Kallikrein 8-Dependent and Independent Synaptic Tagging and Modulation of Long-Term Potentiation: A Quest for the Associated Signaling Pathway(s)

Synaptic tagging is a plausible hypothesis to explain relational memory. However, why and how the tagged synapses can be distinguished from other nonactivated synapses has not yet been clarified. Early-phase long-term potentiation (E-LTP)-related signaling molecules and intracellular molecular trafficking for capturing these toward tagged synapses have been considered essential for the synaptic tagging apparatus. In this chapter, we describe a new mechanism of synaptic tagging that shares the same set of E-LTP induction mechanisms as above; that is, E-LTP-specific proteolysis by Kallikrein 8/neuropsin (KLK8), an extracellular serine protease, is involved in the KLK8-dependent form of synaptic and behavioral tagging.