Regulation of Rab5 function during phagocytosis of live Pseudomonas aeruginosa 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.

Rab26 promotes macrophage phagocytosis through regulation of MFN2 trafficking to mitochondria

Acute respiratory distress syndrome (ARDS) is an inflammatory disorder of the lungs caused by bacterial or viral infection. Timely phagocytosis and clearance of pathogens by macrophages are important in controlling inflammation and alleviating ARDS. However, the precise mechanism of macrophage phagocytosis remains to be explored. Here, we show that the expression of Rab26 is increased in Escherichia coli- or Pseudomonas aeruginosa-stimulated bone marrow-derived macrophages. Knocking out Rab26 reduced phagocytosis and bacterial clearance by macrophages. Rab26 interacts with mitochondrial fusion protein mitofusin-2 (MFN2) and affects mitochondrial reactive oxygen species generation by regulating MFN2 transport. The levels of MFN2 in mitochondria were reduced in Rab26-deficient bone marrow-derived macrophages, and the levels of mitochondrial reactive oxygen species and ATP were significantly decreased. Knocking down MFN2 using small interfering RNA resulted in decreased phagocytosis and killing ability of macrophages. Rab26 knockout reduced phagocytosis and bacterial clearance by macrophages in vivo, significantly increased inflammatory factors, aggravated lung tissue damage, and increased mortality in mice. Our results demonstrate that Rab26 regulates phagocytosis and clearance of bacteria by mediating the transport of MFN2 to mitochondria in macrophages, thus alleviating ARDS in mice and potentially in humans.

Pseudomonas aeruginosa in Chronic Lung Infections: How to Adapt Within the Host?

Bacteria that readily adapt to different natural environments, can also exploit this versatility upon infection of the host to persist. Pseudomonas aeruginosa, a ubiquitous Gram-negative bacterium, is harmless to healthy individuals, and yet a formidable opportunistic pathogen in compromised hosts. When pathogenic, P. aeruginosa causes invasive and highly lethal disease in certain compromised hosts. In others, such as individuals with the genetic disease cystic fibrosis, this pathogen causes chronic lung infections which persist for decades. During chronic lung infections, P. aeruginosa adapts to the host environment by evolving toward a state of reduced bacterial invasiveness that favors bacterial persistence without causing overwhelming host injury. Host responses to chronic P. aeruginosa infections are complex and dynamic, ranging from vigorous activation of innate immune responses that are ineffective at eradicating the infecting bacteria, to relative host tolerance and dampened activation of host immunity. This review will examine how P. aeruginosa subverts host defenses and modulates immune and inflammatory responses during chronic infection. This dynamic interplay between host and pathogen is a major determinant in the pathogenesis of chronic P. aeruginosa lung infections.

Melanin Transferred to Keratinocytes Resides in Nondegradative Endocytic Compartments

Melanin transfer from melanocytes to keratinocytes and subsequent accumulation in the supranuclear region is a critical process in skin pigmentation and protection against UVR. We have previously proposed that the main mode of transfer between melanocytes and keratinocytes is through exo/endocytosis of the melanosome core, termed melanocore. In this study, we developed an in vitro uptake assay using melanocores secreted by melanocytes. We show that the uptake of melanocores, but not melanosomes, by keratinocytes is protease-activated receptor-2-dependent. Furthermore, we found that the silencing of the early endocytic regulator Rab5b, but not the late endocytic regulators Rab7a or Rab9a, significantly impairs melanocore uptake by keratinocytes. After uptake, we observed that melanin accumulates in compartments that are positive for both early and late endocytic markers. We found that melanin does not localize to either highly degradative or acidic organelles, as assessed by LysoTracker and DQ-BSA staining, despite the abundance of these types of organelles within keratinocytes. Therefore, we propose that melanocore uptake leads to storage of melanin within keratinocytes in hybrid endocytic compartments that are not highly acidic or degradative. By avoiding lysosomal degradation, these specialized endosomes may allow melanin to persist within keratinocytes for long periods.

