Selector Gene Research Articles

Non-cell-autonomous mechanism of activity-dependent neurotransmitter switching.

Activity-dependent neurotransmitter switching engages genetic programs regulating transmitter synthesis, but the mechanism by which activity is transduced is unknown. We suppressed activity in single neurons in the embryonic spinal cord to determine whether glutamate-gamma-aminobutyric acid (GABA) switching is cell autonomous. Transmitter respecification did not occur, suggesting that it is homeostatically regulated by the level of activity in surrounding neurons. Graded increase in the number of silenced neurons in cultures led to graded decrease in the number of neurons expressing GABA, supporting non-cell-autonomous transmitter switching. We found that brain-derived neurotrophic factor (BDNF) is expressed in the spinal cord during the period of transmitter respecification and that spike activity causes release of BDNF. Activation of TrkB receptors triggers a signaling cascade involving JNK-mediated activation of cJun that regulates tlx3, a glutamate/GABA selector gene, accounting for calcium-spike BDNF-dependent transmitter switching. Our findings identify a molecular mechanism for activity-dependent respecification of neurotransmitter phenotype in developing spinal neurons.

Maintenance of cell fates through acetylated histone and the histone variant H2A.z in C. elegans.

Maintenance of cell fates is essential for the development and homeostasis of multicellular organisms and involves the preservation of the expression status of selector genes that control many target genes. Epigenetic marks have pivotal roles in the maintenance of gene expression status, as occurs with methylation on lysine 27 of histone H3 (H3K27me) for Hox gene regulation. In contrast, because the levels of histone acetylation decrease during the mitotic phase, acetylated histone has not been believed to contribute to the maintenance of cell fates. Because members of the bromodomain and extra terminal (BET) family bind to acetylated histones localized on mitotic chromosomes, it is possible that they may regulate the transcriptional status of genes throughout the cell cycle. In this commentary, we discuss the recent analyses of C. elegans BET family protein BET-1, which contributes to the maintenance of cell fates through the histone H2A variant HTZ-1/H2A.z. This mechanism represses transcription of selector genes in the genomic region where lysine 27 of histone H3 (H3K27) is demethylated by histone demethylase UTX-1. We discuss the possibility that BET-1 and HTZ-1 maintain the poised state of RNA polymerase II in the cell such that it is ready to respond to differentiation signals.

GATA4 immunolocalization in breast carcinoma as a potent prognostic predictor.

Transcriptional GATA factors are known lineage selector genes and regulate a variety of biological processes including specification and differentiation of tissues. In the present study, we examined expression profiles of six GATA factor genes in invasive ductal carcinomas (IDC) of the breast using microarray analysis (n = 20) and found that GATA4 expression was closely correlated with recurrence in patients. Because the significance of GATA4 has remained largely unknown in breast carcinoma, we further immunolocalized GATA4 in ductal carcinoma in situ (DCIS) of the breast (n = 48) and IDC (n = 163). GATA4 immunoreactivity was detected in the nuclei of carcinoma cells and was positive in 27% of DCIS and 31% of IDC cases. GATA4 status was significantly associated with nuclear grade and van Nuys classification in DCIS and was positively associated with distant metastasis, histological grade and HER2 status, but negatively correlated with progesterone receptor labeling index in IDC. Subsequent multivariate analysis demonstrated that GATA4 status was an independent prognostic factor for both disease-free and breast cancer-specific survival of IDC patients. All of these results indicate that GATA4 plays important roles in the progression of breast carcinoma from an early stage and that immunohistochemical GATA4 status is considered a potent prognostic factor in human breast cancer patients.

