Membrane contact sites between pathogen-containing compartments and host organelles

The novel bacterial effector protein CbEPF1 mediates ER-LD membrane contacts to regulate host lipid droplet metabolism

Effective intracellular communication between cellular organelles is pivotal for maintaining cellular homeostasis. Tether proteins, which are responsible for establishing membrane contact sites between cell organelles, enable direct communication between organelles and ultimately influence organelle function and host cell homeostasis. While recent research has identified tether proteins in several bacterial pathogens, their functions have predominantly been associated with mediating inter-organelle communication specifically between the bacteria containing vacuole (BCV) and the host endoplasmic reticulum (ER). However, this study reveals a novel bacterial effector protein, CbEPF1, which acts as a molecular tether beyond the confines of the BCV and facilitates interactions between host cell organelles. Coxiella burnetii, an obligate intracellular bacterial pathogen, encodes the FFAT motif-containing protein CbEPF1 which localizes to host lipid droplets (LDs). CbEPF1 establishes inter-organelle contact sites between host LDs and the ER through its interactions with VAP family proteins. Intriguingly, CbEPF1 modulates growth of host LDs in a FFAT motif-dependent manner. These findings highlight the potential for bacterial effector proteins to impact host cellular homeostasis by manipulating inter-organelle communication beyond conventional BCVs.

Mitochondria shed their outer membrane in response to infection-induced stress

[Figure: see text].

Contact and competition between mitochondria and microbes

Invading microbes occupy the host cytosol and take up nutrients on which host organelles are also dependent. Thus, host organelles are poised to interact with intracellular microbes. Despite the essential role of host mitochondria in cellular metabolic homeostasis and in mediating cellular responses to microbial infection, we know little of how these organelles interact with intracellular pathogens, and how such interactions affect disease pathogenesis. Here, we give an overview of the different classes of physical and metabolic interactions reported to occur between mitochondria and eukaryotic pathogens. Investigating the underlying molecular mechanisms and functions of such interactions will reveal novel aspects of infection biology.

The role of membrane contact sites at the bacteria-host interface

Membrane contact sites (MCSs) refer to the areas of close proximity between heterologous membranes. A growing body of evidence indicates that MCSs are involved in important cellular functions, such as cellular material transfer, organelle biogenesis, and cell growth. Importantly, the study of MCSs at the bacteria-host interface is an emerging popular research topic. Intracellular bacterial pathogens have evolved a variety of fascinating strategies to interfere with MCSs by injecting effectors into infected host cells. Bacteria-containing vacuoles establish direct physical contact with organelles within the host, ensuring vacuolar membrane integrity and energy supply from host organelles and protecting the vacuoles from the host endocytic pathway and lysosomal degradation. An increasing number of bacterial effectors from various bacterial pathogens hijack components of host MCSs to form the vacuole-organelle MCSs for material exchange. MCS-related events have been identified as new mechanisms of microbial pathogenesis to greatly improve bacterial survival and replication within host cells. In this review, we will discuss the recent advances in MCSs at the bacteria-host interface, focussing on the roles of MCSs mediated by bacterial effectors in microbial pathogenesis.

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.

A Chlamydia effector combining deubiquitination and acetylation activities induces Golgi fragmentation

Pathogenic bacteria are armed with potent effector proteins that subvert host signalling processes during infection¹. The activities of bacterial effectors and their associated roles within the host cell are often poorly understood, particularly for Chlamydia trachomatis², a World Health Organization designated neglected disease pathogen. We identify and explain remarkable dual Lys63-deubiquitinase (DUB) and Lys-acetyltransferase activities in the Chlamydia effector ChlaDUB1. Crystal structures capturing intermediate stages of each reaction reveal how the same catalytic centre of ChlaDUB1 can facilitate such distinct processes, and enable the generation of mutations that uncouple the two activities. Targeted Chlamydia mutant strains allow us to link the DUB activity of ChlaDUB1 and the related, dedicated DUB ChlaDUB2 to fragmentation of the host Golgi apparatus, a key process in Chlamydia infection for which effectors have remained elusive. Our work illustrates the incredible versatility of bacterial effector proteins, and provides important insights towards understanding Chlamydia pathogenesis.

Fighting for Resources: Who Started the Battle? Who Is Winning It?

