Microplastics and Cadmium Co-Exposure Impairs Testicular Function via A Trojan Horse Mechanism: Protective Role of D-Aspartate

Environmental pollutants are increasingly recognized as major contributors to male reproductive dysfunction, with polystyrene microplastics (MPs) and cadmium (Cd) representing emerging co-exposure toxicants. This study investigated their effects on testicular function in sexually mature rats and evaluated the protective potential of D-aspartate (D-Asp). Exposure to MPs and Cd induced marked testicular injury, characterized by MPs accumulation within the seminiferous epithelium, impaired sperm quality, reduced testosterone production, and downregulation of the steroidogenic enzyme 3β-hydroxysteroid dehydrogenase (3β-HSD). These alterations were associated with disruption of the blood–testis barrier through decreased expression of junctional proteins and their regulatory kinases (Src and FAK), accompanied by oxidative stress, apoptosis, cytoskeletal disorganization, and dysregulation of proteins involved in germ cell proliferation and differentiation (PTMA, DAAM1, and PREP). MPs also activated autophagy, as indicated by altered LC3B and p62 expression, suggesting an adaptive response to cellular stress. Combined exposure produced more severe reproductive toxicity than MPs alone but less pronounced effects than Cd exposure, supporting a "Trojan horse" mechanism in which MPs adsorb Cd in the gastrointestinal tract, thereby modulating its bioavailability and systemic toxicity. D-Asp markedly attenuated MPs-induced testicular damage by restoring redox homeostasis, enhancing antioxidant defenses, reducing apoptosis, modulating autophagy, and promoting steroidogenesis and spermatogenesis through the upregulation of StAR, 3β-HSD, 17β-HSD, and germ cell markers (PCNA, SYCP3). Collectively, these findings provide new mechanistic insights into MPs- and Cd-induced reproductive toxicity and identify D-Asp as a promising therapeutic strategy for protecting male reproductive health.