Tag Archives: neuroscience

Reconciling conflicting results in Alzheimer’s disease research

VerghesevCarloAmyloid beta (Aß) deposition is a hallmark pathology in Alzheimer’s disease (AD). The apolipoprotein E (ApoE) gene, which encodes for the ApoE protein, has been established as a strong influence for the development of late-onset AD(1). The link between ApoE and Aß is unclear. One theory postulates that ApoE directly binds with Aß to mediate Aß clearance(2-4), while a second theory suggests the effects of ApoE on Aß clearance are indirect(5-7). Recent papers by Carlo et al(8) and Verghese et al(9) support these two respective contradicting theories.

In line with the first theory of a direct interaction between ApoE and Aß, Carlo et al found that sortilin, and not low-density lipoprotein receptor-related protein 1 (LRP1), mediates the cellular uptake of Aß/ApoE complexes, suggesting that sortilin is a major ApoE receptor that is essential for Aß clearance(8). In contrast, Verghese et al demonstrated that ApoE and Aß rarely bind together and that ApoE competes with Aß to bind with LRP1(9), which plays a role in neuronal Aß uptake(10). Thus, Verghese et al’s results suggest that ApoE inhibits Aß clearance via binding to LRP1(9). Verghese et al’s findings therefore are in accordance with the second theory that ApoE indirectly affects Aß clearance.

It would appear as though the experiments done by Carlo et al and Verghese et al are in contradiction with each other. However, there are important differences between these studies that may contribute to their contrasting results. First, Carlo et al examined the effects of sortilin and LRP1 on Aß/ApoE complexes, while Verghese et al did not use Aß/ApoE complexes in their experiments with LRP1. This might indicate that LRP1 is capable of binding to Aß only when it is not bound to ApoE. While there is an overwhelming amount of evidence that shows a direct interaction between ApoE and Aß(11-16), which supports Carlo et al, Verghese et al pointed out that the majority of studies demonstrating Aß/ApoE binding used synthetically prepared Aß at above physiological concentrations and did not assess the mechanisms behind the effects of Aß/ApoE complexes on Aß metabolism9. Verghese et al challenges previous literature as they show that Aß and ApoE rarely bind together under physiological conditions yet ApoE continues to affect Aß clearance, suggesting that the primary role of ApoE in Aß clearance is unlikely due to ApoE sequestering Aß. This implies that the two groups examined different pathways of ApoE-related Aß clearance: one pathway that depends on Aß/ApoE complexes and one that does not. Thus, the results from the two groups do not necessarily oppose each other as they targeted different pathways that may both contribute to Aß clearance, but the question remains as to which pathway contributes the most to Aß clearance.

In order to reconcile the results from Carlo et al and Verghese et al, several experiments could be done. Aß uptake should be measured in sortilin-expressing, sortilin-deficient, LRP1-expressing, and LRP1- deficient cells incubated with soluble Aß only, soluble Aß with ApoE (with molar ratios in the physiological range, as previously described(9)), and Aß complexed with ApoE (as previously described(8)). This would enable us to determine how free ApoE and Aß-bound ApoE influence Aß clearance with and without sortilin and LRP1. This would also enable us to see whether sortilin is capable of clearing Aß when it is not complexed with ApoE and conversely, whether LRP1 is capable of clearing Aß when complexed with ApoE. These experiments may clarify which pathway has the largest influence on Aß clearance. ApoE expression can be controlled by LXRs(17). Therefore, upregulating and downregulating the expression of ApoE with LXR agonists and antagonists in amyloid mouse models to examine the effects on Aß clearance may also be useful. If Aß clearance is increased by ApoE upregulation, then that would support Carlo et al’s finding that ApoE binding to Aß is essential for the latter’s clearance. If Aß clearance is increased by ApoE downregulation, then that would support Verghese et al’s finding that ApoE inhibits Aß clearance. In fact, there have been conflicting results regarding the effects of increasing or decreasing ApoE expression, as upregulating ApoE has been shown to facilitate Aß clearance(18), yet reduced ApoE expression may reduce Aß levels(19). Therefore, modifying ApoE expression in sortilin-expressing, sortilin knock-outs, LRP1- expressing, and LRP1-knock-outs crossed with amyloid mice could also be done to see how these receptors impact Aß clearance depending on the level of ApoE expression.


1 Poirier, J. et al. Apolipoprotein E polymorphism and Alzheimer’s disease. Lancet 342, 697-699, doi:0140-6736(93)91705-Q [pii] (1993).

