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Aequorin
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Aequorin
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Aequorin 1
Aequorin ribbon diagram from PDB 1ej3 with prosthetic group coelenterazine in blue
Identifiers
OrganismAequorea victoria (Jellyfish)
SymbolN/A
UniProtP07164
Other data
EC number1.13.12.5
Search for
StructuresSwiss-model
DomainsInterPro

Aequorin is a calcium-activated photoprotein isolated from the hydrozoan Aequorea victoria.[1] Its bioluminescence was studied decades before the protein was isolated from the animal by Osamu Shimomura in 1962.[2] In the animal, the protein occurs together with the green fluorescent protein to produce green light by resonant energy transfer, while aequorin by itself generates blue light.

Discussions of "jellyfish DNA" that can make "glowing" animals often refer to transgenic animals that express the green fluorescent protein, not aequorin, although both originally derive from the same animal.

Apoaequorin, the protein portion of aequorin, is an ingredient in the dietary supplement Prevagen. The US Federal Trade Commission (FTC) has charged the maker with false advertising for its memory improvement claims.

Discovery

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Work on aequorin began with E. Newton Harvey in 1921.[3] Though Harvey was unable to demonstrate a classical luciferase-luciferin reaction, he showed that water could produce light from dried photocytes and that light could be produced even in the absence of oxygen. Later, Osamu Shimomura began work into the bioluminescence of Aequorea in 1961. This involved tedious harvesting of tens of thousands of jellyfish from the docks in Friday Harbor, Washington.[1] It was determined that light could be produced from extracts with seawater, and more specifically, with calcium.[2] It was also noted during the extraction the animal creates green light due to the presence of the green fluorescent protein, which changes the native blue light of aequorin to green.[4]

While the main focus of his work was on the bioluminescence,[5] Shimomura and two others, Martin Chalfie and Roger Tsien, were awarded the Nobel Prize in 2008 for their work on green fluorescent proteins.

Structure

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Aequorin is a holoprotein composed of two distinct units, the apoprotein that is called apoaequorin, which has an approximate molecular weight of 21 kDa, and the prosthetic group coelenterazine, the luciferin.[6] This is to say, apoaequorin is the enzyme produced in the photocytes of the animal, and coelenterazine is the substrate whose oxidation the enzyme catalyzes. When coelenterazine is bound, it is called aequorin. Notably, the protein contains three EF hand motifs that function as binding sites for Ca2+ ions.[7] The protein is a member of the superfamily of the calcium-binding proteins, of which there are some 66 subfamilies.[8]

The crystal structure revealed that aequorin binds coelenterazine and oxygen in the form of a peroxide, coelenterazine-2-hydroperoxide.[9] The binding site for the first two calcium atoms show a 20 times greater affinity for calcium than the third site.[10] However, earlier claims that only two EF-hands bind calcium[11] were questioned when later structures indicated that all three sites can indeed bind calcium.[12] Thus, titration studies show that all three calcium-binding sites are active but only two ions are needed to trigger the enzymatic reaction.[13]

Other studies have shown the presence of an internal cysteine bond that maintains the structure of aequorin.[14] This has also explained the need for a thiol reagent like beta mercaptoethanol in the regeneration of the protein since such reagents weaken the sulfhydryl bonds between cysteine residues, expediting the regeneration of the aequorin.

Chemical characterization of aequorin indicates the protein is somewhat resilient to harsh treatments. Aequorin is heat resistant.[15] Held at 95 °C for 2 minutes the protein lost only 25% activity. Denaturants such as 6-M urea or 4-M guanidine hydrochloride did not destroy the protein.

