Nutrient generation and retrieval from the host cell cytosol by intra-vacuolar Legionella pneumophila

Microbial acquisition of nutrients in vivo is a fundamental aspect of infectious diseases, and is a potential target for antimicrobial therapy. Part of the innate host defense against microbial infection is nutritional restriction of access to sources of host nutrients (Abu Kwaik and Bumann, 2013; Eisenreich et al., 2013). Despite this host nutritional restriction, there has been a long held presumption that the host cell cytosol has sufficient nutrients for any intracellular pathogen, although many bacteria fail to grow in the host cytosol if they are microinjected (Goetz et al., 2001). However, recent studies on the two intra-vacuolar pathogens Anaplasma phagocytophilum (Niu et al., 2012) and Legionella pneumophila (Price et al., 2011) and the cytosolic pathogen Francisella tularensis (Steele et al., 2013) have clearly shown that the levels of amino acids in the host cell cytosol are below the threshold sufficient to meet the tremendous demands for carbon, nitrogen and energy to power the robust intracellular proliferation of these pathogens (Abu Kwaik and Bumann, 2013). Therefore, these intracellular pathogens have evolved with efficient strategies to boost the levels of host amino acids to meet their demands for higher levels of carbon, nitrogen and energy sources (Abu Kwaik and Bumann, 2013; Fonseca and Swanson, 2014). There is an emerging paradigm of specific microbial strategies that directly trigger the host cell to boost the cellular levels of essential microbial nutrients, and this paradigm has been designated as “nutritional virulence” (Abu Kwaik and Bumann, 2013). This opinion article is focused on nutritional virulence of L. pneumophila. In the aquatic environment, L. pneumophila proliferates within protozoa, which impact bacterial ecology and pathogenicity (Al-Quadan et al., 2012). Upon transmission to humans, L. pneumophila proliferates in alveolar macrophages within the Legionellacontaining vacuole (LCV) that is ER-derived and evades lysosomal fusion (Figure 1). Within both evolutionarily distant host cells, the Dot/Icm type IV secretion system of L. pneumophila injects ∼300 protein effectors (Zhu et al., 2011; Luo, 2011a) that govern biogenesis of the LCV and modulate a myriad of cellular processes to enable intra-vacuolar proliferation (Figure 1) (Luo, 2011b; Richards et al., 2013). Amino acids are the main sources of carbon, nitrogen and energy for L. pneumophila, which metabolizes them through the TCA cycle (Pine et al., 1979), but also metabolizes minor amounts of glucose in vitro using the Entner-Doudoroff pathway (Eylert et al., 2010; Price et al., 2011). Although L. pneumophila utilizes amino acids as the main sources of carbon and energy, the pathogen is auxotrophic for seven amino acids (Cys, Met, Arg, Thr, Val, Ile, and Leu) (Eylert et al., 2010; Price et al., 2014). Remarkably, there is a high level of synchronization in amino acids auxotrophy between L. pneumophila and its host cells, which has likely played a factor in nutritional evolution of L. pneumophila as an intra-vacuolar pathogen (Price et al., 2014). Interestingly, intra-vacuolar L. pneumophila up-regulates its own amino acids transporters, indicating increased demands for amino acids in the intra-vacuolar environment (Bruggemann et al., 2006; Faucher et al., 2011; Eisenreich et al., 2013). Since the generation time of intra-vacuolar L. pneumophila is ∼40 min, this organism requires high levels of amino acids to be imported from the host cytosol into the LCV lumen (Schunder et al., 2014). A long-held presumption has been that the host cell cytosol is rich in nutrients for invading pathogens. However, recent studies clearly indicate that the basal levels of host cellular amino acids are below the threshold sufficient for the robust intra-vacuolar proliferation of L. pneumophila (Sauer et al., 2005; Wieland et al., 2005) To achieve that needed threshold, L. pneumophila promotes host proteasomal degradation (Price et al., 2011) of LCV-decorated polyubiquitinated proteins (Dorer et al., 2006; Price et al., 2009, 2011; Lomma et al., 2010) mediated by the AnkB effector. Within human macrophages and amoeba, the AnkB translocated effector of L. pneumophila strain AA100/130b is localized to the cytosolic face of the LCV membrane through host-mediated farnesylation of its C-terminal CaaX motif (Figure 1) (Price et al., 2010; AlQuadan et al., 2011; Al-Quadan and Kwaik, 2011). On the LCV membrane, AnkB interacts with the host SCF1 ubiquitin ligase (Figure 1) (Bruckert et al., 2014). As a bona fide F-box effector (Ensminger and Isberg, 2010; Lomma et al., 2010; Price and Abu Kwaik, 2010), AnkB triggers decoration of the LCV with Lys 48-linked polyubiquitinated proteins that are targeted for proteasomal degradation (Figure 1) (Price et al., 2011). The metabolomic profile