MAPT and Parkinson disease: Increased TG2 levels and/or activity, especially the nvolvement of nuclear TG2, have been observed in many neurodegenerative diseases such as Alzheimer's disease (AD), Huntington's disease (HD), and Parkinson's disease (PD).87 AD is characterized by the formation of extracellular neurotoxic aggregates consisting of amyloid-β protein or intracellular neurotoxic aggregates consisting of hyperphosphorylated tau.