Serum Response Factor Orchestrates Nascent Sarcomerogenesis and Silences the Biomineralization Gene Program in the Heart
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Serum response factor orchestrates nascent sarcomerogenesis and silences the biomineralization gene program in the heart Zhiyv Niua,b, Dinakar Iyerc, Simon J. Conwayd, James F. Martine, Kathryn Iveyf, Deepak Srivastavaf, Alfred Nordheimg, and Robert J. Schwartze,1 aCenter for Cardiovascular Development, bSection of Cardiovascular Sciences, and cDepartment of Medicine, Baylor College of Medicine, Houston, TX 77030; dCardiovascular Development Group, Herman B. Wells Center for Pediatric Research, Indiana University School of Medicine, Indianapolis, IN 46202; eCenter for Molecular Development and Disease, Institute of Biosciences and Technology, Texas A&M University System Health Science Center, Houston, TX 77030; fGladstone Institute of Cardiovascular Disease, San Francisco, CA 94158; and gInstitute of Molecular Biology, Tuebingen University, D-72704 Tuebingen, Germany Edited by Eric N. Olson, University of Texas Southwestern Medical Center, Dallas, TX, and approved September 18, 2008 (received for review June 6, 2008) Our conditional serum response factor (SRF) knockout, Srf Cko,in after Cre recombinase mediated ablation of the SRF genetic locus the heart-forming region blocked the appearance of rhythmic (12). To dissect out SRF’s role during early cardiac myocyte beating myocytes, one of the earliest cardiac defects caused by the commitment and differentiation, we generated lineage-specific ablation of a cardiac-enriched transcription factor. The appearance deletion of SRF with our Nkx2.5Cre (13) and Srf LoxP/Loxp mice (14) of Hand1 and Smyd1, transcription and chromatin remodeling in the HFR, well before SRF protein actually accumulated in the factors; Acta1, Acta2, Myl3, and Myom1, myofibril proteins; and heart. calcium-activated potassium-channel gene activity (KCNMB1), the channel protein, were powerfully attenuated in the Srf CKO mutant Results hearts. A requisite role for combinatorial cofactor interactions with The SRF Cardiac-Null Mutant Exhibited Nonbeating and Heart-Looping SRF, as a major determinant for regulating the appearance of Defect. To block the appearance of SRF before the appearance organized sarcomeres, was shown by viral rescue of SRF-null ES of beating cardiac myocytes, we engineered a mouse that cells with SRF point mutants that block cofactor interactions. In the carried both Srf LacZ and Nkx2.5Cre on chromosome 17 which absence of SRF genes associated with biomineralization, GATA-6, was then bred to SRFLox/Lox mice to generate a conditional bone morphogenetic protein 4 (BMP4), and periostin were strongly SRF knockout (Srf Cko) in the heart-forming region. The Srf Cko up-regulated, coinciding with the down regulation of many SRF mutant genotyped as Srf LacZ/Flox:Nkx2.5Cre was first discern- dependent microRNA, including miR1, which exerted robust si- ible at approximately 8.0 dpc (linear heart-tube stage) with a lencer activity over the induction of GATA-6 leading to the down nonbeating heart tube (Fig. 1A). Immunofluorescence staining regulation of BMP4 and periostin. with anti-SRF antibodies showed SRF staining in myocytes of haploid SRF mutant embryos and the absence of SRF in the heart development ͉ microRNA ͉ periostin ͉ cardiogenesis ͉ GATA6 Srf Cko embryo (Fig. 1 B–E). This tubular structure misplaced the anterior portion of the developing out-flow tract in the he heart is the first organ to form in mammals, controlled by an Srf Cko mutant (Fig. 1 F and G). By Ϸ8.5 dpc, severe ventricular Texquisite program that results in the assembly of organized dilation and cranially retained right ventricle/outflow tract sarcomeres that rhythmically beat. First, molecular principles un- were 2 common morphological defects of this motionless derlying sarcomerogenesis were based on the gene-switch paradigm tubular heart (Fig. 1 H and I) The outflow tract derived from in which nonmuscle actins are replaced by smooth muscle and ␣ second heart field (SHF) was undersized, as shown by Wnt11 cardiac -actins in the heart-forming region (1, 2). Serum response expression (Fig. 1 J and K, ref. 15), whereas cardiac field- factor (SRF), identified by Treisman and colleagues (3), and a marker genes Nkx2.5 (16) and Fgf10 (17) appeared unaffected MADS (MCM1, Agamous, Deficiens, serum response factor) box (Fig. 1 L–O). transcription factor may play a critical role in sarcomerogenesis, as deduced from transfection assays demonstrating the essential role SRF Guides Cardiac Myogenesis. Smooth muscle and cardiac ␣-actin of SRF binding sites, or CArG boxes, for switching on cardiac gene gene RNA transcripts emerging at the late cardiac-crescent stage transcription, competition with negative acting YY1 and HOP, and (7.75–8.0 dpc) were blocked in Srf Cko mutant hearts (Fig. 2 A–H). cardiac restricted expression (reviewed in ref. 4). Cardiac progen- itors receiving the