Division of Transplantation, Department of Surgery, University of Wisconsin-Madison School of Medicine and Public Health, Madison, Wisconsin, USA
Correspondence: Jon S. Odorico, M.D., F.A.C.S., Associate Professor of Surgery, Division of Transplantation, Department of Surgery, University of Wisconsin-Madison, School of Medicine and Public Health, H4/756 Clinical Science Center, 600 Highland Avenue, Madison, Wisconsin 53792, USA. Telephone: 608-263-4768; Fax: 608-262-6280; e-mail: email@example.com
Besides its role in exocrine differentiation, pancreas-specific transcription factor 1a (PTF1a) is required for pancreas specification from the foregut endoderm and ultimately for endocrine cell formation. Examining the early role of PTF1a in pancreas development has been challenging due to limiting amounts of embryonic tissue material for study. Embryonic stem cells (ESCs) which can be differentiated in vitro, and without limit to the amount of experimental material, can serve as a model system to study these early developmental events. To this end, we derived and characterized a mouse ESC line with tetracycline-inducible expression of PTF1a (tet-Ptf1a mESCs). We found that transient ectopic expression of PTF1a initiated the pancreatic program in differentiating ESCs causing cells to activate PDX1 expression in bud-like structures resembling pancreatic primordia in vivo. These bud-like structures also expressed progenitor markers characteristic of a developing pancreatic epithelium. The epithelium differentiated to generate a wave of NGN3+ endocrine progenitors, and further formed cells of all three pancreatic lineages. Notably, the insulin+ cells in the cultures were monohormonal, and expressed PDX1 and NKX6.1. PTF1a-induced cultures differentiated into significantly more endocrine and exocrine cells and the ratio of endocrine-to-exocrine cell differentiation could be regulated by retinoic acid (RA) and nicotinamide (Nic) signaling. Moreover, induced cultures treated with RA and Nic exhibited a modest glucose response. Thus, this tet-Ptf1a ESC-based in vitro system is a valuable new tool for interrogating the role of PTF1a in pancreas development and in directing differentiation of ESCs to endocrine cells. Stem Cells2014;32:1195–1207
Integrating the transcriptional and intercellular signals in pancreas development will lead to a better overall understanding of the process and thus allow application of this knowledge to exploit the endogenous regenerative capacity of the pancreas and/or manipulate pluripotent stem cells into therapeutic β cells. Yet we only have a partial understanding of how multiple extrinsic and intrinsic signals act together to regulate various stages of pancreas development. Moreover, investigating the role of transcription factors that are expressed very early in pancreas development and how they combine with tissue inductive events has been challenging due to difficulty in accurately determining the timing and location of their activation, and to limiting amounts of embryonic material for study. Embryonic stem cells (ESCs) are a potentially useful model system to study these early developmental events which are otherwise difficult to study and can provide insights into the gene networks involved.
Pancreas-specific transcription factor 1a (PTF1a) is a basic helix-loop-helix sequence-specific DNA binding subunit of a heterotrimeric transactivator complex called PTF1 whose other components include one of the common E proteins and either the vertebrate suppressor of hairless RBPJ or its paralog RBPJL . PTF1a was originally identified in exocrine cells where it binds and activates the promoters of digestive enzymes and is required for acinar cell development [2-4]. Besides the late role of PTF1a in pancreatic exocrine differentiation, a lineage tracing study demonstrated PTF1a is required very early for pancreas specification from the foregut endoderm  and is expressed in the progenitors of all pancreatic cell types, including acinar, endocrine, and ductal cells. This study also suggested that PTF1a is involved in fate choice decisions between duodenum and pancreas . However, while the role of PTF1a in exocrine differentiation is clear, how PTF1a controls the fate of multipotent progenitors, in turn endocrine development in mammals has not been fully elucidated with different studies presenting conflicting conclusions . For example, by transactivating the pdx1 promoter [7, 8], PTF1a may lead to more committed pancreatic progenitors, and/or may commit PDX1+ progenitors preferentially to pancreatic versus the alternative duodenal fate . Or, as based on a recent study in zebrafish, the dose of PTF1a may dictate endocrine/exocrine fate choice of the progenitors . Other studies in mice seem to point to regulation of endocrine differentiation by the interplay of PTF1a and Delta-Notch signaling  or antagonistic activity between PTF1a and NKX6.1 . Many protocols devised to differentiate ESC into β-cells produce immature multiple-hormone-expressing cells, in vitro and an in vivo environment appear essential for their maturation and amelioration of the diabetic state [12-15]. This could be due to absence/low levels of PTF1a in the cultures leading to improper specification of pancreatic fate and lack of specific transcriptional cues provided by PTF1a in normal pancreatic development. While the precise mechanism by which PTF1a influences endocrine cell specification has yet to be fully resolved, it is clear that PTF1a plays an important role in normal pancreas development and therefore is a critical gene to evaluate in ESC-based pancreatic differentiation cultures.
