TGF-beta isoform signaling regulates secondary transition and mesenchymal-induced endocrine development in the embryonic mouse pancreas

A perfect islet: reviewing recent protocol developments and proposing strategies for stem cell derived functional pancreatic islets

The search for an effective cell replacement therapy for diabetes has driven the development of "perfect" pancreatic islets from human pluripotent stem cells (hPSCs). These hPSC-derived pancreatic islet-like β cells can overcome the limitations for disease modelling, drug development and transplantation therapies in diabetes. Nevertheless, challenges remain in generating fully functional and mature β cells from hPSCs. This review underscores the significant efforts made by researchers to optimize various differentiation protocols aimed at enhancing the efficiency and quality of hPSC-derived pancreatic islets and proposes methods for their improvement. By emulating the natural developmental processes of pancreatic embryogenesis, specific growth factors, signaling molecules and culture conditions are employed to guide hPSCs towards the formation of mature β cells capable of secreting insulin in response to glucose. However, the efficiency of these protocols varies greatly among different human embryonic stem cell (hESC) and induced pluripotent stem cell (hiPSC) lines. This variability poses a particular challenge for generating patient-specific β cells. Despite recent advancements, the ultimate goal remains to develop a highly efficient directed differentiation protocol that is applicable across all genetic backgrounds of hPSCs. Although progress has been made, further research is required to optimize the protocols and characterization methods that could ensure the safety and efficacy of hPSC-derived pancreatic islets before they can be utilized in clinical settings.

Extracellular matrix proteins refine microenvironments for pancreatic organogenesis from induced pluripotent stem cell differentiation

Rationale: The current understanding on manipulating signaling pathways to generate mature human islet organoids with all major hormone-secreting endocrine cell types (i.e., α, β, δ, and γ cells) from induced pluripotent stem cells (iPSCs) is insufficient. However, donor islet shortage necessitates that we produce functional islets in vitro. In this study, we aimed to find decellularized pancreatic extracellular matrix (dpECM) proteins that leverage signaling pathways and promote functional iPSC islet organogenesis. Methods: We performed proteomic analysis to identify key islet promoting factors from porcine and rat dpECM. With this, we identified collagen type II (COL2) as a potential biomaterial cue that endorses islet development from iPSCs. Using global transcriptome profiling, gene set enrichment analysis, immunofluorescence microscopy, flow cytometry, Western blot, and glucose-stimulated hormonal secretion analysis, we examined COL2's role in regulating iPSC pancreatic lineage specification and signaling pathways, critical to islet organogenesis and morphogenesis. Results: We discovered COL2 acts as a functional biomaterial that augments islet development from iPSCs, similar to collagen type V (COL5) as reported in our earlier study. COL2 substantially stimulates the formation of endocrine progenitors and subsequent islet organoids with significantly elevated expressions of pancreatic signature genes and proteins. Furthermore, it enhances islets' glucose sensitivity for hormonal secretion. A cluster of gene expressions associated with various signaling pathways, including but not limited to oxidative phosphorylation, insulin secretion, cell cycle, the canonical WNT, hypoxia, and interferon-γ response, were considerably affected by COL2 and COL5 cues. Conclusion: We demonstrated dpECM's crucial role in refining stem cell differentiation microenvironments for organoid development and maturation. Our findings on biomaterial-stimulated signaling for stem cell specification, organogenesis, and maturation open up a new way to increase the differentiation efficacy of endocrine tissues that can contribute to the production of biologically functional islets.

CFTR represses a PDX1 axis to govern pancreatic ductal cell fate

Inflammation, acinar atrophy, and ductal hyperplasia drive pancreatic remodeling in newborn cystic fibrosis (CF) ferrets lacking a functional cystic fibrosis conductance regulator (CFTR) channel. These changes are associated with a transient phase of glucose intolerance that involves islet destruction and subsequent regeneration near hyperplastic ducts. The phenotypic changes in CF ductal epithelium and their impact on islet function are unknown. Using bulk RNA sequencing (RNA-seq), single-cell RNA sequencing (scRNA-seq), and assay for transposase-accessible chromatin using sequencing (ATAC-seq) on CF ferret models, we demonstrate that ductal CFTR protein constrains PDX1 expression by maintaining PTEN and GSK3β activation. In the absence of CFTR protein, centroacinar cells adopted a bipotent progenitor-like state associated with enhanced WNT/β-Catenin, transforming growth factor β (TGF-β), and AKT signaling. We show that the level of CFTR protein, not its channel function, regulates PDX1 expression. Thus, this study has discovered a cell-autonomous CFTR-dependent mechanism by which CFTR mutations that produced little to no protein could impact pancreatic exocrine/endocrine remodeling in people with CF.

Regulation of multiple signaling pathways promotes the consistent expansion of human pancreatic progenitors in defined conditions

The unlimited expansion of human progenitor cells in vitro could unlock many prospects for regenerative medicine. However, it remains an important challenge as it requires the decoupling of the mechanisms supporting progenitor self-renewal and expansion from those mechanisms promoting their differentiation. This study focuses on the expansion of human pluripotent stem (hPS) cell-derived pancreatic progenitors (PP) to advance novel therapies for diabetes. We obtained mechanistic insights into PP expansion requirements and identified conditions for the robust and unlimited expansion of hPS cell-derived PP cells under GMP-compliant conditions through a hypothesis-driven iterative approach. We show that the combined stimulation of specific mitogenic pathways, suppression of retinoic acid signaling, and inhibition of selected branches of the TGFβ and Wnt signaling pathways are necessary for the effective decoupling of PP proliferation from differentiation. This enabled the reproducible, 2000-fold, over 10 passages and 40-45 d, expansion of PDX1+/SOX9+/NKX6-1+ PP cells. Transcriptome analyses confirmed the stabilization of PP identity and the effective suppression of differentiation. Using these conditions, PDX1+/SOX9+/NKX6-1+ PP cells, derived from different, both XY and XX, hPS cell lines, were enriched to nearly 90% homogeneity and expanded with very similar kinetics and efficiency. Furthermore, non-expanded and expanded PP cells, from different hPS cell lines, were differentiated in microwells into homogeneous islet-like clusters (SC-islets) with very similar efficiency. These clusters contained abundant β-cells of comparable functionality as assessed by glucose-stimulated insulin secretion assays. These findings established the signaling requirements to decouple PP proliferation from differentiation and allowed the consistent expansion of hPS cell-derived PP cells. They will enable the establishment of large banks of GMP-produced PP cells derived from diverse hPS cell lines. This approach will streamline SC-islet production for further development of the differentiation process, diabetes research, personalized medicine, and cell therapies.

