The Burden of Amyloid Plaques in Alzheimer S Disease

1

Diez

Simon Diez

BIOL 303 sect-501

October 30, 2012

Dr. Bert Ely

The Burden of Amyloid Plaques in Alzheimer’s Disease

Alzheimer’s disease (AD) is a neurodegenerative disease that causes dementia, or loss of brain function. It is the most common cause of dementia and is estimated to affect approximately one of every eight Americans over the age of 65 (Hafez et al. 2012). Symptoms of the more rapidly escalating form of AD, early onset Alzheimer’s disease (EOAD), exhibit themselves before the age of 60 while late onset Alzheimer’s disease (LOAD) occurs in people over the age of 60 (Alzheimer’s). While several genetic risk factors have been identified for the rarer familial EOAD, the more common and sporadic form, LOAD, remains more mysterious. The only well-known genetic risk factor for LOAD is the ε4 allele of the apolipoprotein E (APOE) (Hamilton et al. 2012).

The pathology of Alzheimer’s is mainly characterized by the accumulation of deposited amyloid plaques in the brains of AD patients. These plaques result from improperly folded neurotoxic amyloid βs, created from amyloid precursor proteins (APP), and tau associated neurofibrillary tangles. Mutations in four genes, APP, PS1, PS2, and APOE, are estimated to be responsible for up to forty percent of all cases of AD. The associated changes in these genes are predicted to be the cause of the reduced clearance and overproduction of the amyloid βs (Rogaeva et al. 2006). In conjunction with the deposition of amyloid plaques, AD pathology also includes inflammation of the brain (Hazrati et al. 2012). Both of these aspects of AD cause the degeneration of brain tissues and the loss of its function. In order to more fully understand the processes that lead to the deposition amyloid plaques as well as the brain inflammation, the proteins that interact with and modify the amyloid βs are of major concern in the research of AD.

The gamma secretase complex is a multi-subunit intermembrane protease whose most well known substrate is the APP (Hamilton et al. 2012). When APP is cleaved through the amyloidogenic pathway it forms short (17-42) amino acid peptide chains called amyloid βs (A Novel Pathway.). The γ-secretase complex is comprised of four proteins that include both presenilin 1 and 2 (PS1/PS2), whose mutant forms are known risk factors for AD, as well as the PS enhancer 2(PEN2), the anterior pharynx defective (APH-1), and the nicastrin protein (NCSTN) (Hamilton et al. 2012). The study conducted by Hamilton et al. investigates the role of genetic variation at the NCSTN locus in AD. An alternatively spliced form of NCSTN, lacking exon 16, has been shown to have an association with AD in patients that carry the ε4 allele of APOE (Mitsuda et al. 2006). Exon 16 is important as it codes for the juxtamembrane region that interacts with APH-1 (Walker et al. 2006). NCSTN is therefore of interest as it affects the assembly of the γ-secretase complex and could affect the function of the complex through its mutant forms (Yu et al. 2000). The Hamilton et al. study shows that haplotypic variation can be found in tissues of postmortem brains. This variation is seen in the levels of full-length transcripts (not lacking the exon 16 of the NCSTN gene) expressed between haplotypes as well as difference in levels expressed between regions of the brain. Individuals with haplotype (Hap) AA had increased levels of full-length transcripts in the entorhinal cortex and frontal cortex than individuals with HapAB. In individuals with HapAA there is a significantly higher level of full-length transcripts produced in the entorhinal cortex than in the frontal cortex and the hippocampus. In individuals with HapAB there is a nominally significantly higher level of full-length transcripts produced in the hippocampus compared with the frontal cortex and the entorhinal cortex. Although the study did not find any significant functional difference of the γ-secretase complex in cell cultured lines between haplotypes of the NCSTN locus, there is the possibility that tissue specific expression of NCSTN is at play especially given the post mortem brain results. As the alternatively spliced form of NCSTN has been shown to be linked with AD in patients carrying the ε4 allele of APOE, the genetic variation at the NCSTN locus could have an effect on AD through the processing of APP into amyloid βs that is not clearly seen in the cell culture experiment. (Hamilton et al. 2012).

