Syndecan-4 proteoglycan cytoplasmic domain and phosphatidylinositol 4,5-bisphosphate coordinately regulate protein kinase C activity

Signaling networks regulating leukocyte podosome dynamics and function

Podosomes are ventral adhesion structures prominent in cells of the myeloid lineage. A common aspect of these cells is that they are highly motile and must to traverse multiple tissue barriers in order to perform their functions. Recently podosomes have gathered attention from researchers as important cellular structures that can influence cell adhesion, motility and matrix remodeling. Adhesive and soluble ligands act via transmembrane receptors and propagate signals to the leukocyte cytoskeleton via small G proteins of the Rho family, tyrosine kinases and scaffold proteins and are able to induce podosome formation and rearrangements. Manipulation of the signals that regulate podosome formation and dynamics can therefore be a strategy to interfere with leukocyte functions in a multitude of pathological settings, such as infections, atherosclerosis and arthritis. Here, we review the major signaling molecules that act in the formation and regulation of podosomes.

Lysophosphatidylcholine-mediated functional inactivation of syndecan-4 results in decreased adhesion and motility of dendritic cells

Following antigen contact, maturation and migration of DCs into lymphatic tissues are crucial to the developing immune response or maintenance of tolerance. Lysophosphatidylcholine (LysoPC) is generated during apoptosis of cells and acts as a "find-and-eat-me" signal thought to prevent autoimmunity. Moreover, LysoPC can activate PKCδ and initiates a signaling cascade that leads to phosphorylation and inactivation of syndecan-4 (SDC4), a heparansulfate proteoglycan integrin co-receptor. In human monocyte-derived DCs, we recently demonstrated that SDC4 is upregulated during maturation thereby stimulating DC motility. Here, we investigate the effects of LysoPC on DC motility as well as on the involvement of PKCδ phosphorylation-dependent regulation of DC motility by SDC4 and PKCα. Employing a static adhesion assay and videomicroscopy, we show that LysoPC inhibits adhesion of DCs to fibronectin and motility of DCs by decreasing podosome formation. Moreover, DC podosome formation and motility, which both are regulated by SDC4 and subject to control by PKCδ-dependent phosphorylation of SDC4, were inhibited in LysoPC-matured DCs. Thus, these DC are defective in adhesion and migration. Based on our results, we hypothesize that LysoPC released during apoptosis might delay DC migration to lymphoid organs and thus prevent autoimmunity.

Transmembrane signaling proteoglycans

Virtually all metazoan cells contain at least one and usually several types of transmembrane proteoglycans. These are varied in protein structure and type of polysaccharide, but the total number of vertebrate genes encoding transmembrane proteoglycan core proteins is less than 10. Some core proteins, including those of the syndecans, always possess covalently coupled glycosaminoglycans; others do not. Syndecan has a long evolutionary history, as it is present in invertebrates, but many other transmembrane proteoglycans are vertebrate inventions. The variety of proteins and their glycosaminoglycan chains is matched by diverse functions. However, all assume roles as coreceptors, often working alongside high-affinity growth factor receptors or adhesion receptors such as integrins. Other common themes are an ability to signal through their cytoplasmic domains, often to the actin cytoskeleton, and linkage to PDZ protein networks. Many transmembrane proteoglycans associate on the cell surface with metzincin proteases and can be shed by them. Work with model systems in vivo and in vitro reveals roles in growth, adhesion, migration, and metabolism. Furthermore, a wide range of phenotypes for the core proteins has been obtained in mouse knockout experiments. Here some of the latest developments in the field are examined in hopes of stimulating further interest in this fascinating group of molecules.

Serine 34 phosphorylation of rho guanine dissociation inhibitor (RhoGDIalpha) links signaling from conventional protein kinase C to RhoGTPase in cell adhesion

Conventional protein kinase C (PKC) isoforms are essential serine/threonine kinases regulating many signaling networks. At cell adhesion sites, PKCα can impact the actin cytoskeleton through its influence on RhoGTPases, but the intermediate steps are not well known. One important regulator of RhoGTPase function is the multifunctional guanine nucleotide dissociation inhibitor RhoGDIα that sequesters several related RhoGTPases in an inactive form, but it may also target them through interactions with actin-associated proteins. Here, it is demonstrated that conventional PKC phosphorylates RhoGDIα on serine 34, resulting in a specific decrease in affinity for RhoA but not Rac1 or Cdc42. The mechanism of RhoGDIα phosphorylation is distinct, requiring the kinase and phosphatidylinositol 4,5-bisphosphate, consistent with recent evidence that the inositide can activate, localize, and orient PKCα in membranes. Phosphospecific antibodies reveal endogenous phosphorylation in several cell types that is sensitive to adhesion events triggered, for example, by hepatocyte growth factor. Phosphorylation is also sensitive to PKC inhibition. Together with fluorescence resonance energy transfer microscopy sensing GTP-RhoA levels, the data reveal a common pathway in cell adhesion linking two essential mediators, conventional PKC and RhoA.

Podocytes require the engagement of cell surface heparan sulfate proteoglycans for adhesion to extracellular matrices

Podocytes adhere to the glomerular basement membrane by cell surface receptors. Since in other cells these adhesions are enhanced by cell surface proteoglycans, we examined the contribution of these molecules and their glycosaminoglycan side chains to podocyte adhesion by developing immortalized podocyte cell lines with (control) or without (mutant) heparan sulfate glycosaminoglycan chains. In adhesion assays control podocytes attached, spread, and migrated more efficiently compared with mutants, indicating a requirement for heparan sulfate chains in these processes. The proteoglycan syndecan-4 is known to have direct effects on cell attachment, spreading, and cytoskeletal organization. We found it localized to focal adhesions in control podocytes coincident with stress fiber formation. In mutant cells, syndecan-4 was associated with smaller focal contacts and cortical actin organization. Analysis by flow cytometry showed that mutant cells had twice the amount of surface syndecan-4 of control cells. Protein kinase Cα, a signaling molecule bound to and activated by syndecan-4, showed a fourfold increase in membrane localization-activation than that seen in control cells. In vivo, the loss of heparan sulfate glycosaminoglycans in PEXTKO mice led to a loss of glomerular syndecan-4. Overall, our study provides further evidence for a dynamic role of cell surface heparan sulfate glycosaminoglycans in podocyte activity.

Activation of the integrin effector kinase focal adhesion kinase in cancer cells is regulated by crosstalk between protein kinase Calpha and the PDZ adapter protein mda-9/Syntenin

Aberrant adhesion signaling pathways in cancer cells underlie their deadly invasive capabilities. The adhesion-related PDZ adapter protein mda-9/syntenin is a positive regulator of cancer cell progression in breast cancer, melanoma, and other human cancers. In this study, we report that mda-9/syntenin mediates adhesion-mediated activation of protein kinase Calpha (PKCalpha) and focal adhesion kinase (FAK) by fibronectin (FN) in human breast cancer and melanoma cells. FN rapidly stimulated the expression of mda-9/syntenin and the activation of PKCalpha prior to activation of FAK. Inhibiting PKCalpha suppressed basal or FN-induced expression of mda-9/syntenin, as well as cell migration and invasion toward FN stimulated by mda-9/syntenin. Several lines of evidence suggested that activation of PKCalpha and expression of mda-9/syntenin were interdependent. First, mda-9/syntenin inhibition suppressed basal or FN-induced phosphorylation of PKCalpha at Thr(638/641), whereas PKCalpha inhibition suppressed basal or FN-induced expression of mda-9/syntenin. Second, inhibiting either mda-9/syntenin or PKCalpha suppressed FN-induced formation of integrin-beta(1)/FAK/c-Src signaling complexes. Third, inhibiting either mda-9/syntenin or PKCalpha suppressed FN-induced phosphorylation of FAK Tyr(397) and c-Src Tyr(416) and the induction of downstream effector signals to p38 and mitogen-activated protein kinase, Cdc42, and NF-kappaB. In summary, our findings offer evidence that mda-9/syntenin acts as a molecular adaptor linking PKCalpha and FAK activation in a pathway of FN adhesion by human breast cancer and melanoma cells.

