Heparan sulfate-growth factor interactions

Introducing or removing heparan sulfate binding sites does not alter brain uptake of the blood-brain barrier shuttle scFv8D3

The blood-brain barrier (BBB) greatly limits the delivery of protein-based drugs into the brain and is a major obstacle for the treatment of brain disorders. Targeting the transferrin receptor (TfR) is a strategy for transporting protein-based drugs into the brain, which can be utilized by using TfR-binding BBB transporters, such as the TfR-binding antibody 8D3. In this current study, we investigated if binding to heparan sulfate (HS) contributes to the brain uptake of a single chain fragment variable of 8D3 (scFv8D3). We designed and produced a scFv8D3 mutant, engineered with additional HS binding sites, HS(+)scFv8D3, to assess whether increased HS binding would improve brain uptake. Additionally, a mutant with a reduced number of HS binding sites, HS(-)scFv8D3, was also engineered to see if reducing the HS binding sites could also affect brain uptake. Heparin column chromatography showed that only the HS(+)scFv8D3 mutant bound HS in the experimental conditions. Ex vivo results showed that the brain uptake was unaffected by the introduction or removal of HS binding sites, which indicates that scFv8D3 is not dependent on the HS binding sites for brain uptake. Conversely, introducing HS binding sites to scFv8D3 decreased its renal excretion while removing them had the opposite effect.

Defining the versican interactome in lung health and disease

The extracellular matrix (ECM) imparts critical mechanical and biochemical information to cells in the lungs. Proteoglycans are essential constituents of the ECM and play a crucial role in controlling numerous biological processes, including regulating cellular phenotype and function. Versican, a chondroitin sulfate proteoglycan required for embryonic development, is almost absent from mature, healthy lungs and is reexpressed and accumulates in acute and chronic lung disease. Studies using genetically engineered mice show that the versican-enriched matrix can be pro- or anti-inflammatory depending on the cellular source or disease process studied. The mechanisms whereby versican develops a contextual ECM remain largely unknown. The primary goal of this review is to provide an overview of the interaction of versican with its many binding partners, the "versican interactome," and how through these interactions, versican is an integrator of complex extracellular information. Hopefully, the information provided in this review will be used to develop future studies to determine how versican and its binding partners can develop contextual ECMs that control select biological processes. Although this review focuses on versican and the lungs, what is described can be extended to other proteoglycans, tissues, and organs.

Glycosylation in the Tumor Microenvironment: Implications for Tumor Angiogenesis and Metastasis

Just as oncogene activation and tumor suppressor loss are hallmarks of tumor development, emerging evidence indicates that tumor microenvironment-mediated changes in glycosylation play a crucial functional role in tumor progression and metastasis. Hypoxia and inflammatory events regulate protein glycosylation in tumor cells and associated stromal cells in the tumor microenvironment, which facilitates tumor progression and also modulates a patient's response to anti-cancer therapeutics. In this review, we highlight the impact of altered glycosylation on angiogenic signaling and endothelial cell adhesion, and the critical consequences of these changes in tumor behavior.

Proteoglycans as Immunomodulators of the Innate Immune Response to Lung Infection

Proteoglycans (PGs) are complex, multifaceted molecules that participate in diverse interactions vital for physiological and pathological processes. As structural components, they provide a scaffold for cells and structural organization that helps define tissue architecture. Through interactions with water, PGs enable molecular and cellular movement through tissues. Through selective ionic interactions with growth factors, chemokines, cytokines, and proteases, PGs facilitate the ability of these soluble ligands to regulate intracellular signaling events and to influence the inflammatory response. In addition, recent findings now demonstrate that PGs can activate danger-associated molecular patterns (DAMPs) and other signaling pathways to influence production of many of these soluble ligands, indicating a more direct role for PGs in influencing the immune response and tissue inflammation. This review will focus on PGs that are selectively expressed during lung inflammation and will examine the novel emerging concept of PGs as immunomodulatory regulators of the innate immune responses in lungs.

