EXPRESSION OF THE (V-) FIBRONECTIN ISOFORM IS TIGHTLY LINKED TO THE PRESENCE OF A CARTILAGINOUS MATRIX(C)()

Fibronectin is encoded by a single gene, but heterogeneity is introduc ed by alternative splicing of the pre-mRNA. An unique splice variant, designated (V+C)(-), which deletes nucleotides encoding the V, III-15 and I-10 segments, has been identified in articular cartilage. In this study, a ribonuclease protection assay was used to quantitate express ion of the (V+C)(-) isoform in eight canine cartilaginous tissues and in chondrocytes cultured as monolayers or in alginate beads. The (V+C) - fibronectin isoform was detected in all cartilaginous tissues examin ed, ranging from a low of 11% of steady-state fibronectin mRNA in the nucleus pulposus to 71% in the rib. An age dependent increase, from 18 % in the epiphyseal cartilage of a newborn to 54% in the articular car tilage of dogs over 10 months of age, was observed. The ubiquitous pre sence of this isoform in cartilaginous tissues and its absence in all non-cartilaginous tissues examined to date is consistent with a very s trong association of the (V+C)fibronectin isoform with the cartilagino us phenotype. Results from a ribonuclease protection assay using a pro be extending into the V region from III-14 were combined with the quan titative information about (V+C)(-) fibronection expression to develop an over-all profile of splicing within the V region in cartilage.Mono layer culture of articular chondrocytes altered fibronectin splicing p atterns. The (V+C)(-) isoform was rapidly lost and ED-A(+) fibronectin was induced. Three-dimensional culture in alginate beads prevented in duction of ED-A(+) fibronection, but failed to sustain expression of t he (V+C)- isoform. Thus, some matrix component or structure, lost in c ell culture, may be essential to maintain expression of the (V+C)- iso form. The possible relationship of changing patterns of fibronectin is oforms in cultured chondrocytes to maintenance of the differentiated p henotype is discussed.