Yujiro Higashi

Mice Lacking Zfhx1b, the Gene That Codes for Smad-Interacting Protein-1, Reveal a Role for Multiple Neural Crest Cell Defects in the Etiology of Hirschsprung Disease–Mental Retardation Syndrome

Recently, mutations in ZFHX1B, the gene that encodes Smad-interacting protein-1 (SIP1), were found to be implicated in the etiology of a dominant form of Hirschsprung disease-mental retardation syndrome in humans. To clarify the molecular mechanisms underlying the clinical features of SIP1 deficiency, we generated mice that bear a mutation comparable to those found in several human patients. Here, we show that Zfhx1b-knockout mice do not develop postotic vagal neural crest cells, the precursors of the enteric nervous system that is affected in patients with Hirschsprung disease, and they display a delamination arrest of cranial neural crest cells, which form the skeletomuscular elements of the vertebrate head. This suggests that Sip1 is essential for the development of vagal neural crest precursors and the migratory behavior of cranial neural crest in the mouse. Furthermore, we show that Sip1 is involved in the specification of neuroepithelium.

Zeb1 links epithelial-mesenchymal transition and cellular senescence.

Overexpression of zinc finger E-box binding homeobox transcription factor 1 (Zeb1) in cancer leads to epithelial-to-mesenchymal transition (EMT) and increased metastasis. As opposed to overexpression, we show that mutation of Zeb1 in mice causes a mesenchymal-epithelial transition in gene expression characterized by ectopic expression of epithelial genes such as E-cadherin and loss of expression of mesenchymal genes such as vimentin. In contrast to rapid proliferation in cancer cells where Zeb1 is overexpressed, this mesenchymal-epithelial transition in mutant mice is associated with diminished proliferation of progenitor cells at sites of developmental defects, including the forming palate, skeleton and CNS. Zeb1 dosage-dependent deregulation of epithelial and mesenchymal genes extends to mouse embryonic fibroblasts (MEFs), and mutant MEFs also display diminished replicative capacity in culture, leading to premature senescence. Replicative senescence in MEFs is classically triggered by products of the Ink4a (Cdkn2a) gene. However, this Ink4a pathway is not activated during senescence of Zeb1 mutant MEFs. Instead, there is ectopic expression of two other cell cycle inhibitory cyclin-dependent kinase inhibitors, p15Ink4b (Cdkn2b) and p21Cdkn1a (Cdkn1a). We demonstrate that this ectopic expression of p15Ink4b extends in vivo to sites of diminished progenitor cell proliferation and developmental defects in Zeb1-null mice.

Identification of CtBP1 and CtBP2 as Corepressors of Zinc Finger-Homeodomain Factor δEF1

ABSTRACT δEF1, a representative of the zinc finger-homeodomain protein family, is a transcriptional repressor which binds E2-box (CACCTG) and related sequences and counteracts the activators through transrepression mechanisms. It has been shown that the N-proximal region of the protein is involved in the transrepression. Here we demonstrate that δEF1 has a second mechanism of transrepression recruiting CtBP1 or CtBP2 as its corepressor. A two-hybrid screen of mouse cDNAs with various portions of δEF1 identified these proteins, which bind to δEF1 in a manner dependent on the PLDLSL sequence located in the short medial (MS) portion of δEF1. CtBP1 is the mouse orthologue of human CtBP, known as the C-terminal binding protein of adenovirus E1A, while CtBP2 is the second homologue. Fusion of mouse CtBP1 or CtBP2 to Gal4DBD (Gal4 DNA binding domain) made them Gal4 binding site-dependent transcriptional repressors in transfected 10T1/2 cells, indicating their involvement in a transcriptional repression mechanism. When the MS portion of δEF1 was used to Gal4DBD and used to transfect cells, a strong transrepression activity was generated, but this activity was totally dependent on the PLDLSL sequence which served as the site for interaction with endogenous CtBP proteins, indicating that CtBP1 and -2 can act as corepressors. Exogenous CtBP1/2 significantly enhanced transcriptional repression by δEF1, and this enhancement was lost if the PLDLSL sequence was altered, demonstrating that CtBP1 and -2 act as corepressors of δEF1. In the mouse,CtBP1 is expressed from embryo to adult, butCtBP2 is mainly expressed during embryogenesis. In developing embryos, CtBP1 and CtBP2 are expressed broadly with different tissue preferences. Remarkably, their high expression occurs in subsets of δEF1-expressing tissues, e.g., cephalic and dorsal root ganglia, spinal cord, posterior-distal halves of the limb bud mesenchyme, and perichondrium of forming digits, supporting the conclusion that CtBP1 and -2 play crucial roles in the repressor action of δEF1 in these tissues.

