Release and extracellular transit of glycosylphosphatidylinositol proteins

Characterization of soluble folate receptors (folate binding proteins) in humans. Biological roles and clinical potentials in infection and malignancy

This review surveys soluble Folate Receptors (FOLRs) in humans. FOLR1 and FOLR2 are equipped with cellular glycosylphosphatidylinositol (GPI) anchors. FOLR1 is secreted from epithelia with or without a micelle-encapsulated GPI-anchor into milk and other body fluids/secretions, e.g. semen where its interaction with spermatozoa indicates a role in male fertility. FOLR1 and FOLR2 serve as serum biomarkers of various diseases. FOLR3 possesses no GPI-anchor and originates from secretory granules of neutrophil granulocytes; its concentration in serum correlates to the FOLR3 content in leukocytes and rises with increased leukocyte counts (infection, malignancy and pregnancy). FOLR3 exerts anti-microbial and anti-tumor effects by depriving bacteria and tumor cells of natural folates. Megalin receptors mediate reabsorption of ultrafiltered folate-bound FOLR into cells of proximal kidney tubules and of folate-bound FOLR uptake in growing embryos. Megalin receptors overexpressed in malignant tumors could be suitable therapeutic targets for folate-conjugated cytotoxic agents utilizing soluble FOLRs as vectors.

Membrane insertion and intercellular transfer of glycosylphosphatidylinositol-anchored proteins: potential therapeutic applications

Anchorage of a subset of cell surface proteins in eukaryotic cells is mediated by a glycosylphosphatidylinositol (GPI) moiety covalently attached to the carboxy-terminus of the protein moiety. Experimental evidence for the potential of GPI-anchored proteins (GPI-AP) of being released from cells into the extracellular environment has been accumulating, which involves either the loss or retention of the GPI anchor. Release of GPI-AP from donor cells may occur spontaneously or in response to endogenous or environmental signals. The experimental evidence for direct insertion of exogenous GPI-AP equipped with the complete anchor structure into the outer plasma membrane bilayer leaflets of acceptor cells is reviewed as well as the potential underlying molecular mechanisms. Furthermore, promiscuous transfer of certain GPI-AP between plasma membranes of different cells in vivo under certain (patho)physiological conditions has been reported. Engineering of target cell surfaces using chimeric GPI-AP with complete GPI anchor may be useful for therapeutic applications.

Chip-based sensing for release of unprocessed cell surface proteins in vitro and in serum and its (patho)physiological relevance

To study the possibility that certain components of eukaryotic plasma membranes are released under certain (patho)physiological conditions, a chip-based sensor was developed for the detection of cell surface proteins, which are anchored at the outer leaflet of eukaryotic plasma membranes by a covalently attached glycolipid, exclusively, and might be prone to spontaneous or regulated release on the basis of their amphiphilic character. For this, unprocessed, full-length glycosylphosphatidylinositol-anchored proteins (GPI-AP), together with associated phospholipids, were specifically captured and detected by a chip- and microfluidic channel-based sensor, leading to changes in phase and amplitude of surface acoustic waves (SAW) propagating over the chip surface. Unprocessed GPI-AP in complex with lipids were found to be released from rat adipocyte plasma membranes immobilized on the chip, which was dependent on the flow rate and composition of the buffer stream. The complexes were identified in the incubation medium of primary rat adipocytes, in correlation to the cell size, and in rat as well as human serum. With rats, the measured changes in SAW phase shift, reflecting specific mass/size or amount of the unprocessed GPI-AP in complex with lipids, and SAW amplitude, reflecting their viscoelasticity, enabled the differentiation between the lean and obese (high-fat diet) state, and the normal (Wistar) and hyperinsulinemic (Zucker fatty) as well as hyperinsulinemic hyperglycemic (Zucker diabetic fatty) state. Thus chip-based sensing for complexes of unprocessed GPI-AP and lipids reveals the inherently labile anchorage of GPI-AP at plasma membranes and their susceptibility for release in response to (intrinsic/extrinsic) cues of metabolic relevance and may, therefore, be useful for monitoring of (pre-)diabetic disease states.

