Ectodomain shedding, translocation and synthesis of SorLA are stimulated by its ligand head activator

The single transmembrane receptor SorLA is the mammalian orthologue of the head activator-binding protein, HAB, from hydra, The human neuronal precurs or cell line NT2 and the neuroendocrine cell line BON produce head activato r (HA) and respond to HA by entry into mitosis and cell proliferation. They express SorLA, and bind HA with nanomolar affinity, HA coupled to Sepharos e is able to precipitate SorLA specifically proving that SorLA binds HA. Us ing antisera directed against extra- and intracellular epitopes we find Sor LA as membrane receptor and as soluble protein released from cells into the culture medium. Cell lines differ strongly in processing of SorLA, with NT 2 cells expressing SorLA mainly as membrane receptor, whereas release predo minates in BON cells. Soluble SorLA lacks the intracellular domain and is s hed from the transmembrane protein by a metalloprotease, Release from cells and brain slices is stimulated by HA and by phorbol ester, and it is block ed by a metalloprotease inhibitor and by lowering the temperature to 20 deg reesC. Blockade of SorLA shedding and treatment of cells with SorLA antisen se oligonucleotides lead to a decrease in the rate of cell proliferation. F rom this we conclude that SorLA is necessary to mediate the mitogenic effec t of endogenous HA. HA enhances the translocation of SorLA from internal me mbranes to the cell surface and its internalization. In addition, HA stimul ates SorLA synthesis hinting at an autocatalytic feedback loop in which the ligand activates production, processing, and translocation of its receptor .