Francisella subverts innate immune signaling: Focus on PI3K/Akt

Review Article published: 07 February 2011 doi: 10.3389/fmicb.2011.00013 Francisella subverts innate immune signaling: focus on PI3K/Akt Thomas John Cremer, Jonathan P. Butchar and Susheela Tridandapani* Department of Internal Medicine, The Ohio State University Medical Center, Columbus, OH, USA Edited by: Anders Sjostedt, Umeå University, Sweden Reviewed by: Brian Coombes, McMaster University, Canada John T. Belisle, Colorado State University, USA *Correspondence: Susheela Tridandapani, Department of Internal Medicine, Davis Heart and Lung Research Institute, The Ohio State University Medical Center, 415, 473 West, 12th Avenue, Columbus, OH 43210, USA. e-mail: Intracellular bacterial pathogens exploit host cells as a part of their lifecycle, and they do so by manipulating host cell signaling events. Many such bacteria are known to produce effector proteins that promote cell invasion, alter membrane trafficking, and disrupt signaling cascades. This review highlights recent advances in our understanding of signaling pathways involved in host cell responses to Francisella tularensis, a facultative Gram-negative intracellular pathogen that causes tularemia. We highlight several key pathways that are targeted by Francisella, with a focus on the phosphatidylinositol 3-kinase/Akt pathway. Lastly, we discuss the emerging role of microRNAs (miRs), specifically miR-155, as a key regulator of host signaling and defense. Keywords: PI3K, Akt, SHIP, miR-155, host response, Francisella Introduction Successful intracellular pathogens must have mechanisms to usurp their hosts’ normal functions and defenses, thereby creating a favorable environment. For example, Legionella pneumophila secrete type IV effectors that form a permissive niche (Berger et al., 1994) and Yersinia pestis disrupts host nuclear factor kappa B (NF-κB) signaling via the Yop J proteins (Zhou et al., 2005). However, each pathogen can use slightly – or vastly – different mechanisms to conquer the complex signaling pathways that orchestrate the host response. A better understanding of these responses and the methods by which pathogens defeat them will fuel the generation of novel therapeutics. Francisella tularensis is a Gram-negative bacterial pathogen and is the causative agent of tularemia. The highest-virulence F. tularensis subspecies tularensis requires as few as 10 colony forming units to cause disease and death in humans (Sjostedt, 2007). Because of this it has been classified as a category A select agent by the United States Centers for Disease Control. However, there are also lower-virulence subspecies and strains, such as F. tularensis subspecies novicida and the live vaccine strains (LVS) of F. tularensis subspecies holarctica. These subspecies are used extensively for research because they lead to disease in mice that resembles human tularemia, their intracellular lifecycles are similar to that of F. tularensis tularensis, and they present minimal risk to humans. Since 2001 there has been a renewed interest in the study of this Abbreviations: AIM-2, absent in melanoma 2; ERK, extracellular signal-regulated kinase; GSK3β, glycogen synthase kinase 3 beta; LPS, lipopolysaccharide; MHC, major histocompatibility complex; MyD88, myeloid differentiation primary response gene (88); NF-κB, nuclear factor kappa B; PI3K, phosphatidylinositol 3-kinase; PTEN, phosphatase and tensin homolog; ripA, required for intracellular proliferation, factor A; SHIP, SH2 domain-containing inositol 5′-phosphatase 1; SOCS3, suppressor of cytokine signaling 3; Syk, splenic tyrosine kinase; TLR, toll-like receptor. www.frontiersin.org pathogen for biodefense purposes, and tremendous advances have been made in recent years (Oyston et al., 2004; Elkins et al., 2007; Santic et al., 2010). One of the most interesting aspects of infection with F. tularensis is that there is a lack or delay of inflammatory response during the early stages of infection (Andersson et al., 2006; Bosio et al., 2007). Even though this pathogen is Gram-negative, the lipopolysaccharide (LPS) is modified in such a way that it does not activate toll-like receptor (TLR) 4 (Hajjar et al., 2006; Gunn and Ernst, 2007). Additionally, F. tularensis can suppress the host’s ability to respond to pro-inflammatory signals (Telepnev et al., 2003, 2005; Bosio et al., 2007), thus demonstrating that it does not merely evade host cell immune responses. Indeed, genomewide studies on the effect of F. tularensis on human phagocytes shows a broad suppression of numerous signaling pathways including TLR signaling, autophagy, major histocompatibility complex (MHC) presentation, interferon signaling, and phosphatidylinositol 3-kinase (PI3K) signaling (Butchar et al., 2008). Bacterial entry into phagocytes Internalization of Francisella by human phagocytes occurs though a unique process termed “looping phagocytosis” (Clemens and Horwitz, 2007). Optimal phagocytosis of Francisella requires an intact complement pathway, as internalization is dramatically reduced in the absence of complement (Lofgren et al., 1983; Balagopal et al., 2006; Clemens and Horwitz, 2007). However, the bacterium itself is resistant to complement-mediated lysis though its LPS O antigen (Clay et al., 2008). Other receptors such as the scavenger receptor (Pierini, 2006) and mannose receptor (Balagopal et al., 2006; Schulert and Allen, 2006) have been implicated in promoting bacterial uptake. Fc receptors also play a role, aiding both internalization of bacteria (Balagopal et al., 2006) and host protection (Kirimanjeswara et al., 2007). February 2011 | Volume 2 | Article 13 | 1 Cremer et al. Downstream signaling events implicated in the phagocytosis of F. novicida include the splenic tyrosine kinase (Syk) and extracellular signal-regulated kinase (ERK) pathways. Genetic and pharmacologic manipulation of these two kinases established that both pathways are required for macrophage phagocytosis of bacteria under heat-inactivated fetal bovine serum culture conditions (Parsa et al., 2008b). Conflicting reports exist on the role of PI3K signaling during phagocytosis. Phagocytosis of F. tularensis in human monocyte derived macrophages cultured in AB serum is wortmannin sensitive (Clemens and Horwitz, 2007). However, PI3K or downstream Akt signaling in murine macrophages cultured with heatinactivated fetal bovine serum does not have any effect on bacterial internalization (Parsa et al., 2008b; Rajaram et al., 2009). Intracellular replication Intracellular replication requires escape from the phagosome to gain access to the cytosol. Numerous escape/replication deficient mutants of F. novicida have been generated (Lauriano et al., 2004; Santic et al., 2005, 2007; Mohapatra et al., 2008). Studies with these mutants have shown that Francisella manipulates host signaling in at least two distinct ways to promote intracellular proliferation. Firstly, Francisella inhibits protective responses that would promote bacterial (...truncated)