All solid-state GFP sensor

Solid state fluorescence of proteins in high throughput mode and its applications

Direct comparison between fluorescence spectra of a sample in solution and solid state form is valuable to monitor the changes in protein structure when it is “dried” or immobilized on a solid surface (for biocatalysis or sensor applications). We describe here a simple method for recording fluorescence emission spectra of protein powders without using any dedicated accessory for solid samples in a high-throughput format. The 96-well plate used in our studies, was coated black from all the sides and the excitation and emission paths are identical and are from the top of the well. These two features minimize scatter and provide fairly noise free spectra. Even then the fluorescence intensity may be dependent upon many factors such as the extent of protein aggregation, morphology and sizes of the protein particles. Hence, (changes in) λ _max emission may be a more reliable metric in the case of fluorescence spectra of proteins in the solid state. However, any large changes in the intensity could indicate changes in the microenvironment of the fluorophore. The fluorescence emission spectra were blue-shifted (4 to 9 nm), showed an increase in the intensity for different proteins studied upon lyophilization, and were similar to what has been reported by others using available commercial accessories for solid state samples. After validating that our method worked just as well as the dedicated accessories, we applied the method to compare the fluorescence emission spectra of α-chymotrypsin in solution, precipitated form, and the lyophilized powder form. We further examined the fluorescence emission spectra of green fluorescent protein (GFP) in solution and solid form. We also analyzed fluorescence resonance energy transfer (FRET) between tryptophan (Trp57) and the cyclic chromophore of GFP. These findings pointed towards the change in the microenvironment around the cyclic chromophore in GFP upon lyophilization.

Homogeneity and heterogeneity in amylase production by Bacillus subtilis under different growth conditions

Background

Bacillus subtilis is an important cell factory for the biotechnological industry due to its ability to secrete commercially relevant proteins in large amounts directly into the growth medium. However, hyper-secretion of proteins, such as α-amylases, leads to induction of the secretion stress-responsive CssR-CssS regulatory system, resulting in up-regulation of the HtrA and HtrB proteases. These proteases degrade misfolded proteins secreted via the Sec pathway, resulting in a loss of product. The aim of this study was to investigate the secretion stress response in B. subtilis 168 cells overproducing the industrially relevant α-amylase AmyM from Geobacillus stearothermophilus, which was expressed from the strong promoter P(amyQ)-M.

Results

Here we show that activity of the htrB promoter as induced by overproduction of AmyM was “noisy”, which is indicative for heterogeneous activation of the secretion stress pathway. Plasmids were constructed to allow real-time analysis of P(amyQ)-M promoter activity and AmyM production by, respectively, transcriptional and out-of-frame translationally coupled fusions with gfpmut3. Our results show the emergence of distinct sub-populations of high- and low-level AmyM-producing cells, reflecting heterogeneity in the activity of P(amyQ)-M. This most likely explains the heterogeneous secretion stress response. Importantly, more homogenous cell populations with regard to P(amyQ)-M activity were observed for the B. subtilis mutant strain 168degUhy32, and the wild-type strain 168 under optimized growth conditions.

Conclusion

Expression heterogeneity of secretory proteins in B. subtilis can be suppressed by degU mutation and optimized growth conditions. Further, the out-of-frame translational fusion of a gene for a secreted target protein and gfp represents a versatile tool for real-time monitoring of protein production and opens novel avenues for Bacillus production strain improvement.

