Long-term stabilization of PKMζ in dendritic spines in vitro and differential PKMζ protein regulation in environmental enrichment in vivo.

Abstract

The constitutively active Protein kinase C isoform, PKMζ, is both necessary and sufficient for maintaining long-term potentiation (LTP), and is critical for the maintenance of several forms of long-term memory. PKMζmaintains enhanced synaptic function during LTP and long-term memory by retaining GluA2-containing AMPA receptors at the postsynaptic site. Since ZIP inhibition of PKMζirreversibly disrupts longterm synaptic enhancement, we hypothesize that the location of PKMζshould be stable at specific synapses in order for the persistent kinase activity to maintain synapse specific information storage during LTP and memory, and that the stability could potentially be maintained by kinase activity-dependent mechanisms.

In the first study, we examined the stability of PKMζlocalization in dendritic spines of cultured hippocampal neurons and compared that to the rate of movement of PKMζin other compartments of the cell. eGFP-PKMζand eGFP-dnPKMζwere overexpressed in cultured hippocampal neurons and we examined fluorescent PKMζ-fusion protein mobility by fluorescent recovery after photobleaching (FRAP) analysis. We showed that PKMζprotein mobility in dendritic spines is limited, which may be due to kinase activity-dependent and very tight kinase activity-independent protein interactions. We speculate that immobilization of PKMζin the dendritic spine through tight protein interactions may be what keeps PKMζstable at specific synapses to maintain long-lasting synaptic efficacy in vivo.

PKMζplays other roles in brain plasticity. During brain development PKMζ activity affects morphologic stabilization of the dendritic arbor, and PKMζ downregulation was reported to underlie spatial familiarity.

In a second study, we examined the effect of environmental enrichment on PKMζprotein in the hippocampus and showed that PKMζprotein levels are downregulated across the different hippocampal zones and regions of female mice that were housed in an enriched environment (enriched female mice). Interestingly, we found no substantial PKMζdownregulation in enriched male mice, indicating that environmental enrichment affects PKMζin a sex-specific manner. Further, we found that environmental enrichment does not affect the performance of female mice on the hippocampus-dependent place-avoidance task, but did affect anxiety behavior. As this study only provides correlational data, we can only speculate about how environmental enrichment and subsequent changes in PKMζprotein levels affect animal plasticity and behavior. Based on previous studies of PKMζdownregulation, we speculate that enrichment-induced downregulation of PKMζprotein levels may enhance information processing by resetting the hippocampal synapses by means of depotentiation or longterm depression (LTD)-like mechanisms, and/or may underlie the reduced anxiety observed in enriched animals.

These studies yield fundamental new information regarding the role of PKMζin learning and memory. First, we found and quantified stable compartmentalization of PKMζin dendritic spines. Second, we found a potential role for PKMζin enriched environment-induced plasticity.