Distinct distribution of protein disulfide isomerase family proteins in rat tissues

Protein disulfide isomerase as a prosurvival factor in cell therapy for muscular and vascular diseases

Background

Cell therapy for degenerative diseases aims at rescuing tissue damage by delivery of precursor cells. Thus far, this strategy has been mostly unsuccessful due to massive loss of donor cells shortly after transplantation. Several strategies have been applied to increase transplanted cell survival but only with limited success. The endoplasmic reticulum (ER) is an organelle involved in protein folding, calcium homeostasis, and lipid biosynthesis. Protein disulfide isomerase (PDI) is a molecular chaperone induced and activated by ER stress. PDI is induced by hypoxia in neuronal, cardiac, and endothelial cells, supporting increased cell survival to hypoxic stress and protection from apoptosis in response to ischemia.

Methods

We achieved ex vivo PDI gene transfer into luciferase-expressing myoblasts and endothelial cells. We assessed cell engraftment upon intramuscular transplantation into a mouse model of Duchenne muscular dystrophy (mdx mouse) and into a mouse model of ischemic disease.

Results

We observed that loss of full-length dystrophin expression in mdx mice muscle leads to an increase of PDI expression, possibly in response to augmented ER protein folding load. Moreover, we determined that overexpression of PDI confers a survival advantage for muscle cells in vitro and in vivo to human myoblasts injected into murine dystrophic muscle and to endothelial cells administered upon hindlimb ischemia damage, improving the therapeutic outcome of the cell therapy treatment.

Conclusions

Collectively, these results suggest that overexpression of PDI may protect transplanted cells from hypoxia and other possibly occurring ER stresses, and consequently enhance their regenerative properties.

Biosynthesis, structure, and folding of the insulin precursor protein

Insulin synthesis in pancreatic β-cells is initiated as preproinsulin. Prevailing glucose concentrations, which oscillate pre- and postprandially, exert major dynamic variation in preproinsulin biosynthesis. Accompanying upregulated translation of the insulin precursor includes elements of the endoplasmic reticulum (ER) translocation apparatus linked to successful orientation of the signal peptide, translocation and signal peptide cleavage of preproinsulin-all of which are necessary to initiate the pathway of proper proinsulin folding. Evolutionary pressures on the primary structure of proinsulin itself have preserved the efficiency of folding ("foldability"), and remarkably, these evolutionary pressures are distinct from those protecting the ultimate biological activity of insulin. Proinsulin foldability is manifest in the ER, in which the local environment is designed to assist in the overall load of proinsulin folding and to favour its disulphide bond formation (while limiting misfolding), all of which is closely tuned to ER stress response pathways that have complex (beneficial, as well as potentially damaging) effects on pancreatic β-cells. Proinsulin misfolding may occur as a consequence of exuberant proinsulin biosynthetic load in the ER, proinsulin coding sequence mutations, or genetic predispositions that lead to an altered ER folding environment. Proinsulin misfolding is a phenotype that is very much linked to deficient insulin production and diabetes, as is seen in a variety of contexts: rodent models bearing proinsulin-misfolding mutants, human patients with Mutant INS-gene-induced Diabetes of Youth (MIDY), animal models and human patients bearing mutations in critical ER resident proteins, and, quite possibly, in more common variety type 2 diabetes.

Infection with the gastrointestinal nematode Ostertagia ostertagi in cattle affects mucus biosynthesis in the abomasum

The mucus layer in the gastrointestinal (GI) tract is considered to be the first line of defense to the external environment. Alteration in mucus components has been reported to occur during intestinal nematode infection in ruminants, but the role of mucus in response to abomasal parasites remains largely unclear. The aim of the current study was to analyze the effects of an Ostertagia ostertagi infection on the abomasal mucus biosynthesis in cattle. Increased gene expression of MUC1, MUC6 and MUC20 was observed, while MUC5AC did not change during infection. Qualitative changes of mucins, related to sugar composition, were also observed. AB-PAS and HID-AB stainings highlighted a decrease in neutral and an increase in acidic mucins, throughout the infection. Several genes involved in mucin core structure synthesis, branching and oligomerization, such as GCNT3, GCNT4, A4GNT and protein disulphide isomerases were found to be upregulated. Increase in mucin fucosylation was observed using the lectin UEA-I and through the evaluation of fucosyltransferases gene expression levels. Finally, transcription levels of 2 trefoil factors, TFF1 and TFF3, which are co-expressed with mucins in the GI tract, were also found to be significantly upregulated in infected animals. Although the alterations in mucus biosynthesis started early during infection, the biggest effects were found when adult worms were present on the surface of the abomasal mucosa and are likely caused by the alterations in mucosal cell populations, characterized by hyperplasia of mucus secreting cells.

