Selective fragmentation of the trans-Golgi apparatus by Rickettsia rickettsii

Rickettsia rickettsii RoaM negatively regulates expression of a limited number of rickettsial genes

The recently described rickettsial protein RoaM (regulator of actin-based motility) negatively regulates the production of actin tails, and its abrogation induces hyper-spreading behavior in many laboratory-adapted strains of Rickettsia rickettsii. RoaM is not surface exposed; thus, its mechanism of regulating actin-based motility is unclear. Using R. rickettsii strains derived from the virulent Sheila Smith strain that express varying levels of roaM, an RNA-seq experiment was performed. We found that roaM-overexpressing strains downregulate expression of at least six genes which may link the regulatory effects of RoaM to the phenotypic effect on motility. Genes regulated by RoaM were confirmed by RT-qPCR. Among the genes regulated is the secreted effector RarP2, which disrupts the trans-Golgi network. Two of the hypothetical proteins were shown to be secreted via fusion to a glycogen synthase kinase tag, which when phosphorylated reveals exposure to the host-cell cytosol. Taken together, these data support the hypothesis that RoaM affects transcription, downregulating rickettsial genes important for pathogenicity in the mammalian host but which are perhaps otherwise detrimental within the tick vector. To determine how RoaM activity may itself be regulated, we investigated a role of temperature in roaM transcription. RoaM expression itself is not temperature dependent, but many other rickettsial genes are, including some also regulated by RoaM. This suggests that rickettsiae utilize multiple mechanisms to control gene expression in response to environmental signals.

IMPORTANCE

RoaM was previously shown to repress the production of actin tails by unknown mechanisms. The roaM gene is negatively selected for in cell culture resulting in hyper-spreading mutants. This work reveals that rather than specifically regulating motility in Rickettsia rickettsii, a set of rickettsial genes is downregulated that includes the type IV secreted effector, rarP2, as well as two other secreted, putative effectors. Relatively few secreted effectors have been identified in Rickettsia. RoaM appears to be part of a larger biological program encompassing active spreading in mammalian cells and may be a critical component for R. rickettsii to transition from arthropod to mammalian host.

Exploring the relationship between sepsis and Golgi apparatus dysfunction: bioinformatics insights and diagnostic marker discovery

Background

Sepsis, a critical infectious disease, is intricately linked to the dysfunction of the intracellular Golgi apparatus. This study aims to explore the relationship between sepsis and the Golgi apparatus using bioinformatics, offering fresh insights into its diagnosis and treatment.

Methods

To explore the role of Golgi-related genes in sepsis, we analyzed mRNA expression profiles from the NCBI GEO database. We identified differentially expressed genes (DEGs). These DEGs, Golgi-associated genes obtained from the MSigDB database, and key modules identified through WGCNA were intersected to determine Golgi-associated differentially expressed genes (GARGs) linked to sepsis. Subsequently, functional enrichment analyses, including GO, KEGG, and GSEA, were performed to explore the biological significance of the GARGs.A PPI network was constructed to identify core genes, and immune infiltration analysis was performed using the ssGSEA algorithm. To further evaluate immune microenvironmental features, unsupervised clustering was applied to identify immunological subgroups. A diagnostic model was developed using logistic regression, and its performance was validated using ROC curve analysis. Finally, key diagnostic biomarkers were identified and validated across multiple datasets to confirm their diagnostic accuracy.

Results

By intersecting DEGs, WGCNA modules, and Golgi-related gene sets, 53 overlapping GARGs were identified. Functional enrichment analysis indicated that these GARGs were predominantly involved in protein glycosylation and Golgi membrane-related processes. PPI analysis further identified eight hub genes: B3GNT5, FUT11, MFNG, ST3GAL5, MAN1C1, ST6GAL1, C1GALT1C1, and GALNT14. Immune infiltration analysis revealed significant differences in immune cell populations, mainly activated dendritic cells, and effector memory CD8+ T cells, between sepsis patients and healthy controls. A diagnostic model constructed using five pivotal genes (B3GNT5, FUT11, MAN1C1, ST6GAL1, and C1GALT1C1) exhibited predictive accuracy, with AUC values exceeding 0.96 for all genes. Validation with an independent dataset confirmed the differential expression patterns of B3GNT5, C1GALT1C1, and GALNT14, reinforcing their potential as robust diagnostic biomarkers for sepsis.

Conclusion

This study elucidates the link between sepsis and the Golgi apparatus, introduces novel biomarkers for sepsis diagnosis, and offers valuable insights for future research on its pathogenesis and treatment strategies.

Analysis of Acinetobacter P-type type IV secretion system-encoding plasmid diversity uncovers extensive secretion system conservation and diverse antibiotic resistance determinants

Acinetobacter baumannii is globally recognized as a multi-drug-resistant pathogen of critical concern due to its capacity for horizontal gene transfer and resistance to antibiotics. Phylogenetically diverse Acinetobacter species mediate human infection, including many considered as important emerging pathogens. While globally recognized as a pathogen of concern, pathogenesis mechanisms are poorly understood. P-type type IV secretion systems (T4SSs) represent important drivers of pathogen evolution, responsible for horizontal gene transfer and secretion of proteins that mediate host-pathogen interactions, contributing to pathogen survival, antibiotic resistance, virulence, and biofilm formation. Genes encoding a P-type T4SS were previously identified on plasmids harboring the carbapenemase gene blaNDM-1 in several clinically problematic Acinetobacter; however, their prevalence among the genus, geographical distribution, the conservation of T4SS proteins, and full capacity for resistance genes remain unclear. Using systematic analyses, we show that these plasmids belong to a group of 53 P-type T4SS-encoding plasmids in 20 established Acinetobacter species, the majority of clinical relevance, including diverse A. baumannii sequence types and one strain of Providencia rettgeri. The strains were globally distributed in 14 countries spanning five continents, and the conjugative operon's T4SS proteins were highly conserved in most plasmids. A high proportion of plasmids harbored resistance genes, with 17 different genes spanning seven drug classes. Collectively, this demonstrates that P-type T4SS-encoding plasmids are more widespread among the Acinetobacter genus than previously anticipated, including strains of both clinical and environmental importance. This research provides insight into the spread of resistance genes among Acinetobacter and highlights a group of plasmids of importance for future surveillance.

