Protein disulfide isomerase: a multifunctional protein of the endoplasmic reticulum

Updates on Versatile Role of Putative Gasotransmitter Nitric Oxide: Culprit in Neurodegenerative Disease Pathology

Nitric oxide (NO) is a versatile gasotransmitter that contributes in a range of physiological and pathological mechanims depending on its cellular levels. An appropriate concentration of NO is essentially required for cellular physiology; however, its increased level triggers pathological mechanisms like altered cellular redox regulation, functional impairment of mitochondrion, and modifications in cellular proteins and DNA. Its increased levels also exhibit post-translational modifications in protein through S-nitrosylation of their thiol amino acids, which critically affect the cellular physiology. Along with such modifications, NO could also nitrosylate the endoplasmic reticulum (ER)-membrane located sensors of ER stress, which subsequently affect the cellular protein degradation capacity and lead to aggregation of misfolded/unfolded proteins. Since protein aggregation is one of the pathological hallmarks of neurodegenerative disease, NO should be taken into account during development of disease therapies. In this Review, we shed light on the diverse role of NO in both cellular physiology and pathology and discussed its involvement in various pathological events in the context of neurodegenerative diseases.

GPR55 controls functional differentiation of self-renewing epithelial progenitors for salivation

GPR55, a lipid-sensing receptor, is implicated in cell cycle control, malignant cell mobilization, and tissue invasion in cancer. However, a physiological role for GPR55 is virtually unknown for any tissue type. Here, we localize GPR55 to self-renewing ductal epithelial cells and their terminally differentiated progeny in both human and mouse salivary glands. Moreover, we find GPR55 expression downregulated in salivary gland mucoepidermoid carcinomas and GPR55 reinstatement by antitumor irradiation, suggesting that GPR55 controls renegade proliferation. Indeed, GPR55 antagonism increases cell proliferation and function determination in quasiphysiological systems. In addition, Gpr55-/- mice present ~50% enlarged submandibular glands with many more granulated ducts, as well as disordered endoplasmic reticuli and with glycoprotein content. Next, we hypothesized that GPR55 could also modulate salivation and glycoprotein content by entraining differentiated excretory progeny. Accordingly, GPR55 activation facilitated glycoprotein release by itself, inducing low-amplitude Ca2+ oscillations, as well as enhancing acetylcholine-induced Ca2+ responses. Topical application of GPR55 agonists, which are ineffective in Gpr55-/- mice, into adult rodent submandibular glands increased salivation and saliva glycoprotein content. Overall, we propose that GPR55 signaling in epithelial cells ensures both the life-long renewal of ductal cells and the continuous availability of saliva and glycoproteins for oral health and food intake.

Proteomic responses of drought-tolerant and drought-sensitive cotton varieties to drought stress

Drought, one of the most widespread factors reducing agricultural crop productivity, affects biological processes such as development, architecture, flowering and senescence. Although protein analysis techniques and genome sequencing have made facilitated the proteomic study of cotton, information on genetic differences associated with proteomic changes in response to drought between different cotton genotypes is lacking. To determine the effects of drought stress on cotton seedlings, we used two-dimensional polyacrylamide gel electrophoresis (2-DE) and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry to comparatively analyze proteome of drought-responsive proteins during the seedling stage in two cotton (Gossypium hirsutum L.) cultivars, drought-tolerant KK1543 and drought-sensitive Xinluzao26. A total of 110 protein spots were detected on 2-DE maps, of which 56 were identified by MALDI-TOF and MALDI-TOF/TOF mass spectrometry. The identified proteins were mainly associated with metabolism (46.4 %), antioxidants (14.2 %), and transport and cellular structure (23.2 %). Some key proteins had significantly different expression patterns between the two genotypes. In particular, 5-methyltetrahydropteroyltriglutamate-homocysteine methyltransferase, UDP-D-glucose pyrophosphorylase and ascorbate peroxidase were up-regulated in KK1543 compared with Xinluzao26. Under drought stress conditions, the vacuolar H(+)-ATPase catalytic subunit, a 14-3-3g protein, translation initiation factor 5A and pathogenesis-related protein 10 were up-regulated in KK1543, whereas ribosomal protein S12, actin, cytosolic copper/zinc superoxide dismutase, protein disulfide isomerase, S-adenosylmethionine synthase and cysteine synthase were down-regulated in Xinluzao26. This work represents the first characterization of proteomic changes that occur in response to drought in roots of cotton plants. These differentially expressed proteins may be related to biochemical pathways responsible for drought tolerance in KK1543. Although further studies are needed, this proteomic analysis underlines the role of post-translational events. The differentially expressed proteins and their corresponding genes may be used as markers for the breeding of drought tolerance in cotton.

