Zebrafish as a Model to Assess the Teratogenic Potential of Nitrite

Teratogenesis is a process that disrupts the normal development of an embryo or fetus by causing permanent structural and functional abnormalities, growth retardation, or miscarriage in severe cases¹. It can be caused by certain natural agents (teratogens), which interfere with embryonic development in multiple ways². During human fetal development, common teratogens such as radiation, infectious agents, toxic metals, and organic chemicals have been reported to cause defects in epicanthic folds (the skin fold in the upper eye lid) and clinodactyly (curved finger or toe) through morphogenetic errors¹.

Understanding the molecular mechanism of teratogenesis is the first step towards developing treatment and prevention. Several vertebrate models such as the African clawed frog (Xenopus laevis) and zebrafish (Danio rerio) have been used to determine the molecular pathways affected by teratogens. Previous studies have used zebrafish as a model for epidemiology, toxicology and teratogenesis^3-7. Scholz et al. considered zebrafish as a "gold standard" for environmental toxicity assessment. This is due, in part, to the transparency of the zebrafish embryo, which allows researchers to visualize the developmental defect as it occurs⁸. Approximately 70% of human genes have orthologues in zebrafish, making zebrafish a desirable vertebrate model for studying human defects⁹.

Some epidemiological studies have suggested that nitrate and nitrite, commonly present in farm foods and water, are associated with birth defects or spontaneous abortions^10,11, while other studies do not support this association¹². Nitrate (NO₃^–) and nitrite (NO₂^–) are naturally present in soil and water. They are a source of nitrogen for plants and are a part of the nitrogen cycle¹³. Foods such as green beans, carrots, squash, spinach, and beets from farms that use fertilizers high in nitrate have significantly augmented levels of nitrate and nitrite⁷. Milk from cows fed with high nitrate foods and fish in high nitrate water (mainly from soil runoff³⁰) can lead to humans consuming large amounts of nitrate and nitrite¹⁴. Nitrate and nitrite are also commonly used in food preservation, which dramatically increases the amount ingested by humans¹².

Optimal levels of nitrate and nitrite play fundamental roles in physiological processes like vascular homeostasis and function, neurotransmission and immunological host defense mechanisms^13-15. However, exposure to high levels of nitrate and nitrite may lead to adverse effects, especially in infants and children¹⁶. Ingested nitrate is further converted to nitrite in the oral cavity by microflora and in the gastrointestinal tract by intestinal microflora¹⁷.

Nitrate puts infants at a high risk for blue baby syndrome by oxidation of hemoglobin to methemoglobin, impairing hemoglobin from its oxygen carrying ability¹⁸. This results in the blue color of skin that extends to peripheral tissues in more severe cases. Inhibited oxygenation of tissues results in other symptoms, most severely leading to coma and death^19,20. Similar symptoms are observed in babies and adults at higher concentrations of nitrate²¹. Elevated levels of methemoglobin in adults due to nitrite poisoning results in cyanosis, headache, breathing disorders³¹, and death if not treated due to complications related to vital tissue hypoxia^32,33.

Nitrate ingested at higher levels can also result in various health complications. Childhood diabetes, recurrent diarrhea, and recurrent respiratory tract infections in children have been linked with high nitrate intake^11,17,22. Chronic exposure to a high level of nitrate is associated with urination and spleen hemorrhaging. Acute high dose exposure to nitrates can lead to a wide spectrum of medical conditions like abdominal pain, muscle weakness, blood in stools and urine, fainting, and death¹¹. Prenatal exposure to nitrate at high levels has been associated with neural tube and musculoskeletal defect¹¹.

A recent report showed that treating zebrafish embryos with nitrite led to yolk sac edema, craniofacial and axial malformations, and swim bladder noninflation⁵. In this study, we demonstrate a method for treating zebrafish embryos with nitrate and nitrite to determine their teratogenic potential. Embryos were exposed to nitrite at different concentrations and different lengths of time. Ethanol was used as a positive control, since it is an established teratogen²³. Our method showed that both high concentrations and long exposure times to nitrite were detrimental to survival and resulted in various phenotypes, ranging from mild (edema) to severe (gross developmental defects). Therefore, the zebrafish is a useful model for directly exploring the potential teratogenic effects of nitrate and nitrite on embryos to complement epidemiological studies.