Transcriptional biomarkers of microplastic exposure in fish: a tissue-resolved scoping review.

Microplastic exposure in fish has been increasingly assessed at the transcriptional level, yet findings remain dispersed across species, polymers, exposure routes, and tissues, limiting the identification of robust and transferable biomarkers. This scoping review examines which tissue-specific and cross-tissue transcriptional responses recur across available evidence and how they may support multi-gene biomarker development. Relevant experimental fish studies were identified through a structured PubMed search combining microplastic-, fish-, and transcription-related terms, followed by backward reference screening and forward citation chasing. Published studies reporting gene expression and transcriptomic outcomes in microplastic-exposed fish were synthesized with emphasis on tissues that both interface directly with the environment and show particle retention, including gills, gastrointestinal tract, skin, epidermal mucus, and related barrier compartments. The reviewed evidence indicates recurring transcriptional changes in epithelial integrity and junctional architecture, innate and adaptive immune signaling, redox balance and xenobiotic metabolism, ion transport and osmoregulatory control, and endocrine and metabolic regulation. However, the direction and magnitude of these responses vary with particle size, polymer chemistry, morphology, additives, co-contaminants, dose metrics, exposure route, and duration. Because single-gene readouts frequently overlap with responses to diverse stressors, tissue-aware, network-derived multi-gene signatures are proposed as a more specific approach to exposure diagnosis. Their feasibility is supported by the growing availability of RNA-seq datasets and established toxicogenomic workflows for module detection and classifier development. Opportunities for nonlethal biomonitoring include transcriptional signals from epidermal mucus and, prospectively, environmental RNA in surrounding water, although field application will require validation of RNA stability, shedding variability, and sampling procedures.