Protein disulfide isomerase blocks CEBPA translation and is up-regulated during the unfolded protein response in AML

Deregulation of the myeloid key transcription factor CEBPA is a common event in acute myeloid leukemia (AML). We previously reported that the chaperone calreticulin is activated in subgroups of AML patients and that calreticulin binds to the stem loop region of the CEBPA mRNA, thereby blocking CEBPA translation. In this study, we screened for additional CEBPA mRNA binding proteins and we identified protein disulfide isomerase (PDI), an endoplasmic reticulum (ER) resident protein, to bind to the CEBPA mRNA stem loop region. We found that forced PDI expression in myeloid leukemic cells in fact blocked CEBPA translation, but not transcription, whereas abolishing PDI function restored CEBPA protein. In addition, PDI protein displayed direct physical interaction with calreticulin. Induction of ER stress in leukemic HL60 and U937 cells activated PDI expression, thereby decreasing CEBPA protein levels. Finally, leukemic cells from 25.4% of all AML patients displayed activation of the unfolded protein response as a marker for ER stress, and these patients also expressed significantly higher PDI levels. Our results indicate a novel role of PDI as a member of the ER stress-associated complex mediating blocked CEBPA translation and thereby suppressing myeloid differentiation in AML patients with activated unfolded protein response (UPR).

Mouse RIC-3, an endoplasmic reticulum chaperone, promotes assembly of the alpha7 acetylcholine receptor through a cytoplasmic coiled-coil domain

RIC-3 (resistant to inhibitor of cholinesterase) is a transmembrane protein, found in invertebrates and vertebrates, that modulates the surface expression of a variety of nicotinic acetylcholine receptors (nAChRs) in neurons and other cells. To understand its mechanism of action, we investigated the cellular location, transmembrane topology and cellular mechanism by which RIC-3 facilitates alpha7 assembly and surface expression in cultured mammalian cells. We show that the mouse protein is targeted to the ER by the first 31 aa which act as a cleavable signal sequence. The mature protein is a single-pass type I transmembrane protein whose N terminus resides in the lumen of the ER with the coiled-coil domain in the cytoplasm. RIC-3, which binds both unfolded and folded alpha7 subunits, facilitates the surface expression of receptor principally by promoting the folding and assembly of the alpha7 subunits in the ER into fully polymerized receptor. Functional analysis shows that facilitation of surface expression of alpha7 in mammalian cells is reduced in RIC-3 mutants lacking the signal peptide, the lumenal segment or the coiled-coil domain, but not in mutants lacking the long C-terminal region downstream of the coiled-coil domain. We show that the coiled-coil domain of mRIC-3 is not required for the interaction of mRIC-3 with alpha7, but does mediate a homotypic interaction between molecules of mRIC-3. We suggest that efficient assembly of the homomeric alpha7 nAChR may thus require mRIC-3 self-association through the cytoplasmic coiled-coil domain and suggest a model by which this may occur.

