Multiple aspects of Rab protein action in the secretory pathway: focus on Rab3 and Rab6

Rab3A/Rab27A System Silencing Ameliorates High Glucose-Induced Injury in Podocytes

Diabetic nephropathy is a major complication in diabetic patients. Podocytes undergo loss and detachment from the basal membrane. Intra- and intercellular communication through exosomes are key processes for maintaining function, and the Rab3A/Rab27A system is an important counterpart. Previously, we observed significant changes in the Rab3A/Rab27A system in podocytes under glucose overload, demonstrating its important role in podocyte injury. We investigated the implication of silencing the Rab3A/Rab27A system in high glucose-treated podocytes and analysed the effect on differentiation, apoptosis, cytoskeletal organisation, vesicle distribution, and microRNA expression in cells and exosomes. For this, we subjected podocytes to high glucose and transfection through siRNAs, and we isolated extracellular vesicles and performed western blotting, transmission electron microscopy, RT-qPCR, immunofluorescence and flow cytometry assays. We found that silencing RAB3A and RAB27A generally leads to a decrease in podocyte differentiation and cytoskeleton organization and an increase in apoptosis. Moreover, CD63-positive vesicles experienced a pattern distribution change. Under high glucose, Rab3A/Rab27A silencing ameliorates some of these detrimental processes, suggesting a differential influence depending on the presence or absence of cellular stress. We also observed substantial expression changes in miRNAs that were relevant in diabetic nephropathy upon silencing and glucose treatment. Our findings highlight the Rab3A/Rab27A system as a key participant in podocyte injury and vesicular traffic regulation in diabetic nephropathy.

Therapeutic Targeting of Rab GTPases: Relevance for Alzheimer’s Disease

Rab GTPases (Rabs) are small proteins that play crucial roles in vesicle transport and membrane trafficking. Owing to their widespread functions in several steps of vesicle trafficking, Rabs have been implicated in the pathogenesis of several disorders, including cancer, diabetes, and multiple neurodegenerative diseases. As treatments for neurodegenerative conditions are currently rather limited, the identification and validation of novel therapeutic targets, such as Rabs, is of great importance. This review summarises proof-of-concept studies, demonstrating that modulation of Rab GTPases in the context of Alzheimer’s disease (AD) can ameliorate disease-related phenotypes, and provides an overview of the current state of the art for the pharmacological targeting of Rabs. Finally, we also discuss the barriers and challenges of therapeutically targeting these small proteins in humans, especially in the context of AD.

Non-coding RNA derived from extracellular vesicles in cancer immune escape: Biological functions and potential clinical applications

The tumor microenvironment represents a dynamically composed matrix into which cancer cells and many other cell types are embedded to form organ-like structures. The tumor immune microenvironment (TIME), composed of immune cells, is an inseparable part of the tumor microenvironment. Extracellular vesicles (EVs) participate in the occurrence and development of tumors by delivering various biologically active molecules between cells; their role in cancer immune escape in particular has been widely proven. EVs can carry a wide array of cargo, such as non-coding RNAs (ncRNAs), including miRNAs, lncRNAs, and circRNAs, which are selectively loaded by EVs, secreted, and transported to participate in the proliferation of immune cells. Hence, strategies to specifically target EV-ncRNAs could be attractive therapeutic options. In this review, we summarize the current research on the role of EV-ncRNAs in cancer immune escape, and discuss the latest research on the function and regulation mechanism of EV-ncRNAs in cancer immune escape, highlighting and elucidating the potential clinical applications of EV-ncRNAs, including in diagnosis and immunotherapy.

Natural Exogenous Antioxidant Defense against Changes in Human Skin Fibroblast Proteome Disturbed by UVA Radiation

Daily exposure of the skin to UVA radiation causes oxidative modifications to cellular components and biomolecules. These include proteins involved in the metabolism and cytoprotection of fibroblasts, and their modification can contribute to the disruption of cell function and the development of skin disorders. Therefore, there remains a need for highly active cytoprotective compounds with antioxidant properties. The purpose of this study was to investigate the effect of ascorbic acid on the activity of rutin against UVA-induced changes in the proteome of human fibroblasts. All analyses were carried out on fibroblasts cultured in a three-dimensional system exposed to UVA radiation and incubated with rutin and ascorbic acid. Their proteomic profile was analyzed using nano-HPLC, which revealed 150 proteins whose expression was significantly altered between treatment conditions. UVA radiation led to changes in the expression of 82 proteins. However, some of these changes were mitigated by rutin and ascorbic acid separately (23 and 25 proteins, respectively) and rutin and ascorbic acid together (23 proteins). UVA radiation has led to the upregulation of proteins involved in gene expression, catalytic processes and antioxidant pathways, and downregulation of proteins with binding activity. Nevertheless, rutin and ascorbic acid used separately or together have countered these changes to varying degrees. Moreover, rutin and ascorbic acid stimulated fibroblasts irradiated by UVA to increase the expression of the signalling molecules responsible for the opening of the transmembrane channels. In the context of the results obtained, the observed cytoprotective effect of the cooperation of rutin and ascorbic acid results not only from the overlapping properties of the compounds. The effect of rutin alone is probably inhibited by its limited bioavailability. Therefore, its interaction with ascorbic acid increases membrane penetration and improves the cytoprotective effect on skin fibroblasts.

Small GTPases: Structure, biological function and its interaction with nanoparticles

Small GTPase is a kind of GTP-binding protein commonly found in eukaryotic cells. It plays an important role in cytoskeletal reorganization, cell polarity, cell cycle progression, gene expression and many other significant events in cells, such as the interaction with foreign particles. Therefore, it is of great scientific significance to understand the biological properties of small GTPases as well as the GTPase-nano interplay, since more and more nanomedicine are supposed to be used in biomedical field. However, there is no review in this aspect. This review summarizes the small GTPases in terms of the structure, biological function and its interaction with nanoparticles. We briefly introduced the various nanoparticles such as gold/silver nanoparticles, SWCNT, polymeric micelles and other nano delivery systems that interacted with different GTPases. These current nanoparticles exhibited different pharmacological effect modes and various target design concepts in the small GTPases study. This will help to elucidate the conclusion that the therapeutic strategy targeting small GTPases might be a new research direction. It is believed that the in-depth study on the functional mechanism of GTPases can provide insights for the design and study of nanomedicines.

Intracellular cAMP Sensor EPAC: Physiology, Pathophysiology, and Therapeutics Development

This review focuses on one family of the known cAMP receptors, the exchange proteins directly activated by cAMP (EPACs), also known as the cAMP-regulated guanine nucleotide exchange factors (cAMP-GEFs). Although EPAC proteins are fairly new additions to the growing list of cAMP effectors, and relatively "young" in the cAMP discovery timeline, the significance of an EPAC presence in different cell systems is extraordinary. The study of EPACs has considerably expanded the diversity and adaptive nature of cAMP signaling associated with numerous physiological and pathophysiological responses. This review comprehensively covers EPAC protein functions at the molecular, cellular, physiological, and pathophysiological levels; and in turn, the applications of employing EPAC-based biosensors as detection tools for dissecting cAMP signaling and the implications for targeting EPAC proteins for therapeutic development are also discussed.

