Epithelial-to-Mesenchymal Transition Generates Proliferative Human Islet Precursor Cells

Slow and steady is the key to beta-cell replication

The beta-cells of the pancreas are responsible for insulin production and their destruction results in type I diabetes. beta-cell maintenance, growth and regenerative repair is thought to occur predominately, if not exclusively, through the replication of existing beta-cells, not via an adult stem cell. It was recently found that all beta-cells contribute equally to islet growth and maintenance. The fact that all beta-cells replicate homogeneously makes it possible to set up straightforward screens for factors that increase beta-cell replication either In vitro or in vivo. It is possible that a circulating factor may be capable of increasing beta-cell replication or that intrinsic cell cycle regulators may affect beta-cell growth. An improved understanding of the in vivo maintenance and growth of beta-cells will facilitate efforts to expand beta-cells In vitro and may lead to new treatments for diabetes.

Mesenchymal stem cells: Stem cell therapy perspectives for type 1 diabetes

Mesenchymal stem cells (MSCs) are multipotent non-haematopoietic progenitor cells that are being explored as a promising new treatment for tissue regeneration. Although their immunomodulatory properties are not yet completely understood, their low immunogenic potential together with their effects on immune response make them a promising therapeutic tool for severe refractory autoimmune diseases. Type 1 diabetes is characterized by T cell-mediated autoimmune destruction of pancreatic beta cells. While insulin replacement represents the current therapy for type 1 diabetes, its metabolic control remains difficult, as exogenous insulin cannot exactly mimic the physiology of insulin secretion. Pancreatic or islet transplantation can provide exogenous insulin independence, but is limited by its intrinsic complications and the scarcity of organ donors. In this context, stem cell therapy, based on the generation of insulin-producing cells (IPCs) derived from MSCs, represents an attractive possibility. In this review, we provide a brief characterization of MSC immunomodulatory effects, and present the current experimental evidence for the potential therapeutic efficacy of MSC transplantation in diabetes.

ISL-1 is induced in stomach mesenchyme in the presence of pancreatic epithelia

BACKGROUND/PURPOSE

beta-Cell replacement offers a potential cure for type 1 diabetes mellitus in children. We have previously shown that stomach mesenchyme (SM) is competent to derive islet tissue by mesenchymal-to-epithelial transition (iMET). The aim of this study was to further characterize the developmental fate of this SM in the presence of pancreatic epithelia (PE) in SM/PE recombinants. The homeobox ISL-1 was examined in these recombinants because this gene is restricted to the dorsal pancreatic mesenchyme and endocrine cells in early pancreatic development.

METHODS

Chick-quail recombinants of SM + PE (n = 15) and whole stomach controls (n = 8) were cultured for 7 days. In addition, organ blocks were examined after normal development at days 4 to 10 (n = 4 for each stage). Tissues were analyzed using immunochemistry against quail-specific antigen and ISL-1.

RESULTS

Thirteen of 15 SM + PE recombinants expressed the ISL-1 protein in cells from SM origin. Nine of 15 of these recombinants showed iMET and coexpression of insulin, and ISL-1 was recorded.

CONCLUSIONS

Pancreatic epithelium is able to reprogram SM to a more caudal pancreatic fate when cocultured. Islet tissue by mesenchymal-to-epithelial transition observed in recombinants showed coexpression of insulin and ISL-1. These experiments are important to identify the molecular mechanisms behind iMET for potential therapeutic use for treating children with diabetes.

Epithelial cells within the human pancreas do not coexpress mesenchymal antigens: epithelial-mesenchymal transition is an artifact of cell culture

Pancreatic mesenchymal stem cells (MSCs) may be derived from human beta-cells undergoing reversible epithelial-mesenchymal transition (EMT), suggesting that they could be a potential source of new beta-cells. In this study we sought to determine the origin of pancreatic MSCs in the nonendocrine pancreas. Double immunofluorescent (IF) staining and flow cytometry were used to assess the cell phenotype of nonendocrine pancreas tissue following islet procurement, during in vitro expansion of MSCs, and after differentiation. IF staining of paraffin-embedded pancreatic biopsy sections was used to assess cell phenotype in vivo. In this study we demonstrated that: (1) pancreatic epithelial cells do not express MSC antigens in vivo; (2) following islet isolation EpCAM- and CK19-positive epithelial cells coexpressed the MSC antigens CD44 (32+/-8% and 38+/-10%) and CD29 (85+/-4% and 64+/-4%); (3) during in vitro expansion the number of single-positive epithelial and double-positive epithelial/MSCs decreased whereas the number of single-positive MSCs increased and (4) differentiated MSCs do not revert to a true epithelial cell phenotype in our culture conditions, as epithelial cell surface markers (EpCAM, CK19 and E-Cadherin) are not reexpressed, although the MSC phenotype is altered. This study demonstrates that MSCs may be derived in vitro via a pancreatic epithelial cell undergoing EMT, however it is more likely that a small percentage of MSCs that reside in the adult pancreas are proliferating whereas the epithelial cells are negatively selected by the experimental culture conditions.

Investigation and characterization of the duct cell-enriching process during serum-free suspension and monolayer culture using the human exocrine pancreas fraction

OBJECTIVES

We aimed to characterize a serum-free culture system resulting in highly enriched duct cells from human exocrine pancreas. In addition, we tested the effect of vascular endothelial growth factor (VEGF) on endothelial cell proliferation and endocrine differentiation of the duct cells.

METHODS

The exocrine pellet fraction was cultivated in suspension followed by monolayer culture. Time course analysis of multiple acinar and duct cell markers was performed using reverse transcription-polymerase chain reaction and immunocytochemistry. The effects of VEGF and placental growth factor on the quantities of endothelial, duct, and endocrine cells and fibroblasts were investigated using computerized imaging analysis.

RESULTS

Suspension culture of the exocrine material efficiently enriched the cultures for duct cells. Frequent acinar cell death as well as cell selective adherence of acinar cells to the culture dish was the underlying cause of the enrichment. Confocal microscopy demonstrated the virtual absence of cells coexpressing duct cell- and acinar cell-specific markers. The endothelial immunoreactivity of the suspension culture system could be increased 2-fold by VEGF treatment, yet no effect was observed on endocrine cell numbers.

CONCLUSIONS

We have characterized a serum-free in vitro culture system to enrich human duct cells and further show that the contribution of acinoductal transdifferentiation to the enrichment of duct cells is negligible.

