A synaptic vesicle protein with a novel cytoplasmic domain and four transmembrane regions

Genetics of synaptic vesicle function: toward the complete functional anatomy of an organelle

Synaptic transmission starts with the release of neurotransmitters by exocytosis of synaptic vesicles. As a relatively simple organelle with a limited number of components, synaptic vesicles are in principle accessible to complete structural and functional genetic analysis. At present, the majority of synaptic vesicle proteins has been characterized, and many have been genetically analyzed in mice, Drosophila, and Caenorhabditis elegans. These studies have shown that synaptic vesicles contain proteins with diverse structures and functions. Although the genetic studies are as yet unfinished, they promise to lead to a full description of synaptic vesicles as macromolecular machines involved in all aspects of presynaptic neurotransmitter release.

Post-transcriptional regulation of synaptic vesicle protein expression and the developmental control of synaptic vesicle formation

The regulated expression of synaptic vesicle (SV) proteins during development and the assembly of these proteins into functional SVs are critical aspects of nervous system maturation. We have examined the expression patterns of four SV proteins in embryonic hippocampal neurons developing in culture and have found that increases in the levels of these proteins result primarily from post-transcriptional regulation. Synaptotagmin I, vamp 2, and synapsin I proteins are synthesized at nearly constant rates as the neurons develop. However, these proteins are relatively unstable at early times in culture and undergo a progressive increase in half-life with time, possibly as a result of an increase in the efficiency with which they are incorporated into SVs. In contrast, synaptophysin is synthesized at a very low rate at early times in culture, and its rate of synthesis increases dramatically with time. The increase in synaptophysin synthesis is not simply the result of an increase in mRNA level, but is largely attributable to an increase in the rate of translational initiation. Despite the nearly constant rates of synthesis of synaptotagmin I, vamp 2, and synapsin I, we show that the number of SVs in these developing neurons increases, and that SV proteins are more efficiently targeted to SVs at later times in culture. Our results suggest that SV production during development is not limited by the rates of transcription of genes encoding the component proteins, thus allowing control of this process by cytoplasmic mechanisms, without signaling to the nucleus.

A sequential view of neurotransmitter release

Only a few years ago it was thought that a single Ca2+-dependent membrane binding protein might control regulated exocytosis, but it is now clear that the coordinated actions of a large number of proteins and lipids are required for the precise targeting, docking and fusion of vesicles to the plasma membrane. Thinking was focused in 1993 by the SNARE (soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor) hypothesis, which proposed that certain synaptic vesicle membrane proteins combined specifically with particular proteins in the synaptic membrane active zone to form a complex that interacted with synaptoplasmic proteins, ATP and calcium ions to fuse the vesicles with the presynaptic membrane. Much research that has followed has verified the basic predictions of the SNARE hypothesis. However, recent research indicates that SNARE proteins are more widely distributed in secretory systems and that the sequence in which the proteins function may not occur as was originally proposed. That has recently produced a period of deconstruction and reinterpretation of the SNARE hypothesis. Our present state of knowledge is briefly summarized in this review.

Structure and expression of mitsugumin29 gene

Recently mitsugumin29 unique to the triad junction in skeletal muscle was identified as a novel member of the synaptophysin family; the members of this family have four transmembrane segments and are distributed on intracellular vesicles. In this study, we isolated and analyzed mouse mitsugumin29 cDNA and genomic DNA containing the gene. The mitsugumin29 gene mapped to the mouse chromosome 3 F3-H2 is closely related to the synaptophysin gene in exon-intron organization, which indicates their intimate relationship in molecular evolution. RNA blot hybridization and immunoblot analysis revealed that mitsugumin29 is expressed abundantly in skeletal muscle and at lower levels in the kidney. Immunofluorescence microscopy demonstrated that mitsugumin29 exists specifically in cytoplasmic regions of the proximal and distal tubule cells in the kidney. The results obtained may suggest that mitsugumin29 is involved in the formation of specialized endoplasmic reticulum systems in skeletal muscle and renal tubule cells.

Mitsugumin29, a novel synaptophysin family member from the triad junction in skeletal muscle

In skeletal muscle, excitation-contraction (E-C) coupling requires the conversion of the depolarization signal of the invaginated surface membrane, namely the transverse (T-) tubule, to Ca2+ release from the sarcoplasmic reticulum (SR). Signal transduction occurs at the junctional complex between the T-tubule and SR, designated as the triad junction, which contains two components essential for E-C coupling, namely the dihydropyridine receptor as the T-tubular voltage sensor and the ryanodine receptor as the SR Ca2+-release channel. However, functional expression of the two receptors seemed to constitute neither the signal-transduction system nor the junction between the surface and intracellular membranes in cultured cells, suggesting that some as-yet-unidentified molecules participate in both the machinery. In addition, the molecular basis of the formation of the triad junction is totally unknown. It is therefore important to examine the components localized to the triad junction. Here we report the identification using monoclonal antibody and primary structure by cDNA cloning of mitsugumin29, a novel transmembrane protein from the triad junction in skeletal muscle. This protein is homologous in amino acid sequence and shares characteristic structural features with the members of the synaptophysin family. The subcellular distribution and protein structure suggest that mitsugumin29 is involved in communication between the T-tubular and junctional SR membranes.

