Rab35 and its GAP EPI64C in T cells regulate receptor recycling and immunological synapse formation

Rit1-TBC1D10B signaling modulates FcγR-mediated phagosome formation in RAW264 macrophages

Phagocytosis is an important immune response that protects the host from pathogen invasion. Rit1 GTPase is known to be involved in diverse cellular processes. However, its role in FcγR-mediated phagocytosis remains unclear. Our live-cell imaging analysis revealed that Rit1 was localized to the membranes of F-actin-rich phagocytic cups in RAW264 macrophages. Rit1 knockout and expression of the GDP-locked Rit1 mutant suppressed phagosome formation. We also found that TBC1D10B, a GAP for the Rab family GTPases, colocalizes with Rit1 in the membranes of phagocytic cups. Expression and knockout studies have shown that TBC1D10B decreases phagosome formation in both Rab-GAP activity-dependent and -independent manners. Notably, the expression of the GDP-locked Rit1 mutant or Rit1 knockout inhibited the dissociation of TBC1D10B from phagocytic cups. In addition, the expression of the GTP-locked Rit1 mutant promoted the dissociation of TBC1D10B in phagocytic cups and restored the rate of phagosome formation in TBC1D10B-expressing cells. These data suggest that Rit1-TBC1D10B signaling regulates FcγR-mediated phagosome formation in macrophages.

Rab GTPases, Active Members in Antigen-Presenting Cells, and T Lymphocytes

Processes such as cell migration, phagocytosis, endocytosis, and exocytosis refer to the intense exchange of information between the internal and external environment in the cells, known as vesicular trafficking. In eukaryotic cells, these essential cellular crosstalks are controlled by Rab GTPases proteins through diverse adaptor proteins like SNAREs complex, coat proteins, phospholipids, kinases, phosphatases, molecular motors, actin, or tubulin cytoskeleton, among others, all necessary for appropriate mobilization of vesicles and distribution of molecules. Considering these molecular events, Rab GTPases are critical components in specific biological processes of immune cells, and many reports refer primarily to macrophages; therefore, in this review, we address specific functions in immune cells, concretely in the mechanism by which the GTPase contributes in dendritic cells (DCs) and, T/B lymphocytes.

RAB35 is required for murine hippocampal development and functions by regulating neuronal cell distribution

RAB35 is a multifunctional small GTPase that regulates endocytic recycling, cytoskeletal rearrangement, and cytokinesis. However, its physiological functions in mammalian development remain unclear. Here, we generated Rab35-knockout mice and found that RAB35 is essential for early embryogenesis. Interestingly, brain-specific Rab35-knockout mice displayed severe defects in hippocampal lamination owing to impaired distribution of pyramidal neurons, although defects in cerebral cortex formation were not evident. In addition, Rab35-knockout mice exhibited defects in spatial memory and anxiety-related behaviors. Quantitative proteomics indicated that the loss of RAB35 significantly affected the levels of other RAB proteins associated with endocytic trafficking, as well as some neural cell adhesion molecules, such as contactin-2. Collectively, our findings revealed that RAB35 is required for precise neuronal distribution in the developing hippocampus by regulating the expression of cell adhesion molecules, thereby influencing spatial memory.

Targeting Endocytosis and Cell Communications in the Tumor Immune Microenvironment

The existence of multiple endocytic pathways is well known, and their exact biological effects in tumors have been intensively investigated. Endocytosis can affect the connection between tumor cells and determine the fate of tumor cells. Many relationships between endocytosis and tumor cells have been elucidated, but the mechanism of endocytosis between different types of cells in tumors needs to be explored in greater depth. Endocytic receptors sense the environment and are induced by specific ligands to trigger communication between tumor and immune cells. Crosstalk in the tumor microenvironment can occur through direct contact between cell adhesion molecules or indirectly through exosomes. So a better understanding of the endocytic pathways that control cell adhesion molecules and function is expected to lead to new candidates for cancer treatment. In additional, tumor-derived exosomes may changes immune cell function, which may be a key role for tumors to evade immune detection and response. The overall understanding of exosomes through endocytosis is also expected to bring new candidates for therapeutic regulation of tumor immune microenvironment. In this case, endocytic pathways coordinate cell adhesion molecules and exosomes and can be used as targets in the tumor immune microenvironment for cancer treatment.

Transcellular propagation of fibrillar α-synuclein from enteroendocrine to neuronal cells requires cell-to-cell contact and is Rab35-dependent

Parkinson’s disease (PD) is a neurodegenerative condition featured by motor dysfunction, death of midbrain dopaminergic neurons and accumulation of α-synuclein (αSyn) aggregates. Growing evidence suggests that PD diagnosis happens late in the disease progression and that the pathology may originate much earlier in the enteric nervous system (ENS) before advancing to the brain, via autonomic fibers. It was recently described that a specific cell type from the gut epithelium named enteroendocrine cells (EECs) possess many neuron-like properties including αSyn expression. By facing the gut lumen and being directly connected with αSyn-containing enteric neurons in a synaptic manner, EECs form a neural circuit between the gastrointestinal tract and the ENS, thereby being a possible key player in the outcome of PD in the gut. We have characterized the progression and the cellular mechanisms involved in αSyn pre-formed fibrils (PFFs) transfer from EECs to neuronal cells. We show that brain organoids efficiently internalize αSyn PFF seeds which triggers the formation of larger intracellular inclusions. In addition, in the enteroendocrine cell line STC-1 and in the neuronal cell line SH-SY5Y, αSyn PFFs induced intracellular calcium (Ca²⁺) oscillations on an extracellular Ca²⁺ source-dependent manner and triggered αSyn fibrils internalization by endocytosis. We characterized the spread of αSyn PFFs from enteroendocrine to neuronal cells and showed that this process is dependent on physical cell-to-cell contact and on Rab35 GTPase. Lastly, inhibition of Rab35 increases the clearance of αSyn fibrils by redirecting them to the lysosomal compartment. Therefore, our results reveal mechanisms that contribute to the understanding of how seeded αSyn fibrils promote the progression of αSyn pathology from EECs to neuronal cells shifting the focus of PD etiology to the ENS.

Rab35 and glucocorticoids regulate APP and BACE1 trafficking to modulate Aβ production

Chronic stress and elevated glucocorticoids (GCs), the major stress hormones, are risk factors for Alzheimer’s disease (AD) and promote AD pathomechanisms, including overproduction of toxic amyloid-β (Aβ) peptides and intraneuronal accumulation of hyperphosphorylated Tau protein. The latter is linked to downregulation of the small GTPase Rab35, which mediates Tau degradation via the endolysosomal pathway. Whether Rab35 is also involved in Aβ overproduction remains an open question. Here, we find that hippocampal Rab35 levels are decreased not only by stress/GC but also by aging, another AD risk factor. Moreover, we show that Rab35 negatively regulates Aβ production by sorting amyloid precursor protein (APP) and β-secretase (BACE1) out of the endosomal network, where they interact to produce Aβ. Interestingly, Rab35 coordinates distinct intracellular trafficking steps for BACE1 and APP, mediated by its effectors OCRL and ACAP2, respectively. Finally, we demonstrate that Rab35 overexpression prevents the amyloidogenic trafficking of APP and BACE1 induced by high GC levels. These studies identify Rab35 as a key regulator of APP processing and suggest that its downregulation may contribute to stress-related and AD-related amyloidogenesis.

