Amongst them are senescence, autophagy defects, genetic factors (i.e., ApolipoproteinaE-allele4 (APOE4), Triggering receptor expressed on myeloid cells 2 (Trem2), microbiota alterations, lifestyle, cardiovascular and traumatic brain injury, as well as environmental factors (level of education, hypertension, obesity, diabetes, smoking, hearing loss, depression, physical inactivity, social isolation) [5]. Here, TREM2 is linked to major depressive disorder.