Postsynaptic Farp1 organizes presynaptic active zones via SynCAM 1. (A and B) Bassoon intensity is regulated by Farp1 across the synaptic cleft. (A) Confocal images of hippocampal neurons at 14 div. Green, GFP or GFP-Farp1. Red, bassoon detected by immunostaining. Circles mark representative spines apposed to bassoon. (B) Bassoon intensity imaged as in A normalized to staining atop GFP control dendrites (shScramble, n = 15 neurons; shFarp1, 17; GFP, 22; GFP-Farp1, 25; GFP + Ig2+Ig3-GPI, 14; GFP-Farp1 + Ig2+Ig3-GPI, 21; 167, 143, 296, 284, 182, and 286 puncta were analyzed, respectively). (C and D) Farp1 regulates bassoon intensity in dependence on SynCAM 1. (C) Hippocampal neurons from WT and SynCAM 1 KO littermate mice (three mice each) were transfected at 5–7 div to express GFP^myr or GFP-Farp1. Bassoon immunostaining at 21 div was imaged by confocal microscopy. Green, GFP^myr or GFP-Farp1. Red, bassoon. Circles, representative spines apposed to bassoon. (D) Bassoon puncta intensity imaged as in C, normalized to staining atop GFP^myr -expressing WT dendrites. (WT + GFP^myr, n = 173 puncta from 14 neurons; WT + GFP-Farp1, n = 338 from 18 neurons; KO + GFP^myr; n = 309 from 21 neurons; KO + GFP-Farp1, n = 293 from 26 neurons). The broken lines mark the control’s value of AU = 1.0, to which the other values were normalized. Error bars indicate mean ± SEM; ***, P < 0.001. n.s., not significant. Bars, 4 µm.