Physiological roles of Rab27 effectors in regulated exocytosis

Knockdown of TTC9 inhibits the proliferation, migration and invasion, but induces the apoptosis of lung adenocarcinoma cells

Lung adenocarcinoma (LUAD) is one of the most commonly diagnosed subtypes of lung cancer, and one of the deadliest cancers. Tetratricopeptide repeat domain 9A (TTC9) is upregulated and has played an oncogenic role in some malignant tumors. However, the expression and role of TTC9 has not yet been elucidated in LUAD. Here, we investigated the expression profiles, biological functions and potential molecular mechanism of the TTC9 gene in LUAD. TTC9 expression was significantly overexpressed in LUAD tissues compared with that in normal lung tissues. TTC9 expression was closely correlated with gender, lymph node metastasis, and survival status in the TCGA-LUAD cohort. Subsequent cellular function assays demonstrated that knockdown of TTC9 promoted PC9 cell apoptosis and inhibited cell proliferation, migration and invasion, leading to cell cycle arrest in G2 phase. Moreover, inhibition of TTC9 suppressed the tumorigenicity of PC9 cells in nude mice. TTC9 might serve as oncogene in LUAD through cancer-related signaling pathways including p38 MAPK pathway. The expression of TTC9 gene might be modulated by DNA copy number variant and DNA methylation. TTC9 was significantly associated with tumor immune infiltration patterns. Accordingly, TTC9 may be a novel therapeutic target for the treatment of LUAD.

In vivo Roles of Rab27 and Its Effectors in Exocytosis

The monomeric GTPase Rab27 regulates exocytosis of a broad range of vesicles in multicellular organisms. Several effectors bind GTP-bound Rab27a and/or Rab27b on secretory vesicles to execute a series of exocytic steps, such as vesicle maturation, movement along microtubules, anchoring within the peripheral F-actin network, and tethering to the plasma membrane, via interactions with specific proteins and membrane lipids in a local milieu. Although Rab27 effectors generally promote exocytosis, they can also temporarily restrict it when they are involved in the rate-limiting step. Genetic alterations in Rab27-related molecules cause discrete diseases manifesting pigment dilution and immunodeficiency, and can also affect common diseases such as diabetes and cancer in complex ways. Although the function and mechanism of action of these effectors have been explored, it is unclear how multiple effectors act in coordination within a cell to regulate the secretory process as a whole. It seems that Rab27 and various effectors constitutively reside on individual vesicles to perform consecutive exocytic steps. The present review describes the unique properties and in vivo roles of the Rab27 system, and the functional relationship among different effectors coexpressed in single cells, with pancreatic beta cells used as an example.Key words: membrane trafficking, regulated exocytosis, insulin granules, pancreatic beta cells.

Exophilin-5 regulates allergic airway inflammation by controlling IL-33-mediated Th2 responses

A common variant in the RAB27A gene in adults was recently found to be associated with the fractional exhaled nitric oxide level, a marker of eosinophilic airway inflammation. The small GTPase Rab27 is known to regulate intracellular vesicle traffic, although its role in allergic responses is unclear. We demonstrated that exophilin-5, a Rab27-binding protein, was predominantly expressed in both of the major IL-33 producers, lung epithelial cells, and the specialized IL-5 and IL-13 producers in the CD44hiCD62LloCXCR3lo pathogenic Th2 cell population in mice. Exophilin-5 deficiency increased stimulant-dependent damage and IL-33 secretion by lung epithelial cells. Moreover, it enhanced IL-5 and IL-13 production in response to TCR and IL-33 stimulation from a specific subset of pathogenic Th2 cells that expresses a high level of IL-33 receptor, which exacerbated allergic airway inflammation in a mouse model of asthma. Mechanistically, exophilin-5 regulates extracellular superoxide release, intracellular ROS production, and phosphoinositide 3-kinase activity by controlling intracellular trafficking of Nox2-containing vesicles, which seems to prevent the overactivation of pathogenic Th2 cells mediated by IL-33. This is the first report to our knowledge to establish the significance of the Rab27-related protein exophilin-5 in the development of allergic airway inflammation, and provides insights into the pathophysiology of asthma.

Determination of the Rab27-Effector Binding Affinity Using a High-Throughput FRET-Based Assay

Thus far, two Rab27 isoforms (Rab27a and Rab27b) have been identified that interact with their eleven downstream effectors proteins, preferentially in their GTP-bound state. In recent years, a number of studies has suggested roles for Rab27-effector protein interactions in the development of cancer cell invasion and metastasis, and immune and inflammatory responses. Here we develop an in vitro fluorescence resonance energy transfer (FRET)-based protein-protein interaction assay to report Rab27 protein interactions with their effectors. We particularly focus on determining the interaction of mouse (m) Synaptotagmin-like protein (Slp)1 and mSlp2 effector proteins with human (h)Rab27. Green fluorescent protein (GFP)-N-terminus Rab27 binding domains (m-Slp1 and m-Slp2) recombinant proteins were used as donor fluorophores, whereas mCherry-hRab27a/b recombinant proteins were used as acceptor fluorophores. The conditions of this assay were validated and optimized, and the specificity of the assay was confirmed. Accordingly, this assay can be used to assess and identify key determinants and/or candidate inhibitors of Rab27-effector interactions.

Rab27a co-ordinates actin-dependent transport by controlling organelle-associated motors and track assembly proteins

Cell biologists generally consider that microtubules and actin play complementary roles in long- and short-distance transport in animal cells. On the contrary, using melanosomes of melanocytes as a model, we recently discovered that the motor protein myosin-Va works with dynamic actin tracks to drive long-range organelle dispersion in opposition to microtubules. This suggests that in animals, as in yeast and plants, myosin/actin can drive long-range transport. Here, we show that the SPIRE-type actin nucleators (predominantly SPIRE1) are Rab27a effectors that co-operate with formin-1 to generate actin tracks required for myosin-Va-dependent transport in melanocytes. Thus, in addition to melanophilin/myosin-Va, Rab27a can recruit SPIREs to melanosomes, thereby integrating motor and track assembly activity at the organelle membrane. Based on this, we suggest a model in which organelles and force generators (motors and track assemblers) are linked, forming an organelle-based, cell-wide network that allows their collective activity to rapidly disperse the population of organelles long-distance throughout the cytoplasm.