Hence, we established a mouse ESC line with tetracycline-inducible expression of PTF1a (tet-Ptf1a). In this study, we sought to determine the impact of ectopic PTF1a expression on pancreatic exocrine and endocrine lineage differentiation in ESCs in vitro. We found that transient PTF1a induction activated PDX1 expression resulting in PTF1a+PDX1+ progenitor cell formation and that effects on pancreatic lineage differentiation were time- and context-dependent. Notably, induction of PTF1a generated bud-like structures reminiscent of pancreatic primordia formed during development, in vivo. The results also point to the importance of achieving PTF1a expression in differentiating ESC cultures in order to optimize endocrine specification. As such, this tet-Ptf1a ESC line serves as a new tool to interrogate the role of PTF1a in pancreas development and for efficiently directing differentiation of ESCs to the endocrine lineages.
Materials and Methods
The tet-Ptf1a line was generated from the parent Ainv15 ESC line , a gift from M. Kyba/G. Daley and available at ATCC (Manassas, VA http://www.atcc.org/) as described previously .
Tet-Ptf1a and Ainv15 ESCs were maintained in an undifferentiated state and cultured in differentiation media as previously described , with the following modifications. To initiate the formation of embryoid bodies (EBs), ESCs were resuspended at a density of three million cells per siliconized (Sigma-St Louis, MO http://www.sigmaaldrich.com/united-states.html) 60 mm petri dish (BD Biosciences-San Jose, CA http://www.bdbiosciences.com). EBs were transferred to new siliconized petri dishes with fresh media every 24 hours. After 7 days of suspension culture, EBs were removed from the media and plated onto 24-well plates with 0.1% gelatinized glass coverslips at a concentration of 50 EBs per well. Plated EB cultures were maintained in Dulbecco's modified Eagle's medium (DMEM)-high glucose with 100 U penicillin/streptomycin, 2 mM l-glut with 1% serum replacement (Gibco-Grand Island, NY http://www.lifetechnologies.com/ipac/en/home/brands/gibco.html). To induce PTF1a expression, cultures were exposed to 1 μg/ml doxycycline (Dox) renewed every 24 hours. Retinoic acid and nicotinamide treatments were performed after Ptf1a induction using 5 μM of retinoic acid (Sigma-St Louis, MO http://www.sigmaaldrich.com/united-states.html) in carrier Dimethyl Sulfoxide (DMSO) for the 3 days and/or 10 mM nicotinamide (Sigma-St Louis, MO http://www.sigmaaldrich.com/united-states.html) for 21 days beginning at 7 days postplating (EB7 + 7).
Cultures were dissociated by incubating in 0.25% trypsin for 3 minutes followed 0.05% trypsin for 15 minutes and heavy pipetting to make single-cell suspension. Cells were then fixed in 0.1% paraformaldehyde for 10 minutes, pelleted, and resuspended in fluorescence-activated cell sorting (FACS) buffer (2% Fetal Bovine Serum (FBS) and 0.1% sodium azide in Phosphate buffered saline (PBS)). Methanol (90%) was used to permeabilize the cells and cells were then incubated with the primary antibody diluted in the FACS buffer for 2 hours at room temperature. After washing (with FACS buffer+0.1% Triton X-100), cells were incubated in secondary antibody for 1 hour. Samples were read on a FACSCalibur flow cytometer (BD Biosciences-San Jose, CA http://www.bdbiosciences.com).
