Contribution of LicB to the adaptation of Streptococcus pneumoniae to host niche conditions

Choline Binding Proteins (CBP) play a fundamental role in the infection mechanism of S. pneumoniae by promoting adhesion, endocytosis, and evasion of the host immune system (Figure 1) (1–4). CBPs bind to teichoic acids by their phosphorylcholine decoration moieties, a feature that is key to performing their functions. S. pneumoniae is auxotrophic for choline, a necessary precursor for teichoic acid decoration, and the protein that mediates choline import is LicB (Figure 1), a proton-coupled symporter essential for bacterial growth and survival (5,6). Understanding the function of LicB can contribute to the development of new approaches for the treatment and prevention of S. pneumoniae infection (7). In this study, I aimed to uncover the contribution of LicB to S. pneumoniae adaptation to host niche pH conditions. I used solid supported membrane (SSM)- based electrophysiology to investigate LicB function under different pH conditions to assess its impact on choline import mediated by LicB. I showed that low pH conditions boost the transport rate activity of LicB, which might play an essential role in the adaptation mechanism to the physiological pH of the host that ranges from 5.0 to 6.5 (8,9). I also evaluated the impact of the enhanced transport rate activity of LicB on the composition of the cell wall and showed that the phosphorylcholine content of teichoic acids is increased under low pH conditions.
Due to the essential role of LicB in cell wall biosynthesis and S. pneumoniae pathogenicity, obtaining inhibitors against this protein is highly relevant. I have tested the effect of two compounds that are known to inhibit the human choline transporter CHT1: hemicholinium 3 (HC3) and hemicholinium 15 (HC15) (6,10–13). I showed that both compounds inhibit LicB activity in proteoliposomes, however, no inhibitory effect was observed in vivo. I showed that HC3 impacts the S. pneumoniae growth pattern by hindering the transition into the autolysis phase. Autolysis is a crucial process in the infection cycle of S. pneumoniae, as it allows for the release of virulent factors such as pneumolysin, which initiates the inflammatory process (14 16). These results indicate that compounds such as HC3 are valuable molecules that could serve as templates for the synthesis of new molecules for the prevention of detrimental effects of inflammation caused by S. pneumoniae.