The Transcriptional Repressor ZEB Regulates p73 Expression at the Crossroad between Proliferation and Differentiation

ABSTRACT The newly discovered p73 gene encodes a nuclear protein that has high homology with p53. Furthermore, ectopic expression of p73 in p53+/+ and p53−/− cancer cells recapitulates some of the biological activities of p53 such as growth arrest, apoptosis, and differentiation. p73−/−-deficient mice exhibit severe defects in proper development of the central nervous system and pheromone sensory pathway. They also suffer from inflammation and infections. Here we studied the transcriptional regulation of p73 at the crossroad between proliferation and differentiation. p73 mRNA is undetectable in proliferating C2C12 cells and is expressed at very low levels in undifferentiated P19 and HL60 cells. Conversely, it is upregulated during muscle and neuronal differentiation as well as in response to tetradecanoyl phorbol acetate-induced monocytic differentiation of HL60 cells. We identified a 1-kb regulatory fragment located within the first intron of p73, which is positioned immediately upstream to the ATG codon of the second exon. This fragment exerts silencer activity on p73 as well as on heterologous promoters. The p73 intronic fragment contains six consensus binding sites for transcriptional repressor ZEB, which binds these sites in vitro and in vivo. Ectopic expression of dominant-negative ZEB (ZEB-DB) restores p73 expression in proliferating C2C12 and P19 cells. Thus, transcriptional repression of p73 expression by ZEB binding may contribute to the modulation of p73 expression during differentiation.

Ndrg1-Deficient Mice Exhibit a Progressive Demyelinating Disorder of Peripheral Nerves

ABSTRACT NDRG1 is an intracellular protein that is induced under a number of stress and pathological conditions, and it is thought to be associated with cell growth and differentiation. Recently, human NDRG1 was identified as a gene responsible for hereditary motor and sensory neuropathy-Lom (classified as Charcot-Marie-Tooth disease type 4D), which is characterized by early-onset peripheral neuropathy, leading to severe disability in adulthood. In this study, we generated mice lacking Ndrg1 to analyze its function and elucidate the pathogenesis of Charcot-Marie-Tooth disease type 4D. Histological analysis showed that the sciatic nerve of Ndrg1-deficient mice degenerated with demyelination at about 5 weeks of age. However, myelination of Schwann cells in the sciatic nerve was normal for 2 weeks after birth. Ndrg1-deficient mice showed muscle weakness, especially in the hind limbs, but complicated motor skills were retained. In wild-type mice, NDRG1 was abundantly expressed in the cytoplasm of Schwann cells rather than the myelin sheath. These results indicate that NDRG1 deficiency leads to Schwann cell dysfunction, suggesting that NDRG1 is essential for maintenance of the myelin sheaths in peripheral nerves. These mice will be used for future analyses of the mechanisms of myelin maintenance.

Dual-Mode Modulation of Smad Signaling by Smad-Interacting Protein Sip1 Is Required for Myelination in the Central Nervous System

Myelination by oligodendrocytes in the central nervous system (CNS) is essential for proper brain function, yet the molecular determinants that control this process remain poorly understood. The basic helix-loop-helix transcription factors Olig1 and Olig2 promote myelination, whereas bone morphogenetic protein (BMP) and Wnt/β-catenin signaling inhibit myelination. Here we show that these opposing regulators of myelination are functionally linked by the Olig1/2 common target Smad-interacting protein-1 (Sip1). We demonstrate that Sip1 is an essential modulator of CNS myelination. Sip1 represses differentiation inhibitory signals by antagonizing BMP receptor-activated Smad activity while activating crucial oligodendrocyte-promoting factors. Importantly, a key Sip1-activated target, Smad7, is required for oligodendrocyte differentiation and partially rescues differentiation defects caused by Sip1 loss. Smad7 promotes myelination by blocking the BMP- and β-catenin-negative regulatory pathways. Thus, our findings reveal that Sip1-mediated antagonism of inhibitory signaling is critical for promoting CNS myelination and point to new mediators for myelin repair.

Dlx1&2-Dependent Expression of Zfhx1b (Sip1, Zeb2) Regulates the Fate Switch between Cortical and Striatal Interneurons

Mammalian pallial (cortical and hippocampal) and striatal interneurons are both generated in the embryonic subpallium, including the medial ganglionic eminence (MGE). Herein we demonstrate that the Zfhx1b (Sip1, Zeb2) zinc finger homeobox gene is required in the MGE, directly downstream of Dlx1&2, to generate cortical interneurons that express Cxcr7, MafB, and cMaf. In its absence, Nkx2-1 expression is not repressed, and cells that ordinarily would become cortical interneurons appear to transform toward a subtype of GABAergic striatal interneurons. These results show that Zfhx1b is required to generate cortical interneurons, and suggest a mechanism for the epilepsy observed in humans with Zfhx1b mutations (Mowat-Wilson syndrome).

