Guido David

Syndecan-syntenin-ALIX regulates the biogenesis of exosomes

The biogenesis of exosomes, small secreted vesicles involved in signalling processes, remains incompletely understood. Here, we report evidence that the syndecan heparan sulphate proteoglycans and their cytoplasmic adaptor syntenin control the formation of exosomes. Syntenin interacts directly with ALIX through LYPX(n)L motifs, similarly to retroviral proteins, and supports the intraluminal budding of endosomal membranes. Syntenin exosomes depend on the availability of heparan sulphate, syndecans, ALIX and ESCRTs, and impact on the trafficking and confinement of FGF signals. This study identifies a key role for syndecan–syntenin–ALIX in membrane transport and signalling processes.

Perlecan, basal lamina proteoglycan, promotes basic fibroblast growth factor-receptor binding, mitogenesis, and angiogenesis

A survey of defined species of cell surface and extracellular matrix heparan sulfate proteoglycans (HSPG) was performed in a search for cellular proteoglycans that can promote bFGF receptor binding and biological activity. Of the various affinity-purified HSPGs tested, perlecan, the large basement membrane HSPG, is found to induce high affinity binding of bFGF both to cells deficient in HS and to soluble FGF receptors. Heparin-dependent mitogenic activity of bFGF is strongly augmented by perlecan. Monoclonal antibodies to perlecan extract the receptor binding promoting activity from active HSPG preparations. In a rabbit ear model for in vivo angiogenesis, perlecan is a potent inducer of bFGF-mediated neovascularization. These results identify perlecan as a major candidate for a bFGF low affinity, accessory receptor and an angiogenic modulator.

Integral membrane heparan sulfate proteoglycans.

Heparan sulfate is a regulatory polysaccharide. It modulates specific growth factor‐receptor interactions, accelerates the formation of specific proteinase‐proteinase inhibitor complexes, and mediates interactions of the cell surface with several enzymes and structural proteins. It abounds on the surfaces of embryonic cells, respecting or outlining morphogenetic rather than histological boundaries. This cell surface‐associated heparan sulfate is implanted on specific integral membrane proteins, which together constitute two novel molecular families. The first family includes four syndecan‐like integral membrane proteoglycans (SLIPS), with core proteins that span the membrane and shared sequence motifs in highly conserved cytoplasmic domains. The second is made up by two or more glypican‐related integral membrane proteoglycans (GRIPS) that are linked to the cell surface via glycosyl phosphatidylinositol. These proteoglycans show differential expression and turnover patterns, prevailing in distinct cell types, membrane domains, and endocytotic machineries, and are subject to strict developmental controls. This suggests that each of these cell surface proteoglycans functions in a specific context, and that these functions pertain to the transduction of signals that emanate from the continuous interplay between matrix components, growth factors, and proteinases. Caution: beware of loose GRIPS and SLIPS on unsteady cell surfaces.—David, G. Integral membrane heparan sulfate proteoglycans. FASEB J. 7: 1023‐1030; 1993.

Syndecans Serve as Attachment Receptors for Human Immunodeficiency Virus Type 1 on Macrophages

ABSTRACT Macrophages are thought to represent one of the first cell types in the body to be infected during the early stage of human immunodeficiency virus type 1 (HIV-1) transmission and represent a potential viral reservoir in vivo. Thus, an understanding of HIV-1 attachment to these cells is fundamental to the development of novel anti-HIV-1 therapies. Although one of the major targets of HIV-1 in vivo—CD4+ T lymphocytes—express high CD4 levels, other major targets such as macrophages do not. We asked in this study whether this low CD4 level on macrophages is sufficient to support HIV-1 attachment to these cells or whether cell surface proteins other than CD4 are required for this process. We show that CD4 alone is not sufficient to support the initial adsorption of HIV-1 to macrophages. Importantly, we find that heparan sulfate proteoglycans (HSPGs) serve as the main class of attachment receptors for HIV-1 on macrophages. Most importantly, we demonstrate that a single family of HSPGs, the syndecans, efficiently mediates HIV-1 attachment and represents an abundant class of attachment receptors on macrophages.

Syndecan captures, protects, and transmits HIV to T lymphocytes

This study demonstrates that syndecan functions as an in trans HIV receptor. We show that syndecan, when expressed in nonpermissive cells, becomes the major mediator for HIV adsorption. This adsorption is mediated by the binding of gp120 to the heparan sulfate chains of syndecan. Although syndecan does not substitute for HIV entry receptors, it enhances the in trans infectivity of a broad range of primate lentiviruses including primary viruses produced from PBMCs. Furthermore, syndecan preserves virus infectivity for a week, whereas unbound virus loses its infectivity in less than a day. Moreover, we obtain evidence suggesting that the vast syndecan-rich endothelial lining of the vasculature can provide a microenvironment which boosts HIV replication in T cells.

