Differences in the distribution of cytochrome b561 and synaptophysin in dog splenic nerve: a biochemical and immunocytochemical study

Heterologous expression and site-directed mutagenesis of an ascorbate-reducible cytochrome b561

Cytochromes b561 (Cyts b561) are ubiquitous membrane proteins catalyzing ascorbate-mediated trans-membrane electron transfer. A heterologous expression system in Saccharomyces cerevisiae was developed to study their structure-function relationship. Recombinant mouse chromaffin granule Cyt b561 (CGCytb) shows spectral characteristics, ascorbate reducibility, and redox potentials identical to that of the native bovine protein. Moreover, the reconstituted recombinant protein mediated trans-membrane electron transport with kinetic characteristics similar to that of bovine CGCytb. Site-directed mutant analysis supports the presence of two hemes coordinated by the highly conserved His pairs H52/H120 and H86/H159. Reduction of CGCytb by ascorbate showed biphasic kinetics (Kd1: 0.016 +/- 0.005 mM, Kd2: 1.24 +/- 0.19 mM). Mutation of a well-conserved Arg residue (R72) abolished high affinity CGCytb reduction by ascorbate, indicating that this residue may be critical for substrate binding. On the other hand, mutation of a Lys previously suggested to play a role in ascorbate binding (K83), did not affect the ascorbate-mediated reduction of the protein.

Postganglionic, direct axo-axonal contacts on the splenic nerve

The splenic nerve is made up almost exclusively of adrenergic fibers. Consequently it was used as a model system in the study of autonomic sympathetic neurotransmission. The splenic nerve regulates the vasoconstriction and volume reduction of the spleen. Brain-immune interactions via modulation of the splenic nerve activity may regulate peripheral cellular immunity. An inhibition of noradrenaline release by alpha(2)-adrenoceptor activation has been reported. As we were interested in a structurally detailed distribution of synaptophysin, immunocytochemical methods were applied to splenic nerve axons. In 1 microm plastic sections a network of synaptophysin-positive varicosities could be observed all along the splenic nerve. They were also positive for dopamine-beta-hydroxylase and cytochrome b561. At the ultrastructural level the varicosities were seen to establish direct contact with the splenic axons. In normal morphology the varicosities revealed small synaptic vesicles and several dense granules. It is demonstrated that a network of direct symmetric contacts of adrenergic nature is present all along the nerve. These terminals may have an inhibitory effect on the splenic nerve activity via axonal receptors. This finding opens new perspectives for the study of the splenic nerve in general and more particularly for its role in the regulation of peripheral cellular immunity.

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.

A 29 kDa intracellular chloride channel p64H1 is associated with large dense-core vesicles in rat hippocampal neurons

A novel class of intracellular chloride channels, the p64 family, has been found on several types of vesicles. These channels, acting in concert with the electrogenic proton pump, regulate the pH of the vesicle interior, which is critical for vesicular function. Here we describe the molecular cloning of p64H1, a p64 homolog, from both human and cow. Northern blot analysis showed that p64H1 is expressed abundantly in brain and retina, whereas the other members of this family (e.g., p64 and NCC27) are expressed only at low levels in these tissues. Immunohistochemical analysis of p64H1 in rat brain, using an affinity-purified antibody, revealed a high level of expression in the limbic system-the hippocampal formation, the amygdala, the hypothalamus, and the septum. Immunoelectron microscopic analysis of p64H1 in hippocampal neurons demonstrated a striking association between p64H1 and large dense-core vesicles (LDCVs) and microtubules. In contrast, very low p64H1 labeling was found in perikarya or associated with small synaptic vesicles (SSVs) in axonal profiles. Immunoblot analysis confirmed that p64H1 is colocalized with heavy membrane fractions containing LDCVs rather than the fractions containing SSVs. These results suggest that p64H1-mediated Cl- permeability may be involved in the maintenance of low internal pH in LDCVs and in the maturation of LDCVs and the biogenesis of functional neuropeptides.

