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

Planarian LDB and SSDP proteins scaffold transcriptional complexes for regeneration and patterning

Sequence-specific transcription factors often function as components of large regulatory complexes. LIM-domain binding protein (LDB) and single-stranded DNA-binding protein (SSDP) function as core scaffolds of transcriptional complexes in animals and plants. Little is known about potential partners and functions for LDB/SSDP complexes in the context of tissue regeneration. In this work, we find that planarian LDB1 and SSDP2 promote tissue regeneration, with a particular function in anterior regeneration and mediolateral polarity reestablishment. We find that LDB1 and SSDP2 interact with one another and with characterized planarian LIM-HD proteins Arrowhead, Islet1, and Lhx1/5-1. We also show that SSDP2 and LDB1 function with islet1 in polarity reestablishment and with lhx1/5-1 in serotonergic neuron maturation. Finally, we find new roles for LDB1 and SSDP2 in regulating gene expression in the planarian intestine and parenchyma; these functions are likely LIM-HD-independent. Together, our work provides insight into LDB/SSDP complexes in a highly regenerative organism. Further, our work provides a strong starting point for identifying and characterizing potential binding partners of LDB1 and SSDP2 and for exploring roles for these proteins in diverse aspects of planarian physiology.

Temporally resolved single cell transcriptomics in a human model of amniogenesis

Amniogenesis is triggered in a collection of pluripotent epiblast cells as the human embryo implants. To gain insights into the critical but poorly understood transcriptional machinery governing amnion fate determination, we examined the evolving transcriptome of a developing human pluripotent stem cell-derived amnion model at the single cell level. This analysis revealed several continuous amniotic fate progressing states with state-specific markers, which include a previously unrecognized CLDN10+ amnion progenitor state. Strikingly, we found that expression of CLDN10 is restricted to the amnion-epiblast boundary region in the human post-implantation amniotic sac model as well as in a peri-gastrula cynomolgus macaque embryo, bolstering the growing notion that, at this stage, the amnion-epiblast boundary is a site of active amniogenesis. Bioinformatic analysis of published primate peri-gastrula single cell sequencing data further confirmed that CLDN10 is expressed in cells progressing to amnion. Additionally, our loss of function analysis shows that CLDN10 promotes amniotic but suppresses primordial germ cell-like fate. Overall, this study presents a comprehensive amniogenic single cell transcriptomic resource and identifies a previously unrecognized CLDN10+ amnion progenitor population at the amnion-epiblast boundary of the primate peri-gastrula.

Gastrulation: Its Principles and Variations

As epiblast cells initiate development into various somatic cells, they undergo a large-scale reorganization, called gastrulation. The gastrulation of the epiblast cells produces three groups of cells: the endoderm layer, the collection of miscellaneous mesodermal tissues, and the ectodermal layer, which includes the neural, epidermal, and associated tissues. Most studies of gastrulation have focused on the formation of the tissues that provide the primary route for cell reorganization, that is, the primitive streak, in the chicken and mouse. In contrast, how gastrulation alters epiblast-derived cells has remained underinvestigated. This chapter highlights the regulation of cell and tissue fate via the gastrulation process. The roles and regulatory functions of neuromesodermal progenitors (NMPs) in the gastrulation process, elucidated in the last decade, are discussed in depth to resolve points of confusion. Chicken and mouse embryos, which form a primitive streak as the site of mesoderm precursor ingression, have been investigated extensively. However, primitive streak formation is an exception, even among amniotes. The roles of gastrulation processes in generating various somatic tissues will be discussed broadly.

Xenopus Ssbp2 is required for embryonic pronephros morphogenesis and terminal differentiation

The nephron, functional unit of the vertebrate kidney, is specialized in metabolic wastes excretion and body fluids osmoregulation. Given the high evolutionary conservation of gene expression and segmentation patterning between mammalian and amphibian nephrons, the Xenopus laevis pronephric kidney offers a simplified model for studying nephrogenesis. The Lhx1 transcription factor plays several roles during embryogenesis, regulating target genes expression by forming multiprotein complexes with LIM binding protein 1 (Ldb1). However, few Lhx1-Ldb1 cofactors have been identified for kidney organogenesis. By tandem- affinity purification from kidney-induced Xenopus animal caps, we identified single-stranded DNA binding protein 2 (Ssbp2) interacts with the Ldb1-Lhx1 complex. Ssbp2 is expressed in the Xenopus pronephros, and knockdown prevents normal morphogenesis and differentiation of the glomus and the convoluted renal tubules. We demonstrate a role for a member of the Ssbp family in kidney organogenesis and provide evidence of a fundamental function for the Ldb1-Lhx1-Ssbp transcriptional complexes in embryonic development.

The SSBP3 co-regulator is required for glucose homeostasis, pancreatic islet architecture, and beta-cell identity

OBJECTIVE

Transcriptional complex activity drives the development and function of pancreatic islet cells to allow for proper glucose regulation. Prior studies from our lab and others highlighted that the LIM-homeodomain transcription factor (TF), Islet-1 (Isl1), and its interacting co-regulator, Ldb1, are vital effectors of developing and adult β-cells. We further found that a member of the Single Stranded DNA-Binding Protein (SSBP) co-regulator family, SSBP3, interacts with Isl1 and Ldb1 in β-cells and primary islets (mouse and human) to impact β-cell target genes MafA and Glp1R in vitro. Members of the SSBP family stabilize TF complexes by binding directly to Ldb1 and protecting the complex from ubiquitin-mediated turnover. In this study, we hypothesized that SSBP3 has critical roles in pancreatic islet cell function in vivo, similar to the Isl1::Ldb1 complex.

METHODS

We first developed a novel SSBP3 LoxP allele mouse line, where Cre-mediated recombination imparts a predicted early protein termination. We bred this mouse with constitutive Cre lines (Pdx1- and Pax6-driven) to recombine SSBP3 in the developing pancreas and islet (SSBP3ΔPanc and SSBP3ΔIslet), respectively. We assessed glucose tolerance and used immunofluorescence to detect changes in islet cell abundance and markers of β-cell identity and function. Using an inducible Cre system, we also deleted SSBP3 in the adult β-cell, a model termed SSBP3Δβ-cell. We measured glucose tolerance as well as glucose-stimulated insulin secretion (GSIS), both in vivo and in isolated islets in vitro. Using islets from control and SSBP3Δβ-cell we conducted RNA-Seq and compared our results to published datasets for similar β-cell specific Ldb1 and Isl1 knockouts to identify commonly regulated target genes.

RESULTS

SSBP3ΔPanc and SSBP3ΔIslet neonates present with hyperglycemia. SSBP3ΔIslet mice are glucose intolerant by P21 and exhibit a reduction of β-cell maturity markers MafA, Pdx1, and UCN3. We observe disruptions in islet cell architecture with an increase in glucagon+ α-cells and ghrelin+ ε-cells at P10. Inducible loss of β-cell SSBP3 in SSBP3Δβ-cell causes hyperglycemia, glucose intolerance, and reduced GSIS. Transcriptomic analysis of 14-week-old SSBP3Δβ-cell islets revealed a decrease in β-cell function gene expression (Ins, MafA, Ucn3), increased stress and dedifferentiation markers (Neurogenin-3, Aldh1a3, Gastrin), and shared differentially expressed genes between SSBP3, Ldb1, and Isl1 in adult β-cells.

