Inhibition of transcription factor GATA-4 expression blocks in vitro cardiac muscle differentiation

Commitment of mesodermal cells to the cardiac lineage is a very early event that occurs during gastrulation, and differentiation of cardiac muscle cells begins in the presomite stage prior to formation of the beating heart tube. However, the molecular events, including gene products that are required for differentiation of cardiac muscle cells, remain essentially unknown. GATA-4 is a recently characterized cardiac muscle-restricted transcription factor whose properties suggest an important regulatory role in heart development. We tested the role of GATA-4 in cardiac differentiation, using the pluripotent P19 embryonal carcinoma cells, which can be differentiated into beating cardiac muscle cells. In this system, GATA-4 transcripts and protein are restricted to cells committed to the cardiac lineage, and induction of GATA-4 precedes expression of cardiac marker genes and appearance of beating cells. Inhibition of GATA-4 expression by antisense transcripts blocks development of beating cardiac muscle cells and interferes with expression of cardiac muscle markers. These data indicate that GATA-4 is necessary for development of cardiac muscle cells and identify for the first time a tissue-specific transcription factor that may be crucial for early steps of mammalian cardiogenesis.

Spatial regulation of SpMTA metallothionein gene expression in sea urchin embryos by a regulatory cassette in intron 1

The SpMTA metallothionein (MT) gene of the sea urchin Strongylocentrotus purpuratus is restricted in its expression to the aboral ectoderm in gastrulae and pluteus larvae. The proximal 1.6 kb of the 5'-flanking region together with the 1.12-kb first intron of the SpMTA gene are sufficient for its correct cell-type specific expression in transgenic embryos. This restricted spatial expression is largely eliminated by deletion of an interior 405-bp region in the intron. Within this region is a 295-bp, genomically repetitive, transposon-like segment (Nemer et al., 1993), containing several sequence motifs highly homologous to posited regulatory elements in the promoters of other genes (Thiebaud et al., 1990). The P3A and P5 sites in this apparent regulatory cassette were shown through competition to bind with relatively high affinities the same nuclear factors, bound by their counterpart sites in the CyIIIa actin promoter.

Localization of the human B-type natriuretic peptide precursor (NPPB) gene to chromosome 1p36

Cardiac myocytes synthesize and secrete a family of peptide hormones with potent natriuretic, diuretic, and vasodilatory properties. These peptides are derived from precursor molecules that are encoded by two different genes, the atrial natriuretic peptide precursor A (NPPA) and the B-type natriuretic peptide or natriuretic peptide precursor B (NPPB). A human genomic clone for the NPPB gene was used to determine the chromosomal location of the NPPB gene. Analysis of Southern blot hybridization to DNAs from various somatic cell hybrids and fluorescence in situ hybridization allowed assignment of the NPPB locus to human chromosome 1p36. This location coincided with that of the NPPA locus; pulsed-field gel electrophoresis placed NPPA and NPPB within 50 kb of each other. This close chromosomal linkage, together with the conserved primary sequences and structural organization of the two natriuretic peptide precursor genes, suggests that the natriuretic peptide loci may have evolved from a common ancestor gene.

Transcription of brain natriuretic peptide and atrial natriuretic peptide genes in human tissues

We have compared the expression of atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) genes in various human tissues using a quantitative polymerase chain reaction technique. Tissues of three human subjects, obtained at autopsy, were analyzed. BNP transcripts could be detected in the central nervous system, lung, thyroid, adrenal, kidney, spleen, small intestine, ovary, uterus, and striated muscle. ANP transcripts could also be demonstrated in various human extracardiac tissues including several endocrine organs. In all peripheral tissues, the level of both natriuretic peptide transcripts was approximately 1-2 orders of magnitude lower than in cardiac ventricular tissues. This distribution is in marked contrast to the much lower level of ANP and BNP transcripts present in extracardiac rat tissues (generally less than 1/1000 of ventricles). These data suggest differential expression of the two natriuretic peptide genes in cardiac and extracardiac tissues in man. Furthermore, the presence of local synthesis of ANP and BNP in various peripheral organs suggests paracrine and/or autocrine function of these natriuretic peptides.

A hormone-encoding gene identifies a pathway for cardiac but not skeletal muscle gene transcription

In contrast to skeletal muscle, the mechanisms responsible for activation and maintenance of tissue-specific transcription in cardiac muscle remain poorly understood. A family of hormone-encoding genes is expressed in a highly specific manner in cardiac but not skeletal myocytes. This includes the A- and B-type natriuretic peptide (ANP and BNP) genes, which encode peptide hormones with crucial roles in the regulation of blood volume and pressure. Since these genes are markers of cardiac cells, we have used them to probe the mechanisms for cardiac muscle-specific transcription. Cloning and functional analysis of the rat BNP upstream sequences revealed unexpected structural resemblance to erythroid but not to muscle-specific promoters and enhancers, including a requirement for regulatory elements containing GATA motifs. A cDNA clone corresponding to a member of the GATA family of transcription factors was isolated from a cardiomyocyte cDNA library. Transcription of this GATA gene is restricted mostly to the heart and is undetectable in skeletal muscle. Within the heart, GATA transcripts are localized in ANP- and BNP-expressing myocytes, and forced expression of the GATA protein in heterologous cells markedly activates transcription from the natural cardiac muscle-specific ANP and BNP promoters. This GATA-dependent pathway defines the first mechanism for cardiac muscle-specific transcription. Moreover, the present findings reveal striking similarities between the mechanisms controlling gene expression in hematopoietic and cardiac cells and may have important implications for studies of cardiogenesis.

