Role of Blood-Brain Barrier in Alzheimer's Disease.

The blood-brain barrier (BBB) is involved in the pathogenesis of Alzheimer's disease (AD). BBB is a highly selective semipermeable structural and chemical barrier which ensures a stable internal environment of the brain and prevents foreign objects invading the brain tissue. BBB dysfunction induces the failure of Aß transport from brain to the peripheral circulation across the BBB. Especially, decreased levels of LRP-1 (low density lipoprotein receptor-related protein 1) and increased levels of RAGE (receptor for advanced glycation endproducts) at the BBB can cause the failure of Aß transport. The pathogenesis of AD is related to the BBB structural components, including pericytes, astrocytes, vascular endothelial cells, and tight junctions. BBB dysfunction will trigger neuroinflammation and oxidative stress, then enhance the activity of ß-secretase and γ-secretase, and finally promote Aß generation. A progressive accumulation of Aß in brain and BBB dysfunction may become a feedback loop that gives rise to cognitive impairment and the onset of dementia. The correlation between BBB dysfunction and tau pathology has been well-reported. Therefore, regulating BBB function may be a new therapeutic target for treating AD.

AP39, a Mitochondria-Targeted Hydrogen Sulfide Donor, Supports Cellular Bioenergetics and Protects against Alzheimer's Disease by Preserving Mitochondrial Function in APP/PS1 Mice and Neurons.

Increasing evidence suggests that mitochondrial functions are altered in AD and play an important role in AD pathogenesis. It has been established that H2S homeostasis is balanced in AD. The emerging mitochondrial roles of H2S include antioxidation, antiapoptosis, and the modulation of cellular bioenergetics. Here, using primary neurons from the well-characterized APP/PS1 transgenic mouse model, we studied the effects of AP39 (a newly synthesized mitochondrially targeted H2S donor) on mitochondrial function. AP39 increased intracellular H2S levels, mainly in mitochondrial regions. AP39 exerted dose-dependent effects on mitochondrial activity in APP/PS1 neurons, including increased cellular bioenergy metabolism and cell viability at low concentrations (25-100 nM) and decreased energy production and cell viability at a high concentration (250 nM). Furthermore, AP39 (100 nM) increased ATP levels, protected mitochondrial DNA, and decreased ROS generation. AP39 regulated mitochondrial dynamics, shifting from fission toward fusion. After 6 weeks, AP39 administration to APP/PS1 mice significantly ameliorated their spatial memory deficits in the Morris water maze and NORT and reduced Aß deposition in their brains. Additionally, AP39 inhibited brain atrophy in APP/PS1 mice. Based on these results, AP39 was proposed as a promising drug candidate for AD treatment, and its anti-AD mechanism may involve protection against mitochondrial damage.

Hydrogen Sulfide Selectively Inhibits γ-Secretase Activity and Decreases Mitochondrial Aß Production in Neurons from APP/PS1 Transgenic Mice.

Hydrogen sulfide (H2S) is now considered to be a gasotransmitter and may be involved in the pathological process of Alzheimer's disease (AD). A majority of APP is associated with mitochondria and is a substrate for the mitochondrial γ-secretase. The mitochondria-associated APP metabolism where APP intracellular domains (AICD) and Aß are generated locally and may contribute to mitochondrial dysfunction in AD. Here, we aimed to investigate the ability of H2S to mediate APP processing in mitochondria and assessed the possible mechanisms underlying H2S-mediated AD development. We treated neurons from APP/PS1 transgenic mice with a range of sodium hydrosulfide (NaHS) concentrations. NaHS attenuated APP processing and decreased Aß production in mitochondria. Meanwhile, NaHS did not changed BACE-1 and ADAM10 (a disintegrin and metalloprotease 10) protein levels, but NaHS (30 µM) significantly increased the levels of presenilin 1(PS1), PEN-2, and NCT, as well as improved the γ-secretase activity, while NaHS (50 µM) exhibits the opposing effects. Furthermore, the intracellular ATP and the COX IV activity of APP/PS1 neurons were increased after 30 µM NaHS treatment, while the ROS level was decreased and the MMP was stabilized. The effect of NaHS differs from DAPT (a non-selective γ-secretase inhibitor), and it selectively inhibited γ-secretase in vitro, without interacting with Notch and modulating its cleavage. The results indicated that NaHS decreases Aß accumulation in mitochondria by selectively inhibiting γ-secretase. Thus, we provide a mechanistic view of NaHS is a potential anti-AD drug candidate and it may decrease Aß deposition in mitochondria by selectively inhibiting γ-secretase activity and therefore protecting the mitochondrial function during AD conditions.

