Cellular and subcellular localization of protein I in the peripheral nervous system

An updated reappraisal of synapsins: structure, function and role in neurological and psychiatric disorders

Synapsins (Syns) are phosphoproteins strongly involved in neuronal development and neurotransmitter release. Three distinct genes SYN1, SYN2 and SYN3, with elevated evolutionary conservation, have been described to encode for Synapsin I, Synapsin II and Synapsin III, respectively. Syns display a series of common features, but also exhibit distinctive localization, expression pattern, post-translational modifications (PTM). These characteristics enable their interaction with other synaptic proteins, membranes and cytoskeletal components, which is essential for the proper execution of their multiple functions in neuronal cells. These include the control of synapse formation and growth, neuron maturation and renewal, as well as synaptic vesicle mobilization, docking, fusion, recycling. Perturbations in the balanced expression of Syns, alterations of their PTM, mutations and polymorphisms of their encoding genes induce severe dysregulations in brain networks functions leading to the onset of psychiatric or neurological disorders. This review presents what we have learned since the discovery of Syn I in 1977, providing the state of the art on Syns structure, function, physiology and involvement in central nervous system disorders.

Functional distribution of synapsin I in human sperm

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Synapsin I was localized in the human sperm equatorial segment.

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Presence of synapsin I was confirmed by dot and Western blotting techniques.

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Treatment of sperm with anti-synapsin antibodies significantly decreased motility.

Proteins known to function during cell–cell communication and exocytosis in neurons and other secretory cells have recently been reported in human sperm. Synapsins are a group of proteins that have been very well characterized in neurons, but little is known about synapsin function in other cell types. Based upon previous findings and the known function of synapsin, we tested the hypothesis that synapsin I was present in human sperm. Washed, capacitated, and acrosome induced sperm preparations were used to evaluate the functional distribution of synapsin I using immunocytochemistry. Protein extracts from mouse brain, mouse testis/epididymis, and human semen were used for protein blotting techniques. Immunolocalization revealed synapsin I was enriched in the sperm equatorial segment. Protein extracts from mouse brain, mouse testis/epididymis, and human semen were positive for synapsin I using several different antibodies, and dot blot results were confirmed by Western blot analyses. Finally, treatment of capacitated and acrosome reaction induced samples with anti-synapsin antibodies significantly reduced sperm motility. Localization of synapsin I in human sperm is a novel finding. The association of synapsin I with the sperm equatorial segment and effects on motility are suggestive of a role associated with capacitation and/or acrosome reaction, processes that render sperm capable of fertilizing an oocyte.

Pentylenetetrazol-induced epileptiform activity affects basal synaptic transmission and short-term plasticity in monosynaptic connections

Epileptic activity is generally induced in experimental models by local application of epileptogenic drugs, including pentylenetetrazol (PTZ), widely used on both vertebrate and invertebrate neurons. Despite the high prevalence of this neurological disorder and the extensive research on it, the cellular and molecular mechanisms underlying epileptogenesis still remain unclear. In this work, we examined PTZ-induced neuronal changes in Helix monosynaptic circuits formed in vitro, as a simpler experimental model to investigate the effects of epileptiform activity on both basal release and post-tetanic potentiation (PTP), a form of short-term plasticity. We observed a significant enhancement of basal synaptic strength, with kinetics resembling those of previously described use-dependent forms of plasticity, determined by changes in estimated quantal parameters, such as the readily releasable pool and the release probability. Moreover, these neurons exhibited a strong reduction in PTP expression and in its decay time constant, suggesting an impairment in the dynamic reorganization of synaptic vesicle pools following prolonged stimulation of synaptic transmission. In order to explain this imbalance, we determined whether epileptic activity is related to the phosphorylation level of synapsin, which is known to modulate synaptic plasticity. Using western blot and immunocytochemical staining we found a PTZ-dependent increase in synapsin phosphorylation at both PKA/CaMKI/IV and MAPK/Erk sites, both of which are important for modulating synaptic plasticity. Taken together, our findings suggest that prolonged epileptiform activity leads to an increase in the synapsin phosphorylation status, thereby contributing to an alteration of synaptic strength in both basal condition and tetanus-induced potentiation.