Deciphering early human pancreas development at the single-cell level

Understanding pancreas development can provide clues for better treatments of pancreatic diseases. However, the molecular heterogeneity and developmental trajectory of the early human pancreas are poorly explored. Here, we performed large-scale single-cell RNA sequencing and single-cell assay for transposase accessible chromatin sequencing of human embryonic pancreas tissue obtained from first-trimester embryos. We unraveled the molecular heterogeneity, developmental trajectories and regulatory networks of the major cell types. The results reveal that dorsal pancreatic multipotent cells in humans exhibit different gene expression patterns than ventral multipotent cells. Pancreato-biliary progenitors that generate ventral multipotent cells in humans were identified. Notch and MAPK signals from mesenchymal cells regulate the differentiation of multipotent cells into trunk and duct cells. Notably, we identified endocrine progenitor subclusters with different differentiation potentials. Although the developmental trajectories are largely conserved between humans and mice, some distinct gene expression patterns have also been identified. Overall, we provide a comprehensive landscape of early human pancreas development to understand its lineage transitions and molecular complexity.

Determinants and dynamics of pancreatic islet architecture

The islets of Langerhans are highly organized structures that have species-specific, three-dimensional tissue architecture. Islet architecture is critical for proper hormone secretion in response to nutritional stimuli. Islet architecture is disrupted in all types of diabetes mellitus and in cadaveric islets for transplantation during isolation, culture, and perfusion, limiting patient outcomes. Moreover, recapitulating native islet architecture remains a key challenge for in vitro generation of islets from stem cells. In this review, we discuss work that has led to the current understanding of determinants of pancreatic islet architecture, and how this architecture is maintained or disrupted during tissue remodeling in response to normal and pathological metabolic changes. We further discuss both empirical and modeling data that highlight the importance of islet architecture for islet function.

Emerging diabetes therapies: Bringing back the β-cells

BACKGROUND

Stem cell therapies are finally coming of age as a viable alternative to pancreatic islet transplantation for the treatment of insulin-dependent diabetes. Several clinical trials using human embryonic stem cell (hESC)-derived β-like cells are currently underway, with encouraging preliminary results. Remaining challenges notwithstanding, these strategies are widely expected to reduce our reliance on human isolated islets for transplantation procedures, making cell therapies available to millions of diabetic patients. At the same time, advances in our understanding of pancreatic cell plasticity and the molecular mechanisms behind β-cell replication and regeneration have spawned a multitude of translational efforts aimed at inducing β-cell replenishment in situ through pharmacological means, thus circumventing the need for transplantation.

SCOPE OF REVIEW

We discuss here the current state of the art in hESC transplantation, as well as the parallel quest to discover agents capable of either preserving the residual mass of β-cells or inducing their proliferation, transdifferentiation or differentiation from progenitor cells.

MAJOR CONCLUSIONS

Stem cell-based replacement therapies in the mold of islet transplantation are already around the corner, but a permanent cure for type 1 diabetes will likely require the endogenous regeneration of β-cells aided by interventions to restore the immune balance. The promise of current research avenues and a strong pipeline of clinical trials designed to tackle these challenges bode well for the realization of this goal.

Role of TGF-Beta Signaling in Beta Cell Proliferation and Function in Diabetes

Beta (β) cell dysfunction or loss is the common pathological feature in all types of diabetes mellitus (diabetes). Resolving the underlying mechanism may facilitate the treatment of diabetes by preserving the β cell population and function. It is known that TGF-β signaling plays diverse roles in β cell development, function, proliferation, apoptosis, and dedifferentiation. Inhibition of TGF-β signaling expands β cell lineage in the development. However, deletion of Tgfbr1 has no influence on insulin demand-induced but abolishes inflammation-induced β cell proliferation. Among canonical TGF-β signaling, Smad3 but not Smad2 is the predominant repressor of β cell proliferation in response to systemic insulin demand. Deletion of Smad3 simultaneously improves β cell function, apoptosis, and systemic insulin resistance with the consequence of eliminated overt diabetes in diabetic mouse models, revealing Smad3 as a key mediator and ideal therapeutic target for type-2 diabetes. However, Smad7 shows controversial effects on β cell proliferation and glucose homeostasis in animal studies. On the other hand, overexpression of Tgfb1 prevents β cells from autoimmune destruction without influence on β cell function. All these findings reveal the diverse regulatory roles of TGF-β signaling in β cell biology.

Activation of Pancreatic Stellate Cells Is Beneficial for Exocrine but Not Endocrine Cell Differentiation in the Developing Human Pancreas

