The Drosophila miti-mere gene, a member of the POU family, is required for the specification of the RP2/sibling lineage during neurogenesis

A repressor-decay timer for robust temporal patterning in embryonic Drosophila neuroblast lineages

Biological timers synchronize patterning processes during embryonic development. In the Drosophila embryo, neural progenitors (neuroblasts; NBs) produce a sequence of unique neurons whose identities depend on the sequential expression of temporal transcription factors (TTFs). The stereotypy and precision of NB lineages indicate reproducible TTF timer progression. We combine theory and experiments to define the timer mechanism. The TTF timer is commonly described as a relay of activators, but its regulatory circuit is also consistent with a repressor-decay timer, where TTF expression begins when its repressor decays. Theory shows that repressor-decay timers are more robust to parameter variations than activator-relay timers. This motivated us to experimentally compare the relative importance of the relay and decay interactions in vivo. Comparing WT and mutant NBs at high temporal resolution, we show that the TTF sequence progresses primarily by repressor-decay. We suggest that need for robust performance shapes the evolutionary-selected designs of biological circuits.

The Oct1 homolog Nubbin is a repressor of NF-κB-dependent immune gene expression that increases the tolerance to gut microbiota

Background

Innate immune responses are evolutionarily conserved processes that provide crucial protection against invading organisms. Gene activation by potent NF-κB transcription factors is essential both in mammals and Drosophila during infection and stress challenges. If not strictly controlled, this potent defense system can activate autoimmune and inflammatory stress reactions, with deleterious consequences for the organism. Negative regulation to prevent gene activation in healthy organisms, in the presence of the commensal gut flora, is however not well understood.

Results

We show that the Drosophila homolog of mammalian Oct1/POU2F1 transcription factor, called Nubbin (Nub), is a repressor of NF-κB/Relish-driven antimicrobial peptide gene expression in flies. In nub¹ mutants, which lack Nub-PD protein, excessive expression of antimicrobial peptide genes occurs in the absence of infection, leading to a significant reduction of the numbers of cultivatable gut commensal bacteria. This aberrant immune gene expression was effectively blocked by expression of Nub from a transgene. We have identified an upstream regulatory region, containing a cluster of octamer sites, which is required for repression of antimicrobial peptide gene expression in healthy flies. Chromatin immunoprecipitation experiments demonstrated that Nub binds to octamer-containing promoter fragments of several immune genes. Gene expression profiling revealed that Drosophila Nub negatively regulates many genes that are involved in immune and stress responses, while it is a positive regulator of genes involved in differentiation and metabolism.

Conclusions

This study demonstrates that a large number of genes that are activated by NF-κB/Relish in response to infection are normally repressed by the evolutionarily conserved Oct/POU transcription factor Nub. This prevents uncontrolled gene activation and supports the existence of a normal gut flora. We suggest that Nub protein plays an ancient role, shared with mammalian Oct/POU transcription factors, to moderate responses to immune challenge, thereby increasing the tolerance to biotic stress.

The Drosophila CPEB protein Orb2 has a novel expression pattern and is important for asymmetric cell division and nervous system function

Cytoplasmic polyadenylation element binding (CPEB) proteins bind mRNAs to regulate their localization and translation. While the first CPEBs discovered were germline specific, subsequent studies indicate that CPEBs also function in many somatic tissues including the nervous system. Drosophila has two CPEB family members. One of these, orb, plays a key role in the establishment of polarity axes in the developing egg and early embryo, but has no known somatic functions or expression outside of the germline. Here we characterize the other Drosophila CPEB, orb2. Unlike orb, orb2 mRNA and protein are found throughout development in many different somatic tissues. While orb2 mRNA and protein of maternal origin are distributed uniformly in early embryos, this pattern changes as development proceeds and by midembryogenesis the highest levels are found in the CNS and PNS. In the embryonic CNS, Orb2 appears to be concentrated in cell bodies and mostly absent from the longitudinal and commissural axon tracts. In contrast, in the adult brain, the protein is seen in axonal and dendritic terminals. Lethal effects are observed for both RNAi knockdowns and orb2 mutant alleles while surviving adults display locomotion and behavioral defects. We also show that orb2 funtions in asymmetric division of stem cells and precursor cells during the development of the embryonic nervous system and mesoderm.

The Drosophila Hem/Kette/Nap1 protein regulates asymmetric division of neural precursor cells by regulating localization of Inscuteable and Numb

The Hem/Kette/Nap1 protein is involved in many biological processes. We have recently reported that Hem is required for the normal migration of neurons in the Drosophila embryo. In this paper, we report that Hem regulates the asymmetric division of neural precursor cells. We find that a well-studied Hem/Kette mutant allele produces at least two main, but possibly more, phenotypic classes of mutant embryos, and these phenotypes correlate with variable levels of maternal wild type Hem protein in the developing embryo. While the weaker class exhibits weak axon guidance defect and the mis-migration of neurons, the stronger class causes severe axon guidance defects, mis-migration of neurons and symmetric division of ganglion mother cells (GMC) of the RP2/sib lineage. We also show that the basis for the loss of asymmetric division is due to non-localization of Inscuteable and Numb in GMC-1. A non-asymmetric Numb segregates to both daughter cells of GMC-1, which then prevents Notch signaling from specifying a sib fate. This causes both cells to adopt an RP2 fate. Furthermore, loss of function for Abelson tyrosine kinase also causes loss of asymmetric localization of Inscuteable and Numb and symmetric division of GMC-1, the loss of function for WAVE has a very weakly penetrant loss of asymmetry defect. These results define another role for Hem/Kette/Nap1 in a neural precursor cell during neurogenesis.

