Common membrane proteins of chromaffin granules, endocrine and synaptic vesicles: Properties, tissue distribution, membrane topography and regulation of synthesis

Noradrenaline storage function of species-specific protein bodies, markers of monoamine neurons in human locus coeruleus demonstrated by dopamine-beta-hydroxylase immunogold localization

Our histochemical and ultrastructural studies have identified, in human catecholamine locus coeruleus (LC) neurons, abundant and large spherical protein bodies (PB), containing histone-like, arginine-rich proteins, which originate as dense bodies in mitochondria. This species-specific phenotype in the neurons of man is highly intriguing. In the electron microscope PB are disrupted in LC neurons in depressed individuals, where noradrenaline is known to be reduced. This coincidence of ultrastructure and neurochemistry raises the question whether these bodies could qualify as noradrenaline-storing organelles in the human LC. Our rationale was to examine, in known model tissues that contain catecholamines--sympathetic ganglia and tumors of the autonomic nervous system--if vesicles show the same fine structure and histochemistry as the PB of the human LC. Hence, we selected biopsy tissues of five ganglioneuromas and postmortem tissues of LC from 25 control subjects. Since dopamine-beta-hydroxylase (DBH) is a hallmark of noradrenaline identity and present in dense core vesicles, the investigation of DBH localization with the immunogold method constituted the experiment of choice for this study. Histochemical determinations of arginine with Carmoisine L, and of lipids with Rhodamine B complemented the study of similarities between the PB of the human LC and ganglioneuromas. Our results showed, with the colloidal gold method, that DBH immunogold labeling was localized in the core and in the double membranes of the PB, and also in the adjacent mitochondria. These results indicate that protein bodies (a) are unequivocal storage vesicles of noradrenaline, and (b) derive from regular mitochondria and represent a new phenotype in man, which is probably an evolutionary adaptation of amine-storing organelles.

Subcellular localization of chromogranins, calcium channels, amine carriers, and proteins of the exocytotic machinery in bovine splenic nerve

Subcellular fractionation of bovine splenic nerves, which consist mainly of sympathetic nerve fibers, has been useful for characterizing cellular organelles en route to the terminal. In the present study we have characterized the subcellular distribution of both secretory and membrane proteins. A newly discovered chromogranin-like protein, NESP55, was found in large dense-core vesicles. The endogenous processing of NESP55 was comparable to that of chromogranins but more limited than that of secretogranin II and chromogranin B. For membrane proteins three major types of distribution were found. The amine carrier VMAT2 was confined to large dense-core vesicles. VAMP or synaptobrevin was present both in large dense-core vesicles and in lighter vesicles, whereas SNAP-25, syntaxin, and two types (N and L) of Ca2+ channels were found in a special population of lighter vesicles but were not present in large dense-core vesicles or at the most in very low concentrations. The plasma membrane norepinephrine transporter was apparently present in a separate type of vesicle, but this requires further study. These results further characterize vesicles en route to the terminal and establish for the first time that peptides involved in exocytosis (syntaxin, SNAP-25, and N- and L-type Ca2+ channels) are apparently transported to the terminal in a special type of vesicle. The exclusive presence of the amine carrier in large dense-core vesicles indicates that the formation of small dense-core vesicles in the terminals requires a reuse of membrane components of large dense-core vesicles.

Regulation of rat brain vesicular monoamine transporter by chronic treatment with ovarian hormones

Ovarian steroids play an important role in neuroregulation and in the pathophysiology of various neuropsychiatric disorders. Most of the studies focused on the impact of gonadal steroids on post-synaptic receptors and plasma membrane transporters. In the present study, we evaluated the effect of chronic treatment with ovarian steroids on the expression of rat brain vesicular monoamine transporter (VMAT2). Ovariectomized rats were treated for 21 days with estradiol, progesterone or both. VMAT2 gene expression was assessed on the protein level by high affinity [3H]dihydrotetrabenazine ([3H]TBZOH) binding using autoradiography and on the mRNA level by in situ hybridization. Progesterone administration led to a decrease in [3H]TBZOH binding in the middle striatum and in the nucleus accumbens and to a parallel decrease in VMAT2 mRNA level in the substantia nigra pars compacta and dorsal raphè nuclei. Chronic estradiol treatment reduced VMAT2 mRNA level in the dorsal raphè and [3H]TBZOH binding in middle part of the striatum and nucleus accumbens but did not affect VMAT2 mRNA level in the substantia nigra pars compacta. Simultaneous administration of both ovarian steroids did not modulate VMAT2 mRNA in the substantia nigra pars compacta as well as [3H]TBZOH binding in the striatum or the nucleus accumbens but reduced VMAT2 mRNA level in the dorsal raphè. It appears that ovarian steroids may play a crucial role in the regulation of VMAT2 gene expression in the dopamine and serotonin systems. This modulatory activity may be relevant to synaptic and neuronal plasticity as well as to the molecular and cellular pathophysiology of gender-specific neuropsychiatric disorders.

Exocytosis in chromaffin cells of the adrenal medulla

The chromaffin cell has been used as a model to characterize releasable components present in secretory granules and to understand the cellular mechanisms involved in catecholamine release. Recent physiological and biochemical developments have revealed that molecular mechanisms implicated in granule trafficking are conserved in all eukaryotic species: a rise in intracellular calcium triggers regulated exocytosis, and highly conserved proteins are essential elements which interact with each other to form a molecular scaffolding, ensuring the docking of granules at the plasma membrane, and perhaps membrane fusion. However, the mechanisms regulating secretion are multiple and cell specific. They operate at different steps along the life of a granule, from the time of granule biosynthesis up to the last step of exocytosis. With regard to cell specificity, noradrenaline and adrenaline chromaffin cells display different receptor and signaling characteristics that may be important to exocytosis. Characterization of regulated exocytosis in chromaffin cells provides not only fundamental knowledge of neurosecretion but is of additional importance as these cells are used for therapeutic purposes.

