Gsh-1: a novel murine homeobox gene expressed in the central nervous system

Transcriptional control of visual neural circuit development by GS homeobox 1

As essential components of gene expression networks, transcription factors regulate neural circuit assembly. The homeobox transcription factor encoding gene, gs homeobox 1 (gsx1), is expressed in the developing visual system; however, no studies have examined its role in visual system formation. In zebrafish, retinal ganglion cell (RGC) axons that transmit visual information to the brain terminate in ten arborization fields (AFs) in the optic tectum (TeO), pretectum (Pr), and thalamus. Pretectal AFs (AF1-AF9) mediate distinct visual behaviors, yet we understand less about their development compared to AF10 in the TeO. Using gsx1 zebrafish mutants, immunohistochemistry, and transgenic lines, we observed that gsx1 is required for vesicular glutamate transporter, Tg(slc17a6b:DsRed), expression in the Pr, but not overall neuron number. gsx1 mutants have normal eye morphology, yet they exhibit impaired visual ability during prey capture. RGC axon volume in the gsx1 mutant Pr and TeO is reduced, and AF7 that is active during feeding is missing which is consistent with reduced hunting performance. Timed laser ablation of Tg(slc17a6b:DsRed)-positive cells reveals that they are necessary for AF7 formation. This work is the first to implicate gsx1 in establishing cell identity and functional neural circuits in the visual system.

Gsx2, but not Gsx1, is necessary for early forebrain patterning and long-term survival in zebrafish

BACKGROUND

Homeobox transcription factor encoding genes, genomic screen homeobox 1 and 2 (gsx1 and gsx2), are expressed during neurodevelopment in multiple vertebrates. However, we have limited knowledge of the dynamic expression of these genes through developmental time and the gene networks that they regulate in zebrafish.

RESULTS

We confirmed that gsx1 is expressed initially in the hindbrain and diencephalon and later in the optic tectum, pretectum, and cerebellar plate. gsx2 is expressed in the early telencephalon and later in the pallium and olfactory bulb. gsx1 and gsx2 are co-expressed in the hypothalamus, preoptic area, and hindbrain, however, rarely co-localize in the same cells. gsx1 and gsx2 mutant zebrafish were made with TALENs. gsx1 mutants exhibit stunted growth, however, they survive to adulthood and are fertile. gsx2 mutants experience swim bladder inflation failure that prevents survival. We also observed significantly reduced expression of multiple forebrain patterning distal-less homeobox genes in mutants, and expression of foxp2 was not significantly affected.

CONCLUSIONS

This work provides novel tools with which other target genes and functions of Gsx1 and Gsx2 can be characterized across the central nervous system to better understand the unique and overlapping roles of these highly conserved transcription factors.

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.

Conserved Gsx2/Ind homeodomain monomer versus homodimer DNA binding defines regulatory outcomes in flies and mice

How homeodomain proteins gain sufficient specificity to control different cell fates has been a long-standing problem in developmental biology. The conserved Gsx homeodomain proteins regulate specific aspects of neural development in animals from flies to mammals, and yet they belong to a large transcription factor family that bind nearly identical DNA sequences in vitro. Here, we show that the mouse and fly Gsx factors unexpectedly gain DNA binding specificity by forming cooperative homodimers on precisely spaced and oriented DNA sites. High-resolution genomic binding assays revealed that Gsx2 binds both monomer and homodimer sites in the developing mouse ventral telencephalon. Importantly, reporter assays showed that Gsx2 mediates opposing outcomes in a DNA binding site-dependent manner: Monomer Gsx2 binding represses transcription, whereas homodimer binding stimulates gene expression. In Drosophila, the Gsx homolog, Ind, similarly represses or stimulates transcription in a site-dependent manner via an autoregulatory enhancer containing a combination of monomer and homodimer sites. Integrating these findings, we test a model showing how the homodimer to monomer site ratio and the Gsx protein levels defines gene up-regulation versus down-regulation. Altogether, these data serve as a new paradigm for how cooperative homeodomain transcription factor binding can increase target specificity and alter regulatory outcomes.

Agenesis of the putamen and globus pallidus caused by recessive mutations in the homeobox gene GSX2

Basal ganglia are subcortical grey nuclei that play essential roles in controlling voluntary movements, cognition and emotion. While basal ganglia dysfunction is observed in many neurodegenerative or metabolic disorders, congenital malformations are rare. In particular, dysplastic basal ganglia are part of the malformative spectrum of tubulinopathies and X-linked lissencephaly with abnormal genitalia, but neurodevelopmental syndromes characterized by basal ganglia agenesis are not known to date. We ascertained two unrelated children (both female) presenting with spastic tetraparesis, severe generalized dystonia and intellectual impairment, sharing a unique brain malformation characterized by agenesis of putamina and globi pallidi, dysgenesis of the caudate nuclei, olfactory bulbs hypoplasia, and anomaly of the diencephalic-mesencephalic junction with abnormal corticospinal tract course. Whole-exome sequencing identified two novel homozygous variants, c.26C>A; p.(S9*) and c.752A>G; p.(Q251R) in the GSX2 gene, a member of the family of homeobox transcription factors, which are key regulators of embryonic development. GSX2 is highly expressed in neural progenitors of the lateral and median ganglionic eminences, two protrusions of the ventral telencephalon from which the basal ganglia and olfactory tubercles originate, where it promotes neurogenesis while negatively regulating oligodendrogenesis. The truncating variant resulted in complete loss of protein expression, while the missense variant affected a highly conserved residue of the homeobox domain, was consistently predicted as pathogenic by bioinformatic tools, resulted in reduced protein expression and caused impaired structural stability of the homeobox domain and weaker interaction with DNA according to molecular dynamic simulations. Moreover, the nuclear localization of the mutant protein in transfected cells was significantly reduced compared to the wild-type protein. Expression studies on both patients' fibroblasts demonstrated reduced expression of GSX2 itself, likely due to altered transcriptional self-regulation, as well as significant expression changes of related genes such as ASCL1 and PAX6. Whole transcriptome analysis revealed a global deregulation in genes implicated in apoptosis and immunity, two broad pathways known to be involved in brain development. This is the first report of the clinical phenotype and molecular basis associated to basal ganglia agenesis in humans.

Gsx transcription factors control neuronal versus glial specification in ventricular zone progenitors of the mouse lateral ganglionic eminence

The homeobox gene Gsx2 has previously been shown to inhibit oligodendroglial specification in dorsal lateral ganglionic eminence (dLGE) progenitors of the ventral telencephalon. The precocious specification of oligodendrocyte progenitor cells (OPCs) observed in Gsx2 mutants, however, is transient and begins to normalize by late stages of embryogenesis. Interestingly, this normalization correlates with the expansion of Gsx1, a close homolog of Gsx2, in a subset of progenitors in the Gsx2 mutant LGE. Here, we interrogated the mechanisms underlying oligodendroglial specification in Gsx2 mutants in relation to Gsx1. We found that Gsx1/2 double mutant embryos exhibit a more robust expansion of Olig2+ cells (i.e. OPCs) in the subventricular zone (SVZ) of the dLGE than Gsx2 mutants. Moreover, misexpression of Gsx1 throughout telencephalic VZ progenitors from E15 and onward resulted in a significant reduction of cortical OPCs. These results demonstrate redundant roles of Gsx1 and Gsx2 in suppressing early OPC specification in LGE VZ progenitors. However, Gsx1/2 mutants did not show a significant increase in adjacent cortical OPCs at later stages compared to Gsx2 mutants. This is likely due to reduced proliferation of OPCs within the SVZ of the Gsx1/2 double mutant LGE, suggesting a novel role for Gsx1 in expansion of migrating OPCs in the ventral telencephalon. We further investigated the glial specification mechanisms downstream of Gsx2 by generating Olig2/Gsx2 double mutants. Consistent with the known essential role for Olig2 in OPC specification, ectopic production of cortical OPCs observed in Gsx2 mutants disappeared in Olig2/Gsx2 double mutants. These mutants, however, maintained the expanded expression of gliogenic markers Zbtb20 and Bcan in the VZ of the LGE similarly to Gsx2 single mutants, suggesting that Gsx2 suppresses gliogenesis via Olig2-dependent and -independent mechanisms.

