Inhalation exposure to tire rubber particle-sourced pollutant 6PPD-quinone involving basolateral amygdala impairment in male ICR mice

Abstract

Introduction

The extensive use of 6PPD in tire rubber has led to its increase in atmospheric tire wear particles. 6PPD reacts with ozone to form 6PPD-quinone (6PPD-Q), a respiratory toxicant enriched in tire and road wear particles.

Objectives

The aim of this study is to decipher the potential sensitive target organs and toxic actions by inhalation exposure to atmospheric 6PPD-Q.

Methods

This study employed a mouse inhalation exposure model to simulate environmental levels of 6PPD-Q at three concentrations of 0, 0.14, and 14 mg/m³. Using a 28-day exposure period followed by an equivalent recovery phase, we systematically evaluated the toxicological targets and effects of subacute exposure.

Results

The findings revealed that, compared to pulmonary, cardiovascular, and metabolic organ damage, 6PPD-Q-induced neurotoxicity was more persistent and irreversible, particularly characterized by prolonged anxiety-like behaviors. Histopathological analyses of the basolateral amygdala, using Nissl staining and markers of neuronal aging, indicated substantial neuronal degeneration linked to elevated oxidative stress, identifying this region as a critical target of 6PPD-Q neurotoxicity. Transcriptomic analysis uncovered that the expression of Egr1, a transcription factor crucial for neuronal plasticity, was markedly dysregulated. Findings of significant downregulation at the gene level and an upward trend in protein expression suggest that Egr1 expression is influenced by translational efficiency, epigenetic modifications, and post-translational regulatory mechanisms. Egr1 dysregulation disrupted downstream networks involving solute carrier proteins and calcium-binding proteins, contributing to aberrant neurobehavioral outcomes. Notably, the elevation of Egr1 protein levels in the basolateral amygdala but not in the cerebral cortex highlights the region-specific nature of 6PPD-Q’s neurotoxic effects.

Conclusion

This study provides the first insights into the neurotoxicity and irreversibility of inhaled 6PPD-Q exposure, paving the way for future research into the long-term neurological consequences and regulatory mechanisms of 6PPD-Q.