Syndecans are transmembranous heparan sulfate proteoglycans abundant in the surface of all adherent mammalian cells and involved in vital cellular functions. In this study, we found syndecan-1, -2, -3, and -4 to be constitutively expressed by human umbilical vein endothelial cells. The exposure of the ectodomains of syndecan-1 and -4 to the cell surface and their constitutive shedding into the extracellular compartment was measured by immunoassays. In the presence of plasmin and thrombin, shedding was accelerated and monitored by detection and identification of 35 S-labeled proteoglycans. To elucidate the cleavage site of the syndecan ectodomains, we used a cell-free in vitro system with enzyme and substrate as the only reactants. For this purpose, we constructed recombinant fusion proteins of the syndecan-1 and -4 ectodomain together with maltose-binding protein and enhanced yellow fluorescent protein as reporter proteins attached to the N and C termini via oligopeptide linkers. After protease treatment of the fusion proteins, the electrophoretically resolved split products were sequenced and cleavage sites of the ectodomain were identified. Plasmin generated cleavage sites at Lys 114 2Arg The family of syndecans forms a group of transmembrane heparan sulfate proteoglycans that are composed of a core protein with covalently attached glycosaminoglycan chains. The four mammalian syndecan genes (syndecan-1, -2, -3, -4) have been cloned and sequenced and are expressed in most human cells and tissues (for review, see Refs. 1 and 2), including human umbilical vein endothelial cells (3, 4). The syndecans contain an N-terminal extracellular domain or ectodomain, a hydrophobic transmembrane domain, and a short C-terminal cytoplasmic domain. The ectodomain bears, near the N terminus, three consecutive consensus Ser-Gly sequences for heparan sulfate chain attachment and may also contain Ser-Gly sequences near the plasma membrane that serve as attachment sites for chondroitin sulfate side chains. The length of the ectodomains varies markedly among family members, whereas the length of the transmembrane and cytoplasmic domain is highly conserved (5).The function of syndecans includes anchorage of cells to extracellular matrix components with associated heparan sulfate binding domains (6), maintenance of epithelial and endothelial morphology (1), binding to and modulation of the activity of heparan sulfate binding growth factors (7), and modulation of the activity of several proteases and their inhibitors (for review, see Refs. 2 and 8), but they may also serve as signaling molecules (9, 10) and as arterial counterparts for monocyte L-selectin in the vascular endothel (10).The syndecan ectodomains are released from the cell surface in a process commonly known as shedding (10 -12). Shedding of the ectodomain occurs as part of the normal turnover and involves the activity of a not-identified cell surface zinc metalloproteinase that is specifically inhibited by tissue inhibitor of matrix metalloproteinase-3 (8). So far the site...

OBJECTIVE-The use of human fetal pancreatic tissue may provide a potential source of transplantable ␤-cells as a therapy for type 1 diabetes. Human fetal pancreas has a remarkable capacity to grow and differentiate in vivo and has been shown to reverse diabetes in rodents. However, it is known that human fetal pancreas obtained from the second trimester of gestation is immunogenic and is rejected after transplantation. Tissue obtained from earlier stages might prove to be immune privileged, as has been shown for other tissues.RESEARCH DESIGN AND METHODS-In this study, we determined the immunogenicity of human fetal pancreatic tissue obtained from the first trimester of gestation in a humanized mouse model. A microarray study of immunoregulatory gene expression in first-and second-trimester human fetal pancreas was also undertaken.RESULTS-The analysis of transplanted human fetal pancreata revealed a significantly decreased immunogenicity of the firsttrimester tissue. The first-trimester grafts showed only limited cellular infiltration and contained numerous insulin-positive cells, whereas second-trimester tissue was completely infiltrated and rejected. Furthermore an analysis of immunoregulatory genes expressed in first-and second-trimester human fetal pancreas by microarray demonstrated the upregulation of several key immunoregulatory genes in the second-trimester tissue. This might account for the reduced immunogenicity of the younger tissue.CONCLUSIONS-Our results provide the first indication that the use of first-trimester human fetal pancreas for transplantation might increase the survival of the grafts and might decrease the requirement for immunosuppressive drugs. Diabetes 57:627-634, 2008

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