Targeting of tissue plasminogen activator to the regulated pathway of secretion

Tissue plasminogen activator is a serine protease that plays the dominant role in removal of fibrin from the vascular tree by activating plasminogen to the primary fibrinolytic enzyme, plasmin. Tissue plasminogen activator has a widespread neuroendocrine distribution in addition to its expression by endothelial cells. Within neuroendocrine cells, secretory proteins are sorted into one of two pathways: regulated or constitutive. Proteins entering the regulated pathway are concentrated and stored in vesicles, and subsequently released upon stimulation by a secretagogue. In contrast, in the constitutive pathway, newly synthesized protein is not stored but is transported directly to the cell surface and secreted even in the absence of an extracellular signal. The focus of this article is to review recent studies demonstrating that tissue plasminogen activator is targeted to the regulated secretory pathway in neuroendocrine cells and to discuss the physiological implications of the trafficking of tissue plasminogen activator to regulated secretory vesicles.