Threonine synthesis from homoserine as a selectable marker in mammalian cells

Toxic effect and inability of L-homoserine to be a nitrogen source for growth of Escherichia coli resolved by a combination of in vivo evolution engineering and omics analyses

L-homoserine is a pivotal intermediate in the carbon and nitrogen metabolism of E. coli. However, this non-canonical amino acid cannot be used as a nitrogen source for growth. Furthermore, growth of this bacterium in a synthetic media is potently inhibited by L-homoserine. To understand this dual effect, an adapted laboratory evolution (ALE) was applied, which allowed the isolation of a strain able to grow with L-homoserine as the nitrogen source and was, at the same time, desensitized to growth inhibition by this amino acid. Sequencing of this evolved strain identified only four genomic modifications, including a 49 bp truncation starting from the stop codon of thrL. This mutation resulted in a modified thrL locus carrying a thrL* allele encoding a polypeptide 9 amino acids longer than the thrL encoded leader peptide. Remarkably, the replacement of thrL with thrL* in the original strain MG1655 alleviated L-homoserine inhibition to the same extent as strain 4E, but did not allow growth with this amino acid as a nitrogen source. The loss of L-homoserine toxic effect could be explained by the rapid conversion of L-homoserine into threonine via the thrL*-dependent transcriptional activation of the threonine operon thrABC. On the other hand, the growth of E. coli on a mineral medium with L-homoserine required an activation of the threonine degradation pathway II and glycine cleavage system, resulting in the release of ammonium ions that were likely recaptured by NAD(P)-dependent glutamate dehydrogenase. To infer about the direct molecular targets of L-homoserine toxicity, a transcriptomic analysis of wild-type MG1655 in the presence of 10 mM L-homoserine was performed, which notably identified a potent repression of locomotion-motility-chemotaxis process and of branched-chain amino acids synthesis. Since the magnitude of these effects was lower in a ΔthrL mutant, concomitant with a twofold lower sensitivity of this mutant to L-homoserine, it could be argued that growth inhibition by L-homoserine is due to the repression of these biological processes. In addition, L-homoserine induced a strong upregulation of genes in the sulfate reductive assimilation pathway, including those encoding its transport. How this non-canonical amino acid triggers these transcriptomic changes is discussed.

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.

Homoserine toxicity in Saccharomyces cerevisiae and Candida albicans homoserine kinase (thr1Delta) mutants

In addition to threonine auxotrophy, mutation of the Saccharomyces cerevisiae threonine biosynthetic genes THR1 (encoding homoserine kinase) and THR4 (encoding threonine synthase) results in a plethora of other phenotypes. We investigated the basis for these other phenotypes and found that they are dependent on the toxic biosynthetic intermediate homoserine. Moreover, homoserine is also toxic for Candida albicans thr1Delta mutants. Since increasing levels of threonine, but not other amino acids, overcome the homoserine toxicity of thr1Delta mutants, homoserine may act as a toxic threonine analog. Homoserine-mediated lethality of thr1Delta mutants is blocked by cycloheximide, consistent with a role for protein synthesis in this lethality. We identified various proteasome and ubiquitin pathway components that either when mutated or present in high copy numbers suppressed the thr1Delta mutant homoserine toxicity. Since the doa4Delta and proteasome mutants identified have reduced ubiquitin- and/or proteasome-mediated proteolysis, the degradation of a particular protein or subset of proteins likely contributes to homoserine toxicity.

Fungal homoserine kinase (thr1Delta) mutants are attenuated in virulence and die rapidly upon threonine starvation and serum incubation

