Subcellular distribution of 65,000 calmodulin-binding protein (p65) and synaptophysin (p38) in adrenal medulla

The association of dynamin with synaptophysin regulates quantal size and duration of exocytotic events in chromaffin cells

Although synaptophysin is one of the most abundant integral proteins of synaptic vesicle membranes, its contribution to neurotransmitter release remains unclear. One possibility is that through its association with dynamin it controls the fine tuning of transmitter release. To test this hypothesis, we took advantage of amperometric measurements of quantal catecholamine release from chromaffin cells. First, we showed that synaptophysin and dynamin interact in chromaffin granule-rich fractions and that this interaction relies on the C terminal of synaptophysin. Experimental maneuvers that are predicted to disrupt the association between these two proteins, such as injection of antibodies against dynamin or synaptophysin, or peptides homologous to the C terminal of synaptophysin, increased the quantal size and duration of amperometric spikes. In contrast, the amperometric current that precedes the spike remained unchanged, indicating that synaptophysin/dynamin association does not regulate the initial fusion pore, but it appears to target a later step of exocytosis to control the amount of catecholamines released during a single vesicle fusion event.

Role of the vesicular chloride transporter ClC-3 in neuroendocrine tissue

ClC-3 is an intracellular chloride transport protein known to reside on endosomes and synaptic vesicles. The endogenous protein has been notoriously difficult to detect in immunohistological experiments because of the lack of reliable antibodies. Using newly generated antibodies, we now examine its expression pattern at the cellular and subcellular level. In all tissues examined, immunostaining indicated that ClC-3 is a vesicular protein, with a prominent expression in endocrine cells like adrenal chromaffin cells and pancreatic islet cells. In line with a possible function of ClC-3 in regulating vesicle trafficking or exocytosis in those secretory cells, capacitance measurements and amperometry indicated that exocytosis of large dense-core vesicles (LDCVs) was decreased in chromaffin cells from ClC-3 knock-out mice. However, immunohistochemistry complemented with subcellular fractionation showed that ClC-3 is not detectable on LDCVs of endocrine cells, but localizes to endosomes and synaptic-like microvesicles in both adrenal chromaffin and pancreatic beta cells. This observation points to an indirect influence of ClC-3 on LDCV exocytosis in chromaffin cells, possibly by affecting an intracellular trafficking step.

Clinical and Pathological Features Associated With the Testicular Tumor of the Adrenogenital Syndrome

PURPOSE

Testicular tumor of the adrenogenital syndrome is a rare clinical entity found in young men with endocrine disorders. Histologically it resembles Leydig cell tumor. We 1) reviewed the clinical features of testicular tumor of the adrenogenital syndrome and 2) determined if special histopathological features of the tumor and synaptophysin reactivity could distinguish testicular tumor of the adrenogenital syndrome from Leydig cell tumor.

MATERIALS AND METHODS

We reviewed the medical and pathological records for all patients with testicular tumor of the adrenogenital syndrome seen at our institution from 1978 to 2004. These tumors were examined by histological and immunophenotypic methods for comparison to Leydig cell tumor.

RESULTS

A total of 14 males with an endocrine disorder had pathological evidence of testicular tumor of the adrenogenital syndrome. These tumors were often bilateral (93% or 13 of 14 cases), associated with pain (92% or 12 of 13) and refractory to medical management with high dose exogenous steroids (93% or 13 of 14). Testicular tumor of the adrenogenital syndrome was managed by tumor enucleation in 7 patients (54%) and by radical orchiectomy in 6 (46%). All patients had resolution of pain at 3-month followup. Upon histological review features found to be more common to testicular tumor of the adrenogenital syndrome compared with Leydig cell tumor were nuclear pleiomorphism, low mitotic activity, extensive fibrosis, lymphoid aggregates, adipose metaplasia and prominent lipochrome pigment. Synaptophysin (ICN, Costa Mesa, California) reactivity was strong in testicular tumor of the adrenogenital syndrome but rarely observed in Leydig cell tumor.

CONCLUSIONS

In our series medical treatment failed in patients with testicular tumor of the adrenogenital syndrome and conservative surgical therapy was possible in select individuals. We identified special histopathological and immunophenotypic features, including synaptophysin staining, which distinguish testicular tumor of the adrenogenital syndrome from Leydig cell tumor.

