Altered Gene Expression Within the Renin-Angiotensin System in Normal Aging and Dementia.

The renin-angiotensin system (RAS) is dysregulated in Alzheimer's disease (AD). In this study, we have explored the hypothesis that an -age--related imbalance in brain RAS is a trigger for RAS dysregulation in AD. We characterized RAS gene expression in the frontal cortex from (i) a cohort of normal aging (n = 99, age range = 19-96 years) and (ii) a case-control cohort (n = 209) including AD (n = 66), mixed dementia (VaD + AD; n = 50), pure vascular dementia (VaD; n = 42), and age-matched controls (n = 51). The AD, mixed dementia, and age-matched controls were further stratified by Braak tangle stage (BS): BS0-II (n = 48), BSIII-IV (n = 44), and BSV-VI (n = 85). Gene expression was calculated by quantitative PCR (qPCR) for ACE1, AGTR1, AGTR2, ACE2, LNPEP, and MAS1 using the 2-∆∆Cq method, after adjustment for reference genes (RPL13 and UBE2D2) and cell-specific calibrator genes (NEUN, GFAP, PECAM). ACE1 and AGTR1, markers of classical RAS signaling, and AGTR2 gene expression were elevated in normal aging and gene expression in markers of protective downstream regulatory RAS signaling, including ACE2, MAS1, and LNPEP, were unchanged. In AD and mixed dementia, AGTR1 and AGTR2 gene expression were elevated in BSIII-IV and BSV-VI, respectively. MAS1 gene expression was reduced at BSV-VI and was inversely related to parenchymal Aß and tau load. LNPEP gene expression was specifically elevated in VaD. These data provide novel insights into RAS signaling in normal aging and dementia.

Elevated late-life blood pressure may maintain brain oxygenation and slow amyloid-ß accumulation at the expense of cerebral vascular damage.

Hypertension in midlife contributes to cognitive decline and is a modifiable risk factor for dementia. The relationship between late-life hypertension and dementia is less clear. We have investigated the relationship of blood pressure and hypertensive status during late life (after 65 years) to post-mortem markers of Alzheimer's disease (amyloid-ß and tau loads); arteriolosclerosis and cerebral amyloid angiopathy; and to biochemical measures of ante-mortem cerebral oxygenation (the myelin-associated glycoprotein:proteolipid protein-1 ratio, which is reduced in chronically hypoperfused brain tissue, and the level of vascular endothelial growth factor-A, which is upregulated by tissue hypoxia); blood-brain barrier damage (indicated by an increase in parenchymal fibrinogen); and pericyte content (platelet-derived growth factor receptor ß, which declines with pericyte loss), in Alzheimer's disease (n = 75), vascular (n = 20) and mixed dementia (n = 31) cohorts. Systolic and diastolic blood pressure measurements were obtained retrospectively from clinical records. Non-amyloid small vessel disease and cerebral amyloid angiopathy were scored semiquantitatively. Amyloid-ß and tau loads were assessed by field fraction measurement in immunolabelled sections of frontal and parietal lobes. Homogenates of frozen tissue from the contralateral frontal and parietal lobes (cortex and white matter) were used to measure markers of vascular function by enzyme-linked immunosorbent assay. Diastolic (but not systolic) blood pressure was associated with the preservation of cerebral oxygenation, correlating positively with the ratio of myelin-associated glycoprotein to proteolipid protein-1 and negatively with vascular endothelial growth factor-A in both the frontal and parietal cortices. Diastolic blood pressure correlated negatively with parenchymal amyloid-ß in the parietal cortex. In dementia cases, elevated late-life diastolic blood pressure was associated with more severe arteriolosclerosis and cerebral amyloid angiopathy, and diastolic blood pressure correlated positively with parenchymal fibrinogen, indicating blood-brain barrier breakdown in both regions of the cortex. Systolic blood pressure was related to lower platelet-derived growth factor receptor ß in controls in the frontal cortex and in dementia cases in the superficial white matter. We found no association between blood pressure and tau. Our findings demonstrate a complex relationship between late-life blood pressure, disease pathology and vascular function in dementia. We suggest that hypertension helps to reduce cerebral ischaemia (and may slow amyloid-ß accumulation) in the face of increasing cerebral vascular resistance, but exacerbates vascular pathology.

