Regulation of Apterous activity inDrosophilawing development

High hedgehog signaling is transduced by a multikinase-dependent switch controlling the apico-basal distribution of the GPCR smoothened

The oncogenic G-protein-coupled receptor (GPCR) Smoothened (SMO) is a key transducer of the hedgehog (HH) morphogen, which plays an essential role in the patterning of epithelial structures. Here, we examine how HH controls SMO subcellular localization and activity in a polarized epithelium using the Drosophila wing imaginal disc as a model. We provide evidence that HH promotes the stabilization of SMO by switching its fate after endocytosis toward recycling. This effect involves the sequential and additive action of protein kinase A, casein kinase I, and the Fused (FU) kinase. Moreover, in the presence of very high levels of HH, the second effect of FU leads to the local enrichment of SMO in the most basal domain of the cell membrane. Together, these results link the morphogenetic effects of HH to the apico-basal distribution of SMO and provide a novel mechanism for the regulation of a GPCR.

A Novel Neuroprotective Role of Phosphatase of Regenerating Liver-1 against CO2 Stimulation in Drosophila

Neuroprotection is essential for the maintenance of normal physiological functions in the nervous system. This is especially true under stress conditions. Here, we demonstrate a novel protective function of PRL-1 against CO₂ stimulation in Drosophila. In the absence of PRL-1, flies exhibit a permanent held-up wing phenotype upon CO₂ exposure. Knockdown of the CO₂ olfactory receptor, Gr21a, suppresses the phenotype. Our genetic data indicate that the wing phenotype is due to a neural dysfunction. PRL-1 physically interacts with Uex and controls Uex expression levels. Knockdown of Uex alone leads to a similar wing held-up phenotype to that of PRL-1 mutants. Uex acts downstream of PRL-1. Elevated Uex levels in PRL-1 mutants prevent the CO₂-induced phenotype. PRL-1 and Uex are required for a wide range of neurons to maintain neuroprotective functions. Expression of human homologs of PRL-1 could rescue the phenotype in Drosophila, suggesting a similar function in humans.

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PRL-1 functions to protect the nervous system against olfactory CO₂ stimulation

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PRL-1 physically interacts with Uex and controls Uex expression levels

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PRLs may retain a similar neuroprotective function in humans

Lhx2, an evolutionarily conserved, multifunctional regulator of forebrain development

A hundred years after Lhx2 ortholog apterous was identified as a critical regulator of wing development in Drosophila, LIM-HD gene family members have proved to be versatile and powerful components of the molecular machinery that executes the blueprint of embryogenesis across vertebrate and invertebrate species. Here, we focus on the spatio-temporally varied functions of LIM-homeodomain transcription factor LHX2 in the developing mouse forebrain. Right from its earliest known role in telencephalic and eye field patterning, to the control of the neuron-glia cell fate switch, and the regulation of axon pathfinding and dendritic arborization in late embryonic stages, LHX2 has been identified as a fundamental, temporally dynamic, always necessary, and often sufficient factor in a range of critical developmental phenomena. While Lhx2 mutant phenotypes have been characterized in detail in multiple brain structures, only recently have we advanced in our understanding of the molecular mechanisms by which this factor acts. Common themes emerge from how this multifunctional molecule controls a range of developmental steps in distinct forebrain structures. Examining these shared features, and noting unique aspects of LHX2 function is likely to inform our understanding of how a single factor can bring about a diversity of effects and play central and critical roles across systems and stages. The parallels in LHX2 and APTEROUS functions, and the protein complexes they participate in, offer insights into evolutionary strategies that conserve tool kits and deploy them to play new, yet familiar roles in species separated by hundreds of millions of years.