Development of neuroendocrine neurons in the mammalian hypothalamus

The neuroendocrine system consists of a heterogeneous collection of (mostly) neuropeptidergic neurons found in four hypothalamic nuclei and sharing the ability to secrete neurohormones (all of them neuropeptides except dopamine) into the bloodstream. There are, however, abundant hypothalamic non-neuroendocrine neuropeptidergic neurons developing in parallel with the neuroendocrine system, so that both cannot be entirely disentangled. This heterogeneity results from the workings of a network of transcription factors many of which are already known. Olig2 and Fezf2 expressed in the progenitors, acting through mantle-expressed Otp and Sim1, Sim2 and Pou3f2 (Brn2), regulate production of magnocellular and anterior parvocellular neurons. Nkx2-1, Rax, Ascl1, Neurog3 and Dbx1 expressed in the progenitors, acting through mantle-expressed Isl1, Dlx1, Gsx1, Bsx, Hmx2/3, Ikzf1, Nr5a2 (LH-1) and Nr5a1 (SF-1) are responsible for tuberal parvocellular (arcuate nucleus) and other neuropeptidergic neurons. The existence of multiple progenitor domains whose progeny undergoes intricate tangential migrations as one source of complexity in the neuropeptidergic hypothalamus is the focus of much attention. How neurosecretory cells target axons to the medial eminence and posterior hypophysis is gradually becoming clear and exciting progress has been made on the mechanisms underlying neurovascular interface formation. While rat neuroanatomy and targeted mutations in mice have yielded fundamental knowledge about the neuroendocrine system in mammals, experiments on chick and zebrafish are providing key information about cellular and molecular mechanisms. Looking forward, data from every source will be necessary to unravel the ways in which the environment affects neuroendocrine development with consequences for adult health and disease.

Septal contributions to olfactory bulb interneuron diversity in the embryonic mouse telencephalon: role of the homeobox gene Gsx2

Background

Olfactory bulb (OB) interneurons are known to represent diverse neuronal subtypes, which are thought to originate from a number of telencephalic regions including the embryonic dorsal lateral ganglionic eminence (dLGE) and septum. These cells migrate rostrally toward the OB, where they then radially migrate to populate different OB layers including the granule cell layer (GCL) and the outer glomerular layer (GL). Although previous studies have attempted to investigate regional contributions to OB interneuron diversity, few genetic tools have been used to address this question at embryonic time points when the earliest populations are specified.

Methods

In this study, we utilized Zic3-lacZ and Gsx2e-CIE transgenic mice as genetic fate-mapping tools to study OB interneuron contributions derived from septum and LGE, respectively. Moreover, to address the regional (i.e. septal) requirements of the homeobox gene Gsx2 for OB interneuron diversity, we conditionally inactivated Gsx2 in the septum, leaving it largely intact in the dLGE, by recombining the Gsx2 floxed allele using Olig2^Cre/+ mice.

Results

Our fate mapping studies demonstrated that the dLGE and septum gave rise to OB interneuron subtypes differently. Notably, the embryonic septum was found to give rise largely to the calretinin⁺ (CR⁺) GL subtype, while the dLGE was more diverse, generating all major GL subpopulations as well as many GCL interneurons. Moreover, Gsx2 conditional mutants (cKOs), with septum but not dLGE recombination, showed impaired generation of CR⁺ interneurons within the OB GL. These Gsx2 cKOs exhibited reduced proliferation within the septal subventricular zone (SVZ), which correlated well with the reduced number of CR⁺ interneurons observed.

Conclusions

Our findings indicate that the septum and LGE contribute differently to OB interneuron diversity. While the dLGE provides a wide range of OB interneuron subtypes, the septum is more restricted in its contribution to the CR⁺ subtype. Gsx2 is required in septal progenitors for the correct expansion of SVZ progenitors specified toward the CR⁺ subtype. Finally, the septum has been suggested to be the exclusive source of CR⁺ interneurons in postnatal studies. Our results here demonstrate that dLGE progenitors in the embryo also contribute to this OB neuronal subtype.

Electronic supplementary material

The online version of this article (doi:10.1186/s13064-017-0090-5) contains supplementary material, which is available to authorized users.

Lampreys, the jawless vertebrates, contain only two ParaHox gene clusters

ParaHox genes (Gsx, Pdx, and Cdx) are an ancient family of developmental genes closely related to the Hox genes. They play critical roles in the patterning of brain and gut. The basal chordate, amphioxus, contains a single ParaHox cluster comprising one member of each family, whereas nonteleost jawed vertebrates contain four ParaHox genomic loci with six or seven ParaHox genes. Teleosts, which have experienced an additional whole-genome duplication, contain six ParaHox genomic loci with six ParaHox genes. Jawless vertebrates, represented by lampreys and hagfish, are the most ancient group of vertebrates and are crucial for understanding the origin and evolution of vertebrate gene families. We have previously shown that lampreys contain six Hox gene loci. Here we report that lampreys contain only two ParaHox gene clusters (designated as α- and β-clusters) bearing five ParaHox genes (Gsxα, Pdxα, Cdxα, Gsxβ, and Cdxβ). The order and orientation of the three genes in the α-cluster are identical to that of the single cluster in amphioxus. However, the orientation of Gsxβ in the β-cluster is inverted. Interestingly, Gsxβ is expressed in the eye, unlike its homologs in jawed vertebrates, which are expressed mainly in the brain. The lamprey Pdxα is expressed in the pancreas similar to jawed vertebrate Pdx genes, indicating that the pancreatic expression of Pdx was acquired before the divergence of jawless and jawed vertebrate lineages. It is likely that the lamprey Pdxα plays a crucial role in pancreas specification and insulin production similar to the Pdx of jawed vertebrates.

Transcriptional Regulation of Notch1 Expression by Nkx6.1 in Neural Stem/Progenitor Cells during Ventral Spinal Cord Development

Notch1 signaling plays a critical role in maintaining and determining neural stem/progenitor cell (NSPC) fate, yet the transcriptional mechanism controlling Notch1 specific expression in NSPCs remains incomplete. Here, we show transcription factor Nkx6.1 interacts with a cis-element (CR2, an evolutionarily conserved non-coding fragment in the second intron of Notch1 locus) and regulates the expression of Notch1 in ventral NSPCs of the developing spinal cord. We show that the Notch1 expression is modulated by the interaction of Nkx6.1 with a 139 bp enhancer sequence within CR2. Knockdown or overexpression of Nkx6.1 leads to down- or up-regulated Notch1 expression, respectively. In CR2-GFP transgenic mouse, GFP expression was found prominent in the ventricular zone and neural progenitor cells from embryonic day 9.5 to postnatal day 7. GFP+ cells were mainly neural progenitors for interneurons and not for motoneurons or glial cells. Moreover, GFP expression persisted in a subset of ependymal cells in the adult spinal cord, suggesting that CR2 is active in both embryonic and adult NSPCs. Together our data reveal a novel mechanism of Notch1 transcriptional regulation in the ventral spinal cord by Nkx6.1 via its binding with Notch1 enhancer CR2 during embryonic development.

Maintenance of neural stem cell regional identity in culture

Neural stem cells (NSCs) are distributed throughout the ventricular-subventricular zone (V-SVZ) in the adult mouse brain. NSCs located in spatially distinct regions of the V-SVZ generate different types of olfactory bulb (OB) neurons, and the regional expression of specific transcription factors correlates with these differences in NSC developmental potential. In a recent article, we show that Nkx2.1-expressing embryonic precursors give rise to NKX2.1+ NSCs located in the ventral V-SVZ of adult mice. Here we characterize a V-SVZ monolayer culture system that retains regional gene expression and neurogenic potential of NSCs from the dorsal and ventral V-SVZ. In particular, we find that Nkx2.1-lineage V-SVZ NSCs maintain Nkx2.1 expression through serial passage and can generate new neurons in vitro. Thus, V-SVZ NSCs retain key aspects of their in vivo regional identity in culture, providing new experimental opportunities for understanding how such developmental patterns are established and maintained during development.

