Miniaturized Devices for Bioluminescence Imaging in Freely Behaving Animals.

In vivo fluorescence miniature microscopy has recently proven a major advance, enabling cellular imaging in freely behaving animals. However, fluorescence imaging suffers from autofluorescence, phototoxicity, photobleaching and non- homogeneous illumination artifacts. These factors limit the quality and time course of data collection. Bioluminescence provides an alternative kind of activity-dependent light indicator. Bioluminescent calcium indicators do not require light input, instead generating photons through chemiluminescence. As such, limitations inherent to the requirement for light presentation are eliminated. Further, bioluminescent indicators also do not require excitation light optics: the removal of these components should make a lighter and lower cost microscope with fewer assembly parts. While there has been significant recent progress in making brighter and faster bioluminescence indicators, the advances in imaging hardware have not yet been realized. A hardware challenge is that despite potentially higher signal-to-noise of bioluminescence, the signal strength is lower than that of fluorescence. An open question we address in this report is whether fluorescent miniature microscopes can be rendered sensitive enough to detect bioluminescence. We demonstrate this possibility in vitro and in vivo by implementing optimizations of the UCLA fluorescent miniscope v3.2. These optimizations yielded a miniscope (BLmini) which is 22% lighter in weight, has 45% fewer components, is up to 58% less expensive, offers up to 15 times stronger signal and is sensitive enough to capture spatiotemporal dynamics of bioluminescence in the brain with a signal-to-noise ratio of 34 dB.

Point mutations in firefly luciferase C-domain demonstrate its significance in green color of bioluminescence.

Firefly luciferase is a two-domain enzyme that catalyzes the bioluminescent reaction of firefly luciferin oxidation. Color of the emitted light depends on the structure of the enzyme, yet the exact color-tuning mechanism remains unknown by now, and the role of the C-domain in it is rarely discussed, because a very few color-shifting mutations in the C-domain were described. Recently we reported a strong red-shifting mutation E457K in the C-domain; the bioluminescence spectra of this enzyme were independent of temperature or pH. In the present study we investigated the role of the residue E457 in the enzyme using the Luciola mingrelica luciferase with a thermostabilized N-domain as a parent enzyme for site-directed mutagenesis. We obtained a set of mutants and studied their catalytic properties, thermal stability and bioluminescence spectra. Experimental spectra were represented as a sum of two components (bioluminescence spectra of putative "red" and "green" emitters); λmax of these components were constant for all the mutants, but the ratio of these emitters was defined by temperature and mutations in the C-domain. We suggest that each emitter is stabilized by a specific conformation of the active site; thus, enzymes with two forms of the active site coexist in the reactive media. The rigid structure of the C-domain is crucial for maintaining the conformation corresponding to the "green" emitter. We presume that the emitters are the keto- and enol forms of oxyluciferin.

Strategy of mutual compensation of green and red mutants of firefly luciferase identifies a mutation of the highly conservative residue E457 with a strong red shift of bioluminescence.

Bioluminescence spectra of firefly luciferases demonstrate highly pH-sensitive spectra changing the color from green to red light when pH is lowered from alkaline to acidic. This reflects a change of ratio of the green and red emitters in the bimodal spectra of bioluminescence. We show that the mutations strongly stabilizing green (Y35N) or red (H433Y) emission compensate each other leading to the WT color of firefly luciferase. We further used this compensating ability of Y35N to search for strong red-shifting mutations in the C-domain of firefly luciferase by random mutagenesis. The discovered mutation E457K substantially increased the contribution of the red emitter and caused a 12 nm red shift of the green emitter as well. E457 is highly conservative not only in beetle luciferases but also in a whole ANL superfamily of adenylating enzymes and forms a conservative structural hydrogen bond with V471. Our results suggest that the removal of this hydrogen bond only mildly affects luciferase properties and that most of the effect of E457K is caused by the introduction of positive charge. E457 forms a salt bridge with R534 in most ANL enzymes including pH-insensitive luciferases which is absent in pH-sensitive firefly luciferases. The mutant A534R shows that this salt bridge is not important for pH-sensitivity but considerably improves in vivo thermostability. Although E457 is located far from the oxyluciferin-binding site, the properties of the mutant E457K suggest that it affects color by influencing the AMP binding.

