Intravascular infusions of soluble beta-amyloid compromise the blood-brain barrier, activate CNS glial cells and induce peripheral hemorrhage

Relationship Between Amyloid-β Deposition and Blood-Brain Barrier Dysfunction in Alzheimer's Disease

Amyloid-β (Aβ) is the predominant pathologic protein in Alzheimer's disease (AD). The production and deposition of Aβ are important factors affecting AD progression and prognosis. The deposition of neurotoxic Aβ contributes to damage of the blood-brain barrier. However, the BBB is also crucial in maintaining the normal metabolism of Aβ, and dysfunction of the BBB aggravates Aβ deposition. This review characterizes Aβ deposition and BBB damage in AD, summarizes their interactions, and details their respective mechanisms.

Obesity and cognitive decline: role of inflammation and vascular changes

The incidence of obesity in middle age is increasing markedly, and in parallel the prevalence of metabolic disorders including cardiovascular disease and type II diabetes is also rising. Numerous studies have demonstrated that both obesity and metabolic disorders are associated with poorer cognitive performance, cognitive decline, and dementia. In this review we discuss the effects of obesity on cognitive performance, including both clinical and preclinical observations, and discuss some of the potential mechanisms involved, namely inflammation and vascular and metabolic alterations.

Blood-brain barrier drug delivery of IgG fusion proteins with a transferrin receptor monoclonal antibody

INTRODUCTION

Biologic drugs are large molecules that do not cross the blood- brain barrier (BBB). Brain penetration is possible following the re-engineering of the biologic drug as an IgG fusion protein. The IgG domain is a MAb against an endogenous BBB receptor such as the transferrin receptor (TfR). The TfRMAb acts as a molecular Trojan horse to ferry the fused biologic drug into the brain via receptor-mediated transport on the endogenous BBB TfR.

AREAS COVERED

This review discusses TfR isoforms, models of BBB transport of transferrin and TfRMAbs, and the genetic engineering of TfRMAb fusion proteins, including BBB penetrating IgG-neurotrophins, IgG-decoy receptors, IgG-lysosomal enzyme therapeutics and IgG-avidin fusion proteins, as well as BBB transport of bispecific antibodies formed by fusion of a therapeutic antibody to a TfRMAb targeting antibody. Also discussed are quantitative aspects of the plasma pharmacokinetics and brain uptake of TfRMAb fusion proteins, as compared to the brain uptake of small molecules, and therapeutic applications of TfRMAb fusion proteins in mouse models of neural disease, including Parkinson's disease, stroke, Alzheimer's disease and lysosomal storage disorders. The review covers the engineering of TfRMAb-avidin fusion proteins for BBB targeted delivery of biotinylated peptide radiopharmaceuticals, low-affinity TfRMAb Trojan horses and the safety pharmacology of chronic administration of TfRMAb fusion proteins.

EXPERT OPINION

The BBB delivery of biologic drugs is possible following re-engineering as a fusion protein with a molecular Trojan horse such as a TfRMAb. The efficacy of this technology will be determined by the outcome of future clinical trials.

Pinocembrin improves cognition and protects the neurovascular unit in Alzheimer related deficits

Amyloid-β (Aβ) peptides accumulate in the brain and initiate a cascade of pathologic events in Alzheimer's disease. The receptor for advanced glycation end products (RAGE) has been implicated to mediate Aβ-induced perturbations in the neurovascular unit (NVU). We demonstrated that pinocembrin exhibits neuroprotection through inhibition of the Aβ and/or RAGE pathway, but the therapeutic role and mechanism involved are not ascertained. Here, we report that a 3-month treatment with pinocembrin prevents the cognition decline in APP/PS1 transgenic mice without altering Aβ burden and oxidative stress. Instead, pinocembrin is effective in conferring neurovascular protection through maintenance of neuropil ultrastructure, reduction of glial activation and levels of inflammatory mediators, preservation of microvascular function, improving the cholinergic system by conserving the ERK-CREB-BDNF pathway, and modulation of RAGE-mediated transduction. Furthermore, in an in vitro model, pinocembrin provides the NVU protection against fibrillar Aβ₁₋₄₂, accompanied by regulation of neurovascular RAGE pathways. Our findings indicate that pinocembrin improves cognition, at least in part, attributable to the NVU protection, and highlights pinocembrin as a potential therapeutic strategy for the prevention and/or treatment of Alzheimer's disease.

Blood-brain barrier disruption: mechanistic links between Western diet consumption and dementia

Both obesity and Alzheimer's disease (AD) are major health burdens in Western societies. While commonly viewed as having separate etiologies, this review highlights data suggesting that intake of "Western diets", diets high in saturated fatty acids (SFA) and simple carbohydrates, may pose a common environmental risk factor contributing to the development of both of these adverse pathologies. We discuss the effects of Western Diet intake on learning and memory processes that are dependent on the hippocampus, as well as the importance of this brain region in both obesity development and the onset of Alzheimer's and other dementias. A putative mechanism is discussed that mechanistically links Western diet consumption, blood brain barrier (BBB) degradation, and subsequent hippocampal damage and dementia pathology.

Disaggregation of amyloid plaque in brain of Alzheimer's disease transgenic mice with daily subcutaneous administration of a tetravalent bispecific antibody that targets the transferrin receptor and the Abeta amyloid peptide

Anti-amyloid antibodies (AAA) are under development as new therapeutics that disaggregate the amyloid plaque in brain in Alzheimer's disease (AD). However, the AAAs are large molecule drugs that do not cross the blood-brain barrier (BBB), in the absence of BBB disruption. In the present study, an AAA was re-engineered for receptor-mediated transport across the BBB via the endogenous BBB transferrin receptor (TfR). A single chain Fv (ScFv) antibody form of an AAA was fused to the carboxyl terminus of each heavy chain of a chimeric monoclonal antibody (mAb) against the mouse TfR, and this produced a tetravalent bispecific antibody designated the cTfRMAb-ScFv fusion protein. Unlike a conventional AAA, which has a plasma half-time of weeks, the cTfRMAb-ScFv fusion protein is cleared from plasma in mice with a mean residence time of about 3 h. Therefore, a novel protocol was developed for the treatment of one year old presenilin (PS)-1/amyloid precursor protein (APP) AD double transgenic PSAPP mice, which were administered daily subcutaneous (sc) injections of 5 mg/kg of the cTfRMAb-ScFv fusion protein for 12 consecutive weeks. At the end of the treatment, brain amyloid plaques were quantified with confocal microscopy using both Thioflavin-S staining and immunostaining with the 6E10 antibody against Abeta amyloid fibrils. Fusion protein treatment caused a 57% and 61% reduction in amyloid plaque in the cortex and hippocampus, respectively. No increase in plasma immunoreactive Abeta amyloid peptide, and no cerebral microhemorrhage, was observed. Chronic daily sc treatment of the mice with the fusion protein caused no immune reactions and only a low titer antidrug antibody response. In conclusion, re-engineering AAAs for receptor-mediated BBB transport allows for reduction in brain amyloid plaque without cerebral microhemorrhage following daily sc treatment for 12 weeks.

