Heterozygous missense variant in GLI2 impairs human endocrine pancreas development

Missense variants are the most common type of coding genetic variants. Their functional assessment is fundamental for defining any implication in human diseases and may also uncover genes that are essential for human organ development. Here, we apply CRISPR-Cas9 gene editing on human iPSCs to study a heterozygous missense variant in GLI2 identified in two siblings with early-onset and insulin-dependent diabetes of unknown cause. GLI2 is a primary mediator of the Hedgehog pathway, which regulates pancreatic β-cell development in mice. However, neither mutations in GLI2 nor Hedgehog dysregulation have been reported as cause or predisposition to diabetes. We establish and study a set of isogenic iPSC lines harbouring the missense variant for their ability to differentiate into pancreatic β-like cells. Interestingly, iPSCs carrying the missense variant show altered GLI2 transcriptional activity and impaired differentiation of pancreatic progenitors into endocrine cells. RNASeq and network analyses unveil a crosstalk between Hedgehog and WNT pathways, with the dysregulation of non-canonical WNT signaling in pancreatic progenitors carrying the GLI2 missense variant. Collectively, our findings underscore an essential role for GLI2 in human endocrine development and identify a gene variant that may lead to diabetes.

Acinar-ductal cell rearrangement drives branching morphogenesis of the murine pancreas in an IGF/PI3K-dependent manner

During organ formation, progenitor cells need to acquire different cell identities and organize themselves into distinct structural units. How these processes are coordinated and how tissue architecture(s) is preserved despite the dramatic cell rearrangements occurring in developing organs remain unclear. Here, we identified cellular rearrangements between acinar and ductal progenitors as a mechanism to drive branching morphogenesis in the pancreas while preserving the integrity of the acinar-ductal functional unit. Using ex vivo and in vivo mouse models, we found that pancreatic ductal cells form clefts by protruding and pulling on the acinar basement membrane, which leads to acini splitting. Newly formed acini remain connected to the bifurcated branches generated by ductal cell rearrangement. Insulin growth factor (IGF)/phosphatidylinositol 3-kinase (PI3K) pathway finely regulates this process by controlling pancreatic ductal tissue fluidity, with a simultaneous impact on branching and cell fate acquisition. Together, our results explain how acinar structure multiplication and branch bifurcation are synchronized during pancreas organogenesis.

In-Depth Analysis of the Pancreatic Extracellular Matrix during Development for Next-Generation Tissue Engineering

The pancreas is a complex organ consisting of differentiated cells and extracellular matrix (ECM) organized adequately to enable its endocrine and exocrine functions. Although much is known about the intrinsic factors that control pancreas development, very few studies have focused on the microenvironment surrounding pancreatic cells. This environment is composed of various cells and ECM components, which play a critical role in maintaining tissue organization and homeostasis. In this study, we applied mass spectrometry to identify and quantify the ECM composition of the developing pancreas at the embryonic (E) day 14.5 and postnatal (P) day 1 stages. Our proteomic analysis identified 160 ECM proteins that displayed a dynamic expression profile with a shift in collagens and proteoglycans. Furthermore, we used atomic force microscopy to measure the biomechanical properties and found that the pancreatic ECM was soft (≤400 Pa) with no significant change during pancreas maturation. Lastly, we optimized a decellularization protocol for P1 pancreatic tissues, incorporating a preliminary crosslinking step, which effectively preserved the 3D organization of the ECM. The resulting ECM scaffold proved suitable for recellularization studies. Our findings provide insights into the composition and biomechanics of the pancreatic embryonic and perinatal ECM, offering a foundation for future studies investigating the dynamic interactions between the ECM and pancreatic cells.

Understanding cell fate acquisition in stem-cell-derived pancreatic islets using single-cell multiome-inferred regulomes

Pancreatic islet cells derived from human pluripotent stem cells hold great promise for modeling and treating diabetes. Differences between stem-cell-derived and primary islets remain, but molecular insights to inform improvements are limited. Here, we acquire single-cell transcriptomes and accessible chromatin profiles during in vitro islet differentiation and pancreas from childhood and adult donors for comparison. We delineate major cell types, define their regulomes, and describe spatiotemporal gene regulatory relationships between transcription factors. CDX2 emerged as a regulator of enterochromaffin-like cells, which we show resemble a transient, previously unrecognized, serotonin-producing pre-β cell population in fetal pancreas, arguing against a proposed non-pancreatic origin. Furthermore, we observe insufficient activation of signal-dependent transcriptional programs during in vitro β cell maturation and identify sex hormones as drivers of β cell proliferation in childhood. Altogether, our analysis provides a comprehensive understanding of cell fate acquisition in stem-cell-derived islets and a framework for manipulating cell identities and maturity.