SULF1/SULF2 splice variants differentially regulate pancreatic tumour growth progression

This study highlights the highly dynamic nature of SULF1/SULF2 splice variants in different human pancreatic cancers that regulate the activities of multiple cell signalling pathways in development and disease. Most pancreatic tumours expressed variable levels of both SULF1 and SULF2 variants including some expression during inflammation and pancreatitis. Many ductal and centro-acinar cell-derived pancreatic tumours are known to evolve into lethal pancreatic ductal adenocarcinomas but the present study also detected different stages of such tumour progression in the same tissue biopsies of not only acinar cell origin but also islet cell-derived cancers. The examination of caerulein-induced pancreatic injury and tumorigenesis in a Kras-driven mouse model confirmed the activation and gradual increase of SULF1/SULF2 variants during pancreatitis and tumorigenesis but with reduced levels in Stat3 conditional knockout mice with reduced inflammation. The significance of differential spatial and temporal patterns of specific SULF1/SULF2 splice variant expression during cancer growth became further apparent from their differential stimulatory or inhibitory effects on growth factor activities, tumour growth and angiogenesis not only during in vitro but also in vivo growth thus providing possible novel therapeutic targets.

Growth factor-heparan sulfate "switches" regulating stages of branching morphogenesis

The development of branched epithelial organs, such as the kidney, mammary gland, lung, pancreas, and salivary gland, is dependent upon the involvement and interaction of multiple regulatory/modulatory molecules, including soluble growth factors, extracellular matrix components, and their receptors. How the function of these molecules is coordinated to bring about the morphogenetic events that regulate iterative tip-stalk generation (ITSG) during organ development remains to be fully elucidated. A common link to many growth factor-dependent morphogenetic pathways is the involvement of variably sulfated heparan sulfates (HS), the glycosaminoglycan backbone of heparan sulfate proteoglycans (HSPG) on extracellular surfaces. Genetic deletions of HS biosynthetic enzymes (e.g., C5-epimerase, Hs2st), as well as considerable in vitro data, indicate that variably sulfated HS are essential for kidney development, particularly in Wolffian duct budding and early ureteric bud (UB) branching. A role for selective HS modifications by enzymes (e.g., Ext, Ndst, Hs2st) in stages of branching morphogenesis is also strongly supported for mammary gland ductal branching, which is dependent upon a set of growth factors similar to those involved in UB branching. Taken together, these studies provide support for the notion that the specific spatio-temporal HS binding of growth factors during the development of branched epithelial organs (such as the kidney, mammary gland, lung and salivary gland) regulates these complex processes by potentially acting as "morphogenetic switches" during the various stages of budding, branching, and other developmental events central to epithelial organogenesis. It may be that two or more growth factor-selective HS interactions constitute a functionally equivalent morphogenetic switch; this may help to explain the paucity of severe branching phenotypes with individual growth factor knockouts.

Heparan sulfate: a key regulator of embryonic stem cell fate

Heparan sulfate (HS) belongs to a class of glycosaminoglycans and is a highly sulfated, linear polysaccharide. HS biosynthesis and modification involves numerous enzymes. HS exists as part of glycoproteins named HS proteoglycans, which are expressed abundantly on the cell surface and in the extracellular matrix. HS interacts with numerous proteins, including growth factors, morphogens, and adhesion molecules, and thereby regulates important developmental processes in invertebrates and vertebrates. Embryonic stem cells (ESCs) are distinguished by their characteristics of self-renewal and pluripotency. Self-renewal allows ESCs to proliferate indefinitely in their undifferentiated state, whereas pluripotency implies their capacity to differentiate into the three germ layers and ultimately all cell types of the adult body. Both traits are tightly regulated by numerous cell signaling pathways. Recent studies have highlighted the importance of HS in the modulation of ESC functions, specifically their lineage fate. Here, we review the current advances that have been made in understanding the structural changes of HS during ESC differentiation and in deciphering the molecular mechanisms by which HS modulates cell fate. Finally, we discuss the applications of heparinoids and chemical inhibitors of HS biosynthesis for the manipulation of ESC culture and directed differentiation.

