APOE and Alzheimer disease: The activation of the TREM2-APOE pathway is a key transcriptional signature of reactive microglia, conserved in multiple mouse models of neurodegenerative diseases such as AD (APP/PS1 mice), amyotrophic lateral sclerosis (ALS) (SOD1G93A mice) and multiple sclerosis (MS) (experimental autoimmune encephalomyelitis) [34].