Brain and muscle creatine kinase genes contain common TA-rich recognition protein-binding regulatory elements

The myogenic regulatory circuit that controls cardiac/slow twitch troponin C gene transcription in skeletal muscle involves E-box, MEF-2, and MEF-3 motifs

We have characterized the specific DNA regulatory elements responsible for the function of the human cardiac troponin C gene (cTnC) muscle-specific enhancer in myogenic cells. We used functional transient transfection assays with deletional and site-specific mutagenesis to evaluate the role of the conserved sequence elements. Gel electrophoresis mobility shift assays (EMSA) demonstrated the ability of the functional sites to interact with nuclear proteins. We demonstrate that three distinct transcription activator binding sites commonly found in muscle-specific enhancers (a MEF-2 site, a MEF-3 site, and at least four redundant E-box sites) all contribute to full enhancer activity but a CArG box does not. Mutation of either the MEF-2 or MEF-3 sites or deletion of the E-boxes reduces expression by 70% or more. Furthermore, the MEF-2 site and the E-boxes specifically bind, respectively, to MEF-2 and myogenic determination factors derived from nuclear extracts. EMSA assays using a MEF-3 containing oligonucleotide revealed indistinguishable separation patterns with extracts from myogenic cells and nonmyogenic cells. These data suggest that expression of the cTnC gene in slow-twitch skeletal muscle is sustained through complex interactions at the 3'Ile enhancer between muscle-specific and nontissue-specific transcription factors: either a myogenic bHLH complex or MEF-2 can activate transcription but only in the presence of a third transcriptional activator that appears not to be muscle specific. We conclude from these observations that the cTnC 3'Ile element is a composite enhancer that functions through the combined interactions of at least five regulatory elements and their cognate binding factors: three or four E-boxes, a MEF-2 site, and a MEF-3 site. The data support the notion that all of these sites contribute to enhancer function in cell systems in an additive way but that none are absolutely required for enhancer activity. The data imply that the levels of transcription of cTnC in myogenic tissues in which the activities of one of the transcriptional factors is lacking would be partially but not wholly suppressed. Our data support the critical role of E-box sites in conjunction with the adjacent elements. Hence, we assign CTnC gene regulation to the "ordinary" rather than to the "novel" category of transcriptional regulation during skeletal myogenesis.

The role of MEF2 transcription factors in dehydration and anoxia survival in Rana sylvatica skeletal muscle

The wood frog (Rana sylvatica) can endure freezing of up to 65% of total body water during winter. When frozen, wood frogs enter a dormant state characterized by a cessation of vital functions (i.e., no heartbeat, blood circulation, breathing, brain activity, or movement). Wood frogs utilize various behavioural and biochemical adaptations to survive extreme freezing and component anoxia and dehydration stresses, including a global suppression of metabolic functions and gene expression. The stress-responsive myocyte enhancer factor-2 (MEF2) transcription factor family regulates the selective expression of genes involved in glucose transport, protein quality control, and phosphagen homeostasis. This study examined the role of MEF2A and MEF2C proteins as well as select downstream targets (glucose transporter-4, calreticulin, and muscle and brain creatine kinase isozymes) in 40% dehydration and 24 h anoxia exposure at the transcriptional, translational, and post-translational levels using qRT-PCR, immunoblotting, and subcellular localization. Mef2a/c transcript levels remained constant during dehydration and anoxia. Total, cytoplasmic, and nuclear MEF2A/C and phospho-MEF2A/C protein levels remained constant during dehydration, whereas a decrease in total MEF2C levels was observed during rehydration. Total and phospho-MEF2A levels remained constant during anoxia, whereas total MEF2C levels decreased during 24 h anoxia and P-MEF2C levels increased during 4 h anoxia. In contrast, cytoplasmic MEF2A levels and nuclear phospho-MEF2A/C levels were upregulated during anoxia. MEF2 downstream targets remained constant during dehydration and anoxia, with the exception of glut4 which was upregulated during anoxia. These results suggest that the upregulated MEF2 response reported in wood frogs during freezing may in part stem from their cellular responses to surviving prolonged anoxia, rather than dehydration, leading to an increase in GLUT4 expression which may have an important role during anoxia survival.

