Remote memory engrams are controlled by encoding-specific tau phosphorylation
The engram represents the physical trace that encodes a specific memory and enables its recall 1-4. Functional failure of the engram is linked to the progressive memory decline in Alzheimers disease 5. However, it is unknown whether the microtubule-associated protein tau, a central factor in Alzheimers 6,7, has a direct function in the engram. Here, we demonstrate that tau and encoding-associated tau phosphorylation are critical for robust remote memory engrams. Tau is required specifically during memory formation for remote, yet not proximal recall in memory paradigms in mice. Controlled expression of tau exclusively during memory entrainment is necessary and sufficient to restore remote memory deficits in tau knockout mice. Tau is phosphorylated at specific sites during encoding. Gene editing to ablate site-specific phosphorylation at threonine-205 (T205) lowers precision of engram cell recruitment and precludes efficient remote recall. Vector-based engineering of engram cells reveals that T205 phosphorylation of tau is required to engrain memory for recall at remote timepoints. Notably, in the absence of tau, memory is recalled from latency by direct optogenetic activation of engram cells at distal time points but not when natural cues are used, revealing an association-specific gatekeeper function of tau during encoding. Our work delineates a physiologic role of site-specific tau phosphorylation at the inception of episodic memory to support an enduring engram and enable efficient remote recall. Thus, encoding-associated phosphorylation of tau is proximal to the elusive substrate of remote memory and may connect to the basis of amnesia in Alzheimers disease.