Unconventional secretory routes: direct protein export across the plasma membrane of mammalian cells

Active caspase-1 is a regulator of unconventional protein secretion

Mammalian cells export most proteins by the endoplasmic reticulum/Golgi-dependent pathway. However, some proteins are secreted via unconventional, poorly understood mechanisms. The latter include the proinflammatory cytokines interleukin(IL)-1beta, IL-18, and IL-33, which require activation by caspase-1 for biological activity. Caspase-1 itself is activated by innate immune complexes, the inflammasomes. Here we show that secretion of the leaderless proteins proIL-1alpha, caspase-1, and fibroblast growth factor (FGF)-2 depends on caspase-1 activity. Although proIL-1alpha and FGF-2 are not substrates of the protease, we demonstrated their physical interaction. Secretome analysis using iTRAQ proteomics revealed caspase-1-mediated secretion of other leaderless proteins with known or unknown extracellular functions. Strikingly, many of these proteins are involved in inflammation, cytoprotection, or tissue repair. These results provide evidence for an important role of caspase-1 in unconventional protein secretion. By this mechanism, stress-induced activation of caspase-1 directly links inflammation to cytoprotection, cell survival, and regenerative processes.

Unconventional secretion of fibroblast growth factor 2 and galectin-1 does not require shedding of plasma membrane-derived vesicles

Various molecular mechanisms of unconventional secretion of fibroblast growth factor 2 and galectin-1 have been proposed. A non-vesicular pathway that is based on direct translocation across the plasma membrane has been described. In other studies, however, release into the extracellular space of cell-derived vesicles was implicated in both FGF-2 and Gal-1 secretion. Such vesicles were proposed to originate either from plasma membrane shedding or by the release of exosomes. Employing an inhibitor of plasma membrane blebbing and based on a quantitative biochemical analysis of cell culture supernatants for vesicles potentially carrying FGF-2 or Gal-1, we demonstrate that both FGF-2 and Gal-1 are not exported by shedding of plasma membrane-derived vesicles.

Fibroblast growth factor homologous factor 1 (FHF1) is expressed in a subpopulation of calcitonin gene-related peptide-positive nociceptive neurons in the murine dorsal root ganglia

Dorsal root ganglia (DRG) neurons exhibit a wide molecular heterogeneity in relation to the various sensory modalities (mechanoception, thermoception, nociception) that they subserve. Finding markers of subpopulations is an important step in understanding how these neurons convey specific information. We identified fibroblast growth factor homologous factor 1 (FHF1) in a search for markers of subpopulations of DRG neurons. FHFs constitute a family of four factors that share some structural properties with fibroblast growth factors (FGFs) but are functionally distinct. They are expressed in specific subsets of neurons and are involved in the modulation of sodium channel activity. The pattern of expression of FHF1 in the DRG was determined during development, in the adult and after axotomy. We show that in the adult, FHF1 is expressed in two populations, one composed of nociceptors and another in which no neurotrophic factor receptors were detected (panTrk-/c-Ret-). Interestingly, in the nociceptors, FHF1 expression was restricted to a subset of TrkA+/calcitonin gene-related peptide (CGRP)-positive neurons. Neurofilament 200 (NF-200) and peripherin labeling indicates that 70% of the FHF1-expressing neurons contribute to A-fibers and 30% to C-fibers. FHF1 interacts with the Na(v)1.9 sodium channel isoform, which is strongly expressed in cRet+/isolectin-B4 binding neurons, but we show that FHF1 is not expressed in the cRet+/IB4+ subclass and that it does not colocalize with Na(v)1.9. Our results argue strongly against the possibility that FHF1 has a modulatory effect on this channel in cRet+/IB4+ neurons, but FHF1 could play a role in a distinct subset of TrkA+/CGRP+ nociceptors.

Regulation of tumor progression by extracellular galectin-3

The relationship between a tumor cell and its microenvironment is bi-directional. The proteins expressed by the tumor cells alter the signatures on the seemingly normal stromal cells within the microenvironment, while the tumor cell signatures reflect the changes that occur as these cells interact with the host microenvironment. Galectin-3 is a carbohydrate-binding protein that is over-expressed in a variety of tumors and immune cells in response to various stimuli. Ever since its discovery, it has been associated with cell and extracellular matrix interactions. However, in the last decade, an extensive accumulation of data has changed the perspective of this multifunctional protein. The unique structure of this protein, consisting of a carbohydrate-binding domain and a matrix metalloproteinase cleavable domain, enables it to interact with a plethora of ligands in a carbohydrate-dependent or independent manner. It is now becoming evident that galectin-3 is involved with a variety of extracellular functions like cell adhesion, migration, invasion, angiogenesis, immune functions, apoptosis and endocytosis. Galectin-3 is a substrate for matrix metalloproteinases and its cleavage plays an important role in tumor progression and can be used as a surrogate diagnostic marker for in vivo MMP activity.

Release of metabolic enzymes by Giardia in response to interaction with intestinal epithelial cells

Giardia lamblia, an important cause of diarrheal disease, resides in the small intestinal lumen in close apposition to epithelial cells. Since the disease mechanisms underlying giardiasis are poorly understood, elucidating the specific interactions of the parasite with the host epithelium is likely to provide clues to understanding the pathogenesis. Here we tested the hypothesis that contact of Giardia lamblia with intestinal epithelial cells might lead to release of specific proteins. Using established co-culture models, intestinal ligated loops and a proteomics approach, we identified three G. lamblia proteins (arginine deiminase, ornithine carbamoyl transferase and enolase), previously recognized as immunodominant antigens during acute giardiasis. Release was stimulated by cell-cell interactions, since only small amounts of arginine deiminase and enolase were detected in the medium after culturing of G. lamblia alone. The secreted G. lamblia proteins were localized to the cytoplasm and the inside of the plasma membrane of trophozoites. Furthermore, in vitro studies with recombinant arginine deiminase showed that the secreted Giardia proteins can disable host innate immune factors such as nitric oxide production. These results indicate that contact of Giardia with epithelial cells triggers metabolic enzyme release, which might facilitate effective colonization of the human small intestine.

Ovine endometrial expression of fibroblast growth factor (FGF) 2 and conceptus expression of FGF receptors during early pregnancy

In ruminants, conceptus development beyond the blastocyst state requires input from uterine-derived factors. Fibroblast growth factor 2 (FGF2) is expressed by the bovine endometrium throughout the estrus cycle and early pregnancy and stimulates trophectoderm expression of interferon-tau, the maternal recognition of pregnancy factor in ruminants. The objective of this study was to examine the expression of FGF2 in ovine endometrium and peri-attachment conceptuses and FGF receptors (FGFR) in conceptuses. FGF2 mRNA was present in the ovine endometrium with specific localization within the luminal and glandular epithelium. No pregnancy-dependent changes in endometrial FGF2 mRNA abundance were detected until placental attachment was well underway. FGF2 protein was detected in the uterine lumen throughout the estrous cycle and early pregnancy. Concentrations of luminal FGF2 protein did not differ based on pregnancy status. However, uterine luminal FGF2 protein levels increased at days 12-13 after estrus in both cyclic and pregnant ewes. Ovine conceptuses collected at days 14-19 after mating contained transcripts for FGF2 and FGFR types 1, 2 and 3. In summary, FGF2 is expressed by the ovine endometrium and conceptus during early pregnancy, and peri-attachment conceptuses possess several FGFR types. Concentrations of FGF2 protein in the uterine lumen increase coincident with the initiation of pregnancy recognition in ewes. These observations support the concept that FGF2 and potentially other FGFs may affect conceptus development and/or gene expression during early pregnancy in ruminants.

