Nkx2-9 is a novel homeobox transcription factor which demarcates ventral domains in the developing mouse CNS

A detailed review of the spinal accessory nerve and its anatomical variations with cadaveric illustration

The spinal accessory nerve, considered part of the eleventh cranial nerve, provides motor innervation to sternocleidomastoid and trapezius. A comprehensive literature review and two cadaveric dissections were undertaken. The spinal accessory nerve originates from the spinal accessory nucleus. Its rootlets unite and ascend between the denticulate ligament and dorsal spinal rootlets. Thereafter, it can anastomose with spinal roots, such as the McKenzie branch, and/or cranial roots. The spinal accessory nerve courses intracranially via foramen magnum and exits via jugular foramen, within which it usually lies anteriorly. Extracranially, it usually crosses anterior to the internal jugular vein and lies lateral to internal jugular vein deep to posterior belly of digastric. The spinal accessory nerve innervates sternocleidomastoid, receives numerous contributions in the posterior triangle and terminates within trapezius. Its posterior triangle course approximates a perpendicular bisection of the mastoid-mandibular angle line. The spinal accessory nerve contains sensory nociceptive fibres. Its cranial nerve classification is debated due to occasional non-fusion with the cranial root. Surgeons should familiarize themselves with the variable course of the spinal accessory nerve to minimize risk of injury. Patients with spinal accessory nerve injuries might require specialist pain management.

The novel linkage between Fuz and Gpr161 genes regulates sonic hedgehog signaling during mouse embryonic development

Sonic hedgehog (Shh) signaling regulates embryonic morphogenesis utilizing primary cilia, the cell antenna acting as a signaling hub. Fuz, an effector of planar cell polarity (PCP) signaling, involves Shh signaling via cilia formation, while the G protein-coupled receptor 161 (Gpr161) is a negative regulator of Shh signaling. The range of phenotypic malformations observed in mice bearing mutations in either of these two genes is similar; however, their functional relations have not been previously explored. This study identified the genetic and biochemical link between Fuz and Gpr161 in mouse embryonic development. Fuz was genetically epistatic to Gpr161 via Shh signaling during mouse embryonic development. The FUZ biochemically interacted with GPR161, and Fuz regulated Gpr161 ciliary trafficking via β-arrestin2. Our study suggested the novel Gpr161-Fuz axis that regulates Shh signaling during mouse embryonic development.

Transcription factor support for the dual embryological origin of the sternocleidomastoid and trapezius muscles

The embryological origin of the trapezius and sternocleidomastoid muscles has been debated for over a century. To shed light on this issue, the present anatomical study was performed. Five fresh frozen human cadavers, three males and two females, were used for this study. Samples from each specimen's trapezius and sternocleidomastoid were fixed in 10% formalin and placed in paraffin blocks. As Paired like homeodomain 2 (Pitx2) and T-box factor 1(Tbx1) have been implicated in the region and muscle type regulation, we performed Tbx1 and Pitx2 Immunohistochemistry (IHC) on these muscle tissue samples to identify the origin of the trapezius and sternocleidomastoid muscles. We have used the latest version of QuPath, v0.4.3, software to quantify the Tbx and Pitx2 staining. For the sternocleidomastoid muscle, for evaluated samples, the average amount of positively stained Tbx1 and Pitx2 was 25% (range 16%-30%) and 18% (range 12%-23%), respectively. For the trapezius muscles, for evaluated samples, the average amount of positively stained Tbx1 and Pitx2 parts of the samples was 17% (range 15%-20%) and 15% (14%-17%), respectively. Our anatomical findings suggest dual origins of both the trapezius and sternocleidomastoid muscles. Additionally, as neither Pitx2 nor Tbx1 made up all the staining observed for each muscle, other contributions to these structures are likely. Future studies with larger samples are now necessary to confirm these findings.

Nkx2.9 Contributes to Mid-Hindbrain Patterning by Regulation of mdDA Neuronal Cell-Fate and Repression of a Hindbrain-Specific Cell-Fate

Nkx2.9 is a member of the NK homeobox family and resembles Nkx2.2 both in homology and expression pattern. However, while Nkx2.2 is required for development of serotonergic neurons, the role of Nkx2.9 in the mid-hindbrain region is still ill-defined. We have previously shown that Nkx2.9 expression is downregulated upon loss of En1 during development. Here, we determined whether mdDA neurons require Nkx2.9 during their development. We show that Nkx2.9 is strongly expressed in the IsO and in the VZ and SVZ of the embryonic midbrain, and the majority of mdDA neurons expressed Nkx2.9 during their development. Although the expression of Dat and Cck are slightly affected during development, the overall development and cytoarchitecture of TH-expressing neurons is not affected in the adult Nkx2.9-depleted midbrain. Transcriptome analysis at E14.5 indicated that genes involved in mid- and hindbrain development are affected by Nkx2.9-ablation, such as Wnt8b and Tph2. Although the expression of Tph2 extends more rostral into the isthmic area in the Nkx2.9 mutants, the establishment of the IsO is not affected. Taken together, these data point to a minor role for Nkx2.9 in mid-hindbrain patterning by repressing a hindbrain-specific cell-fate in the IsO and by subtle regulation of mdDA neuronal subset specification.

Genome-wide chromatin accessibility and transcriptome profiling show minimal epigenome changes and coordinated transcriptional dysregulation of hedgehog signaling in Danforth's short tail mice

Danforth's short tail (Sd) mice provide an excellent model for investigating the underlying etiology of human caudal birth defects, which affect 1 in 10 000 live births. Sd animals exhibit aberrant axial skeleton, urogenital and gastrointestinal development similar to human caudal malformation syndromes including urorectal septum malformation, caudal regression, vertebral-anal-cardiac-tracheo-esophageal fistula-renal-limb (VACTERL) association and persistent cloaca. Previous studies have shown that the Sd mutation results from an endogenous retroviral (ERV) insertion upstream of the Ptf1a gene resulting in its ectopic expression at E9.5. Though the genetic lesion has been determined, the resulting epigenomic and transcriptomic changes driving the phenotype have not been investigated. Here, we performed ATAC-seq experiments on isolated E9.5 tailbud tissue, which revealed minimal changes in chromatin accessibility in Sd/Sd mutant embryos. Interestingly, chromatin changes were localized to a small interval adjacent to the Sd ERV insertion overlapping a known Ptf1a enhancer region, which is conserved in mice and humans. Furthermore, mRNA-seq experiments revealed increased transcription of Ptf1a target genes and, importantly, downregulation of hedgehog pathway genes. Reduced sonic hedgehog (SHH) signaling was confirmed by in situ hybridization and immunofluorescence suggesting that the Sd phenotype results, in part, from downregulated SHH signaling. Taken together, these data demonstrate substantial transcriptome changes in the Sd mouse, and indicate that the effect of the ERV insertion on Ptf1a expression may be mediated by increased chromatin accessibility at a conserved Ptf1a enhancer. We propose that human caudal dysgenesis disorders may result from dysregulation of hedgehog signaling pathways.