Genetics

[edit]

Aequorin is presumably encoded in the genome of Aequorea. At least four copies of the gene were recovered as cDNA from the animal.[16][17] Because the genome has not been sequenced, it is unclear if the cDNA variants can account for all of the isoforms of the protein.[18]

Mechanism of action

[edit]

Early studies of the bioluminescence of Aequorea by E. Newton Harvey had noted that the bioluminescence appears as a ring around the bell, and occurs even in the absence of air.[19] This was remarkable because most bioluminescence reactions require oxygen, and led to the idea that the animals somehow store oxygen.[20] It was later discovered that the apoprotein can stably bind coelenterazine-2-hydroperoxide, and oxygen is required for the regeneration to this active form of aequorin.[21] However, in the presence of calcium ions, the protein undergoes a conformational change and converts its prosthetic group, coelenterazine-2-hydroperoxide, into excited coelenteramide and CO2.[22] As the excited coelenteramide relaxes to the ground state, blue light (wavelength of 465 nm) is emitted. Before coelenteramide is exchanged out, the entire protein is still fluorescent blue.[23][24] because of the connection between bioluminescence and fluorescence, this property was ultimately important in the discovery of the luciferin coelenterazine.[25]

Applications

[edit]

Since the emitted light can be easily detected with a luminometer, aequorin has become a useful tool in molecular biology for the measurement of intracellular Ca2+ levels.[26] The early successful purification of aequorin led to the first experiments involving the injection of the protein into the tissues of living animals to visualize the physiological release of calcium in the muscle fibers of a barnacle.[27] Since then, the protein has been widely used in many model biological systems, including zebrafish,[28] rats, mice, and cultured cells.[29][30][31][32]

Cultured cells expressing the aequorin gene can effectively synthesize apoaequorin; however, recombinant expression yields only the apoprotein. Therefore it is necessary to add coelenterazine into the culture medium of the cells to obtain a functional protein and thus use its blue light emission to measure Ca2+ concentration. Coelenterazine is a hydrophobic molecule, and therefore is easily taken up across plant and fungal cell walls, as well as the plasma membrane of higher eukaryotes, making aequorin suitable as a Ca2+ reporter in plants, fungi, and mammalian cells.[33][34]

Aequorin has a number of advantages over other Ca2+ indicators. Because the protein is large, it has a low leakage rate from cells compared to lipophilic dyes such as DiI. It lacks phenomena of intracellular compartmentalization or sequestration as is often seen for Voltage-sensitive dyes, and does not disrupt cell functions or embryo development. Moreover, the light emitted by the oxidation of coelenterazine does not depend on any optical excitation, so problems with auto-fluorescence are eliminated.[35] The primary limitation of aequorin is that the prosthetic group coelenterazine is irreversibly consumed to produce light, and requires continuous addition of coelenterazine into the media. Such issues led to developments of other genetically encoded calcium sensors including the calmodulin-based sensor cameleon,[36] developed by Roger Tsien and the troponin-based sensor, TN-XXL, developed by Oliver Griesbeck.[37]

[edit]

Apoaequorin is an ingredient in Prevagen, which is marketed by Quincy Bioscience as a memory supplement. In 2017, the US Federal Trade Commission (FTC) charged the maker with falsely advertising that the product improves memory, provides cognitive benefits, and is "clinically shown" to work.[38][39] According to the FTC, "the marketers of Prevagen preyed on the fears of older consumers experiencing age-related memory loss". Quincy said that it would fight the charges.[40][41][42]

Prior to the suit, a clinical trial run by researchers employed by Quincy Bioscience "found no overall benefit compared to a placebo for its primary endpoints involving memory and cognition", while the company's advertising misleadingly cited a few contested subgroup analyses that showed slight improvements.[43][44]

The suit (Spath, et al. v. Quincy Bioscience Holding Company, Inc., et al., Case No. 18-cv-12416, D. NJ.) was dismissed in the District court, but an appeal seeking to overturn the dismissal was filed. The suit was consolidated with another against Quincy Pharmaceuticals, Vanderwerff v. Quincy Bioscience (Case No. 17-cv-784, D. NJ), which was the lead case.[45]

On February 21, 2019, the United States Court of Appeals for the Second Circuit ruled that the FTC and the state of New York could proceed with their lawsuit against Quincy Bioscience for its claims that Prevagen can improve memory. The order came less than two weeks after the parties argued the case before a three-judge panel of the circuit, where company lawyers admitted they did not "dispute that if you look across the entire 211 people who completed the study there was no statistically significant difference". The court vigorously dismissed allegations by the company lawyers that the FTC pursued its action for political reasons.[46][47]

On March 23, 2020, a federal magistrate judge in the United States District Court for the Southern District of Florida entered a report and recommendations certifying a nationwide class action for the class of consumers who purchased Prevagen over the previous four years.[48] The trial in the case was set for October 2020.[48][49]

