Lack of insulin results in reduced seladin-1 expression in primary cultured neurons and in cerebral cortex of STZ-induced diabetic rats

New insight into the role of altered brain cholesterol metabolism in the pathogenesis of AD: A unifying cholesterol hypothesis and new therapeutic approach for AD

The dysregulation of cholesterol metabolism homeostasis has been universally suggested in the aeotiology of Alzheimer's disease (AD). Initially, studies indicate that alteration of serum cholesterol level might contribute to AD. However, because blood-brain barrier impedes entry of plasma cholesterol, brain cells are not directly influenced by plasma cholesterol. Furthermore, mounting evidences suggest a link between alteration of brain cholesterol metabolism and AD. Interestingly, Amyloid-β proteins (Aβ) can markedly inhibit cellular cholesterol biosynthesis and lower cellular cholesterol content in cultured cells. And Aβ overproduction/overload induces a significant decrease of brain cellular cholesterol content in familial AD (FAD) animals. Importantly, mutations or polymorphisms of genes related to brain cholesterol transportation, such as ApoE4, ATP binding cassette (ABC) transporters, low-density lipoprotein receptor (LDLR) family and Niemann-Pick C disease 1 or 2 (NPC1/2), obviously lead to decreased brain cholesterol transport, resulting in brain cellular cholesterol loss, which could be tightly associated with AD pathological impairments. Additionally, accumulating data show that there are reduction of brain cholesterol biosynthesis and/or disorder of brain cholesterol trafficking in a variety of sporadic AD (SAD) animals and patients. Collectively, compelling evidences indicate that FAD and SAD could share one common and overlapping neurochemical mechanism: brain neuronal/cellular cholesterol deficiency. Therefore, accumulated evidences strongly support a novel hypothesis that deficiency of brain cholesterol contributes to the onset and progression of AD. This review highlights the pivotal role of brain cholesterol deficiency in the pathogenesis of AD. The hypothesis offers valuable insights for the future development of AD treatment.

DHCR24 Knockdown Induces Tau Hyperphosphorylation at Thr181, Ser199, Ser262, and Ser396 Sites via Activation of the Lipid Raft-Dependent Ras/MEK/ERK Signaling Pathway in C8D1A Astrocytes

The synthetase 3β-hydroxysterol-Δ24 reductase (DHCR24) is a key regulator involved in cholesterol synthesis and homeostasis. A growing body of evidence indicates that DHCR24 is downregulated in the brain of various models of Alzheimer's disease (AD), such as astrocytes isolated from AD mice. For the past decades, astrocytic tau pathology has been found in AD patients, while the origin of phosphorylated tau in astrocytes remains unknown. A previous study suggests that downregulation of DHCR24 is associated with neuronal tau hyperphosphorylation. Herein, the present study is to explore whether DHCR24 deficiency can also affect tau phosphorylation in astrocytes. Here, we showed that DHCR24 knockdown could induce tau hyperphosphorylation at Thr181, Ser199, Thr231, Ser262, and Ser396 sites in C8D1A astrocytes. Meanwhile, we found that DHCR24-silencing cells had reduced the level of free cholesterol in the plasma membrane and intracellular organelles, as well as cholesterol esters. Furthermore, reduced cellular cholesterol level caused a decreased level of the caveolae-associated protein, cavin1, which disrupted lipid rafts/caveolae and activated rafts/caveolae-dependent Ras/MEK/ERK signaling pathway. In contrast, overexpression of DHCR24 prevented the overactivation of Ras/MEK/ERK signaling by increasing cellular cholesterol content, therefore decreasing tau hyperphosphorylation in C8D1A astrocytes. Herein, we firstly found that DHCR24 knockdown can lead to tau hyperphosphorylation in the astrocyte itself by activating lipid raft-dependent Ras/MEK/ERK signaling, which might contribute to the pathogenesis of AD and other degenerative tauopathies.

The role of DHCR24 in the pathogenesis of AD: re-cognition of the relationship between cholesterol and AD pathogenesis

Previous studies show that 3β-hydroxysterol-Δ24 reductase (DHCR24) has a remarked decline in the brain of AD patients. In brain cholesterol synthetic metabolism, DHCR24 is known as the heavily key synthetase in cholesterol synthesis. Moreover, mutations of DHCR24 gene result in inhibition of the enzymatic activity of DHCR24, causing brain cholesterol deficiency and desmosterol accumulation. Furthermore, in vitro studies also demonstrated that DHCR24 knockdown lead to the inhibition of cholesterol synthesis, and the decrease of plasma membrane cholesterol and intracellular cholesterol level. Obviously, DHCR24 could play a crucial role in maintaining cholesterol homeostasis via the control of cholesterol synthesis. Over the past two decades, accumulating data suggests that DHCR24 activity is downregulated by major risk factors for AD, suggesting a potential link between DHCR24 downregulation and AD pathogenesis. Thus, the brain cholesterol loss seems to be induced by the major risk factors for AD, suggesting a possible causative link between brain cholesterol loss and AD. According to previous data and our study, we further found that the reduced cholesterol level in plasma membrane and intracellular compartments by the deficiency of DHCR24 activity obviously was involved in β-amyloid generation, tau hyperphosphorylation, apoptosis. Importantly, increasing evidences reveal that the brain cholesterol loss and lipid raft disorganization are obviously linked to neuropathological impairments which are associated with AD pathogenesis. Therefore, based on previous data and research on DHCR24, we suppose that the brain cholesterol deficiency/loss might be involved in the pathogenesis of AD.

