Committing embryonic stem cells to early endocrine pancreas in vitro

Nuclear proteomics and directed differentiation of embryonic stem cells

During the past decade, regenerative medicine has been the subject of intense interest due, in large part, to our growing knowledge of embryonic stem (ES) cell biology. ES cells give rise to cell lineages from the three primordial germ layers--endoderm, mesoderm, and ectoderm. This process needs to be channeled if these cells are to be differentiated efficiently and used subsequently for therapeutic purposes. Indeed, an important area of investigation involves directed differentiation to influence the lineage commitment of these pluripotent cells in vitro. Various strategies involving timely growth factor supplementation, cell co-cultures, and gene transfection are used to drive lineage specific emergence. The underlying goal is to control directly the center of gene expression and cellular programming--the nucleus. Gene expression is enabled, managed, and sustained by the collective actions and interactions of proteins found in the nucleus--the nuclear proteome--in response to extracellular signaling. Nuclear proteomics can inventory these nuclear proteins in differentiating cells and decipher their dynamics during cellular phenotypic commitment. This review details what is currently known about nuclear effectors of stem cell differentiation and describes emerging techniques in the discovery of nuclear proteomics that will illuminate new transcription factors and modulators of gene expression.

Thyroid stem cells: lessons from normal development and thyroid cancer

Ongoing advances in stem cell research have opened new avenues for therapy for many human disorders. Until recently, however, thyroid stem cells have been relatively understudied. Here, we review what is known about thyroid stem cells and explore their utility as models of normal and malignant biological development. We also discuss the cellular origin of thyroid cancer stem cells and explore the clinical implications of cancer stem cells in the thyroid gland. Since thyroid cancer is the most common form of endocrine cancer and that thyroid hormone is needed for the growth and metabolism of each cell in the body, understanding the molecular and the cellular aspects of thyroid stem cell biology will ultimately provide insights into mechanisms underlying human disease.

Translational embryology: using embryonic principles to generate pancreatic endocrine cells from embryonic stem cells

Diseases that affect endodermally derived organs such as the lungs, liver, and pancreas include cystic fibrosis, chronic hepatitis, and diabetes, respectively. Despite the prevalence of these diseases, cures remain elusive. While several promising transplantation-based therapies exist for some diseases such as Type 1 diabetes, they are currently limited by the availability of donor-derived tissues. Embryonic stem cells are a promising and renewable source of tissue for transplantation; however, directing their differentiation into specific, adult cell lineages remains a significant challenge. In this review, we will focus on one endodermally derived organ, the pancreas, and discuss how studies of embryonic pancreas development have been used as the basis for the directed, step-wise differentiation of mouse and human embryonic stem cells into pancreatic endocrine cells that are capable of rescuing Type 1 diabetes in animal models.

Glucagon-like peptide-1 activation of TCF7L2-dependent Wnt signaling enhances pancreatic beta cell proliferation

The insulinotropic hormone GLP-1 (glucagon-like peptide-1) is a new therapeutic agent that preserves or restores pancreatic beta cell mass. We report that GLP-1 and its agonist, exendin-4 (Exd4), induce Wnt signaling in pancreatic beta cells, both isolated islets, and in INS-1 cells. Basal and GLP-1 agonist-induced proliferation of beta cells requires active Wnt signaling. Cyclin D1 and c-Myc, determinants of cell proliferation, are up-regulated by Exd4. Basal endogenous Wnt signaling activity depends on Wnt frizzled receptors and the protein kinases Akt and GSK3beta but not cAMP-dependent protein kinase. In contrast, GLP-1 agonists enhance Wnt signaling via GLP-1 receptor-mediated activation of Akt and beta cell independent of GSK3beta. Inhibition of Wnt signaling by small interfering RNAs to beta-catenin or a dominant-negative TCF7L2 decreases both basal and Exd4-induced beta cell proliferation. Wnt signaling appears to mediate GLP-1-induced beta cell proliferation raising possibilities for novel treatments of diabetes.