Modulation of gene expression by calreticulin binding to the glucocorticoid receptor

Calreticulin is a multifunctional protein that acts as a major Ca(2+)-binding (storage) protein in the lumen of the endoplasmic reticulum. It is also found in the nucleus, suggesting that it may have a role in transcription regulation. Calreticulin has been reported to bind to the synthetic peptide KLGFFKR, which is almost identical to an amino-acid sequence in the DNA-binding domain of the superfamily of nuclear receptors. Could calreticulin interact with the DNA-binding domain of these receptors and affect their function? Here we report that the amino terminus of calreticulin interacts with the DNA-binding domain of the glucocorticoid receptor and prevents the receptor from binding to its specific glucocorticoid response element. Overexpression of calreticulin in mouse L fibroblasts inhibits glucocorticoid-response-mediated transcriptional activation of a glucocorticoid-sensitive reporter gene and of the endogenous, glucocorticoid-sensitive gene encoding cytochrome P450. Together these results indicate that calreticulin may be important in gene transcription, regulating the glucocorticoid receptor and perhaps other members of the super-family of nuclear receptors.

Developmental stage-specific regulation of atrial natriuretic factor gene transcription in cardiac cells

Cardiac myocytes undergo a major genetic switch within the first week of postnatal development, when cell division ceases terminally and many cardiac genes are either activated or silenced. We have developed stage-specific cardiocyte cultures to analyze transcriptional control of the rat atrial natriuretic factor (ANF) gene to identify the mechanisms underlying tissue-specific and developmental regulation of this gene in the heart. The first 700 bp of ANF flanking sequences was sufficient for cardiac muscle- and stage-specific expression in both atrial and ventricular myocytes, and a cardiac muscle-specific enhancer was localized between -136 and -700 bp. Deletion of this enhancer markedly reduced promoter activity in cardiac myocytes and derepressed ANF promoter activity in nonexpressing cells. Two distinct domains of the enhancer appeared to contribute differentially to cardiac specificity depending on the differentiation stage of the myocytes. DNase I footprinting of the enhancer domain active in differentiated cells revealed four putative regulatory elements including an A+T-rich region and a CArG element. Deletion mutagenesis and promoter reconstitution assays revealed an important role for the CArG-containing element exclusively in cardiac cells, where its activity was switched on in differentiated myocytes. Transcriptional activity of the ANF-CArG box correlated with the presence of a cardiac- and stage-specific DNA-binding complex which was not recognized by the c-fos serum response element. Thus, the use of this in vitro model system representing stage-specific cardiac development unraveled the presence of different regulatory mechanisms for transcription of the ANF gene during cardiac differentiation and may be useful for studying the regulatory pathways of other genes that undergo switching during cardiac myogenesis.

A nuclear pathway for alpha 1-adrenergic receptor signaling in cardiac cells

alpha 1-Adrenergic agonists and antagonists constitute an important class of therapeutic agents commonly used for the treatment of various cardiovascular diseases like hypertension, congestive heart failure and supraventricular tachycardia. At the heart level, activation of alpha 1-adrenergic receptors is associated with marked morphological and genetic changes. These include enhancement of contractility, myocardial growth (hypertrophy) and release of the heart major secretory product, atrial natriuretic factor (ANF). However, the signal transduction pathways which link extracellular activation of the receptors to cellular and genetic changes are not well understood. Using primary cardiocyte cultures from neonate rat hearts, an alpha 1-adrenergic regulatory sequence has been identified in the 5' flanking region of the ANF gene. This sequence, which is necessary and sufficient for transcriptional activation in response to the alpha 1-specific agonist phenylephrine, interacts with novel zinc-dependent proteins which are induced by alpha 1-adrenergic stimulation. Consistent with a conserved regulatory mechanism, the alpha 1 response element is highly conserved between rodent, bovine and human ANF genes, and is also present in the promoter region of other alpha 1-responsive cardiac genes. The identification of a nuclear pathway for alpha 1-receptor signaling will be useful for elucidating the intracellular effectors of alpha 1-adrenergic receptors.

Highly identical cassettes of gene regulatory elements, genomically repetitive and present in RNA

A region in the first intron of a metallothionein-encoding gene of the sea urchin Strongylocentrotus purpuratus (SpMTA gene) regulates its 5' promoter activity. Within this region is a 290-bp cassette of six sequence motifs that are present in other genes in this species and posited to operate as regulatory elements. The cassette, present at high multiplicity in the genome, was used to screen genomic DNA clones. Of these, six diverse individuals were partially sequenced and found to have segments 94% identical to the 290-bp cassette in the SpMTA gene. Their next 80 bp diverged from the SpMTA sequence but were highly identical among the six non-SpMTA clones and contained an additional regulatory motif. These diverse clones thus contained 370-bp cassettes of an overall 94% sequence identity and an apparent content of seven regulatory elements. The regulatory cassettes were transposon-like, insofar as the termini of the highly identical regions consisted of 24- to 25-bp inverted repeats, bracketed by 6- to 9-bp direct repeats in the divergent regions. In addition to being in transcripts of the SpMTA intron, the cassette was found in other sea urchin embryo poly(A)+ RNAs, in eggs and embryos, and enriched in pluteus ectoderm. The cassette sequence was present in moderate abundance in transcripts in both sense and antisense orientation. We report the presence of a transposon-like cassette of regulatory elements that is also represented in RNA, which potentially could function differently from previously described transposons.

Detection of C-type natriuretic peptide (CNP) transcript in the rat heart and immune organs

Previous studies suggested the expression of mRNA, coding for CNP, exclusively in the central nervous system. In the present study, using the polymerase chain reaction (PCR) technique instead of the less sensitive Northern blot hybridization, CNP-specific sequences have also been detected in rat atria and ventricles of the heart as well as in organs of the immune system (thymus, spleen and lymph nodes). Parallel PCR-assays documented ANP-mRNA in these tissues. To verify specificity of the PCR-products, Southern blots have been hybridized with a third internal oligonucleotide and amplification products have been sequenced. The relative level of CNP-mRNA in these tissues was estimated to be in the range of 1-9% of total brain CNP transcripts. The results suggest that the peptide may have a peripheral as well as a central site of action. In light of its pronounced effect on cell proliferation, particular interest should focus on a possible role of CNP in the immune system.

