Tau hyperphosphorylation affects Smad 2/3 translocation.

Transforming growth factors beta (TGFbeta) regulate multiple biological activities. TGFbeta activation of the Smad pathway results in activation of genes encoding extracellular matrix molecules, proteases, protease activators and protease inhibitors. In Alzheimer's disease (AD), TGFbeta protein and mRNA levels are raised, which would be expected to be neuroprotective. However, recent observations suggest that TGFbeta-Smad signalling is disrupted by the hyperphosphorylation of tau, the primary component of neurofibrillary tangles: phosphorylated Smad2/3 (pSmad 2/3) co-localises with phosphorylated tau in the neuronal cytoplasm and levels are reduced in the nucleus. We have investigated whether in vitro induction of tau hyperphosphorylation influences pSmad 2/3 localisation in rat primary cortical cells. Treatment with okadaic acid, a protein phosphatase 1 and 2A inhibitor caused hyperphosphorylation of tau at epitopes hyperphosphorylated in AD and disrupted pSmad 2/3 translocation into the nucleus. The disruptive effect of tau phosphorylation on pSmad 2/3 translocation was confirmed by treatment of primary cortical cells with synthetic oligomeric A beta(1-42), a more physiologically relevant model of AD. Our findings suggest that despite the increased level of TGFbeta in AD, the TGFbeta-Smad signalling pathway is impeded within neurones due to sequestration of pSmad 2/3 by hyperphosphorylated tau. This may compromise neuroprotective actions of TGFbeta and contribute to neurodegeneration in AD.

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 Alzheimer's disease (AD): ACE-1 cleaves amyloid-beta peptide (Abeta) 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 Abeta 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 Abeta 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 Abeta 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 Abeta, 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 Abeta 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.

Caveolin-1 and -2 and their relationship to cerebral amyloid angiopathy in Alzheimer's disease.

Cerebral amyloid angiopathy (CAA) affects over 90% of patients with Alzheimer's disease (AD) and increases the risk of cerebral haemorrhage and infarction. Caveolae--cholesterol-enriched plasmalemmal microinvaginations--are implicated in the production of amyloid beta peptide (Abeta). Caveolin-1 (CAV-1) is essential for the formation of caveolae. Caveolin-2 (CAV-2) is expressed at the plasma membrane only when in a stable hetero-oligomeric complex with CAV-1. CAV-1 and CAV-2 are highly co-expressed by endothelium and smooth muscle. Recent studies suggest that down-regulation of CAV-1 causes a reduction in alpha-secretase activity and consequent accumulation of Abeta. We have used quantitative immunohistochemical techniques to assess the relationship between CAV-1 and CAV-2 with respect to Abeta accumulation in the cerebral vasculature in a series of post mortem brains. CAV-1 and CAV-2 were co-expressed within the tunica media and endothelium of cerebral blood vessels. There were regional differences in CAV-1 immunolabelling, which was significantly greater in the frontal cortex and white matter than in the parietal lobe (in both control and AD cases) or the temporal lobe (in AD alone). However, CAV-1 labelling in AD did not differ from that in controls in any of the three lobes examined. Assessment of CAV-1 labelling in relation to the severity of CAA showed CAV-1 to be significantly increased in the frontal white matter in a subgroup of AD cases with absent/mild CAA compared with controls with absent/mild CAA and to AD cases with moderate/severe CAA, but the latter groups did not show significant differences from one another. CAV-1 labelling did not vary with age, gender, APOE genotype, post mortem delay or brain weight. Only segments of blood vessels with particularly abundant Abeta and extensive loss of smooth muscle actin showed loss of CAV-1 and CAV-2 from the tunica media. Within these vessels endothelial CAV-1 was preserved and discontinuous CAV-2 labelling was noted along the outer aspect of the vessel wall. Our findings suggest that alterations in the expression of vascular CAV-1 and CAV-2 are unlikely to play a role in the development of CAA in AD.