Uropathogenic Escherichia coli virulence factor hemolysin A causes programmed cell necrosis by altering mitochondrial

Yongning Lu¹, Amir Rafiq¹, Zhengguo Zhang¹

¹Department of Anatomy and Cell Biology, Unit of Reproductive Biology, Justus-Liebig-University Giessen, Giessen, Germany.

FASEB Journal : Official Publication of the Federation of American Societies for Experimental Biology

|March 1, 2018

Summary

Uropathogenic Escherichia coli (UPEC) is a common cause of urinary tract infections. This study found that UPEC uses a toxin called hemolysin A (HlyA) to trigger a specific type of cell death called necrosis. Unlike typical cell death pathways like apoptosis, HlyA does not activate caspases. Instead, it increases calcium levels in mitochondria, causing mitochondrial dysfunction. This leads to loss of energy production and damage to the cell membrane. The study also found that HlyA causes the release of molecules like HMGB1 and histone 3, which are linked to necrotic cell death. These findings suggest that UPEC uses HlyA to escape immune detection and silently spread within host cells.

Area of Science:

Microbial pathogenesis
Cellular and molecular biology
Infectious disease mechanisms

Background:

Uropathogenic Escherichia coli (UPEC) frequently causes urinary tract infections. Prior research has shown that UPEC employs various virulence factors to manipulate host cells. However, the specific mechanisms by which UPEC evades immune detection remain unclear. Established knowledge suggests that programmed cell death pathways like apoptosis are common in infections. This gap motivated the investigation into UPEC's unique strategies. No prior work had resolved how UPEC might bypass apoptosis. The study aimed to clarify if UPEC uses alternative cell death mechanisms. This uncertainty drove the focus on hemolysin A (HlyA), a known UPEC toxin. The research sought to determine whether HlyA could trigger non-apoptotic cell death.

Purpose Of The Study:

The aim of this study was to investigate how UPEC employs HlyA to manipulate host cell death. UPEC is known to cause infections but the exact role of HlyA in cell death remains unclear. The researchers sought to determine whether HlyA-induced cell death bypasses caspase activation. This question arose from the observation that UPEC-infected cells did not show typical apoptotic markers. The study focused on mitochondrial dynamics and calcium regulation. The motivation was to understand how UPEC evades immune responses. The researchers proposed that HlyA might trigger necrotic cell death. This hypothesis was based on prior findings showing HlyA's role in membrane damage.

Main Methods:

The researchers used UPEC strains expressing hemolysin A (HlyA) to infect host cells. They measured mitochondrial calcium levels using fluorescent indicators. Mitochondrial dynamics were assessed by imaging mitochondrial networks. The study tracked changes in membrane potential and reactive oxygen species. Researchers quantified ATP levels to assess mitochondrial function. Plasma membrane integrity was evaluated using dye exclusion assays. The release of HMGB1 and histone 3 was detected via ELISA. The experimental design included both infected and uninfected control groups.

Main Results:

UPEC strains with HlyA did not activate caspases, indicating non-apoptotic cell death. HlyA increased mitochondrial calcium levels, disrupting normal function. Mitochondrial networks fragmented, impairing energy production. Membrane potential was lost, and reactive oxygen species increased. ATP levels dropped significantly in infected cells. Plasma membrane damage was observed, with HMGB1 and histone 3 released. These findings suggest programmed cell necrosis rather than apoptosis. The results indicate that HlyA manipulates mitochondrial dynamics to trigger necrotic cell death.

Conclusions:

The study concludes that UPEC uses HlyA to induce programmed cell necrosis. This process involves mitochondrial calcium overload and fragmentation. The authors suggest that HlyA bypasses apoptosis by targeting mitochondria. The findings indicate that UPEC evades early immune detection through this mechanism. The release of HMGB1 and histone 3 supports the necrotic cell death model. The researchers propose that UPEC employs HlyA to escape innate immune responses. This strategy allows UPEC to propagate within host cells silently. The authors emphasize the importance of mitochondrial dynamics in UPEC pathogenesis.

Keywords:

ATP HMGB1 apoptosis caspase histone 3 Uropathogenic Escherichia coli Hemolysin A Programmed cell necrosis Mitochondrial dynamics

Frequently Asked Questions

Hemolysin A increases mitochondrial calcium levels, leading to mitochondrial fragmentation and dysfunction. This results in loss of membrane potential and ATP depletion, triggering necrotic cell death.

Mitochondrial dynamics are manipulated by HlyA, causing fragmentation and impairing function. This disruption leads to cell necrosis and supports UPEC's evasion of immune detection.

The absence of caspase activation indicates that UPEC-induced cell death is not apoptosis. Instead, it suggests programmed cell necrosis, a distinct pathway with different implications for immune responses.

HMGB1 and histone 3 are danger-associated molecules released during necrotic cell death. Their release indicates that UPEC triggers necrosis, which may help the bacteria evade immune detection.

Unlike toxins that induce apoptosis, HlyA causes necrotic cell death by targeting mitochondria. This allows UPEC to escape immune responses and propagate within host cells.

The study suggests that UPEC uses HlyA to manipulate mitochondrial dynamics and induce necrotic cell death. This strategy helps the bacteria evade immune detection and propagate within host cells.

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