Requirements for the localization of nesprin-3 at the nuclear envelope and its interaction with plectin

The outer nuclear membrane proteins nesprin-1 and nesprin-2 are retained at the nuclear envelope through an interaction of their klarsicht/ANC-1/syne homology (KASH) domain with Sun proteins present at the inner nuclear membrane. We investigated the requirements for the localization of nesprin-3alpha at the outer nuclear membrane and show that the mechanism by which its localization is mediated is similar to that reported for the localization of nesprin-1 and nesprin-2: the last four amino acids of the nesprin-3alpha KASH domain are essential for its interaction with Sun1 and Sun2. Moreover, deletion of these amino acids or knockdown of the Sun proteins results in a redistribution of nesprin-3alpha away from the nuclear envelope and into the endoplasmic reticulum (ER), where it becomes colocalized with the cytoskeletal crosslinker protein plectin. Both nesprin-3alpha and plectin can form dimers, and dimerization of plectin is required for its interaction with nesprin-3alpha at the nuclear envelope, which is mediated by its N-terminal actin-binding domain. Additionally, overexpression of the plectin actin-binding domain stabilizes the actin cytoskeleton and prevents the recruitment of endogenous plectin to the nuclear envelope. Our studies support a model in which the actin cytoskeleton influences the binding of plectin dimers to dimers of nesprin-3alpha, which in turn are retained at the nuclear envelope through an interaction with Sun proteins.

Thiol/disulfide exchange is required for membrane fusion directed by the Newcastle disease virus fusion protein

Newcastle disease virus (NDV), an avian paramyxovirus, initiates infection with attachment of the viral hemagglutinin-neuraminidase (HN) protein to sialic acid-containing receptors, followed by fusion of viral and cell membranes, which is mediated by the fusion (F) protein. Like all class 1 viral fusion proteins, the paramyxovirus F protein is thought to undergo dramatic conformational changes upon activation. How the F protein accomplishes extensive conformational rearrangements is unclear. Since several viral fusion proteins undergo disulfide bond rearrangement during entry, we asked if similar rearrangements occur in NDV proteins during entry. We found that inhibitors of cell surface thiol/disulfide isomerase activity--5'5-dithio-bis(2-nitrobenzoic acid) (DTNB), bacitracin, and anti-protein disulfide isomerase antibody--inhibited cell-cell fusion and virus entry but had no effect on cell viability, glycoprotein surface expression, or HN protein attachment or neuraminidase activities. These inhibitors altered the conformation of surface-expressed F protein, as detected by conformation-sensitive antibodies. Using biotin maleimide (MPB), a reagent that binds to free thiols, free thiols were detected on surface-expressed F protein, but not HN protein. The inhibitors DTNB and bacitracin blocked the detection of these free thiols. Furthermore, MPB binding inhibited cell-cell fusion. Taken together, our results suggest that one or several disulfide bonds in cell surface F protein are reduced by the protein disulfide isomerase family of isomerases and that F protein exists as a mixture of oxidized and reduced forms. In the presence of HN protein, only the reduced form may proceed to refold into additional intermediates, leading to the fusion of membranes.

Protein disulphide isomerase mediates platelet aggregation and secretion

Platelet surface thiols and disulphides play an important role in platelet responses. Agents that reduce disulphide bonds expose the fibrinogen receptor in platelets and activate the purified glycoprotein (GP) IIbIIIa receptor. Protein disulphide isomerase (PDI), an enzyme that rearranges disulphides bonds, is found on the platelet surface where it is catalytically active. We investigated the role of PDI in platelet responses using (1) rabbit anti-PDI IgG specific for PDI, (2) a competing substrate (scrambled ribonuclease A), and (3) the PDI inhibitor, bacitracin. Fab fragments of the rabbit anti-PDI IgG inhibited platelet responses to the agonists tested (ADP and collagen), whereas Fab fragments prepared identically from normal rabbit IgG had no inhibitory effect. Scrambled ribonuclease A blocked platelet aggregation and secretion, but native ribonuclease A did not. When biphasic platelet aggregation was examined using platelets in citrated plasma, the principle effect of bacitracin was on second phase or irreversible aggregation responses and the accompanying secretion. Using flow cytometry and an antibody specific for activated GPIIbIIIa (PAC-1), the rabbit anti-PDI Fab fragments substantially inhibited activation of GPIIbIIIa when added before, but not after, platelet activation. In summary, we have demonstrated that protein disulphide isomerase mediates platelet aggregation and secretion, and that it activates GPIIbIIIa, suggesting this receptor as the target of the enzyme.

