Reassessing the amyloid cascade hypothesis of Alzheimer's disease

Aging and excitotoxic stress exacerbate neural circuit reorganization in amyloid precursor protein intracellular domain transgenic mice

The cleavage of amyloid precursor protein (APP) by presenilins simultaneously generates amyloid-β (Aβ) and APP intracellular Domain (AICD) peptides. Aβ plays a pivotal role in Alzheimer's disease (AD) pathology and recently AICD was also shown to contribute to AD. Transgenic mice overexpressing AICD show age-dependent tau phosphorylation and aggregation, memory deficits, and neurodegeneration. Moreover, these mice show aberrant electrical activity and silent seizures beginning at 3-4 months of age. Here we show that AICD mice also displayed abnormal mossy fiber sprouting beginning about the same time and that this sprouting intensified as the animals aged. Expression of neuropeptide Y was increased in mossy fiber terminals in aged but not young AICD mice. Importantly, young AICD mice injected with kainic acid showed similar pathology to that observed in aged AICD mice. These data show that elevated levels of AICD render neurons hypersensitive to stress and induce hippocampal circuit reorganization, which can further exacerbate hyperexcitability. These results further demonstrate that AICD, in addition to Aβ, can play a significant role in AD pathogenesis.

Anti-amyloidogenic effect of AA3E2 attenuates beta-amyloid induced toxicity in SK-N-MC cells

Beta-amyloid peptide (Abeta) is believed to play a recognized role in pathogenesis of Alzheimer's disease (AD). Self-association of Abeta peptide into amyloid fibrils causes neurotoxicity. Compounds capable of interfering with Abeta-Abeta interaction through binding to nucleation sites can inhibit Abeta amyloidogenesis and Abeta-induced cytotoxicity. AA3E2 is a triazine-derivative whose anti-amyloidogenic ability has previously been established. In the present study, we evaluated the protective effect of AA3E3 against Abeta(1-42)-induced toxicity in SK-N-MC cell line. The cell exposure to the co-incubated Abeta(1-42) with AA3E2 decreased the cell viability loss dose-dependently, compared to cells exposed to Abeta(1-42) fibrils. Co-incubation with AA3E2 also attenuated the ROS production, activation of caspase-3 and the extent of apoptotic cell death induced by Abeta(1-42) fibril. Moreover, the 3D structure of the molecular associates between Abeta(1-42) and AA3E2 were theoretically determined by docking studies. Our docking data indicated that AA3E2 inhibits the formation of Abeta fibril likely via binding to the nucleation site within the hydrophobic region of Abeta (KLVFF). These observations provide the background for future design of more elegant beta-breaking agents for dissolution of Abeta fibrillar aggregates.

CSF biomarkers in prediction of cerebral and clinical change in mild cognitive impairment and Alzheimer's disease

Brain atrophy and altered CSF levels of amyloid beta (Abeta(42)) and the microtubule-associated protein tau are potent biomarkers of Alzheimer's disease (AD)-related pathology. However, the relationship between CSF biomarkers and brain morphometry is poorly understood. Thus, we addressed the following questions. (1) Can CSF biomarker levels explain the morphometric differences between normal controls (NC) and patients with mild cognitive impairment (MCI) or AD? (2) How are CSF biomarkers related to atrophy across the brain? (3) How closely are CSF biomarkers and morphometry related to clinical change [clinical dementia rating sum of boxes (CDR-sb)]? Three hundred seventy participants (105 NC, 175 MCI, 90 AD) from the Alzheimer's Disease Neuroimaging Initiative were studied, of whom 309 were followed for 1 year and 176 for 2 years. Analyses were performed across the entire cortical surface, as well as for 30 cortical and subcortical regions of interest. Results showed that CSF biomarker levels could not account for group differences in brain morphometry at baseline but that CSF biomarker levels showed moderate relationships to longitudinal atrophy rates in numerous brain areas, not restricted to medial temporal structures. Baseline morphometry was at least as predictive of atrophy as were CSF biomarkers. Even MCI patients with levels of Abeta(42) comparable with controls and of p-tau lower than controls showed more atrophy than the controls. Morphometry predicted change in CDR-sb better than did CSF biomarkers. These results indicate that morphometric changes in MCI and AD are not secondary to CSF biomarker changes and that the two types of biomarkers yield complementary information.

Genetics of Vascular Dementia

Genetic studies are transforming the way we diagnose, evaluate and treat patients. The era of genome-wide association studies promised to discover common risk variants in heterogeneous disorders where previous small-scale association studies had on the whole failed. However, as we enter the post-association era a degree of disappoint is felt regarding the lack of risk factors with large effect for a number of disorders including vascular disease. Vascular disorders are sporadic by nature, though a familial component has been observed. This review will focus on vascular dementia, the genetic risk factors for vascular disorders and highlight how new technologies may overcome the limitations of genome-wide association and nominate those genes that influence disease risk.

