Age-dependent decline of neprilysin in Alzheimer's disease and normal brain: inverse correlation with A beta levels

Comparison of Abeta levels in the brain of familial and sporadic Alzheimer's disease

Mutations in presenilin (PS) and amyloid precursor protein (APP) genes are a predominant cause for early-onset familial Alzheimer disease (AD). Although these mutations are rare, they have in the past decades advanced our understanding of the underlying molecular mechanisms of AD. In the present study, Abeta levels were measured in cortical regions of APPsw and PS1 (M146V) mutation carriers, sporadic AD (SAD) and age-matched non-demented individuals. We found similar levels of soluble Abeta42, insoluble and soluble Abeta40 in both APPsw mutation carriers and SAD. However, lower levels of insoluble Abeta42 were detected in the frontal and temporal cortex of APPsw brain. In PS1 brain, insoluble Abeta40 and Abeta42 levels were significantly lower in all four cortical regions compared with SAD, whilst levels of Abeta40 were lower in frontal and occipital cortex compared with APPsw brain. The insoluble Abeta42/40 ratio was similar in SAD and APPsw but significantly higher in PS1 mutation carriers. Our results indicate that the pattern of Abeta deposition in PS1 mutation carriers differs from that in both APPsw and SAD, whereas the pattern in APPsw mutation carriers is more similar to that in SAD.

Smoking, nicotine and neuropsychiatric disorders

Tobacco smoking is an extremely addictive and harmful form of nicotine (NIC) consumption, but unfortunately also the most prevalent. Although disproportionately high frequencies of smoking and its health consequences among psychiatric patients are widely known, the neurobiological background of this epidemiological association is still obscure. The diverse neuroactive effects of NIC and some other major tobacco smoke constituents in the central nervous system may underlie this association. This present paper summarizes the pharmacology of NIC and its receptors (nAChR) based on a systematic review of the literature. The role of the brain's reward system(s) in NIC addiction and the results of functional and structural neuroimaging studies on smoking-related states and behaviors (i.e. dependence, craving, withdrawal) are also discussed. In addition, the epidemiological, neurobiological, and genetic aspects of smoking in several specific neuropsychiatric disorders are reviewed and the clinical relevance of smoking in these disease states addressed.

Endothelin-converting enzyme-2 is increased in Alzheimer's disease and up-regulated by Abeta

Alzheimer's disease (AD) is thought to be caused by the accumulation of amyloid beta (Abeta) peptide within the brain. Endothelin-converting enzyme-2 (ECE-2), which is expressed in neural tissues, cleaves 'big endothelin' to produce the vasoconstrictor endothelin-1. ECE-2 also degrades Abeta. We have examined ECE-2 expression in the temporal cortex of brain tissue from patients with AD, vascular dementia, and controls. Immunohistochemistry with specific antibodies showed ECE-2 to be abundant within pyramidal neurons in both the hippocampus and neocortex, but also to be present in certain astrocytes and microglia, particularly in AD brains. Quantitative real-time PCR showed ECE-2 mRNA to be markedly elevated in AD but not in vascular dementia. ECE-2 protein concentration, measured by sandwich enzyme-linked immunosorbent assay, was also significantly elevated in AD but not in vascular dementia. Exposure of SH-SY5Y human neuroblastoma cells to monomeric or oligomeric Abeta(1-42) caused an initial decrease in ECE-2 mRNA at 4 hours, but a marked increase by 24 hours. Our findings indicate that Abeta accumulation in AD is unlikely to be caused by ECE-2 deficiency. However, ECE-2 expression is up-regulated, perhaps to minimize Abeta accumulation, but this may also be a mechanism through which endothelin-1 production is increased and cerebral blood flow is reduced in AD. Our findings suggest that endothelin-1 receptor antagonists, already licensed for treating other diseases, could be of benefit in AD therapies.

