Heterologous protein production and delivery systems for Lactococcus lactis

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 live vehicles for the production and delivery of heterologous proteins of vaccinal, medical or technological interest has therefore been extensively investigated. Lactococcus lactis, a LAB species, is a potential candidate for the production of biologically useful proteins. Several delivery systems have been developed to target heterologous proteins to a specific cell location (i.e., cytoplasm, cell wall or extracellular medium). A promising application of L. lactis is its use as an antigen delivery vehicle, for the development of live mucosal vaccines. The expression of heterologous proteins and antigens as well as the various delivery systems developed in L. lactis, and its use as an oral vaccine carrier are discussed.     

乳酸乳球菌作为黏膜免疫活载体疫苗传递抗原的研究进展

乳酸菌是人和动物肠道内的常见细菌,被公认为安全级(generally recognized as safe,GRAS)微生物。近年来,对于乳酸菌作为宿主菌表达外源蛋白或抗原的研究取得了一定进展。乳酸乳球菌(Lactococcus lactis)是乳酸菌的代表菌种,以其生长迅速、易于操作等优点成为表达外源抗原,作为黏膜免疫活载体疫苗的理想菌株。随着对乳酸乳球菌基因工程的研究逐渐深入,已发展了一系列组成型和诱导型乳酸乳球菌表达系统以及蛋白定位系统。破伤风毒素片段C、布氏杆菌L7/L12蛋白等多种病原微生物抗原已成功在乳酸乳球菌中表达,并已证明部分重组乳酸乳球菌作为黏膜免疫疫苗可以同时刺激局部黏膜免疫应答和系统免疫应答。目前,如何使活载体乳酸乳球菌以最佳方式向黏膜免疫系统提呈抗原继而诱导有效免疫反应是该领域的研究热点,也是巨大挑战。实现外源抗原在乳酸乳球菌中的准确定位及与细胞因子的共表达是未来研究的重要方向之一。乳酸乳球菌作为黏膜免疫活载体疫苗传递外源抗原具有广阔的应用前景。     

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.     

Protein secretion in Lactococcus lactis : an efficient way to increase the overall heterologous protein production

Lactococcus lactis, the model lactic acid bacterium (LAB), is a food grade and well-characterized Gram positive bacterium. It is a good candidate for heterologous protein delivery in foodstuff or in the digestive tract. L. lactis can also be used as a protein producer in fermentor. Many heterologous proteins have already been produced in L. lactis but only few reports allow comparing production yields for a given protein either produced intracellularly or secreted in the medium. Here, we review several works evaluating the influence of the localization on the production yields of several heterologous proteins produced in L. lactis. The questions of size limits, conformation, and proteolysis are addressed and discussed with regard to protein yields. These data show that i) secretion is preferable to cytoplasmic production; ii) secretion enhancement (by signal peptide and propeptide optimization) results in increased production yield; iii) protein conformation rather than protein size can impair secretion and thus alter production yields; and iv) fusion of a stable protein can stabilize labile proteins. The role of intracellular proteolysis on heterologous cytoplasmic proteins and precursors is discussed. The new challenges now are the development of food grade systems and the identification and optimization of host factors affecting heterologous protein production not only in L. lactis, but also in other LAB species.     

Campbell-like integration of heterologous plasmid DNA into the chromosome of Lactococcus lactis subsp. lactis

Integrable vectors were constructed based on the plasmid pHV60, which is essentially a pBR322 replicon carrying a chloramphenicol resistance marker, by inserting 1.3-kilobase chromosomal fragments of Lactococcus lactis subsp. lactis MG1363 into this plasmid. Three constructs as well as pHV60 were electroporated to strain MG1363. Transformants were obtained with all constructs, and also with pHV60 (albeit with low frequency). By using Southern hybridizations, it appeared that pHV60 showed homology with the chromosome of MG1363, and that it most probably uses this homology to integrate in a Campbell-like manner. The presence of chromosomal sequences in pHV60 stimulated insertion elsewhere in the chromosome by a factor of 5 to 100. In all cases the integrated plasmids were amplified, at a selective pressure of 5 micrograms of chloramphenicol per ml, to a level of approximately 15 copies per chromosome. Although the amplification was gradually lost under nonselective conditions, one copy remained stably integrated in the chromosome. The results show that a Campbell-like integration strategy can be used to improve the accessibility of the lactococcal chromosome for genetic analysis and is potentially useful in stabilizing unstable genes in lactococci.     

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.     

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.     

Campbell-like integration of heterologous plasmid DNA into the chromosome of Lactococcus lactis subsp. lactis.

