Hyung-Seok Kim

Korea University, Seoul, Seoul, United States

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Publications (10)29.18 Total impact

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    ABSTRACT: Oligodendrocytes are myelinating glial cells in the CNS and are essential for proper neuronal function. During development, oligodendrocyte progenitor cells (OPCs) are specified from the motor neuron precursor domain of the ventral spinal cord and differentiate into myelinating oligodendrocytes after migration to the white matter of the neural tube. Cell cycle control of OPCs influences the balance between immature OPCs and myelinating oligodendrocytes, but the precise mechanism regulating the differentiation of OPCs into myelinating oligodendrocytes is unclear. To understand the mechanisms underlying oligodendrocyte differentiation, an N-ethyl-N-nitrosourea-based mutagenesis screen was performed and a zebrafish leo1 mutant, dalmuri (dal(knu6)) was identified in the current study. Leo1 is a component of the evolutionarily conserved RNA polymerase II-associated factor 1 complex (PAF1C), which is a positive regulator of transcription elongation. The dal(knu6) mutant embryos specified motor neurons and OPCs normally, and at the appropriate time, but OPCs subsequently failed to differentiate into myelinating oligodendrocytes and were eliminated by apoptosis. A loss-of-function study of cdc73, another member of PAF1C, showed the same phenotype in the CNS, indicating that PAF1C function is required for oligodendrocyte differentiation. Interestingly, inhibition of positive transcription elongation factor b (p-TEFb), rescued downregulated gene expression and impaired oligodendrocyte differentiation in the dal(knu6) mutant and Cdc73-deficient embryos. Together, these results indicate that antagonistic regulation of gene expression by PAF1C and p-TEFb plays a crucial role in oligodendrocyte development in the CNS.
    Journal of Neuroscience 06/2012; 32(24):8201-7. · 6.91 Impact Factor
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    ABSTRACT: Previous studies have shown that Notch signaling not only regulates the number of early differentiating neurons, but also maintains proliferating neural precursors in the neural tube. Although it is well known that Notch signaling is closely related to the differentiation of adult neural stem cells, none of transgenic zebrafish provides a tool to figure out the relationship between Notch signaling and the differentiation of neural precursors. The goal of this study was to characterize Her4-positive cells by comparing the expression of a fluorescent Her4 reporter in Tg[her4-dRFP] animals with a GFAP reporter in Tg[gfap-GFP] adult zebrafish. BrdU incorporation indicated that dRFP-positive cells were proliferating and a double labeling assay revealed that a significant fraction of the Her4-dRFP positive population was also GFAP-GFP positive. Our observations suggest that a reporter line with Notch-dependent gene expression can provide a tool to examine proliferating neural precursors and/or neuronal/glial precursors in the development of the adult nervous system to examine the model in which Notch signaling maintains proliferating neural precursors in the neural tube.
    Molecules and Cells 05/2012; 33(6):627-32. · 2.21 Impact Factor
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    ABSTRACT: The generation of various subtypes of neurons and glial cells at the right time and place is crucial for the proper development of the vertebrate CNS. Although the mechanisms and factors for the regulation of neuronal diversity in the CNS have been well studied, the mechanisms regulating the sequential production of neuronal and glial cells from neural precursors remain poorly understood. This study shows that Tcf3, a member of the Lef/Tcf family of proteins, is required to inhibit the premature oligodendroglial fate specification of spinal cord precursors using the transgenic zebrafish, which expresses a dominant repressor form of Tcf3 under the control of a heat-shock inducible promoter. In addition, the data revealed that Tcf3 function in oligodendroglial fate specification is mediated independently of canonical Wnt signaling. Altogether, these results show a novel function for Tcf3 in regulating the timing of oligodendroglial fate specification in the spinal cord.
