Most patients with Alzheimer's disease (AD) present decreased levels of melatonin, a day-night rhythm-related hormone. To investigate the role of melatonin deficiency in AD, we used constant illumination to interrupt melatonin metabolism and measured some of the AD-like alterations in rats. Concomitant with decreased serum melatonin, the rats developed spatial memory deficits, tau hyperphosphorylation at multiple sites, activation of glycogen synthase kinase-3 and protein kinase A, as well as suppression of protein phosphatase-1. Prominent oxidative damage and organelle lesions, demonstrated by increased expression of endoplasmic reticulum (ER) stress-related proteins including BiP/GRP78 and CHOP/GADD153, decreased number of rough ER and free ribosome, thinner synapses, and increased superoxide dismutase and monoamine oxidase were also observed in the light exposed rats. Simultaneous supplement of melatonin partially arrested the behavioral and molecular impairments. It is suggested that melatonin deficiency may be an upstream effector responsible for the AD-like behavioral and molecular pathologies with ER stress-involved mechanisms.

Abnormal hyperphosphorylation of microtubule-associated protein tau is involved in the pathogenesis of several neurodegenerative diseases, including Alzheimer's disease (AD). Endoplasmic reticulum (ER) stress is indicated to play an important role in neurodegeneration and activation of glycogen synthase kinase-3β (GSK-3β), an integral kinase in tau phosphorylation. To explore the effect of ER stress on tau phosphorylation, we treated cultured cells (HEK293 and SH-SY5Y cells) and rat brain with thapsigargin, an ER stress inducer. We found that the phosphorylation level of tau was significantly increased after thapsigargin treatment. By using a cell-free reconstitution system, we also observed that co-culture of the thapsigargin-treated ER fraction from HEK293/wt (without tau) with cytoplasm prepared from HEK293/tau induced an increased tau phosphorylation. Concurrently, activation of GSK-3β as evidenced by an increased phospho-GSK-3β at Tyr-216 and decreased phospho-GSK-3β at Ser-9 both in vitro and in vivo was detected. Application of lithium chloride, a GSK-3 inhibitor, could efficiently attenuate the thapsigargin-induced tau hyperphosphorylation with suppressed activation of GSK-3β in cell cultures and rat brains. Our data provide further evidence supporting the role of ER stress in tau hyperphosphorylation and the protective role of lithium.

We have reported recently that inhibition of protein phosphatase (PP)-2A and PP-1 by calyculin A, a specific inhibitor of PP-2A and PP-1, induced Alzheimer-like hyperphosphorylation of tau and spatial memory retention impairment. In this study, we tested the in vivo effects of melatonin on these Alzheimer-like changes. We found that administration of melatonin intraperitoneally for 9 consecutive days before injection of calyculin A could prevent calyculin A-induced synaptophysin loss, memory retention deficits, as well as hyperphosphorylation of tau and neurofilaments. Furthermore, melatonin partially reversed the phosphorylation of the catalytic subunit of PP-2A at Tyrosine 307 (Y307), a crucial site negatively regulating the activity of PP-2A, and reduced the levels of malondialdehyde, a marker of oxidative stress, induced by calyculin A. These results suggest that melatonin could serve as a potential therapeutic agent for preventing Alzheimer-like pathological changes and behavioral abnormality via modulating the activity of PP-2A and oxidative stress.

Endoplasmic reticulum (ER) stress is involved in Alzheimer's disease (AD), but the mechanism is not fully understood. Here, we injected tunicamycin (TM), a recognized ER stress inducer, into the brain ventricle of Sprague‐Dawley (SD) rats to induce the unfolded protein response (UPR), demonstrated by the enhanced phosphorylation of pancreatic ER kinase (PERK), inositol‐requiring enzyme‐1 (IRE‐1) and activating transcription factor‐6 (ATF‐6). We observed that UPR induced spatial memory deficits and impairments of synaptic plasticity in the rats. After TM treatment, GSK‐3β was activated and phosphorylation of cAMP response element binding protein at Ser129 (pS129‐CREB) was increased with an increased nuclear co‐localization of pY126‐GSK‐3β and pS129‐CREB. Simultaneous inhibition of GSK‐3β by hippocampal infusion of SB216763 (SB) attenuated TM‐induced UPR and spatial memory impairment with restoration of pS129‐CREB and synaptic plasticity. We concluded that UPR induces AD‐like spatial memory deficits with mechanisms involving GSK‐3β/pS129‐CREB pathway.

Glucose-regulated protein 94 is a downstream effector of ER-α36-mediated oestrogen signalling, and may be involved in ER-α36 function during gastric carcinogenesis.