Dissecting the role of the Tir:Nck and Tir:IRTKS/IRSp53 signalling pathways in vivo

Attaching and effacing (A/E) lesions and actin polymerization, the hallmark of enteropathogenic Escherichia coli (EPEC), enterohemorrhagic E. coli (EHEC) and Citrobacter rodentium (CR) infections, are dependent on the effector Tir. Phosphorylation of Tir_EPEC/CR Y474/1 leads to recruitment of Nck and neural Wiskott–Aldrich syndrome protein (N‐WASP) and strong actin polymerization in cultured cells. Tir_EPEC/CR also contains an Asn‐Pro‐Tyr (NPY_454/1) motif, which triggers weak actin polymerization. In EHEC the NPY₄₅₈ actin polymerization pathway is amplified by TccP/EspF_U, which is recruited to Tir via IRSp53 and/or insulin receptor tyrosine kinase substrate (IRTKS). Here we used C. rodentium to investigate the different Tir signalling pathways in vivo. Following infection with wild‐type C. rodentium IRTKS, but not IRSp53, was recruited to the bacterial attachment sites. Similar results were seen after infection of human ileal explants with EHEC. Mutating Y471 or Y451 in Tir_CR abolished recruitment of Nck and IRTKS respectively, but did not affect recruitment of N‐WASP or A/E lesion formation. This suggests that despite their crucial role in actin polymerization in cultured cells the Tir:Nck and Tir:IRTKS pathways are not essential for N‐WASP recruitment or A/E lesion formation in vivo. Importantly, wild‐type C. rodentium out‐competed the tir tyrosine mutants during mixed infections. These results uncouple the Tir:Nck and Tir:IRTKS pathways from A/E lesion formation in vivo but assign them an important in vivo role.