Efficient differentiation of AR42J cells towards insulin-producing cells using pancreatic transcription factors in combination with growth factors

Protein Production and Purification of a Codon-Optimized Human NGN3 Transcription Factor from E. coli

Neurogenin 3 (NGN3) transcription factor is vital for the development of endocrine cells of the intestine and pancreas. NGN3 is also critical for the neural precursor cell determination in the neuroectoderm. Additionally, it is one of the vital transcription factors for deriving human β-cells from specialized somatic cells. In the current study, the production and purification of the human NGN3 protein from Escherichia coli (E. coli) is reported. First, the 642 bp protein-coding nucleotide sequence of the NGN3 gene was codon-optimized to enable enhanced protein expression in E. coli strain BL21(DE3). The codon-optimized NGN3 sequence was fused in-frame to three different fusion tags to enable cell penetration, nuclear translocation, and affinity purification. The gene insert with the fusion tags was subsequently cloned into an expression vector (pET28a( +)) for heterologous expression in BL21(DE3) cells. A suitable genetic construct and the ideal expression conditions were subsequently identified that produced a soluble form of the recombinant NGN3 fusion protein. This NGN3 fusion protein was purified to homogeneity (purity > 90%) under native conditions, and its secondary structure was retained post-purification. This purified protein, when applied to human cells, did not induce cytotoxicity. Further, the cellular uptake and nuclear translocation of the NGN3 fusion protein was demonstrated followed by its biological activity in PANC-1 cells. Prospectively, this recombinant protein can be utilized for various biological applications to investigate its functionality in cell reprogramming, biological processes, and diseases.

CREB Promotes Beta Cell Gene Expression by Targeting Its Coactivators to Tissue-Specific Enhancers

CREB mediates effects of cyclic AMP on cellular gene expression. Ubiquitous CREB target genes are induced following recruitment of CREB and its coactivators to promoter proximal binding sites. We found that CREB stimulates the expression of pancreatic beta cell-specific genes by targeting CBP/p300 to promoter-distal enhancer regions. Subsequent increases in histone acetylation facilitate recruitment of the coactivators CRTC2 and BRD4, leading to release of RNA polymerase II over the target gene body. Indeed, CREB-induced hyperacetylation of chromatin over superenhancers promoted beta cell-restricted gene expression, which is sensitive to inhibitors of CBP/p300 and BRD4 activity. Neurod1 appears critical in establishing nucleosome-free regions for recruitment of CREB to beta cell-specific enhancers. Deletion of a CREB-Neurod1-bound enhancer within the Lrrc10b-Syt7 superenhancer disrupted the expression of both genes and decreased beta cell function. Our results demonstrate how cross talk between signal-dependent and lineage-determining factors promotes the expression of cell-type-specific gene programs in response to extracellular cues.

Emergence of synthetic mRNA: In vitro synthesis of mRNA and its applications in regenerative medicine

The field of gene therapy has evolved over the past two decades after the first introduction of nucleic acid drugs, such as plasmid DNA (pDNA). With the development of in vitro transcription (IVT) methods, synthetic mRNA has become an emerging class of gene therapy. IVT mRNA has several advantages over conventional pDNA for the expression of target proteins. mRNA does not require nuclear localization to mediate protein translation. The intracellular process for protein expression is much simpler and there is no potential risk of insertion mutagenesis. Having these advantages, the level of protein expression is far enhanced as comparable to that of viral expression systems. This makes IVT mRNA a powerful alternative gene expression system for various applications in regenerative medicine. In this review, we highlight the synthesis and preparation of IVT mRNA and its therapeutic applications. The article includes the design and preparation of IVT mRNA, chemical modification of IVT mRNA, and therapeutic applications of IVT mRNA in cellular reprogramming, stem cell engineering, and protein replacement therapy. Finally, future perspectives and challenges of IVT mRNA are discussed.