Characterization of the interaction between Robo1 and heparin and other glycosaminoglycans

Roundabout 1 (Robo1) is the cognate receptor for secreted axon guidance molecule, Slits, which function to direct cellular migration during neuronal development and angiogenesis. The Slit2-Robo1 signaling is modulated by heparan sulfate, a sulfated linear polysaccharide that is abundantly expressed on the cell surface and in the extracellular matrix. Biochemical studies have further shown that heparan sulfate binds to both Slit2 and Robo1 facilitating the ligand-receptor interaction. The structural requirements for heparan sulfate interaction with Robo1 remain unknown. In this report, surface plasmon resonance (SPR) spectroscopy was used to examine the interaction between Robo1 and heparin and other GAGs and determined that heparin binds to Robo1 with an affinity of ~650 nM. SPR solution competition studies with chemically modified heparins further determined that although all sulfo groups on heparin are important for the Robo1-heparin interaction, the N-sulfo and 6-O-sulfo groups are essential for the Robo1-heparin binding. Examination of differently sized heparin oligosaccharides and different GAGs also demonstrated that Robo1 prefers to bind full-length heparin chains and that GAGs with higher sulfation levels show increased Robo1 binding affinities.

Proteoglycans: key regulators of pulmonary inflammation and the innate immune response to lung infection

Exposure to viruses and bacteria results in lung infections and places a significant burden on public health. The innate immune system is an early warning system that recognizes viruses and bacteria, which results in the rapid production of inflammatory mediators such as cytokines and chemokines and the pulmonary recruitment of leukocytes. When leukocytes emigrate from the systemic circulation through the extracellular matrix (ECM) in response to lung infection they encounter proteoglycans, which consist of a core protein and their associated glycosaminoglycans. In this review, we discuss how proteoglycans serve to modify the pulmonary inflammatory response and leukocyte migration through a number of different mechanisms including: (1) The ability of soluble proteoglycans or fragments of glycosaminoglycans to activate Toll-like receptor (TLRs) signaling pathways; (2) The binding and sequestration of cytokines, chemokines, and growth factors by proteoglycans; (3) the ability of proteoglycans and hyaluronan to facilitate leukocyte adhesion and sequestration; and (4) The interactions between proteoglycans and matrix metalloproteinases (MMP) that alter the function of these proteases. In conclusion, proteoglycans fine-tune tissue inflammation through a number of different mechanisms. Clarification of the mechanisms whereby proteoglycans modulate the pulmonary inflammatory response will most likely lead to new therapeutic approaches to inflammatory lung disease and lung infection.

Endothelial heparan sulfate in angiogenesis

Heparan sulfate (HS) is a linear polysaccharide composed of 50-200 glucosamine and uronic acid (glucuronic acid or iduronic acid) disaccharide repeats with epimerization and various sulfation modifications. HS is covalently attached to core proteins to form HS-proteoglycans. Most of the functions of HS-proteoglycans are mediated by their HS moieties. The biosynthesis of HS is initiated by chain polymerization and is followed by stepwise modification reactions, including sulfation and epimerization. These modifications generate ligand-binding sites that modulate cell functions and activities of proteinases and/or proteinase inhibitors. HS is abundantly expressed in developing and mature vasculature, and understanding its roles in vascular biology and related human diseases is an area of intense investigation. In this chapter, we summarize the significant recent advances in our understanding of the roles of HS in developmental and pathological angiogenesis with a major focus on studies using transgenic as well as gene knockout/knockdown models in mice and zebrafish. These studies have revealed that HS critically regulates angiogenesis by playing a proangiogenic role, and this regulatory function critically depends on HS fine structure. The latter is responsible for facilitating cell-surface binding of various proangiogenic growth factors that in turn mediate endothelial growth signaling. In cancer, mouse studies have revealed important roles for endothelial cell-surface HS as well as matrix-associated HS, wherein signaling by multiple growth factors as well as matrix storage of growth factors may be regulated by HS. We also discuss important mediators that may fine-tune such regulation, such as heparanase and sulfatases; and models wherein targeting HS (or core protein) biosynthesis may affect tumor growth and vascularization. Finally, the importance of targeting HS in other human diseases wherein angiogenesis may play pathophysiologic (or even therapeutic) roles is considered.

