CircECE1 promotes osteosarcoma progression through regulating RAB3D by sponging miR-588

BACKGROUND

Circular RNAs (circRNAs) have been confirmed to be involved in cancer pathogenesis. However, the underlying mechanism of circRNA endothelin converting enzyme 1 (circECE1) in osteosarcoma (OS) development is still not understood.

METHODS

The expression levels of circECE1, microRNA-588 (miR-588) and RAB3D, member RAS oncogene family (RAB3D) were gauged by quantitative real-time polymerase chain reaction (qRT-PCR) and western blot. OS cell proliferation was assessed by cell counting kit-8 (CCK-8) assay and 5-ethynyl-2'-deoxyuridine (EdU) assay. OS cell apoptosis rate and metastasis were identified by flow cytometry and transwell assay. Dual-luciferase reporter analysis and RNA immunoprecipitation (RIP) assay were performed to confirm the interactions among circECE1, miR-588 and RAB3D. Xenograft tumor models were established to explore circECE1 function in vivo. Immunohistochemistry (IHC) assay was applied to analyze RAB3D level after circECE1 knockdown.

RESULTS

In OS, circECE1 expression was higher than that in normal chondroma tissues. High levels of circECE1 were positively linked to OS cell viability, proliferation, migration and invasion, and negatively linked to OS cell apoptosis rate. It was found that circECE1 was a miR-588 sponge, and miR-588 inhibitor abrogated the influence of si-circECE1 on OS cells. MiR-588 targeted RAB3D to further regulate the pathological process of OS. Moreover, silencing circECE1 blocked OS tumor growth in vivo.

CONCLUSION

We elucidated the function of a novel circECE1/miR-588/RAB3D axis in OS progression.

Bioinformatics analysis to screen the key genes in pediatric Chronic Active Epstein-Barr Virus Infection

Chronic active EBV infection (CAEBV) is associated with poor prognosis and high mortality. We performed bioinformatics analysis to screen out key genes associated with CAEBV. Weighted gene co-expression network analysis (WGCNA) was used to identify the gene module which was most correlated with pediatric CAEBV. Furthermore, the differentially expressed genes (DEGs) between pediatric acute infectious mononucleosis (AIM) and pediatric CAEBV were investigated. Least absolute shrinkage and selection operator (LASSO) and random forest then were performed to identify the key variables associated with pediatric CAEBV. We also explored the correlation between these hub genes with EBV infection related pathway and immune cell abundance. Compared with pediatric AIM, 1561 DEGs were up-regulated in pediatric CAEBV, and these genes were mainly enriched in inflammatory response and inflammation-related pathways. WGCNA analysis showed that genes in blue module were mostly related to pediatric CAEBV. Genes in the blue module and DEGs are intersected to get 174 genes and these genes are also enriched in inflammatory response-related pathways. The key CAEBV-related genes were selected from these 174 genes by applying the random Forest and LASSO algorithm, resulting in TPST1, TNFSF8 and RAB3GAP1. These three genes showed good diagnostic performance in distinguishing pediatric CAEBV from pediatric AIM. Furthermore, Cibersort and GSEA analysis indicated that these three genes were positively correlated with myeloid cell enrichment and persistent EBV infection pathway, respectively. Our finding systematically analyzed the difference between AIM and CAEBV and identified TPST1, TNFSF8 and RAB3GAP1 were the key genes in the development of CAEBV.

Novel, homozygous RAB3GAP1 c.2606 + 1G>A, p.Glu830ValfsTer9 variant and chromosome 3q29 duplication in a Turkish individual with Warburg micro syndrome

Warburg micro syndrome (WARBM) is a rare, autosomal recessive, neurodevelopmental disorder characterized by microcephaly, cortical dysplasia, corpus callosum hypoplasia, congenital hypotonia leading to subsequent spastic quadriplegia, severe developmental delay and hypogenitalism. Ophthalmologic findings that may affect any ocular segment including characteristic, small, atonic pupils. WARBM is known to be caused by biallelic, pathogenic variants in at least five genes although additional genetic loci may exist. The RAB3GAP1 c.748 + 1G>A, p.Asp250CysfsTer24 founder variant has been described in families of Turkish ancestry. We report the clinical and molecular findings in three, unrelated, Turkish families with WARBM. A novel c.974-2A>G variant causing WARBM in three siblings of Turkish descent was found. Functional studies of the novel, c.2606 + 1G>A variant in patients' mRNA revealed skipping of exon 22 which results in a premature stop codon in exon 23. However, the clinical consequences of this variant are blended given that the individual also had a maternally inherited chromosome 3q29 microduplication.

The spectrum of neurodevelopmental, neuromuscular and neurodegenerative disorders due to defective autophagy

Primary dysfunction of autophagy due to Mendelian defects affecting core components of the autophagy machinery or closely related proteins have recently emerged as an important cause of genetic disease. This novel group of human disorders may present throughout life and comprises severe early-onset neurodevelopmental and more common adult-onset neurodegenerative disorders. Early-onset (or congenital) disorders of autophagy often share a recognizable "clinical signature," including variable combinations of neurological, neuromuscular and multisystem manifestations. Structural CNS abnormalities, cerebellar involvement, spasticity and peripheral nerve pathology are prominent neurological features, indicating a specific vulnerability of certain neuronal populations to autophagic disturbance. A typically biphasic disease course of late-onset neurodegeneration occurring on the background of a neurodevelopmental disorder further supports a role of autophagy in both neuronal development and maintenance. Additionally, an associated myopathy has been characterized in several conditions. The differential diagnosis comprises a wide range of other multisystem disorders, including mitochondrial, glycogen and lysosomal storage disorders, as well as ciliopathies, glycosylation and vesicular trafficking defects. The clinical overlap between the congenital disorders of autophagy and these conditions reflects the multiple roles of the proteins and/or emerging molecular connections between the pathways implicated and suggests an exciting area for future research. Therapy development for congenital disorders of autophagy is still in its infancy but may result in the identification of molecules that target autophagy more specifically than currently available compounds. The close connection with adult-onset neurodegenerative disorders highlights the relevance of research into rare early-onset neurodevelopmental conditions for much more common, age-related human diseases.Abbreviations: AC: anterior commissure; AD: Alzheimer disease; ALR: autophagic lysosomal reformation; ALS: amyotrophic lateral sclerosis; AMBRA1: autophagy and beclin 1 regulator 1; AMPK: AMP-activated protein kinase; ASD: autism spectrum disorder; ATG: autophagy related; BIN1: bridging integrator 1; BPAN: beta-propeller protein associated neurodegeneration; CC: corpus callosum; CHMP2B: charged multivesicular body protein 2B; CHS: Chediak-Higashi syndrome; CMA: chaperone-mediated autophagy; CMT: Charcot-Marie-Tooth disease; CNM: centronuclear myopathy; CNS: central nervous system; DNM2: dynamin 2; DPR: dipeptide repeat protein; DVL3: disheveled segment polarity protein 3; EPG5: ectopic P-granules autophagy protein 5 homolog; ER: endoplasmic reticulum; ESCRT: homotypic fusion and protein sorting complex; FIG4: FIG4 phosphoinositide 5-phosphatase; FTD: frontotemporal dementia; GBA: glucocerebrosidase; GD: Gaucher disease; GRN: progranulin; GSD: glycogen storage disorder; HC: hippocampal commissure; HD: Huntington disease; HOPS: homotypic fusion and protein sorting complex; HSPP: hereditary spastic paraparesis; LAMP2A: lysosomal associated membrane protein 2A; MEAX: X-linked myopathy with excessive autophagy; mHTT: mutant huntingtin; MSS: Marinesco-Sjoegren syndrome; MTM1: myotubularin 1; MTOR: mechanistic target of rapamycin kinase; NBIA: neurodegeneration with brain iron accumulation; NCL: neuronal ceroid lipofuscinosis; NPC1: Niemann-Pick disease type 1; PD: Parkinson disease; PtdIns3P: phosphatidylinositol-3-phosphate; RAB3GAP1: RAB3 GTPase activating protein catalytic subunit 1; RAB3GAP2: RAB3 GTPase activating non-catalytic protein subunit 2; RB1: RB1-inducible coiled-coil protein 1; RHEB: ras homolog, mTORC1 binding; SCAR20: SNX14-related ataxia; SENDA: static encephalopathy of childhood with neurodegeneration in adulthood; SNX14: sorting nexin 14; SPG11: SPG11 vesicle trafficking associated, spatacsin; SQSTM1: sequestosome 1; TBC1D20: TBC1 domain family member 20; TECPR2: tectonin beta-propeller repeat containing 2; TSC1: TSC complex subunit 1; TSC2: TSC complex subunit 2; UBQLN2: ubiquilin 2; VCP: valosin-containing protein; VMA21: vacuolar ATPase assembly factor VMA21; WDFY3/ALFY: WD repeat and FYVE domain containing protein 3; WDR45: WD repeat domain 45; WDR47: WD repeat domain 47; WMS: Warburg Micro syndrome; XLMTM: X-linked myotubular myopathy; ZFYVE26: zinc finger FYVE-type containing 26.