Pathology associated with AAV mediated expression of beta amyloid or C100 in adult mouse hippocampus and cerebellum

Accumulation of beta amyloid (Aβ) in the brain is a primary feature of Alzheimer’s disease (AD) but the exact molecular mechanisms by which Aβ exerts its toxic actions are not yet entirely clear. We documented pathological changes 3 and 6 months after localised injection of recombinant, bi-cistronic adeno-associated viral vectors (rAAV2) expressing human Aβ40-GFP, Aβ42-GFP, C100-GFP or C100^V717F-GFP into the hippocampus and cerebellum of 8 week old male mice. Injection of all rAAV2 vectors resulted in wide-spread transduction within the hippocampus and cerebellum, as shown by expression of transgene mRNA and GFP protein. Despite the lack of accumulation of Aβ protein after injection with AAV vectors, injection of rAAV2-Aβ42-GFP and rAAV2- C100^V717F-GFP into the hippocampus resulted in significantly increased microgliosis and altered permeability of the blood brain barrier, the latter revealed by high levels of immunoglobulin G (IgG) around the injection site and the presence of IgG positive cells. In comparison, injection of rAAV2-Aβ40-GFP and rAAV2-C100-GFP into the hippocampus resulted in substantially less neuropathology. Injection of rAAV2 vectors into the cerebellum resulted in similar types of pathological changes, but to a lesser degree. The use of viral vectors to express different types of Aβ and C100 is a powerful technique with which to examine the direct in vivo consequences of Aβ expression in different regions of the mature nervous system and will allow experimentation and analysis of pathological AD-like changes in a broader range of species other than mouse.

Drug delivery to the brain in Alzheimer's disease: consideration of the blood-brain barrier

The successful treatment of Alzheimer's disease (AD) will require drugs that can negotiate the blood-brain barrier (BBB). However, the BBB is not simply a physical barrier, but a complex interface that is in intimate communication with the rest of the central nervous system (CNS) and influenced by peripheral tissues. This review examines three aspects of the BBB in AD. First, it considers how the BBB may be contributing to the onset and progression of AD. In this regard, the BBB itself is a therapeutic target in the treatment of AD. Second, it examines how the BBB restricts drugs that might otherwise be useful in the treatment of AD and examines strategies being developed to deliver drugs to the CNS for the treatment of AD. Third, it considers how drug penetration across the AD BBB may differ from the BBB of normal aging. In this case, those differences can complicate the treatment of CNS diseases such as depression, delirium, psychoses, and pain control in the AD population.

A diet enriched in docosahexanoic Acid exacerbates brain parenchymal extravasation of apo B lipoproteins induced by chronic ingestion of saturated fats

Chronic ingestion of saturated fatty acids (SFAs) was previously shown to compromise blood-brain barrier integrity, leading to brain parenchymal extravasation of apolipoprotein B (apo B) lipoproteins enriched in amyloid beta. In contrast, diets enriched in mono- or polyunsaturated (PUFA) oils had no detrimental effect. Rather, n3 and n6 oils generally confer protection via suppression of inflammation. This study investigated in wild-type mice if a PUFA diet enriched in docosahexanoic acid (DHA) restored blood-brain barrier integrity and attenuated parenchymal apo B abundance induced by chronic ingestion of SFA. Cerebrovascular leakage of apo B was quantitated utilising immunofluorescent staining. The plasma concentration of brain-derived S100β was measured as a marker of cerebrovascular inflammation. In mice fed SFA for 3 months, provision thereafter of a DHA-enriched diet exacerbated parenchymal apo B retention, concomitant with a significant increase in plasma cholesterol. In contrast, provision of a low-fat diet following chronic SFA feeding had no effect on SFA-induced parenchymal apo B. The findings suggest that in a heightened state of cerebrovascular inflammation, the provision of unsaturated fatty acids may be detrimental, possibly as a consequence of a greater susceptibility for oxidation.

Receptor-mediated abeta amyloid antibody targeting to Alzheimer's disease mouse brain

The goal of this work is the reduction in the Abeta amyloid peptide burden in brain of Alzheimer's disease (AD) transgenic mice without the concomitant elevation in plasma Abeta amyloid peptide. An anti-Abeta amyloid antibody (AAA) was re-engineered as a fusion protein with a blood-brain barrier (BBB) molecular Trojan horse. The AAA was engineered as a single chain Fv (ScFv) antibody, and the ScFv was fused to the heavy chain of a chimeric monoclonal antibody (mAb) against the mouse transferrin receptor (TfR), and this fusion protein was designated cTfRMAb-ScFv. The cTfRMAb-ScFv protein penetrates mouse brain from blood via transport on the BBB TfR, and the brain uptake is 3.5% of injected dose/gram brain following an intravenous administration. Double transgenic APPswe,PSEN1dE9 mice were studied at 12 months of age. The mice were shown to have extensive Abeta amyloid plaques in cerebral cortex based on immunocytochemistry. The mice were treated every 3-4 days by intravenous injections of either saline or the cTfRMAb-ScFv fusion protein at an injection dose of 1 mg/kg for 12 consecutive weeks. The brain Aβ¹⁻⁴² concentration was reduced 40% in the fusion protein treated mice, without any elevation in plasma Aβ¹⁻⁴² concentration. No cerebral microhemorrhage was observed in the treated mice. These results show that brain-penetrating antibody pharmaceutics can be developed for brain disorders such as AD following the re-engineering of the antibody as a fusion protein that is transported across the BBB via receptor-mediated transport.

Transport of drugs across the blood–brain barrier in Alzheimer’s disease

Alzheimer’s disease, a neurodegenerative disorder, is associated with various pathological alterations to the blood–brain barrier, including disruption to the inter-endothelial tight junction proteins, altered expression of transport proteins involved in drug efflux, a reduction in cerebral blood flow and a thickening of the brain capillary basement membrane. There are many conflicting reports on whether such changes alter the ability of endogenous proteins to extravasate into the brain parenchyma, and there are even fewer reports focusing on the potential impact of these changes on drug transport into the CNS. The purpose of this review is to critically evaluate how the reported changes to the blood–brain barrier in Alzheimer’s disease have (or have not) resulted in altered CNS drug delivery, and to highlight the requirement for more rigorous and systematic studies in this field for the benefit of drug discovery and delivery scientists.

