THE NONRECEPTOR TYROSINE KINASE LYN IS LOCALIZED IN THE DEVELOPING MURINE BLOOD-BRAIN-BARRIER

The blood-brain barrier, formed by brain endothelium, is critical for brain function. The development of the blood-brain barrier involves br ain angiogenesis and endothelial cell differentiation, processes which require active signal transduction pathways. The differentiation of b rain endothelial cells to the ''blood-brain-barrier phenotype'' involv es cytoskeletal changes which modulate the tightness of the barrier. I n order to identify signal transduction proteins involved in blood-bra in barrier development, cDNA from bovine and murine brain endothelial cells was used in a polymerase chain reaction for cloning of DNA encod ing Src homology 3 domains. Src homology 3 domains are structural doma ins found in many signal transduction proteins. These domains often me diate interaction of signaling proteins with the cytoskeleton and ther efore may play a role in the regulation of the cytoskeletal changes wh ich occur during blood-brain-barrier development. Unexpectedly, all bo vine and murine clones analyzed from polymerase chain reactions encode d the Src homology 3 domain of one protein, namely the non-receptor ty rosine kinase, Lyn, which is involved in signal transduction in cells of the hemopoietic system. In situ hybridization analyses confirmed th e presence of lyn mRNA in developing blood vessels in embryonic and ea rly post-natal mouse brain, but not in endothelium outside the brain. In bovine brain endothelial cells in primary culture, p53(lyn) is high ly abundant and present in two forms which have different patterns of tyrosine phosphorylation, These data suggest that Lyn may be involved in transduction of growth and differentiation signals required for blo od-brain-barrier development.