Nicotinamide induces differentiation of embryonic stem cells into insulin-secreting cells

The poly(ADP-ribose) polymerase (PARP) inhibitor, nicotinamide, induces differentiation and maturation of fetal pancreatic cells. In addition, we have previously reported evidence that nicotinamide increases the insulin content of cells differentiated from embryonic stem (ES) cells, but the possibility of nicotinamide acting as a differentiating agent on its own has never been completely explored. Islet cell differentiation was studied by: (i) X-gal staining after neomycin selection; (ii) BrdU studies; (iii) single and double immunohistochemistry for insulin, C-peptide and Glut-2; (iv) insulin and C-peptide content and secretion assays; and (v) transplantation of differentiated cells, under the kidney capsule, into streptozotocin (STZ)-diabetic mice. Here we show that undifferentiated mouse ES cells treated with nicotinamide: (i) showed an 80% decrease in cell proliferation; (ii) co-expressed insulin, C-peptide and Glut-2; (iii) had values of insulin and C-peptide corresponding to 10% of normal mouse islets; (iv) released insulin and C-peptide in response to stimulatory glucose concentrations; and (v) after transplantation into diabetic mice, normalized blood glucose levels over 7 weeks. Our data indicate that nicotinamide decreases ES cell proliferation and induces differentiation into insulin-secreting cells. Both aspects are very important when thinking about cell therapy for the treatment of diabetes based on ES cells.

New insights into thyroid stem cells

Stem cells exhibit an extraordinary ability for self-renewal. They also give rise to many specialized cells. The potential of stem cells in regenerative medicine, developmental biology, and drug discovery has been well documented. Although advances in stem cell science have raised broad ethical concerns, it is clear that stem cell technology has revolutionized our thinking in modern biology and medicine and provided the basis for understanding many of the mechanisms controlling basic biological processes and disease mechanisms. This review details the nascent field of thyroid stem cell research, exploring the current status of thyroid stem cell differentiation from the perspectives of both developmental biology and cell replacement therapy. It highlights successes to date in the generation of thyroid follicular cells from embryonic stem cells in the laboratory and the identification and characterization of adult stem cells from human thyroid glands and thyroid cancers. Finally, it outlines future challenges with a focus on potential stem cell therapy for thyroid patients.

Stem/Progenitor cell sources of insulin-producing cells for the treatment of diabetes

Patients with diabetes experience decreased insulin secretion that is linked to a significant reduction in the number of islet cells. Reversal of diabetes can be achieved through islet transplantation, but the scarcity of donor islets severely hinders wide application of this therapeutic modality. Toward that end, embryonic stem cells, adult tissue-residing progenitor cells, and regenerating native beta-cells may serve as sources of islet cell surrogates. Insulin-producing cells generated from stem or progenitor cells display subsets of native beta-cell attributes, indicating the need for further development of methods for differentiation to completely functional beta-cells. Pharmacological approaches aiming at stimulating the in vivo/ex vivo regeneration of beta-cells have also been proposed as a way of augmenting islet cell mass. We review the current state of the generation of insulin-producing cells from different sources with emphasis on embryonic stem cells and adult progenitor cells. Challenges for the clinical use of these sources are also discussed.

Endocrine cells develop within pancreatic bud-like structures derived from mouse ES cells differentiated in response to BMP4 and retinoic acid

