Production and secretion of recombinant proteins in Dictyostelium discoideum

Production and characterization of pharmacologically active recombinant human phosphodiesterase 4B in Dictyostelium discoideum

Phosphodiesterase 4B (PDE4B) is an important therapeutic target for asthma and chronic obstructive pulmonary disease. To identify PDE4 subtype-specific compounds using high-throughput assays, full-length recombinant PDE4 proteins are needed in bulk quantity. In the present study, full-length human PDE4B2 was expressed in the cellular slime mould Dictyostelium discoideum (Dd). A cell density of 2 x 10(7) cells/mL was obtained and up to 1 mg/L recombinant PDE4B2 was purified through Ni-NTA affinity chromatography. The expressed protein was soluble and its activity was comparable to PDE4B2 protein expressed in mammalian cells (K(m)=1.7 microM). The functional significance of the Dd expression system is supported by the demonstration that, in concert with proteins expressed in mammalian systems, there are no major changes in the affinity for PDE4B2 inhibitors and substrates. These findings thus provide the first evidence that Dd can be utilized for the expression and purification of functionally active full-length human PDE4B2 in large amounts required for high-throughput screening of pharmacologically active compounds against this therapeutic target.

Purification of recombinant human phosphodiesterase 7A expressed in Dictyostelium discoideum

Phosphodiesterase plays an important role in regulating inflammatory pathways and T cell function. The development of phosphodiesterase 7 inhibitor may give better efficacy profile over phosphodiesterase 4 inhibitors. However, the recombinant phosphodiesterase 7 is required in large quantity for high-throughput screening of new drugs by in vitro enzymatic assays. In the present study, recombinant human PDE7A1 was expressed in Dictyostelium discoideum under the control of constitutively active actin-15 promoter. The cytosolic localization of the expressed protein was confirmed by immunofluorescence studies. Upto 2 mg of recombinant protein was purified using His-Tag affinity column chromatography followed by ion-exchange Resource Q column purification. The recombinant protein expressed in D. discoideum followed Michaelis-Menten kinetics similar to the protein expressed in mammalian system and showed no major changes in affinity to substrate or inhibitors. Thus, our study clearly demonstrates a robust expression system for successful bulk production of pharmacologically active isoform of human PDE7A1 required for high-throughput assays.

Use of the myosin motor domain as large-affinity tag for the expression and purification of proteins in Dictyostelium discoideum

The cellular slime mold Dictyostelium discoideum is increasingly be used for the overexpression of proteins. Dictyostelium is amenable to classical and molecular genetic approaches and can easily be grown in large quantities. It contains a variety of chaperones and folding enzymes, and is able to perform all kinds of post-translational protein modifications. Here, new expression vectors are presented that have been designed for the production of proteins in large quantities for biochemical and structural studies. The expression cassettes of the most successful vectors are based on a tandem affinity purification tag consisting of an octahistidine tag followed by the myosin motor domain tag. The myosin motor domain not only strongly enhances the production of fused proteins but is also used for a fast affinity purification step through its ATP-dependent binding to actin. The applicability of the new system has been demonstrated for the expression and purification of subunits of the dynein-dynactin motor protein complex from different species.

Optimizing heterologous expression in dictyostelium: importance of 5' codon adaptation

Expression of heterologous proteins in Dictyostelium discoideum presents unique research opportunities, such as the functional analysis of complex human glycoproteins after random mutagenesis. In one study, human chorionic gonadotropin (hCG) and human follicle stimulating hormone were expressed in Dictyostelium. During the course of these experiments, we also investigated the role of codon usage and of the DNA sequence upstream of the ATG start codon. The Dictyostelium genome has a higher AT content than the human, resulting in a different codon preference. The hCG-beta gene contains three clusters with infrequently used codons that were changed to codons that are preferred by Dictyostelium. The results reported here show that optimizing the first 5-17 codons of the hCG gene contributes to 4- to 5-fold increased expression levels, but that further optimization has no significant effect. These observations suggest that optimal codon usage contributes to ribosome stabilization, but does not play an important role during the elongation phase of translation. Furthermore, adapting the 5'-sequence of the hCG gene to the Dictyostelium 'Kozak'-like sequence increased expression levels approximately 1.5-fold. Thus, using both codon optimization and 'Kozak' adaptation, a 6- to 8-fold increase in expression levels could be obtained for hCG.