Transcriptional activation of rat creatine kinase B by 17beta-estradiol in MCF-7 cells involves an estrogen responsive element and GC-rich sites

The rat creatine kinase B (CKB) gene is induced by estrogen in the uterus, and constructs containing rat CKB gene promoter inserts are highly estrogen-responsive in cell culture. Analysis of the upstream -568 to -523 region of the promoter in HeLa cells has identified an imperfect palindromic estrogen response element (ERE) that is required for hormone inducibility. Analysis of the CKB gene promoter in MCF-7 breast cancer cells confirmed that pCKB7 (containing the -568 to -523 promoter insert) was estrogen-responsive in transient transfection studies. However, mutation and deletion analysis of this region of the promoter showed that two GC-rich sites and the concensus ERE were functional cis-elements that bound estrogen receptor alpha (ERalpha)/Sp1 and ERalpha proteins, respectively. The role of these elements was confirmed in gel mobility shift and chromatin immunoprecipitation assays and transfection studies in MDA-MB-231 and Schneider Drosophila SL-2 cells. These results show that transcriptional activation of CKB by estrogen is dependent, in part, on ERalpha/Sp1 action which is cell context-dependent.

Synergistic transcriptional activation by Sox10 and Sp1 family members

Neuronal nicotinic acetylcholine receptors (nAChR) are expressed at specific times during development and in discrete neuronal populations. Transcriptional regulation of the receptor genes clearly plays a key role in the molecular pathway underlying the expression of these critical synaptic components. In an effort to understand this regulation, we focus upon the genes encoding three receptor subunits: alpha3, alpha5 and beta4. These subunits are genomically clustered and constitute the predominant nAChR subtype expressed in the peripheral nervous system. We and others demonstrated that the general transcription factors, Sp1 and Sp3, can transactivate the promoter of each subunit gene. Further, we showed that the regulatory factor Sox10 transactivates the alpha3 and beta4 promoters and does so in a cell-type-specific manner. Interestingly, the Sp- and Sox10-binding sites on the beta4 promoter are located immediately adjacent to each other, raising the possibility that the two sets of factors functionally interact to regulate receptor gene expression. Consistent with this hypothesis, we demonstrated that the proteins can directly interact. Here, we extend these observations and show that Sox10 and the Sp factors functionally interact, leading to synergistic transcriptional activation in a cholinergic cell line. Finally, evidence for the existence of cell-type-specific co-regulators for Sp1 and Sox10 is presented.

Requirement of MEF2D in the induced differentiation of HL60 promyeloid cells

The regulatory role of MEF2 (myocyte enhancer binding factor 2) proteins in nonmuscle tissues has not been well characterized. We examined the expression of MEF2 family members, namely, MEF2A, -B, -C, and -D, in the differentiation of HL60 promyeloid cells and observed the remarkable increase in the expressions of MEF2A and MEF2D proteins during the differentiation process into monocytes. To examine the role of MEF2, we expressed a dominant-negative form of MEF2D, without its transactivation domain, in HL60 cells. When the HL60 cell line expressing the mutant MEF2D was induced to differentiate by VitD(3) treatment, cell surface expression of CD14 and the ability to reduce NBT, which are important characteristics of differentiated monocytes, were significantly decreased compared with control HL60 cells. These results show that MEF2D is required in the differentiation process along the monocyte/macrophage lineage,

Mef2 Proteins, Required for Muscle Differentiation, Bind an Essential Site in the Ig λ Enhancer

The Ig λ light chain gene enhancer has two unique essential motifs, λA and λB. The transcription factors that bind the λB motif have been identified as Pu.1 and Pu.1-interacting partner (Pip). We report here that the λA site includes a binding site for the myocyte-specific enhancer factor 2 (Mef2) family of transcription factors. Mef2 proteins were first described in muscle cells and, in vertebrates, include four known members designated A to D. Using a λA electrophoretic-mobility shift assay (EMSA), in conjunction with a high affinity Mef2 binding site and anti-Mef2 Abs, we show that members of the Mef2 family are present in nuclear extracts of λ-producing B cells and bind the λA site. Functional assays using the chloramphenicol acetyltransferase (CAT) reporter construct containing three copies of the λA motif demonstrate that the λA sequence can function as an enhancer in conjunction with the thymidine kinase (TK) promoter and is regulated by Mef2 proteins. Extrapolating from other systems where transcriptional regulation by Mef2 has been studied, other transcription factors may be involved along with Mef2 in transcriptional regulation at the λA site.

