Overexpression Beadex mutations and loss-of-function heldup-a mutations in Drosophila affect the 3' regulatory and coding components, respectively, of the Dlmo gene

Beadex, the Drosophila LIM only protein, is required for the growth of the larval neuromuscular junction

The appropriate growth of the neurons, accurate organization of their synapses, and successful neurotransmission are indispensable for sensorimotor activities. These processes are highly dynamic and tightly regulated. Extensive genetic, molecular, physiological, and behavioral studies have identified many molecular candidates and investigated their roles in various neuromuscular processes. In this article, we show that Beadex (Bx), the Drosophila LIM only (LMO) protein, is required for motor activities and neuromuscular growth of Drosophila. The larvae bearing Bx7, a null allele of Bx, and the RNAi-mediated neuronal-specific knockdown of Bx show drastically reduced crawling behavior, a diminished synaptic span of the neuromuscular junctions (NMJs) and an increased spontaneous neuronal firing with altered motor patterns in the central pattern generators (CPGs). Microarray studies identified multiple targets of Beadex that are involved in different cellular and molecular pathways, including those associated with the cytoskeleton and mitochondria that could be responsible for the observed neuromuscular defects. With genetic interaction studies, we further show that Highwire (Hiw), a negative regulator of synaptic growth at the NMJs, negatively regulates Bx, as the latter's deficiency was able to rescue the phenotype of the Hiw null mutant, HiwDN. Thus, our data indicate that Beadex functions downstream of Hiw to regulate the larval synaptic growth and physiology.NEW & NOTEWORTHY A novel role for Beadex (Bx) regulates the larval neuromuscular junction (NMJ) structure and function in a tissue-specific manner. Bx is expressed in a subset of Toll-6-expressing neurons and is involved in regulating synaptic span and physiology, possibly through its negative interaction with Highwire (Hiw). The findings of this study provide insights into the molecular mechanisms underlying NMJ development and function and warrant further investigation to understand the role of Bx in these processes fully.

Beadex, a Drosophila LIM domain only protein, function in follicle cells is essential for egg development and fertility

LIM domain, constituted by two tandem C2H2 zinc finger motif, proteins regulate several biological processes. They are usually found associated with various functional domains like Homeodomain, kinase domain and other protein binding domains. LIM proteins that are devoid of other domains are called LIM only proteins (LMO). LMO proteins were first identified in humans and are implicated in development and oncogenesis. They regulate various cell specifications by regulating the activity of respective transcriptional complexes. The Drosophila LMO protein (dLMO), Beadex (Bx), regulates various developmental processes like wing margin development and bristle development. It also regulates Drosophila behavior in response to cocaine and ethanol. We have previously generated Bx null flies and shown its essential function in neurons for multiple aspects of female reproduction. However, it was not known whether Bx affects reproduction through its independent function in ovaries. In this paper we show that female flies null for Bx lay eggs with multiple defects. Further, through knock down studies we demonstrate that function of Bx in follicle cells is required for normal egg development. We also show that function of Bx is particularly required in border cells for Drosophila fertility.

Constitutive scaffolding of multiple Wnt enhanceosome components by Legless/BCL9

Wnt/β-catenin signaling elicits context-dependent transcription switches that determine normal development and oncogenesis. These are mediated by the Wnt enhanceosome, a multiprotein complex binding to the Pygo chromatin reader and acting through TCF/LEF-responsive enhancers. Pygo renders this complex Wnt-responsive, by capturing β-catenin via the Legless/BCL9 adaptor. We used CRISPR/Cas9 genome engineering of Drosophila legless (lgs) and human BCL9 and B9L to show that the C-terminus downstream of their adaptor elements is crucial for Wnt responses. BioID proximity labeling revealed that BCL9 and B9L, like PYGO2, are constitutive components of the Wnt enhanceosome. Wnt-dependent docking of β-catenin to the enhanceosome apparently causes a rearrangement that apposes the BCL9/B9L C-terminus to TCF. This C-terminus binds to the Groucho/TLE co-repressor, and also to the Chip/LDB1-SSDP enhanceosome core complex via an evolutionary conserved element. An unexpected link between BCL9/B9L, PYGO2 and nuclear co-receptor complexes suggests that these β-catenin co-factors may coordinate Wnt and nuclear hormone responses.

Production of Small Noncoding RNAs from the flamenco Locus Is Regulated by the gypsy Retrotransposon of Drosophila melanogaster

Protective mechanisms based on RNA silencing directed against the propagation of transposable elements are highly conserved in eukaryotes. The control of transposable elements is mediated by small noncoding RNAs, which derive from transposon-rich heterochromatic regions that function as small RNA-generating loci. These clusters are transcribed and the precursor transcripts are processed to generate Piwi-interacting RNAs (piRNAs) and endogenous small interfering RNAs (endo-siRNAs), which silence transposable elements in gonads and somatic tissues. The flamenco locus is a Drosophila melanogaster small RNA cluster that controls gypsy and other transposable elements, and has played an important role in understanding how small noncoding RNAs repress transposable elements. In this study, we describe a cosuppression mechanism triggered by new euchromatic gypsy insertions in genetic backgrounds carrying flamenco alleles defective in gypsy suppression. We found that the silencing of gypsy is accompanied by the silencing of other transposons regulated by flamenco, and of specific flamenco sequences from which small RNAs against gypsy originate. This cosuppression mechanism seems to depend on a post-transcriptional regulation that involves both endo-siRNA and piRNA pathways and is associated with the occurrence of developmental defects. In conclusion, we propose that new gypsy euchromatic insertions trigger a post-transcriptional silencing of gypsy sense and antisense sequences, which modifies the flamenco activity. This cosuppression mechanism interferes with some developmental processes, presumably by influencing the expression of specific genes.

Dynamic expression and regulation by Fgf8 and Pou2 of the zebrafish LIM-only gene, lmo4

We report the expression of zebrafish lmo4 during the first 48 h of development. Like its murine ortholog, lmo4 is expressed in somitic mesoderm, branchial arches, otic vesicles, and limb (pectoral fin) buds. In addition, however, we report zebrafish lmo4 expression in the developing eye, cardiovascular tissue, and the neural plate and telencephalon. We demonstrate that expression in the rostral hindbrain requires acerebellar (ace/fgf8) and spiel ohne grenzen (spg/pou2) activity.

