Modulation of endoplasmic reticulum chaperone GRP78 by high glucose in hippocampus of streptozotocin-induced diabetic mice and C6 astrocytic cells

HDAC1 Promotes Mitochondrial Pathway Apoptosis and Inhibits the Endoplasmic Reticulum Stress Response in High Glucose-Treated Schwann Cells via Decreased U4 Spliceosomal RNA

Dysfunction of Schwann cells, including cell apoptosis, autophagy inhibition, dedifferentiation, and pyroptosis, is a pivotal pathogenic factor in induced diabetic peripheral neuropathy (DPN). Histone deacetylases (HDACs) are an important family of proteins that epigenetically regulate gene transcription by affecting chromatin dynamics. Here, we explored the effect of HDAC1 on high glucose-cultured Schwann cells. HDAC1 expression was increased in diabetic mice and high glucose-cultured RSC96 cells, accompanied by cell apoptosis. High glucose also increased the mitochondrial pathway apoptosis-related Bax/Bcl-2 and cleaved caspase-9/caspase-9 ratios and decreased endoplasmic reticulum response-related GRP78, CHOP, and ATF4 expression in RSC96 cells (P < 0.05). Furthermore, overexpression of HDAC1 increased the ratios of Bax/Bcl-2, cleaved caspase-9/caspase-9, and cleaved caspase-3 and reduced the levels of GRP78, CHOP, and ATF4 in RSC96 cells (P < 0.05). In contrast, knockdown of HDAC1 inhibited high glucose-promoted mitochondrial pathway apoptosis and suppressed the endoplasmic reticulum response. Moreover, RNA sequencing revealed that U4 spliceosomal RNA was significantly reduced in HDAC1-overexpressing RSC96 cells. Silencing of U4 spliceosomal RNA led to an increase in Bax/Bcl-2 and cleaved caspase-9 and a decrease in CHOP and ATF4. Conversely, overexpression of U4 spliceosomal RNA blocked HDAC1-promoted mitochondrial pathway apoptosis and inhibited the endoplasmic reticulum response. In addition, alternative splicing analysis of HDAC1-overexpressing RSC96 cells showed that significantly differential intron retention (IR) of Rpl21, Cdc34, and Mtmr11 might be dominant downstream targets that mediate U4 deficiency-induced Schwann cell dysfunction. Taken together, these findings indicate that HDAC1 promotes mitochondrial pathway-mediated apoptosis and inhibits the endoplasmic reticulum stress response in high glucose-cultured Schwann cells by decreasing the U4 spliceosomal RNA/IR of Rpl21, Cdc34, and Mtmr11.

The gut microbiota-astrocyte axis: Implications for type 2 diabetic cognitive dysfunction

BACKGROUND

Diabetic cognitive dysfunction (DCD) is one of the most insidious complications of type 2 diabetes mellitus, which can seriously affect the ability to self-monitoring of blood glucose and the quality of life in the elderly. Previous pathological studies of cognitive dysfunction have focused on neuronal dysfunction, characterized by extracellular beta-amyloid deposition and intracellular tau hyperphosphorylation. In recent years, astrocytes have been recognized as a potential therapeutic target for cognitive dysfunction and important participants in the central control of metabolism. The disorder of gut microbiota and their metabolites have been linked to a series of metabolic diseases such as diabetes mellitus. The imbalance of intestinal flora has the effect of promoting the occurrence and deterioration of several diabetes-related complications. Gut microbes and their metabolites can drive astrocyte activation.

AIMS

We reviewed the pathological progress of DCD related to the "gut microbiota-astrocyte" axis in terms of peripheral and central inflammation, intestinal and blood-brain barrier (BBB) dysfunction, systemic and brain energy metabolism disorders to deepen the pathological research progress of DCD and explore the potential therapeutic targets.

CONCLUSION

"Gut microbiota-astrocyte" axis, unique bidirectional crosstalk in the brain-gut axis, mediates the intermediate pathological process of neurocognitive dysfunction secondary to metabolic disorders in diabetes mellitus.

