A Cell-Free Protein Expression System Derived from Human Primary Peripheral Blood Mononuclear Cells

Historically, some of the first cell-free protein expression systems studied in vitro translation in various human blood cells. However, because of limited knowledge of eukaryotic translation and the advancement of cell line development, interest in these systems decreased. Eukaryotic translation is a complex system of factors that contribute to the overall translation of mRNA to produce proteins. The intracellular translateome of a cell can be modified by various factors and disease states, but it is impossible to individually measure all factors involved when there is no comprehensive understanding of eukaryotic translation. The present work outlines the use of a coupled transcription and translation cell-free protein expression system to produce recombinant proteins derived from human donor peripheral blood mononuclear cells (PBMCs) activated with phytohemagglutinin-M (PHA-M). The methods outlined here could result in tools to aid immunology, gene therapy, cell therapy, and synthetic biology research and provide a convenient and holistic method to study and assess the intracellular translation environment of primary immune cells.

Real-time dissolved carbon dioxide monitoring II: Surface aeration intensification for efficient CO2 removal in shake flasks and mini-bioreactors leads to superior growth and recombinant protein yields

Mass transfer is known to play a critical role in bioprocess performance and henceforth monitoring dissolved O2 (DO) and dissolved CO2 (dCO2 ) is of paramount importance. At bioreactor level these parameters can be monitored online and can be controlled by sparging air/oxygen or stirrer speed. However, traditional small-scale systems such as shake flasks lack real time monitoring and also employ only surface aeration with additional diffusion limitations imposed by the culture plug. Here we present implementation of intensifying surface aeration by sparging air in the headspace of the reaction vessel and real-time monitoring of DO and dCO2 in the bioprocesses to evaluate the impact of intensified surface aeration. We observed that sparging air in the headspace allowed us to keep dCO2 at low level, which significantly improved not only biomass growth but also protein yield. We expect that implementing such controlled smart shake flasks can minimize the process development gap which currently exists in shake flask level and bioreactor level results.

Real-time dissolved carbon dioxide monitoring I: Application of a novel in situ sensor for CO2 monitoring and control

Dissolved carbon dioxide (dCO2 ) is a well-known critical parameter in bioprocesses due to its significant impact on cell metabolism and on product quality attributes. Processes run at small-scale faces many challenges due to limited options for modular sensors for online monitoring and control. Traditional sensors are bulky, costly, and invasive in nature and do not fit in small-scale systems. In this study, we present the implementation of a novel, rate-based technique for real-time monitoring of dCO2 in bioprocesses. A silicone sampling probe that allows the diffusion of CO2 through its wall was inserted inside a shake flask/bioreactor and then flushed with air to remove the CO2 that had diffused into the probe from the culture broth (sensor was calibrated using air as zero-point calibration). The gas inside the probe was then allowed to recirculate through gas-impermeable tubing to a CO2 monitor. We have shown that by measuring the initial diffusion rate of CO2 into the sampling probe we were able to determine the partial pressure of the dCO2 in the culture. This technique can be readily automated, and measurements can be made in minutes. Demonstration experiments conducted with baker's yeast and Yarrowia lipolytica yeast cells in both shake flasks and mini bioreactors showed that it can monitor dCO2 in real-time. Using the proposed sensor, we successfully implemented a dCO2 -based control scheme, which resulted in significant improvement in process performance.

Manufacturing biological medicines on demand: Safety and efficacy of granulocyte colony-stimulating factor in a mouse model of total body irradiation

Protein therapeutics, also known as biologics, are currently manufactured at centralized facilities according to rigorous protocols. The manufacturing process takes months and the delivery of the biological products needs a cold chain. This makes it less responsive to rapid changes in demand. Here, we report on technology application for on-demand biologics manufacturing (Bio-MOD) that can produce safe and effective biologics from cell-free systems at the point of care without the current challenges of long-term storage and cold-chain delivery. The objective of the current study is to establish proof-of-concept safety and efficacy of Bio-MOD-manufactured granulocyte colony-stimulating factor (G-CSF) in a mouse model of total body irradiation at a dose estimated to induce 30% lethality within the first 30 days postexposure. To illustrate on-demand Bio-MOD production feasibility, histidine-tagged G-CSF was manufactured daily under good manufacturing practice-like conditions prior to administration over a 16-day period. Bio-MOD-manufactured G-CSF improved 30-day survival when compared with saline alone (p = .073). In addition to accelerating recovery from neutropenia, the platelet and hemoglobin nadirs were significantly higher in G-CSF-treated animals compared with saline-treated animals (p < .05). The results of this study demonstrate the feasibility of consistently manufacturing safe and effective on-demand biologics suitable for real-time release.

Wood Microfluidics

As nonbiodegradable plastics continue to pollute our land and oceans, countries are starting to ban the use of single-use plastics. In this paper, we demonstrated the fabrication of wood-based microfluidic devices and their adaptability for single-use, point-of-care (POC) applications. These devices are made from easily sourced renewable materials for fabrication while exhibiting all the advantages of plastic devices without the problem of nonbiodegradable waste and cost. To build these wood devices, we utilized laser engraving and traditional mechanical methods and have adapted specific surface coatings to counter the wicking effect of wood. To demonstrate their versatility, wood microfluidic devices were adapted for (i) surface plasmon coupled enhancement (SPCE) of fluorescence for detection of proteins, (ii) T-/Y-geometry microfluidic channel mixers, and (iii) devices for rapid detection of microbial contamination. These provide proof of concept for the use of wooden platforms for POC applications. In this study, we measured the fluorescence intensities of recombinant green fluorescent protein (GFP) standards (ranging from 1.5-25 ng/μL) and 6XHis-G-CSF (ranging from 0.1-100 ng/μL) expressed in cell-free translation systems. All tested devices perform as well as or better than their plastic counterparts.