Regulation of the biosynthesis of large dense-core vesicles in chromaffin cells and neurons

1. The proteins of large dense-core vesicles (LDV) in neuroendocrine tissues are well characterized. Secretory components comprise chromogranins and neuropeptides. Intrinsic membrane proteins include cytochrome b-561, transporters, SV2, synaptotagmin, and synaptobrevin. 2. The effects of stimulation and of second messengers on the biosynthesis of LDV have been studied in detail. 3. Regulation of biosynthesis is complex. The cell can adapt to prolonged stimulation either by producing vesicles of normal size filled with a higher quantum of secretory peptides or by forming larger vesicles. In addition, some components, e.g., enzymes, can be upregulated specifically.

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.

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.

Membrane composition of adrenergic large and small dense cored vesicles and of synaptic vesicles: consequences for their biogenesis

The membrane proteins of adrenergic large dense cored vesicles, in particular those of chromaffin granules, have been characterized in detail. With the exception of the nucleotide carrier all major peptides have been cloned. There has been a controversy whether these vesicles contain antigens like synaptophysin, synaptotagmin and VAMP or synaptobrevin found in high concentration in synaptic vesicles. One can now conclude that large dense core vesicles also contain these peptides although in lower concentrations. The biosynthesis of large dense core vesicles is analogous to that of other peptide secreting vesicles of the regulated pathway. One cannot yet definitely define the biosynthesis of small dense core vesicles which apparently have a very similar membrane composition to that of large dense core vesicles. They may form directly from large dense core vesicles when their membranes have been retrieved after exocytosis. These membranes may become sorted in an endosomal compartment where peptides may be deleted or added. Such an addition could be derived from synaptophysin-rich vesicles present in adrenergic axons. However small dense core vesicle peptides may also be transported axonally independent of large dense core vesicles. For proving one of these possibilities some crucial experiments have been suggested.

Immunocytochemical localization of synaptic proteins at vesicular organelles in PC12 cells

The distribution of the three synaptic vesicle proteins SV2, synaptophysin and synaptotagmin, and of SNAP-25, a component of the docking and fusion complex, was investigated in PC12 cells by immunocytochemistry. Colloidal gold particle-bound secondary antibodies and a preembedding protocol were applied. Granules were labeled for SV2 and synaptotagmin but not for synaptophysin. Electron-lucent vesicles were labeled most intensively for synaptophysin but also for SV2 and to a lesser extent for synaptotagmin. The t-SNARE SNAP-25 was found at the plasma membrane but also at the surface of granules. Labeling of Golgi vesicles was observed for all antigens investigated. Also components of the endosomal pathway such as multivesicular bodies and multilamellar bodies were occasionally marked. The results suggest that the three membrane-integral synaptic vesicle proteins can have a differential distribution between electron-lucent vesicles (of which PC12 cells may possess more than one type) and granules. The membrane compartment of granules appears not to be an immediate precursor of that of electron-lucent vesicles.

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.

Epidermal growth factor induces the differential release of GP2 and amylase from AR4-2J cells

Epidermal growth factor (EGF) stimulates secretion of glycoprotein 2 (GP2) in a time-and concentration-dependent manner from the AR4-2J pancreatoma cell line. Cell differentiation induced by dexamethasone treatment for 3 d, however, did not significantly alter either basal or EGF-stimulated GP2 release. Basal and EGF-stimulated GP2 release were similarly unaffected by caerulein, which promotes amylase secretion by a regulated route. A brief exposure to cycloheximide profoundly blocked EGF-evoked GP2 secretion. Furthermore, EGF-stimulated GP2 release was not accompanied by significant alterations in intracellular ionic calcium levels, in contrast to the stimulatory actions of caerulein. We conclude that EGF-stimulated release of GP2 occurs via a novel secretory pathway that is neither regulated nor constitutive as currently defined.

Biosynthesis of large dense-core vesicles in PC12 cells: effects of depolarization and second messengers on the mRNA levels of their constituents

mRNA levels of various constituents of large dense-core vesicles were determined in PC12 cells during depolarization and/or in the presence of BayK 8644, forskolin or phorbolester. For the soluble (secretory) proteins of the vesicles the mRNAs of chromogranin A and B, secretogranin II, neuropeptide Y and VGF were analyzed. Depolarization in the presence of BayK induced a strong up-regulation of the messages for chromogranin B, neuropeptide Y and VGF. Addition of forskolin enhanced this response for neuropeptide Y and VGF, phorbolester did the same only for VGF. Partly membrane-bound and membrane-spanning components analyzed were carboxypeptidase H, dopamine beta-hydroxylase and glycoprotein III (clusterin), peptidylglycine alpha-amidating mono-oxygenase and cytochrome b-561, respectively. Changes of mRNAs for these components were in general smaller and delayed. Six days of depolarization caused an up-regulation of glycoprotein III, peptidylglycine alpha-amidating mono-oxygenase and carboxypeptidase H mRNA levels which were not further increased by cyclic AMP and phorbolester. The dopamine beta-hydroxylase message increased after 6 days of depolarization, however, addition of phorbolester reduced this effect. For cytochrome b-561 there was no change after any of the conditions employed. These in vitro results are compared with those obtained for the biosynthesis regulation of large dense-core vesicles under in vivo conditions. It is suggested that in vivo acetylcholine and vasoactive intestinal polypeptide released from splanchnic nerve induce a differential change in the biosynthesis of large dense-core vesicles by acting via calcium and protein kinase A and C.

