47 ± 0.11; MMP9/MMP13i, 0.27 ± 0.11; MMP13i, 0.92 ± 0.15; Figures S3C and S3D), indicating that MMP9-dependent cleavage is active not only during periods of elevated neuronal activity, but also under basal conditions. Moreover, we noted that MMP3 inhibitor III (50 μM) induced a partial but nonsignificant decrease in NLG1-NTFs after KCl (KCl + MMP3i, 1.7 ± 0.1, p = 0.092; Figures 3E and 3F). MMPs are secreted as inactive zymogens and require

cleavage of their prodomains to become enzymatically active (Ethell and Ethell, 2007). Thus, the effect of MMP3 inhibition could be due to impaired MMP9 activation. To determine which MMP is the terminal protease-cleaving NLG1, we treated neurons with 4-aminophenylmercuric acetate (APMA), a JQ1 order compound that nonselectively activates all MMPs by cleaving their prodomains (Van Wart and Birkedal-Hansen, Ulixertinib cost 1990) and tested the effect of specific MMP inhibitors. Brief incubation of DIV21 cortical neurons with 0.5 mM APMA for 15 min induced robust generation of NLG1-NTFs (3.8 ± 0.8-fold increase relative to control; Figures 3G and 3H). Coincubation with MMP2/MMP9 inhibitor II (0.3 μM) blocked APMA-induced cleavage (0.6 ± 0.1), whereas MMP2 inhibitor III (50 μM) and MMP3 inhibitor III (50 μM) had no effect (3.8 ± 1.0 and 3.2 ± 0.8, respectively),

indicating that MMP9 is the downstream protease responsible for cleavage of NLG1. To further validate these findings, we tested how NLG1 is regulated by activity in neurons lacking MMP9. KCl depolarization of DIV17 and DIV18 wild-type (WT) mouse cortical cultures for 2 hr resulted in extensive loss of NLG1 (0.36 ± 0.01 relative to control; Figure 3I). By contrast, KCl incubation of MMP9 KO cultures induced no loss of NLG1 (0.90 ± 0.04 relative to control; Figure 3J), confirming that MMP9 is responsible for activity-dependent regulation of NLG1. To characterize the activity-dependent production of NLG1-CTFs, we measured CTF levels in whole cell extracts of DIV21 dissociated

neuron cultures treated with KCl. As expected, KCl incubation increased NLG1-CTF levels (1.6 ± 0.4 compared to control; Figures S3E and S3F). Interestingly, inhibition of the γ-secretase complex with DAPT (20 μM) during KCl treatment resulted in increased accumulation of NLG1-CTFs (KCl+DAPT, 3.7 ± 0.8; Figures S3E and S3F), indicating that NLG1 is processed by the γ-secretase complex following ectodomain cleavage. Deglycosylation of NLG1-NTFs very produces ∼70 kDa species (Figure 2E), which, based on amino acid mass, indicates that proteolysis occurs in the extracellular juxtamembrane region of NLG1. To determine the specific domain targeted for cleavage, we generated a series of mutants with sequential deletions and amino acid (aa) replacements in the juxtamembrane domain (Figure 4A). NLG1 mutants were screened for their resistance to APMA cleavage using biotinylation-based labeling and isolation of NLG1-NTFs in COS7 cells. Brief incubation with APMA resulted in robust shedding of GFP-NLG1 (Figure 4B).