Analysis of the Alternative Promoters that Regulate Tissue-Specific Expression of Human Aromatic l-Amino Acid Decarboxylase

Abstract: Previously we identified two alternative first exons (exon N1 and exon L1) coding for 5' untranslated regions of human aromatic l -amino acid decarboxylase (AADC) and found that their alternative usage produced two types of mRNAs in a tissue-specific manner. To determine the cis -acting element regulating the tissue-specific expression of human AADC, we produced three kinds of transgenic mice harboring 5' flanking regions of the human AADC gene fused to the bacterial chloramphenicol acetyltransferase (CAT) gene. The transgene termed ACA contained −7.0 kb to −30 bp in exon N1, including the entire exon L1; ACN contained −3.6 kb to −30 bp in exon N1; and ACL contained −2.8 kb to −42 bp in exon L1. The ACA transgenic mice expressed CAT at extremely high levels in peripheral nonneuronal tissues, such as pancreas, liver, kidney, small intestine, and colon, that contained endogenous high AADC activity, whereas CAT immunoreactivity was not detected in either catecholaminergic or serotonergic neurons in the CNS. Thus, it was suggested that the ACA transgene contained the major part of cis -regulatory elements for the expression of AADC in peripheral nonneuronal tissues. On the other hand, the ACN transgenic mice moderately expressed CAT in various tissues except for the lung and liver, and the ACL transgenic mice showed moderate CAT expression only in the kidney.     

Identification of a fat cell enhancer: analysis of requirements for adipose tissue-specific gene expression.

The molecular basis for adipose-specific gene expression is not known. To approach the problem of adipocyte gene expression, we have analyzed in detail the capacity of the 5'-flanking region of the adipocyte P2 (aP2) gene to direct cell-type specific gene expression. Although the proximal promoter containing AP-1 and C/EBP binding sites is capable of directing differentiation-dependent gene expression in cultured adipocytes, these constructs are essentially inactive in the tissues of transgenic mice. We found that -5.4 kb of the 5'-flanking region were required to direct heterologous gene (chloramphenicol acetyl transferase; CAT) expression to the adipose tissue of transgenic mice. By deletion analysis, we identified a 520 bp enhancer at -5.4 kb of the aP2 gene. We show that this enhancer can direct high levels of gene expression specifically to the adipose tissue of transgenic mice. This enhancer also functions in a differentiation-dependent manner in cultured adipocytes and cannot be transactivated in preadipocytes by C/EBP. Molecular analysis indicates that several cis- and trans- acting acting elements, though not C/EBP, contribute to the specificity and potency of this enhancer.     

Identification of protein-binding sites in the hepatitis B virus enhancer and core promoter domains.

We have investigated the role of liver-specific trans-acting factor(s) in the regulation of hepatitis B virus (HBV) gene expression. A recorder plasmid (pEcoAluCAT; HBV nucleotides 1 through 1878) was constructed containing the HBV enhancer and the promoter region of the pregenomic RNA, which was ligated to the bacterial chloramphenicol acetyltransferase (CAT) gene. Upon transfecting this plasmid into various cell lines, the CAT gene was expressed only in cells of liver origin. Moreover, competition cotransfections with pEcoAluCAT and plasmids containing HBV enhancer sequences in human hepatoblastoma-derived HepG2 cells indicated the presence of titratable trans-acting factor(s) in these cells. Gel mobility shift assays using HBV enhancer and core promoter domains confirmed the existence of sequence-specific DNA-binding proteins in liver cell nuclear extract which bound to these regions. These binding sites encompass 17- and 12-nucleotide palindromes in the HBV enhancer and core promoter domains, respectively, when mapped by the methylation interference assay.     