Differential cooperative enzymatic activities of protein disulfide isomerase family in protein folding

Endoplasmic reticulum (ER)p61, ERp72, and protein disulfide isomerase (PDI), which are members of the PDI family protein, are ubiquitously present in mammalian cells and are thought to participate in disulfide bond formation and isomerization. However, why the 3 different members need to be colocalized in the ER remains an enigma. We hypothesized that each PDI family protein might have different modes of enzymatic activity in disulfide bond formation and isomerization. We purified PDI, ERp61, and ERp72 proteins from rat liver microsomes and compared the effects of each protein on the folding of bovine pancreatic trypsin inhibitor (BPTI). ERp61 and ERp72 accelerated the initial steps more efficiently than did PDI. ERp61 and ERp72, however, accelerated the rate-limiting step less efficiently than did PDI. PDI or ERp72 did not impede the folding of BPTI by each other but rather catalyzed the folding reaction cooperatively with each other. These data suggest that differential enzymatic activities of ERp proteins and PDI represent a complementary contribution of these enzymes to protein folding in the ER.

Purification and biochemical characterization of native ERp29 from rat liver

ERp29 is a recently characterized resident of the ER (endoplasmic reticulum) lumen that has broad biological significance, being expressed ubiquitously and abundantly in animal cells. As an apparent housekeeper, ERp29 is thought to be a general folding assistant for secretory proteins and to probably function as a PDI (protein disulphide isomerase)-like molecular chaperone. In the present paper, we report the first purification to homogeneity and direct functional analysis of native ERp29, which has led to the unexpected finding that ERp29 lacks PDI-like folding activities. ERp29 was purified 4800-fold in non-denaturing conditions exploiting an unusual affinity for heparin. Two additional biochemical hallmarks that will assist the classification of ERp29 homologues were identified, namely the idiosyncratic behaviours of ERp29 on size-exclusion chromatography (M(r)<globular homodimer) and SDS/PAGE (M(r)>monomeric mass). In contrast with PDI and parallel-purified co-residents (calreticulin, ERp60), native ERp29 lacked classical chaperone, disulphide reductase and isomerase, and calcium-binding activities. In the chaperone assays, ERp29 neither protected substrate proteins against thermal aggregation nor interacted stably with chemically denatured proteins as detected by cross-linking. ERp29 also did not exhibit helper activity toward calreticulin (chaperone) or PDI and ERp60 (disulphide reductase). By refuting long-standing predictions about chaperone activity, these results expose ERp29 as a functionally distinct member of the ER machinery and prompt a revised hypothesis that ERp29 acts as a non-classical folding assistant. The native preparation and biochemical hallmarks established here provide a useful foundation for ongoing efforts to resolve the functional orphan status of ERp29.

The thioredoxin-like fold: hidden domains in protein disulfide isomerases and other chaperone proteins

Although protein disulphide isomerase (PDI) has been known for nearly 40 years, several new PDIs have recently been described that reveal a remarkable diversity in both structure and function. This article reviews our current knowledge of the PDI family members and identifies four novel PDIs in the human genome. These include human transmembrane proteins that have C. elegans or Drosophila orthologues for which a developmental role has been proven. Their role in development, together with other functional roles for PDIs such as conferring resistance to apoptosis under hypoxia and a potential role in the oxygen-sensing apparatus are discussed.

Annexin V, annexin VI, S100A1 and S100B in developing and adult avian skeletal muscles

