Transsynaptic signaling by activity-dependent cleavage of neuroligin-1

Abstract

Adhesive contact between pre- and postsynaptic neurons initiates synapse formation during brain development and provides a natural means of transsynaptic signaling. Numerous adhesion molecules and their role during synapse development have been described in detail. However, once established, the mechanisms of adhesive disassembly and its function in regulating synaptic transmission have been unclear. Here, we report that synaptic activity induces acute proteolytic cleavage of neuroligin-1 (NLG1), a postsynaptic adhesion molecule at glutamatergic synapses. NLG1 cleavage is triggered by NMDA receptor activation, requires Ca2+ /calmodulin-dependent protein kinase, and is mediated by proteolytic activity of matrix metalloprotease 9 (MMP9). Cleavage of NLG1 occurs at single activated spines, is regulated by neural activity in vivo, and causes rapid destabilization of its presynaptic partner neurexin-1β (NRX1β). In turn, NLG1 cleavage depresses synaptic transmission by abruptly reducing presynaptic release probability. Thus, local proteolytic control of synaptic adhesion tunes synaptic transmission during brain development and plasticity.

Figure 1. Neuronal Activity Triggers Loss of Neuroligin-1 from Synapses

(A) Hippocampal neurons (DIV21) incubated in media (Control) or media with 30 mM KCl for 2 h (KCl) immunolabeled for PSD-95 and pan-NLG (NLG1-4). Arrows show NLG1-4 labeling at PSD-95⁺ synapses under control conditions (open) or after KCl (solid). Boxed regions are magnified in lower panels. Scale bars, 5 μm and 1 μm. (B) Means ± SEM of NLG1-4 FL intensity in PSD-95⁺ spines (left) or entire neuron (right) normalized to controls. Control, n = 435 puncta, 8 neurons; KCl, n = 546 puncta, 9 neurons. * p < 0.01. (C) Immunoblot analysis of NLG1 in membrane fractions isolated from control (Ctrl) or KCl-treated cortical neurons (DIV21). EXT, whole cell extract; SPM, synaptic plasma membrane; PSDI,II, III; postsynaptic density fractions. Note that 4-fold less protein by mass was loaded in PSD fraction lanes. See Experimental Procedures for details. (D) Means ± SEM of NLG1 protein levels relative to EXT control. n = 3, *p < 0.05, **p<0.005, ***p<0.001. (E) Biotinylation assay of endocytosis performed on DIV21 cortical neurons incubated in media (Ctrl) or media with 30 mM KCl. Surf- Total surface protein at time zero. See Experimental Procedures for details. (F) Quantitative analysis of NLG1 (left) and GluA1 (right) internalization over time. (G) Immunoblot of NLG1 in lysates of DIV21 cortical cultures following 2 h incubation in medium (Ctrl) or medium with 30 mM KCl alone or supplemented with MG132 (10 μM), leupeptin (200 μM), leupeptin+MG132 (Leup/MG), or GM6001 (10 μM). Note that GM6001 prevents KCl-induced loss of total NLG1. (H) Means ± SEM of total NLG1 under indicated conditions. n = 3, *p < 0.05.

Figure 2. A Soluble N-Terminal Fragment of NLG1 is Abundant in Brain

(A) Diagram of biotinylation-based cleavage assay. Media from surface biotinylated neuronal cultures was collected and soluble biotin conjugates isolated by streptavidin precipitation. (B) Immunoblot analysis of NLG1 reveals time-dependent release of soluble ~90 kDa NLG1-NTFs to culture media. Lys- 10% Total surface protein at time zero. (C) Deglycosylation induces an equivalent mass shift of both full-length NLG1 (Lysate) and NLG1-NTFs (Media) collected after surface biotinylation. (D) Immunoblot analysis of NLG1 in whole homogenates (5 μg, Total) and soluble (50 μg, Sol) fractions obtained from cortical, hippocampal, and cerebellar tissue from P60 WT mice reveals soluble ~90 kDa NLG1-NTFs. (E) Deglycosylation of whole homogenates (total) and soluble fractions from P5 WT mouse cortex results in a ~20 kDa apparent mass shift of NLG1-NTFs. (F) Immunoblot analysis of NLG1 in whole brain homogenates (Total) and soluble fractions (Sol) from WT and NLG1-KO mice reveals absence of NLG1-NTFs in NLG1-KO tissue. (G) Developmental profile of NLG1-NTFs from WT mouse cortical extracts at different postnatal days (P). Total, 10 μg whole cortical homogenates; Sol, 50 μg soluble fractions. (H) Line scan analysis of immunoblots shown in (G) Note that the relative abundance of NLG1-NTFs is higher during early developmental stages. The graph plots signal intensity along the lines shown on the blots on the left. (I) Immunoblot analysis of NLG1-CTFs in total homogenates (Total), soluble fractions (Sol), and membrane fractions (Memb) of P60 WT mouse cortex. (J) Immunoblot analysis of NLG1-CTFs in whole brain homogenates from WT and NLG1-KO mice reveals the absence of ~20 kDa NLG1-CTFs in NLG1-KO tissue.