Chi and dLMO function antagonistically on Notch signaling through directly regulation of fng transcription

Gene apterous (ap), chip (chi) and beadex (bx) play important roles in the dorsal-ventral compartmentalization in Drosophila wing discs. Meanwhile, Notch signaling is essential to the same process. It has been reported that Ap and Chi function as a tetramer to regulate Notch signaling. At the same time, dLMO (the protein product of gene bx) regulates the activity of Ap by competing its binding with Chi. However, the detailed functions of Chi and dLMO on Notch signaling and the relevant mechanisms remain largely unknown. Here, we report the detailed functions of Chi and dLMO on Notch signaling. Different Chi protein levels in adjacent cells could activate Notch signaling mainly in the cells with higher level of Chi. dLMO could induce antagonistical phenotypes on Notch signaling compared to that induced by Chi. These processes depend on their direct regulation of fringe (fng) transcription.

A cis-regulatory mutation in troponin-I of Drosophila reveals the importance of proper stoichiometry of structural proteins during muscle assembly

Rapid and high wing-beat frequencies achieved during insect flight are powered by the indirect flight muscles, the largest group of muscles present in the thorax. Any anomaly during the assembly and/or structural impairment of the indirect flight muscles gives rise to a flightless phenotype. Multiple mutagenesis screens in Drosophila melanogaster for defective flight behavior have led to the isolation and characterization of mutations that have been instrumental in the identification of many proteins and residues that are important for muscle assembly, function, and disease. In this article, we present a molecular-genetic characterization of a flightless mutation, flightless-H (fliH), originally designated as heldup-a (hdp-a). We show that fliH is a cis-regulatory mutation of the wings up A (wupA) gene, which codes for the troponin-I protein, one of the troponin complex proteins, involved in regulation of muscle contraction. The mutation leads to reduced levels of troponin-I transcript and protein. In addition to this, there is also coordinated reduction in transcript and protein levels of other structural protein isoforms that are part of the troponin complex. The altered transcript and protein stoichiometry ultimately culminates in unregulated acto-myosin interactions and a hypercontraction muscle phenotype. Our results shed new insights into the importance of maintaining the stoichiometry of structural proteins during muscle assembly for proper function with implications for the identification of mutations and disease phenotypes in other species, including humans.

Beadex function in the motor neurons is essential for female reproduction in Drosophila melanogaster

Drosophila melanogaster has served as an excellent model system for understanding the neuronal circuits and molecular mechanisms regulating complex behaviors. The Drosophila female reproductive circuits, in particular, are well studied and can be used as a tool to understand the role of novel genes in neuronal function in general and female reproduction in particular. In the present study, the role of Beadex, a transcription co-activator, in Drosophila female reproduction was assessed by generation of mutant and knock down studies. Null allele of Beadex was generated by transposase induced excision of P-element present within an intron of Beadex gene. The mutant showed highly compromised reproductive abilities as evaluated by reduced fecundity and fertility, abnormal oviposition and more importantly, the failure of sperm release from storage organs. However, no defect was found in the overall ovariole development. Tissue specific, targeted knock down of Beadex indicated that its function in neurons is important for efficient female reproduction, since its neuronal knock down led to compromised female reproductive abilities, similar to Beadex null females. Further, different neuronal class specific knock down studies revealed that Beadex function is required in motor neurons for normal fecundity and fertility of females. Thus, the present study attributes a novel and essential role for Beadex in female reproduction through neurons.

An evolutionary conserved role for anaplastic lymphoma kinase in behavioral responses to ethanol

Anaplastic lymphoma kinase (Alk) is a gene expressed in the nervous system that encodes a receptor tyrosine kinase commonly known for its oncogenic function in various human cancers. We have determined that Alk is associated with altered behavioral responses to ethanol in the fruit fly Drosophila melanogaster, in mice, and in humans. Mutant flies containing transposon insertions in dAlk demonstrate increased resistance to the sedating effect of ethanol. Database analyses revealed that Alk expression levels in the brains of recombinant inbred mice are negatively correlated with ethanol-induced ataxia and ethanol consumption. We therefore tested Alk gene knockout mice and found that they sedate longer in response to high doses of ethanol and consume more ethanol than wild-type mice. Finally, sequencing of human ALK led to the discovery of four polymorphisms associated with a low level of response to ethanol, an intermediate phenotype that is predictive of future alcohol use disorders (AUDs). These results suggest that Alk plays an evolutionary conserved role in ethanol-related behaviors. Moreover, ALK may be a novel candidate gene conferring risk for AUDs as well as a potential target for pharmacological intervention.

Modularity of CHIP/LDB transcription complexes regulates cell differentiation

Transcription is the first step through which the cell operates, via its repertoire of transcription complexes, to direct cellular functions and cellular identity by generating the cell-specific transcriptome. The modularity of the composition of constituents of these complexes allows the cell to delicately regulate its transcriptome. In a recent study we have examined the effects of reducing the levels of specific transcription co-factors on the function of two competing transcription complexes, namely CHIP-AP and CHIP-PNR which regulate development of cells in the thorax of Drosophila. We found that changing the availability of these co-factors can shift the balance between these complexes leading to transition from utilization of CHIP-AP to CHIP-PNR. This is reflected in change in the expression profile of target genes, altering developmental cell fates. We propose that such a mechanism may operate in normal fly development. Transcription complexes analogous to CHIP-AP and CHIP-PNR exist in mammals and we discuss how such a shift in the balance between them may operate in normal mammalian development.

Lmo genes regulate behavioral responses to ethanol in Drosophila melanogaster and the mouse

BACKGROUND

Previous work from our laboratory demonstrated a role for the Drosophila Lim-only (dLmo) gene in regulating behavioral responses to cocaine. Herein, we examined whether dLmo influences the flies' sensitivity to ethanol's sedating effects. We also investigated whether 1 of the mammalian homologs of dLmo, Lmo3, is involved in behavioral responses to ethanol in mice.

METHODS

To examine dLmo function in ethanol-induced sedation, mutant flies with reduced or increased dLmo expression were tested using the loss of righting (LOR) assay. To determine whether mouse Lmo3 regulates behavioral responses to ethanol, we generated transgenic mice expressing a short-hairpin RNA targeting Lmo3 for RNA interference-mediated knockdown by lentiviral infection of single cell embryos. Adult founder mice, expressing varying amounts of Lmo3 in the brain, were tested using ethanol loss-of-righting-reflex (LORR) and 2-bottle choice ethanol consumption assays.