The ovary of the desert locust Schistocerca gregaria contains a glycine- and proline-rich peptide that displays sequence similarities with a new class of GPRP proteins from plants

A novel, highly hydrophobic, glycine- and proline-rich peptide was characterized in the ovary of the desert locust, Schistocerca gregaria. The peptide was detected as one of the major peaks in a chromatographic separation of an acidic methanolic extract of 50 mature ovaries. Electrospray mass spectrometry yielded a molecular mass of 6305 Da. The partial amino acid sequence as determined by Edman degradation based automated microsequencing is as follows: Ala-Tyr-Pro-Ala-His-Gln-Gly-Tyr- Pro-Ala-His-Val-Gly-Tyr-Ala-Arg-Val-Gly-Tyr-Gly- Gly-Tyr-Pro-Ser-Tyr-Gly-Tyr-Pro-Ala. Four amino acids (Gly, Pro, Ala, and Tyr) account for more than 80% of the composition of this sequence. Gly-Tyr-Pro is the most important repetitive motif. Ala-Tyr-Pro, Gly-Tyr-Gly and Gly-Tyr-Ala occur as variations of this motif. The novel glycine- and proline-rich insect peptide displays structural characteristics similar to those of a new class of glycine- and proline-rich proteins (GPRP) that have recently been identified in Arabidopsis thaliana (thale cress) and Daucus carota (carrot). The GPRP of A. thaliana contains the same repetitive motifs (except for Ala-Tyr-Pro), the Gly-Tyr-Pro motif also being the most abundant.

Collateral projections from striatonigral neurons to substance P receptor-expressing intrinsic neurons in the striatum of the rat

It is well known that striatonigral neurons produce substance P (SP); however, no SP receptor (SPR) has so far been found in the substantia nigra. On the other hand, a previous study in the rat striatum indicated that SPR was expressed only in cholinergic or somatostatinergic intrinsic neurons (Kaneko et al. [1993] Brain Res. 631:297-303). Thus, it was assumed that SP produced by striatonigral neurons might be released through their intrastriatal axon collaterals to act upon intrinsic neurons in the striatum. To confirm this assumption, the distribution of axon collaterals of striatonigral neurons was examined in the striatum of the rat. The experiments were performed on brain slices by combining retrograde labeling with tetramethylrhodamine-dextran amine, electrophysiological recording, intracellular staining with biocytin, and immunocytochemistry for SPR. The distribution of axons of cholinergic striatal neurons (a group of SP-negative intrinsic striatal neurons) was also examined. It was observed that 16% of varicosities of intrastriatal axon collaterals of striatonigral neurons, as well as 6% of axonal varicosities of cholinergic neurons, were in close apposition to dendrites and cell bodies of SPR-immunoreactive striatal neurons. Since SPR-immunoreactive striatal neurons constituted only 2.7% of the total population of striatal neurons (Kaneko et al. [1993] Brain Res. 631:297-303), it appeared that axonal varicosities of striatonigral neurons were preferentially apposed to SPR-immunoreactive striatal neurons and that the varicosities in close apposition to SPR-immunoreactive neurons were derived more frequently from striatonigral neurons than from cholinergic interneurons. Confocal laser scanning microscopy indicated that axonal varicosities in close apposition to SPR-immunoreactive cells showed synaptophysin immunoreactivity, a marker of synaptic vesicles. In intrastriatal axons of striatonigral neurons, it was further revealed from electron microscopy that axonal varicosities in close apposition to SPR-immunoreactive dendrites, at least a part of them, made synapses of the symmetric type. Striatonigral neurons might release SP preferentially around cholinergic or somatostatinergic intrinsic neurons to regulate them through SP-SPR interactions.

Synapse alteration in hippocampal CA3 field following entorhinal cortex lesion

To model one aspect of the neurodegeneration observed in Alzheimer's disease and to investigate the synaptic alteration of the hippocampus associated with entorhinal cortex lesion, ibotenic acid was used to produce selective unilateral neuronal loss in rat entorhinal cortex. Immunohistological and microdensitometrical analyses confirmed ibotenic acid lesion of the entorhinal cortex after 3 months and showed a decrease of synaptophysin-immunoreactive substances in the stratum lucidum of the CA3 field. This study demonstrates that entorhinal cortex lesion can lead to synaptic alterations and cause damage to presynaptic terminals with projecting area in the disruption of the entorhinal cortex hippocampus relay passage.

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

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

Alterations in synaptic strength preceding axon withdrawal

Permanent removal of axonal input to postsynaptic cells helps shape the pattern of neuronal connections in response to experience, but the process is poorly understood. Intracellular recording from newborn and adult mouse muscle fibers temporarily innervated by two axons showed an increasing disparity in the synaptic strengths of the two inputs before one was eliminated. The connection that survived gained strength by increasing the amount of neurotransmitter released (quantal content), whereas the input that was subsequently removed became progressively weaker, because of a reduction in quantal content and a reduction in quantal efficacy associated with reduced postsynaptic receptor density. Once the synaptic strengths of two inputs began to diverge, complete axonal withdrawal of the weaker input occurred within 1 to 2 days. These experiments provide a link between experience-driven changes in synaptic strength and long-term changes in connectivity in the mammalian nervous system.