Nkx2.2 and Nkx2.9 are the key regulators to determine cell fate of branchial and visceral motor neurons in caudal hindbrain

Cranial motor nerves in vertebrates are comprised of the three principal subtypes of branchial, visceral, and somatic motor neurons, which develop in typical patterns along the anteroposterior and dorsoventral axes of hindbrain. Here we demonstrate that the formation of branchial and visceral motor neurons critically depends on the transcription factors Nkx2.2 and Nkx2.9, which together determine the cell fate of neuronal progenitor cells. Disruption of both genes in mouse embryos results in complete loss of the vagal and spinal accessory motor nerves, and partial loss of the facial and glossopharyngeal motor nerves, while the purely somatic hypoglossal and abducens motor nerves are not diminished. Cell lineage analysis in a genetically marked mouse line reveals that alterations of cranial nerves in Nkx2.2; Nkx2.9 double-deficient mouse embryos result from changes of cell fate in neuronal progenitor cells. As a consequence progenitors of branchiovisceral motor neurons in the ventral p3 domain of hindbrain are transformed to somatic motor neurons, which use ventral exit points to send axon trajectories to their targets. Cell fate transformation is limited to the caudal hindbrain, as the trigeminal nerve is not affected in double-mutant embryos suggesting that Nkx2.2 and Nkx2.9 proteins play no role in the development of branchiovisceral motor neurons in hindbrain rostral to rhombomere 4.

nkx2.1 and nkx2.4 genes function partially redundant during development of the zebrafish hypothalamus, preoptic region, and pallidum

During ventral forebrain development, orthologs of the homeodomain transcription factor Nkx2.1 control patterning of hypothalamus, preoptic region, and ventral telencephalon. However, the relative contributions of Nkx2.1 and Nkx2.4 to prosencephalon development are poorly understood. Therefore, we analyzed functions of the previously uncharacterized nkx2.4-like zgc:171531 as well as of the presumed nkx2.1 orthologs nkx2.1a and nkx2.1b in zebrafish forebrain development. Our results show that zgc:171531 and nkx2.1a display overlapping expression patterns and a high sequence similarity. Together with a high degree of synteny conservation, these findings indicate that both these genes indeed are paralogs of nkx2.4. As a result, we name zgc:171531 now nkx2.4a, and changed the name of nkx2.1a to nkx2.4b, and of nkx2.1b to nkx2.1. In nkx2.1, nkx2.4a, and nkx2.4b triple morpholino knockdown (nkx2TKD) embryos we observed a loss of the rostral part of prosomere 3 and its derivative posterior tubercular and hypothalamic structures. Furthermore, there was a loss of rostral and intermediate hypothalamus, while a residual preoptic region still develops. The reduction of the ventral diencephalon was accompanied by a ventral expansion of the dorsally expressed pax6, revealing a dorsalization of the basal hypothalamus. Within the telencephalon we observed a loss of pallidal markers, while striatum and pallium are forming. At the neuronal level, nkx2TKD morphants lacked several neurosecretory neuron types, including avp, crh, and pomc expressing cells in the hypothalamus, but still form oxt neurons in the preoptic region. Our data reveals that, while nkx2.1, nkx2.4a, and nkx2.4b genes act partially redundant in hypothalamic development, nkx2.1 is specifically involved in the development of rostral ventral forebrain including the pallidum and preoptic regions, whereas nkx2.4a and nkx2.4b control the intermediate and caudal hypothalamus.

Motor neurons and the generation of spinal motor neuron diversity

Motor neurons (MNs) are neuronal cells located in the central nervous system (CNS) controlling a variety of downstream targets. This function infers the existence of MN subtypes matching the identity of the targets they innervate. To illustrate the mechanism involved in the generation of cellular diversity and the acquisition of specific identity, this review will focus on spinal MNs (SpMNs) that have been the core of significant work and discoveries during the last decades. SpMNs are responsible for the contraction of effector muscles in the periphery. Humans possess more than 500 different skeletal muscles capable to work in a precise time and space coordination to generate complex movements such as walking or grasping. To ensure such refined coordination, SpMNs must retain the identity of the muscle they innervate. Within the last two decades, scientists around the world have produced considerable efforts to elucidate several critical steps of SpMNs differentiation. During development, SpMNs emerge from dividing progenitor cells located in the medial portion of the ventral neural tube. MN identities are established by patterning cues working in cooperation with intrinsic sets of transcription factors. As the embryo develop, MNs further differentiate in a stepwise manner to form compact anatomical groups termed pools connecting to a unique muscle target. MN pools are not homogeneous and comprise subtypes according to the muscle fibers they innervate. This article aims to provide a global view of MN classification as well as an up-to-date review of the molecular mechanisms involved in the generation of SpMN diversity. Remaining conundrums will be discussed since a complete understanding of those mechanisms constitutes the foundation required for the elaboration of prospective MN regeneration therapies.

Control of astrocyte progenitor specification, migration and maturation by Nkx6.1 homeodomain transcription factor

Although astrocytes are the most abundant cell type in the central nervous system (CNS), little is known about their molecular specification and differentiation. It has previously been reported that transcription factor Nkx6.1 is expressed in neuroepithelial cells that give rise to astrocyte precursors in the ventral spinal cord. In the present study, we systematically investigated the function of Nkx6.1 in astrocyte development using both conventional and conditional Nkx6.1 mutant mice. At early postnatal stages, Nkx6.1 was expressed in a subpopulation of astrocytes in the ventral spinal cord. In the conventional Nkx6.1KO spinal cord, the initial specification of astrocyte progenitors was affected by the mutation, and subsequent migration and differentiation were disrupted in newborn mice. In addition, the development of VA2 subtype astrocytes was also inhibited in the white matter. Further studies with Nkx6.1 conditional mutants revealed significantly delayed differentiation and disorganized arrangement of fibrous astrocytes in the ventral white matter. Together, our studies indicate that Nkx6.1 plays a vital role in astrocyte specification and differentiation in the ventral spinal cord.

Mesodermal Nkx2.5 is necessary and sufficient for early second heart field development

The vertebrate heart develops from mesoderm and requires inductive signals secreted from early endoderm. During embryogenesis, Nkx2.5 acts as a key transcription factor and plays essential roles for heart formation from Drosophila to human. In mice, Nkx2.5 is expressed in the early first heart field, second heart field pharyngeal mesoderm, as well as pharyngeal endodermal cells underlying the second heart field. Currently, the specific requirements for Nkx2.5 in the endoderm versus mesoderm with regard to early heart formation are incompletely understood. Here, we performed tissue-specific deletion in mice to dissect the roles of Nkx2.5 in the pharyngeal endoderm and mesoderm. We found that heart development appeared normal after endodermal deletion of Nkx2.5 whereas mesodermal deletion engendered cardiac defects almost identical to those observed on Nkx2.5 null embryos (Nkx2.5(-/-)). Furthermore, re-expression of Nkx2.5 in the mesoderm rescued Nkx2.5(-/-) heart defects. Our findings reveal that Nkx2.5 in the mesoderm is essential while endodermal expression is dispensable for early heart formation in mammals.

