Defects of the body plan of mutant embryos lacking Lim1, Otx2 or Hnf3beta activity

The orientation of the anterior-posterior (A-P) axis was examined in gastrula-stage Hnf3beta, Otx2 and Lim1 null mutant embryos that display defective axis development. In situ hybridization analysis of the expression pattern of genes associated with the posterior germ layer tissues and the primitive streak (T, Wnt3 and Fgf8) and anterior endoderm (Cer1 and Sox17) revealed that the A-P axis of mutant embryos remains aligned with the proximo-distal plane of the gastrula. Further analysis revealed that cells which express Chrd activity are either absent in Hnf3beta mutant embryos or localised in heterotopic sites in Lim1 and Otx2 null mutants. Lim1-expressing cells are present in the Hnf3beta mutant embryo albeit in heterotopic sites. In all three mutants, Gsc-expressing cells are missing from the anterior mesendoderm. These findings suggest that although some cells with organizer activity may be present in the mutant embryo, they are not properly localised and fail to contribute to the axial mesoderm of the head. By contrast, in T/T mutant embryos that display normal head fold development, the expression domains of organizer, primitive streak and anterior endoderm genes are regionalised correctly in the gastrula.     

Selective degradation of excess Ldb1 by Rnf12/RLIM confers proper Ldb1 expression levels and Xlim-1/Ldb1 stoichiometry in Xenopus organizer functions

The Xenopus LIM homeodomain (LIM-HD) protein, Xlim-1, is expressed in the Spemann organizer and cooperates with its positive regulator, Ldb1, to activate organizer gene expression. While this activation is presumably mediated through Xlim-1/Ldb1 tetramer formation, the mechanisms regulating proper Xlim-1/Ldb1 stoichiometry remains largely unknown. We isolated the Xenopus ortholog (XRnf12) of the RING finger protein Rnf12/RLIM and explored its functional interactions with Xlim-1 and Ldb1. Although XRnf12 functions as a E3 ubiquitin ligase for Ldb1 and causes proteasome-dependent degradation of Ldb1, we found that co-expression of a high level of Xlim-1 suppresses Ldb1 degradation by XRnf12. This suppression requires both the LIM domains of Xlim-1 and the LIM interaction domain of Ldb1, suggesting that Ldb1, when bound to Xlim-1, escapes degradation by XRnf12. We further show that a high level of Ldb1 suppresses the organizer activity of Xlim-1/Ldb1, suggesting that excess Ldb1 molecules disturb Xlim-1/Ldb1 stoichiometry. Consistent with this, Ldb1 overexpression in the dorsal marginal zone suppresses expression of several organizer genes including postulated Xlim-1 targets, and importantly, this suppression is rescued by co-expression of XRnf12. These data suggest that XRnf12 confers proper Ldb1 protein levels and Xlim-1/Ldb1 stoichiometry for their functions in the organizer. Together with the similarity in the expression pattern of Ldb1 and XRnf12 throughout early embryogenesis, we propose Rnf12/RLIM as a specific regulator of Ldb1 to ensure its proper interactions with LIM-HD proteins and possibly other Ldb1-interacting proteins in the organizer as well as in other tissues.     

Lim1 is required in both primitive streak-derived tissues and visceral endoderm for head formation in the mouse.

Lim1 is a homeobox gene expressed in the extraembryonic anterior visceral endoderm and in primitive streak-derived tissues of early mouse embryos. Mice homozygous for a targeted mutation of Lim1 lack head structures anterior to rhombomere 3 in the hindbrain. To determine in which tissues Lim1 is required for head formation and its mode of action, we have generated chimeric mouse embryos and performed tissue layer recombination explant assays. In chimeric embryos in which the visceral endoderm was composed of predominantly wild-type cells, we found that Lim1(-)(/)(-) cells were able to contribute to the anterior mesendoderm of embryonic day 7.5 chimeric embryos but that embryonic day 9.5 chimeric embryos displayed a range of head defects. In addition, early somite stage chimeras generated by injecting Lim1(-)(/)(-) embryonic stem cells into wild-type tetraploid blastocysts lacked forebrain and midbrain neural tissue. Furthermore, in explant recombination assays, anterior mesendoderm from Lim1(-)(/)(-) embryos was unable to maintain the expression of the anterior neural marker gene Otx2 in wild-type ectoderm. In complementary experiments, embryonic day 9.5 chimeric embryos in which the visceral endoderm was composed of predominantly Lim1(-)(/)(-) cells and the embryo proper of largely wild-type cells, also phenocopied the Lim1(-)(/)(-) headless phenotype. These results indicate that Lim1 is required in both primitive streak-derived tissues and visceral endoderm for head formation and that its inactivation in these tissues produces cell non-autonomous defects. We discuss a double assurance model in which Lim1 regulates sequential signaling events required for head formation in the mouse.     

