Proteoglycans: from structural compounds to signaling molecules. Cell Tissue Res. 2010;339:237-246. [PMID: 19513755 DOI: 10.1007/s00441-009-0821-y]

Small leucine-rich proteoglycans orchestrate receptor crosstalk during inflammation

Inflammation is not only a defensive mechanism against microbial invasion, but also frequently represents a critical response to tissue injury under sterile conditions. It is now well established that tissue injury leads to the release of endogenous molecules of intra- and extracellular origin acting as damage-associated molecular patterns (DAMPs). The small leucine-rich proteoglycans (SLRPs) can act as powerful DAMPs following their proteolytical release from the extracellular matrix. Recent investigations of SLRP signaling networks revealed new levels of complexity, showing that SLRPs can cluster different types of receptors and orchestrate a host of downstream signaling events. This review will summarize the evidence for the multifunctional proinflammatory signaling properties of the two archetypal SLRPs, biglycan and decorin. These secreted proteoglycans link the innate to the adaptive immune response and operate in a broad biological context, encompassing microbial defense, tumor growth and autoimmunity.

Syndecan 4 regulates FGFR1 signaling in endothelial cells by directing macropinocytosis

Fibroblast growth factor 2 (FGF2) induces endothelial cell migration and angiogenesis through two classes of receptors: receptor tyrosine kinases, such as FGF receptor 1 (FGFR1), and heparan sulfate proteoglycans, such as syndecan 4 (S4). We examined the distinct contributions of FGFR1 and S4 in shaping the endothelial response to FGF2. S4 determined the kinetics and magnitude of FGF2-induced mitogen-activated protein kinase (MAPK) signaling by promoting the macropinocytosis of the FGFR1-S4-FGF2 signaling complex. Internalization of the S4 receptor complex was independent of clathrin and dynamin, proceeded from lipid raft-enriched membranes, and required activation of the guanosine triphosphatases RhoG and Rab5. Genetic knockout of S4, disruption of S4 function, or inhibition of Rab5 led to increased endocytosis and MAPK signaling. These data define the mechanism by which FGFR1 and S4 coordinate downstream signaling upon FGF2 stimulation: FGFR1 initiates MAPK signaling, whereas S4-dependent FGFR1 macropinocytosis modulates the kinetics of MAPK activation. Our studies identify S4 as a regulator of MAPK signaling and address the question of how distinct classes of FGFRs individually contribute to signal transduction in endothelial cells.

Chemokines regulate small leucine-rich proteoglycans in the extracellular matrix of the pressure-overloaded right ventricle

Chemokines have been suggested to play a role during development of left ventricular failure, but little is known about their role during right ventricular (RV) remodeling and dysfunction. We have previously shown that the chemokine (C-X-C motif) ligand 13 (CXCL13) regulates small leucine-rich proteoglycans (SLRPs). We hypothesized that chemokines are upregulated in the pressure-overloaded RV, and that they regulate SLRPs. Mice with RV pressure overload following pulmonary banding (PB) had a significant increase in RV weight and an increase in liver weight after 1 wk. Microarray analysis (Affymetrix) of RV tissue from mice with PB revealed that CXCL10, CXCL6, chemokine (C-X3-C motif) ligand 1 (CX3CL1), chemokine (C-C motif) ligand 5 (CCL5), CXCL16, and CCL2 were the most upregulated chemokines. Stimulation of cardiac fibroblasts with these same chemokines showed that CXCL16 increased the expression of the four SLRPs: decorin, lumican, biglycan, and fibromodulin. CCL5 increased the same SLRPs, except decorin, whereas CX3CL1 increased the expression of decorin and lumican. CXCL16, CX3CL1, and CCL5 were also shown to increase the levels of glycosylated decorin and lumican in the medium after stimulation of fibroblasts. In the pressure-overloaded RV tissue, Western blotting revealed an increase in the total protein level of lumican and a glycosylated form of decorin with a higher molecular weight compared with control mice. Both mice with PB and patients with pulmonary stenosis had significantly increased circulating levels of CXCL16 compared with healthy controls measured by enzyme immunoassay. In conclusion, we have found that chemokines are upregulated in the pressure-overloaded RV and that CXCL16, CX3CL1, and CCL5 regulate expression and posttranslational modifications of SLRPs in cardiac fibroblasts. In the pressure-overloaded RV, protein levels of lumican were increased, and a glycosylated form of decorin with a high molecular weight appeared.

Endothelial progenitor cells and integrins: adhesive needs

In the last decade there have been multiple studies concerning the contribution of endothelial progenitor cells (EPCs) to new vessel formation in different physiological and pathological settings. The process by which EPCs contribute to new vessel formation in adults is termed postnatal vasculogenesis and occurs via four inter-related steps. They must respond to chemoattractant signals and mobilize from the bone marrow to the peripheral blood; home in on sites of new vessel formation; invade and migrate at the same sites; and differentiate into mature endothelial cells (ECs) and/or regulate pre-existing ECs via paracrine or juxtacrine signals. During these four steps, EPCs interact with different physiological compartments, namely bone marrow, peripheral blood, blood vessels and homing tissues. The success of each step depends on the ability of EPCs to interact, adapt and respond to multiple molecular cues. The present review summarizes the interactions between integrins expressed by EPCs and their ligands: extracellular matrix components and cell surface proteins present at sites of postnatal vasculogenesis. The data summarized here indicate that integrins represent a major molecular determinant of EPC function, with different integrin subunits regulating different steps of EPC biology. Specifically, integrin α4β1 is a key regulator of EPC retention and/or mobilization from the bone marrow, while integrins α5β1, α6β1, αvβ3 and αvβ5 are major determinants of EPC homing, invasion, differentiation and paracrine factor production. β2 integrins are the major regulators of EPC transendothelial migration. The relevance of integrins in EPC biology is also demonstrated by many studies that use extracellular matrix-based scaffolds as a clinical tool to improve the vasculogenic functions of EPCs. We propose that targeted and tissue-specific manipulation of EPC integrin-mediated interactions may be crucial to further improve the usage of this cell population as a relevant clinical agent.

