The exercise-microbiota-queuine-tRNA axis in Parkinson's disease: evidence, uncertainties, and experimental priorities

Parkinson's disease (PD) is a multisystem neurodegenerative disorder characterized by progressive nigrostriatal dopaminergic degeneration, α-synuclein aggregation, mitochondrial dysfunction, oxidative stress, and neuroinflammatory remodeling. Although these mechanisms have been extensively investigated, how systemic metabolic and microbiota-derived signals intersect with neuronal translational control remains incompletely understood. Queuosine (Q) modification of tRNAs is a distinctive RNA modification because its precursor, queuine, is not synthesized de novo by mammalian cells but is acquired from diet and gut microbial metabolism. Emerging evidence indicates that Q-tRNA modification can influence codon decoding, translational speed, proteostasis, oxidative stress responses, and mitochondrial function, but direct evidence linking Q-tRNA dysregulation to PD remains limited. In this narrative review, we propose a conceptual and hypothesis-generating framework in which the microbiota-queuine-Q-tRNA modification axis may contribute to neuronal translational buffering and stress adaptation in PD. We distinguish established mechanisms, emerging evidence, and speculative links, emphasizing that the complete causal chain from exercise-induced microbiota remodeling to altered queuine availability, Q-tRNA modification, mitochondrial translational recalibration, and dopaminergic neuroprotection has not yet been experimentally demonstrated. We further discuss tRNA-derived fragments (tRFs) as candidate biomarkers and potential effector molecules in PD-associated translational stress, neuroinflammation, and intercellular RNA communication. Finally, we outline experimental priorities for validating this model, including direct Q-tRNA profiling in PD tissues and biofluids, exercise-intervention studies in PD models, microbiota/queuine manipulation, and mechanistic testing of circulating RNA carrier transport across the blood-brain barrier. This framework does not establish a new pathogenic pathway, but provides a structured roadmap for investigating how exercise, microbial metabolism, and RNA modification biology may converge on selective neuronal vulnerability in PD.