Targeting of green fluorescent protein expression to the cell surface

Circular PCR as an efficient and precise umbrella of methods for the generation of circular dsDNA with staggered nicks: Mechanism and types

Here, we introduce the highly versatile circular polymerase chain reaction (CiPCR) technique, propose a mechanism of action, and describe a number of examples demonstrating the versatility of this technique. CiPCR takes place between two fragments of dsDNA with two homologous regions, as long as one of the fragments carries said regions at its 3'- and 5'-ends. Upon hybridization, elongation by a polymerase occurs from all 3'-ends continuously until a 5'-end is reached, leading to stable circular dsDNA with staggered nicks. When both dsDNA fragments carry the homology at their 3'- and 5'-ends (Type I CiPCR), all four 3'-ends effectively prime amplification of the intervening region and CiPCR products can function as template during the reaction. In contrast, when only one of the two dsDNA fragments carries the homologous regions at its 3'- and 5'-ends and the other carries such regions internally (Type II CiPCR), only two 3'-ends can be amplified and CiPCR products possess no template activity. We demonstrate the applicability of both CiPCR types via well-illustrated experimental examples. CiPCR is well adapted to the quick resolution of most of the molecular cloning challenges faced by the biology/biomedicine laboratory, including the generation of insertions, deletions, and mutations.

PET and SPECT

Assessment of gene function following the completion of human genome sequencing may be done using radionuclide imaging procedures. These procedures are needed for the evaluation of genetically manipulated animals or newly designed biomolecules which require a thorough understanding of physiology, biochemistry and pharmacology. The experimental approaches will involve many new technologies, including in-vivo imaging with SPECT and PET. Nuclear medicine procedures may be applied for the determination of gene function and regulation using established and new tracers or using in-vivo reporter genes, such as genes encoding enzymes, receptors, antigens or transporters. Visualization of in-vivo reporter gene expression can be done using radiolabeled substrates, antibodies or ligands. Combinations of specific promoters and in-vivo reporter genes may deliver information about the regulation of the corresponding genes. Furthermore, protein-protein interactions and the activation of signal transduction pathways may be visualized noninvasively. The role of radiolabeled antisense molecules for the analysis of mRNA content has to be investigated. However, possible applications are therapeutic interventions using triplex oligonucleotides with therapeutic isotopes, which can be brought near to specific DNA sequences to induce DNA strand breaks at selected loci. After the identification of new genes, functional information is required to investigate the role of these genes in living organisms. This can be done by analysis of gene expression, protein-protein interaction or the biodistribution of new molecules and may result in new diagnostic and therapeutic procedures, which include visualization of and interference with gene transcription, and the development of new biomolecules to be used for diagnosis and treatment. Furthermore, the characterization of tumor cell-specific properties allows the design of new treatment modalities, such as gene therapy, which circumvent resistance mechanisms towards conventional chemotherapeutic drugs.

Impact of functional genomics and proteomics on radionuclide imaging

The assessment of gene function following the completion of human genome sequencing may be performed using radionuclide imaging procedures. These procedures are needed for the evaluation of genetically manipulated animals or newly designed biomolecules, which requires a thorough understanding of physiology, biochemistry, and pharmacology. The experimental approaches will involve many new technologies, including in vivo imaging with single photon emission computed tomography and positron emission tomography. Nuclear medicine procedures may be applied for the determination of gene function and regulation using established and new tracers, or using in vivo reporter genes, such as genes encoding enzymes, receptors, antigens, or transporters. Visualization of in vivo reporter gene expression can be performed using radiolabeled substrates, antibodies, or ligands. Combinations of specific promoters and in vivo reporter genes may deliver information about the regulation of the corresponding genes. Furthermore, protein-protein interactions and activation of signal transduction pathways may be visualized noninvasively. The role of radiolabeled antisense molecules for the analysis of messenger ribonucleic acid (RNA) content has to be investigated. However, possible applications are therapeutic intervention using triplex oligonucleotides with therapeutic isotopes, which can be brought near to specific deoxyribonucleic acid sequences to induce deoxyribonucleic acid strand breaks at selected loci. Imaging of labeled siRNA makes sense if these are used for therapeutic purposes to assess the delivery of these new drugs to their target tissue. Pharmacogenomics will identify new surrogate markers for therapy monitoring, which may represent potential new tracers for imaging. Drug distribution studies for new therapeutic biomolecules are needed at least during preclinical stages of drug development. New treatment modalities, such as gene therapy with suicide genes, will need procedures for therapy planning and monitoring. Finally, new biomolecules will be developed by bioengineering methods, which may be used for the isotope-based diagnosis and treatment of disease.