Post-prandial lipid metabolism, lipid-modulating agents and cerebrovascular integrity: implications for dementia risk

Amyloid-β (Aβ) is secreted as an apolipoprotein of nascent triglyceride-rich lipoproteins (TRL) derived from both liver and intestine, but is better recognized as the principal protein component of senile plaque in subjects with Alzheimer's disease. Recent studies suggest that exaggerated exposure to plasma Aβ can compromise cerebrovascular integrity, resulting thereafter in blood to brain delivery of plasma proteins including TRL-Aβ. Parenchymal deposits of Aβ show significant immunoreactivity to apolipoprotein B (apo B), consistent with the notion of lipoprotein-Aβ entrapment. In wild type mice chronically fed physiologically relevant diets, saturated fats (SFA) enhance chylomicron-Aβ concomitant with disturbances in blood-brain barrier integrity. Similarly, dietary cholesterol promotes cerebrovascular extravasation of apo B lipoprotein-Aβ. In this study, we investigated the effects of atorvastatin, pravastatin and probucol on dietary-fat induced disturbances in BBB function. Atorvastatin, a lipid soluble HMG-CoA reductase inhibitor prevented SFA induced parenchymal extravasation of apo B-Aβ at 28 days when incorporated into the diet at 20 mg/kg. In contrast, pravastatin a water soluble agent had no effect on BBB integrity at an equivalent dose. In cholesterol supplemented mice, probucol maintained BBB function and extravasation of apo B-Aβ was not evident. The findings suggest that some lipid-modulating agents may be effective in ameliorating the negative effects of saturated fats and cholesterol on cerebrovascular integrity.

IgG-single chain Fv fusion protein therapeutic for Alzheimer's disease: Expression in CHO cells and pharmacokinetics and brain delivery in the rhesus monkey

Monoclonal antibodies (MAb) directed against the Abeta amyloid peptide of Alzheimer's disease (AD) are potential new therapies for AD, since these antibodies disaggregate brain amyloid plaque. However, the MAb is not transported across the blood-brain barrier (BBB). To enable BBB transport, a single chain Fv (ScFv) antibody against the Abeta peptide of AD was re-engineered as a fusion protein with the MAb against the human insulin receptor (HIR). The HIRMAb acts as a molecular Trojan horse to ferry the ScFv therapeutic antibody across the BBB. Chinese hamster ovary (CHO) cells were stably transfected with a tandem vector encoding the heavy and light chains of the HIRMAb-ScFv fusion protein. A high secreting line was isolated following methotrexate amplification and dilutional cloning. The HIRMAb-ScFv fusion protein in conditioned serum-free medium was purified by protein A affinity chromatography. The fusion protein was stable as a liquid formulation, and retained high-affinity binding of both the HIR and the Abeta amyloid peptide. The HIRMAb-ScFv fusion protein was radiolabeled with the (125)I-Bolton-Hunter reagent, followed by measurement of the pharmacokinetics of plasma clearance and brain uptake in the adult Rhesus monkey. The HIRMAb-ScFv fusion protein was rapidly cleared from plasma and was transported across the primate BBB in vivo. In conclusion, the HIRMAb-ScFv fusion protein is a new class of antibody-based therapeutic for AD that has been specifically engineered to cross the human BBB.

The effects of a high-energy diet on hippocampal function and blood-brain barrier integrity in the rat

Cognitive impairment and Alzheimer's disease are linked with intake of a Western diet, characterized by high levels of saturated fats and simple carbohydrates. In rats, these dietary components have been shown to disrupt hippocampal-dependent learning and memory processes, particularly those involving spatial memory. Using a rat model, the present research assessed the degree to which consumption of a high-energy (HE) diet, similar to those found in modern Western cultures, produces a selective impairment in hippocampal function as opposed to a more global cognitive disruption. Learning and memory performance was examined following 90-day consumption of an HE-diet in three nonspatial discrimination learning problems that differed with respect to their dependence on the integrity of the hippocampus. The results showed that consumption of the HE-diet impaired performance in a hippocampal-dependent feature negative discrimination problem relative to chow-fed controls, whereas performance was spared on two discrimination problems that do not rely on the hippocampus. To explore the mechanism whereby consuming HE-diets impairs cognitive function, we investigated the effect of HE-diets on the integrity of the blood-brain barrier (BBB). We found that HE-diet consumption produced a decrease in mRNA expression of tight junction proteins, particularly Claudin-5 and -12, in the choroid plexus and the BBB. Consequently, an increased blood-to-brain permeability of sodium fluorescein was observed in the hippocampus, but not in the striatum and prefrontal cortex following HE-diet access. These results indicate that hippocampal function may be particularly vulnerable to disruption by HE-diets, and this disruption may be related to impaired BBB integrity.

Alzheimer's disease drug development and the problem of the blood-brain barrier

Alzheimer's disease (AD) drug development is limited by the presence of the blood-brain barrier (BBB). More than 98% of all small-molecule drugs, and approximately 100% of all large-molecule drugs, do not cross the BBB. Although the vast majority of AD drug candidates do not cross the BBB, the present-day AD drug-development effort is characterized by an imbalance wherein >99% of the drug-development effort is devoted to central nervous system (CNS) drug discovery, and <1% of drug development is devoted to CNS drug delivery. Future AD drug development needs a concerted effort to incorporate BBB sciences early in the CNS drug discovery process. This goal can be achieved by a reallocation of resources, and an expansion of research efforts in the pure science of BBB biology and the applied science of brain drug-targeting technology.

Amyloid-beta-induced occludin down-regulation and increased permeability in human brain endothelial cells is mediated by MAPK activation

Vascular dysfunction is emerging as a key pathological hallmark in Alzheimer’s disease (AD). A leaky blood–brain barrier (BBB) has been described in AD patient tissue and in vivo AD mouse models. Brain endothelial cells (BECs) are linked together by tight junctional (TJ) proteins, which are a key determinant in restricting the permeability of the BBB. The amyloid β (Aβ) peptides of 1–40 and 1–42 amino acids are believed to be pivotal in AD pathogenesis. We therefore decided to investigate the effect of Aβ 1–40, the Aβ variant found at the highest concentration in human plasma, on the permeability of an immortalized human BEC line, hCMEC/D3. Aβ 1–40 induced a marked increase in hCMEC/D3 cell permeability to the paracellular tracer 70 kD FITC-dextran when compared with cells incubated with the scrambled Aβ 1–40 peptide. Increased permeability was associated with a specific decrease, both at the protein and mRNA level, in the TJ protein occludin, whereas claudin-5 and ZO-1 were unaffected. JNK and p38MAPK inhibition prevented both Aβ 1–40-mediated down-regulation of occludin and the increase in paracellular permeability in hCMEC/D3 cells. Our findings suggest that the JNK and p38MAPK pathways might represent attractive therapeutic targets for preventing BBB dysfunction in AD.