Low-cost customizable microscale toolkit for rapid screening and purification of therapeutic proteins

Biopharmaceutical separations require tremendous amounts of optimization to achieve acceptable product purity. Typically, large volumes of reagents and biological materials are needed for testing different parameters, thus adding to the expense of biopharmaceutical process development. This study demonstrates a versatile and customizable microscale column (µCol) for biopharmaceutical separations using immobilized metal affinity chromatography (IMAC) as an example application to identify key parameters. µCols have excellent precision, efficiency, and reproducibility, can accommodate any affinity, ion-exchange or size-exclusion-based resin and are compatible with any high-performance liquid chromatography (HPLC) system. µCols reduce reagent amounts, provide comparable purification performance and high-throughput, and are easy to automate compared with current conventional resin columns. We provide a detailed description of the fabrication methods, resin packing methods, and µCol validation experiments using a conventional HPLC system. Finite element modeling using COMSOL Multiphysics was used to validate the experimental performance of the µCols. In this study, µCols were used for improving the purification achieved for granulocyte colony stimulating factor (G-CSF) expressed using a cell-free CHO in vitro translation (IVT) system and were compared to a conventional 1 ml IMAC column. Experimental data revealed comparable purity with a 10-fold reduction in the amount of buffer, resin, and purification time for the μCols compared with conventional columns for similar protein yields.

Minimally invasive technique for measuring transdermal glucose with a fluorescent biosensor

There is a need for blood glucose monitoring techniques that eliminate the painful and invasive nature of current methods, while maintaining the reliability and accuracy of established medical technology. This research aims to ultimately address these shortcomings in critically ill pediatric patients. Presented in this work is an alternative, minimally invasive technique that uses microneedles (MN) for the collection of transdermal glucose (TG). Due to their comparable skin properties, diffusion studies were performed on full thickness Yucatan miniature pig skin mounted to an in-line diffusion flow cell and on different skin sites of human subjects. Collected TG samples were measured with a L255C mutant of the E. coli glucose-binding protein (GBP) with an attached fluorescent probe. The binding constant (K_d = 0.67 μM) revealed the micromolar sensitivity and high selectivity of the his-tagged GBP biosensor for glucose, making it suitable for TG measurements. In both the animal and human models, skin permeability and TG diffusion across the skin increased with MN application. For intact and MN-treated human skin, a significant positive linear correlation (r > 0.95, p < 0.01) existed between TG and BG. The micromolar sensitivity of GBP minimized the volume required for interstitial fluid glucose analysis allowing MN application time (30 s) to be shortened compared to other studies. This time reduction can help in eliminating skin irritation issues and improving practical use of the technique by caregivers in the hospital. In addition, the his-tagged optical biosensor used in this work can be immobilized and used with a portable sensing fluorometer device at the point of care (POC) making this minimally invasive technology more ideal for use in the pediatric intensive care unit. Graphical abstract ᅟ.

Rapid recombinant protein expression in cell-free extracts from human blood

Several groups have recently reported on the utility of cell-free expression systems to make therapeutic proteins, most of them employing CHO or E. coli cell-free extracts. Here, we propose an alternative that uses human blood derived leukocyte cell extracts for the expression of recombinant proteins. We demonstrate expression of nano luciferase (Nluc), Granulocyte-colony stimulating factor (G-CSF) and Erythropoietin (EPO) in cell-free leukocyte extracts within two hours. Human blood is readily available from donors and blood banks and leukocyte rich fractions are easy to obtain. The method described here demonstrates the ability to rapidly express recombinant proteins from human cell extracts that could provide the research community with a facile technology to make their target protein. Eventually, we envision that any recombinant protein can be produced from patient-supplied leukocytes, which can then be injected back into the patient. This approach could lead to an alternative model for personalized medicines and vaccines.

Development and characterization of a point-of care rate-based transcutaneous respiratory status monitor

Blood gas measurements provide vital clinical information in critical care. The current "gold standard" for blood gas measurements involves obtaining blood samples, which can be painful and can lead to bleeding, thrombus formation, or infection. Mass transfer equilibrium-based transcutaneous blood gas monitors have been used since the 1970s, but they require heating the skin to ≥42 °C to speed up the transcutaneous gas diffusion. Thus, these devices have a potential risk for skin burns. Here we report a new generation of noninvasive device for respiratory status assessment. Instead of waiting for mass transfer equilibrium, the blood gas levels are monitored by measuring the transcutaneous diffusion rate, which is proportional to blood gas concentration. The startup time of this device is almost independent of skin temperature, so the measurement can be made at any body temperature. The test results show that this device can track the blood gas levels quickly even at normal body temperature.

Improving the recombinant human erythropoietin glycosylation using microsome supplementation in CHO cell-free system

Cell-Free Protein Synthesis (CFPS) offers many advantages for the production of recombinant therapeutic proteins using the CHO cell-free system. However, many complex proteins are still difficult to express using this method. To investigate the current bottlenecks in cell-free glycoprotein production, we chose erythropoietin (40% glycosylated), an essential endogenous hormone which stimulates the development of red blood cells. Here, we report the production of recombinant erythropoietin (EPO) using CHO cell-free system. Using this method, EPO was expressed and purified with a twofold increase in yield when the cell-free reaction was supplemented with CHO microsomes. The protein was purified to near homogeneity using an ion-metal affinity column. We were able to analyze the expressed and purified products (glycosylated cell-free EPO runs at 25-28 kDa, and unglycosylated protein runs at 20 kDa on an SDS-PAGE), identifying the presence of glycan moieties by PNGase shift assay. The purified protein was predicted to have ∼2,300 IU in vitro activity. Additionally, we tested the presence and absence of sugars on the cell-free EPO using a lectin-based assay system. The results obtained in this study indicate that microsomes augmented in vitro production of the glycoprotein is useful for the rapid production of single doses of a therapeutic glycoprotein drug and to rapidly screen glycoprotein constructs in the development of these types of drugs. CFPS is useful for implementing a lectin-based method for rapid screening and detection of glycan moieties, which is a critical quality attribute in the industrial production of therapeutic glycoproteins.

Optimizing cell-free protein expression in CHO: Assessing small molecule mass transfer effects in various reactor configurations

Cell-free protein synthesis (CFPS) is an ideal platform for rapid and convenient protein production. However, bioreactor design remains a critical consideration in optimizing protein expression. Using turbo green fluorescent protein (tGFP) as a model, we tracked small molecule components in a Chinese Hamster Ovary (CHO) CFPS system to optimize protein production. Here, three bioreactors in continuous-exchange cell-free (CECF) format were characterized. A GFP optical sensor was built to monitor the product in real-time. Mass transfer of important substrate and by-product components such as nucleoside triphosphates (NTPs), creatine, and inorganic phosphate (Pi) across a 10-kDa MWCO cellulose membrane was calculated. The highest efficiency measured by tGFP yields were found in a microdialysis device configuration; while a negative effect on yield was observed due to limited mass transfer of NTPs in a dialysis cup configuration. In 24-well plate high-throughput CECF format, addition of up to 40 mM creatine phosphate in the system increased yields by up to ∼60% relative to controls. Direct ATP addition, as opposed to creatine phosphate addition, negatively affected the expression. Pi addition of up to 30 mM to the expression significantly reduced yields by over ∼40% relative to controls. Overall, data presented in this report serves as a valuable reference to optimize the CHO CFPS system for next-generation bioprocessing. Biotechnol. Bioeng. 2017;114: 1478-1486. © 2017 Wiley Periodicals, Inc.