The atrial natriuretic peptide system in rat ovaries

The atrial natriuretic peptide (ANP) gene is expressed in several extracardiac tissues where ANP is thought to be involved in autocrine or paracrine regulation. The current studies were designed to characterize the ANP system in rat ovaries. ANP content in rat ovaries was estimated by RIA to be 240 +/- 70 pg/mg protein. HPLC revealed the presence of the 28-amino acid circulating peptide as well as the 126-amino acid prohormone, suggesting that the ovaries are a site of ANP synthesis. Indeed, ANP messenger RNA was detected in this tissue by RNase mapping. ANP present in ovarian extracts displaced [125I]ANP from bovine adrenal receptors (R1 class) in a dose-dependent manner and in parallel to the synthetic peptide, indicating that it possesses biological activity. Immunocytochemical studies localized ANP to interstitial cells surrounding the follicles; weaker but specific staining was also observed in the ovum. High affinity ANP receptors (dissociation constant, 0.30 +/- 0.06 nM; maximum binding capacity, 160 +/- 40 fmol/mg protein) were identified in ovarian membranes. Unlabeled ANP but not c-atrial natriuretic factor (a specific agonist of ANP clearance receptors) competed with binding of [125I]ANP to ovarian membranes in a dose-dependent manner, suggesting that ovarian ANP receptors are predominantly of the R1 class. This was confirmed by cross-linking studies with [125I]ANP, which detected a single protein band with a molecular size of about 120 kilodaltons, corresponding to that of the guanylate cyclase-coupled R1 class of receptor. Consistent with the presence of biologically active receptors, ANP markedly enhanced cGMP accumulation (by 15-fold) in ovarian cells. The presence of both local ANP synthesis and high affinity transducing receptors in the ovaries indicates that the peptide plays a local role in ovarian growth or steroidogenesis.

Combinatorial regulation by promoter and intron 1 regions of the metallothionein SpMTA gene in the sea urchin embryo

The SpMTA metallothionein gene of the sea urchin Strongylocentrotus purpuratus is regulated developmentally, histospecifically, and by heavy-metal induction. The sequenced 5' flank of the gene can be divided into proximal, middle, and distal regions, each containing a pair of metal response elements (MREs). Canonical 7-bp core sequences are present in all except the middle-region MREs c and d, which contain 1-bp mismatches. Metal-induced expression in transgenic blastulae was increased with each consecutive addition of the middle and distal regions to a chimeric reporter gene construct containing the proximal SpMTA promoter region. Reduced metal induction through point mutation of the distal MREs e and f indicated that the MREs themselves were largely responsible for the transcriptional increase. These activities were further enhanced by SpMTA intron 1, but not when a specific interior region of the intron had been deleted. The atypical MREs c and d did not support induction by themselves, i.e., when present alone with mutated proximal MREs a and b. However, in the presence of intron 1, they were able to substitute for the nullified MREs a and b in the promotion of metal-induced expression. This capability suggests, furthermore, that these atypical MREs, in addition to responding to an intron 1 region, participate cooperatively with the canonical proximal MREs.

fos/jun repression of cardiac-specific transcription in quiescent and growth-stimulated myocytes is targeted at a tissue-specific cis element

Unlike that of skeletal muscle cells in which growth and differentiation appear mutually exclusive, growth stimulation of cardiac cells is characterized by transient expression of early response nuclear proto-oncogenes as well as induction of several cardiac-specific markers. This observation led to the speculation that these proto-oncogenes, particularly c-fos and c-jun, might act as positive regulators of cardiac transcription. We have examined the role of c-jun and c-fos in basal and growth-stimulated cardiac transcription, using the cardiac-specific atrial natriuretic factor (ANF) gene as a marker. The results indicate that c-jun and c-fos are negative regulators of ANF transcription. Inducers of jun and fos activity, such as mitogens and growth factors, inhibited endogenous ANF transcripts. In transient cotransfection assays, jun and fos were able to trans-repress the ANF promoter in both quiescent and alpha 1-adrenergic stimulated myocytes. This repression was specific to myocyte cultures and was not observed in nonmuscle cells. Deletion analysis indicated that repression does not require typical AP-1-binding sites (tetradecanoyl phorbol acetate response elements) or serum response elements but is targeted at a cardiac-specific element within the ANF promoter. Various Fos-related proteins, including Fra-1, Fos B, and v-Fos, were able to trans-repress ANF transcription. In addition, C-terminal c-fos mutants which no longer repress transcription of such early growth response genes as c-fos and EGR-1 retained the ability to repress ANF transcription. Repression by c-jun occurs via the N-terminal activation domain and does not require the DNA-binding domain, suggesting that proto-oncogene repression involves interaction with one or more limiting cardiac-specific coactivators.

Novel glucocorticoid receptor complex with DNA element of the hormone-repressed POMC gene

Previous studies defined a DNA element necessary for glucocorticoid repression of the pro-opiomelanocortin (POMC) gene. The glucocorticoid receptor (GR) binds this negative glucocorticoid response element (nGRE) with an in vitro affinity similar to that of GR for positive GREs. However, whereas GR binds GREs as homodimers, a novel GR complex which forms with nGRE appears to contain three GR molecules. Biochemical characterization of this complex as well as equilibrium binding studies suggest that it is formed by sequential binding of a GR homodimer followed by binding of a GR monomer on the opposite side of the double helix. The DNA-binding domain (DBD) of GR is sufficient for differential binding of GRE and nGRE, as bacterially-expressed DBD formed unique nGRE complexes that contain three GR polypeptides. Thus, the POMC nGRE provides the first example of an interaction between GR and DNA in which GR binds otherwise than as a homodimer. Despite its high affinity for GR, the nGRE differs significantly from GREs in that it does not activate transcription in any context. As the nGRE appears insufficient on its own to confer hormone responsiveness, other POMC promoter elements are likely to be required to mediate glucocorticoid repression.

