| Synaptic dysfunctions in AP-2µ KO mice are mitigated by BACE1 haploinsufficiency.Figure 5(A, B) AP-2α levels normalized to GAPDH are significantly decreased in lysates of iPSC-derived neurons from late-onset AD patients carrying the TREM2 p.R47H (AD-TREM2-2 and AD-TREM2-4) variant compared to healthy controls (CON8 and CON9) set to 100% (ADAP-2α: 52.42±8.04%, p=0.020, N=3 biological replicates). Data information: All graphs show mean ± SEM, statistical analysis was performed by one-sample Student's t‐test n.s.-non-significant.* indicates p≤0.05, ** indicates p≤0.01, *** indicates p≤0.001. (C) Confocal images of either AP-2 WT (AAV-GFPCamkIIα-labelled) or AP-2 KO (AAV-Cre-GFPCamkIIα-labelled) granule neurons in the dentate gyrus of 13-week-old mice. Scale bars: 20µm, 2µm (inserts). (D) Loss of AP-2µ significantly decreases the number of spines on granule neurons (WT: 12.63±0.88, KO: 8.07±0.45, p=0.004, N=4 biological replicates). Data information: All graphs show mean ± SEM, statistical analysis was performed by unpaired two-tailed Student's t‐test n.s.-non-significant.* indicates p≤0.05, ** indicates p≤0.01, *** indicates p≤0.001. (E, F) Overlap coefficient between the presynaptic marker Bassoon and the postsynaptic marker PSD95 is significantly decreased in the entorhinal cortex of AP-2µ KO mice compared to the WT (WT: 0.45±0.02, KO: 0.33±0.06, p=0.034, N=3 biological replicates). Scale bar, 750nm. Data information: All graphs show mean ± SEM, statistical analysis was performed by unpaired two-tailed Student's t‐test n.s.-non-significant.* indicates p≤0.05, ** indicates p≤0.01, *** indicates p≤0.001. Lost preference for the novel object (nov) comparing to the familiar object (fam) in AP-2µ KO mice tested 24h after training in NOR Representative heatmaps are shown in (G). Lost preference for the novel object (nov) comparing to the familiar object (fam) in AP-2µ KO mice tested 24h after training in NOR (WTfam: 27.51±6.77, WTnov: 72.49±6.77, pWTfam vs WTnov=0.000; KOfam: 43.9±13.12, KOnov: 59.91±13.12, pKOfam vs KOnov=0.469, N=7 WT and 8 KO mice). Data information: All graphs show mean ± SEM, statistical analysis was performed by unpaired two-tailed Student's t‐test n.s.-non-significant.* indicates p≤0.05, ** indicates p≤0.01, *** indicates p≤0.001. (I, J) AP-2µ KO mice are significantly more active in the open field compared to the WT (WT: 1663±139.08%, KO: 3976±802.30%, p=0.020, N=7 WT and 8 KO mice). Representative heatmaps are shown in (I). Data information: All graphs show mean ± SEM, statistical analysis was performed by unpaired two-tailed Student's t‐test n.s.-non-significant.* indicates p≤0.05, ** indicates p≤0.01, *** indicates p≤0.001. (K, L) Overlap coefficient between Bassoon and PSD95 is rescued upon BACE1 KD in AP-2µ KO neurons (WTscr: 0.39±0.02; KOscr: 0.24±0.01; WTshBACE1: 0.37±0.02; KOshBACE1: 0.33±0.02; pWTscr vs pKOscr<0.000; pKOscr vs pKOshBACE1=0.010; pWTscr vs pWTshBACE1=0.813; pWTshBACE1 vs pKOshBACE1=0.623, 22-24 neurons for each condition, N=3). Scale bars: 10µm. Data information: All graphs show mean ± SEM, statistical analysis was performed by two-way ANOVA n.s.-non-significant.* indicates p≤0.05, ** indicates p≤0.01, *** indicates p≤0.001. Neuronal loss is rescued in the entorhinal cortex of AP-2µ mice haploinsufficient for BACE1 (AP-2WT/BACEWT: 2.25±0.08 N=9 sections from N=3 for each genotype). Scale bar: 100 μm. Neuronal loss is rescued in the entorhinal cortex of AP-2µ KO mice haploinsufficient for BACE1 AP-2KO/BACEWT: 1.51±0.08, AP-2WT/BACEHET: 1.80±0.07, AP-2KO/BACEHET: 2.00±0.07; pAP-2WT/BACE1WT vs pAP-2KO/BACE1WT<0.000; pAP-2WT/BACE1WT vs pAP-2WT/BACE1HET=0.001; pAP-2KO/BACE1WT vs pAP-2KO/BACE1HET=0.000; p AP-2WT/BACE1HET vs AP-2KO/BACE1HET=0.268, N=9 sections from N=3 for each genotype). Scale bar: 100 μm. Data information: All graphs show mean ± SEM, statistical analysis was performed by two-way ANOVA n.s.-non-significant.* indicates p≤0.05, ** indicates p≤0.01, *** indicates p≤0.001. (P, Q) APP-CTF levels are rescued in the entorhinal cortex of AP-2µ KO mice haploinsufficient for BACE1 (AP-2WT/BACEWT:427574±24021, AP-2KO/BACEWT: 538895±34662, AP-2WT/BACEHET: 389870±19842, AP-2KO/BACEHET: 395389±21853; pAP-2WT/BACE1WT vs pAP-2KO/BACE1WT=0.017; pAP-2KO/BACE1WT vs pAP-2KO/BACE1HET=0.001; p AP-2WT/BACE1HET vs AP-2KO/BACE1HET=0.979; 13-15 sections, N=3 mice for each genotype). Scale bars: 40µm. Data information: All graphs show mean ± SEM, statistical analysis was performed by two-way ANOVA n.s.-non-significant.* indicates p≤0.05, ** indicates p≤0.01, *** indicates p≤0.001.