The late endocytic Rab39a GTPase regulates the interaction between multivesicular bodies and chlamydial inclusions

Hijacking host cell vesicular transport: New insights into the nutrient acquisition mechanism of Chlamydia

Chlamydia infection is an important cause of public health diseases, and no effective vaccine is currently available. Owing to its unique intracellular lifestyle, Chlamydia requires a variety of nutrients and substrates from host cells, particularly sphingomyelin, cholesterol, iron, amino acids, and the mannose-6-phosphate receptor, which are essential for inclusion development. Here, we summarize the recent advances in Chlamydia nutrient acquisition mechanism by hijacking host cell vesicular transport, which plays an important role in chlamydial growth and development. Chlamydia obtains the components necessary to complete its intracellular developmental cycle by recruiting Rab proteins (major vesicular trafficking regulators) and Rab effector proteins to the inclusion, interfering with Rab-mediated multivesicular trafficking, reorienting the nutrition of host cells, and reconstructing the intracellular niche environment. Consequently, exploring the role of vesicular transport in nutrient acquisition offers a novel perspective on new approaches for preventing and treating Chlamydia infection.

The Chlamydia effector CpoS modulates the inclusion microenvironment and restricts the interferon response by acting on Rab35

The obligate intracellular bacterium Chlamydia trachomatis inserts a family of inclusion membrane (Inc) proteins into the membrane of its vacuole (the inclusion). The Inc CpoS is a critical suppressor of host cellular immune surveillance, but the underlying mechanism remained elusive. By complementing a cpoS mutant with various natural orthologs and variants of CpoS, we linked distinct molecular interactions of CpoS to distinct functions. Unexpectedly, we found CpoS to be essential for the formation of inclusion membrane microdomains that control the spatial organization of multiple Incs involved in signaling and modulation of the host cellular cytoskeleton. While the function of CpoS in microdomains was uncoupled from its role in the suppression of host cellular defenses, we found the ability of CpoS to interact with Rab GTPases to be required not only for the manipulation of membrane trafficking, such as to mediate transport of ceramide-derived lipids (sphingolipids) to the inclusion, but also for the inhibition of Stimulator of interferon genes (STING)-dependent type I interferon responses. Indeed, depletion of Rab35 phenocopied the exacerbated interferon responses observed during infection with CpoS-deficient mutants. Overall, our findings highlight the role of Inc-Inc interactions in shaping the inclusion microenvironment and the modulation of membrane trafficking as a pathogenic immune evasion strategy. IMPORTANCE Chlamydia trachomatis is a prevalent bacterial pathogen that causes blinding ocular scarring and urogenital infections that can lead to infertility and pregnancy complications. Because Chlamydia can only grow within its host cell, boosting the intrinsic defenses of human cells may represent a novel strategy to fight pathogen replication and survival. Hence, CpoS, a Chlamydia protein known to block host cellular defenses, or processes regulated by CpoS, could provide new opportunities for therapeutic intervention. By revealing CpoS as a multifunctional virulence factor and by linking its ability to block host cellular immune signaling to the modulation of membrane trafficking, the present work may provide a foundation for such rationale targeting and advances our understanding of how intracellular bacteria can shape and protect their growth niche.

The Small GTPase Rab14 Regulates the Trafficking of Ceramide from Endoplasmic Reticulum to Golgi Apparatus and Facilitates Classical Swine Fever Virus Assembly

Classical swine fever virus (CSFV) is a highly pathogenic RNA virus belonging to the Flaviviridae family that can cause deadly classical swine fever (CSF) in pigs. However, the molecular details of virus replication in the host are still unclear. Our previous studies have reported that several Rab proteins mediate CSFV entry into host cells, but it is unknown whether CSFV hijacks other Rab proteins for effective viral infection. Here, we systematically studied the role of Rab14 protein in regulating lipid metabolism for promoting viral assembly. First, Rab14 knockdown and overexpression significantly affected CSFV replication, indicating the essential role of Rab14 in CSFV infection. Interestingly, Rab14 could significantly affect virus replication in the late stage of infection. Mechanistically, CSFV NS5A recruited Rab14 to the ER, followed by ceramide transportation to the Golgi apparatus, where sphingomyelin was synthesized. The experimental data of small molecule inhibitors, RNA interference, and replenishment assay showed that the phosphatidylinositol-3-kinase (PI3K)/AKT/AS160 signaling pathway regulated the function of Rab14 to affect the transport of ceramide. More importantly, sphingomyelin on the Golgi apparatus contributed to the assembly of viral particles. Blockage of the Rab14 regulatory pathway induced the reduction of the content of sphingomyelin on the Golgi apparatus, impairing the assembly of virus particles. Our study clarifies that Rab14 regulates lipid metabolism and promotes CSFV replication, which provides insight into a novel function of Rab14 in regulating vesicles to transport lipids to the viral assembly factory. IMPORTANCE The Rab protein family members participate in the viral replication of multiple viruses and play important roles in the virus infection cycle. Our previous research focused on Rab5/7/11, which regulated the trafficking of vesicles in the early stage of CSFV infection, especially in viral endocytosis. However, the role of other Rab proteins in CSFV replication is unclear and needs further clarification. Strikingly, we screened some Rabs and found the important role of Rab14 in CSFV infection. Virus infection mobilized Rab14 to regulate the vesicle to transport ceramide from the ER to the Golgi apparatus, further promoting the synthesis of sphingomyelin and facilitating virus assembly. The treatment of inhibitors showed that the lipid transport mediated by Rab14 was regulated by the PI3K/AKT/AS160 signaling pathway. Knockdown of Rab14 or the treatment with PI3K/AKT/AS160 inhibitors reduced the ceramide content in infected cells and hindered virus assembly. Our study is the first to explain that vesicular lipid transport regulated by Rab promotes CSFV assembly, which is conducive to the development of antiviral drugs.