Molecular aspects of tetanus and botulinum neurotoxin poisoning

Clostridial neurotoxins, tetanus and the botulinum toxins A-G, are high molecular weight proteins consisting of a heavy chain which is responsible for the internalisation and a light chain possessing a zinc-dependent proteolytic activity. They exclusively proteolyse either the vesicle membrane protein, synaptobrevin or two integral plasma membrane proteins, SNAP 25 and syntaxin. Together with cytosolic proteins these proteins form the SNARE complex involved in vesicle exocytosis, and their cleavage blocks the latter process. Clostridial neurotoxins have now become powerful tools to investigate the final events occurring during secretion in neuronal, endocrine, and non-neuronal cells. They are applied to dissect the specific interactions of the SNARE protein complex with cytosolic fusogens and other modulators of exocytosis. Whereas exocytosis is not essential for the survival of cells, the organism as a whole will fall victim to a few nanograms since interneuronal and neuromuscular transmission is vital to muscular control, especially in respiration. Although all clostridial neurotoxins by their light chains attack proteins of the SNARE complex, tetanus toxin and the various botulinum toxins differ dramatically in their clinical symptoms. The biological information for this difference resides on the respective heavy chains which select different transport routes carrying the light chain from the place of entrance to the final compartment of action. So far the different transport vesicles used either by the various botulinum neurotoxins or by tetanus toxin are not yet defined. Nevertheless at least one of the botulinum toxins serves as a beneficial drug in the treatment of severe neuromuscular spasms.

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.

Requirements for exocytosis in permeabilized neuroendocrine cells. Possible involvement of heterotrimeric G proteins associated with secretory vesicles

Exocytosis in neuroendocrine cells, such as chromaffin cells, is under the regulatory control of heterotrimeric G proteins. LDCV from bovine adrenal medulla contains alpha o-, beta-, and gamma-subunits of G-protein heterotrimers. Probably G proteins associated with the secretory vesicles control the final steps of secretion. G(o), associated with LDCV, could be the pertussis toxin-sensitive G protein that either inhibits exocytosis in PC12 cells or activates it in chromaffin cells. So far, it is unclear whether the other effects of GTP analogues are mediated by heterotrimeric G proteins or by small GTP-binding proteins. The other type of secretory vesicle, SSV from rat brain, also possesses functional sets of G-protein heterotrimers, each consisting of an alpha-, a beta- and a gamma-subunit. In addition to alpha o-subunits, however, alpha i-subunits were found on SSV. Their functional role remains to be determined. Thus, two types of secretory vesicles of the regulated pathway possess functional sets of G-protein heterotrimers. Besides exocytosis, heterotrimeric G proteins on secretory vesicles may control their maturation, transmitter storage, and endocytotic retrieval. So far, it is unclear whether the pattern of G proteins on LDCV and SSV analogues differs within various types of neuroendocrine cells and whether it will change after neoplastic transformation. An altered G-protein setup, not only at the plasma membrane but also on secretory vesicles, may play a role in pathophysiological processes occurring in neuroendocrine cells and tumors derived from them. Such changes might explain the altered secretion observed in neuroendocrine tumor diseases.

Large dense-core vesicles in rat adrenal after reserpine: levels of mRNAs of soluble and membrane-bound constituents in chromaffin and ganglion cells indicate a biosynthesis of vesicles with higher secretory quanta

Rats were injected with a large dose of reserpine known to stimulate the adrenal medulla. Various times after drug treatment the mRNA levels of several constituents of large dense-core vesicles were determined by northern blot analysis and in situ hybridization. The latter method allowed detection of changes in mRNA levels not only in chromaffin cells, but also in the ganglion cells found in adrenal medulla. Levels of the mRNAs of secretory components of large dense-core vesicles (chromogranins A and B, secretogranin II, VGF, and neuropeptide Y) increased in chromaffin cells by 215-857% after 1-3 days of drug treatment. For partly membrane-bound components (dopamine beta-hydroxylase, prohormone convertase 2, carboxypeptidase H, and peptidylglycine alpha-amidating monooxygenase) the changes ranged from 182 to 315%, whereas for glycoprotein III and for intrinsic membrane proteins (cytochrome b561 and vesicle monoamine transporter 2) no change occurred. In ganglion cells the mRNAs that could be detected for VGF, neuropeptide Y, secretogranin II, carboxypeptidase H, and vesicle monoamine transporter 1 showed an analogous pattern of change, with significant increases for the secretory proteins and no change for the membrane components. From these and previous results we suggest the following concept: Long-lasting stimulation of chromaffin cells or neurons does not induce the biosynthesis of a larger number of vesicles but rather leads to the formation of vesicles containing higher secretory quanta of chromogranins and neuropeptides.

Bovine chromaffin cells: studies on the biosynthesis of phospholipids in chromaffin granules

We have studied the biosynthesis of chromaffin granules by labelling primary cultures of bovine chromaffin cells with either [35S]methionine or various precursors for lipids. After labelling the cells were subjected to subcellular fractionation including density gradient centrifugation. After [35S]methionine significant label (mainly represented by labelled chromogranin A) was found in the soluble proteins of chromaffin granules, whereas the membranes were relatively little labelled. However incorporation into membrane bound dopamine beta-hydroxylase and cytochrome b-561 could be demonstrated. Neither of the used lipid precursors ([3H]glycerol, [3H]choline, [3H]palmitic acid or [3H]arachidonic acid) was incorporated to any significant extent into the soluble components of chromaffin granules. Thus there is no evidence that this secretory compartment contains any lipids or acylated proteins. Incorporation of lipid precursors into the membranes of chromaffin granules was apparently low. After short chases labelled lysolecithin was not present in these organelles. However with prolonged chase times labelled lysolecithin, apparently appeared in chromaffin granules irrespective of whether the cells were stimulated or not. We can conclude that the reusable membranes of chromaffin granules have a very low lipid turnover. Lysolecithin is not transferred into these organelles during biosynthesis but is formed in them during their long life span. This formation of lysolecithin is independent of stimulation of these cells and therefore unlikely to be involved in exocytosis.