Rab3a and SNARE proteins: potential regulators of melanosome movement

Melanosomes are specialized organelles that undergo a dynamic process of transport along the melanocyte dendrite to the dendrite tip and transfer to keratinocytes. We hypothesized that soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNARE), which are involved in membrane fusion, and rab3a, a GTP-binding protein involved in exocytosis in neuronal cells and in SNARE complex assembly, may play a part in melanosome transport and transfer. By reverse transcription-polymerase chain reaction we identified transcripts for rab3a, vesicle-associated membrane protein-2, synaptosome-associated proteins of 23 kDa and 25 kDa, and syntaxin-4 in murine melanocytic cells. We also showed that purified melanosome preparations contain rab3a and SNARE, including vesicle-associated membrane protein-2, syntaxin-4, synaptosome-associated proteins 23 kDa and 25 kDa, and the SNARE accessory protein, alpha-soluble N-ethylmaleimide-sensitive factor attachment protein. Ultraviolet radiation is a potent stimulus for melanosome transport and transfer. We show that ultraviolet radiation rapidly suppresses melanosome-associated rab3a expression and that this occurs at the protein and mRNA level. Finally, we show that vesicle-associated membrane protein-2 and synaptosome-associated protein 23 kDa coimmunoprecipitate from purified melanocytic cell membranes, suggesting that they form complexes. The presence of rab3a and SNARE on melanosomes, and of SNARE complexes in melanocytic cell membranes suggests that these proteins play a part in targeting melanosomes to the plasma membrane, to melanosome transfer to keratinocytes, or both.

RGS1 is expressed in monocytes and acts as a GTPase-activating protein for G-protein-coupled chemoattractant receptors

The leukocyte response to chemoattractants is transduced by the interaction of transmembrane receptors with GTP-binding regulatory proteins (G-proteins). RGS1 is a member of a protein family constituting a newly appreciated and large group of proteins that act as deactivators of G-protein signaling pathways by accelerating the GTPase activity of G-protein alpha subunits. We demonstrate here that RGS1 is expressed in human monocytes; by immunofluorescence and subcellular fractionation RGS1 was localized to the plasma membrane. By using a mixture of RGS1 and plasma membranes, we were able to demonstrate GAP activity of RGS1 on receptor-activated G-proteins; RGS1 did not affect ligand-stimulated GDP-GTP exchange. We found that RGS1 desensitizes a variety of chemotactic receptors including receptors for N-formyl-methionyl-leucyl-phenylalanine, leukotriene B4, and C5a. Interaction of RGS proteins and ligand-induced G-protein signaling can be demonstrated by determining GTPase activity using purified RGS proteins and plasma membranes.

Quantitative measurement of mast cell degranulation using a novel flow cytometric annexin-V binding assay

BACKGROUND

Mast cells are primary mediators of allergic inflammation. Antigen-mediated crosslinking of their cell surface immunoglobulin E (IgE) receptors results in degranulation and the release of proinflammatory mediators including histamine, tumor necrosis factor-alpha, and leukotrienes.

METHODS

Mast cells were stimulated to degranulate by using either IgE crosslinking or ionophore treatment. Exogenously added annexin-V was used to stain exocytosing granules, and the extent of binding was measured flow cytometrically. Release of the enzyme beta-hexosaminidase was used for population-based measurements of degranulation. Two known inhibitors of degranulation, the phosphatidylinositol 3 kinase inhibitor wortmannin and overexpression of a mutant rab3d protein, were used as controls to validate the annexin-V binding assay.

RESULTS

Annexin-V specifically bound to mast cell granules exposed after stimulation in proportion to the extent of degranulation. Annexin-V binding was calcium dependent and was blocked by phosphatidylserine containing liposomes, consistent with specific binding to this membrane lipid. Visualization of annexin-V staining showed granular cell surface patches that colocalized with the exocytic granule marker VAMP-green fluorescent protein (GFP). Wortmannin inhibited both annexin-V binding and beta-hexosaminidase release in RBL-2H3 cells, as did the expression of a dominant negative rab3d mutant protein.

CONCLUSIONS

The annexin-V binding assay represents a powerful new flow cytometric method to monitor mast cell degranulation for functional analysis.

Rabphilin knock-out mice reveal that rabphilin is not required for rab3 function in regulating neurotransmitter release

Rab3A and rab3C are GTP-binding proteins of synaptic vesicles that regulate vesicle exocytosis. Rabphilin is a candidate rab3 effector at the synapse because it binds to rab3s in a GTP-dependent manner, it is co-localized with rab3s on synaptic vesicles, and it dissociates with rab3s from the vesicles during exocytosis. Rabphilin contains two C(2) domains, which could function as Ca(2+) sensors in exocytosis and is phosphorylated as a function of stimulation. However, it is unknown what essential function, if any, rabphilin performs. One controversial question regards the respective roles of rab3s and rabphilin in localizing each other to synaptic vesicles: although rabphilin is mislocalized in rab3A knock-out mice, purified synaptic vesicles were shown to require rabphilin for binding of rab3A but not rab3A for binding of rabphilin. To test whether rabphilin is involved in localizing rab3s to synaptic vesicles and to explore the functions of rabphilin in regulating exocytosis, we have now analyzed knock-out mice for rabphilin. Mice that lack rabphilin are viable and fertile without obvious physiological impairments. In rabphilin-deficient mice, rab3A is targeted to synaptic vesicles normally, whereas in rab3A-deficient mice, rabphilin transport to synapses is impaired. These results show that rabphilin binds to vesicles via rab3s, consistent with an effector function of rabphilin for a synaptic rab3-signal. Surprisingly, however, no abnormalities in synaptic transmission or plasticity were observed in rabphilin-deficient mice; synaptic properties that are impaired in rab3A knock-out mice were unchanged in rabphilin knock-out mice. Our data thus demonstrate that rabphilin is endowed with the properties of a rab3 effector but is not essential for the regulatory functions of rab3 in synaptic transmission.

