Estradiol potentiates 8-OH-DPAT-induced sumoylation of 5-HT_1A receptor: Characterization and subcellular distribution of sumoylated 5-HT_1A receptors

Sumoylation is a recently described post-translational modification and only a few sumoylated neurotransmitter receptors are known. Through the present studies, we discovered that serotonin1A receptors (5-HT_1A-Rs) can be sumoylated by SUMO1 (small-ubiquitin-related modifier 1) protein. The SUMO1-5-HT_1A-R is ~55kDa, is located in the membrane fraction, but not the cytosol, and is distributed in all of the brain regions expressing 5-HT_1A-Rs examined. Acute stimulation of 5-HT_1A-Rs significantly increased SUMO1-5-HT_1A-R in rat hypothalamus. Pre-treatment with estradiol for 2 days, which causes a partial desensitization of 5-HT_1A-R signaling, potentiated agonist-induced increases in SUMO1-5-HT_1A-Rs in the hypothalamus of ovariectomized rats. Using discontinuous gradient centrifugation followed by digitonin treatment, we found that the majority of SUMO1-5-HT_1A-Rs is co-localized with endoplasmic-reticulum and trans-Golgi-network markers. Although a small proportion of SUMO1-5-HT_1A-Rs are located in the detergent resistant microdomain (DRM) that contain active G-protein coupled receptors, their distribution was different from that of the Gαz protein that couples to the receptors. These data suggest that the SUMO1-5-HT_1A-Rs are an inactive form of 5-HT_1A-Rs, a finding further supported by results showing minimal 5-HT_1A-R agonist binding to SUMO1-5-HT_1A-Rs. Furthermore, SUMO1-5-HT_1A-Rs in the DRM were increased by treatment with a 5-HT_1A-R agonist, 8-OH-DPAT ((+)8-hydroxy-2-dipropylaminotetralin). Together, these data suggest that sumoylation of 5-HT_1A-Rs may be related to 5-HT_1A-R trafficking and internalization, which may contribute to 5-HT_1A-R desensitization. Since 5-HT_1A-Rs play an important role in mood regulation, the present results significantly impact on the understanding of the pathogenesis of affective disorders and development of better therapeutic approaches for these diseases.