The Diabetes-Linked Transcription Factor PAX4: From Gene to Functional Consequences

Paired box 4 (PAX4) is a key factor in the generation of insulin producing β-cells during embryonic development. In adult islets, PAX4 expression is sequestered to a subset of β-cells that are prone to proliferation and more resistant to stress-induced apoptosis. The importance of this transcription factor for adequate pancreatic islets functionality has been manifested by the association of mutations in PAX4 with the development of diabetes, independently of its etiology. Overexpression of this factor in adult islets stimulates β-cell proliferation and increases their resistance to apoptosis. Additionally, in an experimental model of autoimmune diabetes, a novel immunomodulatory function for this factor has been suggested. Altogether these data pinpoint at PAX4 as an important target for novel regenerative therapies for diabetes treatment, aiming at the preservation of the remaining β-cells in parallel to the stimulation of their proliferation to replenish the β-cell mass lost during the progression of the disease. However, the adequate development of such therapies requires the knowledge of the molecular mechanisms controlling the expression of PAX4 as well as the downstream effectors that could account for PAX4 action.

The role of microRNAs in islet β-cell development

Cell-based therapies suggest novel treatments to overcome the complication of the current therapeutic approaches in diabetes mellitus type 1. Replacement of the destroyed pancreatic islet β-cells by appropriate alternative cells needs an efficient approach to differentiate the cells into viable and functional insulin producing cells. Small non-coding RNA molecules, microRNAs (miRNA), have critical roles in post-transcriptional regulation of gene expression. Therefore, they can direct the cells toward β-cell like cells and control islet β-cell development. Previous reports showed the manipulation of the miRNA expression on islet β-cell differentiation and regeneration. Likewise, the regulation of epithelial to mesenchymal transi-tion by the miR-30 family and the miR-200 family may be a useful approach to conduct islet β-cell development. Investigation of stem cells differentiation showed that the dynamic expression patterns of miR-375 and miR-7 are similar to developing human fetal pancreas while dynamic expression of miR-146a and miR-34a occurred during the differentiation. Moreover, miR-342 and its both targets, FOXA2 and MAFB, are found in β-cell differentiation and maturation. Because miRNAs can target specific transcription factors during islet β-cell development and differentiation, they could be offerred as alternative regenerative treatment for diabetes mellitus. Considering that the application of these non-coding RNAs remains limited in the literature, in this review article, we present an overview of the roles of miRNAs in the islet β-cell development, focusing on the application of different miRNAs in the experimental protocols.

Making β(-like)-cells from exocrine pancreas

Creating an abundant source of β(-like)-cells has been a major goal in diabetes research for many decades. The concept of cell plasticity has inspired many strategies towards regenerative medicine, but its successes have been limited until very recently. Today, most cell types in the pancreas are considered candidates for the generation of β(-like)-cells through transdifferentiation. While β(-like)-cells that are in vitro differentiated from human embryonic stem cells are already being grafted in patients, β(-like)-cells generated by transdifferentiation are not yet ready for clinical application. These cells would however offer several advantages over the current β(-like)-cells generated by directed differentiation, especially concerning safety issues. In addition, perfect control of the transdifferentiation efficiency would through targeted drug delivery support a non-invasive cell therapy for diabetes. Lastly, focusing on the exocrine pancreas as prime candidate makes sense in view of their abundance and high plasticity. Keeping these hopeful perspectives in mind, it is worth to continue focused research on the mechanisms that control transdifferentiation from pancreas exocrine to β-cells.

Generation of Functional Beta-Like Cells from Human Exocrine Pancreas

Transcription factor mediated lineage reprogramming of human pancreatic exocrine tissue could conceivably provide an unlimited supply of islets for transplantation in the treatment of diabetes. Exocrine tissue can be efficiently reprogrammed to islet-like cells using a cocktail of transcription factors: Pdx1, Ngn3, MafA and Pax4 in combination with growth factors. We show here that overexpression of exogenous Pax4 in combination with suppression of the endogenous transcription factor ARX considerably enhances the production of functional insulin-secreting β-like cells with concomitant suppression of α-cells. The efficiency was further increased by culture on laminin-coated plates in media containing low glucose concentrations. Immunocytochemistry revealed that reprogrammed cultures were composed of ~45% islet-like clusters comprising >80% monohormonal insulin⁺ cells. The resultant β-like cells expressed insulin protein levels at ~15–30% of that in adult human islets, efficiently processed proinsulin and packaged insulin into secretory granules, exhibited glucose responsive insulin secretion, and had an immediate and prolonged effect in normalising blood glucose levels upon transplantation into diabetic mice. We estimate that approximately 3 billion of these cells would have an immediate therapeutic effect following engraftment in type 1 diabetes patients and that one pancreas would provide sufficient tissue for numerous transplants.

