Assessing mechanisms driving phenol isopropylated phosphate (IPP)-induced larval photomotor response deficits in zebrafish
Isopropylated phenyl phosphates (IPP) are an additive organophosphate flame retardant (OPFR) which has been extensively used in furniture, electronics, automobiles, plastics, and children's products to slow down the spread of fire. The processing and distribution of IPP- containing products have been prohibited but its continuous leaching from end use products has retained the concern of its toxicity. The present study was designed to evaluate IPP-induced developmental toxicity using zebrafish embryos. We first performed  range finding experiments with embryonic zebrafish, exposed to 0 - 200 ?M IPP from 6 to 120 h post fertilization and found significant morphological impacts like pericardial edema, yolk sac edema and spinal curvature at higher concentrations. Following this, relying on secondary analyses of our whole embryo mRNA-seq data, we quantified neurotransmitters and found significant increase in the levels of dopamine and its metabolite, 3-methoxytyramine. We then conducted, in vitro retinoic acid receptor (RAR) signaling assay and noticed significant inhibition of RARa, but not RAR? and RAR?. For behavioral readouts, we performed larval photomotor response (LPR) assay at sublethal concentrations and observed hypoactive locomotory behavior in exposed larvae. Whole-mount immunohistochemistry for 5-methylcytosine and global DNA methylation assay showed significant IPP-induced hypermethylation in situ. Finally, based on whole embryo RNA-seq data, we hypothesized that IPP affects the development of brain and eyes. Firstly, we performed global DNA methylation in brain and eyes, but did not find significant effects. Then, we conducted mRNA sequencing on dissected brains and eyes, and found 2 and 135 differentially expressed genes, respectively. Gene ontology revealed that IPP affect voltage gated ion channel activity, synaptic transmission and neurotransmitter signaling. Collectively, our data shows that IPP induces morphological abnormalities and disrupts larval photo motor response, potentially through RA inhibition and methylomic regulation. Finally, we observed that IPP affects gene expression within the developing eye, establishing synaptic transmission, vision and muscle contraction as a potential causative factor for LPR responses. Overall design: Zebrafish embryos were exposed to 0 or 0.2 uM IPP from 6 to 120 hpf, eyes and brains were dissected out, RNA extraction and RNA sequencing using a Novogene Library preparation Pipeline and sequencing on an Novaseq 600 (Illumina, San Diego, CA) (2x150 bp, ~40m reads/sample).