Sequential interaction of the chaperones BiP and GRP94 with immunoglobulin chains in the endoplasmic reticulum

Proteomic screening of glucose-responsive and glucose non-responsive MIN-6 beta cells reveals differential expression of proteins involved in protein folding, secretion and oxidative stress

The glucose-sensitive insulin-secretion (GSIS) phenotype is relatively unstable in long-term culture of beta cells. The purpose of this study was to investigate relative changes in the proteome between glucose-responsive (low passage) and glucose non-responsive (high passage) murine MIN-6 pancreatic beta cells. The 2D-DIGE and subsequent DeCyder analysis detected 3351 protein spots in the pH range of 4-7. Comparing MIN-6(H) to MIN-6(L) and using a threshold of 1.2-fold, the number of proteins with a decrease in expression level was 152 (4.5%), similar was 3140 (93.7%) and increased 59 (1.8%). From the differentially expressed proteins identified in this study, groups of proteins associated with the endoplasmic reticulum (ER) and proteins involved in oxidative stress were found to be significantly decreased in the high-passage (H passage) cells. These proteins included endoplasmic reticulum protein 29 (ERp29); 78-kDa glucose-related protein, (GRP78); 94-kDa glucose-related protein (GRP94); protein disulphide isomerase; carbonyl reductase 3; peroxidoxin 4 and superoxide dismutase 1. These results suggest that non-GSIS MIN-6 cells do not have the same ability/capacity of glucose-responsive MIN-6 cells to successfully fold, modify or secrete proteins and counteract the problems associated with oxidative stress.

Heat shock protein gp96 is a master chaperone for toll-like receptors and is important in the innate function of macrophages

gp96 is an endoplasmic reticulum chaperone for cell-surface Toll-like receptors (TLRs). Little is known about its roles in chaperoning other TLRs or in the biology of macrophage in vivo. We generated a macrophage-specific gp96-deficient mouse. Despite normal development and activation by interferon-gamma, tumor necrosis factor-alpha, and interleukin-1beta, the mutant macrophages failed to respond to ligands of both cell-surface and intracellular TLRs including TLR2, TLR4, TLR5, TLR7, and TLR9. Furthermore, we found that TLR4 and TLR9 preferentially interacted with a super-glycosylated gp96 species. The categorical loss of TLRs in gp96-deficient macrophages operationally created a conditional and cell-specific TLR null mouse. These mice were resistant to endotoxin shock but were highly susceptible to Listeria monocytogenes. Our results demonstrate that gp96 is the master chaperone for TLRs and that macrophages, but not other myeloid cells, are the dominant source of proinflammatory cytokines during endotoxemia and Listeria infections.

Plant endoplasmin supports the protein secretory pathway and has a role in proliferating tissues

Endoplasmin is a molecular chaperone of the heat-shock protein 90 class located in the endoplasmic reticulum and its activity is poorly characterized in plants. We assessed the ability of endoplasmin to alleviate stress via its transient overexpression in tobacco protoplasts treated with tunicamycin, an inhibitor of glycosylation and inducer of the unfolded protein response (UPR). Endoplasmin supported the secretion of a model secretory protein but was less effective than BiP, the endoplasmic reticulum member of the heat-shock protein 70 family. Consistently, immunoprecipitation experiments with in vivo radioactively labelled proteins using an antiserum prepared against Arabidopsis endoplasmin showed that a much smaller number of newly synthesized polypeptides associated with endoplasmin than with BiP. Synthesis of endoplasmin was enhanced by UPR inducers in tobacco seedlings but not protoplasts. As BiP synthesis was induced in both systems, we conclude that the UPR acts differently, at least in part, on the expression of the two chaperones. Endoplasmin was not detectable in extracts of leaves and stems of the Arabidopsis endoplasmin T-DNA insertion mutant shepherd. However, the chaperone is present, albeit at low levels, in shepherd mutant callus, mature roots and tunicamycin-treated seedlings, demonstrating that the mutation is leaky. Reduced endoplasmin in the shepherd mutant has no effect on BiP protein levels in callus or mature roots, leaves and stems, but is compensated by increased BiP in seedlings. This increase occurs in proliferating rather than expanding leaf cells, indicating an important role for endoplasmin in proliferating plant tissues.

Effect of selected insecticides on growth rate and stress protein expression in cultured human A549 and SH-SY5Y cells

Two organochlorines (dienochlor, endosulfan) and one neonicotinoid (imidacloprid) insecticides were investigated as putative cellular aggressors, both as pure chemicals and as commercial formulations, in order to evaluate the additional toxicity due to additives present in the commercial formulations. Toxicity was evaluated on human cells in vitro, by culturing neuronal SH-SY5Y and pulmonary A549 cell lines for 3 days in the presence of increasing concentrations of the selected pesticides. LOEC (lowest observed effect concentration), IC50 (concentration leading to a 50% decrease of cell growth) and expression changes of molecular chaperones involved in cellular protein quality control were determined. The investigated molecular chaperones were the cytosolic resident heat shock proteins (HSP27, HSP72/73, and HSP90) and the glucose regulated proteins (GRP78, GRP94) located in the endoplasmic reticulum (ER). Organochlorines were found to be the most toxic in both A549 and SH-SY5Y cells, IC50 being respectively 0.95 and 0.36 microM for dienochlor, 34 and 20 microM for endosulfan, 1.8 and 1.5 mM for imidacloprid. This shows that neuronal cells were more sensitive than pulmonary cells. LOEC and IC50 appeared at lower concentrations of active molecule when using the commercial formulations Techn'ufan (endosulfan) and Confidor (imidacloprid), indicating an additional adverse effect of additives. Insecticide concentrations higher than IC50 were found to induce an underexpression of all cytosolic HSPs, probably resulting from a general inhibition of protein synthesis. HSP27 was found to be underexpressed at concentrations of imidacloprid or endosulfan (as Techn'ufan) lower than IC50. This underexpression of the anti-apoptotic HSP27 could contribute to the increase of cell mortality. GRP78 was up-regulated by endosulfan in A549, but not in SH-SY5Y cells, suggesting a damaging effect on proteins specific to pulmonary cells. Conversely, HSP72/73 was found to be down-regulated, resulting probably from the ER unfolded protein response (UPR) as previously reported [Skandrani, D., Gaubin, Y., Vincent, C., Beau, B., Murat, J.C., Soleilhavoup, J.P., Croute, F., 2006. Relationship between toxicity of selected insecticides and expression of stress protein (HSP, GRP) in cultured human cells: effects of commercial formulations versus pure active molecules. Biochim. Biophys. Acta 1760 (1), 95-103].

Identification of novel quaternary domain interactions in the Hsp90 chaperone, GRP94

The structural basis for the coupling of ATP binding and hydrolysis to chaperone activity remains a central question in Hsp90 biology. By analogy to MutL, ATP binding to Hsp90 is thought to promote intramolecular N-terminal dimerization, yielding a molecular clamp functioning in substrate protein activation. Though observed in studies with recombinant domains, whether such quaternary states are present in native Hsp90s is unknown. In this study, native subunit interactions in GRP94, the endoplasmic reticulum Hsp90, were analyzed using chemical cross-linking in conjunction with tandem mass spectrometry. We report the identification of two distinct intermolecular interaction sites. Consistent with previous studies, one site comprises the C-terminal dimerization domain. The remaining site represents a novel intermolecular contact between the N-terminal and middle (M) domains of opposing subunits. This N+M domain interaction was present in the nucleotide-empty, ADP-, ATP-, or geldanamycin-bound states and could be selectively disrupted upon addition of synthetic geldanamycin dimers. These results identify a compact, intertwined quaternary conformation of native GRP94 and suggest that intersubunit N+M interactions are integral to the structural biology of Hsp90.

