OptoTrkB activation for in-vivo translation of optogenetic memory engram consolidation

The consolidation of memory engrams relies fundamentally on robust synaptic plasticity, specifically Late-phase Long-Term Potentiation (L-LTP), which is governed by the Brain-Derived Neurotrophic Factor (BDNF) and its high-affinity receptor, TrkB. While neuronal BDNF synthesis is critical, recent evidence suggests that astrocytes play an equally pivotal role in maintaining the pool of bioactive neurotrophins through a p75NTR-dependent recycling mechanism. This doctoral thesis investigates the physiological necessity of astrocytic BDNF recycling for long-term memory (LTM) consolidation and establishes a novel, non-invasive optogenetic strategy to rescue cognitive deficits associated with the disruption of this pathway. Utilizing a conditional p75NTR-flox mouse model, we first demonstrated that impaired astrocytic BDNF recycling selectively abolishes LTM in a context-minimized home-cage Novel Object Recognition (hcNOR) paradigm, while sparing short-term memory. Electrophysiological assessments confirmed that these behavioural deficits correlate with a failure to sustain L-LTP. To bypass the limitations of traditional, invasive optogenetic stimulation, we employed OptoTrkB, a light-sensitive chimeric receptor, to directly modulate TrkB signaling. We validated the functionality of OptoTrkB in cortical neuronal cultures and demonstrated its efficacy in restoring L-LTP in p75-deficient ex vivo preparations. To translate these findings in vivo, we implemented a Bioluminescent-Optogenetic (BL-OG) system. This approach utilizes a luciferase released by astrocytes to activate TrkB signaling non-invasively via the systemic administration of a luciferin substrate. Our results indicate that non-invasive BL-OG activation of OptoTrkB successfully rescues LTM deficits in p75-deficient mice. Furthermore, we established that this rescue is dependent on astrocytic vesicular release mechanisms. Crucially, to dissect the specific contribution of the memory trace, we utilized a custom-designed external LED cage lid system rather than invasive optical fibers. By delivering light during a defined critical time window in the consolidation phase, we achieved a complete, engram-specific rescue of LTM. This pivotal finding demonstrates that temporally precise boosting of TrkB signaling exclusively within the memory trace is sufficient to compensate for the lack of astrocytic support. Collectively, this work elucidates the critical contribution of astrocytic BDNF recycling to support memory engram consolidation. It further validates OptoTrkB-BL-OG as a powerful, non-invasive tool for the precise temporal manipulation of neurotrophic signaling, offering a promising translational avenue for treating cognitive disorders characterized by synaptic dysregulation.