Rab27a negatively regulates phagocytosis by prolongation of the actin-coating stage around phagosomes

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.

In vivo Roles of Rab27 and Its Effectors in Exocytosis

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

The clearance of dead cells by efferocytosis

Multiple modes of cell death have been identified, each with a unique function and each induced in a setting-dependent manner. As billions of cells die during mammalian embryogenesis and daily in adult organisms, clearing dead cells and associated cellular debris is important in physiology. In this Review, we present an overview of the phagocytosis of dead and dying cells, a process known as efferocytosis. Efferocytosis is performed by macrophages and to a lesser extent by other 'professional' phagocytes (such as monocytes and dendritic cells) and 'non-professional' phagocytes, such as epithelial cells. Recent discoveries have shed light on this process and how it functions to maintain tissue homeostasis, tissue repair and organismal health. Here, we outline the mechanisms of efferocytosis, from the recognition of dying cells through to phagocytic engulfment and homeostatic resolution, and highlight the pathophysiological consequences that can arise when this process is abrogated.

Rab7B/42 Is Functionally Involved in Protein Degradation on Melanosomes in Keratinocytes

Keratinocytes uptake melanosomes from melanocytes and retain them in the perinuclear region, where they form melanin caps. Although these processes are crucial to protecting nuclear DNA against ultraviolet injury, the molecular basis of melanosome uptake and decomposition in keratinocytes is poorly understood. One of the major reasons for its being poorly understood is the lack of a specific marker protein that can be used to visualize or monitor melanosomes (or melanosome-containing compartments) that have been incorporated into keratinocytes. In this study, we performed a comprehensive localization screening for mammalian Rab family small GTPases (Rab1-45) and succeeded in identifying 11 Rabs that were enriched around melanosomes that had been incorporated into keratinocytes. We also established a new assay by using a recently developed melanosome probe (called M-INK) as a means of quantitatively assessing the degradation of proteins on incorporated melanosomes in control and each of a series of Rab-knockdown keratinocytes. The results showed that knockdown or CRISPR/Cas9-mediated knockout of Rab7B (also identified as Rab42) in keratinocytes caused strong inhibition of protein degradation on melanosomes. Our findings indicated that Rab7B/42 is recruited to melanosome-containing compartments and that it promotes protein degradation on melanosomes in keratinocytes.Key words: degradation, keratinocytes, melanocytes, melanosome, Rab small GTPase.

Mechanisms of Mixed Th1/Th2 Responses in Mice Induced by Albizia julibrissin Saponin Active Fraction by in Silico Analysis

The purified active fraction of Albizia julibrissin saponin (AJSAF) is an ideal adjuvant candidate that improves antigen-specific both cellular and humoral immune responses and elicits mixed Th1/Th2 responses, but its mechanisms remain unclear. The key features of action of AJSAF were investigated in mice immunized with Newcastle disease virus-based recombinant influenza vaccine (rL-H5) and AJSAF at the same leg (AJSAF+rL-H5) or different legs (AJSAF/rL-H5). The adjuvant activity of AJSAF on rL-H5 is strictly dependent on their spatial colocalization. Serum H5 antigen (H5Ag)-specific IgG, IgG1, IgG2a, and IgG2b antibody titers in AJSAF+rL-H5 group were significantly higher than those in AJSAF/rL-H5 group. The mechanisms of selectivity of Th1 or Th2 in mice induced by AJSAF was explored by the transcriptomic and proteomic profiles of H5Ag-stimulated splenocytes from the immunized mice using gene microarray and two-dimensional difference gel electrophoresis coupled with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Compared to rL-H5 alone, AJSAF/rL-H5 induced more differentially expressed genes (DEGs) than AJSAF+rL-H5, whereas AJSAF+rL-H5 upregulated higher mRNA expression of Th1 (T-bet, IFN-γ, TNF-α, IL-12β, and IL-12Rβ1) and Th2 (IL-10 and AICDA) immune response genes. The neutrophil response and its derived S100A8 and S100A9 might be involved in the AJSAF-mediated Th1 response. Meanwhile, AJSAF might induce the adaptive immune responses by improving a local innate immune microenvironment. These findings expanded the current knowledge on the mechanisms of action of saponin-based adjuvants, and provided new insights into how adjuvants shape adaptive immune responses.

