Aggrecan, the Primary Weight-Bearing Cartilage Proteoglycan, Has Context-Dependent, Cell-Directive Properties in Embryonic Development and Neurogenesis: Aggrecan Glycan Side Chain Modifications Convey Interactive Biodiversity

This review examines aggrecan’s roles in developmental embryonic tissues, in tissues undergoing morphogenetic transition and in mature weight-bearing tissues. Aggrecan is a remarkably versatile and capable proteoglycan (PG) with diverse tissue context-dependent functional attributes beyond its established role as a weight-bearing PG. The aggrecan core protein provides a template which can be variably decorated with a number of glycosaminoglycan (GAG) side chains including keratan sulphate (KS), human natural killer trisaccharide (HNK-1) and chondroitin sulphate (CS). These convey unique tissue-specific functional properties in water imbibition, space-filling, matrix stabilisation or embryonic cellular regulation. Aggrecan also interacts with morphogens and growth factors directing tissue morphogenesis, remodelling and metaplasia. HNK-1 aggrecan glycoforms direct neural crest cell migration in embryonic development and is neuroprotective in perineuronal nets in the brain. The ability of the aggrecan core protein to assemble CS and KS chains at high density equips cartilage aggrecan with its well-known water-imbibing and weight-bearing properties. The importance of specific arrangements of GAG chains on aggrecan in all its forms is also a primary morphogenetic functional determinant providing aggrecan with unique tissue context dependent regulatory properties. The versatility displayed by aggrecan in biodiverse contexts is a function of its GAG side chains.

Optimalized transient transfection of chondrogenic primary cell cultures

We aimed to find a transfection method which provides high efficiency with minimal cytotoxic and/or apoptotic effects for gene transfer into multilayer primary chondrogenic cell cultures. The pEGFP-C1 plasmid was introduced into the cell culture and the efficiency of transformation quantified by GFP fluorescence; the resulting nucleofection was effective but resulted in severe apoptosis. Two liposomal reagents designed to allow transfection into adherent cells did not deliver the plasmids sufficiently and cartilage formation did not occur. In addition, a third liposomal compound, recommended for transfection into either adherent or suspension cell cultures, lead to acceptable transfection efficiency but no cartilage formation. When an amphiphilic reagent was used however, there was acceptable transfection efficiency as well as cartilage formation. The viability of the cells which were transfected using the amphiphilic reagent remained unaffected but proliferation was severely diminished, particularly in the presence of GFP. In addition, the amount of cartilage decreased when GFP was expressed, despite unchanged levels of mRNAs of sox9 and aggrecan core protein, factors reflecting on the efficiency of chondrogenesis. Overexpression of both the constitutively active delta and gamma isoforms of catalytic subunit of calcineurin, a protein phosphatase described as a positive regulator of chondrogenesis, decreased protein level of Sox9 and subsequent cartilage formation. In conclusion, we found that amphiphilic reagent applied prior to the adhesion of cells provides a useful means to transfer plasmids to primary differentiating chondrogenic cells.

Chondrocytes and Fibrochondrocytes Differentially Process Aggrecan During De Novo Extracellular Matrix Assembly

The material properties of articular cartilage and meniscal fibrocartilage depend on the composition and ultrastructure of the extracellular matrix (ECM). Aggrecan is the predominant large proteoglycan in these tissues, and confers compressive stiffness through immobilization of negatively charged sulfated glycosaminoglycans (sGAG). The abundance of sGAG is in part regulated by cell-mediated proteolysis of the aggrecan core protein, and transforming growth factor-β (TGF-β) family cytokines upregulate aggrecan synthesis in chondrocytes and fibrochondrocytes. The function(s) of aggrecan and mechanisms of aggrecan processing in the meniscus, however, are not well understood. The objective of this study was to examine tissue-specific kinetics and mechanisms of TGF-β-induced aggrecan turnover using the cell-agarose culture system. In addition, the tissue-specific functional implications of increased proteoglycan production were evaluated in terms of construct material properties.