We have examined factors affecting the in vitro differentiation of Pdx1(GFP/w) ESCs to pancreatic endocrine cells. Inclusion of Bone Morphogenetic Protein 4 (BMP4) during the first four days of differentiation followed by a 24-hour pulse of retinoic acid (RA) induced the formation of GFP(+) embryoid bodies (EBs). GFP expression was restricted to E-cadherin(+) tubes and GFP bright (GFP(br)) buds, reminiscent of GFP(+) early foregut endoderm and GFP(br) pancreatic buds observed in Pdx1(GFP/w) embryos. These organoid structures developed without further addition of exogenous factors between days 5 and 12, suggesting that day 5 EBs contained a template for the subsequent phase of development. EBs treated with nicotinamide after day 12 of differentiation expressed markers of endocrine and exocrine differentiation, but only in cells within the GFP(br) buds. Analysis of Pdx1(GFP/w) ESCs modified by targeting a dsRed1 gene to the Ins1 locus (Pdx1(GFP/w)Ins1(RFP/w) ESCs) provided corroborating evidence that insulin positive cells arose from GFP(br) buds, mirroring the temporal relationship between pancreatic bud development and the formation of endocrine cells in the developing embryo. The readily detectable co-expression of GFP and RFP in grafts derived from transplanted EBs demonstrated the utility of Pdx1(GFP/w)Ins1(RFP/w) ESCs for investigating pancreatic differentiation in vitro and in vivo.

Hedgehog signals in pancreatic differentiation from embryonic stem cells: revisiting the neglected

Recent demonstrations of insulin expression by progenies of mouse and human embryonic stem (ES) cells have attracted interest in setting up these cells as alternative sources of beta-cells needed in diabetes cell therapy. It is widely acknowledged that information gathered in the field of developmental biology as applied to the pancreas is of relevance for designing in vitro differentiation strategies. However, looking back at the protocols used so far, it appears that the natural route toward the pancreas, which goes via the definitive endoderm, was usually bypassed. As a consequence Hedgehog signaling, the earliest inhibitor of pancreas initiation from the endoderm, was generally not considered. A recall of the status of this pathway during ES cell differentiation appears necessary, especially in the light of findings that Activin A treatment of mouse and human ES cells coax them into definitive endoderm, a lineage showing wide Hedgehog ligands expression with the potential to hinder pancreatic programming.

Generation of insulin-producing islet-like clusters from human embryonic stem cells

Recent success in pancreatic islet transplantation has energized the field to discover an alternative source of stem cells with differentiation potential to beta cells. Generation of glucose-responsive, insulin-producing beta cells from self-renewing, pluripotent human ESCs (hESCs) has immense potential for diabetes treatment. We report here the development of a novel serum-free protocol to generate insulin-producing islet-like clusters (ILCs) from hESCs grown under feeder-free conditions. In this 36-day protocol, hESCs were treated with sodium butyrate and activin A to generate definitive endoderm coexpressing CXCR4 and Sox17, and CXCR4 and Foxa2. The endoderm population was then converted into cellular aggregates and further differentiated to Pdx1-expressing pancreatic endoderm in the presence of epidermal growth factor, basic fibroblast growth factor, and noggin. Soon thereafter, expression of Ptf1a and Ngn3 was detected, indicative of further pancreatic differentiation. The aggregates were finally matured in the presence of insulin-like growth factor II and nicotinamide. The temporal pattern of pancreas-specific gene expression in the hESC-derived ILCs showed considerable similarity to in vivo pancreas development, and the final population contained representatives of the ductal, exocrine, and endocrine pancreas. The hESC-derived ILCs contained 2%-8% human C-peptide-positive cells, as well as glucagon- and somatostatin-positive cells. Insulin content as high as 70 ng of insulin/mug of DNA was measured in the ILCs, representing levels higher than that of human fetal islets. In addition, the hESC-derived ILCs contained numerous secretory granules, as determined by electron microscopy, and secreted human C-peptide in a glucose-dependent manner. Disclosure of potential conflicts of interest is found at the end of this article.

