Author contributions: E.H.: conception and design, collection and/or assembly of data, data analysis and interpretation, and manuscript writing; E.M.L.: manuscript writing; V.R., M.A.T., S.K.V., P.K., and M.B.S.: conception and design; H.X., A.D.A., and J.A.W.: collection and/or assembly of data; R.K.: conception and design, final editing, and approval of manuscript.
Disclosure of potential conflicts of interest is found at the end of this article.
First published online in STEM CELLSEXPRESS August 22, 2012.
Epigenetic and chromatin modifications play particularly important roles in embryonic and induced pluripotent stem cells (ESCs and iPSCs) allowing for the cells to both differentiate and dedifferentiate back to a pluripotent state. We analyzed how the loss of a key chromatin-modifying enzyme, histone deacetylase 1 (HDAC1), affects early and cardiovascular differentiation of both ESCs and iPSCs. We also investigated potential differences between these two cell types when differentiation is induced. Our data indicate an essential role for HDAC1 in deacetylating regulatory regions of key pluripotency-associated genes during early differentiation. Although HDAC1 functions primarily as a HDAC, its loss also affects DNA methylation in ESCs and iPSCs both during pluripotency and differentiation. We show that HDAC1 plays a crucial, nonredundant role in cardiomyocyte differentiation and maturation. Our data also elucidate important differences between ESCs and iPSCs, when levels of this enzyme are reduced, that affect their ability to differentiate into functional cardiomyocytes. As varying levels of chromatin-modifying enzymes are likely to exist in patient-derived iPSCs, understanding the molecular circuitry of these enzymes in ESCs and iPSCs is critical for their potential use in cardiovascular therapeutic applications. STEM CELLS2012;30:2412–2422
The ability to isolate human embryonic stem cells (ESCs) from unused in vitro fertilization embryos opened a door of opportunities and hopes for their many potential uses in drug testing, use as models to help our understanding of various biological processes, and most importantly their therapeutic potential in regenerative medicine. However, ethical, technical, and regulatory issues as well as unavailability of autologous human ESCs for cell therapy applications limit the potential therapeutic utility of ESCs for cardiac repair in humans. Reprogramming of somatic cells into induced pluripotent stem cells (iPSCs) opened a new and exciting door of a cell type with the apparent plasticity of embryonic stem cell and the added advantage of patient specificity [1, 2]. Since then the focus of iPSC biology has shifted toward understanding the epigenetic regulation and cell signaling that underlie somatic cell dedifferentiation processes and the molecular mechanisms that are involved in the maintenance of the newly acquired pluripotent phenotype.
iPSCs pose great potential for their use in clinical research and therapy. However, despite their promises and the new advancements made in the field, some challenges and unknowns still hinder the full realization of the clinical potential of these cells. Critical gaps in our knowledge of iPSC biology include incomplete epigenetic and mechanistic characterization of their reprogramming and directed differentiation processes [3–7]. Additionally, while it is widely accepted that one of the first steps that allows these cells to differentiate is expression of pluripotency-associated genes, such as Oct4, Nanog, and Sox2, how these genes are turned off as differentiation is induced is poorly understood.
A cell's identity is defined by its epigenetic code, modifications of which directly influence gene expression or repression [8–13]. The epigenetic state of pluripotent cells is extremely complex as pluripotency needs to be tightly regulated and maintained during continuous proliferation yet developmental genes should be accessible enough for differentiation to occur rapidly once the differentiation machinery in the cell has started [11–13]. Histone deacetylases (HDACs) have been identified as key players in both reprograming of somatic cells into iPSCs as well as important enzymes during differentiation of ESCs and iPSCs. However, most such studies rely on global inhibitors of HDACs, which in fact have different roles and direct differentiation into different lineages. The epigenetic similarity between iPS and ESCs and their pluripotent potential has also been recently questioned [11, 14].
