Lactococcus lactis as a live vector: heterologous protein production and DNA delivery systems

Lactic acid bacteria (LAB), widely used in the food industry, are present in the intestine of most animals, including humans. The potential use of these bacteria as mucosal delivery vehicles for vaccinal, medical or technological use has been extensively investigated. Lactococcus lactis, a LAB species, is a potential candidate for the production of biologically useful proteins and for plasmid DNA delivery to eukaryotic cells. Several delivery systems have been developed to target heterologous proteins to a specific cell location (i.e., cytoplasm, cell wall or extracellular medium) and more recently to efficiently transfer DNA to eukaryotic cells. A promising application of L. lactis is its use for the development of live mucosal vaccines. Here, we have reviewed the expression of heterologous protein and the various delivery systems developed for L. lactis, as well as its use as an oral vaccine carrier.     

Heterologous expression of Brucella abortus GroEL heat-shock protein in Lactococcus lactis

Background  

Brucella abortus is a facultative intracellular pathogen that mainly infects cattle and humans. Current vaccines rely on live attenuated strains of B. abortus, which can revert to their pathogenic status and thus are not totally safe for use in humans. Therefore, the development of mucosal live vaccines using the food-grade lactic acid bacterium, Lactococcus lactis, as an antigen delivery vector, is an attractive alternative and a safer vaccination strategy against B. abortus. Here, we report the construction of L. lactis strains genetically modified to produce B. abortus GroEL heat-shock protein, a candidate antigen, in two cellular locations, intracellular or secreted.     

Heterologous protein production in the yeast Kluyveromyces lactis

Kluyveromyces lactis is both scientifically and biotechnologically one of the most important non-Saccharomyces yeasts. Its biotechnological significance builds on its history of safe use in the food industry and its well-known ability to produce enzymes like lactase and bovine chymosin on an industrial scale. In this article, we review the various strains, genetic techniques and molecular tools currently available for the use of K. lactis as a host for protein expression. Additionally, we present data illustrating the recent use of proteomics studies to identify cellular bottlenecks that impede heterologous protein expression.     

Controlled Production of Stable Heterologous Proteins in Lactococcus lactis

The use of Lactococcus lactis (the most extensively characterized lactic acid bacterium) as a delivery organism for heterologous proteins is, in some cases, limited by low production levels and poor-quality products due to surface proteolysis. In this study, we combined in one L. lactis strain use of the nisin-inducible promoter P_nisA and inactivation of the extracellular housekeeping protease HtrA. The ability of the mutant strain, designated htrA-NZ9000, to produce high levels of stable proteins was confirmed by using the staphylococcal nuclease (Nuc) and the following four heterologous proteins fused or not fused to Nuc that were initially unstable in wild-type L. lactis strains: (i) Staphylococcus hyicus lipase, (ii) the bovine rotavirus antigen nonstructural protein 4, (iii) human papillomavirus antigen E7, and (iv) Brucella abortus antigen L7/L12. In all cases, protein degradation was significantly lower in strain htrA-NZ9000, demonstrating the usefulness of this strain for stable heterologous protein production.     

Heterologous protein secretion in Lactococcus lactis is enhanced by the Bacillus subtilis chaperone-like protein PrsA

The Bacillus subtilis lipoprotein PrsA enhances the yield of several homologous and heterologous exported proteins in B. subtilis by being involved in the posttranslocational stage of the secretion process. In this work, we have studied the effect of B. subtilis PrsA on the secretion of Bacillus amyloliquefaciens α-amylase (AmyQ), a target protein for PrsA, and Bacillus licheniformis penicillinase (PenP) a nontarget protein for PrsA, in Lactococcus lactis. Two compatible plasmids were constructed and introduced into L. lactis strain NZ9000: one high copy plasmid, expressing the AmyQ gene (amyQ) or the PenP gene (penP), and one low copy plasmid, expressing the PrsA encoding gene (prsA). When amyQ and prsA were simultaneously expressed under the nisin-inducible promoter P _nisA , Western blotting experiments revealed a 15- to 20-fold increase in the total yield of AmyQ and a sixfold increase in secreted AmyQ activity, compared to a control strain lacking prsA. When expressed under the same induction conditions, PrsA had no effect on the secretion or total yield of PenP. These results show that the secretion yield of some heterologous proteins can be significantly increased in L. lactis when coproduced with the B. subtilis PrsA protein.     