Expression and localization of Rab3D in rat parotid gland

Rab3 proteins (isoforms A, B, C and D) are low molecular weight GTP-binding proteins proposed to be involved in regulated exocytosis. In the present study, Rab3 protein expression and localization was examined in rat parotid gland by reverse transcription (rt) PCR, Western blotting and immunocytochemistry. An approximately 200 bp PCR product was obtained from parotid RNA by rtPCR and this fragment was cloned and sequenced. Nucleotide and deduced amino acid sequences obtained from five clones were identical to rab3D. Membrane and cytosolic fractions prepared from parotid acini were immunoblotted with antisera specific for each of the four Rab3 isoforms. A 28 kDa protein was detected with Rab3D-specific antisera in both fractions with staining being more intense in the membrane fraction. No other Rab3 isoforms were detected by immunoblotting, a result consistent with those obtained by rtPCR. Rab3D was enriched in zymogen granule membranes and Triton X-114 extraction revealed that this isoform is predominantly lipid-modified in parotid. Localization of Rab3D was done on frozen sections of parotid gland by immunofluorescence microscopy. Staining was observed primarily in the acinar cells and was adjacent to the acinar lumen. Incubation of dispersed acini with isoproterenol and substance P stimulated amylase secretion 4- and 2-fold above basal, respectively. Isoproterenol, but not substance P, induced redistribution of Rab3D from the cytosol to the membrane fraction in dispersed parotid acini. Consistent with these findings, isoproterenol injections into fasted rats also resulted in increased membrane-associated Rab3D in the parotid acini. These results indicate that Rab3D is: (1) the major Rab3 isoform expressed in rat parotid gland; (2) localized to zymogen granule membranes; and (3) involved with regulated enzyme secretion in acinar cells.

Identification of Rab3A GTPase as an acrosome-associated small GTP-binding protein in rat sperm

The acrosome reaction is a membrane fusion event that is prerequisite for sperm penetration through the zona pellucida. To elucidate the molecular mechanisms involved in membrane fusion, the expression and localization of Rab proteins, a subfamily of small GTPases that have been shown to play key roles in regulation of intracellular membrane traffic and exocytosis, were examined in rat testis and sperm. Reverse transcription polymerase chain reaction, immunoblot analysis, and immunofluorescence microscopy revealed that Rab3A protein, which is thought to be involved in regulation of exocytosis in neurons and endocrine cells, is associated with the sperm acrosome. The protein was undetectable in acrosome-free heads prepared by sucrose density gradient centrifugation. Immunogold electron microscopy performed on ultrathin cryosections provided further evidence that Rab3A protein is associated with the acrosomal membrane. Acrosome reaction assays revealed that synthetic peptide of the Rab3 effector domain inhibited acrosomal exocytosis triggered by calcium ionophore A23187 in a concentration-dependent fashion, suggesting that Rab3A acts as an inhibitory regulator in the acrosome reaction. In view of the putative role of Rab3A protein in membrane fusion systems, these results suggest that Rab3A could be involved in regulating the mammalian acrosome reaction by controlling the membrane fusion system in sperm.

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.

Differential involvement of small G proteins in Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disease characterized by the progressive deterioration of cognitive function and memory in association with the wide-spread presence of senile plaques, neurofibrillary tangles and neuronal cell death. However, its pathophysiology remains unknown. GTP-binding proteins with molecular weights of approximately 20,000 are designated small G proteins. In the present study we quantitatively analyzed the small G proteins, Ras, Rap, Ral and Rab in brains removed at autopsy from controls and AD patients to examine whether small G proteins are equally or differentially affected in AD. Western blot analysis indicated that the protein level of Ras and RalB in both the cytosolic and membranous fractions and that of Rap2 in the cytosolic fraction was significantly decreased, while that of Rab8 in the membranous fraction was significantly increased in AD brains compared with controls. The protein level of other small G proteins was not different between control and AD brains. These results suggest a differential involvement of small G proteins in AD.

Expression of Rab3, a Ras-related GTP-binding protein, in human nontumorous pituitaries and pituitary adenomas

Rab proteins are low molecular weight GTP-binding proteins. Among these proteins, the Rab3 isoforms are considered to be involved in the exocytosis of synaptic vesicles and secretory granules in the central nervous system and anterior pituitary gland. In recent reports, the expression of Rab3 isoforms in anterior pituitary glands of mammalian species was extensively investigated. In the present study, we investigated the localization of Rab3 protein in 5 human nontumorous pituitaries and 114 human pituitary adenomas using immunohistochemical methods. In five human nontumorous pituitaries, Rab3 protein was expressed in the cytoplasm of anterior pituitary cells. Double staining for anterior pituitary hormones revealed the expression of Rab3 in growth hormone-secreting cells, but rare expression was observed in the other anterior pituitary hormone-secreting cells. Among the pituitary adenomas, 71 (62.3%) of 114 pituitary adenomas were positive for Rab3. Among the different pituitary adenoma types, the incidence of Rab3 immunopositivity was highest in growth hormone-secreting adenomas (100%), followed by adrenocorticotropic hormone-secreting adenomas (71.4%), thyroid-stimulating hormone-secreting adenomas (57.1%), nonfunctioning adenomas (56.0%), and prolactin-secreting adenomas (33.3%). After an embedding immunoelectron microscopic study, Rab3 was localized along the limiting membrane of secretory granules in the Rab3-positive pituitary adenomas. Western blotting showed the molecular weight of Rab3 to be 25 kDa in the pituitary adenomas, which were immunohistochemically positive for Rab3 protein. These results suggested that Rab3 might be involved in regulating the exocytosis of secretory granules of the anterior pituitary cells, especially growth hormone-secreting ones, which are particularly characterized by densely granulated cytologic features.

Structural basis of activation and GTP hydrolysis in Rab proteins

BACKGROUND

Rab proteins comprise a large family of GTPases that regulate vesicle trafficking. Despite conservation of critical residues involved in nucleotide binding and hydrolysis, Rab proteins exhibit low sequence identity with other GTPases, and the structural basis for Rab function remains poorly characterized.

RESULTS

The 2. 0 A crystal structure of GppNHp-bound Rab3A reveals the structural determinants that stabilize the active conformation and regulate GTPase activity. The active conformation is stabilized by extensive hydrophobic contacts between the switch I and switch II regions. Serine residues in the phosphate-binding loop (P loop) and switch I region mediate unexpected interactions with the gamma phosphate of GTP that have not been observed in previous GTPase structures. Residues implicated in the interaction with effectors and regulatory factors map to a common face of the protein. The electrostatic potential at the surface of Rab3A indicates a non-uniform distribution of charged and nonpolar residues.

CONCLUSIONS

The major structural determinants of the active conformation involve residues that are conserved throughout the Rab family, indicating a common mode of activation. Novel interactions with the gamma phosphate impose stereochemical constraints on the mechanism of GTP hydrolysis and provide a structural explanation for the large variation of GTPase activity within the Rab family. An asymmetric distribution of charged and nonpolar residues suggests a plausible orientation with respect to vesicle membranes, positioning predominantly hydrophobic surfaces for interaction with membrane-associated effectors and regulatory factors. Thus, the structure of Rab3A establishes a framework for understanding the molecular mechanisms underlying the function of Rab GTPases.