Protein glycosylation: implications for in vivo functions and therapeutic applications

The glycosylation machinery in eukaryotic cells is available to all proteins that enter the secretory pathway. There is a growing interest in diseases caused by defective glycosylation, and in therapeutic glycoproteins produced through recombinant DNA technology route. The choice of a bioprocess for commercial production of recombinant glycoprotein is determined by a variety of factors, such as intrinsic biological properties of the protein being expressed and the purpose for which it is intended, and also the economic target. This review summarizes recent development and understanding related to synthesis of glycans, their functions, diseases, and various expression systems and characterization of glycans. The second section covers processing of N- and O-glycans and the factors that regulate protein glycosylation. The third section deals with in vivo functions of protein glycosylation, which includes protein folding and stability, receptor functioning, cell adhesion and signal transduction. Malfunctioning of glycosylation machinery and the resultant diseases are the subject of the fourth section. The next section covers the various expression systems exploited for the glycoproteins: it includes yeasts, mammalian cells, insect cells, plants and an amoeboid organism. Biopharmaceutical properties of therapeutic proteins are discussed in the sixth section. In vitro protein glycosylation and the characterization of glycan structures are the subject matters for the last two sections, respectively.

Purification by reflux electrophoresis of whey proteins and of a recombinant protein expressed in Dictyostelium discoideum

Protein purification that combines the use of molecular mass exclusion membranes with electrophoresis is particularly powerful as it uses properties inherent to both techniques. The use of membranes allows efficient processing and is easily scaled up, while electrophoresis permits high resolution separation under mild conditions. The Gradiflow apparatus combines these two technologies as it uses polyacrylamide membranes to influence electrokinetic separations. The reflux electrophoresis process consists of a series of cycles incorporating a forward phase and a reverse phase. The forward phase involves collection of a target protein that passes through a separation membrane before trailing proteins in the same solution. The forward phase is repeated following clearance of the membrane in the reverse phase by reversing the current. We have devised a strategy to establish optimal reflux separation parameters, where membranes are chosen for a particular operating range and protein transfer is monitored at different pH values. In addition, forward and reverse phase times are determined during this process. Two examples of the reflux method are described. In the first case, we described the purification strategy for proteins from a complex mixture which contains proteins of higher electrophoretic mobility than the target protein. This is a two-step procedure, where first proteins of higher mobility than the target protein are removed from the solution by a series of reflux cycles, so that the target protein remains as the leading fraction. In the second step the target protein is collected, as it has become the leading fraction of the remaining proteins. In the second example we report the development of a reflux strategy which allowed a rapid one-step preparative purification of a recombinant protein, expressed in Dictyostelium discoideum. These strategies demonstrate that the Gradiflow is amenable to a wide range of applications, as the protein of interest is not necessarily required to be the leading fraction in solution.

Expression of an Arabidopsis plasma membrane aquaporin in Dictyostelium results in hypoosmotic sensitivity and developmental abnormalities

The rd28 gene of Arabidopsis thaliana encodes a water channel protein, or aquaporin, of the plasma membrane. A construct in which transcription of the rd28 cDNA is controlled by the Dictyostelium actin15 promoter was transformed into Dictyostelium discoideum cells. Transformants contained RD28 protein in their plasma membranes. When shifted to a low-osmotic-strength buffer, cells expressing rd28 swelled rapidly and burst, indicating that the plant aquaporin allowed rapid water entry in the amoebae. The rate of osmotic lysis was a function of the osmotic pressure of the buffer. We also selected transformants in which the expression of the rd28 cDNA is driven by the promoter of the prespore cotB gene. These transformants accumulated rd28 mRNA uniquely in prespore cells. In low-osmotic-strength buffer, the cotB::rd28 cells aggregated and formed normally proportioned slugs but failed to form normal fruiting bodies. The number of spores was reduced 20-fold, and the stalks of the fruiting bodies were abnormally short. The consequences of expressing RD28 in prespore cells could be partially overcome by increasing the osmolarity of the medium. Under these conditions, the cotB::rd28 cells formed fruiting bodies of more normal appearance, and the number of viable spores increased slightly. Because prespore cells have to shrink and dehydrate to form spores, it was not unexpected that expression of an aquaporin would disrupt this process, but it was surprising to find that stalk differentiation was also affected by expression of rd28 in prespore cells. It appears that osmotic stress on prespore cells alters their ability to signal terminal differentiation in prestalk cells. The results provide independent confirmation that plant aquaporins can function in the cells of other organisms, and that D. discoideum can be used to study the properties of these water channels.

