Thioredoxin-1 in the SNpc Regulates Subventricular Zone Neural Proliferation and Olfactory Bulb Neural Differentiation in MPTP-Induced Olfactory Dysfunction.

Despite hyposmia being a dominant non-motor manifestation of Parkinson's disease (PD), its underlying driving mechanisms are poorly defined. The redox protein Thioredoxin-1 (Trx-1) offers neuroprotection against various insults; however, its potential involvement in the neural proliferation in the subventricular zone (SVZ) and neural differentiation in the olfactory bulb (OB) related to MPTP-induced olfactory dysfunction have not been previously established. Our research demonstrates that when Trx-1 is downregulated in the substantia nigra pars compacta (SNpc), MPTP-triggered olfactory deficits are significantly intensified. A key anatomical discovery in our study is the existence of projections from the SNpc to the SVZ. We established that the MPTP-driven death of SNpc dopaminergic (DAergic) neurons correlates with decreased dopamine D1 receptor (D1R) levels in the SVZ, an effect that is magnified by the loss of Trx-1. Alongside D1R reductions, MPTP suppressed a cascade of SVZ signaling molecules (phosphorylated PKA, Wnt3a, β-catenin, Pax6, cyclin D1, and CDK4), with Trx-1 deficiency causing even steeper declines. Furthermore, Trx-1 knockdown hindered the generation of immature neurons and disrupted DAergic neuronal differentiation within the OB. Collectively, our findings suggest that reduced Trx-1 expression in the SNpc may contribute to PD-related olfactory deficits, potentially via inhibiting SVZ neural proliferation, decreasing immature and mature neuron populations, and disrupted differentiation of OB immature neurons. By accelerating MPTP-triggered degeneration of DAergic neurons in the SNpc, Trx-1 downregulation reduces the SVZ of DAergic input. This disruption impairs D1R-mediated the neural proliferation in the SVZ, as well as the maturation and differentiation of immature neurons in OB, ultimately driving the progression of olfactory dysfunction in a PD mouse model.