Association of cell surface heparan sulfate proteoglycans of Schwann cells with extracellular matrix proteins

Heparan Sulfate and Heparan Sulfate Proteoglycans in Cancer Initiation and Progression

Heparan sulfate (HS) are complex unbranched carbohydrate chains that are heavily modified by sulfate and exist either conjugated to proteins or as free, unconjugated chains. Proteins with covalently bound Heparan sulfate chains are termed Heparan Sulfate Proteoglycans (HSPGs). Both HS and HSPGs bind to various growth factors and act as co-receptors for different cell surface receptors. They also modulate the dynamics and kinetics of various ligand-receptor interactions, which in turn can influence the duration and potency of the signaling. HS and HSPGs have also been shown to exert a structural role as a component of the extracellular matrix, thereby altering processes such as cell adhesion, immune cell infiltration and angiogenesis. Previous studies have shown that HS are deregulated in a variety of solid tumors and hematological malignancies and regulate key aspects of cancer initiation and progression. HS deregulation in cancer can occur as a result of changes in the level of HSPGs or due to changes in the levels of HS biosynthesis and remodeling enzymes. Here, we describe the major cell-autonomous (proliferation, apoptosis/senescence and differentiation) and cell-non-autonomous (angiogenesis, immune evasion, and matrix remodeling) roles of HS and HSPGs in cancer. Finally, we discuss therapeutic opportunities for targeting deregulated HS biosynthesis and HSPGs as a strategy for cancer treatment.

Sulfatase-mediated manipulation of the astrocyte-Schwann cell interface

Schwann cell (SC) transplantation following spinal cord injury (SCI) may have therapeutic potential. Functional recovery is limited however, due to poor SC interactions with host astrocytes and the induction of astrogliosis. Olfactory ensheathing cells (OECs) are closely related to SCs, but intermix more readily with astrocytes in culture and induce less astrogliosis. We previously demonstrated that OECs express higher levels of sulfatases, enzymes that remove 6-O-sulfate groups from heparan sulphate proteoglycans, than SCs and that RNAi knockdown of sulfatase prevented OEC-astrocyte mixing in vitro. As human OECs are difficult to culture in large numbers we have genetically engineered SCs using lentiviral vectors to express sulfatase 1 and 2 (SC-S1S2) and assessed their ability to interact with astrocytes. We demonstrate that SC-S1S2s have increased integrin-dependent motility in the presence of astrocytes via modulation of NRG and FGF receptor-linked PI3K/AKT intracellular signaling and do not form boundaries with astrocytes in culture. SC-astrocyte mixing is dependent on local NRG concentration and we propose that sulfatase enzymes influence the bioavailability of NRG ligand and thus influence SC behavior. We further demonstrate that injection of sulfatase expressing SCs into spinal cord white matter results in less glial reactivity than control SC injections comparable to that of OEC injections. Our data indicate that sulfatase-mediated modification of the extracellular matrix can influence glial interactions with astrocytes, and that SCs engineered to express sulfatase may be more OEC-like in character. This approach may be beneficial for cell transplant-mediated spinal cord repair. GLIA 2016 GLIA 2017;65:19-33.

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.

Expression of glypican-1, syndecan-1 and syndecan-4 mRNAs protein kinase C-regulated in rat immature Sertoli cells by semi-quantitative RT-PCR analysis

In seminiferous tubules, Sertoli cells provide structural and nutritional support for the developing germinal cells. Cell to cell signalization and cell adhesion require proteoglycans expressed at the cell membrane. A preliminary biochemical and structural approach indicated that cell surface proteoglycans are mostly heparan sulfate (HSPG) in immature rat Sertoli cells. The present study focused on the qualitative and quantitative expression of three membrane HSPG, syndecan-1, syndecan-4 and glypican-1 in Sertoli cells of 20-day-old rat. A semi-quantitative multiplex RT-PCR strategy was developed to appreciate the effect of PKC activation on the mRNA expression of the three HSPG. Our data show that the syndecan-1 and glypican-1 mRNA expression is increased by the phorbol myristate acetate (PMA) suggesting a regulation of their expression by the phosphatidyl inositol pathway, as previously hypothesized (Fagen et al., Biochim. Biophys. Acta, 1472 (1999) 250-261). In addition, a physiological effector of the PKC as ATP gave similar effects. Thus, this over-expression could be related with paracrine factors secreted by germ cells.

Evidence that glypican is a receptor mediating beta-amyloid neurotoxicity in PC12 cells

Docking of beta-amyloid fibrils to neuronal or glial cell membranes may be an early, necessary and intervenable step during the progression of Alzheimer's disease. Formation of neurofibrillary tangles and amyloid plaques as well as neurotoxicity and inflammation may be direct or indirect consequences. In an attempt to find a receptor that mediates those effects, we assessed rat pheochromocytoma PC12 cell 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) reduction after addition of beta-amyloid to the culture medium. Presence of competitive substances in the medium, cell-surface treatment and specific block of cellular synthesis pathways helped to identify the heparan sulphate moiety of a glycosylphosphatidylinositol-anchored protein likely to represent glypican as a possible receptor mediating beta-amyloid neurotoxicity.