Three-dimensional colocalization analysis of plasma-derived apolipoprotein B with amyloid plaques in APP/PS1 transgenic mice

Parenchymal accumulation of amyloid-beta (A beta) is a hallmark pathological feature of Alzheimer's disease. An emerging hypothesis is that blood-to-brain delivery of A beta may increase with compromised blood-brain barrier integrity. In plasma, substantial A beta is associated with triglyceride-rich lipoproteins (TRLs) secreted by the liver and intestine. Utilizing apolipoprotein B as an exclusive marker of hepatic and intestinal TRLs, here we show utilizing an highly sensitive 3-dimensional immuno-microscopy imaging technique, that in APP/PS1 amyloid transgenic mice, concomitant with substantially increased plasma A beta, there is a significant colocalization of apolipoprotein B with cerebral amyloid plaque. The findings are consistent with the possibility that circulating lipoprotein-A beta contributes to cerebral amyloidosis.

Soluble aggregates of the amyloid-beta protein selectively stimulate permeability in human brain microvascular endothelial monolayers

Cerebral amyloid angiopathy associated with Alzheimer's disease is characterized by cerebrovascular deposition of the amyloid-beta protein (Abeta). Abeta elicits a number of morphological and biochemical alterations in the cerebral microvasculature, which culminate in hemorrhagic stroke. Among these changes, compromise of the blood-brain barrier has been described in Alzheimer's disease brain, transgenic animal models of Alzheimer's disease, and cell culture experiments. In the current study, presented data illustrates that isolated soluble Abeta(1-40) aggregates, but not unaggregated monomer or mature fibril, enhance permeability in human brain microvascular endothelial monolayers. Abeta(1-40)-induced changes in permeability are paralleled by both a decrease in transendothelial electrical resistance and a re-localization of the tight junction-associated protein zonula occludin-1 away from cell borders and into the cytoplasm. Small soluble Abeta(1-40) aggregates are confirmed to be the most potent stimulators of endothelial monolayer permeability by establishing an inverse relationship between average aggregate size and stimulated changes in diffusional permeability coefficients. These results support previous findings demonstrating that small soluble Abeta(1-40) aggregates are also primarily responsible for endothelial activation, suggesting that these same species may elicit other changes in the cerebrovasculature associated with cerebral amyloid angiopathy and Alzheimer's disease.

Soluble amyloid-beta peptides potently disrupt hippocampal synaptic plasticity in the absence of cerebrovascular dysfunction in vivo

Long before the onset of clinical Alzheimer's disease non-fibrillar, soluble assembly states of amyloid-beta (Abeta) peptides are believed to cause cognitive problems by disrupting synaptic function in the absence of significant neurodegeneration. Since many of the risk factors for Alzheimer's disease are vascular, impairment of cerebral blood flow by soluble Abeta has been proposed to be critical in triggering these early changes. However, it is not known if soluble Abeta can affect cerebrovascular function at the concentrations required to cause inhibition of synaptic plasticity mechanisms believed to underlie the early cognitive deficits of Alzheimer's disease. Here we developed a new method to simultaneously assess the ability of soluble Abeta to impair plasticity at synapses and to affect resting and activity-dependent local blood flow in the rat hippocampus in vivo. Intracerebroventricular injection of soluble synthetic Abeta(40) dimers rapidly inhibited plasticity of excitatory synaptic transmission at doses (10-42 pmol) comparable to natural Abeta, but failed to affect vascular function measured using laser-Doppler flowmetry (LDF). Like wild-type Abeta(40), the more vasculotropic Abeta produced by people with familial hemorrhagic stroke of the Dutch type (Abeta(40)E22Q), impaired hippocampal plasticity without causing a significant change in local blood flow. Furthermore, neither resting nor activation-evoked hippocampal perfusion was affected by soluble Abeta(42), even at a concentration that markedly (25%) reduced baseline synaptic transmission. These findings demonstrate that the putative synaptotoxic soluble Abeta species of early Alzheimer's disease cause synaptic dysfunction in the absence of detectible changes in local blood flow. This strongly indicates that early cognitive deficits can be caused by soluble Abeta independently of deleterious effects on cerebrovascular dynamics.

Chylomicron amyloid-beta in the aetiology of Alzheimer's disease

Alzheimer's disease is characterized by inflammatory proteinaceous deposits comprised principally of the protein amyloid-beta (Abeta). Presently, the origins of cerebral amyloid deposits are controversial, though pivotal for the prevention of Alzheimer's disease. Recent evidence suggests that in blood, Abeta may serve as a regulating apoprotein of the triglyceride-rich-lipoproteins and we have found that the synthesis of Abeta in enterocytes and thereafter secretion as part of the chylomicron cascade is regulated by dietary fats. It is our contention that chronically elevated plasma levels of Abeta in response to diets rich in saturated fats may lead to disturbances within the cerebrovasculature and exaggerated blood-to-brain delivery of circulating Abeta, thereby exacerbating amyloidosis. Consistent with this hypothesis we show that enterocytic Abeta is increased concomitant with apolipoprotein B48. Furthermore, cerebral extravasation of immunoglobulin G, a surrogate marker of plasma proteins is observed in a murine model of Alzheimer's disease maintained on a saturated-fat diet and there is diminished expression of occludin within the cerebrovasculature, an endothelial tight junction protein.

Apolipoprotein E, amyloid-beta, and blood-brain barrier permeability in Alzheimer disease

There is increasing evidence for blood-brain barrier (BBB) compromise in Alzheimer disease (AD). The presence of the epsilon4 allele of the apolipoprotein E (apoE) gene is a risk factor for sporadic AD. Apolipoprotein E is essential both for maintenance of BBB integrity and for the deposition of fibrillar amyloid-beta (Abeta) that leads to the development of Abeta plaques in AD and to cerebral amyloid angiopathy. This review investigates the relationships between apoE, Abeta, and the BBB in AD. Alterations in the expression and distribution of the BBB Abeta transporters receptor for advanced glycation end-products and low-density lipoprotein receptor-related protein 1 in AD and the potential roles of apoE4 expression in adversely influencing Abeta burden and BBB permeability are also examined. Because both apoE and Abeta are ligands for low-density lipoprotein receptor-related protein 1, all 3 molecules are present in AD plaques, and most AD plaques are located close to the cerebral microvasculature. The interactions of these molecules at the BBB likely influence metabolism and clearance of Abeta and contribute to AD pathogenesis. Therapeutic alternatives targeting apoE/Abeta and sealing a compromised BBB are under development for the treatment of AD.