KIF20A, highly expressed in immature hematopoietic cells, supports the growth of HL60 cell line

A microtubule-associated motor protein, kinesin-like family member 20A (KIF20A; also called MKlp2) is required for cytokinesis and contributes to intracellular vesicular trafficking. KIF20A plays a critical role in the development of several cancers, but its role in blood cells and hematological malignancies have not been studied. In the present study, we focused on the role of KIF20A in hematopoietic cells and possible involvement in myeloid neoplasms. We found that human leukemia cell lines and normal bone marrow CD34-positive cells stimulated by growth factors, but not mature peripheral blood cells, exhibit high KIF20A expression. We further found that HL60 cells, which originally express a large amount of KIF20A, showed decreased KIF20A expression in parallel with both neutrophil-like and macrophage-like differentiation-induction. KIF20A-knockdown using a lentivirus shRNA transfection system led to partial cell cycle arrest at the G2/M phase and frequent appearance of multinucleated cells. Treatment with a KIF20A-selective inhibitor, paprotrain enhanced the multinuclearity of KIF20A-knockdown cell clones and suppressed growth. The present study contributes to our understanding of the role of KIF20A in blood cells and leukemia cells in particular.

Rab27A mediated by NF-κB promotes the stemness of colon cancer cells via up-regulation of cytokine secretion

Recent evidences have unveiled critical roles of cancer stem cells (CSCs) in tumorigenicity, but how interactions between CSC and tumor environments help maintain CSC initiation remains obscure. The small GTPases Rab27A regulates autocrine and paracrine cytokines by monitoring exocytosis of extracellular vesicles, and is reported to promote certain tumor progression. We observe that overexpression of Rab27A increased sphere formation efficiency (SFE) by increasing the proportion of CD44⁺ and PKH26^high cells in HT29 cell lines, and accelerating the growth of colosphere with higher percentage of cells at S phase. Mechanism study revealed that the supernatant derived from HT29 sphere after Rab27A overexpression was able to expand sphere numbers with elevated secretion of VEGF and TGF-β. In tumor implanting nude mice model, tumor initiation rates and tumor sizes were enhanced by Rab27A with obvious angiogenesis. As a contrast, knocking down Rab27A impaired the above effects. More importantly, the correlation between higher p65 level and Rab27A in colon sphere was detected, p65 was sufficient to induce up-regulation of Rab27A and a functional NF-κB binding site in the Rab27A promoter was demonstrated. Altogether, our findings reveal a unique mechanism that tumor environment related NF-κB signaling promotes various colon cancer stem cells (cCSCs) properties via an amplified paracrine mechanism regulated by higher Rab27A level.

iTRAQ-based quantitative proteomic analysis reveals Bai-Hu-Tang enhances phagocytosis and cross-presentation against LPS fever in rabbit

ETHNOPHARMACOLOGICAL RELEVANCE

Bai-Hu-Tang (BHT), a classical anti-febrile Chinese formula comprising of liquorice, anemarrhena rhizome, gypsum and rice, has been traditionally used to anti-febrile treatment and promote the production of body fluid to relieve thirst. In this paper, we aim to explore anti-febrile mechanism of BHT at protein level through analyzing alteration of differentially expressed proteins (DEPs) both lipopolysaccharide (LPS) fever syndrome and that was treated with BHT in rabbits.

MATERIALS AND METHODS

Febrile model was induced by LPS injection (i.v.) in rabbits, and BHT (750mg dry extract/kg body weight) was gavaged to another group of LPS fever rabbits. After sacrifice of animals, total protein of liver tissue was isolated, and two-dimensional liquid chromatography (LC) - tandem mass spectrometry (MS) coupled with isobaric tags for relative and absolute quantification (iTRAQ) labeling analysis was employed to quantitatively identify differentially expressed proteins in two group animals, which were compared with control group. Then bioinformatic analysis of DEPs was conducted through hierarchical Clustering, Venn analysis, gene ontology (GO) annotation enrichment, and kyoto encyclopedia of genes and genomes (KEGG) pathways enrichment.

RESULT

The results demonstrated there were 63 and 109 DEPs in LPS fever group and BHT-treated group, respectively. Enrichment analysis of GO annotations indicated that BHT mainly regulated expression of some extracellular structural proteins for response to stimulus and stress. KEGG analysis showed that ribosome and phagosome were the most significant pathways. Thereinto, several proteins in phagosome pathway were significantly up-regulated by BHT, including F-actin, coronin, Rac, and major histocompatibility complex class I (MHC I), which work in phagocytosis and cross-presentation CONCLUSION: BHT may contribute to pyrogen clearance by boosting antigenic phagocytosis, degradation, and cross presentation in the liver.

