Sequestration of host metabolism by an intracellular pathogen

Biotic interactions as drivers of algal origin and evolution

Contents 670 I. 671 II. 671 III. 676 IV. 678 678 References 678 SUMMARY: Biotic interactions underlie life's diversity and are the lynchpin to understanding its complexity and resilience within an ecological niche. Algal biologists have embraced this paradigm, and studies building on the explosive growth in omics and cell biology methods have facilitated the in-depth analysis of nonmodel organisms and communities from a variety of ecosystems. In turn, these advances have enabled a major revision of our understanding of the origin and evolution of photosynthesis in eukaryotes, bacterial-algal interactions, control of massive algal blooms in the ocean, and the maintenance and degradation of coral reefs. Here, we review some of the most exciting developments in the field of algal biotic interactions and identify challenges for scientists in the coming years. We foresee the development of an algal knowledgebase that integrates ecosystem-wide omics data and the development of molecular tools/resources to perform functional analyses of individuals in isolation and in populations. These assets will allow us to move beyond mechanistic studies of a single species towards understanding the interactions amongst algae and other organisms in both the laboratory and the field.

Chlamydia trachomatis Prevents Apoptosis Via Activation of PDPK1-MYC and Enhanced Mitochondrial Binding of Hexokinase II

The intracellular human bacterial pathogen Chlamydia trachomatis pursues effective strategies to protect infected cells against death-inducing stimuli. Here, we show that Chlamydia trachomatis infection evokes 3-phosphoinositide-dependent protein kinase-1 (PDPK1) signaling to ensure the completion of its developmental cycle, further leading to the phosphorylation and stabilization of MYC. Using biochemical approaches and imaging we demonstrate that Chlamydia-induced PDPK1-MYC signaling induces host hexokinase II (HKII), which becomes enriched and translocated to the mitochondria. Strikingly, preventing the HKII interaction with mitochondria using exogenous peptides triggers apoptosis of infected cells as does inhibiting either PDPK1 or MYC, which also disrupts intracellular development of Chlamydia trachomatis. These findings identify a previously unknown pathway activated by Chlamydia infection, which exhibits pro-carcinogenic features. Targeting the PDPK1-MYC-HKII-axis may provide a strategy to overcome therapeutic resistance of infection.

Crossing the border - Solute entry into the chlamydial inclusion

Chlamydiales comprise important human and animal pathogens as well as endosymbionts of amoebae. Generally, these obligate intracellular living bacteria are characterized by a biphasic developmental cycle, a reduced genome and a restricted metabolic capacity. Because of their metabolic impairment, Chlamydiales essentially rely on the uptake of diverse metabolites from their hosts. Chlamydiales thrive in a special compartment, the inclusion, and hence are surrounded by an additional membrane. Solutes might enter the inclusion through pores and open channels or by redirection of host vesicles, which fuse with the inclusion membrane and release their internal cargo. Recent investigations shed new light on the chlamydia-host interaction and identified an additional way for nutrient uptake into the inclusion. Proteome studies and targeting analyses identified chlamydial and host solute carriers in inclusions of Chlamydia trachomatis infected cells. These transporters are involved in the provision of UDP-glucose and biotin, and probably deliver further metabolites to the inclusion. By the controlled recruitment of specific solute carriers to the inclusion, the chlamydial resident thus can actively manipulate the metabolite availability and composition in the inclusion. This review summarizes recent findings and new ideas on carrier mediated solute uptake into the chlamydial inclusion in the context of the bacterial and host metabolism.

Unexpected genomic features in widespread intracellular bacteria: evidence for motility of marine chlamydiae

Chlamydiae are obligate intracellular bacteria comprising important human pathogens and symbionts of protists. Molecular evidence indicates a tremendous diversity of chlamydiae particularly in marine environments, yet our current knowledge is based mainly on terrestrial representatives. Here we provide first insights into the biology of marine chlamydiae representing three divergent clades. Our analysis of single-cell amplified genomes revealed hallmarks of the chlamydial lifestyle, supporting the ancient origin of their characteristic developmental cycle and major virulence mechanisms. Surprisingly, these chlamydial genomes encode a complete flagellar apparatus, a previously unreported feature. We show that flagella are an ancient trait that was subject to differential gene loss among extant chlamydiae. Together with a chemotaxis system, these marine chlamydiae are likely motile, with flagella potentially playing a role during host cell infection. This study broadens our view on chlamydial biology and indicates a largely underestimated potential to adapt to different hosts and environments.

The Chlamydia trachomatis type III secretion substrates CT142, CT143, and CT144 are secreted into the lumen of the inclusion

Chlamydia trachomatis is a human bacterial pathogen causing ocular and genital infections. It multiplies exclusively within an intracellular membrane-bound vacuole, the inclusion, and uses a type III secretion system to manipulate host cells by injecting them with bacterially-encoded effector proteins. In this work, we characterized the expression and subcellular localization in infected host cells of the C. trachomatis CT142, CT143, and CT144 proteins, which we previously showed to be type III secretion substrates. Transcriptional analyses in C. trachomatis confirmed the prediction that ct142, ct143 and ct144 are organized in an operon and revealed that their expression is likely driven by the main σ factor, σ⁶⁶. In host cells infected by C. trachomatis, production of CT142 and CT143 could be detected by immunoblotting from 20–26 h post-infection. Immunofluorescence microscopy of infected cells revealed that from 20 h post-infection CT143 appeared mostly as globular structures outside of the bacterial cells but within the lumen of the inclusion. Furthermore, immunofluorescence microscopy of cells infected by C. trachomatis strains carrying plasmids producing CT142, CT143, or CT144 under the control of the ct142 promoter and with a C-terminal double hemagglutinin (2HA) epitope tag revealed that CT142-2HA, CT143-2HA or CT144-2HA showed an identical localization to chromosomally-encoded CT143. Moreover, CT142-2HA or CT144-2HA and CT143 produced by the same bacteria co-localized in the lumen of the inclusion. Overall, these data suggest that the CT142, CT143, and CT144 type III secretion substrates are secreted into the lumen of the inclusion where they might form a protein complex.

