Cloning from purified high endothelial venule cells of hevin, a close relative of the antiadhesive extracellular matrix protein SPARC

Molecular cloning and functional analysis of SUT-1, a sulfate transporter from human high endothelial venules

High endothelial venules (HEV) are specialized postcapillary venules found in lymphoid organs and chronically inflamed tissues that support high levels of lymphocyte extravasation from the blood. One of the major characteristics of HEV endothelial cells (HEVEC) is their capacity to incorporate large amounts of sulfate into sialomucin-type counter-receptors for the lymphocyte homing receptor L-selectin. Here, we show that HEVEC express two functional classes of sulfate transporters defined by their differential sensitivity to the anion-exchanger inhibitor 4,4'-diisothiocyanostilbene-2, 2'-disulfonic acid (DIDS), and we report the molecular characterization of a DIDS-resistant sulfate transporter from human HEVEC, designated SUT-1. SUT-1 belongs to the family of Na(+)-coupled anion transporters and exhibits 40-50% amino acid identity with the rat renal Na(+)/sulfate cotransporter, NaSi-1, as well as with the human and rat Na(+)/dicarboxylate cotransporters, NaDC-1/SDCT1 and NaDC-3/SDCT2. Functional expression studies in cRNA-injected Xenopus laevis oocytes showed that SUT-1 mediates high levels of Na(+)-dependent sulfate transport, which is resistant to DIDS inhibition. The SUT-1 gene mapped to human chromosome 7q33. Northern blotting analysis revealed that SUT-1 exhibits a highly restricted tissue distribution, with abundant expression in placenta. Reverse transcription-PCR analysis indicated that SUT-1 and the diastrophic dysplasia sulfate transporter (DTD), one of the two known human DIDS-sensitive sulfate transporters, are coexpressed in HEVEC. SUT-1 and DTD could correspond, respectively, to the DIDS-resistant and DIDS-sensitive components of sulfate uptake in HEVEC. Together, these results demonstrate that SUT-1 is a distinct human Na(+)-coupled sulfate transporter, likely to play a major role in sulfate incorporation in HEV.

Collagen accumulation is decreased in SPARC-null mice with bleomycin-induced pulmonary fibrosis

Secreted protein acidic and rich in cysteine (SPARC) has been shown to be coexpressed with type I collagen in tissues undergoing remodeling and wound repair. We speculated that SPARC is required for the accumulation of collagen in lung injury and that its absence would attenuate collagen accumulation. Accordingly, we have assessed levels of collagen in SPARC-null mice in an intratracheal bleomycin-injury model of pulmonary fibrosis. Eight- to ten-week-old SPARC-null and wild-type (WT) mice received bleomycin (0.0035 U/g) or saline intratracheally and were subsequently killed after 14 days. Relative levels of SPARC mRNA were increased 2.7-fold (P < 0.001) in bleomycin-treated WT lungs in comparison with saline-treated lungs. Protein from bleomycin-treated WT lung contained significantly more hydroxyproline (191.9 microg/lung) than protein from either bleomycin-treated SPARC-null lungs or saline-treated WT and SPARC-null lungs (147.4 microg/lung, 125.4 microg/lung, and 113. 0 microg/lung, respectively; P < 0.03). These results indicate that SPARC is increased in response to lung injury and that accumulation of collagen, as indicated by hydroxyproline content, is attenuated in the absence of SPARC. The properties of SPARC as a matricellular protein associated with cell proliferation and matrix turnover are consistent with its participation in the development of pulmonary fibrosis.

A novel, high endothelial venule-specific sulfotransferase expresses 6-sulfo sialyl Lewis(x), an L-selectin ligand displayed by CD34

L-selectin mediates lymphocyte homing by facilitating lymphocyte adhesion to unique carbohydrate ligands, sulfated sialyl Lewis(x), which are expressed on high endothelial venules (HEV) in secondary lymphoid organs. The nature of the sulfotransferase(s) that contribute to sulfation of such L-selectin counterreceptors has been uncertain. We herein describe a novel L-selectin ligand sulfotransferase, termed LSST, that directs the synthesis of the 6-sulfo sialyl Lewis(x) on L-selectin counterreceptors CD34, GlyCAM-1, and MAdCAM-1. LSST is predominantly expressed in HEV and exhibits striking catalytic preference for core 2-branched mucin-type O-glycans as found in natural L-selectin counterreceptors. LSST enhances L-selectin-mediated adhesion under shear compared to nonsulfated controls. LSST therefore corresponds to an HEV-specific sulfotransferase that contributes to the biosynthesis of L-selectin ligands required for lymphocyte homing.

