Membrane-associated Heparan Sulfate Research Articles

Novel peptide binder to Glypican-3 for targeted radiopharmaceutical therapy of hepatocellular carcinoma.

e16131 Background: Glypican-3 (GPC3) is a membrane-associated heparan sulfate proteoglycan primarily involved in embryonic development, and is barely detectable in normal adult tissues. Significant upregulation of GPC3 protein in hepatocellular carcinomas (HCC) has been observed in multiple immunohistochemistry (IHC) studies with up to 75% positivity rate, and is associated with poor prognosis. GPC3 is not expressed in healthy or non-malignant liver tissue. Targeting GPC3 could fulfill an unmet medical need for HCC, a leading cause of cancer-related deaths worldwide for which efficacious therapies are lacking. The differential expression of GPC3 between tumor and normal tissues provides an opportunity for GPC3-targeted radiopharmaceutical therapy (RPT) to treat HCC. Methods: RAYZ-8009 is comprised of a novel macrocyclic peptide binder to GPC3, a linker, and chelator DOTA that can be complexed with different radioisotopes. The affinity of peptide binders to GPC3 was determined by surface plasma resonance (SPR), as well as a radioligand binding assay in human HCC cell line HepG2. The cross-species binding was assessed by radioligand binding using recombinant mouse, cynomolgus monkey, and human GPC3 proteins. Target-mediated internalization in HepG2 cells was measured using Microbeta at various time points. In vivo biodistribution with 177Lu, and anti-tumor efficacy studies using 225Ac were performed in HepG2 tumor-bearing athymic nude mice. Results: RAYZ-8009 showed high binding affinity to human GPC3 with a KD of 0.7 nM as determined by SPR. Binding affinity was maintained across mouse, cynomolgus monkey and human GPC3. Potent cellular binding was confirmed in GPC3+ HepG2 cells, and was independent of choice of isotope. 177Lu-RAYZ-8009 showed fast and efficient internalization with 42% internalized by 20 minutes in HepG2 cells. In vivo biodistribution of 177Lu-RAYZ-8009 showed tumor uptake of 19.8, 16.6, 16.4, and 8.8 %ID/g at 2, 24, 48, and 96 hours, respectively. Renal uptake was 16.1, 4.7, 1.6, and 0.7 %ID/g at the same timepoints, with tumor/kidney ratios of 1.3, 3.7, 11.3, and 15.0, respectively. Minimal uptake was observed in other normal tissues. Tumor-specific uptake and retention were also observed when tumors were implanted orthotopically with no uptake in non-malignant liver tissue. Furthermore, significant tumor growth inhibition and survival benefit were achieved with 225Ac-RAYZ-8009 in GPC3+ HCC xenografts. Conclusions: Preclinical pharmacodynamic, pharmacokinetic, biodistribution and efficacy data demonstrate the potential of RAYZ-8009 as a RPT agent for the treatment of patients with GPC3-positive HCC.

A 2,5-Dihydroxybenzoic Acid-Gelatin Conjugate Inhibits the Basal and Hsp90-Stimulated Migration and Invasion of Tumor Cells.

The extracellular cell surface-associated and soluble heat shock protein 90 (Hsp90) is known to participate in the migration and invasion of tumor cells. Earlier, we demonstrated that plasma membrane-associated heparan sulfate proteoglycans (HSPGs) bind the extracellular Hsp90 and thereby promote the Hsp90-mediated motility of tumor cells. Here, we showed that a conjugate of 2,5-dihydroxybenzoic acid with gelatin (2,5-DHBA–gelatin), a synthetic polymer with heparin-like properties, suppressed the basal (unstimulated) migration and invasion of human glioblastoma A-172 and fibrosarcoma HT1080 cells, which was accompanied by the detachment of a fraction of Hsp90 from cell surface HSPGs. The polymeric conjugate also inhibited the migration/invasion of cells stimulated by exogenous soluble native Hsp90, which correlated with the inhibition of the attachment of soluble Hsp90 to cell surface HSPGs. The action of the 2,5-DHBA–gelatin conjugate on the motility of A-172 and HT1080 cells was similar to that of heparin. The results demonstrate a potential of the 2,5-DHBA–gelatin polymer for the development of antimetastatic drugs targeting cell motility and a possible role of extracellular Hsp90 in the suppression of the migration and invasion of tumor cells mediated by the 2,5-DHBA–gelatin conjugate and heparin.

