Ventral midbrain (VM) dopaminergic (DA) neurons that project to striatum progressively degenerate to cause Parkinsonís disease (PD). Neurotrophic factors (NTFs) have potential to protect, maintain and/or restore nigrostriatal DA innervation in PD. However, limitations employing NTF ligands in PD brain impedes their effectiveness. Targeting downstream intracellular molecules which regulate DA neurotrophic effects may be a more effective strategy for NTF therapy. To identify such targets, we investigated intracellular molecules regulating DA neurotrophic effects of growth/differentiation factor-5 (GDF5)-BMPR-Smad signalling.
Smad-interacting protein-1 (Sip1) transcription factor negatively regulates Smad signalling during CNS/PNS development. We found that siRNA-mediated knockdown of Sip1 significantly increased neurite growth in SH-SY5Y cells and embryonic VM DA neurons, human DA neuronal models. Knockdown of CtBP and NuRD, components of Sip1 transcriptional repressor complex, also induced significant neurite growth. BMPR activation and Smad transcriptional regulation were both required for neurite growth-promoting effects induced by Sip1 knockdown, suggesting BMPR-Smad signalling induces growth in absence of Sip1-mediated repression. Indeed, Sip1 overexpression repressed GDF5-Smad-induced neurite growth in VM neurons. Furthermore, Sip1 VM expression decreased during DA neuronal outgrowth in vivo, when GDF5/BMPR expression increased. Finally, p300 HAT is a positive regulator of Smad signalling whose activation, using CTPB, had neurotrophic effects in SH-SY5Y cells.
In conclusion, this study identifies Sip1 and p300 as novel regulators of neurite growth, and suggests that Sip1 is a negative regulator of VM DA neurotrophic effects of GDF5-BMPR-Smad signalling. Targeted inhibition of Sip1 mimicked DA neurotrophic effects of GDF5, and may be a viable neurotrophic strategy for PD.