Recombinant cystatins in plants

Dozens of studies have assessed the practical value of plant cystatins as ectopic inhibitors of Cys proteases in biological systems. The potential of these proteins in crop protection to control herbivorous pests and pathogens has been documented extensively over the past 25 years. Their usefulness to regulate endogenous Cys proteases in planta has also been considered recently, notably to implement novel traits of agronomic relevance in crops or to generate protease activity-depleted environments in plants or plant cells used as bioreactors for recombinant proteins. After a brief update on the basic structural characteristics of plant cystatins, we summarize recent advances on the use of these proteins in plant biotechnology. Attention is also paid to the molecular improvement of their structural properties for the improvement of their protease inhibitory effects or the fine-tuning of their biological target range.

Functional proteomics-aided selection of protease inhibitors for herbivore insect control

Studies have reported the potential of protease inhibitors to engineer insect resistance in transgenic plants but the general usefulness of this approach in crop protection still remains to be established. Insects have evolved strategies to cope with dietary protease inhibitors, such as the use of proteases recalcitrant to inhibition, that often make the selection of effective inhibitors very challenging. Here, we used a functional proteomics approach for the ‘capture’ of Cys protease targets in crude protein extracts as a tool to identify promising cystatins for plant improvement. Two cystatins found to differ in their efficiency to capture Cys proteases of the coleopteran pest Leptinotarsa decemlineata also differed in their usefulness to produce transgenic potato lines resistant to this insect. Plants expressing the most potent cystatin at high level had a strong repressing effect on larval growth and leaf intake, while plants expressing the weakest cystatin showed no effect on both two parameters compared to untransformed parental line used for genetic transformation. Our data underline the relevance of considering the whole range of possible protease targets when selecting an inhibitor for plant pest control. They also confirm the feasibility of developing cystatin-expressing transgenics resistant to a major pest of potato.

Companion Protease Inhibitors for the In Situ Protection of Recombinant Proteins in Plants

We previously described a procedure for the use of plant protease inhibitors as "companion" accessory proteins to prevent unwanted proteolysis of clinically useful recombinant proteins in leaf crude protein extracts (Benchabane et al. Methods Mol Biol 483:265-273, 2009). Here we describe the use of these inhibitors for the protection of recombinant proteins in planta, before their extraction from leaf tissues. A procedure is first described involving inhibitors co-expressed along-and co-migrating-with the protein of interest in host plant cells. An alternative, single transgene scheme is then described involving translational fusions of the recombinant protein and companion inhibitor. These approaches may allow for a significant improvement of protein steady-state levels in leaves, comparable to yield improvements observed with protease-deficient strains of less complex protein expression hosts such as E. coli or yeasts.

Cereal cystatins delay sprouting and nutrient loss in tubers of potato, Solanum tuberosum

BACKGROUND

Recent studies have reported agronomically useful ectopic effects for recombinant protease inhibitors expressed in leaves of transgenic plants, including improved tolerance to abiotic stress conditions and partial resistance to necrotrophic pathogens. Here we assessed the effects of these proteins on the post-dormancy sprouting of storage organs, using as a model potato tubers expressing cysteine protease inhibitors of the cystatin protein superfamily.

RESULTS

Sprout emergence and distribution, soluble proteins, starch and soluble sugars were monitored in tubers of cereal cystatin-expressing clones stored for several months at 4 °C. Cystatin expression had a strong repressing effect on sprout growth, associated with an apparent loss of apical dominance and an increased number of small buds at the skin surface. Soluble protein content remained high for up to 48 weeks in cystatin-expressing tubers compared to control (untransformed) tubers, likely explained by a significant stabilization of the major storage protein patatin, decreased hydrolysis of the endogenous protease inhibitor multicystatin and low cystatin-sensitive cysteine protease activity in tuber tissue. Starch content decreased after several months in cystatin-expressing tubers but remained higher than in control tubers, unlike sucrose showing a slower accumulation in the transgenics. Plantlet emergence, storage protein processing and height of growing plants showed similar time-course patterns for control and transgenic tubers, except for a systematic delay of 2 or 3 d in the latter group likely due to limited sprout size at sowing.

CONCLUSIONS

Our data point overall to the onset of metabolic interference effects for cereal cystatins in sprouting potato tubers. They suggest, in practice, the potential of endogenous cysteine proteases as relevant targets for the development of potato varieties with longer storage capabilities.