Sulfotransferases of two specificities function in the reconstitution of high endothelial cell ligands for L-selectin

L-selectin, a lectin-like receptor, mediates rolling of lymphocytes on high endothelial venules (HEVs) in secondary lymphoid organs by interacting with HEV ligands. These ligands consist of a complex of sialomucins, candidates for which are glycosylation- dependent cell adhesion molecule 1 (GlyCAM-1), CD34, and podocalyxin. The ligands must be sialylated, fucosylated, and sulfated for optimal recognition by L-selectin. Our previous structural characterization of GlyCAM-1 has demonstrated two sulfation modifications, Gal-6-sulfate and GlcNAc-6-sulfate in the context of sialyl Lewis x. We now report the cloning of a Gal-6-sulfotransferase and a GlcNAc-6-sulfotransferase, which can modify GlyCAM-1 and CD34. The Gal-6-sulfotransferase shows a wide tissue distribution. In contrast, the GlcNAc-6-sulfotransferase is highly restricted to HEVs, as revealed by Northern analysis and in situ hybridization. Expression of either enzyme in Chinese hamster ovary cells, along with CD34 and fucosyltransferase VII, results in ligand activity, as detected by binding of an L-selectin/IgM chimera. When coexpressed, the two sulfotransferases synergize to produce strongly enhanced chimera binding.

Biochemical Characterization and Molecular Cloning of a Novel Endothelial-Specific Sialomucin

We have generated rat monoclonal antibodies (MoAbs) against cell surface antigens of the mouse endothelioma cell line bEND.3. Three antibodies (V.1A7, V.5C7, and V.7C7) were selected, all of which recognize a 75-kD antigen on bEND.3 cells and bind selectively to endothelial cells in cryostat sections of mouse tissues. A cDNA for the antigen was isolated from a bEND.3 pCDM8 expression library by using transient expression in COS-7 cells and immunoselection with the three MoAbs. This cDNA coded for a novel, type I membrane protein of 248 amino acids with an extracellular domain rich in threonine and serine residues (35%). The protein is sensitive to O-sialoglycoprotein endopeptidase, indicating that it belongs to the class of sialomucin-like proteins. Therefore, we suggest the name endomucin. Treatment of isolated endomucin by sialidase and O-glycosidase reduced the apparent molecular weight to 45 kD and abolished binding of all three antibodies, indicating that carbohydrates are directly or indirectly involved in the formation of the antibody epitopes. Immunohistological analysis of all examined mouse tissues showed that endomucin is an endothelial antigen found in venous endothelium as well as in capillaries, but not on arterial endothelium. Interestingly, high endothelial venules of peripheral and mesenteric lymph nodes as well as of Peyers’s patches were negative for staining with the three MoAbs.

Biochemical Characterization and Molecular Cloning of a Novel Endothelial-Specific Sialomucin

We have generated rat monoclonal antibodies (MoAbs) against cell surface antigens of the mouse endothelioma cell line bEND.3. Three antibodies (V.1A7, V.5C7, and V.7C7) were selected, all of which recognize a 75-kD antigen on bEND.3 cells and bind selectively to endothelial cells in cryostat sections of mouse tissues. A cDNA for the antigen was isolated from a bEND.3 pCDM8 expression library by using transient expression in COS-7 cells and immunoselection with the three MoAbs. This cDNA coded for a novel, type I membrane protein of 248 amino acids with an extracellular domain rich in threonine and serine residues (35%). The protein is sensitive to O-sialoglycoprotein endopeptidase, indicating that it belongs to the class of sialomucin-like proteins. Therefore, we suggest the name endomucin. Treatment of isolated endomucin by sialidase and O-glycosidase reduced the apparent molecular weight to 45 kD and abolished binding of all three antibodies, indicating that carbohydrates are directly or indirectly involved in the formation of the antibody epitopes. Immunohistological analysis of all examined mouse tissues showed that endomucin is an endothelial antigen found in venous endothelium as well as in capillaries, but not on arterial endothelium. Interestingly, high endothelial venules of peripheral and mesenteric lymph nodes as well as of Peyers’s patches were negative for staining with the three MoAbs.

Lymphopoiesis and matrix glycoprotein SC1/ECM2

Rapid progress is being made in characterizing extracellular matrix and other components of the bone marrow microenvironment. New cloning strategies have been particularly helpful in identifying molecules made by marrow stromal cells. Matrix glycoprotein SC1/ECM2 (SC1/ECM2), a calcum-binding secreted protein, is one example and can contribute to the nurturing environment for B lymphocyte precursors. A fusion protein prepared from the SC1/ECM2 and the constant region of human immunoglobulin preferentially bound to B lineage cells in a divalent cation dependent manner. Furthermore, mitogen-dependent proliferation of mature B cells, as well as the cloning of pre-B cells, was increased in a dose dependent manner by addition of the fusion protein. SC1/ECM2 is also capable of augmenting lymphopoiesis when expressed as a transmembrane protein on fibroblasts. While the C-terminal portion of SC1/ECM2 has sequence homology to osteonectin/SPARC, the unique amino-terminal one fifth of the protein was sufficient to augment lymphocyte growth. As additional information accrues about the molecular requirements for lympho-hematopoiesis, it should become possible to engineer more efficient supporting microenvironments.

