Lipopolysaccharide stimulates the secretion of the amyloid precursor protein via a protein kinase C-mediated pathway

Role of prostacyclin signaling in endothelial production of soluble amyloid precursor protein-α in cerebral microvessels

We tested hypothesis that activation of the prostacyclin (PGI2) receptor (IP receptor) signaling pathway in cerebral microvessels plays an important role in the metabolism of amyloid precursor protein (APP). In human brain microvascular endothelial cells activation of IP receptor with the stable analogue of PGI2, iloprost, stimulated expression of amyloid precursor protein and a disintegrin and metalloprotease 10 (ADAM10), resulting in an increased production of the neuroprotective and anticoagulant molecule, soluble APPα (sAPPα). Selective agonist of IP receptor, cicaprost, and adenylyl cyclase activator, forskolin, also enhanced expression of amyloid precursor protein and ADAM10. Notably, in cerebral microvessels of IP receptor knockout mice, protein levels of APP and ADAM10 were reduced. In addition, iloprost increased protein levels of peroxisome proliferator-activated receptor δ (PPARδ) in human brain microvascular endothelial cells. PPARδ-siRNA abolished iloprost-augmented protein expression of ADAM10. In contrast, GW501516 (a selective agonist of PPARδ) upregulated ADAM10 and increased production of sAPPα. Genetic deletion of endothelial PPARδ (ePPARδ-/-) in mice significantly reduced cerebral microvascular expression of ADAM10 and production of sAPPα. In vivo treatment with GW501516 increased sAPPα content in hippocampus of wild type mice but not in hippocampus of ePPARδ-/- mice. Our findings identified previously unrecognized role of IP-PPARδ signal transduction pathway in the production of sAPPα in cerebral microvasculature.

Activation of α7 nicotinic receptor affects APP processing by regulating secretase activity in SH-EP1-α7 nAChR-hAPP695 cells

Multiple lines of evidence have implicated that nicotinic acetylcholine receptor (nAChR) may be an important therapeutic target for the treatment of Alzheimer's disease (AD). Although there are reports suggesting a link between alpha7 nAChR subtype and AD, there has been little report on the mechanism. The present study investigates whether and how α7 nAChR activation affects APP695 processing in SH-EP1 cell model. Cell line co-expressing α7 nAChR gene and human amyloid precursor protein 695 (hAPP695) gene were constructed by stable transfection. Expression of β-amyloid, α-form of secreted APP (αAPPs) and APP1695 was measured by ELISA, western blotting and real-time PCR respectively. Additionally, α, β, and γ-secretase activities were also analyzed in constructed SH-EP1-α7 nAChR-hAPP695 cell line. The results showed that SH-EP1-α7 nAChR-hAPP695 cell line, expressing both hAPP695 gene and α7 nAChR subtype gene, was constructed successfully. The secreted Aβ was decreased and αAPPs was significantly increased by non-selective nAChR agonist nicotine (10 μM) and specific α7 nAChR agonist GTS-21 (1 μM), and APP expression was not affected. Furthermore, specific α7 nAChR antagonist methyllycaconitine (MLA) reversed the alterations induced by activation of α7 nAChR. CTF-α was increased and CTF-γ was decreased when treated with nicotine (10 μM). In addition, the results of enymatic activity analysis showed that nicotine (1μM) and GTS-21 (0.1, 1 μM) decreased γ-secretase activity, but has no effects on α-secretase activity and β-secretase activity. Our findings demonstrate that, through regulating γ-secretase activity, α7 nAChR activation reduces APP processing in amyloidogenic pathway, and at the same time enhances APP processing in non-amyloidogenic pathway. The constructed SH-EP1-α7 nAChR-hAPP695 cell line might be useful for screening specific nAChR agonists against AD.

