sPDGFRß and neuroinflammation are associated with AD biomarkers and differ by race: The ASCEND Study.

INTRODUCTION: There remains an urgent need to identify preclinical pathophysiological mechanisms of Alzheimer's disease (AD) development in high-risk, racially diverse populations. We explored the relationship between cerebrospinal fluid (CSF) markers of vascular injury and neuroinflammation with AD biomarkers in middle-aged Black/African American (B/AA) and non-Hispanic White (NHW) participants. METHODS: Adults (45-65 years) with a parental history of AD were enrolled (n = 82). CSF and blood biomarkers were collected at baseline and year 2. RESULTS: CSF total tau (t-tau), phosphorylated tau (p-tau), and amyloid beta (Aß)40 were elevated at year 2 compared to baseline. CSF soluble platelet-derived growth factor receptor ß (sPDGFRß) levels, a marker of pericyte injury, correlated positively with t-tau, p-tau, Aß40 markers of vascular injury, and cytokines at baseline and year 2. CSF sPDGFRß and tau were significantly lower in B/AA than NHW. DISCUSSION: Vascular dysfunction and neuroinflammation may precede cognitive decline and disease pathology in the very early preclinical stages of AD, and there are race-related differences in these relationships. HIGHLIGHTS: Cerebrospinal fluid (CSF) Alzheimer's disease (AD) biomarkers changed over 2 years in high-risk middle-aged adults. Markers of vascular dysfunction were associated with the CSF biomarkers amyloid beta and tau. AD biomarkers were lower in Black compared to non-Hispanic White individuals. Markers of vascular dysfunction were lower among Black individuals.

VEGFR1 and VEGFR2 in Alzheimer's Disease.

Vascular endothelial growth factor (VEGF) is a potent angiogenic factor. Despite upregulation of VEGF in the brain in Alzheimer's disease (AD), probably in response to amyloid-ß, vasoconstriction, and tissue hypoxia, there is no consequent increase in microvessel density. VEGF binds to and activates VEGF receptor 2 (VEGFR2), but also binds to VEGF receptor 1 (VEGFR1), which exists in less-active membrane-bound and inactive soluble (sVEGFR1) forms and inhibits pro-angiogenic signaling. We have investigated whether altered expression of VEGF receptors might account for the lack of angiogenic response to VEGF in AD. We assessed the cellular distribution and protein level of VEGFR1 and VEGFR2 in parietal cortex from 50 AD and 36 age-matched control brains, and related the findings to measurements of VEGF and von Willebrand factor level (a marker of microvessel density) in the same tissue samples. VEGFR2 was expressed by neurons, astrocytes and endothelial cells. VEGFR1 was expressed predominantly neuronally and was significantly reduced in AD (p = 0.02). Western blot analysis on a subset of brains showed reduction in VEGFR1:sVEGFR1 in AD (p = 0.046). The lack of angiogenesis despite cerebral hypoperfusion in AD is not explained by altered expression of VEGFR2 or total VEGFR1; indeed, the downregulation of VEGFR1 may represent a pro-angiogenic response to the hypoperfusion. However, the relative increase in sVEGFR1 would be expected to have an anti-angiogenic effect which may be a factor in AD.

Overexpression of Kinesin Superfamily Motor Proteins in Alzheimer's Disease.

Defects in motor protein-mediated neuronal transport mechanisms have been implicated in a number of neurodegenerative disorders but remain relatively little studied in Alzheimer's disease (AD). Our aim in the present study was to assess the expression of the anterograde kinesin superfamily motor proteins KIF5A, KIF1B, and KIF21B, and to examine their relationship to levels of hyperphosphorylated tau, amyloid-ß protein precursor (AßPP), and amyloid-ß (Aß) in human brain tissue. We used a combination of qPCR, immunoblotting, and ELISA to perform these analyses in midfrontal cortex from 49 AD and 46 control brains. Expression of KIF5A, KIF1B, and KIF21B at gene and protein level was significantly increased in AD. KIF5A protein expression correlated inversely with the levels of AßPP and soluble Aß in AD brains. Upregulation of KIFs may be an adaptive response to impaired axonal transport in AD.

Endothelin-converting enzymes degrade α-synuclein and are reduced in dementia with Lewy bodies.

