A proteoglycan that activates fibroblast growth factors during early neuronal development is a perlecan variant

The ECM as a driver of heart development and repair

The developing heart is formed of two tissue layers separated by an extracellular matrix (ECM) that provides chemical and physical signals to cardiac cells. While deposition of specific ECM components creates matrix diversity, the cardiac ECM is also dynamic, with modification and degradation playing important roles in ECM maturation and function. In this Review, we discuss the spatiotemporal changes in ECM composition during cardiac development that support distinct aspects of heart morphogenesis. We highlight conserved requirements for specific ECM components in human cardiac development, and discuss emerging evidence of a central role for the ECM in promoting heart regeneration.

Diabetes enhances the efficacy of AAV2 vectors in the retina: therapeutic effect of AAV2 encoding vasoinhibin and soluble VEGF receptor 1

Adeno-associated virus (AAV) vector-mediated delivery of inhibitors of blood-retinal barrier breakdown (BRBB) offers promise for the treatment of diabetic macular edema. Here, we demonstrated a reversal of blood-retinal barrier pathology mediated by AAV type 2 (AAV2) vectors encoding vasoinhibin or soluble VEGF receptor 1 (sFlt-1) when administered intravitreally to diabetic rats. Efficacy and safety of the AAV2 vasoinhibin vector were tested by monitoring its effect on diabetes-induced changes in the retinal vascular bed and thickness, and in the electroretinogram (ERG). Also, the transduction of AAV2 vectors and expression of AAV2 receptors and co-receptors were compared between the diabetic and the non-diabetic rat retinas. AAV2 vasoinhibin or AAV2 sFlt-1 vectors were injected intravitreally before or after enhanced BRBB due to diabetes induced by streptozotocin. The BRBB was examined by the Evans blue method, the vascular bed by fluorescein angiography, expression of the AAV2 EGFP reporter vector by confocal microscopy, and the AAV2 genome, expression of transgenes, receptors, and co-receptors by quantitative PCR. AAV2 vasoinhibin and sFlt-1 vectors inhibited the diabetes-mediated increase in BRBB when injected after, but not before, diabetes was induced. The AAV2 vasoinhibin vector decreased retinal microvascular abnormalities and the diabetes-induced reduction of the B-wave of the ERG, but it had no effect in non-diabetic controls. Also, retinal thickness was not altered by diabetes or by the AAV2 vasoinhibin vector. The AAV2 genome, vasoinhibin and sFlt-1 transgenes, and EGFP levels were higher in the retinas from diabetic rats and were associated with an elevated expression of AAV2 receptors (syndecan, glypican, and perlecan) and co-receptors (fibroblast growth factor receptor 1, αvβ5 integrin, and hepatocyte growth factor receptor). We conclude that retinal transduction and efficacy of AAV2 vectors are enhanced in diabetes, possibly due to their elevated cell entry. AAV2 vectors encoding vasoinhibin and sFlt-1 may be desirable gene therapeutics to target diabetic retinopathy and macular edema.

Neural stem cell survival factors

Neural stem and progenitor cells (NSCs and NPs) give rise to the central nervous system (CNS) during embryonic development. NSCs and NPs differentiate into three main cell-types of the CNS; astrocytes, oligodendrocytes, and neurons. NSCs are present in the adult CNS and are important in maintenance and repair. Adult NSCs hold great promise for endogenous or self-repair of the CNS. Intriguingly, NSCs have been implicated as the cells that give rise to brain tumors. Thus, the balance between survival, growth and differentiation is a critical aspect of NSC biology, during development, in the adult, and in disease processes. In this review, we survey what is known about survival factors that control both embryonic and adult NSCs. We discuss the neurosphere culture system as this is widely used to measure NSC activity and behavior in vitro and emphasize the importance of clonality. We define here NSC survival factors in their broadest sense to include any factor that influences survival and proliferation of NSCs and NPs. NSC survival factors identified to date include growth factors, morphogens, proteoglycans, cytokines, hormones, and neurotransmitters. Understanding NSC and NP interaction in response to these survival factors will provide insight to CNS development, disease and repair.

Mast cells produce novel shorter forms of perlecan that contain functional endorepellin: a role in angiogenesis and wound healing

Mast cells are derived from hematopoietic progenitors that are known to migrate to and reside within connective and mucosal tissues, where they differentiate and respond to various stimuli by releasing pro-inflammatory mediators, including histamine, growth factors, and proteases. This study demonstrated that primary human mast cells as well as the rat and human mast cell lines, RBL-2H3 and HMC-1, produce the heparan sulfate proteoglycan, perlecan, with a molecular mass of 640 kDa as well as smaller molecular mass species of 300 and 130 kDa. Utilizing domain-specific antibodies coupled with N-terminal sequencing, it was confirmed that both forms contained the C-terminal module of the protein core known as endorepellin, which were generated by mast cell-derived proteases. Domain-specific RT-PCR experiments demonstrated that transcripts corresponding to domains I and V, including endorepellin, were present; however, mRNA transcripts corresponding to regions of domain III were not present, suggesting that these cells were capable of producing spliced forms of the protein core. Fractions from mast cell cultures that were enriched for these fragments were shown to bind endothelial cells via the α(2)β(1) integrin and stimulate the migration of cells in "scratch assays," both activities of which were inhibited by incubation with either anti-endorepellin or anti-perlecan antibodies. This study shows for the first time that mast cells secrete and process the extracellular proteoglycan perlecan into fragments containing the endorepellin C-terminal region that regulate angiogenesis and matrix turnover, which are both key events in wound healing.

