Transcriptomic analysis identifies muscle-specific mitochondrial and vesicular transport genes as methylmercury toxicity targets in a Drosophila model of congenital Minamata disease.

Prenatal methylmercury (MeHg) exposure presents a heightened concern in early human development, as has been exemplified in historic cases of congenital minimata disease (CMD). Children who experience CMD characteristically present with various degrees of cognitive and motor symptoms and signs, much like cerebral palsy. MeHg has thus been characterized as a neurotoxicant, where motor deficits are ascribed to central nervous system targets. Skeletal muscle as a post-synaptic MeHg target and contributor to the etiology of CMD has garnered far less attention. Prior studies using Drosophila to model CMD revealed that developmental exposure of MeHg in the larval/pupal stages can elicit graded and latent dose responses affecting adult flight behavior at lower doses (0.4-2.5 ppm in food) and eclosion (emergence from the pupa case) at higher doses (>2.5 ppm in food). The latter phenotype is accompanied by dysmorphogenesis of skeletal muscles. Here, we investigate respective roles for muscle and neural targets in MeHg toxicity. Using RNA-seq analysis, we find that developmental MeHg exposure produces 10 times as many differentially expressed transcripts in indirect flight muscle compared to the ventral nerve cord. Among known MeHg response genes, Nrf2 antioxidant response pathway genes showed muscle-specific MeHg-induced expression changes. Within the muscle transcriptome, the most enriched and significant Gene Ontology terms identified genes required for mitochondrial ribosomal translation at the pupa stage and mitochondrial function (respiratory chain complex I) and vesicle trafficking (ESCRT III) pathways in adults, all showing decreased expression with MeHg exposure. By using an intact, whole-animal developmental model, we identify preferential candidates to evaluate a novel role for muscle-specific mitochondria and intercellular vesicular communication mechanisms as targets in MeHg toxicity and the etiology of CMD.