Current Alzheimer Research - Volume 8, Issue 1, 2011

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At diagnosis, Alzheimer's disease (AD) brains are extensively burdened with plaques and tangles and display a degree of synaptic failure most likely beyond therapeutic treatment. It is therefore crucial to identify early pathological events in the progression of the disease. While it is not currently feasible to identify and study early, pre-clinical stages of AD, transgenic (Tg) models offer a valuable tool in this regard. Here we investigated cognitive, structural and biochemical CNS alterations occurring in our newly developed McGill-Thyl-APP Tg mice (over-expressing the human amyloid precursor protein with the Swedish and Indiana mutations) prior to extracellular plaque deposition. Pre-plaque, 3-month old Tg mice already displayed cognitive deficits concomitant with reorganization of cortical cholinergic pre-synaptic terminals. Conformational specific antibodies revealed the early appearance of intracellular amyloid β (Aβ)-oligomers and fibrillar oligomers in pyramidal neurons of cerebral cortex and hippocampus. At the same age, the cortical levels of insulin degrading enzyme -a well established Aβ-peptidase, were found to be significantly down-regulated. Our results suggest that, in the McGill-Thy1-APP Tg model, functional, structural and biochemical alterations are already present in the CNS at early, pre-plaque stages of the pathology. Accumulation of intraneuronal neurotoxic Aβ-oligomers (possibly caused by a failure in the clearance machinery) is likely to be the culprit of such early, pre-plaque pathology. Similar neuronal alterations might occur prior to clinical diagnosis in AD, during a yet undefined ‘latent’ stage. A better understanding of such pre-clinical AD might yield novel therapeutic targets and or diagnostic tools.

According to recent criteria, Mild Cognitive Impairment (MCI) represents a clinical condition with multiple cognitive presentations (amnesic and non amnesic) that can be supported by different types of brain lesions (mainly vascular and atrophic). In order to asses if the cognitive presentation and the rate of progression differ according to the type of brain pathology, two populations of MCI patients, characterized by hippocampal atrophy (n: 39) and vascular subcortical pathology (n: 36) respectively, on the basis of MRI findings, were investigated. Patients underwent an extensive neuropsychological test battery twice (at baseline and at two years follow-up), which is made up of the MMSE and various tests of episodic memory, short-term memory, visual-spatial abilities, executive functions, language, attention, praxis and psychomotor speed. Atrophic and vascular MCI patients showed a remarkably different pattern of impairment at the baseline. The former were significantly more impaired in episodic memory tasks. The latter were more impaired in an action naming task. At the follow up examination, the rate of progression to dementia was higher in atrophic (14/39) than in vascular (5/36) MCI patients. The comparison between neuropsychological scores obtained at the baseline and at the followup showed that atrophic MCI patients underwent a severe decline in several cognitive domains, whereas vascular MCI patients showed a significant decline only in those tasks requiring executive abilities. Our results confirm that a selective and severe defect of episodic memory is associated with hippocampal atrophy and that MCI patients with atrophic lesions are more likely to convert to Alzheimer's type dementia while MCI patients with vascular lesions are characterized by a slight decline in executive function over time and by a tendency to develop probable vascular forms of dementia.

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In Alzheimers's disease (AD) a disturbed balance between synthesis and removal of Aβ leads to the formation of Aβ deposits and a reaction of the innate immune system. Little evidence exists for a contribution of the adaptive immune response in AD, as no signs of influx of blood borne cells or presence of immunoglobulins in Aβ deposits are apparent. Factors of the complement (C) system and pentraxins act as pattern recognition molecules and mediate uptake of Aβ by glial cells expressing C-receptors (Crec). These interactions may, however, also lead to synthesis and secretion of reactive oxygen species (ROS), cytokines, chemokines and other potentially neurotoxic agents by the glial cells. Virtually all complement factors are produced in brain, and the expression is increased in AD affected brain areas. In AD brain areas with amyloid deposits especially classical pathway C activation products are readily observed. Also C regulatory proteins (Creg) and Crec can be found in the brain parenchyma and are upregulated, especially under acute inflammatory conditions, such as meningitis. However, under chronic low-grade inflammatory conditions, such as in AD, Creg and to some extent Crec expression may remain at a low level, thereby allowing C activation to proceed, leading to sustained activation of glial cells and neurodegenerative changes. In this review evidence from immunohistochemical, in vitro and animal studies pointing to a role for C activation is discussed, with special focus on the disturbed balance between C activators and Cregs in AD.

The Kallikrein-Kinin System (KKS) has been associated to inflammatory and immunogenic responses in the peripheral and central nervous system by the activation of two receptors, namely B1 receptor and B2 receptor. The B1 receptor is absent or under-expressed in physiological conditions, being up-regulated during tissue injury or in the presence of cytokines. The B2 receptor is constitutive and mediates most of the biological effects of kinins. Some authors suggest a link between the KKS and the neuroinflammation in Alzheimer's disease (AD). We have recently described an increase in bradykinin (BK) in the cerebrospinal fluid and in densities of B1 and B2 receptors in brain areas related to memory, after chronic infusion of amyloid-beta (Aβ) peptide in rats, which was accompanied by memory disruption and neuronal loss. Mice lacking B1 or B2 receptors presented reduced cognitive deficits related to the learning process, after acute intracerebroventricular (i.c.v). administration of Aβ. Nevertheless, our group showed an early disruption of cognitive function by i.c.v. chronic infusion of Aβ after a learned task, in the knock-out B2 mice. This suggests a neuroprotective role for B2 receptors. In knock-out B1 mice the memory disruption was absent, implying the participation of this receptor in neurodegenerative processes. The acute or chronic infusion of Aβ can lead to different responses of the brain tissue. In this way, the proper involvement of KKS on neuroinflammation in AD probably depends on the amount of Aβ injected. Though, BK applied to neurons can exert inflammatory effects, whereas in glial cells, BK can have a potential protective role for neurons, by inhibiting proinflammatory cytokines. This review discusses this duality concerning the KKS and neuroinflammation in AD in vivo.

