The role of the teashirt gene in trunk segmental identity in Drosophila

Triggering and modulation of a complex behavior by a single peptidergic command neuron in Drosophila

At the end of their growth phase, Drosophila larvae remodel their bodies, glue themselves to a substrate, and harden their cuticle in preparation for metamorphosis. This process-termed pupariation-is triggered by a surge in the hormone ecdysone. Substrate attachment is achieved by a pupariation subprogram called glue expulsion and spreading behavior (GSB). An epidermis-to-CNS Dilp8-Lgr3 relaxin signaling event that occurs downstream of ecdysone is critical for unlocking progression of the pupariation motor program toward GSB, but the factors and circuits acting downstream of Lgr3 signaling remain unknown. Here, using cell-type-specific RNA interference and behavioral monitoring, we identify Myoinhibiting peptide (Mip) as a neuromodulator of multiple GSB action components, such as tetanic contraction, peristaltic contraction alternation, and head-waving. Mip is required in a pair of brain descending neurons, which act temporally downstream of Dilp8-Lgr3 signaling. Mip modulates GSB via ventral nerve cord neurons expressing its conserved receptor, sex peptide receptor (SPR). Silencing of Mip descending neurons by hyperpolarization completely abrogates GSB, while their optogenetic activation at a restricted competence time window triggers GSB-like behavior. Hence, Mip descending neurons have at least two functions: to act as GSB command neurons and to secrete Mip to modulate GSB action components. Our results provide insight into conserved aspects of Mip-SPR signaling in animals, reveal the complexity of GSB control, and contribute to the understanding of how multistep innate behaviors are coordinated in time and with other developmental processes through command neurons and neuropeptidergic signaling.

Chromatin gene-gene loops support the cross-regulation of genes with related function

Chromatin loops between gene pairs have been observed in diverse contexts in both flies and vertebrates. Combining high-resolution Capture-C, DNA fluorescence in situ hybridization, and genetic perturbations, we dissect the functional role of three loops between genes with related function during Drosophila embryogenesis. By mutating the loop anchor (but not the gene) or the gene (but not loop anchor), we disentangle loop formation and gene expression and show that the 3D proximity of paralogous gene loci supports their co-regulation. Breaking the loop leads to either an attenuation or enhancement of expression and perturbs their relative levels of expression and cross-regulation. Although many loops appear constitutive across embryogenesis, their function can change in different developmental contexts. Taken together, our results indicate that chromatin gene-gene loops act as architectural scaffolds that can be used in different ways in different contexts to fine-tune the coordinated expression of genes with related functions and sustain their cross-regulation.

Senataxin helicase, the causal gene defect in ALS4, is a significant modifier of C9orf72 ALS G4C2 and arginine-containing dipeptide repeat toxicity

Identifying genetic modifiers of familial amyotrophic lateral sclerosis (ALS) may reveal targets for therapeutic modulation with potential application to sporadic ALS. GGGGCC (G4C2) repeat expansions in the C9orf72 gene underlie the most common form of familial ALS, and generate toxic arginine-containing dipeptide repeats (DPRs), which interfere with membraneless organelles, such as the nucleolus. Here we considered senataxin (SETX), the genetic cause of ALS4, as a modifier of C9orf72 ALS, because SETX is a nuclear helicase that may regulate RNA-protein interactions involved in ALS dysfunction. After documenting that decreased SETX expression enhances arginine-containing DPR toxicity and C9orf72 repeat expansion toxicity in HEK293 cells and primary neurons, we generated SETX fly lines and evaluated the effect of SETX in flies expressing either (G4C2)58 repeats or glycine-arginine-50 [GR(50)] DPRs. We observed dramatic suppression of disease phenotypes in (G4C2)58 and GR(50) Drosophila models, and detected a striking relocalization of GR(50) out of the nucleolus in flies co-expressing SETX. Next-generation GR(1000) fly models, that show age-related motor deficits in climbing and movement assays, were similarly rescued with SETX co-expression. We noted that the physical interaction between SETX and arginine-containing DPRs is partially RNA-dependent. Finally, we directly assessed the nucleolus in cells expressing GR-DPRs, confirmed reduced mobility of proteins trafficking to the nucleolus upon GR-DPR expression, and found that SETX dosage modulated nucleolus liquidity in GR-DPR-expressing cells and motor neurons. These findings reveal a hitherto unknown connection between SETX function and cellular processes contributing to neuron demise in the most common form of familial ALS.

Diagnostic yield of exome sequencing in congenital vertical talus

BACKGROUND

Congenital vertical talus (CVT), also known as "rocker-bottom foot", is a rare foot deformity associated with a dislocation of the talonavicular joint. Although genetic causes of CVT have been described in single isolated and syndromic families, whole-exome sequencing (WES) of large cohorts have not yet been reported.

METHODS

In this study, 62 probands with CVT were evaluated for likely causative single nucleotide variants (SNVs) and copy number variants (CNVs) using WES. Segregation of variants within families was determined by Sanger sequencing.

RESULTS

In this cohort, CVT occurred as an isolated anomaly in 75.8% (47/62) and was familial in 19.3% (12/62) of cases. Analysis of WES data led to the identification of likely causative variants in known disease genes in 30.6% (19/62) of all CVT probands. More than one proband had likely causative SNVs in TSHZ1, GDF5, and LMX1B. Only two probands had likely causative CNVs: a chromosome 12q13.13 deletion of the 5' HOXC gene cluster, and a chromosome 18q22.3q23 deletion involving TSHZ1. Familial CVT was strongly predictive of identifying a molecular diagnosis [75% (9/12) of familial cases compared to 20% (10/50) of non-familial cases (Chi-square test, P-value = 0.0002)]. There was no difference in the solved rate based on isolated or syndromic presentation, unilateral or bilateral affectation, or sex.

CONCLUSIONS

CVT is genetically heterogeneous and more often caused by SNVs than CNVs. There is a high yield of WES in familial CVT cases (∼75%). Additional research is needed to identify the causes of sporadic CVT, which had much lower solved rates.

Panarthropod tiptop/teashirt and spalt orthologs and their potential role as "trunk"-selector genes

BACKGROUND

In the vinegar fly Drosophila melanogaster, the homeodomain containing transcription factor Teashirt (Tsh) appears to specify trunk identity in concert with the function of the Hox genes. While in Drosophila there is a second gene closely related to tsh, called tiptop (tio), in other arthropods species only one copy exists (called tio/tsh). The expression of tsh and tio/tsh, respectively, is surprisingly similar among arthropods suggesting that its function as trunk selector gene may be conserved. Other research, for example on the beetle Tribolium castaneum, questions even conservation of Tsh function among insects. The zinc-finger transcription factor Spalt (Sal) is involved in the regulation of Drosophila tsh, but this regulatory interaction does not appear to be conserved in Tribolium either. Whether the function and interaction of tsh and sal as potential trunk-specifiers, however, is conserved is still unclear because comparative studies on sal expression (except for Tribolium) are lacking, and functional data are (if at all existing) restricted to Insecta.

