Glycosylphosphatidylinositol Moiety Research Articles

Structural analysis of Plasmodium falciparum ookinete surface antigen Pfs28 relevant for malaria vaccine design

Pfs28 is a Plasmodium falciparum malaria transmission-blocking vaccine candidate that is anchored to the parasite surface through a C-terminal glycosylphosphatidylinositol (GPI) moiety, and plays a role in parasite survival in the mosquito midgut. Pfs28 contains epidermal growth factor (EGF)-like domains and is part of a family of sexual stage malaria proteins that includes the related vaccine antigen Pfs25. The lack of structural definition of Pfs28 and the immune response to this candidate has limited further malaria vaccine development for this antigen. Here, we present the crystal structure of Pfs28, examine its conservation with P. vivax Pvs28, and evaluate the cross-reactivity of Pfs28 to antibodies that recognize Pfs25. Pfs28 is comprised of four EGF-like domains stabilized by ten disulfide bridges with an overall architecture that highly resembles Pfs25. Despite the high sequence and structural similarity between these antigens, no cross reactivity of Pfs28 to anti-Pfs25 monoclonal antibodies could be demonstrated.

Elimination of GPI2 suppresses glycosylphosphatidylinositol GlcNAc transferase activity and alters GPI glycan modification in Trypanosoma brucei

Many eukaryotic cell-surface proteins are post-translationally modified by a glycosylphosphatidylinositol (GPI) moiety that anchors them to the cell membrane. The biosynthesis of GPI anchors is initiated in the endoplasmic reticulum by transfer of GlcNAc from UDP-GlcNAc to phosphatidylinositol. This reaction is catalyzed by GPI GlcNAc transferase, a multisubunit complex comprising the catalytic subunit Gpi3/PIG-A as well as at least five other subunits, including the hydrophobic protein Gpi2, which is essential for the activity of the complex in yeast and mammals, but the function of which is not known. To investigate the role of Gpi2, we exploited Trypanosoma brucei (Tb), an early diverging eukaryote and important model organism that initially provided the first insights into GPI structure and biosynthesis. We generated insect-stage (procyclic) trypanosomes that lack TbGPI2 and found that in TbGPI2-null parasites, (i) GPI GlcNAc transferase activity is reduced, but not lost, in contrast with yeast and human cells, (ii) the GPI GlcNAc transferase complex persists, but its architecture is affected, with loss of at least the TbGPI1 subunit, and (iii) the GPI anchors of procyclins, the major surface proteins, are underglycosylated when compared with their WT counterparts, indicating the importance of TbGPI2 for reactions that occur in the Golgi apparatus. Immunofluorescence microscopy localized TbGPI2 not only to the endoplasmic reticulum but also to the Golgi apparatus, suggesting that in addition to its expected function as a subunit of the GPI GlcNAc transferase complex, TbGPI2 may have an enigmatic noncanonical role in Golgi-localized GPI anchor modification in trypanosomes.

A knockout cell library of GPI biosynthetic genes for functional studies of GPI-anchored proteins

Over 100 kinds of proteins are expressed as glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) on the cell surface in mammalian cells. GPI-APs possess unique properties in terms of their intracellular trafficking and association with lipid rafts. Although it is clear that GPI-APs play critical roles in various biological phenomena, it is poorly understood how the GPI moiety contributes to these mechanisms. More than 30 genes are involved in the correct biosynthesis of GPI-APs. We here constructed a cell library in which 32 genes involved in GPI biosynthesis were knocked out in human embryonic kidney 293 cells. Using the cell library, the surface expression and sensitivity to phosphatidylinositol-specific phospholipase C of GPI-APs were analyzed. Furthermore, we identified structural motifs of GPIs that are recognized by a GPI-binding toxin, aerolysin. The cell-based GPI-knockout library could be applied not only to basic researches, but also to applications and methodologies related to GPI-APs.

Tissue-Nonspecific Alkaline Phosphatase in Central Nervous System Health and Disease: A Focus on Brain Microvascular Endothelial Cells.

