Expression and phylogeny of claudins in vertebrate primordia

Claudins, the major transmembrane proteins of tight junctions, are members of the tetraspanin superfamily of proteins that mediate cellular adhesion a nd migration. Their functional importance is demonstrated by mutations in c laudin genes that eliminate tight junctions in myelin and the testis, aboli sh Mg2+ resorption in the kidney, and cause autosomal recessive deafness. H ere we report that two paralogs among 15 claudin genes in the zebrafish, Da nio rerio, are expressed in the otic and lateral-line placodes at their ear liest stages of development. Related claudins in amphibians and mammals are expressed in a similar manner in vertebrate primordia such as sensory plac odes, branchial arches, and limb buds. We also show that the claudin gene f amily may have expanded along the chordate stem lineage from urochordates t o gnathostomes, in parallel with the elaboration of vertebrate characters. We propose that tight junctions not only form barriers in mature epithelia, but also participate in vertebrate morphogenesis.