Abstract. The lymphocyte-high endothelial venule (HEV) cell interaction is an essential element of the immune system, as it controls lymphocyte recirculation between blood and lymphoid organs in the body. This interaction involves an 85-95-kD class of lymphocyte surface glycoprotein(s), CD44. A subset of lymphocyte CD44 molecules is modified by covalent linkage to chondroitin sulfate (Jalkanen, S., M. Jalkanen, R. Bargatze, M. Tammi, and E. C. . J. Immunol. 141 :1615-1623 . In this work, we show that removal of chondroitin sulfate by chondroitinase treatment of lymphocytes or incubation of HEV with chondroitin sulfate does not significantly inhibit lymphocyte binding to HEV, suggesting that chondroitin sulfate is not involved in endothelial cell recognition of lymphocytes. Affinity-purified CD44 antigen was, on the other hand, observed to bind native Type I collagen fibrils, laminin, and fibronectin, but not gelatin. Binding to fibronectin was studied more closely, and it was found to be mediated through the chondroitin sulfate-containing form of the molecule. The binding site on fibronec-

Abstract. We describe cDNA clones for a cell surface proteoglycan that bears both heparan sulfate and chondroitin sulfate and that links the cytoskeleton to the interstitial matrix. The eDNA encodes a unique core protein of 32,868 D that contains several structural features consistent with its role as a glycosaminoglycan-containing matrix anchor. The sequence shows discrete cytoplasmic, transmembrane, and NH2-terminal extracellular domains, indicating that the molecule is a type I integral membrane protein. The cytoplasmic domain is small and similar in size but not in sequence to that of the/~-chain of various integrins. The extracellular domain contains a single dibasic sequence adjacent to the extracellular face of the transmembrane domain, potentially serving as the proteasesusceptible site involved in release of this domain from the cell surface. The extracellular domain contains two distinct types of putative glycosaminoglycan attachment sites; one type shows sequence characteristics of the sites previously described for chondroitin sulfate attachment (Bourdon, M. A., T. Krusius, S. Campbell, N. B. Schwartz, and E. Ruoslahti. 1987. Proc. Natl. Acad. Sci. USA. 84:3194-3198), but the other type has newly identified sequence characteristics that potentially correspond to heparan sulfate attachment sites. The single N-linked sugar recognition sequence is within the putative chondroitin sulfate attachment sequence, suggesting asparagine glycosylation as a mechanism for regulating chondroitin sulfate chain addition. Both 5' and 3' regions of this eDNA have sequences substantially identical to analogous regions of the human insulin receptor eDNA: a 99-bp region spanning the 5' untranslated and initial coding sequences is 67 % identical and a 35-bp region in the 3' untranslated region is 81% identical in sequence. mRNA expression is tissue specific; various epithelial tissues show the same two sizes of mRNA (2.6 and 3.4 kb); in the same relative abundance (3:1), the cerebrum shows a single 4.5-kb mRNA. This core protein eDNA describes a new class of molecule, an integral membrane proteoglycan, that we propose to name syndecan (from the Greek syndein, to bind together).

The cell surface proteoglycan on normal murine mammary gland mouse mammary epithelial cells consists of an ectodomain bearing heparan and chondroitin sulfate chains and a lipophilic domain that is presumed to be intercalated into the plasma membrane. Because the ectodomain binds to matrix components produced by stromal cells with specificity and high affinity, we have proposed that the cell surface proteoglycan is a matrix receptor that binds epithelial cells to their underlying basement membrane. We now show that the proteoglycan surrounds cells grown in subconfluent or newly confluent monolayers, but becomes restricted to the basolateral surface of cells that have been confluent for a week or more; Triton X-100 extraction distinguishes three fractions of cell surface proteoglycan: a fraction released by detergent and presumed to be free in the membrane, a fraction bound via a salt-labile linkage, and a nonextractable fraction; the latter two fractions co-localize with actin filament bundles at the basal cell surface; and when proteoglycans at the apical cell surface are cross-linked by antibodies, they initially assimilate into detergent-resistant, immobile clusters that are subsequently aggregated by the cytoskeleton. These findings suggest that the proteoglycan, initially present on the entire surface and free in the plane of the membrane, becomes sequestered at the basolateral cell surface and bound to the actin-rich cytoskeleton as the cells become polarized in vitro. Binding of matrix components may cross-link proteoglycans at the basal cell surface and cause them to associate with the actin cytoskeleton, providing a mechanism by which the cell surface proteoglycan acts as a matrix receptor to stabilize the morphology of epithelial sheets.

We investigated the expression of proteoglycans (PGs) and hyaluronan (HA) during healing of human mucosal wounds. Biopsy specimens of experimental wounds were taken 1, 3, and 7 days after wounding. Frozen sections were used for immunolocalization of CD44, syndecan-1, basement membrane-associated heparan sulfate proteoglycan (BM-HSPG), decorin, and biglycan. HA was localized in paraffin sections with a specific HA-binding probe. Epithelium showed first signs of migration on Day 1, more progressive migration on Day 3, and epithelial sheets confronted on Day 7. CD44 surrounded migrating keratinocytes at all stages of wound healing. In epithelium, CD44 and HA remarkably localized to the same region. Expression of syndecan-1 was switched from the suprabasal cell layer of unwounded epithelium to the basal cell layer of the migrating wound epithelium. BM-HSPG was absent under migrating keratinocytes. It started to reappear at the basement membrane zone on Day 7. The area under the wound epithelium containing newly synthesized collagen fibers first became positive for decorin on Day 7, whereas staining of biglycan was negative. Granulation tissue was also strongly positive for CD44 and hyaluronan. Our results indicate that migrating keratinocytes express both CD44 and syndecan-1 but not BM-HSPG. During differentiation of keratinocytes, expression of CD44 preceded that of syndecan-1. The results suggest that different HSPGs have multiple functions in keratinocyte migration and differentiation during reepithelialization.

The core protein of the proteoglycan at the cell surface of NMuMG mouse mammary epitheial cells bears both heparan and chondroitin sulfate chains and is recognized by the monodonal antibody 281-2. Using this antibody and the peroxidase-antiperoxidase staining technique in adult mouse tissues, we found that the antibody recognizes the antigen in a highly restricted distribution, staining a variety of epithelial cells but no cells derived from embryonic mesoderm or neural crest. The antibody fails to stain any stromal (mesenchymal) or neuronal cells, with the exception ofplasma cells and Leydig cells. Squamous and transitional epitheia stain intensely over their entire surfaces, whereas cuboidal and columnar epithelia stain moderately and only at the lateral surface ofthe basal cells. Within squamous and transitional