Along with the burden of amyloid plaques, inflammation of the brain also greatly enhances the degenerative effects of AD. In AD, inflammation includes that caused by the activation of the complement system (Eikenlenboom et al. 2006). This system is utilized in the body to enhance immune response in the presence of pathogens (Janway et al. 2001). The complement receptor 1 (CR1) has been associated with faster rates of cognitive impairment caused by the increased burden of amyloid plaques (Chibnik et l. 2011). CR1 plays a role in removal of pathogens coated with the C3b and C4b complement components and is known to be activated by amyloid βs (Hazrati et al 2012). CR1 is a transmembrane glycoprotein whose role is believed to be in inflammation caused by the immune system and glial cells, those that surround, support, and destroy dead neurons (Sofka). In the past, single nucleotide polymorphisms (SNPs) of CR1 have been linked to AD, especially in people who carry the ε4 allele of APOE (Hazrati et al. 2012). Further variation in CR1 occurs due to copy number variation of low copy repeat (LCP) 1, which has been linked to AD. Copy number variation at LCP1 gives several isoforms of CR1. Isoform CR1-S has previously been identified as a risk factor for AD and has been shown to be in linkage disequilibrium, or non-random association, with the AD significant SNPs of CR1. CR1-S has been proposed as causing higher inhibition of the complement system as it has more C3b/C4b binding sites than the CR1-F isoform (Brouwers et al. 2012). Hazrati et al. (2012) identified one genotype (F/S) as increasing the chance of AD by a factor of 1.8. Furthermore they showed that while both AD patients and controls exhibited higher expression of the F isoform compared to the S isoform, AD patients had smaller ratios of F to S than the ratio in the controls, meaning that the AD patients showed a higher expression of relative the S isoform than that of the controls. The study also showed that there is little association between the SNPs for this genotype and AD, which further strengthens their conclusion that CR1’s role in AD is due to the copy number variation. However the authors rejected the hypothesis that the CR1-S isoform decreases complement activity through its additional binding sites as complement activity was increased in the brains of AD patients and the CR1-S isoforms are seen in higher rates in AD brains than in healthy brains. This study brings up the idea that the aggregation of amyloid plaques in AD may be partially caused by changes in the systems that regulate amyloid βs, such as CR1, in conjunction with those that cause the amyloid βs overproduction.

Much research in AD has been aimed at understanding the role of the signaling proteins and receptors in the low-density lipoprotein (LDL) family (Hafez et al. 2012). Reelin is of interest as it binds LDL receptors also known to bind APOE (D’Arcangelo et al. 1999; Ashford 2004). Reelin’s binding of certain receptors causes a signaling cascade that regulates the role of key kinases involved in the production of amyloid βs and phosphorylation of the microtubule-associated protein tau, the two the main components of amyloid plaques (Heisbeger et al 1999; Beffert et al. 2002; Ohkubo et al. 2003; Phiel et al. 2003). In AD mouse models reduced reelin has been shown to accelerate the formation of amyloid βs and tau (Knuesel et al. 2009; Kocherhans et al. 2010). Reelin is further connected to AD in that it interacts with APP and through its ability to modify synapses can promote neurite outgrowth (Hoe et al. 2009). Reelin has further been shown to restore reductions in long-term potentiation caused by amyloid βs and to improve the strength of synapses in CA1 pyramidal cells, cells integral to proper functioning of the hippocampus (Durakoglugil et al. 2009). In the Hafez et al. study, F-spondin was used as a proxy for reelin as it is highly homologous to reelin. Like reelin, F-spondin is a secreted extracellular protein expressed in the hippocampus. F-spondin further resembles reelin in its role in development and adulthood of proper neural placement and plasticity, the ability of neurons to change an adapt (Higashijima et al. 1997; Burstyn-Cohen et al. 1999; Feinstein et al. 1999; Andrade et al. 2007). F-spondin functions as a homolog of reelin since it binds two of the same LDL receptors involved in AD as reelin and possesses an N-terminal reelin domain. F-spondin could further play a role in AD as it has been previously shown to regulate processing of APP. In normal mice, F-spondin was shown to reduce amyloid βs in comparison to control mice treated with a green fluorescent protein (GFP). GFP was used in the control groups as it can be easily visualized under UV light to ensure that transfer of the lentiviral vectors (F-spondin for trated mice and GFP for controls) was successful. Furthermore, transgenic mice (ones overexpressing human APP and PS1, which are standard models for AD) showed reduced amyloid plaque deposition in the hippocampus compared to GFP transgenic mice. In normal mice the suppression of reelin has been shown to impair memory whereas reelin stimulation improves memory (Hafez et al. 2012). It is hypothesized that this link between memory and the reelin pathway is due to increased synaptic plasticity. Although F-spondin did not show improved memory in transgenic mice, its role in reducing the burden of amyloid plaques suggests it could possibly be therapeutic to AD. Previous studies have found overexpressed F-spondin in cerebral spinal fluid (CSF) of AD patients, which seems to discredit F-spondin as possibly therapeutic in AD (Ringman et al. 2012). However Hafez et al. states that this could be a form of homeostasis where levels of F-spondin are more highly expressed in some parts of the body, such as the CSF, in order to compensate for reduced levels in another, the parts of the brain where AD plaques are most prevalent. This proposed system highly resembles the lowered levels of amyloid βs in CSF of AD patients when it is clearly over expressed in the brains of AD patients.

