We find that LRRTM1 and LRRTM2 DKD in vivo blocks LTP in neonatal CA1 pyramidal neurons, a deficit that is rescued by wild-type LRRTM2. Further replacement experiments revealed that the extracellular, but not intracellular, domain of LRRTM2 is required for LTP. LTP was not rescued by expression of a mutant LRRTM2 reported to impair binding to Nrxs Tariquidar in vitro (Siddiqui et al., 2010), although whether this mutant quantitatively reaches the surface to the same degree as wild-type LRRTM2 is unknown. Importantly, LRRTM1 and LRRTM2 DKD in adult CA1 pyramidal neurons in vivo

also strongly impaired LTP. These results demonstrate that the block of LTP by LRRTM1 and LRRTM2 DKD is not due to some unknown effect on synapse maturation but rather to a critical role of LRRTMs in LTP at mature synapses. A cell culture model of LTP provided further insight into the mechanisms by which LRRTMs may function in LTP. LRRTM1 and LRRTM2 Compound Library DKD blocked this model of LTP and surprisingly increased the net surface expression of AMPARs under basal conditions. Immunocytochemical and electrophysiological assays revealed that DKD caused an increase in surface expression of extrasynaptic AMPARs while decreasing synaptic AMPARs. Furthermore, the DKD did not affect the initial increase in surface and synaptic AMPAR expression 10 min after

cLTP induction yet caused a decrease in net AMPAR surface expression when measured 20 min after cLTP. All of the effects of the DKD in cultured neurons were reversed by wild-type LRRTM2, suggesting that the phenotypes were Idoxuridine not due to off-target effects. The results in cultured neurons are consistent with the decrease in AMPAR-mediated synaptic transmission caused by LRRTM DKD in vivo in neonatal hippocampus (de Wit et al., 2009 and Soler-Llavina et al., 2011) as well as the time course of the block of LTP in acute slices. They support the hypothesis that LRRTMs are required for maintaining a normal complement of synaptic AMPARs to support basal synaptic transmission but not for the AMPAR exocytosis that occurs after LTP induction. However, in adult CA1 pyramidal neurons, LRRTM1 and LRRTM2 DKD

did not have a detectable effect on basal AMPAR-mediated synaptic transmission (Soler-Llavina et al., 2011). A simple hypothesis to explain all of these results is that in young, developing synapses LRRTMs serve two functions. They help maintain a normal complement of synaptic AMPARs for basal synaptic transmission and, after LTP induction, they contribute to the scaffolding or “slot” complex that stabilizes the newly delivered AMPARs (Malinow and Malenka, 2002 and Opazo and Choquet, 2011). In their absence after LTP induction, AMPARs transiently diffuse into but cannot be maintained within the PSD; they escape to sites at which endocytosis occurs, a process that may have been accelerated by the LTP induction protocol.