Loss of neuronal βPix isoforms impairs neuronal morphology in the hippocampus and causes behavioral defects.

βPix is a guanine nucleotide exchange factor for the Rac1 and Cdc42 small GTPases, which play important roles in dendritic spine morphogenesis by modulating actin cytoskeleton organization. The formation and plasticity of the dendritic spines are essential for normal brain function. Among the alternatively spliced βPix isoforms, βPix-b and βPix-d are expressed specifically in neurons. Our previous studies using cultured hippocampal neurons identified the roles of βPix-b and βPix-d in spine formation and neurite development, respectively. Here, we analyzed the in vivo role of the neuronal βPix isoforms in brain development and function by using βPix neuronal isoform knockout (βPix-NIKO) mice, in which the expression of the βPix-b and βPix-d isoforms is blocked, while the expression of the ubiquitous βPix-a isoform is maintained. Loss of the neuronal βPix isoforms leads to reduced activity of Rac1 and Cdc42, decreased dendritic complexity and spine density, and increased GluN2B and Ca²⁺/calmodulin-dependent protein kinase IIα expression in the hippocampus. The defects in neurite development, dendritic spine maturation, and synaptic density in cultured βPix-NIKO hippocampal neurons were rescued by the expression of βPix-b or βPix-d. In behavioral studies, βPix-NIKO mice exhibited robust deficits in novel object recognition and decreased anxiety levels. Our findings suggest that neuronal morphogenetic signaling by the neuronal βPix isoforms contributes to normal behaviors.