Islet-derived progenitors as a source of in vitro islet regeneration

Current therapies do not prevent the complications of diabetes. Furthermore, these therapies do not address the underlying pathology; the lack of functional beta-cell mass that occurs in both types 1 and 2 diabetes. While pancreas and islet transplantation do serve to increase beta-cell mass, a lack of donor organs limits the therapeutic potential of these treatments. As such, expansion of beta-cell mass from endogenous sources, either in vivo or in vitro, represents an area of increasing interest. One potential source of islet progenitors is the islet proper, via the dedifferentiation, proliferation, and redifferentiation of facultative progenitors residing within the islet. We have developed a tissue culture platform whereby isolated adult human pancreatic islets form proliferative duct-like structures expressing ductal and progenitor markers. Short-term treatment with a peptide fragment of islet neogenesis-associated protein (INGAP) induces these structures to reform islet-like structures that resemble freshly isolated islets with respect to the frequency and distribution of the four endocrine cell types, islet gene expression and hormone production, insulin content, and glucose-responsive insulin secretion. As such, the plasticity of adult human islets has significant implications for islet regeneration.

Can beta-cells be derived from exocrine pancreas?

A major goal of research aiming at improving islet cell replacement therapy is to find the most suitable progenitor cell type from which functional beta-cells can be generated in large numbers. Many possibilities have been raised, including beta-cells themselves, embryonic or adult stem cells and reprogramming of other cell types. Some of these progenitor types may be active or reside in a dormant state in adults in vivo, while others can be rather considered to be products of tissue engineering in vitro. Starting from the available pancreas organs from cadaveric donors, an attractive possibility is to reprogram acinar exocrine cells into beta-cells. Indeed, acinar cells isolated from adult rats display a pronounced plasticity in culture. After an initial step of dedifferentiation, they can be redirected to the beta-cell phenotype by adding agonists of the JAK2/STAT3 signalling pathway to the medium (epidermal growth factor and leukaemia inhibitory factor). The acinar cells that undergo exocrine-to-endocrine transdifferentiation first need to re-express neurogenin-3 and then need to escape inhibition by Notch signalling. The insulin-expressing cells that are generated in this way are glucose-regulated and can normalize glycaemia after transplantation into diabetic immunocompromised mice. It will now be important to translate these findings to human cells.

Expression of islet-specific microRNAs during human pancreatic development

During pancreatic islet development, sequential changes in gene expression are known to be necessary for efficient differentiation and function of the endocrine pancreas. Several studies till now have demonstrated that microRNAs (miRNAs), which regulate translation of gene transcripts, influence gene expression cascades involved in pancreas development. Some of these miRNAs; miR-7 and miR-375 have been known to be expressed at high levels in pancreas and are also known to be involved in Zebrafish pancreas development as well as insulin secretion in mice. We demonstrate here that 4 different islet-specific microRNAs (miR-7, miR-9, miR-375 and miR-376) are expressed at high levels during human pancreatic islet development. Of these, miR-375, is seen to be differentially expressed in human islet beta- as well as non-beta-cells. Though no significant difference in abundance of miR-375 was noted in either cell type, analysis of islet-specific miRNA and mRNA in single cells show that non-beta cells contain higher levels of miR-375. Our data demonstrate that miRNAs that are known to be regulated during Zebrafish pancreatic development may play similar role in human pancreatic islet development.

Dose-dependent requirement of patched homologue 1 in mouse pancreatic beta cell mass

AIMS/HYPOTHESIS

Ectopic activation of hedgehog (HH) signalling in pancreas induces various abnormal morphogenetic events in the pancreas. This study analysed the dose-dependent requirement of patched homologue 1 (PTCH1), a negative regulator of HH signalling on pancreatic development.

METHODS

We used a recessive spontaneous mutant mouse denoted as mes which carries a mutated Ptch1 resulting in deletion of the most carboxy-terminal cytoplasmic domain of the PTCH1 protein. In this study, we analysed pancreatic morphology in Ptch1 ( +/+ ), Ptch1 ( +/mes ), Ptch1 (+/-), Ptch1 ( mes/me ) (s) and Ptch1 (-/mes ) mouse embryos, as well as the islet mass in adult Ptch1 (+/+), Ptch1 (+/mes ) and Ptch1 (+/-) mice.

RESULTS

Until embryonic day (E) 12.5, no obvious abnormality of pancreas was observed in any of the Ptch1 mutants. The levels of PDX1 and glucagon were also not evidently different among the mice genotypes studied. Thereafter, morphological abnormalities appeared in the Ptch1 mutant mice. The beta, alpha and exocrine cell masses decreased at E18.5 in parallel with increased HH signalling, with beta cell mass showing the highest sensitivity to HH signalling with a significant decrease even in Ptch1 (+/mes ) mice. Adult Ptch1 (+/-) mice also showed a significant decrease in beta cell mass compared with wild-type mice.

CONCLUSIONS/INTERPRETATION

Our findings indicate that the carboxy-terminal domain of Ptch1 is essential for pancreatic development. In addition, the loss of Ptch1 function decreases both the endocrine and exocrine cell mass in a dose-dependent manner, with beta cells particularly sensitive to changes in HH signalling.

HES-1 is involved in adaptation of adult human beta-cells to proliferation in vitro

OBJECTIVE

In vitro expansion of beta-cells from adult human islets could solve the tissue shortage for cell replacement therapy of diabetes. Culture of human islet cells typically results in <16 cell doublings and loss of insulin expression. Using cell lineage tracing, we demonstrated that the expanded cell population included cells derived from beta-cells. Understanding the molecular mechanisms involved in beta-cell fate in vitro is crucial for optimizing expansion and redifferentiation of these cells. In the developing pancreas, important cell-fate decisions are regulated by NOTCH receptors, which signal through the hairy and enhancer of split (HES)-1 transcriptional regulator. Here, we investigated the role of the NOTCH signaling pathway in beta-cell dedifferentiation and proliferation in vitro.

RESEARCH DESIGN AND METHODS

Isolated human islets were dissociated into single cells. beta-Cells were genetically labeled using a Cre-lox system delivered by lentiviruses. Cells were analyzed for changes in expression of components of the NOTCH pathway during the initial weeks in culture. HES-1 expression was inhibited by a small hairpin RNA (shRNA), and the effects on beta-cell phenotype were analyzed.

RESULTS

Human beta-cell dedifferentiation and entrance into the cell cycle in vitro correlated with activation of the NOTCH pathway and downregulation of the cell cycle inhibitor p57. Inhibition of HES-1 expression using shRNA resulted in significantly reduced beta-cell replication and dedifferentiation.

CONCLUSIONS

These findings demonstrate that the NOTCH pathway is involved in determining beta-cell fate in vitro and suggest possible molecular targets for induction of beta-cell redifferentiation following in vitro expansion.

