Glycosylphosphatidylinositol-anchored proteins: structure, function, and cleavage by phosphatidylinositol-specific phospholipase C

Insight into the adaptation mechanisms of high hydrostatic pressure in physiology and metabolism of hadal fungi from the deepest ocean sediment

High hydrostatic pressure (HHP) influences the life processes of organisms living at depth in the oceans. While filamentous fungi are one of the essential members of deep-sea microorganisms, few works have explored their piezotolerance to HHP. Here, we obtained three homogeneous Aspergillus sydowii from terrestrial, shallow, and hadal areas, respectively, to compare their pressure resistance. A set of all-around evaluation methods including determination of growth rate, metabolic activity, and microscopic staining observation was established and indicated that A. sydowii DM1 from the hadal area displayed significant piezotolerance. Global analysis of transcriptome data under elevated HHP revealed that A. sydowii DM1 proactively modulated cell membrane permeability, hyphae morphology, and septal quantities for seeking a better livelihood under mild pressure. Besides, differentially expressed genes were mainly enriched in the biosynthesis of amino acids, carbohydrate metabolism, cell process, etc., implying how the filamentous fungi respond to elevated pressure at the molecular level. We speculated that A. sydowii DM1 could acclimatize itself to HHP by adopting several strategies, including environmental response pathway HOG-MAPK, stress proteins, and cellular metabolisms.IMPORTANCEFungi play an ecological and biological function in marine environments, while the physiology of filamentous fungi under high hydrostatic pressure (HHP) is an unknown territory due to current technologies. As filamentous fungi are found in various niches, Aspergillus sp. from deep-sea inspire us to the physiological trait of eukaryotes under HHP, which can be considered as a prospective research model. Here, the evaluation methods we constructed would be universal for most filamentous fungi to assess their pressure resistance, and we found that Aspergillus sydowii DM1 from the hadal area owned better piezotolerance and the active metabolisms under HHP indicated the existence of undiscovered metabolic strategies for hadal fungi. Since pressure-related research of marine fungi has been unexpectedly neglected, our study provided an enlightening strategy for them under HHP; we believed that understanding their adaptation and ecological function in original niches will be accelerated in the perceivable future.

The mechanisms underlying the enrichment and action of glypican-1-positive exosomes in colorectal cancer cells

BACKGROUND

Glypican-1 (GPC1) is overexpressed in several tumors, and GPC1⁺ exosomes have shown the potential to predict early colorectal cancer (CRC). However, the mechanisms underlying the enrichment and action of GPC1⁺ exosomes in CRC remain unknown.

METHODS

The expression of slit guidance ligand 2 (SLIT2), hypoxia-inducible factor (HIF)-1α/2α, and GPC1 in clinical CRC tissues was detected using immunohistochemistry and western blot. Exosomes were isolated from the supernatants of CRC cell cultures. The effects of SLIT2, hypoxia, heparin, and phospholipase C (PLC) on exosomal GPC1 expression and GPC1⁺ exosome enrichment in CRC cells were analyzed with western blot and flow cytometry. CRC cell proliferation was assessed with MTT and colony formation assays. Co-immunoprecipitation was used to detect the binding of GPC1 and SLIT2 in SW480 cells. Nude mice were subcutaneously inoculated with SW480 cells with different treatments. The Wnt signaling was detected.

RESULTS

SLIT2 was poorly expressed and GPC1, HIF-1α, and HIF-2α were highly expressed in human CRC tissues. SLIT2 in CRC cells inhibited GPC1⁺ exosome enrichment and exosomal GPC1 expression. PLC and heparin increased GPC1⁺ exosome enrichment in CRC cells in a concentration-dependent manner. Hypoxia increased the enrichment of GPC1⁺ exosomes in CRC cells depending on HIF-2α expression. GPC1⁺ exosomes stimulated CRC cell proliferation and xenograft tumor growth through activation of Wnt signaling.

CONCLUSIONS

GPC1⁺ exosome enrichment is related to PLC and heparin. Hypoxia increases the enrichment of GPC1⁺ exosomes in CRC cells by activating HIF-2α and downregulating SLIT2. GPC1⁺ exosomes further drive CRC progression by activating Wnt signaling.

Phosphatidylcholine Cation—Tyrosine π Complexes: Motifs for Membrane Binding by a Bacterial Phospholipase C

Phosphatidylinositol-specific phospholipase C (PI-PLC) enzymes are a virulence factor in many Gram-positive organisms. The specific activity of the Bacillus thuringiensis PI-PLC is significantly increased by adding phosphatidylcholine (PC) to vesicles composed of the substrate phosphatidylinositol, in part because the inclusion of PC reduces the apparent Kd for the vesicle binding by as much as 1000-fold when comparing PC-rich vesicles to PI vesicles. This review summarizes (i) the experimental work that localized a site on BtPI-PLC where PC is bound as a PC choline cation—Tyr-π complex and (ii) the computational work (including all-atom molecular dynamics simulations) that refined the original complex and found a second persistent PC cation—Tyr-π complex. Both complexes are critical for vesicle binding. These results have led to a model for PC functioning as an allosteric effector of the enzyme by altering the protein dynamics and stabilizing an ‘open’ active site conformation.

Providing Biological Plausibility for Exposure–Health Relationships for the Mycotoxins Deoxynivalenol (DON) and Fumonisin B1 (FB1) in Humans Using the AOP Framework

Humans are chronically exposed to the mycotoxins deoxynivalenol (DON) and fumonisin B1 (FB1), as indicated by their widespread presence in foods and occasional exposure in the workplace. This exposure is confirmed by human biomonitoring (HBM) studies on (metabolites of) these mycotoxins in human matrices. We evaluated the exposure–health relationship of the mycotoxins in humans by reviewing the available literature. Since human studies did not allow the identification of unequivocal chronic health effects upon exposure to DON and FB1, the adverse outcome pathway (AOP) framework was used to structure additional mechanistic evidence from in vitro and animal studies on the identified adverse effects. In addition to a preliminary AOP for DON resulting in the adverse outcome (AO) ‘reduced body weight gain’, we developed a more elaborated AOP for FB1, from the molecular initiating event (MIE) ‘inhibition of ceramide synthases’ leading to the AO ‘neural tube defects’. The mechanistic evidence from AOPs can be used to support the limited evidence from human studies, to focus FB1- and DON-related research in humans to identify related early biomarkers of effect. In order to establish additional human exposure–health relationships in the future, recommendations are given to maximize the information that can be obtained from HBM.

Sex-stratified gene-by-environment genome-wide interaction study of trauma, posttraumatic-stress, and suicidality

Epidemiologic studies recognize that trauma and posttraumatic stress are associated with heightened suicidal behavior severity, yet examination of these associations from a genetic perspective is limited. We performed a multivariate gene-by-environment genome-wide interaction study (GEWIS) of suicidality in 123,633 individuals using a covariance matrix based on 26 environments related to traumatic experiences, posttraumatic stress, social support, and socioeconomic status. We discovered five suicidality risk loci, including the male-associated rs2367967 (CWC22), which replicated in an independent cohort. All GEWIS-significant loci exhibited interaction effects where at least 5% of the sample had environmental profiles conferring opposite SNP effects from the majority. We identified PTSD as a primary driving environment for GxE at suicidality risk loci. The male suicidality GEWIS was enriched for three middle-temporal-gyrus inhibitory neuron transcriptomic profiles: SCUBE- and PVALB-expressing cells (β = 0.028, p = 3.74 × 10-4), OPRM1-expressing cells (β = 0.030, p = 0.001), and SPAG17-expressing cells (β = 0.029, p = 9.80 × 10-4). Combined with gene-based analyses (CNTN5 p association  = 2.38 × 10-9, p interaction  = 1.51 × 10-3; PSMD14 p association  = 2.04 × 10-7, p interaction  = 7.76 × 10-6; HEPACAM p association  = 2.43 × 10-6, p interaction  = 3.82 × 10-7) including information about brain chromatin interaction profiles (UBE2E3 in male neuron p = 1.07 × 10-5), our GEWIS points to extracellular matrix biology and synaptic plasticity as biological interactors with the effects of potentially modifiable lifetime traumatic experiences on genetic risk for suicidality. Characterization of molecular basis for the effects of traumatic experience and posttraumatic stress on risk of suicidal behaviors may help to identify novel targets for which more effective treatments can be developed for use in high-risk populations.

