Subcellular Fractionation of Brain Tissue Using Free-Flow Electrophoresis

Accurate annotation of protein identifications in organellar proteomics highly depends on the sample quality with special respect to contaminations from other subcellular compartments. In this respect, Free-flow electrophoresis (FFE) offers a valuable alternative to classical centrifugation techniques, since it relies on quite different physical parameters. During the last years, FFE has been successfully used for the separation of various organelles from different tissues, yet is largely unknown in the field of neurobiology. Here we present two separation schemes for the fractionation of a synaptic preparation from rat brain using different modes of FFE. Isotachophoresis (ITP), a focusing technique separating organelles according to their electrophoretic mobilities, was able to distribute the synaptosome sample into different subfractions: mitochondrial cross contaminations showed the highest electrophoretic mobility and migrated nearest to the anode of the FFE instrument; proximate to these, proteins of the presynaptic compartment accumulated, whereas nearest to the cathode of the instrument postsynaptic marker proteins were predominantly found. As a nonfocusing technique, zonal FFE does not possess a separation capacity comparable to ITP; however, due to a continuous separation mode, it is adapted to process higher sample amounts and can be used for large-scale separations. We applied zonal FFE to the same starting material as in ITP and were able to separate mitochondria from synaptic material of the preparation, thus offering a fast alternative to clean synaptosome preparations from residual mitochondrial contaminations.