MATERIALS AND METHODS
Cell Types and Cell Culture
iPSCs were generated from NIH3T3 cells using cell extract-mediated reprogramming. The efficiency of our technique in generating these cells as well as their pluripotent nature has been previously analyzed and reported . C57BL/6 murine ESCs (mESCs) were purchased from ATCC (Manassas, VA, www.atcc.org) (Cat.# SCRC-1002) and iPSCs were cultured in 15%-fetal bovine serum (FBS(, 50 μM Beta-Mercaptoethanol, 1 mM nonessential amino acids, and 100 U/ml Pen/Strep-supplemented Dulbecco's modified Eagle's medium in the presence of leukemia inhibitory factor (10 ng/ml).
Formation of Embryoid Bodies
Differentiation of iPS and ESCs through embryoid body (EB) formation was performed using standard hanging drop method. Briefly, a single-cell suspension of each cell line at a concentration of 2.5 × 105 cell per milliliter in 20 ml of differentiating media (Iscove's modified Dulbecco's medium supplemented with 15% FBS, 100 U/ml Pen/Strep, 200 μg/ml transferrin, 0.5 mM L-ascorbic acid, and 4.5 × 10−4 M monothioglycerol) was deposited in 20 μl hanging drops in 100 × 100 mm2 Petri dishes. After 2 days of being cultured in suspension, the cells were plated onto 0.1% gelatine-coated dishes for continued differentiation.
Real-Time Arrays and mRNA Expression
Expression analysis of epigenetic modifying enzymes and factors was performed using SABiosciences's (Valencia, CA, www.sabiosciences.com) RT2 Profiler PCR Array System according to manufacturer's instructions. Quantitative real-time PCR (Q-RT-PCR or Q-PCR) was performed as described earlier . Q-RT-PCR was performed using gene-specific labeled probes and primers. Array data were verified with independent Q-PCR.
The lentiviral-hdac1 shRNA vectors were purchased from Sigma-Aldrich (St. Louis, MO, www.sigmaaldrich.com) and transduction was performed according to manufacturer's instructions. Stably knocked-down clones were generated by puromycin selection for 2 weeks.
Protein expression analysis through immunofluorescence staining was performed as described earlier .
Calcium studies were performed mostly as previously described . Isolated EBs were loaded with fluo-4AM (Invitrogen, Grand Island, NY, www.invitrogen.com 15 μmol/l, 20 minutes) and placed in an experimental chamber on the stage of a confocal microscope. Media were recirculated for the remainder of the experiment. Laser scanning of cells in beating loci was accomplished with an LSM510 laser scanning confocal microscope (Zeiss Instruments, Thornwood, NY, www.zeiss.com) and allowed measurements of intracellular Ca2+ transients in individual myocytes during the experiment. Data were collected during spontaneous beating and external stimulation (350 ms). Image J was used to visualize the beating profile.
Methylation and Pyrosequencing Verification
Methylation studies were performed as previously described . PCR reactions were carried out using the Hotstart Taq polymerase kit (Qiagen, Valencia, CA, www.qiagen.com,) in 25 μl total volume and with 50 pm of forward primer and reverse primer. For each PCR reaction, 50 ng of the bisulfite converted DNA in 1 μl was used as a template. After 5 minutes of initial denaturation at 95°C, the cycling conditions of 44 cycles consisted of denaturation at 95°C for 15 seconds, annealing at 65°C for 30 seconds (NKX2.5, T, and GATA4) and 60°C for 30 seconds (TBX5), and elongation at 72°C for 45 seconds. The PCR products were stored at 4°C until ready for pyrosequencing. Pyrosequencing was performed using the PyroMark MD Pyrosequencing System (Biotage, Charlotte, NC, www.biotage.com) as described previously. In brief, the PCR product was bound onto streptavidin-Sepharose HP beads (GE Healthcare, Waukesha, WI, www.gehealthcare.com). Beads containing the immobilized PCR product were denatured using a 0.2 M NaOH solution and neutralized. Pyrosequencing primer at a concentration of 0.3 μM was annealed to the purified single-stranded PCR product at 28°C. Methylation quantification was performed using the manufacturer-provided software. The primers used in the PCR runs and pyrosequencing reactions are shown in supporting information Table S1.