Biosynthesis of proteins of large dense-core vesicles in rat PC12 cells: regulation by forskolin and phorbol ester

We have investigated the influence of various second messengers on the biosynthesis of large dense-core vesicle constituents in rat PC12 cells. After treatment with forskolin, phorbol ester or a combination of both substances for up to six days, the messenger RNA levels of several vesicle components were determined by northern blotting. Forskolin increased the expression of messenger RNA encoding the soluble proteins chromogranin B, neuropeptide Y and VGF. Addition of phorbol ester markedly enhanced the effects of forskolin. On the other hand, the expression of two further soluble proteins, chromogranin A and secretogranin II, remained fairly unchanged with all treatments tested. Amongst partly membrane-bound vesicle components, the biosynthesis of glycoprotein III and peptidylglycine alpha-amidating mono-oxygenase was significantly up-regulated by combined treatment with forskolin plus phorbol ester. The carboxypeptidase H messenger RNA increased due to phorbol ester and after long-term application of both drugs. In contrast, phorbol ester alone or plus forskolin down-regulated the expression of dopamine beta-hydroxylase. Essentially the same applies to the intrinsic membrane protein cytochrome b-561, whose messenger RNA level declined in all treatment groups. In conclusion, our results show that forskolin and phorbol ester can regulate the composition of large dense-core vesicles in quite distinct patterns.

Clusterin (SGP-2): a multifunctional glycoprotein with regional expression in astrocytes and neurons of the adult rat brain

Clusterin (SGP-2) is a newly described glycoprotein associated with several putative functions including responses to brain injury. This study reports the regional and cell type expression of clusterin mRNA and its encoded glycoprotein in the rat brain; a limited comparison was also done with the human brain. Using in situ hybridization combined with immunocytochemistry, we found that astrocytes and neurons may express clusterin mRNA in the normal adult brain. While astrocytes throughout the brain contained clusterin mRNA, there was regional selectivity for neuronal clusterin expression. In the striatum, clusterin mRNA was not detected in neurons. Only a subset of substantia nigra dopaminergic neurons or locus ceruleus noradrenergic neurons (tyrosine hydroxylase immunopositive) contained clusterin mRNA. However, neuronal clusterin mRNA was prevalent in pontine nuclei and in the red nucleus of the midbrain tegmentum. Similarly, clusterin mRNA was prevalent in both rat and human hippocampal neuron-specific enolase immunopositive pyramidal neurons, although rat CA1 neurons had less mRNA than CA2-CA3 neurons. Monotypic primary cell cultures from the neonatal rat showed clusterin mRNA in both neurons and astrocytes, but not in microglia. By immunocytochemistry, no clusterin immunopositive glia were observed in any region of the rat brain, confirming previous studies. However, clusterin immunopositive cells (putative neurons) were observed in the Purkinje cell layer of the cerebellum, medial and interposed cerebellar nuclei, trigeminal motor nucleus, and red nucleus. Finally, in vitro studies suggest that astrocytes, but not neurons, secrete clusterin, which is pertinent to clusterin immunodeposits found after experimental lesioning.

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

Compared with neurons of the CNS, the organization of the peripheral adrenergic axon and nerve terminal is more complex because two types of neurotransmitter-containing vesicles, i.e., large (LDVs) and small dense-core vesicles, coexist with the axonal reticulum (AR) and the well-characterized small synaptic vesicles. The AR, which is still poorly examined, is assumed to play some role in neurosecretion. We have studied the subcellular localization of noradrenaline, cytochrome b561, and synaptophysin in control and ligated dog splenic nerve using both biochemical and ultrastructural approaches. Noradrenaline and cytochrome b561 coaccumulated proximal to a ligation, whereas distally only the latter was found. Despite a codistribution with noradrenaline at high densities in sucrose gradients, synaptophysin did not accumulate on either side of the ligation. At the ultrastructural level, cytochrome b561 immunoreactivity was found on LDVs and AR elements, both accumulating proximal to the ligation. Distally, the multivesicular bodies (MVBs), immunolabeled for cytochrome b561, account for the retrograde transport of LDVs and AR membranes retrieved at the nerve terminal. No synaptophysin immunoreactivity could be detected on LDVs, AR, or MVBs. The results obtained from the ligation experiments together with the ultrastructural data clearly illustrate that synaptophysin is absent from LDVs and AR elements in adrenergic axons.

Markers of axonal injury in post mortem human brain

beta-Amyloid precursor protein (beta APP) can be detected immunocytochemically at sites of axonal injury in the brain, and has recently been found to be a useful marker for injured axons in patients who survived for only 3 h after head trauma. It is transported by fast axonal transport and is thought to accumulate in detectable levels where the cytoskeleton breaks down. If this theory is correct, other substances should accumulate here in the same way, so we have used antibodies to other neuronal proteins to compare their efficacy as markers of axonal injury. SNAP-25, chromogranin A and cathepsin D also marked injured axons at all survival times studied (2.5 h-2 weeks), although they were not as sensitive or specific as beta APP. Immunolabelling for the 68-kDa neurofilament subunit (NF68) was present in most uninjured axons, and allowed axonal swellings to be seen in some cases. Synaptophysin, GAP-43, ubiquitin or tau did not label any normal or injured axons in this study. We, therefore, suggest that beta APP should be the immunocytochemical marker of choice for the detection of injured axons. This study also showed that microwave antigen retrieval significantly enhances the immunoreactivity of SNAP-25, chromogranin A, synaptophysin, GAP-43, ubiquitin and tau, in addition to that of beta APP, in formalin-fixed, paraffin-embedded tissue, and reveals NF68 antigenicity where it was not previously detectable.

