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Summary The endosomal system constitutes a highly dynamic vesicle network used to relay materials and signals between the cell and its environment.1 Once internalized, endosomes gradually mature into late acidic compartments and acquire a multivesicular body (MVB) organization through invagination of the limiting membrane (LM) to form intraluminal vesicles (ILVs).2 Cargoes sequestered into ILVs can either be delivered to lysosomes for degradation or secreted following fusion of the MVB with the plasma membrane.3 It has been speculated that commitment to ILVs is not a terminal event, and that a return pathway exists, allowing “back-fusion” or “retrofusion” of intraluminal membranes to the LM.4 The existence of retrofusion as a way to support membrane equilibrium within the MVB has been widely speculated in various cell biological contexts, including exosome uptake5 and major histocompatibility complex class II (MHC class II) antigen presentation.6, 7, 8, 9 Given the small physical scale, retrofusion of ILVs cannot be measured with conventional techniques. To circumvent this, we designed a chemically tunable cell-based system to monitor retrofusion in real time. Using this system, we demonstrate that retrofusion occurs as part of the natural MVB lifestyle, with attributes parallel to those of viral infection. Furthermore, we find that retrofusion and exocytosis coexist in an equilibrium, implying that ILVs inert to retrofusion comprise a significant fraction of exosomes destined for secretion. MVBs thus contain three types of ILVs: those committed to lysosomal degradation, those retrofusing ILVs, and those subject to secretion in the form of exosomes. Video abstract, Graphical abstract, Highlights • MVBs are complex organelles with intraluminal vesicles bound by the limiting membrane • Intraluminal membranes are in a dynamic equilibrium with the limiting membrane • Retrofusion of internal vesicles is controlled by processes used for viral fusion • Exosomes arise from internal MVB vesicles not participating in retrofusion, Multivesicular bodies (MVBs) are complex organelles harboring internal vesicles. Using a chemically controlled system, Perrin et al. visualize a part of these internal vesicles fusing back to the limiting membrane. This process of retrofusion exists in equilibrium with lysosomal degradation and exosome release and is inhibited by antiviral proteins.

HLA class I (HLA-I) glycoproteins drive immune responses by presenting antigens to cognate CD8+ T cells. This process is often hijacked by tumors and pathogens for immune evasion. Because options for restoring HLA-I antigen presentation are limited, we aimed to identify druggable HLA-I pathway targets. Using iterative genome-wide screens, we uncovered that the cell surface glycosphingolipid (GSL) repertoire determines effective HLA-I antigen presentation. We show that absence of the protease SPPL3 augmented B3GNT5 enzyme activity, resulting in upregulation of surface neolacto-series GSLs. These GSLs sterically impeded antibody and receptor interactions with HLA-I and diminished CD8+ T cell activation. Furthermore, a disturbed SPPL3-B3GNT5 pathway in glioma correlated with decreased patient survival. We show that the immunomodulatory effect could be reversed through GSL synthesis inhibition using clinically approved drugs. Overall, our study identifies a GSL signature that inhibits immune recognition and represents a potential therapeutic target in cancer, infection, and autoimmunity.

The invariant chain (Ii, also known as CD74) is a multifunctional regulator of adaptive immune responses and is responsible for sorting major histocompatibility complex class I and class II (MHCI and MHCII, respectively) molecules, as well as other Ii-associated molecules, to a specific endosomal pathway. When Ii is expressed, endosomal maturation and proteolytic degradation of proteins are delayed and, in non-antigen presenting cells, the endosomal size increases, but the molecular mechanisms underlying this are not known. We identified that a SNARE, Vti1b, is essential for regulating these Ii-induced effects. Vti1b binds to Ii and is localized at the contact sites of fusing Ii-positive endosomes. Furthermore, truncated Ii lacking the cytoplasmic tail, which is not internalized from the plasma membrane, relocates Vti1b to the plasma membrane. Knockout of Ii in an antigen-presenting cell line was found to speed up endosomal maturation, whereas silencing of Vti1b inhibits the Ii-induced maturation delay. Our results suggest that Ii, by interacting with the SNARE Vti1b in antigen-presenting cells, directs specific Ii-associated SNARE-mediated fusion in the early part of the endosomal pathway that leads to a slower endosomal maturation for efficient antigen processing and MHC antigen loading.

