Rab22a affects the morphology and function of the endocytic pathway

Rab22B’s role in trans-Golgi network membrane dynamics

The small GTPase Rab22B (or Rab31) has been suspected to be involved in trafficking at trans-Golgi network. However, its exact cellular localization, tissue expression profile, and functions have not been uncharacterized. Specific antibody raised against Rab22B's protein revealed that Rab22B is brain-enriched, but is also present in substantial levels in spleen and intestine. In HeLa cells, endogenous Rab22B is largely associated with the trans-Golgi network (TGN). Over-expression of a GDP-binding mutant (Rab22BSN), but not wild-type Rab22B, specifically disrupts the TGN localization of TGN46, a dynamic marker which cycles between the TGN and the plasma membrane. The TGN resident membrane protein syntaxin 16, cis-Golgi markers such as GM130 and syntaxin 5, as well as the TGN/late endosome marker mannose 6-phosphate receptor (M6PR) are not affected by Rab22BSN, neither was endosomal-TGN transport of the Shiga toxin B subunit. The disruption of TGN46 staining by Rab22BSN could be specifically attributed to a domain at the C-terminal portion of Rab22B, where its sequence deviates the most from Rab22A. Over-expression of Rab22BSN inhibits the cell surface transport of the vesicular stomatitis virus G protein. Thus, Rab22B may have a role in anterograde exit from the TGN.

Involvement of PLEKHM1 in osteoclastic vesicular transport and osteopetrosis in incisors absent rats and humans

This study illustrates that Plekhm1 is an essential protein for bone resorption, as loss-of-function mutations were found to underlie the osteopetrotic phenotype of the incisors absent rat as well as an intermediate type of human osteopetrosis. Electron and confocal microscopic analysis demonstrated that monocytes from a patient homozygous for the mutation differentiated into osteoclasts normally, but when cultured on dentine discs, the osteoclasts failed to form ruffled borders and showed little evidence of bone resorption. The presence of both RUN and pleckstrin homology domains suggests that Plekhm1 may be linked to small GTPase signaling. We found that Plekhm1 colocalized with Rab7 to late endosomal/lysosomal vesicles in HEK293 and osteoclast-like cells, an effect that was dependent on the prenylation of Rab7. In conclusion, we believe PLEKHM1 to be a novel gene implicated in the development of osteopetrosis, with a putative critical function in vesicular transport in the osteoclast.

Rab cascades and tethering factors in the endomembrane system

Rab GTPases are key proteins that determine organelle identity and operate at the center of fusion reactions. Like Ras, they act as switches that are connected to a diverse network of tethering factors, exchange factors and GTPase activating proteins. Recent studies suggest that Rabs are linked to each other via their effectors, thus coordinating protein transport in the endomembrane system. Within this review, we will focus on selected examples that highlight these issues.

Role of endosomal Rab GTPases in cytokinesis

Completion of cytokinesis requires Rab 11-dependent membrane trafficking events to deliver new membrane to the furrow and for abscission. Many Rabs have overlapping endosomal distributions, hence, we examined whether these Rabs also function in cytokinesis. Analysis of the distribution of Rabs 4, 5, 7, 8, 9, 11, 21, and 22 revealed that only Rab 11 was enriched within the furrow of cells in telophase or present within the midbody. By contrast, Rabs 4, 5, 7, 8, and 9 were mainly localised within a peri-nuclear compartment facing away from the furrow. Using RNA interference and dominant negative Rab mutants, we evaluated the role of these Rabs in furrowing and abscission. Consistent with previous work, we find that Rab 11 is intimately involved in abscission. However, we further found that depletion of Rab 4 slowed but did not prevent abscission. Depletion of any other Rab species had little effect on furrowing or abscission. These data suggest that the membrane trafficking events required for completion of cytokinesis are largely controlled by Rab 11 and not other endosomal Rab proteins, and further suggest that the relocation of Rab 11-specific cargo is an integral facet of abscission. Arf6 knockdown was without effect on cytokinesis, but when both Rab 11 and Arf6 were knocked-down, we found the furrow rapidly regressed and the cells were unable to form a stable midbody. We suggest that Rab 11 and Arf6 function synergistically in the switch from furrowing to abscission, as well as in the terminal stage of abscission.

