Activation of caspase-1 by the NLRP3 inflammasome regulates the NADPH oxidase NOX2 to control phagosome function

Anna Sokolovska, Christine E Becker, W K Eddie Ip, Vijay A K Rathinam, Matthew Brudner, Nicholas Paquette, Antoine Tanne, Sivapriya K Vanaja, Kathryn J Moore, Katherine A Fitzgerald, Adam Lacy-Hulbert & Lynda M Stuart. Nature Immunology. (2013) 14, 543-553

Speaker: Cheng-Lun Hsu (徐政倫)                              Time: 14:00~15:00, Oct. 30, 2013

Commentator: Dr. Chin-Peng Chang (張志鵬博士)    Place: Room 601

Abstract:

The internalizaion of particles into organelles called phagosomes, which restrict the growth of pathogen and process antigen for priming responses to T cells. Phagosomal acidification is associated with the clearance of pathogen and maturation of phagosome (1), and it begins immediately after the engulfment of pathogen. In the early acidification, the phagosomal pH is mainly mediated by vacuolar H⁺ ATPase (V-ATPase). However, how the phagosomal pH is regulated at the later stage remains unclear. The author thought that the rapid remodeling of organelles in response to the internalization of pathogen is probably controlled at the post-translational level. To investigate this possibility, they focused on NLRP3 inflammasome whose downstream effectors,caspases, can rapidly modify the selected substrates of host. It is necessary to detect the active caspase-1 by YVAD-FLICA, a caspase-1 specific inhibitor labeled with fluorescence, inStaphylococcus.aureus-infected macrophages. In the analysis of the microscopy and flow cytometry, S. aureus phagocytosis triggered the NLRP3 inflammasome and activation of caspase-1, which accumulated on the S. aureus-containing vacuoles. Furthermore, the inhibition of NLRP3 inflammasome and caspase-1 lead to the defect in acidification, suggesting that the NLRP3 inflammasomeregulates acidification of S. aureus-containing phagosomes. Previous study has shown that the NADPH oxidase, NOX2, can counteract the activity of V-ATPase and neutralize phagosome pH (2). The author identified the potential caspase-1 cleavage site of NOX2 complex, including the Rac subunit and the gp91-phox. The results suggest that caspase-1 hydrolyzes one or more subunits of the NOX2 complex to control the phagosome pH. Unlike the case of Gram-positive bacteria, Gram-negative bacteria can't regulate the phagosomal pH through the NLRP3 inflammasome-caspase-1 pathway. Taken together, these data showed another novel function of caspase-1 in regulating phagosome maturation and function.

References:

1.     E. Sergio Trombetta. et al. (2003) Activation of Lysosomal Function During Dendritic Cell Maturation. Science 299, 1400-1403.

2.     Savina, A. et al. (2006) NOX2 controls phagosomal pH to regulate antigen processing during crosspresentation by dendritic cells. Cell 126, 205–218.