The sonic hedgehog-GLI1 signaling pathway in brain tumor development

INTRODUCTION

The sonic hedgehog (Shh) pathway is a regulatory network involved in development and cancer. Proteins like Ptch, SMO, and Gli are central to the Shh pathway. Other proteins like HHIP, SUFU, Bmi-1, Cyclin D2, Plakoglobin, PAX6, Nkx2.2, and SFRP1 are not so well understood in Shh regulation as Gli-1 downstream target genes.

AREAS COVERED

In this review we try to explain the Shh pathway components and their role in development and cancer, mainly of the brain. A summary of each of the proteins is presented together with an overview of their involvement in cancer.

EXPERT OPINION

Genetic alterations of the Shh pathway have been detected in cancer stem cells, a subgroup of tumor cells implicated in the origin and maintenance of tumors, being responsible for cancer recurrence and chemotherapy resistance. Cancer stem cells constitute a novel target for biomedical researchers. Specifically, the Shh pathway is being explored as a new opportunity for targeted therapies against tumors. Therefore, a better knowledge of every of the regulators of the Shh pathway is needed.

Motor axon exit from the mammalian spinal cord is controlled by the homeodomain protein Nkx2.9 via Robo-Slit signaling

Mammalian motor circuits control voluntary movements by transmitting signals from the central nervous system (CNS) to muscle targets. To form these circuits, motor neurons (MNs) must extend their axons out of the CNS. Although exit from the CNS is an indispensable phase of motor axon pathfinding, the underlying molecular mechanisms remain obscure. Here, we present the first identification of a genetic pathway that regulates motor axon exit from the vertebrate spinal cord, utilizing spinal accessory motor neurons (SACMNs) as a model system. SACMNs are a homogeneous population of spinal MNs with axons that leave the CNS through a discrete lateral exit point (LEP) and can be visualized by the expression of the cell surface protein BEN. We show that the homeodomain transcription factor Nkx2.9 is selectively required for SACMN axon exit and identify the Robo2 guidance receptor as a likely downstream effector of Nkx2.9; loss of Nkx2.9 leads to a reduction in Robo2 mRNA and protein within SACMNs and SACMN axons fail to exit the spinal cord in Robo2-deficient mice. Consistent with short-range interactions between Robo2 and Slit ligands regulating SACMN axon exit, Robo2-expressing SACMN axons normally navigate through LEP-associated Slits as they emerge from the spinal cord, and fail to exit in Slit-deficient mice. Our studies support the view that Nkx2.9 controls SACMN axon exit from the mammalian spinal cord by regulating Robo-Slit signaling.

Crossing the border: molecular control of motor axon exit

Living organisms heavily rely on the function of motor circuits for their survival and for adapting to ever-changing environments. Unique among central nervous system (CNS) neurons, motor neurons (MNs) project their axons out of the CNS. Once in the periphery, motor axons navigate along highly stereotyped trajectories, often at considerable distances from their cell bodies, to innervate appropriate muscle targets. A key decision made by pathfinding motor axons is whether to exit the CNS through dorsal or ventral motor exit points (MEPs). In contrast to the major advances made in understanding the mechanisms that regulate the specification of MN subtypes and the innervation of limb muscles, remarkably little is known about how MN axons project out of the CNS. Nevertheless, a limited number of studies, mainly in Drosophila, have identified transcription factors, and in some cases candidate downstream effector molecules, that are required for motor axons to exit the spinal cord. Notably, specialized neural crest cell derivatives, referred to as Boundary Cap (BC) cells, pre-figure and demarcate MEPs in vertebrates. Surprisingly, however, BC cells are not required for MN axon exit, but rather restrict MN cell bodies from ectopically migrating along their axons out of the CNS. Here, we describe the small set of studies that have addressed motor axon exit in Drosophila and vertebrates, and discuss our fragmentary knowledge of the mechanisms, which guide motor axons out of the CNS.

Transitional Nerve: A New and Original Classification of a Peripheral Nerve Supported by the Nature of the Accessory Nerve (CN XI)

Classically, the accessory nerve is described as having a cranial and a spinal root. Textbooks are inconsistent with regard to the modality of the spinal root of the accessory nerve. Some authors report the spinal root as general somatic efferent (GSE), while others list a special visceral efferent (SVE) modality. We investigated the comparative, anatomical, embryological, and molecular literature to determine which modality of the accessory nerve was accurate and why a discrepancy exists. We traced the origin of the incongruity to the writings of early comparative anatomists who believed the accessory nerve was either branchial or somatic depending on the origin of its target musculature. Both theories were supported entirely by empirical observations of anatomical and embryological dissections. We find ample evidence including very recent molecular experiments to show the cranial and spinal root are separate entities. Furthermore, we determined the modality of the spinal root is neither GSE or SVE, but a unique peripheral nerve with a distinct modality. We propose a new classification of the accessory nerve as a transitional nerve, which demonstrates characteristics of both spinal and cranial nerves.

Embryonic stem cell-derived hemangioblasts remain epigenetically plastic and require PRC1 to prevent neural gene expression

Many lineage-specific developmental regulator genes are transcriptionally primed in embryonic stem (ES) cells; RNA Pol(II) is bound at their promoters but is prevented from productive elongation by the activity of polycomb repressive complexes (PRC) 1 and 2. This epigenetically poised state is thought to enable ES cells to rapidly execute multiple differentiation programs and is recognized by a simultaneous enrichment for trimethylation of lysine 4 and trimethylation of lysine 27 of histone H3 (bivalent chromatin) across promoter regions. Here we show that the chromatin profile of this important cohort of genes is progressively modified as ES cells differentiate toward blood-forming precursors. Surprisingly however, neural specifying genes, such as Nkx2-2, Nkx2-9, and Sox1, remain bivalent and primed even in committed hemangioblasts, as conditional deletion of PRC1 results in overt and inappropriate expression of neural genes in hemangioblasts. These data reinforce the importance of PRC1 for normal hematopoietic differentiation and reveal an unexpected epigenetic plasticity of mesoderm-committed hemangioblasts.

The transcription factors Nkx2.2 and Nkx2.9 play a novel role in floor plate development and commissural axon guidance

The transcription factors Nkx2.2 and Nkx2.9 have been proposed to execute partially overlapping functions in neuronal patterning of the ventral spinal cord in response to graded sonic hedgehog signaling. The present report shows that in mice lacking both Nkx2 proteins, the presumptive progenitor cells in the p3 domain of the neural tube convert to motor neurons (MN) and never acquire the fate of V3 interneurons. This result supports the concept that Nkx2 transcription factors are required to establish V3 progenitor cells by repressing the early MN lineage-specific program, including genes like Olig2. Nkx2.2 and Nkx2.9 proteins also perform an additional, hitherto unknown, function in the development of non-neuronal floor plate cells. Here, we demonstrate that loss of both Nkx2 genes results in an anatomically smaller and functionally impaired floor plate causing severe defects in axonal pathfinding of commissural neurons. Defective floor plates were also seen in Nkx2.2(+/-);Nkx2.9(-/-) compound mutants and even in single Nkx2.9(-/-) mutants, suggesting that floor plate development is sensitive to dose and/or timing of Nkx2 expression. Interestingly, adult Nkx2.2(+/-);Nkx2.9(-/-) compound-mutant mice exhibit abnormal locomotion, including a permanent or intermittent hopping gait. Drug-induced locomotor-like activity in spinal cords of mutant neonates is also affected, demonstrating increased variability of left-right and flexor-extensor coordination. Our data argue that the Nkx2.2 and Nkx2.9 transcription factors contribute crucially to the formation of neuronal networks that function as central pattern generators for locomotor activity in the spinal cord. As both factors affect floor plate development, control of commissural axon trajectories might be the underlying mechanism.

