Identification of a novel slow-muscle-fiber enhancer binding protein, MusTRD1

The molecular mechanisms which are responsible for restricting skeletal muscle gene expression to specific fiber types, either slow or fast twitch, are unknown. As a first step toward defining the components which direct slow-fiber-specific gene expression, we identified the sequence elements of the human troponin I slow upstream enhancer (USE) that bind muscle nuclear proteins. These include an E-box, a MEF2 element, and two other elements, USE B1 and USE C1. In vivo analysis of a mutation that disrupts USE B1 binding activity suggested that the USE B1 element is essential for high-level expression in slow-twitch muscles. This mutation does not, however, abolish slow-fiber specificity. A similar analysis indicated that the USE C1 element may play only a minor role. We report the cloning of a novel human USE B1 binding protein, MusTRD1 (muscle TFII-I repeat domain-containing protein 1), which is expressed predominantly in skeletal muscle. Significantly, MusTRD1 contains two repeat domains which show remarkable homology to the six repeat domains of the recently cloned transcription factor TFII-I. Furthermore, both TFII-I and MusTRD1 bind to similar but distinct sequences, which happen to conform with the initiator (Inr) consensus sequence. Given the roles of MEF2 and basic helix-loop-helix (bHLH) proteins in muscle gene expression, the similarity of TFII-I and MusTRD1 is intriguing, as TFII-I is believed to coordinate the interaction of MADS-box proteins, bHLH proteins, and the general transcription machinery.

The human troponin I slow promoter directs slow fiber-specific expression in transgenic mice

Troponin I (TnI) is a muscle-specific protein involved in the calcium-mediated contraction of striated muscle. Three TnI isoforms have been identified, each encoded by a separate gene and expressed in specific striated muscles in the adult. The slow isoform gene (TnIs) is transcriptionally regulated during skeletal muscle development such that its expression in the adult is restricted to muscle fibers innervated by a slow nerve. To delineate regions of this gene that are responsive to information imparted by the slow nerve, we generated transgenic mice carrying -4,200 to +12 bp of the human TnIs gene linked to the bacterial chloramphenicol acetyltransferase (CAT) coding region. By Northern blot analysis, we detected transgene transcripts only in muscles containing slow-twitch fibers. CAT histochemical analysis revealed that expression of the transgene is restricted solely to slow-twitch fibers as characterized by type I myosin heavy-chain (MyHC) expression. Using regeneration as a model for neural influenced expression, we show that this gene construct also contains sequences necessary to respond to cues from the central nervous system.

Delineation of a slow-twitch-myofiber-specific transcriptional element by using in vivo somatic gene transfer

Contractile proteins are encoded by multigene families, most of whose members are differentially expressed in fast- versus slow-twitch myofibers. This fiber-type-specific gene regulation occurs by unknown mechanisms and does not occur within cultured myocytes. We have developed a transient, whole-animal assay using somatic gene transfer to study this phenomenon and have identified a fiber-type-specific regulatory element within the promoter region of a slow myofiber-specific gene. A plasmid-borne luciferase reporter gene fused to various muscle-specific contractile gene promoters was differentially expressed when injected into slow- versus fast-twitch rat muscle: the luciferase gene was preferentially expressed in slow muscle when fused to a slow troponin I promoter, and conversely, was preferentially expressed in fast muscle when fused to a fast troponin C promoter. In contrast, the luciferase gene was equally well expressed by both muscle types when fused to a nonfiber-type-specific skeletal actin promoter. Deletion analysis of the troponin I promoter region revealed that a 157-bp enhancer conferred slow-muscle-preferential activity upon a minimal thymidine kinase promoter. Transgenic analysis confirmed the role of this enhancer in restricting gene expression to slow-twitch myofibers. Hence, somatic gene transfer may be used to rapidly define elements that direct myofiber-type-specific gene expression prior to the generation of transgenic mice.

Optimization of experimental variables influencing reporter gene expression in hepatoma cells following calcium phosphate transfection

We describe a highly efficient calcium phosphate transfection protocol capable of achieving 100% transfection efficiency of reporter genes transiently expressed in the human hepatoma cell lines HuH7 and HepG2. This procedure, a modification of that described by Chen and Okayama, is reliable, reproducible, and eliminates the requirement for the inclusion of cotransfected internal control plasmids. While Chen and Okayama described the pH of the 2x BBS (N,N-bis[2-hydroxyethyl]-2-aminoethanesulfonic acid-buffered saline) and DNA concentration as being critical factors for optimal transfection efficiency, we show that a reduced and strictly monitored standing time of the DNA/CaCl2/2x BBS cocktail prior to addition to cultured cells is essential for a particular combination of pH and DNA concentration. We also show that the inclusion of internal control plasmids can inhibit reporter gene activity in a promoter- and dose-dependent manner. The method so described is also applicable for the transfection of other mammalian cell lines including COS and HeLa, and conceivably for the generation of stable transfectants at high frequency.

Identification of a liver-specific promoter for the ovine growth hormone receptor

Growth hormone (GH) receptor cDNA clones from several species are characterized by heterogeneity in the 5' untranslated region (5'UT). This has been attributed to different promoters directing the expression of the gene from exons encoding 5'UT's which are alternatively spliced onto a common splice acceptor 11 basepairs (bp) upstream of the initiating AUG on exon 2. The following study identifies exon 1A of the ovine (o) GH receptor gene, corresponding to the 5'UT of a developmentally regulated, liver-specific transcript. Exon 1A spans 206 bp at a position 17 kilobases (kb) upstream of exon 2. Sequencing of the 669 bp region 5' to the transcription initiation site (+1) reveals a TATA box at -31, a CCAAT box at -88, and putative binding sites for several transcription factors involved in liver-specific gene expression. Two repetitive sequence elements are located in the 5' and 3' flanking regions of exon 1A. Functional analysis of the 4.5 kb region upstream of exon 1A was performed by transfecting the human hepatoma cell line HuH7 with luciferase reporter gene constructs. Positive and negative regulatory regions are identified, with basal promoter activity within 473 bp of the transcription initiation site. A 47 bp region containing putative binding sites for the activated glucocorticoid receptor and C/EBP-like proteins, between -180 and -133, is essential for transcriptional activation.

Developmental and tissue-specific regulation of ovine insulin-like growth factor II (IGF-II) mRNA expression

Insulin-like growth factor II (IGF-II) is believed to be involved in the development of the fetus. Northern and dot-blot analysis of RNA isolated from different sheep tissues at various stages of development were undertaken, revealing that the ovine IGF-II gene is expressed as a multitranscript family (6.0, 5.1, 5.0, 4.7, 3.8, 2.9, 2.3, 1.9, 1.6, 1.3 kb). Evidence that the ovine IGF-II gene may be regulated in a developmental, tissue-specific, co-ordinate or independent manner is presented. The developmental profile of IGF-II gene expression correlates with plasma levels (Mesiano et al. (1989) Endocrinology 124, 1485-1491), and suggests that the rapid fall in plasma concentration at term can be attributed to regulation at the transcriptional level. With the exception of the kidney, IGF-II expression was down-regulated at birth in all tissues examined. As in man but not rat, an adult liver-specific transcript was detected and attributed to different 5' untranslated regions in the fetal and adult IGF-II mRNAs. The finding of IGF-II transcripts in all tissues examined supports evidence from other species of autocrine/paracrine roles for IGF-II in the development of the fetus.