Grab recruitment by Rab27A-Rabphilin3a triggers Rab3A activation in human sperm exocytosis

Sperm must undergo the regulated exocytosis of its dense core granule (the acrosome reaction, AR) to fertilize the egg. We have previously described that Rabs3 and 27 are organized in a RabGEF cascade within the signaling pathway elicited by exocytosis stimuli in human sperm. Here, we report the identity and the role of two molecules that link these secretory Rabs in the RabGEF cascade: Rabphilin3a and GRAB. Like Rab3 and Rab27, GRAB and Rabphilin3a are present, localize to the acrosomal region and are required for calcium-triggered exocytosis in human sperm. Sequestration of either protein with specific antibodies introduced into streptolysin O-permeabilized sperm impairs the activation of Rab3 in the acrosomal region elicited by calcium, but not that of Rab27. Biochemical and functional assays indicate that Rabphilin3a behaves as a Rab27 effector during the AR and that GRAB exhibits GEF activity toward Rab3A. Recombinant, active Rab27A pulls down Rabphilin3a and GRAB from human sperm extracts. Conversely, immobilized Rabphilin3a recruits Rab27 and GRAB; the latter promotes Rab3A activation. The enzymatic activity of GRAB toward Rab3A was also suggested by in silico and in vitro assays with purified proteins. In summary, we describe here a signaling module where Rab27A-GTP interacts with Rabphilin3a, which in turn recruits a guanine nucleotide-exchange activity toward Rab3A. This is the first description of the interaction of Rabphilin3a with a GEF. Because the machinery that drives exocytosis is highly conserved, it is tempting to hypothesize that the RabGEF cascade unveiled here might be part of the molecular mechanisms that drive exocytosis in other secretory systems.

Regulation of insulin exocytosis by calcium-dependent protein kinase C in beta cells

The control of insulin release from pancreatic beta cells helps ensure proper blood glucose level, which is critical for human health. Protein kinase C has been shown to be one key control mechanism for this process. After glucose stimulation, calcium influx into beta cells triggers exocytosis of insulin-containing dense-core granules and activates protein kinase C via calcium-dependent phospholipase C-mediated generation of diacylglycerol. Activated protein kinase C potentiates insulin release by enhancing the calcium sensitivity of exocytosis, likely by affecting two main pathways that could be linked: (1) the reorganization of the cortical actin network, and (2) the direct phosphorylation of critical exocytotic proteins such as munc18, SNAP25, and synaptotagmin. Here, we review what is currently known about the molecular mechanisms of protein kinase C action on each of these pathways and how these effects relate to the control of insulin release by exocytosis. We identify remaining challenges in the field and suggest how these challenges might be addressed to advance our understanding of the regulation of insulin release in health and disease.

Rab2a and Rab27a cooperatively regulate the transition from granule maturation to exocytosis through the dual effector Noc2

Exocytosis of secretory granules entails budding from the trans-Golgi network, sorting and maturation of cargo proteins, and trafficking and fusion to the plasma membrane. Rab27a regulates the late steps in this process, such as granule recruitment to the fusion site, whereas Rab2a functions in the early steps, such as granule biogenesis and maturation. Here, we demonstrate that these two small GTPases simultaneously bind to Noc2 (also known as RPH3AL) in a GTP-dependent manner, although Rab2a binds only after Rab27a has bound. In pancreatic β-cells, the ternary Rab2a-Noc2-Rab27a complex specifically localizes on perinuclear immature granules, whereas the binary Noc2-Rab27a complex localizes on peripheral mature granules. In contrast to the wild type, Noc2 mutants defective in binding to Rab2a or Rab27a fail to promote glucose-stimulated insulin secretion. Although knockdown of any component of the ternary complex markedly inhibits insulin secretion, only knockdown of Rab2a or Noc2, and not that of Rab27a, impairs cargo processing from proinsulin to insulin. These results suggest that the dual effector, Noc2, regulates the transition from Rab2a-mediated granule biogenesis to Rab27a-mediated granule exocytosis.

New Insights into How Trafficking Regulates T Cell Receptor Signaling

There is emerging evidence that exocytosis plays an important role in regulating T cell receptor (TCR) signaling. The trafficking molecules involved in lytic granule (LG) secretion in cytotoxic T lymphocytes (CTL) have been well-studied due to the immune disorder known as familial hemophagocytic lymphohistiocytosis (FHLH). However, the knowledge of trafficking machineries regulating the exocytosis of receptors and signaling molecules remains quite limited. In this review, we summarize the reported trafficking molecules involved in the transport of the TCR and downstream signaling molecules to the cell surface. By combining this information with the known knowledge of LG exocytosis and general exocytic trafficking machinery, we attempt to draw a more complete picture of how the TCR signaling network and exocytic trafficking matrix are interconnected to facilitate T cell activation. This also highlights how membrane compartmentalization facilitates the spatiotemporal organization of cellular responses that are essential for immune functions.

Rab27b is Involved in Lysosomal Exocytosis and Proteolipid Protein Trafficking in Oligodendrocytes

Myelination by oligodendrocytes in the central nervous system requires coordinated exocytosis and endocytosis of the major myelin protein, proteolipid protein (PLP). Here, we demonstrated that a small GTPase, Rab27b, is involved in PLP trafficking in oligodendrocytes. We showed that PLP co-localized with Rab27b in late endosomes/lysosomes in oligodendrocytes. Short hairpin-mediated knockdown of Rab27b not only reduced lysosomal exocytosis but also greatly diminished the surface expression of PLP in oligodendrocytes. In addition, knockdown of Rab27b reduced the myelin-like membranes induced by co-culture of oligodendrocytes and neurons. Our data suggest that Rab27b is involved in myelin biogenesis by regulating PLP transport from late endosomes/lysosomes to the cell membrane in oligodendrocytes.

