HSP105 interacts with GRP78 and GSK3 and promotes ER stress-induced caspase-3 activation

Stress of the endoplasmic reticulum (ER stress) is caused by the accumulation of misfolded proteins, which occurs in many neurodegenerative diseases. ER stress can lead to adaptive responses or apoptosis, both of which follow activation of the unfolded protein response (UPR). Heat shock proteins (HSP) support the folding and function of many proteins, and are important components of the ER stress response, but little is known about the role of one of the major large HSPs, HSP105. We identified several new partners of HSP105, including glycogen synthase kinase-3 (GSK3), a promoter of ER stress-induced apoptosis, and GRP78, a key component of the UPR. Knockdown of HSP105 did not alter UPR signaling after ER stress, but blocked caspase-3 activation after ER stress. In contrast, caspase-3 activation induced by genotoxic stress was unaffected by knockdown of HSP105, suggesting ER stress-specificity in the apoptotic action of HSP105. However, knockdown of HSP105 did not alter cell survival after ER stress, but instead diverted signaling to a caspase-3-independent cell death pathway, indicating that HSP105 is necessary for apoptotic signaling after UPR activation by ER stress. Thus, HSP105 appears to chaperone the responses to ER stress through its interactions with GRP78 and GSK3, and without HSP105 cell death following ER stress proceeds by a non-caspase-3-dependent process.     

GRP94 in ER quality control and stress responses

A system of endoplasmic reticulum (ER) chaperones has evolved to optimize the output of properly folded secretory and membrane proteins. An important player in this network is Glucose Regulated Protein 94 (GRP94). Over the last decade, new structural and functional data have begun to delineate the unique characteristics of GRP94 and have solidified its importance in ER quality control pathways. This review describes our current understanding of GRP94 and the four ways in which it contributes to the ER quality control: (1) chaperoning the folding of proteins; (2) interacting with other components of the ER protein folding machinery; (3) storing calcium; and (4) assisting in the targeting of malfolded proteins to ER-associated degradation (ERAD).     

Decreased protein and mRNA expression of ER stress proteins GRP78 and GRP94 in HepG2 cells over-expressing CYP2E1

CYP2E1 causes oxidative stress mediated cell death; the latter is one mechanism for endoplasmic reticulum (ER) stress in the cell. Unfolded proteins accumulate during ER stress and ER resident proteins GRP78 and GRP94 protect cells against ER dysfunction. We examined the possible role of GRP78 and GRP94 as protective factors against CYP2E1-mediated toxicity in HepG2 cells expressing CYP2E1 (E47 cells). E47 cells expressed high levels of CYP2E1 protein and catalytic activity which is associated with increased ROS generation, lipid peroxidation and the elevated presence of ubiquinated and aggregated proteins as compared to control HepG2 C34 cells which do not express CYP2E1. The mRNA and protein expression of GRP78 and GRP94 were decreased in E47 cells compared to the C34 cells, which may explain the accumulation of ubiquinated and aggregated proteins. Expression of these GRP proteins was induced with the ER stress agent thapsigargin in E47 cells, and E47 cells were more resistant to the toxicity caused by thapsigargin and calcimycin, possibly due to this upregulation and also because of the high expression of GSH and antioxidant enzymes in E47 cells. Antioxidants such as trolox and N-acetylcysteine increased GRP78 and GRP94 levels in the E47 cells, suggesting that CYP2E1- derived oxidant stress was responsible for down regulation of these GRPs in the E47 cells. Thapsigargin mediated toxicity was decreased in cells treated with the antioxidant trolox indicating a role for oxidative stress in this toxicity. These results suggest that CYP2E1 mediated oxidative stress downregulates the expression of GRP proteins in HepG2 cells and oxidative stress is an important mechanism in causing ER dysfunction in these cells.     

GRP94, an ER chaperone with protein and peptide binding properties

GRP94 is the ER representative of the HSP90 family of stress-induced proteins. It binds to a limited number of proteins in the secretory pathway, apparently by recognizing advanced folding intermediates or incompletely assembled proteins, GRP94 also binds peptides and can act as a tumor vaccine, delivering the peptides for presentation to T lymphocytes. Here, we review the current data about GRP94 and propose a structural model that integrates the biochemical data and known functions of the protein.     