Diversity and structural-functional insights of alpha-solenoid proteins

Alpha-solenoids are a significant and diverse subset of structured tandem repeat proteins (STRPs) that are important in various domains of life. This review examines their structural and functional diversity and highlights their role in critical cellular processes such as signaling, apoptosis, and transcriptional regulation. Alpha-solenoids can be classified into three geometric folds: low curvature, high curvature, and corkscrew, as well as eight subfolds: ankyrin repeats; Huntingtin, elongation factor 3, protein phosphatase 2A, and target of rapamycin; armadillo repeats; tetratricopeptide repeats; pentatricopeptide repeats; Pumilio repeats; transcription activator-like; and Sel-1 and Sel-1-like repeats. These subfolds represent distinct protein families with unique structural properties and functions, highlighting the versatility of alpha-solenoids. The review also discusses their association with disease, highlighting their potential as therapeutic targets and their role in protein design. Advances in state-of-the-art structure prediction methods provide new opportunities and challenges in the functional characterization and classification of this kind of fold, emphasizing the need for continued development of methods for their identification and proper data curation and deposition in the main databases.

Development of inducible promoter and CRISPRi plasmids functional in Rickettsia rickettsii

Rickettsia rickettsii is an obligate intracellular, tick-borne bacterium that causes Rocky Mountain spotted fever. The demanding nature of cultivating these bacteria within host cells and the labor involved in obtaining clonal isolates have severely limited progress regarding the development of compatible genetic tools to study this pathogen. Conditional expression of genes that might be toxic or have an otherwise undesirable effect is the next logical goal to expand upon the constitutive expression plasmids generated thus far. We describe the construction of an inducible promoter system based on the tet-On system, leveraging design elements from the anhydrotetracycline-inducible promoter system used for Borrelia burgdorferi and one of the few characterized rickettsial promoters for the outer membrane gene, rompB (sca5). The functionality of this promoter is demonstrated via fluorescence of induced mScarlet production and was then used to construct a generalized inducible expression vector for R. rickettsii. The development of a functional inducible promoter was then applied to the construction of a CRISPR interference plasmid as a means to reduce or essentially silence the transcription of targeted genes. We demonstrate the viability of a simplified, single vector CRISPRi system to disrupt gene expression in R. rickettsii targeting the type IV secreted effector rarP2 and autotransporter peptidase rapL as examples.

IMPORTANCE

This work expands upon the genetic toolbox available for R. rickettsii. We describe both an inducible promoter and CRISPRi system compatible with Rickettsia, which may provide key instruments for the development of further tools. The development of an inducible promoter system allows for the overexpression of genes, which might be toxic when expressed constitutively. The CRISPRi system enables the ability to knock down genes with specificity, and critically, genes that may be essential and could not otherwise be knocked out. These developments may provide the foundation for unlocking genetic tools for other pathogens of the order Rickettsiales, such as the Anaplasma, Orientia, and Ehrlichia for which there are currently no inducible promoters or CRISPRi platforms.

Golgiphagy: a novel selective autophagy to the fore

The Golgi apparatus is the central hub of the cellular endocrine pathway and plays a crucial role in processing, transporting, and sorting proteins and lipids. Simultaneously, it is a highly dynamic organelle susceptible to degradation or fragmentation under various physiological or pathological conditions, potentially contributing to the development of numerous human diseases. Autophagy serves as a vital pathway for eukaryotes to manage intracellular and extracellular stress and maintain homeostasis by targeting damaged or redundant organelles for removal. Recent research has revealed that autophagy mechanisms can specifically degrade Golgi components, known as Golgiphagy. This review summarizes recent findings on Golgiphagy while also addressing unanswered questions regarding its mechanisms and regulation, aiming to advance our understanding of the role of Golgiphagy in human disease.

crANKing up the infection: ankyrin domains in Rickettsiales and their role in host manipulation

Intracellular bacteria use secreted effector proteins to modify host biology and facilitate infection. For many of these microbes, a particular eukaryotic domain-the ankyrin repeat (ANK)-plays a central role in specifying the host proteins and pathways targeted by the microbe. While we understand much of how some ANKs function in model organisms like Legionella and Coxiella, the understudied Rickettsiales species harbor many proteins with ANKs, some of which play critical roles during infection. This minireview is meant to organize and summarize the research progress made in understanding some of these Rickettsiales ANKs as well as document some of the techniques that have driven much of this progress.

Host cell manipulation by microsporidia secreted effectors: Insights into intracellular pathogenesis

Microsporidia are prolific producers of effector molecules, encompassing both proteins and nonproteinaceous effectors, such as toxins, small RNAs, and small peptides. These secreted effectors play a pivotal role in the pathogenicity of microsporidia, enabling them to subvert the host's innate immunity and co-opt metabolic pathways to fuel their own growth and proliferation. However, the genomes of microsporidia, despite falling within the size range of bacteria, exhibit significant reductions in both structural and physiological features, thereby affecting the repertoire of secretory effectors to varying extents. This review focuses on recent advances in understanding how microsporidia modulate host cells through the secretion of effectors, highlighting current challenges and proposed solutions in deciphering the complexities of microsporidial secretory effectors.

Cell-selective proteomics reveal novel effectors secreted by an obligate intracellular bacterial pathogen

Pathogenic bacteria secrete protein effectors to hijack host machinery and remodel their infectious niche. Rickettsia spp. are obligate intracellular bacteria that can cause life-threatening disease, but their absolute dependence on the host cell has impeded discovery of rickettsial effectors and their host targets. We implemented bioorthogonal non-canonical amino acid tagging (BONCAT) during R. parkeri infection to selectively label, isolate, and identify effectors delivered into the host cell. As the first use of BONCAT in an obligate intracellular bacterium, our screen more than doubles the number of experimentally validated effectors for the genus. The seven novel secreted rickettsial factors (Srfs) we identified include Rickettsia-specific proteins of unknown function that localize to the host cytoplasm, mitochondria, and ER. We further show that one such effector, SrfD, interacts with the host Sec61 translocon. Altogether, our work uncovers a diverse set of previously uncharacterized rickettsial effectors and lays the foundation for a deeper exploration of the host-pathogen interface.

Cycloheptylprodigiosin from marine bacterium Spartinivicinus ruber MCCC 1K03745T induces a novel form of cell death characterized by Golgi disruption and enhanced secretion of cathepsin D in non-small cell lung cancer cell lines

Prodiginines have been studied extensively for their anticancer activity, however, the majority of the research has focused on prodigiosin. In this study, cycloheptylprodigiosin (S-1) is extracted from marine bacterium Spartinivicinus ruber MCCC 1K03745T, and its anticancer property was investigated. It exhibits remarkable cytotoxicity against a panel of human lung cancer cell lines, with the IC50 values ranging from 84.89 nM to 661.2 nM. After 6 h of treatment, S-1 gradually accumulates on mitochondria and lysosomes. While lower doses of S-1 induce cell cycle arrest, treatment with higher doses results in cell death in apoptotic independent manner in both NCI-H1299 and NCI-H460 cell lines. Interestingly, treatment with S-1 leads to the accumulation of LC3B-II via pathways that vary among different cell lines. In addition to its role as an autophagy inhibitor, S-1 also promotes autophagy initiation as demonstrated by the increment of EGFP fragment in the EGFP-LC3 degradation assay, however, inhibition of autophagy does not rescue cells from death induced by S-1. Mechanistically, S-1 impairs autophagic flux through disrupting acidic lysosomal pH and blocking the maturation of cathepsin D. Moreover, treatment with S-1 enhanced secretion of both pro- and mature forms of cathepsin D, coincident with disintegration of trans-Golgi network. Interestingly, S-1 does not induce ferroptosis, pyroptosis or necroptosis in NCI-H1299 cells. However, treatment of NCI-H460 cells with S-1 induces methuosis, which can be suppressed by Rac1 inhibitor EHT 1864. Our data demonstrate that S-1 is an effective anticancer agent with potential therapeutic application.