RESULTS

We found that in flies, reduced dLmo activity increased sensitivity to ethanol-induced sedation, whereas increased expression of dLmo led to increased resistance to ethanol-induced sedation. In mice, reduced levels of Lmo3 were correlated with increased sedation time in the LORR test and decreased ethanol consumption in the 2-bottle choice protocol.

CONCLUSIONS

These data describe a novel and conserved role for Lmo genes in flies and mice in behavioral responses to ethanol. These studies also demonstrate the feasibility of rapidly translating findings from invertebrate systems to mammalian models of alcohol abuse by combining RNA interference in transgenic mice and behavioral testing.

Transcriptional regulation by CHIP/LDB complexes

It is increasingly clear that transcription factors play versatile roles in turning genes "on" or "off" depending on cellular context via the various transcription complexes they form. This poses a major challenge in unraveling combinatorial transcription complex codes. Here we use the powerful genetics of Drosophila combined with microarray and bioinformatics analyses to tackle this challenge. The nuclear adaptor CHIP/LDB is a major developmental regulator capable of forming tissue-specific transcription complexes with various types of transcription factors and cofactors, making it a valuable model to study the intricacies of gene regulation. To date only few CHIP/LDB complexes target genes have been identified, and possible tissue-dependent crosstalk between these complexes has not been rigorously explored. SSDP proteins protect CHIP/LDB complexes from proteasome dependent degradation and are rate-limiting cofactors for these complexes. By using mutations in SSDP, we identified 189 down-stream targets of CHIP/LDB and show that these genes are enriched for the binding sites of APTEROUS (AP) and PANNIER (PNR), two well studied transcription factors associated with CHIP/LDB complexes. We performed extensive genetic screens and identified target genes that genetically interact with components of CHIP/LDB complexes in directing the development of the wings (28 genes) and thoracic bristles (23 genes). Moreover, by in vivo RNAi silencing we uncovered novel roles for two of the target genes, xbp1 and Gs-alpha, in early development of these structures. Taken together, our results suggest that loss of SSDP disrupts the normal balance between the CHIP-AP and the CHIP-PNR transcription complexes, resulting in down-regulation of CHIP-AP target genes and the concomitant up-regulation of CHIP-PNR target genes. Understanding the combinatorial nature of transcription complexes as presented here is crucial to the study of transcription regulation of gene batteries required for development.

A view from Drosophila: multiple biological functions for individual microRNAs

microRNAs (miRNAs) comprise an extensive class of post-transcriptional regulatory molecules in higher eukaryotes. Intensive research in Drosophila has revealed that miRNAs control myriad developmental and physiological processes. Interestingly, several of the best-studied miRNAs impact multiple biological processes, often by regulating distinct key target genes in each setting. Here we discuss the roles of some of these pleiotropic miRNAs, and their implications for studying and interpreting the roles of miRNAs in gene regulatory networks.

miR-9a prevents apoptosis during wing development by repressing Drosophila LIM-only

Loss of Drosophila mir-9a induces a subtle increase in sensory bristles, but a substantial loss of wing tissue. Here, we establish that the latter phenotype is largely due to ectopic apoptosis in the dorsal wing primordium, and we could rescue wing development in the absence of this microRNA by dorsal-specific inhibition of apoptosis. Such apoptosis was a consequence of de-repressing Drosophila LIM-only (dLMO), which encodes a transcriptional regulator of wing and neural development. We observed cell-autonomous elevation of endogenous dLMO and a GFP-dLMO 3'UTR sensor in mir-9a mutant wing clones, and heterozygosity for dLMO rescued the apoptosis and wing defects of mir-9a mutants. We also provide evidence that dLMO, in addition to senseless, contributes to the bristle defects of the mir-9a mutant. Unexpectedly, the upregulation of dLMO, loss of Cut, and adult wing margin defects seen with mir-9a mutant clones were not recapitulated by clonal loss of the miRNA biogenesis factors Dicer-1 or Pasha, even though these mutant conditions similarly de-repressed miR-9a and dLMO sensor transgenes. Therefore, the failure to observe a phenotype upon conditional knockout of a miRNA processing factor does not reliably indicate the lack of critical roles of miRNAs in a given setting.

Imprinting of the Y chromosome influences dosage compensation in roX1 roX2 Drosophila melanogaster

Drosophila melanogaster males have a well-characterized regulatory system that increases X-linked gene expression. This essential process restores the balance between X-linked and autosomal gene products in males. A complex composed of the male-specific lethal (MSL) proteins and RNA is recruited to the body of transcribed X-linked genes where it modifies chromatin to increase expression. The RNA components of this complex, roX1 and roX2 (RNA on the X1, RNA on the X2), are functionally redundant. Males mutated for both roX genes have dramatically reduced survival. We show that reversal of sex chromosome inheritance suppresses lethality in roX1 roX2 males. Genetic tests indicate that the effect on male survival depends upon the presence and source of the Y chromosome, revealing a germ line imprint that influences dosage compensation. Conventional paternal transmission of the Y chromosome enhances roX1 roX2 lethality, while maternal transmission of the Y chromosome suppresses lethality. roX1 roX2 males with both maternal and paternal Y chromosomes have very low survival, indicating dominance of the paternal imprint. In an otherwise wild-type male, the Y chromosome does not appreciably affect dosage compensation. The influence of the Y chromosome, clearly apparent in roX1 roX2 mutants, thus requires a sensitized genetic background. We believe that the Y chromosome is likely to act through modulation of a process that is defective in roX1 roX2 mutants: X chromosome recognition or chromatin modification by the MSL complex.

Drosophila mir-9a regulates wing development via fine-tuning expression of the LIM only factor, dLMO

MicroRNAs are short non-coding endogenous RNAs that are implicated in regulating various aspects of plants and animal development, however their functions in organogenesis are largely unknown. Here we report that mir-9a belonging to the mir-9 family, regulates Drosophila wing development through a functional target site in the 3' untranslated region of the Drosophila LIM only protein, dLMO. dLMO is a transcription cofactor, that directly inhibits the activity of Apterous, the LIM-HD factor required for the proper dorsal identity of the wings. Deletions of the 3' untranslated region, including the mir-9a site, generate gain-of-function dLMO mutants (Beadex) associated with high levels of dLMO mRNA and protein. Beadex mutants lack wing margins, a phenotype also observed in null mir-9a mutants. We found that mir-9a and dLMO are co-expressed in wing discs and interact genetically for controlling wing development. Lack of mir-9a results in overexpression of dLMO, while gain-of-function mir-9a mutant suppresses dLMO expression. These data indicate that a function of mir-9a is to ensure the appropriate stoichiometry of dLMO during Drosophila wing development. The mir-9a binding site is conserved in the human counterpart LMO2, the T-cell acute leukemia oncogene, suggesting that mir-9 might apply a similar strategy to maintain LMO2 expression under a detrimental threshold.