Is related the hematopoietic stem cells differentiation in the Nile tilapia with GABA exposure?

The signaling mediated by small non-proteinogenic molecules, which probably have the capacity to serve as a bridge amongst complex systems is one of the most exiting challenges for the study. In the current report, stem cells differentiation of the immune system in Nile tilapia treated with sub-basal doses of GABA evaluated as c-kit⁺ and Sca-1⁺ cells disappearance on pronephros, thymus, spleen and peripheral blood mononuclear cells by flow cytometry was assessed. Explanation of biological response was performed by molecular docking approach and multiparametric analysis. Stem cell differentiation depends on a delicate balance of negative and positive interactions of this neurotransmitter with receptors and transcription factors involved in this process. This in turn depends on the type of interaction with hematopoietic niche to differentiate into primordial, early or late hematopoiesis as well as from the dose delivery. In fish treated with the low doses of GABA (0.1% over basal value) primordial hematopoiesis is regulated by interaction of glutamate (Glu) with the Ly-6 antigen. Early hematopoiesis was influenced by the bond of GABA near or adjacent to turns of FLTR3-Ig-IV domain. During late hematopoiesis, negative regulation by structural modifications on PU.1/IRF-4 complex, IL-7Rα and GM-CSFR mainly prevails. Results of molecular docking were in agreement with the percentages of the main blood cells lineages estimated in pronephros by flow cytometry. Current study provides the first evidences about the role of inhibitory and excitatory neurotransmitters such as GABA and Glu, respectively with the most transcriptional factors and receptors involved on hematopoiesis in adult Nile tilapia.

Hypophosphatasia: Canadian update on diagnosis and management

Hypophosphatasia (HPP) is a rare inherited disorder of bone and mineral metabolism caused by loss of function mutations in the ALPL gene. The presentation in children and adults can be extremely variable and natural history is poorly understood particularly in adults. Careful patient evaluation is required with consideration of pharmacologic intervention in individuals meeting criteria for therapy.

INTRODUCTION

The purposes of this review are to present current evidence regarding the diagnosis and management of hypophosphatasia in children and adults and provide evidence-based recommendations for management.

METHOD

A MEDLINE, EMBASE, and Cochrane database search and literature review was completed. The following consensus recommendations were developed based on the highest level of evidence as well as expert opinion.

RESULTS

Hypophosphatasia is a rare inherited disorder of bone and mineral metabolism due to loss of function mutations in the tissue non-specific alkaline phosphatase (ALPL) gene causing reductions in the activity of the tissue non-specific isoenzyme of alkaline phosphatase (TNSALP). Deficient levels of alkaline phosphatase result in elevation of inhibitors of mineralization of the skeleton and teeth, principally inorganic pyrophosphate. The impaired skeletal mineralization may result in elevations in serum calcium and phosphate. Clinical features include premature loss of teeth, metatarsal and subtrochanteric fractures as well as fragility fractures. Poor bone healing post fracture has been observed. Myalgias and muscle weakness may also be present. In infancy and childhood, respiratory and neurologic complications can occur.

CONCLUSIONS

HPP is associated with significant morbidity and mortality. Pharmacologic intervention can result in significant clinical improvement. This Canadian position paper provides an overview of the musculoskeletal, renal, dental, respiratory, and neurologic manifestations of hypophosphatasia. The current state of the art in the diagnosis and management of hypophosphatasia is presented.

Deletion of the GCW13 gene derepresses Gap1-dependent uptake of amino acids in Pichia pastoris grown on methanol as the sole carbon source

In Pichia pastoris, most of the Glycosylphosphatidylinositol (GPI)-anchored proteins are of unknown function. Gcw13, one of these GPI-anchored proteins, was found to exert an inhibitory effect on the growth of the histidine auxotrophic P. pastoris strain GS115 on methanol as the sole carbon source. To investigate the biological function of Gcw13, RNA sequencing (RNA-Seq) was performed to compare the difference of gene expression between GS115 and GCW13-deletion strain D13. RNA-Seq analysis showed that, in strain D13, the expression of genes involved in the methanol utilization pathway or peroxisome biogenesis was not changed, and a high proportion of genes involved in the biosynthesis of amino acids were down-regulated, whereas GAP1, which encodes a general amino acid permease, was significantly up-regulated. Besides, the intracellular concentrations of various amino acids were significantly higher in D13 than that in GS115. We also observed that deletion of GCW13 resulted in more Gap1 presented on the cell surface and more active uptake of the toxic proline analogue l-azetidine-2-carboxylate acid (AzC). These results suggest that Gcw13 suppresses the expression of GAP1 and facilitates the endocytosis of Gap1 on methanol, resulting in decreasing Gap1-dependent uptake of amino acids in P. pastoris, which might contribute to the poor growth of GS115 on methanol.

MARCH1 protects the lipid raft and tetraspanin web from MHCII proteotoxicity in dendritic cells

Dendritic cells (DCs) produce major histocompatibility complex II (MHCII) in large amounts to function as professional antigen presenting cells. Paradoxically, DCs also ubiquitinate and degrade MHCII in a constitutive manner. Mice deficient in the MHCII-ubiquitinating enzyme membrane-anchored RING-CH1, or the ubiquitin-acceptor lysine of MHCII, exhibit a substantial reduction in the number of regulatory T (Treg) cells, but the underlying mechanism was unclear. Here we report that ubiquitin-dependent MHCII turnover is critical to maintain homeostasis of lipid rafts and the tetraspanin web in DCs. Lack of MHCII ubiquitination results in the accumulation of excessive quantities of MHCII in the plasma membrane, and the resulting disruption to lipid rafts and the tetraspanin web leads to significant impairment in the ability of DCs to engage and activate thymocytes for Treg cell differentiation. Thus, ubiquitin-dependent MHCII turnover represents a novel quality-control mechanism by which DCs maintain homeostasis of membrane domains that support DC's Treg cell-selecting function.

Ceratocystis cacaofunesta genome analysis reveals a large expansion of extracellular phosphatidylinositol-specific phospholipase-C genes (PI-PLC)

BACKGROUND

The Ceratocystis genus harbors a large number of phytopathogenic fungi that cause xylem parenchyma degradation and vascular destruction on a broad range of economically important plants. Ceratocystis cacaofunesta is a necrotrophic fungus responsible for lethal wilt disease in cacao. The aim of this work is to analyze the genome of C. cacaofunesta through a comparative approach with genomes of other Sordariomycetes in order to better understand the molecular basis of pathogenicity in the Ceratocystis genus.

