Binding of the viral immunogenic octapeptide VSV8 to native glucose-regulated protein Grp94 (gp96) and its inhibition by the physiological ligands ATP and Ca2+

Ca2+ transport and signalling in enamel cells

Dental enamel is one of the most remarkable examples of matrix-mediated biomineralization. Enamel crystals form de novo in a rich extracellular environment in a stage-dependent manner producing complex microstructural patterns that are visually stunning. This process is orchestrated by specialized epithelial cells known as ameloblasts which themselves undergo striking morphological changes, switching function from a secretory role to a cell primarily engaged in ionic transport. Ameloblasts are supported by a host of cell types which combined represent the enamel organ. Fully mineralized enamel is the hardest tissue found in vertebrates owing its properties partly to the unique mixture of ionic species represented and their highly organized assembly in the crystal lattice. Among the main elements found in enamel, Ca2+ is the most abundant ion, yet how ameloblasts modulate Ca2+ dynamics remains poorly known. This review describes previously proposed models for passive and active Ca2+ transport, the intracellular Ca2+ buffering systems expressed in ameloblasts and provides an up-dated view of current models concerning Ca2+ influx and extrusion mechanisms, where most of the recent advances have been made. We also advance a new model for Ca2+ transport by the enamel organ.

Exogenous heat shock protein gp96 ameliorates CD4+CD62L+ T-cell-mediated transfer colitis

BACKGROUND

The heat shock protein gp96 is an endoplasmic reticulum chaperone involved in endoplasmic reticulum stress reactions. gp96 binds antigens and is secreted into extracellular space on cell stress. After reinternalization by antigen presenting cells, antigens can be transferred to major histocompatibility complex molecules. In recent studies, we found induction of gp96 during differentiation of intestinal macrophages, whereas it was absent in intestinal macrophages of patients with Crohn's disease.

METHODS

To study immuno-modulating effects of gp96 in T-cell transfer colitis BALB/c donor mice were injected with 2 × 100 μg gp96. After 1 week, 2.5 × 10(5) CD4+CD62L+ cells were isolated from spleens and injected into severe combined immunodeficiency recipients. Another group received cells from untreated donors and was treated with 100 μg gp96 after transfer. Control groups received cells from untreated donors, or buffer alone.

RESULTS

After transfer of CD4+CD62L+ T cells from gp96-pretreated donors, mice (TBT gp96) showed an initial weight loss, but after 3 weeks, they recovered and reached the starting weight after 5 weeks. Mice treated with gp96 after transfer (TAT gp96) showed a delayed weight loss in comparison with the CD4+CD62L+ group. The histological scores in CD4CD62L mice were 2.6 ± 0.1, in TBT gp96 mice 1.3 ± 0.3 (CD4+CD62L+ versus TBT gp96: P < 0.05) and in mice treated after transfer 1.9 ± 0.1 (CD4+CD62L+ versus TAT gp96: P < 0.05).

CONCLUSIONS

These findings indicate an essential role of gp96 in the maintenance of tolerance against luminal antigens in the intestinal mucosa. The absence of gp96 in intestinal macrophages of patients with Crohn's disease might provoke loss of this tolerance mediating mechanism.

Secreted heat shock protein gp96-Ig: next-generation vaccines for cancer and infectious diseases

Over the past decade, our laboratory has developed a secreted heat shock protein (HSP), chaperone gp96, cell-based vaccine that generates effective anti-tumor and anti-infectious immunity in vivo. Gp96-peptide complexes were identified as an extremely efficient stimulator of MHC I-mediated antigen cross-presentation, generating CD8 cytotoxic T-lymphocyte responses detectable in blood, spleen, gut and reproductive tract to femto-molar concentrations of antigen. These studies provided the first evidence that cell-based gp96-Ig-secreting vaccines may serve as a potent modality to induce both systemic and mucosal immunity. This approach takes advantage of the combined adjuvant and antigen delivery capacity of gp96 for the generation of cytotoxic immunity against a wide range of antigens in both anti-vial and anti-cancer vaccination. Here, we review the vaccine design that utilizes the unique property/ability of endoplasmic HSP gp96 to bind antigenic peptides and deliver them to antigen-presenting cells.