Vertebrate head induction by anterior primitive endoderm

In vertebrates the antero-posterior organization of the embryonic body axis is thought to result from the activity of two separate centers, the head organizer and the trunk organizer, as operationally defined by Spemann in the 1920s. Current molecular studies have supported the existence of a trunk organizer activity while the presence of a distinct head inducing center has remained elusive. Mainly based on analyses of headless mutants in mice, it has been proposed that the anterior axial mesoderm plays a determining role in head induction. Recent gain- and loss-of-function studies in various organisms, however, provide compelling evidence that a largely ignored region, the anterior primitive endoderm, specifies rostral identity. In this review we discuss the emerging concept that the anterior primitive endoderm, rather than the prechordal plate mesoderm, induces head development in the vertebrate embryo.     

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.     

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.     

HNF3beta and Lim1 interact in the visceral endoderm to regulate primitive streak formation and anterior-posterior polarity in the mouse embryo.

Recent embryological and genetic experiments have suggested that the anterior visceral endoderm and the anterior primitive streak of the early mouse gastrula function as head- and trunk-organising centers, respectively. Here, we report that HNF3beta and Lim1 are coexpressed in both organising centers suggesting synergistic roles of these genes in regulating organiser functions and hence axis development in the mouse embryo. To investigate this possibility, we generated compound HNF3beta and Lim1 mutant embryos. An enlarged primitive streak and a lack of axis formation were observed in HNF3beta (-)(/)(-);Lim1(-)(/)(-), but not in single homozygous mutant embryos. Chimera experiments indicate that the primary defect in these double homozygous mutants is due to loss of activity of HNF3beta and Lim1 in the visceral endoderm. Altogether, these data provide evidence that these genes function synergistically to regulate organiser activity of the anterior visceral endoderm. Moreover, HNF3beta (-)(/)(-);Lim1(-)(/)(-) mutant embryos also exhibit defects in mesoderm patterning that are likely due to lack of specification of anterior primitive streak cells.     

oto is a homeotic locus with a role in anteroposterior development that is partially redundant with Lim1.

Genetic control of mammalian head development involves mechanisms that are shared with trunk development as well as mechanisms that are independent. For example, mutations in the nodal gene disrupt axis formation and head development while mutations in the Otx2 or Lim1 genes block head development without disrupting development of the trunk. We show here that the oto mutation on mouse chromosome 1 defines a locus with a critical role in anterior development. The oto mutation disrupts development of the telencephalic and optic vesicles, the pharyngeal endoderm and the first branchial arch. Also, oto embryos have dose-dependent, posterior homeotic transformations throughout the axial skeleton. To further dissect the role of the oto locus in head development, we crossed mice carrying oto and Lim1 mutations. Interactions between the two mutations indicate that the role of oto in the regulation of head development is partially redundant with that of Lim1. The phenotype of oto embryos points to an early and critical role for oto in the development of forebrain subregions. Transformations of the vertebrae in oto embryos reveal a Lim1-independent role in the establishment of positional information in the trunk.     

1H, 15N and 13C assignments of an intramolecular Lmo2-LIM2/Ldb1-LID complex

Lmo2 is a LIM-only protein involved in hematopoiesis and the development of T-cell acute lymphoblastic leukaemia. Here we report backbone and side chain NMR assignments for an engineered intramolecular complex of the C-terminal LIM domain from Lmo2 tethered to the LIM interaction domain (LID) from LIM domain binding protein 1 (Ldb1).