Characterization of Rab3A, Rab3B and Rab3C: different biochemical properties and intracellular localization in bovine chromaffin cells

Rab3 and synaptotagmin proteins in the regulation of vesicle fusion and neurotransmitter release

Ca²⁺-triggered neurotransmitter release involves complex regulatory mechanisms, including a series of protein-protein interactions. Three proteins, synaptobrevin (VAMP), synaptosomal-associated protein of 25kDa (SNAP-25) and syntaxin, constitute the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) core complex that plays key roles in controlling vesicle fusion and exocytosis. Many other proteins participate in the regulation of the processes via direct and/or indirect interaction with the SNARE complex. Although much effort has been made, the regulatory mechanism for exocytosis is still not completely clear. Accumulated evidence indicates that the small GTPase Rab3 and synaptotagmin proteins play important regulatory roles during vesicle fusion and neurotransmitter release. This review outlines our present understanding of the two regulatory proteins, with the focus on the interaction of Rab3 with synaptotagmin in the regulatory process.

Distinct role of Rab3A and Rab3B in secretory activity of rat melanotrophs

Members of the Rab3 (A-D) subfamily of small GTPases are believed to play a key role in regulated exocytosis. These proteins share approximately 80% identity at amino acid level. The question of whether isoforms of Rab3 are functionally redundant was the subject of this study. We used RT-PCR analysis, in situ hybridization histochemistry, and confocal microscope-based analysis of immunocytochemistry to show that rat melanotrophs contain about equal amounts of Rab3A and Rab3B transcripts as well as proteins. Therefore, these cells are a suitable model to study the subcellular distribution and the role of these paralogous isoforms in regulated exocytosis. Secretory activity of single cells was monitored with patch-clamp capacitance measurements, and the cytosol was dialyzed with a high-calcium-containing patch pipette solution. Preinjection of antisense oligodeoxyribonucleotides specific to Rab3A, but not to Rab3B, induced a specific blockage of calcium-dependent secretory responses, indicating an exclusive requirement for Rab3A in melanotroph cell-regulated secretion. Although the injection of purified Rab3B protein was ineffective, the injection of recombinant Rab3A proteins into rat melanotrophs revealed that regulated secretion was stimulated by a GTP-bound Rab3A with an intact COOH terminus and inhibited by Rab3AT36N, impaired in GTP binding. These results indicate that Rab3A, but not Rab3B, enhances secretory output from rat melanotrophs and that their function is not redundant.

Vesicle pools, docking, priming, and release

The release of neurotransmitter from synaptic vesicles represents the final event by which presynapses send their chemical signal to the receiving postsynapses. Prior to fusion, synaptic vesicles undergo a series of maturation events, most notably the membrane-delimited docking and priming steps. Physiological and optical experiments with high-time resolution have allowed the distinction of vesicles in different maturation states with respect to fusion, the so-called vesicle pools. In this review, we define the various vesicle pools and discuss pathways leading into and out of these pools. We also provide an overview of an array of proteins that have been identified or are speculated to play a role in the transition between the various vesicle pools.

Identification of Necrosis-Associated Genes in Glioblastoma by cDNA Microarray Analysis

PURPOSE

In the field of cancer research, there has been a paucity of interest in necrosis, whereas studies focusing on apoptosis abound. In neuro-oncology, this is particularly surprising because of the importance of necrosis as a hallmark of glioblastoma (GBM), the most malignant and most common primary brain tumor, and the fact that the degree of necrosis has been shown to be inversely related to patient survival. It is therefore of considerable interest and importance to identify genes and gene products related to necrosis formation.

EXPERIMENTAL DESIGN

We used a nylon cDNA microarray to analyze mRNA expression of 588 universal cellular genes in 15 surgically resected human GBM samples with varying degrees of necrosis. Gene expression was correlated with the degree of necrosis using rank correlation coefficients. The expression of identified genes was compared with their expression in tissue samples from 5 anaplastic astrocytomas (AAs). Immunostaining was used to determine whether genes showing the most positive correlation with necrosis were increasingly expressed in tumor tissues, as grade of necrosis increased.

RESULTS

The hybridization results indicated that 26 genes showed significant correlation with the amount of necrosis. All 26 genes had functions associated with either Ras, Akt, tumor necrosis factor alpha, nuclear factor kappaB, apoptosis, procoagulation, or hypoxia. Nine genes were positively correlated with necrosis grade, and 17 genes were negatively correlated with necrosis grade. There were significant differences in the median expression levels of 3 of the 26 genes between grade III necrosis GBM and anaplastic astrocytoma (AA) samples; all but 1 of the genes had elevated expression when comparing necrosis grade III with AA samples. Two factors, the ephrin type A receptor 1 and the prostaglandin E(2) receptor EP4 subtype, not previously considered in this context, were highlighted because of their particularly high (positive) correlation coefficients; immunostaining showed the products of these two genes to be localized in perinecrotic and necrotic regions and to be overexpressed in grade III GBMs, but not AAs. These two molecules also showed significant correlation with survival of GBM patients (P = 0.0034) in a combined model.

CONCLUSIONS

The application of cDNA expression microarray analysis has identified specific genes and patterns of gene expression that may help elucidate the molecular basis of necrogenesis in GBM. Additional studies will be required to further investigate and confirm these findings.