Higher order Rab programming in phagolysosome biogenesis

Phagosomes offer kinetically and morphologically tractable organelles to dissect the control of phagolysosome biogenesis by Rab GTPases. Model phagosomes harboring latex beads undergo a coordinated Rab5–Rab7 exchange, which is akin to the process of endosomal Rab conversion, the control mechanisms of which are unknown. In the process of blocking phagosomal maturation, the intracellular pathogen Mycobacterium tuberculosis prevents Rab7 acquisition, thus, providing a naturally occurring tool to study Rab conversion. We show that M. tuberculosis inhibition of Rab7 acquisition and arrest of phagosomal maturation depends on Rab22a. Four-dimensional microscopy revealed that phagosomes harboring live mycobacteria recruited and retained increasing amounts of Rab22a. Rab22a knockdown in macrophages via siRNA enhanced the maturation of phagosomes with live mycobacteria. Conversely, overexpression of the GTP-locked mutant Rab22aQ64L prevented maturation of phagosomes containing heat-killed mycobacteria, which normally progress into phagolysosomes. Moreover, Rab22a knockdown led to Rab7 acquisition by phagosomes harboring live mycobacteria. Our findings show that Rab22a defines the critical checkpoint for Rab7 conversion on phagosomes, allowing or disallowing organellar transition into a late endosomal compartment. M. tuberculosis parasitizes this process by actively recruiting and maintaining Rab22a on its phagosome, thus, preventing Rab7 acquisition and blocking phagolysosomal biogenesis.

Rab22a regulates the sorting of transferrin to recycling endosomes

Rab22a is a member of the Rab family of small GTPases that localizes in the endocytic pathway. In CHO cells, expression of canine Rab22a (cRab22a) causes a dramatic enlargement of early endocytic compartments. We wondered whether transferrin recycling is altered in these cells. Expression of the wild-type protein and a GTP hydrolysis-deficient mutant led to the redistribution of transferrin receptor to large cRab22a-positive structures in the periphery of the cell and to a significant decrease in the plasma membrane receptor. Kinetic analysis of transferrin uptake indicates that internalization and early recycling were not affected by cRab22a expression. However, recycling from large cRab22a-positive compartments was strongly inhibited. A similar effect on transferrin transport was observed when human but not canine Rab22a was expressed in HeLa cells. After internalization for short periods of time (5 to 8 min) or at a reduced temperature (16 degrees C), transferrin localized with endogenous Rab22a in small vesicles that did not tubulate with brefeldin A, suggesting that the endogenous protein is present in early/sorting endosomes. Rab22a depletion by small interfering RNA disorganized the perinuclear recycling center and strongly inhibited transferrin recycling. We speculate that Rab22a controls the transport of the transferrin receptor from sorting to recycling endosomes.

Interferon-gamma induces internalization of epithelial tight junction proteins via a macropinocytosis-like process

Increased epithelial permeability is observed in inflammatory states. However, the mechanism by which inflammatory mediators such as IFN-gamma increase epithelial permeability is unknown. We recently observed that IFN-gamma induces disassembly of tight junctions (TJ); in this study we asked whether such TJ disassembly is mediated by endocytosis of junctional proteins. The role of three major internalization pathways in disruption of TJ in IFN-gamma-treated intestinal epithelial cells was analyzed using selective inhibitors and markers of the pathways. No role for the clathrin- and caveolar-mediated endocytosis in the IFN-gamma-induced internalization of TJ proteins was observed. However, inhibitors of macropinocytosis blocked internalization of TJ proteins and junctional proteins colocalized with macropinocytosis markers, dextran and phosphatidylinositol-3,4,5-trisphosphate. Internalized TJ proteins were identified in early and recycling endosomes but not in late endosomes/lysosomes. These results for the first time suggest that IFN-gamma produces a leaky epithelial barrier by inducing macropinoytosis of TJ proteins.

Vps9 domain-containing proteins: activators of Rab5 GTPases from yeast to neurons

Endocytosis of cell surface receptors plays an important role in regulating cell signaling cascades. In some cases, internalization of an activated receptor attenuates the signaling process, while in other cases the clustering of activated receptors on early endosomal structures has been proposed to be essential for fully activating signaling cascades. Regulating the movement of receptors and other signaling proteins through the endocytic pathway, therefore, has a direct impact on cellular homeostasis. The small GTPase Rab5 is a crucial regulatory component of the endocytic pathway. Activation of Rab5 is mediated by GDP-GTP exchange factors (GEFs) that generate the Rab5-GTP complex. A large number of proteins have been identified that contain a specific, highly conserved domain (Vps9) that catalyzes nucleotide exchange on Rab5, linking the regulation of cell signaling cascades with intracellular receptor trafficking through the endocytic pathway.