Circulating microparticles in normal pregnancy and pre-eclampsia

Cellular particles may be larger shed microparticles (>or=100 nm, MPs) that are the products of cell activation or necrosis. There are also smaller endocytic nanoparticles (<100 nm), called exosomes, which are internal vesicles of late endosomes or multivesicular bodies and are released into the extracellular milieu upon fusion of the multivesicular body with the cell surface. Both MPs and exosomes can be detected in the circulations of non-pregnant and pregnant women. In the former MPs are increased in conditions associated with systemic inflammation such as sepsis or metabolic syndrome. During normal pregnancy MPs are increased and they increase further with pre-eclampsia. They include not only MPs derived from platelets, endothelium and various leukocytes but also syncytiotrophoblast derived MPs (often called STBMs). STBMs interact with both immune and endothelial cells and may contribute to the systemic inflammation of both normal and pre-eclamptic pregnancies. However inhibitory activity has also been ascribed to trophoblast derived exosomes. In vitro, they down-regulate T cell activity, a T cell change that has been repeatedly observed, ex vivo, during normal pregnancy.

Exosomal transfer of proteins and RNAs at synapses in the nervous system

Background

Many cell types have been reported to secrete small vesicles called exosomes, that are derived from multivesicular bodies and that can also form from endocytic-like lipid raft domains of the plasma membrane. Secretory exosomes contain a characteristic composition of proteins, and a recent report indicates that mast cell exosomes harbor a variety of mRNAs and microRNAs as well. Exosomes express cell recognition molecules on their surface that facilitate their selective targeting and uptake into recipient cells.

Results

In this review, I suggest that exosomal secretion of proteins and RNAs may be a fundamental mode of communication within the nervous system, supplementing the known mechanisms of anterograde and retrograde signaling across synapses. In one specific scenario, exosomes are proposed to bud from the lipid raft region of the postsynaptic membrane adjacent to the postsynaptic density, in a manner that is stimulated by stimuli that elicit long-term potentiation. The exosomes would then transfer newly synthesized synaptic proteins (such as CAM kinase II alpha) and synaptic RNAs to the presynaptic terminal, where they would contribute to synaptic plasticity.

Conclusion

The model is consistent with the known cellular and molecular features of synaptic neurobiology and makes a number of predictions that can be tested in vitro and in vivo.

Open peer review

Reviewed by Etienne Joly, Gaspar Jekely, Juergen Brosius and Eugene Koonin. For the full reviews, please go to the Reviewers' comments section.

Unconventional secretion: an extracellular trap for export of fibroblast growth factor 2

Several secretory proteins are released from cells by mechanisms that are distinct from the classical endoplasmic reticulum (ER)/Golgi-mediated secretory pathway. Recent studies unexpectedly revealed that the interaction between one such protein, fibroblast growth factor 2 (FGF-2), and cell surface heparan sulfate proteoglycans (HSPGs) is essential for secretion. FGF-2 mutants that cannot bind to heparan sulfates are not secreted, and cells that do not express functional HSPGs cannot secrete wild-type FGF-2. FGF-2 appears to be secreted by direct translocation across the plasma membrane in an ATP- and membrane-potential-independent manner. I propose that its translocation across the membrane is a diffusion-controlled process in which cell surface HSPGs function as an extracellular molecular trap that drives directional transport of FGF-2.

Cardioinductive network guiding stem cell differentiation revealed by proteomic cartography of tumor necrosis factor alpha-primed endodermal secretome

In the developing embryo, instructive guidance from the ventral endoderm secures cardiac program induction within the anterolateral mesoderm. Endoderm-guided cardiogenesis, however, has yet to be resolved at the proteome level. Here, through cardiopoietic priming of the endoderm with the reprogramming cytokine tumor necrosis factor alpha (TNFalpha), candidate effectors of embryonic stem cell cardiac differentiation were delineated by comparative proteomics. Differential two-dimensional gel electrophoretic mapping revealed that more than 75% of protein species increased >1.5-fold in the TNFalpha-primed versus unprimed endodermal secretome. Protein spot identification by linear ion trap quadrupole (LTQ) tandem mass spectrometry (MS/MS) and validation by shotgun LTQ-Fourier transform MS/MS following multidimensional chromatography mapped 99 unique proteins from 153 spot assignments. A definitive set of 48 secretome proteins was deduced by iterative bioinformatic screening using algorithms for detection of canonical and noncanonical indices of secretion. Protein-protein interaction analysis, in conjunction with respective expression level changes, revealed a nonstochastic TNFalpha-centric secretome network with a scale-free hierarchical architecture. Cardiovascular development was the primary developmental function of the resolved TNFalpha-anchored network. Functional cooperativity of the derived cardioinductive network was validated through direct application of the TNFalpha-primed secretome on embryonic stem cells, potentiating cardiac commitment and sarcomerogenesis. Conversely, inhibition of primary network hubs negated the procardiogenic effects of TNFalpha priming. Thus, proteomic cartography establishes a systems biology framework for the endodermal secretome network guiding stem cell cardiopoiesis.

S100A11, an dual mediator for growth regulation of human keratinocytes

We previously revealed a novel signal pathway involving S100A11 for inhibition of the growth of normal human keratinocytes (NHK) caused by high Ca(++) or transforming growth factor beta. Exposure to either agent resulted in transfer of S100A11 to nuclei, where it induced p21(WAF1). In contrast, S100A11 has been shown to be overexpressed in many human cancers. To address this apparent discrepancy, we analyzed possible new functions of S100A11, and we provide herein evidence that 1) S100A11 is actively secreted by NHK; 2) extracellular S100A11 acts on NHK to enhance the production of epidermal growth factor family proteins, resulting in growth stimulation; 3) receptor for advanced glycation end products, nuclear factor-kappaB, Akt, and cAMP response element-binding protein are involved in the S100A11-triggered signal transduction; and 4) production and secretion of S100A11 are markedly enhanced in human squamous cancer cells. These findings indicate that S100A11 plays a dual role in growth regulation of epithelial cells.