Figure 3. Neuronal Activity Induces NLG1 Cleavage Through NMDAR, CaMK, and MMP9 Signaling

(A) Isolation and detection of NLG1-NTFs under control conditions or in the presence of TTX (2 μM) or bicuculline (50 μM) plus 4AP (25 μM, Bic/4AP). Surface- 10% of total surface protein at time zero. (B) Means ± SEMs of NLG1-NTFs under indicated conditions normalized to control. n = 4, * p< 0.05. (C) Isolation and detection of NLG1-NTFs under control conditions or in the presence of KCl (30 mM, 2 h) with or without indicated pharmacological agents. (D) Means ± SEMs of NLG1-NTFs levels normalized to control. n = 4, * p< 0.05, ** p < 0.01. (E) Isolation and detection of ~90 kDa NLG1-NTFs under control conditions or in the presence of KCl with or without MMP inhibitors. Note that inhibitors targeting MMP9 abrogate KCl-induced NLG1 cleavage. (F) Means ± SEMs of NLG1-NTFs levels normalized to control. n = 8, *p < 0.005. n.s., not significant. (G) Immunoblot analysis of NLG1 and NLG1-NTFs in biotinylated fractions of DIV21cortical neurons treated with the nonselective MMP activator APMA with or without MMP inhibitors. Lys- Whole lysates; Med- Media. Note that only 10% of precipitate was loaded in Lys lanes. (H) Means ± SEMs of NLG1-NTFs produced under the indicated conditions normalized to control. n = 3, *p < 0.05. (I–J) Immunoblot analysis of NLG1 in whole cell extracts from DIV18 cortical neuron cultures from (I) WT or (J) MMP9 KO mice following 2 h incubation in Neurobasal medium (Control) or medium with 30mM KCl (KCl). Bar graphs on the right represent means ± SEM of total NLG1 levels under the indicated conditions for WT (n = 6, *p < 0.01) and MMP9 KO neurons (n = 6, p > 0.05), respectively.

Figure 4. Local Glutamate Uncaging Triggers Rapid Cleavage of Synaptic NLG1

(A) Model illustrating the NLG1-ΔSD substitution mutants in relation to NLG1 domains. AChED, acetylcholinesterase homology domain; DD, dimerization domain; SD, stalk domain; TM, transmembrane domain; CT, C-terminal domain. Numbers indicate amino acids. (B) Biotinylation-based cleavage assay of COS7 cells expressing the indicated NLG1 mutants. Total lysates (Lys) or media (Med) were collected and subjected to immunoblot analysis following treatment with the MMP activator APMA with or without the broad spectrum MMP inhibitor GM6001 (GM). Both NLG1-ΔSDfull and NLG-ΔSD3 were resistant to APMA-induced cleavage indicating that MMP cleavage sites lie between residues 672 and 695. (C) Time-lapse images of hippocampal neurons expressing GFP-NLG1 and tdTomato before and after glutamate uncaging. The yellow dot represents the uncaging spot. “1” represents the dendritic spine stimulated with glutamate and “2” represents a neighboring spine within 10 μm of spine 1. Dashed white line represents the contour used for fluorescence analysis in (F). Scale bars, 2μm. (D) Same as (C) but glutamate uncaging was performed in the presence of MMP2/9 inhibitor II (0.3 μM). Scale bar, 2μm. (E) Same as (C) but cells express GFP-NLG1-ΔSD3 and tdTomato. Scale bar, 2μm. (F–H) Fluorescence intensity profiles of GFP (green) and tdTomato (red) along the white dashed lines depicted in (C–E). (I) Representation of the dendrite segment depicted in (E) illustrating analysis of GFP and tdTomato fluorescence in dendritic spines and adjacent dendritic shaft regions. (J–L) Means ± SEMs of GFP/tdTomato fluorescence intensity ratio after glutamate uncaging in dendritic spines and adjacent dendritic shaft regions in the conditions represented in (C–E), respectively. n = 10, 11 and 11; *p<0.05, **p<0.001.