CONCLUSIONS

SSBP3 drives proper islet identity and function, where its loss causes altered islet-cell abundance and glucose homeostasis. β-Cell SSBP3 is required for GSIS and glucose homeostasis, at least partially through shared regulation of Ldb1 and Isl1 target genes.

Xenopus Ssbp2 is required for embryonic pronephros morphogenesis and terminal differentiation

The nephron, functional unit of the vertebrate kidney, is specialized in metabolic wastes excretion and body fluids osmoregulation. Given the high evolutionary conservation of gene expression and segmentation patterning between mammalian and amphibian nephrons, the Xenopus laevis pronephric kidney offers a simplified model for studying nephrogenesis. The Lhx1 transcription factor plays several roles during embryogenesis, regulating target genes expression by forming multiprotein complexes with LIM binding protein 1 (Ldb1). However, few Lhx1-Ldb1 cofactors have been identified for kidney organogenesis. By tandem-affinity purification from kidney-induced Xenopus animal caps, we identified s ingle- s tranded DNA b inding p rotein 2 (Ssbp2) interacts with the Ldb1-Lhx1 complex. Ssbp2 is expressed in the Xenopus pronephros, and knockdown prevents normal morphogenesis and differentiation of the glomus and the convoluted renal tubules. We demonstrate a role for a member of the Ssbp family in kidney organogenesis and provide evidence of a fundamental function for the Ldb1-Lhx1-Ssbp transcriptional complexes in embryonic development.

Planarian LDB and SSDP proteins scaffold transcriptional complexes for regeneration and patterning

Sequence-specific transcription factors often function as components of large regulatory complexes. LIM-domain binding protein (LDB) and single-stranded DNA-binding protein (SSDP) function as core scaffolds of transcriptional complexes in animals and plants. Little is known about potential partners and functions for LDB/SSDP complexes in the context of tissue regeneration. In this work, we find that planarian LDB1 and SSDP2 promote tissue regeneration, with a particular function in mediolateral polarity reestablishment. We find that LDB1 and SSDP2 interact with one another and with characterized planarian LIM-HD proteins Arrowhead, Islet1, and Lhx1/5-1. SSDP2 and LDB1 also function with islet1 in polarity reestablishment and with lhx1/5-1 in serotonergic neuron maturation. Finally, we show new roles for LDB1 and SSDP2 in regulating gene expression in the planarian intestine and parenchyma; these functions may be LIM-HD-independent. Together, our work provides insight into LDB/SSDP complexes in a highly regenerative organism. Further, our work provides a strong starting point for identifying and characterizing potential binding partners of LDB1 and SSDP2 and for exploring roles for these proteins in diverse aspects of planarian physiology.

Loss of Single-Stranded DNA Binding Protein 2 Expression Is Associated with Aggressiveness and Poor Overall Survival in Patients with Invasive Breast Carcinoma

Background: Single-stranded DNA binding protein 2 (SSBP2) is involved in the DNA damage response and the maintenance of genome stability. Previous studies have suggested that SSBP2 has a tumor suppressor function or oncogenic function. Loss of SSBP2 expression has been reported in various tumors. However, the role of SSBP2 expression in invasive breast carcinoma has not been reported. Methods: Immunohistochemical staining for SSBP2 was performed on tissue microarrays consisting of 491 invasive breast carcinoma cases. The result of nuclear SSBP2 staining was stratified as either negative or positive. Then, we investigated the correlations between SSBP2 expression and various clinicopathological parameters and patient outcomes. Results: Loss of nuclear SSBP2 expression was observed in 61 cases (12.4%) of 491 invasive breast carcinomas. Loss of nuclear SSBP2 expression was significantly correlated with larger tumor size (p < 0.001, chi-squared test), higher histological grade (p = 0.016, Cochran–Armitage trend test), higher pathological T stage (p < 0.001, Cochran–Armitage trend test), estrogen receptor status (p < 0.001, chi-squared test), and molecular subtype (p < 0.001, chi-squared test). Kaplan–Meier survival analysis revealed that patients with loss of nuclear SSBP2 expression had worse overall survival (p = 0.013, log-rank test). However, loss of nuclear SSBP2 expression was not correlated with recurrence-free survival (p = 0.175, log-rank test). Conclusions: Loss of nuclear SSBP2 expression was associated with adverse clinicopathological characteristics and poor patient outcomes. SSBP2 acts as a tumor suppressor in invasive breast carcinoma and may be used as a prognostic biomarker.

LIM homeodomain proteins and associated partners: Then and now

The field of molecular embryology started around 1990 by identifying new genes and analyzing their functions in early vertebrate embryogenesis. Those genes encode transcription factors, signaling molecules, their regulators, etc. Most of those genes are relatively highly expressed in specific regions or exhibit dramatic phenotypes when ectopically expressed or mutated. This review focuses on one of those genes, Lim1/Lhx1, which encodes a transcription factor. Lim1/Lhx1 is a member of the LIM homeodomain (LIM-HD) protein family, and its intimate partner, Ldb1/NLI, binds to two tandem LIM domains of LIM-HDs. The most ancient LIM-HD protein and its partnership with Ldb1 were innovated in the metazoan ancestor by gene fusion combining LIM domains and a homeodomain and by creating the LIM domain-interacting domain (LID) in ancestral Ldb, respectively. The LIM domain has multiple interacting interphases, and Ldb1 has a dimerization domain (DD), the LID, and other interacting domains that bind to Ssbp2/3/4 and the boundary factor, CTCF. By means of these domains, LIM-HD-Ldb1 functions as a hub protein complex, enabling more intricate and elaborate gene regulation. The common, ancestral role of LIM-HD proteins is neuron cell-type specification. Additionally, Lim1/Lhx1 serves crucial roles in the gastrula organizer and in kidney development. Recent studies using Xenopus embryos have revealed Lim1/Lhx1 functions and regulatory mechanisms during development and regeneration, providing insight into evolutionary developmental biology, functional genomics, gene regulatory networks, and regenerative medicine. In this review, we also discuss recent progress at unraveling participation of Ldb1, Ssbp, and CTCF in enhanceosomes, long-distance enhancer-promoter interactions, and trans-interactions between chromosomes.

Low Expression of Single-stranded DNA Binding Protein 2 (SSBP2) Predicts Unfavourable Postoperative Outcomes in Patients With Clear Cell Renal Cell Carcinoma

BACKGROUND

Single-stranded DNA binding protein 2 (SSBP2) is a subunit of a single-stranded DNA binding complex, which is involved in the maintenance of hematopoietic stem cells and stress responses. Numerous studies have suggested that SSBP2 functions as a tumor suppressor and is silenced through a pathway mediated by promoter hypermethylation. However, the role of SSBP2 in human renal cell carcinoma has not been reported, to date. Herein, we investigated the clinicopathological significance of SSBP2 expression in clear cell renal cell carcinoma (ccRCC).

MATERIALS AND METHODS

We constructed tissue micro arrays consisting of 173 ccRCC tissues, and SSBP2 expression was evaluated semi-quantitatively based on the staining intensity and the proportion of stained cells. Regarding statistical analysis, the tissues were divided into two groups according to SSBP2 expression, and correlation of SSBP2 expression with various clinicopathological characteristics and patient outcomes was evaluated.