Evolution of nubbin function in hemimetabolous and holometabolous insect appendages

Insects display a whole spectrum of morphological diversity, which is especially noticeable in the organization of their appendages. A recent study in a hemipteran, Oncopeltus fasciatus (milkweed bug), showed that nubbin (nub) affects antenna morphogenesis, labial patterning, the length of the femoral segment in legs, and the formation of a limbless abdomen. To further determine the role of this gene in the evolution of insect morphology, we analyzed its functions in two additional hemimetabolous species, Acheta domesticus (house cricket) and Periplaneta americana (cockroach), and re-examined its role in Drosophila melanogaster (fruit fly). While both Acheta and Periplaneta nub-RNAi first nymphs develop crooked antennae, no visible changes are observed in the morphologies of their mouthparts and abdomen. Instead, the main effect is seen in legs. The joint between the tibia and first tarsomere (Ta-1) is lost in Acheta, which in turn, causes a fusion of these two segments and creates a chimeric nub-RNAi tibia-tarsus that retains a tibial identity in its proximal half and acquires a Ta-1 identity in its distal half. Similarly, our re-analysis of nub function in Drosophila reveals that legs lack all true joints and the fly tibia also exhibits a fused tibia and tarsus. Finally, we observe a similar phenotype in Periplaneta except that it encompasses different joints (coxa-trochanter and femur-tibia), and in this species we also show that nub expression in the legs is regulated by Notch signaling, as had previously been reported in flies and spiders. Overall, we propose that nub acts downstream of Notch on the distal part of insect leg segments to promote their development and growth, which in turn is required for joint formation. Our data represent the first functional evidence defining a role for nub in leg segmentation and highlight the varying degrees of its involvement in this process across insects.

Neuralized mediates asymmetric division of neural precursors by two distinct and sequential events: promoting asymmetric localization of Numb and enhancing activation of Notch-signaling

In the CNS, the evolutionarily conserved Notch pathway regulates asymmetric cell fate specification to daughters of ganglion mother cells (GMCs). The E3 Ubiquitin ligase protein Neuralized (Neur) is thought to activate Notch-signaling by the endocytosis of Delta and the Delta-bound extracellular domain of Notch. The intracellular Notch then initiates Notch-signaling. Numb blocks N-signaling in one of the two daughters of a GMC, allowing that cell to adopt a different identity. Numb is asymmetrically localized in a GMC and is segregated to only one of the two daughter cells. In the typical GMC-1→RP2/sib lineage, we found that loss of Neur activity causes symmetric division of GMC-1 into two RP2s. We further found that Neur asymmetrically localizes in a late GMC-1 to the Numb domain and Neur mediates asymmetric division via two distinct, sequential mechanisms: by promoting the asymmetric localization of Numb in a GMC-1 via down-regulation of the transcription factor Pdm1, followed by enhancing the Notch-signaling via trans-potentiation of Notch in a cell committed to become a sib. In neur mutants the GMC-1 identity is not altered but Numb is non-asymmetrically localized due to an up-regulation of Pdm1. Thus, both its daughters inherit Numb, which prevents Notch from specifying a sib identity. Neur also enhances Notch since in neur; numb double mutants, both sibling cells often adopt a mixed fate as opposed to an RP2 fate observed in Notch; numb double mutants. Furthermore, over-expression of Neur can induce both cells to adopt a sib fate similar to gain of function Notch. Our results tie Numb and Notch-signaling through a single player, Neur, thus giving us a more complete picture of the events surrounding asymmetric division of precursor cells. We also show that Neur and Numb are interdependent for their asymmetric-localizations.

Wingless activity in the precursor cells specifies neuronal migratory behavior in the Drosophila nerve cord

Neurons and their precursor cells are formed in different regions within the developing CNS, but they migrate and occupy very specific sites in the mature CNS. The ultimate position of neurons is crucial for establishing proper synaptic connectivity in the brain. In Drosophila, despite its extensive use as a model system to study neurogenesis, we know almost nothing about neuronal migration or its regulation. In this paper, I show that one of the most studied neuronal pairs in the Drosophila nerve cord, RP2/sib, has a complicated migratory route. Based on my studies on Wingless (Wg) signaling, I report that the neuronal migratory pattern is determined at the precursor cell stage level. The results show that Wg activity in the precursor neuroectodermal and neuroblast levels specify neuronal migratory pattern two divisions later, thus, well ahead of the actual migratory event. Moreover, at least two downstream genes, Cut and Zfh1, are involved in this process but their role is at the downstream neuronal level. The functional importance of normal neuronal migration and the requirement of Wg signaling for the process are indicated by the finding that mislocated RP2 neurons in embryos mutant for Wg-signaling fail to properly send out their axon projection.