Pancreatic stellate cells (PaSCs) are non-endocrine, mesenchymal-like cells that reside within the peri-pancreatic tissue of the rodent and human pancreas. PaSCs regulate extracellular matrix (ECM) turnover in maintaining the integrity of pancreatic tissue architecture. Although there is evidence indicating that PaSCs are involved in islet cell survival and function, its role in islet cell differentiation during human pancreatic development remains unclear. The present study examines the expression pattern and functional role of PaSCs in islet cell differentiation of the developing human pancreas from late 1st to 2nd trimester of pregnancy. The presence of PaSCs in human pancreata (8–22 weeks of fetal age) was characterized by ultrastructural, immunohistological, quantitative RT-PCR and western blotting approaches. Using human fetal PaSCs derived from pancreata at 14–16 weeks, freshly isolated human fetal islet-epithelial cell clusters (hIECCs) were co-cultured with active or inactive PaSCs in vitro. Ultrastructural and immunofluorescence analysis demonstrated a population of PaSCs near ducts and newly formed islets that appeared to make complex cell-cell dendritic-like contacts. A small subset of PaSCs co-localized with pancreatic progenitor-associated transcription factors (PDX1, SOX9, and NKX6-1). PaSCs were highly proliferative, with significantly higher mRNA and protein levels of PaSC markers (desmin, αSMA) during the 1st trimester of pregnancy compared to the 2nd trimester. Isolated human fetal PaSCs were identified by expression of stellate cell markers and ECM. Suppression of PaSC activation, using all-trans retinoic acid (ATRA), resulted in reduced PaSC proliferation and ECM proteins. Co-culture of hIECCs, directly on PaSCs or indirectly using Millicell^® Inserts or using PaSC-conditioned medium, resulted in a reduction the number of insulin⁺ cells but a significant increase in the number of amylase⁺ cells. Suppression of PaSC activation or Notch activity during the co-culture resulted in an increase in beta-cell differentiation. This study determined that PaSCs, abundant during the 1st trimester of pancreatic development but decreased in the 2nd trimester, are located near ductal and islet structures. Direct and indirect co-cultures of hIECCs with PaSCs suggest that activation of PaSCs has opposing effects on beta-cell and exocrine cell differentiation during human fetal pancreas development, and that these effects may be dependent on Notch signaling.

TGF-β Signaling in Pancreatic Islet β Cell Development and Function

Pancreatic islet beta cells (β-cells) synthesize and secrete insulin in response to rising glucose levels and thus are a prime target in both major forms of diabetes. Type 1 diabetes ensues due to autoimmune destruction of β-cells. On the other hand, the prevailing insulin resistance and hyperglycemia in type 2 diabetes (T2D) elicits a compensatory response from β-cells that involves increases in β-cell mass and function. However, the sustained metabolic stress results in β-cell failure, characterized by severe β-cell dysfunction and loss of β-cell mass. Dynamic changes to β-cell mass also occur during pancreatic development that involves extensive growth and morphogenesis. These orchestrated events are triggered by multiple signaling pathways, including those representing the transforming growth factor β (TGF-β) superfamily. TGF-β pathway ligands play important roles during endocrine pancreas development, β-cell proliferation, differentiation, and apoptosis. Furthermore, new findings are suggestive of TGF-β's role in regulation of adult β-cell mass and function. Collectively, these findings support the therapeutic utility of targeting TGF-β in diabetes. Summarizing the role of the various TGF-β pathway ligands in β-cell development, growth and function in normal physiology, and during diabetes pathogenesis is the topic of this mini-review.

SMAD7 enhances adult β-cell proliferation without significantly affecting β-cell function in mice

The interplay between the transforming growth factor β (TGF-β) signaling proteins, SMAD family member 2 (SMAD2) and 3 (SMAD3), and the TGF-β-inhibiting SMAD, SMAD7, seems to play a vital role in proper pancreatic endocrine development and also in normal β-cell function in adult pancreatic islets. Here, we generated conditional SMAD7 knockout mice by crossing insulin1^Cre mice with SMAD7^fx/fx mice. We also created a β cell-specific SMAD7-overexpressing mouse line by crossing insulin1^Dre mice with HPRT-SMAD7/RosaGFP mice. We analyzed β-cell function in adult islets when SMAD7 was either absent or overexpressed in β cells. Loss of SMAD7 in β cells inhibited proliferation, and SMAD7 overexpression enhanced cell proliferation. However, alterations in basic glucose homeostasis were not detectable following either SMAD7 deletion or overexpression in β cells. Our results show that both the absence and overexpression of SMAD7 affect TGF-β signaling and modulates β-cell proliferation but does not appear to alter β-cell function. Reversible SMAD7 overexpression may represent an attractive therapeutic option to enhance β-cell proliferation without negative effects on β-cell function.

Expansion and Maintenance of CD133-Expressing Pancreatic Ductal Epithelial Cells by Inhibition of TGF-β Signaling

Restoring β-cell mass by the transplantation of pancreatic islets is an effective diabetes treatment, but it is limited by the shortage of donor organs. CD133-expressing pancreatic ductal epithelial cells (PDECs) have the ability to generate insulin-producing cells. The expansion of these cells is dependent on extrinsic niche factors, but few of those signals have been identified. In this study, CD133-expressing PDECs were purified by sorting from adult wild-type C57BL/6 mice and TGFβRII^null/null mice. Furthermore, using immunofluorescence and transplantation assays, we found that the inhibition of the transforming growth factor-β (TGF-β) pathway promoted the expansion of CD133-expressing PDECs for many generations and maintained the ability of CD133-expressing PDECs to generate insulin-producing cells. Moreover, western blot, qRT-PCR, and dual luciferase assays using TGF-β inhibitors were performed to identify the mechanisms by which TGF-β signaling regulates proliferation and differentiation. The results showed that the inhibition of TGF-β signaling enhanced Id2 binding to the promoter region of the cell proliferation repressor p16 and promoted the expansion of CD133-expressing PDECs, and the increased Id2 binding to NeuroD1 decreased the transcription of Pax6 to maintain CD133-expressing PDECs in the Pdx1-expression stage. Taken together, the effect of TGF-β antagonists on CD133-expressing PDECs reveals a novel paradigm of signaling that explains the balance between the expansion and differentiation of pancreatic duct epithelial progenitors.

