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

Neuropeptides and small-molecule amine transmitters: cooperative signaling in the nervous system

Neuropeptides are expressed in cell-specific patterns throughout mammalian brain. Neuropeptide gene expression has been useful for clustering neurons by phenotype, based on single-cell transcriptomics, and for defining specific functional circuits throughout the brain. How neuropeptides function as first messengers in inter-neuronal communication, in cooperation with classical small-molecule amine transmitters (SMATs) is a current topic of systems neurobiology. Questions include how neuropeptides and SMATs cooperate in neurotransmission at the molecular, cellular and circuit levels; whether neuropeptides and SMATs always co-exist in neurons; where neuropeptides and SMATs are stored in the neuron, released from the neuron and acting, and at which receptors, after release; and how neuropeptides affect 'classical' transmitter function, both directly upon co-release, and indirectly, via long-term regulation of gene transcription and neuronal plasticity. Here, we review an extensive body of data about the distribution of neuropeptides and their receptors, their actions after neuronal release, and their function based on pharmacological and genetic loss- and gain-of-function experiments, that addresses these questions, fundamental to understanding brain function, and development of neuropeptide-based, and potentially combinatorial peptide/SMAT-based, neurotherapeutics.

A secretory vesicle failure in Parkinson's disease occurs in human platelets

Objective

The presence of elevated dopamine (DA) and its major metabolites in the cytosol of neurons has been associated with their vulnerability in Parkinson's disease (PD). Over 99% of the cell's amines are confined to secretory vesicles (SVs), making these structures fundamental in the regulation of cytosolic DA levels. SVs of platelets use similar, if not the same mechanisms to accumulate serotonin in SVs as dopaminergic neurons do to store DA. Hence, any functional defects in platelets probably mirrors events in DA neurons.

Methods

We have isolated fresh platelets from the blood of 75 PD patients, 116 matched controls and 24 patients with Parkinsonism, assaying serotonin handling (basal content, accumulation, secretion and spontaneous leakage).

Results

We found a dramatic decrease in the serotonin content and uptake by SVs, as well as decreased thrombin‐induced release by platelets from PD patients but not in those from most Parkinsonism cases. Platelets from PD patients also failed to retain serotonin in SVs.

Interpretation

These findings indicate a functional impairment in the handling of amines by SVs in PD patients. This defect may serve as a biomarker of PD, and the approach described here may be potentially used for the subclinical detection of PD and to establish a platform to assay disease modifying drugs. ANN NEUROL 2022

The Phosphoprotein Synapsin Ia Regulates the Kinetics of Dense-Core Vesicle Release

Common fusion machinery mediates the Ca²⁺-dependent exocytosis of synaptic vesicles (SVs) and dense-core vesicles (DCVs). Previously, Synapsin Ia (Syn Ia) was found to localize to SVs, essential for mobilizing SVs to the plasma membrane through phosphorylation. However, whether (or how) the phosphoprotein Syn Ia plays a role in regulating DCV exocytosis remains unknown. To answer these questions, we measured the dynamics of DCV exocytosis by using single-vesicle amperometry in PC12 cells (derived from the pheochromocytoma of rats of unknown sex) overexpressing wild-type or phosphodeficient Syn Ia. We found that overexpression of phosphodeficient Syn Ia decreased the DCV secretion rate, specifically via residues previously shown to undergo calmodulin-dependent kinase (CaMK)-mediated phosphorylation (S9, S566, and S603). Moreover, the fusion pore kinetics during DCV exocytosis were found to be differentially regulated by Syn Ia and two phosphodeficient Syn Ia mutants (Syn Ia-S62A and Syn Ia-S9,566,603A). Kinetic analysis suggested that Syn Ia may regulate the closure and dilation of DCV fusion pores via these sites, implying the potential interactions of Syn Ia with certain DCV proteins involved in the regulation of fusion pore dynamics. Furthermore, we predicted the interaction of Syn Ia with several DCV proteins, including Synaptophysin (Syp) and soluble N-ethylmaleimide-sensitive factor attachment receptor (SNARE) proteins. By immunoprecipitation, we found that Syn Ia interacted with Syp via phosphorylation. Moreover, a proximity ligation assay (PLA) confirmed their phosphorylation-dependent, in situ interaction on DCVs. Together, these findings reveal a phosphorylation-mediated regulation of DCV exocytosis by Syn Ia.SIGNIFICANCE STATEMENT Although they exhibit distinct exocytosis dynamics upon stimulation, synaptic vesicles (SVs) and dense-core vesicles (DCVs) may undergo co-release in neurons and neuroendocrine cells through an undefined molecular mechanism. Synapsin Ia (Syn Ia) is known to recruit SVs to the plasma membrane via phosphorylation. Here, we examined whether Syn Ia also affects the dynamics of DCV exocytosis. We showed that Syn Ia regulates the DCV secretion rate and fusion pore kinetics during DCV exocytosis. Moreover, Syn Ia-mediated regulation of DCV exocytosis depends on phosphorylation. We further found that Syn Ia interacts with Synaptophysin (Syp) on DCVs in a phosphorylation-dependent manner. Thus, these results suggest that Syn Ia may regulate the release of DCVs via phosphorylation.

