Tie1: an orphan receptor provides context for angiopoietin-2/Tie2 signaling.

Angiopoietin-1/Tie2 (ANG1/Tie2) signaling is well documented as regulating angiogenesis and vessel maturation. This pathway is complicated by involvement of the orphan receptor Tie1, which has been implicated as both a positive and negative regulator of ANG1/Tie2 signaling, and ANG2, which can serve as both a Tie2 agonist and antagonist, depending on the context. Two papers in this issue of the JCI provide new insight into this complicated pathway. Korhonen et al. reveal that Tie1 acts to modulate the effects of ANG1 and ANG2 on Tie2 in vitro and in vivo. Kim et al. demonstrate that ANG2 acts as a Tie2 agonist in non-pathological conditions, whereas in the setting of inflammation, ANG2 functions as a Tie2 antagonist and promotes vascular dysfunction. Both studies indicate that inflammation promotes cleavage of the ectodomain of Tie1 and that this cleavage event corresponds with the switch of ANG2 from a Tie2 agonist to an antagonist. The results of these studies lay the groundwork for future strategies to therapeutically exploit this pathway in diseases characterized by adverse vascular remodeling and increased permeability.