Rab22a affects the morphology and function of the endocytic pathway

Insights into the complex interactions between Rab22a and extracellular vesicles in cancers

INTRODUCTION

Oncogenic Ras-related GTP-binding proteins, referred to as Rabs, are characterized by their intricate interactions with upstream, downstream molecules, and notably, extracellular vesicles (EVs). While the expansive family of Rabs and their associated signaling pathways have been exhaustively dissected, Rab22a emerges as an entity of outstanding interest, owing to its potent influence in many biological processes and its conspicuous correlation with cancer metastasis and migration. A burgeoning interest in the interactions between Rab22a and EVs in the field of oncology underscores the necessity for more in-depth reviews and scholarly discourses.

METHODS

We performed a review based on published original and review articles related to Rab22a, tumor, microRNA, exosome, microvesicles, EVs, CD147, lysosome, degradation, endosomal recycling, etc. from PubMed, Web of Science and Google Scholar databases.

RESULTS AND CONCLUSIONS

We summarize the regulatory processes governing the expression of Rab22a and the mutants of Rab22a. Notably, the present understanding of complex interactions between Rab22a and EVs are highlighted, encompassing both the impact of Rab22a on the genesis of EVs and the role of EVs that are affected by Rab22a mutants in propelling tumor advancement. The dynamic interaction between Rab22a and EVs plays a significant role in the progression of tumors, and it can provide novel insights into the pathogenesis of cancers and the development of new therapeutic targets.

The Regulation of Exosome Generation and Function in Physiological and Pathological Processes

Exosomes, a type of extracellular vesicle with a diameter of approximately 100 nm that is secreted by all cells, regulate the phenotype and function of recipient cells by carrying molecules such as proteins, nucleic acids, and lipids and are important mediators of intercellular communication. Exosomes are involved in various physiological and pathological processes such as immunomodulation, angiogenesis, tumorigenesis, metastasis, and chemoresistance. Due to their excellent properties, exosomes have shown their potential application in the clinical diagnosis and treatment of disease. The functions of exosomes depend on their biogenesis, uptake, and composition. Thus, a deeper understanding of these processes and regulatory mechanisms can help to find new targets for disease diagnosis and therapy. Therefore, this review summarizes and integrates the recent advances in the regulatory mechanisms of the entire biological process of exosomes, starting from the formation of early-sorting endosomes (ESCs) by plasma membrane invagination to the release of exosomes by fusion of multivesicular bodies (MVBs) with the plasma membrane, as well as the regulatory process of the interactions between exosomes and recipient cells. We also describe and discuss the regulatory mechanisms of exosome production in tumor cells and the potential of exosomes used in cancer diagnosis and therapy.

Attachment, Entry, and Intracellular Trafficking of Classical Swine Fever Virus

Classical swine fever virus (CSFV), which is a positive-sense, single-stranded RNA virus with an envelope, is a member of the Pestivirus genus in the Flaviviridae family. CSFV causes a severe and highly contagious disease in pigs and is prevalent worldwide, threatening the pig farming industry. The detailed mechanisms of the CSFV life cycle have been reported, but are still limited. Some receptors and attachment factors of CSFV, including heparan sulfate (HS), laminin receptor (LamR), complement regulatory protein (CD46), MER tyrosine kinase (MERTK), disintegrin, and metalloproteinase domain-containing protein 17 (ADAM17), were identified. After attachment, CSFV internalizes via clathrin-mediated endocytosis (CME) and/or caveolae/raft-dependent endocytosis (CavME). After internalization, CSFV moves to early and late endosomes before uncoating. During this period, intracellular trafficking of CSFV relies on components of the endosomal sorting complex required for transport (ESCRT) and Rab proteins in the endosome dynamics, with a dependence on the cytoskeleton network. This review summarizes the data on the mechanisms of CSFV attachment, internalization pathways, and intracellular trafficking, and provides a general view of the early events in the CSFV life cycle.

Vps9d1 regulates tubular endosome formation through specific activation of Rab22A

The small GTPase Rab22A is an important regulator of the formation of tubular endosomes, which are one of the types of recycling endosome compartments of the clathrin-independent endocytosis pathway. In order to regulate tubular endosome formation, Rab22A must be activated by a specific guanine-nucleotide-exchange factor (GEF); however, all of the GEFs that have been reported to exhibit Rab22A-GEF activity in vitro also activate Rab5A, an essential regulator of the clathrin-mediated endocytosis pathway, and no Rab22A-specific GEF has ever been identified. Here, we identified Vps9d1, a previously uncharacterized vacuolar protein sorting 9 (VPS9) domain-containing protein, as a novel Rab22A-GEF. The formation of tubular endosome structures was found to be severely impaired in Vps9d1-depleted HeLa cells, but Rab5A localization was unaffected. Expression of a constitutively active Rab22A mutant in Vps9d1-depleted HeLa cells restored tubular endosomes, but expression of a GEF-activity-deficient Vps9d1 mutant did not. Moreover, Vps9d1 depletion altered the distribution of clathrin-independent endocytosed cargos and impaired their recycling. Our findings indicate that Vps9d1 promotes tubular endosome formation by specifically activating Rab22A.

miR-204 suppresses uveal melanoma cell migration and invasion through negative regulation of RAB22A

Uveal melanoma (UM), a frequently seen adulthood primary ocular malignancy, shows high aggressiveness. Accumulating studies have revealed the crucial effects of microRNAs (miRNAs) on tumorigenesis and development in various human tumors. miR-204, the cancer-associated miRNA, shows dysregulation and is related to several human malignancies, but its effect on UM remains unknown. The present work focused on exploring miR-204's effect on UM and elucidating its possible molecular mechanisms. According to our results, miR-204 expression markedly increased within both UM tissues and cell lines. As revealed by functional analysis, miR-204 suppressed UM cell invasion and migration. Besides, RAB22A expression decreased through directly binding miR-204 into the corresponding 3' untranslated region (3'UTR) in UM cells. Furthermore, the RAB22A mRNA level increased, which was negatively related to the miR-204 level within UM samples. As revealed by mechanical research, miR-204 exerted its inhibition on the invasion and migration of UM cells via RAB22A. Taken together, this study suggested the tumor-suppressing effect of miR-204 on UM through down-regulating RAB22A. Thus, miR-204 may serve as the new anti-UM therapeutic target.

Localization of Chicken Rab22a in Cells and Its Relationship to BF or Ii Molecules and Genes

Rab22a is an important small GTPase protein the molecule that is involved in intracellular transportation and regulation of proteins. It also plays an important role in antigens uptake, transportation, regulation of endosome morphology, and also regulates the transport of antigens to MHC (Major Histocompatibility Complex) molecules. To investigate the role of Rab22a, the intracellular co-localization of chicken Rab22a (cRab22a) molecule and its relationship to BF and chicken invariant chain (cIi) molecules was studied. A 3D protein structure of Rab22a was constructed by using informatics tools (DNASTAR 4.0 and DNAMAN). Based on the model, the corresponding recombinant eukaryotic plasmids were constructed by point mutations in the protein's structural domains. HEK 293T cells were co-transfected with plasmids pEGFP-C1-cIi to observe the intracellular co-localization. Secondly, the DC2.4 Mouse Dendritic Cell and Murine RAW 264.7 cells were transfected with recombinant plasmids of pmCherry-cRab22a and pmCherry-mRab22a respectively. Subsequently, the intracellular localization of cRab22a in early and late endosomes was observed with specific antibodies against EEA1 and LAMP1 respectively. For gene expression-based studies, the cRab22a gene was down-regulated and up-regulated in HD11 cells, following the detection of transcription levels of the BFa (MHCIa) and cIi genes by real-time quantitative PCR (RT-qPCR). The interactions of the cRab22a gene with BFa and cIi were detected by co-immunoprecipitation (Co-IP) and Western blot. The results showed that the protein structures of chicken and mouse Rab22a were highly homologous (95.4%), and both localize to the early and late endosomes. Ser41 and Tyr74 are key amino acids in the Switch regions of Rab22a which maintain its intracellular localization. The down-regulation of cRab22a gene expression significantly reduced (p < 0.01) the transcription of BFa (MHCIa) and cIi in HD11 cells. However, when the expression of the cRab22a gene was increased 55 times as compared to control cells, the expression of the BFa (MHCIa) gene was increased 1.7 times compared to the control cells (p < 0.01), while the expression of the cIi gene did not significantly differ from control (p > 0.05). Western blot results showed that cRab22a could not directly bind to BFa and cIi. So, cRab22a can regulate BFa and cIi protein molecules indirectly. It is concluded that cRab22a was localized with cIi in the endosome. The Switch regions of cRab22a are the key domains that affect intracellular localization and colocalization of the cIi molecule.