Annexins and S100 proteins constitute two multigenic families of Ca2+-modulated proteins that have been implicated in the regulation of both intracellular and extracellular activities. Some annexins can interact with certain S100 protein dimers thereby forming heterotetramers in which an S100 dimer crosslinks two copies of the partner annexin. It is suggested that S100 protein binding to an annexin might serve the function of regulating annexin function and annexin binding to an S100 protein might regulate S100 function. In the present study, annexin V, annexin VI (or ANXA5 and ANXA6, respectively, according to a novel nomenclature), S100A1 and S100B were analyzed for their subcellular localization in developing and adult avian skeletal muscles by confocal laser scanning microscopy, immunogold cytochemistry, and western blotting, and for their ability to form annexin-S100 heterocomplex in vivo by immunoprecipitation. These four proteins displayed distinct expression patterns, ANXA5 being the first to be expressed in myotubes (i.e. at embryonic day 8), followed by ANXA6 (at embryonic day 12) and S100A1 and S100B (between embryonic day 12 and embryonic day 15). The two annexins and the two S100 proteins were found associated to different extents with the sarcolemma, membranes of the sarcoplasmic reticulum, and putative transverse tubules where they appeared to be co-localized from embryonic day 18 onwards. No one of these proteins was found associated with the contractile apparatus of the sarcomeres. Immunoprecipitation studies indicated that ANXA6/S100A1 and ANXA6/S100B complexes formed in vivo. Whereas, ANXA5 was not recovered in S100A1 or S100B immunoprecipitates. From our data we suggest that: (i) ANXA5 and ANXA6, and S100A1 and S100B can be used as markers of skeletal muscle development; (ii) ANXA6 and S100A1 and S100B appear strategically located close to or on skeletal muscle membrane organelles that are critically involved in the regulation of Ca2+ fluxes, thus supporting previous in vitro observations implicating S100A1 and ANXA6 in the stimulation of Ca2+-induced Ca2+ release; and (iii) ANXA6/S100A1 and ANXA6/S100B complexes can form in vivo thereby regulating each other activities and/or acting in concert to regulate membrane-associated activities.

Protein-disulfide isomerase is a component of an NBD-cholesterol monomerizing protein complex from hamster small intestine

A rapid in vitro assay was developed for monitoring protein-mediated cholesterol monomerization from bile acid aggregates. This assay uses a fluorescent cholesterol analog, 22-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-23,24-bisnor-5-cholen-3 beta-ol (NBD-cholesterol), which was shown to be absorbed by hamster in a fashion similar to cholesterol. The fluorescence of aggregates of NBD-cholesterol was strongly quenched in 2.5 mM of taurocholic acid. Addition of proteins from enterocytes of hamster small intestine led to a time- and dose-dependent dequenching of NBD-cholesterol fluorescence. Comparable dequenching can be detected with SDS and appears to involve monomerization of the NBD-cholesterol. Purification of enterocyte extract by sequential chromatography revealed an approximately 140-kDa protein complex (p140) able to mediate the monomerization of NBD-cholesterol. Each p140 complex mediated monomerization of 2.7 NBD-cholesterol molecules. The p140 complex appeared to be formed by dimerization of two approximately 58-kDa molecules since SDS-PAGE revealed a single dominant band at 58 kDa (p58). Protein sequence analysis suggested that p58 is protein-disulfide isomerase (PDI), and this conclusion was confirmed by cloning of hamster PDI, and detection of PDI enzyme activity in the purified fraction. Additional studies with either pure PDI or lysates of cells transfected with hamster PDI showed that PDI by itself was not sufficient for monomerizing cholesterol. Further, despite a similar mobility on SDS-PAGE (approximately 58 kDa), the p140 complex appeared approximately 45-kDa larger than pure PDI (approximately 95 kDa) when analyzed by a gel-filtration chromatography. The p140 complex may thus contain an unidentified molecule(s) in addition to PDI that may contribute importantly to cholesterol monomerization.

ERp29 is a ubiquitous resident of the endoplasmic reticulum with a distinct role in secretory protein production

ERp29 was recently characterized biochemically as a novel protein that resides in mammalian endoplasmic reticulum (ER). Here we applied immunochemical procedures at the cellular level to investigate the hypothesized role of ERp29 in secretory protein production. ERp29 was localized exclusively to the ER/nuclear envelope of MDCK cells using confocal immunocytochemistry and comparative markers of the ER lumen, ER/Golgi membrane, nuclei, and mitochondria. A predominant association with rough ER was revealed by sucrose-gradient analysis of rat liver microsomes. Immunohistochemistry showed ERp29 expression in 35 functionally distinct cell types of rat, establishing ERp29 as a general ER marker. The ERp29 expression profile largely paralleled that of protein disulfide isomerase (PDI), the closest relative of ERp29, consistent with a role in secretory protein production. However strikingly different ERp29/PDI ratios were observed in various cell types, suggesting independent regulation and functional roles. Together, these findings associate ERp29 primarily with the early stages of secretory protein production and implicate ERp29 in a distinct functional role that is utilized in most cells. Our identification of several ERp29-enriched cell types suggests a potential selectivity of ERp29 for non-collagenous substrates and provides a physiological foundation for future investigations.

