Three Efficient Methods for Preparation of Coelenterazine Analogues

An optimized bioluminescent substrate for non-invasive imaging in the brain

Bioluminescence imaging (BLI) allows non-invasive visualization of cells and biochemical events in vivo and thus has become an indispensable technique in biomedical research. However, BLI in the central nervous system remains challenging because luciferases show relatively poor performance in the brain with existing substrates. Here, we report the discovery of a NanoLuc substrate with improved brain performance, cephalofurimazine (CFz). CFz paired with Antares luciferase produces greater than 20-fold more signal from the brain than the standard combination of D-luciferin with firefly luciferase. At standard doses, Antares-CFz matches AkaLuc-AkaLumine/TokeOni in brightness, while occasional higher dosing of CFz can be performed to obtain threefold more signal. CFz should allow the growing number of NanoLuc-based indicators to be applied to the brain with high sensitivity. Using CFz, we achieve video-rate non-invasive imaging of Antares in brains of freely moving mice and demonstrate non-invasive calcium imaging of sensory-evoked activity in genetically defined neurons.

Acquisition of bioluminescent trait by non-luminous organisms from luminous organisms through various origins

Bioluminescence is a natural light emitting phenomenon that occurs due to a chemical reaction between luciferin and luciferase. It is primarily an innate and inherited trait in most terrestrial luminous organisms. However, most luminous organisms produce light in the ocean by acquiring luminous symbionts, luciferin (substrate), and/or luciferase (enzyme) through various transmission pathways. For instance, coelenterazine, a well-known luciferin, is obtained by cnidarians, crustaceans, and deep-sea fish through multi-level dietary linkages from coelenterazine producers such as ctenophores, decapods, and copepods. In contrast, some non-luminous Vibrio bacteria became bioluminescent by obtaining lux genes from luminous Vibrio species by horizontal gene transfer. Various examples detailed in this review show how non-luminescent organisms became luminescent by acquiring symbionts, dietary luciferins and luciferases, and genes. This review highlights three modes (symbiosis, ingestion, and horizontal gene transfer) that allow organisms lacking genes for autonomous bioluminescent systems to obtain the ability to produce light. In addition to bioluminescence, this manuscript discusses the acquisition of other traits such as pigments, fluorescence, toxins, and others, to infer the potential processes of acquisition.

Novel furimazine derivatives for nanoluciferase bioluminescence with various C-6 and C-8 substituents

Nanoluciferase (NLuc) is the emerging commercially available luciferase considering its small size and superior bioluminescence performance. Nevertheless, this bioluminescence system has some limitations, including narrow emission wavelength and single substrate. Herein, a series of novel furimazine derivatives at the C-6 and C-8 positions of the imidazopyrazinone core have been designed and synthesized for extension of the bioluminescence substrates. It should be noted that two compounds, molecules A2 (2-(furan-2-ylmethyl)-6-(4-(hydroxymethyl)phenyl)-8-(phenylthio)imidazo[1,2-a]pyrazin-3(7H)-one) and A3 (2-(furan-2-ylmethyl)-6-(4-amino-3-fluorophenyl)-8-(phenylthio)imidazo[1,2-a]pyrazin-3(7H)-one), display reasonable bioluminescence properties for in vitro and in vivo biological evaluations. In particular, compound A3 can broaden the application of NLuc bioluminescence techniques, especially for in vivo bioluminescent imaging.

Novel NanoLuc substrates enable bright two-population bioluminescence imaging in animals

Sensitive detection of two biological events in vivo has long been a goal in bioluminescence imaging. Antares, a fusion of the luciferase NanoLuc to the orange fluorescent protein CyOFP, has emerged as a bright bioluminescent reporter with orthogonal substrate specificity to firefly luciferase (FLuc) and its derivatives such as AkaLuc. However, the brightness of Antares in mice is limited by the poor solubility and bioavailability of the NanoLuc substrate furimazine. Here, we report a new substrate, hydrofurimazine, whose enhanced aqueous solubility allows delivery of higher doses to mice. In the liver, Antares with hydrofurimazine exhibited similar brightness to AkaLuc with its substrate AkaLumine. Further chemical exploration generated a second substrate, fluorofurimazine, with even higher brightness in vivo. We used Antares with fluorofurimazine to track tumor size and AkaLuc with AkaLumine to visualize CAR-T cells within the same mice, demonstrating the ability to perform two-population imaging with these two luciferase systems.