Galectin-1 promotes HIV-1 infectivity in macrophages through stabilization of viral adsorption

Following primary infection with human immunodeficiency virus type-1 (HIV-1), macrophages are thought to play an important role, as they are one of the first target cells the virus encounters and can also sustain a significant production of viruses over extended periods of time. While the interaction between the primary cellular receptor CD4 and the virus-encoded external envelope glycoprotein gp120 initiates the infection process, it has been suggested that various host factors are exploited by HIV-1 to facilitate adsorption onto the cell surface. Macrophages and other cells found at the infection site can secrete a soluble mammalian lectin, galectin-1, which binds to beta-galactoside residues through its carbohydrate recognition domain. Being a dimer, galectin-1 can cross-link ligands expressed on different constituents to mediate adhesion between cells or between cells and pathogens. We report here that galectin-1, but not galectin-3, increased HIV-1 infectivity in monocyte-derived macrophages (MDMs). This phenomenon was likely due to an enhancement of virus adsorption kinetics, which facilitates HIV-1 entry. The fusion inhibitors T-20 and TAK779 remained effective at reducing infection even in the presence of galectin-1, indicating that the galectin-1-mediated effect is occurring at a step prior to fusion. Together, our data suggest that galectin-1 can facilitate HIV-1 infection in MDMs by promoting early events of the virus replicative cycle (i.e. adsorption).

Identification of eukaryotic secreted and cell surface proteins using the yeast secretion trap screen

Secreted and cell surface proteins play essential roles in numerous essential biological processes in eukaryotic organisms, but are often more difficult to isolate and identify than proteins that are localized in intracellular compartments. However, several high-throughput 'gene-trap' techniques have been developed to characterize these 'secretomes', including the yeast secretion trap (YST) screen. This method involves fusing cDNA libraries from the tissue or cell type of interest to a yeast (Saccharomyces cerevisiae) invertase reporter gene, transforming the resulting fusion library into an invertase-deficient yeast strain and plating the transformants on a medium containing sucrose as the sole carbon source. A yeast cell with a transgene encoding a secreted or cell surface protein can synthesize a secreted invertase fusion protein that can rescue the mutant, and the plasmid DNA can then be sequenced to identify the gene that encodes it. We describe a recently improved version of this screen, which allows the identification of genes encoding secreted proteins in 1-2 months.

Contributions of Galectin-3 and -9 to Epithelial Cell Adhesion Analyzed by Single Cell Force Spectroscopy

Galectins are widely expressed in epithelial tissues and have been implicated in a variety of cellular processes, including adhesion and polarization. Here we studied the contributions of galectins in cell adhesion and cyst formation of Madin-Darby canine kidney cells. Quantitative single cell force spectroscopy and standard adhesion assays were employed to study both early (<2 min) and long term (90 min) adhesion of cells to different extracellular matrix components. Inhibitors were used to examine the contribution of integrins and galectins in general and RNA interference to specifically address the role of two abundantly expressed galectins, galectin-3 and -9. We found that both galectin-3 and -9 were required for optimal long term cell adhesion to both collagen I and laminin-111. Early adhesion to laminin was found to be integrin-independent and was instead mediated by carbohydrate interactions and galectin-3 and -9. The opposite was observed for early adhesion to collagen. Although similar, the contributions of galectin-3 and -9 to adhesion appeared to be by distinct processes. These defects in adhesion of the two galectin knockdown cell lines may underlie the epithelial phenotypes observed in the cyst assays. Our findings emphasize the complex regulation of epithelial cell functions by galectins.

GRASPing unconventional secretion

GRASP proteins associate with the Golgi apparatus and have been implicated in the stacking of Golgi cisternae, vesicle tethering, and mitotic progression, but their specific functions are unclear. In this issue, Kinseth et al. (2007) show unexpectedly that a GRASP homolog is required for an unconventional secretory pathway that bypasses the usual route for Golgi-dependent membrane traffic.

The Golgi-Associated Protein GRASP Is Required for Unconventional Protein Secretion during Development

During Dictyostelium development, prespore cells secrete acyl-CoA binding protein (AcbA). Upon release, AcbA is processed to generate a peptide called spore differentiation factor-2 (SDF-2), which triggers terminal differentiation of spore cells. We have found that cells lacking Golgi reassembly stacking protein (GRASP), a protein attached peripherally to the cytoplasmic surface of Golgi membranes, fail to secrete AcbA and, thus, produce inviable spores. Surprisingly, AcbA lacks a signal sequence and is not secreted via the conventional secretory pathway (endoplasmic reticulum-Golgi-cell surface). GRASP is not required for conventional protein secretion, growth, and the viability of vegetative cells. Our findings reveal a physiological role of GRASP and provide a means to understand unconventional secretion and its role in development.

Spitzenkorper localization and intracellular traffic of green fluorescent protein-labeled CHS-3 and CHS-6 chitin synthases in living hyphae of Neurospora crassa

The subcellular location and traffic of two selected chitin synthases (CHS) from Neurospora crassa, CHS-3 and CHS-6, labeled with green fluorescent protein (GFP), were studied by high-resolution confocal laser scanning microscopy. While we found some differences in the overall distribution patterns and appearances of CHS-3-GFP and CHS-6-GFP, most features were similar and were observed consistently. At the hyphal apex, fluorescence congregated into a conspicuous single body corresponding to the location of the Spitzenkörper (Spk). In distal regions (beyond 40 microm from the apex), CHS-GFP revealed a network of large endomembranous compartments that was predominantly comprised of irregular tubular shapes, while some compartments were distinctly spherical. In the distal subapex (20 to 40 microm from the apex), fluorescence was observed in globular bodies that appeared to disintegrate into vesicles as they advanced forward until reaching the proximal subapex (5 to 20 microm from the apex). CHS-GFP was also conspicuously found delineating developing septa. Analysis of fluorescence recovery after photobleaching suggested that the fluorescence of the Spk originated from the advancing population of microvesicles (chitosomes) in the subapex. The inability of brefeldin A to interfere with the traffic of CHS-containing microvesicles and the lack of colocalization of CHS-GFP with the endoplasmic reticulum (ER)-Golgi body fluorescent dyes lend support to the idea that CHS proteins are delivered to the cell surface via an alternative route distinct from the classical ER-Golgi body secretory pathway.

Relevance of partially structured states in the non-classical secretion of acidic fibroblast growth factor

Acidic fibroblast growth factor (aFGF) is a signal peptide-less protein that is secreted into the extracellular compartment as part of a multiprotein release complex, consisting of aFGF, S100A13 (a calcium binding protein), and a 40 kDa (p40) form of synaptotagmin (Syt1), a protein that participates in the docking of a variety of secretory vesicles. p40 Syt1, and specifically its C2A domain, is believed to play a major role in the non-classical secretion of the aFGF release complex mediated by the interaction of aFGF and p40 Syt1with the phospholipids of the cell membrane inner leaflet. In the present study, we investigate the structural characteristics of aFGF and the C2A domain of p40 Syt1 under acidic conditions, using a variety of biophysical techniques including multidimensional NMR spectroscopy. Urea-induced equilibrium unfolding (at pH 3.4) of both aFGF and the C2A domain are non-cooperative and proceed with the accumulation of stable intermediate states. 1-Anilino-8-napthalene sulfonate (ANS) binding and size-exclusion chromatography results suggest that both aFGF and the C2A domain exist as partially structured states under acidic conditions (pH 3.4). Limited trypsin digestion analysis and 1H-15N chemical shift perturbation data reveal that the flexibility of certain portions of the protein backbone is increased in the partially structured state(s) of aFGF. The residues that are perturbed in the partially structured state(s) in aFGF are mostly located at the N- and C-terminal ends of the protein. In marked contrast, most of the interactions stabilizing the native secondary structure are preserved in the partially structured state of the C2A domain. Isothermal titration calorimetry data indicate that the binding affinity between aFGF and the C2A domain is significantly enhanced at pH 3.4. In addition, both aFGF and the C2A domain exhibit much higher lipid binding affinity in their partially structured states. The translocation of the multiprotein FGF release complex across the membrane appears to be facilitated by the formation of partially structured states of aFGF and the C2A domain of p40 Syt1.