Proliferative effects on neural stem/progenitor cells of a sulfated polysaccharide purified from the sea cucumber Stichopus japonicus

To test the effects of a sulfated polysaccharide, Haishen (HS) on the viability and proliferation of neural stem/progenitor cells (NSPCs), we isolated the polysaccharide from the body wall of the sea cucumber Stichopus japonicus by enzymolysis extraction, anion-exchange and gel-permeation chromatography. HS is a highly sulfated fucoidan with a molecular weight of 4.23x10(5) Da. Due to its safety being of invertebrate origin they are less likely to contain infectious agents, the effects of HS on the viability and proliferation of NSPCs in vitro were examined by MTT assay, BrdU labeling and neurosphere formation assay, respectively. Our results showed that HS alone increased NSPC viability in a dose-dependent manner. Moreover, HS acted synergistically with fibroblast growth factor-2 (FGF-2) but not epidermal growth factor (EGF) to enhance the proliferation of NSPCs. Finally, HS did not induce apoptosis of NSPCs. Our findings suggest that HS can serve as an adjuvant for promoting the proliferation of NSPCs.

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.

Dynamic expression patterns of ECM molecules in the developing mouse olfactory pathway

Olfactory sensory neuron (OSN) axons follow stereotypic spatio-temporal paths in the establishment of the olfactory pathway. Extracellular matrix (ECM) molecules are expressed early in the developing pathway and are proposed to have a role in its initial establishment. During later embryonic development, OSNs sort out and target specific glomeruli to form precise, complex topographic projections. We hypothesized that ECM cues may help to establish this complex topography. The aim of this study was to characterize expression of ECM molecules during the period of glomerulogenesis, when synaptic contacts are forming. We examined expression of laminin-1, perlecan, tenascin-C, and CSPGs and found a coordinated pattern of expression of these cues in the pathway. These appear to restrict axons to the pathway while promoting axon outgrowth within. Thus, ECM molecules are present in dynamic spatio-temporal positions to affect OSN axons as they navigate to the olfactory bulb and establish synapses.

A novel use of TAT-EGFP to validate techniques to alter osteosarcoma cell surface glycosaminoglycan expression

Several methods to alter cell surface glycosaminoglycan (GAG) expression have previously been described, including treatments with chlorate to reduce the addition of charged sulfate groups, xyloside compounds to displace GAGs from their core proteins, and GAG lyases, such as heparinase and chondroitinase, to release GAG fragments from the cell layer. While these methods are useful in identifying cellular mechanisms which are dependent on GAGs, they must be stringently validated to assess results in the appropriate context. To determine the most useful technique for the evaluation of GAG function in osteogenesis, MG-63 osteosarcoma cells were systematically treated with these agents and evaluated for changes in cell surface GAGs using a TAT-EGFP fusion protein. TAT, a protein transduction domain from the HIV-1 virus, requires cell surface GAGs to traverse cell membranes. The EGFP component provides a method to assess protein entry into cells in both qualitative and quantitative tests. Here, TAT-EGFP transduction analysis confirmed radiochemical and physiological data that chlorate effectively disrupts GAG expression. TAT-EGFP entry into cells was also inhibited by the exogenous application of commercial heparin and GAGs extracted from MG-63 cells as well as by the pre-treatment of cells with chondroitinase ABC. However, neither heparinase III treatment nor the addition of exogenous chondroitin-6-sulfate affected TAT-EGFP entry into cells. In addition, xyloside-beta-D-naphthol and xyloside-beta-D-cis/trans-decahydro-2-naphthol treatment could not induce significant phenotypic change in these cells, and the unaffected TAT-EGFP transduction confirmed that this was due to an inability to efficiently prime GAG synthesis. The use of TAT-EGFP is thus a useful technique to specifically evaluate cell surface GAG expression in a simple, quantifiable manner, and avoids the complications involved with conventional radiochemical assays or analytical chromatography.

The expression and functions of glycoconjugates in neural stem cells

The mammalian central nervous system is organized by a variety of cells such as neurons and glial cells. These cells are generated from a common progenitor, the neural stem cell (NSC). NSCs are defined as undifferentiated neural cells that are characterized by their high proliferative potential while retaining the capacity for self-renewal and multipotency. Glycoconjugates carrying carbohydrate antigens, including glycoproteins, glycolipids, and proteoglycans, are primarily localized on the plasma-membrane surface of cells and serve as excellent biomarkers at various stages of cellular differentiation. Moreover, they also play important functional roles in determining cell fate such as self-renewal, proliferation, and differentiation. In the present review, we discuss the expression pattern and possible functions of glycoconjugates and carbohydrate antigens in NSCs, with an emphasis on stage-specific embryonic antigen-1, human natural killer antigen-1, polysialic acid-neural cell-adhesion molecule, prominin-1, gp130, chondroitin sulfate proteoglycans, heparan sulfate proteoglycans, cystatin C, galectin-1, glycolipids, and Notch.

Spatial and Temporal Expression of Perlecan in the Early Chick Embryo

Perlecan is a major heparan sulfate proteoglycan that binds growth factors and interacts with various extracellular matrix proteins and cell surface molecules. The expression and spatiotemporal distribution of perlecan was studied by RT-PCR, immunoprecipitation and immunofluorescence in the chick embryo from stages X (morula) to HH17 (29 somites). Combined RT-PCR and immunohistochemistry demonstrated the expression of perlecan as early as stage X and its presence may be fundamental to the first basement membrane assembly on the epiblast ventral surface at stage XIII (blastula). Perlecan fluorescence was intense in the cells ingressing through the primitive streak and was strong lining the epiblast ventral surface lateral to the streak at stage HH3-4 (gastrula). At stage HH5-6 (neurula), perlecan fluorescence was low in the neuroepithelium and stronger in the apical surface of the neural plate. At stage HH10-11 (12 somites), perlecan fluorescence was intense in the neuroepithelium and was then essentially nondetectable in the neuroepithelium, and the intensity had shifted to the basement membranes of encephalic vesicles by stage HH17. Perlecan immunofluorescence was intense in neural crest cells, strong in pharyngeal arches, intense in thymus and lung rudiments, intense in aortic arches and in dorsal aorta, strong in lens and retina and intense in intraretinal space and in optic stalk, strong in the dorsal mesocardium, myocardium and endocardium, strong in dermomyotome, low in sclerotome in somites, intense in mesonephric duct and tubule rudiments, intense in the lining of the gut luminal surface. Inhibition of the function of perlecan by blocking antibodies showed that perlecan is crucial for maintaining basement membrane integrity which mediates the epithelialization, adhesive separation and maintenance of neuroepithelium in brain, somite epithelialization, and tissue architecture during morphogenesis of the heart tube, dorsal aorta and gut. An intriguing possibility is that perlecan, as a signaling molecule that modulates the activity of growth factors and cytokines, participates in the signaling pathways that guide gastrulation movements and neural crest cell migration, proliferation and survival, cardiac cell proliferation and paraxial mesoderm (somitic) cell proliferation and segmentation.