The synapsin gene family in basal chordates: evolutionary perspectives in metazoans

Background

Synapsins are neuronal phosphoproteins involved in several functions correlated with both neurotransmitter release and synaptogenesis. The comprehension of the basal role of the synapsin family is hampered in vertebrates by the existence of multiple synapsin genes. Therefore, studying homologous genes in basal chordates, devoid of genome duplication, could help to achieve a better understanding of the complex functions of these proteins.

Results

In this study we report the cloning and characterization of the Ciona intestinalis and amphioxus Branchiostoma floridae synapsin transcripts and the definition of their gene structure using available C. intestinalis and B. floridae genomic sequences. We demonstrate the occurrence, in both model organisms, of a single member of the synapsin gene family. Full-length synapsin genes were identified in the recently sequenced genomes of phylogenetically diverse metazoans. Comparative genome analysis reveals extensive conservation of the SYN locus in several metazoans. Moreover, developmental expression studies underline that synapsin is a neuronal-specific marker in basal chordates and is expressed in several cell types of PNS and in many, if not all, CNS neurons.

Conclusion

Our study demonstrates that synapsin genes are metazoan genes present in a single copy per genome, except for vertebrates. Moreover, we hypothesize that, during the evolution of synapsin proteins, new domains are added at different stages probably to cope up with the increased complexity in the nervous system organization. Finally, we demonstrate that protochordate synapsin is restricted to the post-mitotic phase of CNS development and thereby is a good marker of postmitotic neurons.

Synapsin-like immunoreactivity is present in hair cells and efferent terminals of the toadfish crista ampullaris

The synapsins are presynaptic membrane-associated proteins involved in neurotransmitter release. They are differentially expressed in tissues and cells of the central and peripheral nervous system. In vestibular end organs of mammals, synapsin I-like immunoreactivity has been reported in efferent and afferent terminals and in afferent nerve calyces surrounding type I hair cells. In addition, synapsin I has recently been described in several non-neural cell lines. The present study was conducted to locate synapsin-like immunoreactivity in the neuronal and non-neuronal cells of the fish crista ampullaris, to examine the possibility that the non-neuronal sensory receptor cells express synapsins in vivo. Synapsin-like immunostaining was visualized by immunofluorescence detection in wholemounts of the toadfish crista ampullaris using multiphoton laser scanning microscopy and by electron microscopic visualization of post-embedding immunogold labeling. The results demonstrate that synapsin-like immunoreactivity is present in vestibular hair cells and efferent boutons of the toadfish crista ampullaris. Afferent endings are not labeled. Staining in hair cells is not associated with the synaptic ribbons, suggesting that there is an additional, non-synaptic role for the synapsins in some non-neuronal cells of vertebrates. Moreover, while the cristae of amniote and anamniote species share many functional attributes, differences in their synaptic vesicle-associated protein profiles appear to reflect their disparate hair cell populations.

Molecular determinants of synapsin targeting to presynaptic terminals

Although synapsins are abundant synaptic vesicle proteins that are widely used as markers of presynaptic terminals, the mechanisms that target synapsins to presynaptic terminals have not been elucidated. We have addressed this question by imaging the targeting of green fluorescent protein-tagged synapsins in cultured hippocampal neurons. Whereas all synapsin isoforms targeted robustly to presynaptic terminals in wild-type neurons, synapsin Ib scarcely targeted in neurons in which all synapsins were knocked-out. Coexpression of other synapsin isoforms significantly strengthened the targeting of synapsin Ib in knock-out neurons, indicating that heterodimerization is required for synapsin Ib to target. Truncation mutagenesis revealed that synapsin Ia targets via distributed binding sites that include domains B, C, and E. Although domain A was not necessary for targeting, its presence enhanced targeting. Domain D inhibited targeting, but this inhibition was overcome by domain E. Thus, multiple intermolecular and intramolecular interactions are required for synapsins to target to presynaptic terminals.

