title: Ehrlichia chaffeensis TRP120 Activates Canonical Notch Signaling To Downregulate TLR2/4 Expression and Promote Intracellular Survival.
abstract: Ehrlichia chaffeensis preferentially targets mononuclear phagocytes and survives through a strategy of subverting innate immune defenses, but the mechanisms are unknown. We have shown E. chaffeensis type 1 secreted tandem repeat protein (TRP) effectors are involved in diverse molecular pathogen-host interactions, such as the TRP120 interaction with the Notch receptor-cleaving metalloprotease ADAM17. In the present study, we demonstrate E. chaffeensis, via the TRP120 effector, activates the canonical Notch signaling pathway to promote intracellular survival. We found that nuclear translocation of the transcriptionally active Notch intracellular domain (NICD) occurs in response to E. chaffeensis or recombinant TRP120, resulting in upregulation of Notch signaling pathway components and target genes notch1, adam17, hes, and hey Significant differences in canonical Notch signaling gene expression levels (>40%) were observed during early and late stages of infection, indicating activation of the Notch pathway. We linked Notch pathway activation specifically to the TRP120 effector, which directly interacts with the Notch metalloprotease ADAM17. Using pharmacological inhibitors and small interfering RNAs (siRNAs) against γ-secretase enzyme, Notch transcription factor complex, Notch1, and ADAM17, we demonstrated that Notch signaling is required for ehrlichial survival. We studied the downstream effects and found that E. chaffeensis TRP120-mediated activation of the Notch pathway causes inhibition of the extracellular signal-regulated kinase 1/2 (ERK1/2) and p38 mitogen-activated protein kinase (MAPK) pathways required for PU.1 and subsequent Toll-like receptor 2/4 (TLR2/4) expression. This investigation reveals a novel mechanism whereby E. chaffeensis exploits the Notch pathway to evade the host innate immune response for intracellular survival.
  
  
  
  E. chaffeensis is an obligately intracellular bacterium and the etiologic agent of human monocytotropic ehrlichiosis (HME), an emerging life-threatening tick-borne zoonosis. Mechanisms by which E. chaffeensis establishes intracellular infection and avoids innate host defenses are not understood, but functionally relevant host-pathogen interactions with type 1 secreted TRP effectors are essential for the ehrlichial cellular reprogramming strategy. This study provides further insight into the molecular strategies used by obligately intracellular pathogens such as E. chaffeensis, which have small genomes and a limited number of effector proteins and exploit evolutionarily conserved host cell programs such as Notch signaling to promote infection and intracellular survival.
authors.affiliation: Department of Pathology, University of Texas Medical Branch, Galveston, Texas, USA.
authors.forename: Taslima T
authors.initials: TT
authors.lastname: Lina
authors.affiliation: Department of Pathology, University of Texas Medical Branch, Galveston, Texas, USA.
authors.forename: Paige S
authors.initials: PS
authors.lastname: Dunphy
authors.affiliation: Department of Pathology, University of Texas Medical Branch, Galveston, Texas, USA.
authors.forename: Tian
authors.initials: T
authors.lastname: Luo
authors.affiliation: Department of Pathology, University of Texas Medical Branch, Galveston, Texas, USA Department of Microbiology and Immunology, University of Texas Medical Branch, Galveston, Texas, USA Center for Biodefense and Emerging Infectious Diseases, University of Texas Medical Branch, Galveston, Texas, USA Sealy Center for Vaccine Development, University of Texas Medical Branch, Galveston, Texas, USA Institute for Human Infections and Immunity, University of Texas Medical Branch, Galveston, Texas, USA jemcbrid@utmb.edu.
authors.forename: Jere W
authors.initials: JW
authors.lastname: McBride
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism*; D002460:Cell Line; D015536:Down-Regulation; D018469:Ehrlichia chaffeensis / Q000472:pathogenicity*; D054884:Host-Pathogen Interactions; D006801:Humans; D057131:Immune Evasion; D050296:Microbial Viability; D009000:Monocytes / Q000276:immunology / Q000382:microbiology; D051881:Receptor, Notch1 / Q000378:metabolism*; D015398:Signal Transduction; D051195:Toll-Like Receptor 2 / Q000096:biosynthesis*; D051197:Toll-Like Receptor 4 / Q000096:biosynthesis*
chemical_list: D001426:Bacterial Proteins; C497761:NOTCH1 protein, human; D051881:Receptor, Notch1; C495342:TLR2 protein, human; C495345:TLR4 protein, human; D051195:Toll-Like Receptor 2; D051197:Toll-Like Receptor 4