TBC1D10C is a cytoskeletal functional linker that modulates cell spreading and phagocytosis in macrophages

Cell spreading and phagocytosis are notably regulated by small GTPases and GAP proteins. TBC1D10C is a dual inhibitory protein with GAP activity. In immune cells, TBC1D10C is one of the elements regulating lymphocyte activation. However, its specific role in macrophages remains unknown. Here, we show that TBC1D10C engages in functions dependent on the cytoskeleton and plasma membrane reorganization. Using ex vivo and in vitro assays, we found that elimination and overexpression of TBC1D10C modified the cytoskeletal architecture of macrophages by decreasing and increasing the spreading ability of these cells, respectively. In addition, TBC1D10C overexpression contributed to higher phagocytic activity against Burkholderia cenocepacia and to increased cell membrane tension. Furthermore, by performing in vitro and in silico analyses, we identified 27 TBC1D10C-interacting proteins, some of which were functionally classified as protein complexes involved in cytoskeletal dynamics. Interestingly, we identified one unreported TBC1D10C-intrinsically disordered region (IDR) with biological potential at the cytoskeleton level. Our results demonstrate that TBC1D10C shapes macrophage activity by inducing reorganization of the cytoskeleton-plasma membrane in cell spreading and phagocytosis. We anticipate our results will be the basis for further studies focused on TBC1D10C. For example, the specific molecular mechanism in Burkholderia cenocepacia phagocytosis and functional analysis of TBC1D10C-IDR are needed to further understand its role in health and disease.

The roles of GTPase-activating proteins in regulated cell death and tumor immunity

GTPase-activating protein (GAP) is a negative regulator of GTPase protein that is thought to promote the conversion of the active GTPase-GTP form to the GTPase-GDP form. Based on its ability to regulate GTPase proteins and other domains, GAPs are directly or indirectly involved in various cell requirement processes. We reviewed the existing evidence of GAPs regulating regulated cell death (RCD), mainly apoptosis and autophagy, as well as some novel RCDs, with particular attention to their association in diseases, especially cancer. We also considered that GAPs could affect tumor immunity and attempted to link GAPs, RCD and tumor immunity. A deeper understanding of the GAPs for regulating these processes could lead to the discovery of new therapeutic targets to avoid pathologic cell loss or to mediate cancer cell death.

The role of endocytic trafficking in antigen T Cell Receptor activation

Antigen T cell receptors (TCR) recognize antigenic peptides displayed by the major histocompatibility complex (pMHC) and play a critical role in T cell activation. The levels of TCR complexes at the cell surface, where signaling is initiated, depend on the balance between TCR synthesis, recycling and degradation. Cell surface TCR interaction with pMHC leads to receptor clustering and formation of a tight T cell-APC contact, the immune synapse, from which the activated TCR is internalized. While TCR internalization from the immune synapse has been initially considered to arrest TCR signaling, recent evidence support the hypothesis that the internalized receptor continues to signal from specialized endosomes. Here, we review the molecular mechanisms of TCR endocytosis and recycling, both in steady state and after T cell activation. We then discuss the experimental evidence in favor of endosomal TCR signaling and its possible consequences on T cell activation.

F-Actin Dynamics in the Regulation of Endosomal Recycling and Immune Synapse Assembly

Membrane proteins endocytosed at the cell surface as vesicular cargoes are sorted at early endosomes for delivery to lysosomes for degradation or alternatively recycled to different cellular destinations. Cargo recycling is orchestrated by multimolecular complexes that include the retromer, retriever, and the WASH complex, which promote the polymerization of new actin filaments at early endosomes. These endosomal actin pools play a key role at different steps of the recycling process, from cargo segregation to specific endosomal subdomains to the generation and mobility of tubulo-vesicular transport carriers. Local F-actin pools also participate in the complex redistribution of endomembranes and organelles that leads to the acquisition of cell polarity. Here, we will present an overview of the contribution of endosomal F-actin to T-cell polarization during assembly of the immune synapse, a specialized membrane domain that T cells form at the contact with cognate antigen-presenting cells.

The dual functions of Rab11 and Rab35 GTPases-regulation of cell division and promotion of tumorigenicity

The broad studies of cancer have led researchers to the creditable understanding of biological and environmental factors that make benign cells to become malignant, as well as the developmental aspects of the tumour cells, known as the "hallmarks of cancer". However, additional research is needed to uncover the features of cancer biology, which would allow to design new and more effective treatment strategies for cancer patients. Since RabGTPases and their effectors are frequently altered in cancer, their role in a regulation of cell division leading to the acquisition of cancer cell-like phenotype has drawn a lot of attention from different research groups in recent years. Both, Rab11 and Rab35 belong to a superfamily of small monomeric GTPases that regulate a diverse array of cellular functions. Lately, Rab11 and Rab35 were declared as oncogenic, and because of their association with abundant cellular functions, a linkage to the induction of cancer, has been proposed. Although the clear connection between the improper regulation of Rab11 or Rab35 and the initiation of tumorigenicity has only beginning to emerge, in this review we will discuss the newest findings regarding the participation of RabGTPases in a control of cell division and promotion of tumorigenesis, trying to link the actual function to the cancer causality.

Microclusters as T Cell Signaling Hubs: Structure, Kinetics, and Regulation

When T cell receptors (TCRs) engage with stimulatory ligands, one of the first microscopically visible events is the formation of microclusters at the site of T cell activation. Since the discovery of these structures almost 20 years ago, they have been studied extensively in live cells using confocal and total internal reflection fluorescence (TIRF) microscopy. However, due to limits in image resolution and acquisition speed, the spatial relationships of signaling components within microclusters, the kinetics of their assembly and disassembly, and the role of vesicular trafficking in microcluster formation and maintenance were not finely characterized. In this review, we will summarize how new microscopy techniques have revealed novel insights into the assembly of these structures. The sub-diffraction organization of microclusters as well as the finely dissected kinetics of recruitment and disassociation of molecules from microclusters will be discussed. The role of cell surface molecules in microcluster formation and the kinetics of molecular recruitment via intracellular vesicular trafficking to microclusters is described. Finally, the role of post-translational modifications such as ubiquitination in the downregulation of cell surface signaling molecules is also discussed. These results will be related to the role of these structures and processes in T cell activation.