Proteomic analysis of Schistosoma mansoni egg secretions

Schistosomiasis remains a largely neglected, global health problem. The morbid pathology of the disease stems from the host's inflammatory response to parasite eggs trapped in host tissues. Long term host/parasite survival is dependent upon the successful modulation of the acute pathological response, which is induced by egg antigens. In this study, using Multidimensional Protein Identification Technology, we identified the Schistosoma mansoni egg secretome consisting of 188 proteins. Notably we identified proteins involved in redox balance, molecular chaperoning and protein folding, development and signaling, scavenging and metabolic pathways, immune response modulation, and 32 novel, previously uncharacterized schistosome proteins. We localized a subset of previously characterized schistosome proteins identified in egg secretions in this study, to the surface of live S. mansoni eggs using the circumoval precipitin reaction. The identification of proteins actively secreted by live schistosome eggs provides important new information for understanding immune modulation and the pathology of schistosomiasis.

Microparticles and immunomodulation in pregnancy and pre-eclampsia

Cellular microparticles are ubiquitously shed from cell membranes or secreted as endocytic vesicles called exosomes. Shed microparticles are >/=100nm in size and are generated during apoptosis or necrosis. In contrast, exosomes are smaller (<100nm), express more limited protein content and are released from late endosomes. Both membrane particles and exosomes can be detected in the circulation in non-pregnant and pregnant women. In the former, they are increased in conditions associated with systemic inflammation such as sepsis or metabolic syndrome. During pregnancy, they are also associated with pre-eclampsia and include not only particles derived from platelets, endothelium and various leukocytes but also syncytiotrophoblast-derived microparticles. Syncytiotrophoblast membrane microparticles (often called STBMs) interact with both immune and endothelial cells. They may contribute to the systemic inflammatory response of both normal and pre-eclamptic pregnancies, although inhibitory activity has also been described. Moreover, trophoblast-derived exosomes may contribute to or cause the downregulation of T cell activity that has been repeatedly observed during pregnancy. Deletion of activate T cells which express Fas ligand by Fas-expressing exosomes derived from trophoblast may contribute to immunoregulation necessary for normal pregnancy.

Cellular localization and function of the antiviral protein, ovine Mx1 (oMx1): I. Ovine Mx1 is secreted by endometrial epithelial cells via an 'unconventional' secretory pathway

PROBLEM

Embryonic loss is a major contributor to infertility. Understanding factors contributing to embryonic loss will aid in development of technologies to improve/regulate fertility in animals and humans.

METHOD OF STUDY

We tested the hypothesis that the antiviral protein, ovine Mx1 (oMx1), is secreted by uterine epithelial cells. Uterine flushes were obtained from cyclic and early pregnant ewes and examined for levels of oMx1 protein. The pathway for ovine Mx1 secretion in ovine glandular epithelial (oGE) cells was determined using brefeldin A (BFA), an inhibitor of the conventional secretory pathway. Effects of BFA were determined using beta2-microglobulin (beta2MG) as a marker for the conventional secretory pathway, and interferon stimulated gene 15 (ISG15) and Galectin-1 (Gal-1) as markers for the unconventional secretory pathways.

RESULTS

Ovine Mx1 protein levels were low in uterine flushes from cyclic ewes and levels increased in pregnant ewes after D 15. Ovine GE cells secreted oMx1 in response to interferon and secretion was not reduced by BFA, suggesting oMx1 was secreted via an unconventional secretory pathway. beta2MG secretion was reduced by BFA, whereas ISG15 and Gal-1 were not.

CONCLUSION

This is the first report that the antiviral protein, oMx1, is secreted and provides evidence that secretion occurs via unconventional secretory pathway(s).

Membrane trafficking of CD98 and its ligand galectin 3 in BeWo cells − implication for placental cell fusion

CD98 heavy chain (CD98hc), expressed at high levels in developing human trophoblasts, is an integral membrane protein with multiple N-linked glycosylation sites and known to be important for cell fusion, adhesion, and amino acid transport. Western blotting and flow cytometry were used to study the effect of brefeldin A, an inhibitor of protein translocation through the Golgi, on CD98hc in the human placental trophoblast cell line BeWo. Although brefeldin A treatment caused increased cell surface expression of CD98hc, a novel partially glycosylated form of the protein was found and, concomitantly, cell fusion was reduced. Western blotting showed that CD98 and galectin 3, a proposed ligand for the glycosylated extracellular domain of CD98hc, co-immunoprecipitated, and double-label immuno-electron microscopy confirmed that CD98hc associated with galectin 3. Furthermore, cell fusion was reduced (specifically) by the disaccharide lactose, a known ligand for the carbohydrate recognition domain of galectin 3, suggesting that the association was functional. Taken together, the data suggest that N-glycosylation of CD98 and subsequent interaction with galectin 3 is critical for aspects of placental cell biology, and provides a rationale for the observation that, in the mouse, truncation of the CD98hc extracellular domain leads to early embryonic lethality [Tsumura H, Suzuki N, Saito H, Kawano M, Otake S, Kozuka Y, Komada H, Tsurudome M & Ito Y (2003) Biochem Biophys Res Commun 308, 847-851].

Antioxidant gene expression and function in in vitro-developing Schistosoma mansoni mother sporocysts: possible role in self-protection

The ability of the larval forms of Schistosoma mansoni to invade and parasitize their molluscan host, Biomphalaria glabrata, is determined by a multitude of factors. In this study we sought to elucidate the possible mechanisms by which the invading larvae are able to counteract the potentially harmful oxidative environment presented by the host upon initial miracidial infection. This was attempted by examining the gene expression profile of parasite antioxidant enzymes of the linked glutathione-(GSH) thioredoxin (Trx) redox pathway during early intramolluscan larval development. Three such enzymes, the peroxiredoxins (Prx1, Prx2 and Prx3) were examined as to their activity and sites of expression within S. mansoni miracidia and in vitro-cultured mother sporocysts. Results of these studies demonstrated that the H(2)O(2)-reducing enzymes Prx1 and 2 are upregulated during early mother sporocyst development compared to miracidia. Immunolocalization studies further indicated that Prx1 and Prx2 proteins are expressed within the apical papillae of miracidia and tegumental syncytium of sporocysts, and are released with parasite excretory-secretory proteins (ESP) during in vitro larval transformation. Removal of Prx1 and Prx2 from larval ESP by immunoabsorption significantly reduced the ability of ESP to breakdown exogenous H(2)O(2), thereby directly linking ESP Prx proteins with H(2)O(2)-scavenging activity. Moreover, exposure of live sporocysts to exogenous H(2)O(2)stimulated an upregulation of Prx1 and 2 gene expression suggesting the involvement of H(2)O(2)-responsive elements in regulating larval Prx gene expression. These data provide evidence that Prx1 and Prx2 may function in the protection of S. mansoni sporocysts during the early stages of infection.