The effect of syndecan-4 and glypican-1 knockdown on the proliferation and differentiation of turkey satellite cells differing in age and growth rates

Posthatch skeletal muscle growth requires myogenic satellite cells and the dynamic expression of cell membrane-associated proteins. The membrane associated heparan sulfate proteoglycans, syndecan-4 and glypican-1, link the satellite cell niche to the intracellular environment. Sydnecan-4 and glypican-1 are differentially expressed with age in turkey satellite cells and their over-expression impacts both satellite cell proliferation and differentiation, but their effect on satellite cells from lines with different growth potentials is not known. The objective of the current study was to determine if syndecan-4 and glypican-1 regulation of satellite cell proliferation and differentiation is affected by age and growth selection. Pectoralis major satellite cells isolated at 1 d, 7 and 16-wk of age from a Randombred Control 2 (RBC2) line and a 16-wk body weight (F) line selected from the RBC2 line turkeys were studied. Syndecan-4 and glypican-1 expression was knocked down in both lines. The F-line cells proliferated faster than RBC2 line cells regardless of age, while differentiation tended to be greater in RBC2 line cells than F-line cells at each age. Syndecan-4 knockdown decreased proliferation at 7- and 16-wk but not 1 d cells, and increased differentiation at 1 d and 7 wk but not 16 wk cells. Glypican-1 knockdown differentially affected proliferation depending on cell age, whereas differentiation was decreased for 7- and 16-wk but not 1 d cells. These data suggest syndecan-4 and glypican-1 differentially affected satellite cell function in an age-dependent manner, but had little impact on differences in proliferation and differentiation due to growth selection.

Advances in the study of oncofetal antigen glypican-3 expression in HBV-related hepatocellular carcinoma.

Early specific diagnosis and effective treatment of hepatocellular carcinoma (HCC) are crucial. Expression of membrane-associated heparan sulfate proteoglycan glypican-3 (GPC-3) was recently found to increase as part of the malignant transformation of hepatocytes, and this increase is especially marked in patients with hepatitis B virus (HBV) infection, periportal cancerous embolus, or extra-hepatic metastasis. According to data from basic and clinical studies, the oncofetal antigen GPC-3 is a highly specific diagnostic biomarker of HCC and an indicator of its prognosis, and GPC-3 is also a promising target molecule for HCC gene therapy since it may play a crucial role in cell proliferation, metastasis, and invasion and it may mediate oncogenesis and oncogenic signaling pathways. This review summarizes recent advances in the use of oncofetal antigen GPC-3 to diagnose HBV-related HCC, estimate its prognosis, and its targeted therapy.

Decoding the Matrix: Instructive Roles of Proteoglycan Receptors.

The extracellular matrix is a dynamic repository harboring instructive cues that embody substantial regulatory dominance over many evolutionarily conserved intracellular activities, including proliferation, apoptosis, migration, motility, and autophagy. The matrix also coordinates and parses hierarchical information, such as angiogenesis, tumorigenesis, and immunological responses, typically providing the critical determinants driving each outcome. We provide the first comprehensive review focused on proteoglycan receptors, that is, signaling transmembrane proteins that use secreted proteoglycans as ligands, in addition to their natural ligands. The majority of these receptors belong to an exclusive subset of receptor tyrosine kinases and assorted cell surface receptors that specifically bind, transduce, and modulate fundamental cellular processes following interactions with proteoglycans. The class of small leucine-rich proteoglycans is the most studied so far and constitutes the best understood example of proteoglycan-receptor interactions. Decorin and biglycan evoke autophagy and immunological responses that deter, suppress, or exacerbate pathological conditions such as tumorigenesis, angiogenesis, and chronic inflammatory disease. Basement membrane-associated heparan sulfate proteoglycans (perlecan, agrin, and collagen XVIII) represent a unique cohort and provide proteolytically cleaved bioactive fragments for modulating cellular behavior. The receptors that bind the genuinely multifactorial and multivalent proteoglycans represent a nexus in understanding basic biological pathways and open new avenues for therapeutic and pharmacological intervention.