Mediators of cerebral hypoperfusion and blood-brain barrier leakiness in Alzheimer's disease, vascular dementia and mixed dementia.

In vascular dementia (VaD) and Alzheimer's disease (AD), cerebral hypoperfusion and blood-brain barrier (BBB) leakiness contribute to brain damage. In this study, we have measured biochemical markers and mediators of cerebral hypoperfusion and BBB in the frontal (BA6) and parietal (BA7) cortex and underlying white matter, to investigate the pathophysiology of vascular dysfunction in AD, VaD and mixed dementia. The ratio of myelin-associated glycoprotein to proteolipid protein-1 (MAG:PLP1), a post-mortem biochemical indicator of the adequacy of ante-mortem cerebral perfusion; the concentration of fibrinogen adjusted for haemoglobin level, a marker of blood-brain barrier (BBB) leakiness; the level of vascular endothelial growth factor-A (VEGF), a marker of tissue hypoxia; and endothelin-1 (EDN1), a potent vasoconstrictor, were measured by ELISA in the frontal and parietal cortex and underlying white matter in 94 AD, 20 VaD, 33 mixed dementia cases and 58 age-matched controls. All cases were assessed neuropathologically for small vessel disease (SVD), cerebral amyloid angiopathy (CAA) severity, Aß and phospho-tau parenchymal load, and Braak tangle stage. Aß40 and Aß42 were measured by ELISA in guanidine-HCl tissue extracts. We found biochemical evidence of cerebral hypoperfusion in AD, VaD and mixed dementia to be associated with SVD, Aß level, plaque load, EDN1 level and Braak tangle stage, and to be most widespread in mixed dementia. There was evidence of BBB leakiness in AD-limited to the cerebral cortex and related to EDN1 level. In conclusion, abnormalities of cerebral perfusion and BBB function in common types of dementia can largely be explained by a combination of arteriolosclerosis, and Aß-, tau- and endothelin-related vascular dysfunction. The relative contributions of these processes vary considerably both between and within the diseases.

Zibotentan, an Endothelin A Receptor Antagonist, Prevents Amyloid-ß-Induced Hypertension and Maintains Cerebral Perfusion.

Cerebral blood flow is reduced in Alzheimer's disease (AD), which is associated with mid-life hypertension. In people with increased cerebral vascular resistance due to vertebral artery or posterior communicating artery hypoplasia, there is evidence that hypertension develops as a protective mechanism to maintain cerebral perfusion. In AD, amyloid-ß (Aß) accumulation may similarly raise cerebral vascular resistance by upregulation of the cerebral endothelin system. The level of endothelin-1 in brain tissue correlates positively with Aß load and negatively with markers of cerebral hypoperfusion such as increased vascular endothelial growth factor. We previously showed that cerebroventricular infusion of Aß40 exacerbated pre-existing hypertension in Dahl rats. We have investigated the effects of 28-day cerebral infusion of Aß40 on blood pressure and heart rate and their variability; carotid flow; endothelin-1; and markers of cerebral oxygenation, in the (normotensive) Wistar rat, and the modulatory influence of the endothelin A receptor antagonist Zibotentan (ZD4054). Cerebral infusion of Aß caused progressive rise in blood pressure (p < 0.0001) (paired t-test: increase of 3 (0.1-5.6) mmHg (p = 0.040)), with evidence of reduced baroreflex responsiveness, and accumulation of Aß and elevated endothelin-1 in the vicinity of the infusion. Oral Zibotentan (3 mg/kg/d, administered for 31 d) abrogated the effects of Aß40 infusion on baroreflex responsiveness and blood pressure, which declined, although without reduction in carotid blood flow, and Zibotentan caused uncoupling of the positive linear relationship between endothelin-1 and vascular endothelial growth factor, which as a sensor of tissue oxygenation would be expected to increase if there were hypoperfusion.

