Recombinant Rat Tie-2 Fc Chimera Protein, CF

Tie-2, also known as Tek, is a 145 kDa, type I transmembrane glycoprotein receptor tyrosine kinase that is a receptor for angiopoietins (1). The 1120 amino acid (aa) rat Tie-2 precursor contains an 18 aa signal sequence, a 723 aa extracellular domain (ECD), a 25 aa transmembrane segment, and a 354 aa cytoplasmic tail (2). The ECD contains two C2 Ig-like domains, three EGF-like motifs, and three fibronectin type III repeats. The cytoplasmic region has a split tyrosine kinase domain and presumably autophosphorylates as a ligand-induced homodimer (3). Rat Tie-2 ECD shares 96%, 90% and 89% aa identity with mouse, human and bovine Tie-2, respectively, and 47% aa identity with rat Tie-1 ECD. Cells known to express Tie-2 include embryonic and adult endothelial cells, hematopoietic stem cells and a circulating population of proangiogenic Tie-2 expressing monocytes (TEM) (4 - 7). A soluble form of Tie-2, most likely the result of proteolytic cleavage, is found in serum (8). The four angiopoietins are ligands of Tie-2. Ang-1 and Ang-4 are Tie-2 activators, while Ang-2 and Ang-3 can be activators or inhibitors, depending on context (1, 9). Tie-2 is said to be important for maintaining vascular integrity. It mediates endothelial cell-smooth muscle cell communication, and inhibits endothelial cell apoptosis, thus maintaining endothelial cell survival (10 - 12). It is also absolutely required for embryonic development of the endocardium (3, 10, 13). While not essential for embryonic hematopoiesis, Ang-1 production by osteoblasts promotes quiescence ofTie-2-expressing bone marrow stem cells. This quiescence is critical for maintaining an ongoing hematopoietic capability (12, 14, 15).