The Role of the Francisella Tularensis Pathogenicity Island in Type VI Secretion, Intracellular Survival, and Modulation of Host Cell Signaling

Abstract

Francisella tularensis is a highly virulent gram-negative intracellular bacterium that causes the zoonotic disease tularemia. Essential for its virulence is the ability to multiply within host cells, in particular monocytic cells. The bacterium has developed intricate means to subvert host immune mechanisms and thereby facilitate its intracellular survival by preventing phagolysosomal fusion followed by escape into the cytosol, where it multiplies. Moreover, it targets and manipulates numerous host cell signaling pathways, thereby ameliorating the otherwise bactericidal capacity. Many of the underlying molecular mechanisms still remain unknown but key elements, directly or indirectly responsible for many of the aforementioned mechanisms, rely on the expression of proteins encoded by the Francisella pathogenicity island (FPI), suggested to constitute a type VI secretion system. We here describe the current knowledge regarding the components of the FPI and the roles that have been ascribed to them.

Keywords: FPI; Francisella tularensis; T6SS; intracellular growth; phagosomal escape.

Figure 1

The Francisella pathogenicity island (FPI). A schematic representation of the Francisella tularensis Schu S4 variant of the FPI with a consensus nomenclature adapted from Ludu et al. (2008). The ORFs of the FPI are essentially the same in all subspecies, with the exception of the anmK–pdpD region. In type A strains like Schu S4 (clade A.I) or WY96-3418 (clade A.II), pdpD encodes a 1,195 aa protein, which is somewhat larger in F. novicida (1,245 aa). The anmK gene product (371 aa in F. novicida) is truncated in WY96-3418, and may be expressed as two separate ORFs in Schu S4 (anmK1 and anmK2). F. tularensis type B strains, e.g., LVS, lack anmK and most of the pdpD gene due to a large deletion within this region. The ORFs are drawn to scale, and their relative sizes indicated. Black arrows indicate gene products with no significant homology to other proteins; while white arrows represent products with homologs in other bacterial systems (see text for details).

Figure 2

Signaling triggered by F. tularensis located in the cytosol. Host cell signaling occurs after F. tularensis escapes from the phagosome to reside and multiply within the cytoplasm of the monocytic cells. The infection eventually leads to apoptosis of the host cell. The engagement of a multitude of signaling pathways have been described, for details see references in the text. Only pathways that have been experimentally verified are included in the figure.

Figure 3

Signaling triggered by F. tularensis located in the phagosome. The signaling occurs during the brief period after uptake of the bacteria by monocytic cells and before they escape into the cytoplasm. Moreover, the signaling also occurs during infection with FPI mutants, e.g., within iglC, which are unable to escape from the phagosome.

Figure 4

A model for the regulation of the FPI locus. Preceding expression of the FPI locus is an unknown environmental stimulus that leads to KdpD-mediated phosphorylation of PmrA, allowing it to bind to promoter regions. The promoter upstream of pdpD has been experimentally confirmed, while the promoter upstream of pdpA (?) is hypothetical. PmrA binding may recruit the MglA/SspA/FevR complex, which directly interacts with the RNA polymerase (RNAP) to initiate transcription. For FevR to efficiently bind to MglA/SspA, it first needs to be activated by the alarmone ppGpp of the stringent response, while fevR expression requires PmrA, MglA, SspA as well as MigR. PmrA and MglA/SspA/FevR also regulate many additional genes outside of the FPI cluster, mainly in a positive manner, however their target genes appear to be different. At an unknown signal, the Hfq repressor specifically inhibits the expression of the putative pdpA to pdpE operon, presumably by binding to the predicted promoter upstream of pdpA. Hfq also regulates expression of genes outside of the FPI, mainly by acting as a repressor.

Figure 5

A comparison of V. cholerae VgrG1 and Francisella VgrG. VgrG1 schematics are as presented by Pukatzki et al. (2007). The gp27- and gp5-like domains are homologous to bacteriophage T4 tail-spike complexes utilized to pierce bacterial membranes. VgrG1 is an evolved VgrG, with a C-terminal extension encoding an actin cross-linking domain. The significantly smaller VgrG of Francisella lacks a C-terminal active domain and aligns to the central parts of VgrG1, showing homology to the predicted cell-puncturing parts of this protein.