Functional ablation of the mouse Ldb1 gene results in severe patterning defects during gastrulation

Lhx5 promotes forebrain development and activates transcription of secreted Wnt antagonists

In vertebrate embryos, induction and patterning of the forebrain require the local inhibition of caudalizing signals, such as Wnts, emanating from the mesendoderm and caudal brain. Here, we report that Lhx5, expressed in the rostral neuroectoderm, regulates the local inhibition of Wnts. Activation of Lhx5 expands forebrain structures, whereas inhibition of Lhx5 function compromises forebrain development in zebrafish embryos. Lhx5 can rescue forebrain deficiencies caused by excess Wnt activity, and inhibition of Lhx5 function results in ectopic activation of Wnt signaling. Lhx5 regulates the expression of two secreted Frizzled-related Wnt antagonists, Sfrp1a and Sfrp5. These Sfrps can reduce the ectopic activation of Wnt signaling and rescue the forebrain deficiencies caused by inhibition of Lhx5 function. Our results demonstrate that Lhx5 is a required factor that promotes forebrain development and inhibits Wnt signaling by activating the transcription of secreted Wnt antagonists.

Isolation and characterization of hematopoietic transcription factor complexes by in vivo biotinylation tagging and mass spectrometry

We have described the application of a simple biotinylation tagging approach for the direct purification of tagged transcription factor complexes, based on the use of artificial short peptide tags that are specifically and efficiently biotinylated by the bacterial BirA biotin ligase, which is co-expressed in cells with the tagged factor. We used this approach to initially characterize complexes formed by the hematopoietic transcription factor GATA-1 in erythroid cells. GATA-1 is essential for the erythroid differentiation, its functions encompassing upregulation of erythroid genes, repression of alternative transcription programs, and suppression of cell proliferation. However, it was not clear how all of these GATA-1 functions are mediated. Our work describes, for the first time, distinct GATA-1 interactions with the essential hematopoietic factor Gfi-1b, the repressive MeCP1 complex, and the chromatin remodeling ACF/WCRF complex, in addition to the known GATA-1/FOG-1 and GATA-1/TAL-1 complexes. We also provide evidence that distinct GATA-1 complexes are associated with specific GATA-1 functions in erythroid differentiation, for example, GATA-1/Gfi-1b with the suppression of cell proliferation and GATA-1/FOG-1/MeCP1 with the repression of other hematopoietic transcription programs. We next applied the biotinylation tag to Ldb-1, a known partner of GATA-1, and characterized a number of novel interaction partners that are essential in erythroid development, in particular, Eto-2, Lmo4, and CdK9. Last, we are in the process of applying the same technology to characterize the factors that are bound to the suppressed gamma-globin promoter in vivo.

Spliced Isoforms of LIM-Domain-Binding Protein (CLIM/NLI/Ldb) Lacking the LIM-Interaction Domain

LIM-domain-binding proteins (CLIM/NLI/Ldb) are nuclear cofactors for LIM homeodomain transcription factors (LIM-HDs) and LIM-only proteins (LMOs). The LIM-interaction domain (LID) of Ldb is located in the carboxy-terminal region and encoded by the last exon (exon 10) of Ldb genes. It is known that the mammalian CLIM1/Ldb2 gene has a splice isoform, named CLIM1b, lacking the LID. However, little is known about the nature of CLIM1b or the evolutionary conservation of this type of alternative splicing in amphibians and teleost fish. Here, we demonstrate that splice isoforms lacking the LID are also present in the Ldb1 genes of mammals, chick, and Xenopus, as well as in fish paralog Ldb4. All these splicing variations occur in intron 9 and exon 10. We observed that Ldb4b (splice isoform lacking LID) is localized in the nucleus when expressed in mammalian culture cells, and binds to Ldb4a (splice isoform containing LID) but not directly to LIM proteins. However, Ldb4b binds to LMO4 via Ldb4a when coexpressed in culture cells. We also found that mouse Ldb1b lacks the ability to activate protein 4.2 promoter, which is stimulated by LMO2 and Ldb1. These findings suggest that splice isoforms of Ldb lacking LID are potential regulators of Ldb function.

Zic3 is critical for early embryonic patterning during gastrulation

Mutations in the zinc finger transcription factor ZIC3 are associated with human left-right patterning abnormalities (X-linked heterotaxy, HTX1, MIM 306955), and mice null for Zic3 show a similar phenotype. However, the developmental function of Zic3 is largely unknown and its expression in early embryonic development suggests a role prior to organ formation. The current study of Zic3 null mice identifies a novel function for Zic3 in the gastrula-stage embryo. Analysis of Zic3 function at early embryonic stages shows that it ensures the fidelity of embryonic patterning, including patterning of the anterior visceral endoderm, the initiation of gastrulation, and positioning of the primitive streak. At later stages, deficiency of Zic3 results in abnormal mesoderm allocation. These results indicate a requirement for Zic3 during early embryogenesis prior to cardiac and visceral organ patterning.