A sensitive S-Trap-based approach to the analysis of T cell lipid raft proteome

The analysis of T cell lipid raft proteome is challenging due to the highly dynamic nature of rafts and the hydrophobic character of raft-resident proteins. We explored an innovative strategy for bottom-up lipid raftomics based on suspension-trapping (S-Trap) sample preparation. Mouse T cells were prepared from splenocytes by negative immunoselection, and rafts were isolated by a detergent-free method and OptiPrep gradient ultracentrifugation. Microdomains enriched in flotillin-1, LAT, and cholesterol were subjected to proteomic analysis through an optimized protocol based on S-Trap and high pH fractionation, followed by nano-LC-MS/MS. Using this method, we identified 2,680 proteins in the raft-rich fraction and established a database of 894 T cell raft proteins. We then performed a differential analysis on the raft-rich fraction from nonstimulated versus anti-CD3/CD28 T cell receptor (TCR)-stimulated T cells. Our results revealed 42 proteins present in one condition and absent in the other. For the first time, we performed a proteomic analysis on rafts from ex vivo T cells obtained from individual mice, before and after TCR activation. This work demonstrates that the proposed method utilizing an S-Trap-based approach for sample preparation increases the specificity and sensitivity of lipid raftomics.

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.

Coordinating Cytoskeleton and Molecular Traffic in T Cell Migration, Activation, and Effector Functions

Dynamic localization of receptors and signaling molecules at the plasma membrane and within intracellular vesicular compartments is crucial for T lymphocyte sensing environmental cues, triggering membrane receptors, recruiting signaling molecules, and fine-tuning of intracellular signals. The orchestrated action of actin and microtubule cytoskeleton and intracellular vesicle traffic plays a key role in all these events that together ensure important steps in T cell physiology. These include extravasation and migration through lymphoid and peripheral tissues, T cell interactions with antigen-presenting cells, T cell receptor (TCR) triggering by cognate antigen-major histocompatibility complex (MHC) complexes, immunological synapse formation, cell activation, and effector functions. Cytoskeletal and vesicle traffic dynamics and their interplay are coordinated by a variety of regulatory molecules. Among them, polarity regulators and membrane-cytoskeleton linkers are master controllers of this interplay. Here, we review the various ways the T cell plasma membrane, receptors, and their signaling machinery interplay with the actin and microtubule cytoskeleton and with intracellular vesicular compartments. We highlight the importance of this fine-tuned crosstalk in three key stages of T cell biology involving cell polarization: T cell migration in response to chemokines, immunological synapse formation in response to antigen cues, and effector functions. Finally, we discuss two examples of perturbation of this interplay in pathological settings, such as HIV-1 infection and mutation of the polarity regulator and tumor suppressor adenomatous polyposis coli (Apc) that leads to familial polyposis and colorectal cancer.

Dysregulated Plasma Membrane Turnover Underlying Dendritic Pathology in Neurodegenerative Diseases

Due to their enormous surface area compared to other cell types, neurons face unique challenges in properly handling supply and retrieval of the plasma membrane (PM)-a process termed PM turnover-in their distal areas. Because of the length and extensiveness of dendritic branches in neurons, the transport of materials needed for PM turnover from soma to distal dendrites will be inefficient and quite burdensome for somatic organelles. To meet local demands, PM turnover in dendrites most likely requires local cellular machinery, such as dendritic endocytic and secretory systems, dysregulation of which may result in dendritic pathology observed in various neurodegenerative diseases (NDs). Supporting this notion, a growing body of literature provides evidence to suggest the pathogenic contribution of dysregulated PM turnover to dendritic pathology in certain NDs. In this article, we present our perspective view that impaired dendritic endocytic and secretory systems may contribute to dendritic pathology by encumbering PM turnover in NDs.

A Conditional Mutation in SCD1 Reveals Linkage Between PIN Protein Trafficking, Auxin Transport, Gravitropism, and Lateral Root Initiation

Auxin is transported in plants with distinct polarity, defined by transport proteins of the PIN-formed (PIN) family. Components of the complex trafficking machinery responsible for polar PIN protein localization have been identified by genetic approaches, but severe developmental phenotypes of trafficking mutants complicate dissection of this pathway. We utilized a temperature sensitive allele of Arabidopsis thaliana SCD1 (stomatal cytokinesis defective1) that encodes a RAB-guanine nucleotide exchange factor. Auxin transport, lateral root initiation, asymmetric auxin-induced gene expression after gravitropic reorientation, and differential gravitropic growth were reduced in the roots of the scd1-1 mutant relative to wild type at the restrictive temperature of 25°C, but not at the permissive temperature of 18°C. In scd1-1 at 25°C, PIN1- and PIN2-GFP accumulated in endomembrane bodies. Transition of seedlings from 18 to 25°C for as little as 20 min resulted in the accumulation of PIN2-GFP in endomembranes, while gravitropism and root developmental defects were not detected until hours after transition to the non-permissive temperature. The endomembrane compartments that accumulated PIN2-GFP in scd1-1 exhibited FM4-64 signal colocalized with ARA7 and ARA6 fluorescent marker proteins, consistent with PIN2 accumulation in the late or multivesicular endosome. These experiments illustrate the power of using a temperature sensitive mutation in the gene encoding SCD1 to study the trafficking of PIN2 between the endosome and the plasma membrane. Using the conditional feature of this mutation, we show that altered trafficking of PIN2 precedes altered auxin transport and defects in gravitropism and lateral root development in this mutant upon transition to the restrictive temperature.

IRAP-dependent endosomal T cell receptor signalling is essential for T cell responses

T cell receptor (TCR) activation is modulated by mechanisms such as TCR endocytosis, which is thought to terminate TCR signalling. Here we show that, upon internalization, TCR continues to signal from a set of specialized endosomes that are crucial for T cell functions. Mechanistically, TCR ligation leads to clathrin-mediated internalization of the TCR-CD3ζ complex, while maintaining CD3ζ signalling, in endosomal vesicles that contain the insulin responsive aminopeptidase (IRAP) and the SNARE protein Syntaxin 6. Destabilization of this compartment through IRAP deletion enhances plasma membrane expression of the TCR-CD3ζ complex, yet compromises overall CD3ζ signalling; moreover, the integrity of this compartment is also crucial for T cell activation and survival after suboptimal TCR activation, as mice engineered with a T cell-specific deletion of IRAP fail to develop efficient polyclonal anti-tumour responses. Our results thus reveal a previously unappreciated function of IRAP-dependent endosomal TCR signalling in T cell activation.

T cell receptors (TCR) are internalized when activated by their ligands. Here the authors show that the internalized TCRs are localized to endosomes expressing IRAP and Syntaxin 6 to maintain intracellular signalling capacity, whose importance is shown by the absence of efficient polyclonal anti-tumour response in mice with T-specific conditional deletion of IRAP.

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.

Epidermal control of axonal attachment via β-spectrin and the GTPase-activating protein TBC-10 prevents axonal degeneration

Neurons are subjected to strain due to body movement and their location within organs and tissues. However, how they withstand these forces over the lifetime of an organism is still poorly understood. Here, focusing on touch receptor neuron-epidermis interactions using Caenorhabditis elegans as a model system, we show that UNC-70/β-spectrin and TBC-10, a conserved GTPase-activating protein, function non-cell-autonomously within the epidermis to dynamically maintain attachment of the axon. We reveal that, in response to strain, UNC-70/β-spectrin and TBC-10 stabilize trans-epidermal hemidesmosome attachment structures which otherwise become lost, causing axonal breakage and degeneration. Furthermore, we show that TBC-10 regulates axonal attachment and maintenance by inactivating RAB-35, and reveal functional conservation of these molecules with their vertebrate orthologs. Finally, we demonstrate that β-spectrin functions in this context non-cell-autonomously. We propose a model in which mechanically resistant epidermal attachment structures are maintained by UNC-70/β-spectrin and TBC-10 during movement, preventing axonal detachment and degeneration.

