Molecular mechanisms of memory maintenance and dysfunction in neural circuits

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How molecular mechanisms modify neuronal networks to maintain long-term memory is a fundamental question in neuroscience, with relevance for disorders of persistent, memory-like dysfunction of brain circuits. Atypical PKCs (aPKC), the persistently active isoform PKMζ and PKCι/λ, are core molecules maintaining late- phase synaptic long-term potentiation (LTP) and several forms of long-term memory. Unlike most PKCs that are active only briefly after stimulation, aPKCs have persistent actions. After strong synaptic stimulation, PKMζ increases by new synthesis, and the persistent increase in the autonomously active isoform enhances synaptic transmission during LTP maintenance and lasts for days to weeks during long-term memory storage. The other aPKC, PKCι/λ, can also maintain LTP and long-term memory, as revealed by PKMζ-knockout (KO) mice. Inhibitors of aPKC disrupt memory even weeks after it is formed and ameliorate persistent symptoms of PTSD, addiction, and chronic neuropathic pain in specific brain circuits in animal models. Conversely, overexpressing PKMζ enhances long-term memory and alleviates persistent deficits in disorders in which decreased PKMζ is implicated. Thus, understanding how aPKCs contribute to maintaining memory by sustaining representations of memory in brain circuits will provide fundamental information to assess their roles in pathological memory. Therefore, our Specific Aims are: Aim 1: Is there a hierarchy of PKCs in memory maintenance that store representations differently in networks of neurons? Spatial memory representations depend on the discharge of hippocampus place cell ensembles. We will examine if the properties of hippocampus place cell ensemble representations of spatial memories differ when maintained by PKMζ or PKCι/λ, and if other PKCs can also maintain spatial memory. Aim 2: How are spatial memory-related place cell ensemble representations modified when memory is erased by inhibiting individual PKCs in wild-type and PKMζ- KO mice? Using novel isoform-selective antagonists and conditional KO (cKO) mice, we will test the necessity of aPKC-mediated enhanced synaptic connectivity for representing spatial memory by examining whether reversing this connectivity concurrently erases memory and destabilizes memory-related place cell ensemble representations. Aim 3: Does persistently increased synthesis of PKMζ maintain very long-term memory? Strong conditioning produces increases in PKMζ in the hippocampus that last a month. We will use PKMζ-antisense and PKMζ-cKOs to determine if these persistent increases are due to persistent increased synthesis and/or decreased degradation. To test sufficiency of PKMζ for maintaining memory and memory- related representations of space, we will use overexpression of PKMζ that prolongs long-term memory to examine if increased PKMζ synthesis also perpetuates memory-related place cell ensemble representations. Our aims will elucidate the persistent molecular mechanisms maintaining long-term memories, causally test their role in memory persistence, and so establish a basis for understanding disorders of pathological memory.