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.

Cloning and characterization of the 5'-flanking region of the Ehox gene

The paired-like homeobox-containing gene Ehox plays a role in embryonic stem cell differentiation and is highly expressed in the developing placenta and thymus. To understand the mechanisms of regulation of Ehox gene expression, the 5'-flanking region of the Ehox gene was isolated from a mouse BAC library. 5'-RACE analysis revealed a single transcriptional start site 130 nucleotides upstream of the translation initiation codon. Transient transfection with a luciferase reporter gene under the control of serially deleted 5'-flanking sequences revealed that the nt -84 to -68 region contained a positive cis-acting element for efficient expression of the Ehox gene. Mutational analysis of this region and oligonucleotide competition in the electrophoretic mobility shift assay revealed the presence of a CCAAT box, which is a target for transcription nuclear factor Y (NFY). NFY is essential for positive gene regulation. No tissue-specific enhancer was identified in the 1.9-kb 5'-flanking region of the Ehox gene. Ehox is expressed during the early stages of embryo development, specifically in the brain at 9.5 dpc, as well as during the late stages of embryo development. These results suggest that NFY is an essential regulatory factor for Ehox transcriptional activity, which is important for the post-implantation stage of the developing embryo.

Structure and functional characterization of single-strand DNA binding protein SSDP1: Carboxyl-terminal of SSDP1 has transcription activity

LIM-homeodomain transcription factors control a wide range of developmental processes, such as early patterning of the embryonic development, organ formations of brain, limbs, and eyes. Molecular mechanisms of the underlying processes involve complicated multiple protein complexes that direct transcription activation of target genes, protein-protein interactions, and transcriptional regulations. Among those molecules, cofactor Ldb1, interacting with LIM/homeobox family transcription factor, defines a tetrameric protein complex in controlling downstream genes of transcriptional regulation. In addition, SSDP specifically involves this complex supported by showing that both SSDP1 and SSDP2 bind to Ldb1 in vivo. Here it has been found that SSDP1 itself is a transcription factor that has transcription activity independently. Furthermore, C-terminal of SSDP1 possessing an entire transcription activity in vivo, confirmed in both yeast and mammalian cells, has been defined. Interestingly, the transcriptional function of SSDP1 was not required for the interaction with Ldb1. Thus, biochemical data of SSDP1 presented by this study provides biochemical evidence for a better understanding of transcriptional regulation.

Beta-catenin signaling marks the prospective site of primitive streak formation in the mouse embryo

Beta-catenin signaling has been shown to be involved in triggering axis formation in several organisms, including Xenopus and zebrafish. Genetic analysis has demonstrated that the Wnt/beta-catenin signaling pathway is also involved in axis formation in the mouse, since a targeted deletion of beta-catenin results in embryos that have a block in anterior-posterior axis formation, fail to initiate gastrulation, and do not form mesoderm. However, because beta-catenin is ubiquitously expressed, the precise time and cell types in which this signaling pathway is active during early embryonic development remain unknown. Thus, to better understand the role of the Wnt/beta-catenin signaling pathway in axis formation and mesoderm specification, we have examined both the distribution and signaling activity of beta-catenin during early embryonic development in the mouse. We show that the N-terminally nonphosphorylated form of beta-catenin as well as beta-catenin signaling is first detectable in the extraembryonic visceral endoderm in day 5.5 embryos. Before the initiation of gastrulation at day 6.0, beta-catenin signaling is asymmetrically distributed within the epiblast and is localized to a small group of cells adjacent to the embryonic--extraembryonic junction. At day 6.5 and onward, beta-catenin signaling was detected in the primitive streak and mature node. Thus, beta-catenin signaling precedes primitive streak formation and is present in epiblast cells that will go on to form the primitive streak. These results support a critical role for the Wnt/beta-catenin pathway in specifying cells to form the primitive streak and node in the mammalian embryo as well as identify a novel domain of Wnt/beta-catenin signaling activity during early embryogenesis.