Maturational gradients in the retina of the ferret

In the present set of studies, we have examined the site for the initiation of retinal maturation in the ferret. A variety of maturational features across the developing inner and outer retina were examined by using standard immunohistochemical, carbocyanine dye labelling, and Nissl-staining techniques, including 1) two indices of early differentiation of the first-born retinal ganglion cells, the presence of beta-tubulin and of neuron-specific enolase; 2) the receding distribution of chondroitin sulfate proteoglycans within the inner retina; 3) the distribution of the first ganglion cells to grow axons along the optic nerve; 4) the emergence of the inner plexiform layer; 5) the emergence of the outer plexiform layer and 6) the onset of synaptophysin immunoreactivity within it; 7) the differentiation of calbindin-immunoreactive horizontal cells; and 8) the cessation of proliferative activity at the ventricular surface. Although we were able to define distinct maturational gradients that are associated with many of these features of inner and outer retinal development (each considered in detail in this report), with dorsal retina maturing before ventral retina, and with peripheral retina maturing last, none showed a clear initiation in the region of the developing area centralis. Rather, maturation began in the peripapillary retina dorsal to the optic nerve head, which is consistent with previous studies on the topography of ganglion cell genesis in the ferret. These results make clear that the order of retinal maturation and the formation of the area centralis are not linked, at least not in the ferret.

Nerve growth factor induced modification of presynaptic elements in adult visual cortex in vivo

Nerve growth factor (NGF) has been shown to play important roles in neuronal survival, growth and differentiation. Recently, we have found that intracortical infusion of NGF into adult cat visual cortex can recreate ocular dominance plasticity, suggesting that NGF is also involved in activity-dependent modification of synaptic connectivity in the adult brain. To further explore the mechanisms of NGF-induced plasticity in adult visual cortex, we studied two presynaptic markers: GAP-43 and synaptophysin. Immunocytochemical staining showed that NGF-treatment of adult visual cortex selectively increased the level of the phosphorylated form of GAP-43, while the total level of GAP-43 was not changed. These results demonstrate that NGF-treatment stimulates phosphorylation processes of GAP-43 in vivo. In addition, NGF-treatment of adult visual cortex increased the level of synaptophysin immunoreactivity. Since the phosphorylated form of GAP-43 is known to be enriched in the membrane skeleton of growth cones and of developing synapses, and the phosphorylation of GAP-43 has been linked with events that underlie synaptic plasticity, and since synaptophysin is a major component of presynaptic vesicles, our results suggest that NGF-treatment of adult visual cortex modulates presynaptic terminals, possibly by inducing axonal sprouting and formation of new synapses, and that these changes may play a role in the NGF-induced functional plasticity.

Targeting of P-selectin to two regulated secretory organelles in PC12 cells

Targeting of P-selectin to the regulated secretory organelles (RSOs) of phaeochromocytoma PC12 cells has been investigated. By expressing from cDNA a chimera composed of HRP and P-selectin, and then following HRP activity through subcellular fractionation, we have discovered that P-selectin contains signals that target HRP to the synaptic-like microvesicles (SLMV) as well as the dense-core granules (DCGs) of these cells. Mutagenesis of the chimera followed by transient expression in PC12 cells shows that at least two different sequences within the carboxy-terminal cytoplasmic tail of P-selectin are necessary, but that neither is sufficient for trafficking to the SLMV. One of these sequences is centred on the 10 amino acids of the membrane-proximal C1 exon that is also implicated in lysosomal targeting. The other sequence needed for trafficking to the SLMV includes the last four amino acids of the protein. The same series of mutations have a different effect on DCG targeting, showing that traffic to the two different RSOs depends on different features within the cytoplasmic domain of P-selectin.

Pantophysin is a ubiquitously expressed synaptophysin homologue and defines constitutive transport vesicles

Certain properties of the highly specialized synaptic transmitter vesicles are shared by constitutively occurring vesicles. We and others have thus identified a cDNA in various nonneuroendocrine cell types of rat and human that is related to synaptophysin, one of the major synaptic vesicle membrane proteins, which we termed pantophysin. Here we characterize the gene structure, mRNA and protein expression, and intracellular distribution of pantophysin. Its mRNA is detected in murine cell types of nonneuroendocrine as well as of neuroendocrine origin. The intron/exon structure of the murine pantophysin gene is identical to that of synaptophysin except for the last intron that is absent in pantophysin. The encoded polypeptide of calculated mol wt 28,926 shares many sequence features with synaptophysin, most notably the four hydrophobic putative transmembrane domains, although the cytoplasmic end domains are completely different. Using antibodies against the unique carboxy terminus pantophysin can be detected by immunofluorescence microscopy in both exocrine and endocrine cells of human pancreas, and in cultured cells, colocalizing with constitutive secretory and endocytotic vesicle markers in nonneuroendocrine cells and with synaptophysin in cDNA-transfected epithelial cells. By immunoelectron microscopy, the majority of pantophysin reactivity is detected at vesicles with a diameter of < 100 nm that have a smooth surface and an electron-translucent interior. Using cell fractionation in combination with immunoisolation, these vesicles are enriched in a light fraction and shown to contain the cellular vSNARE cellubrevin and the ubiquitous SCAMPs in epithelial cells and synaptophysin in neuroendocrine or cDNA-transfected nonneuroendocrine cells and neuroendocrine tissues. Pantophysin is therefore a broadly distributed marker of small cytoplasmic transport vesicles independent of their content.