Tissue-nonspecific alkaline phosphatase (TNAP) is an ectoenzyme bound to the plasma membranes of numerous cells via a glycosylphosphatidylinositol (GPI) moiety. TNAP’s function is well-recognized from earlier studies establishing its important role in bone mineralization. TNAP is also highly expressed in cerebral microvessels; however, its function in brain cerebral microvessels is poorly understood. In recent years, few studies have begun to delineate a role for TNAP in brain microvascular endothelial cells (BMECs)—a key component of cerebral microvessels. This review summarizes important information on the role of BMEC TNAP, and its implication in health and disease. Furthermore, we discuss current models and tools that may assist researchers in elucidating the function of TNAP in BMECs.

Short and sweet: How glycans impact prion conversion, cofactor interactions, and cross-species transmission.

Prion diseases are transmissible and incurable neurodegenerative disorders of humans and animals. Similar to other neurodegenerative disorders, such as Alzheimer’s disease and Parkinson’s disease, prion diseases are characterized by protein aggregation in the central nervous system (CNS). The key molecular event in prion disease is the conformational conversion of the cellular prion protein, PrPC, into a misfolded and aggregated conformer, PrPSc, which templates further PrPC misfolding [1]. Although this same protein misfolding event occurs in all prion diseases, affected individuals expressing identical PrPC sequences can exhibit strikingly heterogeneous clinical and pathological phenotypes. These phenotypic differences have been linked to distinct PrPSc conformations, known as strains, yet the source of prion strain diversity is incompletely understood. One potential contributor to strain diversity lies in the posttranslational modifications (PTMs) on PrPC, which add a layer of structural complexity to an otherwise highly conserved protein. PrPC is posttranslationally modified by the covalent linkage of (i) 0 to 2 N-linked glycans at positions 181 and 197 (human PrP) and (ii) a glycosylphosphatidylinositol (GPI) moiety, which anchors PrPC in the outer leaflet of the plasma membrane [2,3]. The N-linked glycans on PrPC are branched [bi- (51%), tri- (32%), or tetra-antennary (17%)] and terminally sialylated, predominantly via alpha 2,6 linkages [4–6]. While necessary for regulating protein interactions and function, PTMs can profoundly modulate the pathogenesis of neurodegenerative diseases. For example, aberrant hyperphosphorylation of tau protein leads to tau detachment from microtubules and fibrillization into neurofibrillary tangles, a pathologic hallmark of Alzheimer’s disease. Prion diseases are no different, and the presence of PrP PTMs can markedly alter both the disease phenotype and transmission barrier. Here we explore how PrP PTMs impact prion conversion, cross-species transmission, the neuroinflammatory response, and PrP interaction with cofactors.

Lamin is essential for nuclear localization of the GPI synthesis enzyme PIG-B and GPI-anchored protein production in Drosophila.

ABSTRACTMembrane lipid biosynthesis is a complex process that occurs in various intracellular compartments. In Drosophila, phosphatidylinositol glycan-B (PIG-B), which catalyzes addition of the third mannose in glycosylphosphatidylinositol (GPI), localizes to the nuclear envelope (NE). Although this NE localization is essential for Drosophila development, the underlying molecular mechanism remains unknown. To elucidate this mechanism, we identified PIG-B-interacting proteins by performing immunoprecipitation followed by proteomic analysis. We then examined which of these proteins are required for the NE localization of PIG-B. Knockdown of Lamin Dm0, a B-type lamin, led to mislocalization of PIG-B from the NE to the endoplasmic reticulum. Lamin Dm0 associated with PIG-B at the inner nuclear membrane, a process that required the tail domain of Lamin Dm0. Furthermore, GPI moieties were distributed abnormally in the Lamin Dm0 mutant. These data indicate that Lamin Dm0 is involved in the NE localization of PIG-B and is required for proper GPI-anchor modification of proteins.

CD157: From Myeloid Cell Differentiation Marker to Therapeutic Target in Acute Myeloid Leukemia.

Human CD157/BST-1 and CD38 are dual receptor-enzymes derived by gene duplication that belong to the ADP ribosyl cyclase gene family. First identified over 30 years ago as Mo5 myeloid differentiation antigen and 10 years later as Bone Marrow Stromal Cell Antigen 1 (BST-1), CD157 proved not to be restricted to the myeloid compartment and to have a diversified functional repertoire ranging from immunity to cancer and metabolism. Despite being a NAD+-metabolizing ectoenzyme anchored to the cell surface through a glycosylphosphatidylinositol moiety, the functional significance of human CD157 as an enzyme remains unclear, while its receptor role emerged from its discovery and has been clearly delineated with the identification of its high affinity binding to fibronectin. The aim of this review is to provide an overview of the immunoregulatory functions of human CD157/BST-1 in physiological and pathological conditions. We then focus on CD157 expression in hematological tumors highlighting its emerging role in the interaction between acute myeloid leukemia and extracellular matrix proteins and its potential utility for monoclonal antibody targeted therapy in this disease.