RESULTS

We present an analysis of the C. cacaofunesta genome focusing on secreted proteins that might constitute pathogenicity factors. Comparative genome analyses among five Ceratocystidaceae species and 23 other Sordariomycetes fungi showed a strong reduction in gene content of the Ceratocystis genus. However, some gene families displayed a remarkable expansion, in particular, the Phosphatidylinositol specific phospholipases-C (PI-PLC) family. Also, evolutionary rate calculations suggest that the evolution process of this family was guided by positive selection. Interestingly, among the 82 PI-PLCs genes identified in the C. cacaofunesta genome, 70 genes encoding extracellular PI-PLCs are grouped in eight small scaffolds surrounded by transposon fragments and scars that could be involved in the rapid evolution of the PI-PLC family. Experimental secretome using LC-MS/MS validated 24% (86 proteins) of the total predicted secretome (342 proteins), including four PI-PLCs and other important pathogenicity factors.

CONCLUSION

Analysis of the Ceratocystis cacaofunesta genome provides evidence that PI-PLCs may play a role in pathogenicity. Subsequent functional studies will be aimed at evaluating this hypothesis. The observed genetic arsenals, together with the analysis of the PI-PLC family shown in this work, reveal significant differences in the Ceratocystis genome compared to the classical vascular fungi, Verticillium and Fusarium. Altogether, our analyses provide new insights into the evolution and the molecular basis of plant pathogenicity.

Introduction: General Aspects of the Chemical Biology of Glycoproteins

This chapter is meant to serve as an introduction to the remainder of the book by providing general background on the chemical biology of glycoproteins as well as a brief review of the chapters that follow. The purpose here is to introduce some basic concepts common to many forms of glycosylation for those readers who may be unfamiliar with the field. We begin with a discussion of the strategies and methods used to study protein glycosylation. During the overview, an effort is made to highlight a few relevant aspects of chemical glycobiology, including glycoprotein biosynthesis and a brief description of the synthesis and function of glycoproteins. Finally, we have a summary of the contributions from chemical biology over the years. It is our hope that, after reading this introductory chapter, the reader will have a broad view of the chemical glycobiology field as it currently stands and a deeper appreciation for some of the unique ideas that chemical biology brings to the field.

A homozygous intronic branch-point deletion in the ALPL gene causes infantile hypophosphatasia

Hypophosphatasia (HPP) is a multi-systemic inborn disease with an extraordinary spectrum of severity, ranging from the absence of mineralization to high lethality and it involves different organs including bone, muscle, kidney, lung, gastrointestinal tract and the nervous system. The disease is characterized by low levels of serum alkaline phosphatase, caused by loss-of-function mutations within the ALPL gene that encodes the tissue-nonspecific alkaline phosphatase TNAP. Here we present the functional characterization of a gene mutation, detected in intron 7 of the ALPL gene of a boy with infantile HPP in whom routine sequencing of the coding region failed to detect any mutation. The homozygous c.793del-14_33 mutation results in the loss of the branch-point motif, relevant for correct ALPL pre-mRNA splicing. The main transcript skips exon 8 and codes for a C-terminally truncated TNAP protein of 275 amino acids, which was detected in peripheral blood mononuclear cells and serum from the patient. The functional characterization of recombinant TNAP₂₇₅ revealed no enzymatic activity nor any dominant-negative effect, relevant for the heterozygous parents. Nevertheless correct pre-mRNA splicing can take place without the branch-point sequence to a limited extend, as concluded from the ALPL cDNA, obtained from patient's PBMC, and from the low serum AP activity. These data reaffirm that in clear cut clinical cases, where conventional sequencing including the coding sequence and direct exon-intron-boundaries fails to detect mutations, deeper analyses of regulatory important motifs like branch-point sequences are required to establish a genetic diagnosis.

Maternal ENODLs Are Required for Pollen Tube Reception in Arabidopsis

During the angiosperm (flowering-plant) life cycle, double fertilization represents the hallmark between diploid and haploid generations [1]. The success of double fertilization largely depends on compatible communication between the male gametophyte (pollen tube) and the maternal tissues of the flower, culminating in precise pollen tube guidance to the female gametophyte (embryo sac) and its rupture to release sperm cells. Several important factors involved in the pollen tube reception have been identified recently [2-6], but the underlying signaling pathways are far from being understood. Here, we report that a group of female-specific small proteins, early nodulin-like proteins (ENODLs, or ENs), are required for pollen tube reception. ENs are featured with a plastocyanin-like (PCNL) domain, an arabinogalactan (AG) glycomodule, and a predicted glycosylphosphatidylinositol (GPI) anchor motif. We show that ENs are asymmetrically distributed at the plasma membrane of the synergid cells and accumulate at the filiform apparatus, where arriving pollen tubes communicate with the embryo sac. EN14 strongly and specifically interacts with the extracellular domain of the receptor-like kinase FERONIA, localized at the synergid cell surface and known to critically control pollen tube reception [6]. Wild-type pollen tubes failed to arrest growth and to rupture after entering the ovules of quintuple loss-of-function EN mutants, indicating a central role of ENs in male-female communication and pollen tube reception. Moreover, overexpression of EN15 by the endogenous promoter caused disturbed pollen tube guidance and reduced fertility. These data suggest that female-derived GPI-anchored ENODLs play an essential role in male-female communication and fertilization.

Effects of GPI-anchored TNAP on the dynamic structure of model membranes

Tissue-nonspecific alkaline phosphatase (TNAP) plays a crucial role during skeletal mineralization, and TNAP deficiency leads to the soft bone disease hypophosphatasia. TNAP is anchored to the external surface of the plasma membranes by means of a GPI (glycosylphosphatidylinositol) anchor. Membrane-anchored and solubilized TNAP displays different kinetic properties against physiological substrates, indicating that membrane anchoring influences the enzyme function. Here, we used Electron Spin Resonance (ESR) measurements along with spin labeled phospholipids to probe the possible dynamic changes prompted by the interaction of GPI-anchored TNAP with model membranes. The goal was to systematically analyze the ESR data in terms of line shape changes and of alterations in parameters such as rotational diffusion rates and order parameters obtained from non-linear least-squares simulations of the ESR spectra of probes incorporated into DPPC liposomes and proteoliposomes. Overall, the presence of TNAP increased the dynamics and decreased the ordering in the three distinct regions probed by the spin labeled lipids DOPTC (headgroup), and 5- and 16-PCSL (acyl chains). The largest change was observed for 16-PCSL, thus suggesting that GPI-anchored TNAP can give rise to long reaching modifications that could influence membrane processes halfway through the bilayer.

GAS1 is present in the cerebrospinal fluid and is expressed in the choroid plexus of the adult rat

Growth arrest specific 1 (GAS1) is a GPI-anchored protein that inhibits proliferation when overexpressed in tumors but during development it promotes proliferation and survival of different organs and tissues. This dual ability is caused by its capacity to interact both by inhibiting the signaling induced by the glial cell line-derived neurotrophic factor and by facilitating the activity of the sonic hedgehog pathway. GAS1 is expressed as membrane bound in different organs and as a secreted form by glomerular mesangial cells. In the developing central nervous system, GAS1 is found in neural progenitors; however, it continues to be expressed in the adult brain. Here, we demonstrate that soluble GAS1 is present in the cerebrospinal fluid (CSF) and it is expressed in the choroid plexus (CP) of the adult rat, the main producer of CSF. Additionally, we confirm the presence of GAS1 in blood plasma and liver of the adult rat, the principal source of blood plasma proteins. The pattern of expression of GAS1 is perivascular in both the CP and the liver. In vitro studies show that the fibroblast cell line NIH/3T3 expresses one form of GAS1 and releases two soluble forms into the supernatant. Briefly, in the present work, we show the presence of GAS1 in adult rat body fluids focusing in the CSF and the CP, and suggest that secreted GAS1 exists as two different isoforms.

