Identification of a direct Dlx homeodomain target in the developing mouse forebrain and retina by optimization of chromatin immunoprecipitation

DLX genes and proteins in mammalian forebrain development

The vertebrate Dlx gene family encode homeobox transcription factors that are related to the Drosophila Distal-less (Dll) gene and are crucial for development. Over the last ∼35 years detailed information has accrued about the redundant and unique expression and function of the six mammalian Dlx family genes. DLX proteins interact with general transcriptional regulators, and co-bind with other transcription factors to enhancer elements with highly specific activity in the developing forebrain. Integration of the genetic and biochemical data has yielded a foundation for a gene regulatory network governing the differentiation of forebrain GABAergic neurons. In this Primer, we describe the discovery of vertebrate Dlx genes and their crucial roles in embryonic development. We largely focus on the role of Dlx family genes in mammalian forebrain development revealed through studies in mice. Finally, we highlight questions that remain unanswered regarding vertebrate Dlx genes despite over 30 years of research.

DLX gene expression in the developing chick pharyngeal arches and relationship to endothelin signaling and avian jaw patterning

BACKGROUND

A hinged jaw that articulates with the skull base is a striking feature of the vertebrate head and has been greatly modified between, and within, vertebrate classes. Genes belonging to the DLX homeobox family are conserved mediators of local signaling pathways that distinguish the dorsal and ventral aspects of the first pharyngeal arch. Specifically, a subset of DLX genes are expressed in the cranial neural crest-derived mandibular ectomesenchyme in response to ventral endothelin signaling, an important step that confers the first arch with maxillary and mandibular identities. Downstream targets of DLX genes then execute the morphogenetic processes that lead to functional jaws. Identifying lineage-specific variations in DLX gene expression and the regulatory networks downstream of DLX action is necessary to understand how different kinds of jaws evolved.

RESULTS

Here, we describe and compare the expression of all six DLX genes in the chick pharyngeal arches, focusing on the period of active patterning in the first arch. Disruption of endothelin signaling results in the down-regulation of ventral-specific DLX genes and confirms their functional role in avian jaw patterning.

CONCLUSIONS

This expression resource will be important for comparative embryology and for identifying synexpression groups of DLX-regulated genes in the chick.

Genetic Regulation of Vertebrate Forebrain Development by Homeobox Genes

Forebrain development in vertebrates is regulated by transcription factors encoded by homeobox, bHLH and forkhead gene families throughout the progressive and overlapping stages of neural induction and patterning, regional specification and generation of neurons and glia from central nervous system (CNS) progenitor cells. Moreover, cell fate decisions, differentiation and migration of these committed CNS progenitors are controlled by the gene regulatory networks that are regulated by various homeodomain-containing transcription factors, including but not limited to those of the Pax (paired), Nkx, Otx (orthodenticle), Gsx/Gsh (genetic screened), and Dlx (distal-less) homeobox gene families. This comprehensive review outlines the integral role of key homeobox transcription factors and their target genes on forebrain development, focused primarily on the telencephalon. Furthermore, links of these transcription factors to human diseases, such as neurodevelopmental disorders and brain tumors are provided.

Increased Sociability in Mice Lacking Intergenic Dlx Enhancers

The Dlx homeodomain transcription factors play important roles in the differentiation and migration of GABAergic interneuron precursors. The mouse and human genomes each have six Dlx genes organized into three convergently transcribed bigene clusters (Dlx1/2, Dlx3/4, and Dlx5/6) with cis-regulatory elements (CREs) located in the intergenic region of each cluster. Amongst these, the I56i and I12b enhancers from the Dlx1/2 and Dlx5/6 locus, respectively, are active in the developing forebrain. I56i is also a binding site for GTF2I, a transcription factor whose function is associated with increased sociability and Williams-Beuren syndrome. In determining the regulatory roles of these CREs on forebrain development, we have generated mutant mouse-lines where Dlx forebrain intergenic enhancers have been deleted (I56i^(-/-), I12b^(-/-)). Loss of Dlx intergenic enhancers impairs expression of Dlx genes as well as some of their downstream targets or associated genes including Gad2 and Evf2. The loss of the I56i enhancer resulted in a transient decrease in GABA⁺ cells in the developing forebrain. The intergenic enhancer mutants also demonstrate increased sociability and learning deficits in a fear conditioning test. Characterizing mice with mutated Dlx intergenic enhancers will help us to further enhance our understanding of the role of these Dlx genes in forebrain development.

DLX Genes: Roles in Development and Cancer

Simple Summary

DLX homeobox family genes encode transcription factors that have indispensable roles in embryonic and postnatal development. These genes are critically linked to the morphogenesis of craniofacial structures, branchial arches, forebrain, and sensory organs. DLX genes are also involved in postnatal homeostasis, particularly hematopoiesis and, when dysregulated, oncogenesis. DLX1/2, DLX3/4, and DLX5/6 exist as bigenes on different chromosomes, sharing intergenic enhancers between gene pairs, which allows orchestrated spatiotemporal expression. Genomic alterations of human DLX gene enhancers or coding sequences result in congenital disorders such as split-hand/foot malformation. Aberrant postnatal expression of DLX genes is associated with hematological malignancies, including leukemias and lymphomas. In several mouse models of T-cell lymphoma, Dlx5 has been shown to act as an oncogene by cooperating with activated Akt, Notch1/3, and/or Wnt to drive tumor formation. In humans, DLX5 is aberrantly expressed in lung and ovarian carcinomas and holds promise as a therapeutic target.

Abstract

Homeobox genes control body patterning and cell-fate decisions during development. The homeobox genes consist of many families, only some of which have been investigated regarding a possible role in tumorigenesis. Dysregulation of HOX family genes have been widely implicated in cancer etiology. DLX homeobox genes, which belong to the NK-like family, exert dual roles in development and cancer. The DLX genes are the key transcription factors involved in regulating the development of craniofacial structures in vertebrates. The three DLX bigenes have overlapping expression in the branchial arches. Disruption of DLX function has destructive consequences in organogenesis and is associated with certain congenital disorders in humans. The role of DLX genes in oncogenesis is only beginning to emerge. DLX2 diminishes cellular senescence by regulating p53 function, whereas DLX4 has been associated with metastasis in breast cancer. In human ovarian cancer cells, DLX5 is essential for regulating AKT signaling, thereby promoting cell proliferation and survival. We previously implicated Dlx5 as an oncogene in murine T-cell lymphoma driven by a constitutively active form of Akt2. In this mouse model, overexpression of Dlx5 was caused by a chromosomal rearrangement that juxtaposed the Tcr-beta promoter region near the Dlx5 locus. Moreover, transgenic mice overexpressing Dlx5, specifically in immature T-cells, develop spontaneous thymic lymphomas. Oncogenesis in this mouse model involves binding of Dlx5 to the Notch1 and Notch3 gene loci to activate their transcription. Dlx5 also cooperates with Akt signaling to accelerate lymphomagenesis by activating Wnt signaling. We also discuss the fact that human DLX5 is aberrantly expressed in several human malignancies.

