GPI-anchored proteins: now you see 'em, now you don't

Loss of function of the bHLH transcription factor Nrd1 in tomato enhances resistance to Pseudomonas syringae

Basic helix-loop-helix (bHLH) transcription factors constitute a superfamily in eukaryotes, but their roles in plant immunity remain largely uncharacterized. We found that the transcript abundance in tomato (Solanum lycopersicum) leaves of one bHLH transcription factor-encoding gene, negative regulator of resistance to DC3000 1 (Nrd1), increased significantly after treatment with the immunity-inducing flgII-28 peptide. Plants carrying a loss-of-function mutation in Nrd1 (Δnrd1) showed enhanced resistance to Pseudomonas syringae pv. tomato (Pst) DC3000 although early pattern-triggered immunity responses, such as generation of reactive oxygen species and activation of mitogen-activated protein kinases after treatment with flagellin-derived flg22 and flgII-28 peptides, were unaltered compared to wild-type plants. RNA-sequencing (RNA-seq) analysis identified a gene, Arabinogalactan protein 1 (Agp1), whose expression is strongly suppressed in an Nrd1-dependent manner. Agp1 encodes an arabinogalactan protein, and overexpression of the Agp1 gene in Nicotiana benthamiana led to ∼10-fold less Pst growth compared to the control. These results suggest that the Nrd1 protein promotes tomato susceptibility to Pst by suppressing the defense gene Agp1. RNA-seq also revealed that the loss of Nrd1 function has no effect on the transcript abundance of immunity-associated genes, including AvrPtoB tomato-interacting 9 (Bti9), Cold-shock protein receptor (Core), Flagellin sensing 2 (Fls2), Flagellin sensing (Fls3), and Wall-associated kinase 1 (Wak1) upon Pst inoculation, suggesting that the enhanced immunity observed in the Δnrd1 mutants is due to the activation of key PRR signaling components as well as the loss of Nrd1-regulated suppression of Agp1.

A cell-based phenotypic library selection and screening approach for the de novo discovery of novel functional chimeric antigen receptors

Anti-tumor therapies that seek to exploit and redirect the cytotoxic killing and effector potential of autologous or syngeneic T cells have shown extraordinary promise and efficacy in certain clinical settings. Such cells, when engineered to express synthetic chimeric antigen receptors (CARs) acquire novel targeting and activation properties which are governed and orchestrated by, typically, antibody fragments specific for a tumor antigen of interest. However, it is becoming increasingly apparent that not all antibodies are equal in this regard, with a growing appreciation that ‘optimal’ CAR performance requires a consideration of multiple structural and contextual parameters. Thus, antibodies raised by classical approaches and intended for other applications often perform poorly or not at all when repurposed as CARs. With this in mind, we have explored the potential of an in vitro phenotypic CAR library discovery approach that tightly associates antibody-driven bridging of tumor and effector T cells with an informative and functionally relevant CAR activation reporter signal. Critically, we demonstrate the utility of this enrichment methodology for ‘real world’ de novo discovery by isolating several novel anti-mesothelin CAR-active scFv candidates.

Major Surface Antigens in Zoonotic Babesia

Human babesiosis results from a combination of tick tropism for humans, susceptibility of a host to sustain Babesia development, and contact with infected ticks. Climate modifications and increasing diagnostics have led to an expanded number of Babesia species responsible for human babesiosis, although, to date, most cases have been attributed to B. microti and B. divergens. These two species have been extensively studied, and in this review, we mostly focus on the antigens involved in host–parasite interactions. We present features of the major antigens, so-called Bd37 in B. divergens and BmSA1/GPI12 in B. microti, and highlight the roles of these antigens in both host cell invasion and immune response. A comparison of these antigens with the major antigens found in some other Apicomplexa species emphasizes the importance of glycosylphosphatidylinositol-anchored proteins in host–parasite relationships. GPI-anchor cleavage, which is a property of such antigens, leads to soluble and membrane-bound forms of these proteins, with potentially differential recognition by the host immune system. This mechanism is discussed as the structural basis for the protein-embedded immune escape mechanism. In conclusion, the potential consequences of such a mechanism on the management of both human and animal babesiosis is examined.

A Conserved Mitochondrial Chaperone-Protease Complex Involved in Protein Homeostasis

Mitochondria are essential organelles involved in cellular energy production. The inner mitochondrial membrane protein stomatin-like protein 2 (SLP-2) is a member of the SPFH (stomatin, prohibitin, flotilin, and HflK/C) superfamily and binds to the mitochondrial glycerophospholipid cardiolipin, forming cardiolipin-enriched membrane domains to promote the assembly and/or stabilization of protein complexes involved in oxidative phosphorylation. In addition, human SLP-2 anchors a mitochondrial processing complex required for proteolytic regulation of proteins involved in mitochondrial dynamics and quality control. We now show that deletion of the gene encoding the Trypanosoma brucei homolog TbSlp2 has no effect on respiratory protein complex stability and mitochondrial functions under normal culture conditions and is dispensable for growth of T. brucei parasites. In addition, we demonstrate that TbSlp2 binds to the metalloprotease TbYme1 and together they form a large mitochondrial protein complex. The two proteins negatively regulate each other's expression levels by accelerating protein turnover. Furthermore, we show that TbYme1 plays a role in heat-stress resistance, as TbYme1 knock-out parasites displayed mitochondrial fragmentation and loss of viability when cultured at elevated temperatures. Unbiased interaction studies uncovered putative TbYme1 substrates, some of which were differentially affected by the absence of TbYme1. Our results support emerging evidence for the presence of mitochondrial quality control pathways in this ancient eukaryote.

Identification and Clinical Associations of 3 Forms of Circulating T-cadherin in Human Serum

CONTEXT

T-cadherin (T-cad) is a glycosylphosphatidylinositol (GPI)-anchored cadherin that mediates adiponectin to induce exosome biogenesis and secretion, protect cardiovascular tissues, promote muscle regeneration, and stimulate therapeutic heart protection by transplanted mesenchymal stem cells. CDH13, the gene locus of T-cad, affects plasma adiponectin levels most strongly, in addition to affecting cardiovascular disease risk and glucose homeostasis. Recently, it has been suggested that T-cad exists in human serum, although the details are still unclear.

OBJECTIVE

To validate the existence of T-cad forms in human serum and investigate the association with clinical parameters of type 2 diabetes patients.

METHODS

Using newly developed monoclonal antibodies against T-cad, pooled human serum was analyzed, and novel T-cad enzyme-linked immunosorbent assays (ELISAs) were developed. The serum T-cad concentrations of 183 Japanese type 2 diabetes patients were measured in a cross-sectional observational study. The main outcome measure was the existence of soluble T-cad in human serum.