Minireview: pancreatic progenitor cells--recent studies

Past studies of pancreatic progenitor cell biology relied mostly on histological analyses. Recent studies, using genetic labeling and tracing of progenitors, direct single cell analyses, colony assays, and enrichment of the minor population of progenitor cells through the use of cell surface markers, have strongly suggested that pancreatic progenitor cells with various frequency and lineage potentials, including the multipotent progenitors that give rise to endocrine, exocrine, and duct cells, exist in the developing and adult pancreas. In this review, it is therefore proposed that pancreatic progenitor cells may be organized in a hierarchy, in which the most primitive pan-pancreatic multipotent progenitors are at the top and rare, and the monopotent progenitors are at the bottom and abundant. This model may explain why only drastic injuries lead to effective activation of the progenitor cell compartment of the higher hierarchy, whereas under steady state, pregnancy, and milder injuries, recruitment of preexisting mature cells or their immediate monopotent progenitors could be sufficient to restore metabolic homeostasis. It is also proposed that the morphologically defined ductal cells are likely to be functionally heterogeneous and that endocrine progenitor cell activity should be determined based on functional analyses rather than histological locations.

In vivo reprogramming of adult pancreatic exocrine cells to beta-cells

One goal of regenerative medicine is to instructively convert adult cells into other cell types for tissue repair and regeneration. Although isolated examples of adult cell reprogramming are known, there is no general understanding of how to turn one cell type into another in a controlled manner. Here, using a strategy of re-expressing key developmental regulators in vivo, we identify a specific combination of three transcription factors (Ngn3 (also known as Neurog3) Pdx1 and Mafa) that reprograms differentiated pancreatic exocrine cells in adult mice into cells that closely resemble beta-cells. The induced beta-cells are indistinguishable from endogenous islet beta-cells in size, shape and ultrastructure. They express genes essential for beta-cell function and can ameliorate hyperglycaemia by remodelling local vasculature and secreting insulin. This study provides an example of cellular reprogramming using defined factors in an adult organ and suggests a general paradigm for directing cell reprogramming without reversion to a pluripotent stem cell state.

Ex-vivo Expansion of Adult Human Pancreatic Beta-Cells

Ex-vivo generation of human insulin-producing cells is considered a promising approach to providing an abundant source of cells for beta-cell replacement therapy in diabetes. Expansion of adult beta-cells from the limited number of islet donors is an attractive prospect. However, while evidence supports the replicative capacity of both rodent and human beta-cells in vivo, attempts at expanding these cells in tissue culture result in loss of beta-cell phenotype, making it difficult to track their fate during continuous propagation and raising doubts about their therapeutic potential. Recent lineage-tracing studies demonstrate the ability of human beta-cells to survive and replicate to a significant degree in vitro. Beta-cell delamination out of the normal epithelial structure, a process that results in dedifferentiation, seems to be required for significant in-vitro proliferation. Therefore, ways must be found of inducing redifferentiation of the expanded cells ex vivo, or of restoring their function upon transplantation. Elucidation of the signaling pathways altered during beta-cell adaptation to growth in culture may provide clues to cell redifferentiation. In a recent study, we found that human beta-cell dedifferentiation and entrance into the cell cycle in vitro correlated with activation of the Notch pathway and downregulation of the cell cycle inhibitor p57. Inhibition of the Notch downstream target HES1 using short hairpin RNA reduced beta-cell dedifferentiation and replication, suggesting a potential target for inducing cell redifferentiation following expansion in culture. This review critically discusses the potential for using ex-vivo beta-cell replication and redifferentiation in cell replacement therapy in diabetes.

Mesenchymal stem cells derived from human exocrine pancreas express transcription factors implicated in beta-cell development

OBJECTIVES

Transplantation of in vitro generated islets or insulin-producing cells represents an attractive option to overcome organ shortage. The aim of this study was to isolate, expand, and characterize cells from human exocrine pancreas and analyze their potential to differentiate into beta cells.

METHODS

Fibroblast-like cells growing out of human exocrine tissue were characterized by flow cytometry and by their capacity to differentiate into mesenchymal cell lineages. During cell expansion and after differentiation toward beta cells, expression of transcription factors of endocrine pancreatic progenitors was analyzed by reverse transcription polymerase chain reaction.

RESULTS

Cells emerged from 14/18 human pancreatic exocrine fractions and were expanded up to 40 population doublings. These cells displayed surface antigens similar to mesenchymal stem cells from bone marrow. A culture of these cells in adipogenic and chondrogenic differentiation media allowed differentiation into adipocyte- and chondrocyte-like cells. During expansion, cells expressed transcription factors implicated in islet development such as Isl1, Nkx2.2, Nkx6.1, nestin, Ngn3, Pdx1, and NeuroD. Activin A and hepatocyte growth factor induced an expression of insulin, glucagon, and glucokinase.

CONCLUSIONS

Proliferating cells with characteristics of mesenchymal stem cells and endocrine progenitors were isolated from exocrine tissue. Under specific conditions, these cells expressed little insulin. Human pancreatic exocrine tissue might thus be a source of endocrine cell progenitors.

Cellular origins of adult human islet in vitro dedifferentiation

Cultured human islets can be dedifferentiated to duct-like structures composed mainly of cytokeratin+ and nestin+ cells. Given that these structures possess the potential to redifferentiate into islet-like structures, we sought to elucidate their specific cellular origins. Adenoviral vectors were engineered for beta-, alpha-, delta- or PP-cell-specific GFP expression. A double-stranded system was designed whereby cultures were infected with two vectors: one expressed GFP behind the cumate-inducible promoter sequence, and the other expressed the requisite transactivator behind the human insulin, glucagon, somatostatin or pancreatic polypeptide promoter. This system labels hormone+ cells in the islet in a cell-specific manner, allowing these cells to be tracked during the course of transformation from islet to duct-like structure. Post-infection, islets were cultured to induce dedifferentiation. Fluorescence microscopy demonstrated that alpha-, delta- and PP-cells contributed equally to the cytokeratin+ population, with minimal beta-cell contribution, whereas the converse was true for nestin+ cells. Complementary targeted cell ablation studies, using streptozotocin or similar adenoviral expression of the Bax (Bcl2-associated X protein) toxigene, validated these findings and suggested a redundancy between alpha-, delta- and PP-cells with respect to cytokeratin+ cell derivation. These results call into question the traditional understanding of islet cells as being terminally differentiated and provide support for the concept of adult islet morphogenetic plasticity.