Alkaline Phosphatase and Hypophosphatasia

Hypophosphatasia (HPP) results from ALPL mutations leading to deficient activity of the tissue-non-specific alkaline phosphatase isozyme (TNAP) and thereby extracellular accumulation of inorganic pyrophosphate (PPi), a natural substrate of TNAP and potent inhibitor of mineralization. Thus, HPP features rickets or osteomalacia and hypomineralization of teeth. Enzyme replacement using mineral-targeted TNAP from birth prevented severe HPP in TNAP-knockout mice and was then shown to rescue and substantially treat infants and young children with life-threatening HPP. Clinical trials are revealing aspects of HPP pathophysiology not yet fully understood, such as craniosynostosis and muscle weakness when HPP is severe. New treatment approaches are under development to improve patient care.

Membrane Protein Mobility and Orientation Preserved in Supported Bilayers Created Directly from Cell Plasma Membrane Blebs

Membrane protein interactions with lipids are crucial for their native biological behavior, yet traditional characterization methods are often carried out on purified protein in the absence of lipids. We present a simple method to transfer membrane proteins expressed in mammalian cells to an assay-friendly, cushioned, supported lipid bilayer platform using cell blebs as an intermediate. Cell blebs, expressing either GPI-linked yellow fluorescent proteins or neon-green fused transmembrane P2X2 receptors, were induced to rupture on glass surfaces using PEGylated lipid vesicles, which resulted in planar supported membranes with over 50% mobility for multipass transmembrane proteins and over 90% for GPI-linked proteins. Fluorescent proteins were tracked, and their diffusion in supported bilayers characterized, using single molecule tracking and moment scaling spectrum (MSS) analysis. Diffusion was characterized for individual proteins as either free or confined, revealing details of the local lipid membrane heterogeneity surrounding the protein. A particularly useful result of our bilayer formation process is the protein orientation in the supported planar bilayer. For both the GPI-linked and transmembrane proteins used here, an enzymatic assay revealed that protein orientation in the planar bilayer results in the extracellular domains facing toward the bulk, and that the dominant mode of bleb rupture is via the "parachute" mechanism. Mobility, orientation, and preservation of the native lipid environment of the proteins using cell blebs offers advantages over proteoliposome reconstitution or disrupted cell membrane preparations, which necessarily result in significant scrambling of protein orientation and typically immobilized membrane proteins in SLBs. The bleb-based bilayer platform presented here is an important step toward integrating membrane proteomic studies on chip, especially for future studies aimed at understanding fundamental effects of lipid interactions on protein activity and the roles of membrane proteins in disease pathways.

Recent progress on lipid lateral heterogeneity in plasma membranes: From rafts to submicrometric domains

The concept of transient nanometric domains known as lipid rafts has brought interest to reassess the validity of the Singer-Nicolson model of a fluid bilayer for cell membranes. However, this new view is still insufficient to explain the cellular control of surface lipid diversity or membrane deformability. During the past decades, the hypothesis that some lipids form large (submicrometric/mesoscale vs nanometric rafts) and stable (>min vs s) membrane domains has emerged, largely based on indirect methods. Morphological evidence for stable submicrometric lipid domains, well-accepted for artificial and highly specialized biological membranes, was further reported for a variety of living cells from prokaryot es to yeast and mammalian cells. However, results remained questioned based on limitations of available fluorescent tools, use of poor lipid fixatives, and imaging artifacts due to non-resolved membrane projections. In this review, we will discuss recent evidence generated using powerful and innovative approaches such as lipid-specific toxin fragments that support the existence of submicrometric domains. We will integrate documented mechanisms involved in the formation and maintenance of these domains, and provide a perspective on their relevance on membrane deformability and regulation of membrane protein distribution.

Pharmacological Inhibition of Protein Lipidation

Lipid modifications of mammalian proteins are widespread, modifying thousands of targets involved in all aspects of cellular physiology cellular physiology. Broadly, lipidations serve to increase protein hydrophobicity and association with cellular membranes. Often, these modifications are absolutely essential for protein stability and localization, and serve critical roles in dynamic regulation of protein function. A number of lipidated proteins are associated with diseases, including parasite infections, neurological diseases, diabetes, and cancer, suggesting that lipid modifications represent potentially attractive targets for pharmacological intervention. This review briefly describes the various types of posttranslational protein lipid modifications, proteins modified by them, and the enzymatic machinery associated with these. We then discuss several case studies demonstrating successful development of lipidation inhibitors of potential (and more rarely, realized) clinical value. Although this field remains in its infancy, we believe these examples demonstrate the potential utility of targeting protein lipidation as a viable strategy for inhibiting the function of pathogenic proteins.

RPTPs in axons, synapses and neurology

Receptor-like protein tyrosine phosphatases represent a large protein family related to cell adhesion molecules, with diverse roles throughout neural development in vertebrates and invertebrates. This review focuses on their roles in axon growth, guidance and repair, as well as more recent findings demonstrating their key roles in pre-synaptic and post-synaptic maturation and function. These enzymes have been linked to memory and neuropsychiatric defects in loss-of-function rodent models, highlighting their potential as future drug targets.

Newly Identified Cancer-Associated Role of Human Neuronal Growth Regulator 1 (NEGR1)

Neuronal growth regulator 1 (NEGR1) has become a great interest based on the recent findings that its genetic alteration is implicated in human obesity and human dyslexia. By analyzing the gene expression profiles of tumor biopsies and normal tissues, we identified NEGR1 as a commonly down-regulated gene in many types of human cancer tissues. NEGR1 contains a C-terminal GPI anchor attachment site and is primarily localized to cell membrane rafts, especially in cell-to-cell contacting areas. The oncogenic phenotype was clearly attenuated when NEGR1 was overexpressed in the human ovarian cancer cell line SKOV-3. Furthermore, cell aggregation and neurite outgrowth was greatly increased by NEGR1 overexpression. On the contrary, cell migration and invasion was increased in NEGR1-depleted cells, suggesting that NEGR1 may contribute to tumor suppression. Taken together, we suggest that NEGR1 is a raft-associated extracellular protein that may participate in cell recognition and interaction, which is important in growth control and malignant transformation.

Mechanism for the endocytosis of spherical nucleic acid nanoparticle conjugates

Intracellular delivery of nucleic acids as gene regulation agents typically requires the use of cationic carriers or viral vectors, yet issues related to cellular toxicity or immune responses hamper their attractiveness as therapeutic candidates. The discovery that spherical nucleic acids (SNAs), polyanionic structures comprised of densely packed, highly oriented oligonucleotides covalently attached to the surface of nanoparticles, can effectively enter more than 50 different cell types presents a potential strategy for overcoming the limitations of conventional transfection agents. Unfortunately, little is known about the mechanism of endocytosis of SNAs, including the pathway of entry and specific proteins involved. Here, we demonstrate that the rapid cellular uptake kinetics and intracellular transport of SNAs stem from the arrangement of oligonucleotides into a 3D architecture, which supports their targeting of class A scavenger receptors and endocytosis via a lipid-raft-dependent, caveolae-mediated pathway. These results reinforce the notion that SNAs can serve as therapeutic payloads and targeting structures to engage biological pathways not readily accessible with linear oligonucleotides.