The small GTPase RAB-35 facilitates the initiation of phagosome maturation and acts as a robustness factor for apoptotic cell clearance

We recently identified the novel function of the small GTPase RAB-35 in apoptotic cell clearance in Caenorhabditis elegans, a process in which dying cells are engulfed and degraded inside phagosomes. We have found that RAB-35 functions in two separate steps of cell corpse clearance, cell corpse recognition and the initiation of phagosome maturation. During the latter process, RAB-35 facilitates the removal of phosphatidylinositol-4,5-bisphosphate (PI(4,5)P₂) from the membranes of nascent phagosomes and the simultaneous production of phosphatidylinositol-3-P (PI(3)P) on these same membranes, a process that we have coined the PI(4,5)P₂ to PI(3)P shift. RAB-35 also promotes the recruitment of the small GTPase RAB-5 to the phagosomal surface. During these processes, the activity of RAB-35 is controlled by the candidate GTPase-activating protein (GAP) TBC-10 and the candidate guanine nucleotide exchange factor (GEF) FLCN-1. Overall, RAB-35 leads a third pathway during cell corpse clearance that functions in parallel to the two known pathways, one led by the phagocytic receptor CED-1 and the other led by the CED-10/Rac1 GTPase. Here, we further report that RAB-35 acts as a robustness factor that maintains the clearance activity and embryonic viability under conditions of heat stress. Moreover, we obtained additional evidence suggesting that RAB-35 acts upstream of RAB-5 and RAB-7. To establish a precise temporal pattern for its own dissociation from phagosomal surfaces, RAB-35 controls the removal of its own GAP. We propose that RAB-35 defines a largely unexplored initial phase of phagosome maturation.

Molecular alterations induced by Yersinia pestis, dengue virus and Staphylococcal enterotoxin B under severe stress

BACKGROUND

Severe stress can have drastic and systemic effects with dire implications on the health and wellbeing of exposed individuals. Particularly, the effect of stress on the immune response to infection is of interest to public health because of its implications for vaccine efficacy and treatment strategies during stressful scenarios. Severe stress has previously been shown to cause an anergic state in the immune system that persists following exposure to a potent mitogen.

METHODS

Transcriptome and microRNA changes were characterized using blood samples collected from U.S. Army Ranger candidates immediately before and after training, followed by exposure to representative pathogenic agents: Yersinia pestis, dengue virus 2, and Staphylococcal enterotoxin B (SEB). We employed experimental and computational approaches to characterize altered gene expression, processes, pathways, and regulatory networks mediating the host's response towards severe stress; to assess the protective immunity status of the stressed host towards infection; and to identify pathogen-induced biomarkers under severe stress conditions.

RESULTS

We observed predicted inhibition of pathways significantly associated with lymphopoiesis, wound healing, inflammatory response, lymphocyte activation, apoptosis, and predicted activation of oxidative stress. Using weighted correlation network analyses, we demonstrated preservation of these pathways across infection and stress combinations. Regulatory networks comprising a common set of upstream regulators: transcription factors, microRNAs and post-translational regulators (kinases and phosphatases) may be drivers of molecular alterations leading to compromised protective immunity. Other sets of transcripts were persistently altered in both the pre- and post-stress conditions due to the host's response to each pathogenic agent, forming specific molecular signatures with the potential to distinguish infection from that of severe stress.

CONCLUSIONS

Our results suggest that severe stress alters molecules implicated in the development of leukopoietic stem cells, thereby leading to depletion of cellular and molecular repertoires of protective immunity. Suppressed molecules mediating membrane trafficking of recycling endosomes, membrane translocation and localization of the antigen processing mechanisms and cell adhesions indicate suboptimal antigen presentation, impaired formation of productive immunological synapses, and inhibited T-cell activations. These factors may collectively be responsible for compromised protective immunity (infection susceptibility, delayed wound healing, and poor vaccine response) observed in people under severe stress.

A Ciliary View of the Immunological Synapse

The primary cilium has gone from being a vestigial organelle to a crucial signaling hub of growing interest given the association between a group of human disorders, collectively known as ciliopathies, and defects in its structure or function. In recent years many ciliogenesis proteins have been observed at extraciliary sites in cells and likely perform cilium-independent functions ranging from regulation of the cytoskeleton to vesicular trafficking. Perhaps the most striking example is the non-ciliated T lymphocyte, in which components of the ciliary machinery are repurposed for the assembly and function of the immunological synapse even in the absence of a primary cilium. Furthermore, the specialization traits described at the immunological synapse are similar to those seen in the primary cilium. Here, we review common regulators and features shared by the immunological synapse and the primary cilium that document the remarkable homology between these structures.

Orchestration of Immunological Synapse Assembly by Vesicular Trafficking

Ligation of the T-cell antigen receptor (TCR) by cognate peptide bound to the Major Histocompatibility Complex on the surface of an antigen-presenting cell (APC) leads to the spatial reorganization of the TCR and accessory receptors to form a specialized area of intimate contact between T cell and APC, known as the immunological synapse (IS), where signals are deciphered, coordinated, and integrated to promote T cell activation. With the discovery that an endosomal TCR pool contributes to IS assembly and function by undergoing polarized recycling to the IS, recent years have witnessed a shift from a plasma membrane-centric view of the IS to the vesicular trafficking events that occur at this location following the TCR-dependent translocation of the centrosome toward the synaptic membrane. Here we will summarize our current understanding of the trafficking pathways that are responsible for the steady delivery of endosomal TCRs, kinases, and adapters to the IS to sustain signaling, as well as of the endocytic pathways responsible for signal termination. We will also discuss recent evidence highlighting a role for endosomes in sustaining TCR signaling after its internalization at the IS and identifying the IS as a site of formation and release of extracellular vesicles that allow for transcellular communication with the APC.

Somatic support cells regulate germ cell survival through the Baz/aPKC/Par6 complex

Local signals and structural support from the surrounding cellular microenvironment play key roles in directing development in both embryonic organs and adult tissues. In Drosophila, male germ cells are intimately associated and co-differentiate with supporting somatic cells. Here, we show that the function of the Baz/aPKC/Par6 apical polarity complex in somatic cyst cells is required stage specifically for survival of the germ cells they enclose. Although spermatogonia enclosed by cyst cells in which the function of the Par complex had been knocked down survived and proliferated, newly formed spermatocytes enclosed by cyst cells lacking Par complex proteins died soon after onset of meiotic prophase. Loss of Par complex function resulted in stage-specific overactivation of the Jun-kinase (JNK) pathway in cyst cells. Knocking down expression of JNK pathway components or the GTPase Rab35 in cyst cells lacking Par complex function rescued the survival of neighboring spermatocytes, suggesting that action of the apical polarity complex ensures germ cell survival by preventing JNK pathway activation, and that the mechanism by which cyst cells lacking Par complex function kill neighboring spermatocytes requires intracellular trafficking in somatic cyst cells.

