Alzheimer's disease (AD) is caused by the accumulation of neurotoxic amyloid-␤ (A␤) peptides. A␤ is derived from amyloid-␤ protein precursor (A␤PP). In the non-amyloidogenic pathway, A␤PP is cleaved by ␣-secretase and ␥-secretase at the plasma membrane, excluding A␤ production. Alternatively, A␤PP in the plasma membrane is internalized via endocytosis, and delivered to early endosomes and lysosomes, where it is cleaved by ␤-secretase and ␥-secretase. Recent studies have shown that insulin in the periphery crosses the blood-brain barrier, and plays important roles in the brain. Furthermore, impaired insulin signaling has been linked to the progression of AD, and intranasal insulin administration improves memory impairments and cognition. However, the underlying molecular mechanisms of insulin treatment remain largely unknown. To investigate the effects of insulin on A␤PP processing, we tested the effects of insulin on neuroblastoma SH-SY5Y cells overexpressing A␤PP, and cultured rat cortical neurons. We found that insulin increased the level of cell surface A␤PP, decreasing the endocytosis rate of A␤PP. Insulin reduced A␤ generation through upregulation of A␤PP O-GlcNAcylation via Akt insulin signaling. Our present data suggest that insulin affects A␤ production by regulating A␤PP processing through A␤PP O-GlcNAcylation. These results provide mechanistic insight into the beneficial effects of insulin, and a possible link between insulin deficient diabetes and cerebral amyloidosis in the pathogenesis of AD.

β-amyloid precursor protein (APP) can be cleaved by α-, and γ-secretase at plasma membrane producing soluble ectodomain fragment (sAPPα). Alternatively, following endocytosis, APP is cleaved by β-, and γ-secretase at early endosomes generating β-amyloid (Aβ), the main culprit in Alzheimer’s disease (AD). Thus, APP endocytosis is critical for Aβ production. Recently, we reported that Monsonia angustifolia , the indigenous vegetables consumed in Tanzania, improved cognitive function and decreased Aβ production. In this study, we examined the underlying mechanism of justicidin A, the active compound of M. angustifolia , on Aβ production. We found that justicidin A reduced endocytosis of APP, increasing sAPPα level, while decreasing Aβ level in HeLa cells overexpressing human APP with the Swedish mutation. The effect of justicidin A on Aβ production was blocked by endocytosis inhibitors, indicating that the decreased APP endocytosis by justicidin A is the underlying mechanism. Thus, justicidin A, the active compound of M. angustifolia , may be a novel agent for AD treatment.

Accumulation of β-amyloid (Aβ) in the brain has been implicated in the pathology of Alzheimer’s disease (AD). Aβ is produced from the Aβ precursor protein (APP) through the amyloidogenic pathway by β-, and γ-secretase. Alternatively, APP can be cleaved by α-, and γ-secretase, precluding the production of Aβ. Thus, stimulating α-secretase mediated APP processing is considered a therapeutic option not only for decreasing Aβ production but for increasing neuroprotective sAPPα. We have previously reported that 7-deoxy-trans-dihydronarciclasine (E144), the active component of Lycoris chejuensis, decreases Aβ production by attenuating APP level, and retarding APP maturation. It can also improve cognitive function in the AD model mouse. In this study, we further analyzed the activating effect of E144 on α-secretase. Treatment of E144 increased sAPPα, but decreased β-secretase products from HeLa cells stably transfected with APP. E144 directly activated ADAM10 and ADAM17 in a substrate-specific manner both in cell-based and in cell-free assays. The Lineweaver–Burk plot analysis revealed that E144 enhanced the affinities of A Disintegrin and Metalloproteinases (ADAMs) towards the substrate. Consistent with this result, immunoprecipitation analysis showed that interactions of APP with ADAM10 and ADAM17 were increased by E144. Our results indicate that E144 might be a novel agent for AD treatment as a substrate-specific activator of α-secretase.