Increased transcripts for B-type natriuretic peptide in spontaneously hypertensive rats. Quantitative polymerase chain reaction for atrial and brain natriuretic peptide transcripts

The cardiac natriuretic peptide family includes atrial natriuretic factor and brain or B-type natriuretic peptide, also known as iso-atrial natriuretic factor (isoANF). Although these peptides contribute to cardiovascular homeostasis, their respective roles remain unclear. To study regulation of atrial natriuretic factor and isoANF gene expression during progression of hypertension, we developed a quantitative polymerase chain reaction protocol to measure their transcript level in spontaneously hypertensive rat (SHR) hearts. At the onset of hypertension, atrial natriuretic factor transcripts in 5-week-old SHR were 50% of those of age-matched Wistar-Kyoto (WKY) rats, whereas the level of isoANF transcripts was similar in atria and twofold higher in ventricles. Because atria are the major sites of atrial natriuretic factor gene expression and ventricles contribute predominantly to cardiac isoANF synthesis, total atrial natriuretic factor messenger RNA (mRNA) in the hearts of 5-week-old SHR was about 50% of that in WKY rats, and total isoANF mRNA content was already higher than in control rats. In left ventricles and ventricular septa, progression of hypertension led to a maximal increase of twofold and fourfold in atrial natriuretic factor and isoANF mRNA levels, respectively, with no detectable change in right ventricles. In the atria of older SHR, atrial natriuretic factor and isoANF mRNA levels were comparable to those of age-matched controls. These data indicate that, although increased blood pressure stimulates both atrial natriuretic factor and isoANF gene expression, regulation of the two natriuretic peptide genes is not temporally coordinated in all cardiac compartments. Furthermore, isoANF mRNA is already induced in the ventricles at the onset of the hypertensive stage, and in older SHR, the isoANF gene is hyperresponsive to progression of hypertension compared with atrial natriuretic factor. Thus, isoANF might represent a very sensitive marker of cardiac changes in hypertension.

Homodimer formation is rate-limiting for high affinity DNA binding by glucocorticoid receptor

The glucocorticoid receptor (GR) is a hormone-inducible transcription factor which activates transcription of specific genes by binding to a DNA sequence present in the promoters of inducible genes. These glucocorticoid response elements (GREs) have a conserved palindromic sequence. Each half-GRE palindrome binds one subunit of GR. We have assessed the relative affinity of GR monomers and homodimers for GRE and determined whether homodimer formation is rate-limiting for high affinity GRE binding. The in vitro affinity of GRE binding by GR homodimers was approximately 2 x 10(-10) M, whereas it was approximately 1 nM for GR monomers. While homodimer:GRE complexes were very stable, monomer:GRE complexes appeared less stable in vitro. At low receptor concentration, GR preferentially bound GRE as a homodimer. Prior dilution of GR (equilibrium shifted to monomers) before addition to a GRE binding reaction resulted in slower kinetics of binding by comparison to parallel reactions in which concentrated (largely homodimeric) GR was added first. Taken together, these experiments suggest that homodimer formation is rate-limiting for high affinity GRE binding. A GRE mutant which contained only a half-binding site and which was unable to bind GR homodimers was also unable to confer glucocorticoid-inducible transcription. Taken together with previous work, these experiments support the model that GR homodimers are required for hormone-dependent activation of transcription and that receptor homodimer formation is rate-limiting for GRE binding.

Distal cis-acting promoter sequences mediate glucocorticoid stimulation of cardiac atrial natriuretic factor gene transcription

Although receptors for most steroid hormones are present in the heart, few cardiac-specific target genes have been identified and studied at the molecular level. Transcription of the atrial natriuretic factor (ANF) gene, which encodes the major secretory product of the heart, is induced by glucocorticoids. In both atrial and ventricular cardiac cells in primary cultures, ANF mRNA levels are increased 3-4-fold after dexamethasone treatment in a time- and dose-dependent manner. This response to glucocorticoids is completely abolished by the antagonist RU486. Interestingly, ventricular myocytes appear to be more sensitive to glucocorticoids than atrial myocytes. DNA-mediated gene transfer studies indicate that glucocorticoids affect ANF gene transcription via a glucocorticoid response element located in the distal 5'-flanking sequences of the rat ANF gene between -697 and -1,029 base pairs. In vitro DNase I footprinting experiments reveal the presence of two binding sites for purified glucocorticoid receptor within this region. Mobility shift assays and competition experiments show that binding of the glucocorticoid receptor to both ANF sites results in a DNA-protein complex similar in affinity and specificity to that of the well characterized mammary tumor virus glucocorticoid response element. Since glucocorticoid activation of the ANF promoter appears specific to cardiac cells, the interaction between the glucocorticoid receptor binding sites and cardiac-specific regulatory elements of this promoter could provide a model to study a mechanism of hormone-dependent signal transduction in the heart.

Evidence for atrial natriuretic peptide (ANP) synthesis and the presence of ANP-transducing receptors in the rat olfactory bulb

This study demonstrates the presence of both atrial natriuretic peptide (ANP) precursor and ANP transcripts in the rat olfactory bulb (OB), a key brain structure involved in the generation of olfaction-dependent behavior. In addition to synthesizing ANP, the OB contains ANP-transducing receptors coupled to the guanylate cyclase system but it is devoid of ANP "clearance receptors." The characterization of biologically active ANP receptors and the evidence for in situ ANP synthesis in this region of the CNS adds credence to the hypothesis that the peptide plays a putative role in olfaction.