Synergistic effects of high fat feeding and apolipoprotein E deletion on enterocytic amyloid-beta abundance

Background

Amyloid-β (Aβ), a key protein found in amyloid plaques of subjects with Alzheimer's disease is expressed in the absorptive epithelial cells of the small intestine. Ingestion of saturated fat significantly enhances enterocytic Aβ abundance whereas fasting abolishes expression. Apolipoprotein (apo) E has been shown to directly modulate Aβ biogenesis in liver and neuronal cells but it's effect in enterocytes is not known. In addition, apo E modulates villi length, which may indirectly modulate Aβ as a consequence of differences in lipid absorption. This study compared Aβ abundance and villi length in wild-type (WT) and apo E knockout (KO) mice maintained on either a low-fat or high-fat diet. Wild-type C57BL/6J and apo E KO mice were randomised for six-months to a diet containing either 4% (w/w) unsaturated fats, or chow comprising 16% saturated fats and 1% cholesterol. Quantitative immunohistochemistry was used to assess Aβ abundance in small intestinal enterocytes. Apo E KO mice given the low-fat diet had similar enterocytic Aβ abundance compared to WT controls.

Results

The saturated fat diet substantially increased enterocytic Aβ in WT and in apo E KO mice, however the effect was greater in the latter. Villi height was significantly greater in apo E KO mice than for WT controls when given the low-fat diet. However, WT mice had comparable villi length to apo E KO when fed the saturated fat and cholesterol enriched diet. There was no effect of the high-fat diet on villi length in apo E KO mice.

Conclusion

The findings of this study are consistent with the notion that lipid substrate availability modulates enterocytic Aβ. Apo E may influence enterocytic lipid availability by modulating absorptive capacity.

Native and oxidized low-density lipoproteins modulate the vasoactive actions of soluble β-amyloid peptides in rat aorta

Cerebrovascular accumulation of Aβ (β-amyloid) occurs in aging and AD (Alzheimer's disease). Hypercholesterolaemia, which is associated with raised plasma LDL (low-density lipoprotein), may predispose to AD. Soluble Aβ is found in the circulation and enhances vasoconstriction. Under conditions that may favour the formation of short Aβ oligomers, as opposed to more severe polymerization leading to Aβ fibrillogenesis, we investigated the influence of LDLs on the vasoactive actions of soluble Aβ. Thus the actions of Aβ40 and Aβ42 in combination with native or oxidized LDL on vasoconstriction to NA (noradrenaline) and vasodilatation to ACh (acetylcholine) were examined in rat aortic rings. LDL, particularly when oxidized, potentiated NA-induced constriction when combined with soluble Aβ40 and, especially, Aβ42. Soluble Aβ40 reduced relaxation induced by ACh, but Aβ42 was ineffective. Native and oxidized LDL also attenuated relaxation. Synergism occurred between oxidized LDL and Aβ with respect to ACh-induced relaxation, but not between native LDL and Aβ. We have shown for the first time that, under conditions that may result in Aβ oligomer formation, LDL, particularly when oxidized, modulates the vascular actions of soluble Aβ to extents greater than those reported previously for fibrillar Aβ preparations. Mechanisms whereby a treatable condition, namely hypercholesterolaemia, might contribute to the development of the cerebrovascular component of AD are indicated.

Soluble aggregates of the amyloid-beta protein activate endothelial monolayers for adhesion and subsequent transmigration of monocyte cells

Increasing evidence suggests that the deposition of amyloid plaques, composed primarily of the amyloid-beta protein (Abeta), within the cerebrovasculature is a frequent occurrence in Alzheimer's disease and may play a significant role in disease progression. Accordingly, the pathogenic mechanisms by which Abeta can alter vascular function may have therapeutic implications. Despite observations that Abeta elicits a number of physiological responses in endothelial cells, ranging from alteration of protein expression to cell death, the Abeta species accountable for these responses remains unexplored. In the current study, we show that isolated soluble Abeta aggregation intermediates activate human brain microvascular endothelial cells for both adhesion and subsequent transmigration of monocyte cells in the absence of endothelial cell death and monolayer disruption. In contrast, unaggregated Abeta monomer and mature Abeta fibril fail to induce any change in endothelial adhesion or transmigration. Correlations between average Abeta aggregate size and observed increases in adhesion illustrate that smaller soluble aggregates are more potent activators of endothelium. These results support previous studies demonstrating heightened neuronal activity of soluble Abeta aggregates, including Abeta-derived diffusible ligands, oligomers, and protofibrils, and further show that soluble aggregates also selectively exhibit activity in a vascular cell model.

Soluble adhesion molecules and angiotensin-converting enzyme in dementia

We aimed to determine plasma and cerebrospinal fluid (CSF) levels of angiotensin-converting enzyme (ACE) and the soluble forms of intercellular adhesion molecule-1 (sICAM-1), vascular cell adhesion molecule-1 (sVCAM-1) and platelet endothelial cell adhesion molecule-1 (sPECAM-1) as surrogate markers for endothelial cell activation in clinically diagnosed patients with Alzheimer's disease (AD, n=260), dementia with Lewy bodies (DLB, n=39) and non-demented controls (n=34). Plasma sICAM-1 and sPECAM-1 were higher and CSF sVCAM-1 were lower in AD and DLB patients than in controls (p<0.001). DLB patients had higher CSF sICAM-1, but lower CSF sVCAM-1 (p<0.001). No difference in ACE levels was found between the dementia groups and controls. In controls and AD patients CSF sICAM and sVCAM-1 strongly correlated with each other and with blood barrier permeability whereas in DLB group these correlations were weaker. The observed patterns in adhesion molecules may reflect distinctions in the pathophysiological basis of their generation in dementia patients.

Insights into the mechanisms of action of anti-Abeta antibodies in Alzheimer's disease mouse models

A number of hypotheses regarding how anti-Abeta antibodies alter amyloid deposition have been postulated, yet there is no consensus as to how Abeta immunotherapy works. We have examined the in vivo binding properties, pharmacokinetics, brain penetrance, and alterations in Abeta levels after a single peripheral dose of anti-Abeta antibodies to both wild-type (WT) and young non-Abeta depositing APP and BRI-Abeta42 mice. The rapid rise in plasma Abeta observed after antibody (Ab) administration is attributable to prolongation of the half-life of Abeta bound to the Ab. Only a miniscule fraction of Ab enters the brain, and despite dramatic increases in plasma Abeta, we find no evidence that total brain Abeta levels are significantly altered. Surprisingly, cerebral spinal fluid Abeta levels transiently rise, and when Ab:Abeta complex is directly injected into the lateral ventricles of mice, it is rapidly cleared from the brain into the plasma where it remains stable. When viewed in context of daily turnover of Abeta, these data provide a framework to evaluate proposed mechanisms of Abeta attenuation mediated by peripheral administration of an anti-Abeta monoclonal antibody (mAb) effective in passive immunization paradigm. Such quantitative data suggest that the mAbs are either indirectly enhancing clearance of Abeta or targeting a low abundance aggregation intermediate.