Measuring transdermal glucose levels in neonates by passive diffusion: an in vitro porcine skin model

Current glucose monitoring techniques for neonates rely heavily on blood glucose monitors which require intermittent blood collection through skin-penetrating pricks on the heel or fingers. This procedure is painful and often not clinically conducive, which presents a need for a noninvasive method for monitoring glucose in neonates. Our motivation for this study was to develop an in vitro method for measuring passive diffusion of glucose in premature neonatal skin using a porcine skin model. Such a model will allow us to initially test new devices for noninvasive glucose monitoring without having to do in vivo testing of newborns. The in vitro model is demonstrated by comparing uncompromised and tape-stripped skin in an in-line flow-through diffusion apparatus with glucose concentrations that mimic the hypo-, normo-, and hyper-glycemic conditions in the neonate (2.0, 5.0, and 20 mM, respectively). Transepidermal water loss (TEWL) of the tape-stripped skin was approximately 20 g m^-2 h^-1, which closely mimics TEWL for neonatal skin at about 190 days post-conceptional age. The tape-stripped skin showed a >15-fold increase in glucose diffusion compared to the uncompromised skin. The very small concentrations of collected glucose were measured with a highly selective and highly sensitive fluorescent glucose biosensor based on the glucose binding protein (GBP). The demonstrated method of glucose determination is noninvasive and painless, which makes it especially desirable for glucose testing in neonates and children. This study is an important step towards an in vitro model for noninvasive real-time glucose monitoring that may be easily transferred to the clinic for glucose monitoring in neonates. Graphical Abstract Glucose diffusion through model skin was measured using an in-line flow-through diffusion apparatus with glucose solutions mimicking hypo-, normo- and hyperglycemia in the neonate. Phosphate buffered saline was added to the top chamber and the glucose that diffused through the model skin into the buffer was measured using a fluorescent glucose binding protein biosensor.

The effect of pH on the glucose response of the glucose-galactose binding protein L255C labeled with Acrylodan

The glucose-galactose binding protein (GGBP) is used as an optical biosensor in medical and bioprocess applications. This paper investigates the effect of pH on the behavior of GGBP-L255C labeled with Acrylodan for the purpose of finding the optimum conditions for sensing purposes as well as for protein preparation, purification and storage. The Acrylodan-GGBP fluorescence response in absence and presence of glucose was measured under varying buffer and pH conditions. Dissociation constants (Kd) and Gibbs free energies (ΔG) for the protein-glucose binding were calculated. Binding was found to be energetically favored at slightly acidic to neutral conditions, specifically close to the pI of GBP (∼ 5.0). Minimal fluorescence response to glucose was exhibited at pH 3.0 accompanied by a blue shift in the steady state fluorescence spectrum. In contrast, an almost 45% response to glucose was shown at pH 4.5-9.0 with a 13-nm red shift. Frequency domain lifetime measurements and quenching with KI suggest that at highly acidic conditions both the glucose-free and the glucose-bound protein are in a conformation distinct from those observed at higher pH values.

A rate-based transcutaneous CO2 sensor for noninvasive respiration monitoring

The pain and risk of infection associated with invasive blood sampling for blood gas measurements necessitate the search for reliable noninvasive techniques. In this work we developed a novel rate-based noninvasive method for a safe and fast assessment of respiratory status. A small sampler was built to collect the gases diffusing out of the skin. It was connected to a CO2 sensor through gas-impermeable tubing. During a measurement, the CO2 initially present in the sampler was first removed by purging it with nitrogen. The gases in the system were then recirculated between the sampler and the CO2 sensor, and the CO2 diffusion rate into the sampler was measured. Because the measurement is based on the initial transcutaneous diffusion rate, reaching mass transfer equilibrium and heating the skin is no longer required, thus, making it much faster and safer than traditional method. A series of designed experiments were performed to analyze the effect of the measurement parameters such as sampler size, measurement location, subject positions, and movement. After the factor analysis tests, the prototype was sent to a level IV NICU for clinical trial. The results show that the measured initial rate of increase in CO2 partial pressure is linearly correlated with the corresponding arterial blood gas measurements. The new approach can be used as a trending tool, making frequent blood sampling unnecessary for respiratory status monitoring.

Designing a Low-Cost Multifunctional Infant Incubator

Every year, an unacceptably large number of infant deaths occur in developing nations, with premature birth and asphyxia being two of the leading causes. A well-regulated thermal environment is critical for neonatal survival. Advanced incubators currently exist, but they are far too expensive to meet the needs of developing nations. We are developing a thermodynamically advanced low-cost incubator suitable for operation in a low-resource environment. Our design features three innovations: (1) a disposable baby chamber to reduce infant mortality due to nosocomial infections, (2) a passive cooling mechanism using low-cost heat pipes and evaporative cooling from locally found clay pots, and (3) insulated panels and a thermal bank consisting of water that effectively preserve and store heat. We developed a prototype incubator and visited and presented our design to our partnership hospital site in Mysore, India. After obtaining feedback, we have determined realistic, nontrivial design requirements and constraints in order to develop a new prototype incubator for clinical trials in hospitals in India.

Glucose monitoring in neonates: need for accurate and non-invasive methods

Neonatal hypoglycaemia can lead to devastating consequences. Thus, constant, accurate and safe glucose monitoring is imperative in neonatal care. However, point-of-care (POC) devices for glucose testing currently used for neonates were originally designed for adults and do not address issues specific to neonates. This review will address currently available monitoring options and describe new methodologies for non-invasive glucose monitoring in newborns.

