Post-translational Addition of Chondroitin Sulfate Glycosaminoglycans

Structural deciphering of the NG2/CSPG4 proteoglycan multifunctionality

The chondroitin sulfate proteoglycan 4 ( CSPG4) gene encodes a transmembrane proteoglycan (PG) constituting the largest and most structurally complex macromolecule of the human surfaceome. Its transcript shows an extensive evolutionary conservation and, due to the elaborated intracellular processing of the translated protein, it generates an array of glycoforms with the potential to exert variant-specific functions. CSPG4-mediated molecular events are articulated through the interaction with more than 40 putative ligands and the concurrent involvement of the ectodomain and cytoplasmic tail. Alternating inside-out and outside-in signal transductions may thereby be elicited through a tight functional connection of the PG with the cytoskeleton and its regulators. The potential of CSPG4 to influence both types of signaling mechanisms is also asserted by its lateral mobility along the plasma membrane and its intersection with microdomain-restricted internalization and endocytic trafficking. Owing to the multitude of molecular interplays that CSPG4 may engage, and thanks to a differential phosphorylation of its intracellular domain accounted by crosstalking signaling pathways, the PG stands out for its unique capability to affect numerous cellular phenomena, including those purporting pathologic conditions. We discuss here the progresses made in advancing our understanding about the structural-functional bases for the ability of CSPG4 to widely impact on cell behavior, such as to highlight how its multivalency may be exploited to interfere with disease progression.-Tamburini, E., Dallatomasina, A., Quartararo, J., Cortelazzi, B., Mangieri, D., Lazzaretti, M., Perris, R. Structural deciphering of the NG2/CSPG4 proteoglycan multifunctionality.

Prostaglandin E2 increases transforming growth factor-beta type III receptor expression through CCAAT enhancer-binding protein delta in osteoblasts

Variations in individual TGF-beta receptors (TbetaRs) may modify TGF-beta activity and significantly alter its effects on connective tissue growth or repair. Differences in the amount of TbetaR type III (TbetaRIII) relative to signal transducing TbetaRI occur on bone cells during differentiation or in response to other growth regulators. Here we investigated prostaglandin (PG) E2, a potent effector during trauma, inflammation, or mechanical load, on TbetaR expression in primary osteoblast-enriched cultures. PGE2 rapidly increased TbetaRIII mRNA and protein expression and enhanced TbetaRIII gene promoter activity through a discrete region within 0.4 kb of the transcription start site. PGE2 alters osteoblast function through multiple signal-inducing pathways. In this regard, protein kinase A (PKA) activators, PGE1 and forskolin, also enhanced gene expression through the TbetaRIII gene promoter, whereas protein kinase C activators, PGF2alpha and phorbol myristate acetate, did not. The stimulatory effect of PGE2 on TbetaRIII promoter activity was suppressed by a dominant negative PKA-regulatory subunit, but not by dominant negative protein kinase C. PGE2 specifically increased nuclear factor CCAAT enhancer-binding protein delta (C/EBPdelta) binding to a half-binding site upstream of the basal TbetaRIII promoter region, and promoter activity was sensitive to C/EBPdelta overexpression and to dominant-negative C/EBPdelta competition. In parallel with their effect on TbetaRIII expression, activators of PKA decreased TGF-beta-induced activity. In summary, high levels of PGE2 that occur with inflammation or trauma may, through PKA-activated C/EBPdelta, preferentially increase TbetaRIII expression and in this way delay TGF-beta-dependent activation of osteoblasts during the early stabilization phase of bone repair.

Limited Diversity of Human scFv Fragments Isolated by Panning a Synthetic Phage-Display scFv Library with Cultured Human Melanoma Cells

To broaden the specificity of the Abs recognizing human melanoma-associated Ags (MAAs), we have isolated human single-chain fragment of the V region (scFv) fragments by panning the synthetic phage Ab library (#1) with the human melanoma cell lines S5 and SK-MEL-28. All of the isolated scFv fragments reacted with the mouse mAb defined high molecular weight melanoma-associated Ag (HMW-MAA). scFv #70 immunoprecipitates the two characteristic subunits of HMW-MAA, while scFv #28 only immunoprecipitates its large subunit. These results challenge the current view regarding the structure of HMW-MAA and indicate that it consists of two independent subunits. The human scFv fragments share some similarities with the mouse anti-HMW-MAA mAb. Like mAb 149.53 and 225.28, scFv #28 reacts with rat B49 neural cells that express a homologue of HMW-MAA. scFv #70 reacts with a determinant that is spatially close to the one identified by mAbs 149.53, VT68.2, and VT86. Besides suggesting similarities in the recognition of human melanoma cells by the mouse and human Ab repertoire, these results indicate that the Abs isolated from synthetic Ab libraries resemble those that are found in natural Ab repertoires. The restricted diversity of the scFv fragments that were isolated by panning synthetic Ab libraries with different melanoma cell lines suggests that certain Ags, like HMW-MAA, are immunodominant in vitro. This phenomenon, which parallels the in vivo immunodominance of certain Ags, implies that the antigenic profile of the cells used for panning determines the specificity of the preponderant population of isolated Abs.