2 Koistinaho, M. et al. Apolipoprotein E promotes astrocyte colocalization and degradation of deposited amyloid-beta peptides. Nat Med 10, 719-726, doi:10.1038/nm1058nm1058 [pii] (2004).

3 Morikawa, M. et al. Production and characterization of astrocyte-derived human apolipoprotein E isoforms from immortalized astrocytes and their interactions with amyloid-beta. Neurobiol Dis 19, 66-76, doi:S0969-9961(04)00279-7 [pii]10.1016/j.nbd.2004.11.005 (2005).

4 Jiang, Q. et al. ApoE promotes the proteolytic degradation of Abeta. Neuron 58, 681- 693, doi:10.1016/j.neuron.2008.04.010S0896-6273(08)00328-0 [pii] (2008). 5 Kim, J. et al. Overexpression of low-density lipoprotein receptor in the brain

markedly inhibits amyloid deposition and increases extracellular A beta clearance. Neuron 64, 632-644, doi:10.1016/j.neuron.2009.11.013S0896-6273(09)00896-4 [pii] (2009).

6 Basak, J. M., Verghese, P. B., Yoon, H., Kim, J. & Holtzman, D. M. Low-density lipoprotein receptor represents an apolipoprotein E-independent pathway of Abeta uptake and degradation by astrocytes. J Biol Chem 287, 13959-13971, doi:10.1074/jbc.M111.288746M111.288746 [pii] (2012).

7 Katsouri, L. & Georgopoulos, S. Lack of LDL receptor enhances amyloid deposition and decreases glial response in an Alzheimer’s disease mouse model. PLoS One 6, e21880, doi:10.1371/journal.pone.0021880PONE-D-11-05741 [pii] (2011).

8 Carlo, A. S. et al. The pro-neurotrophin receptor sortilin is a major neuronal apolipoprotein E receptor for catabolism of amyloid-beta peptide in the brain. J Neurosci 33, 358-370, doi:10.1523/JNEUROSCI.2425-12.201333/1/358 [pii] (2013).

9 V erghese, P . B. et al. ApoE influences amyloid-beta (Abeta) clearance despite minimal apoE/Abeta association in physiological conditions. Proc Natl Acad Sci U S A 110, E1807-1816, doi:10.1073/pnas.12204841101220484110 [pii] (2013).

10 Kanekiyo, T. et al. Heparan sulphate proteoglycan and the low-density lipoprotein receptor-related protein 1 constitute major pathways for neuronal amyloid-beta uptake. J Neurosci 31, 1644-1651, doi:10.1523/JNEUROSCI.5491-10.201131/5/1644 [pii] (2011).

11 Namba, Y., Tomonaga, M., Kawasaki, H., Otomo, E. & Ikeda, K. Apolipoprotein E immunoreactivity in cerebral amyloid deposits and neurofibrillary tangles in Alzheimer’s disease and kuru plaque amyloid in Creutzfeldt-Jakob disease. Brain Res 541, 163-166, doi:0006-8993(91)91092-F [pii] (1991).

12 Strittmatter, W. J. et al. Apolipoprotein E: high-avidity binding to beta-amyloid and increased frequency of type 4 allele in late-onset familial Alzheimer disease. Proc Natl Acad Sci U S A 90, 1977-1981 (1993).

13 Bales, K. R. et al. Apolipoprotein E is essential for amyloid deposition in the APP(V717F) transgenic mouse model of Alzheimer’s disease. Proc Natl Acad Sci U S A 96, 15233-15238 (1999).

14 LaDu, M. J. et al. Isoform-specific binding of apolipoprotein E to beta-amyloid. J Biol Chem 269, 23403-23406 (1994).

NEUR 602: Carlo et al vs Verghese et al Angela Tam

15 Yang, D. S., Smith, J. D., Zhou, Z., Gandy, S. E. & Martins, R. N. Characterization of the binding of amyloid-beta peptide to cell culture-derived native apolipoprotein E2, E3, and E4 isoforms and to isoforms from human plasma. J Neurochem 68, 721- 725 (1997).

16 Kim, J., Basak, J. M. & Holtzman, D. M. The role of apolipoprotein E in Alzheimer’s disease. Neuron 63, 287-303, doi:10.1016/j.neuron.2009.06.026S0896- 6273(09)00549-2 [pii] (2009).

17 Laffitte, B. A. et al. LXRs control lipid-inducible expression of the apolipoprotein E gene in macrophages and adipocytes. Proc Natl Acad Sci U S A 98, 507-512, doi:10.1073/pnas.021488798021488798 [pii] (2001).