SummaryHLA class I (HLA-I) drives immune responses by presenting antigen-derived peptides to cognate CD8+T cells. This process is often hijacked by tumors and pathogens for immune evasion. Since therapeutic options for restoring HLA-I antigen presentation are limited, we aimed to identify new HLA-I pathway targets. By iterative genome-wide screens we uncovered that the cell surface glycosphingolipid (GSL) repertoire determines effective HLA-I antigen presentation. We show that absence of the protease SPPL3 augments B3GNT5 enzyme activity, resulting in upregulated levels of surface (neo)lacto-series GSLs. These GSLs sterically impede molecular interactions with HLA-I and diminish CD8+T cell activation. In accordance, a disturbed SPPL3-B3GNT5 pathway in glioma associates with decreased patient survival. Importantly, we show that this immunomodulatory effect can be reversed through GSL synthesis inhibition using clinically approved drugs. Overall, our study identifies a GSL signature that functionally inhibits antigen presentation and represents a potential therapeutic target in cancer, infection and autoimmunity.

HLA class I (HLA-I) drives immune responses by presenting antigen-derived peptides to cognate CD8+ T cells. This process is often hijacked by tumors and pathogens for immune evasion. Since therapeutic options for restoring HLA-I antigen presentation are limited, we aimed to identify new HLA-I pathway targets. By iterative genome-wide screens we uncovered that the cell surface glycosphingolipid (GSL) repertoire determines effective HLA-I antigen presentation. We show that absence of the protease SPPL3 augments B3GNT5 enzyme activity, resulting in upregulated levels of surface (neo)lacto-series GSLs. These GSLs sterically impede molecular interactions with HLA-I and diminish CD8+ T cell activation. In accordance, a disturbed SPPL3-B3GNT5 pathway in glioma associates with decreased patient survival. Importantly, we show that this immunomodulatory effect can be reversed through GSL synthesis inhibition using clinically approved drugs. Overall, our study identifies a GSL signature that functionally inhibits antigen presentation and represents a potential therapeutic target in cancer, infection and autoimmunity.

Upon nuclear envelope (NE) fragmentation in the prometaphase, the nuclear and cytosolic proteomes mix and must be redefined to reinstate homeostasis. Here, by using a molecular GFP ladder, we show that in early mitosis, condensed chromatin excludes cytosolic proteins. When the NE reforms tightly around condensed chromatin in late mitosis, large GFP multimers are automatically excluded from the nucleus. This can be circumvented by limiting DNA condensation with Q15, a condensin II inhibitor. Soluble small and other nuclear localization sequence (NLS)-targeted proteins then swiftly enter the expanding nuclear space. We then examined proteasomes, which are located in the cytoplasm and nucleus. A significant fraction of 20S proteasomes is imported by the importin IPO5 within 20 min of reformation of the nucleus, after which import comes to an abrupt halt. This suggests that maintaining the nuclear–cytosol distribution after mitosis requires chromatin condensation to exclude cytosolic material from the nuclear space, and specialized machineries for nuclear import of large protein complexes, such as the proteasome.

C-type lectins are key players in immune regulation by driving distinct functions of antigen-presenting cells. The C-type lectin CLEC16A gene is located at 16p13, a susceptibility locus for several autoimmune diseases, including multiple sclerosis. However, the function of this gene and its potential contribution to these diseases in humans are poorly understood. In this study, we found a strong upregulation of CLEC16A expression in the white matter of multiple sclerosis patients (n = 14) compared to non-demented controls (n = 11), mainly in perivascular leukocyte infiltrates. Moreover, CLEC16A levels were significantly enhanced in peripheral blood mononuclear cells of multiple sclerosis patients (n = 69) versus healthy controls (n = 46). In peripheral blood mononuclear cells, CLEC16A was most abundant in monocyte-derived dendritic cells, in which it strongly co-localized with human leukocyte antigen class II. Treatment of these professional antigen-presenting cells with vitamin D, a key protective environmental factor in multiple sclerosis, downmodulated CLEC16A in parallel with human leukocyte antigen class II. Knockdown of CLEC16A in distinct types of model and primary antigen-presenting cells resulted in severely impaired cytoplasmic distribution and formation of human leucocyte antigen class II-positive late endosomes, as determined by immunofluorescence and electron microscopy. Mechanistically, CLEC16A participated in the molecular machinery of human leukocyte antigen class II-positive late endosome formation and trafficking to perinuclear regions, involving the dynein motor complex. By performing co-immunoprecipitations, we found that CLEC16A directly binds to two critical members of this complex, RILP and the HOPS complex. CLEC16A silencing in antigen-presenting cells disturbed RILP-mediated recruitment of human leukocyte antigen class II-positive late endosomes to perinuclear regions. Together, we identify CLEC16A as a pivotal gene in multiple sclerosis that serves as a direct regulator of the human leukocyte antigen class II pathway in antigen-presenting cells. These findings are a first step in coupling multiple sclerosis-associated genes to the regulation of the strongest genetic factor in multiple sclerosis, human leukocyte antigen class II.

