Biofluorescence

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Fluorescence is the emission of light by a molecule or an atom that has absorbed light or other electromagnetic radiation. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, than the absorbed radiation. A perceptible example of fluorescence occurs when the absorbed radiation is in the ultraviolet region of the electromagnetic spectrum (invisible to the human eye), while the emitted light is in the visible region; this gives the fluorescent substance a distinct color that can only be seen when the substance has been exposed to UV light.

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Biofluorescence is fluorescence emitted by a living organism. Biofluorescence requires an external light source and a biomolecular substance that converts absorbed light into a new one. The fluorescent substance absorbs light at one wavelength, often blue or UV, and emits at another, longer wavelength, green, red, or anything in between. In a living organism, the fluorescent agent often is a protein (or several), but it could be other biomolecules as well.

Since biofluorescence was discovered in Aequorea victoria and the green fluorescent protein structure was resolved, many other organisms have been shown to exhibit biofluorescence and many new fluorescent proteins have been discovered. [1] [2] [3]

Taxonomic range

Plants

Biofluorescence is frequent in plants, and can occur in many of their parts. [4] The biofluorescence in chlorophyll but has been studied since the 1800s. [5] Generally, chlorophyll fluoresces red, [6] and can be used as a measure of photosynthetic capabilities, [7] [6] or general health. [5] After absorbing light, chlorophyll may fluoresce as part of the physiological processes involved in photosynthesis. [6]

Reproductive organs such as pollen, [8] [9] anthers [9] or petals [10] may also fluoresce. These characters may produce a variety of colors depending on the pigment responsible for fluorescence. [10] [8] [5] [9] While it is unclear what the primary function of different kinds of fluorescence are in plants, [4] reproductive characters may biofluoresce as a signal to attract pollinators, [11] [9] However, biofluorescence may also attract prey in predatory plants, [12] or serve no function. [5]

Animals

While biofluorescence was first discovered and extensively characterized in invertebrates, recent work has observed biofluorescence in many vertebrates, with discoveries of biofluorescence have been made in salamanders and frogs, [13] [14] [15] fish, [16] [17] [18] birds, [19] [20] [21] and mammals. [22] [23] [21]

Functions

The function of biofluorescence in each case is not completely known. The fluorescent signal may play a role in inter- and intraspecific communication, such as camouflage (e.g. corals [24] ), attracting mates (e.g. birds [25] and copepods [26] ) and symbionts (e.g. corals [3] ), or deterring predators. [26]

Other explanations are physiological, with bright color being a side-product of a defense from UV (e.g. the protein sandercyanin, [17] and UV protection of genes in pollen [9] ). Bright red fluorescence in the larvae of Acropora millepora coral correlates with the activation of a diapause-like state that may aid in conserving energy and tolerating heat and other stressors during a long dispersal to novel habitats. [27]

Evolution

Most likely biofluorescence arose multiple times by convergent evolution. [3] [28] Reconstruction experiments suggest the original fluorescent protein was green, and had a simple beta-barrel shape with a chromophore hidden inside. Different colors of green fluorescent proteins (GFP), yellow, red, cyan, and amber, are determined by variations in chromophore structure. Red fluorescent proteins chromophore are the most complex and require extra maturation steps. New fluorescent proteins evolved through gene duplication and accumulation of multiple mutations which gradually changed autocatalytic functions and final chromophore structure. [28]

GFP analogs are common, but this is not the only possible structural solution for biofluorescence. In freshwater Japanese eels Anguilla japonica the unique protein UnaG fluoresces by binding bilirubin, a mechanism very distinct from that of green fluorescent protein. [16] UnaG absorbs blue light and emits green only when the complex with bilirubin is formed. This feature makes UnaG attractive for biomedical assays in exploration of bilirubin-dependent cellular processes. [29]

Another non-GFP- like fluorescent protein is a blue protein, sandercyanin, from freshwater fish walleye, Sander vitreus, in the North hemisphere. Sandercyanin is seasonally produced, with production peaking in the late summer, and is thought to be a defense against high UV. Sandercyanin binds biliverdin IXa, and together they form a tetra-homomer which absorbs UV light at 375nm and emits red light at 675nm. [17]

Two species of catsharks, Cephaloscyllium ventriosum, endemic to the eastern Pacific, and Scyliorhinus retifer, from the western Atlantic, fluoresce by a different mechanism. [18] The fluorescence is produced by brominated tryptophan-kynurenine metabolites, small aromatic compounds present in the lighter-colored regions of skin on the fish. Dermal features of the shark skin optically enhance the fluorescent signal. [18]

See also

Related Research Articles

<span class="mw-page-title-main">Fluorescence</span> Emission of light by a substance that has absorbed light

Fluorescence is one of two kinds of emission of light by a substance that has absorbed light or other electromagnetic radiation. When exposed to ultraviolet radiation, many substances will glow (fluoresce) with colored visible light. The color of the light emitted depends on the chemical composition of the substance. Fluorescent materials generally cease to glow nearly immediately when the radiation source stops. This distinguishes them from the other type of light emission, phosphorescence. Phosphorescent materials continue to emit light for some time after the radiation stops.