Drosophila LIM-only is a positive regulator of transcription during thoracic bristle development

The Drosophila LIM-only (LMO) protein DLMO functions as a negative regulator of transcription during development of the fly wing. Here we report a novel role of DLMO as a positive regulator of transcription during the development of thoracic sensory bristles. We isolated new dlmo mutants, which lack some thoracic dorsocentral (DC) bristles. This phenotype is typical of malfunction of a thoracic multiprotein transcription complex, composed of CHIP, PANNIER (PNR), ACHAETE (AC), and DAUGHTERLESS (DA). Genetic interactions reveal that dlmo synergizes with pnr and ac to promote the development of thoracic DC bristles. Moreover, loss-of-function of dlmo reduces the expression of a reporter target gene of this complex in vivo. Using the GAL4-UAS system we also show that dlmo is spatially expressed where this complex is known to be active. Glutathione-S-transferase (GST)-pulldown assays showed that DLMO can physically bind CHIP and PNR through either of the two LIM domains of DLMO, suggesting that DLMO might function as part of this transcription complex in vivo. We propose that DLMO exerts its positive effect on DC bristle development by serving as a bridging molecule between components of the thoracic transcription complex.

Phylogenetic analysis of the plant-specific zinc finger-homeobox and mini zinc finger gene families

Zinc finger-homeodomain proteins (ZHD) are present in many plants; however, the evolutionary history of the ZHD gene family remains largely unknown. We show here that ZHD genes are plant-specific, nearly all intronless, and related to MINI ZINC FINGER (MIF) genes that possess only the zinc finger. Phylogenetic analyses of ZHD genes from representative land plants suggest that non-seed plant ZHD genes occupy basal positions and angiosperm homologs form seven distinct clades. Several clades contain genes from two or more major angiosperm groups, including eudicots, monocots, magnoliids, and other basal angiosperms, indicating that several duplications occurred before the diversification of flowering plants. In addition, specific lineages have experienced more recent duplications. Unlike the ZHD genes, MIFs are found only from seed plants, possibly derived from ZHDs by loss of the homeodomain before the divergence of seed plants. Moreover, the MIF genes have also undergone relatively recent gene duplications. Finally, genome duplication might have contributed substantially to the expansion of family size in angiosperms and caused a high level of functional redundancy/overlap in these genes.

Drosophila, a genetic model system to study cocaine-related behaviors: a review with focus on LIM-only proteins

In the last decade, the fruit fly Drosophila melanogaster, highly accessible to genetic, behavioral and molecular analyses, has been introduced as a novel model organism to help decipher the complex genetic, neurochemical, and neuroanatomical underpinnings of behaviors induced by drugs of abuse. Here we review these data, focusing specifically on cocaine-related behaviors. Several of cocaine's most characteristic properties have been recapitulated in Drosophila. First, cocaine induces motor behaviors in flies that are remarkably similar to those observed in mammals. Second, repeated cocaine administration induces behavioral sensitization a form of behavioral plasticity believed to underlie certain aspects of addiction. Third, a key role for dopaminergic systems in mediating cocaine's effects has been demonstrated through both pharmacological and genetic methods. Finally, and most importantly, unbiased genetic screens, feasible because of the simplicity and scale with which flies can be manipulated in the laboratory, have identified several novel genes and pathways whose role in cocaine behaviors had not been anticipated. Many of these genes and pathways have been validated in mammalian models of drug addiction. We focus in this review on the role of LIM-only proteins in cocaine-induced behaviors.

A gain-of-function suppressor screen for genes involved in dorsal-ventral boundary formation in the Drosophila wing

The Drosophila wing primordium is subdivided into a dorsal (D) and a ventral (V) compartment by the activity of the LIM-homeodomain protein Apterous in D cells. Cell interactions between D and V cells induce the activation of Notch at the DV boundary. Notch is required for the maintenance of the compartment boundary and the growth of the wing primordium. Beadex, a gain-of-function allele of dLMO, results in increased levels of dLMO protein, which interferes with the activity of Apterous and results in defects in DV axis formation. We performed a gain-of-function enhancer-promoter (EP) screen to search for suppressors of Beadex when overexpressed in D cells. We identified 53 lines corresponding to 35 genes. Loci encoding for micro-RNAs and proteins involved in chromatin organization, transcriptional control, and vesicle trafficking were characterized in the context of dLMO activity and DV boundary formation. Our results indicate that a gain-of-function genetic screen in a sensitized background, as opposed to classical loss-of-function-based screenings, is a very efficient way to identify redundant genes involved in a developmental process.

Crystallization of an Lhx3-Isl1 complex

A stable intramolecular complex comprising the LIM domains of the LIM-homeodomain protein Lhx3 tethered to a peptide region of Isl1 has been engineered, purified and crystallized. The monoclinic crystals belong to space group C2, with unit-cell parameters a = 119, b = 62.2, c = 51.9 A, beta = 91.6 degrees , and diffract to 2.05 A resolution.

A positive role for NLI/Ldb1 in long-range beta-globin locus control region function

Long-range interactions between distant regulatory elements, such as enhancers, and their target genes underlie the specificity of gene expression in many developmentally regulated gene families. NLI/Ldb1, a widely expressed nuclear factor, is a potential mediator of long-range interactions. Here, we show that NLI/Ldb1 and erythroid-binding partners GATA-1/SCL/LMO2 bind in vivo to the beta-globin locus control region (LCR). The C-terminal LIM interaction domain of NLI is required for formation of the complex on chromatin. Loss of the LIM domain converts NLI into a dominant-negative inhibitor of globin gene expression, and knockdown of NLI by using shRNA results in failure to activate beta-globin expression. Kinetic studies reveal that the NLI/GATA-1/SCL/LMO2 complex is detected at the beta-globin promoter coincident with RNA Pol II recruitment, beta-globin transcription, and chromatin loop formation during erythroid differentiation, providing evidence that NLI facilitates long-range gene activation.