Rin1 restores host phagocytic activity during invasion by Pseudomonas aeruginosa

Pseudomonas aeruginosa uses a type III secretion system to deliver toxic effector proteins directly into host cells and alter host protein functions. Exoenzyme S (ExoS), a type III effector protein, ADP-ribosylates Rab5 GTPase and impairs early phagocytic events in macrophage cells. In this study, we tested the hypothesis that Rin1, a Ras effector protein and Rab5 guanine nucleotide exchange factor, generates an intrinsic Rab5 activity cycle during phagocytosis of live P. aeruginosa; thus, allowing proper phagocytic killing. We found that Rab5 activity was attenuated at a very early time point (2.5 min) of the phagocytic process of live but not of heat-inactivated P. aeruginosa. However, upon overexpressing Rin1 in macrophages, the Rab5 activity sustained for a prolonged time (∼20 min) counteracting the negative effects during phagocytosis of live P. aeruginosa. Ras, also a substrate of the ADP-ribosyltransferase activity of ExoS, remained active during the early events of phagocytosis of live as well as heat-inactivated P. aeruginosa. Further examinations revealed that the Rin1 : Vps9 domain (the Rab5 nucleotide catalytic domain) and the Rin1 : RA domain (the Ras association domain of Rin1) are both required for optimal Rin1 function. Finally, the time-based analysis of the ADP-ribosylation status of Rab5 and Ras obtained from this study was consistent in the context of the regulation of (i) Rab5 activity by Rin1 : Vps9 domain and (ii) Ras interaction with Rin1 via the Rin1 : RA domain. These observations highlight a novel crosstalk between Rin1-Rab5 and Rin1-Ras complexes that offsets the anti-phagocytic effects of ExoS in macrophages.

The RAB5-GEF function of RIN1 regulates multiple steps during Listeria monocytogenes infection

Listeria monocytogenes is a food-borne pathogenic bacterium that invades intestinal epithelial cells through a phagocytic pathway that relies on the activation of host cell RAB5 GTPases. Listeria monocytogenes must subsequently inhibit RAB5, however, in order to escape lysosome-mediated destruction. Relatively little is known about upstream RAB5 regulators during L. monocytogenes entry and phagosome escape processes in epithelial cells. Here we identify RIN1, a RAS effector and RAB5-directed guanine nucleotide exchange factor (GEF), as a host cell factor in L. monocytogenes infection. RIN1 is rapidly engaged following L. monocytogenes infection and is required for efficient invasion of intestinal epithelial cells. RIN1-mediated RAB5 activation later facilitates the fusion of phagosomes with lysosomes, promoting clearance of bacteria from the host cell. These results suggest that RIN1 is a host cell regulator that performs counterbalancing functions during early and late stages of L. monocytogenes infection, ultimately favoring pathogen clearance.

The ADP-ribosyltransferase domain of the effector protein ExoS inhibits phagocytosis of Pseudomonas aeruginosa during pneumonia

Pseudomonas aeruginosa is a Gram-negative pathogen commonly associated with nosocomial infections such as hospital-acquired pneumonia. It uses a type III secretion system to deliver effector proteins directly into the cytosol of host cells. Type III secretion in P. aeruginosa has been linked to severe disease and worse clinical outcomes in animal and human studies. The majority of P. aeruginosa strains secrete ExoS, a bifunctional toxin with GTPase-activating protein and ADP-ribosyltransferase activities. Numerous in vitro studies have investigated the targets and cellular effects of ExoS, linking both its enzymatic activities with inhibition of bacterial internalization. However, little is known about how this toxin facilitates the progression of infection in vivo. In this study, we used a mouse model to investigate the role of ExoS in inhibiting phagocytosis during pneumonia. We first confirmed previous findings that the ADP-ribosyltransferase activity of ExoS, but not the GTPase-activating protein activity, was responsible for bacterial persistence and decreased host survival in this model. We then used two distinct assays to demonstrate that ExoS inhibited phagocytosis during pneumonia. In contrast to the findings of several in vitro studies, this in vivo inhibition was also dependent on the ADP-ribosyltransferase activity, but not the GTPase-activating protein activity, of ExoS. These results demonstrate for the first time the antiphagocytic function of ExoS in the context of an actual infection and indicate that blocking the ADP-ribosyltransferase activity of ExoS may have potential therapeutic benefit.