Mutant Huntingtin Impairs BDNF Release from Astrocytes by Disrupting Conversion of Rab3a-GTP into Rab3a-GDP

Brain-derived neurotrophic factor (BDNF) is essential for neuronal differentiation and survival. We know that BDNF levels decline in the brains of patients with Huntington's disease (HD), a neurodegenerative disease caused by the expression of mutant huntingtin protein (mHtt), and furthermore that administration of BDNF in HD mice is protective against HD neuropathology. BDNF is produced in neurons, but astrocytes are also an important source of BDNF in the brain. Nonetheless, whether mHtt affects astrocytic BDNF in the HD brain remains unknown. Here we investigated astrocytes from HD140Q knock-in mice and uncovered evidence that mHtt decreases BDNF secretion from astrocytes, which is mediated by exocytosis in astrocytes. Our results demonstrate that mHtt associates with Rab3a, a small GTPase localized on membranes of dense-core vesicles, and prevents GTP-Rab3a from binding to Rab3-GAP1, disrupting the conversion of GTP-Rab3a into GDP-Rab3a and thus impairing the docking of BDNF vesicles on plasma membranes of astrocytes. Importantly, overexpression of Rab3a rescues impaired BDNF vesicle docking and secretion from HD astrocytes. Moreover, ATP release and the number of ATP-containing dense-core vesicles docking are decreased in HD astrocytes, suggesting that the exocytosis of dense-core vesicles is impaired by mHtt in HD astrocytes. Further, Rab3a overexpression reduces reactive astrocytes in the striatum of HD140Q knock-in mice. Our results indicate that compromised exocytosis of BDNF in HD astrocytes contributes to the decreased BDNF levels in HD brains and underscores the importance of improving glial function in the treatment of HD.

SIGNIFICANCE STATEMENT

Huntington's disease (HD) is an inherited neurodegenerative disorder that affects one in every 10,000 Americans. To date, there is no effective treatment for HD, in part because the pathogenic mechanism driving the disease is not fully understood. The dysfunction of astrocytes is known to contribute to the pathogenesis of HD. One important role of astrocytes is to synthesize and release brain-derived neurotrophic factor (BDNF), which is vital for neuronal survival, development, and function. We found that mutant huntingtin protein (mHtt) at the endogenous level decreases BDNF secretion from astrocytes by disrupting the conversion of GTP-Rab3a into GDP-Rab3a and that overexpressing Rab3a can rescue this deficient BDNF release and early neuropathology in HD knock-in mouse brain. Our study suggests that astrocytic Rab3a is a potential therapeutic target for HD treatment.

Rab3a and Rab10 are regulators of lysosome exocytosis and plasma membrane repair

Disruption of the cell plasma membrane can occur due to mechanical damage, pore forming toxins, etc. Resealing or plasma membrane repair (PMR) is the emergency response required for cell survival. It is triggered by Ca²⁺ entering through the disruption, causing organelles such as lysosomes located underneath the plasma membrane to fuse rapidly with the adjacent plasma membrane. We have recently identified some of the molecular traffic machinery that is involved in this vital process. Specifically, we showed that 2 members of the Rab family of small GTPases, Rab3a and Rab10, are essential for lysosome exocytosis and PMR in cells challenged with a bacterial toxin, streptolysin-O (SLO). Additionally, we showed that Rab3a regulates PMR via the interaction with 2 effectors, synaptotagmin-like protein 4a (Slp4-a) and nonmuscle myosin heavy chain IIA (NMHC IIA), the latter being identified for the first time as a Rab3a effector. This tripartite complex is essential for the positioning of the peripheral lysosomes responsible for PMR. In cells lacking any of the components of this tripartite complex, lysosomes were concentrated in the perinuclear region and absent in the periphery culminating with PMR inhibition.

GCAP1, Rab6, and HSP27: Novel Autoantibody Targets in Cancer-Associated Retinopathy and Autoimmune Retinopathy

PURPOSE

Autoantibodies (AAbs) with different retinal specificities were reported in cancer-associated retinopathy (CAR) and autoimmune retinopathy (AR). The goal was to identify the small retinal proteins of apparent molecular mass of 23-kDa often recognized by patients' AAbs.

METHODS

Sera specific for a 23-kDa retinal protein of 173 patients were investigated retrospectively by Western blotting and double immunofluorescence confocal microscopy. A proteomic analysis revealed new 23-kDa protein candidates, including guanylyl cyclase-activating proteins (GCAPs), heat shock protein 27 (HSP27), and Rab6A GTPase (Rab6A).

RESULTS

Among the cohort of 173 patients, only 68 had anti-recoverin AAbs and the remaining 105 reacted with 4 unique proteins, which were identified as a Rab6A, HSP27, GCAP1, and GCAP2. Confocal images from a double labeling study confirmed the reactivity of AAbs with different types of cells in human retina, consistent with the target protein's respective cellular functions. Patients (62/173) had been diagnosed with various kinds of cancer, including 20% of patients who had anti-recoverin, 11% anti-Rab6A, and 5% anti-HSP27 AAbs. Only 50% of recoverin-seropositive patients had cancer and the individuals with anti-recoverin AAbs had a significantly higher likelihood to be diagnosed with cancer than patients with other anti-23-kDa AAbs.

CONCLUSIONS

The newly discovered retinal autoantigens may be involved in pathogenicity of CAR and AR. The recognition of AAbs against various retinal proteins associated with autoimmune retinal degeneration broadens the group of proteins related with these entities.

TRANSLATIONAL RELEVANCE

Patients with anti-recoverin, anti-GCAP1, anti-Rab6A, and anti-HSP27 AAbs represented diverse clinical phenotypes, so the presence of disease-associated AAbs provides important information for molecular diagnosis.

Rab43 regulates the sorting of a subset of membrane protein cargo through the medial Golgi

To evaluate the role of cytoplasmic domains of membrane-spanning proteins in directing trafficking through the secretory pathway, we generated fluorescently tagged VSV G tsO45 with either the native G tail (G) or a cytoplasmic tail derived from the chicken AE1-4 anion exchanger (G(AE)). We previously showed that these two proteins progressed through the Golgi with distinct kinetics. To investigate the basis for the differential sorting of G and G(AE), we analyzed the role of several Golgi-associated small GTP-binding proteins and found that Rab43 differentially regulated their transport through the Golgi. We show that the expression of GFP-Rab43 arrested the anterograde transport of G(AE) in a Rab43-positive medial Golgi compartment. GFP-Rab43 expression also inhibited the acquisition of endoH-resistant sugars and the surface delivery of G(AE), as well as the surface delivery of the AE1-4 anion exchanger. In contrast, GFP-Rab43 expression did not affect the glycosylation or surface delivery of G. Unexpectedly, down-regulation of endogenous Rab43 using small interfering RNA resulted in an increase in the accumulation of G(AE) on the cell surface while having minimal effect on the surface levels of G. Our data demonstrate that Rab43 regulates the sorting of a subset of membrane-spanning cargo as they progress through the medial Golgi.