Cortactin promotes exosome secretion by controlling branched actin dynamics

Sinha et al. show that the cytoskeletal and tumor-overexpressed protein cortactin promotes secretion of exosomes from cancer cells by stabilizing dynamic cortical actin docking sites for multivesicular endosomes, suggesting a potential mechanism by which cortactin may promote tumor aggressiveness.

Exosomes are extracellular vesicles that influence cellular behavior and enhance cancer aggressiveness by carrying bioactive molecules. The mechanisms that regulate exosome secretion are poorly understood. Here, we show that the actin cytoskeletal regulatory protein cortactin promotes exosome secretion. Knockdown or overexpression of cortactin in cancer cells leads to a respective decrease or increase in exosome secretion, without altering exosome cargo content. Live-cell imaging revealed that cortactin controls both trafficking and plasma membrane docking of multivesicular late endosomes (MVEs). Regulation of exosome secretion by cortactin requires binding to the branched actin nucleating Arp2/3 complex and to actin filaments. Furthermore, cortactin, Rab27a, and coronin 1b coordinately control stability of cortical actin MVE docking sites and exosome secretion. Functionally, the addition of purified exosomes to cortactin-knockdown cells rescued defects of those cells in serum-independent growth and invasion. These data suggest a model in which cortactin promotes exosome secretion by stabilizing cortical actin-rich MVE docking sites.

Sequential recruitment of Rab GTPases during early stages of phagocytosis

The phagocytosis and destruction of pathogens and dead cells by macrophages is important for innate immunity and tissue maintenance. Multiple Rab family GTPases engage effector molecules to coordinate the early stages of phagocytosis, which include rapid changes in actin polymerization, membrane phospholipids, trafficking and the activation of receptors. Defining the spatiotemporal, sequential recruitment of these Rabs is critical for insights into how phagocytosis is initiated and coordinated. Here, we screened GFP-tagged Rabs expressed in fixed and live cells to identify and stratify those recruited to early phagocytic membranes at stages defined by phospholipid transitions. We propose a sequence of Rabs 35, 13, 8a, 8b, 27a, 10, and 31 that precedes and accompanies phagocytic cup closure, followed after closure by recruitment of endosomal Rabs 5a, 5b, 5c, 14, and 11. Reducing the expression of individual Rabs by siRNA knockdown, notably Rabs 35 and 13, disrupts phagocytosis prior to phagocytic cup closure, confirming a known role for Rab35 and revealing anew the involvement of Rab13. The results enhance our understanding of innate immune responses in macrophages by revealing the sequence of Rabs that initiates phagocytosis.

Impaired killing of Candida albicans by granulocytes mobilized for transfusion purposes: a role for granule components

Granulocyte transfusions are used to treat neutropenic patients with life-threatening bacterial or fungal infections that do not respond to anti-microbial drugs. Donor neutrophils that have been mobilized with granulocyte-colony stimulating factor (G-CSF) and dexamethasone are functional in terms of antibacterial activity, but less is known about their fungal killing capacity. We investigated the neutrophil-mediated cytotoxic response against C. albicans and A. fumigatus in detail. Whereas G-CSF/dexamethasone-mobilized neutrophils appeared less mature as compared to neutrophils from untreated controls, these cells exhibited normal ROS production by the NADPH oxidase system and an unaltered granule mobilization capacity upon stimulation. G-CSF/dexamethasone-mobilized neutrophils efficiently inhibited A. fumigatus germination and killed Aspergillus and Candida hyphae, but the killing of C. albicans yeasts was distinctly impaired. Following normal Candida phagocytosis, analysis by mass spectrometry of purified phagosomes after fusion with granules demonstrated that major constituents of the antimicrobial granule components, including major basic protein (MBP), were reduced. Purified MBP showed candidacidal activity, and neutrophil-like Crisp-Cas9 NB4-KO-MBP differentiated into phagocytes were impaired in Candida killing. Together, these findings indicate that G-CSF/dexamethasone-mobilized neutrophils for transfusion purposes have a selectively impaired capacity to kill Candida yeasts, as a consequence of an altered neutrophil granular content.

Differential Inhibition of Macrophage Activation by Lymphocytic Choriomeningitis Virus and Pichinde Virus Is Mediated by the Z Protein N-Terminal Domain

Several arenavirus pathogens, such as Lassa and Junin viruses, inhibit macrophage activation, the molecular mechanism of which is unclear. We show that lymphocytic choriomeningitis virus (LCMV) can also inhibit macrophage activation, in contrast to Pichinde and Tacaribe viruses, which are not known to naturally cause human diseases. Using a recombinant Pichinde virus system, we show that the LCMV Z N-terminal domain (NTD) mediates the inhibition of macrophage activation and immune functions.