Radionuclide imaging in the post-genomic era

The assessment of gene function, which follows the completion of human genome sequencing, may be performed using the tools of the genome program. These tools represent high-throughput methods evaluating changes in the expression of many or all genes of an organism at the same time in order to investigate genetic pathways for normal development and disease. They describe proteins on a proteome-wide scale, thereby, creating a new way of doing cell research which results in the determination of three dimensional protein structures and the description of protein networks. These descriptions may then be used for the design of new hypotheses and experiments in the traditional physiological, biochemical, and pharmacological sense. The evaluation of genetically manipulated animals or new designed biomolecules will require a thorough understanding of physiology, biochemistry, and pharmacology and the experimental approaches will involve many new technologies including in vivo imaging with SPECT and positron emission tomography (PET). Nuclear medicine procedures may be applied for the determination of gene function and regulation using established and new tracers or using in vivo reporter genes such as genes encoding enzymes, receptors, antigens, or transporters. Pharmacogenomics will identify new surrogate markers for therapy monitoring which may represent potential new tracers for imaging. Also drug distribution studies for new therapeutic biomolecules are needed at least during preclinical stages of drug development. Finally, new biomolecules will be developed by bioengineering methods, which may be used for isotope-based diagnosis and treatment of disease.

Functional genomics and radioisotope-based imaging procedures

After the sequencing of the human genome has been completed, non-invasive imaging studies are needed to assess the function of new genes in living organisms. The evaluation of genetically manipulated animals or new designed biomolecules will require a thorough understanding of physiology, biochemistry and pharmacology, and the experimental approaches will involve many new technologies including in vivo imaging with single photon emission computed tomography (SPECT) and positron emission tomography (PET). Nuclear medicine procedures may be applied for the determination of gene function and regulation using established and new tracers or using in vivo reporter genes such as enzymes, receptors, antigens or transporters. Pharmacogenomics will identify new surrogate markers for therapy monitoring which may represent potential new tracers for imaging. Also, drug distribution studies for new therapeutic biomolecules are needed at least during preclinical stages of drug development. Clinical gene therapy needs non-invasive tools to evaluate the efficiency of gene transfer. These informations can be used for therapy planning, follow-up studies in treated tumors and as an indicator of prognosis. Therapy planning is performed by the assessment of gene expression for example using radio-labeled specific substrates to determine the activity of suicide enzymes such as the Herpes Simplex Virus thymidine kinase. Follow-up studies with single photon emission tomography or positron emission tomography may be done to evaluate early or late effects of gene therapy on tumor metabolism or proliferation. Finally, new biomolecules will be developed by bioengineering methods which may be used for isotope-based diagnosis and treatment of disease.

Engineering of technetium-99m-binding artificial receptors for imaging gene expression

BACKGROUND

Optimization of gene therapy protocols requires accurate and non-invasive quantification of vector delivery and gene expression. To facilitate non-invasive imaging of gene expression, we have genetically engineered 'artificial receptors', i.e. membrane proteins that bind (99m)Tc-oxotechnetate ((99m)TcOT) via transchelation from a complex with glucoheptonate. The latter is a component of a widely used clinical imaging kit.

METHODS

The engineered marker proteins were designed as type I and II membrane proteins and consisted of (1) an (99m)TcOT-binding domain, metallothionein (MT), and (2) a membrane-anchoring domain. Engineered constructs were used for transfection of COS-1 and 293 cells; the expression of mRNA was verified by RT-PCR.

RESULTS

Immunofluorescent analysis, cell fractionation and immunoblotting revealed expression of marker proteins on plasma membrane. Transfection of cells resulted in strong positive staining of plasma membrane with anti-His-tag antibodies. Scintigraphic imaging in vitro confirmed the ability of transfected cells to bind (99m)TcOT. The fraction of bound radioactivity reached a peak (3.53%) when 0.93 MBq (99m)TcOT was added to transfected COS-1 cells. The experiment-to-control signal ratio was equal to 32 at the same added dose.

CONCLUSIONS

(1) Both types of engineered 'artificial receptors' were expressed on the surface of eukaryotic cells; (2) marker proteins were functional in binding (99m)TcOT; and (3) type II membrane proteins were more efficient in binding (99m)TcOT than type I proteins. We anticipate that the developed approach could be useful for 'tagging' transfected cells with (99m)TcOT enabling imaging of tracking in vivo transduced cells or cell therapies.

