Localization and subcellular distribution of smg p25A, a ras p21-like GTP-binding protein, in rat brain

Molecular Insights into the Roles of Rab Proteins in Intracellular Dynamics and Neurodegenerative Diseases

In eukaryotes, the cellular functions are segregated to membrane-bound organelles. This inherently requires sorting of metabolites to membrane-limited locations. Sorting the metabolites from ribosomes to various organelles along the intracellular trafficking pathways involves several integral cellular processes, including an energy-dependent step, in which the sorting of metabolites between organelles is catalyzed by membrane-anchoring protein Rab-GTPases (Rab). They contribute to relaying the switching of the secretory proteins between hydrophobic and hydrophilic environments. The intracellular trafficking routes include exocytic and endocytic pathways. In these pathways, numerous Rab-GTPases are participating in discrete shuttling of cargoes. Long-distance trafficking of cargoes is essential for neuronal functions, and Rabs are critical for these functions, including the transport of membranes and essential proteins for the development of axons and neurites. Rabs are also the key players in exocytosis of neurotransmitters and recycling of neurotransmitter receptors. Thus, Rabs are critical for maintaining neuronal communication, as well as for normal cellular physiology. Therefore, cellular defects of Rab components involved in neural functions, which severely affect normal brain functions, can produce neurological complications, including several neurodegenerative diseases. In this review, we provide a comprehensive overview of the current understanding of the molecular signaling pathways of Rab proteins and the impact of their defects on different neurodegenerative diseases. The insights gathered into the dynamics of Rabs that are described in this review provide new avenues for developing effective treatments for neurodegenerative diseases-associated with Rab defects.

Electron microscopic localization of M2-muscarinic receptors in cholinergic and noncholinergic neurons of the laterodorsal tegmental and pedunculopontine nuclei of the rat mesopontine tegmentum

Muscarinic m2 receptors (M2Rs) are implicated in autoregulatory control of cholinergic output neurons located within the pedunculopontine (PPT) and laterodorsal tegmental (LTD) nuclei of the mesopontine tegmentum (MPT). However, these nuclei contain many noncholinergic neurons in which activation of M2R heteroceptors may contribute significantly to the decisive role of the LTD and PPT in sleep-wakefulness. We examined the electron microscopic dual immunolabeling of M2Rs and the vesicular acetylcholine transporter (VAchT) in the MPT of rat brain to identify the potential sites for M2R activation. M2R immunogold labeling was predominately seen in somatodendritic profiles throughout the PPT/LTD complex. In somata, M2R immunogold particles were often associated with Golgi lamellae and cytoplasmic endomembrannes, but were rarely in contact with the plasma membrane, as was commonly seen in dendrites. Approximately 36% of the M2R-labeled somata and 16% of the more numerous M2R-labeled dendrites coexpressed VAchT. M2R and M2R/VAchT-labeled dendritic profiles received synapses from inhibitory- and excitatory-type axon terminals, over 88% of which were unlabeled and others contained exclusively M2R or VAchT immunoreactivity. In axonal profiles M2R immunogold was localized to plasmalemmal and cytoplasmic regions and showed a similar distribution in many VAchT-negative glial profiles. These results provide ultrastructural evidence suggestive of somatic endomembrane trafficking of M2Rs, whose activation serves to regulate the postsynaptic excitatory and inhibitory responses in dendrites of cholinergic and noncholinergic neurons in the MPT. They also suggest the possibility that M2Rs in this brain region mediate the effects of acetylcholine on the release of other neurotransmitters and on glial signaling. J. Comp. Neurol. 524:3084-3103, 2016. © 2016 Wiley Periodicals, Inc.

Tissue multicolor STED nanoscopy of presynaptic proteins in the calyx of Held

The calyx of Held, a large glutamatergic terminal in the mammalian auditory brainstem has been extensively employed to study presynaptic structure and function in the central nervous system. Nevertheless, the nanoarchitecture of presynaptic proteins and subcellular components in the calyx terminal and its relation to functional properties of synaptic transmission is only poorly understood. Here, we use stimulated emission depletion (STED) nanoscopy of calyces in thin sections of aldehyde-fixed rat brain tissue to visualize immuno-labeled synaptic proteins including VGluT1, synaptophysin, Rab3A and synapsin with a lateral resolution of approximately 40 nm. Excitation multiplexing of suitable fluorescent dyes deciphered the spatial arrangement of the presynaptic phospho-protein synapsin relative to synaptic vesicles labeled with anti-VGluT1. Both predominantly occupied the same focal volume, yet may exist in exclusive domains containing either VGluT1 or synapsin immunoreactivity. While the latter have been observed with diffraction-limited fluorescence microscopy, STED microscopy for the first time revealed VGluT1-positive domains lacking synapsins. This observation supports the hypothesis that molecularly and structurally distinct synaptic vesicle pools operate in presynaptic nerve terminals.

Cooperative regulation of neurotransmitter release by Rab3a and synapsin II

To understand how the presynaptic proteins synapsin and Rab3a may interact in the regulation of the synaptic vesicle cycle and the release process, we derived a double knockout (DKO) mouse lacking both synapsin II and Rab3a. We found that Rab3a deletion rescued epileptic-like seizures typical for synapsin II gene deleted animals (Syn II(-)). Furthermore, action potential evoked release was drastically reduced in DKO synapses, although spontaneous release remained normal. At low Ca2+ conditions, quantal content was equally reduced in Rab3a(-) and DKO synapses, but as Ca2+ concentration increased, the increase in quantal content was more prominent in Rab3a(-). Electron microscopy analysis revealed that DKO synapses have a combined phenotype, with docked vesicles being reduced similar to Rab3a(-), and intraterminal vesicles being depleted similar to Syn II(-). Consistently, both Syn II(-) and DKO terminals had increased synaptic depression and incomplete recovery. Taken together, our results suggest that synapsin II and Rab3a have separate roles in maintaining the total store of synaptic vesicles and cooperate in promoting the latest steps of neuronal secretion.

Rab3a deletion reduces vesicle docking and transmitter release at the mouse diaphragm synapse

Rab3a is a small GTP binding protein associated with presynaptic vesicles that is thought to regulate vesicle targeting to active zones. Although this rab3a function implies that vesicle docking and action potential-evoked release might be inhibited in rab3a gene-deleted synapses, such inhibition has never been demonstrated. To investigate vesicle docking at the neuromuscular junction of rab3a gene-deleted (rab3a(-)) mice, we performed electron microscopy analysis of the diaphragm slow-fatigue (type I) synapses. We found a significant (26%) reduction in the number of vesicles docked to the presynaptic membrane in rab3a(-) terminals, although intraterminal vesicles were not affected. Aiming to detect possible changes in quantal release due to rab3a gene deletion, we minimized the variability between preparations employing focal recordings of synaptic responses from visualized type I endplates. We found a significant decrease in both evoked (27% reduction in quantal content) and spontaneous (28% reduction in mini frequency) quantal release. The decrease in the evoked release produced by rab3a deletion was most pronounced at reduced extracellular Ca(2+) concentrations (over 50% decrease at 0.5 and 0.2 mM Ca(2+)). By manipulating extracellular calcium, we demonstrated that calcium cooperativity is not altered in rab3a(-) synapses, however calcium sensitivity of quantal release is affected. Thus, we demonstrated that rab3a positively regulates docking and basal quantal release at the mouse neuromuscular junction. This result is consistent with the proposed role of rab3a in trafficking and targeting vesicles to the active zones.

