Rotenone targets midbrain astrocytes to produce glial dysfunction-mediated dopaminergic neurodegeneration

Exposure to pesticides, such as rotenone or paraquat, is an environmental factor that plays an important role in the pathogenesis of Parkinson's disease (PD). Rotenone induces PD-like pathology and is therefore used to develop parkinsonian animal models. Dopaminergic neurotoxicity caused by rotenone has been attributed to the inhibition of mitochondrial complex I, oxidative stress and neuroinflammation; however, the mechanisms underlying selective dopaminergic neurodegeneration by rotenone remain unclear. To resolve this, we focused on glial diversity and examined whether the brain region-specific glial response to rotenone could determine the vulnerability of dopaminergic neurons using primary cultured neurons, astrocytes and microglia from the midbrain and striatum of rat embryos and rotenone-injected PD model mice. Direct neuronal treatment with low-dose rotenone failed to damage dopaminergic neurons. Conversely, rotenone exposure in the presence of midbrain astrocyte and microglia or conditioned media from rotenone-treated midbrain glial cultures containing astrocytes and microglia produced dopaminergic neurotoxicity, but striatal glia did not. Surprisingly, conditioned media from rotenone-treated midbrain astrocytes or microglia monocultures did not affect neuronal survival. We also demonstrated that rotenone targeted midbrain astrocytes prior to microglia to induce dopaminergic neurotoxicity. Rotenone-treated astrocytes produced secreted protein acidic and rich in cysteine (SPARC) extracellularly, which induced microglial proliferation, increase in IL-1β and TNF-α, and NF-κB (p65) nuclear translocation in microglia, resulting in dopaminergic neurodegeneration. In addition, rotenone exposure caused the secretion of NFAT-related inflammatory cytokines and a reduction in the level of an antioxidant metallothionein (MT)-1 from midbrain glia. Furthermore, we observed microglial proliferation and a decrease in the number of MT-positive astrocytes in the substantia nigra, but not the striatum, of low-dose rotenone-injected PD model mice. Our data highlight that rotenone targets midbrain astrocytes, leading to SPARC secretion, which promotes the neurotoxic conversion of microglia and leads to glial dysfunction-mediated dopaminergic neurodegeneration.