Passive Diffusion of Transdermal Glucose: Noninvasive Glucose Sensing Using a Fluorescent Glucose Binding Protein

The motivation for this study was to determine if a statistically significant correlation exists between blood glucose (BG) and transdermal glucose (TG) collected by passive diffusion. A positive outcome will indicate that noninvasive passive TG diffusion is a painless alternative to collecting blood through a break on the skin. Sampling involves placing a small volume of buffer solution on the surface of membrane or skin for 5 minutes. The sample is then assayed with fluorescent GBP. In vitro testing was done on regenerated cellulose and a porcine skin model to determine diffusion of standard glucose solutions. In vivo testing was done on a healthy subject and a subject with type 2 diabetes. Glucose diffused readily through the regenerated cellulose membrane with good correlation between surface and internal glucose concentrations (R ² = .997). But the porcine skin model required a surface prewash to achieve the same good correlation R ² = .943). Based on this, an optimum prewash step was determined for the in vivo studies. The resulting correlation coefficients between TG and BG after a 15-minute prewash in a healthy subject and type 2 subject were .87 and .93, respectively. Removal of the extraneous glucose in the skin by prewashing was an important step in achieving good correlation between TG and BG. The results suggest that passive collection of TG is a noninvasive alternative to current practice of breaking the skin. Further studies are under way to determine the lag time between TG and BG and for the sampling protocol to be more amenable to point-of-care application.

Portable system for the detection of micromolar concentrations of glucose

Glucose in non-invasively collected biofluids is generally in the micromolar range and thus, requires sensing methodologies capable of measuring glucose at these levels. Here, we present a small fluorometer system that can quantify glucose in the range of 0-5 μM with resolution of ~0.07 μM. It relies on the glucose binding protein (GBP) fluorescently labeled with two fluorophores. Fluorescence signals from the dual-labeled GBP are utilized in a ratiometric mode, making the measurements insensitive to variations in protein concentration and other systematic errors. Fluorescence is quantified by a miniature, dedicated ratiometric fluorometer that is powered via USB. Concentration is calculated using an ultra-mobile personal computer (UMPC). The whole system is designed to be pocket sized suitable for point-of-care or bedside applications. Test results suggest that the system is a promising tool for accurate measurements of low glucose concentrations (0.1-10 μM) in biological samples.

A novel approach toward noninvasive monitoring of transcutaneous CO(2)

The continuous monitoring of transcutaneous gases is an integral part of neonatal intensive care. Present monitors measure the equilibrating values of these gases by raising the skin temperature to 42°C or above. Because neonatal skin is very sensitive and delicate, this often leads to serious skin injuries. In this work, we present a new approach to the noninvasive measurement of transcutaneous partial pressure of carbon dioxide (tcpCO2) based on the initial pseudo steady state diffusion rates instead of the mass-transfer equilibrium. Because we are following initial diffusion rates, each measurement takes no more than a few minutes. Additionally, raising the surface temperature is not required, thus, skin irritation and burns are highly unlikely. A dual-chamber diffusion vessel with either porcine skin or dialysis membrane placed between the two chambers was used to mimic neonatal skin. LI-820 CO2 Analyzer was used to measure the CO2 diffusing through the membrane or skin. Initial experiments on adult human skin under varying physical activities, food intake and breathing patterns showed a strong influence of the various conditions on the amount of CO2 diffusing through skin. These initial findings suggest that this method can be used not only on neonates but to a wider population of patients.

Detection of trace glucose on the surface of a semipermeable membrane using a fluorescently labeled glucose-binding protein: a promising approach to noninvasive glucose monitoring

BACKGROUND

Our motivation for this study was to develop a noninvasive glucose sensor for low birth weight neonates. We hypothesized that the underdeveloped skin of neonates will allow for the diffusion of glucose to the surface where it can be sampled noninvasively. On further study, we found that measurable amounts of glucose can also be collected on the skin of adults.

METHOD

Cellulose acetate dialysis membrane was used as surrogate for preterm neonatal skin. Glucose on the surface was collected by saline-moistened swabs and analyzed with glucose-binding protein (GBP). The saline-moistened swab was also tested in the neonatal intensive care unit. Saline was directly applied on adult skin and collected for analysis with two methods: GBP and high-performance anion-exchange chromatography (HPAEC).

RESULTS

The amount of glucose on the membrane surface was found (1) to accumulate with time but gradually level off, (2) to be proportional to the swab dwell time, and (3) the concentration of the glucose solution on the opposite side of the membrane. The swab, however, failed to absorb glucose on neonatal skin. On direct application of saline onto adult skin, we were able to measure by HPAEC and GBP the amount of glucose collected on the surface. Blood glucose appears to track transdermal glucose levels.

CONCLUSIONS

We were able to measure trace amounts of glucose on the skin surface that appear to follow blood glucose levels. The present results show modest correlation with blood glucose. Nonetheless, this method may present a noninvasive alternative to tracking glucose trends.

Study on low-cost calibration-free pH sensing with disposable optical sensors

As labor costs become more expensive, less labor-intensive disposable devices have become more ubiquitous. Similarly, the disposable optical pH sensor developed in our lab could provide a convenient yet cost-effective way for pH sensing in processes that require stringent pH control. This optical pH sensor is prepared in uniform individual lots of 100-200 sensors per lot. Calibration is accomplished on a few randomly selected sensors out of each lot. We show that all others in the same lot can then be used directly without requiring individual calibration. In this paper, a calibration model is derived to include all the factors that affect the signal of the disposable sensor. Experimental results show that the derived calibration model fits the experimental data. The readings of 28 randomly selected disposable sensors with 4 sensors from each of the 7 lots show an error less than 0.1 pH units in the useful sensing range of the sensor. The calibration model indicates that if further improvement on precision is desired, more uniform porous material and more advanced coating techniques will be required. When it comes to the effects of the varying coasters, house-made low-cost fluorometers, the variability in the brightness ratio of the blue-to-violet LEDs is the primary reason for the lack of precision. Other factors like LED light intensity distribution, optical properties of the filters and electronics also contribute to the coaster-to-coaster difference, but to a lesser extent. Two different methods for correcting the instrument variations were introduced. After correction, the collective reading errors for all the tested instruments were reduced to less than 0.2 pH units within the sensor's useful sensing range. Based on this result, our lab is currently implementing further improvements in modifying the coasters to equalize the ratios of blue-to-violet LED brightness.