Pericapillary haem-rich deposits: evidence for microhaemorrhages in aging human cerebral cortex

In this post mortem study, we examined haem-rich deposits (HRDs) in patients with and without dementia, using a histochemical label (Prussian blue) to show haem, autofluorescence to detect red blood cells (RBCs), and immunohistochemistry for clotting-related factors and collagen IV. The patients studied had no clinical or post mortem evidence of macrovascular stroke. To allow examination of the spatial relationships between HRDs and the microvasculature, we cut 45-microm sections. Haem-rich deposits were small (<200 microm diameter). They were rare in younger (<50 years) patients but were more common in older (>70 years) patients, particularly in cerebral cortex, and were most abundant in cases with senile plaques. Wherever HRDs appeared they were perivascular and appeared to form around capillaries or small arterioles. Using a software package (Proxan) developed to outline vessels and HRDs, and to analyse the distances between them, a tight spatial correlation between HRDs and capillaries was shown. In addition, HRDs were rich in von Willebrand factor (vWF), fibrinogen, collagen IV and RBCs. These observations suggest that HRDs are the residua of capillary bleeds (microhaemorrhages), and that microhaemorrhages are a common feature of the aging cerebral cortex, particularly where plaque pathology is present.

Neutral sphingomyelinase activation in endothelial and glial cell death induced by amyloid beta-peptide

We have explored the molecular mechanism underlying amyloid beta-peptide (Abeta)-mediated cytotoxicity in vitro. Exposure of murine cerebral endothelial cells (CECs) or C6 glioma cells to Abeta25-35 resulted in dose-dependent cell death. Ceramide is a pro-apoptotic lipid mediator. Forced elevation of cellular ceramide levels, either by application of an exogenous C2 ceramide analogue or bacterial sphingomyelinase that induces endogenous ceramide release from sphingomyelin, mimicked Abeta25-35 cytotoxicity in both CECs and C6 glioma cells. Abeta25-35-induced synthesis of ceramide was selectively mediated by activation of neutral sphingomyelinase (nSMase), but not acidic sphingomyelinase (aSMase) or ceramide synthase. Both 3-O-Me-SM and N-acetyl-L-cysteine, the selective and nonselective pharmacological inhibitors of nSMase, respectively, suppressed nSMase activation, ceramide production, and cytotoxic action induced by Abeta25-35 in CECs. Furthermore, genetic knockdown of nSMase by an antisense strategy rendered C6 glioma cells specifically resistant to Abeta25-35 cytotoxicity without affecting their vulnerability to serum deprivation. Together, nSMase activation with subsequent ceramide production may contribute, at least partially, to Abeta25-35 cytotoxicity in cell types with cerebral endothelial and glial lineage.

Altered CNS response to injury in the MRL/MpJ mouse

The MRL/MpJ mouse has a greatly enhanced healing response and an absence of scarring compared with other mouse strains. Following lesions to the CNS mammals show a scarring response known as reactive gliosis, and this CNS scar tissue blocks regeneration of cut axons. We have therefore compared reactive gliosis in the MRL/MpJ mouse and the Swiss Webster mouse, which exhibits normal scarring in the periphery. The lesion model was a stab lesion to the cortex, in which reactive gliosis has previously been quantified. Axon regeneration was examined following a cut lesion to the dopaminergic projection from the substantia nigra to the striatum used in previous regeneration experiments. In the MRL/MpJ following the lesion compared with Swiss Webster mice there was greater cell loss around the lesion followed by greater and more widespread and more prolonged cellular proliferation. Early after the lesion there was a greater loss of glial fibrillary acidic protein (GFAP)-positive astrocytes around the injury site in the MRL/MpJ, and an enhancement and prolongation of the microglial inflammatory response. This was accompanied by greater and more widespread blood-brain barrier leakage following injury. RNA levels for the matrix metalloproteinases (MMP)-2 and MMP-9 as well as for the thrombin receptors PAR-1 and PAR-4 were also greater at the MRL/MpJ injury site. All of these differences were transient and by 14 days post-injury there were no differences observed between MRL/MpJ and control mice. No axonal regeneration was observed following axotomy to the nigrostriatal pathway of the MRL/MpJ or the Swiss Webster mice at any time point.

Mechanisms of soluble beta-amyloid impairment of endothelial function

Alzheimer's disease (AD) has been recently associated with vascular risk factors. beta-amyloid peptides (AbetaP), the main component of senile plaques typical of AD, circulate in soluble globular form in bloodstream. Interestingly, AbetaP is able to induce endothelial dysfunction, and this effect may represent the link between vascular and neuronal pathophysiological factors involved in AD. We aimed to clarify the molecular mechanisms underlying globular AbetaP-induced vascular toxicity. Using several methodological approaches, we have observed that in vascular tissues globular AbetaP is unable to induce oxidative stress, one of the mechanisms hypothesized involved in beta-amyloid toxicity. More important, we have demonstrated that globular AbetaP is able to localize on vascular endothelium, where it inhibits eNOS enzymatic activity. In particular, AbetaP enhances eNOS phosphorylation on threonine 495 and serine 116 and reduces acetylcholine-induced phosphorylation on serine 1177. Such an effect depends on a PKC signaling pathway, as suggested by its phosphorylation on serine 660. In fact, selective inhibition of the calcium-dependent group of PKC is able to rescue beta-amyloid-induced alteration of eNOS phosphorylation, NO production, and endothelial vasorelaxation. The activation of these Ca(2+)-dependent pathways is probably due to the ability of AbetaP to evoke Ca(2+) leakage from inositol 1,4,5-triphosphate receptors on endoplasmic reticulum. Our data demonstrate that globular AbetaP-induced endothelial NO dysfunction can be attributed to an alteration of intracellular Ca(2+) homeostasis, which could lead to the activation of calcium-dependent group of PKC with a consequent change of the eNOS phosphorylation pattern. These mechanisms could contribute to shed further light on the toxic effect of beta-amyloid in vascular tissues.

Beta-amyloid peptide-induced blood-brain barrier disruption facilitates T-cell entry into the rat brain

Activated T-lymphocytes can migrate through the blood-brain barrier (BBB) and are able to invade the central nervous system (CNS). In the present study, we investigated whether disruption of the BBB leads to enhanced T-cell migration into the CNS. Amyloid-beta peptide 25-35 (A beta) or tumor necrosis factor-alpha (TNFalpha) were administered into the right common carotid artery of adult male Wistar rats. The agents were administered either alone, or were followed by a cell suspension of exogenously activated T-cells. Rats of other groups received activated or non-stimulated T-lymphocytes only. Sagittal brain sections were analyzed with immunohistochemistry of CD3 to reveal the presence of T-lymphocytes within the CNS parenchyma. Administration of activated T-cells alone led to T-cell migration into the brain. Infusion of either substances (A beta or TNFalpha) resulted in T-cell invasion of the CNS even when no exogenous T-cells were added. Infusion of either of the agents together with T-lymphocytes generated a more intense T-lymphocyte migration than in the other groups. Electron microscopic analysis and Evans-blue extravasation studies confirmed parallel disruption of the BBB. Our study demonstrates that A beta and TNFalpha induce enhanced T-lymphocyte migration towards the brain. This effect may be attributed at least partly to dysfunctioning of the BBB, but other mechanisms are also possible.