Matrine alleviates spatial learning and memory impairment in diabetic mice by inhibiting endoplasmic reticulum stress and through modulation of PK2/PKRs pathway

Clinical and epidemiological studies indicate that diabetic cognitive impairment often occurs in diabetes mellitus patients. Matrine (Mat), an active component of Sophora flavescens Ait root extracts, has widely pharmacological activities including anti-tumor, anti-diabetes, cardioprotective and neuroprotective effects. The present study was designed to elucidate the possibly neuroprotective effects of Mat against diabetic spatial learning and memory impairment caused by high-fat diet and streptozotocin injection in mice. The results showed that Mat treatment significantly ameliorated fasting blood glucose level, impaired glucose tolerance, and lipid metabolism disorder in diabetic mice. In addition, diabetic mice exhibited spatial learning and memory impairment in the Morris water maze test, which could be attenuated by Mat treatment. Moreover, administration of Mat remarkably alleviated histological damage in diabetic hippocampus. Also, further investigations showed that Mat treatment abated endoplasmic reticulum stress induced hippocampal ultra-structure injury as evidenced by increasing the numbers of rough endoplasmic reticulum and mitochondria, as well as down-regulating endoplasmic reticulum stress related protein levels (GRP78, CHOP, ATF6 and Caspase-12). Furthermore, administration of Mat enhanced hippocampal protein expressions of PK2, PKR1 and PKR2, which decreased significantly in diabetic mice. Collectively, these findings suggested that Mat could ameliorate diabetes-induced spatial learning and memory impairment, possibly by alleviating ER stress, and partly through modulation of PK2/PKRs pathway.

Sodium butyrate ameliorates the cognitive impairment of Alzheimer's disease by regulating the metabolism of astrocytes

RATIONALE

Energy metabolism disorder is a widespread feature that exists in the early clinical stages of Alzheimer's disease (AD). Astrocyte is the most numerous and the largest glial cell in the brain. By transporting energetic fuels such as lactate and ketones to neurons, astrocytes play a pivotal role in maintaining the cerebral energy homeostasis. Sodium butyrate (NaB), a type of short-chain fatty acid; its anti-inflammatory effect; and inhibition on histone deacetylases have been widely studied.

METHODS

Spatial memory and cognitive ability of mice were assessed by using behavioral tests. Western blotting and ELISA kits were used to detect related protein levels and other biochemical markers, respectively.

OBJECTIVES

To prove the therapeutic effect of NaB on AD cognitive impairment and provide possible research ideas for mechanism exploration.

RESULTS

Administration of NaB could improve the cognitive impairments induced by Aβ_25-35 in mice. Furthermore, NaB could promote the differentiation of astrocytes towards A2-neuron-protective subtype, astroglial mitochondrial function, and lactate shuttle between astrocytes and neurons.

CONCLUSION

These findings reveal the effect of sodium butyrate on astrocytes, which may improve the pathological status of AD and provide experimental basis for sodium butyrate treatment of AD.

Advanced Glycation End-Product Precursor Methylglyoxal May Lead to Development of Alzheimer's Disease

INTRODUCTION

Diabetes mellitus (DM) is characterized by chronic hyperglycemia and diabetic complications. Exacerbated cortical neuronal degeneration was observed in Alzheimer's disease (AD) patients with DM. In fact, DM is now considered a risk factor of AD, as DM-induced activation of stress responses in the central nervous system (CNS) such as oxidative stress and neuroinflammation may lead to various neurodegenerative disorders. Methylglyoxal (MG) is one of the most reactive advanced glycation end-product (AGE) precursors. Abnormal accumulation of MG is observed in the serum of diabetic patients. As MG is reported to promote brain cells impairment in the CNS, and it is found that AGEs are abnormally increased in the brains of AD patients. Therefore, the effect of MG causing subsequent symptoms of AD was investigated.

METHODS

5-week-old C57BL/6 mice were intraperitoneally injected with MG solution for 11 weeks. The Morris water maze (MWM) was used to examine the spatial learning ability and cognition of mice. After MG treatment, MTT assay, real-time PCR analyses, and Western blot were performed to assess the harvested astrocytes and hippocampi.