GABAergic neuron-specific whole-brain transduction by AAV-PHP.B incorporated with a new GAD65 promoter

GABAergic interneurons play a critical role in tuning neural networks in the central nervous system, and their defects are associated with neuropsychiatric disorders. Currently, the mDlx enhancer is solely used for adeno-associated virus (AAV) vector-mediated transgene delivery into cortical interneurons. Here, we developed a new inhibitory neuron-specific promoter (designated as the mGAD65 promoter), with a length of 2.5 kb, from a mouse genome upstream of exon 1 of the Gad2 gene encoding glutamic acid decarboxylase (GAD) 65. Intravenous infusion of blood-brain barrier-penetrating AAV-PHP.B expressing an enhanced green fluorescent protein under the control of the mGAD65 promoter transduced the whole brain in an inhibitory neuron-specific manner. The specificity and efficiency of the mGAD65 promoter for GABAergic interneurons, which was assessed at the motor cortex, were almost identical to or slightly higher than those of the mDlx enhancer. Immunohistochemical analysis revealed that the mGAD65 promoter preferentially transduced parvalbumin (PV)-expressing interneurons. Notably, the mGAD65 promoter transduced chandelier cells more efficiently than the mDlx enhancer and robustly labeled their synaptic boutons, called the cartridge, targeting the axon initial segments of excitatory pyramidal neurons. To test the ability of the mGAD65 promoter to express a functional molecule, we virally expressed G-CaMP, a fluorescent Ca2+ indicator, in the motor cortex, and this enabled us to monitor spontaneous and drug-induced Ca2+ activity in GABAergic inhibitory neurons. These results suggest that the mGAD65 promoter is useful for AAV-mediated targeting and manipulation of GABAergic neurons with the dominance of cortical PV-expressing neurons, including chandelier cells.

Direct reprogramming of oligodendrocyte precursor cells into GABAergic inhibitory neurons by a single homeodomain transcription factor Dlx2

Oligodendrocyte precursor cells (NG2 glia) are uniformly distributed proliferative cells in the mammalian central nervous system and generate myelinating oligodendrocytes throughout life. A subpopulation of OPCs in the neocortex arises from progenitor cells in the embryonic ganglionic eminences that also produce inhibitory neurons. The neuronal fate of some progenitor cells is sealed before birth as they become committed to the oligodendrocyte lineage, marked by sustained expression of the oligodendrocyte transcription factor Olig2, which represses the interneuron transcription factor Dlx2. Here we show that misexpression of Dlx2 alone in postnatal mouse OPCs caused them to switch their fate to GABAergic neurons within 2 days by downregulating Olig2 and upregulating a network of inhibitory neuron transcripts. After two weeks, some OPC-derived neurons generated trains of action potentials and formed clusters of GABAergic synaptic proteins. Our study revealed that the developmental molecular logic can be applied to promote neuronal reprogramming from OPCs.

Forging our understanding of lncRNAs in the brain

During both development and adulthood, the human brain expresses many thousands of long noncoding RNAs (lncRNAs), and aberrant lncRNA expression has been associated with a wide range of neurological diseases. Although the biological significance of most lncRNAs remains to be discovered, it is now clear that certain lncRNAs carry out important functions in neurodevelopment, neural cell function, and perhaps even diseases of the human brain. Given the relatively inclusive definition of lncRNAs-transcripts longer than 200 nucleotides with essentially no protein coding potential-this class of noncoding transcript is both large and very diverse. Furthermore, emerging data indicate that lncRNA genes can act via multiple, non-mutually exclusive molecular mechanisms, and specific functions are difficult to predict from lncRNA expression or sequence alone. Thus, the different experimental approaches used to explore the role of a lncRNA might each shed light upon distinct facets of its overall molecular mechanism, and combining multiple approaches may be necessary to fully illuminate the function of any particular lncRNA. To understand how lncRNAs affect brain development and neurological disease, in vivo studies of lncRNA function are required. Thus, in this review, we focus our discussion upon a small set of neural lncRNAs that have been experimentally manipulated in mice. Together, these examples illustrate how studies of individual lncRNAs using multiple experimental approaches can help reveal the richness and complexity of lncRNA function in both neurodevelopment and diseases of the brain.

GABAergic Interneuron Differentiation in the Basal Forebrain Is Mediated through Direct Regulation of Glutamic Acid Decarboxylase Isoforms by Dlx Homeobox Transcription Factors

GABA is the key inhibitory neurotransmitter in the cortex but regulation of its synthesis during forebrain development is poorly understood. In the telencephalon, members of the distal-less (Dlx) homeobox gene family are expressed in, and regulate the development of, the basal ganglia primodia from which many GABAergic neurons originate and migrate to other forebrain regions. The Dlx1/Dlx2 double knock-out mice die at birth with abnormal cortical development, including loss of tangential migration of GABAergic inhibitory interneurons to the neocortex (Anderson et al., 1997a). We have discovered that specific promoter regulatory elements of glutamic acid decarboxylase isoforms (Gad1 and Gad2), which regulate GABA synthesis from the excitatory neurotransmitter glutamate, are direct transcriptional targets of both DLX1 and DLX2 homeoproteins in vivo Further gain- and loss-of-function studies in vitro and in vivo demonstrated that both DLX1 and DLX2 are necessary and sufficient for Gad gene expression. DLX1 and/or DLX2 activated the transcription of both Gad genes, and defects in Dlx function disrupted the differentiation of GABAergic interneurons with global reduction in GABA levels in the forebrains of the Dlx1/Dlx2 double knock-out mouse in vivo Identification of Gad genes as direct Dlx transcriptional targets is significant; it extends our understanding of Dlx gene function in the developing forebrain beyond the regulation of tangential interneuron migration to the differentiation of GABAergic interneurons arising from the basal telencephalon, and may help to unravel the pathogenesis of several developmental brain disorders.SIGNIFICANCE STATEMENT GABA is the major inhibitory neurotransmitter in the brain. We show that Dlx1/Dlx2 homeobox genes regulate GABA synthesis during forebrain development through direct activation of glutamic acid decarboxylase enzyme isoforms that convert glutamate to GABA. This discovery helps explain how Dlx mutations result in abnormal forebrain development, due to defective differentiation, in addition to the loss of tangential migration of GABAergic inhibitory interneurons to the neocortex. Reduced numbers or function of cortical GABAergic neurons may lead to hyperactivity states such as seizures (Cobos et al., 2005) or contribute to the pathogenesis of some autism spectrum disorders. GABAergic dysfunction in the basal ganglia could disrupt the learning and development of complex motor and cognitive behaviors (Rubenstein and Merzenich, 2003).