Figure 5. Juxtamembrane Cleavage of Neuroligin Destabilizes Presynaptic Neurexin-1β

(A) Left, schematic of thrombin-cleavable NLG1. Right, DIV21 hippocampal neurons expressing GFP-Thr-NLG1 and mCherry for 3–5 d were imaged before (Pre) and 30 min after (Post) thrombin incubation. Scale bar, 5 μm. (B) Same as (A) but neurons expressed GFP-Thr-NLG1 and PSD-95-mCh. Scale bar, 5 μm. (C) Dually-labeled synapses with postsynaptic GFP-Thr-NLG1 and presynaptic NRX1β-mCh imaged by confocal microscopy before (Pre) and 30 min after (Post) thrombin incubation. Solid arrowheads show loss of presynaptic NRX1β-mCh apposed to postsynaptic GFP-Thr-NLG1 puncta. Open arrowheads show generation of mobile NRX1β-mCh puncta along the axon. Scale bar, 5 μm. (D) Same as (C) but cells were co-transfected with GFP-Thr-NLG1 and synaptophysin-mCh (Sph-mCh). Sph-mCh is not affected GFP-Thr-NLG1 cleavage. Scale bar, 5 μm. (E) Timelapse of the highlighted region depicted in (C). The red arrowhead shows the time of thrombin (Thr) application. Scale bar, 5 μm. (F) Means ± SEM of fluorescence intensity change 30 min after thrombin (Thr) cleavage at synapses expressing GFP-Thr-NLG1. mCherry, n = 307 spines; PSD-95-mCh, n = 403 spines; Sph-mCh, n = 43 synaptic pairs; NRX1β-mCh, n = 87 synaptic pairs, *p <0.05. (G) Fluorescence intensity before and after thrombin application at synapses containing presynaptic NRX1β-mCh and postsynaptic GFP-Thr-NLG1. Arrowhead shows time of thrombin (Thr) application. n = 87 synaptic pairs. (H) Same as (G) but neurons expressed GFP-NLG1 lacking a thrombin cleavage site. n = 76 synaptic pairs.

Figure 6. NLG1 Cleavage Rapidly Reduces Release Probability at Excitatory Synapses

(A) Miniature EPSCs (mEPSCs) recorded before (Baseline) and after (Post) thrombin treatment in hippocampal neurons (DIV15-25) expressing GFP-Thr-NLG1 or GFP-NLG1. (B) Means ± SEM of mEPSC frequency and (C) amplitude change after thrombin incubation in neurons expressing GFP-NLG1 or GFP-Thr-NLG1. n = 8, 5 for GFP-Thr-NLG1 and GFP-NLG1, respectively. *p < 0.01. (D) Paired-pulse evoked EPSCs (eEPSCs) recorded before (Baseline) or after (Post) thrombin treatment in hippocampal neurons (DIV15-25) expressing GFP-Thr-NLG1 or GFL-NLG. Insets depict traces before and after thrombin treatment, normalized to the amplitude of the first EPSP. (E) Means ± SEM of eEPSC amplitude change after thrombin in neurons expressing GFP-NLG1 or GFP-Thr-NLG1. n = 6, 6; *p < 0.05. (F) Means ± SEM of paired-pulse ratio change after thrombin in neurons expressing GFP-NLG1 or GFP-Thr-NLG1. n = 6, 6; *p < 0.05. (G) Means ± SEM of time constants (τ) from stimulus-induced FM4-64 fluorescence decay measurements in presynaptic terminals apposing neurons expressing GFP-NLG1 or GFP-NLG1-ΔSD3. Terminals in GFP-NLG1-ΔSD3 synapses exhibit faster FM4-64 decay kinetics. GFP-NLG1-ΔSD3, n = 396 terminals, 12 neurons; GFP-NLG1, n = 438 terminals, 11 neurons; ***p<0.0001 (H) Cumulative distribution of FM4-64 decay constants (τ) of terminals analyzed in (G). (I) Representative kymographs of FM4-64 intensity in terminals apposing GFP-NLG1 or GFP-NLG1-ΔSD3. Arrowhead indicates time of KCl application.

Figure 7. NLG1 Cleavage is Triggered by Neural Activity and Sensory Experience In Vivo

(A) WT and MMP9 KO mice were injected with pilocarpine (315 mg/kg) or saline (control) for 2 h. (B) Immunoblot analysis of total extracts (5 μg) and soluble fractions (Sol, 50 μg) of hippocampi from mice injected with pilocarpine or saline (control). Arrow indicates increased NLG1-NTFs after PSE in WT mice. (C) Means ± SEMs of NLG1-NTFs levels in indicated conditions normalized to control. n = 9 animals in each WT group; n = 10, 11 in MMP9 KO control and pilocarpine groups, respectively. *p < 0.005. (D) WT and MMP9-KO mice were reared in a normal 12 h day/night cycle (LR) for 26 d or submitted to dark rearing for 5 d from P21-P26 (DR). DR+2hL indicates group submitted to 2 h light exposure (yellow box) after DR. (E) Immunoblot analysis of primary visual cortex soluble fractions of P26 mice reared under different conditions. α-actinin (α-Act) was used as a loading control. (F) Means ± SEM of NLG1-NTFs levels under different conditions normalized to LR groups. n = 27 animals in each WT group; n = 24, 25, 28 animals in MMP9 KO LR, DR and DR+2hL groups, respectively. *p < 0.01.