Structural Remodeling and Shedding of GPI-Anchors

A number of proteins on the plasma membrane are modified by the glycolipid, glycosylphosphatidylinositol (GPI). These proteins are called GPI-anchored proteins (GPI-APs) and approximately 150 proteins exist as GPI-anchored forms in mammals. Characteristics of GPI-APs include: 1) forming membrane domains with specific lipids such as sphingolipids and cholesterol on the cell membrane and 2) cleavage at the GPI moiety to release the protein from the cell surface. Since complete structures of GPI anchors were determined in 1988, genes involved in biosynthetic pathways and structural remodeling of GPI anchors have been identified. In recent years, structural remodeling of GPI has been shown to regulate quality control, transport and localization of GPI-APs. In addition, particular GPI-cleaving enzymes cause shedding of GPI-APs from the cell surface. Here, I introduce recent progress about the structural chemistry and cleaving mechanisms of GPI-anchors.

Anchored Fibromodulin as a Novel Target in Chronic Lymphocytic Leukemia: Diagnostic and Therapeutic Implications.

We have previously reported the aberrant expression of Fibromodulin (FMOD) in patients with chronic lymphocytic leukemia (CLL). Although FMOD has been considered as a cytoplasmic or secretory protein, we discovered the cell surface expression of FMOD in leukemic B cells via anchoring with glycosylphosphatidylinositol (GPI). To evaluate FMOD as a new biomarker in CLL patients in comparison with healthy individuals. A monoclonal antibody was generated against human FMOD. The cell surface expression of FMOD in 52 CLL patients and 45 healthy individuals were compared by flow cytometry. A bacterial phosphatidylinositol-specific phospholipase C (PI-PLC) was used to determine the cell surface localization of FMOD using ELISA and flow cytometry techniques. Annexin V-FITC and propidium iodide (PI) was used to detect apoptosis induction in CLL PBMCs following in vitro incubation with anti-FMOD mAb. The results demonstrated the widespread cell surface expression of GPI-anchored FMOD in CLL patients (median: 79.9 %), although healthy individuals had low FMOD expression (median: 6.2 %) (p≤0.0001). The cut-off value of FMOD expression was estimated with high sensitivity and specificity at 17.9 %. Furthermore, in vitro apoptosis induction of leukemic cells following incubation with anti-FMOD mAb showed a direct apoptosis of CLL cells (27.9%) with very low effect on healthy PBMCs (6%). The membrane-anchoring of FMOD by means of a GPI moiety in leukemic cells supports FMOD as a highly potential diagnostic and therapeutic target in CLL patients.

N-Glycan-dependent protein folding and endoplasmic reticulum retention regulate GPI-anchor processing.

Glycosylphosphatidylinositol (GPI) anchoring of proteins is a conserved posttranslational modification in the endoplasmic reticulum (ER). Soon after GPI is attached, an acyl chain on the GPI inositol is removed by post-GPI attachment to proteins 1 (PGAP1), a GPI-inositol deacylase. This is crucial for switching GPI-anchored proteins (GPI-APs) from protein folding to transport states. We performed haploid genetic screens to identify factors regulating GPI-inositol deacylation, identifying seven genes. In particular, calnexin cycle impairment caused inefficient GPI-inositol deacylation. Calnexin was specifically associated with GPI-APs, dependent on N-glycan and GPI moieties, and assisted efficient GPI-inositol deacylation by PGAP1. Under chronic ER stress caused by misfolded GPI-APs, inositol-acylated GPI-APs were exposed on the cell surface. These results indicated that N-glycans participate in quality control and temporal ER retention of GPI-APs, ensuring their correct folding and GPI processing before exiting from the ER. Once the system is disrupted by ER stress, unprocessed GPI-APs become exposed on the cell surface.