Calcium influences the stability and conformation of rotavirus SA11 glycoprotein VP7 expressed in Dictyostelium discoideum

We have previously reported expression of the rotavirus outer capsid glycoprotein, VP7, in the relatively new expression host, Dictyostelium discoideum. To optimise yields of recombinant VP7, we examined the role of Ca2+ since stability of both VP7 and mature rotavirus during a rotavirus infection are calcium-dependent. Low micromolar levels of free extracellular Ca2+ were required to maximise yields of VP7 in D. discoideum whilst levels of VP7 were reduced following depletion of intracellular Ca2+ reserves using A23187 and EGTA. Immunoblot analysis suggested that VP7 was being degraded in an intracellular compartment. Immunoprecipitation with a conformation-dependent neutralising antibody confirmed that EGTA-induced Ca2+ chelation alters the conformation of VP7. These results suggest that stability of VP7 is dependent on maintaining adequate levels of intracellular Ca2+ and that conformational changes in VP7 which occur following depletion of Ca2+ reserves induce rapid proteolysis of the protein. Since these results establish conditions for expressing optimal levels of VP7 in the correct conformation they have important implications for the development of a subunit vaccine based on recombinant VP7.

Analysis of gene function in Dictyostelium

Over the past ten years, powerful molecular genetic techniques have been developed to analyze gene function in Dictyostelium. DNA-mediated transformation using a variety of selections and vectors has allowed the introduction of wild-type or modified genes that are under various forms of transcriptional control. Homologous recombination is efficient and can be used to modify the genome in precise ways. In addition, it is now possible to clone genes based on their mutant phenotype alone, either by insertional mutagenesis, or by screening antisense expression cDNA libraries. Finally, a nearly complete physical map of the genome is available and so genes are easily mapped by physical techniques. We discuss many of these advances within the context of major research problems presently under study.

From virus to vaccine: developments using the simple eukaryote, Dictyostelium discoideum

Mass vaccination compaigns against viral diseases, both human and anim al, depend on the availability of cheap viral antigens. The eukaryote Dictyostelium discoideum has simple growth requirements and rapid growth rates and forms stable cell lines. These features, together with the possibility of secreting recombinant (glyco)proteins into a defined buffer, make the D. discoideum expression system an attractive option for producing economical recombinant subunit vaccines.

Recombinant glycoprotein production in the slime mould Dictyostelium discoideum

Dictyostelium discoideum is a well known amoeboid organism, with unicellular and multicellular life-cycle stages, that is used for studying cell and developmental biology. With advances in gene-disruption technology and transformation of this organism, many homologous proteins have been expressed either to complement defective proteins or to study basic cell biology. Now, D. discoideum is being used to express heterologous proteins that are difficult to study in other systems, and its unique cell biology is being exploited to facilitate a wide range of protein modifications. In the past year, substantial progress has been made in expressing correctly folded forms of malarial circumsporozoite antigen and rotavirus surface glycoprotein VP7. Exciting developments have also been made in expressing human muscarinic receptors.