Follistatin: a multifunctional regulatory protein

Follistatin was first described in 1987 as a follicle-stimulating hormone inhibiting substance present in ovarian follicular fluid. We now know that this effect of follistatin is only one of its many properties in a number of reproductive and nonreproductive systems. A majority of these functions are facilitated through the affinity of follistatin for activin, where activin's effects are neutralized through its binding to follistatin. As such, the interplay between follistatin and activin represents a powerful regulatory mechanism that impinges on a variety of cellular processes within the body. In this review we focus on the biochemical characteristics of follistatin and its interaction with activin and discuss the emerging role of these proteins as potent tissue regulators in the gonad, pituitary gland, pregnancy membranes, vasculature, and liver. Consideration is also given to the larger family of proteins that contain follistatin-like modules, in particular with regard to their functional and structural implications.

Identification of an N-acetylglucosamine-6-0-sulfotransferase activity specific to lymphoid tissue: an enzyme with a possible role in lymphocyte homing

BACKGROUND

The leukocyte adhesion molecule L-selection participates in the initial attachment of blood-borne lymphocytes to high endothelial venules (HEVs) during lymphocyte homing to secondary lymphoid organs, and contributes to leukocyte adhesion and extravasation in HEV-like vessels at sites of chronic inflammation. The L-selection ligands on lymph mode HEVs are mucin-like glycoproteins adorned with the unusual sulfated carbohydrate epitope, 6-sulfo sialyl Lewis x. Sulfation of this epitope on the N-acetylglucosamine (GlcNAc) residue confers high-avidity L-selection binding, and is thought to be restricted in the vasculature to sites of sustained lymphocyte recruitment. The GlcNAc-6-0 sulfotransferase that installs the sulfate ester may be a key modulator of lymphocyte recruitment to secondary lymphoid organs and sites of chronic inflammation and is therefore a potential target for anti-inflammatory therapy.

RESULTS

A GlcNAc-6-0-sulfotransferase activity was identified within porcine lymph nodes and characterized using a rapid, sensitive, and quantitative assay. We synthesized two unnatural oligosaccharide substrates, GlcNAc beta 1-->6Gal alpha-R and Gal beta 1-->4GlcNAc beta 1-->6Gal alpha-R, that incorporate structural motifs from the native L-selection ligands into an unnatural C-glycosyl hydrocarbon scaffold. The sulfotransferase incorporated greater than tenfold more sulfate into the disaccharide than the trisaccharide, indicating a requirement for a terminal GlcNAc. Activity across tissues was highly restricted to the HEVs within peripheral lymph node.

CONCLUSIONS

The restricted expression of the GlcNAc-6-0-sulfotransferase activity to lymph node HEVs strongly suggestions a role in the biosynthesis of L-selection ligands. In addition, similar sulfated epitopes are known to be expressed on HEV-like vessels of chronically inflamed tissues; indicating that this sulfotransferase may also contribute to inflammatory lymphocyte recruitment. We identified a concise disaccharide motif, GlcNAc beta 1-->6Gal alpha-R, that preserved both recognition and specificity determinants for the GlcNAc-6-0-sulfotransferase. The absence of activity on the trisaccharide Gal beta 1-->6Gal alpha-R indicates a requirement for a substrate with a terminal GlcNAc residue, suggesting that sulfation precedes further biosynthetic assembly of L-selection ligands.

Identification of podocalyxin-like protein as a high endothelial venule ligand for L-selectin: parallels to CD34

The leukocyte adhesion molecule, L-selectin, mediates the recruitment of lymphocytes to secondary lymphoid organs via interactions with specific ligands presented on high endothelial venules (HEV). Although the HEV-derived ligands for L-selectin are still incompletely defined, they share a common sialomucin-like structure which is thought to present clustered oligosaccharides to the lectin domain of L-selectin. Podocalyxin-like protein (PCLP) is a transmembrane sialomucin that is similar in structure to the well-characterized L-selectin ligand CD34. PCLP has been shown previously to be expressed on the foot processes of podocytes in the kidney glomerulus as well as on vascular endothelium at some sites. We have determined that PCLP is present on HEV, where it binds to both recombinant L-selectin and the HEV-specific monoclonal antibody MECA-79. Furthermore, purified HEV-derived PCLP is able to support the tethering and rolling of lymphocytes under physiological flow conditions in vitro. These results suggest a novel function for PCLP as an adhesion molecule and allow the definition of conserved structural features in PCLP and CD34, which may be important for L-selectin ligand function.