Down-regulating glypican-3 expression: molecular-targeted therapy for hepatocellular carcinoma.

Hepatocellular carcinoma (HCC) prognosis is very poor, its early treatment is of the utmost importance. Glypican-3 (GPC-3), a membrane-associated heparan sulfate proteoglycan, plays a crucial role in cell proliferation and metastasis, particularly in progression. GPC-3 mediated oncogenesis involves signaling pathways during the malignant transformation of hepatocyte carcinogenesis, with an increasing expression of GPC-3 observed from non-cancerous- to cancerous-tissues, with brown granule-like staining localized in tumor parts of atypical hyperplasia and HCC formation. GPC-3 expression in HCC tissues or circulating blood was associated with tumor size or HBV infection. Circulation of GPC-3-mRNA was detected in HCC patients with relation to TNM stage, periportal cancerous embolus, and extra-hepatic metastasis. After hepatoma, cells were transfected with shRNA, GPC-3 expression or proliferation was inhibited with promoting apoptosis, cell cycle arrested in G1 phase, alteration of hepatoma cell migration and invasion behaviors with down-regulation of β-catenin, IGF-II, and VEGF, and growth of nude mice xenograft tumors was significantly suppressed with the decreases in β-catenin, p-GSK3β, and cyclinD1 expression, suggesting that GPC-3 not only is a specific biomarker for HCC diagnosis, but also is a valuable molecular-target for HCC gene therapy.

Glypican-3 promotes epithelial-mesenchymal transition of hepatocellular carcinoma cells through ERK signaling pathway.

Glypican-3 (GPC3), a membrane-associated heparan sulfate proteoglycan, is frequently upregulated in hepatocellular carcinoma (HCC). However, how GPC3 contributes to the progress of HCC is largely unclear. The present study investigated the association between GPC3 expression and HCC clinicopathological characteristics, and particularly focused on the role and underlying mechanisms of GPC3 in HCC epithelial-mesenchymal transition (EMT). Remarkably elevated expression of GPC3 was demonstrated in HCC tumor tissues compared with paired non-tumor tissues in 45 patients with HCC by quantitative real-time PCR, immunohistochemistry, and western blotting, respectively. Furthermore, the tissue expression of GPC3 was increased during HCC progression from Barcelona Clinic Liver Cancer stage A or B to stage C. The enhanced levels of GPC3 in HCC tumor tissues were tightly correlated to the expression of the EMT-associated proteins and tumor vascular invasion. Patients with GPC3-high expression in tumor tissues displayed significantly shorter survival time than those with GPC3-low expression (P=0.001). Consistent with the findings in patients, HepG2 cells, which expressed high levels of GPC3, showed stronger capacity of migration and significant EMT-like changes when compared to those HCC cells with low levels of GPC3, e.g., Hep3B and Huh7 in scratch, Transwell assays and western blotting. Furthermore, administration with exogenous GPC3 in HCC cells activated extracellular signal-regulated kinase (ERK) and significantly enhanced cell migration and invasion. The behavior was significantly inhibited by the ERK inhibitor PD98059. Together, our studies show that GPC3 contributes to HCC progression and metastasis through impacting EMT of cancer cells, and the effects of GPC3 are associated with ERK activation.

Abstract 1307: c-Myc is a key mediator of glypican-1 (GPC1)-dependent deregulation of the cell cycle