Regulation of Brn3b by DLX1 and DLX2 is required for retinal ganglion cell differentiation in the vertebrate retina

Regulated retinal ganglion cell (RGC) differentiation and axonal guidance is required for a functional visual system. Homeodomain and basic helix-loop-helix transcription factors are required for retinogenesis, as well as patterning, differentiation and maintenance of specific retinal cell types. We hypothesized that Dlx1, Dlx2 and Brn3b homeobox genes function in parallel intrinsic pathways to determine RGC fate and therefore generated Dlx1/Dlx2/Brn3b triple-knockout mice. A more severe retinal phenotype was found in the Dlx1/Dlx2/Brn3b-null retinas than was predicted by combining features of the Brn3b single- and Dlx1/Dlx2 double-knockout retinas, including near total RGC loss with a marked increase in amacrine cells in the ganglion cell layer. Furthermore, we discovered that DLX1 and DLX2 function as direct transcriptional activators of Brn3b expression. Knockdown of Dlx2 expression in primary embryonic retinal cultures and Dlx2 gain of function in utero strongly support that DLX2 is both necessary and sufficient for Brn3b expression in vivo. We suggest that ATOH7 specifies RGC-committed progenitors and that Dlx1 and Dlx2 function both downstream of ATOH7 and in parallel, but cooperative, pathways that involve regulation of Brn3b expression to determine RGC fate.

Summary:Dlx1/2 homeobox genes regulate retinal ganglion cell (RGC) differentiation by directly activating Brn3b expression; accordingly, Dlx1/Dlx2/Brn3b triple-knockout mice exhibit near complete RGC loss.

DLX4 is associated with orofacial clefting and abnormal jaw development

Cleft lip and/or palate (CL/P) are common structural birth defects in humans. We used exome sequencing to study a patient with bilateral CL/P and identified a single nucleotide deletion in the patient and her similarly affected son—c.546_546delG, predicting p.Gln183Argfs*57 in the Distal-less 4 (DLX4) gene. The sequence variant was absent from databases, predicted to be deleterious and was verified by Sanger sequencing. In mammals, there are three Dlx homeobox clusters with closely located gene pairs (Dlx1/Dlx2, Dlx3/Dlx4, Dlx5/Dlx6). In situ hybridization showed that Dlx4 was expressed in the mesenchyme of the murine palatal shelves at E12.5, prior to palate closure. Wild-type human DLX4, but not mutant DLX4_c.546delG, could activate two murine Dlx conserved regulatory elements, implying that the mutation caused haploinsufficiency. We showed that reduced DLX4 expression after short interfering RNA treatment in a human cell line resulted in significant up-regulation of DLX3, DLX5 and DLX6, with reduced expression of DLX2 and significant up-regulation of BMP4, although the increased BMP4 expression was demonstrated only in HeLa cells. We used antisense morpholino oligonucleotides to target the orthologous Danio rerio gene, dlx4b, and found reduced cranial size and abnormal cartilaginous elements. We sequenced DLX4 in 155 patients with non-syndromic CL/P and CP, but observed no sequence variants. From the published literature, Dlx1/Dlx2 double homozygous null mice and Dlx5 homozygous null mice both have clefts of the secondary palate. This first finding of a DLX4 mutation in a family with CL/P establishes DLX4 as a potential cause of human clefts.

Genomic perspectives of transcriptional regulation in forebrain development

The forebrain is the seat of higher-order brain functions, and many human neuropsychiatric disorders are due to genetic defects affecting forebrain development, making it imperative to understand the underlying genetic circuitry. Recent progress now makes it possible to begin fully elucidating the genomic regulatory mechanisms that control forebrain gene expression. Herein, we discuss the current knowledge of how transcription factors drive gene expression programs through their interactions with cis-acting genomic elements, such as enhancers; how analyses of chromatin and DNA modifications provide insights into gene expression states; and how these approaches yield insights into the evolution of the human brain.

Early growth response 2 (Egr-2) expression is triggered by NF-κB activation

Transcription factors are known to play multiple roles in cellular function. Investigators report that factors such as early growth response (Egr) protein and nuclear factor kappa B (NF-κB) are activated in the brain during cancer, brain injury, inflammation, and/or memory. To explore NF-κB activity further, we investigated the transcriptomes of hippocampal slices following electrical stimulation of NF-κB p50 subunit knockout mice (p50-/-) versus their controls (p50+/+). We found that the early growth response gene Egr-2 was upregulated by NF-κB activation, but only in p50+/+ hippocampal slices. We then stimulated HeLa cells and primary cortical neurons with tumor necrosis factor alpha (TNFα) to activate NF-κB and increase the expression of Egr-2. The Egr-2 promoter sequence was analyzed for NF-κB binding sites and chromatin immunoprecipitation (ChIP) assays were performed to confirm promoter occupancy in vivo. We discovered that NF-κB specifically binds to an NF-κB consensus binding site within the proximal promoter region of Egr-2. Luciferase assay demonstrated that p50 was able to transactivate the Egr-2 promoter in vitro. Small interfering RNA (siRNA)-mediated p50 knockdown corroborated other Egr-2 expression studies. We show for the first time a novel link between NF-κB activation and Egr-2 expression with Egr-2 expression directly controlled by the transcriptional activity of NF-κB.