Divergent functions of neuronal Rab11b in Ca2+-regulated versus constitutive exocytosis

Using PC12 cells that express transfected human growth hormone (hGH) as a secreted reporter protein, we have searched for Rab proteins that function in exocytosis. Among the Rab proteins tested, we found that besides the previously described Rab3 proteins, only members of the Rab11 family (Rab11a, 11b, and 25) impaired Ca2+-induced exocytosis. Rab11b, which is enriched in brain, had the strongest effect. Consistent with a role in exocytosis, Rab11 and Rab3 proteins were colocalized with other vesicle proteins on secretory vesicles in PC12 cells and on mature synaptic vesicles in brain. Rab11b mutants that fix Rab11b in the GTP- or GDP-bound state both effectively inhibited Ca2+-induced exocytosis but seemed to act by distinct mechanisms: whereas GDP-bound Rab11b greatly stimulated constitutive secretion of hGH and depleted hGH stores in secretory vesicles, GTP-bound Rab11b only had a moderate effect on constitutive secretion and no effect on vesicular hGH stores. These results suggest that, consistent with a GTP-dependent regulation of Rab function, GDP-bound Rab11b indirectly inhibits Ca2+-triggered exocytosis by causing the loss of hGH from the PC12 cells, whereas GTP-bound Rab11b directly impairs Ca2+-triggered exocytosis. In contrast to neuroendocrine PC12 cells in which GTP- and GDP-bound Rab11b inhibited Ca2+-induced, but not constitutive, exocytosis, in non-neuronal cells GTP- and GDP-bound Rab11b inhibited constitutive exocytosis and caused an accumulation of cellular hGH. Viewed together, our data suggest that, in addition to other functions, Rab11 has a specific role in neuronal and neuroendocrine but not in non-neuronal cells as a GTP-dependent switch between regulated and constitutive secretory pathways.

Myosins II and V in chromaffin cells: myosin V is a chromaffin vesicle molecular motor involved in secretion

The presence of myosin II and V in chromaffin cells and their subcellular distribution is described. Myosin II and V distribution in sucrose density gradients showed only a strong correlation between the distribution of myosin V and secretory vesicle markers. Confocal microscopy images demonstrated colocalization of myosin V with dopamine beta-hydroxylase, a chromaffin vesicle marker, whereas myosin II was present mainly in the cell cortex. Cell depolarization induced, in a Ca2+ and time-dependent manner, the dissociation of myosin V from chromaffin vesicles suggesting that this association was not permanent but determined by secretory cycle requirements. Myosin II was also found in the crude granule fraction, however, its distribution was not affected by cell depolarization. Myosin V head antibodies were able to inhibit secretion whereas myosin II antibodies had no inhibitory effect. The pattern of inhibition indicated that these treatments interfered with the transport of vesicles from the reserve to the release-ready compartment, suggesting the involvement of myosin V and not myosin II in this transport process. The results described here suggest that myosin V is a molecular motor involved in chromaffin vesicle secretion. However, these results do not discard an indirect role for myosin II in secretion through its interaction with F-actin networks.

Molecular motors involved in chromaffin cell secretion

Neurosecretory cells, including chromaffin cells, possess a mesh of filamentous actin underneath the plasma membrane. It has been proposed that filamentous actin network separates the secretory vesicles into two compartments: the reserve pool and the release-ready vesicle pool. Disassembly of chromaffin cell cortical filamentous actin in response to stimulation allows the movement of vesicles from the reserve pool into the release-ready vesicle pool. Electron microscopy of cytoskeletons revealed the presence of polygonal areas almost devoid of actin filaments in stimulated cells. The percentage of stimulated cells showing disrupted cytoskeleton correlates well with the increase in secretion in these cells. Fine filaments also remain in these areas of disassembly, and these reacted with actin antibodies, as demonstrated by immunogold staining. In addition, the movement of vesicles between pools requires Ca(2+) and ATP, a condition for activation of a molecular motor. Confocal microscopy images demonstrated colocalization of myosin Va with dopamine-beta-hydroxylase. Cell depolarization induced the dissociation of myosin Va from chromaffin vesicles. 2,3-Butadione-2-monoxime (BDM), an inhibitor of myosin ATPase, inhibited secretion, suggesting a blockage for chromaffin vesicle transport between the reserve pool and the release-ready vesicle pool. On the other hand, myosin II subcellular distribution was not affected by cell depolarization. Confocal microscopy images show myosin II to be localized in the cell cortex and in some perinuclear structures. Chromaffin vesicles were not stained by myosin II antibody.