    Molecules and Cells 09/2011; 32(4):383-8. · 2.21 Impact Factor
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    ABSTRACT: During normal forebrain development in vertebrates, rostral neural tissue must be protected from Wnt signals via the actions of locally expressed Wnt antagonistic factors. In zebrafish zygotic oep (Zoep) mutants, forebrain structure is severely disrupted with reduced expression of the Wnt antagonists secreted frizzled related protein1 and dickkopf1. To analyze the temporal effects of Wnt antagonism on forebrain development, we generated transgenic zebrafish that overexpressed the dominant negative form of frizzled8a (DNfz8a) in wild-type and Zoep mutants under the control of a heat-inducible promoter. This model allowed for assessment of the dynamics of Wnt antagonistic signaling during forebrain development. Our results demonstrated that overexpression of DNfz8a in Zoep embryos between 7 and 16hpf increased putative forebrain region demarcated by anf and distal-less2 expressions. These results suggest that normal forebrain development requires continual Wnt antagonism from the early gastrula to the mid-somitogenesis stage.
    Biochemical and Biophysical Research Communications 05/2009; 381(4):717-21. · 2.41 Impact Factor
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    ABSTRACT: We investigated chamber-specific gene expression by isolating a 2.2-kb polymerase chain reaction product containing the 5'-flanking region of the zebrafish ventricular myosin heavy-chain gene (vmhc). Promoter analysis revealed that the fragment, consisting of nucleotides from -301 to +26, is sufficient for vmhc promoter activity. Among several putative cis-acting elements in the region, a GATA-binding site was identified to be crucial for the activity of the promoter, as evidenced by the complete abolishment of promoter activity by a single nucleotide substitution of GATA-binding site (-287, C-->T). Knockdown of GATA-binding proteins 4 and 6 (GATA4 and -6) by their antisense morpholino oligonucleotides resulted in reduced green fluorescent protein (GFP) reporter gene and endogenous vmhc expression. These findings suggest that GATA4 and -6 play a key role in the regulation of vmhc expression in the ventricle. In addition, the vmhc promoter and the transgenic zebrafish (vmhc:gfp) are useful tools to study the formation and function of the ventricle. Developmental Dynamics 238:1574-1581, 2009. (c) 2009 Wiley-Liss, Inc.
    Developmental Dynamics 05/2009; 238(6):1574-81. · 2.59 Impact Factor
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    ABSTRACT: Mind bomb (Mib) facilitates Notch signaling pathway by promoting the endocytosis of Notch ligand. The zebrafish mib ta52b mutant has a defect in its ubiquitin ligase activity which is necessary to inhibit the neurogenesis, resulting in a neuronal hyperplasia. Several genes regulated in the mib ta52b mutant have been well established, however, there were relatively few reports about the transcriptome profile. To identify the genes differentially expressed in the mib ta52b mutant, genome-wide analysis was performed using serial analysis of gene expression. Three hundred and thirty-five transcripts were identified whose expressions were significantly altered in the mib ta52b mutant as compared with the wild-type. Interestingly, it was suggested that the mib ta52b mutation may affect not only neurogenesis but also mesoderm development. These results provide new insights into Notch signaling pathway.
    Molecular and General Genetics 12/2008; 281(1):77-85. · 2.88 Impact Factor
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    ABSTRACT: Wnts have been shown to provide a posteriorizing signal that has to be repressed in the specification of vertebrate forebrain region. Previous studies have shown that Wnt activation by LiCl treatment causes an expansion of optic stalk and mid-hindbrain boundary, whereas eye and ventral diencephalon in the forebrain region were reduced. However, the molecular mechanism, by which inhibits Wnt activity in the forebrain remains poorly defined. To investigate relationship between forebrain specification and Wnt signaling, the zebrafish homologue of secreted frizzled related protein1 (sfrp1) has been characterized. The transcripts of sfrp1 are detected in the presumptive forebrain at gastrula and in the ventral telencephalon, ventral diencephalon, midbrain and optic vesicles at 24h after postfertilization (hpf). Overexpression of sfrp1 causes an anteriorization of embryo, with enlarged head and reduced posterior structure as in the embryo overexpressing dominant-negative form of Frizzled8a or Dkk1. Its overexpression restored the eye defects in the Wnt8b-overexpressing embryos, but not in the LiCl-treated embryos. These results suggest that Sfrp1 expressed in the forebrain and eye field plays a critical role in the extracellular events of antagonizing Wnt activity for the forebrain specification.
