Studies from Dr. Feng Shao’s laboratory at National Institute of Biological Sciences (NIBS), Beijing, reveal the key mechanisms of Burkholderia cenocepacia infection and the inflammatory responses induced by this Gram-negative bacterial pathogen

     April 28, 2016. Studies from Dr. Feng Shao’s laboratory at National Institute of Biological Sciences (NIBS), Beijing, reveal the key mechanisms of Burkholderia cenocepacia infection and the inflammatory responses induced by this Gram-negative bacterial pathogen.

     The innate immune system in mammals senses microbial products and is critical for counteracting pathogen infection. Central to innate immunity is the membrane-bound or cytosolic pattern recognition receptor (PRR). In a previous study (Xu et al., Nature 2014), the Shao group discovered that Pyrin, whose genetic mutation causes a human autoinflammatory disease called familial Mediterranean fever, serves as a PRR to sense inactivating modifications of Rho GTPases by various bacterial toxins and then trigger canonical inflammasome assembly and caspase-1 activation in macrophage anti-bacteria immunity. Recently, they identified a novel type VI secretion system effector in B. cenocepacia that uses an unprecedented deamidase activity to modify Rho GTPases. Such modification and the resulting inactivation of Rho GTPases can also lead to Pyrin inflammasome activation, which mediates lung inflammation and anti-bacterial defenses in mice. The work entitled “A Burkholderia Type VI Effector Deamidates Rho GTPases to Activate the Pyrin Inflammasome and Trigger Inflammation” is published in Cell Host & Microbe on April 28, 2016.

     B. cenocepacia is an opportunistic Gram-negative pathogen that infects lung alveolar microphages. B. cenocepacia infection is self-limiting in healthy individuals but can cause severe and even lethal infection in cystic fibrosis patients. The hallmark of B. cenocepacia infection in both clinics and mouse model is severe lung inflammation, but the underlying mechanism is unknown. In macrophage infection, B. cenocepacia, like many other bacterial pathogens, cause inactivation of Rho GTPases and disruption of actin cytoskeleton structure, resulting in a characteristic cell morphological change, which critically requires bacterial type VI secretion system and is essential for intracellular survival of the bacteria. Taking advantage of above morphology phenotype, the Shao group, in collaboration with Professor Miguel Valvano’s team at the Queen’s University in United Kingdom, performed a transposon-based bacterial genetic screen, and identified a type VI secretion system effector which they named as TecA; B. cenocepacia devoid of the TecA-encoding gene fail to induce cytoskeleton disruption and morphological change of infected macrophages. In the Xu et al. study, Shao and his colleagues previously showed that B. cenocepacia can cause deamidation of Rho GTPases on a specific asparagine residue. Furthering this, they, through a series of biochemical and cell biological experiments, now showed that the type VI secretion system-delivered TecA is both necessary and sufficient for B. cenocepacia-induced asparagines deamidation of Rho GTPases. Bioinformatics analyses of TecA sequence uncovered TecA homologous proteins in other bacterial pathogens; the TecA family bears a conserved Cys-His-Asp catalytic triad characteristic of many proteases and hydrolytic enzymes. When any of the catalytic triad residues was mutated, TecA and its homologues were found to lose the ability of catalyzing Rho deamidation. The Rho family contains three major members, RhoA, Rac1 and Cdc42. The researchers further discovered that deamidation of Rac1 is responsible for infection-induced actin cytoskeleton disruption, and the same modification/inactivation of RhoA determines Pyrin inflammasome activation in B. cenocepacia infected macrophages. In the mouse model of B. cenocepacia infection, the Shao team showed that TecA deamidation of Rho GTPases is responsible for B. cenocepacia-triggered and Pyrin inflammasome-mediated lung inflammation, which plays an important role in bacterial clearance and protecting mice from lethal B. cenocepacia infection.

     The study for the first identifies a type VI secretion effector in Burkholderia species and TecA is also the first bacterial effector with asparagines deamidase activity. Different from most type VI effectors that target bacterial competitors, TecA is one of the few type VI effectors that work inside host cytosol for virulence. The Shao study also reveals the mechanism for B. cenocepacia-induced lung inflammation, answering a long standing question in the field.

     Postdoc fellow Daniel F. Aubert in the Valvano laboratory and postdoc fellows Hao Xu and Jieling Yang from the Shao group are co-first authors of this paper; PhD students Xuyan Shi and Wenqing Gao from the Shao lab also made significant contributions; other contributors Lin Li and Dr. She Chen at NIBS proteomic facility as well as graduate student Fabiana Bisaro from the Valvano laboratory. Professor Miguel Valvano and Dr. Feng Shao are co-corresponding authors of this paper. The research carried in the Shao laboratory at National Institute of Biological Sciences, Beijing was supported by the 973 National High-Tech. Projects, the Beijing Municipal Government, and Howard Hughes Medical Institute in the States.

http://dx.doi.org/10.1016/j.chom.2016.04.004