Summary: The Par polarity complex suppresses JNK pathway activity in Drosophila somatic support cells to allow stage-specific germ cell survival.

The small GTPase RAB-35 defines a third pathway that is required for the recognition and degradation of apoptotic cells

In metazoans, apoptotic cells are swiftly engulfed by phagocytes and degraded inside phagosomes. Multiple small GTPases in the Rab family are known to function in phagosome maturation by regulating vesicle trafficking. We discovered rab-35 as a new gene important for apoptotic cell clearance from a genetic screen targeting putative Rab GTPases in Caenorhabditis elegans. We further identified TBC-10 as a putative GTPase-activating protein (GAP), and FLCN-1 and RME-4 as two putative Guanine Nucleotide Exchange Factors (GEFs), for RAB-35. We found that RAB-35 was required for the efficient incorporation of early endosomes to phagosomes and for the timely degradation of apoptotic cell corpses. More specifically, RAB-35 promotes two essential events that initiate phagosome maturation: the switch of phagosomal membrane phosphatidylinositol species from PtdIns(4,5)P2 to PtdIns(3)P, and the recruitment of the small GTPase RAB-5 to phagosomal surfaces. These functions of RAB-35 were previously unknown. Remarkably, although the phagocytic receptor CED-1 regulates these same events, RAB-35 and CED-1 appear to function independently. Upstream of degradation, RAB-35 also facilitates the recognition of apoptotic cells independently of the known CED-1 and CED-5 pathways. RAB-35 localizes to extending pseudopods and is further enriched on nascent phagosomes, consistent with its dual roles in regulating apoptotic cell-recognition and phagosome maturation. Epistasis analyses indicate that rab-35 acts in parallel to both of the canonical ced-1/6/7 and ced-2/5/10/12 clearance pathways. We propose that RAB-35 acts as a robustness factor, defining a novel pathway that aids these canonical pathways in both the recognition and degradation of apoptotic cells.

Aberrant endocytosis leads to the loss of normal mitotic spindle orientation during epithelial glandular morphogenesis

Epithelial cells form tissues with many functions, including secretion and environmental separation and protection. Glandular epithelial tissues comprise cysts and tubules that are formed from a polarized, single-epithelial cell layer surrounding a central, fluid-filled lumen. The pathways regulating key processes in epithelial tissue morphogenesis such as mitotic spindle formation are incompletely understood, but are important to investigate, as their dysregulation is a signature of epithelial tumors. Here, we describe a signaling axis that manifests in a defect in mitotic spindle orientation during epithelial growth and cystogenesis. We found that activation of the small GTPase ADP-ribosylation factor 6 (ARF6) results in the sustained internalization of cell-surface components such as the cMet receptor and the cell-adhesion molecule E-cadherin. The spindle orientation defect arising from elevated levels of ARF6-GTP required an increase in cMet endocytosis, but was independent of E-cadherin internalization or elevated extracellular signal-regulated kinase (ERK) activity resulting from internalized receptor signaling on endosomes. Misorientation of the mitotic spindle resulted in the development of epithelial cysts with structural abnormalities, the most conspicuous of which was the presence of multiple intercellular lumens. Abnormal mitotic spindle orientation was necessary but insufficient to disrupt glandular development, as blocking the strong prosurvival signal resulting from ERK hyperactivation yielded structurally normal cysts despite continued manifestation of spindle orientation defects. Our findings highlight a previously unknown link between ARF6 activation, cMet receptor internalization, and mitotic spindle orientation during epithelial glandular morphogenesis.

The recycling endosome and bacterial pathogens

Bacterial pathogens have developed a wide range of strategies to survive within human cells. A number of pathogens multiply in a vacuolar compartment, whereas others can rupture the vacuole and replicate in the host cytosol. A common theme among many bacterial pathogens is the use of specialised secretion systems to deliver effector proteins into the host cell. These effectors can manipulate the host's membrane trafficking pathways to remodel the vacuole into a replication-permissive niche and prevent degradation. As master regulators of eukaryotic membrane traffic, Rab GTPases are principal targets of bacterial effectors. This review highlights the manipulation of Rab GTPases that regulate host recycling endocytosis by several bacterial pathogens, including Chlamydia pneumoniae, Chlamydia trachomatis, Shigella flexneri, Salmonella enterica serovar Typhimurium, Uropathogenic Escherichia coli, and Legionella pneumophila. Recycling endocytosis plays key roles in a variety of cellular aspects such as nutrient uptake, immunity, cell division, migration, and adhesion. Though much remains to be understood about the molecular basis and the biological relevance of bacterial pathogens exploiting Rab GTPases, current knowledge supports the notion that endocytic recycling Rab GTPases are differentially targeted to avoid degradation and support bacterial replication. Thus, future studies of the interactions between bacterial pathogens and host endocytic recycling pathways are poised to deepen our understanding of bacterial survival strategies.

The immunological synapse as a pharmacological target

The development of T cell mediated immunity relies on the assembly of a highly specialized interface between T cell and antigen presenting cell (APC), known as the immunological synapse (IS). IS assembly is triggered when the T cell receptor (TCR) binds to specific peptide antigen presented in association to the major histocompatibility complex (MHC) by the APC, and is followed by the spatiotemporal dynamic redistribution of TCR, integrins, co-stimulatory receptors and signaling molecules, allowing for the fine-tuning and integration of the signals that lead to T cell activation. The knowledge acquired to date about the mechanisms of IS assembly underscores this structure as a robust pharmacological target. The activity of molecules involved in IS assembly and function can be targeted by specific compounds to modulate the immune response in a number of disorders, including cancers and autoimmune diseases, or in transplanted patients. Here, we will review the state-of-the art of the current therapies which exploit the IS to modulate the immune response.

The role of the oncogenic Rab35 in cancer invasion, metastasis, and immune evasion, especially in leukemia

The study of cancer has allowed researchers to describe some biological characteristics that tumor cells acquire during their development, known as the “hallmarks of cancer” but more research is needed to expand our knowledge about cancer biology and to generate new strategies of treatment. The role that RabGTPases might play in some hallmarks of cancer represents interesting areas of study since these proteins are frequently altered in cancer. However, their participation is not well known. Recently, Rab35was recognized as an oncogenic RabGTPase and and because of its association with different cellular functions, distinctly important in immune cells, a possible role of Rab35 in leukemia can be suggested. Nevertheless, the involvement of Rab35 in cancer remains poorly understood and its possible specific role in leukemia remains unknown. In this review, we analyze general aspects of the participation of RabGTPases in cancer, and especially, the plausible role of Rab35 in leukemia.