GTP- and GDP-Dependent Rab27a Effectors in Pancreatic Beta-Cells

Small guanosine triphosphatases (GTPases) participate in a wide variety of cellular functions including proliferation, differentiation, adhesion, and intracellular transport. Conventionally, only the guanosine 5'-triphosphate (GTP)-bound small GTPase interacts with effector proteins, and the resulting downstream signals control specific cellular functions. Therefore, the GTP-bound form is regarded as active, and the focus has been on searching for proteins that bind the GTP form to look for their effectors. The Rab family small GTPase Rab27a is highly expressed in some secretory cells and is involved in the control of membrane traffic. The present study reviews recent progress in our understanding of the roles of Rab27a and its effectors in pancreatic beta-cells. In the basal state, GTP-bound Rab27a controls insulin secretion at pre-exocytic stages via its GTP-dependent effectors. We previously identified novel guanosine 5'-diphosphate (GDP)-bound Rab27-interacting proteins. Interestingly, GDP-bound Rab27a controls endocytosis of the secretory membrane via its interaction with these proteins. We also demonstrated that the insulin secretagogue glucose converts Rab27a from its GTP- to GDP-bound forms. Thus, GTP- and GDP-bound Rab27a regulate pre-exocytic and endocytic stages in membrane traffic, respectively. Since the physiological importance of GDP-bound GTPases has been largely overlooked, we consider that the investigation of GDP-dependent effectors for other GTPases is necessary for further understanding of cellular function.

Rab27a/Slp2-a complex is involved in Schwann cell myelination

Myelination of Schwann cells in the peripheral nervous system is an intricate process involving myelin protein trafficking. Recently, the role and mechanism of the endosomal/lysosomal system in myelin formation were emphasized. Our previous results demonstrated that a small GTPase Rab27a regulates lysosomal exocytosis and myelin protein trafficking in Schwann cells. In this present study, we established a dorsal root ganglion (DRG) neuron and Schwann cell co-culture model to identify the signals associated with Rab27a during myelination. First, Slp2-a, as the Rab27a effector, was endogenously expressed in Schwann cells. Second, Rab27a expression significantly increased during Schwann cell myelination. Finally, Rab27a and Slp2-a silencing in Schwann cells not only reduced myelin protein expression, but also impaired formation of myelin-like membranes in DRG neuron and Schwann cell co-cultures. Our findings suggest that the Rab27a/Slp2-a complex affects Schwann cell myelination in vitro.

Inhibition of cancer cell invasion by new ((3,4-dihydroxy benzylidene)hydrazinyl)pyridine-3-sulfonamide analogs

Rab GTPases regulate various types of intracellular membrane trafficking in all eukaryotes. Since Rab27a and its multiple effectors are involved in exocytosis of lysosome-related organelles and play a major role in malignancy, compounds targeting Rab27a could be likely used to inhibit invasive growth and tumor metastasis. Thus, we designed and synthesized several compounds based on the previously reported Rab27a-targeting synthetic compounds identified by virtual screening, and investigated their anti-metastatic effects in MDA-MB231 and A375 cells. Among the synthesized compounds, (E)-N-(3-chlorophenyl)-6-(2-(3,4-dihydroxy benzylidene)hydrazinyl)pyridine-3-sulfonamide (3d) and (E)-N-benzyl-6-(2-(3,4-dihydroxy benzylidene)hydrazinyl)-N-methylpyridine-3-sulfonamide (3f) significantly inhibited the invasiveness of both tumor cell lines. Compounds 3d and 3f also decreased the levels of signature extracellular matrix marker proteins (fibronectin, collagen, and α-smooth muscle actin) and representative mesenchymal cell markers (N-cadherin and vimentin). Taken together, our results suggest that novel sulfonamide analogs have anti-metastatic activity in breast and melanoma cancer cell lines and may be used as therapeutic agents to treat malignant cancer.

PI3K regulates endocytosis after insulin secretion by mediating signaling crosstalk between Arf6 and Rab27a

In secretory cells, endocytosis is coupled to exocytosis to enable proper secretion. Although endocytosis is crucial to maintain cellular homeostasis before and after secretion, knowledge about secretagogue-induced endocytosis in secretory cells is still limited. Here, we searched for proteins that interacted with the Rab27a GTPase-activating protein (GAP) EPI64 (also known as TBC1D10A) and identified the Arf6 guanine-nucleotide-exchange factor (GEF) ARNO (also known as CYTH2) in pancreatic β-cells. We found that the insulin secretagogue glucose promotes phosphatidylinositol (3,4,5)-trisphosphate (PIP3) generation through phosphoinositide 3-kinase (PI3K), thereby recruiting ARNO to the intracellular side of the plasma membrane. Peripheral ARNO promotes clathrin assembly through its GEF activity for Arf6 and regulates the early stage of endocytosis. We also found that peripheral ARNO recruits EPI64 to the same area and that the interaction requires glucose-induced endocytosis in pancreatic β-cells. Given that GTP- and GDP-bound Rab27a regulate exocytosis and the late stage of endocytosis, our results indicate that the glucose-induced activation of PI3K plays a pivotal role in exocytosis-endocytosis coupling, and that ARNO and EPI64 regulate endocytosis at distinct stages.

