MMP9 and Parkinson disease: In addition, in the MPTP mouse model of PD, ghrelin and exendin-4 have been shown to reduce the loss of DA neurons in the substantia nigra and striatum and the activation of microglia by inhibiting the expression of MMP3 (Kim et al., 2009; Moon et al., 2009), and reducing the expression of MMP9 may have the potential to treat PD (Lorenzl et al., 2004; Kim S. Y. et al., 2010; Lee et al., 2010; Broom et al., 2011).