Inter-kingdom Signaling by the Legionella Quorum Sensing Molecule LAI-1 Modulates Cell Migration through an IQGAP1-Cdc42-ARHGEF9-Dependent Pathway

Small molecule signaling promotes the communication between bacteria as well as between bacteria and eukaryotes. The opportunistic pathogenic bacterium Legionella pneumophila employs LAI-1 (3-hydroxypentadecane-4-one) for bacterial cell-cell communication. LAI-1 is produced and detected by the Lqs (Legionella quorum sensing) system, which regulates a variety of processes including natural competence for DNA uptake and pathogen-host cell interactions. In this study, we analyze the role of LAI-1 in inter-kingdom signaling. L. pneumophila lacking the autoinducer synthase LqsA no longer impeded the migration of infected cells, and the defect was complemented by plasmid-borne lqsA. Synthetic LAI-1 dose-dependently inhibited cell migration, without affecting bacterial uptake or cytotoxicity. The forward migration index but not the velocity of LAI-1-treated cells was reduced, and the cell cytoskeleton appeared destabilized. LAI-1-dependent inhibition of cell migration involved the scaffold protein IQGAP1, the small GTPase Cdc42 as well as the Cdc42-specific guanine nucleotide exchange factor ARHGEF9, but not other modulators of Cdc42, or RhoA, Rac1 or Ran GTPase. Upon treatment with LAI-1, Cdc42 was inactivated and IQGAP1 redistributed to the cell cortex regardless of whether Cdc42 was present or not. Furthermore, LAI-1 reversed the inhibition of cell migration by L. pneumophila, suggesting that the compound and the bacteria antagonistically target host signaling pathway(s). Collectively, the results indicate that the L. pneumophila quorum sensing compound LAI-1 modulates migration of eukaryotic cells through a signaling pathway involving IQGAP1, Cdc42 and ARHGEF9.小分子信号传导可促进细菌之间以及细菌与真核生物之间的信息交流。机会致病性病原体嗜肺军团菌（Legionella pneumophila）利用LAI-1（3-羟基十五烷-4-酮）进行细菌间的细胞信号交流。其合成与感知LAI-1的Lqs（军团菌群体感应，Legionella quorum sensing）系统可调控诸多生物学过程，包括DNA摄取的自然感受态以及病原体-宿主细胞互作。本研究旨在解析LAI-1在跨界信号传导中的功能。缺失自诱导合酶LqsA的嗜肺军团菌不再能够抑制受感染细胞的迁移，该缺陷可通过质粒携带的lqsA予以互补恢复。人工合成的LAI-1可呈剂量依赖性地抑制细胞迁移，且不会影响细菌吞噬作用或细胞毒性。经LAI-1处理的细胞，其正向迁移指数（而非迁移速率）出现下降，且细胞骨架呈现不稳定状态。LAI-1介导的细胞迁移抑制作用涉及支架蛋白IQGAP1、小GTP酶Cdc42以及Cdc42特异性鸟苷酸交换因子ARHGEF9，但不涉及其他Cdc42调控因子，亦不涉及RhoA、Rac1或Ran GTPase。经LAI-1处理后，无论细胞是否表达Cdc42，Cdc42均会发生失活，且IQGAP1会重新分布至细胞皮层。此外，LAI-1可逆转嗜肺军团菌对细胞迁移的抑制作用，表明该化合物与细菌可通过拮抗方式靶向宿主信号通路。综上，本研究结果表明，嗜肺军团菌群体感应化合物LAI-1可通过一条涉及IQGAP1、Cdc42与ARHGEF9的信号通路调控真核细胞的迁移。