UV-B radiation induces epithelial tumors in mice lacking DNA polymerase eta and mesenchymal tumors in mice deficient for DNA polymerase iota.

ABSTRACT DNA polymerase η (Pol η) is the product of the Polh gene, which is responsible for the group variant of xeroderma pigmentosum, a rare inherited recessive disease which is characterized by susceptibility to sunlight-induced skin cancer. We recently reported in a study of Polh mutant mice that Pol η is involved in the somatic hypermutation of immunoglobulin genes, but the cancer predisposition of Polh−/− mice has not been examined until very recently. Another translesion synthesis polymerase, Pol ι, a Pol η paralog encoded by the Poli gene, is naturally deficient in the 129 mouse strain, and the function of Pol ι is enigmatic. Here, we generated Polh Poli double-deficient mice and compared the tumor susceptibility of them with Polh- or Poli-deficient animals under the same genetic background. While Pol ι deficiency does not influence the UV sensitivity of mouse fibroblasts irrespective of Polh genotype, Polh Poli double-deficient mice show slightly earlier onset of skin tumor formation. Intriguingly, histological diagnosis after chronic treatment with UV light reveals that Pol ι deficiency leads to the formation of mesenchymal tumors, such as sarcomas, that are not observed in Polh−/− mice. These results suggest the involvement of the Pol η and Pol ι proteins in UV-induced skin carcinogenesis.

Smad-interacting protein-1 (Zfhx1b) acts upstream of Wnt signaling in the mouse hippocampus and controls its formation

Smad-interacting protein-1 (Sip1) [Zinc finger homeobox (Zfhx1b)] is a transcription factor implicated in the genesis of Mowat–Wilson syndrome in humans. Sip1 expression in the dorsal telencephalon of mouse embryos was documented from E12.5. We inactivated the gene specifically in cortical precursors. This resulted in the lack of the entire hippocampal formation. Sip1 mutant mice exhibited death of differentiating cells and decreased proliferation in the region of the prospective hippocampus and dentate gyrus. The expression of the Wnt antagonist Sfrp1 was ectopically activated, whereas the activity of the noncanonical Wnt effector, JNK, was down-regulated in the embryonic hippocampus of mutant mice. In cortical cells, Sip1 protein was detected on the promoter of Sfrp1 gene and both genes showed a mutually exclusive pattern of expression suggesting that Sfrp1 expression is negatively regulated by Sip1. Sip1 is therefore essential to the development of the hippocampus and dentate gyrus, and is able to modulate Wnt signaling in these regions.

Regulation of ocular lens development by Smad-interacting protein 1 involving Foxe3 activation.

Sip1, a Smad-binding zinc-finger homeodomain transcription factor, has essential functions in embryonic development, but its role in individual tissues and the significance of its interaction with Smad proteins have not been fully characterized. In the lens lineage, Sip1 expression is activated after lens placode induction, and as the lens develops, the expression is localized in the lens epithelium and bow region where immature lens fibers reside. The lens-lineage-specific inactivation of the Sip1 gene was performed using mice homozygous for floxed Sip1 that carry a lens-specific Cre recombinase gene. This caused the development of a small hollow lens connected to the surface ectoderm, identifying two Sip1-dependent steps in lens development. The persistence of the lens stalk resembles a defect in Foxe3 mutant mice, and Sip1-defective lenses lose Foxe3 expression, placing Foxe3 downstream of Sip1. In the Sip1-defective lens, β-crystallin-expressing immature lens fiber cells were produced, but γ-crystallin-expressing mature fiber cells were absent, indicating the requirement for Sip1 activity in lens fiber maturation. A 6.2 kb Foxe3 promoter region controlled lacZ transgene expression in the developing lens, where major and minor lens elements were identified upstream of -1.26 kb. Using transfection assays, the Foxe3 promoter was activated by Sip1 and this activation is further augmented by Smad8 in the manner dependent on the Smad-binding domain of Sip1. This Sip1-dependent activation and its augmentation by Smad8 occur using the proximal 1.26 kb promoter, and are separate from lens-specific regulation. This is the first demonstration of the significance of Smad interaction in modulating Sip1 activity.