Antigen-specific electrophoretic cell separation for immunological investigations

Free-flow electrophoresis for purification of plant mitochondria by surface charge

Sample purity is the key for a successful in-depth analysis of any given subcellular proteome. The suitability of free-flow electrophoresis to assist conventional, centrifugation-based techniques in the preparation of plant mitochondria from green and non-green tissue was assessed by various means, including functional assays, immunoblots, electron microscopy and differential gel electrophoresis. Results indicated a significant increase in purity of the mitochondrial samples, highlighted specific contaminants previously reported as mitochondrial proteins, and also pointed to new means for separating plastids and peroxisomes from mitochondria in plant organellar extracts by exploiting differences in surface charge. This approach has the potential to allow a deeper and more comprehensive investigation of the Arabidopsis organellar proteomes, by providing a second dimension of separation based on surface charge in addition to conventional centrifugation purification protocols relying on size and density.

Subcellular fractionation, electromigration analysis and mapping of organelles

Subcellular fractionation has provided the means required to analyze the composition and properties of purified cellular elements. In particular, subcellular fractionation has helped to define membrane boundaries and became necessary for the development of cell-free assays that reconstitute complicated cellular processes. Although cell fractionation techniques have improved over the last decades the purification of organelles to homogeneity is still a barely accessible goal in cell biology. In this article, we will first briefly review the basic principles of subcellular fractionation, and the establishment of different organelle fractions by density centrifugation, using tissue culture cells as a paradigm. Then we will discuss some of the intrinsic problems and will compare gradient purification of cellular extracts with electromigration analysis. Finally, we will describe alternative approaches, such as immunoisolation and flow cytometry to purify organelles from tissue culture cells.

Lymphocyte fractionation using immunomagnetic colloid and a dipole magnet flow cell sorter

The relationship between cell function and surface marker expression is a subject of active investigation in biology and medicine. These investigations require separating cells of a homogeneous subset into multiple fractions of varying marker expression. We have developed a novel cell sorter, the dipole magnet flow sorter (DMFS), which separates selected T lymphocyte subpopulations, targeted by immunomagnetic colloid, into multiple fractions according to cell surface marker expression, as determined by flow cytometry. A narrow stream of cells is introduced into a sheath of carrier fluid in a rectangular channel while subjected to a perpendicular magnetic force. The special design of the pole pieces ensures a constant magnetic force acting on the magnetically labeled cells in the separation area. Cells are spread across the flow in relation to their magnetophoretic mobility. Separation is achieved by control of the positions of the effluent stream boundaries, which separate fluid volumes with cells of different magnetophoretic mobility. CD4 and CD8 T lymphocytes labeled with primary antibody-fluorescein isothiocyanate (FITC) conjugate and anti-FITC-magnetic colloid are the chosen cell systems. Flow cytometry analysis shows that, for CD4 cells, a three-fold increase in total marker number per cell is observed when comparing the highest to the lowest fluorescence fractions. Similarly, a four-fold increase in total marker number is observed for CD8 cells. We also observed the separation of two dissimilar cell types that differed in expression of the CD4 marker, monocytes and T helper lymphocytes. We believe that this type of separation is applicable to any cells in suspension for which a suitable antibody exists and, due to the comparatively gentle nature of the process, is particularly suitable for the sorting of fragile cells.

Isolation of peroxisome subpopulations from rat liver by means of immune free-flow electrophoresis

Immune free-flow electrophoresis (IFFE) has been applied to the separation of peroxisomes (PO). IFFE is a modification of antigen-specific electrophoretic cell separation (ASECS), and combines the advantages of electrophoretic separation with the high selectivity of an immune reaction. It differs from the latter in the pH of the electrophoresis buffer, which was shifted from the physiological range (ASECS) to the pI of IgG molecules (pH approximately 8.0), thus further decreasing the mobility produced by the binding of a specific antibody. This enhances the mobility differences between IgG-coupled particles and those nondecorated, with resultant improved separation. We have now succeeded in isolating different subpopulations of PO by applying IFFE to heavy, light, and post-mitochondrial fractions separated by differential centrifugation of a rat liver homogenate. The obtained PO subfractions differed in their composition of matrix and membrane proteins, as revealed by immunoblotting. This indicates that they indeed represent distinct subpopulations of rat hepatic PO.

Isolation of rat hepatic peroxisomes by means of immune free flow electrophoresis

Rat hepatic peroxisomes (PO) were separated from other cell organelles by free flow electrophoresis (FFE) in combination with immunocomplexing PO prior to FFE with an antibody directed against the cytoplasmic aspect of the peroxisomal membrane protein PMP 70. This novel approach is based on a method termed antigen-specific electrophoretic cell separation (ASECS) which was originally introduced for the isolation of human T and B lymphocyte subpopulations by Hansen and Hannig (J. Immunol. Methods 1982, 51, 197-208). We adapted this technique to PO isolation from a crude peroxisomal fraction, streamlining it by the following modifications: (i) The sandwich-technique recommended to further lower a negative surface charge was renounced. (ii) Instead, the pH of the electrophoresis buffer was raised from 7.2 to 8.0, thus minimizing the electrophoretic mobility of the particles immunocomplexed due to the fact that the isoelectric point (pI) of IgG molecules is close to pH 8.0. PO isolated by this modification, referred to as immune free flow electrophoresis (IFFE), are as pure, intact, and structurally well-preserved as are highly purified PO obtained by density gradient centrifugation. The technique is currently applied for the isolation of peroxisomal subpopulations that are difficult to obtain by means of density gradient centrifugation.

Flow cytometry and cell-separation procedures

A series of small incremental advances characterize the year's technical developments in flow cytometry, and these now extend the use of the technique to even the molecular level. The increasing application of immunomagnetic-bead separation procedures dominates the developments in other cell-separation procedures.