Microglial C5aR (CD88) expression correlates with amyloid-beta deposition in murine models of Alzheimer's disease

Alzheimer's disease (AD), a progressive neurodegenerative disease characterized by the accumulation of amyloid-beta protein and neuronal loss, is the leading cause of age-related dementia in the world today. The disease is also associated with neuroinflammation, robust activation of astrocytes and microglia, and evidence of activation of the complement system, localized with both fibrillar amyloid-beta (fAbeta) plaques and tangles. The observations are consistent with a complement-dependent component of AD progression. We have previously shown that inhibition of the major complement receptor for C5a (CD88) with the antagonist PMX205 results in a significant reduction in pathology in two mouse models of AD. To further characterize the role of complement in AD-related neuroinflammation, we examined the age- and disease-associated expression of CD88 in brain of transgenic mouse models of AD and the influence of PMX205 on the presence of various complement activation products using flow cytometry, western blot, and immunohistochemistry. CD88 was found to be up-regulated in microglia, in the immediate vicinity of amyloid plaques. While thioflavine plaque load and glial recruitment is significantly reduced after treatment with PMX205, C1q remains co-localized with fAbeta plaques and C3 is still expressed by the recruited astrocytes. Thus, with PMX205, potentially beneficial activities of these early complement components may remain intact, while detrimental activities resulting from C5a-CD88 interaction are inhibited. This further supports the targeted inhibition of specific complement mediated activities as an approach for AD therapy.

Poly-N-methylated amyloid beta-peptide (Abeta) C-terminal fragments reduce Abeta toxicity in vitro and in Drosophila melanogaster

Alzheimer's disease (AD), an age related neurodegenerative disorder, threatens to become a major health-economic problem. Assembly of 40- or 42-residue amyloid beta-peptides (Abeta) into neurotoxic oligo-/polymeric beta-sheet structures is an important pathogenic feature in AD, thus, inhibition of this process has been explored to prevent or treat AD. The C-terminal part plays an important role in Abeta aggregation, but most Abeta aggregation inhibitors have targeted the central region around residues 16-23. Herein, we synthesized hexapeptides with varying extents of N-methylation based on residues 32-37 of Abeta, to target its C-terminal region. We measured the peptides' abilities to retard beta-sheet and fibril formation of Abeta and to reduce Abeta neurotoxicity. A penta-N-methylated peptide was more efficient than peptides with 0, 2, or 3 N-methyl groups. This penta-N-methylated peptide moreover increased life span and locomotor activity in Drosophila melanogaster flies overexpressing human Abeta(1-42).

Presenilin transgenic mice as models of Alzheimer's disease

Mutations in presenilin-1 (PS1) and presenilin-2 (PS2) cause familial Alzheimer's disease (FAD). Presenilins influence multiple molecular pathways and are best known for their role in the gamma-secretase cleavage of type I transmembrane proteins including the amyloid precursor protein (APP). PS1 and PS2 FAD mutant transgenic mice have been generated using a variety of promoters. PS1-associated FAD mutations have also been knocked into the endogenous mouse gene. PS FAD mutant mice consistently show elevations of Abeta42 with little if any effect on Abeta40. When crossed with plaque forming APP FAD mutant lines, the PS1 FAD mutants cause earlier and more extensive plaque deposition. Although single transgenic PS1 or PS2 mice do not form plaques, they exhibit a number of pathological features including age-related neuronal and synaptic loss as well as vascular pathology. They also exhibit increased susceptibility to excitotoxic injury most likely on the basis of exaggerated calcium release from the endoplasmic reticulum. Electrophysiologically long-term potentiation in the hippocampus is increased in young PS1 FAD mutant mice but this effect appears to be lost with aging. In most studies neurogenesis in the adult hippocampus is also impaired by PS1 FAD mutants. Mice in which PS1 has been conditionally knocked out in adult forebrain on a PS2 null background (PS1/2 cDKO) develop a striking neurodegeneration that mimics AD neuropathology in being associated with neuronal and synaptic loss, astrogliosis and hyperphosphorylation of tau, although it is not accompanied by plaque deposits. The relevance of PS transgenic mice as models of AD is discussed.