Two-way ANOVA followed by a Bonferroni post hoc test was used to analyze the data. p values of <.05 were used to assign significance.
In order to identify key chromatin-modifying factors and enzymes in mESCs and iPSCs, we performed Q-RT-PCR array-based analysis of the expression patterns of 172 chromatin-modifying enzymes and factors in mESCs, iPSCs, and NIH3T3 cells (Fig. 1A). ESCs and iPSCs had very similar expression profiles with less than 15% of genes showing significant difference between these two cell types. Additionally, the few genes that did show significant difference only showed a twofold to threefold difference. When comparing the two pluripotent cell types to differentiated cells, the majority of epigenetic enzymes and factors, which were differentially expressed, were upregulated in pluripotent cells indicating more dynamic chromatin modifications in these cells (Fig. 1A). We assessed the expression levels of several HDACs from all different HDAC classes. Only HDAC1 and to a lower extent HDAC2 were highly expressed in pluripotent cells. Independent real time PCR analysis verified this data (Fig. 1B, supporting information Fig. S1A–S1E). Based on this array data, we identified HDAC1 as one of the enzymes expressed at high levels in pluripotent cells. Independent Q-RT-PCR and immunoblots confirmed that HDAC1 is expressed at high levels in pluripotent cells and the expression levels significantly go down in somatic cells representative of the three germ layers (Fig. 1B, 1C).
This expression indicated a key role for HDAC1 in the pluripotency and differentiation plasticity of iPSCs. Additionally, it provided a model to study whether slight differences at the chromatin levels in iPSCs would result in a biological deficiency of these cells to differentiate. To elucidate the role of HDAC1 in pluripotent cell differentiation, particularly into cardiomyocyte differentiation, we created shRNA-mediated stable HDAC1-knockdown (HDAC1-KD) cell lines in both ESCs and iPSCs (Fig. 1D). To test whether there was a compensatory elevated expression of other HDACs when HDAC1 is knocked down, we analyzed RNA expression levels of HDAC2 (another class I HDAC) and HDAC5 (a class II HDAC). In both cell types, we did not observe any significant increase in the expression levels of these HDACs (supporting information Fig. S2A).
Interestingly, loss of HDAC1 in either iPS or ESCs did not alter the expression levels of two pluripotency-associated genes (Oct4 and Nanog) under basal undifferentiated and self-renewal conditions (Fig. 1E). Additionally, cell proliferation (Fig. 1F) and cell division parameters did not significantly change between wild-type and HDAC1-KD cells, under basal, undifferentiated conditions (supporting information Fig. S2B, S2C). Even though iPSCs showed slightly different cell numbers as proliferation progresses, this difference was not statistically different (Fig. 1F).
HDAC1 has been widely studied due to its implication in many disorders and has been shown to be important during development [18, 19]. HDAC1 knockout (HDAC1-KO) mice are embryonic lethal at day 9 postfertilization. We next investigated a possible impact of HDAC1 deficiency in cardiovascular cell lineage differentiation of ESCs and iPSCs.
The embryonic lethality in HDAC1-KO mice and the observed high expression of this enzyme in iPS and ESCs indicated that HDAC1 potentially plays a role in the early stages of differentiation. HDAC1 is a HDAC and as such is involved in silencing gene expression. Master regulators of pluripotency, such as OCT4, SOX2, and NANOG, maintain pluripotency by binding to activating regions of promoters of genes important in maintaining pluripotency, autologous regulation, and associate with complexes that keep developmental genes repressed [20–22]. Because lack of HDAC1 during early differentiation could result in persistent high levels of these pluripotency master regulators through lack of deacetylation at the regulatory regions of these genes, we hypothesized that HDAC1 could be involved in silencing pluripotency-associated genes as the cells are induced to differentiate. In order to test this hypothesis, we induced differentiation through EB formation in both sets of pluripotent cells. As differentiation progressed, we analyzed changes in expression levels of pluripotency-associated genes.