Isolation of regulators of Drosophila immune defense genes by a double interaction screen in yeast

Innate immunity is a universal and ancient defense system in metazoans against microorganisms. Antimicrobial peptides, which are synthesized both in insects and humans, constitute an endogenous, gene-encoded defense arsenal. In Drosophila, antimicrobial peptides, such as the potent cecropins, are expressed both constitutively in barrier epithelia, as well as systemically in response to infection. Rel/NF-kappaB proteins are well-known regulators of antimicrobial peptide genes, but very few Rel/NF-kappaB co-factors and/or tissue-specific regulators have been identified. We performed a double interaction screen in yeast to isolate Drosophila cDNAs coding for direct regulators, as well as Dif co-regulators, of the CecropinA1 gene. Three classes of positive cDNA clones corresponding to 15 Drosophila genes were isolated and further characterized. One of the Dif-independent cDNAs encoded the Rel/NF-kappaB protein Relish; a well-known activator of antimicrobial peptide genes in Drosophila, demonstrating the applicability of this type of screen for isolating regulators of immune defense. Most interestingly, three transcription factors belonging to the POU domain class of homeodomain proteins, Pdm1, Pdm2 and Dfr/Vvl were isolated as Dif-interacting partners, and subsequently verified as regulators of CecA1 expression in Drosophila cells. The importance of POU proteins in development and differentiation in Drosophila and mammals is well documented, but their role in regulation of Drosophila immune defense genes is a new and essential finding.

Generating asymmetry: with and without self-renewal

At some point during the history of organismal evolution, unicellular, unipotent and mitotically active cells acquired an ability to undergo a special type of cell division called asymmetric division. By this special type of cell division, these cells could divide to generate two different progeny or to self-renew and at the same time generate a progeny that is committed to become a cell different from the mother cell. This type of cell division, which forms the basis for the functioning of totipotent or multipotent stem cells, underlies the fundamental basis for the developmental evolution of organisms. It is not clear if the asymmetric division without self-renewal preceded the asymmetric division with self-renewal. It is reasonable to assume that the asymmetric division without self-renewal preceded the asymmetric division with self-renewal. In this review we explore the genetic regulation of these two types of asymmetric divisions using the Drosophila central nervous system (CNS) as a model system. The results from recent studies argue that for cells to undergo a self-renewing asymmetric division, certain "stem cell" proteins must be maintained or up-regulated, while genes encoding proteins responsible for differentiation must be repressed or down-regulated. As long as a balance between these two classes of proteins is maintained via asymmetric segregation and activation/repression, the progeny that receives stem cell proteins/maintains stem cell competence will have the potential to undergo self-renewing asymmetric division. The other progeny will commit to differentiate. In non-self-renewing asymmetric division, down-regulation of stem cell proteins/competence combined with asymmetric segregation of cell identity specifying factors (either cell-autonomous or a combination of cell autonomous and non-cell autonomous signals) cause the two progeny to assume different differentiated identities. Identification of mutations that confer a stem cell type of division to nonstem cell precursors, or mutations that eliminate asymmetric division, has led the way in elucidating the molecular basis for these divisions. Given that there is a considerable degree of conservation of genes and their function, these studies should provide clear insight into how the self-renewing asymmetric division of stem cells in neural and other lineages is regulated not only in Drosophila but also in vertebrates including humans.

Differential effects of Drosophila mastermind on asymmetric cell fate specification and neuroblast formation

During neurogenesis in the ventral nerve cord of the Drosophila embryo, Notch signaling participates in the pathway that mediates asymmetric fate specification to daughters of secondary neuronal precursor cells. In the NB4-2 --> GMC-1 --> RP2/sib lineage, a well-studied neuronal lineage in the ventral nerve cord, Notch signaling specifies sib fate to one of the daughter cells of GMC-1. Notch mediates this process via Mastermind (Mam). Loss of function for mam, similar to loss of function for Notch, results in GMC-1 symmetrically dividing to generate two RP2 neurons. Loss of function for mam also results in a severe neurogenic phenotype. In this study, we have undertaken a functional analysis of the Mam protein. We show that while ectopic expression of a truncated Mam protein induces a dominant-negative neurogenic phenotype, it has no effect on asymmetric fate specification. This truncated Mam protein rescues the loss of asymmetric specification phenotype in mam in an allele-specific manner. We also show an interallelic complementation of loss-of-asymmetry defect. Our results suggest that Mam proteins might associate during the asymmetric specification of cell fates and that the N-terminal region of the protein plays a role in this process.

