Testicular thermoregulation with respect to spermatogenesis and contraception.

There has been some evidence that a short heating exposure of the testicles can lead to temporary sterility. However, little attention has been paid to analyzing the temperature/time pattern throughout the testicle and how metabolism and blood perfusion affect the temperature profile. We use the Penne bioheat equation that accounts for the elements of conduction, perfusion, and metabolism along with the COMSOL Finite Element Method for the analysis of this thermoregulation problem. Typically, the testicles were modeled as two uniform spheres and exposed to an increase in surface temperature of 42 °C for 30 min. We found that heat produced by tissue metabolism had a minor effect on the temperature pattern. However, blood flow from the core (at 37 °C) could prevent the region where the testicles touch each other from reaching the target temperature of 42 °C. Further, we found that for animals, as rodents and monkeys that are used for contraception studies, the temperature distribution on heating testicles is much more uniform than for humans. Thus, extrapolation results from animal models to humans should be done with caution.

Protein interaction affinity determination by quantitative FRET technology.

The dissociation constant, K(d) , is an important parameter for characterizing protein-protein interaction affinities. SUMOylation is one of the important protein post-translational modifications and it involves a multi-step enzymatic cascade reaction, resulting in peptide activation and substrate conjugation. Multiple covalent and non-covalent protein-protein interactions are involved in this cascade. Techniques involving Förster resonance energy transfer (FRET) have been widely used in biological studies in vitro and in vivo, and they are very powerful tools for elucidating protein interactions in many regulatory cascades. In our previous studies, we reported the attempt to develop a new method for the determination of the K(d) by FRET assay using the interaction of SUMO1 and its E2 ligase, Ubc9 as a test system. However, the generality and specifications of this new method have not been fully determined. Here we report a systematic approach for determining the dissociation constant (K(d) ) in the SUMOylation cascade and for further sensitivity and accuracy testing by the FRET technology. From a FRET donor to acceptor concentration ratio range of 4-40, the K(d) s of SUMO1 and Ubc9 consistently agree well with values from surface plasmon resonance and isothermal titration calorimetry. These results demonstrate the high sensitivity and accuracy of the FRET-based K(d) determination approach. This technology, therefore, can be used in general for protein-protein interaction dissociation constant determination.

A MEMS affinity glucose sensor using a biocompatible glucose-responsive polymer.

We present a MEMS affinity sensor that can potentially allow long-term continuous monitoring of glucose in subcutaneous tissue for diabetes management. The sensing principle is based on detection of viscosity changes due to affinity binding between glucose and poly(acrylamide-ran-3-acrylamidophenylboronic acid) (PAA-ran-PAAPBA), a biocompatible, glucose-specific polymer. The device uses a magnetically driven vibrating microcantilever as a sensing element, which is fabricated from Parylene and situated in a microchamber. A solution of PAA-ran-PAAPBA fills the microchamber, which is separated from the surroundings by a semi-permeable membrane. Glucose permeates through the membrane and binds reversibly to the phenylboronic acid moiety of the polymer. This results in a viscosity change of the sensing solution, which is obtained by measuring the damped cantilever vibration using an optical lever setup, allowing determination of the glucose concentration. Experimental results demonstrate that the device is capable of detecting glucose at physiologically relevant concentrations from 27 mg/dL to 324 mg/dL. The glucose response time constant of the sensor is approximately 3 min, which can be further improved with device design optimization. Excellent reversibility and stability are observed in sensor responses, as highly desired for long-term, stable continuous glucose monitoring.

Protein-Protein Affinity Determination by Quantitative FRET Quenching.

The molecular dissociation constant, Kd, is a well-established parameter to quantitate the affinity of protein-protein or other molecular interactions. Recently, we reported the theoretical basis and experimental procedure for Kd determination using a quantitative FRET method. Here we report a new development of Kd determination by measuring the reduction in donor fluorescence due to acceptor quenching in FRET. A new method of Kd determination was developed from the quantitative measurement of donor fluorescence quenching. The estimated Kd values of SUMO1-Ubc9 interaction based on this method are in good agreement with those determined by other technologies, including FRET acceptor emission. Thus, the acceptor-quenched approach can be used as a complement to the previously developed acceptor excitation method. The new methodology has more general applications regardless whether the acceptor is an excitable fluorophore or a quencher. Thus, these developments provide a complete methodology for protein or other molecule interaction affinity determinations in solution.

