The Drosophila LIM-only gene, dLMO, is mutated in Beadex alleles and might represent an evolutionarily conserved function in appendage development

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.

The Stage-Based Model of Addiction-Using Drosophila to Investigate Alcohol and Psychostimulant Responses

Addiction is a progressive and complex disease that encompasses a wide range of disorders and symptoms, including substance use disorder (SUD), for which there are few therapeutic treatments. SUD is the uncontrolled and chronic use of substances despite the negative consequences resulting from this use. The progressive nature of addiction is organized into a testable framework, the neurobiological stage-based model, that includes three behavioral stages: (1) binge/intoxication, (2) withdrawal/negative affect, and (3) preoccupation/anticipation. Human studies offer limited opportunities for mechanistic insights into these; therefore, model organisms, like Drosophila melanogaster, are necessary for understanding SUD. Drosophila is a powerful model organism that displays a variety of SUD-like behaviors consistent with human and mammalian substance use, making flies a great candidate to study mechanisms of behavior. Additionally, there are an abundance of genetic tools like the GAL4/UAS and CRISPR/Cas9 systems that can be used to gain insight into the molecular mechanisms underlying the endophenotypes of the three-stage model. This review uses the three-stage framework and discusses how easily testable endophenotypes have been examined with experiments using Drosophila, and it outlines their potential for investigating other endophenotypes.

A Compilation of the Diverse miRNA Functions in Caenorhabditis elegans and Drosophila melanogaster Development

MicroRNAs are critical regulators of post-transcriptional gene expression in a wide range of taxa, including invertebrates, mammals, and plants. Since their discovery in the nematode, Caenorhabditis elegans, miRNA research has exploded, and they are being identified in almost every facet of development. Invertebrate model organisms, particularly C. elegans, and Drosophila melanogaster, are ideal systems for studying miRNA function, and the roles of many miRNAs are known in these animals. In this review, we compiled the functions of many of the miRNAs that are involved in the development of these invertebrate model species. We examine how gene regulation by miRNAs shapes both embryonic and larval development and show that, although many different aspects of development are regulated, several trends are apparent in the nature of their regulation.

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.

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.

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.

HOX-mediated LMO2 expression in embryonic mesoderm is recapitulated in acute leukaemias

The Lim Domain Only 2 (LMO2) leukaemia oncogene encodes an LIM domain transcriptional cofactor required for early haematopoiesis. During embryogenesis, LMO2 is also expressed in developing tail and limb buds, an expression pattern we now show to be recapitulated in transgenic mice by an enhancer in LMO2 intron 4. Limb bud expression depended on a cluster of HOX binding sites, while posterior tail expression required the HOX sites and two E-boxes. Given the importance of both LMO2 and HOX genes in acute leukaemias, we further demonstrated that the regulatory hierarchy of HOX control of LMO2 is activated in leukaemia mouse models as well as in patient samples. Moreover, Lmo2 knock-down impaired the growth of leukaemic cells, and high LMO2 expression at diagnosis correlated with poor survival in cytogenetically normal AML patients. Taken together, these results establish a regulatory hierarchy of HOX control of LMO2 in normal development, which can be resurrected during leukaemia development. Redeployment of embryonic regulatory hierarchies in an aberrant context is likely to be relevant in human pathologies beyond the specific example of ectopic activation of LMO2.

Extent With Modification: Leg Patterning in the Beetle Tribolium castaneum and the Evolution of Serial Homologs

Serial homologs are similar structures that develop at different positions within a body plan. These structures share some, but not all, aspects of developmental patterning, and their evolution is thought to be constrained by shared, pleiotropic gene functions. Here we describe the functions of 17 developmental genes during metamorphic development of the legs in the red flour beetle, Tribolium castaneum. This study provides informative comparisons between appendage development in Drosophila melanogaster and T. castaneum, between embryonic and adult development in T. castaneum, and between the development of serially homologous appendages. The leg gap genes Distal-less and dachshund are conserved in function. Notch signaling, the zinc-finger transcription factors related to odd-skipped, and bric-à-brac have conserved functions in promoting joint development. homothorax knockdown alters the identity of proximal leg segments but does not reduce growth. Lim1 is required for intermediate leg development but not distal tarsus and pretarsus development as in D. melanogaster. Development of the tarsus requires decapentaplegic, rotund, spineless, abrupt, and bric-à-brac and the EGF ligand encoded by Keren. Metathoracic legs of T. castaneum have four tarsomeres, whereas other legs have five. Patterns of gene activity in the tarsus suggest that patterning in the middle of the tarsal region, not the proximal- or distal-most areas, is responsible for this difference in segment number. Through comparisons with other recent studies of T. castaneum appendage development, we test hypotheses for the modularity or interdependence of development during evolution of serial homologs.