Developmental expression of membrane type 4-matrix metalloproteinase (Mt4-mmp/Mmp17) in the mouse embryo.

Matrix metalloproteinases (MMPs) constitute a large group of endoproteases that play important functions during embryonic development, tumor metastasis and angiogenesis by degrading components of the extracellular matrix. Within this family, we focused our study on Mt4-mmp (also called Mmp17) that belongs to a distinct subset that is anchored to the cell surface via a glycosylphosphatidylinositol (GPI) moiety and with the catalytic site exposed to the extracellular space. Information about its function and substrates is very limited to date, and little has been reported on its role in the developing embryo. Here, we report a detailed expression analysis of Mt4-mmp during mouse embryonic development by using a LacZ reporter transgenic mouse line. We showed that Mt4-mmp is detected from early stages of development to postnatal stages following a dynamic and restricted pattern of expression. Mt4-mmp was first detected at E8.5 limited to the intersomitic vascularization, the endocardial endothelium and the dorsal aorta. Mt4-mmpLacZ/+ cells were also observed in the neural crest cells, somites, floor plate and notochord at early stages. From E10.5, expression localized in the limb buds and persists during limb development. A strong expression in the brain begins at E12.5 and continues to postnatal stages. Specifically, staining was observed in the olfactory bulb, cerebral cortex, hippocampus, striatum, septum, dorsal thalamus and the spinal cord. In addition, LacZ-positive cells were also detected during eye development, initially at the hyaloid artery and later on located in the lens and the neural retina. Mt4-mmp expression was confirmed by quantitative RT-PCR and western blot analysis in some embryonic tissues. Our data point to distinct functions for this metalloproteinase during embryonic development, particularly during brain formation, angiogenesis and limb development.

The functional roles of T-cadherin in mammalian biology

T-cadherin is a cadherin and cell adhesion molecule that is anchored to the cell surface membrane through a glycosylphosphatidylinositol moiety. T-cadherin lacks a transmembrane and cytoskeletal domain, suggesting that it must interact with other membrane-bound molecules to elicit cellular signaling to modulate normal cellular functions, and alternatively its absence can be a factor in promoting neoplasia. Moreover, apart from binding to itself it can sequester adiponectin to the cell surface. Consistent with these observations, recent research has expanded the scope of T-cadherin’s role in cancer, neuronal function, metabolism and cardiovascular disease. In this context, we highlight the experimental and genomic evidence that links T-cadherin with these diseases. In particular, we discuss how T-cadherin homophilic and heterophilic interactions impact on signaling pathways and cellular behavior.

HSV1 MicroRNA Modulation of GPI Anchoring and Downstream Immune Evasion

SummaryHerpes simplex virus 1 (HSV1) is a ubiquitous human pathogen that utilizes variable mechanisms to evade immune surveillance. The glycosylphosphatidylinositol (GPI) anchoring pathway is a multistep process in which a myriad of different proteins are covalently attached to a GPI moiety to be presented on the cell surface. Among the different GPI-anchored proteins there are many with immunological importance. We present evidence that the HSV1-encoded miR H8 directly targets PIGT, a member of the protein complex that covalently attaches proteins to GPI in the final step of GPI anchoring. This results in a membrane down-modulation of several different immune-related, GPI-anchored proteins, including ligands for natural killer-activating receptors and the prominent viral restriction factor tetherin. Thus, we suggest that by utilizing just one of dozens of miRNAs encoded by HSV1, the virus can counteract the host immune response at several key points.

Thematic Review Series: Glycosylphosphatidylinositol (GPI) Anchors: Biochemistry and Cell Biology Biosynthesis of GPI-anchored proteins: special emphasis on GPI lipid remodeling