L-3-n-butylphthalide improves cognitive impairment and reduces amyloid-beta in a transgenic model of Alzheimer's disease

Alzheimer's disease (AD) is an age-related, progressive neurodegenerative disorder that occurs gradually and results in memory, behavior, and personality changes. L-3-n-butylphthalide (L-NBP), an extract from seeds of Apium graveolens Linn (Chinese celery), has been demonstrated to have neuroprotective effects on ischemic, vascular dementia, and amyloid-beta (Abeta)-infused animal models. In the current study, we examined the effects of L-NBP on learning and memory in a triple-transgenic AD mouse model (3xTg-AD) that develops both plaques and tangles with aging, as well as cognitive deficits. Ten-month-old 3xTg-AD mice were given 15 mg/kg L-NBP by oral gavage for 18 weeks. L-NBP treatment significantly improved learning deficits, as well as long-term spatial memory, compared with vehicle control treatment. L-NBP treatment significantly reduced total cerebral Abeta plaque deposition and lowered Abeta levels in brain homogenates but had no effect on fibrillar Abeta plaques, suggesting preferential removal of diffuse Abeta deposits. Furthermore, we found that L-NBP markedly enhanced soluble amyloid precursor protein secretion (alphaAPPs), alpha-secretase, and PKCalpha expression but had no effect on steady-state full-length APP. Thus, L-NBP may direct APP processing toward a non-amyloidogenic pathway and preclude Abeta formation in the 3xTg-AD mice. The effect of l-NBP on regulating APP processing was further confirmed in neuroblastoma SK-N-SH cells overexpressing wild-type human APP(695) (SK-N-SH APPwt). L-NBP treatment in 3xTg-AD mice also reduced glial activation and oxidative stress compared with control treatment. L-NBP shows promising preclinical potential as a multitarget drug for the prevention and/or treatment of Alzheimer's disease.

Down syndrome fibroblast model of Alzheimer-related endosome pathology: accelerated endocytosis promotes late endocytic defects

Endocytic dysfunction is an early pathological change in Alzheimer's disease (AD) and Down's syndrome (DS). Using primary fibroblasts from DS individuals, we explored the interactions among endocytic compartments that are altered in AD and assessed their functional consequences in AD pathogenesis. We found that, like neurons in both AD and DS brains, DS fibroblasts exhibit increased endocytic uptake, fusion, and recycling, and trafficking of lysosomal hydrolases to rab5-positive early endosomes. Moreover, late endosomes identified using antibodies to rab7 and lysobisphosphatidic acid increased in number and appeared as enlarged, perinuclear vacuoles, resembling those in neurons of both AD and DS brains. In control fibroblasts, similar enlargement of rab5-, rab7-, and lysobisphosphatidic acid-positive endosomes was induced when endocytosis and endosomal fusion were increased by expression of either a rab5 or an active rab5 mutant, suggesting that persistent endocytic activation results in late endocytic dysfunction. Conversely, expression of a rab5 mutant that inhibits endocytic uptake reversed early and late endosomal abnormalities in DS fibroblasts. Our results indicate that DS fibroblasts recapitulate the neuronal endocytic dysfunction of AD and DS, suggesting that increased trafficking from early endosomes can account, in part, for downstream endocytic perturbations that occur in neurons in both AD and DS brains.

Induction of lactoferrin gene expression by innate immune stimuli in mouse mammary epithelial HC-11 cells

Lactoferrin (LF) is a multifunctional protein. While its functions and mechanism of actions are actively being investigated, the cellular signals that regulate LF expression have not been as explored. We have previously demonstrated that LF is upregulated by estrogen in the reproductive system. In this study, we show that the expression of LF was stimulated by bacterial lipopolysaccharide (LPS) and double-stranded RNA (dsRNA) in normal mouse mammalian HC-11 cells. When cells were exposed to either LPS or dsRNA, the mRNA and protein of LF were increased in a dose- and time-dependent manner, yet the kinetics of LF induction by dsRNA or LPS were different. The LPS and dsRNA-induced LF was mainly released into the culture medium where it blocked TNF-alpha production in exposed cells. We explored the mechanisms of LF induction by LPS and dsRNA using specific inhibitors and found that the induction could be attenuated by inhibitors to PKC, NF-kappaB, p38 and JNK, but not by an inhibitor to PKA. Interestingly, ERK inhibitor was effective against dsRNA but not against LPS induction of LF. These data suggest that LF was induced by LPS and dsRNA through PKC, NF-kappaB and MAPK pathways which in turn play an inhibitory role in the continuation of innate inflammation.

