We have undertaken a comparative study of the interaction of the three mammalian transforming growth factor-␤s (TGF-␤) with heparin and heparan sulfate. TGF-␤1 and -␤2, but not -␤3, bind to heparin and the highly sulfated liver heparan sulfate. These polysaccharides potentiate the biological activity of TGF-␤1 (but not the other isoforms), whereas a low sulfated mucosal heparan sulfate fails to do so. Potentiation is due to antagonism of the binding and inactivation of TGF-␤1 by ␣ 2 -macroglobulin, rather than by modulation of growth factor-receptor interactions. TGF-␤2⅐␣ 2 -macroglobulin complexes are more refractory to heparin/ heparan sulfate, and those involving TGF-␤3 cannot be affected. Comparison of the amino acid sequences of the TGF-␤ isoforms strongly implicates the basic amino acid residue at position 26 of each monomer as being a vital binding determinant. A model is proposed in which polysaccharide binding occurs at two distinct sites on the TGF-␤ dimer. Interaction with heparin and liver heparan sulfate may be most effective because of the ability of the dimer to co-operatively engage two specific sulfated binding sequences, separated by a distance of approximately seven disaccharides, within the same chain.

Embryonic stem (ES) cells can be cultured in conditions thateither maintain pluripotency or allow differentiation to the three embryonic germ layers. Heparan sulfate (HS), a highly polymorphic glycosaminoglycan, is a critical cell surface coreceptor in embryogenesis, and in this paper we describe its structural transition from an unusually low-sulfated variant in ES cells to a more highly sulfated form in fluorescence-activated cell sorting-purified neural progenitor cells. The characteristic domain structure of HS was retained during this transformation. However, qualitative variations in surface sulfation patterns between ES and differentiated cells were revealed using HS epitope-specific antibodies and the HS-binding growth factor fibroblast growth factor 2 (FGF-2). Expression profiles of the HS modification enzymes indicated that both "early" (N-sulfotransferases) and "late" (6O-and 3O-sulfotransferases) sulfotransferases contributed to the alterations in sulfation patterning. An HS-null ES line was used to demonstrate the necessity for HS in neural differentiation. HS is a coreceptor for many of the protein effectors implicated in pluripotency and differentiation (e.g., members of the FGF family, bone morphogenic proteins, and fibronectin). We suggest that the stage-specific activities of these proteins are finely regulated by dynamic changes in sulfation motifs in HS chains.

Mouse embryonic stem (mES) cells express a low sulfated form of heparan sulfate (HS). HS chains displayed by ES cells and their progeny become more complex and more sulfated during progression from pluripotency to neuroectodermal precursors. Sulfated epitopes are important for recognition and binding of a variety of ligands including members of the fibroblast growth factor (FGF) family. We demonstrated previously that mES cells lacking HS cannot undergo neural specification but this activity can be recovered by adding soluble heparin, a highly sulfated glycosaminoglycan (GAG). Therefore, we hypothesized that soluble GAGs might be used to support neural differentiation of HS competent cells and that the mechanisms underlying this activity might provide useful information about the signaling pathways critical for loss of pluripotency and early lineage commitment. In this study, we demonstrate that specific HS/heparin polysaccharides support formation of Sox1 1 neural progenitor cells from wild-type ES cells. This effect is dependent on sulfation pattern, concentration, and length of saccharide. Using a selective inhibitor of FGF signal transduction, we show that heparin modulates signaling events regulating exit from pluripotency and commitment to primitive ectoderm and subsequently neuroectoderm. Interestingly, we were also able to demonstrate that multiple receptor tyrosine kinases were influenced by HS in this system. This suggests roles for additional factors, possibly in cell proliferation or protection from apoptosis, during the process of neural specification. Therefore, we conclude that soluble GAGs or synthetic mimics could be considered as suitable low-cost factors for addition to ES cell differentiation regimes. STEM CELLS 2011;29:629-640 Disclosure of potential conflicts of interest is found at the end of this article.

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The interaction of fibronectin with cell surface heparan sulfate proteoglycans is important biologically in inducing reorganization of the cytoskeleton and the assembly of focal adhesions. The major heparan sulfatebinding site in fibronectin, which is also implicated in these morphological events, is the COOH-terminal Hep-2 domain. We describe the first extensive study of the structural determinants required for the interaction between heparan sulfate/heparin and Hep-2. It is clear that, in heparan sulfate, there is a very prominent role for N-sulfate groups, as opposed to a relatively small apparent contribution from carboxyl groups. Furthermore, a minimal octasaccharide binding sequence appeared to contain at least two 2-O-sulfated iduronate residues, but no 6-O-sulfate groups. However, affinity was enhanced by the presence of 6-O-sulfates, and the interaction with Hep-2 also increased progressively with oligosaccharide size up to a maximum length of a tetradecasaccharide. This overall specificity is compatible with recent information on the structure of Hep-2

