it could disrupt normal processes. Any treatment based on this work would need to target the harmful buildup or its effects while preserving the protein’s useful roles.

Richard Mayeux, chair of Columbia’s Department of Neurology and a project collaborator, says the work offers potential targets and ways to measure whether an intervention is protecting the brain. By tying a major genetic risk factor to early vascular damage, the findings give researchers a more specific question to pursue: whether interrupting this process can preserve the blood-brain barrier in vulnerable people.

The rare FN1 mutation supplied the initial clue. The new study maps a possible route from APOE ε4 to fibronectin accumulation, disrupted cell signaling and barrier leakage. The next test is whether therapies can reproduce that natural protection without interfering with fibronectin’s normal work.

Materials provided by Columbia University Irving Medical Center.

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