L-Selectin is a calcium-dependent lectin-like receptor that is widely distributed on leukocytes. It belongs to the newly emerging selectin family of cell adhesion proteins, which includes E-selectin (ELAM-1) and P-selectin (PADGEM, GMP-140, CD62). L-Selectin was discovered as a lymphocyte homing receptor, involved in the organ-specific adherence of blood-borne lymphocytes to high endothelial venules (HEV) in lymph nodes. L-Selectin is now known to participate in the early stages of the interaction of neutrophils with venular endothelium at sites of acute inflammation. The endothelial and leukocyte ligands for the selectins are all fucosylated and all require sialic acid for functionality. The HEV-associated ligands for L-selectin are sulfated, fucosylated, and sialylated, O-linked glycoproteins. A major objective for future work is a detailed comparison of the carbohydrate structures of the selectin ligands. L-Selectin, in addition to its functions within the blood vascular compartment, can also mediate the attachment of lymphocytes, and possibly other leukocytes, to central nervous system myelinated tracts. This latter interaction may have pathophysiological significance in the etiology of demyelinating diseases. As the selectins participate in a number of leukocyte adhesive interactions pertinent to inflammation, these molecules are being investigated with increasing intensity.

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The Selectins are a family of adhesion molecules which mediate the binding of leukocytes to endothelial cells. ELAM1, a member of this family, is expressed on the surface of endothelial cells in response to cytokines and can facilitate the binding of neutrophils, monocytes, eosinophils, NK cells and CLA⁺ memory T cells. There is evidence that ELAM1 is present in both acute and chronic inflammatory diseases. In addition, there is data that suggest ELAM1 may be involved in metastasis of colon carcinoma. The putative ligand(s) for ELAM1 are discussed in detail, Currently, it is clear that a structure related to, but not necessarily identical to, the carbohydrate Sle^x [SAα2, 3Galβ1, 4(α1, 3Fuc) GlcNAc] is critical in most, if not all ELAM1 interactions. The important parts of this structure are the SA (sialic acid) and the fucose moiety.

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GMP-140, a member of the selectin family of adhesion molecules, is a receptor for neutrophils and monocytes that is expressed on the surface of activated platelets and endothelial cells. The corresponding ligand on leukocytes involves a carbohydrate structure including the CD15 antigen, lacto-N-fucopentaose III. GMP-140 mediated cell binding is likely critical in the hemostatic and inflammatory response to tissue and vascular injury. In addition these interactions may be important in pathologic processes such as atherosclerosis or metastasis. Future investigations will further elucidate structure-function relationships of GMP-140 and its ligand as well as the cellular consequences of adhesion. This data will lead to the design of in vivo studies to determine the biologic significance of these interactions.

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The neuron-glia cell adhesion molecule (Ng-CAM) mediates both neuron-glia and neuron-neuron adhesion. It is synthesized in neurons from a 6kb mRNA as 200kDa forms which are cleaved to yield two components of 135kDa and 80kDa. Amino acid sequence analysis indicates that it contains six immunoglobulin domains and five fibronectin type III repeats, a transmembrane domain and a cytoplasmic region. Ng-CAM mediates neuron-neuron adhesion by a homophilic mechanism and neuron-astrocyte adhesion by a heterophilic mechanism. The protein is expressed on neurons and Schwann cells but not on astrocytes. It is most prevalent during development on cell bodies of migrating neurons and on axons during formation of nerves; Ng-CAM expression is greatly increased following nerve injury. Anti-Ng-CAM antibodies inhibited migration of granule cells along Bergmann glia in the cerebellar explants and fasciculation of neurites in outgrowths from explants of dorsal root ganglia. In summary, Ng-CAM on neurons can bind to Ng-CAM on adjacent neurons and to as yet unidentified ligands on astrocytes.

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Carbohydrate Binding Protein 35 (CBP35) is a galactose-specific lectin belonging to the L-30 subgroup of the S-type family of animal lectins. The polypeptide chain (Mr-35, 000) consists of two distinct domains: a proline- and glycine-rich domain at the amino-terminal half and a carbohydrate recognition domain at the carboxyl-terminal half. The amino acid sequence information also indicates that CBP35 is identical to (within a given species) or homologous with (between species) proteins isolated and studied under other names: (i) L-34, a tumor cell lectin; (ii) human and rat lung lectins, HL-29 and RL-29; (iii) IgE-binding protein, εBP; (iv) LBP, a non-integrin type laminin-binding protein; and (v) Mac-2, a cell surface marker of thioglycollate-elicited macrophages. It is curious that the same polypeptide, under different guises, is found in two topologically distinct compartments of a cell: intracellular (cytosol and nucleus) and extracellular (cell surface and medium). Studies of the possible function (s) of this protein have, in turn, been guided by this dual localization: as a cell surface receptor for carbohydrate-containing ligands, including laminin and IgE, and as a nucleocytoplasmic shuttle in the form of a ribonucleoprotein complex.

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The glycosaminoglycans, heparin and heparan sulfate, are present in tissues in proteoglycan form and participate in a variety of cell-cell and cell-extracellular matrix interactions. Due to their strong negative charge and to their widely varying sequences, these polysaccharides bind to a large number of proteins. Three classes of heparin/heparan sulfate-binding proteins are known to be of physiological importance. These are the heparin/heparan sulfate-binding growth factors, extracellular matrix macromolecules and cell surface binding proteins. In this review, current knowledge in these three areas is described with emphasis given to the cell surface heparin/heparan sulfate-binding proteins which are putative “receptors” that may mediate the varied effects of heparin/heparan sulfate on cell behavior.