Reception of Slit requires only the chondroitin-sulphate-modified extracellular domain of Syndecan at the target cell surface

Syndecan (Sdc) is a conserved transmembrane heparan sulfate proteoglycan (HSPG) bearing additional chondroitin sulfate (CS) modifications on its extracellular domain. In vertebrates, this extracellular domain of Sdc is shed and acts as a soluble effector of cellular communication events, and its cytoplasmic domain participates in intracellular signaling needed to maintain epithelial integrity. In Drosophila, Sdc has been shown to be necessary for Slit signaling-dependent axon and myotube guidance during CNS development and muscle pattern formation. We report that Sdc acts in a cell-autonomous manner in Slit-receiving cells and that its membrane-anchored extracellular domain is sufficient to mediate Slit signaling. Sdc activity can be replaced by the human homolog hsdc2. However, the HSPG Dally-like protein (Dlp), which lacks CS modifications at its extracellular domain, can only partially substitute for Sdc function, and its activity is not restricted to the Slit target cells. Our results suggest that Sdc and Dlp act in a cooperative but nonredundant fashion in axon and myotube guidance. We propose that Dlp, which lacks CS modifications, participates in the transfer of Slit from its site of expression to the target cells, where CS-modified Sdc concentrates and presents the ligand.

Syndecan proteoglycan contributions to cytoskeletal organization and contractility

Cells exert tension on the extracellular matrix through specific receptors that link to the actin cytoskeleton. The best characterized are the integrins, which, when activated and clustered, can link to the extracellular matrix at specialized adhesion zones, known as focal contacts or focal adhesions. However, other transmembrane receptors can also localize there, including one transmembrane proteoglycan, syndecan-4. This heparan sulfate proteoglycan can also link directly to the cytoskeleton through alpha-actinin, and can signal through protein kinase C. In turn, the pathway leads to RhoA and Rho kinases that control actomyosin contractility. Syndecan-4 may, therefore, be a sensor of tension exerted on the matrix. These processes are described here, their significance being potential roles in wound contraction, tumor-stroma interactions, fibrosis and the regulation of motility.

De-sulfation of MG-63 cell glycosaminoglycans delays in vitro osteogenesis, up-regulates cholesterol synthesis and disrupts cell cycle and the actin cytoskeleton

Glycosaminoglycan (GAG) sugars are largely responsible for the bioactivity of the proteoglycan proteins they decorate, and are particularly important for mediating the processes of cell attachment and growth factor signaling. Here, we show that chlorate-induced de-sulfation of GAGs expressed by MG-63 osteosarcoma cells results in delayed cell proliferation when the cells are exposed to chlorate for short or medium periods, but a disrupted mineralization without altered cell proliferation in response to long-term chlorate exposure. Analysis of GAG-binding growth factor activity indicated that chlorate disrupted BMP2/noggin signaling, but not FGF2 activity. Microarray analyses, which were confirmed by subsequent cell-based assays, indicated that chlorate predominantly disrupted the cell cycle and actin cytoskeleton and upregulated cholesterol synthesis, without affecting cell migration or attachment. Furthermore, we observed that disruption of the functions of the proteoglycan syndecan-4 replicated phenotypes induced by chlorate, implicating a primary role for this proteoglycan in providing bioactivity for these cells. J. Cell. Physiol. 219: 572-583, 2009. (c) 2009 Wiley-Liss, Inc.

Structural and cell adhesion properties of zebrafish syndecan-4 are shared with higher vertebrates

The syndecan proteoglycans are an ancient class of receptor, bearing heparan sulfate chains that interact with numerous potential ligands including growth factors, morphogens, and extracellular matrix molecules. The single syndecan of invertebrates appears not to have cell adhesion roles, but these have been described for mammalian paralogues, especially syndecan-4. This member is best understood in terms of interactions, signaling, and structure of its cytoplasmic domain. The zebrafish homologue of syndecan-4 has been genetically linked to cell adhesion and migration in zebrafish embryos, but no molecular and cellular studies have been reported. Here it is demonstrated that key functional attributes of syndecan-4 are common to both zebrafish and mammalian homologues. These include glycosaminoglycan substitution, a NXIP motif in the extracellular domain that promotes integrin-mediated cell adhesion, and a transmembrane GXXXG motif that promotes dimer formation. In addition, despite some amino acid substitutions in the cytoplasmic domain, its ability to form twisted clamp dimers is preserved, as revealed by nuclear magnetic resonance spectroscopy. This technique also showed that phosphatidylinositol 4,5-bisphosphate can interact with the zebrafish syndecan-4 cytoplasmic domain, and that the molecule in its entirety supports focal adhesion formation, and complements the murine null cells to restore a normal actin cytoskeleton identically to the rat homologue. Therefore, the cell adhesion properties of syndecan-4 are consistent across the vertebrate spectrum and reflect an early acquisition of specialization after syndecan gene duplication events at the invertebrate/early chordate boundary.

The oligomeric status of syndecan-4 regulates syndecan-4 interaction with alpha-actinin

Syndecan-4, a cell surface heparan sulfate proteoglycan, is known to regulate the organization of the cytoskeleton, and oligomerization is crucial for syndecan-4 function. We therefore explored a possible regulatory effect of syndecan-4 oligomerization on the cytoskeleton. Glutathione-S-transferase-syndecan-4 proteins were used to show that syndecan-4 interacted specifically with alpha-actinin, but not paxillin, talin, and vinculin. Interestingly, only dimeric, and not monomeric, recombinant syndecan-4 interacted with alpha-actinin in the presence of phosphatidylinositol 4,5-bisphosphate (PIP2), and PIP2 potentiated the interaction of both the cytoplasmic domain syndecan-4 peptide and recombinant syndecan-4 proteins with alpha-actinin, implying that oligomerization of syndecan-4 was important for this interaction. Consistent with this notion, alpha-actinin interaction was largely absent in syndecan-4 mutants defective in transmembrane domain-induced oligomerization, and alpha-actinin-associated focal adhesions were decreased in rat embryo fibroblasts expressing mutant syndecan-4. Besides, this interaction was consistently lower with the phosphorylation-mimicking syndecan-4 mutant S183E which is known to destabilize the oligomerization of the syndecan-4 cytoplasmic domain. Taken together, the data suggest that the oligomeric status of syndecan-4 plays a crucial role in regulating the interaction of syndecan-4 with alpha-actinin.