Neprilysin overexpression inhibits plaque formation but fails to reduce pathogenic Abeta oligomers and associated cognitive deficits in human amyloid precursor protein transgenic mice

The accumulation of amyloid-beta (Abeta) peptides in the brain of patients with Alzheimer's disease (AD) may arise from an imbalance between Abeta production and clearance. Overexpression of the Abeta-degrading enzyme neprilysin in brains of human amyloid precursor protein (hAPP) transgenic mice decreases overall Abeta levels and amyloid plaque burdens. Because AD-related synaptic and cognitive deficits appear to be more closely related to Abeta oligomers than to plaques, it is important to determine whether increased neprilysin activity also diminishes the levels of pathogenic Abeta oligomers and related neuronal deficits in vivo. To address this question, we crossed hAPP transgenic mice with neprilysin transgenic mice and analyzed their offspring. Neprilysin overexpression reduced soluble Abeta levels by 50% and effectively prevented early Abeta deposition in the neocortex and hippocampus. However, it did not reduce levels of Abeta trimers and Abeta*56 or improve deficits in spatial learning and memory. The differential effect of neprilysin on plaques and oligomers suggests that neprilysin-dependent degradation of Abeta affects plaques more than oligomers and that these structures may form through distinct assembly mechanisms. Neprilysin's inability to prevent learning and memory deficits in hAPP mice may be related to its inability to reduce pathogenic Abeta oligomers. Reduction of Abeta oligomers will likely be required for anti-Abeta treatments to improve cognitive functions.

[(11)C]PIB-amyloid binding and levels of Abeta40 and Abeta42 in postmortem brain tissue from Alzheimer patients

beta-Amyloid (Abeta) deposits are one of the major histopathological hallmarks of Alzheimer's disease (AD). The amyloid-imaging positron emission tomography (PET) tracer [(11)C]PIB (N-methyl[(11)C]2-(4'-methylaminophenyl)-6-hydroxy-benzothiazole) is used in the assessment of Abeta deposits in the human brain. [(11)C]PIB-amyloid interaction and insoluble Abeta40 and Abeta42 peptide levels in the brain were quantified in postmortem tissue from nine AD patients and nine age-matched control subjects in the temporal, frontal and parietal cortices and the cerebellum. Autoradiographical studies showed significantly higher densities of specific [(11)C]PIB-amyloid binding in gray matter in the temporal and parietal cortex (62fmol/mg tissue) in AD patients as compared to control subjects, whereas the density was somewhat lower in the frontal cortex (56fmol/mg tissue). No specific binding could be detected in the AD cerebellum or in the tissues from the control subjects (< or =5fmol/mg tissue). Insoluble Abeta40 and total Abeta levels (i.e. sum of Abeta40 and Abeta42) were significantly higher in patients than in controls in all measured cortical regions as determined using ELISA, which was confirmed using immunohistochemistry. The present findings show a more regional selective distribution of [(11)C]PIB amyloid binding than previously reported. Moreover, it is suggested that some of the [(11)C]PIB binding and insoluble Abeta seen in control subjects may be amyloid in the blood vessels.

Consequence of Abeta immunization on the vasculature of human Alzheimer's disease brain

