HDAC9 and Alzheimer disease: Tacedinaline (Tac, a selective class I HDAC inhibitor) counteracted the increase in ER‐Ca2+ retention and mitochondrial Ca2+ accumulation, mitochondrial depolarization and impaired ER‐mitochondria cross‐talk, as observed in 3xTg‐AD mouse hippocampal neurons and AβO‐exposed HT22 cells.