Differential expression of natriuretic peptide genes in cardiac and extracardiac tissues

Cardiac myocytes secrete a family of natriuretic and diuretic peptides, including atrial natriuretic factor (ANF) and brain natriuretic peptide or the rat hormone isoANF. These peptides share structural and functional similarities, but their respective physiological roles have yet to be elucidated. Differential expression of natriuretic peptide genes may reflect distinct physiological or pathophysiological functions for these peptides. To test this hypothesis, we have determined the sites of expression of the ANF and isoANF genes in rat tissues using polymerase chain reaction amplification of ANF and isoANF transcripts. Like ANF mRNA, isoANF mRNA was detected in all heart compartments, and the transcription initiation sites of the isoANF gene, determined from primer extension experiments, were identical in atria and ventricles. In the adult heart, the ventricular isoANF mRNA concentration is only 3 times lower than in atria, and in sharp contrast to ANF, the isoANF gene is constitutively expressed in ventricles during postnatal development. Since ventricles are at least 20 times larger than atria, this implies that isoANF is mostly a ventricular hormone, whereas ANF is essentially an atrial hormone in normal adult hearts. Low levels of isoANF mRNA were found in few extracardiac tissues, including hypothalamus, brain, lung, and aorta. IsoANF transcripts were more abundant than ANF transcripts in aorta, whereas ANF and isoANF mRNA levels were similar in lung. In brain and hypothalamus, ANF transcripts were only 7- and 2-fold greater than isoANF transcripts. The presence of isoANF transcripts in brain and hypothalamus suggests that isoANF is the rat homolog of the human hormone brain natriuretic peptide. Thus, the expression of these two natriuretic peptide genes is not coordinated, suggesting that ANF and isoANF may play different physiological roles in cardiac and extracardiac tissues.

Polyubiquitin RNA characteristics and conditional induction in sea urchin embryos

A cDNA of the sea urchin Strongylocentrotus purpuratus was identified as encoding polyubiquitin and used to detect a single gene with transcripts containing multiple ubiquitin coding units. Polyubiquitin transcripts exist as a 3.2-kb RNA in polyribosomes and as three higher molecular weight RNAs in purified nuclei. The amount of polyubiquitin RNA is essentially constant at 10(4) -10(5) transcripts per embryo during the egg-to-blastula period and then declines during further development. Heat shock elicits a transient increase in the level of polyubiquitin RNA, while Zn(II) ions induce a sustained accumulation, that is influenced by developmental parameters: One round of Zn(II) induction elicits the accumulation of the nuclear 7.6- and 5.6-kb RNAs, as well as the 3.2-kb polysomal RNA; however, a second round of induction yields only the 5.6- and 3.2-kb RNAs, suggestive of a change in pre-mRNA size or processing. Polyubiquitin RNA is expressed equally in ectodermal and mesoendodermal tissues and is induced in both tissue fractions by treatment of pluteus larvae with Zn(II). However, in isolated and cultured tissue fractions, polyubiquitin RNA is not inducible by Zn(II), in contrast to the full inducibility of metallothionein mRNAs. Polyubiquitin RNA induction thus appears to be conditioned by the integrity of the embryo, as well as by previous exposure to inducer.

Structure, spatial, and temporal expression of two sea urchin metallothionein genes, SpMTB1 and SpMTA

The metallothionein-B genes of the sea urchin Strongylocentrotus purpuratus encode a metallothionein (MT) isoform distinguishable from the MTA isoform. The MTB subfamily consists of at least two genes, MTB1 and MTB2, and possibly two to three others. The unique MTB1 and MTA genes have a high degree of identity but diverge in structural detail and expression. Transcripts of the MTA, MTB1, troponin C Spec 1, and CyIIIa actin genes begin simultaneously to accumulate at an early blastula stage. MTB1 mRNA becomes localized in the embryonic gut and oral ectoderm, whereas MTA, Spec 1, and CyIIIa actin mRNAs are spatially restricted to the aboral ectoderm. Several DNA elements are localized at the same positions in the MTB1 and MTA genes: these include respective CATA and TATA boxes, two metal response elements, and three distinct upstream DNA elements that are also present, and in the same order, in the Spec 1 gene promoter. A heptameric sequence, element A, is present at two sites each in the Spec 1 and CyIIIa actin genes, five sites in MTA, but only one site in MTB1. Most strikingly, the first intron of MTA contains elements not found in the MTB1 introns, including a consensus metal response element, an element A, and the P3A site demonstrated in the CyIIIa actin gene to be linked to the regulation of spatial expression.

Effect of prolonged high salt diet on atrial natriuretic factor in rats

Normotensive Sprague-Dawley rats were given 8% NaCl for 5 weeks. This salt load did not affect their blood pressure nor hematocrit, and plasma atrial natriuretic factor (ANF) showed no change at 3 weeks but decreased after 5 weeks of the experimental period when compared with control rats. The responsiveness of particulate guanylate cyclase and formation of cGMP in ANF target organs suggested an augmented baseline activity of the cGMP system but its relative hyporesponsiveness to exogenous ANF following prolonged salt loading. Decreased plasma ANF levels cannot be explained by its altered production since atrial levels of the peptide were comparable in rats with or without salt loading. Atrial ANF mRNA was unaffected by the salt regimen. This study demonstrates that plasma ANF does not increase during long-term NaCl loading and even decreases after 5 weeks of 8% NaCl. The changes in plasma ANF are associated with changes in the functional state of ANF receptors coupled to particulate guanylate cyclase, but in the opposite direction than expected from lowered plasma ANF. Thus, ANF may not play a significant role in the regulation of sodium excretion in response to prolonged high salt consumption or, if it does, it is not reflected by expected changes in its plasma levels.