Alzheimer's disease (AD) is a neurodegenerative disease with major clinical hallmarks of memory loss, dementia, and cognitive impairment. Neuroinflammation is involved in the onset of several neurodegenerative disorders. Astrocyte is the most abundant type of glial cells in the central nervous system (CNS) and appears to be involved in the induction of neuroinflammation. Under stress and injury, astrocytes become astrogliotic leading to an upregulation of the expression of proinflammatory cytokines and chemokines, which are associated with the pathogenesis of AD. Cytokines and related molecules play roles in both neuroprotection and neurodegeneration in the CNS. During early AD pathogenesis, amyloid beta (Aβ), S100B and IL-1β could bring about a vicious cycle of Aβ generation between astrocytes and neurons leading to chronic, sustained and progressive neuroinflammation. In advanced stages of AD, TRAIL secreted from astrocytes have been shown to bind to death receptor 5 (DR5) on neurons to trigger apoptosis in a caspase-8-dependent manner. Furthermore, astrocytes could be reactivated by TGFβ1 to generate more Aβ and to undergo the aggravating astrogliosis. TGFβ2 was also observed to cooperate with Aβ to cause neuronal demise by destroying the stability of lysosomes in neurons. Inflammatory molecules can be either potential biomarkers for diagnosis or target molecules for therapeutic intervention. Understanding their roles and their relationship with activated astrocytes is particularly important for attenuating neuroinflammation in the early stage of AD. The main purpose of this review is to provide a comprehensive insight into the role of astrocytes in the neuroinflammatory pathogenesis of AD.

Cerebrovascular accumulation of amyloid-β protein (Aβ) aggregates in Alzheimer's disease (AD) is proposed to contribute to disease progression and brain inflammation as a result of Aβ-induced increases in endothelial monolayer permeability and stimulation of the endothelium for cellular adhesion and transmigration. These deficiencies facilitate the entry of serum proteins and monocyte-derived microglia into the brain. In the current study, a role for nuclear factor-κB (NF-κB) in the activation of cerebral microvascular endothelial cells by Aβ is explored. Quantitative immunocytochemistry is employed to demonstrate that Aβ1-40 preparations containing isolated soluble aggregates elicit the most pronounced activation and nuclear translocation of NF-κB. This rapid and transient response is observed down to physiological Aβ concentrations and parallels phenotypic changes in endothelial monolayers that are selectively elicited by soluble Aβ1-40 aggregates. While monomeric and fibrillar preparations of Aβ1-40 also activated NF-κB, this response was less pronounced, limited to a small cell population, and not coupled with phenotypic changes. Soluble Aβ1-40 aggregate stimulation of endothelial monolayers for adhesion and subsequent transmigration of monocytes as well as increases in permeability were abrogated by inhibition of NF-κB activation. Together, these results provide additional evidence indicating a role for soluble Aβ aggregates in the activation of the cerebral microvascular endothelium and implicate the involvement of NF-κB signaling pathways in Aβ stimulation of endothelial dysfunction associated with AD.

Widespread neuroinflammation in the central nervous system (CNS) of Alzheimer's disease (AD) patients, involving pro-inflammatory mediators such as complement components, might be responsible for AD associated behavioral symptoms such as anxiety. Vaccinia virus complement control protein (VCP) and curcumin (Cur) are the bioactive compounds of natural origin shown to inhibit the in-vitro complement activation. In order to develop complement regulatory compounds which could be delivered to the CNS by a non-invasive route, VCP, its truncated version (tVCP), and Cur were administered to Wistar rats intranasally. The distribution of these compounds in cerebrospinal fluid (CSF) was studied using an enzyme linked immunosorbent assay (ELISA), using VCP and tVCP as antigens and a modified fluorimetric method (Cur). VCP and tVCP were also detected in the olfactory lobes of the rat brain using immunohistochemical analysis. These compounds were then compared for their ability to attenuate the anxiety levels in APPswePS1δE9 mice using an elevated plus maze (EPM) apparatus. VCP treatment significantly improved the exploratory behavior and reduced the anxiety behavior in APPswePS1δE9 mice. tVCP however showed an opposite effect to VCP, whereas Cur showed no effect on the anxiety behavior of these mice. When these mice were subsequently tested for their cognitive performance in the Morris water maze (MWM), they showed tendencies to collide with the periphery of the walls of MWM. This unusual activity was termed “kissperi” behavior. This newly defined index of anxiety was comparable to the anxiety profile of the VCP and tVCP treated groups on EPM. VCP can thus be delivered to the CNS effectively via intranasal route of administration to attenuate anxiety associated with AD.