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Phosphorylated p38MAPK specific antibodies cross‐react with sarkosyl‐insoluble hyperphosphorylated tau proteins

Neurofibrillary tangles (NFT) accumulated in Alzheimer's diseases and related disorders contain hyperphosphorylated tau and display immunoreactivity for active forms of various kinases. To understand the role of p38MAPK (mitogen‐activated protein kinase) in NFT formation, we have studied a transgenic (Tg) mouse model of tauopathy, JNPL3, that expresses P301L mutant tau, and bigenic mice, TAPP, generated by cross‐breeding of JNPL3 with Tg2576 mice. Age‐matched non‐Tg mice (NTg), wild‐type human tau Tg mice (JN25), and Tg2576 mice were used as controls. Phosphorylated p38MAPK (active form) immunoreactivity was consistently located in NFT and granulovaculolar degeneration in JNPL3 and TAPP mice older than 5 months of age. Unphosphorylated/total‐p38MAPK was not detectable in spinal cord and brain sections from 2‐ to 11‐month‐old mice, even though JNPL3 mice, but not controls had an age‐dependent increase of total‐p38MAPK by western blotting. Spinal cord/brain extracts from mice and human with tauopathy were demonstrated to have insignificant amount of active‐p38MAPK. However, they contained antiactive‐p38MAPK cross‐reactive proteins insoluble in sarkosyl and similar to phosphorylated tau in size. Consistently, antiactive‐p38MAPK immunoprecipitates displayed tau immunoreactivity, but not total‐p38MAPK, and antitau immunoprecipitates displayed active‐p38MAPK immunoreactivity. Together, the results indicate that the cross‐reactivity of antiactive‐p38MAPK antibody with phosphorylated tau is responsible for the immunolabeling of tau‐positive inclusion.