RESULTS

There were 3 forms of soluble T-cad: a 130-kDa form with a prodomain, a 100-kDa mature form, and a 30-kDa prodomain in human serum. Using newly developed ELISAs to measure them simultaneously, we found that the 130-kDa form of T-cad positively correlated with plasma adiponectin (r = 0.28, P < .001), although a physiological interaction with adiponectin was not observed in serum. The unique 30-kDa prodomain was associated with several clinical parameters in diabetes patients.

CONCLUSION

We identified 3 novel forms of soluble T-cad. Their importance as disease markers and/or biomarkers of adiponectin function and the possible bioactivity of the respective molecules require further investigation.

A cryptic non-GPI-anchored cytosolic isoform of CD59 controls insulin exocytosis in pancreatic β-cells by interaction with SNARE proteins

CD59 is a glycosylphosphatidylinositol (GPI)-anchored cell surface inhibitor of the complement membrane attack complex (MAC). We showed previously that CD59 is highly expressed in pancreatic islets but is down-regulated in rodent models of diabetes. CD59 knockdown but not enzymatic removal of cell surface CD59 led to a loss of glucose-stimulated insulin secretion (GSIS), suggesting that an intracellular pool of CD59 is required. In this current paper, we now report that non-GPI-anchored CD59 is present in the cytoplasm, colocalizes with exocytotic protein vesicle-associated membrane protein 2, and completely rescues GSIS in cells lacking endogenous CD59 expression. The involvement of cytosolic non-GPI-anchored CD59 in GSIS is supported in phosphatidylinositol glycan class A knockout GPI anchor-deficient β-cells, in which GSIS is still CD59 dependent. Furthermore, site-directed mutagenesis demonstrated different structural requirements of CD59 for its 2 functions, MAC inhibition and GSIS. Our results suggest that CD59 is retrotranslocated from the endoplasmic reticulum to the cytosol, a process mediated by recognition of trimmed N-linked oligosaccharides, supported by the partial glycosylation of non-GPI-anchored cytosolic CD59 as well as the failure of N-linked glycosylation site mutant CD59 to reach the cytosol or rescue GSIS. This study thus proposes the previously undescribed existence of non-GPI-anchored cytosolic CD59, which is required for insulin secretion.-Golec, E., Rosberg, R., Zhang, E., Renström, E., Blom, A. M., King, B. C. A cryptic non-GPI-anchored cytosolic isoform of CD59 controls insulin exocytosis in pancreatic β-cells by interaction with SNARE proteins.

Glycosylphosphatidylinositol-Anchored Proteins in Arabidopsis and One of Their Common Roles in Signaling Transduction

Diverse proteins are found modified with glycosylphosphatidylinositol (GPI) at their carboxyl terminus in eukaryotes, which allows them to associate with membrane lipid bilayers and anchor on the external surface of the plasma membrane. GPI-anchored proteins (GPI-APs) play crucial roles in various processes, and more and more GPI-APs have been identified and studied. In this review, previous genomic and proteomic predictions of GPI-APs in Arabidopsis have been updated, which reveal their high abundance and complexity. From studies of individual GPI-APs in Arabidopsis, certain GPI-APs have been found associated with partner receptor-like kinases (RLKs), targeting RLKs to their subcellular localization and helping to recognize extracellular signaling polypeptide ligands. Interestingly, the association might also be involved in ligand selection. The analyses suggest that GPI-APs are essential and widely involved in signal transduction through association with RLKs.

Thy-1, a Pathfinder Protein for the Post-genomic Era

Thy-1 is possibly the smallest of cell surface proteins – 110 amino acids folded into an Immunoglobulin variable domain, tethered to the outer leaflet of the cell surface membrane via just the two saturated fatty acids of its glycosylphosphatidylinositol (GPI) anchor. Yet Thy-1 is emerging as a key regulator of differentiation in cells of endodermal, mesodermal, and ectodermal origin, acting as both a ligand (for certain integrins and other receptors), and as a receptor, able to modulate signaling and hence differentiation in the Thy-1-expressing cell. This is an extraordinary diversity of molecular pathways to be controlled by a molecule that does not even cross the cell membrane. Here I review aspects of the cell biology of Thy-1, and studies of its role as deduced from gene knock-out studies, that suggest how this protein can participate in so many different signaling-related functions. While mechanisms differ in molecular detail, it appears overall that Thy-1 dampens down signaling to control function.

A molecular dynamics model for glycosylphosphatidyl-inositol anchors: “flop down” or “lollipop”?

Computational model for GPI anchors tested in DMPC and POPC bilayers. The free anchor rarely occurs as an erected “lollipop-like” conformation, it rather “flops down” onto the bilayer surface. Yet an attached protein (here green fluorescent protein) exhibits extensive orientational flexibility due to the phospho-ethanolamine linker.

Monomeric amyloid-β reduced amyloid-β oligomer-induced synapse damage in neuronal cultures

Alzheimer's disease is a progressive neurodegenerative disease characterized by the accumulation of amyloid-β (Aβ) in the brain. Aβ oligomers are believed to cause synapse damage resulting in the memory deficits that are characteristic of this disease. Since the loss of synaptic proteins in the brain correlates closely with the degree of dementia in Alzheimer's disease, the process of Aβ-induced synapse damage was investigated in cultured neurons by measuring the loss of synaptic proteins. Soluble Aβ oligomers, derived from Alzheimer's-affected brains, caused the loss of cysteine string protein and synaptophysin from neurons. When applied to synaptosomes Aβ oligomers increased cholesterol concentrations and caused aberrant activation of cytoplasmic phospholipase A₂ (cPLA₂). In contrast, Aβ monomer preparations did not affect cholesterol concentrations or activate synaptic cPLA₂, nor did they damage synapses. The Aβ oligomer-induced aggregation of cellular prion proteins (PrP^C) at synapses triggered the activation of cPLA₂ that leads to synapse degeneration. Critically, Aβ monomer preparations did not cause the aggregation of PrP^C; rather they reduced the Aβ oligomer-induced aggregation of PrP^C. The presence of Aβ monomer preparations also inhibited the Aβ oligomer-induced increase in cholesterol concentrations and activation of cPLA₂ in synaptosomes and protected neurons against the Aβ oligomer-induced synapse damage. These results support the hypothesis that Aβ monomers are neuroprotective. We hypothesise that synapse damage may result from a pathological Aβ monomer:oligomer ratio rather than the total concentrations of Aβ within the brain.