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.

Localization of synapsin I in normal fibers and regenerating axonal sprouts of the rat sciatic nerve

The localization of synapsin I, a synaptic vesicle-associated protein, was investigated immunocyto-chemically in normal nerve fibers and regenerating axonal sprouts following crush-injuries to the rat sciatic nerve. In normal myelinated axons, weak synapsin I immunoreactivity was found in the axoplasmic/smooth endoplasmic domains, but not in the cytoskeletal domains comprising neurofilaments and microtubules. In non-myelinated axons without dense cytoskeletal structures, moderate immunoreactivity was distributed diffusely throughout the axoplasm. In the crush-injured nerves, intense synapsin I immunoreactivity was demonstrated by light microscopy in early regenerating sprouts emerging from nodes of Ranvier. These nodal sprouts subsequently elongated as regenerating axons through the space between the basal lamina and the myelin sheath (or Schwann cell plasma membrane). Intense synapsin I immunoreactivity was also found in the growth cones of such long regenerating axons. Electron microscopy revealed that synapsin I immunoreactivity was associated mainly with vesicular organelles in the nodal sprouts and growth cones of regenerating axons. Long regenerating axons exhibited no synapsin I immunoreactivity in the shaft, which contained an abundance of neurofilaments. However, vesicle accumulations remaining in the periphery of the shaft still exhibited intense synapsin I immunoreactivity. Thus, it can be concluded that synapsin I is localized at especially high density in the domains comprising vesicular organelles, which are characteristic of early nodal sprouts, as well as in growth cones of regenerating axons. These findings, together with the proposed functions of synapsin I investigated in other studies, suggest that synapsin I may play important roles in vesicular dynamics including the translocation of vesicles to the plasma membrane in sprouts and growth cones of regenerating axons.

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.

Influence of spinal cord transection on the presence and axonal transport of CGRP-, chromogranin A-, VIP-, synapsin I-, and synaptophysin-like immunoreactivities in rat motor nerve

Using immunofluorescence and cytofluorimetric scanning (CFS), we investigated the short-term (1-7 days) influence of lower thoracic spinal cord transection on lumbar motor neurons. The content of calcitonin gene-related peptide- (CGRP) like immunoreactivity (LI), chromogranin A (Chr A)-LI, vasoactive intestinal polypeptide (VIP)-LI, Syn I-LI, and synaptophysin (p38)-LI in motor perikarya, and the anterograde and retrograde axonal transport of these substances in the sciatic nerve, were studied in nerve crush (6 h) experiments. During the week after transection, CGRP-LI in perikarya decreased, whereas Chr A-LI increased. VIP-LI, co-localized with Chr A-LI in motor perikarya, did not change after transection. The antero- and retrograde transport of CGRP-LI in the sciatic nerve, occurring in both motor and sensory axons, appeared unchanged in cytofluorimetric scanning (CFS) graphs, but the microscopical picture clearly showed that large motor axons had a decreased content of CGRP-LI at 3 and 7 days posttransection, whereas thinner axons were unchanged in fluorescence intensity. The anterograde transport of Chr A-LI, present in both motor and postganglionic adrenergic axons, was decreased 1 and 3 days after lesion, but returned to control by day 7. There was a marked decrease in anterograde transport of VIP-LI, present mainly in postganglionic sympathetic axons, at day 3, but at 7 days transport was normal. The amounts of transported p38, the synaptic vesicle marker, were in the normal range during the whole period. Syn I-LI accumulation anterogradely was somewhat decreased at 3 and 7 days posttransection, and at 1 day the retrograde accumulation was significantly increased. The results suggest that removal of supraspinal input to intact lower motor neurons causes alterations in metabolism and axonal transport of organelle-associated substances, partly probably related to the complex pattern of transmitter leakage from degenerating, descending nerve terminals. These alterations appear to take place also in postganglionic sympathetic neurons in the sciatic nerve, that originate in the lumbar sympathetic chain.