Ontogenic development and distribution of mRNAs of chromogranin A and B, secretogranin II, p65 and synaptin/synaptophysin in rat brain

We have studied by in situ hybridization the mRNA levels of several constituents of transmitter storing vesicles during ontogenic development of rat brain. The following vesicle components were investigated: chromogranin A and B and secretogranin II, representing secretory peptides of large dense core vesicles, and the membrane proteins p65 and synaptin/synaptophysin which are found in both large and small synaptic vesicles but are concentrated in the latter ones. Several ontogenic patterns were observed: concomitant increases of most or all mRNAs in certain brain regions, e.g. in the thalamic nuclei at gestational day 18 or in the cortex at postnatal day 6. For some areas selective increases for the various chromogranin mRNAs occurred, thus throughout development the substantia nigra compacta contained only the chromogranin B mRNA, whereas the lateral and medial geniculate nuclei and the medial tuberal nucleus expressed only secretogranin II mRNA. In the paraventricular hypothalamic nucleus, secretogranin II mRNA declined at P1 and then increased again. In the intermediate cortex there was a rather selective appearance of a high level of chromogranin A mRNA already at gestational day 16. In general the mRNAs for the membrane components become detectable by in situ hybridization together with the chromogranin mRNA, however, in the claustrum a high level of the p65 mRNA is present already at gestational day 16 whereas the chromogranin mRNA only appears at day 20. In some nuclei there was also a differential expression of the membrane components with e.g. the synaptophysin mRNA being present without any concomitant appearance of p65. These results establish that the ontogenic development of the investigated components in many brain areas simply indicate the starting point of biosynthesis of both types of vesicles finally leading to functional synapses. In those cases where a selective dissociation in the biosynthesis of these components occurs, a functional relevance of one component for a certain stage of development might be postulated. Since these data define the time of onset of vesicle biosynthesis in the various brain regions, future studies on single components of these vesicles can be interpreted in the context of the present findings.

Kainic acid seizures in the rat: differential expression of chromogranin A, carboxypeptidase H and peptidylglycine alpha-amidating monooxigenase in subfields of the hippocampal formation

Using in situ hybridization histochemistry concentrations of mRNAs encoding chromogranin A (ChA), carboxypeptidase H (CPH) and peptidylglycine alpha-amidating monooxigenase (PAM) have been investigated in the hippocampus after kainic acid (KA)-induced limbic seizures in the rat. Increased concentrations (by 150%) of ChA and CPH mRNAs were found in the granule cell layer 24 h after KA injection. At the same time PAM mRNA levels were only slightly elevated (by 50%). Whereas the increases in CPH and PAM transcripts were only transient, ChA mRNA concentrations in the granule cell layer were elevated up to 2 months after the initial seizures. In contrast, in the pyramidal cell layers of all hippocampal subfields (CA1 to CA3) ChA mRNA concentrations were significantly reduced (by 40-70%) 1-60 days after KA. PAM and CPH messages were slightly reduced in the pyramidal cell layer of CA1 but not in CA2 and CA3. The experiments demonstrate that KA-induced limbic seizures cause sustained changes in the expression of ChA mRNA. At the same time the expression of two enzymes involved in post-translational processing of neuropeptides, PAM and CPH, becomes only transiently altered. Synthesis of ChA may be regulated differently in the strata granulosum and pyramidale during epileptic seizures.

Glycoprotein III (clusterin, sulfated glycoprotein 2) in endocrine, nervous, and other tissues: immunochemical characterization, subcellular localization, and regulation of biosynthesis

Specific antisera were raised against the A and B chains of glycoprotein III. Immunoblotting revealed that in adrenal medulla both chains migrate very closely together in two-dimensional electrophoresis. Both chains with slightly differing molecular sizes are found in several endocrine tissues and in brain, kidney, liver, and serum. The mRNA has an analogous widespread distribution. In primary cultures of chromaffin cells the level of message becomes significantly increased by treatment with histamine or 12-O-tetradecanoylphorbol 13-acetate/forskolin. However, the increase is small when compared with that of secretogranin II. The subcellular localization of glycoprotein III in endocrine organs and in the posterior pituitary was investigated by subcellular fractionation and immunoelectron microscopy. Glycoprotein III was found to be confined to the large dense-core vesicles of these organs. For a discussion of the function of glycoprotein III, its localization in these organelles has to be taken into account.

Different degrees of processing of secretogranin II in large dense core vesicles of bovine adrenal medulla and sympathetic axons correlate with their content of soluble PC1 and PC2

We investigated the processing of secretogranin II in large dense core vesicles of adrenal medulla and sympathetic nerve. Despite the fact that both types of vesicles have a very similar biochemical composition, the degree of processing of secretogranin II in vesicles from splenic nerve was significantly higher. The endoproteases PC1 and PC2, two likely candidates for secretogranin II cleavage, are found in both types of vesicles, however, relative to secretogranin II the nerve vesicles have a much higher content of these enzymes. This probably explains the fast and more extensive processing of secretogranin II in these vesicles.

Localization of sulfated glycoprotein-2/clusterin mRNA in the rat brain by in situ hybridization

Sulfated glycoprotein-2 (SGP-2) gene expression seems to be constitutively expressed in a variety of tissues and organs, although levels of expression vary widely from one tissue to the other. SGP-2, also known as clusterin, has been reported to be expressed in the central nervous system (CNS). Some possible roles for brain SGP-2 have been postulated. In order to provide a substrate for a better understanding of the functions of this glycoprotein in the CNS, we investigated the detailed anatomical and cellular distribution of SGP-2 mRNA in the adult rat brain as well as the variation in its cellular expression after excitotoxin lesion. Transcripts for SGP-2 were found to be distributed throughout the rat CNS, although regional differences in their prevalence were readily observed. The ependymal lining of the ventricles showed the highest level of expression followed by various gray matter areas, some of which contained very intensively labeled cells. These cells were mostly found among several hypothalamic and brainstem nuclei, the habenular complex, as well as in the ventral horn of the spinal cord, which displayed striking hybridization signals over motoneurons. Occasional cells expressing high levels of SGP-2 transcripts were found in fiber tracts. Highly SGP-2 mRNA-positive resting glial cells were mainly located near the glial limitans and blood vessels. Two areas of relatively low constitutive SGP-2 mRNA expression are shown to produce strong hybridization signals 10 days after the local administration of the excitotoxin kainic acid. This overexpression of SGP-2 transcripts appears to involve GFAP-positive cells. Taken together, these results indicate that in the intact adult rat CNS, various cell populations, including neurons, constitutively express SGP-2 transcripts, whereas in the injured brain, reactive astrocytes become the major producers.