<span class="mw-page-title-main">Green fluorescent protein</span> Protein that exhibits bright green fluorescence when exposed to ultraviolet light

The green fluorescent protein (GFP) is a protein that exhibits green fluorescence when exposed to light in the blue to ultraviolet range. The label GFP traditionally refers to the protein first isolated from the jellyfish Aequorea victoria and is sometimes called avGFP. However, GFPs have been found in other organisms including corals, sea anemones, zoanithids, copepods and lancelets.

<span class="mw-page-title-main">Fluorescent tag</span>

In molecular biology and biotechnology, a fluorescent tag, also known as a fluorescent label or fluorescent probe, is a molecule that is attached chemically to aid in the detection of a biomolecule such as a protein, antibody, or amino acid. Generally, fluorescent tagging, or labeling, uses a reactive derivative of a fluorescent molecule known as a fluorophore. The fluorophore selectively binds to a specific region or functional group on the target molecule and can be attached chemically or biologically. Various labeling techniques such as enzymatic labeling, protein labeling, and genetic labeling are widely utilized. Ethidium bromide, fluorescein and green fluorescent protein are common tags. The most commonly labelled molecules are antibodies, proteins, amino acids and peptides which are then used as specific probes for detection of a particular target.

Phycobilins are light-capturing bilins found in cyanobacteria and in the chloroplasts of red algae, glaucophytes and some cryptomonads. Most of their molecules consist of a chromophore which makes them coloured. They are unique among the photosynthetic pigments in that they are bonded to certain water-soluble proteins, known as phycobiliproteins. Phycobiliproteins then pass the light energy to chlorophylls for photosynthesis.

<span class="mw-page-title-main">Fluorophore</span> Agents that emit light after excitation by light

A fluorophore is a fluorescent chemical compound that can re-emit light upon light excitation. Fluorophores typically contain several combined aromatic groups, or planar or cyclic molecules with several π bonds.

<span class="mw-page-title-main">Förster resonance energy transfer</span> Photochemical energy transfer mechanism

Förster resonance energy transfer (FRET), fluorescence resonance energy transfer, resonance energy transfer (RET) or electronic energy transfer (EET) is a mechanism describing energy transfer between two light-sensitive molecules (chromophores). A donor chromophore, initially in its electronic excited state, may transfer energy to an acceptor chromophore through nonradiative dipole–dipole coupling. The efficiency of this energy transfer is inversely proportional to the sixth power of the distance between donor and acceptor, making FRET extremely sensitive to small changes in distance.

<span class="mw-page-title-main">Fluorescence microscope</span> Optical microscope that uses fluorescence and phosphorescence

A fluorescence microscope is an optical microscope that uses fluorescence instead of, or in addition to, scattering, reflection, and attenuation or absorption, to study the properties of organic or inorganic substances. "Fluorescence microscope" refers to any microscope that uses fluorescence to generate an image, whether it is a simple set up like an epifluorescence microscope or a more complicated design such as a confocal microscope, which uses optical sectioning to get better resolution of the fluorescence image.

<span class="mw-page-title-main">Biliverdin</span> Green bile pigment

Biliverdin is a green tetrapyrrolic bile pigment, and is a product of heme catabolism. It is the pigment responsible for a greenish color sometimes seen in bruises.

Kaede is a photoactivatable fluorescent protein naturally originated from a stony coral, Trachyphyllia geoffroyi. Its name means "maple" in Japanese. With the irradiation of ultraviolet light (350–400 nm), Kaede undergoes irreversible photoconversion from green fluorescence to red fluorescence.

EosFP is a photoactivatable green to red fluorescent protein. Its green fluorescence (516 nm) switches to red (581 nm) upon UV irradiation of ~390 nm due to a photo-induced modification resulting from a break in the peptide backbone near the chromophore. Eos was first discovered as a tetrameric protein in the stony coral Lobophyllia hemprichii. Like other fluorescent proteins, Eos allows for applications such as the tracking of fusion proteins, multicolour labelling and tracking of cell movement. Several variants of Eos have been engineered for use in specific study systems including mEos2, mEos4 and CaMPARI.

Photoactivatable fluorescent proteins (PAFPs) is a type of fluorescent protein that exhibit fluorescence that can be modified by a light-induced chemical reaction.

