Emx1 null mutant mouse phenotype: potential implications for human epilepsy

Abstract

In the last decade, several homeobox-containing genes including members of the empty spiracle (Emxl and Emx2) and orthodenticle (Otxl and Otx2) families, have been found to be expressed in the cerebral cortex of rodents. Among these genes, Emxl shows the most widespread pattern of expression in this part of the brain. It has been recently demonstrated that Emxl is expressed by pyramidal cells, which are the glutamate-containing projection neurons of the cortex, from the time of their origin during embryonic period of neocorticogenesis, and until long after birth. In contrast, non-pyramidal cells, the GABA-containing cortical intemeurons originate in the ventral telencephalon and have been found not to express this homeobox gene. Emxl gene inactivation at a first analysis has not showed to cause gross morphological abnormalities to the developing and the adult brain. We studied the development of the telecephalon in Emxl null embryos and examined the cortices of adult Emxl null mice, using standard histological methods as well as a variety immonocytochemical, in situ hybridization and tracing techniques. I did not find gross abnormalities in the development of Emxl null mutant embryos; therefore, the overall cytological appearance of the adult cortex was indistinguishable between the mutant mice and the wild type littermates. A deeper analysis of the density and distribution of the two principal neuronal cell types of the cortex, pyramidal and nonpyramidal neurons, revealed no significant difference in the number and layering of pyramidal neurons in mutant cortices. However, counts of GABAergic neurons showed statistically significant differences between mutant and wild type littermates. Earlier reports provided evidence in support of the hypothesis that loss of inhibitory GABAergic intemeurons results in neuronal hyperxcitability in the cortex, and eventually leads to seizure activity; more recent investigation confirmed that disruption of GABAergic neurotransmission is implicated in epilepsy