Reprogramming of Pancreatic Exocrine Cells AR42J Into Insulin-producing Cells Using mRNAs for Pdx1, Ngn3, and MafA Transcription Factors

Direct reprogramming of pancreatic nonendocrine cells into insulin-producing β-cells represents a promising approach for the treatment of insulin-dependent diabetes. However, its clinical application is limited by the potential for insertional mutagenesis associated with the viral vectors currently used for cell reprogramming. With the aim of developing a nonintegrative reprogramming strategy for derivation of insulin-producing cells, here, we evaluated a new approach utilizing synthetic messenger RNAs encoding reprogramming transcription factors. Administration of synthetic mRNAs encoding three key transcription regulators of β-cell differentiation-Pdx1, Neurogenin3, and MafA-efficiently reprogrammed the pancreatic exocrine cells into insulin-producing cells. In addition to the insulin genes expression, the synthetic mRNAs also induced the expressions of genes important for proper pancreatic β-cell function, including Sur1, Kir6.2, Pcsk1, and Pcsk2. Pretreating cells with the chromatin-modifying agent 5-Aza-2'-deoxycytidine further enhanced reprogramming efficiency, increasing the proportion of insulin-producing cells from 3.5 ± 0.9 to 14.3 ± 1.9% (n = 4). Moreover, 5-Aza-2'-deoxycytidine pretreatment enabled the reprogrammed cells to respond to glucose challenge with increased insulin secretion. In conclusion, our results support that the reprogramming of pancreatic exocrine cells into insulin-producing cells, induced by synthetic mRNAs encoding pancreatic transcription factors, represents a promising approach for cell-based diabetes therapy.

The Human Endocrine Pancreas: New Insights on Replacement and Regeneration

Islet transplantation is an effective cell therapy for type 1 diabetes (T1D) but its clinical application is limited due to shortage of donors. After a decade-long period of exploration of potential alternative cell sources, the field has only recently zeroed in on two of them as the most likely to replace islets. These are pluripotent stem cells (PSCs) (through directed differentiation) and pancreatic non-endocrine cells (through directed differentiation or reprogramming). Here we review progress in both areas, including the initiation of Phase I/II clinical trials using human embryonic stem cell (hESc)-derived progenitors, advances in hESc differentiation in vitro, novel insights on the developmental plasticity of the pancreas, and groundbreaking new approaches to induce β cell conversion from the non-endocrine compartment without genetic manipulation.

Transcription factor regulation of pancreatic organogenesis, differentiation and maturation

Lineage tracing studies have revealed that transcription factors play a cardinal role in pancreatic development, differentiation and function. Three transitions define pancreatic organogenesis, differentiation and maturation. In the primary transition, when pancreatic organogenesis is initiated, there is active proliferation of pancreatic progenitor cells. During the secondary transition, defined by differentiation, there is growth, branching, differentiation and pancreatic cell lineage allocation. The tertiary transition is characterized by differentiated pancreatic cells that undergo further remodeling, including apoptosis, replication and neogenesis thereby establishing a mature organ. Transcription factors function at multiple levels and may regulate one another and auto-regulate. The interaction between extrinsic signals from non-pancreatic tissues and intrinsic transcription factors form a complex gene regulatory network ultimately culminating in the different cell lineages and tissue types in the developing pancreas. Mutations in these transcription factors clinically manifest as subtypes of diabetes mellitus. Current treatment for diabetes is not curative and thus, developmental biologists and stem cell researchers are utilizing knowledge of normal pancreatic development to explore novel therapeutic alternatives. This review summarizes current knowledge of transcription factors involved in pancreatic development and β-cell differentiation in rodents.

Identification of miRNAs Involved in Reprogramming Acinar Cells into Insulin Producing Cells