3D multicellular systems in disease modelling: From organoids to organ-on-chip

Cell-cell interactions underlay organ formation and function during homeostasis. Changes in communication between cells and their surrounding microenvironment are a feature of numerous human diseases, including metabolic disease and neurological disorders. In the past decade, cross-disciplinary research has been conducted to engineer novel synthetic multicellular organ systems in 3D, including organoids, assembloids, and organ-on-chip models. These model systems, composed of distinct cell types, satisfy the need for a better understanding of complex biological interactions and mechanisms underpinning diseases. In this review, we discuss the emerging field of building 3D multicellular systems and their application for modelling the cellular interactions at play in diseases. We report recent experimental and computational approaches for capturing cell-cell interactions as well as progress in bioengineering approaches for recapitulating these complexities ex vivo. Finally, we explore the value of developing such multicellular systems for modelling metabolic, intestinal, and neurological disorders as major examples of multisystemic diseases, we discuss the advantages and disadvantages of the different approaches and provide some recommendations for further advancing the field.

Functional genomics and the future of iPSCs in disease modeling

Induced pluripotent stem cells (iPSCs) are valuable in disease modeling because of their potential to expand and differentiate into virtually any cell type and recapitulate key aspects of human biology. Functional genomics are genome-wide studies that aim to discover genotype-phenotype relationships, thereby revealing the impact of human genetic diversity on normal and pathophysiology. In this review, we make the case that human iPSCs (hiPSCs) are a powerful tool for functional genomics, since they provide an in vitro platform for the study of population genetics. We describe cutting-edge tools and strategies now available to researchers, including multi-omics technologies, advances in hiPSC culture techniques, and innovations in drug development. Functional genomics approaches based on hiPSCs hold great promise for advancing drug discovery, disease etiology, and the impact of genetic variation on human biology.

Development of a 3D atlas of the embryonic pancreas for topological and quantitative analysis of heterologous cell interactions

Generating comprehensive image maps, while preserving spatial three-dimensional (3D) context, is essential in order to locate and assess quantitatively specific cellular features and cell-cell interactions during organ development. Despite recent advances in 3D imaging approaches, our current knowledge of the spatial organization of distinct cell types in the embryonic pancreatic tissue is still largely based on two-dimensional histological sections. Here, we present a light-sheet fluorescence microscopy approach to image the pancreas in three dimensions and map tissue interactions at key time points in the mouse embryo. We demonstrate the utility of the approach by providing volumetric data, 3D distribution of three main cellular components (epithelial, mesenchymal and endothelial cells) within the developing pancreas, and quantification of their relative cellular abundance within the tissue. Interestingly, our 3D images show that endocrine cells are constantly and increasingly in contact with endothelial cells forming small vessels, whereas the interactions with mesenchymal cells decrease over time. These findings suggest distinct cell-cell interaction requirements for early endocrine cell specification and late differentiation. Lastly, we combine our image data in an open-source online repository (referred to as the Pancreas Embryonic Cell Atlas).

Summary: A light-sheet fluorescence microscopy approach is used for 3D imaging of the pancreas and to quantitatively map its interactions with surrounding tissues at key development time points in the mouse embryo.

The postnatal pancreatic microenvironment guides β cell maturation through BMP4 production

Glucose homeostasis depends on regulated insulin secretion from pancreatic β cells, which acquire their mature phenotype postnatally. The functional maturation of β cells is regulated by a combination of cell-autonomous and exogenous factors; the identity of the latter is mostly unknown. Here, we identify BMP4 as a critical component through which the pancreatic microenvironment regulates β cell function. By combining transgenic mouse models and human iPSCs, we show that BMP4 promotes the expression of core β cell genes and is required for proper insulin production and secretion. We identified pericytes as the primary pancreatic source of BMP4, which start producing this ligand midway through the postnatal period, at the age β cells mature. Overall, our findings show that the islet niche directly promotes β cell functional maturation through the timely production of BMP4. Our study highlights the need to recapitulate the physiological postnatal islet niche for generating fully functional stem-cell-derived β cells for cell replacement therapy for diabetes.