Canine counterpart of senile dementia of the Alzheimer type: amyloid plaques near capillaries but lack of spatial relationship with activated microglia and macrophages

Senile plaques and cerebrovascular amyloidosis are major histopathological lesions in the brains of aged dogs. Different types of amyloid beta protein (A beta) positive plaques are known: diffuse ones and neuritic plaques. Diffuse plaques may contain membrane-bound A beta and/or small amounts of amyloid fibrils. Neuritic plaques are cored plaques with clusters of amyloid fibrils and degenerating neurities. In human amyloid plaques, a pathogenetic role for microglia cells has been described. The aim of this investigation was to study microglia cells in relationship to canine plaques and to investigate the localisation of amyloid plaques in relationship to vasculature. The lesions were studied by hematoxylin and eosin Congo red staining and immunohistochemistry with anti-A beta for plaques, with Mac 387, anti lysozyme and a series of lectins for mononuclear cells, with anti von Willebrand Factor and Lycopersicon esculentum (tomato) lectin for the endothelium of brain capillaries. Diffuse A beta-positive plaques were found in dogs of 10.8 years and older, and cored A beta-positive plaques with birefringent amyloid in Congo red-stained sections in subjects of 15 years and older. Accumulation of microglia cells in relationship to the plaques was not obvious. With anti A beta 8-17 the distribution of the plaques in the cortical layers varied. The younger dogs had primarily diffuse plaques in the deeper layers of the cortical grey matter. The older dogs showed more cored plaques than diffuse plaques which were found throughout all cortical grey matter layers. With anti A beta x-42 more plaques were found positive, especially diffuse ones, whereas staining results of anti A beta x-40 were more confined to amyloid plaques and vascular amyloid. A close spatial relationship was found between the cored plaques and capillaries.

Alzheimer vaccine: amyloid-beta on trial

A new therapeutic approach is being developed for the treatment of Alzheimer's disease (AD). This approach involves the deliberate induction of an autoimmune response to amyloid-beta (Abeta) peptide, the constituent of neuritic plaques that is thought to cause the neurodegeneration and dementia in AD. If this approach is to be effective, antibodies must be produced that can selectively target the toxic forms of Abeta, while leaving the functionally-relevant forms of Abeta and its precursor protein untouched. Furthermore, an approach needs to be found that avoids provoking an acute neuroinflammatory response. The situation is made even more challenging by uncertainty regarding which isoforms of Abeta contribute to the pathogenesis of AD.

Abeta as a bioflocculant: implications for the amyloid hypothesis of Alzheimer's disease

Research into Alzheimer's disease (AD) has been guided by the view that deposits of fibrillar amyloid-beta peptide (Abeta) are neurotoxic and are largely responsible for the neurodegeneration that accompanies the disease. This 'amyloid hypothesis' has claimed support from a wide range of molecular, genetic and animal studies. We critically review these observations and highlight inconsistencies between the predictions of the amyloid hypothesis and the published data. We show that the data provide equal support for a 'bioflocculant hypothesis', which posits that Abeta is normally produced to bind neurotoxic solutes (such as metal ions), while the precipitation of Abeta into plaques may be an efficient means of presenting these toxins to phagocytes. We conclude that if the deposition of Abeta represents a physiological response to injury then therapeutic treatments aimed at reducing the availability of Abeta may hasten the disease process and associated cognitive decline in AD.

Antibodies to beta-amyloid decrease the blood-to-brain transfer of beta-amyloid peptide

Amyloid-beta peptides (Abeta) play an important role in the pathophysiology of dementia of the Alzheimer's type and in amyloid angiopathy. Abeta outside the CNS could contribute to plaque formation in the brain where its entry would involve interactions with the blood-brain barrier (BBB). Effective antibodies to Abeta have been developed in an effort to vaccinate against Alzheimer's disease. These antibodies could interact with Abeta in the peripheral blood, block the passage of Abeta across the BBB, or prevent Abeta deposition within the CNS. To determine whether the blocking antibodies act at the BBB level, we examined the influx of radiolabeled Abeta (125I-Abeta(1-40)) into the brain after ex-vivo incubation with the antibodies. Antibody mAb3D6 (élan Company) reduced the blood-to-brain influx of Abeta after iv bolus injection. It also significantly decreased the accumulation of Abeta in brain parenchyma. To confirm the in-vivo study and examine the specificity of mAb3D6, in-situ brain perfusion in serum-free buffer was performed after incubation of 125I-Abeta(1-40) with another antibody mAbmc1 (DAKO Company). The presence of mAbmc1 also caused significant reduction of the influx of Abeta into the brain after perfusion. Therefore, effective antibodies to Abeta can reduce the influx of Abeta(1-40) into the brain.

Innate immunity: the missing link in neuroprotection and neurodegeneration?

Innate immunity was previously thought to be a nonspecific immunological programme that was engaged by peripheral organs to maintain homeostasis after stress and injury. Emerging evidence indicates that this highly organized response also takes place in the central nervous system. Through the recognition of neuronal fingerprints, the long-term induction of the innate immune response and its transition to an adaptive form might be central to the pathophysiology and aetiology of neurodegenerative disorders. Paradoxically, this response also protects neurons by favouring remyelination and trophic support afforded by glial cells.

Cerebrovascular structure and dementia: new drug targets

Effective pharmacological treatment of cognitive disorders in dementia is lacking despite extensive efforts to produce active therapy aimed at neuronal and vascular targets. In this review, the evidence for the involvement of vascular mechanisms in the pathology and evolution of dementia will be examined and the potential importance of age-related changes in cerebrovascular structure and cerebral blood flow (CBF) autoregulation will be discussed. With a description of recent clinical results (on statins, angiotensin-converting enzyme inhibitors and Ca(2+) channel blockers) and experimental results (on beta-amyloid), the impact of drugs on cerebrovascular targets is examined. The working hypothesis that targeting vascular mechanisms in dementia is an option for future therapy is proposed.

