Deciphering the function of actin remodeling in NLR-mediated innate immune sensing

Innate immunity depends on sensing of conserved pathogen derived structures, referred to as “microbe associated molecular patterns” (MAMPs) by “pattern-recognition receptors” (PRRs) in the host cell. Members of the NLR-protein family are important PRRs that refer inflammatory responses upon activation by microbial stimuli. We have a long standing interesting in the functional characterization of mammalian NLR proteins. A particular focus of our work lies on the NLR-protein NOD1 that senses bacterial peptidoglycan. Although the main concepts of NOD1 activation and signaling are emerging, our knowledge on the cell biology of NOD1 and its fine regulation is still quite limited.  We have shown that NOD1 co-localizes with F-actin in the host cell and in more recent work using HT-screening we now have identified components of the actin regulatory network that contribute to NOD1-mediated inflammatory responses. Notably, many invasive bacteria, that can activate NOD1, can simultaneously affect actin dynamics by the action of virulence proteins which often target small GTPases that indirectly regulate F-actin dynamics. Recent findings, show that subversion of host Rho GTPase activity by bacterial effectors is linked to NOD1-mediated inflammatory responses.

Our recent study revealed that F-actin dynamic is important for NOD1 signaling in human cells. We have identified key components of the regulation of F-actin dynamic, namely the phosphatase SSH1 and its substrate cofilin, as important molecular mediators of this function. This leads us to propose that these proteins are involved in integrating subversion of actin remodeling by bacterial effectors and MAMP sensing.

Here we will expand on our recent findings and address open questions, such as the role of the different orthologs of cofillin and SSH in NLR-meditated responses. Most importantly, we will elucidate the underlying molecular details by identification of the network of proteins involved in this novel pathway. Finally, will address if activation of NOD1 changes actin polymerization in human cells and if this contributes to cell-autonomous anti-bacterial responses. Moreover, we will expand our initial observation made with Shigella flexneri as infection model to other clinically relevant bacterial pathogens and analyze the effect of SSH1/cofilin on additional NOD1-mediated signalling outcomes such as autophagy.

The indirect sensing of bacterial activity by “guarding” host cell actin dynamics is a novel concept in the field of innate immunity in mammals. Deciphering the role of F-actin remodelling in NLR-mediated innate immunity will provide important novel insights into the biology of host-pathogen interactions and in the long run will set the stage for the development of novel therapeutically strategies to treat inflammatory disorders.