GAGA factor isoforms have distinct but overlapping functions in vivo

The Drosophila melanogaster GAGA factor (encoded by the Trithorax-like [Trl] gene) is required for correct chromatin architecture at diverse chromosomal sites. The Trl gene encodes two alternatively spliced isoforms of the GAGA factor (GAGA-519 and GAGA-581) that are identical except for the length and sequence of the C-terminal glutamine-rich (Q) domain. In vitro and tissue culture experiments failed to find any functional difference between the two isoforms. We made a set of transgenes that constitutively express cDNAs coding for either of the isoforms with the goal of elucidating their roles in vivo. Phenotypic analysis of the transgenes in Trl mutant background led us to the conclusion that GAGA-519 and GAGA-581 perform different, albeit largely overlapping, functions. We also expressed a fusion protein with LacZ disrupting the Q domain of GAGA-519. This LacZ fusion protein compensated for the loss of wild-type GAGA factor to a surprisingly large extent. This suggests that the Q domain either is not required for the essential functions performed by the GAGA protein or is exclusively used for tetramer formation. These results are inconsistent with a major role of the Q domain in chromatin remodeling or transcriptional activation. We also found that GAGA-LacZ was able to associate with sites not normally occupied by the GAGA factor, pointing to a role of the Q domain in binding site choice in vivo.

Lineage diversity in the Drosophila nervous system

The detailed descriptions of cellular lineages in the Drosophila nervous system have provided the foundations for an in-depth genetic analysis of the mechanisms that regulate fate decisions at every cell cycle.

Programmed transformations in neuroblast gene expression during Drosophila CNS lineage development

During Drosophila embryonic CNS development, the sequential neuroblast (NB) expression of four proteins, Hunchback (Hb), Pou-homeodomain proteins 1 and 2 (referred to collectively as Pdm), and Castor (Cas), identifies a transcription factor network regulating the temporal development of all ganglia. The Zn-finger proteins Hb and Cas, acting as repressors, confine Pdm expression to a narrow intermediate temporal window; this results in the generation of three panneural domains whose cellular constituents are marked by expression of Hb, Pdm, or Cas (R. Kambadur et al., 1998, Genes Dev. 12, 246-260). Seeking to identify the cellular mechanisms that generate these expression compartments, we studied the lineage development of isolated NBs in culture. We found that the Hb, Pdm, and Cas expression domains are generated by transitions in NB gene expression that are followed by gene product perdurance within sequentially produced sublineages. Our results also indicate that following Cas expression, many CNS NBs continue their asymmetric divisions generating additional progeny, which can be identified by the expression of the bHLH transcription factor Grainyhead (Gh). Gh appears to be a terminal embryonic CNS lineage marker. Taken together, these studies indicate that once NBs initiate lineage development, no additional signaling between NBs and the neuroectoderm and/or mesoderm is required to trigger the temporal progression of Hb --> Pdm --> Cas --> Gh expression during NB outgrowth.

Cell division genes promote asymmetric interaction between Numb and Notch in the Drosophila CNS

Cell intrinsic and cell extrinsic factors mediate asymmetric cell divisions during neurogenesis in the Drosophila embryo. In the NB4-2->GMC-1->RP2/sib lineage, one of the well-studied neuronal lineages in the ventral nerve cord, the Notch (N) signaling interacts with the asymmetrically localized Numb (Nb) to specify sibling neuronal fates to daughter cells of GMC-1. In this current study, we have investigated asymmetric cell fate specifications by N and Nb in the context of cell cycle. We have used loss-of-function mutations in N and nb, cell division mutants cyclinA (cycA), regulator of cyclin A1 (rca1) and string/cdc25 phosphatase (stg), and the microtubule destabilizing agent, nocodazole, to investigate this issue. We report that the loss of cycA, rca1 or stg leads to a block in the division of GMC-1, however, this GMC-1 exclusively adopts an RP2 identity. While the loss of N leads to the specification of RP2 fates to both progeny of GMC-1 and loss of nb results in the specification of sib fates to these daughter cells, the GMC-1 in the double mutant between nb and cycA assumes a sib fate. These epistasis results indicate that both N and nb function downstream of cell division genes and that progression through cell cycle is required for the asymmetric localization of Nb. In the absence of entry to metaphase, the Nb protein prevents the N signaling from specifying sib fate to the RP2/sib precursor. These results are also consistent with our finding that the sib cell is specified as RP2 in N; nb double mutants. Finally, our results show that nocodazole-arrested GMC-1 in wild-type embryos randomly assumes either an RP2 fate or a sib fate. This suggests that microtubules are involved in mediating the antagonistic interaction between Nb and N during RP2 and sib fate specification.

