Intracellular distribution and transcriptional regulation of Atlantic salmon (Salmo salar) Rab5c, 7a and 27a homologs by immune stimuli

Rab GTPases control trafficking of intracellular vesicles and are key regulators of endocytic and secretory pathways. Due to their specific distribution, they may serve as markers for different endolysosomal compartments. Since Rab GTPases are involved in uptake and trafficking of endocytosed ligands and cell receptors, as well as secretion of immune mediators, they have been implicated in diverse immunological processes and their functions are often exploited by intracellular pathogens such as viruses. While Rab proteins have been studied extensively in mammals, their functions in vesicle trafficking in teleosts are not well known. In the present work, Atlantic salmon Rab5c, Rab7a and Rab27a homologs were studied in terms of intracellular distribution and gene expression. Structured illumination microscopy demonstrated that transgenic, GFP-tagged salmon Rab5c and Rab7a are, predominantly, located within early endosomes and late endosomes/lysosomes, respectively. In contrast, Rab27a showed a broader distribution, which indicates that it associates with diverse intracellular vesicles and organelles. Infection of salmon with Salmonid alphavirus subtype 3 (SAV3) enhanced the mRNA levels of all of the studied Rab isoforms in heart and head kidney and most of them were upregulated in spleen. This may reflect the capacity of the virus to exploit the functions of these rab proteins. It is also possible that the transcriptional regulation of Rab proteins in SAV3-infected organs may play a role in the antiviral immune response. The latter was further supported by in vitro experiments with adherent head kidney leukocytes. The expression of Rab5c and Rab27a was upregulated in these cells following stimulation with TLR ligands including CpG oligonucleotides and polyI:C. The expression of most of the analyzed Rab isoforms in the primary leukocytes was also enhanced by stimulation with type I IFN. Interestingly, IFN-gamma had a negative effect on Rab7a expression which may be linked to the priming activity of this cytokine on monocytes and macrophages. Overall, these data demonstrate that the intracellular distribution of Rab5c, Rab7a and Rab27a is phylogenetically conserved within vertebrates and that these molecules might be implicated in viral infections and the regulation of the antiviral immune response in Atlantic salmon.

Azido Pentoses: A New Tool To Efficiently Label Mycobacterium tuberculosis Clinical Isolates

Mycobacterium tuberculosis (Mtb), the main causative agent of tuberculosis (Tb), has a complex cell envelope which forms an efficient barrier to antibiotics, thus contributing to the challenges of anti-tuberculosis therapy. However, the unique Mtb cell wall can be considered an advantage and be utilized to selectively label Mtb bacteria. Here we introduce three azido pentoses as new compounds for metabolic labeling of Mtb: 3-azido arabinose (3AraAz), 3-azido ribose (3RiboAz), and 5-azido arabinofuranose (5AraAz). 5AraAz demonstrated the highest level of Mtb labeling and was efficiently incorporated into the Mtb cell wall. All three azido pentoses can be easily used to label a variety of Mtb clinical isolates without influencing Mtb-dependent phagosomal maturation arrest in infection studies with human macrophages. Thus, this metabolic labeling method offers the opportunity to attach desired molecules to the surface of Mtb bacteria in order to facilitate investigation of the varying virulence characteristics of different Mtb clinical isolates, which influence the outcome of a Tb infection.

The Rab5A gene of marine fish, large yellow croaker (Larimichthys crocea), and its response to the infection of Cryptocaryon irritans

Rab GTPases, members of the Ras superfamily, encode monomeric G-proteins. Rab proteins regulate key steps in membrane traffic transport and endocytic pathway of host immune responses. Rab5A is involved in immune regulation, particularly in T cell migration and macrophage endocytosis in higher vertebrates. However, little is known of the molecular structure of Rab5A gene in marine teleost fish species and its expression profile during the parasite infection. In this study, the full-length cDNA sequence and genomic structure of Rab5A gene of the large yellow croaker (Larimichthys crocea) (LycRab5A), one of the most economical marine fishes, were identified and characterized. The LycRab5A protein, containing the ATPase/GTPase binding motifs and the effector molecules binding motifs, was highly homologous to that of other animals. The expression plasmid containing LycRab5A cDNA fused with GST was engineered and transformed into Escherichia coli to produce recombinant protein GST-LycRab5A, which was purified to prepare a polyclonal antibody specifically against LycRab5A. Subcellular localization revealed that LycRab5A expressed in the membrane and cytoplasm. Based on real-time PCR and Western blot analysis, we found that both mRNA and protein of LycRab5A were expressed in all tissues we examined; especially it was highly expressed in blood and gill. Interestingly, both mRNA and protein of LycRab5A were substantially up-regulated when parasitic ciliate protozoan (Cryptocaryon irritans) was infected. The expression of LycRab5A was reached to the maximal level at 24 h after infection. The line of evidence suggested that LycRab5A might play an important role in large yellow croaker defense against parasite infection. Moreover, on the basis of protein interaction, it was found that the LycRab5A interacted with myosin light chain (designated as LycMLC), a crucial protein in the process of phagocytosis. This discovery might contribute better understanding to the molecular events involved in fish immune responses.