TCF/Lef regulates the Gsx ParaHox gene in central nervous system development in chordates

BACKGROUND

The ParaHox genes play an integral role in the anterior-posterior (A-P) patterning of the nervous system and gut of most animals. The ParaHox cluster is an ideal system in which to study the evolution and regulation of developmental genes and gene clusters, as it displays similar regulatory phenomena to its sister cluster, the Hox cluster, but offers a much simpler system with only three genes.

RESULTS

Using Ciona intestinalis transgenics, we isolated a regulatory element upstream of Branchiostoma floridae Gsx that drives expression within the central nervous system of Ciona embryos. The minimal amphioxus enhancer region required to drive CNS expression has been identified, along with surrounding sequence that increases the efficiency of reporter expression throughout the Ciona CNS. TCF/Lef binding sites were identified and mutagenized and found to be required to drive the CNS expression. Also, individual contributions of TCF/Lef sites varied across the regulatory region, revealing a partial division of function across the Bf-Gsx-Up regulatory element. Finally, when all TCF/Lef binding sites are mutated CNS expression is not only abolished, but a latent repressive function is also unmasked.

CONCLUSIONS

We have identified a B. floridae Gsx upstream regulatory element that drives CNS expression within transgenic Ciona intestinalis, and have shown that this CNS expression is dependent upon TCF/Lef binding sites. We examine the evolutionary and developmental implications of these results, and discuss the possibility of TCF/Lef not only as a regulator of chordate Gsx, but as a deeply conserved regulatory factor controlling all three ParaHox genes across the Metazoa.

The ParaHox gene Gsx patterns the apical organ and central nervous system but not the foregut in scaphopod and cephalopod mollusks

Background

It has been hypothesized that the ParaHox gene Gsx patterned the foregut of the last common bilaterian ancestor. This notion was corroborated by Gsx expression in three out of four lophotrochozoan species, several ecdysozoans, and some deuterostomes. Remarkably, Gsx is also expressed in the bilaterian anterior-most central nervous system (CNS) and the gastropod and annelid apical organ. To infer whether these findings are consistent with other mollusks or even lophotrochozoans, we investigated Gsx expression in developmental stages of representatives of two other molluscan classes, the scaphopod Antalis entalis and the cephalopod Idiosepius notoides.

Results

Gsx is not expressed in the developing digestive tract of Antalis entalis and Idiosepius notoides. Instead, it is expressed in cells of the apical organ in the scaphopod trochophore and in two cells adjacent to this organ. Late-stage trochophores express Aen-Gsx in cells of the developing cerebral and pedal ganglia and in cells close to the pavilion, mantle, and foot. In postmetamorphic specimens, Aen-Gsx is expressed in the cerebral and pedal ganglia, the foot, and the nascent captacula. In early squid embryos, Ino-Gsx is expressed in the cerebral, palliovisceral, and optic ganglia. In late-stage embryos, Ino-Gsx is additionally expressed close to the eyes and in the supraesophageal and posterior subesophageal masses and optic lobes. Developmental stages close to hatching express Ino-Gsx only close to the eyes.

Conclusions

Our results suggest that Gsx expression in the foregut might not be a plesiomorphic trait of the Lophotrochozoa as insinuated previously. Since neither ecdysozoans nor deuterostomes express Gsx in their gut, a role in gut formation in the last common bilaterian ancestor appears unlikely. Gsx is consistently expressed in the bilaterian anterior-most CNS and the apical organ of lophotrochozoan larvae, suggesting a recruitment of Gsx into the formation of this organ in the Lophotrochozoa. The cephalopod posterior subesophageal mass and optic ganglia and the scaphopod pedal ganglia also express Gsx. In summary, Gsx expression only appears to be conserved in the anterior-most brain region during evolution. Accordingly, Gsx appears to have been recruited into the formation of other expression domains, e.g., the apical organ or the foregut, in some lophotrochozoans.

Temporal identity transition from Purkinje cell progenitors to GABAergic interneuron progenitors in the cerebellum

In the cerebellum, all GABAergic neurons are generated from the Ptf1a-expressing ventricular zone (Ptf1a domain). However, the machinery to produce different types of GABAergic neurons remains elusive. Here we show temporal regulation of distinct GABAergic neuron progenitors in the cerebellum. Within the Ptf1a domain at early stages, we find two subpopulations; dorsally and ventrally located progenitors that express Olig2 and Gsx1, respectively. Lineage tracing reveals the former are exclusively Purkinje cell progenitors (PCPs) and the latter Pax2-positive interneuron progenitors (PIPs). As development proceeds, PCPs gradually become PIPs starting from ventral to dorsal. In gain- and loss-of-function mutants for Gsx1 and Olig1/2, we observe abnormal transitioning from PCPs to PIPs at inappropriate developmental stages. Our findings suggest that the temporal identity transition of cerebellar GABAergic neuron progenitors from PCPs to PIPs is negatively regulated by Olig2 and positively by Gsx1, and contributes to understanding temporal control of neuronal progenitor identities.

Gsx1 expression defines neurons required for prepulse inhibition

In schizophrenia, cognitive overload is thought to reflect an inability to suppress non-salient information, a process which is studied using prepulse inhibition (PPI) of the startle response. PPI is reduced in schizophrenia and routinely tested in animal models and preclinical trials of antipsychotic drugs. However, the underlying neuronal circuitry is not well understood. We used a novel genetic screen in larval zebrafish to reveal the molecular identity of neurons that are required for PPI in fish and mice. Ablation or optogenetic silencing of neurons with developmental expression of the transcription factor genomic screen homeobox 1 (gsx1) produced profound defects in PPI in zebrafish, and PPI was similarly impaired in Gsx1 knockout mice. Gsx1-expressing neurons reside in the dorsal brainstem and form synapses closely apposed to neurons that initiate the startle response. Surprisingly, brainstem Gsx1 neurons are primarily glutamatergic despite their role in a functionally inhibitory pathway. As Gsx1 has an important role in regulating interneuron development in the forebrain, these findings reveal a molecular link between control of interneuron specification and circuits that gate sensory information across brain regions.

β-Hydroxybutyric acid inhibits growth hormone-releasing hormone synthesis and secretion through the GPR109A/extracellular signal-regulated 1/2 signalling pathway in the hypothalamus

β-Hydroxybutyric acid (BHBA) has recently been shown to regulate hormone synthesis and secretion in the hypothalamus. However, little is known about the effects of BHBA-mediated hormone regulation or the detailed mechanisms by which BHBA regulates growth hormone-releasing hormone (GHRH) synthesis and secretion. In the present study, we examined the expression of the BHBA receptor GPR109A in primary hypothalamic cell cultures. We hypothesised that BHBA regulates GHRH via GPR109A and its downstream signals. Initial in vivo studies conducted in rats demonstrated that GHRH mRNA expression in the hypothalamus was strongly inversely correlated with BHBA levels in the cerebrospinal fluid during postnatal development (r = -0.89, P < 0.01). Furthermore, i.c.v. administration of BHBA acutely decreased GHRH mRNA expression in rats. Further in vitro studies revealed a decrease in GHRH synthesis and secretion in primary hypothalamic cells after treatment with BHBA; this effect was inhibited when hypothalamic cells were pretreated with pertussis toxin (PTX). BHBA had no effect on GHRH synthesis and secretion in GT1-7 cells, which do not exhibit cell surface expression of GPR109A. Furthermore, BHBA acutely decreased the transcription of the homeobox gene for Gsh-1 in the hypothalamus in both in vivo and in vitro, and this effect was also inhibited by PTX in vitro. In primary hypothalamic cells, BHBA activated the extracellular signal-regulated kinase (ERK)1/2, p38 and c-Jun N-terminal kinase mitogen-activated protein kinase (MAPK) kinases, as shown by western blot analysis. Moreover, inhibition of ERK1/2 with U0126 attenuated the BHBA-mediated reduction in Gsh-1 expression and GHRH synthesis and secretion. These results strongly suggest that BHBA directly regulates GHRH synthesis and secretion via the GPR109A/ERK1/2 MAPK pathway, and also that Gsh-1 is essential for this function.

