Fibronectin mRNA splice variant in articular cartilage lacks bases encoding the V, III-15, and I-10 protein segments

Fibronectin is an extracellular matrix glycoprotein encoded by a single gene. Alternative RNA splicing has been reported at three sites, ED (extra type III domain)-A, ED-B, and the variable or V region. Articular cartilage fibronectin monomers are rarely (ED-A)+, but approximately 25% are (ED-B)+. RNA gel electrophoresis and Northern blot analysis identified two (ED-B)+ and two (ED-B)- fibronectin transcripts in cartilage, each pair differing by approximately 750 bases. This difference results from a previously unreported RNA splicing pattern that eliminates not only the V region but also nucleotides encoding protein segments III-15 and I-10. This new splice variant, which we designate (V+C)-, represents the majority of fibronectin transcripts in equine, canine, and rabbit articular cartilage but is absent in the liver. Reverse transcriptase-polymerase chain reaction analyses of 11 additional equine tissues failed to detect the (V+C)- splice variant, except for very low levels in lymph node, bone, aorta, and skin. Furthermore, chondrocytes grown in monolayer culture maintain high levels of fibronectin expression but stop expressing (V+C)- transcripts over time. The tissue-specific expression pattern of this novel fibronectin isoform suggests that it may have an important function in the matrix organization of cartilage.