The bacterium L. monocytogenes has been the subject of many c-di-AMP related studies: the first crystal structure of a CdaA-type DAC, the most abundant one, was obtained from a truncated L. monocytogenes enzyme (Rosenberg et al., 2015), many c-di-AMP binding proteins, such as CbpA, CbpB, PycA or OpuCA were first identified in this bacterium (Huynh et al., 2016; Sureka et al., 2017). Moreover, L. monocytogenes was the first bacterium were conditions were identified under which c-di-AMP is dispensable (Whiteley et al., 2015).
L. monocytogenes is closely related to B. subtilis, belonging also to the phylum of the Firmicutes.
It was first described in 1926 by Everitt George Dunne Murray as Bacterium monocytogenes and later on named in honor of the British surgeon Joseph Lister and the upon infection occurring monocytosis (Gray & Killinger, 1966; Murray et al., 1926; Pirie, 1940). One interesting aspect of L. monocytogenes is its lifestyle change. On the one hand, the bacterium can be found ubiquitous in nature as a saprophyte and on the other hand it is a facultative foodborne human pathogen (Farber & Peterkin, 1991; Freitag et al., 2009). L. monocytogenes therefore needs to precisely sense its environment to adapt accordingly. Here, the transcription factor PrfA (positive regulator of listeriolysin A), the master regulator of the virulence of L. monocytogenes is the key player.
Upon changes in temperature, nutrient conditions, components of stress response pathways, like the metabolic regulator CodY and effector molecules, such as glutathione, amount and activity state of PrfA is altered (de las Heras et al., 2011; Lobel et al., 2015; Reniere et al., 2015). In turn, PrfA-dependent genes that are important for triggering endocytosis (esp. internalins InlA and InlB), intracellular vacuolar escape (specific lipases PC-PLC, PI-PLC and the lytic enzyme LLO), up-take of nutrients (hexosephosphate transporter Hpt), intracellular traversation (actin polymeriza-tion by ActA) and infecpolymeriza-tion of neighboring cells are expressed (Chico-Calero et al., 2002; Scortti et al., 2007). Another key feature of L. monocytogenes is its ability to actively cross-protection barri-ers inside humans, such as the blood-brain or blood-placenta barrier, making it a dangerous bug for immunocompromised persons with the possibility of high mortality rates after encephalitis or meningitis and for pregnant women with the danger of prenatal infections, leading to severe con-sequences for the fetus, including disabilities, stillbirths or miscarriages (Low and Donachie, 1997;
Vázquez-boland et al., 2001).
The adaptation of L. monocytogenes to such different environmental niches makes it a very inter-esting candidate to study osmohomeostasis. The bacteria must on the one hand be able to with-stand changes in solute concentrations by environmental events such as changes in humidity and on the other hand be able to survive the transition to the intracellular life style, featuring passage through the stomach, exposure to bile salts in the gut and transition from a natrium rich to a potassium rich environment. One big difference between B. subtilis and L. monocytogenes is the inability of L. monocytogenes to form spores as a persistence state that is resistance against many environmental stresses and its ability despite that to be resistant against various pH, temperature or salt stresses (Farber & Peterkin, 1991). The main strategy for L. monocytogenes is hereby the accumulation of the compatible solutes glycine betaine, carnitine, glutamate and oligopeptides with some indications that glycine betaine uptake is also triggered upon cold stress in L. mono-cytogenes (Ko et al., 1994; Smith, 1996; Whiteley et al., 2017). Interestingly, glycine betaine is co-transported with sodium ions, which in turn have to be exported by putative cation-proton anti-porter systems, leading to an increase in internal protons that in turn could be exported again by electron transport system, thereby linking osmolyte uptake to generation of proton motive force that has also been linked to c-di-AMP itself (Wood, et al., 2001; Zeden et al., 2018). Fig. 1.3 shows an overview of the most prominent systems or putative homologs of L. monocytogenes to known
systems of B. subtilis (MrpA-G, NhaK, KimA, KtrCD, MscL, MscS and NrnA) or S. aureus (KdpFABCDE) that are involved in osmoregulation and/or c-di-AMP metabolism.