Subcellular distribution of M2 muscarinic receptors in relation to dopaminergic neurons of the rat ventral tegmental area

Acetylcholine can affect cognitive functions and reward, in part, through activation of muscarinic receptors in the ventral tegmental area (VTA) to evoke changes in mesocorticolimbic dopaminergic transmission. Among the known muscarinic receptor subtypes present in the VTA, the M2 receptor (M2R) is most implicated in autoregulation and also may play a heteroreceptor role in regulation of the output of the dopaminergic neurons. We sought to determine the functionally relevant sites for M2R activation in relation to VTA dopaminergic neurons by examining the electron microscopic immunolabeling of M2R and the dopamine transporter (DAT) in the VTA of rat brain. The M2R was localized to endomembranes in DAT-containing somatodendritic profiles but showed a more prominent, size-dependent plasmalemmal location in nondopaminergic dendrites. M2R also was located on the plasma membrane of morphologically heterogenous axon terminals contacting unlabeled as well as M2R- or DAT-labeled dendrites. Some of these terminals formed asymmetric synapses resembling those of cholinergic terminals in the VTA. The majority, however, formed symmetric, inhibitory-type synapses or were apposed without recognized junctions. Our results provide the first ultrastructural evidence that the M2R is expressed, but largely not available for local activation, on the plasma membrane of VTA dopaminergic neurons. Instead, the M2R in this region has a distribution suggesting more indirect regulation of mesocorticolimbic transmission through autoregulation of acetylcholine release and changes in the physiological activity or release of other, largely inhibitory transmitters. These findings could have implications for understanding the muscarinic control of cognitive and goal-directed behaviors within the VTA.

Activity-dependent dendritic spine structural plasticity is regulated by small GTPase Rap1 and its target AF-6

Activity-dependent remodeling of dendritic spines is essential for neural circuit development and synaptic plasticity, but the mechanisms that coordinate synaptic structural and functional plasticity are not well understood. Here we investigate the signaling pathways that enable excitatory synapses to undergo activity-dependent structural modifications. We report that activation of NMDA receptors in cultured cortical neurons induces spine morphogenesis and activation of the small GTPase Rap1. Rap1 bimodally regulates spine morphology: activated Rap1 recruits the PDZ domain-containing protein AF-6 to the plasma membrane and induces spine neck elongation, while inactive Rap1 dissociates AF-6 from the membrane and induces spine enlargement. Rap1 also regulates spine content of AMPA receptors: thin spines induced by Rap1 activation have reduced GluR1-containing AMPA receptor content, while large spines induced by Rap1 inactivation are rich in AMPA receptors. These results identify a signaling pathway that regulates activity-dependent synaptic structural plasticity and coordinates it with functional plasticity.

Expression and characterization of Rab38, a new member of the Rab small G protein family

Rab38 is a new member of the Rab small G protein family that regulates intracellular vesicle trafficking. Rab38 is expressed in melanocytes and it has been clarified that a point mutation in the postulated GTP-binding domain of Rab38 is the gene responsible for oculocutaneous albinism in chocolate mice. However, basic information regarding recombinant protein production, intracellular location, and tissue-specific expression pattern has not yet been reported. We produced recombinant Rab38 using a baculovirus/insect cell-protein expression system. A combination of Triton X-114 phase separation and nickel-affinity chromatography yielded exclusively prenylated Rab38 that bound [alpha-32P]-GTP. The mRNA and the native protein were expressed in a tissue-specific manner, e.g., in the lung, skin, stomach, liver, and kidney. Freshly isolated rat alveolar type II cells were highly positive for the mRNA signal, but the signal was rapidly lost over time. Immunofluorescence staining demonstrated that expressed GST-tagged Rab38 was mainly co-localized with endoplasmic reticulum-resident protein and also partly with intermittent vesicles between the endoplasmic reticulum and the Golgi complex. These results indicate that Rab38 is expressed non-ubiquitously in specific tissues and regulates early vesicle transport relating to the endoplasmic reticulum, and hence suggest that Rab38 abnormality may cause multiple organ diseases as well as oculocutaneous albinism.

GTP-binding proteins in intracellular transport

One of the most exciting recent discoveries in the area of intracellular protein transport is the finding that many organelles involved in exocytic and endocytic membrane traffic have one or more Ras-like GTP-binding proteins on their cytoplasmic face that are specific for each membranous compartment. These proteins are attractive candidates for regulators of transport vesicle formation and the accurate delivery of transport vesicles to their correct targets.

Nectin-dependent localization of ZO-1 at puncta adhaerentia junctions between the mossy fiber terminals and the dendrites of the pyramidal cells in the CA3 area of adult mouse hippocampus

Nectin and afadin constitute a novel intercellular adhesion system that organizes adherens junctions in cooperation with the cadherin-catenin system in epithelial cells. Nectin is a Ca(2+)-independent immunoglobulin-like adhesion molecule and afadin is an actin filament (F-actin)-binding protein that connects nectin to the actin cytoskeleton. At the puncta adhaerentia junctions (PAs) between the mossy fiber terminals and the dendrites of the pyramidal cells in the CA3 area of the adult mouse hippocampus, the nectin-afadin system also colocalizes with the cadherin-catenin system and has a role in the formation of synapses. ZO-1 is another F-actin-binding protein that localizes at tight junctions (TJs) and connects claudin to the actin cytoskeleton in epithelial cells. The nectin-afadin system is able to recruit ZO-1 to the nectin-based cell-cell adhesion sites in nonepithelial cells that have no TJs. In the present study, we investigated the localization of ZO-1 in the mouse hippocampus. Immunofluorescence and immunoelectron microscopy revealed that ZO-1 also localized at the PAs between the mossy fiber terminals and the dendrites of the pyramidal cells in the CA3 area of the adult mouse hippocampus, as described for afadin. ZO-1 colocalized with afadin during the development of synaptic junctions and PAs. Microbeads coated with the extracellular fragment of nectin, which interacts with cellular nectin, recruited both afadin and ZO-1 to the bead-cell contact sites in cultured rat hippocampal neurons. These results indicate that ZO-1 colocalizes with nectin and afadin at the PAs and that the nectin-afadin system is involved in the localization of ZO-1.

A novel rabconnectin-3-binding protein that directly binds a GDP/GTP exchange protein for Rab3A small G protein implicated in Ca(2+)-dependent exocytosis of neurotransmitter

BACKGROUND

Rab3A, a member of the Rab3 small G protein family, regulates Ca2+-dependent exocytosis of neurotransmitter. The cyclical activation and inactivation of Rab3A are essential for the Rab3A action in exocytosis. GDP-Rab3A is activated to GTP-Rab3A by Rab3 GDP/GTP exchange protein (Rab3 GEP) and GTP-Rab3A is inactivated to GDP-Rab3A by Rab3 GTPase-activating protein (Rab3 GAP). We have recently found a novel protein, named rabconnectin-3, which is co-immunoprecipitated with Rab3 GEP or GAP from the extract of the crude synaptic vesicle (CSV) fraction of rat brain. Rabconnectin-3 is abundantly expressed in the brain where it is associated with synaptic vesicles. We have found that two more proteins are co-immunoprecipitated with Rab3 GEP from the CSV fraction of rat brain. We attempted here to isolate and characterize one of them.

RESULTS

We determined its partial amino acid sequence, cloned its cDNA from a human cDNA library, and determined its primary structure. The protein consisted of 1490 amino acids (aa) and showed a calculated molecular weight of 163808. The protein had 7 WD domains. The protein was abundantly expressed in the brain where it co-localized with rabconnectin-3 on synaptic vesicles. The protein formed a stable complex with rabconnectin-3. We named this protein rabconnectin-3beta and renamed rabconnectin-3 rabconnectin-3alpha. Rabconnectin-3beta, but not rabconnectin-3alpha, directly bound Rab3 GEP. Neither rabconnectin-3alpha nor -3beta directly bound Rab3 GAP.

CONCLUSION

These results indicate that rabconnectin-3 consists of the alpha and beta subunits and binds directly Rab3 GEP through the beta subunit and indirectly Rab3 GAP through an unidentified molecule(s).

