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Molecular Medicine |
From the Laboratorio di Biologia Vascolare e Terapia Genica, Centro Cardiologico Fondazione "I. Monzino" IRCCS (B.I., P.B.), Milan, Italy; Laboratorio di Patologia Vascolare (A.S., L.M., M.C.C., C.G.), Istituto Dermopatico dellImmacolata, IRCCS, Rome, Italy; Laboratorio di Oncogenesi Molecolare (S.N., A.F.), Istituto Regina Elena, Rome, Italy; and Istituto di Neurobiologia e Medicina Molecolare, CNR, Rome, Italy.
Correspondence to Carlo Gaetano, MD, Laboratorio di Patologia Vascolare, IDI-IRCCS, Via dei Monti di Creta 104, 00167, Rome Italy. E-mail gaetano{at}idi.it
| Abstract |
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, platelet-endothelial cell adhesion molecule-1, VEGF receptor 2, myocyte enhancer factor-2C (MEF2C), and
-sarcomeric actin. In this condition, transcription factors MEF2C and Sma/MAD homolog protein 4 could be isolated from SS-treated ES cells complexed with the cAMP response element-binding protein acetyltransferase. These results provide molecular basis for the SS-dependent cardiovascular commitment of mouse ES cells and suggest that laminar flow may be successfully applied for the in vitro production of cardiovascular precursors.
Key Words: shear stress flow embryonic stem cell chromatin differentiation
| Introduction |
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| Materials and Methods |
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Matrigel Assay
ES cells were exposed to SS for 24 hours or kept in static culture; thereafter, they were trypsinized and plated on matrigel-coated plates (Becton Dickinson) in complete medium as described previously.14
Western Blots
Cells were lysed in 1x Laemmli buffer and boiled for 10 minutes. Total extracts were centrifuged for 10 minutes at 14 000 rpm at 4°C. Nuclear histones were detected as described previously.12 Anti-histone antibodies were purchased from Upstate Biotechnology. Anti-VEGF receptor 2 (VEGFR2), anti-PECAM, anti-MyoD, and antigrowth factor receptor-bound protein-2 antibodies were from Santa Cruz. Anti
-sarcomeric actin (
-SA) was from Sigma, antismooth muscle protein 22-
(SM22
) was from Abcam, and antiendothelial NO synthase (eNOS) antibody was from Transduction Laboratories. Normalization of protein loading was obtained either with anti-H1 antibody or antiGRB-2 antibody. All the antibodies were used according to manufacturer instructions.
Determination of Acetyltransferase Activity
ES cells were exposed to SS for 60 minutes. Protein extracts and acetyltransferase activity were obtained as described previously.12
Macroarray Screening
Atlas Mouse 1.2 array (Clontech) was probed with cDNA from ES cells exposed to SS for 12 hours or kept in static conditions. RNA was extracted with TRIZOL reagent (Invitrogen) according to manufacturer instructions; reverse transcription was performed using Superscript II and 18-mers Oligo-dT (Invitrogen) according to manufacturer instruction. cDNA labeling, array probing, and washing were performed according to Atlas instructions. Spots were visualized with Kodak-BIOMAX exposure films.
Reverse TranscriptionPolymerase Chain Reaction
Total RNA was extracted using TRIZOL reagent (Invitrogen). Reverse transcription was performed using Superscript II and 18-mers Oligo-dT (Invitrogen). Polymerase chain reactions (PCRs) were performed using AmplyTaq DNA-Polymerase (PerkinElmer) in 1.5 mmol/L magnesium chloride and 0.2 mmol/L dNTPs. Primer concentration was 1 µmol/L. Primer sequences were: transforming growth factor-ß (TGF-ß)activated kinase (TAK): fwd: 5'-cag tga gat gat cga agc gc-3'; rev: 5'-ctg tga atc agc gct ttg gg-3'; NeuroD1: fwd: gag gca gac aag aaa gag ga-3'; rev: 5'-gac caa att ggt agt ggg ct-3'; glial cell missing: fwd: 5'-gcg gac agg ctt tga aaa ac-3'; rev: 5'-atg act tct tga gga tcc gg-3'; MEF-2C: fwd: 5'-agc cgg aca aac tca gac at-3'; rev: 5'- tat tcc tct gca gag acg gg-3'; T-box gene 3 (TBX-3): fwd: 5'-gag atg gtc atc acg aag tc-3'; rev: 5'-gaa ccg cgg ctg gta ctt at-3'; forkhead homolog-2 (FKH-2): fwd: 5'-ctg gga agg tct aaa tgt cg-3'; rev: gat ggg tga gtt agc aag ga-3'; insulin-like growth factor-2 (IGF-2): fwd: 5'-aag gtc ccc gtt tct gtt-3'; rev: 5'-tct tct gtg gcc cag aaa gc-3'; thrombin receptor: fwd: 5'-act act tct ccg gca ctg at-3'; rev: 5'-caa gaa aga aga tgg cgg ag-3'; angiogenin: fwd: 5'-tca gga tga ctc cag gta ca-3'; rev: 5'-tgt gtg ctt gca agt ggt ga-3'; and GAPDH fwd: 5'-agc aca gtc cat gcc atc ac-3'; rev: 5'-tcc acc acc ctg tcg ctg ta-3'.