The molecular chaperone gp96/GRP94 interacts with Toll-like receptors and integrins via its C-terminal hydrophobic domain

The structural basis for molecular chaperones to discern misfolded proteins has long been an enigma. As the endoplasmic reticulum paralogue of the cytosolic HSP90, gp96 (GRP94, HSP90b1) is an essential molecular chaperone for Toll-like receptors (TLRs) and integrins. However, little is known about its client-binding domain (CBD). Herein, we provide genetic and biochemical evidence to definitively demonstrate that a C-terminal loop structure, formed by residues 652-678, is the critical region of CBD for both TLRs and integrins. Deletion of this region affects neither the intrinsic ATPase activity nor the overall conformation of gp96. However, without it, the chaperoning function of gp96 collapses. We also find a critical Met pair (Met(658)-Met(662)) for the folding of integrins but not TLRs. Moreover, we find that the TLR binding to gp96 is also dependent on the C-terminal dimerization domain but not the N-terminal ATP-binding pocket of gp96. Our study has unveiled surprisingly the exquisite specificity of gp96 in substrate binding and suggests a manipulation of its CBD as an alternative strategy for targeted therapy of a variety of diseases.     

汉滩病毒核蛋白与热休克蛋白GRP94、HSP27的相互作用

为研究汉滩病毒(Hantaan virus, HTNV)感染诱导乳鼠脑组织热休克蛋白GRP94、HSP27与病毒蛋白的相互关系,选出生2～3d的昆明乳鼠实验性感染汉滩病毒,取8d后发病乳鼠脑组织部分制石蜡切片,用免疫组化结合共聚焦显微镜检测组织中病毒抗原及GRP94、HSP27的表达,部分制匀浆液,用ELISA、免疫共沉淀方法分析病毒抗原和GRP94、HSP27的关系.结果示汉滩病毒感染诱导乳鼠脑组织神经细胞高表达GRP94且与细胞内病毒抗原有共定位关系,但未见HSP27诱导高表达;免疫共沉淀显示汉滩病毒核心抗原(HINV-NP)与GRP94、HSP27呈非共价复合物形式存在.该结果为进一步探讨HSPs在病毒感染复制中的作用以及抗病毒感染方面提供了有意义的实验资料.     

汉滩病毒核蛋白与热休克蛋白GRP94、HSP27的相互作用

为研究汉滩病毒(Hantaan virus,HTNV)感染诱导乳鼠脑组织热休克蛋白GRP94、HSP27与病毒蛋白的相互关系,选出生2—3d的昆明乳鼠实验性感染汉滩病毒,取8d后发病乳鼠脑组织部分制石蜡切片,用免疫组化结合共聚焦显微镜检测组织中病毒抗原及GRP94、HSP27的表达,部分制匀浆液,用ELISA、免疫共沉淀方法分析病毒抗原和GRP94、HSP27的关系。结果示汉滩病毒感染诱导乳鼠脑组织神经细胞高表达GRP94且与细胞内病毒抗原有共定位关系,但未见HSP27诱导高表达;免疫共沉淀显示汉滩病毒核心抗原(HINV—NP)与GRP94、HSP27呈非共价复合物形式存在。该结果为进一步探讨HSPs在病毒感染复制中的作用以及抗病毒感染方面提供了有意义的实验资料。     

How cholesterol interacts with proteins and lipids during its intracellular transport

Sterols, as cholesterol in mammalian cells and ergosterol in fungi, are indispensable molecules for proper functioning and nanoscale organization of the plasma membrane. Synthesis, uptake and efflux of cholesterol are regulated by a variety of protein–lipid and protein–protein interactions. Similarly, membrane lipids and their physico-chemical properties directly affect cholesterol partitioning and thereby contribute to the highly heterogeneous intracellular cholesterol distribution. Movement of cholesterol in cells is mediated by vesicle trafficking along the endocytic and secretory pathways as well as by non-vesicular sterol exchange between organelles. In this article, we will review recent progress in elucidating sterol–lipid and sterol–protein interactions contributing to proper sterol transport in living cells. We outline recent biophysical models of cholesterol distribution and dynamics in membranes and explain how such models are related to sterol flux between organelles. An overview of various sterol-transfer proteins is given, and the physico-chemical principles of their function in non-vesicular sterol transport are explained. We also discuss selected experimental approaches for characterization of sterol–protein interactions and for monitoring intracellular sterol transport. Finally, we review recent work on the molecular mechanisms underlying lipoprotein-mediated cholesterol import into mammalian cells and describe the process of cellular cholesterol efflux. Overall, we emphasize how specific protein–lipid and protein–protein interactions help overcoming the extremely low water solubility of cholesterol, thereby controlling intracellular cholesterol movement. This article is part of a Special Issue entitled: Lipid–protein interactions.     