Molecular mechanism and regulation of autophagy

Autophagy is a major cellular pathway for the degradation of long-lived proteins and cytoplasmic organelles in eukaryotic cells. A large number of intracellular/extracellular stimuli, including amino acid starvation and invasion of microorganisms, are able to induce the autophagic response in cells. The discovery of the ATG genes in yeast has greatly advanced our understanding of the molecular mechanisms participating in autophagy and the genes involved in regulating the autophagic pathway. Many yeast genes have mammalian homologs, suggesting that the basic machinery for autophagy has been evolutionarily conserved along the eukaryotic phylum. The regulation of autophagy is a very complex process. Many signaling pathways, including target of rapamycin (TOR) or mammalian target of rapamycin (mTOR), phosphatidylinositol 3-kinase-I (PI3K-I)/PKB, GTPases, calcium and protein synthesis all play important roles in regulating autophagy. The molecular mechanisms and regulation of autophagy are discussed in this review.

Structural basis of family-wide Rab GTPase recognition by rabenosyn-5

Rab GTPases regulate all stages of membrane trafficking, including vesicle budding, cargo sorting, transport, tethering and fusion. In the inactive (GDP-bound) conformation, accessory factors facilitate the targeting of Rab GTPases to intracellular compartments. After nucleotide exchange to the active (GTP-bound) conformation, Rab GTPases interact with functionally diverse effectors including lipid kinases, motor proteins and tethering complexes. How effectors distinguish between homologous Rab GTPases represents an unresolved problem with respect to the specificity of vesicular trafficking. Using a structural proteomic approach, we have determined the specificity and structural basis underlying the interaction of the multivalent effector rabenosyn-5 with the Rab family. The results demonstrate that even the structurally similar effector domains in rabenosyn-5 can achieve highly selective recognition of distinct subsets of Rab GTPases exclusively through interactions with the switch and interswitch regions. The observed specificity is determined at a family-wide level by structural diversity in the active conformation, which governs the spatial disposition of critical conserved recognition determinants, and by a small number of both positive and negative sequence determinants that allow further discrimination between Rab GTPases with similar switch conformations.

Regulation and role of autophagy in mammalian cells

The recent period has witnessed progress in the understanding of the lysosomal autophagic pathway. The discovery of a family of genes conserved from yeast to humans, and involved in the formation of autophagosomes, has unraveled new protein-conjugation systems and has shed light on the importance of autophagy in physiology and pathophysiology. The elucidation of the molecular control of autophagy will also lead to a better understanding of the role of autophagy during cell death. As a great number of extracellular stimuli (starvation, hormonal or therapeutic treatment) as well as intracellular stimuli (accumulation of misfolded proteins, invasion of microorganisms) is able to modulate the autophagic response, it is not surprising that several signaling pathways are involved in the control of autophagy. The mammalian Target of Rapamycin (mTOR) signaling pathway plays a major role in transmitting autophagic stimuli because of its ability to sense nutrient, metabolic and hormonal signals. In addition, autophagy, which is characterized by a flux of membrane from the formation of the autophagosome to the fusion with the lysosome, is regulated by GTPases, similarly to the vesicular transport along the exocytic/endocytic pathway. The aim of the present review is to give an overview of autophagy and to discuss its regulation by activators and effectors of mTOR and GTPases.

Fluorescent Microscopy‐Based Assays to Study the Role of Rab22a in Clathrin‐Independent Endocytosis

The endocytic and trafficking route followed by proteins that enter cells independently of clathrin is not well understood. In HeLa cells, a distinct endocytic and recycling pathway is followed for such clathrin-independent cargo proteins, and we have characterized the role of Rab proteins in this process. Here we describe cell-based flourescence assays to examine the effects of expression and depletion of Rab22 on endosomal morphology and endocytic recycling.