Identification of genes downstream of the Shh signalling in the developing chick wing and syn-expressed with Hoxd13 using microarray and 3D computational analysis

Sonic hedgehog (Shh) signalling by the polarizing region at the posterior margin of the chick wing bud is pivotal in patterning the digits but apart from a few key downstream genes, such as Hoxd13, which is expressed in the posterior region of the wing that gives rise to the digits, the genes that mediate the response to Shh signalling are not known. To find genes that are co-expressed with Hoxd13 in the posterior of chick wing buds and regulated in the same way, we used microarrays to compare gene expression between anterior and posterior thirds of wing buds from normal chick embryos and from polydactylous talpid³ mutant chick embryos, which have defective Shh signalling due to lack of primary cilia. We identified 1070 differentially expressed gene transcripts, which were then clustered. Two clusters contained genes predominantly expressed in posterior thirds of normal wing buds; in one cluster, genes including Hoxd13, were expressed at high levels in anterior and posterior thirds in talpid³ wing buds, in the other cluster, genes including Ptc1, were expressed at low levels in anterior and posterior thirds in talpid³ wing buds. Expression patterns of genes in these two clusters were validated in normal and talpid³ mutant wing buds by in situ hybridisation and demonstrated to be responsive to application of Shh. Expression of several genes in the Hoxd13 cluster was also shown to be responsive to manipulation of protein kinase A (PKA) activity, thus demonstrating regulation by Gli repression. Genes in the Hoxd13 cluster were then sub-clustered by computational comparison of 3D expression patterns in normal wing buds to produce syn-expression groups. Hoxd13 and Sall1 are syn-expressed in the posterior region of early chick wing buds together with 6 novel genes which are likely to be functionally related and represent secondary targets of Shh signalling. Other groups of syn-expressed genes were also identified, including a group of genes involved in vascularisation.

Repression of Lim only protein 4-activated transcription inhibits proliferation and induces apoptosis of normal mammary epithelial cells and breast cancer cells

Lim only protein (LMO) 4 acts as a transcriptional adapter and modulates mammary gland morphogenesis as well as breast oncogenesis in transgenic mice. Yet, the molecular and cellular mechanisms of these effects remain to be fully elucidated. Engrailed LMO4 fusion protein is a powerful dominant repressor of LMO4 activated transcription that was successfully used to discover the role of LMO4 as a transcriptional activator in mammary gland development in our previous studies using mouse models. In this manuscript, we investigated the cellular effects of LMO4 in human normal mammary epithelial cells (HMECs) and breast cancer cell lines using the Engrailed-LMO4 fusion protein. HMEC cell growth was inhibited by the expression of the Engrailed-LMO4 fusion protein. The decrease in cell number was due to both decreased cell proliferation and enhanced apoptosis, suggesting that LMO4 promotes proliferation and survival of normal mammary epithelial cells. The expression of the Engrailed-LMO4 fusion protein also suppressed cell growth, and induced apoptosis in two breast cancer cell lines, MDA-MB-231 and T47D, suggesting that LMO4 contributes to oncogenesis by similar mechanisms of enhanced cell survival and proliferation. Taken together, our data indicate that LMO4 has similar cellular effects in normal mammary epithelial cells and breast cancer cells, and also provide direct evidence for the idea that normal development and carcinogenesis share conserved molecular mechanisms.

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.

