Emma L. Ashby

Angiotensin‐converting enzyme (ACE) levels and activity in Alzheimer's disease, and relationship of perivascular ACE‐1 to cerebral amyloid angiopathy

Aims: Several observations point to the involvement of angiotensin‐converting enzyme‐1 (ACE‐1) in Alzheimers disease (AD): ACE‐1 cleaves amyloid‐β peptide (Aβ) in vitro, the level and activity of ACE‐1 are reportedly increased in AD, and variations in the ACE‐1 gene are associated with AD. We analysed ACE‐1 activity and expression in AD and control brains, particularly in relation to Aβ load and cerebral amyloid angiopathy (CAA). Methods: ACE‐1 activity was measured in the frontal cortex from 58 control and 114 AD cases of known Aβ load and CAA severity. The distribution of ACE‐1 was examined immunohistochemically. In five AD cases with absent or mild CAA, five with moderate to severe CAA and five controls with absent or mild CAA, levels of vascular ACE‐1 were assessed by quantitative immunofluorescence. Results: ACE‐1 activity was increased in AD (P < 0.001) and correlated directly with parenchymal Aβ load (P = 0.05). Immunohistochemistry revealed ACE‐1 in neurones and cortical blood vessels – in the intima but most abundant perivascularly. Cases with moderate to severe CAA had significantly more vessel‐associated ACE‐1 than did those with little or no CAA. Perivascular ACE‐1 did not colocalize with Aβ, smooth muscle actin, glial fibrillary acidic protein, collagen IV, vimentin or laminin, but was similarly distributed to extracellular matrix (ECM) proteins fibronectin and decorin. Conclusions: Our findings indicate that ACE‐1 activity is increased in AD, in direct relationship to parenchymal Aβ load. Increased ACE‐1, probably of neuronal origin, accumulates perivascularly in severe CAA and colocalizes with vascular ECM. The possible relationship of ACE‐1 to the deposition of perivascular ECM remains to be determined.

Pathophysiology of white matter perfusion in Alzheimer’s disease and vascular dementia

The pathophysiology of white matter hypoperfusion is poorly understood. Barker et al. quantify ante-mortem hypoperfusion by measuring myelin proteins differentially susceptible to ischaemia, and assess the extent to which vasoregulatory factors protect from or contribute to ischaemic white matter injury in Alzheimer’s disease and vascular dementia.

Current status of renin-aldosterone angiotensin system-targeting anti-hypertensive drugs as therapeutic options for Alzheimer's disease.

Introduction: Hypertension is a modifiable risk factor for Alzheimers disease (AD) and other dementias. Yet, despite this well-documented association, few of the current strategies to treat AD are directed at this possible target. The renin–aldosterone angiotensin system (RAAS) is a centrally active modifiable pathway that is involved in cerebral blood flow regulation. Currently, three classes of RAAS-targeting drugs are licensed for treatment of peripheral hypertension – angiotensin-converting enzyme inhibitors (ACE-Is), angiotensin II receptor blockers (ARBs) and direct renin inhibitors (DRIs). All of these are generally well tolerated and have been shown to offer varying degrees of protection on aspects of cognition and dementia, thus making them an attractive therapeutic option for AD. Areas covered: This review summarises existing evidence regarding the plausibility of using RAAS-targeting drugs as a strategy to treat AD and highlights unresolved aspects to such approaches, namely the potential impact of altering angiotensin II-mediated processes in the central nervous system. Expert opinion: Continued biochemical research of the RAAS pathway in combination with formal investigation of current RAAS-modifying drugs in randomised clinical trials is now necessary to determine their therapeutic value in AD.

Kallikrein-related peptidase 6 in Alzheimer's disease and vascular dementia.