The role of heat shock proteins in antigen cross presentation

Heat shock proteins (HSPs) are molecular chaperones that bind tumor antigens and mediate their uptake into antigen presenting cells. HSP–antigen complexes are then directed toward either the MHC class I pathway through antigen cross presentation or the conventional class II pathway, leading to activation of T cell subsets. Uptake of HSP-chaperoned polypeptides can involve both receptor-mediated and receptor-independent routes, and mechanisms of antigen sorting between the Class I and II pathways after uptake are currently under investigation. The processes involved in internalization of HSP–antigen complexes differ somewhat from the mechanisms previously determined for (unchaperoned) particulate and free soluble antigens. A number of studies show that HSP-facilitated antigen cross presentation requires uptake of the complexes by scavenger receptors (SR) followed by processing in the proteasome, and loading onto MHC class I molecules. In this review we have examined the roles of HSPs and SR in antigen uptake, sorting, processing, cell signaling, and activation of innate and adaptive immunity.

Conformational dynamics of the molecular chaperone Hsp90

The ubiquitous molecular chaperone Hsp90 makes up 1-2% of cytosolic proteins and is required for viability in eukaryotes. Hsp90 affects the folding and activation of a wide variety of substrate proteins including many involved in signaling and regulatory processes. Some of these substrates are implicated in cancer and other diseases, making Hsp90 an attractive drug target. Structural analyses have shown that Hsp90 is a highly dynamic and flexible molecule that can adopt a wide variety of structurally distinct states. One driving force for these rearrangements is the intrinsic ATPase activity of Hsp90, as seen with other chaperones. However, unlike other chaperones, studies have shown that the ATPase cycle of Hsp90 is not conformationally deterministic. That is, rather than dictating the conformational state, ATP binding and hydrolysis only shift the equilibria between a pre-existing set of conformational states. For bacterial, yeast and human Hsp90, there is a conserved three-state (apo-ATP-ADP) conformational cycle; however; the equilibria between states are species specific. In eukaryotes, cytosolic co-chaperones regulate the in vivo dynamic behavior of Hsp90 by shifting conformational equilibria and affecting the kinetics of structural changes and ATP hydrolysis. In this review, we discuss the structural and biochemical studies leading to our current understanding of the conformational dynamics of Hsp90, as well as the roles that nucleotide, co-chaperones, post-translational modification and substrates play. This view of Hsp90's conformational dynamics was enabled by the use of multiple complementary structural methods including, crystallography, small-angle X-ray scattering (SAXS), electron microscopy, Förster resonance energy transfer (FRET) and NMR. Finally, we discuss the effects of Hsp90 inhibitors on conformation and the potential for developing small molecules that inhibit Hsp90 by disrupting the conformational dynamics.

Organellar calcium buffers

Ca(2+) is an important intracellular messenger affecting many diverse processes. In eukaryotic cells, Ca(2+) storage is achieved within specific intracellular organelles, especially the endoplasmic/sarcoplasmic reticulum, in which Ca(2+) is buffered by specific proteins known as Ca(2+) buffers. Ca(2+) buffers are a diverse group of proteins, varying in their affinities and capacities for Ca(2+), but they typically also carry out other functions within the cell. The wide range of organelles containing Ca(2+) and the evidence supporting cross-talk between these organelles suggest the existence of a dynamic network of organellar Ca(2+) signaling, mediated by a variety of organellar Ca(2+) buffers.