Sulfation in high endothelial venules: cloning and expression of the human PAPS synthetase

High endothelial venules (HEVs) are specialized postcapillary venules found in lymphoid organs and chronically inflamed tissues that support high levels of lymphocyte extravasation from the blood. Studies with chlorate, a metabolic inhibitor of sulfation, had previously revealed that production of PAPS (3'-phosphoadenosine-5'-phosphosulfate), the high-energy donor of sulfate, is required for sulfation and high-affinity recognition of HEV sialomucins GlyCAM-1 and CD34 by the lymphocyte homing receptor L-selectin. Here, we report the molecular characterization of a novel 2.5 kb human cDNA from MECA-79+ HEV-derived endothelial cells that encodes the target of chlorate, PAPS synthetase, a multifunctional enzyme containing domains for both ATP sulfurylase and adenosine-5'-phosphosulfate kinase. Functional expression of the isolated cDNA in Chinese hamster ovary cells results in high levels of PAPS synthesis, which is abolished by treatment of the transfected cells with chlorate. Northern blot analysis reveals a wide tissue distribution of PAPS synthetase mRNA in the human body, suggesting that human PAPS synthetase may be important for sulfation not only of HEV sialomucins, but also of many other molecules, including mucins such as the P-selectin ligand PSGL-1, proteoglycans, hormones, neurotransmitters, drugs, and xenobiotics.

Biosynthesis of sulfated L-selectin ligands in human high endothelial venules (HEV)

High endothelial venules (HEVs) are specialized post-capillary venules found in lymphoid tissues, that support high levels of lymphocyte extravasation from the blood. Lymphocyte L-selectin plays a key role in the initial interaction of lymphocytes with HEVs by recognizing sulfated carbohydrate ligands on HEV mucin-like glycoproteins, GlyCAM-1, CD34 and MAdCAM-1. Sulfation is key to the uniqueness of the HEV ligands since 6 or 6'-sulfated-sLe(x) isoforms have recently been identified as major capping groups of GlyCAM-1 and sulfation of both GlyCAM1 and CD34 has been shown to be required for high-affinity L-selectin binding and recognition by the HEV-specific monoclonal antibody MECA-79. To characterize the molecular mechanisms involved in the biosynthesis of sulfated L-selectin ligands in HEVs, we have started to isolate genes that play a role in sulfate metabolism in HEVs. Studies with chlorate, a selective inhibitor of the synthesis of the high energy donor of sulfate, PAPS (3'-phosphoadénosine 5'-phosphosulfate), had previously revealed that PAPS synthesis is required for sulfation of HEV ligands and recognition by L-selectin. Therefore, we screened an HEV cDNA library in order to isolate cDNAs encoding enzymes involved in PAPS synthesis. This strategy allowed us to isolate a novel cDNA encoding the PAPS synthetase from human HEVs. The molecular characteristics of PAPS synthetase and its role in biosynthesis of sulfated L-selectin ligands in HEVs are discussed.

Human and murine high endothelial venule cells phagocytose apoptotic leukocytes

Apoptotic cell death occurs during normal lymphocyte development and differentiation as well as following lymphocyte exposure to endogenous corticosteroids released during stress, malnutrition, and trauma. Recognition and engulfment of these apoptotic cells is important for the clearance of dying cells before they release potent inflammatory mediators into the vasculature or tissues. Phagocytosis of apoptotic cells is accomplished in part by macrophages. We report for the first time that apoptotic lymphocytes are also phagocytosed by high endothelial venule (HEV) cells. The murine HEV cell line mHEVa rapidly phagocytosed apoptotic lymphoid and myeloid cells with the greatest rate of phagocytosis occurring at 0-6 h. To confirm HEV cell interaction with apoptotic cells, we demonstrated that apoptotic human tonsil lymphocytes were phagocytosed by human tonsil HEV cells in primary cultures. Furthermore, we examined HEV cell phagocytosis in vivo. Mice were treated with a natural corticosterone (4-pregnene-11 beta,21-diol-3,20-dione) at levels detected during stress or malnutrition (93-180 micrograms serum cortisol/dl). At 4-12 h posttreatment, apoptotic lymphocytes were present inside vacuoles of HEV cells in axillary lymph node tissue sections, as determined by transmission electron microscopy. These data suggest that, in addition to macrophages, lymph node HEV cells also play a role in the removal of apoptotic lymphocytes. Moreover, since HEV cells are specialized endothelial cells that regulate lymphocyte migration into peripheral lymphoid tissues, they may provide an important checkpoint for clearance of apoptotic lymphocytes within the vasculature, as well as limiting entrance of nonfunctional lymphocytes into the lymph node.

