Identification and Characterization of a Factor That Binds to Two Human Sarcomeric Actin Promoters

Endothelial-specific ablation of serum response factor causes hemorrhaging, yolk sac vascular failure, and embryonic lethality

Background

Serum response factor (SRF), a member of the MADS box family of nuclear transcription factors, plays an important role in cardiovascular development and function. Numerous studies demonstrate a central role for SRF in regulating smooth and cardiac muscle cell gene expression. Consistent with this, loss of SRF function blocks differentiation of coronary vascular smooth muscle cells from proepicardial precursors, indicating SRF is necessary for coronary vasculogenesis. The role of SRF in endothelial cell contribution during early vascular development, however, has not been addressed. To investigate this, we generated transgenic mice lacking expression of SRF in endothelial cells. Mice expressing Cre recombinase (Tie2Cre⁺) under Tie2 promoter control were bred to mice homozygous for Srf alleles containing loxP recombination sites within the Srf gene (Srf^f/f). Tie2 is a tyrosine kinase receptor expressed predominantly on endothelial cells that mediates signalling during different stages of blood vessel remodelling. Resulting embryos were harvested at specific ages for observation of physical condition and analysis of genotype.

Results

Tie2Cre^+/-Srf^f/f embryos appeared to develop normally compared to wild-type littermates until embryonic day 10.5 (E10.5). Beginning at E11.5, Tie2Cre^+/-Srf^f/f embryos exhibited cerebrovascular hemorrhaging and severely disrupted vascular networks within the yolk sac. Hemorrhaging in mutant embryos became more generalized with age, and by E14.5, most Tie2Cre^+/-Srf^f/f embryos observed were nonviable and grossly necrotic. Hearts of mutant embryos were smaller relative to overall body weight compared to wild-type littermates. Immunohistochemical analysis revealed the presence of vascular endothelial cells; however, vessels failed to undergo appropriate remodelling. Initial analysis by electron microscopy suggested a lack of appropriate cell-cell contacts between endothelial cells. Consistent with this, disrupted E-cadherin staining patterns were observed in mutant embryos.

Conclusion

These results provide the first in vivo evidence in support of a role for SRF in endothelial cell function and strongly suggest SRF is required for appropriate vascular remodelling.

Tissue-specific regulatory elements in mammalian promoters

Transcription factor-binding sites and the cis-regulatory modules they compose are central determinants of gene expression. We previously showed that binding site motifs and modules in proximal promoters can be used to predict a significant portion of mammalian tissue-specific transcription. Here, we report on a systematic analysis of promoters controlling tissue-specific expression in heart, kidney, liver, pancreas, skeletal muscle, testis and CD4 T cells, for both human and mouse. We integrated multiple sources of expression data to compile sets of transcripts with strong evidence for tissue-specific regulation. The analysis of the promoters corresponding to these sets produced a catalog of predicted tissue-specific motifs and modules, and cis-regulatory elements. Predicted regulatory interactions are supported by statistical evidence, and provide a foundation for targeted experiments that will improve our understanding of tissue-specific regulatory networks. In a broader context, methods used to construct the catalog provide a model for the analysis of genomic regions that regulate differentially expressed genes.

Kruppel-like factor 2 transcriptional regulation involves heterogeneous nuclear ribonucleoproteins and acetyltransferases

Kruppel-like factor 2 (KLF2) is expressed in several cell types, and knockout animals have shown that KLF2 gene regulation is involved in multiple biological processes. These include maintaining T-cells in the quiescent state, preventing preadipocytes from differentiating into mature adipocytes, stabilizing blood vessel walls through endothelial cell function, and advancing the later stages of lung development. Defining the regulation of KLF2 expression is important to understand these diverse functions. Promoter analysis of KLF2 has revealed that a region between -138 and -111 base pairs is required for its transcription, and this nucleotide sequence occurs in a region that is highly conserved in evolution. The present study was carried out to identify transcription factors that bind to this region of the KLF2 promoter. Nuclear factors were enriched by DNA affinity chromatography using the conserved nucleotide sequence of the KLF2 promoter. Mass spectrometry analysis of the proteins eluted from the affinity matrix identified several proteins, including glucose regulated protein-78 kDa (GRP-78), heterogeneous nuclear ribonucleoprotein (hnRNP)-U, hnRNP-D, CArG binding factor (CBF), P300/CBP associated factor (PCAF), cAMP response element binding protein (CREB) and SWI/SNF. The binding of these proteins to the highly conserved region of the KLF2 promoter element was tested by electrophoretic mobility supershift assays and chromatin immunoprecipitation analysis. These procedures confirmed that hnRNP-U, hnRNP-D, PCAF, and P-300 bind to the KLF2 promoter. Transactivation experiments demonstrated that these proteins are important for regulating KLF2 transcription. Of special interest is the role of hnRNPs in the transcription of the KLF2 gene.