Neurotransmitter segregation: functional and plastic implications

Synaptic cotransmission is the ability of neurons to use more than one transmitter to convey synaptic signals. Cotransmission was originally described as the presence of a classic transmitter, which conveys main signal, along one or more cotransmitters that modulate transmission, later on, it was found cotransmission of classic transmitters. It has been generally accepted that neurons store and release the same set of transmitters in all their synaptic processes. However, some findings that show axon endings of individual neurons storing and releasing different sets of transmitters, are not in accordance with this assumption, and give support to the hypothesis that neurons can segregate transmitters to different synapses. Here, we review the studies showing segregation of transmitters in invertebrate and mammalian central nervous system neurons, and correlate them with our results obtained in sympathetic neurons. Our data show that these neurons segregate even classic transmitters to separated axons. Based on our data we suggest that segregation is a plastic phenomenon and responds to functional synaptic requirements, and to 'environmental' cues such as neurotrophins. We propose that neurons have the machinery to guide the different molecules required in synaptic transmission through axons and sort them to different axon endings. We believe that transmitter segregation improves neuron interactions during cotransmission and gives them selective and better control of synaptic plasticity.

Subcellular localization and putative role of VPS13A/chorein in dopaminergic neuronal cells

Chorea-acanthocytosis (ChAc) is a rare hereditary neurodegenerative disorder caused by loss of function mutations in the vacuolar protein sorting 13 homolog A (VPS13A) gene encoding chorein. Although a deficiency in chorein function leads to apoptosis of striatal neurons in ChAc model mouse, its detailed subcellular localization and physiological role remain unclear. In this study, we produced two anti-chorein polyclonal antibodies and examined the intracellular localization of endogenous chorein in neuronal cells. Immunocytochemically, chorein was observed in the termini of extended neurites and partially colocalized with synaptotagmin I in differentiated PC12 cells. Subcellular localization analysis by sucrose density gradient fractionation showed that chorein and synaptotagmin I were located in dense-core vesicles (DCVs), which contain dopamine. In addition, PC12 cells stably expressing carboxyterminal fragment of chorein increased K(+)-induced dopamine release. Taken together, these results suggest that chorein is involved in exocytosis of DCV.

Subcellular localization of the antidepressant-sensitive norepinephrine transporter

BACKGROUND

Reuptake of synaptic norepinephrine (NE) via the antidepressant-sensitive NE transporter (NET) supports efficient noradrenergic signaling and presynaptic NE homeostasis. Limited, and somewhat contradictory, information currently describes the axonal transport and localization of NET in neurons.

RESULTS

We elucidate NET localization in brain and superior cervical ganglion (SCG) neurons, aided by a new NET monoclonal antibody, subcellular immunoisolation techniques and quantitative immunofluorescence approaches. We present evidence that axonal NET extensively colocalizes with syntaxin 1A, and to a limited degree with SCAMP2 and synaptophysin. Intracellular NET in SCG axons and boutons also quantitatively segregates from the vesicular monoamine transporter 2 (VMAT2), findings corroborated by organelle isolation studies. At the surface of SCG boutons, NET resides in both lipid raft and non-lipid raft subdomains and colocalizes with syntaxin 1A.

CONCLUSION

Our findings support the hypothesis that SCG NET is segregated prior to transport from the cell body from proteins comprising large dense core vesicles. Once localized to presynaptic boutons, NET does not recycle via VMAT2-positive, small dense core vesicles. Finally, once NET reaches presynaptic plasma membranes, the transporter localizes to syntaxin 1A-rich plasma membrane domains, with a portion found in cholera toxin-demarcated lipid rafts. Our findings indicate that activity-dependent insertion of NET into the SCG plasma membrane derives from vesicles distinct from those that deliver NE. Moreover, NET is localized in presynaptic membranes in a manner that can take advantage of regulatory processes targeting lipid raft subdomains.

Segregation of met-enkephalin from vesicular acetylcholine transporter and choline acetyltransferase in sympathetic preganglionic varicosities mostly lacking synaptophysin and synaptotagmin

Sympathetic preganglionic neurons (SPN) coexpress the acetylcholine (ACh)-synthesizing enzyme choline acetyltransferase and different peptides in their cell bodies, but can express them independently in separate varicosities, indicating that SPN segregate transmitters to different synapses. Consequently, there are populations of preganglionic varicosities (peptidergic and noncholinergic) that store peptides but not ACh. We studied in the cell bodies and axon processes of the rat SPN the expression and the proportional coexpression of the vesicular ACh transporter-like immunoreactivity (VAChT), a specific marker of cholinergic synaptic vesicles or ChAT-like immunoreactivity (ChAT), and the peptide methionine enkephalin-like immunoreactivity (mENK), and confirmed the presence of a population of SPN peptidergic, noncholinergic varicosities. We characterized these varicosities by exploring the occurrence of synaptophysin-like immunoreactivity (Syn), a marker of small clear vesicles, and synaptotagmin-like immunoreactivity (Syt), a preferential marker of large dense core vesicles. We found that (i) VAChT and mENK, like ChAT-mENK, were coexpressed in only 59% of the mENK-containing varicosities, although they colocalized in the SPN cell bodies; and (ii) almost 60% of the population of mENK-containing varicosities did not express Syn or Syt, and over 80% of the mENK-containing varicosities negative for VAChT also lacked Syn. These data prove that SPN segregate mENK from VAChT and ChAT, and show that most of the subset of mENKergic varicosities negative for VAChT also does not express Syn, suggesting the presence of a different vesicular pattern in these sympathetic preganglionic varicosities.