Ultrastructural localization of enkephalin and mu-opioid receptors in the rat ventral tegmental area

Enkephalins are endogenous ligands for opioid receptors whose activation potently modulates the output of mesocorticolimbic dopaminergic neurons within the ventral tegmental area. Many of the reinforcing effects of enkephalins in the mesocorticolimbic system are mediated by mu-opioid receptors. To determine the sites for Leu(5)-enkephalin activation of mu-opioid receptors in the ventral tegmental area, we examined the dual electron microscopic immunocytochemical localization of their respective antigens in this region of rat brain. Enkephalin immunoperoxidase reaction product and mu-opioid receptor immunogold-silver labeling showed similar cellular and subcellular distribution in both the paranigral and parabrachial subdivisions of the ventral tegmental area. Enkephalin immunoreactivity was mainly localized in small unmyelinated axons (50.4%) and in axon terminals (40.4%). The majority of these terminals formed symmetric, inhibitory-type synapses, many of which were on dendrites expressing plasmalemmal mu-opioid receptors. Appositional contacts were also often seen between axons or terminals that were differentially labeled for the two antigens. In addition, some of the enkephalin-labeled terminals and a few somatodendritic profiles showed a plasmalemmal or vesicular localization of mu-opioid receptors. Our results indicate that dendritic targets of inhibitory terminals, as well as nearby axon terminals, are potential sites for enkephalin activation of mu-opioid receptors throughout the ventral tegmental area. Moreover, co-localization of enkephalin and mu-opioid receptors in selective neuronal profiles may indicate an autoregulatory role for these receptors or their internalization along with the bound ligand in this brain region.

Rabconnectin-3, a novel protein that binds both GDP/GTP exchange protein and GTPase-activating protein for Rab3 small G protein family

Rab3A, a member of the Rab3 small G protein family, regulates Ca(2+)-dependent exocytosis of neurotransmitter. The cyclical activation and inactivation of Rab3A are essential for the Rab3A action in exocytosis. GDP-Rab3A is activated to GTP-Rab3A by Rab3 GDP/GTP exchange protein (Rab3 GEP), and GTP-Rab3A is inactivated to GDP-Rab3A by Rab3 GTPase-activating protein (Rab3 GAP). It remains unknown how or in which step of the multiple exocytosis steps these regulators are activated and inactivated. We isolated here a novel protein that was co-immunoprecipitated with Rab3 GEP and GAP by their respective antibodies from the crude synaptic vesicle fraction of rat brain. The protein, named rabconnectin-3, bound both Rab3 GEP and GAP. The cDNA of rabconnectin-3 was cloned from a human cDNA library and its primary structure was determined. Human rabconnectin-3 consisted of 3,036 amino acids and showed a calculated M(r) of 339,753. It had 12 WD domains. Tissue and subcellular distribution analyses in rat indicated that rabconnectin-3 was abundantly expressed in the brain where it was enriched in the synaptic vesicle fraction. Immunofluorescence and immunoelectron microscopy revealed that rabconnectin-3 was concentrated on synaptic vesicles at synapses. These results indicate that rabconnectin-3 serves as a scaffold molecule for both Rab3 GEP and GAP on synaptic vesicles.

Distribution of soluble N-ethylmaleimide fusion protein attachment proteins (SNAPs) in the rat nervous system

Soluble N-ethylmaleimide-sensitive fusion protein (NSF) attachment protein (SNAP) plays an essential role in vesicular transport and the release of neurotransmitters and hormones through associations with NSF and SNAP receptors (SNAREs). Three isoforms (alpha, beta and gamma) of SNAP are expressed in mammals. We have generated isoform-specific antibodies and studied the expression and distribution of these SNAP isoforms in the rat nervous system. Each antibody specifically recognized alpha-, beta- or gamma-SNAP in an isoform-specific manner in immunoblots of brain homogenate. Alpha- and gamma-SNAP were ubiquitously expressed in various tissues, whereas beta-SNAP was expressed only in brain. After subcellular fractionation of brain homogenates, all three isoforms were recovered in both soluble and particulate fractions. Immunohistochemistry revealed that alpha- and beta-SNAP were generally differentially distributed both in synaptic and non-synaptic regions, including brain white matter. The presynaptic location of both alpha- and beta-SNAP was confirmed by immunoelectron microscopy. At the neuromuscular junction, immunoreactive alpha-SNAP was identified in synaptic vesicles, while in the cerebellum, beta-SNAP was present in the presynaptic membranes of basket neuron and mossy fiber terminals. From these results we suggest that both alpha- and beta-SNAP may play an important role in neurotransmitter release as well as in constitutive vesicular transport.

Nectin: an adhesion molecule involved in formation of synapses

The nectin-afadin system is a novel cell-cell adhesion system that organizes adherens junctions cooperatively with the cadherin-catenin system in epithelial cells. Nectin is an immunoglobulin-like adhesion molecule, and afadin is an actin filament-binding protein that connects nectin to the actin cytoskeleton. Nectin has four isoforms (-1, -2, -3, and -4). Each nectin forms a homo-cis-dimer followed by formation of a homo-trans-dimer, but nectin-3 furthermore forms a hetero-trans-dimer with nectin-1 or -2, and the formation of each hetero-trans-dimer is stronger than that of each homo-trans-dimer. We show here that at the synapses between the mossy fiber terminals and dendrites of pyramidal cells in the CA3 area of adult mouse hippocampus, the nectin-afadin system colocalizes with the cadherin-catenin system, and nectin-1 and -3 asymmetrically localize at the pre- and postsynaptic sides of puncta adherentia junctions, respectively. During development, nectin-1 and -3 asymmetrically localize not only at puncta adherentia junctions but also at synaptic junctions. Inhibition of the nectin-based adhesion by an inhibitor of nectin-1 in cultured rat hippocampal neurons results in a decrease in synapse size and a concomitant increase in synapse number. These results indicate an important role of the nectin-afadin system in the formation of synapses.

Plasmalemmal mu-opioid receptor distribution mainly in nondopaminergic neurons in the rat ventral tegmental area

Opiate-evoked reward and motivated behaviors reflect, in part, the enhanced release of dopamine produced by activation of the mu-opioid receptor (muOR) in the ventral tegmental area (VTA). We examined the functional sites for muOR activation and potential interactions with dopaminergic neurons within the rat VTA by using electron microscopy for the immunocytochemical localization of antipeptide antisera raised against muOR and tyrosine hydroxylase (TH), the synthesizing enzyme for catecholamines. The cellular and subcellular distribution of muOR was remarkably similar in the two major VTA subdivisions, the paranigral (VTApn) and parabrachial (VTApb) nuclei. In each region, somatodendritic profiles comprised over 50% of the labeled structures. MuOR immunolabeling was often seen at extrasynaptic/perisynaptic sites on dendritic plasma membranes, and 10% of these dendrites contained TH. MuOR-immunoreactivity was also localized to plasma membranes of axon terminals and small unmyelinated axons, none of which contained TH. The muOR-immunoreactive axon terminals formed either symmetric or asymmetric synapses that are typically associated with inhibitory and excitatory amino acid transmitters. Their targets included unlabeled (30%), muOR-labeled (25%), and TH-labeled (45%) dendrites. Our results suggest that muOR agonists in the VTA affect dopaminergic transmission mainly indirectly through changes in the postsynaptic responsivity and/or presynaptic release from neurons containing other neurotransmitters. They also indicate, however, that muOR agonists directly affect a small population of dopaminergic neurons expressing muOR on their dendrites in VTA and/or terminals in target regions.

Role of Rab3 GDP/GTP exchange protein in synaptic vesicle trafficking at the mouse neuromuscular junction

The Rab3 small G protein family consists of four members, Rab3A, -3B, -3C, and -3D. Of these members, Rab3A regulates Ca(2+)-dependent neurotransmitter release. These small G proteins are activated by Rab3 GDP/GTP exchange protein (Rab3 GEP). To determine the function of Rab3 GEP during neurotransmitter release, we have knocked out Rab3 GEP in mice. Rab3 GEP-/- mice developed normally but died immediately after birth. Embryos at E18.5 showed no evoked action potentials of the diaphragm and gastrocnemius muscles in response to electrical stimulation of the phrenic and sciatic nerves, respectively. In contrast, axonal conduction of the spinal cord and the phrenic nerve was not impaired. Total numbers of synaptic vesicles, especially those docked at the presynaptic plasma membrane, were reduced at the neuromuscular junction approximately 10-fold compared with controls, whereas postsynaptic structures and functions appeared normal. Thus, Rab3 GEP is essential for neurotransmitter release and probably for formation and trafficking of the synaptic vesicles.

