Researchers have found that oxidative stress and neurotoxicity biomarkers increased in tadpoles exposed to synthetic severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike protein peptides.
SARS-CoV-2, the causative pathogen of coronavirus disease 2019 (COVID-19), spreads mainly by the airborne transmission of virus droplets from an infected person. However, the virus can stay alive on various surfaces (also known as 'fomites') for several days. Coming in contact with such contaminated surfaces can also cause virus transmission.
Environmental transmission of the virus can also occur through indirect contact with urine or stool of infected people. Studies around the globe have detected the presence of the virus's ribonucleic acid (RNA) in sewage and wastewater. Household waste from hospitals and large buildings can thus harbor the virus and its ecotoxicological effects are yet unknown.
Thus, there is an urgent need to investigate the effect of the virus on aquatic organisms that live near discharges from such places. However, there are almost no aquatic animal models to conduct trials on and understand the virus's effect on aquatic vertebrates.
SARS-CoV-2 increased oxidative stress in tadpoles
To study how SARS-CoV-2 affects aquatic animals, a team of researchers used previously developed synthetic peptides of the SARS-CoV-2 spike protein to study its effect on a tadpole Physalaemus cuvieri. The team published their results in the bioRxiv* preprint server.
The tadpole is commonly found in freshwater throughout Brazil and South America and has a stable and abundant population. Previous studies have used this animal to study the effects of water pollution.
The authors divided the tadpoles into seven groups of 50 animals each, including a control group. They added two different concentrations of the three synthetic spike protein peptides into water and kept the tadpoles in the spiked water for a day. Using biomarkers for oxidative stress and neurotoxicity, they assessed the toxic effects of the virus on the tadpoles. The team also estimated the binding affinity and mode of the synthesized peptides with the oxidative stress and neurotoxic biomarkers using chemoinformatic screens and molecular docking simulations.
The teams found significant biochemical changes in the tadpoles exposed to two of the peptide after 24 hours. They found an increase in the production of nitrite and hydrogen peroxide, indicating an increase in the oxidative stress processes in the animals. They also found an increase in the amounts of the enzymes catalase and superoxide dismutase, which are antioxidants produced against oxidative stress.
These results are in agreement with previous studies that have shown that SARS-CoV-2 can induce oxidative stress upon infection and show the peptides can induce metabolic changes in the tadpoles.
The antioxidant levels produced were not enough to counter the oxidative stress. The authors think this could be because the peptides induced functional changes in and had an affinity to the antioxidant enzyme; this idea being supported by molecular docking simulations. The increase in nitrite could be because of a standard response to SARS-CoV-2 by the innate immune system, and hence an increase in the proinflammatory cytokines.
Neurotoxicity in tadpoles
The authors evaluated neurotoxicity in the animals by measuring acetylcholinesterase (AChE), a marker for cholinergic function. At the higher concentration, all the peptides caused increased production of AChE, between 200% and 700%.
This result is different from previous studies, which reported suppression in AChE production, possibly because of the degradation in neurotransmission and oxidation damage. The results seen in the tadpoles may be because of an activation of the cholinergic anti-inflammatory pathway. This is seen to aid the prevention of inflammatory conditions in animal models and the pathway can control inflammation by releasing the neurotransmitter acetylcholine. This mechanism has also been reported in COVID-19 patients.
The activation of the tadpoles' cholinergic system could also be because of the direct interaction of the peptides with AChE. Molecular docking analysis showed a strong affinity between the two. However, more studies are needed to determine if this interaction changed the association and catalysis mechanism or increased enzyme efficiency because of increased substrate affinity with the active site.
Thus, the results indicate that SARS-CoV-2 protein fragments have a strong detrimental effect on tadpoles. However, there are still many questions about the effect of the virus in the aquatic environment, such as the effect of longer exposure, the effect on other animal models, and other toxicity biomarkers, which can be addressed in future studies. This knowledge can help in understanding the impact of SARS-CoV-2 on the environment and the ecosystem.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.