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.

B-50, the growth associated protein-43: modulation of cell morphology and communication in the nervous system

The growth-associated protein B-50 (GAP-43) is a presynaptic protein. Its expression is largely restricted to the nervous system. B-50 is frequently used as a marker for sprouting, because it is located in growth cones, maximally expressed during nervous system development and re-induced in injured and regenerating neural tissues. The B-50 gene is highly conserved during evolution. The B-50 gene contains two promoters and three exons which specify functional domains of the protein. The first exon encoding the 1-10 sequence, harbors the palmitoylation site for attachment to the axolemma and the minimal domain for interaction with G0 protein. The second exon contains the "GAP module", including the calmodulin binding and the protein kinase C phosphorylation domain which is shared by the family of IQ proteins. Downstream sequences of the second and non-coding sequences in the third exon encode species variability. The third exon also contains a conserved domain for phosphorylation by casein kinase II. Functional interference experiments using antisense oligonucleotides or antibodies, have shown inhibition of neurite outgrowth and neurotransmitter release. Overexpression of B-50 in cells or transgenic mice results in excessive sprouting. The various interactions, specified by the structural domains, are thought to underlie the role of B-50 in synaptic plasticity, participating in membrane extension during neuritogenesis, in neurotransmitter release and long-term potentiation. Apparently, B-50 null-mutant mice do not display gross phenotypic changes of the nervous system, although the B-50 deletion affects neuronal pathfinding and reduces postnatal survival. The experimental evidence suggests that neuronal morphology and communication are critically modulated by, but not absolutely dependent on, (enhanced) B-50 presence.

Noradrenaline storing vesicles in sympathetic neurons and their role in neurotransmitter release: an historical overview of controversial issues

More than 25 years have passed since the original demonstration that proteins such as chromogranin A and dopamine-beta-hydroxylase, which are co-stored together with noradrenaline in large dense cored vesicles in adrenergic nerves, are released by exocytosis. Despite much evidence in favour, it was for a long time thought that large dense cored vesicles were not eminently involved in the release of noradrenaline. The present review attempts to demonstrate, making use of evidence from different approaches, that the release of noradrenaline from sympathetic neurons occurs ultimately from large dense cored vesicles. A model of the secretory cycle is proposed.

Ultrastructural evidence for a separate, small synaptic vesicle (SSV) pathway in ligated bovine splenic nerves, incubated in vitro

In sympathetic nerves the tubules of the axonal reticulum make up the immature elements of the neurosecretory apparatus. The formation of the mature large dense granules occurs via a less dense tubular intermediate, representing the maturing part. At a terminal small synaptophysin-positive vesicles are found intermingled with the dense granules. The biogenesis of these clear, small synaptic vesicles and their relationship with dense granules remains to be determined. In search for the small synaptic vesicles we undertook a careful ultrastructural examination of the axons proximal to a ligation in bovine splenic nerve incubated in vitro for 3 h. The distended axons were crowded with tubules, granulo-tubular elements and dense granules. Occasionally homogeneous clusters of small, uniform vesicles were detected. They were shown to be positive for synaptophysin and were negative for dopamine-beta-hydroxylase, a marker for the granular pathway. The clusters of small vesicles could be found in close spatial relationship with the maturing and mature elements of granular secretion. Our findings argue for the presence of two separate neurosecretory pathways in sympathetic nerves and favour the idea that both small synaptic vesicles and dense granules are a differentiation product of the axonal reticulum. This configuration can explain the biogenesis of small synaptic vesicles and dense granules both in the cell body and at the nerve terminal.