Rim1 and rabphilin-3 bind Rab3-GTP by composite determinants partially related through N-terminal alpha -helix motifs

Rim1 is a protein of the presynaptic active zone, the area of the plasma membrane specialized for neurotransmitter exocytosis, and interacts with Rab3, a small GTPase implicated in neurotransmitter vesicle dynamics. Here, we have studied the molecular determinants of Rim1 that are responsible for Rab3 binding, employing surface plasmon resonance and recombinant, bacterially expressed Rab3 and Rim1 proteins. A site that binds GTP- but not GDP-saturated Rab3 was localized to a short alpha-helical sequence near the Rim1 N terminus (amino acids 19-55). Rab3 isoforms A, C, and D were bound with similar affinities (K(d) = 1-2 microm). Low affinity binding of Rab6A-GTP was also observed (K(d) = 16 microm), whereas Rab1B, -5, -7, -8, or -11A did not bind. Adjacent sequences up to amino acid 387, encompassing differentially spliced sequences, the zinc finger module, and the SGAWFF motif of Rim1, did not significantly contribute to the strength or the specificity of Rab3 binding, whereas a point mutation within the helix (R33G) abolished binding. This Rab3 binding site of Rim1 is reminiscent of the N-terminal alpha-helix that is part of the Rab3-binding region of rabphilin-3, and indeed we observed low affinity, specific binding of Rab3A (K(d) on the order of magnitude of 10-100 microm) to this region of rabphilin-3 alone (amino acids 40-88), whereas additional sequences up to amino acid 178 are needed for high affinity Rab3A binding to rabphilin-3 (K(d) = 10-20 nm). In contrast, an N-terminal alpha-helix motif in aczonin, with sequence similarity to the Rab3-binding site of Rim1, did not bind Rab3A, -C, or -D or several other Rab proteins. These results were qualitatively confirmed in pull-down experiments with native, prenylated Rab3 from brain lysate in Triton X-100. Munc13 bound to the zinc finger domain of Rim1 but not to the rabphilin-3 or aczonin zinc fingers. Pull-down experiments from brain lysate in the presence of cholate as detergent detected binding to downstream Rim1 sequences, between amino acids 56 and 387, of syntaxin and of Rab3. The latter, however, was inhibited rather than stimulated by GTP.

Rab3a binding and secretion-enhancing domains in Rim1 are separate and unique. Studies in adrenal chromaffin cells

Rim1 was identified in brain by its ability to bind Rab3a-GTP and has been postulated to be a Rab3a effector protein. Like Rabphilin3, it modulates secretion and contains a zinc finger and two C2 domains. We have investigated the structural basis for the ability of Rim1 to bind Rab3a-GTP and to stimulate exocytosis in chromaffin cells. Both full-length and N-terminal Rim1 enhance secretion 40-50% in both intact and permeabilized cells. The abilities of Rim1 to enhance secretion and to bind Rab3a-GTP reside on distinct and relatively small domains that act independently. A approximately 30-amino acid sequence immediately N-terminal of the zinc finger constitutes the minimal Rab3a-GTP binding domain. This short sequence is not found in Rabphilin3 and is entirely different from the zinc finger and flanking regions of Rabphilin3 that bind Rab3a-GTP. The zinc finger domain in Rim1 is unnecessary for Rab3a-GTP binding but, alone, enhances secretion. An analysis of the characteristics of the enhancement of secretion in permeabilized chromaffin cells indicates that N-terminal Rim1 does not alter the sensitivity of secretion to Ca(2+) but, instead, increases the rate of ATP-dependent priming of secretion.

Small GTP-binding proteins

Small GTP-binding proteins (G proteins) exist in eukaryotes from yeast to human and constitute a superfamily consisting of more than 100 members. This superfamily is structurally classified into at least five families: the Ras, Rho, Rab, Sar1/Arf, and Ran families. They regulate a wide variety of cell functions as biological timers (biotimers) that initiate and terminate specific cell functions and determine the periods of time for the continuation of the specific cell functions. They furthermore play key roles in not only temporal but also spatial determination of specific cell functions. The Ras family regulates gene expression, the Rho family regulates cytoskeletal reorganization and gene expression, the Rab and Sar1/Arf families regulate vesicle trafficking, and the Ran family regulates nucleocytoplasmic transport and microtubule organization. Many upstream regulators and downstream effectors of small G proteins have been isolated, and their modes of activation and action have gradually been elucidated. Cascades and cross-talks of small G proteins have also been clarified. In this review, functions of small G proteins and their modes of activation and action are described.

Five geranylgeranyl diphosphate synthases expressed in different organs are localized into three subcellular compartments in Arabidopsis

Geranylgeranyl diphosphate (GGPP) is the precursor for the biosynthesis of gibberellins, carotenoids, chlorophylls, isoprenoid quinones, and geranylgeranylated proteins in plants. There is a small gene family for GGPP synthases encoding five isozymes and one related protein in Arabidopsis, and all homologs have a putative localization signal to translocate into specific subcellular compartments. Using a synthetic green fluorescent protein (sGFP), we studied the subcellular localization of these GGPP synthases. When these fusion proteins were expressed by the cauliflower mosaic virus 35S promoter in Arabidopsis, GGPS1-sGFP and GGPS3-sGFP proteins were translocated into the chloroplast, GGPS2-sGFP and GGPS4-sGFP proteins were localized in the endoplasmic reticulum, and the GGPS6-sGFP protein was localized in the mitochondria. Both GGPS1 and GGPS3 proteins synthesized in vitro were taken up into isolated intact pea chloroplasts and processed to the mature form. RNA-blot and promoter-beta-glucuronidase (GUS) analysis showed that these GGPP synthases genes are organ-specifically expressed in Arabidopsis. GGR and GGPS1 were ubiquitously expressed, while GGPS2, GGPS3, and GGPS4 were expressed specifically in the flower, root, and flower, respectively. These results suggest that each GGPP synthase gene is expressed in different tissues during plant development and GGPP is synthesized by the organelles themselves rather than being transported into the organelles. Therefore, we predict there will be specific pathways of GGPP production in each organelle.

Contributions of immunohistochemistry and in situ hybridization to the functional analysis of pituitary adenomas

Immunohistochemistry (IHC) and recently in situ hybridization (ISH) have elucidated various aspects of human pituitary adenomas, i.e., functional differentiation and classification, transcription factors and mechanism of hormone production, regulation of hormone secretion, and processing of prohormones. Recently, the use of tyramide (catalyzed signal amplification; TSA or CSA) and RT-PCR has been effective for detection of trivial amount of proteins (peptides) and mRNA, respectively. Immunomolecular histochemistry is expected to further clarify the function and biology of human pituitary adenomas.

Three isoforms of synaptic scaffolding molecule and their characterization. Multimerization between the isoforms and their interaction with N-methyl-D-aspartate receptors and SAP90/PSD-95-associated protein

The synaptic scaffolding molecule (S-SCAM) has been identified as a protein interacting with SAP90/PSD-95-associated protein (SAPAP) (also called guanylate kinase-associated protein/hDLG-associated protein). S-SCAM has six PDZ (we have numbered them PDZ-0 to -5), two WW, and one guanylate kinase (GK) domains and interacts with N-methyl-D-aspartate (NMDA) receptor via PDZ-5 and SAPAP via the GK domain. We have identified here shorter isoforms of S-SCAM that start at the 164th or 224th methionine, and we renamed the original one, S-SCAMalpha, the middle one, S-SCAMbeta, and the shortest one, S-SCAM-gamma. S-SCAMbeta and -gamma have five PDZ (PDZ-1 to -5), two WW, and one GK domains. S-SCAMalpha interacted with S-SCAMbeta and -gamma through the region containing PDZ-4 and -5. The region containing both of PDZ-4 and -5 is sufficient for the clustering of NMDA receptors and forms a dimer in gel filtration, suggesting that S-SCAM forms multimers via the interaction between the C-terminal PDZ domains and assembles NMDA receptors into clusters. S-SCAMbeta and -gamma also interacted with SAPAP, suggesting that the N-terminal region of the GK domain is not necessary for the interaction. Finally, we have identified the interaction of the PDZ domains of S-SCAM with the GK domain of PSD-95/SAP90. S-SCAM, PSD-95/SAP90, and SAPAP are colocalized at least in some part in brain. Therefore, S-SCAM, PSD-95/SAP90, and SAPAP may form a complex in vivo.

