The Mitochondria - The Powerhouse against Infectious Diseases

Mitochondria are not just the powerhouse of the cell. Known for their role in energy production and metabolism, they also play a significant role in the immune system. While oxidative phosphorylation is the primary pathway of energy production in higher organisms, this process is oxygen-demanding and relatively slow under stressful conditions. Therefore, mitochondria can switch to glycolysis, a rapid energy-production method independent of oxygen supply. External stressors, like pathogen-specific metabolites, can also alter mitochondrial metabolism to favour pathogen survival and replication.

Notably, mitochondria can facilitate immune responses at the cellular level. They can transition from a catabolic, energy-producing phase to an anabolic phase to maximize the use of metabolites for both adenosine triphosphate (ATP) production (the cellular form of energy) and cellular homeostasis. Additionally, mitochondria generate reactive oxygen species (ROS) that activate inflammatory pathways within the cell to bolster protection against intracellular pathogens. ROS also activates essential immune cell subsets such as macrophages and T cells to control and clear extracellular pathogens. This article will explore the intricate interplay between mitochondrial metabolism and various bacterial and viral infections.

Mitochondria and Bacterial Infections - Legionnaire's Disease

Legionnaire's disease, a severe and potentially fatal bacterial pneumonia, is caused by Legionella pneumophila, an intracellular bacterium. Upon entry into human lung macrophages, L. pneumophila injects pathogenic proteins into the cell to initiate bacterial replication. This interaction has piqued great interest in better understanding Legionella infections since macrophages activation are largely influenced by mitochondrial metabolic responses. In 1983 L. pneumophila was observed to form Legionella-containing vacuoles (LCVs) upon entry into macrophages and later associate with the mitochondria. Recent studies have shed further light on how LCVs can cause mitochondrial fragmentation, an indicator of cell death. Mitochondrial fragmentation renders cells more susceptible to stressors and increases ROS production, where uncontrolled ROS output and oxidative stress can also induce cell death.

Despite this, L. pneumophila did not induce other markers of cell death. Further analyses of macrophages’ metabolic rate after L. pneumophila infection revealed a shift from oxidative phosphorylation to glycolysis, a less sustainable option for long-term cell survival and energy production. One bacterial metabolite, which is injected

into cells upon macrophage entry, Ceg3, may interfere with a component of the oxidative phosphorylation pathway, potentially impacting ATP output and subsequent cellular function.

Metabolic alterations that may favor glycolysis over oxidative phosphorylation resemble the Warburg effect observed in cancer cells, which allows for uncontrolled and deleterious cell division. Understanding the intricacies of this relationship between L. pneumophila infection and mitochondrial involvement provides valuable insights into the mechanisms of Legionnaire’s disease and potential avenues for therapeutic interventions.

Mitochondria and Viral Infections - Long COVID

Long COVID, or post-acute COVID-19 syndrome, is a condition characterized by long-term symptoms appearing after the resolution of a SARS-CoV-2 infection and affects 10% of all infected individuals. The severity of the initial infection is closely tied to the risk of developing long COVID and profoundly impacts the quality of life. Metabolic perturbations, particularly involving iron deposition and mitochondrial association in the liver, are strongly linked to the severity of SARS-CoV-2 disease. Elevated iron levels can lead to tissue damage, and disruptions in mitochondrial oxidative phosphorylation can also disrupt iron metabolism, leading to tissue inflammation in a positive feedback manner.

Aside from external metabolites that serve as an indicator of COVID-19 illness severity, the mitochondrial DNA (mtDNA), a unique set of DNA within the mitochondria which governs metabolic function independently of the cell’s nuclear DNA, may serve as a biomarker for the organism’s ability to fight against infections and diseases. High levels of circulating mtDNA in the blood are associated with inflammation and linked to the severity of the initial SARS-CoV-2 infection. Tissue damage during SARS-CoV-2 infections, may increase oxidative stress, alter mtDNA, and decrease oxidative phosphorylation efficiency, thereby reducing metabolic capacity. Additionally, viral infections can lead to the expulsion of damaged mtDNA from cells, suggesting that individuals with lower mitochondrial reserves may be more susceptible to long COVID.

Host cells can also modulate metabolism to fight against SARS-CoV-2. For instance, manipulating iron metabolism and metabolite consumption via mitochondrial remodelling may be a form of starvation to slow viral replication. Metabolites like succinate, found within oxidative phosphorylation pathways, also play important roles in inflammation, immunomodulation, and anti-viral activity. During hypoxic conditions caused by lung infections, alterations in oxidative phosphorylation lead to succinate buildup, causing generation of excess ROS within the mitochondria. This suggests the importance of mitochondrial health for viral resistance, but also its tolerance against oxidative stress and ability to consume high levels of ROS.

Similar to L. pneumophila infections, SARS-CoV-2 viral proteins can inhibit components of oxidative phosphorylation, disrupt ATP output, enhance mitochondrial fragmentation, and induce cell death. These mitochondrial perturbations suggest a shift towards glycolysis, indicating that SARS-CoV-2 infections may heighten the risk of cancer. Altogether, severe COVID-19 infections may be associated with lung injuries due to mitochondrial damage, but it remains elusive what proportion of these effects are caused by host versus viral effects.

Conclusions

From Legionella bacterial infections to SARS-CoV-2 and post-acute COVID-19 syndrome, mitochondria play crucial roles in shaping disease outcomes. Pathogens can dynamically interact with mitochondria, potentially induce fragmentation, and compromise cellular metabolism. Understanding of how mitochondrial disease impacts immunity has led to hypotheses of treatments options, like transferring healthy stem cells to “donate” mitochondria to host cells garnering the virus and damaged mitochondria. Continued research in the field of mitochondria is encouraged to foster the development of targeted therapeutic approaches.

- Jinny Tsang