The use of chimeric gene constructs to express a bacterial endoglucanase in mammalian cells

Comparison of promoter region constructs for in vivo intramuscular expression

BACKGROUND

High transgene expression is generally expected after gene transfer. However, different level, kinetics and localization of expression might be needed for relevant therapeutic applications. Former studies have compared various promoter regions driving gene expression leading to conflicting results. In the present work, two promoter families have been compared using the efficient in vivo intramuscular electrotransfer technique.

METHODS

Three promoter regions were constructed by associating the strong ubiquitous cytomegalovirus (CMV) enhancer-promoter to its homologous intron A or to a heterologous intron, or to a hybrid intron. Promoter regions derived from the muscle creatine kinase (MCK) promoter were also studied. The expression of the same transgene (SeAP or neurotrophin-3) under control of these different promoters was compared after plasmid electrotransfer in mouse tibialis-cranialis skeletal muscle.

RESULTS

Heterologous intron association to the CMV promoter did not modify gene expression kinetics nor increase gene expression level. Usefulness of intron A or hybrid intron association to the CMV promoter depended on the gene. The various MCK promoters drove efficient gene expression but lower than that obtained with the CMV promoter. Furthermore, peak value was reached earlier with MCK promoter regions (14 days).

CONCLUSION

For applications of gene transfer restricted to skeletal muscle, the MCK promoter or a MCK promoter variant would be a promising alternative to the CMV promoter. Indeed, it has been demonstrated that the use of MCK promoter limits humoral and cell-mediated immune responses. Furthermore, the MCK promoter decreases the initial expression peak that may be detrimental, drives a sustained gene expression, and improves gene transfer safety.

Adenovirus-mediated transfer of a minigene expressing multiple isoforms of VEGF is more effective at inducing angiogenesis than comparable vectors expressing individual VEGF cDNAs

To assess the hypothesis that angiogenic gene therapy with the genomic form of vascular endothelial growth factor (VEGF) expressing the three major isoforms could be more potent than a vector expressing a single isoform, we designed an adenovirus vector (AdVEGF-All) expressing a VEGF cDNA/genomic hybrid gene. AdVEGF-All expressed all three major isoforms (121, 165, 189) in a 2:2:1 ratio. AdVEGF-All was 100-fold more potent than cDNA vectors expressing VEGF 121, 165, or 189 in restoring blood flow to the ischemic mouse hind limb. Interestingly, a mixture of Ad vectors individually expressing the VEGF 121, 165, and 189 cDNAs was equipotent to an equivalent dose of AdVEGF-All. Thus, a mixture of VEGF isoforms provides a more potent angiogenic response than a single isoform, suggesting that the individual isoforms function synergistically, an observation with important implications for gene and recombinant protein therapy.

Vectors for gene expression in mammalian cells

The achievement of robust and regulated protein production in mammalian cells is a complex process that requires careful consideration of many factors, including transcriptional and translational control elements, RNA processing, gene copy number, mRNA stability, the chromosomal site of gene integration, potential toxicity of recombinant proteins to the host cell, and the genetic properties of the host. Gene transfer into mammalian cells may be effected either by infection with virus that carries the recombinant gene of interest, or by direct transfer of plasmid DNA. This chapter discusses the molecular architecture of non-viral vectors for high-level protein production. Virus-based vectors for gene therapy, protein production, vaccine development and other applications are summarized in a table and described.

RNA splicing of bacterial genes in eukaryotes

The presence of intervening sequences or introns in eukaryotic genes has been known for more than 20 years, and the mechanisms underlying RNA splicing have been studied in depth both genetically and biochemically. In recent years, however, an increasing number of bacterial genes have been introduced into higher eukaryotes as important tools for genetic studies. Their gene products are frequently used as an indirect measure for cell type-specific promoter activity, as, for example, in the case of chloramphenicol acetyl transferase (CAT assay) or beta-galactosidase. Here we show that RNA splicing of two prokaryotic genes encoding site-specific DNA recombinases occurs in eukaryotic cells. In one case, splicing is only observed after treatment of cells with the cytokine alpha interferon. We further demonstrate that mutating an intragenic donor splice site in a bacterial gene apparently activates a second, alternative splicing pathway. In conjunction with previous reports, our findings should also be regarded as a warning that splicing of bacterial genes in higher eukaryotes is a more common phenomenon than presently recognized, which may be difficult to overcome and may cause problems in the interpretation of experimental results.