The postsynaptic density 95/disc-large/zona occludens protein syntenin directly interacts with frizzled 7 and supports noncanonical Wnt signaling

Wnt signaling pathways are essential for embryonic patterning, and they are disturbed in a wide spectrum of diseases, including cancer. An unresolved question is how the different Wnt pathways are supported and regulated. We previously established that the postsynaptic density 95/disc-large/zona occludens (PDZ) protein syntenin binds to syndecans, Wnt coreceptors, and known stimulators of protein kinase C (PKC)alpha and CDC42 activity. Here, we show that syntenin also interacts with the C-terminal PDZ binding motif of several Frizzled Wnt receptors, without compromising the recruitment of Dishevelled, a key downstream Wnt-signaling component. Syntenin is coexpressed with cognate Frizzled during early development in Xenopus. Overexpression and down-regulation of syntenin disrupt convergent extension movements, supporting a role for syntenin in noncanonical Wnt signaling. Syntenin stimulates c-jun phosphorylation and modulates Frizzled 7 signaling, in particular the PKCalpha/CDC42 noncanonical Wnt signaling cascade. The syntenin-Frizzled 7 binding mode indicates syntenin can accommodate Frizzled 7-syndecan complexes. We propose that syntenin is a novel component of the Wnt signal transduction cascade and that it might function as a direct intracellular link between Frizzled and syndecans.

Switch in syndecan-1 and syndecan-4 expression controls maturation associated dendritic cell motility

Dendritic cells (DCs) need to mobilize within the extracellular matrix (ECM) during their maturation and concomitant migration from peripheral sites to lymphoid organs. Syndecans are cell surface proteoglycans that mediate the interaction of DCs with the ECM. Here we investigated the influence of syndecans on dendritic cell motility and morphology. Langerhans cells of the epidermis and monocyte-derived DCs were found to undergo a switch in syndecan expression during maturation. Syndecan-1 was downregulated and syndecan-4 was strongly upregulated within the first hours of lipopolysaccharide-induced dendritic cell maturation and during Langerhans cell emigration from human skin, as shown by flow cytometry and qRT-PCR. Syndecan-1 downregulation was inhibited by syndecan-4 siRNA knock-down, indicating a functional interconnection between enhanced syndecan-4 expression and syndecan-1 downregulation. Syndecan-4 upregulation is functionally involved in dendritic cell motility, as inhibition of syndecan-4 function by means of blocking antibodies or through siRNA knock-down decreased dendritic cell motility. In other experiments, the cytoskeletal component a-actinin was observed to be upregulated in DCs as a consequence of the induction of maturation, and was found to colocalize with syndecan-4. Furthermore, lammellopodial spreading by DCs on fibronectin (FN)-coated surfaces was dependent on syndecan-4. This binding of syndecan-4 to FN and its association with the cytoskeleton may be relevant for syndecan-4-dependent dendritic cell motility. We conclude that the switch in syndecan expression during dendritic cell maturation controls the motility of DCs in a way that appears to be crucial for their mobilization from peripheral sites and subsequent migration to lymphoid tissues.

Syndecan-2 induces filopodia and dendritic spine formation via the neurofibromin-PKA-Ena/VASP pathway

Syndecan-2 induced filopodia before spinogenesis; therefore, filopodia formation was used here as a model to study the early downstream signaling of syndecan-2 that leads to spinogenesis. Screening using kinase inhibitors indicated that protein kinase A (PKA) is required for syndecan-2–induced filopodia formation in both human embryonic kidney cells and hippocampal neurons. Because neurofibromin, a syndecan-2–binding partner, activates the cyclic adenosine monophosphate pathway, the role of neurofibromin in syndecan-2–induced filopodia formation was investigated by deletion mutant analysis, RNA interference, and dominant-negative mutant. The results showed that neurofibromin mediates the syndecan-2 signal to PKA. Among actin-associated proteins, Enabled (Ena)/vasodilator-stimulated phosphoprotein (VASP) were predicted as PKA effectors downstream of syndecan-2, as Ena/VASP, which is activated by PKA, induces actin polymerization. Indeed, when the activities of Ena/VASP were blocked, syndecan-2 no longer induced filopodia formation. Finally, in addition to filopodia formation, neurofibromin and Ena/VASP contributed to spinogenesis. This study reveals a novel signaling pathway in which syndecan-2 activates PKA via neurofibromin and PKA consequently phosphorylates Ena/VASP, promoting filopodia and spine formation.

Identification of acidic amino acid residues in the protein kinase C alpha V5 domain that contribute to its insensitivity to diacylglycerol

The protein kinase C (PKC) isoforms are maintained in an inactive and closed conformation by intramolecular interactions. Upon activation these are disrupted by activators, binding proteins and cellular membrane. We have seen that autophosphorylation of two sites in the C-terminal V5 domain is crucial to keep PKC alpha insensitive to the activator diacylglycerol, which presumably is caused by a masking of the diacylglycerol-binding C1a domain. Here we demonstrate that the diacylglycerol sensitivity of the PKC beta isoforms also is suppressed by autophosphorylation of the V5 sites. To analyze conformational differences, a fusion protein ECFP-PKC alpha-EYFP was expressed in cells and the FRET signal was analyzed. The analogous mutant with autophosphorylation sites exchanged for alanine gave rise to a substantially lower FRET signal than wild-type PKC alpha indicating a conformational difference elicited by the mutations. Expression of the isolated PKC alpha V5 domain led to increased diacylglycerol sensitivity of PKC alpha. We identified acidic residues in the V5 domain that, when mutated to alanines or lysines, rendered PKC alpha sensitive to diacylglycerol. Furthermore, mutation to glutamate of four lysines in a lysine-rich cluster in the C2 domain gave a similar effect. Simultaneous reversal of the charges of the acidic residues in the V5 and the lysines in the C2 domain gave rise to a PKC alpha that was insensitive to diacylglycerol. We propose that these structures participate in an intramolecular interaction that maintains PKC alpha in a closed conformation. The disruption of this interaction leads to an unmasking of the C1a domain and thereby increased diacylglycerol sensitivity of PKC alpha.

Integrins and syndecan-4 make distinct, but critical, contributions to adhesion contact formation

During cell adhesion to fibronectin there is a major reorganisation of the actin cytoskeleton and concomitant formation of adhesion complexes. Conflicting studies of adhesion receptors report that either integrin alone, or both integrin and syndecan-4 mediate the formation of vinculin-containing adhesions, and differences in these studies have been attributed to the density and conformational integrity of ligands used. We have endeavoured to resolve these issues by ELISA analysis of immobilised polypeptides, and found that ligands of both integrin alpha(5)beta(1) and syndecan-4 are necessary for focal adhesion formation under conditions of equivalent density of folded ligand. We also demonstrate that integrin and syndecan-4 play quite distinct roles in adhesion contact maturation and are not interchangeable. These results help us to understand how cells respond efficiently to changes in matrix environment, which should prove useful for developing approaches to aid wound healing.

Syndecans in wound healing, inflammation and vascular biology

Syndecans are heparan sulphate proteoglycans consisting of a type I transmembrane core protein modified by heparan sulphate and sometimes chondroitin sulphate chains. They are major proteoglycans of many organs including the vasculature, along with glypicans and matrix proteoglycans. Heparan sulphate chains have potential to interact with a wide array of ligands, including many growth factors, cytokines, chemokines and extracellular matrix molecules relevant to growth regulation in vascular repair, hypoxia, angiogenesis and immune cell function. This is consistent with the phenotypes of syndecan knock-out mice, which while viable and fertile, show deficits in tissue repair. Furthermore, there are potentially important changes in syndecan distribution and function described in a variety of human vascular diseases. The purpose of this review is to describe syndecan structure and function, consider the role of syndecan core proteins in transmembrane signalling and also their roles as co-receptors with other major classes of cell surface molecules. Current debates include potential redundancy between syndecan family members, the significance of multiple heparan sulphate interactions, regulation of the cytoskeleton and cell behaviour and the switch between promoter and inhibitor of important cell functions, resulting from protease-mediated shedding of syndecan ectodomains.