Cholesterol is an essential lipid in eukaryotic cells and is present in membranes of all intracellular compartments. A major source for cellular cholesterol is internalized lipoprotein particles that are transported toward acidic late endosomes (LE) and lysosomes. Here the lipoprotein particles are hydrolyzed, and free cholesterol is redistributed to other organelles. The LE can contain over half of the cellular cholesterol and, as a major sorting station, can contain many cholesterol-binding proteins from the ABCA, STARD, and ORP families. Here, we show that metastatic lymph node 64 (MLN64, STARD3) and oxysterol-binding protein-related protein 1L (ORP1L) define two subpopulations of LE. MLN64 is present on a LE containing the cholesterol transporter ABCA3, whereas ORP1L localizes to another population of LE containing Niemann Pick type C1 (NPC1), a cholesterol exporter. Endocytosed cargo passes through MLN64/ABCA3-positive compartments before it reaches ORP1L/NPC1-positive LE. The MLN64/ABCA3 compartments cycle between LE and plasma membrane and frequently contact “later” ORP1L/NPC1-containing LE. We propose two stages of cholesterol handling in late endosomal compartments: first, cholesterol enters MLN64/ABCA3-positive compartments from where it can be recycled to the plasma membrane, and later, cholesterol enters ORP1L/NPC1 endosomes that mediate cholesterol export to the endoplasmic reticulum.

Summary Through a network of progressively maturing vesicles, the endosomal system connects the cell’s interior with extracellular space. Intriguingly, this network exhibits a bilateral architecture, comprised of a relatively immobile perinuclear vesicle “cloud” and a highly dynamic peripheral contingent. How this spatiotemporal organization is achieved and what function(s) it curates is unclear. Here, we reveal the endoplasmic reticulum (ER)-located ubiquitin ligase Ring finger protein 26 (RNF26) as the global architect of the entire endosomal system, including the trans-Golgi network (TGN). To specify perinuclear vesicle coordinates, catalytically competent RNF26 recruits and ubiquitinates the scaffold p62/sequestosome 1 (p62/SQSTM1), in turn attracting ubiquitin-binding domains (UBDs) of various vesicle adaptors. Consequently, RNF26 restrains fast transport of diverse vesicles through a common molecular mechanism operating at the ER membrane, until the deubiquitinating enzyme USP15 opposes RNF26 activity to allow vesicle release into the cell’s periphery. By drawing the endosomal system’s architecture, RNF26 orchestrates endosomal maturation and trafficking of cargoes, including signaling receptors, in space and time., Graphical Abstract, Highlights • The bulk of the endosomal system and TGN clusters in the perinuclear “cloud” • The ER-located E3 ubiquitin ligase RNF26 retains vesicles in the perinuclear cloud • RNF26 employs the ubiquitin scaffold SQSTM1 to capture specific vesicle adaptors • Opposition between RNF26 and DUB USP15 times release of vesicles for fast transport, The endosomal system exhibits a bilateral architecture, comprised of a relatively immobile perinuclear vesicle “cloud” and a highly dynamic peripheral contingent. How this cloud is organized and what purpose it serves is unknown. Here, we reveal its molecular determinants centered around the ER-located ubiquitin ligase RNF26, capable of retaining the entire endosomal system’s repertoire through a common mechanism operating at the ER membrane. Countered by the deubiquitinating enzyme USP15, RNF26 draws the endosomal system’s architecture, thus orchestrating vesicle maturation and cargo trafficking in space and time.