As of September 21, 2020, Quincy Bioscience agreed to settle the claims that it misrepresented its Prevagen products as supporting brain health and helping with memory loss. Under the terms of the settlement, eligible purchasers applying by October 26, 2020, for purchases made from 2007 through July 31, 2020, could recover refunds of up to $70. [50]

Dr. Harriet Hall, writing for Science-Based Medicine, noted that the Quincy-sponsored study (known as "Madison Memory Study") was negative, but that the company utilized p-hacking to find favorable results. She wrote that their cited safety studies were all rat studies and their claim that apoaequorin crosses the blood–brain barrier was based solely on a dog study[51].[52] The American Pharmacists Association warns that Apoaequorin "is unlikely to be absorbed to a significant degree; instead it degrades into amino acids".[53]

On April 3, 2025, Quincy Bioscience, who was a co-plaintiff with Amazon, won a lawsuit against several sellers of counterfeit Prevagen on the Amazon website.   A Washington federal judge awarded a combined total of $1,895,375.40 in default judgments.[54][55]

Prevagen is purported to be a target for theft in retail stores by organized retail crime groups. Several factors contribute to it being a popular target, including its price, demand, and marketing claims around its effectiveness.  On April 24, 2020, The U.S. Circuit Court of Appeals for the Seventh Circuit upheld a ruling in favor of Quincy finding an online seller of Prevagen liable for selling Prevagen products in damaged condition and products the seller knew or should have known to have been stolen, ordering the seller to pay $480,968.13 in damages.[56]  On August 24, 2023, the Florida Attorney General charged two members of a criminal ring[57] with stealing $10,000 worth of Prevagen and other items.  On May 21, 2024, a multistate theft ring leader[58] was sentenced to federal prison for directing a crime ring that stole an estimated $9 million worth of merchandise, including Prevagen, Abreva, Zantac, and other products, over three years.

References

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Aequorin

View on Grokipedia
from Grokipedia
Aequorin is a calcium-activated photoprotein originally isolated from the bioluminescent Aequorea victoria. It emits blue light through an intramolecular chemiluminescent reaction triggered by the binding of calcium ions, without requiring external excitation light or additional cofactors beyond its bound . Discovered by Japanese chemist Osamu Shimomura in the early 1960s during investigations into , aequorin was the first protein-based calcium sensor identified, paving the way for its recombinant expression and engineering variants with altered spectral properties or sensitivities. The protein's structure, comprising the apoprotein apoaequorin complexed with coelenterazine (the light-emitting substrate) and molecular oxygen, undergoes a rapid oxidation upon calcium coordination, releasing light at approximately 469 nm. Aequorin's primary significance lies in its application as a genetically encodable intracellular calcium reporter, enabling real-time monitoring of calcium dynamics in living cells and organisms through imaging, which offers advantages over fluorescent indicators by avoiding background autofluorescence. Despite the later prominence of (also from A. victoria), aequorin remains a key tool in studies, particularly in scenarios requiring high sensitivity to low calcium concentrations or non-invasive detection in translucent tissues.

Origin and Discovery

Natural Occurrence

Aequorin is a calcium-activated photoprotein naturally produced by the hydrozoan jellyfish Aequorea victoria, commonly known as the crystal jelly, which inhabits the pelagic waters of the northeastern Pacific Ocean. This species ranges from the Bering Sea southward to central California, with documented occurrences in coastal regions including Puget Sound in Washington State and nearshore waters of Oregon bays. The jellyfish medusae, which can reach diameters up to 25 cm, are seasonally abundant in these areas, budding asexually from hydroid colonies in early spring. Within A. victoria, aequorin is localized to the marginal cells of the jellyfish's umbrella, where it forms part of the bioluminescent system triggered by mechanical disturbance or calcium influx. While similar photoproteins exist in other hydrozoans, such as obelin in geniculata, aequorin is specifically isolated from A. victoria and not reported as endogenously occurring in other organisms.