Genomic organisation of the seven ParaHox genes of coelacanths

Human and mouse genomes contain six ParaHox genes implicated in gut and neural patterning. In coelacanths and cartilaginous fish, an additional ParaHox gene exists—Pdx2—that dates back to the genome duplications in early vertebrate evolution. Here we examine the genomic arrangement and flanking genes of all ParaHox genes in coelacanths, to determine the full complement of these genes. We find that coelacanths have seven ParaHox genes in total, in four chromosomal locations, revealing that five gene losses occurred soon after vertebrate genome duplication. Comparison of intergenic sequences reveals that some Pdx1 regulatory regions associated with development of pancreatic islets are older than tetrapods, that Pdx1 and Pdx2 share few if any conserved non-coding elements, and that there is very high sequence conservation between coelacanth species.

Intact cluster and chordate-like expression of ParaHox genes in a sea star

Background

The ParaHox genes are thought to be major players in patterning the gut of several bilaterian taxa. Though this is a fundamental role that these transcription factors play, their activities are not limited to the endoderm and extend to both ectodermal and mesodermal tissues. Three genes compose the ParaHox group: Gsx, Xlox and Cdx. In some taxa (mostly chordates but to some degree also in protostomes) the three genes are arranged into a genomic cluster, in a similar fashion to what has been shown for the better-known Hox genes. Sea urchins possess the full complement of ParaHox genes but they are all dispersed throughout the genome, an arrangement that, perhaps, represented the primitive condition for all echinoderms. In order to understand the evolutionary history of this group of genes we cloned and characterized all ParaHox genes, studied their expression patterns and identified their genomic loci in a member of an earlier branching group of echinoderms, the asteroid Patiria miniata.

Results

We identified the three ParaHox orthologs in the genome of P. miniata. While one of them, PmGsx is provided as maternal message, with no zygotic activation afterwards, the other two, PmLox and PmCdx are expressed during embryogenesis, within restricted domains of both endoderm and ectoderm. Screening of a Patiria bacterial artificial chromosome (BAC) library led to the identification of a clone containing the three genes. The transcriptional directions of PmGsx and PmLox are opposed to that of the PmCdx gene within the cluster.

Conclusions

The identification of P. miniata ParaHox genes has revealed the fact that these genes are clustered in the genome, in contrast to what has been reported for echinoids. Since the presence of an intact cluster, or at least a partial cluster, has been reported in chordates and polychaetes respectively, it becomes clear that within echinoderms, sea urchins have modified the original bilaterian arrangement. Moreover, the sea star ParaHox domains of expression show chordate-like features not found in the sea urchin, confirming that the dynamics of gene expression for the respective genes and their putative regulatory interactions have clearly changed over evolutionary time within the echinoid lineage.

Identification of an intact ParaHox cluster with temporal colinearity but altered spatial colinearity in the hemichordate Ptychodera flava

BACKGROUND

ParaHox and Hox genes are thought to have evolved from a common ancestral ProtoHox cluster or from tandem duplication prior to the divergence of cnidarians and bilaterians. Similar to Hox clusters, chordate ParaHox genes including Gsx, Xlox, and Cdx, are clustered and their expression exhibits temporal and spatial colinearity. In non-chordate animals, however, studies on the genomic organization of ParaHox genes are limited to only a few animal taxa. Hemichordates, such as the Enteropneust acorn worms, have been used to gain insights into the origins of chordate characters. In this study, we investigated the genomic organization and expression of ParaHox genes in the indirect developing hemichordate acorn worm Ptychodera flava.

RESULTS

We found that P. flava contains an intact ParaHox cluster with a similar arrangement to that of chordates. The temporal expression order of the P. flava ParaHox genes is the same as that of the chordate ParaHox genes. During embryogenesis, the spatial expression pattern of PfCdx in the posterior endoderm represents a conserved feature similar to the expression of its orthologs in other animals. On the other hand, PfXlox and PfGsx show a novel expression pattern in the blastopore. Nevertheless, during metamorphosis, PfXlox and PfCdx are expressed in the endoderm in a spatially staggered pattern similar to the situation in chordates.

CONCLUSIONS

Our study shows that P. flava ParaHox genes, despite forming an intact cluster, exhibit temporal colinearity but lose spatial colinearity during embryogenesis. During metamorphosis, partial spatial colinearity is retained in the transforming larva. These results strongly suggest that intact ParaHox gene clustering was retained in the deuterostome ancestor and is correlated with temporal colinearity.

SoxB1-driven transcriptional network underlies neural-specific interpretation of morphogen signals

The reiterative deployment of a small cadre of morphogen signals underlies patterning and growth of most tissues during embyogenesis, but how such inductive events result in tissue-specific responses remains poorly understood. By characterizing cis-regulatory modules (CRMs) associated with genes regulated by Sonic hedgehog (Shh), retinoids, or bone morphogenetic proteins in the CNS, we provide evidence that the neural-specific interpretation of morphogen signaling reflects a direct integration of these pathways with SoxB1 proteins at the CRM level. Moreover, expression of SoxB1 proteins in the limb bud confers on mesodermal cells the potential to activate neural-specific target genes upon Shh, retinoid, or bone morphogenetic protein signaling, and the collocation of binding sites for SoxB1 and morphogen-mediatory transcription factors in CRMs faithfully predicts neural-specific gene activity. Thus, an unexpectedly simple transcriptional paradigm appears to conceptually explain the neural-specific interpretation of pleiotropic signaling during vertebrate development. Importantly, genes induced in a SoxB1-dependent manner appear to constitute repressive gene regulatory networks that are directly interlinked at the CRM level to constrain the regional expression of patterning genes. Accordingly, not only does the topology of SoxB1-driven gene regulatory networks provide a tissue-specific mode of gene activation, but it also determines the spatial expression pattern of target genes within the developing neural tube.

A cis-element in the Notch1 locus is involved in the regulation of gene expression in interneuron progenitors

Interneurons comprise approximately one third of the total cortical neurons in the mammalian cerebral cortex. Studies have revealed many details in the generation of this cell type. However, the mechanism that defines interneuron-lineage specific gene expression is not well understood. Gene regulatory elements, e.g., promoters, enhancers, and trans-acting factors, are essential for the proper control of gene expression. Here, we report that a novel evolutionarily conserved cis-element in the second intron of the Notch1 locus plays an important role in regulating gene expression in interneuron progenitors. The spatiotemporal activity of the cis-element in the developing central nervous system (CNS) was determined by both transient reporter expression in the developing chick and a transgenic mouse model. Its activity is well correlated with neurogenesis in both the chick and mouse and restricted to neural progenitor cells in the ganglionic eminence that are fated to differentiate into GABAergic interneurons of the neocortex. We further demonstrate that the cis-element activity requires the binding motif for trans-acting factors Gsh1/Barx2/Brn3. Deletion of this binding motif abolishes reporter gene expression. Together, these data provide new insights into the regulatory mechanisms of interneuron development in the vertebrate CNS.

Molecular regulation of striatal development: a review

The central nervous system is composed of the brain and the spinal cord. The brain is a complex organ that processes and coordinates activities of the body in bilaterian, higher-order animals. The development of the brain mirrors its complex function as it requires intricate genetic signalling at specific times, and deviations from this can lead to brain malformations such as anencephaly. Research into how the CNS is specified and patterned has been studied extensively in chick, fish, frog, and mice, but findings from the latter will be emphasised here as higher-order mammals show most similarity to the human brain. Specifically, we will focus on the embryonic development of an important forebrain structure, the striatum (also known as the dorsal striatum or neostriatum). Over the past decade, research on striatal development in mice has led to an influx of new information about the genes involved, but the precise orchestration between the genes, signalling molecules, and transcription factors remains unanswered. We aim to summarise what is known to date about the tightly controlled network of interacting genes that control striatal development. This paper will discuss early telencephalon patterning and dorsal ventral patterning with specific reference to the genes involved in striatal development.