Specificity in substrate binding by protein folding catalysts: tyrosine and tryptophan residues are the recognition motifs for the binding of peptides to the pancreas-specific protein disulfide isomerase PDIp

Using a cross-linking approach, we recently demonstrated that radiolabeled peptides or misfolded proteins specifically interact in vitro with two luminal proteins in crude extracts from pancreas microsomes. The proteins were the folding catalysts protein disulfide isomerase (PDI) and PDIp, a glycosylated, PDI-related protein, expressed exclusively in the pancreas. In this study, we explore the specificity of these proteins in binding peptides and related ligands and show that tyrosine and tryptophan residues in peptides are the recognition motifs for their binding by PDIp. This peptide-binding specificity may reflect the selectivity of PDIp in binding regions of unfolded polypeptide during catalysis of protein folding.

Enzymatic activation of immunoglobulin binding factor in female reproductive tract

Human seminal plasma and cervical mucus contains an immunoglobulin binding factor (IgBF) which interacts with IgG as monomers under reducing condition. It may play a role in preventing antibody production against allogeneic sperms in the female reproductive tract. However, since IgBF is secreted as a homodimer that does not bind IgG, in vivo activation systems should be investigated. GSH reduces the inactive native dimer to the active monomer. Protein disulfide isomerase (PDI), a molecular chaperone, alters the configuration of dimers to active monomers. 20S proteasomes produced by activated T cells which cleave the dimers in the presence of GSH to active fragments. All these activating systems are widely distributed as cellular enzymes in vivo. Also PDI mRNAs are expressed in uterine cervix, endometrium and fallopian tube. Since these enzymes are produced upon stimulation by the immune system, we hypothesize that immunocompetent cells interact with allogeneic sperms, leading to the local production of these enzymes that will activate IgBF.

Elevated expression of PDI family proteins during differentiation of mouse F9 teratocarcinoma cells

We investigated the expression of protein disulfide isomerase family proteins (PDI, ERp61, and ERp72) in mouse F9 teratocarcinoma cells during differentiation induced by treatment with retinoic acid and dibutyryl cAMP. Each member of this family was expressed at a constitutive level in undifferentiated F9 cells. During differentiation of F9 cells to parietal or visceral endodermal cells the protein level of all these enzymes increased, although the extent of this increase in both protein and mRNA levels varied among the enzymes. Certain proteins were found to be coimmunoprecipitated with PDI, ERp61, and ERp72 in the presence of a chemical crosslinker. Type IV collagen was significantly coprecipitated with PDI whereas laminin was equally coprecipitated with the three proteins. Furthermore, 210 kDa protein characteristically coprecipitated with ERp72. Thus, the induction of PDI family proteins during the differentiation of F9 cells and their association with different proteins may implicate specific functions of each member of this family despite the common redox activity capable of catalyzing the disulfide bond formation.

NF-Y, a CCAAT box-binding protein, is one of the trans-acting factors necessary for the response of the murine ERp72 gene to protein traffic

The accumulation of incompletely assembled immunoglobulin mu heavy chain in transfected COS cells stimulates the cellular response to protein traffic that results in the increased transcription and elevated synthesis of several ER chaperones, including ERP72, a member of the protein disulfide isomerase family of molecular chaperones. The ERp72 promoter contains an 82 bp ER protein traffic response element (ERPTRE) that is sufficient to mediate this response. Previously, it had been shown that the alteration of a putative AP-2 site and a CCAAT and inverted CCAAT site within the ERPTRE significantly decreased the response of ERp72 promoter to mu chain accumulation. We have extended these findings by demonstrating a role for NF-Y and a potentially novel DNA-binding protein in the regulation of transcription from the ERp72 promoter. The fact that NF-Y binding to the ERPTRE is observed in extracts from both control cells and cells in which the response to protein traffic has been activated indicates that the binding of NF-Y, while necessary, is not sufficient to account for the response. Each of the two CCAAT sites in the ERPTRE can bind NF-Y independently, but both sites must be intact for full ERPTRE function. A second protein can bind to the ERPTRE independently of NF-Y and at a site overlapping or close to the 3' end of the reverse CCAAT site. It is possible that interactions between NF-Y, this protein and perhaps other factors are responsible for the regulation of the protein traffic response.