Autophagy is the main homeostatic pathway guiding cytosolic materials for degradation by the lysosome. Maturation of autophagosomes requires their transport towards the perinuclear region of the cell, with key factors underlying both processes still poorly understood. Here we show that transport and positioning of late autophagosomes depends on cholesterol by way of the cholesterol-sensing Rab7 effector ORP1L. ORP1L localizes to late autophagosomes and—under low-cholesterol conditions—contacts the ER protein VAP-A, forming ER-autophagosome contact sites, which prevent minus-end transport by the Rab7–RILP–dynein complex. ORP1L-mediated contact sites also inhibit localization of PLEKHM1 to Rab7. PLEKHM1, together with RILP, then recruits the homotypic fusion and vacuole protein-sorting (HOPS) complex for fusion of autophagosomes with late endosomes and lysosomes. Thus, ORP1L, via its liganding by lipids and the formation of contacts between autophagic vacuoles and the ER, governs the last steps in autophagy that lead to the lysosomal degradation of cytosolic material., Autophagy requires transport of autophagosomes to the perinuclear region. Here, the authors show that ORP1L localizes to autophagosomes and mediates formation of ER contact sites that prevent autophagosome transport and fusion with endocytic vesicles when cholesterol levels are low.

SummaryMHC class II molecules (MHC-II) present peptides to T helper cells to facilitate immune responses and are strongly linked to autoimmune diseases. To unravel processes controlling MHC-II antigen presentation, we performed a genome-wide flow cytometry-based RNAi screen detecting MHC-II expression and peptide loading followed by additional high-throughput assays. All data sets were integrated to answer two fundamental questions: what regulates tissue-specific MHC-II transcription, and what controls MHC-II transport in dendritic cells? MHC-II transcription was controlled by nine regulators acting in feedback networks with higher-order control by signaling pathways, including TGFβ. MHC-II transport was controlled by the GTPase ARL14/ARF7, which recruits the motor myosin 1E via an effector protein ARF7EP. This complex controls movement of MHC-II vesicles along the actin cytoskeleton in human dendritic cells (DCs). These genome-wide systems analyses have thus identified factors and pathways controlling MHC-II transcription and transport, defining targets for manipulation of MHC-II antigen presentation in infection and autoimmunity.

Late endosomes (LEs) have characteristic intracellular distributions determined by their interactions with various motor proteins. Motor proteins associated to the dynactin subunit p150Glued bind to LEs via the Rab7 effector Rab7-interacting lysosomal protein (RILP) in association with the oxysterol-binding protein ORP1L. We found that cholesterol levels in LEs are sensed by ORP1L and are lower in peripheral vesicles. Under low cholesterol conditions, ORP1L conformation induces the formation of endoplasmic reticulum (ER)–LE membrane contact sites. At these sites, the ER protein VAP (VAMP [vesicle-associated membrane protein]-associated ER protein) can interact in trans with the Rab7–RILP complex to remove p150Glued and associated motors. LEs then move to the microtubule plus end. Under high cholesterol conditions, as in Niemann-Pick type C disease, this process is prevented, and LEs accumulate at the microtubule minus end as the result of dynein motor activity. These data explain how the ER and cholesterol control the association of LEs with motor proteins and their positioning in cells.

The small GTPase Rab7 controls late endocytic transport by the minus end–directed motor protein complex dynein–dynactin, but how it does this is unclear. Rab7-interacting lysosomal protein (RILP) and oxysterol-binding protein–related protein 1L (ORP1L) are two effectors of Rab7. We show that GTP-bound Rab7 simultaneously binds RILP and ORP1L to form a RILP–Rab7–ORP1L complex. RILP interacts directly with the C-terminal 25-kD region of the dynactin projecting arm p150Glued, which is required for dynein motor recruitment to late endocytic compartments (LEs). Still, p150Glued recruitment by Rab7–RILP does not suffice to induce dynein-driven minus-end transport of LEs. ORP1L, as well as βIII spectrin, which is the general receptor for dynactin on vesicles, are essential for dynein motor activity. Our results illustrate that the assembly of microtubule motors on endosomes involves a cascade of linked events. First, Rab7 recruits two effectors, RILP and ORP1L, to form a tripartite complex. Next, RILP directly binds to the p150Glued dynactin subunit to recruit the dynein motor. Finally, the specific dynein motor receptor Rab7–RILP is transferred by ORP1L to βIII spectrin. Dynein will initiate translocation of late endosomes to microtubule minus ends only after interacting with βIII spectrin, which requires the activities of Rab7–RILP and ORP1L.