Metagenome diversity illuminates the origins of pathogen effectors

Recent metagenome-assembled genome (MAG) analyses have profoundly impacted Rickettsiology systematics. The discovery of basal lineages (novel families Mitibacteraceae and Athabascaceae) with predicted extracellular lifestyles exposed an evolutionary timepoint for the transition to host dependency, which seemingly occurred independent of mitochondrial evolution. Notably, these basal rickettsiae carry the Rickettsiales vir homolog (rvh) type IV secretion system and purportedly use rvh to kill congener microbes rather than parasitize host cells as described for later-evolving rickettsial pathogens. MAG analysis also substantially increased diversity for the genus Rickettsia and delineated a sister lineage (the novel genus Tisiphia) that stands to inform on the emergence of human pathogens from protist and invertebrate endosymbionts. Herein, we probed Rickettsiales MAG and genomic diversity for the distribution of Rickettsia rvh effectors to ascertain their origins. A sparse distribution of most Rickettsia rvh effectors outside of Rickettsiaceae lineages illuminates unique rvh evolution from basal extracellular species and other rickettsial families. Remarkably, nearly every effector was found in multiple divergent forms with variable architectures, indicating profound roles for gene duplication and recombination in shaping effector repertoires in Rickettsia pathogens. Lateral gene transfer plays a prominent role in shaping the rvh effector landscape, as evinced by the discovery of many effectors on plasmids and conjugative transposons, as well as pervasive effector gene exchange between Rickettsia and Legionella species. Our study exemplifies how MAGs can yield insight into pathogen effector origins, particularly how effector architectures might become tailored to the discrete host cell functions of different eukaryotic hosts.IMPORTANCEWhile rickettsioses are deadly vector-borne human diseases, factors distinguishing Rickettsia pathogens from the innumerable bevy of environmental rickettsial endosymbionts remain lacking. Recent metagenome-assembled genome (MAG) studies revealed evolutionary timepoints for rickettsial transitions to host dependency. The rvh type IV secretion system was likely repurposed from congener killing in basal extracellular species to parasitizing host cells in later-evolving pathogens. Our analysis of MAG diversity for over two dozen rvh effectors unearthed their presence in some non-pathogens. However, most effectors were found in multiple divergent forms with variable architectures, indicating gene duplication and recombination-fashioned effector repertoires of Rickettsia pathogens. Lateral gene transfer substantially shaped pathogen effector arsenals, evinced by the discovery of effectors on plasmids and conjugative transposons, as well as pervasive effector gene exchanges between Rickettsia and Legionella species. Our study exemplifies how MAGs yield insight into pathogen effector origins and evolutionary processes tailoring effectors to eukaryotic host cell biology.

Rickettsia rickettsii virulence determinants RARP2 and RapL mitigate IFN- signaling in primary human dermal microvascular endothelial cells

We compared the growth characteristics of a virulent Rickettsia rickettsii strain (Sheila Smith) to an attenuated R. rickettsii stain (Iowa) and a non-pathogenic species (R. montanensis) in primary human dermal microvascular endothelial cells (HDMEC). All replicated in Vero cells, however, only the Sheila Smith strain productively replicated in HDMECs. The Iowa strain showed minimal replication over a 24-h period, while R. montanensis lost viability and induced lysis of the HDMECs via a rapid programmed cell death response. Both the virulent and attenuated R. rickettsii strains, but not R. montanensis, induced an interferon-1 response, although the response was of lesser magnitude and delayed in the Sheila Smith strain. IFN-β secretion correlated with increased host cell lysis, and treatment with anti-IFNAR2 antibody decreased lysis from Iowa-infected but not Sheila Smith-infected cells. Both Sheila Smith- and Iowa-infected cells eventually lysed, although the response from Sheila Smith was delayed and showed characteristics of apoptosis. We, therefore, examined whether reconstitution of the Iowa strain with two recently described putative virulence determinants might enhance survival of Iowa within HDMECs. Reconstitution with RARP2, which is inhibitory to anterograde trafficking through the Golgi apparatus, reduced IFN-β secretion but had no effect on cell lysis. RapL, which proteolytically processes surface exposed autotransporters and enhances replication of Iowa in Guinea pigs, suppressed both IFN-β production and host cell lysis. These findings suggest distinct mechanisms by which virulent spotted fever group rickettsiae may enhance intracellular survival and replication.IMPORTANCEWe examined a naturally occurring non-pathogenic rickettsial species, R. montanensis, a laboratory-attenuated R. rickettsii strain (Iowa), and a fully virulent R. rickettsii strain (Sheila Smith) for growth in human dermal microvascular endothelial cells. The two avirulent strains replicated poorly or not at all. Only the virulent Sheila Smith strain replicated. IFN-β production correlated with the inhibition of R. rickettsii Iowa. Reconstitution of Iowa with either of two recently described putative virulence determinants altered the IFN-β response. A rickettsial ankyrin repeat protein, RARP2, disrupts the trans-Golgi network and inhibits IFN-β secretion. An autotransporter peptidase, RapL, restores proteolytic maturation of outer membrane autotransporters and diminishes the IFN-β response to enhance cell survival and permit replication of the recombinant strain. These studies point the way toward discovery of mechanisms for innate immune response avoidance by virulent rickettsia.

