Second Messenger Systems in Alzheimer's Disease: A Quantitative Autoradiographic Study

Quantitative ligand binding autoradiography was used to map key components of second messenger systems in the CNS. [3H]-Forskolin binding to Gs-adenylate cyclase and [3H]-phorbol 12,13 dibutyrate (PDBu) binding to protein kinase C was investigated in human postmortem brain of control patients and patients with Alzheimer's Disease (AD). Disruption of glutamatergic and cholinergic systems may contribute to the pathology of AD. In view of this, alterations in ligand binding sites following selective lesions of glutamatergic and cholinergic pathways in rat brain were used as a framework on which to elucidate possible plastic modifications of second messenger systems in AD. Since the primary lesion in AD occurs within the cortex, ligand binding to second messenger systems was investigated following excitotoxic lesion of the rat cerebral cortex. Second Messenger Ligand Binding in Alzheimer's Disease:In two separate series, [3H]-forskolin binding was investigated in a total of 15 controls and 16 age-matched patients dying with AD in middle frontal and temporal cortices and in the hippocampal formation. AD brains contained numerous neuritic plaques in both cortical areas and the hippocampal region, whilst controls had minimal neuritic plaques. Choline acetyltransferase (ChAT) activity was significantly reduced (>50%) in AD compared to control subjects in both cortex and the hippocampus. [3H]-Forskolin binding was significantly reduced by approximately 50% in all layers of the middle frontal cortex in AD brain compared to controls. There was a positive correlation between [3H]-forskolin binding and ChAT activity in each layer of frontal cortex (correlation coefficient, r = 0.662 - 0.712) when data from control and AD brain were combined. [3H]-Forskolin binding was minimally altered in 1 of the 11 discrete regions examined in the hippocampus in AD brain compared to control. ChAT activity and [3H]-forskolin binding were unrelated in any region of the hippocampus (r = 0.42 - 0.6). In the temporal cortex and the molecular layer of the dentate gyrus, there was evidence that [3 H]-forskolin binding was lower in AD patients compared to control subjects. Whether these changes achieved the probability level of 5% was a reflection of group size, variability of measurements, and the errors of sampling heterogeneous populations. There was no association between the number of neuritic plaques and [3H]-forskolin binding in any brain region examined. The effect of 5'guanylimidodiphosphate (Gpp(NH)p) on [3H]-forskolin binding was examined in adjacent sections from the same group of control and AD patients. In control brain, basal levels of [3H]-forskolin binding were significantly increased in layers I-III of middle frontal cortex (28%) and middle temporal cortex (30%) in the presence of Gpp(NH)p. In AD brain, the ability of Gpp(NH)p to enhance [3H]-forskolin binding from basal levels in cortical layers (I-III) was conserved. Gpp(NH)p had no effect on the level of [3H]-forskolin binding within each region of the hippocampus in the control or AD group. In a separate study, both quantitative autoradiography and homogenate binding to particulate and cytosolic fractions were employed to investigate [3H]-PDBu binding in middle frontal and temporal cortices, and the hippocampal region of nine control and nine AD subjects. All AD brains exhibited extensive signs of the pathology classically associated with the disease, namely numerous neuritic plaques and a profound reduction in ChAT activity ( 60%) in both cortical areas and the hippocampus. Quantitative autoradiographic analysis of [3H]-PDBu binding showed there was no significant difference between control and AD sections in all areas examined within the middle frontal and temporal cortices and hippocampal formation. In adjacent sections to those used for [3H]-PDBu autoradiography, [3H]-forskolin binding was markedly reduced in all layers of middle frontal and temporal cortex (at least 30%) and in the molecular layer of the dentate gyrus (38%) in AD when compared with control subjects. In a parallel study, [3H]-PDBu binding to homogenate preparations of control and AD brain confirmed that there was no significant difference in [3H]-PDBu binding in either the particulate or cytosolic fraction. (Abstract shortened by ProQuest.)

