Tyrosine kinase gene expression in the mouse small intestine

To identify tyrosine kinases that may regulate regeneration of the mammalian intestinal epithelium, we amplified portions of the catalytic domains of protein kinases expressed in intestinal crypt cells, using the polymerase chain reaction technique with primers directed against two invariant amino acid sequence motifs found in all kinases. These fragments were cloned and a library of kinase catalytic domains was generated. Sequence analysis of unique clones resulted in the identification of the catalytic domains of several characterized tyrosine kinases, including lyn, hck, c-fgr, tec, JAK2, itk, and the putative receptor kinase ryk, and expression of these kinases has not previously been described in the intestine. We compared the levels of mRNA encoding these kinases in multiple tissues using RNase protection assays, and we localized the expression of hck, lyn, and JAK2 in the intestine using in situ hybridization. In addition, we identified two novel putative catalytic domain sequences. One of these, which we have named sik (src-related intestinal kinase), is expressed at high levels in the gastrointestinal tract and may play a specific role in signal transduction in epithelial tissues.