SUMOylation at K340 inhibits tau degradation through deregulating its phosphorylation and ubiquitination.

Intracellular accumulation of the abnormally modified tau is hallmark pathology of Alzheimer's disease (AD), but the mechanism leading to tau aggregation is not fully characterized. Here, we studied the effects of tau SUMOylation on its phosphorylation, ubiquitination, and degradation. We show that tau SUMOylation induces tau hyperphosphorylation at multiple AD-associated sites, whereas site-specific mutagenesis of tau at K340R (the SUMOylation site) or simultaneous inhibition of tau SUMOylation by ginkgolic acid abolishes the effect of small ubiquitin-like modifier protein 1 (SUMO-1). Conversely, tau hyperphosphorylation promotes its SUMOylation; the latter in turn inhibits tau degradation with reduction of solubility and ubiquitination of tau proteins. Furthermore, the enhanced SUMO-immunoreactivity, costained with the hyperphosphorylated tau, is detected in cerebral cortex of the AD brains, and ß-amyloid exposure of rat primary hippocampal neurons induces a dose-dependent SUMOylation of the hyperphosphorylated tau. Our findings suggest that tau SUMOylation reciprocally stimulates its phosphorylation and inhibits the ubiquitination-mediated tau degradation, which provides a new insight into the AD-like tau accumulation.

Neuroglobin attenuates Alzheimer-like tau hyperphosphorylation by activating Akt signaling.

Neuroglobin (Ngb) is a recently identified member of hemoglobin family, distributed mainly in central and peripheral nervous systems. Recent studies suggest that Ngb can protect neural cells from ß-amyloid-induced toxicity in Alzheimer disease (AD). Hyperphosphorylation of tau is another characterized pathological hallmark in the AD brains; however, it is not reported whether Ngb also affects tau phosphorylation. In this study, we found that the level of Ngb was significantly reduced in Tg2576 mice (a recognized mouse model of AD) and TgMAPt mice, and the level of Ngb was negatively correlated with tau phosphorylation. Over-expression of Ngb attenuates tau hyperphosphorylation at multiple AD-related sites induced by up-regulation of glycogen synthase kinase-3ß (GSK-3ß), a crucial tau kinase. While Ngb activates Akt and thus inhibits GSK-3ß, simultaneously inhibition of Akt abolishes the effects of Ngb on GSK-3ß inhibition and tau hyperphosphorylation. Our data indicate that Ngb may attenuate tau hyperphosphorylation through activating Akt signaling pathway, implying a therapeutic target for AD.

Opposite effects of two estrogen receptors on tau phosphorylation through disparate effects on the miR-218/PTPA pathway.

The two estrogen receptors (ERs), ERα and ERß, mediate the diverse biological functions of estradiol. Opposite effects of ERα and ERß have been found in estrogen-induced cancer cell proliferation and differentiation as well as in memory-related tasks. However, whether these opposite effects are implicated in the pathogenesis of Alzheimer's disease (AD) remains unclear. Here, we find that ERα and ERß play contrasting roles in regulating tau phosphorylation, which is a pathological hallmark of AD. ERα increases the expression of miR-218 to suppress the protein levels of its specific target, protein tyrosine phosphatase α (PTPα). The downregulation of PTPα results in the abnormal tyrosine hyperphosphorylation of glycogen synthase kinase-3ß (resulting in activation) and protein phosphatase 2A (resulting in inactivation), the major tau kinase and phosphatase. Suppressing the increased expression of miR-218 inhibits the ERα-induced tau hyperphosphorylation as well as the PTPα decline. In contrast, ERß inhibits tau phosphorylation by limiting miR-218 levels and restoring the miR-218 levels antagonized the attenuation of tau phosphorylation by ERß. These data reveal for the first time opposing roles for ERα and ERß in AD pathogenesis and suggest potential therapeutic targets for AD.

Inhibition of glycogen synthase kinase-3 reverses tau hyperphosphorylation induced by Pin1 down-regulation.

One of the neuropathological hallmarks of Alzheimer's disease (AD) is the occurrence of neurofibrillary tangles (NFTs) that are composed of abnormally hyperphosphorylated microtubule-associated protein tau. Abnormal tau hyperphosphorylation is mainly induced due to the imbalance between protein kinases and phosphatases. In the tanglerich subregions of the hippocampus and parietal cortex in the brain of AD patients, the levels of the phosphorylationdependent protein peptidyl-prolyl cis-trans isomerase (Pin1) were found to be low. Although Pin1 can regulate tau phosphorylation, it is not clear whether the inhibition of glycogen synthase kinase 3 (GSK-3), the primary mediator of tau phosphorylation in AD, could reverse tau hyperphosphorylation induced due to the down-regulation of Pin1. We found that while suppression of Pin1, either by using its inhibitor Juglone or a shRNA plasmid against Pin1, induces tau hyperphosphorylation and GSK-3ß activation both in vivo and in vitro, inhibition of GSK-3ß by SB216763 or LiCl reverses tau hyperphosphorylation. Our data suggest that GSK-3ß activation plays an important role in tau hyperphosphorylation induced by the down-regulation of Pin1, and the inhibition of GSK-3ß might be a potential therapeutic approach for AD pathology.

Melatonin attenuates scopolamine-induced memory/synaptic disorder by rescuing EPACs/miR-124/Egr1 pathway.

Alzheimer's disease (AD) is the most prevalent type of dementia in elderly people. There are decreased melatonin levels in the serum of AD patients, and melatonin supplements are able to reverse AD pathology and memory deficits in many animal experiments and clinical trials. However, the underlying mechanism regarding how melatonin rescues the AD-like memory/synaptic disorder remains unknown. Here, we use the Morris water maze, step-down inhibitory avoidance task, in vivo long-term potentiation recording, and Golgi staining and report that intraperitoneal injection of melatonin (1 mg/kg/day) for 14 days in rats effectively reverses the memory and synaptic impairment in scopolamine-induced amnesia, a well-recognized dementia animal model. Using real-time polymerase chain reaction and western blotting experiments, we further determined that melatonin rescues the EPACs/miR-124/Egr1 signal pathway, which is important in learning and memory, as reported recently. Our studies provide a novel underlying epigenetic mechanism for melatonin to attenuate the synaptic disorder and could benefit drug discovery in neurodegenerative diseases.