Signal Peptide and Propeptide Optimization for Heterologous Protein Secretion in Lactococcus lactis

Lactic acid bacteria are food-grade microorganisms that are potentially good candidates for production of heterologous proteins of therapeutical or technological interest. We developed a model for heterologous protein secretion in Lactococcus lactis using the staphylococcal nuclease (Nuc). The effects on protein secretion of alterations in either (i) signal peptide or (ii) propeptide sequences were examined. (i) Replacement of the native Nuc signal peptide (SP_Nuc) by that of L. lactis protein Usp45 (SP_Usp) resulted in greatly improved secretion efficiency (SE). Pulse-chase experiments showed that Nuc secretion kinetics was better when directed by SP_Usp than when directed by SP_Nuc. This SP_Usp effect on Nuc secretion is not due to a better antifolding activity, since SP_Usp:Nuc precursor proteins display enzymatic activity in vitro, while SP_Nuc:Nuc precursor proteins do not. (ii) Deletion of the native Nuc propeptide dramatically reduces Nuc SE, regardless of which SP is used. We previously reported that a synthetic propeptide, LEISSTCDA, could efficiently replace the native Nuc propeptide to promote heterologous protein secretion in L. lactis (Y. Le Loir, A. Gruss, S. D. Ehrlich, and P. Langella, J. Bacteriol. 180:1895–1903, 1998). To determine whether the LEISSTCDA effect is due to its acidic residues, specific substitutions were introduced, resulting in neutral or basic propeptides. Effects of these two new propeptides and of a different acidic synthetic propeptide were tested. Acidic and neutral propeptides were equally effective in enhancing Nuc SE and also increased Nuc yields. In contrast, the basic propeptide strongly reduced both SE and the quantity of secreted Nuc. We have shown that the combination of the native SP_Usp and a neutral or acidic synthetic propeptide leads to a significant improvement in SE and in the quantity of synthesized Nuc. These observations will be valuable in the production of heterologous proteins in L. lactis.     

Heterologous gene expression of bovine plasmin in Lactococcus lactis

Heterologous production of bovine plasmin was studied in the industrially relevant bacterium Lactococcus lactis. Two sets of lactococcal gene expression signals were coupled to the region of the plasmin gene coding for the serine protease domain. When the promoter region of the prtP gene was used, plasmin was detected mainly intracellularly in strain BPL25 by Western blot hybridization. The intracellular presence of plasmin led to physiological stress. Expression of the plasmin gene driven by the promoter and complete signal sequence of the lactococcal usp45 gene resulted in efficient plasmin secretion in strain BPL420. Cell lysis was observed in strains producing plasmin fragments including the catalytic domain, but not in control strains, which only produced a non-catalytic region of plasmin. The plasmin produced was shown to be biologically active. Received: 2 December 1996 / Received revision: 17 March 1997 / Accepted: 27 April 1997     

Lactococcus lactis, an efficient cell factory for recombinant protein production and secretion

The use of Gram-positive bacteria for heterologous protein production proves to be a useful choice due to easy protein secretion and purification. The lactic acid bacterium Lactococcus lactis emerges as an attractive alternative to the Gram-positive model Bacillus subtilis. Here, we review recent work on the expression and secretion systems available for heterologous protein secretion in L. lactis, including promoters, signal peptides and mutant host strains known to overcome some bottlenecks of the process. Among the tools developed in our laboratory, inactivation of HtrA, the unique housekeeping protease at the cell surface, or complementation of the Sec machinery with B. subtilis SecDF accessory protein each result in the increase in heterologous protein yield. Furthermore, our lactococcal expression/secretion system, using both P(Zn)zitR, an expression cassette tightly controlled by environmental zinc, and a consensus signal peptide, SP(Exp4), allows efficient production and secretion of the staphylococcal nuclease, as evidenced by protein yields (protein amount/biomass) comparable to those obtained using NICE or P170 expression systems under similar laboratory conditions. Finally, the toolbox we are developing should contribute to enlarge the use of L. lactis as a protein cell factory.     

Heterologous leaky production of transglutaminase in Lactococcus lactis significantly enhances the growth performance of the host

This study describes a novel strategy to improve the growth performance of Lactococcus lactis by heterologous production of food-grade transglutaminase. The mtg gene from Streptoverticillium mobaraense that encodes the transglutaminase mature protein was cloned into a nisin-inducible expression vector and transformed into L. lactis subsp. cremoris NZ9000. The leaky expression of the mtg gene from the nisA promoter resulted in ammonia formation and carbon flux redistribution at the pyruvate branch. As a consequence, medium acidification was lessened and energy utilization was improved. This led to significantly higher biomass production under aerobic conditions and particularly under non-pH-controlled conditions (up to a 12-fold increase). The results presented here provide a novel way to enhance the growth yield of L. lactis, which is an important step for the purposes of producing proteins of commercial interest using L. lactis as a host.     

