Triphenyl Phosphate (TPHP)-Induced Neurotoxicity in Adult Male Chinese Rare Minnows ( Gobiocypris rarus)

Triphenyl Phosphate Alters Methyltransferase Expression and Induces Genome-Wide Aberrant DNA Methylation in Zebrafish Larvae

Emerging environmental contaminants, organophosphate flame retardants (OPFRs), pose significant threats to ecosystems and human health. Despite numerous studies reporting the toxic effects of OPFRs, research on their epigenetic alterations remains limited. In this study, we investigated the effects of exposure to 2-ethylhexyl diphenyl phosphate (EHDPP), tricresyl phosphate (TMPP), and triphenyl phosphate (TPHP) on DNA methylation patterns during zebrafish embryonic development. We assessed general toxicity and morphological changes, measured global DNA methylation and hydroxymethylation levels, and evaluated DNA methyltransferase (DNMT) enzyme activity, as well as mRNA expression of DNMTs and ten-eleven translocation (TET) methylcytosine dioxygenase genes. Additionally, we analyzed genome-wide methylation patterns in zebrafish larvae using reduced-representation bisulfite sequencing. Our morphological assessment revealed no general toxicity, but a statistically significant yet subtle decrease in body length following exposure to TMPP and EHDPP, along with a reduction in head height after TPHP exposure, was observed. Eye diameter and head width were unaffected by any of the OPFRs. There were no significant changes in global DNA methylation levels in any exposure group, and TMPP showed no clear effect on DNMT expression. However, EHDPP significantly decreased only DNMT1 expression, while TPHP exposure reduced the expression of several DNMT orthologues and TETs in zebrafish larvae, leading to genome-wide aberrant DNA methylation. Differential methylation occurred primarily in introns (43%) and intergenic regions (37%), with 9% and 10% occurring in exons and promoter regions, respectively. Pathway enrichment analysis of differentially methylated region-associated genes indicated that TPHP exposure enhanced several biological and molecular functions corresponding to metabolism and neurological development. KEGG enrichment analysis further revealed TPHP-mediated potential effects on several signaling pathways including TGFβ, cytokine, and insulin signaling. This study identifies specific changes in DNA methylation in zebrafish larvae after TPHP exposure and brings novel insights into the epigenetic mode of action of TPHP.

Species-specific metabolism of triphenyl phosphate and its mono-hydroxylated product by human and rat CYP2E1 and the carp ortholog

Organophosphorus flame retardants (PFRs) are a class of flame retardants and environmental pollutants with various biological effects. Recentstudies have evidenced activation of some PFRs by human CYP enzymes (including CYP2E1) for genotoxic effects. However, the activity of CYPs in fish species toward PFR metabolism remains unclear. This study was aimed on comparing the metabolism of triphenyl phosphate (TPHP) and 4-OH-TPHP in human, rat, and common carp, and the involvement of human CYP2E1 and its orthologs in the metabolism, by using fomepizole (4-MP, CYP2E1 inhibitor) as a modulator, in silico molecular docking and dynamics analyses. The rate of TPHP metabolism was apparently faster with human and rat, microsomes than with fish microsomes, the major metabolites were phosphodiester and hydroxylated phosphate, with 30-80 % of TPHP forming unidentified metabolites in the system of each species. 4-OH-TPHP was readily metabolized by both human and rat microsomes, whereas it was hardly metabolized in carp assays. Meanwhile, with 4-MP the transformation of TPHP to 4-OH-TPHP was enhanced in the human/rat systems while suppressed in the carp system. Moreover, the formation of unidentified metabolites in human and rat systems was mostly inhibited by 4-MP. Through molecular dynamics analysis TPHP and its primary metabolites showed high affinity for human and rat CYP2E1, as well as the carp ortholog (CYP2G1-like enzyme), however, the 4-OH-TPHP bond to the latter was too far from the heme to permit a biochemical reaction. This study suggests that the metabolism/activation of TPHP might be favored in mammals rather than carp, a fish species.