Trafficking of lysosomal cathepsin B-green fluorescent protein to the surface of thyroid epithelial cells involves the endosomal/lysosomal compartment

Cathepsin B, a lysosomal cysteine proteinase, is involved in limited proteolysis of thyroglobulin with thyroxine liberation at the apical surface of thyroid epithelial cells. To analyze the trafficking of lysosomal enzymes to extracellular locations of thyroid epithelial cells, we have expressed a chimeric protein consisting of rat cathepsin B and green fluorescent protein. Heterologous expression in CHO cells validated the integrity of the structural motifs of the chimeric protein for targeting to endocytic compartments. Homologous expression, colocalization and transport experiments with rat thyroid epithelial cell lines FRT or FRTL-5 demonstrated the correct sorting of the chimeric protein into the lumen of the endoplasmic reticulum, and its subsequent transport via the Golgi apparatus and the trans-Golgi network to endosomes and lysosomes. In addition, the chimeras were secreted as active enzymes from FRTL-5 cells in a thyroid-stimulating-hormone-dependent manner. Immunoprecipitation experiments after pulse-chase radiolabeling showed that secreted chimeras lacked the propeptide of cathepsin B. Thus, the results suggest that cathepsin B is first transported to endosomes/lysosomes from where its matured form is retrieved before being secreted, supporting the view that endosome/lysosome-derived cathepsin B contributes to the potential of extracellular proteolysis in the thyroid.

Balancing GFP reporter plasmid quantity in large-scale transient transfections for recombinant anti-human Rhesus-D IgG1 synthesis

Using transient expression, high amounts (>20 mg/mL) of secreted anti-human Rhesus-D IgG1 were produced in a suspension-adapted HEK293 EBNA cell line (Meissner et al., Biotechnol Bioeng 75: 197-203, 2001). Time of harvest was 3 days after transfection. For the estimation of transfection efficiencies, we routinely co-transfected EGFP reporter DNA. At higher reporter plasmid concentrations, >2% of total transfecting plasmid DNA, a substantial reduction of recombinant antibody synthesis, was observed. This phenomenon was investigated in detail by co-expressing various green fluorescent protein (GFP) reporter constructs, which were targeted at different subcellular locations. Enhanced and humanized GFPs targeted to either the endoplasmic reticulum, the cytosol, or the nucleus reduced recombinant antibody production by 30 to 40% when present at higher concentrations in the transfection solution. The most severe effects were observed when the co-transfected EGFP was targeted to the endoplasmic reticulum, leading to a reduction of up to 80% in the presence of only 5% of reporter DNA. Interestingly, one nuclear-targeted GFP variant that was not codon optimized for expression in human cell lines could be added, to up to almost half of the total amount of transfecting DNA, without adverse effect on antibody production. Although the minimum amount of this reporter DNA needed for fluorescence reading was 10 times higher than for the other variants, it provided a much broader quantity range within which the transfection process could be studied without being negatively affected.

Imaging Techniques for Gene Therapy: SPECT, PET, and MRI

Gene therapy by the transfer and expression of suicide genes is performed using genes coding for nonmammalian enzymes that transform nontoxic prodrugs into toxic metabolites. Employing radiolabeled specific substrates and scintigraphic procedures to determine the functional activity of the recombinant enzyme in vivo, a therapeutic window of maximal gene expression and consecutive drug administration may be defined. If the gene therapy approach is based on the transduction of receptor genes, the recombinant gene expression in tumor cells can be monitored with radiolabeled ligands. Transfer of transporter genes as the sodium iodide transporter may also lead to the visualization of transduction via accumulation of iodide or pertechnetate. Furthermore, imaging based on transchelation of oxotechnetate to a polypeptide motif from a biocompatible complex with a higher dissociation constant than that of a diglycilcysteine complex or tyrosinase gene transfer for metal ion scavenging have been described. In addition, therapy effects may be assessed by the evaluation of the morphological changes of the tumor using magnetic resonance imaging or, more effectively, by the measurement of changes in metabolism with positron emission tomography employing tracers of tumor metabolism and proliferation. Finally, enzyme or receptor genes may serve as noninvasive reporter genes, if applied in the context of bicistronic vectors leading to coexpression of the therapeutic gene and the noninvasive reporter gene.