Inhibitors of N-linked oligosaccharide processing glucosidases interfere with oligodendrocyte differentiation in culture

Previous studies have demonstrated that inhibitors of glycoprotein processing glucosidases interfere with the development of oligodendrocyte properties in primary cultures of embryonic rat brain cells (Bhat, J Neurosci Res 20:158-164, 1988). The present study examines the effect of castanospermine, an inhibitor of the processing glucosidases, on the development and differentiation of isolated oligodendrocyte progenitor cells. Treatment of oligodendrocyte progenitors with castanospermine did not affect the developmental progression of the precursors to become committed oligodendrocytes as revealed by comparable increases in the percentages of cells positive for galactocerebroside (a surface marker for terminally differentiated oligodendrocytes) in control and drug-treated cultures. On the other hand, there was an impairment of the expression of differentiated properties of oligodendrocytes [i.e., sulfolipid synthesis, myelin basic protein (MBP)] and 2'3'-cyclic nucleotide 3'-phosphohydrolase in the drug-treated cultures. Immunocytochemical analysis with anti-MBP antibodies revealed a reduced number of MBP-positive cells in inhibitor-treated cultures. Furthermore, a majority of MBP-positive cells in such cultures displayed immunoreactive MBP in their cell body and not the processes, unlike in control cultures where both cell body and the processes of oligodendrocytes stained intensely for MBP. The strong inhibitory effect of castanospermine on the expression of oligodendrocyte-specific activities was contrasted with a relatively smaller effect of swainsonine, a mannosidase inhibitor on oligodendrocyte differentiation. Both castanospermine and swainsonine, however, effectively blocked the formation of complex-type oligosaccharides, suggesting thereby a lack of correlation between the inhibition of the formation of complex-type oligosaccharides and oligodendrocyte differentiation. It is suggested, therefore, that early trimming reactions involving the removal of glucose residues from the high mannose oligosaccharides in the endoplasmic reticulum may be essential for the cell surface localization and function of glycoproteins critically involved in surface interactions of oligodendrocytes with each other and/or with the substratum.

Effects of cycloheximide, brefeldin A, suramin, heparin and primaquine on proteoglycan and glycosaminoglycan biosynthesis in human embryonic skin fibroblasts

(1) We have isolated radiolabelled proteoglycans and glycosaminoglycans produced by human embryonic skin fibroblasts in the presence of (a) cycloheximide to inhibit protein synthesis or (b) brefeldin A to impede transport between the endoplasmic reticulum and the Golgi complex or (c) suramin, heparin or primaquine to interfere with internalization, recycling and degradation. Effects on glycosaminoglycan synthesis were assayed separately by using exogenous p-nitrophenyl beta-D-xylopyranoside (and [3H]galactose) or 125I-labelled p-hydroxyphenyl beta-D-xylopyranoside as initiators. (2) Inhibition of protein synthesis or blocking of transport to the Golgi complex prevented production of most of the proteoglycans with one exception: Cell-associated heparan sulphate-proteoglycan was still produced at 20% of the control level. (3) Treatment with suramin or heparin resulted in decreased deposition of proteoglycan in the pericellular matrix but increased accumulation of cell-associated proteoglycan. Primaquine blocked all proteoglycan synthesis. (4) In the presence of cycloheximide, exogenous beta-D-xyloside initiated galactosaminoglycan production. In contrast, in brefeldin A-treated cells, synthesis was completely abolished. Not even formation of the linkage-region trisaccharide could be detected. (5) These results suggest that exogenous xyloside enters the endoplasmic reticulum and is subsequently transported to the trans-Golgi complex where all further steps involved in glycosaminoglycan assembly takes place. (6) Heparan sulphate proteoglycan produced by brefeldin A-treated cells could be derived from (a) an intracellular pool of preformed core protein located to the trans-Golgi complex, or (b) resident proteoglycan that was either deglycanated/reglycanated or chain-extended. As combined treatment with suramin and brefeldin A markedly reduced cell-associated proteoglycan production, the latter possibility is favoured.

