HCAR2 and Cognitive impairment: Considering that the upregulation of AA formation precedes the development of clinical symptoms of AD and is associated with the improvement of episodic and working memory [65], the present review suggests that the upregulation of the formation of AA, a putative endogenous GPR109A agonist, is a compensatory (adaptive) response aimed at attenuating the development of cognitive impairment in schizophrenia, MCI, and AD (Figure 3).