As depicted, L. monocytogenes encodes homologs of two high-affinity potassium uptake systems, KimA and KdpABC, that are translational or transcriptional controlled by c-di-AMP in B. subtilis and S. aureus respectively. Interestingly, the kimA riboswitch seems to be absent in L. monocytogenes (Nelson et al., 2013). L. monocytogenes also encodes a voltage gated potassium transporter KvLm (Lmo2059; Santos et al., 2008; Santos et al., 2012) and homologs of two potential sodium export system Lmo0803 (NhaK) and Lmo2378-2374 (MrpA-G). Furthermore, L. monocytogenes possesses two uptake system for glycine betaine, GbuABC and BetL, an uptake system for oligopeptides (Op-pABCDF), an uptake system for carnitine (OpuC) and a bile salt specific exporter (BilE), demon-strating a broad set of different osmolyte transport systems (for further details see chapters above). Of those Gbu, Opp and Opu have been directly or indirectly implicated to be regulated by c-di-AMP (Fig. 1.3). Genes for homologs of two mechanosensitive channels Lmo2064 and Lmo1013 (MscL and MscS, respectively) and the c-di-AMP binding proteins, PycA, CbpA, CbpB and PstA (DarA) are also present. Several c-di-AMP secreting multi drug efflux systems (MdrMTAC) and two PDEs PdeA (GdpP) and PgpH and a potential homolog of the nano-RNase NrnA (Lmo1575) are
Fig. 1.3 c-di-AMP signaling and osmoregulation in L. monocytogenes. c-di-AMP is synthesized in L. monocytogenes by the sole DAC CdaA that is regulated by CdaR and GlmM in L. monocytogenes. c-di-AMP has been shown to be secreted by the multidrug resistance transporters and degraded by the PDE PdeA (GdpP) and PgpH. L. monocytogenes also har-bors a homolog of the nano-RNase NrnA that has been shown to degrade pApA further to AMP. c-di-AMP binds to the proteins of unknown function CbpA, CbpB and PstA (DarA). It has also been shown to inhibit the pyruvate carboxylase PycA and the osmolyte (carnitine) uptake system OpuC in L. monocytogenes. c-di-AMP has also been shown to inhibit uptake of glycine betaine in other organisms via homologs of the Gbu osmolyte transporter, but no binding could be identified for L. monocytogenes. In other bacteria, c-di-AMP has been shown to inhibit potassium uptake of the KtrCD low affinity potassium transporter and to inhibit transcription of the KdpABC high affinity transporter by binding to KdpD of the KdpDE two-component system. Despite those c-di-AMP-related systems, L. monocytogenes possesses other proteins that have been shown to play a role in un- or specific osmoregulation. Those are the mechanosensitive channels MscL and MscS (large and small conductance, respectively), the glycine betaine uptake system BetL, the volt-age gated potassium exporter Lmo2059 (KvLm), homologs of the B. subtilis sodium exporter NhaK and MrpA-G. L. mon-ocytogenes also expressed a bile salt specific exporter that is important in virulence (BilE) and an oligopeptide trans-porter, whose expression is regulated by CodY and the stringent response. In L. monocytogenes c-di-AMP signaling re-lated suppressor mutations have been found in CbpB, PstA, KtrD, RelA, Gbu system, Opp system and PycA, indicating a functional relationship between them and c-di-AMP signaling (based on Commichau et al., 2018).
KtrD
Unknown Unknown / not in Listeria /
suppressor mutations*
present, as well. Concerning the synthesis of c-di-AMP, L. monocytogenes harbors only on DAC domain containing enzyme, CdaA, or also called DacA, encoded together with its regulator CdaR and also the GlmM enzyme (Commichau et al., 2018; Kaplan Zeevi et al., 2013; Rismondo et al., 2016; Rosenberg et al., 2015; Sureka et al., 2014; Whiteley et al., 2015; Whiteley et al., 2017;
Woodward et al., 2010).
L. monocytogenes has a broad set-up of osmolyte transport systems, interesting life-style changes and is known for its adaptiveness. The bacterium encodes only one DAC, of the most abundant CdaA-type that is a prime target for finding novel antibiotics. These characteristics make L. mono-cytogenes an interesting subject to study c-di-AMP and its effect on osmoregulation and especially how synthesis of c-di-AMP might be regulated and coupled to sensing alterations in the osmotic state of the environment.