Brain postsynaptic densities: the relationship to glial and neuronal filaments.

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Abstract
Preparations of isolated rat brain postsynaptic densities (PSD) contain a characteristic set of proteins. The most prominent has a MW of .apprx. 50,000. Since this major PSD protein may relate to a similarly sized component of neurofilaments (F. Blomberg et al.), evidence of neurofilament proteins among the PSD polypeptides was investigated. A novel technique for detecting protein antigens in SDS[sodium dodecyl sulfate]-polyacrylamide gels (immunoblotting) and an antiserum that was selective for neurofilaments in immunohistochemical tests were used. As a control, an antiserum against glial filament protein (GFAP) was used because antisera against GFAP stain only glial cells in immunohistochemical tests. They were not expected to react with PSD occurring only in neurons. PSD contained neuronal and glial filament proteins at higher concentrations than synaptic plasma membranes, myelin or myelinated axons. Immunoperoxidase staining of histological sections with the same 2 antisera was contradictory, implying PSD in intact brain tissue contained no neuronal or glial filament proteins and the intermediate filament proteins in isolated PSD preparations were contaminants. The proteins of isolated brain intermediate filaments were labeled with 125I and added to brain tissue at the start of a subcellular fractionation schedule. Neuronal and glial filament proteins stuck selectively to PSD during the isolation procedure. The stickiness of PSD for brain cytoplasmic proteins implied that biochemical analysis of subcellular fractions was insufficient to establish a given protein as a synaptic junctional component. An immunohistochemical localization at PSD in intact tissue, which is available for tubulin, phosphoprotein I and calmodulin, appears essential for confirmation. Isolated preparations of brain intermediate filaments apparently contain neuronal and glial filaments.