Circular RNA circ_0029589 promotes ox-LDL-induced endothelial cell injury through regulating RAB22A by serving as a sponge of miR-1197

BACKGROUND

Dysfunction of endothelial cells is now considered a vital contributor to the pathogenesis of atherosclerosis (AS). Moreover, circular RNA (circRNA) circ_0029589 has been found to be involved in the regulation of oxidized low-density lipoprotein (ox-LDL)-induced endothelial cell damage. Nevertheless, its molecular mechanism in ox-LDL-triggered endothelial cell injury is poorly defined.

METHODS

Human umbilical vein endothelial cells (HUVECs) treated with ox-LDL were applied as cell models of AS. Circ_0029589, microRNA-1197 (miR-1197), and Ras-related protein Rab-22A (RAB22A) expression were detected using real-time quantitative polymerase chain reaction (RT-qPCR). Cell proliferation, apoptosis, angiogenesis, and invasion were detected using 3-(4, 5-dimethyl-2-thiazolyl)-2, 5-diphenyl-2-H-tetrazolium bromide (MTT), 5-ethynyl-2'-deoxyuridine (EdU), flow cytometry, tube formation, and transwell assays. Western blot analysis of Cleaved-caspase-3, B-cell lymphoma-2 related X protein (Bax), and RAB22A. IL-6, IL-1β, and Tumor necrosis factor α (TNF-α) levels were gauged using ELISA kits. Superoxide Dismutase (SOD) activity and Malondiahyde (MDA) level were assessed using special kits. Bioinformatics software predicted the binding between miR-1197 and circ_0029589 or RAB22A, which was proved using dual-luciferase reporter and RNA pull-down assays.

RESULTS

Circ_0029589 and RAB22A expression were strengthened, and miR-1197 was reduced in ox-LDL-treated HUVECs. Importantly, circ_0029589 silencing ameliorated ox-LDL-triggered HUVEC damage via promoting cell proliferation, tube formation ability, invasion, and repressing cell apoptosis, inflammation, and oxidative stress. Mechanical analysis suggested that circ_0029589 might affect RAB22A content through sponging miR-1197.

CONCLUSION

Circ_0090231 might protect against ox-LDL-mediated HUVEC injury via the miR-1197/RAB22A axis, which provides a therapeutic strategy for endothelial cell damage of AS.

A Novel Membrane Targeting Domain Mediates the Endosomal or Golgi Localization Specificity of Small GTPases Rab22 and Rab31

Rab22 and Rab31 belong to the Rab5 subfamily of GTPases that regulates endocytic traffic and endosomal sorting. Rab22 and Rab31 (a.k.a. Rab22b) are closely related and share 87% amino acid sequence similarity, but they show distinct intracellular localization and function in the cell. Rab22 is localized to early endosomes and regulates early endosomal recycling, while Rab31 is mostly localized to the Golgi complex with only a small fraction in the endosomes at steady state. The specific determinants that affect this differential localization, however, are unclear. In this study, we identify a novel membrane targeting domain (MTD) consisting of the C-terminal hypervariable domain (HVD), inter-switch loop (ISL), and N-terminal domain as a major determinant of endosomal localization for Rab22 and Rab31, as well as Rab5. Rab22 and Rab31 share the same N-terminal domain, but we find Rab22 chimeras with Rab31 HVD exhibit phenotypic Rab31 localization to the Golgi complex while Rab31 chimeras with the Rab22 HVD localize to early endosomes, similar to wild type Rab22. We also find that the Rab22 HVD favors interaction with the early endosomal effector protein Rabenosyn-5, which may stabilize the Rab localization to the endosomes. The importance of effector interaction in endosomal localization is further demonstrated by the disruption of Rab22 endosomal localization in Rabenosyn-5 knockout cells and by the shift of Rab31 to the endosomes in Rabenosyn-5 overexpressing cells. Taken together, we have identified a novel MTD that mediates localization of Rab5 subfamily members to early endosomes via interaction with an effector such as Rabenosyn-5.

Rab22a Promotes Epithelial-Mesenchymal Transition in Papillary Thyroid Carcinoma by Activating PI3K/AKT/mTOR Signaling Pathway

Background

Rab22a is a member of the RAS superfamily, involved in early endosome formation and intracellular vesicle transport. Rab22a is significantly upregulated in a variety of malignant tumors. However, its function in thyroid cancer has never been addressed.

Methods

The expression of Rab22a in paraffin sections of 101 patients was detected by immunohistochemical staining. By upregulating and downregulating the expression of Rab22a in thyroid cancer cell lines, the effect of Rab22a on cell proliferation, invasion, and migration was analyzed. Co-IP was employed, and the interaction between Rab22a and PI3Kp85α was shown. The function of Rab22a on PI3K/AKT/mTOR signaling and epithelial-mesenchymal transition (EMT) was further studied by western blot analysis.

Results

Immunostaining showed that Rab22a was significantly overexpressed in thyroid cancer tissues but negative in adjacent normal tissues or nodular goiters. The proliferation, migration, invasion, and EMT in papillary thyroid carcinoma cell lines were enhanced upon Rab22a overexpression but inhibited after knocking down Rab22a. The co-IP assay demonstrated an interaction between Rab22a and PI3K85α, an effector of PI3K. We further found that Rab22a can activate the PI3K/AKT/mTOR signaling pathway. However, the ability of Rab22a to promote the proliferation, invasion, migration, and EMT of papillary thyroid carcinoma cells was significantly inhibited after being treated with LY294002, a PI3K inhibitor.

Conclusions

Rab22a can promote the EMT process and enhance proliferation, migration, and invasion of papillary thyroid carcinoma cells by activating the PI3K/AKT/mTOR signaling pathway. Our study provides new pathological diagnosis clues and clinical treatment targets for thyroid cancer.

Cancer-driving mutations and variants of components of the membrane trafficking core machinery

The core machinery for vesicular membrane trafficking broadly comprises of coat proteins, RABs, tethering complexes and SNAREs. As cellular membrane traffic modulates key processes of mitogenic signaling, cell migration, cell death and autophagy, its dysregulation could potentially results in increased cell proliferation and survival, or enhanced migration and invasion. Changes in the levels of some components of the core machinery of vesicular membrane trafficking, likely due to gene amplifications and/or alterations in epigenetic factors (such as DNA methylation and micro RNA) have been extensively associated with human cancers. Here, we provide an overview of association of membrane trafficking with cancer, with a focus on mutations and variants of coat proteins, RABs, tethering complex components and SNAREs that have been uncovered in human cancer cells/tissues. The major cellular and molecular cancer-driving or suppression mechanisms associated with these components of the core membrane trafficking machinery shall be discussed.

