Neurodegeneration at the crossroads: the gut-brain axis and blood-brain barrier in Parkinson’s disease — a review

Parkinson's disease (PD), which is one of the most common neurodegenerative illnesses, involves abnormal deposition of α-Synuclein and loss of dopaminergic neurons in the substantia nigra. Beyond this, there is increasing evidence that the gut-brain axis (GBA) and blood-brain barrier (BBB) interfere in disease initiation and progression. Dysbiosis of the gut microbiota affects the intestine and the BBB, allowing microbial metabolites and proinflammatory mediators to enter the CNS, causing neuroinflammation and neurodegeneration. Studies show that α-Synuclein pathology can originate in the gut and reach the brain via the vagus nerve. This review summarizes the connections among GBA, BBB, and PD, focusing on oxidative damage, inflammatory cascades, decreased expression of tight junction proteins, and signaling pathways such as TLR4/MyD88/NF-κB. In addition, we discuss therapeutic strategies that target the microbiota-BBB axis, such as probiotics, fecal microbiota transplantation, natural compounds (e.g., piperine, anethole, polymannuronic acid, Paeonia lactiflora), and stem cell therapy, which have demonstrated neuroprotective potential in animal models. Overall, the literature emphasizes the importance of restoring gut homeostasis and BBB integrity, and suggests that getting this axis right may offer novel opportunities for PD treatment. Future research is crucial to validate the efficacy of this approach clinically and to develop tailored therapies to prevent or delay PD progression.