PPAR-α agonist regulates amyloid-ß generation via inhibiting BACE-1 activity in human neuroblastoma SH-SY5Y cells transfected with APPswe gene.

Alzheimer's disease is a neuroinflammatory disease and is the most common cause of dementia in the elderly. Studies have shown the beneficial effects of the peroxisome proliferator-activated receptor alpha (PPAR-α) agonists on the treatment of neuroinflammatory diseases. The aim of the present study is to examine the ability of GW7647 (a PPAR-α agonist) to regulate amyloid precursor protein (APP) amyloidogenic processing in human neuroblastoma SH-SY5Y cells transfected with APPswe gene. After administration of GW7647 for 24 h, the levels of APP, soluble APPß (sAPPß), and presenilin 1 (PS-1) were assessed by Western blot. Cellular culture medium levels of amyloid-ß 42 (Aß42) were analyzed by ELISA, and the activity of beta-site APP cleaving enzyme 1 (BACE-1) was measured by fluorometric assay. We found that GW7647 decreased the expression of sAPPß and the activity of BACE-1, and also reduced Aß42 release. However, GW7647 did not modify the levels of APP and PS-1. Furthermore, LY294002, the phosphoinositide 3-kinase (PI3-K) inhibitor, reversed the effects of GW7647 on the BACE-1 activity and the levels of sAPPß and Aß42. Our data demonstrate that GW7647 may reduce Aß production via inhibiting BACE-1 activity, and this may involve in PI3-K pathway.

Hydrogen sulfide-induced processing of the amyloid precursor protein in SH-SY5Y human neuroblastoma cells involves the PI3-K/Akt signaling pathway.

Hydrogen sulfide (H2S) has been recently categorized as a gasotransmitter, and it may be involved in the pathology of Alzheimer's disease. However, whether H2S induces amyloid precursor protein (APP) processing remains unknown. In the present study, we tested the ability of H2S to mediate APP processing in SH-SY5Y human neuroblastoma cells. We treated SH-SY5Y human neuroblastoma cells with a range of sodium hydrosulfide (H2S donor) concentrations. Western blot analysis showed that H2S increased the generation of C83 and decreased the production of C99. Meanwhile, H2S increased the levels of a disintegrin and metalloprotease 10 (ADAM10) mRNA and protein, but had no effect on TNF-α-converting enzyme (TACE, also known as ADAM17) mRNA and protein levels. H2S also induced a significant decrease of extracellular amyloid-ß42 (Aß42). Furthermore, SH-SY5Y human neuroblastoma cells were assayed for activation of the phosphoinositide 3-kinase (PI3-K) pathway. H2S activated the PI3-K pathway. Using specific inhibitor of PI3-K, we determined that the effects of H2S on APP processing and Aß42 were blocked by LY 294002 (PI3-K inhibitor). These data indicate that H2S can induce APP processing, and this effect is dependent on activation of the PI3-K signaling pathway.

Fenofibrate reduces amyloidogenic processing of APP in APP/PS1 transgenic mice via PPAR-α/PI3-K pathway.

The peroxisome proliferator-activated receptor alpha (PPAR-α), a member of the family of ligand-activated nuclear hormone receptors, plays a relevant role in the development of Alzheimer's disease (AD). To better understand the role of PPAR-α in AD, we examined the ability of fenofibrate (a PPAR-α agonist) to regulate amyloid precursor protein (APP) processing in APP/PS1 transgenic mice. After intragastric administration of fenofibrate into 3-month-old APP/PS1 transgenic mice for 6 months, and the levels of relative proteins were quantified by quantitative reverse transcription-PCR, Western blotting and ELISA. We found that fenofibrate increased the expression of PPAR-α, and decreased beta-site amyloid precursor protein cleaving enzyme 1 (BACE-1) mRNA and protein levels, and also reduced soluble APPß (sAPPß) and amyloid-ß 42 (Aß42) releases. However. fenofibrate did not modify the levels of APP and presenilin 1 (PS1). Furthermore, LY294002, the phosphoinositide 3-kinase (PI3-K) inhibitor, suppressed the effects of fenofibrate on BACE-1, sAPPß, and Aß42, but not PPAR-α. Our data suggest that fenofibrate may reduce the amyloidogenic processing of APP in APP/PS1 transgenic mice via PPAR-α/PI3-K pathway.