Expression and regulation of RAB3 proteins in osteoclasts and their precursors

The ruffled membrane, the resorptive organelle of the osteoclast, is generated by fusion of intracytoplasmic acidifying vesicles with the plasma membrane, an event analogous to regulated exocytosis. While the ruffled membrane is essential to the bone resorptive process, the mechanisms governing its generation are unknown. However, regulated exocytosis is mediated, in part, by isoforms of the Rab3 subset of Rab GTPases. Because of similarities between exocytosis and ruffled membrane formation, we asked if Rab3 proteins are expressed by osteoclasts or their precursors, and if so, are these molecules regulated by agents known to prompt the osteoclast phenotype? We find murine osteoclast precursors, in the form of bone marrow macrophages (BMMs), express at least two Rab3 isoforms, namely A and B/C, which are individually enhanced by a variety of hematopoietic cytokines. Consistent with the osteoclastogenic properties of a number of these cytokines, differentiation of BMMs into osteoclasts, in vitro, is associated with increased expression of both isoforms, particularly Rab3B/C. Finally, Rab3B/C localizes with the avian osteoclast H+ATPase (vacuolar proton pump) and pp60c-src, both intracellularly and within acidifying vesicles derived largely from the ruffled membrane. Thus, expression of specific rab3 proteins, an event which may control formation of the osteoclast ruffled membrane, is modulated by cytokines during osteoclastogenesis.

Localization of the delta-opioid receptor and dopamine transporter in the nucleus accumbens shell: implications for opiate and psychostimulant cross-sensitization

Opiate- and psychostimulant-induced modulation of dopamine transmission in the nucleus accumbens shell (AcbSh) is thought to play a key role in their potent reinforcing and locomotor effects. To investigate the cellular basis for potential functional interactions involving opiates active at the delta-opioid receptor (DOR) and psychostimulants that bind selectively to the dopamine transporter (DAT), we examined the electron microscopic localization of their respective antisera in rat AcbSh. DOR immunoperoxidase labeling was seen primarily, and DAT immunogold particles exclusively, in axon terminals. In these terminals, DOR immunoreactivity was prominently associated with discrete segments of the plasma membrane and the membranes of nearby small synaptic and large dense core vesicles. DAT immunogold particles were almost exclusively distributed along nonsynaptic axonal plasma membranes. Thirty-nine percent DOR-labeled profiles (221/566) either apposed DAT-immunoreactive terminals or also contained DAT. Of these 221 DOR-labeled profiles, 13% were axon terminals containing DAT and 15% were dendritic spines apposed to DAT-immunoreactive terminals. In contrast, 70% were morphologically heterogeneous axon terminals and small axons apposed to DAT-immunoreactive terminals. Our results indicate that DOR agonists in the AcbSh can directly modulate the release of dopamine, as well as postsynaptic responses in spiny neurons that receive dopaminergic input, but act principally to control the presynaptic secretion of other neurotransmitters whose release may influence or be influenced by extracellular dopamine. Thus, while opiates and psychostimulants mainly have differential sites of action, cross-sensitization of their addictive properties may occur through common neuronal targets.

Comparison of the effects on secretion in chromaffin and PC12 cells of Rab3 family members and mutants. Evidence that inhibitory effects are independent of direct interaction with Rabphilin3

The Rab class of low molecular weight GTPases has been implicated in the regulation of vesicular trafficking between membrane compartments in eukaryotic cells. The Rab3 family consisting of four highly homologous isoforms is associated with secretory granules and synaptic vesicles. Many different types of experiments indicate that Rab3a is a negative regulator of exocytosis and that its GTP-bound form interacts with Rabphilin3, a possible effector. Overexpression of Rabphilin3 in chromaffin cells enhances secretion. We have investigated the expression, localization, and effects on secretion of the various members of the Rab3 family in bovine chromaffin and PC12 cells. We found that Rab3a, Rab3b, Rab3c, and Rab3d are expressed to varying degrees in PC12 cells and in a fraction enriched in chromaffin granule membranes from the adrenal medulla. Immunocytochemistry revealed that all members of the family when overexpressed in PC12 cells localize to secretory granules. Binding constants for the interaction of the GTP-bound forms of Rab3a, Rab3b, Rab3c, and Rab3d with Rabphilin3 were comparable (Kd = 10-20 nM). Overexpression of each of the four members of the Rab3 family inhibited secretion. Mutations in Rab3a were identified that strongly impaired the ability of the GTP-bound form to interact with Rabphilin3. The mutated proteins inhibited secretion similarly to wild type Rab3a. Although Rab3a and Rabphilin3 are located on the same secretory granule or secretory vesicle and interact both in vitro and in situ, it is concluded that the inhibition of secretion by overexpression of Rab3a is unrelated to its ability to interact with Rabphilin3.

SNARE complex proteins, including the cognate pair VAMP-2 and syntaxin-4, are expressed in cultured oligodendrocytes

Myelin membrane synthesis in the CNS by oligodendrocytes (OLs) involves directed intracellular transport and targeting of copious amounts of specialized lipids and proteins over a relatively short time span. As in other plasma membrane-directed fusion, this process is expected to use specific trafficking and vesicle fusion proteins characteristic of the SNARE model. We have investigated the developmental expression of SNARE proteins in highly enriched primary cultures of OLs at discrete stages of differentiation. VAMP-2/synaptobrevin-2, syntaxin-2 and -4, nsec-1/munc-18-1, Rab3a, synaptophysin, and synapsin were expressed. During differentiation, expression of the vesicular SNARE VAMP-2, the small GTP-binding protein Rab3a, and the target SNARE syntaxin-4 were up-regulated. VAMP-2 and Rab3 proteins detected immunocytochemically in cultured OLs were localized within the developing process network; in situ anti-VAMP-2 antibody stained the perikarya of rows of cells with the distribution and appearance of OLs. We discuss the potential involvement of SNARE complex proteins in a plasma membrane-directed transport mechanism targeting nascent myelin vesicles to the forming myelin sheath.

Rab3D, a small GTPase, is localized on mast cell secretory granules and translocates to the plasma membrane upon exocytosis

Although mast cell secretion has been intensively studied because of its pivotal role in allergic reactions and its advantages as a physiologic model, the molecular composition of the secretory machine is virtually unknown. In view of the guanine-nucleotide dependency of mast cell exocytosis and the participation of Rab3 proteins in synaptic vesicle release, we hypothesized that a Rab3 isoform regulates mast cell secretion. Fragments of Rab3A, 3B, and 3D were cloned from RBL-2H3 mast cells by reverse transcription- polymerase chain reaction (RT-PCR). Northern blot analysis revealed Rab3D transcripts to be relatively abundant, Rab3B substantially less so, and Rab3A and 3C undetectable. By ribonuclease (RNase) protection assay, Rab3D transcripts were at least 10-fold more abundant than those of other isoforms, and by immunoblot analysis, Rab3D protein was at least 60-fold more abundant than that of Rab3B. Rab3D was more abundant in RBL cells than in brain, but the total mass of Rab3 proteins in RBL cells was 10-fold less than in brain. Rab3D only partly colocalized with secretory granules in RBL cells, but fully colocalized in mature peritoneal mast cells. There was a descending concentration gradient of Rab3D from peripheral to central granules, and no cytoplasmic pool was detectable in resting mast cells. Following exocytotic degranulation, Rab3D translocated to the plasma membrane and remained there for at least 15 min. These studies suggest that Rab3D is a component of the regulated exocytotic machine of mast cells, and identify differences between mast cells and neurons in Rab3 expression and trafficking.