Abstract Human gliomas show extraordinary cellular and genomic heterogeneity while the heparan sulfate proteoglycan glypican-1 (GPC1) is nearly universally overexpressed in human gliomas but absent in normal glia. Cell membrane-associated heparan sulfate proteoglycans, including GPC1, can enhance the activity of many heparan sulfate-binding growth factors and cytokines to promote cell growth and development. Our previous studies using normal human astrocytes and U87-MG, a human glioma cell line carrying relatively low basal levels of GPC1, showed that transduction of GPC1 dramatically stimulates G1-S cell cycle progression and DNA replication, ultimately leading to DNA re-replication and DNA damage. This cell cycle effect of GPC1 appears to be at least partially attributable to the deregulation of G1/S checkpoint resulting from significant downregulation of pRb, p21Cip/p27Kip and CDH1 (FZR1) and significant upregulation of cyclin E, cyclin-dependent kinase 2 (CDK2) and Skp2 by GPC1 {Qiao, D., et al., Mol. Cell. Biol. 2013, 33(22):4408}. Here we further report that c-Myc acts as a key mediator for GPC1-mediated G1-S cell cycle deregulation. Both c-Myc protein and transactivation activity can be significantly induced by ectopic expression of GPC1 in a cell type-independent fashion. Inhibition of GPC1-induced c-Myc transactivation by the c-Myc/Max-specific inhibitor 10058-F4 almost completely abolishes GPC1-induced E2F activation and DNA re-replication. Consistent with these results, inhibition of Myc activity completely restores the protein levels of cyclin E, CDK2, pRb, p27Kip and Skp2 which were significantly altered by expression of GPC1. GPC1-induced reduction of p21Cip was partially restored upon inhibition of c-Myc. Expression of GPC1 also induced cell detachment and rounding, accompanied by a significant downregulation of the β1 and β3 integrin subunits. Inhibition of Myc effectively blocked GPC1 induction of both cell detachment/rounding and downregulation of β1 and β3 integrins. These results suggest a key role for c-Myc in GPC-1-mediated cellular effects and cell signaling. In addition to direct transcriptional activation of cyclin E, E2Fs and Skp2, c-Myc may also modulate specific protein degradation activities. The c-Myc-mediated transcriptional repression of p21Cip/p27Kip and of integrins may also contribute to GPC1-induced effects on the cell cycle and on cell morphology. Given the potent oncogenic activity of c-Myc and the fact that c-Myc is required for the maintenance of stemness of glioma stem cells, our data imply that GPC1 may play a significant role in both glioma tumorigenesis and growth. Further in vitro and in vivo studies are required to fully understand the role and mechanisms of both GPC1 and c-Myc in human gliomas. Citation Format: Dianhua Qiao, Kristy Meyer, Andreas Friedl. c-Myc is a key mediator of glypican-1 (GPC1)-dependent deregulation of the cell cycle. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1307. doi:10.1158/1538-7445.AM2014-1307

Glypican-3 as an emerging molecular target for hepatocellular carcinoma gene therapy

Glypican-3 (GPC-3), a membrane-associated heparan sulfate proteoglycan, plays a crucial role in cell proliferation and metastasis, particularly in hepatocellular carcinoma (HCC) progression, and perhaps is a valuable target for its gene therapy. However, its mechanism remains to be explored. In the present study, the biological behaviors of HCC cells were investigated by interfering GPC-3 gene transcription. After the cells were transfected with specific GPC-3 short hairpin RNA (shRNA), the inhibition of GPC-3 expression was 75.6 % in MHCC-97H or 73.8 % in Huh7 cells at mRNA level; the rates of proliferation and apoptosis were 53.6 and 60.5 % in MHCC-97H or 54.9 and 54.4 % in Huh7 cells, with the cell cycles arrested in the G1 phase; the incidences of cell migration, metastasis, and invasion inhibition were 80.1, 56.4, and 69.1 % in MHCC-97H or 80.9, 59.6, and 58.3 % in Huh7 cells, respectively. The cell biological behaviors were altered by silencing GPC-3 with down-regulation of β-catenin, insulin-like growth factor-II and vascular endothelial growth factor, and Gli1 up-regulation. The cell proliferation was significantly inhibited (up to 95.11 %) by shRNA plus anti-cancer drugs, suggesting that GPC-3 gene should be a potential target for promoting hepatoma cell apoptosis and inhibiting metastasis through the Wnt/β-catenin and Hh singling pathways.