Long non-coding RNA MEG3 mediates high glucose-induced endothelial cell dysfunction

Long noncoding RNAs (lncRNAs) are implicated in the progression of diabetes mellitus (DM) and diabetes-induced endothelial dysfunction. Maternally expressed gene 3 (MEG3) encodes an lncRNA which is suggested to function as a tumor suppressor. Therefore, the aim of the present study was to investigate whether MEG3 is a potential regulator and molecular biomarker of high glucose-induced endothelial dysfunction. LncRNA Meg3-specific small interfering RNA (siRNA) and scrambled (Scr) siRNA were transfected for MEG3 dysfunction studies. RNA and protein expression were examined by quantitative RT-PCR (qPCR) and Western blot, respectively. The percentage of apoptotic cells was measured by flow cytometry. Cell viability was determined through MTT assay. This study demonstrates involvement of lncRNA MEG3 in high glucose-induced endothelial dysfunction. MEG3 is significantly downregulated in an endothelial cell model of hyperglycemia. In addition, MEG3 knockdown could exacerbate inflammatory damage in endothelial cells. Interestingly, MEG3 knockdown in HUVECs significantly induced proliferation and inhibited apoptosis by upregulating Bcl-2 and downregulating Bax, caspase-3, and P53. It should be noted that MEG3 knockdown could activate the TGF-β signaling pathway via upregulating TGF-β1, SMAD2, and SMAD7 and activate the Wnt/β-catenin signaling pathway via upregulating β-catenin and Cyclin D1 and downregulating TCF7L2. Our results indicate that MEG3 can be regarded as a novel therapeutic target and molecular biomarker for high glucose-induced endothelial dysfunction.

TGF-β Family Signaling in Ductal Differentiation and Branching Morphogenesis

Epithelial cells contribute to the development of various vital organs by generating tubular and/or glandular architectures. The fully developed forms of ductal organs depend on processes of branching morphogenesis, whereby frequency, total number, and complexity of the branching tissue define the final architecture in the organ. Some ductal tissues, like the mammary gland during pregnancy and lactation, disintegrate and regenerate through periodic cycles. Differentiation of branched epithelia is driven by antagonistic actions of parallel growth factor systems that mediate epithelial-mesenchymal communication. Transforming growth factor-β (TGF-β) family members and their extracellular antagonists are prominently involved in both normal and disease-associated (e.g., malignant or fibrotic) ductal tissue patterning. Here, we discuss collective knowledge that permeates the roles of TGF-β family members in the control of the ductal tissues in the vertebrate body.

Cellular and molecular mechanisms coordinating pancreas development

The pancreas is an endoderm-derived glandular organ that participates in the regulation of systemic glucose metabolism and food digestion through the function of its endocrine and exocrine compartments, respectively. While intensive research has explored the signaling pathways and transcriptional programs that govern pancreas development, much remains to be discovered regarding the cellular processes that orchestrate pancreas morphogenesis. Here, we discuss the developmental mechanisms and principles that are known to underlie pancreas development, from induction and lineage formation to morphogenesis and organogenesis. Elucidating such principles will help to identify novel candidate disease genes and unravel the pathogenesis of pancreas-related diseases, such as diabetes, pancreatitis and cancer.

A radial axis defined by semaphorin-to-neuropilin signaling controls pancreatic islet morphogenesis

The islets of Langerhans are endocrine organs characteristically dispersed throughout the pancreas. During development, endocrine progenitors delaminate, migrate radially and cluster to form islets. Despite the distinctive distribution of islets, spatially localized signals that control islet morphogenesis have not been discovered. Here, we identify a radial signaling axis that instructs developing islet cells to disperse throughout the pancreas. A screen of pancreatic extracellular signals identified factors that stimulated islet cell development. These included semaphorin 3a, a guidance cue in neural development without known functions in the pancreas. In the fetal pancreas, peripheral mesenchymal cells expressed Sema3a, while central nascent islet cells produced the semaphorin receptor neuropilin 2 (Nrp2). Nrp2 mutant islet cells developed in proper numbers, but had defects in migration and were unresponsive to purified Sema3a. Mutant Nrp2 islets aggregated centrally and failed to disperse radially. Thus, Sema3a-Nrp2 signaling along an unrecognized pancreatic developmental axis constitutes a chemoattractant system essential for generating the hallmark morphogenetic properties of pancreatic islets. Unexpectedly, Sema3a- and Nrp2-mediated control of islet morphogenesis is strikingly homologous to mechanisms that regulate radial neuronal migration and cortical lamination in the developing mammalian brain.

A synopsis of factors regulating beta cell development and beta cell mass

The insulin-secreting beta cells in the endocrine pancreas regulate blood glucose levels, and loss of functional beta cells leads to insulin deficiency, hyperglycemia (high blood glucose) and diabetes mellitus. Current treatment strategies for type-1 (autoimmune) diabetes are islet transplantation, which has significant risks and limitations, or normalization of blood glucose with insulin injections, which is clearly not ideal. The type-1 patients can lack insulin counter-regulatory mechanism; therefore, hypoglycemia is a potential risk. Hence, a cell-based therapy offers a better alternative for the treatment of diabetes. Past research was focused on attempting to generate replacement beta cells from stem cells; however, recently there has been an increasing interest in identifying mechanisms that will lead to the conversion of pre-existing differentiated endocrine cells into beta cells. The goal of this review is to provide an overview of several of the key factors that regulate new beta cell formation (neogenesis) and beta cell proliferation.

Inhibition of TGF-β Signaling Promotes Human Pancreatic β-Cell Replication

Diabetes is associated with loss of functional pancreatic β-cells, and restoration of β-cells is a major goal for regenerative therapies. Endogenous regeneration of β-cells via β-cell replication has the potential to restore cellular mass; however, pharmacological agents that promote regeneration or expansion of endogenous β-cells have been elusive. The regenerative capacity of β-cells declines rapidly with age, due to accumulation of p16(INK4a), resulting in limited capacity for adult endocrine pancreas regeneration. Here, we show that transforming growth factor-β (TGF-β) signaling via Smad3 integrates with the trithorax complex to activate and maintain Ink4a expression to prevent β-cell replication. Importantly, inhibition of TGF-β signaling can result in repression of the Ink4a/Arf locus, resulting in increased β-cell replication in adult mice. Furthermore, small molecule inhibitors of the TGF-β pathway promote β-cell replication in human islets transplanted into NOD-scid IL-2Rg(null) mice. These data reveal a novel role for TGF-β signaling in the regulation of the Ink4a/Arf locus and highlight the potential of using small molecule inhibitors of TGF-β signaling to promote human β-cell replication.