RESULTS

Low SSBP2 expression was observed in 114 of 175 (65.9%) of ccRCC cases, and low SSBP2 expression was significantly correlated with larger tumor size (p=0.005, Chi-square test), higher WHO/ISUP histological grade (p<0.001, Chi-square test), tumor necrosis (p=0.008, Chi-square test), sarcomatoid change (p=0.021, Chi-square test), and higher pT AJCC stage (p=0.002, Chi-square test). Kaplan-Meier survival curves revealed that patients with low SSBP2 expression had worse recurrence-free survival (p=0.041, log-rank test).

CONCLUSION

ccRCC with low SSBP2 expression was associated with adverse clinicopathological characteristics and poor patient outcomes.

Crystal structure of human LDB1 in complex with SSBP2

The Lim domain binding proteins (LDB1 and LDB2 in human and Chip in Drosophila) play critical roles in cell fate decisions through partnership with multiple Lim-homeobox and Lim-only proteins in diverse developmental systems including cardiogenesis, neurogenesis, and hematopoiesis. In mammalian erythroid cells, LDB1 dimerization supports long-range connections between enhancers and genes involved in erythropoiesis, including the β-globin genes. Single-stranded DNA binding proteins (SSBPs) interact specifically with the LDB/Chip conserved domain (LCCD) of LDB proteins and stabilize LDBs by preventing their proteasomal degradation, thus promoting their functions in gene regulation. The structural basis for LDB1 self-interaction and interface with SSBPs is unclear. Here we report a crystal structure of the human LDB1/SSBP2 complex at 2.8-Å resolution. The LDB1 dimerization domain (DD) contains an N-terminal nuclear transport factor 2 (NTF2)-like subdomain and a small helix 4-helix 5 subdomain, which together form the LDB1 dimerization interface. The 2 LCCDs in the symmetric LDB1 dimer flank the core DDs, with each LCCD forming extensive interactions with an SSBP2 dimer. The conserved linker between LDB1 DD and LCCD covers a potential ligand-binding pocket of the LDB1 NTF2-like subdomain and may serve as a regulatory site for LDB1 structure and function. Our structural and biochemical data provide a much-anticipated structural basis for understanding how LDB1 and the LDB1/SSBP interactions form the structural core of diverse complexes mediating cell choice decisions and long-range enhancer-promoter interactions.

Rotational symmetry of the structured Chip/LDB-SSDP core module of the Wnt enhanceosome

The Chip/LIM-domain binding protein (LDB)-single-stranded DNA-binding protein (SSDP) (ChiLS) complex controls numerous cell-fate decisions in animal cells, by mediating transcription of developmental control genes via remote enhancers. ChiLS is recruited to these enhancers by lineage-specific LIM-domain proteins that bind to its Chip/LDB subunit. ChiLS recently emerged as the core module of the Wnt enhanceosome, a multiprotein complex that primes developmental control genes for timely Wnt responses. ChiLS binds to NPFxD motifs within Pygopus (Pygo) and the Osa/ARID1A subunit of the BAF chromatin remodeling complex, which could synergize with LIM proteins in tethering ChiLS to enhancers. Chip/LDB and SSDP both contain N-terminal dimerization domains that constitute the bulk of their structured cores. Here, we report the crystal structures of these dimerization domains, in part aided by DARPin chaperones. We conducted systematic surface scanning by structure-designed mutations, followed by in vitro and in vivo binding assays, to determine conserved surface residues required for binding between Chip/LDB, SSDP, and Pygo-NPFxD. Based on this, and on the 4:2 (SSDP-Chip/LDB) stoichiometry of ChiLS, we derive a highly constrained structural model for this complex, which adopts a rotationally symmetrical SSDP2-LDB2-SSDP2 architecture. Integrity of ChiLS is essential for Pygo binding, and our mutational analysis places the NPFxD pockets on either side of the Chip/LDB dimer, each flanked by an SSDP dimer. The symmetry and multivalency of ChiLS underpin its function as an enhancer module integrating Wnt signals with lineage-specific factors to operate context-dependent transcriptional switches that are pivotal for normal development and cancer.

Mechanistic insights from the LHX1-driven molecular network in building the embryonic head

Development of an embryo is driven by a series of molecular instructions that control the differentiation of tissue precursor cells and shape the tissues into major body parts. LIM homeobox 1 (LHX1) is a transcription factor that plays a major role in the development of the embryonic head of the mouse. Loss of LHX1 function disrupts the morphogenetic movement of head tissue precursors and impacts on the function of molecular factors in modulating the activity of the WNT signaling pathway. LHX1 acts with a transcription factor complex to regulate the transcription of target genes in multiple phases of development and in a range of embryonic tissues of the mouse and Xenopus. Determining the interacting factors and transcriptional targets of LHX1 will be key to unraveling the ensemble of factors involved in head development and building a head gene regulatory network.

Possible Role of Single Stranded DNA Binding Protein 3 on Skin Hydration by Regulating Epidermal Differentiation

Background

Skin hydration is a common problem both in elderly and young people as dry skin may cause irritation, dermatological disorders, and wrinkles. While both genetic and environmental factors seem to influence skin hydration, thorough genetic studies on skin hydration have not yet been conducted.

Objective

We used a genome-wide association study (GWAS) to explore the genetic elements underlying skin hydration by regulating epidermal differentiation and skin barrier function.

Methods

A GWAS was conducted to investigate the genetic factors influencing skin hydration in 100 Korean females along with molecular studies of genes in human epidermal keratinocytes for functional study in vitro.

Results

Among several single nucleotide polymorphisms identified in GWAS, we focused on Single Stranded DNA Binding Protein 3 (SSBP3) which is associated with DNA replication and DNA damage repair. To better understand the role of SSBP3 in skin cells, we introduced a calcium-induced differentiation keratinocyte culture system model and found that SSBP3 was upregulated in keratinocytes in a differentiation dependent manner. When SSBP3 was overexpressed using a recombinant adenovirus, the expression of differentiation-related genes such as loricrin and involucrin was markedly increased.

Conclusion

Taken together, our results suggest that genetic variants in the intronic region of SSBP3 could be determinants in skin hydration of Korean females. SSBP3 represents a new candidate gene to evaluate the molecular basis of the hydration ability in individuals.

PtaB, a lim-domain binding protein in Aspergillus fumigatus regulates biofilm formation and conidiation through distinct pathways

The exopolysaccharide galactosaminogalactan (GAG) plays an important role in mediating adhesion, biofilm formation, and virulence in the pathogenic fungus Aspergillus fumigatus. The developmental modifiers MedA, StuA, and SomA regulate GAG biosynthesis, but the mechanisms underlying this regulation are poorly understood. PtaB is a lim-domain binding protein that interacts with the transcription factor SomA and is required for normal conidiation and biofilm formation. Disruption of ptaB resulted in impaired GAG production and conidiation in association with a markedly reduced expression of GAG biosynthetic genes (uge3 and agd3), developmental regulators (medA and stuA), and genes involved in the core conidiation pathway. Overexpression of medA and dual overexpression of uge3 and agd3 in the ΔptaB mutant increased biofilm formation but not conidiation, whereas overexpression of core conidiation genes rescued conidiation but not biofilm formation. Overexpression of stuA modestly increased both conidiation and biofilm formation. Analysis of ptaB truncation mutants revealed that overexpression of the lim-domain binding region restored conidiation but not biofilm formation, suggesting that ptaB may govern these processes by interacting with different partners. These studies establish that PtaB governs GAG biosynthesis at the level of substrate availability and polymer deacetylation and that PtaB-mediated biofilm formation and conidiation are largely independent pathways.