Heparan sulfate regulation of progenitor cell fate

Currently there is an intense effort being made to elucidate the factors that control stem and progenitor cell fate. Developments in our understanding of the FGF/FGFR pathway and its role as an effector of stem cell pluripotency have heightened expectations that a therapeutic use for stem cells will move from a possibility to a probability. Mounting evidence is revealing the molecular mechanisms by which fibroblast growth factor (FGF) signaling, together with a large number of other growth and adhesive factors, is controlled by the extracellular sugar, heparan sulfate (HS). What has resulted is a novel means of augmenting and thus regulating the growth factor control of stem and progenitor cell fate. Here, we review the numerous bioactivities of HS, and the development of strategies to implement HS-induced control of cell fate decisions.

Selective assembly of fibulin-1 splice variants reveals distinct extracellular matrix networks and novel functions for perlecan/UNC-52 splice variants

Fibulin-1C and fibulin-1D splice variants have been conserved throughout metazoan evolution and have distinct functions in Caenorhabditis elegans development. Both splice variants are required for the assembly of hemidesmosome-mediated mechanosensory neuron and uterine attachments, although the molecular associations that underlie their distinct functions at these locations are not known. Here, we show that the assembly of fibulin-1C and fibulin-1D splice variants at these anchorages is dependent upon distinct components of the extracellular matrix (ECM): Fibulin-1D assembly at uterine and mechanosensory neurons attachments is dependent upon a perlecan/ UNC-52 splice variant that includes alternately spliced IG8-IG10, whereas the assembly of fibulin-1C at mechanosensory neuron attachments is dependent upon laminin/ EPI-1. These data not only indicate that fibulin-1C and fibulin-1D are components of distinct networks of ECM but also demonstrates a novel function for a major class of perlecan splice variants found in C. elegans and mouse. In addition, we demonstrate that overexpression of another ECM protein, collagen XVIII, can suppress gonad morphogenesis defects associated with loss of fibulin-1C, suggesting that some genetic defects that result in a weakened basement membrane can be compensated by overexpression of genes for ECM components that stabilize basement membranes.

FGF2 plays a key role in embryonic cerebrospinal fluid trophic properties over chick embryo neuroepithelial stem cells

During early stages of brain development, neuroepithelial stem cells undergo intense proliferation as neurogenesis begins. Fibroblast growth factor 2 (FGF2) has been involved in the regulation of these processes, and although it has been suggested that they work in an autocrine-paracrine mode, there is no general agreement on this because the behavior of neuroepithelial cells is not self-sufficient in explants cultured in vitro. In this work, we show that during early stages of development in chick embryos there is another source of FGF2, besides that of the neuroepithelium, which affects the brain primordium, since the cerebrospinal fluid (E-CSF) contains several isoforms of this factor. We also demonstrate, both in vitro and in vivo, that the FGF2 from the E-CSF has an effect on the regulation of neuroepithelial cell behavior, including cell proliferation and neurogenesis. In order to clarify putative sources of FGF2 in embryonic tissues, we detected by in situ hybridization high levels of mRNA expression in notochord, mesonephros and hepatic primordia, and low levels in brain neuroectoderm, corroborated by semiquantitative PCR analysis. Furthermore, we show that the notochord segregates several FGF2 isoforms which modify the behavior of the neuroepithelial cells in vitro. In addition, we show that the FGF2 ligand is present in the embryonic serum; and, by means of labeled FGF2, we prove that this factor passes via the neuroepithelium from the embryonic serum to the E-CSF in vivo. Considering all these results, we propose that, in chick embryos, the behavior of brain neuroepithelial stem cells at the earliest stages of development is influenced by the action of the FGF2 contained within the E-CSF which could have an extraneural origin, thus suggesting a new and complementary way of regulating brain development.

Identification and Functions of Chondroitin Sulfate in the Milieu of Neural Stem Cells

The behavior of cells is generally considered to be regulated by environmental factors, but the molecules in the milieu of neural stem cells have been little studied. We found by immunohistochemistry that chondroitin sulfate (CS) existed in the surroundings of nestin-positive cells or neural stem/progenitor cells in the rat ventricular zone of the telencephalon at embryonic day 14. Brain-specific chondroitin sulfate proteoglycans (CSPGs), including neurocan, phosphacan/receptor-type protein-tyrosine phosphatase beta, and neuroglycan C, were detected in the ventricular zone. Neurospheres formed by cells from the fetal telencephalon also expressed these CSPGs and NG2 proteoglycan. To examine the structural features and functions of CS polysaccharides in the milieu of neural stem cells, we isolated and purified CS from embryonic day 14 telencephalons. The CS preparation consisted of two fractions differing in size and extent of sulfation: small CS polysaccharides with low sulfation and large CS polysaccharides with high sulfation. Interestingly, both CS polysaccharides and commercial preparations of dermatan sulfate CS-B and an E-type of highly sulfated CS promoted the fibroblast growth factor-2-mediated proliferation of neural stem/progenitor cells. None of these CS preparations promoted the epidermal growth factor-mediated neural stem cell proliferation. These results suggest that these CSPGs are involved in the proliferation of neural stem cells as a group of cell microenvironmental factors.

