Cellular and subcellular localization of PKMζ

In contrast to protein kinases that participate in long-term potentiation (LTP) induction and memory consolidation, the autonomously active atypical protein kinase C isoform, protein kinase Mzeta (PKMζ), functions in the core molecular mechanism of LTP maintenance and long-term memory storage. Here, using multiple complementary techniques for light and electron microscopic immunolocalization, we present the first detailed characterization of the cellular and subcellular distribution of PKMζ in rat hippocampus and neocortex. We find that PKMζ is widely expressed in forebrain with prominent immunostaining in hippocampal and neocortical grey matter, and weak label in white matter. In hippocampal and cortical pyramidal cells, PKMζ expression is predominantly somatodendritic, and electron microscopy highlights the kinase at postsynaptic densities and in clusters within spines. In addition, nuclear label and striking punctate immunopositive structures in a paranuclear and dendritic distribution are seen by confocal microscopy, occasionally at dendritic bifurcations. PKMζ immunoreactive granules are observed by electron microscopy in cell bodies and dendrites, including endoplasmic reticulum. The widespread distribution of PKMζ in nuclei, nucleoli and endoplasmic reticulum suggests potential roles of this kinase in cell-wide mechanisms involving gene expression, biogenesis of ribosomes and new protein synthesis. The localization of PKMζ within postsynaptic densities and spines suggests sites where the kinase stores information during LTP maintenance and long-term memory.