Hipk promotes photoreceptor differentiation through the repression of Twin of eyeless and Eyeless expression

Organogenesis is a complex developmental process, which requires tight regulation of selector gene expression to specify individual organ types. The Pax6 homolog Eyeless (Ey) is an example of such a factor and its expression pattern reveals it is dynamically controlled during development. Ey׳s paralog Twin of eyeless (Toy) induces its expression during embryogenesis, and the two genes are expressed in nearly identical patterns during the larval stages of development. While Ey must be expressed to initiate retinal specification, it must subsequently be repressed behind the morphogenetic furrow to allow for neuronal differentiation. Thus far, a few factors have been implicated in this repression including the signaling pathways Hedgehog (Hh) and Decapentaplegic (Dpp), and more recently downstream components of the retinal determination gene network (RDGN) Sine oculis (So), Eyes absent (Eya), and Dachshund (Dac). Homeodomain-interacting protein kinase (Hipk), a conserved serine–threonine kinase, regulates numerous factors during tissue patterning and development, including the Hh pathway. Using genetic analyses we identify Hipk as a repressor of both Toy and Ey and show that it may do so, in part, through Hh signaling. We also provide evidence that Ey repression is a critical step in ectopic eye development and that Hipk plays an important role in this process. Because Ey repression within the retinal field is a critical step in eye development, we propose that Hipk is a key link between eye specification and patterning.

Homothorax and Extradenticle alter the transcription factor network in Drosophila ommatidia at the dorsal rim of the retina

A narrow band of ommatidia in the dorsal periphery of the Drosophila retina called the dorsal rim area (DRA) act as detectors for polarized light. The transcription factor Homothorax (Hth) is expressed in DRA inner photoreceptors R7 and R8 and is both necessary and sufficient to induce the DRA fate, including specialized morphology and unique Rhodopsin expression. Hth expression is the result of Wingless (Wg) pathway activity at the eye margins and restriction to the dorsal eye by the selector genes of the Iroquois complex (Iro-C). However, how the DRA is limited to exactly one or two ommatidial rows is not known. Although several factors regulating the Drosophila retinal mosaic are expressed in DRA ommatidia, the role of Hth in this transcriptional network is uncharacterized. Here we show that Hth functions together with its co-factor Extradenticle (Exd) to repress the R8-specific factor Senseless (Sens) in DRA R8 cells, allowing expression of an ultraviolet-sensitive R7 Rhodopsin (Rh3). Furthermore, Hth/Exd act in concert with the transcriptional activators Orthodenticle (Otd) and Spalt (Sal), to activate expression of Rh3 in the DRA. The resulting monochromatic coupling of Rh3 between R7 and R8 in DRA ommatidia is important for comparing celestial e-vector orientation rather than wavelengths. Finally, we show that Hth expression expands to many ommatidial rows in regulatory mutants of optomotorblind (omb), a transcription factor transducing Wg signaling at the dorsal and ventral eye poles. Therefore, locally restricted recruitment of the DRA-specific factor Hth alters the transcriptional network that regulates Rhodopsin expression across ommatidia.

Role of mef2ca in developmental buffering of the zebrafish larval hyoid dermal skeleton

Phenotypic robustness requires a process of developmental buffering that is largely not understood, but which can be disrupted by mutations. Here we show that in mef2cab1086 loss of function mutant embryos and early larvae, development of craniofacial hyoid bones, the opercle (Op) and branchiostegal ray (BR), becomes remarkably unstable; the large magnitude of the instability serves as a positive attribute to learn about features of this developmental buffering. The OpBR mutant phenotype variably includes bone expansion and fusion, Op duplication, and BR homeosis. Formation of a novel bone strut, or a bone bridge connecting the Op and BR together occurs frequently. We find no evidence that the phenotypic stability in the wild type is provided by redundancy between mef2ca and its co-ortholog mef2cb, or that it is related to the selector (homeotic) gene function of mef2ca. Changes in dorsal–ventral patterning of the hyoid arch also might not contribute to phenotypic instability in mutants. However, subsequent development of the bone lineage itself, including osteoblast differentiation and morphogenetic outgrowth, shows marked variation. Hence, steps along the developmental trajectory appear differentially sensitive to the loss of buffering, providing focus for the future study.