Synergistic activation of the N-methyl-D-aspartate receptor subunit 1 promoter by myocyte enhancer factor 2C and Sp1

The N-methyl-D-aspartate (NMDA) subtype of glutamate receptor plays important roles in neuronal development, plasticity, and cell death. NMDA receptor subunit 1 (NR1) is an essential subunit of the NMDA receptor and is developmentally expressed in postnatal neurons of the central nervous system. Here we identify on the NR1 promoter a binding site for myocyte enhancer factor 2C (MEF2C), a developmentally expressed neuron/muscle transcription factor found in cerebrocortical neurons, and study its regulation of the NR1 gene. Co-expression of MEF2C and Sp1 cDNAs in primary neurons or cell lines synergistically activates the NR1 promoter. Disruption of the MEF2 site or the MEF2C DNA binding domain moderately reduces this synergism. Mutation of the Sp1 sites or the activation domains of Sp1 protein strongly reduces the synergism. Results of yeast two-hybrid and co-immunoprecipitation experiments reveal a physical interaction between MEF2C and Sp1 proteins. The MEF2C DNA binding domain is sufficient for this interaction. Dominant-negative MEF2C interferes with expression of NR1 mRNA in neuronally differentiated P19 cells. Growth factors, including epidermal growth factor and basic fibroblast growth factor, can up-regulate NR1 promoter activity in stably transfected PC12 cells, even in the absence of the MEF2 site, but the Sp1 sites are necessary for this growth factor regulation, suggesting that Sp1 sites may mediate these effects.

Molecular characterization of the creatine kinases and some historical perspectives

Over the last 15 years, molecular characterization of the creatine kinase (CK) gene family has paralleled the molecular revolution of understanding gene structure, function, and regulation. In this review, we present a summary of advances in molecular analysis of the CK gene family with a few vignettes of historical interest. We describe how the muscle CK gene provided an essential model system to examine myogenic regulatory mechanisms, leading to the discovery of the binding site for the MyoD family of basic helix-loop-helix transcription factors essential in skeletal myogenesis and the characterization of the MEF2 family of factors with an A/T rich consensus binding site essential in skeletal myogenesis and cardiogenesis. Cloning and characterization of the four mRNAs and nuclear genes encoding the cytosolic CKs, muscle and brain CKs, and the mitochondrial (Mt) CKs, sarcomeric MtCK and ubiquitous MtCK, has allowed intriguing study of tissue-specific and cell-specific expression of the different CKs and analysis of structural, functional, regulatory, and evolutionary relationships among both the four CK proteins and genes. Current and future studies focus on understanding both cellular energetics facilitated by the CK enzymes, especially energy channelling from the site of production, the mitochondrial matrix and inner membrane, to various cytosolic foci of utilization, and regulation of MtCK gene expression at the cell and tissue-specific level as models of regulation of energy producing genes.

Evidence that the homeodomain protein Gtx is involved in the regulation of oligodendrocyte myelination

We have investigated the patterns of postnatal brain expression and DNA binding of Gtx, a homeodomain transcription factor. Gtx mRNA accumulates in parallel with the RNAs encoding the major structural proteins of myelin, myelin basic protein (MBP), and proteolipid protein (PLP) during postnatal brain development; Gtx mRNA decreases in parallel with MBP and PLP mRNAs in the brains of myelin-deficient rats, which have a point mutation in the PLP gene. Gtx mRNA is expressed in differentiated, postmitotic oligodendrocytes but is not found in oligodendrocyte precursors or astrocytes. These data thus demonstrate that Gtx is expressed uniquely in differentiated oligodendrocytes in postnatal rodent brain and that its expression is regulated in parallel with the major myelin protein mRNAs, encoding MBP and PLP, under a variety of physiologically relevant circumstances. Using a Gtx fusion protein produced in bacteria, we have confirmed that Gtx is a sequence-specific DNA-binding protein, which binds DNA sequences containing a core AT-rich homeodomain binding site. Immunoprecipitation of labeled DNA fragments encoding either the MBP or PLP promoter regions with this fusion protein has identified several Gtx-binding fragments, and we have confirmed these data using an electrophoretic mobility shift assay. In this way we have identified four Gtx binding sites within the first 750 bp of the MBP promoter and four Gtx binding sites within the first 1. 3 kb of the PLP promoter. In addition, inspection of the PLP promoter sequence demonstrates the presence of six additional Gtx binding sites. These data, taken together, strongly suggest that Gtx is important for the function of differentiated oligodendrocytes and may be involved in the regulation of myelin-specific gene expression.

