SARS-CoV-2, the virus responsible for the COVID-19 pandemic, has demonstrated a profound ability to induce cell death in the human airway epithelium.
Cell death, a fundamental biological process, can occur through several pathways, including apoptosis, necroptosis, and pyroptosis. Each pathway plays a unique role in maintaining cellular homeostasis and responding to pathogenic threats.
Cell death pathways have long been the focus of research, given their critical roles in disease and health. Understanding these pathways becomes even more pertinent in the context of viral infections.
The SARS-CoV-2 pandemic has underscored the need for detailed knowledge of how viruses manipulate host cell processes to cause disease [1].
Historically, apoptosis was the first cell death pathway to be extensively studied. Necroptosis and pyroptosis, which are more inflammatory, have gained attention more recently, especially concerning infectious diseases and cancer.
Apoptosis: The programmed cell death
Apoptosis is a highly regulated process that allows cells to die in a controlled manner, preventing the release of harmful substances into the surrounding tissue.
This process is typically marked by cellular shrinkage, DNA fragmentation, and the formation of apoptotic bodies, which are then engulfed and digested by phagocytic cells [1].
Necroptosis: The inflammatory cell death
Necroptosis, on the other hand, is a form of programmed necrosis that releases cellular contents, causing inflammation.
This pathway is characterized by the formation of pores in the cell membrane through the action of mixed lineage kinase domain-like pseudokinase (MLKL) following its phosphorylation by receptor-interacting protein kinases (RIPK1 and RIPK3) [1].
Pyroptosis: The fiery cell death
Pyroptosis is an inflammatory form of cell death driven by the activation of inflammasomes and the cleavage of gasdermin proteins. These proteins create pores in the cell membrane, leading to cell lysis and the release of pro-inflammatory cytokines such as IL-1β [1].
SARS-CoV-2 and cell death pathways
Research led by Kaixin Liang and colleagues has uncovered that SARS-CoV-2 activates all three major cell death pathways—apoptosis, necroptosis, and pyroptosis—in the human airway epithelium.
These findings are critical for understanding how the virus causes damage and contributes to severe COVID-19 symptoms [1].
Study findings
The study utilized a combination of primary human airway epithelial cultures and animal models to investigate the cell death response to SARS-CoV-2 infection.
It was observed that SARS-CoV-2 induces necroptosis primarily in infected cells, while uninfected bystander cells undergo apoptosis. Pyroptosis occurs later in the infection process [1].
The key findings include:
- Necroptosis: Infected cells showed significant phosphorylation of MLKL, indicating active necroptosis. This process is triggered by the viral Z-RNA binding to Z-DNA-binding protein 1 (ZBP1), which interacts with RIPK3 to initiate the necroptotic cascade [1].
- Apoptosis: Bystander cells that were not directly infected by the virus exhibited markers of apoptosis, such as caspase-3 cleavage. This indicates that apoptosis might be a secondary effect of the viral infection, possibly due to the inflammatory environment created by the necroptotic cells [1].
- Pyroptosis: This form of cell death, marked by gasdermin D (GSDMD) cleavage, occurs later in the infection timeline, contributing to the inflammatory response seen in severe COVID-19 cases [1].
The activation of necroptosis, in particular, suggests potential therapeutic targets. Inhibitors of RIPK1 and MLKL, key proteins in the necroptotic pathway, could be explored as treatments to prevent the extensive tissue damage observed in severe COVID-19 cases.
Additionally, modulating the inflammasome activation and pyroptosis could reduce the inflammatory response, potentially alleviating some of the severe symptoms associated with the virus [1].
While these findings are promising, they highlight the complexity of the body’s response to SARS-CoV-2.
The interplay between different cell death pathways and the timing of their activation presents challenges for developing effective treatments.
Further research is needed to understand these pathways’ precise triggers and regulatory mechanisms in the context of COVID-19 [1].
[1] Science Immunology. Initiator cell death event induced by SARS-CoV-2 in the human airway epithelium