Permeability of proteins at the blood-brain barrier in the normal adult mouse and double transgenic mouse model of Alzheimer's disease

The permeability of albumin, insulin, and human A beta 1--40 at the blood-brain barrier (BBB) was determined in the normal adult mouse (B6/SJL) and in the double transgenic Alzheimer mouse (APP, PS1) by using an I.V. bolus injection technique to quantify the permeability coefficient-surface area (PS) product for each protein after correction for the residual plasma volume (V(p)) occupied by the protein in the blood vessels of different brain regions using a second aliquot of the same protein radiolabeled with a different isotope of iodine ((125)I vs (131)I) as a vascular space marker. This technology for quantifying BBB permeability of proteins was adapted from the rat to the mouse and involved catheterizing the femoral artery and vein of the mouse instead of the brachial artery and vein as for the rat. Because of the smaller blood volume in the mouse, serial sampling (20 microl) of blood from the femoral artery of the mouse was performed and directly TCA precipitated to generate a whole blood washout curve for the intact protein. When similar blood sampling techniques were used in the rat, the PS values for albumin and insulin at the BBB were similar in these two species. In the double transgenic mouse, the V(p) values for albumin were significantly increased 1.4- to 1.6-fold in five of six brain regions compared to the normal adult mouse, which indicated increased adherence of albumin to vessel walls. As a result, the PS values were significantly decreased, from 1.4- to 3.2-fold, which likely reflected decreased transport of albumin by passive diffusion. In contrast, insulin, which is taken up into the brain by a receptor-mediated transport mechanism at the BBB, showed no significant difference in the V(p) values but a significant increase in the PS values in four of six brain regions. This suggests a compensatory mechanism in the Alzheimer's transgenic brain whereby there is an increased permeability to insulin at the BBB. Surprisingly, there was no significant difference in the V(p) or PS values for human A beta 1--40 at the BBB in the double transgenic Alzheimer mouse at 24, 32, or 52 weeks of age, when there is both significant A beta levels in the plasma and amyloid burden in the brains of these animals. These data suggest that there is not an alteration in permeability to human A beta 1--40 at the BBB with increasing amyloid burden in the double transgenic Alzheimer mouse. Although these observations suggest structural alterations at the BBB, they do not support the concept of extensive BBB damage with substantial increases in BBB permeability in Alzheimer's disease.

The functions of the amyloid precursor protein gene

The amyloid precursor protein (APP) gene and its protein products have multiple functions in the central nervous system and fulfil criteria as neuractive peptides: presence, release and identity of action. There is increased understanding of the role of secretases (proteases) in the metabolism of APP and the production of its peptide fragments. The APP gene and its products have physiological roles in synaptic action, development of the brain, and in the response to stress and injury. These functions reveal the strategic importance of APP in the workings of the brain and point to its evolutionary significance.

Designing drugs to stop the formation of prion aggregates and other amyloids

Amyloid protein aggregates are implicated in many neurodegenerative diseases, including Alzheimer's disease and the prion diseases. Therapeutics to block amyloid formation are often tested in vitro, but it is not clear how to extrapolate from these experiments to a clinical setting, where the effective drug dose may be much lower. Here we address this question using a theoretical kinetic model to calculate the growth rate of protein aggregates as a function of the dose of each of three categories of drug. We find that therapeutics which block the growing ends of amyloids are the most promising, as alternative strategies may be ineffective or even accelerate amyloid formation at low drug concentrations. Our mathematical model can be used to identify and optimise an end-blocking drug in vitro. Our model also suggests an alternative explanation for data previously thought to prove the existence of an entity known as protein X.

Permeability of the blood-brain barrier to albumin and insulin in the young and aged SAMP8 mouse

The decrease in the insulin cerebrospinal fluid/serum ratio seen in Alzheimer's disease has been suggested as a mechanism by which brain glucose utilization could be perturbed. Insulin is transported across the blood-brain barrier (BBB) by a system that is altered by pathophysiological events. We used SAMP8 mice, a strain that by 8-12 months of age develops severe deficits in learning and memory, to determine whether the insulin transporter or BBB integrity was altered with aging. BBB integrity was measured by injecting radioactive albumin intravenously, washing out the vascular space up to 17 hours later, and measuring brain/serum ratios. This very sensitive method found no increase in the permeability of the BBB to albumin in young and aged SAMP8 mice. This compares with previous studies in humans with Alzheimer's disease and in other colonies of SAMP8 mice that have found evidence for BBB disruption. For radioactively labeled insulin, we used multiple-time regression analysis to measure both the unidirectional influx rate (Ki) and the reversible binding to brain endothelium (Vi). A non-significant decrease in the transport rate for whole brain occurred in aged SAMP8 mice. Ki and Vi values significantly varied among brain regions and the Ki for the thalamus and the Vi for the cerebellum and thalamus were higher in aged mice. We conclude that alterations in BBB integrity or the activity of the BBB insulin transporter do not underlie the deficits in learning and memory seen in the aged SAMP8 mouse.

Alzheimer's disease and inflammation: a review of cellular and therapeutic mechanisms

1. Of the neurodegenerative diseases that cause dementia, Alzheimer's disease (AD) is the most common. Three major pathologies characterize the disease: senile plaques, neurofibrillary tangles and inflammation. We review the literature on events contributing to the inflammation and the treatments thought to target this pathology. 2. The senile plaques of AD consist primarily of complexes of the beta-amyloid protein. This protein is central to the pathogenesis of the disease. 3. Inflammatory microglia are consistently associated with senile plaques in AD, although the classic inflammatory response (immunoglobulin and leucocyte infiltration) is absent. beta-Amyloid fragments appear to mediate such inflammatory mechanisms by activating the complement pathway in a similar fashion to immunoglobulin. 4. Epidemiological studies have identified a reduced risk of AD in patients with arthritis and in leprosy patients treated with anti-inflammatory drugs. Longitudinal studies have shown that the consumption of anti-inflammatory medications reduces the risk of AD only in younger patients (< 75 years). 5. There is a considerable body of in vitro evidence indicating that the inflammatory response of microglial cells is reduced by non-steroidal anti-inflammatory drugs (NSAID). However, no published data are available concerning the effects of these medications on brain pathology in AD. 6. Cyclo-oxygenase 2 enzyme is constitutively expressed in neurons and is up-regulated in degenerative brain regions in AD. Non-steroidal anti-inflammatory drugs may reduce this expression. 7. Platelets are a source of beta-amyloid and increased platelet activation and increased circulating beta-amyloid have been identified in AD. Anti-platelet medication (including NSAID) would prevent such activation and its potentially harmful consequences. 8. Increased levels of luminal beta-amyloid permeabilizes the blood-brain barrier (BBB) and increases vasoconstriction of arterial vessels, paralleling the alterations observed with infection and inflammation. Cerebral amyloidosis is highly prevalent in AD, compromising the BBB and vasoactivity. Anti-inflammatory medications may alleviate these problems.