Misexpression Experiments Research Articles

Dimerization partners determine the activity of the Twist bHLH protein during Drosophila mesoderm development.

The basic helix-loop-helix transcription factor Twist regulates a series of distinct cell fate decisions within the Drosophila mesodermal lineage. These twist functions are reflected in its dynamic pattern of expression, which is characterized by initial uniform expression during mesoderm induction, followed by modulated expression at high and low levels in each mesodermal segment, and finally restricted expression in adult muscle progenitors. We show two distinct partner-dependent functions for Twist that are crucial for cell fate choice. We find that Twist can form homodimers and heterodimers with the Drosophila E protein homologue, Daughterless, in vitro. Using tethered dimers to assess directly the function of these two particular dimers in vivo, we show that Twist homodimers specify mesoderm and the subsequent allocation of mesodermal cells to the somatic muscle fate. Misexpression of Twist-tethered homodimers in the ectoderm or mesoderm leads to ectopic somatic muscle formation overriding other developmental cell fates. In addition, expression of tethered Twist homodimers in embryos null for twist can rescue mesoderm induction as well as somatic muscle development. Loss of function analyses, misexpression and dosage experiments, and biochemical studies indicate that heterodimers of Twist and Daughterless repress genes required for somatic myogenesis. We propose that these two opposing roles explain how modulated Twist levels promote the allocation of cells to the somatic muscle fate during the subdivision of the mesoderm. Moreover, this work provides a paradigm for understanding how the same protein controls a sequence of events within a single lineage.

Oh, for some simple guidance

Although I don't feel very old, I must say that things certainly seemed simpler when I was younger: take axon guidance, for example. It had long been proposed that axons were guided by signals secreted from target tissues. By assaying for axon-attracting activities, several candidate signalling molecules were isolated, of which the most well known are probably the vertebrate Netrins and their Drosophila (Netrin-A/B) and Caenorhabditis elegans (Unc-6) orthologues. It was hypothesized that Netrin-1 protein, secreted from the ventral region of the developing vertebrate neural tube, establishes a gradient of attracting activity that directs the ventral migration of the axons of dorsally located neurons. Consistent with this idea, mutations in the mouse netrin 1 gene disrupt the ventral migration of these neurons and mutations in a candidate Netrin receptor, deleted in colorectal cancer (DCC), which is expressed on the migrating axons, producing a similar axon guidance phenotype. Recent reports, however, suggest that Netrin signalling might not be as straightforward as previously thought.Genetic evidence from Drosophila indicates that the activity of the ADAM metalloprotease, Kuzbanian, is required for correct axon guidance. Galko and Tessier-Levigne1xFunction of an axonal chemoattractant modulated by metalloprotease activity. Galko, M.J. and Tessier-Levigne, M. Science. 2000; 289: 1365–1367Crossref | PubMed | Scopus (136)See all References provide support for this in the vertebrate system by demonstrating that a previously identified netrin-synergizing activity (NSA) bears all the hallmarks of being a metalloprotease; NSA can be faithfully mimicked by IC-3, a specific inhibitor of metalloprotease activity. Moreover, they show that the netrin receptor DCC is a metalloprotease substrate, cleavage of which is efficiently blocked by IC-3.In another paper, Hiramoto et al.2xThe Drosophila Netrin receptor Frazzled guides axons by controlling Netrin distribution. Hiramoto, M. et al. Nature. 2000; 406: 886–889Crossref | PubMed | Scopus (89)See all References report that the product of the Drosophila frazzled gene, the fly orthologue of Netrin receptor DCC, is not uniformly distributed in the membrane of expressing cells and that this distribution is controlled by the cytoplasmic domain of the protein. Perhaps more surprisingly, the authors show that Frazzled is not required in the neurons whose migration is disrupted in frazzled mutants. Through a series of elegant misexpression experiments, the authors provide evidence that Frazzled does indeed bind Drosophila Netrins, but rather than transducing a signal, as would a canonical receptor, Frazzled presents the bound Netrin to other receptors. As DCC has been shown to bind Netrin, the authors suggest that a similar process might take place during guidance of axons in vertebrates.Thus, the control of axon guidance by Netrin/DCC/Frazzled is not just a matter of target tissues secreting signals that are recognised and transduced by receptors in the migrating neurons. It appears more likely that a fine balance is struck between the availability of active and proteolytically inactivated DCC/Frazzled and that the precise subcellular localization of these proteins controls the ability of receptor–Netrin complexes to be recognised by a second class of currently unidentified receptors on the migrating axonal growth cones. Not such a simple story after all.

