S6K/p70S6K1 protects against tau-mediated neurodegeneration by decreasing the level of tau phosphorylated at Ser262 in a Drosophila model of tauopathy

Orexin-A/OX1R is involved in regulation of autophagy to promote cortisol secretion in adrenocortical cell

BACKGROUND

Hypercortisolism has emerged as a prominent clinical condition worldwide caused by biochemical cortisol excess in patients, and optimization treatment is needed urgently in the clinic. Previously, we observed that orexin-A/orexin type 1 receptor (OX1R) promoted cell proliferation, inhibited apoptosis, and increased cortisol release in adrenocortical cells. However, the functions of orexin-A/OX1R on autophagy and its molecular mechanism are not known.

METHODS

Transmission electron microscopy and confocal microscope were performed to detect autophagosomes. Western blot were performed to detect autophagy proteins. The cortisol concentration was assessed with an ELISA.

FINDINGS

Our data demonstrated that orexin-A/OX1R activated the mammalian target of rapamycin/p70 ribosomal protein S6 kinase-1 pathway, thereby inhibiting autophagy in H295R cells and Y-1 cells. Furthermore, the orexin-A/OX1R-mediated suppression of autophagy played a crucial role in cortisol secretion. Mechanistically, the expression of 3β-hydroxysteroid dehydrogenase/isomerase, the rate-limiting enzyme in cortisol synthesis, was increased with autophagy inhibition mediated by orexin-A/OX1R.

INTERPRETATION

This study provided the evidence that orexin-A/OX1R participated in modulating mTOR/p70S6K1/autophagy signaling pathway to promote cortisol secretion in adrenocortical cell. The findings suggest the mechanistic basis for disorders of cortisol secretion, providing the potential therapeutic targets for hypercortisolism treatment. FUND: This work was supported by National Natural Science Foundation of China (32170603, 31871286), the Doctoral Start-up Foundation of Liaoning Province (20180540008, 2019-BS-298), the Natural Science Foundation of Liaoning Province (2019-ZD-0779), and Shenyang Science and Technology Plan Fund Projects (21-173-9-28).

Causal Association Between mTOR-Dependent Protein Levels and Alzheimer's Disease: A Mendelian Randomization Study

BACKGROUND

Most previous studies supported that the mammalian target of rapamycin (mTOR) is over-activated in Alzheimer's disease (AD) and exacerbates the development of AD. It is unclear whether the causal associations between the mTOR signaling-related protein and the risk for AD exist.

OBJECTIVE

This study aims to investigate the causal effects of the mTOR signaling targets on AD.

METHODS

We explored whether the risk of AD varied with genetically predicted AKT, RP-S6K, EIF4E-BP, eIF4E, eIF4A, and eIF4G circulating levels using a two-sample Mendelian randomization analysis. The summary data for targets of the mTOR signaling were acquired from published genome-wide association studies for the INTERVAL study. Genetic associations with AD were retrieved from the International Genomics of Alzheimer's Project. We utilized the inverse variance weighted as the primary approach to calculate the effect estimates.

RESULTS

The elevated levels of AKT (OR = 0.910, 95% CI=0.840-0.986, p = 0.02) and RP-S6K (OR = 0.910, 95% CI=0.840-0.986, p = 0.02) may decrease the AD risk. In contrast, the elevated eIF4E levels (OR = 1.805, 95% CI=1.002-1.174, p = 0.045) may genetically increase the AD risk. No statistical significance was identified for levels of EIF4-BP, eIF4A, and eIF4G with AD risk (p > 0.05).

CONCLUSION

There was a causal relationship between the mTOR signaling and the risk for AD. Activating AKT and RP-S6K, or inhibiting eIF4E may be potentially beneficial to the prevention and treatment of AD.

cPKCγ Deficiency Exacerbates Autophagy Impairment and Hyperphosphorylated Tau Buildup through the AMPK/mTOR Pathway in Mice with Type 1 Diabetes Mellitus

