TREM2 and early-onset autosomal dominant Alzheimer disease: The ectodomain of TREM2 directly interacts with the environment to regulate microglial function.[21, 22, 23, 24, 25] TREM2 has been found to play a key role in the onset and progression of Alzheimer's disease (AD).[26, 27] However, it remains unknown whether TREM2 plays a role in the pathogenesis of sporadic PD.