Embryonic Exposure to Organophosphate Flame Retardants (OPFRs) Differentially Induces Cardiotoxicity in Rare Minnow (Gobiocypris rarus)

Organophosphorus flame retardants (OPFRs) such as triphenyl phosphate (TPHP) and tris(1,3-dichloro-2-propyl) phosphate (TDCIPP) were reported to impair cardiac function in fish. However, limited information is available regarding their cardiotoxic mechanisms. Using rare minnow (Gobiocypris rarus) as a model, we found that both TPHP and TDCIPP exposures decreased heart rate at 96 h postfertilization (hpf) in embryos. Atropine (an mAChR antagonist) can significantly attenuate the bradycardia caused by TPHP, but only marginally attenuated in TDCIPP treatment, suggesting that TDCIPP-induced bradycardia is independent of mAChR. Unlike TDCIPP, although TPHP-induced bradycardia could be reversed by transferring larvae to a clean medium, the inhibitory effect of AChE activity persisted compared to 96 hpf, indicating the existence of other bradycardia regulatory mechanisms. Transcriptome profiling revealed cardiotoxicity-related pathways in treatments at 24 and 72 hpf in embryos/larvae. Similar transcriptional alterations were also confirmed in the hearts of adult fish. Further studies verified that TPHP and TDCIPP can interfere with Na+/Ca2+ transport and lead to disorders of cardiac excitation-contraction coupling in larvae. Our findings provide useful clues for unveiling the differential cardiotoxic mechanisms of OPFRs and identifying abnormal Na+/Ca2+ transport as one of a select few known factors sufficient to impair fish cardiac function.

Astaxanthin activates the Nrf2/Keap1/HO-1 pathway to inhibit oxidative stress and ferroptosis, reducing triphenyl phosphate (TPhP)-induced neurodevelopmental toxicity

Triphenyl phosphate (TPhP) serves as a major organophosphorus flame retardant, and its induced neurodevelopmental toxicity has attracted widespread attention, but the mechanism remains unclear. In this study, we involved zebrafish to explore the new mechanism of TPhP inducing oxidative stress and ferroptosis to promote neurodevelopmental toxicity. The results suggested that TPhP affected the embryonic development, reduced the number of new neurons, and led to abnormal neural behavior in zebrafish larvae. TPhP also induced ROS accumulation, activated the antioxidant defense signal Nrf2 and Keap1, and significantly changed the activities of Acetylcholinesterase (AChE), Adenosine triphosphatase (ATPase) and glutathione S-transferase (GST). In addition, TPhP induced ferroptosis in zebrafish, which was reflected in the increase of Fe2+ content, the abnormal expression of GPX4 protein and genes related to iron metabolism (gpx4a, slc7a11, acsl4b, tfa, slc40a1, fth1b, tfr2, tfr1a, tfr1b and ncoa4). Astaxanthin intervention specifically inhibited ROS levels, and reversed SLC7A11 and GPX4 expression levels and Fe2+ metabolism thus alleviating ferroptosis induced by TPhP. Astaxanthin also partially reversed the activity of AChE, GST and the expression of neurodevelopmental-related genes (gap43, gfap, neurog1 and syn2a), so as to partially rescue the embryonic developmental abnormalities and motor behavior disorders induced by TPhP. More interestingly, the expression of mitochondrial apoptosis-related protein BAX, anti-apoptotic protein BCL-2, Caspase3 and Caspase9 was significantly altered in the TPhP exposed group, which could be also reversed by Astaxanthin intervention. In summary, our results suggested that TPhP exposure can induce oxidative stress and ferroptosis, thereby causing neurodevelopment toxicity to zebrafish, while Astaxanthin can partially reverse oxidative stress and reduce the neurodevelopmental toxicity of zebrafish larvae by activating Nrf2/Keap1/HO-1 signaling pathway.

Neural System Impairment and Involved Microglia-Neuron Regulation of Broflanilide in Zebrafish Larvae