A new A-P compartment boundary and organizer in holometabolous insect wings

Decades of research on the highly modified wings of Drosophila melanogaster has suggested that insect wings are divided into two Anterior-Posterior (A-P) compartments separated by an axis of symmetry. This axis of symmetry is created by a developmental organizer that establishes symmetrical patterns of gene expression that in turn pattern the A-P axis of the wing. Butterflies possess more typical insect wings and butterfly wing colour patterns provide many landmarks for studies of wing structure and development. Using eyespot colour pattern variation in Vanessa butterflies, here we show an additional A-P axis of symmetry running between wing sectors 3 and 4. Boundaries of Drosophila mitotic clones suggest the existence of a previously undetected Far-Posterior (F-P) compartment boundary that coincides with this additional A-P axis. A similar compartment boundary is evident in butterfly mosaic gynandromorphs. We suggest that this additional compartment boundary and its associated developmental organizer create an axis of wing colour pattern symmetry and a gene expression-based combinatorial code, permitting each insect wing compartment to acquire a unique identity and allowing for the individuation of butterfly eyespots.

The stage-dependent roles of Ldb1 and functional redundancy with Ldb2 in mammalian retinogenesis

The Lim domain-binding proteins are key co-factor proteins that assemble with LIM domains of the LMO/LIM-HD family to form functional complexes that regulate cell proliferation and differentiation. Using conditional mutagenesis and comparative phenotypic analysis, we analyze the function of Ldb1 and Ldb2 in mouse retinal development, and demonstrate overlapping and specific functions of both proteins. Ldb1 interacts with Lhx2 in the embryonic retina and both Ldb1 and Ldb2 play a key role in maintaining the pool of retinal progenitor cells. This is accomplished by controlling the expression of the Vsx2 and Rax, and components of the Notch and Hedgehog signaling pathways. Furthermore, the Ldb1/Ldb2-mediated complex is essential for generation of early-born photoreceptors through the regulation of Rax and Crx. Finally, we demonstrate functional redundancy between Ldb1 and Ldb2. Ldb1 can fully compensate the loss of Ldb2 during all phases of retinal development, whereas Ldb2 alone is sufficient to sustain activity of Lhx2 in both early- and late-stage RPCs and in Müller glia. By contrast, loss of Ldb1 disrupts activity of the LIM domain factors in neuronal precursors. An intricate regulatory network exists that is mediated by Ldb1 and Ldb2, and promotes RPC proliferation and multipotency; it also controls specification of mammalian retina cells.

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.

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.

Seizure evoked regulation of LIM-HD genes and co-factors in the postnatal and adult hippocampus

The LIM-homeodomain (LIM-HD) family of transcription factors is well known for its functions during several developmental processes including cell fate specification, cell migration and axon guidance, and its members play fundamental roles in hippocampal development. The hippocampus is a structure that displays striking activity dependent plasticity.  We examined whether LIM-HD genes and their co-factors are regulated during kainic acid induced seizure in the adult rat hippocampus as well as in early postnatal rats, when the hippocampal circuitry is not fully developed.  We report a distinct and field-specific regulation of LIM-HD genes Lhx1,Lhx2, and Lhx9, LIM-only gene Lmo4, and cofactor Clim1a in the adult hippocampus after seizure induction. In contrast none of these genes displayed altered levels upon induction of seizure in postnatal animals.  Our results provide evidence of temporal and spatial seizure mediated regulation of LIM-HD family members and suggest that LIM-HD gene function may be involved in activity dependent plasticity in the adult hippocampus

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.

The transcriptional co-factor Chip acts with LIM-homeodomain proteins to set the boundary of the eye field in Drosophila

Development involves the establishment of boundaries between fields specified to differentiate into distinct tissues. The Drosophila larval eye-antennal imaginal disc must be subdivided into regions that differentiate into the adult eye, antenna and head cuticle. We have found that the transcriptional co-factor Chip is required for cells at the ventral eye-antennal disc border to take on a head cuticle fate; clones of Chip mutant cells in this region instead form outgrowths that differentiate into ectopic eye tissue. Chip acts independently of the transcription factor Homothorax, which was previously shown to promote head cuticle development in the same region. Chip and its vertebrate CLIM homologues have been shown to form complexes with LIM-homeodomain transcription factors, and the domain of Chip that mediates these interactions is required for its ability to suppress the eye fate. We show that two LIM-homeodomain proteins, Arrowhead and Lim1, are expressed in the region of the eye-antennal disc affected in Chip mutants, and that both require Chip for their ability to suppress photoreceptor differentiation when misexpressed in the eye field. Loss-of-function studies support the model that Arrowhead and Lim1 act redundantly, using Chip as a co-factor, to prevent retinal differentiation in regions of the eye disc destined to become ventral head tissue.