Several vacuolar bacteria and parasites, such as Legionella, Chlamydia, and Toxoplasma, have been reported to grow associated with host mitochondria. The reason behind this phenomenon remains elusive. In this issue of Cell Metabolism, Pernas et al. (2018) propose that fusion of host mitochondria limits the availability of fatty acids needed for Toxoplasma gondii replication.

Mitochondria Restrict Growth of the Intracellular Parasite Toxoplasma gondii by Limiting Its Uptake of Fatty Acids

How intracellular pathogens acquire essential non-diffusible host metabolites and whether the host cell counteracts the siphoning of these nutrients by its invaders are open questions. Here we show that host mitochondria fuse during infection by the intracellular parasite Toxoplasma gondii to limit its uptake of fatty acids (FAs). A combination of genetics and imaging of FA trafficking indicates that Toxoplasma infection triggers lipophagy, the autophagy of host lipid droplets (LDs), to secure cellular FAs essential for its proliferation. Indeed, Toxoplasma FA siphoning and growth are reduced in host cells genetically deficient for autophagy or triglyceride depots. Conversely, Toxoplasma FA uptake and proliferation are increased in host cells lacking mitochondrial fusion, required for efficient mitochondrial FA oxidation, or where mitochondrial FA oxidation is pharmacologically inhibited. Thus, mitochondrial fusion can be regarded as a cellular defense mechanism against intracellular parasites, by limiting Toxoplasma access to host nutrients liberated by lipophagy.

Ubc7/Ube2g2 ortholog in Entamoeba histolytica: connection with the plasma membrane and phagocytosis

Endoplasmic reticulum (ER)-associated degradation (ERAD) and unfolded protein response (UPR) pathways are important for quality and quantity control of membrane and secretory proteins. We have identified orthologs of ER-associated ubiquitin conjugating enzymes (E2s) Ubc6/Ube2j2 and Ubc7/Ube2g2, ubiquitin ligases (E3) Hrd1 and GP78/AMFR, and sensor of UPR, Ire1 in E. histolytica that show conservation of important features of these proteins. Biochemical characterization of the ortholog of ERAD E2, Ubc7/Ube2g2 (termed as EhUbc7), was carried out. This E2 was transcriptionally upregulated several folds upon induction of UPR with tunicamycin. Ire1 ortholog was also upregulated upon UPR induction suggesting a linked UPR and ERAD pathway in this organism. EhUbc7 showed enzymatic activity and, similar to its orthologs in higher eukaryotes, formed polyubiquitin chains in vitro and localized to both cytoplasm and membranes. However, unlike its ortholog in higher eukaryotes, it also showed localization to the plasma membrane along with calreticulin. Inactivation of EhUbc7 significantly inhibited erythrophagocytosis, suggesting a novel function that has not been reported before for this E2. No change in growth, motility, or cell-surface expression of Gal/GalNAC lectin was observed due to inactivation of EhUbc7. The protein was present in the phagocytic cups but not in the phagosomes. A significant decrease in the number of phagocytic cups in inactive EhUbc7 expressing cells was observed, suggesting altered kinetics of phagocytosis. These findings have implications for evolutionary and mechanistic understanding of connection between phagocytosis and ER-associated proteins.

Phosphatidylserine decarboxylase CT699, lysophospholipid acyltransferase CT775, and acyl-ACP synthase CT776 provide membrane lipid diversity to Chlamydia trachomatis

De novo lipid synthesis and scavenging of fatty acids (FA) are processes essential for the formation of the membrane of the human pathogen Chlamydia trachomatis (C.t.). Host FA are assimilated via esterification by the bacterial acyl-acyl carrier protein (ACP) synthase AasC but inhibitors of the host acyl-CoA synthetase enymes ACSL also impaired growth of C.t. in human cells. In E. coli, activity of AasC was sensitive to triacsin C and rosiglitazone G. The absence of a triacsin C-insensitive pathway and the increased inhibition by rosiglitazone G confirmed the sensitivity of the bacterial acyl-ACP synthase to these drugs in infected human cells. We found no evidence that the human ACSL enzymes are required for lipid formation by C.t. The broad substrate specificity of acyltransferase CT775 provides C.t. with the capacity to incorporate straight-chain and bacterial specific branched-chain fatty acids. CT775 accepts both acyl-ACP and acyl-CoA as acyl donors and, 1- or 2-acyl isomers of lysophosphoplipids as acyl acceptors. The enzyme responsible for remodeling of human phosphatidylserine to bacterial phosphatidylethanolamine was identified as CT699. These findings provide evidence that the pathogen has the ability to extend the lipid diversity of its membrane.

