[Show abstract][Hide abstract] ABSTRACT: Upf1 is a highly conserved RNA helicase essential for nonsense-mediated mRNA decay (NMD), an mRNA quality-control mechanism
that degrades aberrant mRNAs harboring premature termination codons (PTCs). For the activation of NMD, UPF1 interacts first
with a translation–terminating ribosome and then with a downstream exon–junction complex (EJC), which is deposited at exon–exon
junctions during splicing. Although the helicase activity of Upf1 is indispensable for NMD, its roles and substrates have
yet to be fully elucidated. Here we show that stable RNA secondary structures between a PTC and a downstream exon–exon junction
increase the levels of potential NMD substrates. We also demonstrate that a stable secondary structure within the 3′-untranslated
region (UTR) induces the binding of Upf1 to mRNA in a translation-dependent manner and that the Upf1-related molecules are
accumulated at the 5′-side of such a structure. Furthermore, we present evidence that the helicase activity of Upf1 is used
to bridge the spatial gap between a translation–termination codon and a downstream exon–exon junction for the activation of
NMD. Based on these findings, we propose a model that the Upf1-related molecular motor scans the 3′-UTR in the 5′-to-3′ direction
for the mRNA-binding factors including EJCs to ensure mRNA integrity.
Preview · Article · May 2012 · Nucleic Acids Research
[Show abstract][Hide abstract] ABSTRACT: Among the insertional mutagenesis techniques used in the current international knockout mouse project (KOMP) on the inactivation
of all mouse genes in embryonic stem (ES) cells, random gene trapping has been playing a major role. Gene-targeting experiments
have also been performed to individually and conditionally knockout the remaining ‘difficult-to-trap’ genes. Here, we show
that transcriptionally silent genes in ES cells are severely underrepresented among the randomly trapped genes in KOMP. Our
conditional poly(A)-trapping vector with a common retroviral backbone also has a strong bias to be integrated into constitutively
transcribed genome loci. Most importantly, conditional gene disruption could not be successfully accomplished by using the
retrovirus vector because of the frequent development of intra-vector deletions/rearrangements. We found that one of the cut
and paste-type DNA transposons, Tol2, can serve as an ideal platform for gene-trap vectors that ensures identification and conditional disruption of a broad spectrum
of genes in ES cells. We also solved a long-standing problem associated with multiple vector integration into the genome of
a single cell by incorporating a mixture of differentially tagged Tol2 transposons. We believe our strategy indicates a straightforward approach to mass-production of conditionally disrupted alleles
for genes in the target cells.
Full-text · Article · Mar 2012 · Nucleic Acids Research
[Show abstract][Hide abstract] ABSTRACT: Mouse embryonic stem cells (ESCs) require transcriptional regulation to ensure rapid proliferation that allows for self-renewal.
However, the molecular mechanism by which transcriptional factors regulate this rapid proliferation remains largely unknown.
Here we present data showing that CIBZ, a BTB domain zinc finger transcriptional factor, is a key transcriptional regulator
for regulation of ESC proliferation. Here we show that deletion or siRNA knockdown of CIBZ inhibits ESC proliferation. Cell
cycle analysis shows that loss of CIBZ delays the progression of ESCs through the G1 to S phase transition. Conversely, constitutive
ectopic expression of exogenous CIBZ in ESCs promotes proliferation and accelerates G1/S transition. These findings suggest
that regulation of the G1/S transition explains, in part, CIBZ-associated ESC proliferation. Our data suggest that CIBZ acts
through the post-transcriptionally regulates the expression of Nanog, a positive regulator of ESC proliferation and G1/S transition,
but does not affect Oct3/4 and Sox2 protein expression. Notably, constitutive overexpression of Nanog partially rescued the
proliferation defect caused by CIBZ knockdown, indicating the role of CIBZ in ESC proliferation and G1/S transition at least
in part depends on the Nanog protein level.