HB-GAM inhibits proliferation and enhances differentiation of neural stem cells

Proliferation of neural stem cells in the embryonic cerebral cortex is regulated by many growth factors and their receptors. Among the key molecules stimulating stem cell proliferation are FGF-2 and the FGF receptor-1. This ligand-receptor system is highly dependent on the surrounding heparan sulfates. We have found that heparin-binding growth-associated molecule (HB-GAM, also designated as pleiotrophin) regulates neural stem cell proliferation in vivo and in vitro. Deficiency of HB-GAM results in a pronounced, up to 50% increase in neuronal density in the adult mouse cerebral cortex. This phenotype arises during cortical neurogenesis, when HB-GAM knockout embryos display an enhanced proliferation rate as compared to wild-type embryos. Further, our in vitro studies show that exogenously added HB-GAM inhibits formation and growth of FGF-2, but not EGF, stimulated neurospheres, restricts the number of nestin-positive neural stem cells, and inhibits FGF receptor phosphorylation. We propose that HB-GAM functions as an endogenous inhibitor of FGF-2 in stem cell proliferation in the developing cortex.

Decorin is one of the proteoglycans expressed in Walker 256 rat mammary carcinoma

Proteoglycan and glycosaminoglycan content was analyzed in a model of rat mammary carcinoma to study the roles of these compounds in tumorigenesis. Hyaluronic acid and proteoglycans bearing chondroitin and/or dermatan sulfate chains were detected in solid tumors obtained after subcutaneous inoculation of Walker 256 rat carcinoma cells. About 10% of sulfated glycosaminoglycan chains corresponded to heparan sulfate. The small leucine-rich proteoglycan, decorin, was identified as one of the proteoglycans, in addition to others of higher molecular weight, by cross-reaction with an antiserum raised against pig laryngeal decorin and by N-terminal amino acid sequencing. Decorin was separated from other proteoglycans by hydrophobic chromatography and its complete structure was determined. It has a molecular weight of about 85 kDa and a dermatan chain of 45 kDa with 4-sulfated disaccharides. After degradation of the glycosaminoglycan chain, three core proteins of different molecular weight (36, 46 and 56 kDa) were identified. The presence of hyaluronic acid and decorin has been reported in a variety of tumors and tumor cells. In the Walker 256 mammary carcinoma model, hyaluronic acid may play an important role in tumor progression, since it provides a more hydrated extracellular matrix. On the other hand, decorin, which is expressed by stromal cells, represents a host defense response to tumor growth.

Localisation of specific heparan sulfate proteoglycans during the proliferative phase of brain development

Early brain development is characterised by the proliferation of neural precursor cells. Several families of signalling molecules such as the fibroblast growth factors (FGFs) and Wnts are known to play important roles in this early phase of brain development. Accumulating evidence demonstrates that signalling of these molecules requires the presence of heparan sulfate chains attached to a proteoglycan core protein (HSPG). However, the specific identity of the HSPG components in the developing brain is unknown. To determine which HSPGs might be involved at this early phase, we analysed the expression of the major cell surface HSPG families in the developing brain at a time of most active proliferation. Syndecan-1 and glypican-4 were the most highly expressed in the developing brain during the time of peak proliferation and localise to ventricular regions of the brain, where the precursor cells are proliferating. Syndecan-4, although less abundant, also localises to cells in the ventricular zone. We have also examined HSPG involvement in brain development using cultures of embryonic neural precursor cells. We find that FGF2 stimulation of proliferation is inhibited in the presence of sodium chlorate, an inhibitor of heparan sulfate synthesis, and is rescued by addition of exogenous heparan sulfate. These data support a requirement for heparan sulfate in FGF signalling for proliferation of brain precursor cells. The expression of these specific HSPGs within the proliferative zone of the brain suggests that they may be involved in regulation of early brain development, such as FGF-stimulated proliferation.

The heparan sulfate proteoglycan agrin modulates neurite outgrowth mediated by FGF-2

Although the role of agrin in the formation of the neuromuscular junction is well established, other functions for agrin have remained elusive. The present study was undertaken to assess the role of agrin in neurite outgrowth mediated by the heparin-binding growth factor basic fibroblast growth factor (FGF-2), which we have shown previously to bind to agrin with high affinity and that has been shown to mediate neurite outgrowth from a number of neuronal cell types. Using both an established neuronal cell line, PC12 cells, and primary chick retina neuronal cultures, we find that agrin potentiates the ability of FGF-2 to stimulate neurite outgrowth. In PC12 cells and retinal neurons agrin increases the efficacy of FGF-2 stimulation of neurite outgrowth mediated by the FGF receptor, as an inhibitor of the FGF receptor abolished neurite outgrowth in the presence of agrin and FGF-2. We also examined possible mechanisms by which agrin may modulate neurite outgrowth, analyzing ERK phosphorylation and c-fos phosphorylation. These studies indicate that agrin augments a transient early phosphorylation of ERK in the presence of FGF-2, and augments and sustains FGF-2 mediated increases in c-fos phosphorylation. These data are consistent with established mechanisms where heparan sulfate proteoglycans such as agrin may increase the affinity between FGF-2 and the FGF receptor. In summary, our studies suggest that neural agrin contributes to the establishment of axon pathways by modulating the function of neurite promoting molecules such as FGF-2.

Perlecan protein core interacts with extracellular matrix protein 1 (ECM1), a glycoprotein involved in bone formation and angiogenesis

The goal of this study was to discover novel partners for perlecan, a major heparan sulfate proteoglycan of basement membranes, and to examine new interactions through which perlecan may influence cell behavior. We employed the yeast two-hybrid system and used perlecan domain V as bait to screen a human keratinocyte cDNA library. Among the strongest interacting clones, we isolated a approximately 1.6-kb cDNA insert that encoded extracellular matrix protein 1 (ECM1), a secreted glycoprotein involved in bone formation and angiogenesis. The sequencing of the clone revealed the existence of a novel splice variant that we name ECM1c. The interaction was validated by co-immunoprecipitation studies, using both cell-free systems and mammalian cells, and the specific binding site within each molecule was identified employing various deletion mutants. The C terminus of ECM1 interacted specifically with the epidermal growth factor-like modules flanking the LG2 subdomain of perlecan domain V. Perlecan and ECM1 were also co-expressed by a variety of normal and transformed cells, and immunohistochemical studies showed a partial expression overlap, particularly around dermal blood vessels and adnexal epithelia. ECM1 has been shown to regulate endochondral bone formation, stimulate the proliferation of endothelial cells, and induce angiogenesis. Similarly, perlecan plays an important role in chondrogenesis and skeletal development, as well as harboring pro- and anti-angiogenic activities. Thus, a physiological interaction could also occur in vivo during development and in pathological events, including tissue remodeling and tumor progression.