Myocyte-specific enhancer binding factor 2C expression in fetal mouse brain development

We have previously found that myocyte-specific enhancer binding factor 2C (MEF2C) is expressed in the brain, where it is found at high levels in the developing cerebral cortex. We have now examined MEF2C expression in fetal mouse brain by in situ hybridization and by immunohistochemistry from E11 to E17, the period when most cortical neurons are born. The distribution of MEF2C mRNA detected by in situ hybridization closely resembles that of MEF2C immunoreactivity. MEF2C is not present in proliferative zones in the brain. It is present at high levels in cells that have migrated to the subplate and cortical plate. MEF2C is also found in the olfactory blub at high levels and at lower levels in hippocampus, basal forebrain, striatum, cerebellum, and inferior colliculus, and in some nuclei of the hypothalamus, thalamus and brainstem. The pattern of expression suggests that MEF2C is expressed in a subset of postmitotic neurons in the brain and that it may therefore function to promote terminal differentiation of the cells that express it.

MEF2 protein expression, DNA binding specificity and complex composition, and transcriptional activity in muscle and non-muscle cells

Tissue-specific gene expression can be mediated by complex transcriptional regulatory mechanisms. Based on the dichotomy of the ubiquitous distribution of the myocyte enhancer factor 2 (MEF2) gene mRNAs compared to their cell type-restricted activity, we investigated the basis for their tissue specificity. Electrophoretic mobility shift assays using the muscle creatine kinase MEF2 DNA binding site as a probe showed that HeLa, Schneider, L6E9 muscle, and C2C12 muscle cells have a functional MEF2 binding activity that is indistinguishable based on competition analysis. Interestingly, chloramphenicol acetyltransferase reporter assays showed MEF2 site-dependent trans-activation in myogenic C2C12 cells but no trans-activation by the endogenous MEF2 proteins in HeLa cells. By immunofluorescence, we detected abundant nuclear localized MEF2A and MEF2D protein expression in HeLa cells and C2C12 muscle cells. Using immuno-gel shift analysis and also co-immunoprecipitation studies, we show that the predominant MEF2 DNA binding complex bound to MEF2 sites from either the muscle creatine kinase or c-jun regulatory regions in C2C12 muscle cells is comprised of a MEF2A homodimer, whereas in HeLa cells, it is a MEF2A:MEF2D heterodimer. Thus, the presence of MEF2 DNA binding complexes is not necessarily coupled with trans-activation of target genes. The ability of the MEF2 proteins to activate transcription in vivo correlates with the specific dimer composition of the DNA binding complex and the cellular context.

Myocyte-specific enhancer binding factor 2C expression in gerbil brain following global cerebral ischemia

Myocyte-specific enhancer binding factor 2 (MEF2C) is a transcription factor expressed at high levels in brain. In this study, the distribution of MEF2C expression in brain was studied in normal adult gerbils and in adult gerbils subjected to 10 min of global cerebral ischemia. In normal animals, MEF2C-immunoreactivity and messenger RNA expression were detected in cortex, hippocampus, caudate-putamen, thalamus, hypothalamus, and amygdala. Within the hippocampus, MEF2C-immunoreactivity and MEF2C messenger RNA were found in interneurons scattered through the CA fields, a subset of which are parvalbumin-immunoreactive. MEF2C-immunoreactivity and MEF2C messenger RNA were also present in granule cells in the dentate gyrus. MEF2C-immunoreactivity was also detected in microglia in the hippocampus. After transient forebrain ischemia, CA1 pyramidal neurons, which are MEF2C-negative, degenerate whereas MEF2C-positive interneurons survive. Our results thus indicate that MEF2C is a marker for hippocampal neurons that are resistant to ischemia. It remains to be determined whether MEF2C plays a direct role in protecting the neurons that express it from ischemic injury. In addition, MEF2C-immunoreactivity is present in microglia, and, after ischemia, there were increased numbers of MEF2C-immunoreactive microglia in CA1, so MEF2C-immunoreactivity is a marker of both resting and activated microglia.