The Obligate Intracellular Bacterial Pathogen Anaplasma phagocytophilum Exploits Host Cell Multivesicular Body Biogenesis for Proliferation and Dissemination

Anaplasma phagocytophilum is the etiologic agent of the emerging infection, granulocytic anaplasmosis. This obligate intracellular bacterium lives in a host cell-derived vacuole that receives membrane traffic from multiple organelles to fuel its proliferation and from which it must ultimately exit to disseminate infection. Understanding of these essential pathogenic mechanisms has remained poor. Multivesicular bodies (MVBs) are late endosomal compartments that receive biomolecules from other organelles and encapsulate them into intralumenal vesicles (ILVs) using endosomal sorting complexes required for transport (ESCRT) machinery and ESCRT-independent machinery. Association of the ESCRT-independent protein, ALIX, directs MVBs to the plasma membrane where they release ILVs as exosomes. We report that the A. phagocytophilum vacuole (ApV) is acidified and enriched in lysobisphosphatidic acid, a lipid that is abundant in MVBs. ESCRT-0 and ESCRT-III components along with ALIX localize to the ApV membrane. siRNA-mediated inactivation of ESCRT-0 and ALIX together impairs A. phagocytophilum proliferation and infectious progeny production. RNA silencing of ESCRT-III, which regulates ILV scission, pronouncedly reduces ILV formation in ApVs and halts infection by arresting bacterial growth. Rab27a and its effector Munc13-4, which drive MVB trafficking to the plasma membrane and subsequent exosome release, localize to the ApV. Treatment with Nexinhib20, a small molecule inhibitor that specifically targets Rab27a to block MVB exocytosis, abrogates A. phagocytophilum infectious progeny release. Thus, A. phagocytophilum exploits MVB biogenesis and exosome release to benefit each major stage of its intracellular infection cycle: intravacuolar growth, conversion to the infectious form, and exit from the host cell. IMPORTANCE Anaplasma phagocytophilum causes granulocytic anaplasmosis, a globally emerging zoonosis that can be severe, even fatal, and for which antibiotic treatment options are limited. A. phagocytophilum lives in an endosomal-like compartment that interfaces with multiple organelles and from which it must ultimately exit to spread within the host. How the bacterium accomplishes these tasks is poorly understood. Multivesicular bodies (MVBs) are intermediates in the endolysosomal pathway that package biomolecular cargo from other organelles as intralumenal vesicles for release at the plasma membrane as exosomes. We discovered that A. phagocytophilum exploits MVB biogenesis and trafficking to benefit all aspects of its intracellular infection cycle: proliferation, conversion to its infectious form, and release of infectious progeny. The ability of a small molecule inhibitor of MVB exocytosis to impede A. phagocytophilum dissemination indicates the potential of this pathway as a novel host-directed therapeutic target for granulocytic anaplasmosis.

Functions of Sphingolipids in Pathogenesis During Host-Pathogen Interactions

Sphingolipids are a class of membrane lipids that serve as vital structural and signaling bioactive molecules in organisms ranging from yeast to animals. Recent studies have emphasized the importance of sphingolipids as signaling molecules in the development and pathogenicity of microbial pathogens including bacteria, fungi, and viruses. In particular, sphingolipids play key roles in regulating the delicate balance between microbes and hosts during microbial pathogenesis. Some pathogens, such as bacteria and viruses, harness host sphingolipids to promote development and infection, whereas sphingolipids from both the host and pathogen are involved in fungus-host interactions. Moreover, a regulatory role for sphingolipids has been described, but their effects on host physiology and metabolism remain to be elucidated. Here, we summarize the current state of knowledge about the roles of sphingolipids in pathogenesis and interactions with host factors, including how sphingolipids modify pathogen and host metabolism with a focus on pathogenesis regulators and relevant metabolic enzymes. In addition, we discuss emerging perspectives on targeting sphingolipids that function in host-microbe interactions as new therapeutic strategies for infectious diseases.