Overexpression of Rab22a hampers the transport between endosomes and the Golgi apparatus

The transport and sorting of soluble and membrane-associated macromolecules arriving at endosomal compartments require a complex set of Rab proteins. Rab22a has been localized to the endocytic compartment; however, very little is known about the function of Rab22a and inconsistent results have been reported in studies performed in different cell lines. To characterize the function of Rab22a in endocytic transport, the wild-type protein (Rab22a WT), a hydrolysis-deficient mutant (Rab22a Q64L), and a mutant with reduced affinity for GTP (Rab22a S19N) were expressed in CHO cells. None of the three Rab22a constructs affected the transport of rhodamine-dextran to lysosomes, the digestion of internalized proteins, or the lysosomal localization of cathepsin D. In contrast with the mild effect of Rab22a on the endosome-lysosome route, cells expressing Rab22a WT and Rab22a Q64L presented a strong delay in the retrograde transport of cholera toxin from endosomes to the Golgi apparatus. Moreover, these cells accumulated the cation independent mannose 6-phosphate receptor in endosomes. These observations indicate that Rab22a can affect the trafficking from endosomes to the Golgi apparatus probably by promoting fusion among endosomes and impairing the proper segregation of membrane domains required for targeting to the trans-Golgi network (TGN).

A role for the small GTPase Rab21 in the early endocytic pathway

Rab proteins comprise a family of monomeric GTPases that control cellular membrane traffic. Rab21 is a poorly characterised member with no known function. Human Rab21 cDNA from K562 cells was subcloned into GFP expression vectors to generate Rab21 and Rab21 mutants defective in either GTP hydrolysis (Rab21 Q78L) or binding (Rab21 T33N) for transfection studies in HeLa cells. Confocal fluorescence microscopy and ultrastructural studies revealed Rab21 to be predominantly localised to the early endocytic pathway, on vesicles containing earlyendosomal antigen 1 EEA1, transferrin receptor and internalised ligands. EEA1 was localised to enlarged endosomes in Rab21 wild-type expressing cells but the GTP hydrolysis and GDP binding mutants had unique phenotypes labelling tubular reticular structures and the trans-Golgi network, respectively. Early endosome localisation for Rab21 was confirmed in a hepatoma cell line that allowed analysis of the subcellular distribution of the endogenous protein. Comparison of the localisation of Rab21 with other Rabs revealed extensive colocalisation with early endocytic variants Rab4, Rab5, Rab17 and Rab22 but much less overlap with those associated with late endosomes, recycling endosomes and the early secretory pathway. Cells expressing Rab21 T33N had defects in endocytosis of transferrin and epidermal growth factor and failed to effectively deliver the latter ligand to late endosomes and lysosomes for degradation. Collectively, our data provide the first characterisation of Rab21 function in early endosome dynamics.

Structure, Exchange Determinants, and Family-Wide Rab Specificity of the Tandem Helical Bundle and Vps9 Domains of Rabex-5

The Rab5 GTPase, an essential regulator of endocytosis and endosome biogenesis, is activated by guanine-nucleotide exchange factors (GEFs) that contain a Vps9 domain. Here, we show that the catalytic core of the Rab GEF Rabex-5 has a tandem architecture consisting of a Vps9 domain stabilized by an indispensable helical bundle. A family-wide analysis of Rab specificity demonstrates high selectivity for Rab5 subfamily GTPases. Conserved exchange determinants map to a common surface of the Vps9 domain, which recognizes invariant aromatic residues in the switch regions of Rab GTPases and selects for the Rab5 subfamily by requiring a small nonacidic residue preceding a critical phenylalanine in the switch I region. These and other observations reveal unexpected similarity with the Arf exchange site in the Sec7 domain.

Rab22a regulates the recycling of membrane proteins internalized independently of clathrin

Plasma membrane proteins that are internalized independently of clathrin, such as major histocompatibility complex class I (MHCI), are internalized in vesicles that fuse with the early endosomes containing clathrin-derived cargo. From there, MHCI is either transported to the late endosome for degradation or is recycled back to the plasma membrane via tubular structures that lack clathrin-dependent recycling cargo, e.g., transferrin. Here, we show that the small GTPase Rab22a is associated with these tubular recycling intermediates containing MHCI. Expression of a dominant negative mutant of Rab22a or small interfering RNA-mediated depletion of Rab22a inhibited both formation of the recycling tubules and MHCI recycling. By contrast, cells expressing the constitutively active mutant of Rab22a exhibited prominent recycling tubules and accumulated vesicles at the periphery, but MHCI recycling was still blocked. These results suggest that Rab22a activation is required for tubule formation and Rab22a inactivation for final fusion of recycling membranes with the surface. The trafficking of transferrin was only modestly affected by these treatments. Dominant negative mutant of Rab11a also inhibited recycling of MHCI but not the formation of recycling tubules, suggesting that Rab22a and Rab11a might coordinate different steps of MHCI recycling.