Preview · Article · Feb 2012 · Journal of Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: Postnatal growth and regeneration of skeletal muscle are carried out mainly by satellite cells, which, upon stimulation, begin to express myogenin (Myog), the critical determinant of myogenic differentiation. DNA methylation status has been associated with the expression of Myog, but the causative mechanism remains almost unknown. Here, we report that the level of CIBZ, a methyl-CpG-binding protein, decreases upon myogenic differentiation of satellite-derived C2C12 cells, and during skeletal muscle regeneration in mice. We present data showing that the loss of CIBZ promotes myogenic differentiation, whereas exogenous expression of CIBZ impairs it, in cultured cells. CIBZ binds to a Myog promoter-proximal region and inhibits Myog transcription in a methylation-dependent manner. These data suggest that the suppression of myogenic differentiation by CIBZ is dependent, at least in part, on the regulation of Myog. Our data show that the methylation status of this proximal Myog promoter inversely correlates with Myog transcription in cells and tissues, and during postnatal growth of skeletal muscle. Notably, induction of Myog transcription by CIBZ suppression is independent of the demethylation of CpG sites in the Myog promoter. These observations provide the first reported molecular mechanism illustrating how Myog transcription is coordinately regulated by a methyl-CpG-binding protein and the methylation status of the proximal Myog promoter.
[Show abstract][Hide abstract] ABSTRACT: We previously identified and characterized a murine BTB domain-containing protein, CIBZ (ZBTB38 in human), that interacts with CtBP and binds to methylated CpGs. However, its physiological function remained unknown. As CtBP is reportedly involved in p53-independent programmed cell death, we examine here whether CIBZ is associated with apoptosis. We found that CIBZ was highly expressed in proliferating C2C12 cells but that its expression levels decreased upon induction of apoptosis by serum starvation. Knockdown of CIBZ by small interfering RNA in C2C12 cells induced apoptosis, as determined by an increase of annexin V/propidium iodide labeling, activation of caspase-3, and cleavage of poly(ADP-ribose) polymerase. CIBZ inhibition also activated caspase-7 and caspase-9, suggesting that CIBZ-associated apoptosis occurs through the mitochondrial pathway. Notably, knockdown of CIBZ in p53(-/-) mouse embryonic fibroblast cells also activated caspase-3 and cleavage of poly(ADP-ribose) polymerase, indicating that CIBZ-associated apoptosis is mediated by a p53-independent pathway; however, because both common and distinct targets are regulated by CIBZ- and CtBP-associated apoptosis, we conclude that more than one pathway is involved. Finally, using mutagenesis and an in vitro caspase cleavage assay, we show that CIBZ is a novel substrate of caspase-3 and identify two caspase-3 recognition sites. These findings indicate, collectively, that CIBZ plays an important role by participating in the negative regulation of apoptosis in murine cells.
No preview · Article · Jun 2008 · Journal of Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: The transcriptional corepressor C-terminal binding protein (CtBP) is thought to be involved in development and oncogenesis, but the regulation of its corepressor activity is largely unknown. We show here that a novel BTB-zinc finger protein, CIBZ (CtBP-interacting BTB zinc finger protein; a mouse ortholog of rat ZENON that was recently identified as an e-box/dyad binding protein), redistributes CtBP to pericentromeric foci from a diffuse nuclear localization in interphase cells. CIBZ physically associates with CtBP via a conserved CtBP binding motif, PLDLR. When heterologously targeted to DNA, CIBZ represses transcription via two independent repression domains, an N-terminal BTB domain and a PLDLR motif-containing RD2 region, in a histone deacetylase-independent and -dependent manner, respectively. Mutation in the PLDLR motif abolishes the CIBZ-CtBP interaction and transcriptional repression activity of RD2, but does not affect the repression activity of the BTB domain. Furthermore, this PLDLR-mutated CIBZ cannot target CtBP to pericentromeric foci, although it is localized to the pericentromeric foci itself. These results suggest that at least one repression mechanism mediated by CIBZ is recruitment of the CtBP/HDAC complex to pericentromeric foci, and that CIBZ may regulate pericentromeric targeting of CtBP.