Akt Phosphorylation Influences Persistent Chlamydial Infection and Chlamydia-Induced Golgi Fragmentation Without Involving Rab14

Chlamydia trachomatis is an obligate intracellular bacterium that causes multiple diseases involving the eyes, gastrointestinal tract, and genitourinary system. Previous studies have identified that in acute chlamydial infection, C. trachomatis requires Akt pathway phosphorylation and Rab14-positive vesicles to transmit essential lipids from the Golgi apparatus in survival and replication. However, the roles that Akt phosphorylation and Rab14 play in persistent chlamydial infection remain unclear. Here, we discovered that the level of Akt phosphorylation was lower in persistent chlamydial infection, and positively correlated with the effect of activating the development of Chlamydia but did not change the infectivity and 16s rRNA gene expression. Rab14 was found to exert a limited effect on persistent infection. Akt phosphorylation might regulate Chlamydia development and Chlamydia-induced Golgi fragmentation in persistent infection without involving Rab14. Our results provide a new insight regarding the potential of synergistic repressive effects of an Akt inhibitor with antibiotics in the treatment of persistent chlamydial infection induced by penicillin.

Autophagy and Extracellular Vesicles, Connected to rabGTPase Family, Support Aggressiveness in Cancer Stem Cells

Even though cancers have been widely studied and real advances in therapeutic care have been made in the last few decades, relapses are still frequently observed, often due to therapeutic resistance. Cancer Stem Cells (CSCs) are, in part, responsible for this resistance. They are able to survive harsh conditions such as hypoxia or nutrient deprivation. Autophagy and Extracellular Vesicles (EVs) secretion are cellular processes that help CSC survival. Autophagy is a recycling process and EVs secretion is essential for cell-to-cell communication. Their roles in stemness maintenance have been well described. A common pathway involved in these processes is vesicular trafficking, and subsequently, regulation by Rab GTPases. In this review, we analyze the role played by Rab GTPases in stemness status, either directly or through their regulation of autophagy and EVs secretion.

Chlamydia trachomatis Infection Impairs MHC-I Intracellular Trafficking and Antigen Cross-Presentation by Dendritic Cells

During cross-presentation, exogenous antigens (i.e. intracellular pathogens or tumor cells) are internalized and processed within the endocytic system and also by the proteasome in the cytosol. Then, antigenic peptides are associated with Major Histocompatibility Complex (MHC) class I molecules and these complexes transit to the plasma membrane in order to trigger cytotoxic immune responses through the activation of CD8+ T lymphocytes. Dendritic cells (DCs) are particularly adapted to achieve efficient antigen cross-presentation and their endocytic network displays important roles during this process, including a sophisticated MHC-I transport dependent on recycling compartments. In this study, we show that C. trachomatis, an obligate intracellular pathogen that exhibits multiple strategies to evade the immune system, is able to induce productive infections in the murine DC line JAWS-II. Our results show that when C. trachomatis infects these cells, the bacteria-containing vacuole strongly recruits host cell recycling vesicles, but no other endosomal compartments. Furthermore, we found that chlamydial infection causes significant alterations of MHC-I trafficking in JAWS-II DCs: reduced levels of MHC-I expression at the cell surface, disruption of the perinuclear MHC-I intracellular pool, and impairment of MHC-I endocytic recycling to the plasma membrane. We observed that all these modifications lead to a hampered cross-presentation ability of soluble and particulate antigens by JAWS-II DCs and primary bone marrow-derived DCs. In summary, our findings provide substantial evidence that C. trachomatis hijacks the DC endocytic recycling system, causing detrimental changes on MHC-I intracellular transport, which are relevant for competent antigen cross-presentation.