Influence of N-linked oligosaccharides on the processing and neurite-promoting activity of proteoglycans released by neurons in vitro

Inhibitors of enzymes involved in processing of N-linked oligosaccharides were used to examine biosynthesis and the neurite-promoting activity of proteoglycans produced by and released from dissociated chick embryo spinal cord neurons in vitro. In the cell compartment and in conditioned medium both castanospermine and swainsonine inhibited 3H-glucosamine incorporation into glycoprotein but only castanospermine reduced 3H-glucosamine incorporation into heparan sulphate and chondroitin sulphate proteoglycans. All of the neurite-promoting activity of neuron-conditioned medium that complexed to laminin was associated with heparan sulphate proteoglycans as determined by heparitinase digestion. Neuron-conditioned medium prepared in the presence of castanospermine displayed a 38 +/- 6% (mean +/- SD) reduction in 3H-glucosamine incorporation into heparan sulphate proteoglycans and a 30 +/- 5% reduction in substrate-attached neurite-promoting activity compared to control conditioned medium and to conditioned medium prepared in the presence of swainsonine. When neurons were coincubated with castanospermine, neurite growth on a laminin substrate was 50 +/- 10% of control growth or growth in the presence of swainsonine. However, when neuron-conditioned medium was used to pretreat the laminin substrate the inhibitory effect on neurite growth produced by castanospermine coincubation was reversed. Influences on neuronal processing of N-linked oligosaccharides alter neurite growth directly and also alter the neurite-promoting activity of neuron-conditioned medium by inhibiting the synthesis of heparan sulphate proteoglycans. These studies provide further evidence for an autocrine role for heparan sulphate proteoglycans in neurite growth.

A novel prostate carcinoma-associated glycoprotein complex (PAC) recognized by monoclonal antibody TURP-27

A prostate carcinoma-associated antigen recognized by MAb TURP-27 was characterized immunohistochemically and biochemically. TURP-27 antigen was found localized in the cell membrane and cytoplasm of the ductal epithelial cells of normal (10%), benign (75-100%) and malignant (20-100%) prostate cells. Fetal prostate tissues were also found to express the TURP-27 antigen, suggesting expression early in development. This antigen was not expressed by non-prostate tumors examined, but significant cross-reactivity was observed in myelinated nerves while minor cross-reactivity was seen in certain lymphocyte subsets, cells in the adrenal medulla and chief cells of stomach. Immunoblotting and biochemical data demonstrated that the TURP-27 antigen is a sialic-acid-containing glycoprotein complex with major molecular species in prostate tissues of 310-250, 180, 140, 115, 95-90, 69, and 40- to 35-kDa. Immunoblotting patterns similar to those observed for prostate tissues were also seen in CNS extracts with the exception of the 69 and 40- to 35-kDa proteins. This prostate carcinoma-associated sialoglycoprotein complex (PAC) recognized by MAb TURP-27 is likely to represent a novel tumor antigen expressed by prostate tumors.

Effect of inhibitors of N-linked oligosaccharide processing on the cell surface expression of a melanoma integrin

The role of trimming and processing of N-linked oligosaccharides on the cell surface expression of the melanoma vitronectin receptor, a member of the integrin family of cell adhesion receptors, was examined by using specific glucosidase and mannosidase inhibitors. Inhibition of glucosidases I and II by castanospermine or N-methyldeoxynojirimycin delayed the vitronectin receptor alpha/beta chain heterodimer assembly and alpha chain cleavage and resulted in a decrease in the level of expression cell surface receptor. Conversely, the vitronectin receptor synthesized in the presence of the mannosidase I and II inhibitors, 1-deoxymannojirimycin and swainsonine, was transported normally to the cell surface with its alpha chain N-linked oligosaccharides in an endoglycosidase H-sensitive form. In the presence of swainsonine, time course studies of the cell surface replacement of control, endoglycosidase H-resistant receptor with an endoglycosidase H-sensitive form demonstrated a vitronectin receptor half-life of approximately 15-16 h. These studies provide evidence that the rates of assembly, proteolytic cleavage, and cell surface expression of the melanoma vitronectin receptor are dependent on the initial trimming of glucosyl residues from the alpha chain N-linked oligosaccharides.