Broflanilide is widely used to control pests and has attracted attention due to its adverse effects on aquatic organisms. Our previous study showed that broflanilide has a negative impact on the central nervous system (CNS) at lethal dosages; however, its neural effects under practical situations and the underlying mechanisms remain unknown. To elucidate how broflanilide affects the CNS, we exposed zebrafish larvae to broflanilide at 16.9 and 88.0 μg/L (the environmentally relevant concentrations) for 120 h. Zebrafish locomotion was significantly disturbed at 88.0 μg/L, with a decreased moving distance and velocity accompanied by an inhibited neurotransmitter level. In vivo neuroimaging analysis indicated that the nerves of zebrafish larvae, including the axons, myelin sheaths, and neurons, were impaired. The number of neurons was significantly reduced after exposure, with an impaired morphological structure. These changes were accompanied by the abnormal transcription of genes involved in early CNS development. In addition, an increased total number of microglia and an elevated proportion of amoeboid microglia were observed after 88.0 μg/L broflanilide exposure, pointing out to an upstream role of microglia activation in mediating broflanilide neurotoxicity. Meanwhile, increased inflammatory cytokine levels and brain neutrophil numbers were observed, implicating significant inflammatory response and immune toxicity. Our findings indicate that broflanilide interferes with microglia-neuron regulation and induces neurodevelopmental disorders.

Neurodevelopmental toxicity of organophosphate flame retardant triphenyl phosphate (TPhP) on zebrafish (Danio rerio) at different life stages

As a substitute for polybrominated diphenyl ethers (PBDEs), organophosphate flame retardant triphenyl phosphate (TPhP) is widely used in our daily products and diffusely exists in our living surroundings, but there is a paucity of information concerning its neurodevelopmental toxicity. Herein, we investigated the effects of TPhP exposure on developmental parameters, locomotor behavior, oxidative stress, apoptosis and transcriptional levels in zebrafish at different developmental stages, so as to explore the effects of TPhP exposure on zebrafish neural development and the underlying molecular mechanisms. TPhP concentration gradient exposure reduced the survival rate, hatchability, heart rate, body length and eye distance of zebrafish embryos/larvae, and caused malformations of zebrafish larvae. TPhP also leads to abnormal locomotor behaviors, such as reduced swimming distance and swimming speed, and impaired panic avoidance reflex to high light stimulation. TPhP caused ROS accumulation in 96 hpf larvae and induced Nrf2-antioxidant response in zebrafish. In addition, TPhP further activated mitochondrial signaling pathways, which affected apoptosis in the zebrafish eye region, resulting in visual impairment. Neurodevelopmental (mbpa, syn2a, foxo3a and pax6a), Retinoid acid metabolism (cyp26a1, raraa, rbp5, rdh1, crabp1a and rbp2a) and apoptosis-related genes (bcl2a, baxa and casp9) revealed the molecular mechanism of abnormal behavior and phenotypic symptoms, and also indicated that 96 hpf larvae are more sensitive than 7 dpf larvae. Thus, in the present study, we revealed the neurotoxic effects of TPhP at different early life stages in zebrafish, and zebrafish locomotor behavior impairments induced by TPhP exposure are attributed to co-regulation of visuomotor dysfunction and neuro-related genes. These results suggest that the safety of TPhP in organisms and even in humans needs to be further studied.

Long-Term Exposure to SSRI Citalopram Induces Neurotoxic Effects in Zebrafish

Residual antidepressants are of increasing concern worldwide, yet critical information on their long-term neurotoxic impacts on nontarget aquatic animals is lacking. Here, we investigated the long-term effects (from 0 to 150 days postfertilization) of the selective serotonin reuptake inhibitor citalopram (0.1-100 μg/L) on motor function, learning, and memory in zebrafish over two generations and explored the reversibility of the effect in F1 larvae. Unlike F0⁺ larvae, we found that F1⁺ larvae displayed decreased sensorimotor performance when continuously exposed to citalopram at 100 μg/L. No adverse effects were found in F1^- larvae after they were transferred to a clean medium. Whole-mount immunofluorescence assays suggested that the motor impairments were related to axonal projections of the spinal motor neurons (MNs). For F0⁺ adults, long-term citalopram exposure mainly caused male-specific declines in motor, learning, and memory performance. Analysis of serotonergic and cholinergic MNs revealed no significant changes in the male zebrafish spinal cord. In contrast, the number of glutamatergic spinal MNs decreased, likely associated with the impairment of motor function. Additionally, treatment with 100 μg/L citalopram significantly reduced the number of dopaminergic neurons, but no significant neuronal apoptosis was observed in the adult telencephalon. Overall, this study provides neurobehavioral evidence and novel insights into the neurotoxic mechanisms of long-term citalopram exposure and may facilitate the assessment of the environmental and health risks posed by citalopram-containing antidepressant drugs.