Canine tissue-specific expression of multiple small leucine rich proteoglycans

Small leucine rich proteoglycans (SLRPs) are important constituents of extracellular matrix (ECM) and contribute to the production, organization and remodelling of collagen and elastin through complex biological systems. The relative expression and distribution of SLRPs in a variety of different mammalian tissues is poorly characterized. The aim of this study was to map the expression of seven SLRPs (biglycan, versican, prolargin, fibromodulin, osteoglycin, decorin and lumican) in seven tissues (bone, cartilage, cruciate ligament, skin, ventricular myocardium, mitral valve and cornea) in young adult dogs using a combination of quantitative real-time PCR, immunohistochemistry and protein immunoblotting. Clear and consistent patterns of SLRP expression and distribution were identified for the seven tissues examined, with the greatest SLRP expression in cartilage, skin, cornea and mitral valve, and the least expression in myocardium. In general, lumican and prolargin had the greatest expression across the seven tissues whilst osteoglycin was the least abundantly expressed SLRP. These data provide a SLRP profile for different canine tissues which can inform future studies of SLRP expression in development and disease.

A comparative evaluation of the small leucine-rich proteoglycans of pathological human intervertebral discs

PURPOSE

Proteoglycans are important to the functioning of the intervertebral disc. In addition to aggrecan there are the small leucine-rich proteoglycans (SLRPs). These are less common but in other locations their functions include collagen organisation, sequestering growth factors and stimulating inflammation. We have performed a comparative analysis of the SLRP core protein species present in intervertebral discs with various pathologies.

METHODS

Eighteen intervertebral discs from patients with scoliosis (n = 7, 19-53 years), degenerative disc disease (n = 6, 35-51 years) and herniations (n = 5, 33-58 years) were used in this study. Proteoglycans were dissociatively extracted from disc tissues and the SLRPs (biglycan, decorin, fibromodulin, keratocan and lumican) assessed by Western blotting following deglycosylation with chondroitinase ABC and keratanase.

RESULTS

Intact SLRP core proteins and a number of core protein fragments were identified in most of the discs examined. Biglycan and fibromodulin were the most extensively fragmented. Keratocan generally occurred as two bands, one representing the intact core protein, the other a smaller fragment. The intact core protein of lumican was detected in all samples with fragmentation evident in only one of the older scoliotic discs. Decorin was less obvious in the disc samples and showed little fragmentation.

CONCLUSION

In this cohort of pathological intervertebral discs, fragmentation of certain SLRP core proteins was common, indicating that some SLRPs are extensively processed during the pathological process. Identification of specific SLRP fragments which correlate with disc pathology may not only help understand their aetiopathogeneses, but also provide biomarkers which can be used to monitor disease progression or to identify particular disc disorders.

Artificial extracellular matrices composed of collagen I and sulfated hyaluronan with adsorbed transforming growth factor β1 promote collagen synthesis of human mesenchymal stromal cells

Sulfated glycosaminoglycans (GAG) are multifunctional components of the extracellular matrix and are involved in the regulation of adhesion, proliferation and differentiation of cells. The effects of GAG are mediated in general by their interactions with cations and water, and in particular by their binding to growth factors. The aim of this study was to generate artificial extracellular matrices (aECM) containing collagen I and hyaluronan sulfate (HyaS), which are capable of adsorbing and releasing transforming growth factor β1 (TGF-β1), and to promote collagen synthesis of cultured human mesenchymal stromal cells (hMSC). For the preparation of aECM, monosulfated Hya (HyaS1) or trisulfated Hya (HyaS3) were used; the natural chondroitin-4-sulfate was used as a control. As applied for the in vitro experiments, the resulting matrices were composed of 93-98% collagen I and 2-7% GAG derivative. Adsorption of TGF-β1 to the aECM and release from the aECM was dependent on the degree of sulfation of hyaluronan. Collagen synthesis of hMSC was promoted only by aECM with adsorbed TGF-β1; the bare aECM had a slightly inhibitory effect on collagen synthesis. The promoting effect did not correlate either to the amount of adsorbed TGF-β1 nor to the release of TGF-β1, indicating that the correct presentation of TGF-β1 to the cells might be critical. The results indicate that sulfated hyaluronan-containing aECM have the potential to control both the adsorption and release of TGF-β1, and thereby promote collagen synthesis of hMSC. Thus, these aECM might be a useful tool for different tissue-engineering applications to enhance bone formation when used for biomaterial coating.