Synaptotagmin I-DeltaC2B. A novel synaptotagmin isoform with a single C2 domain in the bovine adrenal medulla

Synaptotagmin I is a 65 kDa type 1 membrane glycoprotein found in secretory organelles that plays a key role in regulated exocytosis. We have characterised two forms (long and short) of synaptotagmin I that are present in the bovine adrenal medulla. The long form is a type I integral membrane protein which has two cytoplasmic C2 domains and corresponds to the previously characterised full-length synaptotagmin I isoform. The short-form synaptotagmin I-DeltaC2B has the same structure in the lumenal and transmembrane sequences, but synaptotagmin I-DeltaC2B is truncated such that it only has a single cytoplasmic C2 domain. Analysis of synaptotagmin I-DeltaC2B expression indicates that synaptotagmin I-DeltaC2B is preferentially expressed in the bovine adrenal medulla. However, it is absent from the dense core chromaffin granules. Furthermore, when expressed in the rat pheochromocytoma cell line PC12 bovine synaptotagmin I-DeltaC2B is largely absent from dense core granules and synaptic-like microvesicles. Instead, indirect immunofluorescence microscopy reveals the intracellular location of synaptotagmin I-DeltaC2B to be the plasma membrane.

NSF and SNAP are present on adrenal chromaffin granules

N-ethylmaleimide sensitive fusion protein (NSF) and soluble NSF attachment proteins (SNAPs) are involved in many vesicular transport steps. It has been proposed that SNAPs and NSF associate with their membrane receptors only when vesicles dock on the target membrane. Analysis of NSF and alpha-SNAP distribution in fractionation of organelles from adrenal medulla indicated that a substantial amount of both proteins distributed with chromaffin granules. Further fractionation of intact granules and lysed granule membranes showed exact overlap of NSF and alpha-SNAP distribution with chromaffin granules. These results suggest that NSF and alpha-SNAP are associated with chromaffin granules and support the idea that they function prior to docking of the granules on the plasma membrane.

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.

Synaptic vesicle proteins in cells of the sympathoadrenal lineage

The cells of sympathoadrenal lineage display different characteristics after differentiation, although they stem from the same neural crest precursor during embryonic development. In the present study we compared the distribution patterns of several synaptic vesicle proteins in the superior cervical ganglion (SCG) and the adrenal medulla. Using indirect immunofluorescence combined with confocal laser scanning microscopy, it was observed that antisera against integral synaptic vesicle membrane proteins (SV2, synaptotagmin I, synaptobrevin II and synaptophysin) induced strong immunoreactivities in these cells, but anti-synaptobrevin I caused only a faint fluorescence. Immunoreactivities of the synaptic vesicle-associated proteins Rab3a and SNAP25 were also observed in the cells. Synapsin-Ia-reactive material appeared absent from chromaffin cells but present in small amounts in sympathetic neurons in the SCG and iris terminals. On the other hand, synapsin IIa immunoreactive material was strong in most SCG neurons and in adrenergic iris terminals. The neural specific clatrin light chain was detected in the SCG cells and in ganglion cells of the adrenal, but only weak traces could be observed in chromaffin cells. One of the vesicular monoamine transmitter transporters, VMAT2, which is expressed in catecholamine neurons in the brain stem, was observed in most cells in the SCG and also in groups of cells in the adrenal medulla, where the VMAT2-positive small chromaffin cells were PNMT-negative. SIF cells in the SCG contained most of the synaptic vesicle proteins investigated. The results show that after differentiation, sympathetic neurons, SIF cells and adrenal chromaffin cells still share many vesicle proteins even though their physiology is different.

Botulinum neurotoxin C1 cleaves both syntaxin and SNAP-25 in intact and permeabilized chromaffin cells: correlation with its blockade of catecholamine release