Waterborne and Dietary Bioaccumulation of Organophosphate Esters in Zooplankton Daphnia magna

Organophosphate esters (OPEs) are widely used as an additive in flame retardants, plasticizers, lubricants, consumer chemicals, and foaming agents. They can accumulate in aquatic organisms from water (waterborne exposure) and food (dietary exposure). However, the bioaccumulation characteristics and relative importance of different exposure routes to the bioaccumulation of OPEs are relatively poorly understood. In this study, Daphnia magna were exposed to fo typical OPEs (tris(2-chloroethyl) phosphate (TCEP), tris(1,3-dichloro-2-propyl) phosphate (TDCIPP), tris(2-butoxyethyl) phosphate (TBOEP), and triphenyl phosphate (TPHP)), and their toxicokinetics under waterborne and dietary exposure routes were analyzed. For the waterborne exposure route, the bioconcentration factors (BCFs) increased in the order of TBOEP, TCEP, TDCPP, and TPHP, which were consistent with their uptake rate constants. TPHP might have the most substantial accumulation potential while TBOEP may have the smallest potential. In dietary exposure, the depuration rate constants of four OPEs were different from those in the waterborne experiment, which may indicate other depuration mechanisms in two exposure routes. The biomagnification factors (BMFs) of fur OPEs were all below 1, suggesting trophic dilution in the transfer of four OPEs from Scenedesmus obliquus to D. magna. Except for TBOEP, the contributions of dietary exposure were generally lower than waterborne exposure in D. magna under two exposure concentrations. This study provides information on the bioaccumulation and contribution of OPEs in D. magna via different exposure routes and highlights the importance of considering different exposure routes in assessing the risk of OPEs.

A single-cell survey unveils cellular heterogeneity and sensitive responses in mouse cortices induced by oral exposure to triphenyl phosphate

Triphenyl phosphate (TPhP) is a non-halogenated organophosphorus flame retardant, and there is a higher exposure risk in children. TPhP has been found to be neurotoxic upon developmental exposure, yet the specific mechanism remains unclear. To characterize the cellular responses underlying TPhP-induced developmental neurotoxicity, we administered TPhP (0.5, 5 or 50 mg/kg/day) to neonatal mice from postnatal day 10 (P10)-P70. A total of 17,229 cells and 26,338 genes were identified in cortical samples from control and low-dose (the internal doses of metabolite DPhP comparable to human exposure level) groups using single-cell RNA sequencing (scRNA-seq). TPhP exposure led to heterogeneous transcriptional alterations and intercellular crosstalk among neurons, neural stem/progenitor cells (NSPCs), endothelial cells, and immunocytes. Deprivation of NSPCs, loss of mature neurons, and concomitant neuroinflammation mediated by extrinsic and intrinsic immunocytes were found in TPhP-exposed cortices. In addition, we observed blood-brain barrier destruction prior to the anxiety/depression-like neurobehavioral changes. These results reveal the distinctive cellular processes in TPhP's neurodevelopmental toxicity and uncover that the impeded neurogenesis, disrupted vascular barrier, and concomitant neuroinflammation are the sensitive responses to TPhP exposure. Our study paves the way for the application of scRNA-seq in toxicity assessments for emerging neurotoxic pollutants.

Penetration of Organophosphate Triesters and Diesters across the Blood-Cerebrospinal Fluid Barrier: Efficiencies, Impact Factors, and Mechanisms

The penetration of organophosphate triesters (tri-OPEs) and diesters (di-OPEs) across the blood-brain barrier and their influencing factors remain unclear in humans. In this study, 21 tri-OPEs and 8 di-OPEs were measured in 288 paired serum and cerebrospinal fluid (CSF) samples collected in Jinan, China. Six tri-OPEs were frequently detected in both serum and CSF, with median concentrations ranging from 0.062 to 1.62 and 0.042-1.11 ng/mL, respectively. Their penetration efficiencies across the blood-CSF barrier (BCSFB) (R_CSF/serum, C_CSF/C_serum) were calculated at 0.667-2.80, and these efficiencies first increased and then decreased with their log K_ow values. The reduced penetration efficiencies of triphenyl phosphate (TPHP) and 2-ethylhexyl diphenyl phosphate (EHDPP) may be attributed to their strong binding affinities for human serum albumin and p-glycoprotein due to their high hydrophobicity and aryl structure, as indicated by molecular docking. This suggests that active efflux transport may be involved in the penetration of TPHP and EHDPP in addition to passive diffusion similar to the other four tri-OPEs. Di-OPEs were found in few serum samples and even fewer CSF samples, indicating their limited BCSFB permeability. This may be due to their high polarity, low hydrophobicity, and ionic state in blood. This study has important implications for understanding the neurotoxicity of tri-OPEs and di-OPEs and the underlying mechanisms.