The links between the reelin pathway, the CR1 associated complement system, and the functional variations of NCSTN with AD could very well be explained through their roles and interactions with amyloid βs. All three proteins have been shown to be associated with the most well known genetic risk factor for LOAD, the ε4 allele of APOE. The deposition of amyloid plaques could therefore be viewed the result of multiple steps that are not functionally normal in the brains of AD patients, but that could have some yet unknown link possibly through their connectedness with APOE. Plaque deposition could follow a multi-step process that requires multiple genetic mutations in order to create the abnormalities seen in AD. Firstly, abnormalities in the proteins that make up amyloid plaques, APP and tau, could make a person more susceptible to AD. Some of these genetic links have already been established, such as the mutant form of APP’s role in LOAD. Furthermore, genetic variation in the slew of proteins that process APP, of which NCSTN is one, and tau could cause changes that would promote the formation of neurotoxic forms of these proteins, amyloid βs and incorrectly processed tau. In addition the suppression of the systems set in place to destroy these toxic forms, such as the immune response of CR1, would further increase a person’s susceptibility to AD. The effects of the immune response are more complicated though, as the AD significant forms of CR1 have been seen to be increased in the CSF of AD patients. This could be due to the homeostatic response that was mentioned in the Hazrati et al. study or the over expression of the CR1 complement system could be also occurring in the brain as a response to the over-production of amyloid βs. Increased amyloid βs production could increase the activation of the CR1 system and therefore cause the CR1 related inflammation in the brain that is observed in AD patients. In this case there is still the fact that over produced amyloid βs accumulate in spite of increased CR1 complement immune response. This suggests that a functional variation in CR1 caused by a yet unidentified genetic link, could reduce CR1’s ability to clear the brain of amyloid βs or that there is another factor at play that inhibits the CR1 complement system from doing its job correctly.

In whatever mechanism the accumulation of amyloid plaques occurs, AD’s pathology is clearly very complex. The genetic factors that affect AD are equally as complex and as they are not fully understood, the mechanism of the disease is still a mystery. One thing that can be said about AD is that it is not caused by one single factor, either genetic or otherwise, but it is most likely the effect of a combination of factors of which genetics clearly plays a role. While some of these factors have been identified and confirmed, there are still links to AD that are being researched. The relative strengths of each of the known genetic risk variables are also a mystery.

Works Cited

"Alzheimer's Disease." PubMed Health. Ed. Luc Jasmin and David Zieve. U.S. National Library

of Medicine, 18 Nov. 0000. Web. 25 Oct. 2012. <http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001767/>.

Andrade N, Komnenovic V, Blake SM, Jossin Y, Howell B, Goffinet A, Schneider WJ, Nimpf J

(2007) ApoER2/VLDL receptor and Dab1 in the rostral migratory stream function in postnatal neuronal migration independently of Reelin. Proc Nat Acad Sci U S A 104:8508–8513.