Current Alzheimer Research - Volume 7, Issue 7, 2010

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The ATP binding cassette transporter-2 (ABCA2) has been genetically linked to Alzheimer's disease but the molecular mechanisms are unknown. In this study, the effects of expression of human ABCA2 on endogenous amyloid precursor protein (APP) expression, trafficking and processing were examined in mouse N2a neuronal cells. ABCA2 expression increased the steady-state APP mRNA levels through transcription. ABCA2 also induced increased synthesis of APP holoprotein and altered APP processing and metabolite generation. ABCA2 expression promoted β-secretase (BACE1) cleavage of APP not at the common Asp1 amino acid site (β-site) of Aβ in APP but at the Glu11 site (β´-site) to increase C89 carboxyl-terminal fragment levels (β´-CTF/C89). The levels of N-terminally truncated Aβ11-40 peptides were also increased by ABCA2 expression. The delivery of newly synthesized APP to the cell surface through the secretary pathway was not perturbed by ABCA2 expression; however, ABCA2 expression increased the amount of APP in earlyendosomal compartments, which also contained the highest levels of β´-CTF/C89 and is likely the site of increased BACE1 processing of APP. This report identifies ABCA2 as a key regulator of endogenous APP expression and processing and suggests a possible biochemical mechanism linking ABCA2 expression, APP processing and Alzheimer's disease.

Alzheimer's disease (AD) is the most common form of dementia in the elderly with more than 26 million people worldwide living with the disease. Besides the main neuropathological hallmarks of AD, provoked by the accumulation of amyloid-β (Aβ) and tau hyperphosphorylation, other cells and cellular systems such as microglia and the neurovascular unit establishing the blood-brain-barrier (BBB) have been implicated to play a role in AD etiopathology. Insulating the brain from the blood stream, the BBB facilitates supply and disposal of nutrients and metabolites by the expression of transporters and transcytotic receptors at the polarized endothelial cell (EC) surface. Recently, several proteins involved in Aβ transport across the BBB have been identified in in vitro and in vivo studies. In this review, we summarize recent evidence of receptor- and transporter-mediated Aβ clearance across the BBB. Furthermore, we discuss the models used to identify and characterize Aβ transport across the BBB in regard to barrier properties and their suitability for experimental investigation of Aβ clearance mechanisms at an EC barrier.

Although Alzheimer's disease (AD) is considered a neurodegenerative disorder, in the last few years a large amount of evidence has suggested that it is also a vascular pathology characterized by increased capillary density and expression of angiogenic factors. In AD the endothelium degenerates, promoting local neuroinflammation and activation of brain endothelium, perivascular microglia, pericytes, astrocytes. Excess tumor necrosis factor (TNF) in the cerebrospinal fluid (CSF), at a concentration of 25 times higher than in the control group, has been demonstrated in AD. Recent studies provide evidence that treatment with TNF-α antagonists may result in a rapid cognitive improvement in AD patients. In the present work we investigated the role of astrocytes in AD angiogenesis and neuroinflammation by means of conditioned media of untreated and Aβ-treated rat hippocampal astrocytes (RHAs) on rat microvascular endothelial cells (RCECs). The results demonstrated that RHA media increase RCEC proliferation and capillary-like structure formation. Moreover RHAs secrete IL-1β and, only after the Aβ1-42 treatment, TNF-α promotes RCEC release of IL-1β, IL-6 and TNF-α. The removal of IL-1β, TNF-α and/or VEGF, a strong angiogenic inducer highly over-expressed in AD brains, by means of specific antibody-coated beads in RHA media affects RCEC release of IL-1β, IL-6 and TNF-α. We hypothesised that astrocytes contribute to AD angiogenesis and neuroinflammation by the direct release of pro-inflammatory cytokines. The effect of an anti-inflammatory agent, such as etanercept, decreased RCEC in vitro cytokine release. This could be compared to the effect found in our experiments with antibody anti TNF-α-coated beads.

Amyloid β-protein (Aβ) assembly into toxic fibrillar structures is seminal in development of senile plaques, the pathological hallmark of Alzheimer's disease. Blocking this process could have a therapeutic value. β-sheet breaker peptides (βSBP) decrease Aβ fibrillogenesis and neurotoxicity by preventing or dissolving misfolded Aβ aggregates. The present study investigated the effects of SBPs on A 40-related neuropathology, memory impairment in 8-armed radial maze and expression of A 40 in brain and serum. A 40 was injected into amygdaloid nucleus followed 8 days later by octapeptide 7bgr;SBPs 15-22, 16-23 and 17-24. Aβ40 was detected not only in amygdala, but also in serum. Aβ40 induced cellular changes in amygdala and additionally in hippocampus. A7bgr;40 decreased correct choices, whereas increased errors (both number of arms revisited and total number of revisits) and latency of completing the maze test. The βSBPs decreased Aβ40-induced pathological changes, memory impairment and Aβ40 expression in serum. The 7bgr;SBP15-22 distinctively decreased the total errors on day 14. The present results show that octapeptide βSBPs corrected Aβ40-induced memory impairment, and support investigation of βSBPs as a promising treatment of diseases characterized by neurodegeneration and memory impairment such as Alzheimer's disease.