Islet-to-LMO stoichiometries control the function of transcription complexes that specify motor neuron and V2a interneuron identity

LIM transcription factors bind to nuclear LIM interactor (Ldb/NLI/Clim) in specific ratios to form higher-order complexes that regulate gene expression. Here we examined how the dosage of LIM homeodomain proteins Isl1 and Isl2 and LIM-only protein Lmo4 influences the assembly and function of complexes involved in the generation of spinal motor neurons (MNs) and V2a interneurons (INs). Reducing the levels of Islet proteins using a graded series of mutations favored V2a IN differentiation at the expense of MN formation. Although LIM-only proteins (LMOs) are predicted to antagonize the function of Islet proteins, we found that the presence or absence of Lmo4 had little influence on MN or V2a IN specification. We did find, however, that the loss of MNs resulting from reduced Islet levels was rescued by eliminating Lmo4, unmasking a functional interaction between these proteins. Our findings demonstrate that MN and V2a IN fates are specified by distinct complexes that are sensitive to the relative stoichiometries of the constituent factors and we present a model to explain how LIM domain proteins modulate these complexes and, thereby, this binary-cell-fate decision.

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.

Chip is required for posteclosion behavior in Drosophila

Neurons acquire their molecular, neurochemical, and connectional features during development as a result of complex regulatory mechanisms. Here, we show that a ubiquitous, multifunctional protein cofactor, Chip, plays a critical role in a set of neurons in Drosophila that control the well described posteclosion behavior. Newly eclosed flies normally expand their wings and display tanning and hardening of their cuticle. Using multiple approaches to interfere with Chip function, we find that these processes do not occur without normal activity of this protein. Furthermore, we identified the nature of the deficit to be an absence of Bursicon in the hemolymph of newly eclosed flies, whereas the responsivity to Bursicon in these flies remains normal. Chip interacts with transcription factors of the LIM-HD (LIM-homeodomain) family, and we identified one member, dIslet, as a potential partner of Chip in this process. Our findings provide the first evidence of transcriptional mechanisms involved in the development of the neuronal circuit that regulates posteclosion behavior in Drosophila.

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.

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 can interact with Smad proteins and modulate transforming growth factor-beta signaling in epithelial cells

LIM-only protein 4 (LMO4) plays critical roles in mammalian development, and has been proposed to play roles in epithelial oncogenesis, including breast cancer. As LMO4 is highly expressed in the epithelial compartments at locations of active mesenchymal-epithelial interactions, we reasoned that LMO4 might act by modulating signaling pathways involved in mesenchymal-epithelial signaling. One such candidate signal is the transforming growth factor-beta (TGFbeta) cytokine pathway, which plays important roles both in development and cancer. We show here that the transcriptional response to TGFbeta in epithelial cells is sensitive to LMO4 levels; both up- and downregulation of LMO4 can enhance TGFbeta signaling as assessed by a TGFbeta-responsive reporter gene. Furthermore, LMO4 can interact with the MH1 and linker domains of receptor-mediated Smad proteins, and associate with the endogenous TGFbeta-responsive Plasminogen Activator Inhibitor-1 gene promoter in a TGFbeta-dependent manner, suggesting that such interactions may mediate the effects of LMO4 on TGFbeta signaling. When introduced into mammary epithelial cells, LMO4 potentiated the growth-inhibitory effects of TGFbeta in those cells. These results define a new function for LMO4 as a coactivator in TGFbeta signaling, and provide a potential novel mechanism for LMO4-mediated regulation in development and oncogenesis.

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.

Human Bex2 interacts with LMO2 and regulates the transcriptional activity of a novel DNA-binding complex

Human Bex2 (brain expressed X-linked, hBex2) is highly expressed in the embryonic brain, but its function remains unknown. We have identified that LMO2, a LIM-domain containing transcriptional factor, specifically interacts with hBex2 but not with mouse Bex1 and Bex2. The interaction was confirmed both by pull-down with GST-hBex2 and by coimmunoprecipitation assays in vivo. Using electrophoretic mobility shift assay, we have demonstrated the physical interaction of hBex2 and LMO2 as part of a DNA-binding protein complex. We have also shown that hBex2 can enhance the transcriptional activity of LMO2 in vivo. Furthermore, using mammalian two-hybrid analysis, we have identified a neuronal bHLH protein, NSCL2, as a novel binding partner for LMO2. We then showed that LMO2 could up-regulate NSCL2-dependent transcriptional activity, and hBex2 augmented this effect. Thus, hBex2 may act as a specific regulator during embryonic development by modulating the transcriptional activity of a novel E-box sequence-binding complex that contains hBex2, LMO2, NSCL2 and LDB1.