Rab27a GTPase modulates L-type Ca2+ channel function via interaction with the II-III linker of CaV1.3 subunit

In a variety of cells, secretory processes require the activation of both Rab27a and L-type channels of the Ca(V)1.3 subtype. In the retinal pigment epithelium (RPE), Rab27a and Ca(V)1.3 channels regulate growth-factor secretion towards its basolateral side. Analysis of murine retina sections revealed a co-localization of both Rab27a and Ca(V)1.3 at the basolateral membrane of the RPE. Heterologously expressed Ca(V)1.3/β3/α2δ1 channels showed negatively shifted voltage-dependence and decreased current density of about 70% when co-expressed with Rab27a. However, co-localization analysis using α(5)β(1) integrin as a membrane marker revealed that Rab27a co-expression reduced the surface expression of Ca(V)1.3 only about 10%. Physical binding of heterologously expressed Rab27a with Ca(V)1.3 channels was shown by co-localization in immunocytochemistry as well as co-immunoprecipitation which was abolished after deletion of a MyRIP-homologous amino acid sequence at the II-III linker of the Ca(V)1.3 subunit. Rab27a over-expression in ARPE-19 cells positively shifted the voltage dependence, decreased current density of endogenous Ca(V)1.3 channels and reduced VEGF-A secretion. We show the first evidence of a direct functional modulation of an ion channel by Rab27a suggesting a new mechanism of Rab and ion channel interaction in the control of VEGF-A secretion in the RPE.

Slow degradation in phagocytic astrocytes can be enhanced by lysosomal acidification

Inefficient lysosomal degradation is central in the development of various brain disorders, but the underlying mechanisms and the involvement of different cell types remains elusive. We have previously shown that astrocytes effectively engulf dead cells, but then store, rather than degrade the ingested material. In the present study we identify reasons for the slow digestion and ways to accelerate degradation in primary astrocytes. Our results show that actin-rings surround the phagosomes for long periods of time, which physically inhibit the phago-lysosome fusion. Furthermore, astrocytes express high levels of Rab27a, a protein known to reduce the acidity of lysosomes by Nox2 recruitment, in order to preserve antigens for presentation. We found that Nox2 colocalizes with the ingested material, indicating that it may influence antigen processing also in astrocytes, as they express MHC class II. By inducing long-time acidification of astrocytic lysosomes using acidic nanoparticles, we could increase the digestion of astrocyte-ingested, dead cells. The degradation was, however, normalized over time, indicating that inhibitory pathways are up-regulated in response to the enhanced acidification. GLIA 2015;63:1997-2009.

Rab27A regulates transport of cell surface receptors modulating multinucleation and lysosome-related organelles in osteoclasts

Rab27A regulates transport of lysosome-related organelles (LROs) and release of secretory granules in various types of cells. Here, we identified up-regulation of Rab27A during differentiation of osteoclasts (OCLs) from bone-marrow macrophages (BMMs), by DNA microarray analysis. Rab27A deficiency in OCLs, using small interfering RNA (siRNA) knockdown in RAW-D cell line or BMMs derived from ashen mice, which display genetic defects in Rab27A expression, induced multinucleated and giant cells. Upon stimulation with macrophage-colony stimulating factor (M-CSF) and receptor activator of nuclear factor kappa-B ligand (RANKL), essential cytokines for OCL differentiation, phosphorylation levels of extracellular signal-regulated kinase (Erk), proto-oncogene tyrosine-protein kinase (Src), and p-38 were slightly enhanced in ashen BMMs than in wild-type BMMs. The cell surface level of c-fms, an M-CSF receptor, was slightly higher in ashen BMMs than in wild-type BMMs, and down-regulation of RANK, a RANKL receptor, was delayed. In addition to receptors, OCLs derived from ashen mice exhibited aberrant actin ring formation, abnormal subcellular localization of lysosome-associated membrane protein (LAMP2) and cathepsin K (CTSK), and marked reduction in resorbing activity. Thus, these findings suggest that Rab27A regulates normal transport of cell surface receptors modulating multinucleation and LROs in OCLs.

The role of Rab27a in the regulation of neutrophil function

Neutrophils are central regulators of the innate immune response and help shape the adaptive immune response. Malfunction and unregulated neutrophil activation leads to disease and inflammation. During the host response to infection, neutrophils display several mechanisms of defense mediated by their arsenal of granular proteins. Regulation of granular trafficking, docking and fusion is at the core of the neutrophil defense response to pathogens. The small GTPase Rab27a has emerged as a central regulator of the neutrophil response through its tight control of vesicular trafficking and degranulation. This review focuses on the latest research that has led to the characterization of Rab27a as an essential regulator of neutrophil function.