The syndecans, tuners of transmembrane signaling

Syndecans, a family of transmembrane proteoglycans, are putative integrators of extracellular signals. The interaction of syndecans with extracellular ligands via particular motifs in their heparan sulfate chains, their clustering, association with particular cytoskeletal structures, binding to cytoplasmic effectors, and intracellular phosphorylation represent as many means to bring this role to a successful conclusion. In this review, we will briefly address the characteristics of syndecans as heparan sulfate proteoglycans (HSPGs) and focus mainly on the properties, binding interactions, and potential signaling functions of the cytoplasmic domains of these molecules.—Zimmermann, P., David, G. The syndecans, tuners of transmembrane signaling. FASEB J. 13 (Suppl.), S91–S100 (1999)

PIP2-PDZ Domain Binding Controls the Association of Syntenin with the Plasma Membrane

PDZ proteins organize multiprotein signaling complexes. According to current views, PDZ domains engage in protein-protein interactions. Here we show that the PDZ domains of several proteins bind phosphatidylinositol 4,5-bisphosphate (PIP(2)). High-affinity binding of syntenin to PIP(2)-containing lipid layers requires both PDZ domains of this protein. Competition and mutagenesis experiments reveal that the protein and the PIP(2) binding sites in the PDZ domains overlap. Overlay assays indicate that the two PDZ domains of syntenin cooperate in binding to cognate peptides and PIP(2). Experiments on living cells demonstrate PIP(2)-dependent and peptide-dependent modes of plasma membrane association of the PDZ domains of syntenin. These observations suggest that local changes in phosphoinositide concentration control the association of PDZ proteins with their target receptors at the plasma membrane.

Mammalian heparanase: what is the message?

•  Heparan sulphate proteoglycans ‐  Heparan sulphate ‐  Heparan sulphate interactions ‐  Core proteins •  Heparanase ‐  Molecular properties of human heparanase‐1 ‐  Working mechanism and substrate specificity of human heparanase‐1 ‐  Expression of human heparanase‐1 ‐  Cellular activation and localization of human heparanase‐1 •  Involvement of human heparanase‐1 in normal and pathological processes •  Tumour metastasis ‐  Tumour angiogenesis ‐  Other pathologies ‐  Normal development ‐  Heparanase‐2 •  Conclusions

Cell-Free Human Immunodeficiency Virus Type 1 Transcytosis through Primary Genital Epithelial Cells

ABSTRACT Although the transport of human immunodeficiency virus type 1 (HIV-1) through the epithelium is critical for HIV-1 colonization, the mechanisms controlling this process remain obscure. In the present study, we investigated the transcellular migration of HIV-1 as a cell-free virus through primary genital epithelial cells (PGECs). The absence of CD4 on PGECs implicates an unusual entry pathway for HIV-1. We found that syndecans are abundantly expressed on PGECs and promote the initial attachment and subsequent entry of HIV-1 through PGECs. Although CXCR4 and CCR5 do not contribute to HIV-1 attachment, they enhance viral entry and transcytosis through PGECs. Importantly, HIV-1 exploits both syndecans and chemokine receptors to ensure successful cell-free transport through the genital epithelium. HIV-1-syndecan interactions rely on specific residues in the V3 of gp120 and specific sulfations within syndecans. We found no obvious correlation between coreceptor usage and the capacity of the virus to transcytose. Since viruses isolated after sexual transmission are mainly R5 viruses, this suggests that the properties conferring virus replication after transmission are distinct from those conferring cell-free virus transcytosis through the genital epithelium. Although we found that cell-free HIV-1 crosses PGECs as infectious particles, the efficiency of transcytosis is extremely poor (less than 0.02% of the initial inoculum). This demonstrates that the genital epithelium serves as a major barrier against HIV-1. Although one cannot exclude the possibility that limited passage of cell-free HIV-1 transcytosis through an intact genital epithelium occurs in vivo, it is likely that the establishment of infection via cell-free HIV-1 transmigration is a rare event.

A new chromosome anomaly in acute lymphoblastic leukemia (ALL)

SummaryA new chromosome anomaly in acute lymphoblastic leukemia (ALL) is reported. Three, possibly four, patients showed an identical karyotype anomaly, characterized by a (4;11)(q13;q22) reciprocal translocation. This anomaly has not so far been found in lymphoproliferative disorders other than ALL. Two of the patients had congenital leukemia, but the anomaly described appears to be more characteristic of ALL than of congenital leukemia, and may help the clinician in establishing the diagnosis of ALL.