Deep blue light-emitting diodes based on Cd1-xZnx S @ ZnS quantum dots

We demonstrate deep blue light-emitting diodes based on chemically synthesized Cd(1-x)Zn(x) S @ ZnS quantum dots (QDs). Composite films of poly-(N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine) (poly-TPD) and 4,4',N,N'-diphenylcarbazole (CBP) are employed for facilitated hole injection into Cd(1-x)Zn(x) S @ ZnS QDs and uniform QD deposition. The fabricated devices possess moderate turn-on voltage (6 V) and external quantum efficiency (0.1-0.3%), and exhibit color-saturated blue emission with a narrow spectral bandwidth of full width at half maximum <25 nm (Commission Internationale de l'Eclairage (CIE) coordinates of (0.169, 0.024) and (0.156, 0.028) for devices with electroluminescence (EL) lambda(max) at 434 and 454 nm, respectively). Most of the emission originates from the Cd(1-x)Zn(x) S @ ZnS QD layers (99% of the total EL emission).

Photodiode-based detection system for biosensors

Many sensors and biosensors are based on the detection of optical changes in the sensing phase. In order to build a stand-alone sensing device, a miniature and low-cost detection system is critical. Here, the method for manufacturing the most critical part (the photodetector) is described in detail. The receiver is based on a photodiode. The practical design of such device is presented here. By using it, it is possible to construct an optical sensor for fluorescence or absorption-based measurement. Discussed here are suitable methods for generation and modulation of probing light, as well as the possible optical configurations. Practical advice is given on the selection of the bandwidth, noise, amplification, etc. The described system was used for detection of green red fluorescent protein in E. coli during fermentation.

Prediction of GFP spectral properties using artificial neural network

The prediction of the excitation and the emission maxima of green fluorescent protein (GFP) chromophores were investigated by a quantitative structure-property relationship study. A data set of 19 GFP color variants and an additional data set consisting of 29 synthetic GFP chromophores were collected from the literature. Artificial neural network implementing the back-propagation algorithm was employed. The proposed computational approach reliably predicted the excitation and the emission maxima of GFP chromophores with correlation coefficient exceeding 0.9. The usefulness of quantum chemical descriptors was revealed by a comparative study with other molecular descriptors. Assignment of appropriate protonation state of the chromophore for the GFP color variants data set was shown to be necessary for good predictive performance. Results suggest that the confinement of the GFP chromophore has no significant influence on the predictive performance of the data set used. A comparative investigation with the traditional modeling methods, particularly multiple linear regression and partial least squares, reveals that artificial neural network is the most suitable modeling approach for the GFP spectral properties. It is anticipated that this methodology has great potential in accelerating the design and engineering of novel GFP color variants of scientific or industrial interest.

Validation of an optical sensor-based high-throughput bioreactor system for mammalian cell culture

Cell culture optimization is a labor-intensive process requiring a large number of experiments to be conducted under varying conditions. Here we describe a high-throughput bioreactor system that allows 12 mini stirred-tank bioreactors to be operated simultaneously. All bioreactors are monitored by low-cost minimally invasive optical sensors for pH and dissolved oxygen. The sensors consist of single-use patches affixed inside the bioreactors and monitored optically from the outside. Experimental results show that different sensing patches with the same composition respond consistently. The discrepancy between different pH sensors is less than 0.1 pH units over most of their responsive range. The discrepancy between different dissolved oxygen sensors is less than 10% over the whole range from 0% to 100% dissolved oxygen. The consistency of the sensing system ensures that only an initial one-time calibration is required for the sensing patches. After that, a calibration code is generated and sensing patches of the same composition can be used directly. This greatly reduces the time and cost required for monitored multi-bioreactor operations. We used SP2/0 myeloma/mouse hybridoma cell cultures to demonstrate reactor performance consistency. Transcriptional profiling, HPLC analysis, viable cell count, and viability inspection show that the presence of sensing patches and the use of optical monitoring have no apparent effect on the metabolism of the cells.