Interplay between Rab27a effectors in pancreatic β-cells

The small GTPase Rab27a is a member of the Rab family that is involved in membrane trafficking in various kinds of cells. Rab27a has GTP- and GDP-bound forms, and their interconversion regulates intracellular signaling pathways. Typically, only a GTP-bound GTPase binds its specific effectors with the resulting downstream signals controlling specific cellular functions. We previously identified novel Rab27a-interacting proteins. Surprisingly, some of these proteins interacted with GDP-bound Rab27a. The present study reviews recent progress in our understanding of the roles of Rab27a and its effectors in the secretory process. In pancreatic β-cells, GTP-bound Rab27a regulates insulin secretion at the pre-exocytotic stages via its GTP-specific effectors such as Exophilin8/Slac2-c/MyRIP and Slp4/Granuphilin. Glucose stimulation causes insulin exocytosis. Glucose stimulation also converts Rab27a from its GTP- to its GDP-bound form. GDP-bound Rab27a interacts with GDP-specific effectors and controls endocytosis of the secretory membrane. Thus, Rab27a cycling between GTP- and GDP-bound forms synchronizes with the recycling of secretory membrane to re-use the membrane and keep the β-cell volume constant.

Quantitative analysis of the TNF-α-induced phosphoproteome reveals AEG-1/MTDH/LYRIC as an IKKβ substrate

The inhibitor of the nuclear factor-κB (IκB) kinase (IKK) complex is a key regulator of the canonical NF-κB signalling cascade and is crucial for fundamental cellular functions, including stress and immune responses. The majority of IKK complex functions are attributed to NF-κB activation; however, there is increasing evidence for NF-κB pathway-independent signalling. Here we combine quantitative mass spectrometry with random forest bioinformatics to dissect the TNF-α-IKKβ-induced phosphoproteome in MCF-7 breast cancer cells. In total, we identify over 20,000 phosphorylation sites, of which ∼1% are regulated up on TNF-α stimulation. We identify various potential novel IKKβ substrates including kinases and regulators of cellular trafficking. Moreover, we show that one of the candidates, AEG-1/MTDH/LYRIC, is directly phosphorylated by IKKβ on serine 298. We provide evidence that IKKβ-mediated AEG-1 phosphorylation is essential for IκBα degradation as well as NF-κB-dependent gene expression and cell proliferation, which correlate with cancer patient survival in vivo.

Inflammatory cytokines such as TNF-α influence inflammation, apoptosis and tumour development through regulation of the kinase IKKβ. Krishnan and Nolte et al. apply quantitative proteomics to identify potential IKKβ targets, and reveal phosphorylation of AEG-1 by IKKβ as a mechanism controlling NF-κB signalling.

Signaling mechanisms of glucose-induced F-actin remodeling in pancreatic islet β cells

The maintenance of whole-body glucose homeostasis is critical for survival, and is controlled by the coordination of multiple organs and endocrine systems. Pancreatic islet β cells secrete insulin in response to nutrient stimuli, and insulin then travels through the circulation promoting glucose uptake into insulin-responsive tissues such as liver, skeletal muscle and adipose. Many of the genes identified in human genome-wide association studies of diabetic individuals are directly associated with β cell survival and function, giving credence to the idea that β-cell dysfunction is central to the development of type 2 diabetes. As such, investigations into the mechanisms by which β cells sense glucose and secrete insulin in a regulated manner are a major focus of current diabetes research. In particular, recent discoveries of the detailed role and requirements for reorganization/remodeling of filamentous actin (F-actin) in the regulation of insulin release from the β cell have appeared at the forefront of islet function research, having lapsed in prior years due to technical limitations. Recent advances in live-cell imaging and specialized reagents have revealed localized F-actin remodeling to be a requisite for the normal biphasic pattern of nutrient-stimulated insulin secretion. This review will provide an historical look at the emergent focus on the role of the actin cytoskeleton and its regulation of insulin secretion, leading up to the cutting-edge research in progress in the field today.

Genome-wide association study for Marek's disease mortality in layer chickens

A genome-wide association study (GWAS) using Bayesian variable selection was performed to determine genomic regions associated with mortality due to Marek's disease virus (MDV) infection in layers. Mortality (%) under experimental disease challenge (500 plaque-forming units of a very virulent plus MDV strain) was recorded for progeny groups (average 15.5 birds; range 3 to 30) of 253 genotyped sires from four generations of a brown-egg layer line. An additional generation of 43 sires with progeny data was used to validate results. Sires were genotyped with a 42K Illumina single-nucleotide polymorphism (SNP) chip. Methods BayesB (pi = 0.995) and BayesCpi, with or without weighting residuals by the size of progeny groups were applied. The proportion of genetic variance contributed by SNPs within each 1-megabase (Mb) genomic region was quantified. Average mortality was 33% but differed significantly between generations. Genetic markers explained about 11% of phenotypic variation in mortality. Correlations between genomic estimated breeding values and percentage of progeny mortality for the validation generation (sons of individuals in training) were 0.12, 0.17, 0.02, and 0.16 for BayesB, weighted BayesB, BayesCpi, and weighted BayesCpi, respectively, when using the whole genome, and 0.03, 0.20, -0.06, and 0.14, when using only SNP from the 10, 1-Mb regions, explaining the largest proportion of genetic variance according to each method. Results suggest that regions on chromosomes 2, 3, 4, 9, 15, 18, and 21 are associated with Marek's disease resistance and can be used for selection and that accounting for the size of progeny groups has a large impact on correct localization of such genomic regions.

Rab27a and melanosomes: a model to investigate the membrane targeting of Rabs

Rab proteins constitute the largest family within the Ras superfamily of small GTPases (>60 in mammals) and are essential regulators of transport between intracellular organelles. Key to this activity is their targeting to specific compartments within the cell. However, although great strides have been made over the last 25 years in assigning functions to individual Rabs and identifying their downstream effectors, the mechanism(s) regulating their targeting to specific subcellular membranes remains less well understood. In the present paper, we review the evidence supporting the proposed mechanisms of Rab targeting and highlight insights into this process provided by studies of Rab27a.