Effects of entorhinal cortex lesion on learning behavior and on hippocampus in the rat

The initial stage of Alzheimer's disease is characterized by a neuropathological change in the entorhinal cortex. In a previous study it was shown that rats with excitotoxic lesion of entorhinal cortex showed an impaired acquisition of passive and active avoidance responses. In this study a rat with excitotoxic lesion of the entorhinal cortex was tested for 'more operant' behavioral learning (i.e., positive reinforcement operant learning). The hippocampus was also examined histologically as acetylcholinesterase-stained sections, and as synaptophysin immunostained sections and examined biochemically by liquid chromatography. Eight weeks after operation, the bilateral entorhinal cortex lesioned rats showed an impaired acquisition of positive reinforcement operant learning. The lesioned side of unilateral entorhinal cortex lesioned rats showed a decrease of acetylcholinesterase-positive fibers in the CA3, the dentate gyrus, and of synaptophysin-positive substances in the CA3. Biochemical study showed a decreased level of acetylcholine in the CA3, and in the dentate gyrus. The histological and biochemical findings are interpreted as indicating that the entorhinal cortex of the rat provides the major extrinsic synaptic input to the hippocampal formation via the circuit which serves as a relay passage through the dentate gyrus and via direct projections into the hippocampus. Behavioral findings confirmed the importance of the entorhinal cortex in memory acquisition and indicated that rats with a partial neuronal loss in the entorhinal cortex may be a useful model for the memory disturbance of Alzheimer's disease.

Synaptophysin, a major synaptic vesicle protein, is not essential for neurotransmitter release

Synaptophysin (syp I) is a synaptic vesicle membrane protein that constitutes approximately 7% of the total vesicle protein. Multiple lines of evidence implicate syp I in a number of nerve terminal functions. To test these, we have disrupted the murine Syp I gene. Mutant mice lacking syp I were viable and fertile. No changes in the structure and protein composition of the mutant brains were observed except for a decrease in synaptobrevin/VAMP II. Synaptic transmission was normal with no detectable changes in synaptic plasticity or the probability of release. Our data demonstrate that one of the major synaptic vesicle membrane proteins is not essential for synaptic transmission, suggesting that its function is either redundant or that it has a more subtle function not apparent in the assays used.

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.

Molecular characterization of the gene coding for GPRP, a class of proteins rich in glycine and proline interacting with membranes in Arabidopsis thaliana

The gene coding for a new class of proteins rich in glycine and proline (GPRP) was cloned in Arabidopsis thaliana. In the protein sequence, five amino acids - glycine, proline, alanine, tyrosine and histidine - account for 79.4% of the total composition. The protein has two different glycine-rich domains interrupted by a hydrophobic segment having a high probability of helix formation. The protein synthesized in vitro interacts with microsomes possibly through the hydrophobic domain. The gene in Arabidopsis has two introns, one in the coding region and the other one in the 5' non-coding region. The later one is 778 bp long. Homologous sequences are found in carrot, tomato and tobacco. GPRP mRNA is found in the different organs of the plant analyzed except in mature seeds and anthers, and mostly in epidermal and vascular tissues. Possible hypotheses about the function of GPRP are discussed.

PC12 cells show immunoreactivity to a number of proteins and peptides, including vasostatin

N-terminal chromogranin A (CGA) contains peptides with vasoinhibitory properties, called vasostatin I (VST) and II [CGA (1-76) and (1-113) in human and bovine; (1-128) in rat]. Three fragments of VST were synthesized and antisera raised: human CGA (68-76) (VST I) rat CGA (121-128) (VST II fragment 2), and bovine/human CGA (83-91) (VST II, fragment 3). Strong immunoreactivity was observed in PC12 cells with antisera to VST II, fragment 3, VST I, and neuron-specific enolase. Little or no immunoreactivity was observed using antisera to synaptophysin, whole molecule CGA, pancreastatin, protein gene product 9.5, somatostatin, pancreatic polypeptide, or with antibodies 875 and 876 to VST II, fragment 2. Most of the VST antisera cross-reacted, with a species of molecular weight, 61 kDa but one, 874, cross-reacted with two species of molecular weights, 7.2 and 12 kDa. Our results show the presence of N-terminally processed CGA in PC12 cells.

A unifying hypothesis for acetylcholine release

Mediatophore is the only nerve terminal membrane protein known to translocate acetylcholine upon calcium action. It is localized at the active zone. In this review we attempted to describe its role in relation to the vesicular and membrane protein complexes that are formed at the active zone. The model pictures a possible set of sequential steps that lead to exocytosis. The smallest quantal events are attributed to mediatophore opening momentarily, while synaptic vesicles synchronize release by controlling the calcium microdomain. A clear distinction is made between sub-quantal ACh release preserved after Botulinum toxin action, and exocytosis of vesicular contents. A cybernetic model for release and exocytosis related to protein interactions is presented for future works.