We have examined the roles of the endothelin-converting enzyme-1 and -2 (ECE-1 and ECE-2) in the homeostasis of α-synuclein (α-syn) and pathogenesis of Lewy body disease. The ECEs are named for their ability to convert inactive big endothelin to the vasoactive peptide endothelin-1 (EDN1). We have found that ECE-1 and ECE-2 cleave and degrade α-syn in vitro and siRNA-mediated knockdown of ECE-1 and ECE-2 in SH-SY5Y neuroblastoma cells significantly increased α-syn both intracellularly (within the cell lysate) (p < 0.05 for both ECE-1 and -2) and extracellularly (in the surrounding medium) (p < 0.05 for ECE-1 and p = 0.07 for ECE-2). Double immunofluorescent labelling showed co-localization of ECE-1 and ECE-2 with α-syn within the endolysosomal system (confirmed by a proximity ligation assay). To assess the possible relevance of these findings to human Lewy body disease, we measured ECE-1 and ECE-2 levels by sandwich ELISA in post-mortem samples of cingulate cortex (a region with a predilection for Lewy body pathology) in dementia with Lewy bodies (DLB) and age-matched controls. ECE-1 (p < 0.001) and ECE-2 (p < 0.01) levels were significantly reduced in DLB and both enzymes correlated inversely with the severity of Lewy body pathology as indicated by the level of α-syn phosphorylated at Ser129 (r = -0.54, p < 0.01 for ECE-1 and r = -0.49, p < 0.05 for ECE-2). Our novel findings suggest a role for ECEs in the metabolism of α-syn that could contribute to the development and progression of DLB.

Differential changes in Aß42 and Aß40 with age.

Amyloid-ß peptide (Aß), the cerebral accumulation of which is thought to cause Alzheimer's disease (AD), is produced throughout life. The level of insoluble Aß rises with age and is further increased in AD. In contrast, we showed previously that in mid-frontal cortex in a cohort without neurological disease, soluble Aß declined progressively between 16 and 95 y. We speculated that the divergent changes in the levels of soluble and insoluble Aß with age might reflect an increasing tendency to favor the production or retention within the brain of Aß42 over Aß40, leading to elevation of Aß(42:40) even as total soluble Aß decreased. We have now measured Aß40 and Aß42 in soluble and insoluble (guanidine-extractable) fractions of human postmortem brain tissue from the same cohort studied previously. Although in normal brains the absolute level of Aß40 in both soluble and insoluble fractions and that of Aß42 in the soluble fraction declined with age, those declines were predominantly before about 50 y, after which Aß(42:40) tended to increase in both the soluble and insoluble fractions. Insoluble Aß42 increased progressively with age. Differential production or retention of Aß40 and Aß42 in the over-50 s is likely to contribute to the influence of age on the risk of sporadic AD.

Aß-degrading enzymes: potential for treatment of Alzheimer disease.

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), insulin-degrading enzyme, and endothelin-converting enzyme reduce Aß levels and protect against cognitive impairment in mouse models of AD. The activity of several Aß-degrading enzymes rises with age and increases still further in AD, perhaps as a physiological response to minimize the buildup of Aß. The age- and disease-related changes in expression of more recently recognized Aß-degrading enzymes (e.g. NEP-2 and cathepsin B) remain to be investigated, and there is strong evidence that reduced NEP activity contributes to the development of cerebral amyloid angiopathy. Regardless of the role of Aß-degrading enzymes in the development of AD, experimental data indicate that increasing the activity of these enzymes (NEP in particular) has therapeutic potential in AD, although targeting their delivery to the brain remains a major challenge. The most promising current approaches include the peripheral administration of agents that enhance the activity of Aß-degrading enzymes and the direct intracerebral delivery of NEP by convection-enhanced delivery. In the longer term, genetic approaches to increasing the intracerebral expression of NEP or other Aß-degrading enzymes may offer advantages.

Neprilysin protects against cerebral amyloid angiopathy and Aß-induced degeneration of cerebrovascular smooth muscle cells.