A new anti-mesothelin antibody targets selectively the membrane-associated form

Mesothelin is a glycosylphosphatidylinositol (GPI)-anchored membrane protein that shows promise as a target for antibody-directed cancer therapy. High levels of soluble forms of the antigen represent a barrier to directing therapy to cellular targets. The ability to develop antibodies that can selectively discriminate between membrane-bound and soluble conformations of a specific protein, and thus target only the membrane-associated antigen, is a substantive issue. We show that use of a tolerance protocol provides a route to such discrimination. Mice were tolerized with soluble mesothelin and a second round of immunizations was performed using mesothelin transfected P815 cells. RNA extracted from splenocytes was used in phage display to obtain mesothelin-specific antigen-binding fragments (Fabs) that were subsequently screened by flow cytometry and ELISA. This approach generated 147 different Fabs in 34 VH-CDR3 families. Utilizing competition assays with soluble protein and mesothelin-containing serum obtained from metastatic cancer patients, 10 of these 34 VH-CDR3 families were found to bind exclusively to the membrane-associated form of mesothelin. Epitope mapping performed for the 1H7 clone showed that it does not recognize GPI anchor. VH-CDR3 sequence analysis of all Fabs showed significant differences between Fabs selective for the membrane-associated form of the antigen and those that recognize both membrane bound and soluble forms. This work demonstrates the potential to generate an antibody specific to the membrane-bound form of mesothelin. 1H7 offers potential for therapeutic application against mesothelin-bearing tumors, which would be largely unaffected by the presence of the soluble antigen.

The role of complement inhibitors beyond controlling inflammation

The complement system is an arm of innate immunity that aids in the removal of pathogens and dying cells. Due to its harmful, pro-inflammatory potential, complement is controlled by several soluble and membrane-bound inhibitors. This family of complement regulators has been recently extended by the discovery of several new members, and it is becoming apparent that these proteins harbour additional functions. In this review, the current state of knowledge of the physiological functions of four complement regulators will be described: cartilage oligomeric matrix protein (COMP), CUB and sushi multiple domains 1 (CSMD1), sushi domain-containing protein 4 (SUSD4) and CD59. Complement activation is involved in both the development of and defence against cancer. COMP expression is pro-oncogenic, whereas CSMD1 and SUSD4 act as tumour suppressors. These effects may be related in part to the complex influence of complement on cancer but also depend on unrelated functions such as the protection of cells from endoplasmic reticulum stress conveyed by intracellular COMP. CD59 is the main inhibitor of the membrane attack complex, and its deficiency leads to complement attack on erythrocytes and severe haemolytic anaemia, which is now amenable to treatment with an inhibitor of C5 cleavage. Unexpectedly, the intracellular pool of CD59 is crucial for insulin secretion from pancreatic β-cells. This finding is one of several relating to the intracellular functions of complement proteins, which until recently were only considered to be present in the extracellular space. Understanding the alternative functions of complement inhibitors may unravel unexpected links between complement and other physiological systems, but is also important for better design of therapeutic complement inhibition.

Array comparative genomic hybridization and genomic sequencing in the diagnostics of the causes of congenital anomalies

The aim of this review is to provide the current state of knowledge about the usefulness of modern genetic technologies in uncovering the causality of isolated and multiple congenital anomalies. Array comparative genomic hybridization and next-generation sequencing have revolutionized the clinical approach to patients with these phenotypes. Both technologies enable early diagnosis, especially in clinically challenging newborn populations, and help to uncover genetic defects associated with various phenotypes. The application of both complementary methods could assist in identifying many variants that may simultaneously be involved in the development of a number of isolated or multiple congenital anomalies. Both technologies carry serious variant misinterpretation risks as well. Therefore, the methods of variant classification and accessible variant databases are mentioned. A useful strategy of clinical genetic testing with the application of both methodologies is presented. Finally, future directions and challenges are briefly commented on in this review.

Differential subcellular distribution of four phospholipase C isoforms and secretion of GPI-PLC activity

Phospholipase C (PLC) is an important enzyme of signal transduction pathways by generation of second messengers from membrane lipids. PLCs are also indicated to cleave glycosylphosphatidylinositol (GPI)-anchors of surface proteins thus releasing these into the environment. However, it remains unknown whether this enzymatic activity on the surface is due to distinct PLC isoforms in higher eukaryotes. Ciliates have, in contrast to other unicellular eukaryotes, multiple PLC isoforms as mammals do. Thus, Paramecium represents a perfect model to study subcellular distribution and potential surface activity of PLC isoforms. We have identified distinct subcellular localizations of four PLC isoforms indicating functional specialization. The association with different calcium release channels (CRCs) argues for distinct subcellular functions. They may serve as PI-PLCs in microdomains for local second messenger responses rather than free floating IP₃. In addition, all isoforms can be found on the cell surface and they are found together with GPI-cleaved surface proteins in salt/ethanol washes of cells. We can moreover show them in medium supernatants of living cells where they have access to GPI-anchored surface proteins. Among the isoforms we cannot assign GPI-PLC activity to specific PLC isoforms; rather each PLC is potentially responsible for the release of GPI-anchored proteins from the surface.

Biomedical applications of glycosylphosphatidylinositol-anchored proteins

Glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) use a unique posttranslational modification to link proteins to lipid bilayer membranes. The anchoring structure consists of both a lipid and carbohydrate portion and is highly conserved in eukaryotic organisms regarding its basic characteristics, yet highly variable in its molecular details. The strong membrane targeting property has made the anchors an interesting tool for biotechnological modification of lipid membrane-covered entities from cells through extracellular vesicles to enveloped virus particles. In this review, we will take a closer look at the mechanisms and fields of application for GPI-APs in lipid bilayer membrane engineering and discuss their advantages and disadvantages for biomedicine.

Characterization of a midgut mucin-like glycoconjugate of Lutzomyia longipalpis with a potential role in Leishmania attachment

Background

Leishmania parasites are transmitted by phlebotomine sand flies and a crucial step in their life-cycle is the binding to the sand fly midgut. Laboratory studies on sand fly competence to Leishmania parasites suggest that the sand flies fall into two groups: several species are termed “specific/restricted” vectors that support the development of one Leishmania species only, while the others belong to so-called “permissive” vectors susceptible to a wide range of Leishmania species. In a previous study we revealed a correlation between specificity vs permissivity of the vector and glycosylation of its midgut proteins. Lutzomyia longipalpis and other four permissive species tested possessed O-linked glycoproteins whereas none were detected in three specific vectors examined.

Results

We used a combination of biochemical, molecular and parasitological approaches to characterize biochemical and biological properties of O-linked glycoprotein of Lu. longipalpis. Lectin blotting and mass spectrometry revealed that this molecule with an apparent molecular weight about 45–50 kDa corresponds to a putative 19 kDa protein with unknown function detected in a midgut cDNA library of Lu. longipalpis. We produced a recombinant glycoprotein rLuloG with molecular weight around 45 kDa. Anti-rLuloG antibodies localize the native glycoprotein on epithelial midgut surface of Lu. longipalpis. Although we could not prove involvement of LuloG in Leishmania attachment by blocking the native protein with anti-rLuloG during sand fly infections, we demonstrated strong binding of rLuloG to whole surface of Leishmania promastigotes.