Kinesin superfamily-associated protein 3 is preferentially expressed in glutamatergic neurons and contributes to the excitatory control of female puberty

It was earlier shown that expression of kinesin superfamily-associated protein 3 (KAP3), involved in the neuronal anterograde, microtubule-dependent transport of membrane organelles, increases in the hypothalamus of female rats during the juvenile phase of sexual development. KAP3 mRNA is abundant in the hypothalamus, suggesting that it might be expressed in broadly disseminated neuronal systems controlling neuroendocrine function. The present study identifies one of these systems and provides evidence for an involvement of KAP3 in the excitatory control of female puberty. In situ hybridization and immunohistofluorescence studies revealed that the KAP3 gene is expressed in glutamatergic neurons but not in GABAergic or GnRH neurons. Hypothalamic KAP3 mRNA levels increase during the juvenile period of female prepubertal development, remaining elevated throughout puberty. These changes appear to be, at least in part, estradiol dependent because ovariectomy decreases and estradiol increases KAP3 mRNA abundance. Lowering hypothalamic KAP3 protein levels via intraventricular administration of an antisense oligodeoxynucleotide resulted in reduced release of both glutamate and GnRH from the median eminence and delayed the onset of puberty. The median eminence content of vesicular glutamate transporter 2, a glutamate neuron-selective synaptic protein, and synaptophysin, a synaptic vesicle marker, were also reduced, suggesting that the loss of KAP3 diminishes the anterograde transport of these proteins. Altogether, these results support the view that decreased KAP3 synthesis diminishes GnRH output and delays female sexual development by compromising hypothalamic release of glutamate.

Distinct roles of lymphotoxin-β signaling and the homeodomain transcription factor Nkx2.3 in the ontogeny of endothelial compartments in spleen

The formation of peripheral lymphoid tissues is indispensable for the efficient recognition and elimination of external antigens by lymphoid and accessory cells of the adaptive immune system. The spleen is structurally arranged around various vascular beds with distinct endothelial phenotypes. Using immunohistochemistry, we investigated the postnatal developmental characteristics of the marginal sinus and its relationship with various red-pulp sinus subsets. We also determined the importance of the lymphotoxin beta receptor (LT beta R) and the role of the Nkx2.3 transcription factor for the formation of the splenic vasculature. Both the administration of soluble LT beta R-Ig fusion protein to neonates and the deletion of LT beta R or downstream signaling components (RelB and p52) of the NF-kappaB family inhibited the phenotypic maturation of marginal sinus but had no effect on the vascular compartmentalization of the red pulp. The integrity of the marginal sinus and the proper vascular segregation of the red pulp appeared to be controlled by Nkx2.3, as Nkx2.3-deficient mice exhibited an abnormal distribution of IBL-7/1(hi)/IBL-9/2(-) sinuses and a lack of IBL-7/1(lo)/IBL-9/2(+) vessels. Our data suggest that phenotypic heterogeneity among different vascular elements within distinct anatomical regions of the spleen differentially depends on developmental factors such as lymphotoxin signaling or Nkx2.3, whereas the marginal sinus is controlled by both pathways.

Gene expression patterns in primary neuronal clusters of the Drosophila embryonic brain

The brain of Drosophila is formed by approximately 100 lineages, each lineage being derived from a stem cell-like neuroblast that segregates from the procephalic neurectoderm of the early embryo. A neuroblast map has been established in great detail for the early embryo, and a suite of molecular markers has been defined for all neuroblasts included in this map [Urbach, R., Technau, G.M. (2003a) Molecular markers for identified neuroblasts in the developing brain of Drosophila. Development 130, 3621-3637]. However, the expression of these markers was not followed into later embryonic or larval stages, mainly due to the fact that anatomical landmarks to which expression patterns could be related had not been defined. Such markers, in the form of stereotyped clusters of neurons whose axons project along cohesive bundles ("primary axon bundles" or "PABs") are now available [Younossi-Hartenstein, A., Nguyen, B., Shy, D., Hartenstein, V. 2006. Embryonic origin of the Drosophila brain neuropile. J. Comp. Neurol. 497, 981-998]. In the present study we have mapped the expression of molecular markers in relationship to primary neuronal clusters and their PABs. The markers we analyzed include many of the genes involved in patterning of the brain along the anteroposterior axis (cephalic gap genes, segment polarity genes) and dorso-ventral axis (columnar patterning genes), as well as genes expressed in the dorsal protocerebrum and visual system (early eye genes). Our analysis represents an important step along the way to identify neuronal lineages of the mature brain with genes expressed in the early embryo in discrete neuroblasts. Furthermore, the analysis helped us to reconstruct the morphogenetic movements that transform the two-dimensional neuroblast layer of the early embryo into the three-dimensional larval brain and provides the basis for deeper understanding of how the embryonic brain develops.

Cloning of zebrafish nkx6.2 and a comprehensive analysis of the conserved transcriptional response to Hedgehog/Gli signaling in the zebrafish neural tube

Sonic Hedgehog (Shh) signaling helps pattern the vertebrate neural tube, in part by regulating the dorsal/ventral expression of a number of homeodomain containing transcription factors. These Hh responsive genes have been divided into two classes, with Class II genes being activated by Hh signaling and Class I genes being repressed by Hh signaling. While the transcriptional response to varying Hh levels is well defined in chick and mouse, it is only partially described in zebrafish, despite the fact that zebrafish has emerged as a powerful genetic system for the study of neural patterning. To better characterize the Hh response in the zebrafish neural tube, we cloned the zebrafish Class II Hh target genes nkx2.9 and nkx6.2. We then analyzed the expression of a number of Class I and Class II Hh responsive genes in wild type, Hh mutant, and Hh over-expressing zebrafish embryos. We show that expression of Class I and Class II genes is highly conserved in the vertebrate neural tube. Further, ventral-most Class II gene expression was completely lost in all Hh pathway mutants analyzed, indicating high levels of Hh signaling are blocked in all of these mutants. In contrast, more dorsally expressed genes were variably affected in different Hh pathway mutants, indicating mid-levels of Hh signaling are differentially affected. This comprehensive expression study provides an important tool for the characterization of Hh signaling in zebrafish and provides a sensitive assay for determining the degree to which newly identified zebrafish mutants affect Hh signaling.

