Zn finger homeodomain 2
ATBF1 is a transcription factor containing four homeodomains and 17 zinc fingers. Since the Drosophila homolog, ZFH-2, is implicated in neurogenesis, ATBF1 expression was examined in developing mouse brain and in P19 mouse embryonal carcinoma cells during differentiation. Pre- and post-natal mouse brains express high levels of ATBF1 mRNA, but the adult brain contains only a small amount of ATBF1 transcripts. In P19 cells, ATBF1 transcripts are undetectable before differentiation; however, 1 day after induction of neuronal differentiation with retinoic acid, ATBF1 mRNA is expressed at a high level. This increased level reaches a maximum on the 4th day and then declines. No comparable level of ATBF1 mRNA is expressed when P19 cells are treated with dimethyl sulfoxide to induce muscle cells. These temporal patterns of ATBF1 expression in vivo and in vitro suggest that ATBF1 may play a role in neuronal differentiation (Ido, 1994).
zfh-4 has been identified as a new member of the zinc finger-homeodomain (zfh) family of transcription factors. Zfh-4 expression is prominent in developing muscle and brain. In both tissues, zfh-4 RNA levels are highest embryonically, then decrease gradually to barely detectable levels in adults. In myogenic cell lines, far more zfh-4 is expressed in proliferating myoblasts than in myotubes, suggesting a cellular basis for the developmental regulation observed in vivo. In contrast, zfh-4 RNA in brain is more abundant in postmitotic cells of the marginal zone than in proliferating cells of the ventricular zone. Within the brain, zfh-4 RNA is regionally localized: expression is highest in midbrain, readily detectable in hindbrain, and very low in cerebral cortex. Its patterns of expression, and its homology to known DNA binding proteins, support the idea that zfh-4 may be a regulator of gene expression in developing brain and muscle (Kostich, 1995).
The human ATBF1-B gene encodes a 306-kDa protein containing 4 homeodomains and 18 zinc fingers including one pseudo zinc finger motif. A second ATBF1 cDNA, 12 kilobase pairs long, termed ATBF1-A, has been cloned. The deduced ATBF1-A protein is 404 kDa in size and differs from ATBF1-B by a 920-amino acid extention at the N terminus. Analysis of 5'-genomic sequences shows that the 5'-noncoding sequences specific to ATBF1-A and ATBF1-B transcripts are contained in distinct exons that can splice to a downstream exon common to the ATBF1-A and ATBF1-B mRNAs. The expression of ATBF1-A transcripts increases to high levels when P19 and NT2/D1 cells are treated with retinoic acid to induce neuronal differentiation. Preferential expression of ATBF1-A transcripts is also observed in developing mouse brain. Transient transfection assays show that the 5.5-kilobase pair sequence upstream of the ATBF1-A-specific exon (exon 2) supports expression of the linked chloramphenicol acetyltransferase gene in neuronal cells derived from P19 cells but not in undifferentiated P19 or in F9 cells, which do not differentiate into neurons. These results show that ATBF1-A and ATBF1-B transcripts are generated by alternative promoter usage combined with alternative splicing and that the ATBF1-A-specific promoter is activated during neuronal differentiation (Miura, 1995).
A mouse ATBF1 cDNA has been isolated that is 12-kb long and capable of encoding a 406-kDa protein containing four homeodomains and 23 zinc-finger motifs. Mouse ATBF1 is 94% homologous to the human ATBF1-A transcription factor. Northern blot and RNase protection analysis have shown that levels of ATBF1 transcripts are low in adult mouse tissues, but high in developing brain, consistent with a role for ATBF1 in neuronal differentiation (Ido, 1996).
Using the yeast two-hybrid system, the transcription factor ATBF1 was identified as v-Myb- and c-Myb-binding protein. Deletion mutagenesis revealed amino acids 2484-2520 in human ATBF1 and 279-300 in v-Myb as regions required for in vitro binding of both proteins. Further experiments identified leucines Leu325 and Leu332 of the Myb leucine zipper motif as additional amino acid residues important for efficient ATBF1-Myb interaction in vitro. In co-transfection experiments, the full-length ATBF1 was found to form in vivo complexes with v-Myb and inhibit v-Myb transcriptional activity. Both ATBF1 2484-2520 and Myb 279-300 regions are required for the inhibitory effect. Finally, the chicken ATBF1 was identified, showing a high degree of amino acid sequence homology with human and murine proteins. These data reveal Myb proteins as the first ATBF1 partners detected so far and identify amino acids 279-300 in v-Myb as a novel protein-protein interaction interface through which Myb transcriptional activity can be regulated (Kaspar, 1999).
