Reelin deficiency causes specific defects in the molecular composition of the synapses in the adult brain

The extracellular protein Reelin regulates radial neuronal migration in the embryonic brain, promotes dendrite outgrowth in the developing postnatal forebrain, and strengthens synaptic transmission in the adult brain. Heterozygous reeler mice expressing reduced levels of Reelin are grossly normal but exhibit behavioral and physiological abnormalities. We previously demonstrated that dendritic spine density is reduced in the developing hippocampus of these mice. In this study, we investigated the consequence of Reelin deficiency on synapse formation in adult heterozygous reeler mice using imaging and biochemical approaches. Using a reeler colony that expresses yellow fluorescent protein in selected neurons, we analyzed spine density in hippocampal area CA1 by confocal microscopy and found modest abnormalities in heterozygous reeler mice. However, biochemical analysis of synaptic composition revealed specific postsynaptic defects in scaffolding proteins, neurotransmitter receptors, and signaling proteins. Using whole brain homogenates and purified pre- and postsynaptic fractions, we found that the defects were localized to the postsynaptic compartment of heterozygous reeler synapses. Decreased levels of postsynaptic density-95 (PSD-95), the N-methyl d-aspartate (NMDA) receptor subunits NR2A and NR2B, and the phosphatase PTEN were found specifically in the postsynaptic density fraction obtained from these mice. Furthermore, we found that PSD-95, NR2A, and PTEN interact with each other at the synapse. Finally, we show that levels of NR2A are reduced in conditional Pten knock out mice, demonstrating that the PTEN phosphatase regulates NMDA receptor expression at the synapse in vivo. These studies may provide insights into the etiology of cognitive disorders associated with deficiencies in Reelin signaling and PTEN dysfunction.