Regulatory elements of the pro-opiomelanocortin gene: pituitary specificity and glucocorticoid repression

A short 543-bp fragment o f the pro-opiomelanocortin gene is sufficient for pituitary-specific expression and, in the anterior pituitary gland, for repression of pro-opiomelanocortin transcription by glucocorticoids. Within this 5'-flanking fragment of the gene, multiple regulatory elements contribute to tissue-specific expression and a single glucocorticoid receptor binding site acts as a "negative glucocorticoid response element." The current model for glucocorticoid repression depends on the mutually exclusive binding of the glucocorticoid receptor and of a positive transcription factor, the chicken ovalbumin upstream promoter element transcription factor, to overlapping DNA sequences within the negative glucocorticoid response element.

Glucocorticoid receptor binding to a specific DNA sequence is required for hormone-dependent repression of pro-opiomelanocortin gene transcription

Glucocorticoids rapidly and specifically inhibit transcription of the pro-opiomelanocortin (POMC) gene in the anterior pituitary, thus offering a model for studying negative control of transcription in mammals. We have defined an element within the rat POMC gene 5'-flanking region that is required for glucocorticoid inhibition of POMC gene transcription in POMC-expressing pituitary tumor cells (AtT-20). This element contains an in vitro binding site for purified glucocorticoid receptor. Site-directed mutagenesis revealed that binding of the receptor to this site located at position base pair -63 is essential for glucocorticoid repression of transcription. Although related to the well-defined glucocorticoid response element (GRE) found in glucocorticoid-inducible genes, the DNA sequence of the POMC negative glucocorticoid response element (nGRE) differs significantly from the GRE consensus; this sequence divergence may result in different receptor-DNA interactions and may account at least in part for the opposite transcriptional properties of these elements. Hormone-dependent repression of POMC gene transcription may be due to binding of the receptor over a positive regulatory element of the promoter. Thus, repression may result from mutually exclusive binding of two DNA-binding proteins to overlapping DNA sequences.

Structure of an ectodermally expressed sea urchin metallothionein gene and characterization of its metal-responsive region

The metallothionein-A gene in the metallothionein gene family of the sea urchin Strongylocentrotus purpuratus (SpMTA gene) was sequenced and found to contain three coding exons plus a 3' entirely noncoding exon. Putative alpha and beta MT domains were encoded, by its exons 2 and 3, respectively, in reverse of the order in vertebrate metallothionein genes. The SpMTA promoter was characterized through the expression of recombinant constructs containing various portions of the proximal 678-base-pair (bp) 5'-flanking region of the SpMTA gene. Zygotes injected with constructs were cultured to the blastula stage in the presence of a heavy-metal chelator and then incubated in the presence or absence of cadmium. The longest constructs were expressed only when heavy-metal ion was present. Two putative metal-responsive elements (MREs a and b) within 240 bp of the transcription start site resembled mammalian MREs in their critical 8-bp cores (TGCRCNCS) and in their locations relative to each other and to the TATA box. Elimination of activity by site-specific mutations in MREs a and b, separately or in both, identified them as metal regulatory elements. Thus, MRE recognition in this invertebrate resembles that in vertebrates. Upstream sites with single-mismatched MREs neither acted as MREs nor amplified the activity of MREs a and b. The SpMTA, Spec1, and CyIIIa actin genes, which have the same ectodermal specificity, have common DNA elements at relatively similar locations in their promoter regions; however, these elements are insufficient in themselves to promote gene expression.

Ventricles as a major site of atrial natriuretic factor synthesis and release in cardiomyopathic hamsters with heart failure

The aim of the present study was to correlate in cardiomyopathic hamsters with congestive heart failure the levels of atrial and ventricular atrial natriuretic factor (ANF) messenger RNA (mRNA) with immunoreactive ANF (IR-ANF) plasma levels and the relative amount of IR-ANF released by the whole heart versus isolated ventricles in the Langendorff preparation. High-performance liquid chromatography analysis of the forms of ANF present in plasma and in the Langendorff effluent of whole heart versus isolated ventricles was also performed. As previously found for cardiac IR-ANF, the levels of ANF mRNA decreased gradually in atria and increased in an analogous fashion in ventricles with the severity of congestive heart failure. Plasma IR-ANF levels (C-terminal) were more elevated in moderate than in severe congestive heart failure, as were the IR-ANF levels in the Langendorff effluent of the whole heart. On the contrary, the effluent of isolated ventricles from animals in severe heart failure yielded more IR-ANF than that from hamsters in moderate heart failure. Thus, while the isolated ventricles from controls contributed 35.8% of IR-ANF released by the whole heart, ventricles from hamsters in moderate heart failure contributed 17.5%, and those from hamsters in severe heart failure contributed 73.9%. These results indicate that atrial cardiocytes contribute more IR-ANF than their ventricular counterpart in moderate heart failure and that ventricles are a major source of plasma IR-ANF in severe heart failure. Analysis of IR-ANF from plasma and the Langendorff effluent from whole hearts and isolated ventricles revealed that the ventricles are the major source of the propeptide (and of its cleaved products) found in the circulation of cardiomyopathic hamsters. These results suggest that ANF synthesis and secretion do not increase conjointly in atria but do increase in ventricles during congestive heart failure.