Intraislet Pancreatic Ducts Can Give Rise to Insulin-Positive Cells

A key question in diabetes research is whether new β-cells can be derived from endogenous, nonendocrine cells. The potential for pancreatic ductal cells to convert into β-cells is a highly debated issue. To date, it remains unclear what anatomical process would result in duct-derived cells coming to exist within preexisting islets. We used a whole-mount technique to directly visualize the pancreatic ductal network in young wild-type mice, young humans, and wild-type and transgenic mice after partial pancreatectomy. Pancreatic ductal networks, originating from the main ductal tree, were found to reside deep within islets in young mice and humans but not in mature mice or humans. These networks were also not present in normal adult mice after partial pancreatectomy, but TGF-β receptor mutant mice demonstrated formation of these intraislet duct structures after partial pancreatectomy. Genetic and viral lineage tracings were used to determine whether endocrine cells were derived from pancreatic ducts. Lineage tracing confirmed that pancreatic ductal cells can typically convert into new β-cells in normal young developing mice as well as in adult TGF-β signaling mutant mice after partial pancreatectomy. Here the direct visual evidence of ducts growing into islets, along with lineage tracing, not only represents strong evidence for duct cells giving rise to β-cells in the postnatal pancreas but also importantly implicates TGF-β signaling in this process.

A hybrid computational method for the discovery of novel reproduction-related genes

Uncovering the molecular mechanisms underlying reproduction is of great importance to infertility treatment and to the generation of healthy offspring. In this study, we discovered novel reproduction-related genes with a hybrid computational method, integrating three different types of method, which offered new clues for further reproduction research. This method was first executed on a weighted graph, constructed based on known protein-protein interactions, to search the shortest paths connecting any two known reproduction-related genes. Genes occurring in these paths were deemed to have a special relationship with reproduction. These newly discovered genes were filtered with a randomization test. Then, the remaining genes were further selected according to their associations with known reproduction-related genes measured by protein-protein interaction score and alignment score obtained by BLAST. The in-depth analysis of the high confidence novel reproduction genes revealed hidden mechanisms of reproduction and provided guidelines for further experimental validations.

Pancreatic Islet-Like Three-Dimensional Aggregates Derived From Human Embryonic Stem Cells Ameliorate Hyperglycemia in Streptozotocin-Induced Diabetic Mice

We previously reported the in vitro differentiation of human embryonic stem cells (hESCs) into pancreatic endoderm. Here we demonstrate that islet-like three-dimensional (3D) aggregates can be derived from the pancreatic endoderm by optimizing our previous protocol. Sequential treatment with Wnt3a, activin A, and noggin induced a transient upregulation of T and MixL1, followed by increased expression of endodermal genes, including FOXA2, SOX17, and CXCR4. Subsequent treatment with retinoic acid highly upregulated PDX1 expression. We also show that inhibition of sonic hedgehog signaling by bFGF/activin βB and cotreatment with VEGF and FGF7 produced many 3D cellular clusters that express both SOX17 and PDX1. We found for the first time that proteoglycans and vimentin(+) mesenchymal cells were mainly localized in hESC-derived PDX1(+) clusters. Importantly, treatment with chlorate, an inhibitor of proteoglycan sulfation, together with inhibition of Notch signaling significantly increased the expression of Neurog3 and NeuroD1, promoting a transition from PDX1(+) progenitor cells toward mature pancreatic endocrine cells. Purified dithizone(+) 3D aggregates generated by our refined protocol produced pancreatic hormones and released insulin in response to both glucose and pharmacological drugs in vitro. Furthermore, the islet-like 3D aggregates decreased blood glucose levels and continued to exhibit pancreatic features after transplantation into diabetic mice. Generation of islet-like 3D cell aggregates from human pluripotent stem cells may overcome the shortage of cadaveric donor islets for future cases of clinical islet transplantation.

Generation of β cells from human pluripotent stem cells: are we there yet?

In 1998, the landmark paper describing the isolation and culture of human embryonic stem cells (ESCs) was published. Since that time, the main goal of many diabetes researchers has been to derive β cells from ESCs as a renewable cell-based therapy for the treatment of patients with diabetes. In working toward this goal, numerous protocols that attempt to recapitulate normal pancreatic development have been published that result in the formation of pancreatic cell types from human pluripotent cells. This review examines stem cell differentiation methods and places them within the context of pancreatic development. We additionally compare strategies that are currently being used to generate pancreatic cell types and contrast them with approaches that have been used to generate functional cell types in different lineages. In doing this, we aim to identify how new approaches might be used to improve yield and functionality of in vitro-derived pancreatic β cells as an eventual cell-based therapy for type 1 diabetes.

A smad signaling network regulates islet cell proliferation

Pancreatic β-cell loss and dysfunction are critical components of all types of diabetes. Human and rodent β-cells are able to proliferate, and this proliferation is an important defense against the evolution and progression of diabetes. Transforming growth factor-β (TGF-β) signaling has been shown to affect β-cell development, proliferation, and function, but β-cell proliferation is thought to be the only source of new β-cells in the adult. Recently, β-cell dedifferentiation has been shown to be an important contributory mechanism to β-cell failure. In this study, we tie together these two pathways by showing that a network of intracellular TGF-β regulators, smads 7, 2, and 3, control β-cell proliferation after β-cell loss, and specifically, smad7 is necessary for that β-cell proliferation. Importantly, this smad7-mediated proliferation appears to entail passing through a transient, nonpathologic dedifferentiation of β-cells to a pancreatic polypeptide-fold hormone-positive state. TGF-β receptor II appears to be a receptor important for controlling the status of the smad network in β-cells. These studies should help our understanding of properly regulated β-cell replication.