A Pitx transcription factor controls the establishment and maintenance of the serotonergic lineage in planarians

In contrast to adult vertebrates, which have limited capacities for neurogenesis, adult planarians undergo constitutive cellular turnover during homeostasis and are even able to regenerate a whole brain after decapitation. This enormous plasticity derives from pluripotent stem cells residing in the planarian body in large numbers. It is still obscure how these stem cells are programmed for differentiation into specific cell lineages and how lineage identity is maintained. Here we identify a Pitx transcription factor of crucial importance for planarian regeneration. In addition to patterning defects that are co-dependent on the LIM homeobox transcription factor gene islet1, which is expressed with pitx at anterior and posterior regeneration poles, RNAi against pitx results in islet1-independent specific loss of serotonergic (SN) neurons during regeneration. Besides its expression in terminally differentiated SN neurons we found pitx in stem cell progeny committed to the SN fate. Also, intact pitx RNAi animals gradually lose SN markers, a phenotype that depends neither on increased apoptosis nor on stem cell-based turnover or transdifferentiation into other neurons. We propose that pitx is a terminal selector gene for SN neurons in planarians that controls not only their maturation but also their identity by regulating the expression of the Serotonin production and transport machinery. Finally, we made use of this function of pitx and compared the transcriptomes of regenerating planarians with and without functional SN neurons, identifying at least three new neuronal targets of Pitx.

Dynamic Rewiring of the Drosophila Retinal Determination Network Switches Its Function from Selector to Differentiation

Organ development is directed by selector gene networks. Eye development in the fruit fly Drosophila melanogaster is driven by the highly conserved selector gene network referred to as the “retinal determination gene network,” composed of approximately 20 factors, whose core comprises twin of eyeless (toy), eyeless (ey), sine oculis (so), dachshund (dac), and eyes absent (eya). These genes encode transcriptional regulators that are each necessary for normal eye development, and sufficient to direct ectopic eye development when misexpressed. While it is well documented that the downstream genes so, eya, and dac are necessary not only during early growth and determination stages but also during the differentiation phase of retinal development, it remains unknown how the retinal determination gene network terminates its functions in determination and begins to promote differentiation. Here, we identify a switch in the regulation of ey by the downstream retinal determination genes, which is essential for the transition from determination to differentiation. We found that central to the transition is a switch from positive regulation of ey transcription to negative regulation and that both types of regulation require so. Our results suggest a model in which the retinal determination gene network is rewired to end the growth and determination stage of eye development and trigger terminal differentiation. We conclude that changes in the regulatory relationships among members of the retinal determination gene network are a driving force for key transitions in retinal development.

Role of Phox2b in assembly of the circuits of the autonomic nervous system

The wiring of the brain, the way in which vast numbers of neuronal types are produced, migrate and grow neurites in precise spatial arrangements and connect to their proper target, and the prominent role of transcription factors in these phenomena still represent major questions in neurobiology. Using the reflex circuits of the autonomic nervous system of vertebrates and their master transcription regulator the homeodomain protein Phox2b as a model we explored this issue from several angles. From a molecular point of view we have shown that Phox2b is required in visceral neurons after it has specified them, providing one of the first examples for a “terminal selector gene” in vertebrates.

Distal‐Less Regulates Eyespot Patterns and Melanization in Bicyclus Butterflies

Butterfly eyespots represent novel complex traits that display substantial diversity in number and size within and across species. Correlative gene expression studies have implicated a large suite of transcription factors, including Distal-less (Dll), Engrailed (En), and Spalt (Sal), in eyespot development in butterflies, but direct evidence testing the function of any of these proteins is still missing. Here we show that the characteristic two-eyespot pattern of wildtype Bicyclus anynana forewings is correlated with dynamic progression of Dll, En, and Sal expression in larval wings from four spots to two spots, whereas no such decline in gene expression ensues in a four-eyespot mutant. We then conduct transgenic experiments testing whether over-expression of any of these genes in a wild-type genetic background is sufficient to induce eyespot differentiation in these pre-patterned wing compartments. We also produce a Dll-RNAi transgenic line to test how Dll down-regulation affects eyespot development. Finally we test how ectopic expression of these genes during the pupal stages of development alters adults color patters. We show that over-expressing Dll in larvae is sufficient to induce the differentiation of additional eyespots and increase the size of eyespots, whereas down-regulating Dll leads to a decrease in eyespot size. Furthermore, ectopic expression of Dll in the early pupal wing led to the appearance of ectopic patches of black scales. We conclude that Dll is a positive regulator of focal differentiation and eyespot signaling and that this gene is also a possible selector gene for scale melanization in butterflies.

