A molecular biological approach to reducing dietary amino acid needs

Expression of threonine-biosynthetic genes in mammalian cells and transgenic mice

Threonine is a nutritionally essential amino acid (EAA) for the growth and development of humans and other nonruminant animals and must be provided in diets to sustain life. The aim of this study was to synthesize threonine in mammalian cells through transgenic techniques. To achieve this goal, we combined the genes involved in bacterial threonine biosynthesis pathways into a single open reading frame separated by self-cleaving peptides (2A) and then linked it into a transposon system (piggyBac). The plasmids pEF1a-IRES-GFP-E2F-his and pEF1a-IRES-GFP-M2F-his expressed Escherichia coli homoserine kinase and threonine synthase efficiently in mouse cells and enabled cells to synthesize threonine from homoserine. This biosynthetic pathway occurred with a low level of efficiency in transgenic mice. Three transgenic mice were identified by Southern blot from 72 newborn mice, raising the possibility that a high level of expression of these genes in mouse embryos might be lethal. The results indicated that it is feasible to synthesize threonine in animal cells using genetic engineering technology. Further work is required to improve the efficiency of this method for introducing genes into mammals. We propose that the transgenic technology provides a promising means to enhance the synthesis of nutritionally EAAs in farm animals and to eliminate or reduce supplementation of these nutrients in diets for livestock, poultry and fish.

Transgene-mediated modifications to animal biochemistry

The application of gene-transfer technology to domestic animals provides a way for the introduction of genes encoding biochemical pathways that are currently nonfunctional in these animals. This might provide a mechanism for increasing the availability of specific substrates that currently limit certain production characteristics, such as the production of wool. The progress and problems associated with recent attempts to transfer a cysteine biosynthetic pathway and a glyoxylate cycle to sheep are discussed, in addition to the extension of this concept to other biochemical pathways.

The commercial and agricultural applications of animal transgenesis

The potential for commercial application of transgenic technologies in domestic animals is discussed in relation to the areas where a significant impact on agriculture might be expected. These are the endocrine system, novel biochemical pathways, structural proteins of milk and of textile fibers, and the immune system. Manipulation of the endocrine system has been investigated for some years and it is clear that very accurate control is needed over gene expression if this approach is to prove commercially useful. The area most advanced in commercial application is the production of high-value pharmaceutical proteins in the mammary glands of domestic animals. Other applications that are discussed remain to be proven in larger animals despite being demonstrated laboratory test animals. These include a functional cysteine biosynthetic pathway and a functional glyoxylate cycle transferred from bacteria to mice, and alterations to the proteins of hair that change the physical properties of the resultant fibers. Research is also actively directed toward novel approaches for providing domestic animals with resistance to insects.

The biosynthesis of threonine by mammalian cells: expression of a complete bacterial biosynthetic pathway in an animal cell

The coding regions for the Escherichia coli gene for aspartokinase I/homoserine dehydrogenase I (thrA) and the Corynebacterium glutamicum gene for aspartic semialdehyde dehydrogenase (asd) have been subcloned into a Simian Virus 40 (SV40)-based mammalian expression vector. Both enzyme activities are expressed in mouse 3T3 cells after transfer of the corresponding chimaeric gene. The kinetic parameters are similar to those of the native bacterial enzymes, and aspartokinase I/homoserine dehydrogenase I retains its allosteric regulation by threonine. An extract of the cells expressing aspartokinase I/homoserine dehydrogenase I, mixed with one from cells expressing aspartic semialdehyde dehydrogenase, produced homoserine when the mixture was incubated with aspartic acid, ATP and NADPH. The thrA and asd expression cassettes were combined into a single plasmid which, when transfected into 3T3 cells, enabled them to produce homoserine from aspartic acid. Homoserine-producing 3T3 cells were transfected with the plasmid pSVthrB/C (homoserine kinase and threonine synthase) and selected for growth on homoserine. Cell lines isolated from these cells expressed the complete bacterial threonine pathway, were independent of threonine for growth and could be maintained in medium which contained no free threonine. The threonine in the proteins of these cells became enriched in 15N when the culture medium contained [15N]aspartic acid. The production of homoserine and the growth of cells was at a maximum when there was more than 2.5 mM aspartate in the medium. Below this concentration the high Km of aspartokinase limited the flux through the pathway. In the presence of additional aspartic acid the new pathway could sustain a cell cycle time close to that of the same cells cultured in threonine-containing medium.

