Although the cause of Alzheimer's disease (AD) remains unknown, there are various theories implicating the accumulation of toxic amyloid beta as a possible cause. AD is a fatal neurodegenerative disease characterised by neurofibrillary tangles (NFTs) and or senile plaques (SPs). Studies have shown that the major constituents of NFTs and SPs are toxic ïï¢ï€±ï€ï€´° and 1-42. Rival and colleagues aimed to identify pathways and intermediates that are critical for Aï¢ mediated toxicity in vivo. A drosophila model is the model of choice for studying the pathogenesis of neurodegenerative diseases in parcticular Alzheimer's disease (Crowther et al., 2009). Several researchers including Lui et al., (2004), Bilen et al., (2005) and Finelli et al., (2003) have used a drosophila model to study the pathogenesis of AD and Rival and colleagues did the same. Drosophila is used because so much of its genetics is already known, thus it is easily manipulated to establish a cause and effect relationship. In this case a transgenic drosophila model expressing human ïï¢1-42 was used to study the implications of Aß1-42 aggregation and its consequent toxicity. The paper starts off by stating that the smaller aggregates of ï¢ï€ amyloid peptide are the major pathogenic species in Alzheimer's disease (AD), but the mechanism by which they mediate their toxicity remains unknown. It then goes on to suggest possible mechanisms for toxicity. One such suggestion states that toxicity of Aï¢ could be due to reactive oxidative species. Several researchers including Goodman et al., (1995) and Yatin et al., (2000) concur that oxidative species play a major role in the toxicity of Aï¢ï€®ï€
Main methods and analytical techniques used
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Various methods were implicated by Rival and colleagues. Drosophila stocks used were UAS-CAT, UAS-SOD1, and USAS-MITSOD2. Furthermore ferritin 1 heavy chain was isolated from EST clone GH 2406 and ferritin 2 light chain was isolated from EST clone ATI6780 respectively. A genome wide gene-expression analysis and complementary genetic screen in a Drosophila model of AD was carried out. A genetic screen and a microarray analysis were used in conjunction with each other; the latter was used to identify the changes in the expression of genes affected by oxidative stress. A control was used to compare Arctic ïï¢ï€±-42. Gene ontology was used to determine the function of genes following the results of the microarray. Following from this a hypergeometric test was executed to establish whether any gene sets or functional groups were significantly represented more frequently compared to the other genes in the list from the microarray. This allowed the association of function in a list of genes. P element screen was used to establish factors that modify the toxicity of ï¢ amyloid. The effect of Gal4 activated GS element was determined by crossing elav-Gal4 on the X chromosome. Furthermore the arctic Aß1-42 transgene on the 2nd chromosome was crossed with virgin females with a floating GS element. GS element without Gal-4 was used as a control. Each GS line consisted of 17 lines. Longevity was determined by the median survival rate of the flies in which the GS element had extended their life. Any life span more than 2SD more than the mean was classed as significant. A second longevity assay was €used to confirm the results of the GS element. It was also used to establish the effect of GS element on flies that did not express Aï¢ï€®ï€ A secondary screen containing the longevity assay and where all flies expressed the GS element was used to compare survival rates of flies expressing GS element versus flies not expressing GS element. The role of oxidative stress was tested by seeing how flies responded to H2O2 in their foods. The sensorimotor effect of flies was also determined using food treatment and statistical analysis.
The results of the microarray analysis provided sufficient evidence for the role of oxidation stress-related genes in the brain of flies expressing Aï¢ï€®ï€ The role of oxidative stress was more observable in the early stages of Aï¢ 1-42 expression. In the screen of GS inserts, the authors reported that there was a higher percentage of GS inserts leading to an increase in median survival rate and there was also a lower percentage of GS inserts decreased survival. Selected suppressor GS elements also increased survival rate but had no effect on the control flies. They found that ferritin is effective in reversing the oxidative stress mediated by Aß42 and treatment with clioquinol an iron chelating compound extended the life of flies expressing Arctic Aß42. It was found that clioquinol had no effect on the longevity of flies expressing non toxic AB 1-40 and flies expressing toxic ïï¢ï€ levels of zinc in flies.
Main conclusions drawn by authors
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Oxidative stress as a result of hydrogen peroxide can cause AD and thus supports metal chelating therapies used in AD. Their findings agree with the studies of post-mortem report and they were keen to point this out. Rival and colleagues identified three main sources of oxidative stress. The first was the generation of hydroxyl radical via Fenton reaction was the most important toxic cause of oxidative stress. Secondly via mitochondrial superoxide levels and thirdly reactive aldehydes species produced by lipid oxidation. They also concluded that oxidative stress can be prevented by influencing iron metabolism. Prevention of oxidative stress would reduce AB toxicity in AD.
Figure 1. Drosophila 'fruit fly' model is used to study the pathogenesis of AD. (Crowther et al., 2009).
The Drosophila model provided a more detail observation and understanding of the oxidative pathways that are implicated in the pathology of neurodegenerative disease The Drosophila model is used by many researchers to study the pathogenesis of many neurodegenerative diseases. The fruit fly model allowed the authors to view the generic toxicity of Aï¢1-40 in vivo. Gotz et al., 1994 concluded that Drosophilia offers some advantage over other models when studying the pathogenesis of neurodegenerative diseases. In my opinion when choosing any biological organism to study human diseases the level of conservation between that organism and humans should be high. Analysis of Drosophilia has revealed 70% of human related disease genes, this implies high conservation. Another advantage of using flies is the relatively short life span, late onset disorders such AD can be observed in a short space of time. In terms of the paper I think that the research done by the authors has significantly improved our understanding and knowledge of the oxidative stress pathways as well as improving the need for more research into metal chelating therapies. Chelating therapies could potentially either prevent and/or treat oxidation hence reducing the rapid progression of AD and ultimately prolonging life.