Immobilized metal ion affinity partitioning of cells in aqueous two-phase systems: erythrocytes as a model

Recent advances in drug delivery applications of aqueous two-phase systems

Aqueous two-phase systems (ATPSs) are liquid-liquid equilibria between two aqueous phases that usually contain over 70% water content each, which results in a nontoxic organic solvent-free environment for biological compounds and biomolecules. ATPSs have attracted significant interest in applications for formulating carriers (microparticles, nanoparticles, hydrogels, and polymersomes) which can be prepared using the spontaneous phase separation of ATPSs as a driving force, and loaded with a wide range of bioactive materials, including small molecule drugs, proteins, and cells, for delivery applications. This review provides a detailed analysis of various ATPSs, including strategies employed for particle formation, polymerization of droplets in ATPSs, phase-guided block copolymer assemblies, and stimulus-responsive carriers. Processes for loading various bioactive payloads are discussed, and applications of these systems for drug delivery are summarized and discussed.

Development of separation technique for stem cells

In recent years, human embryonic stem cells have been established, and somatic stem cells derived from various adult organs have been identified and characterized to differentiate into various kinds of functional cells. There have been attempts to use functional cells induced from such stem cells for tissue regeneration and cell therapy. The method is expected to become an important treatment for intractable diseases in the near future. Since tissues and organs generally contain only a small quantity of somatic stem cells, and since it is necessary to separate functional cells generated from stem cells for use in therapy, an effective method for specific cell separation is crucial to the practical application of regenerative medicine. For the specific separation of cells, a fluorescence activated cell sorter using specific antibodies is a powerful tool, but the method is not suitable for large-scale processing and a special device is required. Although a magnetic cell separation system using immuno-magnetic fine particles is also commercially available, the system still needs special apparatus for large-scale processing. We developed a novel method for the separation of specific cells in an aqueous two-phase system using antibodies modified with a temperature-responsive polymer. The method enables the processing of a large quantity of cells without the requirement of a special device.

Combined hydrophobic-metal binding fusion tags for applications in aqueous two-phase partitioning

In this work, we studied the influence of fusion affinity tags containing both hydrophobic and histidines residues on the partitioning of the green fluorescent protein, GFPuv, in aqueous two-phase system. The tags were fused to the N-terminal of GFPuv and tested by immobilized metal affinity partitioning, in a PEG/salt system. The presence of both types of residues in the tag increased the partitioning greatly. Particularly, four engineered tags (H6, FH6, WH6, and YH6) containing a hexa-histidine sequence as well as different hydrophobic residues, all increased partitioning more than twice, reaching K values around 20, as compared to another construct (His6-GFP) containing an isolated hexa-histidine sequence. YH6, also proved be beneficial for protein expression.

Type-specific separation of animal cells in aqueous two-phase systems using antibody conjugates with temperature-sensitive polymers

A new type of aqueous two-phase system (ATPS) has been developed in which a temperature-sensitive polymer, poly-N-isopropylacrylamide [poly (NIPAM)] was used as a ligand carrier for the specific separation of animal cells. Monoclonal antibodies were modified with itaconic anhydride and copolymerized with N-isopropylacrylamide, and the ligand-conjugated carriers were added to the polyethylene glycol 8000-dextran T500 aqueous two-phase systems. The antibody-polymer conjugates were partitioned to the top phase in the absence or presence of 0.15 M NaCl. When ligand-conjugated carriers were used, more than 80% of the cells were specifically partitioned to the top phase in the presence of NaCl up to 0.1 M. The cells were partitioned almost completely to the bottom phase at 0.1 M NaCl or above, when no antibody-conjugate was added in the ATPS. As a model system, CD34-positive human acute myeloid leukemia cells (KG-1) were specifically separated from human T lymphoma cells (Jurkat) by applying anti-CD34 conjugated with poly-N-isopropylacrylamide in the aqueous two-phase system. By the temperature-induced precipitation of the polymer, about 90% of the antibody-polymer conjugates were recovered from the top phase, which gave approximately 75% cell separating efficiency in the next cycle of reuse.