Spatiotemporal distribution of SPARC/osteonectin in developing and mature chicken retina

Expression of SPARC (Secreted Protein, Acidic, Rich in Cysteine), a counteradhesive, calcium-binding extracellular matrix (ECM) glycoprotein, is associated with several morphogenetic events during early development. In this study, changes in the spatiotemporal distribution of SPARC transcripts and the protein during chicken retinal development were documented by in situ hybridization and indirect immunofluorescence microscopy. SPARC transcripts were first detected within the proliferating neural ectoderm at embryonic day 4. 5 (E4.5), followed short thereafter (E5) by appearance of SPARC. SPARC was enriched within the inner plexiform layer (IPL) by E10 and within the outer plexiform layer (OPL) by E14, several days after these layers became morphologically distinct. Significant levels of SPARC transcripts were first observed within the ganglion cell layer (GCL) at E17 prior to accumulation of SPARC within the nerve fiber layer, seen first at E20. SPARC protein was first detected within the developing retinal pigment epithelium (RPE) at E10 and increased significantly at RPE cells ceased to proliferate and continued differentiating. Of special note was the restriction of SPARC to the basal-half of the RPE cells. SPARC transcripts were similarly distributed in the adult retina, but at lower levels than in the period just prior to hatching. In the adult retina SPARC was retained in the nerve fiber layer and present in the inner nuclear layer (INL) and outer nuclear layer (ONL), but lost from the IPL and OPL. These changes in expression pattern with time indicate that SPARC is developmentally regulated and therefore may have important function(s) in both morphological development of the retina and functioning of the mature eye.

Matrix Glycoprotein SC1/ECM2 Augments B Lymphopoiesis

The extracellular matrix produced by stromal cells plays a critical role in lympho-hematopoiesis. It was recently discovered that matrix glycoprotein SC1/ECM2 is a component of that matrix and preliminary evidence suggested that it could contribute to the nurturing environment for B-lymphocyte precursors. A fusion protein prepared from the amino terminal portion of SC1/ECM2 and the constant region of human Ig preferentially bound to pre-B cells. Furthermore, the cloning efficiency of interleukin-7–dependent B-cell precursors was increased in a dose-dependent manner by addition of this fusion protein. We now report the complete cDNA sequence for murine SC1/ECM2 and its localization to the central region of chromosome 5. A fusion protein prepared from the full length of SC1/ECM2 and Ig was found to recognize pre-B cells in a divalent cation-dependent manner, and to augment mitogen-dependent proliferation of mature B cells, as well as the cloning of pre-B cells, but to have no influence on myeloid progenitor cells. Although SC1/ECM2 is normally a secreted protein, we show that it is also capable of augmenting lymphopoiesis when expressed as a transmembrane protein on fibroblasts. Although the C-terminal portion of SC1/ECM2 has sequence homology to osteonectin/SPARC, the unique N-terminal one fifth of the protein was sufficient to augment lymphocyte growth.

Properties of the extracellular calcium binding module of the proteoglycan testican

The extracellular calcium-binding (EC) module of human testican (115 residues) was obtained in native form by recombinant production in mammalian cell culture and thus shown to represent an independently folding domain. This module showed a large loss in alpha-helix upon calcium depletion. Apparently only one of the two EF hands binds calcium, with a moderate affinity (Kd =68 microM) about 100-fold lower than in the homologous BM-40 protein. No clear evidence was obtained for collagen binding, indicating that EC modules found in different proteins may not share similar functions.

Organ-selective regulation of vascular adhesion protein-1 expression in man

Vascular adhesion protein-1 (VAP-1) is an endothelial molecule which mediates lymphocyte binding to endothelium in peripheral lymph nodes and at certain sites of inflammation. The expression of VAP-1 in vivo is strongly up-regulated in inflamed tissues, such as gut and skin. The purpose of this work was to examine the factors responsible for this induction of VAP-1. Since the expression of VAP-1 could not be induced in cultured endothelial cells with a large panel of mediators, we used an organ culture technique for the investigation of the regulation of VAP-1 expression in a more physiological micromilieu. Indeed, we found that the expression of endothelial VAP-1 could be up-regulated in human tonsillar tissue with interleukin (IL)-1, IL-4, tumor necrosis factor (TNF-alpha), interferon (IFN)-gamma and lipopolysaccharide, whereas histamine, thrombin, dibutyryl cAMP, N-formyl-Met-Leu-Phe (fMLP) and phorbol 12-myristate 13-acetate (PMA) had no effect. The induced VAP-1 protein was similar in molecular weight to the non-induced VAP-1, suggesting that VAP-1 synthesized de novo carries appropriate carbohydrate moieties. In contrast to tonsil organ culture, similar inductions performed with human appendix showed no up-regulation of VAP-1 expression, indicating that the regulation of VAP-1 expression exhibits organ-selective characteristics. Furthermore, in these tissues the smooth muscle cells, which constitutively express VAP-1, could not be stimulated to alter their level of expression of this molecule. In conclusion, the expression of VAP-1 can be markedly up-regulated with several mediators in tonsil but not in appendix organ culture, whereas cultured endothelial cells cannot be induced to express VAP-1. These results indicate that the expression of VAP-1 is regulated in a tissue- and cell type-selective manner, and a correct micromilieu is required for the up-regulation to occur.