Characterization of the Caenorhabditis elegans Islet LIM-homeodomain ortholog, lim-7

lim-7 is one of seven Caenorhabditis elegans LIM-homeodomain (LIM-HD)-encoding genes and the sole Islet ortholog. LIM-HD transcription factors, including Islets, function in neuronal and non-neuronal development across diverse phyla. Our results show that a lim-7 deletion allele causes early larval lethality with terminal phenotypes including uncoordination, detached pharynx, constipation and morphological defects. A lim-7(+) transgene rescues lethality but not adult sterility. A lim-7(+) reporter in the full genomic context is expressed in all gonadal sheath cells, URA neurons, and additional cells in the pharyngeal region. Finally, we identify a 45-bp regulatory element in the first intron that is necessary and sufficient for lim-7 gonadal sheath expression.

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.

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.

Single-stranded DNA-binding proteins regulate the abundance and function of the LIM-homeodomain transcription factor LHX2 in pituitary cells

A family of single-stranded DNA-binding proteins (or SSBPs) has been shown to augment the function of LIM-homeodomain (LIM-HD) transcription factors in embryogenesis by interaction with LIM domain-binding protein-1 (LDB1). No DNA-binding complex has been described, however, containing a LIM-HD protein, LDB1, and SSBP, and the mechanism by which SSBPs affect LIM-HD function had not been elucidated. Through use of electrophoretic mobility shift, antibody supershift, and ChIP analyses, we show that an Lhx2-Ldb1-Ssbp3 complex binds a specific element in the Lhx2 target gene Cga (encoding the alpha subunit of glycoprotein hormones) in the alphaT3-1 pituitary cell line. Using overexpression and knockdown approaches, we demonstrate that SSBP3 inhibits Lhx2 and Ldb1 turnover, stimulates assembly of this DNA-binding complex, promotes its recruitment to the Cga promoter, and enhances Cga transcription. These studies provide novel insights into the regulation of pituitary gene expression and LIM-HD function more generally.

dAtaxin-2 mediates expanded Ataxin-1-induced neurodegeneration in a Drosophila model of SCA1

Spinocerebellar ataxias (SCAs) are a genetically heterogeneous group of neurodegenerative disorders sharing atrophy of the cerebellum as a common feature. SCA1 and SCA2 are two ataxias caused by expansion of polyglutamine tracts in Ataxin-1 (ATXN1) and Ataxin-2 (ATXN2), respectively, two proteins that are otherwise unrelated. Here, we use a Drosophila model of SCA1 to unveil molecular mechanisms linking Ataxin-1 with Ataxin-2 during SCA1 pathogenesis. We show that wild-type Drosophila Ataxin-2 (dAtx2) is a major genetic modifier of human expanded Ataxin-1 (Ataxin-1[82Q]) toxicity. Increased dAtx2 levels enhance, and more importantly, decreased dAtx2 levels suppress Ataxin-1[82Q]-induced neurodegeneration, thereby ruling out a pathogenic mechanism by depletion of dAtx2. Although Ataxin-2 is normally cytoplasmic and Ataxin-1 nuclear, we show that both dAtx2 and hAtaxin-2 physically interact with Ataxin-1. Furthermore, we show that expanded Ataxin-1 induces intranuclear accumulation of dAtx2/hAtaxin-2 in both Drosophila and SCA1 postmortem neurons. These observations suggest that nuclear accumulation of Ataxin-2 contributes to expanded Ataxin-1-induced toxicity. We tested this hypothesis engineering dAtx2 transgenes with nuclear localization signal (NLS) and nuclear export signal (NES). We find that NLS-dAtx2, but not NES-dAtx2, mimics the neurodegenerative phenotypes caused by Ataxin-1[82Q], including repression of the proneural factor Senseless. Altogether, these findings reveal a previously unknown functional link between neurodegenerative disorders with common clinical features but different etiology.