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

Type 1 diabetes is characterized by the selective destruction of pancreatic β-cells caused by an autoimmune attack. Type 2 diabetes is a more complex pathology which, in addition to β-cell loss caused by apoptotic programs, includes β-cell dedifferentiation and peripheric insulin resistance. β-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 pancreaticderived insulin secretion exerts on the body’s glycemia. Restoration of damaged β-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 β-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 β-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.

Beta-catenin signalling in mesenchymal islet-derived precursor cells

OBJECTIVES

Previously, we characterized human islet-derived precursor cells (hIPCs) as mesenchymal stem cells that migrate out from islets in vitro and can differentiate into functional islet-like structures following proliferative expansion. Here, we investigate the role of beta-catenin signalling in derivation and proliferation of hIPCs.

MATERIALS AND METHODS

Localization of beta-catenin was performed using confocal microscopy. Expression levels of beta-catenin target genes were measured by quantitative real-time polymerase chain reaction. Loss-of-function studies were performed using specific short interfering RNAs.

RESULTS

Immunostaining of islet outgrowths revealed translocation of beta-catenin from plasma membranes in intact islets to the nucleus in cells migrating out. There were no nuclear beta-catenin-positive cells in intact islets whereas between 35% and 70% of cells in established hIPC cultures exhibited nuclear beta-catenin. Transcripts for beta-catenin target genes were increased in hIPCs compared to those in islets. Beta-catenin translocated to the cell membrane when hIPCs formed epithelial cell clusters. In proliferating hIPCs, there was a strong correlation between markers of proliferation and nuclear beta-catenin. Treatment of hIPCs with the glycogen synthase kinase-3beta inhibitor (2'Z,3'E)-6-Bromoindirubin-3'-oxime increased intracellular beta-catenin but reduced nuclear beta-catenin, and was associated with reduced cell proliferation. Finally, knockdown of beta-catenin decreased beta-catenin target gene expression and hIPC proliferation.

CONCLUSIONS

These results support a functional role for beta-catenin during proliferation of hIPCs and suggest that activated beta-catenin signalling may also be important during hIPC derivation from islets.

Identification and characterization of a novel expandable adult stem/progenitor cell population in the human exocrine pancreas

It is a general opinion that tissue-specific stem cells are present in adult tissues but their specific properties remain elusive. They are rare in tissues and heterogeneous; in addition, their identification and the characterization of their progeny has encountered technical difficulties. In particular, the existence of pancreatic stem cells remains elusive because specific markers for their identification are not available. We established a method for the isolation of a population of stem/progenitor cells from the human exocrine pancreas, and propose it as a model for other human compact organs. We also used markers that identified and finally characterized these cells. Spheroids with self-replicative potential were obtained from all specimens. The isolated population contained a subset of CD34+ CD45- cells and was able to generate, in appropriate conditions, colonies that produce insulin. We obtained evidence that most freshly isolated spheroids, when co-cultured with the c-kit positive neuroblastoma cell line LAN 5, produced a c-kit positive progeny of cells larger in their cytoplasmic content than the original spheroid population, with elongated morphology resembling the neuronal phenotype. We identified a novel predominant functional type of stem/progenitor cell within the human exocrine pancreas, able to generate insulin-producing cells and potentially non-pancreatic cells.

In vitro proliferation of cells derived from adult human beta-cells revealed by cell-lineage tracing

OBJECTIVE

Expansion of insulin-producing beta-cells from adult human islets could alleviate donor shortage for cell-replacement therapy of diabetes. A major obstacle to development of effective expansion protocols is the rapid loss of beta-cell markers in the cultured cells. Here, we report a genetic cell-lineage tracing approach for following the fate of cultured beta-cells.

RESEARCH DESIGN AND METHODS

Cells dissociated from isolated human islets were infected with two lentiviruses, one expressing Cre recombinase under control of the insulin promoter and the other, a reporter cassette with the structure cytomegalovirus promoter-loxP-DsRed2-loxP-eGFP.

RESULTS

Beta-cells were efficiently and specifically labeled by the dual virus system. Label(+), insulin(-) cells derived from beta-cells were shown to proliferate for a maximum of 16 population doublings, with an approximate doubling time of 7 days. Isolated labeled cells could be expanded in the absence of other pancreas cell types if provided with medium conditioned by pancreatic non-beta-cells. Analysis of mouse islet cells by the same method revealed a much lower proliferation of labeled cells under similar culture conditions.

CONCLUSIONS

Our findings provide direct evidence for survival and dedifferentiation of cultured adult human beta-cells and demonstrate that the dedifferentiated cells significantly proliferate in vitro. The findings confirm the difference between mouse and human beta-cell proliferation under our culture conditions. These findings demonstrate the feasibility of cell-specific labeling of cultured primary human cells using a genetic recombination approach that was previously restricted to transgenic animals.

Isolation of major pancreatic cell types and long-term culture-initiating cells using novel human surface markers

We have developed a novel panel of cell-surface markers for the isolation and study of all major cell types of the human pancreas. Hybridomas were selected after subtractive immunization of Balb/C mice with intact or dissociated human islets and assessed for cell-type specificity and cell-surface reactivity by immunohistochemistry and flow cytometry. Antibodies were identified by specific binding of surface antigens on islet (panendocrine or alpha-specific) and nonislet pancreatic cell subsets (exocrine and duct). These antibodies were used individually or in combination to isolate populations of alpha, beta, exocrine, or duct cells from primary human pancreas by FACS and to characterize the detailed cell composition of human islet preparations. They were also employed to show that human islet expansion cultures originated from nonendocrine cells and that insulin expression levels could be increased to up to 1% of normal islet cells by subpopulation sorting and overexpression of the transcription factors Pdx-1 and ngn3, an improvement over previous results with this culture system. These methods permit the analysis and isolation of functionally distinct pancreatic cell populations with potential for cell therapy.

Diabetes mellitus: an opportunity for therapy with stem cells?

In both Type 1 and 2 diabetes, insufficient numbers of insulin-producing beta-cells are a major cause of defective control of blood glucose and its complications. Restoration of damaged beta-cells by endocrine pancreas regeneration would be an ideal therapeutic option. The possibility of generating insulin-secreting cells with adult pancreatic stem or progenitor cells has been investigated extensively. The conversion of differentiated cells such as hepatocytes into beta-cells is being attempted using molecular insights into the transcriptional make-up of beta-cells. Additionally, the enhanced proliferation of beta-cells in vivo or in vitro is being pursued as a strategy for regenerative medicine for diabetes. Advances have also been made in directing the differentiation of embryonic stem cells into beta-cells. Although progress is encouraging, major gaps in our understanding of developmental biology of the pancreas and adult beta-cell dynamics remain to be bridged before a therapeutic application is made possible.