A novel muscle-specific enhancer identified within the deletion overlap region of two XLDC patients lacking muscle exon 1 of the human dystrophin gene

Previous studies point to the involvement of several discrete transcriptional enhancers in the modulation of dystrophin gene expression in skeletal and cardiac muscle. Analysis of deletion breakpoints in two X-linked dilated cardiomyopathy patients with mutations that remove muscle exon 1 identified a 3.2-kb deletion overlap region (XLDC3.2) between -1199 and +2057 bp predicted to contain regulatory elements essential for dystrophin gene expression in cardiac muscle. A novel-sequence-based search strategy was used to identify a 543-bp region downstream of muscle exon 1 rich in cardiac-specific transcriptional elements. Designated dystrophin muscle enhancer 2 (DME2), this candidate enhancer was seen to function in a position- and orientation-independent manner in muscle cell lines but not in fibroblasts. As only modest activity was observed in primary neonatal rat cardiomyocytes, DME2 is thought to play a role in dystrophin gene regulation at later stages of cardiac muscle development.

Induction of smooth muscle alpha-actin in vascular smooth muscle cells by arginine vasopressin is mediated by c-Jun amino-terminal kinases and p38 mitogen-activated protein kinase

Exposure of vascular smooth muscle cells to arginine vasopressin (AVP) increases smooth muscle alpha-actin (SM-alpha-actin) expression through activation of the SM- alpha-actin promoter. The goal of this study was to determine the role of the mitogen-activated protein kinase (MAP kinase) family in regulation of SM-alpha-actin expression. AVP activated all three MAP kinase family members: ERKs, JNKs, and p38 MAP kinase. Inhibition of JNKs or p38 decreased AVP-stimulated SM-alpha-actin promoter activity, whereas inhibition of ERKs had no effect. A 150-base pair region of the promoter containing two CArG boxes was sufficient to mediate regulation by vasoconstrictors. Mutations in either CArG box decreased AVP-stimulated promoter activity. Electrophoretic mobility shift assays using oligonucleotides corresponding to either CArG box resulted in a complex of similar mobility whose intensity was increased by AVP. Antibodies against serum response factor (SRF) completely super-shifted this complex, indicating that SRF binds to both CArG boxes. Overexpression of SRF increased basal promoter activity, but activity was still stimulated by AVP. AVP stimulation rapidly increased SRF phosphorylation. These data indicate that both JNKs and p38 participate in regulation of SM- alpha-actin expression. SRF, which binds to two critical CArG boxes in the promoter, represents a potential target of these kinases.

SRF protein is upregulated during stretch-induced hypertrophy of rooster ALD muscle

Serum response element 1 has previously been reported to be necessary and sufficient for activation of the skeletal alpha-actin promoter during hypertrophy of the anterior latissimus dorsi (ALD) muscle of roosters [J. A. Carson, R. J. Schwartz, and F. W. Booth. Am. J. Physiol. 270 (Cell Physiol. 39): C1624-C1633, 1996]. Serum response factor (SRF) protein is the transcription factor that binds as a homodimer to serum response element 1 and activates the skeletal alpha-actin promoter. An increased expression of exogenous SRF protein in replicating C2C12 myoblasts induced a three- to fourfold activation of the skeletal alpha-actin promoter (L. Wei, W. Zhou, J. D. Croissant, F.-E. Johansen, R. Prywes, A. Balasubramamyan, and R. J. Schwartz. J. Biol. Chem. 273: 30287-30294, 1998). Thus we hypothesized that SRF protein concentration would be increased during hypertrophy of skeletal muscle. In the present study, 10% of the rooster's body weight was attached to the left wing to induce enlargement of the ALD muscle compared with the contralateral muscle. With Western analysis, a significant increase in SRF protein per gram of wet weight of the ALD muscle was noted at 7 and 13 days of hypertrophy. Furthermore, the increase in SRF protein occurred in both crude nuclear protein and cytoplasmic fractions in 7-day stretched ALD muscles. This is the first report showing increased protein concentration for a transcription factor whose regulatory element in the skeletal alpha-actin promoter has previously been shown to be required for the transduction of a hypertrophy signal in overloaded skeletal muscle of an animal.

A muscle-specific enhancer within intron 1 of the human dystrophin gene is functionally dependent on single MEF-1/E box and MEF-2/AT-rich sequence motifs

In previous studies we have described a 5.0 kb Hin dIII fragment downstream of muscle exon 1 that exhibits properties consistent with a muscle-specific transcriptional enhancer. The goal of this study has been to identify the sequence elements responsible for muscle-specific enhancer activity. Functional studies indicated that this enhancer is active in pre- and post-differentiated H9C2(2-1) myoblasts but functions poorly in L6 and C2C12 myotubes. The core enhancer region was delimited to a 195 bp Spe I- Acc I fragment and sequence analysis identified three MEF-1/E box and two MEF-2/AT-rich motifs as potential muscle-specific regulatory domains. EMSA competition and DNase footprinting indicated that sequences within a 30 bp region containing single adjoining MEF-1/E box and MEF-2/AT-rich motifs are target binding sites for trans -acting factors expressed in H9C2(2-1) myotubes but not in L6 or C2C12 myotubes. Site-specific mutations within these motifs resulted in a significant reduction in enhancer activity in H9C2(2-1) myotubes. These results suggest that the mechanisms governing DMD gene expression in muscle are similar to those identified in other muscle-specific genes. However, the myogenic profile of enhancer activity and trans -acting factor binding suggests a more specialized role for this enhancer that is consistent with its potential involvement in dystrophin gene regulation in cardiac muscle.