Single-stranded DNA binding protein Ssbp3 induces differentiation of mouse embryonic stem cells into trophoblast-like cells

BACKGROUND

Intrinsic factors and extrinsic signals which control unlimited self-renewal and developmental pluripotency in embryonic stem cells (ESCs) have been extensively investigated. However, a much smaller number of factors involved in extra-embryonic trophoblast differentiation from ESCs have been studied. In this study, we investigated the role of the single-stranded DNA binding protein, Ssbp3, for the induction of trophoblast-like differentiation from mouse ESCs.

METHODS

Gain- and loss-of-function experiments were carried out through overexpression or knockdown of Ssbp3 in mouse ESCs under self-renewal culture conditions. Expression levels of pluripotency and lineage markers were detected by real-time quantitative reverse-transcription polymerase chain reaction (qRT-PCR) analyses. The global gene expression profile in Ssbp3-overexpressing cells was determined by affymetrix microarray. Gene ontology and pathway terms were analyzed and further validated by qRT-PCR and Western blotting. The methylation status of the Elf5 promoter in Ssbp3-overexpressing cells was detected by bisulfite sequencing. The trophoblast-like phenotype induced by Ssbp3 was also evaluated by teratoma formation and early embryo injection assays.

RESULTS

Forced expression of Ssbp3 in mouse ESCs upregulated expression levels of lineage-associated genes, with trophoblast cell markers being the highest. In contrast, depletion of Ssbp3 attenuated the expression of trophoblast lineage marker genes induced by downregulation of Oct4 or treatment with BMP4 and bFGF in ESCs. Interestingly, global gene expression profiling analysis indicated that Ssbp3 overexpression did not significantly alter the transcript levels of pluripotency-associated transcription factors. Instead, Ssbp3 promoted the expression of early trophectoderm transcription factors such as Cdx2 and activated MAPK/Erk1/2 and TGF-β pathways. Furthermore, overexpression of Ssbp3 reduced the methylation level of the Elf5 promoter and promoted the generation of teratomas with internal hemorrhage, indicative of the presence of trophoblast cells.

CONCLUSIONS

This study identifies Ssbp3, a single-stranded DNA binding protein, as a regulator for mouse ESCs to differentiate into trophoblast-like cells. This finding is helpful to understand the regulatory networks for ESC differentiation into extra-embryonic lineages.

Single-stranded DNA binding proteins are required for LIM complexes to induce transcriptionally active chromatin and specify spinal neuronal identities

LIM homeodomain factors regulate the development of many cell types. However, transcriptional coactivators that mediate their developmental function remain poorly defined. To address these, we examined how two related NLI-dependent LIM complexes, which govern the development of spinal motor neurons and V2a interneurons, activate the transcription in the embryonic spinal cord. We found that single-stranded DNA-binding proteins are recruited to these LIM complexes via NLI, and enhance their transcriptional activation potential. Ssdp1 and Ssdp2 (Ssdp1/2) are highly expressed in the neural tube and promote motor neuron differentiation in the embryonic spinal cord and P19 stem cells. Inhibition of Ssdp1/2 activity in mouse and chick embryos suppresses the generation of motor neurons and V2a interneurons. Furthermore, Ssdp1/2 recruit histone-modifying enzymes to the motor neuron-specifying LIM complex and trigger acetylation and lysine 4 trimethylation of histone H3, which are well-established chromatin marks for active transcription. Our results suggest that Ssdp1/2 function as crucial transcriptional coactivators for LIM complexes to specify spinal neuronal identities during development.

Lhx1 functions together with Otx2, Foxa2, and Ldb1 to govern anterior mesendoderm, node, and midline development

Costello et al. demonstrate that Smad4/Eomes-dependent Lhx1 expression in the epiblast marks the entire definitive endoderm lineage, the anterior mesendoderm, and midline progenitors. In proteomic experiments, they characterize a complex comprised of Lhx1, Otx2, and Foxa2 as well as the chromatin-looping protein Ldb1.

Gene regulatory networks controlling functional activities of spatially and temporally distinct endodermal cell populations in the early mouse embryo remain ill defined. The T-box transcription factor Eomes, acting downstream from Nodal/Smad signals, directly activates the LIM domain homeobox transcription factor Lhx1 in the visceral endoderm. Here we demonstrate Smad4/Eomes-dependent Lhx1 expression in the epiblast marks the entire definitive endoderm lineage, the anterior mesendoderm, and midline progenitors. Conditional inactivation of Lhx1 disrupts anterior definitive endoderm development and impedes node and midline morphogenesis in part due to severe disturbances in visceral endoderm displacement. Transcriptional profiling and ChIP-seq (chromatin immunoprecipitation [ChIP] followed by high-throughput sequencing) experiments identified Lhx1 target genes, including numerous anterior definitive endoderm markers and components of the Wnt signaling pathway. Interestingly, Lhx1-binding sites were enriched at enhancers, including the Nodal-proximal epiblast enhancer element and enhancer regions controlling Otx2 and Foxa2 expression. Moreover, in proteomic experiments, we characterized a complex comprised of Lhx1, Otx2, and Foxa2 as well as the chromatin-looping protein Ldb1. These partnerships cooperatively regulate development of the anterior mesendoderm, node, and midline cell populations responsible for establishment of the left–right body axis and head formation.

Systems genetic analysis of hippocampal neuroanatomy and spatial learning in mice

Variation in hippocampal neuroanatomy correlates well with spatial learning ability in mice. Here, we have studied both hippocampal neuroanatomy and behavior in 53 isogenic BXD recombinant strains derived from C57BL/6J and DBA/2J parents. A combination of experimental, neuroinformatic and systems genetics methods was used to test the genetic bases of variation and covariation among traits. Data were collected on seven hippocampal subregions in CA3 and CA4 after testing spatial memory in an eight-arm radial maze task. Quantitative trait loci were identified for hippocampal structure, including the areas of the intra- and infrapyramidal mossy fibers (IIPMFs), stratum radiatum and stratum pyramidale, and for a spatial learning parameter, error rate. We identified multiple loci and gene variants linked to either structural differences or behavior. Gpc4 and Tenm2 are strong candidate genes that may modulate IIPMF areas. Analysis of gene expression networks and trait correlations highlight several processes influencing morphometrical variation and spatial learning.