Krox-20 controls SCIP expression, cell cycle exit and susceptibility to apoptosis in developing myelinating Schwann cells

The transcription factors Krox-20 and SCIP each play important roles in the differentiation of Schwann cells. However, the genes encoding these two proteins exhibit distinct time courses of expression and yield distinct cellular phenotypes upon mutation. SCIP is expressed prior to the initial appearance of Krox-20, and is transient in both the myelinating and non-myelinating Schwann cell lineages; while in contrast, Krox-20 appears approximately 24 hours after SCIP and then only within the myelinating lineage, where its expression is stably maintained into adulthood. Similarly, differentiation of SCIP−/− Schwann cells appears to transiently stall at the promyelinating stage that precedes myelination, whereas Krox-20(−/−) cells are, by morphological criteria, arrested at this stage. These observations led us to examine SCIP regulation and Schwann cell phenotype in Krox-20 mouse mutants. We find that in Krox-20(−/−) Schwann cells, SCIP expression is converted from transient to sustained. We further observe that both Schwann cell proliferation and apoptosis, which are normal features of SCIP+ cells, are also markedly increased late in postnatal development in Krox-20 mutants relative to wild type, and that the levels of cell division and apoptosis are balanced to yield a stable number of Schwann cells within peripheral nerves. These data demonstrate that the loss of Krox-20 in myelinating Schwann cells arrests differentiation at the promyelinating stage, as assessed by SCIP expression, mitotic activity and susceptibility to apoptosis.

Gene regulatory functions of Drosophila fish-hook, a high mobility group domain Sox protein

In this study we investigate the gene regulatory functions of Drosophila Fish-hook (Fish), a high mobility group (HMG) Sox protein that is essential for embryonic segmentation. We show that the Fish HMG domain binds to the vertebrate Sox protein consensus DNA binding sites, AACAAT and AACAAAG, and that this binding induces an 85 degrees DNA bend. In addition, we use a heterologous yeast system to show that the NH2-terminal portion of Fish protein can function as a transcriptional activator. Fish directly regulates the expression of the pair rule gene, even-skipped (eve), by binding to multiple sites located in downstream regulatory regions that direct formation of eve stripes 1, 4, 5, and 6. Fish may function along with the Drosophila POU domain proteins Pdm-1 and Pdm-2 to regulate eve transcription, as genetic interactions were detected between fish and pdm mutants. Finally, we determined that Fish protein is expressed in a dynamic pattern throughout embryogenesis, and is present in nuclear and cytoplasmic compartments.

Regulation of POU genes by castor and hunchback establishes layered compartments in the Drosophila CNS

POU transcription factors participate in cell-identity decisions during nervous system development, yet little is known about the regulatory networks controlling their expression. We report all known Drosophila POU genes require castor (cas) for correct CNS expression. drifter and I-POU depend on cas for full expression, whereas pdm-1 and pdm-2 are negatively regulated. cas encodes a zinc finger protein that shares DNA-binding specificity with another pdm repressor: the gap segmentation gene regulator Hunchback (Hb). Our studies reveal that the embryonic CNS contains sequentially generated neuroblast sublineages that can be distinguished by their expression of either Hb, Pdm-1, or Cas. Hb and Cas may directly silence pdm expression in early and late developing sublineages, given that pdm-1 cis-regulatory DNA contains >=32 Hb/Cas-binding sites and its enhancer(s) are ectopically activated in cas- neuroblasts. In addition, the targeted misexpression of Cas in all neuroblast lineages reduces Pdm-1 expression without altering Hb expression. By ensuring correct POU gene expression boundaries, hb and cas maintain temporal subdivisions in the cell-identity circuitry controlling CNS development.

Requirement for engrailed and invected genes reveals novel regulatory interactions between engrailed/invected, patched, gooseberry and wingless during Drosophila neurogenesis

During neurogenesis, the transmembrane protein Patched (Ptc) promotes a wingless (wg)-mediated specification of a neuronal precursor cell, NB4-2, by repressing gooseberry (gsb). In this study, novel interactions of these genes with engrailed (en) and invected (inv) during neurogenesis have been uncovered. While in row 4 cells Ptc represses gsb and wg, in row 5 cells en/inv relieve Ptc repression of gsb by a non-autonomous mechanism that does not involve hedgehog (hh). This differential regulation of gsb leads to the specification of NB5-3 and NB4-2 identities to two distinct neuroblasts. The uncoupling of the ptc-gsb regulatory circuit also enables gsb to promote Wg expression in row 5 cells. Our results suggest that the en/inv-->ptc-->gsb-->wg pathway uncovered here and the hh-->wg are distinct pathways that function to maintain wild-type level of Wg. Our results also indicate that Hh is not the only ligand for Ptc and similarly Ptc is not the only receptor for Hh.

XIPOU 2 is a potential regulator of Spemann's Organizer

XIPOU 2, a member of the class III POU-domain family, is expressed initially at mid-blastula transition (MBT) and during gastrulation in the entire marginal zone mesoderm, including Spemann's Organizer (the Organizer). To identify potential targets of XIPOU 2, the interaction of XIPOU 2 with other genes co-expressed in the Organizer was examined by microinjecting XIPOU 2's mRNA into the lineage of cells that contributes to the Organizer, head mesenchyme and prechordal plate. XIPOU 2 suppresses the expression of a number of dorsal mesoderm-specific genes, including gsc, Xlim-1, Xotx2, noggin and chordin, but not Xnot. As a consequence of the suppression of dorsal mesoderm gene expression, bone morphogenetic factor-4 (Bmp-4), a potent inducer of ventral mesoderm, is activated in the Organizer. Gsc is a potential target of XIPOU 2. XIPOU 2 is capable of binding a class III POU protein binding site (CATTAAT) that is located within the gsc promoter, in the activin-inducible (distal) element. Furthermore, XIPOU 2 suppresses the activation of the gsc promoter by activin signaling. At the neurula and tailbud stages, dorsoanterior structures are affected: embryos displayed micropthalmia and the loss of the first branchial arch, as detected by the expression of pax-6, Xotx2 and en-2. By examining events downstream from the Wnt and chordin pathways, we determined that XIPOU 2, when overexpressed, acts specifically in the Organizer, downstream from GSK-3beta of the Wnt pathway and upstream from chordin. The interference in dorsalizing events caused by XIPOU 2 was rescued by chordin. Thus, in addition to its direct neuralizing ability, in a different context, XIPOU 2 has the potential to antagonize dorsalizing events in the Organizer.