Immunoadhesins containing pre-S domains of hepatitis B virus large envelope protein are secreted and inhibit virus infection

Hepatitis B virus (HBV) replication produces three envelope proteins (L, M, and S) that have a common C terminus. L, the largest, contains a domain, pre-S1, not present on M. Similarly M contains a domain, pre-S2, not present on S. The pre-S1 region has important functions in the HBV life cycle. Thus, as an approach to studying these roles, the pre-S1 and/or pre-S2 sequences of HBV (serotype adw2, genotype A) were expressed as N-terminal fusions to the Fc domain of a rabbit immunoglobulin G chain. Such proteins, known as immunoadhesins (IA), were highly expressed following transfection of cultured cells and, when the pre-S1 region was present, >80% were secreted. The IA were myristoylated at a glycine penultimate to the N terminus, although mutation studies showed that this modification was not needed for secretion. As few as 30 amino acids from the N terminus of pre-S1 were both necessary and sufficient to drive secretion of IA. Even expression of pre-S1 plus pre-S2, in the absence of an immunoglobulin chain, led to efficient secretion. Overall, these studies demonstrate an unexpected ability of the N terminus of pre-S1 to promote protein secretion. In addition, some of these secreted IA, at nanomolar concentrations, inhibited infection of primary human hepatocytes either by hepatitis delta virus (HDV), a subviral agent that uses HBV envelope proteins, or HBV. These IA have potential to be part of antiviral therapies against chronic HDV and HBV, and may help understand the attachment and entry mechanisms used by these important human pathogens.

Apical Sorting by Galectin-3-Dependent Glycoprotein Clustering

Epithelial cells are characterized by their polarized organization based on an apical membrane that is separated from the basolateral membrane domain by tight junctions. Maintenance of this morphology is guaranteed by highly specific sorting machinery that separates lipids and proteins into different carrier populations for the apical or basolateral cell surface. Lipid-raft-independent apical carrier vesicles harbour the beta-galactoside-binding lectin galectin-3, which interacts directly with apical cargo in a glycan-dependent manner. These glycoproteins are mistargeted to the basolateral membrane in galectin-3-depleted cells, dedicating a central role to this lectin in raft-independent sorting as apical receptor. Here, we demonstrate that high-molecular-weight clusters are exclusively formed in the presence of galectin-3. Their stability is sensitive to increased carbohydrate concentrations, and cluster formation as well as apical sorting are perturbed in glycosylation-deficient Madin-Darby canine kidney (MDCK) II cells. Together, our data suggest that glycoprotein cross-linking by galectin-3 is required for apical sorting of non-raft-associated cargo.

Caspase-1-dependent processing of pro-interleukin-1beta is cytosolic and precedes cell death

The pro-inflammatory cytokine interleukin-1beta is a key mediator of inflammation and is implicated in the pathogenesis of diverse disease states. Despite its biological importance, the mechanisms of its processing to an active form and its trafficking to the extracellular compartment remain poorly understood. Interleukin-1beta secretion is proposed to occur via several distinct mechanisms including microvesicle shedding and the regulated secretion of lysosomes. In this study, we report for the first time that caspase-1-dependent processing of pro-interleukin-1beta can occur in the cytosol following activation of P2X7-receptor. We also provide evidence that the pathway of secretion in this model is independent of the lysosomal trafficking regulator, a protein involved in lysosome secretion. Although release of interleukin-1beta occurred before the appearance of significant levels of lactate dehydrogenase in the supernatant, the cells ultimately died. It is clear that structural changes preceding cell death, occurring after caspase-1 activation, promote the cellular release of interleukin-1beta. We investigated the involvement of lipid rafts in this process and discovered that depleting the plasma membrane of cholesterol did not adversely affect interleukin-1beta secretion in response to ATP. We propose that, in macrophages, ATP-induced interleukin-1beta processing occurs in the cytosol by a mechanism that resembles pyroptosis.

Cellular Localization and Function of the Antiviral Protein, Ovine Mx1 (oMx1): II. The oMx1 Protein Is a Regulator of Secretion in an Ovine Glandular Epithelial Cell Line

PROBLEM

Embryonic loss is a major contributor to infertility. Understanding factors affecting embryonic loss will help increase fertility.

METHOD OF STUDY

We investigated if ovine Mx1 (oMx1) mediated secretion by ovine glandular epithelial (oGE) cells using small interfering RNA (siRNA). Effects on secretion were examined through the conventional endoplasmic reticulum-Golgi pathway using beta2- microglobulin (beta2MG) as a marker, and interferon-stimulated gene 15 (ISG15) as a marker for unconventional secretion.

RESULTS

Mx1 siRNA reduced oMx1 mRNA levels at 12 and 24 hr after IFN-tau treatment (P < 0.05), without affecting levels of oMx2, ISG15, 2',5'-oligoadenylate synthetas or beta2MG. Mx1 siRNA reduced Mx1 protein levels at 48 and 120 hr after treatment (P < 0.05) and protein levels remained low at 120 hr. Transient oMx1 knock-down reduced secretion of oMx1 (P < 0.01). ISG15 protein in secretions was reduced without affecting intracellular levels (P < 0.05). Levels of beta2MG in secretions were not affected by Mx1 siRNA.

CONCLUSION

We showed that oMx1 protein is secreted by oGE cells and that reduction in oMx1 protein levels by siRNA reduced secretion of ISG15, but not beta2MG. Results support the hypothesis that oMx1 is a regulator of secretion through unconventional secretory pathway(s).

STI1 promotes glioma proliferation through MAPK and PI3K pathways

Gliomas are tumors derived from glia or their precursors within the central nervous system. Clinically, gliomas are divided into four grades and the glioblastoma multiforme (GBM), also referred as grade IV astrocytoma, is the most aggressive and the most common glioma in humans. The prognosis for patients with GBM remains dismal, with a median survival of 9-12 months. Despite their striking heterogeneity, common alterations in specific cellular signal transduction pathways occur within most GBMs. Previous work from our group identified the co-chaperone stress-inducible protein 1 (STI1) as a cell surface ligand for cellular prion (PrP(C)), which leads to the activation of several signal transduction pathways, some of which modulate cell survival. In the present work, we used thymidine incorporation assays to investigate the effect of STI1 upon proliferation of the human glioblastoma-derived cell line A172. Here we report that STI1 is secreted by and induces proliferation in tumor cells, an effect that is modulated by the Erk and PI3K pathways, and that, in contrast to glioma cells, STI1 does not induce proliferation of normal glia. In addition, our data suggest the involvement of PrP(C) in STI1-induced proliferation of A172 cells. These results provide initial evidence of a new functional role for STI1 on the physiology of human gliomas, and may lead to the identification of new therapeutic targets in these tumors.