Huperzine A regulates amyloid precursor protein processing via protein kinase C and mitogen-activated protein kinase pathways in neuroblastoma SK-N-SH cells over-expressing wild type human amyloid precursor protein 695

Alpha-secretase (alpha-secretase), cleaves the amyloid precursor protein (APP) within the amyloid-beta (Abeta) sequence, resulting in the release of a secreted fragment of APP (alphaAPPs) and precluding Abeta generation. We investigated the effects of the acetylcholinesterase inhibitor, huperzine A (Hup A), on APP processing and Abeta generation in human neuroblastoma SK-N-SH cells overexpressing wild-type human APP695. Hup A dose-dependently (0-10 microM) increased alphaAPPs release. Therefore, we evaluated two alpha-secretase candidates, a disintegrin and metalloprotease (ADAM) 10 and ADAM17 in Hup A-induced non-amyloidogenic APP metabolism. Hup A enhanced the level of ADAM10, and the inhibitor of tumor necrosis factor-alpha converting enzyme (TACE)/ADAM17 inhibited the Hup A-induced rise in alphaAPPs levels, further suggesting Hup A directed APP metabolism toward the non-amyloidogenic alpha-secretase pathway. Hup A had no effect on Abeta generation in this cell line. The steady-state levels of full-length APP and cell viability were unaffected by Hup A. Alpha-APPs release induced by Hup A treatment was significantly reduced by muscarinic acetylcholine receptor antagonists (particularly by an M1 antagonist), protein kinase C (PKC) inhibitors, GF109203X and calphostin C, and the mitogen-activated kinase kinase (MEK) inhibitors, U0126 and PD98059. Furthermore, Hup A markedly increased the phosphorylation of p44/p42 mitogen-activated protein (MAP) kinase, which was blocked by treatment with U0126 and PD98059. In addition, Hup A inhibited acetylcholinesterase activity by 20% in neuroblastoma cells. Our results indicate that the activation of muscarinic acetylcholine receptors, PKC and MAP kinase may be involved in Hup A-induced alphaAPPs secretion in neuroblastoma cells and suggest multiple pharmacological mechanisms of Hup A regarding the treatment of Alzheimer's disease (AD).

Effects of huperzine A on amyloid precursor protein processing and beta-amyloid generation in human embryonic kidney 293 APP Swedish mutant cells

The amyloid precursor protein (APP) is cleaved enzymatically by nonamyloidogenic and amyloidogenic pathways. alpha-Secretase (alpha-secretase), cleaves APP within the beta-amyloid (Abeta) sequence, resulting in the release of a secreted fragment of APP (alphaAPPs) and precluding Abeta generation. In this study, we investigated the effects of an acetylcholinesterase inhibitor, huperzine A (Hup A), on APP processing and Abeta generation in human embryonic kidney 293 cells transfected with human APP bearing the Swedish mutation (HEK293 APPsw). Hup A dose dependently (0-10 microM) increased alphaAPPs release and membrane-coupled APP CTF-C83, suggesting increased APP metabolism toward the nonamyloidogenic alpha-secretase pathway. The metalloprotease inhibitor TAPI-2 inhibited the Hup A-induced increase in alphaAPPs release, further suggesting a modulatory effect of Hup A on alpha-secretase activity. The synthesis of full-length APP and cell viability were unchanged after Hup A incubation, whereas the level of Abeta(Total) was significantly decreased, suggesting an inhibitory effect of Hup A on Abeta production. Hup A-induced alphaAPPs release was significantly reduced by the protein kinase C (PKC) inhibitors GF109203X and Calphostin C. These data, together with the finding that the PKCalpha level was enhanced prior to the increase of alphaAPPs secretion, indicate that PKC may be involved in Hup A-induced alphaAPPs secretion by HEK293 APPsw cells. Our data suggest alternative pharmacological mechanisms of Hup A relevant to the treatment of Alzheimer's disease.