Abstract
Neurodevelopmental disorders (NDDs) are a diverse group of complex disorders that impact one in six children in the United States. NDDs often have both genetic and environmental etiologies; however, relatively little is known about how these factors may converge to influence the onset of NDDs. Bisphenols are a group of chemicals that have been implicated as a potential environmental risk factor for NDDs. These chemicals are used in the manufacturing processes of polycarbonate plastics and epoxy resins and can be found in many consumer products. The most infamous bisphenol, bisphenol A (BPA), is an endocrine disrupting chemical that impacts a variety of human systems, ranging from the reproductive system to central nervous system. This has led to a reduction of BPA in plastic manufacturing processes and a corresponding increase in the use of BPA-analogs, like bisphenol F (BPF). BPF has undergone far less toxicological testing than BPA, but due to structural similarities, BPF may have similar negative health impacts. With this study, we aimed to determine how BPF exposure would affect neurodevelopment in both a wild-type genetic background and in a fragile X mental retardation 1 (FMR1) mutant background using Drosophila melanogaster as a model. FMR1 is an NDD-risk gene in humans and Drosophila have a functional ortholog referred to as dfmr1 (Drosophila FMR1). We exposed the wild-type w1118 strain and two dfmr1 null mutant strains to 0.1 mM and 1 mM BPF from embryonic development through adulthood. We assessed how BPF exposure impacted synapse formation at the neuromuscular junction (NMJ) in larvae, locomotory activity in larvae, and grooming activity in adults. We used immunofluorescence and confocal microscopy to examine NMJ synaptic structures in body wall muscle groups 4 and 6/7. We found both genetic strains were affected by BPF, but only by the 1mM dose and the observed impacts varied across the two genetic strains. In the w1118 strain, type Ib boutons were increased at muscle group 4 but decreased at muscle groups 6/7, while axon branching was unaffected. In the dfmr1 mutant strain, BPF exposure increased formation of type Is synaptic boutons and increased axon branching at muscle group 4, but there was no impact on muscle group 4 type Ib synaptic boutons. There was also no impact on axon branching in muscle group 4 or synapse formation at muscle groups 6/7. We used standard locomotor activity and grooming activity assays to observe and quantify three different metrics of locomotor activity (direction changes, peristaltic contractions, and distance traveled) and two different metrics of grooming activity (grooming bouts and time spent grooming). Locomotory and grooming activity were both decreased in the w1118 strain, but in response to 0.1 mM BPF and not 1 mM BPF. The only behavior affected by BPF exposure in the dfmr1 mutants was larval peristaltic contractions, which were reduced in response to 1 mM BPF—other measures of larval locomotion and grooming activity were not affected. In summation, our results indicate that exposure to BPF differentially impacts neurodevelopment. The w1118 strain was impacted by both 0.1 and 1 mM BPF concentrations depending on the assay, while dfmr1 null mutant strains were only impacted by 1 mM BPF. This study provides evidence for the importance of evaluating toxicology in distinct genetic backgrounds and supports the need for further risk assessment of BPF and other BPA analogs.