Insulin-expressing colonies developed from murine embryonic stem cell-derived progenitors

Previous studies describe a unique culture method for the commitment of murine embryonic stem cells to early endocrine pancreata. In this report, early pancreatic-like beta-cell progenitors were enriched and a colony assay devised to allow these progenitors to differentiate into insulin-expressing colonies in vitro. An embryonic stem cell line with enhanced green fluorescent protein (EGFP) inserted into one allele of neurogenin 3 (Ngn3), a marker for pancreatic endocrine progenitors, was differentiated. During the late stage of culture, 20-30% of cells were Ngn3-EGFP(+). Gene expression profiling using the PancChip microarray platform demonstrated that Ngn3-EGFP(+) cells differentially express endocrine-related genes. A novel semisolid culture method was developed to support the formation of individual insulin/C-peptide-expressing colonies from dissociated single cells. Approximately 0.1-0.6% of Ngn3-EGFP(+) cells gave rise to insulin-expressing colonies, a three- to fivefold enrichment of beta-cell-like progenitors, or insulin-expressing colony-forming units (ICFUs), compared with nonsorted cells. All of the single colonies expressed insulin II, while 69% coexpressed insulin I and 44% coexpressed glucagon. Some single colonies expressed insulin I, insulin II, and Pdx-1 (pancreatic duodenal homeobox-1), but not glucagon. In other colonies, glucagon expression overlapped with C-peptide II in double immunostaining analysis, suggesting heterogeneity among the ICFUs and their resulting colonies. Together, these results demonstrate that progenitors that have the potential to give rise to insulin-expressing cells can be derived from murine embryonic stem cells.

Enhancement of insulin-producing cell differentiation from embryonic stem cells using pax4-nucleofection method

AIM: To enhance the differentiation of insulin producing cell (IPC) ability from embryonic stem (ES) cells in vitro.

METHODS: Four-day embryoid body (EB)-formatted ES cells were dissociated as single cells for the followed plasmid DNA delivery. The use of Nucleofector™electroporator (Amaxa biosystems, Germany) in combination with medium-contained G418 provided a high efficiency of gene delivery for advanced selection. Neucleofected cells were plated on the top of fibronectin-coated Petri dishes. Addition of Ly294002 and raised the glucose in medium at 24 h before examination. The differentiation status of these cells was monitored by semi-quantitative PCR (SQ-PCR) detection of the expression of relative genes, such as oct-4, sox-17, foxa2, mixl1, pdx-1, insulin 1, glucagons and somatostatin. The percentage of IPC population on d 18 of the experiment was investigated by immunohistochemistry (IHC), and the content/secretion of insulin was estimated by ELISA assay. The mice with severe combined immunodeficiency disease (SCID) pretreated with streptozotocin (STZ) were used to eliminate plasma glucose restoration after pax4⁺ ES implantation.

RESULTS: A high efficiency of gene delivery was demonstrated when neucleofection was used in the present study; approximately 70% cells showed DsRed expression 2 d after neucleofection. By selection of medium-contained G418, the percentage of DsRed expressing cells kept high till the end of study. The pancreatic differentiation seemed to be accelerated by pax4 nucleofection. When compared to the group of cells with mock control, foxa2, mixl1, pdx1, higher insulin and somatostatin levels were detected by SQ-PCR 4 d after nucleofection in the group of pax4 expressing plasmid delivery. Approximately 55% of neucleofected cells showed insulin expression 18 d after neucleofection, and only 18% of cells showed insulin expression in mock control. The disturbance was shown by nucleofected pax4 RNAi vector; only 8% of cells expressed insulin 18 d after nucleofection. A higher IPC population was also detected in the insulin content by ELISA assay, and the glucose dependency was demonstrated in insulin secretion level. In the animal model, improvement of average plasma glucose concentration was observed in the group of pax-4 expressed ES of SCID mice pretreated with STZ, but no significant difference was observed in the group of STZ-pretreated SCID mice who were transplanted ES with mock plasmid.

CONCLUSION: Enhancement of IPC differentiation from EB-dissociated ES cells can be revealed by simply using pax4 expressing plasmid delivery. Not only more IPCs but also pancreatic differentiation-related genes can be detected by SQ-PCR. Expression of relative genes, such as foxa 2, mixl 1, pdx-1, insulin 1 and somatostatin after nucleofection, suggests that pax4 accelerates the whole differentiation progress. The higher insulin production with glucose dependent modulation suggests that pax4 expression can drive more mature IPCs. Although further determination of the entire mechanism is required, the potential of pax-4-nucleofected cells in medical treatment is promising.

