Infoscience

Thesis

Characterization of the 5HT3 receptor in live cells and in vitro: heterologous expression, isolation and reconstitution in lipid membranes

The serotonin type 3 receptor (5HT3R) is a homopentameric, cation selective channel that mediates fast excitatory transmission in the nervous system. It is a member of the Cys-loop family of receptors that include the nicotinic acetylcholine receptor (nAChR), the γ-amino butyric acid (GABA) type A and C and glycine receptors. The 5HT3 receptor is involved in behavioral and digestive disorders and it is therefore an important therapeutic target. The 5-HT3 receptor was expressed in mammalian HEK cells (chapter 2) stably and transiently, and its expression was optimized to obtain 1.6 x 106 receptors per cell for its solubilisation (chapter 3) and purification (chapter 4). Solubilisation of the receptor was evaluated under different conditions such as using CHAPS and C12E9 detergents and homogenization procedures such as vortexing and thurraxing the membranes. Radioligand binding assays indicated that the amount of solubilised receptor was similar in both cases. The stability of the receptor at different temperatures was evaluated. The receptor was more stable when kept at -20 °C compared to 4 or 37 °C. Purification of the 5-HT3 receptor from HEK cells was performed by tandem IMAC (Immobilized Metal ion Affinity chromatography)/IMAC purification and IMAC/streptactin flow column chromatography. The 5-HT3R was obtained 50% pure and a yield of 10 pmoles of receptor binding sites per gram of starting material (dry cells) was obtained. Purified receptor preparations were homogenous. Finally, the receptor was concentrated 5.5 times, which allowed the incorporation of the receptor in lipid vesicles for further studies. Detergent solubilised 5-HT3R was incorporated in lipid vesicles and the functionality of the receptor was assessed by studying calcium ion permeability of the channel using calcium ion sensitive fluorophores (chapter 5). The effect of the cAMP dependent kinase (PKA), on the 5-HT3R was investigated by measuring the KD of the radioactive ligand [3H]GR65630 to the receptor in cells where PKA was activated or inhibited (chapter 6). Finally a dose-response curve of the binding activity of the 5-HT3R upon activation of the PKA was assayed in both HEK and CHO cells (chapter 6). Radioligand binding assays indicated that upon activation of the PKA, there was a decrease of the KD of 30% indicating an increase in the receptor binding affinity to 5-HT upon activation of the PKA (chapter 6). A kinase and cytoskeleton binding protein PDLI1 that was co-purified with the receptor was found phosphorylated upon PK activation (chapter 6). As PDLI1-like proteins are adaptor proteins between kinases, cytoskeleton proteins and sometimes receptors, the latter may be involved in the kinase activation of the receptor. These results contribute to the optimization of the receptor expression, solubilisation and purification in mammalian cells for its incorporation in lipid vesicles and for further structural studies. In addition, this work gave new insights to the mechanisms of channel regulation by PKA phosphorylation at the cell membrane and the intermediate proteins that are involved.

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