Syndecan-3 and perlecan are differentially expressed by progenitors and mature oligodendrocytes and accumulate in the extracellular matrix

Oligodendrocyte progenitors originate in the subventricular zone, proliferate, migrate to their final destinations, differentiate, and interact with axons to produce multilamellar myelin sheaths. These processes are regulated by a variety of environmental signals, including growth factors, the extracellular matrix, and adhesion molecules. Heparan sulfate proteoglycans are premier candidates as participants in this regulation by virtue of their structural diversity and their capacity to function as coreceptors for both growth factors and extracellular matrix molecules. Consistently with this, we have previously shown that oligodendrocyte progenitors are unable to proliferate in response to fibroblast growth factor-2 (FGF-2) in the absence of sulfated heparan sulfate proteoglycan. Here we show that members of three families of heparan sulfate proteoglycans, syndecan, perlecan, and glypican, are developmentally and posttranscriptionally regulated during oligodendrocyte-lineage progression: Syndecan-3 is synthesized by oligodendrocyte progenitors (but not terminally differentiated oligodendrocytes) and is up-regulated by FGF-2; perlecan synthesis increases as oligodendrocytes undergo terminal differentiation; glypican-1 is expressed by both progenitors and differentiated oligodendrocytes. Astrocytes express glypican-1 and perlecan but not syndecan-3. All three of these heparan sulfate proteoglycans are shed from the cell surface and bind to specific substrates. The developmentally regulated expression of these heparan sulfate proteoglycans is indicative of their participation in events involving growth factor receptors and the extracellular matrix that may regulate oligodendrocyte progenitor proliferation, migration, and adhesion phenomena.

Hyperplastic conotruncal endocardial cushions and transposition of great arteries in perlecan-null mice

Perlecan is a heparan-sulfate proteoglycan abundantly expressed in pericellular matrices and basement membranes during development. Inactivation of the perlecan gene in mice is lethal at two developmental stages: around E10 and around birth. We report a high incidence of malformations of the cardiac outflow tract in perlecan-deficient embryos. Complete transposition of great arteries was diagnosed in 11 out of 15 late embryos studied (73%). Three of these 11 embryos also showed malformations of semilunar valves. Mesenchymal cells in the outflow tract were abnormally abundant in mutant embryos by E9.5, when the endocardial-mesenchymal transformation starts in wild-type embryos. At E10.5, mutant embryos lacked well-defined spiral endocardial ridges, and the excess of mesenchymal cells obstructed sometimes the outflow tract lumen. Most of this anomalous mesenchyme expressed the smooth muscle cell-specific alpha-actin isoform, a marker of the neural crest in the outflow tract of the mouse. In wild-type embryos, perlecan is present in the basal surface of myocardium and endocardium, as well as surrounding presumptive neural crest cells. We suggest that the excess of mesenchyme at the earlier stages of conotruncal development precludes the formation of the spiral ridges and the rotation of the septation complex in order to achieve a concordant ventriculoarterial connection. The observed mesenchymal overpopulation might be due to an uncontrolled migration of neural crest cells, which would arrive prematurely to the heart. Thus, perlecan is involved in the control of the outflow tract mesenchymal population size, underscoring the importance of the extracellular matrix in cardiac morphogenesis.

Extracellular matrix in development of the early embryo

The extracellular matrix interacts with cells and promotes and regulates cellular functions such as adhesion, migration, proliferation, differentiation, and morphogenesis. Extracellular molecules are linked to one another by multiple binding domains and form a stable, multifunctional matrix. Cells respond to the extracellular matrix through plasma membrane receptors, which include integrin and non-integrin receptors. The regulation of these interactions requires the coordination of a multiplicity of signals both spatially and temporally.

Heparan sulfate proteoglycans in the nervous system: their diverse roles in neurogenesis, axon guidance, and synaptogenesis

Development of the mammalian nervous system involves generation of neurons from neural stem cells, migration of generated neurons toward genetically determined locations, extension of axons and dendrites, and establishment of neuronal connectivity. Recent progresses revealed diverse role of heparan sulfate proteoglycans in these processes. This article reviews our current knowledge about the functional roles of heparan sulfate proteoglycans in three critical events in mammalian neural development, namely neurogenesis, axon guidance, and synapse development.

Fibroblast growth factor-binding protein is a novel partner for perlecan protein core

Perlecan, a widespread heparan sulfate proteoglycan, functions as a bioactive reservoir for growth factors by stabilizing them against misfolding or proteolysis. These factors, chiefly members of the fibroblast growth factor (FGF) gene family, are coupled to the N-terminal heparan sulfate chains, which augment high affinity binding and receptor activation. However, rather little is known about biological partners of the protein core. The major goal of this study was to identify novel proteins that interact with the protein core of perlecan. Using the yeast two-hybrid system and domain III of perlecan as bait, we screened approximately 0.5 10(6) cDNA clones from a keratinocyte library and identified a strongly interactive clone. This cDNA corresponded to FGF-binding protein (FGF-BP), a secreted protein previously shown to bind acidic and basic FGF and to modulate their activities. Using a panel of deletion mutants, FGF-BP binding was localized to the second EGF repeat of domain III, a region very close to the binding site for FGF7. FGF-BP could be coimmunoprecipitated with an antibody against perlecan and bound in solution to recombinant domain III-alkaline phosphatase fusion protein. Immunohistochemical analyses revealed colocalization of FGF-BP and perlecan in the pericellular stroma of various squamous cell carcinomas suggesting a potential in vivo interaction. Thus, FGF-BP should be considered a novel biological ligand for perlecan, an interaction that could influence cancer growth and tissue remodeling.