Loss of Nkx2.8 deregulates progenitor cells in the large airways and leads to dysplasia

Nkx2.8, a homeodomain transcription factor, has been characterized in liver cancer and in the developing central nervous system. We now show that this factor is also expressed in the lung, where it localizes in adults to a discrete population of tracheobronchial basal cells. To target the mouse gene, the first exon was replaced by a LacZ marker gene joined to the intact 5'-untranslated region. Marker expression was observed throughout the lower respiratory tract, beginning on E11 in a few cells of the distal lung buds. The region of expression then spread upward. By neonatal day 1, expression was greatest in the large airways and the Nkx2.8-/- mice exhibited generalized tracheobronchial hyperplasia. Bromodeoxyuridine (BrdUrd) labeling studies showed that a higher rate of bronchial cell proliferation persisted at 6 to 8 months. In adults, Nkx2.8 marker expression decreased with progressive differentiation into ciliated and secretory cells. The cell localizations and patterns of coexpression with BrdUrd and differentiation markers suggest a progenitor relationship: the cells that most strongly express Nkx2.8 seem to function as tracheobronchial stem cells. Moreover, Nkx2.8 acts to limit the number of these progenitor cells because the marker-expressing population was greatly expanded in Nkx2.8-/- mice. Increased proliferation and an altered progenitor relationship caused progressive bronchial pathology, which manifested as widespread dysplasia in the large airways of 1-year-old Nkx2.8-/- mice.

Discontinuous organization and specification of the lateral floor plate in zebrafish

The floor plate is a signaling center in the ventral neural tube of vertebrates with important functions during neural patterning and axon guidance. It is composed of a centrally located medial floor plate (MFP) and a bilaterally positioned lateral floor plate (LFP). While the role of the MFP as source of signaling molecules like, e.g., Sonic Hedgehog (Shh) is well understood, the exact organization and function of the LFP are currently unclear. Based on expression analyses, the one cell wide LFP in zebrafish has been postulated to be a homogenous structure. We instead show that the zebrafish trunk LFP is discontinuously arranged. Single LFP cells alternate with p3 neuronal precursor cells, which develop V3 interneurons along the anteroposterior (AP) axis. Our mutant analyses indicate that both, formation of LFP and p3 cells require Delta-Notch signaling. Importantly, however, the two cell types are differentially regulated by Hedgehog (HH) and Nkx2.2 activities. This implicates a novel mechanism of neural tube patterning, in which distinct cell populations within one domain of the ventral neural tube are differently specified along the AP axis. We conclude that different levels of HH and Nkx2.2 activities are responsible for the alternating appearance of LFP and p3 neuronal progenitor cells in the zebrafish ventral neural tube.

Genomewide expression profiling in the zebrafish embryo identifies target genes regulated by Hedgehog signaling during vertebrate development

Hedgehog proteins play critical roles in organizing the embryonic development of animals, largely through modulation of target gene expression. Little is currently known, however, about the kinds and numbers of genes whose expression is controlled, directly or indirectly, by Hedgehog activity. Using techniques to globally repress or activate Hedgehog signaling in zebrafish embryos followed by microarray-based expression profiling, we have discovered a cohort of genes whose expression responds significantly to loss or gain of Hedgehog function. We have confirmed the Hedgehog responsiveness of a representative set of these genes with whole-mount in situ hybridization as well as real time PCR. In addition, we show that the consensus Gli-binding motif is enriched within the putative regulatory elements of a sizeable proportion of genes that showed positive regulation in our assay, indicating that their expression is directly induced by Hedgehog. Finally, we provide evidence that the Hedgehog-dependent spatially restricted transcription of one such gene, nkx2.9, is indeed mediated by Gli1 through a single Gli recognition site located within an evolutionarily conserved enhancer fragment. Taken together, this study represents the first comprehensive survey of target genes regulated by the Hedgehog pathway during vertebrate development. Our data also demonstrate for the first time the functionality of the Gli-binding motif in the control of Hedgehog signaling-induced gene expression in the zebrafish embryo.

Evolution of striated muscle: Jellyfish and the origin of triploblasty

The larval and polyp stages of extant Cnidaria are bi-layered with an absence of mesoderm and its differentiation products. This anatomy originally prompted the diploblast classification of the cnidarian phylum. The medusa stage, or jellyfish, however, has a more complex anatomy characterized by a swimming bell with a well-developed striated muscle layer. Based on developmental histology of the hydrozoan medusa this muscle derives from the entocodon, a mesoderm-like third cell layer established at the onset of medusa formation. According to recent molecular studies cnidarian homologs to bilaterian mesoderm and myogenic regulators are expressed in the larval and polyp stages as well as in the entocodon and derived striated muscle. Moreover striated and smooth muscle cells may have evolved directly and independently from non-muscle cells as indicated by phylogenetic analysis of myosin heavy chain genes (MHC class II). To accommodate all evidences we propose that striated muscle-based locomotion coevolved with the nervous and digestive systems in a basic metazoan Bauplan from which the ancestors of the Ctenophora (comb jellyfish), Cnidaria (jellyfish and polyps), as well as the Bilateria are derived. We argue for a motile tri-layered cnidarian ancestor and a monophyletic descent of striated muscle in Cnidaria and Bilateria. As a consequence, diploblasty evolved secondarily in cnidarian larvae and polyps.

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.

Ascl1/Mash1 is required for the development of central serotonergic neurons

The transcriptional control of the differentiation of central serotonergic (5-HT) neurons in vertebrates has recently come under scrutiny and has been shown to involve the homeobox genes Nkx2-2 and Lmx1b, the Ets-domain gene Pet1 (also known as Fev) and the zinc-finger gene Gata3. The basic helix-loop-helix (bHLH) gene Ascl1 (also known as Mash1) is coexpressed with Nkx2-2 in the neuroepithelial domain of the hindbrain, which gives rise to 5-HT neurons. Here we show in the mouse that Ascl1 is essential for the birth of 5-HT neurons, both as a proneural gene for the production of postmitotic neuronal precursors and as a determinant of the serotonergic phenotype for the parallel activation of Gata3, Lmx1b and Pet1. Thus Ascl1, which is essential for noradrenergic differentiation, is also a determinant of the serotonergic phenotype.

Retinoic acid and the control of dorsoventral patterning in the avian spinal cord

The development of neural subtypes in the dorsoventral (DV) axis of the vertebrate central nervous system (CNS) involves the integration of signalling pathways coupled with the combinatorial expression of homeodomain transcription factors. Previous studies have implicated a role for retinoic acid in the specification of a subtype of motor neurons (MN) and in the patterning of a group of interneurons within the ventral spinal cord. In this study, we use the vitamin A-deficient (VAD) quail model to further investigate the role of retinoids in the patterning of the neural tube. Using genetic markers specific to neuronal cell populations, we demonstrate that in the absence of retinoic acid, there is a disruption to the molecular mechanisms associated with the dorsoventral patterning of the spinal cord. In particular, we observe an uneven dorsal expansion of ventral-specific genes, accompanied by a reduction in the domain of roof plate and dorsal patterning genes, both of which are rescued upon addition of retinoids during development. In addition, there is a loss of V1 interneuron-specific gene expression and a decrease in the ventricular zone expression of motor neuron patterning genes. Interestingly, these effects are localised to the rostral half of the spinal cord, indicating that RA is integrated in both anteroposterior (AP) and dorsoventral patterning processes. Using differential display techniques, we have isolated 27 retinoic acid-regulated genes within the spinal cord that together reveal several interesting potential biological functions for retinoids within the avian neural tube. In summary, we propose that retinoids have an essential role in the patterning of the dorsoventral axis of the spinal cord, and are also required for the correct integration of anteroposterior patterning signals with dorsoventral determinants in the rostral spinal cord.