Peripodial Cells Regulate Proliferation and Patterning of Drosophila Imaginal Discs

Cells employ a diverse array of signaling mechanisms to establish spatial patterns during development. Nowhere is this better understood than in Drosophila, where the limbs and eyes arise from discrete epithelial sacs called imaginal discs. Molecular-genetic analyses of pattern formation have generally treated discs as single epithelial sheets. Anatomically, however, discs comprise a columnar cell monolayer covered by a squamous epithelium known as the peripodial membrane. Here we demonstrate that during development, peripodial cells signal to disc columnar cells via microtubule-based apical extensions. Ablation and targeted gene misexpression experiments demonstrate that peripodial cell signaling contributes to growth control and pattern formation in the eye and wing primordia. These findings challenge the traditional view of discs as monolayers and provide foundational evidence for peripodial cell function in Drosophila appendage development.

Dual roles of Wnt signaling during chondrogenesis in the chicken limb.

Long bones of the appendicular skeleton are formed from a cartilage template in a process known as endochondral bone development. Chondrocytes within this template undergo a progressive program of differentiation from proliferating to postmitotic prehypertrophic to hypertrophic chondrocytes, while mesenchymal cells immediately surrounding the early cartilage template form the perichondrium. Recently, members of the Wnt family of secreted signaling molecules have been implicated in regulating chondrocyte differentiation. We find that Wnt-5a, Wnt-5b and Wnt-4 genes are expressed in chondrogenic regions of the chicken limb: Wnt-5a is expressed in the perichondrium, Wnt-5b is expressed in a subpopulation of prehypertrophic chondrocytes and in the outermost cell layer of the perichondrium, and Wnt-4 is expressed in cells of the joint region. Misexpression experiments demonstrate that two of these Wnt molecules, Wnt-5a and Wnt-4, have opposing effects on the differentiation of chondrocytes and that these effects are mediated through divergent signaling pathways. Specifically, Wnt-5a misexpression delays the maturation of chondrocytes and the onset of bone collar formation, while Wnt-4 misexpression accelerates these two processes. Misexpression of a stabilized form of beta-catenin also results in accelerated chondrogenesis, suggesting that a beta-catenin/TCF-LEF complex is involved in mediating the positive regulatory effect of Wnt-4. A number of the genes involved in Wnt signal tranduction, including two members of the Frizzled gene family, which are believed to encode Wnt-receptors, show very dynamic and distinct expression patterns in cartilaginous elements of developing chicken limbs. Misexpression of putative dominant-negative forms of the two Frizzled proteins results in severe shortening of the infected cartilage elements due to a delay in chondrocyte maturation, indicating that an endogenous Wnt signal does indeed function to promote chondrogenic differentiation.

Pax6 defines the di-mesencephalic boundary by repressing En1 and Pax2.

Transcriptional factors and signaling molecules are responsible for regionalization of the central nervous system. In the early stage of neural development, Pax6 is expressed in the prosencephalon, while En1 and Pax2 are expressed in the mesencephalon. Here, we misexpressed Pax6 in the mesencephalon to elucidate the mechanism of the di-mesencephalic boundary formation. Histological analysis, expression patterns of diencephalic marker genes, and fiber trajectory of the posterior commissure indicated that Pax6 misexpression caused a caudal shift of the di-mesencephalic boundary. Pax6 repressed En1, Pax2 and other tectum (mesencephalon)-related genes such as En2, Pax5, Pax7, but induced Tcf4, a diencephalon marker gene. To know how Pax6 represses En1 and Pax2, we ectopically expressed a dominant-active or negative form of Pax6. The dominant-active form of Pax6 showed a similar but more severe phenotype than Pax6, while the dominant-negative form showed an opposite phenotype, suggesting that Pax6 acts as a transcriptional activator. Thus Pax6 may repress tectum-related genes by activating an intervening repressor. The results of misexpression experiments, together with normal expression patterns of Pax6, En1 and Pax2, suggest that repressive interaction between Pax6 and En1/Pax2 defines the di-mesencephalic boundary.

Cato Encodes a Basic Helix-Loop-Helix Transcription Factor Implicated in the Correct Differentiation of Drosophila Sense Organs

In Drosophila neurogenesis, proneural genes encode bHLH proteins that are required for neural precursor selection. But many vertebrate homologues are expressed later and are postulated to have multiple roles during neurogenesis. We have isolated a new Drosophila gene, cato, which encodes a protein with a bHLH domain that is closely related to that of the proneural protein Atonal. cato expression is restricted to the developing PNS, where it is expressed in between the stages of precursor selection and terminal differentiation (and therefore later than the proneural genes). We present evidence from loss-of-function and misexpression experiments that cato is involved in sensory neurone morphology. Moreover, in prospero mutants, in which axon and dendrite outgrowth is defective, cato is strongly derepressed in the developing CNS.