Regulation of phagolysosomal activity by miR-204 critically influences structure and function of retinal pigment epithelium/retina

MicroRNA-204 (miR-204) is expressed in pulmonary, renal, mammary and eye tissue, and its reduction can result in multiple diseases including cancer. We first generated miR-204-/- mice to study the impact of miR-204 loss on retinal and retinal pigment epithelium (RPE) structure and function. The RPE is fundamentally important for maintaining the health and integrity of the retinal photoreceptors. miR-204-/- eyes evidenced areas of hyper-autofluorescence and defective photoreceptor digestion, along with increased microglia migration to the RPE. Migratory Iba1+ microglial cells were localized to the RPE apical surface where they participated in the phagocytosis of photoreceptor outer segments (POSs) and contributed to a persistent build-up of rhodopsin. These structural, molecular and cellular outcomes were accompanied by decreased light-evoked electrical responses from the retina and RPE. In parallel experiments, we suppressed miR-204 expression in primary cultures of human RPE using anti-miR-204. In vitro suppression of miR-204 in human RPE similarly showed abnormal POS clearance and altered expression of autophagy-related proteins and Rab22a, a regulator of endosome maturation. Together, these in vitro and in vivo experiments suggest that the normally high levels of miR-204 in RPE can mitigate disease onset by preventing generation of oxidative stress and inflammation originating from intracellular accumulation of undigested photoreactive POS lipids. More generally, these results implicate RPE miR-204-mediated regulation of autophagy and endolysosomal interaction as a critical determinant of normal RPE/retina structure and function.

Rab22a regulates the establishment of epithelial polarity

Membrane trafficking establishes and maintains epithelial polarity. Rab22a has a polarized distribution in activated T-cells, but its role in epithelial polarity has not been investigated. We showed previously that Rab14 acts upstream of Arf6 to establish the apical membrane initiation site (AMIS), but its interaction with Rab22a is unknown. Here we show that Rab14 and Rab22a colocalize in endosomes of both unpolarized and polarized MDCK cells and Rab22a localizes to the cell:cell interface of polarizing cell pairs. Knockdown of Rab22a results in a multi-lumen phenotype in three-dimensional culture. Further, overexpression of Rab22a in Rab14 knockdown cells rescues the multi-lumen phenotype observed with Rab14 knockdown, suggesting that Rab22a is downstream of Rab14. Because of the relationship between Rab14 and Arf6, we investigated the effect of Rab22a knockdown on Arf6. We find that Rab22a knockdown results in decreased active Arf6 and that Rab22a co-immunoprecipitates with the Arf6 GEF EFA6. In addition, EFA6 is retained in intracellular puncta in Rab22a KD cells. These results suggest that Rab22a acts downstream of Rab14 to traffic EFA6 to the AMIS to regulate Arf6 in the establishment of polarity.

MicroRNA-193b acts as a tumor suppressor in colon cancer progression via targeting RAB22A

To explore microRNA (miR)-193b expression and its potential role in colon cancer, reverse transcription-quantitative polymerase chain reaction was performed to detect the miR-193b expression levels in 62 colon cancer tissues and normal adjacent tissues. The miR-193b-overexpressed cell line SW620 was used to study the role of miR-193b in colon cancer. Subsequently, a Transwell assay and cell cycle assay were performed to observe the functional cell changes in the in vitro expression levels of miR-193b. Results indicated that miR-193b expression levels were significantly decreased in colon cancer tissues compared with adjacent normal tissue (P<0.001) and the expression of miR-193b was significantly correlated with TNM staging (P=0.03) and lymph node invasion (P=0.007). Furthermore, overexpression of miR-193b significantly decreased colon cancer cell cycle progression and its migration ability. In addition, the present findings suggested that the increased expression of miR-193b by RAB22A, inhibited downstream proteins involved in the Ras signaling pathway, including the Ras and extracellular signal-related kinase which may inhibit cancer proliferation and migration. In conclusion, the aim was to clarify the association of miR-193b expression with colon cancer, and to explore the mechanism of miR-193b in colon cancer proliferation and cell migration. The preliminary findings revealed that miR-193b may have an important role in the process in colon cancer cell cycle and migration by the RAB22A-Ras signaling pathway, thus providing a theoretical basis for miR-193b as a potential molecular target for colon cancer treatment.

Screening of microRNAs for a repressor of hepatitis B virus replication

Background

Hepatitis B virus (HBV) infection is a leading cause of persistent liver diseases, cirrhosis and hepatocellular carcinoma (HCC) worldwide. Since deregulation of microRNA (miRNA) expression by HBV infection contributes to enhanced viral replication and pathogenesis, modulation of miRNA activity can be a novel therapeutic approach towards HBV eradication. As the effects of the vast majority of miRNAs on HBV replication have not been empirically investigated, here, we aim to identify novel therapeutic targets that have a strong antiviral effect on HBV.

Methods

HepG2-hNTCP-C4 cells were infected with HBV, and then were individually transfected with the library mimics of 2048 miRNAs. To assess the amount of intracellular and extracellular DNA and HBsAg, qPCR and ELISA were performed respectively.

Results

From miRNA library screening, we identified 39 miRNAs as candidate repressors of HBV replication. Among them, 9 miRNAs, including miR-204, strongly decreased both HBV DNA and HBsAg in culture supernatant of HepG2-hNTCP-C4 cells. Furthermore, we also showed that inhibition of Rab22a, one of the targets of miR-204, also suppressed intracellular and extracellular HBV DNA expression in HepG2.2.15.7 cells.

Conclusions

Our findings contribute to the understanding of the roles of miRNAs underlying HBV replication and show the possibility of developing a novel strategy for miRNA-mediated HBV treatment.

The enigmatic endosome - sorting the ins and outs of endocytic trafficking

The early endosome (EE), also known as the sorting endosome (SE) is a crucial station for the sorting of cargoes, such as receptors and lipids, through the endocytic pathways. The term endosome relates to the receptacle-like nature of this organelle, to which endocytosed cargoes are funneled upon internalization from the plasma membrane. Having been delivered by the fusion of internalized vesicles with the EE or SE, cargo molecules are then sorted to a variety of endocytic pathways, including the endo-lysosomal pathway for degradation, direct or rapid recycling to the plasma membrane, and to a slower recycling pathway that involves a specialized form of endosome known as a recycling endosome (RE), often localized to the perinuclear endocytic recycling compartment (ERC). It is striking that 'the endosome', which plays such essential cellular roles, has managed to avoid a precise description, and its characteristics remain ambiguous and heterogeneous. Moreover, despite the rapid advances in scientific methodologies, including breakthroughs in light microscopy, overall, the endosome remains poorly defined. This Review will attempt to collate key characteristics of the different types of endosomes and provide a platform for discussion of this unique and fascinating collection of organelles. Moreover, under-developed, poorly understood and important open questions will be discussed.

Rab22a: A novel regulator of immune functions

Dendritic cells (DCs) trigger CD8 + T cell responses after the internalization of exogenous antigens in a process called cross-presentation. Multiple intracellular transport events within the endocytic and secretory routes take place in order to accomplish this fundamental immunological process. The endomembrane system can be envisioned as a complex network of membrane domains coordinately working in the fusion of organelles, the budding of vesicles and tubules, and modifying the molecular composition of the limiting membranes. In this context of tightly regulated and dynamic endomembrane transport, small GTPases of the Rab family display a pivotal role by organizing membrane microdomains and defining specific identities to the different intracellular compartments. In this review, we synthesize and update the current knowledge about Rab22a, which has been involved in several immune functions. In this way, we analyze the intracellular localization of Rab22a and its important role in the endocytic recycling, including its relevance during MHC-I trafficking, antigen cross-presentation by DCs and the formation of T cell conjugates. We also describe how different pathogenic microorganisms hijack Rab22a functions to achieve efficient infection and intracellular survival strategies. Furthermore, we examine the oncogenic properties of Rab22a and how its expression determines the progression of many tumors. In summary, we highlight the role of Rab22a as a key effector of the intracellular trafficking that could be exploited in future therapies to modulate the immune system.