Amplifications were performed in a GeneAmp 2004 Thermalcycler (Perkin Elmer) using the following programs: 1 cycle 5'- 94°C, 5'- Tann; 35 cycles 1'- 72°C, 30'- 94°C, 1'- Tann; 1 cycle 7'- 72°C.
Annealing temperatures (Tann) were: 55°C for FKH-2, thrombin receptor, angiogenin, NeuroD1, TAK, and GAPDH; and 60°C for TBX-3, IGF-2, glial cell missing, and MEF-2C. Amplified transcripts were loaded on a 2% agarose gel.
RNase Protection Assay
RNA was extracted with TRIZOL reagent (Invitrogen) according to manufacturer instructions. RNase protection assay (RPA) was performed using RiboQuant RNase Protection Assay System (Pharmingen).
MEF-2C probe was obtained by amplifying murine MEF-2C coding region from nucleotide 1186 to nucleotide 1492 using the following oligonucleotides: fwd 5'-catgccgccatctgccctcagtcag-3'; rev 5'-ccctttcgtcggcgaaggtc-3'. Amplification was performed with Platinum Pfx DNA Polymerase (Invitrogen) according to manufacturer instruction in a GeneAmp 2400 Thermalcycler using the following program: 1 cycle: 5 minutes-94°C, 5 minutes-55°C; 30 cyles: 2 minutes-72°C, 30 s-94°C, 1 minute-55°C; 1 cycle: 10 minutes-72°C. PCR product was cloned in TOPO vector (Invitrogen) and subcloned in pBluScript plasmid. The construct was linearized with BamHI, gel-purified, and ethanol precipitated. Labeling was obtained by using the in vitro transcription kit (Pharmingen) according to manufacturer instructions. Protected bands were resolved on a 5% polyacrylamide/UREA gel.
Transient Transfections and Luciferase Assays
Transient transfections were performed using Lipofectamine Plus reagent (Invitrogen), and luciferase assays were performed using the Luciferase Assay Kit (Promega) as described previously.15
Stable Transfections and FACS Analysis
Stable clones were obtained by electroporating 107 cells with 20 µg of 442/+297GFP reporter construct in 0.8 mL of PBS. Electroporation parameters were: 250 V, 200
, and 960 µF. Cells were selected using 250 µg/mL of G418 (Life Technologies). FACS analysis was performed as described previously.15
Pull-Down Assay
Nuclear extracts and pull-down assay were performed as described previously.12 MEF2C consensus binding site was: 5'-gatcgctctaaaaataaccctgtcg-3'. Smad4 consensus binding site was: 5'-tcgagagccagacaaaaagccagacatttagccagacac-3'. MEF2C, Smad4, and CREB-binding protein (CBP)/p300 were detected by Western blot using anti-MEF2C, anti-Smad4 and anti-CBP antibodies (Santa Cruz Biotechnology) according to manufacturer instructions.
Immunofluorescence
ES cells were exposed to SS for 24 hours or kept in static conditions. Cells were immediately fixed in 4% paraformaldehyde solution for 10 minutes at room temperature, washed 3x for 5 minutes with PBS, and blocked for 1 hour in PBS containing 8% BSA. Cells fields were incubated with primary antibodies in PBS containing 1% BSA O/N, at +4°C. Plates were washed twice for 5 minutes with PBS and incubated with
-rabbittetramethylrhodamine B isothiocyanate,
-mouse fluorescein isothiocyanate, and
-goatfluorescein isothiocyanate secondary antibodies for 1 hour at room temperature in the dark. After 3 washes in PBS for 5 minutes, fields were incubated with 1 µg/mL Hoechst, 20 minutes at room temperature in the dark, and then washed 3x with PBS for 5 minutes. A total of 100 µL of DAKO cytomation-mounting fluorescent medium was used. Staining was visualized with an Axioplan 2 microscope; pictures were obtained by using a Axiocam and analyzed with a KS 300 3.0 acquisition software (Zeiss).