Glypican-4 is an FGF2-binding heparan sulfate proteoglycan expressed in neural precursor cells

FGF2 is a crucial mitogen for neural precursor cells in the developing cerebral cortex. Heparan sulfate proteoglycans (HSPGs) are thought to play a role in cortical neurogenesis by regulating the action of FGF2 on neural precursor cells. In this article, we present data indicating that glypican-4 (K-glypican), a GPI-anchored cell surface HSPG, is involved in these processes. In the developing mouse brain, glypican-4 mRNA is expressed predominantly in the ventricular zone of the telencephalon. Neither the outer layers of the telencephalic wall nor the ventricular zone of other parts of the developing brain express significant levels of glypican-4, with the exception of the ventricular zone of the tectum. In cultures of E13 rat cortical precursor cells, glypican-4 is expressed in cells immunoreactive for nestin and the D1.1 antigen, markers of neural precursor cells. Glypican-4 expression was not detected in early postmitotic or fully differentiated neurons. Recombinant glypican-4 produced in immortalized neural precursor cells binds FGF2 through its heparan sulfate chains and suppressed the mitogenic effect of FGF2 on E13 cortical precursor cells. The spatiotemporal expression pattern of glypican-4 in the developing cerebral wall significantly overlaps with that of FGF2. These results suggest that glypican-4 plays a critical role in the regulation of FGF2 action during cortical neurogenesis.

Nervous system proteoglycans as modulators of neurite outgrowth

The proteoglycans are multifunctional macromolecules composed of a core polypeptide and a variable number of glycosaminoglycan chains. The structural diversity and complexities of proteoglycan expression in the developing and adult Nervous System underlies the variety of biological functions that these molecules fulfill. Thus, in the Nervous System, proteoglycans regulate the structural organisation of the extracellular matrix, modulate growth factor activities and cellular adhesive and motility events, such as cell migration and axon outgrowth. This review summarises the evidences indicating that proteoglycans have an important role as modulators of neurite outgrowth and neuronal polarity. Special emphasis will be placed on those studies that have shown that proteoglycans of certain subtypes inhibit neurite extension either during the development and/or the regeneration of the vertebrate Central Nervous System.

Differential ability of heparan sulfate proteoglycans to assemble the fibroblast growth factor receptor complex in situ

Fibroblast growth factors (FGFs) require heparan sulfate proteoglycans (HSPGs) as cofactors for signaling. The heparan sulfate chains (HS) mediate stable high affinity binding of FGFs to their receptor tyrosine kinases (FR) and may specifically regulate FGF activity. A novel in situ binding assay was developed to examine the ability of HSPGs to promote FGF/FR binding using a soluble FR fusion construct (FR1-AP). This fusion protein probe forms a dimer in solution, simulating the dimerization or oligomerization that is thought to occur at the cell surface physiologically. In frozen sections of human skin, FGF-2 binds to keratinocytes and basement membranes of epidermis and dermal blood vessels. In contrast, in skin preincubated with FGF-2, FR1-AP binds avidly to FGF-2 immobilized on keratinocyte cell surfaces, but fails to bind to basement membranes at the dermo-epidermal junction or dermal microvessels despite the fact that these structures bind large amounts of FGF-2. Apparently, basement membrane and cell surface HSPGs differ in their ability to mediate the assembly of a FGF/FR signaling complex presumably due to structural differences of the heparan sulfate chains.

Phenotypic alterations in Kaposi's sarcoma cells by antisense reduction of perlecan

Metastasis is a sequence of events including proliferation, migration, adhesion, invasion and subsequent metastatic growth of tumour cells in distant organs. We previously showed that highly metastatic variants of murine melanoma cells express higher levels of the basement membrane proteoglycan perlecan than low or non metastatic variants and expression of an antisense perlecan can reduce metastatic potential. In contrast, antisense expression of perlecan in fibrosarcoma cells was reported to enhance tumorigenesis. To better understand the role of perlecan in angiogenesis we have transfected KS-IMM, an immortalized cell line derived from a human Kaposi s sarcoma, with an antisense perlecan construct and investigated the positive/negative role of perlecan in KS. KS-IMM cells were transfected with either empty vector (neo) or the antisense perlecan construct and clones were isolated. Immuno-blot analysis showed a reduction of perlecan levels in two (AP3 and AP4) isolated clones, in Northern blot analysis endogenous perlecan was undetectable in the AP3 and AP4 clones, while it was present in the neo control clones. AP clones had a reduced migration to HGF in Boyden chambers as compared to neo clones. Proliferation in low serum or serum-free conditions was strongly reduced in the AP clones as compared to the neo control cells. The neotransfected cells showed rapid proliferation in low serum supplemented with HGF and VEGF, while antisense transfected clones showed little response. Finally, AP-trasfected KS-IMM cells had significantly reduced migration to VEGF and HGF with respect to controls. In contrast, when the AP transfected cells were injected in nude mice they paradoxically showed enhanced tumor growth as compared to controls. Our preliminary data indicate that perlecan reduction plays a crucial role on Kaposi s sarcoma cell migration and proliferation in vitro. However, in vivo KS-IMM depleted of perlecan had a growth advantage. A possible hypothesis is that perlecan is necessary for growth of KS-IMM cells in vitro, however its down-regulation might promote angiogenesis through increased angiogenic growth factor diffusion, resulting in enhanced tumor growth in vivo.

Heparin differentially regulates the interaction of fibroblast growth factor-4 with FGF receptors 1 and 2

Fibroblast growth factor-4 (FGF4), like other FGFs, shares a high affinity for the anionic glycosaminoglycans heparin and heparan sulfate (HS), which in turn enhance FGF-receptor (FGFR) binding and activation. Here we demonstrate using a cell free system that, at low concentrations of heparin, FGF4 binds only to FGFR-2, while much higher heparin levels are required for binding to FGFR-1. Chemical crosslinking of radiolabeled FGF4 to the soluble FGF receptors confirms the preferential formation of FGF4-FGFR-2 complexes under restricted heparin availability, with maximal ligand-receptor interactions at almost 20-fold lower heparin concentrations then those required for the affinity labeling of FGFR-1. In accordance, HS-deficient cells expressing FGFR-2 proliferate in response to FGF4 at extremely low exogenous heparin concentrations, while FGFR-1 expressing cells are completely unresponsive under the same conditions. We suggest that FGFR-2 is the preferred receptor for FGF4 under restricted HS conditions and that the bioavailability of structurally distinct HS motifs may differentially control receptor specificity of FGF4 in vivo.