Engineering life in synthetic systems

The second EMBO-EMBL Symposium 'Synthetic Morphogenesis: From Gene Circuits to Tissue Architecture' was held virtually in March 2021, with participants from all over the world joining from the comfort of their sofas to discuss synthetic morphogenesis at large. Leading scientists from a range of disciplines, including developmental biology, physics, chemistry and computer science, covered a gamut of topics from the principles of cell and tissue organization, patterning and gene regulatory networks, to synthetic approaches for exploring evolutionary and developmental biology principles. Here, we describe some of the high points.

Pancreatic cell fate specification: insights into developmental mechanisms and their application for lineage reprogramming

Diabetes is a group of metabolic disorders, which results from insufficient functional pancreatic β-cell mass either due to the autoimmune destruction of insulin producing β-cells, or their death or de-differentiation as compensation for insulin resistance. The ability to reprogram cell types within close developmental proximity to β-cells offers a strategy to replenish β-cell mass and a future possible treatment of diabetes. Here, we review recent advances in the fields of pancreas development and lineage reprogramming. We also probe the possibility of using reprogrammed cells as an approach by which to further understand developmental mechanisms, in particular roadblocks to changing cell identity. Finally, we highlight fundamental challenges that need to be overcome to advance lineage reprogramming for generating pancreatic cells.

Employing core regulatory circuits to define cell identity

The interplay between extrinsic signaling and downstream gene networks controls the establishment of cell identity during development and its maintenance in adult life. Advances in next-generation sequencing and single-cell technologies have revealed additional layers of complexity in cell identity. Here, we review our current understanding of transcription factor (TF) networks as key determinants of cell identity. We discuss the concept of the core regulatory circuit as a set of TFs and interacting factors that together define the gene expression profile of the cell. We propose the core regulatory circuit as a comprehensive conceptual framework for defining cellular identity and discuss its connections to cell function in different contexts.

The transCampus Metabolic Training Programme Explores the Link of SARS-CoV-2 Virus to Metabolic Disease

Currently, we are experiencing a true pandemic of a communicable disease by the virus SARS-CoV-2 holding the whole world firmly in its grasp. Amazingly and unfortunately, this virus uses a metabolic and endocrine pathway via ACE2 to enter our cells causing damage and disease. Our international research training programme funded by the German Research Foundation has a clear mission to train the best students wherever they may come from to learn to tackle the enormous challenges of diabetes and its complications for our society. A modern training programme in diabetes and metabolism does not only involve a thorough understanding of classical physiology, biology and clinical diabetology but has to bring together an interdisciplinary team. With the arrival of the coronavirus pandemic, this prestigious and unique metabolic training programme is facing new challenges but also new opportunities. The consortium of the training programme has recognized early on the need for a guidance and for practical recommendations to cope with the COVID-19 pandemic for the community of patients with metabolic disease, obesity and diabetes. This involves the optimal management from surgical obesity programmes to medications and insulin replacement. We also established a global registry analyzing the dimension and role of metabolic disease including new onset diabetes potentially triggered by the virus. We have involved experts of infectious disease and virology to our faculty with this metabolic training programme to offer the full breadth and scope of expertise needed to meet these scientific challenges. We have all learned that this pandemic does not respect or heed any national borders and that we have to work together as a global community. We believe that this transCampus metabolic training programme provides a prime example how an international team of established experts in the field of metabolism can work together with students from all over the world to address a new pandemic.

A Specialized Niche in the Pancreatic Microenvironment Promotes Endocrine Differentiation

The interplay between pancreatic epithelium and the surrounding microenvironment is pivotal for pancreas formation and differentiation as well as adult organ homeostasis. The mesenchyme is the main component of the embryonic pancreatic microenvironment, yet its cellular identity is broadly defined, and whether it comprises functionally distinct cell subsets is not known. Using genetic lineage tracing, transcriptome, and functional studies, we identified mesenchymal populations with different roles during pancreatic development. Moreover, we showed that Pbx transcription factors act within the mouse pancreatic mesenchyme to define a pro-endocrine specialized niche. Pbx directs differentiation of endocrine progenitors into insulin- and glucagon-positive cells through non-cell-autonomous regulation of ECM-integrin interactions and soluble molecules. Next, we measured functional conservation between mouse and human pancreatic mesenchyme by testing identified mesenchymal factors in an iPSC-based differentiation model. Our findings provide insights into how lineage-specific crosstalk between epithelium and neighboring mesenchymal cells underpin the generation of different pancreatic cell types.

