Identification of synaptic vesicle, pre- and postsynaptic proteins in human cerebrospinal fluid using liquid-phase isoelectric focusing

Synaptic pathology is central in the pathogenesis of several psychiatric disorders, for example in Alzheimer's disease (AD) and schizophrenia. Quantification of specific synaptic proteins has proved to be a useful method to estimate synapitc density in the brain. Using this approach, several synaptic proteins have been demonstrated to be altered in both AD and schizophrenia. Until recently, the analysis of synaptic pathology has been limited to postmortem tissue. In living subjects, these synaptic proteins may be studied through analysis of cerebrospinal fluid (CSF). In an earlier study performed by us, one synaptic vesicle specific protein, synaptotagmin, was detected in CSF for the first time using a procedure based on affinity chromatography, reversed-phase chromatography, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and chemiluminescence immunoblotting. However, other synaptic proteins were not detectable with this procedure. Therefore, we have developed a procedure including precipitation of CSF proteins with trichloroacetic acid, followed by liquid-phase isoelectric focusing using the Rotofor Cell, and finally analysis of Rotofor fractions by Western blotting for identification of synaptic proteins in CSF. Five synaptic proteins, rab3a, synaptotagmin, growth-associated protein (GAP-43), synaptosomal-associated protein (SNAP-25) and neurogranin, have been demonstrated in CSF using this method. The major advantage of liquid-phase isoelectric focusing (IEF) using the Rotofor cell is that it provides synaptic proteins from CSF in sufficient quantities for identification. This method may also be suitable for identification of other types of trace amounts of brain-specific proteins in CSF. These results demonstrate that several synaptic proteins can be identified and measured in CSF to study synaptic function and pathology in degenerative disorders.