title: GRAF1a is a brain-specific protein that promotes lipid droplet clustering and growth, and is enriched at lipid droplet junctions.
abstract: Lipid droplets are found in all cell types. Normally present at low levels in the brain, they accumulate in tumours and are associated with neurodegenerative diseases. However, little is known about the mechanisms controlling their homeostasis in the brain. We found that GRAF1a, the longest GRAF1 isoform (GRAF1 is also known as ARHGAP26), was enriched in the brains of neonates. Endogenous GRAF1a was found on lipid droplets in oleic-acid-fed primary glial cells. Exclusive localization required a GRAF1a-specific hydrophobic segment and two membrane-binding regions, a BAR and a PH domain. Overexpression of GRAF1a promoted lipid droplet clustering, inhibited droplet mobility and severely perturbed lipolysis following the chase of cells overloaded with fatty acids. Under these conditions, GRAF1a concentrated at the interface between lipid droplets. Although GRAF1-knockout mice did not show any gross abnormal phenotype, the total lipid droplet volume that accumulated in GRAF1(-/-) primary glia upon incubation with fatty acids was reduced compared to GRAF1(+/+) cells. These results provide additional insights into the mechanisms contributing to lipid droplet growth in non-adipocyte cells, and suggest that proteins with membrane sculpting BAR domains play a role in droplet homeostasis.
authors.affiliation: MRC Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge CB2 0QH, UK slucken@mrc-lmb.cam.ac.uk hmm@mrc-lmb.cam.ac.uk.
authors.forename: Safa
authors.initials: S
authors.lastname: Lucken-Ardjomande Häsler
authors.affiliation: MRC Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge CB2 0QH, UK.
authors.forename: Yvonne
authors.initials: Y
authors.lastname: Vallis
authors.affiliation: MRC Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge CB2 0QH, UK.
authors.forename: Helen E
authors.initials: HE
authors.lastname: Jolin
authors.affiliation: MRC Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge CB2 0QH, UK.
authors.forename: Andrew N
authors.initials: AN
authors.lastname: McKenzie
authors.affiliation: MRC Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge CB2 0QH, UK slucken@mrc-lmb.cam.ac.uk hmm@mrc-lmb.cam.ac.uk.
authors.forename: Harvey T
authors.initials: HT
authors.lastname: McMahon
mesh_terms: D000818:Animals; D015153:Blotting, Western; D001921:Brain / Q000378:metabolism*; D002254:Carbonates / Q000494:pharmacology; D002458:Cell Fractionation; D002478:Cells, Cultured; D020690:GTPase-Activating Proteins / Q000235:genetics / Q000378:metabolism*; D006367:HeLa Cells; D006801:Humans; D051379:Mice; D008817:Mice, Mutant Strains; D009457:Neuroglia / Q000187:drug effects / Q000378:metabolism
chemical_list: D002254:Carbonates; C000593136:GRAF1 protein, mouse; D020690:GTPase-Activating Proteins; C005686:sodium carbonate
keywords: ARHGAP26; BAR protein; GRAF1; Lipid droplet