Tag Archives: brain

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.

References

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).

Review: Inhibition of RNA lariat debranching enzyme suppresses TDP-43 toxicity in ALS disease models

Neurodegenerative diseases are often associated with accumulation of misfolded proteins. For instance, amyotrophic lateral sclerosis (ALS) affects 5 out of 100000 people worldwide. The most notable ALS cases are Stephen Hawkin and Lou Gehrig, hence the disease is known as Lou Gehrig disease. In 20% of the ALS cases, it is due to a mutation in SOD1, while other cases without Sod1 mutation are associated with TDP-43 protein accumulation in the cytoplasm of spinal cord neurons. This study has shed light on treating ALS by accumulating RNA introns or simply delivering oligonucleotides.

Before you read on: this is a decent paper published in Nature; easy to understand; straight-forward protocols

In this study, the researchers performed a genome wide screen to see which genes are responsible for suppressing TDP-43 toxicity. They narrowed down on dbr1, which suppresses the TDP-43 and TDP-43 mutant toxicity. This suppression is, however, not due to a lowered expression of TDP-43. When human neuronal cell line is transfected with siRNA against dbr1, the toxicity caused by TDP-43 is relieved. The author also proved that Dbr1 knockdown reduces TDP-43 toxicity in primary rat neurons. Using DBR1 mutants that do not have lariat debranching enzymatic activity in yeast spotting assay (look at the figure below if you don’t know what DBR1 does: responsible for debranching the lariat, and subsequently degrading the introns to avoid accumulation of the junk DNA) , TDP-43 toxicity is reduced. As Dbr1 is responsible for debranching lariats following splicing, the knockdown of Dbr1 should increase the amount of introns in the cell. This group incorporated MS2 RNA binding protein into the intron and GFP-tagged MS2-CP protein to visualize the localization of intron. In Dbr1 null cell, intron is colocalized with TDP-43.

Significance:

The accumulation of introns alleviated the TDP-43 toxicity, which suggest that the accumulation may be a way to treat ALS cases. There are currently no direct therapies against TDP43. So how can we go around this problem? Delivering oligonucleotides into ALS models may be a possible therapy in the near future. In fact, researchers had already used antisense oligonucleotide against SOD1 to treat animal models of ALS which showed slowed disease progression.

Take home message: Without Dbr1, a lariat debranching enzyme, introns accumulate in the cytoplasm which is correlated with lowered TDP-43 toxicity and TDP-43 colocalization with DBR1.

Reference:
Nat Genet. 2012 Oct 28. doi: 10.1038/ng.2434. [Epub ahead of print]
Inhibition of RNA lariat debranching enzyme suppresses TDP-43 toxicity in ALS disease models.
Armakola M, Higgins MJ, Figley MD, Barmada SJ, Scarborough EA, Diaz Z, Fang X, Shorter J, Krogan NJ, Finkbeiner S, Farese RV Jr, Gitler AD.

Fight against Multiple Sclerosis: a battle against demyelination

You can take the train to Chicago or you can take the plane to Chicago. So what happens if you somehow make a hybrid model of train and plane (call it trane) to get to Chicago? Do you get there faster or slower? Today, we are going to discuss a neuroscience paper from Hannover, Germany. Demyelination is the loss of myelin sheath in neurons. Myelin sheath is important in conducting signals in the complex neural network. If the myelin sheath is damaged, impairment in movement, coordination is observed. What is an example of demyelination disease? Multiple sclerosis is a prime example – it is one of the most common neurological disorders in youth adults. It is an immune-mediated demyelinating disease in the central nervous system (CNS).

In the paper, the authors ask what happens to the brain if you put two demyelinating agents together. Do you accelerate the process of demyelination?

There are two demyelinating agents tested in this paper: cuprizone and theiler virus. First, cuprizone is a chemical that chelates copper in the central nervous system. This leads to a lack of copper supply to neuronal cells. As copper is required for oligodendrocyte metabolism, oligodendrocytes (responsible for myelination) are dead. Thus, cuprizone leads to the death of oligodendrocytes, which indirectly shuts down the production of myelination. In contrast, theiler virus is a positive sense RNA virus that is grouped under the family of picornaviruses. Theiler virus DA strain causes chronic demyelination. During early phase of infection, the virus replicates in the gray matter of CNS. Later in infection, the virus persists in macrophage. At this stage, the virus induces demyelinating lesion, axonal damage, etc which resemble MS in human.

The paper had done a couple of experiments that support cuprizone in alleviating the CNS demyelination caused by Theiler virus. Cuprizone alone can cause demyelination in a specific region, known as corpus callosum. However, if you put cuprizone with theiler virus, the combination has a reduced demyelination in the thoracic spinal cord comparing to theiler virus infection alone. Look at the figure below, the bottom MBP panel shows you the presence of myelin basic proteins. Myelin basic proteins are responsible for myelination. You can see that there is a dramatic increase of myelin basic proteins in TMEV/CPZ (co-administration) comparing to TMEV (theiler alone). The co-administration also correlates with a better performance on the Rotarod scale (the Rotarod scale: place the mouse on a suspended rotating rod. This enables scientists to test the alertness, balance and the brain function of the mouse). Cuprizone also reduces the detectable amount of immune cells, such as B, T and macrophage during theiler infection. Since the immune system is down, scientists then ask if that allow the virus to replicate faster. The answer is no. In fact, the replication of theiler virus remains the same. This interesting paper has several insights in neuroinflammation. One, the cause of neuroinflammation must be caused by several factors. Two, theiler virus does not require neuroinflammation to remain persistent. Lastly, cuprizone reduces inflammation but does not affect virus replication.

Take home message:

The tug of war between cuprizone and theiler virus does sound fascinating. It is interesting to see if you put two demyelinating agents together, the effect turns out to be less effective than having theiler infection alone. It is hard to say who wins this tug of war, but it is definitely an innovative approach to study neurodegeneration using virus and chemical to induce demyelination.

Reference:

Cuprizone inhibits demyelinating leukomyelitis by reducing immune responses without virus exacerbation in an infectious model of multiple sclerosis.

Herder V, Hansmann F, Stangel M, Schaudien D, Rohn K, Baumgärtner W, Beineke A.

J Neuroimmunol. 2012 Mar;244(1-2):84-93. Epub 2012 Feb 12.

PMID: 22329906