Epstein-Barr virus latent membrane protein 1 promotes concentration in multivesicular bodies of fibroblast growth factor 2 and its release through exosomes

FGF-2, a potent angiogenic factor that is involved in tumor invasion, is known to be released extracellularly by a nonclassical secretory pathway. Recently it has become clear that Epstein-Barr virus, specifically its oncoprotein LMP1, can induce expression of angiogenic factors. Among these factors is FGF-2. LMP1 not only promotes expression of FGF-2, but also the release extracellularly of its 18-kDa isoform. We analyzed the mechanism of FGF-2 release induced by LMP1. Confocal immunofluorescence microscopy revealed colocalization of FGF-2 with LMP1 in small dots also stained positively for CD63 and cathepsin D, markers of late endosomes or multivesicular bodies. Biochemical analysis and immunoelectron microscopy of purified exosomal fractions from cotransfected cells demonstrated increased release of exosomes and the concentration of LMP1 and FGF-2 in these structures. Moreover, cotransfection appeared to induce partial redistribution of the Na(+)/K(+)-ATPase, which participates in FGF-2 release, from the plasma membrane to the intracellular LMP1/FGF-2 positive dots. Treatment with ouabain, which inhibits Na(+)/K(+)-ATPase activity, partially suppressed FGF-2 secretion via exosomes in a dose-dependent manner. The results suggest that exosomes may represent a previously unrecognized mechanism for FGF-2 release mediated by LMP1, and that this pathway involves the activity of Na(+)/K(+)-ATPase.

Functions of the histone chaperone nucleolin in diseases

Alteration of nuclear morphology is often used by pathologist as diagnostic marker for malignancies like cancer. In particular, the staining of cells by the silver staining methods (AgNOR) has been proved to be an important tool for predicting the clinical outcome of some cancer diseases. Two major argyrophilic proteins responsible for the strong staining of cells in interphase are the nucleophosmin (B23) and the nucleolin (C23) nucleolar proteins. Interestingly these two proteins have been described as chromatin associated proteins with histone chaperone activities and also as proteins able to regulate chromatin transcription. Nucleolin seems to be over-expressed in highly proliferative cells and is involved in many aspect of gene expression: chromatin remodeling, DNA recombination and replication, RNA transcription by RNA polymerase I and II, rRNA processing, mRNA stabilisation, cytokinesis and apoptosis. Interestingly, nucleolin is also found on the cell surface in a wide range of cancer cells, a property which is being used as a marker for the diagnosis of cancer and for the development of anti-cancer drugs to inhibit proliferation of cancer cells. In addition to its implication in cancer, nucleolin has been described not only as a marker or as a protein being involved in many diseases like viral infections, autoimmune diseases, Alzheimer's disease pathology but also in drug resistance. In this review we will focus on the chromatin associated functions of nucleolin and discuss the functions of nucleolin or its use as diagnostic marker and as a target for therapy

Rhamm-/- fibroblasts are defective in CD44-mediated ERK1,2 motogenic signaling, leading to defective skin wound repair

Rhamm (receptor for hyaluronan-mediated motility) is an hyaluronan binding protein with limited expression in normal tissues and high expression in advanced cancers. To understand its physiological functions and identify the molecular mechanisms underlying these functions, we created mice with a genetic deletion of Rhamm. We show that Rhamm^−/− fibroblasts fail to resurface scratch wounds >3 mm or invade hyaluronan-supplemented collagen gels in culture. We identify a requirement for Rhamm in the localization of CD44 to the cell surface, formation of CD44–ERK1,2 (extracellular-regulated kinase 1,2) complexes, and activation/subcellular targeting of ERK1,2 to the cell nucleus. We also show that cell surface Rhamm, restricted to the extracellular compartment by linking recombinant protein to beads, and expression of mutant active mitogen-activated kinase kinase 1 (Mek1) are sufficient to rescue aberrant signaling through CD44–ERK1,2 complexes in Rh^−/− fibroblasts. ERK1,2 activation and fibroblast migration/differentiation is also defective during repair of Rh^−/− excisional skin wounds and results in aberrant granulation tissue in vivo. These results identify Rhamm as an essential regulator of CD44–ERK1,2 fibroblast motogenic signaling required for wound repair.

A functional screen to characterize the secretomes of eukaryotic pathogens and their hosts in planta

Complex suites of proteins that are secreted by plants and phytopathogens into the plant apoplast play crucial roles in surveillance, assault, defense, and counter-defense. High-throughput genome-scale strategies are being developed to better understand the nature of these "secretomes" and the identity of pathogen-derived effector proteins that subvert plant defenses and promote pathogenicity. Although combined bioinformatic and experimental approaches recently have provided comprehensive coverage of secreted proteins from bacterial phytopathogens, far less is known about the secretomes and batteries of effectors of eukaryotic phytopathogens; notably fungi and oomycetes. The yeast secretion trap (YST) represents a potentially valuable technique to simultaneously target pathogen and host secretomes in infected plant material. A YST screen, using a new vector system, was applied to study the interaction between tomato (Solanum lycopersicum) and the oomycete Phytophthora infestans, revealing sets of genes encoding secreted proteins from both pathogen and host. Most of those from the oomycete had no identifiable function and were detectable in planta only during pathogenesis, underlining the value of YST as a tool to identify new candidate effectors and pathogenicity factors. In addition, the majority of the P. infestans proteins had homologs in the genomes of the related oomycetes R. sojae and P. ramorum.

Cell-surface heparan sulfate proteoglycans are essential components of the unconventional export machinery of FGF-2

FGF-2 is an unconventionally secreted lectin that transmits proangiogenic signals through a ternary complex with high-affinity FGF receptors and heparan sulfate proteoglycans (HSPGs). Although FGF-2 signal transduction is understood in great detail, its mechanism of release from cells, which is independent of the classical secretory pathway, remains elusive. To test the hypothesis that FGF-2 secretion is linked to its cell-surface ligands, we studied FGF-2 release using mutants defective for HSPG binding and cells with impaired HSPG biosynthesis. Here, we report that a functional interaction between FGF-2 and HSPGs is required for net export of FGF-2 from mammalian cells. FGF-2 release requires extracellular, membrane-proximal HSPGs. We propose that extracellular HSPGs form a molecular trap that drives FGF-2 translocation across the plasma membrane.

Thrombin induces rapid PAR1-mediated non-classical FGF1 release

Thrombin induces cell proliferation and migration during vascular injury. We report that thrombin rapidly stimulated expression and release of the pro-angiogenic polypeptide fibroblast growth factor 1 (FGF1). Thrombin failed to induce FGF1 release from protease-activated receptor 1 (PAR1) null fibroblasts, indicating that this effect was dependent on PAR1. Similarly to thrombin, FGF1 expression and release were induced by TRAP, a specific oligopeptide agonist of PAR1. These results identify a novel aspect of the crosstalk between FGF and thrombin signaling pathways which both play important roles in tissue repair and angiogenesis.