RESULTS

Here, we provide additional data on arthropod tsh expression, show the first data on onychophoran tio/tsh expression, and provide a comprehensive investigation on sal expression patterns in arthropods and an onychophoran.

CONCLUSIONS

Our data support the idea that tio/tsh genes are involved in the development of "trunk" segments by regulating limb development. Our data suggest further that the function of Sal is indeed unlikely to be conserved in trunk vs head development like in Drosophila, but early expression of sal is in line with a potential homeotic function, at least in Arthropoda.

A single ortholog of teashirt and tiptop regulates larval pigmentation and adult appendage patterning in Bombyx mori

Two paralogous genes, teashirt (tsh) and tiptop (tio), encode zinc-finger transcription factors and play important roles in insect growth and development. In the fruit fly, Drosophila melanogaster, tsh promotes trunk segmental identities and contributes to the patterning of other tissues during the embryonic stage. During the adult stage, tsh contributes to the specification and patterning of appendages, including the leg, wing and eye. While tio acts redundantly with tsh, flies lacking tio function are viable without deleterious phenotypes. This gene pair is present in the genomes of all Drosophila species but only as a single homologue in several other insect species. In Oncopeltus fasciatus and Tribolium castaneum, tsh/tio has been functionally characterized as specifying the identity of the leg during the adult stage. However, in lepidopteran insects which include large numbers of pests in agriculture and forestry, as well as the major silk producer silkworm Bombyx mori, the biological functions of tsh/tio are still poorly understood. In the current study, we performed functional analysis of tsh/tio by using both CRISPR/Cas9-mediated mutagenesis and transposon-mediated ectopic expression in B. mori. The results show that loss of tsh/tio function affected pigmentation during the larval stage and appendage pattering during the adult stage. RNA-seq analysis and subsequent q-RT-PCR analysis revealed that depletion of tsh/tio significantly elevated the expression of the kynurenine 3-monooxygenase gene, as well as melanin synthase-related genes during the larval stage. Furthermore, ubiquitous ectopic expression of tsh/tio induces developmental retardation and eventually larval lethality. These data reveal evolutionarily conserved functions of tsh/tio in controlling adult appendage patterning, as well as the novel function of regulating larval pigmentation in B. mori, providing novel insights into how tsh/tio regulates insect growth and development.

Teashirt 1 (Tshz1) is essential for the development, survival and function of hypoglossal and phrenic motor neurons in mouse

Feeding and breathing are essential motor functions and rely on the activity of hypoglossal and phrenic motor neurons that innervate the tongue and diaphragm, respectively. Little is known about the genetic programs that control the development of these neuronal subtypes. The transcription factor Tshz1 is strongly and persistently expressed in developing hypoglossal and phrenic motor neurons. We used conditional mutation of Tshz1 in the progenitor zone of motor neurons (Tshz1^{MN Δ}) to show that Tshz1 is essential for survival and function of hypoglossal and phrenic motor neurons. Hypoglossal and phrenic motor neurons are born in correct numbers, but many die between embryonic day 13.5 and 14.5 in Tshz1^{MN Δ} mutant mice. In addition, innervation and electrophysiological properties of phrenic and hypoglossal motor neurons are altered. Severe feeding and breathing problems accompany this developmental deficit. Although motor neuron survival can be rescued by elimination of the pro-apoptotic factor Bax, innervation, feeding and breathing defects persist in Bax^-/-; Tshz1^{MN Δ} mutants. We conclude that Tshz1 is an essential transcription factor for the development and physiological function of phrenic and hypoglossal motor neurons.

The evolution of selector gene function: Expression dynamics and regulatory interactions of tiptop/teashirt across Arthropoda

The transcription factors spineless (ss) and tiptop/teashirt (tio/tsh) have been shown to be selectors of distal appendage identity in an insect, but it is unknown how they regulate one another. Here, we examined the regulatory relationships between these two determinants in the milkweed bug Oncopeltus faciatus, using maternal RNA interference (RNAi). We show that Ofas-ss RNAi embryos bear distally transformed antennal buds with heterogeneous Ofas-tio/tsh expression domains comparable to wild type legs. In the reciprocal experiment, Ofas-tio/tsh RNAi embryos bear distally transformed walking limb buds with ectopic expression of Ofas-ss in the distal leg primordia. These data suggest that Ofas-ss is required for the maintenance of Ofas-tio/tsh expression in the distal antenna, whereas Ofas-tio/tsh represses Ofas-ss in the leg primordia. To assess whether expression boundaries of tio/tsh are associated with the trunk region more generally, we surveyed the expression of one myriapod and two chelicerate tio/tsh homologs. Our expression survey suggests that tio/tsh could play a role in specifying distal appendage identity across Arthropoda, but Hox regulation of tio/tsh homologs has been evolutionarily labile.

The functional organization of descending sensory-motor pathways in Drosophila

In most animals, the brain controls the body via a set of descending neurons (DNs) that traverse the neck. DN activity activates, maintains or modulates locomotion and other behaviors. Individual DNs have been well-studied in species from insects to primates, but little is known about overall connectivity patterns across the DN population. We systematically investigated DN anatomy in Drosophila melanogaster and created over 100 transgenic lines targeting individual cell types. We identified roughly half of all Drosophila DNs and comprehensively map connectivity between sensory and motor neuropils in the brain and nerve cord, respectively. We find the nerve cord is a layered system of neuropils reflecting the fly's capability for two largely independent means of locomotion -- walking and flight -- using distinct sets of appendages. Our results reveal the basic functional map of descending pathways in flies and provide tools for systematic interrogation of neural circuits.

Characterization of the Sexually Dimorphic fruitless Neurons That Regulate Copulation Duration

Male courtship in Drosophila melanogaster is a sexually dimorphic innate behavior that is hardwired in the nervous system. Understanding the neural mechanism of courtship behavior requires the anatomical and functional characterization of all the neurons involved. Courtship involves a series of distinctive behavioral patterns, culminating in the final copulation step, where sperms from the male are transferred to the female. The duration of this process is tightly controlled by multiple genes. The fruitless (fru) gene is one of the factors that regulate the duration of copulation. Using several intersectional genetic combinations to restrict the labeling of GAL4 lines, we found that a subset of a serotonergic cluster of fru neurons co-express the dopamine-synthesizing enzyme, tyrosine hydroxylase, and provide behavioral and immunological evidence that these neurons are involved in the regulation of copulation duration.

Abolishment of Spontaneous Flight Turns in Visually Responsive Drosophila

Animals react rapidly to external stimuli, such as an approaching predator, but in other circumstances, they seem to act spontaneously, without any obvious external trigger. How do the neural processes mediating the execution of reflexive and spontaneous actions differ? We studied this question in tethered, flying Drosophila. We found that silencing a large but genetically defined set of non-motor neurons virtually eliminates spontaneous flight turns while preserving the tethered flies' ability to perform two types of visually evoked turns, demonstrating that, at least in flies, these two modes of action are almost completely dissociable.

