The Neglected Subiculum: Decoding and Manipulating Memory Retrieval with Multiphoton Holography

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Memory impairment emerges in diverse, clinically-significant conditions spanning all stages of life. After a memory has been encoded, its retrieval is believed to occur through the reactivation of an engram, the physiological substrate of memory. Engrams are partitioned into neuronal ensembles—small groups of functionally-related neurons—that are dispersed throughout the brain. Direct optogenetic stimulation of engram cells in the hippocampus has proven sufficient to trigger the coordinated reactivation of these dispersed ensembles. The primary output node of the hippocampus, the subiculum, has proven critical to the integrity of memory retrieval and is robustly associated with the memory impairments observed in Alzheimer’s disease. However, despite a rich history of memory research in the hippocampal formation, the subiculum and its role in memory remain critically understudied. Remarkably, recent evidence indicates the integrity of the subiculum may be critical for memory retrieval without being necessary for encoding. Identifying the unique role of the subiculum in memory retrieval is thus a central question in memory research. To address this gap in knowledge, I will leverage large-scale two-photon calcium imaging alongside holographic photostimulation to decode and manipulate subicular activity during memory retrieval in mice. I will test the hypothesis that conjunctive-coding and recurrent-modularity in the subiculum facilitate the coordinated reactivation of neuronal ensembles during memory retrieval. I have constructed an experimental setup in which I am able to simultaneously measure (two-photon calcium imaging) and manipulate neural activity (two- photon holographic optogenetics) in mice during a closed-loop fear conditioning assay. I have found that partially-overlapping neuronal ensembles in the subiculum encode a variety of task-relevant features during retrieval, including the temporal gap between the retrieval cue and the anticipated aversive stimuli. In the F99 phase, I will test the hypothesis that the activation of subsets of the identified neuronal ensembles preferentially drive intra-ensemble activity. To do so, I will be trained in and use two-photon holographic optogenetics to selectively stimulate arbitrary subsets of intra-ensemble and random neurons. Simultaneously, I will record the activity of these neurons using two-photon calcium imaging in order to determine whether stimulation drives persistent activity on an order of magnitude longer than photostimulation duration and whether such activity is specific to ensemble neurons. In the K00 phase, I will investigate the coordination of subicular and cortical activity during memory retrieval, and its perturbation in disease. This proposal will establish a causal understanding of subicular dynamics during memory retrieval, a deeper understanding of the coordination of memory retrieval across brain regions, and nurture the development of an independent researcher from a disadvantaged background at the cutting-edge of memory research.