Rab3-GEF Controls Active Zone Development at the Drosophila Neuromuscular Junction

Synaptic signaling involves the release of neurotransmitter from presynaptic active zones (AZs). Proteins that regulate vesicle exocytosis cluster at AZs, composing the cytomatrix at the active zone (CAZ). At the Drosophila neuromuscular junction (NMJ), the small GTPase Rab3 controls the distribution of CAZ proteins across release sites, thereby regulating the efficacy of individual AZs. Here we identify Rab3-GEF as a second protein that acts in conjunction with Rab3 to control AZ protein composition. At rab3-GEF mutant NMJs, Bruchpilot (Brp) and Ca²⁺ channels are enriched at a subset of AZs, leaving the remaining sites devoid of key CAZ components in a manner that is indistinguishable from rab3 mutant NMJs. As the Drosophila homologue of mammalian DENN/MADD and Caenorhabditis elegans AEX-3, Rab3-GEF is a guanine nucleotide exchange factor (GEF) for Rab3 that stimulates GDP to GTP exchange. Mechanistic studies reveal that although Rab3 and Rab3-GEF act within the same mechanism to control AZ development, Rab3-GEF is involved in multiple roles. We show that Rab3-GEF is required for transport of Rab3. However, the synaptic phenotype in the rab3-GEF mutant cannot be fully explained by defective transport and loss of GEF activity. A transgenically expressed GTP-locked variant of Rab3 accumulates at the NMJ at wild-type levels and fully rescues the rab3 mutant but is unable to rescue the rab3-GEF mutant. Our results suggest that although Rab3-GEF acts upstream of Rab3 to control Rab3 localization and likely GTP-binding, it also acts downstream to regulate CAZ development, potentially as a Rab3 effector at the synapse.

High-content analysis of Rab protein function at the ER-Golgi interface

The Rab family of small GTPases play fundamental roles in the regulation of trafficking pathways between intracellular membranes in eukaryotic cells. In this short commentary we highlight a recent high-content screening study that investigates the roles of Rab proteins in retrograde trafficking from the Golgi complex to the endoplasmic reticulum, and we discuss how the findings of this work and other literature might influence our thoughts on how the architecture of the Golgi complex is regulated.

miR-5100 promotes tumor growth in lung cancer by targeting Rab6

Our previous study demonstrated that microRNA 5100 (miR-5100) is overexpressed in lung cancer tissues; however, the function of miR-5100 remained elusive. In this study, we demonstrate that miR-5100 is highly expressed in a wide variety of lung cancer tissues and lung cancer cell lines. Exogenous expression of miR-5100 in A549 and H1299 lung cancer cells enhanced proliferation and colony formation, and conversely, suppression of miR-5100 exhibited inhibitory effects. Furthermore, we demonstrate that miR-5100 promotes tumor growth in nude mice. These effects may result from the ability of miR-5100 to promote G1/S transition and downregulate cyclin D1 and cyclin-dependent kinases 2 (CDK2) expressions in lung cancer stable cells. Using a bioinformatics target prediction tool, we identified Rab6 as a potential target of miR-5100. Consistently, overexpression of miR-5100 specifically reduced the expression of a luciferase reporter containing the predicted binding site from the 3'untranslated region (3'UTR) of Rab6 and decreased the accumulation of endogenous Rab6 in A549 and H1299 cells. Moreover, exogenous expression of Rab6 compromised the effects of miR-5100 on cell proliferation and colony formation. Our data suggest that miR-5100 promotes tumor growth by facilitating the G1/S transition and targeting Rab6.

RAB GTPases and RAB-interacting proteins and their role in the control of cognitive functions

A RAS-related class of small monomeric G proteins, the RAB GTPases, is emerging as of key biological importance in compartment specific directional control of vesicles formation, transport and fusion. Thanks to human genetic observation and to the consequent dedicated biochemical work, substantial progress has been made on the understanding of the role played by RAB GTPases and their effector proteins on neuronal development and the shaping of cognitive functions. This review is highlighting these initial elements to broaden the current scope of research on developmental cognitive deficits and take the point of view of RAB GTPases control on membrane transport in neurons and astrocytes.

Rab3a is critical for trapping alpha-MSH granules in the high Ca²⁺-affinity pool by preventing constitutive exocytosis

Rab3a is a small GTPase of the Rab3 subfamily that acts during late stages of Ca²⁺-regulated exocytosis. Previous functional analysis in pituitary melanotrophs described Rab3a as a positive regulator of Ca²⁺-dependent exocytosis. However, the precise role of the Rab3a isoform on the kinetics and intracellular [Ca²⁺] sensitivity of regulated exocytosis, which may affect the availability of two major peptide hormones, α-melanocyte stimulating hormone (α-MSH) and β-endorphin in plasma, remain elusive. We employed Rab3a knock-out mice (Rab3a KO) to explore the secretory phenotype in melanotrophs from fresh pituitary tissue slices. High resolution capacitance measurements showed that Rab3a KO melanotrophs possessed impaired Ca²⁺-triggered secretory activity as compared to wild-type cells. The hampered secretion was associated with the absence of cAMP-guanine exchange factor II/ Epac2-dependent secretory component. This component has been attributed to high Ca²⁺-sensitive release-ready vesicles as determined by slow photo-release of caged Ca²⁺. Radioimmunoassay revealed that α-MSH, but not β-endorphin, was elevated in the plasma of Rab3a KO mice, indicating increased constitutive exocytosis of α-MSH. Increased constitutive secretion of α-MSH from incubated tissue slices was associated with reduced α-MSH cellular content in Rab3a-deficient pituitary cells. Viral re-expression of the Rab3a protein in vitro rescued the secretory phenotype of melanotrophs from Rab3a KO mice. In conclusion, we suggest that Rab3a deficiency promotes constitutive secretion and underlies selective impairment of Ca²⁺-dependent release of α-MSH.

Identification and characterization of multiple novel Rab-myosin Va interactions

A systematic screen of the entire human Rab GTPase family for interactions with myosin Va identified 10 novel Rab partners for myosin Va, all of which belong to the endocytic recycling and post-Golgi secretory membrane network. However, Rab10 and Rab11 appear to be the major determinants of its recruitment to intracellular membranes.

