CDK5 and Alzheimer disease: Recently, the administration of some natural drugs to AD models has been shown to inhibit the hyperphosphorylation of the tau protein by regulating the activity of cyclin-dependent kinase-5 (Gao et al., 2014; Ma et al., 2015), glycogen synthase kinase-3 (Li et al., 2011a) or serine/threonine protein phosphatase-2a (Karakani et al., 2015) to improve the symptoms of AD model rats.