Dvl regulates endo- and exocytotic processes through binding to synaptotagmin

Dvl, an important component of the Wnt signalling pathway, is thought to be involved in synaptogenesis. In this study, we investigated whether Dvl regulates neurotransmitter release. Knockdown of Dvl in PC12 cells suppressed K(+)-induced dopamine release, and this phenotype was restored by expression of Dvl-1. We identified synaptotagmin (Syt) I, which is involved in neurotransmitter release, as a Dvl-binding protein. Dvl directly bound to the C2B domain of Syt I. Dvl colocalized with Syt I at the tip of neurites of differentiated PC12 cells and of neurons in the rat dorsal root ganglion. Dvl and Syt I was located in large dense-core vesicles, which contain dopamine. In addition, endocytosis of vesicles containing Syt I was suppressed in Dvl knockdown PC12 cells. Dvl inhibited the binding of Syt I to the complex consisting of syntaxin-1A and SNAP-25. Furthermore, micro2-adaptin of AP-2, which is known to play a role in endocytosis, formed a complex with Dvl and Syt I. Taken together, these results suggest that Dvl is involved in endo- and exocytotic processes through the binding to Syt I.

Regenerating supernumerary axons are cholinergic and emerge from both autonomic and motor neurons in the rat spinal cord

Multipolar neurons in the mammalian nervous system normally exhibit one axon and several dendrites. However, in response to an axonal injury, adult motoneurons may regenerate supernumerary axons. Supernumerary axons emerge from the cell body or dendritic trees in addition to the stem motor axon. It is not known whether these regenerating axons contain neurotransmitters for synaptic transmission at their terminals. Here, using immunohistochemistry for choline acetyltransferase, an enzyme that synthesizes acetylcholine, we demonstrate the emergence of cholinergic supernumerary axons at 6 weeks after a unilateral L5-S2 ventral root avulsion and acute implantation of the avulsed L6 ventral root into the adult rat spinal cord. Light microscopic serial reconstruction of choline acetyltransferase immunoreactive arbors shows that these supernumerary axons originate from both autonomic and motor neurons. The supernumerary axons emerge from the cell body or dendrites, exhibit an abnormal projection pattern within the intramedullary gray and white matters, make frequent abrupt turns in direction, and form bouton-like swellings as well as growth cone-like terminals. Double labeling immunohistochemistry studies show that the choline acetyltransferase immunoreactive supernumerary axons co-localized with two proteins associated with axonal growth and elongation, growth-associated protein 43 and p75, the low affinity neurotrophic factor receptor. Our findings suggest that regenerating supernumerary axons selectively transport and store choline acetyltransferase, supporting the notion that supernumerary axons may develop functional and active synaptic transmission. Therefore, regenerating supernumerary axons may contribute to the plasticity in neural circuits following injury in the adult nervous system.

Protein kinase A affects trafficking of the vesicular monoamine transporters in PC12 cells

Previous studies have shown that the vesicular monoamine transporter 2 (VMAT2) is localized to both large dense core vesicles and synaptic vesicles in vivo. However, when exogenously expressed in PC12 cells, VMAT2 localizes only to large dense core vesicles. This distribution is similar to that of the endogenous vesicular monoamine transporter 1 (VMAT1) in PC12 cells. When VMAT2 was expressed in a protein kinase A (PKA)-deficient PC12 cell line it localized to synaptic-like microvesicles. Expression of recombinant VMAT1 in the same cell line showed a heterogeneous distribution to both large dense core vesicles and synaptic-like microvesicles. Coexpression of the PKA catalytic subunit partially restored trafficking of both VMAT2 and VMAT1 to large dense core vesicles; treatment of wild-type PC12 cells with the PKA inhibitor H89 increased VMAT2 on synaptic-like microvesicles. The VMAT1 and VMAT2 in large dense core vesicles exhibit a larger molecular size than those located on synaptic-like microvesicles. This difference is due to differential N-linked glycosylation. In vitro phosphorylation experiments show that PKA does not phosphorylate VMAT2. A chimera containing the VMAT2 cytoplasmic C-terminus fused to vesicular acetylcholine transporter (VAChT) shows mislocalization to synaptic-like microvesicles and VAChT-like glycosylation in the PKA-deficient cell line. However, coexpression with PKA changes the chimera's trafficking to large dense core vesicles and increases the molecular size. These results suggest that protein kinase A affects the formation and/or composition of VMAT trafficking complexes.