DHCR24 Knock-Down Induced Tau Hyperphosphorylation at Thr181, Ser199, Thr231, Ser262, Ser396 Epitopes and Inhibition of Autophagy by Overactivation of GSK3β/mTOR Signaling

Accumulating evidences supported that knock-down of DHCR24 is linked to the pathological risk factors of AD, suggesting a potential role of DHCR24 in AD pathogenesis. However, the molecular mechanism link between DHCR24 and tauopathy remains unknown. Here, in order to elucidate the relationship between DHCR24 and tauopathy, we will focus on the effect of DHCR24 on the tau hyperphosphorylation at some toxic sites. In present study, we found that DHCR24 knock-down significantly lead to the hyperphosphorylation of tau sites at Thr181, Ser199, Thr231, Ser262, Ser396. Moreover, DHCR24 knock-down also increase the accumulation of p62 protein, simultaneously decreased the ratio of LC3-II/LC3-I and the number of autophagosome compared to the control groups, suggesting the inhibition of autophagy activity. In contrast, DHCR24 knock-in obviously abolished the effect of DHCR24 knock-down on tau hyperphosphrylation and autophagy. In addition, to elucidate the association between DHCR24 and tauopathy, we further showed that the level of plasma membrane cholesterol, lipid raft-anchored protein caveolin-1, and concomitantly total I class PI3-K (p110α), phospho-Akt (Thr308 and Ser473) were significantly decreased, resulting in the disruption of lipid raft/caveola and inhibition of PI3-K/Akt signaling in silencing DHCR24 SH-SY5Y cells compared to control groups. At the same time, DHCR24 knock-down simultaneously decreased the level of phosphorylated GSK3β at Ser9 (inactive form) and increased the level of phosphorylated mTOR at Ser2448 (active form), leading to overactivation of GSK3β and mTOR signaling. On the contrary, DHCR24 knock-in largely increased the level of membrane cholesterol and caveolin-1, suggesting the enhancement of lipid raft/caveola. And synchronously DHCR24 knock-in also abolished the effect of DHCR24 knock-down on the inhibition of PI3-K/Akt signaling as well as the overactivation of GSK3β and mTOR signaling. Collectively, our data strongly supported DHCR24 knock-down lead to tau hyperphosphorylation and the inhibition of autophagy by a lipid raft-dependent PI3-K/Akt-mediated GSK3β and mTOR signaling. Taking together, our results firstly demonstrated that the decrease of plasma membrane cholesterol mediated by DHCR24 deficiency might contribute to the tauopathy in AD and other tauopathies.

Serum seladin-1 levels in diabetes mellitus and Alzheimer's disease patients

BACKGROUND

Diabetes mellitus (DM) has been shown to increase the risk of Alzheimer's disease (AD). Downregulation of selective Alzheimer's disease indicator-1 (seladin-1) occurs in the cerebral regions affected by AD. However, inconsistent results have been reported for the relationship between seladin-1 levels and AD. The effect of DM on serum seladin-1 levels in AD is unknown. The present study is aimed to investigate serum seladin-1 levels in diabetic and non-diabetic patients with AD.

METHODS

Forty-six patients with AD and 25 healthy volunteers over 65 years of age were included in this study. The patients were divided into three groups-those with AD only, those with DM and AD, and control groups. Demographic characteristics and serum seladin-1 levels were compared among the groups.

RESULTS

There was no statistically significant difference in seladin-1 levels in the AD only group compared to the control group (p = 0.376). However, seladin-1 levels were significantly lower in the DM and AD group compared to the AD only and control groups (p = 0.002, p = 0.001; respectively). Negative correlations were present between seladin-1 and fasting glucose, postprandial glucose, HbA1c, and insulin (p < 0.05; all).

CONCLUSION

Decreased serum seladin-1 values in the presence of DM and inverse correlations with diabetic parameters in patients with AD, together with a non-significant difference from the control group, suggests that seladin-1 may be altered only in the presence of DM in patients with AD. Lower serum seladin-1 levels may also play a role in the pathogenesis of AD in patients with DM.

