Differential cellular and subcellular localization of protein phosphatase 1 isoforms in brain

Protein phosphatase 1 (PP1) is a gene family with a number of important functions in brain. Association with a wide variety of regulatory/targeting subunits is thought to be instrumental in directing the phosphatase to specific subcellular locations and substrates. By using antibodies directed against specific PP1 isoforms, we asked whether PP1 isoforms are differentially distributed in brain. Immunoblotting detects in brain the PP1γ2 isoform, which had previously been thought to be testis specific, in addition to α, β, and γ1 isoforms. PP1 isoform expression varies modestly in extracts from different subdissected brain regions and is relatively constant during postnatal development, except for an about twofold increase in PP1γ2. By immunohistochemical analyses of rat brain, PP1β and PP1γ1 cellular expression is widespread but quite distinct from one another. Subcellular fractionation studies demonstrate that PP1β and PP1γ1 are selectively associated with different cytoskeletal elements: PP1β with microtubules, PP1γ1 with the actin cytoskeleton. Double‐immunofluorescence labeling of cultured cortical neurons further reveals a strikingly different and nonoverlapping localization of PP1β and PP1γ1: whereas PP1β localizes to a discrete area of the soma, PP1γ1 is highly enriched in dendritic spines and presynaptic terminals of cultured neurons. These results show that PP1 isoforms are targeted to different neuronal cytoskeletal compartments with a high degree of specificity, presumably by isoform‐specific association with regulatory/targeting proteins. Furthermore, the synaptic localization of PP1γ1 indicates that it is this isoform that is involved in the regulation of synaptic phosphoproteins such as neurotransmitter receptors and ion channels implicated in synaptic plasticity. J. Comp. Neurol. 413:373–384, 1999. © 1999 Wiley‐Liss, Inc.