Cellular factors binding to a novel cis-acting element mediate steroid hormone responsiveness of mouse mammary tumor virus promoter

Steroid hormone receptors regulate mouse mammary tumor virus (MMTV) gene expression by binding to hormone response DNA elements present in the long terminal repeat. Tissue-specific expression of MMTV is unlikely to be regulated by steroid hormone-receptor complex alone, and mammary cell-specific factors might play a role in the hormone-induced transcriptional activation. In this report we have investigated the function of a novel cis-acting element designated Kil (-204 to -188) which is located adjacent to the distal glucocorticoid response element, in steroid hormone-induced transcription of MMTV. Electrophoretic mobility shift assays indicate that cellular factors bind to the Kil element, and dexamethasone stimulation results in alterations in the binding pattern of proteins in this region. By transient transfection assays using wild type and deletion mutants of the Kil element, we show that this novel cis-acting element is necessary for hormone-induced transcription of MMTV and functions in mammary tumor cells but not in NIH/3T3 cells. Mutagenesis of the Kil sequence suggests that the entire Kil element functioning as one unit is necessary for hormone-induced transcription of MMTV. When placed in the context of heterologous promoters, neither Kil element nor glucocorticoid response element is able to induce significant hormone-induced transcription of MMTV. The presence of both the DNA elements in tandem results in optimal induction of transcription in the presence of steroid hormones. Our results also indicate that the Kil element functions in human breast carcinoma cell lines such as T47D and MCF-7. These results suggest that Kil element in combination with distal glucocorticoid response element functions as a mammary cell-specific enhancer to regulate MMTV transcription.

Muscle gene E-box control elements. Evidence for quantitatively different transcriptional activities and the binding of distinct regulatory factors

The muscle creatine kinase gene enhancer contains two regulatory elements (MCK-R and MCK-L) with the consensus E-box sequence (CAnnTG). A myocyte specific protein complex, MEF1, binds the MCK-R site. MEF1 contains several basic H-L-H myogenic determination factors (MDFs), each dimerized with ubiquitous members of the bH-L-H family (e.g. E12/E47). We now demonstrate that the ubiquitous bH-L-H factor E2-2 is a major component of the endogenous MCK-R site specific complex. Previous studies described the MCK-L site as a similar but low affinity MDF/bH-L-H heterodimer binding site. However, we find that the MCK-L site exhibits preferential binding of an unknown ubiquitous factor which contains neither E12/E47 nor E2-2, and that it exhibits differential transcriptional activity with muscle and non-muscle cells. The differential behavior of the MCK-L and MCK-R sites may be a general trait of E-box elements since one among several E-boxes in the MLC 1/3 enhancer also binds preferentially to the MCK-L factor. From our studies we now propose separate consensus sequences for MCK-R and MCK-L E-box types: AACAc/gc/gTGCa/t and GGa/cCANGTGGc/gNa/g. Our results suggest that while many muscle gene E-boxes are capable of binding the previously characterized spectrum of MDF/bH-L-H heterodimers in vitro, MCK-L type E-boxes probably bind qualitatively different factors in vivo.

The non-histone chromosomal protein HMG-I(Y) contributes to repression of the immunoglobulin heavy chain germ-line epsilon RNA promoter

The rate of germ-line RNA transcription correlates with the rate of immunoglobulin heavy chain isotype switching. A promoter element for the transcription of RNA from the germ-line mouse immunoglobulin epsilon heavy chain constant region gene is induced by interleukin(IL)-4 and lipopolysaccharide, and is bound at its transcription initiation sites by an IL-4-inducible nuclear protein, NF-BRE. To examine the function of the binding site for this IL-4-inducible complex, substitution mutations were introduced in the promoter. These binding site mutations increased promoter activity and decreased binding of NF-BRE. To investigate the paradox of an IL-4-inducible protein binding to a repressor site in an IL-4-inducible promoter, we determined that the non-histone chromosomal protein HMG-I(Y) binds at the transcription initiation sites of the germ-line epsilon promoter. Assays with antisera against HMG-I(Y) revealed monomeric HMG-I(Y) in nuclear extracts. Cotransfection of an expression construct directing the synthesis of anti-sense HMG-I(Y) RNA also increased promoter activity, consistent with a repressor function of HMG-I(Y). Thus, the data are most consistent with a model in which HMG-I(Y) participates in repression of promoter activity. The effects of IL-4 may include derepression at this site.