The Chlamydia effector CT622/TaiP targets a nonautophagy related function of ATG16L1

The obligate intracellular bacteria Chlamydia trachomatis, the causative agent of trachoma and sexually transmitted diseases, multiply in a vacuolar compartment, the inclusion. From this niche, they secrete "effector" proteins, that modify cellular activities to enable bacterial survival and proliferation. Here, we show that the host autophagy-related protein 16-1 (ATG16L1) restricts inclusion growth and that this effect is counteracted by the secretion of the bacterial effector CT622/TaiP (translocated ATG16L1 interacting protein). ATG16L1 is mostly known for its role in the lipidation of the human homologs of ATG8 (i.e., LC3 and homologs) on double membranes during autophagy as well as on single membranes during LC3-associated phagocytosis and other LC3-lipidation events. Unexpectedly, the LC3-lipidation-related functions of ATG16L1 are not required for restricting inclusion development. We show that the carboxyl-terminal domain of TaiP exposes a mimic of an eukaryotic ATG16L1-binding motif that binds to ATG16L1's WD40 domain. By doing so, TaiP prevents ATG16L1 interaction with the integral membrane protein TMEM59 and allows the rerouting of Rab6-positive compartments toward the inclusion. The discovery that one bacterial effector evolved to target ATG16L1's engagement in intracellular traffic rather than in LC3 lipidation brings this "secondary" activity of ATG16L1 in full light and emphasizes its importance for maintaining host cell homeostasis.

Rab GTPase Mediating Regulation of NALP3 in Colorectal Cancer

The NALP3 inflammasome signaling contributes to inflammation within tumor tissues. This inflammation may be promoted by the vesicle trafficking of inflammasome components and cytokines. Rab5, Rab7 and Rab11 regulate vesicle trafficking. However, the role of these proteins in the regulation of inflammasomes remains largely unknown. To elucidate the role of these Rab proteins in inflammasome regulation, HCT-116, a colorectal cancer (CRC) cell line expressing pDsRed-Rab5 wild type (WT), pDsRed-Rab5 dominant-negative (DN), pDsRed-Rab7 WT, pDsRed-Rab7 DN, pDsRed-Rab11 WT and pDsRed-Rab11 DN were treated with lipopolysaccharide (LPS)/nigericin. Inflammasome activation was analyzed by measuring the mRNA expression of NLRP3, Pro-CASP1, RAB39A and Pro-IL-1β, conducting immunofluorescence imaging and western blotting of caspase-1 and analysing the secretion levels of IL-1β using enzyme-linked immunosorbent assay (ELISA). The effects of Rabs on cytokine release were evaluated using MILLIPLEX MAP Human Cytokine/Chemokine Magnetic Bead Panel-Premixed 41 Plex. The findings showed that LPS/nigericin-treated cells expressing Rab5-WT indicated increased NALP3 expression and secretion of the IL-1β as compared to Rab5-DN cells. Caspase-1 was localized in the nucleus and cytosol of Rab5-WT cells but was localized in the cytosol in Rab5-DN cells. There were no any effects of Rab7 and Rab11 expression on the regulation of inflammasomes. Our results suggest that Rab5 may be a potential target for the regulation of NALP3 in the treatment of the CRC inflammation.

Rab Proteins: Insights into Intracellular Trafficking in Endometrium

Rab proteins belong to the Ras superfamily of small monomeric GTPases. These G proteins are the main controllers of vesicular transport in every tissue, among them, the endometrium. They are in charge of to the functional subcellular compartmentalization and cargo transport between organelles and the plasma membrane. In turn, intracellular trafficking contributes to endometrial changes during the menstrual cycle, secretion to the uterine fluid, and trophoblast implantation; however, few reports analyze the role of Rab proteins in the uterus. In general, Rab proteins control the release of cytokines, growth factors, enzymes, hormones, cell adhesion molecules, and mucus. Further, the secretion of multiple compounds into the uterine cavity is required for successful implantation. Therefore, alterations in Rab-controlled intracellular transport likely impair secretory processes to the uterine fluid that may correlate with abnormal endometrial development and failed reproductive outcomes. Overall, they could explain recurrent miscarriages, female infertility, and/or assisted reproductive failure. Interestingly, estrogen (E2) and progesterone (P) regulate gene expression of Rab proteins involved in secretory pathways. This review aims to gather information regarding the role of Rab proteins and intracellular trafficking in the endometrium during the different menstrual phases, and in the generation of a receptive stage for embryo implantation, modulated by E2 and P. This knowledge might be useful for the development of novel reproductive therapies that overcome low implantation rates of assisted reproductive procedures.

The GTPase Rab39a promotes phagosome maturation into MHC-I antigen-presenting compartments

For CD8 T lymphocytes to mount responses to cancer and virally-infected cells, dendritic cells must capture antigens present in tissues and display them as peptides bound to MHC-I molecules. This is most often accomplished through a pathway called antigen cross-presentation (XPT). Here, we report that the vesicular trafficking protein Rab39a is needed for optimal cross-presentation by dendritic cells in vitro and cross-priming of CD8 T cells in vivo. Without Rab39a, MHC-I presentation of intraphagosomal peptides is inhibited, indicating that Rab39a converts phagosomes into peptide-loading compartments. In this process, Rab39a promotes the delivery of MHC-I molecules from the endoplasmic reticulum (ER) to phagosomes, and increases the levels of peptide-empty MHC-I conformers that can be loaded with peptide in this compartment. Rab39a also increases the levels of Sec22b and NOX2, previously recognized to participate in cross-presentation, on phagosomes, thereby filling in a missing link into how phagosomes mature into cross-presenting vesicles.