High Resolution Non-contact Fluorescence Based Temperature Sensor for Neonatal Care

To date, thermistors are used to continuously monitor the body temperature of newborn babies in the neonatal intensive care unit. The thermistor probe is attached to the body with a strong adhesive tape to ensure that the probe stays in place. However, these strong adhesives are shown to increase microbial growth and cause serious skin injuries via epidermal stripping. The latter compromises the skin's ability to serve as a protective barrier leading to increase in water loss and further microbial infections. In this article a new approach is introduced that eliminates the need for an adhesive. Instead, two kinds of fluorophores are entrapped in a skin friendly chitosan gel that can be easily wiped on and off of the skin, and has antimicrobial properties as well. A CCD camera is used to detect the temperature dependent fluorescence of the fluorophore, tris(1,10-phenthroline)ruthenium(II) while 8-aminopyrene-1,3,6-trisulfonic acid serves as the reference. This temperature sensor was found to have a resolution of at least 0.13°C.

Dual optical sensor for oxygen and temperature based on the combination of time domain and frequency domain techniques

In measuring specific conditions in the real world, there are many situations where both the oxygen concentration and the temperature have to be determined simultaneously. Here we describe a dual optical sensor for oxygen and temperature that can be adapted for different applications. The measurement principle of this sensor is based on the luminescence decay times of the oxygen-sensitive ruthenium complex tris-4,7-diphenyl-1,10-phenanthroline ruthenium(III) [Rudpp] and the temperature-sensitive europium complex tris(dibenzoylmethane) mono(5-amino-1,10-phenanthroline)europium(III) [Eudatp]. The excitation and emission spectra of the two luminophores overlap significantly and cannot be discriminated in the conventional way using band pass filters or other optical components. However, by applying both the frequency and time domain techniques, we can separate the signals from the individual decay time of the complexes. The europium complex is entrapped in a poly(methyl methacrylate) (PMMA) layer and the ruthenium complex is physically adsorbed on silica gel and incorporated in a silicone layer. The two layers are attached to each other by a double sided silicone based tape. The europium sensing film was found to be temperature-sensitive between 10 and 70°C and the ruthenium oxygen-sensitive layer can reliably measure between 0 and 21% oxygen.

A luminescence lifetime assisted ratiometric fluorimeter for biological applications

In general, the most difficult task in developing devices for fluorescence ratiometric sensing is the isolation of signals from overlapping emission wavelengths. Wavelength discrimination can be achieved by using monochromators or bandpass filters, which often lead to decreased signal intensities. The result is a device that is both complex and expensive. Here we present an alternative system--a low-cost standalone optical fluorimeter based on luminescence lifetime assisted ratiometric sensing (LARS). This paper describes the principle of this technique and the overall design of the sensor device. The most significant innovation of LARS is the ability to discriminate between two overlapping luminescence signals based on differences in their luminescence decay rates. Thus, minimal filtering is required and the two signals can be isolated despite significant overlap of luminescence spectra. The result is a device that is both simple and inexpensive. The electronic circuit employs the lock-in amplification technique for the signal processing and the system is controlled by an onboard microcontroller. In addition, the system is designed to communicate with external devices via Bluetooth.

Tumor necrosis factor alpha and interferon gamma cooperatively induce oxidative stress and motoneuron death in rat spinal cord embryonic explants

The accumulation of reactive microglia in the degenerating areas of amyotrophic lateral sclerosis (ALS) tissue is a key cellular event creating a chronic inflammatory environment that results in motoneuron death. We have developed a new culture system that consists in rat spinal cord embryonic explants in which motoneurons migrate outside the explant, growing as a monolayer in the presence of glial cells. The proinflammatory cytokines tumor necrosis factor alpha (TNF-alpha) and interferon gamma (IFN-gamma) have been proposed to be involved in ALS-linked microglial activation. In our explants, the combined exposure to these cytokines resulted in an increased expression of the pro-oxidative enzymes inducible nitric oxide synthase (iNOS), the catalytic subunit of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, gp91(phox) and cyclooxygenase-2 (COX-2), as compared to each cytokine alone. This effect was related to their cooperation in the activation of the transcription factor nuclear factor kappa B (NF-kappaB). TNF-alpha and IFN-gamma also cooperated to promote protein oxidation and nitration, thus increasing the percentage of motoneurons immunoreactive for nitrotyrosine. Apoptotic motoneuron death, measured through annexin V-Cy3 and active caspase-3 immunoreactivities, was also found cooperatively induced by TNF-alpha and IFN-gamma. Interestingly, these cytokines did not affect the viability of purified spinal cord motoneurons in the absence of glial cells. It is proposed that the proinflammatory cytokines TNF-alpha and IFN-gamma have cooperative/complementary roles in inflammation-induced motoneuron death.

Development and application of an excitation ratiometric optical pH sensor for bioprocess monitoring

The development of a fluorescent excitation ratiometric pH sensor (AHQ-PEG) using a novel allylhydroxyquinolinium (AHQ) derivative copolymerized with polyethylene glycol dimethacrylate (PEG) is described. The AHQ-PEG sensor film is shown to be suitable for real-time, noninvasive, continuous, online pH monitoring of bioprocesses. Optical ratiometric measurements are generally more reliable, robust, inexpensive, and insensitive to experimental errors such as fluctuations in the source intensity and fluorophore photobleaching. The sensor AHQ-PEG in deionized water was shown to exhibit two excitation maxima at 375 and 425 nm with a single emission peak at 520 nm. Excitation spectra of AHQ-PEG show a decrease in emission at the 360 nm excitation and an increase at the 420 nm excitation with increasing pH. Accordingly, the ratio of emission at 420:360 nm excitation showed a maximum change between pH 5 and 8 with an apparent pK(a) of 6.40. The low pK(a) value is suitable for monitoring the fermentation of most industrially important microorganisms. Additionally, the AHQ-PEG sensor was shown to have minimal sensitivity to ionic strength and temperature. Because AHQ is covalently attached to PEG, the film shows no probe leaching and is sterilizable by steam and alcohol. It shows rapid (approximately 2 min) and reversible response to pH over many cycles without any photobleaching. Subsequently, the AHQ-PEG sensor film was tested for its suitability in monitoring the pH of S. cereviseae (yeast) fermentation. The observed pH using AHQ-PEG film is in agreement with a conventional glass pH electrode. However, unlike the glass electrode, the present sensor is easily adaptable to noninvasive monitoring of sterilized, closed bioprocess environments without the awkward wire connections that electrodes require. In addition, the AHQ-PEG sensor is easily miniaturized to fit in microwell plates and microbioreactors for high-throughput cell culture applications.