Trafficking of cargo through the endosomal system depends on endosomal fusion events mediated by SNARE proteins, Rab-GTPases, and multisubunit tethering complexes. The CORVET and HOPS tethering complexes, respectively, regulate early and late endosomal tethering and have been characterized in detail in yeast where their sequential membrane targeting and assembly is well understood. Mammalian CORVET and HOPS subunits significantly differ from their yeast homologues, and novel proteins with high homology to CORVET/HOPS subunits have evolved. However, an analysis of the molecular interactions between these subunits in mammals is lacking. Here, we provide a detailed analysis of interactions within the mammalian CORVET and HOPS as well as an additional endosomal-targeting complex (VIPAS39-VPS33B) that does not exist in yeast. We show that core interactions within CORVET and HOPS are largely conserved but that the membrane-targeting module in HOPS has significantly changed to accommodate binding to mammalian-specific RAB7 interacting lysosomal protein (RILP). Arthrogryposis-renal dysfunction-cholestasis (ARC) syndrome-associated mutations in VPS33B selectively disrupt recruitment to late endosomes by RILP or binding to its partner VIPAS39. Within the shared core of CORVET/HOPS, we find that VPS11 acts as a molecular switch that binds either CORVET-specific TGFBRAP1 or HOPS-specific VPS39/RILP thereby allowing selective targeting of these tethering complexes to early or late endosomes to time fusion events in the endo/lysosomal pathway.

The Rac-specific GEF (guanine-nucleotide exchange factor) Tiam1 (T-lymphoma invasion and metastasis 1) regulates migration, cell–matrix and cell–cell adhesion by modulating the actin cytoskeleton through the GTPase, Rac1. Using yeast two-hybrid screening and biochemical assays, we found that Tiam1 interacts with the p21-Arc [Arp (actin-related protein) complex] subunit of the Arp2/3 complex. Association occurred through the N-terminal pleckstrin homology domain and the adjacent coiled-coil region of Tiam1. As a result, Tiam1 co-localizes with the Arp2/3 complex at sites of actin polymerization, such as epithelial cell–cell contacts and membrane ruffles. Deletion of the p21-Arc-binding domain in Tiam1 impairs its subcellular localization and capacity to activate Rac1, suggesting that binding to the Arp2/3 complex is important for the function of Tiam1. Indeed, blocking Arp2/3 activation with a WASP (Wiskott–Aldrich syndrome protein) inhibitor leads to subcellular relocalization of Tiam1 and decreased Rac activation. Conversely, functionally active Tiam1, but not a GEF-deficient mutant, promotes activation of the Arp2/3 complex and its association with cytoskeletal components, indicating that Tiam1 and Arp2/3 are mutually dependent for their correct localization and signalling. Our data suggests a model in which the Arp2/3 complex acts as a scaffold to localize Tiam1, and thereby Rac activity, which are both required for activation of the Arp2/3 complex and further Arp2/3 recruitment. This ‘self-amplifying’ signalling module involving Tiam1, Rac and the Arp2/3 complex could thus drive actin polymerization at specific sites in cells that are required for dynamic morphological changes.

The small GTPase Rab7 controls fusion and transport of late endocytic compartments. A critical mediator is the Rab7 effector RILP that recruits the minus-end dynein-dynactin motor complex to these compartments. We identified a natural occurring splice variant of RILP (RILPsv) lacking only 27 amino acids encoded by exon VII. Both variants bind Rab7, prolong its GTP-bound state, and induce clustering of late endocytic compartments. However, RILPsv does not recruit the dynein-dynactin complex, implicating exon VII in motor recruitment. Clustering might still occur via dimerization, since both RILP and RILPsv are able to form hetero- and homo-dimers. Moreover, both effectors compete for Rab7 binding but with different outcome for dynein-dynactin recruitment and transport. Hence, RILPsv provides an extra dimension to the control of vesicle fusion and transport by the small GTPase Rab7.

Salmonella typhimurium survives and replicates intracellular in a membrane-bound compartment, the Salmonella-containing vacuole (SCV). In HeLa cells, the SCV matures through interactions with the endocytic pathway, but Salmonella avoids fusion with mature lysosomes. The exact mechanism of the inhibition of phagolysosomal fusion is not understood. Rab GTPases control several proteins involved in membrane fusion and vesicular transport. The small GTPase Rab7 regulates the transport of and fusion between late endosomes and lysosomes and associates with the SCV. We show that the Rab7 GTPase cycle is not affected on the SCV. We then manipulated a pathway downstream of the small GTPase Rab7 in HeLa cells infected with Salmonella. Expression of the Rab7 effector RILP induces recruitment of the dynein/dynactin motor complex to the SCV. Subsequently, SCV fuse with lysosomes. As a result, the intracellular replication of Salmonella is inhibited. Activation of dynein-mediated vesicle transport can thus control intracellular survival of Salmonella.

