Dominant role of the cytosolic C-terminal domain of the rat 5-HT1B receptor in axonal-apical targeting

Abstract

The 5-HT1A and 5-HT1B receptors for serotonin exhibit a different membrane localization to either soma and dendrites (5-HT1AR) or axons and terminals (5-HT1BR) of neurons in the CNS. The mechanisms responsible for their differential targeting were investigated previously by transfecting various 5-HT1AR/5-HT1BR chimeras in the epithelial Lilly pork kidney (LLC-PK1) cell line. This first study suggested that a specific targeting signal is located in the C-terminal portion (comprising the last two transmembrane and the cytoplasmic C-terminal domains) of the 5-HT1A and/or 5-HT1B receptors. In the present study, the role of the cytosolic C-terminal tail of the receptors was further investigated by transfecting truncated receptors and 5-HT1AR/5-HT1BR chimeras in both the epithelial LLC-PK1 cells and rat hippocampal neurons in primary culture. Confocal microscopic analysis of immunofluorescence with specific anti-5-HTR antibodies and anti-microtubule-associated protein 2 or anti-neurofilament 200k antibodies showed that substitution of the cytosolic C-terminal tail of the 5-HT1BR in the 5-HT1AR addressed the resulting chimera to the axon of neurons and to the apical domain of LLC-PK1 cells. Therefore, the short tail of the 5-HT1BR presents an apical targeting signal that can also act as an axonal targeting signal. In addition, a domain within the third intracytoplasmic loop of the 5-HT1BR, responsible for its Golgi sequestration in LLC-PK1 cells, appeared to act as another axonal targeting signal in hippocampal neurons.