Phenotype reversion in fetal human liver epithelial cells identifies the role of an intermediate meso-endodermal stage before hepatic maturation

Understanding the biological potential of fetal stem/progenitor cells will help define mechanisms in liver development and homeostasis. We isolated epithelial fetal human liver cells and established phenotype-specific changes in gene expression during continuous culture conditions. Fetal human liver epithelial cells displayed stem cell properties with multilineage gene expression, extensive proliferation and generation of mesenchymal lineage cells, although the initial epithelial phenotype was rapidly supplanted by meso-endodermal phenotype in culture. This meso-endodermal phenotype was genetically regulated through cytokine signaling, including transforming growth factor beta, bone morphogenetic protein, fibroblast growth factor and other signaling pathways. Reactivation of HNF3alpha (FOXA1) transcription factor, a driver of hepatic specification in the primitive endoderm, indicated that the meso-endodermal phenotype represented an earlier developmental stage of cells. We found that fetal liver epithelial cells formed mature hepatocytes in vivo, including after genetic manipulation using lentiviral vectors, offering convenient assays for analysis of further cell differentiation and fate. Taken together, these studies demonstrate plasticity in fetal liver epithelial stem cells, offer paradigms for defining mechanisms regulating lineage switching in stem cells, and provide potential avenues for regulating cell phenotypes for applications of stem cells, such as for cell therapy.

PAX6 is expressed in pancreatic adenocarcinoma and is downregulated during induction of terminal differentiation

Tumors of the exocrine pancreas are a major cause of cancer death and have among the poorest prognosis of any malignancy. Following the "cancer stem cell hypothesis," where tumors are believed to originate in tissue specific stem cells, we screened primary ductal pancreatic carcinomas and cell lines for the expression of possible stem cell factors. We find 32/46 (70%) of primary tumors and 9/10 (90%) of cell lines express PAX6. PAX6 is a transcription factor expressed throughout the pancreatic bud during embryogenesis but not in the mature exocrine pancreas. PAX proteins have also been implicated in maintaining stem cells in a committed but undifferentiated state but a role for PAX proteins in putative pancreas stem cells is not known. We induced a pancreatic carcinoma cell line, Panc-1, to differentiate by transfecting wild-type p53 and treating the cells with differentiation agents gastrin or butyrate. This treatment induces cells to terminally differentiate into a growth-arrested cell with neurite-like processes, express the terminal differentiation marker somatostatin and downregulate PAX6. This phenotype can be replicated by directly inhibiting PAX6 expression. These data support a model where PAX proteins are aberrantly expressed in tumors and downregulation leads to differentiation.

Mesenchymal stem cells: biology and clinical potential in type 1 diabetes therapy

Mesenchymal stem cells (MSCs) can be derived from adult bone marrow, fat and several foetal tissues. In vitro, MSCs have the capacity to differentiate into multiple mesodermal and non-mesodermal cell lineages. Besides, MSCs possess immunosuppressive effects by modulating the immune function of the major cell populations involved in alloantigen recognition and elimination. The intriguing biology of MSCs makes them strong candidates for cell-based therapy against various human diseases. Type 1 diabetes is caused by a cell-mediated autoimmune destruction of pancreatic β-cells. While insulin replacement remains the cornerstone treatment for type 1 diabetes, the transplantation of pancreatic islets of Langerhans provides a cure for this disorder. And yet, islet transplantation is limited by the lack of donor pancreas. Generation of insulin-producing cells (IPCs) from MSCs represents an attractive alternative. On the one hand, MSCs from pancreas, bone marrow, adipose tissue, umbilical cord blood and cord tissue have the potential to differentiate into IPCs by genetic modification and/or defined culture conditions In vitro. On the other hand, MSCs are able to serve as a cellular vehicle for the expression of human insulin gene. Moreover, protein transduction technology could offer a novel approach for generating IPCs from stem cells including MSCs. In this review, we first summarize the current knowledge on the biological characterization of MSCs. Next, we consider MSCs as surrogate β-cell source for islet transplantation, and present some basic requirements for these replacement cells. Finally, MSCs-mediated therapeutic neovascularization in type 1 diabetes is discussed.

Stem cells, cell transplantation and liver repopulation

Liver transplantation is currently the only therapeutic option for patients with end-stage chronic liver disease and for severe acute liver failure. Because of limited donor availability, attention has been focused on the possibility to restore liver mass and function through cell transplantation. Stem cells are a promising source for liver repopulation after cell transplantation, but whether or not the adult mammalian liver contains hepatic stem cells is highly controversial. Part of the problem is that proliferation of mature adult hepatocytes is sufficient to regenerate the liver after two-thirds partial hepatectomy or acute toxic liver injury and participation of stem cells is not required. However, under conditions in which hepatocyte proliferation is blocked, undifferentiated epithelial cells in the periportal areas, called "oval cells", proliferate, differentiate into hepatocytes and restore liver mass. These cells are referred to as facultative liver stem cells, but they do not repopulate the normal liver after their transplantation. In contrast, epithelial cells isolated from the early fetal liver can effectively repopulate the normal liver, but they are already traversing the hepatic lineage and may not be true stem cells. Mesenchymal stem cells and embryonic stem cells can be induced to differentiate along the hepatic lineage in culture, but at present these cells are inefficient in repopulating the liver. This review will characterize these various cell types and compare the properties of these cells and the conditions under which they do or do not repopulate the liver following their transplantation.

Spontaneous myogenic differentiation of Flk-1-positive cells from adult pancreas and other nonmuscle tissues

At the embryonic or fetal stages, autonomously myogenic cells (AMCs), i.e., cells able to spontaneously differentiate into skeletal myotubes, have been identified from several different sites other than skeletal muscle, including the vascular compartment. However, in the adult animal, AMCs from skeletal muscle-devoid tissues have been described in only two cases. One is represented by thymic myoid cells, a restricted population of committed myogenic progenitors of unknown derivation present in the thymic medulla; the other is represented by a small subset of adipose tissue-associated cells, which we recently identified. In the present study we report, for the first time, the presence of spontaneously differentiating myogenic precursors in the pancreas and in other skeletal muscle-devoid organs such as spleen and stomach, as well as in the periaortic tissue of adult mice. Immunomagnetic selection procedures indicate that AMCs derive from Flk-1(+) progenitors. Individual clones of myogenic cells from nonmuscle organs are morphologically and functionally indistinguishable from skeletal muscle-derived primary myoblasts. Moreover, they can be induced to proliferate in vitro and are able to participate in muscle regeneration in vivo. Thus, we provide evidence that fully competent myogenic progenitors can be derived from the Flk-1(+) compartment of several adult tissues that are embryologically unrelated to skeletal muscle.