Chondroitin Sulfate is the Primary Receptor for a Peptide-Modified AAV That Targets Brain Vascular Endothelium In Vivo

Recently, we described a peptide-modified AAV2 vector (AAV-GMN) containing a capsid-displayed peptide that directs in vivo brain vascular targeting and transduction when delivered intravenously. In this study, we sought to identify the receptor that mediates transduction by AAV-GMN. We found that AAV-GMN, but not AAV2, readily transduces the murine brain endothelial cell line bEnd.3, a result that mirrors previously observed in vivo transduction profiles of brain vasculature. Studies in vitro revealed that the glycosaminoglycan, chondroitin sulfate C, acts as the primary receptor for AAV-GMN. Unlike AAV2, chondroitin sulfate expression is required for cell transduction by AAV-GMN, and soluble chondroitin sulfate C can robustly inhibit AAV-GMN transduction of brain endothelial cells. Interestingly, AAV-GMN retains heparin-binding properties, though in contrast to AAV2, it poorly transduces cells that express heparan sulfate but not chondroitin sulfate, indicating that the peptide insertion negatively impacts heparan-mediated transduction. Lastly, when delivered directly, this modified virus can transduce multiple brain regions, indicating that the potential of AAV-GMN as a therapeutic gene delivery vector for central nervous system disorders is not restricted to brain vascular endothelium.

Changes in proliferation, differentiation, fibroblast growth factor 2 responsiveness and expression of syndecan‐4 and glypican‐1 with turkey satellite cell age

Myogenic satellite cells are heterogeneous multipotential stem cells that are required for muscle repair, maintenance, and growth. The membrane-associated heparan sulfate proteoglycans syndecan-4 and glypican-1 differentially regulate satellite cell proliferation, differentiation, fibroblast growth factor 2 (FGF2) signal transduction, and expression of the myogenic regulatory factors MyoD and myogenin. The objective of the current study was to determine the effect of age on syndecan-4 and glypican-1 satellite cell populations, proliferation, differentiation, FGF2 responsiveness, and expression of syndecan-4, glypican-1, MyoD, and myogenin using satellite cells isolated from the pectoralis major muscle of 1-day-old, 7-week-old and 16-week-old turkeys. Proliferation was significantly reduced in the 16-week-old satellite cells, while differentiation was decreased in the 7-week-old and the 16-week-old cells beginning at 48 h of differentiation. Fibroblast growth factor 2 responsiveness was highest in the 1-day-old and 7-week-old cells during proliferation; during differentiation there was an age-dependent response to FGF2. Syndecan-4 and glypican-1 satellite cell populations decreased with age, but syndecan-4 and glypican-1 were differentially expressed with age during proliferation and differentiation. MyoD and myogenin mRNA expression was significantly decreased in 16-week-old cells compared to the 1-day-old and 7-week-old cells. MyoD and myogenin protein expression was higher during proliferation in the 16-week-old cells and decreased with differentiation. These data demonstrate an age-dependent effect on syndecan-4 and glypican-1 satellite cell subpopulations, which may be associated with age-related changes in proliferation, differentiation, FGF2 responsiveness, and the expression of the myogenic regulatory factors MyoD and myogenin.

MicroRNA‐520c‐3p inhibits hepatocellular carcinoma cell proliferation and invasion through induction of cell apoptosis by targeting glypican‐3

Glypican-3 (GPC3) is a membrane-associated heparan sulfate proteoglycan involved in regulation of cell proliferation, cell survival, cell migration and differentiation process. MicroRNAs (miRNAs) are single-stranded, non-coding functional RNAs that are important in many biological processes. GPC3 and miRNAs have been found to play essential roles in the development and progression of hepatocellular carcinoma (HCC). However, little information about the relationship between GPC3 and miRNAs is available nowadays. Therefore, this study aims to examine the relationship between GPC3 and miRNAs. Dual-luciferase reporter assay was used to validate the direct target of GPC3. Fluorescence quantitative PCR and Western blotting were used to examined the gene expression at mRNA and protein levels. Cell apoptosis was evaluated by flow cytometric analysis and Annexin V-FITC staining. Invasion of cells was evaluated by Transwell matrigel assay. The results showed that miR-520c-3p could specifically target GPC3 in HCC cells. GPC3 protein levels decreased with unchanged transcription efficiency after miRNA transfection, and there was negative correlation of miR-520c-3p expression in HCC in relate to GPC3 protein levels. Moreover, miR-520c-3p not only induced HCC cell apoptosis, but also inhibited the growth and invasion of the cells. Interestingly, overexpression of GPC3 could effectively reverse apoptosis induced by miR-520c-3p transfection in HCC. Taken together, these results supported that miR-520c-3p may decrease GPC3 protein levels to inhibit proliferation of HCC cells. Therefore, GPC3 could be a new target for genetic diagnosis and treatment of HCC.