Western Blot Analyzes
Tetptf1a and Ainv15 cells were differentiated in 60 mm dishes to different time points and harvested using 1 ml of Nonidet P40 lysis buffer supplemented with Halt protease inhibitor cocktail (Thermo Scientific-Waltham, MA http://www.thermoscientific.com). Forty micrograms of each sample lysate was run on 12% polyacrylamide gels (Biorad-Hercules, CA http://www.bio-rad.com/) and transferred onto nitrocellulose membranes (Biorad-Hercules, CA http://www.bio-rad.com/) in the cold. Following electroblotting, membranes were blocked with 5% milk in Phosphate buffered saline+0.1% Triton X-100 (PBST) and incubated overnight with the primary antibody. Horseradish peroxidase-conjugated secondary antibodies (Pierce-Rockford, IL http://www.piercenet.com/) and chemiluminescent substrate (SuperSignal West Femto, Pierce-Rockford, IL http://www.piercenet.com/) were used to detect the protein bands in a luminescent image analyzer (Fujifilm LAS-4000 Stamford, CT http://www.fujimed.com). Primary antibody: rabbit anti-PTF1A (Chris Wright), 1:6,000. Secondary antibody: HRP-conjugated goat anti-rabbit (1:20,000). Blots were stripped and reprobed for GAPDH (mouse anti-GAPDH HRP-conjugated 1:10,000, Abcam 9482-Cambridge, MA http://www.abcam.com).
TetPtf1a cells grown on p60 culture dishes to EB7 + 28 were fixed for 45 minutes in 3% glutaraldehyde, 1% paraformaldehyde, and 0.1 M cacodylate buffer solution. Fixed tissues were then prepared and sectioned as previously described . Sections were then examined using a Hitachi H-7000 electron microscope (Dallas, TX http://www.hitachi-hta.com/).
Insulin Content and Release Assays
Insulin content was determined by acid ethanol extraction followed by a mouse C-peptide ELISA (Biovendor-Asheville, NC http://www.biovendor.com/). For insulin release assay, cultures were grown in 60 mm dishes and media were changed to DMEM-low glucose the previous night. After preincubation in Krebs-Ringer buffer with 3.3 mM glucose at 37°C for 120 minutes, the cells were then incubated in Krebs-Ringer buffer containing 25 mM glucose, at 37°C for an additional 120 minutes. The supernatants were collected and the C-peptide levels were normalized to total cell number in the cultures by trypsinization and counting parallel samples treated similarly.
Validation of Murine ESC Line Possessing Inducible Expression of Ptf1a
Using the tet-on system developed by Kyba et al. , a murine ESC line (tet-Ptf1a) with tetracycline-inducible Ptf1a expression was generated . To validate the cell line, tet-Ptf1a ESCs were treated with doxycycline (Dox) for 3 days. Most cells expressed PTF1a in the presence of Dox while very few cells stained positively in the absence of Dox (Fig. 1A). Similarly, Dox treatment resulted in strong PTF1a expression detected by Western blot analysis (Fig. 1B). Hence, ectopic expression of Ptf1a can be induced and shutdown robustly in the tet-Ptf1a cell line.
Transient Ectopic Ptf1a Expression Early in ESC Differentiation Significantly Increases Insulin Transcript Expression
As a starting point for Ptf1a-activated pancreatic differentiation studies, we first tested several different Ptf1a induction regimens by adding Dox at different ESC differentiation stages to identify an optimal context to observe Ptf1a-induced effects. We treated cultures of differentiating tet-Ptf1a cells with Dox during each of the following time intervals: (a) for 7 days of EB formation (EB0-EB7), (b) for the initial 7 days after plating EBs (EB7 + 0 to EB7 + 7), (c) for 3 days from EB7 + 4 to EB7 + 7, or (d) for 28 days continuously after plating (EB7 + 0 to EB7 + 28). The inducibility of the transgene was confirmed at all the time points tested. Time course analysis of Ptf1a transcript in each induction regimen showed similar induction levels (Supporting Information Fig. S2I). The short 3-day induction resulted in significantly more transcript levels of islet hormones (insulin, glucagon, and somatostatin) when compared with other treatment regimens, uninduced (No Dox) tet-Ptf1a cells and Ainv15 control cells at the end of our culture period (EB7 + 28) (Fig. 1C). Ins transcript levels in the 3-day induced cells were 200-fold higher than in Ainv15 control cells (p < .01). Since Ptf1a is a direct transcriptional activator of many exocrine enzyme genes, we also tested whether induction of Ptf1a increased expression of a classic exocrine gene, amylase-2 (Amy2) (Fig. 1D). We observed a time-dependent activation of both endocrine and exocrine genes in ESCs induced to ectopically express Ptf1a (Fig. 1C, 1D). For example, whereas increased islet hormone and amylase mRNA expression was seen when Ptf1a was expressed after plating, Ptf1a over-expression during EB formation had little effect on endocrine and exocrine transcript abundance. Furthermore, prolonged (28-day) Pft1a expression led to predominantly increased Amy2 expression whereas, transient Ptf1a expression (3-day) resulted in proportionately greater hormone transcript expression. These results led us to use this culture protocol as the basis for further experiments.