C3d fragment of complement interacts with laminin and binds to basement membranes of glomerulus and trophoblast

Two mouse monoclonal antibodies generated against human placental homogenate were found to react specifically with human complement component C3. In immunofluorescence of human tissues, these antibodies gave a bright linear staining outlining the glomerular basement membrane of the adult kidney and the trophoblast basement membrane of placenta. An identical staining pattern was observed with a rabbit C3d antiserum which also prevented binding of the monoclonal antibodies to tissue sections. Only negligible basement membrane staining was observed in the same tissues with antisera to human C3c, C5, IgG, IgA, or IgM. When interactions of C3 with basement membrane proteins were tested in enzyme immunoassays and column chromatography, C3(H2O) was found to bind efficiently to solid-phase laminin. Native C3 from fresh plasma did not bind to laminin but C3 from plasma treated with methylamine bound efficiently. When C3 was cleaved with trypsin, C3b and C3d but not C3c bound to laminin-Sepharose. The interaction of C3 and laminin was inhibited by soluble laminin and by high ionic strength. The results indicate that C3d, a biologically active breakdown product of C3, can be found in glomerular and placental basement membranes in the absence of signs for ongoing local complement activation or immune complex deposition. It is possible that binding affinities between C3 and basement membrane molecules, especially laminin, are involved in the retention of C3d at these sites. Such interactions between C3 and components of the glomerular basement membrane could play important roles in complement-related pathological processes of the glomerulus.     

黄芪注射液对网腔钙结合蛋白基因沉默阿霉素损伤心肌细胞内质网应激伴侣蛋白GRP78,GRP94 mRNA的作用

目的：研究黄芪注射液对网腔钙结合蛋白（calumenin）基因沉默阿霉素损伤心肌细胞内质网应激伴侣蛋白GRP78,GRP94 mRNA的作用。方法：实验将体外培养的1~3 d乳鼠心肌细胞分为5组：对照组、模型组（正常细胞+3 mg/L阿霉素）、calumenin基因沉默模型组（慢病毒感染细胞+3 mg/L阿霉素）、黄芪组1（正常细胞+3 mg/L阿霉素+200 g/L黄芪）、黄芪组2（慢病毒感染细胞+3 mg/L阿霉素+200 g/L黄芪）。构建慢病毒-calumenin质粒,转染乳鼠培养心肌细胞,采用实时荧光定量分析（real-time PCR）检测各组心肌细胞calumenin及内质网应激伴侣蛋白GRP78、GRP94 mRNA表达。结果：①与对照组比较,模型组心肌细胞calumenin mRNA表达减少（P<0.05）,而calumenin基因沉默模型组及黄芪组2心肌细胞calumenin mRNA表达明显减少（P<0.01）;与模型组比较,黄芪组1心肌细胞calumenin mRNA表达增加（P<0.05）;与calumenin基因沉默模型组比较,黄芪组2心肌细胞calumenin mRNA表达明显增加（P<0.01）。②与对照组相比较,模型组及calumenin基因沉默模型组心肌细胞内质网应激伴侣蛋白GRP78、GRP94 mRNA表达明显增多（P<0.01）;与模型组比较,黄芪组1心肌细胞GRP78、GRP94 mRNA表达明显减少（P<0.01）;与calumenin基因沉默模型组比较,黄芪组2心肌细胞内质网应激伴侣蛋白GRP78、GRP94 mRNA表达明显减少（P<0.01）。结论：①阿霉素损伤可引起心肌细胞calumenin表达减少。②Calumenin可缓解阿霉素损伤所诱导心肌细胞内质网应激。黄芪注射液可抑制阿霉素损伤所诱导心肌细胞内质网应激,这种作用可能系通过calumenin介导实现的。     

The endoplasmic reticulum stress protein GRP94, in addition to BiP, associates with unassembled immunoglobulin chains.

The molecular chaperone BiP/GRP78 associates with various polypeptides in the endoplasmic reticulum, including immunoglobulin chains. We now show, using chemical cross-linking, that another endoplasmic reticulum stress protein, GRP94, associates with newly synthesized immunoglobulin light and heavy chains. We demonstrate the presence of ternary complexes composed of immunoglobulin chains, BiP and GRP94. Because both BiP and GRP94 associate far less with fully assembled immunoglobulin than with unassembled subunits, our data suggest that GRP94, like BiP, functions as a molecular chaperone. The presence of both BiP and GRP94 in the same complex further suggests that the two stress proteins work in concert during the folding and assembly of immunoglobulins.     

The AKT3 potassium channel protein interacts with the AtPP2CA protein phosphatase 2C

The AKT3 potassium channel protein was identified as a strongly interacting partner of the Arabidopsis thaliana protein phosphatase 2C (AtPP2CA) in a yeast two-hybrid screen. A deletion analysis indicated that the catalytic domain of AtPP2CA was essential for the interaction with AKT3. Furthermore, the related PP2C phosphatase ABI1 did not interact with AKT3 in yeast.     