Perspectives of immobilized-metal affinity chromatography

Immobilized Metal-Affinity Chromatography (IMAC) represents a relatively new separation technique that is primarily appropriate for the purification of proteins with natural surface-exposed histidine residues and for recombinant proteins with engineered histidine tags or histidine clusters. Because the method has gained broad popularity in recent years, the main recent developments in the field of new sorbents, techniques and possible applications are discussed in this article. Advantages of the method and new prospects are described as well as the problems and concerns that appear when the method is to be used for production of pharmaceutical-grade proteins.

Comparative studies of recombinant human granulocyte-colony stimulating factor, its Ser-17 and (His)6-tagged forms interaction with metal ions by means of immobilized metal ion affinity partitioning. Effect of chelated nickel and mercuric ions on extraction and refolding of proteins from inclusion bodies

The chelation capability of the reactive dye Light Resistant Yellow 2KT towards metal ions, particularly mercury(II) was evaluated in the pH range 5.0-7.0, and it was shown that the dye-Hg(II) complex has a free site for the interaction with human recombinant granulocyte-colony stimulating factor (rhG-CSF) from Escherichia coli. Affinity partitioning of three rhG-CSF forms--native, rhG-CSF[Cys17--->Ser17] and (His)6-rhG-CSF was studied in aqueous two-phase systems, which contained metal ions--Cu(II), Ni(II) and Hg(II)--chelated by dye-poly(ethylene glycol) at pH 5.0 and 7.0, in the presence or absence of many selected agents. It was determined, that chelated Ni(II) ions exhibited stronger interaction with the hexahistidine-tagged protein form, while the extraction power of Cu(II) ions was found to be of comparable order of magnitude for all three protein forms at pH 7.0. A comparative study of rhG-CSF and both its forms partitioning in the presence of chelated Hg(II) ions at pH 7.0 and 5.0 revealed possible direct interaction between Hg(II) ions and unpaired Cys-17 of rhG-CSF. The partitioning of three rhG-CSF forms inclusion body extract was studied in the presence of chelated Ni(II) and Hg(II) ions thus explaining the efficiency of targeted proteins renaturation gained upon their inclusion body forms interactions with chelated metal ions.

Effects of specific binding reactions on the partitioning behavior of biomaterials

Affinity partitioning is a special branch of biomaterials separations using aqueous two-phase systems. It combines the capability of diverse biomolecules to partition in aqueous two-phase systems using the principle of biorecognition. As a result, the macromolecule exhibiting affinity for a certain ligand is transferred to that phase where the ligand is present. This chapter describes the present status of the theoretical background of this approach and the properties of various natural and artificial compounds which act as affinity ligands in liquid-liquid systems. The affinity partitioning of proteins (enzymes and plasma proteins), cell membranes, cells, and nucleic acids are described as typical examples. The results are discussed in terms of theoretical understanding and practical application.

Concerning the separation of mammalian cells in immobilized metal ion affinity partitioning systems: a matter of selectivity

The effect of introducing an immobilized metal ion ligand in the lower phase of the PEG/Dextran system was studied on the erythrocytes and lymphocytes partition. The ligand in the lower phase was added as an insoluble form [Sepharose-IDA-M(II)] with or without a ligand in the upper phase. We first checked that the addition of the insoluble ligand in the system did not affect the phase volume and settling, and also that Sepharose-IDA-M(II) partitioned strictly in the lower phase. Then we studied the partition of cells with various concentrations of ligand in the lower and upper phases. We clearly demonstrate here that the partition in immobilized metal ion affinity partitioning (IMAP) systems is correlated with the affinity between the cell surface and the ligand. Cells are attracted to the ligand-containing phase. This fact is important not only for the greater understanding of IMAP, but could also for the separation of some types of cells.

Study of human cord blood lymphocytes by immobilized metal ion affinity partitioning

The potential of immobilized metal ion affinity partitioning (IMAP) using dextran-PEG+PEG-IDA-M(II) systems to separate mononuclear cells from cord blood has been evaluated. The distribution of B cells, T cells, monocytes and hematopoietic stem cells between PEG and dextran phases was determined by flow cytometry with fluorochrome-labelled specific antibodies. Comparing these values with the post-Ficoll repartition resulted in the determination of enrichment factors, for each subpopulation, in the different phases. We were able to distinguish the partition pattern of B cells, T cells, monocytes and stem cells in different IMAP systems. Their partition was affected by the nature and the concentration of the metal used, but no specificity in distribution for the subpopulations was found.