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.

Immunocytochemical localization of synapsin I in the adrenal medulla of rats

The localization of synapsin I in the rat adrenal medulla was studied using the light- and electronmicroscopic immunohistochemistry. By light microscopy, many dot-like reaction products for synapsin I were recognized to be distributed throughout the medullary tissue. The immunoelectron microscopy clearly revealed that gold particles for synapsin I accumulated in abundance in the nerve terminals forming synapses with the chromaffin cell, while the particles were not localized in the chromaffin cells at all. In the nerve terminal, the gold particles were localized exclusively in the region occupied by synaptic vesicles except for the region just beneath the presynaptic plasma membrane. The synaptic vesicles were frequently linked with the adjacent ones by filamentous structures implicated in synapsin I. It is concluded morphologically that synapsin I is a highly-specific protein for the genuine neuron, and is not detected even in the chromaffin cell which originates from the neural crest.

Synapsin I expression in the rat retina during postnatal development

The expression of the synapsin I gene was studied during postnatal development of the rat retina at the mRNA and protein levels. In situ hybridization histochemistry showed that synapsin I mRNA was expressed already in nerve cells in the ganglion cell layer of the neonatal retina, while it appeared in neurons of the inner nuclear layer from postnatal day 4 onward. Maximal expression of synapsin I mRNA was observed at P12 in ganglion cells and in neurons of the inner nuclear layer followed by moderate expression in the adult. At the protein level a shift of synapsin I appearance was observed from cytoplasmic to terminal localization during retinal development by immunohistochemistry. In early stages (P4 and P8), synapsin I was seen in neurons of the ganglion cell layer and in neurons of the developing inner nuclear layer as well as in the developing inner plexiform layer. In the developing outer plexiform layer synapsin I was localized only in horizontal cells and in their processes. Its early appearance at P4 indicated the early maturation of this cell type. A shift and strong increase of labelling to the plexiform layers at P12 indicated the localization of synapsin I in synaptic terminals. The inner plexiform layer exhibited a characteristic stratified pattern. Photoreceptor cells never exhibited synapsin I mRNA or synapsin I protein throughout development.

Neuronal phosphoproteins. Mediators of signal transduction

This article summarizes some of our knowledge concerning intracellular protein phosphorylation pathways in nerve cells. It also summarizes, very briefly, recent direct experimental evidence involving intracellular injection of protein kinases, protein kinase inhibitors, and substrates, indicating that protein phosphorylation mediates the actions of a variety of neurotransmitters on their target cells. Finally, it summarizes in somewhat greater detail the results of studies of three different types of substrate proteins that appear to regulate different types of biological responses in nerve cells: synapsin I, a substrate protein present in virtually all nerve terminals, which appears to regulate neurotransmitter release from those nerve terminals; the acetylcholine receptor, the phosphorylation of which regulates its rate of desensitization in the presence of acetylcholine; and DARPP-32, the phosphorylation of which converts it into a very potent phosphoprotein phosphatase inhibitor that may be involved in the regulation by the neuromodulator dopamine of the effects of the neurotransmitter glutamate. The identification and characterization of additional neuronal phosphoproteins can be expected to lead to the clarification of numerous additional molecular mechanisms by which signal transduction is carried out in nerve cells.

Synapsin I, a phosphoprotein associated with synaptic vesicles: possible role in regulation of neurotransmitter release

The data presented here provide evidence that the study of neuronal phosphoproteins can lead to the identification of previously unknown proteins and that these proteins may play important roles in neuronal communication. Specifically, in the case of synapsin I, direct evidence has been obtained that this phosphoprotein is involved in regulating neurotransmitter release. A tentative explanation of the results obtained in the micro-injection studies is as follows: synapsin I, in the dephosphostate, is bound to the cytoplasmic surface of synaptic vesicles and inhibits the ability of the vesicle to interact with the plasma membrane; increases in intracellular calcium activate calmodulin kinase II which in turn phosphorylates synapsin I and the phosphorylated synapsin I dissociates from the synaptic vesicle thus removing a constraint on the release of neurotransmitter. Clearly, more studies need to be done to rigorously test this hypothesis. Nevertheless these studies of synapsin I suggest that the study of previously unknown phosphoproteins will lead to the elucidation of previously unknown regulatory processes in neurons.

