Regulation of 5-HT2A serotonin receptor sorting and signaling in cortical pyramidal neurons

The 5-HT_2A serotonin receptor is an important G protein-coupled receptor that mediates the effects of hallucinogens and the therapeutic actions of a number of commonly prescribed medications, including atypical antipsychotics, antidepressants and anxiolytics. The 5-HT_2A receptor, the most abundant serotonin receptor in the cerebral cortex, is enriched in apical dendrites as well as in dendritic spines of pyramidal neurons. A canonical Type I PDZ-binding motif at the carboxyl terminus of the 5-HT_2A receptor has been predicted to mediate 5-HT_2A receptor interaction with components of the sorting apparatus responsible for establishing neuronal polarity. We discovered that disrupting the PDZ-binding motif on the 5-HT _2A receptor greatly attenuated 5-HT_2A receptor targeting to dendrites (without interfering with its axonal exclusion), suggesting that this protein-protein interaction motif is a necessary dendritic targeting signal in cultured cortical pyramidal neurons. Co-immunoprecipitation and immunofluorescent studies subsequently revealed that the PDZ-binding motif mediates the interaction between the 5-HT_2A receptor and PSD-95, a prototypic PDZ domain-containing protein and a major regulator of polarized protein sorting in neurons. The association with PSD-95 potentiated 5-HT _2A receptor-mediated signal transduction at least in part by inhibiting the agonist-induced internalization and promoting the cell surface clustering of 5-HT_2A receptors in HEK-293 cells. DOI (a 5-HT₂ agonist and hallucinogen) and neuronal depolarization promote 5-HT_2A receptor trafficking to PSD-95-enriched sites. Additionally, we found that the PDZ-binding motif mediates the neuronal activity-evoked targeting of 5-HT_2A receptors to distal dendrites, concomitant with an elevation in 5-HT _2A receptor-mediated signal transduction. Taken together, my studies elucidate a novel role for PDZ-binding domains in the activity-dependent regulation of 5-HT_2A receptor signaling and trafficking. These studies will be relevant for understanding how diverse classes of drugs that target the 5-HT_2A receptor exert their profound effects on human cognition, emotion and perception.