Neprilysin (NEP), which degrades amyloid-ß (Aß), is expressed by neurons and cerebrovascular smooth muscle cells (CVSMCs). NEP immunolabeling is reduced within cerebral blood vessels of Alzheimer's disease (AD) patients with cerebral amyloid angiopathy (CAA). We have now measured NEP enzyme activity in leptomeningeal and purified cerebral cortical blood vessel preparations from control and AD patients with and without CAA. Measurements were adjusted for smooth muscle actin (SMA) to control for variations in CVSMC content. NEP activity was reduced in CAA, in both controls and AD. In leptomeningeal vessels, NEP activity was related to APOE genotype, being highest in ε2-positive and lowest in ε4-positive brains. To assess the role of NEP in protecting CVSMCs from Aß toxicity, we measured cell death in primary human adult CVSMCs exposed to Aß(1-40) , Aß(1-42) or Aß(1-40(Dutch variant)) . Aß(1-42) was most cytotoxic to CVSMCs. Aß(1-42) -mediated cell death was increased following siRNA-mediated knockdown or thiorphan-mediated inhibition of NEP activity; conversely Aß(1-42) -mediated cytotoxicity was reduced by the addition of somatostatin and NEP over-expression following transfection with NEP cDNA. Our findings suggest that NEP protects CVSMCs from Aß toxicity and protects cerebral blood vessels from the development and complications of CAA.

Accumulation of insoluble amyloid-ß in down's syndrome is associated with increased BACE-1 and neprilysin activities.

We previously reported age- and Alzheimer's disease (AD)-related increases in the activities of ß-secretase (BACE-1) and Aß-degrading enzymes including neprilysin (NEP) and angiotensin-converting enzyme (ACE) in the frontal cortex. We suggested that these increases were secondary to the accumulation of insoluble amyloid-ß (Aß) and a decline in soluble Aß. We have further tested this hypothesis by examination of frontal cortex obtained postmortem from individuals with Down's syndrome (DS), in whom AD-like neuropathological changes occur in association with early-onset dementia. We measured total soluble and insoluble (guanidine-extractable) Aß, BACE-1 activity, and the concentrations and activities of NEP and ACE in two independent DS cohorts: an initial, Bristol cohort (9 DS cases, 8 controls matched for age-at-death) and a validation Newcastle cohort (20 DS, 18 controls with a wider spectrum of age-at-death). In both cohorts the level of insoluble (but not soluble) Aß was significantly higher in DS than controls and was comparable to previously measured levels in AD. NEP protein concentration and activity were significantly increased in DS; a trend towards increased BACE-1 activity was observed in DS but did not reach statistical significance. Both NEP and BACE-1 correlated with the level of insoluble Aß. The concentration of ACE in DS was elevated in the pilot cohort only and ACE activity was unchanged. These findings provide strong support that BACE-1 and NEP activities, but not ACE, increase in response to the accumulation of insoluble Aß within 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.

Abeta-degrading enzymes in Alzheimer's disease.

In Alzheimer's disease (AD) Abeta accumulates because of imbalance between the production of Abeta and its removal from the brain. There is increasing evidence that in most sporadic forms of AD, the accumulation of Abeta is partly, if not in some cases solely, because of defects in its removal--mediated through a combination of diffusion along perivascular extracellular matrix, transport across vessel walls into the blood stream and enzymatic degradation. Multiple enzymes within the central nervous system (CNS) are capable of degrading Abeta. Most are produced by neurons or glia, but some are expressed in the cerebral vasculature, where reduced Abeta-degrading activity may contribute to the development of cerebral amyloid angiopathy (CAA). Neprilysin and insulin-degrading enzyme (IDE), which have been most extensively studied, are expressed both neuronally and within the vasculature. The levels of both of these enzymes are reduced in AD although the correlation with enzyme activity is still not entirely clear. Other enzymes shown capable of degrading Abetain vitro or in animal studies include plasmin; endothelin-converting enzymes ECE-1 and -2; matrix metalloproteinases MMP-2, -3 and -9; and angiotensin-converting enzyme (ACE). The levels of plasmin and plasminogen activators (uPA and tPA) and ECE-2 are reported to be reduced in AD. Reductions in neprilysin, IDE and plasmin in AD have been associated with possession of APOEepsilon4. We found no change in the level or activity of MMP-2, -3 or -9 in AD. The level and activity of ACE are increased, the level being directly related to Abeta plaque load. Up-regulation of some Abeta-degrading enzymes may initially compensate for declining activity of others, but as age, genetic factors and diseases such as hypertension and diabetes diminish the effectiveness of other Abeta-clearance pathways, reductions in the activity of particular Abeta-degrading enzymes may become critical, leading to the development of AD and CAA.

