APOE and Alzheimer disease: The sigma-2 (σ2) receptor has been positively identified as the ER-resident transmembrane protein 97 (TMEM97) [42], and the σ2 receptor/TMEM97, progesterone receptor membrane component 1 (PGRMC1) and low-density lipoprotein receptor (LDLR) have been found to form a trimeric complex and regulate the uptake of lipoproteins such as LDL and apolipoprotein E (ApoE) [43,44], whose E4 allele (ApoE ε4) is the greatest risk factor for AD development [45].