Insulin-releasing or insulin-sensitizing drugs in Parkinson's disease? Choosing a pathway

Insulin Enhances Gene Expression of Midnolin, a Novel Genetic Risk Factor for Parkinson's Disease, via Extracellular Signal-Regulated Kinase, Phosphoinositide 3-Kinase and Multiple Transcription Factors in SH-SY5Y Cells

Parkinson's disease (PD) is the second most common neurodegenerative disease. Although many monogenic variants have been identified that cause familial PD, most cases are sporadic and the mechanisms of sporadic PD onset remain unclear. We previously identified midnolin (MIDN) as a novel genetic risk factor for PD in a Japanese population. MIDN copy number loss was strongly associated with sporadic PD, which was replicated in a British population. Furthermore, suppression of MIDN expression in rat pheochromocytoma cells inhibits neurite outgrowth and expression of Parkin ubiquitin ligase. However, the detailed molecular mechanisms of MIDN expression are unknown. We, therefore, investigated the molecular mechanism of MIDN expression in human neuroblastoma SH-SY5Y cells. We found that MIDN expression was promoted by insulin via extracellular-signal regulated kinase1/2 and phosphoinositide 3-kinase-dependent pathways. In addition, MIDN promoter activity was enhanced by mutations at transcription factor AP-2 consensus sequences and reduced by mutations at cAMP response element-binding protein and activator protein 1 (AP-1) consensus sequences. The dominant-negative cAMP response element-binding protein mutant did not block MIDN promoter activity, but both the pharmacological inhibitor and decoy oligodeoxynucleotide for AP-1 significantly blocked its activity. Additionally, DNA binding of c-FOS and c-JUN to the AP-1 consensus sequence in the MIDN promoter was enhanced by insulin as determined by chromatin immunoprecipitation, which suggested that AP-1 positively regulated MIDN expression. Taken together, this study reveals molecular mechanisms of MIDN gene expression induced by insulin in neuronal cells, and drugs which promote MIDN expression may have potential to be a novel medicine for PD. SIGNIFICANCE STATEMENT: We demonstrated that insulin promotes midnolin expression via extracellular-signal regulated kinase 1/2 and phosphoinositide 3-kinase pathways. Furthermore, we identified the important region of the MIDN promoter and showed that transcription factors, including activator protein 1, positively regulate MIDN expression, whereas transcription factor AP-2 negatively regulates basal and insulin-induced MIDN expression. We believe that our observations are important and that they contribute to the development of novel drugs to treat Parkinson's disease.