Distribution of synaptic vesicle proteins in the mammalian retina identifies obligatory and facultative components of ribbon synapses

The mammalian retina contains two synaptic layers. The outer plexiform layer (OPL) is primarily composed of ribbon synapses while the inner plexiform layer (IPL) comprises largely conventional synapses. In presynaptic terminals of ribbon synapses, electron-dense projections called ribbons are present at the synaptic plasma membranes. Ribbons bind synaptic vesicles and guide them to the synaptic membrane for fusion. In this manner, ribbons are thought to accelerate the delivery of vesicles for continuous exocytosis. In recent years, a large number of synaptic proteins has been described but it is not known if these protein colocalize in the same types of synapses. In previous studies, several proteins essential for synaptic function were not detected in ribbon synapses, suggesting that the mechanism of synaptic vesicle exocytosis may be very different in ribbon and conventional synapses. Using confocal laser scanning microscopy, we have now systematically investigated the protein composition of ribbon synapses. Our results show that, of the 19 synaptic proteins investigated, all except synapsin and rabphilin are obligatorily present in ribbon synapses. For example, rab3 which was reported to be absent from ribbon synapses, was found in bovine, rat and mouse ribbon synapses using multiple independent antibodies. In addition, we found staining in these synapses for PSD-95 and NMDA receptors, which suggested a similar design for the postsynaptic component in ribbon and conventional synapses. Our data show that ribbon synapses are more conventional in composition than reported, that most synaptic proteins are colocalized to the same type of synapse, and that synapsin and rabphilin are likely to be dispensible for basic synaptic functions.

Cytosolic RAB3D is associated with RAB escort protein (REP), not RAB-GDP dissociation inhibitor (GDI), in dispersed chief cells from guinea pig stomach

Rab3D, a low-molecular-weight GTP-binding protein believed to be involved with regulated exocytosis, is associated with secretory granules in gastric chief cells. Although Rab3D is predominantly membrane associated, a significant fraction is cytosolic. Rab proteins are geranylgeranylated on their C-terminal cysteine motifs by geranylgeranyltransferase (GGTase). Rab escort protein (REP) is required to present Rab proteins to GGTase and may accompany newly modified Rab proteins to their target membrane. In most tissues, cytosolic Rab proteins are complexed with rab-GDP dissociation inhibitor (rab-GDI). In the present study, we examined the interactions of Rab3D with cytosolic proteins in dispersed chief cells. Two REP isoforms and at least two GDI isoforms are present in chief cell and brain cytosol. When chief cell cytosol was fractionated by gel filtration chromatography, Rab3D eluted with REP at >150 kDa, whereas rab-GDI eluted as a separate 65-kDa peak, suggesting that Rab3D exists as a complex with REP, but not with rab-GDI. In addition, a small fraction of Rab3D eluted as a monomer at 29 kDa. As has been demonstrated previously, in brain cytosol, Rab3 proteins co-elute with rab-GDI at approx. 90 kDa, suggesting that Rab3 proteins undergo active cycling between membrane and cytosolic compartments in this tissue. In vitro experiments revealed that Rab3D remains associated with REP after geranylgeranylation. Our findings suggest that, in gastric chief cells, Rab3D remains associated with REP after geranylgeranylation until it is presented to its target membrane.

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.

Immunohistochemical demonstration of exocytosis-regulating proteins within rat molar dentinal tubules

No morphologically defined synaptic structures have so far been detected between nerve terminals and the dentine-producing odontoblasts. Recent studies of the molecular mechanisms in neuronal exocytosis have identified several proteins that participate in synaptic-vesicle exocytosis. By localizing these proteins with immunohistochemical methods, information about the capacity for synaptic exocytosis should be obtained. Here, antibodies directed against some of the exocytosis-related proteins were used to investigate whether they are present in nerve fibers within the dentinal tubules in rat molars. Antibodies against synaptosome-associated protein of 25 kDa, Rab 3, synaptotagmin and synapsin all produced a punctuate staining pattern, suggesting that the proteins are accumulated in bouton-like elements. The results demonstrate that a set of exocytosis-related proteins is accumulated in the dentinal tubules, most probably within the intradentinal nerves. This finding is consistent with the hypothesis that intradentinal nerves can mediate efferent signals.

Identification of synaptic vesicle, pre- and postsynaptic proteins in human cerebrospinal fluid using liquid-phase isoelectric focusing

Synaptic pathology is central in the pathogenesis of several psychiatric disorders, for example in Alzheimer's disease (AD) and schizophrenia. Quantification of specific synaptic proteins has proved to be a useful method to estimate synapitc density in the brain. Using this approach, several synaptic proteins have been demonstrated to be altered in both AD and schizophrenia. Until recently, the analysis of synaptic pathology has been limited to postmortem tissue. In living subjects, these synaptic proteins may be studied through analysis of cerebrospinal fluid (CSF). In an earlier study performed by us, one synaptic vesicle specific protein, synaptotagmin, was detected in CSF for the first time using a procedure based on affinity chromatography, reversed-phase chromatography, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and chemiluminescence immunoblotting. However, other synaptic proteins were not detectable with this procedure. Therefore, we have developed a procedure including precipitation of CSF proteins with trichloroacetic acid, followed by liquid-phase isoelectric focusing using the Rotofor Cell, and finally analysis of Rotofor fractions by Western blotting for identification of synaptic proteins in CSF. Five synaptic proteins, rab3a, synaptotagmin, growth-associated protein (GAP-43), synaptosomal-associated protein (SNAP-25) and neurogranin, have been demonstrated in CSF using this method. The major advantage of liquid-phase isoelectric focusing (IEF) using the Rotofor cell is that it provides synaptic proteins from CSF in sufficient quantities for identification. This method may also be suitable for identification of other types of trace amounts of brain-specific proteins in CSF. These results demonstrate that several synaptic proteins can be identified and measured in CSF to study synaptic function and pathology in degenerative disorders.

Rab3 is present on endosomes from bovine chromaffin cells in primary culture

Rab3a, a small GTP-binding protein, is believed to mediate Ca2+-dependent exocytosis. Consistent with such a role was the previously reported specific association of Rab3a with synaptic vesicles in neurons and secretory granules in adrenal chromaffin cells. Secretory vesicles are believed to be the final point of Rab3a membrane association, as it was shown by several groups that Rab3a dissociates from the secretory vesicle membrane during stimulated exocytosis. In chromaffin cells, Rab3a is not exclusively localized on secretory granules since a fraction is present on a previously unidentified subcellular compartment equilibrating at light sucrose density. This ‘light’ membraneous structure could be the starting point for reassociation of Rab3a with membranes involved in granule formation, or it could be a structure unrelated to granules. The present study used several subcellular fractionation techniques and immunomicroscopy to unravel the nature of the ‘light’ Rab3a-containing structures from bovine chromaffin cells in primary culture. After stimulation, amounts of both Rab3a-d and the granule marker dopamine-beta-hydroxylase (DbetaH) increase transiently in sucrose gradient fractions enriched in endosomal markers. A diaminobenzidine-induced density shift of endosomes alters the distribution of DbetaH and Rab3a-d. At the ultrastructural level, subplasmalemmal pleiomorphic organelles were detected by Rab3a-d-immunogold labelling. Taken together our data provide for the first time evidence that internalised secretory granule membranes go through an endosomal stage where Rab3a is present, resembling the neuronal synaptic vesicle cycle. This indicates that the endosome is an important trafficking route in the biogenesis/recycling of secretory vesicles in chromaffin cells, in which Rab3a could have an as yet unknown regulatory function, and could point to the existence of alternative recycling pathways for the chromaffin granule membrane.

Cloning and localization of Rab3 isoforms in bovine, rat, and human parathyroid glands

Rab3 proteins are small GTP-binding proteins known to play a role in regulated exocytosis processes. This study examines the expression of Rab3 mRNA and protein in bovine, rat and human parathyroid glands. mRNAs of several Rab3 isoforms were detected in bovine (Rab3A, Rab3B and Rab3C) and rat (Rab3A, Rab3B and Rab3D) parathyroid glands by RT-PCR and sequencing. Rab3A protein was detected in the cytosolic extract from bovine parathyroid gland by Western blotting using a monoclonal antibody for Rab3A. Rab3A protein was localized to parathyroid hormone-containing chief cells by immunohistochemical staining. Subcellular localization of Rab3A protein by immunogold electron microscopy revealed that the majority of Rab3A protein was not associated with dense-core vesicles, but localized in the cytosol of the chief cells. Altogether, our results demonstrate that Rab3 isoforms are expressed in parathyroid chief cells, suggesting that they may play a role in regulated exocytosis in these cells.