Ssdp1 regulates head morphogenesis of mouse embryos by activating the Lim1-Ldb1 complex

The transcriptional activity of LIM-homeodomain (LIM-HD) proteins is regulated by their interactions with various factors that bind to the LIM domain. We show that reduced expression of single-stranded DNA-binding protein 1 (Ssdp1), which encodes a co-factor of LIM domain interacting protein 1 (Ldb1), in the mouse mutant headshrinker (hsk) disrupts anterior head development by partially mimicking Lim1 mutants. Although the anterior visceral endoderm and the anterior definitive endoderm, which together comprise the head organizer, were able to form normally in Ssdp1(hsk/hsk) mutants, development of the prechordal plate was compromised. Head development is partially initiated in Ssdp1(hsk/hsk) mutants, but neuroectoderm tissue anterior to the midbrain-hindbrain boundary is lost, without a concomitant increase in apoptosis. Cell proliferation is globally reduced in Ssdp1(hsk/hsk) mutants, and approximately half also exhibit smaller body size, similar to the phenotype observed in Lim1 and Ldb1 mutants. We also show that Ssdp1 contains an activation domain and is able to enhance transcriptional activation through a Lim1-Ldb1 complex in transfected cells, and that Ssdp1 interacts genetically with Lim1 and Ldb1 in both head development and body growth. These results suggest that Ssdp1 regulates the development of late head organizer tissues and body growth by functioning as an essential activator component of a Lim1 complex through interaction with Ldb1.

Gastrula organiser and embryonic patterning in the mouse

Embryonic patterning of the mouse during gastrulation and early organogenesis engenders the specification of anterior versus posterior structures and body laterality by the interaction of signalling and modulating activities. A group of cells in the mouse gastrula, characterised by the expression of a repertoire of "organiser" genes, acts as a source and the conduit for allocation of the axial mesoderm, floor plate and definitive endoderm. The organiser and its derivatives provide the antagonistic activity that modulates WNT and TGFbeta signalling. Recent findings show that the organiser activity is augmented by morphogenetic activity of the extraembryonic and embryonic endoderm, suggesting embryonic patterning is not solely the function of the organiser.

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.

Emerging Asymmetry and Embryonic Patterning in Early Mouse Development

Recent studies have revealed asymmetries in the mouse zygote and preimplantation embryo, well before the establishment of anterior-posterior polarity after implantation. Whether these asymmetries are causally related to embryonic patterning or are coincidental outcomes of the topology of normal development remains uncertain.

Defective neural tube closure and anteroposterior patterning in mice lacking the LIM protein LMO4 or its interacting partner Deaf-1

LMO4 belongs to a family of transcriptional regulators that comprises two zinc-binding LIM domains. LIM-only (LMO) proteins appear to function as docking sites for other factors, leading to the assembly of multiprotein complexes. The transcription factor Deaf-1/NUDR has been identified as one partner protein of LMO4. We have disrupted the Lmo4 and Deaf-1 genes in mice to define their biological function in vivo. All Lmo4 mutants died shortly after birth and showed defects within the presphenoid bone, with 50% of mice also exhibiting exencephaly. Homeotic transformations were observed in Lmo4-null embryos and newborn mice, but with incomplete penetrance. These included skeletal defects in cervical vertebrae and the rib cage. Furthermore, fusions of cranial nerves IX and X and defects in cranial nerve V were apparent in some Lmo4(-/-) and Lmo4(+/-) mice. Remarkably, Deaf-1 mutants displayed phenotypic abnormalities similar to those observed in Lmo4 mutants. These included exencephaly, transformation of cervical segments, and rib cage abnormalities. In contrast to Lmo4 nullizygous mice, nonexencephalic Deaf-1 mutants remained healthy. No defects in the sphenoid bone or cranial nerves were apparent. Thus, Lmo4 and Deaf-1 mutant mice exhibit overlapping as well as distinct phenotypes. Our data indicate an important role for these two transcriptional regulators in pathways affecting neural tube closure and skeletal patterning, most likely reflecting their presence in a functional complex in vivo.

SCL: from the origin of hematopoiesis to stem cells and leukemia

In the hematopoietic system, lineage commitment and differentiation is controlled by the combinatorial action of transcription factors from diverse families. SCL is a basic helix-loop-helix transcription factor that is an essential regulator at several levels in the hematopoietic hierarchy and whose inappropriate regulation frequently contributes to the development of pediatric T-cell acute lymphoblastic leukemia. This review discusses advances that have shed important light on the functions played by SCL during normal hematopoiesis and leukemogenesis and have revealed an unexpected robustness of hematopoietic stem cell function. Molecular studies have unraveled a mechanism through which gene expression is tightly controlled, as SCL functions within multifactorial complexes that exhibit an all-or-none switch-like behavior in transcription activation, arguing for a quantal process that depends on the concurrent occupation of target loci by all members of the complex. Finally, variations in composition of SCL-containing complexes may ensure flexibility and specificity in the regulation of lineage-specific programs of gene expression, thus providing the molecular basis through which SCL exerts its essential functions at several branch points of the hematopoietic hierarchy.