Myosin Va is a widely expressed actin-based motor protein that binds members of the Rab GTPase family (3A, 8A, 10, 11A, 27A) and is implicated in many intracellular trafficking processes. To our knowledge, myosin Va has not been tested in a systematic screen for interactions with the entire Rab GTPase family. To that end, we report a yeast two-hybrid screen of all human Rabs for myosin Va-binding ability and reveal 10 novel interactions (3B, 3C, 3D, 6A, 6A′, 6B, 11B, 14, 25, 39B), which include interactions with three new Rab subfamilies (Rab6, Rab14, Rab39B). Of interest, myosin Va interacts with only a subset of the Rabs associated with the endocytic recycling and post-Golgi secretory systems. We demonstrate that myosin Va has three distinct Rab-binding domains on disparate regions of the motor (central stalk, an alternatively spliced exon, and the globular tail). Although the total pool of myosin Va is shared by several Rabs, Rab10 and Rab11 appear to be the major determinants of its recruitment to intracellular membranes. We also present evidence that myosin Va is necessary for maintaining a peripheral distribution of Rab11- and Rab14-positive endosomes.

Rab3a ablation related changes in morphology of secretory vesicles in major endocrine pancreatic cells, pituitary melanotroph cells and adrenal gland chromaffin cells in mice

In this work we have compared the ultrastructural characteristics of major pancreatic endocrine cells, pituitary melanotrophs and adrenal chromaffin cells in the normal mouse strain (wild type, WT) and mice with a known secretory deficit, the Rab3a knockout strain (Rab3a KO). For this purpose, pancreata, pituitary glands and adrenal glands from the Rab3a KO and from the WT mice were analysed, using conventional transmission electron microscopy (TEM). In order to assess the significance of the presence of Rab3a proteins in the relevant cells, we focused primarily on their secretory vesicle morphology and distribution. Our results showed a comparable general morphology in Rab3a KO and WT in all assessed endocrine cell types. In all studied cell types, the distribution of secretory granules along the plasma membrane (number of docked and almost-docked vesicles) was comparable between Rab3a KO and WT mice. Specific differences were found in the diameters of their secretory vesicles, diameters of their electron-dense cores and the presence of autophagic structures in the cells of Rab3A KO mice only. Occasionally, individual electron-dense round vesicles were present inside autophagosome-like structures; these were possibly secretory vesicles or their remnants. The differences found in the diameters of the secretory vesicles confirm the key role of Rab3a proteins in controlling the balance between secretory vesicle biogenesis and degradation, and suggest that the ablation of this protein probably changes the nature of the reservoir of secretory vesicles available for regulated exocytosis.

Regulation of the Golgi complex by phospholipid remodeling enzymes

The mammalian Golgi complex is a highly dynamic organelle consisting of stacks of flattened cisternae with associated coated vesicles and membrane tubules that contribute to cargo import and export, intra-cisternal trafficking, and overall Golgi architecture. At the morphological level, all of these structures are continuously remodeled to carry out these trafficking functions. Recent advances have shown that continual phospholipid remodeling by phospholipase A (PLA) and lysophospholipid acyltransferase (LPAT) enzymes, which deacylate and reacylate Golgi phospholipids, respectively, contributes to this morphological remodeling. Here we review the identification and characterization of four cytoplasmic PLA enzymes and one integral membrane LPAT that participate in the dynamic functional organization of the Golgi complex, and how some of these enzymes are integrated to determine the relative abundance of COPI vesicle and membrane tubule formation. This article is part of a Special Issue entitled Lipids and Vesicular Transport.

Electron tomography reveals Rab6 is essential to the trafficking of trans-Golgi clathrin and COPI-coated vesicles and the maintenance of Golgi cisternal number

We have shown previously that Rab6, a small, trans-Golgi-localized GTPase, acts upstream of the conserved oligomeric Golgi complex (COG) and ZW10/RINT1 retrograde tether complexes to maintain Golgi homeostasis. In this article, we present evidence from the unbiased and high-resolution approach of electron microscopy and electron tomography that Rab6 is essential to the trans-Golgi trafficking of two morphological classes of coated vesicles; the larger corresponds to clathrin-coated vesicles and the smaller to coat protein I (COPI)-coated vesicles. On the basis of the site of coated vesicle accumulation, cisternal dilation and the normal kinetics of cargo transport from the endoplasmic reticulum (ER) to Golgi followed by delayed Golgi to cell surface transport, we suggest that Golgi function in cargo transport is preferentially inhibited at the trans-Golgi/trans-Golgi network (TGN). The >50% increase in Golgi cisternae number in Rab6-depleted HeLa cells that we observed may well be coupled to the trans-Golgi accumulation of COPI-coated vesicles; depletion of the individual Rab6 effector, myosin IIA, produced an accumulation of uncoated vesicles with if anything a decrease in cisternal number. These results are the first evidence for a Rab6-dependent protein machine affecting Golgi-proximal, coated vesicle accumulation and probably transport at the trans-Golgi and the first example of concomitant cisternal proliferation and increased Golgi stack organization under inhibited transport conditions.

Crystal structure of inactive form of Rab3B

Rab proteins are the largest family of ras-related GTPases in eukaryotic cells. They act as directional molecular switches at membrane trafficking, including vesicle budding, cargo sorting, transport, tethering, and fusion. Here, we generated and crystallized the Rab3B:GDP complex. The structure of the complex was solved to 1.9Å resolution and the structural base comparison with other Rab3 members provides a structural basis for the GDP/GTP switch in controlling the activity of small GTPase. The comparison of charge distribution among the members of Rab3 also indicates their different roles in vesicular trafficking.

A PLA1-2 punch regulates the Golgi complex

The mammalian Golgi complex, trans Golgi network (TGN) and ER-Golgi intermediate compartment (ERGIC) are comprised of membrane cisternae, coated vesicles and membrane tubules, all of which contribute to membrane trafficking and maintenance of their unique architectures. Recently, a new cast of players was discovered to regulate the Golgi and ERGIC: four unrelated cytoplasmic phospholipase A (PLA) enzymes, cPLA(2)α (GIVA cPLA(2)), PAFAH Ib (GVIII PLA(2)), iPLA(2)-β (GVIA-2 iPLA(2)) and iPLA(1)γ. These ubiquitously expressed enzymes regulate membrane trafficking from specific Golgi subcompartments, although there is evidence for some functional redundancy between PAFAH Ib and cPLA(2)α. Three of these enzymes, PAFAH Ib, cPLA(2)α and iPLA(2)-β, exert effects on Golgi structure and function by inducing the formation of membrane tubules. We review our current understanding of how PLA enzymes regulate Golgi and ERGIC morphology and function.