Beta-cell replacement for insulin-dependent diabetes mellitus

Beta-cell replacement is considered the optimal treatment for type 1 diabetes, however, it is hindered by a shortage of human organ donors. Given the difficulty of expanding adult beta cells in vitro, stem/progenitor cells, which can be expanded in tissue culture and induced to differentiate into multiple cell types, represent an attractive source for generation of cells with beta-cell properties. In the absence of well-characterized human pancreas progenitor cells, investigators are exploring the use of embryonic stem cells and stem/progenitor cells from other tissues. Once abundant surrogate beta cells are available, the challenge will be to protect them from recurring autoimmunity.

Induction of immune tolerance to facilitate beta cell regeneration in type 1 diabetes

A definitive cure for type 1 diabetes is currently being pursued with enormous effort by the scientific community. Different strategies are followed to restore physiologic production of insulin in diabetic patients. Restoration of self-tolerance remains the milestone that must be reached in order to move a step further and recover a cell source capable of independent and functional insulin production. Multiple strategies aimed at modulation of both central and peripheral immunity must be considered. Promising results now show that the immune system can be modulated in a way that acquisition of a "diabetes-suppressive" phenotype is possible. Once self-tolerance is achieved, reversal of the disease may be obtained by simply allowing physiologic rescue and/or regeneration of the beta cells to take place. Given that these outcomes have already been confirmed in humans, refinement of existing protocols along with novel methods adapted to T1DM reversal will allow translation into clinical trials.

Proliferation of sorted human and rat beta cells

AIMS/HYPOTHESIS

The aim of the study was to determine whether purified beta cells can replicate in vitro and whether this is enhanced by extracellular matrix (ECM) and growth factors.

METHODS

Human beta cells were purified by FACS by virtue of their high zinc content using Newport Green, and excluding ductal and dead cells. Rat beta cells were sorted by autofluorescence or using the same method developed for human cells. Cells were plated on poly-L-lysine or ECMs from rat or human bladder carcinoma cells or bovine corneal ECM and incubated in the presence of BrdU with or without growth factors.

RESULTS

The newly developed method for sorting human beta cells yields a population containing 91.4 +/- 2.8% insulin-positive cells with a low level of spontaneous apoptosis and a robust secretory response to glucose. Beta cells from 8-week-old rats proliferated in culture and this was increased by ECM. Among growth factors, only human growth hormone (hGH) and the glucagon-like peptide-1 analogue liraglutide enhanced proliferation of rat beta cells, with a significant increase on both poly-L-lysine and ECM. By contrast, sorted adult human beta cells from 16 donors aged 48.9 +/- 14.3 years (range 16-64 years) failed to replicate demonstrably in vitro regardless of the substratum or growth factors used.

CONCLUSIONS/INTERPRETATION

These findings indicate that, in our conditions, the fully differentiated human adult insulin-producing beta cell was unable to proliferate in vitro. This has important implications for any attempt to expand cells from pancreases of donors of this age group. By contrast, the rat beta cells used here were able to divide in vitro, and this was enhanced by ECM, hGH and liraglutide.

New pancreas from old: microregulators of pancreas regeneration

MicroRNAs (miRNAs) are 18-22 nucleotide RNA molecules that mediate post-transcriptional gene silencing, primarily by binding to the 3' untranslated region of their target mRNA. Several studies have demonstrated the role of miRNAs in mouse pancreas development (miR-124a, miR-503, miR-541, miR-214) as well as in insulin secretion (miR-375, miR-9). Pancreatic transcription factors that are temporally expressed during early pancreas development are re-expressed during pancreas regeneration following pancreatectomy in mice. The only exception to this is Neurogenin3 (NGN3). Here, we discuss recent evidence for miRNA-mediated silencing of ngn3, which inhibits endocrine cell development via the classical 'stem cell pathway' during mouse pancreatic regeneration, thereby favoring beta-cell regeneration.

Enhanced Oxygenation Promotes β-Cell Differentiation In Vitro

Despite progress in our knowledge about pancreatic islet specification, most attempts at differentiating stem/progenitor cells into functional, transplantable beta cells have met only with moderate success thus far. A major challenge is the intrinsic simplicity of in vitro culture systems, which cannot approximate the physiological complexity of in vivo microenvironments. Oxygenation is a critical limitation of standard culture methods, and one of special relevance for the development of beta cells, known for their high O(2) requirements. Based on our understanding of islet physiology, we have tested the hypothesis that enhanced O(2) delivery (as provided by novel perfluorocarbon-based culture devices) may result in higher levels of beta-cell differentiation from progenitor cells in vitro. Using a mouse model of pancreatic development, we demonstrate that a physiological-like mode of O(2) delivery results in a very significant upregulation of endocrine differentiation markers (up to 30-fold for insulin one and 2), comparable to relevant in vivo controls. This effect was not observed by merely increasing environmental O(2) concentrations in conventional settings. Our findings indicate that O(2) plays an important role in the differentiation of beta cells from their progenitors and may open the door to more efficient islet differentiation protocols from embryonic and/or adult stem cells. Disclosure of potential conflicts of interest is found at the end of this article.

The replication of beta cells in normal physiology, in disease and for therapy

Replication of beta cells is an important source of beta-cell expansion in early childhood. The recent linkage of type 2 diabetes with several transcription factors involved in cell cycle regulation implies that growth of the beta-cell mass in early childhood might be an important determinant of risk for type 2 diabetes. Under some circumstances, including obesity and pregnancy, the beta-cell mass is adaptively increased in adult humans. The mechanisms by which this adaptive growth occurs and the relative contributions of beta-cell replication or of mechanisms independent of beta-cell replication are unknown. Also, although there is interest in the potential for beta-cell regeneration as a therapeutic approach in both type 1 and 2 diabetes, little is yet known about the potential sources of new beta cells in adult humans. In common with other cell types, replicating beta cells have an increased vulnerability to apoptosis, which is likely to limit the therapeutic value of inducing beta-cell replication in the proapoptotic environment of type 1 and 2 diabetes unless applied in conjunction with a strategy to suppress increased apoptosis.

Regeneration in liver and pancreas: time to cut the umbilical cord?