Chlamydial Infection From Outside to Inside

Chlamydia are obligate intracellular bacteria, characterized by a unique biphasic developmental cycle. Specific interactions with the host cell are crucial for the bacteria’s survival and amplification because of the reduced chlamydial genome. At the start of infection, pathogen-host interactions are set in place in order for Chlamydia to enter the host cell and reach the nutrient-rich peri-Golgi region. Once intracellular localization is established, interactions with organelles and pathways of the host cell enable the necessary hijacking of host-derived nutrients. Detailed information on the aforementioned processes will increase our understanding on the intracellular pathogenesis of chlamydiae and hence might lead to new strategies to battle chlamydial infection. This review summarizes how chlamydiae generate their intracellular niche in the host cell, acquire host-derived nutrients in order to enable their growth and finally exit the host cell in order to infect new cells. Moreover, the evolution in the development of molecular genetic tools, necessary for studying the chlamydial infection biology in more depth, is discussed in great detail.

The multiple functions of the numerous Chlamydia trachomatis secreted proteins: the tip of the iceberg

Chlamydia trachomatis serovars are obligate intracellular bacterial pathogens mainly causing ocular and urogenital infections that affect millions of people worldwide and which can lead to blindness or sterility. They reside and multiply intracellularly within a membrane-bound vacuolar compartment, known as inclusion, and are characterized by a developmental cycle involving two morphologically and physiologically distinct chlamydial forms. Completion of the developmental cycle involves the secretion of > 70 C. trachomatis proteins that function in the host cell cytoplasm and nucleus, in the inclusion membrane and lumen, and in the extracellular milieu. These proteins can, for example, interfere with the host cell cytoskeleton, vesicular and non-vesicular transport, metabolism, and immune signalling. Generally, this promotes C. trachomatis invasion into, and escape from, host cells, the acquisition of nutrients by the chlamydiae, and evasion of cell-autonomous, humoral and cellular innate immunity. Here, we present an in-depth review on the current knowledge and outstanding questions about these C. trachomatis secreted proteins.

Rab39a and Rab39b Display Different Intracellular Distribution and Function in Sphingolipids and Phospholipids Transport

Rab GTPases define the identity and destiny of vesicles. Some of these small GTPases present isoforms that are expressed differentially along developmental stages or in a tissue-specific manner, hence comparative analysis is difficult to achieve. Here, we describe the intracellular distribution and function in lipid transport of the poorly characterized Rab39 isoforms using typical cell biology experimental tools and new ones developed in our laboratory. We show that, despite their amino acid sequence similarity, Rab39a and Rab39b display non-overlapping intracellular distribution. Rab39a localizes in the late endocytic pathway, mainly at multivesicular bodies. In contrast, Rab39b distributes in the secretory network, at the endoplasmic reticulum/cis-Golgi interface. Therefore, Rab39a controls trafficking of lipids (sphingomyelin and phospholipids) segregated at multivesicular bodies, whereas Rab39b transports sphingolipids biosynthesized at the endoplasmic reticulum-Golgi factory. Interestingly, lyso bis-phosphatidic acid is exclusively transported by Rab39a, indicating that both isoforms do not exert identical functions in lipid transport. Conveniently, the requirement of eukaryotic lipids by the intracellular pathogen Chlamydia trachomatis rendered useful for dissecting and distinguishing Rab39a- and Rab39b-controlled trafficking pathways. Our findings provide comparative insights about the different subcellular distribution and function in lipid transport of the two Rab39 isoforms.

Akt/AS160 Signaling Pathway Inhibition Impairs Infection by Decreasing Rab14-Controlled Sphingolipids Delivery to Chlamydial Inclusions