RESULTS

Significantly longer escape latency and reduced percentage time spent in the target quadrant were observed in the 9-week-MG-treated mice. We have found in both in vitro and in vivo models that MG induced astrogliosis, pro-inflammatory cytokines, AD-related markers, and ERK activation. Further, trend of normalization of the tested markers mRNA expressions were observed after ERK inhibition.

CONCLUSION

Our in vivo results suggested that MG could induce AD symptoms and in vitro results implied that ERK may regulate the promotion of inflammation and Aβ formation in MG-induced reactive astrocytes. Taken together, MG may participate in the dysfunction of brain cells resulting in possible diabetes-related neurodegeneration by promoting astrogliosis, Aβ production, and neuroinflammation through the ERK pathway. Our findings provide insight of targeting ERK as a therapeutic application for diabetes-induced AD.

The parkinsonian LRRK2 R1441G mutation shows macroautophagy-mitophagy dysregulation concomitant with endoplasmic reticulum stress

Autophagy is a mechanism responsible for the degradation of cellular components to maintain their homeostasis. However, autophagy is commonly altered and compromised in several diseases, including neurodegenerative disorders. Parkinson's disease (PD) can be considered a multifactorial disease because environmental factors, genetic factors, and aging are involved. Several genes are involved in PD pathology, among which the LRRK2 gene and its mutations, inherited in an autosomal dominant manner, are responsible for most genetic PD cases. The R1441G LRRK2 mutation is, after G2019S, the most important in PD pathogenesis. Our results demonstrate a relationship between the R1441G LRRK2 mutation and a mechanistic dysregulation of autophagy that compromises cell viability. This altered autophagy mechanism is associated with organellar stress including mitochondrial (which induces mitophagy) and endoplasmic reticulum (ER) stress, consistent with the fact that patients with this mutation are more vulnerable to toxins related to PD, such as MPP⁺.

Trichostatin A increases BDNF protein expression by improving XBP-1s/ATF6/GRP78 axis in Schwann cells of diabetic peripheral neuropathy

Diabetic peripheral neuropathy (DPN) is the common complication of diabetes mellitus. Histone deacetylase (HDAC) inhibitor trichostatin A (TSA) is reported to ameliorate the peripheral nerves degeneration of DPN. However, the exact mechanism is still not well elucidated. Here, we first revealed that TSA promoted nerve conduction and brain derived neurotrophic factor (BDNF) expression in the sciatic nerves of diabetic mice. In line, TSA also reversed high glucose-reduced mature BDNF expression in vitro cultured rat Schwann cells (RSC96). Then unexpectedly, the downstream targets of TSA HDAC1 and HDAC5 were not involved in TSA-improved BDNF expression. Furthermore, unfolded protein response (UPR) chaperone GRP78 was revealed to be downregulated with high glucose stimulation in RSC96 cells, which was avoided with TSA treatment. Also, GRP78 upregulation mediated TSA-improved mature BDNF expression in high glucose-cultured RSC96 cells by binding with BDNF. As well, TSA treatment enhanced the binding of GRP78 with BDNF in RSC96 cells. Again, UPR-associated transcription factors XBP-1s and ATF6 were involved in TSA-increased GRP78 expression in high glucose-stimulated RSC96 cells. Finally, conditioned medium from high glucose-cultured RSC96 cells delayed neuron SH-SY5Y differentiation and that from TSA-treated high glucose-cultured RSC96 cells promoted SH-SY5Y cell differentiation. Taken together, our findings suggested that TSA increased BDNF expression to ameliorate DPN by improving XBP-1s/ATF6/GRP78 axis in Schwann cells.