appropriate developmental cues switch on Immunofluorescence staining confirmed the absence of smooth ␣ Cko several cardiac-restricted transcription factors such as Nkx2–5, muscle and striated -actin in the hearts of Srf embryos (Fig. 2 GATA-4, and myocardin that interact with SRF to activate many C, D, G, and H). Expression of Myl2 and Myom1 components of the cardiac and smooth-muscle structural genes (reviewed in refs. 4, 5). thick filament and M-band of sarcomeres were dependent on SRF SRF target genes are also involved with contractility, cell move- expression (Fig. 2 I–L). Analysis with transmission electron micros- ment, and cell growth signaling (6, 7) and the recently discovered microRNAs, required for normal heart development (8). Author contributions: Z.N., D.I., S.J.C., J.F.M., K.I., D.S., and R.J.S. designed research; Z.N., The function of SRF in heart development in vivo has been D.I., S.J.C., J.F.M., and K.I. performed research; A.N. contributed new reagents/analytic obscured by the early lethality of SRF null mice before the onset of tools; Z.N., D.I., S.J.C., J.F.M., K.I., D.S., and R.J.S. analyzed data; and Z.N. and R.J.S. wrote the cardiogenesis (9). Even recent SRF inactivation studies in the heart, paper. performed through a conditional knockout strategy by using Cre The authors declare no conflict of interest. recombinase driven by late expressing transgenic promoters such This article is a PNAS Direct Submission. as, SM22␣, and or ␣/ myosin-heavy chains, failed to reveal an 1To whom correspondence should be addressed. E- mail: [email protected]. obligatory role for SRF in controlling the appearance of beating This article contains supporting information online at www.pnas.org/cgi/content/full/ myocytes (10–12). That failure is because SRF induced during early 0805491105/DCSupplemental. cardiogenesis is relatively stable and sarcomeres appeared even © 2008 by The National Academy of Sciences of the USA 17824–17829 ͉ PNAS ͉ November 18, 2008 ͉ vol. 105 ͉ no. 46 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0805491105 Downloaded by guest on September 30, 2021 A SRF B C da da da da h h ctr cko D E FG ctr cko ctr cko Wnt11 Fig. 1. Early cardiac restricted Srf knockout exhib- ited nonbeating and cardiac looping defects. (A) H I JK Quantitative summary of the number of beating hearts observed for control and Srf Cko embryos at Ϸ8.25 dpc. (B–E) Immunofluorescent staining of con- trol Srf Lox/ϩ and Srf Cko embryo sections anti-SRF (green) antibodies. Boxed areas in B and C were cko ctr magnified in panels D and E.(F–I) Control (ctr) ctr cko Srf Lox/ϩ and Srf Cko embryos at Ϸ8.25 dpc. Cardiac Nkx2.5 Fgf10 looping was affected in Srf cko embryo. Red dashed line, body middle line. Whole mount lacZ staining LMNO revealed the right ventricle and outflow tract (arrow head) and the dilated left ventricle (open arrow) in Srf Cko embryo. (J and K) Reduced Wnt11 RNA tran- scripts in the Srf Cko embryo indicated an undersized outflow tract and a SHF at 7.75 dpc. (L–O) The first heart field appeared unaffected, as shown by the similar levels of Nkx2.5 and Fgf10 RNA transcripts in ctr cko ctr cko both the control and Srf Cko embryo at 7.75 dpc. copy indicated that neither aligned filaments nor Z disks that were The appearance of Smyd1, a cardiac and skeletal muscle- formed in multiple Srf Cko cardiac mutants correlated well with the specific chromatin-remodeling factor (18) failed to appear in nonbeating heart (Fig. 2 M–Q). These ‘‘paralyzed’’ mutant hearts the nascent Srf Cko mutant myocytes (Fig. 3 A and B). Expres- did not display any sarcomere signatures in multiple Srf Cko mutant sion of Tgf1/1, a LIM protein that serves as a cofactor with the samples. androgen receptor and p300 (19), was also blocked in the SM -actin SM -actin ABCDda M N da da da h h Fig. 2. SRF insufficiency shuts down cardiac myocyte ctr cko ctr cko ctr cko differentiation. (A–H) Whole mount in situ hybridiza- CA -actin CA -actin tion and immunofluorescence staining showed the ab- EF G OP sence of Acta2 (smooth muscle ␣-actin) and Actc1 (car- H diac ␣-actin) expression in the Srf Cko embryo in comparison to control embryos at 8.5 dpc. (I–L) Whole mount in situ hybridization showed down-regulation of Myl2 and Myom1 in Srf Cko embryos. (M–P) Transmis- h h sion EM revealed organized sarcomere structure in BIOLOGY control (M and O) but not Srf Cko sections (N and P). (Q) ctr cko ctr cko ctr cko DEVELOPMENTAL Quantitative summary of contractile structures recog- Myl2 Myom1 Q nized in multiple control and Srf Cko samples. In control 100% samples (M and O) Z-lines were observed at high inci- IJKL dence (n ϭ 10:21 myocytes), whereas aligned filament 62% bundles were observed in all control samples. ‘‘Disar- 48% 41% rayed’’ filament structures shown within the dotted lines were observed in less than half of the Srf cko 0% 0% 0% mutant cells (n ϭ 11:27 cells) which shared a diameter ctr cko ctr cko at 10 nm, same as noncontractile intermediated fila- ments (N and P).