EJ-ras oncogene transfection of endothelial cells upregulates the expression of syndecan-4 and downregulates heparan sulfate sulfotransferases and epimerase

The EC rabbit endothelial cell line was transfected with the EJ-ras oncogene (EJ-ras EC). EJ-ras EC cells display over expression of the Ras oncogene, morphological changes and deregulation of the cell cycle, becoming more densely populated and serum-independent. In addition, EJ-ras-transfectant cells show higher levels of the syndecan-4 mRNA. In addition to the increase in the core protein, a parallel increase in the glycosylation of the syndecan-4 protein, a proteoglycan that bears heparan sulfate chains, also occurs. This increase is observed both for the heparan sulfate proteoglycan synthesized by the cells and for that secreted to the culture medium. This enhancement in heparan sulfate synthesis was observed through metabolic labeling of the cells, immunoprecipitation of syndecan-4 and heparitinases treatment. Furthermore, the EJ-ras-transfectant cells do not exhibit decreased synthesis of heparan sulfate during the G(1)-S phase transition, as observed for the parental cell line. Also, heparan sulfate synthesis is not stimulated by PMA as displayed by parental endothelial cells. Significant structural changes of heparan sulfate, such as decreased O-sulfation, were observed in the EJ-ras-transfected cells. Decreases in the mRNA levels of some enzymes (glucuronosyl C-5 epimerase, iduronosyl-2-O-sulfotransferase, glucosaminyl-6-O-sulfotransferase-1 and N-deacetylase/N-sulfotransferase-1), involved in the biosynthetic pathway of heparan sulfate, were also observed. The results suggest that overexpression of the EJ-ras oncogene alters the cell cycle, through signal transduction cascades, upregulates the expression of syndecan-4, and downregulates enzymes involved in the heparan sulfate biosynthesis related to chain modification, leading to the structural changes of the heparan sulfate syndecan-4 proteoglycan in endothelial cells.

Protein kinase Calpha can undergo membrane localization via an alternative phosphatidylinositol 4,5-bisphosphate-dependent pathway

Protein kinase Calpha (PKCalpha) activation is known to be dependent on the metabolic product of phosphatidylinositol 4,5-bisphosphate (PIP2) by phospholipase C (PLC). Here we report that fibroblasts may have an additional PIP2-dependent mechanism for membrane localization of PKCalpha. We observed PKCalpha membrane localization in both wild type and PLCgamma1 -/- mouse embryonic fibroblasts. Treatment of cells with a specific PLC inhibitor U73122 resulted in increased PIP2 levels and enhanced membrane localization of PKCalpha. PKCalpha levels in the membrane fraction decreased following incubation with PLCgamma, but increased following treatment with U73122 or addition of exogenous PIP2 in vitro. In addition, PKCalpha interacted with PIP2-conjugate bead and mixed micelles containing PIP2. Finally, we found that PIP2 is involved in syndecan-4-mediated membrane localization of PKCalpha. Taken together, these data suggest that PIP2 might contribute to directly regulating the membrane localization of PKCalpha.

The role of syndecans in disease and wound healing

Syndecans are a family of transmembrane heparan sulfate proteoglycans widely expressed in both developing and adult tissues. Until recently, their role in pathogenesis was largely unexplored. In this review, we discuss the reported involvement of syndecans in human cancers, infectious diseases, obesity, wound healing and angiogenesis. In some cancers, syndecan expression has been shown to regulate tumor cell function (e.g. proliferation, adhesion, and motility) and serve as a prognostic marker for tumor progression and patient survival. The ectodomains and heparan sulfate glycosaminoglycan chains of syndecans can also act as receptors/co-receptors for some bacterial and viral pathogens, mediating infection. In addition, syndecans bind to obesity-related factors and regulate their signaling, in turn modulating food consumption and weight balance. In vivo animal models of tissue injury and in vitro data also implicate syndecans in processes necessary for wound healing, including fibroblast and endothelial proliferation, cell motility, angiogenesis, and extracellular matrix organization. These new insights into the involvement of syndecans in disease and tissue repair coupled with the emergence of syndecan-specific molecular tools may lead to novel therapies for a variety of human diseases.

Beta1 and beta3 integrins cooperate to induce syndecan-4-containing cross-linked actin networks in human trabecular meshwork cells

PURPOSE

To characterize the molecular composition of cross-linked actin networks (CLANs) and the regulation of their formation by integrins in normal human trabecular meshwork (TM) cells. CLANs have been observed in steroid-treated and glaucomatous TM cells and have been suggested to contribute to decreased outflow facility by altering the contractility of the TM.

METHODS

Immunofluorescence microscopy was used to identify molecular components of CLANs and quantitate CLAN formation in HTM cells plated on coverslips coated with various extracellular matrix (ECM) proteins (fibronectin, types I and IV collagen, and vitronectin), vascular cell adhesion molecule (VCAM)-1, or activating antibodies against beta1, beta3, or alpha2beta1 integrins. These integrin antibodies were also used as soluble ligands.

RESULTS

CLAN vertices contained the actin-binding proteins alpha-actinin and filamin and the signaling molecules syndecan-4 and PIP2. CLANs lacked Arp3 and cortactin. CLAN formation was dependent on the ECM substrate and was significantly higher on fibronectin and VCAM-1 compared with vitronectin, types I or IV collagen. Adsorbed beta1 integrin antibodies also induced CLANs, whereas adsorbed beta3 or alpha2beta1 integrin antibodies did not. Soluble beta3 integrin antibodies, however, induced CLANs and actually enhanced CLAN formation in cells spread on fibronectin, VCAM-1, type I or type IV collagen, or beta1 integrin antibodies.

CONCLUSIONS

CLANs are unique actin-branched networks whose formation can be regulated by beta1 and beta3 integrin signaling pathways. Thus, integrin-mediated signaling events can modulate the organization of the actin cytoskeleton in TM cells and hence could participate in regulating cytoskeletal events previously demonstrated to be involved in controlling outflow facility.

Syndecan-1 regulates alphavbeta5 integrin activity in B82L fibroblasts

B82L mouse fibroblasts respond to fibronectin or vitronectin via a syndecan-1-mediated activation of the alphavbeta5 integrin. Cells attached to syndecan-1-specific antibody display only filopodial extension. However, the syndecan-anchored cells extend lamellipodia when the antibody-substratum is supplemented with serum, or low concentrations of adsorbed vitronectin or fibronectin, that are not sufficient to activate the integrin when plated alone. Integrin activation is blocked by treatment with (Arg-Gly-Asp)-containing peptides and function-blocking antibodies that target alphav integrins, as well as by siRNA-mediated silencing of beta5 integrin expression. In addition, alphavbeta5-mediated cell attachment and spreading on high concentrations of vitronectin is blocked by competition with recombinant syndecan-1 ectodomain core protein and by downregulation of mouse syndecan-1 expression by mouse-specific siRNA. Taking advantage of the species-specificity of the siRNA, rescue experiments in which human syndecan-1 constructs are expressed trace the activation site to the syndecan-1 ectodomain. Moreover, both full-length mouse and human syndecan-1 co-immunoprecipitate with the beta5 integrin subunit, but fail to do so if the syndecan is displaced by competition with soluble, recombinant syndecan-1 ectodomain. These results suggest that the ectodomain of the syndecan-1 core protein contains an active site that assembles into a complex with the alphavbeta5 integrin and regulates alphavbeta5 integrin activity.