Molecular cloning of cDNA encoding human Rab3D whose expression is upregulated with myeloid differentiation

To identify genes expressed in myeloid differentiation, we isolated a cDNA fragment by differential display using RNA prepared from HT93A cells, a human cell line capable of differentiating into neutrophil and eosinophil lineages in response to retinoic acid (RA). Evaluation of the full-length clone isolated from an HT93A cDNA library showed that it encoded a 24 kDa protein comprised of several domains conserved in the Ras superfamily. Comparison of the deduced amino acid sequence of this clone with Rab proteins revealed that it had highest homology to a small GTP-binding protein, murine Rab3D. The mRNA expression of human Rab3D was upregulated in the course of myeloid differentiation, and it was preferentially expressed in granulocytes. These results suggest that human Rab3D may play a specific role in granulocytes, for example in exocytosis of neutrophil-specific granules or in degranulation of both eosinophils and basophils.

Structural basis of Rab effector specificity: crystal structure of the small G protein Rab3A complexed with the effector domain of rabphilin-3A

The small G protein Rab3A plays an important role in the regulation of neurotransmitter release. The crystal structure of activated Rab3A/GTP/Mg2+ bound to the effector domain of rabphilin-3A was solved to 2.6 A resolution. Rabphilin-3A contacts Rab3A in two distinct areas. The first interface involves the Rab3A switch I and switch II regions, which are sensitive to the nucleotide-binding state of Rab3A. The second interface consists of a deep pocket in Rab3A that interacts with a SGAWFF structural element of rabphilin-3A. Sequence and structure analysis, and biochemical data suggest that this pocket, or Rab complementarity-determining region (RabCDR), establishes a specific interaction between each Rab protein and its effectors. RabCDRs could be major determinants of effector specificity during vesicle trafficking and fusion.

Subcellular distribution and function of Rab3A, B, C, and D isoforms in insulin-secreting cells

Insulin-secreting cells express four GTPases of the Rab3 family. After separation of extracts of INS-1 cells on a sucrose density gradient, the bulk of the A, B, and C isoforms was recovered in the fractions enriched in insulin-containing secretory granules. Rab3D was also mainly associated with secretory granules, but a fraction of this isoform was localized on lighter organelles. Analyses by confocal microscopy of immunostained HIT-T15 cells transfected with epitope-tagged constructs confirmed the distribution of the Rab3 isoforms. Transfection of HIT-T15 cells with GTPase-deficient mutants of the Rab3 isoforms decreased nutrient-induced insulin release to different degrees (D>B>A>>C), while overexpression of Rab3 wild types had minor or no effects. Expression of the same Rab3 mutants in PC12 cells provoked an inhibition of K+-stimulated secretion of dense core vesicles, indicating that, in beta-cells and neuroendocrine cells, the four Rab3 isoforms play a similar role in exocytosis. A Rab3A/C chimera in which the carboxyterminal domain of A was replaced with the corresponding region of C inhibited insulin secretion as Rab3A. In contrast, a Rab3C/A chimera containing the amino-terminal domain of C was less potent and reduced exocytosis as Rab3C. This suggests that the degree of inhibition obtained after transfection of the Rab3 isoforms is determined by differences in the variable amino-terminal region.

Characterization of beta cells developed in vitro from rat embryonic pancreatic epithelium

The present study evaluates the development and functional properties of beta cells differentiated in vitro. The authors have previously demonstrated that when E12.5 rat pancreatic rudiments are cultured in vitro in the absence of mesenchyme, the majority of the epithelial cells differentiate into endocrine beta cells. Thus, depletion of the mesenchyme provokes the expansion of endocrine tissue at the expense of exocrine tissue. The potential use of this procedure for the production of beta cells led the authors to characterize the beta cells differentiated in this model and to compare their properties with those of the endocrine cells of the embryonic and adult pancreas. This study shows that the beta cells that differentiate in vitro in the absence of mesenchyme express the homeodomain protein Nkx6.1, a transcription factor that is characteristic of adult mature beta cells. Further, electron microscopy analysis shows that these beta cells are highly granulated, and the ultrastructural analysis of the granules shows that they are characteristic of mature beta cells. The maturity of these granules was confirmed by a double-immunofluorescence study that demonstrated that Rab3A and SNAP-25, two proteins associated with the secretory pathway of insulin, are strongly expressed. Finally, the maturity of the differentiated beta cells in this model was confirmed when the cells responded to stimulation with 16 mM glucose by a 5-fold increase in insulin release. The authors conclude that the beta cells differentiated in vitro from rat embryonic pancreatic rudiments devoid of mesenchyme are mature beta cells.

Co-localization of Rab3B and oxytocin to electron dense granules of the sheep corpus luteum during the estrous cycle

Oxytocin and its carrier protein, neurophysin, are both associated with luteal secretory granules which migrate from the paranuclear region to the cell membrane where exocytosis takes place. Rab3 proteins are thought to be associated with membrane vesicles or granules undergoing exocytotic fusion with the plasma membrane. The objective of this study was to determine whether Rab3B is co-localized with oxytocin within the same secretory granules of large luteal cells obtained from corpora lutea of 16 Merino cross ewes at day 3, 7, 12 or 15 of the estrous cycle using immunocytochemistry. The mean granule density (granules/microm3) was not significantly different (P > 0.05) between the days examined. Electron microscopic immunocytochemistry showed that oxytocin and Rab3B were co-localized to the secretory granules on all days evaluated. Rab3B immunostaining was primarily located within secretory granules scattered throughout the cytoplasm. The mean intensity of labelling (number of gold particles) for oxytocin per microm2 cytoplasmic luteal tissue was significantly decreased on day 15 compared to those observed on days 3, 7 and 12 of estrous cycle. No significant changes were observed in the mean intensity of the Rab3B label at the different times of the cycle. The present study provides evidence that a member of the subfamily of Rab proteins, Rab3B, is present and co-localized with oxytocin in the same secretory granules of the ovine corpus luteum. These results implicate Rab3B protein directly or indirectly in the hormone secretory pathway of ovarian tissue.

Insulin promotes phosphorylation and activation of geranylgeranyltransferase II. Studies with geranylgeranylation of rab-3 and rab-4

Rab proteins play a crucial role in the trafficking of intracellular vesicles. Rab proteins are GTPases that cycle between an inactive GDP-bound form and an active GTP-bound conformation. A prerequisite to Rab activation by GTP loading is its post-translational modification by the addition of geranylgeranyl moieties to highly conserved C-terminal cysteine residues. We examined the effect of insulin on the activity of geranylgeranyltransferase II (GGTase II) in 3T3-L1 fibroblasts and adipocytes. In fibroblasts, insulin increased the enzymatic activity of GGTase II 2.5-fold after 1 h of incubation, an effect that is blocked by perillyl alcohol, an inhibitor of prenyltransferases, but not by the geranylgeranyltransferase I inhibitor, GGTI-298, or the farnesyltransferase inhibitor, alpha-hydroxyfarnesylphosphonic acid. Concomitantly, insulin stimulated the phosphorylation of the GGTase II alpha-subunit without any effect on the GGTase II beta-subunit. At the same time, insulin also increased the amounts of geranylgeranylated Rab-3 in 3T3-L1 fibroblasts from 44 +/- 1.2% in control cells to 63 +/- 3.8 and 64 +/- 6.1% after 1 and 24 h of incubation, respectively. In adipocytes, insulin increased the amounts of geranylgeranylated Rab-4 from 38 +/- 0.6% in control cells to 56 +/- 1.7 and 60 +/- 2.6% after 1 and 24 h of incubation, respectively. In both fibroblasts and adipocytes, the presence of perillyl alcohol blocked the ability of insulin to increase geranylgeranylation of Rab-4, whereas GGTI-298 and alpha-hydroxyfarnesylphosphonic acid were without effect, indicating that insulin activates GGTase II. In summary, insulin promotes phosphorylation and activation of GGTase II in both 3T3 L1 fibroblasts and adipocytes and increases the amounts of geranylgeranylated Rab-3 and Rab-4 proteins.