The seven types (A--G) of botulinum neurotoxin (BoNT) are Zn2+ -dependent endoproteases that potently block neurosecretion. Syntaxin is presently thought to be the sole substrate for BoNT/C1, and synaptosomal-associated protein of Mr = 25 000 (SNAP-25) is selectively proteolyzed by types A and E. In this study, the effects of C1 on Ca2+ -regulated exocytosis of dense core granules from adreno-chromaffin cells were examined together with its underlying molecular action. Intact chromaffin cells were exposed to the toxin, and catecholamine release therefrom was then measured in conjunction with the monitoring of syntaxin cleavage by Western blotting. A good correlation was obtained between degradation of syntaxin 1A/B and reduction in Ca2+- or Ba2+-dependent secretion. However, blotting with antibodies against a C-terminal peptide of SNAP-25 revealed the additional disappearance of immunoreactivity, with the same toxin concentration dependency as syntaxin breakdown. Notably, the cleaved SNAP-25 product was similar in size to that produced by BoNT/A; however, contamination of BoNT/C1 by serotypes A or E was eliminated. Therefore, it is concluded that syntaxin 1A/B and SNAP-25 are cleaved in intact cells poisoned with only C1. Notably, C1 treatment of chromaffin cells abolished Ca2+ -evoked secretion following digitonin permeabilization, compared with partial inhibition by BoNT/A, suggesting the importance of syntaxin for catecholamine release. Unexpectedly, C1 failed to proteolyze a soluble recombinant SNAP-25, even though it served as an efficient substrate for BoNT/A. These interesting observations suggest that C1 can only efficiently cleave SNAP-25 in intact cells, possibly due to the existence therein of a unique conformation and/or the participation of accessory factors.

Peptidergic transmission: from morphological correlates to functional implications

Like non-peptidergic transmitters, neuropeptides and their receptors display a wide distribution in specific cell types of the nervous system. The peptides are synthesized, typically as part of a larger precursor molecule, on the rough endoplasmic reticulum in the cell body. In the trans-Golgi network, they are sorted to the regulated secretory pathway, packaged into so-called large dense-core vesicles, and concentrated. Large dense-core vesicles are preferentially located at sites distant from active zones of synapses. Exocytosis may occur not only at synaptic specializations in axonal terminals but frequently also at nonsynaptic release sites throughout the neuron. Large dense-core vesicles are distinguished from small, clear synaptic vesicles, which contain "classical' transmitters, by their morphological appearance and, partially, their biochemical composition, the mode of stimulation required for release, the type of calcium channels involved in the exocytotic process, and the time course of recovery after stimulation. The frequently observed "diffuse' release of neuropeptides and their occurrence also in areas distant to release sites is paralleled by the existence of pronounced peptide-peptide receptor mismatches found at the light microscopic and ultrastructural level. Coexistence of neuropeptides with other peptidergic and non-peptidergic substances within the same neuron or even within the same vesicle has been established for numerous neuronal systems. In addition to exerting excitatory and inhibitory transmitter-like effects and modulating the release of other neuroactive substances in the nervous system, several neuropeptides are involved in the regulation of neuronal development.

Identification of subcellular compartments containing peptidylglycine alpha-amidating monooxygenase in rat anterior pituitary

Both soluble and integral membrane forms of peptidylglycine alpha-amidating monooxygenase (PAM) are expressed in the rat anterior pituitary, making it an ideal model system for studying the routing of proteins into secretory granules. To identify the subcellular compartments involved in the routing of integral membrane PAM, we used subcellular fractionation, metabolic labeling and immunoblot analysis. Mature secretory granules were found to contain full-length integral membrane PAM along with a significant amount of soluble PAM generated by endoproteolytic cleavage. PAM proteins were not co-distributed with tyrosylprotein sulfotransferase activity during sucrose gradient centrifugation, indicating that the trans-Golgi/TGN is not a major PAM-containing compartment at steady state. Fractionation of the 4,000 g and 10,000 g pellets obtained by differential centrifugation identified a significant amount of integral membrane PAM in a light fraction lacking soluble secretory granule proteins. Metabolic labeling experiments with primary anterior pituitary cells demonstrated that integral membrane PAM enters a light compartment with similar properties only after exit from the trans-Golgi/TGN. Comparison of the metabolic labeling and immunoblot analyses suggests that PAM in this post-trans-Golgi/TGN compartment is in organelles involved in the intracellular recycling of integral membrane PAM. Small amounts of full-length integral membrane PAM were also recovered in fractions containing internalized transferrin and may be in an endosomal compartment following retrieval from the cell surface.

