Unexpected metabolic disorders induced by endocrine disruptors in Xenopus tropicalis provide new lead for understanding amphibian decline

Abstract

Despite numerous studies suggesting that amphibians are highly sensitive to endocrine disruptors (EDs), both their role in the decline of populations and the underlying mechanisms remain unclear. This study showed that frogs exposed throughout their life cycle to ED concentrations low enough to be considered safe for drinking water, developed a prediabetes phenotype and, more commonly, a metabolic syndrome. Female exposed from tadpole stage to benzo()pyrene or triclosan at concentrations of 50 ng⋅L displayed glucose intolerance syndrome, liver steatosis, liver mitochondrial dysfunction, liver transcriptomic signature, and pancreatic insulin hypersecretion, all typical of a prediabetes state. This metabolic syndrome led to progeny whose metamorphosis was delayed and occurred while the individuals were both smaller and lighter, all factors that have been linked to reduced adult recruitment and likelihood of reproduction. We found that F animals did indeed have reduced reproductive success, demonstrating a lower fitness in ED-exposed Moreover, after 1 year of depuration, that had been exposed to benzo()pyrene still displayed hepatic disorders and a marked insulin secretory defect resulting in glucose intolerance. Our results demonstrate that amphibians are highly sensitive to EDs at concentrations well below the thresholds reported to induce stress in other vertebrates. This study introduces EDs as a possible key contributing factor to amphibian population decline through metabolism disruption. Overall, our results show that EDs cause metabolic disorders, which is in agreement with epidemiological studies suggesting that environmental EDs might be one of the principal causes of metabolic disease in humans.

Concomitant exposure to benzo[a]pyrene and triclosan at environmentally relevant concentrations induces metabolic syndrome with multigenerational consequences in Silurana (Xenopus) tropicalis

“…The control frogs were exposed to dimethyl sulfoxide (DMSO, vehicle) at a concentration of 4/1000 (v:v). As described in Regnault et al (2018), pollutant concentrations in the medium were continually monitored using GC-MS/MS, and confirmed that the average weekly BaP/TCS concentrations were both equal to 50 ng.L -1 . These concentrations match the BaP and TCS concentrations found in polluted ponds (Trapido and Veldre, 1996;Lindström et al, 2002;Blair et al, 2013;Olivares-Rubio et al, 2015).…”

“…The hepatosomatic index (HSI) was calculated by dividing the weight of the liver by the body weight and hepatoxicity was evaluated by measuring alanine aminotransferase activity in serum as previously described in Regnault et al (2018).…”

“…The complete method is largely detailed in Regnault et al (2018). Briefly, for each biological replicate, the RNAqueous®-4PCR Kit (Ambion, USA) was used to extract the total RNA from 15 mg of liver and according to the manufacturer's instructions.…”

“…were validated with several RNAseq experiments (Regnault et al, 2014(Regnault et al, , 2018. The ΔΔCT method (Pfaffl, 2001) taking Into account PCR efficiency was used for data analysis was used for data analysis.…”

“…More recently, two independent controlled exposures of S. tropicalis throughout their life cycle to these two ED at environmentally relevant concentrations induced a metabolic syndrome featuring a pre-diabetes state. The exposed animals produced progeny that metamorphosed later, were smaller and lighter at metamorphosis, and had reduced reproductive success, thus linking metabolic disorders to putative population decline (Regnault et al, 2018).…”