Ptf1a Expression in Differentiating ESCs Activates Pdx1 Expression and Initiates Formation of Pdx1+Ptf1a+ Bud-Like Structures
Given our findings that ectopic expression of Ptf1a in a narrow window period increased expression of both endocrine and exocrine genes and prior reports of Ptf1a transactivating the Pdx1 promoter in vivo and in pancreatic cell lines [7, 8], we sought to determine whether Ptf1a was inducing expression of Pdx1 in ESC cultures. We first investigated the Ptf1a expression profile through the course of differentiation. Transcript levels (Fig. 2A) and protein (Fig. 2B) levels peaked during induction period and were very low/undetectable at other time points or in untreated differentiated tet-Ptf1a cells (Fig. 2A, 2F) and in differentiated Ainv15 cells treated with Dox (Fig. 2B) indicating that the expression of Ptf1a protein was tightly controlled by induced transgene expression. We found that transient Ptf1a expression from EB7 + 4 to EB7 + 7 resulted in extensive Pdx1 mRNA expression (Fig. 2A), and this did not occur in the absence of Dox in ESC cultures at this stage. Importantly, the effects of ectopic Ptf1a expression appeared to be context-dependent. We observed that ectopic Ptf1a activated PDX1 protein expression in EB7 + 4 cells but not in undifferentiated ESCs (Fig. 2C, 2D) or other contexts (data not shown). We then sought to perform a quantitative analysis of PDX1+ cells at different time points by flow cytometry (Fig. 2G, 2H). The proportion of PDX1+ cells was high in induced cultures at EB7 + 14 (ranging between 15% and 25%, Fig. 2G). Normalizing to this data value, we found 70% fewer PDX1+ cells in uninduced control cultures and ∼98% fewer in Ainv15 cultures (Fig. 2G, 2H). Thus, our findings indicate that ectopic Ptf1a expression in differentiating ESC progeny activates PDX1 expression in a context- and time-dependent manner.
The most striking result of ectopic Ptf1a expression was the outgrowth and expansion of unique PDX1+ bud-like structures, reminiscent of morphological features occurring during in vivo development. Before induction, there were very few PDX1+ or PTF1a+ cells (Fig. 3A), but after Ptf1a induction PDX1 expression appeared in a subset of PTF1a+ cells in bud-like outgrowths (Fig. 3B) beginning within 2–3 days of ectopically expressing Ptf1a transgene. The PDX1+ cell population continued to enlarge and the bud-like structures continued to expand long after Ptf1a transgene expression was extinguished (Fig. 3D). These bud-like growths contain cells which predominantly coexpress both PTF1a and PDX1, whereas neither bud-like growths nor coexpressing cells are present at any time in No Dox-treated cultures (Fig. 3C, 3E, 3G). By EB7 + 14, the buds have grown into structures that resemble pancreatic epithelium coexpressing PTF1a and PDX1 (Fig. 3D). There were significantly more Ptf1a+Pdx1+ bud-like structures in Dox-induced cultures when compared with uninduced cells regardless of the stage of differentiation (Fig. 3H). On categorizing these buds based on size as small and large, small buds were predominant in early Dox-treated cultures and their number decreased with time. In contrast, the number of large buds increased with time suggesting these bud-like structures expand over time (Fig. 3H). In cells induced to express Ptf1a, insulin+ cells are more prominently detected (starting at ∼EB7 + 14) than in cultures not expressing Ptf1a, gradually increasing in number until EB7 + 28 when PDX1 staining becomes primarily associated with insulin staining (Supporting Information Fig. S1). Insulin+ cells in the cultures coexpress C-peptide (Fig. 5F).