Synaptotagmin I is present mainly in autonomic and sensory neurons of the rat peripheral nervous system

The distribution of synaptotagmin I in the peripheral nervous system of the rat was investigated by immunofluorescence and confocal laser scanning microscopy. After crushing of the sciatic nerve, synaptotagmin I-like immunoreactivity accumulated proximally as well as distally to the crushes in thin and medium-sized axons. Double labelling studies revealed that synaptotagmin I co-localized with tyrosine hydroxylase, a marker of sympathetic adrenergic neurons, and with substance P, a marker for sensory neurons. No synaptotagmin I-like immunoreactivity was found in large axons, while accumulations of the synaptic vesicle proteins synaptophysin and synapsin I were found in all types of axons. Furthermore, no synaptotagmin I-like immunoreactivity was detected in motor endplates. In contrast, the protein was found in muscle spindles of young rats and in perivascular terminals, where it co-localized with synaptophysin and synapsin I. Lumbar sympathectomy resulted in a marked reduction of the amount and intensity of synaptotagmin I-like immunoreactivity in sciatic nerve. High magnification revealed that synaptotagmin I-like immunoreactivity was mainly distributed in a fine granular pattern, but large, brightly fluorescent granules which were not labelled by anti-synaptophysin or anti-synapsin I were occasionally observed. We conclude that synaptotagmin I is mainly expressed in adrenergic and sensory neurons and is absent from, or below detection levels, in motoneurons.

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.

Cytoskeleton and molecular mechanisms in neurotransmitter release by neurosecretory cells

The process of exocytosis is a fascinating interplay between secretory vesicles and cellular components. Secretory vesicles are true organelles which not only store and protect neurotransmitters from inactivation but also provide the cell with efficient carriers of material for export. Different types of secretory vesicles are described and their membrane components compared. Associations of several cytoplasmic proteins and cytoskeletal components with secretory vesicles and the importance of such associations in the mechanism of secretion are discussed. A description of possible sites of action for Ca2+ as well as possible roles for calmodulin, G-proteins and protein kinase C in secretion are also presented. Important aspects of the cytoskeleton of neurosecretory cells are discussed. The cytoskeleton undergoes dynamic changes as a result of cell stimulation. These changes (i.e. actin filament disassembly) which are a prelude to exocytosis, play a central role in secretion. Moreover, advanced electrophysiological techniques which allow the study of secretory vesicle-plasma membrane fusion in real-time resolution and at the level of the single secretory vesicle, have also provided a better understanding of the secretory process.

Vesamicol binding to subcellular membranes that are distinct from catecholaminergic vesicles in PC12 cells

We have examined PC12 cells for the localization of binding sites for vesamicol [l-2-(4-phenylpiperidino) cyclohexanol], a compound that has previously been shown to bind to cholinergic vesicles and to inhibit the uptake of acetylcholine. Initial studies presented in this article demonstrate the existence of a specific, saturable vesamicol binding site in PC12 cells. Subsequent experiments show that these binding sites reside in a membrane population that is distinct from catecholamine-containing compartments with respect to density and antigenic composition. In particular, vesamicol binding compartments have a lower density than catecholaminergic vesicles and, unlike these latter vesicles, do not appear to contain the vesicle-specific proteins synaptophysin and SV2 as part of the same membrane. These results suggest that vesicular transport proteins for acetylcholine and catecholamines are differentially sorted to distinct membrane compartments in PC12 cells.

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.

Glycosylation and transmembrane topography of bovine chromaffin granule p65

The bovine homologue of p65, a calmodulin-binding protein located in the membranes of synaptic vesicles and endocrine secretory granules, has been studied by the use of monoclonal antibodies directed against this antigen and against dopamine beta-mono-oxygenase. The protein (apparent molecular mass 67 kDa; pI = 5.5-6.2) is partially degraded by treatment with neuraminidase or endoglycosidase F. Trypsin treatment of intact adrenal chromaffin granules or of granule membranes releases a soluble 39 kDa fragment of p65 which corresponds to the whole of its cytoplasmic domain. This domain contains both the epitope for the monoclonal antibody cgm67 and the calmodulin-binding site. The 20 amino acids at the N-terminus of this fragment are identical to part of the rat p65 sequence.

The role of protein kinase C and its neuronal substrates dephosphin, B-50, and MARCKS in neurotransmitter release

