Development and Structure of Renin Secretory Vesicles

ly playing a role in granule formation, the glycosylation state of renin may also be important for mannose-6phosphate receptor-mediated uptake of renin or prorenin in other tissues, such as the heart (62, 93). Protogranules that are destined for the regulated secretory pathway pinch off from the trans-Golgi network. They contain prorenin and proteases, such as prohormone convertases (1, 5) and cathepsin B (53, 58), that have the ability to cleave off the prosegment. Other proteases, such as kallikrein and plasmin, are also able to cleave the prosegment, but there is no evidence that they play a physiological role in the process. In dense core secretory granules, an acidic intracellular pH is created by vacuolar H+-ATPases. The low pH is optimal for the protease activity that activates prorenin to renin by splitting off the 43amino acid NH2-terminal propeptide. Activation of the proteases from their own pro-forms by autoactivation is also dependent on a low pH of 4–6. The low pH of the renin secretory vesicles has been used experimentally to stain them with substances like the fluorescent dye quinacrin (1, 9, 63). When using quinacrine for dynamic visualization of live cells, it is possible to observe disappearance of fluorescence from individual secretory granules, representing exocytosis, a sudden increase in granule pH, or both (63). The early secretory granules have the ability to take up extracellular markers, such as horseradish peroxidise, which is targeted to early granules without transit through the Golgi apparatus (88). Such retrograde transport linking early/recycling endosomes to the trans-Golgi network is a well known cellular phenomenon (e.g., Ref. 2) involving SNARE complexes and clathrin (74). The direct uptake route into the secretory granules could potentially be used to target the activation of renin.