Chlamydia trachomatis, an obligate intracellular bacterium, intercepts different trafficking pathways of the host cell to acquire essential lipids for its survival and replication, particularly from the Golgi apparatus via a Rab14-mediated transport. Molecular mechanisms underlying how these bacteria manipulate intracellular transport are a matter of intense study. Here, we show that C. trachomatis utilizes Akt/AS160 signaling pathway to promote sphingolipids delivery to the chlamydial inclusion through Rab14-controlled vesicular transport. C. trachomatis provokes Akt phosphorylation along its entire developmental life cycle and recruits phosphorylated Akt (pAkt) to the inclusion membrane. As a consequence, Akt Substrate of 160 kDa (AS160), also known as TBC1D4, a GTPase Activating Protein (GAP) for Rab14, is phosphorylated and therefore inactivated. Phosphorylated AS160 (pAS160) loses its ability to promote GTP hydrolysis, favoring Rab14 binding to GTP. Akt inhibition by an allosteric isoform-specific Akt inhibitor (iAkt) prevents AS160 phosphorylation and reduces Rab14 recruitment to chlamydial inclusions. iAkt further impairs sphingolipids acquisition by C. trachomatis-inclusion and provokes lipid retention at the Golgi apparatus. Consequently, treatment with iAkt decreases chlamydial inclusion size, bacterial multiplication, and infectivity in a dose-dependent manner. Similar results were found in AS160-depleted cells. By electron microscopy, we observed that iAkt generates abnormal bacterial forms as those reported after sphingolipids deprivation or Rab14 silencing. Taken together, our findings indicate that targeting the Akt/AS160/Rab14 axis could constitute a novel strategy to limit chlamydial infections, mainly for those caused by antibiotic-resistant bacteria.

Life inside and out: making and breaking protein disulfide bonds in Chlamydia

Disulphide bonds are widely used among all domains of life to provide structural stability to proteins and to regulate enzyme activity. Chlamydia spp. are obligate intracellular bacteria that are especially dependent on the formation and degradation of protein disulphide bonds. Members of the genus Chlamydia have a unique biphasic developmental cycle alternating between two distinct cell types; the extracellular infectious elementary body (EB) and the intracellular replicating reticulate body. The proteins in the envelope of the EB are heavily cross-linked with disulphides and this is known to be critical for this infectious phase. In this review, we provide a comprehensive summary of what is known about the redox state of chlamydial envelope proteins throughout the developmental cycle. We focus especially on the factors responsible for degradation and formation of disulphide bonds in Chlamydia and how this system compares with redox regulation in other organisms. Focussing on the unique biology of Chlamydia enables us to provide important insights into how specialized suites of disulphide bond (Dsb) proteins cater for specific bacterial environments and lifecycles.

Male genital tract immune response against Chlamydia trachomatis infection

Chlamydia trachomatis is the most commonly reported agent of sexually transmitted bacterial infections worldwide. This pathogen frequently leads to persistent, long-term, subclinical infections, which in turn may cause severe pathology in susceptible hosts. This is in part due to the strategies that Chlamydia trachomatis uses to survive within epithelial cells and to evade the host immune response, such as subverting intracellular trafficking, interfering signaling pathways and preventing apoptosis. Innate immune receptors such as toll-like receptors expressed on epithelial and immune cells in the genital tract mediate the recognition of chlamydial molecular patterns. After bacterial recognition, a subset of pro-inflammatory cytokines and chemokines are continuously released by epithelial cells. The innate immune response is followed by the initiation of the adaptive response against Chlamydia trachomatis, which in turn may result in T helper 1-mediated protection or in T helper 2-mediated immunopathology. Understanding the molecular mechanisms developed by Chlamydia trachomatis to avoid killing and host immune response would be crucial for designing new therapeutic approaches and developing protective vaccines. In this review, we focus on chlamydial survival strategies and the elicited immune responses in male genital tract infections.

Emerging Role of Retromer in Modulating Pathogen Growth

Intracellular pathogens have developed elegant mechanisms to modulate host endosomal trafficking. The highly conserved retromer pathway has emerged as an important target of viruses and intravacuolar bacteria. Some pathogens require retromer function to survive. For others, retromer activity restricts intracellular growth; these pathogens must disrupt retromer function to survive. In this review, we discuss recent paradigm changes to the current model for retromer assembly and cargo selection. We highlight how the study of pathogen effectors has contributed to these fundamental insights, with a special focus on the biology and structure of two recently described bacterial effectors, Chlamydia trachomatis IncE and Legionella pneumophila RidL. These two pathogens employ distinct strategies to target retromer components and overcome restriction of intracellular growth imposed by retromer.

Consequences of Rab GTPase dysfunction in genetic or acquired human diseases

Rab GTPases are important regulators of intracellular membrane trafficking in eukaryotes. Both activating and inactivating mutations in Rab genes have been identified and implicated in human diseases ranging from neurological disorders to cancer. In addition, altered Rab expression is often associated with disease prognosis. As such, the study of diseases associated with Rabs or Rab-interacting proteins has shed light on the important role of intracellular membrane trafficking in disease etiology. In this review, we cover recent advances in the field with an emphasis on cellular mechanisms.