Heparan sulfate proteoglycans (HSPG) encompass some of the most abundant macromolecules on the surface of almost every cell type. Heparan sulfate (HS) chains provide a key interaction surface for the binding of numerous proteins such as growth factors and morphogens, helping to define the ability of a cell to respond selectively to environmental cues. The specificity of HSprotein interactions are governed predominantly by the order and positioning of sulfate groups, with distinct cell types expressing unique sets of HS epitopes. Embryos deficient in HS-synthesis (Ext1 ؊/؊ ) exhibit pre-gastrulation lethality and lack recognizable organized mesoderm and extraembryonic tissues. Here we demonstrate that embryonic stem cells (ESCs) derived from Ext1 ؊/؊ embryos are unable to differentiate into hematopoietic lineages, instead retaining ESC marker expression throughout embryoid body (EB) culture. However hematopoietic differentiation can be restored by the addition of soluble heparin. Consistent with specific size and composition requirements for HS:growth factor signaling, chains measuring at least 12 saccharides were required for partial rescue of hematopoiesis with longer chains (18 saccharides or more) required for complete rescue. Critically N-and 6-O-sulfate groups were essential for rescue. Heparin addition restored the activity of multiple signaling pathways including bone morphogenic protein (BMP) with activation of phosphoSMADs re-established by the addition of heparin. Heparin addition to wild-type cultures also altered the outcome of differentiation, promoting hematopoiesis at low concentrations, yet inhibiting blood formation at high concentrations. Thus altering the levels of HS and HS sulfation within differentiating ESC cultures provides an attractive and accessible mechanism for influencing cell fate.Cells have adopted elaborate systems to identify growth factor and morphogen cues in order to elicit an appropriate response throughout embryonic development and adulthood. A key component of this process is the sugar polymer heparan sulfate (HS), 2 which coats the outer surface of almost every cell within the body enabling the selective detection of environmental signals. HS polymerization is catalyzed by a heterodimer consisting of EXT1 and EXT2 enzymes resulting in the addition of alternating glucuronic acid (GlcA) and N-acetylglucosamine (GlcNAc) residues. This backbone is then enzymatically modified by de-N-acetylation and N-sulfation of selected GlcNAc residues to form N-sulfoglucosamine (GlcNS), epimerization of a subset of GlcA residues to iduronic acid (IdoUA) and the addition of sulfate groups at the 2-O-position of GlcA/IdoUA and 6-O-and rarely 3-O-position of GlcNS/GlcNAc. Modification is highly regulated and not complete, forming a cell-type specific pattern of highly sulfated domains (S-domains) interspersed by unmodified regions with the length, position and patterning of sulfate groups ultimately determining HS:ligand binding. The related GAG heparin is virtually fully sulfated along its l...

Background: Heparan sulfate (HS) is an essential regulator of multiple angiogenic growth factors. Results: Down-regulation of 6-O-sulfation in endothelial cell HS affects FGF2-and VEGF-mediated endothelial cell functions. Conclusion:The level of 6-O-sulfation in specific HS domains regulates endothelial cell responses to angiogenic growth factors. Significance: The relationships between 6-O-sulfation and endothelial phenotypes could help to design HS sequences inhibiting angiogenic growth factors.

We have previously shown that (i) human skin fibroblasts of fetal and adult origin display distinctive migratory phenotypes, (ii) this difference in cell behavior results from the production of a soluble "migration stimulating factor" (MSF) by fetal cells, and (iii) skin fibroblasts from breast cancer patients commonly resemble fetal fibroblasts both in migratory phenotype and in production of MSF. Data are now presented indicating that MSF present in the conditioned medium of fetal and cancer patient fibroblasts is precipitated at 10% saturation ammonium sulfate and binds to heparin and cation-exchange resins. Based on this information, we have devised a scheme for the purification of MSF involving the sequential application of ammonium sulfate precipitation, heparin affinity, gel filtration, and reverse-phase chromatography. Purified MSF has an estimated molecular mass of 70 kDa; amino acid analysis reveals a relatively high level of proline (13.34 residues per 100). Our results further suggest that skin fibroblasts from breast cancer patients produce an additional factor with migration stimulating activity; this factor is precipitated at higher concentrations of ammonium sulfate and binds to anion-exchange resins. We have previously discussed the possible direct involvement of fetal-like fibroblasts in cancer pathogenesis. The availability of MSF obtained from cancer patient fibroblasts provides a potential means with which to examine the complex cellular interactions contributing to this process as well as develop a screening regime for identifying individuals at elevated risk of developing cancer.