Genome-wide association study of cardiotoxicity in the NCCTG N9831 (Alliance) adjuvant trastuzumab trial

OBJECTIVES

The major clinical side effect of the ERBB2-targeted breast cancer therapy, trastuzumab, is a decline in the left ventricular ejection fraction (LVEF). Improved markers are needed to better identify patients susceptible to cardiotoxicity.

METHODS

The NCCTG N9831 trial compared adjuvant doxorubicin and cyclophosphamide followed by either weekly paclitaxel (arm A); paclitaxel then trastuzumab (arm B); or concurrent paclitaxel and trastuzumab (arm C) in patients with HER2-positive breast cancer. A genome-wide association study was performed on all patients with available DNA (N=1446). We used linear regression to identify single nucleotide polymorphisms (SNPs) associated with decline in LVEF, adjusting for age, baseline LVEF, antihypertensive medications, and the first two principle components.

RESULTS

In total, 618 863 SNPs passed quality control and DNA from 1191 patients passed genotyping quality control and were identified as Whites of non-Hispanic origin. SNPs at six loci were associated with a decline in LVEF (P=7.73×10 to 8.93×10), LDB2, BRINP1, chr6 intergenic, RAB22A, TRPC6, and LINC01060, in patients who received chemotherapy plus trastuzumab (arms BC, N=800). None of these loci were significant in patients who received chemotherapy only (arm A, N=391) and did not increase in significance in the combined analysis of all patients. We did not observe association, P<0.05, with SNPs previously associated with trastuzumab-induced cardiotoxicity at ERBB2, I655V, and P1170A. We replicated association, P<0.05, with SNPs previously associated with anthracycline-induced cardiotoxicity at CBR3 and ABCB1.

CONCLUSION

Our study identified six putative novel cardiotoxicity loci in patients treated with combination chemotherapy and trastuzumab that require further investigation and confirmed known associations of anthracycline-induced cardiotoxicity.

RAB22A overexpression promotes the tumor growth of melanoma

Malignant melanoma is the most aggressive type of skin cancer. RAB22A, a member of RAS oncogene family, has been found to be significantly upregulated in multiple human cancers. In the present study, we found that RAB22A mRNA expression was significantly upregulated in melanoma tissues (including 60 primary melanomas and 84 metastatic melanomas) compared to benign nevi (n = 20), which were significantly higher in metastatic melanoma tissues than primary tissues. Immunohistochemistry data further showed that the positive immunoreactivity of RAB22A was detected in 66% (95/144) melanoma tissues, but not in benign nevi. Moreover, high expression of RAB22A was significantly associated with advanced clinical stage in melanoma. Furthermore, patients with high RAB22A expression had shorter overall survival compared those with low expression of RAB22A. In-vitro study showed that RAB22A was also upregulated in melanoma cell lines WM35, A375, WM451, and SK-MEL-1, when compared with the normal melanocyte HM cells. Knockdown of RAB22A significantly reduced the proliferation, migration and invasion of melanoma A375 cells, while overexpression of RAB22A significantly promoted these malignant phenotypes. In addition, RAB22A was found to be a target of miR-203, a tumor suppressive miRNA in melanoma. Besides, miR-203 was downregulated in melanoma tissues and cell lines, when compared with benign nevi and HM cells, respectively. Taken these findings together, our study could validate an oncogenic role of RAB22A in melanoma, suggesting that RAB22A may be a potential therapeutic target for melanoma.

Differential requirement of Rab22a for the recruitment of ER-derived proteins to phagosomes and endosomes in dendritic cells

The recruitment of endoplasmic reticulum (ER) components to dendritic cell (DC) phagosomes and endosomes is a crucial event to achieve efficient cross-presentation of exogenous antigens. We have previously identified the small GTPase Rab22a as a key regulator of MHC-I trafficking and antigen cross-presentation by DCs. In this study we show that low expression of Rab22a does not prevent the normal delivery of ER-derived proteins to DC phagosomes. In contrast, the presence of these proteins was diminished in endosomes labelled with a fluid phase marker. These observations were confirmed by a functional assay that assesses the translocation of a soluble protein to the cytosol. Interestingly, we also demonstrate that early endosomal maturation is altered in Rab22a deficient DCs. Our results indicate that Rab22a plays a major role in endosomal function and highlight the importance of studying the endocytic and phagocytic pathways separately in DCs.

Consequences of Rab GTPase dysfunction in genetic or acquired human diseases

Rab GTPases are important regulators of intracellular membrane trafficking in eukaryotes. Both activating and inactivating mutations in Rab genes have been identified and implicated in human diseases ranging from neurological disorders to cancer. In addition, altered Rab expression is often associated with disease prognosis. As such, the study of diseases associated with Rabs or Rab-interacting proteins has shed light on the important role of intracellular membrane trafficking in disease etiology. In this review, we cover recent advances in the field with an emphasis on cellular mechanisms.

Rab22a enhances CD147 recycling and is required for lung cancer cell migration and invasion

Rab22a is a member of the Ras-related small GTPase family, which plays a key role in regulating the recycling of cargo proteins entering cells through clathrin-independent endocytosis (CIE). Rab22a is overexpressed in different cancer types, including liver cancer, malignant melanoma, ovarian cancer and osteosarcoma. However, its oncogenic role remains unknown. In this study, we found that silencing of Rab22a suppressed the migration and invasion of lung cancer cells. Furthermore, Rab22a interacts with CD147, and knockdown of Rab22a blocks CD147 recycling and promotes CD147 degradation. Taken together, our findings indicate that Rab22a enhances recycling of CD147, which is required for lung cancer cell migration and invasion,and targeting CD147 recycling may be a rational strategy for lung cancer therapy.

Rab22a controls MHC-I intracellular trafficking and antigen cross-presentation by dendritic cells

Cross-presentation by MHC class I molecules allows the detection of exogenous antigens by CD8⁺ T lymphocytes. This process is crucial to initiate cytotoxic immune responses against many pathogens (i.e., Toxoplasma gondii) and tumors. To achieve efficient cross-presentation, dendritic cells (DCs) have specialized endocytic pathways; however, the molecular effectors involved are poorly understood. In this work, we identify the small GTPase Rab22a as a key regulator of MHC-I trafficking and antigen cross-presentation by DCs. Our results demonstrate that Rab22a is recruited to DC endosomes and phagosomes, as well as to the vacuole containing T. gondii parasites. The silencing of Rab22a expression did not affect the uptake of exogenous antigens or parasite invasion, but it drastically reduced the intracellular pool and the recycling of MHC-I molecules. The knockdown of Rab22a also hampered the cross-presentation of soluble, particulate and T. gondii-associated antigens, but not the endogenous MHC-I antigen presentation through the classical secretory pathway. Our findings provide compelling evidence that Rab22a plays a central role in the MHC-I endocytic trafficking, which is crucial for efficient cross-presentation by DCs.