SSBP3 Interacts With Islet-1 and Ldb1 to Impact Pancreatic β-Cell Target Genes

Islet-1 (Isl1) is a Lin11, Isl1, Mec3 (LIM)-homeodomain transcription factor important for pancreatic islet cell development, maturation, and function, which largely requires interaction with the LIM domain-binding protein 1 (Ldb1) coregulator. In other tissues, Ldb1 and Isl1 interact with additional factors to mediate target gene transcription, yet few protein partners are known in β-cells. Therefore, we hypothesize that Ldb1 and Isl1 participate in larger regulatory complexes to impact β-cell gene expression. To test this, we used cross-linked immunoprecipitation and mass spectrometry to identify interacting proteins from mouse β-cells. Proteomic datasets revealed numerous interacting candidates, including a member of the single-stranded DNA-binding protein (SSBP) coregulator family, SSBP3. SSBPs potentiate LIM transcription factor complex activity and stability in other tissues. However, nothing was known of SSBP3 interaction, expression, or activity in β-cells. Our analyses confirmed that SSBP3 interacts with Ldb1 and Isl1 in β-cell lines and in mouse and human islets and demonstrated SSBP3 coexpression with Ldb1 and Isl1 pancreas tissue. Furthermore, β-cell line SSBP3 knockdown imparted mRNA deficiencies similar to those observed upon Ldb1 reduction in vitro or in vivo. This appears to be (at least) due to SSBP3 occupancy of known Ldb1-Isl1 target promoters, including MafA and Glp1r. This study collectively demonstrates that SSBP3 is a critical component of Ldb1-Isl1 regulatory complexes, required for expression of critical β-cell target genes.

Context-specific function of the LIM homeobox 1 transcription factor in head formation of the mouse embryo

Lhx1 encodes a LIM homeobox transcription factor that is expressed in the primitive streak, mesoderm and anterior mesendoderm of the mouse embryo. Using a conditional Lhx1 flox mutation and three different Cre deleters, we demonstrated that LHX1 is required in the anterior mesendoderm, but not in the mesoderm, for formation of the head. LHX1 enables the morphogenetic movement of cells that accompanies the formation of the anterior mesendoderm, in part through regulation of Pcdh7 expression. LHX1 also regulates, in the anterior mesendoderm, the transcription of genes encoding negative regulators of WNT signalling, such as Dkk1, Hesx1, Cer1 and Gsc. Embryos carrying mutations in Pcdh7, generated using CRISPR-Cas9 technology, and embryos without Lhx1 function specifically in the anterior mesendoderm displayed head defects that partially phenocopied the truncation defects of Lhx1-null mutants. Therefore, disruption of Lhx1-dependent movement of the anterior mesendoderm cells and failure to modulate WNT signalling both resulted in the truncation of head structures. Compound mutants of Lhx1, Dkk1 and Ctnnb1 show an enhanced head truncation phenotype, pointing to a functional link between LHX1 transcriptional activity and the regulation of WNT signalling. Collectively, these results provide comprehensive insight into the context-specific function of LHX1 in head formation: LHX1 enables the formation of the anterior mesendoderm that is instrumental for mediating the inductive interaction with the anterior neuroectoderm and LHX1 also regulates the expression of factors in the signalling cascade that modulate the level of WNT activity.

Taspase1-dependent TFIIA cleavage coordinates head morphogenesis by limiting Cdkn2a locus transcription

Head morphogenesis requires complex signal relays to enable precisely coordinated proliferation, migration, and patterning. Here, we demonstrate that, during mouse head formation, taspase1-mediated (TASP1-mediated) cleavage of the general transcription factor TFIIA ensures proper coordination of rapid cell proliferation and morphogenesis by maintaining limited transcription of the negative cell cycle regulators p16Ink4a and p19Arf from the Cdkn2a locus. In mice, loss of TASP1 function led to catastrophic craniofacial malformations that were associated with inadequate cell proliferation. Compound deficiency of Cdkn2a, especially p16Ink4a deficiency, markedly reduced the craniofacial anomalies of TASP1-deficent mice. Furthermore, evaluation of mice expressing noncleavable TASP1 targets revealed that TFIIA is the principal TASP1 substrate that orchestrates craniofacial morphogenesis. ChIP analyses determined that noncleaved TFIIA accumulates at the p16Ink4a and p19Arf promoters to drive transcription of these negative regulators. In summary, our study elucidates a regulatory circuit comprising proteolysis, transcription, and proliferation that is pivotal for construction of the mammalian head.

Requirement for ssbp2 in hematopoietic stem cell maintenance and stress response

Transcriptional mechanisms governing hematopoietic stem cell (HSC) quiescence, self-renewal, and differentiation are not fully understood. Sequence-specific ssDNA-binding protein 2 (SSBP2) is a candidate acute myelogenous leukemia (AML) suppressor gene located at chromosome 5q14. SSBP2 binds the transcriptional adaptor protein Lim domain-binding protein 1 (LDB1) and enhances LDB1 stability to regulate gene expression. Notably, Ldb1 is essential for HSC specification during early development and maintenance in adults. We previously reported shortened lifespan and greater susceptibility to B cell lymphomas and carcinomas in Ssbp2(-/-) mice. However, whether Ssbp2 plays a regulatory role in normal HSC function and leukemogenesis is unknown. In this study, we provide several lines of evidence to demonstrate a requirement for Ssbp2 in the function and transcriptional program of hematopoietic stem and progenitor cells (HSPCs) in vivo. We found that hematopoietic tissues were hypoplastic in Ssbp2(-/-) mice, and the frequency of lymphoid-primed multipotent progenitor cells in bone marrow was reduced. Other significant features of these mice were delayed recovery from 5-fluorouracil treatment and diminished multilineage reconstitution in lethally irradiated bone marrow recipients. Dramatic reduction of Notch1 transcripts and increased expression of transcripts encoding the transcription factor E2a and its downstream target Cdkn1a also distinguished Ssbp2(-/-) HSPCs from wild-type HSPCs. Finally, a tendency toward coordinated expression of SSBP2 and the AML suppressor NOTCH1 in a subset of the Cancer Genome Atlas AML cases suggested a role for SSBP2 in AML pathogenesis. Collectively, our results uncovered a critical regulatory function for SSBP2 in HSPC gene expression and function.

Initiating head development in mouse embryos: integrating signalling and transcriptional activity

The generation of an embryonic body plan is the outcome of inductive interactions between the progenitor tissues that underpin their specification, regionalization and morphogenesis. The intercellular signalling activity driving these processes is deployed in a time- and site-specific manner, and the signal strength must be precisely controlled. Receptor and ligand functions are modulated by secreted antagonists to impose a dynamic pattern of globally controlled and locally graded signals onto the tissues of early post-implantation mouse embryo. In response to the WNT, Nodal and Bone Morphogenetic Protein (BMP) signalling cascades, the embryo acquires its body plan, which manifests as differences in the developmental fate of cells located at different positions in the anterior–posterior body axis. The initial formation of the anterior (head) structures in the mouse embryo is critically dependent on the morphogenetic activity emanating from two signalling centres that are juxtaposed with the progenitor tissues of the head. A common property of these centres is that they are the source of antagonistic factors and the hub of transcriptional activities that negatively modulate the function of WNT, Nodal and BMP signalling cascades. These events generate the scaffold of the embryonic head by the early-somite stage of development. Beyond this, additional tissue interactions continue to support the growth, regionalization, differentiation and morphogenesis required for the elaboration of the structure recognizable as the embryonic head.