Conclusions

We characterized a novel O-glycoprotein from sand fly Lutzomyia longipalpis. It has mucin-like properties and is localized on the luminal side of the midgut epithelium. Recombinant form of the protein binds to Leishmania parasites in vitro. We propose a role of this molecule in Leishmania attachment to sand fly midgut.

With or without rafts? Alternative views on cell membranes

The fundamental mechanisms of protein and lipid organization at the plasma membrane have continued to engage researchers for decades. Among proposed models, one idea has been particularly successful which assumes that sterol-dependent nanoscopic phases of different lipid chain order compartmentalize proteins, thereby modulating protein functionality. This model of membrane rafts has sustainably sparked the fields of membrane biophysics and biology, and shifted membrane lipids into the spotlight of research; by now, rafts have become an integral part of our terminology to describe a variety of cell biological processes. But is the evidence clear enough to continue supporting a theoretical concept which has resisted direct proof by observation for nearly twenty years? In this essay, we revisit findings that gave rise to and substantiated the raft hypothesis, discuss its impact on recent studies, and present alternative mechanisms to account for plasma membrane heterogeneity.

Hyaluronidase 2 (HYAL2) is expressed in endothelial cells, as well as some specialized epithelial cells, and is required for normal hyaluronan catabolism

Hyaluronidase 2 (HYAL2) is a membrane-anchored protein that is proposed to initiate the degradation of hyaluronan (HA) in the extracellular matrix. The distribution of HYAL2 in tissues, and of HA in tissues lacking HYAL2, is largely unexplored despite the importance of HA metabolism in several disease processes. Herein, we use immunoblot and histochemical analyses to detect HYAL2 and HA in mouse tissues, as well as agarose gel electrophoresis to examine the size of HA. HYAL2 was detected in all tissues that were examined, including the brain. It was localized to the surface and cytoplasm of endothelial cells, as well as specialized epithelial cells in several tissues, including the skin. Accumulated HA, often of higher molecular mass than that in control tissues, was detected in tissues from Hyal2 (-/-) mice. The accumulating HA was located near to where HYAL2 is normally found, although in some tissues, it was distant from the site of HYAL2 localization. Overall, HYAL2 was highest in tissues that remove HA from the circulation (liver, lymph node and spleen), but the levels of HA accumulation in Hyal2 (-/-) mice were highest in tissues that catabolize locally synthesized HA. Our results support HYAL2's role as an extracellular enzyme that initiates HA breakdown in somatic tissues. However, our findings also suggest that HYAL2 contributes to HA degradation through other routes, perhaps as a soluble or secreted form.

Monoacylated Cellular Prion Proteins Reduce Amyloid-β-Induced Activation of Cytoplasmic Phospholipase A2 and Synapse Damage

Alzheimer’s disease (AD) is a progressive neurodegenerative disease characterized by the accumulation of amyloid-β (Aβ) and the loss of synapses. Aggregation of the cellular prion protein (PrP^C) by Aβ oligomers induced synapse damage in cultured neurons. PrP^C is attached to membranes via a glycosylphosphatidylinositol (GPI) anchor, the composition of which affects protein targeting and cell signaling. Monoacylated PrP^C incorporated into neurons bound “natural Aβ”, sequestering Aβ outside lipid rafts and preventing its accumulation at synapses. The presence of monoacylated PrP^C reduced the Aβ-induced activation of cytoplasmic phospholipase A₂ (cPLA₂) and Aβ-induced synapse damage. This protective effect was stimulus specific, as treated neurons remained sensitive to α-synuclein, a protein associated with synapse damage in Parkinson’s disease. In synaptosomes, the aggregation of PrP^C by Aβ oligomers triggered the formation of a signaling complex containing the cPLA₂.a process, disrupted by monoacylated PrP^C. We propose that monoacylated PrP^C acts as a molecular sponge, binding Aβ oligomers at the neuronal perikarya without activating cPLA₂ or triggering synapse damage.

A model for targeting colon carcinoma cells using single-chain variable fragments anchored on virus-like particles via glycosyl phosphatidylinositol anchor

PURPOSE

VLPs displaying tumor targeting single-chain variable fragments (VLP-rscFvs) which targets tumor-associated glycoprotein-72 (TAG-72) marker protein have a potential for immunotherapy against colon carcinoma tumors. In this study, scFvs anchored on VLPs using glycosylphosphatidylinositol (GPI) were prepared to target colon carcinoma spheroids in vitro.

METHODS

VLPs-rscFvs were produced by co-injecting two types of Bombyx mori nucleopolyhedrovirus (BmNPV) bacmids, encoding RSV-gag and rscFvs cDNA into silkworm larvae. Large unilamellar vesicles (LUVs) of 100 nm in diameter were made using 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and packaged with Sulforhodamine B (SRB). LUV-SRB was used to associate with VLP-rscFvs assisted by GP64 present on VLP-rscFvs to produce VLP-rscFv associated SRB (VLP-rscFvs-SRB) at pH 7.5.

RESULTS

The antigenicity of the purified VLPs-rScFvs was confirmed by enzyme-linked immunosorbent assay (ELISA) using TAG-72 as antigen. LUV-SRB made of DOPC was used to associate with 100 μg of VLP-rscFvs to produce VLP-rscFv-SRB. Specific delivery and penetration of SRB up to 100 μm into the spheroids shows the potential of the new model.

CONCLUSIONS

The current study demonstrated the display, expression and purification of VLP-rscFvs efficiently. As a test model VLP-rscFv-SRB were prepared which can be used for immunotherapy. rscFvs provide the specificity needed to target tumors and VLPs serve as carrier transporting the dye to target.

ESAT-6-gpi DNA vaccine augmented the specific antitumour efficacy induced by the tumour vaccine B16F10-ESAT-6-gpi/IL-21 in a mouse model

In this study, we hypothesized that the mice immunized with the glycosylphosphatidylinositol (GPI) anchored 6-kDa early-secreted antigenic target (ESAT-6) DNA vaccine (ESAT-6-gpi) and the tumour vaccine B16F10-ESAT-6-gpi/IL-21 might significantly enhance immune responses and antimelanoma efficacy. Our experimental results indicated that the anti-ESAT-6 antibody induced by the DNA vaccine ESAT-6-gpi bound ESAT-6 to the surface of tumour vaccine to activate a complement classical pathway and resulted in the B16F10 tumour cell lysis and apoptosis, which served as a potential trigger for breaking melanomatous immune tolerance to elicit an initiation of natural antimelanoma immunity. Our innovative approach of using the DNA vaccine ESAT-6-gpi priming and the tumour vaccine B16F10-ESAT-6-gpi/IL-21 boosting induced strong antimelanoma immunity that inhibited melanomatous growth. These findings highlighted the DNA vaccine ESAT-6-gpi as an immune enhancer to augment the immune efficacy of the tumour vaccine B16F10-ESAT -6-gpi/IL-21 against melanoma in a mouse model.