Inhibition of glioma cell lysosome exocytosis inhibits glioma invasion

Cancer cells invade by secreting enzymes that degrade the extracellular matrix and these are sequestered in lysosomal vesicles. In this study, the effects of the selective lysosome lysing drug GPN and the lysosome exocytosis inhibitor vacuolin-1 on lysosome exocytosis were studied to determine their effect on glioma cell migration and invasion. Both GPN and vacuolin-1 evidently inhibited migration and invasion in transwell experiments and scratch experiments. There are more lysosomes located on the cell membrane of glioma cells than of astrocytes. GPN decreased the lysosome number on the cell membrane. We found that rab27A was expressed in glioma cells, and colocalized with cathepsin D in lysosome. RNAi-Rab27A inhibited lysosome cathepsin D exocytosis and glioma cell invasion in an in vitro assay. Inhibition of cathepsin D inhibited glioma cell migration. The data suggest that the inhibition of lysosome exocytosis from glioma cells plays an important modulatory role in their migration and invasion.

Deregulation of Rab and Rab effector genes in bladder cancer

Growing evidence indicates that Rab GTPases, key regulators of intracellular transport in eukaryotic cells, play an important role in cancer. We analysed the deregulation at the transcriptional level of the genes encoding Rab proteins and Rab-interacting proteins in bladder cancer pathogenesis, distinguishing between the two main progression pathways so far identified in bladder cancer: the Ta pathway characterized by a high frequency of FGFR3 mutation and the carcinoma in situ pathway where no or infrequent FGFR3 mutations have been identified. A systematic literature search identified 61 genes encoding Rab proteins and 223 genes encoding Rab-interacting proteins. Transcriptomic data were obtained for normal urothelium samples and for two independent bladder cancer data sets corresponding to 152 and 75 tumors. Gene deregulation was analysed with the SAM (significant analysis of microarray) test or the binomial test. Overall, 30 genes were down-regulated, and 13 were up-regulated in the tumor samples. Five of these deregulated genes (LEPRE1, MICAL2, RAB23, STXBP1, SYTL1) were specifically deregulated in FGFR3-non-mutated muscle-invasive tumors. No gene encoding a Rab or Rab-interacting protein was found to be specifically deregulated in FGFR3-mutated tumors. Cluster analysis showed that the RAB27 gene cluster (comprising the genes encoding RAB27 and its interacting partners) was deregulated and that this deregulation was associated with both pathways of bladder cancer pathogenesis. Finally, we found that the expression of KIF20A and ZWINT was associated with that of proliferation markers and that the expression of MLPH, MYO5B, RAB11A, RAB11FIP1, RAB20 and SYTL2 was associated with that of urothelial cell differentiation markers. This systematic analysis of Rab and Rab effector gene deregulation in bladder cancer, taking relevant tumor subgroups into account, provides insight into the possible roles of Rab proteins and their effectors in bladder cancer pathogenesis. This approach is applicable to other group of genes and types of cancer.

Differential expression of Rab27A/B correlates with clinical outcome in hepatocellular carcinoma

AIM: To investigate the association of Rab27A and Rab27B expression with clinicopathological characteristics and prognosis of hepatocellular carcinoma (HCC).

METHODS: We used reverse transcription polymerase chain reaction (RT-PCR), real-time PCR, and Western blotting to detect Rab27A and Rab27B mRNA and protein expression in 5 human HCC lines and the immortalized hepatic HL-7702 cell line. We further examined 148 primary HCC samples matched with adjacent normal tissue and 80 non-HCC specimens by immunohistochemistry to evaluate the correlation of Rab27A and Rab27B expression with clinicopathological features and prognosis.

RESULTS: Our data showed that Rab27A and Rab27B were differentially expressed in cell lines and primary HCC tumors. Rab27A mRNA and protein were detected in 67% (4/6) of human cell lines and 80% (4/5) of HCC cell lines, while Rab27B was found in 50% (3/6) of human lines and 40% (2/5) of HCC lines. Rab27A expression was higher in primary HCC (46.2%, 66/143) than in matched adjacent tissue (24.3%, 33/136, P < 0.001), whereas immunopositivity for Rab27B was lower in primary HCC (57.4%, 81/141) than in matched adjacent tissue (87.5%, 119/136, P < 0.001). Analysis of clinicopathological characteristics of 148 HCC specimens revealed significant correlations between Rab27A and Rab27B expression and tumor tumor-node-metastasis (TNM) classification (P = 0.046 and P = 0.027, respectively), and between strong Rab27A expression and tumor differentiation grade (P = 0.008). Survival analyses revealed that patients with Rab27A⁺ or Rab27B⁺ tumors had significantly reduced overall survival compared with that of patients with Rab27A^- or Rab27B^- tumors (P = 0.015 and P = 0.005, respectively). Risk analyses revealed that Rab27B⁺ and TNM III-IV were independent poor prognosis factors associated with a 3.36- and 3.37- fold higher relative risk of death, respectively.

CONCLUSION: Rab27A and Rab27B expression were closely correlated with tumor progression and can be valuable prognostic indicators for HCC patients.

Rab27a-mediated protease release regulates neutrophil recruitment by allowing uropod detachment

Neutrophil migration is vital for immunity and precedes effector functions such as pathogen killing. Here, we report that this process is regulated by the Rab27a GTPase, a protein known to control granule exocytosis. Rab27a-deficient (Rab27a KO) neutrophils exhibit migration defects in vitro and in vivo, and live-cell microscopy suggests that delayed uropod detachment causes the migratory defect. Surface expression of CD11b, a key adhesion molecule, is increased in chemokine-stimulated Rab27a KO neutrophils compared with the control, suggesting a turnover delay caused by a defect in elastase secretion from azurophilic granules at the rear of bone marrow polymorphonuclear leukocytes (BM-PMNs). We suggest that Rab27a-dependent protease secretion regulates neutrophil migration through proteolysis-dependent de-adhesion of uropods, a mechanism that could be conserved in cell migration and invasion.

