Focal switching of photochromic fluorescent proteins enables multiphoton microscopy with superior image contrast.

Department of Chemistry, Columbia University, New York, NY 10027, USA.
Biomedical Optics Express (Impact Factor: 3.5). 08/2012; 3(8):1955-63. DOI: 10.1364/BOE.3.001955
Source: PubMed

ABSTRACT Probing biological structures and functions deep inside live organisms with light is highly desirable. Among the current optical imaging modalities, multiphoton fluorescence microscopy exhibits the best contrast for imaging scattering samples by employing a spatially confined nonlinear excitation. However, as the incident laser power drops exponentially with imaging depth into the sample due to the scattering loss, the out-of-focus background eventually overwhelms the in-focus signal, which defines a fundamental imaging-depth limit. Herein we significantly improve the image contrast for deep scattering samples by harnessing reversibly switchable fluorescent proteins (RSFPs) which can be cycled between bright and dark states upon light illumination. Two distinct techniques, multiphoton deactivation and imaging (MPDI) and multiphoton activation and imaging (MPAI), are demonstrated on tissue phantoms labeled with Dronpa protein. Such a focal switch approach can generate pseudo background-free images. Conceptually different from wave-based approaches that try to reduce light scattering in turbid samples, our work represents a molecule-based strategy that focused on imaging probes.

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    ABSTRACT: Förster resonance energy transfer (FRET) and bioluminescence resonance energy transfer (BRET) are two major biophysical techniques for studying nanometer-scale motion dynamics within living cells. Both techniques read photoemission from the transient RET-excited acceptor, which makes RET and detection processes inseparable. We here report a novel hybrid strategy, bioluminescence assisted switching and fluorescence imaging (BASFI) using a bioluminescent Renilla luciferase RLuc8 as the donor and a photochromic fluorescent protein Dronpa as the acceptor. When in close proximity, RET from RLuc8 switches Dronpa from its original dark state to a stable bright state, whose fluorescence is imaged subsequently with an external laser. Such decoupling between RET and imaging processes in BASFI promises high photon flux as in FRET and minimal bleedthroughs as in BRET. We demonstrated BASFI with Dronpa-RLuc8 fusion constructs and drug-inducible intermolecular FKBP-FRB protein–protein interactions in live cells with high sensitivity, resolution, and specificity. Integrating the advantages of FRET and BRET, BASFI will be a valuable tool for various biophysical studies.
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    ABSTRACT: Two-photon excited fluorescence microscopy (TPFM) offers the highest penetration depth with subcellular resolution in light microscopy, due to its unique advantage of nonlinear excitation. However, a fundamental imaging-depth limit, accompanied by a vanishing signal-to-background contrast, still exists for TPFM when imaging deep into scattering samples. Formally, the focusing depth, at which the in-focus signal and the out-of-focus background are equal to each other, is defined as the fundamental imaging-depth limit. To go beyond this imaging-depth limit of TPFM, we report a new class of super-nonlinear fluorescence microscopy for high-contrast deep tissue imaging, including multiphoton activation and imaging (MPAI) harnessing novel photo-activatable fluorophores, stimulated emission reduced fluorescence (SERF) microscopy by adding a weak laser beam for stimulated emission, and two-photon induced focal saturation imaging with preferential depletion of ground-state fluorophores at focus. The resulting image contrasts all exhibit a higher-order (third- or fourth- order) nonlinear signal dependence on laser intensity than that in the standard TPFM. Both the physical principles and the imaging demonstrations will be provided for each super-nonlinear microscopy. In all these techniques, the created super-nonlinearity significantly enhances the imaging contrast and concurrently extends the imaging depth-limit of TPFM. Conceptually different from conventional multiphoton processes mediated by virtual states, our strategy constitutes a new class of fluorescence microscopy where high-order nonlinearity is mediated by real population transfer.

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