Rab27a is essential for the formation of neutrophil extracellular traps (NETs) in neutrophil-like differentiated HL60 cells

Neutrophils play a crucial role in host defence. In response to a variety of inflammatory stimulation, they form neutrophil extracellular traps (NETs). NETs are extracellular structures composed of chromatin fibers decorated with antimicrobial proteins and developing studies indicate that NETs contribute to extracellular microbial killing. While the intracellular signaling pathways that regulate NET formation remain largely unknown, there is growing evidence that generation of reactive oxygen species (ROS) is a key event for NET formation. The Rab family small GTPase Rab27a is an important component of the secretory machinery of azurophilic granules in neutrophils. However, the precise mechanism of NET formation and whether or not Rab27a contributes to this process are unknown. Using neutrophil-like differentiated HL60 cells, we show here that Rab27a plays an essential role in both phorbol myristate acetate (PMA)- and Candida albicans-induced NET formation by regulating ROS production. Rab27a-knockdown inhibited ROS-positive phagosome formation during complement-mediated phagocytosis. To investigate the role of Rab27a in neutrophil function in detail, both primary human neutrophils and neutrophil-like differentiated HL60 cells were treated with PMA, and NET formation process was assessed by measurement of release of histone H3 into the medium, citrullination of the arginine in position 3 of histone H4 and chase of the nuclear change of the living cells in the co-existence of both cell-permeable and -impermeable nuclear indicators. PMA-induced NET formation occured sequentially in both neutrophil-like differentiated HL60 cells and primary neutrophils, and Rab27a-knockdown clearly inhibited NET formation in association with reduced ROS production. We also found that serum-treated Candida albicans triggers NET formation in a ROS-dependent manner, and that Rab27a-knockdown inhibits this process as well. Our findings demonstrate that Rab27a plays an important role in NET formation induced by both Candida albicans infection and PMA treatment by regulating ROS production.

Rab1a and Rab5a preferentially bind to binary lipid compositions with higher stored curvature elastic energy

Rab proteins are a large family of GTP-binding proteins that regulate cellular membrane traffic and organelle identity. Rab proteins cycle between association with membranes and binding to RabGDI. Bound on membranes, each Rab has a very specific cellular location and it is this remarkable degree of specificity with which Rab GTPases recognize distinct subsets of intracellular membranes that forms the basis of their ability to act as key cellular regulators, determining the recruitment of downstream effectors to the correct membrane at the correct time. The molecular mechanisms controlling Rab localization remain poorly understood. Here, we present a fluorescence-based assay to investigate Rab GTPase membrane extraction and delivery by RabGDI. Using EGFP-Rab fusion proteins the amount of Rab:GDI complex obtained by GDI extraction of Rab proteins from HEK293 membranes could be determined, enabling control of complex concentration. Subsequent partitioning of the Rab GTPases into vesicles made up of artificial binary lipid mixtures showed for the first time, that the composition of the target membrane plays a key role in the localization of Rab proteins by sensing the stored curvature elastic energy in the membrane.

Functional role(s) of phagosomal Rab GTPases

Rab GTPases are at the central node of the machinery that regulates trafficking of organelles, including phagosomes. Thanks to the unique combination of high quality phagosome purification with highly sensitive proteomic studies, the network of Rab proteins that are dynamically associated with phagosomes during the process of maturation of this organelle is relatively well known. Whereas the phagosomal functions of many of the Rab proteins associated with phagosomes are characterized, the role(s) of most of these trafficking regulators remains to be identified. In some cases, even when the function in the context of phagosome biology is described, phagosomal Rab proteins seem to have similar roles. This review summarizes the current knowledge about the identity and function of phagosomal Rab GTPases, with a particular emphasis on new evidence that clarify these seemingly overlapping Rab functions during phagosome maturation.