Cell-Autonomous and Non-cell-autonomous Function of Hox Genes Specify Segmental Neuroblast Identity in the Gnathal Region of the Embryonic CNS in Drosophila

During central nervous system (CNS) development neural stem cells (Neuroblasts, NBs) have to acquire an identity appropriate to their location. In thoracic and abdominal segments of Drosophila, the expression pattern of Bithorax-Complex Hox genes is known to specify the segmental identity of NBs prior to their delamination from the neuroectoderm. Compared to the thoracic, ground state segmental units in the head region are derived to different degrees, and the precise mechanism of segmental specification of NBs in this region is still unclear. We identified and characterized a set of serially homologous NB-lineages in the gnathal segments and used one of them (NB6-4 lineage) as a model to investigate the mechanism conferring segment-specific identities to gnathal NBs. We show that NB6-4 is primarily determined by the cell-autonomous function of the Hox gene Deformed (Dfd). Interestingly, however, it also requires a non-cell-autonomous function of labial and Antennapedia that are expressed in adjacent anterior or posterior compartments. We identify the secreted molecule Amalgam (Ama) as a downstream target of the Antennapedia-Complex Hox genes labial, Dfd, Sex combs reduced and Antennapedia. In conjunction with its receptor Neurotactin (Nrt) and the effector kinase Abelson tyrosine kinase (Abl), Ama is necessary in parallel to the cell-autonomous Dfd pathway for the correct specification of the maxillary identity of NB6-4. Both pathways repress CyclinE (CycE) and loss of function of either of these pathways leads to a partial transformation (40%), whereas simultaneous mutation of both pathways leads to a complete transformation (100%) of NB6-4 segmental identity. Finally, we provide genetic evidences, that the Ama-Nrt-Abl-pathway regulates CycE expression by altering the function of the Hippo effector Yorkie in embryonic NBs. The disclosure of a non-cell-autonomous influence of Hox genes on neural stem cells provides new insight into the process of segmental patterning in the developing CNS.

The central nervous system (CNS) needs to be subdivided into functionally specified regions. In the developing CNS of Drosophila, each neural stem cell, called neuroblasts (NB), acquires a unique identity according to its anterior-posterior and dorso-ventral position to generate a specific cell lineage. Along the anterior-posterior body axis, Hox genes of the Bithorax-Complex convey segmental identities to NBs in the trunk segments. In the derived gnathal and brain segments, the mechanisms specifying segmental NB identities are largely unknown. We investigated the role of Hox genes of the Antennapedia-Complex in the gnathal CNS. In addition to cell-autonomous Hox gene function, we unexpectedly uncovered a parallel non-cell-autonomous pathway in mediating segmental specification of embryonic NBs in gnathal segments. Both pathways restrict the expression of the cell cycle gene CyclinE, ensuring the proper specification of a glial cell lineage. Whereas the Hox gene Deformed mediates this cell-autonomously, labial and Antennapedia influence the identity via transcriptional regulation of the secreted molecule Amalgam (and its downstream pathway) in a non-cell-autonomous manner. These findings shed new light on the role of the highly conserved Hox genes during segmental patterning of neural stem cells in the CNS.

The effect of neurospecific knockdown of candidate genes for locomotor behavior and sound production in Drosophila melanogaster

Molecular mechanisms underlying the functioning of central pattern generators (CPGs) are poorly understood. Investigations using genetic approaches in the model organism Drosophila may help to identify unknown molecular players participating in the formation or control of motor patterns. Here we report Drosophila genes as candidates for involvement in the neural mechanisms responsible for motor functions, such as locomotion and courtship song. Twenty-two Drosophila lines, used for gene identification, were isolated from a previously created collection of 1064 lines, each carrying a P element insertion in one of the autosomes. The lines displayed extreme deviations in locomotor and/or courtship song parameters compared with the whole collection. The behavioral consequences of CNS-specific RNAi-mediated knockdowns for 10 identified genes were estimated. The most prominent changes in the courtship song interpulse interval (IPI) were seen in flies with Sps2 or CG15630 knockdown. Glia-specific knockdown of these genes produced no effect on the IPI. Estrogen-induced knockdown of CG15630 in adults reduced the IPI. The product of the CNS-specific gene, CG15630 (a predicted cell surface receptor), is likely to be directly involved in the functioning of the CPG generating the pulse song pattern. Future studies should ascertain its functional role in the neurons that constitute the song CPG. Other genes (Sps2, CG34460), whose CNS-specific knockdown resulted in IPI reduction, are also worthy of detailed examination.

The Hippo pathway effector Yki downregulates Wg signaling to promote retinal differentiation in the Drosophila eye

The evolutionarily conserved Hippo signaling pathway is known to regulate cell proliferation and maintain tissue homeostasis during development. We found that activation of Yorkie (Yki), the effector of the Hippo signaling pathway, causes separable effects on growth and differentiation of the Drosophila eye. We present evidence supporting a role for Yki in suppressing eye fate by downregulation of the core retinal determination genes. Other upstream regulators of the Hippo pathway mediate this effect of Yki on retinal differentiation. Here, we show that, in the developing eye, Yki can prevent retinal differentiation by blocking morphogenetic furrow (MF) progression and R8 specification. The inhibition of MF progression is due to ectopic induction of Wingless (Wg) signaling and Homothorax (Hth), the negative regulators of eye development. Modulating Wg signaling can modify Yki-mediated suppression of eye fate. Furthermore, ectopic Hth induction due to Yki activation in the eye is dependent on Wg. Last, using Cut (Ct), a marker for the antennal fate, we show that suppression of eye fate by hyperactivation of yki does not change the cell fate (from eye to antenna-specific fate). In summary, we provide the genetic mechanism by which yki plays a role in cell fate specification and differentiation - a novel aspect of Yki function that is emerging from multiple model organisms.

teashirt is required for head-versus-tail regeneration polarity in planarians

Regeneration requires that the identities of new cells are properly specified to replace missing tissues. The Wnt signaling pathway serves a central role in specifying posterior cell fates during planarian regeneration. We identified a gene encoding a homolog of the Teashirt family of zinc-finger proteins in the planarian Schmidtea mediterranea to be a target of Wnt signaling in intact animals and at posterior-facing wounds. Inhibition of Smed-teashirt (teashirt) by RNA interference (RNAi) resulted in the regeneration of heads in place of tails, a phenotype previously observed with RNAi of the Wnt pathway genes β-catenin-1, wnt1, Dvl-1/2 or wntless. teashirt was required for β-catenin-1-dependent activation of posterior genes during regeneration. These findings identify teashirt as a transcriptional target of Wnt signaling required for Wnt-mediated specification of posterior blastemas.