Factors affecting the amount and the mode of merocyanine 540 binding to the membrane of human erythrocytes. A comparison with the binding to leukemia cells

In the presence of albumin Merocyanine 540 (MC540) exhibits a very limited binding to the outer surface of the membrane of normal erythrocytes, whereas pronounced binding is observed to leukemia cells. To find out whether this difference is due to differences in the composition or structural organization of the cell membrane we analyzed effects of a number of covalent and non-covalent perturbations of the red cell membrane on the binding and fluorescence characteristics of membrane-bound MC540. It is shown that exposure of the cells to cationic chlorpromazine, neuraminidase or photodynamic treatment with AlPcS4 as sensitizer caused a limited increase (30-50%) of MC540 binding, together with a red shift of the fluorescence emission maximum and an increase of the relative fluorescence quantum yield of membrane-bound MC540. Other forms of perturbation of the membrane structure, like hyperthermia (48 degrees C) and treatments that produce a decrease of phospholipid asymmetry in addition to accelerated flip-flop, did not result in increased MC540 binding, but did cause a red shift of the fluorescence emission maximum and an increase of the relative fluorescence quantum yield. These changes in fluorescence properties indicate a penetration of the dye into more hydrophobic regions in the membrane. MC540, bound to Brown Norway myelocytic leukemia cells, exhibited a red shift of the fluorescence emission maximum and an increased relative fluorescence quantum yield as compared to MC540 bound to untreated erythrocytes. These changes were of the same order of magnitude as in photodynamically treated red blood cells. Dye binding per surface area, however, was about 3-times higher with these leukemia cells than with photodynamically treated red blood cells. This demonstrates that certain perturbations of the erythrocyte membrane evoked a MC540 binding that became qualitatively comparable to the dye binding to leukemia cells, although dye binding per surface area was still significantly lower.

Segregation of normal and pathological human red blood cells, lymphocytes and fibroblasts by immobilized metal-ion affinity partitioning

Immobilized metal-ion affinity partitioning (IMAP) is shown to be useful as a preliminary screening test and for the separation of different cell populations based upon recognition of the differences in the proteins on cell surfaces. The feasibility of using IMAP to segregate a spectrum of normal human cells (red blood cells, lymphocytes and fibroblasts) from their counterpart pathological cells has been demonstrated. A clear segregation between normal and sickle-cell anemia red blood cells (RBC), or malaria (Plasmodium vivax) infected RBCs was obtained. Further, the partition differences were found to depend on the nature and the concentrations of metal used. Cells from breast cancer and those from the lung adenocarcinoma showed differences in their partition pattern as compared to normal fibroblasts when PEG-IDA-M(II) was added to the phase system. Maximum differences between the three cell populations were observed in the presence of 10% PEG-IDA-Ni(II). Normal lymphocytes and Burkitt's lymphoma cells (Raji and Namalwa cell lines) were shown to partition differently in the presence of PEG-IDA-M(II) in the phase system. Normal lymphocytes could be distinguished from the Burkitt's lymphoma cell lines in all three phases (top, interface and bottom), in the presence of 10% PEG-IDA-Ni(II) in the system. These differences in the partition behavior could mainly be attributed to the density, surface exposure and micro-environment of histidine residues of cell membrane-associated proteins. These data, along with those obtained for normal and pathological human cells show that IMAP could be a simple and versatile tool for the segregation and study of cells.

Immobilized metal-ion affinity partitioning of NAD(+)-dependent dehydrogenases in poly(ethylene glycol)-dextran two-phase systems

Affinity partitioning of yeast alcohol dehydrogenase (YADH), lactate dehydrogenase from rabbit muscle (MLDH) and lactate and malate dehydrogenases from pig heart (HLDH and HMDH, respectively) were studied in aqueous two-phase systems containing metal ions (Cu2+, Ni2+, Zn2+ and Cd2+) chelated by iminodiacetate-poly(ethylene glycol) (IDA-PEG). The partitioning behaviour of the enzymes in the presence of Cu(II)-IDA-PEG was studied as a function of the concentration of NaCl, the pH of the medium and the concentration of added selected agents. It was demonstrated that the partition effect (delta log K) of dehydrogenases in the presence of Cu(II)-IDA-PEG and the affinity of enzymes for immobilized Cu2+ ions increases in the order MLDH > YADH > HMDH > or = HLDH. It was shown that the determined variations in the enzyme affinities for Cu(II)-IDA-PEG might be related to the differences in the content of histidine residues accessible to the solvent.