Key Markers of Pancreatic Developmental Program Are Expressed in Ptf1a-Induced Cultures
To support the hypothesis that the bud-like structures developing in Ptf1a-induced ESCs represent pancreatic differentiation, other markers of pancreas development were investigated at various stages. Induced and uninduced cultures were costained with PDX1 and early pancreatic endoderm markers such as HNF6, FOXA2, and MNX1 immediately after induction (EB7 + 7) to determine whether the emerging PDX1+ bud-like clusters correspond to early budding and commitment to the pancreatic fate [19, 20]. We found the majority of cells contained within these bud-like regions of the Ptf1a-induced cultures coexpressed PDX1 and FOXA2, PDX1 and HNF6, and PDX1 and MNX1 (Supporting Information Fig. S2E, S2G, S2H). FOXA2 and HNF6 were expressed in a larger domain than was observed for PDX1, and PDX1+ cells emerged from within these FOXA2 and HNF6 regions (Supporting Information Fig. S2E, S2G). Apart from a few PDX1+MNX1+ (Supporting Information Fig. S2H, inset) bud-like structures, few isolated MNX1+ (Supporting Information Fig. S2H, arrow) cells were seen, possibly of neuronal lineage . HNF4a+ cells were rarely observed, and did not coexpress PDX1; one such area is shown (Supporting Information Fig. S2F), and the HNF4a+PDX1− cells could represent a population of liver progenitors . In contrast, uninduced cells did not express PDX1, and had very little FOXA2 and HNF6 (Supporting Information Fig. S2A, S2C), and more widespread MNX1 and HNF4a at the same stage (Supporting Information Fig. S2B, S2D). SOX9 was expressed in most PDX1+ cells (Fig. 4A) as is the case in the pancreatic epithelium during early pancreas development . These observations demonstrate transient Ptf1a over-expression gives rise to bud-like outgrowths that express many early pancreatic endoderm markers. By acting at the early pancreas specification stage, ectopic Ptf1a expression may potentiate further pancreatic lineage differentiation.
Downstream pancreatic markers, including NKX6.1 (epithelial-endocrine progenitor), and Ngn3 (endocrine committed) were then evaluated. A time course analysis of NKX6.1 staining revealed its expression within a subset of PDX1+ cells. NKX6.1 expression was widespread at EB7 + 10 (Supporting Information Fig. S3A–S3C), became restricted to the central cells of branching-like epithelium by EB7 + 14 (Fig. 4B; Supporting Information Fig. S3D–S3F), and eventually was present only in Insulin+ PDX1+ cells (Supporting Information Fig. S3G–S3I). This pattern of NKX6.1 expression is similar to that normally seen in the developing pancreas in vivo [11, 24].
Ngn3 is expressed transiently in islet progenitors appearing within the PDX1+ pancreatic epithelium and is critical to initiate endocrine specification of the secondary transition . Hence, we investigated whether Ptf1a over-expression and PDX1+ pancreatic epithelium-like cells demonstrated Ngn3 gene activation. First, we found Ngn3 mRNA levels in Ptf1a-induced cultures were transiently increased ∼20-fold over undifferentiated ESCs, while uninduced cultures did not show a similar increase (Fig. 4C). In concert with transient Ngn3 transcript expression, immunoflourescent staining of cultures demonstrated transient NGN3 protein expression within the interior/trunk regions of PDX1+ branching-like epithelium in the Ptf1a-induced cultures with NGN3+ cells most prominent at ∼EB7 + 16 (Fig. 4E). Also, the tip marker CPA1 was not coexpressed with NGN3 (Supporting Information Fig. S4). It was generally observed that NGN3 was expressed in PDX1lo cells (Fig. 4, insets), and it is interesting to note that several other studies have reported this observation in vivo [25, 26]. In contrast, NGN3+ cells were not detected in uninduced cells at any stage (data not shown). The appearance of NGN3+ cells among the PTF1a-expressing PDX1+ cell population demonstrates the capacity of ESC-derived pancreatic epithelial-like buds for endocrine specification.