The role of homeobox genes in retinal development and disease

Homeobox genes are an evolutionarily conserved class of transcription factors that are critical for development of many organ systems, including the brain and eye. During retinogenesis, homeodomain-containing transcription factors, which are encoded by homeobox genes, play essential roles in the regionalization and patterning of the optic neuroepithelium, specification of retinal progenitors and differentiation of all seven of the retinal cell classes that derive from a common progenitor. Homeodomain transcription factors control retinal cell fate by regulating the expression of target genes required for retinal progenitor cell fate decisions and for terminal differentiation of specific retinal cell types. The essential role of homeobox genes during retinal development is demonstrated by the number of human eye diseases, including colobomas and anophthalmia, which are attributed to homeobox gene mutations. In the following review, we highlight the role of homeodomain transcription factors during retinogenesis and regulation of their gene targets. Understanding the complexities of vertebrate retina development will enhance our ability to drive differentiation of specific retinal cell types towards novel cell-based replacement therapies for retinal degenerative diseases.

Mechanisms regulating GABAergic neuron development

Neurons using gamma-aminobutyric acid (GABA) as their neurotransmitter are the main inhibitory neurons in the mature central nervous system (CNS) and show great variation in their form and function. GABAergic neurons are produced in all of the main domains of the CNS, where they develop from discrete regions of the neuroepithelium. Here, we review the gene expression and regulatory mechanisms controlling the main steps of GABAergic neuron development: early patterning of the proliferative neuroepithelium, production of postmitotic neural precursors, establishment of their identity and migration. By comparing the molecular regulation of these events across CNS, we broadly identify three regions utilizing distinct molecular toolkits for GABAergic fate determination: telencephalon-anterior diencephalon (DLX2 type), posterior diencephalon-midbrain (GATA2 type) and hindbrain-spinal cord (PTF1A and TAL1 types). Similarities and differences in the molecular regulatory mechanisms reveal the core determinants of a GABAergic neuron as well as provide insights into generation of the vast diversity of these neurons.

The ascl1a and dlx genes have a regulatory role in the development of GABAergic interneurons in the zebrafish diencephalon

During development of the mouse forebrain interneurons, the Dlx genes play a key role in a gene regulatory network (GRN) that leads to the GABAergic phenotype. Here, we have examined the regulatory relationships between the ascl1a, dlx, and gad1b genes in the zebrafish forebrain. Expression of ascl1a overlaps with dlx1a in the telencephalon and diencephalon during early forebrain development. The loss of Ascl1a function results in a loss of dlx expression, and subsequent losses of dlx5a and gad1b expression in the diencephalic prethalamus and hypothalamus. Loss of Dlx1a and Dlx2a function, and, to a lesser extent, of Dlx5a and Dlx6a, impairs gad1b expression in the prethalamus and hypothalamus. We conclude that dlx1a/2a act downstream of ascl1a but upstream of dlx5a/dlx6a and gad1b to activate GABAergic specification. This pathway is conserved in the diencephalon, but has diverged between mammals and teleosts in the telencephalon.

Roles of homeobox genes in retinal ganglion cell differentiation and axonal guidance

In this review, we have described four major families of homeobox genes which play important roles in RGC differentiation as well as axonal pathfinding. The mechanism underlying how these HD TFs affect axonal pathfinding is not entirely known. One possibility is that the downstream targets directly regulated by these HD TF are responsible for axonal guidance. Examples of this are the repression of EphB1 by Isl2, and Vax1/Vax2 regulation of EphB2/EphB3 expression. The roles of Dlx homeobox genes in RGC axonal guidance have not yet been reported. However, in the mouse telencephalon, Dlx1 and Dlx2 promote the tangential migration of GABAergic interneurons by repressing axonal growth (Cobos et al. 2007) and inhibiting Neuropilin-2 expression (Le et al. 2007). It is possible that the genetic program defining RGC identity also encodes a unique "sensory" network for their axons, determining how and where RGC axons respond to guidance cues en route to CNS targets.

Brn-3b inhibits generation of amacrine cells by binding to and negatively regulating DLX1/2 in developing retina

During retinogenesis, the basic helix-loop-helix proneural gene math5 (atoh7) initiates the generation of the first-born neurons, retinal ganglion cells (RGCs), by activating a network of RGC transcription factors, including Brn-3b (POU4F2). Herein, we show that the expression of DLX1 and DLX2 is significantly down-regulated in math5-null retina but is markedly increased in Brn-3b-null retina. Interestingly, Brn-3b interacts with DLX1 through its homeodomain, and this interaction represses DLX1 activity. Retrovirus-mediated mis-expression of DLX1 or DLX2 dramatically increases the number of amacrine/bipolar cells and concurrently reduces rod photoreceptors. Conversely, combined ectopic expression of Brn-3b with DLX1 or DLX2 promotes the production of RGCs and inhibits amacrine cell differentiation. Thus, DLX1/2 play an essential role in cell fate selection between amacrine and RGCs. Brn-3b suppresses the role of DLX1/2 through physical interaction and biases the competent precursors toward RGC fates.

Hierarchical interactions of homeodomain and forkhead transcription factors in regulating odontogenic gene expression

FoxJ1 is a forkhead transcription factor expressed in multiple tissues during development and a major regulator of cilia development. FoxJ1(-/-) mice present with defects in odontogenesis, and we correlate these defects to hierarchical interactions between homeodomain factors Pitx2 and Dlx2 with FoxJ1 in regulating their expression through direct physical interactions. Chromatin immunoprecipitation assays reveal endogenous Pitx2 and Dlx2 binding to the Dlx2 promoter and Dlx2 binding to the FoxJ1 promoter as well as Dlx2 and FoxJ1 binding to the amelogenin promoter. PITX2 activation of the Dlx2 promoter is attenuated by a direct Dlx2 physical interaction with PITX2. Dlx2 autoregulates its promoter, and Dlx2 transcriptionally activates the downstream gene FoxJ1. Dlx2 and FoxJ1 physically interact and synergistically regulate both Dlx2 and FoxJ1 promoters. Dlx2 and FoxJ1 also activate the amelogenin promoter, and amelogenin is required for enamel formation and late stage tooth development. FoxJ1(-/-) mice maxillary and mandibular incisors are reduced in length and width and have reduced amelogenin expression. FoxJ1(-/-) mice show a reduced and defective ameloblast layer, revealing a biological effect of these transcription factor hierarchies during tooth morphogenesis. These transcriptional mechanisms may contribute to other developmental processes such as neuronal, pituitary, and heart development.

