2 September 2011. The blood-brain barrier (BBB) falters in Alzheimer’s disease, and conventional wisdom has it that the leakiness arises from vascular cell death triggered by neuroinflammation and hypoxia. A paper published August 31 in PLoS ONE offers an alternative idea. In their analyses of AD mice and postmortem human tissue, researchers led by Wilfred Jefferies at the University of British Columbia in Vancouver, Canada, found the disrupted blood-brain barrier linked to markers of angiogenesis, not of apoptosis, as might have been presumed. The authors propose that APP, or more likely a fragment of APP, possibly Aβ, spurs new, yet leaky blood vessels, which leads to weakening of the blood-brain barrier. “We think this is a fertile area for research, and potentially for therapeutic intervention,” Jefferies told ARF. “The data suggest that the AD field should redirect some of its efforts toward therapeutic approaches that modify blood vessel growth and permeability.”
Most AD patients rack up Aβ in their brain’s blood vessels. Damage to the blood-brain barrier (BBB) is a common AD clinical feature (Farrall and Wardlaw, 2009) that also appears in AD mice (see Ujiie et al., 2003; Paul et al., 2007). What causes this BBB permeability is not clear, but a prevailing notion holds that Aβ-clogged vessels lead to inflammation and oxygen deprivation that eventually choke the life out of BBB endothelial cells.
With this assumption, first author Kaan Biron and colleagues looked in Tg2576 AD mice for dying vasculature that would explain the increased BBB permeability previously reported in these animals. The team homed in on occludin and ZO-1—which are expressed at capillary endothelial cell tight junctions that seal the vessels from the brain parenchyma. They figured that amyloid pathology might influence BBB integrity at the level of these two proteins. By confocal microscopy, the hippocampus and neocortex from old Tg2576 mice (18-24 months of age) showed a higher proportion of blood vessels with abnormal ZO-1 expression relative to brain tissue from young (five-month-old) Tg2576 or age-matched wild-type mice. Moreover, Western blots revealed low occludin levels in the brains of old Tg2576 mice, compared to the other animals. The data thus far seemed to fit with the inflammation/hypoxia theory.
But when the researchers looked at activated caspase-3, a marker of apoptosis, they found no staining in the brain endothelium of wild-type or AD mice, young or old. All of the mice did have caspase-3-immunoreactive hippocampal neurons, though. “We see dying neurons,” Jefferies said. “We just don’t see dying vasculature.”
Lacking evidence of apoptosis in the brain endothelium, the team explored another potential mechanism for increased BBB permeability in AD mice—angiogenesis. When vascular cells divide to form new vessels, they disrupt and re-form existing tight junctions, and this “hypervascularization” could disrupt BBB integrity, Jefferies suggested. To test that idea, Biron and colleagues used confocal microscopy to measure blood vessel density in the brains of young and old wild-type and Tg2576 mice, using the endothelial protein CD105 as a marker. They found the brain vasculature in old Tg2576 mice to be about twice as dense as that of age-matched wild-type mice. By Western blot analysis of the neocortex and hippocampus, old AD mice had twice as much CD105 as did wild-type controls. Furthermore, postmortem tissue from four AD patients showed a similar increase in microvascular density, relative to brain tissue from the same number of healthy elderly, suggesting the findings may have clinical relevance, though that needs to be confirmed with a larger number of samples.
This extensive angiogenesis in the current study parallels the hypervascularity in age-related macular degeneration, an eye disorder involving disintegration of the blood-eye barrier, Jefferies said.
In “our working model,” the amyloid precursor protein (APP), or a fragment of it, drives the angiogenic process, Jefferies said, noting that APP is the sole difference between wild-type and Tg2576 mice. A secondary follow-on, he said, is the possibility that “creating a permeable BBB allows more Aβ to enter and be deposited as plaques, which then gives rise to neurotoxicity.” The influence of APP on blood vessel formation is complex, however. A study in AD mice suggested that brain Aβ is anti-angiogenic (see ARF related news story on Paris et al., 2010), whereas another analysis hinted that APP might support tumor growth because it is angiogenic (see Venkataramani et al., 2010).
The paper is an “important contribution,” Berislav Zlokovic of the University of Rochester, New York, told ARF. “It points to aberrant angiogenesis as an underappreciated problem in AD.” Zlokovic’s lab identified a homeobox gene (MEOX2) that supports vascularization, helps clear brain Aβ, and is reduced in late-stage AD patients (ARF related news story on Wu et al., 2005). The current study would be strengthened by functional data showing “whether the vessels are perfusing at all,” Zlokovic said. Though Jefferies and colleagues saw no evidence of cell death at the vasculature, “maybe the vessels are dysfunctional and would eventually die,” Zlokovic said. Studies of AD patients and mouse models have shown dysregulated cerebral blood flow early in the disease process, calling into question whether "hypervascular" areas are, in fact, functional, Zlokovic noted.
Jefferies said his lab is currently investigating whether modulating angiogenesis can influence pathophysiology in Tg2576 mice.—Esther Landhuis.
Biron KE, Dickstein DL, Gopaul R, Jefferies WA. Amyloid Triggers Extensive Cerebral Angiogenesis Causing Blood Brain Barrier Permeability and Hypervascularity in Alzheimer’s Disease. PLoS ONE. August 2011. Abstract