Determination of the sequence requirements for the expression of a Xenopus borealis embryonic/larval skeletal actin gene

In this study, we demonstrate that all sequences necessary and sufficient for the expression of a Xenopus borealis alpha 3B embryonic/larval skeletal actin gene, reside in a 156-nucleotide fragment of the promoter that spans nucleotides -197 to -42. This region of the promoter contains three imperfect repeats of the CC(A/T)6GG (CArG) box motif that have been demonstrated to be important in the expression of other sarcomeric actin genes. Deletion of the actin promoter, using Xenopus microinjection techniques as a transient assay system for promoter activity, shows that the most distal CArG box (CArG box 3) is essential for the full expression of the gene. Under our assay conditions, the most proximal CArG box (CArG box 1) exhibits two binding activities using bandshift analysis. One of these binding activities contains components antigenically related to a serum-response factor (transcription factor), whilst the second does not. In contrast, CArG box3 produces only a single retarded band using electrophoretic mobility-shift analysis. Although the shifted complex coelectrophoreses with the CArG box 1/serum-response factor complex, the band produced by CArG box3 appears to be distinct from SRF. In addition to the CArG motifs, a further upstream regulatory element has been identified in the actin promoter between nucleotides -197 and -167. In the actin promoter, a downstream region can apparently fulfil this function.

Expression of the sarcomeric actin isogenes in the rat heart with development and senescence

Sarcomeric actin genes, alpha-cardiac and alpha-skeletal, are coexpressed in neonatal rodent hearts and are regulated in response to hormonal and hemodynamic stimuli; however, their precise developmental pattern of expression has not been determined, and it is unknown whether they are coexpressed during senescence. We have, therefore, investigated the accumulation of sarcomeric actin transcripts in rat heart during fetal and postnatal development and with senescence by two different techniques: primer extension analysis with an oligonucleotide common to both sarcomeric actins and RNA hybridization with specific cardiac alpha-actin cRNA probes. We found that at 17-19 days in utero both isogenes are coexpressed and alpha-skeletal actin mRNAs represent 28.0 +/- 0.8% of the sarcomeric actin mRNA total. Skeletal actin mRNAs increase to 40% of the total 1 week after birth (NS, p = 0.15), remain constant for 3 weeks, and decrease to less than 20% of the total in ventricles and atria of 1-month-old rats. The alpha-skeletal actin transcripts further decline to less than 5% of the total at 2 months of age and do not reaccumulate in senescent animals. There was no significant difference between male and female rat ventricles. By comparison with the known accumulations of alpha- and beta-myosin heavy chain mRNAs, our results demonstrate that whatever the developmental stage the kinetics of expression for the sarcomeric myosin and actin multigene families are independent.

The SRF and MCM1 transcription factors

The mammalian transcription factor SRF (serum-response factor) and the related Saccharomyces cerevisiae transcription factor MCM1 are the prototypes of a new class of dimeric DNA-binding proteins. Their function is regulated in part by the interactions of their DNA-binding domains with accessory proteins. Recent work has advanced the functional characterization of the contributions of SRF and MCM1, and their accessory proteins to transcriptional activation.

Muscle-specific expression of the cardiac alpha-actin gene requires MyoD1, CArG-box binding factor, and Sp1

Expression of the human cardiac alpha-actin gene (HCA) depends on the interactions of multiple transcriptional regulators with promoter elements. We report here that the tissue-specific expression of this promoter is determined by the simultaneous interaction of at least three specific protein-DNA complexes. The myogenic determinant gene MyoD1 activated the transcription of transfected HCA-CAT promoter constructs in nonmuscle cells, including CV-1 and HeLa cells. Gel mobility-shift and footprinting assays revealed that MyoD1 specifically interacted with a single consensus core sequence, CANNTG, at -50. Previously characterized sites interact with a protein identical with or related to the serum response factor (SRF) at -100 and Sp1 at -70. All three elements must be intact to support transcription in muscle cells: site-specific mutation within any one of these three elements eliminated transcriptional expression by the promoter. Furthermore, expression of the promoter in embryonic Drosophila melanogaster cells that lack MyoD1 and Sp1 is strictly dependent on all three sites remaining intact and on the presence of exogenously supplied Sp1 and MyoD1. These experiments suggest that the presence of three sequence-specific binding proteins, including MyoD1, and their intact target DNA sequences are minimal requirements for muscle-specific expression of the HCA gene.