Immunocapture-based fluorometric assay for the measurement of insulin-degrading enzyme activity in brain tissue homogenates.

Internally quenched fluorogenic substrates are commonly used for measuring enzyme activity in biological samples and allow high sensitivity and continuous real-time measurement that is well suited for high throughput analysis. We describe the development and optimisation of an immunocapture-based assay that uses the fluorogenic peptide substrate (Mca-RPPGFSAFK(Dnp)) and allows the specific measurement of insulin-degrading enzyme (IDE) activity in brain tissue homogenates. This fluorogenic substrate can be cleaved by a number of enzymes including neprilysin (NEP), endothelin-converting enzyme-1 (ECE-1) and angiotensin-converting enzyme (ACE), as well as IDE, and we have previously shown that discrimination between these individual enzymes is not readily achieved in tissue homogenates, even in the presence of selective inhibitors and pH conditions. We tested a panel of IDE antibodies to isolate and capture IDE from brain tissue homogenates and found that immunocapture with antibody to the inactive domain of IDE prior to the addition of fluorogenic substrate allows sensitive (linear at 156-2500ng/ml) and specific measurement of IDE activity and negligible cross-reactivity with NEP, ACE or ECE-1. This assay should allow the measurement of IDE enzyme levels in a variety of biological tissues and may be useful in study of diseases such as Alzheimer's disease and insulin-dependent diabetes.

Immunocapture-based fluorometric assay for the measurement of neprilysin-specific enzyme activity in brain tissue homogenates and cerebrospinal fluid.

Neprilysin, a zinc-metalloendopeptidase, has important roles in the physiology and pathology of many diseases such as hypertension, cancer and Alzheimer's disease. We have developed an immunocapture assay to measure the specific enzyme activity of neprilysin in brain tissue homogenates and cerebrospinal fluid (CSF). The assay uses a neprilysin-specific antibody, previously used in a commercially available ELISA kit, to isolate and immobilise NEP from brain homogenates and CSF, prior to the addition of a fluorogenic peptide substrate (Mca-RPPGFSAFK(Dnp)). This fluorogenic substrate is ordinarily cleaved by multiple enzymes. We have shown that without the immunocapture phase, even under reaction conditions reported to be specific for neprilysin - i.e. in the presence of thiorphan, at pH above 7 - the fluorogenic peptide substrate does not allow neprilysin activity in brain homogenates and CSF to be discriminated from that of other closely related enzymes. The specificity of the immunocapture enzyme activity assay was confirmed by >80% inhibition of substrate cleavage in brain homogenates and CSF in the presence of thiorphan. The assay allows high-throughput analysis and, critically, also ensures a high level of enzyme specificity even when assaying crude tissue homogenates or CSF.

Decreased expression and activity of neprilysin in Alzheimer disease are associated with cerebral amyloid angiopathy.

Neprilysin (NEP) degrades amyloid-beta (Abeta) and is thought to contribute to its clearance from the brain. In Alzheimer disease (AD), downregulation of NEP has been suggested to contribute to the development of cerebral amyloid angiopathy (CAA). We examined the relationship among NEP, CAA, and APOE status in AD and elderly control cases. NEP was most abundant in the tunica media of cerebrocortical blood vessels and in pyramidal neurons. In homogenates of the frontal cortex, NEP protein levels were reduced in AD but not significantly; NEP enzymatic activity was significantly reduced in AD. Immunohistochemistry revealed a reduction of both vascular and parenchymal NEP. The loss of vessel-associated NEP in AD was inversely related to the severity of CAA, and analysis of cases with severe CAA showed that levels of vascular NEP were reduced to the same extent in Abeta-free and Abeta-laden vessels, strongly suggesting that the reduction in NEP is not simply secondary to CAA. Possession of APOE epsilon4 was associated with significantly lower levels of both parenchymal and vascular NEP. Colinearity of epsilon4 with the presence of moderate to severe CAA precluded assessment of the independence of this association from NEP levels. However, logistic regression analysis showed low NEP levels to be a significant independent predictor of moderate to severe CAA.