Direct Lineage Reprogramming: Harnessing Cell Plasticity between Liver and Pancreas

Direct lineage reprogramming of abundant and accessible cells into therapeutically useful cell types holds tremendous potential in regenerative medicine. To date, a number of different cell types have been generated by lineage reprogramming methods, including cells from the neural, cardiac, hepatic, and pancreatic lineages. The success of this strategy relies on developmental biology and the knowledge of cell-fate-defining transcriptional networks. Hepatocytes represent a prime target for β cell conversion for numerous reasons, including close developmental origin, accessibility, and regenerative potential. We present here an overview of pancreatic and hepatic development, with a particular focus on the mechanisms underlying the divergence between the two cell lineages. Additionally, we discuss to what extent this lineage relationship can be exploited in efforts to reprogram one cell type into the other and whether such an approach may provide a suitable strategy for regenerative therapies of diabetes.

A whole organism small molecule screen identifies novel regulators of pancreatic endocrine development

An early step in pancreas development is marked by the expression of the transcription factor Pdx1 within the pancreatic endoderm, where it is required for the specification of all endocrine cell types. Subsequently, Pdx1 expression becomes restricted to the β-cell lineage, where it plays a central role in β-cell function. This pivotal role of Pdx1 at various stages of pancreas development makes it an attractive target to enhance pancreatic β-cell differentiation and increase β-cell function. In this study, we used a newly generated zebrafish reporter to screen over 8000 small molecules for modulators of pdx1 expression. We found four hit compounds and validated their efficacy at different stages of pancreas development. Notably, valproic acid treatment increased pancreatic endoderm formation, while inhibition of TGFβ signaling led to α-cell to β-cell transdifferentiation. HC toxin, another HDAC inhibitor, enhances β-cell function in primary mouse and human islets. Thus, using a whole organism screening strategy, this study identified new pdx1 expression modulators that can be used to influence different steps in pancreas and β-cell development.

Robo signalling controls pancreatic progenitor identity by regulating Tead transcription factors

A complex interplay of intrinsic factors and extrinsic signalling pathways controls both cell lineage commitment and maintenance of cell identity. Loss of defined cellular states is the cause of many different cancers, including pancreatic cancer. Recent findings suggest a clinical role for the conserved SLIT/ROBO signalling pathway in pancreatic cancer. However, whilst this pathway has been extensively studied in many processes, a role for Slit and Robo genes in pancreas cell identity and plasticity has not been established yet. Here, we identify Slit/Robo signalling as a key regulator of pancreatic progenitor identity. We find that Robo1 and Robo2 are required for preserving pancreatic cell identity shortly after fate induction and, subsequently, for expansion of the pancreatic progenitor pool in the mouse. Furthermore, we show that Robo receptors control the expression of Tead transcription factors as well as its downstream transcriptional activity. Our work identifies an interplay between Slit/Robo pathway and Tead intrinsic regulators, functioning as gatekeeper of pancreatic cell identity.

Xenopus as a model system for studying pancreatic development and diabetes

Diabetes is a chronic disease caused by the loss or dysfunction of the insulin-producing β-cells in the pancreas. To date, much of our knowledge about β-cells in humans comes from studying rare monogenic forms of diabetes. Importantly, the majority of mutations so far associated to monogenic diabetes are in genes that exert a regulatory role in pancreatic development and/or β-cell function. Thus, the identification and study of novel mutations open an unprecedented window into human pancreatic development. In this review, we summarize major advances in the genetic dissection of different types of monogenic diabetes and the insights gained from a developmental perspective. We highlight future challenges to bridge the gap between the fast accumulation of genetic data through next-generation sequencing and the need of functional insights into disease mechanisms. Lastly, we discuss the relevance and advantages of studying candidate gene variants in vivo using the Xenopus as model system.