Chromosome mapping and identification of amphiphilic proteins of hexaploid wheat kernels

Amphiphilic proteomic analysis was carried out on the ITMI (International Triticae Mapping Population) population resulting from a cross between "Synthetic", i.e.: "W7984" and "Opata". Out of a total of 446 spots, 170 were specific to either of the two parents, and 276 were common to both. Preliminary analysis, which was performed on 80 progenies (Amiour et al. 2002a), was completed here using a total of 101 selfed lines. Seventy two Loci of amphiphilic spots placed at LOD = 5 were conclusively assigned to 15 chromosomes. Some spots mapped during the first analysis were eliminated because of the significant distortion segregation observed in the second analysis. Group-1 chromosomes had by far the greatest number of mapped spots (51). Using the Quantitative Trait Loci (QTLs) approach, analysis of the quantitative variation of each spot revealed that 96 spots out of the 170 specific ones showed at least one Protein Quantity Locus (PQL). These PQLs were distributed throughout the genome. With Matrix Laser Desorption Ionisation Time Of Flight (MALDI-TOF) spectrometry and Database interrogation, a total of 93 specific and 41 common spots were identified. This enabled us to show that the majority of these proteins are associated with membranes and/or play a role in plant defence against external invasions. Using multiple-regression analysis, other amphiphilic proteins, in addition to puroindolines, were shown to be involved in variation in kernel hardness in the ITMI population.

Rab proteins and endocytic trafficking: potential targets for therapeutic intervention

Rab GTPases serve as master regulators of vesicular membrane transport on both the exo- and endocytic pathways. In their active forms, rab proteins serve in cargo selection and as scaffolds for the sequential assembly of effectors requisite for vesicle budding, cytoskeletal transport, and target membrane fusion. Rab protein function is in turn tightly regulated at the level of protein expression, localization, membrane association, and activation. Alterations in the rab GTPases and associated regulatory proteins or effectors have increasingly been implicated in causing human disease. Some diseases such as those resulting in bleeding and pigmentation disorders (Griscelli syndrome), mental retardation, neuropathy (Charcot-Marie-Tooth), kidney disease (tuberous sclerosis), and blindness (choroideremia) arise from direct loss of function mutations of rab GTPases or associated regulatory molecules. In contrast, in a number of cancers (prostate, liver, breast) as well as vascular, lung, and thyroid diseases, the overexpression of select rab GTPases have been tightly correlated with disease pathogenesis. Unique therapeutic opportunities lie ahead in developing strategies that target rab proteins and modulate the endocytic pathway.

Autophagy: a barrier or an adaptive response to cancer

Macroautophagy or autophagy is a degradative pathway terminating in the lysosomal compartment after the formation of a cytoplasmic vacuole that engulfs macromolecules and organelles. The recent discovery of the molecular controls of autophagy that are common to eukaryotic cells from yeast to human suggests that the role of autophagy in cell functioning is far beyond its nonselective degradative capacity. The involvement of proteins with properties of tumor suppressor and oncogenic properties at different steps of the pathway implies that autophagy must be considered in tumor progression. Autophagy as a stress response mechanism protects cancer cells from low nutrient supply or therapeutic insults. Autophagy is also involved in the elimination of cancer cells by triggering a non-apoptotic cell death program, suggesting a negative role in tumor development. These two aspects of autophagy will be discussed in this review.

Diversity of signaling controls of macroautophagy in mammalian cells

Macroautophagy is a major lysosomal catabolic process conserved from yeast to human. The formation of autophagic vacuoles is stimulated by a variety of intracellular and extracellular stress situations including amino acid starvation, aggregation of misfolded proteins, and accumulation of damaged organelles. Several signaling pathways control the formation of autophagic vacuoles. As some of them are engaged in the control of protein synthesis or cell survival this suggests that macroautophagy is intimately associated with the execution of cell proliferation and cell death programs. Whether or not these different signaling pathways converge to a unique point to trigger the formation of autophagic vacuole remains an open question.