Ptf1a-Induced Cultures Can Further Generate Adult Pancreatic Cell Types
Given the increased number of both exocrine and endocrine progenitors (CPA+ cells and NGN3+ cells) in the Ptf1a-induced cultures, we sought to determine whether adult pancreatic cell types are produced. Large numbers of Amy+ cells were found in Dox-treated cultures (Fig. 5A). In particular, the amylase-expressing tissue generated by transient Ptf1a-induction exhibited tissue morphology similar to the “grape-like” or acinar structures typically seen in more mature pancreatic tissue. These tissues contained polarized epithelial cells possessing zymogen granules, a basement membrane, and a central lumen thus forming “acinar” units (Supporting Information Fig. S5B, S5C). Dolichos Biflorus Agglutinin (DBA), a marker of duct epithelium  was found intertwined with the Amy-expressing cell clusters (Fig. 5B, 5D). Hormone+ cells, expressing insulin, glucagon or somatostatin were also found embedded within these regions (Fig. 5C, 5D). H&E sections of the Ptf1a-induced cultures depict regions possessing epithelial morphology with areas containing insulin+ cells (Supporting Information Fig. S6). In cultures where Ptf1a was not induced, such hormone-positive and enzyme-expressing cells are rare (data not shown). In summary, all pancreatic lineages, including acinar, endocrine, and ductal cells are generated upon Ptf1a-induction in differentiating ESC cultures.
Insulin+ Cells in Ptf1a-Induced Cultures Possess Some Typical Features of Mature β Cells
Individual hormone-expressing cells were further investigated by staining and quantified in late stage cultures. Cells individually expressing insulin, glucagon, and somatostatin were observed (Fig. 5E, 5G, 5H). Insulin+ cells almost uniformly coexpressed PDX1 and C-peptide (Fig. 5E, 5F). Large populations of these insulin/C-peptide costaining cells formed clusters with apparent direct cell-cell contact that was not generally observed in uninduced or wild-type ESC cultures. These insulin+ cells coexpress NKX6.1 (Supporting Information Fig. S3G–S3I) but do not coexpress glucagon (97% of insulin+ cells did not express glucagon and none expressed somatostatin) (Fig. 5H). These insulin+ cells were then examined further for ultrastructural morphology. Electron microscopic evaluation of these late stage cells showed electron-dense secretory granules with peripheral halos similar to those seen in β cells (Supporting Information Fig. S5D). Thus, the insulin+ cells generated after further differentiation of ESC-derived PDX1+PTF1a+ progenitors express many characteristic β cell features.
Treatment with Nicotinamide and Retinoic Acid Enhances Endocrine Differentiation in Ptf1a-Induced Cultures
Although β cells appeared to form spontaneously in cultures comprised of PDX1+PTF1a+ progenitors, we desired to increase the yield of β cells as well as test whether ESC-derived pancreatic progenitors in vitro are responsive to defined differentiation factors. Nicotinamide (Nic) has been shown to enhance further differentiation and glucose responsiveness of fetal pancreatic tissue and possibly stimulate proliferation of β cells [28, 29], and to promote ESC differentiation to insulin-secreting cells  possibly in part due to its ability to inhibit histone deacetylases  and/or its anti-inflammatory effects . Retinoid signaling contributes at multiple stages to pancreas differentiation in the embryo [33, 34], and from ESCs . Retinoic acid (RA) has been shown to induce generation of NGN3+ cells and further promote their differentiation to β cells in a mouse pancreatic explant assay . Based on these data, we treated tet-Ptf1a cells sequentially with Dox to induce Ptf1a expression, followed immediately by RA and Nic treatment according to the protocol described in the methods and Figure 6A. We then compared these cultures to those which had been treated with No Dox, Dox alone, or Dox+Nic. Whereas PTF1a induction alone modestly increased islet hormone and amylase transcripts (Fig. 6B) and the number of endocrine cells (Fig. 6C), addition of Nic to induced tet-Ptf1a cells increased insulin+ cells significantly (approximately three- to fourfold or ∼1,000 cells per coverslip) over that obtained by PTF1a induction alone (Fig. 6C). Treatment with both RA and Nic resulted in further increase in insulin transcript expression (Fig. 6B) and the number of insulin+ cells (∼2,300 per coverslip); 1.15% of total cells compared to 0.04% in No