The ductal origin of structural and functional heterogeneity between pancreatic islets

Islets form in the pancreas after the first endocrine cells have arisen as either single cells or small cell clusters in the epithelial cords. These cords constitute the developing pancreas in one of its earliest recognizable stages. Islet formation begins at the time the cords transform into a branching ductal system, continues while the ductal system expands, and finally stops before the exocrine tissue of ducts and acini reaches its final expansion. Thus, islets continuously arise from founder cells located in the branching and ramifying ducts. Islets arising from proximal duct cells locate between the exocrine lobules, develop strong autonomic and sensory innervations, and pass their blood to efferent veins (insulo-venous efferent system). Islets arising from cells of more distal ducts locate within the exocrine lobules, respond to nerve impulses ending at neighbouring blood vessels, and pass their blood to the surrounding acini (insulo-acinar portal system). Consequently, the section of the ductal system from which an islet arises determines to a large extent its future neighbouring tissue, architecture, properties, and functions. We note that islets interlobular in position are frequently found in rodents (rats and mice), whereas intralobularly-located, peripheral duct islets prevail in humans and cattle. Also, we expound on bovine foetal Laguesse islets as a prominent foetal type of type 1 interlobular neuro-insular complexes, similar to neuro-insular associations frequently found in rodents. Finally, we consider the probable physiological and pathophysiological implications of the different islet positions within and between species.

Factors expressed by murine embryonic pancreatic mesenchyme enhance generation of insulin-producing cells from hESCs

Islet transplantation has proven to be a successful strategy to restore normoglycemia in patients with type 1 diabetes (T1D). However, the dearth of cadaveric islets available for transplantation hampers the widespread application of this treatment option. Although human embryonic stem cells and induced pluripotent stem cells are capable of generating insulin-producing cells in vitro when provided with the appropriate inductive cues, the insulin-expressing cells that develop behave more like immature β-cells with minimal sensitivity to glucose stimulation. Here, we identify a set of signaling factors expressed in mouse embryonic mesenchyme during the time when foregut and pancreatic progenitors are specified and test their activities during in vitro differentiation of human embryonic stem cells. Several of the identified factors work in concert to expand the pancreatic progenitor pool. Interestingly, transforming growth factor (TGF)-β ligands, most potent in inducing pancreatic progenitors, display strong inhibitory effects on subsequent endocrine cell differentiation. Treatment with TGF-β ligands, followed by the addition of a TGF-β receptor antagonist, dramatically increased the number of insulin-producing cells in vitro, demonstrating the need for dynamic temporal regulation of TGF-β signaling during in vitro differentiation. These studies illustrate the need to precisely mimic the in vivo conditions to fully recapitulate pancreatic lineage specification in vitro.

TGF-β Signaling Regulates Pancreatic β-Cell Proliferation through Control of Cell Cycle Regulator p27 Expression

Proliferation of pancreatic β-cells is an important mechanism underlying β-cell mass adaptation to metabolic demands. Increasing β-cell mass by regeneration may ameliorate or correct both type 1 and type 2 diabetes, which both result from inadequate production of insulin by β-cells of the pancreatic islet. Transforming growth factor β (TGF-β) signaling is essential for fetal development and growth of pancreatic islets. In this study, we exposed HIT-T15, a clonal pancreatic β-cell line, to TGF-β signaling. We found that inhibition of TGF-β signaling promotes proliferation of the cells significantly, while TGF-β signaling stimulation inhibits proliferation of the cells remarkably. We confirmed that this proliferative regulation by TGF-β signaling is due to the changed expression of the cell cycle regulator p27. Furthermore, we demonstrated that there is no observed effect on transcriptional activity of p27 by TGF-β signaling. Our data show that TGF-β signaling mediates the cell-cycle progression of pancreatic β-cells by regulating the nuclear localization of CDK inhibitor, p27. Inhibition of TGF-β signaling reduces the nuclear accumulation of p27, and as a result this inhibition promotes proliferation of β-cells.

Smad signaling pathways regulate pancreatic endocrine development

Expansion of the pancreatic endocrine cell population occurs during both embryonic development and during post-natal pancreatic growth and regeneration. Mechanisms of the expansion of endocrine cells during embryonic development are not completely understood, and no clear mechanistic link has been established between growth of the embryonic endocrine pancreas and the islet cell replication that occurs in an adult animal. We found that transforming growth factor-beta (TGF-β) superfamily signaling, which has been implicated in many developmental processes, plays a key role in regulating pancreatic endocrine maturation and development. Specifically, the intracellular mediators of TGF-β signaling, smad2 and smad3, along with their inhibitor smad7, appear to mediate this process. Smad2, smad3 and smad7 were all broadly expressed throughout the early embryonic pancreatic epithelium. However, during later stages of development, smad2 and smad3 became strongly localized to the nuclei of the endocrine positive cells, whereas the inhibitory smad7 became absent in the endocrine component. Genetic inactivation of smad2 and smad3 led to a significant expansion of the embryonic endocrine compartment, whereas genetic inactivation of smad7 led to a significant decrease in the endocrine compartment. In vitro antisense studies further corroborated these results and supported the possibility that interplay between the inhibitory smad7 and the intracellular mediators smad2/3 is a control point for pancreatic endocrine development. These results should provide a better understanding of the key control mechanisms for β-cell development.

TGFβ receptor signaling is essential for inflammation-induced but not β-cell workload-induced β-cell proliferation

Protection and restoration of a functional β-cell mass are fundamental strategies for prevention and treatment of diabetes. Consequently, knowledge of signals that determine the functional β-cell mass is of immense clinical relevance. Transforming growth factor β (TGFβ) superfamily signaling pathways play a critical role in development and tissue specification. Nevertheless, the role of these pathways in adult β-cell homeostasis is not well defined. Here, we ablated TGFβ receptor I and II genes in mice undergoing two surgical β-cell replication models (partial pancreatectomy or partial duct ligation), representing two triggers for β-cell proliferation, increased β-cell workload and local inflammation, respectively. Our data suggest that TGFβ receptor signaling is necessary for baseline β-cell proliferation. By either provision of excess glucose or treatment with exogenous insulin, we further demonstrated that inflammation and increased β-cell workload are both stimulants for β-cell proliferation but are TGFβ receptor signaling dependent and independent, respectively. Collectively, by using a pancreas-specific TGFβ receptor-deleted mouse model, we have identified two distinct pathways that regulate adult β-cell proliferation. Our study thus provides important information for understanding β-cell proliferation during normal growth and in pancreatic diseases.