Localization and subcellular distribution of N-copine in mouse brain

N-Copine is a novel protein with two C2 domains. Its expression is brain specific and up-regulated by neuronal activity such as kainate stimulation and tetanus stimulation evoking hippocampal CA1 long-term potentiation. We examined the localization and subcellular distribution of N-copine in mouse brain. In situ hybridization analysis showed that N-copine mRNA was expressed exclusively in neurons of the hippocampus and in the main and accessory olfactory bulb, where various forms of synaptic plasticity and memory formation are known to occur. In immunohistochemical analyses, N-copine was detected mainly in the cell bodies and dendrites in the neurons, whereas presynaptic proteins such as synaptotagmin I and rab3A were detected in the regions where axons pass through. In fractionation experiments of brain homogenate, N-copine was associated with the membrane fraction in the presence of Ca2+ but not in its absence. As a GST-fusion protein with the second C2 domain of N-copine showed Ca2+-dependent binding to phosphatidylserine, this domain was considered to be responsible for the Ca2+-dependent association of N-copine with the membrane. Thus, N-copine may have a role as a Ca2+ sensor in postsynaptic events, in contrast to the known roles of "double C2 domain-containing proteins," including synaptotagmin I, in presynaptic events.

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.

Retinal targets for calmodulin include proteins implicated in synaptic transmission

Ca2+ influxes regulate multiple events in photoreceptor cells including phototransduction and synaptic transmission. An important Ca2+ sensor in Drosophila vision appears to be calmodulin since a reduction in levels of retinal calmodulin causes defects in adaptation and termination of the photoresponse. These functions of calmodulin appear to be mediated, at least in part, by four previously identified calmodulin-binding proteins: the TRP and TRPL ion channels, NINAC and INAD. To identify additional calmodulin-binding proteins that may function in phototransduction and/or synaptic transmission, we conducted a screen for retinal calmodulin-binding proteins. We found eight additional calmodulin-binding proteins that were expressed in the Drosophila retina. These included six targets that were related to proteins implicated in synaptic transmission. Among these six were a homolog of the diacylglycerol-binding protein, UNC13, and a protein, CRAG, related to Rab3 GTPase exchange proteins. Two other calmodulin-binding proteins included Pollux, a protein with similarity to a portion of a yeast Rab GTPase activating protein, and Calossin, an enormous protein of unknown function conserved throughout animal phylogeny. Thus, it appears that calmodulin functions as a Ca2+ sensor for a broad diversity of retinal proteins, some of which are implicated in synaptic transmission.

rab3 mediates cortical granule exocytosis in the sea urchin egg

Egg activation at fertilization in the sea urchin results in the exocytosis of approximately 15,000 cortical granules that are docked at the plasma membrane. Previously, we reported that several integral membrane proteins modeled in the SNARE hypothesis, synaptotagmin, VAMP, and syntaxin, in addition to a small GTPase of the ras superfamily, rab3, were present on cortical granules (Conner, S., Leaf, D., and Wessel, G., Mol. Reprod. Dev. 48, 1-13, 1997). Here we report that rab3 is associated with cortical granules throughout oogenesis, during cortical granule translocation, and while docked at the egg plasma membrane. Following cortical granule exocytosis, however, rab3 reassociates with a different population of vesicles, at least some of which are of endocytic origin. Because of its selective association with cortical granules in eggs and oocytes, we hypothesize that rab3 functions in cortical granule exocytosis. To test this hypothesis, we used a strategy of interfering with rab3 function by peptide competition with its effector domain, a conserved region within specific rab types. We first identified the effector domain sequence in Lytechinus variegatus eggs and find the sequence 94% identical to the effector domain of rab3 in Stronglocentrotus purpuratus. Then, with synthetic peptides to different regions of the rab3 protein, we find that cortical granule exocytosis is inhibited in eggs injected with effector domain peptides, but not with peptides from the hypervariable region or with a scrambled effector peptide. Additionally, effector-peptide-injected eggs injected with IP3 are blocked in their ability to exocytose cortical granules, suggesting that the inhibition is directly on the membrane fusion event and not the result of interference with the signal transduction mechanism leading to calcium release. We interpret these results to mean that rab3 functions in the regulation of cortical granule exocytosis following vesicle docking.

Mechanism of action of rab3A in mossy fiber LTP

In mossy fiber synapses of the hippocampal CA3 region, LTP is induced by cAMP and requires the synaptic vesicle protein rab3A. In contrast, CA1-region synapses do not exhibit this type of LTP. We now show that cAMP enhances glutamate release from CA3 but not CA1 synaptosomes by (1) increasing the readily releasable pool as tested by hypertonic sucrose; (2) potentiating release evoked by KCl depolarization, which opens voltage-gated Ca2+ channels; and (3) by enhancing Ca2+ action on the secretory apparatus as monitored by the Ca2+-ionophore ionomycin. In rab3A-deficient synaptosomes, forskolin still enhances KCl- and sucrose-induced glutamate release but not ionomycin-induced release. Our results show that cAMP has multiple actions in mossy fiber synapses, of which only the direct activation of the secretory apparatus requires rab3A and functions in mfLTP.

Regulation of the Ca2+ sensitivity of exocytosis by Rab3a

Ca2+ ions trigger the release of hormones and neurotransmitters and contribute to making the secretory vesicles competent for fusion. Here, we present evidence for the involvement of the GTP-binding protein Rab3a in the sensitivity of the exocytotic process to internal [Ca2+]. The secretory activity of bovine adrenal chromaffin cells was elicited by Ca2+ dialysis through a patch-clamp pipette and assayed by monitoring changes in cell membrane capacitance. Microinjection of antisense oligonucleotides directed to rab3a mRNA increased the secretory activity observed at low (0.2-4 microM) [Ca2+], but did not change the maximal activity observed at 10 microM free [Ca2+]. Moreover, after a train of depolarizing stimuli, the secretory activity of antisense-injected cells dialyzed with 10 microM [Ca2+] was increased significantly compared with that of control cells. This result suggests that the activity of either Rab3a or its partners might change upon stimulation. We conclude that Rab3a, together with its partners, participates in the Ca2+ dependence of exocytosis and that its activity is modulated further in a stimulus-dependent manner. These findings should provide some clues to elucidate the role of Rab3a in synaptic plasticity.

Carboxyl methylation of rab3D is developmentally regulated in the rat pancreas: correlation with exocrine function

Several GTPases of the rab family, including rab3A, are methylesterifled on their carboxy-terminal prenylcysteine residue. The significance of this reversible posttranslational modification for the function of rab proteins is unknown, although it has been postulated that carboxyl methylation facilitates the membrane association of prenylated proteins through a hydrophobic mechanism. We here demonstrate, that pancreatic rab3D undergoes developmentally regulated carboxyl methylation concurrently with the maturation of the regulated secretory apparatus in pancreatic acinar cells: in fetal glands, which are refractive to hormone stimulation, the majority of the rab3D protein was methylated, whereas in neonatal and adult glands, which are secretory competent, only 50% was methylated. The methylated form of rab3D was also predominant in a transplantable acinar cell tumor which displays impaired secretory responsiveness and morphological characteristics reminiscent of the fetal pancreas. In addition, treatment of AR42J pancreatic acinar tumor cells with dexamethasone to induce a regulated secretory pathway, led to a significant increase in the size of the unmethylated pool of a rab3-like protein. Strikingly, membrane preparations from adult pancreata and parotid glands contained both methylated and unmethylated forms of rab3D indiscriminately. These results suggest that the acquisition of stimulus-secretion coupling by the exocrine pancreas correlates with the methylation state of rab3D, and that carboxyl methylation plays no significant role in enhancing the membrane association or determining the subcellular distribution of rab3D.