Cloning and expression of murine SC1, a gene product homologous to SPARC

A number of cDNAs (SC1, QR1, and hevin) have been shown to be similar to SPARC (secreted protein acidic and rich in cysteine), a matricellular protein that regulates cell adhesion, cell cycle, and matrix assembly and remodeling. These proteins are 61-65% identical in the final 200 residues of their C-termini; their N-terminal sequences are related but more divergent. All have an overall acidic pl, with a follistatin-like region that is rich in cysteine, and a Ca+2 binding consensus sequence at the C-terminus. Using degenerate primers representing the most highly conserved region in SPARC, SC1, and QR1, we identified a 300-BP SC1 clone in a primary polymerase chain reaction (PCR) screen of a mouse brain cDNA library. This cDNA was used to obtain a full-length clone, which hybridized to a 2.8-KB RNA abundant in brain. Mouse SC1 displays a similarity of 70% to mouse SPARC at the amino acid level. Northern blot and RNAse protection assays revealed a 2.8-KB mRNA expressed at moderate levels (relative to brain) in mouse heart, adrenal gland, epididymis, and lung, and at low levels in kidney, eye, liver, spleen, submandibular gland, and testis. In contrast to SPARC, in situ hybridization showed expression of SC1 mRNA in the tunica media and/or adventitia of medium and large vessels; transcripts were not detected in capillaries, venules, or large lymphatics. The distribution of transcripts for SC1 was also different from that of SPARC in several organs, including adrenal gland, lung, heart, liver, and spleen. Moreover, SC1 mRNA was not evident in endothelium cultured from rat heart, bovine fetal and adult aorta, mouse aorta, human omentum, and bovine retina. Cultured smooth muscle cells and fibroblasts also failed to express SC1 mRNA. The absence of SC1 transcript in cultured cells indicates that the SC1 gene is potentially sensitive to regulatory factors in serum or to a three-dimensional architecture conferred by the extracellular matrix that is lacking in vitro. In conclusion, the expression of SPARC and SC1 appears to be coincident in specific tissues (e.g., adrenal gland and brain), but these proteins exhibit distinct expression patterns in most organs of the mouse. Because SC1 and SPARC are structurally similar and exhibit counteradhesive effects on cultured cells, their overlapping and/or adjacent expression in most tissues predicts that one protein might compensate functionally, at least in part, for the other.

Sialylated, fucosylated ligands for L-selectin expressed on leukocytes mediate tethering and rolling adhesions in physiologic flow conditions

Interaction of leukocytes in flow with adherent leukocytes may contribute to their accumulation at sites of inflammation. Using L-selectin immobilized in a flow chamber, a model system that mimics presentation of L-selectin by adherent leukocytes, we characterize ligands for L-selectin on leukocytes and show that they mediate tethering and rolling in shear flow. We demonstrate the presence of L-selectin ligands on granulocytes, monocytes, and myeloid and lymphoid cell lines, and not on peripheral blood T lymphocytes. These ligands are calcium dependent, sensitive to protease and neuraminidase, and structurally distinct from previously described ligands for L-selectin on high endothelial venules (HEV). Differential sensitivity to O-sialo-glycoprotease provides evidence for ligand activity on both mucin-like and nonmucin-like structures. Transfection with fucosyltransferase induces expression of functional L-selectin ligands on both a lymphoid cell line and a nonhematopoietic cell line. L-selectin presented on adherent cells is also capable of supporting tethering and rolling interactions in physiologic shear flow. L-selectin ligands on leukocytes may be important in promoting leukocyte-leukocyte and subsequent leukocyte endothelial interactions in vivo, thereby enhancing leukocyte localization at sites of inflammation.

The exon structure of the mouse Sc1 gene is very similar to the mouse Sparc gene

Sc1 and Sparc are two extracellular proteins sharing similarity in their carboxyl terminus, with 63% identity over a 232-amino-acid region. We have cloned and mapped the genomic locus of mouse Sc1. The mouse Sc1 gene contains 11 exons spanning approximately 35 kb of DNA. The genomic structure (exon/intron boundaries) of Sc1 exons 6 to exon 11 is identical to those of the similar portion of the Sparc gene. This suggests that Sc1 and Sparc originated from a common ancestral gene. Using fluorescence in situ hybridization analysis, Sc1 was localized to band 5E4 of mouse chromosome 5.

Renaturation of SPARC expressed in Escherichia coli requires isomerization of disulfide bonds for recovery of biological activity