Human kallikrein-related peptidase 6 (KLK6) is highly expressed in the central nervous system. Although the physiological roles of this serine protease are unknown, in vitro substrates include amyloid precursor protein and components of the extracellular matrix, which are altered in neurological disease, particularly Alzheimers disease (AD). We have compared KLK6 expression in post-mortem brain tissue in AD, vascular dementia (VaD) and controls. We studied the distribution of KLK6 in the temporal cortex and white matter by immunohistochemistry, and measured KLK6 mRNA and protein levels in the frontal and temporal cortex from 15 AD, 15 VaD and 15 control brains. Immunohistochemistry showed KLK6 to be restricted to endothelial cells. After adjustment for variations in vessel density by measurement of factor VIII-related antigen, we found KLK6 protein and mRNA levels to be significantly decreased in the frontal but not the temporal cortex in AD. In VaD, KLK6 protein level was significantly increased in the frontal cortex. Our findings suggest that an altered KLK6 expression may contribute to vascular abnormalities in AD and VaD.

Aβ degradation or cerebral perfusion? Divergent effects of multifunctional enzymes

There is increasing evidence that deficient clearance of β-amyloid (Aβ) contributes to its accumulation in late-onset Alzheimer disease (AD). Several Aβ-degrading enzymes, including neprilysin (NEP), endothelin-converting enzyme (ECE), and angiotensin-converting enzyme (ACE) reduce Aβ 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 Aβ-degrading enzymes rise with age and increases still further in AD, perhaps as a physiological response that helps to minimize the build-up of Aβ. 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 Aβ-degrading enzymes, ischemia and Aβ in AD: ischemia has been shown to increase Aβ production both in vitro and in vivo, whereas increased Aβ 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 Aβ in cerebral blood vessels and lessening its toxicity to vascular smooth muscle cells. In assessing the role of Aβ-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.

Assessment of activation of the plasma kallikrein-kinin system in frontal and temporal cortex in Alzheimer's disease and vascular dementia

Decreased cerebral blood flow and blood-brain barrier disruption are features of Alzheimers disease (AD). The plasma kallikrein-kinin system modulates cerebrovascular tone through release of vasoactive bradykinin (BK). Cerebroventricular infusion of Aβ1-40 enhances BK release, suggesting that the activity of this system may be elevated in AD. We investigated the profile of the activating protease of this system, plasma kallikrein (PK), in frontal and temporal brain tissue from postmortem confirmed cases of AD, vascular dementia (VaD), and controls. Measurements of neuron specific enolase messenger ribonucleic acid (mRNA) and protein were used to adjust for neuronal loss. Adjusted PK mRNA was significantly increased in the frontal cortex in AD, and the frontal and temporal cortex in VaD. Similar trends were seen for PK protein level in AD and VaD. PK activity was significantly increased in the frontal and temporal cortex in AD. Increased PK activity in AD is likely to contribute to increased BK release and may thereby influence cerebral blood flow and vascular permeability.

Effects of Hypertension and Anti-Hypertensive Treatment on Amyloid-β (Aβ) Plaque Load and Aβ-Synthesizing and Aβ-Degrading Enzymes in Frontal Cortex

Epidemiological data associate hypertension with a predisposition to Alzheimers disease (AD), and a number of postmortem and in vivo studies also demonstrate that hypertension increases amyloid-β (Aβ) pathology. In contrast, anti-hypertensive medications reportedly improve cognition and decrease the risk of AD, while certain classes of anti-hypertensive drugs are associated with decreased AD-related pathology. We investigated the effects of hypertension and anti-hypertensive treatment on Aβ plaque load in postmortem frontal cortex in AD. Aβ load was significantly increased in hypertensive (n = 20) relative to normotensive cases (n = 62) and was also significantly higher in treated (n = 9) than untreated hypertensives (n = 11). We then looked into mechanisms by which hypertension and treatment might increase Aβ load, focusing on Aβ-synthesizing enzymes, β- and γ-secretase, and Aβ-degrading enzymes, angiotensin-converting enzyme (ACE), insulin-degrading enzyme (IDE) and neprilysin. ACE and IDE protein levels were significantly lower in hypertensive (n = 21) than normotensive cases (n = 64), perhaps translating to decreased Aβ catabolism in hypertensives. ACE level was significantly higher in treated (n = 9) than untreated hypertensives (n = 12), possibly reflecting feedback upregulation of the renin-angiotensin system. Prospective studies in larger cohorts stratified according to anti-hypertensive drug class are needed to confirm these initial findings and to elucidate the interactions between hypertension, anti-hypertensive treatments, and Aβ metabolism.

