PM2.5 exposure contributes to anxiety and depression-like behaviors via phenyl-containing compounds interfering with dopamine receptor

Abstract

As a global problem, fine particulate matter (PM_2.5) really needs local fixes. Considering the increasing epidemiological relevance to anxiety and depression but inconsistent toxicological results, the most important question is to clarify whether and how PM_2.5 causally contributes to these mental disorders and which components are the most dangerous for crucial mitigation in a particular place. In the present study, we chronically subjected male mice to a real-world PM_2.5 exposure system throughout the winter heating period in a coal combustion area and revealed that PM_2.5 caused anxiety and depression-like behaviors in adults such as restricted activity, diminished exploratory interest, enhanced repetitive stereotypy, and elevated acquired immobility, through behavioral tests including open field, elevated plus maze, marble-burying, and forced swimming tests. Importantly, we found that dopamine signaling was perturbed using mRNA transcriptional profile and bioinformatics analysis, with Drd1 as a potential target. Subsequently, we developed the Drd1 expression-directed multifraction isolating and nontarget identifying framework and identified a total of 209 compounds in PM_2.5 organic extracts capable of reducing Drd1 expression. Furthermore, by applying hierarchical characteristic fragment analysis and molecular docking and dynamics simulation, we clarified that phenyl-containing compounds competitively bound to DRD1 and interfered with dopamine signaling, thereby contributing to mental disorders. Taken together, this work provides experimental evidence for researchers and clinicians to identify hazardous factors in PM_2.5 and prevent adverse health outcomes and for local governments and municipalities to control source emissions for diminishing specific disease burdens.

Keywords: PM2.5 exposure; anxiety and depression-like behavior; dopamine signaling system; organic component; toxicogenic structure.

Fig. 1.

Behavioral profiles and physiological parameters in male adult mice following the real-world PM_2.5 exposure. (A) Schematic representation of PM_2.5 exposure model and behavioral tests used. UA: unconcentrated air (the control group); CA: concentrated air (PM_2.5-exposed group). OFT: the open field test; EPMT: elevated plus maze test; MBT: marble-burying test; FST: forced swimming test. (B) PM_2.5 concentrations monitored in the chambers during the exposure period, with the histogram showing the mean PM_2.5 concentrations. Anxiety and depression-like behavior tests in mice evaluated by (C) total distance, (D) time in the center, (E) time in the open arms, (F) time in the closed arms, (G) marbles buried, and (H) immobility time (n = 12 to 15). Alterations of (I) body weight and (J) brain weight (n = 10). (K) Histological images of the cortex stained with H&E (Bar: 100 µm, 200× magnification). (L) Changes of cortical thickness (n = 3). The data in the histograms are presented as mean ± SEM and were analyzed by a two-tailed t test. *P < 0.05 and ***P < 0.001 indicate significance between different groups.

Fig. 2.

The mRNA expression profile and biological processes in the cortex of male adult mice following PM_2.5 exposure. (A) Heatmap showing the expression levels of DEGs. (B) DEGs-disease enrichment analysis showing the relationship between target genes and predicted diseases in CTD. The size and color of the squares represent inference scores between target genes and predicted diseases. (C) The KEGG enrichment analysis of DEGs. The size of the circle represents the number of annotated genes and the color represents the -Log10 P-value. (D) The relationship between DEGs and the enriched KEGG pathways. (E) The relationship between 12 GO terms related to the nervous system and their annotated genes. The size of the circle represents the number of annotated genes and the color represents the -Log10 P-value. (F) The clustering of GO terms in the cortex.

Fig. 3.

Drd1 acting as a target for dopamine signaling disruption in the cortex of male adult mice in response to PM_2.5 exposure. (A) Intersection of neurologically relevant DEGs in GO terms (blue), KEGG pathways (green), and CTD (red). (B) mRNA expression of eight genes (n = 6 to 10). (C) Disruption of the dopamine system, with red representing the up-regulated genes (Th and Rgs9) and blue representing the down-regulated genes (Avp and Drd1). (D) The expression of Drd1 in the cortex detected by immunofluorescence staining (Bar: 50 µm, 200× magnification). The data are presented as mean ± SEM and were analyzed by a two-tailed t test. *P < 0.05 indicates significance between different groups.

Fig. 4.

Extraction and identification of organic fragments in PM_2.5 using Drd1 expression as a target. (A) Schematic diagram of organic components in PM_2.5 extraction and fractionation. (B) Fractionation chromatogram of the total organic fraction. (C) Effects of primary organic fractions (F1~F9) on Drd1 expression (n = 6 to 8). (D) Fractionation chromatogram of four primary organic fractions (F2, F6, F7, and F9). (E) Effects of secondary organic fractions on Drd1 expression (n = 6 to 8). (F) Plots of molecular mass vs. retention time (RT) of 209 organic compounds. The data were analyzed using one-way ANOVA, followed by the Fisher’s least significant difference (LSD) test. *P < 0.05 and ***P < 0.001 indicate significance between different groups.

Fig. 5.

Prediction of characteristic fragments of suspect compounds in PM_2.5 based on DRD1 activity. (A) Flow chart of the extraction of hierarchical characteristic fragments (Left) and the list of primary and secondary fragments extracted using ToxCast/Tox21 data (Right). (B) Plot of molecular mass vs. retention time (RT) showing the predicted molecular characteristics of 95 active compounds based on primary and secondary characterization fragments. Color of circles represents the different compositions of organic compounds. (C) Sankey diagram showing the relationship between 95 active compounds and their characteristic fragments. (D) The relationship between aromaticity equivalent (Xc) of 209 suspect active compounds and DRD1 activity.

Fig. 6.

Interactions between active compounds and DRD1 measured by MD and MDS. MD results and the binding modes between (A) dopamine and (B) 10 compounds to DRD1. DRD1 is represented by the gray cartoon model. The ligand molecules are represented by the blue bat model. The key residues are represented by the yellow stick model, and the hydrogen bond is represented by the red dotted line. (C) RMSD changes of the active compounds-DRD1 system (top 10 compounds with docking scores) during the 50 ns MD simulation. (D) Ligand-binding free-energy components between tested compounds and DRD1 (kJ/mol). The size of the circle represents the energy level, with red representing negative energy and blue representing positive energy.

Fig. 7.

Toxicological mechanism and toxicogenic structure for real-world PM_2.5 exposure contributing to anxiety and depression-like behaviors in a coal combustion area.