Extracellular o-linked N-acetylglucosamine is enriched in stem cells derived from human umbilical cord blood

Stem cells have a unique ability to self-renew and differentiate into diverse cell types. Currently, stem cells from various sources are being explored as a promising new treatment for a variety of human diseases. A diverse set of functional and phenotypical markers are used in the characterization of specific therapeutic stem cell populations. The glycans on the stem cell surface respond rapidly to alterations in cellular state and signaling and are therefore ideal for identifying even minor changes in cell populations. Many stem cell markers are based on cell surface glycan epitopes including the widely used markers SSEA-3, SSEA-4, Tra 1-60, and Tra 1-81. We have now discovered by mRNA analysis that a novel glycosyltranferase, epidermal growth factor (EGF) domain-specific O-linked GlcNAc transferase (EOGT), is highly expressed in stem cells. EOGT is responsible for adding O-linked N-acetylglucosamine (O-GlcNAc) to folded EGF domains on extracellular proteins, such as those on the Notch receptors. We were able to show by immunological assays that human umbilical cord blood–derived mesenchymal stromal cells display O-GlcNAc, the product of EOGT, and that O-GlcNAc is further elongated with galactose to form O-linked N-acetyllactosamine. We suggest that these novel glycans are involved in the fine tuning of Notch receptor signaling pathways in stem cells.

Production of a recombinant antibody specific for i blood group antigen, a mesenchymal stem cell marker

Multipotent mesenchymal stem/stromal cells (MSCs) offer great promise for future regenerative and anti-inflammatory therapies. Panels of functional and phenotypical markers are currently used in characterization of different therapeutic stem cell populations from various sources. The i antigen (linear poly-N-acetyllactosamine) from the Ii blood group system has been suggested as a marker for MSCs derived from umbilical cord blood (UCB). However, there are currently no commercially available antibodies recognizing the i antigen. In the present study, we describe the use of antibody phage display technology to produce recombinant antibodies recognizing a structure from the surface of mesenchymal stem cells. We constructed IgM phage display libraries from the lymphocytes of a donor with an elevated serum anti-i titer. Antibody phage display technology is not dependent on immunization and thus allows the generation of antibodies against poorly immunogenic molecules, such as carbohydrates. Agglutination assays utilizing i antigen–positive red blood cells (RBCs) from UCB revealed six promising single-chain variable fragment (scFv) antibodies, three of which recognized epitopes from the surface of UCB-MSCs in flow cytometric assays. The amino acid sequence of the V_H gene segment of B12.2 scFv was highly similar to the V_H4.21 gene segment required to encode anti-i specificities. Further characterization of binding properties revealed that the binding of B12.2 hyperphage was inhibited by soluble linear lactosamine oligosaccharide. Based on these findings, we suggest that the B12.2 scFv we have generated is a prominent anti-i antibody that recognizes i antigen on the surface of both UCB-MSCs and RBCs. This binder can thus be utilized in UCB-MSC detection and isolation as well as in blood group serology.

Metabolic glycoengineering of mesenchymal stromal cells with N-propanoylmannosamine

There is an increasing interest in the modification of cell surface glycosylation to improve the properties of therapeutic cells. For example, glycosylation affects the biodistribution of mesenchymal stromal cells (MSCs). Metabolic glycoengineering is an efficient way to modify the cell surface. The mammalian biosynthetic machinery tolerates the unnatural sialic acid precursor, N-propanoylmannosamine (ManNProp), and incorporates it into cell surface glycoconjugates. We show here by mass spectrometric analysis of cell surface N-glycans that about half of N-acetylneuraminic acid was replaced by N-propanoylneuraminic acid in the N-glycans of human umbilical cord blood-derived MSCs supplemented with ManNProp. In addition, the N-glycan profile was altered. ManNProp-supplemented cells had more multiply fucosylated N-glycan species than control cells. The fucosylated epitopes were shown in tandem mass spectrometric analysis to be Lewis x or blood group H epitopes, but not sialyl Lewis x (sLex). The amounts of tri- and tetra-antennary and polylactosamine-containing N-glycans also increased in ManNProp supplementation. In accordance with previous studies of other cell types, increased expression of the sLex epitope in ManNProp-supplemented MSCs was demonstrated by flow cytometry. In light of the N-glycan analysis, the sLex epitope in these cells is likely to be carried by O-glycans or glycolipids. sLex has been shown to target MSCs to bone marrow, which may be desirable in therapeutic applications. The present results represent the first structural analysis of an N-glycome of ManNProp-supplemented cells and demonstrate the feasibility of modifying cell surface glycosylation of therapeutic cells by this type of metabolic glycoengineering.