Human pluripotent stem cells: an emerging model in developmental biology

Developmental biology has long benefited from studies of classic model organisms. Recently, human pluripotent stem cells (hPSCs), including human embryonic stem cells and human induced pluripotent stem cells, have emerged as a new model system that offers unique advantages for developmental studies. Here, we discuss how studies of hPSCs can complement classic approaches using model organisms, and how hPSCs can be used to recapitulate aspects of human embryonic development 'in a dish'. We also summarize some of the recently developed genetic tools that greatly facilitate the interrogation of gene function during hPSC differentiation. With the development of high-throughput screening technologies, hPSCs have the potential to revolutionize gene discovery in mammalian development.

Rapid and simplified purification of recombinant adeno-associated virus

Preclinical gene therapy studies both in vitro and in vivo require high purity preparations of adeno-associated virus (AAV). Current methods for purification of AAV entail the use of centrifugation over either a CsCl or iodixanol gradient, or the use of chromatography. These methods can be cumbersome and expensive, necessitating ultrahigh speed gradient centrifugation or, for chromatography the use of other expensive equipment. In addition, these methods are time consuming, and the viral yield is not high. Currently no commercial purification kits are available for other than AAV serotype 2. A simplified method was used for the purification of AAV, with a viral yield that is able to be used effectively in adult and embryo mice. The method does not require ultrahigh speed gradient centrifugation nor chromatography. Instead, polyethylene glycol (PEG)/aqueous two-phase partitioning is used to remove soluble proteins from the PEG8000 precipitated virus-protein mixture. The procedure obtained rapidly up to 95% recovery of high quality purified AAV. The entire purification process, including HEK293 cell transfection, can be completed readily within one week, with purity seemingly higher than that obtained after one round of CsCl gradient purification.

Transforming growth factor-β induces epithelial to mesenchymal transition and suppresses the proliferation and transdifferentiation of cultured human pancreatic duct cells

Pancreatic duct cells are considered a potential source of β-cell regeneration, and transforming growth factor-β (TGF-β) has been suggested to perform an important role in these processes, but the underlying mechanism of the signal pathways, especially in humans, remains poorly understood. To evaluate the role of TGF-β1, pancreatic duct cells were isolated from three brain-dead organ donors. Pancreatic cell clusters harvested after islet isolation were dispersed to single cells and cultured in monolayers, then treated with TGF-β1. We analyzed the characteristics of the cultured cells, the TGF-β1 intracellular signaling pathway, the proliferation, and transdifferentiation rates of the duct cells. We also evaluated the genes and protein expression patterns after TGF-β1 treatment. After TGF-β1 treatment, typical morphologic changes representative of EMT were observed and Erk1/2, JNK, and AKT phosphorylation, Ras downstream effectors, were increased. β cell-specific transcription factors including PDX-1, Beta2/NeuroD, Ist-1, and NGN3 were markedly suppressed and the rate of transdifferentiation into β cells was also suppressed. Genomic and proteomic analyses suggested that TGF-β1 induces marked changes in a variety of structural genes and proteins associated with EMT. In conclusion, TGF-β1 induces EMT in cultured human pancreatic duct cells, but suppresses its proliferation and transdifferentiation into β cells. Our results are the first report of TGF-β1 effects for EMT and ductal cell transdifferentiation and proliferation at the protein level in human pancreatic duct cells.

Production of functional glucagon-secreting α-cells from human embryonic stem cells

OBJECTIVE

Differentiation of human embryonic stem (hES) cells to fully developed cell types holds great therapeutic promise. Despite significant progress, the conversion of hES cells to stable, fully differentiated endocrine cells that exhibit physiologically regulated hormone secretion has not yet been achieved. Here we describe an efficient differentiation protocol for the in vitro conversion of hES cells to functional glucagon-producing α- cells.

RESEARCH DESIGN AND METHODS

Using a combination of small molecule screening and empirical testing, we developed a six-stage differentiation protocol for creating functional α-cells. An extensive in vitro and in vivo characterization of the differentiated cells was performed.

RESULTS

A high rate of synaptophysin expression (>75%) and robust expression of glucagon and the α-cell transcription factor ARX was achieved. After a transient polyhormonal state in which cells coexpress glucagon and insulin, maturation in vitro or in vivo resulted in depletion of insulin and other β-cell markers with concomitant enrichment of α-cell markers. After transplantation, these cells secreted fully processed, biologically active glucagon in response to physiologic stimuli including prolonged fasting and amino acid challenge. Moreover, glucagon release from transplanted cells was sufficient to reduce demand for pancreatic glucagon, resulting in a significant decrease in pancreatic α-cell mass.

CONCLUSIONS

These results indicate that fully differentiated pancreatic endocrine cells can be created via stepwise differentiation of hES cells. These cells may serve as a useful screening tool for the identification of compounds that modulate glucagon secretion as well as those that promote the transdifferentiation of α-cells to β-cells.

Functional signature of human islet-derived precursor cells compared to bone marrow-derived mesenchymal stem cells

Pancreatic islet beta-cell replenishment can be driven by epithelial cells from exocrine pancreas via epithelial-mesenchymal transition (EMT) and the reverse process MET, while specified pancreatic mesenchymal cells control islet cell development and maintenance. The role of human islet-derived precursor cells (hIPCs) in regeneration and support of endocrine islets is under investigation. Here, we analyzed hIPCs as to their immunophenotype, multilineage differentiation capacity, and gene profiling, in comparison to human bone marrow-derived mesenchymal stem cells (hBM-MSCs). hIPCs and hBM-MSCs display a common mesenchymal character and express lineage-specific marker genes upon induction toward pancreatic endocrine and mesenchymal pathways of differentiation. hIPCs can go further along endocrine pathways while lacking some core mesenchymal differentiation attributes. Significance analysis of microarray (SAM) from 5 hBM-MSC and 3 hIPC donors mirrored such differences. Candidate gene cluster analysis disclosed differential expression of key lineage regulators, indicated a HoxA gene-associated positional memory in hIPCs and hBM-MSCs, and showed as well a clear transition state from mesenchyme to epithelium or vice versa in hIPCs. Our findings raise new research platforms to further clarify the potential of hIPCs to undergo complete MET thus contributing to islet cell replenishment, maintenance, and function.