Contactin 4, -5 and -6 differentially regulate neuritogenesis while they display identical PTPRG binding sites

The neural cell-adhesion molecules contactin 4, contactin 5 and contactin 6 are involved in brain development, and disruptions in contactin genes may confer increased risk for autism spectrum disorders (ASD). We describe a co-culture of rat cortical neurons and HEK293 cells overexpressing and delivering the secreted forms of rat contactin 4-6. We quantified their effects on the length and branching of neurites. Contactin 4-6 effects were different depending on the contactin member and duration of co-culture. At 4 days in culture, contactin 4 and -6 increased the length of neurites, while contactin 5 increased the number of roots. Up to 8 days in culture, contactin 6 progressively increased the length of neurites while contactin 5 was more efficient on neurite branching. We studied the molecular sites of interaction between human contactin 4, -5 or -6 and the human Protein Tyrosine Phosphatase Receptor Gamma (PTPRG), a contactin partner, by modeling their 3D structures. As compared to contactin 4, we observed differences in the Ig2 and Ig3 domains of contactin 5 and -6 with the appearance of an omega loop that could adopt three distinct conformations. However, interactive residues between human contactin 4-6 and PTPRG were strictly conserved. We did not observe any differences in PTPRG binding on contactin 5 and -6 either. Our data suggest that the differential contactin effects on neurite outgrowth do not result from distinct interactions with PTPRG. A better understanding of the contactin cellular properties should help elucidate their roles in ASD.

Antigenic characterization of an intrinsically unstructured protein, Plasmodium falciparum merozoite surface protein 2

Merozoite surface protein 2 (MSP2) is an abundant glycosylphosphatidylinositol (GPI)-anchored protein of Plasmodium falciparum, which is a potential component of a malaria vaccine. As all forms of MSP2 can be categorized into two allelic families, a vaccine containing two representative forms of MSP2 may overcome the problem of diversity in this highly polymorphic protein. Monomeric recombinant MSP2 is an intrinsically unstructured protein, but its conformational properties on the merozoite surface are unknown. This question is addressed here by analyzing the 3D7 and FC27 forms of recombinant and parasite MSP2 using a panel of monoclonal antibodies raised against recombinant MSP2. The epitopes of all antibodies, mapped using both a peptide array and by nuclear magnetic resonance (NMR) spectroscopy on full-length recombinant MSP2, were shown to be linear. The antibodies revealed antigenic differences, which indicate that the conserved N- and C-terminal regions, but not the central variable region, are less accessible in the parasite antigen. This appears to be an intrinsic property of parasite MSP2 and is not dependent on interactions with other merozoite surface proteins as the loss of some conserved-region epitopes seen using the immunofluorescence assay (IFA) on parasite smears was also seen on Western blot analyses of parasite lysates. Further studies of the structural basis of these antigenic differences are required in order to optimize recombinant MSP2 constructs being evaluated as potential vaccine components.

Cellular function and molecular structure of ecto-nucleotidases

Ecto-nucleotidases play a pivotal role in purinergic signal transmission. They hydrolyze extracellular nucleotides and thus can control their availability at purinergic P2 receptors. They generate extracellular nucleosides for cellular reuptake and salvage via nucleoside transporters of the plasma membrane. The extracellular adenosine formed acts as an agonist of purinergic P1 receptors. They also can produce and hydrolyze extracellular inorganic pyrophosphate that is of major relevance in the control of bone mineralization. This review discusses and compares four major groups of ecto-nucleotidases: the ecto-nucleoside triphosphate diphosphohydrolases, ecto-5'-nucleotidase, ecto-nucleotide pyrophosphatase/phosphodiesterases, and alkaline phosphatases. Only recently and based on crystal structures, detailed information regarding the spatial structures and catalytic mechanisms has become available for members of these four ecto-nucleotidase families. This permits detailed predictions of their catalytic mechanisms and a comparison between the individual enzyme groups. The review focuses on the principal biochemical, cell biological, catalytic, and structural properties of the enzymes and provides brief reference to tissue distribution, and physiological and pathophysiological functions.

Drosophila GPI-mannosyltransferase 2 is required for GPI anchor attachment and surface expression of chaoptin

Glycosylphosphatidylinositol (GPI) anchors are critical for the membrane attachment of a wide variety of essential signaling and cell adhesion proteins. The GPI anchor is a complex glycolipid structure that utilizes glycosylphosphatidylinositol-mannosyltransferases (GPI-MTs) for the addition of three core mannose residues during its biosynthesis. Here, we demonstrate that Drosophila GPI-MT2 is required for the GPI-mediated membrane attachment of several GPI-anchored proteins, including the photoreceptor-specific cell adhesion molecule, chaoptin. Mutations in gpi-mt2 lead to defects in chaoptin trafficking to the plasma membrane in Drosophila photoreceptor cells. In gpi-mt2 mutants, loss of sufficient chaoptin in the membrane leads to microvillar instability, photoreceptor cell pathology, and retinal degeneration. Finally, using site-directed mutagenesis, we have identified key amino acids that are essential for GPI-MT2 function and cell viability in Drosophila. Our findings on GPI-MT2 provide a mechanistic link between GPI anchor biosynthesis and protein trafficking in Drosophila and shed light on a novel mechanism for inherited retinal degeneration.

The highly conserved spermatophyte cell wall DUF642 protein family: phylogeny and first evidence of interaction with cell wall polysaccharides in vitro

The evolution of spermatophyte plants involved fundamental changes in cell wall structure and function which resulted from diversification of carbohydrates and proteins. Cell wall proteomic analyses identified a novel family of proteins of yet unknown function, the DUF642 (Domain of Unknown Function 642) proteins. To investigate the evolution of the DUF642 gene family, 154 gene sequences from 24 plant species were analyzed, and phylogenetic inferences were conducted using the Maximum Likelihood and Bayesian Inference methods. Orthologous genes were detected in spermatophyte species and absent in non-seed known plant genomes. Protein sequences shared conserved motifs that defined the signature of the family. Distribution of conserved motifs indicated an ancestral intragenic duplication event. Gene phylogeny documented paleoduplication events originating three or four clades, depending on root position. When based on mid-point rooting, it retrieved four monophyletic clades: A, B, C, and D. A glycosylphosphatidylinositol (GPI)-anchor site and one or two galactose-binding domains-like (GBDLs) could be predicted for some DUF642 proteins. The B, C, and D clades grouped the predicted GPI-anchored proteins. First evidence of in vitro interaction of a DUF642 protein with a cell wall polysaccharide fraction is provided. A competition assay with cellulose prevented this interaction. The degree of diversification and the conservation of the family suggested that DUF642 proteins are key components in seed plant evolution.

Stem cell antigen-1 deficiency enhances the chemopreventive effect of peroxisome proliferator-activated receptorγ activation

Stem cell antigen-1 (Sca-1, Ly6A) is a glycerophosphatidylinositol (GPI)-anchored protein that was identified as a murine marker of bone marrow stem cells. Although Sca-1 is widely used to enrich for stem and progenitor cells in various tissues, little is known about its function and associated signaling pathways in normal and malignant cells. Here, we report that the absence of Sca-1 in the mammary gland resulted in higher levels of PPARγ and PTEN, and a reduction of pSer84PPARγ, pERK1/2, and PPARδ. This phenotype correlated with markedly increased sensitivity of Sca-1 null mice to PPARγ agonist GW7845 and insensitivity to PPARδ agonist GW501516. Reduction of Sca-1 expression in mammary tumor cells by RNA interference resulted in a phenotype similar to the Sca-1 deficient mammary gland, as evidenced by increased PPARγ expression and transcriptional activity, resulting in part from a lesser susceptibility to proteasomal degradation. These data implicate Sca-1 as a negative regulator of the tumor suppressor effects of PPARγ.