Prospects and Pitfalls of Pregnancy-Associated Malaria Vaccination Based on the Natural Immune Response to Plasmodium falciparum VAR2CSA-Expressing Parasites

Pregnancy-associated malaria, a manifestation of severe malaria, is the cause of up to 200,000 infant deaths a year, through the effects of placental insufficiency leading to growth restriction and preterm delivery. Development of a vaccine is one strategy for control. Plasmodium falciparum-infected red blood cells accumulate in the placenta through specific binding of pregnancy-associated parasite variants that express the VAR2CSA antigen to chondroitin sulphate A on the surface of syncytiotrophoblast cells. Parasite accumulation, accompanied by an inflammatory infiltrate, disrupts the cytokine balance of pregnancy with the potential to cause placental damage and compromise foetal growth. Multigravid women develop immunity towards VAR2CSA-expressing parasites in a gravidity-dependent manner which prevents unfavourable pregnancy outcomes. Although current vaccine design, targeting VAR2CSA antigens, has succeeded in inducing antibodies artificially, this candidate may not provide protection during the first trimester and may only protect those women living in areas endemic for malaria. It is concluded that while insufficient information about placental-parasite interactions is presently available to produce an effective vaccine, incremental progress is being made towards achieving this goal.

In vivo scintigraphic imaging of proteoglycans

In this chapter, we present the methods developed in our lab for the scintigraphic imaging and direct quantitative evaluation of proteoglycan (PG) distribution in vivo. These methods relate to (1) the synthesis and radiolabeling of the NTP 15-5 with (99m)Tc, (2) preclinical scintigraphic imaging using laboratory animals, and (3) quantitative analysis of scintigraphic images.

Overview of the matrisome--an inventory of extracellular matrix constituents and functions

Completion of genome sequences for many organisms allows a reasonably complete definition of the complement of extracellular matrix (ECM) proteins. In mammals this "core matrisome" comprises ∼300 proteins. In addition there are large numbers of ECM-modifying enzymes, ECM-binding growth factors, and other ECM-associated proteins. These different categories of ECM and ECM-associated proteins cooperate to assemble and remodel extracellular matrices and bind to cells through ECM receptors. Together with receptors for ECM-bound growth factors, they provide multiple inputs into cells to control survival, proliferation, differentiation, shape, polarity, and motility of cells. The evolution of ECM proteins was key in the transition to multicellularity, the arrangement of cells into tissue layers, and the elaboration of novel structures during vertebrate evolution. This key role of ECM is reflected in the diversity of ECM proteins and the modular domain structures of ECM proteins both allow their multiple interactions and, during evolution, development of novel protein architectures.

Mechanisms of polyethylenimine-mediated DNA delivery: free carrier helps to overcome the barrier of cell-surface glycosaminoglycans

BACKGROUND

Polyethylenimine (PEI) polyplexes mediate efficient gene transfer only at high +/- charge ratios at which free noncomplexed PEI is present. The excess of PEI gives polyplexes a positive surface charge that plays a role in polyplex binding on the cell membrane. Although positively charged PEI polyplexes are known to interact with anionic cell-surface glycosaminoglycans (GAGs), the exact role of free PEI in such interactions is unclear.

METHODS

Chinese hamster ovary wild-type cells and mutants lacking cell-surface GAGs were transfected with marker genes using PEI polyplexes with and without free PEI. The total amount of cell-associated plasmid DNA (pDNA) delivered by polyplexes was determined by quantitative real-time PCR and transgene expression was determined using β-galactosidase and luciferase assays.

RESULTS

Transfection activity of polyplexes without free PEI in cells expressing cell-surface GAGs was low even though pDNA was delivered to cells. In the absence of cell-surface GAGs, polyplexes without free PEI had high transfection efficacy. This indicates that the cell-surface GAGs inhibit transfection by purified polyplexes. PEI polyplexes with free carrier mediated transfection in both normal and GAG-deficient cells because free PEI overcomes the inhibitory effect of cell-surface GAGs on transfection. The intracellular elimination of pDNA was faster in the presence of GAGs and, despite improved transfection, free PEI reduced pDNA association with the cells.

CONCLUSIONS

Free PEI is essential for minimizing the undesirable binding of polyplexes to cell-surface GAGs that have a negative impact on transfection. The same mechanism may be important in transfections with other polyplexes that require high charge ratios for transfection.

On the selectivity and efficacy of defense peptides with respect to cancer cells

Here, we review potential determinants of the anticancer efficacy of innate immune peptides (ACPs) for cancer cells. These determinants include membrane-based factors, such as receptors, phosphatidylserine, sialic acid residues, and sulfated glycans, and peptide-based factors, such as residue composition, sequence length, net charge, hydrophobic arc size, hydrophobicity, and amphiphilicity. Each of these factors may contribute to the anticancer action of ACPs, but no single factor(s) makes an overriding contribution to their overall selectivity and toxicity. Differences between the anticancer actions of ACPs seem to relate to different levels of interplay between these peptide and membrane-based factors.

Small leucine-rich proteoglycans in kidney disease

Research over the past 2 decades provides ample evidence that small leucine-rich proteoglycans (SLRPs; such as decorin, biglycan, fibromodulin, and lumican) of the extracellular matrix are deeply involved in the regulation of inflammatory and fibrotic renal disorders. Initial efforts in SLRP research focused on the interaction between decorin and TGF-β because it had been unequivocally demonstrated that decorin treatment exerts beneficial effects in fibrotic disorders involving TGF-β overproduction in the kidney. This was followed by a paradigm shift in our understanding of SLRP biology, with new evidence showing that in addition to their role as structural matrix components, soluble SLRPs also act as signaling molecules regulating various complex biologic processes in a molecule- and cell-specific manner. With the identification of SLRP-derived endogenous ligands of Toll-like receptors, the general question regarding the mechanisms of SLRP-derived signaling in pathogen-dependent and independent renal inflammation arose. This led to the fascinating concept of SLRPs as autonomous triggers of sterile renal inflammation in response to renal stress or injury. This review focuses on the key biologic roles of SLRPs in the normal and diseased kidney with special emphasis on newly described signaling events triggered by these proteoglycans.

