DISCOVERY OF TIE1, TIE2, AND THE ANGIOPOIETINS

In the early 1990s, numerous groups used an reverse transcription polymerase chain reaction (RT-PCR) approach to identify new tyrosine kinases. This approach, based on the use of degenerate primers complementary to highly conserved regions of the kinase domain, was first described by the lab of Andrew Wilks (1,2). The small RT-PCR fragments produced using this approach and RNA isolated from several sources, such as K-562 cells, embryonic mouse heart tissue, differentiated embryonic stem cells and hematopoietic stem cells (3–10), provided the starting material for the cloning and characterization of many tyrosine kinases including new families of receptor tyrosine kinases (RTK). One of these families has just two members, Tie1 and Tie2. These receptors were first identified by RT-PCR of RNA isolated from the megakaryoblastic cell line K-562 (11). cDNAs coding for these receptors were described in 1992 and 1993, respectively, and were originally named Tie (Tie1) and Tek/Hyk (Tie 2) (4,6,11).

The Tie receptors remained orphan receptors until 1996 when angiopoietin-1 (Ang1) was identified as a ligand for Tie2 (12). Ang1 was cloned using a unique secretion trap cloning method, in which a receptor body was used to probe cells transfected with a cDNA expression library. Subsequent to the identification of Ang1, several other angiopoietins (Ang2, 3, and 4) were cloned by low-stringency hybridization (13,14). Ang3, which is isolated from mouse, and Ang4, which is of human origin, are thought to be interspecies orthologues (14). Both Ang3 and Ang4 bind to and activate Tie2 within their respected species (15).