Effect of high glucose condition on glucose metabolism in primary astrocytes

In the brain, glucose enters astrocytes through glucose transporter (GLUT1) and either enters glycolysis or the glycogen shunt. Astrocytes meet the energy needs of neurons by building up and breaking down their glycogen supply. High glucose exposure causes astrocyte dysregulation, but its effects on glucose metabolism are relatively unknown. We hypothesized that high glucose conditioning induces a glycogenic state in the astrocyte, resulting in an inefficient mobilization of substrates when challenged with glucose deprivation. Using neonatal rat astrocytes, we used normal glucose (NG, 5.5 mM) vs. high glucose (HG, 25 mM) feeding media and measured cell membrane GLUT1 expression, glucose analog uptake, glycogen content, and cellular bioenergetics. This study demonstrates that HG conditioning causes increased glucose analog uptake (p < 0.05) without affecting GLUT1 membrane expression when compared to NG conditioned astrocytes. Increased glucose uptake in HG astrocytes is associated with higher baseline glycogen content compared to NG exposed astrocytes (p < 0.05). When challenged with glucose deprivation, HG astrocytes break down more than double the amount of glycogen molecules compared to NG astrocytes, although they break down a similar percentage of the starting glycogen stores (NG = 62%, HG = 55%). Additionally, HG conditioning negatively impacts astrocyte maximal respiration and glycolytic reserve capacity assessed by the Seahorse mitochondrial stress test and glycolytic stress test, respectively (p < 0.05). These results suggest that HG conditioning shifts astrocytes towards glycogen storage at baseline. Despite increased glycogen storage, HG astrocytes demonstrate decreased metabolic efficiency and capacity putting them at higher risk during extended periods of glucose deprivation.

Methylglyoxal-induced neuroinflammatory response in in vitro astrocytic cultures and hippocampus of experimental animals

Diabetes mellitus is characterized by chronic hyperglycemia and its diverse complications. Hyperglycemia is associated with inflammatory responses in different organs and diabetic patients have a higher risk of developing neurodegenerative disorders. Methylglyoxal is a reactive advanced glycation end product precursor that accumulates in diabetic patients. It induces various stress responses in the central nervous system and causes neuronal dysfunction. Astrocytes are actively involved in maintaining neuronal homeostasis and possibly play a role in protecting the brain against neurodegeneration. However it is not clear whether methylglyoxal exerts any adverse effects towards these astrocytes. In the present study we investigated the effects of methylglyoxal in astrocytic cultures and hippocampi of experimental animals. The cells from the astrocytic line DITNC1 were treated with methylglyoxal for 1 to 24 h. For the in vivo model, 3 months old C57BL/6 mice were treated with methylglyoxal solution for 6 weeks by intraperitoneal injection. Following the treatment, both astrocytes and hippocampi were harvested for MTT assay, Western blot and real time PCR analyses. We found that methylglyoxal induced astrogliosis in DITNC1 astrocytic cultures and C57BL/6 mice. Further, activation of the pro-inflammatory JNK signaling pathway and its downstream effectors c-Jun were observed. Furthermore, increased gene expression of pro-inflammatory cytokines and astrocytic markers were observed from real time PCR analyses. In addition, inhibition of JNK activities resulted in down-regulation of TNF-α gene expression in methylglyoxal treated astrocytes. Our results suggest that methylglyoxal may contribute to the progression of diabetes related neurodegeneration through JNK pathway activation in astrocytes and the subsequent neuroinflammatory responses in the central nervous system.

Targeting Glial Mitochondrial Function for Protection from Cerebral Ischemia: Relevance, Mechanisms, and the Role of MicroRNAs

Astrocytes and microglia play crucial roles in the response to cerebral ischemia and are effective targets for stroke therapy in animal models. MicroRNAs (miRs) are important posttranscriptional regulators of gene expression that function by inhibiting the translation of select target genes. In astrocytes, miR expression patterns regulate mitochondrial function in response to oxidative stress via targeting of Bcl2 and heat shock protein 70 family members. Mitochondria play an active role in microglial activation, and miRs regulate the microglial neuroinflammatory response. As endogenous miR expression patterns can be altered with exogenous mimics and inhibitors, miR-targeted therapies represent a viable intervention to optimize glial mitochondrial function and improve clinical outcome following cerebral ischemia. In the present article, we review the role that astrocytes and microglia play in neuronal function and fate following ischemic stress, discuss the relevance of mitochondria in the glial response to injury, and present current evidence implicating miRs as critical regulators in the glial mitochondrial response to cerebral ischemia.

