This concludes a three-part series. See also Part 1 and Part 2.
8 December 2010. Cholesterol not only clogs up arteries and leads to heart disease, but it can be bad for your brain as well. Evidence for cholesterol’s role in Alzheimer’s disease has been growing in recent years, and many cardiovascular risk factors have popped up also (for a recent review of cholesterol genes in AD, see Wollmer, 2010). Apolipoprotein E, a cholesterol transporter, is the primary genetic variant associated with sporadic AD risk, supporting a central role for the sterol in the disease. Yet despite a large body of research since 1993 (see, e.g., ARF Live Discussion from 2002 and ARF related news story and ARF news story), this role remains mysterious. The 2010 Society for Neuroscience annual meeting, held 13-17 November in San Diego, California, showcased the cholesterol-AD link in a nanosymposium, where researchers explored multiple aspects of the relationship between ApoE, cholesterol, and AD. Yet again, in 2010, the talks failed to add up to a unified picture of disease mechanisms; rather, they presented diverse ideas. Some researchers discussed roles for ApoE in AD pathogenesis that have nothing to do with cholesterol transport, while others focused directly on the effects and location of cholesterol in the brain. Overall, the symposium provided more questions than answers, but pointed to some intriguing new directions for research.
Is It the Coziness Between Cholesterol and Aβ?
Where does cholesterol go in the brain? First author Santiago Solé Domènech and Björn Johansson at the Karolinska Institute in Stockholm, together with Peter Sjövall at the SP Technical Research Institute of Sweden, Boras, described a new method for visualizing cholesterol and Aβ deposits in AD brains of both humans and 3xTgAD mouse model. Solé Domènech and colleagues first identified Abeta deposits using an amyloid-specific fluorescent probe (p-FTAA) and confocal laser scanning microscopy. Then they performed time-of-flight secondary ion mass spectrometry (ToF-SIMS) on adjacent brain sections to determine cholesterol content. The sections analyzed by ToF-SIMS were then incubated with p-FTAA to detect amyloid. By overlaying ToF-SIMS and fluorescent images from the same section, Solé Domènech and colleagues could examine the relationship between Abeta and cholesterol. They found that cholesterol accumulations often surround Abeta plaques at a distance of 0 to 50 microns from the plaque center. At least some of this cholesterol is extracellular, Solé Domènech said. In addition, p-FTAA fluorescence imaging revealed structural differences between human and mouse amyloid deposits. Solé Domènech hypothesized that the cholesterol deposits around plaques might be released primarily by astrocytes, the main providers of ApoE and cholesterol in the central nervous system, an idea he is testing in ongoing experiments. The findings dovetail with recent work led by Christian Duyckaerts at the Pitié-Salpêtrière Hospital in Paris, France, that shows increased cholesterol concentrations in human AD plaques by mass spectroscopy (see Panchal et al., 2010). At the SfN meeting, researchers led by Tara Spires-Jones at Massachusetts General Hospital, Charlestown, also reported that ApoE also colocalized with oligomeric Aβ (see Part 1 of this series).
Is It Inflammation?
Why is the ApoE4 allele a risk factor for AD? Robert Bell, working with Berislav Zlokovic at the University of Rochester, New York, presented data that suggested ApoE4 fails to repress a proinflammatory cytokine, cyclophilin A, that damages blood vessels. Previously, the lab had shown that the ApoE4 allele slows the clearance of Aβ across the blood-brain barrier (see Deane et al., 2008). In the current work, Bell focused instead on ApoE’s direct effect on the vasculature. Bell showed how ApoE knockout mice, as well as mice with the ApoE4 allele, lose blood-brain barrier integrity and have poorer cerebral blood flow. Overexpression of ApoE2 or ApoE3 alleles rescued this phenotype, but the ApoE4 allele did not.
Looking for the mechanism behind this effect, Bell turned to cyclophilin A. This cytokine has been shown to promote atherosclerosis and vascular damage in mouse models (see Jin et al., 2004 and Satoh et al., 2010). Bell and colleagues found higher cyclophilin A levels in the microvasculature of ApoE knockouts and ApoE4-expressing mice compared to wild-type animals. In primary brain endothelial cell cultures from ApoE knockout mice, ApoE2 and ApoE3 repressed cyclophilin A expression, but ApoE4 did not. Inhibition or knockout of cyclophilin A restored blood-brain barrier function, and improved cerebral blood flow and synaptic connections in ApoE knockout mice. The authors also showed that cyclophilin A directly injures mouse brain endothelial cells in culture, but not cortical neurons, implying that the cytokine exerts its effects by damaging endothelial cells. Bell suggested that since breakdown of the blood-brain barrier and reductions in microvasculature could lead to neurodegeneration, inhibition of cyclophilin A could be a promising therapeutic strategy in people who carry the ApoE4 allele, because it could help prevent vascular damage and neurodegeneration. Prior studies have linked microhemorrhages to dementia, while cerebral amyloid angiopathy is a major form of the disease (see, e.g., ARF related news story and ARF news story).
