TGF-β1 Neuroprotection via Inhibition of Microglial Activation in a Rat Model of Parkinson's Disease

Transforming growth factor (TGF)-beta 1 is a pleiotropic cytokine with immunosuppressive and anti-inflammatory properties. Recently we have shown that TGF-beta 1 pretreatment in vitro protects against 1-methyl-4-phenylpyridinium (MPP+)-induced dopaminergic neuronal loss that characterizes in Parkinson's disease (PD). Herein, we aimed to demonstrate that TGF-beta 1 administration in vivo after MPP+ toxicity has neuroprotection that is achieved by a mediation of microglia. A rat model of PD was prepared by injecting MPP+ unilaterally in the striatum. At 14 days after MPP+ injection, TGF-beta 1 was administrated in the right lateral cerebral ventricle. Primary ventral mesencephalic (VM) neurons and cerebral cortical microglia were treated by MPP+, respectively, and TGF-beta 1 was applied to neuronal or microglial cultures at 1 h after MPP+ treatment. As expected, MPP+ resulted in decrease in TGF-beta 1 production in the substantia nigra and in primary VM neurons and microglia. TGF-beta 1 intracerebroventricular administration alleviated MPP+-induced PD-like changes in pathology, motor coordination and behavior. Meanwhile, TGF-beta 1 ameliorated MPP+-induced microglial activation and inflammatory cytokine production in vivo. Interestingly, TGF-beta 1 treatment was not able to ameliorate MPP+-induced dopaminergic neuronal loss and caspase-3/9 activation in mono-neuron cultures, but TGF-beta 1 alleviated MPP+-induced microglial activation and inflammatory cytokine production in microglia-enriched cultures. This effect of TGF-beta 1 inhibiting microglial inflammatory response was blocked by Smad3 inhibitor SIS3. Importantly, neuronal exposure to supernatants of primary microglia that had been treated with TGF-beta 1 reduced dopaminergic neuronal loss and caspase-3/9 activation induced by MPP+-treated microglial supernatants. These findings establish that TGF-beta 1 exerts neuroprotective property in PD by inhibiting microglial inflammatory response via Smad3 signaling.