Evaluating the influence of selection markers on obtaining selected pools and stable cell lines in human cells

Selection markers are common genetic elements used in recombinant cell line development. While several selection systems exist for use in mammalian cell lines, no previous study has comprehensively evaluated their performance in the isolation of recombinant populations and cell lines. Here we examine four antibiotics, hygromycin B, neomycin, puromycin, and Zeocin™, and their corresponding selector genes, using a green fluorescent protein (GFP) as a reporter in two model cell lines, HT1080 and HEK293. We identify Zeocin™ as the best selection agent for cell line development in human cells. In comparison to the other selection systems, Zeocin™ is able to identify populations with higher fluorescence levels, which in turn leads to the isolation of better clonal populations and less false positives. Furthermore, Zeocin™-resistant populations exhibit better transgene stability in the absence of selection pressure compared to other selection agents. All isolated Zeocin™-resistant clones, regardless of cell type, exhibited GFP expression. By comparison, only 79% of hygromycin B-resistant, 47% of neomycin-resistant, and 14% of puromycin-resistant clones expressed GFP. Based on these results, we rank Zeocin™ > hygromycin B ∼ puromycin > neomycin for cell line development in human cells. Furthermore, this study demonstrates that selection marker choice does indeed impact cell line development.

Prdm13 Mediates the Balance of Inhibitory and Excitatory Neurons in Somatosensory Circuits

Generating a balanced network of inhibitory and excitatory neurons during development requires precise transcriptional control. In the dorsal spinal cord, Ptf1a, a basic helix-loop-helix (bHLH) transcription activator, maintains this delicate balance by inducing homeodomain (HD) transcription factors such as Pax2 to specify the inhibitory lineage while suppressing HD factors such as Tlx1/3 that specify the excitatory lineage. We uncover the mechanism by which Ptf1a represses excitatory cell fate in the inhibitory lineage. We identify Prdm13 as a direct target of Ptf1a and reveal that Prdm13 actively represses excitatory cell fate by binding to regulatory sequences near the Tlx1 and Tlx3 genes to silence their expression. Prdm13 acts through multiple mechanisms, including interactions with the bHLH factor Ascl1, to repress Ascl1 activation of Tlx3. Thus, Prdm13 is a key component of a highly coordinated transcriptional network that determines the balance of inhibitory versus excitatory neurons in the dorsal spinal cord.

Modular development of the teleost trunk along the dorsoventral axis and zic1/zic4 as selector genes in the dorsal module

Teleost fish exhibit remarkable diversity in morphology, such as fins and coloration, particularly on the dorsal side. These structures are evolutionary adaptive because their back is highly visible to other individuals. However, owing to the late phenotypic appearance (from larva to adult) and lack of appropriate mutants, the genetic mechanisms that regulate these dorsoventrally asymmetric external patterns are largely unknown. To address this, we have analyzed the spontaneous medaka mutant Double anal fin (Da), which exhibits a mirror-image duplication of the ventral half across the lateral midline from larva to adult. Da is an enhancer mutant for zic1 and zic4 in which their expression in dorsal somites is lost. We show that the dorsoventral polarity in Da somites is lost and then demonstrate using transplantation techniques that somites and their derived tissues globally determine the multiple dorsal-specific characteristics of the body (fin morphology and pigmentation) from embryo to adult. Intriguingly, the zic1/zic4 expression in the wild type persists throughout life in the dorsal parts of somite derivatives, i.e. the myotome, dermis and vertebrae, forming a broad dorsal domain in the trunk. Comparative analysis further implies a central role for zic1/zic4 in morphological diversification of the teleost body. Taken together, we propose that the teleost trunk consists of dorsal/ventral developmental modules and that zic1/zic4 in somites function as selector genes in the dorsal module to regulate multiple dorsal morphologies.