Polysulphated glycosaminoglycans modulate transcription of interleukin-1β treated chondrocytes in monolayer culture

SummaryThe ability of polysulphated glycosaminoglycans (PSGAGs, Adequan®) to modulate the transcription of major articular cartilage matrix proteins and enzymes was examined. Northern blot analyses were used to compare steady-state mRNA levels of type-II procollagen, aggrecan core protein, matrix metalloproteinase (MMP)-1, MMP-3, tissue inhibitor of metalloproteinase (TIMP)-1, and TIMP-3 in equine chondrocytes grown in monolayer culture. The groups included: control, low-dose (0.1mg/ml) Adequan®, high-dose (1.0mg/ml) Adequan®, IL-1 β (10ng/ml), IL-1 β with low-dose Adequan®, and IL-1 β with high-dose Adequan®. The responses (p<0.05) to treatment were compared with one-way analysis of variance and post hoc least significant difference test. Interleukin-1 β significantly decreased steady-state levels of type-II procollagen (0.5x) and aggrecan core protein (to undetectable levels) and significantly increased MMP-1 (11.7x), MMP-3 (1.8x), TIMP-1 (1.9x), and TIMP-3 (5.1 x). Adequan® alone significantly increased steady-state mRNA levels of type-II procollagen (1.6x) and decreased MMP-1 (0.3x at high-dose only), MMP-3 (0.5x), and TIMP-1 (0.3x at high-dose only). In IL-1 β-treated chondrocytes, Adequan® significantly increased steady-state mRNA levels of type-II procollagen (1.4x of IL-1 β-treated levels), aggrecan core protein (IL-1 β -treated had no detectable expression), and MMP-3 (3.6 x at high dose) and significantly decreased steady-state mRNA levels of MMP-1 (0.3 x), and TIMP-1 (0.2x). This study is the first to demonstrate that Adequan® alters steady-state mRNA levels of key matrix proteins and enzymes; that it counteracts some of the deleterious effects of IL-1 β; and that these effects are in part dose-dependent.

Hyaluronate degradation as an alternative mechanism for proteoglycan release from cartilage during interleukin-1β-stimulated catabolism

Data presented previously suggest that release of components of the cartilage matrix, in response to catabolic agents, cannot be accounted for by proteolytic mechanisms alone. In the present study, the release of glycosaminoglycan-containing components from bovine nasal cartilage cultured in the presence of interleukin-1β, and from bovine nasal, fetal bovine epiphyseal and adult human articular cartilage cultured in the presence of retinoic acid, was accompanied by the loss of link protein and hyaluronate into the culture medium. Chromatographic analysis of the released hyaluronate showed it to be markedly reduced in size relative to that extracted from the corresponding tissue. It is proposed that, under stimulation by catabolic agents, two independent, but concurrent, mechanisms act to promote the release of aggrecan from the cartilage matrix. First, proteolytic cleavage of the aggrecan core protein results in the production of glycosaminoglycan-containing fragments that are free to diffuse from the tissue. Secondly, cleavage of hyaluronate renders portions of the proteoglycan aggregate small enough so that complexes of aggrecan (or fragments containing its G1 domain) and link protein are released from the tissue. It is likely that both mechanisms contribute to cartilage metabolism in normal physiology and pathology.

Chondrocyte-mediated catabolism of aggrecan: aggrecanase-dependent cleavage induced by interleukin-1 or retinoic acid can be inhibited by glucosamine

A rat chondrosarcoma cell line and bovine cartilage explants have been used to study the control of aggrecan degradation by chondrocytes treated with interleukin-1 (IL-1) or retinoic acid (RA). Aggrecan fragment analysis with anti-neo-epitope antibodies suggests that aggrecanase (an as yet unidentified enzyme) is the only aggrecan-degrading proteinase active in these cultures. With rat cells, aggrecanase converts the aggrecan core protein into two major G1-domain-bearing products (60 kDa with a C-terminal Glu-373, and 220 kDa with a C-terminal Glu-1459). Both products were quantified on a standardized Western analysis system with a G1-specific antibody. Immunoblots were analysed by scanning densitometry and the sensitivity, linearity and reproducibility of the assay were established. With rat cells the aggrecanase response to IL-1 was optimal at about 2 mM glutamine, but was progressively inhibited at higher concentrations, with about 90% inhibition at 10 mM glutamine. Such inhibition by glutamine was not, however, observed with bovine explants. On the other hand, marked inhibition of aggrecanase-dependent cleavage was observed with both rat cells and bovine explants when d(+)-glucosamine was included at concentrations above 2 mM. Inhibition was apparently not due to cytotoxicity or interference with IL-1 signalling, since biosynthetic activity was not inhibited and inhibition of the aggrecanase response was also obtained when RA was used as the catabolic stimulator. Possible mechanisms for the inhibition of the aggrecanase response by glucosamine in chondrocytes treated with IL-1 or RA are discussed.