4D, middle panels) than in wild-type neurons (Fig. 4D, upper panels). Addition of HA-Cbln1 to the culture medium restored accumulation of endogenous NRXs associated with GluD2 puncta on cbln1-null Purkinje cell dendrites (Fig. 4D, lower panels). Together, these results indicate that Cbln1/GluD2 serves as a presynaptic CDK and cancer organizer by directly accumulating its presynaptic receptor NRXs(S4+). Cbln1 also serves as a postsynaptic organizer that induces clustering of GluD2 and its

associated proteins at the postsynaptic site. To examine whether NRX functions as a postsynaptic organizer by forming a tripartite complex with Cbln1 and GluD2, we cultured HEK293 cells expressing GluD2 with beads coated with NRX1β. GluD2 clustering was induced around beads coated with NRX1β(S4+) only when HA-Cbln1 was added to the culture medium (Fig. 5A). However, beads coated with NRX1β(S4−) did not cause clustering of GluD2 even in the presence of HA-Cbln1 (Fig. 5A), suggesting that NRX1β(S4+) caused GluD2 clustering in HEK293 cells by forming a complex with Cbln1. The C-terminus of GluD2 interacts directly with several intracellular molecules in neurons; many of these serve as scaffolds for other postsynaptic molecules. Thus, to examine whether NRX also functions MK-2206 as a postsynaptic organizer in neurons,

we cultured cbln1-null Purkinje cells with beads

coated with NRX1β(S4+) from 10 to 13 DIV. Immunocytochemical analyses showed that GluD2 clustering was induced around beads only in the presence of HA-Cbln1 (Fig. 5B). Similarly, shank2, a scaffold protein that binds to the C-terminus of GluD2, clustered around beads coated with NRX1β(S4+) (Fig. 5B). In contrast, beads coated with NRX1β(S4−) did not cause clustering of GluD2 or shank2 even in the presence of HA-Cbln1 (Fig. 5B). Coimmunostaining of presynaptic synapsin I and postsynaptic GluD2 showed that Lepirudin GluD2 puncta induced by beads coated with NRX1β(S4+) in the presence of HA-Cbln1 were not associated with synapsin I-positive presynaptic terminals (Fig. 5C), indicating that NRX1β(S4+)-beads directly induced GluD2 clustering at the contact sites. These results indicated that the tripartite complex consisting of NRX, Cbln1 and GluD2 serves as a bidirectional synaptic organizer. Of the Cbln family members, Cbln1, Cbln2 and Cbln4 mRNAs are expressed in various brain regions outside the cerebellum, including the olfactory bulb, entorhinal cortex and certain thalamic nuclei (Miura et al., 2006). As NRXs(S4+) are also highly expressed in these regions (Ichtchenko et al., 1995), Cbln family members may also be involved in synapse formation by forming complexes with NRXs.