Identification of Cis-regulatory Elements in the Mouse Pax9/Nkx2-9 Genomic Region: Implication for Evolutionary Conserved Synteny

We previously reported close physical linkage between Pax9 and Nkx2-9 in the human, mouse, and pufferfish (Fugu rubripes) genomes. In this study, we analyzed cis-regulatory elements of the two genes by comparative sequencing in the three species and by transgenesis in the mouse. We identified two regions including conserved noncoding sequences that possessed specific enhancer activities for expression of Pax9 in the medial nasal process and of Nkx2-9 in the ventral neural tube. Remarkably, the latter contained the consensus Gli-binding motif. Interestingly, the identified Pax9 cis-regulatory sequences were located in an intron of the neighboring gene Slc25a21. Close examination of an extended genomic interval around Pax9 revealed the presence of strong synteny conservation in the human, mouse, and Fugu genomes. We propose such an intersecting organization of cis-regulatory sequences in multigenic regions as a possible mechanism that maintains evolutionary conserved synteny.

Undulated short-tail Deletion Mutation in the Mouse Ablates Pax1 and Leads to Ectopic Activation of Neighboring Nkx2-2 in Domains That Normally Express Pax1

Previous studies have indicated that the Undulated short-tail deletion mutation in mouse Pax1 (Pax1Un-s) not only ablates Pax1, but also disturbs a gene or genes nearby Pax1. However, which gene(s) is involved and how the Pax1Un-s phenotype is confined to the Pax1-positive tissues remain unknown. In the present study, we determined the Pax1Un-s deletion interval to be 125 kb and characterized genes around Pax1. We show that the Pax1Un-s mutation affects four physically linked genes within or near the deletion, including Pax1, Nkx2-2, and their potential antisense genes. Remarkably, Nkx2-2 is ectopically activated in the sclerotome and limb buds of Pax1Un-s embryos, both of which normally express Pax1. This result suggests that the Pax1Un-s deletion leads to an illegitimate interaction between remotely located Pax1 enhancers and the Nkx2-2 promoter by disrupting an insulation mechanism between Pax1 and Nkx2-2. Furthermore, we show that expression of Bapx1, a downstream target of Pax1, is more strongly affected in Pax1Un-s mutants than in Pax1-null mutants, suggesting that the ectopic expression of Nkx2-2 interferes with the Pax1-Bapx1 pathway. Taken together, we propose that a combination of a loss-of-function mutation of Pax1 and a gain-of-function mutation of Nkx2-2 is the molecular basis of the Pax1Un-s mutation.

Distinct regulators of Shh transcription in the floor plate and notochord indicate separate origins for these tissues in the mouse node

The establishment of the floor plate at the ventral midline of the CNS is dependent on an inductive signaling process mediated by the secreted protein Sonic hedgehog (Shh). To understand molecularly how floor plate induction proceeds we identified a Shh-responsive regulatory element that directs transgene reporter expression to the ventral midline of the CNS and notochord in a Shh-like manner and characterized critical cis-acting sequences regulating this element. Cross-species comparisons narrowed the activity of the Shh floor plate enhancer to an 88-bp sequence within intron 2 of Shh that included highly conserved binding sites matching the consensus for homeodomain, Tbx and Foxa transcription factors. Mutational analysis revealed that the homeodomain and Foxa binding sites are each required for activation of the Shh floor plate enhancer, whereas the Tbx site was required for repression in regions of the CNS where Shh is not normally expressed. We further show that Shh enhancer activity was detected in the mouse node from where the floor plate and notochord precursors derive. Shh reporter expression was restricted to the ventral (mesodermal) layer of the node in a pattern similar to endogenous Shh. X-gal-positive cells emerging from the node were only detected in the notochord lineage, suggesting that the floor plate and notochord arise from distinct precursors in the mouse node.

Rearrangement in the PITX2 and MIPOL1 genes in a patient with a t(4;14) chromosome

We report the molecular characterization of a patient with mild craniofacial and acallosal central nervous system midline defects and a t(4;14)(q25)(q13) chromosome. With the use of flow sorted chromosomes, the translocation breakpoint junction was defined within a 100 kb region with markers mapping to chromosomes 4q25 and 14q13. Analysis of genomic sequences demonstrated that the breakpoint junction at 14q13 was within the third intron of the 5' untranslated region of the MIPOL1 gene (GI: 22048098). On chromosome 4q25, two breakpoint junctions were found. One was about 47 kb distal to the 5' end of a putative gene (GI: 8923996) with unknown function but with partial similarity to kinases, and a second breakpoint was within the 3' end of the PITX2 gene (GI: 21361182) that resulted in the deletion of exons 6 and 7 of this gene. We also searched for microdeletions in a panel of candidate genes mapping within 2 Mb of the translocation breakpoint junction on chromosomes 4 and 14, however, no evidence for deletions or rearrangements was found. The finding of two breaks on chromosome 4q25 suggests a complex microrearrangement, such as an inversion, in addition to a translocation in this patient.

Targeted disruption of the homeobox gene Nkx2.9 reveals a role in development of the spinal accessory nerve

The homeodomain-containing transcription factor Nkx2.9 is expressed in the ventralmost neural progenitor domain of the neural tube together with the related protein Nkx2.2 during early mouse embryogenesis. Cells within this region give rise to V3 interneurons and visceral motoneurons in spinal cord and hindbrain, respectively. To investigate the role of the Nkx2.9 gene, we generated a mutant mouse by targeted gene disruption. Homozygous mutant animals lacking Nkx2.9 were viable and fertile with no apparent morphological or behavioral phenotype. The distribution of neuronal progenitor cells and differentiated neurons in spinal cord was unaffected in Nkx2.9-deficient animals. This finding is in contrast to Nkx2.2-null mutants, which have been shown to exhibit ventral to dorsal transformation of neuronal cell fates in spinal cord. Our results suggest that specification of V3 interneurons in the posterior CNS does not require Nkx2.9, most probably because of functional redundancy with the co-expressed Nkx2.2 protein. In hindbrain, however, absence of Nkx2.9 resulted in a significantly altered morphology of the spinal accessory nerve (XIth), which appeared considerably shorter and thinner than in wild-type animals. Consistent with this phenotype, immature branchial motoneurons of the spinal accessory nerve, which normally migrate from a ventromedial to a dorsolateral position within the neural tube, were markedly reduced in Nkx2.9-deficient embryos at E10.5, while ventromedial motor column cells were increased in numbers. In addition, the vagal and glossopharyngeal nerves appeared abnormal in approximately 50% of mutant embryos, which may be related to the observed reduction of Phox2b expression in the nucleus ambiguus of adult mutant mice. From these observations, we conclude that Nkx2.9 has a specific function in the hindbrain as determinant of the branchial motoneuron precursor cells for the spinal accessory nerve and possibly other nerves of the branchial-motor column. Like other Nkx genes expressed in the CNS, Nkx2.9 seems to be involved in converting positional information into cell fate decisions.