Ceramide-1-phosphate is a regulator of Golgi structure and is co-opted by the obligate intracellular bacterial pathogen Anaplasma phagocytophilum

Many intracellular pathogens structurally disrupt the Golgi apparatus as an evolutionarily conserved promicrobial strategy. Yet, the host factors and signaling processes involved are often poorly understood, particularly for Anaplasma phagocytophilum, the agent of human granulocytic anaplasmosis. We found that A. phagocytophilum elevated cellular levels of the bioactive sphingolipid, ceramide-1-phosphate (C1P), to promote Golgi fragmentation that enables bacterial proliferation, conversion from its non-infectious to infectious form, and productive infection. A. phagocytophilum poorly infected mice deficient in ceramide kinase, the Golgi-localized enzyme responsible for C1P biosynthesis. C1P regulated Golgi morphology via activation of a PKCα/Cdc42/JNK signaling axis that culminates in phosphorylation of Golgi structural proteins, GRASP55 and GRASP65. siRNA-mediated depletion of Cdc42 blocked A. phagocytophilum from altering Golgi morphology, which impaired anterograde trafficking of trans-Golgi vesicles into and maturation of the pathogen-occupied vacuole. Cells overexpressing phosphorylation-resistant versions of GRASP55 and GRASP65 presented with suppressed C1P- and A. phagocytophilum-induced Golgi fragmentation and poorly supported infection by the bacterium. By studying A. phagocytophilum, we identify C1P as a regulator of Golgi structure and a host factor that is relevant to disease progression associated with Golgi fragmentation.IMPORTANCECeramide-1-phosphate (C1P), a bioactive sphingolipid that regulates diverse processes vital to mammalian physiology, is linked to disease states such as cancer, inflammation, and wound healing. By studying the obligate intracellular bacterium Anaplasma phagocytophilum, we discovered that C1P is a major regulator of Golgi morphology. A. phagocytophilum elevated C1P levels to induce signaling events that promote Golgi fragmentation and increase vesicular traffic into the pathogen-occupied vacuole that the bacterium parasitizes. As several intracellular microbial pathogens destabilize the Golgi to drive their infection cycles and changes in Golgi morphology is also linked to cancer and neurodegenerative disorder progression, this study identifies C1P as a potential broad-spectrum therapeutic target for infectious and non-infectious diseases.

Pathogenic Rickettsia spp. as emerging models for bacterial biology

Our understanding of free-living bacterial models like Escherichia coli far outpaces that of obligate intracellular bacteria, which cannot be cultured axenically. All obligate intracellular bacteria are host-associated, and many cause serious human diseases. Their constant exposure to the distinct biochemical niche of the host has driven the evolution of numerous specialized bacteriological and genetic adaptations, as well as innovative molecular mechanisms of infection. Here, we review the history and use of pathogenic Rickettsia species, which cause an array of vector-borne vascular illnesses, as model systems to probe microbial biology. Although many challenges remain in our studies of these organisms, the rich pathogenic and biological diversity of Rickettsia spp. constitutes a unique backdrop to investigate how microbes survive and thrive in host and vector cells. We take a bacterial-focused perspective and highlight emerging insights that relate to new host-pathogen interactions, bacterial physiology, and evolution. The transformation of Rickettsia spp. from pathogens to models demonstrates how recalcitrant microbes may be leveraged in the lab to tap unmined bacterial diversity for new discoveries. Rickettsia spp. hold great promise as model systems not only to understand other obligate intracellular pathogens but also to discover new biology across and beyond bacteria.

Split but merge: Golgi fragmentation in physiological and pathological conditions

The Golgi complex is a highly dynamic and tightly regulated cellular organelle with essential roles in the processing as well as the sorting of proteins and lipids. Its structure undergoes rapid disassembly and reassembly during normal physiological processes, including cell division, migration, polarization, differentiation, and cell death. Golgi dispersal or fragmentation also occurs in pathological conditions, such as neurodegenerative diseases, infectious diseases, congenital disorders of glycosylation diseases, and cancer. In this review, current knowledge about both structural organization and morphological alterations in the Golgi in physiological and pathological conditions is summarized together with the methodologies that help to reveal its structure.

Orientia tsutsugamushi: comprehensive analysis of the mobilome of a highly fragmented and repetitive genome reveals the capacity for ongoing lateral gene transfer in an obligate intracellular bacterium

IMPORTANCE

Obligate intracellular bacteria, or those only capable of growth inside other living cells, have limited opportunities for horizontal gene transfer with other microbes due to their isolated replicative niche. The human pathogen Ot, an obligate intracellular bacterium causing scrub typhus, encodes an unusually high copy number of a ~40 gene mobile genetic element that typically facilitates genetic transfer across microbes. This proliferated element is heavily degraded in Ot and previously assumed to be inactive. Here, we conducted a detailed analysis of this element in eight Ot strains and discovered two strains with at least one intact copy. This implies that the element is still capable of moving across Ot populations and suggests that the genome of this bacterium may be even more dynamic than previously appreciated. Our work raises questions about intracellular microbial evolution and sounds an alarm for gene-based efforts focused on diagnosing and combatting scrub typhus.

Cell-selective proteomics reveal novel effectors secreted by an obligate intracellular bacterial pathogen

Pathogenic bacteria secrete protein effectors to hijack host machinery and remodel their infectious niche. Rickettsia spp. are obligate intracellular bacteria that can cause life-threatening disease, but their absolute dependence on the host cell environment has impeded discovery of rickettsial effectors and their host targets. We implemented bioorthogonal non-canonical amino acid tagging (BONCAT) during R. parkeri infection to selectively label, isolate, and identify secreted effectors. As the first use of BONCAT in an obligate intracellular bacterium, our screen more than doubles the number of experimentally validated effectors for R. parkeri. The novel secreted rickettsial factors (Srfs) we identified include Rickettsia-specific proteins of unknown function that localize to the host cytoplasm, mitochondria, and ER. We further show that one such effector, SrfD, interacts with the host Sec61 translocon. Altogether, our work uncovers a diverse set of previously uncharacterized rickettsial effectors and lays the foundation for a deeper exploration of the host-pathogen interface.

Streptococcus pneumoniae disrupts the structure of the golgi apparatus and subsequent epithelial cytokine response in an H2O2-dependent manner

BACKGROUND

Lung infections caused by Streptococcus pneumonia are a global leading cause of death. The reactive oxygen species H2O2 is one of the virulence factors of Streptococcus pneumoniae. The Golgi apparatus is essential for the inflammatory response of a eukaryotic cell. Golgi fragmentation was previously shown to be induced by bacterial pathogens and in response to H2O2 treatment. This led us to investigate whether the Golgi apparatus is actively involved and targeted in host-pathogen interactions during pneumococcal infections.

METHODS

Following in vitro infection of BEAS-2B bronchial epithelial cells with Streptococcus pneumoniae for 16 h, the structure of the Golgi apparatus was assessed by fluorescence staining of the Golgi-associated protein, Golgin-97. To investigate the effect of H2O2 production on Golgi structure, BEAS-2B cells were treated with H2O2 or the H2O2 degrading enzyme Catalase, prior to Golgi staining. Artificial disruption of the Golgi apparatus was induced by treatment of cells with the GBF1 inhibitor, Golgicide A. A proinflammatory cellular response was induced by treatment of cells with the bacterial cell wall component and TLR4 ligand lipoteichoic acid.