Myocyte-specific enhancer binding factor 2C expression in human brain development

Myocyte-specific enhancer binding factor 2C (MEF2C) activates transcription by binding to the myocyte-specific enhancer binding factor 2 (MEF2) regulatory element and has been shown previously to be expressed in muscle and in the brain. We have now studied MEF2C expression in human brain using an antiserum raised against amino acids 140-238 of MEF2C. Western blotting demonstrated that, in fetal brain, MEF2C-immunoreactive bands have the same apparent molecular weight as those in extracts of COS cells transfected with MEF2C complementary DNA. In adult brain, however, MEF2C-immunoreactive bands have a higher molecular weight. In the cerebral cortex, MEF2C immunoreactivity is present in the cortical plate, and is not found in the intermediate zone or ventricular zone. At 14 weeks of gestation, the earliest age examined, MEF2C immunoreactivity is present in cell nuclei throughout the cortical plate. Subsequently, MEF2C immunoreactivity develops a bilaminate and then a trilaminate distribution, and ultimately is expressed preferentially in layers II, IV and VI of mature neocortex. MEF2C immunoreactivity is also found in entorhinal cortex, hippocampus, claustrum, cerebellum and amygdala, and in scattered cells in the thalamus. These findings suggest a role for MEF2C in postmitotic neuronal differentiation, in particular, in the development of certain cortical layers, but also in differentiation of other neurons as well.

Protooncogene c-ski is expressed in both proliferating and postmitotic neuronal populations

The cellular protooncogene, c-ski, is expressed in all cells of the developing mouse at low but detectable levels. In situ hybridization and Northern blot analyses reveal that some cells and tissues express this gene at higher levels at certain stages of embryonic and postnatal development. RT-PCR results indicate that alternative splicing of exon 2, known to occur in chickens (Sutrave and Hughes [1989] Mol. Cell. Biol. 9:4046-4051; Grimes et al. [1993] Oncogene 8:2863-2868) does not occur in adult mouse tissues. In the embryo, neural crest cells express the c-ski gene during migration at 8.5 to 9.5 days post coitum (p.c.). Neural crest derivatives such as dorsal root ganglia and melanocytes stain positively with an antibody to the ski protein. At 9 days p.c., the entire neural tube has high levels of c-ski gene expression. By 12-13.5 days only the ependymal layer expresses c-ski above background levels. At 14-16 days p.c., c-ski mRNAs are detected at high levels in the cortical layers of the brain and in the olfactory bulb. In 2 week and 6 week postnatal brains, c-ski gene transcripts are also detected in the hippocampus and in the granule cell layer of the cerebellum. The allantois and placenta exhibit high levels of c-ski mRNAs. Neonatal lung tissue increases c-ski gene expression approximately two-fold compared to prenatal levels. These results suggest that ski plays a role in both the proliferation and differentiation of specific cell populations of the central and peripheral nervous systems and of other tissues.

Creatine kinase activity as an indicator of unopposed estrogen action in the mouse uterus associated with anti-progesterone treatment

The brain isozyme of creatine kinase (CKB) is a major component of the estrogen-induced proteins in the rat uterus. Hormonal specificity of this response was studied in cotransfection assays using the rat CKB promoter linked to the bacterial chloramphenicol acetyltransferase gene. Response was specific for estrogen as 17 beta-estradiol in the presence of estrogen receptor dramatically stimulated the CKB promoter. This induction was completely blocked by the estrogen antagonist ICI 164,384. Nuclear receptors for progesterone, androgen, glucocorticoid and vitamin D did not significantly activate the CKB promoter in the presence of their respective ligands. Creatine kinase (CK) activity was analyzed in decidualized mouse uterus to assess estrogenic activity in vivo. Upon oil stimulation, uterine horns of day 4 pseudopregnant mice underwent a dramatic outgrowth in response to endogenous progesterone. This response was accompanied by a significant decrease in CK activity from a control value of 1.44 +/- 0.25 to 0.38 +/- 0.08 IU/mg protein (P < 0.001), indicating that the action of estrogen was suppressed. Treatment of females one day prior to oil-stimulation with progesterone receptor antagonists, RU486 (Mifepristone) or ZK299 (Onapristone), or with a monoclonal antibody to progesterone (DB3), abolished decidualization, and also restored the CK activity to the control value. These results suggest that CK can be used as a specific cellular marker to detect unopposed estrogen action in the mouse uterus associated with progesterone withdrawal or receptor blockade.