Historical Isolation and Characterization

Aequorin was first isolated in the summer of 1961 by Osamu Shimomura, in collaboration with Frank H. Johnson and Y. Saiga, from the photophores of the Aequorea victoria collected in near . The work began after Shimomura's arrival at in 1960, where Johnson provided specimens demonstrating the jellyfish's , prompting systematic extraction efforts. Approximately 10,000 jellyfish were processed to obtain sufficient material, with light-emitting marginal organs dissected and homogenized in a cold, acidic buffer (pH 4) containing EDTA to chelate calcium ions and prevent premature luminescence during extraction. Neutralization with then yielded a crude extract containing the photoprotein. Purification involved repeated cycles of acidification, neutralization, and dialysis against calcium-free buffers to remove contaminants, followed by salt fractionation and on diethylaminoethyl (DEAE)- columns, achieving a preparation with high . The process was scaled using a custom mechanical device to excise photophore rings at a rate of 600 per hour, enabling isolation of milligram quantities of the protein. This method, detailed in their 1962 publication, marked the first successful extraction of a , calcium-triggered photoprotein from a marine organism, distinguishing it from traditional luciferase-luciferin systems observed in other bioluminescent species. Initial characterization revealed aequorin as a single polypeptide of approximately 20,000–21,000 daltons molecular weight, with an around pH 4.5 and high stability in the apoprotein form at low temperatures (retaining activity for months at 0°C). , peaking at 469 nm (blue light), was triggered stoichiometrically by micromolar concentrations of Ca²⁺ (threshold ~10⁻⁶ M), without requiring additional cofactors beyond trace oxygen, and proceeded irreversibly with consumption of the protein (one per ). Unlike enzymatic luciferases, aequorin functioned as a pre-charged photoprotein, binding its (later identified as coelenterazine) covalently, which decomposed upon Ca²⁺ activation to yield coelenteramide, CO₂, and apo-aequorin. These properties positioned aequorin as a bioluminescent system, enabling early applications in calcium detection.

Structural and Genetic Properties

Protein Structure

Apoaequorin, the apoprotein component of aequorin, consists of a single polypeptide chain of 196 with a calculated of 22,514 Da. It lacks residues and thus forms no bonds, contributing to its stability in recombinant forms. The protein is highly helical, with secondary structure dominated by alpha-helices arranged in the characteristic EF-hand motifs. The tertiary , resolved by at 2.3 resolution (PDB ID: 1EJ3), reveals a compact globular fold featuring four helix-loop-helix EF-hand domains. Three of these domains (EF-hands 1, 2, and 4) possess canonical calcium-binding loops capable of coordinating Ca²⁺ ions via oxygen atoms from aspartate, glutamate, and backbone carbonyl groups, while the third EF-hand is degenerate and non-functional for metal binding. The core contains a hydrophobic cavity that sequesters the coelenterazine-2-hydroperoxide , shielded from solvent and positioned near the calcium-binding sites to enable conformational changes upon Ca²⁺ binding. Aequorin functions as a , with no evidence of in its native state. Subsequent structures of variants, such as semi-synthetic aequorins, confirm near-identical folds with resolutions up to 1.6 .

Gene Sequence and Expression

The apoaequorin gene from Aequorea victoria encodes a polypeptide of 189 amino acids, with a molecular weight of approximately 21 kDa, featuring three EF-hand motifs for calcium binding. The complete cDNA sequence was first cloned in 1985 using a mixed synthetic oligonucleotide probe based on partial amino acid sequences of the purified protein, yielding multiple recombinant plasmids from a A. victoria cDNA library. Sequence analysis revealed the open reading frame starting with an initiating methionine and including a C-terminal proline, consistent with the protein's role in forming the calcium-activated photoprotein complex. Comparative sequencing of five distinct apoaequorin cDNAs identified three isoforms, differing primarily in the third EF-hand domain, which influences calcium sensitivity and stability. These isoforms, designated aequorin-1, -2, and -3, share over 95% identity, suggesting evolutionary divergence within the for fine-tuning bioluminescent responses. The genomic organization includes introns, as evidenced by cloned genomic fragments aligning with cDNA sequences, though full details remain partially characterized. In native A. victoria, apoaequorin expression is localized to photocytes in the umbrella margin, where the protein assembles with coelenterazine and molecular oxygen to form functional aequorin, enabling calcium-triggered during mechanical stimulation. Regulatory mechanisms of expression in the are not fully elucidated, but mRNA abundance correlates with maturation stages, peaking in sexually mature specimens. Recombinant expression of apoaequorin cDNA, initially in Escherichia coli using plasmids like pAEQ1, produces inclusion bodies requiring denaturation and refolding for activity, achieving yields sufficient for biochemical studies after prosthetic group reconstitution. Subsequent optimizations in eukaryotic systems, such as yeast or mammalian cells, enable cytosolic or targeted expression for calcium imaging, with intracellular concentrations reaching ~1 μM in stably transfected lines.

