The presynaptic protein synaptophysin is not affected by NLG1 cleavage, indicating that shedding of NLG1 is not accompanied by gross structural changes in presynaptic terminals (Figures 5D and 5F). Considering the expanding set of transsynaptic NRX binding partners, these results suggest that postsynaptic NLG1 is an important regulator of NRX1β stability at synapses. Previous studies have shown that deletion of αNRXs in mice reduces action-potential-evoked

VX-770 mw neurotransmitter release due to impaired presynaptic Ca2+ channel function (Missler et al., 2003). However, due to the role of NRXs in synapse maturation, it has been unclear whether this effect is due to indirect developmental effects or reflects an ongoing role of NRXs in neurotransmitter release. Using the highly selective thrombin-induced XAV-939 in vivo cleavage of NLG1, we show an overall reduction in excitatory transmission and release probability concurrent with NRX1β

loss (Figure 6). These findings support the notion that the NLG-NRX complex is a critical regulator of neurotransmitter release (Futai et al., 2007; Missler et al., 2003; Zhang et al., 2005) and provide evidence that NLG1-dependent regulation of presynaptic function can occur over time scales of minutes. In mature hippocampal and cortical cultures, postsynaptic receptor blockade increases mEPSC frequency (Burrone et al., 2002; Thiagarajan et al., 2005; Wierenga et al., 2006) and augments presynaptic terminal size and release probability (Murthy et al., 2001; Thiagarajan et al., 2005). By contrast, local stimulation of dendrites acutely reduces release probability of contacting presynaptic terminals (Branco et al., 2008). These data have implied the existence of local transsynaptic negative feedback signals capable of modifying presynaptic function based on postsynaptic activity. Our next results indicate

that NLG1 cleavage is bidirectionally regulated by activity (Figures 3A and 3B). Moreover, acute NLG1 cleavage decreases release probability, whereas expression of a cleavage-resistant NLG1 isoform induces the opposite effect (Figure 6). Together, these data support a model where increased or decreased local NLG1 cleavage alternately dampens or augments presynaptic function based on postsynaptic activity, thereby contributing to overall levels of neuronal excitability. The developmental profile of NLG1-NTFs in the brain (Figures 2G and 2H) indicates that NLG1 cleavage is upregulated during early stages of development, a time when activity-dependent mechanisms sculpt new circuits (Hensch, 2004). Moreover, our results indicate that MMP9-dependent cleavage of NLG1 is regulated by sensory experience during visual cortex maturation (Figures 7E and 7F). Interestingly, tissue plasminogen activator, a robust activator of MMP9 (Wang et al., 2003), regulates synapse maintenance during cortical development (Mataga et al.