Immunoblot analyses showed that NRX1β(S4+)-Flag bound to the columns conjugated with HA-Cbln2 as well as HA-Cbln1, whereas it did not bind to columns conjugated with Cbln4 or CS-Cbln1 (Fig. 6A). Similarly, HA-Cbln1 and HA-Cbln2 but not HA-Cbln4 or HA-CS-Cbln1 bound to columns that immobilized NRX1β(S4+)-Fc, whereas none of the Cbln family members bound to NRX1β(S4−)-Fc columns (Fig. 6B). Furthermore, beads coated with HA-Cbln1 or HA-Cbln2, but not those coated with HA-Cbln4 or HA-CS-Cbln1, GSI-IX nmr caused clustering of NRX1β(S4+) expressed in HEK293 cells (Supporting Information Fig. S3). These results indicate that, like Cbln1, Cbln2 also binds and accumulates NRXs carrying the splice site 4 insert. To examine whether

Cbln family proteins had direct synaptogenic activities in cerebellar granule cells, we performed artificial synapse-forming

assays using beads coated with HA-Cbln1, HA-CS-Cbln1, HA-Cbln2 or HA-Cbln4. Beads were incubated with cbln1-null cerebellar granule cells for 3 days and presynaptic terminals were immunostained with synapsin I. Like HA-Cbln1, HA-Cbln2 significantly induced clustering of synapsin I-positive presynaptic terminals on the beads (Fig. 6C). Although the amount of Cbln proteins on the beads was adjusted, the intensity of synapsin I immunoreactivity on Cbln2-coated beads was weaker than that on Cbln1-coated beads (P = 0.015; Fig. 6C). Thus, Cbln2 may have weaker affinity to NRX1β(S4+) (Fig. 6A and B) and weaker synaptogenic activity (Fig. 6C) than Cbln1. Consistent with the finding that HA-Cbln4 did

EGFR inhibitor not bind to NRXs (Fig. 6A oxyclozanide and B), HA-Cbln4 as well as HA-CS-Cbln1 did not induce accumulation of presynaptic terminals of cbln1-null granule cells (Fig. 6C). The NRX(S4+) is widely expressed in the central nervous system, including the hippocampus and cerebral cortex (Ichtchenko et al., 1995). Although GluD2 is specifically expressed in Purkinje cells, its family member δ1 glutamate receptor (GluD1), which also binds to Cbln1 (Matsuda et al., 2010), is highly expressed in various brain regions, such as the striatum, especially during development (Lomeli et al., 1993). Indeed, Cbln1 is expressed in the thalamic parafascicular nucleus that sends axons to the striatal neurons (Kusnoor et al., 2010). Thus, the NRX/Cbln1/GluD1 complex might be involved in synaptic functions in these brain regions. As a first step to explore this possibility, we performed artificial synapse-forming assays using HEK293 cells and wild-type hippocampal neurons as a model system, taking advantage of the fact that hippocampal neurons do not express endogenous Cbln1 (Miura et al., 2006). Immunocytochemical analyses showed that HEK293 cells expressing GluD2 but not those expressing GluD2ΔNTD accumulated synaptophysin-positive presynaptic terminals of hippocampal neurons only when recombinant HA-Cbln1 protein was added to the culture medium (Fig. 7A).