One of the first steps in the differentiation process is the silencing of pluripotency-associated genes. While levels of these genes vary slightly under self-renewal conditions (Fig. 1E), we observed that upon the induction of differentiation expression levels of Oct4, Sox2, and Nanog dramatically decreased in wild-type iPS and ESCs (Fig. 2A, 2B). However, in HDAC1-KD cells, persistent high levels of these pluripotency-associated genes were observed (Fig. 2A, 2B). Since lack of HDAC1 could affect deacetylation of pluripotency-associated genes, we checked acetylation levels at regulatory regions of these genes to test whether the persistent high levels of expression were due to failure of these regulatory regions to get deacetylated by HDAC1. We analyzed the extent of acetylation of Histone H3 at lysine 9 (H3AcK9) in day 6 differentiating EBs of wt and HDAC1-KD cells by chromatin immune-precipitation. Because expression levels of pluripotency-associated genes dramatically decrease in wt cells after differentiation has been induced, we chose day 6 of differentiation so as to be able to directly compare acetylation levels at these promoters between wt and HDAC1-KD cells. At later days of differentiation, acetylation levels of pluripotency-associated genes in wt cells are undetectable. As expected, acetylation levels of all four regulatory regions analyzed for Oct4 were very high in HDAC1-KD cells (Fig. 2C, 2D). Based on expression data (Fig. 1A), acetylation levels of these genes would be expected to stay high as differentiation progresses. Rather interestingly, acetylation of these regions in the iPS-HDAC1-KD cells was lower than in mES-HDAC1-KD cells (Fig. 2D). Acetylation levels of Nanog and Sox2 promoter regions were also higher in mES and iPS HDAC1-KD cells compared to wt cells (Fig. 2E). Differences in acetylation levels between wt iPS and ESCs were insignificant. Acetylation levels at promoter regions of Nanog and Sox2 follow the same pattern as with Oct4. However, iPS-HDAC1 KD cells show lower levels of acetylation at these promoters compared to ES-HDAC1 KD cells. This data suggest that HDAC1 plays a crucial role in deacetylating regulatory regions of pluripotency-associated genes during differentiation, resulting in their repression. We further assessed a direct physical association of HDAC1 with OCT4 in both mESCs and iPSCs in their pluripotent, undifferentiated state (supporting information Fig. S2F). HDAC1 is known to associate with two complexes, the NuRD and the NODE complex [20, 23]. This change is likely to occur through association with the NuRD complex, rather than with the NODE complex, since key members of the latter would not be present during differentiation [20, 23]. The lack of HDAC1 leads to the deregulated suppression of pluripotency-associated genes thereby inhibiting mESC and iPSC differentiation.
Next we investigated whether this could lead to a higher differentiating potential of iPSCs even when HDAC1 had been knocked down. In the first stages of differentiation, we observed EBs from cells in which HDAC1 had been knocked down failed to expand and grow compared to their respective wt counterparts (supporting information Fig. S2D). In order to better visualize differentiation within the EB, we stained for alkaline phosphatase, an enzyme expressed in pluripotent cells. As an EB expands and differentiates, cells in the periphery are more differentiated than cells in the core of the EB. As EBs derived from ESCs and iPSCs grew and differentiated, they lost expression of alkaline phosphatase, an enzyme expressed in pluripotent cells. We observed higher expression, even in the periphery of ESCs when compared with their respective wt cells (Fig. 2F). However, iPSCs in which HDAC1 had been knocked down showed a pattern of alkaline phosphatase loss similar to their respective wt. This indicated a retention of limited differentiation ability in iPSCs in which HDAC1 had been knocked down compared to ES-HDAC1 KD cells.
