Synaptic scaffolding molecule α is a scaffold to mediate N-methyl-D-aspartate receptor-dependent RhoA activation in dendrites

Synaptic scaffolding molecule (S-SCAM) interacts with a wide variety of molecules at excitatory and inhibitory synapses. It comprises three alternative splicing variants, S-SCAM alpha, -beta, and -gamma. We generated mutant mice lacking specifically S-SCAM alpha. S-SCAM alpha-deficient mice breathe and feed normally but die within 24 h after birth. Primary cultured hippocampal neurons from mutant mice have abnormally elongated dendritic spines. Exogenously expressed S-SCAM alpha corrects this abnormal morphology, while S-SCAMP and -gamma have no effect. Active RhoA decreases in cortical neurons from mutant mice. Constitutively active RhoA and ROCKII shift the length of dendritic spines toward the normal level, whereas ROCK inhibitor (Y27632) blocks the effect by S-SCAMa. S-SCAM alpha fails to correct the abnormal spine morphology under the treatment of N-methyl-D-aspartate (NMDA) receptor inhibitor (AP-5), Ca2+/calmodulin kinase inhibitor (KN-62), or tyrosine kinase inhibitor (PP2). NMDA treatment increases active RhoA in dendrites in wild-type hippocampal neurons, but not in mutant neurons. The ectopic expression of S-SCAM alpha, but not -beta, recovers the NMDA-responsive accumulation of active RhoA in dendrites. Phosphorylation of extracellular signal-regulated kinase 1/2 and Akt and calcium influx in response to NMDA are not impaired in mutant neurons. These data indicate that S-SCAM alpha is a scaffold required to activate RhoA protein in response to NMDA receptor signaling in dendrites.