Characterization of the 5HT3 receptor in live cells and in vitro

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.

A Biochemical and Biophysical Investigation of 5-HT3 Receptor Stability

Menno Berend Tol

The 5-hydroxytryptamine 3 receptor (5-HT3R) is a member of the pentameric ligand-gated ion channel (pLGIC) family, that plays an important role in fast signal transduction and cell-cell communication in synapses: They convert a chemical signal from a neurotransmitter into the electrical signal that represents the action potential. Activation of 5-HT3R with serotonin triggers a range of molecular reorganizations that lead to channel opening and subsequent membrane depolarization. Because of their role, pLGICs are involved in a wide range of diseases and disorders and are therefore important drug targets. A typical pLGIC is composed of a β -sheet rich extracellular domain (ECD) where ligand binding takes place, a transmembrane domain that is composed of four α-helices (TM1-TM4). In vertebrates pLGICs often contain a third intracellular domain (ICD), located between TM3 and TM4. The 5-HT3R is active as a homopentameric receptor of five identical A subunits, or as a heteropentamer containing one or more of several subunits (B to E) together with an A subunit. The 5-HT3 receptor is naturally located in the cell membrane in low amounts. For in vitro experiments such as biophysical investigation, the receptor has to be overproduced and purified in a detergent-solubilized state. Therefore, a robust and scalable expression system for homo- and heteropentameric 5-HT3 receptors was developed, building upon a tetracycline inducible cell line and Strep-tag affinity purification technology. Production of solubilized and purified 5-HT3R is well suited for use in the structural and functional characterization of the receptor. The stability of the 5-HT3 receptor is an important parameter during structural and functional experimentation. Using thermal unfolding the stability of 5-HT3R was investigated in plasma membranes as well as during detergent-extraction, purification and reconstitution into artificial lipid bilayers. A large loss in thermostability was found that correlates with the loss of the lipid bilayer during membrane solubilization and purification. Thermal unfolding of 5-HT3R occurred in consecutive steps at distinct protein locations: First a loss of ligand binding is detected, followed by formation of different transient low oligomeric states of receptor pentamers, followed by partial unfolding of helical parts of the protein, which finally leads to the formation large receptor aggregates. It was also found that structural destabilization of the receptor in detergents could be partially reversed by reconstituting the receptor into lipid bilayers. Thermal unfolding leads to the irreversible formation of aggregates which is known to prevent effective refolding. Therefore, an further investigation of 5-HT3R was made by unfolding the receptor in urea and sodium dodecyl sulfate (SDS). The results indicate iii iv ABSTRACT that, unlike thermal unfolding, unfolding in urea and SDS does not result in the formation of aggregates. Unfolding by urea shows a 50% loss of helical content while keeping the characteristic pentamer intact. Contrastingly, unfolding by SDS shows dissociation of the pentamer, but no noticeable change in secondary structure. Still, refolding from urea and SDS proved to be impossible. Two weak fluorophores, crystal violet (CrV) and ethidium bromide (EtBr), bind to the homologous nicotinic acetylcholine receptor (nAChR). Because the fluorescence of these compounds is also sensitive towards their environment they can be used as a molecular beacon for the functional state of a receptor. When applied to 5-HT3R, it was found that CrV and EtBr bind with micromolar affinity to the desensitized state of the receptor – similar to binding reported for the nAChR. It appeared that binding of CrV or EtBr to the 5-HT3R induces a certain functional state (resting, open or desensitized). Because no detectable change in fluorescence occurs, this prevents their use as molecular beacons. Still, non-competitive binding to the 5-HT3R was little reported on before and could be an important target in pharmacological research. The ICD of the 5-HT3R is predicted to contain a stretch of ∼60 disordered or flexible residues which might be involved in subunit assembly and membrane trafficking. These unstructured regions of a 5-HT3R were probed using limited proteolysis. Thereby two distinct fragments were formed that remain tightly associated with each other, while a small stretch of 35 residues between TM3 and TM4 was removed. Furthermore, it was shown that the thermal stability of the helical core is unchanged compared to untreated 5-HT3R, but that the extracellular ligand-binding domain is very sensitive to cleavage in the ICD. By probing the 5-HT3R with limited proteolysis a rigid core was identified, indicating the important role of the transmembrane domain for holding together the pLGIC scaffold. The results presented here contribute a significant body of knowledge to membrane proteins in general and to the 5-HT3 receptor in particular, upon which further research can be based leading to advances in the fields of biochemistry, biophysics and pharmacology.

