Metalloendopeptidase EC3.4.24.15 is constitutively released from the exofacial leaflet of lipid rafts in GT1-7 cells

Metalloendopeptidase EC3.4.24.15 (EP24.15) is a physiologically important neuropeptide-degrading enzyme involved in the metabolism of multiple neuropeptides. The mechanism of release of EP24.15 from neuronal cells is multimodal, being both constitutive and stimulatable. Previous studies have characterized stimulated EP24.15 secretion, yet little is understood concerning constitutive release of the peptidase. Utilizing the mouse hypothalamic neuronal GT1-7 cell line, we demonstrate that EP24.15 exists within lipid rafts in the plasma membrane, and that the enzyme is localized to the exofacial leaflet of lipid rafts. Further, we have found that biotinylated EP24.15 on the extracellular surface is released into the cell media in a fashion similar to constitutive release. In addition, classical and non-classical secretion pathway inhibitors were employed to understand the release of EP24.15 into surrounding cell media. The non-classical secretion inhibitor glyburide, a blocker of ATP-sensitive K+ channels, decreased the amount of constitutively released EP24.15 in cell media of GT1-7 cells. With these data, we conclude that EP24.15 association with lipid rafts on the extracellular surface precedes constitutive release of the peptidase into the extracellular milieu for its action on neuropeptides.