Systemic infection modifies the neuroinflammatory response in late stage Alzheimer's disease.

Clinical studies indicate that systemic infections accelerate cognitive decline in Alzheimer's disease. Animal models suggest that this may be due to enhanced pro-inflammatory changes in the brain. We have performed a post-mortem human study to determine whether systemic infection modifies the neuropathology and in particular, neuroinflammation, in the late-stage of the disease.Sections of cerebral cortex and underlying white matter from controls and Alzheimer's patients who died with or without a terminal systemic infection were immunolabelled and quantified for: (i) Αß and phosphorylated-tau; (ii) the inflammation-related proteins Iba1, CD68, HLA-DR, FcγRs (CD64, CD32a, CD32b, CD16), CHIL3L1, IL4R and CCR2; and (iii) T-cell marker CD3. In Alzheimer's disease, the synaptic proteins synaptophysin and PSD-95 were quantified by ELISA, and the inflammatory proteins and mRNAs by MesoScale Discovery Multiplex Assays and qPCR, respectively.Systemic infection in Alzheimer's disease was associated with decreased CD16 (p = 0.027, grey matter) and CD68 (p = 0.015, white matter); increased CD64 (p = 0.017, white matter) as well as increased protein expression of IL6 (p = 0.047) and decreased IL5 (p = 0.007), IL7 (p = 0.002), IL12/IL23p40 (p = 0.001), IL15 (p = 0.008), IL16 (p < 0.001) and IL17A (p < 0.001). Increased expression of anti-inflammatory genes CHI3L1 (p = 0.012) and IL4R (p = 0.004) were detected in this group. T-cell recruitment to the brain was reduced when systemic infection was present. However, exposure to systemic infection did not modify the pathology. In Alzheimer's disease, CD68 (p = 0.026), CD64 (p = 0.002), CHI3L1 (p = 0.016), IL4R (p = 0.005) and CCR2 (p = 0.010) were increased independently of systemic infection.Our findings suggest that systemic infections modify neuroinflammatory processes in Alzheimer's disease. However, rather than promoting pro-inflammatory changes, as observed in experimental models, they seem to promote an anti-inflammatory, potentially immunosuppressive, environment in the human brain.

Cerebral Aß40 and systemic hypertension.

Mid-life hypertension and cerebral hypoperfusion may be preclinical abnormalities in people who later develop Alzheimer's disease. Although accumulation of amyloid-beta (Aß) is characteristic of Alzheimer's disease and is associated with upregulation of the vasoconstrictor peptide endothelin-1 within the brain, it is unclear how this affects systemic arterial pressure. We have investigated whether infusion of Aß40 into ventricular cerebrospinal fluid modulates blood pressure in the Dahl salt-sensitive rat. The Dahl salt-sensitive rat develops hypertension if given a high-salt diet. Intracerebroventricular infusion of Aß induced a progressive rise in blood pressure in rats with pre-existing hypertension produced by a high-salt diet ( p < 0.0001), but no change in blood pressure in normotensive rats. The elevation in arterial pressure in high-salt rats was associated with an increase in low frequency spectral density in systolic blood pressure, suggesting autonomic imbalance, and reduced cardiac baroreflex gain. Our results demonstrate the potential for intracerebral Aß to exacerbate hypertension, through modulation of autonomic activity. Present findings raise the possibility that mid-life hypertension in people who subsequently develop Alzheimer's disease may in some cases be a physiological response to reduced cerebral perfusion complicating the accumulation of Aß within the brain.

Synaptic protein levels altered in vascular dementia.

