DIETARY METHYLMERCURY AND SELENIUM INTERACTIONS IN RAINBOW TROUT JUVENILES Oncorhynchus mykiss

Introduction

Mercury (Hg), especially the organic,  methylated form (MeHg), is a toxic heavy  metal that can accumulate in large amounts in fish  through time in various organ s, like liver and brain. Brain, with neurons, is the pri ncipal target of MeHg toxicity (Ni et al. 2012 ). MeHg induces its toxicity through  induction of oxidative stress and inflammation

. Selenium (Se), a n essential micronutrient mainly involved in antioxidant protection , has been shown to reduce MeHg toxicity.  However, Se and MeHg interaction s are far to be totally understood. As relatively high levels of these two elements can be found  in marine ingredients, a better understanding of their interactions  is  important to improve aquafeed formulations .

Material and Methods

Six plant protein-rich diets were formulated to contain  two different MeHg  levels (0 and 2.5 mg Hg/kg  in HgC and HgH diets ) combined with 3 different  Se levels and forms (0.3 mg Se/kg in SeC diets and 2.6 mg Se/kg supplied either as sodium selenite in SeI diets or  as L-selenomethionine  from Orffa  in SeO diets).  Each diet was distributed to 3 replicate tanks of 50 fish (initial mean body weight: 26 ±1 g) to apparent satiation over a 3-week feeding trial at a water temperature of 17.5°C. Fish g rowth performance (n=3 tanks/diet) was analysed as well as total Hg and Se contents (n=3 individuals/diet) and relative gene expression levels (RT-PCR, n=9 individuals/diet ) in liver and brain.

Results

 Reduced growth performance of fish fed MeHg was recorded only within  the non-Se supplemented SeC-group (Table 1). Feeding MeHg significantly increased total Hg concentrations in both liver and brain but to a significant smaller extent in liver when fish were supplemented with Se. Along with the hepatic increase of total Hg, dietary MeHg supplementation decreased total Se content in liver. Dietary inorganic Se supplementation increased total Se concentrations in liver to a larger extent than dietary organic Se supplementation but inorganic Se intake did not increase significantly Se content in brain contrary to organic Se intake.

 In liver, MeHg intake increased transcript levels of enzymes involved in cellular stress response and antioxidant defences such as GSTπ and MsrB3 (Table 1) . Both  dietary organic and inorganic Se supplementation increased hepatic  transcript levels of the selenoprotein GPX1a and also SelPa1 but only in HgC groups. In brain, dietary MeHg supplementation  decreased transcript levels of RBFOX3, a neural marker, and  up-regulated expression of IL-1β, a pro-inflammatory cytokine.

Discussion and conclusion

 The growth inhibi tion by MeHg after only 21 days  of feeding  could  be due to the prooxidant and proinflammatory properties highlighted by gene expression analysis in brain and liver.  Fighting against MeHg-induced oxidative stress and inflammation consumes energy that is thus less available for growth (Morcillo et al. 2017). The mitigation by dietary Se supplementation that promoted antioxidant genes lends further support to this hypothesis.

 Indeed,  at the transcript level we found that MeHg induced inflammation in brain and antioxidant defences in liver to fight against oxidative stress .  The  antioxidant support by d ietary Se supplementation  in plant-based diets at the transcript level, in agreement with previous studies (Fontagné-Dicharry et al., 2020), could provide better protection against xenobiotics like MeHg in rainbow trout juveniles , even if the controlled genes by dietary MeHg and Se are not systematically similar and deserved further investigation for a better understanding.

 The present  results provide evidence of interactions between dietary MeHg and Se on tissue Hg and Se concentrations, especially in liver. The difference of Hg and Se storage between the two tissues suggest post-absorptive interacting effects between Hg and Se  maybe at the depuration level  due to their strong affinity for each other  as suggested by Amlund et al. (2015) in zebrafish  but do not exclude interactive effects at the digestive or absorption level that deserve further investigation. As muscle, the edible part, has been shown to readily accumulate MeHg (Berntssen et al. 2004), this tissue should be included in the study to ascertain this finding with different dietary Se levels and forms.

Our data  showed that the  dietary  level of 2 mg Hg/kg diet  supplied  as organic mercury in duced adverse effects as early as 21 days of feeding, especially  in Se-deficient diets , in agreement with the threshold levels of dietary MeHg set as 5  mg Hg/kg by Berntssen et al. (2004) for salmonid diets. In brain, our RBFOX3 results confirm the high sensitivity of neurons to MeHg toxicity (Ni et al. 2014). Longer exposure, for example up to 6 months, should be assessed in rainbow trout juveniles for a better understanding on the  adverse effects of MeHg and the interacting effects with  dietary Se.

Acknowledgements

 This project  was supported by the French National Research Agency (ANR-18-CE34-0004), the Region Nouvelle-Aquitaine (BENESEL) and I-site E2S  UPPA (contract 2020-32) and  has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska- Curie grant agreement No. 101007962.

References

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