Improved "optical highlighter" probes derived from discosoma red fluorescent protein

Multiphoton Bleaching of Red Fluorescent Proteins and the Ways to Reduce It

Red fluorescent proteins and biosensors built upon them are potentially beneficial for two-photon laser microscopy (TPLM) because they can image deeper layers of tissue, compared to green fluorescent proteins. However, some publications report on their very fast photobleaching, especially upon excitation at 750-800 nm. Here we study the multiphoton bleaching properties of mCherry, mPlum, tdTomato, and jREX-GECO1, measuring power dependences of photobleaching rates K at different excitation wavelengths across the whole two-photon absorption spectrum. Although all these proteins contain the chromophore with the same chemical structure, the mechanisms of their multiphoton bleaching are different. The number of photons required to initiate a photochemical reaction varies, depending on wavelength and power, from 2 (all four proteins) to 3 (jREX-GECO1) to 4 (mCherry, mPlum, tdTomato), and even up to 8 (tdTomato). We found that at sufficiently low excitation power P, the rate K often follows a quadratic power dependence, that turns into higher order dependence (K~Pα with α > 2) when the power surpasses a particular threshold P*. An optimum intensity for TPLM is close to the P*, because it provides the highest signal-to-background ratio and any further reduction of laser intensity would not improve the fluorescence/bleaching rate ratio. Additionally, one should avoid using wavelengths shorter than a particular threshold to avoid fast bleaching due to multiphoton ionization.

Multiphoton photochemistry of red fluorescent proteins in solution and live cells

Genetically encoded fluorescent proteins (FPs), and biosensors based on them, provide new insights into how living cells and tissues function. Ultimately, the goal of the bioimaging community is to use these probes deep in tissues and even in entire organisms, and this will require two-photon laser scanning microscopy (TPLSM), with its greater tissue penetration, lower autofluorescence background, and minimum photodamage in the out-of-focus volume. However, the extremely high instantaneous light intensities of femtosecond pulses in the focal volume dramatically increase the probability of further stepwise resonant photon absorption, leading to highly excited, ionizable and reactive states, often resulting in fast bleaching of fluorescent proteins in TPLSM. Here, we show that the femtosecond multiphoton excitation of red FPs (DsRed2 and mFruits), both in solution and live cells, results in a chain of consecutive, partially reversible reactions, with individual rates driven by a high-order (3-5 photon) absorption. The first step of this process corresponds to a three- (DsRed2) or four-photon (mFruits) induced fast isomerization of the chromophore, yielding intermediate fluorescent forms, which then subsequently transform into nonfluorescent products. Our experimental data and model calculations are consistent with a mechanism in which ultrafast electron transfer from the chromophore to a neighboring positively charged amino acid residue triggers the first step of multiphoton chromophore transformations in DsRed2 and mFruits, consisting of decarboxylation of a nearby deprotonated glutamic acid residue.

From green to red--To more dead? Autofluorescent proteins as photosensitizers

Inactive compounds like autofluorescent proteins can absorb visible daylight (around 500-700 nm) and can emit active electrons producing reactive oxygen species (ROS) leading to an increase in photokilling processes in bacteria. The endogenously originated ROS create single strand breaks in the cells DNA. These various types of breaks can be partially repaired by different cellular repair systems but a high number of breaks leads to cell death. A dramatic increase in cell killing can be observed from green, via yellow to red color emission. This was tested by colony forming ability. The generation of ROS and the bacterial protection mechanisms are discussed. We outline some possibilities for use the protein's properties for treatment of antibiotic multi-resistant and difficult to treat bacteria like the methicillin-resistant Staphylococcus aureus (MRSA).

Absolute two-photon absorption spectra and two-photon brightness of orange and red fluorescent proteins

Fluorescent proteins with long emission wavelengths are particularly attractive for deep tissue two-photon microscopy. Surprisingly, little is known about their two-photon absorption (2PA) properties. We present absolute 2PA spectra of a number of orange and red fluorescent proteins, including DsRed2, mRFP, TagRFP, and several mFruit proteins, in a wide range of excitation wavelengths (640-1400 nm). To evaluate 2PA cross section (sigma(2)), we use a new method relying only on the optical properties of the intact mature chromophore. In the tuning range of a mode-locked Ti:sapphire laser, 700-1000 nm, TagRFP possesses the highest two-photon cross section, sigma(2) = 315 GM, and brightness, sigma(2)phi = 130 GM, where phi is the fluorescence quantum yield. At longer wavelengths, 1000-1100 nm, tdTomato has the largest values, sigma(2) = 216 GM and sigma(2)phi = 120 GM, per protein chain. Compared to the benchmark EGFP, these proteins present 3-4 times improvement in two-photon brightness.