LncRNA HOTAIR controls the expression of Rab22a by sponging miR-373 in ovarian cancer

Increasing evidence suggests that the long non-coding RNA, HOX transcript antisense intergenic RNA (HOTAIR) is widely involved in the progression and metastasis of cancer. However, the specific role of HOTAIR in ovarian carcinogenesis remains to be fully elucidated. In the present study, the levels of HOTAIR were detected in 30 paired cancer and noncancer tissues using reverse transcription-quantitative polymerase chain reaction analysis. The effect of HOTAIR on the ovarian cancer cells was examined by overexpression or small interfering RNA interference experiments. To examine the competitive endogenous RNA (ceRNAs) mechanism, a luciferase reporter assay was used. In patients with ovarian cancer, HOTAIR was significantly upregulated. Furthermore, the upregulation of HOTAIR increased the proliferation, migration and invasion of ovarian cancer cells. By contrast, the knockdown of HOTAIR repressed cell invasion and viability. HOTAIR functioned as a ceRNA, and acted as a sink for microRNA (miR)‑373, thereby regulating the expression of Rab22a. The upregulation of HOTAIR contributed to the malignant progression of ovarian cancer cells. Therefore, the positive regulation between HOTAIR and Rab22a can be partially attributed to the ceRNA regulatory network through miR-373.

Plasmodium falciparum Rab1A Localizes to Rhoptries in Schizonts

Over-expression of a GFP-PfRab1A fusion protein in Plasmodium falciparum schizonts produces a punctate pattern of fluorescence typical of rhoptries, secretory organelles involved in host cell invasion. The GFP-positive bodies were purified by a combination of differential and density gradient centrifugation and their protein content determined by MS/MS sequencing. Consistent with the GFP rhoptry-like pattern of transgenic parasites, four of the 19 proteins identified have been previously described to be rhoptry-associated and another four are ER or ER-associated proteins. Confirmation that GFP-PfRab1A decorates rhoptries was obtained by its co-localization with Rap1 and Ron4 in late phase schizonts. We conclude that PfRab1A potentially regulates vesicular traffic from the endoplasmic reticulum to the rhoptries in Apicomplexa parasites.

Plasmodium Rab5b is secreted to the cytoplasmic face of the tubovesicular network in infected red blood cells together with N-acylated adenylate kinase 2

BACKGROUND

Rab5 GTPase regulates membrane trafficking between the plasma membrane and endosomes and harbours a conserved C-terminal isoprenyl modification that is necessary for membrane recruitment. Plasmodium falciparum encodes three Rab5 isotypes, and one of these, Rab5b (PfRab5b), lacks the C-terminal modification but possesses the N-terminal myristoylation motif. PfRab5b was reported to localize to the parasite periphery. However, the trafficking pathway regulated by PfRab5b is unknown.

METHODS

A complementation analysis of Rab5 isotypes was performed in Plasmodium berghei. A constitutively active PfRab5b mutant was expressed under the regulation of a ligand-dependent destabilization domain (DD)-tag system in P. falciparum. The localization of PfRab5b was evaluated after removing the ligand followed by selective permeabilization of the membrane with different detergents. Furthermore, P. falciparum N-terminally myristoylated adenylate kinase 2 (PfAK2) was co-expressed with PfRab5b, and trafficking of PfAK2 to the parasitophorous vacuole membrane was examined by confocal microscopy.

RESULTS

PfRab5b complemented the function of PbRab5b, however, the conventional C-terminally isoprenylated Rab5, PbRab5a or PbRab5c, did not. The constitutively active PfRab5b mutant localized to the cytosol of the parasite and the tubovesicular network (TVN), a region that extends from the parasitophorous vacuole membrane (PVM) in infected red blood cells (iRBCs). By removing the DD-ligand, parasite cytosolic PfRab5b signal disappeared and a punctate structure adjacent to the endoplasmic reticulum (ER) and parasite periphery accumulated. The peripheral PfRab5b was sensitive to extracellular proteolysis after treatment with streptolysin O, which selectively permeabilizes the red blood cell plasma membrane, indicating that PfRab5b localized on the iRBC cytoplasmic face of the TVN. Transport of PfAK2 to the PVM was abrogated by overexpression of PfRab5b, and PfAK2 accumulated in the punctate structure together with PfRab5b.

CONCLUSION

N-myristoylated Plasmodium Rab5b plays a role that is distinct from that of conventional mammalian Rab5 isotypes. PfRab5b localizes to a compartment close to the ER, translocated to the lumen of the organelle, and co-localizes with PfAK2. PfRab5b and PfAK2 are then transported to the TVN, and PfRab5b localizes on the iRBC cytoplasmic face of TVN. These data demonstrate that PfRab5b is transported from the parasite cytosol to TVN together with N-myristoylated PfAK2 via an uncharacterized membrane-trafficking pathway.

MiR-204 inhibits the proliferation and invasion of renal cell carcinoma by inhibiting RAB22A expression

While miR-204 expression may be linked to renal cell carcinoma (RCC) progression, the detailed mechanisms remain unclear. In the present study, we demonstrated that miR-204 was differentially expressed in RCC tissues when compared with surrounding normal kidney tissues. Ectopic overexpression of miR-204 in human RCC cells suppressed cell proliferation and invasion in vitro and in vivo. Mechanism dissection revealed that miR-204 may function through RAB22A signals to inhibit RCC proliferation and invasion. Overexpression of RAB22A by oe-RAB22A was able to partially reverse the miR-204-mediated suppression of RCC tumor progression. Together, these results revealed that miR-204 suppressed RCC proliferation and invasion by directly targeting the RAB22A gene. Targeting newly identified RAB22A with miR-204 may aid in the suppression of RCC proliferation and invasion.

Occupation of nucleotide in the binding pocket is critical to the stability of Rab11A

The Ras superfamily of small G proteins is a family of guanosine triphosphatases (GTPases) and each GTPase has conserved amino acid sequences in the enzymatic active site that are responsible for specific interactions with GDP and GTP molecules. Rab GTPases, which belong to the Ras superfamily, are key regulators of intracellular vesicle trafficking via the recruitment of effector molecules. Here, we purified wild type, active mutant and inactive mutant of Rab11A. In this process, we found that the inactive mutant (Rab11A S25N) had low stability compared with wild type and other mutants. Further analysis revealed that the stability of Rab11A S25N is dependent on the occupation of GDP in the nucleotide binding pocket of the protein. We found that the stability of Rab11A S25N is affected by the presence of GDP, not other nucleotides, and is independent of pH or salt in FPLC buffer. Our results provide a better understanding of how GTPase can be stable under in vitro conditions without effector proteins and how proper substrate/cofactor coordination is crucial to the stability of Rab11A. Successful purification and proposed purification methods will provide a valuable guide for investigation of other small GTPase proteins.

MiR-373 targeting of the Rab22a oncogene suppresses tumor invasion and metastasis in ovarian cancer

Metastasis is major cause of mortality in patients with ovarian cancer. MiR-373 has been shown to play pivotal roles in tumorigenesis and metastasis; however, a role for miR-373 in ovarian cancer has not been investigated. In this study, we show that the miR-373 expression is down-regulated in human epithelial ovarian cancer (EOC) and inversely correlated with clinical stage and histological grade. Ectopic overexpression of miR-373 in human EOC cells suppressed cell invasion in vitro and metastasis in vivo, and the epithelial–mesenchymal transition process. Silencing the expression of miR-373 resulted in an increased migration and invasion of EOC cells. Using integrated bioinformatics analysis, gene expression arrays, and luciferase assay, we identified Rab22a as a direct and functional target of miR-373 in EOC cells. Expression levels of miR-373 were inversely correlated with Rab22a protein levels in human EOC tissues. Rab22a knockdown inhibited invasion and migration of EOC cells, increased E-cadherin expression, and suppressed the expression of N-cadherin. Moreover, overexpression of Rab22a abrogated miR-373-induced invasion and migration of EOC cells. Taken together, these results demonstrate that miR-373 suppresses EOC invasion and metastasis by directly targeting Rab22a gene, a new potential therapeutic target in EOC.