Proteasome inhibitors induce a terminal unfolded protein response in multiple myeloma cells

Multiple myeloma (MM) is an incurable plasma cell malignancy. The 26S proteasome inhibitor, bortezomib, selectively induces apoptosis in MM cells; however, the nature of its selectivity remains unknown. Here we demonstrate that 5 different MM cell lines display similar patterns of sensitivity to 3 proteasome inhibitors (PIs) but respond differently to specific NF-kappaB inhibition. We further show that PIs initiate the unfolded protein response (UPR), a signaling pathway activated by the accumulation of misfolded proteins within the endoplasmic reticulum (ER). Consistent with reports that prosurvival/physiologic UPR components are required for B-cell differentiation into antibody-secreting cells, we found that MM cells inherently expressed the ER chaperones GRP78/Bip and GRP94/gp96. However, bortezomib rapidly induced components of the proapoptotic/terminal UPR, including PERK, the ER stress-specific eIF-2alpha kinase; ATF4, an ER stress-induced transcription factor; and its proapoptotic target, CHOP/GADD153. Consistent with our hypothesis that PIs induce the accumulation of misfolded ER-processed proteins, we found that the amount of immunoglobulin subunits retained within MM cells correlated with their sensitivity to PIs. These findings suggest that MM cells have a lower threshold for PI-induced UPR induction and ER stress-induced apoptosis because they constitutively express ER stress survival factors to function as secretory cells.

Proteasome inhibitors induce a terminal unfolded protein response in multiple myeloma cells

Multiple myeloma (MM) is an incurable plasma cell malignancy. The 26S proteasome inhibitor, bortezomib, selectively induces apoptosis in MM cells; however, the nature of its selectivity remains unknown. Here we demonstrate that 5 different MM cell lines display similar patterns of sensitivity to 3 proteasome inhibitors (PIs) but respond differently to specific NF-kappaB inhibition. We further show that PIs initiate the unfolded protein response (UPR), a signaling pathway activated by the accumulation of misfolded proteins within the endoplasmic reticulum (ER). Consistent with reports that prosurvival/physiologic UPR components are required for B-cell differentiation into antibody-secreting cells, we found that MM cells inherently expressed the ER chaperones GRP78/Bip and GRP94/gp96. However, bortezomib rapidly induced components of the proapoptotic/terminal UPR, including PERK, the ER stress-specific eIF-2alpha kinase; ATF4, an ER stress-induced transcription factor; and its proapoptotic target, CHOP/GADD153. Consistent with our hypothesis that PIs induce the accumulation of misfolded ER-processed proteins, we found that the amount of immunoglobulin subunits retained within MM cells correlated with their sensitivity to PIs. These findings suggest that MM cells have a lower threshold for PI-induced UPR induction and ER stress-induced apoptosis because they constitutively express ER stress survival factors to function as secretory cells.

Proteomic analysis of enriched microsomal fractions from GS-NS0 murine myeloma cells with varying secreted recombinant monoclonal antibody productivities

The folding, transport and modification of recombinant proteins in the constitutive secretory pathway of eukaryotic cell expression systems are reported to be a bottleneck in their production. We have utilised a proteomic approach to investigate the processes catalysed by proteins constituting the secretory pathway to further our understanding of those processes involved in high-level antibody secretion. We used GS-NS0 cell populations differing in qmAb to prepare enriched microsome fractions from each cell population at mid-exponential growth phase. These were analysed by 2-D PAGE to characterise the microsome protein component and test the hypothesis that bottlenecks in recombinant protein synthesis exist in these compartments, which are alleviated in high producers by the up-regulation of key secretory pathway proteins. Proteins whose abundance changed in a statistically significant manner with increasing qmAb were involved in a range of cellular functions: energy metabolism, mAb folding/assembly, cytoskeletal organisation and protein turnover. Amongst these were BiP and PDI, chaperones resident in the ER that interact with nascent immunoglobulins during their folding/assembly. However, our results suggest that there are diverse mechanisms by which these cells achieve qmAb. The results imply that cell-engineering strategies for improving qmAb should target proteins associated with altered functional phenotype identified in this study.

Relationship between toxicity of selected insecticides and expression of stress proteins (HSP, GRP) in cultured human cells: Effects of commercial formulations versus pure active molecules

Three carbamate (formetanate, methomyl, pyrimicarb) and one pyrethroid (bifenthrin) insecticides were investigated both as pure chemicals and as commercial formulations in order to unveil possible toxic effects of additives and solvents present in the commercial formulations and to evaluate the cellular stress response as a defense mechanism. Toxic effects were evaluated on A549 cells, derived from a human lung carcinoma, by measuring (1) threshold concentrations leading to a decrease of the growth rate (LOEC), (2) sublethal concentrations (SC) which arrested growth without killing the cells, and (3) expression levels of several stress proteins, i.e., HSP27, HSP72/73, HSP90, GRP78, and GRP94. As compared to the pure active molecule, LOEC appeared at lower concentrations when using the commercial formulations, i.e., Dicarzol (formetanate), Lannate20 (methomyl) and Talstar or Kiros EV (bifenthrin). Propylene glycol and propylene glycol monomethyl ether, respectively, present in Talstar and kiros, do not account for the high toxicity of these commercial formulations and do not potentiate the toxicity of bifenthrin. Additive but not synergistic adverse effects were observed when cells are exposed to a mixture of 4 different commercial formulations. Our results show that the concentrations of active molecules recommended in flori-cultural general use or for spray preparations are much higher than SC concentrations, as determined on A549 pulmonary cells. GRP78 was up-regulated by all the insecticides, commercial preparations being more efficient to trigger the stress reaction. This suggests that insecticides and additives present in commercial formulations disrupt ER functions. Conversely, HSP72/73 was found to be down-regulated by all the insecticides. This seems to be related with a decrease of protein synthesis in the cytosol, as a result of the ER unfolded protein response. Indeed, tunicamycin, known to inhibit N-linked glycosylation in the ER, was found to induce a similar inverse correlation between GRP78 overexpression and HSP72/73 under-expression. Expression of GRP94 was found to be increased and HSP27 lowered by the highest concentrations of bifenthrin commercial formulations. Methomyl and Lannate20 only induced an under-expression of HSP90.

Versatility of the endoplasmic reticulum protein folding factory

The endoplasmic reticulum (ER) is dedicated to import, folding and assembly of all proteins that travel along or reside in the secretory pathway of eukaryotic cells. Folding in the ER is special. For instance, newly synthesized proteins are N-glycosylated and by default form disulfide bonds in the ER, but not elsewhere in the cell. In this review, we discuss which features distinguish the ER as an efficient folding factory, how the ER monitors its output and how it disposes of folding failures.

Engineering mammalian cell factories for improved recombinant monoclonal antibody production: lessons from nature?

In this review we consider how cell specific recombinant monoclonal antibody (Mab) production by engineered mammalian cells can be improved. Whilst it is generally recognized that Mab production is limited post-transcriptionally at folding and assembly reactions, genetic engineering strategies based on overexpression of individual chaperones or foldases in mammalian cells have not reliably increased cell specific Mab production. Given that recent studies have established that chaperones and foldases themselves exist in a large multiprotein complex, which may coordinate the sequential processing of Mabs, we propose that global expansion of all components of the secretory pathway will likely be necessary to generically improve recombinant Mab production by mammalian cells. In this context, what can be learnt from nature? Important recent studies have delineated some of the main cellular pathways involved in the differentiation of B-cells into nature's own high level Mab producers, plasma cells. This is achieved by a dramatic re-programming of cellular function where the coordinated expansion of metabolic and secretory machinery precedes Ig production, then is maintained by induction of a key intracellular signaling pathway, the unfolded protein response (UPR). Here we review genetic engineering strategies to increase cell specific production rate and discuss whether manipulation of intracellular signaling systems such as the UPR will provide a novel means to engineer mammalian cells for high level recombinant Mab production.

Approaches to the isolation and characterization of molecular chaperones

Molecular chaperones are integral components of the cellular machinery involved in ensuring correct protein folding and the continued maintenance of protein structure. An understanding of these ubiquitous molecules is key to finding cures to protein misfolding diseases such as Alzheimer's and Creutzfeldt-Jacob diseases. In addition, further understanding of chaperones will enhance our comprehension of the way the body copes with the environmental stresses that humans encounter daily. Our laboratory and our collaborators specialize in the production and characterization of chaperones from a wide variety of sources in order to gain a fuller understanding of how chaperones function in the cell. In this review, we primarily use the Hsp70/Hsp40 chaperone pair as an example to discuss recent advances in technology and reductions in cost that lend themselves to chaperone purification from both native and recombinant sources. Common assays to assess purified chaperone activity are also discussed.