The histone methyltransferase Setd7 promotes pancreatic progenitor identity

Cell fate specification depends on transcriptional activation driven by lineage-specific transcription factors as well as changes in chromatin organization. To date, the interplay between transcription factors and chromatin modifiers during development is not well understood. We focus here on the initiation of the pancreatic program from multipotent endodermal progenitors. Transcription factors that play key roles in regulating pancreatic progenitor state have been identified, but the chromatin regulators that help establishing and maintaining pancreatic fate are less well known. Using a comparative approach, we identify a critical role for the histone methyltransferase Setd7 in establishing pancreatic cell identity. We show that Setd7 is expressed in the prospective pancreatic endoderm of Xenopus and mouse embryos prior to Pdx1 induction. Importantly, we demonstrate that setd7 is sufficient and required for pancreatic cell fate specification in Xenopus. Functional and biochemical approaches in Xenopus and mouse endoderm support that Setd7 modulates methylation marks at pancreatic regulatory regions, possibly through interaction with the transcription factor Foxa2. Together, these results demonstrate that Setd7 acts as a central component of the transcription complex initiating the pancreatic program.

In vivo reprogramming for tissue repair

Vital organs such as the pancreas and the brain lack the capacity for effective regeneration. To overcome this limitation, an emerging strategy consists of converting resident tissue-specific cells into the cell types that are lost due to disease by a process called in vivo lineage reprogramming. Here we discuss recent breakthroughs in regenerating pancreatic β-cells and neurons from various cell types, and highlight fundamental challenges that need to be overcome for the translation of in vivo lineage reprogramming into therapy.

Recessive mutations in PCBD1 cause a new type of early-onset diabetes

Mutations in several genes cause nonautoimmune diabetes, but numerous patients still have unclear genetic defects, hampering our understanding of the development of the disease and preventing pathogenesis-oriented treatment. We used whole-genome sequencing with linkage analysis to study a consanguineous family with early-onset antibody-negative diabetes and identified a novel deletion in PCBD1 (pterin-4 α-carbinolamine dehydratase/dimerization cofactor of hepatocyte nuclear factor 1 α), a gene that was recently proposed as a likely cause of diabetes. A subsequent reevaluation of patients with mild neonatal hyperphenylalaninemia due to mutations in PCBD1 from the BIODEF database identified three additional patients who had developed HNF1A-like diabetes in puberty, indicating early β-cell failure. We found that Pcbd1 is expressed in the developing pancreas of both mouse and Xenopus embryos from early specification onward showing colocalization with insulin. Importantly, a morpholino-mediated knockdown in Xenopus revealed that pcbd1 activity is required for the proper establishment of early pancreatic fate within the endoderm. We provide the first genetic evidence that PCBD1 mutations can cause early-onset nonautoimmune diabetes with features similar to dominantly inherited HNF1A-diabetes. This condition responds to and can be treated with oral drugs instead of insulin, which is important clinical information for these patients. Finally, patients at risk can be detected through a newborn screening for phenylketonuria.

Glimpse into Hox and tale regulation of cell differentiation and reprogramming

During embryonic development, cells become gradually restricted in their developmental potential and start elaborating lineage-specific transcriptional networks to ultimately acquire a unique differentiated state. Hox genes play a central role in specifying regional identities, thereby providing the cell with critical information on positional value along its differentiation path. The exquisite DNA-binding specificity of the Hox proteins is frequently dependent upon their interaction with members of the TALE family of homeodomain proteins. In addition to their function as Hox-cofactors, TALE homeoproteins control multiple crucial developmental processes through Hox-independent mechanisms. Here, we will review recent findings on the function of both Hox and TALE proteins in cell differentiation, referring mostly to vertebrate species. In addition, we will discuss the direct implications of this knowledge on cell plasticity and cell reprogramming.

Mutually exclusive signaling signatures define the hepatic and pancreatic progenitor cell lineage divergence

A key question in stem cell biology is how distinct cell types arise from common multipotent progenitor cells. It is unknown how liver and pancreas cells diverge from a common endoderm progenitor population and adopt specific fates. Using RNA-seq, Spagnoli and colleagues define the gene expression programs of liver and pancreas progenitors and identify the noncanonical Wnt pathway as a potential developmental regulator of this fate decision. Furthermore, this study provides a framework for lineage-reprogramming strategies to convert adult hepatic cells into pancreatic cells.