Re-examination of CD91 function in GRP94 (glycoprotein 96) surface binding, uptake, and peptide cross-presentation

GRP94 (gp96)-peptide complexes can be internalized by APCs and their associated peptides cross-presented to yield activation of CD8(+) T cells. Investigations into the identity (or identities) of GRP94 surface receptors have yielded conflicting results, particularly with respect to CD91 (LRP1), which has been proposed to be essential for GRP94 recognition and uptake. To assess CD91 function in GRP94 surface binding and endocytosis, these parameters were examined in mouse embryonic fibroblast (MEF) cell lines whose expression of CD91 was either reduced via RNA interference or eliminated by genetic disruption of the CD91 locus. Reduction or loss of CD91 expression abrogated the binding and uptake of receptor-associated protein, an established CD91 ligand. Surface binding and uptake of an N-terminal domain of GRP94 (GRP94.NTD) was unaffected. GRP94.NTD surface binding was markedly suppressed after treatment of MEF cell lines with heparin, sodium chlorate, or heparinase II, demonstrating that heparin sulfate proteoglycans can function in GRP94.NTD surface binding. The role of CD91 in the cross-presentation of GRP94-associated peptides was examined in the DC2.4 dendritic cell line. In DC2.4 cells, which express CD91, GRP94.NTD-peptide cross-presentation was insensitive to the CD91 ligands receptor-associated protein or activated α(2)-macroglobulin and occurred primarily via a fluid-phase, rather than receptor-mediated, uptake pathway. These data clarify conflicting data on CD91 function in GRP94 surface binding, endocytosis, and peptide cross-presentation and identify a role for heparin sulfate proteoglycans in GRP94 surface binding.

Efficient cross-priming of antiviral CD8+ T cells by antigen donor cells is GRP94 independent

Cross-priming, the activation of naive CD8+ T cells by dendritic cells presenting Ags synthesized by other cells, is believed to play an important role in the generation of antiviral and antitumor responses. The molecular mechanism(s) underlying cross-priming remain poorly defined and highly controversial. GRP94 (gp96), an abundant endoplasmic reticulum chaperone with innate immune-activating capacity, has been widely reported to play a major role in cross-priming. In this study, we show that cells whose expression of GRP94 is silenced via transient or stable transfection with GRP94-directed small interfering RNAs demonstrate no reduction in their abilities to generate class I peptide complexes in cultured cells or to prime antiviral CD8+ T cell responses in vivo. In demonstrating the dispensability of GRP94, our finding points to the importance of alternative mechanisms for generation of class I peptide complexes from endogenous and exogenous Ags and immunogens.

The exception that reinforces the rule: crosspriming by cytosolic peptides that escape degradation

The nature of crosspriming immunogens for CD8(+) T cell responses is highly controversial. By using a panel of T cell receptor-like antibodies specific for viral peptides bound to mouse D(b) major histocompatibility complex class I molecules, we show that an exceptional peptide (PA(224-233)) expressed as a viral minigene product formed a sizeable cytosolic pool continuously presented for hours after protein synthesis was inhibited. PA(224-233) pool formation required active cytosolic heat-shock protein 90 but not ER g96 and uniquely enabled crosspriming by this peptide. These findings demonstrate that exceptional class I binding oligopeptides that escape proteolytic degradation are potent crosspriming agents. Thus, the feeble immunogenicity of natural proteasome products in crosspriming can be attributed to their evanescence in donor cells and not an absolute inability of cytosolic oligopeptides to be transferred to and presented by professional antigen-presenting cells.

Secreted heat shock protein gp96-Ig: an innovative vaccine approach

Heat shock proteins (HSPs) are a large family of proteins with different molecular weights and different intracellular localizations. These proteins undertake crucial functions in maintaining cell homeostasis, and therefore they have been conserved during evolution. HSP gp96 also known as glucose-regulated protein grp94, is the primary chaperone of the endoplasmatic reticulum. Gp96/grp94, because of its peptide chaperone capacity and its ability to interact actively with professional antigen-presenting cells (APCs), is also endowed with crucial immunological functions such as natural adjuvant for priming innate and adaptive immunity. To make gp96 accessible to the immune system without biochemical purification and without cell lysis, we generated a secreted form of gp96. The immunological properties of secreted gp96 and its implications for vaccine in human cancer and infectious diseases will be discussed.