Interleukin-3 (IL-3) regulates the proliferation, survival and differentiation of haematopoietic cells via interaction with specific cell-surface receptors. IL-3 is expressed in several non-hematopoietic cell types. Studies have demonstrated the presence of IL-3 in the central nervous system, however, its physiological role in these cells is poorly understood. Previously we have been demonstrated that IL-3 prevents neuronal death induced by fibrillary β amyloid in these cells, by PI 3-kinase and Jak/STAT pathway activation. In this study, we demonstrated that IL-3 significantly reduced Aβ- promoted neurite degeneration and toxicity. Thus, this cytokine provides cellular protection against Aβ neurotoxicity in primary cortical neuronal cells, by modulating microtubular dynamics and prevention of tau cleavage and hyperphosphorylation. We also demonstrates that IL-3 is expressed in the “in vivo” mouse model of AD, Tg2576, which also expresses human AβPP with the Swedish mutation. In summary, these results suggest that IL-3 could play a neuroprotective role in AD.

We have investigated physical properties of microvasculature and vessel association with microglial clusters in cortical tissue from Alzheimer disease individuals, classified as severe (ADsev) or mild (ADmild), and nondemented controls (ND). Immunostaining with laminin or von Willebrand factor demonstrated numbers of microvessels and microvascular density were significantly higher in ADsev cases compared with levels in ADmild or ND cases suggesting proangiogenic activity in ADsev brain. Evidence for extravascular laminin immunoreactivity was found in ADsev tissue and was largely absent in ADmild and ND cases suggesting vascular remodeling in ADsev brain included abnormalities in blood vessels. Microgliosis was progressively increased from ND to ADmild to ADsev with the latter demonstrating areas of clustered microglia (groupings of three or more cells) rarely observed in ADmild or ND cases. Microglial clusters in ADsev brain were in close proximity with extravascular laminin and also plasma protein, fibrinogen, implicating vascular perturbation as a component of inflammatory reactivity. ADsev brain also exhibited elevated levels of the proinflammatory/ angiogenic factors tumor necrosis factor-α (TNF-α) and vascular endothelial growth factor (VEGF) in association, relative to non-association, with microglial clusters. The presence of extravascular laminin and fibrinogen and the vascular modifying factors, TNF-α and VEGF in localization with clusters of activated microglia, is consistent with microglial- induced vascular remodeling in ADsev brain. Microglial-vascular reciprocal interactions could serve a critical role in the amplification and perpetuation of inflammatory reactivity in AD brain.

The increasing rate of failed trails found in mood and anxiety disorders is now being seen in Alzheimer's studies. Factors related to the administration of clinician rating scales, such as poor inter-rater reliability, poor interview quality and rater bias may be a contributing factor. Studies have found inter-rater reliability to be problematic in Alzheimer's studies, even with less subjective outcome measures. Lack of standardization of administration and scoring procedures has been identified as a major contributing factor. Remediation through better training procedures has been found to be successful, although ongoing calibration is needed to prevent rater drift. Expectancy bias and baseline score inflation is more difficult to remediate. Inflation of baseline scores increases placebo response, since lower severity has been found to be associated with higher placebo response. The use of centralized raters that are independent from study sites may help ameliorate these issues. Increased methodological research examining new approaches to these problems is warranted. The increased costs associated with this research should offset the time and expense of continuing with ‘business as usual’.

Recent studies of Alzheimer's disease (AD) and other neuropsychiatric drug developments raise questions whether failures of some drugs occur due to flaws in methods. In three case studies of recent AD drug development failures with phenserine, metrifonate, and tarenflurbil we identified methodological lapses able to account for the failures. Errors in complex systems such as drug developments are both almost inescapable due to human mistakes and most frequently hidden at the time of occurrence and thereafter. We propose preemptive error management as a preventive strategy to exclude or control error intrusions into neuropsychiatric drug developments. We illustrate the functions we anticipate for a preemptive error management preventive strategy with a checklist and identify the limitations of this aspect of the proposal with three drug examples. This strategy applies core scientific practices to insure the quality of data within the current context of AD drug development practices.

This is with reference to the article entitled, “Diabetes NEP-Like Endopeptidases and Alzheimer's Disease”, by R.A. Marr and B.J. Spencer, published in Current Alzheimer Research, Vol. 7, No. 3, pp. 223-229. In the article the title was unintentionally published as “Diabetes NEP-Like Endopeptidases and Alzheimer's Disease”. The correct title is “NEP-Like Endopeptidases and Alzheimer's Disease”. The revised title of the article will read as follows: NEP-Like Endopeptidases and Alzheimer's Disease R.A. Marr and B.J. Spencer