Swimming in Plastamination: Polylactic Acid (PLA) Nanoplastics Affect Bull Sperm Functions

Plastic contamination (plastamination) is one of the main challenges of the 21st century. Microplastics (MPs, 5 mm-1 µm) and nanoplastics (NPs, <1 µm) are mainly produced from the environmental degradation of plastic waste and enter the food chain, bypass biological barriers, and bioaccumulate in tissues where they exert toxic/inflammatory effects. In the male reproductive system, MP/NPs can cross the blood-testis barrier, impair spermatogenesis, and affect semen quality parameters. While biodegradable polymers like polylactic acid (PLA) may represent an ecofriendly alternative to carbon fossil polymers, their real effects in reproduction have been poorly investigated. In the present manuscript, the effects of increasing doses of rhodamine B-loaded PLA-NPs (170 ± 20 nm mean size) on the physiology of frozen-thawed bovine sperm were investigated using a computer-assisted sperm analyzer (CASA) and flow cytometry. Sperm kinetics revealed a significant effect of PLA-NPs on progressive motility at 60 min, with higher values at 200 and 300 µg/mL doses (p < 0.05 vs. control). Flow cytometry showed that PLA-NP exposure did not alter acrosomal integrity (PNA488+); however, the proportion of spermatozoa showing high mitochondrial potential (MitoDR+) was significantly reduced compared to controls at a 300 µg/mL PLA-NP dose after 120 min of incubation. The proportion of spermatozoa metabolically active (MitoDR+) with contextual membrane destabilization (M540+) was significantly lower in samples with 200 and 300 µg/mL PLA, independently of the incubation time. Finally, immunofluorescence analysis revealed the internalization of PLA-NPs within spermatozoa at the longest exposure time. In conclusion, PLA-NPs are internalized in spermatozoa, with specific localization in mitochondria, and slightly affect progressive motility mainly by mitochondrial interference and plasma membrane destabilization. Despite limited effects on post-thaw bull sperm motility even at high doses, plastamination warrants consideration, and further studies on the biological effects of biodegradable plastics are recommended to preserve sperm physiology.