Β-catenin-dependent control of positional information along the AP body axis in planarians involves a teashirt family member

Wnt/β-catenin signaling regulates tissue homeostasis and regeneration in metazoans. In planarians-flatworms with high regenerative potential-Wnt ligands are thought to control tissue polarity by shaping a β-catenin activity gradient along the anterior-posterior axis, yet the downstream mechanisms are poorly understood. We performed an RNA sequencing (RNA-seq)-based screen and identified hundreds of β-catenin-dependent transcripts, of which several were expressed in muscle tissue and stem cells in a graded fashion. In particular, a teashirt (tsh) ortholog was induced in a β-catenin-dependent manner during regeneration in planarians and zebrafish, and RNAi resulted in two-headed planarians. Strikingly, intact planarians depleted of tsh induced anterior markers and slowly transformed their tail into a head, reminiscent of β-catenin RNAi phenotypes. Given that β-catenin RNAi enhanced the formation of muscle cells expressing anterior determinants in tail regions, our study suggests that this pathway controls tissue polarity through regulating the identity of differentiating cells during homeostasis and regeneration.

Building and specializing epithelial tubular organs: the Drosophila salivary gland as a model system for revealing how epithelial organs are specified, form and specialize

The past two decades have witnessed incredible progress toward understanding the genetic and cellular mechanisms of organogenesis. Among the organs that have provided key insight into how patterning information is integrated to specify and build functional body parts is the Drosophila salivary gland, a relatively simple epithelial organ specialized for the synthesis and secretion of high levels of protein. Here, we discuss what the past couple of decades of research have revealed about organ specification, development, specialization, and death, and what general principles emerge from these studies.

Octopamine neuromodulation regulates Gr32a-linked aggression and courtship pathways in Drosophila males

Chemosensory pheromonal information regulates aggression and reproduction in many species, but how pheromonal signals are transduced to reliably produce behavior is not well understood. Here we demonstrate that the pheromonal signals detected by Gr32a-expressing chemosensory neurons to enhance male aggression are filtered through octopamine (OA, invertebrate equivalent of norepinephrine) neurons. Using behavioral assays, we find males lacking both octopamine and Gr32a gustatory receptors exhibit parallel delays in the onset of aggression and reductions in aggression. Physiological and anatomical experiments identify Gr32a to octopamine neuron synaptic and functional connections in the suboesophageal ganglion. Refining the Gr32a-expressing population indicates that mouth Gr32a neurons promote male aggression and form synaptic contacts with OA neurons. By restricting the monoamine neuron target population, we show that three previously identified OA-Fru^M neurons involved in behavioral choice are among the Gr32a-OA connections. Our findings demonstrate that octopaminergic neuromodulatory neurons function as early as a second-order step in this chemosensory-driven male social behavior pathway.

To mate or fight? When meeting other members of their species, male fruit flies must determine whether a second fly is male or female and proceed with the appropriate behavioral patterns. The taste receptor, Gr32a, has been reported to respond to chemical messages (pheromones) that are important for gender recognition, as eliminating Gr32a function impairs both male courtship and aggressive behavior. Here we demonstrate that different subsets of Gr32a-expressing neuron populations mediate these mutually exclusive behaviors and the male Gr32a-mediated behavioral response is amplified through neurons that contain the neuromodulator octopamine (OA, an invertebrate equivalent of norepinephrine). Gr32a-expressing neurons connect functionally and synaptically with distinct OA neurons indicating these amine neurons may function as early as a second-order step in a chemosensory-driven circuit. Our results contribute to understanding how an organism selects an appropriate behavioral response upon receiving external sensory signals.

The tiptop/teashirt genes regulate cell differentiation and renal physiology in Drosophila

The physiological activities of organs are underpinned by an interplay between the distinct cell types they contain. However, little is known about the genetic control of patterned cell differentiation during organ development. We show that the conserved Teashirt transcription factors are decisive for the differentiation of a subset of secretory cells, stellate cells, in Drosophila melanogaster renal tubules. Teashirt controls the expression of the water channel Drip, the chloride conductance channel CLC-a and the Leukokinin receptor (LKR), all of which characterise differentiated stellate cells and are required for primary urine production and responsiveness to diuretic stimuli. Teashirt also controls a dramatic transformation in cell morphology, from cuboidal to the eponymous stellate shape, during metamorphosis. teashirt interacts with cut, which encodes a transcription factor that underlies the differentiation of the primary, principal secretory cells, establishing a reciprocal negative-feedback loop that ensures the full differentiation of both cell types. Loss of teashirt leads to ineffective urine production, failure of homeostasis and premature lethality. Stellate cell-specific expression of the teashirt paralogue tiptop, which is not normally expressed in larval or adult stellate cells, almost completely rescues teashirt loss of expression from stellate cells. We demonstrate conservation in the expression of the family of tiptop/teashirt genes in lower insects and establish conservation in the targets of Teashirt transcription factors in mouse embryonic kidney.

Narrowing the critical region for congenital vertical talus in patients with interstitial 18q deletions

Interstitial deletions of 18q lead to a number of phenotypic features, including multiple types of foot deformities. Many of these associated phenotypes have had their critical regions narrowly defined. Here we report on three patients with small overlapping deletions of chromosome 18q determined by microarray analysis (chr18:72493281-73512553 hg19 coordinates). All of the patients have congenital vertical talus (CVT). Based on these findings and previous reports in the literature and databases, we narrow the critical region for CVT to a minimum of five genes (ZNF407, ZADH2, TSHZ1, C18orf62, and ZNF516), and propose that TSHZ1 is the likely causative gene for CVT in 18q deletion syndrome.

Homeotic functions of the Teashirt transcription factor during adult Drosophila development

During Drosophila development region-specific regulation of target genes by Hox proteins is modulated by genetic interactions with various cofactors and genetic collaborators. During embryogenesis one such modulator of Hox target specificity is the zinc-finger transcription factor Teashirt (Tsh) that is expressed in the developing trunk and cooperatively functions with trunk-specific Hox proteins to promote appropriate segment fate. This embryonic function of Tsh is characterized as homeotic since loss of embryonic Tsh activity leads to transformation of trunk segments toward head identity. In addition to this embryonic homeotic role, Tsh also performs vital Hox-independent functions through patterning numerous embryonic, larval and adult structures. Here we address whether the homeotic function of Tsh is maintained throughout development by investigating its contribution to patterning the adult abdomen. We show that Tsh is expressed throughout the developing abdomen and that this expression is dependent on the three Bithorax Hox proteins Ultrabithorax, Abdominal-A and Abdominal-B. Conditional reduction of Tsh activity during pupation reveals broad homeotic roles for this transcription factor throughout the adult abdomen. Additionally we show that, as during embryogenesis, the tsh paralog tiptop (tio) plays a partially redundant role in this homeotic activity.