Activation of Nrf2-Regulated Glutathione Pathway Genes by Ischemic Preconditioning

Prophylactic pharmacological activation of astrocytic gene expression driven by the transcription factor Nrf2 boosts antioxidant defences and protects against neuronal loss in ischemia and other disease models. However, the role of Nrf2 in mediating endogenous neuroprotective responses is less clear. We recently showed that Nrf2 is activated by mild oxidative stress in both rodent and human astrocytes. Moreover, brief exposure to ischemic conditions was found to activate Nrf2 both in vivo and in vitro, and this was found to contribute to neuroprotective ischemic preconditioning. Here we show that transient ischemic conditions in vitro and in vivo cause an increase in the expression of Nrf2 target genes associated with the glutathione pathway, including those involved in glutathione biosynthesis and cystine uptake. Taken together, these studies indicate that astrocytic Nrf2 may represent an important mediator of endogenous neuroprotective preconditioning pathways

Chronic cerebral hypoperfusion alters amyloid-β peptide pools leading to cerebral amyloid angiopathy, microinfarcts and hemorrhages in Tg-SwDI mice

Cerebral hypoperfusion is an early feature of Alzheimer’s disease (AD) that influences the progression from mild cognitive impairment to dementia. Understanding the mechanism is of critical importance in the search for new effective therapies. We hypothesized that cerebral hypoperfusion promotes the accumulation of amyloid-β (Aβ) and degenerative changes in the brain and is a potential mechanism contributing to development of dementia. To address this, we studied the effects of chronic cerebral hypoperfusion induced by bilateral carotid artery stenosis on Aβ peptide pools in a transgenic mouse model of AD (transgenic mice with Swedish, Dutch and Iowa mutations in human amyloid precursor protein (APP) (Tg-SwDI)). Cerebrovascular integrity was characterized by quantifying the occurrence of microinfarcts and haemorrhages and compared with wild-type mice without Aβ. A significant increase in soluble Aβ peptides (Aβ40/42) was detected after 1 month of hypoperfusion in the parenchyma in parallel with elevated APP and APP proteolytic products. Following 3 months, a significant increase in insoluble Aβ40/42 was determined in the parenchyma and vasculature. Microinfarct load was significantly increased in the Tg-SwDI as compared with wild-type mice and further exacerbated by hypoperfusion at 1 and 3 months. In addition, the number of Tg-SwDI hypoperfused mice with haemorrhages was increased compared with hypoperfused wild-type mice. Soluble parenchymal Aβ was associated with elevated NADPH oxidase-2 (NOX2) which was exacerbated by 1-month hypoperfusion. We suggest that in response to hypoperfusion, increased Aβ production/deposition may contribute to degenerative processes by triggering oxidative stress promoting cerebrovascular disruption and the development of microinfarcts.</jats:p

Angiotensin II-inhibition:effect on Alzheimer's pathology in the aged triple transgenic mouse

Reducing excessive accumulation of amyloid-β (Aβ) in Alzheimer's disease (AD) is a key objective of most AD therapies, and inhibition of angiotensin-converting enzyme (ACE) may delay onset or progression of AD. The effects of an ACE-inhibitor (ACE-I) and an angiotensin II receptor blocker (ARB) on Aβ and tau pathology in a triple transgenic (3xTGAD) mouse model of AD were investigated. 9-10month 3xTGAD mice were treated with ARB, ACE-I or vehicle for 6 months. Mean arterial blood pressure (MABP) was measured periodically and mice were assessed behaviourally. Aβ, phospho-tau, amyloid precursor protein (APP) and ACE activity were analysed. MABP was significantly reduced at 2 weeks and 3 months in the ACE-I group and at 3 months in the ARB group, compared to vehicle. Neither drug altered performance of 3xTGAD mice in Morris Water Maze or T-maze, nor were Aβ, tau immunolabelling or APP levels altered. ACE-I significantly reduced ACE activity in kidney. Prolonged treatment with ACE-I or ARB does not affect Aβ or phospho-tau accumulation in brains of aged 3xTGAD mice

Hippocampal capillary pericytes in post-stroke and vascular dementias and Alzheimer’s disease and experimental chronic cerebral hypoperfusion