Heparin is a unique marker of progenitors in the glial cell lineage

The oligodendrocyte-type-2 astrocyte progenitor cells (precursors of oligodendrocytes and type-2 astrocytes) are an excellent system in which to study differentiation as they can be manipulated in vitro. Maintenance of oligodendrocyte-type-2 astrocyte progenitor cells requires basic fibroblast growth factor, a growth factor whose action normally depends on a heparan sulfate coreceptor. Biochemical analysis revealed a most surprising result: that the oligodendrocyte-type-2 astrocyte progenitors did not synthesize heparan sulfate, the near ubiquitous N-sulfated cell surface polysaccharide, but the chemically related heparin in a form that was almost completely N- and O-sulfated. The heparin was detected in the pericellular fraction of the cells and the culture medium. In contrast the differentiated glial subpopulations (oligodendrocytes and type-2 astrocytes) synthesized typical heparan sulfate but with distinctive fine structural features for each cell type. Thus heparin is a unique differentiation marker in the glial lineage. Previously heparin has been found only in a subset of mature mast cells called the connective tissue mast cells. Its presence within the developing nervous system on a precise population of progenitors may confer specific and essential recognition properties on those cells in relation to binding soluble growth and/or differentiation factors and the extracellular matrix.

Heparan sulphates upregulate regeneration of transected sciatic nerves of adult guinea-pigs

The increased content of soluble glycosaminoglycan-containing forms in sciatic nerves during recovery from crush injury [Shum & Chau (1996) J. Neurosci. Res., 46, 465] suggests that the glycosaminoglycans modulate the environment for post-traumatic tissue remodelling and axonal regrowth. To test this, defined amounts of soluble heparan sulphates from bovine kidney or guinea-pig nerve were introduced into the regenerating environment via silicone conduits that bridged 8-mm gaps of transected sciatic nerves of adult guinea-pigs. Controls were bridged using the phosphate-buffered saline (PBS) vehicle or a chondroition sulphate preparation from whale cartilage. After timed periods of recovery, the animals were assessed for electromyographic signals at the target gastrocnemius muscle to determine the conduction velocity across the bridged nerve. Sections of the bridge were also histologically examined for nerve fibres. Transected sciatic nerves bridged with heparan sulphates or chondroitin sulphate showed earlier stimulated myelination of axons (week 5-6) than PBS-bridged nerves (week 9). Initial electromyographic indication of reconnection with the target was at week 9 post-transection. In the course of 20 weeks, transected sections of the bridge indicated similar numbers of unmyelinated axons irrespective of bridge material, but distinctly higher numbers of myelinated axons in heparan sulphate-bridged nerves than either PBS- or chondroitin sulphate-bridged nerves. At the end of the same period, heparan sulphate-bridged nerves resumed normal conduction velocities, but both PBS- and chondroitin sulphate-bridged nerves remained at 50% of that of the intact contralateral nerves. These results are the first to demonstrate that supplementation of soluble heparan sulphate to the fluid regenerative neural environment can restore functional, axonal reconnection of the severed nerve with the target muscle.

Matrix proteoglycans: from molecular design to cellular function

The proteoglycan superfamily now contains more than 30 full-time molecules that fulfill a variety of biological functions. Proteoglycans act as tissue organizers, influence cell growth and the maturation of specialized tissues, play a role as biological filters and modulate growth-factor activities, regulate collagen fibrillogenesis and skin tensile strength, affect tumor cell growth and invasion, and influence corneal transparency and neurite outgrowth. Additional roles, derived from studies of mutant animals, indicate that certain proteoglycans are essential to life whereas others might be redundant. The review focuses on the most recent genetic and molecular biological studies of the matrix proteoglycans, broadly defined as proteoglycans secreted into the pericellular matrix. Special emphasis is placed on the molecular organization of the protein core, the utilization of protein modules, the gene structure and transcriptional control, and the functional roles of the various proteoglycans. When possible, proteoglycans have been grouped into distinct gene families and subfamilies offering a simplified nomenclature based on their protein core design. The structure-function relationship of some paradigmatic proteoglycans is discussed in depth and novel aspects of their biology are examined.

Regulation of neural stem cell differentiation in the forebrain

In the developing forebrain, mounting evidence suggests that neural stem cell proliferation and differentiation is regulated by growth factors. In vitro in the presence of serum, stem cell proliferation is predominantly mediated by fibroblast growth factor-2 (FGF-2) whereas neuronal differentiation can be triggered by FGF-1 in association with a specific heparan sulphate proteoglycan. On the other hand, astrocyte differentiation in vivo and in vitro appears to be dependent on signalling through the leukaemia inhibitory factor receptor (LIFR). The evidence suggests that in the absence of LIFR signalling, the stem cell population is present at approximately the same frequency and can generate neurons but is blocked from producing astrocytes that express glial fibrillary acidic protein (GFAP) or have trophic functions. The block can be overcome by other growth factors such as BMP-2/4 or interferon-gamma, providing further evidence that the inhibition to astrocyte development does not result from loss of a precursor population. Signalling through the LIFR, in addition to stimulating astrocyte differentiation, may also inhibit neuronal differentiation, which may explain why this receptor is expressed at the earliest stages of neurogenesis. Another signalling system which also exerts its influence on neurogenesis through active inhibition is Delta-Notch. We show in vitro that at high cell densities which impede neuronal production by FGF-1, lowering the levels of expression of the receptor Notch by antisense oligonucleotide results in a significant increase in neuronal production. Thus, stem cell differentiation appears to be dependent on the outcome of interactions between a number of signalling pathways, some which promote specific lineages and some which inhibit.