On the design of low-cost fluorescent protein biosensors

There is a large body of knowledge on proteins and their ligands that is available to the sensor researcher for the successful design of fluorescent biosensors. Chemically synthesized receptors rarely match the sensitivity and selectivity of proteins.Additionally, proteins are easily produced and manipulated through recombinant protein techniques. Although limitations exist in the prediction of signal response of proteins labeled with fluorescent probes, thoughtful experimentation can lead to useful, highly responsive fluorescent protein assays. Conversion of these assays into sensor devices may require additional manipulation of the fluorescence properties of the labeled proteins. We have shown that this can be achieved by a second fluorophore serving as a reference for ratiometric measurements. The choice of reference is contingent on the low-cost, miniaturized design of the device. Accordingly, the reference fluorophore is excitable with the same LED as the signal transducing probe and has a fluorescence decay lifetime that is orders of magnitude longer.Alternating illumination with intensity modulated light at two frequencies allows for ratiometric sensing without the need for bulky filter wheels while collecting the signals over a wide range of emission wavelengths. The result is a simple optoelectronics design that is cost-effective and small enough to be portable.In summary, the process of designing protein-based fluorescent biosensors for practical applications requires the systematic collaboration of a cross-disciplinary group of molecular biologists, chemists and engineers.

Low-cost optical lifetime assisted ratiometric glutamine sensor based on glutamine binding protein

Here we report a reagentless fluorescence sensing technique for glutamine in the submicromolar range based on the glutamine binding protein (QBP). The S179C mutant is labeled with the short-lived acrylodan (lifetime<5ns) and the long-lived tris(dibenzoylmethane) mono(5-amino-1,10-phenanthroline)europium(III) (lifetime > 300 micros) at the -SH and the N-terminal positions, respectively. In the presence of glutamine the fluorescence of acrylodan is quenched, while the fluorescence of europium complex remains constant. In this report we describe an innovative technique, the so called lifetime assisted ratiometric sensing to discriminate the two fluorescence signals using minimal optics and power requirements. This method exploits the large difference between the fluorescence lifetimes of the two fluorophores to isolate the individual fluorescence from each other by alternating the modulation frequency of the excitation light between 300 Hz and 10 kHz. The result is a ratiometric optical method that does not require expensive and highly attenuating band pass filters for each of the dyes, but only one long pass filter for both. Thus, the signal to noise ratio is enhanced, and at the same time, the optical setup is simplified. The end product is a simple sensing device suitable for low-cost applications such as point-of-care diagnostics or in-the-field analysis.

On the possibility of real-time monitoring of glucose in cell culture by microdialysis using a fluorescent glucose binding protein sensor

Although glucose sensors with millimolar sensitivity are still the norm, there is now a developing interest in glucose sensors with micromolar sensitivity for applications in minimally invasive sampling techniques such as fast microdialysis and extraction of interstitial fluid by iontophoresis and laser poration. In this regard, the glucose binding protein (GBP) with a binding constant for glucose in the micromolar range is of particular relevance. GBP is one of the soluble binding proteins found in the periplasmic space of Gram-negative bacteria. Because of its hinge-like tertiary structure, glucose binding induces a large conformational change, which can be used for glucose sensing by attaching a polarity sensitive fluorescent probe to a site on the protein that is allosterically responsive to glucose binding. Correspondingly, the resulting optical biosensor has micromolar sensitivity to glucose. Because binding is reversible, the biosensor is reusable and can be stored at 4 degrees C for 6 months without losing its sensitivity. In this paper, we show the feasibility of using the GBP biosensor to monitor glucose in microdialysis. The effect of perfusion rate, bulk glucose concentration and temperature on microdialysis efficiency was determined. Additionally, the glucose concentrations in mammalian cell culture were monitored to demonstrate the applicability of this sensor in complex and dynamic processes over a period of time. As the sensor is sensitive to micromolar glucose, high dialysis efficiency is not required when the bulk glucose concentration is within the millimolar physiological range. Thus, a perfusion rate of 10 microL/min or faster can be used, resulting in delay times of 1 min or less.

Comparing the performance of the optical glucose assay based on glucose binding protein with high-performance anion-exchange chromatography with pulsed electrochemical detection: efforts to design a low-cost point-of-care glucose sensor

BACKGROUND

The glucose binding protein (GBP) is one of many soluble binding proteins found in the periplasmic space of gram-negative bacteria. These proteins are responsible for chemotactic responses and active transport of chemical species across the membrane. Upon ligand binding, binding proteins undergo a large conformational change, which is the basis for converting these proteins into optical biosensors.

METHODS

The GBP biosensor was prepared by attaching a polarity-sensitive fluorescent probe to a single cysteine mutation at a site on the protein that is allosterically responsive to glucose binding. The fluorescence response of the resulting sensor was validated against high-performance anion-exchange chromatography (HPAEC) with pulsed electrochemical detection. Finally, a simple fluorescence reader was built using a lifetime-assisted ratiometric technique.

RESULTS

The GBP assay has a linear range of quantification of 0.100-2.00 microM and a sensitivity of 0.164 microM(-1) under the specified experimental conditions. The comparison between GBP and HPAEC readings for nine blind samples indicates that there is no statistical difference between the analytical results of the two methods at the 95% confidence level. Although the methods of fluorescence detection are based on different principles, the response of the homemade device to glucose concentrations was comparable to the response of the larger and more expensive tabletop fluorescence spectrophotometer.

CONCLUSIONS

A glucose binding protein labeled with a polarity-sensitive probe can be used for measuring micromolar amounts of glucose. Using a lifetime-assisted ratiometric technique, a low-cost GBP-based micromolar glucose monitor could be built.

Directional surface plasmon-coupled emission from a 3 nm green fluorescent protein monolayer

High-sensitivity detection schemes are of great interest for a number of applications. Unfortunately, such schemes are usually high-cost. We demonstrate a low-cost approach to a high-sensitivity detection scheme based on surface plasmon-coupled emission (SPCE). The SPCE of a monomolecular layer of green fluorescent protein (GFP) is reported here. The protein was electrostatically attached to a thin, SiO(2)-protected silver film deposited on a quartz substrate. The visible, directional emission of GFP was observed at a sharp, well-defined angle of 47.5 degrees from the normal to the coupling prism, and the spectrum corresponded to that of GFP. The SPCE resulting from the reverse Kretschmann configuration showed a 12-fold enhancement over the free space fluorescence. The directional emission was 97% p-polarized. The directionality and high polarization can be coupled with the intrinsic spectral resolution of SPCE to be used in the design miniaturized spectrofluorometers. The observation of SPCE in the visible region of the spectrum from a monolayer of protein opens up new possibilities in protein-based sensing.