Robustness and stability of the gene regulatory network involved in DV boundary formation in the Drosophila wing

Gene regulatory networks have been conserved during evolution. The Drosophila wing and the vertebrate hindbrain share the gene network involved in the establishment of the boundary between dorsal and ventral compartments in the wing and adjacent rhombomeres in the hindbrain. A positive feedback-loop between boundary and non-boundary cells and mediated by the activities of Notch and Wingless/Wnt-1 leads to the establishment of a Notch dependent organizer at the boundary. By means of a Systems Biology approach that combines mathematical modeling and both in silico and in vivo experiments in the Drosophila wing primordium, we modeled and tested this regulatory network and present evidence that a novel property, namely refractoriness to the Wingless signaling molecule, is required in boundary cells for the formation of a stable dorsal-ventral boundary. This new property has been validated in vivo, promotes mutually exclusive domains of Notch and Wingless activities and confers stability to the dorsal-ventral boundary. A robustness analysis of the regulatory network complements our results and ensures its biological plausibility.

LMO4 mRNA stability is regulated by extracellular ATP in F11 cells

LIM only domain protein 4 (LMO4) interacts with many signaling and transcription factors to regulate cellular proliferation, differentiation and plasticity. In Drosophila, mutations in the 3' untranslated region (UTR) of the homologue dLMO cause a gain of function by increasing mRNA stability. LMO4 3'UTR contains several AU-rich elements (ARE) and is highly conserved among vertebrates, suggesting that RNA destabilizing mechanisms are evolutionarily conserved. Here, we found that extracellular ATP stabilized LMO4 mRNA in F11 cells. The LMO4 3'UTR added to a luciferase reporter markedly reduced reporter activity under basal conditions, but increased activity with ATP treatment. Two ARE motifs were characterized in the LMO4 3'UTR. ATP increased binding of HuD protein to ARE1. ARE1 conferred ATP and HuD-dependent mRNA stabilization. In contrast, sequences flanking ARE2 bound CUGBP1 and ATP destabilized this complex. Thus, our results suggest that ATP modulates recruitment of RNA-binding proteins to the 3'UTR to stabilize LMO4 mRNA.

Identification of the Key LMO2-binding Determinants on Ldb1

The overexpression of LIM-only protein 2 (LMO2) in T-cells, as a result of chromosomal translocations, retroviral insertion during gene therapy, or in transgenic mice models, leads to the onset of T-cell leukemias. LMO2 comprises two protein-binding LIM domains that allow LMO2 to interact with multiple protein partners, including LIM domain-binding protein 1 (Ldb1, also known as CLIM2 and NLI), an essential cofactor for LMO proteins. Sequestration of Ldb1 by LMO2 in T-cells may prevent it binding other key partners, such as LMO4. Here, we show using protein engineering and enzyme-linked immunosorbent assay (ELISA) methodologies that LMO2 binds Ldb1 with a twofold lower affinity than does LMO4. Thus, excess LMO2 rather than an intrinsically higher binding affinity would lead to sequestration of Ldb1. Both LIM domains of LMO2 are required for high-affinity binding to Ldb1 (K(D) = 2.0 x 10(-8) M). However, the first LIM domain of LMO2 is primarily responsible for binding to Ldb1 (K(D) = 2.3 x 10(-7) M), whereas the second LIM domain increases binding by an order of magnitude. We used mutagenesis in combination with yeast two-hybrid analysis, and phage display selection to identify LMO2-binding "hot spots" within Ldb1 that locate to the LIM1-binding region. The delineation of this region reveals some specific differences when compared to the equivalent LMO4:Ldb1 interaction that hold promise for the development of reagents to specifically bind LMO2 in the treatment of leukemia.

Osa modulates the expression of Apterous target genes in the Drosophila wing

The establishment of the dorsal-ventral axis of the Drosophila wing depends on the activity of the LIM-homeodomain protein Apterous. Apterous activity depends on the formation of a higher order complex with its cofactor Chip to induce the expression of its target genes. Apterous activity levels are modulated during development by dLMO. Expression of dLMO in the Drosophila wing is regulated by two distinct Chip dependent mechanisms. Early in development, Chip bridges two molecules of Apterous to induce expression of dLMO in the dorsal compartment. Later in development, Chip, independently of Apterous, is required for expression of dLMO in the wing pouch. We have conducted a modular P-element based EP (enhancer/promoter) misexpression screen to look for genes involved in Apterous activity. We have found Osa, a member of the Brahma chromatin-remodeling complex, as a positive modulator of Apterous activity in the Drosophila wing. Osa mediates activation of some Apterous target genes and repression of others, including dLMO. Osa has been shown to bind Chip. We propose that Chip recruits Osa to the Apterous target genes, thus mediating activation or repression of their expression.

Lmo mutants reveal a novel role for circadian pacemaker neurons in cocaine-induced behaviors

Drosophila has been developed recently as a model system to investigate the molecular and neural mechanisms underlying responses to drugs of abuse. Genetic screens for mutants with altered drug-induced behaviors thus provide an unbiased approach to define novel molecules involved in the process. We identified mutations in the Drosophila LIM-only (LMO) gene, encoding a regulator of LIM-homeodomain proteins, in a genetic screen for mutants with altered cocaine sensitivity. Reduced Lmo function increases behavioral responses to cocaine, while Lmo overexpression causes the opposite effect, reduced cocaine responsiveness. Expression of Lmo in the principal Drosophila circadian pacemaker cells, the PDF-expressing ventral lateral neurons (LN(v)s), is sufficient to confer normal cocaine sensitivity. Consistent with a role for Lmo in LN(v)function,Lmomutants also show defects in circadian rhythms of behavior. However, the role for LN(v)s in modulating cocaine responses is separable from their role as pacemaker neurons: ablation or functional silencing of the LN(v)s reduces cocaine sensitivity, while loss of the principal circadian neurotransmitter PDF has no effect. Together, these results reveal a novel role for Lmo in modulating acute cocaine sensitivity and circadian locomotor rhythmicity, and add to growing evidence that these behaviors are regulated by shared molecular mechanisms. The finding that the degree of cocaine responsiveness is controlled by the Drosophila pacemaker neurons provides a neuroanatomical basis for this overlap. We propose that Lmo controls the responsiveness of LN(v)s to cocaine, which in turn regulate the flies' behavioral sensitivity to the drug.