Semi-automated analysis of organelle movement and membrane content: understanding rab-motor complex transport function

Organelle motility is an essential cellular function that is regulated by molecular motors, and their adaptors and activators. Here we established a new method that allows more direct investigation of the function of these peripheral membrane proteins in organelle motility than is possible by analysis of the organelle movement alone. This method uses multi-channel time-lapse microscopy to record the movement of organelles and associated fluorescent proteins, and automatic organelle tracking, to compare organelle movement parameters with the association of membrane proteins. This approach allowed large-scale, unbiased analysis of the contribution of organelle-associated proteins and cytoskeleton tracks in motility. Using this strategy, we addressed the role of membrane recruitment of Rab GTPases and effectors in organelle dynamics, using the melanosome as a model. We found that Rab27a and Rab32/38 were mainly recruited to sub-populations of slow-moving/static and fast-moving melanosomes, respectively. The correlation of Rab27a recruitment with slow movement/docking was dependent on the effector melanophilin. Meanwhile, using cytoskeleton-disrupting drugs, we observed that this speed:Rab content relationship corresponded to a decreased frequency of microtubule (MT)-based transport and an increased frequency of actin-dependent slow movement/docking. Overall, our data indicate the ability of Rab27a and effector recruitment to switch melanosomes from MT- to actin-based tethering and suggest that a network of Rab signalling may integrate melanosome biogenesis and transport.

Melanosomes on the move: a model to understand organelle dynamics

Advances in live-cell microscopy have revealed the extraordinarily dynamic nature of intracellular organelles. Moreover, movement appears to be critical in establishing and maintaining intracellular organization and organellar and cellular function. Motility is regulated by the activity of organelle-associated motor proteins, kinesins, dyneins and myosins, which move cargo along polar MT (microtubule) and actin tracks. However, in most instances, the motors that move specific organelles remain mysterious. Over recent years, pigment granules, or melanosomes, within pigment cells have provided an excellent model for understanding the molecular mechanisms by which motor proteins associate with and move intracellular organelles. In the present paper, we discuss recent discoveries that shed light on the mechanisms of melanosome transport and highlight future prospects for the use of pigment cells in unravelling general molecular mechanisms of intracellular transport.

Exophilin8 transiently clusters insulin granules at the actin-rich cell cortex prior to exocytosis

Exophilin8/MyRIP/Slac2-c is an effector protein of the small GTPase Rab27a and is specifically localized on retinal melanosomes and secretory granules. We investigated the role of exophilin8 in insulin granule trafficking. Exogenous expression of exophilin8 in pancreatic β cells or their cell line, MIN6, polarized (exophilin8-positive) insulin granules at the cell corners, where both cortical actin and the microtubule plus-end-binding protein, EB1, were present. Mutation analyses indicated that the ability of exophilin8 to act as a linker between Rab27a and myosin Va is essential for its granule-clustering activity. Moreover, exophilin8 and exophilin8-associated insulin granules were markedly stable and immobile. Total internal reflection fluorescence microscopy indicated that exophilin8 restricts the motion of insulin granules at a region deeper than that where another Rab27a effector, granuphilin, accumulates docked granules directly attached to the plasma membrane. However, the exophilin8-induced immobility of insulin granules was eliminated upon secretagogue stimulation and did not inhibit evoked exocytosis. Furthermore, exophilin8 depletion prevents insulin granules from being transported close to the plasma membrane and inhibits their fusion. These findings indicate that exophilin8 transiently traps insulin granules into the cortical actin network close to the microtubule plus-ends and supplies them for release during the stimulation.

Loss of Rab27 function results in abnormal lung epithelium structure in mice

Rab27 small GTPases regulate secretion and movement of lysosome-related organelles such as T cell cytolytic granules and platelet-dense granules. Previous studies indicated that Rab27a and Rab27b are expressed in the murine lung suggesting that they regulate secretory processes in the lung. Consistent with those studies, we found that Rab27a and Rab27b are expressed in cell types that contain secretory granules: alveolar epithelial type II (AEII) and Clara cells. We then used Rab27a/Rab27b double knockout (DKO) mice to examine the functional consequence of loss of Rab27 proteins in the murine lung. Light and electron microscopy revealed a number of morphological changes in lungs from DKO mice when compared with those in control animals. In aged DKO mice we observed atrophy of the bronchiolar and alveolar epithelium with reduction of cells numbers, thinning of the bronchiolar epithelium and alveolar walls, and enlargement of alveolar airspaces. In these samples we also observed increased numbers of activated foamy alveolar macrophages and granulocyte containing infiltrates together with reduction in the numbers of Clara cells and AEII cells compared with control. At the ultrastructural level we observed accumulation of cytoplasmic membranes and vesicles in Clara cells. Meanwhile, AEII cells in DKO accumulated large mature lamellar bodies and lacked immature/precursor lamellar bodies. We hypothesize that the morphological changes observed at the ultrastructural level in DKO samples result from secretory defects in AEII and Clara cells and that over time these defects lead to atrophy of the epithelium.

Rab GTPases implicated in inherited and acquired disorders

The endocytotic machinery imports, transports and exports receptors and associated molecules between the plasma membrane and various cytoplasmic chambers resulting in selective recycling, degradation, or secretion of molecules and signaling complexes. Trafficking of receptors, growth factors, nutrients, cytokines, integrins as well as pathogens dictates the kinetics and magnitude of signal transduction cascades. Understandably, alterations in the 'fate' of such cargo complexes have profound physiologic and pathophysiologic implications. Rab GTPases regulate endocytosis by decorating intracellular vesicles and targeting these vesicles along with their cargoes to appropriate subcellular compartments. In the last decade, the number of genetic diseases driven by germline mutations in Rab GTPases or their interacting proteins, has increased and there is growing evidence of aberrant Rab GTPase function in acquired pathophysiologies such as immune deficiency, infection, obesity, diabetes and cancer.