Intracellular proteins are degraded by the proteasome, and resulting peptides surviving cytoplasmic peptidase activity can be presented by MHC class I molecules. Here, we show that intracellular aminopeptidases degrade peptides within seconds, almost irrespectively of amino acid sequence. N- but not C-terminal extension increases the half-life of peptides until they are 15 amino acids long. Beyond 15 amino acids, peptides are exclusively trimmed by the peptidase TPPII, which displays both exo- and endopeptidase activity. Surprisingly, most proteasomal degradation products are handled by TPPII before presentation by MHC class I molecules. We define three distinct proteolytic activities during antigen processing in vivo. Proteasome-generated peptides relevant for antigen presentation are mostly 15 amino acids or longer. These require TPPII activity for further trimming before becoming substrates for other peptidases and MHC class I. The heterogeneous pool of aminopeptidases will process TPPII products into MHC class I peptides and beyond.

The topoisomerase II poisons doxorubicin and etoposide constitute longstanding cornerstones of chemotherapy. Despite their extensive clinical use, many patients do not respond to these drugs. Using a genome-wide gene knockout approach, we identified Keap1, the SWI/SNF complex, and C9orf82 (CAAP1) as independent factors capable of driving drug resistance through diverse molecular mechanisms, all converging on the DNA double-strand break (DSB) and repair pathway. Loss of Keap1 or the SWI/SNF complex inhibits generation of DSB by attenuating expression and activity of topoisomerase IIα, respectively, whereas deletion of C9orf82 augments subsequent DSB repair. Their corresponding genes, frequently mutated or deleted in human tumors, may impact drug sensitivity, as exemplified by triple-negative breast cancer patients with diminished SWI/SNF core member expression who exhibit reduced responsiveness to chemotherapy regimens containing doxorubicin. Collectively, our work identifies genes that may predict the response of cancer patients to the broadly used topoisomerase II poisons and defines alternative pathways that could be therapeutically exploited in treatment-resistant patients. Cancer Res; 75(19); 4176–87. ©2015 AACR.

MHC class I molecules present a variable but limited repertoire of antigenic peptides for T-cell recognition. Understanding how peptide selection is achieved requires mechanistic insights into the interactions between the MHC I and candidate peptides. We find that, at first encounter, MHC I H-2K(b) considers a wide range of peptides, including those with expanded N termini and unfitting anchor residues. Discrimination occurs in the second step, when noncanonical peptides dissociate with faster exchange rates. This second step exhibits remarkable temperature sensitivity, as illustrated by numerous noncanonical peptides presented by H-2K(b) in cells cultured at 26 °C relative to 37 °C. Crystallographic analyses of H-2K(b)-peptide complexes suggest that a conformational adaptation of H-2K(b) drives the decisive step in peptide selection. We propose that MHC class I molecules consider initially a large peptide pool, subsequently refined by a temperature-sensitive induced-fit mechanism to retain the canonical peptide repertoire.

Meeting abstracts Anti-tumor immune responses are often hampered by an excess of coinhibition of T cells, but also by a lack of MHC antigen presentation and costimulation. The latter are not yet successfully exploited as targets for immunotherapy, while augmenting MHC and costimulatory molecules

Many intracellular compartments, including MHC class II-containing lysosomes [1], melanosomes [2], and phagosomes [3], move along microtubules in a bidirectional manner and in a stop-and-go fashion due to the alternating activities of a plus-end directed kinesin motor and a minus-end directed dynein-dynactin motor [4]. It is largely unclear how motor proteins are targeted specifically to different compartments. Rab GTPases recruit and/or activate several proteins involved in membrane fusion and vesicular transport [5, 6]. They associate with specific compartments after activation, which makes Rab GTPases ideal candidates for controlling motor protein binding to specific membranes. We and others [7] have identified a protein, called RILP (for Rab7-interacting lysosomal protein), that interacts with active Rab7 on late endosomes and lysosomes. Here we show that RILP prevents further cycling of Rab7. RILP expression induces the recruitment of functional dynein-dynactin motor complexes to Rab7-containing late endosomes and lysosomes. Consequently, these compartments are transported by these motors toward the minus end of microtubules, effectively inhibiting their transport toward the cell periphery. This signaling cascade may be responsible for timed and selective dynein motor recruitment onto late endosomes and lysosomes.

