Amyloid builds up in the brains of people with Alzheimer’s disease, and in the more common, sporadic form of AD, inefficient clearance of Aβ may be the main culprit. A study published in the November 28 issue of JAMA Neurology lends credence to this idea. Researchers led by Jeffrey Iliff at the Oregon Health & Science University, Portland, report that a water channel called aquaporin 4 (Aqp4) goes missing from its normal site surrounding blood vessels in AD, suggesting that one of the brain’s clearance systems, the “glymphatic” pathway, may be compromised.
A brain-wide glymphatic pathway was defined in 2012 by Maiken Nedergaard of the University of Rochester Medical Center in New York and her colleagues, including Iliff. The pathway allows interstitial fluid to circulate quickly and deeply within the brain, aided by the flow of water through Aqp4 channels. Aqp4 is strategically positioned on the end-feet of astrocytes that form a space around blood vessels through which the fluid flows (Iliff et al., 2012). “Aqp4 creates a preferential pathway, a low-resistance route for water transport,” explained Iliff. The flow is thought to start along the outside of arteries and arterioles, move through the brain tissue and then out along veins and venules, whisking away waste particles lodged between cells, including Aβ.
Suspecting that aging might take a toll on the glymphatic pathway, Iliff and co-workers examined its function in mice. Previously, they had reported an age-dependent loss of perivascular Aqp4 that correlated with impaired clearance of Aβ injected into the brains of otherwise normal mice (Kress et al., 2014). The findings were consistent with earlier studies from Carol Colton at Duke University in Durham, North Carolina, who found less Aqp4 in astrocytic end feet in four mouse models of cerebral amyloid angiopathy than in wild-type controls. (Wilcock et al., 2009). The findings were intriguing because they suggested a breakdown of the glymphatic pathway that could play a role in AD, said Iliff.
Would this hold true in people? Colton’s 2009 study hinted it might since she found a negative correlation between AD severity and postmortem perivascular Aqp4 in cerebral cortex tissue. Others have reported higher global Aqp4 expression in AD compared to healthy controls (Moftakhar et al., 2010) or that Aqp4 levels vary around plaques (Hoshi et al., 2012).
To expand on this, Iliff and colleagues used immunofluorescence and western blots to track Aqp4 expression levels and localization, and quantitatively assess how they correlated with age, amyloid plaque density, and AD pathology. Analyzing postmortem tissues from the frontal cortices of 21 people with AD and 58 healthy volunteers between 33 and 105 years old, the researchers found that older people and those with AD had a moderate increase in total Aqp4 expression as assessed by immunostaining. Western blots showed no difference in Aqp4 expression between the groups, but this may be due to differences between the areas of tissue sampled with each technique, suggested the authors.
More strikingly, however, the researchers found drastic differences in the location of Aqp4 among young and old brains and healthy and AD brains. Aqp4 uniformly distributed from the cortical surface to the subcortical white matter boundary in normal people younger than 60. In people 60-85 and in people with AD, Aqp4 was intensely expressed only in a sparse population of cortical astrocytes. Large patches of tissue showed faint Aqp4 staining, particularly in the deep cortical layers. At higher magnification, the researchers found that perivascular Aqp4 in particular was strongly reduced in AD brains, and, though to a lesser extent, in aged brains. Loss of perivascular Aqp4 in AD tissue tracked with both Aβ plaque density and Braak staging. Also, in AD brains, the researchers saw bright foci of Aqp4 associated with Aβ plaques, although, as in aged individuals, Aqp4 staining across most of the deep cortical expanse was lower than normal (see image above).
This corroborates the rodent data and supports further exploration of the role of the glymphatic system and aquaporin in AD pathology, said Iliff. The study also offered some clues about people who remain sharp far into old age. Remarkably, when the researchers examined brain tissue obtained from such people older than 85, they found Aqp4 staining patterns that were much more like those of healthy, young people than those of the 65- to 80-year-olds or people with AD. “We might be seeing a survivor effect,” said Iliff. This Aqp4 preservation could be protective, but it could also be an epiphenomenon.
What this loss of perivascular Aqp4 in AD means remains uncertain. “It’s a chicken-and-egg question,” noted Costantino Iadecola at the Weill Cornell Medical College in New York. “What comes first: Is it the aquaporin loss that leads to reduced Aβ clearance? Or is the reduced Aβ clearance resulting in loss of aquaporin? That’s really the million-dollar question.”
Iliff suspects there’s a little of both chicken and egg. There could be a nasty feed-forward cycle, where loss of perivascular Aqp4 causes a buildup of Aβ, which in turn causes more Aqp4 to be lost. However, he noted that the drop in perivascular Aqp4 occurred in some people without visible plaques, suggesting it may happen early in disease. It could be that vascular dysfunction sets the stage for the pathological cycle. That dovetails with other studies placing low blood flow due to atherosclerosis or increased vascular stiffness, or low-level breakdown in the blood-brain barrier, into the cascade that leads to AD pathology.
Others cautioned that much remains unknown about the basics of the glymphatic system. That the equilibration of CSF with interstitial fluid in the brain parenchyma depends on the presence of Aqp4 in astrocyte end-feet has been well established, noted Roy Weller, University of Southampton, England, in an email to Alzforum (see Rennels 1985; Iliff et al., 2012). “What is not certain is the perivenous pathway that this group has suggested as the route for elimination of CSF and tracers that have entered the brain,” he wrote. As a next step, Weller suggested measuring Aqp4 around different blood vessel types—arteries, capillaries, and veins—to determine which are most affected by aging and AD pathology.
Colton suggested looking beyond Aqp4 to see in what other ways astrocytes are altered and why Aqp4 vanishes from their end-feet. She had found certain potassium channels, as well as dystrophin, a protein that anchors channels to cell membranes, to be reduced at perivascular sites. Perhaps the loss of dystrophin sets several membrane channels loose, including Aqp4, so that they are no longer tethered to their normal sites of action, she said. Iliff plans to explore the molecular underpinnings of the change in Aqp4 localization and to search for upstream changes that might make targets for therapeutic intervention.
“The most important step will be to measure this [Aqp4] biology in living, breathing humans,” said Iliff. He wants to use MRI and PET to measure the dynamics of contrast agents injected into the brain and to develop ligands to track particular molecules. A PET ligand for Aqp4 is already under development (Nakamura et al., 2011).—Marina Chicurel
Marina Chicurel is a science writer based in Santa Cruz, California.
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No Available Further Reading
- Zeppenfeld DM, Simon M, Haswell JD, D'Abreo D, Murchison C, Quinn JF, Grafe MR, Woltjer RL, Kaye J, Iliff JJ. Association of Perivascular Localization of Aquaporin-4 With Cognition and Alzheimer Disease in Aging Brains. JAMA Neurol. 2017 Jan 1;74(1):91-99. PubMed.