Localization versus function of Rab3 proteins. Evidence for a common regulatory role in controlling fusion

Rab3A, Rab3B, Rab3C, and Rab3D constitute a family of GTP-binding proteins that are implicated in regulated exocytosis. Various localizations and distinct functions have been proposed for different and occasionally even for the same Rab3 protein. This is exemplified by studies demonstrating that deletion of Rab3A in knock-out mice results in dysregulation of the final stages of exocytosis, whereas overexpression of Rab3A in neuroendocrine cells causes nearly complete inhibition of Ca(2+)-triggered exocytosis. We have now examined the properties of all Rab3 proteins in the same assays, with the long-term goal of identifying a common conceptual framework for their functions. Using quantitative immunoblotting, we found that all four Rab3 proteins were expressed in brain and endocrine tissues, although at widely different levels. Rab3A, Rab3B, and Rab3C co-localized to synaptic and secretory vesicles consistent with potential redundancy, whereas Rab3D was expressed at high levels only in the endocrine pituitary (where it was more abundant than Rab3A, Rab3B, and Rab3C combined), in exocrine glands, and in adipose tissue. In transfected PC12 cells, all four Rab3 proteins strongly inhibited Ca(2+)-triggered exocytosis. Except for a mutation that fixes Rab3 into a permanently GDP-bound state, all Rab3 mutations tested had no effect on this inhibition, including a mutation in the calmodulin-binding site that was described as inactivating (Coppola, T., Perret-Menoud, V., Lüthi, S., Farnsworth, C. C., Glomset, J. A., and Regazzi, R. (1999) EMBO J. 18, 5885-5891). Unexpectedly, overexpression of wild type Rab3A and permanently GTP-bound mutant Rab3A in PC12 cells caused a loss of secretory vesicles and an increase in constitutive, Ca(2+)-independent exocytosis that correlated with the inhibition of regulated Ca(2+)-triggered exocytosis. Our data indicate that overexpression of Rab3 in PC12 cells impairs the normal control of the final step in exocytosis, thereby converting the regulated secretory pathway into a constitutive pathway. These results offer an hypothesis that reconciles Rab3 transfection and knock-out studies by suggesting that Rab3 functions as a gatekeeper of a late stage in exocytosis.

Rab3B in human platelet is membrane bound and interacts with Ca(2+)/calmodulin

The subcellular distribution of Rab3B in fresh and aged platelets was determined and majority of the protein was localized with the particulate fraction with only a minor amount detected in the cytosol. Rab3B was pulled out from platelet particulate fraction with GST-RabGDI-alpha fusion protein. Using GST-Rab3B in in vitro pull-down experiments, the binding of calmodulin from platelet cytosol to Rab3B was demonstrated. In the reverse experiment, binding of Rab3B from platelet particulate and cytosolic fractions to Sepharose-CaM beads was also observed. The interaction between Rab3B and calmodulin was Ca(2+)-dependent but independent of the guanine nucleotide status of Rab3B. These findings provide evidence that Rab3B is primarily localized with the particulate fraction and that Ca(2+)/calmodulin could regulate function of this GTPase in the platelet.

Rab3B regulates ZO-1 targeting and actin organization in PC12 neuroendocrine cells

Rab3B is a monomeric GTPase that modulates norepinephrine secretion when expressed in PC12 neuroendocrine cells. In the present study we determined whether rab3B also regulates the organization of intercellular junctions, since this GTPase localizes to regions of cell contact in multiple cell types. The stable expression of rab3B, but not the closely related rab3A, led to two morphological phenotypes in PC12 cells: (i) reorganization of F-actin into long filopodia and (ii) redistribution of the junction-associated protein ZO-1. ZO-1 localization was not appreciably affected by the expression of a GTP binding mutant of rab3B (N135I) that stimulates norepinephrine secretion by PC12 cells. The apparent diversity of these rab3B phenotypes implies that this GTPase is capable of influencing cell signaling pathways that in turn modulate the cytoskeleton and junction organization. In support of this hypothesis we observed that rab3B expression also altered the profile of proteins that interact with the signaling molecule, phosphatidylinositol 3-kinase (PI3-kinase). The effect of rab3B on protein interactions with PI3-kinase was reversed by inhibitors of this kinase. Furthermore, PI3-kinase inhibitors virtually abolished ZO-1 localization at the surfaces of cells that express rab3B, but not rab3A, whereas these inhibitors had no effect on rab3B-dependent norepinephrine secretion. Our results indicate that rab3B can influence junctional protein targeting and secretion by distinct mechanisms.

High-resolution crystal structure of S. cerevisiae Ypt51(DeltaC15)-GppNHp, a small GTP-binding protein involved in regulation of endocytosis

Ypt/Rab proteins are membrane-associated small GTP-binding proteins which play a central role in the coordination, activation and regulation of vesicle-mediated transport in eukaryotic cells. We present the 1.5 A high-resolution crystal structure of Ypt51 in its active, GppNHp-bound conformation. Ypt51 is an important regulator involved in the endocytic membrane traffic of Saccharomyces cerevisiae. The structure reveals small but significant structural differences compared with H-Ras p21. The effector loop and the catalytic loop are well defined and stabilized by extensive hydrophobic interactions. The switch I and switch II regions form a well-defined epitope for hypothetical effector protein binding. Sequence comparisons between the different isoforms Ypt51, Ypt52 and Ypt53 provide the first insights into determinants for specific effector binding and for fine-tuning of the intrinsic GTP-hydrolysis rate.

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.

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.

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.