Transforming growth factor-beta1 induces heparan sulfate 6-O-endosulfatase 1 expression in vitro and in vivo

Transforming growth factor (TGF)-beta1 plays an important role in the development of pulmonary fibrosis. In this study we examined the relationship between TGF-beta1 stimulation and the expression of heparan sulfate (HS) 6-O-endosulfatase 1 (Sulf1) in cultured normal human lung fibroblasts (NHLFs) and in murine lungs in vivo. By removing 6-O-sulfates from specific HS intrachain sites on the cell surface, Sulf1 has been shown to modulate the activities of many HS binding growth factors and morphogens including fibroblast growth factor (FGF)-2. Real time reverse transcription-PCR analysis revealed that TGF-beta1 increased Sulf1 expression in NHLFs in a dose- and time-dependent manner which was accompanied by a decrease in 6-O-sulfated disaccharides as revealed by high performance liquid chromatography analysis. Decreased ERK activation after FGF-2 stimulation was observed in TGF-beta1-treated NHLFs compared with control cells without changes in HS-dependent FGF-2 binding or FGF-2.FR1c complex formation. To study the function of Sulf1, negative control or Sulf1-specific small interference RNA (siRNA)-transfected NHLFs were stimulated with TGF-beta1. Enhanced Smad2/3 phosphorylation and elevated total Smad2 protein level were observed in Sulf1 siRNA-transfected cells and were accompanied by enhanced expression of alpha-smooth muscle actin and fibronectin. In addition, Sulf1 siRNA transfection enhanced the anti-proliferative effect of TGF-beta1. Finally Sulf1 expression was up-regulated in the lungs of mice treated with adenovirus encoding active TGF-beta1. Taken together, our data indicate that Sulf1 is a TGF-beta1-responsive gene both in vitro and in vivo and may function as a negative regulator of TGF-beta1-induced fibrogenesis.

Heparan sulphate proteoglycans in glia and in the normal and injured CNS: expression of sulphotransferases and changes in sulphation

Heparan sulphate proteoglycans (HSPGs) have multiple functions relevant to the control of the CNS injury response, particularly in modulating the effects of growth factors and localizing molecules that affect axon growth. We examined the pattern of expression and glycanation of HSPGs in the normal and damaged CNS, and in astrocytes and oligodendrocyte precursors because of their participation in the injury reaction. The composition of HS glycosaminoglycan (GAG) chains was analysed by biochemical analysis and by the binding of antibodies that recognize sulphated epitopes. We also measured levels of HS sulphotransferases and syndecans. Compared with oligodendrocytes, oligodendrocyte precursors have more 2-O-sulphation in their HS GAG. This is accompanied by higher expression of the enzyme responsible for 2-O-sulphation, HS 2-O-sulphotransferase (HS2ST) and a fall in syndecan-1. Astrocytes treated with tumour growth factor (TGF)alpha or TGFbeta to mimic the injury response showed upregulation of syndecan-1 and HS2ST correlating with an increase in 2-O-sulphate residues in their HS GAGs. This also correlated with increased staining with AO4B08 anti-GAG antibody that recognizes high sulphation, and reduced staining with RB4EA12 recognizing low sulphation. After injury to the adult rat brain there was an overall increase in the quantity of HSPG around the injury site, mRNA for HS2ST was increased, and the changes in staining with sulphation-specific antibodies were consistent with an increase in 2-O-sulphated HS. Syndecan-1 was upregulated in astrocytes. The major injury-related change, seen in injured brain and cultured glia, was an increase in 2-O-sulphated HS and increased syndecan-1, suggesting novel approaches to modulating scar formation.