Integrable vectors were constructed based on the plasmid pHV60, which is essentially a pBR322 replicon carrying a chloramphenicol resistance marker, by inserting 1.3-kilobase chromosomal fragments of Lactococcus lactis subsp. lactis MG1363 into this plasmid. Three constructs as well as pHV60 were electroporated to strain MG1363. Transformants were obtained with all constructs, and also with pHV60 (albeit with low frequency). By using Southern hybridizations, it appeared that pHV60 showed homology with the chromosome of MG1363, and that it most probably uses this homology to integrate in a Campbell-like manner. The presence of chromosomal sequences in pHV60 stimulated insertion elsewhere in the chromosome by a factor of 5 to 100. In all cases the integrated plasmids were amplified, at a selective pressure of 5 micrograms of chloramphenicol per ml, to a level of approximately 15 copies per chromosome. Although the amplification was gradually lost under nonselective conditions, one copy remained stably integrated in the chromosome. The results show that a Campbell-like integration strategy can be used to improve the accessibility of the lactococcal chromosome for genetic analysis and is potentially useful in stabilizing unstable genes in lactococci.     

Overproduction of heterologous mannitol 1-phosphatase: a key factor for engineering mannitol production by Lactococcus lactis

To achieve high mannitol production by Lactococcus lactis, the mannitol 1-phosphatase gene of Eimeria tenella and the mannitol 1-phosphate dehydrogenase gene mtlD of Lactobacillus plantarum were cloned in the nisin-dependent L. lactis NICE overexpression system. As predicted by a kinetic L. lactis glycolysis model, increase in mannitol 1-phosphate dehydrogenase and mannitol 1-phosphatase activities resulted in increased mannitol production. Overexpression of both genes in growing cells resulted in glucose-mannitol conversions of 11, 21, and 27% by the L. lactis parental strain, a strain with reduced phosphofructokinase activity, and a lactate dehydrogenase-deficient strain, respectively. Improved induction conditions and increased substrate concentrations resulted in an even higher glucose-to-mannitol conversion of 50% by the lactate dehydrogenase-deficient L. lactis strain, close to the theoretical mannitol yield of 67%. Moreover, a clear correlation between mannitol 1-phosphatase activity and mannitol production was shown, demonstrating the usefulness of this metabolic engineering approach.     

IS946-mediated integration of heterologous DNA into the genome of Lactococcus lactis subsp. lactis.

The lactococcal insertion sequence IS946 was used to construct suicide vectors for insertion of heterologous DNA into chromosomal and plasmid sequences of Lactococcus lactis subsp. lactis. Electroporation of L. lactis strains, including the recombination-deficient strain MMS362, with the suicide vector pTRK145 yielded 10(1) to 10(3) transformants per micrograms of DNA. pTRK145 insertions occurred primarily in the chromosome, with one insertion detected in a resident plasmid. Vector-specific probes identified junction fragments that varied among transformants, indicating random insertions of pTRK145.     

IS946-mediated integration of heterologous DNA into the genome of Lactococcus lactis subsp. lactis.

The lactococcal insertion sequence IS946 was used to construct suicide vectors for insertion of heterologous DNA into chromosomal and plasmid sequences of Lactococcus lactis subsp. lactis. Electroporation of L. lactis strains, including the recombination-deficient strain MMS362, with the suicide vector pTRK145 yielded 10(1) to 10(3) transformants per micrograms of DNA. pTRK145 insertions occurred primarily in the chromosome, with one insertion detected in a resident plasmid. Vector-specific probes identified junction fragments that varied among transformants, indicating random insertions of pTRK145.     

Evaluation of pKD1-based plasmid systems for heterologous protein production in Kluyveromyces lactis

The stability of pKD1-based vectors was evaluated during the synthesis of intracellular and extracellular gene products in the yeast Kluyveromyces lactis. The Escherichia coli lacZ and MFalpha1 leader-BPTI (bovine pancreatic trypsin inhibitor) cassettes were placed under the control of the inducible K. lactis LAC4 promoter and inserted into the pKD1-based plasmids. To induce gene expression while maintaining inducer level, a gratuitous gal1-209 K. lactis strain was employed. Selective medium containing 5 g glucose/l and 0.5 g galactose (inducer)/l allowed optimum expression and secretion of heterologous products without a significant effect on the growth of the recombinant cells. During long-term sequential batch cultures (60 generations), plasmid instability was mainly the result of structural instability. The expression and secretion of BPTI resulted in greater structural instability relative to the intracellular beta-galactosidase. For both products, vectors carrying the pKD1 replication origin and the cis-acting stability locus (partial-pKD1 vectors) were more stable than vectors carrying the full pKD1 sequence (full-pKD1 vectors). However, after 55 generations, the beta-galactosidase and BPTI activities were still higher with the full-pKD1 vectors. This was due to the significantly higher initial beta-galactosidase and BPTI activities for the full-pKD1 vectors (approximately 85% and 47% higher, respectively) relative to the partial-pKDI vectors. Southern blots confirmed that these increases were due to the higher copy number of the vectors carrying the full pKD1 sequence. In contrast to our previously reported results for the secretion of invertase, full-pKD1 vectors were preferred for the expression/secretion of beta-galactosidase and BPTI for at least 55 generations. Due to their structural stability, partial-pKD1 vectors will be advantageous for very long cultivation times.     

