Inhibition of sortase A by chalcone prevents Listeria monocytogenes infection

Abstract

The critical role of sortase A in gram-positive bacterial pathogenicity makes this protein a good potential target for antimicrobial therapy. In this study, we report for the first time the crystal structure of Listeria monocytogenes sortase A and identify the active sites that mediate its transpeptidase activity. We also used a sortase A (SrtA) enzyme activity inhibition assay, simulation, and isothermal titration calorimetry analysis to discover that chalcone, an agent with little anti-L. monocytogenes activity, could significantly inhibit sortase A activity with an IC₅₀ of 28.41 ± 5.34 μM by occupying the active site of SrtA. The addition of chalcone to a co-culture of L. monocytogenes and Caco-2 cells significantly inhibited bacterial entry into the cells and L. monocytogenes-mediated cytotoxicity. Additionally, chalcone treatment decreased the mortality of infected mice, the bacterial burden in target organs, and the pathological damage to L. monocytogenes-infected mice. In conclusion, these findings suggest that chalcone is a promising candidate for the development of treatment against L. monocytogenes infection.

Introduction

The food-borne pathogen Listeria monocytogenes (L. monocytogenes), a gram-positive facultative intracellular bacterium, is the causative agent of listeriosis, which is characterized by gastroenteritis, meningitis, encephalitis, bacteremia, soft tissue and parenchymal infections, and mother-to-fetus transmission [1], [2]. Upon the ingestion of contaminated foods by the host, the invasion of L. monocytogenes into host cells enables the bacteria to evade the humoral immune system, which is a critical step in the establishment of the infection [2]. During infection, L. monocytogenes has evolved elaborate molecular strategies to enter, survive, and multiply inside both professional and non-professional phagocytic cells, thus complicating the treatment of L. monocytogenes infections [3]. Previous studies have shown that different types of cell wall anchored proteins (internalin A, internalin B, VIP, etc.) and secreted pore-forming cytolysins (listeriolysin) are required for cell entry, suggesting that this process is quite complex [3], [4], [5]. Thus, interference with the surface proteins involved in the infection process would be a promising direction for controlling L. monocytogenes infections.

Many surface proteins in gram-positive bacteria are anchored to the cell wall envelope by the transpeptidase enzyme sortase A (SrtA, encoded by srtA), which recognizes the conserved LPXTG motif, where X is any amino acid [6]. After a protein precursor is targeted for translocation across the membrane by its N-terminal secretion signal, the intermediate is covalently linked to the cell wall by the attack of an amine nucleophile, and then SrtA cleaves the protein precursor on its LPXTG motif between the threonine (T) and glycine (G) residues [7]. The thiolate group of the essential active site Cys in SrtA then attacks the scissile Thr-Gly bond on the precursor protein. Two additional absolutely conserved residues, His127 and Arg197, are also located in the active pocket of the transpeptidase protein [8], [9], [10]. Sortases have been well characterized as ideal targets of anti-infective drug development, because many of the surface proteins they display are virulence factors required for infection [11]. Mazmanian’s group showed that Staphylococcus aureus SrtA mutants are defective in anchoring surface proteins and in the pathogenesis of animal infections [12]. It was also shown recently that the virulence of Streptococcus pneumoniae [13] and Bacillus anthracis [14] SrtA mutants was reduced by infection of tissue culture cells. In L. monocytogenes, the inactivation of SrtA did not affect the expression of LPXTG-containing proteins, but SrtA mutation inhibited the anchorage of these proteins, which affected bacterial virulence [15]. In their recent study, Zhang et al. [16] and Huang et al. [17] showed that SrtA inhibitors could provide robust protection against S. aureus or Streptococcus mutans infections. However, to our knowledge, no study examining the implications of L. monocytogenes SrtA inhibition has been reported.

Natural products are a major source of chemical and functional diversity and have provided a variety of therapeutic agents with bacteriostatic, bactericidal, or anti-virulence factor activity against many bacterial infections [18]. In this study, we have identified chalcone, a precursor molecule of many flavonoids that is found in traditional Chinese medicine, as a potential inhibitor of L. monocytogenes SrtA. Chalcone and its derivatives have been reported to exert a broad spectrum of pharmacological activities including anti-malarial [19], anti-cancer, immunomodulatory, antiviral, and antimicrobial properties [20].

Based on the above description, we systematically evaluated the inhibitory effects of chalcone on L. monocytogenes virulence in vitro and in vivo. Furthermore, we simulated the interaction between the SrtA active site and its peptide substrate, LPTTG. We verified the binding site residues through which chalcone interacts with SrtA and the mechanism by which chalcone inhibits sortase A. Taken together, the data in the present study suggest that chalcone provides robust protection against L. monocytogenes infection by blocking the active site of SrtA and that chalcone is a promising candidate for the development of an anti-infective drug against listeriosis.