Bioluminescence Profiling of NanoKAZ/NanoLuc Luciferase Using a Chemical Library of Coelenterazine Analogues

We describe here an extensive structure-bioluminescence relationship study of a chemical library of analogues of coelenterazine, using nanoKAZ/NanoLuc, a mutated luciferase originated from the catalytic subunit of the deep-sea shrimp Oplophorus gracilirostris. Out of the 135 O-acetylated precursors that were prepared by using our recently reported synthesis and following their hydrolysis to give solutions of the corresponding luciferins, notable bioluminescence improvements were achieved in comparison with furimazine, which is currently amongst the best substrates of nanoKAZ/NanoLuc. For instance, the rather more lipophilic analogue 8-(2,3-difluorobenzyl)-2-((5-methylfuran-2-yl)methyl)-6-phenylimidazo[1,2-a]pyrazin-3(7H)-one provided a 1.5-fold improvement of the total light output over a 2 h period, a close to threefold increase of the initial signal intensity and a signal-to-background ratio five times greater than furimazine. The kinetic parameters for the enzymatic reaction were obtained for a selection of luciferin analogues and provided unexpected insights into the luciferase activity. Most prominently, along with a general substrate-dependent and irreversible inactivation of this enzyme, in the case of the optimized luciferin mentioned above, the consumption of 2664 molecules was found to be required for the detection of a single Relative Light Unit (RLU; a luminometer-dependent fraction of a photon).

Phosphorus-containing amino acids with a P–C bond in the side chain or a P–O, P–S or P–N bond: from synthesis to applications

Strategies for the preparation of phosphorus-containing amino acids and their utility in the organic chemistry, physico-chemistry, agrochemistry, and pharmacology fields are reported.

ATP-Independent Bioluminescent Reporter Variants To Improve in Vivo Imaging

Coelenterazine (CTZ)-utilizing marine luciferases and their derivatives have attracted significant attention because of their ATP-independency, fast enzymatic turnover, and high bioluminescence brightness. However, marine luciferases typically emit blue photons and their substrates, including CTZ and the recently developed diphenylterazine (DTZ), have poor water solubility, hindering their in vivo applications. Herein, we report a family of pyridyl CTZ and DTZ analogs that exhibit spectrally shifted emission and improved water solubility. Through directed evolution, we engineered a LumiLuc luciferase with broad substrate specificity. In the presence of corresponding pyridyl substrates (i.e., pyCTZ, 6pyDTZ, or 8pyDTZ), LumiLuc generates highly bright blue, teal, or yellow bioluminescence. We compared our LumiLuc-8pyDTZ pair with several benchmark reporters in a tumor xenograft mouse model. Our new pair, which does not need organic cosolvents for in vivo administration, surpasses other reporters by detecting early tumors. We further fused LumiLuc to a red fluorescent protein, resulting in a LumiScarlet reporter with further red-shifted emission and enhanced tissue penetration. LumiScarlet-8pyDTZ was comparable to Akaluc-AkaLumine, the brightest ATP-dependent luciferase-luciferin pair, for detecting cells in deep tissues of mice. In summary, we have engineered a new family of ATP-independent bioluminescent reporters, which will have broad applications because of their ATP-independency, excellent biocompatibility, and superior in vivo sensitivity.

