Subcellular distribution of spinophilin immunolabeling in primate prefrontal cortex: Localization to and within dendritic spines

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Abstract
Signal transduction in the nervous system depends on kinases and phosphatases, whose localization is regulated by a large group of scaffolding proteins. In particular, protein phosphatase-1 mediates dopamine's actions on a variety of substrates, including glutamate receptors, and this, in turn, depends on the binding of protein phosphatase-1 to its binding protein spinophilin. To better understand spinophilin's role in targeting protein phosphatase-1 within neurons, we used a combination of preembedding immunoperoxidase and postembedding immunogold labeling and electron microscopy to determine the localization of this scaffolding protein in macaque prefrontal cortex. Consistent with previous reports, spinophilin was found predominantly in dendritic spines, but a significant number of labeled dendritic shafts and, less frequently, glia and preterminal axons were also identified. By using the postembedding immunogold method, we further examined the distribution of spinophilin within dendritic spines. Spinophilin immunoreactivity was present throughout the spine, but the density of label was heterogeneous and defined two domains. The highest density of label was associated with the postsynaptic density and the 100 nm immediately subjacent to it. The deeper region of the spine, further than 100 nm from the postsynaptic density, had a lower density of spinophilin label. The distribution of spinophilin reported in this study supports its role in modulating glutamatergic neurotransmission but also suggests the possibility that spinophilin may target protein phosphatase-1 to other sites within the spine or to other neuronal or glial compartments.