Brain aging

How the main Alzheimer's risk gene may make the brain's blood vessels leak

People who carry the APOE4 gene variant are at higher risk of Alzheimer's disease and tend to have leakier blood vessels in the brain. Using donated human brains, lab-grown human cells, zebrafish and mice, a team traced part of that leak to a sticky scaffold protein, fibronectin, piling up around the vessels. Most of the cause-and-effect evidence comes from cells and animals.

The brain is protected by the blood-brain barrier: the walls of its smallest blood vessels are sealed tightly so that most substances in the blood can't slip into brain tissue. The seal depends on teamwork between the cells lining the vessels and astrocytes, star-shaped support cells whose “feet” wrap around the vessels. In Alzheimer's disease this barrier starts to leak early, and it leaks more in people who carry APOE4, a version of the gene APOE that is the strongest common genetic risk factor for late-onset Alzheimer's. Why APOE4 does this has been unclear.

The team, at Columbia University in New York, had earlier found that APOE4 carriers with Alzheimer's build up fibronectin around brain vessels. Fibronectin is a protein of the extracellular matrix, the mesh that holds cells together; it is useful in wound healing. They had also reported that a variant that disables the fibronectin gene was linked to a lower risk of Alzheimer's. This paper asks how fibronectin could connect APOE4 to a leaky barrier.

What they did

The team combined many kinds of evidence:

  • Donated human brains: they stained vessels in brain tissue from 23 people, grouped as healthy, Alzheimer's, damage to small vessels, or both, and compared smaller groups of Alzheimer's brains from people with two APOE4 or two APOE3 copies (4 each).
  • Spinal fluid: fibronectin levels in fluid from 156 people with Alzheimer's and 139 without.
  • Lab-grown human cells: astrocytes and blood-vessel models grown from stem cells that were identical except for carrying APOE4 or APOE3.
  • Animals: aged mice (21–22 months) carrying human APOE4 or APOE3 genes (6 each), and zebrafish in which the fibronectin gene was switched off, or in which astrocytes were made to produce extra human fibronectin.

What they found

  • More fibronectin around vessels. In brains with small-vessel damage, fibronectin at vessels was about 47% higher than in healthy brains; Alzheimer's brains showed a smaller rise that did not reach significance on its own. Alzheimer's brains with two APOE4 copies had about 31% more than those with two APOE3 copies. Spinal fluid of people with Alzheimer's also held more fibronectin.
  • APOE4 mice: twice the fibronectin, leakier vessels. Old APOE4 mice had about twice as much fibronectin at brain vessels as APOE3 mice, and more of the blood protein fibrinogen leaking into brain tissue.
  • Astrocytes are a main source. Human APOE4 astrocytes made more fibronectin and laid it down in clumps. Inflammatory signals raised fibronectin production; amyloid-beta, the protein that forms plaques in Alzheimer's, did so in zebrafish.
  • Too much fibronectin is enough to cause leaks. Zebrafish larvae whose astrocytes made extra human fibronectin had leaky brain vessels.
  • It blocks repair signals. Excess fibronectin dampened a chain of growth signals (VEGF, HB-EGF and IGF-1) that keeps the barrier healthy. Blocking the step through which cells sense fibronectin restored VEGF in zebrafish and in human APOE4 astrocytes, and adding HB-EGF and IGF-1 restored the vessel seals in zebrafish whose VEGF signalling had been blocked.

What to keep in mind

  • Mostly cells and animals. The human evidence (donated brains and spinal fluid) shows that fibronectin and Alzheimer's go together; the evidence that fibronectin causes leaks comes from zebrafish and lab cultures. Zebrafish brains differ in many ways from ours.
  • Small human groups. The brain comparisons used a few donors per group, and the authors say the findings should be checked in more human brains.
  • Other diseases mixed in. The authors note that common conditions such as diabetes and high blood pressure could contribute to the links they saw.
  • Memory not tested. The study did not measure whether lowering fibronectin improves thinking or memory; the authors list this as a key open question.
  • No treatment yet. The drugs used to block steps of the pathway were research tools in cells and fish, not medicines.

Why it matters

Much Alzheimer's research has focused on amyloid plaques. This study looks at a different angle: the health of the brain's blood vessels, and why they fail sooner in APOE4 carriers. It points to fibronectin as one link in that chain and as a possible future drug target, which fits with the earlier finding that people with a disabled fibronectin gene had lower Alzheimer's risk. Whether reducing fibronectin would help people is still a question for future studies.

Sources