The secretory granule-associated protein CAPS2 regulates neurotrophin release and cell survival

Neurotrophins are key modulators of various neuronal functions, including differentiation, survival, and synaptic plasticity, but the molecules that regulate their secretion are poorly understood. We isolated a clone that is predominantly expressed in granule cells of postnatally developing mouse cerebellum, which turned out to be a paralog of CAPS (Ca2+-dependent activator protein for secretion), and named CAPS2. CAPS2 is enriched on vesicular structures of presynaptic parallel fiber terminals of granule cells connecting postsynaptic spines of Purkinje cell dendrites. Vesicle factions affinity-purified by the CAPS2 antibody from mouse cerebella contained significant amounts of neurotrophin-3 (NT-3), brain-derived neurotrophic factor (BDNF), and chromogranin B but not marker proteins for synaptic vesicle synaptophysin and synaptotagmin. In cerebellar primary cultures, punctate CAPS2 immunoreactivities are primarily colocalized with those of NT-3 and BDNF and near those of a postsynaptic marker, postsynaptic density-95, around dendritic arborization of Purkinje cells. Exogenously expressed CAPS2 enhanced release of exogenous NT-3 and BDNF from PC12 cells and endogenous NT-3 from cultured granule cells in a depolarization-dependent manner. Moreover, the overexpression of CAPS2 in granule cells promotes the survival of Purkinje cells in cerebellar cultures. Thus, we suggest that CAPS2 mediates the depolarization-dependent release of NT-3 and BDNF from granule cells, leading to regulation in cell differentiation and survival during cerebellar development.

Protein kinase C is involved in neurokinin receptor modulation of N- and L-type Ca2+ channels in DRG neurons of the adult rat

Whole cell patch-clamp techniques were used to examine neurokinin receptor modulation of Ca2+ channels in small to medium size dorsal root ganglia neurons (<40 pF) that express mainly N- and L-type Ca2+ currents. Low concentrations of substance P enhanced Ca2+ currents (5-40%, <0.2 microM), while higher concentrations applied cumulatively reversed these enhancements (5-28% reductions, >0.5 microM). This apparent inhibition by high concentrations of substance P was blocked by the administration of the NK3 antagonist SB 235,375 (0.2 microM). The NK1 agonist, [Sar9,Met11]-substance P (0.05 to 1.0 microM) did not alter Ca2+ currents; whereas the NK2 agonist, [betaAla8]-neurokinin A (4-10), enhanced Ca2+ currents (5-36% increase, 0.05-0.5 microM). The enhancement was reversed by the NK2 antagonist MEN 10,376 (0.2 microM) but unaffected by the NK3 antagonist SB 235,375 (0.2 microM). The NK3 agonist [MePhe7]-neurokinin B (0.5-1.0 microM) inhibited Ca2+ currents (6-24% decrease). This inhibition was not prevented by the NK2 antagonist MEN 10,376 (0.2 microM) but was blocked by the NK3 antagonist SB 235,375 (0.2 microM). Both the enhancement and inhibition of Ca2+ currents by neurokinin agonists were reversed by the protein kinase C inhibitor bisindolylmaleimide I HCl (0.2-0.5 microM). Following inhibition of Ca2+ channels by [MePhe7]-neurokinin the facilitatory effect of BayK 8644 (5 microM) was increased and the inhibitory effect of the N-type Ca2+ channel blocker w -conotoxin GVIA (1 microM) was diminished, suggesting that the NK3 agonist inhibits N-type Ca2+ channels. Similarly, block of all but N-type Ca2+ channels, revealed that [betaAla8]-neurokinin A (4-10) enhanced the currents while [MePhe7]-neurokinin B inhibited the currents. Inhibition of all but L-type Ca2+ channels, revealed that [betaAla8]-neurokinin A (4-10) enhanced the currents while [MePhe7]-neurokinin B had no effect. Activation of protein kinase C with low concentrations of phorbol-12,13-dibutyrate enhanced Ca2+ currents, but high concentrations inhibited N- and L-type Ca2+ currents. In summary, these data suggest that in adult rat dorsal root ganglia neurons, NK2 receptors enhance both L- and N-type Ca2+ channels and NK3 receptors inhibit N-type Ca2+ channels and that these effects are mediated by protein kinase C phosphorylation of Ca2+ channels.

Heterogeneous expression of SNAP-25 and synaptic vesicle proteins by central and peripheral inputs to sympathetic neurons

Neurons in prevertebral sympathetic ganglia receive convergent synaptic inputs from peripheral enteric neurons in addition to inputs from spinal preganglionic neurons. Although all inputs are functionally cholinergic, inputs from these two sources have distinctive neurochemical and functional profiles. We used multiple-labeling immunofluorescence, quantitative confocal microscopy, ultrastructural immunocytochemistry, and intracellular electrophysiologic recordings to examine whether populations of inputs to the guinea pig coeliac ganglion express different levels of synaptic proteins that could influence synaptic strength. Boutons of enteric intestinofugal inputs, identified by immunoreactivity to vasoactive intestinal peptide, showed considerable heterogeneity in their immunoreactivity to synaptosome-associated protein of 25 kDa (SNAP-25), synapsin, synaptophysin, choline acetyltransferase, and vesicular acetylcholine transporter. Mean levels of immunoreactivity to these proteins were significantly lower in terminals of intestinofugal inputs compared with terminals of spinal preganglionic inputs. Nevertheless, many boutons with undetectable levels of SNAP-25 immunoreactivity formed morphologically normal synapses with target neurons. Treatment with botulinum neurotoxin type A (20-50 nM for 2 hours in vitro) generated significant cleavage of SNAP-25 and produced similar dose- and time-dependent inhibitions of synaptic transmission from all classes of inputs, regardless of their mean level of SNAP-25 expression. The simplest interpretation of these results is that only synaptic boutons with detectable levels of SNAP-25 immunoreactivity contribute significantly to fast cholinergic transmission. Consequently, the low synaptic strength of intestinofugal inputs to final motor neurons in sympathetic pathways may be due in part to the low proportion of their boutons that express SNAP-25 and other synaptic proteins.