Up-regulation of homeodomain genes, DLX1 and DLX2, by FLT3 signaling

BACKGROUND

Activating mutations in fms-like tyrosine kinase-3 (FLT3) are frequent in acute myeloid leukemia and represent both a poor prognostic feature and a therapeutic target. We have identified a previously unrecognized downstream effect of FLT3 activation, namely up-regulation of the homeodomain genes, DLX1 and DLX2.

DESIGN AND METHODS

MV4;11 cells with FLT3-internal tandem duplication mutation, RS4;11 cells with wild-type FLT3 and blasts from patients with acute myeloid leukemia were used to pursue the relation between FLT3, DLX1/2 and transforming growth factor-β (TGFβ). Real-time quantitative reverse transcriptase polymerase chain reaction, western blot and reverse-phase protein array were performed to detect changes in gene and protein expression. RNA interference and MTS assays were used to study the interaction of PKC412, FLT3 inhibitor and TGFβ1.

RESULTS

A direct relationship between FLT3 activity and DLX1/2 expression was revealed by both inhibition and up-regulation of FLT3 signaling in MV4;11 and RS4;11 cell lines, respectively, in isolated blast cells from patients with acute myeloid leukemia, and in reverse-phase protein array assays of samples from patients with acute myeloid leukemia. Mechanistically, the link between FLT3 and DLX1 expression appears to involve MAPK signaling through the ERK and JNK pathways. To determine whether elevated DLX1 had a functional consequence, we explored the reported inhibition by DLX1 on TGFβ/Smad signaling. Indeed, TGFβ responses were blunted by FLT3 activation in a DLX1-dependent manner and FLT3 inhibition resulted in a time-dependent increase in nuclear phospho-Smad2.

CONCLUSIONS

These findings suggest that alterations in DLX1/2 contribute to the biological consequences of FLT3 activation.

Identification of direct downstream targets of Dlx5 during early inner ear development

Dlx5, a homeobox transcription factor, plays a key role in the development of many organ systems. It is a candidate gene for human split-hand/split-foot type 1 malformation associated with sensorineural hearing loss. A deletion of one of its enhancers has been implicated in human craniofacial defects/hearing loss and it has also been associated with autism. However, little is known of how Dlx5 exerts its regulatory effects. We identified direct targets of Dlx5 in the mouse inner ear by gene expression profiling wild-type and Dlx5 null otic vesicles from embryonic stages E10 and E10.5. Four hundred genes were differentially expressed. We examined the genomic DNA sequences in the promoter regions of these genes for (i) previously described Dlx5 binding sites, (ii) novel 12 bp long motifs with a canonical homeodomain element shared by two or more genes and (iii) 100% conservation of these motifs in promoters of human orthologs. Forty genes passed these filters, 12 of which are expressed in the otic vesicle in domains that overlap with Dlx5. Chromatin immunoprecipitation using a Dlx5 antibody confirmed direct binding of Dlx5 to promoters of seven of these (Atbf1, Bmper, Large, Lrrtm1, Msx1, Ebf1 and Lhx1) in a cell line over-expressing Dlx5. Bmper and Lrrtm1 were up-regulated in this cell line, further supporting their identification as targets of Dlx5 in the inner ear and potentially in other organs. These direct targets support a model in which Bmp signaling is downstream of Dlx5 in the early inner ear and provide new insights into how the Dlx5 regulatory cascade is initiated.

Mutually exclusive expression of DLX2 and DLX5/6 is associated with the metastatic potential of the human breast cancer cell line MDA-MB-231

Background

The DLX gene family encodes for homeobox transcription factors involved in the control of morphogenesis and tissue homeostasis. Their expression can be regulated by Endothelin1 (ET1), a peptide associated with breast cancer invasive phenotype. Deregulation of DLX gene expression was found in human solid tumors and hematologic malignancies. In particular, DLX4 overexpression represents a possible prognostic marker in ovarian cancer. We have investigated the role of DLX genes in human breast cancer progression.

Methods

MDA-MB-231 human breast carcinoma cells were grown in vitro or injected in nude mice, either subcutaneously, to mimic primary tumor growth, or intravenously, to mimic metastatic spreading. Expression of DLX2, DLX5 and DLX6 was assessed in cultured cells, either treated or not with ET1, tumors and metastases by RT-PCR. In situ hybridization was used to confirm DLX gene expression in primary tumors and in lung and bone metastases. The expression of DLX2 and DLX5 was evaluated in 408 primary human breast cancers examining the GSE1456 and GSE3494 microarray datasets. Kaplan-Meier estimates for disease-free survival were calculated for the patients grouped on the basis of DLX2/DLX5 expression.

Results

Before injection, or after subcutaneous growth, MDA-MB-231 cells expressed DLX2 but neither DLX5 nor DLX6. Instead, in bone and lung metastases resulting from intravenous injection we detected expression of DLX5/6 but not of DLX2, suggesting that DLX5/6 are activated during metastasis formation, and that their expression is alternative to that of DLX2. The in vitro treatment of MDA-MB-231 cells with ET1, resulted in switch from DLX2 to DLX5 expression. By data mining in microarray datasets we found that expression of DLX2 occurred in 21.6% of patients, and was significantly correlated with prolonged disease-free survival and reduced incidence of relapse. Instead, DLX5 was expressed in a small subset of cases, 2.2% of total, displaying reduced disease-free survival and high incidence of relapse which was, however, non-significantly different from the other groups due to the small size of the DLX+ cohort. In all cases, we found mutually exclusive expression of DLX2 and DLX5.

Conclusions

Our studies indicate that DLX genes are involved in human breast cancer progression, and that DLX2 and DLX5 genes might serve as prognostic markers.

The relationship between dlx and gad1 expression indicates highly conserved genetic pathways in the zebrafish forebrain

The Dlx genes encode a family of transcription factors important for the development of the vertebrate forebrain. These genes have very similar expression domains during the development of the telencephalon in mice and play a role in gamma-aminobutyric acid (GABAergic) interneuron differentiation. We have used triple fluorescent in situ hybridization to study the relative expression domains of the dlx and gad1 genes in the zebrafish telencephalon and diencephalon. We also generated transgenic zebrafish with regulatory elements from the zebrafish dlx1a/2a locus. The zebrafish dlx regulatory elements recapitulated dlx expression in the forebrain and mimicked the relationship between the expression of the dlx genes and gad1. Finally, we show that a putative enhancer located downstream of dlx2b can also activate reporter gene expression in a tissue-specific manner similar to endogenous dlx2b expression. Our results indicate the dlx genes are regulated by an evolutionarily conserved genetic pathway and may play a role in GABAergic interneuron differentiation in the zebrafish forebrain.