Helicobacter pylori and toll-like receptor agonists induce syndecan-4 expression in an NF-kappaB-dependent manner

The syndecans are a family of transmembrane heparan sulfate proteoglycans (HSPG) that have been implicated in a wide variety of biological functions including the regulation of growth factor signaling, adhesion, tumorigenesis, and inflammation. In the current studies, we examined the regulation of syndecan-4 gene expression in gastric epithelial cells and macrophages in response to infection with live Helicobacter pylori and purified toll-like receptor (TLR) agonists. H. pylori, PAM3CSK4 (a TLR2 agonist), and Escherichia coli flagellin (a TLR5 agonist) all induced the rapid expression of syndecan-4 mRNA in MKN45 gastric epithelial cells. Similarly, lipopolysaccharide (LPS) (a TLR4 agonist) also induced the expression of syndecan-4 in macrophages. The H. pylori- and TLR-induced increase in syndecan-4 mRNA was blocked by the proteosome inhibitor MG-132 suggesting a role for nuclear factor kappaB (NF-kappaB) in the regulation of syndecan-4 gene expression. An 895-bp fragment of the human syndecan-4 promoter was cloned upstream of the luciferase reporter. When transfected into MKN45 cells, the activity of this promoter was inducible by H. pylori and TLR agonists. Inducible activity of the syndecan-4 promoter was blocked by cotransfection with a dominant negative IkappaBalpha expression plasmid. Electrophoretic mobility shift assays (EMSA) demonstrated the presence of a highly conserved NF-kappaB-binding site. Mutation of this site within the context of the full-length syndecan-4 promoter resulted in a complete loss of responsiveness to H. pylori and TLR agonists. These results thus demonstrate that the response of the syndecan-4 gene to infectious agents, or their products, is a direct result of NF-kappaB binding to the promoter and induction of de novo transcription.

Structural basis of syndecan-4 phosphorylation as a molecular switch to regulate signaling

The syndecan transmembrane proteoglycans are involved in the organization of the actin cytoskeleton and have important roles as cell surface receptors during cell-matrix interactions. We have shown that the syndecan-4 cytoplasmic domain (4L) forms oligomeric complexes that bind to and stimulate PKCalpha activity in the presence of PtdIns(4,5)P2, emphasizing the importance of multimerization in the regulation of PKCalpha activation. Oligomerization of the cytoplasmic domain of syndecan-4 is regulated either positively by PtdIns(4,5)P2 or negatively by phosphorylation of serine 183. Phosphorylation results in reduced PKCalpha activity by inhibiting PtdIns(4,5)P2-dependent oligomerization of the syndecan-4 cytoplasmic domain. Data from NMR and gel-filtration chromatography show that the phosphorylated cytoplasmic domain (p-4L) exists as a dimer, similar to 4L, but not as higher-order oligomers. NMR analysis showed that the overall conformation of p-4L is a compact intertwined dimer with an unusually symmetric clamp shape, and its molecular surface is mostly positively charged. The two parallel strands form a cavity in the center of the dimeric twist. An especially marked effect of phosphorylation of the syndecan-4 cytoplasmic domain is a dramatic conformational change near the C2 region that ablates an interaction site with the PDZ domain of syntenin. Wound healing studies further suggest that syndecan-4 phosphorylation might influence cell migration behavior. We conclude that the phosphorylation (Ser183) of syndecan-4 can play a critical role as a molecular switch to regulate its functions through conformational change.

Transmembrane domain-induced oligomerization is crucial for the functions of syndecan-2 and syndecan-4

The syndecans are known to form homologous oligomers that may be important for their functions. We have therefore determined the role of oligomerization of syndecan-2 and syndecan-4. A series of glutathione S-transferase-syndecan-2 and syndecan-4 chimeric proteins showed that all syndecan constructs containing the transmembrane domain formed SDS-resistant dimers, but not those lacking it. SDS-resistant dimer formation was hardly seen in the syndecan chimeras where each transmembrane domain was substituted with that of platelet-derived growth factor receptor (PDGFR). Increased MAPK activity was detected in HEK293T cells transfected with syndecan/PDGFR chimeras in a syndecan transmembrane domain-dependent fashion. The chimera-induced MAPK activation was independent of both ligand and extracellular domain, implying that the transmembrane domain is sufficient to induce dimerization/oligomerization in vivo. Furthermore, the syndecan chimeras were defective in syndecan-4-mediated focal adhesion formation and protein kinase Calpha activation or in syndecan-2-mediated cell migration. Taken together, these data suggest that the transmembrane domains are sufficient for inducing dimerization and that transmembrane domain-induced oligomerization is crucial for syndecan-2 and syndecan-4 functions.

Syndecans: new kids on the signaling block

Cell-associated proteoglycans provide highly complex and sophisticated systems to control interactions of extracellular cell matrix components and soluble ligands with the cell surface. Syndecans, a conserved family of heparan- and chondroitin-sulfate carrying transmembrane proteins, are emerging as central players in these interactions. Recent studies have demonstrated the essential role of syndecans in modulating cellular signaling in embryonic development, tumorigenesis, and angiogenesis. In this review, we focus on new advances in our understanding of syndecan-mediated cell signaling.

Fibronectin's central cell-binding domain supports focal adhesion formation and Rho signal transduction

Fibroblast adhesion to fibronectin (FN) induces formation of focal adhesions (FAs), structures that have significant effect on cell migration and signaling. FA formation requires actomyosin-based contractility that is regulated by Rho-dependent myosin light chain (MLC) phosphorylation. Previous studies indicated that the FN central cell-binding (and integrin-binding) domain (CBD) is insufficient for FA formation and that the major heparin-binding domain (HepII) facilitates FA formation in a Rho-dependent manner. We describe here conditions under which FN CBD alone is sufficient for FA formation in both human dermal fibroblasts and the FN-null murine fibroblasts. CBD-mediated FA formation is dependent on its surface adsorption and the adhesion activity of the cells. Attachment of FN-null fibroblasts to CBD elicits the same biphasic regulation of Rho activity as seen on intact FN, whereas adhesion to HepII alone does not activate Rho. Activation of Rho requires high levels of integrin occupancy. However, FN or CBD may induce FAs without increased activation of Rho (i.e. the basal level of GTP-Rho induces sufficient phospho-MLC for FA assembly under this condition). In contrast, adhesion to HepII alone does not sustain MLC phosphorylation. Pulse stimulation of cells on CBD or HepII with lysophosphatidic acid elevates Rho GTP loading to the same level, but the lysophosphatidic acid-stimulated MLC phosphorylation is significantly lower in cells on HepII than on CBD. Coating HepII with suboptimal concentrations of CBD induces FAs without increased activation of Rho. Therefore, FN CBD can support FA formation and generate contraction by activating Rho or by facilitating Rho downstream signaling.

