Sorting of synaptophysin into special vesicles in nonneuroendocrine epithelial cells

Synaptophysin is a major transmembrane glycoprotein of a type of small vesicle with an electron-translucent content (SET vesicles), including the approximately 50-nm presynaptic vesicles in neuronal cells, and of similar, somewhat larger (< or = approximately 90 nm) vesicles (SLMV) in neuroendocrine (NE) cells. When certain epithelial non-NE cells, such as human hepatocellular carcinoma PLC cells, were cDNA transfected to synthesize synaptophysin, the new molecules appeared in specific SET vesicles. As this was in contrast to other reports that only NE cells were able to sort synaptophysin away from other plasma membrane proteins into presynaptic- or SLMV-type vesicles, we have further characterized the vesicles containing synaptophysin in transfected PLC cells. Using fractionation and immunoisolation techniques, we have separated different kinds of vesicles, and we have identified a distinct type of synaptophysin-rich, small (30-90-nm) vesicle that contains little, if any, protein of the constitutive secretory pathway marker hepatitis B surface antigen, of the fluid phase endocytosis marker HRP, and of the plasma membrane recycling endosomal marker transferrin receptor. In addition, we have found variously sized vesicles that contained both synaptophysin and transferrin receptor. A corresponding result was also obtained by direct visualization, using double-label immunofluorescence microscopy for the endocytotic markers and synaptophysin in confocal laser scan microscopy and in double-immunogold label electron microscopy. We conclude that diverse non-NE cells of epithelial nature are able to enrich the "foreign" molecule synaptophysin in a category of SET vesicles that are morphologically indistinguishable from SLMV of NE cells, including one type of vesicle in which synaptophysin is sorted away from endosomal marker proteins. Possible mechanisms of this sorting are discussed.