An SNP in an ultraconserved regulatory element affects Dlx5/Dlx6 regulation in the forebrain

Dlx homeobox genes play a crucial role in the migration and differentiation of the subpallial precursor cells that give rise to various subtypes of gamma-aminobutyric acid (GABA)-expressing neurons of the forebrain, including local-circuit cortical interneurons. Aberrant development of GABAergic interneurons has been linked to several neurodevelopmental disorders, including epilepsy, schizophrenia, Rett syndrome and autism. Here, we report in mice that a single-nucleotide polymorphism (SNP) found in an autistic proband falls within a functional protein binding site in an ultraconserved cis-regulatory element. This element, I56i, is involved in regulating Dlx5/Dlx6 homeobox gene expression in the developing forebrain. We show that the SNP results in reduced I56i activity, predominantly in the medial and caudal ganglionic eminences and in streams of neurons tangentially migrating to the cortex. Reduced activity is also observed in GABAergic interneurons of the adult somatosensory cortex. The SNP affects the affinity of Dlx proteins for their binding site in vitro and reduces the transcriptional activation of the enhancer by Dlx proteins. Affinity purification using I56i sequences led to the identification of a novel regulator of Dlx gene expression, general transcription factor 2 I (Gtf2i), which is among the genes most often deleted in Williams-Beuren syndrome, a neurodevelopmental disorder. This study illustrates the clear functional consequences of a single nucleotide variation in an ultraconserved non-coding sequence in the context of developmental abnormalities associated with disease.

Role for TGF-beta superfamily signaling in telencephalic GABAergic neuron development

Signaling mechanisms mediated by the Transforming Growth Factor-β (TGF-β) superfamily regulate a variety of developmental processes. Here we show that components of both bone morphogenetic protein/growth differentiation factor and TGF-β/activin/Nodal branches of TGF-β superfamily signaling are expressed in the developing subpallium. Furthermore, Smad proteins, transcriptional effectors of TGF-β signaling, are co-expressed and physically interact in the basal ganglia with Dlx homeodomain transcription factors, which are critical regulators of the differentiation, migration and survival of telencephalic GABAergic neurons. We also show that Dlx and Smad proteins localize to promoters/enhancers of a number of common telencephalic genes in vivo and that Smad proteins co-activate transcription with Dlx family members, except with certain mutated human DLX proteins identified in autistic individuals. In agreement with these observations, expression of dominant-negative Smads in the developing basal ganglia phenocopies the cell migration defects observed in Dlx1/2-deficient mice. Together, these results suggest that TGF-β superfamily signaling plays a role in telencephalic GABAergic neuron development through functional interactions with Dlx transcription factors.

The organization of the transcriptional network in specific neuronal classes

Genome-wide expression profiling has aided the understanding of the molecular basis of neuronal diversity, but achieving broad functional insight remains a considerable challenge. Here, we perform the first systems-level analysis of microarray data from single neuronal populations using weighted gene co-expression network analysis to examine how neuronal transcriptome organization relates to neuronal function and diversity. We systematically validate network predictions using published proteomic and genomic data. Several network modules of co-expressed genes correspond to interneuron development programs, in which the hub genes are known to be critical for interneuron specification. Other co-expression modules relate to fundamental cellular functions, such as energy production, firing rate, trafficking, and synapses, suggesting that fundamental aspects of neuronal diversity are produced by quantitative variation in basic metabolic processes. We identify two transcriptionally distinct mitochondrial modules and demonstrate that one corresponds to mitochondria enriched in neuronal processes and synapses, whereas the other represents a population restricted to the soma. Finally, we show that galectin-1 is a new interneuron marker, and we validate network predictions in vivo using Rgs4 and Dlx1/2 knockout mice. These analyses provide a basis for understanding how specific aspects of neuronal phenotypic diversity are organized at the transcriptional level.

Balanced gene regulation by an embryonic brain ncRNA is critical for adult hippocampal GABA circuitry

Genomic studies demonstrate that while the majority of the mammalian genome is transcribed, only about 2% of these transcripts are protein coding. We have been investigating how the long, polyadenylated Evf2 non-coding RNA regulates transcription of homeodomain transcription factors DLX5 and DLX6 in the developing mouse forebrain. Here we show that in developing ventral forebrain, Evf2 recruits DLX and MECP2 transcription factors to key DNA regulatory elements in the Dlx 5/6 intergenic region and controls Dlx5, Dlx6, and GAD67 expression through trans and cis-acting mechanisms. Evf2 mouse mutants have reduced numbers of GABAergic interneurons in early post-natal hippocampus and dentate gyrus. Although the numbers of GABAergic interneurons and GAD67 RNA levels return to normal in Evf2 mutant adult hippocampus, reduced synaptic inhibition occurs. These results suggest that non-coding RNA-dependent balanced gene regulation in embryonic brain is critical for proper formation of GABA-dependent neuronal circuitry in adult brain.

Generation of Cre-transgenic mice using Dlx1/Dlx2 enhancers and their characterization in GABAergic interneurons

DLX1 and DLX2 transcription factors are necessary for forebrain GABAergic neuron differentiation, migration, and survival. We generated transgenic mice that express Cre-recombinase under the control of two ultra-conserved DNA elements near the Dlx1 and 2 locus termed I12b and URE2. We show that Cre-recombinase is active in a "Dlx-pattern" in the embryonic forebrain of transgenic mice. I12b-Cre is more active than URE2-Cre in the medial ganglionic eminences and its derivatives. Fate-mapping of EGFP+ cells in adult Cre;Z/EG animals demonstrated that GABAergic neurons, but not glia, are labeled. Most NPY+, nNOS+, parvalbumin+, and somatostatin+ cells are marked by I12b-Cre in the cortex and hippocampus, while 25-40% of these interneuron subtypes are labeled by URE2-Cre. Labeling of neurons generated between E12.5 to E15.5 indicated differences in birth-dates of EGFP+ cells that populate the olfactory bulb, hippocampus, and cortex. Finally, we provide the first in vivo evidence that both I12b and URE2 are direct targets of DLX2 and require Dlx1 and Dlx2 expression for proper activity.