Prominent role of RAB39A-RXRB axis in cancer development and stemness

In this study, we found that RAB39A, a member of the RAS oncogene family, was selectively expressed in cancer cells of different histotypes, by analyzing gene expression in human osteosarcoma cells and the cancer stem cells (CSCs) and by comparing them with normal cells through global transcriptomics and principal component analyses. We further validated RAB39A as a therapeutic target, by silencing its expression. The silencing impaired cancer stemness and spherogenic ability in vitro, as well as tumorigenesis in vivo. RNA-seq analyses in the silenced spheres suggested that RAB39A is associated downstream with RXRB and KLF4. Notably, RXRB expression was inhibited in RAB39A-silenced CSCs. Induced overexpression of RXRB in RAB39A-silenced cells restored spherogenic ability and tumorigenesis, confirming RXRB as a major effector of RAB39A. Quantitative RT-PCR analysis of ∼400 human cancer tissues showed that RAB39A was highly expressed in sarcomas and in malignancies of lymphoid, adrenal and testicular tissues. Our data provide the rationale for targeting of the RAB39A-RXRB axis as a therapy for aggressive cancers.

Consequences of Rab GTPase dysfunction in genetic or acquired human diseases

Rab GTPases are important regulators of intracellular membrane trafficking in eukaryotes. Both activating and inactivating mutations in Rab genes have been identified and implicated in human diseases ranging from neurological disorders to cancer. In addition, altered Rab expression is often associated with disease prognosis. As such, the study of diseases associated with Rabs or Rab-interacting proteins has shed light on the important role of intracellular membrane trafficking in disease etiology. In this review, we cover recent advances in the field with an emphasis on cellular mechanisms.

Taking control: reorganization of the host cytoskeleton by Chlamydia

Both actin and microtubules are major cytoskeletal elements in eukaryotic cells that participate in many cellular processes, including cell division and motility, vesicle and organelle movement, and the maintenance of cell shape. Inside its host cell, the human pathogen Chlamydia trachomatis manipulates the cytoskeleton to promote its survival and enhance its pathogenicity. In particular, Chlamydia induces the drastic rearrangement of both actin and microtubules, which is vital for its entry, inclusion structure and development, and host cell exit. As significant progress in Chlamydia genetics has greatly enhanced our understanding of how this pathogen co-opts the host cytoskeleton, we will discuss the machinery used by Chlamydia to coordinate the reorganization of actin and microtubules.

Chlamydia trachomatis and its interaction with the cellular retromer

Chlamydia trachomatis is an important human pathogen. This obligate intracellular bacterium grows inside the eukaryotic cell in a membrane-bound compartment, the inclusion. Recent global approaches describe the interactions of C. trachomatis with its host cell and indicate the inclusion is an intracellular trafficking hub embedded into the cellular vesicular trafficking pathways recruiting subunits of the retromer protein complex of the host cell. Here we review these recent developments in deciphering Chlamydia-host cell interactions with emphasis on the role of the retromer complex.

Diverted recycling-Shigella subversion of Rabs

Small GTPases of the Rab protein family control intracellular vesicular trafficking to allow their communication and maintenance. It is a common strategy for intracellular bacteria to exploit these pathways to shape their respective niches for survival. The subversion of Rabs for the generation of an intracellular environment favoring the pathogen has been described almost exclusively for intracellular bacteria that reside within bacterial containing vacuoles (BCVs). However, less is known about Rab subversion for bacteria that rupture the BCV to reach the host cytoplasm. Here, we provide recent examples of Rab targeting by both groups of intracellular bacteria with a special focus on Shigella, the causative agent of bacillary dysentery. Shigella recruits Rab11, the hallmark of the perinuclear recycling compartment to in situ formed macropinosomes at the entry foci via the bacterial effector IpgD. This leads to efficient BCV rupture and cytosolic escape. We discuss the concept of diverted recycling through host Rab GTPases that emerges as a novel pathogen strategy.

C9orf72's Interaction with Rab GTPases-Modulation of Membrane Traffic and Autophagy

Hexanucleotide repeat expansion in an intron of Chromosome 9 open reading frame 72 (C9orf72) is the most common genetic cause of Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Dementia (FTD). While functional haploinsufficiency of C9orf72 resulting from the mutation may play a role in ALS/FTD, the actual cellular role of the protein has been unclear. Recent findings have now shown that C9orf72 physically and functionally interacts with multiple members of the Rab small GTPases family, consequently exerting important influences on cellular membrane traffic and the process of autophagy. Loss of C9orf72 impairs endocytosis in neuronal cell lines, and attenuated autophagosome formation. Interestingly, C9orf72 could influence autophagy both as part of a Guanine nucleotide exchange factor (GEF) complex, or as a Rab effector that facilitates transport of the Unc-51-like Autophagy Activating Kinase 1 (Ulk1) autophagy initiation complex. The cellular function of C9orf72 is discussed in the light of these recent findings.