Transcription factors in cardiogenesis: the combinations that unlock the mysteries of the heart

Heart formation is one of the first signs of organogenesis within the developing embryo and this process is conserved from flies to man. Completing the genetic roadmap of the molecular mechanisms that control the cell specification and differentiation of cells that form the developing heart has been an exciting and fast-moving area of research in the fields of molecular and developmental biology. At the core of these studies is an interest in the transcription factors that are responsible for initiation of a pluripotent cell to become programmed to the cardiac lineage and the subsequent transcription factors that implement the instructions set up by the cells commitment decision. To gain a better understanding of these pathways, cardiac-expressed transcription factors have been identified, cloned, overexpressed, and mutated to try to determine function. Although results vary depending on the gene in question, it is clear that there is a striking evolutionary conservation of the cardiogenic program among species. As we move up the evolutionary ladder toward man, we encounter cases of functional redundancy and combinatorial interactions that reflect the complex networks of gene expression that orchestrate heart development. This review focuses on what is known about the transcription factors implicated in heart formation and the role they play in this intricate genetic program.

Sequence-specific DNA binding by the vnd/NK-2 homeodomain of Drosophila

The ventral nervous system defective (vnd)/NK-2 homeodomain and some flanking amino acid residues were expressed in Escherichia coli, purified to homogeneity, and the protein was covalently coupled to Sepharose. Oligodeoxynucleotides that contained 16-bp random sequences were purified by vnd/NK-2 affinity column chromatography, cloned, and sequenced. The consensus nucleotide sequence of the vnd/NK-2 homeodomain binding site was shown to be T(T/C)AAGTG(G/C). The apparent equilibrium dissociation constant (K(D)) of the vnd/NK-2 homeodomain for the consensus sequence is 1.9 x 10(-10) M. In addition, results of competition between oligodeoxynucleotides for binding to the vnd/NK-2 homeodomain and determination of the apparent K(D) values of oligodeoxynucleotides that differ from the consensus sequence by only a single base pair demonstrate that the four central nucleotides, AAGT, in this sequence play a major role in determining the affinity of binding.

Formation of neuroblasts in the embryonic central nervous system of Drosophila melanogaster is controlled by SoxNeuro

Sox proteins form a family of HMG-box transcription factors related to the mammalian testis determining factor SRY. Sox-mediated modulation of gene expression plays an important role in various developmental contexts. Drosophila SoxNeuro, a putative ortholog of the vertebrate Sox1, Sox2 and Sox3 proteins, is one of the earliest transcription factors to be expressed pan-neuroectodermally. We demonstrate that SoxNeuro is essential for the formation of the neural progenitor cells in central nervous system. We show that loss of function mutations of SoxNeuro are associated with a spatially restricted hypoplasia: neuroblast formation is severely affected in the lateral and intermediate regions of the central nervous system, whereas ventral neuroblast formation is almost normal. We present evidence that a requirement for SoxNeuro in ventral neuroblast formation is masked by a functional redundancy with Dichaete, a second Sox protein whose expression partially overlaps that of SoxNeuro. Genetic interactions of SoxNeuro and the dorsoventral patterning genes ventral nerve chord defective and intermediate neuroblasts defective underlie ventral and intermediate neuroblast formation. Finally, the expression of the Achaete-Scute gene complex suggests that SoxNeuro acts upstream and in parallel with the proneural genes.

The Sox-domain containing geneDichaete/fish-hookacts in concert withvndandindto regulate cell fate in theDrosophilaneuroectoderm

In the Drosophila embryonic central nervous system, neural stem cells, called neuroblasts, acquire fates in a position-specific manner. Recent work has identified a set of genes that functions along the dorsoventral axis to enable neuroblasts that develop in different dorsoventral domains to acquire distinct fates. These genes include the evolutionarily conserved transcription factors ventral nerve cord defective and intermediate neuroblasts defective, as well as the Drosophila EGF receptor. We show that the Sox-domain-containing gene Dichaete/fish-hook also plays a crucial role to pattern the neuroectoderm along the DV axis. Dichaete is expressed in the medial and intermediate columns of the neuroectoderm, and mutant analysis indicates that Dichaete regulates cell fate and neuroblast formation in these domains. Molecular epistasis tests, double mutant analysis and dosage-sensitive interactions demonstrate that during these processes, Dichaete functions in parallel with ventral nerve cord defective and intermediate neuroblasts defective, and downstream of EGF receptor signaling to mediate its effect on development. These results identify Dichaete as an important regulator of dorsoventral pattern in the neuroectoderm, and indicate that Dichaete acts in concert with ventral nerve cord defective and intermediate neuroblasts defective to regulate pattern and cell fate in the neuroectoderm.

Mice lacking the Nkx6.2 (Gtx) homeodomain transcription factor develop and reproduce normally

The Nkx homeobox genes are expressed in a variety of developing tissues and have been implicated in controlling tissue patterning and cell differentiation. Expression of Nkx6.2 (Gtx) was previously observed in the embryonic neural tube, testis, and differentiating oligodendrocytes. To investigate the role of Nkx6.2 in the control of cell specification and differentiation, we generated mice with null mutations in Nkx6.2 using the standard gene targeting approach. Null mutant mice were viable and fertile without apparent histological and immunohistochemical changes in the central nervous systems and testis. The absence of detectable phenotypes suggests a redundant function of Nkx6.2 in mouse development.

Homeobox gene mutations and brain-stem developmental disorders: learning from knockout mice

Analysis of mice that carry targeted inactivations of Hox, Nkx and Phox2 homeobox genes revealed their involvement in regional patterning of brain-stem territories, in specification of neuronal identity, in establishment of appropriate patterns of connectivity and in control of neurotransmission. The specific abnormalities generated by such mutations may provide clues to the genetic basis and cellular mechanisms that are involved in human brain-stem developmental disorders.

TTF-1, a homeodomain gene required for diencephalic morphogenesis, is postnatally expressed in the neuroendocrine brain in a developmentally regulated and cell-specific fashion

TTF-1 is a member of the Nkx family of homeodomain genes required for morphogenesis of the hypothalamus. Whether TTF-1, or other Nkx genes, contributes to regulating differentiated hypothalamic functions is not known. We now report that postnatal hypothalamic TTF-1 expression is developmentally regulated and associated with the neuroendocrine process of female sexual development. Lesions of the hypothalamus that cause sexual precocity transiently activate neuronal TTF-1 expression near the lesion site. In intact animals, hypothalamic TTF-1 mRNA content also increases transiently, preceding the initiation of puberty. Postnatal expression of the TTF-1 gene was limited to subsets of hypothalamic neurons, including LHRH neurons, which control sexual maturation, and preproenkephalinergic neurons of the lateroventromedial nucleus of the basal hypothalamus, which restrain sexual maturation and facilitate reproductive behavior. TTF-1 mRNA was also detected in astrocytes of the median eminence and ependymal/subependymal cells of the third ventricle, where it colocalized with erbB-2, a receptor involved in facilitating sexual development. TTF-1 binds to and transactivates the erbB-2 and LHRH promoters, but represses transcription of the preproenkephalin gene. The singular increase in hypothalamic TTF-1 gene expression that precedes the initiation of puberty, its highly specific pattern of cellular expression, and its transcriptional actions on genes directly involved in neuroendocrine reproductive regulation suggest that TTF-1 may represent one of the controlling factors that set in motion early events underlying the central activation of mammalian puberty.

