Fluorescence fluctuation spectroscopy approaches to the study of receptors in live cells

Characterization of clostridium botulinum neurotoxin serotype A (BoNT/A) and fibroblast growth factor receptor interactions using novel receptor dimerization assay

Clostridium botulinum neurotoxin serotype A (BoNT/A) is a potent neurotoxin that serves as an effective therapeutic for several neuromuscular disorders via induction of temporary muscular paralysis. Specific binding and internalization of BoNT/A into neuronal cells is mediated by its binding domain (H_C/A), which binds to gangliosides, including GT1b, and protein cell surface receptors, including SV2. Previously, recombinant H_C/A was also shown to bind to FGFR3. As FGFR dimerization is an indirect measure of ligand-receptor binding, an FCS & TIRF receptor dimerization assay was developed to measure rH_C/A-induced dimerization of fluorescently tagged FGFR subtypes (FGFR1-3) in cells. rH_C/A dimerized FGFR subtypes in the rank order FGFR3c (EC₅₀ ≈ 27 nM) > FGFR2b (EC₅₀ ≈ 70 nM) > FGFR1c (EC₅₀ ≈ 163 nM); rH_C/A dimerized FGFR3c with similar potency as the native FGFR3c ligand, FGF9 (EC₅₀ ≈ 18 nM). Mutating the ganglioside binding site in H_C/A, or removal of GT1b from the media, resulted in decreased dimerization. Interestingly, reduced dimerization was also observed with an SV2 mutant variant of H_C/A. Overall, the results suggest that the FCS & TIRF receptor dimerization assay can assess FGFR dimerization with known and novel ligands and support a model wherein H_C/A, either directly or indirectly, interacts with FGFRs and induces receptor dimerization.

Spatial and temporal resolution of mORC4 fluorescent variants reveals structural requirements for achieving higher order self-association and pronuclei entry

The Origin Replication Complex (ORC), which is a multi-subunit protein complex composed of six proteins ORC1-6, is essential for initiating licensing at DNA replication origins. We have previously reported that ORC4 has an alternative function wherein it forms a cage surrounding the extruded chromatin in female meiosis and is required for polar body extrusion (PBE). As this is a highly unexpected finding for protein that normally binds DNA, we tested whether ORC4 can actually form larger, higher order structures, which would be necessary to form a cage-like structure. We generated two fluorescent constructs of mouse ORC4, mORC4-EGFP and mORC4-FlAsH, to examine its spatial dynamics during oocyte activation in live cells. We show that both constructs were primarily monomeric throughout the embryo but self-association into larger units was detected with both probes. However, mORC4-FlAsH clearly showed higher order self-association and unique spatial distribution while mORC4-EGFP failed to form large structures during Anaphase II. Interestingly, both variants were found in the pronuclei suggesting that its role in DNA licensing is still functional. Our results with both constructs support the prediction that ORC4 can form higher order structures in the cytoplasm, suggesting that it is possible to form a cage-like structure. The finding that FlAsH labeled ORC4 formed demonstrably larger higher order structures than ORC4-GFP suggests that ORC4 oligomerization is sensitive to the bulky addition of GFP at its carboxy terminus.

Application of three-photon excitation FCS to the study of protein oligomerization

Three-photon excitation fluorescence correlation spectroscopy was used to detect oligomerization equilibria of rat liver phosphofructokinase. The fluorescence intensity produced by the three-photon excitation of tryptophan was collected using the DIVER microscope. In this home-built upright microscope, a large area photomultiplier, placed directly below the sample, is used as the detector. The lack of optical elements in the microscope detection path results in a significantly improved detection efficiency in the UV region down to about 300 nm, which encompasses the fluorescence emission from tryptophan. The three-photon excitation autocorrelation decays obtained for phosphofructokinase in the presence of F6P showed the presence of large oligomers. Substitution of F6P with ATP in the buffer medium results in dissociation of the large oligomers, which is reported by the decreased autocorrelation amplitude. The three-photon excitation process was verified from the slope of the log–log plot of intensity against laser power.