Towards stem-cell therapy in the endocrine pancreas

Many approaches of stem-cell therapy for the treatment of diabetes have been described. One is the application of stem cells for replacement of nonfunctional islet cells in the native endogenous pancreas; another one is the use of stem cells as an inexhaustible source for islet-cell transplantation. During recent years three types of stem cells have been investigated: embryonic stem cells, bone-marrow-derived stem cells and organ-bound stem cells. We discuss the advantages and limitations of these different cell types. The applicability for the treatment of dysfunction of beta cells in the pancreas has been demonstrated for all three cell types, but more-detailed understanding of the sequence of events during differentiation is required to produce fully functional insulin-producing cells.

Stem cells for the treatment of diabetes

Diabetes mellitus is a devastating disease and over 6% of the population is affected worldwide. The success achieved over the last few years with islet transplantation suggest that diabetes can be cured by the replenishment of deficient beta cells. These observations are proof of concept and have intensified interest in treating diabetes or other diseases not only by cell transplantation but also by stem cells. Work with ES cells has not yet produced cells with the phenotype of true beta cells, but there has been recent progress in directing ES cells to the endoderm. Bone marrow-derived stem cells could initiate pancreatic regeneration. Pancreatic stem/progenitor cells have been identified, and the formation of new beta cells from duct, acinar and liver cells is an active area of investigation. Some agents including glucagon-like peptide-1/exendin-4 can stimulate the regeneration of beta cells in vivo. Overexpression of embryonic transcription factors in stem cells could efficiently induce their differentiation into insulin-expressing cells. New technology, known as protein transduction technology, facilitates the differentiation of stem cells into insulin-producing cells. Recent progress in the search for new sources of beta cells has opened up several possibilities for the development of new treatments for diabetes.

Isolating endoderm and understanding developmental signals: defining sequential steps of embryonic stem cell differentiation to β cells

PURPOSE OF REVIEW

False starts have marked early work towards efficiently differentiating embryonic stem cells to β cells. Recent research has returned to foundations of developmental biology by focusing first on achieving definitive endoderm differentiation as a requisite step to ultimately producing high-quality islet endocrine cells. The present review will highlight recent demonstrations of definitive endoderm differentiation from embryonic stem cells and emphasize how mesenchymal signals, learned from embryological studies, may be used to further induce pancreatic specification from embryonic stem cells.

RECENT FINDINGS

Recently, advances have been made in the identification and purification of cells having the expected characteristics of definitive endoderm, from which all pancreatic cell types are derived. These studies have defined putative markers of definitive endoderm and provided insights into embryological signals controlling endoderm formation in mice and humans. Emerging data about the relationship between developing definitive endoderm and the surrounding mesenchyme now suggest possible methods of replicating the pancreatic developmental process in embryonic stem cells in vitro.

SUMMARY

Lessons learned from understanding developmental mechanisms of definitive endoderm formation and pancreas specification in lower organisms and adapted in application to human embryonic stem cells will drive further advances in this field.

Sodium butyrate activates genes of early pancreatic development in embryonic stem cells

Embryonic stem (ES) cells can differentiate into any tissue, including pancreatic islet cell types. Protocols for the efficient generation of these cells in vitro could have therapeutic applications for type I diabetes. Here we describe a simple method for the differentiation of mouse ES cells into epithelial cells with a gene expression profile consistent with that expected of early pancreatic progenitors (PP). It is based on the addition of sodium butyrate, an agent known to induce chromatin rearrangements. Variations on the length of exposure to butyrate result in the generation of hepatocytes or PP-like cells. qRT-PCR indicates that butyrate induces mesendoderm/definitive endoderm, but not neuroectoderm differentiation. PPlike cells show a strong upregulation of Ipf1/Pdx1, p48, Isl-1 and Nkx6.1, but not Ngn3, NeuroD/ Beta2 or Pax4. PP-like cells also express the epithelial marker E-cadherin. Taken together, our observations suggest that butyrate stimulates early events of pancreatic specification, prior to the onset of endocrine differentiation. These findings are discussed in the context of the development of protocols for the in vitro differentiation of islets.