Rab27 effectors, pleiotropic regulators in secretory pathways

Rab27, a member of the small GTPase Rab family, is widely conserved in metazoan, and two Rab27 isoforms, Rab27A and Rab27B, are present in vertebrates. Rab27A was the first Rab protein whose dysfunction was found to cause a human hereditary disease, type 2 Griscelli syndrome, which is characterized by silvery hair and immunodeficiency. The discovery in the 21st century of three distinct types of mammalian Rab27A effectors [synaptotagmin-like protein (Slp), Slp homologue lacking C2 domains (Slac2), and Munc13-4] that specifically bind active Rab27A has greatly accelerated our understanding not only of the molecular mechanisms of Rab27A-mediated membrane traffic (e.g. melanosome transport and regulated secretion) but of the symptoms of Griscelli syndrome patients at the molecular level. Because Rab27B is widely expressed in various tissues together with Rab27A and has been found to have the ability to bind all of the Rab27A effectors that have been tested, Rab27A and Rab27B were initially thought to function redundantly by sharing common Rab27 effectors. However, recent evidence has indicated that by interacting with different Rab27 effectors Rab27A and Rab27B play different roles in special types of secretion (e.g. exosome secretion and mast cell secretion) even within the same cell type. In this review article, I describe the current state of our understanding of the functions of Rab27 effectors in secretory pathways.

Tubulin polymerization-promoting protein (TPPP/p25α) promotes unconventional secretion of α-synuclein through exophagy by impairing autophagosome-lysosome fusion

Aggregation of α-synuclein can be promoted by the tubulin polymerization-promoting protein/p25α, which we have used here as a tool to study the role of autophagy in the clearance of α-synuclein. In NGF-differentiated PC12 catecholaminergic nerve cells, we show that de novo expressed p25α co-localizes with α-synuclein and causes its aggregation and distribution into autophagosomes. However, p25α also lowered the mobility of autophagosomes and hindered the final maturation of autophagosomes by preventing their fusion with lysosomes for the final degradation of α-synuclein. Instead, p25α caused a 4-fold increase in the basal level of α-synuclein secreted into the medium. Secretion was strictly dependent on autophagy and could be up-regulated (trehalose and Rab1A) or down-regulated (3-methyladenine and ATG5 shRNA) by enhancers or inhibitors of autophagy or by modulating minus-end-directed (HDAC6 shRNA) or plus-end-directed (Rab8) trafficking of autophagosomes along microtubules. Finally, we show in the absence of tubulin polymerization-promoting protein/p25α that α-synuclein release was modulated by dominant mutants of Rab27A, known to regulate exocytosis of late endosomal (and amphisomal) elements, and that both lysosomal fusion block and secretion of α-synuclein could be replicated by knockdown of the p25α target, HDAC6, the predominant cytosolic deacetylase in neurons. Our data indicate that unconventional secretion of α-synuclein can be mediated through exophagy and that factors, which increase the pool of autophagosomes/amphisomes (e.g. lysosomal disturbance) or alter the polarity of vesicular transport of autophagosomes on microtubules, can result in an increased release of α-synuclein monomer and aggregates to the surroundings.

Regulation of vesicular trafficking and leukocyte function by Rab27 GTPases and their effectors

The Rab27 family of GTPases regulates the efficiency and specificity of exocytosis in hematopoietic cells, including neutrophils, CTLs, NK cells, and mast cells. However, the mechanisms regulated by Rab27 GTPases are cell-specific, as they depend on the differential expression and function of particular effector molecules that are recruited by the GTPases. In addition, Rab27 GTPases participate in multiple steps of the regulation of the secretory process, including priming, tethering, docking, and fusion through sequential interaction with multiple effector molecules. Finally, recent reports suggest that Rab27 GTPases and their effectors regulate vesicular trafficking mechanisms other than exocytosis, including endocytosis and phagocytosis. This review focuses on the latest discoveries on the function of Rab27 GTPases and their effectors Munc13-4 and Slp1 in neutrophil function comparatively to their functions in other leukocytes.

Knockdown of Coronin-1C disrupts Rac1 activation and impairs tumorigenic potential in hepatocellular carcinoma cells

Coronin-1C is an important F-actin binding protein which is critical for cell motility. Furthermore, the expression of this protein was found to be increased in diffuse tumors and was correlated with the degree of tumor malignancy. However, the mechanism(s) through which this protein enhances malignancy in hepatocellular carcinoma (HCC) is poorly understood. In this study, we found that Coronin-1C was overexpressed in human HCC tissues compared with the adjacent non-tumor tissues. Overexpression of Coronin-1C enhanced the cell migration in the human HCC cell line BEL-7402, whereas suppressed cell migration and proliferation were observed in Coronin-1C-knockdown BEL-7402 cells together with impaired cell polarity, disrupted cytoskeleton and decreased Rac-1 activation. Moreover, the Coronin-1C knockdown cells displayed a lower degree of malignancy by inducing smaller tumors in nude mice. Thus, we demonstrated a relationship between Coronin-1C overexpression and human HCC growth through enhancement of tumor cell proliferation and migration, which are correlated with Rac-1 activation.

