Down-regulation of dendritic spine and glutamic acid decarboxylase 67 expressions in the reelin haploinsufficient heterozygous reeler mouse

Heterozygous reeler mice (HRM) haploinsufficient for reelin express ≈50% of the brain reelin content of wild-type mice, but are phenotypically different from both wild-type mice and homozygous reeler mice. They exhibit, (i) a down-regulation of glutamic acid decarboxylase 67 (GAD67)-positive neurons in some but not every cortical layer of frontoparietal cortex (FPC), (ii) an increase of neuronal packing density and a decrease of cortical thickness because of neuropil hypoplasia, (iii) a decrease of dendritic spine expression density on basal and apical dendritic branches of motor FPC layer III pyramidal neurons, and (iv) a similar decrease in dendritic spines expressed on the basal dendrite branches of CA1 pyramidal neurons of the hippocampus. To establish whether the defect of GAD67 down-regulation observed in HRM is responsible for neuropil hypoplasia and decreased dendritic spine density, we studied heterozygous GAD67 knockout mice (HG67M). These mice exhibited a down-regulation of GAD67 mRNA expression in FPC (about 50%), but they expressed normal amounts of reelin and had no neuropil hypoplasia or down-regulation of dendritic spine expression. These findings, coupled with electron-microscopic observations that reelin colocalizes with integrin receptors on dendritic spines, suggest that reelin may be a factor in the dynamic expression of cortical dendritic spines perhaps by promoting integrin receptor clustering. These findings are interesting because the brain neurochemical and neuroanatomical phenotypic traits exhibited by the HRM are in several ways similar to those found in postmortem brains of psychotic patients.

Homologous-pairing activity of the human DNA-repair proteins Xrcc3⋅Rad51C

The human Xrcc3 protein is involved in the repair of damaged DNA through homologous recombination, in which homologous pairing is a key step. The Rad51 protein is believed to be the only protein factor that promotes homologous pairing in recombinational DNA repair in mitotic cells. In the brain, however, Rad51 expression is extremely low, whereas XRCC3, a human homologue of Saccharomyces cerevisiae RAD57 that activates the Rad51-dependent homologous pairing with the yeast Rad55 protein, is expressed. In this study, a two-hybrid analysis conducted with the use of a human brain cDNA library revealed that the major Xrcc3-interacting protein is a Rad51 paralog, Rad51C/Rad51L2. The purified Xrcc3⋅Rad51C complex, which shows apparent 1:1 stoichiometry, was found to catalyze the homologous pairing. Although the activity is reduced, the Rad51C protein alone also catalyzed homologous pairing, suggesting that Rad51C is a catalytic subunit for homologous pairing. The DNA-binding activity of Xrcc3⋅Rad51C was drastically decreased in the absence of Xrcc3, indicating that Xrcc3 is important for the DNA binding of Xrcc3⋅Rad51C. Electron microscopic observations revealed that Xrcc3⋅Rad51C and Rad51C formed similar filamentous structures with circular single-stranded DNA.