Growth and sex effects on the expression of syndecan-4 and glypican-1 in turkey myogenic satellite cell populations

The adult skeletal muscle stem cells, satellite cells, are responsible for skeletal muscle growth and regeneration. Satellite cells represent a heterogeneous cell population that differentially express cell surface markers. The membrane-associated heparan sulfate proteoglycans, syndecan-4, and glypican-1, are differentially expressed by satellite cells during the proliferation and differentiation stages of satellite cells. However, how the population of syndecan-4- or glypican-1-positive satellite cells changes during proliferation and differentiation, and how sex and muscle growth potential affect the expression of these genes is unknown. Differences in the amount of satellite cells positive for syndecan-4 or glypican-1 would affect the process of proliferation and differentiation which would impact both muscle mass accretion and the regeneration of muscle. In the current study, the percentage of satellite cells positive for syndecan-4 or glypican-1 from male and female turkeys from a Randombred Control Line 2 and a line (F) selected for increased 16-week body weight were measured during proliferation and differentiation. Growth selection altered the population of syndecan-4- and glypican-1-positive satellite cells and there were sex differences in the percentage of syndecan-4- and glypican-1-positive satellite cells. This study provides new information on dynamic changes in syndecan-4- and glypican-1-positive satellite cells showing that they are differentially expressed during myogenesis and growth selection and sex affects their expression.

Knockdown of GPC3 inhibits the proliferation of Huh7 hepatocellular carcinoma cells through down‐regulation of YAP

Glypican-3 (GPC3), a membrane-associated heparan sulfate proteoglycan, is frequently upregulated in hepatocellular carcinoma (HCC). Yes-associated protein (YAP) is also found over-expressed in HCC and has been identified as a key effector molecule in Hippo pathway, which could control the organ size in animals through the regulation of cell proliferation and apoptosis and plays an important role in the development of malignant tumors. Studies have reported that GPC3 and YAP might collaborate to regulate the development of HCC. To elucidate the role of GPC3 in the development of HCC and its relationship with YAP, siRNA technique was employed to knock down GPC3 in Huh7 HCC cells. Moreover, recombinant human YAP-1 was used to examine the effects of GPC3 on Huh7 cells. The results of flow cytometric analysis and Annexin-V-FLUOS apoptosis assay showed that knockdown of GPC3-induced apoptosis in Huh7 cells, resulting in inhibition of cell proliferation as examined by EdU incorporation assay, migration, and invasion. GPC3 knockdown also suppressed the expression of YAP in mRNA and protein levels, as examined by fluorescence quantitative PCR and Western blot analysis. Moreover, addition of recombinant human YAP-1 effectively rescued the cells from apoptosis triggered by GPC3 knockdown. Taken together, our findings suggest that GPC3 regulates HCC cell proliferation with the involvement of Hippo pathway.

Migration of turkey muscle satellite cells is enhanced by the syndecan-4 cytoplasmic domain through the activation of RhoA