Hyaluronan-mediated angiogenesis in vascular disease: uncovering RHAMM and CD44 receptor signaling pathways

The correct formation of new blood vessels from existing vasculature (angiogenesis) is essential for embryogenesis and the effective repair of damaged or wounded tissues. However, excessive and detrimental vascularization also occurs in neoplasia, promoting tumour growth and metastasis, as well as in proliferative diabetic retinopathy and atherosclerosis. Greater understanding of the mechanisms controlling the angiogenic process will allow optimization of wound healing, and provide mechanisms to inhibit vascularization in tumours and other diseases. Evidence supports a cascade of events in which the perturbation of one of the steps is sufficient to significantly inhibit neovascularization. The extracellular macromolecules, notably glycosaminoglycans (GAGs), are important mediators of angiogenesis. Hyaluronan (HA), a large, non-sulphated GAG, was first discovered in the vitreous of the eye [.], and is ubiquitously expressed in the extracellular matrix (ECM) of tissues. Native high molecular weight HA (n-HA) is anti-angiogenic, whereas HA degradation products (o-HA; 3-10 disaccharides) stimulate endothelial cell (EC) proliferation, migration and tube formation following activation of specific HA receptors in particular, CD44 and Receptor for HA-Mediated Motility (RHAMM, CD168). The involvement of HA in the regulation of angiogenesis makes it an attractive therapeutic target. We review the role of o-HA in modulation of angiogenesis during tissue injury, and vascular disease, focusing on receptor-mediated signal transduction pathways that have been evaluated.

Proteomic inventory of "anchorless" proteins on the colon adenocarcinoma cell surface

Surface proteins play important pathophysiological roles in health and disease, and accumulating proteomics-based studies suggest that several "non-membrane" proteins are sorted to the cell surface by unconventional mechanisms. Importantly, these proteins may comprise attractive therapeutic targets and novel disease markers for colon cancer. To perform a proteomics-based inventory of these so-called "anchorless" surface proteins, intact colon adenocarcinoma SW480 cells were labeled with membrane-impermeable biotin after which only soluble biotinylated proteins were isolated and identified by nanoLC-MS/MS. Computer-assisted analysis predicted that only 9 of the 97 identified surface-exposed proteins have predicted secretory signal peptides, whereas 2 other proteins have a putative transmembrane segment. Of the 9 proteins with putative signal peptides, 1 was predicted to be retained at the cell surface by a GPI-anchor, whereas 5 other proteins contained an ER-retention motif (KDEL) that should prevent them from being sorted to the cell surface. The remaining 86 soluble "surface" proteins lack known export signals and the possibility that these proteins are candidate substrates of non-classical transporters or exported by unconventional mechanisms is discussed. Alternatively, the large number of "intracellular" and ER-resident proteins may imply that biotinylation approaches are not only specific for surface proteins, but also biased against a certain subset of non-surface proteins. This underscores the importance of post-proteomic verification of proteomics-based inventories on surface-exposed proteins, which eventually should reveal to which extent non-classical export and retention mechanisms contribute to the sorting of "anchorless" proteins to the surface of colon tumor cells.

Release of FGF1 and p40 synaptotagmin 1 correlates with their membrane destabilizing ability

Fibroblast growth factor (FGF)1 is released from cells as a constituent of a complex that contains the small calcium binding protein S100A13, and the p40 kDa form of synaptotagmin (Syt)1, through an ER-Golgi-independent stress-induced pathway. FGF1 and the other components of its secretory complex are signal peptide-less proteins. We examined their capability to interact with lipid bilayers by studying protein-induced carboxyfluorescein release from liposomes of different phospholipid (pL) compositions. FGF1, p40 Syt1, and S100A13 induced destabilization of liposomes composed of acidic but not of zwitterionic pL. We produced mutants of FGF1 and p40 Syt1, in which specific basic amino acid residues in the regions that bind acidic pL were substituted. The ability of these mutants to induce liposomes destabilization was strongly attenuated, and they exhibited drastically diminished spontaneous and stress-induced release. Apparently, the non-classical release of FGF1 and p40 Syt1 involves destabilization of membranes containing acidic pL.

Galectin-1: a small protein with major functions

Galectins are a family of carbohydrate-binding proteins with an affinity for beta-galactosides. Galectin-1 (Gal-1) is differentially expressed by various normal and pathological tissues and appears to be functionally polyvalent, with a wide range of biological activity. The intracellular and extracellular activity of Gal-1 has been described. Evidence points to Gal-1 and its ligands as one of the master regulators of such immune responses as T-cell homeostasis and survival, T-cell immune disorders, inflammation and allergies as well as host-pathogen interactions. Gal-1 expression or overexpression in tumors and/or the tissue surrounding them must be considered as a sign of the malignant tumor progression that is often related to the long-range dissemination of tumoral cells (metastasis), to their dissemination into the surrounding normal tissue, and to tumor immune-escape. Gal-1 in its oxidized form plays a number of important roles in the regeneration of the central nervous system after injury. The targeted overexpression (or delivery) of Gal-1 should be considered as a method of choice for the treatment of some kinds of inflammation-related diseases, neurodegenerative pathologies and muscular dystrophies. In contrast, the targeted inhibition of Gal-1 expression is what should be developed for therapeutic applications against cancer progression. Gal-1 is thus a promising molecular target for the development of new and original therapeutic tools.

Genetic screen for signal peptides in Hydra reveals novel secreted proteins and evidence for non-classical protein secretion

We have screened a Hydra cDNA library for sequences encoding N-terminal signal peptides using the yeast invertase secretion vector pSUC [Jacobs et al., 1997. A genetic selection for isolating cDNAs encoding secreted proteins. Gene 198, 289-296]. We isolated and sequenced 907 positive clones; 88% encoded signal peptides; 12% lacked signal peptides. By searching the Hydra EST database we identified full-length sequences for the selected clones. These encoded 37 known proteins with signal peptides and 40 novel Hydra-specific proteins with signal peptides. Localization of two signal peptide-containing sequences, VEGF and ferritin, to the secretory pathway was confirmed with GFP fusion proteins. In addition, we isolated 105 clones which lacked signal peptides but which supported invertase secretion from yeast. Isolation of plasmids from these clones and retransformation in invertase-negative yeast cells confirmed the phenotype. A GFP fusion protein of one such clone encoding the foot morphogen pedibin was localized to the cytoplasm in transfected Hydra cells and did not enter the ER/Golgi secretory pathway. Secretion of pedibin and other proteins lacking signal peptides appears to occur by a non-classical protein secretion route.

