CONTEXT: Blood-based analytes may be indicators of pathological processes in Alzheimer disease (AD). OBJECTIVE: To identify plasma proteins associated with AD pathology using a combined proteomic and neuroimaging approach.
DESIGN: Discovery-phase proteomics to identify plasma proteins associated with correlates of AD pathology. Confirmation and validation using immunodetection in a replication set and an animal model.
SETTING: A multicenter European study (AddNeuroMed) and the Baltimore Longitudinal Study of Aging.
PARTICIPANTS: Patients with AD, subjects with mild cognitive impairment, and healthy controls with standardized clinical assessments and structural neuroimaging.
MAIN OUTCOME MEASURES: Association of plasma proteins with brain atrophy, disease severity, and rate of clinical progression. Extension studies in humans and transgenic mice tested the association between plasma proteins and brain amyloid. RESULTS: Clusterin/apolipoprotein J was associated with atrophy of the entorhinal cortex, baseline disease severity, and rapid clinical progression in AD. Increased plasma concentration of clusterin was predictive of greater fibrillar amyloid-beta burden in the medial temporal lobe. Subjects with AD had increased clusterin messenger RNA in blood, but there was no effect of single-nucleotide polymorphisms in the gene encoding clusterin with gene or protein expression. APP/PS1 transgenic mice showed increased plasma clusterin, age-dependent increase in brain clusterin, as well as amyloid and clusterin colocalization in plaques. CONCLUSIONS: These results demonstrate an important role of clusterin in the pathogenesis of AD and suggest that alterations in amyloid chaperone proteins may be a biologically relevant peripheral signature of AD.

Thambisetty M, Simmons A, Velayudhan L, et al. Association of plasma clusterin concentration with severity, pathology, and progression in Alzheimer disease. Arch Gen Psychiatry. 2010;67(7):739-48.

Studies have shown that clusterin (also called apolipoprotein J) can influence the structure and toxicity of amyloid-β (Aβ) in vitro. To determine whether endogenous clusterin plays a role in influencing Aβ deposition, structure, and toxicity in vivo, we bred PDAPP mice, a transgenic mouse model of Alzheimer's disease, to clusterin−/− mice. By 12 months of age, PDAPP, clusterin−/− mice had similar levels of brain Aβ deposition as did PDAPP, clusterin+/+ mice. Although Aβ deposition was similar, PDAPP, clusterin−/− mice had significantly fewer fibrillar Aβ (amyloid) deposits than PDAPP mice expressing clusterin. In the absence of clusterin, neuritic dystrophy associated with the deposited amyloid was markedly reduced, resulting in a dissociation between fibrillar amyloid formation and neuritic dystrophy. These findings demonstrate that clusterin markedly influences Aβ structure and neuritic toxicity in vivo and is likely to play an important role in Alzheimer's disease pathogenesis.

Demattos RB, O'dell MA, Parsadanian M, et al. Clusterin promotes amyloid plaque formation and is critical for neuritic toxicity in a mouse model of Alzheimer's disease. Proc Natl Acad Sci USA. 2002;99(16):10843-8.