Endoplasmic reticulum calcium release is modulated by actin polymerization

Intracellular calcium ions regulate the structure and functions of cytoskeletal proteins. On the other hand, recent studies have shown that the cytoskeleton, and actin filaments in particular, can modulate calcium influx through plasma membrane ligand- and voltage-gated channels. We now report that calcium release from inositol trisphosphate (IP3) and ryanodine-sensitive endoplasmic reticulum (ER) stores is modulated by polymerization and depolymerization of actin filaments in cultured hippocampal neurons. Depolymerization of actin filaments with cytochalasin D attenuates calcium release induced by carbamylcholine (CCh; a muscarinic agonist for IP3 pathway), caffeine (a ryanodine receptor agonist) and thapsigargin (an inhibitor of the ER calcium- ATPase) in both the presence and absence of extracellular calcium. Conversely, the actin polymerizing agent jasplakinolide potentiates calcium release induced by CCh, caffeine and thapsigargin. Cytochalasin D attenuated, while jasplakinolide augmented, thapsigargin-induced JNK activation and neuronal cell death. Our data show that the actin cytoskeleton regulates ER calcium release, suggesting roles for actin in the various physiological and pathological processes that involve calcium release.     

内质网应激

内质网应激表现为内质网腔内错误折叠与未折叠蛋白聚集以及Ca^2 平衡紊乱,可激活未折叠蛋白反应、内质网超负荷反应和caspase-12介导的凋亡通路等信号途径,既能诱导糖调节蛋白(glucose-regulated protein 78kD,GRP78)、GRP94等内质网分子伴侣表达而产生保护效应,亦能独立地诱导细胞凋亡。内质网应激直接影响应激细胞的转归,如适应、损伤或凋亡。     

鼻病毒非结构蛋白2B 诱导细胞发生内质网应激

为探讨鼻病毒非结构蛋白2B诱导内质网应激和细胞凋亡的机制,本研究构建了鼻病毒非结构蛋白2B的真核表达载体p2B‐GFP ,通过转染BHK‐21细胞检测相关标志蛋白的变化情况。结果显示,非结构蛋白2B定位表达于BHK‐21细胞内质网,诱导内质网应激标志蛋白Grp78、CHOP的表达增加,并使活化转录因子6（ATF6）的转录活性增加,还诱导BHK‐21细胞发生核浓缩而凋亡,使凋亡标志蛋白PARP发生降解而减少。结果提示,鼻病毒非结构蛋白2B可诱导细胞发生内质网应激,并经该途径诱导细胞凋亡。     

Endoplasmic reticulum stress and apoptosis

Cell death is an essential event in normal life and development, as well as in the pathophysiological processes that lead to disease. It has become clear that each of the main cellular organelles can participate in cell death signalling pathways, and recent advances have highlighted the importance of the endoplasmic reticulum (ER) in cell death processes. In cells, the ER functions as the organelle where proteins mature, and as such, is very responsive to extracellular-intracellular changes of environment. This short overview focuses on the known pathways of programmed cell death triggering from or involving the ER.     

Endoplasmic reticulum stress response is involved in the pathogenesis of stress induced gastric lesions in rats

Stress gastric ulcer is a serious complication, but the mechanism involved is not fully clarified. It is well known that mucosal cell apoptosis plays a crucial role in the pathogenesis of gastric ulceration. Recent studies have shown that endoplasmic reticulum (ER) stress is an important pathway leading to cellular apoptosis. To investigate the role of ER stress in the pathogenesis of stress gastric ulcer, we studied the alteration in the expression of ER stress markers GRP78 (glucose-regulated protein 78) and caspase-12 (an ER stress-specific proapoptotic molecule) and their relations with gastric mucosal apoptosis during development of stress gastric lesions in the water-immersion and restraint stress (WRS) model in rats. Rats developed severe gastric lesions after 6 h of WRS. Typical apoptosis was observed at the edge cells of WRS induced gastric lesions. Western blot analysis showed that GRP78 and activated caspase-12 were over-expressed in the gastric tissues of WRS rats. Immunohistochemical analysis demonstrated that increased GRP78 and caspase-12 were distributed only under the lesions. In addition, dithiothreitol and tunicamycin (ER stress inducers), which increased the expression of GRP78 and activated caspase-12, caused gastric mucosal injury and mucosal cell apoptosis in vitro. These findings suggest that ER stress might be involved in the development of stress gastric ulcer through an apoptotic mechanism.     

Endoplasmic reticulum stress and kidney dysfunction

Chronic kidney disease (CKD) affects millions of persons worldwide and constitutes a major public health problem. Therefore, understanding the molecular basis of CKD is a key challenge for the development of preventive and therapeutic strategies. A major contributor to chronic histological damage associated with CKD is acute kidney injury (AKI). At the cellular level, kidney injuries are associated with microenvironmental alterations, forcing cells to activate adaptive biological processes that eliminate the stressor and generate alarm signals. These signalling pathways actively participate in tissue remodelling by promoting inflammation and fibrogenesis, ultimately leading to CKD. Many stresses that are encountered upon kidney injury are prone to trigger endoplasmic reticulum (ER) stress. In the kidney, ER stress both participates in acute and chronic histological damages, but also promotes cellular adaptation and nephroprotection. In this review, we will discuss the implication of ER stress in the pathophysiology of AKI and CKD progression, and we will give a critical analysis of the current experimental and clinical evidence that support ER stress as a mediator of kidney damage.     