A potent far-upstream enhancer in the mouse pro alpha 2(I) collagen gene regulates expression of reporter genes in transgenic mice

We have identified three DNase I-hypersensitive sites in chromatin between 15 and 17 kb upstream of the mouse pro alpha 2 (I) collagen gene. These sites were detected in cells that produce type I collagen but not in cells that do not express these genes. A construction containing the sequences from -17 kb to +54 bp of the mouse pro alpha 2 (I) collagen gene, cloned upstream of either the Escherichia coli beta- galactosidase or the firefly luciferase reporter gene, showed strong enhancer activity in transgenic mice when compared with the levels seen previously in animals harboring shorter promoter fragments. Especially high levels of expression of the reporter gene were seen in dermis, fascia, and the fibrous layers of many internal organs. High levels of expression could also be detected in some osteoblastic cells. When various fragments of the 5' flanking sequences were cloned upstream of the 350-bp proximal pro alpha 2(I) collagen promoter linked to the lacZ gene, the cis-acting elements responsible for enhancement were localized in the region between -13.5 and -19.5 kb, the same region that contains the three DNase I-hypersensitive sites. Moreover, the DNA segment from -13.5 to -19.5 kb was also able to drive the cell-specific expression of a 220-bp mouse pro alpha 1(I) collagen promoter, which is silent in transgenic mice. Hence, our data suggest that a far-upstream enhancer element plays a role in regulating high levels of expression of the mouse pro alpha 2(I) collagen gene.     

An E box comprises a positional sensor for regional differences in skeletal muscle gene expression and methylation.

To dissect the molecular mechanisms conferring positional information in skeletal muscles, we characterized the control elements responsible for the positionally restricted expression patterns of a muscle-specific transgene reporter, driven by regulatory sequences from the MLC1/3 locus. These sequences have previously been shown to generate graded transgene expression in the segmented axial muscles and their myotomal precursors, fortuitously marking their positional address. An evolutionarily conserved E box in the MLC enhancer core, not recognized by MyoD, is a target for a nuclear protein complex, present in a variety of tissues, which includes Hox proteins and Zbu1, a DNA-binding member of the SW12/SNF2 gene family. Mutation of this E box in the MLC enhancer has only a modest positive effect on linked CAT gene expression in transfected muscle cells, but when introduced into transgenic mice the same mutation elevates CAT transgene expression in skeletal muscles, specifically releasing the rostral restriction on MLC-CAT transgene expression in the segmented axial musculature. Increased transgene activity resulting from the E box mutation in the MLC enhancer correlates with reduced DNA methylation of the distal transgenic MLC1 promoter as well as in the enhancer itself. These results identify an E box and the proteins that bind to it as a positional sensor responsible for regional differences in axial skeletal muscle gene expression and accessibility.     

Promotor and enhancer like activity at the 5'-end of normal and T24 Ha-ras1 genes

We have used a short-term transfection technique, in which we monitor the ability of DNA fragments to induce the expression of the chloramphenicol acetyltransferase (CAT) gene in rat 208F fibroblast cells. Using appropriate vectors we have assayed for promoter or enhancer activity of the 0.8 kb SstI fragment located within the 5'-flanking sequences of the first coding exon of the human T24 and normal Ha-ras1 genes. We find that this fragment contains promoter and enhancer activities in both the normal and the T24 Ha-ras1 gene.     

Upstream element of the sea urchin arylsulfatase gene serves as an insulator.

Insulator DNAs functionally isolate neighboring genes by blocking interactions between distal cis-regulatory elements and promoters. Here we report that a DNA fragment located in the upstream region of sea urchin, H. pulcherrimus, arylsulfatase (HpArs) gene blocks the interaction of the Ars enhancer when positioned between the enhancer and the target promoter, in an orientation dependent manner. The Ars insulator works only 3' to 5' direction and has no significant stimulatory or inhibitory effects on its own promoter. In transgenic Drosophila, the Ars insulator blocks the interaction between even-skipped stripe enhancer and its target promoter. The insulation mechanism operates also unidirectionally in Drosophila. We also show that the efficiency of transformation of HeLa cells is enhanced when the integrated gene is flanked by the Ars insulator, suggesting the sea urchin insulator overcomes the position-dependent transgene expression in mammalian cells. These results demonstrate that the mechanism of action of the insulator has been conserved throughout evolution.