Reserpine causes differential changes in the mRNA levels of chromogranin B, secretogranin II, carboxypeptidase H, alpha-amidating monooxygenase, the vesicular amine transporter and of synaptin/synaptophysin in rat brain

Brains of rats treated with a high dose of reserpine were analyzed by in situ hybridization. The mRNA levels of several components of large dense core and small synaptic vesicles were determined. After drug treatment the secretogranin II message was elevated in the parvocellular neurons of the paraventricular nucleus, in the zona incerta, dorsal raphe, locus coeruleus and in the nucleus tractus solitarius. The levels of chromogranin B mRNA were increased in the dorsal raphe and in the substantia nigra compacta. In control animals messages for synaptin/synaptophysin could be found in most of the nuclei investigated, that of the vesicular amine transporter was only detectable in substantia nigra compacta, the dorsal raphe and the locus coeruleus whereas those of carboxypeptidase H and the alpha-amidating monooxygenase could only be determined in the paraventricular nucleus. All these messages were not changed after reserpine. We conclude that the chromogranin B/secretogranin II messages are regulated concomitantly with various neuropeptides. They represent useful general markers to identify stimulated neurons. Our results are consistent with the concept that stimulation of neurons leads to an increased synthesis of secretory peptides and consequently to large dense core vesicles filled with higher quanta of peptides.

Catecholamines are present in a synaptic-like microvesicle-enriched fraction from bovine adrenal medulla

"Synaptic-like microvesicles" are present in all neuroendocrine cells and cell lines. Despite their resemblance to small synaptic vesicles of the CNS, a thorough biochemical characterization is lacking. Moreover, the subcellular distribution of synaptophysin, the most abundant integral membrane protein of small synaptic vesicles, in adrenal medulla is still controversial. Using gradient centrifugation, we were able to compare the distribution of several markers for small synaptic vesicles and chromaffin granules. Synaptophysin was found at a high density (1.16 g/ml), purifying away from dopamine beta-hydroxylase and cytochrome b561. Both noradrenaline and adrenaline showed a parallel distribution with synaptophysin, suggesting their presence in synaptic-like microvesicles. Experiments in the presence of tetrabenazine did not influence the catecholamine content. Additionally, tetrabenazine binding showed a consistent shoulder in the region of synaptophysin. [3H]Noradrenaline uptake was blocked by tetrabenazine, but not by desipramine. Also chromogranin A parallels the distribution of synaptophysin; however, a localization in the Golgi cannot be ruled out. Synaptophysin was shown to undergo very fast phosphorylation, together with another triplet protein of approximately 18 kDa. In contrast, the latter showed a rather bimodal distribution coinciding with synaptophysin and dopamine beta-hydroxylase. Immunoelectron microscopy of synaptic-like microvesicle fractions showed an intense labeling for synaptophysin on 60-90-nm organelles. Whereas abundant gold labeling for cytochrome b561 was found over the entire surface of chromaffin granules, synaptophysin labeling was encountered mostly on vesicles adsorbed to granules. We conclude that catecholamines might be stored in synaptic-like microvesicles of the chromaffin cell.

Synaptophysin--a common constituent of presumptive secretory microvesicles in the mammalian pinealocyte: a study of rat and gerbil pineal glands

Recent studies have established that pinealocytes of the mammalian pineal gland contain marker molecules of neuroendocrine cells or paraneurons like the synaptic vesicle-associated protein synaptophysin (p38). The objective of this study was to identify the subcellular synaptophysin-positive compartment and to characterize in detail the intracellular distribution of this protein in rat and gerbil pinealocytes. An analysis of serial semithin sections of plastic-embedded pineals immunostained for synaptophysin, including computer-assisted optical density measurements of synaptophysin immunoreactivities, demonstrated unequivocally that synaptophysin was highly concentrated in dilated process terminals of the pinealocytes. More than 75% of these process terminals were found to border or lie within the pericapillary space. At the ultrastructural level, they contained accumulations of small clear vesicles of variable size that turned out to be the site of synaptophysin immunoreactivity when immunogold staining was performed. In addition, microvesicles surrounding synaptic ribbons were also immunolabeled. Hence, the pinealocyte is the first neuroendocrine cell type that has now been shown to concentrate synaptophysin-positive microvesicles in perivascular process endings. This observation lends strong support to the hypothesis that small clear vesicles in neuroendocrine cells in general, and in pinealocytes in particular, serve secretory functions. The quantitative analysis of completely sectioned process endings revealed that the microvesicles outnumber by far the amount of dense core vesicles and therefore cannot arise by endocytosis of dense core vesicle membranes. Thus, small synaptic-like vesicles probably constitute an independent secretory pathway of the paraneuronal pinealocytes. In the present study, we could also establish the absence of immunoreactivity for synapsin I (belonging to a family of neuron-specific nerve terminal phosphoproteins) from pinealocytes. Synapsin I immunoreactivity was only detectable in intrapineal nerve terminals and varicosities. Taken together, the immunostaining patterns of the pineal gland obtained with antibodies directed against synaptic vesicle-associated proteins render the mammalian pinealocyte a very special type of neuroendocrine cell or paraneuron rather than a "classic" neuron.