Disruption of teashirt zinc finger homeobox 1 is associated with congenital aural atresia in humans

Congenital aural atresia (CAA) can occur as an isolated congenital malformation or in the context of a number of monogenic and chromosomal syndromes. CAA is frequently seen in individuals with an 18q deletion, which is characterized by intellectual disability, reduced white-matter myelination, foot deformities, and distinctive facial features. Previous work has indicated that a critical region for CAA is located in 18q22.3. We studied four individuals (from two families) with CAA and other features suggestive of an 18q deletion, and we detected overlapping microdeletions in 18q22.3 in both families. The minimal region of deletion overlap (72.9-73.4 Mb) contained only one known gene, TSHZ1, which was recently shown to be important for murine middle-ear development. Sequence analysis of the coding exons in TSHZ1 in a cohort of 11 individuals with isolated, nonsyndromic bilateral CAA revealed two mutations, c.723G>A (p.Trp241X) and c.946_947delinsA (p.Pro316ThrfsX16), and both mutations predicted a loss of function. Together, these results demonstrate that hemizygosity of TSHZ1 leads to congenital aural atresia as a result of haploinsufficiency.

A dissection of the teashirt and tiptop genes reveals a novel mechanism for regulating transcription factor activity

In the Drosophila eye the retinal determination (RD) network controls both tissue specification and cell proliferation. Mutations in network members result in severe reductions in the size of the eye primordium and the transformation of the eye field into head cuticle. The zinc-finger transcription factor Teashirt (Tsh) plays a role in promoting cell proliferation in the anterior most portions of the eye field as well as in inducing ectopic eye formation in forced expression assays. Tiptop (Tio) is a recently discovered paralog of Tsh. It is distributed in an identical pattern to Tsh within the retina and can also promote ectopic eye development. In a previous study we demonstrated that Tio can induce ectopic eye formation in a broader range of cell populations than Tsh and is also a more potent inducer of cell proliferation. Here we have focused on understanding the molecular and biochemical basis that underlies these differences. The two paralogs are structurally similar but differ in one significant aspect: Tsh contains three zinc finger motifs while Tio has four such domains. We used a series of deletion and chimeric proteins to identify the zinc finger domains that are selectively used for either promoting cell proliferation or inducing eye formation. Our results indicate that for both proteins the second zinc finger is essential to the proper functioning of the protein while the remaining zinc finger domains appear to contribute but are not absolutely required. Interestingly, these domains antagonize each other to balance the overall activity of the protein. This appears to be a novel internal mechanism for regulating the activity of a transcription factor. We also demonstrate that both Tsh and Tio bind to C-terminal Binding Protein (CtBP) and that this interaction is important for promoting both cell proliferation and eye development. And finally we report that the physical interaction that has been described for Tsh and Homothorax (Hth) do not occur through the zinc finger domains.

Zebrafish Tshz3b negatively regulates Hox function in the developing hindbrain

In flies, the zinc-finger protein Teashirt promotes trunk segmental identities, in part, by repressing the expression and function of anterior hox paralog group (PG) 1-4 genes that specify head fates. Anterior-posterior patterning of the vertebrate hindbrain also requires Hox PG 1-4 function, but the role of vertebrate teashirt-related genes in this process has not been investigated. In this work, we use overexpression and structure-function analyses to show that zebrafish tshz3b antagonizes Hox-dependent hindbrain segmentation. Ectopic Tshz3b perturbs the specification of rhombomere identities and leads to the caudal expansion of r1, the only rhombomere whose identity is specified independently of Hox function. This overexpression phenotype does not require the homeodomain and C-terminal zinc fingers that are unique to vertebrate Teashirt-related proteins, but does require that Tshz3b function as a repressor. Together, these results argue that the negative regulation of Hox PG 1-4 function is a conserved characteristic of Teashirt-related proteins.

Teashirt-3, a novel regulator of muscle differentiation, associates with BRG1-associated factor 57 (BAF57) to inhibit myogenin gene expression

In adult muscles and under normal physiological conditions, satellite cells are found in a quiescent state but can be induced to enter the cell cycle by signals resulting from exercise, injury-induced muscle regeneration, or specific disease states. Once activated, satellite cells proliferate, self-renew, and differentiate to form myofibers. In the present study, we found that the zinc finger-containing factor Teashirt-3 (TSHZ3) was expressed in quiescent satellite cells of adult mouse skeletal muscles. We showed that following treatment with cardiotoxin TSHZ3 was strongly expressed in satellite cells of regenerating muscles. Moreover, immunohistochemical analysis indicated that TSHZ3 was expressed in both quiescent and activated satellite cells on intact myofibers in culture. TSHZ3 expression was maintained in myoblasts but disappeared with myotube formation. In C2C12 myoblasts, we showed that overexpression of Tshz3 impaired myogenic differentiation and promoted the down-regulation of myogenin (Myog) and up-regulation of paired-box factor 7 (Pax7). Moreover, knockdown experiments revealed a selective effect of Tshz3 on Myog regulation, and transcriptional reporter experiments indicated that TSHZ3 repressed Myog promoter. We identified the BRG1-associated factor 57 (BAF57), a subunit of the SWI/SNF complex, as a partner of TSHZ3. We showed that TSHZ3 cooperated with BAF57 to repress MYOD-dependent Myog expression. These results suggest a novel mechanism for transcriptional repression by TSHZ3 in which TSHZ3 and BAF57 cooperate to modulate MyoD activity on the Myog promoter to regulate skeletal muscle differentiation.

Phylogeny of the teashirt-related zinc finger (tshz) gene family and analysis of the developmental expression of tshz2 and tshz3b in the zebrafish

The tshz genes comprise a family of evolutionarily conserved transcription factors. However, despite the major role played by Drosophila tsh during the development of the fruit fly, the expression and function of other tshz genes have been analyzed in a very limited set of organisms and, therefore, our current knowledge of these genes is still fragmentary. In this study, we perform detailed phylogenetic analyses of the tshz genes, identify the members of this gene family in zebrafish and describe the developmental expressions of two of them, tshz2 and tshz3b, and compare them with meis1, meis2.1, meis2.2, pax6a, and pax6b expression patterns. The expression patterns of these genes define a complex set of coexpression domains in the developing zebrafish brain where their gene products have the potential to interact.

Genetic feminization of the thoracic nervous system disrupts courtship song in male Drosophila melanogaster

Despite the growing research investigating the sex-specific organization of courtship behavior in Drosophila melanogaster, much remains to be understood about the sex-specific organization of the motor circuit that drives this behavior. To investigate the sex-specification of a tightly patterned component of courtship behavior, courtship song, the authors used the GAL4/UAS targeted gene expression system to feminize the ventral ganglia in male Drosophila and analyzed the acoustic properties of courtship song. More specifically, the authors used the thoracic-specifying teashirt promoter (tsh(GAL4)) to express feminizing transgenes specifically in the ventral ganglia. When tsh(GAL4) drove expression of transformer (tra), males were unable to produce prolonged wing extensions. Transgenic expression of an RNAi construct directed against male-specific fruitless (fru(M)) transcripts resulted in normal wing extension, but highly defective courtship song, with 58% of males failing to generate detectable courtship song. Of those that did sing, widths of individual pulses were significantly broader than controls, suggesting thoracic fru(M) function serves to mediate proprioceptive-dependent wing vibration damping during pulse song. However, the most critical signal in the song, the interpulse interval, remained intact. The inability to phenocopy this effect by reducing fru(M) expression in motor neurons and proprioceptive neurons suggests thoracic interneurons require fru(M) for proper pulse song execution and patterning of pulse structure, but not for pulse timing. This provides evidence that genes establishing sex-specific activation of complex behaviors may also be used in establishing pattern-generating motor networks underlying these sex-specific behaviors.