Organization, 5'-flanking sequence and promoter activity of the rat GPC1 gene

Glypicans are a member of a family of glycosylphosphatidylinositol anchored heparan sulfate proteoglycans that are expressed in cell and development specific patterns. Rat GPC1 cDNA probes were used to screen rat genomic libraries. Three overlapping genomic clones that contained the entire rat GPC1 gene were isolated. The rat GPC1 gene is approximately 15kb in length and consists of eight exons interrupted by introns of varying lengths. Two of the introns are quite short, with lengths of 41 and 43 base pairs. Each exon-intron splice junction exhibited the consensus splice site sequence. Exon 1 encodes the putative signal peptide and the serine residue of the first putative heparan sulfate attachment site. The last exon encodes the cluster of three potential COOH-terminal heparan sulfate attachment sites, the putative GPI anchor and polypeptide cleavage site, and the 3'-untranslated region including the polyadenylation signal. One of the genomic clones extended approximately 2.8 kb 5' of the exon 1 coding sequence, and is thus likely to contain sequences that regulate GPC1 gene expression. Sequence analysis of the 5'-flanking sequence revealed a lack of consensus TATA and CAAT boxes. A search for potential transcription factor binding sites revealed a number of such motifs, including Sp1 (GC box), NF-kappaB, and MyoD (E-box). This region of the rat GPC1 gene shows significant sequence homology to the 5'-flanking region of the human GPC3 gene. Functional promoter activity of the rat GPC1 sequence was demonstrated by its ability to drive the expression of a luciferase reporter gene in several cell types.

Post-translational modifications of alpha5beta1 integrin by glycosaminoglycan chains. The alpha5beta1 integrin is a facultative proteoglycan

Cell-fibronectin interactions, mediated through several different receptors, have been implicated in a wide variety of cellular properties. Among the cell surface receptors for fibronectin, integrins are the best characterized, particularly the prototype alpha5beta1 integrin. Using [125I]iodine cell surface labeling or metabolic radiolabeling with sodium [35S]sulfate, we identified alpha5beta1 integrin as the only sulfated integrin among beta1 integrin heterodimers expressed by the human melanoma cell line Mel-85. This facultative sulfation was confirmed not only by immunoprecipitation reactions using specific monoclonal antibodies but also by fibronectin affinity chromatography, two-dimensional electrophoresis, and chemical reduction. The covalent nature of alpha5beta1 integrin sulfation was evidenced by its resistance to treatments with high ionic, chaotrophic, and denaturing agents such as 4 M NaCl, 4 M MgCl2, 8 M urea, and 6 M guanidine HCl. Based on deglycosylation procedures as chemical beta-elimination, proteinase K digestion, and susceptibility to glycosaminoglycan lyases (chondroitinase ABC and heparitinases I and II), it was demonstrated that the alpha5beta1 heterodimer and alpha5 and beta1 integrin subunits were proteoglycans. The importance of alpha5beta1 sulfation was strengthened by the finding that this molecule is also sulfated in MG-63 (human osteosarcoma) and HCT-8 (human colon adenocarcinoma) cells.

Schwann cells secrete a novel collagen-like adhesive protein that binds N-syndecan

A heparin-binding glycoprotein was purified from conditioned medium of cultured rat Schwann cells. The protein, p200, which has an apparent molecular mass of approximately 200 kDa, was identified by its ability to bind the cell surface heparan sulfate proteoglycan N-syndecan (syndecan-3) in a membrane overlay assay. Soluble heparin but not chondroitin sulfate inhibited the binding, suggesting the involvement of heparan sulfate chains of proteoglycan in the interaction. Purified p200 promoted the attachment and spreading of Schwann cells. Adhesion to p200 was blocked by heparin, suggesting that heparan sulfate proteoglycans are cell surface receptors for p200. The tissue distribution of p200 was determined by immunoblot analysis with anti-p200 antibodies. Among neonatal rat tissues examined p200 was detected only in sciatic nerve and, at lower levels, in skeletal muscle. p200 expression in sciatic nerve was detectable only during the first 2-3 weeks of postnatal development and was not detected in adult rats. Immunofluorescent staining of rat sciatic nerve showed that p200 was localized in the extracellular matrix surrounding individual Schwann cells-axon units. Two tryptic peptides from p200 were purified and sequenced. These contained multiple GXX collagen-like repeats. Bacterial collagenase digestion of p200 produced a product with an apparent molecular mass of approximately 90 kDa. These data suggest that Schwann cells secrete an apparently novel collagen-like adhesive protein that interacts with cells through cell surface heparan sulfate proteoglycans.

