Neurodegeneration by polyglutamine Atrophin is not rescued by induction of autophagy

Abstract

Polyglutamine pathologies are neurodegenerative diseases that manifest both general polyglutamine toxicity and mutant protein-specific effects. Dentatorubral-pallidoluysian Atrophy (DRPLA) is one of these disorders caused by mutations in the Atrophin-1 protein. We have generated several models for DRPLA in Drosophila and analysed the mechanisms of cellular and organism toxicity. Our genetic and ultrastructural analysis of neurodegeneration suggests that autophagy may have a role in cellular degeneration when polyglutamine proteins are overexpressed in neuronal and glial cells. Clearance of autophagic organelles is blocked at the lysosomal level after correct fusion between autophagosomes and lysosomes. This leads to accumulation of autofluorescent pigments and proteinaceous residues usually degraded by the autophagy-lysosome system. Under these circumstances, further pharmacological and genetic induction of autophagy does not rescue neurodegeneration by polyglutamine Atrophins, in contrast to many other neurodegenerative conditions. Our data thus provide a crucial insight into the specific mechanism of a polyglutamine disease and reveal important differences in the role of autophagy with respect to other diseases of the same family. PolyQ-expanded proteins misfold and accumulate in large aggregates, which have been initially described as toxic, 1 and more recently as a positive prognosis factor in neuronal survival.2 The molecular and cellular mechanisms of toxicity due to polyQ proteins are not yet fully characterised and many of the principal aspects are under intense scrutiny. In particular, many reports have described autophagy as a protective cellular mechanism in neurodegeneration, 3 although its full contribution to the pathogenesis, including cell killing, is still poorly understood. The full cycle of autophagic degradation of cellular components and recycle of simpler constituents is not involved in cell killing. However, many dysfunctions at different steps of this cycle that have been reported upon expression of toxic proteins, including polyQ proteins, can lead to cell degeneration and death. 4 Over the past few years a growing number of studies have focused on identifying the interplay between polyQ effects and protein-specific misfunctions, 5-7 with the assumption that polyQ pathologies combine general polyQ toxicity with disease-specific effects due to the proteins affected. The fruitfly Drosophila melanogaster has proved to be a valuable model organism for polyQ diseases and neurodegeneration. New models have also moved on from initial basic observations, uncovering the complex interplay between polyQ effects and RNA or protein-specific misfunction. 5,7,9 We generated several Drosophila models of DRPLA ( Figure 1a and Supplementary Figure 1), a polyQ disease caused by mutations in atrophin-1.10,11 Atrophins are transcriptional cofactors conserved from Drosophila to mammals, 12-15 providing an ideal background for the dissection of polyQ effects and specific Atrophin functions through Drosophila ...

iFly: The eye of the fruit fly as a model to study autophagy and related trafficking pathways

“…The former work showed that lithium-chloride treatment or GSK3b inhibition results in a partial suppression of mutant ataxin 3 toxicity in the eye (Jia et al, 2013). Another polyQ disease study that modeled Dentatorubral-pallidoluysian Atrophy (DRPLA) found a slightly different role for autophagy (Nisoli et al, 2010). Mutant Atrophin expression also led to a rough eye phenotype possible to visualize rhabdoms without having to dissect the retina, as the illuminating light enters into the eye from behind, travels via the rhabdoms and leaves by crossing the optical elements.…”

Regulation of long-range planar cell polarity by Fat-Dachsous signaling

“…This neurodegeneration can be reduced by loss of d or yki, while loss of wts or sav causes degeneration. atro and Hippo signaling both regulate autophagy (Dutta and Baehrecke, 2008;Nisoli et al, 2010), and degenerating atro-overexpressing or ft mutant eyes in show an increased number of autophagic vesicles.…”

Time Flies: Autophagy During Ageing in Drosophila

“…Both latter manipulations are commonly used to model neurodegeneration [88] Macroautophagy in flies can also be upregulated by Rab5 over-expression and this approach also mitigates polyQ-huntingtin mediated degeneration in the eye [89]. However, there is a fine line between beneficial and detrimental consequences of autophagy activation in the context of neurodegeneration as shown in a dentatorubralpallidoluysian atrophy (DRPLA) fly model [90]. This model is built upon the expression of atrophin with a polyQ expansion and is characterized by lysosomal dysfunction and blocked autophagosome-lysosome fusion, hence reduced autophagic flux [90].…”

“…However, there is a fine line between beneficial and detrimental consequences of autophagy activation in the context of neurodegeneration as shown in a dentatorubralpallidoluysian atrophy (DRPLA) fly model [90]. This model is built upon the expression of atrophin with a polyQ expansion and is characterized by lysosomal dysfunction and blocked autophagosome-lysosome fusion, hence reduced autophagic flux [90]. Even though introduction of a mutant form of Atg1 intensified the neurodegenerative phenotype, upregulation of autophagy in this system had no rescuing effect but, in some case, even had the opposite outcome and increased neurodegeneration [90].…”

“…This model is built upon the expression of atrophin with a polyQ expansion and is characterized by lysosomal dysfunction and blocked autophagosome-lysosome fusion, hence reduced autophagic flux [90]. Even though introduction of a mutant form of Atg1 intensified the neurodegenerative phenotype, upregulation of autophagy in this system had no rescuing effect but, in some case, even had the opposite outcome and increased neurodegeneration [90]. In other words, autophagy plays an important role in scavenging polyQ atrophin from the cytosol, but is only of beneficial nature as long as autophagy can proceed all the way to lysosomal degradation.…”