Proteins of the Rho family regulate cytoskeletal rearrangements in response to receptor stimulation and are involved in the establishment and maintenance of epithelial cell morphology. We recently showed that Rac is able to downregulate Rho activity and that the reciprocal balance between Rac and Rho activity is a major determinant of cellular morphology and motility in NIH3T3 fibroblasts. Using biochemical pull-down assays, we analyzed the effect of transient and sustained oncogenic Ras signaling on the activation state of Rac and Rho in epithelial MDCK cells. In contrast to the activation of Rac by growth factor-induced Ras signaling, we found that sustained signaling by oncogenic RasV12 permanently downregulates Rac activity, which leads to upregulation of Rho activity and epithelial–mesenchymal transition. Oncogenic Ras decreases Rac activity through sustained Raf/MAP kinase signaling, which causes transcriptional downregulation of Tiam1, an activator of Rac in epithelial cells. Reconstitution of Rac activity by expression of Tiam1 or RacV12 leads to downregulation of Rho activity and restores an epithelial phenotype in mesenchymal RasV12- or RafCAAX-transformed cells. The present data reveal a novel mechanism by which oncogenic Ras is able to interfere with the balance between Rac and Rho activity to achieve morphological transformation of epithelial cells.

Late endosomes and lysosomes are dynamic organelles that constantly move and fuse to acquire cargo from early endosomes, phagosomes and autophagosome. Defects in lysosomal dynamics cause severe neurodegenerative and developmental diseases, such as Niemann-Pick type C disease and ARC syndrome, yet little is known about the regulation of late endosomal fusion in a mammalian system. Mammalian endosomes destined for fusion need to be transported over very long distances before they tether to initiate contact. Here, we describe that lysosomal tethering and transport are combined processes co-regulated by one multi-protein complex: RAB7-RILP-ORP1L. We show that RILP directly and concomitantly binds the tethering HOPS complex and the p150(Glued) subunit of the dynein motor. ORP1L then functions as a cholesterol-sensing switch controlling RILP-HOPS-p150(Glued) interactions. We show that RILP and ORP1L control Ebola virus infection, a process dependent on late endosomal fusion. By combining recruitment and regulation of both the dynein motor and HOPS complex into a single multiprotein complex, the RAB7-RILP-ORP1L complex efficiently couples and regulates the timing of microtubule minus-end transport and fusion, two major events in endosomal biology.

DNA topoisomerase II inhibitors are a major class of cancer chemotherapeutics, which are thought to eliminate cancer cells by inducing DNA double-strand breaks. Here we identify a novel activity for the anthracycline class of DNA topoisomerase II inhibitors: histone eviction from open chromosomal areas. We show that anthracyclines promote histone eviction irrespective of their ability to induce DNA double-strand breaks. The histone variant H2AX, which is a key component of the DNA damage response, is also evicted by anthracyclines, and H2AX eviction is associated with attenuated DNA repair. Histone eviction deregulates the transcriptome in cancer cells and organs such as the heart, and can drive apoptosis of topoisomerase-negative acute myeloid leukaemia blasts in patients. We define a novel mechanism of action of anthracycline anticancer drugs doxorubicin and daunorubicin on chromatin biology, with important consequences for DNA damage responses, epigenetics, transcription, side effects and cancer therapy., Anthracycline-based drugs can kill cancer cells by inhibiting topoisomerase II and promoting DNA double-strand breaks. Pang et al. show that anthracyclines also induce eviction of histones from open chromatin regions and, in doing so, modulate DNA repair and apoptosis in human cancer cells.

CD70 is a TNF-related transmembrane molecule expressed by mature dendritic cells (DCs), which present antigens to T cells via major histocompatibility complex (MHC) molecules. In DCs, CD70 localizes with MHC class II molecules in late endosomal vesicles, known as MHC class II compartments (MIICs). MIICs are transported to the immune synapse when a DC contacts an antigen-specific CD4+ T cell. Consequently, MHC class II and CD70 are simultaneously exposed to the T cell. Thereby, T-cell activation via the antigen receptor and CD70-mediated co-stimulation are synchronized, apparently to optimize the proliferative response. We report here that the invariant chain (Ii), a chaperone known to transport MHC class II to MIICs, performs a similar function for CD70. CD70 was found to travel by default to the plasma membrane, whereas Ii coexpression directed it to late endosomes and/or lysosomes. In cells containing the MHC class II presentation pathway, CD70 localized to MIICs. This localization relied on Ii, since transport of CD70 from the Golgi to MIICs was impeded in Ii-deficient DCs. Biophysical and biochemical studies revealed that CD70 and Ii participate in an MHC-class-II-independent complex. Thus, Ii supports transport of both MHC class II and CD70 to MIICs and thereby coordinates their delivery to CD4+ T cells.