Differential expression of agrin in renal basement membranes as revealed by domain-specific antibodies

We determined the specificity of two hamster monoclonal antibodies and a sheep polyclonal antiserum against heparan sulfate proteoglycan isolated from rat glomerular basement membrane. The antibodies were characterized by enzyme-linked immunosorbent assay on various basement membrane components and immunoprecipitation with heparan sulfate proteoglycan with or without heparitinase pre-treatment. These experiments showed that the antibodies specifically recognize approximately 150-, 105-, and 70-kDa core proteins of rat glomerular basement membrane heparan sulfate proteoglycan. Recently, we showed that agrin is a major heparan sulfate proteoglycan in the glomerular basement membrane (Groffen, A. J. A., Ruegg, M. A., Dijkman, H. B. P. M., Van der Velden, T. J., Buskens, C. A., van den Born, J., Assmann, K. J. M., Monnens, L. A. H., Veerkamp, J. H., and van den Heuvel, L. P. W. J. (1998) J. Histochem. Cytochem. 46, 19-27). Therefore, we tested whether our antibodies recognize agrin. To this end, we evaluated staining of Chinese hamster ovary cells transfected with constructs encoding full-length or the C-terminal half of rat agrin by analysis on a fluorescence-activated cell sorter. Both hamster monoclonals and the sheep antiserum clearly stained cells transfected with the construct encoding full-length agrin, whereas wild type cells and cells transfected with the construct encoding the C-terminal part of agrin were not recognized. A panel of previously characterized monoclonals, directed against C-terminal agrin, clearly stained cells transfected with either of the constructs but not wild type cells. This indicates that both hamster monoclonals and the sheep antiserum recognize epitopes on the N-terminal half of agrin. By immunohistochemistry on rat renal tissue, we compared distribution of N-terminal agrin with that of C-terminal agrin. The monoclonal antibodies against C-terminal agrin stained almost exclusively the glomerular basement membrane, whereas the anti-N-terminal agrin antibodies recognized all renal basement membranes, including tubular basement membranes. Based on these results, we hypothesize that full-length agrin is predominantly expressed in the glomerular basement membrane, whereas in most other renal basement membranes a truncated isoform of agrin is predominantly found that misses (part of) the C terminus, which might be due to alternative splicing and/or posttranslational processing. The possible significance of this finding is discussed.

The cell surface proteoglycan syndecan-1 mediates fibroblast growth factor-2 binding and activity

Binding of fibroblast growth factors (FGFs) to receptor tyrosine kinases (FGFRs) and signaling is facilitated by binding of FGF to heparan sulfate proteoglycans (HSPGs). There are multiple families of HSPGs, including extracellular and cell surface forms. An important and potentially controversial question is whether cell surface forms of HSPGs act as positive or negative regulators of FGF signaling. This study examines the ability of the cell surface HSPG syndecan-1 to regulate FGF binding and signaling. HSPG-deficient Raji lymphoma cells, expressing a transfected syndecan-1 cDNA (Raji S1 cells), were used as HSPG "donor" cells. BaF3 cells, expressing an FGFR1 cDNA (FR1C-11 cells), were used as FGFR "reporter" cells. Using Raji S1 cells preincubated with FGF, it was found that they formed heterotypic aggregates with FR1C-11 cells in the presence of FGF-2, but not FGF-1. In addition, the FR1C-11 cells demonstrated FGF-2, but not FGF-1, dependent survival when cultured on fixed Raji S1 cells. Thus, Raji syndecan-1 1) differentially regulates the binding and signaling of FGFs 1 and 2 and 2) acts as a positive regulator of FGF-2 signaling.

Structural modification of fibroblast growth factor-binding heparan sulfate at a determinative stage of neural development

Heparan sulfate (HS) glycosaminoglycans are essential modulators of fibroblast growth factor (FGF) activity and appear to act by coupling particular forms of FGF to appropriate FGF receptors. During neural development, one particular HS proteoglycan is able to rapidly switch its potentiating activity from FGF-2, as neural precursor cell proliferation occurs, to FGF-1, as neuronal differentiation occurs. Using various analytical techniques, including chemical and enzymatic cleavage, low pressure chromatography, and strong anion-exchange high performance liquid chromatography, we have analyzed the different HSs expressed during these crucial developmental stages. There are distinct alterations in patterns of 6-O-sulfation, total chain length, and the number of sulfated domains of the HS from the more mature embryonic brain. These changes correlate with a switch in the ability of the HS to potentiate the actions of FGF-1 in triggering cell differentiation. It thus appears that each HS pool is designed to function in the modulation of an intricate interaction with a specific growth factor and its cognate receptor, and suggests tightly regulated expression of specific, bioactive disaccharide sequences. The data can be used to construct a simple model of controlled variations in HS chain structure which have functional consequences at a crucial stage of neuronal maturation.

Essential role of heparan sulfates in axon navigation and targeting in the developing visual system

Heparan sulfate (HS) is abundant in the developing brain and is a required co-factor for many types of fibroblast growth factor (FGF) signaling in vitro. We report that some HSs, when added exogenously to the developing Xenopus optic pathway, severely disrupt target recognition causing axons from the retina to bypass their primary target, the optic tectum. Significantly, HS sidechains from a neuroepithelial perlecan variant that preferentially bind FGF-2, HS(FGF-2), cause aberrant targeting, whereas those that preferentially bind FGF-1 do not. Charge-matched fragments of HS(FGF-2) show that the mistargeting activity associates with the FGF-binding fragments. Heparitinase removal of native HSs at the beginning of optic tract formation retards retinal axon elongation; addition of FGF-2 restores axon extension but axons lose directionality. Late HS removal, after axons have extended through the tract, elicits a tectal bypass phenotype indicating a growth promoting and guidance function for native HSs. Our results demonstrate that different HS sidechains from the same core protein differentially affect axon growth in vivo, possibly due to their distinct FGF-binding preferences, and suggest that growth factors and HSs are important partners in regulating axon growth and guidance in the developing visual system.