The mouse zfh-4 cDNA is 12 kb long and capable of encoding a 3,550-amino acid protein containing four homeodomains and 22 zinc fingers including two pseudo zinc finger motifs. The mouse ZFH-4 is 51% homologous to the mouse ATBF1 and 23% to the Drosophila ZFH-2. The homeodomain and zinc finger regions are highly conserved between ZFH-4 and ATBF1 except that one zinc finger is missing in ZFH-4. Analysis of partial genomic sequences shows that the mouse zfh-4 and ATBF1 genes are similar in exon-intron organization. RT-PCR analysis of zfh-4 transcripts in adult mouse tissues shows that zfh-4 expression is low but reproducibly detectable in brain, heart, lung and muscle. In these mouse tissues, ATBF1 transcripts were poorly amplified by PCR under the conditions where zfh-4 transcripts were amplified, suggesting that the expression of zfh-4 mRNA is higher than that of ATBF1 mRNA. Other comparative analysis suggests functional similarities and dissimilarities between ZFH-4 and ATBF1 (Sakata, 2000).
The ATBF1 gene encodes two protein isoforms, the 404-kDa ATBF1-A, possessing four homeodomains and 23 zinc fingers, and the 306-kDa ATBF1-B, lacking a 920-amino acid N-terminal region of ATBF1-A which contains 5 zinc fingers. In vitro, ATBF1-A is expressed in proliferating C2C12 myoblasts, but its expression levels decrease upon induction of myogenic differentiation in low serum medium. Forced expression of ATBF1-A in C2C12 cells results in repression of MyoD and myogenin expression and elevation of Id3 and cyclin D1 expression, leading to inhibition of myogenic differentiation in low serum. In contrast, transfection of C2C12 cells with the ATBF1-B isoform leads to an acceleration of myogenic differentiation, as indicated by an earlier onset of myosin heavy chain expression and formation of a higher percentage of multinucleated myotubes. The fourth homeodomain of ATBF1-A binds to an AT-rich element adjacent to the E1 E-box of the muscle regulatory factor 4 promoter mediating transcriptional repression. The ATBF1-A-specific N-terminal region possesses general transcription repressor activity. These results suggest that ATBF1-A plays a role in the maintenance of the undifferentiated myoblast state, and its down-regulation is a prerequisite to initiate terminal differentiation of C2C12 cells (Berry, 2001).
Ptosis is defined as drooping of the upper eyelid and can impair full visual acuity. It occurs in a number of forms including congenital bilateral isolated ptosis, which may be familial and for which two linkage groups are known on chromosomes 1p32-34.1 and Xq24-27.1. The analysis is described of the chromosome breakpoints in a patient with congenital bilateral isolated ptosis and a de novo balanced translocation 46,XY,t(1;8)(p34.3;q21.12). The 1p breakpoint lies ~13 Mb distal to the previously reported linkage locus at 1p32-1p34.1 and does not disrupt a coding sequence, whereas the chromosome 8 breakpoint disrupts a gene homologous to the mouse zfh-4 gene. Murine zfh-4 codes for a zinc finger homeodomain protein and is a transcription factor expressed in both muscle and nerve tissue. Human ZFH-4 is therefore a candidate gene for congenital bilateral isolated ptosis (McMullan, 2002).
The ATBF1 gene encodes transcription factors containing four homeodomains and multiple zinc finger motifs. However, the gene products have yet to be identified and the role remains unknown in vivo. In this study, an antiserum was raised for ATBF1; high levels of expression of ATBF1 were found in developing rat brain. Western and Northern blot analyses detected a 400 kDa protein and 12.5 kb mRNA in developing rat brain, respectively; both corresponding to ATBF1-A but not the B isoform. The protein is highly expressed in the midbrain and diencephalon and mRNA is highly expressed in the brainstem, mostly in embryo and neonatal brain. Immunohistochemistry identified postmitotic neurons in the brainstem as the major site of ATBF1 expression, and the expression levels vary depending on age of and location in the brain. Expression is transient and weak in the precursor cells at early neurogenesis. ATBF1 decreases postnatally, but remains in mature neurons, including those expressing DOPA decarboxylase (DDC). High levels of ATBF1 are expressed in precursor cells in accordance with neurogenesis and are continued to the mature neurons in specific areas such as the inferior colliculus. Expression is not significant from precursor cells to mature neurons in the cerebral cortex and hippocampus. ATBF1 and its Drosophila homolog, Zfh-2, are known to regulate cell differentiation and proliferation via the interaction with either of the basic helix-loop-helix transcription factors, c-myb, or the DDC gene. Together with these reported functions the expression features detected in this study suggest that ATBF1 may participate in the regulation of neuronal cell maturation or region-specific central nervous system differentiation (Ishii, 2003).
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