Statistical Analysis
Results were analyzed by 1-way ANOVA. Post hoc tests according to the Student-NewmanKeuls method were used to assess statistically significant differences among different groups. A value of P<0.05 was considered statistically significant.
| Results |
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Previous studies suggested that the level of methylated histone H3 on lysine at position 79 (K79) is a marker of active chromatin regions.16,17 We found that SS enhanced K79 methylation on histone H3 with a peak at 60 minutes. Although TSA treatment reduced the level of SS-induced K79 methylation, nonetheless, the overall level of histone methylation on this specific residue was increased compared with static controls (Figure 1F). Moreover, the overall acetyltransferase activity was enhanced in ES cells under SS conditions (Figure 1G). Notably, SS failed to induce chromatin remodeling and HAT activation in nonvascular mouse NIH-3T3 cells (data not shown). These results indicate that mouse ES cells are sensitive to SS becoming susceptible to the activation of chromatin remodeling.
SS Enhances Transcription From VEGFR2 Promoter
Previous work demonstrated that in adult endothelial cells, HAT molecules are required for activation of VEGFR2 promoter.15 To correlate SS effect on histone acetylation with the expression of vascular genes in ES cells, cells were transfected with a 442/+297 VEGFR2luc promoter construct15 and exposed to SS in the presence or absence of TSA. As shown in Figure 2A, SS enhances TSA-induced luciferase activity. This result was confirmed using stable ES cell clones expressing the green fluorescent protein (GFP) under control of the same VEGFR2 promoter region (Figure 2B). In fact, SS and TSA enhanced the number and the fluorescence intensity of ESGFP cells. The requirement of TSA to fully induce VEGFR2 promoter activation underlies the importance of histone acetylation to achieve gene expression in ES.
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SS Enhances the Expression of Cardiovascular Markers in ES Cells
The results obtained by transfection analysis and the evidence that mechanical stimuli may have profound effects on cardiomyocytes19 and heart formation in zebrafish embryos11 prompted us to investigate whether cell-specific markers were modulated by SS. Expression of cardiovascular markers in undifferentiated ES cells was evaluated after 24 hours of SS treatment. Figure 3A shows that in presence (top right) or absence (top left) of SS, ES cell shape did not visibly change, thus indicating that ES cells, at their early stage of differentiation, are unable to adapt their morphology in presence of biomechanical stimuli. However, SS-treated ES cells plated on matrigel organized in tubular-like structures, becoming visible after 3 to 6 hours from plating (bottom right). Control cells kept in static culture were predominantly organized in round clamps (bottom left). In this condition, the endothelial cell markers VEGFR2 and PECAM were expressed by ES cells exposed to SS, as revealed by immunocytochemistry and Western blot analysis (Figure 3B and 3E). Interestingly, cardiac and smooth muscle markers
-SA, MEF2C, smooth muscle actin (SMA), and SM22
were also upregulated (Figure 3C through 3E). The expression of all markers tested was retained at for least 24 hours after replacing ES cells in static conditions (Figure 3B through 3D). However, PECAM and SM22
were found downregulated at the 72-hour time point (Figure 3B and 3C). On the contrary, control cells showed a barely detectable expression of the same markers at the 24-hour time point. Notably, during these experiments, the skeletal musclespecific marker MyoD remained undetectable in static or SS condition (Figure 3F). Moreover, by a macroarray (data not shown) and RT-PCR screening (Figure 4A), genes belonging to the cardiovascular/mesodermic lineage were found upregulated in ES cells exposed to SS. Specifically, MEF-2C,20 thrombin receptor,21 TBX-3,22,23 FKH-2,24 angiogenin,25 IGF-2,26 and TAK27 were reproducibly positive. On the contrary, 2 neurospecific genes NeuroD128 and glial cell missing29 were not modulated. Altogether, these data indicate that SS stimulates expression of lineage-specific markers, possibly underlying activation of a cardiovascular differentiation program.
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Laminar SS Promotes MEF2C and Smad4 Association With CBP/p300
The transcription factor MEF-2C is highly important for cardiovascular development.810 RPAs showed that SS treatment upregulates MEF2C expression in ES cells (Figure 4B, left). In this condition, DNA pull-down experiments indicate that SS induced formation of active MEF2C/CBP complexes (Figure 4B, right). Because SS activates Smad proteins,30,31 which bind HATs and may associate to members of the MEF2 transcription factor family, modulating their activity,7 we performed a pull-down assay to detect SS-dependent nuclear translocation and HAT association of Smad4 transcription factor. Remarkably, the presence of active a Smad4/CBP complexes was observed in nuclear extracts obtained from SS-treated ES cells (Figure 4C), indicating that SS may contribute to the formation of transcription complexes important for ES cell differentiation into cardiovascular precursor cells.
| Discussion |
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| Acknowledgments |
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| Footnotes |
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| References |
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