Chromogranin A: localization and stoichiometry in large dense core catecholamine storage vesicles from sympathetic nerve

Abstract

Chromogranin A is present in both adrenal medullary chromaffin granules and sympathetic nerve large dense core catecholamine storage vesicles (LDVs), yet selective stimulation of sympathetic axons provokes only minor changes in chromogranin A in the circulation. We therefore examined the stoichiometry of chromogranin A storage in purified LDVs as compared to chromaffin granules. Chromogranin A was found in LDVs on immunocytochemical sections of sympathetic axons. Sedimentation of sympathetic axon homogenates on sucrose-D 2O gradients localized chromogranin A, norepinephrine, enkephalins and dopamine-β-hydroxylase to the same gradient particulate fractions, suggesting that they inhabit a particle of the same buoyant density, the LDV. Chromogranin A was identified in LDV by radioimmunoassay, immunoblotting and immunocytochemistry. Purified LDVs contained 17.8 ±4.8% of cell total chromogranin A, at 27.9 ± 3.5-fold enrichment over the original axon homogenate. When LDVs were lysed, all of the chromogranin A immunoreactivity originated from the soluble vesicle core rather than the LDV membrane. Although chromogranin A/catecholamine ratios were similar in LDVs and adrenal chromaffin granules, chromogranin A was a quantitatively minor protein in LDVs, accounting for only 0.16 ± 0.015% of total LDV protein, as compared to 35.2 ± 1.4% of total chromaffin granule protein. Each LDV particle contained approximately 0.94 ± 0.09 chromogranin A molecules. Immunocytochemical data suggested that chromogranin A is costored in large dense core noradrenergic vesicles in subpopulations of sympathetic axons, analogous to enkephalins and neuropeptide Y. Thus, only profound changes in exocytotic catecholamine release from sympathetic axon LDVs would be expected to perturb circulating chromogranin A concentrations. Since chromogranin A-derived peptides also seem to modulate catecholamine release, these results may have functional consequences for secretory control in sympathetic axons.