Bioluminescence calcium imaging of network dynamics and their cholinergic modulation in slices of cerebral cortex from male rats

The activity of neuronal ensembles was monitored in neocortical slices from male rats using wide-field bioluminescence imaging of a calcium sensor formed with the fusion of green fluorescent protein and aequorin (GA) and expressed through viral transfer. GA expression was restricted to pyramidal neurons and did not conspicuously alter neuronal morphology or neocortical cytoarchitecture. Removal of extracellular magnesium or addition of GABA receptor antagonists triggered epileptiform flashes of variable amplitude and spatial extent, indicating that the excitatory and inhibitory networks were functionally preserved in GA-expressing slices. We found that agonists of muscarinic acetylcholine receptors largely increased the peak bioluminescence response to local electrical stimulation in layer I or white matter, and gave rise to a slowly decaying response persisting for tens of seconds. The peak increase involved layers II/III and V and did not result in marked alteration of response spatial properties. The persistent response involved essentially layer V and followed the time course of the muscarinic afterdischarge depolarizing plateau in layer V pyramidal cells. This plateau potential triggered spike firing in layer V, but not layer II/III pyramidal cells, and was accompanied by recurrent synaptic excitation in layer V. Our results indicate that wide-field imaging of GA bioluminescence is well suited to monitor local and global network activity patterns, involving different mechanisms of intracellular calcium increase, and occurring on various timescales.

Gram-scale synthesis of luciferins derived from coelenterazine and original insights into their bioluminescence properties

An original three component synthetic access to coelenterazine and analogues can lead to grams of marine luciferins which are extensively used in bioluminescence-based assays.

Quantification of receptor activation by oxytocin and vasopressin in endocytosis-coupled bioluminescence reduction assay using nanoKAZ

Oxytocin (OXT) and arginine vasopressin (AVP) are structurally similar neuropeptide hormones that function as neurotransmitters in the brain, and have opposite key roles in social behaviors. These peptides bind to their G protein-coupled receptors (OXTR and AVPRs), inducing calcium ion-dependent signaling pathways and endocytosis of these receptors. Because selective agonists and antagonists for these receptors have been developed as therapeutic and diagnostic agents for diseases such as psychiatric disorders, facile methods are in demand for the evaluation of selectivity between these receptors. In this study, we developed a quantitative assay for OXT- and AVP-induced endocytosis of their receptors. The mutated Oplophorus luciferase, nanoKAZ, was fused to OXTR and AVPRs to enable rapid quantification of agonist-induced endocytosis by bioluminescence reduction. Agonist stimulation significantly decreases bioluminescence of nanoKAZ-fused receptors in living cells. Using this system, we evaluated clinically used OXTR antagonist atosiban and a reported pyrazinyltriazole derivative, hereby designated as PF13. Atosiban acted as an antagonist of AVPR1a, as well as an agonist for AVPR1b, whereas PF13 antagonized OXTR more selectively than atosiban, as reported previously. This paper shows a strategy for quantification of agonist-induced endocytosis of OXTR and AVPRs, and confirms its potent utility in the evaluation of agonists and antagonists.

Coelenterazine analogues emit red-shifted bioluminescence with NanoLuc

We report the synthesis and characterization of novel coelenterazine analogues that demonstrate a red-shift in their bioluminescent emission with NanoLuc luciferase. These coelenterazines can be tuned to shift the bioluminescent emission from blue light in the native system. In particular, direct attachment of an aryl moiety to the imidazopyrazinone core of furimazine at the C8 position provides a significant red-shift while maintaining reasonable light output. In addition, modification of the C6 aryl moiety provided additive red-shifts, and by combining the most promising modifications we report a coelenterazine with a maximum emission near 600 nm with NanoLuc. Finally, we show that this new bioluminescent system is capable of efficient BRET to far-red fluorophores. We anticipate these new principles of NanoLuc substrate design will impact applications that depend on shifting the colour of emission to the red, most notably in vivo bioluminescent imaging.

Bioluminescent Probes for Imaging Biology beyond the Culture Dish

Bioluminescence with luciferase-luciferin pairs is an attractive method for surveying cells in live tissues and whole organisms. Recent advances in luciferin chemistry and luciferase engineering are further expanding the scope of the technology. It is now possible to spy on cells in a variety of deep tissues and visualize multicellular interactions, feats that are enabling new questions to be asked and new ideas to be explored. This perspective piece highlights recent successes in bioluminescent probe development and their applications to imaging in live cells, tissues, and animals.