Organisms that are capable of robust tissue regeneration, including the urodele amphibians, use mechanisms that recapitulate embryonic development to regrow organs. Although mammals are not so adept at regeneration, several adult tissues exhibit partial or complete regrowth after injury. An ability to influence growth in mammalian tissues has become more imperative with the emergence of "regenerative medicine" as a discipline. For this field to fulfill its promise of providing functional tissues for clinical use, a more detailed picture will be required of how adult human tissues maintain mass during normal homeostasis and after injury. Studies of developing and regenerating liver and pancreas now suggest that mammals use distinct programs to regulate tissue growth during embryogenesis and adulthood.

Molecular and cellular key players in human islet transplantation

Human islet transplantation could represent an attractive alternative to insulin injections for the treatment of diabetes type 1. However, such an approach requires a better understanding of the molecular and cellular switches controlling β-cell function in general as well as after transplantation into the liver. Although much research has been done into the suitability of stem or progenitor cells to generate a limitless supply of human β-cells, a reproducible and efficient protocol for the differentiation of such cells into stably insulin-secreting β-cells suitable for transplantation has yet to be reported. Fueled by recent findings showing that mature β-cells are able to regenerate, many efforts have been undertaken to expand this cell pool. Unfortunately, also these approaches had problems to yield sufficiently differentiated human islet cells. The aim of this review is to summarize recent findings describing some of the molecular and cellular key players of islet biology. A more complete understanding of their orchestration and the use of new methods such as real time confocal imaging for the assessment of islet quality may yield the necessary advancements for more successful human islet transplantation.

Recovery from diabetes in mice by beta cell regeneration

The mechanisms that regulate pancreatic beta cell mass are poorly understood. While autoimmune and pharmacological destruction of insulin-producing beta cells is often irreversible, adult beta cell mass does fluctuate in response to physiological cues including pregnancy and insulin resistance. This plasticity points to the possibility of harnessing the regenerative capacity of the beta cell to treat diabetes. We developed a transgenic mouse model to study the dynamics of beta cell regeneration from a diabetic state. Following doxycycline administration, transgenic mice expressed diphtheria toxin in beta cells, resulting in apoptosis of 70%-80% of beta cells, destruction of islet architecture, and diabetes. Withdrawal of doxycycline resulted in a spontaneous normalization of blood glucose levels and islet architecture and a significant regeneration of beta cell mass with no apparent toxicity of transient hyperglycemia. Lineage tracing analysis indicated that enhanced proliferation of surviving beta cells played the major role in regeneration. Surprisingly, treatment with Sirolimus and Tacrolimus, immunosuppressants used in the Edmonton protocol for human islet transplantation, inhibited beta cell regeneration and prevented the normalization of glucose homeostasis. These results suggest that regenerative therapy for type 1 diabetes may be achieved if autoimmunity is halted using regeneration-compatible drugs.

The human insulin gene displays transcriptionally active epigenetic marks in islet-derived mesenchymal precursor cells in the absence of insulin expression

Human islet-derived precursor cells (hIPCs), mesenchymal cells derived in vitro from adult pancreas, proliferate freely and do not express insulin but can be differentiated to epithelial cells that express insulin. hIPCs have been studied with the goal of obtaining large quantities of insulin-producing cells suitable for transplantation into patients suffering from type 1 diabetes. It appeared that undifferentiated hIPCs are "committed" to a pancreatic endocrine phenotype through multiple cell divisions, suggesting that epigenetic modifications at the insulin locus could be responsible. We determined patterns of histone modifications over the insulin gene in human islets and hIPCs and compared them with HeLa and human bone marrow-derived mesenchymal stem cells (hBM-MSCs), neither of which expresses insulin. The insulin gene in islets displays high levels of histone modifications (H4 hyperacetylation and dimethylation of H3 lysine 4) typical of active genes. These are not present in HeLa and hBM-MSCs, which instead have elevated levels of H3 lysine 9 dimethylation, a mark of inactive genes. hIPCs, in contrast, show significant levels of active chromatin modifications, as much as half those seen in islets, and show no measurable H3 K9 methylation. Cells expanded from a minor population of mesenchymal stromal cells found in islets exhibit the same histone modifications as established hIPCs. We conclude that hIPCs, which do not express the insulin gene, nonetheless uniquely exhibit epigenetic marks that could poise them for activation of insulin expression. This epigenetic signature may be a general mechanism whereby tissue-derived precursor cells are committed to a distinct specification. Disclosure of potential conflicts of interest is found at the end of this article.

Human islet-derived precursor cells are mesenchymal stromal cells that differentiate and mature to hormone-expressing cells in vivo

Islet transplantation offers improved glucose homeostasis in diabetic patients, but transplantation of islets is limited by the supply of donor pancreases. Undifferentiated precursors hold promise for cell therapy because they can expand before differentiation to produce a large supply of functional insulin-producing cells. Previously, we described proliferative populations of human islet-derived precursor cells (hIPCs) from adult islets. To show the differentiation potential of hIPCs, which do not express insulin mRNA after at least 1,000-fold expansion, we generated epithelial cell clusters (ECCs) during 4 days of differentiation in vitro. After transplantation into mice, 22 of 35 ECC preparations differentiated and matured into functional cells that secreted human C-peptide in response to glucose. Transcripts for insulin, glucagon, and somatostatin in recovered ECC grafts increased with time in vivo, reaching levels approximately 1% of those in adult islets. We show that hIPCs are mesenchymal stromal cells (MSCs) that adhere to plastic, express CD73, CD90, and CD105, and can differentiate in vitro into adipocytes, chondrocytes, and osteocytes. Moreover, we find a minor population of CD105(+)/CD73(+)/CD90(+) cells in adult human islets (prior to incubation in vitro) that express insulin mRNA at low levels. We conclude that hIPCs are a specific type of pancreas-derived MSC that are capable of differentiating into hormone-expressing cells. Their ability to mature into functional insulin-secreting cells in vivo identifies them as an important adult precursor or stem cell population that could offer a virtually unlimited supply of human islet-like cells for replacement therapy in type 1 diabetes. Disclosure of potential conflicts of interest is found at the end of this article.