Dual immunofluorescence staining of proteoglycans in human temporal bones

OBJECTIVES/HYPOTHESIS

Immunofluorescence staining methods have been developed to study the distribution of macromolecules in archival formalin-fixed celloidin-embedded human temporal bone tissues. The aim of this study was to investigate the feasibility of utilizing this approach to evaluate the codistribution of more than one molecule of interest in a single tissue section.

STUDY DESIGN

Retrospective study of proteoglycan codistribution in archival human temporal bone tissues.

METHODS

The chondroitin sulfate and keratan sulfate proteoglycans were selected for evaluating this methodology. Human tissues with known proteoglycan staining patterns were studied as controls. Thirty-one formalin-fixed celloidin-embedded archival human temporal bones were evaluated, and the observations in 11 specimens are described. A dual immunofluorescence staining method was developed using primary antibodies of differing isotypes and secondary antibodies labeled with fluorophores having nonoverlapping emission characteristics.

RESULTS

The specificity of the dual immunofluorescence technique for chondroitin sulfate and keratan sulfate proteoglycans was demonstrated in control tissues and confirmed through inhibition studies. The normal human tectorial membrane exhibited intense chondroitin sulfate staining. Cochlear and vestibular hair cells exhibited predominantly keratan sulfate staining. Keratan sulfate staining predominated in spiral ganglion cell bodies and fibers. Alterations in the normal distribution pattern of proteoglycans were observed in cases of presbycusis and otosclerosis.

CONCLUSIONS

The dual immunofluorescence staining methodology can be used to study archival formalin-fixed celloidin-embedded human temporal bone tissues. This technique may be applied to the evaluation of other molecules in archival human temporal bone tissues and lead to improvement in our understanding of the function of these molecules and their role in disease processes.

Proteoglycans in cancer biology, tumour microenvironment and angiogenesis

Proteoglycans, key molecular effectors of cell surface and pericellular microenvironments, perform multiple functions in cancer and angiogenesis by virtue of their polyhedric nature and their ability to interact with both ligands and receptors that regulate neoplastic growth and neovascularization. Some proteoglycans such as perlecan, have pro- and anti-angiogenic activities, whereas other proteoglycans, such as syndecans and glypicans, can also directly affect cancer growth by modulating key signalling pathways. The bioactivity of these proteoglycans is further modulated by several classes of enzymes within the tumour microenvironment: (i) sheddases that cleave transmembrane or cell-associated syndecans and glypicans, (ii) various proteinases that cleave the protein core of pericellular proteoglycans and (iii) heparanases and endosulfatases which modify the structure and bioactivity of various heparan sulphate proteoglycans and their bound growth factors. In contrast, some of the small leucine-rich proteoglycans, such as decorin and lumican, act as tumour repressors by physically antagonizing receptor tyrosine kinases including the epidermal growth factor and the Met receptors or integrin receptors thereby evoking anti-survival and pro-apoptotic pathways. In this review we will critically assess the expanding repertoire of molecular interactions attributed to various proteoglycans and will discuss novel proteoglycan functions modulating cancer progression, invasion and metastasis and how these factors regulate the tumour microenvironment.

Lymphangiogenesis in post-natal tissue remodeling: lymphatic endothelial cell connection with its environment

The main physiological function of the lymphatic vasculature is to maintain tissue fluid homeostasis. Lymphangiogenesis or de novo lymphatic formation is closely associated with tissue inflammation in adults (i.e. wound healing, allograft rejection, tumor metastasis). Until recently, research on lymphangiogenesis focused mainly on growth factor/growth factor-receptor pathways governing this process. One of the lymphatic vessel features is the incomplete or absence of basement membrane. This close association of endothelial cells with the underlying interstitial matrix suggests that cell-matrix interactions play an important role in lymphangiogenesis and lymphatic functions. However, the exploration of interaction between extracellular matrix (ECM) components and lymphatic endothelial cells is in its infancy. Herein, we describe ECM-cell and cell-cell interactions on lymphatic system function and their modification occurring in pathologies including cancer metastasis.

Role of glomerular proteoglycans in IgA nephropathy

Mesangial matrix expansion is a prominent feature of the most common form of glomerulonephritis, IgA nephropathy (IgAN). To find molecular markers and improve the understanding of the disease, the gene and protein expression of proteoglycans were investigated in biopsies from IgAN patients and correlated to clinical and morphological data. We collected and microdissected renal biopsies from IgAN patients (n = 19) and from healthy kidney donors (n = 14). Patients were followed for an average time of 4 years and blood pressure was according to target guidelines. Distinct patterns of gene expression were seen in glomerular and tubulo-interstitial cells. Three of the proteoglycans investigated were found to be of special interest and upregulated in glomeruli: perlecan, decorin and biglycan. Perlecan gene expression negatively correlated to albumin excretion and progress of the disease. Abundant decorin protein expression was found in sclerotic glomeruli, but not in unaffected glomeruli from IgAN patients or in controls. Transforming growth factor beta (TGF-β), known to interact with perlecan, decorin and biglycan, were upregulated both on gene and protein level in the glomeruli. This study provides further insight into the molecular mechanisms involved in mesangial matrix expansion in IgAN. We conclude that perlecan is a possible prognostic marker for patients with IgAN. In addition, the up-regulation of biglycan and decorin, as well as TGF-β itself, indicate that regulation of TGF-β, and other profibrotic markers plays a role in IgAN pathology.

