PKMζ Maintains Postsynaptic Structures in Dissociated Hippocampal Neurons: A Mechanism for Memory Maintenance.

Abstract

Protein kinase Mζ (PKMζ), the brain-specific, constitutively active PKC atypical isoform, has been shown to maintain long-term potentiation (LTP) and long-memory storage. Inhibition of PKMζ activity by the selective ζ-inhibitory peptide (ZIP) disrupts LTP maintenance. Similarly, infusion of ZIP into the brain appears to permanently erase established memory without preventing future learning. These findings indicate that PKMζ plays a pivotal role in memory storage, and PKMζ activity is required for maintenance of memory. To explore the underlying mechanisms of PKMζ in synaptic plasticity, I investigated its effect on spine morphology, thought to be involved in memory storage. It has been previously established that LTP can induce increases in dendritic spine size, which corresponds proportionally to the strength of the synapse. In addition, larger spines are more stable. Immunolabeling showed that PKMζ protein increased with forskolin, an agent known to induce “chemical LTP”. PKMζ inhibition with 10 μM ZIP or 10 μM chelerythrine, another PKMζ-selective inhibitor, reversed the chemical LTP induced increase in spine size. Overexpression of PKMζ increased dendritic spine size. PKMζ inhibition also decreased spine size of control neurons, thereby making PKMζ both necessary and sufficient for maintenance of spine size. Immunostaining for postsynaptic density-95 (PSD-95), the major component of the postsynaptic density, decreased with PKMζ inhibition, and PKMζ overexpression increased, PSD-95 clustering. Similarly, inhibition of PKMζ significantly reduced postsynaptic alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptor (AMPARs) subunit GluR2, the main postsynaptic receptors mediating fast excitatory transmission. Furthermore, overexpression of PKMζ increased postsynaptic GluR2 receptors. Blocking palmitoylation reverses PKMζ-mediated PSD-95 aggregation and synaptic GluR2 localization. Therefore, PKMζ plays a role in maintaining postsynaptic structures in dendritic spines.