The complex interplay between ligand binding and conformational structure of the folate binding protein (folate receptor): Biological perspectives

This review analyzes how interplay between folate binding and changes in folate binding protein (FBP) conformation/self-association affects the biological function of FBP. Concentration-dependent, reversible self-association of hydrophobic apo-FBP at pI=7.4 is associated with decreased affinity for folate, probably due to shielding of binding sites between interacting hydrophobic patches. Titration with folate removes apo-monomers, favoring dissociation of self-associated apo-FBP into apo-monomers. Folate anchors to FBP through a network of hydrogen bonds and hydrophobic interactions, and the binding induces a conformational change with formation of hydrophilic and stable holo-FBP. Holo-FBP exhibits a ligand-mediated concentration-dependent self-association into multimers of great thermal and chemical stability due to strong intermolecular forces. Both ligand and FBP are thus protected against biological/physicochemical decomposition. In biological fluids with low FBP concentrations, e.g., saliva, semen and plasma, hydrophobic apo-monomers and hydrophilic holo-monomers associate into stable asymmetrical complexes with aberrant binding kinetics unless detergents, e.g., cholesterol or phospholipids are present.

Role of glycosylphosphatidylinositol-specific phospholipase D in the homing of umbilical cord blood, mobilized peripheral blood and bone marrow-derived hematopoietic stem/progenitor cells

Recent studies suggested that glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) correlated with tumor malignancy and prognosis of certain tumors. As hematopoietic stem/progenitor cells (HS/PC) homing was similar to tumor invasion and metastasis in some mechanisms, which arose our interests in whether GPI-PLD contribution to the homing of HS/PC. In this study, CD34(+) cells from umbilical cord blood (UCB), mobilized peripheral blood (MPB), and bone marrow (BM) were assayed for their differences in adhesion, migration, respectively. The expression of GPI-anchored proteins (CD48, CD90) on the cells were analyzed by flow cytometry. Semi-quantitive RT-PCR was used to detect GPI-PLD expression in the three different CD34(+) cells. The results showed that GPI-PLD had no effect on the adhesion of CD34(+) cells. While, spontaneous and SDF-1 induced migration of UCB and MPB, but not BM CD34(+) cells were decreased after 1,10-phenanthroline (an inhibitor of GPI-PLD) pretreatment. Furthermore, we found little difference in GPI-anchored adhesion molecules (CD48, CD90) expression between untreated and pretreated CD34(+) cells. GPI-PLD mRNA was low expressed in MPB and undetected in UCB and BM CD34(+) cells. Our results suggested that GPI-PLD probably had no contribution to HS/PC homing, which may due to its low or no expression in UCB, BM and MPB CD34(+) cells.

Hitch-hiking between cells on lipoprotein particles

Cell surface proteins containing covalently linked lipids associate with specialized membrane domains. Morphogens like Hedgehog and Wnt use their lipid anchors to bind to lipoprotein particles and employ lipoproteins to travel through tissues. Removal of their lipid anchors or decreasing lipoprotein levels give rise to adverse Hedgehog and Wnt signaling. Some parasites can also transfer their glycosylphosphatidylinositol-anchored surface proteins to host lipoprotein particles. These antigen-loaded lipoproteins spread throughout the circulation, and probably hamper an adequate immune response by killing neutrophils. Together, these findings imply a widespread role for lipoproteins in intercellular transfer of lipid-anchored surface proteins, and may have various physiological consequences. Here, we discuss how lipid-modified proteins may be transferred to and from lipoproteins at the cellular level.

How Glycosylphosphatidylinositol-phospholipase D acts in homing of hematopoietic stem/progenitor cells?

Homing of hematopoietic stem/progenitor cells (HS/PC) to the bone marrow (BM) microenvironment is the first and essential step in HS/PC engraftment and initiation of the marrow reconstitution during clinical hematopoietic stem cell transplantation (HSCT). How to improve the homing efficiencies and make full use of HS/PC resources, especially umbilical cord blood (UCB), are of great importance in clinical practice. However, the cellular and molecular mechanisms that govern this process are poorly understood. Glycosylphosphatidylinositol-phospholipase D (GPI-PLD) is an enzyme which can regulate the expression of Glycosylphosphatidylinositol (GPI)-anchored proteins and modulate their correspondent functions by releasing GPI-anchored proteins from cell membrane. Recent studies suggested that the mechanisms of the malignancy and prognosis of certain tumors were correlated with GPI-PLD. HS/PC homing was similar to tumor invasion and metastasis in some process. Here we proposed the hypothesis that GPI-PLD might also has played a role in the homing of HS/PC by modulating the adhesion and migration of these cells. If GPI-PLD did participate in HS/PC homing, maybe the mechanisms of homing can herefrom be partly elucidated, which would benefit transplantation in clinical practice.