Axonal transport of synapsin I- and cholinergic synaptic vesicle-like material; further immunohistochemical evidence for transport of axonal cholinergic transmitter vesicles in motor neurons

The axonal transport of organelles in motor axons in the sympathectomized rat sciatic has been studied using two antisera which recognize specific components of synaptic vesicles. Anti-synapsin I recognizes synapsin I (SYN I) which is affiliated with the external membrane of synaptic vesicles, while rabbit-anti-synaptic vesicle antiserum (RASVA) recognizes integral membrane glycoproteins in cholinergic synaptic vesicles. Immunofluorescence studies, including cytofluorimetric scanning, show that immunoreactive (IR) material recognized by both antisera: rapidly accumulate proximal to a crush; the material has a granular appearance in the microscope; is redistributed in an isolated segment, and that the transport of the material is sensitive to vinblastine. Thus the proximodistal transport has the characteristics of fast axonal transport. Furthermore, recycling organelles, accumulating on the distal side of a crush are recognized by RASVA, but carry only very little SYN I-IR. The results give further support to the hypothesis that motor cholinergic axons transport axonal cholinergic vesicles towards the motor endplates.

Subcellular storage and axonal transport of neuropeptide Y (NPY) in relation to catecholamines in the cat

The subcellular storage of neuropeptide Y-like immunoreactivity (NPY-LI) in peripheral sympathetic neurons and adrenal gland as well as its axonal transport in the sciatic nerve was studied in relation to catecholamines in the cat. In the subcellular fractions from different parts of sympathetic neurons, i.e. cell bodies (coeliac ganglia), axons (sciatic nerve) and terminals (spleen), the NPY-LI was found together with noradrenaline (NA) in heavy fractions assumed to contain large dense-cored vesicles. In addition, minor lighter fractions in the coeliac ganglion contained NPY-LI. The molar ratio between vesicular NA and NPY was high in the terminal regions (150 to 1) and much lower in axons and cell bodies (10 to 1), thus reflecting the different mechanisms of resupply for classical transmitter and peptide. In the adrenal gland the NPY-LI was mainly located in the catecholamine-storing chromaffin-granule fraction and also to a smaller extent in lighter fractions. Using reversed-phase HPLC, one molecular form of NPY-LI corresponding to porcine NPY was found in the coeliac ganglion, while the adrenal medulla also contained minor peaks with NPY-LI in addition to the main form, which co-eluted with porcine NPY. NA was stored both in light and heavy fractions in the spleen, while it was mainly found in heavier fractions in the sciatic nerve. In the coeliac ganglion, most of the noradrenaline was present in a non-particulate form. The anterograde transport rate for NPY-LI in the sciatic nerve was estimated to be about 9 mm h-1. A minor retrograde transport of NPY-LI was also detected. In conclusion, the present data suggest that NPY, a peptide with sympathoactive actions, is co-stored with NA in heavy fractions corresponding to large dense-cored vesicles, while light fractions with small dense-cored vesicles probably contain NA but not NPY-LI. The main resupply of NPY to terminals is, in contrast to NA, most likely by axonal transport, which implicates differences in the storage, turnover and release of these co-existing substances in the sympathoadrenal system.