Production of pharmaceutical proteins in solanaceae food crops

The benefits of increased safety and cost-effectiveness make vegetable crops appropriate systems for the production and delivery of pharmaceutical proteins. In particular, Solanaceae edible crops could be inexpensive biofactories for oral vaccines and other pharmaceutical proteins that can be ingested as minimally processed extracts or as partially purified products. The field of crop plant biotechnology is advancing rapidly due to novel developments in genetic and genomic tools being made available today for the scientific community. In this review, we briefly summarize data now available regarding genomic resources for the Solanaceae family. In addition, we describe novel strategies developed for the expression of foreign proteins in vegetable crops and the utilization of these techniques to manufacture pharmaceutical proteins.

Beneficial 'unintended effects' of a cereal cystatin in transgenic lines of potato, Solanum tuberosum

BACKGROUND

Studies reported unintended pleiotropic effects for a number of pesticidal proteins ectopically expressed in transgenic crops, but the nature and significance of such effects in planta remain poorly understood. Here we assessed the effects of corn cystatin II (CCII), a potent inhibitor of C1A cysteine (Cys) proteases considered for insect and pathogen control, on the leaf proteome and pathogen resistance status of potato lines constitutively expressing this protein.

RESULTS

The leaf proteome of lines accumulating CCII at different levels was resolved by 2-dimensional gel electrophoresis and compared with the leaf proteome of a control (parental) line. Out of ca. 700 proteins monitored on 2-D gels, 23 were significantly up- or downregulated in CCII-expressing leaves, including 14 proteins detected de novo or up-regulated by more than five-fold compared to the control. Most up-regulated proteins were abiotic or biotic stress-responsive proteins, including different secretory peroxidases, wound inducible protease inhibitors and pathogenesis-related proteins. Accordingly, infection of leaf tissues by the fungal necrotroph Botryris cinerea was prevented in CCII-expressing plants, despite a null impact of CCII on growth of this pathogen and the absence of extracellular Cys protease targets for the inhibitor.

CONCLUSIONS

These data point to the onset of pleiotropic effects altering the leaf proteome in transgenic plants expressing recombinant protease inhibitors. They also show the potential of these proteins as ectopic modulators of stress responses in planta, useful to engineer biotic or abiotic stress tolerance in crop plants of economic significance.

[Current status in the commercialization and application of genetically modified plants and their effects on human and livestock health and phytoremediation]

Developments in the use of genetically modified plants for human and livestock health and phytoremediation were surveyed using information retrieved from Entrez PubMed, Chemical Abstracts Service, Google, congress abstracts and proceedings of related scientific societies, scientific journals, etc. Information obtained was classified into 8 categories according to the research objective and the usage of the transgenic plants as 1: nutraceuticals (functional foods), 2: oral vaccines, 3: edible curatives, 4: vaccine antigens, 5: therapeutic antibodies, 6: curatives, 7: diagnostic agents and reagents, and 8: phytoremediation. In total, 405 cases were collected from 2006 to 2010. The numbers of cases were 120 for nutraceuticals, 65 for oral vaccines, 25 for edible curatives, 36 for vaccine antigens, 36 for therapeutic antibodies, 76 for curatives, 15 for diagnostic agents and reagents, and 40 for phytoremediation (sum of each cases was 413 because some reports were related to several categories). Nutraceuticals, oral vaccines and curatives were predominant. The most frequently used edible crop was rice (51 cases), and tomato (28 cases), lettuce (22 cases), potato (18 cases), corn (15 cases) followed.

A protease activity-depleted environment for heterologous proteins migrating towards the leaf cell apoplast

Recombinant proteins face major constraints along the plant cell secretory pathway, including proteolytic processing compromising their structural integrity. Here, we demonstrate the potential of protease inhibitors as in situ stabilizing agents for recombinant proteins migrating towards the leaf apoplast. Genomic data for Arabidopsis, rice and Nicotiana spp. were assessed to determine the relative incidence of protease families in the cell secretory pathway. Transient expression assays with the model platform Nicotiana benthamiana were then performed to test the efficiency of protease inhibitors in stabilizing proteins targeted to the apoplast. Current genomic data suggest the occurrence of proteases from several families along the secretory pathway, including A1 and A22 Asp proteases; C1A and C13 Cys proteases; and S1, S8 and S10 Ser proteases. In vitro protease assays confirmed the presence of various proteases in N. benthamiana leaves, notably pointing to the deposition of A1- and S1-type activities preferentially in the apoplast. Accordingly, transient expression and secretion of the A1/S1 protease inhibitor, tomato cathepsin D inhibitor (SlCDI), negatively altered A1 and S1 protease activities in this cell compartment, while increasing the leaf apoplast protein content by ∼45% and improving the accumulation of a murine diagnostic antibody, C5-1, co-secreted in the apoplast. SlCYS9, an inhibitor of C1A and C13 Cys proteases, had no impact on the apoplast proteases and protein content, but stabilized C5-1 in planta, presumably upstream in the secretory pathway. These data confirm, overall, the potential of protease inhibitors for the in situ protection of recombinant proteins along the plant cell secretory pathway.