The LIM domain-only protein LMO4 is required for neural tube closure

Nuclear LIM domain-only proteins (LMOs), which consist of two closely spaced 50 amino acid Zn2+-finger protein interaction modules mediate interactions between several classes of transcription factors important for development. LMO2 is necessary for development of the entire hematopoietic system and overexpression of LMO1 or LMO2 results in human acute T cell leukemia. LMO4 is the most widely expressed LMO but its normal function is unknown. During development, LMO4 is expressed in dividing neuroepithelial cells within the ventricular zone along the entire rostrocaudal axis of the nervous system. In telencephalic and spinal cord regions of the CNS, LMO4 is highly expressed in ventral but is low in dorsal proliferating neuroepithelial cells. To understand the role of LMO4 during mouse development, we generated a homozygous null mutation in the gene. We found that LMO4 is required for proper closure of the anterior neural tube. In the absence of LMO4, elevation, bending, and proliferation of the ventral neural epithelium and consequent fusion of the prospective dorsal ends of the neural tube do not occur. LMO4 mutant mice die embryonically and exhibit exencephaly, which is associated with abnormal patterns of cell proliferation and with high levels of apoptotic cell death within the neuroepithelium. LMO4 is thus essential for normal patterns of proliferation and for survival of neural epithelial cells in the rostral neural tube. LMO4 is also expressed in Schwann cell progenitors after these contact neurites, a process mediated in part by neuregulin (Nrg).

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.

Expression of an engrailed-LMO4 fusion protein in mammary epithelial cells inhibits mammary gland development in mice

LIM domain factors and associated cofactors are important developmental regulators in pattern formation and organogenesis. In addition, overexpression of two LIM-only factors (LMOs) causes acute lymphocytic leukemia. The more recently discovered LMO factor LMO4 is highly expressed in proliferating epithelial cells, and frequently overexpressed in breast carcinoma. Here we show that while LMO4 is expressed throughout mammary gland development, it is dramatically upregulated in mammary epithelial cells during midpregnancy. The LMO coactivator Clim2/Ldb1/NLI showed a similar expression pattern, consistent with the idea that LMO4 and Clim2 act as a complex in mammary epithelial cells. In MCF-7 cells, LMO4 transcripts were upregulated by heregulin, an activator of ErbB receptors that are known to be important in mammary gland development and breast cancer. To test the hypothesis that LMO4 plays roles in mammary gland development, we created an engrailed-LMO4 fusion protein. This fusion protein maintains the ability to interact with Clim2, but acts as a dominant repressor of both basal and activated transcription when recruited to a DNA-regulatory region. When the engrailed-LMO4 fusion protein was expressed under control of the MMTV promoter in transgenic mice, both ductular development in virgin mice and alveolar development in pregnant mice were inhibited. These results suggest that LMO4 plays a role in promoting mammary gland development.

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.

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.

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.

neuralized functions cell-autonomously to regulate a subset of notch-dependent processes during adult Drosophila development

neuralized (neu) represents one of the strong neurogenic mutants in Drosophila. Mutants of this class display, among other phenotypes, a strong overcommitment to neural fates at the expense of epidermal fates. We analyzed the role of neu during adult development by using mutant clonal analysis, misexpression of wild-type and truncated forms of Neu, and examination of genetic interactions with N-pathway mutations. We find that neu is required cell-autonomously for lateral inhibition during peripheral neurogenesis and for multiple asymmetric cell divisions in the sensory lineage. In contrast, neu is apparently dispensable for other N-mediated processes, including lateral inhibition during wing vein development and wing margin induction. Misexpression of wild-type Neu causes defects in both peripheral neurogenesis and wing vein development, while a truncated form lacking the RING finger is further capable of inhibiting formation of the wing margin. In addition, the phenotypes produced by misexpression of wild-type and truncated Neu proteins are sensitive to the dosage of several N-pathway components. Finally, using epitope-tagged Neu proteins, we localize Neu to the plasma membrane and reveal a novel morphology to the sensory organ precursor cells of wing imaginal discs. Collectively, these data indicate a key role for neu in the reception of the lateral inhibitory signal during peripheral neurogenesis.