Viruses and tetraspanins: lessons from single molecule approaches

Tetraspanins are four-span membrane proteins that are widely distributed in multi-cellular organisms and involved in several infectious diseases. They have the unique property to form a network of protein-protein interaction within the plasma membrane, due to the lateral associations with one another and with other membrane proteins. Tracking tetraspanins at the single molecule level using fluorescence microscopy has revealed the membrane behavior of the tetraspanins CD9 and CD81 in epithelial cell lines, providing a first dynamic view of this network. Single molecule tracking highlighted that these 2 proteins can freely diffuse within the plasma membrane but can also be trapped, permanently or transiently, in tetraspanin-enriched areas. More recently, a similar strategy has been used to investigate tetraspanin membrane behavior in the context of human immunodeficiency virus type 1 (HIV-1) and hepatitis C virus (HCV) infection. In this review we summarize the main results emphasizing the relationship in terms of membrane partitioning between tetraspanins, some of their partners such as Claudin-1 and EWI-2, and viral proteins during infection. These results will be analyzed in the context of other membrane microdomains, stressing the difference between raft and tetraspanin-enriched microdomains, but also in comparison with virus diffusion at the cell surface. New advanced single molecule techniques that could help to further explore tetraspanin assemblies will be also discussed.

A mutation associated with centronuclear myopathy enhances the size and stability of dynamin 2 complexes in cells

BACKGROUND

Dynamin 2 (Dyn2) is a ~100kDa GTPase that assembles around the necks of nascent endocytic and Golgi vesicles and catalyzes membrane scission. Mutations in Dyn2 that cause centronuclear myopathy (CNM) have been shown to stabilize Dyn2 polymers against GTP-dependent disassembly in vitro. Precisely timed regulation of assembly and disassembly is believed to be critical for Dyn2 function in membrane vesiculation, and the CNM mutations interfere with this regulation by shifting the equilibrium toward the assembled state.

METHODS

In this study we use two fluorescence fluctuation spectroscopy (FFS) approaches to show that a CNM mutant form of Dyn2 also has a greater propensity to self-assemble in the cytosol and on the plasma membrane of living cells.

RESULTS

Results obtained using brightness analysis indicate that unassembled wild-type Dyn2 is predominantly tetrameric in the cytosol, although different oligomeric species are observed, depending on the concentration of expressed protein. In contrast, an R369W mutant identified in CNM patients forms higher-order oligomers at concentrations above 1μM. Investigation of Dyn2-R369W by Total Internal Reflection Fluorescence (TIRF) FFS reveals that this mutant forms larger and more stable clathrin-containing structures on the plasma membrane than wild-type Dyn2.

CONCLUSIONS AND GENERAL SIGNIFICANCE

These observations may explain defects in membrane trafficking reported in CNM patient cells and in heterologous systems expressing CNM-associated Dyn2 mutants.

Fluorescence correlation spectroscopy and photon-counting histogram analysis of receptor-receptor interactions

Fluorescence correlation spectroscopy (FCS) performed using a laser scanning confocal microscope is a technique with single-molecule sensitivity that is becoming more accessible to cell biologists. In this chapter, we describe the use of FCS for the analysis of diffusion coefficients and receptor-receptor interactions in live cells in culture. In particular, we describe a protocol to collect fluorescence fluctuation data from fluorescence-tagged receptors as they diffuse into an out of a small laser-illuminated observation volume using a commercially available system such as the Zeiss ConfoCor 3 or LSM-780 microscope. Autocorrelation analysis of the fluctuations in fluorescence intensity provides information about the diffusion time and number of fluorescent molecules in the observation volume. A photon-counting histogram can be used to examine the relationship between fluorescence intensity and the number of fluorescent molecules to estimate the average molecular brightness of the sample. Since molecular brightness is directly proportional to the number of fluorescent molecules, it can be used to monitor receptor-receptor interactions and to decode the number of receptor monomers present in an oligomeric complex.