The how and why of exocytic vesicles

The purpose of this review is to draw the attention of general readers to the importance of cellular exocytic vesiculation as a normal mechanism of development and subsequent adjustment to changing conditions, focusing on red cell (RBC) vesiculation. Recent studies have emphasized the possible role of these microparticles as diagnostic and investigative tools. RBCs lose membrane, both in vivo and during ex vivo storage, by the blebbing of microvesicles from the tips of echinocytic spicules. Microvesicles shed by RBCs in vivo are rapidly removed by the reticuloendothelial system. During storage, this loss of membrane contributes to the storage lesion and the accumulation of the microvesicles are believed to be thrombogenic and thus to be clinically important.

Different distributions of human bone alkaline phosphatase isoforms in serum and bone tissue extracts

BACKGROUND

In vitro, bone alkaline phosphatase (BALP) is released from the osteoblast membrane with its glycosylphosphatidylinositol (GPI) anchor still attached (i.e., in an anchor-intact form); however, in vivo, BALP circulates as a variable mixture of anchorless isoforms, which can be identified by high-performance liquid chromatography (HPLC). Previous studies have shown that the relative abundance of these BALP isoforms in serum may be clinically useful for the diagnosis and management of metabolic bone disease.

METHODS

In the current studies, we describe a method for the determination of anchorless BALP isoforms in extracts of bone and we present novel data on the conversion of anchor-intact to anchorless BALP by incubation with endogenous circulating GPI-specific phospholipase D (GPI-PLD).

RESULTS

A 72-h extraction with 0.1% Triton X-100 released approximately 90% of the BALP activity from powdered bone. An average of 19% of this activity was anchorless, but essentially all of the activity could be converted to the anchorless form by incubation with partially purified GPI-PLD from human serum. Using HPLC, we detected four BALP isoforms (B/I, B1x, B1, and B2) in these GPI-PLD-treated extracts of bone. An additional BALP fraction was also detected in the samples during the initial phase of GPI-PLD treatment.

CONCLUSIONS

The abundance of the BALP isoforms differed between bone and serum, particularly for the B/I isoform, which comprised, on average, 18% of the BALP in GPI-PLD-treated extracts of healthy bone tissue, but only 6% of the total BALP activity in serum from healthy individuals. Based on our recent finding of differences in the number of sialic acid residues between the BALP isoforms, we hypothesize that this difference between BALP isoforms in serum and extracts of bone is due to the different patterns of glycosylation, which results in different biological half-lives in the circulation. A preliminary application of our method to the extraction of BALP isoforms from a small number of human bone samples suggests that the method should be useful for studies of human skeletal site-specific and metabolic bone disease-specific differences in the amounts and distributions of the BALP isoforms in bone.

Glycosylphosphatidylinositol-anchored proteins are not required for crosslinking-mediated endocytosis or transfection of avidin bioconjugates into biotinylated cells

Even though glycosylphosphatidylinositol (GPI)-anchored proteins lack direct structural contact with the intracellular space, these ubiquitously expressed surface receptors activate signaling cascades and endocytosis when crosslinked by extracellular ligands. Such properties may be due to their association with membrane microdomains composed of glycosphingolipids, cholesterol and some signaling proteins. In this study, we hypothesize that GPI proteins may be required for crosslinking-mediated endocytosis of extracellular bioconjugates. To test this hypothesis, we first biotinylated the surface membranes of native K562 erythroleukemia cells versus K562 cells incapable of surface GPI protein expression. We then compared the entry of fluorescently labeled avidin or DNA condensed on polyethylenimine-avidin bioconjugates into the two biotinylated cell populations. Using fluorescence microscopy, nearly 100% efficiency of fluorescent avidin endocytosis was demonstrated in both cell types over a 24 h period. Surprisingly, plasmid DNA transfer was slightly more efficient among the biotinylated GPI-negative cells as measured by the expression of green fluorescence protein. Our findings that GPI proteins are not required for the endocytosis of avidin bioconjugates into biotinylated cells suggest that endocytosis associated with general membrane crosslinking may be due to overall reorganization of the membrane domains rather than GPI protein-specific interactions.