The Proneural Gene amos Promotes Multiple Dendritic Neuron Formation in the Drosophila Peripheral Nervous System

In the Drosophila peripheral nervous system, proneural genes direct the formation of different types of sensory organs. Here, we show that amos is a novel proneural gene that promotes multiple dendritic (MD) neuron formation. amos encodes a basic-helix-loop-helix (bHLH) protein of the Atonal family. During embryonic development, amos is expressed in patches of ectodermal cells, and the expression is quickly restricted to sensory organ precursors. Loss of amos function eliminates MD neurons that remain in ASC;atonal mutants. Misexpression of amos generates ectopic MD and other types of neurons. Amos interacts with the ubiquitously expressed Daughterless protein in vivo and in vitro. Our final misexpression experiments suggest that a domain located outside the DNA-binding domain of Amos determines the MD neuronal specificity.

The knirps and knirps-related genes organize development of the second wing vein in Drosophila.

The neighboring homologous knirps (kni) and knirps-related (knrl) genes in Drosophila encode transcription factors in the steroid hormone receptor superfamily. During early embryogenesis, kni functions as a gap gene to control expression of segmentation genes within the abdominal region of the embryo. In this study, we present evidence that kni and knrl link A/P positional information in larval wing imaginal discs to morphogenesis of the second longitudinal wing vein (L2). We show that kni and knrl are expressed in similar narrow stripes corresponding to the position of the L2 primordium. The kni and knrl L2 stripes abut the anterior border of the broad central expression domain of the Dpp target gene spalt major (salm). We provide evidence that radius incompletus (ri), a well-known viable mutant lacking the L2 vein, is a regulatory mutant of the kni/knrl locus. In ri mutant wing discs, kni and knrl fail to be expressed in the L2 primordium. In addition, the positions of molecular breakpoints in the kni/knrl locus indicate that the ri function is provided by cis-acting sequences upstream of the kni transcription unit. Epistasis tests reveal that the kni/knrl locus functions downstream of spalt major (salm) and upstream of genes required to initiate vein-versus-intervein differentiation. Mis-expression experiments suggest that kni and knrl expressing cells inhibit neighboring cells from becoming vein cells. Finally, kni and knrl are likely to refine the L2 position by positively auto-regulating their own expression and by providing negative feedback to repress salm expression. We propose a model in which the combined activities of kni and knrl organize development of the L2 vein in the appropriate position.

Functional analysis of the fibrinogen-related scabrous gene from Drosophila melanogaster identifies potential effector and stimulatory protein domains.

The scabrous (sca) gene encodes a secreted dimeric glycoprotein with putative coiled-coil domains N-terminally and a C-terminal region related to the blood clot protein fibrinogen. Homozygous sca mutants have extra bristle organs and rough eyes. We describe a GAL4-based expression system for testing rescue of the sca mutant phenotype by altered SCA proteins and for misexpression. We find that deletion of the fibrinogen-related domain (FReD) greatly decreases SCA function, confirming the importance of this conserved region. SCA function could not be restored by FReDs from human fibrinogen chain genes. However, proteins lacking any FReD still showed some function in both rescue and misexpression experiments, suggesting that putative effector-binding regions lie outside this domain. Consistent with this, proteins expressing only the FReD had no rescuing activity but were recessive negative; i.e., they enhanced the phenotype of sca mutations but had no phenotype in the presence of a wild-type sca allele. This suggests that the FReD contributes to SCA function by binding to other components of the bristle determination pathway, increasing the activity of the linked N-terminal region.

A subset of notch functions during Drosophila eye development require Su(H) and the E(spl) gene complex.

The Notch signalling pathway is involved in many processes where cell fate is decided. Previous work showed that Notch is required at successive steps during R8 specification in the Drosophila eye. Initially, Notch enhances atonal expression and promotes atonal function. After atonal autoregulation has been established, Notch signalling represses atonal expression during lateral specification. In this paper we investigate which known components of the Notch pathway are involved in each signalling process. Using clonal analysis we show that a ligand of Notch, Delta, is required along with Notch for both proneural enhancement and lateral specification, while the downstream components Suppressor-of-Hairless and Enhancer-of-Split are involved only in lateral specification. Our data point to a distinct signal transduction pathway during proneural enhancement by Notch. Using misexpression experiments we also show that particular Enhancer-of-split bHLH genes can differ greatly in their contribution to lateral specification.

