The hippocampal memory system processes episodic and semantic memory, containing information about who, what, where and when, that are critical in contextualizing experiences to form memories. Cellular consolidation mechanisms in long-term memory have been well studied within the hippocampus using rodent models, uncovering regulation of immediate early genes and target effectors required for synaptic plasticity. The systems consolidation hypothesis posits that the hippocampus, however, becomes disengaged over time, identifying cortical regions, including the medial prefrontal cortex (mPFC), as a putative site of long term memory processing and storage. This redistribution process can take up to years in humans, but in rat models of contextual fear memories takes about 2-4 weeks. The molecular mechanisms that enable the mPFC to process and store memories across this extended time frame are unknown. The Alberini lab has recently uncovered novel mechanisms in the prelimbic (PL) subregion of the mPFC in rats in a related cognitive process, memory reconsolidation, wherein memory recall can result in memory enhancement. Specifically, postsynaptic cell adhesion molecules neuroligin 1 (NLGN1) and NLGN2, isolated to excitatory and inhibitory synapses, respectively, simultaneously support memory strengthening while also suppressing memory extinction. These data inform the overarching hypothesis of this proposal that NLGN1 and NLGN2 play a critical role in long term memory consolidation by balancing excitatory and inhibitory synapses in the PL cortex with experience. Research from other laboratories indicates that learning-dependent disinhibition of fast- spiking parvalbumin-positive interneurons (PVINs) in the PL cortex synchronize excitatory cell output and is crucial for fear memory expression. Moreover, Nlgn2 knockout in mice preferentially degrades PVIN neurotransmission but spares other inhibitory neuron subtypes. Taken together, I propose that fear memory consolidation requires experience dependent NLGN2 regulation on PVINs in the PL cortex to create a disinhibitory circuit that regulates excitatory neuron output, in part mediated by NLGN1 regulation. I will test this hypothesis using biochemical and behavioral techniques, with a particular interest in NLGN2 on PVINs as a disinhibitory mechanism.
Aim 1 will determine the temporal profile of NLGN1 and NLGN2 protein regulation following learning, and subsequently block their function in vivo to test whether they are required for long term memory.
Aim 2 will measure learning-induced NLGN1 and NLGN2 changes on individual neuron types, and then knockout Nlgn2 in PVINs in the PL cortex using a novel cre-dependent CRISPR-Cas9 approach in PV-cre rats to determine an effect on long term memory formation and maintenance. These results will provide a proposed molecular substrate for PL mechanisms of systems consolidation. Memory deficits are shared among many neuropsychiatric diseases; therefore, identifying molecular pathways underlying memory consolidation will provide promising targets for therapeutic interventions.
Very little is known about the molecular mechanisms required in the medial prefrontal cortex (mPFC) for long- term memory consolidation. This project will use molecular and cellular analyses as well as molecular interference approaches to identify the roles of specific synaptic adhesion molecules regulated by learning and required for memory consolidation in the prelimbic subregion of the mPFC. This understanding will identify potential targets for neuropsychiatric disorders wherein memory deficits and prefrontal cortex malfunction are common phenotypes.