MUNC13-4 protein regulates the oxidative response and is essential for phagosomal maturation and bacterial killing in neutrophils

Neutrophils use diverse mechanisms to kill pathogens including phagocytosis, exocytosis, generation of reactive oxygen species (ROS), and neutrophil extracellular traps. These mechanisms rely on their ability to mobilize intracellular organelles and to deliver granular cargoes to specific cellular compartments or into the extracellular milieu, but the molecular mechanisms regulating vesicular trafficking in neutrophils are not well understood. MUNC13-4 is a RAB27A effector that coordinates exocytosis in hematopoietic cells, and its deficiency is associated with the human immunodeficiency familial hemophagocytic lymphohistiocytosis type 3. In this work, we have established an essential role for MUNC13-4 in selective vesicular trafficking, phagosomal maturation, and intracellular bacterial killing in neutrophils. Using neutrophils from munc13-4 knock-out (KO) mice, we show that MUNC13-4 is necessary for the regulation of p22(phox)-expressing granule trafficking to the plasma membrane and regulates extracellular ROS production. MUNC13-4 was also essential for the regulation of intracellular ROS production induced by Pseudomonas aeruginosa despite normal trafficking of p22(phox)-expressing vesicles toward the phagosome. Importantly, in the absence of MUNC13-4, phagosomal maturation was impaired as observed by the defective delivery of azurophilic granules and multivesicular bodies to the phagosome. Significantly, this mechanism was intact in RAB27A KO neutrophils. Intracellular bacterial killing was markedly impaired in MUNC13-4 KO neutrophils. MUNC13-4-deficient cells showed a significant increase in neutrophil extracellular trap formation but were unable to compensate for the impaired bacterial killing. Altogether, these findings characterize novel functions of MUNC13-4 in the innate immune response of the neutrophil and have direct implications for the understanding of immunodeficiencies in patients with MUNC13-4 deficiency.

Genome-wide association identifies multiple genomic regions associated with susceptibility to and control of ovine lentivirus

BACKGROUND

Like human immunodeficiency virus (HIV), ovine lentivirus (OvLV) is macrophage-tropic and causes lifelong infection. OvLV infects one quarter of U.S. sheep and induces pneumonia and body condition wasting. There is no vaccine to prevent OvLV infection and no cost-effective treatment for infected animals. However, breed differences in prevalence and proviral concentration have indicated a genetic basis for susceptibility to OvLV. A recent study identified TMEM154 variants in OvLV susceptibility. The objective here was to identify additional loci associated with odds and/or control of OvLV infection.

METHODOLOGY/PRINCIPAL FINDINGS

This genome-wide association study (GWAS) included 964 sheep from Rambouillet, Polypay, and Columbia breeds with serological status and proviral concentration phenotypes. Analytic models accounted for breed and age, as well as genotype. This approach identified TMEM154 (nominal P=9.2×10(-7); empirical P=0.13), provided 12 additional genomic regions associated with odds of infection, and provided 13 regions associated with control of infection (all nominal P<1 × 10(-5)). Rapid decline of linkage disequilibrium with distance suggested many regions included few genes each. Genes in regions associated with odds of infection included DPPA2/DPPA4 (empirical P=0.006), and SYTL3 (P=0.051). Genes in regions associated with control of infection included a zinc finger cluster (ZNF192, ZSCAN16, ZNF389, and ZNF165; P=0.001), C19orf42/TMEM38A (P=0.047), and DLGAP1 (P=0.092).

CONCLUSIONS/SIGNIFICANCE

These associations provide targets for mutation discovery in sheep susceptibility to OvLV. Aside from TMEM154, these genes have not been associated previously with lentiviral infection in any species, to our knowledge. Further, data from other species suggest functional hypotheses for future testing of these genes in OvLV and other lentiviral infections. Specifically, SYTL3 binds and may regulate RAB27A, which is required for enveloped virus assembly of human cytomegalovirus. Zinc finger transcription factors have been associated with positive selection for repression of retroviral replication. DLGAP1 binds and may regulate DLG1, a known regulator of HIV infectivity.