Pseudomonas aeruginosa is a major cause of hospital-acquired infections. To cause severe disease, this bacterium uses a type III secretion system that delivers four effector proteins, ExoS, ExoT, ExoU, and ExoY, into host cells. The majority of P. aeruginosa strains secrete ExoS, a bifunctional toxin with GTPase-activating protein and ADP-ribosyltransferase activities. In cell culture models, both enzymatic activities have been associated with decreased bacterial internalization. However, our study is the first to examine a role for ExoS in blocking phagocytosis in an animal model. We report that ExoS does inhibit phagocytosis during pneumonia. The ADP-ribosyltransferase activity, but not the GTPase-activating protein activity, of ExoS is necessary for this effect. Our findings highlight the ability of P. aeruginosa to manipulate the inflammatory response during pneumonia to facilitate bacterial survival.

Bacterial factors exploit eukaryotic Rho GTPase signaling cascades to promote invasion and proliferation within their host

Actin cytoskeleton is a main target of many bacterial pathogens. Among the multiple regulation steps of the actin cytoskeleton, bacterial factors interact preferentially with RhoGTPases. Pathogens secrete either toxins which diffuse in the surrounding environment, or directly inject virulence factors into target cells. Bacterial toxins, which interfere with RhoGTPases, and to some extent with RasGTPases, catalyze a covalent modification (ADPribosylation, glucosylation, deamidation, adenylation, proteolysis) blocking these molecules in their active or inactive state, resulting in alteration of epithelial and/or endothelial barriers, which contributes to dissemination of bacteria in the host. Injected bacterial virulence factors preferentially manipulate the RhoGTPase signaling cascade by mimicry of eukaryotic regulatory proteins leading to local actin cytoskeleton rearrangement, which mediates bacterial entry into host cells or in contrast escape to phagocytosis and immune defense. Invasive bacteria can also manipulate RhoGTPase signaling through recognition and stimulation of cell surface receptor(s). Changes in RhoGTPase activation state is sensed by the innate immunity pathways and allows the host cell to adapt an appropriate defense response.

Vinculin and Rab5 complex is required [correction of requited]for uptake of Staphylococcus aureus and interleukin-6 expression

Vinculin, a 116-kDa membrane cytoskeletal protein, is an important molecule for cell adhesion; however, little is known about its other cellular functions. Here, we demonstrated that vinculin binds to Rab5 and is required for Staphylococcus aureus (S. aureus) uptake in cells. Viunculin directly bound to Rab5 and enhanced the activation of S. aureus uptake. Over-expression of active vinculin mutants enhanced S. aureus uptake, whereas over-expression of an inactive vinculin mutant decreased S. aureus uptake. Vinculin bound to Rab5 at the N-terminal region (1-258) of vinculin. Vinculin and Rab5 were involved in the S. aureus-induced phosphorylation of MAP kinases (p38, Erk, and JNK) and IL-6 expression. Finally, vinculin and Rab5 knockdown reduced infection of S. aureus, phosphorylation of MAPKs and IL-6 expression in murine lungs. Our results suggest that vinculin binds to Rab5 and that these two molecules cooperatively enhance bacterial infection and the inflammatory response.

Exoenzyme S ADP-ribosylates Rab5 effector sites to uncouple intracellular trafficking

Pseudomonas aeruginosa exoenzyme S (ExoS) ADP-ribosylates multiple eukaryotic targets to promote cytopathology and bacterial colonization. ADP-ribosylation of the small GTPase Rab5 has previously been shown to block fluid-phase endocytosis and trafficking of plasma membrane receptors to the early endosomes as well as inhibit phagocytosis of the bacterium. In this study, ExoS is shown to be capable of ADP-ribosylating 6 candidate arginine residues that are located in the effector binding region or in the C terminus of Rab5. Two Rab5 derivatives were engineered, which contained Arg→Ala mutations at four Arg residues within the effector binding region (EF) or two Arg residues within the C-terminal tail (TL). Expression of Rab5(TL) does not affect the ability of ExoS to modify intracellular trafficking, while expression of Rab5(EF) rescued the ability of ExoS to inhibit intracellular trafficking. ADP-ribosylation of effector arginines likely uncouples Rab5 signaling to downstream effectors. This is a different mechanism for inhibition than observed for the ADP-ribosylation of Ras by ExoS, where ADP-ribosylated Ras loses the ability to bind guanine nucleotide exchange factor (GEF). Other experiments showed that expression of dominant negative Rab5(Ser34Asn) does not inhibit ExoS trafficking to the perinuclear region of intoxicated cells. This study provides insight into a mechanism for how ExoS ADP-ribosylation of Rab5 inhibits Rab5 function.