Targeting the bile acid receptor TGR5 with Gentiopicroside to activate Nrf2 antioxidant signaling and mitigate Parkinson’s disease in an MPTP mouse model

Parkinson's disease (PD) is a common neurodegenerative disorder characterized by classical symptoms including bradykinesia, rest tremor and rigidity. Oxidative stress and mitochondrial dysfunction are recognized as pivotal factors in PD progression. Gentiopicroside (GPS), a secoiridoid derived from Gentiana manshurica Kitagawa, exhibits antioxidant and mitophagy induction properties. Nonetheless, the effects and mechanisms by which GPS mitigates neurodegeneration in PD remain to be thoroughly elucidated. The goal of this study was to investigate the neuroprotective effects and mechanisms of GPS in PD models. We established the MPTP/MPP+-induced PD models to measure the neuroprotection of GPS. Transcriptomic analysis, oxidative biochemical kits, western blot and cell immunofluorescence were conducted to elucidate the fundamental mechanisms at play. Subsequently, the targeting and activation of the transmembrane G protein-coupled receptor-5 (TGR5) by GPS were measured by molecular docking, cellular thermal shift assay, microscale thermophoresis (MST) and cyclic adenosine monophosphate (cAMP) quantitation. Finally, we verified whether the neuroprotective and antioxidant effects of GPS were dependent on TGR5 by using specific small interfering RNA (siRNA), pharmacological antagonist and knockout mice. GPS significantly attenuated dopaminergic (DAergic) neuron loss and restored motor function in the MPTP-induced PD mouse model. Whole-genome RNA sequencing and subsequent mechanistic investigations revealed that GPS enhanced the expression and facilitated nuclear entry of factor erythroid-related 2-factor 2 (Nrf2), and reduced oxidative stress and mitochondrial dysfunction stimulated by neurotoxin. Additionally, GPS could target TGR5 and prevent its downregulation in PD model. TGR5's silencing or inhibition weakened the neuroprotective effect of GPS and blocked GPS-mediated activation of Nrf2 antioxidant signaling in PD model. Moreover, the therapeutic effect of GPS in mitigating motor deficits and neurodegeneration was also abolished in Tgr5 knockout mice. These findings collectively indicated that GPS targeted TGR5 to activate Nrf2 antioxidant signaling and ultimately ameliorated the pathological progression of PD.