Structure of synaptogyrin (p29) defines novel synaptic vesicle protein

Synaptogyrin (p29) is a synaptic vesicle protein that is uniformly distributed in the nervous system (Baumert et al., 1990). We have cloned and sequenced the cDNA encoding synaptogyrin, and the sequence predicts a protein with a molecular mass of 25,900 D with four membrane-spanning domains. The topology of the protein was confirmed by limited proteolysis using domain-specific antibodies. Database searches revealed several cDNA sequences coding polypeptides with sequence identities ranging from 32 to 46%, suggesting that synaptogyrin is a member of a multigene family. When the synaptogyrin cDNA is expressed in COS cells, the generated protein is indistinguishable from native synaptogyrin. To study intracellular sorting, synaptogyrin was expressed in CHO cells that revealed a punctate staining that was very similar to that of synaptophysin and endogenously expressed cellubrevin. Significant overlap with transferrin staining was also observed, suggesting that synaptogyrin is targeted to a recycling compartment involved in membrane traffic to and from the plasma membrane.

Mice lacking synaptophysin reproduce and form typical synaptic vesicles

Synaptophysin is one of the major integral membrane proteins of the small (30-50nm diameter) electron-translucent transmitter-containing vesicles in neurons and of similar vesicles in neuroendocrine cells. Since its expression is tightly linked to the occurrence of these vesicle types, we mutated the X-chromosomally located synaptophysin gene in embryonic stem cells for the generation of synaptophysin-deficient mice in order to study the consequence of synaptophysin ablation for the formation and function of such vesicles in vivo. The behavior and appearance of mice lacking synaptophysin was indistinguishable from that of their litter mates and reproductive capacity was comparable to normal mice. Furthermore, no drastic compensatory changes were noted in the expression of several other neuronal polypeptides or in the mRNA levels of synaptophysin isoforms, the closely related neuronal synaptoporin/synaptophysinII, and the ubiquitous pantophysin. Immunofluorescence microscopy of several neuronal and neuroendocrine tissues showed that overall tissue architecture was maintained in the absence of synaptophysin, and that the distribution of other synaptic vesicle components was not visibly affected. In electron-microscopic preparations, large numbers of vesicles with a diameter of 39.9nm and an electron-translucent interior were seen in synaptic regions of synaptophysin-deficient mice; these vesicles could be labeled by antibodies against synaptic vesicle proteins, such as synaptobrevin 2.

Synaptic plasticity: stairway to memory

Since the idea that memory is associated with alterations in synaptic strength was accepted, studies on the cellular and molecular mechanisms responsible for the plastic changes in neurons have attracted wide interest in the scientific community. Recent studies on memory processes have also pointed out some unifying themes emerging from a wide range of nervous systems, suggesting that regardless of the species or brain regions, a common denominator for memory may exist. Thus, the present review attempted to create a hypothetical and universal synaptic model valid for a variety of nervous systems, ranging from molluscs to mammals. The cellular and molecular events leading to short- and long-term modifications of memory have been described in a sequential order, from the triggering signals to the gene expression, synthesis of new proteins and neuronal growth. These events are thought to represent the late phases of memory consolidation leading to persistent modifications in synaptic plasticity, thereby facilitating the permanent storage of acquired information throughout the individual's life.

Evaluation of the evidence for ion channels in synaptic vesicles

Synaptic vesicles (SVs) have been the focus of much research for many years, however only recently have ion channels from SV membranes been reported. There is now convincing evidence that SVs contain ion channels. This conclusion is based on direct experimental results from several different laboratories using the patch clamp or planar lipid bilayer technique on SVs and neurosecretory granules (NSG). Some limitations of patch clamping and of fusing synamptic vesicles to a bilayer are described and the advantages of the nystatin/ergosterol fusion method are presented. Six different channels appear to exist in SV (or NSG) membranes. Two large channels (250 and 154 pS) have been observed in SVs isolated from mammalian brain, two channels (180 and 13 pS) from Torpedo electric organ, and two channels (130 and 30-40 pS) from NSG. The three larger channels from each set (250, 180 and 130 pS7) are novel in that they have a subconductance state. The 154 pS channel has been identified as synaptophysin but the identity and function of the other channels is unknown. Although some of the channels are gated by voltage, only the 130 pS channel is modulated by Ca2+. Further knowledge of what regulates these channels is mandatory if we are to determine the physiological significance of these channels.

Characterization of uvi15+, a stress-inducible gene from Schizosaccharomyces pombe

The uvi15+ gene of Schizosaccharomyces pombe is a member of a group of stress-inducible genes transcription levels of which increase in response to DNA-damaging agents or heat shock. It encodes a polypeptide of calculated molecular mass 11641 Da, with no significant sequence similarity to other known heat shock proteins. The steady-state level of the uvi15+ gene product of about 12 kDa was increased by heat shock and canavanine, an amino acid analog. This gene also showed a transient increase in expression as cells moved into diauxic shift phase. Although deletion of the uvi15+ gene did not affect the mitotic growth or thermotolerance of cells, the mutant cells rapidly lost viability in stationary phase and under starvation conditions. These cells also showed a defect in sporulation ability. These results suggest that the uvi15+ gene encodes a stress response protein involved in the maintenance of cell viability during entry into stationary phase or under starvation conditions.

