Localization of human growth hormone to a sub-set of cytoplasmic vesicles in transfected PC12 cells.

Two distinct population of vesicles can be identified in PC12 cells by subcellular fractionation and immunofluorescence. Density gradient separation reveals one population of dense vesicles that contains the transgenic regulated secretory protein hGH (human growth hormone) along with the endogenous neurotransmitter norepinephrine. Some of the neuronal vesicle marker synaptophysin (P38) is also associated with these vesicles. A second population of low-density vesicles contains synaptophysin but not hGH or norepinephrine. Immunofluorescence localization of hGH revealed a pattern consistent with packaging into catecholaminergic vesicles: the staining is punctate and most concentrated in the tips of the neuritic processes, with secondary accumulation in the perinuclear region. Double-staining of cells for hGH and synaptophysin confirms that these proteins do not co-localize but rather are spatially segregated within the cell. The observed distribution of vesicle markers is inconsistent with simple models for the generation of one type of vesicle from the other, suggesting that the vesicles are products of two divergent biosynthetic pathways. While the hGH is clearly contained in regulated secretory vesicles, the function of the second population of vesicles remains uncertain.