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Activation of JNK Signaling Mediates Amyloid-ß-Dependent Cell Death

Roswell Park Cancer Institute, United States of America
PLoS ONE (Impact Factor: 3.23). 09/2011; 6(9):e24361. DOI: 10.1371/journal.pone.0024361
Source: PubMed

ABSTRACT Alzheimer's disease (AD) is an age related progressive neurodegenerative disorder. One of the reasons for Alzheimer's neuropathology is the generation of large aggregates of Aß42 that are toxic in nature and induce oxidative stress, aberrant signaling and many other cellular alterations that trigger neuronal cell death. However, the exact mechanisms leading to cell death are not clearly understood.
We employed a Drosophila eye model of AD to study how Aß42 causes cell death. Misexpression of higher levels of Aß42 in the differentiating photoreceptors of fly retina rapidly induced aberrant cellular phenotypes and cell death. We found that blocking caspase-dependent cell death initially blocked cell death but did not lead to a significant rescue in the adult eye. However, blocking the levels of c-Jun NH(2)-terminal kinase (JNK) signaling pathway significantly rescued the neurodegeneration phenotype of Aß42 misexpression both in eye imaginal disc as well as the adult eye. Misexpression of Aß42 induced transcriptional upregulation of puckered (puc), a downstream target and functional read out of JNK signaling. Moreover, a three-fold increase in phospho-Jun (activated Jun) protein levels was seen in Aß42 retina as compared to the wild-type retina. When we blocked both caspases and JNK signaling simultaneously in the fly retina, the rescue of the neurodegenerative phenotype is comparable to that caused by blocking JNK signaling pathway alone.
Our data suggests that (i) accumulation of Aß42 plaques induces JNK signaling in neurons and (ii) induction of JNK contributes to Aß42 mediated cell death. Therefore, inappropriate JNK activation may indeed be relevant to the AD neuropathology, thus making JNK a key target for AD therapies.

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    • "Using a targeted misexpression approach [27], we misexpressed higher levels of human Aβ42 gene in the differentiating photoreceptors of the developing eye using a Glass Multiple Repeat (GMR) Gal4 driver [28]. Misexpression of human Aβ42 in the differentiating photoreceptor neurons of the fly retina exhibits the progressive neurodegenerative phenotypes which mimic the neuropathology of AD patients [26]. In order to discern the genetic basis of Aβ42 mediated neurodegeneration, we have employed our transgenic fly model to analyze the toxic effect of Aβ42 accumulation on signaling pathways through a forward genetic screen. "
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    ABSTRACT: Alzheimer's disease (AD) is a debilitating age related progressive neurodegenerative disorder characterized by the loss of cognition, and eventual death of the affected individual. One of the major causes of AD is the accumulation of Amyloid-beta 42 (Aβ42) polypeptides formed by the improper cleavage of amyloid precursor protein (APP) in the brain. These plaques disrupt normal cellular processes through oxidative stress and aberrant signaling resulting in the loss of synaptic activity and death of the neurons. However, the detailed genetic mechanism(s) responsible for this neurodegeneration still remain elusive. We have generated a transgenic Drosophila eye model where high levels of human Aβ42 is misexpressed in the differentiating photoreceptor neurons of the developing eye, which phenocopy Alzheimer's like neuropathology in the neural retina. We have utilized this model for a gain of function screen using members of various signaling pathways involved in the development of the fly eye to identify downstream targets or modifiers of Aβ42 mediated neurodegeneration. We have identified the homeotic gene teashirt (tsh) as a suppressor of the Aβ42 mediated neurodegenerative phenotype. Targeted misexpression of tsh with Aβ42 in the differentiating retina can significantly rescue neurodegeneration by blocking cell death. We found that Tsh protein is absent/ downregulated in the neural retina at this stage. The structure function analysis revealed that the PLDLS domain of Tsh acts as an inhibitor of the neuroprotective function of tsh in the Drosophila eye model. Lastly, we found that the tsh paralog, tiptop (tio) can also rescue Aβ42 mediated neurodegeneration. We have identified tsh and tio as new genetic modifiers of Aβ42 mediated neurodegeneration. Our studies demonstrate a novel neuroprotective function of tsh and its paralog tio in Aβ42 mediated neurodegeneration. The neuroprotective function of tsh is independent of its role in retinal determination.
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    • "The adult fly cultures were maintained at 25°C, while the egg laying (progeny) were transferred to 29°C. Misexpression of Aβ42 in the differentiating retina (GMRGal4>UAS-Aβ42, GMR>Aβ42) exhibits a stronger neurodegenerative phenotype at 29°C [3]. All the targeted misexpression experiments were conducted using the Glass Multiple Repeat driver line (GMR-Gal4), which directs expression of transgenes in the differentiating retinal precursor cells of the developing eye imaginal disc and pupal retina [22]. "
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    ABSTRACT: Alzheimer's disease (AD, OMIM: 104300), a progressive neurodegenerative disorder with no cure to date, is caused by the generation of amyloid-beta-42 (Aβ42) aggregates that trigger neuronal cell death by unknown mechanism(s). We have developed a transgenic Drosophila eye model where misexpression of human Aβ42 results in AD-like neuropathology in the neural retina. We have identified an apical-basal polarity gene crumbs (crb) as a genetic modifier of Aβ42-mediated-neuropathology. Misexpression of Aβ42 caused upregulation of Crb expression, whereas downregulation of Crb either by RNAi or null allele approach rescued the Aβ42-mediated-neurodegeneration. Co-expression of full length Crb with Aβ42 increased severity of Aβ42-mediated-neurodegeneration, due to three fold induction of cell death in comparison to the wild type. Higher Crb levels affect axonal targeting from the retina to the brain. The structure function analysis identified intracellular domain of Crb to be required for Aβ42-mediated-neurodegeneration. We demonstrate a novel neuroprotective role of Crb in Aβ42-mediated-neurodegeneration.
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    • "Depending on the cellular context and on the identity of the stimulus, Erk, JNK, or p38 MAPkinase (MAPKs) is responsible for S36 phosphorylation (Le et al. 2001; Okada et al. 1997), though alternate mechanisms do exist (Khanday et al. 2006a). Stress-activated protein kinases (SAPK), such as c-jun N-terminal kinase (JNK) and p38MAP kinase, are ectopically activated and participate in the pathogenesis of AD (Hensley et al. 1999; Zhu et al. 2003; Tare et al. 2011; Corrêa and Eales 2012). However, the involvement of these SAPKs in b- Amyloid-mediated events is only just being unraveled. "
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