Biochemical Mechanism

Photoprotein Complex Formation

Apoaequorin, the apoprotein component of aequorin consisting of 196 residues, binds coelenterazine—a with a molecular weight of approximately 400 Da—in the presence of molecular oxygen to form the active photoprotein complex. This binding occurs within a hydrophobic cavity of the protein, where coelenterazine is accommodated in a non-covalent but tight association, as evidenced by concentration-dependent quenching of the intrinsic fluorescence of apoaequorin and apo-obelin. The process requires oxygen for the subsequent auto-oxidation of bound coelenterazine to 2-hydroperoxycoelenterazine, which constitutes the light-emitting stabilized in the complex. The formation of the complex is kinetically slow, reflecting apoaequorin's low catalytic efficiency as an oxidase, with a reported turnover number of 1–2 coelenterazine molecules per hour under standard conditions. This sluggish rate suggests that complex assembly proceeds via a mechanism involving initial substrate docking followed by intramolecular oxygen insertion, rather than rapid enzymatic turnover. Studies using stopped-flow techniques on recombinant aequorin and related photoproteins like obelin have further characterized the kinetics of emitter formation, confirming the rate-limiting oxidation step post-binding. The resulting complex is highly stable, resisting dissociation under physiological conditions and requiring denaturants for disassembly, which underscores the tight integration of the hydroperoxycoelenterazine within the protein's EF-hand-like binding sites. Variations in coelenterazine analogs, such as coelenterazine h, can enhance complex formation efficiency and stability, yielding photoproteins with increased intensity—up to 10-fold higher than native—and greater calcium sensitivity. However, the core mechanism remains conserved across natural and semi-synthetic forms, with binding specificity dictated by the protein's geometry, as revealed in structures of related apo-photoproteins and semi-synthetic complexes. This formation enables aequorin's role as a single-turnover bioluminescent , where each complex emits light only upon calcium-triggered decomposition of the bound .

Calcium-Dependent Luminescence

Aequorin's luminescence is triggered by the binding of two calcium ions (Ca²⁺) to the photoprotein complex, which comprises the apoaequorin polypeptide, coelenterazine luciferin, and molecular oxygen. This binding induces a conformational change in the protein, destabilizing the 2-hydroperoxycoelenterazine adduct formed during complex assembly and initiating its rapid oxidation to coelenteramide, carbon dioxide, and an excited-state intermediate that emits blue light at a maximum wavelength of 469 nm. The reaction proceeds via an intramolecular electron transfer facilitated by the protein's EF-hand calcium-binding sites, with the luminescence intensity exhibiting a sigmoidal dependence on Ca²⁺ concentration due to cooperative binding kinetics, enabling sensitive detection over a range of approximately 0.1 to 10 μM Ca²⁺. Unlike enzymatic luciferases, aequorin functions as a single-turnover photoprotein, consuming one equivalent of coelenterazine per light-emitting event without catalytic regeneration. Structural studies of Ca²⁺-loaded apo-aequorin reveal three high-affinity binding sites, though functional triggering requires only two Ca²⁺ ions, highlighting the role of partial occupancy in initiating the luminescent cascade while the third site modulates stability. The emitted photon's quantum yield is approximately 0.21, with the flash decaying rapidly (half-time ~0.2 seconds at saturating Ca²⁺), reflecting the transient nature of the excited coelenteramide species. This mechanism underscores aequorin's utility as a ratiometric indicator for transient Ca²⁺ elevations in biological systems.