Physiological regulation of epithelial sodium channel by proteolysis

PURPOSE OF REVIEW

Activation of epithelial sodium channel (ENaC) by proteolysis appears to be relevant for day-to-day physiological regulation of channel activity in kidney and other epithelial tissues. Pathophysiogical, proteolytic activation of ENaC in kidney has been demonstrated in proteinuric disease.

RECENT FINDINGS

A variation in sodium and potassium intake or plasma aldosterone changes the number of cleaved α and γ-ENaC subunits and is associated with changes in ENaC currents. The protease furin mediates intracellular cleavage, whereas the channel-activating protease prostasin (CAP-1), which is glycophosphatidylinositol-anchored to the apical cell surface, mediates important extracellular cleavage. Soluble protease activity is very low in urine under physiological conditions but rises in proteinuria. In nephrotic syndrome, the dominant soluble protease activity is plasmin, which is formed from filtered plasminogen via urokinase-type plasminogen activator. Plasmin activates ENaC directly at high concentrations and through prostasin at lower concentrations.

SUMMARY

The discovery of serine protease-mediated activation of renal ENaC in physiological and pathophysiological conditions opens the way for new understanding of the pathogenesis of proteinuric sodium retention, which may involve plasmin and present several potential new drug targets.

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.

The modulation of gap-junctional intercellular communication by lipid rafts

Lipid rafts are specific microdomains of plasma membrane which are enriched in cholesterol and sphingolipids. These domains seem to favour the interactions of particular proteins and the regulation of signalling pathways in the cells. Recent data have shown that among the proteins, which are preferentially localized in lipid rafts, are connexins that are the structural proteins of gap junctions. Since gap junctional intercellular communication is involved in various cellular processes and pathologies such as cancer, we were interested to review the various observations concerning this specific localization of connexins in lipid rafts and its consequences on gap junctional intercellular communication capacity. In particular, we will focus our discussion on the role of the lipid raft-connexin connection in cancer progression. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Melanotransferrin: search for a function

BACKGROUND

Melanotransferrin was discovered in the 1980s as one of the first melanoma tumour antigens. The molecule is a transferrin homologue that is found predominantly bound to the cell membrane by a glycosyl-phosphatidylinositol anchor. MTf was described as an oncofoetal antigen expressed in only small quantities in normal tissues, but in much larger amounts in neoplastic cells. Several diseases are associated with expression of melanotransferrin, including melanoma and Alzheimer's disease, although the significance of the protein to the pathogenesis of these conditions remains unclear.

SCOPE OF REVIEW

In this review, we discuss the roles of melanotransferrin in physiological and pathological processes and its potential use as an immunotherapy.

MAJOR CONCLUSIONS

Although the exact biological functions of melanotransferrin remain elusive, a growing number of roles have been attributed to the protein, including iron transport/metabolism, angiogenesis, proliferation, cellular migration and tumourigenesis.

GENERAL SIGNIFICANCE

The high expression of melanotransferrin in several disease states, particularly malignant melanoma, remains intriguing and may have clinical significance. Further studies on the biology of this protein may provide new insights as well as potential therapeutic avenues for cancer treatment. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

GPIHBP1, an endothelial cell transporter for lipoprotein lipase

Interest in lipolysis and the metabolism of triglyceride-rich lipoproteins was recently reignited by the discovery of severe hypertriglyceridemia (chylomicronemia) in glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1)-deficient mice. GPIHBP1 is expressed exclusively in capillary endothelial cells and binds lipoprotein lipase (LPL) avidly. These findings prompted speculation that GPIHBP1 serves as a binding site for LPL in the capillary lumen, creating "a platform for lipolysis." More recent studies have identified a second and more intriguing role for GPIHBP1-picking up LPL in the subendothelial spaces and transporting it across endothelial cells to the capillary lumen. Here, we review the studies that revealed that GPIHBP1 is the LPL transporter and discuss which amino acid sequences are required for GPIHBP1-LPL interactions. We also discuss the human genetics of LPL transport, focusing on cases of chylomicronemia caused by GPIHBP1 mutations that abolish GPIHBP1's ability to bind LPL, and LPL mutations that prevent LPL binding to GPIHBP1.

Proteomic profiling of lipid rafts in a human breast cancer model of tumorigenic progression

Tumor biomarkers assist in the early detection of cancer, act as therapeutic targets for intervention, and function as diagnostic indicators for the evaluation of therapeutic responses. To identify novel human breast cancer biomarkers, we have analyzed the protein content of lipid rafts isolated from a series of human mammary epithelial cell lines with increasing tumorigenic potential. Since lipid rafts function as platforms for protein interaction critical to several biological processes, we hypothesized that the abundance of proteins associated with proliferation, invasion and metastasis would be dysregulated in highly transformed cells. For this purpose, the MCF10A epithelial lineage, which include benign MCF10A cells, premalignant AT and TG3B cells, and malignant CA1a tumor cells, was utilized. Detergent-resistant membranes were isolated from each line and proteins were identified and relatively quantitated using iTRAQ™ reagents and tandem mass spectrometry. 57 proteins were identified, and 1667 peptide identifications, mapping to 49 proteins, contained sufficient information for semi-quantitative analysis. When comparing malignant to benign cells, we observed consistent alterations in groups of proteins, such as a 5.7-fold average decrease in G protein content (n = 5), 2.7-fold decrease in glycosylphosphatidylinositol-linked proteins (n = 7) and 3.3-fold increase in intermediate filaments (n = 9). Several of the identified proteins, including caveolin-1, filamin A, keratins 5, 6 and 17, and vimentin, are bona fide or candidate biomarkers in clinical studies, underscoring the usefulness of the MCF10A series as a model to better understand the biological mechanisms underlying cancer progression.

Is prenatal myo-inositol deficiency a mechanism of CNS injury in galactosemia?

Classic Galactosemia due to galactose-1-phosphate uridyltransferase (GALT) deficiency is associated with apparent diet-independent complications including cognitive impairment, learning problems and speech defects. As both galactose-1-phosphate and galactitol may be elevated in cord blood erythrocytes and amniotic fluid despite a maternal lactose-free diet, endogenous production of galactose may be responsible for the elevated fetal galactose metabolites, as well as postnatal CNS complications. A prenatal deficiency of myo-inositol due to an accumulation of both galactose-1- phosphate and galactitol may play a role in the production of the postnatal CNS dysfunction. Two independent mechanisms may result in fetal myo-inositol deficiency: competitive inhibition of the inositol monophosphatase1 (IMPA1)-mediated hydrolysis of inositol monophosphate by high galactose-1- phosphate levels leading to a sequestration of cellular myo-inositol as inositol monophosphate and galactitol-induced reduction in SMIT1-mediated myo-inositol transport. The subsequent reduction of myo-inositol within fetal brain cells could lead to inositide deficiencies with resultant perturbations in calcium and protein kinase C signaling, the AKT/mTOR/ cell growth and development pathway, cell migration, insulin sensitivity, vescular trafficking, endocytosis and exocytosis, actin cytoskeletal remodeling, nuclear metabolism, mRNA export and nuclear pore complex regulation, phosphatidylinositol-anchored proteins, protein phosphorylation and/or endogenous iron "chelation". Using a knockout animal model we have shown that a marked deficiency of myo-inositol in utero is lethal but the phenotype can be rescued by supplementing the drinking water of the pregnant mouse. If myo-inositol deficiency is found to exist in the GALT-deficient fetal brain, then the use of myo-inositol to treat the fetus via oral supplementation of the pregnant female may warrant consideration.