Glycosylphosphatidylinositols (GPIs) act as membrane anchors of many eukaryotic cell surface proteins. GPIs in various organisms have a common backbone consisting of ethanolamine phosphate (EtNP), three mannoses (Mans), one non-N-acetylated glucosamine, and inositol phospholipid, whose structure is EtNP-6Manα-2Manα-6Manα-4GlNα-6myoinositol-P-lipid. The lipid part is either phosphatidylinositol of diacyl or 1-alkyl-2-acyl form, or inositol phosphoceramide. GPIs are attached to proteins via an amide bond between the C-terminal carboxyl group and an amino group of EtNP. Fatty chains of inositol phospholipids are inserted into the outer leaflet of the plasma membrane. More than 150 different human proteins are GPI anchored, whose functions include enzymes, adhesion molecules, receptors, protease inhibitors, transcytotic transporters, and complement regulators. GPI modification imparts proteins with unique characteristics, such as association with membrane microdomains or rafts, transient homodimerization, release from the membrane by cleavage in the GPI moiety, and apical sorting in polarized cells. GPI anchoring is essential for mammalian embryogenesis, development, neurogenesis, fertilization, and immune system. Mutations in genes involved in remodeling of the GPI lipid moiety cause human diseases characterized by neurological abnormalities. Yeast Saccharomyces cerevisiae has >60 GPI-anchored proteins (GPI-APs). GPI is essential for growth of yeast. In this review, we discuss biosynthesis of GPI-APs in mammalian cells and yeast with emphasis on the lipid moiety.

Long-Term Outcome of Allogeneic Stem Cell Transplantation in Patients with Combined Paroxysmal Nocturnal Hemoglobinuria and Aplastic Anemia (AA/PNH syndrome)

Background:Paroxysmal nocturnal hemoglobinuria (PNH) is a nonmalignant clonal disorder of hematopoietic stem cells characterized by a somatic mutation in the PIG-A gene, encoding the glycosyl phosphatidylinositol (GPI) moiety. PNH clones lack GPI-anchored proteins (GPI-AP) which inhibit the activation and cytolytic functions of complement. Recently, Eculizumab, humanized monoclonal antibody directed against complement component C5, has used increasingly for the patients with hemolytic PNH. However, the patients with PNH clone and bone marrow failure syndrome (i.e. aplastic anemia) should be treated as their predominant clinical manifestation. Allogeneic stem cell transplantation (SCT) can be curative treatment option especially for PNH patients with combined aplastic anemia (AA). The aim of the present study was to evaluate long-term outcome of allogeneic SCT in patients with AA/PNH.Methods: Total of 27 patients with PNH clones underwent allogeneic SCT at our institution between Jan 1998 and Mar 2014. Among them, seven patients had classic PNH and 20 patients with cytopenia had AA/PNH (with bone marrow evidence of a concomitant AA). We analyzed long-term transplant outcomes in 20 patients with AA/PNH.Results: There were 12 male and 8 female patients with a median age of 34 years (range, 13-51 years). The median interval from the diagnosis to transplantation was 8 months (range; 1-201 months). The median transfusions prior to SCT were 33 units (range; 8-208 units). Pre-transplant GPI-AP deficient neutrophils and erythrocytes were 46% (0-99) and 15.6% (0-88), respectively. Median white blood cell, absolute neutrophil count, hemoglobin, and platelet at transplant were 2.3×109/L, 0.7×109/L, 7.9 g/dL, and 21×109/L, respectively. Median LDH level was 714 U/L (range; 273-6499 U/L) and 11 (55%) patients had LDH ≥1.5x upper normal limit. PNH patients with SAA (n=14), VSAA (n=4), or non-SAA (n=2) received SCT from sibling (s) donor (n=15) or unrelated (u) donor (n=5). The conditioning regimen for s-SCT consisted of fludarabine (180 mg/m2) + cyclophosphamide (CY, 100 mg/kg) + ATG (10 mg/kg) (n=11), or busulfex (12.8 mg/kg) + CY (120mg/kg) (n=4). The conditioning regimen for u-SCT was TBI (fractionated, 800 cGy) + CY (100-120 mg/kg) ± ATG (2.5 mg/kg). GVHD prophylaxis consisted of CsA + MTX in s-SCT and FK506 + mini-MTX in u-SCT, respectively. After a median follow-up of 57 months (range 4.7-122.1), the 5-year estimated OS rates were 90.0 ± 6.7%. Two patients died of treatment-related mortality (TRM), including acute GVHD (n=1) and cerebral hemorrhage (n=1), respectively. Except one patient with early TRM, 19 patients engrafted with no secondary graft-failure. The cumulative incidence of acute GVHD (≥grade II) and chronic GVHD was 25.0 ± 1.0% and 26.3 ± 10.4%, respectively. PNH clones disappeared at median 1.8 months (range 0.9-11.9) after SCT and reemerging of PNH clone was not observed in all patients.Conclusion: This study showed that long-term transplant outcome in patients with AA/PNH were comparable to that of allogeneic SCT in SAA (the 3-year estimated OS rates were 92.7 and 89 % for s-SCT and u-SCT, respectively) at our institution (ASH Annual Meeting Abstracts 2012;120:4151). Therefore, application of allogeneic SCT should be considered in PNH patients with AA in case of availability of well matched donor. DisclosuresNo relevant conflicts of interest to declare.