EPFL2012

Advanced Fluorescence Microscopy to Study Plasma Membrane Protein Dynamics

Joachim Piguet

Membrane protein dynamics is of great importance for living organisms. The precise localization of proteins composing a synapse on the membrane facing a nerve terminus is essential for proper functioning of the nervous system. In muscle fibers, the nicotinic acetylcholine is densely packed under the motor nerve termini. A receptor associated protein, rapsyn, acts as a linker between the receptor and the other components of the synaptic suramolecular assembly. Advances in fluorescence microscopy have allowed to measure the behavior of a single receptor in the cell membrane. In this work single-molecule microscopy was used to track the motion of ionotropic acetylcholine (nAChR) and serotonin (5HT3R) receptors in the plasma membrane of cells. We present methods for measuring single-molecule diffusion and their analysis. Single molecule tracking has shown a high dependence of acetylcholine receptors diffusion on its associated protein rapsyn. Comparing muscle cells that either express rapsyn or are devoid of it, we found that rapsyn plays an important role on receptor immobilization. A three-fold increase of receptor mobility was observed in muscle cells devoid of rapsyn. However, in these cells, a certain fraction of immobilized receptors was also found immobile. Furthermore, nAChR were strongly confined in membrane domains of few tens of nanometers. This showed that membrane composition and membrane associated proteins influence on receptor localization. During muscle cell differentiation, the fraction of immobile nAChR diminished along with the decreasing nAChR and stable rapsyn expression levels. The importance of rapsyn in nAChR immobilization has been further confirmed by measurements in HEK 293 cells, where co-expression of rapsyn increased immobilization of the receptor. nAChR is a ligand-gated ion-channel of the Cys-loop family. In mammals, members of this receptor family share general structural and functional features. They are homo- or hetero-pentamers and form a membrane-spanning ion channel. Subunits have three major regions, an extracellular ligand binding domain, a transmembrane channel and a large intracellular loop. 5HT3R was used as a model to study the effect of this loop on receptor mobility. Single-molecule tracking experiments on receptors with progressively larger deletions in the intracellular loop did not show a dependence of the size of the loop on the diffusion coefficient of mobile receptors. However, two regions were identified to play a role in receptor mobility by changing the fractions of immobile and directed receptors. Interestingly, a prokaryotic homologue of cys-loop receptors, ELIC, devoid of a large cytoplasmic loop was found to be immobile or to show directed diffusion similar as the wild-type 5HT3R. The scaffolding protein rapsyn stabilizes nAChR clusters in a concentration dependent manner. We have measured the density and self-interactions of rapsyn using FRET microscopy. Point-mutations of rapsyn, known to provoke myopathies, destabilized rapsyn self-interactions. Rapsyn-N88K, and R91L were found at high concentration in the cytoplasm suggesting that this modification disturbs membrane association of rapsyn. A25V was found to accumulate in the endoplasmic reticulum. Fluorescent tools to measure intracellular concentration of calcium ions are of great value to study the function of neurons. Rapsyn is highly abundant at the neuromuscular junction and thus is a genuine synaptic marker. A fusion protein of rapsyn with a genetically encoded ratiometric calcium sensor has been made to probe synapse activity. This thesis has shown that the combined use of biologically relevant system and modern fluorescence microscopy techniques deliver important information on pLGIC behaviour in the cell membrane.