Role of CD73 in Renal Sympathetic Neurotransmission in the Mouse Kidney

Adenosine formed during renal sympathetic nerve stimulation (RSNS) enhances, by activating A₁ receptors, the postjunctional effects of released norepinephrine and participates in renal sympathetic neurotransmission. Because in many cell types CD73 (ecto-5'-nucleotidase) is important for the conversion of 5'-AMP to adenosine, we investigated whether CD73 is necessary for normal renal sympathetic neurotransmission. In isolated kidneys from CD73 wild-type mice (CD73 +/+; n=17) perfused at a constant rate with Tyrode's solution, RSNS increased perfusion pressure by 17±4, 36±8 and 44±10 mm Hg at 3, 5 and 7 Hz, respectively. Similar responses were elicited from kidneys isolated from CD73 knockout mice (CD73 -/-; n=13; 28±11, 43±10 and 44±10 mm Hg at 3, 5 and 7 Hz, respectively); and a high concentration (100 μmol/L) of α,β-methyleneadenosine 5'-diphosphate (CD73 inhibitor) did not alter responses to RSNS in C57BL/6 mouse kidneys (n=5; 21±5, 36±8 and 43±9 at 3, 5 and 7 Hz, respectively). Measurements of renal venous adenosine and inosine (adenosine metabolite) by liquid chromatography-tandem mass spectrometry demonstrated that the metabolism of exogenous 5'-AMP to adenosine and inosine was similar in CD73 -/- versus CD73 +/+ kidneys. A₁ receptor mRNA expression was increased in CD73 -/- kidneys, and 2-chloro-N⁶-cyclopentyladenosine (0.1 μmol/L; A₁ receptor agonist) enhanced renovascular responses to norepinephrine more in CD73 -/- versus CD73 +/+ kidneys. We conclude that CD73 is not essential for renal sympathetic neurotransmission because in the absence of renal CD73 other enzymes metabolize 5'-AMP to adenosine and because of compensatory upregulation of postjunctional coincident signaling between norepinephrine and adenosine.

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.

Implications of lipid moiety in oligomerization and immunoreactivities of GPI-anchored proteins

Glycosylphosphatidylinositol (GPI) enriches GPI-anchored proteins (GPI-AP) in lipid rafts by intimate interaction of its lipid moiety with sphingolipids and cholesterol. In addition to such lipid-lipid interactions, it has been reported that GPI may interact with protein moiety linked to GPI and affect protein conformations because GPI delipidation reduced immunoreactivities of protein. Here, we report that GPI-APs that have not undergone fatty acid remodeling exhibit reduced immunoreactivities in Western blotting, similar to delipidated proteins, compared with normal remodeled GPI-APs. In contrast, immunostaining in flow cytometry and immunoprecipitation did not show significant differences between remodeled and unremodeled GPI-APs. Moreover, detection with premixed primary/secondary antibody complexes or Fab fragments eliminated this difference in Western blotting. These results indicate that normally remodeled GPI enhanced oligomerization of GPI-APs and that inefficient oligomerization of unremodeled GPI-APs was responsible for reduced immunoreactivities. Moreover, the reduction in immunoreactivities of delipidated GPI-APs was most likely caused by the same effect. Finally, by chemical cross-linking of surface proteins in living cells and cell killing assay using a pore-forming bacterial toxin, we showed that enhanced oligomerization by GPI-remodeling occurs under a physiological membrane environment. Thus, this study clarifies the significance of GPI fatty acid remodeling in oligomerization of GPI-APs and provides useful information for technical studies of these cell components.

Structural features affecting trafficking, processing, and secretion of Trypanosoma cruzi mucins

Trypanosoma cruzi is wrapped by a dense coat of mucin-type molecules encoded by complex gene families termed TcSMUG and TcMUC, which are expressed in the insect- and mammal-dwelling forms of the parasite, respectively. Here, we dissect the contribution of distinct post-translational modifications on the trafficking of these glycoconjugates. In vivo tracing and characterization of tagged-variants expressed by transfected epimastigotes indicate that although the N-terminal signal peptide is responsible for targeting TcSMUG products to the endoplasmic reticulum (ER), the glycosyl phosphatidylinositol (GPI)-anchor likely functions as a forward transport signal for their timely progression along the secretory pathway. GPI-minus variants accumulate in the ER, with only a minor fraction being ultimately released to the medium as anchorless products. Secreted products, but not ER-accumulated ones, display several diagnostic features of mature mucin-type molecules including extensive O-type glycosylation, Galf-based epitopes recognized by monoclonal antibodies, and terminal Galp residues that become readily sialylated upon addition of parasite trans-sialidases. Processing of N-glycosylation site(s) is dispensable for the overall TcSMUG mucin-type maturation and secretion. Despite undergoing different O-glycosylation elaboration, TcMUC reporters yielded quite similar results, thus indicating that (i) molecular trafficking signals are structurally and functionally conserved between mucin families, and (ii) TcMUC and TcSMUG products are recognized and processed by a distinct repertoire of stage-specific glycosyltransferases. Thus, using the fidelity of a homologous expression system, we have defined some biosynthetic aspects of T. cruzi mucins, key molecules involved in parasite protection and virulence.

Glycosylphosphatidylinositol anchors regulate glycosphingolipid levels

Glycosylphosphatidylinositol (GPI) anchor biosynthesis takes place in the endoplasmic reticulum (ER). After protein attachment, the GPI anchor is transported to the Golgi where it undergoes fatty acid remodeling. The ER exit of GPI-anchored proteins is controlled by glycan remodeling and p24 complexes act as cargo receptors for GPI anchor sorting into COPII vesicles. In this study, we have characterized the lipid profile of mammalian cell lines that have a defect in GPI anchor biosynthesis. Depending on which step of GPI anchor biosynthesis the cells were defective, we observed sphingolipid changes predominantly for very long chain monoglycosylated ceramides (HexCer). We found that the structure of the GPI anchor plays an important role in the control of HexCer levels. GPI anchor-deficient cells that generate short truncated GPI anchor intermediates showed a decrease in very long chain HexCer levels. Cells that synthesize GPI anchors but have a defect in GPI anchor remodeling in the ER have a general increase in HexCer levels. GPI-transamidase-deficient cells that produce no GPI-anchored proteins but generate complete free GPI anchors had unchanged levels of HexCer. In contrast, sphingomyelin levels were mostly unaffected. We therefore propose a model in which the transport of very long chain ceramide from the ER to Golgi is regulated by the transport of GPI anchor molecules.