Understanding how distinct cell types arise from multipotent progenitor cells is a major quest in stem cell biology. The liver and pancreas share many aspects of their early development and possibly originate from a common progenitor. However, how liver and pancreas cells diverge from a common endoderm progenitor population and adopt specific fates remains elusive. Using RNA sequencing (RNA-seq), we defined the molecular identity of liver and pancreas progenitors that were isolated from the mouse embryo at two time points, spanning the period when the lineage decision is made. The integration of temporal and spatial gene expression profiles unveiled mutually exclusive signaling signatures in hepatic and pancreatic progenitors. Importantly, we identified the noncanonical Wnt pathway as a potential developmental regulator of this fate decision and capable of inducing the pancreas program in endoderm and liver cells. Our study offers an unprecedented view of gene expression programs in liver and pancreas progenitors and forms the basis for formulating lineage-reprogramming strategies to convert adult hepatic cells into pancreatic cells.

RhoGAP control of pancreas development: putting cells in the right place at the right time

Recent evidences suggested that growth and differentiation of pancreatic cell lineages, including the insulin-producing β-cells, depend on proper tissue-architecture, epithelial remodeling and cell positioning within the branching pancreatic epithelium. We recently found that Rho GTPase and its regulator, Stard13 RhoGAP, coordinate morphogenesis with growth in the developing pancreas. Conditional mutation of Stard13 in the mouse pancreas hampers epithelial remodeling and distal tip domain formation, affecting proliferation and expansion of pancreatic progenitors. These defects eventually result in pancreatic hypoplasia at birth. Stard13 acts by regulating Rho signaling spatially and temporally during pancreas development. In line with this, pharmacological activation or inhibition of Rho mimics or rescues, respectively, the defects observed in Stard13-deficient embryos and pancreatic organ cultures. Furthermore, in the absence of Stard13 uninhibited Rho activity affects the actomyosin contractile network, disrupting its apical distribution and hampering coordinated cell-shape changes. These results unveil therefore the crucial role of actin cytoskeletal dynamics during the onset of pancreatic branching morphogenesis. Finally, our findings define a reciprocal interaction between the actin-MAL/SRF and the MAPK signaling to locally regulate progenitor cell proliferation in the pancreas.

Rho signalling restriction by the RhoGAP Stard13 integrates growth and morphogenesis in the pancreas

The development of functional organ architecture relies on coordinated morphogenesis and growth. In the developing pancreas, the branching epithelium is organised in discrete domains, delineating one specific domain of progenitor cells at the tip of the branches. The molecular mechanisms underlying the coordinated action of branching and proliferation in organ formation are largely unknown. Here, we identify the RhoGAP protein Stard13 as an essential regulator of pancreas tissue architecture in the mammalian embryo. Conditional ablation of Stard13 expression in the pancreas disrupts epithelial morphogenesis and tip-domain organisation, resulting in hampered proliferation of tip progenitors and subsequent organ hypoplasia. Stard13 acts by regulating Rho signalling spatially and temporally during pancreas development. Our findings provide new insights into the mechanisms that shape pancreatic epithelium to create a mature organ and establish a functional link between Rho-mediated control of epithelial remodelling and organ size determination, involving reciprocal interaction of actin-MAL/SRF and MAPK signalling pathways.

A system for ex vivo culturing of embryonic pancreas

The pancreas controls vital functions of our body, including the production of digestive enzymes and regulation of blood sugar levels. Although in the past decade many studies have contributed to a solid foundation for understanding pancreatic organogenesis, important gaps persist in our knowledge of early pancreas formation. A complete understanding of these early events will provide insight into the development of this organ, but also into incurable diseases that target the pancreas, such as diabetes or pancreatic cancer. Finally, this information will generate a blueprint for developing cell-replacement therapies in the context of diabetes. During embryogenesis, the pancreas originates from distinct embryonic outgrowths of the dorsal and ventral foregut endoderm at embryonic day (E) 9.5 in the mouse embryo. Both outgrowths evaginate into the surrounding mesenchyme as solid epithelial buds, which undergo proliferation, branching and differentiation to generate a fully mature organ. Recent evidences have suggested that growth and differentiation of pancreatic cell lineages, including the insulin-producing β-cells, depends on proper tissue-architecture, epithelial remodeling and cell positioning within the branching pancreatic epithelium. However, how branching morphogenesis occurs and is coordinated with proliferation and differentiation in the pancreas is largely unknown. This is in part due to the fact that current knowledge about these developmental processes has relied almost exclusively on analysis of fixed specimens, while morphogenetic events are highly dynamic. Here, we report a method for dissecting and culturing mouse embryonic pancreatic buds ex vivo on glass bottom dishes, which allow direct visualization of the developing pancreas (Figure 1). This culture system is ideally devised for confocal laser scanning microscopy and, in particular, live-cell imaging. Pancreatic explants can be prepared not only from wild-type mouse embryos, but also from genetically engineered mouse strains (e.g. transgenic or knockout), allowing real-time studies of mutant phenotypes. Moreover, this ex vivo culture system is valuable to study the effects of chemical compounds on pancreatic development, enabling to obtain quantitative data about proliferation and growth, elongation, branching, tubulogenesis and differentiation. In conclusion, the development of an ex vivo pancreatic explant culture method combined with high-resolution imaging provides a strong platform for observing morphogenetic and differentiation events as they occur within the developing mouse embryo.

