Spatial Intensity Distribution Analysis: Studies of G Protein-Coupled Receptor Oligomerisation

Abstract

Spatial intensity distribution analysis (SpIDA) is a recently developed approach for determining quaternary structure information on fluorophore-labelled proteins of interest in situ. It can be applied to live or fixed cells and native tissue. Using confocal images, SpIDA generates fluorescence intensity histograms that are analysed by super-Poissonian distribution functions to obtain density and quantal brightness values of the fluorophore-labelled protein of interest. This allows both expression level and oligomerisation state of the protein to be determined. We describe the application of SpIDA to investigate the oligomeric state of G protein-coupled receptors (GPCRs) at steady state and following cellular challenge, and consider how SpIDA may be used to explore GPCR quaternary organisation in pathophysiology and to stratify medicines.

Keywords: G protein-coupled receptor; SpIDA; dimerisation; ligand regulation; quaternary structure.

Figure 1

SpIDA Procedure. Schematic representation of the procedure to perform SpIDA measurements as described in Box 1 and in more detail in Ward et al.. (A) The G protein-coupled receptor of interest is modified by the incorporation of a fluorophore, for example, monomeric (A²⁰⁶K) enhanced GFP (i) or the SNAP-tag (ii), for which labelled substrates are available , , at the carboxyl (intracellular) or amino (extracellular) terminus of the receptor, respectively. (B) The receptor construct is expressed stably in a heterologous mammalian cell system such as Flp-In T-REx 293 or CHO-K1 cells. (C) Laser scanner confocal images are collected from the glass coverslip attached to the basolateral membrane of the cells. (D) RoIs (red square) are selected and analysed using MATLAB graphical user interface programme. (E) Mean fluorescence intensity and QB values are normalised to the QB of the fluorophore label in its monomeric state and provides information on the density (expressed as particles per μm²) and the oligomeric state (expressed as MEUs) of the fluorophore-tagged protein. (i) The expression levels and calculated oligomeric state of human dopamine D₃ receptor linked to mEGFP (hD₃–mEGFP) expressed in Flp-In T-REx 293 cells is shown as an example . The vertical broken line corresponds to the mean receptor number per μm² and the horizontal dotted line corresponds to 1.406 MEU which represents mean +2 standard deviations of the data set (see Marsango et al. for details). (ii) RoIs characterised by QB MEU values >1.406 were considered to contain a prevalence of hD₃–mEGFP in a dimeric/oligomeric state. Data are adapted from Marsango et al.. Abbreviations: mEGFP, monomeric enhanced GFP; MEU, monomeric equivalent unit; QB, quantal brightness; RoI, region of interest; SpIDA, spatial intensity distribution analysis.

Figure I

Monomeric EGFP Expression and Localisation to the Plasma Membrane. Schematic representation of a single molecule of mEGFP or a tandem construct of two molecules of mEGFP linked by a short polypeptide. Each was fused to the carboxyl-terminal tail of CD86 (A, i), or to a palmitoylation + myristoylation sequence, derived from the Lyn non-receptor tyrosine kinase, (B, i) and expressed. Laser scanning confocal images of cells expressing a single mEGFP linked to CD86 (A, ii section and A, iii basolateral membrane) or the palmitoylation + myristoylation sequence (B, ii section and B, iii basolateral membrane). Representative images are shown. Abbreviation: mEGFP, monomeric enhanced GFP.

Figure 2

Effect of Expression Level and Ligand Treatment on GPCR Quaternary Structure. (A) The oligomeric state of the secretin receptor–mEGFP expressed in untreated CHO-K1 cells (blue) or cells treated overnight with 5 mM sodium butyrate (yellow). Vertical broken lines represent mean receptors per μm² for control and sodium-butyrate-treated cells. The horizontal dotted lines represent 1.48 and 3.0 MEU respectively (see Ward et al. for determination of these values). RoIs characterised by QB MEU >1.48 were considered to contain a prevalence of secretin–mEGFP receptor in larger than monomeric state, while those characterised by QB MEU >3.0 were considered to contain a prevalence of secretin–mEGFP receptor in an oligomeric, rather than dimeric, state. Proportions of outcomes corresponding to monomer, dimer, or oligomer are shown. Data are adapted from Ward et al.. (B) The oligomeric state of hD₃R–mEGFP expressed as QB MEU in HEK293 derived cells untreated (grey) or treated overnight with 10 μM spiperone (yellow) or haloperidol (blue) is shown. Vertical broken lines represent the mean receptor per μm² for untreated and treated cells (i). RoIs characterised by QB MEU >1.406 were considered to contain a prevalence of hD₃R–mEGFP in a dimeric/oligomeric state (ii). Data are adapted from Marsango et al.. Abbreviations: mEGFP, monomeric enhanced GFP; hD₃R, human dopamine D₃ receptor; MEU, monomeric equivalent unit; NT, not treated; QB, quantal brightness; RoI, region of interest.