Aberrant differentiation of neuromuscular junctions in mice lacking s-laminin/laminin beta 2

Synapse formation requires a complex interchange of information between the pre- and postsynaptic partners. At the skeletal neuromuscular junction, some of this information is contained in the basal lamina (BL), which runs through the synaptic cleft between the motor nerve terminal and the muscle fibre. During regeneration following injury, components of synaptic BL can trigger several features of postsynaptic differentiation in the absence of the nerve terminal, and of presynaptic differentiation in the absence of the muscle fibre. One nerve-derived component of synaptic BL, agrin, is known to affect postsynaptic differentiation, but no muscle-derived components have yet been shown to influence motor nerve terminals. A candidate for such a role is s-laminin (also called laminin beta 2), a homologue of the B1 (beta 1) chain of the widely distributed BL glycoprotein, laminin. s-Laminin is synthesized by muscle cells and concentrated in synaptic BL. In vitro, recombinant s-laminin fragments are selectively adhesive for motor neuron-like cells, inhibit neurite outgrowth promoted by other matrix molecules, and act as a 'stop signal' for growing neurites. By generating and characterizing mice with a targeted mutation of the s-laminin gene, we show here that s-laminin regulates formation of motor nerve terminals.

The topogenic fate of the polytopic transmembrane proteins, synaptophysin and connexin, is determined by their membrane-spanning domains

The synaptophysins and connexins are polytopic transmembrane proteins of similar secondary structure that accumulate as multiple homo-oligomers in specialized membrane regions, the presynaptic transmitter vesicles or gap junctions. Transfection and expression of the respective genes in cultured epithelial cells results in the de novo formation of either small cytoplasmic, synaptophysin-rich vesicles, or functional gap junctions consisting of clustered connexin molecules. To examine the molecular requirements for the specific enrichment and topogenesis of both types of molecule, chimeric cDNAs were constructed composed of different parts of the rat synaptophysin and rat liver connexin32 genes. Expression of the encoded chimeric polypeptides in hepatocellular carcinoma-derived cells showed that only chimeras with all four transmembrane domains from either parent molecule were delivered to their specific destination. In contrast, chimeras with transmembrane domains from both connexin32 and synaptophysin were always retained in the endoplasmic reticulum. The topogenic nature of the transmembrane domains was further demonstrated by deletion mutagenesis, indicating that removal of cytoplasmic end domains or intravesicular loops does not abolish targeting. On the other hand, excision of individual transmembrane domains or introduction of point mutations in transmembrane segments resulted in retention in the endoplasmic reticulum.

Synaptic vesicle proteins and regulated exocytosis

The recent identification of novel proteins associated with the membranes of synaptic vesicles has ignited the field of molecular neurobiology to probe the function of these molecules. Evidence is mounting that the vesicle proteins vamp (synaptobrevin), rab3A, synaptophysin, synaptotagmin (p65) and SV2 play an important role in regulated exocytosis, by regulating neurotransmitter uptake, vesicle targeting and fusion with the presynaptic plasma membrane.

The synaptic vesicle and its targets

Synaptic vesicles play the central role in synaptic transmission. They are regarded as key organelles involved in synaptic functions such as uptake, storage and stimulus-dependent release of neurotransmitter. In the last few years our knowledge concerning the molecular components involved in the functioning of synaptic vesicles has grown impressively. Combined biochemical and molecular genetic approaches characterize many constituents of synaptic vesicles in molecular detail and contribute to an elaborate understanding of the organelle responsible for fast neuronal signalling. By studying synaptic vesicles from the electric organ of electric rays and from the mammalian cerebral cortex several proteins have been characterized as functional carriers of vesicle function, including proteins involved in the molecular cascade of exocytosis. The synaptic vesicle specific proteins, their presumptive function and targets of synaptic vesicle proteins will be discussed. This paper focuses on the small synaptic vesicles responsible for fast neuronal transmission. Comparing synaptic vesicles from the peripheral and central nervous systems strengthens the view of a high conservation in the overall composition of synaptic vesicles with a unique set of proteins attributed to this cellular compartment. Synaptic vesicle proteins belong to gene families encoding multiple isoforms present in subpopulations of neurons. The overall architecture of synaptic vesicle proteins is highly conserved during evolution and homologues of these proteins govern the constitutive secretion in yeast. Neurotoxins from different sources helped to identify target proteins of synaptic vesicles and to elucidate the molecular machinery of docking and fusion. Synaptic vesicle proteins and their markers are useful tools for the understanding of the complex life cycle of synaptic vesicles.