Neurovascular unit mural cells called ‘pericytes’ maintain the blood-brain barrier and local cerebral blood flow.Pathological changes in the hippocampus predispose to cognitive impairment and dementia. The role of hippocampal pericytes in dementia is largely unknown. We investigated hippocampal pericytes in 90 postmortem brains from post-stroke dementia (PSD), vascular dementia (VaD), Alzheimer’s disease (AD), and AD-VaD (Mixed) subjects, and post-stroke non-demented survivors as well as similar age controls. We used collagen IV immunohistochemistry to determine pericyte densities and a mouse model of VaD to validate the effects of chronic cerebral hypoperfusion. Despite increased trends in hippocampal microvascular densities across all dementias, mean pericyte densities were reduced by ~25–40% in PSD, VaD and AD subjects compared to those in controls, which calculated to 14.1 ± 0.7 per mm capillary length, specifically in the cornu ammonis (CA) 1 region (P = 0.01). In mice with chronic bilateral carotid artery occlusion, hippocampal pericyte loss was ~60% relative to controls (P &lt; 0.001). Pericyte densities were correlated with CA1 volumes (r = 0.54, P = 0.006) but not in any other sub-region. However, mice subjected to the full-time environmental enrichment (EE) paradigm showed remarkable attenuation of hippocampal CA1 pericyte loss in tandem with CA1 atrophy. Our results suggest loss of hippocampal microvascular pericytes across common dementias is explained by a vascular aetiology, whilst the EE paradigm offers significant protection

Chronic cerebral hypoperfusion:a key mechanism leading to vascular cognitive impairment and dementia. Closing the translational gap between rodent models and human vascular cognitive impairment and dementia

Increasing evidence suggests that vascular risk factors contribute to neurodegeneration, cognitive impairment and dementia. While there is considerable overlap between features of vascular cognitive impairment and dementia (VCID) and Alzheimer’s disease (AD), it appears that cerebral hypoperfusion is the common underlying pathophysiological mechanism which is a major contributor to cognitive decline and degenerative processes leading to dementia. Sustained cerebral hypoperfusion is suggested to be the cause of white matter attenuation, a key feature common to both AD and dementia associated with cerebral small vessel disease (SVD). White matter changes increase the risk for stroke, dementia and disability. A major gap has been the lack of mechanistic insights into the evolution and progress of VCID. However, this gap is closing with the recent refinement of rodent models which replicate chronic cerebral hypoperfusion. In this review, we discuss the relevance and advantages of these models in elucidating the pathogenesis of VCID and explore the interplay between hypoperfusion and the deposition of amyloid β (Aβ) protein, as it relates to AD. We use examples of our recent investigations to illustrate the utility of the model in preclinical testing of candidate drugs and lifestyle factors. We propose that the use of such models is necessary for tackling the urgently needed translational gap from preclinical models to clinical treatments.</jats:p

The Subtype of GluN2 C-terminal Domain Determines the Response to Excitotoxic Insults

It is currently unclear whether the GluN2 subtype influences NMDA receptor (NMDAR) excitotoxicity. We report that the toxicity of NMDAR-mediated Ca(2+) influx is differentially controlled by the cytoplasmic C-terminal domains of GluN2B (CTD(2B)) and GluN2A (CTD(2A)). Studying the effects of acute expression of GluN2A/2B-based chimeric subunits with reciprocal exchanges of their CTDs revealed that CTD(2B) enhances NMDAR toxicity, compared to CTD(2A). Furthermore, the vulnerability of forebrain neurons in vitro and in vivo to NMDAR-dependent Ca(2+) influx is lowered by replacing the CTD of GluN2B with that of GluN2A by targeted exon exchange in a mouse knockin model. Mechanistically, CTD(2B) exhibits stronger physical/functional coupling to the PSD-95-nNOS pathway, which suppresses protective CREB activation. Dependence of NMDAR excitotoxicity on the GluN2 CTD subtype can be overcome by inducing high levels of NMDAR activity. Thus, the identity (2A versus 2B) of the GluN2 CTD controls the toxicity dose-response to episodes of NMDAR activity