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Mice lacking the Dlx1 and Dlx2 homeobox genes (Dlx1/2 mutants) have severe deficits in subpallial differentiation, including overexpression of the Gsx1 and Gsx2 homeobox genes. To investigate whether Gsx overexpression contributes to the Dlx1/2 mutant phenotypes, we made compound loss-of-function mutants. Eliminating Gsx2 function from the Dlx1/2 mutants rescued the increased expression of Ascl1 and Hes5 (Notch signaling mediators) and Olig2 (oligodendrogenesis mediator). In addition, Dlx1/2;Gsx2 mutants, like Dlx1/2;Ascl1 mutants, exacerbated the Gsx2 and Dlx1/2 patterning and differentiation phenotypes, particularly in the lateral ganglionic eminence (LGE) caudal ganglionic eminence (CGE), and septum, including loss of GAD1 expression. On the other hand, eliminating Gsx1 function from the Dlx1/2 mutants (Dlx1/2;Gsx1 mutants) did not severely exacerbate their phenotype; on the contrary, it resulted in a partial rescue of medial ganglionic eminence (MGE) properties, including interneuron migration to the cortex. Thus, despite their redundant properties, Gsx1 and -2 have distinct interactions with Dlx1 and -2. Gsx2 interaction is strongest in the LGE, CGE, and septum, whereas the Gsx1 interaction is strongest in the MGE. From these studies, and earlier studies, we present a model of the transcriptional network that regulates early steps of subcortical development., (Copyright © 2012 Wiley Periodicals, Inc.)

Lhx6 and Lhx8 transcription factor coexpression in early-born MGE neurons is required to induce neuronal Shh expression. We provide evidence that these transcription factors regulate expression of a Shh enhancer in MGE neurons. Lhx6 and Lhx8 are also required to prevent Nkx2-1 expression in a subset of pallial interneurons. Shh function in early-born MGE neurons was determined by genetically eliminating Shh expression in the MGE mantle zone (MZ). This mutant had reduced SHH signaling in the overlying progenitor zone, which led to reduced Lhx6, Lhx8, and Nkx2-1 expression in the rostrodorsal MGE and a preferential reduction of late-born somatostatin(+) and parvalbumin(+) cortical interneurons. Thus, Lhx6 and Lhx8 regulate MGE development through autonomous and nonautonomous mechanisms, the latter by promoting Shh expression in MGE neurons, which in turn feeds forward to promote the developmental program of the rostrodorsal MGE., (Copyright © 2011 Elsevier Inc. All rights reserved.)

Background: Bicoid (Bcd) is a Drosophila morphogenetic protein responsible for patterning the anterior structures in embryos. Recent experimental studies have revealed important insights into the behavior of this morphogen gradient, making it necessary to develop a model that can recapitulate the biological features of the system, including its dynamic and scaling properties., Methodology/principal Findings: We present a biologically realistic 2-D model of the dynamics of the Bcd gradient in Drosophila embryos. This model is based on equilibrium binding of Bcd molecules to non-specific, low affinity DNA sites throughout the Drosophila genome. It considers both the diffusion media within which the Bcd gradient is formed and the dynamic and other relevant properties of bcd mRNA from which Bcd protein is produced. Our model recapitulates key features of the Bcd protein gradient observed experimentally, including its scaling properties and the stability of its nuclear concentrations during development. Our simulation model also allows us to evaluate the effects of other biological activities on Bcd gradient formation, including the dynamic redistribution of bcd mRNA in early embryos. Our simulation results suggest that, in our model, Bcd protein diffusion is important for the formation of an exponential gradient in embryos., Conclusions/significance: The 2-D model described in this report is a simple and versatile simulation procedure, providing a quantitative evaluation of the Bcd gradient system. Our results suggest an important role of Bcd binding to non-specific, low-affinity DNA sites in proper formation of the Bcd gradient in our model. They demonstrate that highly complex biological systems can be effectively modeled with relatively few parameters.