A fourth human MEF2 transcription factor, hMEF2D, is an early marker of the myogenic lineage

The transition from multipotent mesodermal precursor to committed myoblast and its differentiation into a mature myocyte involve molecular events that enable the cell to activate muscle-specific genes. Among the participants in this process is the myocyte-specific enhancer factor 2 (MEF2) family of tissue-restricted transcription factors. These factors, which share a highly conserved DNA-binding domain including a MADS box, are essential for the expression of multiple muscle genes with cognate target MEF2 sites in cis. We report here a new human MEF2 factor, hMEF2D, which is unique among the members of this family in that it is present not only in myotubes but also in undifferentiated myoblasts, even before the appearance of myogenin. hMEF2D comprises several alternatively spliced products of a single gene, one of which is the human homolog of the Xenopus SRF-related factor SL-1. Like its relatives, cloned hMEF2D is capable of activating transcription via sequence-specific binding to the MEF2 site, recapitulating endogenous tissue-specific MEF2 activity. Indeed, while MEF2D mRNAs are ubiquitous, the protein is highly restricted to those cell types that contain this activity, implicating posttranscriptional mechanisms in the regulation of MEF2D expression. Alternative splicing may be important in this process: two alternative MEF2D domains, at least one of which is specifically included during myogenic differentiation, also correlate precisely with endogenous MEF2 activity. These findings provide compelling evidence that MEF2D is an integral link in the regulatory network for muscle gene expression. Its presence in undifferentiated myoblasts further suggests that it may be a mediator of commitment in the myogenic lineage.

Prednisolone enhances myogenesis and dystrophin-related protein in skeletal muscle cell cultures from mdx mouse

The differentiation of skeletal muscle cells from mdx mice which lack dystrophin expression was examined after glucocorticoid treatment, namely alpha-methylprednisolone (PDN). Primary skeletal muscle cell cultures were established from newborn mdx, congenic C57BL/10, and allogenic BALB/C mice. We show that PDN promotes the myogenesis of both mdx- and control mice-derived cultures as determined by 1) the number of myotubes, 2) acetylcholine receptors, and 3) dystrophin and dystrophin-related protein levels. These results support the hypothesis that PDN could enhance the myogenesis of satellite cells and increase dystrophin-related protein expression in DMD treated patients.

Expression of the rat brain creatine kinase gene in C6 glioma cells

We have recently shown that while brain creatine kinase (CKB) mRNA was detectable in RNA from cultured primary rat brain neurons, CKB mRNA was about 15-fold higher in primary astrocytes and 17-fold higher in oligodendrocytes (Molloy et al., J Neurochem 59:1925-1932, 1992). To begin to understand the molecular mechanisms responsible for brain glial cells containing the highest levels of CKB mRNA in the body, we have examined the expression of rat CKB mRNA in established C6 glioma cells. RNase-protection analysis showed the endogenous CKB mRNA levels in exponentially growing C6 were high and measured 50% of that in total RNA from rat brain lysate and 60% of that in cultured primary astrocytes and oligodendrocytes. The 5' and 3' ends of CKB mRNA in C6 were mapped to the same nucleotides as CKB mRNA from rat brain, indicating that the sites of in vivo transcription initiation and termination/polyadenylation of CKB mRNA in C6 are the same as in total rat brain RNA. The level of CKB enzyme activity in C6 whole cell lysates was among the highest of the glial cell lines which we measured. All creatine kinase enzyme activity present in C6 was found in the dimeric CKB isoform (BB), which is characteristic of CKB expression in the brain. A 2.9 kb gene fragment containing the basal CKB promoter and far-upstream 5' sequences was cloned upstream of the chloramphenicol acetyltransferase (CAT) gene and transfected into C6 cells. CAT activity was readily detectable in C6 and mapping of the 5' end of the CAT mRNA showed that transcription was directed from the correct initiation site. Since we found C6 cells were difficult to transfect, conditions were established which both maximized transfection efficiency and maintained normal C6 cell morphology. These results should permit the future identification of the nuclear trans-acting factors and the cognate cis-acting regulatory elements responsible for high CKB mRNA expression in brain glial cells.