Mannose receptor (MR) engagement by mesothelin GPI anchor polarizes tumor-associated macrophages and is blocked by anti-MR human recombinant antibody

Tumor-infiltrating macrophages respond to microenvironmental signals by developing a tumor-associated phenotype characterized by high expression of mannose receptor (MR, CD206). Antibody cross-linking of CD206 triggers anergy in dendritic cells and CD206 engagement by tumoral mucins activates an immune suppressive phenotype in tumor-associated macrophages (TAMs). Many tumor antigens are heavily glycosylated, such as tumoral mucins, and/or attached to tumor cells by mannose residue-containing glycolipids (GPI anchors), as for example mesothelin and the family of carcinoembryonic antigen (CEA). However, the binding to mannose receptor of soluble tumor antigen GPI anchors via mannose residues has not been systematically studied. To address this question, we analyzed the binding of tumor-released mesothelin to ascites-infiltrating macrophages from ovarian cancer patients. We also modeled functional interactions between macrophages and soluble mesothelin using an in vitro system of co-culture in transwells of healthy donor macrophages with human ovarian cancer cell lines. We found that soluble mesothelin bound to human macrophages and that the binding depended on the presence of GPI anchor and of mannose receptor. We next challenged the system with antibodies directed against the mannose receptor domain 4 (CDR4-MR). We isolated three novel anti-CDR4-MR human recombinant antibodies (scFv) using a yeast-display library of human scFv. Anti-CDR4-MR scFv #G11 could block mesothelin binding to macrophages and prevent tumor-induced phenotype polarization of CD206(low) macrophages towards TAMs. Our findings indicate that tumor-released mesothelin is linked to GPI anchor, engages macrophage mannose receptor, and contributes to macrophage polarization towards TAMs. We propose that compounds able to block tumor antigen GPI anchor/CD206 interactions, such as our novel anti-CRD4-MR scFv, could prevent tumor-induced TAM polarization and have therapeutic potential against ovarian cancer, through polarization control of tumor-infiltrating innate immune cells.

Enhancement of specific cellular immune response induced by glycosyl-phosphatidylinositol-anchored BCR/ABL and mIL-12

bcr/abl fusion gene is thought to be a promising target for chronic myelogenous leukemia (CML) patients to enhance immune response after attaining complete remission. In this study, we sought to enhance cellular immunity by co-expression of BCR/ABL and murine IL-12 gene on the tumor cell surface as a glycosyl-phosphatidylinositol (GPI)-form. The successfully constructed plasmid pBudCE4.1-BCR/ABL-GPI-mIL12 resulted in high levels of splenocyte proliferative responses, significant levels of IL-2 and IFNγ, and strong cytotoxic T-lymphocyte (CTL) responses in vitro. In a murine transplant model, the vaccinated mice showed decreased infiltration of leukemia cells and reduced expression of BCR/ABL transcripts and protein in bone marrow cells. Results of the present study indicated that this novel immunization strategy is useful in enhancing immune protection in mice, which would provide new insights into the development of effective vaccines for treating CML.

The alpha2delta subunits of voltage-gated calcium channels form GPI-anchored proteins, a posttranslational modification essential for function

Voltage-gated calcium channels are thought to exist in the plasma membrane as heteromeric proteins, in which the alpha1 subunit is associated with two auxiliary subunits, the intracellular beta subunit and the alpha(2)delta subunit; both of these subunits influence the trafficking and properties of Ca(V)1 and Ca(V)2 channels. The alpha(2)delta subunits have been described as type I transmembrane proteins, because they have an N-terminal signal peptide and a C-terminal hydrophobic and potentially transmembrane region. However, because they have very short C-terminal cytoplasmic domains, we hypothesized that the alpha(2)delta proteins might be associated with the plasma membrane through a glycosylphosphatidylinositol (GPI) anchor attached to delta rather than a transmembrane domain. Here, we provide biochemical, immunocytochemical, and mutational evidence to show that all of the alpha(2)delta subunits studied, alpha(2)delta-1, alpha(2)delta-2, and alpha(2)delta-3, show all of the properties expected of GPI-anchored proteins, both when heterologously expressed and in native tissues. They are substrates for prokaryotic phosphatidylinositol-phospholipase C (PI-PLC) and trypanosomal GPI-PLC, which release the alpha(2)delta proteins from membranes and intact cells and expose a cross-reacting determinant epitope. PI-PLC does not affect control transmembrane or membrane-associated proteins. Furthermore, mutation of the predicted GPI-anchor sites markedly reduced plasma membrane and detergent-resistant membrane localization of alpha(2)delta subunits. We also show that GPI anchoring of alpha(2)delta subunits is necessary for their function to enhance calcium currents, and PI-PLC treatment only reduces calcium current density when alpha(2)delta subunits are coexpressed. In conclusion, this study redefines our understanding of alpha(2)delta subunits, both in terms of their role in calcium-channel function and other roles in synaptogenesis.

Plasmodium falciparum merozoite surface protein 2 is unstructured and forms amyloid-like fibrils

Several merozoite surface proteins are being assessed as potential components of a vaccine against Plasmodium falciparum, the cause of the most serious form of human malaria. One of these proteins, merozoite surface protein 2 (MSP2), is unusually hydrophilic and contains tandem sequence repeats, characteristics of intrinsically unstructured proteins. A range of physicochemical studies has confirmed that recombinant forms of MSP2 are largely unstructured. Both dimorphic types of MSP2 (3D7 and FC27) are equivalently extended in solution and form amyloid-like fibrils although with different kinetics and structural characteristics. These fibrils have a regular underlying beta-sheet structure and both fibril types stain with Congo Red, but only the FC27 fibrils stain with Thioflavin T. 3D7 MSP2 fibrils seeded the growth of fibrils from 3D7 or FC27 MSP2 monomer indicating the involvement of a conserved region of MSP2 in fibril formation. Consistent with this, digestion of fibrils with proteinase K generated resistant peptides, which included the N-terminal conserved region of MSP2. A monoclonal antibody that reacted preferentially with monomeric recombinant MSP2 did not react with the antigen in situ on the merozoite surface. Glutaraldehyde cross-linking of infected erythrocytes generated MSP2 oligomers similar to those formed by polymeric recombinant MSP2. We conclude that MSP2 oligomers containing intermolecular beta-strand interactions similar to those in amyloid fibrils may be a component of the fibrillar surface coat on P. falciparum merozoites.