Induction of apoptosis by hypertension via endoplasmic reticulum stress

BACKGROUND/AIMS

Endoplasmic reticulum (ER) stress is one of the intrinsic apoptosis pathways, and cardiac apoptosis can occur in cardiovascular diseases, such as hypertension. However, the mechanisms by which ER stress leads to apoptosis remain enigmatic, particularly in the progression from cardiac hypertrophy to diastolic heart failure due to hypertension.

METHODS

We used spontaneously hypertensive rats (SHRs) to investigate possible signalling pathways for ER stress.

RESULTS

We found that cardiac protein and mRNA levels of glucoseregulated protein 78 were up-regulated. In addition, the CHOP- and caspase-12-dependent pathways, but not that of JNK, were activated in the SHR rats.

CONCLUSIONS

These results suggest that ER stress can contribute to myocardial apoptosis during hypertensive disease.

Hyperglycemia-Induced Oxidative-Nitrosative Stress Induces Inflammation and Neurodegeneration via Augmented Tuberous Sclerosis Complex-2 (TSC-2) Activation in Neuronal Cells

Diabetes is a systemic disease mainly characterized by chronic hyperglycemia and with extensive and long-lasting spiteful complications in central nervous systems (CNS). Astrocytes play an important role in the defense mechanism of CNS, with great ability of withstanding accumulation of toxic substances. Apart from functional disorders, hyperglycemia leads to slow progressive structural abnormalities in the CNS through oxidative stress pathways. However, the molecular mechanism by which neurons die under oxidative stress induced by high glucose (HG) remains largely unclear. Here, we report that HG-induced inflammation and neurodegeneration in brain tissues, brain astrocytes (C6), and pheochromocytoma (PC-12) cells are cultured in HG conditions. Our results show that the increases in phosphorylation of Akt and ERK1/2MAPK are associated with increased accumulations of reactive oxygen species (ROS) in neuronal cells, which simultaneously enhanced phosphorylations of tuberous sclerosis complex-2 (TSC-2) and mammalian target of rapamycin (mTOR) in the diabetic brain and in HG-exposed neuronal cells. Pharmacologic inhibition of Akt or ERK1/2 or siRNA-mediated gene silencing of TSC-2 suppressed the strong downregulation of TSC-2-mTOR activation. Findings of this study also demonstrate that HG resulted in phosphorylation of NF-κB, coinciding with the increased production of inflammatory mediators and activation of neurodegenerative markers. Pretreatment of cells with antioxidants, phosphoinositide3-kinase (PI3-K)/Akt, and ERK1/2 inhibitors significantly reduced HG-induced TSC-2 phosphorylation and restored NF-κB protein expression leading to decreased production of inflammatory mediators and neurodegenerative markers. These results illustrate that ROS functions as a key signaling component in the regulatory pathway induced by elevated glucose in neuronal cell activation leading to inflammation and neurodegeneration.

Calcium homeostasis and ER stress in control of autophagy in cancer cells

Autophagy is a basic catabolic process, serving as an internal engine during responses to various cellular stresses. As regards cancer, autophagy may play a tumor suppressive role by preserving cellular integrity during tumor development and by possible contribution to cell death. However, autophagy may also exert oncogenic effects by promoting tumor cell survival and preventing cell death, for example, upon anticancer treatment. The major factors influencing autophagy are Ca²⁺ homeostasis perturbation and starvation. Several Ca²⁺ channels like voltage-gated T- and L-type channels, IP3 receptors, or CRAC are involved in autophagy regulation. Glucose transporters, mainly from GLUT family, which are often upregulated in cancer, are also prominent targets for autophagy induction. Signals from both Ca²⁺ perturbations and glucose transport blockage might be integrated at UPR and ER stress activation. Molecular pathways such as IRE 1-JNK-Bcl-2, PERK-eIF2α-ATF4, or ATF6-XBP 1-ATG are related to autophagy induced through ER stress. Moreover ER molecular chaperones such as GRP78/BiP and transcription factors like CHOP participate in regulation of ER stress-mediated autophagy. Autophagy modulation might be promising in anticancer therapies; however, it is a context-dependent matter whether inhibition or activation of autophagy leads to tumor cell death.