Assistant Professor of Immunology & Microbiology, University of Colorado Anschutz School of Medicine
The major focus of my research program is to elucidate pathways of innate immunity that can distinguish harmless microbes from pathogens, thereby enabling the host to mount responses that are commensurate with the threat.
Our innate immune system orchestrates critical immediate defenses against microbial invasion. To distinguish microbes from self, the innate immune system relies on the detection of microbe-associated molecular patterns (MAMPs) by pattern recognition receptors (PRRs). However, MAMPs are present in both harmless and pathogenic microbes. Detection of MAMPs is therefore not always sufficient to mount a response that is commensurate with the threat. To distinguish between virulent microbes and ones with lower disease-causing potential, the mammalian innate immune system can detect pathogen-induced processes by mechanisms that are not fully resolved. One pathogen-induced process sensed by the innate immune system is the ability to access the host cell cytosol. Enteric pathogens gain cytosolic access by utilizing a type III secretion system (T3SS) and remain either in a pathogen-containing vacuole (e.g. Salmonella enterica serotype Typhimurium [S. Typhimurium]), escape from the pathogen-containing vacuole after entering epithelial cells (e.g. Shigella flexneri) or remain in the intestinal lumen (e.g. Citrobacter rodentium and enteropathogenic Escherichia coli [EPEC]). The function of T3SSs is to deliver proteins, termed effectors, into host cells. Here effector proteins target numerous cellular signaling pathways to mediate invasion of (S. Typhimurium and S. flexneri) or attachment to (C. rodentium and EPEC) the epithelium. However, translocation of effector proteins also induces NF-κB activation and inflammatory responses, suggesting that this process is detected as a pathogen-induced process through unknown mechanisms. Effector proteins directly implicated in NF-κB activation include EspT, EspM2 and Map from EPEC and C. rodentium, IpgB1, IpgB2 and OspB from S. flexneri and SipA and SopE from S. Typhimurium. One thing these effector proteins have in common is that they serve as nuclear exchange factors for the small Rho GTPase family consisting of Rac1, Cdc42 and RhoA. Furthermore, the small Rho GTPases are required for effector-mediated NF-κB activation. However, the signaling pathway downstream of Rho GTPases that leads to NF-κB activation is poorly understood. Our studies indicate that activation of Rac1 by SopE triggers the NOD1 signaling pathway with consequent Rip2-mediated induction of NF-κB-dependent inflammatory responses both in tissue culture cells as well as in the mouse colitis model. Others and we have shown recently that IpgB2 and SipA-mediated NF-κB activation proceeds through NOD/Rip2, thereby implicating this signaling pathway in the detection of cytosolic access by a T3SS. But how do effector proteins generate a conserved “pattern of pathogenesis” that activates the NOD1/2 signaling pathway? What is the role of the small Rho GTPases in inducing effector-mediated NF-κB activation and what is the connection to NOD1? These results suggest that manipulation of the actin cytoskeleton by injecting proteins that alter the activity of small Rho GTPases is a common theme in microbial pathogenesis.
A second focus of my research program is the involvement of Nodosome activation in ER stress and the Unfolded Protein Response (UPR). NOD1 and NOD2 have been associated with metabolic diseases/inflammatory disorders such as inflammatory bowel disease (IBD) and type 2 diabetes. Other genes associated with these diseases are for example ATG16L and XBP1 that are important factors in the essential cellular processes autophagy and the unfolded protein response (UPR), respectively. The ATG16L1 protein is recruited to the cell membrane by NOD1 and NOD2 at the site of bacterial invasion, thereby linking autophagy and NOD activation. The UPR is activated when stress in the ER occurs as a result of the accumulation of unfolded protein in the lumen of the ER. Activation of the UPR will restore the normal function of the cell by inhibiting protein synthesis and activation of signaling pathways. Three transmembrane receptors, ATF6, PERK and IRE1, are disassociated from the ER chaperone protein BiP, and this disassociation results in dimerization and phosphorylation of these receptors to an active state. Activated IRE1 recruits the tumor necrosis factor (TNF) receptor-associated factor 2 (TRAF2) that leads to the activation of MAP kinases and NF-κB and the subsequent production of pro-inflammatory cytokines such as IL-6 and TNFα. Interestingly, NOD1 and NOD2 contain major TRAF2 binding motifs, and TRAF2 has been reported to be essential in NOD1 induced NF-κB activation. We therefor investigated whether NOD1 and/or NOD2 are involved in the NF-kB activation as a result of ER stress. Our research shows that activation of the pro-inflammatory signaling pathway within the UPR induced by ER stress is dependent on the innate immune sensors NOD1 and NOD2. The association of NOD1 and NOD2 with the ER stress induced pro-inflammatory responses may have great implications in the understanding of metabolic diseases/inflammatory disorders such as inflammatory bowel disease (IBD) and type 2 diabetes.
In summary, the host recognizes distinct cellular processes either induced by pathogens for example through the activation of small Rho GTPases, or by self-inflicted damage by disruption of the ER structure and induction of the UPR. Our research program will continue to test our central hypothesis that the nodosome helps monitoring the integrity of the host cell cytosol by integrating cellular signals to induce pro-inflammatory responses.