18 Riddell, D. R. et al. The LXR agonist TO901317 selectively lowers hippocampal Abeta42 and improves memory in the Tg2576 mouse model of Alzheimer’s disease. Mol Cell Neurosci 34, 621-628, doi:S1044-7431(07)00021-8 [pii]10.1016/j.mcn.2007.01.011 (2007).

19 Bien-Ly, N., Gillespie, A. K., Walker, D., Yoon, S. Y. & Huang, Y. Reducing human apolipoprotein E levels attenuates age-dependent Abeta accumulation in mutant human amyloid precursor protein transgenic mice. J Neurosci 32, 4803-4811, doi:10.1523/JNEUROSCI.0033-12.201232/14/4803 [pii] (2012).

How flu protein hijacks and suppresses immune response?

The 1918 Spanish flu claims an estimated 50 million deaths worldwide within a short period of time. It is clear how we have to understand influenza virus better in order to prevent another pandemic in the future. We are equipped with a sophisticated immune system. At the molecular level within a single cell, there are sensors that can detect components of the virus. For instance, MDA5 protein that floats around in the cytoplasm to detect double stranded RNA. As double stranded RNA is not produced in the cell but virus, MDA5 can activate the host immune response. Today, we will discuss two papers that have the same theme: how influenza virus suppresses immune response.

I am not going into too much experimental detail. The first story found that the C-terminal of influena virus protein PB1-F2 protein can interact with MAVS. MAVS is an adaptor protein (shown on the right) that gets activated by sensor proteins, such as the previously mentioned MDA5 and RIG-1 (senses 5’ phosphate in negative sense RNA viruses). After activation, it signals a downstream cascade that leads to the phosphorylation of regulatory proteins (IRF3,IRF7). Phosphorylated IRF3 and IRF7 then can enter the nucleus to activate transcription of inflammatory genes by binding to specific enhancer regions (ISRE) near the promoter. The interaction between PB1-F2 and MAVS is able to reduce the transcription of IFN-b (inflammatory gene). Furthermore, PB1-F2 reduces the membrane potential in the mitochondria during infection. As maintaining the membrane potential is essential for MAVS activation, it suggests that PB1-F2 reduces the potential to suppress further activation by MAVS. However, it is yet clear whether the interaction between MAVS and PB1-F2, and the function to reduce mitochondrial potential is linked (see below). Other viruses have also shown to suppress immune response by targeting MAVS adaptor protein. For example, Hepatitis C virus NS3/4A protease cleaves MAVS. This cleavage leads to the loss of membrane potential and results in reduction of inflammatory response.

Why can’t we get rid of influenza virus?

The second story is the NS1 protein of influenza virus. The scientists from NY Mount Sinai found that there is a little stretch of histone-like sequence on NS1. DNA normally wraps around histones (see right). The wrapping has some benefits to it: if you roll up a long piece of tape, you take up less space. It’s the same idea here. Wrapping around histone facilitates space-saving. Wrapping, however, inhibits transcription as transcription machinery can’t get into the specific region for transcription. Thus, modification on the histone by either methylation or acetylation signals for wrapping or unwrapping. In this paper, the little stretch of NS1 is found to act like histone. NS1 can get methylated. It can also bind to histone-binding factors, such as PAF1, CHD1. PAF1 is responsible for activating the transcription of antiviral genes. When they incubate PAF1 and NS1 together on a chromatin template, less amount of RNA transcripts is detected comparing to PAF1 alone. Thus, NS1 can get methylated like histones. Using this mechanism, it can interact with histone-binding factors to take them away from activating the transcription of inflammatory genes.

I think both stories converge on the same theme that viruses have evolved to hijack the host’s system. And we have to understand those hijacking mechanisms better before we can eradicate or at least provide a better cure against viral infection.


Nature. 2012 Mar 14;483(7390):428-33. doi: 10.1038/nature10892. Suppression of the antiviral response by an influenza histone mimic. Marazzi I, Ho JS, Kim J, Manicassamy B, Dewell S, Albrecht RA, Seibert CW, Schaefer U, Jeffrey KL, Prinjha RK, Lee K, García-Sastre A, Roeder RG, Tarakhovsky A.

J Virol. 2012 Aug;86(16):8359-66. Epub 2012 Jun 6. Influenza Virus Protein PB1-F2 Inhibits the Induction of Type I Interferon by Binding to MAVS and Decreasing Mitochondrial Membrane Potential. Varga ZT, Grant A, Manicassamy B, Palese P.