Malignant mesothelioma--a connective tissue tumor with proteoglycan-dependent differentiation

Malignant mesothelioma (MM) is a connective tissue tumor with partial epithelioid differentiation. The pattern of proteoglycan (PG) expression by epithelioid and fibroblast-like (sarcomatoid) MM cells differ; cell surface PGs being more abundant in the former phenotype and matrix PGs in the latter. The differentiation as well as much of the malignant nature of these tumors is dependent on the expression of surface PGs. The syndecans, however, also translocate to the nucleus for an as yet unknown function.

Novel extracellular matrix structures in the neural stem cell niche capture the neurogenic factor fibroblast growth factor 2 from the extracellular milieu

The novel extracellular matrix structures called fractones are found in the lateral ventricle walls, the principal adult brain stem cell niche. By electron microscopy, fractones were shown to contact neural stem and progenitor cells (NSPC), suggesting a role in neurogenesis. Here, we investigated spatial relationships between proliferating NSPC and fractones and identified basic components and the first function of fractones. Using bromodeoxyuridine (BrdU) for birth-dating cells in the adult mouse lateral ventricle wall, we found most mitotic cells next to fractones, although some cells emerged next to capillaries. Like capillary basement membranes, fractones were immunoreactive for laminin beta1 and gamma1, collagen IV, nidogen, and perlecan, but not laminin-alpha1, in the adult rat, mouse, and human. Intriguingly, N-sulfate heparan sulfate proteoglycan (HSPG) immunoreactivity was restricted to fractone subpopulations and infrequent subependymal capillaries. Double immunolabel for BrdU and N-sulfate HSPG revealed preferential mitosis next to N-sulfate HSPG immunoreactive fractones. To determine whether N sulfate HSPG immunoreactivity within fractones reflects a potential for binding neurogenic growth factors, we identified biotinylated fibroblast growth factor 2 (FGF-2) binding sites in situ on frozen sections, and in vivo after intracerebroventricular injection of biotinylated FGF-2 in the adult rat or mouse. Both binding assays revealed biotinylated FGF-2 on fractone subpopulations and on infrequent subependymal capillaries. The binding of biotinylated FGF-2 was specific and dependent upon HSPG, as demonstrated in vitro and in vivo by inhibition with heparatinase and by the concomitant disappearance of N-sulfate HSPG immunoreactivity. These results strongly suggest that fractones promote growth factor activity in the neural stem cell niche.

FGF2 binding, signaling, and angiogenesis are modulated by heparanase in metastatic melanoma cells

Heparanase (HPSE) and fibroblast growth factor-2 (FGF2) are critical regulators of melanoma angiogenesis and metastasis. Elevated HPSE expression contributes to melanoma progression; however, further augmentation of HPSE presence can inhibit tumorigenicity. HPSE enzymatically cleaves heparan sulfate glycosaminoglycan chains (HS) from proteoglycans. HS act as both low-affinity FGF2 receptors and coreceptors in the formation of high-affinity FGF2 receptors. We have investigated HPSE's ability to modulate FGF2 activity through HS remodeling. Extensive HPSE degradation of human metastatic melanoma cells (70W) inhibited FGF2 binding. Unexpectedly, treatment of 70W cells with low HPSE concentrations enhanced FGF2 binding. In addition, HPSE-unexposed cells did not phosphorylate extracellular signal-related kinase (ERK) or focal adhesion kinase (FAK) in response to FGF2. Conversely, in cells treated with HPSE, FGF2 stimulated ERK and FAK phosphorylation. Secondly, the presence of soluble HPSE-degraded HS enhanced FGF2 binding and ERK phosphorylation at low HS concentrations. Higher concentrations of soluble HS inhibited FGF2 binding, but FGF2 signaling through ERK remained enhanced. Soluble HS were unable to support FGF2-stimulated FAK phosphorylation irrespective of HPSE treatment. Finally, cell exposure to HPSE or to HPSE-degraded HS modulated FGF2-induced angiogenesis in melanoma. In conclusion, these effects suggest relevant mechanisms for the HPSE modulation of melanoma growth factor responsiveness and tumorigenicity.