Pluripotent ESCs and iPSCs are a promising source for potential therapeutic applications for regenerative medicine including cardiovascular repair and regeneration. In order to better understand the differentiation ability of iPS HDAC1-KD cells and how that compared to ES HDAC1-KD cells, we looked at iPS-HDAC1 KD cells potential to differentiate specifically into fully functional cardiomyocytes. Lack of HDAC1 also reduced the expression of early endodermal and to some extent ectodermal markers (supporting information Fig. S2E); however, we were most interested in the effect of HDAC1 on cardiovascular differentiation, partly due to inconsistencies in the current knowledge about the role of HDAC1 in cardiovascular differentiation. HDAC1-KO mice are embryonic lethal with defects in heart formation but cardiac-specific HDAC1 knockout mice (under myosine heavy chain promoter:- a late marker of cardiac differentiation) do not present any overt cardiac phenotype although double HDAC1/HDAC2 cardiac-specific KO mice did show arrhythmias, shortly after birth. Thus, we investigated the role of HDAC1 in the differentiation of iPS-HDAC1 KD cells into fully functional cardiomyocytes and how their differentiation compared to that of ES-HDAC1-KD cells.
We determined the comparative effects of HDAC1 silencing on cardiomyocyte differentiation in both mES-HDAC1-KD and iPS-HDAC1-KD cells. As EBs differentiates, differentiated cardiomyocytes show spontaneous beating. Although wt mESCs and iPSCs differentiated similarly and showed similar kinetics for spontaneous beating, their HDAC1-KD counterparts displayed either complete loss or significantly reduced and delayed beating loci (Fig. 3A). While approximately 30% of EBs generated from wt mESC and iPSC shows loci of spontaneous beating, none of the EBs generated from ES-HDAC1-KD showed any spontaneous beating (Fig. 3A, supporting information Videos 1, 2). However, some iPS-HDAC1-KD cells did spontaneously beat, albeit the beating was delayed and significantly reduced when compared with wt iPSCs (supporting information Videos 1, 2). This data show that some iPSCs even under very low/absent HDAC1 levels are able to differentiate into spontaneously beating cardiomyocytes.
We wanted to investigate whether iPS-HDAC1 KD cells, unlike their mESC counterpart, retained the ability to differentiate into fully functional cardiomyocytes and express cardiomyocyte-specific markers. Expression of early mesodermal genes was significantly lower in both iPS and mESCs in which HDAC1 has been knocked down compared to the respective wild-type (Fig. 3B). Expression levels of key cardiomyocyte markers were consistently lower in HDAC1-KD cells and the lack of expression of key markers was more pronounced in mES-HDAC1-KD cells (Fig. 3C, 3D).
Expression of mature cardiomyocyte markers such as Cardiac Troponin T (CTT) and other cardiovascular-specific proteins was also higher in iPS-HDAC1-KD cells than in mES-HDAC1-KD cells (Fig. 4A, 4B). Additionally, while mES-HDAC1-KD cells showed no expression of mature cardiomyocyte proteins essential for spontaneous contractility such as CTT and α-Sarcomeric Actinin, few of the iPS-HDAC1-KD cell-derived cardiomyocytes did (Fig. 4C). In iPS-HDAC1-KD cells, fewer EBs developed spontaneously beating loci, thus global expression of cardiomyocyte-specific proteins was lower than in iPS-HDAC1-KD-derived EBs. However, expression of these proteins within beating loci in iPS-HDAC1-KD cell-derived EBs is comparable to that of wt iPSC-derived EBs (Fig. 4C). This data indicate that while ESCs lose their ability to differentiate into cardiomyocytes or other cardiovascular lineages when HDAC1 is knocked down, iPSCs, to a certain extent, retain the ability to differentiate under the same conditions and cope better with the drastically reduced levels of HDAC1 during differentiation.
In order to investigate whether the observed effect and difference in behavior between iPSCs and ESCs under reduced levels of HDAC1 was specific to our iPSC type, we repeated these experiments in an independently generated iPSC line, NC2. We tested whether NC2 would also show restricted cardiovascular differentiation with the loss of HDAC1 compared to mES-HDAC1-KD cells, which consistently show very little differentiation. Similarly to the initial iPSC line we analyzed, NC2 (iPS)-HDAC1-KD cells also showed delayed and reduced beating and repressed differentiation when compared with the wt cells, and unlike mES-HDAC1-KD cells, NC2 (iPS)-HDAC1-KD cells show some beating and differentiation (Fig. 5A–5D). This is more apparent at the protein level, where the few beating loci within EBs derived from NC2 (iPS)-HDAC1-KD cells show robust expression of key cardiomyocyte proteins (Fig. 5E) as opposed to mES-HDAC1-KD cells that show none at the protein level (Fig. 4C).