Insulin - producing cells derived from stem cells: recent progress and future directions

Type 1 diabetes is characterized by the selective destruction of pancreatic beta-cells caused by an autoimmune attack. Type 2 diabetes is a more complex pathology which, in addition to beta-cell loss caused by apoptotic programs, includes beta-cell dedifferentiation and peripheric insulin resistance. beta-Cells are responsible for insulin production, storage and secretion in accordance to the demanding concentrations of glucose and fatty acids. The absence of insulin results in death and therefore diabetic patients require daily injections of the hormone for survival. However, they cannot avoid the appearance of secondary complications affecting the peripheral nerves as well as the eyes, kidneys and cardiovascular system. These afflictions are caused by the fact that external insulin injection does not mimic the tight control that pancreatic-derived insulin secretion exerts on the body's glycemia. Restoration of damaged beta-cells by transplantation from exogenous sources or by endocrine pancreas regeneration would be ideal therapeutic options. In this context, stem cells of both embryonic and adult origin (including beta-cell/islet progenitors) offer some interesting alternatives, taking into account the recent data indicating that these cells could be the building blocks from which insulin secreting cells could be generated in vitro under appropriate culture conditions. Although in many cases insulin-producing cells derived from stem cells have been shown to reverse experimentally induced diabetes in animal models, several concerns need to be solved before finding a definite medical application. These refer mainly to the obtainment of a cell population as similar as possible to pancreatic beta-cells, and to the problems related with the immune compatibility and tumor formation. This review will summarize the different approaches that have been used to obtain insulin-producing cells from embryonic and adult stem cells, and the main problems that hamper the clinical applications of this technology.

Generation of insulin-secreting cells from human embryonic stem cells

A growth factor-mediated selection method was used to obtained insulin-secreting cells from human embryonic stem cells (hESC; Royan H1). Our resultant cells were positive for dithizone, a zinc-chelating agent known to selectively stain pancreatic beta cells and immunoreactive for antibodies against insulin, glucagon, and C-peptide. Semi-quantitative reverse transcription-polymerase chain reaction detected expression of proinsulin, insulin and other pancreatic beta-cell-related genes, such as Nkx6.1, Is11, Glut2, Pax4, and prohormone convertase2 (PC2). Moreover, glucagon, somatostatin, K(ATP)-channel genes KIR6.2 and SUR1, islet amyloid polypeptide (IAPP), PC1/3, and glucokinase (GCK) were expressed in the differentiating hESC in a developmental stage-dependent manner. Also, the addition of glucose to the culture medium triggered insulin release from differentiated cells, but transmission electron microscopy of the differentiated cells did not show typical beta-cell granules, even though secretary granules were detected. The results showed that hESC have the ability to transcribe and process insulin, but further improvements of the current method are required to generate a sufficient source of true beta cells for the treatment of diabetes mellitus.

Long-term self-renewal and directed differentiation of human embryonic stem cells in chemically defined conditions

Chemically defined medium (CDM) conditions for controlling human embryonic stem cell (hESC) fate will not only facilitate the practical application of hESCs in research and therapy but also provide an excellent system for studying the molecular mechanisms underlying self-renewal and differentiation, without the multiple unknown and variable factors associated with feeder cells and serum. Here we report a simple CDM that supports efficient self-renewal of hESCs grown on a Matrigel-coated surface over multiple passages. Expanded hESCs under such conditions maintain expression of multiple hESC-specific markers, retain the characteristic hESC morphology, possess a normal karyotype in vitro, as well as develop teratomas in vivo. Additionally, several growth factors were found to selectively induce monolayer differentiation of hESC cultures toward neural, definitive endoderm/pancreatic and early cardiac muscle cells, respectively, in our CDM conditions. Therefore, this CDM condition provides a basic platform for further characterization of hESC self-renewal and directed differentiation, as well as the development of novel therapies.