The i blood group antigen as a marker for umbilical cord blood-derived mesenchymal stem cells

Multipotent mesenchymal stem cells (MSCs) offer great promise for future regenerative and anti-inflammatory therapies. However, there is a lack of methods to quickly and efficiently isolate, characterize, and ex vivo expand desired cell populations for therapeutic purposes. Single markers to identify cell populations have not been characterized; instead, all characterizations rely on panels of functional and phenotypical properties. Glycan epitopes can be used for identifying and isolating specific cell types from heterogeneous populations, on the basis of their cell-type specific expression and prominent cell surface localization. We have now studied in detail the cell surface expression of the blood group i epitope (linear poly-N-acetyllactosamine chain) in umbilical cord blood (UCB)-derived MSCs. We used flow cytometry and mass spectrometric glycan analysis and discovered that linear poly-N-acetyllactosamine structures are expressed in UCB-derived MSCs, but not in cells differentiated from them. We further verified the findings by mass spectrometric glycan analysis. Gene expression analysis indicated that the stem-cell specific expression of the i antigen is determined by β3-N-acetylglucosaminyltransferase 5. The i antigen is a ligand for the galectin family of soluble lectins. We found concomitant cell surface expression of galectin-3, which has been reported to mediate the immunosuppressive effects exerted by MSCs. The i antigen may serve as an endogenous ligand for this immunosuppressive agent in the MSC microenvironment. Based on these findings, we suggest that linear poly-N-acetyllactosamine could be used as a novel UCB-MSC marker either alone or within an array of MSC markers.

Ezrin-dependent regulation of the actin cytoskeleton by -dystroglycan

Dystroglycan is part of an adhesion receptor complex linking the extracellular matrix to the actin cytoskeleton. Previous studies have implicated dystroglycan in basement membrane formation and as a crucial link between dystrophin and laminin in muscle. We report here a further novel function for dystroglycan which appears to be in addition to its role as an adhesion molecule. beta-dystroglycan has been localized to microvilli structures in a number of cell types where it associates with the cytoskeletal adaptor ezrin, through which it is able to modulate the actin cytoskeleton and induce peripheral filopodia and microvilli. Ezrin is able to interact with dystroglycan through a cluster of basic residues in the juxtamembrane region of dystroglycan, and mutation of these residues both prevents ezrin binding and the induction of actin-rich surface protrusions. These studies reveal novel functions and additional signalling roles for dystroglycan, raising the possibility of new avenues for therapeutic intervention in diseases such as Duchenne muscular dystrophy.

Myotilin, the limb-girdle muscular dystrophy 1A (LGMD1A) protein, cross-links actin filaments and controls sarcomere assembly