Triphenyl phosphate-induced pericardial edema is associated with elevated epidermal ionocytes within zebrafish embryos

Triphenyl phosphate (TPHP) is an organophosphate ester-based plasticizer and flame retardant. The objective of this study was to identify the potential role of epidermal ionocytes in mediating TPHP-induced pericardial edema within zebrafish embryos. Exposure to TPHP from 24 to 72 h post fertilization (hpf) resulted in a significant increase in pericardial edema and the number of ionocytes at 72 hpf relative to time-matched embryos treated with vehicle. In addition, co-exposure of embryos to mannitol (an osmotic diuretic) blocked TPHP-induced pericardial edema and effects on ionocyte abundance. However, knockdown of ATPase1a1.4 - an abundant Na⁺/K⁺-ATPase localized to epidermal ionocytes - mitigated TPHP-induced effects on ionocyte abundance but not pericardial edema, whereas co-exposure of embryos to ouabain - a Na⁺/K⁺-ATPase inhibitor - enhanced TPHP-induced pericardial edema but not ionocyte abundance. Overall, our findings suggest that TPHP may have multiple mechanisms of toxicity leading to an increase in ionocyte abundance and pericardial edema within developing zebrafish embryos.

Beyond Traditional Organophosphate Triesters: Prevalence of Emerging Organophosphate Triesters and Organophosphate Diesters in Indoor Dust from a Mega E-waste Recycling Industrial Park in South China

Numerous studies have reported the environmental contamination with traditional organophosphate triesters (tri-OPEs), but there is very little information on emerging tri-OPEs and organophosphate diesters (di-OPEs), especially in e-waste recycling areas. In this study, we conducted a comprehensive survey to monitor a broad suite of 11 traditional tri-OPEs, 12 emerging OPEs, and 10 di-OPEs in indoor dust collected from the workshops of (n = 42) and residential homes adjacent to (n = 24) a mega e-waste recycling industrial park in South China. In addition to traditional tri-OPEs, all of the emerging OPEs and di-OPEs were frequently detected in the dust samples. Total concentrations of emerging tri-OPEs and di-OPEs were in the range of 1210-62 900 and 2010-55 600 ng/g in the workshop dust and 435-23 700 and 186-4350 ng/g in the local home dust, respectively, which were comparable to those of traditional tri-OPEs (1160-61 500 and 370-13 900 ng/g, respectively). Most OPEs exhibited significantly higher concentrations in workshop dust versus local home dust (p < 0.05), indicating that e-waste dismantling activities contributed to the high residues of OPEs in indoor dust. Correlation analysis revealed that tri-OPEs have some common emission sources, i.e., e-waste and household products, while di-OPEs could originate from different sources, e.g., tri-OPE degradation, direct commercial application, and impurities in tri-OPE formulas. For both occupational workers and local adults, the median estimated daily intake values of emerging tri-OPEs (7.5 and 1.7 ng/kg bw/day, respectively) and di-OPEs (3.9 and 0.2 ng/kg bw/day, respectively) were comparable to that of traditional tri-OPEs (4.3 and 1.0 ng/kg bw/day, respectively), which suggests the important contribution of the emerging tri-OPEs and di-OPEs to the overall risks of human external exposure to OPE chemicals.