CD157 at the intersection between leukocyte trafficking and epithelial ovarian cancer invasion

CD157 is a member of the ADP-ribosyl cyclase gene family that is involved in the metabolism of NAD. CD157 behaves both as an ectoenzyme and as a receptor. Though CD157 is anchored to the membrane by a glycosylphosphatidylinositol moiety, which makes it unsuitable to transduce signals on its own, it exploits its localization in selected membrane microdomains and its proclivity to interact with integrins to accomplish receptor functions. Initially characterized as a stromal and myeloid antigen involved in the control of leukocyte adhesion, migration and diapedesis, CD157 was subsequently found to have a far wider distribution. In particular, CD157 was found to be expressed by epithelial ovarian cancer cells where it is involved in interactions among tumor cells, extracellular matrix proteins and mesothelium. The overall picture inferred from experimental and clinical observations is that CD157 is a critical player both in leukocyte trafficking and in ovarian cancer invasion and metastasis formation. In this review, we will discuss the biological mechanisms underpinning the role of CD157 in the control of leukocyte migration and ovarian cancer dissemination.

Multi-Dimensional Analysis of Fluorescence Fluctuation Spectroscopy of Lipid Anchored Proteins in Live Cells Reveal Complex Organization in the Plasma Membrane

Membrane organization of lipid anchored proteins plays a fundamental role in proper cell signaling behavior. Several important signaling proteins, such as members of the Src family of kinases, are anchored to the membrane by covalent acyl, alkyl, and glycosylphosphatidylinositol (GPI) moieties. A convenient approach to studying this lipid anchor organization in cells is to image genetically encoded lipidation motifs of various signaling proteins fused to a fluorescent protein. However, previous efforts to reduce this lateral organization to a simple model often ignore the complexity inherent in cell membranes and have contributed to an inconsistent picture of membrane organization. We use two-color fluorescence fluctuation spectroscopy to study the spatial organization of fluorescently tagged lipid anchors in live cell membranes. In order to retain the complexity of this organization, we simultaneously observe several modes of lipid anchor dynamics by employing a multi-dimensional analysis of the time-resolved fluorescence data. Time-correlated single photon counting (TCSPC) data is gathered on a 3-detector system optimized for simultaneous polarization and two-color fluorescence correlation spectroscopy (FCS). The degree of dynamic colocalization of eGFP- and mCherry-fused lipid anchors can be quantified by pulse-interleaved excitation fluorescence cross-correlation spectroscopy (PIE-FCCS), while single color FCS evaluates the translational mobility of the lipid anchors. The photon counting method also allows for time-resolved fluorescence anisotropy (TRFA) measurements, evaluating the rotational diffusion of the fluorescent protein, as well as fluorescence lifetime measurements to determine energy transfer. Taken together, this simultaneous multi-dimensional analysis of time-resolved fluorescence data offers a powerful approach towards determining the complex organizations of lipid anchors in live cell membranes.

Loss of T-cadherin (CDH-13) regulates AKT signaling and desensitizes cells to apoptosis in melanoma