Interferon-γ-inducible Rab20 regulates endosomal morphology and EGFR degradation in macrophages

IFN-γ is able to modulate endosome dynamics in myelocytic cells, but the molecular mechanisms behind this process remain to be elucidated. Rab20 is identified as part of the molecular machinery that links immune activation and control of endocytic function in macrophages.

Little is known about the molecular players that regulate changes in the endocytic pathway during immune activation. Here we investigate the role of Rab20 in the endocytic pathway during activation of macrophages. Rab20 is associated with endocytic structures, but the function of this Rab GTPase in the endocytic pathway remains poorly characterized. We find that in macrophages, Rab20 expression and endosomal association significantly increase after interferon-γ (IFN-γ) treatment. Moreover, IFN-γ and Rab20 expression induce a dramatic enlargement of endosomes. These enlarged endosomes are the result of homotypic fusion promoted by Rab20 expression. The expression of Rab20 or the dominant-negative mutant Rab20T19N does not affect transferrin or dextran 70 kDa uptake. However, knockdown of Rab20 accelerates epidermal growth factor (EGF) trafficking to LAMP-2–positive compartments and EGF receptor degradation. Thus this work defines a function for Rab20 in the endocytic pathway during immune activation of macrophages.

Decreased Expression of MiRNA-204-5p Contributes to Glioma Progression and Promotes Glioma Cell Growth, Migration and Invasion

Gliomas are the most common malignant primary brain tumors in adults and exhibit a spectrum of aberrantly aggressive phenotype. Although increasing evidence indicated that the deregulation of microRNAs (miRNAs) contributes to tumorigenesis and invasion, little is known about the roles of miR-204-5p in human gliomas. In the present study, the expression of miR-204-5p in clinical glioma tissues was measured by qRT-PCR. The effects of miR-204-5p on glioma cell growth and metastasis were examined by overexpressing or inhibiting miR-204-5p. We found that the expression level of miR-204-5p was significantly reduced in clinical glioma tissues compared with normal brain tissues. Moreover, we revealed that the introduction of miR-204-5p dramatically suppressed glioma cell growth, migration and invasion. Furthermore, mechanistic investigations revealed that RAB22A, a member of the RAS oncogene family, is a direct functional target of miR-204-5p in gliomas. In vivo, restoring miR-204-5p expression in glioma cells suppressed tumorigenesis and increased overall host survival. Our findings suggest that miR-204-5p is a cancer suppressor miRNA and overexpression of miR-204-5p is a novel glioma treatment strategy.

The multivesicular body is the major internal site of prion conversion

The conversion of the properly folded prion protein, PrPc, to its misfolded amyloid form, PrPsc, occurs as the two proteins traffic along the endocytic pathway and PrPc is exposed to PrPsc. To determine the specific site of prion conversion, we knocked down various proteins in the endocytic pathway including Rab7a, Tsg101 and Hrs (also known as HGS). PrPsc was markedly reduced in two chronically infected cell lines by preventing the maturation of the multivesicular body, a process that begins in the early endosome and ends with the sorting of cargo to the lysosome. By contrast, knocking down proteins in the retromer complex, which diverts cargo away from the multivesicular body caused an increase in PrPsc levels. These results suggest that the multivesicular body is the major site for intracellular conversion of PrPc to PrPsc.

miR-204-5p inhibits proliferation and invasion and enhances chemotherapeutic sensitivity of colorectal cancer cells by downregulating RAB22A

PURPOSE

miR-204-5p was found to be downregulated in colorectal cancer tissues in our preliminary microarray analyses. However, the function of miR-204-5p in colorectal cancer remains unknown. We therefore investigated the role, mechanism, and clinical significance of miR-204-5p in colorectal cancer development and progression.

EXPERIMENTAL DESIGN

We measured the expression of miR-204-5p and determined its correlation with patient prognoses. Ectopic expression in colorectal cancer cells, xenografts, and pulmonary metastasis models was used to evaluate the effects of miR-204-5p on proliferation, migration, and chemotherapy sensitivity. Luciferase assay and Western blotting were performed to validate the potential targets of miR-204-5p after the preliminary screening by a microarray analysis and computer-aided algorithms.

RESULTS

miR-204-5p is frequently downregulated in colorectal cancer tissues, and survival analysis showed that the downregulation of miR-204-5p in colorectal cancer was associated with poor prognoses. Ectopic miR-204-5p expression repressed colorectal cancer cell growth both in vitro and in vivo. Moreover, restoring miR-204-5p expression inhibited colorectal cancer migration and invasion and promoted tumor sensitivity to chemotherapy. Mechanistic investigations revealed that RAB22A, a member of the RAS oncogene family, is a direct functional target of miR-204-5p in colorectal cancer. Furthermore, RAB22A protein levels in colorectal cancer tissues were frequently increased and negatively associated with miR-204-5p levels and survival time.

CONCLUSIONS

Our results demonstrate for the first time that miR-204-5p acts as a tumor suppressor in colorectal cancer through inhibiting RAB22A and reveal RAB22A to be a new oncogene and prognostic factor for colorectal cancer.

Human liver cell trafficking mutants: characterization and whole exome sequencing

The HuH7 liver cell mutant Trf1 is defective in membrane trafficking and is complemented by the casein kinase 2α subunit CK2α''. Here we identify characteristic morphologies, trafficking and mutational changes in six additional HuH7 mutants Trf2-Trf7. Trf1 cells were previously shown to be severely defective in gap junction functions. Using a Lucifer yellow transfer assay, remarkable attenuation of gap junction communication was revealed in each of the mutants Trf2-Trf7. Electron microscopy and light microscopy of thiamine pyrophosphatase showed that several mutants exhibited fragmented Golgi apparatus cisternae compared to parental HuH7 cells. Intracellular trafficking was investigated using assays of transferrin endocytosis and recycling and VSV G secretion. Surface binding of transferrin was reduced in all six Trf2-Trf7 mutants, which generally correlated with the degree of reduced expression of the transferrin receptor at the cell surface. The mutants displayed the same transferrin influx rates as HuH7, and for efflux rate, only Trf6 differed, having a slower transferrin efflux rate than HuH7. The kinetics of VSV G transport along the exocytic pathway were altered in Trf2 and Trf5 mutants. Genetic changes unique to particular Trf mutants were identified by exome sequencing, and one was investigated in depth. The novel mutation Ile34Phe in the GTPase RAB22A was identified in Trf4. RNA interference knockdown of RAB22A or overexpression of RAB22AI34F in HuH7 cells caused phenotypic changes characteristic of the Trf4 mutant. In addition, the Ile34Phe mutation reduced both guanine nucleotide binding and hydrolysis activities of RAB22A. Thus, the RAB22A Ile34Phe mutation appears to contribute to the Trf4 mutant phenotype.

Polymeric immunoglobulin receptor traffics through two distinct apically targeted pathways in primary lacrimal gland acinar cells

The polymeric immunoglobulin receptor (pIgR) mediates transcytosis of dimeric immunoglobulin A (dIgA) and its release into mucosal secretions. The present study reveals the complexity of the trafficking of pIgR to the apical plasma membrane in epithelial cells with exocrine secretory functions; in rabbit lacrimal gland acinar cells (LGACs), trafficking of pIgR involves both the transcytotic pathway and one arm of the regulated secretory pathway. By specifically tracking pIgR endocytosed from the basolateral membrane, we show here that the Rab11a-regulated transcytotic pathway mediates the basal-to-apical transport of pIgR, and that pIgR sorted into the transcytotic pathway does not access the regulated secretory pathway. However, previous work in LGACs expanded in the present study has shown that some pIgR is localized to Rab3D-enriched mature secretory vesicles (SVs). Myosin Vb and myosin Vc motors modulate release of proteins from the Rab11a-regulated transcytotic pathway and the Rab3D-enriched secretory pathway in LGACs, respectively. Confocal fluorescence microscopy and biochemical assays showed that inhibition of myosin Vb and myosin Vc activity by overexpression of their dominant-negative mutants each significantly but differentially impaired aspects of apically targeted pIgR trafficking and secretory component release, suggesting that these motors function to regulate pIgR trafficking in both the transcytotic and exocytotic pathways. Intriguingly, a second mature SV population enriched in Rab27b was devoid of pIgR cargo, suggesting the specialization of Rab3D-enriched mature SVs to carry a particular subset of cargo proteins from the trans-Golgi network to the apical plasma membrane.