Controlling Listeria monocytogenes in Cottage cheese through heterologous production of enterocin A by Lactococcus lactis

Aims: Enterocin A is an example of a class IIa bacteriocin with potent anti‐listerial activity. This study was initiated with a view to harnessing this activity, through heterologous production by a lactococcal starter strain, to limit levels of Listeria monocytogenes in a food (Cottage cheese). Methods and Results: Plasmid pEnt02 (containing entA, I, T and D genes under the control of a constitutive promoter) was introduced into a Lactococcus lactis strain capable of fermenting lactose. When this bacteriocin‐producing starter was used in combination with a non‐enterocin A producer, thereby compensating for an associated reduction in acid production, during a Cottage cheese fermentation, a decrease in L. monocytogenes (tagged with lux genes for convenience) levels was evident. Conclusions: Enterocin A, heterologously produced by a food grade lactic acid bacteria (LAB), was therefore shown to have potential for use as a biocontrol agent in food. Significance and Impact of the Study: Many of the most active anti‐listerial compounds identified to date are enterocins. However, because of Enterococcus‐associated concerns, the use of these antimicrobials in a food setting has been curtailed. Although enterocins have been heterologously produced in LAB to overcome this problem, this study represents the first occasion upon which the benefits of such heterologous production have been demonstrated in a food context.     

The immune response to Lactococcus lactis: Implications for its use as a vaccine delivery vehicle

Abstract The development of hydrogenotrophic bacteria in the rumen of lambs was investigated by culture and labeling experiments. ¹⁴CO₂ and ¹³CO₂ incorporation by the rumen microflora of a 24-h-old lamb showed that while there was no labeled methane, double-labeled acetate was formed indicating the presence of hydrogen-dependent acetogenesis. In vitro counts from rumen fluid of 20-h-old lambs confirmed an extensive colonization of acetogenic bacteria while methanogens were absent. Methanogens appeared in the rumen of 30-h-old lambs, and as they developed there was a proportional decrease in the numbers of acetogens, indicating a competition for hydrogen between these two groups. Hydrogen-utilizing sulfate-reducing bacteria, which were established by the 3rd day after birth, did not seem to be affected by this competition.     

Identification of a RecA-like protein in Lactococcus lactis

We have identified in Lactococcus lactis, an analogue of Escherichia coli RecA protein. Physiological responses such as ultraviolet (UV) and chemical mutagenesis and induction of prophage have been characterized and suggest the existence of RecA-like functions in this commercially important species. The putative RecA protein was detected at the position of an apparent molecular weight of 39 kDa by Western blot analysis by using antiserum against E coli RecA protein. In addition, the protein level is significantly increased after UV irradiation in a wild-type strain compared to the recombination deficient mutant strain.     

Dual recombinant Lactococcus lactis for enhanced delivery of DNA vaccine reporter plasmid pPERDBY

Food grade Lactococcus lactis has been widely used as an antigen and DNA delivery vehicle. We have previously reported the use of non‐invasive L. lactis to deliver the newly constructed immunostimulatory DNA vaccine reporter plasmid, pPERDBY. In the present report, construction of dual recombinant L. lactis expressing internalin A of Listeria monocytogenes and harboring pPERDBY (LL InlA + pPERDBY) to enhance the efficiency of delivery of DNA by L. lactis is outlined. After confirmation and validation of LL InlA + pPERDBY, its DNA delivery potential was compared with previously developed non‐invasive r‐ L. lactis::pPERDBY. The use of invasive L. lactis resulted in around threefold increases in the number of enhanced green fluorescent protein‐expressing Caco‐2 cells. These findings reinforce the prospective application of invasive strain of L. lactis for delivery of DNA/RNA and antigens.     

Lactococcus lactis as a vehicle for the heterologous expression of fungal ribotoxin variants with reduced IgE-binding affinity

Fungal ribotoxins are a family of extracellular ribonucleases which inhibit protein biosynthesis by inactivating the ribosomes. This inactivation results in the induction of cell death by apoptosis. Ribotoxins show antitumoral properties based on their ability to cross the membrane of some transformed cells. Unfortunately, they also show an unspecific cytotoxicity which has greatly impaired their potential clinical uses. alpha-Sarcin, produced by Aspergillus giganteus, is the best-characterized ribotoxin. Asp f 1, another ribotoxin produced by A. fumigatus, is indeed one of its major allergens. In this work, the Lactococcus lactis MG1363 strain has been engineered to produce and secrete not only wild-type Asp f 1 and alpha-sarcin but also three different mutants with reduced cytotoxicity and/or IgE-binding affinity. The proteins were secreted in native and active form when the extracellular medium employed was buffered at pH values around 8.0. Strains producing the wild-type natural alpha-sarcin were proved to be innocuous when administered intragastrically to mice for a period of 14 days. Overall, the results presented are discussed in terms of its potential application as a vehicle of oral delivery of hypoallergenic variants as well as a starting point to approach the design of strategies to accomplish the safe delivery of these proteins as antitumoral agents.