Evolution of invertebrate deuterostomes and Hox/ParaHox genes

Transcription factors encoded by Antennapedia-class homeobox genes play crucial roles in controlling development of animals, and are often found clustered in animal genomes. The Hox and ParaHox gene clusters have been regarded as evolutionary sisters and evolved from a putative common ancestral gene complex, the ProtoHox cluster, prior to the divergence of the Cnidaria and Bilateria (bilaterally symmetrical animals). The Deuterostomia is a monophyletic group of animals that belongs to the Bilateria, and a sister group to the Protostomia. The deuterostomes include the vertebrates (to which we belong), invertebrate chordates, hemichordates, echinoderms and possibly xenoturbellids, as well as acoelomorphs. The studies of Hox and ParaHox genes provide insights into the origin and subsequent evolution of the bilaterian animals. Recently, it becomes apparent that among the Hox and ParaHox genes, there are significant variations in organization on the chromosome, expression pattern, and function. In this review, focusing on invertebrate deuterostomes, I first summarize recent findings about Hox and ParaHox genes. Next, citing unsolved issues, I try to provide clues that might allow us to reconstruct the common ancestor of deuterostomes, as well as understand the roles of Hox and ParaHox genes in the development and evolution of deuterostomes.

Homeobox genes Gsx1 and Gsx2 differentially regulate telencephalic progenitor maturation

Homeobox genes Gsx1 and Gsx2 (formerly Gsh1 and Gsh2) are among the earliest transcription factors expressed in neuronal progenitors of the lateral ganglionic eminence (LGE) in the ventral telencephalon. Gsx2 is required for the early specification of LGE progenitor cells and recently has been shown to specify different LGE neuronal subtypes at distinct time points. In Gsx2 mutants, Gsx1 compensates, at least in part, for the loss of Gsx2 in the specification of LGE neuronal subtypes. Because no specific phenotype has been described in Gsx1 mutants, it is unclear what role this factor plays in the development of the ventral telencephalon. Here, we used a gain-of-function approach to express either Gsx1 or Gsx2 throughout the telencephalon and found that Gsx1 functions similarly to Gsx2 in the specification of LGE identity. However, our results show that Gsx1 and Gsx2 differentially regulate the maturation of LGE progenitors. Specifically, Gsx2 maintains LGE progenitors in an undifferentiated state, whereas Gsx1 promotes progenitor maturation and the acquisition of neuronal phenotypes, at least in part, through the down-regulation of Gsx2. These unique results indicate that the two closely related Gsx genes similarly regulate LGE patterning but oppositely control the balance between proliferation and differentiation in the neuronal progenitor pool.

The Gli3 hypomorphic mutation Pdn causes selective impairment in the growth, patterning, and axon guidance capability of the lateral ganglionic eminence

Previous studies have defined a requirement for Sonic hedgehog (Shh) signaling in patterning the ventral telencephalon, a major source of the neuronal diversity found in the mature telencephalon. The zinc finger transcription factor Gli3 is a critical component of the Shh signaling pathway and its loss causes major defects in telencephalic development. Gli3 is expressed in a graded manner along the dorsoventral axis of the telencephalon but it is unknown whether Gli3 expression levels are important for dorsoventral telencephalic patterning. To address this, we used the Gli3 hypomorphic mouse mutant Polydactyly Nagoya (Pdn). We show that in Pdn/Pdn embryos, the telencephalic expression of Gli3 remains graded, but Gli3 mRNA and protein levels are reduced, resulting in an upregulation of Shh expression and signaling. These changes mainly affect the development of the lateral ganglionic eminence (LGE), with some disorganization of the medial ganglionic eminence mantle zone. The pallial/subpallial boundary is shifted dorsally and the production of postmitotic neurons is reduced. Moreover, LGE pioneer neurons that guide corticofugal axons into the LGE do not form properly, delaying the entry of corticofugal axons into the ventral telencephalon. Pdn/Pdn mutants also show severe pathfinding defects of thalamocortical axons in the ventral telencephalon. Transplantation experiments demonstrate that the intrinsic ability of the Pdn ventral telencephalon to guide thalamocortical axons is compromised. We conclude that correct Gli3 levels are particularly important for the LGE's growth, patterning, and development of axon guidance capabilities.

Conservation of ParaHox genes' function in patterning of the digestive tract of the marine gastropod Gibbula varia

BACKGROUND

Presence of all three ParaHox genes has been described in deuterostomes and lophotrochozoans, but to date one of these three genes, Xlox has not been reported from any ecdysozoan taxa and both Xlox and Gsx are absent in nematodes. There is evidence that the ParaHox genes were ancestrally a single chromosomal cluster. Colinear expression of the ParaHox genes in anterior, middle, and posterior tissues of several species studied so far suggest that these genes may be responsible for axial patterning of the digestive tract. So far, there are no data on expression of these genes in molluscs.

RESULTS

We isolated the complete coding sequences of the three Gibbula varia ParaHox genes, and then tested their expression in larval and postlarval development. In Gibbula varia, the ParaHox genes participate in patterning of the digestive tract and are expressed in some cells of the neuroectoderm. The expression of these genes coincides with the gradual formation of the gut in the larva. Gva-Gsx patterns potential neural precursors of cerebral ganglia as well as of the apical sensory organ. During larval development this gene is involved in the formation of the mouth and during postlarval development it is expressed in the precursor cells involved in secretion of the radula, the odontoblasts. Gva-Xolx and Gva-Cdx are involved in gut patterning in the middle and posterior parts of digestive tract, respectively. Both genes are expressed in some ventral neuroectodermal cells; however the expression of Gva-Cdx fades in later larval stages while the expression of Gva-Xolx in these cells persists.

CONCLUSIONS

In Gibbula varia the ParaHox genes are expressed during anterior-posterior patterning of the digestive system. This colinearity is not easy to spot during early larval stages because the differentiated endothelial cells within the yolk permanently migrate to their destinations in the gut. After torsion, Gsx patterns the mouth and foregut, Xlox the midgut gland or digestive gland, and Cdx the hindgut. ParaHox genes of Gibbula are also expressed during specification of cerebral and ventral neuroectodermal cells. Our results provide additional support for the ancestral complexity of Gsx expression and its ancestral role in mouth patterning in protostomes, which was secondarily lost or simplified in some species.

Features of the ancestral bilaterian inferred from Platynereis dumerilii ParaHox genes

Background

The ParaHox gene cluster is the evolutionary sister to the Hox cluster. Whilst the role of the Hox cluster in patterning the anterior-posterior axis of bilaterian animals is well established, and the organisation of vertebrate Hox clusters is intimately linked to gene regulation, much less is known about the more recently discovered ParaHox cluster. ParaHox gene clustering, and its relationship to expression, has only been described in deuterostomes. Conventional protostome models (Drosophila melanogaster and Caenorhabditis elegans) are secondarily derived with respect to ParaHox genes, suffering gene loss and cluster break-up.

Results

We provide the first evidence for ParaHox gene clustering from a less-derived protostome animal, the annelid Platynereis dumerilii. Clustering of these genes is thus not a sole preserve of the deuterostome lineage within Bilateria. This protostome ParaHox cluster is not entirely intact however, with Pdu-Cdx being on the opposite end of the same chromosome arm from Pdu-Gsx and Pdu-Xlox. From the genomic sequence around the P. dumerilii ParaHox genes the neighbouring genes are identified, compared with other taxa, and the ancestral arrangement deduced.

Conclusion

We relate the organisation of the ParaHox genes to their expression, and from comparisons with other taxa hypothesise that a relatively complex pattern of ParaHox gene expression existed in the protostome-deuterostome ancestor, which was secondarily simplified along several invertebrate lineages. Detailed comparisons of the gene content around the ParaHox genes enables the reconstruction of the genome surrounding the ParaHox cluster of the protostome-deuterostome ancestor, which existed over 550 million years ago.

Ascl1 is a required downstream effector of Gsx gene function in the embryonic mouse telencephalon

Background

The homeobox gene Gsx2 (formerly Gsh2) is known to regulate patterning in the lateral ganglionic eminence (LGE) of the embryonic telencephalon. In its absence, the closely related gene Gsx1 (previously known as Gsh1) can partially compensate in the patterning and differentiation of ventral telencephalic structures, such as the striatum. However, the cellular and molecular mechanisms underlying this compensation remain unclear.