Changes in protein expression profiles between a low phytic acid rice ( Oryza sativa L. Ssp. japonica) line and its parental line: a proteomic and bioinformatic approach

The seed proteome of a low phytic acid (lpa) rice line (Os-lpa-XS110-1), developed as a novel food source, was compared to that of its parental line, Xiushui 110 (XS-110). Analysis by surfaced enhanced laser desorption ionization-time-of-flight mass spectrometry (SELDI-TOF MS) and two-dimensional gel electrophoresis (2-DE) allowed the detection of a potential low molecular weight biomarker and identification of 23 differentially expressed proteins that include stress-related proteins, storage proteins, and potential allergens. Bioinformatic analyses revealed that triose phosphate isomerase (TPI) and fructose bisphosphatealdolase (FBA), two major differentially expressed proteins, are involved in myo-inositol metabolism. Accumulation of globulin was also significantly decreased in the lpa line. This study demonstrates the potential of proteomic and bioinformatic profiling techniques for safety assessment of novel foods. Furthermore, these techniques provide powerful tools for studying functional genomics due to the possibility of identifying genes related to the mutated traits.

A companion protease inhibitor for the protection of cytosol-targeted recombinant proteins in plants

We reported earlier the potential of tomato cathepsin D inhibitor (SlCDI) as an in-built stabilizing agent for the protection of recombinant proteins in transgenic plant leaf crude extracts (Plant Biotechnol J.4, 359-368). Here we document the potential of SlCDI for the in situ protection of proteins in potato leaves. Total protein assays with control and SlCDI-expressing potato lines indicated a positive impact of slcdi transgene expression on leaf protein content, with a mean relative increase of 35%-40% depending on the light regime. Out of approximately 700 proteins detected on 2-D gels, only 20 exhibited a significantly altered level on a protein-specific basis, whereas most proteins were up-regulated on a leaf fresh weight basis, albeit at variable rates. Quantitative reverse trancriptase-PCR assays for rubisco activase showed similar transcript levels in leaves of test and control lines despite protein levels increased by two- to threefold in SlCDI-expressing lines. These observations, along with the unrelated biological functions assigned to MS-identified proteins up-regulated in leaves and protease assays showing slightly increased proteasome activity in protein extracts of SlCDI-expressing lines, suggest a general, proteasome-independent protein stabilizing effect of SlCDI in planta. Transient expression assays with human alpha(1)-antichymotrypsin also showed a stabilizing effect for SlCDI on heterologous proteins, leading to net levels of the human protein increased by approximately 2.5-fold in SlCDI-expressing plants. These data illustrate, overall, the potential of SlCDI as an in vivo protein-stabilizing agent in transgenic plant systems, useful to improve protein levels and recombinant protein accumulation.

Identification of Anoectochilus based on rDNA ITS sequences alignment and SELDI-TOF-MS

The internal transcribed spacer (ITS) sequences alignment and proteomic difference of Anoectochilus interspecies have been studied by means of ITS molecular identification and surface enhanced laser desorption ionization time of flight mass spectrography. Results showed that variety certification on Anoectochilus by ITS sequences can not determine species, and there is proteomic difference among Anoectochilus interspecies. Moreover, proteomic finger printings of five Anoectochilus species have been established for identifying species, and genetic relationships of five species within Anoectochilus have been deduced according to proteomic differences among five species.

On-chip detection of low-molecular-weight recombinant proteins in plant crude extracts by SELDI-TOF MS

This chapter presents a general procedure for the on-chip detection and quantitation of low-molecular-weight recombinant proteins in transgenic plant crude extracts by surface-enhanced laser desorption ionization time-of-flight mass spectrometry (SELDI-TOF MS). A protocol is first described to detect the protein of interest in crude protein extracts of transgenic plant lines, by differential protein mapping against similar extracts from a control, nontransgenic line. A complementary protocol is then presented to generate a standard curve with the SELDI system, allowing the protein to be quantified in different transgenic lines. Overall, this procedure may be carried out within a few hours, without the need for prior purification or enrichment of the recombinant protein.