DNA binding chelates for nonviral gene delivery imaging

Noninvasive in vivo monitoring of gene delivery would provide a critically important information regarding the spatial distribution, local concentration, kinetics of removal and/or biodegradation of the expression vector. We developed a novel approach to noninvasive gene delivery imaging using heterobifunctional peptide-based chelates (PBC) bearing double-stranded DNA-binding groups and a technetium-binding amino acid motif. One of such chelates: Gly-Cys(Acm)-Gly-Cys(Acm)-Gly-Lys(4)-Lys-(N-epsilon-[4-(psoralen-8-yloxy)]butyrate)-NH(2) has been characterized and labeled with reduced (99m)Tc pertechnetate (oxotechnetate). The psoralen moiety (a DNA binding group of PBC) allowed linking to double-stranded DNA upon short-term irradiation with the near UV range light (>320 nm). Approximately 30-40% of added (99m)Tc-labeled PBC was nonextractable and co-eluted with a model pCMV-GFP vector during the gel-permeation chromatography. Nuclear imaging of "naked" DNA and DNA complexes with lipid-based transfection reagents ("lipoplexes") has been performed after systemic or local administration of (99m)Tc-PBC-labeled DNA in mice. Imaging results were corroborated with the biodistribution using (99m)Tc-PBC and (32)P-labeled DNA and lipoplexes. A markedly different biodistribution of (99m)Tc PBC-labeled DNA and lipoplexes was observed with the latter being rapidly trapped in the liver, spleen and lung. (99m)Tc PBC-DNA was used as an imaging tracer during in vivo transfection of B16 melanoma by local injection of "naked" (99m)Tc PBC-DNA and corresponding lipoplexes. As demonstrated by nuclear imaging, (99m)Tc PBC-DNA lipoplexes showed a slower elimination from the site of injection than (99m)Tc PBC-DNA alone. This result correlated with a higher expression of marker mRNA and green fluorescent protein as determined using RT-PCR and immunohistochemistry, respectively.

Display of green fluorescent protein on Escherichia coli cell surface

In this study, expression of green fluorescence protein (GFP) on the external surface of Escherichia coli was achieved by construction of a fusion protein between Lpp-OmpA hybrid and a GFP variant, GFPmut2. The GFP was fused in frame to the carboxyl-terminus of Lpp-OmpA fusion previously shown to direct various other heterologous proteins to E. coli cell surface. Western blot analysis of membrane fractions identified the Lpp-OmpA-GFP fusion protein with the expected size (43 kDa). Immunofluorescence microscopy, immunoelectron microscopy, protease and extracellular pH sensitivity assays further confirmed that GFP is anchored on the outer membrane. The GFP displayed on the E. coli outer surface retained its fluorescence and was not susceptible to the indigenous outer membrane protease OmpT even though there are two putative OmpT proteolytic sites present in GFP. Optimization of the expression conditions was conducted using fluorometry, eliminating cumbersome immuno-labeling procedures. Surface-displayed GFP could be used in a variety of applications including screening of polypeptide libraries, development of live vaccines, construction of whole cell allosteric biosensors, and signal transduction studies.

In vivo imaging of gene expression:

With the ability to readily engineer genes, create knock-in and knock-out models of human disease, and replace and insert genes in clinical trials of gene therapy, it has become clear that imaging will play a critical role in these fields. Imaging is particularly helpful in recording temporal and spatial resolution of gene expression in vivo, determining vector distribution, and, ultimately, understanding endogenous gene expression during disease development. While endeavors are under way to image targets ranging from DNA to entire phenotypes in vivo, this short review focuses on in vivo imaging of gene expression with magnetic resonance and optical techniques.

New approaches for imaging in gene therapy

Gene therapy is increasingly used experimentally and clinically to replace defective genes and/or impart new functions to cells and tissues. With the recent advances in vector design, improvements in transgene and prodrug activation strategies, gene therapy has been applied to a wide variety of diseases, tissues and organ systems. It is now clear that our specialty will play a critical role in gene therapy research and its clinical applications. Three aspects of gene therapy are of particular interest to imaging. The first is in delivering genes and vector products by minimally invasive interventional techniques. The second is in quantitating gene and DNA deliveries, for example, by nuclear imaging. Finally, imaging can be used to monitor the levels of transgene expression in vivo. A variety of imaging techniques including PET imaging, nuclear imaging, MR imaging and optical imaging can potentially be used to achieve the latter. This brief introductory overview is intended to summarize current strategies and illustrate the role that radiology will play in this field.