A Double-Switch Vector System Positively Regulates Transgene Expression by Endogenous microRNA Expression (miR-ON Vector)

To better understand and exploit microRNA (miR) regulation, a more precise characterization of miR expression patterns within a tissue or a lineage during development, differentiation, and homeostasis is needed. We previously showed that lentiviral vectors (LV) can be made responsive to miR to stringently control transgene expression as well as to report miR activity "live" and at the single-cell level. Although very useful, this approach reports miR activity by transgene suppression, hampering the direct identification and selection of miR-expressing cells. Here, we describe a strategy to couple transgene expression to the activity of the miR of interest. To this aim, we generated LV encoding two in-series OFF switches: a transcriptional repressor tagged with miR target sequences and a reporter cassette under the control of the repressor. Reporter expression is ON only when the miR is active and represses translation of the transcriptional repressor. We successfully applied this design to different types of repressors, multiple gene encoding vectors and delivered the system either by two separate or a self-contained vector. We demonstrated its performance by live monitoring of two miRs in different stages of human primary hematopoietic stem/progenitor cell differentiation in vivo. Further applications of this approach include imaging of rare miR-expressing cells and positive regulation of a therapeutic or selector gene in target cells identified by the expression of selected miRs.

RaxIs a Selector Gene for Mediobasal Hypothalamic Cell Types

The brain plays a central role in controlling energy, glucose, and lipid homeostasis, with specialized neurons within nuclei of the mediobasal hypothalamus, namely the arcuate (ARC) and ventromedial (VMH), tasked with proper signal integration. Exactly how the exquisite cytoarchitecture and underlying circuitry becomes established within these nuclei remains largely unknown, in part because hypothalamic developmental programs are just beginning to be elucidated. Here, we demonstrate that the Retina and anterior neural fold homeobox (Rax) gene plays a key role in establishing ARC and VMH nuclei in mice. First, we show that Rax is expressed in ARC and VMH progenitors throughout development, consistent with genetic fate mapping studies demonstrating that Rax+ lineages give rise to VMH neurons. Second, the conditional ablation of Rax in a subset of VMH progenitors using a Shh::Cre driver leads to a fate switch from a VMH neuronal phenotype to a hypothalamic but non-VMH identity, suggesting that Rax is a selector gene for VMH cellular fates. Finally, the broader elimination of Rax throughout ARC/VMH progenitors using Six3::Cre leads to a severe loss of both VMH and ARC cellular phenotypes, demonstrating a role for Rax in both VMH and ARC fate specification. Combined, our study illustrates that Rax is required in ARC/VMH progenitors to specify neuronal phenotypes within this hypothalamic brain region. Rax thus provides a molecular entry point for further study of the ontology and establishment of hypothalamic feeding circuits.

Imposing Order in the Drosophila Eye by the Retinal Determination Network and the Morphogenetic Furrow