Phylogenetic survey of proteins related to synapsin I and biochemical analysis of four such proteins from fish brain

A phylogenetic survey of proteins immunologically related to Synapsin I, a major synaptic vesicle-associated phosphoprotein in mammals was carried out. Proteins antigenically related to Synapsin I were found by use of radioimmunoassay and other radioimmunochemical techniques in the nervous systems of several vertebrate and invertebrate species, which included birds, reptiles, amphibians, fish, echinoderms, arthropods, and mollusks. Four proteins present in fish brain, antigenically related to Synapsin I, were further studied and found to resemble mammalian Synapsin I in several respects. Like Synapsin I, the fish proteins were present in high amounts in nervous tissue, were enriched in synaptosomal fractions of brain where they were substrates for endogenous protein kinases, were acid extractable, and were sensitive to digestion by collagenase. In addition, two-dimensional peptide-mapping analysis revealed some homology between major phosphopeptide fragments of Synapsin I and the fish proteins. The results indicate that proteins related to Synapsin I are wide-spread in the animal kingdom.

Synapsin I (protein I) in different brain regions in senile dementia of Alzheimer type and in multi-infarct dementia

Synapsin I (Protein I), a neuron-specific phosphoprotein enriched in presynaptic nerve terminals, has been used as a quantitative marker for the density of nerve terminals in five brain regions (caudate nucleus, cingulate gyrus, hippocampus, mesencephalon and putamen) from patients who had suffered from Alzheimer disease/senile dementia of Alzheimer type (AD/SDAT), from patients with multi-infarct dementia (MID), and from age-matched controls. Samples were obtained at autopsy. Lower levels of Synapsin I were observed in the hippocampus of patients with AD/SDAT but not with MID. There were no significant differences in Synapsin I levels between patients and controls in any of the other four brain regions examined.

Occurrence, storage and release of neurotensin-like immunoreactivity from the adrenal gland

In the present study we have investigated the presence of radioimmunoassayable neurotensin-like immunoreactivity (NTLI) in extracts of adrenal glands from hen, guinea-pig, rat, cat, dog and man. In addition, gel filtration chromatography and studies on the storage site of NTLI, using subcellular fractionation techniques, were performed. Finally, NTLI release in relation to noradrenaline (NA) from cat adrenal glands upon splanchnic nerve activation was studied in vivo. In all species investigated except guinea-pig and man, the NTLI levels in the adrenal glands were higher than the levels in the heart (control tissue) but lower than in the ileum. Gel filtration of NTLI from cat adrenal glands revealed presence of roughly equal amounts of the whole neurotensin molecule, i.e. NT(1-13), and its main metabolite the N-terminal octapeptide NT(1-8), while in plasma from the adrenal vein almost exclusively NT(1-13) was revealed after electrical stimulation of the splanchnic nerve. The release of NTLI and NA upon splanchnic stimulation was antagonized by hexamethonium. Subcellular fractionation of cat adrenal glands indicated that NTLI was stored in a large subcellular organelle, co-migrating with chromaffin granules. We suggest that NT may be a new adrenal gland hormone or transmitter, possibly co-stored with NA in chromaffin cells. However, at the present stage it cannot be distinguished whether NT exerts its actions locally within the adrenal gland or after release into the systemic circulation.

Protein phosphorylation in the brain

Protein phosphorylation represents an approach, sometimes the only approach available, to study the molecular basis for a wide variety of neurophysiological phenomena. The injection of protein kinases or protein kinase inhibitors into neurones has provided direct evidence that activation of protein kinases has an obligatory role in the mechanisms by which numerous extracellular signals produce specific physiological responses in neurones. A diversity of substrate proteins for the kinases have already been found. In several instances, the identity and functional role of these substrate proteins have been established.

Synapsin I (protein I), a nerve terminal-specific phosphoprotein. III. Its association with synaptic vesicles studied in a highly purified synaptic vesicle preparation

