Using a novel approach, including affinity chromatography, reversed-phase chromatography, and chemiluminescence immunoblotting, we have for the first time been able to demonstrate one of the small synaptic vesicle
proteins,
synaptotagmin I, in cerebrospinal fluid (CSF). Two other small synaptic vesicle
proteins, rab3a and
synaptophysin, were not detectable. The approximate molecular weight of CSF-
synaptotagmin was 65 kDa, as determined by
sodium dodecyl sulfate-
polyacrylamide gel electrophoresis (SDS-PAGE). Further characterization of CSF
synaptotagmin by high-performance capillary electrophoresis (HPCE) showed a single peak. These findings support that the whole
synaptotagmin molecule is present in CSF, without significant proteolytic degradation. After high-speed centrifugation of CSF,
synaptotagmin was exclusively found in the supernatant, suggesting that
synaptotagmin is present in CSF as a free
protein, and not as a constituent of synaptic vesicles. In a preliminary study, we found a marked reduction of CSF
synaptotagmin in patients with
early onset Alzheimer disease (EAD) as compared with age-matched healthy individuals. To elucidate the
biological relevance of this finding, we also quantified
synaptotagmin in brain tissue. A marked reduction in
synaptotagmin was found both in the hippocampus and frontal cortex of EAD, suggesting that a decrease in
synaptotagmin in the brain is followed by a concomitant decrease in the CSF. Analysis of CSF
synaptotagmin might provide a tool to study synaptic function and pathology in the human brain.