With the emergence of multidrug resistant (MDR) bacteria, it is imperative to develop new intervention strategies. Current antibiotics typically target pathogen rather than host-specific biochemical pathways. Here we have developed kinase inhibitors that prevent intracellular growth of unrelated pathogens such as Salmonella typhimurium and Mycobacterium tuberculosis. An RNA interference screen of the human kinome using automated microscopy revealed several host kinases capable of inhibiting intracellular growth of S. typhimurium. The kinases identified clustered in one network around AKT1 (also known as PKB). Inhibitors of AKT1 prevent intracellular growth of various bacteria including MDR-M. tuberculosis. AKT1 is activated by the S. typhimurium effector SopB, which promotes intracellular survival by controlling actin dynamics through PAK4, and phagosome-lysosome fusion through the AS160 (also known as TBC1D4)-RAB14 pathway. AKT1 inhibitors counteract the bacterial manipulation of host signalling processes, thus controlling intracellular growth of bacteria. By using a reciprocal chemical genetics approach, we identified kinase inhibitors with antibiotic properties and their host targets, and we determined host signalling networks that are activated by intracellular bacteria for survival.

Ag presentation by MHC class I is a highly inefficient process because cytosolic peptidases destroy most peptides after proteasomal generation. Various mechanisms shape the MHC class I peptidome. We define a new one: intracellular peptide stability. Peptides with two N-terminal basic amino acids are more stable than other peptides. Such peptides should be overrepresented in the peptidome of MHC class I-associated peptides. HLA-B27 binding peptides use anchor residue R at P2 and, although most amino acids are allowed, particular amino acids are overrepresented at P1, including R and K. We show that such N-terminal dibasic peptides are indeed more efficiently presented by HLA-B27. This suggests that HLA-B27 can present peptides from Ags present in fewer copies than required for successful peptide generation for other MHC class I molecules.

Rho GTPases are crucial regulators of the actin cytoskeleton and they play a role in the control of membrane trafficking. In contrast to the close family members RhoA and RhoC, RhoB localises to endosomes and delays epidermal growth factor receptor traffic. Here, we show that activated RhoB induces the peripheral distribution of endosomes, which align along subcortical actin stress fibres and are surrounded by an actin coat. The Diaphanous-related formin, Dia1, is recruited to endosomes by activated RhoB. Dia1 is required for the formation of the actin coat around endosomes downstream of RhoB, connecting membrane trafficking with the regulation of actin dynamics.

Publisher Summary This chapter explains the expression of Rho guanosine 5'-triphosphates (GTPases) using retroviral vectors. The introduction of wild-type or mutant Rho-like GTPases into cells or tissues facilitates the study of the proteins in diverse cellular processes. Proteins can be transiently administered by direct injection of purified proteins or by injection or transfection of constructs that encode the proteins. Stable expressions are achieved by transfection of constructs, followed by selection of positive cells using a selection marker. The chapter describes the features and the design of the retroviral plasmids and describes the way in which they are used for the generation of stable expressing cells. The retroviral plasmids facilitate the cloning of cDNA constructs and allow the generation of directionally cloned cDNA libraries. The chapter concludes with a discussion of infection protocol.

SummaryMajor Histocompatibility Complex (MHC) class II molecules, including Human Leukocyte Antigen (HLA)-DR, present peptide fragments from proteins degraded in the endocytic pathway. HLA-DR is targeted to late-endocytic structures named MHC class II-containing Compartments (MIIC), where it interacts with HLA-DM. This chaperone stabilizes HLA-DR during peptide exchange and is critical for successful peptide loading. To follow this process in living cells, we have generated cells containing HLA-DR3/Cyan Fluorescent Protein (CFP), HLA-DM/Yellow Fluorescent Protein (YFP), and invariant chain. HLA-DR/DM interactions were observed by Fluorescence Resonance Energy Transfer (FRET). These interactions were pH insensitive, yet occurred only in internal structures and not at the limiting membrane of MIIC. In a cellular model of infection, phagosomes formed a limiting membrane surrounding internalized Salmonella. HLA-DR and HLA-DM did not interact in Salmonella-induced vacuoles, and HLA-DR was not loaded with antigens. The absence of HLA-DR/DM interactions at the limiting membrane prevents local loading of MHC class II molecules in phagosomes. This may allow these bacteria to successfully evade the immune system.