Localization of the Rab3 small G protein regulators in nerve terminals and their involvement in Ca2+-dependent exocytosis

The Rab3 small G protein subfamily (Rab3) consists of four members, Rab3A, -B, -C, and -D. We have recently isolated and characterized the Rab3 regulators, GDP/GTP exchange protein (GEP) and GTPase activating protein (GAP), both of which are specific for the Rab3 subfamily. Rab3 GEP stimulates the conversion of the GDP-bound inactive form to the GTP-bound active form, whereas Rab3 GAP stimulates the reverse reaction. Of the four members of the Rab3 subfamily, evidence is accumulating that Rab3A is involved in Ca2+-dependent exocytosis, particularly in neurotransmitter release. We first analyzed the subcellular localization of Rab3 GEP and GAP in rat brain. Subcellular fractionation analysis showed that both Rab3 GEP and GAP were enriched in the synaptic soluble fraction. Immunocytochemical analysis in primary cultured rat hippocampal neurons showed that both Rab3 GEP and GAP were concentrated at the presynaptic nerve terminals. We then examined whether Rab3 GEP and GAP were involved in Ca2+-dependent exocytosis by use of human growth hormone (GH) co-expression assay system of cultured PC12 cells. Overexpression of the deletion mutant of Rab3 GEP possessing the catalytic activity reduced the high K+-induced GH release without affecting the basal GH release, whereas that of the deletion mutant lacking the catalytic activity showed no effect on the high K+-induced GH release. In contrast, overexpression of Rab3 GAP or its deletion mutant possessing the catalytic activity did not affect the high K+-induced GH release or the basal GH release. These results indicate that Rab3 GEP and GAP are colocalized with Rab3A at the synaptic release sites and suggest that they regulate the activity of Rab3A and are involved in Ca2+-dependent exocytosis.

Molecular cloning and characterization of the noncatalytic subunit of the Rab3 subfamily-specific GTPase-activating protein

We recently purified and characterized from rat brain a GTPase-activating protein (GAP) specific for the Rab3 small G protein subfamily implicated in Ca2+-dependent exocytosis. Rab3 GAP showed two bands with Mr of about 130,000 (p130) and 150,000 (p150) on SDS-polyacrylamide gel electrophoresis. p130, but not p150, showed the catalytic activity. Because p150 was likely the subunit of Rab3 GAP, here we cloned the cDNA of p150, determined its primary structure, and characterized it. The tissue and subcellular distribution patterns of p150 and p130 were similar, and both the proteins were enriched in the synaptic soluble fraction. p150 was co-immunoprecipitated with p130 from this fraction. Recombinant p150 formed a heterodimer with recombinant p130 as estimated by sucrose density gradient ultracentrifugation. Recombinant p150 neither showed the Rab3A GAP activity nor affected the activity of recombinant p130. When p150 and p130 were co-expressed in the cells, the subcellular localization of each protein did not change. These results indicate that p150 is the noncatalytic subunit of Rab3 GAP.

Tomosyn: a syntaxin-1-binding protein that forms a novel complex in the neurotransmitter release process

Syntaxin-1 is a component of the synaptic vesicle docking and/or membrane fusion soluble N-ethylmaleimide-sensitive factor attachment receptor (SNARE) complex (7S and 20S complexes) in nerve terminals. Syntaxin-1 also forms a heterodimer with Munc18/n-Sec1/rbSec1 in a complex that is distinct from the 7S and 20S complexes. In this report, we identify a novel syntaxin-1-binding protein, tomosyn, that is capable of dissociating Munc18 from syntaxin-1 and forming a novel 10S complex with syntaxin-1, soluble N-etyhlmaleimide-sensitive factor attachment (SNAP) 25, and synaptotagmin. The 130 kDa isoform of tomosyn is specifically expressed in brain, where its distribution partly overlaps with that of syntaxin-1 in nerve terminals. High level expression of either syntaxin-1 or tomosyn results in a specific reduction in Ca2+-dependent exocytosis from PC12 cells. These results suggest that tomosyn is an important component in the neurotransmitter release process where it may stimulate SNARE complex formation.

Neurabin-II/spinophilin. An actin filament-binding protein with one pdz domain localized at cadherin-based cell-cell adhesion sites

In a preceding paper, we reported a novel actin filament (F-actin)-binding protein, named neurabin, which was specifically expressed in neural tissue and implicated in neurite formation. We purified from rat brain another F-actin-binding protein, which had a domain organization similar to that of neurabin but was ubiquitously expressed, and named it neurabin-II. The original neurabin, renamed neurabin-I, had 1095 amino acids and a calculated Mr of 122,729, whereas neurabin-II had 817 amino acids and a calculated Mr of 89, 642. Both neurabin-I and -II had one F-actin-binding domain at the N-terminal region, one PDZ domain at the middle region, a domain known to interact with transmembrane proteins, and domains predicted to form coiled-coil structures at the C-terminal region. Both neurabin-I and -II bound along the sides of F-actin and showed F-actin-cross-linking activity. The subcellular distribution analysis indicated that neurabin-II was enriched at the postsynaptic density fraction in rat brain and the adherens junction fraction in rat liver. Immunofluorescence microscopic analysis revealed that neurabin-II was highly concentrated at the synapse in primary cultured rat hippocampal neurons and at the cadherin-based cell-cell adhesion sites in Madin-Darby canine kidney cells. Neurabin-II turned out to be the same as a recently reported protein phosphatase 1-binding protein named spinophilin. These results suggest that neurabin-II/spinophilin plays an important role in linking the actin cytoskeleton to the plasma membrane.

Small GTP-binding proteins in the brain-corpus cardiacum-corpus allatum complex of the silkworm, Bombyx mori: involvement in the secretion of prothoracicotropic hormone

At least three GTP-binding proteins (G-proteins), 28, 25, and 21 kDa, were found in the brain-corpus cardiacum-corpus allatum complex (BR-CC-CA) of the silkworm, Bombyx mori. They bound to GTP and GDP specifically among nucleotides tested, indicating that these proteins are small G-proteins. The 25 kDa G-protein showed a cross-reactivity to anti-rab3A antibody, while it did not cross-react with anti-rhoA, rab3B, and anti-ras antibodies. On the other hand, the 28 and 21 kDa G-proteins showed no cross-reactivity to any of those antibodies tested. Immunoblot analysis using the anti-rab3A antibody demonstrated that the 25 kDa G-protein was detected preferentially in the BR-CC-CA, and to some extent in the suboesophageal ganglion, but not in the salivary gland, fat body, prothoracic gland, and oesophagus. These results suggested that the 25 kDa G-protein was a member of the rab family of G-proteins. Furthermore, 1 mM GTP gamma S capable of activating G-proteins induced BR-CC-CA to release PTTH under the conditions that stimulation of the PTTH release with hetero-trimeric G-protein was suppressed. These results indicated that the small G-proteins may possibly contribute to PTTH release in Bombyx mori.

Role of Rab GTPases in membrane traffic

Small GTPases of the Rab subfamily have been known to be key regulators of intracellular membrane traffic since the late 1980s. Today this protein group amounts to more than 40 members in mammalian cells which localize to distinct membrane compartments and exert functions in different trafficking steps on the biosynthetic and endocytic pathways. Recent studies indicate that cycles of GTP binding and hydrolysis by the Rab proteins are linked to the recruitment of specific effector molecules on cellular membranes, which in turn impact on membrane docking/fusion processes. Different Rabs may, nevertheless, have slightly different principles of action. Studies performed in yeast suggest that connections between the Rabs and the SNARE machinery play a central role in membrane docking/fusion. Further elucidation of this linkage is required in order to fully understand the functional mechanisms of Rab GTPases in membrane traffic.

Rab proteins and post-Golgi trafficking of rhodopsin in photoreceptor cells

Polarized sorting of rhodopsin in retinal rod photoreceptors is mediated by post-Golgi vesicles that bud from the trans-Golgi network and fuse with the specialized domain of the plasma membrane in the rod inner segment. This domain surrounds the cilium that connects the inner segment and the rod outer segment to which mature rhodopsin is delivered. To dissect the sorting machinery that regulates budding, targeting, and fusion of rhodopsin carrier vesicles, their GTP-binding protein composition has been studied using multiple means including high-resolution two-dimensional gel electrophoresis and [32P]GTP overlays of renatured proteins. These studies indicate a succession on rhodopsin-bearing vesicles of rab6, rab11, rab3 and rab8, all members of the small GTP-binding protein family of the known regulators of membrane trafficking. In this review the role of rab proteins in post-Golgi trafficking of rhodopsin is discussed.