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NG2 is a large chondroitin sulfate proteoglycan which was originally identified on rat neural cell lines that could not be classified as being strictly neuronal or glial based on their electrophysiological properties. Immunocytochemical studies of tissue sections and cells in primary cultures revealed the expression of NG2 on dividing progenitor cells of the 02A glial lineage and on immature cells of mesenchymal origin. From biochemical analysis, NG2 was shown to be initially synthesized as a 260kDa core protein which was glycosylated to yield a mature core glycoprotein of 300kDa. It is estimated that each core contains 3-4 chondroitin sulfate chains. The primary structure of the NG2 core protein was determined from cDNA clones. The open reading frame codes for a protein of 2325 amino acids which can be divided into a large extracellular domain, a transmembrane domain, and a short cytoplasmic domain. The ectodomain can be further divided into three subdomains: two cysteine-rich regions separated by a region rich in serine-glycine pairs. One particular motif of 200 amino acids is repeated four times in the ectodomain. A portion of the sequence resembles the putative calcium-binding region of the cadherins. The overall sequence of NG2 has little homology with other known sequences, suggesting that NG2 is a novel integral membrane proteoglycan.

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Most proteins found in mammalians are glycoproteins. Their carbohydrate side chains play important roles in the biological selectivity, in particular, in recognition processes on membranes. All glycoproteins contain glycosidic linkages between the carbohydrate side chains and the peptide backbone. Therefore, the synthesis of glycopeptides requires methods for the stereoselective formation of these glycosidic linkages and selective protecting techniques which do not affect the glycosidic bonds. The benzylic protecting groups and their hydrogenolytic removal fulfil these demands. Since these protections, however, often are required for the blocking of the carbohydrate hydroxy functions, the functional groups of the peptide portion have to be protected with groups which are selectively removable and orthogonally stable towards the benzylic groups. In this sense, the fluorenylmethoxycarbonyl (Fmoc) group and its removal with the weak base morpholine and the allyloxycarbonyl (Aloc) groups removable by palladium (0) catalyzed allyl transfer revealed to be valuable amino protecting groups in glycopeptide chemistry. The allyl ester using the analogous palladium (0) chemistry and the acidolytically cleavable tert-butyl ester proved to be useful for the carboxy protection. The benzylic and the allylic (HYCRAM^™) ester principle are successfully demonstrated as the anchoring systems in solid phase syntheses of glycopeptides.

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Extracellular matrices in which cells migrate and proliferate during embryonic development are enriched in hyaluronan. Hyaluronan-binding proteins, termed hyaladherins, are a group of related proteins that serve as structural components of these extracellular matrices and as receptors at the surface of the cells therein. Recent evidence indicates that these proteins are involved in assembly of hyaluronan-rich, pericellular matrices and in aspects of cell behavior, especially cell migration and aggregation. Mesodermal cells of the early developing limb are surrounded by a hyaluronan-rich matrix, the production of which is regulated by basic fibroblast growth factor (FGF). When the cells of the chondrogenic and myogenic regions of the mesoderm condense, the cells produce less hyaluronan and become separated by a smaller volume of matrix. FGF levels diminish in the limb at this time but an FGF-like factor produced by the ectoderm maintains the high level of hyaluronan present in the periphery of the limb. Also at the condensation stage, the mesodermal cells exhibit increased levels of hyaluronanbinding sites on their surface. Endogenous cell surface hyaluronan bound to these sites can crossbridge adjacent cells, adhering them to one another, due to multivalent binding. Thus condensation can be explained, at least in part, by three facets of hyaluronan-cell interactions: 1) receptor-mediated endocytosis of pre-existing pericellular hyaluronan; 2) decreased hyaluronan production leading to cessation of coat production; 3) stabilization of the condensate by hyaluronan-receptor crossbridging. During differentiation of the condensed mesoderm to cartilage, hyaluronan-binding sites persist on the cell surface. These sites are also essential for assembly of the pericellular matrix surrounding chondrocytes.

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The biological roles of cell surface glycosphingolipids are being determined due to improvements in the methods for their purification, structural and functional analyses. The hypothesis that this major family of cell surface complex carbohydrates participates in important cell recognition and cell regulation phenomena has now been repeatedly demonstrated. As specific glycosphingolipids are implicated in a greater variety of cell regulation events, there is a continuing need for studies to elucidate the mechanisms of their actions. These are likely to converge on two general schemes: Interactions of glycosphingolipids with complementary receptors on apposing membranes (trans recognition), and modulation of activities of proteins in the same membrane (cis regulation). A combination of these mechanisms may link glycosphingolipid recognition to transmembrane control of cell physiology.

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In mammals the uptake and maintenance of glucose in plasma is achieved in part by the action of specific glucose transporters. These are of two types: the sodium-dependent glucose transporters (SGLTs) which actively transport glucose against its concentration gradient by coupling with Na⁺; and the facilitated glucose transporters (GLUTs) which operate by facilitated diffusion. These transporters have been suggested to play roles in differentiation, tumorigenesis and development of several diseases, including diabetes, but the mechanisms underlying these processes are unclear. The use of molecular biological techniques, which began with the cloning of the cDNA of erythroid-type glucose transporter (GLUT-1) in 1985, has resulted in the cloning of five isoforms of GLUTs and of an SGLT within 5 years, and much has been learned about their chromosomal locations and amino acid sequences as well as how these molecules are oriented in the plasma membrane. These exciting advances provide a new dimension to the study of regulation mechanism of of glucose transport at the molecular level.

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