Developmental mechanisms directing early anterior forebrain specification in vertebrates

Research from the last 15 years has provided a working model for how the anterior forebrain is induced and specified during the early stages of embryogenesis. This model relies on three basic processes: (1) induction of the neural plate from naive ectoderm requires the inhibition of BMP/TGFβ signaling; (2) induced neural tissue initially acquires an anterior identity (i.e., anterior forebrain); (3) maintenance and expansion of the anterior forebrain depends on the antagonism of posteriorizing signals that would otherwise transform this tissue into posterior neural fates. In this review, we present a historical perspective examining some of the significant experiments that have helped to delineate this molecular model. In addition, we discuss the function of the relevant tissues that act prior to and during gastrulation to ensure proper anterior forebrain formation. Finally, we elaborate data, mainly obtained from the analyses of mouse mutants, supporting a role for transcriptional repressors in the regulation of cell competence within the anterior forebrain. The aim of this review is to provide the reader with a general overview of the signals as well as the signaling centers that control the development of the anterior neural plate.

Uncovering genes required for neuronal morphology by morphology-based gene trap screening with a revertible retrovirus vector

The molecular mechanisms of neuronal morphology and synaptic vesicle transport have been largely elusive, and only a few of the molecules involved in these processes have been identified. Here, we developed a novel morphology-based gene trap method, which is theoretically applicable to all cell lines, to easily and rapidly identify the responsible genes. Using this method, we selected several gene-trapped clones of rat pheochromocytoma PC12 cells, which displayed abnormal morphology and distribution of synaptic vesicle-like microvesicles (SLMVs). We identified several genes responsible for the phenotypes and analyzed three genes in more detail. The first gene was BTB/POZ domain-containing protein 9 (Btbd9), which is associated with restless legs syndrome. The second gene was cytokine receptor-like factor 3 (Crlf3), whose involvement in the nervous system remains unknown. The third gene was single-stranded DNA-binding protein 3 (Ssbp3), a gene known to regulate head morphogenesis. These results suggest that Btbd9, Crlf3, and Ssbp3 regulate neuronal morphology and the biogenesis/transport of synaptic vesicles. Because our novel morphology-based gene trap method is generally applicable, this method is promising for uncovering novel genes involved in the function of interest in any cell lines.—Hashimoto, Y., Muramatsu, K., Kunii, M., Yoshimura, S., Yamada, M., Sato, T., Ishida, Y., Harada, R., Harada, A. Uncovering genes required for neuronal morphology by morphology-based gene trap screening with a revertible retrovirus vector.

Dynamic in vivo binding of transcription factors to cis-regulatory modules of cer and gsc in the stepwise formation of the Spemann-Mangold organizer

How multiple developmental cues are integrated on cis-regulatory modules (CRMs) for cell fate decisions remains uncertain. The Spemann-Mangold organizer in Xenopus embryos expresses the transcription factors Lim1/Lhx1, Otx2, Mix1, Siamois (Sia) and VegT. Reporter analyses using sperm nuclear transplantation and DNA injection showed that cerberus (cer) and goosecoid (gsc) are activated by the aforementioned transcription factors through CRMs conserved between X. laevis and X. tropicalis. ChIP-qPCR analysis for the five transcription factors revealed that cer and gsc CRMs are initially bound by both Sia and VegT at the late blastula stage, and subsequently bound by all five factors at the gastrula stage. At the neurula stage, only binding of Lim1 and Otx2 to the gsc CRM, among others, persists, which corresponds to their co-expression in the prechordal plate. Based on these data, together with detailed expression pattern analysis, we propose a new model of stepwise formation of the organizer, in which (1) maternal VegT and Wnt-induced Sia first bind to CRMs at the blastula stage; then (2) Nodal-inducible Lim1, Otx2, Mix1 and zygotic VegT are bound to CRMs in the dorsal endodermal and mesodermal regions where all these genes are co-expressed; and (3) these two regions are combined at the gastrula stage to form the organizer. Thus, the in vivo dynamics of multiple transcription factors highlight their roles in the initiation and maintenance of gene expression, and also reveal the stepwise integration of maternal, Nodal and Wnt signaling on CRMs of organizer genes to generate the organizer.

Foxa2 mediates critical functions of prechordal plate in patterning and morphogenesis and is cell autonomously required for early ventral endoderm morphogenesis

Axial mesendoderm is comprised of prechordal plate and notochord. Lack of a suitable Cre driver has hampered the ability to genetically dissect the requirement for each of these components, or genes expressed within them, to anterior patterning. Here, we have utilized Isl1-Cre to investigate roles of the winged helix transcription factor Foxa2 specifically in prechordal plate and ventral endoderm. Foxa2loxP/loxP; Isl1-Cre mutants died at 13.5 dpc, exhibiting aberrations in anterior neural tube and forebrain patterning, and in ventral foregut morphogenesis and cardiac fusion. Molecular analysis of Foxa2loxP/loxP; Isl1-Cre mutants indicated that Foxa2 is required in Isl1 lineages for expression of notochord and dorsal foregut endoderm markers, Shh. Brachyury, and Hlxb9. Our results support a requirement for Foxa2 in prechordal plate for notochord morphogenesis, axial patterning, and patterning of dorsal foregut endoderm. Loss of Foxa2 in ventral endoderm resulted in reduced expression of Sox17, Gata4, and ZO proteins, accounting at least in part for observed lack of foregut fusion, cardia bifida, and increased apoptosis of ventral endoderm.

Foxa2 mediates critical functions of prechordal plate in patterning and morphogenesis and is cell autonomously required for early ventral endoderm morphogenesis

Axial mesendoderm is comprised of prechordal plate and notochord. Lack of a suitable Cre driver has hampered the ability to genetically dissect the requirement for each of these components, or genes expressed within them, to anterior patterning. Here, we have utilized Isl1-Cre to investigate roles of the winged helix transcription factor Foxa2 specifically in prechordal plate and ventral endoderm. Foxa2^loxP/loxP; Isl1-Cre mutants died at 13.5 dpc, exhibiting aberrations in anterior neural tube and forebrain patterning, and in ventral foregut morphogenesis and cardiac fusion. Molecular analysis of Foxa2^loxP/loxP; Isl1-Cre mutants indicated that Foxa2 is required in Isl1 lineages for expression of notochord and dorsal foregut endoderm markers, Shh. Brachyury, and Hlxb9. Our results support a requirement for Foxa2 in prechordal plate for notochord morphogenesis, axial patterning, and patterning of dorsal foregut endoderm. Loss of Foxa2 in ventral endoderm resulted in reduced expression of Sox17, Gata4, and ZO proteins, accounting at least in part for observed lack of foregut fusion, cardia bifida, and increased apoptosis of ventral endoderm.

Modularity of CHIP/LDB transcription complexes regulates cell differentiation

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

Loss of Lhx1 activity impacts on the localization of primordial germ cells in the mouse

Mouse embryos lacking Lhx1 (Lim1) activity display defective gastrulation and are deficient of primordial germ cells (PGCs) (Tsang et al. [2001] International Journal of Developmental Biology 45:549-555). To dissect the specific role of Lhx1 in germ cell development, we studied embryos with conditional inactivation of Lhx1 activity in epiblast derivatives, which, in contrast to completely null embryos, develop normally through gastrulation before manifesting a head truncation phenotype. Initially, PGCs are localized properly to the definitive endoderm of the posterior gut in the conditional mutant embryos, but they depart from the embryonic gut prematurely. The early exit of PGCs from the gut is accompanied by the failure to maintain a strong expression of Ifitm1 in the mesoderm enveloping the gut, which may mediate the repulsive activity that facilitates the retention of PGCs in the hindgut during early organogenesis. Lhx1 therefore may influence the localization of PGCs by modulating Ifitm1-mediated repulsive activity.