SPARC (secreted protein acidic and rich in cysteine, also known as osteonectin and BM-40) belongs to a group of secreted macromolecules that modulate cellular interactions with the extracellular matrix. During vertebrate embryogenesis, as well as in tissues undergoing remodeling and repair, the expression pattern of SPARC is consistent with a fundamental role for this protein in tissue morphogenesis and cellular differentiation. Human SPARC was cloned by the polymerase chain reaction from an endothelial cell cDNA library and was expressed in Escherichia coli as a biologically active protein. Two forms of recombinant SPARC (rSPARC) were recovered from BL21(DE3) cells after transformation with the plasmid pSPARCwt: a soluble, monomeric form that is biologically active (Bassuk et al., 1996, Archiv. Biochem. Biophys. 325, 8-19), and an insoluble form sequestered in inclusion bodies. Aggregated rSPARC was unfolded by urea treatment, purified by nickel-chelate affinity chromatography, and renatured by gradual removal of the denaturant. Proper isomerization of the disulfide bonds was achieved in the presence of a glutathione redox couple. After final purification by high resolution gel filtration chromatography, a monomeric form of rSPARC displaying biological activity was obtained. The recombinant protein inhibited the spreading and synthesis of DNA by endothelial cells, two properties characteristic of the native protein. We conclude that the information for the correct folding of rSPARC resides in the primary structure of the protein, and suggest that post-translational modifications are required neither for folding nor for biological activity.

SC1, a brain extracellular matrix glycoprotein related to SPARC and follistatin, is expressed by rat cerebellar astrocytes following injury and during development

In the nervous system, extracellular matrix components are believed to influence cell shape, proliferation and migration during development and following injury. SC1 is a secreted glycoprotein expressed during neural development and in the adult brain. The molecule shows partial sequence homology to the anti-adhesive extracellular matrix molecule SPARC/osteonectin and to follistatin. We have made a surgical lesion in the adult rat cerebellum and examined changes in SC1 expression at 1 to 14 days after injury. Dual in situ hybridization/immunohistochemistry demonstrated that SC1 mRNA was induced in astrocytes surrounding the wound, reaching maximal levels at 10 days post-lesion. Immunohistochemistry revealed changes in the deposition of SC1 protein in radial fibres of Bergmann glia. SC1 protein was also detected at the border of the lesion, suggesting an association with the glial scar. Double immunohistochemistry with the astrocytic marker GFAP demonstrated that astrocytes also express SC1 during postnatal development.

CD44 and hyaluronan-dependent rolling interactions of lymphocytes on tonsillar stroma

Little is known about how lymphocytes migrate within secondary lymphoid organs. Stromal cells and their associated reticular fibers form a network of fibers that radiate from high endothelial venules to all areas of the lymph node and may provide a scaffold for lymphocyte migration. We studied interactions of lymphocytes with cultured human tonsillar stromal cells and their extracellular matrix using shear stress to distinguish transient interactions from firm adhesion. Tonsillar lymphocytes and SKW3 T lymphoma cells tethered and rolled on monolayers of cultured tonsillar stromal cells and their matrix. A significant proportion of these rolling interactions were independent of divalent cations and were mediated by CD44 binding to hyaluronan, as shown by inhibition with mAb to CD44, soluble hyaluronan, as hyaluronidase treatment of the substrate, and O-glycoprotease treatment of the rolling cells. O-glycoprotease treatment of the substrate also blocked binding completely to stromal matrix and partially to stromal monolayers. SKW3 cells tethered and rolled on plastic-immobilized hyaluronan, confirming the specificity of this interaction. By contrast, monolayers of resting or stimulated human umbilical vein endothelial cells failed to support CD44- and hyaluronan-dependent rolling. SKW3 cells added under flow conditions to frozen sections of human tonsil bound and rolled along reticular fibers in the presence of EDTA. Rolling was blocked by either CD44 mAb or hyaluronan. We propose that lymphocytes migrating through secondary lymphoid organs may use CD44 to bind to hyaluronan immobilized on stromal cells and reticular fibers.

SC1, a SPARC-related glycoprotein, exhibits features of an ECM component in the developing and adult brain

Although extracellular matrix (ECM) components have been shown to play important roles in the development of the CNS, expression generally decreases in the adult brain. This study examines the expression of the SPARC-related glycoprotein SC1 in the rat brain during postnatal development and in the adult. In situ hybridization analysis indicates that expression of SC1 mRNA increases in a caudal to rostral manner as postnatal neural development proceeds and is found at near maximal levels in the adult brain. SC1 mRNA is expressed in glial-enriched areas of the brain at postnatal day 1 (P1) and P5. Between P10 and P20, SC1 mRNA increases in neuron-enriched regions of the hippocampus, dentate gyrus, and cerebral cortex. Immunohistochemistry in the adult shows that SC1 protein is localized to neurons in these regions and to scattered glial cells. Subcellular fractionation demonstrates that the SC1 116/120 kDa doublet is associated with synaptosomes. SC1 is present in the aqueous phase following extraction of membranes with TX-114, suggesting that it is not a transmembrane protein, a property consistent with other adult brain ECM components. Furthermore in cerebellar granule cells grown in culture, high levels of the 120 kDa component are secreted into the media. These results are consistent with the hypothesis that SC1 is an ECM glycoprotein expressed in both the developing and adult brain.