Lipid-Membrane Affinity of Chimeric Metal-binding Green Fluorescent Protein

The Green Fluorescent Protein (GFP) is a useful marker to trace the expression of cellular proteins. However, little is known about changes in protein interaction properties after fusion to GFP. In this study, we present evidence for a binding affinity of chimeric cadmium-binding green fluorescent proteins to lipid membrane. This affinity has been observed in both cellular membranes and artificial lipid monolayers and bilayers. At the cellular level, the presence of Cd-binding peptide promoted the association of the chimeric GFP onto the lipid membrane, which declined the fluorescence emission of the engineered cells. Binding affinity to lipid membranes was further investigated using artificial lipid bilayers and monolayers. Small amounts of the chimeric GFP were found to incorporate into the lipid vesicles due to the high surface pressure of bilayer lipids. At low interfacial pressure of the lipid monolayer, incorporation of the chimeric Cd-binding GFP onto the lipid monolayer was revealed. From the measured lipid isotherms, we conclude that Cd-binding GFP mediates an increase in membrane fluidity and an expansion of the surface area of the lipid film. This evidence was strongly supported by epifluorescence microscopy, showing that the chimeric Cd-binding GFP preferentially binds to fluid-phase areas and defect parts of the lipid monolayer. All these findings demonstrate the hydrophobicity of the GFP constructs is mainly influenced by the fusion partner. Thus, the example of a metal-binding unit used here shines new light on the biophysical properties of GFP constructs.

Binding of chimeric metal-binding green fluorescent protein to lipid monolayer

Membrane-based bioanalytical devices for metal determination using green fluorescent protein as the sensor molecule may be a useful future biomimetic material. However, in order to develop such a device, it is necessary first to understand the interaction of the protein with lipid membranes. Thus we have investigated the interaction between chimeric cadmium-binding green fluorescent proteins (CdBPGFPs) and lipid monolayers, using a film-balance technique complemented with epifluorescence microscopy. The binding avidity was monitored from the surface pressure vs. area isotherms or from the measured increase in the lateral pressure upon injection of the chimeric CdBPGFPs beneath the lipid monolayer. Increased fluidization as well as expansion of the surface area were shown to depend on the concentration of the CdBPGFPs. The kinetics of the protein-induced increase in lateral pressure was found to be biphasic. The chimeric CdBPGFPs possessed high affinity to the 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monolayer with a dissociation constant of Kd = 10(-8)M. Epifluorescence measurements showed that this affinity is due to the presence of the Cd-binding peptide, which caused the GFP to incorporate preferentially to the liquid phase and defect part of the rigid domain at low interfacial pressure. At high compression, the Cd-binding peptide could neither incorporate nor remain in the lipid core. However, specific orientation of the chimeric CdBPGFPs underneath the air-water interface was achieved, even under high surface pressure, when the proteins were applied to the metal-chelating lipid-containing surfaces. This specific binding could be controlled reversibly by the addition of metal ions or metal chelator. The reversible binding of the chimeric CdBPGFPs to metal-chelating lipids provided a potential approach for immobilization, orientation and lateral organization of a protein at the membrane interface. Furthermore, the feasibility of applying the chelator lipids for the codetermination of metal ions with specific ligands was also revealed. Our finding clearly demonstrates that a strong interaction, particularly with fluid lipid domains, could potentially be used for sensor development in the future.

Evaluation of the GFP signal and its aptitude for novel on-line monitoring strategies of recombinant fermentation processes

A high number of economically important recombinant proteins are produced in Escherichia coli based host/vector systems. The major obstacle for improving current processes is a lack of appropriate on-line in situ methods for the monitoring of metabolic burden and critical state variables. Here, a pre-evaluation of the reporter green fluorescent protein (GFP) was undertaken to assess its use as a reporter of stress associated promoter regulation. The investigation of GFP and its blue fluorescent variant BFP was done in model fermentations using E. coli HMS 174(DE3)/pET11 aGFPmut3.1 and E. coli HMS174(DE3)/pET1aBFP host/vector systems cultured in fed-batch and chemostat regime. Our results prove the suitability of the fluorescent reporter proteins for the design of new strategies of on-line bioprocess monitoring. GFPmut3.1 variant can be detected after a short lag-phase of only 10 min, it shows a high fluorescence yield in relation to the amount of reporter protein, a good signal to noise ratio and a low detection limit. The fluorescence-signal and the amount of fluorescent protein, determined by ELISA, showed a close correlation in all fermentations performed. A combination of reporter technology with state of the art sensors helps to develop new strategies for efficient on-line monitoring needed for industrial process optimisation. The development of efficient monitoring will contribute to advanced control of recombinant protein production and accelerate the development of optimised production processes.