Mutations in GDI1 are responsible for X-linked non-specific mental retardation

Rab GDP-dissociation inhibitors (GDI) are evolutionarily conserved proteins that play an essential role in the recycling of Rab GTPases required for vesicular transport through the secretory pathway. We have found mutations in the GDI1 gene (which encodes uGDI) in two families affected with X-linked non-specific mental retardation. One of the mutations caused a non-conservative substitution (L92P) which reduced binding and recycling of RAB3A, the second was a null mutation. Our results show that both functional and developmental alterations in the neuron may account for the severe impairment of learning abilities as a consequence of mutations in GDI1, emphasizing its critical role in development of human intellectual and learning abilities.

Calcium-dependent regulation of rab3 in short-term plasticity

The Rab3 proteins are monomeric GTP-binding proteins associated with secretory vesicles. In their active GTP-bound state, Rab3 proteins are involved in the regulation of hormone secretion and neurotransmitter release. This action is thought to involve specific effectors, including two Ca2+-binding proteins, Rabphilin and Rim. Rab3 acts late in the exocytotic process, in a cell domain in which the intracellular Ca2+ concentration is susceptible to rapid changes. Therefore, we examined the possible Ca2+-dependency of the regulatory action of GTP-bound Rab3 and wild-type Rab3 on neuroexocytosis at identified cholinergic synapses in Aplysia californica. The effects of recombinant GTPase-deficient Aplysia-Rab3 (apRab3-Q80L) or wild-type apRab3 were studied on evoked acetylcholine release. Intraneuronal application of apRab3-Q80L in identified neurons of the buccal ganglion of Aplysia led to inhibition of neurotransmission; wild-type apRab3 was less effective. Intracellular chelation of Ca2+ ions by EGTA greatly potentiated the inhibitory action of apRab3-Q80L. Train and paired-pulse facilitation, two Ca2+-dependent forms of short-term plasticity induced by a rise in intraterminal Ca2+ concentration, were increased after injection of apRab3-Q80L. This result suggests that the inhibition exerted by GTP-bound Rab3 on neuroexocytosis is reduced during transient augmentations of intracellular Ca2+ concentration. Therefore, a Ca2+-dependent modulation of GTP-bound Rab3 function may contribute to short-term plasticity.

Rab3B is essential for GnRH-induced gonadotrophin release from anterior pituitary cells

Gonadotrophin-releasing hormone (GnRH) induces the release of gonadotrophins via an increase in cytosolic Ca2+ concentration ([Ca2+]). Rab3B, a member of the small GTP-binding protein Rab family, is known to be involved in Ca(2+)-regulated exocytosis in pituitary cells. However, it is not known whether Rab3B functions in the physiological process regulated by GnRH in gonadotrophs. In this study using antisense oligonucleotide against Rab3B (AS-Rab3B) we determined that Rab3B is involved in GnRH-induced gonadotrophin release. Rab3B immunopositive cells were reduced in 24% of pituitary cells by AS-Rab3B. This treatment did not affect the population of gonadotrophs or the intracellular contents of gonadotrophins. However, AS-Rab3B significantly inhibited the total amount of basal and GnRH-induced gonadotrophin released from pituitary cells. These results show that Rab3B is involved in basal and GnRH-induced gonadotrophins release but not the storage of gonadotrophins. Next, the changes in [Ca2+] and exocytosis in gonadotrophs treated with AS-Rab3B were compared among Rab3B-positive and -negative cells. The change in [Ca2+] was not different in the two groups, but exocytosis was significantly inhibited in Rab3B-negative cells. These results suggest that Rab3B is essential for GnRH-regulated exocytosis downstream of cytosolic Ca2+ in gonadotrophs.

RAB3 and synaptotagmin: the yin and yang of synaptic membrane fusion

Synaptic vesicle exocytosis occurs in consecutive steps: docking, which specifically attaches vesicles to the active zone; priming, which makes the vesicles competent for Ca(2+)-triggered release and may involve a partial fusion reaction; and the final Ca(2+)-regulated step that completes fusion. Recent evidence suggests that the critical regulation of the last step in the reaction is mediated by two proteins with opposite actions: synaptotagmin, a Ca(2+)-binding protein that is essential for Ca(2+)-triggered release and probably serves as the Ca(2+)-sensor in fusion, and rab3, which limits the number of vesicles that can be fused as a function of Ca2+ in order to allow a temporally limited, repeatable signal.

Expression of Rab3A GTPase and other synaptic proteins is induced in differentiated NT2N neurons

Postmitotic NT2N cells, which are derived from human NT2 teratocarcinoma cells by treatment with retinoic acid (RA) and mitotic inhibitors, are viewed as a good in vitro model for mature neurons of the human central nervous system. Although NT2N cells exhibit many morphological and biochemical characteristics of neurons, the expression of key protein components involved in regulated exocytosis have not been firmly established. Here we show by immunoblot analysis that mature morphologically differentiated NT2N cells contain readily detectable quantities of the synaptic vesicle-associated proteins, synaptobrevin, synapsin, and synaptophysin. They also express the presynaptic plasma membrane protein, SNAP-25, and a Rab GTPase implicated in the control of Ca(2+)-dependent exocytosis, Rab3A. These proteins were not detected in untreated NT2 cells or cells exposed to RA for only 6 d. The induction of an array of proteins known to be involved in the docking and fusion of synaptic vesicles with the plasma membrane provides further support for the validity of NT2N cells as a model for human cortical neurons and suggests that these cells may be useful for in vitro molecular studies of the Ca(2+)-regulated exocytic pathway in nerve terminals.

Follistatin regulates the relative proportions of endocrine versus exocrine tissue during pancreatic development

In this study, we have investigated the role of the embryonic mesenchyme in the development of the pancreas. We have compared the development in vitro of E12.5 rat pancreatic rudiments grown in the presence or absence of mesenchyme. When the E12.5 pancreatic epithelial rudiment is cultured in the presence of its surrounding mesenchyme, both morphogenesis and cytodifferentiation of the exocrine component of the pancreas are completely achieved, while only a few immature endocrine cells develop. The pancreatic rudiments grown in the absence of mesenchyme develop in a completely different way; the exocrine tissue develops poorly and fails to undergo acinar morphogenesis, while the endocrine tissue develops actively. Four times more insulin-positive cells develop after removal of the mesenchyme than in the cultures performed in the presence of mesenchyme. Moreover, the insulin-expressing cells developed in the mesenchyme-depleted rudiments appear mature since they do not coexpress glucagon, express the glucose transporter Glut-2 and express Rab3A, a molecule associated with the secretory granules. Moreover, these endocrine cells are able to associate and form true islets. Both the inductive effect of the mesenchyme on the proper development of the exocrine tissue and its repressive effect on the development of the endocrine cells are mediated by soluble factors. Follistatin, which is expressed by E12.5 pancreatic mesenchyme, can mimic both inductive and repressive effects of the mesenchyme. Follistatin could thus represent one of the mesenchymal factors required for the development of the exocrine tissue while exerting a repressive role on the differentiation of the endocrine cells.