The human NKG2D ligand ULBP2 can be expressed at the cell surface with or without a GPI anchor and both forms can activate NK cells

The activating immune receptor NKG2D binds to several stress-induced ligands that are structurally different. MHC-class-I-related chain (MIC) A/B molecules have a transmembrane domain, whereas most UL16 binding proteins (ULBPs) are glycosylphosphatidylinositol (GPI)-linked molecules. The significance of this variability in membrane anchors is unclear. Here, we demonstrate that ULBP2, but not ULBP1 or ULBP3, can reach the cell surface without the GPI modification. Several proteins are expressed at the cell surface as both transmembrane and GPI-linked molecules, either via alternative splicing or by the expression of linked genes. However, to our knowledge, ULBP2 is the first single mammalian cDNA that can be expressed as either a transmembrane or a GPI-anchored protein. The rate of maturation and the levels of cell surface expression of the non-GPI-linked form were lower than those of the GPI-linked ULBP2. Nonetheless, non-GPI ULBP2 was recognised by NKG2D and triggered NK cell cytotoxicity. These data show that differences in membrane attachment by NKG2D ligands are more important for regulation of their surface expression than for cytotoxic recognition by NKG2D and emphasise that detailed characterisation of the cell biology of individual NKG2D ligands will be necessary to allow targeted modulation of this system.

Fc gammaRIIIb triggers raft-dependent calcium influx in IgG-mediated responses in human neutrophils

Human neutrophils constitutively express a unique combination of FcγRs, namely FcγRIIa and FcγRIIIb. Numerous lines of evidence support the concept that these FcγRs generate only partially characterized intracellular signals. However, despite the fact that both receptors are likely to be engaged simultaneously in a physiological setting, no recent publications have investigated the distinct, although partially convergent, results of their joint activation in IgG-dependent responses. To examine the significance of the co-expression of FcγRIIa and FcγRIIIb on human neutrophils, we analyzed the neutrophil responses to stimuli that engage these FcγRs, namely the phagocytosis of human IgG-opsonized zymosan and the responses to heat-aggregated IgGs. Blocking antibodies to either FcγR significantly decreased the phagocytic index and the stimulated production of superoxide anions. Both receptors are required for optimal IgG-dependent responses by human neutrophils. On the other hand, only blocking antibodies to FcγRIIIb, but not to FcγRIIa, inhibited the mobilization of calcium in response to heat-aggregated IgGs. Furthermore, phagocytosis of IgG-opsonized zymosan by human neutrophils required an extracellular influx of calcium that was blocked only by antibodies against FcγRIIIb. We also observed that this calcium influx as well as the IgG-dependent phagocytosis were dependent on the integrity of the plasma membrane detergent-resistant microdomains to which both isoforms were recruited following stimulation by heat-aggregated IgGs. These data clarify the mechanisms that regulate the FcγRs constitutively expressed on human neutrophils, describe a specific contribution of FcγRIIIb at the level of the mobilization of calcium, and provide evidence for a crucial role of detergent-resistant microdomains in this process.

Binding of pro-prion to filamin A: by design or an unfortunate blunder

Over the last decades, cancer research has focused on tumor suppressor genes and oncogenes. Genes in other cellular pathways has received less attention. Between 0.5% to 1% of the mammalian genome encodes for proteins that are tethered on the cell membrane via a glycosylphosphatidylinositol (GPI)-anchor. The GPI modification pathway is complex and not completely understood. Prion (PrP), a GPI-anchored protein, is infamous for being the only normal protein that when misfolded can cause and transmit a deadly disease. Though widely expressed and highly conserved, little is known about the functions of PrP. Pancreatic cancer and melanoma cell lines express PrP. However, in these cell lines the PrP exists as a pro-PrP as defined by retaining its GPI anchor peptide signal sequence (GPI-PSS). Unexpectedly, the GPI-PSS of PrP has a filamin A (FLNA) binding motif and binds FLNA. FLNA is a cytolinker protein, and an integrator of cell mechanics and signaling. Binding of pro-PrP to FLNA disrupts the normal FLNA functions. Although normal pancreatic ductal cells lack PrP, about 40% of patients with pancreatic ductal cell adenocarcinoma express PrP in their cancers. These patients have significantly shorter survival time compared with patients whose cancers lack PrP. Pro-PrP is also detected in melanoma in situ but is undetectable in normal melanocyte, and invasive melanoma expresses more pro-PrP. In this review, we will discuss the underlying mechanisms by which binding of pro-PrP to FLNA disrupts normal cellular physiology and contributes to tumorigenesis, and the potential mechanisms that cause the accumulation of pro-PrP in cancer cells.

Cholesterol interaction with proteins that partition into membrane domains: an overview

Biological membranes are complex structures composed largely of proteins and lipids. These components have very different structural and physical properties and consequently they do not form a single homogeneous mixture. Rather components of the mixture are more enriched in some regions than in others. This can be demonstrated with simple lipid mixtures that spontaneously segregate components so as to form different lipid phases that are immiscible with one another. The segregation of molecular components of biological membranes also involves proteins. One driving force that would promote the segregation of membrane components is the preferential interaction between a protein and certain lipid components. Among the varied lipid components of mammalian membranes, the structure and physical properties of cholesterol is quite different from that of other major membrane lipids. It would therefore be expected that in many cases proteins would have very different energies of interaction with cholesterol vs. those of other membrane lipids. This would be sufficient to cause segregation of components in membranes. The factors that facilitate the interaction of proteins with cholesterol are varied and are not yet completely understood. However, there are certain groups that are present in some proteins that facilitate interaction of the protein with cholesterol. These groups include saturated acyl chains of lipidated proteins, as well as certain amino acid sequences. Although there is some understanding as to why these particular groups favour interaction with cholesterol, our knowledge of these molecular features is not sufficiently developed to allow for the design of agents that will modify such binding.

Prostasin-dependent activation of epithelial Na+ channels by low plasmin concentrations

Several pathophysiological conditions, including nephrotic syndrome, are characterized by increased renal activity of the epithelial Na(+) channel (ENaC). We recently identified plasmin in nephrotic urine as a stimulator of ENaC activity and undertook this study to investigate the mechanism by which plasmin stimulates ENaC activity. Cy3-labeled plasmin was found to bind to the surface of the mouse cortical collecting duct cell line, M-1. Binding depended on a glycosylphosphatidylinositol (GPI)-anchored protein. Biotin-label transfer showed that plasmin interacted with the GPI-anchored protein prostasin on M-1 cells and that plasmin cleaved prostasin. Prostasin activates ENaC by cleavage of the gamma-subunit, which releases an inhibitory peptide from the extracellular domain. Removal of GPI-anchored proteins from the M-1 cells with phosphatidylinositol-specific phospholipase C (PI-PLC) inhibited plasmin-stimulated ENaC current in monolayers of M-1 cells at low plasmin concentration (1-4 microg/ml). At a high plasmin concentration of 30 microg/ml, there was no difference between cell layers treated with or without PI-PLC. Knockdown of prostasin attenuated binding of plasmin to M1 cells and blocked plasmin-stimulated ENaC current in single M-1 cells, as measured by whole-cell patch clamp. In M-1 cells expressing heterologous FLAG-tagged prostasin, gammaENaC and prostasin were colocalized. A monoclonal antibody directed against the inhibitory peptide of gammaENaC produced specific immunofluorescence labeling of M-1 cells. Pretreatment with plasmin abolished labeling of M-1 cells in a prostasin-dependent way. We conclude that, at low concentrations, plasmin interacts with GPI-anchored prostasin, which leads to cleavage of the gamma-subunit and activation of ENaC, while at higher concentrations, plasmin directly activates ENaC.