Overexpression of myosin motor domains in Dictyostelium: screening of transformants and purification of the affinity tagged protein

The eukaryotic organism Dictyostelium discoideum has become one of the organisms of choice for the overexpression of recombinant myosins and myosin fragments. Here, we describe a protocol that facilitates the screening of cells that have been transformed with myosin expression constructs and allows the rapid purification of recombinant myosins. Depletion of cellular ATP is used to recruit most of the endogenous and recombinant myosin into a rigor-like complex with actin. Following cell lysis the insoluble actomyosin complex is precipitated by centrifugation, washed, and Mg(2+)-ATP is added to extract the recombinant protein from the pellet. More than 90% of the protein in the resulting supernatant corresponds to actin, myosin, and the recombinant myosin fragments. Therefore, it is easy to detect any differences in expression level between individual myosin constructs on SDS-polyacrylamide gels. Additionally, the dependence of expression on external factors, such as cell density, can be readily determined. Furthermore, the presence of a band corresponding to the recombinant protein indicates that the overexpressed protein has at least some of the functional properties that are characteristic for a myosin motor. This rapid and selective extraction protocol can also be utilized to facilitate the purification of recombinant myosin motors on a preparative scale and has proved particularly useful in the purification of myosin head fragments, that are tagged with histidine residues, by Ni(2+)-chelate affinity chromatography.

Expression of the rotavirus SA11 protein VP7 in the simple eukaryote Dictyostelium discoideum

The outer capsid protein of rotavirus, VP7, is a major neutralization antigen and is considered a necessary component of any subunit vaccine developed against rotavirus infection. For this reason, significant effort has been directed towards producing recombinant VP7 that maintains the antigenic characteristics of the native molecule. Using a relatively new expression system, the simple eukaryote Dictyostelium discoideum, we have cloned the portion of simian rotavirus SA11 genome segment 9, encoding the mature VP7 protein, downstream of a native D. discoideum secretion signal sequence in a high-copy-number extrachromosomal vector. The majority of the recombinant VP7 expressed by transformants was intracellular and was detected by Western immunoblot following gel electrophoresis as two or three bands with an apparent molecular mass of 35.5 to 37.5 kDa. A small amount of VP7 having an apparent molecular mass of 37.5 kDa was secreted. Both the intracellular VP7 and the secreted VP7 were N glycosylated and sensitive to endoglycosidase H digestion. Under nonreducing electrophoresis conditions, over half the intracellular VP7 migrated as a monomer while the remainder migrated with an apparent molecular mass approximately twice that of the monomeric form. In an enzyme-linked immunosorbent assay, intracellular VP7 reacted with both nonneutralizing and neutralizing antibodies. The monoclonal antibody recognition pattern paralleled that found with VP7 expressed in either vaccinia virus or herpes simplex virus type 1 and confirms that D. discoideum-expressed VP7 is able to form the major neutralization domains present on viral VP7. Because D. discoideum cells are easy and cheap to grow, this expression system provides a valuable alternative for the large-scale production of recombinant VP7 protein.

Expression, purification and characterisation of secreted recombinant glycoprotein PsA in Dictyostelium discoideum

Dictyostelium discoideum is a newly developed eukaryotic expression system which is an alternative to tissue cultures for the production of recombinant proteins requiring eukaryotic folding and post-translational modifications. The homologous glycoprotein PsA (prespore specific antigen) is a glycosyl phosphatidylinositol (GPI) anchored membrane protein from D. discoideum. A truncated form of PsA has been expressed in D. discoideum and secreted into a peptone based broth at levels of 10 mg per 1 growth medium. A simple purification protocol for recombinant PsA (rPsA) involved three steps: the concentration of the culture supernatant by ammonium sulfate precipitation, Mono Q anion-exchange chromatography, followed by size exclusion chromatography on Superdex 75. 20 mg of rPsA was purified to 98% purity from 37.1 culture supernatant. Purified rPsA was characterised. The molecular mass of the purified rPsA is 15.6 kDa, which suggests that the molecule is secreted as a monomer and contains 12% (w/w) carbohydrate. The protein sequence of rPsA proved identical to that of the predicted DNA construct. Although the recombinant form of PsA is expressed at a different developmental stage from the native molecule, the same Thr residues that are O-glycosylated in the authentic molecule are glycosylated in the recombinant protein.