The Gata5 target, TGIF2, defines the pancreatic region by modulating BMP signals within the endoderm

Mechanisms underlying regional specification of distinct organ precursors within the endoderm, including the liver and pancreas, are still poorly understood. This is particularly true for stages between endoderm formation and the initiation of organogenesis. In this report, we have investigated these intermediate steps downstream of the early endodermal factor Gata5, which progressively lead to the induction of pancreatic fate. We have identified TGIF2 as a novel Gata5 target and demonstrate its function in the establishment of the pancreatic region within dorsal endoderm in Xenopus. TGIF2 acts primarily by restricting BMP signaling in the endoderm to allow pancreatic formation. Consistently, we found that blocking BMP signaling by independent means also perturbs the establishment of pancreatic identity in the endoderm. Previous findings demonstrated a crucial role for BMP signaling in determining dorsal/ventral fates in ectoderm and mesoderm. Our results now extend this trend to the endoderm and identify TGIF2 as the molecular link between dorsoventral patterning of the endoderm and pancreatic specification.

From endoderm to pancreas: a multistep journey

The formation of the vertebrate pancreas is a complex process that typifies the basic steps of embryonic development. It involves the establishment of competence, specification, signaling from neighboring tissues, morphogenesis, and the elaboration of tissue-specific genetic networks. A full analysis of this multistep process will help us to understand classic principles of embryonic development. Furthermore, this will provide the blueprint for experimental programming of pancreas formation from embryonic stem cells in the context of diabetes cell-therapy. Although in the past decade many studies have contributed to a solid foundation for understanding pancreatogenesis, important gaps persist in our knowledge of early pancreas formation. This review will summarize the current understanding of the early mechanisms coming into play to pattern the "pre-pancreatic" region within the endoderm and, gradually, specify the pancreatic tissue.

Balancing BMP signaling through integrated inputs into the Smad1 linker

FGF and other Ras/MAPK pathway activators counterbalance BMP action during neurogenesis, bone formation, and other aspects of vertebrate development and homeostasis. BMP receptors signal through C-terminal phosphorylation and nuclear translocation of the transcription factor Smad1, whereas MAPKs catalyze inhibitory phosphorylation in the Smad1 linker region. Here we show that linker phosphorylation restricts Smad1 activity by enabling Smad1 recognition by the HECT-domain ubiquitin ligase Smurf1. Besides causing Smad1 polyubiquitination, Smurf1 binding inhibits the interaction of Smad1 with the nuclear translocation factor Nup214. Consequently, MAPK-dependent Smurf1 binding leads Smad1 alternatively to degradation or cytoplasmic retention. Smad1 linker phosphorylation and Smurf1 act as interdependent inputs to control BMP signaling during mouse osteoblast differentiation and Xenopus neural development. Linker phosphorylation is triggered also by BMP, providing feedback control. The interplay between linker phosphorylation, Smurf-dependent ubiquitination, and nucleoporin exclusion enables regulation of BMP action by diverse signals and biological contexts.