INTRODUCTION: Cerebral ischaemia is the defining pathophysiological abnormality in most forms of vascular dementia (VAD), but the pathogenesis of the dementia remains poorly understood. In Alzheimer's disease (AD), there is early loss of synaptic proteins, but these have been little studied in VAD. MATERIALS AND METHODS: We measured synaptophysin, postsynaptic density protein 95 (PSD-95), drebrin, synaptosomal-associated protein 25 (SNAP-25) and vascular endothelial growth factor (VEGF) by enzyme-linked immunosorbent assays in superior temporal cortex from 11 patients with VAD and, initially, 11 non-dementia controls. We corrected for neuronal content by measurement of neuron-specific enolase. A further 11 controls were subsequently used in a validation study. Simulation of post-mortem delay found that PSD-95 was stable at 4°C but declined slightly at RT. SNAP-25 and drebrin showed good post-mortem stability. Previous studies had shown good post-mortem preservation of synaptophysin and VEGF. RESULTS: The VAD cases had lower synaptophysin (but P > 0.05 in initial study), significantly lower SNAP-25 (P = 0.024) and significantly higher drebrin (P = 0.020). On comparison with the second control group, the reduction in synaptophysin was significant (P = 0.008), and the other results were confirmed. CONCLUSION: There is probably a reduction in presynaptic proteins in the temporal cortex in VAD, although not as marked as in AD. In VAD, there is also an increase in drebrin, which may be a response to reduced synaptic input.

Aß degradation or cerebral perfusion? Divergent effects of multifunctional enzymes.

There is increasing evidence that deficient clearance of ß-amyloid (Aß) contributes to its accumulation in late-onset Alzheimer disease (AD). Several Aß-degrading enzymes, including neprilysin (NEP), endothelin-converting enzyme (ECE), and angiotensin-converting enzyme (ACE) reduce Aß levels and protect against cognitive impairment in mouse models of AD. In post-mortem human brain tissue we have found that the activity of these Aß-degrading enzymes rise with age and increases still further in AD, perhaps as a physiological response that helps to minimize the build-up of Aß. ECE-1/-2 and ACE are also rate-limiting enzymes in the production of endothelin-1 (ET-1) and angiotensin II (Ang II), two potent vasoconstrictors, increases in the levels of which are likely to contribute to reduced blood flow in AD. This review considers the possible interdependence between Aß-degrading enzymes, ischemia and Aß in AD: ischemia has been shown to increase Aß production both in vitro and in vivo, whereas increased Aß probably enhances ischemia by vasoconstriction, mediated at least in part by increased ECE and ACE activity. In contrast, NEP activity may help to maintain cerebral perfusion, by reducing the accumulation of Aß in cerebral blood vessels and lessening its toxicity to vascular smooth muscle cells. In assessing the role of Aß-degrading proteases in the pathogenesis of AD and, particularly, their potential as therapeutic agents, it is important to bear in mind the multifunctional nature of these enzymes and to consider their effects on other substrates and pathways.

Endothelin-converting enzyme-1 activity, endothelin-1 production, and free radical-dependent vasoconstriction in Alzheimer's disease.

Alzheimer's disease (AD) patients have reduced cerebral blood flow. This precedes dementia and may contribute to its progression. In mice that overexpress amyloid-ß protein precursor, cerebral blood flow declines before the development of plaques or cognitive abnormalities. In the brain, endothelin-1 (ET-1) is a locally acting vasoconstrictor, produced in neurons by endothelin-converting enzyme (ECE)-2 and in endothelial cells by ECE-1. Both ECEs are also capable of cleaving amyloid-ß (Aß). We previously showed ECE-2 and ET-1 to be elevated in postmortem temporal cortex from AD patients, and ECE-2 expression and ET-1 release to be upregulated by Aß42 in vitro. We have now studied isolated leptomeningeal blood vessels from postmortem brains and found that although ECE-1 level is reduced, ECE-1 activity and ET-1 level are significantly elevated in AD vessels. This is specific to AD as there is no specific change in vascular dementia vessels. In primary cultures of human brain endothelial cells, both Aß40 and Aß42 caused a significant increase in ET-1 release, the increase being particularly pronounced with Aß40. In view of previous studies implicating free radicals in the endothelial dysfunction caused by Aß40, we examined whether Aß-mediated ET-1 release could be prevented by the antioxidant superoxide dismutase. Addition of superoxide dismutase to cells exposed to Aß40 prevented the increase in the concentration of ET-1. Our findings indicate that cerebral vasoconstriction induced by Aß results in part from a free radical-mediated increase in ECE-1 activity and ET-1 production.