Endoplasmic reticulum stress controls M2 macrophage differentiation and foam cell formation

Macrophages are essential in atherosclerosis progression, but regulation of the M1 versus M2 phenotype and their role in cholesterol deposition are unclear. We demonstrate that endoplasmic reticulum (ER) stress is a key regulator of macrophage differentiation and cholesterol deposition. Macrophages from diabetic patients were classically or alternatively stimulated and then exposed to oxidized LDL. Alternative stimulation into M2 macrophages lead to increased foam cell formation by inducing scavenger receptor CD36 and SR-A1 expression. ER stress induced by alternative stimulation was necessary to generate the M2 phenotype through JNK activation and increased PPARγ expression. The absence of CD36 or SR-A1 signaling independently of modified cholesterol uptake decreased ER stress and prevented the M2 differentiation typically induced by alternative stimulation. Moreover, suppression of ER stress shifted differentiated M2 macrophages toward an M1 phenotype and subsequently suppressed foam cell formation by increasing HDL- and apoA-1-induced cholesterol efflux indicating suppression of macrophage ER stress as a potential therapy for atherosclerosis.     

Lipid peroxidation,antioxidant activities and stress protein (HSP72/73, GRP94) expression in kidney and liver of rats under lithium treatment

The present work was aimed at studying the effects of a subchronic lithium treatment on rat liver and kidneys, paying attention to the relationship between lithium toxicity, oxidative stress, and stress protein expression. Male rats were submitted to lithium treatment by adding 2 g of lithium carbonate/kg of food for different durations up to 1 month. This treatment led to serum concentrations ranging from 0.5 mM (day 7) to 1.34 mM (day 28) and renal insufficiency highlighted by an increase of blood creatinine and urea levels and a decrease of urea excretion. Lithium treatment was found to trigger an oxidative stress both in kidney and liver, leading to an increase of lipid peroxidation level (TBARS) and of superoxide dismutase and catalase activities. Conversely, glutathione peroxidase activity was reduced. Constitutive HSP73 (heat shock protein 73) expression was not modified by lithium treatment, whereas inducible HSP72 was down-regulated in kidney. GRP94 (glucose regulated protein 94) appeared as two isoforms of 92 and 98 kDa: the 98-kDa protein being overexpressed in kidney by lithium treatment whereas 92-kDa protein was underexpressed both in kidney and liver.     

M2 macrophage polarisation is associated with alveolar formation during postnatal lung development

Background

Macrophages are traditionally associated with inflammation and host defence, however a greater understanding of macrophage heterogeneity is revealing their essential roles in non-immune functions such as development, homeostasis and regeneration. In organs including the brain, kidney, mammary gland and pancreas, macrophages reside in large numbers and provide essential regulatory functions that shape organ development and maturation. However, the role of macrophages in lung development and the potential implications of macrophage modulation in the promotion of lung maturation have not yet been ascertained.

Methods

Embryonic day (E)12.5 mouse lungs were cultured as explants and macrophages associated with branching morphogenesis were visualised by wholemount immunofluorescence microscopy. Postnatal lung development and the correlation with macrophage number and phenotype were examined using Colony-stimulating factor-1 receptor-enhanced green fluorescent protein (Csf1r-EGFP) reporter mice. Structural histological examination was complemented with whole-body plethysmography assessment of postnatal lung functional maturation over time.Flow cytometry, real-time (q)PCR and immunofluorescence microscopy were performed to characterise macrophage number, phenotype and localisation in the lung during postnatal development. To assess the impact of developmental macrophage modulation, CSF-1 was administered to neonatal mice at postnatal day (P)1, 2 and 3, and lung macrophage number and phenotype were assessed at P5. EGFP transgene expression and in situ hybridisation was performed to assess CSF-1R location in the developing lung.

Results

Macrophages in embryonic lungs were abundant and densely located within branch points during branching morphogenesis. During postnatal development, structural and functional maturation of the lung was associated with an increase in lung macrophage number. In particular, the period of alveolarisation from P14-21 was associated with increased number of Csf1r-EGFP+ macrophages and upregulated expression of Arginase 1 (Arg1), Mannose receptor 1 (Mrc1) and Chemokine C-C motif ligand 17 (Ccl17), indicative of an M2 or tissue remodelling macrophage phenotype. Administration of CSF-1 to neonatal mice increased trophic macrophages during development and was associated with increased expression of the M2-associated gene Found in inflammatory zone (Fizz)1 and the growth regulator Insulin-like growth factor (Igf)1. The effects of CSF-1 were identified as macrophage-mediated, as the CSF-1R was found to be exclusively expressed on interstitial myeloid cells.

Conclusions

This study identifies the presence of CSF-1R+ M2-polarised macrophages localising to sites of branching morphogenesis and increasing in number during the alveolarisation stage of normal lung development. Improved understanding of the role of macrophages in lung developmental regulation has clinical relevance for addressing neonatal inflammatory perturbation of development and highlights macrophage modulation as a potential intervention to promote lung development.