Cdx2 regulates endo-lysosomal function and epithelial cell polarity

In contrast to our significant understanding of signaling cascades that determine cell polarity in lower eukaryotic or immortalized cells, little is known about the transcriptional program that governs mammalian epithelial polarization in vivo. Here we show, using conditional gene ablation and three-dimensional tissue culture, that the homeobox transcription factor Cdx2 controls apical-basolateral polarity in mouse enterocytes and human colonic epithelial cells. Cdx2 regulates a comprehensive gene network involved in endo-lysosomal maturation and protein transport. In the absence of Cdx2, defective protein trafficking impairs apical-basal transport and induces ectopic lumen formation. These defects are partially recapitulated by suppression of key apical transport components, Rab11a and Kif3b, which are regulated by Cdx2. Furthermore, Cdx2 deficiency affects components that control the organization of microvillus actin cytoskeleton, leading to severe microvillus atrophy. These results demonstrate that Cdx2 regulates epithelial cell polarity and morphogenesis through control of apical protein transport and endo-lysosomal function.

Synaptotagmin-mediated vesicle fusion regulates cell migration

Chemokines and other chemoattractants direct leukocyte migration and are essential for the development and delivery of immune and inflammatory responses. To probe the molecular mechanisms that underlie chemoattractant-guided migration, we did an RNA-mediated interference screen that identified several members of the synaptotagmin family of calcium-sensing vesicle-fusion proteins as mediators of cell migration: SYT7 and SYTL5 were positive regulators of chemotaxis, whereas SYT2 was a negative regulator of chemotaxis. SYT7-deficient leukocytes showed less migration in vitro and in a gout model in vivo. Chemoattractant-induced calcium-dependent lysosomal fusion was impaired in SYT7-deficient neutrophils. In a chemokine gradient, SYT7-deficient lymphocytes accumulated lysosomes in their uropods and had impaired uropod release. Our data identify a molecular pathway required for chemotaxis that links chemoattractant-induced calcium flux to exocytosis and uropod release.

Loss-of-function mutations in Rab escort protein 1 (REP-1) affect intracellular transport in fibroblasts and monocytes of choroideremia patients

Background

Choroideremia (CHM) is a progressive X-linked retinopathy caused by mutations in the CHM gene, which encodes Rab escort protein-1 (REP-1), an escort protein involved in the prenylation of Rabs. Under-prenylation of certain Rabs, as a result of loss of function mutations in REP-1, could affect vesicular trafficking, exocytosis and secretion in peripheral cells of CHM patients.

Methodology/Principal Findings

To evaluate this hypothesis, intracellular vesicle transport, lysosomal acidification and rates of proteolytic degradation were studied in monocytes (CD14+ fraction) and primary skin fibroblasts from the nine age-matched controls and thirteen CHM patients carrying 10 different loss-of-function mutations. With the use of pHrodo™ BioParticles® conjugated with E. coli, collagen I coated FluoSpheres beads and fluorescent DQ™ ovalbumin with BODYPY FL dye, we demonstrated for the first time that lysosomal pH was increased in monocytes of CHM patients and, as a consequence, the rates of proteolytic degradation were slowed. Microarray analysis of gene expression revealed that some genes involved in the immune response, small GTPase regulation, transcription, cell adhesion and the regulation of exocytosis were significantly up and down regulated in cells from CHM patients compared to controls. Finally, CHM fibroblasts secreted significantly lower levels of cytokine/growth factors such as macrophage chemoattractant protein-1 (MCP-1), pigment epithelial derived factor (PEDF), tumor necrosis factor (TNF) alpha, fibroblast growth factor (FGF) beta and interleukin (lL)-8.

Conclusions/Significance

We demonstrated for the first time that peripheral cells of CHM patients had increased pH levels in lysosomes, reduced rates of proteolytic degradation and altered secretion of cytokines. Peripheral cells from CHM patients expose characteristics that were not previously recognized and could used as an alternative models to study the effects of different mutations in the REP-1 gene on mechanism of CHM development in human population.

Rab27a and Rab27b regulate neutrophil azurophilic granule exocytosis and NADPH oxidase activity by independent mechanisms

Neutrophils rely on exocytosis to mobilize receptors and adhesion molecules and to release microbicidal factors. This process should be strictly regulated because uncontrolled release of toxic proteins would be injurious to the host. In vivo studies showed that the small GTPase Rab27a regulates azurophilic granule exocytosis. Using mouse neutrophils deficient in Rab27a (Rab27a(ash/ash)), Rab27b [Rab27b knockout (KO)] or both [Rab27a/b double KO (DoKo)], we investigated the role of the Rab27 isoforms in neutrophils. We found that both Rab27a and Rab27b deficiencies impaired azurophilic granule exocytosis. Rab27a(ash/ash) neutrophils showed upregulation of Rab27b expression which did not compensate for the secretory defects observed in Rab27a-deficient cells, suggesting that Rab27 isoforms play independent roles in neutrophil exocytosis. Total internal reflection fluorescence microscopy analysis showed that Rab27a(ash/ash) and Rab27b KO neutrophils have a decreased number of azurophilic granules near the plasma membrane. The effect was exacerbated in Rab27a/b DoKo neutrophils. Rab27-deficient neutrophils showed impaired activation of the reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase at the plasma membrane although intraphagosomal reactive oxygen species (ROS) production was not affected. Exocytosis of secretory vesicles in Rab27-deficient neutrophils was functional, suggesting that Rab27 GTPases selectively control the exocytosis of neutrophil granules.

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.