RESULTS

In vitro infection of bronchial epithelial cells with wild type Streptococcus pneumoniae led to a disruption of normal Golgi structure. Golgi fragmentation was not observed after deletion of the pneumococcal H2O2-producing gene, spxB, or neutralization of H2O2 by catalase treatment, but could be induced by H2O2 treatment. Streptococcus pneumoniae infection significantly reduced host cell protein glycosylation and artificial disruption of Golgi structure significantly reduced bacterial adherence, but increased bacterial counts in the supernatant. To understand if this effect depended on cell-contact or soluble factors, pneumococci were treated with cell-supernatant of cells treated with Golgicide A and/or lipoteichoic acid. This approach revealed that lipoteichoic acid conditioned medium inhibits bacterial replication in presence of host cells. In contrast, artificial Golgi fragmentation by Golgicide A treatment prior to lipoteichoic acid treatment rescued bacterial replication. This effect was associated with an increase of IL-6 and IL-8 in the supernatant of lipoteichoic acid treated cells. The increased cytokine release was abolished if cells were treated with Golgicide A prior to lipoteichoic acid treatment.

CONCLUSION

Streptococcus pneumoniae disrupts the Golgi apparatus in an H2O2-dependent manner, thereby inhibiting paracrine anti-infective mechanisms. Video Abstract.

PE_PGRS45 (Rv2615c) protein of Mycobacterium tuberculosis perturbs mitochondria of macrophages

The PE_PGRS proteins have coevolved with the antigenic ESX-V secretory system and are abundant in pathogenic Mycobacterium. Only a few PE_PGRS proteins have been characterized, and research suggests their role in organelle targeting, cell death pathways, calcium (Ca2+ ) homeostasis and disease pathogenesis. The PE_PGRS45 (Rv2615c) protein was predicted to contain mitochondria targeting sequences by in silico evaluation. Therefore, we investigated the targeting of the Rv2615c protein to host mitochondria and its effect on mitochondrial functions. In vitro experiments showed the Rv2615c protein colocalized with the mitochondria and led to morphological mitochondrial perturbations. Recombinant Rv2615c was observed to cause increased levels of intracellular reactive oxygen species and the adenosine diphosphate-to-adenosine triphosphate ratio. The Rv2615c protein also induced mitochondrial membrane depolarization and the generation of mitochondrial superoxide. We observed the release of cytochrome C into the cytoplasm and increased expression of proapoptotic genes Bax and Bim with no significant change in anti-apoptotic Bcl2 in Rv2615c-stimulated THP1 macrophages. Ca2+ is a key signaling molecule in tuberculosis pathogenesis, modulating host cell responses. As reported for other PE_PGRS proteins, Rv2615c also has Ca2+ -binding motifs and thus can modulate calcium homeostasis in the host. We also observed a high level of Ca2+ influx in THP1 macrophages stimulated with Rv2615c. Based on these findings, we suggest that Rv2615c may be an effector protein that could contribute to disease pathogenesis by targeting host mitochondria.

Orientia tsutsugamushi: analysis of the mobilome of a highly fragmented and repetitive genome reveals ongoing lateral gene transfer in an obligate intracellular bacterium

The rickettsial human pathogen Orientia tsutsugamushi (Ot) is an obligate intracellular Gram-negative bacterium with one of the most highly fragmented and repetitive genomes of any organism. Around 50% of its ~2.3 Mb genome is comprised of repetitive DNA that is derived from the highly proliferated Rickettsiales amplified genetic element (RAGE). RAGE is an integrative and conjugative element (ICE) that is present in a single Ot genome in up to 92 copies, most of which are partially or heavily degraded. In this report, we analysed RAGEs in eight fully sequenced Ot genomes and manually curated and reannotated all RAGE-associated genes, including those encoding DNA mobilisation proteins, P-type (vir) and F-type (tra) type IV secretion system (T4SS) components, Ankyrin repeat- and tetratricopeptide repeat-containing effectors, and other piggybacking cargo. Originally, the heavily degraded Ot RAGEs led to speculation that they are remnants of historical ICEs that are no longer active. Our analysis, however, identified two Ot genomes harbouring one or more intact RAGEs with complete F-T4SS genes essential for mediating ICE DNA transfer. As similar ICEs have been identified in unrelated rickettsial species, we assert that RAGEs play an ongoing role in lateral gene transfer within the Rickettsiales. Remarkably, we also identified in several Ot genomes remnants of prophages with no similarity to other rickettsial prophages. Together these findings indicate that, despite their obligate intracellular lifestyle and host range restricted to mites, rodents and humans, Ot genomes are highly dynamic and shaped through ongoing invasions by mobile genetic elements and viruses.

Metagenome diversity illuminates origins of pathogen effectors

Recent metagenome assembled genome (MAG) analyses have profoundly impacted Rickettsiology systematics. Discovery of basal lineages (Mitibacteraceae and Athabascaceae) with predicted extracellular lifestyles reveals an evolutionary timepoint for the transition to host dependency, which occurred independent of mitochondrial evolution. Notably, these basal rickettsiae carry the Rickettsiales vir homolog (rvh) type IV secretion system (T4SS) and purportedly use rvh to kill congener microbes rather than parasitize host cells as described for derived rickettsial pathogens. MAG analysis also substantially increased diversity for genus Rickettsia and delineated a basal lineage (Tisiphia) that stands to inform on the rise of human pathogens from protist and invertebrate endosymbionts. Herein, we probed Rickettsiales MAG and genomic diversity for the distribution of Rickettsia rvh effectors to ascertain their origins. A sparse distribution of most Rickettsia rvh effectors outside of Rickettsiaceae lineages indicates unique rvh evolution from basal extracellular species and other rickettsial families. Remarkably, nearly every effector was found in multiple divergent forms with variable architectures, illuminating profound roles for gene duplication and recombination in shaping effector repertoires in Rickettsia pathogens. Lateral gene transfer plays a prominent role shaping the rvh effector landscape, as evinced by the discover of many effectors on plasmids and conjugative transposons, as well as pervasive effector gene exchange between Rickettsia and Legionella species. Our study exemplifies how MAGs can provide incredible insight on the origins of pathogen effectors and how their architectural modifications become tailored to eukaryotic host cell biology.