Protein Stability and Transcription Factor Complex Assembly Determined by the SCL-LMO2 Interaction

Gene expression programs are established by networks of interacting transcription factors. The basic helix-loop-helix factor SCL and the LIM-only protein LMO2 are components of transcription factor complexes that are essential for hematopoiesis. Here we show that LMO2 and SCL are predominant interaction partners in hematopoietic cells and that this interaction occurs through a conserved interface residing in the loop and helix 2 of SCL. This interaction nucleates the assembly of SCL complexes on DNA and is required for target gene induction and for the stimulation of erythroid and megakaryocytic differentiation. We also demonstrate that SCL determines LMO2 protein levels in hematopoietic cells and reveal that interaction with SCL prevents LMO2 degradation by the proteasome. We propose that the SCL-LMO2 interaction couples protein stabilization with higher order protein complex assembly, thus providing a powerful means of modulating the stoichiometry and spatiotemporal activity of SCL complexes. This interaction likely provides a rate-limiting step in the transcriptional control of hematopoiesis and leukemia, and similar mechanisms may operate to control the assembly of diverse protein modules.

Proteasomal selection of multiprotein complexes recruited by LIM homeodomain transcription factors

Complexes composed of multiple proteins regulate most cellular functions. However, our knowledge about the molecular mechanisms governing the assembly and dynamics of these complexes in cells remains limited. The in vivo activity of LIM homeodomain (LIM-HD) proteins, a class of transcription factors that regulates neuronal development, depends on the high-affinity association of their LIM domains with cofactor of LIM homeodomain proteins (LIM-HDs) (CLIM, also known as Ldb or NLI). CLIM cofactors recruit single-stranded DNA-binding protein 1 (SSDP1, also known as SSBP3), and this interaction is important for the activation of the LIM-HD/CLIM protein complex in vivo. Here, we identify a cascade of specific protein interactions that protect LIM-HD multiprotein complexes from proteasomal degradation. In this cascade, CLIM stabilizes LIM-HDs, and SSDP1 stabilizes CLIM. Furthermore, we show that stabilizing cofactors prevent binding of ubiquitin ligases to multiple protein interaction domains in LIM-HD recruited protein complexes. Together, our results indicate a combinatorial code that selects specific multiprotein complexes via proteasomal degradation in cells with broad implications for the assembly and specificity of multiprotein complexes.

Single-stranded DNA-binding proteins regulate the abundance of LIM domain and LIM domain-binding proteins

The LIM domain-binding protein Ldb1 is an essential cofactor of LIM-homeodomain (LIM-HD) and LIM-only (LMO) proteins in development. The stoichiometry of Ldb1, LIM-HD, and LMO proteins is tightly controlled in the cell and is likely a critical determinant of their biological actions. Single-stranded DNA-binding proteins (SSBPs) were recently shown to interact with Ldb1 and are also important in developmental programs. We establish here that two mammalian SSBPs, SSBP2 and SSBP3, contribute to an erythroid DNA-binding complex that contains the transcription factors Tal1 and GATA-1, the LIM domain protein Lmo2, and Ldb1 and binds a bipartite E-box-GATA DNA sequence motif. In addition, SSBP2 was found to augment transcription of the Protein 4.2 (P4.2) gene, a direct target of the E-box-GATA-binding complex, in an Ldb1-dependent manner and to increase endogenous Ldb1 and Lmo2 protein levels, E-box-GATA DNA-binding activity, and P4.2 and beta-globin expression in erythroid progenitors. Finally, SSBP2 was demonstrated to inhibit Ldb1 and Lmo2 interaction with the E3 ubiquitin ligase RLIM, prevent RLIM-mediated Ldb1 ubiquitination, and protect Ldb1 and Lmo2 from proteasomal degradation. These results define a novel biochemical function for SSBPs in regulating the abundance of LIM domain and LIM domain-binding proteins.