Mechanism of tissue-specific transcription: interplay between positive and negative regulatory factors

At least four regulatory cis-acting DNA sequences, CCAAAAGTGG (element A), TTATTTTTA (element B), TATTTATT (element C), and TATTACCTTTAT (element S), were identified in cardiac myosin light chain-2 (MLC2) proximal promoter as target sites for sequence-specific binding of nuclear proteins. For muscle-specific transcription, the proximal promoter (-53 to +1) consisting only of elements B and C is required. Addition of element A to this promoter results in a muscle-specific up-regulation, whereas the addition of element S exerts a negative effect on transcription. The negative and positive regulatory effects of elements S and A respectively were demonstrated by site-specific mutations of the promoter following transient transfection of cardiac muscle cells in culture. Elements S and A interact separately with distinct nuclear protein factor present in both muscle and non-muscle cells, even though their regulatory activities are restricted to muscle cells. Among the multiple complexes resulting from the interaction of nuclear proteins and elements S and A DNAs, one requires both S and A sequences together for binding. Element B, which exerts a muscle-specific positive effect on transcription, binds to a nuclear protein present in cardiac muscle, but not in non-muscle cells. DNA-protein binding assays and mutational analysis of the MLC2 promoter suggest that the contribution of the functionally opposed cis-elements depends upon an interplay between the positively and negatively acting DNA-binding proteins via protein-protein interactions to mediate opposite regulatory effects on gene transcription.

Analysis of upstream regulatory regions required for the activities of two promoters of the rat aldolase A gene

Rat aldolase A gene has 2 promoters with different tissue specificities (M- and AH promoters). The M promoter is active only in adult skeletal muscle and induced during myogenesis, whereas the AH promoter is active ubiquitously in many tissues, including various cancer cells. Regulatory sequences for these promoters were investigated through assays for transient expression after introduction into myogenic and nonmyogenic cells. When M promoter-CAT fusion genes were transfected into primary cultures of chicken myoblasts, expression of CAT activity was drastically induced during myotube formation. The region comprising 202 to 85 base pairs (bp) upstream from the transcription initiation site was found to be necessary for the induction and an enhancer activity whose region includes the AT-rich recognition sequence (MEF-2 binding site). On the other hand, 2 upstream regions were found to be responsible for AH promoter activity expressed in HepG2 cells. The distal region (-280 to -260) of the promoter includes the AP1 binding sequence, whereas the proximal region (-207 to -180) contains a novel inverted repeat consisting of 22 bp but does not contain known promoter and enhancer sequences.

An upstream regulatory element of the NCAM promoter contains a binding site for homeodomains

In the present study, we have analyzed an upstream regulatory element of the neural cell adhesion molecule (NCAM) promoter which is required for full promoter activity. It contains an ATTATTA motif that resembles the core recognition sequence of homeodomain (HD) proteins of the Antennapedia (Antp) and related types. Electrophoretic mobility shift (EMSA) and DNase I footprinting analyses revealed that the Drosophila HDs coded by the Antp and the zerknüllt (zen) genes bind this site in vitro. In contrast, the engrailed (en) protein did not produce a detectable footprint. The functional relevance of the ATTATTA motif was demonstrated by showing that a two-nucleotide exchange curtailed stimulation of an heterologous promoter. An oligonucleotide known to be recognized with high affinity by Antp-like HDs efficiently competed for endogenous factor binding. These results suggest that the NCAM gene may be a target for HD proteins.

Expression of the gene encoding human brain creatine kinase depends on sequences immediately following the transcription start point

To localize sequences that are important for regulation of the gene (CK-B) encoding human brain creatine kinase (CK-B), we have functionally dissected the region comprising 1.8 kb of DNA upstream from the main transcription start point (tsp) and the first exon and intron, and made a detailed comparison with the situation in the rat CK-B gene. Upon using the transient chloramphenicol acetyltransferase (CAT) assay in human HeLa and mouse neuroblastoma cells, we have delimited the basal promoter in the human CK-B gene to a segment of 150 nucleotides (nt) immediately preceding the major mRNA cap site. No other essential regulatory sequence is located further upstream. Both from tsp mapping and from mutational inactivation studies, we conclude that of the two T + A-rich motifs in the promoter region, the TTAA motif between nt positions -28 to -25 is of major importance for transcriptional activity. Moreover, and most notably, a region spanning 22 nt of the first exon has a strong stimulatory effect on CK-B/CAT synthesis.