The glycosylphosphatidylinositol anchor: a complex membrane-anchoring structure for proteins

Positioned at the C-terminus of many eukaryotic proteins, the glycosylphosphatidylinositol (GPI) anchor is a posttranslational modification that anchors the modified protein in the outer leaflet of the cell membrane. The GPI anchor is a complex structure comprising a phosphoethanolamine linker, glycan core, and phospholipid tail. GPI-anchored proteins are structurally and functionally diverse and play vital roles in numerous biological processes. While several GPI-anchored proteins have been characterized, the biological functions of the GPI anchor have yet to be elucidated at a molecular level. This review discusses the structural diversity of the GPI anchor and its putative cellular functions, including involvement in lipid raft partitioning, signal transduction, targeting to the apical membrane, and prion disease pathogenesis. We specifically highlight studies in which chemically synthesized GPI anchors and analogues have been employed to study the roles of this unique posttranslational modification.

Isolation, purification, characterization and antigenic evaluation of GPI-anchored membrane proteins from Leishmania (Viannia) braziliensis

GPI-anchored proteins from the plasma membrane of Leishmania (Viannia) braziliensis promastigotes were isolated, characterized and their migration pattern compared with those from other Leishmania species. In all cases the SDS-PAGE migration patterns were obtained under reducing and non-reducing conditions, using DL-dithiothreitol (DTT) as a reducer agent. Our results reveal that under reducing conditions the SDS-PAGE migration pattern is modified as a consequence of the disruption of disulphur-bonds and protein transformation. This is demonstrated when in non-reducing conditions the L. (V.) braziliensis-GPI-anchored proteins pattern showed a group of bands over the 100kDa, and two more bands of 52kDa and 50kDa in four different isolates, whereas under reducing conditions the major GPI-anchored protein fractions were detected as bands of 63kDa, 50kDa and an increase of peptides between 34kDa and 22kDa. Similar modifications were detected in the SDS-PAGE migration patterns of GPI-anchored protein fractions from L. (Leishmania) donovani, L. (L.) mexicana and L. (L.) amazonensis run under the same reducing conditions. Antigenic evaluation carried out by Western blot revealed the presence of two very specific L. (V.) braziliensis-GPI-anchored protein bands of 50kDa and 28kDa. These bands were specifically recognized by anti-L. (V.) braziliensis-GPI-anchored protein serum from experimentally immunized animals. These two peptides were not detected when GPI-anchored protein fractions from L. (L.) donovani, L. (L.) mexicana and L. (L.) amazonensis, were challenged with the same anti-serum. The present results lead us to suggest the use of these two peptides as biochemical markers to identify and differentiate leishmaniasis caused by L. (V.) braziliensis. The lack of immunogenicity observed here with the peptide gp63, a very common protein detected in Leishmania species, is considered.

Development and in vitro validation of anti-mesothelin biobodies that prevent CA125/Mesothelin-dependent cell attachment

Preventing peritoneal implantation of carcinoma cells could prolong ovarian cancer patient remission and survival. Peritoneal cells constitutively express mesothelin, a ligand for CA125 that is expressed by tumor cells. Thus blocking CA125/mesothelin-dependent cell attachment may prevent or delay peritoneal metastatic recurrence. We developed a high-throughput screening system for reagents able to block CA125/mesothelin-dependent cell attachment with a sensitive quantitative readout. Using a novel yeast-expression system to produce secreted, in vivo biotinylated proteins and biobodies (Bbs), we generated anti-mesothelin Bbs. Anti-mesothelin Bbs derived from high affinity yeast-display scFv detected both membrane-bound and soluble mesothelins and inhibited CA125/mesothelin-dependent cell attachment.

The human airway trypsin-like protease modulates the urokinase receptor (uPAR, CD87) structure and functions

The human airway trypsin-like protease (HAT) is a respiratory epithelium-associated, type II transmembrane serine protease, which is also detected as an extracellular enzyme in lung fluids during airway inflammatory disorders. We have evaluated its capacity to affect the urokinase-type plasminogen activator receptor (uPAR), a membrane glycolipid-anchored, three-domain (D1D2D3) glycoprotein that plays a crucial role in innate immunity and inflammation by supporting cell migration and matrix degradation, with structure and biological properties that can be regulated via limited endoproteolysis. With the use of immunoblotting, flow immunocytometry, and ELISA analyses applied to a recombinant uPAR protein and to uPAR-expressing monocytic and human bronchial epithelial cells, it was shown that exposure of uPAR to soluble HAT in the range of 10-500 nM resulted in the proteolytic processing of the full-length (D1D2D3) into the truncated (D2D3) species, with cleavage occurring in the D1 to D2 linker sequence after arginine residues at position 83 and 89. Using immunohistochemistry, we found that both HAT and uPAR were expressed in the human bronchial epithelium. Moreover, transient cotransfection in epithelial cells showed that membrane coexpression of the two partners produced a constitutive and extensive shedding of the D1 domain, occurring for membrane-associated HAT concentrations in the nanomolar range. Because the truncated receptor was found to be unable to bind two of the major uPAR ligands, the adhesive matrix protein vitronectin and the serine protease urokinase, it thus appears that proteolytic regulation of uPAR by HAT is likely to modulate cell adherence and motility, as well as tissue remodeling during the inflammatory response in the airways.

The European lipidomics initiative: enabling technologies

Lipidomics is a new term to describe a scientific field that is a lot broader than lipidology, the science of lipids. Besides lipidology, lipidomics covers the lipid-metabolizing enzymes and lipid transporters, their genes and regulation; the quantitative determination of lipids in space and time, and the study of lipid function. Because lipidomics is concerned with all lipids and their enzymes and genes, it faces the formidable challenge to develop enabling technologies to comprehensively measure the expression, location, and regulation of lipids, enzymes, and genes in time, including high-throughput applications. The second challenge is to devise information technology that allows the construction of metabolic maps by browsing through connected databases containing the subsets of data in lipid structure, lipid metabolomics, proteomics, and genomics. In addition, to understand lipid function, on the one hand we need a broad range of imaging techniques to define where exactly the relevant events happen in the body, cells, and subcellular organelles; on the other hand, we need a thorough understanding of how lipids physically interact, especially with proteins. The final challenge is to apply this knowledge in the diagnosis, monitoring, and cure of lipid-related diseases.

Regulation of ecto-5'-nucleotidase activity via Ca2+-dependent, annexin 2-mediated membrane rearrangement?

The spatial segregation of the plasma membrane plays a prominent role in distinguishing and sorting a large number of signals a cell receives simultaneously. The plasma membrane comprises regions known as lipid rafts, which serve as signal-transduction hubs and platforms for sorting membrane-associated proteins. Ca(2+)-binding proteins of the annexin family have been ascribed a role in the regulation of raft dynamics. Glycosylphosphatidylinositol-anchored 5'-nucleotidase is an extracellular, raft-associated enzyme responsible for conversion of extracellular ATP into adenosine. Our results point to a regulation of ecto-5'-nucleotidase activity by Ca(2+)-dependent, annexin-mediated stabilization of membrane rafts.

