Association between frontal cortex oxidative damage and beta-amyloid as a function of age in Down syndrome

Abstract

Down syndrome (DS) is the most common genetic cause of intellectual disability in children, and the number of adults with DS reaching old age is increasing. By the age of 40 years, virtually all people with DS have sufficient neuropathology for a postmortem diagnosis of Alzheimer disease (AD). Trisomy 21 in DS leads to an overexpression of many proteins, of which at least two are involved in oxidative stress and AD: superoxide dismutase 1 (SOD1) and amyloid precursor protein (APP). In this study, we tested the hypothesis that DS brains with neuropathological hallmarks of AD have more oxidative and nitrosative stress than those with DS but without significant AD pathology, as compared with similarly aged-matched non-DS controls. The frontal cortex was examined in 70 autopsy cases (n=29 control and n=41 DS). By ELISA, we quantified soluble and insoluble Aβ40 and Aβ42, as well as oligomers. Oxidative and nitrosative stress levels (protein carbonyls, 4-hydroxy-2-trans-nonenal (HNE)-bound proteins, and 3-nitrotyrosine) were measured by slot-blot. We found that soluble and insoluble amyloid beta peptide (Aβ) and oligomers increase as a function of age in DS frontal cortex. Of the oxidative stress markers, HNE-bound proteins were increased overall in DS. Protein carbonyls were correlated with Aβ40 levels. These results suggest that oxidative damage, but not nitrosative stress, may contribute to the onset and progression of AD pathogenesis in DS. Conceivably, treatment with antioxidants may provide a point of intervention to slow pathological alterations in DS.

Figure 1

Levels of oxidative and nitrosative stress markers in control, DS, and DS with AD cases. Protein was extracted from frontal cortex and loaded on nitrocellulose membranes in a slot-blot apparatus. Membranes were probed with either anti-DNP protein adducts polyclonal antibody (Figure 1A), anti-nitrotyrosine polyclonal antibody (Figure 1B), or anti-HNE-bound protein polyclonal antibody (Figure 1C). Data are expressed as mean ± SEM (*p=0.02).

Figure 2

Soluble and insoluble Aβ40 and Aβ42 as a function of genotype and age group. PBS-, SDS-, and FA-extracted Aβ40 increased with age in both DS and controls (A, B, C). SDS- and FA-extracted Aβ40 were also significantly higher in DS (B, C) and highest in DS with AD. No genotype or age effects were noted for PBS-extracted Aβ42 (D). SDS-extracted Aβ42 was higher overall with age, with DS and DS with AD cases showing the highest levels overall (E). FA-extracted Aβ42 was higher in older cases with or without DS (F). Bars represent mean ± SEM. Closed circles indicate individual data points.

Figure 3

Correlations between age and Aβ as a function of genotype. PBS-, SDS-, and FA-extracted Aβ40 increased with age in DS (A, B, C). However, in control cases, only SDS-extracted Aβ40 increased with age. SDS- and FA-extracted Aβ40 also increased with age in DS (E,F), although but PBS-extracted Aβ42 did not (D). As with SDS-extracted Aβ40, Aβ42 increased with age in control cases, although levels were not as high as in those with DS (E).

Figure 4

Aβ Oligomer accumulation with age and DS and correlations between oxidative damage and Aβ. Oligomer accumulation was significantly higher in DS cases over the age of 40 years with significant AD neuropathology (A). Further, oligomers increased as a function of age in DS but not in control cases (B). Higher levels of SDS-extracted Aβ40 (D) and FA-extracted Aβ40 (E) were significantly correlated with higher levels of PCs. A similar trend was observed between PBS-extracted Aβ40 and PCs (C). Bars represent mean ± SEM. Closed circles indicate individual data points.