Functional Depletion of Mahogunin by Cytosolically Exposed Prion Protein Contributes to Neurodegeneration

ArticleinCell 137(6):1136-47 · July 2009with17 Reads
DOI: 10.1016/j.cell.2009.03.042 · Source: PubMed
Abstract
The pathways leading from aberrant Prion protein (PrP) metabolism to neurodegeneration are poorly understood. Some familial PrP mutants generate increased (Ctm)PrP, a transmembrane isoform associated with disease. In other disease situations, a potentially toxic cytosolic form (termed cyPrP) might be produced. However, the mechanisms by which (Ctm)PrP or cyPrP cause selective neuronal dysfunction are unknown. Here, we show that both (Ctm)PrP and cyPrP can interact with and disrupt the function of Mahogunin (Mgrn), a cytosolic ubiquitin ligase whose loss causes spongiform neurodegeneration. Cultured cells and transgenic mice expressing either (Ctm)PrP-producing mutants or cyPrP partially phenocopy Mgrn depletion, displaying aberrant lysosomal morphology and loss of Mgrn in selected brain regions. These effects were rescued by either Mgrn overexpression, competition for PrP-binding sites, or prevention of cytosolic PrP exposure. Thus, transient or partial exposure of PrP to the cytosol leads to inappropriate Mgrn sequestration that contributes to neuronal dysfunction and disease.
    • "*, p b 0.05 compared with (pcDNA or Scrambled siRNA) expressed cells. interacts with cytosolic prion proteins (PrPs) (Chakrabarti and Hegde, 2009; He et al., 2003). Melanocortin receptors (MCRs) are also ubiquitinated as substrates of MGRN1 and are regulated by MGRN1 (Cooray et al., 2011 ). "
    [Show abstract] [Hide abstract] ABSTRACT: Proteotoxicity of misfolded, disease-causing proteins is deeply implicated in the pathomechanisms for neurodegenerative diseases including copper-zinc superoxide dismutase (SOD1)-linked amyotrophic lateral sclerosis (ALS). However, the precise cellular quality control (QC) mechanisms against aggregation of misfolded mutant SOD1 proteins remain elusive. Here, we found that the Mahogunin ring finger-1 (MGRN1) E3 ubiquitin ligase, which catalyzes mono-ubiquitination to the substrate, was dysregulated in the cellular and mouse models of ALS and that it preferentially interacted with various mutant forms of SOD1. Intriguingly, the motor neurons of presymptomatic ALS mice have diminished MGRN1 cytoplasmic distribution. MGRN1 was partially recruited to mutant SOD1 inclusions where they were positive for p62 and Lamp2. Moreover, overexpression of MGRN1 reduced mutant SOD1 aggregation and alleviated its proteotoxic effects on cells. Taken together, our findings suggest that MGRN1 contributes to the clearance of toxic mutant SOD1 inclusions likely through autophagic pathway, and, most likely, the sequestration of MGRN1 sensitizes motor neurons to degeneration in the ALS mouse model. Furthermore, the present study identifies the MGRN1-mediated protein QC mechanism as a novel therapeutic target in neurodegenerative diseases.
    Article · Nov 2015
    • "It has been demonstrated that the mislocalized PrP may bind to and aggregate intracellular proteins which are not its physiological partners [rev. in [11]]. CytoPrP interacts with cytosolic proteins such as tubulin, Bcl-2 and ubiquitin ligase mahogunin, leading to loss of their functions121314 . These excessive or non-physiological interactions may underlay the molecular mechanism of cytoPrP neurotoxicity in TSE [rev. "
    [Show abstract] [Hide abstract] ABSTRACT: Prion protein (PrP) mislocalized in the cytosol has been presumed to be the toxic entity responsible for the neurodegenerative process in transmissible spongiform encephalopathies (TSE), also called prion diseases. The mechanism underlying the neurotoxicity of cytosolic PrP (cytoPrP) remains, however, unresolved. In this study we analyze toxic effects of the cell-penetrating PrP fragment, PrP1-30 - encompassing residues responsible for binding and aggregation of tubulin. We have found that intracellularly localized PrP1-30 disassembles microtubular cytoskeleton of primary neurons, which leads to the loss of neurites and, eventually, necrotic cell death. Accordingly, stabilization of microtubules by taxol reduced deleterious effects of cytosolic PrP1-30. Furthermore, we have demonstrated that decreased phosphorylation level of microtubule-associated proteins (MAPs), which also increases stability of microtubular cytoskeleton protects neurons from the toxic effects of PrP1-30. CHIR98014 and LiCl - inhibitors of glycogen synthase kinase 3 (GSK-3), a major kinase responsible for phosphorylation of MAPs, inhibited PrP1-30-induced disruption of microtubular cytoskeleton and increased viability of peptide-treated neurons. We have also shown that the N-terminal fragment of cytoPrP may cause the loss of dendritic spines. PrP1-30-induced changes at the level of spines have also been prevented by stabilization of microtubules by taxol as well as LiCl. These observations indicate that the neurotoxicty of cytoPrP is tightly linked to the disruption of microtubular cytoskeleton. Importantly, this study implies that lithium, the commonly used mood stabilizer, may be a promising therapeutic agent in TSE, particularly in case of the disease forms associated with accumulation of cytoPrP. Copyright © 2015. Published by Elsevier B.V.
    Article · Jul 2015
    • "CD59 was detected using rabbit polyclonal anti-CD59 antibodies (Abcam, #ab69084). Plasmids expressing HA-tagged PrP proteins were previously described (Hegde et al., 1998; Rane et al., 2004; Chakrabarti and Hegde, 2009 ). Transiently expressed PrP proteins were probed using mouse monoclonal anti-HA antibodies (Covance/BioLegend, #MMS-101P). "
    [Show abstract] [Hide abstract] ABSTRACT: Glycophosphatidylinositol-anchored proteins (GPI-APs) play essential roles in physiology, but their biogenesis and trafficking have not been systematically characterized. Here, we took advantage of the recently available haploid genetics approach to dissect GPI-AP pathways in human cells using prion protein (PrP) and CD59 as model molecules. Our screens recovered a large number of common and unexpectedly specialized factors in the GPI-AP pathways. PIGN, PGAP2, and PIGF, which encode GPI anchor-modifying enzymes, were selectively isolated in the CD59 screen, suggesting that GPI anchor composition significantly influences the biogenesis of GPI-APs in a substrate-dependent manner. SEC62 and SEC63, which encode components of the ER-targeting machinery, were selectively recovered in the PrP screen, indicating that they do not constitute a universal route for the biogenesis of mammalian GPI-APs. Together, these comparative haploid genetic screens demonstrate that, despite their similarity in overall architecture and subcellular localization, GPI-APs follow markedly distinct biosynthetic and trafficking pathways. Copyright © 2015 The Authors. Published by Elsevier Inc. All rights reserved.
    Full-text · Article · Jun 2015
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