Gut microbiota perturbation and subsequent oxidative stress in gut and kidney tissues of zebrafish after individual and combined exposure to inorganic arsenic and fluoride.

Inorganic arsenic (iAs) and fluoride (F) are present worldwide (Wen et al., 2013). They are co-exposed at different geographic regions including Argentina (Alcaine et al., 2020), Mexico (Alarcón-Herrera et al., 2013), Pakistan (Parvaiz et al., 2020), India (Bhattacharya et al., 2020) and China (He et al., 2020). Not only by drinking contaminated water and taking foods, humans are also exposed to F by F-containing cosmetics, toothpastes, and to As by different medicines used to treat cancer and ulcer (Liu et al., 2008). The permissible limits of As and F in drinking water are 10 ppb (WHO, 2003; Mondal et al., 2021) and 1.5 ppm (World Health Organization, 2004; Mukherjee et al., 2023), respectively. Both As and F are potential toxicants and impose toxic stress on different organs in fish (Li et al., 2019; Liang et al., 2020). Arsenic imposes toxic manifestation by a potent means of oxidative stress which can alter gut barrier function contributing to occurrence of many GI tract related diseases (Bhattacharyya et al., 2014). F induced oxidative stress in mice liver and kidney tissues are reported by Sarkar et al. (2017).

Ingested As and F are mostly absorbed by the junctional proteins of GI tract while kidney plays pivotal role in excretion of excess toxicants from body (Calatayud et al., 2012). GI tract hosts for a collective genome of near about 100 trillion microbes, which is recognized as ‘second brain’ to regulate host’s regular physiological functions including digestion, metabolism, elimination of toxins, biosynthesis of vitamins and shielding against pathogenic overgrowth (Lynch and Pedersen, 2016). The relatively steady configuration of adult gut microbiota gets perturbed due to some external factors like diet, health status, exposure of toxicants (Ba et al., 2017; Meng et al., 2018). Perturbation from the steady state of gut bacterial dynamic accelerates to a number of life-threatening diseases like obesity, cardiovascular diseases, type II diabetes, inflammatory bowel disease (Peng et al., 2019). Gut microbiota has long been regarded as a pivotal object for ecotoxicological studies.

Reports are available on iAs and F mediated gut microbiota disruption (Chi et al., 2016; Yasuda et al., 2017). Dahan et al. (2018) showed in his report that As exposure not only disrupted gut microbiome construction at the abundance level but also significantly changed its metabolic status at functional level in larval zebrafish. Similar study suggested that excessive ingestion of F leads to damage of intestinal barrier and loss of intestinal microflora integrity in exposed mice (Liu et al., 2019). Despite few reports on the independent fallouts of As and F on gut microbiota, very less work have been done on their combined effects. Microbiota disruption and related neurodevelopmental impairment in offspring rats due to mixture upshot of As and F was addressed in a recent report (Qiu et al., 2020). In another study by Yan et al. (2021) revealed that collective subjection to As and F is responsible for gut bacterial perturbation and related cardiovascular defects in offspring rats.

On the other hand, alteration of gut bacterial composition of zebrafish can lead to kidney diseases which signifies the importance of ‘Gut-Kidney axis’ in maintaining normal kidney function (Bu et al., 2023; Jia et al., 2023). Loss of intestinal permeability leads to hike in circulatory LPS, produced by gut bacteria, which binds to toll like receptors in kidney and boosts up the production of ROS, breaks down renal defense strategy thus, promoting kidney diseases (do Nascimento et al., 2020).

Our present study endeavored to explore the gut microbiota perturbation caused by both independent and simultaneous administration of As and F at their respective environmentally relevant concentrations by metagenomic analysis and also to gauge the level of toxic insults imposed on both gut and kidney tissues by evaluating oxidative stress parameters and histoarchitectural studies. Further, immunofluorescence study was performed to observe translocation and measuring expression level of oxidative stress responsive protein Nrf2 inside enterocytic and renal cells. Zebrafish was used as a model organism here, as it possesses 70 % orthologous genetic resemblance with humans so investigating its cellular feedback can dispense near exact perception about what could be the exact scenario in human cell (Bonomo et al., 2020). Furthermore, both As and F are aquatic pollutant and thus using zebrafish in this aspect can guide in comprehending the mechanism of As and F induced toxicity. In short, we tried to find out any possible link between As and F mediated alteration of gut microbiota homeostasis and oxidative stress in gut and kidney tissues.