A genome-wide association study identifies a potential novel gene locus for keratoconus, one of the commonest causes for corneal transplantation in developed countries

Keratoconus is a condition in which the cornea progressively thins over time, and is a major cause for cornea transplantation. To identify keratoconus susceptibility regions, we performed a comprehensive genome-wide association study (GWAS) using a discovery and replication design. A discovery panel of 222 keratoconus Caucasian patients and 3324 Caucasian controls was genotyped using Illumina 370K beadchips. Further associated and fine-mapping single nucleotide polymorphisms (SNPs) (n= 4905) were genotyped in an independent replication case-control panel of 304 cases and 518 controls and a family panel of 307 subjects in 70 families. Logistic regression models implemented in PLINK were performed to test associations in case-control samples with and without principal component (PC) adjustments. Generalized estimation equation models accounting for familial correlations implemented in GWAF were used for association testing in families. No genome-wide associations were identified in the discovery GWAS panel. From the initial testing without adjustments for PCs, the top three SNPs located at 3p26 (rs6442925), 2q21.3 (rs4954218) and 19q13.3 (rs1428642) were identified with unadjusted P-values of 6.5 × 10(-8), 2.4 × 10(-7) and 3.1 × 10(-7), respectively. After adjustments for PCs, rs1428642 became the most significant through the genome with a P-value of 1.4 × 10(-6), while rs6442925 and rs4954218 were less significant (P= 1.9 × 10(-5) and 2.6 × 10(-4)). SNP rs4954218 was confirmed in two independent replication panels with P-values of 0.004 and 0.009, respectively. Meta-analysis revealed a highest association at rs4954218 with adjusted P= 1.6 × 10(-7) (unadjusted P= 1.2 × 10(-9)). These findings suggest SNP rs4954218, located near the RAB3GAP1 gene, previously reported to be associated with corneal malformation, is a potential susceptibility locus for keratoconus.

Structural aspects of Rab6–effector complexes

The small GTPase Rab6 regulates vesicle trafficking at the level of Golgi. Recently, the crystal structures of Rab6 in complexes with two unrelated effectors have been determined. The structure of Rab6a-GTP in complex with a 378-residue internal fragment of the effector Rab6IP1 (Rab6-interacting protein 1) has been solved. In addition, the structure of Rab6 with the golgin, GCC185, has also been determined. In both complexes, two α-helices from the effector mediate binding to switch I, switch II and the interswitch region of Rab6. Comparisons of the complexes reveal significant conformational changes in the conserved hydrophobic triad of Rab6. Thus conformational flexibility in the triad mediates recognition of compositionally distinct α-helical coiled coils, providing a rationale for the promiscuity of Rab6 in effector recruitment.

Silencing of the CHM gene alters phagocytic and secretory pathways in the retinal pigment epithelium

PURPOSE

Choroideremia (CHM) is an X-linked progressive degeneration of the retinal pigment epithelium (RPE), photoreceptors, and choroid caused by mutations in the CHM gene, which encodes Rab escort-protein-1 (REP-1). REP-1 enables posttranslational isoprenyl modification of Rab GTPases, proteins that control vesicle formation, movement, docking, and fusion. The aim of this study was to determine the effect of REP-1 depletion on vesicular trafficking in phagocytic and secretory pathways of human RPE.

METHODS

In vitro, REP-1 expression was inhibited in human fetal RPE (hfRPE) cells by siRNA knockdown and its effects measured on the uptake of bovine photoreceptor outer segments (POS), proteolysis of POS rhodopsin, phagosomal pH, phagosome fusion with early and late endosomes/lysosomes, and polarized secretion of cytokines.

RESULTS

Depletion of REP-1 in human RPE cells did not affect POS internalization but reduced phagosomal acidification and delayed POS protein clearance. REP-1 depletion also caused a decrease in the association of POS-containing phagosomes with late endosomal markers (Rab7, LAMP-1) and increases in the secretion of monocyte chemotactic protein (MCP-1) and interleukin (IL)-8 by hfRPE cells.

CONCLUSIONS

Lack of REP-1 protein expression in hfRPE cells leads to reduced degradation of POS most likely because of the inhibition of phagosome-lysosome fusion events and increased constitutive secretion of MCP-1 and IL-8. These observations may explain the accumulation of unprocessed outer segments within the phagolysosomes of RPE cells and the presence of inflammatory cells in the choroid of patients with CHM.

Proteomics of regulated secretory organelles

Regulated secretory organelles are important subcellular structures of living cells that allow the release in the extracellular space of crucial compounds, such as hormones and neurotransmitters. Therefore, the regulation of biogenesis, trafficking, and exocytosis of regulated secretory organelles has been intensively studied during the last 30 years. However, due to the large number of different regulated secretory organelles, only a few of them have been specifically characterized. New insights into regulated secretory organelles open crucial perspectives for a better comprehension of the mechanisms that govern cell secretion. The combination of subcellular fractionation, protein separation, and mass spectrometry is also possible to study regulated secretory organelles at the proteome level. In this review, we present different strategies used to isolate regulated secretory organelles, separate their protein content, and identify the proteins by mass spectrometry. The biological significance of regulated secretory organelles-proteomic analysis is discussed as well.

The GTPase Rab3b/3c-positive recycling vesicles are involved in cross-presentation in dendritic cells

Antigen cross-presentation in dendritic cells is a complex intracellular membrane transport process, but the underlying molecular mechanisms remain to be thoroughly investigated. In this study, we examined the effect of siRNA-mediated knockdown of 57 Rab GTPases, the key regulators of membrane trafficking, on antigen cross-presentation. Twelve Rab GTPases were identified to be associated with antigen cross-presentation, and Rab3b/3c was indicated to be colocalized with MHC class I molecules at perinuclear tubular structure. Tracing with fluorescence protein-tagged beta(2)-microglobulin demonstrated that the MHC class I molecules were internalized from the plasma membrane to Rab3b/3c-positive compartments, which were also colocalized with the internalized transferrin. Moreover, depletion of Rab3b/3c strongly reduced the fast phase recycling rate of transferrin receptors. Furthermore, the Rab3b/3c-positive compartments were colocalized with a fraction of Rab27a at a juxtaposition of phagosomes. Together, these data demonstrate that Rab3b/3c-positive recycling vesicles are involved in and may constitute one of the recycling compartments in exogenous antigen cross-presentation.

Identification and characterization of RBEL1 subfamily of GTPases in the Ras superfamily involved in cell growth regulation

Recently, we reported the identification of a novel gene named RBEL1 (Rab-like protein 1) and characterized its two encoded isoforms, RBEL1A and RBEL1B, that function as novel GTPases of Ras superfamily. Here we report the identification of two additional splice variants of RBEL1 that we have named RBEL1C and -D. All four RBEL1 isoforms (A, B, C, and D) have identical N termini harboring the Rab-like GTPase domains but contain variable C termini. Although all isoforms can be detected in both cytoplasm and nucleus, RBEL1A is predominantly cytoplasmic, whereas RBEL1B is mostly nuclear. RBEL1C and -D, by contrast, are evenly distributed between the cytoplasm and nucleus. Furthermore, all four RBEL1 proteins are also capable of associating with cellular membrane. The RBEL1 proteins also exhibit a unique nucleotide-binding potential and, whereas the larger A and B isoforms are mainly GTP-bound, the smaller C and D variants bind to both GTP and GDP. Furthermore, a regulatory region at amino acid position 236-302 immediately adjacent to the GTP-binding domain is important for GTP-binding potential of RBEL1A, because deletion of this region converts RBEL1A from predominantly GTP-bound to GDP-bound. RBEL1 knockdown via RNA interference results in marked cell growth suppression, which is associated with morphological and biochemical features of apoptosis as well as inhibition of extracellular signal-regulated kinase phosphorylation. Taken together, our results indicate that RBEL1 proteins are linked to cell growth and survival and possess unique biochemical, cellular, and functional characteristics and, therefore, appear to form a novel subfamily of GTPases within the Ras superfamily.