The Drosophila fish-hook gene encodes a HMG domain protein essential for segmentation and CNS development

We describe the isolation and analysis of the Drosophila fish-hook (fish) gene, which encodes a novel member of the SOX subgroup of High Mobility Group (HMG) domain proteins that exhibit similarity to the mammalian testis determining factor, SRY. The fish gene is initially expressed in a pair-rule-like pattern which is rapidly replaced by strong neuroectoderm expression. fish null mutants exhibit severe segmentation defects, including loss and/or fusion of abdominal denticle belts and stripe-specific defects in pair-rule and segment polarity gene expression.fish mutant embryos also exhibit loss of specific neurons, fusion of adjacent ventral nerve cord ganglia and aberrant axon scaffold organization. These results indicate an essential role for fish in anterior/posterior pattern formation and nervous system development, and suggest a potential function in modulating the activities of gap and pair-rule proteins.

The patched signaling pathway mediates repression of gooseberry allowing neuroblast specification by wingless during Drosophila neurogenesis

The Drosophila signaling molecule Wingless (Wg) plays crucial roles in cell-cell communications during development. In the developing nervous system, a previous study has shown that Wg acts non-autonomously to specify the fate of a specific neuronal precursor, NB4-2 (Q. Chu-LaGraff and C. Q. Doe (1993) Science 261, 1594–1597). The lack of autocrine specification of NB4-2 in Wg-expressing cells suggests that the response to Wg is spatially restricted, presumably through the activity of the Wg-receptor. I show that two other proteins, a transcription factor Gooseberry (Gsb) and a transmembrane protein Patched (Ptc), participate in the Wg-mediated specification of NB4-2 by controlling the response to the Wg signal. In gsb mutants, Wg-positive NB5-3 is transformed to NB4-2 in a Wg-dependent manner, suggesting that Gsb normally represses the capacity to respond to the Wg signal. In ptc mutants, Gsb is ectopically expressed in normally Wg-responsive cells, thus preventing the Wg response and consequently the correct specification of NB4-2 does not take place. This conclusion is supported by the observation that NB4-2 can be specified in gsb;ptc double mutants in a Wg-dependent manner. Moreover, ectopic expression of Gsb from the hsp7O-gsb transgene also blocks the response to the Wg signal. I propose that the responsiveness to the Wg signal is controlled by sequential negative regulation, ptc-->gsb-->Wg receptor. The timing of the response to Gsb suggests that the specification of neuroblast identities takes place within the neuroectoderm, prior to neuroblast delamination.

The GAGA factor is required in the early Drosophila embryo not only for transcriptional regulation but also for nuclear division

The GAGA protein of Drosophila was first identified as a stimulatory factor in in vitro transcription assays using the engrailed and Ultrabithorax promoters. Subsequent studies have suggested that the GAGA factor promotes transcription by blocking the repressive effects of histones; moreover, it has been shown to function in chromatin remodeling, acting together with other factors in the formation of nuclease hypersensitive sites in vitro. The GAGA factor is encoded by the Trithorax-like locus and in the studies reported here we have used the maternal effect allele Trl13C to examine the functions of the protein during embryogenesis. We find that GAGA is required for the proper expression of a variety of developmental loci that contain GAGA binding sites in their upstream regulatory regions. The observed disruptions in gene expression are consistent with those expected for a factor involved in chromatin remodeling. In addition to facilitating gene expression, the GAGA factor appears to have a more global role in chromosome structure and function. This is suggested by the spectrum of nuclear cleavage cycle defects observed in Trl13C embryos. These defects include asynchrony in the cleavage cycles, failure in chromosome condensation, abnormal chromosome segregation and chromosome fragmentation. These defects are likely to be related to the association of the GAGA protein with heterochromatic satellite sequences which is observed throughout the cell cycle.