Neurabin: a novel neural tissue-specific actin filament-binding protein involved in neurite formation

We purified from rat brain a novel actin filament (F-actin)-binding protein of approximately 180 kD (p180), which was specifically expressed in neural tissue. We named p180 neurabin (neural tissue-specific F-actin- binding protein). We moreover cloned the cDNA of neurabin from a rat brain cDNA library and characterized native and recombinant proteins. Neurabin was a protein of 1,095 amino acids with a calculated molecular mass of 122,729. Neurabin had one F-actin-binding domain at the NH2-terminal region, one PSD-95, DlgA, ZO-1-like domain at the middle region, a domain known to interact with transmembrane proteins, and domains predicted to form coiled-coil structures at the COOH-terminal region. Neurabin bound along the sides of F-actin and showed F-actin-cross-linking activity. Immunofluorescence microscopic analysis revealed that neurabin was highly concentrated in the synapse of the developed neurons. Neurabin was also concentrated in the lamellipodia of the growth cone during the development of neurons. Moreover, a study on suppression of endogenous neurabin in primary cultured rat hippocampal neurons by treatment with an antisense oligonucleotide showed that neurabin was involved in the neurite formation. Neurabin is a candidate for key molecules in the synapse formation and function.

Isolation and characterization of a GTPase activating protein specific for the Rab3 subfamily of small G proteins

The Rab small G protein family, consisting of nearly 30 members, is implicated in intracellular vesicle trafficking. They cycle between the GDP-bound and GTP-bound forms, and the latter is converted to the former by the action of a GTPase activating protein (GAP). No GAP specific for each Rab family member or Rab subfamily has been isolated in mammal. Here we purified a GAP with Rab3A as a substrate from rat brain. The purified protein was specifically active on the Rab3 subfamily members (Rab3A, -B, -C, and -D). Of this subfamily, Rab3A and -C are implicated in Ca2+-dependent exocytosis, particularly in neurotransmitter release. This GAP, named Rab3 GAP, was active on the lipid-modified form, but not on the lipid-unmodified form. Rab3 GAP showed a minimum molecular mass of about 130 kDa on SDS-polyacrylamide gel electrophoresis. We cloned its cDNA from a human brain cDNA library, and the isolated cDNA encoded a protein with a Mr of 110,521 and 981 amino acids, which showed no homology to any known protein. The recombinant protein exhibited GAP activity toward the Rab3 subfamily members, and the catalytic domain was located at the C-terminal region. Northern blot analysis indicated that Rab3 GAP was ubiquitously expressed.

Isolation and characterization of a GDP/GTP exchange protein specific for the Rab3 subfamily small G proteins

The Rab small G protein family, consisting of nearly 30 members, is implicated in intracellular vesicle trafficking. They cycle between the GDP-bound inactive and GTP-bound active forms, and the former is converted to the latter by the action of a GDP/GTP exchange protein (GEP). No GEP specific for each Rab family member or Rab subfamily has been isolated. Here we purified a GEP from rat brain with lipid-modified Rab3A as a substrate. The purified protein was specifically active on Rab3A, Rab3C, and Rab3D of the Rab3 subfamily. Of these subfamily members, Rab3A and Rab3C are implicated in Ca2+-dependent exocytosis, particularly in neurotransmitter release. This GEP (Rab3 GEP) was active on the lipid-modified form, but not on the lipid-unmodified form. Rab3 GEP showed a minimum molecular mass of about 200 kDa on SDS-polyacrylamide gel electrophoresis. We cloned its cDNA from a rat brain cDNA library and determined its primary structure. The isolated cDNA encoded a protein with a Mr of 177,982 and 1,602 amino acids, which showed no homology to any known protein. The recombinant protein exhibited GEP activity toward Rab3A, Rab3C, and Rab3D. Northern blot and Western blot analyses indicated that Rab3 GEP was expressed in all the rat tissues examined with the highest expression in brain.

Physical and functional interaction of rabphilin-3A with alpha-actinin

Rabphilin-3A is a downstream target molecule of Rab3A small GTP-binding protein and implicated in Ca2+-dependent neurotransmitter release. Here we have isolated a rabphilin-3A-interacting molecule from a human brain cDNA library by the yeast two-hybrid method and identified it to be alpha-actinin, known to cross-link actin filaments into a bundle. alpha-Actinin interacts with the N-terminal region of rabphilin-3A, with which GTP-Rab3A interacts, and this interaction stimulates the activity of alpha-actinin to cross-link actin filaments into a bundle. The interaction of rabphilin-3A with alpha-actinin is inhibited by guanosine 5'-(3-O-thio)triphosphate-Rab3A. These results suggest that the Rab3A-rabphilin-3A system regulates the alpha-actinin-regulated reorganization of actin filaments. It has been shown that reorganization of actin filaments is also involved in Ca2+-dependent exocytosis. Therefore, rabphilin-3A may serve as a linker for Rab3A and cytoskeleton.

Differential expression of rab3 isoforms in oligodendrocytes and astrocytes

The small GTP-binding protein Rab3a is involved in regulated secretory pathways and is enriched in synaptic and neuroendocrine secretory vesicles. We have reported previously the developmental regulation of Rab3a in oligodendrocytes in culture and purified central nervous system myelin (Huber et al.: FEBS Lett 347: 273-278, 1994). Since multiple rab3 isoforms exist in the brain and may be associated with different secretory pathways, we have investigated the differential expression of the rab3 isoforms in oligodendrocytes, astrocytes, and Schwann cell line RT4-D6P2T. The expression of specific rab3 isoforms (rab3a-c) was detected by polymerase chain reaction (PCR) amplification and confirmed by sequence analyses. These data show that in addition to the previously reported expression in neurons, the two macroglial populations, astrocytes and oligodendrocytes, also express rab3 isoforms. Rab3b was preferentially amplified from purified, cultured astrocytes, while rab3a and rab3c were preferentially amplified from highly enriched populations of both cultured oligodendrocytes and those isolated directly from the brain by immunopanning. No novel rab3 isoform was detected in glia. These results indicate that glial cells in the brain express specific isoforms of the vesicular trafficking Rab3 protein family.

Vesicle-associated membrane protein 2 is essential for cAMP-regulated exocytosis in rat parotid acinar cells. The inhibition of cAMP-dependent amylase release by botulinum neurotoxin B

Amylase exocytosis of the parotid gland is mediated by intracellular cAMP. To investigate whether cAMP-dependent secretion has a mechanism similar to that of regulated neuroexocytosis, we examined the expression of synaptosome-associated proteins. In rat parotid acinar cells, we found 25 (p25) and 18 kDa (p18) proteins reacted with antibodies against Rab3A and vesicle-associated membrane protein 2 (VAMP-2), respectively. On the other hand, syntaxin 1 and SNAP-25, which interact with VAMP-2 at synapses, were undetectable. Rab3A-like p25 and VAMP-2-like p18 were also expressed in other exocrine acinar cells. The latter was localized at secretory granule membranes, and the former was detected in secretory granule and cytosolic fractions. The antibody against VAMP-2 used in this study did not react with cellubrevin, and p18 was cleaved with botulinum neurotoxin B. Thus, we identified p18 as VAMP-2. Botulinum neurotoxin B inhibited the cAMP-induced amylase release from streptolysin O-permeabilized acinar cells. Therefore, VAMP-2 is required for cAMP-regulated amylase release in rat parotid acinar cells. This is the first report that VAMP-2 is involved in regulated exocytosis that is independent of Ca2+.