Protein disulfide isomerase assists protein folding as both an isomerase and a chaperone

Protein disulfide isomerase (PDI) is the physiological catalyst of native disulfide bond formation of nascent peptides in the cells. As a foldase, PDI has both isomerase and chaperone activities. The chaperone activity is intrinsic and independent of its isomerase activity. Both chaperone and isomerase activities are required for PDI to assist folding of denatured and reduced disulfide-containing proteins. PDI may have great applications in protein production by bioengineering for its function as a foldase.

The Selective Inhibition of β1 and β7 Integrin-Mediated Lymphocyte Adhesion by Bacitracin

Integrins play an important role in lymphocyte adhesion to cellular and extracellular components of their microenvironment. The regulation of such adhesion often involves changes in the functional state of the integrins rather than alterations in their expression levels. Although the functional basis for such transitions is unknown, a possible role for disulfide exchange might be postulated based on the observations that integrin function can be activated by bifunctional reducing agents or by Abs that react with areas adjacent to predicted long-range disulfide bonds in integrins. Recently, it has been reported that enzymes that catalyze disulfide exchanges such as protein disulfide isomerase (PDI) are present on the surface of lymphoid cells, raising the possibility that such enzymes might be involved in the control of lymphocyte adhesion. A number of inhibitors of PDI function were examined for their effects on integrin-mediated adherence of T cells. The results did not support role for PDI in the regulation of integrin function, as the inhibitors somatostatin A, tocinoic acid, dithiobisnitrobenzoic acid, and anti-PDI mAb did not interfere with adherence. However, one of the PDI inhibitors, bacitracin, selectively interfered with the β1 integrin-mediated adherence of lymphoid cells to collagen, fibronectin, laminin, and VCAM-1, and with α4β7-dependent adherence to fibronectin and to VCAM-1. In contrast, αvβ3- and αLβ2-mediated adherence were not inhibited. Thus, it appears that bacitracin may be a selective inhibitor of β1 and β7 integrin functions by an as yet unknown mechanism.

An ERp60-like protein from the filarial parasite Dirofilaria immitis has both transglutaminase and protein disulfide isomerase activity

Transglutaminases (TGases; EC 2.3.2.13) are a family of enzymes that catalyze calcium-dependent covalent cross-linking of cellular proteins by establishing epsilon-(gamma-glutamyl)lysine isopeptide bonds. These covalent isopeptide bonds are of great physiological significance because they are highly resistant to proteolysis, denaturants, and reducing agents. Prior studies have demonstrated the presence of isopeptide bonds in the sheath and cuticle of filarial parasites, suggesting an important role for TGase-catalyzed reactions during the growth and development of filarial nematodes. Herein we report the identification and cloning of a cDNA encoding a TGase from the dog heartworm Dirofilaria immitis (DiTG). The DiTG expressed in Escherichia coli (recombinant DiTG) was able to catalyze calcium-dependent cross-linking reactions. The derived amino acid sequence of the DiTG cDNA (pDiTG) predicts a protein of 57.1 kDa and includes an N-terminal hydrophobic signal peptide. The pDiTG has no sequence similarity with any of the known TGases, but it has significant homology to protein disulfide isomerase (PDI) and, particularly, to the PDI-related endoplasmic reticulum protein ERp60, a PDI isoform found in the lumen of endoplasmic reticulum. As predicted from the amino acid sequence homology, recombinant DiTG catalyzed the isomerization of intramolecular disulfide/sulfhydryl bonds in denatured RNase in vitro as effectively as did mammalian PDI. Conversely, purified PDI from bovine liver could catalyze protein cross-linking reactions in a Ca(2+)-dependent manner. This report describes the dual catalytic activity of TGase and PDI in post- and/or cotranslational modification of newly synthesized proteins. These TGase-catalyzed posttranslational modifications may play a pivotal role in the synthesis of new cuticle during the growth and maturation of filarial parasites.