Mechanisms of cell surface trafficking and potential functions of extracellular annexin a2

Abstract

Annexin A2 (AnxA2) belongs to a family of Ca²⁺-dependent phospholipid binding proteins. AnxA2 is abundantly expressed in the cytosol of many different cell types and is translocated to the outer surface of the plasma membrane in response to elevated concentrations of intracellular Ca ²⁺. It is known that the binding of Ca²⁺ to the C-terminal Ca²⁺-binding domains is essential for the recruitment of AnxA2 to the inner leaflet of the plasma membrane. However, the mechanism by which AnxA2 is translocated from the inner leaflet of the plasma membrane to the outer leaflet remains unknown. Since, AnxA2 does not possess a signal sequence that is essential for the classical ER-Golgi secretion; we believe that the protein follows a non-classical pathway of protein secretion. Here, we show that Ca²⁺-induced translocation of AnxA2 to the cell surface is a multi-step pathway that involves association of AnxA2 with specific domains of the plasma membrane called lipid rafts and its recruitment to the intracellular membranes of the endosomal pathway followed by extracellular secretion by association with the secretory vesicles called exosomes. In our studies, we also investigated the role of AnxA2 in inducing neovascular responses. We have shown that angiogenic insults like hypoxia upregulate the expression of AnxA2. AnxA2-induced angiogenic responses were identified to be regulated by the N-terminus which is important for the binding of several proteolytic components including, tissue plasminogen activator (tPA), plasminogen, procathepsin B and tenascin C. Targeting the N-terminus of AnxA2 with a competitive hexapeptide that prevents the binding of proteolytic components inhibited the AnxA2-mediated neovascular responses. In summary, our data suggests that AnxA2 is transported to the cell surface following an unconventional pathway of protein secretion and extracellular AnxA2 is an active proteolytic receptor that can have potential roles in pathological conditions associated with excessive extracellular proteolytic events.