GPNMB and Parkinson disease: The expression levels of glycoprotein GPNMB (Moloney et al., 2018), SNc free water (Guttuso et al., 2018), and iron accumulation (Barbosa et al., 2015; An et al., 2018) were significantly elevated; and mitochondrial function was altered (Reeve et al., 2018) in PD.