硬脂酰-辅酶A脱氢酶基因在食品级乳酸菌中的表达

为了实现硬脂酰-辅酶A脱氢酶1编码基因在乳酸乳球菌中的表达,采用PCR技术扩增获得人类scd1的编码序列。Nco I和Xba I双酶切后定向插入到食品级表达载体pNZ8149中,构建表达载体pNZ8149-scd1。电转化乳酸乳球菌NZ3900,经菌落PCR和测序鉴定scd1基因成功插入到乳酸乳球菌中。在乳链菌肽诱导下进行scd1的表达,转化株提取脂肪酸,进行脂肪酸含量的气相色谱分析。结果显示,SCD1转化菌株中的C16∶1n-7和C18∶1n-7脂肪酸组分比转化pNZ8149的对照组乳酸菌分别提高了92%~169%和53%~127%。文中以scd1基因为例,尝试并证明了脂肪酸脱氢酶类基因能够在食品级乳酸菌中有效表达,为后续研究奠定了基础。     

Heterologous expression of azurin from Pseudomonas aeruginosa in food-grade Lactococcus lactis

Abstract

In this study, azurin, a bacteriocin with anticancer property, was produced by food-grade Lactococcus lactis using the Nisin Controlled Gene Expression (NICE) System. In addition, the antibacterial and cytotoxic properties of recombinant azurin in the culture supernatant were also investigated. Azurin gene from Pseudomonas aeruginosa was cloned into the pNZ8149 vector and the resulting recombinant DNA was transformed into food grade L. lactis NZ3900. The expression of azurin protein was induced by the optimum concentration of nisin for 3?h. Inhibition zones for Escherichia coli and Bacillus cereus were observed at 5.0 and 10?mg/mL concentrations of lyophilized supernatants containing azurin, but no inhibition zone at azurin-free lyophilized supernatants. When HUVEC, HT29, HCT116, and MCF7 cell lines were treated with lyophilized culture supernatants with azurin or without azurin, cell viability decreased with increasing concentrations of the supernatant. Furthermore, the supernatants containing azurin showed more anti-proliferative effect than the azurin-free supernatants. This work provides a practicable method to produce recombinant azurin in the food grade L. lactis strain. As a result, the recombinant L. lactis strain, producing azurin, can be used in the investigation of food biopreservatives and in the development of a therapeutic probiotic.     

Heterologous expression of the plant coumarate: CoA ligase in Lactococcus lactis

AIMS: To demonstrate the expression of coumarate : CoA ligase of Arabidopsis thaliana in Lactococcus lactis as a first step of cloning the vanillin pathway. METHODS AND RESULTS: The 4CL gene was amplified from a cDNA library of A. thaliana by PCR and subcloned into a multicopy lactococcal vector where the expression is under the nisA promoter. The maximum yield of the protein in the recombinant strain of L. lactis was obtained 3 h after induction with 10 ng ml(-1) of nisin. However, these levels were only fraction of those detected in cell extracts of Pseudomonas fluorescens AN103 strain which naturally expresses its own enzyme when grown in the presence of ferulic acid as a carbon source. Among different substrates examined, the enzyme was most active against coumaric acid. CONCLUSIONS: The gene encoding coumarate : CoA ligase in A. thaliana was isolated, sequenced, cloned and expressed in L. lactis. SIGNIFICANCE AND IMPACT OF THE STUDY: This study represents the first of the two steps for genetic engineering of the vanillin pathway in the GRAS (generally recognized as safe) organism L. lactis.     

Host specificity of nisin production by Lactococcus lactis

Kinetics of nisin production have been investigated in terms of endogenous features of the producer organism, Lactococcus lactis. Nisin-producing transposons (Tn Nip) were transferred to different hosts by conjugation. Constructs were cultivated in batch cultures and nisin produced was measured. The proteinase function of C2Prt (Tn Nip)-1 was eliminated by plasmid curing, resulting in the construct C2Prt - (Tn Nip)-1. C2Prt - (Tn Nip)-1 produced nisin to a higher concentration compared to C2Prt (Tn Nip)-1 and was able to maintain the maximum concentration till the end of cultivation. The final concentration of nisin produced was host-specific, because when different constructs carrying the same Tn Nip were cultivated they produced nisin to different concentrations. However, when the same host carried Tn Nip transposons derived from different donors the concentration of nisin produced was similar, suggesting that the two Tn Nip transposons may be similar.     

Eukaryotic membrane protein overproduction in Lactococcus lactis

Eukaryotic membrane proteins play many vital roles in the cell and are important drug targets. Approximately 25% of all genes identified in the genome are known to encode membrane proteins, but the vast majority have no assigned function. Although the generation of structures of soluble proteins has entered the high-throughput stage, for eukaryotic membrane proteins only a dozen high-resolution structures have been obtained so far. One major bottleneck for the functional and structural characterisation of membrane proteins is the overproduction of biologically active material. Recent advances in the development of the Lactococcus lactis expression system have opened the way for the high-throughput functional expression of eukaryotic membrane proteins.     