Restriction of ectopic eye formation by Drosophila teashirt and tiptop to the developing antenna

In Drosophila, the retinal determination network comprises a set of nuclear factors whose loss-of-function phenotypes often include the complete or near total elimination of the developing eye. These genes also share the ability of being able to induce ectopic eye formation when forcibly expressed in nonretinal tissues such as the antennae, legs, halteres, wings, and genitals. However, it appears that the ability to redirect and transform tissue fates is limited; not all tissues and cell populations can be forced into adopting an eye fate. In this report, we demonstrate that ectopic eye formation by teashirt and its paralog tiptop, a potential new eye specification gene, is restricted to the developing antennae. Of interest, tiptop appears to be a more effective inducer of retinal formation than teashirt. A genetic screen for interacting proteins failed to identify paralog-specific relationships suggesting that the differences between these two genes may be attributed instead to structural differences between the duplicates. We also demonstrate that in addition to being expressed in coincident patterns within the developing eye, both paralogs are transcribed at very similar levels.

Zinc-finger paralogues tsh and tio are functionally equivalent during imaginal development in Drosophila and maintain their expression levels through auto- and cross-negative feedback loops

teashirt (tsh) and tiptop (tio) are two Drosophila gene paralogues encoding zinc-finger transcription factors. While tsh is an important developmental regulator, tio null individuals are viable and fertile. Here, we show that tio and tsh have coincident expression domains in the imaginal discs, the precursors of the adult body, and that both genes show similar functional properties when expressed ectopically. Furthermore, tio is able to rescue the development of tsh mutants, indicating that both genes are functionally equivalent during imaginal development. Of interest, the transcriptional regulation of tio and tsh is linked by a negative feedback loop. This mechanism might be required to maintain a tight control on the total levels of tio/tsh and could help explaining why Drosophila keeps an apparently dispensable gene.

Dynamic regulation by polycomb group protein complexes controls pattern formation and the cell cycle in Drosophila

Polycomb group (PcG) proteins form conserved regulatory complexes that modify chromatin to repress transcription. Here, we report genome-wide binding profiles of PhoRC, the Drosophila PcG protein complex containing the DNA-binding factor Pho/dYY1 and dSfmbt. PhoRC constitutively occupies short Polycomb response elements (PREs) of a large set of developmental regulator genes in both embryos and larvae. The majority of these PREs are co-occupied by the PcG complexes PRC1 and PRC2. Analysis of PcG mutants shows that the PcG system represses genes required for anteroposterior, dorsoventral, and proximodistal patterning of imaginal discs and that it also represses cell cycle regulator genes. Many of these genes are regulated in a dynamic manner, and our results suggest that the PcG system restricts signaling-mediated activation of target genes to appropriate cells. Analysis of cell cycle regulators indicates that the PcG system also dynamically modulates the expression levels of certain genes, providing a possible explanation for the tumor phenotype of PcG mutants.

The Drosophila gap gene giant has an anterior segment identity function mediated through disconnected and teashirt

The C2H2 zinc-finger-containing transcription factors encoded by the disconnected (disco) and teashirt (tsh) genes contribute to the regionalization of the Drosophila embryo by establishing fields in which specific Homeotic complex (Hom-C) proteins can function. In Drosophila embryos, disco and the paralogous disco-related (disco-r) are expressed throughout most of the epidermis of the head segments, but only in small patches in the trunk segments. Conversely, tsh is expressed extensively in the trunk segments, with little or no accumulation in the head segments. Little is known about the regulation of these genes; for example, what limits their expression to these domains? Here, we report the regulatory effects of gap genes on the spatial expression of disco, disco-r, and tsh during Drosophila embryogenesis. The data shed new light on how mutations in giant (gt) affect patterning within the anterior gt domain, demonstrating homeotic function in this domain. However, the homeosis does not occur through altered expression of the Hom-C genes but through changes in the regulation of disco and tsh.

Do teashirt family genes specify trunk identity? Insights from the single tiptop/teashirt homolog of Tribolium castaneum

The Drosophila teashirt gene acts in concert with the homeotic selector (Hox) genes to specify trunk (thorax and abdomen) identity. There has been speculation that this trunk-specifying function might be very ancient, dating back to the common ancestor of insects and vertebrates. However, other evidence suggests that the role of teashirt in trunk identity is not well conserved even within the Insecta. To address this issue, we have analyzed the function of Tc-tiotsh, the lone teashirt family member in the red flour beetle, Tribolium castaneum. Although Tc-tiotsh is important for aspects of both embryonic and imaginal development including some trunk features, we find no evidence that it acts as a trunk identity gene. We discuss this finding in the context of recent insights into the evolution and function of the Drosophila teashirt family genes.

Expression patterns of the three Teashirt-related genes define specific boundaries in the developing and postnatal mouse forebrain

We compare the expression patterns of the three mouse Teashirt (mTsh) genes during development of the forebrain and at a postnatal stage. During development, mTsh genes are expressed in domains that are restricted both dorsoventrally and rostrocaudally, with major changes in expression level coinciding with compartment boundaries. Striking complementarities in the distribution of mTsh transcripts were observed in the developing diencephalon, telencephalon, and olfactory bulb (OB). A mTsh1-positive cell population is part of the DLX-positive population localized in the dorsalmost portion of the lateral ganglionic eminence (dLGE). Comparison of the mTsh1 expression domain with the domains of Er81 and Islet1, which mark two distinct progenitor populations in the subventricular zone of the LGE, suggests that mTsh1 marks OB interneuron progenitors. Furthermore, the distinct expression patterns of mTsh1 and mTsh2 in the ventral LGE and the dLGE highlight the differential contributions of these structures to the striatum and the amydaloid complex. For Sey/Sey mutants, we show that Pax6 function is critical for the correct specification of the mTsh1+ population in the dLGE during embryogenesis. At postnatal stages in the OB, mTsh1 is expressed in granule and periglomerular cells, which originate from the subpallium during development. Furthermore, mTsh1+ cells line the walls of the anterior lateral ventricle, a region that gives rise to the interneurons that migrate in the rostral migratory streams and populate the OB postnatally. Our results suggest a role for mTsh genes in the establishment of regional identity and specification of cell types in the developing and adult forebrain.