Ex vivo monitoring of protein production in baculovirus-infected Trichoplusia ni larvae with a GFP-specific optical probe

Trichoplusia ni larvae were infected with baculoviruses containing genes for the expression of ultraviolet optimized green fluorescent protein (GFPuv) and several product proteins. A GFP-specific optical probe was used to both excite the green fluorescent protein (lambda(ex) = 385 nm), and subsequently monitor fluorescence emission (lambda(em) = 514 nm) from outside the infected larvae. The probe's photodetector was connected to a voltmeter, which was used to quantify the amount of GFPuv expressed in infected larvae. Voltage readings were significantly higher for infected vs. uninfected larvae and, by Western analysis, linear with the amount of GFPuv produced. In addition, the probe sensitivity and range were sufficient to delineate infection efficiency and recombinant protein production for model proteins, chloramphenicol acetyltransferase and human interleukin-2. This work represents a critical step in developing an automated process for the production of recombinant proteins in insect larvae.

RNA interference as a metabolic engineering tool: potential for in vivo control of protein expression in an insect larval model

Many ex vivo factors influence the yield of recombinant protein produced via AcMNPV (Autographa californica multiple nucleocapsid nuclear polyhedrosis virus) in Trichoplusia ni (T. ni) larvae. Among these are: the method of infection, the time of infection, the virus load, and the time of harvest. In vivo strategies, however, that attempt to manipulate host function in this and other expression systems have largely been ignored. In this work, RNA interference (RNAi) is shown as an effective metabolic engineering controller to downregulate targeted gene expression. Specifically, RNAi was made to virus-encoded gfp(uv) and was found to inhibit the production of GFPuv in larvae when injected within an 18-h window (before and after) of baculovirus infection. The level of inhibition was found to depend, both in duration and extent, on the concentration of injected RNAi. That relatively low levels of RNAi can inhibit protein synthesis driven by the strong polyhedrin (polh) promoter of AcMNPV, suggests that RNAi will find utility as an in vivo metabolic controller in metabolic engineering studies such as this one pertaining to protein expression.

Monitoring of recombinant protein production using bioluminescence in a semiautomated fermentation process

On-line optimization of fermentation processes can be greatly aided by the availability of information on the physiological state of the cell. The goal of our "BioLux" research project was to design a recombinant cell capable of intracellular monitoring of product synthesis and to use it as part of an automated fermentation system. A recombinant plasmid was constructed containing an inducible promoter that controls the gene coding for a model protein and the genes necessary for bioluminescence. The cells were cultured in microfermenters equipped with an on-line turbidity sensor and a specially designed on-line light sensor capable of continuous measurement of bioluminescence. Initial studies were done under simple culture conditions, and a linear correlation between luminescence and protein production was obtained. Such specially designed recombinant bioluminescent cells can potentially be applied for model-based inference of intracellular product formation, as well as for optimization and control of recombinant fermentation processes.

Low-cost microbioreactor for high-throughput bioprocessing

The design of a microbioreactor is described. An optical sensing system was used for continuous measurements of pH, dissolved oxygen, and optical density in a 2 mL working volume. The K(L)a of the microbioreactor was evaluated under different conditions. An Escherichia coli fermentation in both the microbioreactor and a standard 1 L bioreactor showed similar pH, dissolved oxygen, and optical density profiles.%The low cost of the microbioreactor, detection system, and the small volume of the fermentation broth provide a basis for development of a multiple-bioreactor system for high-throughput bioprocess optimization.