Thirty-fifth anniversary of the optical affinity sensor for glucose: a personal retrospective.

Since 1962 when Clark introduced the enzyme electrode, research has been intense for a robust implantable glucose sensor. An alternative "optical affinity sensor" was introduced by Jerome Schultz in 1979. The evolution of this sensor technology into a new methodology is reviewed. The approach integrates a variety of disparate concepts: the selectivity of immunoassays-selectivity for glucose was obtained with concanavalin A, detection sensitivity was obtained with fluorescence (FITC-Dextran), and miniaturization was achieved by the use of an optical fiber readout system. Refinements of Schultz's optical affinity sensor approach over the past 35 years have led to a number of configurations that show great promise to meet the needs of a successful implantable continuous monitoring device for diabetics, some of which are currently being tested clinically.

Genetically engineered fluorescent cell marker for labeling CD34+ hematopoietic stem cells.

To address the challenge of labeling and tracking stem cells in vivo, we have engineered a fluorescent cell marker CD34EGFP by utilizing the mechanism of the cell-specific activity of CD34 promoter in CD34(+) stem cells. A retroviral vector derived from a murine stem cell virus was constructed to integrate the CD34EGFP gene into the genome of the cells for labeling. Our experiment demonstrates that the 454 bp segment upstream of the murine CD34 cDNA sequence has full function of promoter activity and can serve as a cell-specific promoter for driving the expression of EGFP in CD34(+) hematopoietic stem cells (HSC), providing a living color for labeling stem cells. The CD34EGFP marker was tested in various types of cells, including terminally differentiated cells, CD34(+) mouse myeloid leukemia progenitor cells, CD34(-) hematopoietic cells, and CD34(+) HSCs. We show that the engineered CD34EGFP cell marker is expressed in the CD34(+) stem or progenitor cells but not in CD34(-) or terminally differentiated cells. RT-PCR assay indicates that the transcription level of the CD34EGFP gene from CD34 promoter is almost the same as that from CMV promoter in CD34(+) progenitor cells. The approach we present here offers a framework for genetic engineering of fluorescent cell markers for labeling and tracking stem cells in vivo. We anticipate that a variety of cell markers could be generated by coupling variants of fluorescent proteins with various cell-specific promoters.

Effect of pH on infectivity and morphology of ecotropic moloney murine leukemia virus.

A number of factors affect the infectivity of retroviruses. The effect of pH on infectivity and morphology of ecotropic moloney murine leukemia virus (MoMuLV) was determined in this work. The ecotropic MoMuLVs were found to remain infectious at a narrow pH range from 5.5 to 8.0. Our experiments indicated that the viruses were inactivated swiftly at lower or higher pH. Within 5 min of exposure to pH 4 about 95% of the viruses lost infectiousness. The viruses were completely inactivated after exposure to pH < 3 or pH >11 for 5 min. The inactivation of MoMuLV was irreversible. Electron microscopy revealed that ecotropic MoMuLV remained round-shaped at pH between 7.0 and 5. They became irregular with a convex head at pH < 4. At pH 2, virtually all virion particles were penetrated by stains, causing the accumulation of heavy metals inside the particles. The penetration of heavy metal inside the particles indicated the disassociation of the lipid bilayer of the viruses at low pH. A FACS-based screening strategy for selecting high-titer retrovirus producing cell lines is also presented in this report.

Fluorescence enhancement of warfarin induced by interaction with beta-cyclodextrin.