Reprogramming acinar cells into insulin producing cells using adenoviral (Ad)-mediated delivery of Pdx1, Ngn3 and MafA (PNM) is an innovative approach for the treatment of diabetes. Here, we aimed to investigate the molecular mechanisms involved in this process and in particular, the role of microRNAs. To this end, we performed a comparative study of acinar-to-β cell reprogramming efficiency in the rat acinar cell line AR42J and its subclone B13 after transduction with Ad-PNM. B13 cells were more efficiently reprogrammed than AR42J cells, which was demonstrated by a strong activation of β cell markers (Ins1, Ins2, IAPP, NeuroD1 and Pax4). miRNome panels were used to analyze differentially expressed miRNAs in acinar cells under four experimental conditions (i) non-transduced AR42J cells, (ii) non-transduced B13 cells, (iii) B13 cells transduced with Ad-GFP vectors and (iv) B13 cells transduced with Ad-PNM vectors. A total of 59 miRNAs were found to be differentially expressed between non-transduced AR42J and B13 cells. Specifically, the miR-200 family was completely repressed in B13 cells, suggesting that these cells exist in a less differentiated state than AR42J cells and as a consequence they present a greater plasticity. Adenoviral transduction per se induced dedifferentiation of acinar cells and 11 miRNAs were putatively involved in this process, whereas 8 miRNAs were found to be associated with PNM expression. Of note, Ad-PNM reprogrammed B13 cells presented the same levels of miR-137-3p, miR-135a-5p, miR-204-5p and miR-210-3p of those detected in islets, highlighting their role in the process. In conclusion, this study led to the identification of miRNAs that might be of compelling importance to improve acinar-to-β cell conversion for the future treatment of diabetes.

Reprogramming Mouse Cells With a Pancreatic Duct Phenotype to Insulin-Producing β-Like Cells

Reprogramming technology has opened the possibility of converting one cell type into another by forced expression of transgenes. Transduction of adenoviral vectors encoding 3 pancreatic transcription factors, Pdx1, Ngn3, and MafA, into mouse pancreas results in direct reprogramming of exocrine cells to insulin-producing β-like cells. We hypothesized that cultured adult pancreatic duct cells could be reprogrammed to become insulin-producing β-cells by adenoviral-mediated expression of this same combination of factors. Exocrine were isolated from adult mouse insulin 1 promoter (MIP)-green fluorescent protein (GFP) transgenic mice to allow new insulin-expressing cells to be detected by GFP fluorescence. Cultured cells were transduced by an adenoviral vector carrying a polycistronic construct Ngn3/Pdx1/MafA/mCherry (Ad-M3C) or mCherry sequence alone as a control vector. In addition, the effects of glucagon-like peptide-1 (GLP-1) receptor agonist, exendin-4 (Ex-4) on the reprogramming process were examined. GFP(+) cells appeared 2 days after Ad-M3C transduction; the reprogramming efficiency was 8.6 ± 2.6% by day 4 after transduction. Ad-M3C also resulted in increased expression of β-cell markers insulin 1 and 2, with enhancement by Ex-4. Expression of other β-cell markers, neuroD and GLP-1 receptor, were also significantly up-regulated. The amount of insulin release into the media and insulin content of the cells were significantly higher in the Ad-M3C-transduced cells; this too was enhanced by Ex-4. The transduced cells did not secrete insulin in response to increased glucose, indicating incomplete differentiation to β-cells. Thus, cultured murine adult pancreatic cells with a duct phenotype can be directly reprogrammed to insulin-producing β-like cells by adenoviral delivery of 3 pancreatic transcription factors.

An expandable donor-free supply of functional hepatocytes for toxicology

The B-13 cell is a readily expandable rat pancreatic acinar-like cell that differentiates on simple plastic culture substrata into replicatively-senescent hepatocyte-like (B-13/H) cells in response to glucocorticoid exposure. B-13/H cells express a variety of liver-enriched and liver-specific genes, many at levels similar to hepatocytes in vivo. Furthermore, the B-13/H phenotype is maintained for at least several weeks in vitro, in contrast to normal hepatocytes which rapidly de-differentiate under the same simple – or even under more complex – culture conditions. The origin of the B-13 cell line and the current state of knowledge regarding differentiation to B-13/H cells are presented, followed by a review of recent advances in the use of B-13/H cells in a variety of toxicity endpoints. B-13 cells therefore offer Toxicologists a cost-effective and easy to use system to study a range of toxicologically-related questions. Dissecting the mechanism(s) regulating the formation of B-13/H cell may also increase the likelihood of engineering a human equivalent, providing Toxicologists with an expandable donor-free supply of functional rat and human hepatocytes, invaluable additions to the tool kit of in vitro toxicity tests.