Structure of unliganded GRP94, the endoplasmic reticulum Hsp90. Basis for nucleotide-induced conformational change

GRP94, the endoplasmic reticulum paralog of Hsp90, is regulated by adenosine nucleotides that bind to its N-terminal regulatory domain. Because of its weak affinity for nucleotides, the functionally relevant transition in GRP94 is likely to be between the unliganded and nucleotide-bound states. We have determined the structure of the unliganded GRP94 N-domain. The helix 1-4-5 subdomain of the unliganded protein adopts the closed conformation seen in the structure of the protein in complex with inhibitors. This conformation is distinct from the open conformation of the subdomain seen when the protein is bound to ATP or ADP. ADP soaked into crystals of the unliganded protein reveals an intermediate conformation midway between the open and closed states and demonstrates that in GRP94 the conversion between the open and closed states is driven by ligand binding. The direction of the observed movement in GRP94 shows that nucleotides act to open the subdomain elements rather than close them, which is contrary to the motion proposed for Hsp90. These observations support a model where ATP binding dictates the conformation of the N-domain and regulates its ability to form quaternary structural interactions.

Shiga toxin is transported from the endoplasmic reticulum following interaction with the luminal chaperone HEDJ/ERdj3

Shiga toxin (Stx) follows a complex intracellular pathway in order to kill susceptible cells. After binding to cell surface glycolipids, the toxin is internalized and trafficked in retrograde fashion to the endoplasmic reticulum (ER). From the ER lumen, the toxin must gain access to the cytoplasm, where it enzymatically inactivates the 28S rRNA, inhibiting protein synthesis. The host molecules involved in this pathway and the mechanisms utilized by the toxin to access the cytoplasm from the ER are largely unknown. We found that Stx is capable of energy-dependent transport across the ER lumen, as has recently been demonstrated for the cholera and ricin toxins. Genetic screening for molecules involved in Shiga toxin trafficking yielded a cDNA encoding a prematurely truncated protein. Characterization of this cDNA revealed that it encodes a novel Hsp40 chaperone, designated HEDJ or ERdj3, localized to the ER lumen, where it interacts with BiP, a molecule known to be involved in protein retrotranslocation out of the ER. We demonstrated that within the ER lumen Stx interacts with HEDJ and other chaperones known to be involved in retrotranslocation of proteins across the ER membrane. Moreover, sequential immunoprecipitation revealed that Shiga toxin was present in a complex that included HEDJ and Sec61, the translocon through which proteins are retrotranslocated to the cytoplasm. These findings suggest that HEDJ is a component of the ER quality control system and that Stx utilizes HEDJ and other ER-localized chaperones for transport from the ER lumen to the cytosol.

ER stress and the unfolded protein response

Conformational diseases are caused by mutations altering the folding pathway or final conformation of a protein. Many conformational diseases are caused by mutations in secretory proteins and reach from metabolic diseases, e.g. diabetes, to developmental and neurological diseases, e.g. Alzheimer's disease. Expression of mutant proteins disrupts protein folding in the endoplasmic reticulum (ER), causes ER stress, and activates a signaling network called the unfolded protein response (UPR). The UPR increases the biosynthetic capacity of the secretory pathway through upregulation of ER chaperone and foldase expression. In addition, the UPR decreases the biosynthetic burden of the secretory pathway by downregulating expression of genes encoding secreted proteins. Here we review our current understanding of how an unfolded protein signal is generated, sensed, transmitted across the ER membrane, and how downstream events in this stress response are regulated. We propose a model in which the activity of UPR signaling pathways reflects the biosynthetic activity of the ER. We summarize data that shows that this information is integrated into control of cellular events, which were previously not considered to be under control of ER signaling pathways, e.g. execution of differentiation and starvation programs.

GRP78 Compartmentalized Redistribution in Pb-treated Glia: Role of GRP78 in Lead-induced Oxidative Stress

Glucose-regulated protein of 78 kDa (GRP78) is an endoplasmic reticulum (ER) molecular chaperone functioning in protein folding, assembly and trafficking. GRP78 also plays a role in protection against cytotoxicity and apoptosis induced by environmental insults. Our previous study showed that lead (Pb) directly targets GRP78 by binding to the protein and increasing GRP78 levels. In this study, the effect of Pb on compartmentalized distribution of GRP78 in living cells was examined. A rat GRP78-EGFP fusion protein and EGFP were transiently expressed in astrocytoma cells exposed to 5 microM Pb acetate or 50 microM CuSO4 and fluorescence signals were captured. Control cells expressing EGFP showed a homogeneous distribution of EGFP that was not changed by Pb or Cu treatment. Cells expressing GRP78-EGFP showed a compartmentalized, non-homogeneous distribution of GRP78-EGFP in the cytosol. The redistribution of GRP78-EGFP fluorescent bodies was observed in cells exposed to Pb for 10 h, but not 5 h. Redistribution was also observed in cells exposed to 50 microM Cu for 5 or 10 h. To assess GRP78 function, GRP78 was depleted with dsRNAi oligos in rat C6 glioma cells. GRP78 depletion significantly increased the sensitivity of cells to Pb exposure as indicated by the generation of reactive oxygen species (ROS). These data suggest that Pb directly targets GRP78 and induces its compartmentalized redistribution in living cells and that GRP78 plays a protective role in Pb neurotoxicity.

Protein folding and quality control in the endoplasmic reticulum

The endoplasmic reticulum (ER) is a highly versatile protein factory that is equipped with chaperones and folding enzymes essential for protein folding. ER quality control guided by these chaperones is essential for life. Whereas correctly folded proteins are exported from the ER, misfolded proteins are retained and selectively degraded. At least two main chaperone classes, BiP and calnexin/calreticulin, are active in ER quality control. Folding factors usually are found in complexes. Recent work emphasises more than ever that chaperones act in concert with co-factors and with each other.

Overexpression of endoplasmic reticulum molecular chaperone GRP94 and GRP78 in human lung cancer tissues and its significance

BACKGROUND

To investigate the relationship between the expression of glucose-regulated protein94 (GRP94) and GRP78 at the level of mRNA and protein in vivo and in human lung cancer.

METHODS

RT-PCR, real-time PCR, immunohistochemistry and/or Western blot were used in 54 cases of lung cancer and corresponding normal lung tissue.

RESULTS

The expression pattern of GRP94 and GRP78 was similar. There was a significant overexpression of GRP94 and GRP78 at both mRNA and protein levels in cancer tissues as compared to normal tissues. The relative levels of GRP94 and GRP78 mRNA evaluated by RT-PCR in cancer and normal lung tissue were: GRP94: 3.48+/-2.06 versus 2.01+/-1.83; GRP78: 3.64+/-1.87 versus 2.21+/-1.54; by real-time PCR were: GRP94: 2.89+/-0.64 versus 1.12+/-0.54; GRP78: 2.56+/-0.82 versus 0.96+/-0.42. The relative level of GRP94 and GRP78 protein by Western blot in cancer and normal lung tissue were: GRP94: 3.46+/-1.72 versus 1.81+/-0.92; GRP78: 4.84+/-2.55 versus 1.91+/-1.15, indicating an approximate 2-fold and a 3-fold increase in GRP94 and GRP78 protein in cancer tissue as compared with normal tissue. Immunohistochemistry result for GRP94 and GRP78 in cancer and normal tissue was similar, that is: a stronger stain was observed in cancer tissue (main intensity of staining ++ to +++) compared to normal tissue (main intensity of staining + to ++). All the difference for GRP94 and GRP78 between the two tissues were significant (p<0.05). Furthermore, the overexpression of GRP94 and GRP78 in the cancer tissue correlated with grade of differentiation and stage of tumors. There was stronger expression in poorly differentiated tumors than in well-moderately differentiated tumors (p<0.05). There was also stronger expression in stage III than in stages I and II tumors (p<0.05). No statistically significant differences were found among various pathologic types of tumors. Correlation analysis showed that there is a positive correlation between GRP94 and GRP78.