Post-translational signal peptide cleavage controls differential epitope recognition in the QP-rich domain of recombinant Theileria parva PIM

The presence of the schizont stage of the obligate intracellular parasites Theileria parva or T. annulata in the cytoplasm of an infected leukocyte results in host cell transformation via a mechanism that has not yet been elucidated. Proteins, secreted by the schizont, or expressed on its surface, are of interest as they can interact with host cell molecules that regulate host cell proliferation and/or survival. The major schizont surface protein is the polymorphic immunodominant molecule, PIM, which contains a large glutamine- and proline-rich domain (QP-rd) that protrudes into the host cell cytoplasm. Analyzing QP-rd generated by in vitro transcription/translation, we found that the signal peptide was efficiently cleaved post-translationally upon addition of T cell lysate or canine pancreatic microsomes, whereas signal peptide cleavage of a control protein only occurred cotranslationally and in the presence of microsomal membranes. The QP-rd of PIM migrated anomalously in SDS-PAGE and removal of the 19 amino acids corresponding to the predicted signal peptide caused a decrease in apparent molecular mass of 24kDa. The molecule was analyzed using monoclonal antibodies that recognize a set of previously defined PIM epitopes. Depending on the presence or the absence of the signal peptide, two conformational states could be demonstrated that are differentially recognized, with N-terminal epitopes becoming readily accessible upon signal peptide removal, and C-terminal epitopes becoming masked. Similar observations were made when the QP-rd of PIM was expressed in bacteria. Our observations could also be of relevance to other schizont proteins. A recent analysis of the proteomes of T. parva and T. annulata revealed the presence of a large family of potentially secreted proteins, characterized by the presence of large stretches of amino acids that are also particularly rich in QP-residues.

GPI-specific phospholipase D (GPI-PLD) is expressed during mouse development and is localized to the extracellular matrix of the developing mouse skeleton

Glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) is abundant in serum and has a well-characterized biochemistry; however, its physiological role is completely unknown. Previous investigations into GPI-PLD have focused on the adult animal or on in vitro systems and a putative role in development has been neither proposed nor investigated. We describe the first evidence of GPI-PLD expression during mouse embryonic ossification. GPI-PLD expression was detected predominantly at sites of skeletal development, increasing during the course of gestation. GPI-PLD was observed during both intramembraneous and endochondral ossification and localized predominantly to the extracellular matrix of chondrocytes and to primary trabeculae of the skeleton. In addition, the mouse chondrocyte cell line ATDC5 expressed GPI-PLD after experimental induction of differentiation. These results implicate GPI-PLD in the process of bone formation during mouse embryogenesis.

Effects of a mosquitocidal toxin on a mammalian epithelial cell line expressing its target receptor

The spread of diseases transmitted by Anopheles and Culex mosquitoes, such as malaria and West Nile fever, is a growing concern for human health. Bacillus sphaericus binary toxin (Bin) is one of the few available bioinsecticides able to control populations of these mosquitoes efficiently. We previously showed that Bin binds to Cpm1, an alpha-glucosidase located on the apical side of Culex larval midgut epithelium. We analysed the effects of Bin by expressing a construct encoding Cpm1 in the mammalian epithelial MDCK cell line. Cpm1 is targeted to the apical side of polarized MDCK, where it is anchored by glycosylphosphatidylinositol (GPI) and displays alpha-glucosidase activity. Bin bound to transfected cells and induced a non-specific current presumably related to the opening of pores. The formation of these pores may be related to the location of the toxin/receptor complex in lipid raft microdomains. Finally, Bin promoted the time-dependent appearance of intracytoplasmic vacuoles but did not drive cell lysis. Thus, the dual functionality (enzyme/toxin receptor) of Cpm1 is fully conserved in MDCK cells and Cpm1 is an essential target protein for Bin cytotoxicity in Culex mosquitoes.

Phosphorylation of glycosyl-phosphatidylinositol by phosphatidylinositol 3-kinase changes its properties as a substrate for phospholipases

Phosphatidylinositol 3-kinases (PI3K) phosphorylate the 3-position of the inositol ring of phosphatidylinositol-4,5-bisphosphate to produce phosphatidylinositol-3,4,5-trisphosphate. It is not clear whether PI3K can phosphorylate the inositol group in other biomolecules. We sought to determine whether PI3K was able to use glycosyl-phosphatidylinositol (GPI) as a substrate. This phospholipid may exist either in free form (GPIfree) or forming a lipid anchor (GPIanchor) for the attachment of extracellular proteins to the plasma membrane. We demonstrate the specific PI3K-mediated phosphorylation of the inositol 3-hydroxyl group within both types of GPI by incubating this phospholipid with immunoprecipitated PI3K. The phosphorylated product behaves in HPLC as a derivative of a PI3K lipid product. To our knowledge, this is the first demonstration that PI3K uses lipid substrates other than phosphoinositides. Further, we show that this has potential functional consequences. When GPIfree is phosphorylated, it becomes a poorer substrate for GPI-specific phospholipase D, but a better substrate for phosphatidylinositol-specific phospholipase C. These phosphorylation events may constitute the basis of a previously undescribed signal transduction mechanism.

Crystal structure of human carboxypeptidase M, a membrane-bound enzyme that regulates peptide hormone activity

Carboxypeptidase M (CPM), an extracellular glycosylphosphatidyl-inositol(GPI)-anchored membrane glycoprotein belonging to the CPN/E subfamily of "regulatory" metallo-carboxypeptidases, specifically removes C-terminal basic residues from peptides and proteins. Due to its wide distribution in human tissues, CPM is believed to play important roles in the control of peptide hormone and growth factor activity at the cell surface, and in the membrane-localized degradation of extracellular proteins. We have crystallized human GPI-free CPM, and have determined and refined its 3.0A crystal structure. The structure analysis reveals that CPM consists of a 295 residue N-terminal catalytic domain similar to that of duck CPD-2 (but only distantly related to CPA/B), an adjacent 86 residue beta-sandwich C-terminal domain characteristic of the CPN/E family but more conically shaped than the equivalent domain in CPD-2, and a unique, partially disordered 25 residue C-terminal extension to which the GPI membrane-anchor is post-translationally attached. Through this GPI anchor, and presumably via some positively charged side-chains of the C-terminal domain, the CPM molecule may interact with the membrane in such a way that its active centre will face alongside, i.e. well suited to interact with other membrane-bound protein substrates or small peptides. Modelling of the C-terminal part of the natural substrate Arg(6)-Met-enkephalin into the active site shows that the S1' pocket of CPM is particularly well designed to accommodate P1'-Arg residues, in agreement with the preference of CPM for cleaving C-terminal Arg.