The ER phagosome connection in the era of membrane contact sites

Phagocytosis is an essential mechanism through which innate immune cells ingest foreign material that is either destroyed or used to generate and present antigens and initiate adaptive immune responses. While a role for the ER during phagosome biogenesis has been recognized, whether fusion with ER cisternae or vesicular derivatives occurs has been the source of much contention. Membrane contact sites (MCS) are tight appositions between ER membranes and various organelles that coordinate multiple functions including localized signalling, lipid transfer and trafficking. The discovery that MCS form between the ER and phagosomes now begs the question of whether MCS play a role in connecting the ER to phagosomes under different contexts. In this review, we consider the implications of MCS between the ER and phagosomes during cross-presentation and infection with intracellular pathogens. We also discuss the similarities between these contacts and those between the ER and plasma membrane and acidic organelles such as endosomes and lysosomes. This article is part of a Special Issue entitled: Membrane Contact Sites edited by Christian Ungermann and Benoit Kornmann.

Phosphorylation of Golgi Peripheral Membrane Protein Grasp65 Is an Integral Step in the Formation of the Human Cytomegalovirus Cytoplasmic Assembly Compartment

Human cytomegalovirus (HCMV) is the largest member of the Herpesviridae and represents a significant cause of disease. During virus replication, HCMV alters cellular functions to facilitate its replication, including significant reorganization of the secretory and endocytic pathways of the infected cell. A defining morphologic change of the infected cell is the formation of a membranous structure in the cytoplasm that is designated the virion assembly compartment (AC), which consists of virion structural proteins surrounded by cellular membranes. The loss of normal Golgi compartment morphology and its relocalization from a juxtanuclear ribbonlike structure to a series of concentric rings on the periphery of the AC represents a readily recognized reorganization of cellular membranes in the HCMV-infected cell. Although trafficking of viral proteins to this compartment is required for the assembly of infectious virions, the functional significance of the reorganization of intracellular membranes like the Golgi membranes into the AC in the assembly of infectious virus remains understudied. In this study, we determined that Golgi membrane ribbon fragmentation increased during the early cytoplasmic phase of virion assembly and that Golgi membrane fragmentation in infected cells was dependent on the phosphorylation of an integral cis-Golgi protein, Grasp65. Inhibition of Golgi membrane fragmentation and of its reorganization into the AC resulted in decreased production of infectious particles and alteration of the incorporation of an essential protein into the envelope of the mature virion. These results demonstrated the complexity of the virus-host cell interactions required for efficient assembly of this large DNA virus.

The human cytomegalovirus (HCMV)-induced reorganization of intracellular membranes that is required for the formation of the viral assembly compartment (AC) has been an area of study over the last 20 years. The significance of this virus-induced structure has been evinced by the results of several studies which showed that relocalization of viral proteins to the AC was required for efficient assembly of infectious virus. In this study, we have identified a mechanism for the fragmentation of the Golgi ribbon in the infected cell en route to AC morphogenesis. Identification of this fundamental process during HCMV replication allowed us to propose that the functional role of Golgi membrane reorganization during HCMV infection was the concentration of viral structural proteins and subviral structures into a single intracellular compartment in order to facilitate efficient protein-protein interactions and the virion protein trafficking required for the assembly of this large and structurally complex virus.

Host Lipid Bodies as Platforms for Intracellular Survival of Protozoan Parasites

Pathogens induce several changes in the host cell signaling and trafficking mechanisms in order to evade and manipulate the immune response. One prominent pathogen-mediated change is the formation of lipid-rich organelles, termed lipid bodies (LBs) or lipid droplets, in the host cell cytoplasm. Protozoan parasites, which contribute expressively to the burden of infectious diseases worldwide, are able to induce LB genesis in non-immune and immune cells, mainly macrophages, key players in the initial resistance to the infection. Under host–parasite interaction, LBs not only accumulate in the host cytoplasm but also relocate around and move into parasitophorous vacuoles. There is increasing evidence that protozoan parasites may target host-derived LBs either for gaining nutrients or for escaping the host immune response. Newly formed, parasite-induced LBs may serve as lipid sources for parasite growth and also produce inflammatory mediators that potentially act in the host immune response deactivation. In this mini review, we summarize current knowledge on the formation and role of host LBs as sites exploited by intracellular protozoan parasites as a strategy to maintain their own survival.