Substrate specificity and domain functions of extracellular heparan sulfate 6-O-endosulfatases, QSulf1 and QSulf2

The extracellular sulfatases (Sulfs) are an evolutionally conserved family of heparan sulfate (HS)-specific 6-O-endosulfatases. These enzymes remodel the 6-O-sulfation of cell surface HS chains to promote Wnt signaling and inhibit growth factor signaling for embryonic tissue patterning and control of tumor growth. In this study we demonstrate that the avian HS endosulfatases, QSulf1 and QSulf2, exhibit the same substrate specificity toward a subset of trisulfated disaccharides internal to HS chains. Further, we show that both QSulfs associate exclusively with cell membrane and are enzymatically active on the cell surface to desulfate both cell surface and cell matrix HS. Mutagenesis studies reveal that conserved amino acid regions in the hydrophilic domains of QSulf1 and QSulf2 have multiple functions, to anchor Sulf to the cell surface, bind to HS substrates, and to mediate HS 6-O-endosulfatase enzymatic activity. Results of our current studies establish the hydrophilic domain (HD) of Sulf enzymes as an essential multifunctional domain for their unique endosulfatase activities and also demonstrate the extracellular activity of Sulfs for desulfation of cell surface and cell matrix HS in the control of extracellular signaling for embryonic development and tumor progression.

A synthetic glycosaminoglycan mimetic (RGTA) modifies natural glycosaminoglycan species during myogenesis

Crucial events in myogenesis rely on the highly regulated spatiotemporal distribution of cell surface heparan sulfate proteoglycans to which are associated growth factors, thus creating a specific microenvironment around muscle cells. Most growth factors involved in control of myoblast growth and differentiation are stored in the extracellular matrix through interaction with specific sequences of glycosaminoglycan oligosaccharides, mainly heparan sulfate (HS). Different HS subspecies revealed by specific antibodies, have been shown to provide spatiotemporal regulation during muscle development. We have previously shown that glycosaminoglycan (GAG) mimetics called RGTA (ReGeneraTing Agent), stimulate muscle precursor cell growth and differentiation. These data suggest an important role of GAGs during myogenesis; however, little is yet known about the different species of GAGs synthesized during myogenesis and their metabolic regulation. We therefore quantified GAGs during myogenesis of C2.7 cells and show that the composition of GAG species was modified during myogenic differentiation. In particular, HS levels were increased during this process. In addition, the GAG mimetic RGTA, which stimulated both growth and differentiation of C2.7 cells, increased the total amount of GAG produced by these cells without significantly altering their rate of sulfation. RGTA treatment further enhanced HS levels and changed its sub-species composition. Although mRNA levels of the enzymes involved in HS biosynthesis were almost unchanged during myogenic differentiation, heparanase mRNA levels decreased. RGTA did not markedly alter these levels. Here we show that the effects of RGTA on myoblast growth and differentiation are in part mediated through an alteration of GAG species and provide an important insight into the role of these molecules in normal or pathologic myogenic processes.

Fgf9induces proliferation and nuclear localization of FGFR2 in Sertoli precursors during male sex determination

Recently, we demonstrated that loss of Fgf9 results in a block of testis development and a male to female sex-reversed phenotype; however, the function of Fgf9 in sex determination was unknown. We now show that Fgf9 is necessary for two steps of testis development just downstream of the male sex-determining gene, Sry: (1) for the proliferation of a population of cells that give rise to Sertoli progenitors; and (2) for the nuclear localization of an FGF receptor (FGFR2) in Sertoli cell precursors. The nuclear localization of FGFR2 coincides with the initiation of Sry expression and the nuclear localization of SOX9 during the early differentiation of Sertoli cells and the determination of male fate.