Sorting of synaptophysin into special vesicles in nonneuroendocrine epithelial cells

Synaptophysin is a major transmembrane glycoprotein of a type of small vesicle with an electron-translucent content (SET vesicles), including the approximately 50-nm presynaptic vesicles in neuronal cells, and of similar, somewhat larger (< or = approximately 90 nm) vesicles (SLMV) in neuroendocrine (NE) cells. When certain epithelial non-NE cells, such as human hepatocellular carcinoma PLC cells, were cDNA transfected to synthesize synaptophysin, the new molecules appeared in specific SET vesicles. As this was in contrast to other reports that only NE cells were able to sort synaptophysin away from other plasma membrane proteins into presynaptic- or SLMV-type vesicles, we have further characterized the vesicles containing synaptophysin in transfected PLC cells. Using fractionation and immunoisolation techniques, we have separated different kinds of vesicles, and we have identified a distinct type of synaptophysin-rich, small (30-90-nm) vesicle that contains little, if any, protein of the constitutive secretory pathway marker hepatitis B surface antigen, of the fluid phase endocytosis marker HRP, and of the plasma membrane recycling endosomal marker transferrin receptor. In addition, we have found variously sized vesicles that contained both synaptophysin and transferrin receptor. A corresponding result was also obtained by direct visualization, using double-label immunofluorescence microscopy for the endocytotic markers and synaptophysin in confocal laser scan microscopy and in double-immunogold label electron microscopy. We conclude that diverse non-NE cells of epithelial nature are able to enrich the "foreign" molecule synaptophysin in a category of SET vesicles that are morphologically indistinguishable from SLMV of NE cells, including one type of vesicle in which synaptophysin is sorted away from endosomal marker proteins. Possible mechanisms of this sorting are discussed.

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.

Fast and slow axonal transport-different methodological approaches give complementary information: contributions of the stop-flow/crush approach

This 'minireview' describes experiments in short term crush operated rat nerves, to study endogenous substances in anterograde and retrograde fast axonal transport. Immunofluorescence was used to recognize transported antigens, and cytofluorimetric scanning was employed to quantitate different antigens which had accumulated proximal and distal to the crushes. Vesicle membrane components p38 (synaptophysin) and SV2 accumulated on both sides of a crush. This was expected from a number of studies from different laboratories. Surface associated molecules, however, like synapsins and rab3a, have been studied by other groups with biochemical methods, and suggested to be transported with slow transport. The crush method, however, revealed that a considerable fraction of these two substances are transported with the fast transport system, and, thus, associated with fast transported organelles in the living neuron. Evidently, more than one technique is required to give a more complete picture of intraneuronal transport related events.

Decreased synaptic density in aged brains and its prevention by rearing under enriched environment as revealed by synaptophysin contents

Changes in synaptic density in various brain regions were assessed among different age groups of rats maintained in ordinary small cages, as determined by synaptophysin assay. The synaptophysin content in hippocampus decreases as early as in the adult stage. The most remarkable decrement occurs in occipital cortex. In other regions, synaptophysin contents decrease in senescence to 60-77% of the respective peak values during young and adult stages. The other rat group reared under enriched environment in a large cage until 30 months of age was examined for synaptic density, and was revealed to maintain the similar levels as in young, or even higher levels in frontal, temporal, entorhinal cortices and hippocampus. These results indicate that the synaptic density in cerebrum decreases in senescence and this decrease can be prevented by rearing under enriched environment.

Prejunctional regulatory actions of androgens on a hormone sensitive muscle

The influence of androgens and time course of effects induced by hormone deprivation were examined on the spontaneous transmitter release in the levator ani (LA) muscle of 30-180-day-old male rats. The resting membrane potential (RMP) and miniature endplate potentials (mepps) were recorded intracellularly from LA muscle fibers of intact animals or gonadectomized at different ages. In intact animals, the frequency of mepps increased proportionately to the muscle fiber growth up to 60 days, stabilizing thereafter. Gonadectomy at any age did not affect the RMP, but increased the frequency of mepps by 65% to 140%. The effect was detected after 15 days and was unrelated to the degree of muscle atrophy. Independently of the age of gonadectomy control values of mepp frequency were restored after 90 days, while the accompanying postjunctional changes persisted. These results indicate that androgens exert a prejunctional inhibitory influence on the spontaneous transmitter release in the rat LA muscle. The transient nature of the prejunctional effect induced by hormone deprivation indicates an adjustment of nerve terminals to persistent postjunctional alterations.

The expression of GAP-43 and synaptophysin in the developing rat retina

We have undertaken a detailed study of the expression of GAP-43 and synaptophysin immunoreactivity in the developing postnatal rat retina. We found that these two 'presynaptic' proteins have quite different expression patterns. GAP-43 was first expressed in the optic nerve and the optic fiber layer of the retina, where it disappeared between the 8th and 16th postnatal day. From the 5th postnatal day on, GAP-43 also appeared in the inner plexiform layer, where it was present in three distinct bands. This expression changed little in postnatal development and persisted in the adult retina. GAP-43 was not detected in the outer plexiform layer of the retina. Synaptophysin was absent from the optic nerve and optic fibers at all postnatal stages. It was first expressed in the developing outer plexiform and, with reduced intensity, in the outer nuclear layer between postnatal days 2 and 5. In the inner plexiform layer, synaptophysin could be first detected between postnatal days 8 and 12. The intensity of staining increased during postnatal development in both plexiform layers. The developmental sequence of synaptophysin expression can be correlated with the maturation of presynaptic terminals of photoreceptors and bipolar cells. The rather complex pattern of GAP-43 expression is not easily compatible with a single model of GAP-43 function, and suggests diverse functions of this molecule in the retina.