Rab20 regulates phagosome maturation in RAW264 macrophages during Fc gamma receptor-mediated phagocytosis

Rab20, a member of the Rab GTPase family, is known to be involved in membrane trafficking, however its implication in FcγR-mediated phagocytosis is unclear. We examined the spatiotemporal localization of Rab20 during phagocytosis of IgG-opsonized erythrocytes (IgG-Es) in RAW264 macrophages. By the live-cell imaging of fluorescent protein-fused Rab20, it was shown that Rab20 was transiently associated with the phagosomal membranes. During the early stage of phagosome formation, Rab20 was not localized on the membranes of phagocytic cups, but was gradually recruited to the newly formed phagosomes. Although Rab20 was colocalized with Rab5 to some extent, the association of Rab20 with the phagosomes persisted even after the loss of Rab5 from the phagosomal membranes. Then, Rab20 was colocalized with Rab7 and Lamp1, late endosomal/lysosomal markers, on the internalized phagosomes. Moreover, our analysis of Rab20 mutant expression revealed that the maturation of phagosomes was significantly delayed in cells expressing the GDP-bound mutant Rab20-T19N. These data suggest that Rab20 is an important component of phagosome and regulates the phagosome maturation during FcγR-mediated phagocytosis.

Double-label immunoelectron microscopy for studying the colocalization of proteins in cultured cells

Multiple label immunoelectron microscopy localizes and detects multiple antigens in cells and tissues. In double labeling, two kinds of primary antibodies from different animal species are used after being mixed in a single solution. To distinguish the different antigens, secondary antibodies should be labeled with colloidal gold particles of different diameter. Generally, the secondary antibody that is used for detecting the antigen with lower distribution density is labeled with smaller-sized gold particles. In this chapter, double-label immunoelectron microscopy of gelatin-embedded cultured cells using the cryosectioning technique is described.

Coronin-1 expression in T lymphocytes: insights into protein function during T cell development and activation

Coronin has been described as an actin-binding protein of Dictyostelium discoideum, and it has been demonstrated to play a role in cell migration, cytokinesis and phagocytosis. Coronin-related proteins are found in many eukaryotic species, including Coronin-1 in mammals whose expression is enriched in the hematopoietic tissues. Here, we characterize Coronin-1 gene and protein expression in mouse embryonic and adult T lymphocytes. Coronin-1 is expressed throughout T cell ontogeny and in peripheral alphabeta T cells. Expression varies along thymic cell development, with maximum levels observed in embryonic early thymocytes and, in the adults, the selected TCRalphabeta(+) single-positive thymocytes. Subcellular localization analysis indicates that Coronin-1 is in equilibrium between the cytosol and the cell cortex, where it accumulates in F-actin-rich membrane protrusions induced by polarized activation of TCR-CD3-stimulated T cells. These data are consistent with a role of Coronin-1 in T cell differentiation/activation events involving membrane dynamisms and the cortical actin cytoskeleton.

Rab5-mediated endosome-endosome fusion regulates hemoglobin endocytosis in Leishmania donovani

To understand the trafficking of endocytosed hemoglobin (Hb) in Leishmania, we investigated the characteristics of in vitro fusion between endosomes containing biotinylated Hb (BHb) and avidin-horseradish peroxidase (AHRP). We showed that early endosome fusion in Leishmania is temperature and cytosol dependent and is inhibited by ATP depletion, ATPgammaS, GTPgammaS and N-ethylmaleimide treatment. The Rab5 homolog from Leishmania donovani, LdRab5, was cloned and expressed. Our results showed that homotypic fusion between the early endosomes in Leishmania is Rab5 dependent. Early endosomes containing BHb fused efficiently with late endosomes in a process regulated by Rab7, whereas no fusion between early and late endosomes was detected using fluid phase markers. Pre-treatment of early endosomes containing BHb with monoclonal antibody specific for the C-terminus of the Hb receptor (HbR) or the addition of the C-terminal cytoplasmic fragment of the HbR specifically inhibited the fusion with late endosomes, suggesting that signal(s) mediated through the HbR cytoplasmic tail promotes the fusion of early endosomes containing Hb with late endosomes.