Ectopic expression of Tollo/Toll-8 antagonizes Dpp signaling and induces cell sorting in the Drosophila wing

The wing imaginal disc of Drosophila consists of the primordia for the adult wing and the body wall. The zinc-finger transcription factor Teashirt (Tsh) is expressed in the region proximal to the wing primordium and regulates the formation of the wing-body wall boundary. Here, we report that Tollo/Toll-8, a member of Toll family transmembrane proteins, is also expressed proximal to the wing domain. Ectopic expression of Decapentaplegic (Dpp), a morphogen for wing development, represses tollo expression in the proximal domain. Likewise, misexpression of Tollo in the presumptive wing strongly antagonizes the effects of Dpp signaling. The extracellular domain of Tollo containing the Leucine-Rich Repeats (LRR) is required for the inhibition of Dpp signaling in the wing. Furthermore, clones of cells with Tollo overexpression are sorted out from the surrounding wild-type cells, resulting in the formation of epithelial folds around the clone boundaries. Tsh is ectopically induced at the border of Tollo-expressing clones. Despite the strong effects of Tollo overexpression on Dpp signaling and cell sorting, loss-of-function tollo mutants are viable with normal external morphology. Our data suggest that Tollo function might be redundant but is sufficient to antagonize Dpp signaling and induce sorting of Tollo expressing cells from the wing cells to develop proximal cell fate.

Ligand-binding and signaling properties of the Ax[M1] form of Notch

The Abruptex class of Notch alleles has attracted interest because they exhibit some properties that are best explained in terms of increased activity and others that are best explained in terms of reduced activity in vivo. Here, we report a comparison of the properties of Abruptex[M1] and wild-type Notch as ligand binding receptors. Abruptex[M1] showed less activity than wild-type Notch in its ability to bind Delta and Serrate and was expressed at reduced levels on the cell surface. When differences in expression level were taken into account, Abruptex[M1] was comparable to Notch in its sensitivity to ligand-induced activation of reporter gene expression. Abruptex[M1] was also comparable to Notch in its requirement for modification by Fringe and in being sensitive to cis-dowregulation by co-expressed ligands. By the available criteria Abruptex[M1] exhibits less activity than Notch. To explain the ectopic activity of Abruptex[M1] in vivo we suggest that it may be necessary to invoke an altered response to an as yet unidentified ligand or cofactor.

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.

The LIM domain: from the cytoskeleton to the nucleus

First described 15 years ago as a cysteine-rich sequence that was common to a small group of homeodomain transcription factors, the LIM domain is now recognized as a tandem zinc-finger structure that functions as a modular protein-binding interface. LIM domains are present in many proteins that have diverse cellular roles as regulators of gene expression, cytoarchitecture, cell adhesion, cell motility and signal transduction. An emerging theme is that LIM proteins might function as biosensors that mediate communication between the cytosolic and the nuclear compartments.

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.

Lhx9andLhx9α: Differential Biochemical Properties and Effects on Neuronal Differentiation

The Lhx9 LIM-homeodomain transcription factor and its truncated isoform Lhx9alpha are generated by alternative splicing of the Lhx9 gene. Here we investigated the differential functional properties of these two isoforms. Lhx9alpha, which lacks parts of the homeodomain, was unable to bind DNA in EMSA experiments, but was able to associate with CLIM cofactors in GST pull-down assays. In transfection experiments in PC12 cells, Lhx9alpha fusion constructs systematically showed a nuclear localization, as opposed to Lhx9 fusion constructs, which also localized to the cytoplasm. Moreover, Lhx9 increased NGF-induced neuronal differentiation of PC12 cells. Lhx9alpha, on the other hand, did not significantly increase neuronal differentiation but had an effect on the morphology of PC12 cells. Finally, as tested by RT-PCR experiments on transfected PC12 cells, Lhx9 was not able to induce the transcription of Lhx9alpha. Our results show significantly different functional properties for Lhx9 and Lhx9alpha, and suggest that Lhx9alpha can compete away limiting amounts of nuclear CLIM cofactors. Thus, Lhx9 and Lhx9alpha isoforms could be implicated in regulating various aspects of neuronal differentiation.