Atrial natriuretic factor in the impulse-conduction system of rat cardiac ventricles

A complex network of atrial natriuretic factor-producing cells has been delineated by biochemical and morphological techniques in the rat ventricular myocardium. The chordae tendineae spuriae (CTS; false tendons) contain ANF mRNA and the ANF propeptide (Asn 1-Tyr 126) as assessed by Northern blot analysis, high-pressure liquid chromatography and immunohisto- and -cytochemistry, using three different affinity-purified antibodies: monoclonal and polyclonal antibodies against C-terminal ANF (Arg 101-Tyr 126) and polyclonal antibodies against N-terminal ANF (Asp 11-Ala 37). Two types of cells harboring ANF-containing secretory granules constitute the CTS: the majority (Purkinje type I) have ultrastructural similarities with both atrial and classical Purkinje fibers. Purkinje type-II fibers resemble working ventricular cardiocytes. Both cell types harbor a large paranuclear Golgi complex. The subendocardial Purkinje network is also made up of these two cell types. In this location, Purkinje type-I fibers form cable-like structures while Purkinje type-II fibers are either located beneath the former or abut directly on the endocardium. The latter are not separated from adjacent working ventricular cardiocytes by connective tissue septa. Coronary arteries and arterioles, as in birds, are surrounded by a cushion of Purkinje type-II fibers which blend with the surrounding myocardium. These results indicate that, in the rat, the entire intraventricular conduction system is constituted of endocrine cells producing ANF.

Lung is an important source of atrial natriuretic factor in experimental cardiomyopathy

The regulation of water and electrolyte homeostasis is multifactorial and includes the heart and kidneys as important regulatory centers. Within the heart, a recently discovered hormone, atrial natriuretic factor (ANF), has been implicated in the maintenance of water and salt balance. Primarily found in mammalian atria, ANF has been detected in low amounts in several tissues, including lungs. A disorder of the ANF system has been demonstrated in genetically cardiomyopathic hamsters, a model for human congestive cardiomyopathy. Atrial ANF gene expression and storage are decreased during development of this disease, while paradoxically, circulating levels of ANF are increased. We have hypothesized that an extracardiac source may contribute to ANF production in these pathological conditions. In this paper we provide evidence that ANF synthesis is stimulated in the lungs of hamsters during development of cardiomyopathy as revealed by increased ANF mRNA and peptide levels. Furthermore, we show that ANF synthesized in lungs is secreted and has identical chromatographic and biological properties to circulating ANF. The increased production of ANF in lungs may be physiologically important in preventing pulmonary edema. Alternatively, during cardiac dysfunction, lungs may play a compensatory role by increasing their contribution to plasma ANF levels.

Tissue-specific activity of the pro-opiomelanocortin (POMC) gene and repression by glucocorticoids

The gene encoding pro-opiomelanocortin (POMC) is specifically expressed in two different cell types of the pituitary gland. We have defined the regulatory DNA sequences of the POMC gene that are responsible for this cell-specific expression. In addition, we have defined a regulatory element, located in the proximal region of the POMC promoter, that confers glucocorticoid repression in the anterior pituitary. Using DNA-mediated gene transfer into transgenic mice and tissue culture cells, the POMC regulatory sequences required for cell-specific expression and glucocorticoid repression were localized within a 543-bp fragment in the 5'-flanking region of the gene. Multiple regulatory elements that bind nuclear proteins are present within this region. In particular, a sequence that binds the glucocorticoid receptor and behaves as a "negative glucocorticoid response element" (nGRE) also binds nuclear proteins of the COUP (chicken ovalbumin upstream promoter) family of transcription factors. Thus, glucocorticoid repression of POMC transcription may result from the mutually exclusive binding of the glucocorticoid receptor and the COUP transcription factor to the POMC nGRE.

Glucocorticoid repression of pro-opiomelanocortin gene transcription

Transcription of the pro-opiomelanocortin (POMC) gene is repressed by glucocorticoids in the anterior pituitary gland. We have defined an element within the POMC promoter which is responsible for this regulatory feedback. This element, the "negative glucocorticoid response element" (nGRE), was localized in the proximal region of the POMC promoter and it contains a binding site for the glucocorticoid receptor. Receptor binding to the nGRE was correlated to hormone-dependent repression by using promoter mutagenesis. The nGRE was also shown to contain a binding site for a nuclear protein of the COUP family of transcription factors. Since the binding sites for COUP and the glucocorticoid receptor overlap, glucocorticoid-dependent repression of POMC transcription may result from mutually exclusive binding of these two nuclear transcription factors.

Atrial natriuretic factor in sympathetic ganglia of the rat: dependence on cholinergic innervation

Atrial natriuretic factor is detectable in the peripheral autonomic ganglia of the rat by radioimmunoassay and immunohistochemistry. In the present study, surgical and neurochemical methods were used to evaluate the source of this peptide in sympathetic ganglia. Decentralization of the ganglia and/or central administration of colchicine diminished the atrial natriuretic factor content in para- and prevertebral ganglia. Axotomy did not affect levels of ganglionic atrial natriuretic factor. A messenger ribonucleic acid species hybridizing with rat atrial natriuretic factor complementary deoxyribonucleic acid was not found within the total ribonucleic acid extracted from superior cervical ganglia. These results indicate a direct dependence of ganglionic atrial natriuretic factor on cholinergic innervation.

Sea-urchin RNAs displaying differences in developmental regulation and in complementarity to a collagen exon probe

Sea-urchin embryo RNAs of 9 kb and 7 kb hybridise with a collagen-coding probe. The delta Tm of the hybrids indicates a 70% sequence identity between these RNA regions. Both RNAs are localised in the pluteus endomesoderm, but accumulate over different developmental periods: the 9 kb RNA first appears in the blastula and reaches a maximum concentration during the gastrula stages, while the 7 kb RNA is first detected in the gastrula and is at maximal concentration in the pluteus larva. Animalization by transient exposure of the early stage embryo to Zn2+ alters the developmental profile of the 9 kb collagen mRNA in a way that is clearly different from responses of other mRNAs whose accumulations are initiated during the blastula stage (Nemer, M. (1986) Dev. Biol. 114, 214-224).