Results

We show here that in the Gsx2 mutants Gsx1 is expressed in only a subset of the ventral telencephalic progenitors that normally express Gsx2. Based on the similarities in the expression of Gsx1 and Ascl1 (Mash1) within the Gsx2 mutant LGE, we examined whether Ascl1 plays an integral part in the Gsx1-based recovery. Ascl1 mutants show only modest alterations in striatal development; however, in Gsx2;Ascl1 double mutants, striatal development is severely affected, similar to that seen in the Gsx1;Gsx2 double mutants. This is despite the fact that Gsx1 is expressed, and even expands, in the Gsx2;Ascl1 mutant LGE, comparable to that seen in the Gsx2 mutant. Finally, Notch signaling has recently been suggested to be required for normal striatal development. In spite of the fact that Notch signaling is severely disrupted in Ascl1 mutants, it actually appears to be improved in the Gsx2;Ascl1 double mutants.

Conclusion

These results, therefore, reveal a non-proneural requirement of Ascl1 that together with Gsx1 compensates for the loss of Gsx2 in a subset of LGE progenitors.

Hox, Cdx, and anteroposterior patterning in the mouse embryo

Cdx and Hox gene families descend from the same ProtoHox cluster, already present in the common ancestors of bilaterians and cnidarians, and thought to act by providing anteroposterior (A-P) positional identity to axial tissues in all bilaterians. Mouse Cdx and Hox genes still exhibit common features in their early expression and function. The initiation and early shaping of Hox and Cdx transcriptional domains in mouse embryos are very similar, in keeping with their common involvement in conveying A-P information to the nascent tissues during embryonic axial elongation. Considerations of the impact on axial patterning of the early expression phase of these genes that correlates with the temporally collinear expression of 3'-5'Hox genes suggest that it is concerned with the acquisition of A-P information by the three germ layers as the axis extends. This early A-P information acquired by all cells emerging from the primitive streak or tailbud and their neighbors in the caudal neural plate gets further modulated by the second phase of gene expression occurring later as the tissues mature and differentiate along the growing axis. We discuss the possibility that regulatory phase 1, common to all Cdx and Hox genes, is inherent to the concerted mechanism sequentially turning on 3'-5'Hox genes at early stages, and keeping expression of the initiated genes subsequently in the new materials added posteriorly at the axis extends. The posterior Hox gene expression domain would be subsequently complemented by Hox regulatory phase 2, consisting in a variety of gene-specific, region-specific, and/or tissue-specific gene expression controls. We also touch on the unanswered question whether vertebrate Cdx gene expression delivers A-P positional information in its own right, as Caudal does in Drosophila, or whether it does so exclusively by upregulating Hox genes.

Expression study of cadherin7 and cadherin20 in the embryonic and adult rat central nervous system

BACKGROUND

Vertebrate classic cadherins are divided into type I and type II subtypes, which are individually expressed in brain subdivisions (e.g., prosomeres, rhombomeres, and progenitor domains) and in specific neuronal circuits in region-specific manners. We reported previously the expression of cadherin19 (cad19) in Schwann cell precursors. Cad19 is a type II classic cadherin closely clustered on a chromosome with cad7 and cad20. The expression patterns of cad7 and cad20 have been reported previously in chick embryo but not in the developing and adult central nervous system of mammals. In this study, we identified rat cad7 and cad20 and analyzed their expression patterns in embryonic and adult rat brains.

RESULTS

Rat cad7 protein showed 92% similarity to chick cad7, while rat cad20 protein had 76% similarity to Xenopus F-cadherin. Rat cad7 mRNA was initially expressed in the anterior neural plate including presumptive forebrain and midbrain regions, and then accumulated in cells of the dorsal neural tube and in rhombomere boundary cells of the hindbrain. Expression of rat cad20 mRNA was specifically localized in the anterior neural region and rhombomere 2 in the early neural plate, and later in longitudinally defined ventral cells of the hindbrain. The expression boundaries of cad7 and cad20 corresponded to those of region-specific transcription factors such as Six3, Irx3 and Otx2 in the neural plate, and Dbx2 and Gsh1 in the hindbrain. At later stages, the expression of cad7 and cad20 disappeared from neuroepithelial cells in the hindbrain, and was almost restricted to postmitotic cells, e.g. somatic motor neurons and precerebellar neurons. These results emphasized the diversity of cad7 and cad20 expression patterns in different vertebrate species, i.e. birds and rodents.

CONCLUSION

Taken together, our findings suggest that the expression of cad7 and cad20 demarcates the compartments, boundaries, progenitor domains, specific nuclei and specific neural circuits during mammalian brain development.

ParaHox gene expression in larval and postlarval development of the polychaete Nereis virens (Annelida, Lophotrochozoa)

Background

Transcription factors that encode ANTP-class homeobox genes play crucial roles in determining the body plan organization and specification of different organs and tissues in bilaterian animals. The three-gene ParaHox family descends from an ancestral gene cluster that existed before the evolution of the Bilateria. All three ParaHox genes are reported from deuterostomes and lophotrochozoans, but not to date from any ecdysozoan taxa, and there is evidence that the ParaHox genes, like the related Hox genes, were ancestrally a single chromosomal cluster. However, unlike the Hox genes, there is as yet no strong evidence that the ParaHox genes are expressed in spatial and temporal order during embryogenesis.

Results

We isolated fragments of the three Nereis virens ParaHox genes, then used these as probes for whole-mount in situ hybridization in larval and postlarval worms. In Nereis virens the ParaHox genes participate in antero-posterior patterning of ectodermal and endodermal regions of the digestive tract and are expressed in some cells in the segment ganglia. The expression of these genes occurs in larval development in accordance with the position of these cells along the main body axis and in postlarval development in accordance with the position of cells in ganglia along the antero-posterior axis of each segment. In none of these tissues does expression of the three ParaHox genes follow the rule of temporal collinearity.

Conclusion

In Nereis virens the ParaHox genes are expressed during antero-posterior patterning of the digestive system (ectodermal foregut and hindgut, and endodermal midgut) of Nereis virens. These genes are also expressed during axial specification of ventral neuroectodermal cell domains, where the expression domains of each gene are re-iterated in each neuromere except for the first parapodial segment. These expression domains are probably predetermined and may be directed on the antero-posterior axis by the Hox genes, whose expression starts much earlier during embryogenesis. Our results support the hypothesis that the ParaHox genes are involved in antero-posterior patterning of the developing embryo, but they do not support the notion that these genes function only in the patterning of endodermal tissues.

Conserved properties of the Drosophila homeodomain protein, Ind

Ind-Gsh-type homeodomain proteins are critical to patterning of intermediate domains in the developing CNS; yet, the molecular basis for the activities of these homeodomain proteins is not well understood. Here we identify domains within the Ind protein that are responsible for transcriptional repression, as well as those required for its interaction with the co-repressor, Groucho. To do this, we utilized a combination of chimeric transient transfection assays, co-immunoprecipitation and in vivo expression assays. We show that Ind's candidate Eh1 domain is essential to the embryonic repression activity of this protein, and that Groucho interacts with Ind via this domain. However, when activity is assayed in transient transfection assays using Ind-Gal4 DNA binding domain chimeras to determine domain activity, the repression activity of the Eh1 domain is minimal. This result is similar to previous results on the transcription factors, Vnd and Engrailed. Furthermore, the Eh1 domain is necessary, but not sufficient, for binding to Groucho; the C terminus of Ind, including the homeodomain also affects the interaction with this co-repressor in co-immunoprecipitations. Finally, we show that aspects of the cross-repressive activities of Ind/Gsh2-Ey/Pax6 are evolutionarily conserved. Taken together, these results point to conserved mechanisms used by Gsh/Ind-type homeodomain protein in regulating the expression of target genes.

Cloning and functional analysis of hypothalamic homeobox gene Bsx1a and its isoform, Bsx1b

The hypothalamus is a key regulatory unit of the neuroendocrine system and plays an essential role in energy balance and reproduction. Despite its important role, the molecular mechanisms underlying hypothalamic development are not fully understood. Here, we report molecular analyses of a newly identified murine homeobox gene, Bsx/Bsx1a, that is expressed in the developing and postnatal hypothalamus. We demonstrate that BSX1A is a DNA binding protein and a transcriptional activator. Transcriptional reporter assays identified the C-terminal region of BSX1A as an activation domain. We have isolated an alternative splice form of Bsx1a, designated Bsx1b, which retains the N-terminal region but lacks the homeodomain. Analyses of subcellular localization using transfected cell lines revealed that BSX1A and BSX1B localize in the nuclei and cytoplasm, respectively. Immunohistochemical analyses suggested that both BSX1A and BSX1B are expressed in the neonatal hypothalamus. Taking these data together, we propose that alternative RNA splicing is involved in hypothalamic development/function.