Rho proteins are localized with different membrane compartments involved in vesicular trafficking in anterior pituitary cells

In order to explore the role of certain GTP binding proteins in the rat anterior pituitary, we have analyzed the subcellular distribution of the proteins rho and rab. They were found in both membrane and cytosolic fractions. Rab1 and rab2 were localized in both Golgi and endoplasmic reticulum (ER) membranes, while rab4 and rab6 were found in fractions enriched with Golgi and plasma membranes, implicating these proteins in the control of vesicular intracellular trafficking as described in other systems. Rab3 was localized like a fraction of synaptophysin, suggesting a role for rab3 in the targeting of "synaptic-like' microvesicles. We have identified three substrates of C. botulinum exoenzyme C3. A 26-kDa substrate with an isoelectric point (pI) of 5.2, probably rhoB, was localized in the lightest fractions such as rab3 and synaptophysin proteins. Two other 23-24 kDa substrates with pI of 5.5-5.8, probably rhoA and/or rhoC, were found in both fractions enriched with ER and secretory granules. Rho proteins have been implicated in the control of actin polymerization. Their localization in anterior pituitary suggests that rhoB could control the association of synaptic-like microvesicles and plasma membrane, and that rhoA/rhoC could play a role in secretory granule exocytosis; these two pathways being involved in cytoskeleton protein reorganisation in response to extracellular signals.

Two synpatic vesicle proteins of 25 kDa: a comparison of the molecular properties and tissue distribution of svp25 and o-rab3

Two synaptic vesicle proteins of the electric ray Torpedo--svp25 and o-rab3--are compared with respect to their biochemical properties and tissue distribution. On SDS-PAGE both proteins migrate to the same position of about 25 kDa. As revealed by application of monospecific antibodies and subcellular fractionation both proteins comigrate and cofractionate with the synaptic vesicle compartment. o-Rab3 and svp25 can be separated by lectin chromatography; svp25 is highly glycosylated and binds to concanavalin A sepharose. Upon deglycosylation using glycopeptidase F and O-glycosidase its apparent molecular mass is reduced to about 14 kDa. Partial amino acid sequences obtained by direct microsequencing of purified and deglycosylated svp25 revealed that svp25 is a novel protein that has not yet been characterized in molecular terms. Whereas svp25 was detected in all brain areas investigated, the expression of o-rab3 was found to be restricted to specific regions. An immunoblot analysis demonstrates an exclusive association of both proteins with neural tissues. Our results suggest that cholinergic synaptic vesicles from electric ray electric organ contain at least two membrane-associated proteins of an apparent molecular mass of 25 kDa, the membrane associated o-rab3 and the membrane integral protein svp25. The two proteins can be separated by lectin chromatography for assessment of their biochemical properties.

Possible involvement of Rab11 p24, a Ras-like small GTP-binding protein, in intracellular vesicular transport of isolated pancreatic acini

Rab11 p24 is a Ras-like small guanosine triphosphate (GTP)-binding protein, and specific antibodies against it were newly developed to explore its function. Using the antibody, Rab11 p24 was shown to be abundant in rat pancreas as well as in most rat tissues. To explore the involvement of Rab11 p24 into the exocytotic process, the subcellular distribution of Rab11 p24 in rat pancreatic acini was evaluated also by use of the antibody. When the isolated acini were incubated with 1 x 10(-10) M cholecystokinin octapeptide (CCK-8) that induced the maximal stimulation, the amount of Rab11 p24 increased in the fractions of plasma membrane and zymogen granules, but decreased in the cytosol fraction. This redistribution was time-dependent and occurred within 1 min after the CCK-8 stimulation and reached a maximal level within 2 min after the stimulation. Moreover, a light microscopic immunolabeling technique on the isolated rat pancreatic acini also revealed that higher immunoreactivity with Rab11 p24 was observed over the zymogen granule membrane under CCK-8 stimulation. The present results indicate that Rab11 p24 is translocated from cytosol to the membrane fraction during stimulation with CCK-8 and suggest that Rab11 p24 is involved in the intracellular vesicular transport of isolated acini.

Effect of guanine nucleotide binding on the intrinsic tryptophan fluorescence properties of Rab5

To gain further insight into structural elements involved in Rab5 function, differences in the intrinsic tryptophan fluorescence of the GDP- and guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S)-bound forms of the protein were examined. When excited at 290 nm, Rab5 displays emission maxima at 339.7 nm for the GDP-bound and 336.7 nm for the GTP gamma S-bound forms. The tryptophan fluorescence intensity is quenched by approximately 25% in the GTP gamma S-bound form relative to the GDP-bound conformation. Variant Rab5 molecules were created by site-directed mutagenesis to convert the protein's two tryptophans to phenylalanine residues. Fluorescence studies reveal that the observed changes upon GDP/GTP gamma S exchange are due to a blue shift in the emission spectra for both Trp74 (342.0 to 339.5 nm) and Trp114 (335.3 to 333.7 nm) and fluorescence quenching of Trp114. Consistent with the blue shift in the emission spectra, both tryptophans are more resistant to oxidation by N-bromosuccinimide in the GTP gamma S-bound state. These data indicate that both of Rab5's tryptophans are brought into a more sequestered, hydrophobic environment upon conformational changes promoted by guanine nucleotide exchange. Since Trp74 lies adjacent to Rab5's cognate switch II domain, local conformational changes would be predicted based on the known structure of Ras. However, Trp114 lies within a region of Rab5 potentially related to the switch III domain unique to heterotrimeric G alpha t. Thus, changes in the fluorescence properties of Trp114 upon guanine nucleotide exchange suggest that Rab proteins may have structure-function relationships similar to those described for heterotrimeric GTP-binding proteins.

Analysis of a family of ypt genes and their products from Chlamydomonas reinhardtii

Small G-proteins encoded by the ras-like ypt genes are ubiquitous in eukaryotic cells. They have been shown to play an essential role in membrane vesicle transport. We have isolated four ypt genes, yptC1, yptC4, yptC5 and yptC6, from Chlamydomonas reinhardtii (Cr) genomic and cDNA libraries. Three of them, yptC1, yptC4 and yptC5, are close homologues of ypt genes previously found in the multicellular alga Volvox carteri (Vc), the fourth, yptC6, is new. Each yptC gene is present as a single copy in the genome. Comparisons of genomic and cDNA sequences revealed that the coding regions are interrupted by five (yptC5), six (yptC6), seven (yptC4) and eight (yptC1) introns, respectively. Cr ypt genes and the closely related Vc ypt genes have identical exon-intron structures, but the corresponding intron sequences are completely different. Polyadenylation is signalled by UAUAA, UGUAG and UGUAA. The deduced amino acid (aa) sequence of YptC6 exhibited 79% identity with HRab2; YptC1, YptC4 and YptC5 exhibited over 90% identity with their Vc homologues. Primary structures of the 9-aa 'effector domain' and the contiguous 'helix3-loop7' motif (approx. 30 aa) are 'diagnostic' features for functional assignment. Recombinant YptC proteins, overproduced in Escherichia coli and purified to near homogeneity, displayed strong and specific binding of GTP, but not of GMP or ATP. The four Cr Ypt proteins showed immunochemical cross reactions to their Vc counterparts. Moreover, Western blots demonstrated at least six types of Ypt in both Cr and Vc, suggesting that these Ypt are used for household functions responsible for vesicle transport rather than for cellular differentiation.

Double in situ hybridization reveals overlapping neuronal populations expressing the low molecular weight GTPases Rab3a and Rab3b in Rat brain

The ras-related Rab3 gene subfamily codes for small GTP-binding proteins which control a late step of exocytosis during which vesicles become docked to the plasma membrane. Rab3a and Rab3b are the most abundant Rab3 isoforms expressed in the CNS of mammals. We have shown previously that the Rab3a protein was selectively distributed and expressed in various regions of the rat brain. Here we have determined the pattern of expression of Rab3b mRNA in the brain and compared it with that of Rab3a mRNA. In addition, we examined the co-expression of these two Rab within individual neurons. In general the Rab3b transcript was detected in many regions which also express Rab3a mRNA but at a lower level than Rab3a, except in the olfactory bulb and in the pituitary where the Rab3b hybridization signal was similar and higher respectively. Double in situ hybridization revealed that Rab3a and Rab3b mRNAs were co-localized in most neurons, in all brain areas examined. However, in each of these areas, subsets of neurons appeared to preferentially express either Rab3b or Rab3a, or some neurons did not express either Rab3 homologue at detectable levels. These data support the view of a functional specialization of Rab3a and Rab3b in the control of exocytosis in neuronal and neuroendocrine cells.