Identification of Arx targets unveils new candidates for controlling cortical interneuron migration and differentiation

Mutations in the homeobox transcription factor ARX have been found to be responsible for a wide spectrum of disorders extending from phenotypes with severe neuronal migration defects, such as lissencephaly, to mild forms of intellectual disabilities without apparent brain abnormalities, but with associated features of dystonia and epilepsy. Arx expression is mainly restricted to populations of GABA-containing neurons. Studies of the effects of ARX loss of function, either in humans or mutant mice, revealed varying defects, suggesting multiple roles of this gene in brain patterning, neuronal proliferation and migration, cell maturation and differentiation, as well as axonal outgrowth and connectivity. However, to date, little is known about how Arx functions as a transcription factor or which genes it binds and regulates. Recently, we combined chromatin immunoprecipitation and mRNA expression with microarray analysis and identified approximately 1000 gene promoters bound by Arx in transfected neuroblastoma N2a cells and mouse embryonic brain. To narrow the analysis of Arx targets to those most likely to control cortical interneuron migration and/or differentiation, we compare here our data to previously published studies searching for genes enriched or down-regulated in cortical interneurons between E13.5 and E15.5. We thus identified 14 Arx-target genes enriched (Cxcr7, Meis1, Ppap2a, Slc 12a5, Ets2, Phlda1, Egr1, Igf1, Lmo3, Sema6, Lgi1, Alk, Tgfb3, and Napb) and 5 genes specifically down-regulated (Hmgn3, Lmo1, Ebf3, Rasgef1b, and Slit2) in cortical migrating neurons. In this review, we present these genes and discuss how their possible regulation by Arx may lead to the dysfunction of GABAergic neurons, resulting in mental retardation and epilepsy

Mutations in ARX Result in Several Defects Involving GABAergic Neurons

Genetic investigations of X-linked mental retardation have demonstrated the implication of ARX in a wide spectrum of disorders extending from phenotypes with severe neuronal migration defects, such as lissencephaly, to mild or moderate forms of mental retardation without apparent brain abnormalities, but with associated features of dystonia and epilepsy. These investigations have in recent years directed attention to the role of this gene in brain development. Analysis of its spatio-temporal localization profile revealed expression in telencephalic structures at all stages of development, mainly restricted to populations of GABA-containing neurons. Furthermore, studies of the effects of ARX loss of function either in humans or in lines of mutant mice revealed varying defects, suggesting multiple roles of this gene during development. In particular, Arx has been shown to contribute to almost all fundamental processes of brain development: patterning, neuronal proliferation and migration, cell maturation and differentiation, as well as axonal outgrowth and connectivity. In this review, we will present and discuss recent findings concerning the role of ARX in brain development and how this information will be useful to better understand the pathophysiological mechanisms of mental retardation and epilepsy associated with ARX mutations

Molecules and mechanisms involved in the generation and migration of cortical interneurons

The GABA (γ-aminobutyric acid)-containing interneurons of the neocortex are largely derived from the ganglionic eminences in the subpallium. Numerous studies have previously defined the migratory paths travelled by these neurons from their origins to their destinations in the cortex. We review here results of studies that have identified many of the genes expressed in the subpallium that are involved in the specification of the subtypes of cortical interneurons, and the numerous transcription factors, motogenic factors and guidance molecules that are involved in their migration

Thalamic Gap Junctions Control Local Neuronal Synchrony and Influence Macroscopic Oscillation Amplitude during EEG Alpha Rhythms

Although EEG alpha (α; 8–13 Hz) rhythms are often considered to reflect an “idling” brain state, numerous studies indicate that they are also related to many aspects of perception. Recently, we outlined a potential cellular substrate by which such aspects of perception might be linked to basic α rhythm mechanisms. This scheme relies on a specialized subset of rhythmically bursting thalamocortical (TC) neurons (high-threshold bursting cells) in the lateral geniculate nucleus (LGN) which are interconnected by gap junctions (GJs). By engaging GABAergic interneurons, that in turn inhibit conventional relay-mode TC neurons, these cells can lead to an effective temporal framing of thalamic relay-mode output. Although the role of GJs is pivotal in this scheme, evidence for their involvement in thalamic α rhythms has thus far mainly derived from experiments in in vitro slice preparations. In addition, direct anatomical evidence of neuronal GJs in the LGN is currently lacking. To address the first of these issues we tested the effects of the GJ inhibitors, carbenoxolone (CBX), and 18β-glycyrrhetinic acid (18β-GA), given directly to the LGN via reverse microdialysis, on spontaneous LGN and EEG α rhythms in behaving cats. We also examined the effect of CBX on α rhythm-related LGN unit activity. Indicative of a role for thalamic GJs in these activities, 18β-GA and CBX reversibly suppressed both LGN and EEG α rhythms, with CBX also decreasing neuronal synchrony. To address the second point, we used electron microscopy to obtain definitive ultrastructural evidence for the presence of GJs between neurons in the cat LGN. As interneurons show no phenotypic evidence of GJ coupling (i.e., dye-coupling and spikelets) we conclude that these GJs must belong to TC neurons. The potential significance of these findings for relating macroscopic changes in α rhythms to basic cellular processes is discussed

The Role of Robo3 in the Development of Cortical Interneurons

A number of studies in recent years have shown that members of the Roundabout (Robo) receptor family, Robo1 and Robo2, play significant roles in the formation of axonal tracks in the developing forebrain and in the migration and morphological differentiation of cortical interneurons. Here, we investigated the expression and function of Robo3 in the developing cortex. We found that this receptor is strongly expressed in the preplate layer and cortical hem of the early cortex where it colocalizes with markers of Cajal–Retzius cells and interneurons. Analysis of Robo3 mutant mice at early (embryonic day [E] 13.5) and late (E18.5) stages of corticogenesis revealed no significant change in the number of interneurons, but a change in their morphology at E13.5. However, preliminary analysis on a small number of mice that lacked all 3 Robo receptors indicated a marked reduction in the number of cortical interneurons, but only a limited effect on their morphology. These observations and the results of other recent studies suggest a complex interplay between the 3 Robo receptors in regulating the number, migration and morphological differentiation of cortical interneurons