Hyaluronan: From Biomimetic to Industrial Business Strategy

Hyaluronan (hyaluronic acid) is a naturally occurring polysaccharide of a linear repeating disaccharide unit consisting of β-(1→4)-linked D-glucopyranuronic acid and β-(1→3)-linked 2-acetamido-2-deoxy-D-glucopyranose, which is present in extracellular matrices, the synovial fluid of joints, and scaffolding that comprises cartilage.

In its mechanism of synthesis, its size, and its physico-chemical properties, hyaluronan is unique amongst other glycosaminoglycans. The network-forming, viscoelastic and its charge characteristics are important to many biochemical properties of living tissues. It is an important pericellular and cell surface constituent; its interaction with other macromolecules such as proteins, participates in regulating cell behavior during numerous morphogenic, restorative, and pathological processes in the body. The knowledge of HA in diseases such as various forms of cancers, arthritis and osteoporosis has led to new impetus in research and development in the preparation of biomaterials for surgical implants and drug conjugates for targeted delivery.

A concise and focused review on hyaluronan is timely. This review will cover the following important aspects of hyaluronan: (i) biological functions and synthesis in nature; (ii) current industrial production and potential biosynthetic processes of hyaluronan; (iii) chemical modifications of hyaluronan leading to products of commercial significance; and (iv) and the global market position and manufacturers of hyaluronan.

Malaria in pregnancy: small babies, big problem

Placental malaria is hypothesized to lead to placental insufficiency, which causes fetal growth restriction (FGR). In this review, recent discoveries regarding the mechanisms of pathogenesis by which malaria causes FGR are discussed in the wider context of placental function and fetal growth. Placental malaria and associated host responses can induce changes in placental structure and function, affecting pregnancy-associated growth-regulating hormones and predisposing the offspring to hypertension and vascular dysfunction. Risk factors associated with FGR are highlighted, and potential interventions and studies to uncover remaining mechanisms of pathogenesis are proposed. Together, these strategies aim to decrease the burden of FGR associated with malaria in pregnancy.

Heparan sulfate modification of the transmembrane receptor CD47 is necessary for inhibition of T cell receptor signaling by thrombospondin-1

Cell surface proteoglycans on T cells contribute to retroviral infection, binding of chemokines and other proteins, and are necessary for some T cell responses to the matricellular glycoprotein thrombospondin-1. The major cell surface proteoglycans expressed by primary T cells and Jurkat T cells have an apparent M(r) > 200,000 and are modified with chondroitin sulfate and heparan sulfate chains. Thrombospondin-1 bound in a heparin-inhibitable manner to this proteoglycan and to a soluble form released into the medium. Based on mass spectrometry, knockdown, and immunochemical analyses, the proteoglycan contains two major core proteins as follows: amyloid precursor-like protein-2 (APLP2, apparent M(r) 230,000) and CD47 (apparent M(r) > 250,000). CD47 is a known thrombospondin-1 receptor but was not previously reported to be a proteoglycan. This proteoglycan isoform of CD47 is widely expressed on vascular cells. Mutagenesis identified glycosaminoglycan modification of CD47 at Ser(64) and Ser(79). Inhibition of T cell receptor signaling by thrombospondin-1 was lost in CD47-deficient T cells that express the proteoglycan isoform of APLP2, indicating that binding to APLP2 is not sufficient. Inhibition of CD69 induction was restored in CD47-deficient cells by re-expressing CD47 or an S79A mutant but not by the S64A mutant. Therefore, inhibition of T cell receptor signaling by thrombospondin-1 is mediated by CD47 and requires its modification at Ser(64).

Ephrin-B3 binds to a sulfated cell-surface receptor

The ephrins are a family of proteins known to bind the Eph (erythropoietin-producing hepatocellular) receptor tyrosine kinase family. In the present paper, we provide data showing that ephrin-B3 binds a sulfated cell-surface protein on HEK-293T (human embryonic kidney-293 cells expressing the large T-antigen of simian virus 40) and HeLa cells, a binding that is nearly completely blocked by treatment of these cell lines with chlorate or heparinase, or by addition of the heavily sulfated glycosaminoglycan heparin. This indicates that heparan sulfate on these cells is essential for cell-surface binding of ephrin-B3. Heparin did not affect ephrin-B3 binding to EphB receptors expressed on transfected HEK-293T cells, indicating further that ephrin-B3 binds an alternative receptor which is a heparan sulfate proteoglycan. Site-directed mutagenesis analysis revealed that Arg178 and Lys179 are important for heparin binding of ephrin-B3 and also for ephrin-B3 binding to cells. These amino acids, when introduced in the non-heparin-binding ephrin-B1, conferred the heparin-binding property. Functional studies reveal that ephrin-B3 binding to cells induces cellular signalling and influences cell rounding and cell spreading. In conclusion, our data provide evidence for an unknown ephrin-B3-binding cell-surface proteoglycan involved in cellular signalling.