The assembly and maintenance of the muscle sarcomere requires a complex interplay of actin- and myosin-associated proteins. Myotilin is a thin filament-associated Z-disc protein that consists of two Ig-domains flanked by a unique serine-rich amino-terminus and a short carboxy-terminal tail. It binds to alpha-actinin and filamin c and is mutated in limb girdle muscular dystrophy 1A (LGMD1A). Here we show that myotilin also directly binds F-actin, efficiently cross-links actin filaments alone or in concert with alpha-actinin and prevents filament disassembly induced by Latrunculin A. Myotilin forms dimers via its carboxy-terminal half, which may be necessary for the actin-bundling activity. Overexpression of full-length myotilin but not the carboxy-terminal half induces formation of thick actin cables in non-muscle cells devoid of endogenous myotilin. The expression of myotilin in muscle cells is tightly regulated to the later stages of in vitro myofibrillogenesis, when preassembled myofibrils begin to align. Expression of either amino- or carboxy-terminally truncated myotilin fragments but not wild-type myotilin in differentiating myocytes leads to myofibril disarray. The disease association and functional characteristics indicate an indispensable role for myotilin in stabilization and anchorage of thin filaments, which may be a prerequisite for correct Z-disc organization.

Characterization of human palladin, a microfilament-associated protein

Actin-containing microfilaments control cell shape, adhesion, and contraction. In striated muscle, alpha-actinin and other Z-disk proteins coordinate the organization and functions of actin filaments. In smooth muscle and nonmuscle cells, periodic structures termed dense bodies and dense regions, respectively, are thought to serve functions analogous to Z-discs. We describe here identification and characterization of human palladin, a protein expressed mainly in smooth muscle and nonmuscle and distributed along microfilaments in a periodic manner consistent with dense regions/bodies. Palladin contains three Ig-domains most homologous to the sarcomeric Z-disk protein myotilin. The N terminus includes an FPPPP motif recognized by the Ena-Vasp homology domain 1 domain in Ena/vasodilatator-stimulated phosphoprotein (VASP)/Wiscott-Aldrich syndrome protein (WASP) protein family. Cytoskeletal proteins with FPPPP motif target Ena/VASP/WASP proteins to sites of actin modulation. We identified palladin in a yeast two-hybrid search as an ezrin-associated protein. An interaction between palladin and ezrin was further verified by affinity precipitation and blot overlay assays. The interaction was mediated by the alpha-helical domain of ezrin and by Ig-domains 2-3 of palladin. Ezrin is typically a component of the cortical cytoskeleton, but in smooth muscle cells it is localized along microfilaments. These cells express palladin abundantly and thus palladin may be involved in the microfilament localization of ezrin. Palladin expression was up-regulated in differentiating dendritic cells (DCs), coinciding with major cytoskeletal and morphological alterations. In immature DCs, palladin localized in actin-containing podosomes and in mature DCs along actin filaments. The regulated expression and localization suggest a role for palladin in the assembly of DC cytoskeleton.

The cytoskeletal association of CD11/CD18 leukocyte integrins in phorbol ester-activated cells correlates with CD18 phosphorylation

Leukocyte adhesion is a regulated process, which involves CD11/CD18 leukocyte integrins. CD11/CD18 acidity may be regulated intracellularly, and the CD18 polypeptide has previously been shown to become phosphorylated on serine and threonine after phorbol ester activation of T cells. Increased adhesiveness is believed to be mediated by regulating the overall avidity of cellular contact. CD11/CD18 integrins have earlier been reported to interact with several cytoskeletal proteins. We have now studied the involvement of the CD18 phosphorylation in cytoskeletal associations. We have investigated the distribution of phosphorylated CD18 between soluble, cytoskeletal and nuclear fractions of T cell detergent lysates. A significant amount of phosphorylated CD18 polypeptides was observed to fraction along with the cytoskeleton, while the majority of the cell surface CD18 molecules remained in the soluble fraction. Putative candidates for this altered cytoskeletal binding of CD11/CD18 were shown to be talin and filamin, which were observed to bind to CD18 cytoplasmic peptides and co-precipitate with CD18. The importance of the CD18 cytoplasmic domain in the regulation of the leukocyte adhesion was further strengthened by inhibition of phorbol ester-induced T cell adhesion with a phosphorylated lipopeptide corresponding to the cytoplasmic portion of the CD18. These results indicate that the induced CD18 phosphorylation and the altered cytoskeletal binding of the phosphorylated integrin complex may contribute to the increased avidity of CD11/CD18-mediated leukocyte adhesion.