Since we observed delayed and reduced cardiomyocyte differentiation in iPS-HDAC1-KD cells, we investigated whether the few iPS-HDAC1-KD cells that showed some beating were physiologically competent and had the ability to become fully functional cardiomyocytes. To test this, we monitored calcium handling of the cells during beating in real time. Fully mature cardiomyocytes possess the ability to beat in synchrony with adjacent cells and beat at the rate determined by the pacemakers. Thus, the beating colonies were analyzed for these two crucial characteristics of mature cardiomyocytes: (a) the ability to beat in synchrony and (b) the ability to respond to external stimuli. All wt mESC- and iPSC-derived beating cardiomyocytes analyzed had a synchronized intrinsic rate, responded very well to external stimuli (electric pulse at 350 ms) and recovered back to the initial intrinsic rate when the stimulus ceased (Fig. 6A; supporting information Videos 3, 4). Some iPS-HDAC1-KD-derived beating EBs showed aberrant and nonsynchronous calcium handling and did not respond to external stimuli (Fig. 6A, 6B; supporting information Videos 5, 6). While all analyzed beating loci derived from wt iPSCs showed 100% synchronization and response to the external stimulus, approximately 80% of beating loci derived from iPS-HDAC1-KD cells responded to the external stimulus and only 50% beat in synchrony (Fig. 6B). We used electric stimulation to induce beating in the ES-HDAC1-KD-derived EBs; however, after several exposures these EBs did not show contraction. To investigate whether the reason for the poor synchronization and calcium handling in iPS-HDAC1-KD-derived EBs is due to aberrant expression or localization of gap junction proteins, we analyzed the protein expression level and pattern of Connexin-43 (CX-43) in beating loci derived from these cells. In the wt cells, CX-43 is both highly expressed and adequately organized in the periphery of the cells (Fig. 6C, 6D). In iPSCs in which HDAC1 had been knocked down expression of CX-43 was substantially reduced and disorganized (Fig. 6C, 6D). This data suggest that while some iPSCs are able to overcome the need for HDAC1 in the first early stages of differentiation, HDAC1 is important for these cells to fully mature and maintain a cardiomyocyte phenotype.
Clearly, iPS-HDAC1-KD cells coped better than mES-HDAC1-KD cells in terms of cardiomyocyte differentiation. In an attempt to explain this differential characteristic of iPSCs and mESCs, we analyzed whether iPSCs had an epigenetic memory that allowed them to cope better with the lack of HDAC1 during differentiation. Because our iPSCs used in these experiments are derived from fibroblasts (of mesodermal origin), we tested promoter regions of four different mesodermal and cardiovascular genes to determine any differences in methylation patterns between mESC, iPSC, and their respective HDAC1 KD counterparts, before and during differentiation. In NIH3T3 cells, as expected, these promoters were highly methylated (Fig. 7A). However, the methylation pattern of these promoters in the four pluripotent cell types prior to and during differentiation was interesting. Unlike a recent report , we did not see any differences in the methylation pattern at any of these promoters when comparing wt iPS and mESCs (Fig. 7B–7D). Even during the undifferentiated state, methylation levels of the promoters of all the genes analyzed were lower in HDAC1-KD cells compared to wt cells (Fig. 7B–7D). In the wt cells, methylation levels of these genes went down as the cells differentiated, whereas HDAC1-KD cells maintained similar levels of methylation with very little change between pluripotent and differentiated states. While there was no expression of these genes in the pluripotent state of these cells, the low methylation levels indicate a crosstalk between the histone acetylation and DNA methylation at these promoters during early differentiation.