The sphingosine and diacylglycerol kinase superfamily of signaling kinases: localization as a key to signaling function

The sphingosine and diacylglycerol kinases form a superfamily of structurally related lipid signaling kinases. One of the striking features of these kinases is that although they are clearly involved in agonist-mediated signaling, this signaling is accomplished with only a moderate (and sometimes no) increase in the enzymatic activity of the enzymes. Here, we summarize findings that indicate that signaling by these kinases is strongly dependent on their localization to specific intracellular sites rather than on increases in enzyme activity. Both the substrates and products of these enzymes are bioactive lipids. Moreover, many of the metabolic enzymes that act on these lipids are found in specific organelles. Therefore, changes in the membrane localization of these signaling kinases have profound effects not only on the production of signaling lipid phosphates but also on the metabolism of the upstream signaling lipids.

Lipid raft organization and function in brush borders of epithelial cells

Polarized epithelial cells of multicellular organisms confront the environment with a highly specialized apical cell membrane that differs in composition and function from that facing the internal milieu. In the case of absorptive cells, such as the small intestinal enterocyte and the kidney proximal tubule cell, the apical cell membrane is formed as a brush border, composed of regular, dense arrays of microvilli. Hydrolytic ectoenzymes make up the bulk of the microvillar membrane proteins, endowing the brush border with a huge digestive capacity. Several of the major enzymes are localized in lipid rafts, which, for the enterocyte in particular, are organized in a unique fashion. Glycolipids, rather than cholesterol, together with the divalent lectin galectin-4, define these rafts, which are stable and probably quite large. The architecture of these rafts supports a digestive/absorptive strategy for nutrient assimilation, but also serves as a portal for a large number of pathogens. Caveolae are well-known vehicles for internalization of lipid rafts, but in the enterocyte brush border, binding of cholera toxin is followed by uptake via a clathrin-dependent mechanism. Recently, 'anti-glycosyl' antibodies were shown to be deposited in the enterocyte brush border. When the antibodies were removed from the membrane, other carbohydrate-binding proteins, including cholera toxin, increased their binding to the brush border. Thus, anti-glycosyl antibodies may serve as guardians of glycolipid-based rafts, protecting them from lumenal pathogens and in this way be part of an ongoing 'cross-talk' between indigenous bacteria and the host.

Voltage-Dependent N-Type Ca2+ Channel Activity Regulates the Interaction Between FGF-1 and S100A13 for Stress-Induced Non-Vesicular Release

1. The Ca(2+)-mediated regulation of interaction between FGF-1 and S100A13 in NG108-15 cells was studied. When the stress by depriving B27 supplement from the culture was given, cellular levels of both proteins were decreased, while their releases were significantly increased within 3 h. These stress-induced changes were all abolished by amlexanox, an anti-allergic drug. 2. These releases were significantly inhibited by the addition of EGTA or BAPTA-AM, cellular or extracellular Ca(2+)-chelating agent, respectively. The addition of omega-conotoxin GVIA, a N-type Ca(2+)-channel blocker caused a complete inhibition of the release, while increased the cytosolic levels of both proteins. However, omega-conotoxin MVIIC, the non-N-type Ca(2+)-channel blocker was ineffective. 3. In NG108-15 cells, which had been transfected with Venus-FGF-1 and CFP-S100A13, the supplement-deprivation stress caused several spike-type fluorescence resonance energy transfer (FRET) signals, suggesting that both proteins showing interaction would be immediately released. These spikes were completely abolished by the addition of omega-conotoxin GVIA. However, the addition of amlexanox caused bell-shaped FRET signals without spikes. 4. Thus, it is suggested that the interaction between FGF-1 and S100A13 responsible for stress-induced non-vesicular release is dependent of Ca(2+)-influx through N-type Ca(2+)-channels.

Evidence for serum-deprivation-induced co-release of FGF-1 and S100A13 from astrocytes

Since fibroblast growth factor (FGF)-1 lacks conventional amino-terminal signal peptide essential for endoplasmic reticulum (ER)-Golgi pathway, the mode of release of this polypeptide remains to be fully understood. We attempted to characterize the non-classical (non-vesicular) mode of FGF-1 release in the analyses using immunocytochemistry and immunoblot of conditioned medium (CM) from astrocytes. FGF-1 was completely released from astrocytes upon serum-deprivation stress in a Brefeldin A-insensitive manner. In the immunoprecipitation study using anti-FGF-1 IgG, S100A13 was identified to be the major protein co-eluted with FGF-1. The interaction between GST-FGF-1 and Strep-tag II-S100A13 was found to be Ca(2+)-sensitive, and to require the C-terminal 11 amino acid peptide sequence of S100A13. The overexpression of Delta88-98 mutant of S100A13 selectively inhibited the serum-deprivation stress-induced release of FGF-1, but not the release of S100A13 mutant from C6 glioma cells. However, amlexanox, anti-allergic drug whose target is S100A13, completely inhibited the stress-induced release of FGF-1 as well as S100A13. The stress-induced release of both proteins was also abolished by BAPTA-AM, an intracellular Ca(2+) chelating agent. The serum-deprivation caused Ca(2+) spikes in omega-conotoxin GVIA and thapsigargin-sensitive manner. All these results suggest that S100A13 is a cargo molecule for the serum-deprivation stress-induced non-classical release of FGF-1, and that its driving force of protein-protein interaction and release is possibly mediated by Ca(2+)-induced Ca(2+) release (CICR) coupled to N-type Ca(2+) channel activity.

Carbohydrate-induced modulation of cell membrane. VIII. Agglutination with mammalian lectin galectin-1 increases osmofragility and membrane fluidity of trypsinized erythrocytes

Interaction of lectins with cell surface determinants may alter membrane properties. Using trypsinized rabbit erythrocytes as model we tested the capacity of an endogenous lectin in this respect. Galectin-1 is a member of an adhesion/growth-regulatory family known to interact for example with ganglioside GM(1) and also the hydrophobic tail of oncogenic H-Ras. Assays on membrane fluidity and osmofragility detect galectin-1's capacity to increase the parameters. Moreover, it increases susceptibility of erythrocytes to radical damage. These observations indicate the potential of this endogenous lectin to affect membrane properties beyond the immediate interaction with cell surface epitopes.

Cell surface counter receptors are essential components of the unconventional export machinery of galectin-1

Galectin-1 is a component of the extracellular matrix as well as a ligand of cell surface counter receptors such as β-galactoside–containing glycolipids, however, the molecular mechanism of galectin-1 secretion has remained elusive. Based on a nonbiased screen for galectin-1 export mutants we have identified 26 single amino acid changes that cause a defect of both export and binding to counter receptors. When wild-type galectin-1 was analyzed in CHO clone 13 cells, a mutant cell line incapable of expressing functional galectin-1 counter receptors, secretion was blocked. Intriguingly, we also find that a distant relative of galectin-1, the fungal lectin CGL-2, is a substrate for nonclassical export from Chinese hamster ovary (CHO) cells. Alike mammalian galectin-1, a CGL-2 mutant defective in β-galactoside binding, does not get exported from CHO cells. We conclude that the β-galactoside binding site represents the primary targeting motif of galectins defining a galectin export machinery that makes use of β-galactoside–containing surface molecules as export receptors for intracellular galectin-1.