Expression of a bacterial endo (1-4)-beta-glucanase gene in mammalian cells and post translational modification of the gene product

An endo (1-4)-beta-glucanase gene C6.5 from Bacillus subtilis has been expressed in Chinese hamster ovary (CHO) cells and pancreatic 266-6 cells. The fusion gene, stably transfected into CHO cells consisted of the mouse Amy-2.2 signal peptide coding sequence and the endoglucanase gene C6.5 transcribed from the early SV40 promoter/enhancer, using the dihydrofolate reductase gene as a selective marker. The gene construct transfected into pancreatic 266-6 cells consisted of the mouse Amy-2.2 promoter/enhancer and signal peptide coding sequence and the same C6.5 sequences using the xanthine-guanine phosphoribosyl transferase gene (gpt) as the selective marker. The stably transfected CHO cells synthesized endoglucanase at 1.1 U/mg cell protein in a 72 h culture, with 89% of the activity secreted into the culture fluid in a glycosylated form of 66 kDa as compared with the unglycosylated 53 kDa form expressed in E. coli. Glycosylation did not change the specific activity, protease resistance, or cellulose binding of the endoglucanase as compared to the unglycosylated form of the enzyme from E. coli. The level of expression in the stably transfected pancreatic cells was substantially lower at 0.3 mU/mg cell protein with all detectable activity present in the culture fluid. The secreted enzyme from pancreatic cells was glycosylated with a mass similar to that secreted from CHO cells.

Evidence for a general role for non-catalytic thermostabilizing domains in xylanases from thermophilic bacteria

A genomic library of Clostridium thermocellum DNA constructed in lambda ZAPII was screened for xylanase-expressing clones. Cross-hybridization experiments revealed a new xylanase gene isolated from the gene library, which was designated xyn Y. The encoded enzyme, xylanase Y (XYLY), displayed features characteristic of an endo-beta1,4-xylanase: the enzyme rapidly hydrolysed oat spelt, wheat and rye arabinoxylans and was active against methyl-umbelliferyl-beta-D-cellobioside, but did not hydrolyse any cellulosic substrates. The pH and temperature optima of the enzyme were 6.8 and 75 degrees C respectively, and the recombinant XYLY, expressed by Escherichia coli had a maximum Mr of 116000. The nucleotide sequence of xyn Y contained an open reading frame of 3228 bp encoding a protein of predicted Mr 120 105. The encoded enzyme contained a typical N-terminal 26-residue signal peptide, followed by a 164 amino acid sequence, designated domain A, that was not essential for catalytic activity. Downstream of domain A was a 351-residue xylanase Family F catalytic domain, followed by a 180-residue sequence that exhibited 28% sequence identity with a thermostable domain of Thermoanaerobacterium saccharolyticum xylanase A. The C-terminal portion of XYLY comprised the 23-residue duplicated docking sequence found in all other C. thermocellum plant cell wall hydrolases that are constituents of the bacterium's multienzyme complex, termed the cellulosome, followed by a 286-residue domain which exhibited 32% sequence identity with the N-terminal region of C. thermocellum xylanase Z. The enzyme did not contain linker sequences found in other C. thermocellum plant cell wall hydrolases. Analysis of truncated forms of XYLY and hybrid proteins, comprising segments of XYLY fused to the E. coli maltose binding domain, confirmed that XYLY contained a central catalytic domain and an adjacent thermostable domain. The C-terminal domain did not bind to cellulose or xylan. Western blot analysis using antiserum raised against XYLY showed that the xylanase was located in the cellulosome and did not appear to be extensively glycosylated. The non-catalytic domains of XYLY are discussed in relation to the general stability of thermophilic xylanases.

Manipulation of the repertoire of digestive enzymes secreted into the gastrointestinal tract of transgenic mice

In non-ruminant livestock the energy which can be derived from dietary cellulose and xylan is limited by the inefficient microbial fermentation of these polymers in the hind-gut. Furthermore, in poultry, cereal-derived plant structural polysaccharides impair normal digestive function through the formation of gel-like structures, which trap nutrients rendering them unavailable to the animal. The nutrition of non-ruminant livestock could be significantly improved by the depolymerization of plant structural polysaccharides, through the introduction of cellulase activity into the small intestines of these animals. Here we describe the expression of Clostridium thermocellum endoglucanase E in the exocrine pancreas of transgenic mice. A non-glycosylated active enzyme is secreted into the small intestines, and is resistant to proteolytic inactivation, demonstrating the feasibility of generating non-ruminant animals with the endogenous capacity to depolymerize plant structural polysaccharides in the small intestines.