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.

The LIM domain: regulation by association

The LIM domain is a zinc finger structure that is present in several types of proteins, including homeodomain transcription factors, kinases and proteins that consist of several LIM domains. Proteins containing LIM domains have been discovered to play important roles in a variety of fundamental biological processes including cytoskeleton organization, cell lineage specification and organ development, but also for pathological functions such as oncogenesis, leading to human disease. The LIM domain has been demonstrated to be a protein-protein interaction motif that is critically involved in these processes. The recent isolation and analysis of more LIM domain-containing proteins from several species have confirmed and broadened our knowledge about LIM protein function. Furthermore, the identification and characterization of factors that interact with LIM domains illuminates mechanisms of combinatorial developmental regulation.

The oncogenic LIM-only transcription factor Lmo2 regulates angiogenesis but not vasculogenesis in mice

The LMO2 gene is activated by chromosomal translocations in human T cell acute leukemias, but in mouse embryogenesis, Lmo2 is essential for initiation of yolk sac and definitive hematopoiesis. The LMO2 protein comprises two LIM-zinc-finger-like protein interaction modules and functions by interaction with specific partners in DNA-binding transcription complexes. We have now investigated the role of Lmo2-associated transcription complexes in the formation of the vascular system by following the fate of Lmo2-null embryonic stem (ES) cells in mouse chimeras. Lmo2 is expressed in vascular endothelium, and Lmo2-null ES cells contributed to the capillary network normally until around embryonic day 9. However, after this time, marked disorganization of the vascular system was observed in those chimeric mice that have a high contribution of Lmo2-null ES cells. Moreover, Lmo2-null ES cells do not contribute to endothelial cells of large vessel walls of surviving chimeric mice after embryonic day 10. These results show that Lmo2 is not needed for de novo capillary formation from mesoderm but is necessary for angiogenic remodeling of the existing capillary network into mature vasculature. Thus, Lmo2-mediated transcription complexes not only regulate distinct phases of hematopoiesis but also angiogenesis, presumably by Lmo2 interacting with distinct partners in the different settings.

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.

Mouse deformed epidermal autoregulatory factor 1 recruits a LIM domain factor, LMO-4, and CLIM coregulators

Nuclear LIM domains interact with a family of coregulators referred to as Clim/Ldb/Nli. Although one family member, Clim-2/Ldb-1/Nli, is highly expressed in epidermal keratinocytes, no nuclear LIM domain factor is known to be expressed in epidermis. Therefore, we used the conserved LIM-interaction domain of Clim coregulators to screen for LIM domain factors in adult and embryonic mouse skin expression libraries and isolated a factor that is highly homologous to the previously described LIM-only proteins LMO-1, -2, and -3. This factor, referred to as LMO-4, is expressed in overlapping manner with Clim-2 in epidermis and in several other regions, including epithelial cells of the gastrointestinal, respiratory and genitourinary tracts, developing cartilage, pituitary gland, and discrete regions of the central and peripheral nervous system. Like LMO-2, LMO-4 interacts strongly with Clim factors via its LIM domain. Because LMO/Clim complexes are thought to regulate gene expression by associating with DNA-binding proteins, we used LMO-4 as a bait to screen for such DNA-binding proteins in epidermis and isolated the mouse homologue of Drosophila Deformed epidermal autoregulatory factor 1 (DEAF-1), a DNA-binding protein that interacts with regulatory sequences first described in the Deformed epidermal autoregulatory element. The interaction between LMO-4 and mouse DEAF-1 maps to a proline-rich C-terminal domain of mouse DEAF-1, distinct from the helix-loop-helix and GATA domains previously shown to interact with LMOs, thus defining an additional LIM-interacting domain.