The DUF582 Proteins of Chlamydia trachomatis Bind to Components of the ESCRT Machinery, Which Is Dispensable for Bacterial Growth In vitro

Chlamydiae are Gram negative bacteria that develop exclusively inside eukaryotic host cells, within a membrane-bounded compartment. Members of the family Chlamydiaceae, such as Chlamydia trachomatis, are pathogenic species infecting vertebrates. They have a very reduced genome and exploit the capacities of their host for their own development, mainly through the secretion of proteins tailored to interfere with eukaryotic processes, called effector proteins. All Chlamydiaceae possess genes coding for four to five effectors that share a domain of unknown function (DUF582). Here we show that four of these effectors, which represent the conserved set in all Chlamydiaceae, accumulate in the infectious form of C. trachomatis, and are therefore likely involved in an early step of the developmental cycle. The fifth member of the family, CT621, is specific to C. trachomatis, and is secreted during the growth phase. Using a two-hybrid screen in yeast we identified an interaction between the host protein Hrs and the DUF582, which we confirmed by co-immunoprecipitations in co-transfected mammalian cells. Furthermore, we provide biochemical evidence that a second domain of one of the DUF582 proteins, CT619, binds the host protein Tsg101. Hrs and Tsg101 are both implicated in a well conserved machinery of the eukaryotic cell called the ESCRT machinery, which is involved in several cellular processes requiring membrane constriction. Using RNA interference targeting proteins implicated at different stages of ESCRT-driven processes, or inhibition by expression of a dominant negative mutant of VPS4, we demonstrated that this machinery was dispensable for bacterial entry, multiplication and differentiation into infectious progeny, and for uptake of glycogen into the parasitophorous vacuole. In light of these observations we discuss how the DUF582 proteins might target the ESCRT machinery during infection.

Chlamydial Plasmid-Dependent Pathogenicity

Most Chlamydia species carry a 7.5kb plasmid encoding eight open reading frames conventionally called plasmid glycoproteins 1-8 or pGP1-8. Although the plasmid is not critical for chlamydial growth in vitro, its role in chlamydial pathogenesis is clearly demonstrated in the genital tracts of mice infected with Chlamydia muridarum, a model for investigating the human pathogen Chlamydia trachomatis. Plasmid-free C. trachomatis is also attenuated in both the mouse genital tract and nonhuman primate ocular tissue. Deficiency in pGP3 alone, which is regulated by pGP4, largely reproduced the in vivo but not in vitro phenotypes of the plasmid-free organisms, suggesting that pGP3 is a key in vivo virulence factor. The positive and negative regulations of some chromosomal genes by pGP4 and pGP5, respectively, may allow the plasmid to promote chlamydial adaptation to varied animal tissue environments. The focus of this review is to summarize the progress on the pathogenic functions of the plasmid-encoded open reading frames, which may motivate further investigation of the molecular mechanisms of chlamydial pathogenicity and development of medical utility of the chlamydial plasmid system.

Was the Chlamydial Adaptative Strategy to Tryptophan Starvation an Early Determinant of Plastid Endosymbiosis?

Chlamydiales were recently proposed to have sheltered the future cyanobacterial ancestor of plastids in a common inclusion. The intracellular pathogens are thought to have donated those critical transporters that triggered the efflux of photosynthetic carbon and the consequent onset of symbiosis. Chlamydiales are also suspected to have encoded glycogen metabolism TTS (Type Three Secretion) effectors responsible for photosynthetic carbon assimilation in the eukaryotic cytosol. We now review the reasons underlying other chlamydial lateral gene transfers evidenced in the descendants of plastid endosymbiosis. In particular we show that half of the genes encoding enzymes of tryptophan synthesis in Archaeplastida are of chlamydial origin. Tryptophan concentration is an essential cue triggering two alternative modes of replication in Chlamydiales. In addition, sophisticated tryptophan starvation mechanisms are known to act as antibacterial defenses in animal hosts. We propose that Chlamydiales have donated their tryptophan operon to the emerging plastid to ensure increased synthesis of tryptophan by the plastid ancestor. This would have allowed massive expression of the tryptophan rich chlamydial transporters responsible for symbiosis. It would also have allowed possible export of this valuable amino-acid in the inclusion of the tryptophan hungry pathogens. Free-living single cell cyanobacteria are devoid of proteins able to transport this amino-acid. We therefore investigated the phylogeny of the Tyr/Trp transporters homologous to E. coli TyrP/Mre and found yet another LGT from Chlamydiales to Archaeplastida thereby considerably strengthening our proposal.

One Face of Chlamydia trachomatis: The Infectious Elementary Body

The lifestyle of Chlamydiae is unique: the bacteria alternate between two morphologically distinct forms, an infectious non-replicative elementary body (EB), and a replicative, non-infectious reticulate body (RB). This review focuses on recent advances in understanding the structure and function of the infectious form of the best-studied member of the phylum, the human pathogen Chlamydia trachomatis. Once considered as an inert particle of little functional capacity, the EB is now perceived as a sophisticated entity that encounters at least three different environments during each infectious cycle. We review current knowledge on its composition and morphology, and emerging metabolic activities. These features confer resistance to the extracellular environment, the ability to penetrate a host cell and ultimately enable the EB to establish a niche enabling bacterial survival and growth. The bacterial and host molecules involved in these processes are beginning to emerge.