Dox cultures (Fig. 6C). Comparatively, RA+Nic treatment dramatically reduced amylase gene expression in PTF1a-induced cultures (Fig. 6B), thus suggesting a shift in the differentiation of progenitors away from exocrine fate. This observation is corroborated by immunoflourescent staining for amylase, DBA, and hormones shown in Figure 7. Whereas induction of PTF1a alone led to more Amy+ cells, more DBA+ cells, and a modest increase in hormone+ cells (Fig. 7A-7D), addition of both RA and Nic to Ptf1a over-expressing cells greatly enhanced the number of hormone+ cells (Fig. 7I–7L). Further staining for insulin specifically and amylase yielded similar results; combined RA+Nic treatment led to better insulin expression and endocrine differentiation (Supporting Information Fig. S7E vs. S7C, and S7G, S7H). Conversely, neither Nic nor RA alone significantly increased the number of hormone+ cells (Fig. 7E–7H, 7M–7P). To confirm the authenticity of insulin+ cells, the cultures were costained with C-peptide showing nearly all insulin+ cells expressed C-peptide (Supporting Information Fig. S7H). Importantly, treatment of other ESC lines with RA+Nic did not considerably increase insulin expression (Supporting Information Fig. S7F) indicating that prior Ptf1a expression preconditions cells to have the ability to respond to these morphogens. These findings suggest that Nic and RA regulate the in vitro development of Ptf1a-induced ESCs with the major effects being a shift toward endocrine differentiation and concomitant reduction of acinar and ductal differentiation.
To correlate phenotypic endocrine differentiation with functional assessments, insulin content and glucose-stimulated insulin release were evaluated by C-peptide ELISAs. C-peptide content of cells subjected to Ptf1a over-expression followed by differentiation with Nic and RA treatment was significantly higher than in uninduced controls (Fig. 6D). Glucose-stimulated C-peptide release assays showed that the Dox+RA+Nic cultures were marginally responsive to glucose, releasing significantly more C-peptide after exposure to high glucose than low glucose (Fig. 6E). Moreover, C-peptide secretion into the media under both basal and stimulated conditions was significantly greater for Dox+RA+Nic-treated cells compared to untreated, differentiated cells (Fig. 6E).
Efficient differentiation of ESCs to insulin-expressing β cells in vitro has remained elusive, due to incomplete understanding of the transcriptional networks and their timing of activation, and the difficulty in reproducing suitable niches in a dish. In this regard, directed differentiation by induced expression of many pancreatic genes has been studied. Adenoviral-mediated PTF1a and MIST1 expressions in mouse ESCs were used to generate acinar cells . Besides the late role of PTF1a in exocrine differentiation, it has been established that PTF1a is required very early in development for specification of the pancreas from the foregut endoderm [5, 38]. However, a study exploiting this early function of PTF1a to facilitate differentiation of ESCs to the pancreatic lineage has not been performed. Here, we demonstrate transient ectopic expression of PTF1a alone (without the requirement for PDX1) in ESCs drives the whole pancreatic program exhibiting many of the characteristic morphological and molecular signatures of pancreas development, in vitro.
One of the salient features of this system is the activation of Pdx1 expression by PTF1a that has not been previously reported in ESCs. PTF1a-induced Pdx1 activation is context-dependent and is consistent with prior studies that showed that PTF1a binds to Area III of the Pdx1 promoter that is only active early in development [7, 8]. These studies are also consistent with prior mouse studies showing that PTF1a is a crucial determinant of pancreatic fate besides PDX1 as most PDX1+ progenitors that lack PTF1a expression fail to differentiate into pancreatic cells and acquire a duodenal fate . Ineffectiveness of current protocols of pancreatic differentiation of human and mouse ESCs could be attributed to suboptimal PTF1a coexpression in PDX1+ cells and consequently resulting in some PDX1+ cells giving rise to intestinal, bile duct, and/or stomach cells in culture. This model system provides a means of generating progenitors that express both PTF1a and PDX1, and thus committing the cultures to pancreatic fate, by the ectopic expression of one factor, that is, PTF1a alone.