Endoplasmic reticulum stress contributes to the cell death induced by UCH-L1 inhibitor

At the neuropathological level, Parkinson's disease (PD) is characterized by the accumulation of misfolded proteins, which can trigger the unfolded protein response (UPR). UCH-L1 is a component of ubiquitin proteasome system (UPS). It is reported that the loss of its function will impair ubiquitin proteasome system and cause toxicity to cells. But its mechanism has not been illustrated. In this study, we detected the protein expression of Bip/Grp78 and the spliced form of XBP-1 to examine the activation of unfolded protein response after SK-N-SH cells being treated with LDN-57444, a UCH-L1 inhibitor which could inhibit UCH-L1 hydrolase activity. Our data showed that UCH-L1 inhibitor was able to cause cell death through the apoptosis pathway by decreasing the activity of ubiquitin proteasome system and increasing the levels of highly ubiquitinated proteins, both of which can activate unfolded protein response. There is a lot of evidence that unfolded protein response is activated as a protective response at the early stage of the stress; this protective response can switch to a pro-apoptotic response when the stress persists. In this study, we demonstrated this switch by detecting the upregulation of CHOP/Gadd153. Taken together, our data indicated that the apoptosis induced by UCH-L1 inhibitor may be triggered by the activation of endoplasmic reticulum stress (ERS). Moreover, we provide a new cell model for studying the roles of UCH-L1 in Parkinson's disease.     

内质网应激与糖尿病动脉粥样硬化

动脉粥样硬化是糖尿病常见的并发症,80%的糖尿病患者死于动脉粥样硬化。近年来内质网应激在糖尿病动脉粥样硬化发生、发展过程中的作用受到了广泛关注。本文就内质网应激及其在糖尿病促发动脉粥样硬化中的作用机制作一概述。     

Endoplasmic reticulum stress induced by tunicamycin and thapsigargin protects against transient ischemic brain injury

Transient cerebral ischemia leads to endoplasmic reticulum (ER) stress. However, the contributions of ER stress to cerebral ischemia are not clear. To address this issue, the ER stress activators tunicamycin (TM) and thapsigargin (TG) were administered to transient middle cerebral artery occluded (tMCAO) mice and oxygen-glucose deprivation-reperfusion (OGD-Rep.)-treated neurons. Both TM and TG showed significant protection against ischemia-induced brain injury, as revealed by reduced brain infarct volume and increased glucose uptake rate in ischemic tissue. In OGD-Rep.-treated neurons, 4-PBA, the ER stress releasing mechanism, counteracted the neuronal protection of TM and TG, which also supports a protective role of ER stress in transient brain ischemia. Knocking down the ER stress sensor Eif2s1, which is further activated by TM and TG, reduced the OGD-Rep.-induced neuronal cell death. In addition, both TM and TG prevented PARK2 loss, promoted its recruitment to mitochondria, and activated mitophagy during reperfusion after ischemia. The neuroprotection of TM and TG was reversed by autophagy inhibition (3-methyladenine and Atg7 knockdown) as well as Park2 silencing. The neuroprotection was also diminished in Park2^+/? mice. Moreover, Eif2s1 and downstream Atf4 silencing reduced PARK2 expression, impaired mitophagy induction, and counteracted the neuroprotection. Taken together, the present investigation demonstrates that the ER stress induced by TM and TG protects against the transient ischemic brain injury. The PARK2-mediated mitophagy may be underlying the protection of ER stress. These findings may provide a new strategy to rescue ischemic brains by inducing mitophagy through ER stress activation.     

Endoplasmic reticulum stress triggers autophagy

Eukaryotic cells have evolved strategies to respond to stress conditions. For example, autophagy in yeast is primarily a response to the stress of nutrient limitation. Autophagy is a catabolic process for the degradation and recycling of cytosolic, long lived, or aggregated proteins and excess or defective organelles. In this study, we demonstrate a new pathway for the induction of autophagy. In the endoplasmic reticulum (ER), accumulation of misfolded proteins causes stress and activates the unfolded protein response to induce the expression of chaperones and proteins involved in the recovery process. ER stress stimulated the assembly of the pre-autophagosomal structure. In addition, autophagosome formation and transport to the vacuole were stimulated in an Atg protein-dependent manner. Finally, Atg1 kinase activity reflects both the nutritional status and autophagic state of the cell; starvation-induced autophagy results in increased Atg1 kinase activity. We found that Atg1 had high kinase activity during ER stress-induced autophagy. Together, these results indicate that ER stress can induce an autophagic response.     