Allogeneic tumor-cell-based vaccines secreting endoplasmic reticulum chaperone gp96

Heat-shock proteins are chaperones for proteins including tumor antigens. Heat-shock protein gp96, also known as glucose-regulated protein grp94, is the primary chaperone of the endoplasmic reticulum and a natural adjuvant for priming the innate and adaptive immune system. By transfecting tumor cells with a genetically modified secretory form of gp96, the tumor cells are transformed into vaccine cells. Gp96 vaccines in murine studies trigger robust innate and antigen-specific cellular immune responses and cause tumor rejection followed by long-lasting tumor immunity. The authors briefly review here the generation of cytotoxic T lymphocyte responses by gp96 and the most up to date clinical data in the use of gp96-based cancer vaccines.

The peptide-binding activity of GRP94 is regulated by calcium

GRP94 (glucose-regulated protein of 94 kDa) is a major luminal constituent of the endoplasmic reticulum with known high capacity for calcium in vivo and a peptide-binding activity in vitro. In the present study, we show that Ca2+ regulates the ability of GRP94 to bind peptides. This effect is due to a Ca2+-binding site located in the charged linker domain of GRP94, which, when occupied, enhances the association of peptides with the peptide-binding site in the N-terminal domain of the protein. We further show that grp94-/- cells are hypersensitive to perturbation of intracellular calcium and thus GRP94 is important for cellular Ca2+ storage.

A glycoproteome database of normal human liver tissue

PURPOSE

To extensively investigate the glycoproteins of normal human liver tissue, constructing the glycoprotein profile and database of the normal human liver tissue.

METHODS

The total proteins were extracted from the normal human liver tissue and then subjected to two-dimensional electrophoresis (2-DE). Finally, 2-DE gels were stained according to the methods of multiplexed proteomics (MP) technology. Glycoprotein spots were excised from 2-DE gel and then characterized by matrix assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF-MS).

RESULTS

The PDQuest software detected 1,011 glycoprotein spots and 1,923 total protein spots in the 2-DE gels of sample from the normal human liver tissue. Furthermore, 116 species of glycoproteins were successfully identified via peptide mass profiling using MALDI-TOF-MS/MS and annotated to our databases. In addition, we also applied bioinformatics softwares to predict N- or O-glycosylation sites of identified glycoproteins.

CONCLUSION

This study demonstrates the feasibility of a novel technological platform to contruct glycoprotein databases. These results lay the foundation for future physiological and pathological studies of the human liver.

Concerted mutation of Phe residues belonging to the ?-dystroglycan ectodomain strongly inhibits the interaction with ?-dystroglycan in�vitro

The dystroglycan adhesion complex consists of two noncovalently interacting proteins: alpha-dystroglycan, a peripheral extracellular subunit that is extensively glycosylated, and the transmembrane beta-dystroglycan, whose cytosolic tail interacts with dystrophin, thus linking the F-actin cytoskeleton to the extracellular matrix. Dystroglycan is thought to play a crucial role in the stability of the plasmalemma, and forms strong contacts between the extracellular matrix and the cytoskeleton in a wide variety of tissues. Abnormal membrane targeting of dystroglycan subunits and/or their aberrant post-translational modification are often associated with several pathologic conditions, ranging from neuromuscular disorders to carcinomas. A putative functional hotspot of dystroglycan is represented by its intersubunit surface, which is contributed by two amino acid stretches: approximately 30 amino acids of beta-dystroglycan (691-719), and approximately 15 amino acids of alpha-dystroglycan (550-565). Exploiting alanine scanning, we have produced a panel of site-directed mutants of our two consolidated recombinant peptides beta-dystroglycan (654-750), corresponding to the ectodomain of beta-dystroglycan, and alpha-dystroglycan (485-630), spanning the C-terminal domain of alpha-dystroglycan. By solid-phase binding assays and surface plasmon resonance, we have determined the binding affinities of mutated peptides in comparison to those of wild-type alpha-dystroglycan and beta-dystroglycan, and shown the crucial role of two beta-dystroglycan phenylalanines, namely Phe692 and Phe718, for the alpha-beta interaction. Substitution of the alpha-dystroglycan residues Trp551, Phe554 and Asn555 by Ala does not affect the interaction between dystroglycan subunits in vitro. As a preliminary analysis of the possible effects of the aforementioned mutations in vivo, detection through immunofluorescence and western blot of the two dystroglycan subunits was pursued in dystroglycan-transfected 293-Ebna cells.