Tandem LIM domains provide synergistic binding in the LMO4:Ldb1 complex

Nuclear LIM-only (LMO) and LIM-homeodomain (LIM-HD) proteins have important roles in cell fate determination, organ development and oncogenesis. These proteins contain tandemly arrayed LIM domains that bind the LIM interaction domain (LID) of the nuclear adaptor protein LIM domain-binding protein-1 (Ldb1). We have determined a high-resolution X-ray crystal structure of LMO4, a putative breast oncoprotein, in complex with Ldb1-LID, providing the first example of a tandem LIM:Ldb1-LID complex and the first structure of a type-B LIM domain. The complex possesses a highly modular structure with Ldb1-LID binding in an extended manner across both LIM domains of LMO4. The interface contains extensive hydrophobic and electrostatic interactions and multiple backbone-backbone hydrogen bonds. A mutagenic screen of Ldb1-LID, assessed by yeast two-hybrid and competition ELISA analysis, identified key features at the interface and revealed that the interaction is tolerant to mutation. These combined properties provide a mechanism for the binding of Ldb1 to numerous LMO and LIM-HD proteins. Furthermore, the modular extended interface may form a general mode of binding to tandem LIM domains.

Chip-mediated partnerships of the homeodomain proteins Bar and Aristaless with the LIM-HOM proteins Apterous and Lim1 regulate distal leg development

Proximodistal patterning in Drosophila requires division of the developing leg into increasingly smaller, discrete domains of gene function. The LIM-HOM transcription factors apterous (ap) and Lim1 (also known as dlim1), and the homeobox genes Bar and aristaless (al) are part of the gene battery required for the development of specific leg segments. Our genetic results show that there are posttranslational interactions between Ap, Bar and the LIM-domain binding protein Chip in tarsus four, and between Al, Lim1 and Chip in the pretarsus, and that these interactions depend on the presence of balanced amounts of such proteins. We also observe in vitro protein binding between Bar and Chip, Bar and Ap, Lim1 and Chip, and Al and Chip. Together with the previous evidence for interactions between Ap and Chip, these results suggest that these transcription factors form protein complexes during leg development. We propose that the different developmental outcomes of LIM-HOM function are due to the precise identity and dosage of the interacting partners present in a given cell.

Xenopus Xlmo4 is a GATA cofactor during ventral mesoderm formation and regulates Ldb1 availability at the dorsal mesoderm and the neural plate

We have identified and functionally characterized the Xenopus Xlmo4 gene, which encodes a member of the LIM-domain-only protein family. Xlmo4 is activated at gastrula stages in the mesodermal marginal zone probably in response to BMP4 signaling. Soon after, Xlmo4 is downregulated in the dorsal region of the mesoderm. This repression seems to be mediated by organizer-expressed repressors, such as Gsc. Xlmo4 downregulation is necessary for the proper formation of this territory. Increasing Xlmo4 function in this region downregulates Spemman Organizer genes and suppresses dorsal-anterior structures. By binding to Ldb1, Xlmo4 may restrict the availability of this cofactor for transcription factors expressed at the Spemman Organizer. In the ventral mesoderm, Xlmo4 is required to establish the identity of this territory by acting as a positive cofactor of GATA factors. In the neural ectoderm, Xlmo4 expression depends on Xiro homeoprotein activity. In this region, Xlmo4 suppresses differentiation of primary neurons and interferes with gene expression at the Isthmic Organizer, most likely by displacing Ldb1 from active transcription factor complexes required for these processes. Together, our data suggest that Xlmo4 uses distinct mechanisms to participate in different processes during development.

LIM-domain-binding protein 1: a multifunctional cofactor that interacts with diverse proteins

The ubiquitous nuclear adaptor protein LIM-domain-binding protein 1 (Ldb1) was originally identified as a cofactor for LIM-homeodomain and LIM-only (LMO) proteins that have fundamental roles in development. In parallel, Ldb1 has been shown to have essential functions in diverse biological processes in different organisms. The recent targeting of this gene in mice has revealed roles for Ldb1 in neural patterning and development that have been conserved throughout evolution. Furthermore, the elucidation of the three-dimensional structures of LIM-Ldb1 complexes has provided insight into the molecular basis for the ability of Ldb1 to contact diverse LIM-domain proteins. It has become evident that Ldb1 is a multi-adaptor protein that mediates interactions between different classes of transcription factors and their co-regulators and that the nature of these complexes determines cell fate and differentiation.

Mutational analysis of the LMO4 gene, encoding a BRCA1-interacting protein, in breast carcinomas

The LIM domain-only genes LMO1 and LMO2 are translocated in acute T cell leukemia (T-ALL) and have been shown to be oncogenes in T lymphoid cells. LMO4, the fourth member of this family, is overexpressed in more than 50% of sporadic breast cancers, suggesting a role in breast oncogenesis. We recently found that LMO4 interacts with the breast/ovarian tumor suppressor BRCA1 and that LMO4 can repress its transcriptional activity. Since proto-oncogene deregulation can result from activating mutations in their coding or regulatory sequences, we explored whether the LMO4 gene undergoes somatic mutagenesis in breast cancer. Mutation analysis of the coding and 3' untranslated regions of the LMO4 gene was performed on 82 primary breast and 22 tumor cell lines. A somatic mutation was detected in one primary breast cancer, at the 3' end of exon 2, but was not present in normal DNA derived from the same patient. This mutation causes a frame-shift and potentially results in a truncated LMO4 polypeptide, LIM1(mut), lacking the second LIM domain. This mutant protein could still bind Ldb1 but no longer associated with CtIP or BRCA1. Our results show that somatic mutations within the LMO4 gene do occur in breast cancer but at a very low frequency. Thus, the primary mechanism by which LMO4 is deregulated in breast cancers appears to reflect overexpression of the gene rather than the acquisition of activating genetic mutations.

Ssdp proteins bind to LIM-interacting co-factors and regulate the activity of LIM-homeodomain protein complexes in vivo

LIM-homeodomain transcription factors control a variety of developmental processes, and are assembled into functional complexes with the LIM-binding co-factor Ldb1 (in mouse) or Chip (in Drosophila). We describe the identification and characterization of members of the Ssdp family of proteins, which we show to interact with Ldb1 and Chip. The N terminus of Ssdp is highly conserved among species and binds a highly conserved domain within Ldb1/Chip that is distinct from the domains required for LIM binding and self-dimerization. In Drosophila, Ssdp is expressed in the developing nervous system and imaginal tissues, and it is capable of modifying the in vivo activity of complexes comprised of Chip and the LIM-homeodomain protein Apterous. Null mutations of the ssdp gene are cell-lethal in clones of cells within the developing wing disc. However, clones mutant for a hypomorphic allele give rise to ectopic margins, wing outgrowth and cell identity defects similar to those produced by mutant clones of Chip or apterous. Ssdp and Ldb/Chip each show structural similarity to two Arabidopsis proteins that cooperate with one another to regulate gene expression during flower development, suggesting that the molecular interactions between Ssdp and Ldb/Chip proteins are evolutionarily ancient and supply a fundamental function in the regulated control of transcription.