SARS-CoV-2 accessory proteins ORF7a and ORF3a use distinct mechanisms to down-regulate MHC-I surface expression

Major histocompatibility complex class I (MHC-I) molecules, which are dimers of a glycosylated polymorphic transmembrane heavy chain and the small-protein β2-microglobulin (β2m), bind peptides in the endoplasmic reticulum that are generated by the cytosolic turnover of cellular proteins. In virus-infected cells, these peptides may include those derived from viral proteins. Peptide-MHC-I complexes then traffic through the secretory pathway and are displayed at the cell surface where those containing viral peptides can be detected by CD8+ T lymphocytes that kill infected cells. Many viruses enhance their in vivo survival by encoding genes that down-regulate MHC-I expression to avoid CD8+ T cell recognition. Here, we report that two accessory proteins encoded by SARS-CoV-2, the causative agent of the ongoing COVID-19 pandemic, down-regulate MHC-I expression using distinct mechanisms. First, ORF3a, a viroporin, reduces the global trafficking of proteins, including MHC-I, through the secretory pathway. The second, ORF7a, interacts specifically with the MHC-I heavy chain, acting as a molecular mimic of β2m to inhibit its association. This slows the exit of properly assembled MHC-I molecules from the endoplasmic reticulum. We demonstrate that ORF7a reduces antigen presentation by the human MHC-I allele HLA-A*02:01. Thus, both ORF3a and ORF7a act post-translationally in the secretory pathway to lower surface MHC-I expression, with ORF7a exhibiting a specific mechanism that allows immune evasion by SARS-CoV-2.

Quantifying Golgi Apparatus Fragmentation Using Imaging Flow Cytometry

Unlike the common conception of the Golgi apparatus as a static organelle, it is, in fact, a dynamic structure, as well as a sensitive sensor for the cellular status. In response to various stimuli, the intact Golgi structure undergoes fragmentation. This fragmentation can yield either partial fragmentation, resulting in several separated chunks, or complete vesiculation of the organelle. These distinct morphologies form the basis of several methods for the quantification of the Golgi status. In this chapter, we describe our imaging flow cytometry-based method for quantifying changes in the Golgi architecture. This method has all the benefits of imaging flow cytometry-namely, it is rapid, high-throughput, and robust-while affording easy implementation and analysis capabilities.

Analysis of the Type 4 Effectome across the Genus Rickettsia

Rickettsia are obligate intracellular bacteria primarily carried by arthropod hosts. The genus Rickettsia contains several vertebrate pathogens vectored by hematophagous arthropods. Despite the potential for disease, our understanding of Rickettsias are limited by the difficulties associated with growing and manipulating obligate intracellular bacteria. To aid with this, our lab conducted an analysis of eight genomes and three plasmids from across the genus Rickettsia. Using OPT4e, a learning algorithm-based program designed to identify effector proteins secreted by the type 4 secretion system, we generated a putative effectome for the genus. We then consolidated effectors into homolog sets to identify effectors unique to Rickettsia with different life strategies or evolutionary histories. We also compared predicted effectors to non-effectors for differences in G+C content and gene splitting. Based on this analysis, we predicted 1571 effectors across the genus, resulting in 604 homolog sets. Each species had unique homolog sets, while 42 were present in all eight species analyzed. Effectors were flagged in association with pathogenic, tick and flea-borne Rickettsia. Predicted effectors also varied in G+C content and frequency of gene splitting as compared to non-effectors. Species effector repertoires show signs of expansion, degradation, and horizontal acquisition associated with lifestyle and lineage.

Pathogenicity and virulence of Rickettsia

Rickettsiae include diverse Gram-negative microbial species that exhibit obligatory intracellular lifecycles between mammalian hosts and arthropod vectors. Human infections with arthropod-borne Rickettsia continue to cause significant morbidity and mortality as recent environmental changes foster the proliferation of arthropod vectors and increased exposure to humans. However, the technical difficulties in working with Rickettsia have delayed our progress in understanding the molecular mechanisms involved in rickettsial pathogenesis and disease transmission. Recent advances in developing genetic tools for Rickettsia have enabled investigators to identify virulence genes, uncover molecular functions, and characterize host responses to rickettsial determinants. Therefore, continued efforts to determine virulence genes and their biological functions will help us understand the underlying mechanisms associated with arthropod-borne rickettsioses.

Subversion of host cell signaling: The arsenal of Rickettsial species

Rickettsia is a genus of nonmotile, Gram-negative, non-spore-forming, highly pleomorphic bacteria that cause severe epidemic rickettsioses. The spotted fever group and typhi group are major members of the genus Rickettsia. Rickettsial species from the two groups subvert diverse host cellular processes, including membrane dynamics, actin cytoskeleton dynamics, phosphoinositide metabolism, intracellular trafficking, and immune defense, to promote their host colonization and intercellular transmission through secreted effectors (virulence factors). However, lineage-specific rickettsiae have exploited divergent strategies to accomplish such challenging tasks and these elaborated strategies focus on distinct host cell processes. In the present review, we summarized current understandings of how different rickettsial species employ their effectors' arsenal to affect host cellular processes in order to promote their own replication or to avoid destruction.

A Vaccine for Canine Rocky Mountain Spotted Fever: An Unmet One Health Need

Outbreaks of life-threatening Rocky Mountain spotted fever in humans and dogs associated with a canine-tick maintenance cycle constitute an important One Health opportunity. The reality of the problem has been observed strikingly in Mexico, Brazil, Colombia, and Native American tribal lands in Arizona. The brown dog tick, Rhipicephalus sanguineus sensu lato, acquires the rickettsia from bacteremic dogs and can maintain the bacterium transtadially to the next tick stage. The subsequent adult tick can then transmit infection to a new host, as shown by guinea pig models. These brown dog ticks maintain spotted fever group rickettsiae transovarially through many generations, thus serving as both vector and reservoir. Vaccine containing whole-killed R. rickettsii does not stimulate sufficient immunity. Studies of Rickettsia subunit antigens have demonstrated that conformationally preserved outer-membrane autotransporter proteins A and B are the leading vaccine candidates. The possibility of a potentially safe and effective live attenuated vaccine has only begun to be explored as gene knockout methods are applied to these obligately intracellular pathogens.

PRRSV Infection Induces Gasdermin D-Driven Pyroptosis of Porcine Alveolar Macrophages through NLRP3 Inflammasome Activation

For more than 3 decades, mounting evidence has associated porcine reproductive and respiratory syndrome virus (PRRSV) infection with late-term abortions and stillbirths in sows and respiratory disease in piglets, causing enormous economic losses to the global swine industry. However, to date, the underlying mechanisms of PRRSV-triggered cell death have not been well clarified, especially in the pulmonary inflammatory injury characterized by the massive release of pro-inflammatory factors. Here, we demonstrated that PRRSV infection triggered gasdermin D-mediated host pyroptosis in vitro and in vivo. Mechanistically, PRRSV infection triggered disassembly of the trans-Golgi network (TGN); the dispersed TGN then acted as a scaffold for NLRP3 activation through phosphatidylinositol-4-phosphate. In addition, PRRSV replication-transcription complex (RTC) formation stimulated TGN dispersion and pyroptotic cell death. Furthermore, our results indicated that TMEM41B, an endoplasmic reticulum (ER)-resident host protein, functioned as a crucial host factor in the formation of PRRSV RTC, which is surrounded by the intermediate filament network. Collectively, these findings uncover new insights into clinical features as previously unrecognized mechanisms for PRRSV-induced pathological effects, which may be conducive to providing treatment options for PRRSV-associated diseases and may be conserved during infection by other highly pathogenic viruses. IMPORTANCE Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the pathogens responsible for major economic losses in the global swine industry. Characterizing the detailed process by which PRRSV induces cell death pathways will help us better understand viral pathogenesis and provide implications for therapeutic intervention against PRRSV. Here, we showed that PRRSV infection induces GSDMD-driven host pyroptosis and IL-1β secretion through NOD-, LRR- and pyrin domain-containing protein 3 (NLRP3) inflammasome activation in vitro and in vivo. Furthermore, the molecular mechanisms of PRRSV-induced NLRP3 inflammasome activation and pyroptosis are elucidated here. The dispersed trans-Golgi network (TGN) induced by PRRSV serves as a scaffold for NLRP3 aggregation into multiple puncta via phosphatidylinositol 4-phosphate (PtdIns4P). Moreover, the formation of PRRSV replication-transcription complex is essential for TGN dispersion and host pyroptosis. This research advances our understanding of the PRRSV-mediated inflammatory response and cell death pathways, paving the way for the development of effective treatments for PRRSV diseases.