Caveolin targeting to late endosome/lysosomal membranes is induced by perturbations of lysosomal pH and cholesterol content

Caveolin-1 traffics to late endosomal/lysosomal membranes in response to manipulations of the cholesterol content of cells, suggesting that caveolin functions in the egress of cholesterol from this organelle. Cavicles associate with the periphery of the lysosome as they do with caveosomes, but these are separate organelles.

Caveolin-1 is an integral membrane protein of plasma membrane caveolae. Here we report that caveolin-1 collects at the cytosolic surface of lysosomal membranes when cells are serum starved. This is due to an elevation of the intralysosomal pH, since ionophores and proton pump inhibitors that dissipate the lysosomal pH gradient also trapped caveolin-1 on late endosome/lysosomes. Accumulation is both saturable and reversible. At least a portion of the caveolin-1 goes to the plasma membrane upon reversal. Several studies suggest that caveolin-1 is involved in cholesterol transport within the cell. Strikingly, we find that blocking cholesterol export from lysosomes with progesterone or U18666A or treating cells with low concentrations of cyclodextrin also caused caveolin-1 to accumulate on late endosome/lysosomal membranes. Under these conditions, however, live-cell imaging shows cavicles actively docking with lysosomes, suggesting that these structures might be involved in delivering caveolin-1. Targeting of caveolin-1 to late endosome/lysosomes is not observed normally, and the degradation rate of caveolin-1 is not altered by any of these conditions, indicating that caveolin-1 accumulation is not a consequence of blocked degradation. We conclude that caveolin-1 normally traffics to and from the cytoplasmic surface of lysosomes during intracellular cholesterol trafficking.

Thousands of rab GTPases for the cell biologist

Rab proteins are small GTPases that act as essential regulators of vesicular trafficking. 44 subfamilies are known in humans, performing specific sets of functions at distinct subcellular localisations and tissues. Rab function is conserved even amongst distant orthologs. Hence, the annotation of Rabs yields functional predictions about the cell biology of trafficking. So far, annotating Rabs has been a laborious manual task not feasible for current and future genomic output of deep sequencing technologies. We developed, validated and benchmarked the Rabifier, an automated bioinformatic pipeline for the identification and classification of Rabs, which achieves up to 90% classification accuracy. We cataloged roughly 8.000 Rabs from 247 genomes covering the entire eukaryotic tree. The full Rab database and a web tool implementing the pipeline are publicly available at www.RabDB.org. For the first time, we describe and analyse the evolution of Rabs in a dataset covering the whole eukaryotic phylogeny. We found a highly dynamic family undergoing frequent taxon-specific expansions and losses. We dated the origin of human subfamilies using phylogenetic profiling, which enlarged the Rab repertoire of the Last Eukaryotic Common Ancestor with Rab14, 32 and RabL4. Furthermore, a detailed analysis of the Choanoflagellate Monosiga brevicollis Rab family pinpointed the changes that accompanied the emergence of Metazoan multicellularity, mainly an important expansion and specialisation of the secretory pathway. Lastly, we experimentally establish tissue specificity in expression of mouse Rabs and show that neo-functionalisation best explains the emergence of new human Rab subfamilies. With the Rabifier and RabDB, we provide tools that easily allows non-bioinformaticians to integrate thousands of Rabs in their analyses. RabDB is designed to enable the cell biology community to keep pace with the increasing number of fully-sequenced genomes and change the scale at which we perform comparative analysis in cell biology.

Intracellular compartmentalisation via membrane-delimited organelles is a fundamental feature of the eukaryotic cell. Understanding its origins and specialisation into functionally distinct compartments is a major challenge in evolutionary cell biology. We focus on the Rab enzymes, critical organisers of the trafficking pathways that link the endomembrane system. Rabs form a large family of evolutionarily related proteins, regulating distinct steps in vesicle transport. They mark pathways and organelles due to their specific subcellular and tissue localisation. We propose a solution to the problem of identifying and annotating Rabs in hundreds of sequenced genomes. We developed an accurate bioinformatics pipeline that is able to take into account pre-existing and often inconsistent, manual annotations. We made it available to the community in form of a web tool, as well as a database containing thousands of Rabs assigned to sub-families, which yields clear functional predictions. Thousands of Rabs allow for a new level of analysis. We illustrate this by characterising for the first time the global evolutionary dynamics of the Rab family. We dated the emergence of subfamilies and suggest that the Rab family expands by duplicates acquiring new functions.

Rab GTPases as regulators of endocytosis, targets of disease and therapeutic opportunities

Rab GTPases are well-recognized targets in human disease, although are underexplored therapeutically. Elucidation of how mutant or dysregulated Rab GTPases and accessory proteins contribute to organ specific and systemic disease remains an area of intensive study and an essential foundation for effective drug targeting. Mutation of Rab GTPases or associated regulatory proteins causes numerous human genetic diseases. Cancer, neurodegeneration and diabetes represent examples of acquired human diseases resulting from the up- or downregulation or aberrant function of Rab GTPases. The broad range of physiologic processes and organ systems affected by altered Rab GTPase activity is based on pivotal roles in responding to cell signaling and metabolic demand through the coordinated regulation of membrane trafficking. The Rab-regulated processes of cargo sorting, cytoskeletal translocation of vesicles and appropriate fusion with the target membranes control cell metabolism, viability, growth and differentiation. In this review, we focus on Rab GTPase roles in endocytosis to illustrate normal function and the consequences of dysregulation resulting in human disease. Selected examples are designed to illustrate how defects in Rab GTPase cascades alter endocytic trafficking that underlie neurologic, lipid storage, and metabolic bone disorders as well as cancer. Perspectives on potential therapeutic modulation of GTPase activity through small molecule interventions are provided.

Characterization of RIN3 as a guanine nucleotide exchange factor for the Rab5 subfamily GTPase Rab31

The small GTPase Rab5, which cycles between GDP-bound inactive and GTP-bound active forms, plays essential roles in membrane budding and trafficking in the early endocytic pathway. Rab5 is activated by various vacuolar protein sorting 9 (VPS9) domain-containing guanine nucleotide exchange factors. Rab21, Rab22, and Rab31 (members of the Rab5 subfamily) are also involved in the trafficking of early endosomes. Mechanisms controlling the activation Rab5 subfamily members remain unclear. RIN (Ras and Rab interactor) represents a family of multifunctional proteins that have a VPS9 domain in addition to Src homology 2 (SH2) and Ras association domains. We investigated whether RIN family members act as guanine nucleotide exchange factors (GEFs) for the Rab5 subfamily on biochemical and cell morphological levels. RIN3 stimulated the formation of GTP-bound Rab31 in cell-free and in cell GEF activity assays. RIN3 also formed enlarged vesicles and tubular structures, where it colocalized with Rab31 in HeLa cells. In contrast, RIN3 did not exhibit any apparent effects on Rab21. We also found that serine to alanine substitutions in the sequences between SH2 and RIN family homology domain of RIN3 specifically abolished its GEF action on Rab31 but not Rab5. We examined whether RIN3 affects localization of the cation-dependent mannose 6-phosphate receptor (CD-MPR), which is transported between trans-Golgi network and endocytic compartments. We found that RIN3 partially translocates CD-MPR from the trans-Golgi network to peripheral vesicles and that this is dependent on its Rab31-GEF activity. These results indicate that RIN3 specifically acts as a GEF for Rab31.

