Cell Biological mechanisms of memory
We study the genes and signaling pathways involved in the consolidation of long-term memories, in particular memories of aversive events associated with environmental contexts. The long-term goal of our research is to understand the molecular mechanisms that determine how these contextual memories regulate the acquisition and extinction of fear responses and how are they influenced by stressful experiences. The work pursues the hypothesis that disruption of molecular signaling within the hippocampal brain area results in abnormal fear of threatening environments resembling symptoms of anxiety disorders. Our current research predominantly focuses on the characterization of the roles of the hippocampal serine/threonine kinases cyclin-dependent kinase 5 (Cdk5) and extracellular signal-regulated kinases (Erk 1/2) in memory formation. We combine behavioral mouse models with molecular, biochemical and immunohistochemical approaches to unravel the roles of these signaling pathways in vivo.

Figure 1. Localization of activated Erk-1/2 during acquisition of context-dependent fear. The levels of double phosphorylated Erk-1/2 (pErk-1/2) were determined 1 hr after context-dependent fear conditioning of mice. The localization of pErk-1/2 (green) was compared with the localization of the phosphorylated cAMP-dependent regulatory element binding protein (pCREB). In the hippocampal CA3 subfield (a) pCREB is localized in the nuclei of pyramidal neurons whereas pErk-1/2 is predominantly detected in the mossy fibers. On the other hand, in the central amygdala (b) both proteins typically co-localize in the nucleus. These patterns suggest a unique role of pErk-1/2 signaling in the hippocampus during memory consolidation of context-dependent fear.

We demonstrated that Cdk5 and its neuronal co-activator p35 within the cholinergic septo-hippocampal system are required for long-term memory consolidation of context-dependent conditioned fear. Analysis of different mouse strains revealed a conserved role of Cdk5/p35 in associative learning, however, the regulation of baseline and inducible Cdk5 gene expression was strain-specific and probably dependent on Sp transcription factor levels. The regulation of septo-hippocampal Cdk5 activity by upstream activators is currently investigated.

We have been studying in detail the regulation of Erk-1/2 activity and phosphorylation by upstream kinases as well as the activation of downstream Erk-1/2 substrates during fear conditioning and stress-enhanced fear conditioning. We demonstrated cytoplasmic interactions between Erk-1/2 an Elk-1 followed by nuclear translocation of their substrate p90Rsk-1 but not Elk-1 in hippocampal pyramidal neurons. This pattern was specific for the hippocampal area and contrasted findings from other regions, such as the basolateral and central amygdala, demonstrating nuclear localization of Erk-1/2 and Elk-1. It has been assumed that the mitogen activated and extracellular signal-regulated kinase (Mek) is the main activator of Erk-1/2. Unexpectedly, we observed that cAMP-dependent protein kinase (PKA) and protein kinase C (PKC) but not Mek are the major regulators of hippocampal Erk-1/2 phosphorylation during fear conditioning. Notably, a Mek-dependent Erk-1/2 activation after stress-enhanced fear conditioning that was triggered by hippocampal CRF2 receptors. The role of the described mechanisms in fear extinction has been currently studied. Another line of experiments is aimed to delineate the mechanisms of acquisition and extinction of context-dependent conditioned fear. Contrary to the expectation that extinction processes are based on protein-synthesis-dependent learning, we demonstrated that some forms of fear extinction might predominantly depend on cytoskeletal rearrangement. The molecular aspects of extinction involving the hippocampal formation are currently being investigated in detail.