Bacillus subtilis alkaline phosphatase IV acquires activity only late at the stationary phase when produced in Escherichia coli. Overexpression and characterization of the recombinant enzyme.

The availability of recombinant monomeric alkaline phosphatase (AP) is highly desirable in analytical applications involving AP fusion proteins. The cobalt-dependant alkaline phosphatase IV from Bacillus subtilis (BSAP), which was reported to be strongly monomeric, was overexpressed in Escherichia coli using pET autoinduction system as a cytoplasmic protein without export signal sequence. After 1 day of growth, when the E. coli culture was near the stationary phase (standard time to harvest protein in this expression method), high amounts of BSAP were produced but the soluble fraction of BSAP was nearly inactive: the AP activity in the cell-free extract was near the background level. However, further incubation of bacterial culture lead to a tremendous increase in AP activity which was maximal at the 3rd day of incubation and was 48-100 times higher than at the 1st day of growth. The recombinant BSAP was purified by metal-chelate chromatography and characterized. Typically, 90-140 mg of active protein was produced in 1L of culture (20 g wet cells). BSAP shows 515 U/mg activity at optimum conditions (pH 11 and 0.8-2M NaCl). Contrary to the previous report on the native enzyme, BSAP was found to be dimeric and showed only negligible diesterase activity. The observed unusual late activation of BSAP indicates that prolonged incubation at the stationary phase may be useful for functional expression of some problematic proteins in E. coli.

Approaches to engineer stability of beetle luciferases.

Luciferase enzymes from fireflies and other beetles have many important applications in molecular biology, biotechnology, analytical chemistry and several other areas. Many novel beetle luciferases with promising properties have been reported in the recent years. However, actual and potential applications of wild-type beetle luciferases are often limited by insufficient stability or decrease in activity of the enzyme at the conditions of a particular assay. Various examples of genetic engineering of the enhanced beetle luciferases have been reported that successfully solve or alleviate many of these limitations. This mini-review summarizes the recent advances in development of mutant luciferases with improved stability and activity characteristics. It discusses the common limitations of wild-type luciferases in different applications and presents the efficient approaches that can be used to address these problems.

Thermostabilization of firefly luciferase by in vivo directed evolution.

Firefly luciferase is widely used in a number of areas of biotechnology and molecular biology. However, rapid inactivation of wild-type (WT) luciferases at elevated temperatures often hampers their application. A simple non-lethal in vivo screening scheme was used to identify thermostable mutants of luciferase in Escherichia coli colonies. This scheme allowed carrying out each cycle of mutagenesis in a rapid and efficient manner. Four rounds of directed evolution were conducted on a part of the gene coding for amino acid residues 130-390 of Luciola mingrelica luciferase. The resultant mutant designated 4TS had a half-life of 10 h at 42°C, which is 65-fold higher compared with the WT luciferase. Moreover, the mutant 4TS showed a 1.9-fold increase in specific activity, 5.7-fold reduction of K(m) for ATP and a higher-temperature optimum compared with the WT enzyme. 4TS contains eight mutations, four of which are suggested to be mainly responsible for the enhancement of thermostability: R211L, A217V, E356K and S364C. Thus, directed evolution with non-lethal colony screening for in vivo bioluminescence activity proved to be an effective and efficient approach for increasing thermal stability of luciferase while retaining high catalytic activity.

Triple substitution G216N/A217L/S398M leads to the active and thermostable Luciola mingrelica firefly luciferase.

Insufficient thermal stability of firefly luciferases often limits their application in a wide range of fields. The substitution A217L is known to greatly increase thermal stability of many firefly luciferases. However, for Hotaria parvula firefly luciferase, that shares 98% degree of homology with Luciola mingrelica luciferase, the A217L mutation is known to dramatically decrease catalytic activity. We analyzed the environment of A217 in the 3D-structure of L. mingrelica luciferase with the purpose of identifying possible additional mutations that would allow retention of the high thermal stability of the A217L mutant while preserving the high level of activity. The G216N/A217L double mutant of L. mingrelica luciferase demonstrated significantly improved stability at 42 and 45 °C but retained only 10% of activity; the loss in activity was accompanied by a large red shift of bioluminescence emission maximum from 566 to 611 nm compared with the wild-type enzyme. The triple mutant G216N/A217L/S398M exhibited high thermal stability of the double mutant together with the high activity and bioluminescence spectra close to that of the wild-type luciferase.