Signaling pathways in reactive astrocytes, a genetic perspective

Reactive astrocytes are associated with a vast array of central nervous system (CNS) pathologies. The activation of astrocytes is characterized by changes in their molecular and morphological features, and depending on the type of damage can also be accompanied by inflammatory responses, neuronal damage, and in severe cases, scar formation. Although reactive astrogliosis is the normal physiological response essential for containing damage, it can also have detrimental effects on neuronal survival and axon regeneration, particularly in neurodegenerative diseases. It is believed that progressive changes in astrocytes as they become reactive are finely regulated by complex intercellular and intracellular signaling mechanisms. However, these have yet to be sorted out. Much has been learned from gain-of-function approaches in vivo and culture paradigms, but in most cases, loss-of-function genetic studies, which are a critical complementary approach, have been lacking. Understanding which signaling pathways are required to control different aspects of astrogliosis will be necessary for designing therapeutic strategies to improve their beneficial effects and limit their detrimental ones in CNS pathologies. In this article, we review recent advances in the mechanisms underlying the regulation of aspects of astrogliosis, with the main focus on the signaling pathways that have been studied using loss-of-function genetic mouse models.

Efficient secretion of biologically active Chondroitinase ABC from mammalian cells in the absence of an N-terminal signal peptide

Proteoglycans carrying chondroitin sulfate side chains have been shown to fulfill important biological functions in development, disease, and signaling. One area of considerable interest is the functional importance of chondroitin sulfates as inhibitors of the regeneration of axonal projections in the mammalian central nervous system. In animal models of spinal cord injury, injections of the enzyme Chondroitinase ABC from the bacterium Proteus vulgaris into the lesion site leads to degradation of chondroitin sulfates, and promotes axonal regeneration and significant functional recovery. Here, a mammalian expression system of an epitope-tagged Chondroitinase ABC protein is described. It is demonstrated that the addition of a eukaryotic secretion signal sequence to the N-terminus of the bacterial Chondroitinase ABC sequence allowed secretion, but interfered with function of the secreted enzyme. In contrast, expression of the Chondroitinase ABC gene without N-terminal eukaryotic secretion sequence or bacterial hydrophobic leader sequence led to efficient secretion of a biologically active Chondroitinase ABC protein from both immortalized and primary cells. Moreover, the C-terminal epitope tag could be utilized to follow expression of this protein. This novel Chondroitinase ABC gene is a valuable tool for a better understanding of the in vivo roles of chondroitin sulfates in mammalian development and disease, as well as in gene therapy approaches, including the treatment of spinal chord injuries.

Decorin antagonizes Met receptor activity and down-regulates {beta}-catenin and Myc levels

A theme emerging during the past few years is that members of the small leucine-rich proteoglycan gene family affect cell growth by interacting with multiple receptor tyrosine kinases (RTKs), mostly by a physical down-regulation of the receptors, thereby depriving tumor cells of pro-survival signals. Decorin binds and down-regulates several RTKs, including Met, the receptor for hepatocyte growth factor. Here we demonstrate that decorin blocks several biological activities mediated by the Met signaling axis, including cell scatter, evasion, and migration. These effects were mediated by a profound down-regulation of noncanonical β-catenin levels. In addition, Myc, a downstream target of β-catenin, was markedly down-regulated by decorin, whereas phosphorylation of Myc at threonine 58 was markedly induced. The latter is known to destabilize Myc and target it for proteasomal degradation. We also discovered that systemic delivery of decorin using three distinct tumor xenograft models caused down-regulation of Met and a concurrent suppression of β-catenin and Myc levels. We found that decorin protein core labeled with the near infrared dye IR800 specifically targeted the tumor cells expressing Met. Even 68-h post-injection, decorin was found to reside within the tumor xenografts with little or no binding to other tissues. Collectively, our results indicate a role for a secreted proteoglycan in suppressing the expression of key oncogenic factors required for tumor progression.

Pregnancy-specific glycoprotein 1 induces endothelial tubulogenesis through interaction with cell surface proteoglycans

Pregnancy-specific β1 glycoproteins (PSGs) are the most abundant fetal proteins in the maternal bloodstream in late pregnancy. They are secreted by the syncytiotrophoblast and are detected around day 14 postfertilization. There are 11 human PSG genes, which encode a family of proteins exhibiting significant conservation at the amino acid level. We and others have proposed that PSGs have an immune modulatory function. In addition, we recently postulated that they are proangiogenic due to their ability to induce the secretion of VEGF-A and the formation of tubes by endothelial cells. The cellular receptor(s) for human PSGs remain unknown. Therefore, we conducted these studies to identify the receptor for PSG1, the highest expressed member of the family. We show that removal of cell surface glycosaminoglycans (GAGs) by enzymatic or chemical treatment of cells or competition with heparin completely inhibited binding of PSG1. In addition, PSG1 did not bind to cells lacking heparan or chondroitin sulfate on their surface, and binding was restored upon transfection with all four syndecans and glypican-1. Importantly, the presence of GAGs on the surface of endothelial cells was required for the ability of PSG1 to induce tube formation. This finding indicates that the PSG1-GAG interaction mediates at least some of the PSG1 proposed functions.

The proteoglycan biglycan regulates expression of the B cell chemoattractant CXCL13 and aggravates murine lupus nephritis

CXCL13 is a key B cell chemoattractant and marker of disease activity in patients with SLE; however, the mechanism of its induction has not been identified yet. Here, we have shown that the proteoglycan biglycan triggers CXCL13 expression via TLR2/4 in macrophages and dendritic cells. In vivo, levels of biglycan were markedly elevated in the plasma and kidneys of human SLE patients and lupus-prone (MRL/lpr) mice. Overexpression of soluble biglycan in MRL/lpr mice raised plasma and renal levels of CXCL13 and caused accumulation of B cells with an enhanced B1/B cell ratio in the kidney, worsening of organ damage, and albuminuria. Importantly, biglycan also triggered CXCL13 expression and B cell infiltration in the healthy kidney. Conversely, biglycan deficiency improved systemic and renal outcome in lupus-prone mice, with lower levels of autoantibodies, less enlargement of the spleen and lymph nodes, and reduction in renal damage and albuminuria. This correlated with a marked decline in circulating and renal CXCL13 and a reduction in the number of B cells in the kidney. Collectively, our results describe what we believe to be a novel mechanism for the regulation of CXCL13 by biglycan, a host-derived ligand for TLR2/4. Blocking biglycan-TLR2/4 interactions might be a promising strategy for the management of SLE and other B cell-mediated inflammatory disease entities.

