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      Impact of Drosophila Models in the Study and Treatment of Friedreich’s Ataxia

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          Abstract

          Drosophila melanogaster has been for over a century the model of choice of several neurobiologists to decipher the formation and development of the nervous system as well as to mirror the pathophysiological conditions of many human neurodegenerative diseases. The rare disease Friedreich’s ataxia (FRDA) is not an exception. Since the isolation of the responsible gene more than two decades ago, the analysis of the fly orthologue has proven to be an excellent avenue to understand the development and progression of the disease, to unravel pivotal mechanisms underpinning the pathology and to identify genes and molecules that might well be either disease biomarkers or promising targets for therapeutic interventions. In this review, we aim to summarize the collection of findings provided by the Drosophila models but also to go one step beyond and propose the implications of these discoveries for the study and cure of this disorder. We will present the physiological, cellular and molecular phenotypes described in the fly, highlighting those that have given insight into the pathology and we will show how the ability of Drosophila to perform genetic and pharmacological screens has provided valuable information that is not easily within reach of other cellular or mammalian models.

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          Most cited references212

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          Disturbance of endoplasmic reticulum proteostasis in neurodegenerative diseases.

          The unfolded protein response (UPR) is a homeostatic mechanism by which cells regulate levels of misfolded proteins in the endoplasmic reticulum (ER). Although it is well characterized in non-neuronal cells, a proliferation of papers over the past few years has revealed a key role for the UPR in normal neuronal function and as an important driver of neurodegenerative diseases. A complex scenario is emerging in which distinct UPR signalling modules have specific and even opposite effects on neurodegeneration depending on the disease context. Here, we provide an overview of the most recent findings addressing the biological relevance of ER stress in the nervous system.
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            The Glia-Neuron Lactate Shuttle and Elevated ROS Promote Lipid Synthesis in Neurons and Lipid Droplet Accumulation in Glia via APOE/D

            Elevated reactive oxygen species (ROS) induce the formation of lipids in neurons that are transferred to glia where they form lipid droplets (LD). We show that glial and neuronal monocarboxylate transporters (MCTs), fatty acid transport proteins (FATP), and apolipoproteins are critical for glial LD formation. MCTs enable glia to secrete and neurons to absorb lactate, which is converted to pyruvate and acetyl-CoA in neurons. Lactate metabolites provide a substrate for synthesis of fatty acids, which are processed and transferred to glia by FATP and apolipoproteins. In the presence of high ROS, inhibiting lactate transfer or lowering FATP or apolipoprotein levels all decrease glial LD accumulation in flies and in primary mouse glial-neuronal cultures. We show that human APOE can substitute for a fly glial apolipoprotein and that APOE4, an Alzheimer’s Disease susceptibility allele, is impaired in lipid transport and promotes neurodegeneration, providing insights into disease mechanisms. Liu et al. unravel an evolutionarily conserved mechanism which brings neuron-glia metabolic cooperation full circle. They show that glial lactate can fuel neuronal lipogenesis in response to ROS; in turn, neuronal lipids are transported and stored in glia as lipid droplets. The inability to transport lipids to glia for lipid droplet formation leads to accelerated neurodegeneration under stress.
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              Aconitase and mitochondrial iron-sulphur protein deficiency in Friedreich ataxia.

              Friedreich ataxia (FRDA) is a common autosomal recessive degenerative disease (1/50,000 live births) characterized by a progressive-gait and limb ataxia with lack of tendon reflexes in the legs, dysarthria and pyramidal weakness of the inferior limbs. Hypertrophic cardiomyopathy is observed in most FRDA patients. The gene associated with the disease has been mapped to chromosome 9q13 (ref. 3) and encodes a 210-amino-acid protein, frataxin. FRDA is caused primarily by a GAA repeat expansion within the first intron of the frataxin gene, which accounts for 98% of mutant alleles. The function of the protein is unknown, but an increased iron content has been reported in hearts of FRDA patients and in mitochondria of yeast strains carrying a deleted frataxin gene counterpart (YFH1), suggesting that frataxin plays a major role in regulating mitochondrial iron transport. Here, we report a deficient activity of the iron-sulphur (Fe-S) cluster-containing subunits of mitochondrial respiratory complexes I, II and III in the endomyocardial biopsy of two unrelated FRDA patients. Aconitase, an iron-sulphur protein involved in iron homeostasis, was found to be deficient as well. Moreover, disruption of the YFH1 gene resulted in multiple Fe-S-dependent enzyme deficiencies in yeast. The deficiency of Fe-S-dependent enzyme activities in both FRDA patients and yeast should be related to mitochondrial iron accumulation, especially as Fe-S proteins are remarkably sensitive to free radicals. Mutated frataxin triggers aconitase and mitochondrial Fe-S respiratory enzyme deficiency in FRDA, which should therefore be regarded as a mitochondrial disorder.
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                Author and article information

                Journal
                Int J Mol Sci
                Int J Mol Sci
                ijms
                International Journal of Molecular Sciences
                MDPI
                1422-0067
                07 July 2018
                July 2018
                : 19
                : 7
                : 1989
                Affiliations
                [1 ]Unité de Biologie Fonctionnelle et Adaptative (BFA), Sorbonne Paris Cité, Université Paris Diderot, UMR8251 CNRS, 75013 Paris, France
                [2 ]Department of Genetics, University of Valencia, Campus of Burjassot, 96100 Valencia, Spain
                [3 ]Lehrstuhl für Entwicklungsbiologie, Universität Regensburg, 93040 Regensburg, Germany
                Author notes
                [* ]Correspondence: veronique.monnier@ 123456univ-paris-diderot.fr (V.M.); J.Vicente.Llorens@ 123456uv.es (J.V.L.); juan.navarro@ 123456ur.de (J.A.N.); Tel.: +49-941-943-3083 (J.A.N.)
                [†]

                These authors contributed equally to this work.

                Author information
                https://orcid.org/0000-0001-9823-6246
                https://orcid.org/0000-0003-2266-728X
                Article
                ijms-19-01989
                10.3390/ijms19071989
                6073496
                29986523
                0dfd3a02-a519-40ca-8987-83c29704a11f
                © 2018 by the authors.

                Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/).

                History
                : 28 May 2018
                : 03 July 2018
                Categories
                Review

                Molecular biology
                drosophila melanogaster,friedreich’s ataxia,frataxin,iron,oxidative stress,metal homeostasis,lipid metabolism,endoplasmic reticulum,genetic screens,drug screens

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