Modulation of endothelial cell adhesion by hevin, an acidic protein associated with high endothelial venules

High endothelial venules (HEV) are specialized plump postcapillary venules in lymphoid tissues that support high levels of lymphocyte extravasation from the blood. We have recently identified a novel human transcript, expressed to high levels in HEV, that encodes a secreted, acidic protein closely related to the anti-adhesive extracellular matrix protein known as BM-40, osteonectin, and SPARC (secreted protein acidic and rich in cysteine). Here, we show that this protein, designated hevin, is associated with basal, lateral, and apical surfaces of HEV cells, and unlike MECA-79 antigen, is not expressed on the underlying basement membrane. In contrast to fibronectin or other adhesive extracellular matrix proteins, purified hevin does not support endothelial cell adhesion in vitro. Moreover, addition of soluble exogenous hevin inhibits attachment and spreading of endothelial cells on fibronectin substrates. Hevin-treated cells do not form focal adhesions and exhibit a rounded morphology. Together, these results suggest that hevin is an abundant extracellular protein that modulates high endothelial cell adhesion to the basement membrane.

QRI, a retina-specific gene, encodes an extracellular matrix protein exclusively expressed during neural retina differentiation

Neural retina development results from growth arrest of neuroectodermal precursors and differentiation of postmitotic cells. The QRI gene is specifically expressed in Müller retinal glial cells. Its expression coincides with the stage of withdrawal from the cell cycle and establishment of differentiation and is repressed upon induction of retinal cell proliferation by the v-src gene product. In this report, we show that the QR1 gene encodes several glycosylated proteins that are secreted and can either associate with the extracellular matrix or remain diffusible in the medium. By using pulse-chase experiments, the 100-103 kDa forms seem to appear first and are specifically incorporated into the extracellular matrix, whereas the 108 and 60 kDa polypeptides appear later and are detected as soluble forms in the culture medium. We also report that expression of the QR1 gene is developmentally regulated in the chicken. Its mRNA is first detectable at embryonic day 10, reaches a maximal level at embryonic day 15 and is no longer detected at embryonic day 18. Immunolocalization of the QR1 protein in chicken retina sections during development shows that expression of the protein parallels the differentiation pattern of post-miotic cells (in particular Müller cells and rods), corresponding to the two differentiation gradients in the retina: from the ganglion cell layer to the inner nuclear layer and outer nuclear layer, and from the optic nerve to the iris. At embryonic day 10, expression of the QR1 protein(s) is restricted to the optic nerve region and the inner nuclear layer, colocalizing with Müller cell bodies. As development proceeds, QR1 protein localization spreads towards the iris and towards the outer nuclear layer, following Müller cell elongations towards the photoreceptors. Between embryonic days 16 and 18, the QR1 protein is no longer detectable in the optic nerve region and is concentrated around the basal segment of the photoreceptors in the peripheral retina. Our results suggest a role for the QR1 gene product in the process of growth arrest and establishment of photoreceptor differentiation.

Sialomucin CD34 is the major L-selectin ligand in human tonsil high endothelial venules

Peripheral node addressin (PNAd) is a complex mixture of glycoproteins with L-selectin ligand activity that functions in lymphocyte homing. We have investigated the contribution of the sialomucin CD34 relative to other components of PNAd in lymphocyte tethering and rolling in in vitro laminar flow assays. PNAd was isolated with MECA-79 mAb-Sepharose from tonsillar stroma, and the CD34 component (PNAd,CD34+) and CD34-negative component (PNAd,CD34-) separated on CD34 mAb-Sepharose. Lymphocytes on the PNAd,CD34- fraction tether less efficiently, roll faster and are less resistant to shear detachment than on PNAd. The PNAd,CD34+ fraction constitutes about half the total functional activity. These studies show that CD34 is a major functional component of PNAd. Ligand activity in both the PNAd,CD34+ and PNAd,CD34- fractions is expressed on mucin-like domains, as shown with O-sialoglycoprotease. The CD34 component of PNAd has about four times higher tethering efficiency than total tonsillar CD34. CD34 from spleen shows no lymphocyte tethering. Although less efficient than the PNAd,CD34+ fraction from tonsil, CD34 from the KG1a hematopoietic cell line is functionally active as an L-selectin ligand despite lack of reactivity with MECA-79 mAb, which binds to a sulfation-dependent epitope. All four forms of CD34 are active in binding to E-selectin. KG1a CD34 but not spleen CD34 are active as L-selectin ligands, yet both lack MECA-79 reactivity and possess E-selectin ligand activity. This suggests that L-selectin ligands and E-selectin ligands differ in more respects than presence of the MECA-79 epitope.

High endothelial venules (HEVs): specialized endothelium for lymphocyte migration

High endothelial venules (HEVs) are specialized postcapillary venules found in lymphoid tissues that support high levels of lymphocyte extravasation from the blood. Here, Jean-Philippe Girard and Timothy Springer highlight the unique properties of HEV endothelium, discuss the molecular mechanisms controlling HEV specialization and review evidence suggesting that HEVs could play an important role in the pathogenesis of chronic inflammatory diseases.