Dlx5 and Dlx6 homeobox genes are expressed in developing and mature cortical interneurons. Simultaneous deletion of Dlx5 and 6 results in exencephaly of the anterior brain; despite this defect, prenatal basal ganglia differentiation appeared largely intact, while tangential migration of Lhx6(+) and Mafb(+) interneurons to the cortex was reduced and disordered. The migration deficits were associated with reduced CXCR4 expression. Transplantation of mutant immature interneurons into a wild-type brain demonstrated that loss of either Dlx5 or Dlx5&6 preferentially reduced the number of mature parvalbumin(+) interneurons; those parvalbumin(+) interneurons that were present had increased dendritic branching. Dlx5/6(+/-) mice, which appear normal histologically, show spontaneous electrographic seizures and reduced power of gamma oscillations. Thus, Dlx5&6 appeared to be required for development and function of somal innervating (parvalbumin(+)) neocortical interneurons. This contrasts with Dlx1, whose function is required for dendrite innervating (calretinin(+), somatostatin(+), and neuropeptide Y(+)) interneurons (Cobos et al., 2005).

Here we define the expression of approximately 100 transcription factors (TFs) in progenitors and neurons of the developing mouse medial and caudal ganglionic eminences, anlage of the basal ganglia and pallial interneurons. We have begun to elucidate the transcriptional hierarchy of these genes with respect to the Dlx homeodomain genes, which are essential for differentiation of most gamma-aminobutyric acidergic projection neurons of the basal ganglia. This analysis identified Dlx-dependent and Dlx-independent pathways. The Dlx-independent pathway depends in part on the function of the Mash1 basic helix-loop-helix (b-HLH) TF. These analyses define core transcriptional components that differentially specify the identity and differentiation of the globus pallidus, basal telencephalon, and pallial interneurons.

Here we define the expression of approximately 100 transcription factors in progenitors and neurons of the developing basal ganglia. We have begun to elucidate the transcriptional hierarchy of these genes with respect to the Dlx homeodomain genes, which are essential for differentiation of most GABAergic projection neurons of the basal ganglia. This analysis identified Dlx-dependent and Dlx-independent pathways. The Dlx-independent pathway depends in part on the function of the Mash1 b-HLH transcription factor. These analyses define core transcriptional components that differentially specify the identity and differentiation of the striatum, nucleus accumbens, and septum., (2008 Wiley-Liss, Inc.)

A remarkable feature of development is its reproducibility, the ability to correct embryo-to-embryo variations and instruct precise patterning. In Drosophila, embryonic patterning along the anterior-posterior axis is controlled by the morphogen gradient Bicoid (Bcd). In this article, we describe quantitative studies of the native Bcd gradient and its target Hunchback (Hb). We show that the native Bcd gradient is highly reproducible and is itself scaled with embryo length. While a precise Bcd gradient is necessary for precise Hb expression, it still has positional errors greater than Hb expression. We describe analyses further probing mechanisms for Bcd gradient scaling and correction of its residual positional errors. Our results suggest a simple model of a robust Bcd gradient sufficient to achieve scaled and precise activation of its targets. The robustness of this gradient is conferred by its intrinsic properties of "self-correcting" the inevitable input variations to achieve a precise and reproducible output.

Here we analyze the role of the Lhx6 lim-homeobox transcription factor in regulating the development of subsets of neocortical, hippocampal, and striatal interneurons. An Lhx6 loss-of-function allele, which expresses placental alkaline phosphatase (PLAP), allowed analysis of the development and fate of Lhx6-expressing interneurons in mice lacking this homeobox transcription factor. There are Lhx6+;Dlx+ and Lhx6-;Dlx+ subtypes of tangentially migrating interneurons. Most interneurons in Lhx6(PLAP/PLAP) mutants migrate to the cortex, although less efficiently, and exhibit defects in populating the marginal zone and superficial parts of the neocortical plate. By contrast, migration to superficial parts of the hippocampus is not seriously affected. Furthermore, whereas parvalbumin+ and somatostatin+ interneurons do not differentiate, NPY+ interneurons are present; we suggest that these NPY+ interneurons are derived from the Lhx6-;Dlx+ subtype. Striatal interneurons show deficits distinct from pallial interneurons, including a reduction in the NPY+ subtype. We provide evidence that Lhx6 mediates these effects through promoting expression of receptors that regulate interneuron migration (ErbB4, CXCR4, and CXCR7), and through promoting the expression of transcription factors either known (Arx) or implicated (bMaf, Cux2, and NPAS1) in controlling interneuron development., ((c) 2008 Wiley-Liss, Inc.)