Pre-sorting endosomal transport of the GPI-anchored protein, CD59, is regulated by EHD1

EHD1 regulates the trafficking of multiple receptors from the endocytic recycling compartment (ERC) to the plasma membrane. However, the potential role of EHD1 in regulating the family of glycosylphosphatidylinositol-anchored proteins (GPI-APs) has not been determined. Here we demonstrate a novel role for EHD1 in regulating the trafficking of CD59, an endogenous GPI-AP, at early stages of trafficking through the endocytic pathway. EHD1 displays significant colocalization with newly internalized CD59. Upon EHD1 depletion, there is a rapid Rab5-independent coalescence of CD59 in the ERC region. However, expression of an active Arf6 mutant (Q67L), which traps internalized pre-sorting endosomal cargo in phosphatidylinositol(4,5)-bisphosphate enriched vacuoles, prevents this coalescence. It is of interest that sustained PKC activation leads to a similar coalescence of CD59 at the ERC, and treatment of EHD1-depleted cells with a PKC inhibitor (Go6976) blocked this rapid relocation of CD59. However, unlike sustained PKC activation, EHD1 depletion does not induce the translocation of PKCα to ERC. The results presented herein provide evidence that EHD1 is involved in the control of CD59 transport from pre-sorting endosomes to the ERC in a PKC-dependent manner. However, the mechanisms of EHD1-induced coalescence of CD59 at the ERC differ from those induced by sustained PKC activation.

The late stage of autophagy: cellular events and molecular regulation

Autophagy is an intracellular degradation system that delivers cytoplasmic contents to the lysosome for degradation. It is a "self-eating" process and plays a "house-cleaner" role in cells. The complex process consists of several sequential steps-induction, autophagosome formation, fusion of lysosome and autophagosome, degradation, efflux transportation of degradation products, and autophagic lysosome reformation. In this review, the cellular and molecular regulations of late stage of autophagy, including cellular events after fusion step, are summarized.

Macroautophagy signaling and regulation

Macroautophagy is a vacuolar degradation pathway that terminates in the lysosomal compartment. Macroautophagy is a multistep process involving: (1) signaling events that occur upstream of the molecular machinery of autophagy; (2) molecular machinery involved in the formation of the autophagosome, the initial multimembrane-bound compartment formed in the autophagic pathway; and (3) maturation of autophagosomes, which acquire acidic and degradative capacities. In this chapter we summarize what is known about the regulation of the different steps involved in autophagy, and we also discuss how macroautophagy can be manipulated using drugs or genetic approaches that affect macroautophagy signaling, and the subsequent formation and maturation of the autophagosomes. Modulating autophagy offers a promising new therapeutic approach to human diseases that involve macroautophagy.

Vaccinia virus p37 interacts with host proteins associated with LE-derived transport vesicle biogenesis

BACKGROUND

Proteins associated with the late endosome (LE) appear to play a central role in the envelopment of a number of taxonomically diverse viruses. How viral proteins interact with LE-associated proteins to facilitate envelopment is not well understood. LE-derived transport vesicles form through the interaction of Rab9 GTPase with cargo proteins, and TIP47, a Rab9-specific effector protein. Vaccinia virus (VV) induces a wrapping complex derived from intracellular host membranes to envelope intracellular mature virus particles producing egress-competent forms of virus.

RESULTS

We show that VV p37 protein associates with TIP47-, Rab9-, and CI-MPR-containing membranes. Mutation of a di-aromatic motif in p37 blocks association with TIP47 and inhibits plaque formation. ST-246, a specific inhibitor of p37 function, inhibits these interactions and also blocks wrapped virus particle formation. Vaccinia virus expressing p37 variants with reduced ST-246 susceptibility associates with Rab9 and co-localizes with CI-MPR in the presence and absence of compound.

CONCLUSION

These results suggest that p37 localizes to the LE and interacts with proteins associated with LE-derived transport vesicle biogenesis to facilitate assembly of extracellular forms of virus.

A role for EHD4 in the regulation of early endosomal transport

All four of the C-terminal Eps15 homology domain (EHD) proteins have been implicated in the regulation of endocytic trafficking. However, the high level of amino acid sequence identity among these proteins has made it challenging to elucidate the precise function of individual EHD proteins. We demonstrate here with specific peptide antibodies that endogenous EHD4 localizes to Rab5-, early embryonic antigen 1 (EEA1)- and Arf6-containing endosomes and colocalizes with internalized transferrin in the cell periphery. Knock-down of EHD4 expression by both small interfering RNA and short hairpin RNA leads to the generation of enlarged early endosomal structures that contain Rab5 and EEA1 as well as internalized transferrin or major histocompatibility complex class I molecules. In addition, cargo destined for degradation, such as internalized low-density lipoprotein, also accumulates in the enlarged early endosomes in EHD4-depleted cells. Moreover, we have demonstrated that these enlarged early endosomes are enriched in levels of the activated GTP-bound Rab5. Finally, we show that endogenous EHD4 and EHD1 interact in cells, suggesting coordinated involvement in the regulation of receptor transport along the early endosome to endocytic recycling compartment axis. The results presented herein provide evidence that EHD4 is involved in the control of trafficking at the early endosome and regulates exit of cargo toward both the recycling compartment and the late endocytic pathway.

The internalization of the M2 and M4 muscarinic acetylcholine receptors involves distinct subsets of small G-proteins

Multiple mechanisms exist for the endocytosis of receptors from the cell surface. While the M1, M3, and M4 subtypes of muscarinic acetylcholine receptor and M4 receptors transduce their signals through the same second messengers but internalize though different pathways, we tested the ability of several small G-proteins to regulate the agonist-induced endocytosis of M2 and M4 in JEG-3 human choriocarcinoma cells. Dominant-negative Rab5 as well as both wild-type and dominant-negative Rab11 inhibited M4 but not M2 endocytosis. In contrast, a dominant-negative Arf6 as well as wild-type Rab22 increased M2 but not M4 endocytosis. We used immunocytochemistry to show that in unstimulated cells, the M2 and M4 receptors co-localize on the cell surface, whereas after stimulation M2 and M4 are in distinct vesicular compartments. In this study, we demonstrate that agonist-induced internalization of the M2 receptor utilizes an Arf6, Rab22 dependent pathway, while the M4 receptor undergoes agonist-induced internalization through a Rab5, Rab11 dependent pathway. Additionally, we show that Rab15 and RhoA are not involved in either pathway in JEG-3 cells.

Phagocytosis and host-pathogen interactions in Dictyostelium with a look at macrophages

Research into phagocytosis and host-pathogen interactions in the lower eukaryote Dictyostelium discoideum has flourished in recent years. This chapter presents a glimpse of where this research stands, with emphasis on the cell biology of the phagocytic process and on the wealth of molecular genetic data that have been gathered. The basic mechanistic machinery and most of the underlying genes appear to be evolutionarily conserved, reflecting the fact that phagocytosis arose as an efficient way to ingest food in single protozoan cells devoid of a rigid cell wall. In spite of some differences, the signal transduction pathways regulating phagosome biogenesis are also emerging as ultimately similar between Dictyostelium and macrophages. Both cell types are hosts for many pathogenic invasive bacteria, which exploit phagocytosis to grow intracellularly. We present an overwiew, based on the analysis of mutants, on how Dictyostelium contributes as a genetic model system to decipher the complexity of host-pathogen interactions.