Neurogenesis in the insect central nervous system

Recent advances provide an increasingly sophisticated understanding of Drosophila CNS development. First, genes have been identified that specify unique neuroblast cell fates. Second, neuroblasts have been found to use a novel mechanism for asymmetric localization of proteins into one daughter cell at mitosis. Third, a gene controlling the choice between glial/neuronal determination has been discovered. And finally, new cell-lineage methods are being used to determine the entire lineage of identified neuroblasts, including axon projections and synaptic contacts.

huckebein specifies aspects of CNS precursor identity required for motoneuron axon pathfinding

huckebein encodes a putative zinc finger protein expressed in a subset of Drosophila CNS precursors, including the NB 4-2/GMC 4-2a/RP2 cell lineage. In huckebein mutant embryos, GMC 4-2a does not express the cell fate marker EVEN-SKIPPED; conversely, huckebein overexpression produces a duplicate EVEN-SKIPPED-positive GMC 4-2a. We use Dil to trace the entire NB 4-2 lineage in wild-type and huckebein mutant embryos. Loss of huckebein does not affect the number, position, or type of neurons in the NB 4-2 lineage; however, all motoneurons show axon pathfinding defects and never terminate at the correct muscle. Thus, huckebein regulates aspects of GMC and neuronal identity required for proper motoneuron axon pathfinding in the NB 4-2 lineage.

New neuroblast markers and the origin of the aCC/pCC neurons in the Drosophila central nervous system

Drosophila is an ideal system for identifying genes that control central nervous system (CNS) development. Particularly useful tools include molecular markers for subsets of neural precursors (neuroblasts) and the simple expression pattern of the even-skipped (eve) gene in a subset of neurons. Here we provide additional molecular markers for identified neuroblasts, including several with near single cell specificity. In addition, we use these new markers to trace the development of several eve+ neurons. Our results shows that the eve+ aCC/pCC neurons develop from a different neuroblast than previously thought, and have led us to assign new names for several neuroblasts. These results are supported by DiI cell lineage analysis of neuroblasts identified in vivo.

Tag team specification of a neural precursor in the Drosophila embryonic central nervous system

The development of vertebrate and invertebrate nervous systems requires the production of thousands to millions of uniquely specified neurons from progenitor neural stem cells. A central question focuses on the elucidation of the developmental mechanisms that function within neural stem cell lineages to impart unique identities to neurons. A recent report details the roles that two genes, pdm-1 and pdm-2, play within an identified neural stem cell lineage in the Drosophila embryonic central nervous system. The results show that pdm-1 and pdm-2 are coexpressed in an identified neural precursor and function redundantly to specify the fate of this cell. As such this report offers an initial view of the genetic programs that create neural diversity.

The role of the cell cycle and cytokinesis in regulating neuroblast sublineage gene expression in the Drosophila CNS

The precise temporal control of gene expression is critical for specifying neuronal identity in the Drosophila central nervous system (CNS). A particularly interesting class of genes are those expressed at stereotyped times during the cell lineage of identified neural precursors (neuroblasts): these are termed ‘sublineage’ genes. Although sublineage gene function is vital for CNS development, the temporal regulation of this class of genes has not been studied. Here we show that four genes (ming, even-skipped, unplugged and achaete) are expressed in specific neuroblast sublineages. We show that these neuroblasts can be identified in embryos lacking both neuroblast cytokinesis and cell cycle progression (string mutants) and in embryos lacking only neuroblast cytokinesis (pebble mutants). We find that the unplugged and achaete genes are expressed normally in string and pebble mutant embryos, indicating that temporal control is independent of neuroblast cytokinesis or counting cell cycles. In contrast, neuroblasts require cytokinesis to activate sublineage ming expression, while a single, identified neuroblast requires cell cycle progression to activate even-skipped expression. These results suggest that neuroblasts have an intrinsic gene regulatory hierarchy controlling unplugged and achaete expression, but that cell cycle- or cytokinesis-dependent mechanisms are required for ming and eve CNS expression.

Expression domains of the Cf1a POU domain protein during Drosophila development

We have used antibodies directed against a unique portion of the Drosophila POU domain protein Cfla to localize its sites of expression in developing embryos. Cfla protein is first detected during germ band extension in the tracheal placodes and in the midline mesectoderm cells. Tracheal expression continues throughout embryonic development, especially in the main longitudinal tracheal trunks. Additional sites of high Cfla expression are in the anterior portion of the hindgut, the roof of the stomodeum, a subset of central nervous system cells, the oenocytes, and the ring gland. In addition, Cfla expression was localized in embryos mutant for several loci involved in determining fate along the midline of the CNS and the tracheal system. Cfla midline cell expression is dependent on proper single-minded gene function, and Cfla either regulates or acts in parallel to the genes pointed and rhomboid during midline CNS and tracheal development.

On the functional overlap between two Drosophila POU homeo domain genes and the cell fate specification of a CNS neural precursor