Is It Estrogen?
Qun Zhao, in the lab of Carol Colton at Duke University in Durham, North Carolina, took a different approach to studying ApoE effects. Zhao wondered whether ApoE genotype could help explain the contradictory data on estrogen replacement therapy, which has shown a cognitive benefit in some studies but not in others (see, e.g., a review by Gleason et al., 2005, and a report from the Women’s Health Initiative Memory Study by Shumaker et al., 2003). Zhao suggested that several factors may influence estrogen’s effects. One of these is the timing of hormone replacement. For example, a recent study found that hormone replacement therapy in midlife protects against dementia, while in late life it increases dementia risk (see Whitmer et al., 2010). Other factors, Zhao said, include the hormone type and a woman’s ApoE genotype. An earlier study on atherosclerosis found that women with ApoE2 or ApoE3 alleles benefited from estrogen therapy, while women with an ApoE4 allele did not (see Lehtimäki et al., 2002). Zhao and colleagues wondered if ApoE status may also impact estrogen’s effect on cognition. “In general, this has been a neglected area of research that may have a great deal of public health impact,” Colton wrote in an e-mail to ARF.
Previous work in the Colton lab demonstrated that 17β-estradiol has weaker anti-inflammatory effects on microglia from mice that have their ApoE gene replaced by a human ApoE4 gene, compared to microglia from ApoE3 targeted replacement mice (see Brown et al., 2008). In vitro studies showed that ApoE binds to the estrogen receptor (ER), suggesting it may act as an estradiol coactivator. Zhao tested this idea in cell cultures using the mouse neuroblastoma cell line N2a, which expresses only ERα, and a mouse catecholaminergic neuronal cell line, CAD, which expresses only ERβ. Co-immunoprecipitation studies showed that ApoE bound to ERβ but not ERα. Zhao is now studying the effect of the ApoE allele in cell cultures containing both estrogen receptors using reporter assays, and will look for any difference in estrogen response in the presence of ApoE3 or ApoE4 alleles. She speculated that ApoE may modulate the estrogen response by directly interacting with ERβ and indirectly affecting downstream ERα pathways involved in neuroprotection. (For more on estrogen, see ARF Live Discussion from 2006.)
Is It the Diet?
Other talks focused directly on the effects of cholesterol. Lifestyle factors such as cholesterol intake and exercise are known to alter AD risk. Amanda Kiliaan of Radboud University Nijmegen Medical Center in The Netherlands described published work that examined the effects of diet on cerebral blood volume, Aβ deposition, and learning and behavior (see Hooijmans et al., 2009), as well as recent unpublished experiments. Kiliaan and colleagues fed APP/PS1ΔE9 and wild-type mice either a standard diet of rodent chow, a cholesterol-rich diet resembling a typical Western diet, or a diet rich in docosahexaenoic acid (DHA), a omega-3 fatty acid found in fish oil and believed to be good for brain health (see ARF related news story and ARF news story). The first differences between mice on alternate diets appeared at 15 months, with AD mice eating high-cholesterol chow showing greater Aβ deposition than AD mice on standard diets. AD mice receiving DHA, on the other hand, demonstrated improved spatial memory, greater cerebral blood volume, and fewer Aβ deposits than those on standard chow.
Kiliaan also presented new work, in which AD mice were fed enhanced DHA diets that included various B vitamins, antioxidants, and other essential precursors and cofactors, in a formulation called Fortasyn ConnectTM (see ARF related news story on human trials of this mixture). On Fortasyn, six- to eight-month-old AD mice showed behavioral improvement compared to mice on standard diets or given DHA alone, demonstrating better memory and less anxiety. Kiliaan speculated that Fortasyn may target more mechanisms than simple DHA, and may improve the health of cell membranes. Kiliaan and colleagues are also analyzing data from ApoE knockout mice and ApoE4 targeted replacement mice fed cholesterol-rich or Fortasyn-supplemented diets.
Kiliaan’s results agree with a recent paper from researchers led by Christian Humpel at Innsbruck Medical University in Austria, in which rats fed a high-cholesterol diet had poorer spatial memory than rats on normal chow (see Ullrich et al., 2010). Rats eating cholesterol-rich food had higher levels of Aβ42 and phosphorylated tau in the cortex, as well as increased inflammation and more cortical microbleeds, Humpel found. These rats also lost cholinergic neurons. Overall, the effects of high cholesterol on the brain resemble those of AD, Humpel and colleagues concluded.—Madolyn Bowman Rogers.
This concludes a three-part series. See also Part 1 and Part 2.