Engineering and Modifications

Recombinant Production

Recombinant production of aequorin typically involves the of its apoprotein form, known as apoaequorin, which is subsequently reconstituted with the coelenterazine to yield the functional calcium-sensitive photoprotein. The gene encoding apoaequorin from was first cloned in 1985 via (cDNA) synthesis from mRNA, enabling initial expression attempts. This cloning facilitated the production of recombinant apoaequorin in prokaryotic systems, overcoming limitations of natural extraction from , which yields low quantities and varies seasonally. The predominant host for recombinant expression is , where apoaequorin cDNA is inserted into expression vectors such as pET systems or periplasmic-targeting plasmids to promote proper folding and disulfide bond formation essential for luminescence activity. Early expressions in E. coli cytoplasm yielded low soluble protein due to inclusion body formation, but targeting to the periplasmic space improved recovery by mimicking the oxidative environment of eukaryotic secretion pathways. In situ regeneration within periplasmic E. coli cells, followed by extraction, has achieved high yields of active aequorin, with protocols involving incubation with coelenterazine under controlled pH and oxygen conditions to form the photoprotein complex. Challenges in soluble expression persist, as overexpressed apoaequorin often aggregates into in E. coli, reducing active yields to below 10% of total protein in unoptimized systems. Optimizations include lowering culture temperatures to 18–25°C to slow folding kinetics, co-expression with chaperones like artemin to enhance (increasing active yields up to 5-fold), and fusion tags for stabilization. Purification typically employs immobilized metal (IMAC) for His-tagged variants, achieving >90% purity in a single step, followed by reconstitution and activity verification via calcium-triggered assays. Alternative hosts, such as yeast () or filamentous fungi (), have been explored for higher eukaryotic folding fidelity, with synthetic codon-optimized genes yielding up to 100-fold more apoaequorin than native sequences in fungal systems. These methods support scalable production for research and biosensors, with commercial recombinant apoaequorin available from suppliers using E. coli-based processes.

Variant Developments

Variants of aequorin have been engineered primarily to modulate calcium sensitivity, intensity, kinetic properties, , and emission spectra, addressing limitations in native forms for diverse biosensing applications. Mutations target key residues in the EF-hand calcium-binding motifs, substrate-binding pocket, and structural domains, often guided by structural modeling and . These developments enable finer control over luminescence responses, such as shifting activation thresholds from micromolar to nanomolar calcium levels or extending signal duration. Early efforts focused on enhancing bioluminescence properties through at residues including His16, Met19, Tyr82, Trp86, Trp108, Phe113, and Tyr132, yielding 42 mutants with altered light emission rates and total output; for example, substitutions like H16A increased initial intensity but reduced overall yield, while W108L variants showed prolonged glow phases. Double mutations, such as those combining alterations in the (e.g., Ser137Ala with other pocket residues), have demonstrated synergistic or antagonistic effects on kinetics, with some pairs accelerating rise times by up to 50% while others dampen peak amplitude, highlighting the trade-offs in optimizing for specific cellular contexts. To adjust calcium affinity, "Bright" mutations (e.g., Asp119Ala) shift sensitivity curves rightward, enabling detection in high-calcium environments like mitochondria, whereas "SloDK" variants (e.g., Asp11Asn, Lys55Met) further slow decay rates for sustained signaling, though at the cost of reduced peak brightness. Conversely, high-sensitivity mutants like RedquorinXS, incorporating red-shifted fluorophores and targeted EF-hand tweaks, lower the to sub-micromolar levels for cytosolic imaging, with one variant (e.g., incorporating 4-sulfo-L-phenylalanine) extending emission by factors of 2-5 compared to wild-type. Low-sensitivity apoaequorin mutants, such as those patented with substitutions reducing EF-hand affinity (e.g., Glu18Gln), activate only above 10 μM calcium, suiting probes where basal leaks would otherwise trigger false signals. Thermostability improvements have been achieved by rigidifying flexible loops via mutations like those stabilizing α-helices (e.g., introducing or charged pairs), increasing at 37°C from minutes to hours in one 2021 study, facilitating recombinant expression and use without rapid denaturation. Spectral variants incorporate non-canonical (e.g., via amber suppression) to red-shift emission peaks from 470 nm to ~500-550 nm, enhancing tissue penetration and compatibility with red-shifted filters, with activity confirmed in bacterial and mammalian cells. These engineered forms, often expressed recombinantly in E. coli or , underpin advanced biosensors but require coelenterazine charging, limiting some applications compared to fully genetic reporters.