Proteoglycans in health and disease: novel regulatory signaling mechanisms evoked by the small leucine-rich proteoglycans

The small leucine-rich proteoglycans (SLRPs) are involved in many aspects of mammalian biology, both in health and disease. They are now being recognized as key signaling molecules with an expanding repertoire of molecular interactions affecting not only growth factors, but also various receptors involved in controlling cell growth, morphogenesis and immunity. The complexity of SLRP signaling and the multitude of affected signaling pathways can be reconciled with a hierarchical affinity-based interaction of various SLRPs in a cell- and tissue-specific context. Here, we review this interacting network, describe new relationships of the SLRPs with tyrosine kinase and Toll-like receptors and critically assess their roles in cancer and innate immunity.

Proteoglycans in health and disease: novel roles for proteoglycans in malignancy and their pharmacological targeting

The expression of proteoglycans (PGs), essential macromolecules of the tumor microenvironment, is markedly altered during malignant transformation and tumor progression. Synthesis of stromal PGs is affected by factors secreted by cancer cells and the unique tumor-modified extracellular matrix may either facilitate or counteract the growth of solid tumors. The emerging theme is that this dual activity has intrinsic tissue specificity. Matrix-accumulated PGs, such as versican, perlecan and small leucine-rich PGs, affect cancer cell signaling, growth and survival, cell adhesion, migration and angiogenesis. Furthermore, expression of cell-surface-associated PGs, such as syndecans and glypicans, is also modulated in both tumor and stromal cells. Cell-surface-associated PGs bind various factors that are involved in cell signaling, thereby affecting cell proliferation, adhesion and motility. An important mechanism of action is offered by a proteolytic processing of cell-surface PGs known as ectodomain shedding of syndecans; this facilitates cancer and endothelial cell motility, protects matrix proteases and provides a chemotactic gradient of mitogens. However, syndecans on stromal cells may be important for stromal cell/cancer cell interplay and may promote stromal cell proliferation, migration and angiogenesis. Finally, abnormal PG expression in cancer and stromal cells may serve as a biomarker for tumor progression and patient survival. Enhanced understanding of the regulation of PG metabolism and the involvement of PGs in cancer may offer a novel approach to cancer therapy by targeting the tumor microenvironment. In this minireview, the implication of PGs in cancer development and progression, as well as their pharmacological targeting in malignancy, are presented and discussed.

Structural and mechanistic classification of uronic acid-containing polysaccharide lyases

Polysaccharide lyases (PLs) have been assigned to 21 families based on their sequences, with ~ 50 singletons awaiting further classification. For 19 of these families, the structure of at least one protein is known. In this review, we have analyzed the available structural information and show that presently known PL families belong to six general folds. Only two general catalytic mechanisms have been observed among these PLs: (1) metal-assisted neutralization of the acidic group of the sugar next to the cleaved bond, with, rather unusually, arginine or lysine playing the role of Brønsted base and (2) neutralization of the acidic group on the sugar by a close approach of an amino or acidic group forcing its protonation and Tyr or Tyr-His acting as the Brønsted base and acid.

Matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs): Positive and negative regulators in tumor cell adhesion

Cells adhere to one another and/or to matrices that surround them. Regulation of cell-cell (intercellular) and cell-matrix adhesion is tightly controlled in normal cells, however, defects in cell adhesion are common in the majority of human cancers. Multilateral communication among tumor cells with the extracellular matrix (ECM) and neighbor cells is accomplished through adhesion molecules, ECM components, proteolytic enzymes and their endogenous inhibitors. There is sufficient evidence to suggest that reduced adherence is a tumor cell property engaged during tumor progression. Tumor cells acquire the ability to change shape, detach and easily move through spaces disorganizing the normal tissue architecture. This property is due to changes in expression levels of adhesion molecules and/or due to elevated levels of secreted proteolytic enzymes, including matrix metalloproteinases (MMPs). Among other roles, MMPs degrade the ECM and, therefore, prepare the path for tumor cells to migrate, invade and spread to distant secondary areas, where they form metastasis. Tissue inhibitors of metalloproteinases or TIMPs control MMP activities and, therefore, minimize matrix degradation. Both MMPs and TIMPs are involved in tissue remodeling and decisively regulate tumor cell progression including tumor angiogenesis. In this review, we describe and discuss data that support the important role of MMPs and TIMPs in cancer cell adhesion and tumor progression.

Matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs): Positive and negative regulators in tumor cell adhesion

Cells adhere to one another and/or to matrices that surround them. Regulation of cell-cell (intercellular) and cell-matrix adhesion is tightly controlled in normal cells, however, defects in cell adhesion are common in the majority of human cancers. Multilateral communication among tumor cells with the extracellular matrix (ECM) and neighbor cells is accomplished through adhesion molecules, ECM components, proteolytic enzymes and their endogenous inhibitors. There is sufficient evidence to suggest that reduced adherence is a tumor cell property engaged during tumor progression. Tumor cells acquire the ability to change shape, detach and easily move through spaces disorganizing the normal tissue architecture. This property is due to changes in expression levels of adhesion molecules and/or due to elevated levels of secreted proteolytic enzymes, including matrix metalloproteinases (MMPs). Among other roles, MMPs degrade the ECM and, therefore, prepare the path for tumor cells to migrate, invade and spread to distant secondary areas, where they form metastasis. Tissue inhibitors of metalloproteinases or TIMPs control MMP activities and, therefore, minimize matrix degradation. Both MMPs and TIMPs are involved in tissue remodeling and decisively regulate tumor cell progression including tumor angiogenesis. In this review, we describe and discuss data that support the important role of MMPs and TIMPs in cancer cell adhesion and tumor progression.

The roles of chondroitin-4-sulfotransferase-1 in development and disease

The glycosaminoglycan chondroitin sulfate (CS) consists of long linear chains of repeating disaccharide units, which are covalently attached to core proteins to form CS-proteoglycans. These molecules have been shown to fulfill important biological functions in development, disease, and signaling. Biosynthesis of CS takes place in the Golgi apparatus. Concomitant to chondroitin chain elongation, sulfation of specific carbon residues by chondroitin sulfotransferase enzymes takes place. The sulfation balance and pattern of CS on specific carbon residues are tightly regulated during development, injury, and disease, with the temporal and spatial expression of chondroitin sulfotransferase genes believed to be a crucial determinant of this fine balance of chondroitin sulfation. Chondroitin-4-sulfotransferase-1 (C4ST-1)/carbohydrate sulfotransferase 11 (CHST11) is one of the enzymes involved in the sulfation of chondroitin by catalyzing the transfer of sulfate groups from a sulfate donor to the carbon-4 position of the N-acetylgalactosamine sugar of the repeating disaccharide units. Here, I summarize the significant recent advances in our understanding of the roles of C4ST-1 in vertebrate development, disease, and signaling pathways, and the transcriptional regulation of the C4ST-1 gene. Proper 4-sulfation of chondroitin by C4ST-1 plays a crucial role in the skeletal development and signaling events, and new evidence is suggestive of a potential role for C4ST-1 in human disease, including cancer.

Auxiliary and autonomous proteoglycan signaling networks

Proteoglycans represent a structurally heterogeneous family of proteins that typically undergo extensive posttranslational modification with sulfated sugar chains. Although historically believed to affect signaling pathways exclusively as growth factor coreceptors, proteoglycans are now understood to initiate and modulate signal transduction cascades independently of other receptors. From within the extracellular matrix, proteoglycans are able to shield protein growth factors from circulating proteases and establish gradients that guide cell migration. Extracellular proteoglycans are also critical in the maintenance of growth factor stores and are thus instrumental in modulating paracrine signaling. At the cell membrane, proteoglycans stabilize ligand-receptor interactions, creating potentiated ternary signaling complexes that regulate cell proliferation, endocytosis, migration, growth factor sensitivity, and matrix adhesion. In some cases, proteoglycans are able to independently activate various signaling cascades, attenuate the signaling of growth factors, or orchestrate multimeric intracellular signaling complexes. Signaling between cells is also modulated by proteoglycan activity at the cell membrane, as exemplified by the proteoglycan requirement for effective synaptogenesis between neurons. Finally, proteoglycans are able to regulate signaling from intracellular compartments, particularly in the context of storage granule formation and maintenance. These proteoglycans are also major determinants of exocytic vesicle fate and other vesicular trafficking pathways. In contrast to the mechanisms underlying classical ligand-receptor signaling, proteoglycan signaling is frequently characterized by ligand promiscuity and low-affinity binding; likewise, these events commonly do not exhibit the same degree of reliance on intermolecular structure or charge configurations as other ligand-receptor interactions. Such unique features often defy conventional mechanisms of signal transduction, and present unique challenges to the study of their indispensable roles within cell signaling networks.

The matricellular functions of small leucine-rich proteoglycans (SLRPs)

The small leucine-rich proteoglycans (SLRPs) are biologically active components of the extracellular matrix (ECM), consisting of a protein core with leucine rich-repeat (LRR) motifs covalently linked to glycosaminoglycan (GAG) side chains. The diversity in composition resulting from the various combinations of protein cores substituted with one or more GAG chains along with their pericellular localization enables SLRPs to interact with a host of different cell surface receptors, cytokines, growth factors, and other ECM components, leading to modulation of cellular functions. SLRPs are capable of binding to: (i) different types of collagens, thereby regulating fibril assembly, organization, and degradation; (ii) Toll-like receptors (TLRs), complement C1q, and tumor necrosis factor-alpha (TNFalpha), regulating innate immunity and inflammation; (iii) epidermal growth factor receptor (EGF-R), insulin-like growth factor receptor (IGF-IR), and c-Met, influencing cellular proliferation, survival, adhesion, migration, tumor growth and metastasis as well as synthesis of other ECM components; (iv) low-density lipoprotein receptor-related protein (LRP-1) and TGF-beta, modulating cytokine activity and fibrogenesis; and (v) growth factors such as bone morphogenic protein (BMP-4) and Wnt-I-induced secreted protein-1 (WISP-1), controlling cell proliferation and differentiation. Thus, the ability of SLRPs, as ECM components, to directly or indirectly regulate cell-matrix crosstalk, resulting in the modulation of various biological processes, aptly qualifies these compounds as matricellular proteins.