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pH-responsive substrate switching in mycobacterial type VII ESX secretion
Owen A Collars1, Richard L Hernandez1,2, Simon D Weaver1,2
1Department of Biological Sciences, University of Notre Dame, Notre Dame, Indiana, USA.
Abstract:
During infection, pathogenic mycobacteria reside within phagosomes of varying acidity based on the macrophage activation state. The ESX-1 secretion system (early secreted antigen 6 kilodaltons [ESAT-6] system 1) delivers protein virulence factors essential for phagosome lysis, facilitating infection. The mechanisms underlying ESX-1 lytic activity in heterogeneous environments remain unknown. Here, we show that the canonical Type VII secretion system, ESX-1, orchestrates substrate switching in response to different environments. Growing Mycobacterium marinum at acidic pH resulted in substrate switching in vitro. Substrate switching was accompanied by significant changes to the levels of ESX-1 substrate transcripts and to the levels of both ESX-1 substrates and chaperones at the protein level. We showed that specific ESX-1 transcripts were significantly upregulated in vivo and that distinct substrate sets are required in an acidic infection model.
Importance:
Pathogenic mycobacteria cause chronic and acute disease. Mycobacterial pathogens promote infection by transporting bacterial proteins into the host using ESX/Type VII secretion systems. The ESX-1 system secretes proteins into the phagosome that release the bacteria into the cytoplasm and promote bacterial survival in the macrophage. We show that Mycobacterium marinum, an animal pathogen and model for studying ESX-1 and tuberculosis, switches which ESX-1 proteins are secreted in response to acidic pH, an infection relevant signal. We demonstrate that protein secretion reflects changes in substrate transcripts and in substrate and chaperone protein levels. Finally, we leveraged two infection models to support that ESX-1 substrate switching likely occurs during infection. Our findings support a model in which mycobacterial pathogens use different proteins to lyse macrophage phagosomes of different pH.
Insights
Pathogenic mycobacteria like Mycobacterium marinum adapt to host environments by switching the proteins secreted by the ESX-1 system. This substrate switching, triggered by acidic pH, helps bacteria lyse phagosomes and promote infection.
Area of Science:
- Microbiology
- Cell Biology
- Pathogen-Host Interactions
Background:
- Pathogenic mycobacteria, including those causing tuberculosis, reside in host macrophages within phagosomes.
- The ESX-1 secretion system (early secreted antigen 6 kilodaltons [ESAT-6] system 1) is crucial for mycobacterial virulence, enabling phagosome lysis and bacterial escape.
- The mechanisms by which ESX-1 functions in diverse phagosomal environments are not fully understood.
Purpose of the Study:
- To investigate how the ESX-1 secretion system functions in varying pH conditions relevant to macrophage infection.
- To determine if mycobacteria switch the proteins they secrete via ESX-1 in response to environmental cues like acidic pH.
- To elucidate the molecular changes associated with ESX-1 substrate switching and its relevance during infection.
Main Methods:
- Culturing *Mycobacterium marinum* at different pH levels *in vitro* to observe substrate secretion.
- Analyzing changes in ESX-1 substrate and chaperone transcript and protein levels.
- Utilizing *in vivo* infection models to assess the role of ESX-1 substrate switching during infection.
Main Results:
- *Mycobacterium marinum* exhibits ESX-1 substrate switching when grown at acidic pH *in vitro*.
- Substrate switching correlates with significant alterations in ESX-1 substrate and chaperone transcript and protein expression.
- *In vivo* studies revealed upregulation of specific ESX-1 transcripts and demonstrated the requirement for distinct substrate sets in an acidic infection model.
Conclusions:
- The ESX-1 secretion system dynamically orchestrates substrate switching in response to environmental pH.
- Mycobacterial pathogens likely employ distinct ESX-1 protein sets to lyse macrophage phagosomes of differing acidity during infection.
- Understanding ESX-1 substrate switching provides insights into mycobacterial pathogenesis and potential therapeutic targets.
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