Three-dimensional migration of human adult dermal fibroblasts from collagen lattices into fibrin/fibronectin gels requires syndecan-4 proteoglycan

Fibroblast migration from the peri-wound collagenous stroma into the fibrin-laden wound is critical for granulation tissue formation and subsequent healing. Previously we found that fibroblast transmigration from a collagen matrix into a fibrin matrix required fibronectin (FN). Integrins alpha4beta1, alpha5beta1, and alphavbeta3 and dermatan sulfate CD44 were required for this invasive migration. Here we demonstrated that syndecan-4, a transmembrane heparan sulfate (HS) proteoglycan, known to bind FN, is also required for fibroblast invasive migration of a fibrin/FN gel. This conclusion was based on fibroblast migration using two independent means of disrupting syndecan-4: heparinase degradation of HS glycosaminoglycans or suppression of syndecan-4 core protein with antisense oligodeoxynucleotides. Isolated syndecan-4 from these fibroblasts bound Hep II recombinant constructs FN III12-V15>FN III12-15>FN III12-14 but did not bind the IIICS (V) domain. Furthermore, platelet-derived growth factor (PDGF), which is required to stimulate fibroblast migration, markedly increased cell levels of syndecan-4 core protein in a time and concentration-dependent fashion. PDGF also induced upregulation of syndecan-4 at transcriptional level as determined by RT-PCR. These results demonstrate that syndecan-4 is essential for fibroblast invasive migration into fibrin clot and that PDGF, the stimulus for migration, induces increased syndecan-4 core protein expression.

Mammalian and Drosophila cells adhere to the laminin alpha4 LG4 domain through syndecans, but not glypicans

We have previously shown that the LG4 (laminin G-like) domain of the laminin alpha4 chain is responsible for the significantly higher affinity of the alpha4 chain to heparin than found for other alpha chains [Yamaguchi, Yamashita, Mori, Okazaki, Nomizu, Beck and Kitagawa (2000) J. Biol. Chem. 275, 29458-29465]; four basic residues were identified to be essential for this activity [Yamashita, Beck and Kitagawa (2004) J. Mol. Biol. 335, 1145-1149]. By creating GST (glutathione S-transferase)-fused LG1, LG2, LG4 and LG5 proteins, we found that only LG4 is active for the adhesion of human HT1080 cells, human umbilical vein endothelial cells and Drosophila haemocytes Kc167 with a half-saturating concentration of 20 microg/ml. Adhesion was counteracted by treatment of the cells with heparin, heparan sulphate and heparitinase I. Upon mutating the four basic residues essential for heparin binding within LG4, the adhesion activity was abolished. Pull-down experiments using glutathione beads/GST-fusion proteins indicate a direct interaction of LG4 with syndecan-4, which might be the major receptor for cell adhesion. Neither the release of glypican-1 by treating human cells with phosphatidylinositol-specific phospholipase C nor targeted knockdown of dally or dally-like protein impaired the cell-adhesion activity. As the LG4-LG5 domain of the alpha4 chain is cleaved in vivo from the main body of laminin-8 (alpha4beta1gamma1), we suggest that the heparan sulphate proteoglycan-binding activity of LG4 is significant in modulating the signalling of Wnt, Decapentaplegic and fibroblast growth factors.

The kinetics of FGF-2 binding to heparan sulfate proteoglycans and MAP kinase signaling

Binding of growth factors to specific cell surface receptors is the first step in initiating cell signaling cascades that ultimately result in diverse activities such as proliferation, differentiation, and apoptosis. Dimerization and phosphorylation of tyrosine kinase transmembrane receptors is the typical paradigm for this activation but, for many growth factors, cell surface interactions are not limited to a single receptor type. In particular, heparin-binding growth factors, such as fibroblast growth factor-2 (FGF-2), bind to heparan sulfate proteoglycans (HSPG) on the cell surface and within the extracellular matrix (ECM), and these molecules have been viewed as accessory co-receptors serving to facilitate tyrosine kinase receptor binding. Recent studies, however, have indicated that HSPG can directly participate in signal transduction in response to FGF-2 binding. Thus, in the present study, we used mathematical modeling to examine whether the kinetics of formation of the various FGF-2 bound complexes on the cell surface correlate with the activation of the downstream mediators of FGF-2 response, Erk1/2. We find that FGF-2 binding to its receptor correlates well with Erk1/2 activation and that HSPG can modulate this response through its ability to stabilize these ligand receptor complexes. Moreover, we also observed that FGF-2 binding to HSPG correlates strongly with Erk1/2 activation under conditions where there is a loss of receptor activity, and we demonstrate that the relative amounts of signaling and non-signaling HSPG on the cell surface, as well as the presence of competing HSPG in the ECM, can impact the signal potential via this pathway. Thus, the selective regulation of specific HSPG might provide a mechanism for fine tuned modulation of heparin-binding growth factor signaling in cells where signal intensity and duration could direct cellular response toward growth, migration or differentiation.

Distribution of syndecans 1-4 within the anterior segment of the human eye: expression of a variant syndecan-3 and matrix-associated syndecan-2

Control of the actomyosin network plays a role in regulating the movement of aqueous humor through the anterior segment of the eye. Receptors that could control its activity are unknown. In this study, we show that all four members of the syndecan family, which can regulate the actomyosin network, are present within the anterior segment. In both sections of human anterior segments and cultures of human trabecular meshwork (HTM), Schlemm's canal (HSC) and the ciliary muscle (HCM) cells from the anterior segment, syndecans-3 and -4 were the predominant family members. They were widely distributed throughout the anterior segment. Syndecan-3 within the anterior segment was a novel, recently described variant 55 kDa form. Low levels of syndecans-1 and -2 were also observed in situ and in all three cultures. Their expression was weaker and more localized than that observed for syndecans-3 and -4. Staining for syndecan-1 in HCM cultures was variable. In HTM and HSC cultures, syndecan-2 also co-distributed with fibronectin, laminin and type IV collagen suggesting that it was shed and associated with the extracellular matrix. Western blots supported this idea and showed syndecan-2 ectodomains in lysates from anterior segments.

Distribution and Clinical Significance of Heparan Sulfate Proteoglycans in Ovarian Cancer

PURPOSE

Heparan sulfate proteoglycans have been implicated in cancer cell growth, invasion, metastasis, and angiogenesis. This study was designed to compare their expression in normal ovary and ovarian tumors and then to examine their prognostic significance in ovarian cancer.

EXPERIMENTAL DESIGN

The expression of syndecan-1, -2, -3, and -4, glypican-1, and perlecan was assessed by immunohistochemistry in 147 biopsies that included normal ovary and benign, borderline, and malignant ovarian tumors. Clinical data, including tumor stage, performance status, treatment, and survival, were collected. Univariate and multivariate analyses were performed to evaluate prognostic significance.

RESULTS

The expression patterns of syndecan-1 and perlecan were altered in ovarian tumors compared with normal ovary. Syndecan-1 was not detected in normal ovary but was present in the epithelial and stromal cells of benign and borderline tumors and in ovarian adenocarcinomas. Perlecan expression was decreased in basement membranes that were disrupted by cancer cells but maintained in the basement membranes of blood vessels. Syndecan-2, -3, and -4, and glypican-1 were expressed in normal ovary and benign and malignant ovarian tumors. Stromal expression of syndecan-1 and glypican-1 were poor prognostic factors for survival in univariate analysis.

CONCLUSION

We report for the first time distinct patterns of expression of cell surface and extracellular matrix heparan sulfate proteoglycans in normal ovary compared with ovarian tumors. These data reinforce the role of the tumor stroma in ovarian adenocarcinoma and suggest that stromal induction of syndecan-1 contributes to the pathogenesis of this malignancy.