Neonatal exposure to organophosphorus flame retardant TDCPP elicits neurotoxicity in mouse hippocampus via microglia-mediated inflammation in vivo and in vitro

Tris(1,3-dichloro-2-propyl) phosphate (TDCPP) is a phosphorus-based flame retardant common in consumer goods and baby products. Concerns have been raised about TDCPP exposure and neurodevelopmental toxicity. However, the mechanism and early response for TDCPP-induced neurotoxicity are poorly understood. This study investigates the role of microglia-mediated neuroinflammation in TDCPP-induced neurotoxicity in mice and primary cells. TDCPP was administered to C57BL/6 pups (0, 5, or 50 mg/kg/day) via an oral gavage from postnatal days 10-38 (28 days). The results showed that TDCPP exposure for 28 days altered the gene expression of neuronal markers Tubb3, Nefh, and Nes, and led to apoptosis in the hippocampus. The mRNA levels of pro-inflammatory factors Il-1β, Tnfα and Ccl2 dose dependently increased in the hippocampus at both 24 h and 28 days following exposure, accompanied by microglia activation characterized by an amoeboid-like phenotype. In in vitro studies using the primary microglia isolated from neonatal mice, exposure to TDCPP (0-100 μM) for 24 h resulted in cellular activation. It also increased the expression of genes responsible for inflammatory responses including surface markers and pro-inflammatory cytokines. These changes occurred in a dose-dependent fashion. Neurite outgrowth of primary mouse hippocampal neurons was inhibited by treatment with the conditioned medium harvested from microglia exposed to TDCPP. These results reveal that neonatal exposure to TDCPP induces neuronal damage through microglia-mediated inflammation. This provides insight into the mechanism of TDCPP's neurodevelopmental toxicity, and suggests that microglial cell is a sensitive responder for OPFRs exposure.

Bisphenol F-Induced Neurotoxicity toward Zebrafish Embryos

In this study, the influence of bisphenol F (BPF) toward central nervous system (CNS) was assessed using zebrafish embryos. We found that BPF could induce significant neurotoxicity toward zebrafish embryos, including inhibited locomotion, reduced moving distance, and CNS cell apoptosis at an effective concentration of 0.0005 mg/L. Immunofluorescence assay showed that both microglia and astrocyte in zebrafish brain were significantly activated by BPF, indicating the existence of neuroinflammatory response. Peripheral motor neuron development was significantly inhibited by BPF at 72 hpf. RNA-seq data indicated that neuronal developmental processes and cell apoptosis pathways were significantly affected by BPF exposure, which was consistent with the phenotypic results. Chip-seq assay implied that the transcriptional changes were not mediated by ERα. Additionally, no significant change was found in neurotransmitter levels (5-hydroxytryptamine, dopamine, and acetylcholine) or acetylcholinesterase (Ache) enzyme activity after BPF exposure, indicating that BPF may not affect neurotransmission. In conclusion, BPF could lead to abnormal neural outcomes during zebrafish early life stage through inducing neuroinflammation and CNS cell apoptosis even at environmentally relevant concentration.

mRNA-Sequencing Identifies Liver as a Potential Target Organ for Triphenyl Phosphate in Embryonic Zebrafish

Triphenyl phosphate (TPHP) is a commonly used organophosphate flame retardant and plasticizer in the United States. Using zebrafish as a model, the overall objective of this study was to identify potential organs that might be targeted by TPHP during embryonic development. Based on mRNA-sequencing, TPHP exposure from 24 to 30 h post fertilization (hpf) and 24 to 48 hpf significantly affected the abundance of 305 and 274 transcripts, respectively, relative to vehicle (0.1% DMSO) controls. In addition to minor effects on cardiotoxicity- and nephrotoxicity-related pathways, Ingenuity Pathway Analysis (IPA) of significantly affected transcripts within 30- and 48-hpf embryos revealed that hepatotoxicity-related pathways were strongly affected following exposure to TPHP alone. Moreover, while pre-treatment with fenretinide (a retinoic acid receptor agonist) mitigated TPHP-induced pericardial edema and liver enlargement at 72 hpf and 128 hpf, respectively, IPA revealed that fenretinide was unable to block TPHP-induced effects on cardiotoxicity-, nephrotoxicity-, and hepatotoxicity-related pathways at 48 hpf, suggesting that TPHP-induced effects on the transcriptome were not associated with toxicity later in development. In addition, based on Oil Red O staining, we found that exposure to TPHP nearly abolished neutral lipids from the embryonic head and trunk and, based on metabolomics, significantly decreased the total abundance of metabolites - including betaine, a known osmoprotectant - at 48 and 72 hpf. Overall, our data suggest that, in addition to the heart, TPHP exposure during early development results in adverse effects on the liver, lipid utilization, and osmoregulation within embryonic zebrafish.