Geon Dendritic Filopodia:
the "Tags" for the Formation of Enduring Memory



A compelling question regarding the mechanism of memory consolidation is how specific memory units are “tagged” for strengthening. This paper shows that the tags could be "dendritic filopodia" created by the Ca2+ influx through NMDA receptors during learning. Ryan et al. (2015) proposed that a specific pattern of connectivity between engram cells could be crucial for memory information storage. This paper suggests that the engram cell connectivity could be strengthened by synaptogenesis within memory units, rather than the persistence of high level of synaptic AMPA receptors (AMPARs). The hypothesis agrees with the dual roles of Arc (also known as Arg3.1) which can promote long-term depression (LTD) by enhancing AMPAR endocytosis, and yet Arc also facilitates the formation of enduring memory. Tagging allows the axon to branch out and grow toward only the memory units that have been potentiated by learning. This can be achieved by creating filopodia in the potentiated memory units. Filopodia may search for axon partners through the interaction between axon guidance molecules (e.g., ephrin-A5) and their receptors. This notion is supported by the finding that ephrin-A5 plays a necessary and activity-independent role in the initiation of the early phase of synaptogenesis.


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