Metformin Ameliorates Aβ Pathology by Insulin-Degrading Enzyme in a Transgenic Mouse Model of Alzheimer's Disease

Alzheimer's disease (AD) is the most common neurodegenerative disease. The accumulation of amyloid beta (Aβ) is the main pathology of AD. Metformin, a well-known antidiabetic drug, has been reported to have AD-protective effect. However, the mechanism is still unclear. In this study, we tried to figure out whether metformin could activate insulin-degrading enzyme (IDE) to ameliorate Aβ-induced pathology. Morris water maze and Y-maze results indicated that metformin could improve the learning and memory ability in APP^swe/PS1^dE9 (APP/PS1) transgenic mice. ¹⁸F-FDG PET-CT result showed that metformin could ameliorate the neural dysfunction in APP/PS1 transgenic mice. PCR analysis showed that metformin could effectively improve the mRNA expression level of nerve and synapse-related genes (Syp, Ngf, and Bdnf) in the brain. Metformin decreased oxidative stress (malondialdehyde and superoxide dismutase) and neuroinflammation (IL-1β and IL-6) in APP/PS1 mice. In addition, metformin obviously reduced the Aβ level in the brain of APP/PS1 mice. Metformin did not affect the enzyme activities and mRNA expression levels of Aβ-related secretases (ADAM10, BACE1, and PS1). Meanwhile, metformin also did not affect the mRNA expression levels of Aβ-related transporters (LRP1 and RAGE). Metformin increased the protein levels of p-AMPK and IDE in the brain of APP/PS1 mice, which might be the key mechanism of metformin on AD. In conclusion, the well-known antidiabetic drug, metformin, could be a promising drug for AD treatment.

Effects of chronic hypoxia on the expression of seladin-1/Tuj1 and the number of dark neurons of hippocampus

BACKGROUND

There are evidences showing the relation between chronic hypoxia and Alzheimer's disease (AD) as a metabolic neurodegenerative disease. This study was designed to evaluate the effects of chronic hypoxia on factors which characterized in AD to introduce a new model of AD-dementia.

METHODS AND MATERIALS

Twenty-four male rats were randomly divided in three groups: Control group (Co), Sham group (Sh), Hypoxia induction group (Hx, exposed to hypoxic chamber [oxygen 8% and nitrogen 92%] for 30 days, 4 h/day). Spatial learning and memory were analyzed using the Morris water maze task. At day 30 after hypoxia period, animals were sacrificed and serum was gathered for pro-inflammatory cytokines (interleukin-1β and tumor necrosis factor) measurements and brains were used for molecular and histopathological investigations.

RESULTS

According to behavioral studies, a significant impairment was seen in Hx group (P < 0.05). TNF-α and IL-1β showed a significant enhanced in Hx group comparing with Co group and Sh group (P < 0.05). As well, the gene expression of seladin-1, Tuj1 and the number of seladin-1+, Tuj1+neurons significantly decreased and also the mean number of dark neurons significantly increased in CA1 and CA3 regions of hippocampus.

CONCLUSIONS

In this study, a new model of AD was developed which showed the underlying mechanisms of AD and its relations with chronic hypoxia. Hypoxia for 30 days decreased seladin-1, Tuj1 expression, increased the number of dark neurons, and also induced memory impairment. These results indicated that chronic hypoxia mediated the dementia underlying AD and AD-related pathogenesis in rat.

Insulin deprivation decreases insulin degrading enzyme levels in primary cultured cortical neurons and in the cerebral cortex of rats with streptozotocin-induced diabetes

BACKGROUND

Many studies have indicated a relationship between diabetes and Alzheimer's disease (AD). However, the molecular mechanism underlying this association has not been clarified. Among several factors, insulin degrading enzyme (IDE), which plays roles in the degradation of both insulin and amyloid β (Aβ), has gained interest as a potential target in efforts to solve this puzzle. This study sought to examine the effects of varying insulin and/or glucose concentrations on IDE expression.

METHODS

Experiments were performed on primary cultured rat neurons and cortices of rats with streptozotocin (STZ)-induced diabetes. IDE protein and mRNA expression levels were measured by western blot and RT-PCR, respectively.

RESULTS

In primary cultured cortical neurons, removal of insulin for 5days reduced the expression of IDE. A five-day treatment with a high concentration of glucose in insulin-free media reduced IDE levels, while a high concentration of glucose in the presence of insulin had no effect. In groups treated with glucose or insulin intermittently, the reduction in IDE levels was observed only in neurons exposed to high glucose together with no insulin for 5days. Shorter incubation periods (48h), either continuously or intermittently, did not affect IDE levels. IDE expression in the cortex of rats with STZ-induced diabetes was found to be decreased.

CONCLUSION

Our data suggest that insulin deprivation, rather than high glucose, is a significant determinant of IDE regulation. As evidence indicates potential roles for IDE in diabetes and AD, understanding the mechanisms regulating IDE expression may be important in developing new treatment strategies.