Syndecan-4 (S4) is a cell membrane-associated heparan sulfate proteoglycan that forms oligomers in muscle satellite cells. The S4 oligomers activate protein kinase Cα (PKCα) through the S4 cytoplasmic domain and may regulate the activation of ras homolog gene family member A (RhoA), a signal transduction molecule down-stream of PKCα which is thought to influence cell migration. However, little is known about the function of the S4 cytoplasmic domain in satellite cell migration and RhoA activation. The objective of the current study was to determine the function of S4 and its cytoplasmic domain in cell migration and RhoA activation. To study the objective, clones of S4 and S4 without the cytoplasmic domain (S4C) were used in overexpression studies, and small interference RNAs targeting S4 or RhoA were used in knockdown studies. Satellite cell migration was increased by S4 overexpression, but decreased by the knockdown or deletion of the S4 cytoplasmic domain. The RhoA protein was activated by the overexpression of S4, but not with the deletion of the S4 cytoplasmic domain. The treatment of Rho activator II or the knockdown of RhoA also modulated satellite cell migration. Finally, co-transfection (S4 overexpression and RhoA knockdown) and rescue (the knockdown of S4 and the treatment with Rho activator II) studies demonstrated that S4-mediated satellite cell migration was regulated through the activation of RhoA. The cytoplasmic domain of S4 is required for cell migration and RhoA activation which will affect muscle fiber formation.

The Stalk Domain and the Glycosylation Status of the Activating Natural Killer Cell Receptor NKp30 Are Important for Ligand Binding

The natural cytotoxicity receptors are a unique set of activating proteins expressed mainly on the surface of natural killer (NK) cells. The human natural cytotoxicity receptor family comprises the three type I membrane proteins NKp30, NKp44, and NKp46. Especially NKp30 is critical for the cytotoxicity of NK cells against different targets including tumor, virus-infected, and immature dendritic cells. Although the crystal structure of NKp30 was recently solved (Li, Y., Wang, Q., and Mariuzza, R. A. (2011) J. Exp. Med. 208, 703-714; Joyce, M. G., Tran, P., Zhuravleva, M. A., Jaw, J., Colonna, M., and Sun, P. D. (2011) Proc. Natl. Acad. Sci. U.S.A. 108, 6223-6228), a key question, how NKp30 recognizes several non-related ligands, remains unclear. Therefore, we investigated the parameters that impact ligand recognition of NKp30. Based on various NKp30-hIgG1-Fc fusion proteins, which were optimized for minimal background binding to cellular Fcγ receptors, we identified the flexible stalk region of NKp30 as an important but so far neglected module for ligand recognition and related signaling of the corresponding full-length receptor proteins. Moreover, we found that the ectodomain of NKp30 is N-linked glycosylated at three different sites. Mutational analyses revealed differential binding affinities and signaling capacities of mono-, di-, or triglycosylated NKp30, suggesting that the degree of glycosylation could provide a switch to modulate the ligand binding properties of NKp30 and NK cell cytotoxicity.

Cell Membrane–associated Heparan Sulfate Is a Receptor for Prototype Foamy Virus in Human, Monkey, and Rodent Cells

Foamy viruses (FVs) (spumaretroviruses) are good alternative to retroviruses as gene therapy vector. Despite four decades since the discovery of FV, its receptor molecule is still unknown. FV vector transduction of human CD34(+) cells was inhibited by culture with fibronectin. Because fibronectin contains heparin-binding domain, the interactions of fibronectin with heparan sulfate (HS) on cells might be inhibitory to FV transduction. These observations led us to investigate whether HS is a receptor for FV. Two mutant CHO cell lines (but not parental wild type) lacking cell surface HS but not chondroitin sulfate (CS) were largely resistant to FV attachment and transduction. Inhibition of HS expression using enzymes or chemicals greatly reduced FV transduction in human, monkey, and rodent cells. Raji cells, which lack HS and were largely resistant to FV, were rendered more permissive through ectopic expression of syndecan-1, which contains HS. In contrast, mutant syndecan-1-expressing cells were largely resistant to FV. Our findings indicate that cellular HS is a receptor for FV. Identifying FV receptor will enable better understanding of its entry process and optimal use as gene therapy vector to treat inherited and pathogenic diseases.