Ultrastructural localization of B-50/growth-associated protein-43 to anterogradely transported synaptophysin-positive and calcitonin gene-related peptide-negative vesicles in the regenerating rat sciatic nerve

The growth-associated protein-43/B-50 (B-50/GAP-43) is conveyed from the neuronal soma into the axon by fast axonal transport and moved to the nerve terminal. To visualize and determine the type of vesicles by which B-50/GAP-43 is anterogradely transported in the regenerating rat sciatic nerve, we have investigated Lowicryl HM20 embedded nerve pieces dissected from the proximal side of a collection ligature. Ultrastructurally, numerous vesicular profiles of various sizes, tubules and mitochondria were seen to accumulate proximal to the collection ligature. Both, in unmyelinated and myelinated axons, B-50/GAP-43 immunoreactivity was associated with vesicular profiles which had a diameter of 50 nm. A fraction of the B-50/GAP-43 label co-localized with the small vesicle marker synaptophysin. Co-localization of B-50/GAP-43 was not detected with the large dense-core vesicle marker calcitonin gene-related peptide. These results indicate that, in rat sciatic nerve axons, B-50/GAP-43 is anterogradely transported in small 50 nm vesicles of the constitutive pathway. These transport vesicles were distinguished in two types. We suggest that one type carrying, both, B-50 GAP-43 and synaptophysin has as destination the nerve terminal, whereas the second type, which only contains B-50/GAP-43 and no synaptophysin, may be primarily targeted to the axolemma for local membrane fusion.

Subcellular localization of synaptophysin in noradrenergic nerve terminals: a biochemical and morphological study

The subcellular localization of synaptophysin was investigated in noradrenergic nerve terminals of bovine vas deferens and dog spleen and compared with membrane-bound and soluble markers of noradrenergic storage vesicles. At the light microscopical level chromogranin A- and cytochrome b561-immunoreactivity revealed an identical and very dense innervation of the entire vas deferens. In the case of synaptophysin, most immunoreactivity was found only in the outmost varicosities closest to the lumen, which were also positive for chromogranin A. Small dense-core vesicles of dog spleen were purified using a combination of velocity gradient centrifugation and size exclusion chromatography. Small dense-core vesicles were enriched 64 times as measured by the noradrenaline content. Enrichments for dopamine-beta-hydroxylase were in a similar range. Synaptophysin-containing vesicles were smaller in size and they did not contain the typical noradrenergic markers dopamine-beta-hydroxylase, cytochrome b561, and noradrenaline. Instead, they might store adenosine triphosphate (ATP). A greater part of synaptophysin immunoreactivity was consistently found at high sucrose densities at the position of large dense-core vesicles. We conclude that in the noradrenergic nerve terminal: (1) small dense-core vesicles have a membrane composition similar to large dense-core vesicles, indicating that the former are derived from the latter, and (2) synaptophysin seems not to be present on small dense-core vesicles. We suggest the possibility that synaptophysin-containing vesicles form a residual population whose role in neurotransmission has been taken over by large and small dense-core vesicles following noradrenergic differentiation.

Neurosecretory vesicles can be hybrids of synaptic vesicles and secretory granules

We have investigated the relationship of the so-called small dense core vesicle (SDCV), the major catecholamine-containing neurosecretory vesicle of sympathetic neurons, to synaptic vesicles containing classic neurotransmitters and secretory granules containing neuropeptides. SDCVs contain membrane proteins characteristic of synaptic vesicles such as synaptophysin and synaptoporin. However, SDCVs also contain membrane proteins characteristic of certain secretory granules like the vesicular monoamine transporter and the membrane-bound form of dopamine beta-hydroxylase. In neurites of sympathetic neurons, synaptophysin and dopamine beta-hydroxylase are found in distinct vesicles, consistent with their transport from the trans-Golgi network to the site of SDCV formation in constitutive secretory vesicles and secretory granules, respectively. Hence, SDCVs constitute a distinct type of neurosecretory vesicle that is a hybrid of the synaptic vesicle and the secretory granule membranes and that originates from the contribution of both the constitutive and the regulated pathway of protein secretion.