Pancreas cell fate

Diabetes is characterized by decreased function of insulin-producing beta cells and insufficient insulin output resulting from an absolute (Type 1) or relative (Type 2) inadequate functional beta cell mass. Both forms of the disease would greatly benefit from treatment strategies that could enhance beta cell regeneration and/or function. Successful and reliable methods of generating beta cells or whole islets from progenitor cells in vivo or in vitro could lead to restoration of beta cell mass in individuals with Type 1 diabetes and enhanced beta cell compensation in Type 2 patients. A thorough understanding of the normal developmental processes that occur during pancreatic organogenesis, for example, transcription factors, cell signaling molecules, and cell-cell interactions that regulate endocrine differentiation from the embryonic pancreatic epithelium, is required in order to successfully reach these goals. This review summarizes our current understanding of pancreas development, with particular emphasis on factors intrinsic or extrinsic to the pancreatic epithelium that are involved in regulating the development and differentiation of the various pancreatic cell types. We also discuss the recent progress in generating insulin-producing cells from progenitor sources.

Overcoming self-destruction in the pancreas

Type 1 diabetes (T1D) is an autoimmune disease where insulin producing pancreatic beta cells are progressively destroyed. In the absence of a cure, exogenous insulin is given to maintain glucose homeostasis. Tolerogenic strategies to halt destruction and facilitate recovery of beta cells are being explored. This disease is under polygenic control; the identification of specific candidate pathways to target for drug discovery or corrective therapy would enhance therapeutic options. Whilst islet transplantation combined with immune suppression has shown some efficacy, the availability of beta cells restricts its general use. Differentiation protocols directing cultured stem cells into pancreatic tissue are under development. The availability of embryonic stem cells from a mouse model of T1D should facilitate dissection of the causes and progression of this disease.

Meta-analysis of gene expression in human pancreatic islets after in vitro expansion

Pancreatic islet transplantation as a potential cure for type 1 diabetes (T1D) cannot be scaled up due to a scarcity of human pancreas donors. In vitro expansion of beta-cells from mature human pancreatic islets provides an alternative source of insulin-producing cells. The exact nature of the expanded cells produced by diverse expansion protocols and their potential for differentiation into functional beta-cells remain elusive. We performed a large-scale meta-analysis of gene expression in human pancreatic islet cells, which were processed using three different previously described protocols for expansion and for which redifferentiation was attempted. All three expansion protocols induced dramatic changes in the expression profiles of pancreatic islets; many of these changes are shared among the three protocols. Attempts at redifferentiation of expanded cells induce a limited number of gene expression changes. Nevertheless, these fail to restore a pancreatic islet-like gene expression pattern. Comparison with a collection of public microarray datasets confirmed that expanded cells are highly comparable to mesenchymal stem cells. Genes induced in expanded cells are also enriched for targets of transcription factors important for pluripotency induction. The present data increase our understanding of the active pathways in expanded and redifferentiated islets. Knowledge of the mesenchymal stem cell potential may help development of drug therapeutics to restore beta-cell mass in T1D patients.

Transforming growth factor-beta/Smad3 signaling regulates insulin gene transcription and pancreatic islet beta-cell function

Pancreatic islet beta-cell dysfunction is a signature feature of Type 2 diabetes pathogenesis. Consequently, knowledge of signals that regulate beta-cell function is of immense clinical relevance. Transforming growth factor (TGF)-beta signaling plays a critical role in pancreatic development although the role of this pathway in the adult pancreas is obscure. Here, we define an important role of the TGF-beta pathway in regulation of insulin gene transcription and beta-cell function. We identify insulin as a TGF-beta target gene and show that the TGF-beta signaling effector Smad3 occupies the insulin gene promoter and represses insulin gene transcription. In contrast, Smad3 small interfering RNAs relieve insulin transcriptional repression and enhance insulin levels. Transduction of adenoviral Smad3 into primary human and non-human primate islets suppresses insulin content, whereas, dominant-negative Smad3 enhances insulin levels. Consistent with this, Smad3-deficient mice exhibit moderate hyperinsulinemia and mild hypoglycemia. Moreover, Smad3 deficiency results in improved glucose tolerance and enhanced glucose-stimulated insulin secretion in vivo. In ex vivo perifusion assays, Smad3-deficient islets exhibit improved glucose-stimulated insulin release. Interestingly, Smad3-deficient islets harbor an activated insulin-receptor signaling pathway and TGF-beta signaling regulates expression of genes involved in beta-cell function. Together, these studies emphasize TGF-beta/Smad3 signaling as an important regulator of insulin gene transcription and beta-cell function and suggest that components of the TGF-beta signaling pathway may be dysregulated in diabetes.

Developmental biology of the pancreas: a comprehensive review

Pancreatic development represents a fascinating process in which two morphologically distinct tissue types must derive from one simple epithelium. These two tissue types, exocrine (including acinar cells, centro-acinar cells, and ducts) and endocrine cells serve disparate functions, and have entirely different morphology. In addition, the endocrine tissue must become disconnected from the epithelial lining during its development. The pancreatic development field has exploded in recent years, and numerous published reviews have dealt specifically with only recent findings, or specifically with certain aspects of pancreatic development. Here I wish to present a more comprehensive review of all aspects of pancreatic development, though still there is not a room for discussion of stem cell differentiation to pancreas, nor for discussion of post-natal regeneration phenomena, two important fields closely related to pancreatic development.