EPFL2010

Structural characterisation and membrane insertion of M13 procoat, M13 coat and Pf3 coat proteins

Membrane proteins fulfill many central functions in the biological membrane. The insertion process of these proteins and their structure, which are intimately linked to their function, are not yet well understood. As a model we studied three proteins reconstituted into lipid bilayers (which serve as a model for the biological membrane): the major coat proteins of phages Pf3 and M13 and the precursor of the latter, M13 procoat protein, which spans the inner membrane two times and serves as a model for a multi-spanning membrane protein. Whereas Pf3 coat protein can insert into the membrane in its mature form, M13 coat protein needs the signal sequence present in the precursor to insert. A new, simple and efficient purification method has been developed for M13 procoat, M13 and Pf3 coat proteins. Homogenous preparations were obtained in isopropanol/0.l% TFA, where M13 coat protein is found to be dissolved in a monomeric form, and the two other proteins as dimers. The conformations of these particular proteins in different environments have been determined by circular dichroism and infrared spectroscopy. In organic solvents, the proteins adopt a conformation with an average helix content of 90%. In lipid bilayers composed of phospatidylcholine and phosphatidylglycerol lipids, the average helix content is 50% for M13 procoat protein, 60% for M13 coat protein and 75% for Pf3 coat protein. In order to characterize the orientational distribution the orientational order parameter Sα of the protein helices in planar lipid bilayers have been determined by polarized infrared measurements in the amide I spectral range. The helices of the three proteins are oriented preferentially parallel to the membrane normal, with Sα = 0.63 for M13 procoat protein, Sα = 0.58 for Pf3 coat protein and a distinctly higher value of Sα = 0.81 for M13 coat protein. The topology of M13 and Pf3 coat proteins was investigated using limited proteolysis in lipid vesicles with proteinase K combined with mass spectroscopy to analyze the lytic fragments. The C-terminal part of both proteins was found to be inaccessible to proteinase K, only 14 (Pf3) or 15 (M13) residues on the N-terminal were protease accessible from the outside of the vesicles. Furthermore, the standard free energy change, ΔGo, of a membrane-inserting protein with a leader sequence has been determined experimentally for the first time, using M13 procoat protein as an example. The partition coefficient for the distribution of this protein between the aqueous phase and the membrane phase of preformed lipid vesicles yielded a value of Γ = 6.5×105 M-1, corresponding to a ΔGo of -10.4 kcal/mol, based on measurements of the fluorescence energy transfer between the intrinsic tryptophan of the protein and a suitably labeled lipid membrane of POPC. For comparison, the partition coefficient of the M13 coat protein between the aqueous and the POPC lipid bilayer phase was determined to be distinctly lower: Γ = 1×105 M-1 (ΔGo = -9.3 kcal/mol). Proteinase K digestion experiments have been performed, showing that 20% of the procoat protein bound to lipid vesicles spontaneously integrate in a transbilayer form, whereas 80% remain inserted in the interfacial membrane region. By taking together these results, an upper limit for the free energy change of the transmembrane insertion of procoat protein was estimated to be -14.8 kcal/mol. In order to distinguish further the contribution arising from insertion of the procoat protein into the membrane interfacial region from that due to transmembrane insertion, the partition coefficient of a mutant procoat protein OM30R (which contains a positively charged amino acid in its mature hydrophobic segment (exchange of a Val to an Arg residue at position 30)) was determined, yielding Γ = 0.3×105 M-1 (ΔGo = -8.6 kcal/mol). Previously reported in vivo experiments have shown that the OM30R mutant protein is not translocated across Escherichia coli inner membrane, but only binds to the inner surface. The results presented here indicate that although the insertion of the procoat protein into the interfacial region of the lipid bilayer contributes the major part to ΔGo, it is the final energy gain of the interaction of the hydrophobic portions of the folded pre-protein with the lipid chains which drives the transmembrane insertion of the M13 procoat protein. Neither the leader sequence nor the mature coat protein alone yields this free energy gain. For the different proteins investigated here, spontaneous membrane insertion occurs only for fluid lipid bilayers, but not for membranes in the crystalline lipid phase. Furthermore, by using lipid bilayers with negative membrane surface charges, it was shown that both procoat and coat proteins are electrostatically attracted to the surface of the lipid membrane, though only to a small extent, with apparent partition coefficients of the same order of magnitude as for the phosphatidylcholine lipid membrane. This work has yielded new information about the structure and the insertion of the phage coat proteins. The new methods developed in this work might be of general applicability to study the structural dynamics of membrane protein assembly.

EPFL1996