CONCLUSION

The expression pattern of GRP94 and GRP78 was similar in human lung cancer. They both were related with the differentiation and progression of the cancer. The expression at mRNA and protein level may be valuable in evaluating the grade of differentiation and clinical stage of human lung cancer.

Ligand-induced conformational shift in the N-terminal domain of GRP94, an Hsp90 chaperone

GRP94 is the endoplasmic reticulum paralog of cytoplasmic Hsp90. Models of Hsp90 action posit an ATP-dependent conformational switch in the N-terminal ligand regulatory domain of the chaperone. However, crystal structures of the isolated N-domain of Hsp90 in complex with a variety of ligands have yet to demonstrate such a conformational change. We have determined the structure of the N-domain of GRP94 in complex with ATP, ADP, and AMP. Compared with the N-ethylcarboxamidoadenosine and radicicol-bound forms, these structures reveal a large conformational rearrangement in the protein. The nucleotide-bound form exposes new surfaces that interact to form a biochemically plausible dimer that is reminiscent of those seen in structures of MutL and DNA gyrase. Weak ATP binding and a conformational change in response to ligand identity are distinctive mechanistic features of GRP94 and suggest a model for how GRP94 functions in the absence of co-chaperones and ATP hydrolysis.

Efficient eukaryotic expression of fluorescent scFv fusion proteins directed against CD antigens for FACS applications

Two sets of expression vectors were constructed that permitted the efficient expression of single-chain Fv fragments (scFvs) fused N-terminally to an enhanced mutant of the green fluorescent protein GFP+ or the red fluorescent protein DsRed in insect and mammalian cells. The vectors allowed rapid cloning of scFv fragments and secretion of the fusion proteins in a native conformation. Fluorescent scFv fusion proteins directed against a series of cluster of differentiation (CD) antigens were efficiently secreted by transiently transfected mammalian cells and insect cells infected with baculoviral expression constructs. Yields of the secreted proteins varied from 100 microg/l to 3 mg/l. The purified proteins were functionally active in flow cytometry, immunofluorescent microscopy, and competition binding experiments performed to delineate the epitopes recognized by different monoclonal antibodies against the same polypeptide. The use of two different scFv fragments fused with red and green fluorescent proteins and reacting with T- and B-cell lineage markers (CD7 and CD19), respectively, allowed a simplified quantitation of both subsets in two-color flow cytometry experiments with mixed populations of T- and B-lymphoid cells. Due to the lack of Fc domains in the scFv proteins, the fluorescent fusion proteins showed more than 20-fold reduced background fluorescence compared with whole antibodies of the same specificity in experiments with effector cells expressing the high affinity FcgammaRI receptor CD64. Thus, for a number of analytical applications, fluorescent scFv fusion proteins offer advantages over the use of complete primary antibodies and chemically labeled fluorescent secondary antibodies.

Comparative proteomic analysis of GS-NS0 murine myeloma cell lines with varying recombinant monoclonal antibody production rate

We have employed an inverse engineering strategy based on quantitative proteome analysis to identify changes in intracellular protein abundance that correlate with increased specific recombinant monoclonal antibody production (qMab) by engineered murine myeloma (NS0) cells. Four homogeneous NS0 cell lines differing in qMab were isolated from a pool of primary transfectants. The proteome of each stably transfected cell line was analyzed at mid-exponential growth phase by two-dimensional gel electrophoresis (2D-PAGE) and individual protein spot volume data derived from digitized gel images were compared statistically. To identify changes in protein abundance associated with qMab datasets were screened for proteins that exhibited either a linear correlation with cell line qMab or a conserved change in abundance specific only to the cell line with highest qMab. Several proteins with altered abundance were identified by mass spectrometry. Proteins exhibiting a significant increase in abundance with increasing qMab included molecular chaperones known to interact directly with nascent immunoglobulins during their folding and assembly (e.g., BiP, endoplasmin, protein disulfide isomerase). 2D-PAGE analysis showed that in all cell lines Mab light chain was more abundant than heavy chain, indicating that this is a likely prerequisite for efficient Mab production. In summary, these data reveal both the adaptive responses and molecular mechanisms enabling mammalian cells in culture to achieve high-level recombinant monoclonal antibody production.

Structure of the N-terminal domain of GRP94. Basis for ligand specificity and regulation

GRP94, the endoplasmic reticulum (ER) paralog of the chaperone Hsp90, plays an essential role in the structural maturation or secretion of a subset of proteins destined for transport to the cell surface, such as the Toll-like receptors 2 and 4, and IgG, respectively. GRP94 differs from cytoplasmic Hsp90 by exhibiting very weak ATP binding and hydrolysis activity. GRP94 also binds selectively to a series of substituted adenosine analogs. The high resolution crystal structures at 1.75-2.1 A of the N-terminal and adjacent charged domains of GRP94 in complex with N-ethylcarboxamidoadenosine, radicicol, and 2-chlorodideoxyadenosine reveals a structural mechanism for ligand discrimination among hsp90 family members. The structures also identify a putative subdomain that may act as a ligand-responsive switch. The residues of the charged region fold into a disordered loop whose termini are ordered and continue the twisted beta sheet that forms the structural core of the N-domain. This continuation of the beta sheet past the charged domain suggests a structural basis for the association of the N-terminal and middle domains of the full-length chaperone.

The paranodal complex of F3/contactin and caspr/paranodin traffics to the cell surface via a non-conventional pathway

During myelination, membrane-specialized domains are generated by complex interactions between axon and glial cells. The cell adhesion molecules caspr/paranodin and F3/contactin play a crucial role in the generation of functional septate-like junctions at paranodes. We have previously demonstrated that association with the glycosylphosphatidylinositol-linked F3/contactin is required for the recruitment of caspr/paranodin into the lipid rafts and its targeting to the cell surface. When transfected alone in neuroblastoma N2a cells, caspr/paranodin is retained in the endoplasmic reticulum (ER). Using chimerical constructs, we show that the cytoplasmic region does not contain any ER retention signal, whereas the ectodomain plays a crucial role in caspr/paranodin trafficking. A series of truncations encompassing the extracellular region of caspr/paranodin was unable to abolish ER retention. We show that N-glycosylation and quality control by the lectin-chaperone calnexin are required for the cell surface delivery of caspr/paranodin. Cell surface transport of F3/contactin and caspr/paranodin is insensitive to brefeldin A and the two glycoproteins are endoglycosidase H-sensitive when associated in complex, recruited into the lipid rafts, and expressed on the cell surface. Our results indicate a Golgi-independent pathway for the paranodal cell adhesion complex that may be implicated in the segregation of axonal subdomains.

Thiol-mediated protein retention in the endoplasmic reticulum: the role of ERp44

Formation of disulfide bonds, an essential step for the maturation and exit of secretory proteins from the endoplasmic reticulum (ER), is controlled by specific ER-resident enzymes. A pivotal element in this process is Ero1alpha, an oxidoreductin that lacks known ER retention motifs. Here we show that ERp44 mediates Ero1alpha ER localization through the formation of reversible mixed disulfides. ERp44 also prevents the secretion of an unassembled cargo protein with unpaired cysteines. We conclude that ERp44 is a key element in thiol-mediated retention. It might also favour the maturation of disulfide-linked oligomeric proteins and their quality control.

