Analysis of Cellular Functions by Multipoint Fluorescence Correlation Spectroscopy

Characterization of the fluorescence correlation spectroscopy (FCS) standard rhodamine 6G and calibration of its diffusion coefficient in aqueous solutions

Precise diffusion measurements of rhodamine 6G (Rh6G) dissolved in D2O at concentrations between 50 and 200 μM were carried out in the temperature range from 280 to 320 K using pulsed field gradient nuclear magnetic resonance (PFG-NMR). The obtained diffusion coefficients can be used as a calibration reference in fluorescence correlation spectroscopy (FCS). Besides measuring the diffusivity of Rh6G, the diffusion coefficient of the solvent in the same system could be determined in parallel by PFG-NMR as the resonances of water and Rh6G are well separated in the (1)H NMR spectrum. To analyze the differences due to the isotope effect of the solvent (D2O vs. H2O), the correlation time τD of Rh6G was measured by FCS in both D2O and H2O. The obtained isotopic correction factor, τD(D2O)/τD(H2O) = 1.24, reflects the isotope effect of the solvent´s self-diffusion coefficients as determined previously by PFG-NMR.

Site-specific multipoint fluorescence measurement system with end-capped optical fibers

We present the development and implementation of a spatially and spectrally resolved multipoint fluorescence correlation spectroscopy (FCS) system utilizing multiple end-capped optical fibers and an inexpensive laser source. Specially prepared end-capped optical fibers placed in an image plane were used to both collect fluorescence signals from the sample and to deliver signals to the detectors. The placement of independently selected optical fibers on the image plane was done by monitoring the end-capped fiber tips at the focus using a CCD, and fluorescence from specific positions of a sample were collected by an end-capped fiber, which could accurately represent light intensities or spectral data without incurring any disturbance. A fast multipoint spectroscopy system with a time resolution of ∼1.5 ms was then implemented using a prism and an electron multiplying charge coupled device with a pixel binning for the region of interest. The accuracy of our proposed system was subsequently confirmed by experimental results, based on an FCS analysis of microspheres in distilled water. We expect that the proposed multipoint site-specific fluorescence measurement system can be used as an inexpensive fluorescence measurement tool to study many intracellular and molecular dynamics in cell biology.

Multipoint fluorescence correlation spectroscopy with total internal reflection fluorescence microscope

We report simultaneous determination of diffusion coefficients at different points of a cell membrane using a multipoint fluorescence correlation spectroscopy (FCS) system. A system carrying seven detection areas in the evanescent field is achieved by using seven optical fibers on the image plane in the detection port of an objective-type total internal reflection FCS (TIR-FCS) system. Fluctuation of fluorescence intensity is monitored and evaluated using seven photomultiplier tubes (PMTs) and a newly constructed multichannel correlator. We demonstrate simultaneous-multipoint FCS, with a 3-mus time resolution, to investigate heterogeneous structures such as cell membranes and membrane-binding molecular dynamics near glass surfaces in live cells.

Detection of polyglutamine protein oligomers in cells by fluorescence correlation spectroscopy

Abnormal aggregation of misfolded proteins and their deposition as inclusion bodies in the brain have been implicated as a common molecular pathogenesis of neurodegenerative diseases including Alzheimer, Parkinson, and the polyglutamine (poly(Q)) diseases, which are collectively called the conformational diseases. The poly(Q) diseases, including Huntington disease and various types of spinocerebellar ataxia, are caused by abnormal expansions of the poly(Q) stretch within disease-causing proteins, which triggers the disease-causing proteins to aggregate into insoluble beta-sheet-rich amyloid fibrils. Although oligomeric structures formed in vitro are believed to be more toxic than mature amyloid fibrils in these diseases, the existence of oligomers in vivo has remained controversial. To explore oligomer formation in cells, we employed fluorescence correlation spectroscopy (FCS), which is a highly sensitive technique for investigating the dynamics of fluorescent molecules in solution. Here we demonstrate direct evidence for oligomer formation of poly(Q)-green fluorescent protein (GFP) fusion proteins expressed in cultured cells, by showing a time-dependent increase in their diffusion time and particle size by FCS. We show that the poly(Q)-binding peptide QBP1 inhibits poly(Q)-GFP oligomer formation, whereas Congo red only inhibits the growth of oligomers, but not the initial formation of the poly(Q)-GFP oligomers, suggesting that FCS is capable of identifying poly(Q) oligomer inhibitors. We therefore conclude that FCS is a useful technique to monitor the oligomerization of disease-causing proteins in cells as well as its inhibition in the conformational diseases.

A Homogeneous Cell-Based Assay to Measure Nuclear Translocation Using β-Galactosidase Enzyme Fragment Complementation

Positional complementation describes the use of homogeneous assays using beta- galactosidase (beta gal) enzyme fragment complementation to detect cellular protein translocation. This phenomenon occurs when the protein of interest, recombinantly expressed as a fusion protein with a modified alpha fragment of beta gal, translocates to a cellular compartment expressing an enzyme acceptor fragment of the enzyme. When these fragments interact, high-affinity complementation occurs, and a signal is generated that is then detected upon cell lysis. In the present paper the use of positional complementation is exemplified by measuring nuclear translocation of the glucocorticoid receptor in Chinese hamster ovary-K1 cells. The approach thus provides for homogeneous protocols, in an endpoint microtiter plate assay format, without the use of either imaging or reporter gene techniques. Consequently, these characteristics suggest that the technique is suitable for automated instrumentation protocols used in high throughput screening campaigns designed to identify activators or inhibitors of nuclear translocation.

Cellular imaging in drug discovery

Traditional screening paradigms often focus on single targets. To facilitate drug discovery in the more complex physiological environment of a cell or organism, powerful cellular imaging systems have been developed. The emergence of these detection technologies allows the quantitative analysis of cellular events and visualization of relevant cellular phenotypes. Cellular imaging facilitates the integration of complex biology into the screening process, and addresses both high-content and high-throughput needs. This review describes how cellular imaging technologies contribute to the drug discovery process.