The specificity of proneural genes in determining Drosophila sense organ identity

The proneural genes ( atonal and the genes of the achaete–scute complex (AS-C)) are required for the selection of sense organ precursors. They also endow these precursors with sense organ subtype information. In most of the ectoderm, atonal is required for precursors of chordotonal sense organs, whereas AS-C are required for those of most external sense organs, such as bristles. To address the question of how proneural genes influence subtype identity, we have made use of the Gal4/UAS system of misexpression. Unlike previous misexpression experiments, we found that under specific conditions of misexpression, atonal shows high subtype specificity of ectopic sense organ formation. Moreover, atonal can even transform wild-type external sense organs to chordotonal organs, although scute cannot perform the reciprocal transformation. Our evidence demonstrates that atonal's subtype determining role is not to activate directly chordotonal fate, but to repress the activation of cut, a gene that is necessary for external sense organ fate, thereby freeing its precursors to follow the alternative chordotonal organ fate.

Pitx2 Participates in the Late Phase of the Pathway Controlling Left-Right Asymmetry

Pitx2, a member of the bicoid-related family of homeobox-containing genes, is asymmetrically expressed in the left lateral plate mesoderm and derived tissues during chick and mouse development. Modifications of Pitx2 pattern of expression in the iv mouse mutation correlate with the situs alterations characteristic of the mutation. Misexpression experiments demonstrate that Shh and nodal positively regulate Pitx2 expression. Our results are compatible with a Pitx2 function in the late phase of the gene cascade controlling laterality.

The Transition to Endoreduplication in Trophoblast Giant Cells Is Regulated by the mSNA Zinc Finger Transcription Factor

Terminal cell differentiation is usually associated with cell cycle exit. In some lineages, however, cells undergo continued rounds of DNA synthesis without intervening mitoses (endoreduplication) resulting in polyploid nuclei. This is striking in rodent trophoblast giant cells which contain up to 1000N of DNA. InDrosophila,theEscargotgene has been implicated in regulating the transition from mitotic cell cycles to endocycles during development. We found that a murine homologue,mSna,was expressed in mouse trophoblast and was downregulated during giant cell differentiation. The mSNA zinc finger protein bound to E-box DNA elements and, in transfected C3H10T1/2 fibroblasts, acted as a transcriptional repressor. The maximal repressive effect was dependent on both the zinc finger DNA-binding domain and the N-terminal, seven-amino-acid SNAG domain. Misexpression experiments in Rcho-1 trophoblast cells revealed thatmSnaregulates the transition from replicating precursor cells to committed giant cells: overexpression blocked, whereas antisense RNA-mediated underexpression promoted trophoblast giant cell differentiation. Overexpression ofmSnain precursor cells had no effect on cell cycle kinetics, but did increase cyclin A and B levels, implying actions during G2. These effects were dependent on both the zinc finger and SNAG domains. Together, these data suggest that mSNA has an ESCARGOT-like function to repress the transcription of genes that promote the transition from mitotic to endoreduplicative cell cycles in rodent trophoblast.

Multiple delta genes and lateral inhibition in zebrafish primary neurogenesis.

In Drosophila, cells are thought to be singled out for a neural fate through a competitive mechanism based on lateral inhibition mediated by Delta-Notch signalling. In tetrapod vertebrates, nascent neurons express the Delta1 gene and thereby deliver lateral inhibition to their neighbours, but it is not clear how these cells are singled out within the neurectoderm in the first place. We have found four Delta homologues in the zebrafish--twice as many as reported in any tetrapod vertebrate. Three of these--deltaA, deltaB and deltaD--are involved in primary neurogenesis, while two--deltaC and deltaD--appear to be involved in somite development. In the neural plate, deltaA and deltaD, unlike Delta1 in tetrapods, are expressed in large patches of contiguous cells, within which scattered individuals expressing deltaB become singled out as primary neurons. By gene misexpression experiments, we show: (1) that the singling-out of primary neurons, including the unique Mauthner cell on each side of the hindbrain, depends on Delta-Notch-mediated lateral inhibition, (2) that deltaA, deltaB and deltaD all have products that can deliver lateral inhibition and (3) that all three of these genes are themselves subject to negative regulation by lateral inhibition. These properties imply that competitive lateral inhibition, mediated by coordinated activities of deltaA, deltaB and deltaD, is sufficient to explain how primary neurons emerge from proneural clusters of neuroepithelial cells in the zebrafish.