A mobile endocytic network connects clathrin-independent receptor endocytosis to recycling and promotes T cell activation

Endocytosis of surface receptors and their polarized recycling back to the plasma membrane are central to many cellular processes, such as cell migration, cytokinesis, basolateral polarity of epithelial cells and T cell activation. Little is known about the mechanisms that control the organization of recycling endosomes and how they connect to receptor endocytosis. Here, we follow the endocytic journey of the T cell receptor (TCR), from internalization at the plasma membrane to recycling back to the immunological synapse. We show that TCR triggering leads to its rapid uptake through a clathrin-independent pathway. Immediately after internalization, TCR is incorporated into a mobile and long-lived endocytic network demarked by the membrane-organizing proteins flotillins. Although flotillins are not required for TCR internalization, they are necessary for its recycling to the immunological synapse. We further show that flotillins are essential for T cell activation, supporting TCR nanoscale organization and signaling.

Rab35 GTPase and cancer: Linking membrane trafficking to tumorigenesis

Rab35 is a small GTPase that is involved in many cellular processes, including membrane trafficking, cell polarity, lipid homeostasis, immunity, phagocytosis and cytokinesis. Recent studies showed that activating mutations confer Rab35 with oncogenic properties. Conversely, downregulation of Rab35 inverts apico-basal cell polarity and promotes cell migration. Here we review Rab35's known functions in membrane trafficking and signaling, cell division and cell migration in cancer cells and discuss the importance of Rab35-dependent membrane trafficking in cancer progression.

Prolyl 4-hydroxylase 2 promotes B-cell lymphoma progression via hydroxylation of Carabin

B-cell lymphomas are heterogeneous blood disorders with limited therapeutic options, largely because of their propensity to relapse and become refractory to treatments. Carabin, a key suppressor of B-cell receptor signaling and proliferation, is inactivated in B-cell lymphoma by unknown mechanisms. Here, we identify prolyl 4-hydroxylase 2 (P4HA2) as a specific proline hydroxylase of Carabin. Carabin hydroxylation leads to its proteasomal degradation, thereby activating the Ras/extracellular signal-regulated kinase pathway and increasing B-cell lymphoma proliferation. P4HA2 is undetectable in normal B cells but upregulated in the diffuse large B-cell lymphoma (DLBCL), driving Carabin inactivation and lymphoma proliferation. Our results indicate that P4HA2 is a potential prognosis marker for DLBCL and a promising pharmacological target for developing treatment of molecularly stratified B-cell lymphomas.

Role of the EHD Family of Endocytic Recycling Regulators for TCR Recycling and T Cell Function

T cells use the endocytic pathway for key cell biological functions, including receptor turnover and maintenance of the immunological synapse. Some of the established players include the Rab GTPases, the SNARE complex proteins, and others, which function together with EPS-15 homology domain-containing (EHD) proteins in non-T cell systems. To date, the role of the EHD protein family in T cell function remains unexplored. We generated conditional EHD1/3/4 knockout mice using CD4-Cre and crossed these with mice bearing a myelin oligodendrocyte glycoprotein-specific TCR transgene. We found that CD4⁺ T cells from these mice exhibited reduced Ag-driven proliferation and IL-2 secretion in vitro. In vivo, these mice exhibited reduced severity of experimental autoimmune encephalomyelitis. Further analyses showed that recycling of the TCR-CD3 complex was impaired, leading to increased lysosomal targeting and reduced surface levels on CD4⁺ T cells of EHD1/3/4 knockout mice. Our studies reveal a novel role of the EHD family of endocytic recycling regulatory proteins in TCR-mediated T cell functions.

Armed for destruction: formation, function and trafficking of neutrophil granules

Neutrophils respond nearly instantly to infection, rapidly deploying a potent enzymatic and chemical arsenal immediately upon entering an infected site. This capacity for rapid and potent responses is endowed by stores of antimicrobial proteins contained in readily mobilizable granules. These granules contain the proteins necessary to mediate the recruitment, chemotaxis, antimicrobial function and NET formation of neutrophils. Four granule types exist, and are sequentially deployed as neutrophils enter infected sites. Secretory vesicles are released first, enabling recruitment of neutrophils out of the blood. Next, specific and gelatinase granules are released to enable neutrophil migration and begin the formation of an antimicrobial environment. Finally, azurophilic granules release potent antimicrobial proteins at the site of infection and into phagosomes. The step-wise mobilization of these granules is regulated by calcium signaling, while specific trafficking regulators and membrane fusion complexes ensure the delivery of granules to the correct subcellular site. In this review, we describe neutrophil granules from their formation through to their deployment at the site of infection, focusing on recent developments in our understanding of the signaling pathways and vesicular trafficking mechanisms which mediate neutrophil degranulation.

Transcellular communication at the immunological synapse: a vesicular traffic-mediated mutual exchange

The cell’s ability to communicate with the extracellular environment, with other cells, and with itself is a crucial feature of eukaryotic organisms. In the immune system, T lymphocytes assemble a specialized structure upon contact with antigen-presenting cells bearing a peptide-major histocompatibility complex ligand, known as the immunological synapse (IS). The IS has been extensively characterized as a signaling platform essential for T-cell activation. Moreover, emerging evidence identifies the IS as a device for vesicular traffic-mediated cell-to-cell communication as well as an active release site of soluble molecules. Here, we will review recent advances in the role of vesicular trafficking in IS assembly and focused secretion of microvesicles at the synaptic area in naïve T cells and discuss the role of the IS in transcellular communication.

SCD1 and SCD2 Form a Complex That Functions with the Exocyst and RabE1 in Exocytosis and Cytokinesis

Although exocytosis is critical for the proper trafficking of materials to the plasma membrane, relatively little is known about the mechanistic details of post-Golgi trafficking in plants. Here, we demonstrate that the DENN (Differentially Expressed in Normal and Neoplastic cells) domain protein STOMATAL CYTOKINESIS DEFECTIVE1 (SCD1) and SCD2 form a previously unknown protein complex, the SCD complex, that functionally interacts with subunits of the exocyst complex and the RabE1 family of GTPases in Arabidopsis thaliana Consistent with a role in post-Golgi trafficking, scd1 and scd2 mutants display defects in exocytosis and recycling of PIN2-GFP. Perturbation of exocytosis using the small molecule Endosidin2 results in growth inhibition and PIN2-GFP trafficking defects in scd1 and scd2 mutants. In addition to the exocyst, the SCD complex binds in a nucleotide state-specific manner with Sec4p/Rab8-related RabE1 GTPases and overexpression of wild-type RabE1 rescues scd1 temperature-sensitive mutants. Furthermore, SCD1 colocalizes with the exocyst subunit, SEC15B, and RabE1 at the cell plate and in distinct punctae at or near the plasma membrane. Our findings reveal a mechanism for plant exocytosis, through the identification and characterization of a protein interaction network that includes the SCD complex, RabE1, and the exocyst.

Folliculin Interacts with Rab35 to Regulate EGF-Induced EGFR Degradation

Aims and Hypothesis: This study aims to investigate the mechanism involved in intracellular regulation of EGFR degradation induced by EGF.

Methods: Phosphorylation of proteins related to EGFR signaling was examined by western blot analysis. Activation, connection between Rab35 and folliculin (FLCN) were assessed by pulldown, coimmunoprecipitation assays separately. The relationship between FLCN and cell growth was detected using gene overexpression and knock-down techniques.