Structural basis for recruitment of Rab6-interacting protein 1 to Golgi via a RUN domain

Small GTPase Rab6 regulates vesicle trafficking at the level of Golgi via recruitment of numerous and unrelated effectors. The crystal structure of Rab6a(GTP) in complex with a 378-residue internal fragment of the effector Rab6IP1 was solved at 3.2 angstroms resolution. This Rab6IP1 region encompasses an all alpha-helical RUN domain followed in tandem by a PLAT domain that adopts a beta sandwich fold. The structure reveals that the first and last alpha helices of the RUN domain mediate binding to switch I, switch II, and the interswitch region of Rab6. It represents the largest Rab-effector complex determined to date. Comparisons with the recent structure of Rab6 in complex with an unrelated effector, human golgin GCC185, reveals significant conformational changes in the conserved hydrophobic triad of Rab6. Flexibility in the switch and interswitch regions of Rab6 mediates recognition of compositionally distinct alpha-helical coiled coils, thereby contributing to Rab6 promiscuity in effector recruitment.

Small G proteins as key regulators of pancreatic digestive enzyme secretion

Small GTP-binding (G) proteins act as molecular switches to regulate a number of cellular processes, including vesicular transport. Emerging evidence indicates that small G proteins regulate a number of steps in the secretion of pancreatic acinar cells. Diverse small G proteins have been localized at discrete compartments along the secretory pathway and particularly on the secretory granule. Rab3D, Rab27B, and Rap1 are present on the granule membrane and play a role in the steps leading up to exocytosis. Whether the function of these G proteins is simply to ensure appropriate targeting or if they are involved as regulatory molecules is discussed. Most evidence suggests that Rab3D and Rab27B play a role in tethering the secretory granule to its target membrane. Other Rabs have been identified on the secretory granule that are associated with different steps in the secretory pathway. The Rho family small G proteins RhoA and Rac1 also regulate secretion through remodeling of the actin cytoskeleton. Possible mechanisms for regulation of these G proteins and their effector molecules are considered.

Herpes simplex virus utilizes the large secretory vesicle pathway for anterograde transport of tegument and envelope proteins and for viral exocytosis from growth cones of human fetal axons

Axonal transport of herpes simplex virus (HSV-1) is essential for viral infection and spread in the peripheral nervous system of the host. Therefore, the virus probably utilizes existing active transport and targeting mechanisms in neurons for virus assembly and spread from neurons to skin. In the present study, we used transmission immunoelectron microscopy to investigate the nature and origin of vesicles involved in the anterograde axonal transport of HSV-1 tegument and envelope proteins and of vesicles surrounding partially and fully enveloped capsids in growth cones. This study aimed to elucidate the mechanism of virus assembly and exit from axons of human fetal dorsal root ganglia neurons. We demonstrated that viral tegument and envelope proteins can travel in axons independently of viral capsids and were transported to the axon terminus in two types of transport vesicles, tubulovesicular membrane structures and large dense-cored vesicles. These vesicles and membrane carriers were derived from the trans-Golgi network (TGN) and contained key proteins, such as Rab3A, SNAP-25, GAP-43, and kinesin-1, involved in the secretory and exocytic pathways in axons. These proteins were also observed on fully and partially enveloped capsids in growth cones and on extracellular virions. Our findings provide further evidence to the subassembly model of separate transport in axons of unenveloped capsids from envelope and tegument proteins with final virus assembly occurring at the axon terminus. We postulate that HSV-1 capsids invaginate tegument- and envelope-bearing TGN-derived vesicles and utilize the large secretory vesicle pathway of exocytosis for exit from axons.

Achaete-scute complex homologue 1 regulates tumor-initiating capacity in human small cell lung cancer

The basic helix-loop-helix transcription factor achaete-scute complex homologue 1 (ASCL1) is essential for the development of normal lung neuroendocrine cells as well as other endocrine and neural tissues. Small cell lung cancer (SCLC) and non-SCLC with neuroendocrine features express ASCL1, where the factor may play a role in the virulence and primitive neuroendocrine phenotype of these tumors. In this study, RNA interference knockdown of ASCL1 in cultured SCLC resulted in inhibition of soft agar clonogenic capacity and induction of apoptosis. cDNA microarray analyses bolstered by expression studies, flow cytometry, and chromatin immunoprecipitation identified two candidate stem cell marker genes, CD133 and aldehyde dehydrogenase 1A1 (ALDH1A1), to be directly regulated by ASCL1 in SCLC. In SCLC direct xenograft tumors, we detected a relatively abundant CD133(high)-ASCL1(high)-ALDH1(high) subpopulation with markedly enhanced tumorigenicity compared with cells with weak CD133 expression. Tumorigenicity in the CD133(high) subpopulation depended on continued ASCL1 expression. Whereas CD133(high) cells readily reconstituted the range of CD133 expression seen in the original xenograft tumor, CD133(low) cells could not. Our findings suggest that a broad range of SCLC cells has tumorigenic capacity rather than a small discrete population. Intrinsic tumor cell heterogeneity, including variation in key regulatory factors such as ASCL1, can modulate tumorigenicity in SCLC.

Heterotypic tubular connections at the endoplasmic reticulum-Golgi complex interface

Electron microscopy and cryoimmunocytochemistry were used to characterize tubular connections in the secretory pathway using rat spermatids as model. Our results support the existence of a complex tubular network enriched in the Golgi matrix protein GM130 that transiently joins the cis-Golgi side and the endoplasmic reticulum. These tubules occasionally contain the endoplasmic reticulum resident protein PDI but not COPII complexes or KDEL receptor. At the lateral edges of the stacks tubules were seen to connect cisternae belonging to the same or adjacent stacks. These connections were observed in all cisternae but preferentially on the cis side. Giantin, Gos28 and Rab6 were detected in the tubules; importantly, we reported the presence of cis-trans heterotypic connections between cisternae. On the trans-Golgi side, we occasionally observed tubules highly immunoreactive for Rab6 connecting the stack with the forming acrosome. Together, our results support the existence of transient continuities throughout the secretory pathways.