Warfarin is the most common agent used for control and prevention of venous as well as arterial thromboembolism (blood clots). In aqueous media, warfarin forms inclusion complexes with a family of cyclic oligosaccharides, alpha, beta, gamma-cyclodextrins (CD). The formation of these complexes results in enhancement of the fluorescence of warfarin. Such spectroscopic changes offer a venue for the development of bioanalytical methodologies for warfarin quantification in biological liquids. We characterized the photophysical properties of warfarin in solvents with varying polarity and viscosity. The fluorescence quantum yield of warfarin correlated: (1) strongly with the solvent viscosity (R = 0.979) and (2) weakly with the solvent polarity (R = 0.118). These findings indicate that it is the change of the viscosity, rather than polarity, of the microenvironment that causes the fluorescence enhancement of warfarin upon binding to beta-CD. Utilizing the observed fluorescence enhancement in fluorescence titration measurements, the binding constants of warfarin to beta-CD were obtained (2.6 x 10(2) M(-1)-3.7 x 10(2) M(-1)). Using multivariable linear analysis, we extracted the stoichiometry of warfarin-beta-CD interaction (1:1).

Fabrication of antibody arrays using thermally responsive elastin fusion proteins.

A general method has been developed to immobilize antibodies onto an array surface by employing fusion proteins consisting of an elastin domain with tunable hydrophobic properties and an antibody-binding domain with high binding affinity and specificity for antibodies. Antibodies conjugated with the elastin fusion proteins can be directly printed on a self-assembled monolayer-modified glass slide in a functionally active orientation with a spatially defined pattern. An antibody array sensor for detection of tumor markers was fabricated to demonstrate the utility of the method. We expect that the method presented here could be a simple and universal platform to immobilize antibodies for the fabrication of a variety of antibody array sensors.

Genetic engineering of an allosterically based glucose indicator protein for continuous glucose monitoring by fluorescence resonance energy transfer.

Real-time monitoring of blood glucose could vastly reduce a number of the long-term complications associated with diabetes. In this article, we present a novel approach that relies on a glucose-binding protein engineered such that a 20% reduction in fluorescence due to the fluorescence resonance energy transfer occurs as a result of glucose binding. This change in fluorescence provides a signal for the optical detection of glucose. The novel glucose indicator protein (GIP) was created by fusing two fluorescent reporter proteins (green fluorescent proteins) to each end of an Escherichia coli glucose-binding protein in such a manner that the spatial separation between the fluorescent moieties changes when glucose binds, thus generating a distinct optical signal that can be used for glucose detection. By placing the GIP within a dialysis hollow fiber sensor, a microsensor has been developed for continuous monitoring of glucose. The sensor had a response time to sudden glucose changes within 100 s and was reversible. The sensor was shown to have an optional range on the order of 10 microM of glucose.

Tagging retrovirus vectors with a metal binding peptide and one-step purification by immobilized metal affinity chromatography.

Retroviral vectors produced from packaging cells are invariably contaminated by protein, nucleic acid, and other substances introduced in the manufacturing process. Elimination of these contaminants from retroviral vector preparations is helpful to reduce unwanted side effects, and purified vector preparations are desirable to improve reproducibility of therapeutic effect. Here we report a novel approach to engineer a metal binding peptide (MBP)-tagged murine leukemia virus (MuLV), allowing for one-step purification of retroviral vectors by immobilized metal affinity chromatography (IMAC). We inserted a His6 peptide into an ecotropic envelope protein (Env) by replacing part of its hypervariable region sequence with a sequence encoding the His6 peptide. Display of the His6 tag on the surface of Env endowed the vectors with a high affinity for immobilized metal ions, such as nickel. We demonstrated that the His6-tagged MuLV could be produced to high titers and could be highly purified by one-step IMAC. The protein and DNA contaminants in the purified vector supernatants were below 7 microg/ml and 25 pg/ml, respectively, indicating a 1,229-fold reduction in protein contaminant level and a 6,800-fold reduction in DNA contaminant level. About 56% of the viral vectors were recovered in the IMAC purification. The purified vectors retained their functionality and infectivity. These results establish that an MBP can be functionally displayed on the surface of ecotropic retroviruses without interfering with their integrity, and MBP-tagged retroviral vectors can be highly purified by one-step IMAC.