Redistribution of secretory granule components precedes that of synaptic vesicle proteins during differentiation of a neuronal cell line in serum-free medium

Incubation of the rat sensory neuron-derived cell line ND7 in serum-free medium results in the arrest of mitosis and the appearance of numerous neuronal processes. During this differentiation event, secretory granule components such as chromogranins, neuropeptide Y and the C-flanking peptide of pro-neuropeptide Y move to the tips of the majority of the neuronal processes regardless of process length. In contrast, the synaptic vesicle component, synaptophysin, is found only at the tips of the very long processes which appear following prolonged periods of culture in serum-free medium. A similar restriction of synaptophysin to long processes is also observed following differentiation and process formation induced by other treatments such as incubation in reduced serum or treatment with cyclic AMP or phorbol myristate acetate. Hence the regulated secretory pathway associated with the chromogranins and neuropeptides appears to be segregated into the processes at an earlier stage of ND7 differentiation than the synaptophysin-associated synaptic vesicle pathway. ND7 cells therefore provide a model system for studying the processes regulating these pathways and the redistribution of their components during neuronal differentiation.

Temporal lobe epilepsy of the rat: differential expression of mRNAs of chromogranin B, secretogranin II, synaptin/synaptophysin and p65 in subfield of the hippocampus

We have investigated by in situ hybridization changes in the content of mRNAs encoding for chromogranin B, secretogranin II, synaptin/synaptophysin and p65 after kainic acid-induced seizures and pentylenetetrazol kindling. Kainic acid seizures resulted in marked but transient increases in secretogranin II mRNA concentrations in the granule cell layer and throughout the pyramidal cell layers of the hippocampus (by 100-500%) as well as in various areas of the cerebral cortex (by up to 900%) and the thalamus (up to 300%) 12 h after injection of the toxin. Chromogranin B mRNA concentrations were persistently increased in granule cells (but not in pyramidal cells) of the hippocampus (suprapyramidal blade, 450%) and in cortical areas (250%) at all time intervals after kainic acid injection (12 h to 60 days). Accordingly chromogranin B immunoreactivity was enhanced in the terminal field of mossy fibers and in the inner part of the molecular layer 30 days after kainic acid. Secretogranin II immunoreactivity was also markedly increased in CA1, the paraventricular thalamic nucleus and in the central amygdala. In rats kindled with pentylenetetrazol only chromogranin B (by 200%) but not secretogranin II mRNA was increased in dentate granule cells. In contrast to the mRNAs of these secretory proteins concentrations of mRNAs encoding synaptin/synaptophysin and p65, two membrane proteins of synaptic vesicles, were not altered in any of these brain structures. These data demonstrate that in brain the biosynthesis of chromogranin B and secretogranin II is regulated like that of neuropeptides which is consistent with a role of these secretory polypeptides as precursors of functional peptides. Activation of neurons induces an increased synthesis of neuropeptides but not a concomitant synthesis of membrane proteins of synaptic vesicle. This might lead to an increased quantal content available for transmission.

Biogenesis of secretory granules

The biogenesis of secretory granules in endocrine, neuroendocrine, and exocrine cells is thought to involve a selective aggregation of the regulated secretory proteins into a dense-cored structure. The dense-core is then enveloped by membrane in the trans-Golgi network and buds, forming an immature secretory granule. The immature secretory granule then undergoes a maturation process which gives rise to the mature secretory granule. The recent data on the processes of aggregation, budding and maturation are summarized here. In addition, the current knowledge about the mature secretory granule is reviewed with emphasis on the biogenesis of the membrane of this organelle.

Differential subcellular distribution of PC1, PC2 and furin in bovine adrenal medulla and secretion of PC1 and PC2 from this tissue

The subcellular distribution of PC1, PC2 and furin was determined in bovine adrenal medulla by immunoblotting of fractions obtained by density gradient centrifugation. PC1 and PC2 were found to be confined to chromaffin granules whereas furin (C-terminal-peptide) was absent from these organelles. Stimulation of bovine adrenal medulla by carbamoylcholine chloride induced the secretion of PC1 and PC2. The secreted enzymes had the same molecular size as PC1 and PC2 present in chromaffin granules.

Effects of nerve growth factor on the biosynthesis of chromogranin A and B, secretogranin II and carboxypeptidase H in rat PC12 cells

We investigated the biosynthesis of various constituents (chromogranins A and B, secretogranin II, carboxypeptidase H and synaptin/synaptophysin) of large dense core and small vesicles in PC12 cells. These cells were treated for up to 18 days with nerve growth factor. Peptide levels were determined by quantitative immunoblotting, their mRNAs by Northern blotting. Nerve growth factor treatment changed the levels of the various peptides investigated and their mRNAs in three patterns. Peptide and mRNA levels for chromogranin A and chromogranin B were increased on day 1 and then declined. Synaptin/synaptophysin levels slightly decreased from day 1 onwards. On the other hand secretogranin II increased steadily up to 217% for peptide levels and 257% for mRNA levels. For carboxypeptidase H for which only the mRNA could be determined an analogous behaviour was seen. Its mRNA after 14 days of nerve growth factor treatment was 459% of controls. These results establish that the biosynthesis of the secretory proteins chromogranin A, chromogranin B and secretogranin II is regulated differentially during nerve growth factor treatment. We suggest that neuronal differentiation is accompanied by an increased biosynthesis of secretogranin II. For carboxypeptidase H, the marked increase in mRNA levels after nerve growth factor treatment is the first example that the biosynthesis of this peptide is significantly up-regulated. Synaptin/synaptophysin biosynthesis is not increased although this peptide is a major constituent of small vesicles which increase in number during nerve growth factor treatment.

