CDK5 and Alzheimer disease: Recent phosphoproteomics studies revealed a significant number of proline-directed Ser/Thr sites that are differentially hyperphosphorylated in neurofilaments from AD brains, suggesting the involvement of proline-directed kinases, such as Cdk5, glycogen synthase kinese 3 beta (GSK3beta), ERK1/2, p38 MAP kinase, or stress-activated protein kinases (SAPK/JNK), and PP2A phosphatase in the regulation of the organization and dynamics of NF networks [87-89].