Fatty acid sensor for low-cost lifetime-assisted ratiometric sensing using a fluorescent fatty acid binding protein

Elevated free fatty acid (FA) levels lead to insulin resistance, hypertension, and microangiopathy, all of which are associated with type 2 diabetes. On the other hand, deficiencies of FA are indicative of certain neurodegenerative diseases, including autism. Thus, free FA levels are a diagnostic indicator for a variety of disorders. Here we describe the use of a commercially available FA binding protein labeled with acrylodan (ADIFAB), which we modified with a ruthenium metal-ligand complex with the intention of creating a low-cost FA sensor. The dual-labeled FA binding protein was used in lifetime-assisted ratiometric sensing (LARS) of oleic acid. For both steady-state and time-resolved luminescence decay experiments, the protein is responsive to oleic acid in the range of 0.02-4.7 microM. The emission at 432 nm, which is associated with the acrylodan occupying the FA binding site, decreases in intensity and red shifts to 505 nm on the addition of oleic acid. The intensities of the 505-nm peak due to the acrylodan displaced from the binding site by FA and of the 610-nm emission peak of ruthenium remained nearly unchanged. Fitting of the fluorescence decay data using the method of least squares revealed three emitting components with lifetimes of approximately 0.60, 4.00, and 370 ns. Fractional intensities of the emitting species indicate that changes in modulation between 2 and 10 MHz on binding of the protein with oleic acid are due mainly to the 4.00-ns component. The 0.60- and 370-ns components are assigned to acrylodan (505 nm) and ruthenium, respectively. Note that because ruthenium has a lifetime that is two orders of magnitude longer than that of acrylodan, the FA measurements were carried out at excitation frequencies lower than what can be done with acrylodan alone. Thus, low-cost instrumentation can be designed for a practical FA sensor without sacrificing the quality of measurements.

Genetically engineered binding proteins as biosensors for fermentation and cell culture

The signal-transduction properties and the potential applications of two engineered binding proteins from E. coli were extensively studied. Both proteins have a single cysteine mutation in their polypeptide chains, which allow the introduction of an environmentally sensitive fluorophore: ANS for glucose-binding protein (GBP) and acrylodan for glutamine-binding protein (QBP). Both proteins respond to their ligands in the micromolar range. The proteins can be stored at 4 degrees C for at least 5 months. Apparent binding constant, protein concentration, and fluorophore are three major factors that affect the biosensor's responsive ranges. The binding of the ligand is quick and reversible in solution, but the unfavorable dissociation equilibrium and mass-transfer resistance for encapsulated proteins can delay the response to several minutes and the recovery to hours. Simulated results show that using dialysis tubing with a diameter of 1 mm or less is possible to reduce the recovery time to less than 30 minutes. The potential applications of GBP were studied in yeast fermentation and E. coli fermentations in three different scales: 150 mL, 5 mL, and 100 microL. The results were compared with an YSI 2700 Chemistry Analyzer. Although the latter could not give reliable results for the E. coli fermentations as the glucose concentration in LB medium is close to its lower detection limit, the glucose biosensor presented here was successfully applied to each situation. Glutamine-binding protein was tested in cell cultures of two different scales (100 mL and 100 microL) and the results were also compared with those obtained with YSI. Both QBP and YSI gave good results for the 100-mL cell culture, but the relatively large sample volume requirement of YSI (at least 5 microL) prevented it from being used in the 100-microL cell culture. Because of their small sample volume requirements (less than 1 microL) and high sensitivity, the assays described here might find wide applications in high-throughput bioprocessing.

Dual-Labeled Glucose Binding Protein for Ratiometric Measurements of Glucose

Highly sensitive glucose monitoring has potential applications in conditions where the glucose levels are below the detection limit of currently available technology. Examples include bioprocess monitoring of bacterial cultures and measurement of minute amounts of human interstitial fluid extracted by iontophoresis. Here we describe a ratiometric glucose sensor capable of measuring micromolar levels of glucose. This sensor is based on an E. coli glucose binding protein (GBP) labeled with two fluorophores. The L255C mutant of GBP was labeled with the environment-sensitive fluorophore, acrylodan, at the cysteine mutation and a long-lived metal ligand complex of ruthenium (ruthenium bis(2,2'-bipyridyl)-1, 10-phenanthroline-9-isothiocyanate) at the N-terminal. The acrylodan emission is quenched in the presence of glucose while the ruthenium emission remained constant, thereby serving as a reference. The sensitivity of the sensor is in the micromolar range (K(d) = 0.4-1.4 microM). Thus, it is possible to measure glucose concentrations at micromolar levels and higher (with dilution). Calculations of the fluorescence energy-transfer efficiency between acrylodan and ruthenium gave an approximate distance of 25 A between the two fluorophores, consistent with X-ray crystallographic data. The effect of temperature on glucose binding was measured and analyzed. Maximum signal changes and apparent binding constants increase with temperature. The enthalpy change for glucose binding as calculated from the apparent binding constants is approximately 43.1 kJ/mol. In addition to ratiometric measurements, the presence of the long-lived ruthenium metal ligand complex allows for low-cost modulation-based sensing.

Reagentless optical sensing of glutamine using a dual-emitting glutamine-binding protein

Glutamine is a major source of nitrogen and carbon in cell culture media. Thus, glutamine monitoring is important in bioprocess control. Here we report a reagentless fluorescence sensing for glutamine based on the Escherichia coli glutamine-binding protein (GlnBP) that is sensitive in the submicromolar ranges. The S179C variant of GlnBP was labeled at the -SH and N-terminal positions with acrylodan and ruthenium bis-(2,2'-bipyridyl)-1,10-phenanthroline-9-isothiocyanate, respectively. The acrylodan emission is quenched in the presence of glutamine while the ruthenium acts as a nonresponsive long-lived reference. The apparent binding constant, K'(d), of 0.72 microM was calculated from the ratio of emission intensities of acrylodan and ruthenium (I(515)/I(610)). The presence of the long-lived ruthenium allowed for modulation sensing at lower frequencies (1-10 MHz) approaching an accuracy of +/-0.02 microM glutamine. Dual-frequency ratiometric sensing was also demonstrated. Finally, the extraordinary sensitivity of GlnBP allows for dilution of the sample, thereby eliminating the effects of background fluorescence from the culture media.