Expression profiling of Rab GTPases reveals the involvement of Rab20 and Rab32 in acute brain inflammation in mice

Rab GTPases have emerged as central regulators of vesicle trafficking and are essential for cytokine production during the pathogenesis of neuroinflammation. To characterize the roles of different Rab proteins in brain inflammation, we used quantitative PCR (qPCR) to examine the expression profiles of all members of the Rab family in an experimental model of brain inflammation in mice. We found that Rab20 and Rab32 were substantially up-regulated during the acute phase of inflammation. The increased expression of Rab20 was also confirmed by immunostaining of inflamed brains at different timepoints. The concomitant overexpression of Rabs (Rab20 and Rab32) and early response proinflammatory cytokines (TNF-α and IL-1β) suggested that these Rabs may be important for subsequent inflammatory responses in brain. Furthermore, we found that the expression of certain Rabs was dramatically reduced in cultured primary microglia, which was not observed in the in vivo profiling. In N9, a microglial cell line, however, there was no increase in the expression of Rab20 or Rab32, but Rab3c was significantly overexpressed. These results collectively indicate that Rabs may participate in inflammatory response in microglia during brain inflammation. The differential regulation of individual Rabs in different experimental systems is a caveat for the analysis of Rab functions.

The balance in the delivery of ER components and the vacuolar proton pump to the phagosome depends on myosin IK in Dictyostelium

In Dictyostelium, the cytoskeletal proteins Actin binding protein 1 (Abp1) and the class I myosin MyoK directly interact and couple actin dynamics to membrane deformation during phagocytosis. Together with the kinase PakB, they build a regulatory switch that controls the efficiency of uptake of large particles. As a basis for further functional dissection, exhaustive phagosome proteomics was performed and established that about 1300 proteins participate in phagosome biogenesis. Then, quantitative and comparative proteomic analysis of phagosome maturation was performed to investigate the impact of the absence of MyoK or Abp1. Immunoblots and two-dimensional differential gel electrophoresis of phagosomes isolated from myoK-null and abp1-null cells were used to determine the relative abundance of proteins during the course of maturation. Immunoblot profiling showed that absence of Abp1 alters the maturation profile of its direct binding partners such as actin and the Arp2/3 complex, suggesting that Abp1 directly regulates actin dynamics at the phagosome. Comparative two-dimensional differential gel electrophoresis analysis resulted in the quantification of mutant-to-wild type abundance ratios at all stages of maturation for over one hundred identified proteins. Coordinated temporal changes in these ratio profiles determined the classification of identified proteins into functional groups. Ratio profiling revealed that the early delivery of ER proteins to the phagosome was affected by the absence of MyoK and was coupled to a reciprocal imbalance in the delivery of the vacuolar proton pump and Rab11 GTPases. As direct functional consequences, a delayed acidification and a reduced intraphagosomal proteolysis were demonstrated in vivo in myoK-null cells. In conclusion, the absence of MyoK alters the balance of the contributions of the ER and an endo-lysosomal compartment, and slows down phagosome acidification as well as the speed and efficiency of particle degradation inside the phagosome.

Rab27b regulates c-kit expression by controlling the secretion of stem cell factor

Rab27b, a subfamily of Rab27 small GTPases, was originally identified in platelets. However, the role of Rab27b in megakaryocytic lineage cells remains unknown. Here, using a human megakaryoblastic cell line, CMK, we show that Rab27b negatively regulates c-kit-expression. We found that transfection of shRNA-Rab27b into CMK cells led to specific increase in the amount of the receptor-type tyrosine kinase c-kit. To elucidate the molecular mechanisms by which Rab27b regulates c-kit expression, we analyzed the dynamics of c-kit by the stimulation with its ligand, stem cell factor (SCF). We found that cell surface expression of c-kit was promptly reduced and rapidly degraded in both CMK and Rab27b-knockdown CMK cells. Pretreatment with a lysosome inhibitor bafilomycin suppressed the degradation of c-kit, indicating that c-kit expression is controlled by SCF-induced endolysosomal degradation system. We therefore focused on the potential involvement of SCF in Rab27b-mediated effects on c-kit expression levels. We found that autocrine secretion of SCF was downregulated in Rab27b-knockdown cells as compared with parental CMK cells. These results suggest that Rab27b negatively regulates the cell surface expression of c-kit via secretion of SCF and that ligation of SCF leads to the endolysosomal degradation system of c-kit.

Unraveling the enigma: progress towards understanding the coronin family of actin regulators

Coronins are a conserved family of actin cytoskeleton regulators that promote cell motility and modulate other actin-dependent processes. Although these proteins have been known for 20 years, substantial progress has been made in the past 5 years towards their understanding. In this review, we examine this progress, place it into the context of what was already known, and pose several questions that remain to be addressed. In particular, we cover the emerging consensus about the role of Type I coronins in coordinating the function of Arp2/3 complex and ADF/cofilin proteins. This coordination plays an important role in leading-edge actin dynamics and overall cell motility. Finally, we discuss the roles played by the more exotic coronins of the Type II and III classes in cellular processes away from the leading edge.