Linking pattern formation to cell-type specification: Dichaete and Ind directly repress achaete gene expression in the Drosophila CNS

Mechanisms regulating CNS pattern formation and neural precursor formation are remarkably conserved between Drosophila and vertebrates. However, to date, few direct connections have been made between genes that pattern the early CNS and those that trigger neural precursor formation. Here, we use Drosophila to link directly the function of two evolutionarily conserved regulators of CNS pattern along the dorsoventral axis, the homeodomain protein Ind and the Sox-domain protein Dichaete, to the spatial regulation of the proneural gene achaete (ac) in the embryonic CNS. We identify a minimal achaete regulatory region that recapitulates half of the wild-type ac expression pattern in the CNS and find multiple putative Dichaete-, Ind-, and Vnd-binding sites within this region. Consensus Dichaete sites are often found adjacent to those for Vnd and Ind, suggesting that Dichaete associates with Ind or Vnd on target promoters. Consistent with this finding, we observe that Dichaete can physically interact with Ind and Vnd. Finally, we demonstrate the in vivo requirement of adjacent Dichaete and Ind sites in the repression of ac gene expression in the CNS. Our data identify a direct link between the molecules that pattern the CNS and those that specify distinct cell-types.

Identification of an upstream regulatory element reveals a novel requirement for Ind activity in maintaining ind expression

A maternally established gradient of nuclear Dorsal protein is the first step in subdivision of the Drosophila neurectoderm into stripes of homeodomain gene expression. Dorsal in combination with the EGF and TGFbeta signaling pathways are key regulators of the expression of the genes ventral nervous system defective (vnd), intermediate neuroblasts defective (ind), and muscle segment homeobox (msh) in the developing neurectoderm. These three genes encode homeodomain transcription factors that can repress each other, which ensures adjacent, non-overlapping expression domains. Expression of vnd, ind, and msh is maintained after decline in EGF and TGFbeta signaling, but the relevant positive transcriptional regulators have not yet been defined. Here, we show that Ind can bind DNA with the same sequence specificity as its murine ortholog Gsh1. We have identified a novel upstream regulatory element at the ind locus containing predicted Ind binding sites, and we show that Ind activity is both necessary and sufficient for reporter gene expression from this element. We conclude that Ind can act as a transcriptional activator, and that positive autoregulation of Ind is a mechanism for persistent ind expression within the developing embryonic nervous system.

Breakup of a homeobox cluster after genome duplication in teleosts

Several families of homeobox genes are arranged in genomic clusters in metazoan genomes, including the Hox, ParaHox, NK, Rhox, and Iroquois gene clusters. The selective pressures responsible for maintenance of these gene clusters are poorly understood. The ParaHox gene cluster is evolutionarily conserved between amphioxus and human but is fragmented in teleost fishes. We show that two basal ray-finned fish, Polypterus and Amia, each possess an intact ParaHox cluster; this implies that the selective pressure maintaining clustering was lost after whole-genome duplication in teleosts. Cluster breakup is because of gene loss, not transposition or inversion, and the total number of ParaHox genes is the same in teleosts, human, mouse, and frog. We propose that this homeobox gene cluster is held together in chordates by the existence of interdigitated control regions that could be separated after locus duplication in the teleost fish.

Pdx1-related homeodomain transcription factors are distinctly expressed in mouse adult pancreatic islets

Complex gene networks are responsible for the proper operation of the endocrine pancreas. A central member of such networks, the homeodomain transcription factor Pdx1, belongs to the ParaHox gene cluster, an array of Hox-like homeobox genes. With a combination of mRNA in situ hybridisation and immunodetection, we have found that the rest of ParaHox cluster genes, Cdx1, Cdx2/3, and Cdx4, and Gsh1 and Gsh2, are all expressed in specific islet cell types of the endocrine pancreas. To our knowledge, this is the first report that locates ParaHox genes other than Pdx1 and Cdx2/3 in a place as to be involved in the pancreatic transcriptional regulatory networks, potentially regulating glucagon-insulin homeostasis.

Neuronal migration and ventral subtype identity in the telencephalon depend on SOX1

Little is known about the molecular mechanisms and intrinsic factors that are responsible for the emergence of neuronal subtype identity. Several transcription factors that are expressed mainly in precursors of the ventral telencephalon have been shown to control neuronal specification, but it has been unclear whether subtype identity is also specified in these precursors, or if this happens in postmitotic neurons, and whether it involves the same or different factors. SOX1, an HMG box transcription factor, is expressed widely in neural precursors along with the two other SOXB1 subfamily members, SOX2 and SOX3, and all three have been implicated in neurogenesis. SOX1 is also uniquely expressed at a high level in the majority of telencephalic neurons that constitute the ventral striatum (VS). These neurons are missing in Sox1-null mutant mice. In the present study, we have addressed the requirement for SOX1 at a cellular level, revealing both the nature and timing of the defect. By generating a novel Sox1-null allele expressing beta-galactosidase, we found that the VS precursors and their early neuronal differentiation are unaffected in the absence of SOX1, but the prospective neurons fail to migrate to their appropriate position. Furthermore, the migration of non-Sox1-expressing VS neurons (such as those expressing Pax6) was also affected in the absence of SOX1, suggesting that Sox1-expressing neurons play a role in structuring the area of the VS. To test whether SOX1 is required in postmitotic cells for the emergence of VS neuronal identity, we generated mice in which Sox1 expression was directed to all ventral telencephalic precursors, but to only a very few VS neurons. These mice again lacked most of the VS, indicating that SOX1 expression in precursors is not sufficient for VS development. Conversely, the few neurons in which Sox1 expression was maintained were able to migrate to the VS. In conclusion, Sox1 expression in precursors is not sufficient for VS neuronal identity and migration, but this is accomplished in postmitotic cells, which require the continued presence of SOX1. Our data also suggest that other SOXB1 members showing expression in specific neuronal populations are likely to play continuous roles from the establishment of precursors to their final differentiation.

gsh1 demarcates hypothalamus and intermediate spinal cord in zebrafish

We have cloned and characterized the expression of zebrafish genetic screen homeobox 1 (gsh1). Early expression is confined to hindbrain rhombomeres; by mid-somitogenesis gsh1 is expressed in precise domains within the mesencephalon and diencephalon, as well as in intermediate spinal cord. Double-label experiments revealed that the diencephalic domain is coincident with hypothalamus and that spinal cord expression is in a region that generates interneurons. These data suggest gsh1 may play a role in patterning cell types generated in these domains.

Multiple Dorsoventral Origins of Oligodendrocyte Generation in the Spinal Cord and Hindbrain

Studies have indicated that oligodendrocytes in the spinal cord originate from a ventral progenitor domain defined by expression of the oligodendrocyte-determining bHLH proteins Olig1 and Olig2. Here, we provide evidence that progenitors in the dorsal spinal cord and hindbrain also produce oligodendrocytes and that the specification of these cells may result from a dorsal evasion of BMP signaling over time. Moreover, we show that the generation of ventral oligodendrocytes in the spinal cord depends on Nkx6.1 and Nkx6.2 function, while these homeodomain proteins in the anterior hindbrain instead suppress oligodendrocyte specification. The opposing roles for Nkx6 proteins in the spinal cord and hindbrain, in turn, appear to reflect that oligodendrocytes are produced by distinct ventral progenitor domains at these axial levels. Based on these findings, we propose that oligodendrocytes derive from several distinct positional origins and that the activation of Olig1/2 at different positions is controlled by distinct genetic programs.