In vitro reprogramming of pancreatic alpha cells towards a beta cell phenotype following ectopic HNF4α expression

There is currently a shortage of organ donors available for pancreatic beta cell transplantation into diabetic patients. An alternative source of beta cells is pre-existing pancreatic cells. While we know that beta cells can arise directly from alpha cells during pancreatic regeneration we do not understand the molecular basis for the switch in phenotype. The aim of the present study was to investigate if hepatocyte nuclear factor 4 alpha (HNF4α), a transcription factor essential for a normal beta cell phenotype, could induce the reprogramming of alpha cells towards potential beta cells. We utilised an in vitro model of pancreatic alpha cells, the murine αTC1-9 cell line. We initially characterised the αTC1-9 cell line before and following adenovirus-mediated ectopic expression of HNF4α. We analysed the phenotype at transcript and protein level and assessed its glucose-responsiveness. Ectopic HNF4α expression in the αTC1-9 cell line induced a change in morphology (1.7-fold increase in size), suppressed glucagon expression, induced key beta cell-specific markers (insulin, C-peptide, glucokinase, GLUT2 and Pax4) and pancreatic polypeptide (PP) and enabled the cells to secrete insulin in a glucose-regulated manner. In conclusion, HNF4α reprograms alpha cells to beta-like cells.

Krüppel-Like Factor 4 Overexpression Initiates a Mesenchymal-to-Epithelial Transition and Redifferentiation of Human Pancreatic Cells following Expansion in Long Term Adherent Culture

A replenishable source of insulin-producing cells has the potential to cure type 1 diabetes. Attempts to culture and expand pancreatic β-cells in vitro have resulted in their transition from insulin-producing epithelial cells to mesenchymal stromal cells (MSCs) with high proliferative capacity but devoid of any hormone production. The aim of this study was to determine whether the transcription factor Krüppel-like factor 4 (KLF4), could induce a mesenchymal-to-epithelial transition (MET) of the cultured cells. Islet-enriched pancreatic cells, allowed to dedifferentiate and expand in adherent cell culture, were transduced with an adenovirus containing KLF4 (Ad-Klf4). Cells were subsequently analysed for changes in cell morphology by light microscopy, and for the presence of epithelial and pancreatic markers by immunocytochemistry and quantitative RT/PCR. Infection with Ad-Klf4 resulted in morphological changes, down-regulation of mesenchymal markers, and re-expression of both epithelial and pancreatic cell markers including insulin and transcription factors specific to β-cells. This effect was further enhanced by culturing cells in suspension. However, the effects of Ad-KLf4 were transient and this was shown to be due to increased apoptosis in Klf4-expressing cells. Klf4 has been recently identified as a pioneer factor with the ability to modulate the structure of chromatin and enhance reprogramming/transdifferentiation. Our results show that Klf4 may have a role in the redifferentiation of expanded pancreatic cells in culture, but before this can be achieved the off-target effects that result in increased apoptosis would need to be overcome.

Cell replacement strategies aimed at reconstitution of the β-cell compartment in type 1 diabetes

Emerging technologies in regenerative medicine have the potential to restore the β-cell compartment in diabetic patients, thereby overcoming the inadequacies of current treatment strategies and organ supply. Novel approaches include: 1) Encapsulation technology that protects islet transplants from host immune surveillance; 2) stem cell therapies and cellular reprogramming, which seek to regenerate the depleted β-cell compartment; and 3) whole-organ bioengineering, which capitalizes on the innate properties of the pancreas extracellular matrix to drive cellular repopulation. Collaborative efforts across these subfields of regenerative medicine seek to ultimately produce a bioengineered pancreas capable of restoring endocrine function in patients with insulin-dependent diabetes.

Suppression of epithelial-to-mesenchymal transitioning enhances ex vivo reprogramming of human exocrine pancreatic tissue toward functional insulin-producing β-like cells