Results: Here, we demonstrate that interfering with FLCN, a tumor suppressor, reduces the rate of EGF-induced EGFR degradation, resulting in prolonged activation of downstream signaling. Rab35 is also involved in these processes. Moreover, C-terminal of FLCN binds to and activates Rab35. Of special interest is the observation that erlotinib, a selective EGFR inhibitor, not only obstructs the EGFR-mediated cellular signaling, but also abolishes EGF-stimulated EGFR degradation. Further results reveal that EGF facilitates the activation of Rab35, and FLCN modulates EGF-dependent Rab35 activation and cell growth.

Conclusions: Taken together, our study proposes a negative-feedback regulation model in which FLCN mediates EGF-induced Rab35 activation, thereby increasing EGFR degradation and attenuating EGFR signaling.

Legionella Effector AnkX Disrupts Host Cell Endocytic Recycling in a Phosphocholination-Dependent Manner

The facultative intracellular bacterium Legionella pneumophila proliferates within amoebae and human alveolar macrophages, and it is the causative agent of Legionnaires' disease, a life-threatening pneumonia. Within host cells, L. pneumophila establishes a replicative haven by delivering numerous effector proteins into the host cytosol, many of which target membrane trafficking by manipulating the function of Rab GTPases. The Legionella effector AnkX is a phosphocholine transferase that covalently modifies host Rab1 and Rab35. However, a detailed understanding of the biological consequence of Rab GTPase phosphocholination remains elusive. Here, we broaden the understanding of AnkX function by presenting three lines of evidence that it interferes with host endocytic recycling. First, using immunogold transmission electron microscopy, we determined that GFP-tagged AnkX ectopically produced in mammalian cells localizes at the plasma membrane and tubular membrane compartments, sites consistent with targeting the endocytic recycling pathway. Furthermore, the C-terminal region of AnkX was responsible for association with the plasma membrane, and we determined that this region was also able to bind the phosphoinositide lipids PI(3)P and PI(4)P in vitro. Second, we observed that mCherry-AnkX co-localized with Rab35, a regulator of recycling endocytosis and with major histocompatibility class I protein (MHC-I), a key immunoregulatory protein whose recycling from and back to the plasma membrane is Rab35-dependent. Third, we report that during infection of macrophages, AnkX is responsible for the disruption of endocytic recycling of transferrin, and AnkX's phosphocholination activity is critical for this function. These results support the hypothesis that AnkX targets endocytic recycling during host cell infection. Finally, we have demonstrated that the phosphocholination activity of AnkX is also critical for inhibiting fusion of the Legionella-containing vacuole (LCV) with lysosomes.

Molecular mechanisms regulating secretory organelles and endosomes in neutrophils and their implications for inflammation

Neutrophils constitute the first line of cellular defense against invading microorganisms and modulate the subsequent innate and adaptive immune responses. In order to execute a rapid and precise response to infections, neutrophils rely on preformed effector molecules stored in a variety of intracellular granules. Neutrophil granules contain microbicidal factors, the membrane-bound components of the respiratory burst oxidase, membrane-bound adhesion molecules, and receptors that facilitate the execution of all neutrophil functions including adhesion, transmigration, phagocytosis, degranulation, and neutrophil extracellular trap formation. The rapid mobilization of intracellular organelles is regulated by vesicular trafficking mechanisms controlled by effector molecules that include small GTPases and their interacting proteins. In this review, we focus on recent discoveries of mechanistic processes that are at center stage of the regulation of neutrophil function, highlighting the discrete and selective pathways controlled by trafficking modulators. In particular, we describe novel pathways controlled by the Rab27a effectors JFC1 and Munc13-4 in the regulation of degranulation, reactive oxygen species and neutrophil extracellular trap production, and endolysosomal signaling. Finally, we discuss the importance of understanding these molecular mechanisms in order to design novel approaches to modulate neutrophil-mediated inflammatory processes in a targeted fashion.

Arf6 and Rab22 mediate T cell conjugate formation by regulating clathrin-independent endosomal membrane trafficking

Endosomal trafficking can influence the composition of the plasma membrane and the ability of cells to polarize their membranes. Here, we examined whether trafficking through clathrin-independent endocytosis (CIE) affects the ability of T cells to form a cell-cell conjugate with antigen-presenting cells (APCs). We show that CIE occurs in both the Jurkat T cell line and primary human T cells. In Jurkat cells, the activities of two guanine nucleotide binding proteins, Arf6 and Rab22 (also known as Rab22a), influence CIE and conjugate formation. Expression of the constitutively active form of Arf6, Arf6Q67L, inhibits CIE and conjugate formation, and results in the accumulation of vacuoles containing lymphocyte function-associated antigen 1 (LFA-1) and CD4, molecules important for T cell interaction with the APC. Moreover, expression of the GTP-binding defective mutant of Rab22, Rab22S19N, inhibits CIE and conjugate formation, suggesting that Rab22 function is required for these activities. Furthermore, Jurkat cells expressing Rab22S19N were impaired in spreading onto coverslips coated with T cell receptor-activating antibodies. These observations support a role for CIE, Arf6 and Rab22 in conjugate formation between T cells and APCs.

Rab35 protein regulates evoked exocytosis of endothelial Weibel-Palade bodies

Weibel–Palade bodies (WPB) are secretory organelles of endothelial cells that undergo evoked exocytosis following intracellular Ca²⁺ or cAMP elevation, thereby supplying the vasculature with factors controlling hemostasis. Several cytosolic and membrane-associated proteins, including the Rab family members Rab3, Rab15, and Rab27a, have been implicated in regulating the acute exocytosis of WPB. Here, we carried out a genome-wide screen to identify Rab pathways affecting WPB exocytosis. Overexpression of a specific subset of Rab GTPase–activating proteins (RabGAPs) inhibited histamine-evoked, Ca²⁺-dependent WPB exocytosis, presumably by inactivating the target Rab GTPases. Among these RabGAPs, we concentrated on TBC1D10A and showed that the inhibitory effect depends on its GAP activity. We confirmed that Rab35 was a target Rab of TBC1D10A in human endothelial cells; Rab35 interacted with TBC1D10A, and expression of the GAP-insensitive Rab35(Q67A) mutant rescued the inhibitory effect of TBC1D10A overexpression on WPB exocytosis. Furthermore, knockdown of Rab35 and expression of a dominant-negative Rab35 mutant both inhibited histamine-evoked secretion of the WPB cargos von Willebrand factor and P-selectin. Pulldown and co-immunoprecipitation experiments identified the ArfGAP with coiled-coil, Ank repeat, and pleckstrin homology domain–containing protein ACAP2 as an Rab35 effector in endothelial cells, and depletion as well as overexpression approaches revealed that ACAP2 acts as a negative regulator of WPB exocytosis. Interestingly, a known ACAP2 target, the small GTPase Arf6, supported histamine-evoked WPB exocytosis, as shown by knockdown and overexpression of a dominant-negative Arf6 mutant. Our data identify Rab35 as a novel regulator of WPB exocytosis, most likely acting through the downstream effectors ACAP2 and Arf6.

