Periodic Reporting for period 1 - HOPE (Reverse engineering the assembly of the hippocampal scaffold with novel optical and transgenic strategies) Reporting period: 2021-07-01 to 2022-12-31 Summary of the context and overall objectives of the project We aim at shedding a unique light on hippocampal function at circuit level through the design of a novel method to trace the construction of brain circuits in health and disease based on hybrid multiphoton in vivo longitudinal colour imaging of the dynamics and structure of genetically-tagged neuronal clones from birth into adulthood. This is made possible by the exceptional interdisciplinary collaboration of three labs with expertise in advanced optical microscopy, genetic engineering and systems developmental neuroscience. Our novel methodology, applied here to the hippocampus, will pioneer a new way to track the emergence and plasticity of heterogeneous neuronal circuits as these progressively give rise to function in physiological and pathological contexts, thus bridging the gap between early development and adult circuit physio-pathology. This will enable us to revisit adult hippocampal function from a developmental perspective. The hippocampus is classically described as providing a cognitive map of space, involved in navigation, learning, and episodic memory. However, a more computational and less representational vision of its role presently emerges in which its circuits are best described as producing sequences of neuronal activation arising from the interaction between external contextual inputs and internally-generated preconfigured dynamics. Converging work including ours leads us to hypothesize that internally preconfigured assemblies, shaped by their developmental journey, are the basic modules of hippocampal function. In this context, we will use our new approach to address three interrelated questions: 1) Are assemblies shaped by specific circuits in the adult hippocampus (objective A)? 2) Are they prewired or shaped by experience (objective B)? 3) What is their patho-physiological significance (objective C)? Addressing these major issues raises a timely challenge in both optics and genetic engineering at the core of this synergistic proposal. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far Methodological approaches for long-term and large-scale imaging are being implemented and validated. First in vivo functional imaging data have been obtained. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) HOPE aims to shed new light on the function of the hippocampus and the role of its neuronal circuits through the design of a new, non-invasive and universal method to monitor the growth and construction of brain circuits located deep in the brain, from their neurogenesis to adulthood, under normal and pathological conditions.