PACAP and NGF regulate common and distinct traits of the sympathoadrenal lineage: effects on electrical properties, gene markers and transcription factors in differentiating PC12 cells

To determine the possible role of pituitary adenylate cyclase-activating polypeptide (PACAP) in the development of the sympathoadrenal cell lineage, we have examined the effects of this neurotrophic peptide, in comparison to nerve growth factor (NGF), on the morphology, electrophysiological properties, expression of neuronal and neuroendocrine marker genes, and activity of transcription factors during differentiation of sympathoadrenal-derived cells, using the rat pheochromocytoma PC12 cell model. Both PACAP and NGF elicited rapid neurite outgrowth, which was accompanied by induction of cell excitability and the development of both sodium and calcium currents. Concurrently, PACAP and NGF increased the expression of a marker of synaptic vesicles. By contrast, PACAP, but not NGF, regulated the expression of different constituents of neuroendocrine large dense core vesicles in PC12 cells. Furthermore, PACAP and NGF differentially regulated the expression of mammalian achaete-scute homologue and paired homeobox 2b genes, transcription factors instrumental for sympathoadrenal development. To compare downstream effectors activated by PACAP and NGF, we studied the effects of these factors on the binding activity of consensus 12-O-tetradecanoylphorbol-13-acetate- and cAMP-responsive elements to nuclear extracts of differentiating PC12 cells. We found that both PACAP and NGF markedly increase the binding activity of these cis-regulatory sequences and that PACAP preferentially recruits activator protein-1-like transcription factors to these elements. Taken together, these results show that PACAP and NGF exert common as well as different effects on neuronal and neuroendocrine traits in differentiating PC12 cells, strongly suggesting that these two trophic factors could play complementary roles in the development of the sympathoadrenal cell lineage.

Identification of a novel sorting determinant for the regulated pathway in the secretory protein chromogranin A

Chromogranin A (CgA) is the index member of the chromogranin/secretogranin (or 'granin') family of regulated secretory proteins that are ubiquitously distributed in amine- and peptide-containing secretory granules of endocrine, neuroendocrine and neuronal cells. Because of their abundance and such widespread occurrence, granins have often been used as prototype proteins to elucidate mechanisms of protein targeting into dense-core secretory granules. In this study, we used a series of full-length, point mutant or truncated CgA-green fluorescent protein (GFP) chimeras to explore routing of CgA in neuroendocrine PC12 cells. Using sucrose gradient fractionation and 3D deconvolution microscopy to determine the subcellular localization of the GFP chimeras, as well as secretagogue-stimulated release, the present study establishes that a CgA-GFP fusion protein expressed in neuroendocrine PC12 cells is trafficked to the dense core secretory granule and thereby sorted to the regulated pathway for exocytosis. We show that information necessary for such trafficking is contained within the N-terminal but not the C-terminal region of CgA. We find that CgA's conserved N-terminal hydrophobic Cys(17)-Cys(38) loop structure may not be sufficient for sorting of CgA into dense-core secretory granules, nor is its stabilization by a disulfide bond necessary for such sorting. Moreover, our data reveal for the first time that the CgA(77-115) domain of the mature protein may be necessary (though perhaps not sufficient) for trafficking CgA into the regulated pathway of secretion.

A conserved mechanism of synaptogyrin localization

We have studied the localization of synaptogyrin family members in vivo. Both native and green fluorescent protein (GFP)-tagged Caenorhabditis elegans synaptogyrin (SNG-1) are expressed in neurons and synaptically localized. Deletion and mutational analysis with the use of GFP-tagged SNG-1 has defined a 38 amino acid sequence within the C terminus of SNG-1 and a single arginine in the cytoplasmic loop between transmembrane domain 2 and 3 that are required for SNG-1 localization. These domains may represent components of signals that target synaptogyrin for endocytosis from the plasma membrane and direct synaptogyrin to synaptic vesicles, respectively. In chimeric studies, these regions were sufficient to relocalize cellugyrin, a nonneuronal form of synaptogyrin, from nonsynaptic regions such as the sensory dendrites and the cell body to synaptic vesicles. Furthermore, GFP-tagged rat synaptogyrin is synaptically localized in neurons of C. elegans and in cultured hippocampal neurons. Similarly, the C-terminal domain of rat synaptogyrin is necessary for localization in hippocampal neurons. Our study suggests that the mechanisms for synaptogyrin localization are likely to be conserved from C. elegans to vertebrates.