Investigation of Aβ phosphorylated at serine 8 (pAβ) in Alzheimer's disease, dementia with Lewy bodies and vascular dementia

Deposition of amyloid beta (Aβ) in the brain is one of the defining abnormalities of Alzheimers disease (AD). Phosphorylation of Aβ at serine 8 (pAβ) has been implicated in its aggregation in vitro and pAβ level has been shown to be significantly elevated in AD. We aimed to assess the specificity of pAβ for AD and have investigated associations of pAβ with parenchymal and cerebrovascular accumulation of Aβ, disease progression, angiotensin‐converting enzyme activity and APOE genotype.

Angiotensin-Converting Enzyme in Cerebrospinal Fluid and Risk of Brain Atrophy

BACKGROUND Higher angiotensin-converting enzyme (ACE) activity might increase the risk of Alzheimers disease by increasing blood pressure, and subsequent development of cerebral small vessel disease (CSVD). Yet, it may also decrease this risk, as it functions to degrade amyloid-β, thereby reducing brain atrophy. OBJECTIVE To examine the cross-sectional associations of serum and cerebrospinal fluid (CSF) ACE protein levels and activity with brain atrophy and CSVD in a memory clinic cohort. METHODS In 118 subjects from the memory clinic based Amsterdam Dementia Cohort (mean age 66 ± 8 years), ACE protein levels (ng/ml) and activity in CSF and serum were investigated. Poisson regression analyses were used to associate ACE measurements with rated global cortical atrophy, medial temporal lobe atrophy, lacunar infarcts, white matter hyperintensities, and microbleeds on brain MRI. RESULTS Higher CSF ACE activity was associated with a reduced risk of global brain atrophy. The relative risk (95% CI) of having global cortical atrophy ≥2 per SD increase in CSF ACE activity was 0.67 (0.49; 0.93). ACE levels were not significantly related to measures of CSVD. CONCLUSIONS These results show that high ACE might have protective effects on the brain. This could suggest that ACE inhibitors, which may lower CSF ACE levels, are not preferred as antihypertensive treatment in patients at risk for Alzheimers disease.

Altered Expression of Human Mitochondrial Branched Chain Aminotransferase in Dementia with Lewy Bodies and Vascular Dementia

Cytosolic and mitochondrial human branched chain aminotransferase (hBCATc and hBCATm, respectively) play an integral role in brain glutamate metabolism. Regional increased levels of hBCATc in the CA1 and CA4 region of Alzheimer’s disease (AD) brain together with increased levels of hBCATm in frontal and temporal cortex of AD brains, suggest a role for these proteins in glutamate excitotoxicity. Glutamate toxicity is a key pathogenic feature of several neurological disorders including epilepsy associated dementia, AD, vascular dementia (VaD) and dementia with Lewy bodies (DLB). To further understand if these increases are specific to AD, the expression profiles of hBCATc and hBCATm were examined in other forms of dementia including DLB and VaD. Similar to AD, levels of hBCATm were significantly increased in the frontal and temporal cortex of VaD cases and in frontal cortex of DLB cases compared to controls, however there were no observed differences in hBCATc between groups in these areas. Moreover, multiple forms of hBCATm were observed that were particular to the disease state relative to matched controls. Real-time PCR revealed similar expression of hBCATm mRNA in frontal and temporal cortex for all cohort comparisons, whereas hBCATc mRNA expression was significantly increased in VaD cases compared to controls. Collectively our results suggest that hBCATm protein expression is significantly increased in the brains of DLB and VaD cases, similar to those reported in AD brain. These findings indicate a more global response to altered glutamate metabolism and suggest common metabolic responses that might reflect shared neurodegenerative mechanisms across several forms of dementia.