The fungally conserved subset of amino acid biosynthetic enzymes not present in humans offer exciting potential as an unexploited class of antifungal drug targets. Since threonine biosynthesis is essential in Cryptococcus neoformans, we further explored the potential of threonine biosynthetic enzymes as antifungal drug targets by determining the survival in mice of Saccharomyces cerevisiae homoserine kinase (thr1Delta) and threonine synthase (thr4Delta) mutants. In striking contrast to aspartate kinase (hom3Delta) mutants, S. cerevisiae thr1Delta and thr4Delta mutants were severely depleted after only 4 h in vivo. Similarly, Candida albicans thr1Delta mutants, but not hom3Delta mutants, were significantly attenuated in virulence. Consistent with the in vivo phenotypes, S. cerevisiae thr1Delta and thr4Delta mutants as well as C. albicans thr1Delta mutants were extremely serum sensitive. In both species, serum sensitivity was suppressed by the addition of threonine, a feedback inhibitor of Hom3p. Because mutation of the HOM3 and HOM6 genes, required for the production of the toxic pathway intermediate homoserine, also suppressed serum sensitivity, we hypothesize that serum sensitivity is a consequence of homoserine accumulation. Serum survival is critical for dissemination, an important virulence determinant: thus, together with the essential nature of C. neoformans threonine synthesis, the cross-species serum sensitivity of thr1Delta mutants makes the fungus-specific Thr1p, and likely Thr4p, ideal antifungal drug targets.

Threonine biosynthetic genes are essential in Cryptococcus neoformans

We identified and attempted to disrupt the Cryptococcus neoformans homoserine and/or threonine biosynthetic genes encoding aspartate kinase (HOM3), homoserine kinase (THR1) and threonine synthase (THR4); however, each gene proved recalcitrant to disruption. By replacing the endogenous promoters of HOM3 and THR1 with the copper-repressible CTR4-1 promoter, we showed that HOM3 and THR1 were essential for the growth of C. neoformans in rich media, when ammonium was the nitrogen source, or when threonine was supplied as an amino acid instead of a dipeptide. Moreover, the severity of the growth defect associated with HOM3 or THR1 repression increased with increasing incubation temperature. We believe this to be the first demonstration of threonine biosynthetic genes being essential in a fungus. The necessity of these genes for C. neoformans growth, particularly at physiologically relevant temperatures, makes threonine biosynthetic genes ideal anti-cryptococcal drug targets.

L-Homoserine: a novel excreted metabolic marker of hepatitis B abnormally produced in liver from methionine

The hepatitis B infection leads to various profound pathological processes in liver metabolism. Some biochemical alterations detectable by blood analysis are currently used for a preliminary evaluation of the infection. Based on existing data we present here evidence that non-protein amino acid L-homoserine is a pathological, hepatitis B-induced metabolite that is formed and excreted into urine from methionine via splitting S-adenosylmethionine. The urine L-homoserine is proposed as a new marker in the pre-diagnosis examinations that is easier for the clinical analysis than currently used blood test, and is applicable to large-scale epidemiological surveys of the probability of hepatitis B.

Maternal protein deficiency causes hypermethylation of DNA in the livers of rat fetuses

Maternal protein deficiency during pregnancy is associated with changes in glucose tolerance and hypertension in the offspring of rats. In this study the growth of rat fetuses was examined when the dams were fed diets containing 18% casein, 9% casein or 8% casein supplemented with threonine. The extra threonine was added to reverse the decrease in circulating threonine concentrations that occurs when pregnant rats are fed protein-deficient diets. The fetuses of the group fed the low protein diet supplemented with threonine were significantly smaller than those of the control group and not significantly different from those fed low protein. Homogenates prepared from the livers of dams fed the diet containing 9% casein oxidized threonine at approximately twice the rate of homogenates prepared from dams fed the diet containing 18% casein. We conclude that circulating levels of threonine fall as a consequence of an increase in the activity of the pathway that metabolizes homocysteine produced by the transulfuration of methionine. Serum homocysteine was unaffected in the dams fed low protein diets compared with controls, but was significantly greater in dams fed the low protein diet supplemented with threonine. Elevated levels of homocysteine are associated with changes in the methylation of DNA. The endogenous methylation of DNA was greater than that of controls in the livers of fetuses from dams fed the 9% protein diets and increased further when the diet was supplemented with threonine. Our results suggest that changes in methionine metabolism increase homocysteine production, which leads to changes in DNA methylation in the fetus. An increase in maternal homocysteine may compromise fetal development, leading to the onset of glucose intolerance and hypertension in adult life.

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.

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.