Type 1 diabetes mellitus (T1DM)-induced cognitive dysfunction is common, but its underlying mechanisms are still poorly understood. In this study, we found that knockout of conventional protein kinase C (cPKC)γ significantly increased the phosphorylation of Tau at Ser214 and neurofibrillary tangles, but did not affect the activities of GSK-3β and PP2A in the hippocampal neurons of T1DM mice. cPKCγ deficiency significantly decreased the level of autophagy in the hippocampal neurons of T1DM mice. Activation of autophagy greatly alleviated the cognitive impairment induced by cPKCγ deficiency in T1DM mice. Moreover, cPKCγ deficiency reduced the AMPK phosphorylation levels and increased the phosphorylation levels of mTOR in vivo and in vitro. The high glucose-induced Tau phosphorylation at Ser214 was further increased by the autophagy inhibitor and was significantly decreased by an mTOR inhibitor. In conclusion, these results indicated that cPKCγ promotes autophagy through the AMPK/mTOR signaling pathway, thus reducing the level of phosphorylated Tau at Ser214 and neurofibrillary tangles.

Drosophila as a model to study autophagy in neurodegenerative diseases and digestive tract

Autophagy regulates cellular homeostasis by degrading and recycling cytosolic components and damaged organelles. Disruption of autophagic flux has been shown to induce or facilitate neurodegeneration and accumulation of autophagic vesicles is overt in neurodegenerative diseases. The fruit fly Drosophila has been used as a model system to identify new factors that regulate physiology and disease. Here we provide a historical perspective of how the fly models have offered mechanistic evidence to understand the role of autophagy in neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, Charcot-Marie-Tooth neuropathy, and polyglutamine disorders. Autophagy also plays a pivotal role in maintaining tissue homeostasis and protecting organism health. The gastrointestinal tract regulates organism health by modulating food intake, energy balance, and immunity. Growing evidence is strengthening the link between autophagy and digestive tract health in recent years. Here, we also discuss how the fly models have advanced the understanding of digestive physiology regulated by autophagy.

Tau Abnormalities and Autophagic Defects in Neurodegenerative Disorders; A Feed-forward Cycle

Abnormal deposition of misfolded proteins is a neuropathological characteristic shared by many neurodegenerative disorders including Alzheimer’s disease (AD). Generation of excessive amounts of aggregated proteins and impairment of degradation systems for misfolded proteins such as autophagy can lead to accumulation of proteins in diseased neurons. Molecules that contribute to both these effects are emerging as critical players in disease pathogenesis. Furthermore, impairment of autophagy under disease conditions can be both a cause and a consequence of abnormal protein accumulation. Specifically, disease-causing proteins can impair autophagy, which further enhances the accumulation of abnormal proteins. In this short review, we focus on the relationship between the microtubule-associated protein tau and autophagy to highlight a feed-forward mechanism in disease pathogenesis.

Cdk5 increases MARK4 activity and augments pathological tau accumulation and toxicity through tau phosphorylation at Ser262

Hyperphosphorylation of the microtubule-associated protein tau is associated with many neurodegenerative diseases, including Alzheimer’s disease. Microtubule affinity-regulating kinases (MARK) 1–4 and cyclin-dependent kinase 5 (Cdk5) are tau kinases under physiological and pathological conditions. However, their functional relationship remains elusive. Here, we report a novel mechanism by which Cdk5 activates MARK4 and augments tau phosphorylation, accumulation and toxicity. MARK4 is highly phosphorylated at multiple sites in the brain and in cultured neurons, and inhibition of Cdk5 activity reduces phosphorylation levels of MARK4. MARK4 is known to be activated by phosphorylation at its activation loop by liver kinase B1 (LKB1). In contrast, Cdk5 increased phosphorylation of MARK4 in the spacer domain, but not in the activation loop, and enhanced its kinase activity, suggesting a novel mechanism by which Cdk5 regulates MARK4 activity. We also demonstrated that co-expression of Cdk5 and MARK4 in mammalian cultured cells significantly increased the levels of tau phosphorylation at both Cdk5 target sites (SP/TP sites) and MARK target sites (Ser262), as well as the levels of total tau. Furthermore, using a Drosophila model of tau toxicity, we demonstrated that Cdk5 promoted tau accumulation and tau-induced neurodegeneration via increasing tau phosphorylation levels at Ser262 by a fly ortholog of MARK, Par-1. This study suggests a novel mechanism by which Cdk5 and MARK4 synergistically increase tau phosphorylation and accumulation, consequently promoting neurodegeneration in disease pathogenesis.