2008 Articles

Distinct Pools of ␤-Amyloid in Alzheimer Disease–Affected Brain

Steinerman, Joshua R; Irizarry, Michael; Scarmeas, Nikolaos; Raju, Susan; Brandt, Jason; Albert, Marilyn; Blacker, Deborah; Hyman, Bradley; Stern, Yaakov

Objective: To determine whether β-amyloid (Aβ) peptides segregated into distinct biochemical compartments would differentially correlate with clinical severity of Alzheimer disease (AD).

Design: Clinicopathologic correlation study.

Participants: Twenty-seven patients from a longitudinal study of AD and 13 age- and sex-matched controls without a known history of cognitive impairment or dementia were included in this study.

Interventions: Temporal and cingulate neocortex were processed using a 4-step extraction, yielding biochemical fractions that are hypothesized to be enriched with proteins from distinct anatomical compartments: TRIS (extracellular soluble), Triton (intracellular soluble), sodium dodecyl sulfate (SDS) (membrane associated), and formic acid (extracellular insoluble). Levels of Aβ40 and Aβ42 were quantified in each biochemical compartment by enzyme-linked immunosorbent assay.

Results: The Aβ42 level in all biochemical compartments was significantly elevated in patients with AD vs controls (P < .01). The Aβ40 levels in the TRIS and formic acid fractions were elevated in patients with AD (temporal, P < .01; cingulate, P = .03); however, Triton and SDS Aβ40 levels were similar in patients with AD and in controls. Functional impairment proximal to death correlated with Triton Aβ42 (r = 0.48, P = .02) and SDS Aβ42 (r = 0.41, P = .04) in the temporal cortex. Faster cognitive decline was associated with elevated temporal SDS Aβ42 levels (P < .001), whereas slower decline was associated with elevated cingulate formic acid Aβ42 and SDS Aβ42 levels (P = .02 and P = .01, respectively).

Conclusion: Intracellular and membrane-associated Aβ, especially Aβ42 in the temporal neocortex, may be more closely related to AD symptoms than other measured Aβ species.

A critical role of the β-amyloid (Aβ) peptide in the pathogenesis of Alzheimer disease (AD) has been supported by human, animal, and in vitro studies.1 Most measures of Aβ are markedly elevated in the AD-affected brain, yet the extent of total Aβ accumulation tends to correlate poorly with ADseverity.2-4 Because there is evidence that specific biochemical forms of Aβ (eg, Aβ42, soluble Aβ, and oligomeric Aβ) selectively lead to neuronal dysfunction and neurodegeneration5-7 and can be more reliable correlates of clinical status,8,9 identification and reliable measurement of these toxic Aβ species should enhance their utility as biological markers of disease.

Clarifying the dynamics of Aβ production and compartmentalization is also necessary to explain AD pathogenesis. Specific Aβ species may preferentially exert toxic effects as a function of their cellular location. Although established histologic techniques identify primarily insoluble extracellular and vascular amyloid deposits, novel methods can enhance detection of intraneuronal Aβ, distinguish Aβ pools, and measure changes in Aβ concentration and location over time.10-14

The Aβ in the brain can be segregated into distinct biochemical compartments defined by sequential extraction procedures. In this study of brain autopsy samples from a well-characterized longitudinal cohort of patients with AD and matched controls, we quantified Aβ40 and Aβ42 in biochemical compartments defined by their solubility in 4 solutions. Proteins in these biochemical pools are predicted to derive from distinct anatomical compartments within the cerebral cortex: extracellular soluble, intracellular, membrane associated, and extracellular insoluble.7 We hypothesized that these measures would differentially correlate with disease diagnosis, progression, and severity.