The Ankyrin Repeat Protein RARP-1 Is a Periplasmic Factor That Supports Rickettsia parkeri Growth and Host Cell Invasion

Rickettsia spp. are obligate intracellular bacterial pathogens that have evolved a variety of strategies to exploit their host cell niche. However, the bacterial factors that contribute to this intracellular lifestyle are poorly understood. Here, we show that the conserved ankyrin repeat protein RARP-1 supports Rickettsia parkeri infection. Specifically, RARP-1 promotes efficient host cell entry and growth within the host cytoplasm, but it is not necessary for cell-to-cell spread or evasion of host autophagy. We further demonstrate that RARP-1 is not secreted into the host cytoplasm by R. parkeri. Instead, RARP-1 resides in the periplasm, and we identify several binding partners that are predicted to work in concert with RARP-1 during infection. Altogether, our data reveal that RARP-1 plays a critical role in the rickettsial life cycle. IMPORTANCE Rickettsia spp. are obligate intracellular bacterial pathogens that pose a growing threat to human health. Nevertheless, their strict reliance on a host cell niche has hindered investigation of the molecular mechanisms driving rickettsial infection. This study yields much-needed insight into the Rickettsia ankyrin repeat protein RARP-1, which is conserved across the genus but has not yet been functionally characterized. Earlier work had suggested that RARP-1 is secreted into the host cytoplasm. However, the results from this work demonstrate that R. parkeri RARP-1 resides in the periplasm and is important both for invasion of host cells and for growth in the host cell cytoplasm. These results reveal RARP-1 as a novel regulator of the rickettsial life cycle.

SARS-CoV-2 accessory proteins ORF7a and ORF3a use distinct mechanisms to downregulate MHC-I surface expression

Major histocompatibility complex class I (MHC-I) molecules, which are dimers of a glycosylated polymorphic transmembrane heavy chain and the small protein β 2 -microglobulin (β 2 m), bind peptides in the endoplasmic reticulum that are generated by the cytosolic turnover of cellular proteins. In virus-infected cells these peptides may include those derived from viral proteins. Peptide-MHC-I complexes then traffic through the secretory pathway and are displayed at the cell surface where those containing viral peptides can be detected by CD8 + T lymphocytes that kill infected cells. Many viruses enhance their in vivo survival by encoding genes that downregulate MHC-I expression to avoid CD8 + T cell recognition. Here we report that two accessory proteins encoded by SARS-CoV-2, the causative agent of the ongoing COVID-19 pandemic, downregulate MHC-I expression using distinct mechanisms. One, ORF3a, a viroporin, reduces global trafficking of proteins, including MHC-I, through the secretory pathway. The second, ORF7a, interacts specifically with the MHC-I heavy chain, acting as a molecular mimic of β 2 m to inhibit its association. This slows the exit of properly assembled MHC-I molecules from the endoplasmic reticulum. We demonstrate that ORF7a reduces antigen presentation by the human MHC-I allele HLA-A*02:01. Thus, both ORF3a and ORF7a act post-translationally in the secretory pathway to lower surface MHC-I expression, with ORF7a exhibiting a novel and specific mechanism that allows immune evasion by SARS-CoV-2.

Significance Statement

Viruses may down-regulate MHC class I expression on infected cells to avoid elimination by cytotoxic T cells. We report that the accessory proteins ORF7a and ORF3a of SARS-CoV-2 mediate this function and delineate the two distinct mechanisms involved. While ORF3a inhibits global protein trafficking to the cell surface, ORF7a acts specifically on MHC-I by competing with β 2 m for binding to the MHC-I heavy chain. This is the first account of molecular mimicry of β 2 m as a viral mechanism of MHC-I down-regulation to facilitate immune evasion.

Nitric Oxide Inhibition of Rickettsia rickettsii

Rickettsia rickettsii, the causative agent of Rocky Mountain spotted fever, is an enzootic, obligate, intracellular bacterial pathogen. Nitric oxide (NO) synthesized by the inducible NO synthase (iNOS) is a potent antimicrobial component of innate immunity and has been implicated in the control of virulent Rickettsia spp. in diverse cell types. In this study, we examined the antibacterial role of NO on R. rickettsii. Our results indicate that NO challenge dramatically reduces R. rickettsii adhesion through the disruption of bacterial energetics. Additionally, NO-treated R. rickettsii cells were unable to synthesize protein or replicate in permissive cells. Activated, NO-producing macrophages restricted R. rickettsii infections, but inhibition of iNOS ablated the inhibition of bacterial growth. These data indicate that NO is a potent antirickettsial effector of innate immunity that targets energy generation in these pathogenic bacteria to prevent growth and subversion of infected host cells.

Manipulation of Host Cell Organelles by Intracellular Pathogens

Pathogenic intracellular bacteria, parasites and viruses have evolved sophisticated mechanisms to manipulate mammalian host cells to serve as niches for persistence and proliferation. The intracellular lifestyles of pathogens involve the manipulation of membrane-bound organellar compartments of host cells. In this review, we described how normal structural organization and cellular functions of endosomes, endoplasmic reticulum, Golgi apparatus, mitochondria, or lipid droplets are targeted by microbial virulence mechanisms. We focus on the specific interactions of Salmonella, Legionella pneumophila, Rickettsia rickettsii, Chlamydia spp. and Mycobacterium tuberculosis representing intracellular bacterial pathogens, and of Plasmodium spp. and Toxoplasma gondii representing intracellular parasites. The replication strategies of various viruses, i.e., Influenza A virus, Poliovirus, Brome mosaic virus, Epstein-Barr Virus, Hepatitis C virus, severe acute respiratory syndrome virus (SARS), Dengue virus, Zika virus, and others are presented with focus on the specific manipulation of the organelle compartments. We compare the specific features of intracellular lifestyle and replication cycles, and highlight the communalities in mechanisms of manipulation deployed.

