Dissertation
Mechanistic insights into Staphylococcus aureus β-toxin and superantigen involvement in angiogenesis and its contribution to infective endocarditis
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Autumn 2021
DOI: 10.17077/etd.006305
Abstract
Staphylococcus aureus is the leading causative agent of infective endocarditis (IE) in high-income countries. IE is an infection of the cardiac endothelium characterized by “vegetative” lesions on the heart valves. If left untreated, staphylococcal IE has high morbidity and a 40-50% mortality rate. Staphylococcal IE is distinct from other causes of IE in that the non-healing lesions are tissue destructive. This suggests S. aureus may interfere with angiogenesis, a tissue repair process. S. aureus produces a plethora of virulence factors, and several have been implicated in IE. Recent evidence has shown β-toxin and superantigens (SAgs) extensively contribute to potentiating IE. Functional studies into the mechanisms underlying β-toxin and SAg contribution to IE have been limited.
β-toxin was shown to promote vegetation growth and increase lethality via its sphingomyelinase (SMase) and aggregation activities in rabbit IE models. However, most clinical isolates contain a phage inserted into the β-toxin encoding gene, hlb, thereby inactivating its transcription. Thus, it is debated whether β-toxin is produced during infection and whether it is a relevant virulence factor in vivo. The work in this thesis demonstrates that β-toxin is produced by the clinical isolate MW2 via a phage regulatory switch. We found that phage excision events increased during biofilm growth and in the presence of hydrogen peroxide. We concluded that β-toxin is tightly regulated and highly produced during conditions encountered during infections. This suggests β-toxin is a significant contributing virulence factor to infections, such as IE.
β-toxin SMase activity results in production of ceramide, a potent second messenger molecule, in eukaryotic cells. Ceramide can mediate cell fate and angiogenesis signaling pathways, therefore we sought to investigate whether β-toxin perturbed wound healing processes as a potential mechanism for IE pathogenesis. Utilizing in vitro and ex vivo analyses, we show that β-toxin inhibits wound healing from human aortic endothelial cells (HAECs) and human umbilical vein endothelial cells (HUVECs) and abolished sprout formation in a rabbit aorta explant model. To accomplish this, β-toxin prevents cell proliferation and migration. Evidence suggests that β-toxin may also induce currently undetermined endothelial dysfunction. Thus, β-toxin contributes to overall IE pathogenesis by perturbing proper tissue repair.
S. aureus SAgs are essential for IE pathogenesis. We recently demonstrated that staphylococcal enterotoxin C (SEC) superantigenicity did not contribute to IE in rabbit models. Furthermore, the SAg toxic shock syndrome toxin-1 (TSST-1) directly interacts with HAECs. Together, these data suggest these exotoxins possess SAg-independent functions where they are able to engage with non-hematopoietic cells. This thesis sought to further elucidate how interactions with HAECs along with the endothelium may affect angiogenesis to potentiate IE. Both TSST-1 and SEC prevented sprout formation and increased extracellular matrix degradation from rabbit aortic rings. We show that TSST-1, but not SEC, inhibits wound healing which correlated with aberrant production of several growth factors. This shows that SAgs more widely contribute to S. aureus infections than previously recognized.
Altogether, this body of work demonstrates the multifunctionality of two different S. aureus secreted virulence factors and proposes mechanisms by which they may promote IE. Both β-toxin and SAgs inhibit sprout formation, and thus angiogenesis, seemingly through different interactions and signaling events. This may then promote vegetation growth as the host will continue to deposit platelets and fibrous proteins in an attempt to repair the tissue.
Details
- Title: Subtitle
- Mechanistic insights into Staphylococcus aureus β-toxin and superantigen involvement in angiogenesis and its contribution to infective endocarditis
- Creators
- Phuong Tran
- Contributors
- Wilmara Salgado-Pabon (Advisor)Mary Weber (Advisor)David Weiss (Committee Member)Brad Jones (Committee Member)Aloysius J Klingelhutz (Committee Member)Stefan Strack (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Microbiology
- Date degree season
- Autumn 2021
- DOI
- 10.17077/etd.006305
- Publisher
- University of Iowa
- Number of pages
- xii, 137 pages
- Copyright
- Copyright 2021 Phuong Tran
- Language
- English
- Description illustrations
- illustrations (some color)
- Description bibliographic
- Includes bibliographical references (pages 106-123).
- Public Abstract (ETD)
Staphylococcus aureus are bacteria that cause multiple different infections that affect the skin, heart, lungs, and bones. S. aureus mediates these infections by producing a wide array of host-targeting proteins, such as β-toxin and the superantigens. From previous studies, we know that β-toxin and superantigens play important roles in a heart disease called infective endocarditis. However, we do not know what these proteins do to help cause infective endocarditis. We know that infective endocarditis infections do not properly heal on their own, which is what makes these infections so dangerous. Therefore, we wanted to determine if β-toxin and superantigens somehow inhibited this healing process. We found that both β-toxin and superantigens prevented healing by different mechanisms. β-toxin prevented endothelial cells from growing while superantigens caused tissue damage. Additionally, we found specific proteins were produced during exposure to these toxins, which could help aid in diagnosing endocarditis from patient sera. Overall, these findings help inform on our understanding of how S. aureus causes invasive infections like endocarditis.
- Academic Unit
- Microbiology and Immunology
- Record Identifier
- 9984210641602771
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