Strategies for Developing Therapeutic Application of Human Embryonic Stem Cells

The ongoing debate on human embryonic stem cells (hESC) is fuelled by ethical concerns but also by the legitimate hope that hESC could one day be used for the cure of presently untreatable human diseases. Here we discuss current approaches to and constraints upon hESC differentiation and describe their potential application in clinical medicine.

Ciliated cells differentiated from mouse embryonic stem cells

In the present study, we demonstrated that the mouse embryonic stem cells were differentiated into ciliated epithelial cells, with characteristics of normal ciliated cells. These cells expressed ciliary marker proteins, such as beta-tubulin IV and hepatocyte nuclear factor-3/forkhead homolog 4 (HFH-4), and processed microtubules were arranged in the 9 + 2 structure, which is the same specific alignment observed in normal ciliary microtubules. The cilia of these cells were beating at a frequency of 17-20 Hz. The differentiated embryoid bodies (EBs) containing these ciliated cells expressed respiratory marker genes such as thyroid transcription factor-1 and surfactant protein-C. For the induction of ciliated cells, culture of EBs in serum-free medium during the initial 2 days of the attachment was indispensable. When EBs were treated with bone morphogenetic proteins, the expression of HFH-4 was decreased, and the ciliated cells were scarcely differentiated. Previous methods for inducing ciliated cells in vitro from embryonic or adult tissues involved an air-liquid interface. The system used in this study more closely mimics the normal development of ciliated cells; thus, an added advantage of the system is as a tool for studying the differentiation mechanism of normal ciliated epithelial cells.

New sources of pancreatic beta-cells

Two major initiatives are under way to correct the beta-cell deficit of diabetes: one would generate beta-cells ex vivo that are suitable for transplantation, and the second would stimulate regeneration of beta-cells in the pancreas. Studies of ex vivo expansion suggest that beta-cells have a potential for dedifferentiation, expansion, and redifferentiation. Work with mouse and human embryonic stem (ES) cells has not yet produced cells with the phenotype of true beta-cells, but there has been recent progress in directing ES cells to endoderm. Putative islet stem/progenitor cells have been identified in mouse pancreas, and formation of new beta-cells from duct, acinar and liver cells is an active area of investigation. Peptides, including glucagon-like peptide-1/exendin-4 and the combination of epidermal growth factor and gastrin, can stimulate regeneration of beta-cells in vivo. Recent progress in the search for new sources of beta-cells has opened promising new opportunities and spawned clinical trials.

Identification and expansion of pancreatic stem/progenitor cells

Pancreatic islet transplantation represents an attractive approach for the treatment of diabetes. However, the limited availability of donor islets has largely hampered this approach. In this respect, the use of alternative sources of islets such as the ex vivo expansion and differentiation of functional endocrine cells for treating diabetes has become the major focus of diabetes research. Adult pancreatic stem cells /progenitor cells have yet to be recognized because limited markers exist for their identification. While the pancreas has the capacity to regenerate under certain circumstances, questions where adult pancreatic stem/progenitor cells are localized, how they are regulated, and even if the pancreas harbors a stem cell population need to be resolved. In this article, we review the recent achievements both in the identification as well as in the expansion of pancreatic stem/progenitor cells.

Embryonic stem cell differentiation: emergence of a new era in biology and medicine

The discovery of mouse embryonic stem (ES) cells >20 years ago represented a major advance in biology and experimental medicine, as it enabled the routine manipulation of the mouse genome. Along with the capacity to induce genetic modifications, ES cells provided the basis for establishing an in vitro model of early mammalian development and represented a putative new source of differentiated cell types for cell replacement therapy. While ES cells have been used extensively for creating mouse mutants for more than a decade, their application as a model for developmental biology has been limited and their use in cell replacement therapy remains a goal for many in the field. Recent advances in our understanding of ES cell differentiation, detailed in this review, have provided new insights essential for establishing ES cell-based developmental models and for the generation of clinically relevant populations for cell therapy.