Zebrafish and fly Nkx6 proteins have similar CNS expression patterns and regulate motoneuron formation

Genes belonging to the Nkx, Gsh and Msx families are expressed in similar dorsovental spatial domains of the insect and vertebrate central nervous system (CNS), suggesting the bilaterian ancestor used this genetic program during CNS development. We have investigated the significance of these similar expression patterns by testing whether Nkx6 proteins expressed in ventral CNS of zebrafish and flies have similar functions. In zebrafish, Nkx6.1 is expressed in early-born primary and later-born secondary motoneurons. In the absence of Nkx6.1, there are fewer secondary motoneurons and supernumerary ventral interneurons, suggesting Nkx6.1 promotes motoneuron and suppresses interneuron formation. Overexpression of fish or fly Nkx6 is sufficient to generate supernumerary motoneurons in both zebrafish and flies. These results suggest that one ancestral function of Nkx6 proteins was to promote motoneuron development.

Hmx2 and Hmx3 Homeobox Genes Direct Development of the Murine Inner Ear and Hypothalamus and Can Be Functionally Replaced by Drosophila Hmx

The Hmx homeobox gene family appears to play a conserved role in CNS development in all animal species examined, and in higher vertebrates has an additional role in sensory organ development. Here, we show that murine Hmx2 and Hmx3 have both overlapping and distinct functions in the development of the inner ear's vestibular system, whereas their functions in the hypothalamic/pituitary axis of the CNS appear to be interchangeable. As in analogous knockin studies of Otx and En function, Drosophila Hmx can rescue conserved functions in the murine CNS. However, in contrast to Otx and En, Drosophila Hmx also rescues significant vertebrate-specific functions outside the CNS. Our work suggests that the evolution of the vertebrate inner ear may have involved (1) the redeployment of ancient Hmx activities to regulate the cell proliferation of structural components and (2) the acquisition of additional, vertebrate-specific Hmx activities to regulate the sensory epithelia.

Compromised generation of GABAergic interneurons in the brains of Vax1-/- mice

The subcortical telencephalon is the major source of GABAergic interneurons that, during development, tangentially migrate to the cerebral cortex, where they modulate the glutamatergic excitatory action of pyramidal cells. The transcription factor Vax1, an intracellular mediator of both Shh and Fgf signaling, is expressed at high levels in the medial and lateral ganglionic eminences (MGE and LGE, respectively), in the septal area (SA), in the anterior entopeduncular area (AEP) and in the preoptic area (POA). We show that Vax1 expression in the neuroepithelium is graded: low in the ventricular zone (VZ) and high in the subventricular zone (SVZ), in a pattern that closely reproduces that of several members of the Dlx and Gsh family of homeobox transcription factors. We provide evidence that Vax1 plays an important role in proliferation and differentiation of MGE, POA/AEP and septum, and that the last structure is completely absent in Vax1-/- mice. We show that the absence of Vax1 causes a severe depletion of GABAergic neurons in the neocortex, ranging from 30% to 44%, depending on the cortical areas considered. Taken together, our data indicate that a loss of function mutation in the Vax1 gene generates abnormalities in basal ganglia subventricular zone development and that it prevents the formation of the septum, impairing GABAergic interneuron generation.

Neuroblast formation and patterning during early brain development inDrosophila

The Drosophila embryo provides a useful model system to study the mechanisms that lead to pattern and cell diversity in the central nervous system (CNS). The Drosophila CNS, which encompasses the brain and the ventral nerve cord, develops from a bilaterally symmetrical neuroectoderm, which gives rise to neural stem cells, called neuroblasts. The structure of the embryonic ventral nerve cord is relatively simple, consisting of a sequence of repeated segmental units (neuromeres), and the mechanisms controlling the formation and specification of the neuroblasts that form these neuromeres are quite well understood. Owing to the much higher complexity and hidden segmental organization of the brain, our understanding of its development is still rudimentary. Recent investigations on the expression and function of proneural genes, segmentation genes, dorsoventral-patterning genes and a number of other genes have provided new insight into the principles of neuroblast formation and patterning during embryonic development of the fly brain. Comparisons with the same processes in the trunk help us to understand what makes the brain different from the ventral nerve cord. Several parallels in early brain patterning between the fly and the vertebrate systems have become evident.

Combinatorial function of the homeodomain proteins Nkx2.1 and Gsh2 in ventral telencephalic patterning

Regional patterning of the mammalian telencephalon requires the function of three homeodomain-containing transcription factors, Pax6, Gsh2 and Nkx2.1. These factors are required for the development of the dorsal, lateral and medial domains of the telencephalon, respectively. Previous work has indicated that two of the genes encoding these factors, Pax6 and Gsh2, cross-repress one another in the formation of the border between dorsal and lateral region of the telencephalon. Here, we examine whether similar interactions are responsible for the establishment of other boundaries of telencephalic gene expression. Surprisingly, despite the fact that, at specific times in development, both Pax6 and Gsh2 maintain a complementary pattern of expression with Nkx2.1, in neither case are these boundaries maintained through a similar cross-repressive mechanism. Rather, as revealed by analysis of double-mutant mice, Nkx2.1 and Gsh2 act cooperatively in many aspects to pattern the ventral telencephalon. By contrast, as indicated by both loss- and gain-of-function analysis, Gsh2 expression in the medial ganglionic eminence after E10.5 may negatively regulate Nkx2.1 dependent specification of oligodendrocytes. Therefore, both integrative and antagonistic interactions between homeodomain-containing transcription factors contribute to the patterning of the telencephalon.

Segment polarity and DV patterning gene expression reveals segmental organization of the Drosophila brain

The insect brain is traditionally subdivided into the trito-, deuto- and protocerebrum. However, both the neuromeric status and the course of the borders between these regions are unclear. The Drosophila embryonic brain develops from the procephalic neurogenic region of the ectoderm, which gives rise to a bilaterally symmetrical array of about 100 neuronal precursor cells, called neuroblasts. Based on a detailed description of the spatiotemporal development of the entire population of embryonic brain neuroblasts, we carried out a comprehensive analysis of the expression of segment polarity genes (engrailed, wingless, hedgehog, gooseberry distal, mirror) and DV patterning genes (muscle segment homeobox, intermediate neuroblast defective, ventral nervous system defective) in the procephalic neuroectoderm and the neuroblast layer (until stage 11, when all neuroblasts are formed). The data provide new insight into the segmental organization of the procephalic neuroectodem and evolving brain. The expression patterns allow the drawing of clear demarcations between trito-, deuto- and protocerebrum at the level of identified neuroblasts. Furthermore, we provide evidence indicating that the protocerebrum (most anterior part of the brain) is composed of two neuromeres that belong to the ocular and labral segment, respectively. These protocerebral neuromeres are much more derived compared with the trito- and deutocerebrum. The labral neuromere is confined to the posterior segmental compartment. Finally, similarities in the expression of DV patterning genes between the Drosophila and vertebrate brains are discussed.

Gfi-1 attaches to the nuclear matrix, associates with ETO (MTG8) and histone deacetylase proteins, and represses transcription using a TSA-sensitive mechanism

Gfi-1 and Gfi-1B can repress transcription and play important roles in hematopoietic cell survival and differentiation. Although these proteins are known to bind DNA through a C-terminal zinc-finger domain and may require an N-terminal SNAG domain (SNAIL/Gfi-1) to repress transcription, the mechanism by which Gfi-1 and Gfi-1B act is unknown. A first step towards understanding the mechanism by which these proteins repress transcription is to identify interacting proteins that could contribute to transcriptional repression. ETO (also termed MTG8), was first identified through its involvement in the (8;21) translocation associated with acute myelogenous leukemia. It attaches to the nuclear matrix and associates with histone deacetylases and the co-repressors N-CoR, SMRT, and mSin3A, and may act as a co-repressor for site-specific transcriptions factors. In this report we demonstrate that Gfi-1 interacts with ETO and related proteins both in vitro and in vivo and with histone deacetylase proteins in vivo. We observed that a portion of Gfi-1 and Gfi-1B associated with the nuclear matrix, as is the case with ETO. Moreover, Gfi-1 and ETO co-localize to punctate subnuclear structures. When co-expressed in mammalian cells, Gfi-1 associates with histone deacetylse-1 (HDAC-1), HDAC-2, and HDAC-3. These data identify ETO as a partner for Gfi-1 and Gfi-1B, and suggest that Gfi-1 proteins repress transcription through recruitment of histone deacetylase-containing complexes.