Fluorescence diffuse tomography for detection of red fluorescent protein expressed tumors in small animals

A fluorescence diffuse tomography (FDT) setup for monitoring tumor growth in small animals has been created. In this setup an animal is scanned in the transilluminative configuration by a single source and detector pair. To remove stray light in the detection system, we used a combination of interferometric and absorption filters. To reduce the scanning time, an experimental animal was scanned using the following algorithm: (1) large-step scanning to obtain a general view of the animal (source and detector move synchronously); (2) selection of the fluorescing region; and (3) small-step scanning of the selected region and different relative shifts between the source and detector to obtain sufficient information for 3D reconstruction. We created a reconstruction algorithm based on the Holder norm to estimate the fluorophore distribution. This algorithm converges to the solution with a minimum number of fluorescing zones. The use of tumor cell lines transfected with fluorescent proteins allowed us to conduct intravital monitoring studies. Cell lines of human melanomas Mel-P, Mel-Ibr, Mel-Kor, and human embryonic kidney HEK293 Phoenix were transfected with DsRed-Express and Turbo-RFP genes. The emission of red fluorescent proteins (RFPs) in the long-wave optical range permits detection of deep-seated tumors. In vivo experiments were conducted immediately after subcutaneous injection of fluorescing cells into small animals.

Visualizing cytoskeleton dynamics in mammalian cells using a humanized variant of monomeric red fluorescent protein

Fluorescent proteins are versatile tools for live cell imaging studies. In particular, recent progress was achieved in the development of monomeric red fluorescent proteins (mRFPs) that show improved properties in respect to maturation and intracellular fluorescence. mRFPmars, a red fluorescent protein designed especially for the use in Dictyostelium, proved to be a brilliant label for different cytoskeletal elements. Here we report on the synthesis of a humanized version of a monomeric RFP, mRFPruby, which differs in sequence from mRFPmars in four amino acids and has a codon usage that is optimized for the application in mammalian cells. In order to demonstrate the usefulness of this new mRFP variant, mRFPruby fused to beta-actin was expressed in different mouse cell lines and used to visualize actin cytoskeleton dynamics by live cell microscopy.

Recent advances in all-protein chromophore technology

The green fluorescent protein (GFP) is the foundation of a powerful technology that has revolutionized the way in which the life scientist carries out experiments in the living cell. The technology is continually evolving and improving through the development of existing proteins and discovery of new members of the all-protein chromophore (APC) family. This review gives an overview of the more recent advances in the technology with a particular focus on APCs having optical properties that are significantly red-shifted relative to those variants derived from Aequorea victoria GFP.

Evidence for the Isomerization and Decarboxylation in the Photoconversion of the Red Fluorescent Protein DsRed

Recently, it has been shown that the red fluorescent protein DsRed undergoes photoconversion on intense irradiation, but the mechanism of the conversion has not yet been elucidated. Upon irradiation with a nanosecond-pulsed laser at 532 nm, the chromophore of DsRed absorbing at 559 nm and emitting at 583 nm (R form) converts into a super red (SR) form absorbing at 574 nm and emitting at 595 nm. This conversion leads to a significant change in the fluorescence quantum yield from 0.7 to 0.01. Here we demonstrate that the photoconversion is the result of structural changes of the chromophore and one amino acid. Absorption, fluorescence, and vibrational spectroscopy as well as mass spectrometry suggest that a cis-to-trans isomerization of the chromophore and decarboxylation of a glutamate (E215) take place upon irradiation to form SR. At the same time, another photoproduct (B) with an absorption maximum at 386 nm appears upon irradiation. This species is assigned as a protonated form of the DsRed chromophore. It might be a mixture of several protonated DsRed forms as there is at least two ways of formation. Furthermore, the photoconversion of DsRed is proven to occur through a consecutive two-photon absorption process. Our results demonstrate the importance of the chromophore conformation in the ground state on the brightness of the protein as well as the importance of the photon flux to control/avoid the photoconversion process.