Ubiquitination of pro-IL-1β in response to GAS infection of macrophages

Abstract

Ubiquitination is a post-translational modification process that has a very important role in the homeostasis of eukaryotic cells. Linkage-specific ubiquitination determines the final destination of modified protein. Ubiquitination has also been reported to regulate inflammasome function and IL-1β degradation. Secretion of IL-1β by macrophages in response to inflammasome activation is a key process in the innate immune system in order to develop inflammatory responses during GAS infection.
The aim of this study was to investigate whether GAS is able to manipulate the ubiquitination status of pro-IL-1β and inflammasome outcome.
We used a set of isogenic GAS mutants and a bone marrow-derived macrophage infection model to investigate the process of GAS induced pro-IL-1β ubiquitination. We were able to demonstrate that pro-IL-1β ubiquitination in BMDMs infected by GAS is dependent on GAS secreted SLO and is facilitated by SLO pore formation. We also showed that pro-IL-1β ubiquitination is not required for inflammasome activation and IL-1β release, but might have a role in inducing the degradation of that potent pro-inflammatory cytokine. Our results indicate that SLO is a crucial virulence factor and we propose that SLO, by inducing pro-IL-1β ubiquitination, could manipulate immune responses generated by the host benefiting the bacteria.

Popular Abstract

Streptococcus pyogenes, also known as group A Streptococcus (GAS), is a human specific Gram-positive bacterium. GAS causes diseases ranging from mild skin and throat infections to serious infections such as flesh eating disease and streptococcal toxic shock syndrome, and may also lead to severe sequelae such as rheumatic heart disease. Transmission occurs via respiratory droplets and person-to-person skin contact with infected person. GAS is responsible for over half a million deaths per year globally. A number of GAS virulence determinants are identified to be involved in the pathogenesis processes within the human host. Streptolysin O (SLO) is one of the main virulence factors having an important role in inducing host cell death by compromising host cell membranes and in translocating other bacterial substances into host cell. GAS remains sensitive to penicillin and treatment relies on penicillin.

Ubiquitination is a post-translational modification process in which small protein molecules called ubiquitins are attached to the target protein. The attachment of ubiquitin molecules to target proteins results in certain cellular processes and determines the functional outcome of the ubiquitination process. IL-1β is a major pro-inflammatory cytokine driving inflammation and the subsequent immune response that helps eliminating pathogenic bacteria such as GAS. IL-1β is expressed as an inactive pro-form that requires processing by, in the case of GAS infection, the so-called inflammasome. Once IL-1β is cleaved as a result of inflammasome activation it will be released by the host cells via a poorly understood mechanism. Since inflammasome activation, IL-1β release and IL-1β degradation have all been described to be regulated by ubiquitination, we set out to investigate whether GAS has the ability to influence that ubiquitination status of inflammasome proteins upon infection of macrophages.

For this study, we used a set of isogenic GAS mutants and a bone marrow-derived macrophage (BMDMs) infection model to assess the effect of GAS on pro-IL-1β ubiquitination and ubiquitination of inflammasome related proteins. In our result, we were able to demonstrate that GAS induces pro-IL-1β ubiquitination in an SLO dependent manner. Our results further suggested that GAS induced IL-1β ubiquitination is not required for inflammasome activation or cytokine release but might have a role in inducing the degradation of IL-1β. We suggest that pro-IL-1β ubiquitination could be the strategy employed by GAS to manipulate immune responses generated by the host during GAS infection that could potentially favor the bacteria.

Advisors: Jenny J. Perssson and & Dóra Hancz
Master´s Degree project 60 credits in Molecular Biology (Microbiology)
Department of Biology, Lund University

• author: Melkamu, Getachew
• supervisor: Jenny J Persson
• organization: Degree Projects in Molecular Biology
• course: MOBT01 20171
• year: 2017
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 8925608
• date added to LUP: 2017-09-14 08:40:26
• date last changed: 2017-09-14 08:40:26