Atrial natriuretic factor (ANF) gene expression in the Brattleboro rat

Atrial natriuretic factor (ANF) is a 28-amino acid peptide hormone of cardiac origin. It has natriuretic, diuretic and vasorelaxant properties and inhibits several cardiovascular modulators. Because of the possible effects of arginine vasopressin (AVP) on ANF secretion, we have investigated ANF gene expression in Brattleboro rats which are genetically deficient in AVP. Our results indicate that cardiac ANF mRNA and ANF content are higher in Brattleboro rats compared to Long-Evans controls, whereas the plasma levels are similar in both groups. Typical secretory granules containing immunoreactive ANF are present in ventricular cardiocytes of Brattleboro but not of Long-Evans rats. These data suggest that ANF release may be uncoupled from its synthesis in the absence of AVP.

Synonymous nucleotide substitution rates of beta-tubulin and histone genes conform to high overall genomic rates in rodents but not in sea urchins

Sea urchin and rodent genomes have been posited to evolve rapidly as indicated by divergences in single copy nuclear DNA sequences. We have examined whether the synonymous substitution rates of three highly conserved genes, beta-tubulin, histone H4, and histone H3, adhere to these high genomic substitution rates by comparing sequences between two sea urchins, Strongylocentrotus purpuratus and Lytechinus pictus, and between rodents and humans. Whereas the rate of change between the 3' untranslated regions of the beta-tubulin cDNA of S. purpuratus (Sp-beta 1), sequenced in this study, and of L. pictus (Lp-beta 3) was consistent with the overall rate of change estimated from previous DNA hybridization results between these species, the synonymous substitution rates for the carboxyl domains of these beta-tubulins, as well as for the late histones H4 and H3, were significantly depressed. In contrast, synonymous nucleotide substitution rates between rodents and between rodent and human for the carboxyl domain proper of identical beta-tubulin isotypes and for histone H4 and H3.1 did not differ from the overall rate of change for the rodent genomes. Moreover, an analysis of paralogous human and mouse beta-tubulin sequences supported the conclusion that the synonymous substitution rates in the mouse were higher than those in the human. Differences in constraint on evolutionary change were not evident strictly from the conserved amino acid sequences and base compositions of these genes. Other constraining influences seemed more relevant to the departure of the synonymous substitution rates of the sea urchin beta-tubulin and histone coding regions from the average genomic rate.

Pro-opiomelanocortin gene: a model for negative regulation of transcription by glucocorticoids

The gene encoding pro-opiomelanocortin (POMC) offers an interesting model system to study negative control of transcription in eucaryotes. Indeed, glucocorticoid hormones specifically inhibit transcription of the POMC gene in the anterior pituitary. The POMC gene is predominantly expressed in the anterior and intermediate lobes of the pituitary. However, only anterior pituitary POMC transcription is inhibited by glucocorticoids and stimulated by corticotropin-releasing hormone (CRH). Rat POMC promoter sequences required for anterior pituitary-specific expression were localized between positions -480 and -34 base pairs (bp) by DNA-mediated gene transfer into the POMC-expressing tumor cells. AtT-20. These POMC promoter sequences also confer glucocorticoid inhibition of transcription. While two of the six in vitro binding sites for purified glucocorticoid receptor identified in the rat POMC gene are within these sequences, only one is required for glucocorticoid inhibition; this binding site is located at position -63 bp in the promoter and overlaps a putative CCAAT box sequence. The DNA sequence of the POMC -63 bp receptor binding site is homologous to receptor binding sites identified in the glucocorticoid responsive element (GRE) of glucocorticoid-inducible genes. However, DNA sequence divergencies between these sites, in particular within the conserved hexanucleotide sequence 5'-TGTYCT-3', may be involved in their opposite transcriptional activity. Alternatively, binding of the receptor in the promoter proximal region of the POMC gene may inhibit transcription by a hormone-dependent repressor mechanism.

Coordinate and selective beta-tubulin gene expression associated with cilium formation in sea urchin embryos

beta-Tubulin mRNAs associated with cilium formation in Strongylocentrotus purpurpatus sea urchin embryos are expressed selectively from a multiple gene family. The accumulations of three beta-tubulin mRNAs (beta 1, beta 2, and beta 3) are temporally coordinated with ciliogenesis during blastula development and with the regeneration of cilia after their amputation. In contrast, another beta-tubulin mRNA, beta 4, is not induced in either case. The zinc-animalized embryo with its exaggerated blastula phenotype forms longer cilia through a protracted period of ciliogenesis, in which the beta-tubulin mRNAs, principally beta 1, accumulate to higher than normal levels. The rate of beta-tubulin transcription per nucleus in the animalized embryo is greater than that of the normal embryo and is not changed through deciliation, although the tubulin mRNAs accumulate to higher levels. However, deciliation raises the beta-tubulin transcription rate in the normal embryo to that in the animalized embryo. Thus, the induction of beta-tubulin mRNA by cilium amputation is regulated transcriptionally in the normal embryo, but post-transcriptionally in the zinc-animalized embryos. Moreover, the beta-tubulin genes that are expressed in association with cilium formation appear to be induced selectively within the framework of ectodermal cell-type specificity.

Thyroid hormone stimulates rat pro-natriodilatin mRNA levels in primary cardiocyte cultures

Pro-natriodilatin (PND) is the precursor for atrial natriuretic peptide (ANP), a hormone which plays an important role in cardiovascular homeostasis. Since the effects of thyroid hormone (T3) on the cardiovascular and renal systems appear to mimic those elicited by ANP, we studied the effect of T3 on PND gene expression using rat neonatal cardiocytes in primary cultures. Treatment of cardiocytes for 48 h with T3 (5 X 10(-9) M) results in a maximal increase in PND mRNA levels; this increase is two fold in atrial and four fold in ventricular cell cultures. These results taken together with a previous report showing decreased plasma ANP in hypothyroid and increased plasma ANP in hyperthyroid rats suggest that at least some of the cardiovascular and renal effects of T3 may be mediated by a T3-dependent increase in PND gene expression.