Drosophila tufted is a gain-of-function allele of the proneural gene amos

Tufted is a classical Drosophila mutant characterized by a large number of ectopic mechanosensory bristles on the dorsal mesothorax. Unlike other ectopic bristle mutants, Tufted is epistatic to achaete and scute, the proneural genes that normally control the development of these sensory organs. In this report, I present genetic and molecular evidence that Tufted is a gain-of-function allele of the proneural gene amos that ectopically activates mechanosensory neurogenesis. I also systematically examine the ability of the various proneural bHLH proteins to cross-activate each other and find that their ability to do so is in general relatively limited, despite their common ability to induce the formation of mechanosensory bristles. This phenomenon seems instead to be related to their shared ability to activate Asense and Senseless.

Interaction between apterous and early expression of vestigial in formation of the dorso-ventral compartments in the Drosophila wing disc

BACKGROUND

Compartment formation is a developmental process that requires the existence of barriers against intermixing between cell groups. In the Drosophila wing disc, the dorso-ventral (D/V) compartment boundary is defined by the expression of the apterous (ap) selector gene in the dorsal compartment. AP activity is under control of dLMO which destabilizes the formation of the AP-CHIP complex.

RESULTS

We report that D/V boundary formation in the wing disc also depends on early expression of vestigial (vg). Our data suggest that vg is already required for wing cell proliferation before D/V compartmentalization. In addition, we show that over-expression of vg can, to some extent, rescue the effect of the absence of ap on D/V boundary formation. Early VG product regulates AP activity by inducing dLMO and thus indirectly regulating ap target genes such as fringe and the PSalpha1 and PSalpha2 integrins.

CONCLUSION

Normal cell proliferation is necessary for ap expression at the level of the D/V boundary. This would be mediated by vg, which interacts in a dose-dependent way with ap.

Ssdp proteins interact with the LIM-domain-binding protein Ldb1 to regulate development

The LIM-domain-binding protein Ldb1 is a key factor in the assembly of transcriptional complexes involving LIM-homeodomain proteins and other transcription factors that regulate animal development. We identified Ssdp proteins (previously described as sequence-specific, single-stranded-DNA-binding proteins) as components of Ldb1-associated nuclear complexes in HeLa cells. Ssdp proteins are associated with Ldb1 in a variety of additional mammalian cell types. This association is specific, does not depend on the presence of nucleic acids, and is functionally significant. Genes encoding Ssdp proteins are well conserved in evolution from Drosophila to humans. Whereas the vertebrate Ssdp gene family has several closely related members, the Drosophila Ssdp gene is unique. In Xenopus, Ssdp encoded by Drosophila Ssdp or mouse Ssdp1 mRNA enhances axis induction by Ldb1 in conjunction with the LIM-homeobox gene Xlim1. Furthermore, we were able to demonstrate an interaction between Ssdp and Chip (the fly homolog of Ldb1) in Drosophila wing development. These findings indicate functional conservation of Ssdp as a cofactor of Ldb1 during invertebrate and vertebrate development.

A short history of LIM domains (1993-2002): from protein interaction to degradation

The LIM domain is a cysteine-rich zinc-finger motif found in a large family of proteins. In LIM-homeodomain (LIM-hd) transcription factors and LIM-only (LMO) factors, the LIM domains are responsible for key interactions with co-activators, co-repressors, competitors, and other transcription factors, and are therefore of considerable importance for the regulation of associated transcriptional activity. In this review, the authors describe the progressive discoveries of NLI/Ldb/CLIM, LMO and RLIM, and discuss how the field was very recently updated by the finding that LIM-hd transcriptional activity is controlled by regulated degradation of cofactors and LIM-hd themselves.

Regulation of Apterous activity inDrosophilawing development

Apterous is a LIM-homeodomain protein that confers dorsal compartment identity in Drosophila wing development. Apterous activity requires formation of a complex with a co-factor, Chip/dLDB. Apterous activity is regulated during wing development by dLMO, which competes with Apterous for complex formation. Here, we present evidence that complex formation between Apterous, Chip and DNA stabilizes Apterous protein in vivo. We also report that a difference in the ability of Chip to bind the LIM domains of Apterous and dLMO contributes to regulation of activity levels in vivo.

Apterous mediates development of direct flight muscles autonomously and indirect flight muscles through epidermal cues

Two physiologically distinct types of muscles, the direct and indirect flight muscles, develop from myoblasts associated with the Drosophila wing disc. We show that the direct flight muscles are specified by the expression of Apterous, a Lim homeodomain protein, in groups of myoblasts. This suggests a mechanism of cell-fate specification by labelling groups of fusion competent myoblasts, in contrast to mechanisms in the embryo, where muscle cell fate is specified by single founder myoblasts. In addition, Apterous is expressed in the developing adult epidermal muscle attachment sites. Here, it functions to regulate the expression of stripe, a gene that is an important element of early patterning of muscle fibres, from the epidermis. Our results, which may have broad implications, suggest novel mechanisms of muscle patterning in the adult, in contrast to embryonic myogenesis.

Deletion of a conserved regulatory element in the Drosophila Adh gene leads to increased alcohol dehydrogenase activity but also delays development

In vivo levels of enzymatic activity may be increased through either structural or regulatory changes. Here we use Drosophila melanogaster alcohol dehydrogenase (ADH) in an experimental test for selective differences between these two mechanisms. The well-known ADH-Slow (S)/Fast (F) amino acid replacement leads to a twofold increase in activity by increasing the catalytic efficiency of the enzyme. Disruption of a highly conserved, negative regulatory element in the Adh 3' UTR also leads to a twofold increase in activity, although this is achieved by increasing in vivo Adh mRNA and protein concentrations. These two changes appear to be under different types of selection, with positive selection favoring the amino acid replacement and purifying selection maintaining the 3' UTR sequence. Using transgenic experiments we show that deletion of the conserved 3' UTR element increases adult and larval Adh expression in both the ADH-F and ADH-S genetic backgrounds. However, the 3' UTR deletion also leads to a significant increase in developmental time in both backgrounds. ADH allozyme type has no detectable effect on development. These results demonstrate a negative fitness effect associated with Adh overexpression. This provides a mechanism whereby natural selection can discriminate between alternative pathways of increasing enzymatic activity.