Self-association of N-syndecan (syndecan-3) core protein is mediated by a novel structural motif in the transmembrane domain and ectodomain flanking region

We expressed domains of the core protein of the transmembrane heparan sulfate proteoglycan N-syndecan (syndecan-3) either individually or as maltose-binding protein fusion proteins. Biochemical characterization of the purified proteins revealed that some of them were capable of self-association and formed stable, noncovalent multimeric complexes. The formation of N-syndecan core protein complexes was also demonstrated in mammalian cells by in situ cross-linking. Identification of structural motifs in the core protein of N-syndecan responsible for the formation of these complexes was accomplished by analyzing a series of constructs comprising different regions of the protein as well as site-directed mutants. Self-association was assayed by SDS-polyacrylamide gel electrophoresis, glutaraldehyde cross-linking, and size-exclusion high pressure liquid chromatography. Our results indicated that (i) the transmembrane domain of the N-syndecan core protein was required but not sufficient for the formation of stable complexes; (ii) the minimal amino acid sequence that conferred the ability of the N-syndecan core protein to form multimeric complexes included the last four amino acids (ERKE) of the extracellular domain plus the transmembrane domain; (iii) point mutations that changed the basic residues in this sequence to alanine residues either partially or completely abolished the ability of the N-syndecan core protein to form complexes; and (iv) replacement of conserved glycine residues in the transmembrane domain with leucines abolished complex formation. This property is similar to the oligomerization activity of other transmembrane receptors and suggests that regulated self-association may be important for the biological activity of transmembrane proteoglycans.

Molecular cloning and characterization of an isoprenylated 67 kDa protein

The cDNA coding for a 67 kDa protein (p67) was isolated from a rat Schwann cell library. A recombinant form of p67 expressed in bacteria was used to produce polyclonal anti-p67 antibodies. By immunoblot analysis p67 was found to be expressed in most tissues and cell lines examined. Inspection of the deduced amino acid sequence revealed a COOH-terminal consensus sequence for isoprenylation. Consistent with this finding, p67 was a substrate for isoprenylation in vitro by geranylgeranylpyrophosphate. p67 was associated predominantly with the particulate fraction of rat smooth muscle cells. The rat p67 sequence was highly homologous to a family of recently described human and mouse gamma-interferon inducible, guanine nucleotide binding proteins.

Nervous tissue proteoglycans

The structure, biosynthesis, localization, and possible functional roles of nervous tissue glycosaminoglycans and proteoglycans were last reviewed several years ago. Since that time, there has been an exponential increase in publications on the neurobiology of proteoglycans. This review will therefore focus on reports which have appeared in the period after 1988, and especially on those concerning the properties of individual characterized nervous tissue proteoglycans. Related areas such as the regulation of glycosaminoglycan biosynthesis and the roles of cell surface proteoglycans in adhesion and growth control are covered in other contributions to this special topic issue.

Nervous tissue proteoglycans

The structure, biosynthesis, localization, and possible functional roles of nervous tissue glycosaminoglycans and proteoglycans were last reviewed several years ago. Since that time, there has been an exponential increase in publications on the neurobiology of proteoglycans. This review will therefore focus on reports which have appeared in the period after 1988, and especially on those concerning the properties of individual characterized nervous tissue proteoglycans. Related areas such as the regulation of glycosaminoglycan biosynthesis and the roles of cell surface proteoglycans in adhesion and growth control are covered in other contributions to this special topic issue.

Inhibitory effect of fibronectin and its recombinant polypeptides on the adhesion of metastatic melanoma cells to laminin

We have utilized recombinant fibronectin fragments with cell-binding domain (C-274), heparin-binding domain (H-271) or CS1 peptide in type III connecting segment (IIICS) and their fusion polypeptides such as CH-296 (containing C-274, H-271 and CS1), CH-271 (containing C-274 and H-271) and C-CS1 (containing C-274 and CS1) to investigate the mechanism of the fibronectin-mediated inhibitory effect on tumor cell adhesion to laminin as well as fibronectin. These fragments retained cell adhesion-promoting and/or heparin-binding properties when they were immobilized on a surface. Pretreatment of tumor cells with CH-296 or CH-271 suppressed cell adhesion to both laminin and fibronectin. H-271 at the high concentration of 500 micrograms/ml slightly inhibited cell adhesion to laminin (but not to fibronectin), whereas C-274, C-CS1 or a mixture of C-274, H-271 and CS1 (similar molar ratio to CH-296) inhibited cell adhesion to fibronectin but not to laminin. On the other hand, tumor cell adhesion to laminin-substrate was also inhibited by heparin or heparan sulfate, which were able to bind to laminin, suggesting that heparin-like molecules on the cell surface may be included among the laminin receptors. These results indicated that the co-presence of cell- and heparin-binding domains of fibronectin may be required for the fibronectin-mediated inhibitory effect on tumor cell adhesion to laminin, and that the interaction of the heparin-binding domain of fibronectin with the cell surface leads to the inhibition of the cell adhesion to laminin.