Ventral neural patterning by Nkx homeobox genes: Nkx6.1 controls somatic motor neuron and ventral interneuron fates

There is growing evidence that sonic hedgehog (Shh) signaling regulates ventral neuronal fate in the vertebrate central nervous system through Nkx-class homeodomain proteins. We have examined the patterns of neurogenesis in mice carrying a targeted mutation in Nkx6.1. These mutants show a dorsal-to-ventral switch in the identity of progenitors and in the fate of postmitotic neurons. At many axial levels there is a complete block in the generation of V2 interneurons and motor neurons and a compensatory ventral expansion in the domain of generation of V1 neurons, demonstrating the essential functions of Nkx6.1 in regional patterning and neuronal fate determination.

A homeodomain protein code specifies progenitor cell identity and neuronal fate in the ventral neural tube

Distinct classes of neurons are generated at defined positions in the ventral neural tube in response to a gradient of Sonic Hedgehog (Shh) activity. A set of homeodomain transcription factors expressed by neural progenitors act as intermediaries in Shh-dependent neural patterning. These homeodomain factors fall into two classes: class I proteins are repressed by Shh and class II proteins require Shh signaling for their expression. The profile of class I and class II protein expression defines five progenitor domains, each of which generates a distinct class of postmitotic neurons. Cross-repressive interactions between class I and class II proteins appear to refine and maintain these progenitor domains. The combinatorial expression of three of these proteins--Nkx6.1, Nkx2.2, and Irx3--specifies the identity of three classes of neurons generated in the ventral third of the neural tube.

Vnd/nkx, ind/gsh, and msh/msx: conserved regulators of dorsoventral neural patterning?

Expression of vnd in ventral, ind in intermediate, and msh in dorsal columns of fly neurectoderm, and of homologous gene families in corresponding domains of vertebrate neurectoderm, suggests that elements of dorsoventral neural patterning have been evolutionarily conserved. However, upstream signaling pathways regulating this columnar gene expression pattern appear to have diverged significantly throughout evolution. In addition, while recent loss-of-function studies in flies and mice indicate that these three genes may have a conserved role in regional specification, there is no obvious conservation of the particular cell fates deriving from corresponding domains. The three-column expression pattern may thus represent a developmental mechanism that is more resistant to evolutionary changes than genetic events upstream or downstream of it.

Expression and regulation of the chickenNkx-6.2 homeobox gene suggest its possible involvement in the ventral neural patterning and cell fate specification

Rapid accumulating evidence has suggested that the homeodomain transcription factors of the Nkx family play important roles in controlling vertebrate organ patterning and differentiation. In this study, we report the cloning, expression and regulation of a novel chicken homeobox gene, cNkx-6.2, whose expression is similar, but not identical, to that of mouse Nkx-6.2. The earliest expression of cNkx-6.2 was detected at the neural plate stage in the prospective midbrain and hindbrain regions. As the neural development proceeds, cNkx-6.2 expression was restricted in the ventral region of the entire neural axis except the forebrain region. At late stages of development, cNkx-6.2 expression is downregulated in the ventricular neuroepithelial cells, but subsequently upregulated in a sub-population of cells. Tissue recombination and explant culture experiments demonstrated that expression of cNkx-6.2 can be induced by the notochord signal and purified SHH protein, and repressed by BMP-4 and -7, indicating that the cNkx-6.2 expression can be influenced by both ventral and dorsal midline signals. Taken together, these studies have suggested two different roles for the cNkx-6.2 transcription factor: participating in the Shh-initiated ventral patterning during early CNS development and controlling cell fate specification and differentiation during late development.

At the nexus between pattern formation and cell-type specification: the generation of individual neuroblast fates in the Drosophila embryonic central nervous system

The specification of specific and often unique fates to individual cells as a function of their position within a developing organism is a fundamental process during the development of multicellular organisms. The development of the Drosophila embryonic central nervous system serves as an excellent model system in which to clarify the developmental mechanisms that link pattern formation to cell-type specification. The Drosophila embryonic central nervous system develops from a set of neural stem cells termed neuroblasts. Neuroblasts arise from the ectoderm in an invariant pattern, and each neuroblast acquires a unique fate based on its position within this pattern. Two groups of genes recently have been demonstrated to govern the individual fate specification of neuroblasts. One group, the segment polarity genes, enables neuroblasts that develop in different anteroposterior positions to acquire different fates. The second group, referred to as the columnar genes, ensures that neuroblasts that develop in different dorsoventral domains assume different fates. When integrated, the activities of the segment polarity and columnar genes create a Cartesian coordinate system that bestows unique fates to individual neuroblasts as a function of their position of formation within the ectoderm. BioEssays 1999;21:922-931.

Loss of Nkx2.1 homeobox gene function results in a ventral to dorsal molecular respecification within the basal telencephalon: evidence for a transformation of the pallidum into the striatum

The telencephalon is organized into distinct longitudinal domains: the cerebral cortex and the basal ganglia. The basal ganglia primarily consists of a dorsal region (striatum) and a ventral region (pallidum). Within the telencephalon, the anlage of the pallidum expresses the Nkx2.1 homeobox gene. A mouse deficient in Nkx2.1 function does not form pallidal structures, lacks basal forebrain TrkA-positive neurons (probable cholinergic neurons) and has reduced numbers of cortical cells expressing GABA, DLX2 and calbindin that migrate from the pallidum through the striatum and into the cortex. We present evidence that these phenotypes result from a ventral-to-dorsal transformation of the pallidal primordium into a striatal-like anlage.

Homeobox gene Nkx2.2 and specification of neuronal identity by graded Sonic hedgehog signalling

During vertebrate development, the specification of distinct cell types is thought to be controlled by inductive signals acting at different concentration thresholds. The degree of receptor activation in response to these signals is a known determinant of cell fate, but the later steps at which graded signals are converted into all-or-none distinctions in cell identity remain poorly resolved. In the ventral neural tube, motor neuron and interneuron generation depends on the graded activity of the signalling protein Sonic hedgehog (Shh). These neuronal subtypes derive from distinct progenitor cell populations that express the homeodomain proteins Nkx2.2 or Pax6 in response to graded Shh signalling. In mice lacking Pax6, progenitor cells generate neurons characteristic of exposure to greater Shh activity. However, Nkx2.2 expression expands dosally in Pax6 mutants, raising the possibility that Pax6 controls neuronal pattern indirectly. Here we provide evidence that Nkx2.2 has a primary role in ventral neuronal patterning. In Nkx2.2 mutants, Pax6 expression is unchanged but cells undergo a ventral-to-dorsal transformation in fate and generate motor neurons rather than interneurons. Thus, Nkx2.2 has an essential role in interpreting graded Shh signals and selecting neuronal identity.