Molecular mechanisms of neurotransmitter release

The release of neurotransmitter from neurons represents one of the pivotal events in synaptic transmission. Neurotransmitters are released from synaptic vesicles in presynaptic neurons in response to neural activity, diffuse across the synaptic cleft, and bind specific receptors in order to bring about changes in postsynaptic neurons. Some of the molecular processes that govern neurotransmitter release are now becoming better understood. The steps involved can be broken down into two partially overlapping presynaptic cycles, the neurotransmitter cycle and the synaptic vesicle cycle. The neurotransmitter cycle involves transmitter biosynthesis, storage, reuptake, and degradation. The synaptic vesicle cycle involves targeting to the nerve terminal, docking, fusion, endocytosis, and recycling. Biochemical and structural studies have yielded important insight into our understanding of each of these two cycles. Further, both pharmacological and genetic interference with either of these cycles results in profound alterations in synaptic transmission and behavior, demonstrating the crucial role of neurotransmitter release.

Electron microscopic evidence for multiple types of secretory vesicles in bovine chromaffin cells

It has been previously shown that the neuron-like chromaffin cells from the bovine adrenal medulla are heterogeneous. Among other differences, the cells also differed in secretory vesicles represented in their cytoplasm. The present study investigates the types of secretory vesicles in bovine chromaffin cells by electron microscopy. Morphometric analysis revealed five types of electron-dense secretory vesicles in chromaffin cells. These were as follows: elementary large catecholamine-storing chromaffin granules of rounded shape, large dense core vesicles of ovoid and rod-like shapes, small dense core vesicles as well as ribosome-coated vesicles of intermediate density. Among the electron-lucent vesicles there were small synaptic-like microvesicles, endocytotic clathrin-coated vesicles, growth cone vesicles, and emptied large light core vesicles. The structural and functional backgrounds of different types of secretory vesicles are described, focusing on their formation and potential role.

Isolation and characterization of substance P-containing dense core vesicles from rabbit optic nerve and termini

In neurons, neuropeptides and other synaptic components are transported down the axon to the synapse in vesicles using molecular motors of the kinesin family. In the synapse, these neuropeptides are found in dense core vesicles (DCVs), and, following calcium-mediated exocytosis, they interact with receptors on the target cell. We have developed a rapid, large-scale technique for purifying peptide-containing DCVs from specific nuclei in the central nervous system. By using differential velocity gradient and equilibrium gradient centrifugation, neuropeptide-containing DCVs can be separated by size and density from optic nerve (ON) and its termini, the lateral geniculate nuclei and the superior colliculi. Isolated DCVs contain neuropeptides (substance P and brain-derived neurotrophic factor), synaptic vesicle (SV) membrane proteins (SV2, synaptotagmins, synaptophysin, Rab3 and synaptobrevin), SV-associated proteins (alpha-synuclein), secretory markers for DCVs previously isolated (secretogranin II), and beta-amyloid precursor protein. By using electron microscopic techniques, DCV were also visualized and shown to be immunoreactive for neuropeptides, neurotrophins, and SV membrane proteins. Because of the interesting group of physiological and potentially pathophysiological proteins associated with these vesicles; this isolation procedure, applicable to other CNS nuclei, should represent an important research tool.

Lithium ions Up-regulate mRNAs encoding dense-core vesicle proteins in nerve growth factor-differentiated PC12 cells

We recently reported that lithium ions induced an up-regulation of cysteine string protein (CSP) gene expression in nerve growth factor (NGF)-differentiated PC12 cells but not in undifferentiated cells. Concomitantly, expression of two other proteins of regulated secretory pathways, synaptophysin (SY) and SNAP-25, was unaffected by lithium. To assess further the specificity of this effect of lithium, we used cDNA arrays. Our data indicate that lithium ions increase the level of mRNA for proteins such as secretogranin II and vesicular monoamine transporter 1 that are preferentially associated with large densecore secretory vesicles (LDCVs) without affecting mRNAs for proteins predominantly affiliated with small synaptic-like vesicles, including the vesicular acetylcholine transporter and SY. This action of lithium is detected in NGF-differentiated PC12 cells but not in undifferentiated cells. These observations suggest that lithium ions modulate the turnover of LDCVs, and this may play a role in mediating the therapeutic action of lithium in manic-depressive illness.

Effects of S-nitroso-cysteine on proteins that regulate exocytosis in PC12 cells: inhibitory effects on translocation of synaptophysin and ADP-ribosylation of GTP-binding proteins

S-Nitroso-cysteine (SNC) inhibits Ca2+-induced noradrenaline (NA) release from PC12 cells. Since SNC stimulated Ca2+ mobilization from intracellular Ca2+ pools and SNC-induced inhibition of NA release was not washed-out, SNC may modify exocytosis-related proteins that overcome Ca2+ mobilization. In the present study, we investigated the effects of SNC on exocytosis-related proteins in PC12 cells. Ionomycin stimulated NA release and increased the immunoreactivity of synaptophysin in the cytosol fraction. A 25-kDa synaptosome-associated protein (SNAP-25), which localizes to plasma membranes and vesicles, increased in the cytosol fraction after stimulation. The increases in these proteins by ionomycin were inhibited in PC12 cells treated with 0.6 mM SNC. Synaptobrevin and synapsin-1 in the cytosol fraction, and syntaxin and 43 kDa growth-associated protein in the membrane fraction were not affected by ionomycin or SNC. Incubation of each protein with SNC did not affect antibody immunoreactivity. [32P]ADP-ribosylation of GTP-binding proteins (Gi/Go) by pertussis toxin, but not Gs by cholera toxin, was inhibited in SNC-treated PC12 cells and by co-addition of SNC to the assay mixture. These findings suggest that 1) SNC inhibits translocation of vesicles containing synaptophysin and SNAP-25, and 2) SNC reacts with cysteine residues in Gi/Go, causing inhibition of ADP-ribosylation by pertussis toxin.