In summary, our data indicate that loss of HDAC1 in mESCs and iPSCs inhibits their ability to differentiate by suppressing the histone deacetylation of promoters of pluripotency-associated genes, therefore resulting in their sustained expression and as a consequence repressed lineage-specific differentiation. While mESCs show no differentiation, iPSCs show some ability to differentiate even when HDAC1 has been knocked down. Other reports have indicated a key role for HDACs in ESC differentiation through global inhibition of HDACs using Trichostatin A (TSA) [22, 24]. Although these reports have greatly extended the body of knowledge around HDACs and differentiation, they were not designed to recognize and determine crucial differences between different members of the HDAC family. ESCs have very dynamic chromatin maintenance and modification machinery. Treatment with TSA, which inhibits all class I and II HDACs, results in more acetylated histones and thus a globally more active transcription state, which can result in both inhibition and promotion of differentiation of ESCs, depending on the time of the treatment. This may explain contradicting reports on the role of HDACs in inhibiting differentiation or in promoting differentiation, as both could be possible through different HDACs or different complexes they associate with [18, 19, 22, 24, 25]. Our data using HDAC1-KD pluripotent cells suggest that HDAC1 is specifically important in the early differentiation as it is required to deacetylate pluripotency-associated genes when differentiation is induced. The prolonged expression of these pluripotent genes during differentiation results in delayed/absent expression of early differentiation genes.
Lack of HDAC1 during differentiation results in reduced cardiovascular differentiation and decreased or absent spontaneous contraction in differentiating ESCs and iPSCs. In accordance with recent reports, we saw a difference in differentiation ability between ESCs and iPSCs . While mES-HDAC1-KD cells do not show any spontaneous beating during differentiation, iPS-HDAC1-KD cells do. Some of these cells show expression of cardiovascular markers and some differentiation into cardiomyocytes. However, those cells that do differentiate into cardiomyocytes show partial absence of synchrony and do not always respond to external stimuli. This indicates a role for HDAC1 in the maturation to a fully functional cardiomyocyte phenotype by regulating expression of Gap junction protein CX-43 in iPSCs. Thus, even though these cells are able to differentiate when HDAC1 had been knocked down, they are unable to maintain a functional cardiomyocyte phenotype in its absence.
Recent reports have also shown differences in the methylation pattern of different gene regions between iPSCs and mESCs . When we compared the methylation pattern of specific cardiovascular promoters (short regions in CpG islands close to promoters), we observed no difference between our iPS and mESCs. We did however see a difference in the methylation of cardiomyocyte genes in HDAC1-KD cells both before and during differentiation.
The process of repressing or expressing a gene involves histone modifications, DNA methylation, and expression of various transcription factors and enhancers, which not only act in synchrony but also interact and crosstalk to each other. HDAC1 affects histone acetylation and indirectly DNA methylation. Our observations indicate that analysis of epigenetic molecular mechanisms important in maintaining pluripotency is crucial to our understanding of what makes pluripotent cells pluripotent and what governs their differentiation. This body of knowledge, in the future, could lead to the development of a better translational strategy for the use of these powerful cells in regenerative medicine, including postinjury cardiovascular repair and regeneration.
ESCs and iPSCs carry great potential for therapeutic use. The epigenetic of these molecules however is not fully understood. We showed that HDAC1, a key chromatin-modifying enzyme, is important in deacetylating pluripotency-associated genes during differentiation in both these cell types. We also show that this molecule plays nonredundant role during pluripotent cell-derived cardiomyocyte differentiation and maturation. Unlike ES-HDAC1-KD cells that do not show any cardiomyocyte differentiation ability, iPS-HDAC1 KD cells retain some ability to differentiate, albeit the derived cardiomyocytes are electro-physiologically incompetent. These data expand our knowledge of the chromatin modifications involved in the differentiation of ESCs and iPSCs as well as elucidate differences in differentiation plasticity that are observed when changes at the epigenetic level exist.
This work was supported in part by National Institute of Health Grants HL091983, HL105597, HL095874, HL053354, and HL108795 to R.K. and American Heart Association's predoctoral fellowship Grant 11PRE7360065 to E.H.
DISCLOSURE OF POTENTIAL CONFLICTS OF INTEREST
The authors indicate no potential conflicts of interest.