Homeobox genes in vertebrate forebrain development and disease

Homeobox genes are an evolutionarily conserved class of transcription factors that are key regulators of developmental processes such as regional specification, patterning, migration and differentiation. In both mouse and humans, the developing forebrain is marked by distinct boundaries of homeobox gene expression at different developmental time points. These genes regulate the patterning of the forebrain along the dorsal/ventral and rostral/caudal axes and are also essential for the differentiation of specific neuronal subtypes. Inhibitory interneurons that arise from the ganglionic eminences and migrate tangentially to the neocortex and hippocampus are dramatically affected by mutations in several homeobox genes. In this review, we discuss the identification, expression patterns, loss- and/or gain-of-function models, and confirmed transcriptional targets for a set of homeobox genes required for the correct development of the forebrain in the mouse. In humans, mutations of homeobox genes expressed in the forebrain have been shown to result in mental retardation, epilepsy or movement disorders. The number of homeobox genes currently linked to human nervous system disease is surprisingly low, perhaps reflecting the essential functions of these genes throughout embryogenesis or the degree of functional redundancy during central nervous system development.

Dlx2 homeobox gene transcriptional regulation of Trkb neurotrophin receptor expression during mouse retinal development

Dlx homeobox genes are first expressed in embryonic retina at E11.5. The Dlx1/Dlx2 null retina has a reduced ganglion cell layer (GCL), with loss of late-born differentiated retinal ganglion cells (RGCs) due to increased apoptosis. TrkB signaling is proposed to regulate the dynamics of RGC apoptosis throughout development. DLX2 expression markedly precedes the onset of TrkB expression in the GCL; TrkB co-expression with Dlx2 and RGC markers is well-established by E13.5. In the Dlx1/Dlx2 null retina, TrkB expression is significantly reduced by E16.5. We demonstrated that DLX2 binds to a specific region of the TrkB promoter in retinal neuroepithelium during embryogenesis. In vitro confirmation and the functional consequences of DLX2 binding to this TrkB regulatory region support TrkB as a Dlx2 transcriptional target. Furthermore, ectopic Dlx2 expression in retinal explants activates TrkB expression and Dlx2 knockdown in primary retinal cultures results in reduced TrkB expression. RGC differentiation and survival require the coordinated expression of transcription factors. This study establishes a direct transcriptional relationship between a homeodomain protein involved in RGC differentiation and a neurotrophin receptor implicated in RGC survival. Signaling mediated by TrkB may contribute to survival of late-born RGCs whose terminal differentiation is regulated by Dlx gene function.

DLX5 and DLX6 expression is biallelic and not modulated by MeCP2 deficiency

Mutations in MECP2 and Mecp2 (encoding methyl-CpG binding protein 2 [MeCP2]) cause distinct neurological phenotypes in humans and mice, respectively, but the molecular pathology is unclear. Recent literature claimed that the developmental homeobox gene DLX5 is imprinted and that its imprinting status is modulated by MeCP2, leading to biallelic expression in Rett syndrome and twofold overexpression of Dlx5 and Dlx6 in Mecp2-null mice. The conclusion that DLX5 is a direct target of MeCP2 has implications for research on the molecular bases of Rett syndrome, autism, and genomic imprinting. Attempting to replicate the reported data, we evaluated allele-specific expression of DLX5 and DLX6 in mouse x human somatic cell hybrids, lymphoblastoid cell lines, and frontal cortex from controls and individuals with MECP2 mutations. We identified novel single-nucleotide polymorphisms in DLX5 and DLX6, enabling the first imprinting studies of DLX6. We found that DLX5 and DLX6 are biallelically expressed in somatic cell hybrids and in human cell lines and brain, with no differences between affected and control samples. We also determined expression levels of Dlx5 and Dlx6 in forebrain from seven male Mecp2-mutant mice and eight wild-type littermates by real-time quantitative reverse-transcriptase polymerase chain reaction assays. Expression of Dlx5 and Dlx6, as well as of the imprinted gene Peg3, in mouse forebrain was highly variable, with no consistent differences between Mecp2-null mutants and controls. We conclude that DLX5 and DLX6 are not imprinted in humans and are not likely to be direct targets of MeCP2 modulation. In contrast, the imprinting status of PEG3 and PEG10 is maintained in MeCP2-deficient tissues. Our results confirm that MeCP2 plays no role in the maintenance of genomic imprinting and add PEG3 and PEG10 to the list of studied imprinted genes.

Distinct cis-regulatory elements from the Dlx1/Dlx2 locus mark different progenitor cell populations in the ganglionic eminences and different subtypes of adult cortical 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.

The proneural determinant MASH1 regulates forebrain Dlx1/2expression through the I12b intergenic enhancer

Establishment of neuronal networks is an extremely complex process involving the interaction of a diversity of neuronal cells. During mammalian development, these highly organized networks are formed through the differentiation of multipotent neuronal progenitors into multiple neuronal cell lineages. In the developing forebrain of mammals, the combined function of the Dlx1, Dlx2, Dlx5 and Dlx6 homeobox genes is necessary for the differentiation of the GABAergic interneurons born in the ventricular and subventricular zones of the ventral telencephalon, as well as for the migration of these neurons to the hippocampus, cerebral cortex and olfactory bulbs. The 437 bp I12b enhancer sequence in the intergenic region of the Dlx1/2 bigene cluster is involved in the forebrain regulation of Dlx1/2. Using DNase I footprinting, we identified six regions of I12b potentially bound by transcription factors. Mutagenesis of each binding site affected the expression of reporter constructs in transgenic mice. However,the effects of impairing protein-DNA interactions were not uniform across the forebrain Dlx1/2 expression domains, suggesting that distinct regulatory interactions are taking place in the different populations of neuronal precursors. Analyses of protein-DNA interactions provide evidence of a direct role for MASH1 in Dlx1/2 regulation in the forebrain. DLX proteins play a crucial role in the maintenance of their own expression, as shown by transgenic and co-transfection experiments. These studies suggest that the seemingly continuous domains of Dlx gene expression in the telencephalon and diencephalon are in fact the combination of distinct cell populations within which different genetic regulatory interactions take place.