Synapsin I (protein I) is a neuron-specific phosphoprotein, which is a substrate for cAMP-dependent and Ca/calmodulin-dependent protein kinases. In two accompanying studies (De Camilli, P., R. Cameron, and P. Greengard, and De Camilli, P., S. M. Harris, Jr., W. B. Huttner, and P. Greengard, 1983, J. Cell Biol. 96:1337-1354 and 1355-1373) we have shown, by immunocytochemical techniques at the light microscopic and electron microscopic levels, that synapsin I is present in the majority of, and possibly in all, nerve terminals, where it is primarily associated with synaptic vesicles. In the present study we have prepared a highly purified synaptic vesicle fraction from rat brain by a procedure that involves permeation chromatography on controlled-pore glass as a final purification step. Using immunological methods, synapsin I concentrations were determined in various subcellular fractions obtained in the course of vesicle purification. Synapsin I was found to copurify with synaptic vesicles and to represent approximately 6% of the total protein in the highly purified synaptic vesicle fraction. The copurification of synapsin I with synaptic vesicles was dependent on the use of low ionic strength media throughout the purification. Synapsin I was released into the soluble phase by increased ionic strength at neutral pH, but not by nonionic detergents. The highly purified synaptic vesicle fraction contained a calcium-dependent protein kinase that phosphorylated endogenous synapsin I in its collagenase-sensitive tail region. The phosphorylation of this region appeared to facilitate the dissociation of synapsin I from synaptic vesicles under the experimental conditions used.

Synapsin I (protein I), a nerve terminal-specific phosphoprotein. I. Its general distribution in synapses of the central and peripheral nervous system demonstrated by immunofluorescence in frozen and plastic sections

Synapsin I (formerly referred to as protein I) is the collective name for two almost identical phosphoproteins, synapsin Ia and synapsin Ib (protein Ia and protein Ib), present in the nervous system. Synapsin I has previously been shown by immunoperoxidase studies (De Camilli, P., T. Ueda, F. E. Bloom, E. Battenberg, and P. Greengard, 1979, Proc. Natl. Acad. Sci. USA, 76:5977-5981; Bloom, F. E., T. Ueda, E. Battenberg, and P. Greengard, 1979, Proc. Natl. Acad. Sci. USA 76:5982-5986) to be a neuron-specific protein, present in both the central and peripheral nervous systems and concentrated in the synaptic region of nerve cells. In those preliminary studies, the occurrence of synapsin I could be demonstrated in only a portion of synapses. We have now carried out a detailed examination of the distribution of synapsin I immunoreactivity in the central and peripheral nervous systems. In this study we have attempted to maximize the level of resolution of immunohistochemical light microscopy images in order to estimate the proportion of immunoreactive synapses and to establish their precise distribution. Optimal results were obtained by the use of immunofluorescence in semithin sections (approximately 1 micron) prepared from Epon-embedded nonosmicated tissues after the Epon had been removed. Our results confirm the previous observations on the specific localization of synapsin I in nerve cells and synapses. In addition, the results strongly suggest that, with a few possible exceptions involving highly specialized neurons, all synapses contain synapsin I. Finally, immunocytochemical experiments indicate that synapsin I appearance in the various regions of the developing nervous system correlates topographically and temporally with the appearance of synapses. In two accompanying papers (De Camilli, P., S. M. Harris, Jr., W. B. Huttner, and P. Greengard, and Huttner, W. B., W. Schiebler, P. Greengard, and P. De Camilli, 1983, J. Cell Biol. 96:1355-1373 and 1374-1388, respectively), evidence is presented that synapsin I is specifically associated with synaptic vesicles in nerve endings.

Regulation of phosphorylation of proteins I, IIIa, and IIIb in rat neurohypophysis in vitro by electrical stimulation and by neuroactive agents

The state of phosphorylation of proteins I, IIIa, and IIIb--neuron-specific phosphoproteins--was studied in neurosecretory endings of the neurohypophysis in vitro. Brief periods (a few seconds) of electrical stimulation caused large increases in the state of phosphorylation of all three proteins. The three proteins were dephosphorylated within 1 min after termination of the stimulation. High potassium, 8-bromo-cAMP, and dopamine also stimulated the phosphorylation of the three proteins. The effect of dopamine was blocked by the dopamine antagonist fluphenazine. Peptide mapping of protein I revealed that electrical stimulation or high potassium increased the state of phosphorylation of two regions of the molecule, whereas 8-bromo-cAMP and dopamine increased the state of phosphorylation of only one of these regions.