Research Applications

Intracellular Calcium Imaging

Aequorin serves as a bioluminescent reporter for intracellular calcium dynamics, emitting blue light upon binding Ca²⁺ ions in the physiological range of approximately 0.1–10 μM. Purified aequorin was first microinjected into cells in the early 1970s, enabling pioneering measurements of cytosolic Ca²⁺ transients in preparations such as barnacle muscle fibers and mammalian cells via hypo-osmotic loading or pressure injection. Recombinant apo-aequorin, expressed through or in transgenic organisms, is reconstituted intracellularly with coelenterazine to form the active photoprotein, facilitating targeted in specific compartments like , endoplasmic reticulum, or mitochondria via appended signal sequences. This approach avoids direct protein injection, reducing cell perturbation, and has been applied in diverse systems including plant cells for monitoring agonist-induced Ca²⁺ signatures and animal models for organelle-specific fluxes. The method's high stems from negligible basal and low autofluorescence background, allowing detection with tubes or intensified cameras for down to milliseconds in single cells. However, aequorin's stoichiometric Ca²⁺ consumption limits it to integrative rather than continuous monitoring, often requiring full discharge with agents like ionomycin for absolute quantification via calibration curves relating photon counts to Ca²⁺ concentration. Spatial resolution remains inferior to fluorescent dyes like fura-2, as yields sparse photons, though imaging photon-counting systems mitigate this for subcellular localization. To address limitations, hybrid sensors fuse aequorin with fluorescent proteins such as GFP, enabling dual bioluminescent-fluorescent readouts for ratiometric calibration and enhanced imaging in tissues where light scattering hampers pure . These variants, like GFP-aequorin protein (GAP), extend utility to high-Ca²⁺ environments such as stores, with applications in real-time tracking of Ca²⁺ waves in neurons and high-throughput assays. Early adoption in cardiac and revealed contraction-coupled Ca²⁺ elevations, while targeted recombinant forms have elucidated organelle crosstalk, such as mitochondrial Ca²⁺ uptake during stress, underscoring aequorin's enduring role despite competition from genetically encoded fluorescent indicators.

Biosensors and Assays

Aequorin functions as a bioluminescent by emitting light upon binding Ca²⁺ ions, enabling real-time detection of calcium fluxes in biological systems with a spanning 10 nM to 100 μM. Its application in assays relies on the photoprotein's reconstitution with coelenterazine, followed by calcium-triggered measured via detectors or systems. This non-ratiometric indicator offers high sensitivity and , avoiding interference with cellular , unlike some fluorescent dyes. In functional assays for G-protein-coupled receptors (GPCRs), aequorin detects calcium mobilization downstream of receptor activation, supporting of agonists and antagonists. Automated luminometric platforms using recombinant aequorin in mammalian cells, such as or HEK293 lines, quantify dose-response curves with subnanomolar sensitivity, as demonstrated in 1999 protocols processing up to 96 samples per run. These assays have been adapted for orphan GPCRs, identifying ligands through transient transfections combining receptor and apoaequorin expression. Genetically targeted aequorin variants enable compartmentalized calcium assays, such as intramitochondrial [Ca²⁺] monitoring via mitochondrially directed probes in combination with coelenterazine derivatives for enhanced signal. Fusions like GFP-aequorin protein (GAP), developed in 2014, provide dual-mode detection—luminescence for high dynamic range and for —facilitating organelle-specific in living cells without exogenous dyes. Similarly, aequorin-based indicators for acidic compartments, reviewed in 2023, track proton-coupled calcium dynamics in lysosomes and Golgi, using low-pH tolerant variants to overcome signal quenching. Enhanced variants, such as aequorinXS introduced for higher Ca²⁺ affinity and brighter emission peaking at 465 nm, improve performance in low-calcium environments, with applications in neuronal and muscular assays. In non-mammalian systems, aequorin assays screen for mutants, as in 2021 Arabidopsis protocols using luminometry to identify osmotic stress responses, or in for engineering calcium-responsive strains as of 2024. These tools have also probed in pathogens like , where codon-optimized aequorin revealed antifungal-induced fluxes in 2021 studies.

