Interactive effects of fluoride and seleno-l-methionine at environmental related concentrations on zebrafish (Danio rerio) liver via the gut-liver axis.

Introduction

Fluoride (F) is a ubiquitous aquatic environmental pollutant [1]. Although low F level (< 1.5 mg/L) in drinking water is beneficial to health [2], natural and anthropogenic activities increase the F levels in the aquatic environments, causing serious environmental and human health hazards [3]. F content in F-contaminated aquatic environments can often reach 30–50 mg/L, up to 690 mg/L in some lakes, including certain geographic regions in China, Pakistan, and Afghanistan North America [4,5]. Aquatic organisms spend their entire lives in water and can take up F via both dissolved and dietary routes [6]. Previous studies have reported the toxic effects of F on aquatic organisms, including growth retardation, oxidative stress, endocrine disruption, and histopathological alterations of liver and thyroid [7,8]. Therefore, F pollution has exerted great ecological pressure on aquatic ecosystems.

However, F often co-exists with other pollutants in natural water bodies [9]. They can interact with each, and exacerbate or antagonize their toxicity in exposed organisms depending on their content and chemical speciation. Selenium (Se), an essential micronutrient for animals, is beneficial at low levels yet toxic at high levels [10]. In aquatic environments, Se can co-exist with F at various levels. For example, F and Se have both been found in many rivers and lakes worldwide, including Baikal, the Tana River in Kenya, Northwest Texas, and the Yellow River Basin in China [[11], [12], [13]]. Seleno-l-methionine (Se-Met) is the principal source of Se accumulation in aquatic organisms, and it has also attracted special attention due to its trophic transfer and biomagnification [14]. The threshold level for adverse effects of Se in fish is estimated to be 3–4 mg/kg for dietary Se [10]. The interactions between Se and metals may affect metal accumulation and toxicity in organisms [15]. Moreover, recent studies have indicated the protective effects of low doses of Se against the toxicity of some metals (such as Cd and Hg) in fish [10,16]. However, the interactive effects of Se-Met and F at environmental related concentrations in aquatic organisms remains enigmatic.

The liver is a key organ for systemic regulation in fish. It plays an important role in the metabolism and synthesis of nutrients, immunity and detoxification of toxicants [17]. Excessive F can damage the hepatic structure and function, and induce apoptosis and inflammation [3,18]. In addition, the co-administration with Se could reverse the toxic effects of heavy metals (ie, Cd, Cr) on the liver in fish [19]. However, the effects of F and Se-Met co-treatment on the liver in fish remain largely unknown. Recently, the emergence of the “gut-liver axis” theory has offered a new perspective for studying liver toxicity, which has shown the close relationship between liver toxicity and intestinal mucosal barrier damage, intestinal microbiota imbalance, and inflammatory response [20]. It is believed that exposure to pollutants (i.e., F and Se-Met) can induce intestinal physiological and pathological changes, intestinal microbiota dysbiosis, resulting in intestinal inflammation [21]. Once the intestinal barrier is destroyed, harmful bacteria and their metabolites, such as lipopolysaccharides (LPS), can directly translocate into the liver through the damaged gut-liver axis, thereby exacerbating liver damage [22]. However, whether intestinal microbial imbalance and mucosal damage induced by F and Se-Met cause liver injury in fish by activating the gut-liver axis has yet to be reported.

Zebrafish (Danio rerio) is a small freshwater teleost used as a laboratory model in the evaluation of toxicity [23]. The objective of this study was to determine the interactive effects of F and Se-Met on zebrafish liver, and whether its potential effects were related to the gut-liver axis. Because vitellogenin and estrogen are extremely abundant in the liver of breeding female zebrafish and can interfere with the normal function of the liver [24], male fish were selected for this study. This study can provide new insights into the mechanisms underlying the interactive effects of F and Se-Met on aquatic organisms.