Temporal regulation of apterous activity during development of the Drosophila wing

Dorsoventral axis formation in the Drosophila wing depends on the activity of the selector gene apterous. Although selector genes are usually thought of as binary developmental switches, we find that Apterous activity is negatively regulated during wing development by its target gene dLMO. Apterous-dependent expression of Serrate and fringe in dorsal cells leads to the restricted activation of Notch along the dorsoventral compartment boundary. We present evidence that the ability of cells to participate in this Apterous-dependent cell-interaction is under spatial and temporal control. Apterous-dependent expression of dLMO causes downregulation of Serrate and fringe and allows expression of delta in dorsal cells. This limits the time window during which dorsoventral cell interactions can lead to localized activation of Notch and induction of the dorsoventral organizer. Overactivation of Apterous in the absence of dLMO leads to overexpression of Serrate, reduced expression of delta and concomitant defects in differentiation and cell survival in the wing primordium. Thus, downregulation of Apterous activity is needed to allow normal wing development.

A gain-of-function screen for genes that affect the development of the Drosophila adult external sensory organ

The Drosophila adult external sensory organ, comprising a neuron and its support cells, is derived from a single precursor cell via several asymmetric cell divisions. To identify molecules involved in sensory organ development, we conducted a tissue-specific gain-of-function screen. We screened 2293 independent P-element lines established by P. Rorth and identified 105 lines, carrying insertions at 78 distinct loci, that produced misexpression phenotypes with changes in number, fate, or morphology of cells of the adult external sensory organ. On the basis of the gain-of-function phenotypes of both internal and external support cells, we subdivided the candidate lines into three classes. The first class (52 lines, 40 loci) exhibits partial or complete loss of adult external sensory organs. The second class (38 lines, 28 loci) is associated with increased numbers of entire adult external sensory organs or subsets of sensory organ cells. The third class (15 lines, 10 loci) results in potential cell fate transformations. Genetic and molecular characterization of these candidate lines reveals that some loci identified in this screen correspond to genes known to function in the formation of the peripheral nervous system, such as big brain, extra macrochaetae, and numb. Also emerging from the screen are a large group of previously uncharacterized genes and several known genes that have not yet been implicated in the development of the peripheral nervous system.

Chip is an essential cofactor for apterous in the regulation of axon guidance in Drosophila

LIM-homeodomain transcription factors are expressed in subsets of neurons and are required for correct axon guidance and neurotransmitter identity. The LIM-homeodomain family member Apterous requires the LIM-binding protein Chip to execute patterned outgrowth of the Drosophila wing. To determine whether Chip is a general cofactor for diverse LIM-homeodomain functions in vivo, we studied its role in the embryonic nervous system. Loss-of-function Chip mutations cause defects in neurotransmitter production that mimic apterous and islet mutants. Chip is also required cell-autonomously by Apterous-expressing neurons for proper axon guidance, and requires both a homodimerization domain and a LIM interaction domain to function appropriately. Using a Chip/Apterous chimeric molecule lacking domains normally required for their interaction, we reconstituted the complex and rescued the axon guidance defects of apterous mutants, of Chip mutants and of embryos doubly mutant for both apterous and Chip. Our results indicate that Chip participates in a range of developmental programs controlled by LIM-homeodomain proteins and that a tetrameric complex comprising two Apterous molecules bridged by a Chip homodimer is the functional unit through which Apterous acts during neuronal differentiation.

Chip interacts with diverse homeodomain proteins and potentiates bicoid activity in vivo

The Drosophila protein Chip potentiates activation by several enhancers and is required for embryonic segmentation. Chip and its mammalian homologs interact with and promote dimerization of nuclear LIM proteins. No known Drosophila LIM proteins, however, are required for segmentation, nor for expression of most genes known to be regulated by Chip. Here we show that Chip also interacts with diverse homeodomain proteins using residues distinct from those that interact with LIM proteins, and that Chip potentiates activity of one of these homeodomain proteins in Drosophila embryos and in yeast. These and other observations help explain the roles of Chip in segmentation and suggest a model to explain how Chip potentiates activation by diverse enhancers.

Distant liaisons: long-range enhancer-promoter interactions in Drosophila

Transcriptional activation of many developmentally regulated genes is mediated by proteins binding to enhancer sequences located several kilobases from the promoter. Existing models for how activator proteins function do not adequately explain long-range activation. Recent experiments in Drosophila on insulators that block enhancer-promoter interactions, interchromosomal activation, and mutants deficient in long-range activation are consistent with models in which facilitator factors that function between enhancers and promoters bring them into physical proximity of each other.

Chip and apterous physically interact to form a functional complex during Drosophila development

LIM homeodomain (LIM-HD) proteins play key roles in a variety of developmental processes throughout the animal kingdom. Here we show that the LIM-binding protein Chip acts as a cofactor for the Drosophila LIM-HD family member Apterous (Ap) in wing development. We define the domains of Chip required for LIM-HD binding and for homodimerization and show that mutant proteins deleted for these domains act in a dominant-negative fashion to disrupt Ap function. Our results support a model for multimeric complexes containing Chip and Ap in transcriptional regulation. This model is confirmed by the activity of a chimeric fusion between Chip and Ap that reconstitutes the complex and rescues the ap mutant phenotype.

Regulation of LIM homeodomain activity in vivo: a tetramer of dLDB and apterous confers activity and capacity for regulation by dLMO

Dorsal-ventral axis formation in the Drosophila wing depends on the activity of the LIM homeodomain transcription factor Apterous and its cofactor, dLDB/Chip. We present evidence that Apterous activity depends on the formation of a LIM homeodomain dimer bridged by a dimer of cofactor. We show that Apterous activity levels are regulated in vivo by dLMO, an antagonist of homodimer formation. Making use of a constitutively active form of Apterous and dominant-negative forms of Apterous and dLDB/Chip, we show that the normal function of dLMO is to downregulate Apterous activity and that the dLMO mutant phenotype is due to excess Apterous activity. These findings may point to a general mechanism for regulation of LIM homeodomain protein activity.