Expression and evolution of the mammalian brain gene Ttyh1

Homologues of the Drosophila melanogaster tweety (tty) gene are present in mammals and Caenorhabditis elegans. The encoded proteins have five predicted membrane-spanning regions and recent findings suggest that some family members may be chloride channels. Phylogenetic analysis of the tty family including novel members from slime mould Entamoeba and plants has revealed the occurrence of independent gene duplication events in different lineages. expressed sequence tag data indicate that expression of the mammalian Ttyh1 gene is restricted mainly to neural tissue and is up-regulated in astrocytoma, glioma and several other cancers. In this study, mammalian expression vectors were used to investigate the subcellular localization and the effect of over-expression of Ttyh1 in human epithelial kidney cells. The results confirm that Ttyh1 is a membrane protein and show that it is deposited on the substratum along the migration paths of motile cells above the alpha5beta1-integrin complex. The ectopic expression of Ttyh1 also induced long filopodia, which were branched and dynamic in both stationary and migratory cells. The filopodia contained F-actin and occurred at the ends of microtubules which were polarized towards the membrane. Upon contact with nearby cells some filopodia stabilized and filled with F-actin, whereas Ttyh1 was highly concentrated at the cell-cell interface. Ttyh1 N- and C-terminal antipeptide antibodies detected Ttyh1 along the axons of neurones in primary rat hippocampal cell cultures, and in situ in whole rat brain slices around the hippocampus and occasionally between cells. These data suggest a role for Ttyh1 in process formation, cell adhesion and possibly as a transmembrane receptor.