Because of the lack of tissue available for islet transplantation, new sources of β-cells have been sought for the treatment of type 1 diabetes. The aim of this study was to determine whether the human exocrine-enriched fraction from the islet isolation procedure could be reprogrammed to provide additional islet tissue for transplantation. The exocrine-enriched cells rapidly dedifferentiated in culture and grew as a mesenchymal monolayer. Genetic lineage tracing confirmed that these mesenchymal cells arose, in part, through a process of epithelial-to-mesenchymal transitioning (EMT). A protocol was developed whereby transduction of these mesenchymal cells with adenoviruses containing Pdx1, Ngn3, MafA, and Pax4 generated a population of cells that were enriched in glucagon-secreting α-like cells. Transdifferentiation or reprogramming toward insulin-secreting β-cells was enhanced, however, when using unpassaged cells in combination with inhibition of EMT by inclusion of Rho-associated kinase (ROCK) and transforming growth factor-β1 inhibitors. Resultant cells were able to secrete insulin in response to glucose and on transplantation were able to normalize blood glucose levels in streptozotocin diabetic NOD/SCID mice. In conclusion, reprogramming of human exocrine-enriched tissue can be best achieved using fresh material under conditions whereby EMT is inhibited, rather than allowing the culture to expand as a mesenchymal monolayer.

A Small Molecule Swertisin from Enicostemma littorale Differentiates NIH3T3 Cells into Islet-Like Clusters and Restores Normoglycemia upon Transplantation in Diabetic Balb/c Mice

Aim. Stem cell therapy is one of the upcoming therapies for the treatment of diabetes. Discovery of potent differentiating agents is a prerequisite for increasing islet mass. The present study is an attempt to screen the potential of novel small biomolecules for their differentiating property into pancreatic islet cells using NIH3T3, as representative of extra pancreatic stem cells/progenitors. Methods. To identify new agents that stimulate islet differentiation, we screened various compounds isolated from Enicostemma littorale using NIH3T3 cells and morphological changes were observed. Characterization was performed by semiquantitative RT-PCR, Q-PCR, immunocytochemistry, immunoblotting, and insulin secretion assay for functional response in newly generated islet-like cell clusters (ILCC). Reversal of hyperglycemia was monitored after transplanting ILCC in STZ-induced diabetic mice. Results. Among various compounds tested, swertisin, an isolated flavonoid, was the most effective in differentiating NIH3T3 into endocrine cells. Swertisin efficiently changed the morphology of NIH3T3 cells from fibroblastic to round aggregate cell cluster in huge numbers. Dithizone (DTZ) stain primarily confirmed differentiation and gene expression studies signified rapid onset of differentiation signaling cascade in swertisin-induced ILCC. Molecular imaging and immunoblotting further confirmed presence of islet specific proteins. Moreover, glucose induced insulin release (in vitro) and decreased fasting blood glucose (FBG) (in vivo) in transplanted diabetic BALB/c mice depicted functional maturity of ILCC. Insulin and glucagon expression in excised islet grafts illustrated survival and functional integrity. Conclusions. Rapid induction for islet differentiation by swertisin, a novel herbal biomolecule, provides low cost and readily available differentiating agent that can be translated as a therapeutic tool for effective treatment in diabetes.

Pancreatic transcription factors containing protein transduction domains drive mouse embryonic stem cells towards endocrine pancreas

Protein transduction domains (PTDs), such as the HIV1-TAT peptide, have been previously used to promote the uptake of proteins into a range of cell types, including stem cells. Here we generated pancreatic transcription factors containing PTD sequences and administered these to endoderm enriched mouse embryonic stem (ES) cells under conditions that were designed to mimic the pattern of expression of these factors in the developing pancreas. The ES cells were first cultured as embryoid bodies and treated with Activin A and Bone morphogenetic protein 4 (BMP4) to promote formation of definitive endoderm. Cells were subsequently plated as a monolayer and treated with different combinations of the modified recombinant transcription factors Pdx1 and MafA. The results demonstrate that each transcription factor was efficiently taken up by the cells, where they were localized in the nuclei. RT-qPCR was used to measure the expression levels of pancreatic markers. After the addition of Pdx1 alone for a period of five days, followed by the combination of Pdx1 and TAT-MafA in a second phase, up-regulation of insulin 1, insulin 2, Pdx1, Glut2, Pax4 and Nkx6.1 was observed. As assessed by immunocytochemistry, double positive insulin and Pdx1 cells were detected in the differentiated cultures. Although the pattern of pancreatic markers expression in these cultures was comparable to that of a mouse transformed β-cell line (MIN-6) and human islets, the expression levels of insulin observed in the differentiated ES cell cultures were several orders of magnitude lower. This suggests that, although PTD-TFs may prove useful in studying the role of exogenous TFs in the differentiation of ES cells towards islets and other pancreatic lineages, the amount of insulin generated is well below that required for therapeutically useful cells.