Co-expression of molecular chaperones does not improve the heterologous expression of mammalian G-protein coupled receptor expression in yeast

The limitations to high-level expression of integral membrane proteins are not well understood. The human A(2)a adenosine receptor (A(2)a) and mouse Substance P receptor (SPR) were individually expressed in S. cerevisiae to identify potential cellular bottlenecks for G-protein coupled receptors. In the yeast system, A(2)a was not N-linked glycosylated but was functional and plasma membrane-localized. A(2)a also contained an intramolecular disulfide bond. Substance P receptor was also not N-linked glycosylated in yeast, but, unlike A(2)a, SPR was intracellularly retained, nonfunctional, and did not appear to contain an intramolecular disulfide bond. Since both receptors contain N-linked glycosylation and disulfide bonds in mammalian systems, machinery responsible for interacting with these modifications was investigated-specifically, the potential interactions between the nascent receptor and ER-resident proteins were explored. The chaperones calnexin and protein disulfide isomerase were co-overexpressed with the GPCRs to determine the effect on total and active yields of A(2)a and SPR, as well as on receptor trafficking. The effect of co-expressing the chaperone BiP on the total yields of A(2)a as well as intracellular fates of both receptors were determined. The co-expression of ER resident proteins did not improve A(2)a yields nor did they restore SPR activity or improve SPR cell surface expression. Taken together, these results indicate that an ER-folding bottleneck does not limit the expression of the mammalian receptors in yeast.

Quality Control and Protein Folding in the Secretory Pathway

The biosynthesis of secretory and membrane proteins in the endoplasmic reticulum (ER) yields mostly properly folded and assembled structures with full biological activity. Such fidelity is maintained by quality control (QC) mechanisms that avoid the production of nonnative structures. QC relies on chaperone systems in the ER that monitor and assist in the folding process. When folding promotion is not sufficient, proteins are retained in the ER and eventually retranslocated to the cytosol for degradation by the ubiquitin proteasome pathway. Retention of proteins that fail QC can sometimes occur beyond the ER, and degradation can take place in lysosomes. Several diseases are associated with proteins that do not pass QC, fail to be degraded efficiently, and accumulate as aggregates. In other cases, pathology arises from the downregulation of mutated but potentially functional proteins that are retained and degraded by the QC system.

Profibrillin-1 maturation by human dermal fibroblasts: proteolytic processing and molecular chaperones

Fibrillin-1 is synthesized as a proprotein that undergoes proteolytic processing in the unique C-terminal domain by a member of the PACE/furin family of endoproteases. This family of endoproteases is active in the trans-Golgi network (TGN), but metabolic labeling studies have been controversial as to whether profibrillin-1 is processed intracellularly or after secretion. This report provides evidence that profibrillin-1 processing is not an intracellular event. Bafilomycin A(1) and incubation of dermal fibroblasts at 22 degrees C were used to block secretion in the TGN to confirm that profibrillin-1 processing did not occur in this compartment. Profibrillin-1 immunoprecipitation studies revealed that two endoplasmic reticulum-resident molecular chaperones, BiP and GRP94, interacted with profibrillin-1. To determine the proprotein convertase responsible for processing profibrillin-1, a specific inhibitor of furin, alpha-1-antitrypsin, Portland variant, was both expressed in the cells and added to cells exogenously. In both cases, the inhibitor blocked the processing of profibrillin-1, providing evidence that furin is the enzyme responsible for profibrillin-1 processing. These studies delineate the secretion and proteolytic processing of profibrillin-1, and identify the proteins that interact with profibrillin-1 in the secretory pathway.

Regulatory mechanisms that determine the development and function of plasma cells

Plasma cells are terminally differentiated final effectors of the humoral immune response. Plasma cells that result from antigen activation of B-1 and marginal zone B cells provide the first, rapid response to antigen. Plasma cells that develop after a germinal center reaction provide higher-affinity antibody and often survive many months in the bone marrow. Transcription factors Bcl-6 and Pax5, which are required for germinal center B cells, block plasmacytic differentiation and repress Blimp-1 and XBP-1, respectively. When Bcl-6-dependent repression of Blimp-1 is relieved, Blimp-1 ensures that plasmacytic development is irreversible by repressing BCL-6 and PAX5. In plasma cells, Blimp-1, XBP-1, IRF4, and other regulators cause cessation of cell cycle, decrease signaling from the B cell receptor and communication with T cells, inhibit isotype switching and somatic hypermutation, downregulate CXCR5, and induce copious immunoglobulin synthesis and secretion. Thus, commitment to plasmacytic differentiation involves inhibition of activities associated with earlier B cell developmental stages as well as expression of the plasma cell phenotype.

Cell-cycle control of plasma cell differentiation and tumorigenesis

Cell-cycle control is a major determinant of homeostasis during B-cell development, differentiation, and tumorigenesis. The generation of an antibody response requires activation and expansion of antigen-specific B cells and terminal differentiation of these cells into plasma cells. Plasma cells arrest in the G1 phase of the cell cycle, but the mechanism that underlies timely cell-cycle entry and exit in the humoral immune response is not known. The mammalian cell-cycle is regulated primarily at the G1 to S transition by the balance between positive regulators, the cyclin-dependent kinases (CDK) together with cyclins, and negative regulators, the CDK inhibitors. One such inhibitor, p18INK4c, has been shown to be required for cell-cycle termination and final differentiation of non-secreting plasmacytoid cells to antibody-secreting plasma cells. This finding provides the first direct evidence for cell-cycle control of B-cell immunity. It also raises important questions regarding cell-cycle control of cellular differentiation, apoptosis, and earlier steps of B-cell terminal differentiation. This article discusses the biochemical mechanism of cell-cycle control in the context of antibody response and plasma cell differentiation along with the role of cell-cycle dysregulation in the pathogenesis of multiple myeloma, the plasma cell cancer.

Modulation of the ATPase cycle of BiP by peptides and proteins

BiP, the Hsp70 homologue of the endoplasmic reticulum, interacts with its non-native substrate proteins in an ATP-dependent manner. This interaction is coupled to the ATPase cycle of the chaperone. Binding of short, synthetic peptides stimulate the ATPase activity of BiP. In previous work, we showed that a stably unfolded antibody domain forms a binary complex with BiP. In this study we made use of this complex to analyse the effect of substrate proteins on the ATPase cycle of BiP. Kinetic constants of the partial reactions of the ATPase cycle were determined without substrate, in the presence of a short binding peptide and in the presence of the antibody domain. We show that, in contrast to smaller peptides, the non-native protein domain decelerates the rate limiting hydrolysis step of the ATPase cycle.

Differential increase in the expression of heat shock protein family members during sleep deprivation and during sleep

Although sleep is thought to be restorative from prior wakeful activities, it is not clear what is being restored. To determine whether the synthesis of macromolecules is increased in the cerebral cortex during sleep, we subjected C57BL/6 mice to 6 hours of sleep deprivation and then screened the expression of 1176 genes of known function by using cDNA arrays. The expression of the heat shock proteins (HSP), endoplasmic reticulum protein (ERp72) and glucose-regulated protein (GRp78), was among the genes whose expression was significantly elevated in the cortex during sleep deprivation, whereas GRp78 and GRp94 mRNAs were elevated in the cortex during recovery sleep after sleep deprivation, as confirmed by conventional and quantitative real-time polymerase chain reaction and/or Northern analyses. A systematic evaluation of the expression of six heat shock protein family members (ERP72, GRp78, GRp94, HSP27, HSP70-1, and HSP84) in seven brain regions revealed increased mRNA levels in cortex, basal forebrain, hypothalamus, cerebellum and medulla during sleep deprivation, whereas increased mRNA levels during recovery sleep were limited to the cortex and medulla. Immunohistochemical studies identified increased numbers of GRp78-, GRp94-, and ERp72-immunoreactive cells in the dorsal and lateral cortex during sleep deprivation but, during recovery sleep, elevated numbers of these cells were found only in the lateral cortex. In the medulla, increased numbers of GRp94-immunoreactive cells were observed in nucleus tractus solitarius, dorsal motor nucleus of the vagus and the rostroventrolateral medulla during recovery sleep. The widespread increase of heat shock protein family mRNAs in brain during sleep deprivation may be a neuroprotective response to prolonged wakefulness. In contrast, the relatively limited heat shock protein family mRNA expression during recovery sleep may be related to the role of heat shock proteins in protein biogenesis and thus to the restorative function of sleep.