Event Abstract Back to Event Imposing Order in the Drosophila Eye by the Retinal Determination Network and the Morphogenetic Furrow Justin P. Kumar1* 1 Indiana University, Biology, United States The developing eye of the fruit fly Drosophila melanogaster consists of several thousand non-patterned and undifferentiated cells all of which must first adopt a retinal fate and then be organized into rows of evenly spaced unit eyes or ommatidia. The first task is accomplished by the activity of a set of fourteen nuclear proteins that comprise the evolutionarily conserved retinal determination network. Mutations that disrupt the functioning of this network lead to a loss of the compound eyes. Conversely, forced activation of the network is sufficient to induce eye formation in non-retinal tissues. These results indicate that the network is tasked with promoting an eye fate. Recently we have demonstrated that another responsibility of the network is to prevent antennal and head capsule selector genes from being expressed within the eye field. When the network is disrupted the loss of retinal tissue is accompanied by ectopic expression of non-retinal selector genes and a transformation of the eye field into either antenna or head epidermis. During ectopic eye formation we also observe an inhibition of endogenous selector gene expression when the retinal determination network is activated. Later in development, after cells have committed themselves to an eye fate, the morphogenetic furrow sweeps across the retinal epithelium and transforms the eye field into a crystalline-like lattice of evenly arranged unit eyes. Mutations that block the initiation of the furrow or delay/stop its progression lead to the production of adult eyes that either completely lack or have considerably fewer unit eyes. The acceleration of the furrow, a phenotype that is seen in retinas that are mutant for extramacrochaetae (emc), can have equally devastating effects. The faster moving furrow progresses and patterns the eye field before the tissue has had a chance to reach its full size. We have recently demonstrated that Emc regulates the velocity of pattern formation by promoting the partial degradation of full-length Cubitus Interruptus (Ci) into a smaller isoform that blocks activation of Hedgehog target genes ahead of the advancing furrow. Reductions in emc lead to the stabilization of full-length Ci that then functions to activate Hedgehog dependent target genes. We have determined this to be the primary cause underlying the acceleration of the furrow. In this talk I will briefly summarize our recent findings on how the retinal determination network and the morphogenetic furrow impose order on the developing retina. References Spratford, C.M. and Kumar, J.P. (2013) Extramacrochaetae imposes order on the Drosophila retina by refining the activity of the Hedgehog signaling gradient. Development 140: 1994-2004 Weasner, B.M. and Kumar, J.P. (2013) Competition amongst gene regulatory networks imposes order within the eye-antennal disc of Drosophila. Development 140: 205-215 Anderson, A.M., Weasner, B.M., Weasner, B.P. and Kumar, J.P. (2012) Dual transcriptional activities of Sine Oculis define its roles in normal and ectopic eye development. Development 139: 991-1000 Keywords: Drosophila, Retina, morphogenetic furrow, retinal determination network, extramacrochaetae, Hedgehog Conference: International Conference on Invertebrate Vision, Fjälkinge, Sweden, 1 Aug - 8 Aug, 2013. Presentation Type: Oral presentation preferred Topic: Development and evolution Citation: Kumar JP (2019). Imposing Order in the Drosophila Eye by the Retinal Determination Network and the Morphogenetic Furrow. Front. Physiol. Conference Abstract: International Conference on Invertebrate Vision. doi: 10.3389/conf.fphys.2013.25.00008 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 29 Apr 2013; Published Online: 09 Dec 2019. * Correspondence: Prof. Justin P Kumar, Indiana University, Biology, Bloomington, IN, 47405, United States, jkumar@indiana.edu Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Justin P Kumar Google Justin P Kumar Google Scholar Justin P Kumar PubMed Justin P Kumar Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Competition among gene regulatory networks imposes order within the eye-antennal disc of Drosophila

The eye-antennal disc of Drosophila gives rise to numerous adult tissues, including the compound eyes, ocelli, antennae, maxillary palps and surrounding head capsule. The fate of each tissue is governed by the activity of unique gene regulatory networks (GRNs). The fate of the eye, for example, is controlled by a set of fourteen interlocking genes called the retinal determination (RD) network. Mutations within network members lead to replacement of the eyes with head capsule. Several studies have suggested that in these instances all retinal progenitor and precursor cells are eliminated via apoptosis and as a result the surrounding head capsule proliferates to compensate for retinal tissue loss. This model implies that the sole responsibility of the RD network is to promote the fate of the eye. We have re-analyzed eyes absent mutant discs and propose an alternative model. Our data suggests that in addition to promoting an eye fate the RD network simultaneously functions to actively repress GRNs that are responsible for directing antennal and head capsule fates. Compromising the RD network leads to the inappropriate expression of several head capsule selector genes such as cut, Lim1 and wingless. Instead of undergoing apoptosis, a population of mutant retinal progenitors and precursor cells adopt a head capsule fate. This transformation is accompanied by an adjustment of cell proliferation rates such that just enough head capsule is generated to produce an intact adult head. We propose that GRNs simultaneously promote primary fates, inhibit alternative fates and establish cell proliferation states.