Molecular analysis of vesicular amine transporter function and targeting to secretory organelles

Vesicular transporters are responsible for the loading of neurotransmitters into specialized secretory organelles in neurons and neuroendocrine cells to make them available for regulated neurosecretion. The exocytotic release of neurotransmitter therefore depends on the functional activity of the vesicular transporters and their efficient sorting to these secretory organelles. Molecular analysis of vesicular transport proteins has revealed important information regarding structural domains responsible for their functional properties, including substrate specificity and trafficking to various classes of secretory vesicles. These studies have established the existence of an important functional relationship between transporter activity and presynaptic quantal neurosecretion.

Quantal release of serotonin

We have studied the origin of quantal variability for small synaptic vesicles (SSVs) and large dense-cored vesicles (LDCVs). As a model, we used serotonergic Retzius neurons of leech that allow for combined amperometrical and morphological analyses of quantal transmitter release. We find that the transmitter amount released by a SSV varies proportionally to the volume of the vesicle, suggesting that serotonin is stored at a constant intravesicular concentration and is completely discharged during exocytosis. Transmitter discharge from LDCVs shows a higher degree of variability than is expected from their size distribution, and bulk release from LDCVs is slower than release from SSVs. On average, differences in the transmitter amount released from SSVs and LDCVs are proportional to the size differences of the organelles, suggesting that transmitter is stored at similar concentrations in SSVs and LDCVs.

The tractable contribution of synapses and their component molecules to individual differences in learning

Though once of central importance to psychologists and neurophysiologists alike, the elucidation of neural substrates for individual differences in learning no longer attracts a broad research effort and occupies a place of largely historical interest to the contemporary disciplines. The decline in interest in this subject ensued in part from the perception, arrived at decades ago, that individual differences in learning were not quantified as easily as had once been presumed. Furthermore, the dominant hypotheses in the field defied testing within the constraints imposed by the complex and largely inaccessible vertebrate nervous system. Using a 'model systems' approach where the individual cells and synaptic interactions that comprise a neural network can be identified, we have returned to this question and have established a framework by which we can begin to discern the basis for much of the variability between individuals in their capacity to learn. In the marine mollusc Hermissenda, we have found that a common influence on transmitter exocytosis is expressed homogeneously throughout the nervous system regardless of transmitter system or receptor class. Though uniformly expressed within an individual, this influence on synaptic efficacy is differentially expressed between animals. Importantly, the basal efficiency of exocytosis expressed in an individual nervous system is strongly correlated with the degree to which activity-dependent forms of neuronal/synaptic facilitation can be induced in that nervous system, and predicts the capacity for the intact animal to learn a Pavlovian association. Furthermore, we have established that a decline in basal synaptic efficacy in aged animals, arising from chronic presynaptic Ca(2+) 'leak', may contribute to age-related learning impairments. Because certain fundamental components of the exocytotic cascade are conserved widely across cell types, transmitter systems and species, the principles that we describe may have broad implications for understanding normal variability in learning, but also, in the development of specific strategies to compensate for mild learning deficits and age-related cognitive decline.

Targeting of synaptotagmin to neurite terminals in neuronally differentiated PC12 cells

We have investigated structural elements that determine the accumulation of synaptotagmin, a major synaptic vesicle protein, in neurite terminals of neuronally differentiated neuroendocrine pheochromocytoma PC12 cells. We performed extensive deletion and point mutagenesis of rat synaptotagmin II, expressed mutant proteins in PC12 cells differentiated by nerve growth factor (NGF) and monitored their intracellular distribution by immunofluorescence. We found a structural element located at the carboxy-terminal domain of the synaptotagmin molecule, which is necessary for its accumulation at the terminal. Using alanine-scanning mutagenesis, we have identified two amino acids in this element, tryptophan W405 and leucine L408, that are critical for correct targeting of synaptotagmin II to neurite terminals. Changing either one of them to alanine prevents the accumulation of the protein at the terminals. These amino acids are evolutionarily conserved throughout the entire synaptotagmin family and also among synaptotagmin-related proteins, suggesting that different synaptotagmins may have similar mechanisms of targeting to neuronal cell terminals.

Synaptic vesicle biogenesis

Synaptic vesicles, which have been a paradigm for the fusion of a vesicle with its target membrane, also serve as a model for understanding the formation of a vesicle from its donor membrane. Synaptic vesicles, which are formed and recycled at the periphery of the neuron, contain a highly restricted set of neuronal proteins. Insight into the trafficking of synaptic vesicle proteins has come from studying not only neurons but also neuroendocrine cells, which form synaptic-like microvesicles (SLMVs). Formation and recycling of synaptic vesicles/SLMVs takes place from the early endosome and the plasma membrane. The cytoplasmic machinery of synaptic vesicle/SLMV formation and recycling has been studied by a variety of experimental approaches, in particular using cell-free systems. This has revealed distinct machineries for membrane budding and fission. Budding is mediated by clathrin and clathrin adaptors, whereas fission is mediated by dynamin and its interacting protein SH3p4, a lysophosphatidic acid acyl transferase.

