Identification of Host Proteins Involved in Rickettsial Invasion of Tick Cells

Abstract

Tick-borne spotted fever group (SFG) Rickettsia species are obligate intracellular bacteria capable of infecting both vertebrate and invertebrate host cells, an essential process for subsequent bacterial survival in distinct hosts. The host cell signaling molecules involved in the uptake of Rickettsia into mammalian and Drosophila cells have been identified; however, invasion into tick cells is understudied. Considering the movement of SFG Rickettsia between vertebrate and invertebrate hosts, the hypothesis is that conserved mechanisms are utilized for host cell invasion. The current study employed biochemical inhibition assays to determine the tick proteins involved in Rickettsia montanensis infection of tick-derived cells from a natural host, Dermacentor variabilis. The results revealed several tick proteins important for rickettsial invasion, including actin filaments, actin-related protein 2/3 complex, phosphatidylinositol-3=-kinase, protein tyrosine kinases (PTKs), Src family PTK, focal adhesion kinase, Rho GTPase Rac1, and neural Wiskott-Aldrich syndrome protein. Delineating the molecular mechanisms of rickettsial infection is critical to a thorough understanding of rickettsial transmission in tick populations and the ecology of tick-borne rickettsial diseases.T ick-borne rickettsial diseases caused by pathogenic bacteria belonging to the genera Rickettsia, Anaplasma, and Ehrlichia are steadily increasing in the United States (1). Infection associated with these obligate intracellular bacteria ranges from mild, self-limiting to severe, including death (2). In the typical transmission cycle, spotted fever group (SFG) rickettsiae transmit vertically during the tick life cycle stages and between vectors and vertebrate hosts during tick acquisition of a blood meal. In vertebrate hosts, mechanisms of SFG Rickettsia infection, including invasion, escape from the phagosome, intracellular growth, intracellular movement, and cell-to-cell spread, have been studied (3). Tick acquisition of SFG Rickettsia from rickettsemic hosts is poorly described, and despite the essential role for tick hosts in maintenance and transmission of SFG Rickettsia to vertebrate hosts, the biology of the tick cell-SFG Rickettsia interaction is understudied.The obligate intracellular nature of Rickettsia requires bacteria to invade both vertebrate and invertebrate host cells, and this process, host cell invasion by SFG Rickettsia, has been studied in mammalian and Drosophila cell lines. Mammalian ligands Ku70 and ␣ 2 ␤ 1 integrin interact with rickettsial outer membrane proteins B and A (OmpB and OmpA), respectively, and are involved in Rickettsia conorii invasion of mammalian host cells (4, 5). After binding to rickettsial OmpB, R. conorii induces signaling cascades in which Ku70 is initially ubiquitinated by c-Cbl ubiquitin ligase. Subsequently, signaling molecules, including Cdc42, protein tyrosine kinases (PTKs), phosphatidylinositol-3=-kinase (PI3-kinase), Src family tyrosine kinases (Src), focal adhesion kinase (FAK), and cortactin, c...

Role of Sca2 and RickA in the Dissemination of Rickettsia parkeri in Amblyomma maculatum

“…Together, the data suggest that R. parkeri strains deficient in functional RickA or Sca2 display conserved phenotypes in arthropod and mammalian cells in vitro . Observed deficiencies in ABM were expected, as the host protein complexes utilized for actin-based motility, e.g., Arp2/3 complex, are functional in tick cells ( 20 , 21 ). Interestingly, compared to wild-type R. parkeri , decreased late-phase actin polymerization was observed for R. parkeri rickA :: tn , coinciding with reduced cell-to-cell spread.…”

Counterattacking the tick bite: towards a rational design of anti-tick vaccines targeting pathogen transmission

“…More knowledge on these specific interactions could perhaps be used in order to interfere with cell to cell spread of specific bacteria. The protein complex Arp2/3 is a major component in the regulation of the actin cytoskeleton of most eukaryotic cells [142] and various studies - both in mammalian and tick cell lines - have shown this complex is recruited by SFG Rickettsia in order to enter their host’s cells through endocytosis [134, 142–144]. Studies using varying concentrations of an Arp2/3 complex inhibitor and transcriptional profiles of infected versus uninfected Dermacentor variabilis cells established its importance for R. montanensis invasion [142, 144].…”

“…The protein complex Arp2/3 is a major component in the regulation of the actin cytoskeleton of most eukaryotic cells [142] and various studies - both in mammalian and tick cell lines - have shown this complex is recruited by SFG Rickettsia in order to enter their host’s cells through endocytosis [134, 142–144]. Studies using varying concentrations of an Arp2/3 complex inhibitor and transcriptional profiles of infected versus uninfected Dermacentor variabilis cells established its importance for R. montanensis invasion [142, 144]. Similar results have been shown for R. monacensis , R. conorii and R. rickettsii by examining rickettsial proteins that interact with the Arp2/3 complex, such as RickA [143, 145, 146].…”

“…Similar results have been shown for R. monacensis , R. conorii and R. rickettsii by examining rickettsial proteins that interact with the Arp2/3 complex, such as RickA [143, 145, 146]. Other host proteins involved in rickettsial cell invasion, such as Cdc42, PI 3-kinases, phosphotyrosine kinase (PTK), c-Src, focal adhesion kinase (FAK), Ku70, V-ATPase, α-catenin, Rho GTPases Rac1 and N-WASP have also been investigated to a lesser extent [134, 142, 143, 147, 148]. However, differences were observed between Rickettsia species.…”

“…However, differences were observed between Rickettsia species. For example, Rho GTPases Rac1 were found to play an important role in the internalization of R. montanensis into D. variabilis cells, while they were found unnecessary for R. conorii invasion of VERO cells [142, 143]. These results could be accredited to differences in the rickettsial-host interaction between rickettsial species.…”