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Arx acts as a key selector gene of the ventral telencephalon mainly through its repression transcriptional activity

ABSTRACT: The homeobox containing gene Arx is expressed during ventral telencephalon development and it is required for correct GABAergic interneuron tangential migration from the ganglionic eminences to the olfactory bulbs, cerebral cortex and striatum. Its human ortholog is associated with a variety of neurological clinical manifestations whose syntoms are compatible with a loss of cortical interneurons and altered basal ganglia related-activities in humans. Herein, we reported the identification by global expression profiling of a group of genes whose expression is consistently altered in Arx mutant ganglionic eminences. Following analysis revealed the striking ectopic expression in the ganglionic eminences of a number of genes normally not, or only marginally, expressed in the ventral telencephalon. Among them, we functionally analyzed Ebf3, whose ectopic expression in ventral telencephalon is preventingneuronal tangential migration. Further, we showed that Arx is sufficient to repress Ebf3 endogenous expression and that its silencing in Arx mutant tissue might marginally rescue tangential cell movements. Together, these data provide an initial analysis of the molecular pathways regulated by Arx and how their networking might regulate those specific cellular processes during telencephalon development strongly altered by loss of Arx.

ORGANISM(S): Mus musculus

PROVIDER: GSE12956 | GEO | 2010/02/26

SECONDARY ACCESSION(S): PRJNA110939

REPOSITORIES: GEO

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Project description:Malformations of cortical development are the underlying eitiology of many cases of Mental Retardation and Epilepsy. Subtle, below the resolution of current MRI, cortical dysplasias are probably involved in many cases of MR, Epilepsy and Autism for which no diagnosis can currently be made. Therefore, understanding the process of cortical development will be vital in diagnosing and eventual treatment of many patients with these conditions. More specifically, the cortex forms from two major populations of neuroblasts which reach their final destination in the cortex by differerent mechanisms. One is radial migration from ventricular neuroblasts to the cortical plate. These cells are excititory projection neurons and glia. The second pathway is from the ventral ganglionic eminences and tangential migration of the interneuronal population of primarily inhibitory neurons. Much less is known about the control of the latter process, and many of these currently undiagnosed subtle malformations may stem from abnormalities of this tangential migration. This project focuses on the understanding the control of the tangentially migrating inhibitory interneurons. We aim to uncover the transcriptional differences between two distinct neuronal populations, GFP positive cells in the ganglionic emience and GFP positive cells within the cortical plate, in order to understand the genetic control of tangential migration. We will acomplish this aim by targeting two different labeled transcripts againts the affymetrix mouse genome arrays. We hypothesis that there will be distinct differences in mRNA transcription profiles between the ganglionic eminence and cortical plate within developing interneurons. The differences in transcript profiles will be genes that regulate the migration and differentiation of these developing neurons. Transgenic mice have been generated which express green flourscent protien (GFP) based on the activity of the Dlx 5-6 promotor. Dlx 5-6 are highly regulated genes, and are expressed only in developing interneurons with in the embryonic brain. The expression of this gene begins at around E8 within the medial and lateral ganglionic eminence (GE) and then remain present as cells born within these regions migrate out into the developing cortex. Therefore, we can take advantage of these mice by harvesting embryos at a midpoint in their development (E13.5-14.5) and dissect out GFP positive cells from the ganglionic eminence and from the cortical plate. In order to obtain enough cells to extract sufficent mRNA, we have choosen to perform microdisction of the entire GE and entire cortical plate then break up the tissue into single cell suspension and Florescent activated cell sort (FACs) the populations of GFP + and GFP - cells. We then will extract the total RNA from the GFP postive cell populations using a Trizol based method and purify the RNA with a Quiagen column purification method. The purified RNA will then be amplified using the Affymetrix 1 or 2 round T7 based amplification procedure in order to generate the microgram quantities of labelled cRNA. The labelled biotin cRNA will be sent to the microarray consortium for hybridization againt the Affy mouse genome chip. Three embryos will be pooled in order to obtain sufficent RNA and 6 GFP positive GE and cortical plate samples will be used. The six replicates will be obtained from at least 3 different pregnant mice.

Project description:Arx is a paired-box homeodomain transcription factor and the vertebrate ortholog to the Drosophila aristaless (al) gene. Mutations in Arx are associated with a variety of human diseases, including X-linked infantile spasm syndrome (OMIM: 308350), X-linked myoclonic epilepsy with mental retardation and spasticity (OMIM: 300432), X-linked lissencephaly with ambiguous genitalia (OMIM: 300215), X-linked mental retardation 54 (OMIM: 300419), and agenesis of the corpus callosum with abnormal genitalia (OMIM: 300004). Arx-deficient mice exhibit a complex, pleiotrophic phenotype, including decreased proliferation of neuroepithelial cells of the cortex, dysgenesis of the thalamus and olfactory bulbs, and abnormal nonradial migration of GABAergic interneurons. It has been suggested that deficits in interneuron specification, migration, or function lead to loss of inhibitory neurotransmission, which then fails to control excitatory activity and leads to epilepsy or spasticities. Given that Arx mutations are associated with developmental disorders in which epilepsy and spasticity predominate and that Arx-deficient mice exhibit deficits in interneuron migration, understanding the function of Arx in interneuron migration will prove crucial to understanding the pathology underlying interneuronopathies. Yet, downstream transcriptional targets of Arx, to date, remain unidentified. The aim of this project is to identify bona fide transcriptional targets for the Arx, a transcription factor required for normal migration of interneurons from the ganglionic eminences to the cortex, and to investigate the functions of these genes in the Arx-dependent pathway regulating nonradial neuronal migration. We hypothesize that the genes regulated by the Arx transcription factor will play a critical role in the nonradial migration of interneurons and that the results of this study will provide novel insights into the molecular mechanisms of nonradial neuronal migration, in particular, and possibly the molecular and biochemical pathogenesis underlying epilepsy, mental retardation, infantile spasm syndromes, and other so-called interneuronopathies;; We have recently generated a transgenic mouse with a floxed Arx allele (Arxflox). We have generated conditional knockouts in which Arx is removed specifically from the brain by mating Arxflox mice with transgenic mice expressing Cre behind the neural tube-specific transcriptional regulatory elements of the POU domain, class 3, transcription factor 4 promoter. Preliminary analyses of these mice suggest that conditional knockout mice recapitulate the nonradial migration defects associated with conventional knockout mice. We will compare the gene expression profiles of ganglion eminences (GEs; the anatomical source for nonradially migrating interneurons) from male Arxflox mice that express Pou3f-Cre to those from male mice without Arxflox allele. Animals will be prepared and sacrificed following our institutional protocol. Tissue will be rapidly dissected from E14.5 (the temporal peak of interneuron migration) GEs (MGE and LGE from both left and right hemispheres). Preliminary experiments suggest that the amounts of RNA that can be isolated from a pair of GEs is in the range of 2000-3500 ng, which should be sufficient for microarray analysis following linear amplification of RNA. The GEs from each animal will be combined, snap frozen in liquid nitrogen, and stored at -80 C until RNA is extracted. Total RNA will be extracted using Trizol followed by RNA purification with the RNeasy cleanup kit. We will be providing total RNA samples from four wildtype and four transgenic animals (true biological replicates) from three separate litters to mitigate any expression differences resulting from mouse to mouse or litter to litter variation.

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Arx acts as a key selector gene of the ventral telencephalon mainly through its repression transcriptional activity

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