Oxidative balance in Alzheimer's disease: relationship to APOE, Braak tangle stage, and the concentrations of soluble and insoluble amyloid-ß.

Oxidative damage is greater in brain tissue from patients with Alzheimer's disease (AD) than age-matched controls. The timing of this damage in relation to other pathogenic processes in AD remains unclear. We have examined the relationship of lipid peroxidation (thiobarbituric acid-reactive substances; TBARS) and antioxidant capacity (Trolox-equivalent) to APOE status, Braak tangle stage, amyloid-ß (Aß) plaque load, and the concentration of soluble and insoluble forms of Aß, post-synaptic and dendritic spine proteins PSD95 and drebrin, ß-secretase and Aß-degrading enzymes neprilysin (NEP), insulin-degrading enzyme (IDE), and angiotensin-converting enzyme (ACE), in frontal, temporal, and parietal cortex from AD and control brains. Antioxidant capacity was significantly elevated in AD and directly related to disease severity as indicated by Braak tangle stage and the amount of insoluble Aß. APOE ε4 was associated with increased antioxidant capacity in AD but not controls. In contrast, apart from a reduction in TBARS in Braak stages III-IV in frontal cortex, this measure of oxidative damage did not change significantly with any indicator of disease severity. It was, however, higher in APOE ε4-positive than ε4-negative AD patients and correlated with ß-secretase activity. Neither antioxidant capacity nor oxidative damage was related to the level of PSD95 or drebin or the activity of NEP, IDE, or ACE. Antioxidant capacity in AD is closely related to the level of insoluble Aß and increases with pathological progression of the disease. Increased ß-secretase activity associated with oxidative stress is likely to contribute to the accumulation of Aß and this, in turn, to induce antioxidant capacity.

Distribution and expression of picalm in Alzheimer disease.

PICALM, the gene encoding phosphatidylinositol-binding clathrin assembly (picalm) protein, was recently shown to be associated with risk of Alzheimer disease (AD). Picalm is a key component of clathrin-mediated endocytosis. It recruits clathrin and adaptor protein 2 (AP-2) to the plasma membrane and, along with, AP-2 recognizes target proteins. The attached clathrin triskelions cause membrane deformation around the target proteins enclosing them within clathrin-coated vesicles to be processed in lysosomes or endosomes. We examined the distribution of picalm in control and AD brain tissue and measured levels of picalm messenger RNA (mRNA) by real-time polymerase chain reaction. Immunolabeling of brain tissue showed that picalm is predominately present in endothelial cells. This was further supported by the demonstration of picalm in human cerebral microvascular cells grown in culture. Picalm mRNA was elevated in relation to glyceraldehyde-3-phosphate dehydrogenase but not factor VIII-related antigen or CD31 mRNA in the frontal cortex in AD. No change was seen in the temporal cortex or thalamus. The transport of Aß across vessel walls and into the bloodstream is a major pathway of Aß removal from the brain and picalm is ideally situated within endothelial cells to participate in this process. Further research is needed to determine whether PICALM expression is influenced by Aß levels and whether it affects Aß uptake and transport by endothelial cells.

Fatty acid composition of frontal, temporal and parietal neocortex in the normal human brain and in Alzheimer's disease.

Dietary omega3-polyunsaturated fatty acids are thought to influence the risk of Alzheimer's disease (AD), and supplemental docosahexaenoic acid (DHA; 22:6n-3) has been reported to reduce neurodegeneration in mouse models of AD. We have analysed the fatty acid composition of frontal, temporal and parietal neocortex in 58 normal and 114 AD brains. Significant reductions were found for stearic acid (18:0) in frontal and temporal cortex and arachidonic acid (20:4n-6) in temporal cortex in AD, and increases in oleic acid in frontal and temporal cortex (18:1n-9) and palmitic acid (16:0) in parietal cortex. DHA level varied more in AD than controls but the mean values were not significantly different. Fatty acid composition was not related to APOE genotype, age, gender or post-mortem delay. Further research is needed to distinguish between alterations that are secondary to AD and those that contribute to the disease process.