Syndecan-4 regulates localization, activity and stability of protein kinase C-alpha

During cell-matrix adhesion, syndecan-4 transmembrane heparan sulphate proteoglycan plays a critical role in the formation of focal adhesions and stress fibres. We have shown previously that the syndecan-4 cytoplasmic domain directly binds to and activates PKC-alpha (protein kinase C-alpha) in vitro [Oh, Woods and Couchman (1997) J. Biol. Chem. 272, 8133-8136]. However, whether syndecan-4 has the same activity in vivo needs to be addressed. Using mammalian two-hybrid assays, we showed that syndecan-4 interacted with PKC-alpha in vivo and that this interaction was mediated through syndecan-4 cytoplasmic domain. Furthermore, the activation of PKC increased the extent of interaction between syndecan-4 and PKC-alpha. Overexpression of syndecan-4, but not a mutant lacking its cytoplasmic domain, specifically increased the level of endogenous PKC-alpha and enhanced the translocation of PKC-alpha into both detergent-insoluble and membrane fractions. In addition, rat embryo fibroblasts overexpressing syndecan-4 exhibited a slowed down-regulation of PKC-alpha in response either to a prolonged treatment with PMA or to maintaining cells in suspension culture. PKC-alpha immunocomplex kinase assays also showed that syndecan-4 overexpression increased the activity of membrane PKC-alpha. Taken together, these results suggest that syndecan-4 interacts with PKC-alpha in vivo and regulates its localization, activity and stability.

Syndecan-4-dependent migration of human eosinophils

BACKGROUND

Heparan sulphate proteoglycans (HSPGs) are important participants in cell surface signalling and critical in controlling cell behaviour. They modulate inflammatory cell maturation and activation, leucocyte rolling, adhesion to endothelium as well as extravasation and chemotaxis. Whether eosinophil's function is affected has not yet been reported.

OBJECTIVE

We investigated the effects of transgenic, recombinant anti-thrombin III and Kybernin P, an anti-thrombin III concentrate, as HSPG ligands on spontaneous and chemokine-triggered migration of normal eosinophils from human peripheral blood in modified Boyden chamber micropore filter assays.

METHODS

Eosinophils from human peripheral blood were purified using magnetic antibody cell sorting. The signalling mechanisms required for anti-thrombin-dependent migration were studied using signalling enzyme blockers. Expression of HSPG core protein mRNA was studied by PCR.

RESULTS

Pre-treatment of eosinophils with anti-thrombin III inhibited chemotaxis toward optimal concentrations of eotaxin or RANTES (regulated upon activation normal T cell expressed and activated). In the absence of the chemokines, direct exposure to gradients of anti-thrombin III stimulated eosinophil migration. The effects of anti-thrombin III were abolished by pre-treating cells with heparinase-1, chondroitinase, sodium chlorate and anti-syndecan-4 antibodies. Syndecan-4 gene expression in eosinophils was confirmed in PCR. In the presence of pentasaccharide, anti-thrombin III lost its effect on the cells. Functional responses were also abrogated by inhibition of protein kinase C, phosphatidylinositol 3-kinase and phosphodiesterase.

CONCLUSION

Data indicate that anti-thrombin III affects eosinophil motility via the effects of its heparin-binding site on cell surface syndecan-4. Ligation of syndecan-4 with anti-thrombin III induces eosinophil migration and deactivates motility toward chemokines. These observations suggest that syndecan-4-dependent signalling may control eosinophil locomotion.

Syndecans in tumor cell adhesion and signaling

Anchorage of cells to "heparin"--binding domains that are prevalent in extracellular matrix (ECM) components is thought to occur primarily through the syndecans, a four-member family of transmembrane heparan sulfate proteoglycans that communicate environmental cues from the ECM to the cytoskeleton and the signaling apparatus of the cell. Known activities of the syndecans trace to their highly conserved cytoplasmic domains and to their heparan sulfate chains, which can serve to regulate the signaling of growth factors and morphogens. However, several emerging studies point to critical roles for the syndecans' extracellular protein domains in tumor cell behavior to include cell adhesion and invasion. Although the mechanisms of these activities remain largely unknown, one possibility involves "co-receptor" interactions with integrins that may regulate integrin function and the cell adhesion-signaling phenotype. Thus, alterations in syndecan expression, leading to either overexpression or loss of expression, both of which take place in tumor cells, may have dramatic effects on tumor cell invasion.

Syndecan-3 and syndecan-4 are enriched in Schwann cell perinodal processes

Background

Nodes of Ranvier correspond to specialized axonal domains where voltage-gated sodium channels are highly concentrated. In the peripheral nervous system, they are covered by Schwann cells microvilli, where three homologous cytoskeletal-associated proteins, ezrin, radixin and moesin (ERM proteins) have been found, to be enriched. These glial processes are thought to play a crucial role in organizing axonal nodal domains during development. However, little is known about the molecules present in Schwann cell processes that could mediate axoglial interactions. The aim of this study is to identify by immunocytochemistry transmembrane proteins enriched in Schwann cells processes that could interact, directly or indirectly, with axonal proteins.

Results

We show that syndecan-3 (S3) and syndecan-4 (S4), two proteoglycans expressed in Schwann cells, are enriched in perinodal processes in rat sciatic nerves. S3 labeling was localized in close vicinity of sodium channels as early as post-natal day 2, and highly concentrated at nodes of Ranvier in the adult. S4 immunoreactivity accumulated at nodes later, and was also prominent in internodal regions of myelinated fibers. Both S3 and S4 were co-localized with ezrin in perinodal processes.

Conclusions

Our data identify S3 and S4 as transmembrane proteins specifically enriched in Schwann cell perinodal processes, and suggest that S3 may be involved in early axoglial interactions during development.

Syndecan-1-mediated cell spreading requires signaling by alphavbeta3 integrins in human breast carcinoma cells

Syndecans are cell surface heparan sulfate proteoglycans with regulatory roles in cell adhesion, proliferation, and differentiation [Annu. Rev. Biochem. 68 (1999) 729]. While the syndecan heparan sulfate chains are essential for matrix binding, less is known about the signaling role of their core proteins. To mimic syndecan-specific adhesion, MDA-MB-231 mammary carcinoma cells were plated on antibodies against syndecan-4 or syndecan-1. While cells adherent via syndecan-4 spread, cells adherent via syndecan-1 do not. However, cells adherent via syndecan-1 can be induced to spread by Mn(2+), suggesting that activation of a beta(1) or beta(3) integrin partner is required. Surprisingly, pretreatment of cells with a function-activating beta(1) antibody does not induce spreading, whereas function-blocking beta(1) integrin antibodies do, suggesting involvement of a beta(1)-to-beta(3) integrin cross-talk. Indeed, blockade of beta(1) integrin activation induces alpha(v)beta(3) integrin activation detectable by soluble fibrinogen binding. Spreading in response to syndecan-1 is independent of integrin-ligand binding. Furthermore, competition with soluble murine syndecan-1 ectodomain, which does not disrupt cell adhesion, nonetheless blocks the spreading mechanism. These data suggest that the ectodomain of the syndecan-1 core protein directly participates in the formation of a signaling complex that signals in cooperation with alpha(v)beta(3) integrins; signaling via this complex is negatively regulated by beta(1) integrins.