Dlx homeobox genes promote cortical interneuron migration from the basal forebrain by direct repression of the semaphorin receptor neuropilin-2

Dlx homeobox genes play an important role in vertebrate forebrain development. Dlx1/Dlx2 null mice die at birth with an abnormal cortical phenotype, including impaired differentiation and migration of GABAergic interneurons to the neocortex. However, the molecular basis for these defects downstream of loss of Dlx1/Dlx2 function is unknown. Neuropilin-2 (NRP-2) is a receptor for Class III semaphorins, which inhibit neuronal migration. Herein, we show that Neuropilin-2 is a specific DLX1 and DLX2 transcriptional target by applying chromatin immunoprecipitation to embryonic forebrain tissues. Both homeobox proteins repress Nrp-2 expression in vitro, confirming the functional significance of DLX binding. Furthermore, the homeodomain of DLX1 and DLX2 is necessary for DNA binding and this binding is essential for Dlx repression of Nrp-2 expression. Of importance, there is up-regulated and aberrant expression of NRP-2 in the forebrains of Dlx1/Dlx2 null mice. This is the first report that DLX1 or DLX2 can function as transcriptional repressors. Our data show that DLX proteins specifically mediate the repression of Neuropilin-2 in the developing forebrain. As well, our results support the hypothesis that down-regulation of Neuropilin-2 expression may facilitate tangential interneuron migration from the basal forebrain.

Functional interactions between Dlx2 and lymphoid enhancer factor regulate Msx2

Dlx2, Lymphoid Enhancer Factor (Lef-1) and Msx2 transcription factors are required for several developmental processes. To understand the control of gene expression by these factors, chromatin immunoprecipitation (ChIP) assays identified Msx2 as a downstream target of Dlx2 and Lef-1. Dlx2 activates the Msx2 promoter in several cell lines and binds DNA as a monomer and dimer. A Lef-1 beta-catenin-dependent isoform minimally activates the Msx2 promoter and a Lef-1 beta-catenin-independent isoform is inactive, however co-expression of Dlx2 and both Lef-1 isoforms synergistically activate the Msx2 promoter. Co-immunoprecipitation and protein pull-down experiments demonstrate Lef-1 physically interacts with Dlx2. Deletion analyses of the Lef-1 protein reveal specific regions required for synergism with Dlx2. The Lef-1 beta-catenin binding domain (betaDB) is not required for its interaction with Dlx2. Msx2 can auto-regulate its promoter and repress Dlx2 activation. Msx2 repression of Dlx2 activation is dose-specific and both bind a common DNA-binding element. These transcriptional mechanisms correlate with the temporal and spatial expression of these factors and may provide a mechanism for the control of several developmental processes. We demonstrate new transcriptional activities for Dlx2, Msx2 and Lef-1 through protein interactions and identification of downstream targets.

Luman/CREB3 induces transcription of the endoplasmic reticulum (ER) stress response protein Herp through an ER stress response element

Luman/CREB3 (also called LZIP) is an endoplasmic reticulum (ER) membrane-bound transcription factor which is believed to undergo regulated intramembrane proteolysis in response to cellular cues. We previously found that Luman activates transcription from the unfolded protein response element. Here we report the identification of Herp, a gene involved in ER stress-associated protein degradation (ERAD), as a direct target of Luman. We found that Luman was transcriptionally induced and proteolytically activated by the ER stress inducer thaspsigargin. Overexpression of Luman activated transcription of cellular Herp via ER stress response element II (ERSE-II; ATTGG-N-CCACG) in the promoter region. Mutagenesis studies and chromatin immunoprecipitation assays showed that Luman physically associates with the Herp promoter, specifically the second half-site (CCACG) of ERSE-II. Luman was also necessary for the full activation of Herp during the ER stress response, since Luman small interfering RNA knockdown or functional repression by a dominant negative mutant attenuated Herp gene expression. Like Herp, overexpression of Luman protected cells against ER stress-induced apoptosis. With Luman structurally similar to ATF6 but resembling XBP1 in DNA-binding specificities, we propose that Luman is a novel factor that plays a role in ERAD and a converging point for various signaling pathways channeling through the ER.

The Evf-2 noncoding RNA is transcribed from the Dlx-5/6 ultraconserved region and functions as a Dlx-2 transcriptional coactivator

The identification of ultraconserved noncoding sequences in vertebrates has been associated with developmental regulators and DNA-binding proteins. One of the first of these was identified in the intergenic region between the Dlx-5 and Dlx-6 genes, members of the Dlx/dll homeodomain-containing protein family. In previous experiments, we showed that Sonic hedgehog treatment of forebrain neural explants results in the activation of Dlx-2 and the novel noncoding RNA (ncRNA), Evf-1. In this report, we show that the Dlx-5/6 ultraconserved region is transcribed to generate an alternatively spliced form of Evf-1, the ncRNA Evf-2. Evf-2 specifically cooperates with Dlx-2 to increase the transcriptional activity of the Dlx-5/6 enhancer in a target and homeodomain-specific manner. A stable complex containing the Evf-2 ncRNA and the Dlx-2 protein forms in vivo, suggesting that the Evf-2 ncRNA activates transcriptional activity by directly influencing Dlx-2 activity. These experiments identify a novel mechanism whereby transcription is controlled by the cooperative actions of an ncRNA and a homeodomain protein. The possibility that a subset of vertebrate ultraconserved regions may function at both the DNA and RNA level to control key developmental regulators may explain why ultraconserved sequences exhibit 90% or more conservation even after 450 million years of vertebrate evolution.

Identification and characterization of a novel transcript down-regulated in Dlx1/Dlx2 and up-regulated in Pax6 mutant telencephalon

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.

Dlx1 and Dlx2 function is necessary for terminal differentiation and survival of late-born retinal ganglion cells in the developing mouse retina

Dlx homeobox genes, the vertebrate homologs of Distal-less, play important roles in the development of the vertebrate forebrain, craniofacial structures and limbs. Members of the Dlx gene family are also expressed in retinal ganglion cells (RGC), amacrine and horizontal cells of the developing and postnatal retina. Expression begins at embryonic day 12.5 and is maintained until late embryogenesis for Dlx1, while Dlx2 expression extends to adulthood. We have assessed the retinal phenotype of the Dlx1/Dlx2 double knockout mouse, which dies at birth. The Dlx1/2 null retina displays a reduced ganglion cell layer (GCL), with loss of differentiated RGCs due to increased apoptosis, and corresponding thinning of the optic nerve. Ectopic expression of Crx, the cone and rod photoreceptor homeobox gene, in the GCL and neuroblastic layers of the mutants may signify altered cell fate of uncommitted RGC progenitors. However, amacrine and horizontal cell differentiation is relatively unaffected in the Dlx1/2 null retina. Herein, we propose a model whereby early-born RGCs are Dlx1 and Dlx2 independent, but Dlx function is necessary for terminal differentiation of late-born RGC progenitors.