title: Down-regulation of seladin-1 increases BACE1 levels and activity through enhanced GGA3 depletion during apoptosis.
abstract: Seladin-1 is a neuroprotective protein selectively down-regulated in brain regions affected in Alzheimer disease (AD). Seladin-1 protects cells against beta-amyloid (Abeta) peptide 42- and oxidative stress-induced apoptosis activated by caspase-3, a key mediator of apoptosis. Here, we have employed RNA interference to assess the molecular effects of seladin-1 down-regulation on the beta-secretase (BACE1) function and beta-amyloid precursor protein (APP) processing in SH-SY5Y human neuroblastoma cells in both normal and apoptotic conditions. Our results show that approximately 60% reduction in seladin-1 protein levels, resembling the decrease observed in AD brain, did not significantly affect APP processing or Abeta secretion in normal growth conditions. However, under apoptosis, seladin-1 small interfering RNA (siRNA)-transfected cells showed increased caspase-3 activity on average by 2-fold when compared with control siRNA-transfected cells. Increased caspase-3 activity coincided with a significant depletion of the BACE1-sorting protein, GGA3 (Golgi-localized gamma-ear-containing ADP-ribosylation factor-binding protein), and subsequently augmented BACE1 protein levels and activity. Augmented BACE1 activity in turn correlated with the enhanced beta-amyloidogenic processing of APP and ultimately increased Abeta production. These adverse changes associated with decreased cell viability in seladin-1 siRNA-transfected cells under apoptosis. No changes in GGA3 or BACE1 levels were found after seladin-1 knockdown in normal growth conditions. Collectively, our results suggest that under stress conditions, reduced seladin-1 expression results in enhanced GGA3 depletion, which further leads to augmented post-translational stabilization of BACE1 and increased beta-amyloidogenic processing of APP. These mechanistic findings related to seladin-1 down-regulation are important in the context of AD as the oxidative stress-induced apoptosis plays a key role in the disease pathogenesis.
authors.affiliation: Department of Neurology, University of Kuopio and Kuopio University Hospital, FIN-70211 Kuopio, Finland.
authors.forename: Timo
authors.initials: T
authors.lastname: Sarajärvi
authors.forename: Annakaisa
authors.initials: A
authors.lastname: Haapasalo
authors.forename: Jayashree
authors.initials: J
authors.lastname: Viswanathan
authors.forename: Petra
authors.initials: P
authors.lastname: Mäkinen
authors.forename: Marjo
authors.initials: M
authors.lastname: Laitinen
authors.forename: Hilkka
authors.initials: H
authors.lastname: Soininen
authors.forename: Mikko
authors.initials: M
authors.lastname: Hiltunen
mesh_terms: D033942:Adaptor Proteins, Vesicular Transport / Q000378:metabolism*; D053829:Amyloid Precursor Protein Secretases / Q000096:biosynthesis*; D016229:Amyloid beta-Peptides / Q000737:chemistry; D017209:Apoptosis; D016282:Aspartic Acid Endopeptidases / Q000096:biosynthesis*; D045744:Cell Line, Tumor; D017077:Culture Media, Conditioned / Q000378:metabolism; D015536:Down-Regulation; D005786:Gene Expression Regulation; D006801:Humans; D018613:Microscopy, Confocal / Q000379:methods; D008954:Models, Biological; D009419:Nerve Tissue Proteins / Q000096:biosynthesis*; D018384:Oxidative Stress; D044925:Oxidoreductases Acting on CH-CH Group Donors / Q000096:biosynthesis*; D017434:Protein Structure, Tertiary; D014162:Transfection
chemical_list: D033942:Adaptor Proteins, Vesicular Transport; D016229:Amyloid beta-Peptides; D017077:Culture Media, Conditioned; C407306:GGA adaptor proteins; D009419:Nerve Tissue Proteins; D044925:Oxidoreductases Acting on CH-CH Group Donors; C499055:DHCR24 protein, human; D053829:Amyloid Precursor Protein Secretases; D016282:Aspartic Acid Endopeptidases; C509280:BACE1 protein, human