Syndecan-4 modulates basic fibroblast growth factor 2 signaling in vivo

Syndecan-4 is one of the principal heparan sulfate-carrying proteins on the cell surface. Unlike other members of syndecan family, syndecan-4 mediates phosphatidylinositol 4,5-bisphosphate 2 (PIP(2))-dependent PKC-alpha activation, and overexpression of syndecan-4 in vitro results in enhanced FGF2 signaling. The present study was designed to test the functional effect of increased syndecan-4 expression in endothelial cells in transgenic mice. Several transgenic mice lines expressing syndecan-4 cDNA under control of human endothelial nitric oxide (NO) synthase (eNOS) promoter were generated. Exogenous syndecan-4 was mainly expressed in the heart, brain, and lungs. In particular, the heart demonstrated the greatest increase in the ratio of transgenic-to-native syndecan-4 gene expression. Vessels from the eNOS-syndecan-4 mice demonstrated more pronounced vasodilation to FGF2 but not to VEGF-A(165), sodium nitroprusside, and A 23187 compared with wild-type mice. To elucidate the mechanism of this effect, we measured NO release from primary cardiac endothelial cells isolated from transgenic or wild-type adult mice. Cells from the eNOS-syndecan-4 transgenic mice had a significant increase in FGF2- and VEGF-A(165)-induced NO release compared with endothelial cells from the wild-type mice. However, the absolute magnitude of this increase was higher for FGF2 than VEGF-A(165). In conclusion, enhanced syndecan-4 expression in mouse cardiac endothelial cells results in preferential augmentation of FGF2 but not VEGF-A(165)-induced NO release.

Direct binding of syndecan-4 cytoplasmic domain to the catalytic domain of protein kinase C alpha (PKC alpha) increases focal adhesion localization of PKC alpha

Syndecan-4 is a transmembrane heparan sulfate proteoglycan that acts as a coreceptor with integrins in focal adhesion formation. The central region of syndecan-4 cytoplasmic domain (4V; LGKKPIYKK) binds phosphatidylinositol 4,5-bisphosphate, and together they regulate protein kinase C alpha (PKC alpha) activity. Syndecan 4V peptide directly potentiates PKC alpha activity, leading to "superactivation" of the enzyme, apparently through an interaction with its catalytic domain. We now have performed yeast two-hybrid and in vitro binding assays to determine the interaction sites between 4V and PKC alpha. Full-length PKC alpha weakly interacted with 4V by yeast two-hybrid assays, but PKC alpha constructs that lack the pseudosubstrate region or constructs of the whole catalytic domain interacted more strongly. A mutated 4V sequence (4V(YF): LGKKPIFKK) did not interact with PKC alpha, indicating that tyrosine 192 in the syndecan-4 cytoplasmic domain might be critical for this interaction. Further assays identified a novel interaction site in the C terminus of the catalytic domain of PKC alpha (amino acid sequence 513-672). This encompasses the autophosphorylation sites, which are implicated in activation and stability. Yeast two-hybrid data were confirmed by in vitro binding and coimmunoprecipitation assays. The interaction of syndecan-4 with PKC alpha appears unique since PKC delta and epsilon did not interact with 4V in yeast two-hybrid assays or coimmunoprecipitate with syndecan-4. Finally, overexpression of syndecan-4 in rat embryo fibroblast cells, but not expression of the YF mutant, increased PKC alpha localization to focal adhesions. The data support a mechanism where syndecan-4 binds PKC alpha and localizes it to focal adhesions, whose assembly may be regulated by the kinase.

Nuclear localization of basic fibroblast growth factor is mediated by heparan sulfate proteoglycans through protein kinase C signaling

Understanding the process of wound healing will provide valuable insight for the development of new strategies to treat diseases associated with improper regeneration, such as blindness induced by corneal scarring. Heparan sulfate proteoglycans (HSPG) are not normally expressed in the corneal stroma, but their presence at sites of injury suggests their involvement in the wound healing response. Primary cultured corneal stromal fibroblasts constitutively express HSPG and represent an injured phenotype. Recently, nuclear localization of HSPG was shown to increase in corneal stromal fibroblasts plated on fibronectin (FN), an extracellular matrix protein whose appearance in the corneal stroma correlates with injury. One possible role for the nuclear localization of HSPG is to function as a shuttle for the nuclear transport of heparin-binding growth factors, such as basic fibroblast growth factor (FGF-2). Once in the nucleus, these growth factors might directly modulate cellular activities. To investigate this hypothesis, cells were treated with (125)I-labelled FGF-2 under various conditions and fractionated. Our results show that nuclear localization of FGF-2 was increased in cells plated on FN compared to those on collagen type I (CO). Interestingly, FGF-2-stimulated proliferation was increased in cells plated on FN compared to CO and this effect was absent in the presence of heparinase III. Furthermore, pre-treatment with heparinase III decreased nuclear FGF-2, and CHO cells defective in the ability to properly synthesize heparan sulfate chains showed reduced nuclear FGF-2 indicating that the heparan sulfate chains of HSPG are critical for this process. HSPG signaling, particularly through the cytoplasmic tails of syndecans, was investigated as a potential mechanism for the nuclear localization of FGF-2. Treatment with phorbol 12-myristate-13-acetate (PMA), under conditions that caused downregulation of protein kinase Calpha (PKCalpha), decreased nuclear FGF-2. Using pharmacological inhibitors of specific PKC isozymes, we elucidated a potential mode of regulation whereby PKCalpha mediates the nuclear localization of FGF-2 and PKCdelta inhibits it. Our studies suggest a novel mechanism in which FGF-2 translocates to the nucleus in response to injury.

Integrin-specific signaling pathways controlling focal adhesion formation and cell migration

The fibronectin (FN)-binding integrins alpha4beta1 and alpha5beta1 confer different cell adhesive properties, particularly with respect to focal adhesion formation and migration. After analyses of alpha4+/alpha5+ A375-SM melanoma cell adhesion to fragments of FN that interact selectively with alpha4beta1 and alpha5beta1, we now report two differences in the signals transduced by each receptor that underpin their specific adhesive properties. First, alpha5beta1 and alpha4beta1 have a differential requirement for cell surface proteoglycan engagement for focal adhesion formation and migration; alpha5beta1 requires a proteoglycan coreceptor (syndecan-4), and alpha4beta1 does not. Second, adhesion via alpha5beta1 caused an eightfold increase in protein kinase Calpha (PKCalpha) activation, but only basal PKCalpha activity was observed after adhesion via alpha4beta1. Pharmacological inhibition of PKCalpha and transient expression of dominant-negative PKCalpha, but not dominant-negative PKCdelta or PKCzeta constructs, suppressed focal adhesion formation and cell migration mediated by alpha5beta1, but had no effect on alpha4beta1. These findings demonstrate that different integrins can signal to induce focal adhesion formation and migration by different mechanisms, and they identify PKCalpha signaling as central to the functional differences between alpha4beta1 and alpha5beta1.

A new link between the c-Abl tyrosine kinase and phosphoinositide signalling through PLC-gamma1

The c-Abl tyrosine (Tyr) kinase is activated after platelet-derived-growth factor receptor (PDGFR) stimulation in a manner that is partially dependent on Src kinase activity. However, the activity of Src kinases alone is not sufficient for activation of c-Abl by PDGFR. Here we show that functional phospholipase C-gamma1 (PLC-gamma1) is required for c-Abl activation by PDGFR. Decreasing cellular levels of phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2) by PLC-gamma1-mediated hydrolysis or dephosphorylation by an inositol polyphosphate 5-phosphatase (Inp54) results in increased Abl kinase activity. c-Abl functions downstream of PLC-gamma1, as expression of kinase-inactive c-Abl blocks PLC-gamma1-induced chemotaxis towards PDGF-BB. PLC-gamma1 and c-Abl form a complex in cells that is enhanced by PDGF stimulation. After activation, c-Abl phosphorylates PLC-gamma1 and negatively modulates its function in vivo. These findings uncover a newly discovered functional interdependence between non-receptor Tyr kinase and lipid signalling pathways.