Emerging roles of MICAL family proteins - from actin oxidation to membrane trafficking during cytokinesis

Cytokinetic abscission is the terminal step of cell division, leading to the physical separation of the two daughter cells. The exact mechanism mediating the final scission of the intercellular bridge connecting the dividing cells is not fully understood, but requires the local constriction of endosomal sorting complex required for transport (ESCRT)-III-dependent helices, as well as remodelling of lipids and the cytoskeleton at the site of abscission. In particular, microtubules and actin filaments must be locally disassembled for successful abscission. However, the mechanism that actively removes actin during abscission is poorly understood. In this Commentary, we will focus on the latest findings regarding the emerging role of the MICAL family of oxidoreductases in F-actin disassembly and describe how Rab GTPases regulate their enzymatic activity. We will also discuss the recently reported role of MICAL1 in controlling F-actin clearance in the ESCRT-III-mediated step of cytokinetic abscission. In addition, we will highlight how two other members of the MICAL family (MICAL3 and MICAL-L1) contribute to cytokinesis by regulating membrane trafficking. Taken together, these findings establish the MICAL family as a key regulator of actin cytoskeleton dynamics and membrane trafficking during cell division.

Rab11-FIP3 Regulation of Lck Endosomal Traffic Controls TCR Signal Transduction

The role of endosomes in receptor signal transduction is a long-standing question, which remains largely unanswered. The T cell Ag receptor and various components of its proximal signaling machinery are associated with distinct endosomal compartments, but how endosomal traffic affects T cell signaling remains ill-defined. In this article, we demonstrate in human T cells that the subcellular localization and function of the protein tyrosine kinase Lck depends on the Rab11 effector FIP3 (Rab11 family interacting protein-3). FIP3 overexpression or silencing and its ability to interact with Rab11 modify Lck subcellular localization and its delivery to the immunological synapse. Importantly, FIP3-dependent Lck localization controls early TCR signaling events, such as tyrosine phosphorylation of TCRζ, ZAP70, and LAT and intracellular calcium concentration, as well as IL-2 gene expression. Interestingly, FIP3 controls both steady-state and poststimulation phosphotyrosine and calcium levels. Finally, our findings indicate that FIP3 modulates TCR-CD3 cell surface expression via the regulation of steady-state Lck-mediated TCRζ phosphorylation, which in turn controls TCRζ protein levels. This may influence long-term T cell activation in response to TCR-CD3 stimulation. Therefore, our data underscore the importance of finely regulated endosomal traffic in TCR signal transduction and T cell activation leading to IL-2 production.

Rabs set the stage for polarity

Cell polarity refers to the asymmetric localization of cellular components that allows cells to carry out their specialized functions, be they epithelial barrier function, transmission of action potentials in nerve cells, or modulation of the immune response. The establishment and maintenance of cell polarity requires the directed trafficking of membrane proteins and lipids - essential processes that are mediated by Rab GTPases. Interestingly, several of the Rabs that impact polarity are present in the earliest eukaryotes, and the Rab polarity repertoire has expanded as cells have become more complex. There is a substantial conservation of Rab function across diverse cell types. Rabs act through an assortment of effector proteins that include scaffolding proteins, cytoskeletal motors, and other small GTPases. In this review we highlight the similarities and differences in Rab function for the instruction of polarity in diverse cell types.

The Monomeric GTPase Rab35 Regulates Phagocytic Cup Formation and Phagosomal Maturation in Entamoeba histolytica

One of the hallmarks of amoebic colitis is the detection of Entamoeba histolytica (Eh) trophozoites with ingested erythrocytes. Therefore, erythrophagocytosis is traditionally considered as one of the most important criteria to identify the pathogenic behavior of the amoebic trophozoites. Phagocytosis is an essential process for the proliferation and virulence of this parasite. Phagocytic cargo, upon internalization, follows a defined trafficking route to amoebic lysosomal degradation machinery. Here, we demonstrated the role of EhRab35 in the early and late phases of erythrophagocytosis by the amoeba. EhRab35 showed large vacuolar as well as punctate vesicular localization. The spatiotemporal dynamics of vacuolar EhRab35 and its exchange with soluble cytosolic pool were monitored by fluorescence recovery after photobleaching experiments. Using extensive microscopy and biochemical methods, we demonstrated that upon incubation with RBCs EhRab35 is recruited to the site of phagocytic cups as well as to the nascent phagosomes that harbor Gal/GalNAc lectin and actin. Overexpression of a dominant negative mutant of EhRab35 reduced phagocytic cup formation and thereby reduced RBC internalization, suggesting a potential role of the Rab GTPase in the cup formation. Furthermore, we also performed a phagosomal maturation assay and observed that the activated form of EhRab35 significantly increased the rate of RBC degradation. Interestingly, this mutant also significantly enhanced the number of acidic compartments in the trophozoites. Taken together, our results suggest that EhRab35 is involved in the initial stage of phagocytosis as well as in the phagolysosomal biogenesis in E. histolytica and thus contributes to the pathogenicity of the parasite.

The integration of autophagy and cellular trafficking pathways via RAB GAPs

Macroautophagy is a conserved degradative pathway in which a double-membrane compartment sequesters cytoplasmic cargo and delivers the contents to lysosomes for degradation. Efficient formation and maturation of autophagic vesicles, so-called phagophores that are precursors to autophagosomes, and their subsequent trafficking to lysosomes relies on the activity of small RAB GTPases, which are essential factors of cellular vesicle transport systems. The activity of RAB GTPases is coordinated by upstream factors, which include guanine nucleotide exchange factors (RAB GEFs) and RAB GTPase activating proteins (RAB GAPs). A role in macroautophagy regulation for different TRE2-BUB2-CDC16 (TBC) domain-containing RAB GAPs has been established. Recently, however, a positive modulation of macroautophagy has also been demonstrated for the TBC domain-free RAB3GAP1/2, adding to the family of RAB GAPs that coordinate macroautophagy and additional cellular trafficking pathways.

Ankyrin-B is a PI3P effector that promotes polarized α5β1-integrin recycling via recruiting RabGAP1L to early endosomes

Endosomal membrane trafficking requires coordination between phosphoinositide lipids, Rab GTPases, and microtubule-based motors to dynamically determine endosome identity and promote long-range organelle transport. Here we report that ankyrin-B (AnkB), through integrating all three systems, functions as a critical node in the protein circuitry underlying polarized recycling of α5β1-integrin in mouse embryonic fibroblasts, which enables persistent fibroblast migration along fibronectin gradients. AnkB associates with phosphatidylinositol 3-phosphate (PI3P)-positive organelles in fibroblasts and binds dynactin to promote their long-range motility. We demonstrate that AnkB binds to Rab GTPase Activating Protein 1-Like (RabGAP1L) and recruits it to PI3P-positive organelles, where RabGAP1L inactivates Rab22A, and promotes polarized trafficking to the leading edge of migrating fibroblasts. We further determine that α5β1-integrin depends on an AnkB/RabGAP1L complex for polarized recycling. Our results reveal AnkB as an unexpected key element in coordinating polarized transport of α5β1-integrin and likely of other specialized endocytic cargos.

New Insights into How Trafficking Regulates T Cell Receptor Signaling

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