Expression of glucose-regulation protein 94 in gastric mucosa infected with Helicobacter pylori

AIM

To study the expression of glucose regulation protein 94 in gastric mucosa infected with Helicobacter pylori.

METHODS

Semi-quantitative RT-PCR method was used to demonstrate mRNA expression of Grp94 in H.pylori (Hp) infected and non-infected gastric mucosa. Western blot was used to detect the expression level of Grp94 protein in the tissues.

RESULTS

Hp negative group had 28 cases, and the expression amount of Grp94 mRNA was 0.424±0.055. Hp infected group had 32 cases and the expression amount of Grp94 mRNA was 0.882±0.082. The expression amounts of Grp94 protein were 0.427±0.036, 0.671±0.072 respectively in Hp-negative and positive groups. The expression amounts of Grp94 mRNA and Grp94 protein in infected group showed a significant increase (P<0.01) respectively.

CONCLUSION

H.pylori infection may increase the expression of Grp94 at mRNA level in gastric mucosa. Meanwhile, it also increases synthesis of the protein.

The Hsp70 and TRiC/CCT chaperone systems cooperate in vivo to assemble the von Hippel-Lindau tumor suppressor complex

The degree of cooperation and redundancy between different chaperones is an important problem in understanding how proteins fold in the cell. Here we use the yeast Saccharomyces cerevisiae as a model system to examine in vivo the chaperone requirements for assembly of the von Hippel-Lindau protein (VHL)-elongin BC (VBC) tumor suppressor complex. VHL and elongin BC expressed in yeast assembled into a correctly folded VBC complex that resembles the complex from mammalian cells. Unassembled VHL did not fold and remained associated with the cytosolic chaperones Hsp70 and TRiC/CCT, in agreement with results from mammalian cells. Analysis of the folding reaction in yeast strains carrying conditional chaperone mutants indicates that incorporation of VHL into VBC requires both functional TRiC and Hsp70. VBC assembly was defective in cells carrying either a temperature-sensitive ssa1 gene as their sole source of cytosolic Hsp70/SSA function or a temperature-sensitive mutation in CCT4, a subunit of the TRiC/CCT complex. Analysis of the VHL-chaperone interactions in these strains revealed that the cct4ts mutation decreased binding to TRiC but did not affect the interaction with Hsp70. In contrast, loss of Hsp70 function disrupted the interaction of VHL with both Hsp70 and TRiC. We conclude that, in vivo, folding of some polypeptides requires the cooperation of Hsp70 and TRiC and that Hsp70 acts to promote substrate binding to TRiC.

The heat shock protein Gp96 binds to human neutrophils and monocytes and stimulates effector functions

The endoplasmic reticulum (ER)-resident heat shock protein Gp96 is involved in protein folding and is released into the extracellular space after necrotic cell death. In this context, Gp96 has immunostimulatory properties: it activates dendritic cells or macrophages and delivers associated peptides into the antigen presentation pathway, resulting in the induction of specific T-cell responses. The inflammatory response after necrotic tissue damage leads to the recruitment of polymorphonuclear neutrophils (PMNs) and monocytes, allowing them to make their first encounter with Gp96. We therefore investigated whether PMNs and monocytes interact with Gp96. We were able to show that PMNs and monocytes specifically bind fluorescein isothiocyanate (FITC)-conjugated Gp96. The binding of Gp96-FITC was competed by lipopolysaccharide (LPS) or fucoidan, a known inhibitor of scavenger receptors. Interestingly, the binding of LPS-FITC was also competed not only by fucoidan, but by Gp96, suggesting that LPS and Gp96 share a common receptor on PMNs. One important effector function of PMNs is the clearance of an inflammatory site by phagocytosis. We therefore assessed the influence of Gp96 on phagocytic activity using fluorochrome-labeled polystyrene beads. We found a marked enhancement of phagocytosis in the presence of Gp96 and concluded that PMNs not only bind Gp96, but are also activated by it. Additionally, Gp96-stimulated PMNs and especially monocytes release large amounts of interleukin-8, a potent neutrophil-attracting chemokine. In conclusion, we demonstrate that Gp96 specifically binds to and activates PMNs and monocytes, extending the function of Gp96 as a danger signal to additional members of the innate immune system.

Restoration of Ig secretion: mutation of germline-encoded residues in T15L chains leads to secretion of free light chains and assembled antibody complexes bearing secretion-impaired heavy chains

We previously described T15H chain mutants that were impaired in assembly with L chain and in ability to be secreted from the cell. The unmutated T15L chain is unusual in that it is secretion-impaired in the absence of assembly with H chain. The T15L chain preferentially pairs with T15H in vivo, suggesting that if we introduced mutations that would allow secretion of free T15L chain, they might also lead to the secretion of the complex with the defective H chain. We mutated four positions in the germline T15L that had amino acids infrequently found in other kappa-chains. Mutation to the most frequently occurring amino acid at three of the four positions allowed secretion of free L chain, while the combination of two secretion-restoring mutations was synergistic. Coexpression of secretion-restored mutant L chains with the secretion-defective mutant H chains rescued secretion of the assembled H(2)L(2) complex, suggesting that during somatic hypermutation in vivo, deleterious mutations at the H chain may be compensated by mutations on the L chain. To our knowledge, this is the first example of mutations in IgL chains that are able to restore secretion-defective H chains to secretion competence in mammalian cells.

Sequential waves of functionally related proteins are expressed when B cells prepare for antibody secretion

Upon encounter with antigen, B lymphocytes differentiate into Ig-secreting plasma cells. This step involves a massive development of secretory organelles, most notably the endoplasmic reticulum. To analyze the relationship between organelle reshaping and Ig secretion, we performed a dynamic proteomics study of B lymphoma cells undergoing in vitro terminal differentiation. By clustering proteins according to temporal expression patterns, it appeared that B cells anticipate their secretory role in a multistep process. Metabolic capacity and secretory machinery expand first to accommodate the mass production of IgM that follows.

ER-resident chaperone interactions with recombinant antibodies in transgenic plants

In this study, we demonstrate that the folding and assembly of IgG in transgenic tobacco plants is orchestrated by BiP (binding protein), an endoplasmic reticulum resident chaperone. Expression of BiP and calreticulin was examined in transgenic tobacco plants that express immunoglobulin chains, either singly or in combination to form IgG antibody. BiP mRNA expression was lowest in wild-type nontransformed plants and those that expressed immunoglobulin light chain alone. Higher mRNA levels were detected in plants expressing fully assembled immunoglobulin (light and heavy chains), and the most abundant levels of RNA transcript were found in those plants that expressed immunoglobulin heavy chain alone. Estimation of total BiP demonstrated a similar pattern, with the highest levels detected in plants expressing immunoglobulin heavy chain alone. Immunoprecipitation studies demonstrated that BiP was associated with immunoglobulin chains extracted from protoplast lysates, but not from secreted fluids. Again, most BiP was coprecipitated from plants expressing heavy chain only and those that produced full length IgG. The binding of BiP to Ig heavy chains was ATP-sensitive. Co-expression of heavy and light chain resulted in IgG assembly and displacement of BiP from the heavy chain as the amount of light chain increased. Although calreticulin mRNA and total protein levels varied in a similar manner to those of BiP in the transgenic plants, there was no evidence for association between calreticulin and Ig chains, by coimmunoprecipitation. The results indicate that BiP, but not calreticulin, takes part in immunoglobulin folding and assembly in transgenic plants.

A subset of chaperones and folding enzymes form multiprotein complexes in endoplasmic reticulum to bind nascent proteins

We demonstrate the existence of a large endoplasmic reticulum (ER)-localized multiprotein complex that is comprised of the molecular chaperones BiP; GRP94; CaBP1; protein disulfide isomerase (PDI); ERdj3, a recently identified ER Hsp40 cochaperone; cyclophilin B; ERp72; GRP170; UDP-glucosyltransferase; and SDF2-L1. This complex is associated with unassembled, incompletely folded immunoglobulin heavy chains. Except for ERdj3, and to a lesser extent PDI, this complex also forms in the absence of nascent protein synthesis and is found in a variety of cell types. Cross-linking studies reveal that the majority of these chaperones are included in the complex. Our data suggest that this subset of ER chaperones forms an ER network that can bind to unfolded protein substrates instead of existing as free pools that assembled onto substrate proteins. It is noticeable that most of the components of the calnexin/calreticulin system, which include some of the most abundant chaperones inside the ER, are either not detected in this complex or only very poorly represented. This study demonstrates an organization of ER chaperones and folding enzymes that has not been previously appreciated and suggests a spatial separation of the two chaperone systems that may account for the temporal interactions observed in other studies.