An understanding of signaling pathways is a basic requirement for the treatment of melanoma. Currently, kinases are at the center of melanoma therapies. According to our research, additional alternative molecules are equally important for development of melanoma. In this regard, cancer progression is, among other factors, driven by an altered adhesion via cadherins. For instance, the de-regulated expression of the adhesion molecule T-cadherin is found in various cancer types, including melanoma, and influences migration and invasion. T-cadherin is thought to affect cellular function largely through its signaling and not its adhesion properties because the molecule is anchored into the cell membrane by a glycosylphosphatidylinositol (GPI) moiety. However, detailed knowledge about the consequences of the loss of T-cadherin in melanoma is currently lacking. For this reason, we were interested in assessing which signaling pathways are initiated by T-cadherin. The tumor growth of subcutaneously injected T-cadherin-positive melanoma cells was diminished compared with T-cadherin-negative cells in nude mice. The difference in tumor volume was not due to decreased proliferation but rather due to increased apoptosis. After the expression of T-cadherin was induced, we detected V-AKT murine thymoma viral oncogene homolog (AKT) and FoxO3a hypophosphorylation accompanied by the downregulation of the antiapoptotic molecules BCL-2, BCL-x and Clusterin. Furthermore, we detected a diminished transcriptional activity of CREB and AP-1. We demonstrated that T-cadherin functions as a pro-apoptotic tumor suppressor that antagonizes AKT/CREB/AP-1/FoxO3a signaling, whereas NFκB, TCF/LEF and mTOR are not part of the T-cadherin signaling pathway. Notably, we found that the restoration of T-cadherin in melanoma cells causes sensitization to apoptosis induced by CD95/Fas antibody CH-11.

Effect of the GPI anchor of human Thy-1 on antibody recognition and function

Thymocyte differentiation antigen-1 (Thy-1) is a glycosylphosphatidylinositol (GPI)-linked cell surface glycoprotein expressed on numerous cell types, which regulates signals affecting cell adhesion, migration, differentiation, and survival. In addition, Thy-1 has been detected in the serum, cerebral spinal fluid, wound fluid from venous ulcers, synovial fluid from joints in rheumatoid arthritis, and, more recently, urine. We previously detected Thy-1 in the conditioned media of cytokine-stimulated lung fibroblasts, suggesting that Thy-1 shedding may be a response to cellular stress. Soluble and membrane-bound forms of Thy-1 from in vivo sources have been shown to be identical in size when deglycosylated, suggesting that soluble Thy-1 is separated from the diacyl glycerol portion of its GPI anchor by hydrolysis within the GPI moiety. For Thy-1- and other GPI-anchored proteins, delipidation induces a stable change in conformation that manifests itself in a change in antibody affinity for soluble forms. Using epitope-tagged recombinant soluble Thy-1, we report that widely available monoclonal antibodies to human Thy-1 are unable to detect soluble Thy-1 by immunoblotting. We re-evaluated the Thy-1 that we previously reported in the conditioned media of normal human lung fibroblasts and found it to be entirely insoluble. These findings suggest that most Thy-1 reported in body fluids retains its GPI anchor and may be associated with membrane fragments or vesicles. This phenomenon should be considered in the generation of antibodies and controls for Thy-1 bioassays. Furthermore, the changes in Thy-1 conformation with delipidation, beyond affecting antibody affinity, likely affect the ligand affinity and biological function of soluble vs released membrane-associated forms.

Structural Organization of Mammalian Prions as Probed by Limited Proteolysis

Elucidation of the structure of PrPSc continues to be one major challenge in prion research. The mechanism of propagation of these infectious agents will not be understood until their structure is solved. Given that high resolution techniques such as NMR or X-ray crystallography cannot be used, a number of lower resolution analytical approaches have been attempted. Thus, limited proteolysis has been successfully used to pinpoint flexible regions within prion multimers (PrPSc). However, the presence of covalently attached sugar antennae and glycosylphosphatidylinositol (GPI) moieties makes mass spectrometry-based analysis impractical. In order to surmount these difficulties we analyzed PrPSc from transgenic mice expressing prion protein (PrP) lacking the GPI membrane anchor. Such animals produce prions that are devoid of the GPI anchor and sugar antennae, and, thereby, permit the detection and location of flexible, proteinase K (PK) susceptible regions by Western blot and mass spectrometry-based analysis. GPI-less PrPSc samples were digested with PK. PK-resistant peptides were identified, and found to correspond to molecules cleaved at positions 81, 85, 89, 116, 118, 133, 134, 141, 152, 153, 162, 169 and 179. The first 10 peptides (to position 153), match very well with PK cleavage sites we previously identified in wild type PrPSc. These results reinforce the hypothesis that the structure of PrPSc consists of a series of highly PK-resistant β-sheet strands connected by short flexible PK-sensitive loops and turns. A sizeable C-terminal stretch of PrPSc is highly resistant to PK and therefore perhaps also contains β-sheet secondary structure.