Though ApoE4 is the most prevalent genetic risk factor for Alzheimer disease, its role in pathology has been tricky to pin down. Regarding the other common forms of the allele, E3 seems to be benign and E2 to even protect against AD. Because ApoE is involved in many disturbances characteristic of AD (Aβ accumulation, neuronal survival, inflammation, and synaptic plasticity), it has been difficult to propose an integrated hypothesis that could explain the mechanisms by which ApoE4 increases the susceptibility for AD (see ARF related news story).
Daniel Michaelson from Tel Aviv University, Israel, and colleagues address this issue in the April 30 Journal of Neuroscience. With coauthor Eliezer Masliah of the University of California, San Diego, the Israeli researchers found that activating the amyloid cascade by inhibiting the Aβ-degrading enzyme neprilysin triggers the specific degeneration of hippocampal CA1 neurons, septal neurons, and the entorhinal cortex of ApoE4 mice but does not affect those neuron sets in ApoE3 mice. This is relevant since it reflects the areas of the brain affected in AD patients early on.
Brain amyloid levels are elevated in ApoE4-positive patients, which led to the hypothesis that pathological effects of ApoE4 are mediated by cross-talk interactions with Aβ. Supporting this, previous in vitro studies have found that ApoE4 can bind under appropriate conditions to specific forms of Aβ (see also Strittmatter et al., 1993; LaDu et al., 1994).
“We wanted to explore how ApoE4 promotes the aggregation and deposition of Aβ or retards its clearance under physiologic conditions. And, does this play a role in pathology? Admittedly, this is not a new question, but we take a new approach to answering it,” Michaelson says.
First author Haim Belinson used targeted replacement mice with either human ApoE3 or ApoE4 gene knocked into the mouse locus, and blocked Aβ degradation with the neprilysin inhibitor thiorphan to mimic the amyloid cascade. Within a week, he observed ApoE4-dependent and Aβ-mediated neurodegeneration. The neuropathological effects correlated with impairments in learning and memory of the ApoE4 mice. “We use a ‘fast forward’ in vivo model. It takes days to see results, not months as it would in a double-transgenic mouse model, such as amyloid-β precursor protein (APP) X apoE,” Michaelson explained.
In ApoE4 knock-in mice, neuronal degeneration was most pronounced in hippocampal CA1 neurons and was associated with accumulation of Aβ. Furthermore, accumulation of Aβ in the affected neurons was associated with elevated intraneuronal ApoE levels that were also most prominent in CA1 neurons, as well as with enhanced lysosomal activation. These results were more pronounced in ApoE4 than E3 mice.
A clue to what might be going wrong in those neurons may be found in previous cell culture studies, which found that ApoE4 triggers lysosomal activation and leakage in an Aβ-dependent process, resulting in neuronal death (see Ji et al., 2006). “We propose that once ApoE and Aβ get into the neurons, the cells cannot effectively deal with these proteins. Instead of eliminating them, the proteins clog the system, causing cell death,” Michaelson says. But he adds further studies will be required to understand this and other mechanisms involved in the pathological effects of ApoE4 and Aβ.
Though the scientists observed both intraneuronal and extracellular accumulation of Aβ in the ApoE4 mice, they suggest that the pathological effects of ApoE4 are initiated by the former, which then triggers neuronal loss and cognitive impairments. This conclusion is based on the correlation between kinetic and spatial accumulation of Aβ in CA1 and entorhinal neurons and their degeneration. The extracellular Aβ deposits did not correlate spatially or temporally with neuropathology, leading the scientists to suggest that extracellular Aβ deposits do not play a primary role in the neuronal and cognitive deficits of these mice.
These results have generated some excitement in the field. Yadong Huang from the J. David Gladstone Institutes, San Francisco, California, was not involved in this research but has been working on ApoE for many years. “Previous studies in mouse models did not clearly show neuronal death as a result of ApoE4 and Aβ interaction. One major difference in this study is that the researchers induced pathology in ApoE4 mice by increasing Aβ accumulation through blocking its degradation. This is accomplished without the overexpression of amyloid precursor protein (APP),” Huang said (see also Holtzman et al., 2000; Holtzman, 2004).
These results raise new questions. For instance, a lot remains unknown about the link between lysosomal activation induced by Aβ/ApoE4 accumulation and neuronal and behavioral deficits. Huang said it is unclear whether lysosomal activation is a cause or an outcome. “The other question that arises with this study is whether ApoE4 found in neurons is from extracellular uptake, or ApoE4 expression is turned on in neurons. Interestingly, this study showed that the intracellular ApoE4 and Aβ in CA1 neurons are not always co-localized, suggesting they might come from different pathways. Normally, ApoE is produced primarily by astrocytes in the brain, but we and others have found that ApoE is expressed in neurons in response to neuronal challenge. My guess is it might be generated in neurons in response to intracellular Aβ accumulation,” he said (see also Xu et al., 2008). John Fryer from the Baylor College of Medicine in Houston, Texas, was not involved in the Michaelson research, either. He took an alternative view. “Danny Michaelson's current work nicely links previous studies (see Cirrito et al., 2008; Farris et al., 2007) by showing that a semi-acute inhibition of neprilysin function from thiorphan treatment results in increased Aβ and ApoE4 uptake into neurons via an endosomal/lysosomal pathway and results in neuropathological and behavioral changes. Further work with this paradigm might be able to sort out which receptors are responsible for this uptake of Aβ and/or ApoE during a semi-acute increase in Aβ via inhibition of neprilysin.” (See ARF related news story.)
A further challenge is to understand how this mechanism fits into the larger picture of the other roles ApoE may be playing in phenotypes such as inflammation and plasticity, Michaelson says. Given the genetic importance of ApoE in AD, it certainly deserves the attention.—Nadia Halim
Nadia Halim is a science writer based in Bridgewater, New Jersey.
- Enabling Technologies for Alzheimer Disease Research: Seventh Bar Harbor Workshop, 2007, Part 2
- Lack of Lipoprotein Receptor Boosts Brain ApoE, but Not Aβ
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