MHCI requires CaN to activate MEF2 and limit synapse density. A, Treatment of neurons with the CaN inhibitors FK506/CsA (FK/CsA) blocks the MHCI-induced increase in MEF2 activity; n ≥ 42 cells per condition, four experiments. B, FK/CsA rescues the MHCI-mediated decrease in synapse density; n ≥ 12 dendrites per condition (4 cells), one experiment. C, Similarly, coexpression of a dominant-negative form of CaN (dnCaN, CaN-H151A-YFP) also rescues the MHCI-mediated decrease in synapse density; n ≥ 51 dendrites (17 cells) per condition, two experiments. D, CaN binding to NFAT does not alter synapse density, nor is it required for the MHCI-induced decrease in synapse density. Neurons were transfected with the indicated constructs at 6 DIV, and synapse density was quantified 48 h later. E, Inhibiting CaN activity does not increase synapse density above that seen by sMHCI KD alone. β2m siRNAs were transfected at 5 DIV with or without FK/CsA, and synapse density was analyzed at 8 DIV; n ≥ 57 dendrites (19 cells) per condition, three experiments. *p < 0.05, **p < 0.01, ***p < 0.001; ns, not significant.