Isoforms of bone alkaline phosphatase: characterization and origin in human trabecular and cortical bone

Alkaline phosphatase (ALP) is a glycoprotein and functions as an ectoenzyme attached to the cell membrane by a hydrophobic glycosyl-phosphatidylinositol (GPI) anchor. Three bone ALP (BALP) isoforms in human serum were separated and quantitated by high-performance liquid chromatography. B/I, a minor fraction, is composed on average of bone (70%) and intestinal (30%) ALP, and two major isoforms, B1 and B2. Treatment with GPI-specific phospholipase C (GPI-PLC) did not influence the activities or retention times for B1 and B2, indicating that the biochemical differences between B1 and B2 are likely to be due to different glycosylation patterns. The B/I fraction in serum, on average 4% of total ALP, was found to be composed of B1 and B2 isoforms, each with an intact hydrophobic GPI cell membrane anchor. We investigated the origin of these three BALP isoforms and osteocalcin in human femora from five healthy individuals (four males), mean age 51 years, obtained from a tissue bank. Bone was sampled from three sites: cortical bone, trabecular bone from the diaphysis, and trabecular bone from the greater trochanter. Trabecular bone, from both sites, had higher BALP activities compared with cortical bone. Conversely, the osteocalcin content of cortical bone was more than 3-fold greater than that of trabecular bone. Cortical bone had approximately 2-fold higher activity of B1 compared with B2, whereas trabecular bone had approximately 2-fold higher activity of B2 compared with B1. We observed a previously undescribed BALP isoform (B1x) in all bone samples. B1x was also observed in sera from some patients (60%) with severe renal insufficiency and on chronic dialysis therapy (n = 20). The isoforms of BALP may provide information relating to bone metabolism within specific bone compartments.

Protection of cells from complement-mediated attack: CD59 receptor clustering for the entry of macromolecules into hematopoietic cells

CD59 is a glycosylphosphatidylinositol anchored protein (GPI-protein) that is expressed on surface membranes to protect host cells from complement-mediated attack. CD59 may also serve as a receptor for the endocytosis of macromolecules into nucleated cells. Here we investigate the effects of primary clustering of CD59 with anti-CD59 monoclonal antibody and the secondary clustering of biotinylated anti-CD59 with avidin on red blood cells and erythroleukemic K562 cells. On red blood cells, CD59-targeted antibodies remained evenly distributed on the external membranes. In contrast, clustering, capping and endocytosis of the CD59-targeted complexes was detected on K562 cells. Secondary clustering appeared more efficient and resulted in endosomal localization of the fluorescently labeled complexes within 2 hours. The endocytosis of CD59-bound complexes did not affect K562 cell viability or growth and the surface level of CD59 was constant during the process. These result suggest clustering and subsequent endocytosis of CD59 may enable the entry of macromolecules to the endosomal compartments of hematopoietic cells.

Variegated transfer of recombinant glycosylphosphatidylinositol-anchored CD4 among cultured cells: correlation of flow cytometric and microscopic observations

Glycosylphosphatidylinositol-anchored proteins (GPI-proteins) expressed on the outer leaflet of cell membranes are involved in diverse physiologic as well as pathologic processes in humans. Previously, we demonstrated the intercellular transfer of overexpressed CD4-GPI in vitro from transduced HeLa cells to their parental cell line. In this report we present further information on the transfer process and the nature of the transferred GPI-proteins. In mixed-cell populations, the transfer of CD4-GPI was detectable within minutes at levels proportional to the ratio of donor and recipient cells. The amount of CD4-GPI detected with flow cytometry on the surface of the recipient cells varied according to cell type. Microscopy of mixed cell populations revealed discrete CD4-GPI containing aggregates on the target cells, whereas colocalized transfer of cytoplasm was not detected. Separation of cocultivated cells by semipermeable membranes largely prevented CD4-GPI transfer, but aggregates containing CD4-GPI were demonstrated by electron microscopy in supernatants passed through filters of 0.4-mm pore size.