Co-localization of rab4 with endocytosis-related proteins in the rat parotid glands

Small GTP-binding proteins have been implicated in the regulation of vesicular traffic. We investigated the localization of Rab4 in the rat parotid glands by Western blotting and light-microscopic immunohistochemistry. Rab4 was localized mainly on the intracellular membranes in the subapical-actin terminal web, but was not present in the basolateral region both in acinar and ductal cells. Actin, alpha-adaptin, Rab5A and aquaporin5 were detected in the Rab4-containing intracellular membrane fraction prepared using anti-Rab4 antibody covalently coupled to magnetic beads. Detection of actin indicated that the Rab4-containing intracellular membranes were attached to the actin filaments. Although alpha-adaptin was immunohistochemically distributed along the plasma membrane, this protein coexisted with Rab4 only at the apical region. Rab5A immunoreactivity was distributed all around the cytoplasm. These findings suggested that Rab4 participates in endocytosis at the apical membrane of parotid glands.

Positive selection of self-MHC-reactive T cells by individual peptide-MHC class II complexes

If T cells require specific interactions with MHC-bound peptides during positive selection, then the specificities of T cells selected by one peptide should be distinct from those selected by another. We have examined positive selection of CD4 T cells in four strains of mice, each overexpressing a different peptide-1-A(b)(A(b)) complex. We show that a subset of CD4 T cells is selected by the overexpressed peptide and that the specificities of the CD4 T cells, as measured by reactivity to wild-type antigen-presenting cells, vary greatly depending on which peptide is overexpressed. These differences in specificity are mediated through positive selection not negative selection. Each of the four peptide-A(b) complexes appears to adopt a different conformation, and these differences correlate with the differences in reactivity. Our results suggest that individual peptide-MHC complexes positively select different subsets of self-MHC-reactive T cells and that the conformation of the peptide-MHC complex may contribute to this process.

Deviant expression of Rab5 on phagosomes containing the intracellular pathogens Mycobacterium tuberculosis and Legionella pneumophila is associated with altered phagosomal fate

The intracellular human pathogens Legionella pneumophila and Mycobacterium tuberculosis reside in altered phagosomes that do not fuse with lysosomes and are only mildly acidified. The L. pneumophila phagosome exists completely outside the endolysosomal pathway, and the M. tuberculosis phagosome displays a maturational arrest at an early endosomal stage along this pathway. Rab5 plays a critical role in regulating membrane trafficking involving endosomes and phagosomes. To determine whether an alteration in the function or delivery of Rab5 could play a role in the aberrant development of L. pneumophila and M. tuberculosis phagosomes, we have examined the distribution of the small GTPase, Rab5c, in infected HeLa cells overexpressing Rab5c. Both pathogens formed phagosomes in HeLa cells with molecular characteristics similar to their phagosomes in human macrophages and multiplied in these host cells. Phagosomes containing virulent wild-type L. pneumophila never acquired immunogold staining for Rab5c, whereas phagosomes containing an avirulent mutant L. pneumophila (which ultimately fused with lysosomes) transiently acquired staining for Rab5c after phagocytosis. In contrast, M. tuberculosis phagosomes exhibited abundant staining for Rab5c throughout its life cycle. To verify that the overexpressed, recombinant Rab5c observed on the bacterial phagosomes was biologically active, we examined the phagosomes in HeLa cells expressing Rab5c Q79L, a fusion-promoting mutant. Such HeLa cells formed giant vacuoles, and after incubation with various particles, the giant vacuoles acquired large numbers of latex beads, M. tuberculosis, and avirulent L. pneumophila but not wild-type L. pneumophila, which consistently remained in tight phagosomes that did not fuse with the giant vacuoles. These results indicate that whereas Rab5 is absent from wild-type L. pneumophila phagosomes, functional Rab5 persists on M. tuberculosis phagosomes. The absence of Rab5 on the L. pneumophila phagosome may underlie its lack of interaction with endocytic compartments. The persistence of functional Rab5 on the M. tuberculosis phagosomes may enable the phagosome to retard its own maturation at an early endosomal stage.