One of the limitations on imaging fluorescent proteins within living cells is that they are usually present in small numbers and need to be detected over a large background. We have developed the means to isolate specific fluorescence signals from background by using lock-in detection of the modulated fluorescence of a class of optical probe termed ''optical switches.'' This optical lock-in detection (OLID) approach involves modulating the fluorescence emission of the probe through deterministic, optical control of its fluorescent and nonfluorescent states, and subsequently applying a lock-in detection method to isolate the modulated signal of interest from nonmodulated background signals. Cross-correlation analysis provides a measure of correlation between the total fluorescence emission within single pixels of an image detected over several cycles of optical switching and a reference waveform detected within the same image over the same switching cycles. This approach to imaging provides a means to selectively detect the emission from optical switch probes among a larger population of conventional fluorescent probes and is compatible with conventional microscopes. OLID using nitrospirobenzopyran-based probes and the genetically encoded Dronpa fluorescent protein are shown to generate high-contrast images of specific structures and proteins in labeled cells in cultured and explanted neurons and in live Xenopus embryos and zebrafish larvae.high-contrast ͉ optical switches ͉ "ac"-imaging ͉ fluorescence microscopy U nderstanding the molecular basis for the regulation of complex biological processes such as cell motility and proliferation requires analysis of the distribution and dynamics of protein interactions within living cells in culture and in intact tissue (1). Tremendous advances have been made toward the development of new optical probes (2, 3) and imaging techniques that are capable of detecting proteins down to the level of single molecules (4-11). However, in living cells, such detection is compromised by autofluorescence, which can amount to several thousand equivalents of fluorescein per cell (12), as well as by light scattering (13). A major challenge in live-cell imaging, therefore, is to develop classes of probes and imaging techniques that are capable of resolving fluorescence signals from synthetic probes or genetically encoded fluorescent proteins in living cells and tissue against large background signals that may vary in time and space.A simple and highly-effective approach for isolating a specific fluorescence signal from a large background is to reversibly modulate the fluorescence intensity of only a probe of interest that is bound to a specific protein by applying a uniform, rapid and specific perturbation (e.g., a change in temperature (14), pressure (15), or voltage (16) to which that probe is uniquely attuned. The modulated fluorescence can be isolated from other steady sources of background fluorescence by lock-in detection, making it possible to specifically extract the probe fluoresc...
The Förster resonance energy transfer (FRET) technique is widely used for studying protein interactions within live cells. The effectiveness and sensitivity of determining FRET, however, can be reduced by photobleaching, cross talk, autofluorescence, and unlabeled, endogenous proteins. We present a FRET imaging method using an optical switch probe, Nitrobenzospiropyran (NitroBIPS), which substantially improves the sensitivity of detection to <1% FRET efficiency. Through orthogonal optical control of the colorful merocyanine and colorless spiro states of the NitroBIPS acceptor, donor fluorescence can be measured both in the absence and presence of FRET in the same FRET pair in the same cell. A SNAP-tag approach is used to generate a green fluorescent protein-alkylguaninetransferase fusion protein (GFP-AGT) that is labeled with benzylguanine-NitroBIPS. In vivo imaging studies on this green fluorescent protein-alkylguaninetransferase (GFP-AGT) (NitroBIPS) complex, employing optical lock-in detection of FRET, allow unambiguous resolution of FRET efficiencies below 1%, equivalent to a few percent of donor-tagged proteins in complexes with acceptor-tagged proteins.
Few to single molecule imaging of fluorescent probe molecules can provide information on the distribution, dynamics, interactions and activity of specific fluorescently tagged proteins during cellular processes. Unfortunately, these imaging studies are made challenging in living cells because of fluorescence signals from endogenous cofactors. Moreover, related background signals within multi-cell systems and intact tissue are even higher and reduce signal contrast even for ensemble populations of probe molecules. High-contrast optical imaging within high-background environments will therefore require new ideas on the design of fluorescence probes, and the way their fluorescence signals are generated and analysed to form an image. To this end, in the present review we describe recent studies on a new family of fluorescent probe called optical switches, with descriptions of the mechanisms that underlie their ability to undergo rapid and reversible transitions between two distinct states. Optical manipulation of the fluorescent and non-fluorescent states of an optical switch probe generates a modulated fluorescence signal that can be isolated from a larger unmodulated background by using OLID (optical lock-in detection) techniques. The present review concludes with a discussion on select applications of synthetic and genetically encoded optical switch probes and OLID microscopy for high-contrast imaging of specific proteins and membrane structures within living systems.
A major challenge in cell biology is to elucidate molecular mechanisms that underlie the spatio-temporal control of cellular processes. These studies require microscope imaging techniques and associated optical probes that provide high-contrast and high-resolution images of specific proteins and their complexes. Auto-fluorescence however, can severely compromise image contrast and represents a fundamental limitation for imaging proteins within living cells. We have previously shown that optical switch probes and optical lock-in detection (OLID) image microscopy improve image contrast in high background environments. Here, we present the design, synthesis and characterization of amino- reactive and cell permeable optical switches that integrate the highly fluorescent fluorophore, tetramethylrhodamine (TMR) and spironaphthoxazine (NISO), a highly efficient optical switch. The NISO moiety in TMR-NISO undergoes rapid and reversible, excited-state driven transitions between a colorless spiro (SP)-state and a colored merocyanine (MC)-state in response to irradiation with 365 nm and >530 nm light. In the MC-state, the TMR (donor) emission is almost completely extinguished by Förster resonance energy transfer (FRET) to the MC probe (acceptor), whereas in the colorless SP-state, the quantum yield for TMR fluorescence is maximal. Irradiation of TMR-NISO with a defined sequence of 365 nm and 546 nm manipulates the levels of SP and MC with concomitant modulation of FRET efficiency and the TMR fluorescence signal. High fidelity optical switching of TMR fluorescence is shown for TMR-NISO probes in vitro and for membrane permeable TMR-NISO within living cells.
The 7-(4-aminomethyl-1H-1,2,3-triazol-1-yl) analogue of kabiramide C (5) was synthesized by using the Mitsunobu reaction and 1,3-dipolar cycloaddition. This compound and the intermediate compounds 2 and 4 were shown to bind tightly to G-actin in a 1:1 complex and exhibited the same degree of cytotoxicity as 1. Compound 5 serves as a key intermediate for the synthesis of actin-directed optical probes and drugs.
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