The organisation of the axonal reticulum at a ligation, in in vitro incubated bovine splenic nerves

From previous studies we concluded that in noradrenergic neurons the axonal reticulum can be considered to be an extension of the Golgi apparatus, directly involved in the condensation and packaging of neurosecretion. But the precise ultrastructure of the organisation of the axonal reticulum in relation to neurosecretory granule formation remained to be elucidaded. This conversion was studied in ligated bovine splenic nerve incubated in vitro for three hours. The ultrastructure of the material accumulating proximally and distally was examined and its nature was determined by phosphotungstic acid staining and immunocytochemistry on glycolmethacrylate sections. Proximal to the ligation predominantly electron-lucent vesicles and tubules were found. Tubules of intermediate electron density appeared in between. The latter, especially in thicker sections, were seen to form complexes with tubules and granules of high electron density. All those elements were shown to be positive for dopamine-beta-hydroxylase and cytochrome b561. In the distal part multivesicular bodies accumulated and they were also positive for both enzymes. From these findings it is concluded that the different types of structures accumulating proximally belong to a neurosecretory axonal reticulum. At a block the axonal reticulum is triggered to generate a reticular differentiation, in which granular densities of different size are found. This configuration compares well with that in nerve terminals and strongly suggests that granule formation is basically a local process.

Evidence against differential release of noradrenaline, neuropeptide Y, and dopamine-beta-hydroxylase from adrenergic nerves in the isolated perfused sheep spleen

The subcellular storage and release of noradrenaline (NA), dopamine-beta-hydroxylase (D beta H), and neuropeptide Y (NPY) was studied in the isolated perfused sheep spleen. Subcellular distribution studies showed a bimodal distribution for NA which was well reflected by D beta H and indicated the occurrence of two types of NA storage vesicles. The most dense, presumably large dense-cored vesicles (LDV), contain both membrane-bound and soluble D beta H; the less dense presumably corresponds to small dense-cored vesicles (SDV) and at least does not contain soluble D beta H. The distribution of NPY is extended but shows a peak only at the position of LDV, indicating that LDV contain NPY. Continuous electrical stimulation of the splenic nerve at 2 Hz, 5 Hz, 10 Hz, and 20 Hz or at 20 hz with bursts induced the release of NA, NPY, and D beta H. The ratio among these components was constant. The fractional release of D beta H and NA was comparable at all frequencies used; that of NPY was 10-20 times lower, suggesting the occurrence of a large nonreleasable NPY pool. The present data argue against a high frequency stimulation or intermittent stimulation-induced preferential release of NPY from adrenergic neurons and question the concept of frequency-dependent chemical coding of sympathetic transmission in general. The simplest interpretation of our data is that NA and NPY are released at all frequencies from a single pool. The present finding might signify that only large dense-cored vesicles are involved in the sympathetic stimulation-evoked secretion of catecholamines from adrenergic nerve terminals of the isolated sheep spleen.

Ultrastructural localization of neuropeptide Y-immunoreactivity in the axonal reticulum elements, accumulating anterogradely in transected rat sciatic nerve

The detection of dopamine-beta-hydroxylase and cytochrome B561 on the membranes of the axonal reticulum demonstrated that in sympathetic neurons the different compartments of the axonal reticulum participate in the formation of neurosecretory vesicles. In the present study we tried to reveal that the components of the vesicular content are also channeled along the axonal reticulum, by examining whether neuropeptide Y could be localized in elements of the axonal reticulum. Therefore 6 h transected rat sciatic nerve was embedded in glycolmethacrylate and an immunogold labeling was performed. Counterstaining with phosphotungstic acid at low pH selectively contrasted the accumulated axonal reticulum elements and associated granules. In the non-myelinated axons gold labeling was localized on granules and on tubular and granular profiles, demonstrating the presence of neuropeptide Y in the accumulated axonal reticulum elements. This indicates that neuropeptides are indeed transported via the axonal reticulum to the nerve ending and suggests that the accumulation of large dense-cored vesicles at a block is mainly due to local new formation rather than down transport.