Ab in Lens in Down Syndrome Figure 7. Ab potentiates lens protein aggregation and Rayleigh light scattering in vitro. (A) Brightfield photomicroscopic image of a Congo red-stained protein aggregate (arrow) formed during incubation of human lens protein extract with synthetic human Ab. (B) Same Congo red- stained lens protein aggregate demonstrating classical amyloid birefringence under cross-polarized illumination. (C) Anti-Ab/anti-aB-crystallin double immunogold electron microscopic analysis of protein aggregates formed during incubation of human lens protein with synthetic human Ab. Larger immunogold particles (15 nm diameter) detect Ab. Smaller immunogold particles (10 nm diameter) detect aB-crystallin. Box indicates region of detail highlighted in Fig. 7D. Scale bar = 200 nm. Inset, hetero-oligomeric composition noted in a single protein aggregate demonstrated by double immunogold (anti-Ab and anti-aB-crystallin) electron microscopy. Analysis was performed on human lens protein incubated with synthetic human Ab. Larger immunogold particles (15 nm diameter) detect Ab (black arrow). Smaller immunogold particles (10 nm diameter) detect aB-crystallin (white arrow). Scale bar = 50 nm. (D) Quasi-elastic light scattering (QLS) analysis detects increased backscattered light due to concentration-and time- dependent protein aggregation during incubation of human lens protein with synthetic human Ab. Experimental samples studied by QLS were analyzed by amyloid histochemistry (Fig. 7A,B) and double (anti-Ab and anti-aB-crystallin) immunogold electron microscopy (Fig. 7C). doi:10.1371/journal.pone.0010659.g007 The DS-associated cataract phenotype is not observed in other non-AD neurodegenerative diseases, nor in normal aged controls. In this context, it is important to note that DS-associated cataracts are phenotypically distinct from age-related cataracts (ARC) that commonly emerge starting in the fifth decade of life and typically localize in the central nuclear region. In contrast to the lens pathology in ARC, the DS lens phenotype is often clinically detectable early in life, and in some cases, may be present at birth [20,22,23,24,26,43,44]. The distinctive lens phenotype associated with DS is further distinguished from ARC by supranuclear Ab histopathology, a pathological feature shared with late-onset AD [28]. The results of the present study support a DS-associated pathogenic pathway linking progressive age-dependent Ab accumulation in the lens and supranuclear cataractogenesis with corresponding cerebral Ab accumulation and neuropathology in the brain. We propose the following mechanistic pathway leading to expression of the charac- teristic lens and brain phenotypes associated with DS (Fig. 9). Increased APP gene dosage resulting from chromosome 21 triplication (APP, 21q21) leads to increased expression of the amyloid precursor protein (APP) and elevated levels of the pathogenic Ab cleavage peptides in affected tissues. In the brain, Ab accumulation leads to early-onset neuropathology and progressive age-dependent neurocog- PLoS ONE | www.plosone.org 10 May 2010 | Volume 5 | Issue 5 | e10659