Angiotensin-converting enzyme levels and activity in Alzheimer's disease: differences in brain and CSF ACE and association with ACE1 genotypes.

Angiotensin-converting enzyme (ACE) has been implicated in Alzheimer's disease (AD): ACE1 variations influence plasma ACE and risk of AD, and ACE is increased in AD brain. We measured frontal ACE level and activity in 89 AD and 51 control brains, and post-mortem CSF from 101 cases and 19 controls. Neuron-specific enolase (NSE) level and Braak stage were used to indicate neuronal preservation and disease progression. We genotyped the common ACE insertion/deletion polymorphism, rs4343, rs1800764 and rs4921. ACE activity was elevated in AD and correlated with Braak stage. Crude ACE levels were unchanged but adjustment for NSE suggested increased neuronal ACE production with Braak stage. Exposing SH-SY-5Y neurons to oligomeric Abeta1-42 increased ACE level and activity, suggesting Abeta may upregulate ACE in AD. In CSF, ACE level but not activity was reduced in AD. ACE1 genotype did not predict ACE level or activity in brain or CSF. ACE activity and neuronal production increase in AD brain, possibly in response to Abeta. Peripheral measurements do not reflect ACE activity in the brain.

Neprilysin and insulin-degrading enzyme levels are increased in Alzheimer disease in relation to disease severity.

Experimental reduction of neprilysin (NEP) or insulin-degrading enzyme (IDE) in vivo exacerbates beta-amyloid accumulation in the brain. The level of these enzymes is reportedly reduced during aging and in postmortem brains of patients with sporadic Alzheimer disease (AD). To distinguish between primary decreases in NEP and IDE activity that might contribute to beta-amyloid accumulation and decreases secondary to neurodegenerative changes in AD, we measured NEP and IDE levels by indirect sandwich ELISA and enzyme activities by immunocapture-based fluorogenic assays in postmortem frontal cortex from patients of different ages and at different pathological stages of AD, as indicated by Braak tangle stage. The ELISA measurements of neuron-specific enolase were used to adjust for neuronal loss. Both unadjusted and neuron-specific enolase-adjusted NEP levels and activity were significantly increased in AD and positively correlated with Braak stage but negatively with age in AD patients. Insulin-degrading enzyme activity was higher in AD than controls; this was significant after adjustment for neuron-specific enolase level; unadjusted IDE protein level was decreased in AD but not after adjustment. Our findings suggest that reduction in NEP and IDE activity is not the primary cause of beta-amyloid accumulation in AD, but rather a late-stage phenomenon secondary to neurodegeneration.

Low-temperature improved-throughput method for analysis of brain fatty acids and assessment of their post-mortem stability.

Deficiency of docosahexaenoic acid (DHA) and other omega-3 (omega3) fatty acids may constitute an alterable risk factor for Alzheimer's disease (AD). Mechanisms of potential involvement of DHA in the disease process have been postulated primarily from studies in vitro and in mouse models of AD. Information on the fatty acid profile of the brain in AD itself is limited and in some respects contradictory. Interpretation of the findings is complicated by the diversity of methods used in previous studies and a lack of information as to the effect of post-mortem delay on the results. Here we report the development of a simple and highly reproducible method that enables relatively high-throughput measurement of the fatty acid composition in samples of brain tissue and using this method we have demonstrated that there is no significant change in fatty acid composition under conditions designed to model post-mortem delay of up to 3 days at 4 degrees C (or even at room temperature). The development of this method and the observation that delay of up to 3 days has no effect on fatty acid content will facilitate further studies of fatty acid composition on large cohorts of post-mortem brains.