A subtractive approach to characterize genes with regionalized expression in the gliogenic ventral neuroepithelium: identification of chick Sulfatase 1 as a new oligodendrocyte lineage gene

To address the question of the origin of glial cells and the mechanisms leading to their specification, we have sought to identify novel genes expressed in glial progenitors. We adopted suppression subtractive hybridization (SSH) to establish a chick cDNA library enriched for genes specifically expressed at 6 days of incubation (E6) in the ventral neuroepithelium, a tissue previously shown to contain glial progenitors. Screens were then undertaken to select differentially expressed cDNAs, and out of 82 unique SSH clones, 21 were confirmed to display a regionalized expression along the dorsoventral axis of the E6 ventral neuroepithelium. Among these, we identified a transcript coding for the chick orthologue of Sulf1, a recently identified cell surface sulfatase, as a new, early marker of oligodendrocyte (OL) precursors in the chick embryonic spinal cord. This study provides groundwork for the further identification of genes involved in glial specification.

QSulf1, a heparan sulfate 6-O-endosulfatase, inhibits fibroblast growth factor signaling in mesoderm induction and angiogenesis

The signaling activities of multiple developmental ligands require sulfated heparan sulfate (HS) proteoglycans as coreceptors. QSulf1 and its mammalian orthologs are cell surface HS 6-O-endosulfatases that are expressed in embryonic mesodermal and neural progenitors and promote Wnt signal transduction. In this study, we have investigated the function of QSulf1 in fibroblast growth factor (FGF) signaling, which requires 6-O-sulfated HS for FGF receptor (FGFR) dimerization and tyrosine kinase activation. Here, we report that QSulf1 inhibits FGF2- and FGF4-induced mesoderm formation in the Xenopus embryo and FGF-dependent angiogenesis in the chicken embryo through 6-O-desulfation of cell surface HS. QSulf1 regulates FGF signaling through inhibition of HS-mediated FGFR1 activation by interfering with FGF-HS-FGFR1 ternary complex formation. Furthermore, QSulf1 can produce enzymatically modified soluble heparin that acts as a potent inhibitor of FGF2-induced angiogenesis in the chicken embryo. QSulf1, therefore, has dual regulatory functions as a negative regulator of FGF signaling and a positive regulator of Wnt signaling. Therefore, QSulf1 provides another reagent to produce enzymatically modified heparin compounds, in vivo and in vitro, to modulate cellular signaling in stem cell-based therapies to promote tissue regeneration and in cancer therapies to control cell growth and block angiogenesis.

Spatial and temporal expression of heparan sulfate in mouse development regulates FGF and FGF receptor assembly

Heparan sulfate (HS) interacts with diverse growth factors, including Wnt, Hh, BMP, VEGF, EGF, and FGF family members, and is a necessary component for their signaling. These proteins regulate multiple cellular processes that are critical during development. However, a major question is whether developmental changes occur in HS that regulate the activity of these factors. Using a ligand and carbohydrate engagement assay, and focusing on FGF1 and FGF8b interactions with FGF receptor (FR)2c and FR3c, this paper reveals global changes in HS expression in mouse embryos during development that regulate FGF and FR complex assembly. Furthermore, distinct HS requirements are identified for both complex formation and signaling for each FGF and FR pair. Overall, these results suggest that changes in HS act as critical temporal regulators of growth factor and morphogen signaling during embryogenesis.

