Abstract
Neurons and endocrine cells use a complex array of signaling molecules to communicate with each other and with various targets. The majority of these signaling molecules are stored in specialized organelles awaiting release on demand: 40-60 nm vesicles carry conventional or small molecule neurotransmitters, and 200-400 nm granules contain bioactive peptides. The supply of small molecule neurotransmitters is tightly regulated by local feedback of synthetic rates and transport processes at sites of release. The larger granules that contain bioactive peptides present the secretory cell with special challenges, as the peptide precursors are inserted into the lumen of the secretory pathway in the cell soma and undergo biosynthetic processing while being transported to distant sites for eventual secretion. One solution to this dilemma in information handling has been to employ proteolytic cleavage of secretory granule membrane proteins to produce cytosolic fragments that can signal to the nucleus, affecting gene expression. The use of regulated intramembrane proteolysis to signal from secretory granules to the nucleus is compared to its much better understood role in relaying information from the endoplasmic reticulum by SREBP and ATF6 and from the plasma membrane by cadherins, Notch and ErbB4.