Prospects for the genetic manipulation of dairy cattle: opportunities beyond BST

The dairy industry, with regulatory approvals of recombinant chymosin and bovine somatotropin (BST), has been at the forefront of food and agricultural biotechnology. The commercial fate of these products is one of several factors that may affect the success of future genetic manipulations in dairy cattle and dairy products. Other factors include technical and reproductive constraints in cattle and the cost of producing transgenic cattle. Early applications of genetic manipulation in cattle, for reasons of cost recoupment, may favor production of heterologous proteins in milk for pharmaceutical and medical use. Such applications could benefit genetic modification of milk and milk proteins for food use by providing advance knowledge and experience in mammalian protein expression. Other research opportunity areas that could affect prospects for genetic manipulation of dairy cattle include genome mapping, metabolic pathways, growth and development, and cattle/microbe interactions.

The expression of Escherichia coli diaminopimelate decarboxylase in mouse 3T3 cells

We have subcloned the coding sequence for the Escherichia coli lysA gene coding for diaminopimelic acid decarboxylase (DAP decarboxylase) into a eukaryotic expression vector based on the SV40 early promoter. The activities of a series of constructs with different lengths of non-coding DNA at the 5' and 3' ends of the coding region have been compared by measuring the synthesis of lysine from diaminopimelic acid (DAP) in mouse 3T3 cells. A short non-coding sequence at the 3' end reduced the expression of enzyme activity. Stable lines of 3T3 cells have been produced by co-transfection of the chimeric gene with a plasmid coding for G-418 resistance. Cells were grown in medium containing G-418 and resistant clones were screened for an ability to synthesise lysine from DAP. [3H]Lysine produced from [3H]DAP was incorporated into cell proteins. An enzyme extract from a cell line which had incorporated two copies of the gene synthesised 0.082 nmol of lysine/min per mg protein. In the intact cell the rate of lysine synthesis is limited by the uptake of DAP which is taken up at only 5% of the rate of lysine. lysA has a potential as a reporter gene in studies of gene expression in mammalian cells.

The expression of Escherichia coli threonine synthase and the production of threonine from homoserine in mouse 3T3 cells

We have subcloned the coding sequence for the Escherichia coli threonine synthase gene into a eukaryotic expression vector based on the simian-virus-40 early promoter. When mouse 3T3 cells which already expressed homoserine kinase were transfected with the new plasmid, the cells were able to incorporate radioactivity from [14C]homoserine into their cell proteins. Stable cell lines were established by co-transfecting 3T3 cells with the plasmid coding for threonine synthase and another coding for homoserine kinase and G-418 (Geneticin) resistance. Cells were selected for G-418 resistance and then screened for an ability to synthesize threonine from homoserine and incorporate it into the cell protein. A cell line which expressed both the homoserine kinase and threonine synthase genes was capable of growth in a threonine-deficient medium containing homoserine.

Expression of Escherichia coli homoserine kinase in mouse 3T3 cells

The Escherichia coli gene for homoserine kinase (thrB) has been cloned into a simian-virus-40-based eukaryotic expression vector which also includes a neomycin-resistance gene. Mouse 3T3 cells transfected with this plasmid were selected for resistance and screened for homoserine kinase activity. It has thus been possible to isolate clones which are capable of accumulating homoserine O-phosphate when supplied with homoserine. In broken-cell preparations the kinetic constants for the production of homoserine O-phosphate were similar to those of the wild-type E. coli enzyme. These experiments demonstrate that E. coli homoserine kinase can be expressed in an animal cell and that it can successfully phosphorylate L-homoserine in the intact cell utilizing endogenous ATP.