Targeting intracellular A?42 for Alzheimer's disease drug discovery

Advances in blood-based protein biomarkers for Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder that accounts for the majority of dementia cases. While research over the past decades has made advances into understanding disease pathology, definite AD diagnosis currently relies on confirmation by autopsy. The anticipated dramatic rise in affected individuals over the next decades necessitates the development of diagnostic tests applicable to living individuals, which depends on identification of disease biomarkers. Diagnostics based on blood protein biomarkers are particularly desired since these would allow for economical, rapid and non-invasive analysis of individual biomarker profiles. Research is actively ongoing in this field and has led to the identification of autoantibodies and various proteins in the blood that may represent a disease-specific blood signature of AD. This review provides an overview on the progress in the field of identification of AD-specific blood protein biomarkers.

Morphologically distinct types of amyloid plaques point the way to a better understanding of Alzheimer's disease pathogenesis

The details of the sequence of pathological events leading to neuron death in Alzheimer's disease (AD) are not known. Even the formation of amyloid plaques, one of the major histopathological hallmarks of AD, is not clearly understood; both the origin of the amyloid and the means of its deposition remain unclear. It is still widely considered, however, that amyloid plaques undergo gradual growth in the interstitial space of the brain via continual extracellular deposition of amyloid beta peptides at "seeding sites," and that these growing plaques encroach progressively on neurons and their axons and dendritic processes, eventually leading to neuronal death. Actually, histopathological evidence to support this mechanism is sparse and of uncertain validity. The fact that the amyloid deposits in AD brains that are collectively referred to as plaques are of multiple types and that each seems to have a different origin often is overlooked. We have shown experimentally that many of the so-called "diffuse amyloid plaques," which lack associated inflammatory cells, are either the result of leaks of amyloid from blood vessels at focal sites of blood-brain barrier breaches or are artifacts resulting from grazing sections through the margins of dense core plaques. In addition, we have provided experimental evidence that neuronal death via necrosis leaves a residue that takes the form of a spheroid "cloud" of amyloid, released by cell lysis, surrounding a dense core that often contains neuronal nuclear material. Support for a neuronal origin for these "dense core amyloid plaques" includes their ability to attract inflammatory cells (microglia and immigrant macrophages) and that they contain nuclear and cytoplasmic components that are somewhat resistant to proteolysis by lysosomes released during neuronal cell lysis. We discuss here the clinical and therapeutic importance of recognizing that amyloid deposition occurs both within neurons (intracellular) and in the interstitial (extracellular) space of the brain. For dense core plaques, we propose that the latter location largely follows from the former. This scenario suggests that blocking intraneuronal amyloid deposition should be a primary therapeutic target. This strategy also would be effective for blocking the gradual compromise of neuronal function resulting from this intraneuronal deposition, and the eventual death and lysis of these amyloid-burdened neurons that leads to amyloid release and the appearance of dense core amyloid plaques in the interstitium of AD brains.

Neuronal nicotinic acetylcholine receptors serve as sensitive targets that mediate beta-amyloid neurotoxicity

Alzheimer's disease (AD) is the most common form of brain dementia characterized by the accumulation of beta-amyloid peptides (Abeta) and loss of forebrain cholinergic neurons. Abeta accumulation and aggregation are thought to contribute to cholinergic neuronal degeneration, in turn causing learning and memory deficits, but the specific targets that mediate Abeta neurotoxicity remain elusive. Recently, accumulating lines of evidence have demonstrated that Abeta directly modulates the function of neuronal nicotinic acetylcholine receptors (nAChRs), which leads to the new hypothesis that neuronal nAChRs may serve as important targets that mediate Abeta neurotoxicity. In this review, we summarize current studies performed in our laboratory and in others to address the question of how Abeta modulates neuronal nAChRs, especially nAChR subunit function.

The microglial phagocytic role with specific plaque types in the Alzheimer disease brain

Alzheimer disease (AD) involves glial inflammation associated with amyloid plaques. The role of the microglial cells in the AD brain is controversial, as it remains unclear if the microglia form the amyloid fibrils of plaques or react to them in a macrophage-phagocytic role. Also, it is not known why microglia are preferentially associated with some amyloid plaque types. This review will provide substantial evidence to support the phagocytic role of microglia in the brain as well as explain why microglia are generally associated with specific plaque types that may be explained through their unique mechanisms of formation. In summary, the data presented suggests that plaque associated microglial activation is typically subsequent to specific amyloid plaque formations in the AD brain.

Evidence linking neuronal cell death to autoimmunity in Alzheimer's disease

The catastrophic loss of cerebral neurons in Alzheimer's disease (AD) is not fully understood. Since serum proteins are known to extravasate into the brain parenchyma in AD due to blood-brain barrier (BBB) dysfunction, this study was designed to explore the possibility that neuronal cell death may be the consequence of the anomalous presence of serum proteins in the brain. As compared to age-matched, non-demented 'control' brain tissues, highly significant increases of immunoglobulins (Igs) were detected in parenchyma, which were associated with vessels in the AD brain tissues. Also, there were dramatic increases of +Ig-neurons in areas with greater parenchymal Ig reactivity. The Ig labeling extended throughout the cell, which showed neurodegenerative apoptotic features that were not observed in -Ig-neurons. Thus, the presence of +Ig-neurons in AD brains implies a critical link between the faulty BBB and neuronal death through an autoimmune mechanism.

The use of formic acid to embellish amyloid plaque detection in Alzheimer's disease tissues misguides key observations

We compared our heat pretreatment method to the widely used formic acid pretreatment technique to immunohistochemically detect amyloid in control and Alzheimer's disease brain tissues. Both methods detected amyloid in plaques, neurons, ependymal cells, circulating monocytes, vascular smooth muscle and endothelial cells. Although there were no observable differences in the intensity of the amyloid labeling in these cell types using both pretreatment methods, there were considerable differences in the intensity of amyloid immunolabeling in the plaques. The formic acid produced much more intense amyloid labeling in the plaques than the heat method. With the heat method, the intensity of the amyloid labeling in the plaques was similar to that detected in nearby neurons suggesting a neuronal origin of plaques. Conversely, the intensity of the amyloid in nearby neurons and plaques was drastically different using the formic acid suggesting unique origins of amyloid. The obvious benefits of formic acid for increasing the sensitivity of amyloid plaque immunolabeling may artifactually emphasize plaques over amyloid-containing cells during analyses.