Mechanisms of biphasic insulin-granule exocytosis - roles of the cytoskeleton, small GTPases and SNARE proteins

The release of insulin from pancreatic islets requires negative regulation to ensure low levels of insulin release under resting conditions, as well as positive regulation to facilitate robust responsiveness to conditions of elevated fuel or glucose. The first phase of release involves the plasma-membrane fusion of a small pool of granules, termed the readily releasable pool; these granules are already at the membrane under basal conditions, and discharge their cargo in response to nutrient and also non-nutrient secretagogues. By contrast, second-phase secretion is evoked exclusively by nutrients, and involves the mobilization of intracellular granules to t-SNARE sites at the plasma membrane to enable the distal docking and fusion steps of insulin exocytosis. Nearly 40 years ago, the actin cytoskeleton was first recognized as a key mediator of biphasic insulin release, and was originally presumed to act as a barrier to block granule docking at the cell periphery. More recently, however, the discovery of cycling GTPases that are involved in F-actin reorganization in the islet beta-cell, combined with the availability of reagents that are more specific and tools with which to study the mechanisms that underlie granule movement, have contributed greatly to our understanding of the role of the cytoskeleton in regulating biphasic insulin secretion. Herein, we provide historical perspective and review recent progress that has been made towards integrating cytoskeletal reorganization and cycling of small Rho-, Rab- and Ras-family GTPases into our current models of stimulus-secretion coupling and second-phase insulin release.

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.

Rab27a regulates exocytosis of tertiary and specific granules in human neutrophils

The correct mobilization of cytoplasmic granules is essential for the proper functioning of human neutrophils in host defense and inflammation. In this study, we have found that human peripheral blood neutrophils expressed high levels of Rab27a, whereas Rab27b expression was much lower. This indicates that Rab27a is the predominant Rab27 isoform present in human neutrophils. Rab27a was up-regulated during neutrophil differentiation of HL-60 cells. Subcellular fractionation and immunoelectron microscopy studies of resting human neutrophils showed that Rab27a was mainly located in the membranes of specific and gelatinase-enriched tertiary granules, with a minor localization in azurophil granules. Rab27a was largely absent from CD35-enriched secretory vesicles. Tertiary and specific granule-located Rab27a population was translocated to the cell surface upon neutrophil activation with PMA that induced exocytosis of both tertiary and specific granules. Specific Abs against Rab27a inhibited Ca(2+) and GTP-gamma-S activation and PMA-induced exocytosis of CD66b-enriched tertiary and specific granules in electropermeabilized neutrophils, whereas secretion of CD63-enriched azurophil granules was scarcely affected. Human neutrophils lacked or expressed low levels of most Slp/Slac2 proteins, putative Rab27 effectors, suggesting that additional proteins should act as Rab27a effectors in human neutrophils. Our data indicate that Rab27a is a major component of the exocytic machinery of human neutrophils, modulating the secretion of tertiary and specific granules that are readily mobilized upon neutrophil activation.

Differential expression and activation of Rab27A in human eosinophils: relationship to blood eosinophilia

Eosinophil degranulation is thought to play a pathophysiological role in asthma. Rab27A is a GTP-binding protein that is known to be essential for the degranulation of several leukocyte subsets and thus may be essential for eosinophil granule exocytosis. Here, we show that Rab27A mRNA and protein are expressed in human eosinophils. We have developed a novel assay to assess Rab27A activation and have found a similar activation pattern of this protein upon stimulation of eosinophils, neutrophils and NK cells suggesting a similar function in these cell types. Interestingly, Rab27A expression was elevated in eosinophils from asthmatic donors. Furthermore, eosinophils from eosinophilic donors displayed more rapid Rab27A activation kinetics than those from donors with lower eosinophil counts. Given that elevated blood eosinophil numbers correlate with increased priming of eosinophils, this pattern of Rab27A activation suggests differential protein expression in activated cells may allow eosinophils to degranulate more rapidly upon stimulation.

Purification, crystallization and preliminary X-ray crystallographic analysis of Rab27a GTPase in complex with exophilin4/Slp2-a effector

By switching between GTP-active and GDP-inactive conformations, small Ras GTPases partly regulate membrane trafficking, cell growth and cytoskeleton dynamics. Among Rab GTPases, the Rab27 subfamily, which comprises Rab27a and Rab27b, controls the proper targeting of secretory vesicles to the plasma membrane. GppNHp-bound Rab27a in complex with the Rab27-binding domain of exophilin4/Slp2-a effector has been purified and crystallized for structural studies. The crystals belong to space group P2(1)2(1)2(1) and a complete data set was collected to a resolution of 1.8 A. Eventually, the structural characterization of the Rab27a-exophilin4/Slp2-a complex will clarify Rab27 recognition by its effectors prior to vesicle tethering and docking.

Docking is not a prerequisite but a temporal constraint for fusion of secretory granules

We examined secretory granule dynamics using total internal reflection fluorescence microscopy in normal pancreatic beta cells and their mutants devoid of Rab27a and/or its effector, granuphilin, which play critical roles in the docking and recruitment of insulin granules to the plasma membrane. In the early phase of glucose stimulation in wild-type cells, we observed marked fusion of granules recruited from a relatively distant area, in parallel with that from granules located underneath the plasma membrane. Furthermore, despite a lack of granules directly attached to the plasma membrane, both spontaneous and evoked fusion was increased in granuphilin-null cells. In addition to these granuphilin-null phenotypes, Rab27a/granuphilin doubly deficient cells showed the decreases in granules located next to the docked area and in fusion from granules near the plasma membrane in the early phase of glucose-stimulated secretion, similar to Rab27a-mutated cells. Thus, the two proteins play nonoverlapping roles in insulin exocytosis: granuphilin acts on the granules underneath the plasma membrane, whereas Rab27a acts on those in a more distal area. These findings demonstrate that, in contrast to our conventional understanding, stable attachment of secretory granules to the plasma membrane is not prerequisite but temporally inhibitory for both spontaneous and evoked fusion.