FIBRONECTIN MESSENGER-RNA SPLICE VARIANT IN ARTICULAR-CARTILAGE LACKSBASES ENCODING THE V, III-15, AND I-10 PROTEIN SEGMENTS

Fibronectin is an extracellular matrix glycoprotein encoded by a singl e gene. Alternative RNA splicing has been reported at three sites, ED (extra type III domain)-A, ED-B, and the variable or V region. Articul ar cartilage fibronectin monomers are rarely (ED-A)(+), but approximat ely 25% are (ED-B)(+). RNA gel electrophoresis and Northern blot analy sis identified two (ED-B)(+) and two (ED-B)(-) fibronectin transcripts in cartilage, each pair differing by similar to 750 bases, This diffe rence results from a previously unreported RNA splicing pattern that e liminates 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, R everse transcriptase-polymerase chain reaction analyses of II addition al 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 fib ronectin expression but stop expressing (V+C)(-) transcripts over time . The tissue-specific expression pattern of this novel fibronectin iso form suggests that it may have an important function in the matrix org anization of cartilage.