The cholesterol-synthesizing enzyme seladin-1, encoded by the Dhcr24 gene, is a flavin adenine dinucleotidedependent oxidoreductase and regulates responses to oncogenic and oxidative stimuli. It has a role in neuroprotection and is downregulated in affected neurons in Alzheimer's disease (AD). Here we show that seladin-1-deficient mouse brains had reduced levels of cholesterol and disorganized cholesterol-rich detergent-resistant membrane domains (DRMs). This was associated with inefficient plasminogen binding and plasmin activation, the displacement of b-secretase (BACE) from DRMs to APP-containing membrane fractions, increased b-cleavage of APP and high levels of Ab peptides. In contrast, overexpression of seladin-1 increased both cholesterol and the recruitment of DRM components into DRM fractions, induced plasmin activation and reduced both BACE processing of APP and Ab formation. These results establish a role of seladin-1 in the formation of DRMs and suggest that seladin-1-dependent cholesterol synthesis is involved in lowering Ab levels. Pharmacological enhancement of seladin-1 activity may be a novel Ab-lowering approach for the treatment of AD.

DHCR24/seladin-1, a crucial enzyme in sterol synthesis, is of lower abundance in brain areas affected by Alzheimer's disease. While high levels of DHCR24/seladin-1 exert antiapoptotic function by conferring resistance against oxidative stress, the molecular mechanism for this protective effect is not fully understood. Here we show that DHCR24/seladin-1 expression is up-regulated in an acute response and down-regulated in a chronic response to oxidative stress. High levels of DHCR24/seladin-1 were associated with elevated cholesterol concentrations and a general increase in cholesterol biosynthesis upon oxidative stress exposure in neuroblastoma SH-SY5Y cells. DHCR24/seladin-1 overexpression conferred resistance to oxidative stress in a cholesterol-dependent manner. Mutating the reductase activity within DHCR24/seladin-1 abolished this protective effect. Conversely, DHCR24/seladin-1 levels diminished upon chronic exposure to oxidative stress. Low levels of DHCR24/seladin-1 were associated with reduced p53 levels, independent of DHCR24 activity and cholesterol concentrations. Additionally, ablation of DHCR24/seladin-1 prevented apoptosis of primary neurons in a p53-dependent manner and reduced the response of critical p53 targets due to deficient stabilization of p53 and therefore elevated p53 ubiquitination and degradation. Our findings reveal a dual capacity of DHCR24/seladin-1, which appears to be involved in two mechanistically independent prosurvival effects, exerting an acute response and a chronic response to oxidative stress.The 3␤-hydroxysterol-⌬24 reductase (DHCR24) is a broadly expressed oxidoreductase, sharing homologies with a family of flavin adenine dinucleotide-dependent reductases (26). The dhcr24 gene is the human orthologue of the diminuto/dwarf1, initially identified in plants, where it is required for the synthesis of brassinosteroids, a group of plant sterols that are essential for normal growth and development (4,18,23). In mammals, DHCR24 plays an indispensable role in cholesterol biosynthesis, catalyzing the conversion of desmosterol to cholesterol (6, 26, 27). However, DHCR24 was also described in a different context: dhcr24 expression was shown to be downregulated in brain areas affected by Alzheimer's disease (11) and was therefore named seladin-1, the selective Alzheimer's disease indicator 1, suggesting an association of DHCR24/ seladin-1 with the selective vulnerability of neurons in the affected brain areas. Conversely, high levels of DHCR24/seladin-1 exert protective functions, conferring resistance against oxidative stress and protecting cells from apoptotic cell death (2,9,11,16). Endogenous DHCR24/seladin-1 levels are highly up-regulated upon acute oxidative stress (3, 28), while expression declines to very low levels upon chronic exposure (3), suggesting that DHCR24/seladin-1 plays a role in integrating cellular responses to oxidative stress. However, the precise molecular mechanism for this protective effect is not known. Intriguingly, recent findings identified an interaction...

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The accumulation of the β-amyloid peptide (Aβ) is a central event in the pathogenesis of Alzheimer’s disease (AD). Aβ removal from the brain by immune therapy shows promising potential for the treatment of patients with AD, although the mechanisms of the antibody action are incompletely understood. In this study we compared the biological activities of antibodies raised against various Aβ fragments for Aβ reduction in vitro and in vivo. Antibodies against Aβ enhanced the uptake of Aβ₄₂ aggregates up to 6-fold by primary microglial cells in vitro. The kinetics of Aβ₄₂ uptake varied considerably among antibodies. Based on the activity to mediate Aβ₄₂ uptake by microglial cells, we identified a bioactive antibody that significantly reduced Aβ₄₂ levels in the brains of transgenic mice with neuronal expression of an AD-related mutated amyloid precursor protein. This effect depended on the epitopes recognized by the antibody. Our data suggest that the ability to facilitate Aβ₄₂ uptake by primary microglia cells in vitro can be used to predict the biological activity of the antibody by passive immunization in vivo. This protocol may prove useful for the rapid validation of the activity of antibodies designed to be used in immune therapy of AD.

Accumulation in brain of the b-amyloid peptide (Ab) is considered as crucial pathogenic event causing AlzheimerÕs disease (AD). Anti-Ab immune therapy is a powerful means for Ab clearance from the brain. We recently showed that intravenous injections of anti-Ab antibodies led to reduction, elevation or no change in brain Ab 42 concentrations of an AD mouse model. We report here, in a second passive immunization protocol, a different bioactivity of same antibodies to alter brain Ab 42 concentrations. Comparing the bioactivity of anti-Ab antibodies in these two passive immunization paradigms underscores the potential of immune therapy for AD treatment and suggests that both the epitope recognized by the antibody and the mode of antibody administration are crucial for its biological activity.