Adaptor protein complex 4 deficiency: a paradigm of childhood-onset hereditary spastic paraplegia caused by defective protein trafficking.

Abstract

Deficiency of the adaptor protein complex 4 (AP-4) leads to childhood-onset hereditary spastic paraplegia (AP-4-HSP): SPG47 (AP4B1), SPG50 (AP4M1), SPG51 (AP4E1) and SPG52 (AP4S1). This study aims to evaluate the impact of loss-of-function variants in AP-4 subunits on intracellular protein trafficking using patient-derived cells. We investigated 15 patient-derived fibroblast lines and generated six lines of induced pluripotent stem cell (iPSC)-derived neurons covering a wide range of AP-4 variants. All patient-derived fibroblasts showed reduced levels of the AP4E1 subunit, a surrogate for levels of the AP-4 complex. The autophagy protein ATG9A accumulated in the trans-Golgi network and was depleted from peripheral compartments. Western blot analysis demonstrated a 3-5-fold increase in ATG9A expression in patient lines. ATG9A was redistributed upon re-expression of AP4B1 arguing that mistrafficking of ATG9A is AP-4-dependent. Examining the downstream effects of ATG9A mislocalization, we found that autophagic flux was intact in patient-derived fibroblasts both under nutrient-rich conditions and when autophagy is stimulated. Mitochondrial metabolism and intracellular iron content remained unchanged. In iPSC-derived cortical neurons from patients with AP4B1-associated SPG47, AP-4 subunit levels were reduced while ATG9A accumulated in the trans-Golgi network. Levels of the autophagy marker LC3-II were reduced, suggesting a neuron-specific alteration in autophagosome turnover. Neurite outgrowth and branching were reduced in AP-4-HSP neurons pointing to a role of AP-4-mediated protein trafficking in neuronal development. Collectively, our results establish ATG9A mislocalization as a key marker of AP-4 deficiency in patient-derived cells, including the first human neuron model of AP-4-HSP, which will aid diagnostic and therapeutic studies.

• Authors: Robert Behne, Julian Teinert, Miriam Wimmer, Angelica D'Amore, Alexandra K Davies, Joseph M Scarrott, Kathrin Eberhardt, Barbara Brechmann, Ivy Pin-Fang Chen, Elizabeth D Buttermore, Lee Barrett, Sean Dwyer, Teresa Chen, Jennifer Hirst, Antje Wiesener, Devorah Segal, Andrea Martinuzzi, Sofia T Duarte, James T Bennett, Thomas Bourinaris, Henry Houlden, Agathe Roubertie, Filippo M Santorelli, Margaret Robinson, Mimoun Azzouz, Jonathan O Lipton, Georg H H Borner, Mustafa Sahin, Darius Ebrahimi-Fakhari
• Journal: Human molecular genetics (Hum Mol Genet) Vol. 29 Issue 2 Pg. 320-334 (01 15 2020) ISSN: 1460-2083 [Electronic] England
• PMID: 31915823 (Publication Type: Journal Article, Research Support, N.I.H., Extramural, Research Support, Non-U.S. Gov't)
• Copyright: © The Author(s) 2020. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected].

Chemical References

• AP4B1 protein, human
• Adaptor Protein Complex 4
• Adaptor Protein Complex beta Subunits
• Atg9a protein, human
• Autophagy-Related Proteins
• MAP1LC3B protein, human
• Membrane Proteins
• Microtubule-Associated Proteins
• Vesicular Transport Proteins
• Iron

Topics

• Adaptor Protein Complex 4 (deficiency, genetics, metabolism)
• Adaptor Protein Complex beta Subunits (metabolism)
• Adolescent
• Autophagosomes (metabolism)
• Autophagy (genetics)
• Autophagy-Related Proteins (metabolism)
• Cell Line
• Child
• Child, Preschool
• Female
• Fibroblasts (metabolism)
• Humans
• Induced Pluripotent Stem Cells (metabolism)
• Iron (metabolism)
• Loss of Function Mutation
• Male
• Membrane Proteins (metabolism)
• Microtubule-Associated Proteins (metabolism)
• Mitochondria (metabolism)
• Neurogenesis (genetics)
• Neurons (metabolism)
• Protein Transport (genetics)
• Spastic Paraplegia, Hereditary (genetics, metabolism)
• Vesicular Transport Proteins (metabolism)
• trans-Golgi Network (genetics, metabolism)