MEAT SCIENCE AND MUSCLE BIOLOGY SYMPOSIUM: Extracellular matrix regulation of skeletal muscle formation1,2

Skeletal muscle development and growth is a complex process that involves the interaction of muscle cells with their extracellular environment. Because muscle development involves the interaction of the cell surface and extracellular matrix molecules, research focus has been placed on the proteoglycans. Proteoglycans are macromolecules containing a central core protein with attached carbohydrates, called glycosaminoglycans, that are located at both the cell surface and the extracellular matrix. Research focus has been placed on understanding the mechanisms of the membrane-associated heparan sulfate proteoglycans, syndecan-4 and glypican-1, which are both capable of regulating cellular responsiveness to fibroblast growth factor 2 (FGF2). Fibroblast growth factor 2 is a potent stimulator of muscle cell proliferation and a strong inhibitor of differentiation. Studies on syndecan-4 and glypican-1 show that these proteoglycans differentially regulate muscle cell proliferation, differentiation, and cellular responsiveness to FGF2 with syndecan-4 predominantly modulating muscle cell proliferation and glypican-1 modulating differentiation. Site-directed mutagenesis approaches were used to define the effect of the syndecan-4 and glypican-1 covalently attached side chains on their activity. In general, a functional association was found between the glycosaminoglycan and N-glycosylated chains attached to the central core proteins of syndecan-4 and glypican-1 affecting their regulation of muscle cell proliferation, differentiation, and FGF2 responsiveness. Current research efforts are directed at identifying the cellular signaling pathways modulated by syndecan-4 and glypican-1.

Cell Penetrating Peptides. How do they Cross Membranes?

Cell penetrating peptides (CPPs) are cationic peptides which, when linked to proteins, genes, or nanoparticles, facilitate the transport of these entities across the cell membrane. Different models have been suggested for their mode of action. One possibility is the binding of CPPs to negatively charged lipids and the formation of non-bilayer structures. We have studied the binding of two CPPs (R9, HIV-1 TAT) to model membranes with isothermal titration calorimetry and observe indeed a strong binding to the membrane. However, the bilayer remains intact and non-bilayer structures can be excluded based on 2H- and 31P-NMR spectroscopy. A second mechanism is the binding of CPPs to sulfated sugars at the membrane surface, followed by endocytosis. We have characterized the binding of a large variety of CPPs to heparin sulfate (HS), heparin, and related sulfated glycosaminoglycans (GAGs) and find binding constants in the order of 106 - 107 M−1 per binding site. An even stronger interaction is found between CPPs and DNA with binding constants in 107 - 108 M−1. We have further synthesized a fluorescent derivative of the HIV-1 TAT protein transduction domain (Fg-CPPTAT(PTD)) and have observed its uptake into non-fixated living fibroblasts with time-lapse confocal microscopy. We find that Fg-CPPTAT(PTD) enters the cytoplasm and nucleus of non-fixated fibroblasts within seconds. With differential interference contrast microscopy we furthermore detect dense aggregates on the cell surface. Several observations suggest that these aggregates consist of Fg-CPPTAT(PTD) bound to membrane-associated heparan sulfate (HS). Finally a pore-forming peptide, melittin, also binds to sulfated GAGs but not magainin 2 or nisin Z. The binding of the melittin to lipid bilayers is furthermore an excellent model system to elucidate the thermodynamic parameters of membrane-induced α-helix and β-sheet formation..

The Effect of Fibroblast Growth Factor 2 on the In Vitro Expression of Syndecan-4 and Glypican-1 in Turkey Satellite Cells

The membrane-associated heparan sulfate proteoglycan families, consisting of the syndecans and glypicans, are low-affinity receptors for fibroblast growth factor 2 (FGF2) that are essential in regulating the cellular response to FGF2. Fibroblast growth factor 2 is a potent stimulator of skeletal muscle cell proliferation and a strong inhibitor of differentiation. The regulation of the expression of the syndecans and glypicans will likely play a role in modulating the effects of FGF2 on cellular growth properties. In the present study, the effect of FGF2 on the expression of syndecan-4 and glypican-1 was measured by real-time PCR during turkey myogenic satellite cell proliferation and differentiation in vitro. Both syndecan-4 and glypican-1 transcription were influenced by the addition of exogenous FGF2. Syndecan-4 mRNA expression was reduced only during proliferation, whereas glypican-1 expression was reduced during both proliferation and differentiation. These results suggest that FGF2 growth factor signaling is, in part, regulated by an autoregulatory loop involving FGF2 regulation of syndecan-4 and glypican-1 expression and will affect the growth of skeletal muscle by modulating the proliferation and differentiation of satellite cells.