Direct interaction between Rab3D and the polymeric immunoglobulin receptor and trafficking through regulated secretory vesicles in lacrimal gland acinar cells

The lacrimal gland is responsible for tear production, and a major protein found in tears is secretory component (SC), the proteolytically cleaved fragment of the extracellular domain of the polymeric Ig receptor (pIgR), which is the receptor mediating the basal-to-apical transcytosis of polymeric immunoglobulins across epithelial cells. Immunofluorescent labeling of rabbit lacrimal gland acinar cells (LGACs) revealed that the small GTPase Rab3D, a regulated secretory vesicle marker, and the pIgR are colocalized in subapical membrane vesicles. In addition, the secretion of SC from primary cultures of LGACs was stimulated by the cholinergic agonist carbachol (CCH), and its release rate was very similar to that of other regulated secretory proteins in LGACs. In pull-down assays from resting LGACs, recombinant wild-type Rab3D (Rab3DWT) or the GDP-locked mutant Rab3DT36N both pulled down pIgR, but the GTP-locked mutant Rab3DQ81L did not. When the pull-down assays were performed in the presence of guanosine-5'-(gamma-thio)-triphosphate, GTP, or guanosine-5'-O-(2-thiodiphosphate), binding of Rab3DWT to pIgR was inhibited. In blot overlays, recombinant Rab3DWT bound to immunoprecipitated pIgR, suggesting that Rab3D and pIgR may interact directly. Adenovirus-mediated overexpression of mutant Rab3DT36N in LGACs inhibited CCH-stimulated SC release, and, in CCH-stimulated LGACs, pull down of pIgR with Rab3DWT and colocalization of pIgR with endogenous Rab3D were decreased relative to resting cells, suggesting that the pIgR-Rab3D interaction may be modulated by secretagogues. These data suggest that the novel localization of pIgR to the regulated secretory pathway of LGACs and its secretion therefrom may be affected by its novel interaction with Rab3D.

RBEL1 is a novel gene that encodes a nucleocytoplasmic Ras superfamily GTP-binding protein and is overexpressed in breast cancer

Rab family proteins are generally known as regulators of protein transport and trafficking. A number of Rab proteins have been implicated in cancer development and/or progression. Here we report the identification of a novel Rab-like protein, which we have named RBEL1 (Rab-like protein 1) for its higher similarity to the Rab subfamily members. We have characterized two isoforms of RBEL1 including the predominant RBEL1A and the less abundant RBEL1B that results from alternative splicing. Both isoforms harbor conserved N-terminal guanine trinucleotide phosphate (GTP) binding domains and, accordingly, are capable of binding to GTP. Both isoforms contain variable C termini and exhibit differential subcellular localization patterns. Unlike known Rabs that are mostly cytosolic, RBEL1B predominantly resides in the nucleus, whereas RBEL1A is localized primarily to the cytosol. Interestingly, a point mutation affecting RBEL1B GTP binding also alters the ability of mutant protein to accumulate in the nucleus, suggesting GTP binding potential to be important for RBEL1B nuclear localization. Our results also indicate that RBEL1A is overexpressed in about 67% of primary breast tumors. Thus, RBEL1A and RBEL1B are novel Rab-like proteins that localize in the nucleus and cytosol and may play an important role in breast tumorigenesis.

The role of Rab3a in secretory vesicle docking requires association/dissociation of guanidine phosphates and Munc18-1

Rab3a is a small GTPase that binds selectively to secretory vesicles and switches between active, GTP-bound and inactive, GDP-bound conformations. In yeast, Rab and SM-genes interact genetically to promote vesicle targeting/fusion. We tested different Rab3a conformations and genetic interactions with the SM-gene munc18-1 on the docking function of Rab3a in mammalian chromaffin cells. We expressed Rab3a mutants locked in the GTP- or GDP-bound form in wild-type and munc18-1 null mutant cells and analyzed secretory vesicle distribution. We confirmed that wild-type Rab3a promotes vesicle docking in wild-type cells. Unexpectedly, both GTP- and GDP-locked Rab3a mutants did not promote docking. Furthermore, wild-type Rab3a did not promote docking in munc18-1 null cells and GTP- and GDP-Rab3a both decreased the amount of docked vesicles. The results show that GTP- and GDP-locked conformations do not support a Munc18-1 dependent role of Rab3a in docking. This suggests that nucleotide cycling is required to support docking and that this action of Rab3a is upstream of Munc18-1.

Identification of novel genes expressed in hypoxic brain condition by fluorescence differential display

Fluorescence differential display (FDD) and comparative RT-PCR have been used extensively to detect differentially expressed genes. We investigated hypoxia-induced gene expression in the brain by FDD-PCR and comparative RT-PCR. Mice were anaesthetized after which hypoxia was induced by neck ligation for 1 min or 25 min, then were killed by decapitation, and the brains were dissected either immediately or 30 min after death (Group A1-0, Group A25-0, Group A1-30 and Group A25-30). Control mice without ligation of the neck were killed by decapitation under anaesthesia immediately (Group C-0) or 30 min after death (Group C-30). FDD-PCR, sequence analysis and comparative RT-PCR revealed that mitochondrial thymidine kinase 2, Rab6, selenoprotein T and two novel cDNAs were enhanced in Group A25-0 and Group A25-30 compared with the other groups. In Group A25-30, only RAP2 interacting protein and another novel cDNA were induced. In Group A25-0, one novel gene was detected. These findings were consistent with the results of statistical analysis by ANOVA. No differences of band pattern were observed among Groups A1-0, A1-30, C-0 and C-30. The genes exhibiting altered expression were associated with vital cellular functions, e.g., intracellular signaling and mitochondrial metabolism. In addition, we identified four novel genes, expressed after extended hypoxic conditions in mouse brain with suffocation. These results may contribute to clarify the pathophysiology of asphyxia in the brain and aid in the diagnosis of suffocation.

Rab18 inhibits secretory activity in neuroendocrine cells by interacting with secretory granules

Rab proteins comprise a complex family of small GTPases involved in the regulation of intracellular membrane trafficking and reorganization. In this study, we identified Rab18 as a new inhibitory player of the secretory pathway in neuroendocrine cells. In adrenal chromaffin PC12 cells and pituitary AtT20 cells, Rab18 is located at the cytosol but associates with a subpopulation of secretory granules after stimulation of the regulated secretory pathway, strongly suggesting that induction of secretion provokes Rab18 activation and recruitment to these organelles. In support of this, a dominant-inactive Rab18 mutant was found to distribute diffusely in the cytosol, whereas a dominant-active Rab18 mutant was predominantly associated to secretory granules. Furthermore, interaction of Rab18 with secretory granules was associated to an inhibition in the secretory activity of PC12 and AtT20 cells in response to stimulatory challenges. Association of Rab18 with secretory granules was also observed by immunoelectron microscopy in normal, non-tumoral endocrine cells (pituitary melanotropes), wherein Rab18 protein content is inversely correlated to the level of secretory activity of cells. Taken together, these findings suggest that, in neuroendocrine cells, Rab18 acts as a negative regulator of secretory activity, likely by impairing secretory granule transport.