Notably, the developmental phenotype observed with transient PTF1a induction was different from a similar cell line with PDX1 induction . Ectopic PDX1 expression led to significantly lower endocrine and exocrine gene expression than did PTF1a induction and did not result in the bud-like structures seen in Ptf1a-induced cultures. Other studies that have used either transient or biphasic PDX1 expression in mouse ESCs  or constitutive PDX1 expression in human ESCs  have reported some endocrine differentiation but no morphological changes. The distinctive PDX1+PTF1a+ bud-like structures differ from islet-like clusters reported by many groups  in that they are of epithelial nature and closely resemble pancreatic buds that arise from the foregut endoderm. These structures are unique to Ptf1a-induced cultures, and might be derived from direct downstream genes targets of Ptf1a since they appear immediately after induction. There is strong coexpression of PDX1 and PTF1a along with NKX6.1 in these epithelial structures, and hence mark a fully specified pancreas progenitor unlike other reports of PDX1+ buds . NKX6.1 expression domain emerges as a subset of PDX1+ population, and gets restricted to the endocrine lineage over time. NGN3+ cells arise within the PDX1 domain, some of which were PDX1lo cells, in agreement with previous reports in mouse embryos [25, 26]. All three pancreatic cell types are produced which are organized into striking acinar-like cell clusters expressing amylase and DBA+ duct-like cells with interspersed hormone+ cells, as observed in mature pancreas. More importantly, the insulin+ cells in these cultures were monohormonal in contrast to those generated by popular β-cell differentiation protocols, despite their higher efficiency [12-15]. Finally, the Ptf1a-induced ESC-derived tissue is responsive to endocrine promoting effects of Nic/RA signaling.
Although it may seem intuitive that an endoderm population would be a better starting material than EB-based cultures to evaluate propancreatic effects of PTF1a, we observed that a highly enriched definitive endoderm population, generated from Nodal/Activin/BMP4/ILV treatments, was not a sufficient context to observe the pancreatic and endocrine differentiation promoting effects of ectopic Ptf1a expression (data not shown). This suggests that a more committed foregut endoderm context and/or an additional “inductive” cell type(s) may be necessary. Hence, we conclude an EB-based system that is more heterogeneous than a pure endoderm population may contain the correct combination of cells and extrinsic signals to be able to respond to Ptf1a.
The cell culture model described here possesses many features reminiscent of pancreatic organogenesis in vivo, and therefore would make an excellent model system to study Ptf1a-initiated pancreatic development. This model system could potentially be used for assessing Ptf1a dose control on endocrine versus exocrine fate decisions, and investigating early events of pancreas specification activated by PTF1a that are difficult to access in an embryo, including the transcriptional networks surrounding PTF1a (i.e., direct and indirect targets).
In sum, we have demonstrated Ptf1a-driven differentiation of pancreatic tissues from ESCs. This work establishes Ptf1a as a sufficient initiator of pancreatic differentiation leading to the subsequent formation of exocrine, endocrine and ductal cell types after progressing through developmental stages that are phenotypically and morphologically similar to those that occur in vivo. We anticipate the results of this study will warrant the importance of activating Ptf1a in the development of better differentiation protocols for generating transplantable ESC-derived insulin-expressing tissues.
We would like to thank the following individuals for providing access to important reagents: Chris Wright for anti-Pdx1 antibody; Ray Macdonald for anti-Ptf1a antibody, Samuel Pfaff for anti-hlxb9 antibody, BCBC for anti-Ngn3 antibody, and Michael Kyba for Ainv15 cell line. We would also like to acknowledge the CMN Waisman Core facility for training and use of their A1R-Si Nikon Confocal and FACS Calibur machines. This work was supported by the following funding: NIH ARRA DK-78889-1A2, JDRF Research Grant #2007-75, ADA Innovative Award, Department of Surgery Surgical Associates Award, Vilas Associate Professorship Award, and ROTRF/JDRF Award #71612962. R.K.V. is currently affiliated with the Division of Gastroenterology, Department of Medicine, Virginia Commonwealth University Medical Center, Richmond, VA.
G.N.: designed and performed experiments, wrote manuscript, and analyzed data; R.K.V.: designed and performed experiments, reviewed and edited manuscript, and analyzed data; J.S.O.: designed the experiments, analyzed data, and wrote and edited manuscript.
Disclosure of Potential Conflicts of Interest
The authors indicate no potential conflicts of interest.