Lactococcus lactis as a cell factory for high-level diacetyl production

We report the engineering of Lactococcus lactis for the efficient conversion of sugar into diacetyl by combining NADH-oxidase overproduction and alpha-acetolactate decarboxylase inactivation. Eighty percent of the carbon flux was found to be rerouted via alpha-acetolactate to the production of diacetyl by preloading the cells with NADH-oxidase before their use as a cell factory.     

Optimization of fed-batch production of the model recombinant protein GFP in Lactococcus lactis

Optimization of recombinant protein production using lactic acid bacteria (LAB) remains an important obstacle on the road to realizing LAB as oral vaccine delivery vehicles. Despite this, there have been few published investigations to explore the higher limits of LAB recombinant protein expression in fed-batch fermentations. In this study, results from response surface experiments suggested an optimal set of conditions for expression of green fluorescent protein (GFP), a model recombinant protein, in bench-scale, fed-batch Lactococcus lactis IL1403 fermentations. The 48 4-L fed-batch fermentations in this set of experiments, along with preliminary studies, investigated the effects of pH, temperature, hemin concentration, concentration of the nisin inducer per cell, and time of induction. Cell densities in this data set ranged from 2.9 to 7.4 g/L and maximum GFP expression per cell ranged from 0.1 to 4.4 relative fluorescence units (RFU)/g. The optimal 4-L, fed-batch fermentation process found here yields growth and protein expression values that dramatically improve upon results from traditional test tube and flask processes. Relative to the traditional process, the experimental optimum conditions yield 4.9 times the cell density, 1.6 times the protein per cell mass, and 8 times the total protein concentration. Unexpectedly, experiments also revealed that the compound hemin, known previously to improve growth and survival of Lactococcus lactis (L. lactis), negatively impacted recombinant protein production when added in concentrations from 5 to 20 microg/mL with this strain. The improvement in protein expression over traditional processes demonstrated here is an important step toward commercial development of LAB for oral delivery of recombinant vaccines and therapeutic proteins.     

Multivitamin production in Lactococcus lactis using metabolic engineering

The dairy starter bacterium Lactococcus lactis has the potential to synthesize both folate (vitamin B11) and riboflavin (vitamin B2). By directed mutagenesis followed by selection and metabolic engineering we have modified two complicated biosynthetic pathways in L. lactis resulting in simultaneous overproduction of both folate and riboflavin: Following exposure to the riboflavin analogue roseoflavin we have isolated a spontaneous mutant of L. lactis strain NZ9000 that was changed from a riboflavin consumer into a riboflavin producer. This mutant contained a single base change in the regulatory region upstream of the riboflavin biosynthetic genes. By the constitutive overproduction of GTP cyclohydrolase I in this riboflavin-producing strain, the production of folate was increased as well. Novel foods, enriched through fermentation using these multivitamin-producing starters, could compensate the B-vitamin-deficiencies that are common even in highly developed countries and could specifically be used in dietary foods for the large fraction of the Caucasian people (10-15%) with mutations in the methylene tetrahydrofolate reductase (MTHFR).     

A Nine-Residue Synthetic Propeptide Enhances Secretion Efficiency of Heterologous Proteins in Lactococcus lactis

Lactococcus lactis, a gram-positive organism widely used in the food industry, is a potential candidate for the secretion of biologically useful proteins. We examined the secretion efficiency and capacity of L. lactis by using the Staphylococcus aureus nuclease (Nuc) as a heterologous model protein. When expressed in L. lactis from an efficient lactococcal promoter and its native signal peptide, only ~60% of total Nuc was present in a secreted form at ~5 mg per liter. The remaining 40% was found in a cell-associated precursor form. The secretion efficiency was reduced further to ~30% by the deletion of 17 residues of the Nuc native propeptide (resulting in NucT). We identified a modification which improved secretion efficiency of both native Nuc and NucT. A 9-residue synthetic propeptide, LEISSTCDA, which adds two negative charges at the +2 and +8 positions, was fused immediately after the signal peptide cleavage site. In the case of Nuc, secretion efficiency was increased to ~80% by LEISSTCDA insertion without altering the signal peptide cleavage site, and the yield was increased two- to fourfold (up to ~20 mg per liter). The improvement of NucT secretion efficiency was even more marked and rose from 30 to 90%. Similarly, the secretion efficiency of a third protein, the α-amylase of Bacillus stearothermophilus, was also improved by LEISSTCDA. These data indicate that the LEISSTCDA synthetic propeptide improves secretion of different heterologous proteins in L. lactis.