A major feature of Alzheimer's disease is the accumulation of amyloid-beta peptide (Abeta) in the brain both in the form of plaques in the cerebral cortex and in blood vessel as cerebral amyloid angiopathy (CAA). Experimental models and human clinical trials have shown that accumulation of Abeta plaques can be reversed by immunotherapy. In this study, we hypothesized that Abeta in plaques is solubilized by antibodies generated by immunization and drains via the perivascular pathway, detectable as an increase in cerebrovascular Abeta. We have performed a follow up study of Alzheimer's disease patients immunized against Abeta42. Neuropathological examination was performed on nine patients who died between four months and five years after their first immunization. Immunostaining for Abeta40 and Abeta42 was quantified and compared with that in unimmunized Alzheimer's disease controls (n = 11). Overall, compared with these controls, the group of immunized patients had approximately 14 times as many blood vessels containing Abeta42 in the cerebral cortex (P<0.001) and seven times more in the leptomeninges (P = 0.013); among the affected blood vessels in the immunized cases, most of them had full thickness and full circumference involvement of the vessel wall in the cortex (P = 0.001), and in the leptomeninges (P = 0.015). There was also a significantly higher level of cerebrovascular Abeta40 in the immunized cases than in the unimmunized cases (cortex: P = 0.009 and leptomeninges: P = 0.002). In addition, the immunized patients showed a higher density of cortical microhaemorrhages and microvascular lesions than the unimmunized controls, though none had major CAA-related intracerebral haemorrhages. The changes in cerebral vascular Abeta load did not appear to substantially influence the structural proteins of the blood vessels. Unlike most of the immunized patients, two of the longest survivors, four to five years after first immunization, had virtually complete absence of both plaques and CAA, raising the possibility that, given time, Abeta is eventually cleared from the cerebral vasculature. The findings are consistent with the hypothesis that Abeta immunization results in solubilization of plaque Abeta42 which, at least in part, exits the brain via the perivascular pathway, causing a transient increase in the severity of CAA. The extent to which these vascular alterations following Abeta immunization in Alzheimer's disease are reflected in changes in cognitive function remains to be determined.

Error-protein metabolism and ageing

Ageing and many associated pathologies are accompanied by accumulation of altered proteins. It is suggested that erroneous polypeptide biosynthesis, cytosolic and mitochondrial, is not an insignificant source of aberrant protein in growing and non-mitotic cells. It is proposed that (i) synthesis of sufficient proteases and chaperone proteins necessary for rapid elimination of altered proteins, from cytoplasmic and mitochondrial compartments, is related to cellular protein biosynthetic potential, and (ii) cells growing slowly, or not at all, automatically generate lower levels of protease/chaperone molecules than cells growing rapidly, due to decreased general rate of protein synthesis and lowered amount of error-protein produced per cell. Hence the increased vulnerability of mature organisms may be explained, at least in part, by the decline in constitutive protease/chaperone protein biosynthesis. Upregulation of mitochondria biogenesis, induced by dietary restriction or aerobic exercise, may also increase protease/chaperone protein synthesis, which would improve cellular ability to degrade both error-proteins and proteins damaged post-synthetically by reactive oxygen species etc. These proposals may help explain, in part, the latency of those age-related pathologies where altered proteins accumulate only late in life, and the beneficial effects of aerobic exercise and dietary restriction.

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.

Loss of neprilysin function promotes amyloid plaque formation and causes cerebral amyloid angiopathy

Cerebral deposition of the amyloid beta protein (Abeta), an invariant feature of Alzheimer's disease, reflects an imbalance between the rates of Abeta production and clearance. The causes of Abeta elevation in the common late-onset form of Alzheimer's disease (LOAD) are largely unknown. There is evidence that the Abeta-degrading protease neprilysin (NEP) is down-regulated in normal aging and LOAD. We asked whether a decrease in endogenous NEP levels can prolong the half-life of Abeta in vivo and promote development of the classic amyloid neuropathology of Alzheimer's disease. We examined the brains and plasma of young and old mice expressing relatively low levels of human amyloid precursor protein and having one or both NEP genes silenced. NEP loss of function 1) elevated whole-brain and plasma levels of human Abeta(40) and Abeta(42), 2) prolonged the half-life of soluble Abeta in brain interstitial fluid of awake animals, 3) raised the concentration of Abeta dimers, 4) markedly increased hippocampal amyloid plaque burden, and 5) led to the development of amyloid angiopathy. A approximately 50% reduction in NEP levels, similar to that reported in some LOAD brains, was sufficient to increase amyloid neuropathology. These findings demonstrate an important role for proteolysis in determining the levels of Abeta and Abeta-associated neuropathology in vivo and support the hypothesis that primary defects in Abeta clearance can cause or contribute to LOAD pathogenesis.