Effect of fetal size on fetal placental hyaluronan and hyaluronoglucosaminidases throughout gestation in the pig

The trophoblast-endometrial epithelial cell bilayer of porcine placenta undergoes microscopic folding during gestation, and the folded bilayer is embedded in fetal placental stroma. We hypothesized that hyaluronan was a component of fetal placental stroma, and that hyaluronoglucosaminidases played a role in bilayer folding. Gilts were unilaterally hysterectomized-ovariectomized (UHO) at 160 days of age, mated at estrus and killed on days 25, 45, 65, 85 or 105 of gestation. Fetal placental tissues were collected to evaluate hyaluronan and hyaluronoglucosaminidase content. Fetal placental hyaluronan concentration increased (P<0.01) between day 25 and 45 of gestation, remained high throughout gestation, and was greater (P<0.05) in the fetal placenta of the smallest compared to the largest fetuses on day 105 of gestation. Hyaluronan was localized to fetal placental stroma. Three cDNAs for hyaluronoglucosaminidase 1 (two 1379 and one 1552bp) and one cDNA (1421bp) for hyaluronoglucosaminidase 2 were cloned from day-85 fetal placental RNA. Gene expression analysis indicated that the 1379bp form of hyaluronoglucosaminidase 1 mRNA did not differ, the 1552bp form increased, and the 1421bp form of hyaluronoglucosaminidase 2 decreased during pregnancy. Amount of all three mRNAs was greater (P<0.05) in fetal placenta of the smallest compared to the largest fetuses. Zymography indicated 70 and 55kd protein isoforms of hyaluronoglucosaminidase in fetal placental tissue. Both forms increased with advancing gestation and were greater in fetal placenta of the smallest compared to the largest fetuses (P<0.05). These results are consistent with a role for hyaluronan and hyaluronoglucosaminidases in the development of the microscopic folds of the pig placenta during gestation.

A New World primate deficient in tetherin-mediated restriction of human immunodeficiency virus type 1

Human immunodeficiency virus type 1 (HIV-1) does not replicate in primary cells of New World primates. To better understand this restriction, we expressed owl monkey (Aotus nancymaae) CD4 and CXCR4 in the owl monkey kidney cell line, OMK. An HIV-1 variant modified to evade the owl monkey restriction factor TRIM-cyp replicated efficiently in these cells but could not replicate in primary A. nancymaae CD4-positive T cells. To understand this difference, we examined APOBEC3G and tetherin orthologs from OMK cells and primary A. nancymaae cells. We observed that OMK cells expressed substantially lower levels of APOBEC3G than did A. nancymaae cells. A. nancymaae, but not marmoset (Callithrix jacchus), APOBEC3G was partially downregulated by HIV-1 vif and reduced but did not abolish HIV-1 replication when stably expressed in OMK cells. The functional difference between A. nancymaae and marmoset APOBEC3Gs mapped to residue 128, previously shown to distinguish African green monkey from human APOBEC3G. We also characterized tetherin orthologs from OMK and A. nancymaae cells. The A. nancymaae tetherin ortholog, but not OMK tetherin, prevented HIV-1 release. Alteration of threonine 181 of OMK tetherin rescued its function and its efficient N glycosylation. All alleles of Aotus lemurinus griseimembra examined, but none of A. nancymaae or Aotus vociferans, encoded this nonfunctional tetherin ortholog. Our data indicate that HIV-1 replication in owl monkeys is not restricted at entry but can be limited by APOBEC3G and tetherin. Further, A. lemurinus griseimembra does not restrict HIV-1 replication via tetherin, a property likely useful for the study of tetherin-restricted viruses.

Functional consequences of the subdomain organization of the sulfs

Sulf-1 and Sulf-2 are novel extracellular sulfatases that act on internal glucosamine 6-O-sulfate modifications within heparan sulfate proteoglycans and regulate their interactions with various signaling molecules, including Wnt ligands. Although the Sulfs are multidomain proteins, there is limited information available about how the subdomains contribute to their enzymatic and signaling activities. In this study, we found that both human Sulfs were synthesized as prepro-enzymes and cleaved by a furin-type proteinase to form disulfide-bond linked heterodimers of 75- and 50-kDa subunits. The mature Sulfs were secreted into conditioned medium, as well as retained on the cell membrane. Although the catalytic center resides in the N-terminal 75-kDa subunit, the C-terminal 50-kDa subunit was indispensable for both arylsufatase and glucosamine 6-O-sulfate-endosulfatase activity. We found that the hydrophilic regions of the Sulfs were essential for endosulfatase activity but not for arylsulfatase activity. Using Edman sequencing, we identified furin-type proteinase cleavage sites in Sulf-1 and Sulf-2. Deletion of these sequences resulted in uncleavable forms of Sulfs. The uncleavable Sulfs retained enzymatic activity. However, they were unable to potentiate Wnt signaling, which may be due to their defective localization into lipid rafts on the plasma membrane.

A novel cell-surface protein CSP82 on bone marrow stem cells and a cytosolic phosphoprotein DP58 (ankyrinRD 34B) are involved in promyeloid progenitor induction

The molecular events associated with the development of common myeloid progenitor (CMP) remain largely unknown. This study reports that a novel glycosylphosphatidylinositol (GPI)-anchored lactoferrin CSP82 on uninitiated mouse bone marrow cells (BMC) may be involved in inducing pro-DC from CMP. By peptide mass fingerprinting, CSP82 has been identified as the mouse lactoferrin precursor, but unlike the latter, it occurs as a GPI-linked cell-surface protein. The GPI-linkage was demonstrated on BMC-derived immunoprecipitates and by other techniques. Furthermore, BMC and hematopoietic stem BM cells following incubation with either CSP82 peptide antibody or purified Reagent A yielded CMP-like progenitors (BM4 cells). These progenitors expressed a previously reported cytosolic phosphoprotein DP58 (AnkRD 34B protein). Continued cultivation of BMC in media containing only anti-CSP82 antibody led to DC-like cells, that bore phenotypic and endocytic resemblance with those obtained using GM-CSF. The results suggest that a receptor lactoferrin on BMC may be an important non-cytokine mechanism for early promyeloid progenitor differentiation.

Signal transduction by CD58: the transmembrane isoform transmits signals outside lipid rafts independently of the GPI-anchored isoform

The adhesion molecule CD58 is natively expressed in both a glycosylphosphatidylinositol (GPI)-anchored form and a transmembrane form. We previously demonstrated that the two isoforms of CD58 are differentially distributed in the cell membrane. The GPI-linked form resides in lipid rafts while the transmembrane form resides outside lipid rafts. Following cross-linking a fraction of transmembrane CD58 redistributes to lipid rafts. It has also been demonstrated that ligand binding to CD58 induces biological functions such as cytokine production and immunoglobulin isotype switching, indicating that cell-cell interactions result in CD58-mediated signal transduction. However, the signaling pathways involved in these activation processes are poorly defined. Here we show for the first time that cross-linking of CD58 induces protein tyrosine phosphorylation of BLNK, Syk and PLCgamma, and activation of ERK and Akt/PKB. In addition, we studied how these signaling events relate to the distinct membrane localization of the two isoforms of CD58. We demonstrate that cross-linking of CD58 triggers signaling that is predominantly associated with transmembrane CD58 in nonraft microdomains. Moreover, signaling through transmembrane CD58 does not depend on coexpression of the GPI-linked isoform. Thus, despite the residence of its GPI-anchored isoform in lipid rafts and the translocation of a fraction of its transmembrane isoform to lipid rafts, CD58 signaling is triggered by the transmembrane isoform outside lipid rafts. These findings corroborate signaling outside lipid rafts, as opposed to the established notion that rafts function as essential platforms for signaling.