Voltage-dependent calcium channel subtypes controlling somatic substance P release in the peripheral nervous system

1. Isolated rat dorsal root ganglia (DRG) neurones support vesicular, non synaptic release of substance P in a depolarisation and Ca2+ dependent manner. 2. In vivo this process may mediate cross-communication between DRG cells in some neuropathological conditions and is therefore a putative area for drug intervention. 3. The authors investigated the voltage-dependent Ca2+ channel (VDCC) subtypes involved in somatic release of substance P. Fresh (< 1 day) cultures of DRG neurones were incubated with high K+ depolarising saline in the presence and absence of subtype selective VDCC blockers. Substance P released into the external media was collected and quantified using a radioimmunoassay. 4. The results show that L-type and N-type, but not P-type, VDCCs play an important role in high K+ evoked substance P release from rat DRG neurones.

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

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

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.

Proteolytic processing, axonal transport and differential distribution of chromogranins A and B, and secretogranin II (secretoneurin) in rat sciatic nerve and spinal cord

The chromogranin family comprises chromogranin A and B, and secretogranin II. The present study has focused on the axonal transport of chromogranins/secretogranin II and their detailed distribution in peripheral nerves and the spinal cord. With radioimmunoassay (RIA) and column chromatography, we first studied the processing of chromogranin B and secretogranin II during axonal transport. No larger precursors of these peptides were detected in the sciatic nerves, indicating that they are already processed to a high degree early during axonal transport. We also analysed nerve segments above and below a crush, using RIA, in order to compare these accumulation data with those obtained by the cytofluorimetric-scanning (CFS) technique. For the latter technique, the amounts of accumulation distal to the crush (presumably representing recycling and retrogradely transported peptides) were 30-40% of the amounts in the proximal accumulation for chromogranin A and secretoneurin, in contrast to chromogranin B, which showed 15% recycling. With the RIA, the corresponding values for secretoneurin and PE-11 (antibody against chromogranin B) were 42% and 14%, respectively. Therefore, the data obtained by CFS were in excellent agreement with those obtained by RIA. In crushed sciatic nerves, chromogranin A was present in large axons as well as in small- and medium-sized axons. Chromogranin B was mainly restricted to large axons, while secretoneurin was localized to bundles of small axons. This differential distribution was also found in the spinal roots and in the peripheral terminals. Chromogranin A was present in both ventral and dorsal roots, and chromogranin B was detected in ventral roots and in large sensory axons in the dorsal roots. Secretoneurin was dominant in the dorsal root. Double-labelling studies with antibodies against choline acetyltransferase/vesicular acetylcholine transporter, or against tyrosine hydroxylase, confirmed that chromogranin A was distributed in cholinergic, sensory, as well as adrenergic neurons. Chromogranin B was mainly present in cholinergic motor neurons and large sensory neurons, and secretoneurin was restricted to adrenergic and sensory neurons. The present study demonstrates that chromogranins A and B, and secretoneurin are transported with fast axonal transport in the peripheral nerves, with different amounts of recycling, and that they are differentially distributed in different types of neurons in the peripheral nervous system and the spinal cord, suggesting that each of them may play a special role in subsets of neurons.

Pig fetal septal neurons implanted into the hippocampus of aged or cholinergic deafferented rats grow axons and form cross-species synapses in appropriate target regions

The anatomical specificity of axon growth from fetal pig septal xenografts was studied by transplanting septal cells from E30-35 pig fetuses into cholinergic deafferented (192-IgG-saporin-infused) rats or into aged rats (> 18 months). Cell suspensions (100,000 cells/microl) were injected bilaterally into the dorsal and ventral hippocampus of immunosuppressed rats (10 mg/kg/day cyclosporine A). To assess axonal growth and synapse formation, acetylcholinesterase histochemistry, an antibody to choline acetyltransferase (ChAT), and three pig-positive/rat-negative antibodies: bovine 70kD neurofilament (NF70), human low-affinity NGF receptor (hNGFr), and human synaptobrevin (hSB) were used. In rats with surviving grafts at 6 months, NF70 axonal labeling was more extensive than either ChAT or hNGFr labeling. All three markers demonstrated graft axons extending selectively through the hippocampal CA fields and the molecular layer of the dentate gyrus. Graft axons did not extend into adjacent entorhinal cortex or neocortex. The distribution of pig hSB-positive synapses correlated with AChE-positive fiber outgrowth in to the host. Electron microscopic analysis of hSB-immunostained hippocampal sections revealed pig presynaptic terminals in contact with normal rat postsynaptic structures in the CA fields and the dentate gyrus. These data demonstrate target-appropriate growth of pig cholinergic axons and the formation of cross-species synapses in the deafferented or aged rat hippocampus.

Neurotransmitter release: variations on a theme

Similarities between the ways that synaptic vesicles and large dense-core vesicles release their contents have been emphasized, but recent studies have revealed important mechanistic differences between these two exocytotic processes.