Loss of procollagen IIA from the anterior mesendoderm disrupts the development of mouse embryonic forebrain

Morphogenesis of the mammalian forebrain is influenced by the patterning activity of signals emanating from the anterior mesendoderm. In this study, we show that procollagen IIA (IIA), an isoform of the cartilage extracellular matrix protein encoded by an alternatively spliced transcript of Col2a1, is expressed in the prechordal plate and the anterior definitive endoderm. In the absence of IIA activity, the null mutants displayed a partially penetrant phenotype of loss of head tissues, holoprosencephaly, and loss of mid-facial structures, which is associated with reduced sonic hedgehog (Shh) expression in the prechordal mesoderm. Genetic interaction studies reveal that IIA function in forebrain and face development does not involve bone morphogenetic protein receptor 1A (BMPR1A)- or NODAL-mediated signaling activity.

Regulation of C. elegans presynaptic differentiation and neurite branching via a novel signaling pathway initiated by SAM-10

Little is known about transcriptional control of neurite branching or presynaptic differentiation, events that occur relatively late in neuronal development. Using the Caenorhabditis elegans mechanosensory circuit as an in vivo model, we show that SAM-10, an ortholog of mammalian single-stranded DNA-binding protein (SSDP), functions cell-autonomously in the nucleus to regulate synaptic differentiation, as well as positioning of, a single neurite branch. PLM mechanosensory neurons in sam-10 mutants exhibit abnormal placement of the neurite branch point, and defective synaptogenesis, characterized by an overextended synaptic varicosity, underdeveloped synaptic morphology and disrupted colocalization of active zone and synaptic vesicles. SAM-10 functions coordinately with Lim domain-binding protein 1 (LDB-1), demonstrated by our observations that: (1) mutations in either gene show similar defects in PLM neurons; and (2) LDB-1 is required for SAM-10 nuclear localization. SAM-10 regulates PLM synaptic differentiation by suppressing transcription of prk-2, which encodes an ortholog of the mammalian Pim kinase family. PRK-2-mediated activities of SAM-10 are specifically involved in PLM synaptic differentiation, but not other sam-10 phenotypes such as neurite branching. Thus, these data reveal a novel transcriptional signaling pathway that regulates neuronal specification of neurite branching and presynaptic differentiation.

SSDP cofactors regulate neural patterning and differentiation of specific axonal projections

The developmental activity of LIM homeodomain transcription factors (LIM-HDs) is critically controlled by LIM domain-interacting cofactors of LIM-HDs (CLIM, also known as NLI or LDB). CLIM cofactors associate with single-stranded DNA binding proteins (SSDPs, also known as SSBPs) thereby recruiting SSDP1 and/or SSDP2 to LIM-HD/CLIM complexes. Although evidence has been presented that SSDPs are important for the activity of specific LIM-HD/CLIM complexes, the developmental roles of SSDPs are unclear. We show that SSDP1a and SSDP1b mRNAs are widely expressed early during zebrafish development with conspicuous expression of SSDP1b in sensory trigeminal and Rohon-Beard neurons. SSDP1 and CLIM immunoreactivity co-localize in these neuronal cell types and in other structures. Over-expression of the N-terminal portion of SSDP1 (N-SSDP1), which contains the CLIM-interaction domain, increases endogenous CLIM protein levels in vivo and impairs the formation of eyes and midbrain-hindbrain boundary. In addition, manipulation of SSDP1 via N-SSDP1 over-expression or SSDP1b knock down impairs trigeminal and Rohon-Beard sensory axon growth. We show that N-SSDP1 is able to partially rescue the inhibition of axon growth induced by a dominant-negative form of CLIM (DN-CLIM). These results reveal specific functions of SSDP in neural patterning and sensory axon growth, in part due to the stabilization of LIM-HD/CLIM complexes.

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 pronotum LIM-HD gene tailup is both a positive and a negative regulator of the proneural genes achaete and scute of Drosophila

Early in the development of the imaginal wing disc of Drosophila, the LIM-HD gene tailup (islet), together with the HD genes of the iroquois complex, specify the notum territory of the disc. Later, tailup has been shown to act as a prepattern gene that antagonizes formation of sensory bristles on the notum of this fly. It has been proposed that Tailup downregulates the expression of the proneural genes achaete and scute by interfering with factors needed to activate these genes in the dorsocentral and scutellar regions of the disc. By means of a clonal analysis performed with tailup null alleles, here we show that, on the one hand, tailup is necessary to prevent formation of extra macrochaetae on most of the 11 sites where these landmark bristles arise on the fly notum. On the other hand, tailup is required to activate achaete and scute at the dorsocentral region, probably by acting as an hexameric complex with the cofactor Chip and the transcriptional activator Sspd on the dorsocentral enhancer of the achaete-scute complex. In contrast, in the scutellar region Tailup acts downstream of achaete-scute, antagonizing the proneural function of these genes probably in cooperation with Chip. We conclude that tailup acts on bristle development by several, even antagonistic, mechanisms.

Development of head organizer of the mouse embryo depends on a high level of mitochondrial metabolism

Mouse genetic studies have defined a set of signaling molecules and transcription factors that are necessary to induce the forebrain. Here we describe an ENU-induced mouse mutation, nearly headless (nehe), that was identified based on the specific absence of most of the forebrain at midgestation. Positional cloning and genetic analysis show that, unlike other mouse mutants that disrupt specification of the forebrain, the nehe mutation disrupts mitochondrial metabolism. nehe is a hypomorphic allele of Lipoic acid Synthetase (Lias), the enzyme that catalyzes the synthesis of lipoic acid, an essential cofactor for several mitochondrial multienzyme complexes required for oxidative metabolism. The defect in forebrain development in nehe mutants is apparent as soon as the forebrain is specified, without a concomitant increase in apoptosis. Two tissues required for forebrain specification, the anterior visceral endoderm and the anterior definitive endoderm, develop normally in nehe mutants. However, a third head organizer tissue, the prechordal plate, fails to express markers of cell type determination and shows abnormal morphology in the mutants. We find that the level of phosphorylated (active) AMPK, a cellular energy sensor that affects cell polarity, is up-regulated in nehe mutants at the time when the prechordal plate is normally specified. The results suggest that the nehe phenotype arises because high levels of energy production are required for the specialized morphogenetic movements that generate the prechordal plate, which is required for normal development of the mammalian forebrain. We suggest that a requirement for high levels of ATP for early forebrain patterning may contribute to certain human microcephaly syndromes.

