Oligomerization of the serotonin(1A) receptor in live cells: a time-resolved fluorescence anisotropy approach

The serotonin(1A) receptor is a representative member of the G-protein coupled receptor (GPCR) superfamily and serves as an important target in the development of therapeutic agents for neuropsychiatric disorders. Oligomerization of GPCRs is an important contemporary issue since it is believed to be a crucial determinant for cellular signaling. In this work, we monitored the oligomerization status of the serotonin(1A) receptor tagged to enhanced yellow fluorescent protein (5-HT(1A)R-EYFP) in live cells utilizing time-resolved fluorescence anisotropy decay. We interpret the unresolved fast component of the observed anisotropy decay as fluorescence resonance energy transfer (FRET) between 5-HT(1A)R-EYFP molecules (homo-FRET). Homo-FRET enjoys certain advantages over hetero-FRET in the analysis of receptor oligomerization. Our results reveal the presence of constitutive oligomers of the serotonin(1A) receptor in live cells. We further show that the oligomerization status of the receptor is independent of ligand stimulation and sphingolipid depletion. Interestingly, acute (but not chronic) cholesterol depletion appears to enhance the oligomerization process. Importantly, our results are independent of receptor expression level, thereby ruling out complications arising due to high expression. These results have potential implications in future therapeutic strategies in pathophysiological conditions in which serotonin(1A) receptors are implicated.