The Aristaless-related homeobox gene in mouse brain development

The production and integration of GABAergic interneurons into the cortex is a crucial element of brain development. These neurons play important roles in controlling and modulating neural firing patterns by completing local circuits. The loss of the rhythm generation, synchronization and inhibition of neural signaling provided by interneurons likely leads to severe deficits in the brain, which in human patients are believed to result in intellectual disability (ID) and epilepsy. Mutations in the transcription factor Aristaless-related homeobox gene (ARX) are also associated with these clinical phenotypes, as well as defects in interneuron populations. However, the normal function of ARX and the mechanism of disease in patients with mutations are unknown. In Chapter 2, I studied the role of Arx in interneurons, using in vitro and in vivo techniques, with emphasis on understanding the mechanism of disease of an expansion of the first polyalanine tract in Arx (ArxE). This mutation is associated with ID, Infantile Spasm Syndrome and other early epileptic phenotypes in patients. Expression of ArxE in a mouse model in which Arx has been deleted in cells either in the ventral forebrain or in the entire forebrain demonstrated that this mutation causes dysfunction in the non-radial cell migration (NRCM) of ventrally-born interneurons moving to the cortex, but not in neurons born in dorsal proliferative areas migrating radially to form the cortical plate. Arx is unable to repress specific targets through a context-specific loss of binding to its repressive cofactor Tle1 at those targets. In Chapter 3, I investigated the effect of Arx deletion on interneuron subpopulations, which showed that calbindin-positive neurons are decreased in dorsal areas of the embryonic brain, and increased in ventral areas, while Nkx2.1-positive cells appear to migrate abnormally from the subpallium dorsally into the pallium. This suggests that Arx regulates guidance factors that are important for proper localization of various subsets of interneurons. Therefore, normal transcriptional repression by Arx is necessary in interneurons, and a polyalanine tract expansion leads to a partial loss of this repressive function, which is sufficient to disrupt NRCM

Developmental interneuron subtype deficits after targeted loss of Arx

Background: Aristaless-related homeobox (ARX) is a paired-like homeodomain transcription factor that functions primarily as a transcriptional repressor and has been implicated in neocortical interneuron specification and migration. Given the role interneurons appear to play in numerous human conditions including those associated with ARX mutations, it is essential to understand the consequences of mutations in this gene on neocortical interneurons. Previous studies have examined the effect of germline loss of Arx, or targeted mutations in Arx, on interneuron development. We now present the effect of conditional loss of Arx on interneuron development. Results: To further elucidate the role of Arx in forebrain development we performed a series of anatomical and developmental studies to determine the effect of conditional loss of Arx specifically from developing interneurons in the neocortex and hippocampus. Analysis and cell counts were performed from mouse brains using immunohistochemical and in situ hybridization assays at 4 times points across development. Our data indicate that early in development, instead of a loss of ventral precursors, there is a shift of these precursors to more ventral locations, a deficit that persists in the adult nervous system. The result of this developmental shift is a reduced number of interneurons (all subtypes) at early postnatal and later time periods. In addition, we find that X inactivation is stochastic, and occurs at the level of the neural progenitors. Conclusion: These data provide further support that the role of Arx in interneuron development is to direct appropriate migration of ventral neuronal precursors into the dorsal cortex and that the loss of Arx results in a failure of interneurons to reach the cortex and thus a deficiency in interneurons