Dystrophic neurites of senile plaques are defective in proteins involved in exocytosis and neurotransmission

Dystrophic neurites are major components of neuritic (both immature and mature) senile plaques in Alzheimer disease. Previous studies have shown strong immunoreactivity for different neuropeptides, and chromogranin A, a protein associated with dense-core vesicles, in dystrophic neurites. In the present study, antibodies to synaptophysin, synapsin, Rab3a and synaptotagmin (synaptic vesicle proteins), and SNAP-25 (synaptosomal-associated protein of 25 kD) and syntaxin (presynaptic plasma membrane proteins) have been used to learn about the dystrophic neurite equipment of proteins that are necessary for the docking and fusion of synaptic vesicles, and then for exocytosis and neurotransmission. The present results have shown that, although most neuritic senile plaques have chromogranin A- and SNAP-25-immunoreactive dystrophic neurites, only a percentage of them contain synaptophysin, and a minority contain synaptotagmin and Rab3a. Dystrophic neurites do not contain synapsin and syntaxin. These results show that dystrophic neurites of senile plaques are defective in proteins that control exocytosis and neurotransmission.

Expression of Rab3D N135I inhibits regulated secretion of ACTH in AtT-20 cells

Rab proteins are small molecular weight GTPases that control vesicular traffic in eucaryotic cells. A subset of Rab proteins, the Rab3 proteins are thought to play an important role in regulated exocytosis of vesicles. In transfected AtT-20 cells expressing wild-type Rab3D, we find that a fraction of the protein is associated with dense core granules. In the same cells, expression of a mutated isoform of Rab3D, Rab3D N135I, inhibits positioning of dense core granules near the plasma membrane, blocks regulated secretion of mature ACTH, and impairs association of Rab3A to membranes. Expression of Rab3D N135I does not change the levels of ACTH precursor or the efficiency with which the precursor is processed into ACTH hormone and packaged into dense core granules. We also find that cells expressing mutated Rab3D differentiate to the same extent as untransfected AtT-20 cells. We conclude that expression of Rab3D N135I specifically impairs late membrane trafficking events necessary for ACTH hormone secretion.

Expression of Rab3A in the cortical region in mouse metaphase II eggs

Rab3A, a member of the small GTP-binding protein superfamily, has been implicated in regulated exocytosis at presynapses. At fertilization, sperm-egg fusion induces cytosolic calcium mobilization and cortical granule (CG) exocytosis in the egg. However, it is not yet clear whether Rab3A is involved in this process. We previously reported that Rabphilin-3A functions in calcium-dependent CG exocytosis. Rabphilin-3A is known to bind with Rab3A, Rab3B, and Rab3C. In this study, we clarified which member of the Rab3 was expressed in mouse metaphase II eggs. Messenger RNA encoding Rab3A but not Rab3B or Rab3C, was detected in unfertilized metaphase II eggs by RT-PCR. Rab3A protein was also detected in unfertilized metaphase II eggs by immunoblot analysis. Next the expression and the localization of Rab3A in eggs at various stages of development were determined by immunofluorescence analysis using confocal laser scanning microscopy. Rab3A protein was specifically distributed in the cortical region in eggs from before fertilization to the two-cell stage. However, it was not detected at the three- or four-cell stage 40 hr after fertilization. These results suggest that Rab3A may function with Rabphilin-3A in CG exocytosis.

Overexpression of Rab3D enhances regulated amylase secretion from pancreatic acini of transgenic mice

Rab3D, a member of the ras-related GTP-binding protein Rab family, is localized to secretory granules of various exocrine tissues such as acinar cells of the pancreas, chief cells of the stomach, and parotid and lacrimal secretory cells. To elucidate the function of Rab3D in exocytosis, we have generated transgenic mice that over-express Rab3D specifically in pancreatic acinar cells. Hemagglutinin-tagged Rab3D was localized to zymogen granules by immunohistochemistry, and was shown to be present on zymogen granule membranes by Western blotting; both results are similar to previous studies of endogenous Rab3D. Secretion measurements in isolated acinar preparations showed that overexpression of Rab3D enhanced amylase release. Amylase secretion from intact acini of transgenic mice 5 min after 10 pM cholecystokinin octapeptide (CCK) stimulation was enhanced by 160% of control. In streptolysin-O-permeabilized acini of transgenic mice, amylase secretion induced by 100 microM GTP-gamma-S was enhanced by 150%, and 10 microM Ca2+-stimulated amylase secretion was augmented by 206% of that of the control. To further elucidate Rab3D involvement in stimulus-secretion coupling, we examined the effect of CCK on the rate of GTP binding to Rab3D. Stimulation of permeabilized acini with 10 pM CCK increased the incorporation of radiolabeled GTP into HA-tagged Rab3D. These results indicate that overexpression of Rab3D enhances secretagogue-stimulated amylase secretion through both calcium and GTP pathways. We conclude that Rab3D protein on zymogen granules plays a stimulatory role in regulated amylase secretion from pancreatic acini.

Immunocytochemical analysis of the synaptic proteins SNAP-25 and Rab3A in human pituitary adenomas. Overexpression of SNAP-25 in the mammmosomatotroph lineages

SNAP-25 and Rab3A were originally identified as synaptic proteins involved in neuronal membrane traffic. Recently, both proteins have been detected in several mammalian endocrine cell types and have been proposed as essential components of the exocytotic pathway in neuroendocrine cells. In this study, the expression of SNAP-25 and Rab3A was analysed in biopsied human anterior pituitary tumours (21 cases) by immunocytochemical methods. No differences in Rab3A immunoreactivity were observed between tumour and normal pituitary cells. Strong SNAP-25 immunoreactivity was detected in tumour cells of prolactinomas (n = 3). Several growth hormone (GH)/prolactin (PRL) tumours also displayed intense SNAP-25 immunolabelling (n = 3), whereas the remaining GH-secreting adenomas (n = 4) exhibited moderate to weak SNAP-25 immunoreactivity. In contrast, SNAP-25 near-background immunostaining was observed in tumour cells of adrenocorticotrophic hormone (ACTH)-secreting tumours (n = 4) and non-secreting tumours (n = 7), as well as in normal pituitary cells. Since SNAP-25 and Rab3A have been shown to be involved in exocytotic events in rodent endocrine cells, overexpression of SNAP-25 protein in PRL and GH/PRL tumour cells might be implicated in the mechanism of exocytosis of the neoplastic human mammosomatotroph lineages.

Noc2, a putative zinc finger protein involved in exocytosis in endocrine cells

We have cloned a cDNA encoding a novel protein of 302 amino acids (designated Noc2, no C2 domain) that has 40.7% amino acid identity with and 77.9% similarity to the N-terminal region of rabphilin-3A, a target molecule of Rab3A. However, unlike rabphilin-3A, Noc2 lacks two C2 domains that are thought to interact with Ca2+ and phospholipids. Noc2 is expressed predominantly in endocrine tissues and hormone-secreting cell lines and at very low levels in brain. Immunoblot analysis of subcellular fractions of the insulin-secreting cell line MIN6 and immunocytochemistry reveal that Noc2 is a 38-kDa protein present in the cytoplasm. Overexpression of Noc2 in PC12 cells cotransfected with growth hormone enhances high K+-induced growth hormone secretion. Screening a mouse embryonic cDNA library with the yeast two-hybrid system shows that Noc2 interacts with the LIM domain-containing protein zyxin, a component of the cytoskeleton, and this interaction is further confirmed by the coimmunoprecipitation experiment. Accordingly, Noc2 is probably involved in regulated exocytosis in endocrine cells by interacting with the cytoskeleton.