Recognition and separation of isoenzymes by metal chelates. Immobilized metal ion affinity partitioning of lactate dehydrogenase isoenzymes

Poly(ethylene glycol) (PEG)-bound chelated metal ions partition preferentially into the top, PEG-rich, phase of a PEG-salt or PEG-dextran aqueous two-phase system. Extraction by this soluble affinity ligand of proteins is due to a selective interaction of the chelated metal ion with accessible histidine residues on the protein surface. Using Cu-iminodiacetate-PEG (Cu-IDA-PEG) the surface of lactate dehydrogenase (LDH) isoenzymes from different species was probed for the presence of metal chelate binding sites. It was demonstrated that the homotetramers (LDH-1)(H4) from rabbit, bovine and pig displayed weak binding to chelated copper whereas the M4-type isoenzymes (LDH-5) bound strongly to this ligand. The binding of the different heterotetramers increases as the number of M-type subunits increases. In contrast, the human isoenzymes are bound to chelated copper in a reversed sequence. The comparison of the affinity partitioning effect of Cu-IDA-PEG in PEG-salt and PEG-dextran systems revealed that the discriminatory effect of copper is promoted by high salt concentrations. Resolution of isoenzymes by multiple extraction using counter-current distribution provides valuable data on the partitioning of enzymes relative to that of the bulk proteins. The efficacy of metal chelate affinity partitioning for the purification of LDH from tissue samples by batchwise extraction was also demonstrated.

Immobilized metal ion affinity electrophoresis. A study with several model proteins containing histidine

Immobilized metal ion affinity electrophoresis (IMA-Elec) is one among the many methods derived from the immobilized metal ion affinity chromatography. Two approaches for incorporating the metal ligand, were studied. One was in the form of insoluble particulate material based on Sepharose 6B and the other in the form of soluble polymer based on polyethylene glycol (PEG) 5000. Both the polymers coupled with iminodiacetate and metallized with copper or zinc were used as ligands, incorporated into soluble agarose as the electrophoretic gel. Several histidine-containing model proteins were studied with both the systems and their metal binding strengths were determined as the dissociation constants, Kd. The results clearly demonstrated that the mechanism of protein recognition by immobilized copper or zinc via the accessible histidyl residues was maintained in the IMA-Elec system. Proteins with increasing numbers of histidine residues showed increasing binding strength (lower Kd values). While this basic mechanism was conserved, the supporting polymers (Sepharose 6B and the PEG 5000) showed significant differences in the metal binding to the protein. The polysaccharide Sepharose 6B enhanced the binding strength compared with PEG 5000. The optimum electrophoretic parameters were determined to be current intensities up to 20 mA and pH ca. 7.0. At pH greater than 8.0, a significant decrease in the affinity was observed, this decrease being greater with PEG 5000 than Sepharose 6B as supporting material.

Ligand-receptor interactions in affinity cell partitioning. Studies with transferrin covalently linked to monomethoxypoly(ethylene glycol) and rat reticulocytes

The partitioning of rat reticulocytes in poly(ethylene glycol) (PEG)-dextran two-phase systems increases into the PEG-rich top phase when the cells are incubated with transferrin covalently modified with monomethoxy-PEG (MPEG-transferrin) prior to partitioning. Two observations support the suggestion that such an increase in top-phase partitioning is due to the specific interaction of the MPEG-transferrin conjugate with the transferrin receptor on the surface of the reticulocyte: first, the MPEG-transferrin conjugate competes with [125I]transferrin for the transferrin receptor on reticulocytes (Ka = 6.28 x 10(6) l mol-1); and second, the MPEG-modified transferrin is unable to change the partitioning of rat erythrocytes, cells lacking the transferrin receptor. This example illustrates the feasibility of manipulating the partitioning of a selected cell population when ligand-receptor interactions are exploited. The increase in the partitioning of the reticulocytes takes place within a narrow range of MPEG-transferrin bound per cell, viz., 10.2-11.3 fg per cell. The latter range corresponds to ca. 80,000-89,000 molecules of MPEG-transferrin bound per cell.