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.

Identification of a TAL1 target gene reveals a positive role for the LIM domain-binding protein Ldb1 in erythroid gene expression and differentiation

The TAL1 (or SCL) gene, originally identified from its involvement by a recurrent chromosomal translocation, encodes a basic helix-loop-helix transcription factor essential for erythropoiesis. Although presumed to regulate transcription, its target genes are largely unknown. We show here that a nuclear complex containing TAL1, its DNA-binding partner E47, zinc finger transcription factor GATA-1, LIM domain protein LMO2, and LIM domain-binding protein Ldb1 transactivates the protein 4.2 (P4.2) gene through two E box GATA elements in its proximal promoter. Binding of this complex to DNA was dependent on the integrity of both E box and GATA sites and was demonstrated to occur on the P4.2 promoter in cells. Maximal transcription in transiently transfected cells required both E box GATA elements and expression of all five components of the complex. This complex was shown, in addition, to be capable of linking in solution double-stranded oligonucleotides corresponding to the two P4.2 E box GATA elements. This DNA-linking activity required Ldb1 and increased with dimethyl sulfoxide-induced differentiation of murine erythroleukemia (MEL) cells. In contrast, enforced expression in MEL cells of dimerization-defective mutant Ldb1, as well as wild-type Ldb1, significantly decreased E box GATA DNA-binding activities, P4.2 promoter activity, and accumulation of P4.2 and beta-globin mRNAs. These studies define a physiologic target for a TAL1- and GATA-1-containing ternary complex and reveal a positive role for Ldb1 in erythroid gene expression and differentiation.

Multiple functions of LIM domain-binding CLIM/NLI/Ldb cofactors during zebrafish development

The crucial involvement of CLIM/NLI/Ldb cofactors for the exertion of the biological activity of LIM homeodomain transcription factors (LIM-HD) has been demonstrated. In this paper we show that CLIM cofactors are widely expressed during zebrafish development with high protein levels in specific neuronal cell types where LIM-HD proteins of the Isl class are synthesized. The overexpression of a dominant-negative CLIM molecule (DN-CLIM) that contains the LIM interaction domain (LID) during early developmental stages of zebrafish embryos results in an impairment of eye and midbrain-hindbrain boundary (MHB) development and disturbances in the formation of the anterior midline. On a cellular level we show that the outgrowth of peripheral but not central axons from Rohon Beard (RB) and trigeminal sensory neurons is inhibited by DN-CLIM overexpression. We demonstrate a further critical role of CLIM cofactors for axonal outgrowth of motor neurons. Additionally, DN-CLIM overexpression causes an increase of Isl-protein expression levels in specific neuronal cell types, likely due to a protection of the DN-CLIM/LIM-HD complex from proteasomal degradation. Our results demonstrate multiple roles of the CLIM cofactor family for the development of entire organs, axonal outgrowth of specific neurons and protein expression levels.

Short-range cell interactions and cell survival in the Drosophila wing

During development of multicellular organisms, cells are often eliminated by apoptosis if they fail to receive appropriate signals from their surroundings. Here, we report on short-range cell interactions that support cell survival in the Drosophila wing imaginal disc. We present evidence showing that cells incorrectly specified for their position undergo apoptosis because they fail to express specific proteins that are found on surrounding cells, including the LRR transmembrane proteins Capricious and Tartan. Interestingly, only the extracellular domains of Capricious and Tartan are required, suggesting that a bidirectional process of cell communication is involved in triggering apoptosis. We also present evidence showing that activation of the Notch signal transduction pathway is involved in triggering apoptosis of cells misspecified for their dorsal-ventral position.