Inner ear hair cells produced in vitro by a mesenchymal-to-epithelial transition

Sensory hair cell loss is a major contributor to disabling hearing and balance deficits that affect >250 million people worldwide. Sound exposures, infections, drug toxicity, genetic disorders, and aging all can cause hair cell loss and lead to permanent sensory deficits. Progress toward treatments for these deficits has been limited, in part because hair cells have only been obtainable via microdissection of the anatomically complex internal ear. Attempts to produce hair cells in vitro have resulted in reports of some success but have required transplantation into embryonic ears or coculturing with other tissues. Here, we show that avian inner ear cells can be cultured and passaged for months, frozen, and expanded to large numbers without other tissues. At any point from passage 6 up to at least passage 23, these cultures can be fully dissociated and then aggregated in suspension to induce a mesenchymal-to-epithelial transition that reliably yields new polarized sensory epithelia. Those epithelia develop numerous hair cells that are crowned by hair bundles, composed of a single kinocilium and an asymmetric array of stereocilia. These hair cells exhibit rapid permeance to FM1-43, a dye that passes through open mechanotransducing channels. Because a vial of frozen cells can now provide the capacity to produce bona fide hair cells completely in vitro, these discoveries should open new avenues of research that may ultimately contribute to better treatments for hearing loss and other inner ear disorders.

New sources of pancreatic beta cells

In both type 1 and type 2 diabetes, insufficient numbers of insulin-producing beta cells are a major cause of defective control of blood glucose and its complications. Accordingly, therapies that increase functional beta-cell mass may offer a cure for diabetes. Efforts to achieve this goal explore several directions. Based on the realization that beta cells are capable of significant proliferation throughout adult life, the enhanced proliferation of beta cells in vivo or in vitro is pursued as a strategy for regenerative medicine for diabetes. Alternatively, the conversion of differentiated cells such as hepatocytes into beta cells is being attempted using molecular insights into the transcriptional makeup of beta cells. Advances were also made in directing the differentiation of embryonic stem cells into beta cells. Although progress is encouraging, major gaps in our understanding of developmental biology of the pancreas and adult beta-cell dynamics remain to be closed before a therapeutic application is made possible.

Design and analysis of a long-term live-cell imaging chamber for tracking cellular dynamics within cultured human islets of Langerhans

A means of expanding islet cell mass is urgently needed to supplement the limited availability of donor islets of Langerhans for transplant. Live cell imaging of human islets in culture has the potential to identify the specific cells and processes involved in islet expansion. A novel imaging chamber was developed to facilitate long-term three-dimensional imaging of human islets during transformation. Islets have been induced to transform into duct-like epithelial cystic structures and revert back to glucose responsive endocrine cells under appropriate conditions (Jamal et al. Cell Death Differ. 2005 12:702-712). Here we aim to further our understanding by characterizing the process at a single cell level over time-essentially constructing a high resolution recorded history of each cell and its progeny during transformation and reversion. The imaging chamber enables high resolution imaging of three-dimensional islets while maintaining the structure of the islet cells and intercellular matrix components. A mathematical model was developed to validate the imaging chamber design by determining the required chamber dimensions to avoid introduction of oxygen and nutrient transport limitations. Human islets were embedded in collagen in the imaging chamber and differential interference contrast time course images were obtained at 3 min intervals. Immunofluorescent imaging confirmed that islet phenotype was maintained for at least 5 days during imaging. Analysis of the time courses confirms our ability to identify and track individual cells over time and to observe cell death and phenotype transformation in isolated human islets.

Generation and expansion of multipotent mesenchymal progenitor cells from cultured human pancreatic islets

Cellular models and culture conditions for in vitro expansion of insulin-producing cells represent a key element to develop cell therapy for diabetes. Initial evidence that human beta-cells could be expanded after undergoing a reversible epithelial-mesenchymal transition has been recently negated by genetic lineage tracing studies in mice. Here, we report that culturing human pancreatic islets in the presence of serum resulted in the emergence of a population of nestin-positive cells. These proliferating cells were mainly C-peptide negative, although in the first week in culture, proliferating cells, insulin promoter factor-1 (Ipf-1) positive, were observed. Later passages of islet-derived cells were Ipf-1 negative and displayed a mesenchymal phenotype. These human pancreatic islet-derived mesenchymal (hPIDM) cells were expanded up to 10(14) cells and were able to differentiate toward adipocytes, osteocytes and chondrocytes, similarly to mesenchymal stem/precursor cells. Interestingly, however, under serum-free conditions, hPIDM cells lost the mesenchymal phenotype, formed islet-like clusters (ILCs) and were able to produce and secrete insulin. These data suggest that, although these cells are likely to result from preexisting mesenchymal cells rather than beta-cells, hPIDM cells represent a valuable model for further developments toward future replacement therapy in diabetes.

Differentiation of affinity-purified human pancreatic duct cells to beta-cells

To test whether pancreatic duct cells are in vitro progenitors, they were purified from dispersed islet-depleted human pancreatic tissue using CA19-9 antibody. The purified fraction was almost entirely CK19+ with no insulin+ cells, whereas the unpurified cells (crude duct) were 56% CK19+ and 0.4% insulin+ of total cells (0.7% of CK19+ cells). These cells were expanded as monolayers, aggregated under serum-free conditions, and transplanted into normoglycemic NOD/SCID mice. In crude duct grafts, insulin+ cells increased to 6.1% of CK19+ cells. Purified duct cells had slow expansion and poor aggregation, as well as engraftment. The addition of 0.1% cultured stromal cells improved these parameters. These stromal cells contained no CK19+ cells and no insulin by either quantitative RT-PCR or immunohistochemistry; stromal cell aggregates and grafts contained no insulin+ cells. Aggregation of purified duct plus stromal preparations induced insulin+ cells (0.1% of CK19+ cells), with further increase to 1.1% in grafts. Insulin mRNA mirrored these changes. In these grafts, all insulin+ cells were in duct-like structures, while in crude duct grafts, 85% were. Some insulin+ cells coexpressed duct markers (CK19 and CA19-9) and heat shock protein (HSP)27, a marker of nonislet cells, suggesting the transition from duct. Thus, purified duct cells from adult human pancreas can differentiate to insulin-producing cells.

Growth and regeneration of adult beta cells does not involve specialized progenitors

Cellular progenitors remain poorly characterized in many adult tissues, limited in part by the lack of unbiased techniques to identify progenitors and their progeny. To address this fundamental problem, we developed a novel DNA analog-based lineage-tracing technique to detect multiple rounds of cell division in vivo. Here, we apply this technique to determine the adult lineage mechanism of the insulin-secreting beta cells of pancreatic islets, an important unresolved question in diabetes research. As expected, gastrointestinal and skin epithelia involve specialized progenitors that repeatedly divide to give rise to postmitotic cells. In contrast, specialized progenitors do not contribute to adult beta cells, not even during acute beta cell regeneration. Instead, beta cells are the products of uniform self-renewal, slowed by a replication refractory period that prevents beta cells from immediately redividing. Our approach provides unbiased resolution of previously inaccessible developmental niches and can elucidate lineage mechanisms without candidate markers.