Ectodermal commitment of insulin‐producing cells derived from mouse embryonic stem cells

Embryonic stem cells possess the ability to differentiate in vitro into a variety of cell lineages, including insulin-producing cells. Pancreatic beta-cells derive from foregut endoderm during embryonic development. However, previous reports using transgenic mice strongly indicate that insulin-positive cells may be generated also through the neuroectoderm pathway. To analyze this point, a culture system was performed in which only ectoderm committed cells were present. Based on published work, we achieved this by maintaining transfected clonal R1 mouse embryonic stem cells in monolayer in the absence of LIF. Contrary to differentiation protocols via embryoid body formation, monolayer cultured cells displayed ectodermal fates according to the marker gene expression pattern. Under these particular conditions, neomycin was added in order to select insulin-expressing cells. The cell lineage obtained expressed Pdx1, Pax6, Isl1, AChE, MBP, TH, and GS genes, confirming ectodermal commitment, even though some of these factors are also expressed in endoderm. In addition these cells displayed excitatory properties similar to astrocytes. Co-expression of insulin II and nestin was observed in monolayer culture and in the presence of specific conditioned media. No expression of early endodermal markers was detected along monolayer cultures. Altogether, these observations suggest that cells with ectoderm fates could participate in vitro in the derivation of insulin-producing cells. These results have implications for insulin gene regulation and hormone secretion in order to generate insulin-producing cells for replacement protocols in the treatment of diabetes.

Human embryonic stem cells: possibilities for human cell transplantation

Human embryonic stem (ES) cells serve as a potentially unlimited renewable source for cell transplantation targeted to treat several diseases. One advantage of embryonic stem (ES) cells over other stem cells under research is their apparently indefinite self-renewal capacity if cultured appropriately, and their ready differentiation into various cell phenotypes of all three germ layers. To date, a number of studies have reported the derivation of specific functional derivatives from human ES cells in vitro. While there have been clinical trials of human embryonal carcinoma (EC) cell-derived neurons in humans there has been no attempt as yet using human ES cell derivatives. However, the latter have been transplanted into recipient animals. In some cases ES-derived cells were shown to undergo further maturation, displayed integration with host tissue and even ameliorated the disease condition in the animal model. Recently, it has been reported that human ES cells can be genetically manipulated. Such procedures could be used to direct differentiation to a specific cell type or to reduce graft rejections by the modification of immune responses. This review highlights some of the recent advances in the field and the challenges that lie ahead before clinical trials using ES-derived cells can be contemplated.

Stem cells in the treatment of diabetes

Clinical islet transplantation trials based on cadaveric allogenic islets have demonstrated that it is indeed possible to restore near-physiological insulin secretion capacity in a type 1 diabetic patient through transplantation of insulin-producing cells. In order to develop this form of therapy to become available for the vast majority of patients with diabetes, new sources of transplantable insulin-producing cells need to be identified. Stem cells provide the best potential to achieve this goal. Controversial results have been presented concerning the existence and nature of pancreatic islet stem or precursor cells. An increasing body of evidence suggests that the pancreatic and hepatic cell types (hepatocytes, islet, acinar and ductal cells) have remarkable plasticity and can de- and trans-differentiate into each other under appropriate conditions. Elucidation of the molecular mechanisms regulating these processes could lead to clinically applicable ways of either inducing pancreatic islet regeneration in situ or to expanding the insulin-producing cells in vitro for transplantation. The emergence of human embryonic stem cells has led to an active area of research aiming to achieve targeted differentiation of these cells into a safely transplantable beta-like cell. After initial excitement, it appears that much basic research is still required before this goal could be achieved.