Sp1 modifies leg-to-wing transdetermination in Drosophila

During Drosophila development, the transcription factor Sp1 is necessary for proper leg growth and also to repress wing development. Here we test the role of Sp1 during imaginal disc regeneration. Ubiquitous expression of wg induces a regeneration blastema in the dorsal aspect of the leg disc. Within this outgrowth, the wing selector gene vg is activated in some cells, changing their fate to wing identity in a process known as transdetermination. In this report we demonstrate that reducing the gene copy number of Sp1 significantly increases both the frequency and the area of transdetermination in regenerating leg discs. By examining the expression of known Sp1 target genes, we also show that the proximo-distal patterning gene dachshund is downregulated dorsally, leading to a break in its normal ring-shaped expression pattern. We further report that transdetermination, as evidenced by Vg expression, is only observed when there is a broken ring of Dachshund expression. Combined, these studies establish a role for Sp1 in leg-to-wing transdetermination.

The selector gene Pax7 dictates alternate pituitary cell fates through its pioneer action on chromatin remodeling

The anterior and intermediate lobes of the pituitary gland derive from the surface ectoderm. They provide a simple system to assess mechanisms of developmental identity established by tissue determinants. Each lobe contains a lineage expressing the hormone precursor pro-opiomelanocortin (POMC): the corticotropes and melanotropes. The T-box transcription factor Tpit controls terminal differentiation of both lineages. We now report on the unique role of Pax7 as a selector of intermediate lobe and melanotrope identity. Inactivation of the Pax7 gene results in loss of melanotrope gene expression and derepression of corticotrope genes. Pax7 acts by remodeling chromatin and allowing Tpit binding to a new subset of enhancers for activation of melanotrope-specific genes. Thus, the selector function of Pax7 is exerted through pioneer transcription factor activity. Genome-wide, the Pax7 pioneer activity is preferentially associated with composite binding sites that include paired and homeodomain motifs. Pax7 expression is conserved in human and dog melanotropes and defines two subtypes of pituitary adenomas causing Cushing's disease. In summary, expression of Pax7 provides a unique tissue identity to the pituitary intermediate lobe that alters Tpit-driven differentiation through pioneer and classical transcription factor activities.

Transcriptional regulatory mechanisms underlying the GABAergic neuron fate in different diencephalic prosomeres

Diverse mechanisms regulate development of GABAergic neurons in different regions of the central nervous system. We have addressed the roles of a proneural gene, Ascl1, and a postmitotic selector gene, Gata2, in the differentiation of GABAergic neuron subpopulations in three diencephalic prosomeres: prethalamus (P3), thalamus (P2) and pretectum (P1). Although the different proliferative progenitor populations of GABAergic neurons commonly express Ascl1, they have distinct requirements for it in promotion of cell-cycle exit and GABAergic neuron identity. Subsequently, Gata2 is activated as postmitotic GABAergic precursors are born. In P1, Gata2 regulates the neurotransmitter identity by promoting GABAergic and inhibiting glutamatergic neuron differentiation. Interestingly, Gata2 defines instead the subtype of GABAergic neurons in the rostral thalamus (pTh-R), which is a subpopulation of P2. Without Gata2, the GABAergic precursors born in the pTh-R fail to activate subtype-specific markers, but start to express genes typical of GABAergic precursors in the neighbouring P3 domain. Thus, our results demonstrate diverse mechanisms regulating differentiation of GABAergic neuron subpopulations and suggest a role for Gata2 as a selector gene of both GABAergic neuron neurotransmitter and prosomere subtype identities in the developing diencephalon. Our results demonstrate for the first time that neuronal identities between distinct prosomeres can still be transformed in postmitotic neuronal precursors.