LIM family transcription factors regulate the subtype-specific morphogenesis of retinal horizontal cells at post-migratory stages

In the nervous system, transcription factor expression in progenitor and/or nascent neurons regulates cell type specification. Although the functions of these transcription factors at early stages are well established, whether or not they are required during late developmental periods remains an open question. To address this issue, we conditionally manipulated gene expression using a recently developed transposon-mediated gene transfer system combined with in ovo electroporation. In chicken retinas, horizontal cells are classified into three subtypes according to their characteristic neuronal morphology. Two LIM family transcription factors, Lim1 and Isl1, begin to be expressed in a distinct subset of nascent retinal neurons, which results in complementary expression of these genes in mature retinas in type I and type II/III horizontal cells, respectively. Overexpression of Isl1 in post-migratory horizontal cells represses endogenous Lim1 expression and increases the number of neurons with a dendritic morphology characteristic of type II horizontal cells, which normally express Isl1. Inhibition of Lim1 function by expression of a dominant negative form Lim1 perturbs axonal morphogenesis of type I horizontal cells. Therefore, we propose that LIM family transcription factors are required for subtype-specific morphogenesis of horizontal cells at later stages of retinal development.

Intracellular localization of porcine single-strand binding protein 2

We have previously cloned cofactor CLIM2 (Ldb1/NL1) as a binding protein for LIM homeodomain transcription factor and now seek a protein interacting with CLIM2. Ultimately, SSBP2 was cloned as CLIM2 binding protein from the adult porcine pituitary cDNA library by the Yeast Two-Hybrid System. The amino acid sequence of porcine SSBP2 shows a high identity (99%) with those of other mammalian species, man, and mouse. Using fluorescence protein-fused SSBP2 and its deletion mutants, we observed that SSBP2 overexpressed in CHO cells predominantly localizes in mitochondria. Expression of mutant SSBP2s demonstrated that the first 241 amino acid residues are responsive for the mitochondrial localization. When CLIM2 vector was co-transfected, SSBP2 changed its location to nuclei. The similar translocation was also observed when CLIM2 vector was transfected 17 h after the transfection of SSBP2 vector. The first 120 residues of SSBP2 are responsible for the nuclear localization by guidance with CLIM2. RT-PCR demonstrated that SSBP2 was expressed in the porcine pituitary from fetal 40 days to postnatal 230 days in both genders and in the variety of pituitary and non-pituitary tumor cell lines, indicating that SSBP2 is present ubiquitously and plays a universal function during fetal and postnatal pituitary development.

Short limbs, cleft palate, and delayed formation of flat proliferative chondrocytes in mice with targeted disruption of a putative protein kinase gene, Pkdcc (AW548124)

During long bone development, round proliferative chondrocytes (RPCs) differentiate into flat proliferative chondrocytes (FPCs), and then into hypertrophic chondrocytes (HCs). FPCs and HCs support longitudinal bone growth. Here we show that a putative protein kinase gene, Pkdcc (AW548124), is required for longitudinal bone growth. We originally found Pkdcc expressed in the head organizer, but it is also expressed throughout embryogenesis and in various adult tissues. Pkdcc-/- embryos had no head organizer-related defects, but showed various morphological abnormalities at birth, including short limbs, cleft palate, sternal dysraphia, and shortened intestine. In the long bones of the limbs, only the mineralized regions were shortened, and the cartilage length was normal. In the humerus, Pkdcc was strongly expressed in the early FPCs, and FPC and HC formation was delayed in Pkdcc-/- mutants. Together, these data indicate that Pkdcc encodes a protein kinase that is required for the appropriate timing of FPC differentiation.

Tyrosine phosphorylation controls nuclear localization and transcriptional activity of Ssdp1 in mammalian cells

The LIM-HD proteins interact with different cofactors, including Ssdp1 to regulate development in a diverse range of species. The single stranded DNA binding protein (Ssdp1) is a member of an evolutionarily conserved family of proteins that regulate critical transcriptional processes during embryonic development. Ssdp1 is localized predominantly in the cytoplasm of 293T cells but is translocated to the nucleus when co-transfected with Lck, a member of the Src family of non-receptor tyrosine kinases. The Src tyrosine kinase inhibitor PP2 blocked the nuclear translocation of Ssdp1. Western blot analysis showed that co-expression of Ssdp1 and Lck in 293T cells induces Ssdp1 phosphorylation. Mutation of the Ssdp1 N terminal tyrosine residues 23 and 25 markedly reduced both the phosphorylation and the nuclear localization of Ssdp1. Lck enhanced the transcriptional activity of Ssdp1 in the context of known components of a LIM-homeodomain (LIM-HD)/cofactor complex. We propose that phosphorylation involving N-terminal tyrosine residues of Ssdp1 is a means of regulating its nuclear localization and subsequent transcriptional activation of LIM-HD complexes.

A regulatory network to segregate the identity of neuronal subtypes

Spinal motor neurons (MNs) and V2 interneurons (V2-INs) are specified by two related LIM-complexes, MN-hexamer and V2-tetramer, respectively. Here we show how multiple parallel and complementary feedback loops are integrated to assign these two cell fates accurately. While MN-hexamer response elements (REs) are specific to MN-hexamer, V2-tetramer-REs can bind both LIM-complexes. In embryonic MNs, however, two factors cooperatively suppress the aberrant activation of V2-tetramer-REs. First, LMO4 blocks V2-tetramer assembly. Second, MN-hexamer induces a repressor, Hb9, which binds V2-tetramer-REs and suppresses their activation. V2-INs use a similar approach; V2-tetramer induces a repressor, Chx10, which binds MN-hexamer-REs and blocks their activation. Thus, our study uncovers a regulatory network to segregate related cell fates, which involves reciprocal feedforward gene regulatory loops.

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.

Dkk1andWnt3interact to control head morphogenesis in the mouse

Loss of Dkk1 results in ectopic WNT/β-catenin signalling activity in the anterior germ layer tissues and impairs cell movement in the endoderm of the mouse gastrula. The juxtaposition of the expression domains of Dkk1 and Wnt3 is suggestive of an antagonist-agonist interaction. The downregulation of Dkk1 when Wnt3 activity is reduced reveals a feedback mechanism for regulating WNT signalling. Compound Dkk1;Wnt3 heterozygous mutant embryos display head truncation and trunk malformation, which are not found in either Dkk1+/- or Wnt3+/- embryos. Reducing the dose of Wnt3 gene in Dkk1-/- embryos partially rescues the truncated head phenotype. These findings highlight that head development is sensitive to the level of WNT3 signalling and that DKK1 is the key antagonist that modulates WNT3 activity during anterior morphogenesis.

Regionalisation of the endoderm progenitors and morphogenesis of the gut portals of the mouse embryo

This fate-mapping study reveals that the progenitors of all major parts of the embryonic gut are already present in endoderm of the early-head-fold to early-somite stage (1-9 somites) mouse embryo. The anterior endoderm contributes primarily to the anterior intestinal portal of the early-organogenesis stage (16-19 somites) embryo. Endoderm cells around and lateral to the node are allocated to the open "midgut" region of the embryonic gut. The posterior (post-nodal) endoderm contributes not only to the posterior intestinal portal but also the open "midgut". Descendants of the posterior endoderm span a length of the gut from the level of the 3rd-5th somites to the posterior end of the embryonic gut. The formation of the anterior and posterior intestinal portals is accompanied by similar repertoires of morphogenetic tissue movement. We also discovered that cells on contralateral sides of the anterior endoderm are distributed asymmetrically to the dorsal and ventral sides of the anterior intestinal portal, heralding the acquisition of laterality by the embryonic foregut.