Commercialization and Challenges

Biotechnology Utilization

Aequorin is employed in for calcium-sensitive biosensors and (HTS) assays, particularly targeting (GPCR) signaling via intracellular calcium mobilization. Commercial recombinant cell lines, such as Revvity's AequoScreen series, co-express aequorin with specific receptors like the human α2A in CHO-K1 cells, enabling luminescent detection of ligand-induced calcium fluxes without fluorescent dyes or wash steps. These assays support by quantifying agonist or antagonist effects with low background noise and high sensitivity, suitable for 384- or 1536-well formats. Biotech firms like Euroscreen (now integrated with ) offer aequorin-based services for GPCR hit identification and profiling, leveraging the photoprotein's ability to handle diverse experimental setups including and whole-organism studies. provides ChemiScreen Glow Aequorin calcium-optimized cell lines, such as those stably transfected for assays, to evaluate activity on calcium responses. Instrumentation from Molecular Devices, including the FLIPR Tetra system with specialized ICCD cameras, facilitates aequorin readouts alongside for integrated screening workflows. Recombinant aequorin photoprotein is commercially supplied in lyophilized or liquid forms by Nanolight Technology, produced via E. coli expression and purification, for integration into custom biosensors or assays. These applications underscore aequorin's utility in pharmaceutical HTS, where it outperforms traditional fluorescent indicators in for certain calcium-dependent pathways, though adoption remains niche compared to synthetic dyes due to the need for coelenterazine substrate.

Regulatory and Marketing Disputes

Quincy Bioscience, manufacturer of the Prevagen containing recombinant apoaequorin derived from the photoprotein aequorin, has faced significant regulatory challenges over of and health benefits. The product, marketed since 2009 with prices ranging from $24 to $68 per 30-pill bottle, promoted apoaequorin as stabilizing proteins and enhancing , relying primarily on a single 2016 double-blind study of 218 participants showing minor improvements in certain cognitive tests but criticized for methodological flaws including small sample size and lack of replication. Independent analyses, including those by the FTC, concluded the evidence did not substantiate claims of reducing loss or supporting function in healthy adults or those with mild impairment. In January 2017, the U.S. (FTC) and New York Attorney General jointly sued Quincy Bioscience, alleging deceptive advertising under Section 5 of the FTC Act and New York consumer protection laws, asserting that the company lacked competent and reliable scientific evidence for its efficacy claims. A 2019 district court dismissal was overturned on appeal, leading to trials; in May 2024, a New York jury found Quincy liable for , deceptive practices, and repeated related to Prevagen's memory benefits. On December 6, 2024, a federal court ruled in favor of the FTC, permanently enjoining Quincy from claiming Prevagen improves memory, cognitive function, or brain health, and requiring cessation of misleading testimonials and packaging statements. The U.S. (FDA) raised separate safety concerns, issuing a 2012 warning letter stating apoaequorin was unapproved as a and that structure/function claims risked classifying Prevagen as an unapproved new . Quincy submitted a New Dietary Ingredient notification in 2008 and a (GRAS) notice in 2014, but withdrew the latter in October 2015 amid FDA review questioning its safety due to potential digestibility in the rendering it ineffective and risks of allergic reactions or s. By 2019, the FDA received over 1,000 reports linked to Prevagen, including seizures, strokes, and cardiac issues, prompting development of methods to detect undeclared aequorin proteins in supplements. Despite these disputes, Prevagen generated hundreds of millions in sales, highlighting enforcement gaps for dietary supplements under the and Education Act of 1994, which does not require pre-market FDA approval for safety or efficacy.

References

  1. https://www.sciencedirect.com/topics/neuroscience/aequorin
  2. https://www.nobelprize.org/prizes/chemistry/2008/popular-information/
  3. https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/aequorea
  4. https://onlinelibrary.wiley.com/doi/10.1111/php.12682
  5. https://pmc.ncbi.nlm.nih.gov/articles/PMC16533/
  6. https://pmc.ncbi.nlm.nih.gov/articles/PMC3037093/
  7. https://www.sciencedirect.com/topics/immunology-and-microbiology/aequorea-victoria
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