The phosphatonin pathway: New insights in phosphate homeostasis

Serum phosphate concentrations are maintained within a defined range by processes that regulate the intestinal absorption and renal excretion of inorganic phosphate. The hormones currently believed to influence these processes are parathyroid hormone (PTH) and the active metabolite of vitamin D, 1alpha,25-dihydroxyvitamin D (1alpha,25(OH)2D). A new class of phosphate-regulating factors, collectively known as the phosphatonins, have been shown to be associated with the hypophosphatemic diseases, tumor-induced osteomalacia (TIO), X-linked hypophosphatemic rickets (XLH), and autosomal-dominant hypophosphatemic rickets (ADHR). These factors, which include fibroblast growth factor 23 (FGF23) and secreted frizzled-related protein 4 (FRP4), decrease extracellular fluid phosphate concentrations by directly reducing renal phosphate reabsorption and by suppressing 1alpha,25(OH)2D formation through the inhibition of 25-hydroxyvitamin D 1alpha-hydroxylase. The role of these substances under normal or pathologic conditions is not yet clear. For example, it is unknown whether any of the phosphatonins are directly responsible for the decreased concentrations of 1alpha,25(OH)2D observed in chronic and end-stage kidney disease or whether they are induced in an attempt to correct the hyperphosphatemia seen in late stages of chronic renal failure. Future experiments should clarify their physiologic and pathologic roles in phosphate metabolism.

QSulf1 remodels the 6-O sulfation states of cell surface heparan sulfate proteoglycans to promote Wnt signaling

The 6-O sulfation states of cell surface heparan sulfate proteoglycans (HSPGs) are dynamically regulated to control the growth and specification of embryonic progenitor lineages. However, mechanisms for regulation of HSPG sulfation have been unknown. Here, we report on the biochemical and Wnt signaling activities of QSulf1, a novel cell surface sulfatase. Biochemical studies establish that QSulf1 is a heparan sulfate (HS) 6-O endosulfatase with preference, in particular, toward trisulfated IdoA2S-GlcNS6S disaccharide units within HS chains. In cells, QSulf1 can function cell autonomously to remodel the sulfation of cell surface HS and promote Wnt signaling when localized either on the cell surface or in the Golgi apparatus. QSulf1 6-O desulfation reduces XWnt binding to heparin and HS chains of Glypican1, whereas heparin binds with high affinity to XWnt8 and inhibits Wnt signaling. CHO cells mutant for HS biosynthesis are defective in Wnt-dependent Frizzled receptor activation, establishing that HS is required for Frizzled receptor function. Together, these findings suggest a two-state "catch or present" model for QSulf1 regulation of Wnt signaling in which QSulf1 removes 6-O sulfates from HS chains to promote the formation of low affinity HS-Wnt complexes that can functionally interact with Frizzled receptors to initiate Wnt signal transduction.

Cell signaling events: a view from the matrix

Cellular activities are primarily initiated, modulated and sustained by multifunctional molecules (cytokines and growth factors) that are secreted into the extracellular space and that signal through membrane-bound, high-affinity receptors. In contrast to the fairly well understood mechanisms that mediate the specificity of signal transduction within the confined and compartmentalized environment of the cell, significantly less is known about the mechanisms that regulate the availability of signaling molecules in the extracellular milieu. Recent findings have implicated the participation of extracellular protein macroaggregates in signaling events controlling patterning and morphogenesis. The results suggest a functional coupling between the tissue-specific organization of collagenous and elastic macroaggregates and their ability to perform instructive as well as structural functions. These observations open the way to a novel understanding in these poorly understood and critically important areas of cell and developmental biology.

Unlocking the secrets of syndecans: transgenic organisms as a potential key

Heparan sulfate proteoglycans are known to modulate the activity of a large number of extracellular ligands thereby having the potential to regulate a great diversity of biological processes. The long-term studies in our laboratory have focused on the syndecans, one of the major cell surface heparan sulfate proteoglycan families. Most early work on syndecans involved biochemical studies that provided initial information on their structure and putative biological roles. In recent years, the development of transgenic organisms has allowed a more complete understanding of syndecan function. Studies with transgenic syndecan-1 and syndecan-3 mice have demonstrated an unforeseen role for syndecans in the regulation of feeding behavior. Syndecan-1 knockout mice display a reduced susceptibility to both Wnt-induced tumorigenesis and microbial pathogenesis. Experiments with Drosophila show that syndecan is first expressed upon cellularization in the early embryo, and may play a role in the early developmental stages of the fly. This review focuses on these diverse functions of the syndecans that have been elucidated by the use of transgenic mice and Drosophila as model systems.