Expression of the synaptophysin gene family is not restricted to neuronal and neuroendocrine differentiation in rat and human

The integral membrane protein synaptophysin is one of the major polypeptide components of the small, electron-translucent, transmitter-containing vesicles in neurons and of similar vesicles in neuroendocrine (NE) cells. In an attempt to identify synaptophysin-related molecules, such as synaptoporin, it was noticed in polymerase chain reaction (PCR) experiments that products having the expected size could be amplified not only from neuronal and NE cells, but also from non-NE cells. Northern blot hybridization analyses demonstrated that certain non-NE cells express low amounts of synaptophysin mRNA although the encoded polypeptide could not be detected. These observations, however, did not explain the consistent amplification of cDNA fragments regardless of cell type. PCR products were therefore cloned and a novel type of cDNA was identified in rat and human. The partial human cDNA was completed by isolation of phage cDNA clones constructed from a human keratinocyte cell line (HaCaT) and by PCR. When used in hybridization experiments with genomic DNA, this clone recognized a single gene. The 2106 bp cDNA contains an open reading frame coding for a polypeptide of calculated molecular weight 28,565 and having an isoelectric point of 8.45. This polypeptide is very similar to synaptophysin in the four transmembrane domains and the connecting loop regions but lacks the characteristic cytoplasmic tail. Extensive PCR analyses and Northern blot hybridization experiments demonstrated that the synaptophysin-related gene is ubiquitously expressed in vitro and in vivo. To stress the ubiquity of expression in contrast to the restricted distribution of synaptophysin and synaptoporin, I propose to refer to the encoded polypeptide as pantophysin.

Isolation and characterization of peptides derived from the cleavage of the cytoplasmic domain of synaptophysin in frog brain

Three overlapping peptides have been isolated from the whole brain of the European green frog, Rana ridibunda and identified as fragments of the cytoplasmic domain of the synaptic vesicle membrane protein, synaptophysin. The primary structures of the peptides define the amino acid sequence of the 58 residues at the COOH-terminus of the protein and indicate that fragments arose from proteolytic cleavages at the COOH-terminal side of aspartyl residues. The overall conservation of structure of the cytoplasmic domain between the mammalian and amphibian proteins is relatively poor (58% sequence identity between frog and rat synaptophysin). However, the pattern of repeating tyrosine residues that is present in mammalian and Torpedo synaptophysins is also found in the frog protein suggesting that this structural motif is necessary for the, as yet unknown, function of the protein. The cytoplasmic region of frog synaptophysin is also rich in glutamine and glycine residues, but the putative consensus repeating sequence Tyr-Gly-Pro/Gln-Gln-Gly in mammalian synaptophysins does not occur in the frog protein. The data raise the possibility that synaptophysin, like the chromogranins and secretogranin II, may function as the precursor of biologically active peptides.

Premature arrest of myelin formation in transgenic mice with increased proteolipid protein gene dosage

Proteolipid protein (PLP) is an integral membrane protein of CNS myelin. Mutations of the X chromosome-linked PLP gene cause glial cell death and myelin deficiency in jimpy mice and other neurological mutants. As part of an attempt to rescue these mutants by transgenic complementation, we generated normal mouse lines expressing autosomal copies of the entire wild-type PLP gene. Surprisingly, increase of the PLP gene dosage in nonmutant mice with only 2-fold transcriptional overexpression results in a novel phenotype characterized by severe hypomyelination and astrocytosis, seizures, and premature death. This demonstrates that precise control of the PLP gene is a critical determinant of terminal oligodendrocyte differentiation. Dysmyelination of PLP transgenic mice provides experimental evidence that Pelizaeus-Merzbacher disease, previously associated with a partial duplication of the human X chromosome, can be caused by doubling of the PLP gene dosage.

Synaptophysin and chromogranin A immunoreactivities of Lewy bodies in Parkinson's disease brains

Lewy bodies commonly observed in brains with Parkinson's disease (PD) histochemically contain both protein and lipid as chemical components. Ultrastructurally, they are composed of filamentous, vesicular and granular structures. We investigated PD brains with light and electron microscopic immunohistochemistry using antibodies against two marker proteins for neuronal secretory vesicles, synaptophysin and chromogranin A. Both antibodies immunolabeled the peripheral zones and occasionally central cores of Lewy bodies of the classical and intraneuritic types. In addition, the diffuse immunolabeling was observed in Lewy bodies of the cortical type. Furthermore, the ultrastructural immuno-decoration was found mainly in the vesicular structures, and also in the filamentous and granular structures of Lewy bodies. Immuno-blot analysis of each antibody showed no difference between PD and normal control brains. The present observations suggest that vesicular profiles of Lewy bodies represent presynaptic and dense core secretory vesicles, and therefore that the lipid elements of Lewy bodies are derived from membrane lipids of these vesicles.