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 similar to 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 II 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 main tain 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.