Prevailing notions of cerebral vascularization imply that blood vessels sprout passively into the brain parenchyma from pial vascular plexuses to meet metabolic needs of growing neuronal populations. Endothelial cells, building blocks of blood vessels, are thought to be homogeneous in the brain with respect to their origins, gene expression patterns and developmental mechanisms. These current notions that cerebral angiogenesis is regulated by local environmental signals contrast with current models of cell-autonomous regulation of neuronal development. Here we demonstrate that telencephalic angiogenesis in mice progresses in an orderly, ventral-to-dorsal gradient regulated by compartment-specific homeobox transcription factors. Our data offer new perspectives on intrinsic regulation of angiogenesis in the embryonic telencephalon, call for a revision of the current models of telencephalic angiogenesis and support novel roles for endothelial cells in brain development.

The subventricular zone (SVZ) of the postnatal brain continuously generates olfactory bulb (OB) interneurons. We show that calretinin+, calbindin+, and dopaminergic (TH+) periglomerular OB interneurons correspond to distinct subtypes of GABAergic cells; all were produced in the postnatal mouse brain, but they matured and were eliminated at different rates. The embryonic lateral ganglionic eminence (LGE) is thought to be the site of origin of postnatal SVZ neural progenitors. Consistently, grafts of the embryonic LGE into the adult brain SVZ generated many OB interneurons, including TH+ and calbindin+ periglomerular interneurons. However, calretinin+ cells were not produced from these LGE grafts. Surprisingly, pallial and septal embryonic progenitors transplanted into the adult brain SVZ also resulted in the generation of OB interneurons, including calretinin+ cells. A subset of Dlx2+ OB interneurons was derived from cells expressing Emx1, a transcription factor largely restricted to the pallium during development. Emx1 lineage-derived cells contributed a substantial portion of GABAergic cells in the OB, including calretinin+ interneurons. This is in contrast to cortex, in which Emx1 lineage-derived cells do not differentiate into GABAergic neurons. Our results suggest that some OB interneurons are derived from progenitors outside the LGE and that precursors expressing what has classically been considered a pallial transcription factor generate GABAergic interneurons.

Distinct subtypes of cortical GABAergic interneurons provide inhibitory signals that are indispensable for neural network function. The Dlx homeobox genes have a central role in regulating their development and function. We have characterized the activity of three cis-regulatory sequences involved in forebrain expression of vertebrate Dlx genes: upstream regulatory element 2 (URE2), I12b, and I56i. The three regulatory elements display regional and temporal differences in their activities within the lateral ganglionic eminence (LGE), medial ganglionic eminence (MGE), and caudal ganglionic eminence (CGE) and label distinct populations of tangentially migrating neurons at embryonic day 12.5 (E12.5) and E13.5. We provide evidence that the dorsomedial and ventral MGE are distinct sources of tangentially migrating neurons during midgestation. In the adult cortex, URE2 and I12b/I56i are differentially expressed in parvalbumin-, calretinin-, neuropeptide Y-, and neuronal nitric oxide synthase-positive interneurons; I12b and I56i were specifically active in somatostatin-, vasoactive intestinal peptide-, and calbindin-positive interneurons. These data suggest that interneuron subtypes use distinct combinations of Dlx1/Dlx2 enhancers from the time they are specified through adulthood.

