The developmentally regulated avian Ch21 lipocalin is an extracellular fatty acid-binding protein.

Ch21, a developmentally regulated extracellular protein expressed in chick embryos and in cultured chondrocytes, was expressed in the baculovirus system, and the recombinant protein was purified to homogeneity by gel-filtration chromatography. Separation of two isoforms was achieved on an ion-exchange column. Previous work had shown that Ch21 belongs to the superfamily of lipocalins, which are transport proteins for small hydrophobic molecules. Studies were performed to identify the Ch21 ligand. By analysis of recombinant Ch21 on native polyacrylamide gel electrophoresis and by Lipidex assay, the binding of fatty acid to the protein was shown and a preferential binding of long-chain unsaturated fatty acids was observed. Both isoforms had the same behavior. The binding was saturable. Stoichiometry was about 0.7 mol of ligand/mol of protein. The protein binds the ligand in its monomeric form. Calculated dissociation constants were 2 X 10(-7) M for unsaturated fatty acids and 5 X 10(-7) M for stearic acid. The binding was specific; other hydrophobic molecules, as retinoic acid, progesterone, prostaglandins, and long-chain alcohols and aldehydes did not bind to the protein. Short-chain fatty acids did not bind to the protein. Ch21, also present in chicken serum, represents the first extracellular protein able to selectively bind and transport fatty acid in extracellular fluids and serum. We propose to rename the Ch21 protein as extracellular fatty acid-binding protein (Ex-FABP)

Articular cartilage and changes in Arthritis: Cell biology of osteoarthritis

The reaction patterns of chondrocytes in osteoarthritis can be summarized in five categories: (1) proliferation and cell death (apoptosis); changes in (2) synthetic activity and (3) degradation; (4) phenotypic modulation of the articular chondrocytes; and (5) formation of osteophytes. In osteoarthritis, the primary responses are reinitiation of synthesis of cartilage macromolecules, the initiation of synthesis of types IIA and III procollagens as markers of a more primitive phenotype, and synthesis of active proteolytic enzymes. Reversion to a fibroblast-like phenotype, known as 'dedifferentiation', does not appear to be an important component. Proliferation plays a role in forming characteristic chondrocyte clusters near the surface, while apoptosis probably occurs primarily in the calcified cartilage

Bone formation via cartilage models: the borderline chondrocyte

ncreasing evidence substantiates the view that death is not necessarily the only fate of hypertrophic chondrocytes and that, when exposed to the right microenvironment, these cells can further differentiate to osteoblast-like cells and contribute to initial bone formation. In vitro, when replated as adherent cells in the presence of ascorbic acid, hypertrophic chondrocytes resume cell proliferation, switch from the synthesis of the cartilage-characteristic type II and X collagens to the synthesis of type I collagen, and organize a mineralizing bone matrix. In vivo, expression of bone specific markers by growth plate chondrocytes occurs initially in early hypertrophic cells located at the mid-diaphysis and directly facing the osteogenic perichondrium. In bones formed via cartilage models, the first mineralized bone matrix (the earliest bony collar preceding vascular invasion and the onset of endochondral bone formation) is deposited at the outer aspect of the mid-diaphysis between rows of early hypertrophic chondrocytes and osteoblasts, which are arranged in a peculiar "vis à vis" fashion. The "vis à vis" organization of perichondrial osteogenic cells and peripheral early hypertrophic chondrocytes suggests that the latter cells are exposed -- compared to their cognate, the central hypertrophic chondrocytes -- to a specific microenvironment composed of unique matrix-originating signals and cellular cross-talks. A major role in the differentiation control of, and interaction between, hypertrophic chondrocytes and osteogenic perichondrial cells is certainly played by the Indian Hedgehog/PTHrP signalling system. We propose that all early hypertrophic chondrocytes have the inherent potential to differentiate to osteoblast-like cells and to contribute to initial bone formation, but that only chondrocytes positioned at the "borderland" between cartilage and (non-cartilage) osteogenic tissues undergo further differentiation to bone producing cells. We call these hypertrophic chondrocytes "borderline chondrocytes" to emphasize both their specific location and their dual differentiation potential. Hypertrophic chondrocytes located in different cartilage areas are exposed to an inappropriate matrix and endocrine/paracrine environment, cannot differentiate to osteoblast-like cells and therefore undergo apoptosis