Intracellular redistribution of neuropeptides and secretory proteins during differentiation of neuronal cell lines

We have demonstrated that the mouse neuroblastoma N18Tg2 cell line and several clones of hybrid ND cells (ND7, ND9 and ND21), derived from the fusion of neonatal rat sensory neurons with that neuroblastoma, show immunostaining to protein gene product 9.5, neuropeptide Y, C-flanking peptide of neuropeptide Y, tyrosine hydroxylase and chromogranins. Synaptophysin could only be detected in ND cells. Immunoreactivities to substance P, calcitonin gene-related peptide, galanin and somatostatin could not be detected in any of these cell lines. After three days of incubation in a differentiation medium, cell processes of various lengths were observed both in neuroblastoma and ND cell cultures. In ND7 cells there was also a redistribution of neuropeptide Y and its C-flanking peptide to the tips of cell processes. The differentiation of cell processes was also accompanied by the appearance of immunostaining to rat chromogranins in their tips. In contrast, synaptophysin expression was found mainly in cell bodies. Neuropeptide Y, its C-flanking peptide and chromogranins have been associated with secretory granules, whereas synaptophysin is a marker for small synaptic-like vesicles. Therefore, our morphological findings further support and expand the view that these markers are primarily associated with different subcellular structures. Moreover, they indicate that the regulated secretory pathway associated with chromogranins is segregated into nerve processes at an early stage of differentiation, when the synaptophysin-associated pathway is not yet mature. ND7 cells thus provide a useful model system for studying changes in the distribution of neuropeptides, cytoskeletal elements and proteins associated with cell secretion during neuronal differentiation.

Immunohistochemical study of synaptophysin distribution in the superior cervical ganglion of newborn and adult rats

Intense synaptophysin immunoreactivity was observed around neuronal cell bodies and in the neuropil of the superior cervical ganglion of adult rats. In newborn rats synaptophysin was comparatively less concentrated and restricted to small interstitial spaces. In contrast, in newborns, consistent traces of positivity were found in the Golgi-like area of larger neurons, in agreement with the higher neonatal rate of synaptophysin synthesis. A few clusters of small neurons, numerically more expanded in adult rats, exhibited a dense reaction product filling the whole cytoplasm. No positivity was found in intraganglionic fibres.

Synaptic pathology in Alzheimer's disease: immunological data for markers of synaptic and large dense-core vesicles

We have analysed several markers for small synaptic vesicles (synaptin-synaptophysin, p65 and SV2) and large dense-core vesicles (chromogranin A, secretogranin II/chromogranin C) in the brains of patients with Alzheimer's disease, and normal controls by immunoblotting and immunohistochemistry. In comparison to age-matched controls the levels of all three synaptic vesicle markers were decreased in temporal cortex of Alzheimer patients. On the other hand, the levels of chromogranin A were increased, and those of secretogranin II lowered. This resulted in a significant increase of the ratios of chromogranin A to synaptophysin, p65 or SV2 and of that for chromogranin A to secretogranin II. These increases were significantly correlated to clinical severity of dementia and extent of neuropathological changes. By immunohistochemistry a high percentage of senile plaques was found to contain chromogranin A-reactive dystrophic neurites, whereas synaptophysin reactivity within plaques was rare. These results indicate that the number of synaptic vesicles is lowered in Alzheimer's disease, and that one component of large dense-core vesicles, i.e. chromogranin A, is elevated. We, thus, suggest that in Alzheimer's brain distinct changes occur for both types of synaptic organelles.

Synaptin/synaptophysin, p65 and SV2: their presence in adrenal chromaffin granules and sympathetic large dense core vesicles

The subcellular distribution of three proteins of synaptic vesicles (synaptin/synaptophysin, p65 and SV2) was determined in bovine adrenal medulla and sympathetic nerve axons. In adrenals most p65 and SV2 is confined to chromaffin granules. Part of synaptin/synaptophysin is apparently also present in these organelles, but a considerable portion is found in a light vesicle which does not contain significant concentrations of typical markers of chromaffin granules (cytochrome b-561, dopamine beta-hydroxylase or the amine carrier). An analogous finding was obtained for sympathetic axons. The large dense core vesicles contain most p65 and also SV2 but only a smaller portion of synaptin/synaptophysin. A lighter vesicle containing this latter antigen and some SV2 has also been found. These results establish that in adrenal medulla and sympathetic axons three typical antigens of synaptic vesicles are not restricted to light vesicles. Apparently, a varying part of these antigens is found in chromaffin granules and large dense core vesicles. On the other hand, the light vesicles do not contain significant concentrations of functional antigens of chromaffin granules. Thus, the biogenesis of small presynaptic vesicles which contain all three antigens as well as functional components like the amine carrier is likely to involve considerable membrane sorting.

Sympathetic axons and nerve terminals: the protein composition of small and large dense-core and of a third type of vesicles

Homogenates of bovine splenic nerve and of vas deferens were subjected to differential and density gradient centrifugation to investigate their noradrenaline-storing organelles. The subcellular fractions obtained were analysed by immunoblotting in order to define the presence of various antigens in small dense-core and large dense-core vesicles. In both large granule and microsomal fractions from splenic nerve only one type of noradrenaline-storing vesicle was found, which represents the large dense-core vesicles. These organelles contained chromogranin A, chromogranin B, cytochrome b-561, carboxypeptidase H, glycoprotein II, glycoprotein III, dopamine beta-hydroxylase and the monoamine carrier which are also present in adrenal chromaffin granules. The subcellular distribution of synaptin/synatophysin was more complex since this protein was apparently present in two organelles: in a light vesicle which did not contain significant amounts of antigens found in large dense-core vesicles (dopamine beta-hydroxylase, cytochrome b-561 and the monoamine carrier) and in the dense fractions of the gradient, possibly within large dense-core vesicles. In the microsomal gradient from vas deferens several markers (catecholamines, synaptin/synaptophysin and dopamine beta-hydroxylase) were found in a bimodal distribution, which is consistent with their presence in small and large dense-core vesicles. When the larger granules were removed with higher centrifugation speed a microsomal fraction containing only light vesicles was obtained. After gradient centrifugation of this fraction several components (catecholamines, dopamine beta-hydroxylase, cytochrome b-561, the monoamine carrier and synaptin/synaptophysin) were concentrated in a peak at low density; apparently only small dense-core vesicles were now present.(ABSTRACT TRUNCATED AT 250 WORDS)