The enigmatic biology of rickettsiae: recent advances, open questions and outlook

Rickettsiae are obligate intracellular bacteria that can cause life-threatening illnesses and are among the oldest known vector-borne pathogens. Members of this genus are extraordinarily diverse and exhibit a broad host range. To establish intracellular infection, Rickettsia species undergo complex, multistep life cycles that are encoded by heavily streamlined genomes. As a result of reductive genome evolution, rickettsiae are exquisitely tailored to their host cell environment but cannot survive extracellularly. This host-cell dependence makes for a compelling system to uncover novel host-pathogen biology, but it has also hindered experimental progress. Consequently, the molecular details of rickettsial biology and pathogenesis remain poorly understood. With recent advances in molecular biology and genetics, the field is poised to start unraveling the molecular mechanisms of these host-pathogen interactions. Here, we review recent discoveries that have shed light on key aspects of rickettsial biology. These studies have revealed that rickettsiae subvert host cells using mechanisms that are distinct from other better-studied pathogens, underscoring the great potential of the Rickettsia genus for revealing novel biology. We also highlight several open questions as promising areas for future study and discuss the path toward solving the fundamental mysteries of this neglected and emerging human pathogen.

Rickettsia-host interaction: strategies of intracytosolic host colonization

Bacterial infection is a highly complex biological process involving a dynamic interaction between the invading microorganism and the host. Specifically, intracellular pathogens seize control over the host cellular processes including membrane dynamics, actin cytoskeleton, phosphoinositide metabolism, intracellular trafficking and immune defense mechanisms to promote their host colonization. To accomplish such challenging tasks, virulent bacteria deploy unique species-specific secreted effectors to evade and/or subvert cellular defense surveillance mechanisms to establish a replication niche. However, despite superficially similar infection strategies, diverse Rickettsia species utilize different effector repertoires to promote host colonization. This review will discuss our current understandings on how different Rickettsia species deploy their effector arsenal to manipulate host cellular processes to promote their intracytosolic life within the mammalian host.

The role of the Golgi apparatus in disease (Review)

The Golgi apparatus is known to underpin many important cellular homeostatic functions, including trafficking, sorting and modifications of proteins or lipids. These functions are dysregulated in neurodegenerative diseases, cancer, infectious diseases and cardiovascular diseases, and the number of disease-related genes associated with Golgi apparatus is on the increase. Recently, many studies have suggested that the mutations in the genes encoding Golgi resident proteins can trigger the occurrence of diseases. By summarizing the pathogenesis of these genetic diseases, it was found that most of these diseases have defects in membrane trafficking. Such defects typically result in mislocalization of proteins, impaired glycosylation of proteins, and the accumulation of undegraded proteins. In the present review, we aim to understand the patterns of mutations in the genes encoding Golgi resident proteins and decipher the interplay between Golgi resident proteins and membrane trafficking pathway in cells. Furthermore, the detection of Golgi resident protein in human serum samples has the potential to be used as a diagnostic tool for diseases, and its central role in membrane trafficking pathways provides possible targets for disease therapy. Thus, we also introduced the clinical value of Golgi apparatus in the present review.

Mutations in GDAP1 Influence Structure and Function of the Trans-Golgi Network

Charcot-Marie-Tooth disease (CMT) is a heritable neurodegenerative disease that displays great genetic heterogeneity. The genes and mutations that underlie this heterogeneity have been extensively characterized by molecular genetics. However, the molecular pathogenesis of the vast majority of CMT subtypes remains terra incognita. Any attempts to perform experimental therapy for CMT disease are limited by a lack of understanding of the pathogenesis at a molecular level. In this study, we aim to identify the molecular pathways that are disturbed by mutations in the gene encoding GDAP1 using both yeast and human cell, based models of CMT-GDAP1 disease. We found that some mutations in GDAP1 led to a reduced expression of the GDAP1 protein and resulted in a selective disruption of the Golgi apparatus. These structural alterations are accompanied by functional disturbances within the Golgi. We screened over 1500 drugs that are available on the market using our yeast-based CMT-GDAP1 model. Drugs were identified that had both positive and negative effects on cell phenotypes. To the best of our knowledge, this study is the first report of the Golgi apparatus playing a role in the pathology of CMT disorders. The drugs we identified, using our yeast-based CMT-GDAP1 model, may be further used in translational research.

Comparative transcriptomic analysis of Rickettsia conorii during in vitro infection of human and tick host cells

BACKGROUND

Pathogenic Rickettsia species belonging to the spotted fever group are arthropod-borne, obligate intracellular bacteria which exhibit preferential tropism for host microvascular endothelium in the mammalian hosts, resulting in disease manifestations attributed primarily to endothelial damage or dysfunction. Although rickettsiae are known to undergo evolution through genomic reduction, the mechanisms by which these pathogens regulate their transcriptome to ensure survival in tick vectors and maintenance by transovarial/transstadial transmission, in contrast to their ability to cause debilitating infections in human hosts remain unknown. In this study, we compare the expression profiles of rickettsial sRNAome/transcriptome and determine the transcriptional start sites (TSSs) of R. conorii transcripts during in vitro infection of human and tick host cells.

RESULTS

We performed deep sequencing on total RNA from Amblyomma americanum AAE2 cells and human microvascular endothelial cells (HMECs) infected with R. conorii. Strand-specific RNA sequencing of R. conorii transcripts revealed the expression 32 small RNAs (Rc_sR's), which were preferentially expressed above the limit of detection during tick cell infection, and confirmed the expression of Rc_sR61, sR71, and sR74 by quantitative RT-PCR. Intriguingly, a total of 305 and 132 R. conorii coding genes were differentially upregulated (> 2-fold) in AAE2 cells and HMECs, respectively. Further, enrichment for primary transcripts by treatment with Terminator 5'-Phosphate-dependent Exonuclease resulted in the identification of 3903 and 2555 transcription start sites (TSSs), including 214 and 181 primary TSSs in R. conorii during the infection to tick and human host cells, respectively. Seventy-five coding genes exhibited different TSSs depending on the host environment. Finally, we also observed differential expression of 6S RNA during host-pathogen and vector-pathogen interactions in vitro, implicating an important role for this noncoding RNA in the regulation of rickettsial transcriptome depending on the supportive host niche.

CONCLUSIONS

In sum, the findings of this study authenticate the presence of novel Rc_sR's in R. conorii, reveal the first evidence for differential expression of coding transcripts and utilization of alternate transcriptional start sites depending on the host niche, and implicate a role for 6S RNA in the regulation of coding transcriptome during tripartite host-pathogen-vector interactions.