ARX loss-of-function mutations cause X-linked lissencephaly with ambiguous genitalia (XLAG), a severe neurological condition that results in profound brain malformations, including microcephaly, absence of corpus callosum, and impairment of the basal ganglia. Despite such dramatic defects, their nature and origin remain largely unknown. Here, we used Arx mutant mice as a model to characterize the cellular and molecular mechanisms underlying the basal ganglia alterations. In these animals, the early differentiation of this tissue appeared normal, whereas subsequent differentiation was impaired, leading to the periventricular accumulation of immature neurons in both the lateral ganglionic eminence and medial ganglionic eminence (MGE). Both tangential migration toward the cortex and striatum and radial migration to the globus pallidus and striatum were greatly reduced in the mutants, causing a periventricular accumulation of NPY+ or calretinin+ neurons in the MGE. Arx mutant neurons retained their differentiation potential in vitro but exhibited deficits in morphology and migration ability. These findings imply that cell-autonomous defects in migration underlie the neuronal localization defects. Furthermore, Arx mutants lacked a large fraction of cholinergic neurons and displayed a strong impairment of thalamocortical projections, in which major axon fiber tracts failed to traverse the basal ganglia. Altogether, these results highlight the critical functions of Arx in promoting neural migration and regulating basal ganglia differentiation in mice, consistent with the phenotype of XLAG patients.

Olfactory bulb interneuron development is a complex multistep process that involves cell specification in the ventral telencephalon, tangential migration into the olfactory bulb, and local neuronal maturation. Although several transcription factors have been implicated in this process, how or when they act remains to be elucidated. Here we explore the mechanisms that result in olfactory bulb interneuron defects in Dlx1&2-/- (distal-less homeobox 1 and 2) and Mash1-/- (mammalian achaete-schute homolog 1) mutants. We provide evidence that Dlx1&2 and Mash1 regulate parallel molecular pathways that are required for the generation of these cells, thereby providing new insights into the mechanisms underlying olfactory bulb development. The analysis also defined distinct anatomical zones related to olfactory bulb development. Finally we show that Dlx1&2 are required for promoting tangential migration to the olfactory bulb, potentially via regulating the expression of ErbB4 (v-erb-a erythroblastic leukemia viral oncogene homolog 4), Robo2 (roundabout homolog 2), Slit1 (slit homolog 1), and PK2 (prokineticin 2), which have all been shown to play essential roles in this migration.

By using a custom-made array containing cDNAs preferentially expressed in the mouse embryonic telencephalon (Porteus et al. [1992] Brain Res Mol Brain Res 12:7-22; and Alessandro Bulfone, unpublished data), we studied the gene expression profile of the Dlx1/Dlx2(-/-) subpallium and Pax6(-/-) pallium. We identified a transcript corresponding to Unigene Cluster Mm.94021 and rat Evf-1, which is down-regulated in the Dlx1/Dlx2(-/-) subpallium and up-regulated in the Pax6(-/-) pallium. Here, we report the expression pattern of this transcript, designated mouse Evf1 (mEvf1), in the prenatal forebrain of wild-type, Dlx1/Dlx2(-/-) and Pax6(-/-) mice using RNA in situ hybridization and reverse transcriptase-polymerase chain reaction. In the wild-type forebrain mEvf1 expression is restricted to the ventral thalamus, hypothalamus, and subpallial telencephalon (caudal, lateral, and medial ganglionic eminences and septal primordia), whereas it is down-regulated in the Dlx1/Dlx2(-/-) subpallium (mainly in caudal, lateral, and medial ganglionic eminences), and up-regulated in the Pax6(-/-) lateral and ventral pallium at embryonic day 12.5 and in the dorsal, lateral, and ventral pallium at embryonic day 14.5., ((c) 2004 Wiley-Liss, Inc.)

Induction, neurogenesis, and synaptogenesis of the olfactory bulb are thought to require interactions with the olfactory epithelium. The Dlx family of homeobox genes is expressed in both the olfactory bulb and olfactory epithelium. In particular, Dlx5 is expressed in the olfactory placode, olfactory epithelium, and local circuit neurons of the olfactory bulb. Here we analyzed mice lacking DLX5 function. The Dlx5-/- mutation reduces the size of the olfactory epithelium. Although some olfactory neurons are formed, they fail to generate olfactory axons that innervate the olfactory bulb. Despite the lack of innervation, the olfactory bulb forms, and neurogenesis of projection and local circuit neurons proceeds. However, the mutation has a cell-autonomous effect on the ability of neural progenitors to produce olfactory bulb local circuit neurons, with granule cells more severely affected than periglomerular cells. In addition, the mutation has a noncell-autonomous effect on the morphogenesis of mitral cells.