Arx Is a Direct Target of Dlx2 and Thereby Contributes to the Tangential Migration of GABAergic Interneurons

Abstract

The Arx transcription factor is expressed in the developing ventral telencephalon and subsets of its derivatives. Mutation of human ARX ortholog causes neurological disorders including epilepsy, lissencephaly, and mental retardation. We have isolated the mouse Arx endogenous enhancer modules that control its tightly compartmentalized forebrain expression. Interestingly, they are scattered downstream of its coding region and partially included within the introns of the downstream PolA1 gene. These enhancers are ultraconserved noncoding sequences that are highly conserved throughout the vertebrate phylum. Functional characterization of the Arx GABAergic enhancer element revealed its strict dependence on the activity of Dlx transcription factors. Dlx overexpression induces ectopic expression of endogenous Arx and its isolated enhancer, whereas loss of Dlx expression results in reduced Arx expression, suggesting that Arx is a key mediator of Dlx function. To further elucidate the mechanisms involved, a combination of gain-of-function studies in mutant Arx or Dlx tissues was pursued. This analysis provided evidence that, although Arx is necessary for the Dlx-dependent promotion of interneuron migration, it is not required for the GABAergic cell fate commitment mediated by Dlx factors. Although Arx has additional functions independent of the Dlx pathway, we have established a direct genetic relationship that controls critical steps in the development of telencephalic GABAergic neurons. These findings contribute elucidating the genetic hierarchy that likely underlies the etiology of a variety of human neurodevelopmental disorders.

Prox1Regulates the Subtype-Specific Development of Caudal Ganglionic Eminence-Derived GABAergic Cortical Interneurons

“…Previous work has shown that Dlx1/2 activity induces the expression of the transcription factor Arx (Cobos et al, 2005a;Colasante et al, 2008), promoting migration by suppressing neurite growth mediated by Pak3, a p21-activated serine/ threonine kinase that acts as a downstream effector of the Rho family of GTPases (Cobos et al, 2007). However, in our microarray analysis comparing gene expression levels in control versus Prox1 loss-of-function cells, we did not reveal any measurable change in Pak3, Arx, or any of the Dlx-gene family members (data not shown).…”

“…However, aside from transcription factors, Dlx, Arx, and CoupTF, which participate in the early development of both MGE and CGE cortical interneuron subtypes (Cobos et al, 2005b;Colasante et al, 2008;Friocourt et al, 2008;Kanatani et al, 2008;Lodato et al, 2011), a distinct genetic cascade that regulates CGE-derived cortical interneuron development has not yet been characterized.…”

“…This analysis revealed that the expression of Prox1 is highly enriched (ϫ12.4; Table 1) within CGE-derived versus MGE-derived cortical interneurons at E18.5. Prox1 is a mammalian homolog of Prospero, which determines cell fate through its asymmetric localization in dividing neuroblasts within the fruit fly larva (Choksi et al, 2006;Doe, 2008) as well as in photoreceptor progenitors (Cook et al, 2003). Similarly, in the hippocampus, Prox1 has been shown to play important roles in cell proliferation (Kaltezioti et al, 2010;Lavado et al, 2010) and fate determination (Iwano et al, 2012).…”

Islet-1 Regulates Arx Transcription during Pancreatic Islet α-Cell Development

“…If no defect is observed, it is possible that Re1 might play a redundant role in mediating Arx transcription in developing islet ␣-cells. Outside of the pancreas, Arx expression is also detected in the developing ventral telencephalon of the brain, where its expression is directly regulated by the homeodomain factor Dlx2 (37,38). Two Dlx2 binding sites (HD-1 and HD-2) have been identified in the highly conserved noncoding sequence of the Arx locus (referred to as mUAS3, 37), which encompasses the Re2 sequences.…”

Malformations of Cortical Development and Epilepsy

“…ARX is an extremely important gene in cortical development as ARX regulates genes involved in cell migration, axonal guidance, neurogenesis, and transcription regulation (Colombo et al 2007;Colasante et al 2008Colasante et al , 2013Fulp et al 2008;Okazaki et al 2008), although the mechanisms are not yet known. Because ARX interacts with many different targets during development, mutations cause a wide range of abnormalities, including mental retardation (syndromic and nonsyndromic), infantile spasms without brain malformations, dyskinesia, agenesis of the corpus callosum with abnormal genitalia, variant lissencephaly with dys- morphic basal ganglia, and hydranencephaly (Kato et al 2004;Okazaki et al 2008).…”