Noninvasive measurement of dissolved oxygen in shake flasks

Shake flasks are ubiquitous in cell culture and fermentation. However, conventional devices for measuring oxygen concentrations are impractical in these systems. Thus, there is no definitive information on the oxygen supply of growing cells. Here we report the noninvasive, nonintrusive monitoring of dissolved oxygen (DO) in shake flasks using a low-cost optical sensor. The oxygen-sensitive element is a thin, luminescent patch affixed to the inside bottom of the flask. The sensitivity and accuracy of this device is maximal up to 60% DO, within the range that is critical to cell culture applications. By measuring actual oxygen levels every 1 or 5 min throughout the course of yeast and E. coli fermentations, we found that a modest increase in shaker speed and a decrease in culture volume slowed the onset of oxygen limitation and reduced its duration. This is the first time that in situ oxygen limitation is reported in shake flasks. The same data is unattainable with a Clark type electrode because the presence of the intrusive probe itself changes the actual conditions. Available fiber optic oxygen sensors require cumbersome external connections and recalibration when autoclaved.

On the Possibility of Glucose Sensing Using Boronic Acid and a Luminescent Ruthenium Metal-Ligand Complex

We describe a new approach to optical sensing of glucose based on the competitive interactions between a ruthenium metal ligand complex, a boronic acid derivative and glucose. The metal-ligand complex [Ru(2,2'-bipyridme)2(5,6-dihydroxy-1,10-phenanthrolme)](PF6)2 at pH 8 forms a reversible complex with 2-toluylboronic acid or 2-methoxyphenyl boronic acid. Complexation is accompanied by a several-fold increase in the luminescent intensity of the ruthenium complex. Addition of glucose results in decreased luminescent intensity, which appears to be the result of decreased binding between the metal-ligand complex and the boronic acid. Ruthenium metal-ligand complexes are convenient for optical sensing because their long luminescent decay times allow lifetime-based sensing with simple instrumentation.

Polarization-based sensing of glucose using an oriented reference film

We describe a new approach to glucose sensing using polarization measurements in the presence of a stretch-oriented reference film. The method relies on measurement of the polarized emission from the reference film and of a fluorophore which changes intensity in response to glucose. A glucose-sensitive fluorescent signal was provided by the glucose/galactose binding protein from E. coli. This protein was labeled with an environmentally sensitive fluorophore at a single genetically inserted cysteine residue, and displayed decreased fluorescence upon glucose binding. Using the protein and the reference film we observed glucose-sensitive polarization values for micromolar glucose concentrations. This method of polarization-based sensing is generic and can be used for any sensing fluorophore which displays a change in intensity. In principle, one can construct simple and economical devices for this type of glucose measurement. © 1999 Society of Photo-Optical Instrumentation Engineers.

Polarization-Based Sensing with a Self-Referenced Sample

We describe a new method of fluorescence sensing based on fluorescence polarization. The sensor consists of two compartments, both of which contain the sensing fluorophore. One side of the sensor contains a constant concentration of analyte, and the other contains the unknown concentration. Emission from both sides is observed through polarizers, with the polarization from the sample being rotated 90° from that of the reference. Changes in the fluorescence intensity of the sample result in changes in the measured polarization for the combined emission. We show that this approach can be used to measure glucose and calcium using fluorophores which show analyte-dependent intensity changes, and no change in the spectral shape. Only a single fluorophore is required, this being the sensing fluorophore in both sides of the sensor. We also show that polarization sensing of glucose and calcium can be performed with visual detection of the polarization. In this case the only electronic component is the light source. These simple schemes can be used with a variety of analytes. The only requirement is a change in fluorescence intensity in response to the analyte.

Glucose sensor for low-cost lifetime-based sensing using a genetically engineered protein

We describe a glucose sensor based on a mutant glucose/galactose binding protein (GGBP) and phase-modulation fluorometry. The GGBP from Escherichia coli was mutated to contain a single cysteine residue at position 26. When labeled with a sulfhydryl-reactive probe 2-(4'-iodoacetamidoanilino)naphthalene-6-sulfonic acid, the labeled protein displayed a twofold decrease in intensity in response to glucose, with a dissociation constant near 1 microM glucose. The ANS-labeled protein displayed only a modest change in lifetime, precluding lifetime-based sensing of glucose. A modulation sensor was created by combining ANS26-GGBP with a long-lifetime ruthenium (Ru) metal-ligand complex on the surface of the cuvette. Binding of glucose changed the relative intensity of ANS26-GGBP and the Ru complex, resulting in a dramatic change in modulation at a low frequency of 2.1 MHz. Modulation measurements at 2.1 MHz were shown to accurately determine the glucose concentration. These results suggest an approach to glucose sensing with simple devices.

ADVANCES IN FLUORESCENCE SPECTROSCOPY: MULTI-PHOTON EXCITATION, ENGINEERED PROTEINS, MODULATION SENSING AND MICROSECOND RHENIUM METAL-LIGAND COMPLEXES

The technology and applications of fluorescence spectroscopy are rapidly advancing. In this overview presentation we summarize some recent developments from this laboratory. Two and three-photon excitation have been observed for a wide variety of intrinsic and extrinsic fluorophores, including tryptophan, tyrosine, DNA stains, membrane probes, and even alkanes. It has been possible to observe multi-photon excitation of biopolymers without obvious photochemical or photo-thermal effects. Although not de-scribed in our lecture, another area of increasing interest is the use of engineered proteins for chemical and clinical sensing. We show results for the glucose-galactose binding protein from E. coli. The labeled protein shows spectral changes in response to micromolar concentrations of glucose. This protein was used with a novel sensing method based on the modulated emission of the labeled proteins and a long lifetime reference fluorophore. And finally, we describe a recently developed rhenium complex which displays a lifetime near 3 µs in oxygenated aqueous solution. Such long life-time probes allow detection of microsecond dynamic processes, bypassing the usual nanosecond timescale limit of fluorescence. The result of these developments in protein engineering, sensing methods, and metal-ligand probe chemistry will be the increased use of fluorescence in clinical chemistry and point-of-care analyses.