The Making and Taking of Lipids: The Role of Bacterial Lipid Synthesis and the Harnessing of Host Lipids in Bacterial Pathogenesis

In order to survive environmental stressors, including those induced by growth in the human host, bacterial pathogens will adjust their membrane physiology accordingly. These physiological changes also include the use of host-derived lipids to alter their own membranes and feed central metabolic pathways. Within the host, the pathogen is exposed to many stressful stimuli. A resulting adaptation is for pathogens to scavenge the host environment for readily available lipid sources. The pathogen takes advantage of these host-derived lipids to increase or decrease the rigidity of their own membranes, to provide themselves with valuable precursors to feed central metabolic pathways, or to impact host signalling and processes. Within, we review the diverse mechanisms that both extracellular and intracellular pathogens employ to alter their own membranes as well as their use of host-derived lipids in membrane synthesis and modification, in order to increase survival and perpetuate disease within the human host. Furthermore, we discuss how pathogen employed mechanistic utilization of host-derived lipids allows for their persistence, survival and potentiation of disease. A more thorough understanding of all of these mechanisms will have direct consequences for the development of new therapeutics, and specifically, therapeutics that target pathogens, while preserving normal flora.

Chlamydia cell biology and pathogenesis

Chlamydia spp. are important causes of human disease for which no effective vaccine exists. These obligate intracellular pathogens replicate in a specialized membrane compartment and use a large arsenal of secreted effectors to survive in the hostile intracellular environment of the host. In this Review, we summarize the progress in decoding the interactions between Chlamydia spp. and their hosts that has been made possible by recent technological advances in chlamydial proteomics and genetics. The field is now poised to decipher the molecular mechanisms that underlie the intimate interactions between Chlamydia spp. and their hosts, which will open up many exciting avenues of research for these medically important pathogens.

Extracellular vesicles: masters of intercellular communication and potential clinical interventions

Intercellular signaling via extracellular vesicles (EVs) is an underappreciated modality of cell-cell crosstalk that enables cells to convey packages of complex instructions to specific recipient cells. EVs transmit these instructions through their cargoes of multiple proteins, nucleic acids, and specialized lipids, which are derived from their cells of origin and allow for combinatorial effects upon recipient cells. This Review series brings together the recent progress in our understanding of EV signaling in physiological and pathophysiological conditions, highlighting how certain EVs, particularly exosomes, can promote or regulate infections, host immune responses, development, and various diseases - notably cancer. Given the diverse nature of EVs and their abilities to profoundly modulate host cells, this series puts particular emphasis on the clinical applications of EVs as therapeutics and as diagnostic biomarkers.

Extracellular vesicles and infectious diseases: new complexity to an old story

Exosomes and other extracellular microvesicles (ExMVs) have important functions in intercellular communication and regulation. During the course of infection, these vesicles can convey pathogen molecules that serve as antigens or agonists of innate immune receptors to induce host defense and immunity, or that serve as regulators of host defense and mediators of immune evasion. These molecules may include proteins, nucleic acids, lipids, and carbohydrates. Pathogen molecules may be disseminated by incorporation into vesicles that are created and shed by host cells, or they may be incorporated into vesicles shed from microbial cells. Involvement of ExMVs in the induction of immunity and host defense is widespread among many pathogens, whereas their involvement in immune evasion mechanisms is prominent among pathogens that establish chronic infection and is found in some that cause acute infection. Because of their immunogenicity and enrichment of pathogen molecules, exosomes may also have potential in vaccine preparations and as diagnostic markers. Additionally, the ability of exosomes to deliver molecules to recipient cells raises the possibility of their use for drug/therapy delivery. Thus, ExMVs play a major role in the pathogenesis of infection and provide exciting potential for the development of novel diagnostic and therapeutic approaches.

Membrane contact sites between pathogen-containing compartments and host organelles

Intracellular pathogens survive and replicate within specialised membrane-bound compartments that can be considered as pseudo-organelles. Using the obligate intracellular bacterium Chlamydia as an illustrative example, we consider the modes of lipid transport between pathogen-containing compartments and host organelles, including the formation of static membrane contact sites. We discuss how lipid scavenging can be mediated via the reprogramming of cellular transporters at these interfaces and describe recent data suggesting that pathogen effectors modulate the formation of specific membrane contacts. Further study of these emerging mechanisms is likely to yield new insights into the cell biology of lipid transport and organelle communication, which highlights potential new targets and strategies for future therapeutics. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.