Endoplasmic reticulum stress activates telomerase

Telomerase contributes to cell proliferation and survival through both telomere‐dependent and telomere‐independent mechanisms. In this report, we discovered that endoplasmic reticulum (ER) stress transiently activates the catalytic components of telomerase (TERT) expression in human cancer cell lines and murine primary neural cells. Importantly, we show that depletion of hTERT sensitizes cells to undergo apoptosis under ER stress, whereas increased hTERT expression reduces ER stress‐induced cell death independent of catalytically active enzyme or DNA damage signaling. Our findings establish a functional link between ER stress and telomerase, both of which have important implications in the pathologies associated with aging and cancer.     

内质网应激与心肌肥大

肌浆网是调控心肌细胞钙稳态、蛋白质合成和细胞凋亡的重要亚细胞器。内质网应激是指内质网理化环境改变和过负荷等因素导致未折叠/误折叠蛋白在内质网聚集和钙稳态失衡等内质网功能紊乱状态。适度的内质网应激有利于心肌细胞代偿,持续而严重的内质网应激则触发内质网应激相关细胞凋亡,造成肥大心肌由代偿转向衰竭,是影响心肌肥大发生、发展的重要因素。本文综述了内质网应激反应在心肌肥大发生、发展中的作用。     

Endoplasmic reticulum stress and fungal pathogenesis

The gateway to the secretory pathway is the endoplasmic reticulum (ER), an organelle that is responsible for the accurate folding, post-translational modification and final assembly of up to a third of the cellular proteome. When secretion levels are high, errors in protein biogenesis can lead to the accumulation of abnormally folded proteins, which threaten ER homeostasis. The unfolded protein response (UPR) is an adaptive signaling pathway that counters a buildup in misfolded and unfolded proteins by increasing the expression of genes that support ER protein folding capacity. Fungi, like other eukaryotic cells that are specialized for secretion, rely upon the UPR to buffer ER stress caused by fluctuations in secretory demand. However, emerging evidence is also implicating the UPR as a central regulator of fungal pathogenesis. In this review, we discuss how diverse fungal pathogens have adapted ER stress response pathways to support the expression of virulence-related traits that are necessary in the host environment.     

Endoplasmic reticulum stress is implicated in retinal inflammation and diabetic retinopathy

Diabetic retinopathy is a chronic low-grade inflammatory disease; however, the mechanisms remain elusive. In the present study, we demonstrated that endoplasmic reticulum (ER) stress was activated in the retina in animal models of diabetes and oxygen-induced retinopathy (OIR). Induction of ER stress by tunicamycin resulted in significantly increased expression of inflammatory molecules in the retina. Inhibition of ER stress by chemical chaperone 4-phenyl butyric acid ameliorated inflammation in cultured human retinal endothelial cells exposed to hypoxia, and in the retinas of diabetic and OIR mice. These findings indicate that ER stress is a potential mediator of retinal inflammation in diabetic retinopathy.     

The oligomeric state of Derlin-1 is modulated by endoplasmic reticulum stress

The endoplasmic reticulum (ER) is a major site of protein synthesis in eukaryotes. Newly synthesized proteins are monitored by a process of quality control, which removes misfolded or unassembled polypeptides from the ER for degradation by the proteasome. This requires the retrotranslocation of the misfolded proteins from the ER lumen into the cytosol via a pathway that, for some substrates, involves members of the recently discovered Derlin family. The Derlin-1 isoform is present as a dimer in the ER, and we now show that its dimerization is modulated by ER stress. Three distinct types of chemically-induced ER stress substantially reduce the levels of Derlin-1 dimer as assayed by both cross-linking and co-immunoprecipitation. The potential function of the different Derlin-1 populations with respect to ER quality control is investigated by analysing their capacity to associate with a misfolded membrane protein fragment. We show for the first time that Derlin-1 can associate with an aberrant membrane protein fragment in the absence of the viral component US11, and conclude that it is the monomeric form of Derlin-1 that interacts with this potential ER-associated degradation substrate. On the basis of these data we propose a model where the pool of active Derlin-1 in the ER membrane can be modulated in response to ER stress.     

Endoplasmic reticulum stress, obesity and diabetes

The endoplasmic reticulum (ER) stress response, also commonly known as the unfolded protein response (UPR), is an adaptive response used to align ER functional capacity with demand. It is activated in various tissues under conditions related to obesity and type 2 diabetes. Hypothalamic ER stress contributes to inflammation and leptin/insulin resistance. Hepatic ER stress contributes to the development of steatosis and insulin resistance, and components of the UPR regulate liver lipid metabolism. ER stress in enlarged fat tissues induces inflammation and modifies adipokine secretion, and saturated fats cause ER stress in muscle. Finally, prolonged ER stress impairs insulin synthesis and causes pancreatic β cell apoptosis. In this review, we discuss ways in which ER stress operates as a common molecular pathway in the pathogenesis of obesity and diabetes.