Dietary fats, cerebrovasculature integrity and Alzheimer's disease risk

An emerging body of evidence is consistent with the hypothesis that dietary fats influence Alzheimer's disease (AD) risk, but less clear is the mechanisms by which this occurs. Alzheimer's is an inflammatory disorder, many consider in response to fibrillar formation and extracellular deposition of amyloid-beta (Abeta). Alternatively, amyloidosis could notionally be a secondary phenomenon to inflammation, because some studies suggest that cerebrovascular disturbances precede amyloid plaque formation. Hence, dietary fats may influence AD risk by either modulating Abeta metabolism, or via Abeta independent pathways. This review explores these two possibilities taking into consideration; (i) the substantial affinity of Abeta for lipids and its ordinary metabolism as an apolipoprotein; (ii) evidence that Abeta has potent vasoactive properties and (iii) studies which show that dietary fats modulate Abeta biogenesis and secretion. We discuss accumulating evidence that dietary fats significantly influence cerebrovascular integrity and as a consequence altered Abeta kinetics across the blood-brain barrier (BBB). Specifically, chronic ingestion of saturated fats or cholesterol appears to results in BBB dysfunction and exaggerated delivery from blood-to-brain of peripheral Abeta associated with lipoproteins of intestinal and hepatic origin. Interestingly, the pattern of saturated fat/cholesterol induced cerebrovascular disturbances in otherwise normal wild-type animal strains is analogous to established models of AD genetically modified to overproduce Abeta, consistent with a causal association. Saturated fats and cholesterol may exacerbate Abeta induced cerebrovascular disturbances by enhancing exposure of vessels of circulating Abeta. However, presently there is no evidence to support this contention. Rather, SFA and cholesterol appear to more broadly compromise BBB integrity with the consequence of plasma protein leakage into brain, including lipoprotein associated Abeta. The latter findings are consistent with the concept that AD is a dietary-fat induced phenotype of vascular dementia, reflecting the extraordinary entrapment of peripherally derived lipoproteins endogenously enriched in Abeta. Rather than being the initiating trigger for inflammation in AD, accumulation of extracellular lipoprotein-Abeta may be a secondary amplifier of dietary induced inflammation, or possibly, simply be consequential. Clearly, delineating the mechanisms by which dietary fats increase AD risk may be informative in developing new strategies for prevention and treatment of AD.

Alzheimer's disease-like pathological features in transgenic mice expressing the APP intracellular domain

The hypothesis that amyloid-beta (Abeta) peptides are the primary cause of Alzheimer's disease (AD) remains the best supported theory of AD pathogenesis. Yet, many observations are inconsistent with the hypothesis. Abeta peptides are generated when amyloid precursor protein (APP) is cleaved by presenilins, a process that also produces APP intracellular domain (AICD). We previously generated AICD-overexpressing transgenic mice that showed abnormal activation of GSK-3beta, a pathological feature of AD. We now report that these mice exhibit additional AD-like characteristics, including hyperphosphorylation and aggregation of tau, neurodegeneration and working memory deficits that are prevented by treatment with lithium, a GSK-3beta inhibitor. Consistent with its potential role in AD pathogenesis, we find AICD levels to be elevated in brains from AD patients. The in vivo findings that AICD can contribute to AD pathology independently of Abeta have important therapeutic implications and may explain some observations that are discordant with the amyloid hypothesis.

Recent insights into the molecular genetics of dementia

Our understanding of the molecular genetic basis of two common neurodegenerative dementias, Alzheimer's disease (AD) and frontotemporal lobar degeneration (FTLD), has greatly advanced in recent years. Progranulin mutations were identified as a major cause of FTLD and a potential susceptibility factor for other forms of dementia. In addition, through copy-number analyses of previously identified disease genes and the study of microRNA regulation in dementia, new evidence emerged to support the view that subtle variability in the expression of known disease proteins could increase the risk for sporadic forms of dementia. Finally, in late-onset AD populations, the first genome-wide association studies were performed and novel potential AD susceptibility genes reported. These exciting findings provide novel insights into the disease mechanisms underlying dementia and hold promise for the development of potential treatments.

Protective effect of a triazine-derivative (AA3E2) on beta-amyloid-induced damages in SK-N-MC cells

The role of beta-amyloid (A beta) in the pathogenesis of Alzheimer's disease (AD) is frequently reported in the literature. Though the exact mode of action is not known, it is suggested that A beta induces cell death through induction of oxidative stress possibly through hydrogen peroxide generation. In that case, antioxidants should be capable of attenuating the A beta-induced cytotoxicities. In that regard, we evaluated the effect(s) of a triazine-derivative, AA3E2, with established antioxidant activity. Pretreatment of SK-N-MC neuroblastoma cells with AA3E2, followed by exposure to A beta(1-42) showed 28.3% higher viability relative to the control cells which has not been treated with AA3E2. In addition, AA3E2 inhibited caspase-3 activation caused by A beta(1-42) and it attenuated A beta(1-42)-induced intracellular ROS (reactive oxygen species) accumulation. The lower level of intracellular free radicals was further confirmed by higher and lower activities of intracellular catalase and superoxide dismutase, respectively. These observations, parallel to the literature data, reconfirm the oxidative stress disrupting role of A beta(1-42) peptide. Thus, sequestration of this role by potential antioxidants such as AA3E2 might happen to be a suitable strategy for future treatments of AD.

The rat as an animal model of Alzheimer's disease

As a disease model, the laboratory rat has contributed enormously to neuroscience research over the years. It has also been a popular animal model for Alzheimer's disease but its popularity has diminished during the last decade, as techniques for genetic manipulation in rats have lagged behind that of mice. In recent years, the rat has been making a comeback as an Alzheimer's disease model and the appearance of increasing numbers of transgenic rats will be a welcome and valuable complement to the existing mouse models. This review summarizes the contributions and current status of the rat as an animal model of Alzheimer's disease.