Secretory vesicle-specific antibodies in the confocal study of exo-endocytosis dynamics.

The study of secretory vesicle dynamics is a continuing challenge. Classically it was studied using biochemical techniques, such as subcellular fractionation and immunoprecipitation, combined with time-consuming electron microscopy studies. The recent development of confocal microscopy, giving in-focus optical section images throughout the thickness of a fluorescently labeled sample, allows scientists to study the key events in the secretory cycle at the level of light microscopy. This study demonstrates the use of specific antibodies against marker proteins of two different secretory vesicles (synaptic vesicles and large dense-cored vesicles) to follow their exo-endocytosis dynamics in peripheral adrenergic neurons. Only in recent years has insight grown regarding the presence of both exocytosis pathways in the same neuron. Confocal microscopy is a suitable technique to study aspects of exocytosis, endocytosis, and intracellular sorting and as such improves our knowledge on the interaction between both secretory pathways.

Rab3 is present on endosomes from bovine chromaffin cells in primary culture.

Rab3a, a small GTP-binding protein, is believed to mediate Ca2+-dependent exocytosis. Consistent with such a role was the previously reported specific association of Rab3a with synaptic vesicles in neurons and secretory granules in adrenal chromaffin cells. Secretory vesicles are believed to be the final point of Rab3a membrane association, as it was shown by several groups that Rab3a dissociates from the secretory vesicle membrane during stimulated exocytosis. In chromaffin cells, Rab3a is not exclusively localized on secretory granules since a fraction is present on a previously unidentified subcellular compartment equilibrating at light sucrose density. This 'light' membraneous structure could be the starting point for reassociation of Rab3a with membranes involved in granule formation, or it could be a structure unrelated to granules. The present study used several subcellular fractionation techniques and immunomicroscopy to unravel the nature of the 'light' Rab3a-containing structures from bovine chromaffin cells in primary culture. After stimulation, amounts of both Rab3a-d and the granule marker dopamine-beta-hydroxylase (DbetaH) increase transiently in sucrose gradient fractions enriched in endosomal markers. A diaminobenzidine-induced density shift of endosomes alters the distribution of DbetaH and Rab3a-d. At the ultrastructural level, subplasmalemmal pleiomorphic organelles were detected by Rab3a-d-immunogold labelling. Taken together our data provide for the first time evidence that internalised secretory granule membranes go through an endosomal stage where Rab3a is present, resembling the neuronal synaptic vesicle cycle. This indicates that the endosome is an important trafficking route in the biogenesis/recycling of secretory vesicles in chromaffin cells, in which Rab3a could have an as yet unknown regulatory function, and could point to the existence of alternative recycling pathways for the chromaffin granule membrane.

Retrieved constituents of large dense-cored vesicles and synaptic vesicles intermix in stimulation-induced early endosomes of noradrenergic neurons.

Two storage compartments in cultured noradrenergic neurons derived from the superior cervical ganglion from fetal pig have been defined using sucrose density gradient centrifugation and electron microscopy: (1) large dense-cored vesicles (LDV) contain noradrenaline and dopamine-beta-hydroxylase (DbetaH); (2) small electron-lucent vesicles contain acetylcholine and p38 and represent the noradrenergic small synaptic vesicles (SSV); no small dense-cored vesicles (SDV) could be detected. Our results demonstrate that internalized LDV membrane constituents are retrieved into early endosomes, as shown by the colocalization of retrieved DbetaH with the endosomal markers Rab5 and HRP in sucrose density gradients and on confocal microscopical images. Recycling of the SSV membranes via an endosomal intermediate is also confirmed in noradrenergic neurons. Finally, colocalization of retrieved DbetaH and retrieved p38 in stimulated neurons indicates that the two sets of constituents intermix. These data provide the first experimental evidence for a common early endosome in which SSV and LDV membrane constituents are internalized after exocytosis and imply that endosomal sorting is an important process for the generation of different secretory vesicles in the noradrenergic nerve terminal.

Rab3 dissociation and clathrin-mediated endocytosis, two key steps in the exo-endocytotic pathway of large dense-cored vesicles in primary cultures of superior cervical ganglia.

In this study we have used primary cultures of porcine superior cervical ganglia as a model system to study exo-endocytosis in sympathetic neurons. Pure neuronal cultures with a defined noradrenergic phenotype can be obtained when antimitotics are included in the culture medium, and the high yield from prenatal piglets allows a biochemical approach in addition to morphological studies. Release of large dense-cored vesicles (LDCVs) was visualized by exposing stimulated neurons to anti-dopamine-beta-hydroxylase (D beta H) prior to fixation. Double immunofluorescent staining revealed that synaptotagmin was colocalized with anti-D beta H labeled exocytotic spots, but not the small GTP-binding protein rab3. Density gradient centrifugation of a postmitochondrial supernatant of cultured cells confirmed the dissociation of rab3 from a population of mature LDCVs. As expected, rab3 did not reassociate with the lighter D beta H-positive membrane fraction in the gradient representing internalized LDCV membranes. The fluid phase marker horseradish peroxidase colocalized with retrieved LDCV membranes. Indirect immunofluorescence demonstrated the colocalization of clathrin with D beta H-positive exocytotic spots on the plasma membrane. At the ultrastructural level, depolarization of neurons resulted in the abrupt loss of LDCVs and concomitant appearance of clathrin-coated vesicles and coated omega profiles. The size distribution of coated structures overlapped strongly with that of LDCVs. Taken together, these data clearly suggest that two key regulatory events in the process of exo-endocytosis, i.e. rab3 dissociation and clathrin-mediated internalization, are not only reminiscent of small synaptic vesicles, but also are a feature of the LDCV pathway at the presynaptic plasma membrane of sympathetic neurons.

Expression of angiotensinogen mRNA and localization of angiotensin II and renin in peripheral adrenergic neurons in primary culture.

Although the presence of a local renin-angiotensin system has been established in a number of tissues, evidence for the existence and expression of the components of the renin-angiotensin system in peripheral adrenergic neurons is still missing. Here we provide the results showing the expression of angiotensinogen mRNA and localization of renin and angiotensin II in primary cultured fetal pig superior cervical ganglion neurons but not in non-neuronal cells by in situ hybridization with biotinylated oligodeoxynucleotide probe and immunocytochemical approaches. Our confocal laser scanning microscopical results confirm that angiotensinogen mRNA is expressed in the renin containing neurons. These results, for the first time, provide direct evidence that peripheral adrenergic neurons are not only target cells of angiotensin, but are also a local angiotensin generating source.