The approximately 200 distinct neurons comprising each hemisegment of the Drosophila embryonic CNS are derived from a stereotypic array of approximately 30 progenitor stem cells, called neuroblasts (NBs). Each NB undergoes repeated asymmetric divisions to produce several smaller ganglion mother cells (GMCs), each of which, in turn, divides to produce two neurons and/or glia cells. To understand the process by which cell type diversity is generated in the CNS, we are focusing on identifying genes that affect cell identity in the NB4-2 lineage from which the RP2 motoneuron is derived. We show here that within the early part of the NB4-2 lineage, two closely linked and structurally related POU homeo domain genes, pdm-2 (dPOU28) and pdm-1 (dPOU19), both encode proteins that accumulate to high levels only in the first GMC (GMC4-2a) and not in its progeny, the RP2 motoneuron. Our results from the genetic and developmental analysis of pdm-1 and pdm-2 demonstrate that these genes are not required for the birth of GMC4-2a; however, they are both involved in specifying the identity of GMC4-2a and, ultimately, in the genesis of RP2 neurons, with pdm-2 being the more dominant player in this process. In mutant animals where both pdm-1 and pdm-2 functions are removed, GMC4-2a fails to express markers consistent with a GMC4-2a identity and no mature (Eve protein expressing) RP2 neurons are produced. We demonstrate that in some mutant combinations in which no mature RP2 neurons are produced, some GMC4-2a cells can nevertheless divide. Hence, the failure of the POU mutants to produce mature RP2 neurons is not attributable to a block in GMC4-2a cell division per se but, rather, because the GMC4-2a cells fail to acquire their correct cellular identity.

XIPOU 2, a noggin-inducible gene, has direct neuralizing activity

XIPOU 2, a member of the class III POU domain family, is expressed initially in Spemann's organizer, and later, in discrete regions of the developing nervous system in Xenopus laevis. XIPOU 2 may act downstream from initial neural induction events, since it is activated by the neural inducer, noggin. To determine if XIPOU 2 participates in the early events of neurogenesis, synthetic mRNA was microinjected into specific blastomeres of the 32-cell stage embryo. Misexpression of XIPOU 2 in the epidermis causes a direct switch in cell fate from an epidermal to a neuronal phenotype. In the absence of mesoderm induction, XIPOU 2 has the ability to induce a neuronal phenotype in uncommitted ectoderm. These data demonstrate the potential of XIPOU 2 to act as a master regulator of neurogenesis.

Nubbin encodes a POU-domain protein required for proximal-distal patterning in the Drosophila wing

The nubbin gene is required for normal growth and patterning of the wing in Drosophila. We report here that nubbin encodes a member of the POU family of transcription factors. Regulatory mutants which selectively remove nubbin expression from wing imaginal discs lead to loss of wing structures. Although nubbin is expressed throughout the wing primordium, analysis of genetic mosaics suggests a localized requirement for nubbin activity in the wing hinge. These observations suggest the existence of a novel proximal-distal growth control center in the wing hinge, which is required in addition to the well characterized anterior-posterior and dorsal-ventral compartment boundary organizing centers.

drifter, a Drosophila POU-domain transcription factor, is required for correct differentiation and migration of tracheal cells and midline glia

The Drosophila drifter (dfr) gene, previously referred to as Cf1a, encodes a POU-domain DNA-binding protein implicated as a neuron-specific regulator in the developing central nervous system (CNS). We have isolated full-length dfr cDNA clones that encode a 46-kD protein containing the conserved POU-domain DNA-binding domain. The use of alternate polyadenylation sites produces two dfr mRNA transcripts that are first expressed in stage 10 embryos at 5- to 6-hr of development. A specific anti-dfr polyclonal antiserum generated against a dfr-glutathione S-transferase fusion protein recognizes a 46-kD protein on Western blots and has been used to analyze the cell-specific distribution of dfr protein during embryonic development. dfr protein is distributed in a complex expression pattern including the tracheal system, the middle pair of midline glia, and selected CNS neurons. We have carried out a genetic characterization of the dfr locus, previously localized to region 65D of the third chromosome, by generating a series of overlapping deficiencies between 65A and 65E1 that were used to isolate dfrE82, an EMS-induced lethal allele. Analysis of dfrE82 mutant embryos shows a disruption of the developing tracheal tree as well as commissural defects in the developing CNS. Based on an examination of a cell-specific marker for tracheal cells and midline glia, these defects appear to be caused by a failure of these cells to follow their characteristic routes of migration. The dfrE82 tracheal phenotype is rescued by a dfr minigene present as a P-element transposon expressing wild-type dfr protein in tracheal cells. These results suggest that the dfr protein plays a fundamental role in the differentiation of tracheal cells and midline glia possibly by regulating the expression of essential cell-surface proteins required for cell-cell interactions involved in directed cell migrations.

Conservation of complex expression domains of the pdm-2 POU domain gene between Drosophila virilis and Drosophila melanogaster

The closely linked pdm-1 and pdm-2 genes of Drosophila are expressed in complex patterns that suggest diverse roles in segmentation and nervous system development. A D. virilis pdm cDNA clone was isolated and sequenced. It shares sequence similarity with just the POU domain region of pdm-1, but shares extensive sequence similarity with the largest exon of pdm-2. In situ hybridization to D. virilis embryos shows that virtually all aspects of pdm-2 expression are conserved between D. virilis and D. melanogaster. This includes initial expression in a gap gene-like posterior domain, expression in ectodermal stripes during germ band extension, broad expression in the neurectoderm followed by limitation to discrete subsets of CNS cells, and expression in specific PNS neurons and support cells. The conservation of these expression domains supports the idea that the pdm genes are important for a variety of cell fate decisions in Drosophila development.