    Neuroscience Letters 03/2007; 414(1):26-9. · 2.03 Impact Factor
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    ABSTRACT: Notch activation inhibits neuronal differentiation during development of the nervous system; however, the dynamic role of Notch signaling in individual cell lineages remains poorly understood. We have characterized 3.4 kb 5'-regulatory sequence of a Notch target gene, her4, and used it to drive fluorescent gene expression in transgenic lines where the spatiotemporal pattern of Notch activation can be examined in vivo. The 3.4 kb her4 promoter contains five predicted Su(H) binding sites of which two proximal sites were confirmed to be required for Notch-mediated transcriptional activation. Without Notch, Su(H) effectively represses transcription regulated by the promoter. Analyses of transgenic zebrafish showed that while the expression of proneural genes and Notch activation were both critical for endogenous her4 expression, reporter gene expression was primarily regulated by Notch activity. This study also showed that her4 may be differently regulated in sensory cranial ganglia, where Notch activity is not essential for her4 expression and where Su(H) may repress her4 expression. The establishment of a reporter line with Notch-Su(H)-dependent fluorescent gene expression provides a tool to explore the complex role of Notch signaling in the development of vertebrate nervous system.
    Developmental Biology 02/2007; 301(2):555-67. · 3.87 Impact Factor
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    ABSTRACT: A complete zebrafish mespo cDNA encoding a protein of 131 amino acids with a bHLH domain in the C-terminal has been isolated. The bHLH domain of zebrafish Mespo is highly similar to those in the mouse, chick and Xenopus, sharing 82.4%, 80.4% and 74.5% amino acid identity, respectively. At 50% epiboly, the zebrafish mespo is first detected in the marginal zone of the blastoderm but excluding the prospective shield. Subsequently, mespo expression is intensified in the involuting mesoderm at 60% epiboly, and then restricted to the presomitic mesoderm (PSM) at 95% epiboly. At the 1-somite stage, mespo expression becomes reduced in the most rostral PSM. During segmentation, mespo expression is gradually downregulated at the most rostral segmental plate where cells are being coalesced to form somites. In spadetail mutant embryos, most of mespo-expressing cells were missing.
    Development Genes and Evolution 06/2003; 213(4):203-6. · 1.70 Impact Factor
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    ABSTRACT: A complete cDNA of a novel zebrafish gene named onecut has been isolated; this gene encodes a protein of 446 amino acids with a Cut domain (73 amino acid residues) and a homeodomain. The Cut domain of zebrafish Onecut is highly similar to those in mammalian hepatocyte nuclear factor-6 and Drosophila Onecut, sharing 90 and 88% amino acid identity, respectively. The expression of zebrafish onecut is restricted to neuronal cells, being first detected in trigeminal ganglia neurons at the end of gastrulation. By the 1-somite stage, onecut expression has begun in primary neurons of the lateral stripes in the neural plate, and appeared in neuronal cells of the medial stripes at the 2-somite stage. By the 4-somite stage, onecut expression expanded to the intermediate stripes and to subsets of neuronal cells in the midbrain and hindbrain. Subsequently, onecut expression intensified in the lateral region of midbrain and hindbrain, yet no onecut-positive cells were seen in the telencephalon. By 24hpf, onecut transcripts remained abundant in the spinal cord but were no longer detectable in differentiated Rohon-Beard sensory neurons. The expression of onecut was greatly increased in the neural mutant mindbomb, while being decreased in narrowminded.
    Mechanisms of Development 04/2002; 112(1-2):199-202. · 2.38 Impact Factor