Water channels and zymogen granules in salivary glands

Salivary secretion occurs in response to stimulation by neurotransmitters released from autonomic nerve endings. The molecular mechanisms underlying the secretion of water, a main component of saliva, from salivary glands are not known; the plasma membrane is a major barrier to water transport. A 28-kDa integral membrane protein, distributed in highly water-permeable tissues, was identified as a water channel protein, aquaporin (AQP). Thirteen AQPs (AQP0 - AQP12) have been identified in mammals. AQP5 is localized in lipid rafts under unstimulated conditions and translocates to the apical plasma membrane in rat parotid glands upon stimulation by muscarinic agonists. The importance of increases in intracellular calcium concentration [Ca(2+)](i) and the nitric oxide synthase and protein kinase G signaling pathway in the translocation of AQP5 is reviewed in section I. Signals generated by the activation of Ca(2+) mobilizing receptors simultaneously trigger and regulate exocytosis. Zymogen granule exocytosis occurs under the control of essential process, stimulus-secretion coupling, in salivary glands. Ca(2+) signaling is a principal signal in both protein and water secretion from salivary glands induced by cholinergic stimulation. On the other hand, the cyclic adenosine monophosphate (cAMP)/cAMP-dependent protein kinase system has a major role in zymogen granule exocytosis without significant increases in [Ca(2+)](i). In section II, the mechanisms underlying the control of salivary protein secretion and its dysfunction are reviewed.

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.

Rabphilin regulates SNARE-dependent re-priming of synaptic vesicles for fusion

Synaptic vesicle fusion is catalyzed by assembly of synaptic SNARE complexes, and is regulated by the synaptic vesicle GTP-binding protein Rab3 that binds to RIM and to rabphilin. RIM is a known physiological regulator of fusion, but the role of rabphilin remains obscure. We now show that rabphilin regulates recovery of synaptic vesicles from use-dependent depression, probably by a direct interaction with the SNARE protein SNAP-25. Deletion of rabphilin dramatically accelerates recovery of depressed synaptic responses; this phenotype is rescued by viral expression of wild-type rabphilin, but not of mutant rabphilin lacking the second rabphilin C2 domain that binds to SNAP-25. Moreover, deletion of rabphilin also increases the size of synaptic responses in synapses lacking the vesicular SNARE protein synaptobrevin in which synaptic responses are severely depressed. Our data suggest that binding of rabphilin to SNAP-25 regulates exocytosis of synaptic vesicles after the readily releasable pool has either been physiologically exhausted by use-dependent depression, or has been artificially depleted by deletion of synaptobrevin.

Rab3A deletion selectively reduces spontaneous neurotransmitter release at the mouse neuromuscular synapse

Rab3A is a synaptic vesicle-associated GTP-binding protein thought to be involved in modulation of presynaptic transmitter release through regulation of vesicle trafficking and membrane fusion. Electrophysiological studies at central nervous system synapses of Rab3A null-mutant mice have indicated that nerve stimulation-evoked transmitter release and its short- and long-term modulation are partly dependent on Rab3A, whereas spontaneous uniquantal release is completely independent of it. Here, we studied the acetylcholine (ACh) release at the neuromuscular junction (NMJ) of diaphragm and soleus muscles from Rab3A-deficient mice with intracellular microelectrode methods. Surprisingly, we found 20-40% reduction of spontaneous ACh release but completely intact nerve action potential-evoked release at both high- and low-rate stimulation and during recovery from intense release. The ACh release induced by hypertonic medium was also unchanged, indicating that the pool of vesicles for immediate release is unaltered at the Rab3A-deficient NMJ. These results indicate a selective role of Rab3A in spontaneous transmitter release at the NMJ which cannot or only partly be taken over by the closely related Rab3B, Rab3C, or Rab3D isoforms when Rab3A is deleted. It has been hypothesized that Rab3A mutation underlies human presynaptic myasthenic syndromes, in which severely reduced nerve action potential-evoked ACh release at the NMJ causes paralysis. Our observation that Rab3A deletion does not reduce evoked ACh release at any stimulation rate at the mouse NMJ, argues against this hypothesis.

Physical and functional interaction of noc2/rab3 in exocytosis

Rab, monomeric small Ras-like GTPase, regulates intracellular membrane trafficking in eukaryotic cells. Rab3 is involved in the exocytotic process in a variety of secretory cells including neuronal, neuroendocrine, endocrine, and exocrine cells. Noc2, originally identified as a molecule homologous to Rabphilin-3, is a putative effector of Rab3. Noc2 interacts with the active (GTP-bound) form of Rab3 and regulates hormone secretion in neuroendocrine and endocrine cells and enzyme release in exocrine cells. This chapter describes two kinds of interaction assay by which the association of Noc2 with Rab3 is analyzed: a yeast two-hybrid assay to detect the interaction of Noc2 with the active form of Rab3 in intact cells and a pull-down assay using GST-fused Noc2 protein to ascertain the physical interaction of Noc2 and Rab3 in vitro. Thus, the Noc2 knockout mouse is a useful model for studying the functional consequences of disruption of the interaction.

Molecular mechanisms of lacrimal acinar secretory vesicle exocytosis

The acinar epithelial cells of the lacrimal gland are responsible for the production, packaging and regulated exocytosis of tear proteins into ocular surface fluid. This review summarizes new findings on the mechanisms of exocytosis in these cells. Participating proteins are discussed within the context of different categories of trafficking effectors including targeting and specificity factors (rabs, SNAREs) and transport factors (microtubules, actin filaments and motor proteins). Recent information describing fundamental changes in basic exocytotic mechanisms in the NOD mouse, an animal model of Sjögren's syndrome, is presented.

Functional assays for the investigation of the role of Rab GTPase effectors in dense core granule release

Rab3 and Rab27 GTPases control late events in the secretory pathway of mammalian cells including docking and fusion of secretory vesicles with the plasma membrane. The action of Rab3 and Rab27 on the exocytotic process is exerted through the activation of specific effectors. Several proteins with the capacity to interact in a GTP-dependent manner with Rab3 and Rab27 have been identified. However, for most of these potential Rab effectors a precise function in the secretory process has not yet been attributed. In this chapter we describe a series of approaches that can be applied to assess the properties of Rab3 and Rab27 effectors and their potential role in the regulation of dense core granule release.

T cells use two directionally distinct pathways for cytokine secretion

Activated T helper cells produce many cytokines, some of which are secreted through the immunological synapse toward the antigen-presenting cell. Here we have used immunocytochemistry, live-cell imaging and a surface-mediated secretion assay to show that there are two cytokine export pathways in T helper cells. Some cytokines, including interleukin 2 and interferon-gamma, were secreted into the synapse, whereas others, including tumor necrosis factor and the chemokine CCL3 (MIP-1alpha), were released multidirectionally. Each secretion pathway was associated with different trafficking proteins, indicating that they are molecularly distinct processes. These data suggest that T helper cells release some cytokines into the immunological synapse to impart specific communication and others multidirectionally to promote inflammation and to establish chemokine gradients.