Project Summary/Abstract Stimulation of hippocampal NMDA-type glutamate receptors (NMDARs) can induce long-term potentiation (LTP) or depression (LTD), two opposing forms of synaptic plasticity that are thought to be required for higher brain functions such as learning, memory and cognition. This proposal will focus on the molecular computation that mediates the decision between these two forms of plasticity. LTP has long been known to require the Ca2+/calmodulin(CaM)-dependent protein kinase II (CaMKII) and its autophosphorylation at T286 (pT286), which generates Ca2+-independent “autonomous” CaMKII activity (~20% of the maximal Ca2+/CaM-stimulated activity). In the first funding period of this proposal, we have shown that CaMKII and pT286 are additionally required for LTD. But how can pT286 CaMKII possibly mediate both of these two opposing forms of plasticity? We have started to address this question in the second funding period: LTP additionally requires CaMKII binding to the NMDA-type glutamate receptor (NMDAR) subunit GluN2B, and two mechanisms suppressed this binding during LTD. By contrast, LTD instead requires a distinct additional autophosphorylation, the inhibitory pT305/306, which blocks Ca2+/CaM binding, limits further pT286, and also directly inhibits GluN2B binding. The three regulatory CaMKII mechanisms (pT286, pT305/306, and GluN2B binding) are all thought to require the CaMKII holoenzyme structure and all cross-regulate each other. In order to elucidate how this cross- regulation works to enable the LTP vs LTD decision by CaMKII, we will here determine the specific holoenzyme rules underlying each mechanism. In three related but independent aims, the project will determine the holoenzyme rules for CaMKII phosphorylation at T286, phosphorylation at T305/306, and GluN2B interaction. A fourth aim will test effects on neuronal functions, which are expected to differ for LTP versus LTD.