Radicicol-sensitive peptide binding to the N-terminal portion of GRP94

GRP94 is a molecular chaperone that carries immunologically relevant peptides from cell to cell, transferring them to major histocompatibility proteins for presentation to T cells. Here we examine the binding of several peptides to recombinant GRP94 and study the regulation and site of peptide binding. We show that GRP94 contains a peptide-binding site in its N-terminal 355 amino acids. A number of peptides bind to this site with low on- and off-rates and with specificity that is distinct from that of another endoplasmic reticulum chaperone, BiP/GRP78. Binding to the N-terminal fragment is sufficient to account for the peptide binding activity of the entire molecule. Peptide binding is inhibited by radicicol, a known inhibitor of the chaperone activities of HSP90-family proteins. However, the peptide-binding site is distinct from the radicicol-binding pocket, because both can bind to the N-terminal fragment simultaneously. Furthermore, peptide binding does not cause the same conformational change as does binding of radicicol. When the latter binds to the N-terminal domain, it induces a conformational change in the downstream, acidic domain of GRP94, as measured by altered gel mobility and loss of an antibody epitope. These results relate the peptide-binding activity of GRP94 to its other function as a chaperone.

Association of the thyrotropin receptor with calnexin, calreticulin and BiP. Efects on the maturation of the receptor

The thyrotropin receptor (TSHR) is a member of the G protein-coupled receptor superfamily. It has by now been clearly established that the maturation of the glycoproteins synthesized in the endoplasmic reticulum involves interactions with molecular chaperones, which promote the folding and assembly of the glycoproteins. In this study, we investigated whether calnexin (CNX), calreticulin (CRT) and BiP, three of the main molecular chaperones present in the endoplasmic reticulum, interact with the TSHR and what effects these interactions might have on the folding of the receptor. In the first set of experiments, we observed that in a K562 cell line expressing TSHR, about 50% of the receptor synthesized was degraded by the proteasome after ubiquitination. In order to determine whether TSHR interact with CNX, CRT and BiP, coimmunoprecipitation experiments were performed. TSHR was found to be associated with all three molecular chaperones. To study the role of the interactions between CNX and CRT and the TSHR, we used castanospermine, a glucosidase I and II inhibitor that blocks the interactions between these chaperones and glycoproteins. In K562 cells expressing the TSHR, these drugs led to a faster degradation of the receptor, which indicates that these interactions contribute to stabilizing the receptor after its synthesis. The overexpression of calnexin and calreticulin in these cells stabilizes the receptor during the first hour after its synthesis, whereas the degradation of TSHR increased in a cell line overexpressing BiP and the quantity of TSHR able to acquire complex type oligosaccharides decreased. These results show that calnexin, calreticulin and BiP all interact with TSHR and that the choice made between these two chaperone systems is crucial because each of them has distinct effects on the folding and stability of this receptor at the endoplasmic reticulum level.

Defective assembly of the B-cell receptor chains accounts for its low expression in B-chronic lymphocytic leukaemia

B-cell chronic lymphocytic leukaemia (B-CLL) characteristically displays low amounts of B-cell receptor (BCR), which mainly consists of the heterodimer CD79a/CD79b bound non-covalently with the surface immunoglobulin (SIg). This heterodimer is required for SIg expression and BCR signalling. To better define the mechanisms related to low BCR expression, we have investigated transcription, protein synthesis, assembly and transport of the BCR in B-CLL cells. Our results demonstrated that: (1) there was no major defect in transcriptional expression of the B29 (CD79b) gene; (2) the BCR components were intracellularly detected, thus adequately synthesized, in almost all patients; (3) neither a genetic defect in the transmembrane region of SIg, which associated with CD79a/CD79b, nor a genetic abnormality in the chaperone protein calnexin that is involved in folding and assembly of the BCR were found; (4) a constant defect in the assembly of IgM and CD79b chains occurred leading to abnormal accumulation of both chains in different intracellular compartments; (5) in a majority of CLL patients all of the nascent IgM failed to be processed into mature chains and remained unsuitable for transport. These findings demonstrated that a post-transcriptional defect located at the BCR intracellular assembly and/or trafficking levels could be involved in its low surface expression in B-CLL.

CDK inhibitor p18(INK4c) is required for the generation of functional plasma cells

B cell terminal differentiation is associated with the onset of high-level antibody secretion and cell cycle arrest. Here the cyclin-dependent kinase (CDK) inhibitor p18(INK4c) is shown to be required within B cells for both terminating cell proliferation and differentiation of functional plasma cells. In its absence, B cells hyperproliferate in germinal centers and extrafollicular foci in response to T-dependent antigens but serum antibody titers are severely reduced, despite unimpaired germinal center formation, class switch recombination, variable region-directed hypermutation, and differentiation to antibody-containing plasmacytoid cells. The novel link between cell cycle control and plasma cell differentiation may, at least in part, relate to p18(INK4c) inhibition of CDK6. Cell cycle arrest mediated by p18(INK4C) is therefore requisite for the generation of functional plasma cells.

Human platelets express heat shock protein receptors and regulate dendritic cell maturation

Immunizations using the endoplasmic reticulum-resident heat shock protein Gp96 induce specific immune responses. Specificity is based on the major histocompatibility complex class I-restricted cross-presentation of Gp96-associated peptides derived from endogenous proteins. Initiation of the immune response depends on the ability of Gp96 to induce the production of proinflammatory cytokines by macrophages and dendritic cells (DCs) and of their maturation in a fashion presumably independent of associated peptide. Both events are mediated by Gp96 receptors on antigen-presenting cells. It is known that Gp96 is released from cells at necrosis induced, for example, by virus infection. Although this event supports the efficient induction of immune responses, it might also interfere with processes that are susceptible to chronic inflammation, such as wound healing after tissue damage. Therefore, Gp96-mediated stimulation of the immune system requires tight regulation. Here we show that human thrombocytes specifically interact with Gp96 and that binding of Gp96 to platelets is enhanced more than 10-fold on activation by thrombin. Gp96 interferes with neither thrombin-induced platelet activation nor platelet aggregation. However, the presence of platelets during Gp96-mediated DC activation reduces the secretion of proinflammatory cytokines and the activation of DCs. This effect is independent of soluble platelet factors and cell-to-cell contact between DCs and thrombocytes. Thus, we provide evidence for a regulatory mechanism that neutralizes Gp96 molecules systemically, especially in the blood. This effect might be of significance in wounds in which chronic inflammation and immune responses against autoantigens have to be prevented.

Regulation of ER stress proteins by valproate: therapeutic implications

OBJECTIVES

This paper reviews results of our studies examining the regulation of endoplasmic reticulum (ER) stress proteins by valproate (VPA). and discusses the possible implications in bipolar disorder.

METHODS

Our previous studies in the field are reviewed along with relevant literature.

RESULTS

Using differential display PCR, we identified GRP78 as a VPA-regulated gene in rat cerebral cortex. We also showed that other members of the ER stress proteins family, GRP94 and calreticulin, are also upregulated by VPA. Immunohistochemistry identified that ER stress proteins are increased in frontal and parietal cortex, as well as regions of the hippocampus in rat brain following chronic treatment with VPA.

CONCLUSIONS

Regulation of ER stress proteins by VPA may prove to be important to the mechanism of action of the drug. The neuroprotective role of these proteins may also prove to be involved in the pathophysiology of bipolar disorder.