<span class="mw-page-title-main">Fluorescence in the life sciences</span> Scientific investigative technique

Fluorescence is used in the life sciences generally as a non-destructive way of tracking or analysing biological molecules. Some proteins or small molecules in cells are naturally fluorescent, which is called intrinsic fluorescence or autofluorescence. The intrinsic DNA fluorescence is very weak.Alternatively, specific or general proteins, nucleic acids, lipids or small molecules can be "labelled" with an extrinsic fluorophore, a fluorescent dye which can be a small molecule, protein or quantum dot. Several techniques exist to exploit additional properties of fluorophores, such as fluorescence resonance energy transfer, where the energy is passed non-radiatively to a particular neighbouring dye, allowing proximity or protein activation to be detected; another is the change in properties, such as intensity, of certain dyes depending on their environment allowing their use in structural studies.

A chromoprotein is a conjugated protein that contains a pigmented prosthetic group. A common example is haemoglobin, which contains a heme cofactor, which is the iron-containing molecule that makes oxygenated blood appear red. Other examples of chromoproteins include other hemochromes, cytochromes, phytochromes and flavoproteins.

<span class="mw-page-title-main">GCaMP</span> Genetically encoded calcium indicator

GCaMP is a genetically encoded calcium indicator (GECI) initially developed in 2001 by Junichi Nakai. It is a synthetic fusion of green fluorescent protein (GFP), calmodulin (CaM), and M13, a peptide sequence from myosin light-chain kinase. When bound to Ca2+, GCaMP fluoresces green with a peak excitation wavelength of 480 nm and a peak emission wavelength of 510 nm. It is used in biological research to measure intracellular Ca2+ levels both in vitro and in vivo using virally transfected or transgenic cell and animal lines. The genetic sequence encoding GCaMP can be inserted under the control of promoters exclusive to certain cell types, allowing for cell-type specific expression of GCaMP. Since Ca2+ is a second messenger that contributes to many cellular mechanisms and signaling pathways, GCaMP allows researchers to quantify the activity of Ca2+-based mechanisms and study the role of Ca2+ ions in biological processes of interest.

mCherry is a member of the mFruits family of monomeric red fluorescent proteins (mRFPs). As an RFP, mCherry was derived from DsRed of Discosoma sea anemones, unlike green fluorescent proteins (GFPs) which are often derived from Aequorea victoria jellyfish. Fluorescent proteins are used to tag components in cells so that they can be studied using fluorescence spectroscopy and fluorescence microscopy. mCherry absorbs light between 540 and 590 nm and emits light in the range of 550-650 nm. mCherry belongs to the group of fluorescent protein chromophores used as instruments to visualize genes and analyze their functions in experiments. Genome editing has been improved greatly through the precise insertion of these fluorescent protein tags into the genetic material of many diverse organisms. Most comparisons between the brightness and photostability of different fluorescent proteins have been made in vitro, removed from biological variables that affect protein performance in cells or organisms. It is hard to perfectly simulate cellular environments in vitro, and the difference in environment could have an effect on the brightness and photostability.

<span class="mw-page-title-main">Dronpa</span>

Dronpa is a reversibly switchable photoactivatable fluorescent protein that is 2.5 times as bright as EGFP. Dronpa gets switched off by strong illumination with 488 nm (blue) light and this can be reversed by weak 405 nm UV light. A single dronpa molecule can be switched on and off over 100 times. It has an excitation peak at 503 nm and an emission peak at 518 nm.

<span class="mw-page-title-main">FMN-binding fluorescent protein</span>

A FMN-binding fluorescent protein (FbFP), also known as a LOV-based fluorescent protein, is a small, oxygen-independent fluorescent protein that binds flavin mononucleotide (FMN) as a chromophore.

<span class="mw-page-title-main">SmURFP</span>

Small ultra red fluorescent protein (smURFP) is a class of far-red fluorescent protein evolved from a cyanobacterial phycobiliprotein, α-allophycocyanin. Native α-allophycocyanin requires an exogenous protein, known as a lyase, to attach the chromophore, phycocyanobilin. Phycocyanobilin is not present in mammalian cells. smURFP was evolved to covalently attach phycocyanobilin without a lyase and fluoresce, covalently attach biliverdin and fluoresce, blue-shift fluorescence to match the organic fluorophore, Cy5, and not inhibit E. coli growth. smURFP was found after 12 rounds of random mutagenesis and manually screening 10,000,000 bacterial colonies.

<span class="mw-page-title-main">Red fluorescent protein</span>

Red fluorescent protein (RFP) is a protein which acts as a fluorophore, fluorescing red-orange when excited. The original variant occurs naturally in the coral genus Discosoma, and is named DsRed. Several new variants have been developed using directed mutagenesis which fluoresce orange, red, and far-red.

<span class="mw-page-title-main">Fluorescence imaging</span> Type of non-invasive imaging technique

Fluorescence imaging is a type of non-invasive imaging technique that can help visualize biological processes taking place in a living organism. Images can be produced from a variety of methods including: microscopy, imaging probes, and spectroscopy.

References

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