Collections of cells called engrams are thought to represent memories. Although there has been progress in identifying and manipulating single engrams, little is known about how multiple engrams interact to influence memory. In lateral amygdala (LA), neurons with increased excitability during training outcompete their neighbors for allocation to an engram. We examined whether competition based on neuronal excitability also governs the interaction between engrams. Mice received two distinct fear conditioning events separated by different intervals. LA neuron excitability was optogenetically manipulated and revealed a transient competitive process that integrates memories for events occurring closely in time (coallocating overlapping populations of neurons to both engrams) and separates memories for events occurring at distal times (disallocating nonoverlapping populations to each engram).

Remodeling of cortical connectivity is thought to allow initially hippocampus-dependent memories to be expressed independently of the hippocampus at remote time points. Consistent with this, consolidation of a contextual fear memory is associated with dendritic spine growth in neurons of the anterior cingulate cortex (aCC). To directly test whether such cortical structural remodeling is necessary for memory consolidation, we disrupted spine growth in the aCC at different times following contextual fear conditioning in mice. We took advantage of previous studies showing that the transcription factor myocyte enhancer factor 2 (MEF2) negatively regulates spinogenesis both in vitro and in vivo. We found that increasing MEF2-dependent transcription in the aCC during a critical posttraining window (but not at later time points) blocked both the consolidation-associated dendritic spine growth and subsequent memory expression. Together, these data strengthen the causal link between cortical structural remodeling and memory consolidation and, further, identify MEF2 as a key regulator of these processes.structural plasticity | remote memory | viral vector I n experimental animals, damage to the hippocampus disproportionately impacts recently acquired memories, with relative sparing of remote memories (1-8). Such observations have led to the idea that the hippocampus is transiently required for memory expression, with remote memory expression being exclusively dependent on the cortex (9). According to one model (10), posttraining hippocampal activity coordinates reactivation of memory traces in the cortex. This reactivation leads to the remodeling of cortical connections, allowing the memory to eventually be expressed independently of the hippocampus. A recent study in mice (5) provided correlative evidence for posttraining remodeling of neurons in the anterior cingulate cortex (aCC), a subregion of the prefrontal cortex that plays an essential role in remote memory expression (11). Increases in dendritic spine density on layer 2/3 pyramidal aCC neurons were observed 1 mo, but not 1 d, following contextual fear conditioning (5). As layer 2/3 pyramidal neurons send and receive long-range cortical connections (12), such changes may contribute to increased functional connectivity between the aCC and other cortical areas important for remote memory expression (13,14). However, whether this increase in aCC spine density is necessary for memory consolidation is not known. To test this, it would be necessary to evaluate whether preventing posttraining spinogenesis, specifically in this region, disrupts memory consolidation.The transcription factor myocyte enhancer factor 2 (MEF2) negatively regulates spinogenesis in an activity-dependent manner and therefore provides a tool to address this question. For example, increasing MEF2 function decreases the number of dendritic spines and excitatory synapses in vitro (15) and blocks increases in spine density normally observed following repeated cocaine administration in rat medium spiny...

Memory formation is thought to be mediated by dendritic-spine growth and restructuring. Myocyte enhancer factor 2 (MEF2) restricts spine growth in vitro, suggesting that this transcription factor negatively regulates the spine remodeling necessary for memory formation. Here we show that memory formation in adult mice was associated with changes in endogenous MEF2 levels and function. Locally and acutely increasing MEF2 function in the dentate gyrus blocked both learning-induced increases in spine density and spatial-memory formation. Increasing MEF2 function in amygdala disrupted fear-memory formation. We rescued MEF2-induced memory disruption by interfering with AMPA receptor endocytosis, suggesting that AMPA receptor trafficking is a key mechanism underlying the effects of MEF2. In contrast, decreasing MEF2 function in dentate gyrus and amygdala facilitated the formation of spatial and fear memory, respectively. These bidirectional effects indicate that MEF2 is a key regulator of plasticity and that relieving the suppressive effects of MEF2-mediated transcription permits memory formation.

The FoxO family of transcription factors is known to slow aging downstream from the insulin/IGF (insulin-like growth factor) signaling pathway. The most recently discovered FoxO isoform in mammals, FoxO6, is highly enriched in the adult hippocampus. However, the importance of FoxO factors in cognition is largely unknown. Here we generated mice lacking FoxO6 and found that these mice display normal learning but impaired memory consolidation in contextual fear conditioning and novel object recognition. Using stereotactic injection of viruses into the hippocampus of adult wild-type mice, we found that FoxO6 activity in the adult hippocampus is required for memory consolidation. Genome-wide approaches revealed that FoxO6 regulates a program of genes involved in synaptic function upon learning in the hippocampus. Consistently, FoxO6 deficiency results in decreased dendritic spine density in hippocampal neurons in vitro and in vivo. Thus, FoxO6 may promote memory consolidation by regulating a program coordinating neuronal connectivity in the hippocampus, which could have important implications for physiological and pathological age-dependent decline in memory.