Direct interaction between Teashirt and Sex combs reduced proteins, via Tsh's acidic domain, is essential for specifying the identity of the prothorax in Drosophila

teashirt (tsh) encodes a zinc-finger protein that is thought to be part of a network that contributes to regionalization of the Drosophila embryo and establishes the domains of Hox protein function. tsh and the Hox gene Sex combs reduced (Scr) are essential to establish the identity of the first thoracic segment. We used the development of the first thoracic segment as a paradigm for Scr dependent regional morphological distinctions. In this specific context, we asked whether Tsh protein could have a direct influence on Scr activity. Here we present evidence that Tsh interacts directly with Scr and this interaction depends in part on the presence of a short domain located in the N-terminal half of Teashirt called "acidic domain". In vivo, expression of full length Tsh can rescue the tsh null phenotype throughout the trunk whereas Tsh lacking the Scr interacting domain rescues all the trunk defects except in the prothorax. We suggest this provides insights into the mechanism by which Tsh, in concert with Scr, specifies the prothoracic identity.

The appendage role of insect disco genes and possible implications on the evolution of the maggot larval form

Though initially identified as necessary for neural migration, Disconnected and its partially redundant paralog, Disco-related, are required for proper head segment identity during Drosophila embryogenesis. Here, we present evidence that these genes are also required for proper ventral appendage development during development of the adult fly, where they specify medial to distal appendage development. Cells lacking the disco genes cannot contribute to the medial and distal portions of ventral appendages. Further, ectopic disco transforms dorsal appendages toward ventral fates; in wing discs, the medial and distal leg development pathways are activated. Interestingly, this appendage role is conserved in the red flour beetle, Tribolium (where legs develop during embryogenesis), yet in the beetle we found no evidence for a head segmentation role. The lack of an embryonic head specification role in Tribolium could be interpreted as a loss of the head segmentation function in Tribolium or gain of this function during evolution of flies. However, we suggest an alternative explanation. We propose that the disco genes always function as appendage factors, but their appendage nature is masked during Drosophila embryogenesis due to the reduction of limb fields in the maggot style Drosophila larva.

Tshz1 is required for axial skeleton, soft palate and middle ear development in mice

Members of the Tshz gene family encode putative zinc fingers transcription factors that are broadly expressed during mouse embryogenesis. Tshz1 is detected from E9.5 in the somites, the spinal cord, the limb buds and the branchial arches. In order to assess the function of Tshz1 during mouse development, we generated Tshz1-deficient mice. Tshz1 inactivation leads to neonatal lethality and causes multiple developmental defects. In the craniofacial region, loss of Tshz1 function leads to specific malformations of middle ear components, including the malleus and the tympanic ring. Tshz1(-/-) mice exhibited Hox-like vertebral malformations and homeotic transformations in the cervical and thoracic regions, suggesting that Tshz1 and Hox genes are involved in common pathways to control skeletal morphogenesis. Finally, we demonstrate that Tshz1 is required for the development of the soft palate.

XTsh3 is an essential enhancing factor of canonical Wnt signaling in Xenopus axial determination

In Xenopus, an asymmetric distribution of Wnt activity that follows cortical rotation in the fertilized egg leads to the dorsal-ventral (DV) axis establishment. However, how a clear DV polarity develops from the initial difference in Wnt activity still remains elusive. We report here that the Teashirt-class Zn-finger factor XTsh3 plays an essential role in dorsal determination by enhancing canonical Wnt signaling. Knockdown of the XTsh3 function causes ventralization in the Xenopus embryo. Both in vivo and in vitro studies show that XTsh3 substantially enhances Wnt signaling activity in a beta-catenin-dependent manner. XTsh3 cooperatively promotes the formation of a secondary axis on the ventral side when combined with weak Wnt activity, whereas XTsh3 alone has little axis-inducing ability. Furthermore, Wnt1 requires XTsh3 for its dorsalizing activity in vivo. Immunostaining and protein analyses indicate that XTsh3 is a nuclear protein that physically associates with beta-catenin and efficiently increases the level of beta-catenin in the nucleus. We discuss the role of XTsh3 as an essential amplifying factor of canonical Wnt signaling in embryonic dorsal determination.

Teashirt 3 expression in the chick embryo reveals a remarkable association with tendon development

Drosophila teashirt (tsh) is involved in the patterning of the trunk identity together with the Hox genes. In addition, it is also a player in the Wingless and the Hedgehog pathways. In birds and mammals, three Tshz genes are identified and the expression patterns for mouse Tshz1 and Tshz2 have been reported during embryogenesis. Recently, we showed that all three mouse Tshz genes can rescue the Drosophila tsh loss-of-function phenotype, indicating that the function of the teashirt genes has been conserved during evolution. Here we describe the expression pattern of chick TSHZ3 during embryogenesis. Chick TSHZ3 is expressed in several tissues including mesodermal derivatives, the central and peripheral nervous systems. Emphasis is laid on the dynamic expression occurring in regions of the somites and limbs where tendons develop. We show that TSHZ3 is activated in the somites by FGF8, a known inducer of the tendon marker SCX.

Isolation and expression of zebrafish zinc-finger transcription factor gene tsh1

We report the expression patterns of tsh1, a zebrafish homologue of the Drosophila homeotic gene teashirt. Expression of tsh1 is first detected at the 2-somite stage (10h post-fertilization, hpf) at the anterior end of the spinal cord. Expression expands toward the posterior spinal cord, and by the prim-5 stage (24 hpf) tsh1 transcripts are detected throughout spinal cord. Between the 14- and 25-somite stage (16-24 hpf), spinal cord expression shows a clear anterior boundary at the rostral margin of rhombomere 7. Around the prim-25 stage (36 hpf), while the spinal expression of tsh1 decreases, new expression is detected in the pectoral fin buds and dorsal forebrain. By the long-pec stage (48 hpf), spinal cord expression is undetectable, but strong expression is observed in the rhombencephalon, telencephalon, tectum opticum, midbrain-hindbrain boundary, in the first pharyngeal arch and in the eyes. This expression persists at least until the larval stages. Retinoic acid signaling influences tsh1 expression. Zebrafish tsh1 expression was induced in the anterior neural tube in embryos treated briefly with exogenous retinoic acid. Furthermore, tsh1 expression was down-regulated in the spinal cord in the zebrafish neckless mutant in which RA signaling is disrupted due to a missense mutation in the gene encoding retinaldehyde dehydrogenase type 2.