SCL assembles a multifactorial complex that determines glycophorin A expression

SCL/TAL1 is a hematopoietic-specific transcription factor of the basic helix-loop-helix (bHLH) family that is essential for erythropoiesis. Here we identify the erythroid cell-specific glycophorin A gene (GPA) as a target of SCL in primary hematopoietic cells and show that SCL occupies the GPA locus in vivo. GPA promoter activation is dependent on the assembly of a multifactorial complex containing SCL as well as ubiquitous (E47, Sp1, and Ldb1) and tissue-specific (LMO2 and GATA-1) transcription factors. In addition, our observations suggest functional specialization within this complex, as SCL provides its HLH protein interaction motif, GATA-1 exerts a DNA-tethering function through its binding to a critical GATA element in the GPA promoter, and E47 requires its N-terminal moiety (most likely entailing a transactivation function). Finally, endogenous GPA expression is disrupted in hematopoietic cells through the dominant-inhibitory effect of a truncated form of E47 (E47-bHLH) on E-protein activity or of FOG (Friend of GATA) on GATA activity or when LMO2 or Ldb-1 protein levels are decreased. Together, these observations reveal the functional complementarities of transcription factors within the SCL complex and the essential role of SCL as a nucleation factor within a higher-order complex required to activate gene GPA expression.

Regulation of expression of mouse interferon-induced transmembrane protein like gene-3,Ifitm3 (mil-1, fragilis), in germ cells

Mouse interferon-induced transmembrane protein (IFITM) gene, Ifitm3 (previously known as mil-1 and fragilis), is expressed in primordial germ cells (PGCs), in their precursors, and in germ cells of the fetal gonads (Saitou et al. [2002] Nature 418:293-300; Tanaka and Matsui [2002] Mech Dev 119S:S261-S267). By examining the expression of green fluorescent protein transgene under the control of DNA sequences flanking exon 1, we have identified domains that direct Ifitm3 transcription in PGCs and their precursors in gastrula stage and 13.5 days post coitum embryos. Germ cell-specific expression is achieved by the activity of a consensus element unique to the Ifitm genes, which may act to suppress Ifitm3 expression in somatic tissues. The lack of any influence of the interferon-stimulable response elements on transgene expression in the germ-line suggests that interferon-mediated response is not critical for activating Ifitm3.

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.

Lineage choice and differentiation in mouse embryos and embryonic stem cells

The use of embryonic stem (ES) cells for generating healthy tissues has the potential to revolutionize therapies for human disease or injury, for which there are currently no effective treatments. Strategies for manipulating stem cell differentiation should be based on knowledge of the mechanisms by which lineage decisions are made during early embryogenesis. Here, we review current research into the factors influencing lineage differentiation in the mouse embryo and the application of this knowledge to in vitro differentiation of ES cells. In the mouse embryo, specification of tissue lineages requires cell-cell interactions that are influenced by coordinated cell migration and cellular neighborhood mediated by the key WNT, FGF, and TGFbeta signaling pathways. Mimicking the cellular interactions of the embryo by providing appropriate signaling molecules in culture has enabled the differentiation of ES cells to be directed predominately toward particular lineages. Multistep strategies incorporating the provision of soluble factors known to influence lineage choices in the embryo, coculture with other cells or tissues, genetic modification, and selection for desirable cell types have allowed the production of ES cell derivatives that produce beneficial effects in animal models. Increasing the efficiency of this process can only result from a better understanding of the molecular control of cell lineage determination in the embryo.

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.

Synchronization of neurogenesis and motor neuron specification by direct coupling of bHLH and homeodomain transcription factors

Inductive signaling leads to the coactivation of regulatory pathways for specifying general neuronal traits in parallel with instructions for neuronal subtype specification. Nevertheless, the mechanisms that ensure that these pathways are synchronized have not been defined. To address this, we examined how bHLH proteins Ngn2 and NeuroM controlling neurogenesis functionally converge with LIM-homeodomain (LIM-HD) factors Isl1 and Lhx3 involved in motor neuron subtype specification. We found that Ngn2 and NeuroM transcriptionally synergize with Isl1 and Lhx3 to specify motor neurons in the embryonic spinal cord and in P19 stem cells. The mechanism underlying this cooperativity is based on interactions that directly couple the activity of the bHLH and LIM-HD proteins, mediated by the adaptor protein NLI. This functional link acts to synchronize neuronal subtype specification with neurogenesis.

Failure to produce mitochondrial DNA results in embryonic lethality in Rnaseh1 null mice

Although ribonucleases H (RNases H) have long been implicated in DNA metabolism, they are not required for viability in prokaryotes or unicellular eukaryotes. We generated Rnaseh1(-/-) mice to investigate the role of RNase H1 in mammals and observed developmental arrest at E8.5 in null embryos. A fraction of the mainly nuclear RNase H1 was targeted to mitochondria, and its absence in embryos resulted in a significant decrease in mitochondrial DNA content, leading to apoptotic cell death. This report links RNase H1 to generation of mitochondrial DNA, providing direct support for the strand-coupled mechanism of mitochondrial DNA replication. These findings also have important implications for therapy of mitochondrial dysfunctions and drug development for the structurally related RNase H of HIV.