Folding of newly translated proteins in vivo: the role of molecular chaperones

Recent years have witnessed dramatic advances in our understanding of how newly translated proteins fold in the cell and the contribution of molecular chaperones to this process. Folding in the cell must be achieved in a highly crowded macromolecular environment, in which release of nonnative polypeptides into the cytosolic solution might lead to formation of potentially toxic aggregates. Here I review the cellular mechanisms that ensure efficient folding of newly translated proteins in vivo. De novo protein folding appears to occur in a protected environment created by a highly processive chaperone machinery that is directly coupled to translation. Genetic and biochemical analysis shows that several distinct chaperone systems, including Hsp70 and the cylindrical chaperonins, assist the folding of proteins upon translation in the cytosol of both prokaryotic and eukaryotic cells. The cellular chaperone machinery is specifically recruited to bind to ribosomes and protects nascent chains and folding intermediates from nonproductive interactions. In addition, initiation of folding during translation appears to be important for efficient folding of multidomain proteins.

The prosequence of human lactase-phlorizin hydrolase modulates the folding of the mature enzyme

The efficient transport of proteins along the secretory pathway requires that the polypeptide adopts a stably folded conformation to egress the endoplasmic reticulum (ER). The transport-competent precursor of the brush border enzyme LPH, pro-LPH, undergoes an intracellular cleavage process in the trans-Golgi network between Arg(734) and Leu(735) to yield LPH beta(initial). The role of the prodomain comprising the N-terminally located 734 amino acids of pro-LPH, LPH alpha, in the folding events of LPH beta(initial) has been analyzed by the individual expression of both forms in COS-1 cells. Following synthesis at 37 degrees C LPH beta(initial) acquires a misfolded and enzymatically inactive conformation that is degraded by trypsin. A temperature shift to 20 degrees C generates a stable, trypsin-resistant, and enzymatically active LPH beta(initial) indicating that the individual expression of LPH beta(initial) results in a temperature-sensitive conformation. This form interacts at non-permissive temperatures sequentially with the ER chaperones immunoglobulin-binding protein and calnexin resulting in an ER retention. The LPH alpha prodomain resides in the ER when individually expressed. It reveals compact structural features that are stabilized by disulfide bridges. LPH alpha and LPH beta(initial) readily interact with each other upon coexpression, and this interaction appears to trigger the formation of a trypsin-resistant, correctly folded, enzymatically active, and transport-competent LPH beta(initial) polypeptide. These data clearly demonstrate that the proregion of pro-LPH is an intramolecular chaperone that is critically essential in facilitating the folding of the intermediate form LPH beta(initial) in the context of the pro-LPH polypeptide.

SHEPHERD is the Arabidopsis GRP94 responsible for the formation of functional CLAVATA proteins

The Arabidopsis shepherd (shd) mutant shows expanded shoot apical meristems (SAM) and floral meristems (FM), disorganized root apical meristems, and defects in pollen tube elongation. We have discovered that SHD encodes an ortholog of GRP94, an ER-resident HSP90-like protein. Since the shd phenotypes in SAM and FM are similar to those of the clavata (clv) mutants, we have explored the possibility that CLV complex members could be SHD targets. The SAM and FM morphology of shd clv double mutants are indistinguishable from those of clv single mutants, and the wuschel (wus) mutation is completely epistatic to the shd mutation, indicating that SHD and CLV act in the same genetic pathway to suppress WUS function. Moreover, the effects of CLV3 overexpression that result in the elimination of SAM activity were abolished in the shd mutant, indicating that CLV function is dependent on SHD function. Therefore, we conclude that the SHD protein is required for the correct folding and/or complex formation of CLV proteins.

Stimulation of the weak ATPase activity of human hsp90 by a client protein

Heat shock protein 90 (Hsp90) is a molecular chaperone involved in the folding and assembly of a limited set of "client" proteins, many of which are involved in signal transduction pathways. In vivo, it is found in complex with additional proteins, including the chaperones Hsp70, Hsp40, Hip and Hop (Hsp-interacting and Hsp-organising proteins, respectively), as well as high molecular mass immunophilins, such as FKBP59, and the small acidic protein p23. The role of these proteins in Hsp90-mediated assembly processes is poorly understood. It is known that ATP binding and hydrolysis are essential for Hsp90 function in vivo and in vitro. Here we show, for the first time, that human Hsp90 has ATPase activity in vitro. The ATPase activity is characterised using a sensitive assay based on a chemically modified form of the phosphate-binding protein from Escherichia coli. Human Hsp90 is a very weak ATPase, its activity is significantly lower than that of the yeast homologue, and it has a half-life of ATP hydrolysis of eight minutes at 37 degrees C. Using a physiological substrate of Hsp90, the ligand-binding domain of the glucocorticoid receptor, we show that this "client" protein can stimulate the ATPase activity up to 200-fold. This effect is highly specific and unfolded or partially folded proteins, which are known to bind to Hsp90, do not affect the ATPase activity. In addition, the peroxisome proliferator-activated receptor, which is related in both sequence and structure to the glucocorticoid receptor but which does not bind Hsp90, has no observable effect on the ATPase activity. We establish the effect of the co-chaperones Hop, FKBP59 and p23 on the basal ATPase activity as well as the client protein-stimulated ATPase activity of human Hsp90. In contrast with the yeast system, human Hop has little effect on the basal rate of ATP hydrolysis but significantly inhibits the client-protein stimulated rate. Similarly, FKBP59 has little effect on the basal rate but stimulates the client-protein stimulated rate further. In contrast, p23 inhibits both the basal and stimulated rates of ATP hydrolysis. Our results show that the ATPase activity of human Hsp90 is highly regulated by both client protein and co-chaperone binding. We suggest that the rate of ATP hydrolysis is critical to the mode of action of Hsp90, consistent with results that have shown that both over and under-active ATPase mutants of yeast Hsp90 have impaired function in vivo. We suggest that the tight regulation of the ATPase activity of Hsp90 is important and allows the client protein to remain bound to Hsp90 for sufficient time for activation to occur.

The action of molecular chaperones in the early secretory pathway

The endoplasmic reticulum (ER) serves as a way-station during the biogenesis of nearly all secreted proteins, and associated with or housed within the ER are factors required to catalyze their import into the ER and facilitate their folding. To ensure that only properly folded proteins are secreted and to temper the effects of cellular stress, the ER can target aberrant proteins for degradation and/or adapt to the accumulation of misfolded proteins. Molecular chaperones play critical roles in each of these phenomena.

HSP90 as a new therapeutic target for cancer therapy: the story unfolds

Current anticancer drug development strategies involve identifying novel molecular targets which are crucial for tumourigenesis. The molecular chaperone heat shock protein (HSP) 90 is of interest as an anticancer drug target because of its importance in maintaining the conformation, stability and function of key oncogenic client proteins involved in signal transduction pathways leading to proliferation, cell cycle progression and apoptosis, as well as other features of the malignant phenotype such as invasion, angiogenesis and metastasis. The natural product HSP90 inhibitors geldanamycin and radicicol exert their antitumour effect by inhibiting the intrinsic ATPase activity of HSP90, resulting in degradation of HSP90 client proteins via the ubiquitin proteosome pathway. Anticancer selectivity may derive from the simultaneous combinatorial effects of HSP90 inhibitors on multiple cancer targets and pathways. 17-allylamino, 17-demethoxygeldanamycin (17AAG), a geldanamycin derivative, showed good activity and cancer selectivity in preclinical models and has now progressed to Phase I clinical trial in cancer patients with encouraging initial results. Phase II trials including combination studies with cytotoxic agents are now being planned and these should allow the therapeutic activity of 17AAG to be determined. Second generation HSP90 inhibitors may be designed to overcome some of the drawbacks of 17AAG, including limited oral bioavailability and solubility. They could also be engineered to target specific functions of HSP90, which may not only provide greater molecular selectivity and clinical benefit but may also increase understanding of the complex functions of this molecular chaperone. HSP90 inhibitors provide proof of concept for drugs directed at HSP90 and protein folding and this principle may be applicable to other medical conditions involving protein aggregation and stability.