Xenopus Teashirt1 regulates posterior identity in brain and cranial neural crest

We have isolated two related Xenopus homologues of the homeotic zinc finger protein Teashirt1 (Tsh1), XTsh1a and XTsh1b. While Drosophila teashirt specifies trunk identity in the fly, the developmental relevance of vertebrate Tsh homologues is unknown. XTsh1a/b are expressed in prospective trunk CNS throughout early neurula stages and later in the migrating cranial neural crest (CNC) of the third arch. In postmigratory CNC, XTsh1a/b is uniformly activated in the posterior arches. Gain- and loss-of-function experiments reveal that reduction or increase of XTsh1 levels selectively inhibits specification of the hindbrain and mid/hindbrain boundary in Xenopus embryos. In addition, both overexpression and depletion of XTsh1 interfere with the determination of CNC segment identity. In transplantation assays, ectopic XTsh1a inhibits the routing of posterior, but not of mandibular CNC streams. The loss of function phenotype could be rescued with low amounts either of XTsh1a or murine Tsh3. Our results demonstrate that proper expression of XTsh1 is essential for segmentally restricted gene expression in the posterior brain and CNC and suggest for the first time that teashirt genes act as positional factors also in vertebrate development.

Assisting Hox proteins in controlling body form: are there new lessons from flies (and mammals)?

Hox proteins regulate specific sets of target genes to give rise to morphological distinctions along the anterior-posterior body axis of metazoans. Though they have high developmental specificity, Hox proteins have low DNA binding specificity, so how they select the appropriate target genes has remained enigmatic. There is general agreement that cofactors provide additional specificity, but a comprehensive model of Hox control of gene expression has not emerged. There is now evidence that a global network of zinc finger transcription factors contributes to patterning of the Drosophila embryo. These zinc finger proteins appear to establish fields in which certain Hox proteins can function. Though the nature of these fields is uncertain at this time, it is possible that these zinc finger proteins are Hox cofactors, providing additional specificity during Hox target-gene selection. Furthermore, these zinc finger proteins are conserved, as are aspects of their anterior-posterior expression, suggesting that their roles might be conserved, as well. Perhaps this layer in the genetic control of body patterning will help bridge some of the chasms that remain in our understanding of the genetic control of pattern formation.

A critical role of teashirt for patterning the ventral epidermis is masked by ectopic expression of tiptop, a paralog of teashirt in Drosophila

The teashirt gene encodes a protein with three widely spaced zinc finger motifs that is crucial for specifying trunk identity in Drosophila embryos. Here, we describe a gene called tiptop, which encodes a protein highly similar to Teashirt. We have analyzed the expression patterns and functions of these two genes in the trunk of the embryo. Initially, teashirt and tiptop expressions are detected in distinct domains; teashirt in the trunk and tiptop in parts of the head and tail. In different mutant situations, we show that, in the trunk and head, they repress each other's expression. Unlike teashirt, we found that deletion of tiptop is homozygous viable and fertile. However, embryos lacking both gene activities display a more severe trunk phenotype than teashirt mutant embryos alone. Ectopic expression of either gene produces an almost identical phenotype, indicating that Teashirt and Tiptop have, on the whole, common activities. We conclude that Teashirt and Tiptop repress each other's expression and that Teashirt has a crucial role for trunk patterning that is in part masked by ectopic expression of Tiptop.

Xenopus XsalF

Here we show that XsalF, a frog homolog of the Drosophila homeotic selector spalt, plays an essential role for the forebrain/midbrain determination in Xenopus. XsalF overexpression expands the domain of forebrain/midbrain genes and suppresses midbrain/hindbrain boundary (MHB) markers and anterior hindbrain genes. Loss-of-function studies show that XsalF is essential for the expression of the forebrain/midbrain genes and for the repression of the caudal genes. Interestingly, XsalF functions by antagonizing canonical Wnt signaling, which promotes caudalization of neural tissues. XsalF is required for anterior-specific expressions of GSK3beta and Tcf3, genes encoding antagonistic effectors of Wnt signaling. Loss-of-function phenotypes of GSK3beta and Tcf3 mimic those of XsalF while injections of GSK3beta and Tcf3 rescue loss-of-function phenotypes of XsalF. These findings suggest that the forebrain/midbrain-specific gene XsalF negatively controls cellular responsiveness to posteriorizing Wnt signals by regulating region-specific GSK3beta and Tcf3 expression.

Temporally dynamic response to Wingless directs the sequential elaboration of the proximodistal axis of the Drosophila wing

The Drosophila wing imaginal disc gives rise to three main regions along the proximodistal axis of the dorsal mesothoracic segment: the notum, proximal wing, and wing blade. Development of the wing blade requires the Notch and wingless signalling pathways to activate vestigial at the dorsoventral boundary. However, in the proximal wing, Wingless activates a different subset of genes, e.g., homothorax. This raises the question of how the downstream response to Wingless signalling differentiates between proximal and distal fate specification. Here, we show that a temporally dynamic response to Wingless signalling sequentially elaborates the proximodistal axis. In the second instar, Wingless activates genes involved in proximal wing development; later in the third instar, Wingless acts to direct the differentiation of the distal wing blade. The expression of a novel marker for proximal wing fate, zfh-2, is initially activated by Wingless throughout the "wing primordium," but later is repressed by the activity of Vestigial and Nubbin, which together define a more distal domain. Thus, activation of a distal developmental program is antagonistic to previously established proximal fate. In addition, Wingless is required early to establish proximal fate, but later when Wingless activates distal differentiation, development of proximal fate becomes independent of Wingless signalling. Since P-element insertions in the zfh-2 gene result in a revertable proximal wing deletion phenotype, it appears that zfh-2 activity is required for correct proximal wing development. Our data are consistent with a model in which Wingless first establishes a proximal appendage fate over notum, then the downstream response changes to direct the differentiation of a more distal fate over proximal. Thus, the proximodistal domains are patterned in sequence and show a distal dominance.

Distinct functions of homothorax in leg development in Drosophila

The Drosophila leg is subdivided into two mutually antagonistic proximal and distal domains. The proximal domain is defined by the activity of the homeobox genes homothorax and extradenticle and the distal one by the Dpp/Wg targets Distal-less (Dll) and dachshund (dac). It is known that hth/exd function prevents the activity of Dpp and Wg response genes and that cells deficient for exd activity in the proximal domain differentiate pattern elements corresponding to more distal leg regions. We report new results on the role of hth/exd antagonising the Dpp pathway. In cells expressing hth in the distal leg, there is a debilitation of the Dpp pathway which is reflected in lower levels of Mad phosphorylation and in increased levels of the receptor thick veins. Ectopic hth expression in the distal leg results in JNK-mediated apoptosis, decreased growth and pattern abnormalities. It also causes a general proximalisation of the appendage, which can be explained by interference with the Dpp and Wg pathways. We also report that the repression by hth/exd of the Dpp and Wg target Distal-less is not achieved at the level of transcription but preventing the activation of Dll target genes. We propose that hth/exd function contributes to the normal identity of proximal cells both by limiting the influence of the Dpp and Wg pathways and by activating proximal genes like teashirt (tsh) and aristaless (al).

Tribolium Hox genes repress antennal development in the gnathos and trunk

Evidence from Drosophila suggests that Hox genes not only specify regional identity, but have the additional function of repressing antennal development within their normal domains. This is dramatically demonstrated by a series of Hox mutants in the red flour beetle, Tribolium castaneum, and is likely an ancient function of Hox genes in insects.