[Show abstract][Hide abstract] ABSTRACT: CD4(+)CD25(+) T cells have immunoregulatory and suppressive functions and are responsible for suppressing self-reactive cells and maintaining self-tolerance. In addition to CD4(+)CD25(+) T cells, there is some evidence that a fraction of CD4(+)CD25(-) T cells exhibit suppressive activity in vitro or in vivo. We have shown, using aged mice, that aging not only leads to a decline in the ability to mount CD4(+)CD25(-) T cell responses, but, at the same time, renders aged CD4(+)CD25(-) T cells suppressive. In this study we report two newly established mAbs that could abrogate the suppressive function of aged CD4(+)CD25(-) T cells. These mAbs recognized the same protein, the transmembrane phosphatase CD45. Cross-linking of CD45 on aged CD4(+)CD25(-) T cells was required for the disruption of their suppressive activity. Surprisingly, these mAbs also abrogated the suppressive action of CD4(+)CD25(+) T cells in vitro. Our results demonstrate an unexpected function of CD45 as a negative regulator neutralizing the suppressive activity of aged CD4(+)CD25(-) and young CD4(+)CD25(+) T cells.
Preview · Article · May 2005 · The Journal of Immunology
[Show abstract][Hide abstract] ABSTRACT: An international collaborative project has been proposed to inactivate all mouse genes in embryonic stem (ES) cells using
a combination of random and targeted insertional mutagenesis techniques. Random gene trapping will be the first choice in
the initial phase, and gene-targeting experiments will then be carried out to individually knockout the remaining ‘difficult-to-trap’
genes. One of the most favored techniques of random insertional mutagenesis is promoter trapping, which only disrupts actively
transcribed genes. Polyadenylation (poly-A) trapping, on the other hand, can capture a broader spectrum of genes including
those not expressed in the target cells, but we noticed that it inevitably selects for the vector integration into the last
introns of the trapped genes. Here, we present evidence that this remarkable skewing is caused by the degradation of a selectable-marker
mRNA used for poly-A trapping via an mRNA-surveillance mechanism, nonsense-mediated mRNA decay (NMD). We also report the development
of a novel poly-A-trap strategy, UPATrap, which suppresses NMD of the selectable-marker mRNA and permits the trapping of transcriptionally
silent genes without a bias in the vector-integration site. We believe the UPATrap technology enables a simple and straightforward
approach to the unbiased inactivation of all mouse genes in ES cells.
Preview · Article · Feb 2005 · Nucleic Acids Research
[Show abstract][Hide abstract] ABSTRACT: DNA arrays are capable of profiling the expression patterns of many genes in a single experiment. After finding a gene of interest in a DNA array, however, labor-intensive gene-targeting experiments sometimes must be performed for the in vivo analysis of the gene function. With random gene trapping, on the other hand, it is relatively easy to disrupt and retrieve hundreds of genes/gene candidates in mouse embryonic stem (ES) cells, but one could overlook potentially important gene-disruption events if only the nucleotide sequences and not the expression patterns of the trapped DNA segments are analyzed. To combine the benefits of the above two experimental systems, we first created approximately 900 genetrapped mouse ES cell clones and then constructed arrays of cDNAs derived from the disrupted genes. By using these arrays, we identified a novel gene predominantly expressed in the mouse brain, and the corresponding ES cell clone was used to produced mice homozygous for the disrupted allele of the gene. Detailed analysis of the knockout mice revealed that the gene trap vector completely abolished gene expression downstream of its integration site. Therefore, identification of a gene or novel gene candidate with an interesting expression pattern by using this type of DNA array immediately allows the production of knockout mice from an ES cell clone with a disrupted allele of the sequence of interest.
Preview · Article · Apr 2004 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: CD25(+)CD4(+) regulatory T cells in normal animals are engaged in the maintenance of immunological self-tolerance. We show here that glucocorticoid-induced tumor necrosis factor receptor family-related gene (GITR, also known as TNFRSF18)--a member of the tumor necrosis factor-nerve growth factor (TNF-NGF) receptor gene superfamily--is predominantly expressed on CD25(+)CD4(+) T cells and on CD25(+)CD4(+)CD8(-) thymocytes in normal naïve mice. We found that stimulation of GITR abrogated CD25(+)CD4(+) T cell-mediated suppression. In addition, removal of GITR-expressing T cells or administration of a monoclonal antibody to GITR produced organ-specific autoimmune disease in otherwise normal mice. Thus, GITR plays a key role in dominant immunological self-tolerance maintained by CD25(+)CD4(+) regulatory T cells and could be a suitable molecular target for preventing or treating autoimmune disease.
No preview · Article · Mar 2002 · Nature Immunology