This article focuses on the role of protein phosphorylation, especially that mediated by protein kinase C (PKC), in neurotransmitter release. In the first part of the article, the evidence linking PKC activation to neurotransmitter release is evaluated. Neurotransmitter release can be elicited in at least two manners that may involve distinct mechanisms: Evoked release is stimulated by calcium influx following chemical or electrical depolarization, whereas enhanced release is stimulated by direct application of phorbol ester or fatty acid activators of PKC. A markedly distinct sensitivity of the two pathways to PKC inhibitors or to PKC downregulation suggests that only enhanced release is directly PKC-mediated. In the second part of the article, a framework is provided for understanding the complex and apparently contrasting effects of PKC inhibitors. A model is proposed whereby the site of interaction of a PKC inhibitor with the enzyme dictates the apparent potency of the inhibitor, since the multiple activators also interact with these distinct sites on the enzyme. Appropriate PKC inhibitors can now be selected on the basis of both the PKC activator used and the site of inhibitor interaction with PKC. In the third part of the article, the known nerve terminal substrates of PKC are examined. Only four have been identified, tyrosine hydroxylase, MARCKS, B-50, and dephosphin, and the latter two may be associated with neurotransmitter release. Phosphorylation of the first three of these proteins by PKC accompanies release. B-50 may be associated with evoked release since antibodies delivered into permeabilized synaptosomes block evoked, but not enhanced release. Dephosphin and its PKC phosphorylation may also be associated with evoked release, but in a unique manner. Dephosphin is a phosphoprotein concentrated in nerve terminals, which, upon stimulation of release, is rapidly dephosphorylated by a calcium-stimulated phosphatase (possibly calcineurin [CN]). Upon termination of the rise in intracellular calcium, dephosphin is phosphorylated by PKC. A priming model of neurotransmitter release is proposed where PKC-mediated phosphorylation of such a protein is an obligatory step that primes the release apparatus, in preparation for a calcium influx signal. Protein dephosphorylation may therefore be as important as protein phosphorylation in neurotransmitter release.

Serotonin organelles of rabbit platelets contain synaptophysin

Synaptophysin, an integral membrane protein of synaptic vesicles in nerve terminals and a class of small translucent vesicles in neuroendocrine cells, was detected in intact rabbit platelets by immunoblotting, immunofluorescence staining and immuno-electron microscopy. In a highly purified preparation of serotonin organelles isolated from rabbit platelets, synaptophysin was enriched approximately 10-15-fold over platelet homogenate. About 80% of total platelet synaptophysin was present in this purified fraction. The apparent molecular mass (approximately 38 kDa) and the extent of glycosylation of platelet-derived synaptophysin was more similar to the neuronal than to the neuroendocrine form of the protein. Immunofluorescence microscopy revealed that synaptophysin was compartmentalized in intact rabbit platelets and immuno-electron microscopy of subcellular fractions showed that it was localized exclusively to the membrane surface of serotonin organelles. No synaptophysin-like immunoreactivity was detected in platelets from other species such as human, guinea pig and rat. Another integral membrane protein of synaptic vesicles, p65, and a family of synaptic vesicle-associated phosphoproteins, the synapsins, were not detected in platelets of any species tested. These results provide evidence that serotonin organelles from rabbit platelets share a subset of protein components with synaptic vesicles from neurons. Synaptophysin in serotonin organelles from rabbit platelets, as suggested for small synaptic vesicles in neurons, might play a role in the formation of protein channels for the exocytotic release of serotonin.

Neurotransmitter release

Axon terminals release more than one physiologically active substance. Synaptic messengers may be stored in two different types of vesicles. Small electron-lucent vesicles mainly store classical low molecular weight transmitter substances and the larger electron-dense granules store and release proteins and peptides. Release of the two types of substances underlies different physiological control. Release of messenger molecules from axon terminals is triggered by influx of Ca2+ through voltage sensitive Ca2+ channels and a rise in cytosolic Ca2+ concentrations. Neither the immediate Ca2+ target(s) nor the molecular species involved in synaptic vesicle docking, fusion and retrieval are known. It is, however, likely that steps involved in the molecular cascade of transmitter release include liberation of vesicles from their association with the cytonet and phosphorylation by protein kinase C of proteins which have the ability to alter between membrane bound and cytoplasmic forms and thus facilitate or initiate the molecular interaction between synaptic vesicles and the plasma membrane.

Expression of scinderin, an actin filament-severing protein, in different tissues

Scinderin is a calcium-dependent actin filament-severing protein recently discovered in the chromaffin cells of adrenal medulla. In view of the wide tissue distribution of gelsolin, another actin filament-severing protein, experiments were performed to determine the tissue expression of scinderin. Extracts prepared from different bovine tissues were tested by actin-DNase I Sepharose 4B-binding procedure and immunoprecipitation followed by immunoblotting with scinderin and gelsolin antibodies. Among the tissues tested, scinderin was found to be present in the adrenal medulla, brain, anterior and posterior pituitaries, kidney, salivary gland and testis. Scinderin was not found in liver, plasma, skeletal and heart muscles. Gelsolin was expressed in all of the above tissues. The results suggest that scinderin seems to be restricted to tissues with high secretory activity.