Snail controls differentiation of hepatocytes by repressing HNF4alpha expression

Epithelial-to-mesenchymal transition (EMT) is a coordinated process, occurring both during morphogenesis and tumor progression, that allows epithelial cells to dissociate from initial contacts and migrate to secondary sites. The transcriptional repressors of the Snail family induce EMT in different epithelial cell lines and their expression is strictly correlated with EMT during the development and progression of carcinomas. We have previously shown that EMT in hepatocytes correlates with the downregulation of hepatic differentiation key factors HNFs (hepatocyte nuclear factors), and in particular of HNF4alpha. Here, we demonstrate that Snail overexpression is sufficient (i) to induce EMT in hepatocytes with conversion of morphology, downregulation of several epithelial adhesion molecules, reduction of proliferation and induction of matrix metalloproteinase 2 expression and, (ii) most relevantly, to repress the transcription of the HNF4alpha gene through a direct binding to its promoter. These finding demonstrate that Snail is at the crossroads of the regulation of EMT in hepatocytes by a dual control of epithelial morphogenesis and differentiation.

Guiding embryonic stem cells towards differentiation: lessons from molecular embryology

Embryonic stem cells are uniquely endowed with the capacity of self-renewal and the potential to give rise to all possible cell types, including germ cells. These qualities have made mouse embryonic stem cells a valuable resource for genetic manipulation of the mouse genome. In addition, they present a powerful system for the in vitro dissection of mammalian embryonic development. The recent isolation of human embryonic stem cells has raised a lot of interest for the potential of transposing our knowledge of lineage-specific differentiation of embryonic stem cells to cell-based therapy of human disease. Recent reports have provided insights into the specific differentiation of embryonic stem cells to different cell types of the embryo. However, progress in this direction seems to depend on the knowledge of the mechanisms controlling lineage decisions during embryogenesis.

The RNA-binding protein, Vg1RBP, is required for pancreatic fate specification

Signaling mechanisms underlying the induction of the pre-pancreatic tissue within the endoderm remain poorly understood. Through an expression cloning strategy, we have identified a previously uncharacterized pancreatic factor that we named Shirin. Interestingly, the non-coding RNA regulatory sequence (3 UTR) of Shirin is sufficient to induce insulin expression in Xenopus embryos. Biochemical studies demonstrate that this RNA sequence is able to bind directly to a trans-acting factor, Vg1RBP, which was previously shown to be involved in the localization of endodermal determinant factors. Loss-of-function analysis indicates that Vg1RBP is required for establishment of pancreatic fate within the endoderm, suggesting a synergism between Vg1RBP and Shirin in the embryo. This study argues for a central role of post-transcriptional mechanisms in establishing pancreatic fate, where a 3 UTR may recruit factors necessary for pancreatic development, and highlights an unknown embryological activity of Vg1RBP.

A full menu for stem-cell research

A report on the Stem Cell EuroConference, Paris, France, 9-10 December 2004.

Inhibition of MMH (Met murine hepatocyte) cell differentiation by TGF(beta) is abrogated by pre-treatment with the heritable differentiation effector FGF1

MMH (Met murine hepatocyte) liver cells derived from transgenic mice expressing a truncated constitutively active form of human c-Met are non-transformed immortalized cell lines. We have previously shown that they harbor: (1) epithelial cells that express the liver-enriched transcription factors HNF4 and HNF1(alpha), and that can be stably induced by FGF1 to express liver functions, and (2) fibroblast-like bi-potential palmate cells that can differentiate into bile duct-like structures in Matrigel cultures, or into epithelial cells competent to express hepatic functions. Low concentrations of TGF(beta) have been found to inhibit growth and differentiation of MMH cells. The factor stabilized the palmate cell phenotype, and it provoked epithelial cells to acquire palmate-like morphological characteristics, in parallel with down-regulation of expression of HNF4 and HNF1(alpha) and activation of Snail transcripts. The effects of TGF(beta) were dominant if it was added with FGF1, but the effects on differentiation were abrogated if cells had been pre-treated with FGF1. This work identifies TGF(beta) as a factor that could be implicated in maintaining bi-potential precursor cells in the liver, FGF1 as one that could over-ride the TGF(beta) effects and Snail as a candidate for mediation of the signal.

Identification of a bipotential precursor cell in hepatic cell lines derived from transgenic mice expressing cyto-Met in the liver

Met murine hepatocyte (MMH) lines were established from livers of transgenic mice expressing constitutively active human Met. These lines harbor two cell types: epithelial cells resembling the parental populations and flattened cells with multiple projections and a dispersed growth habit that are designated palmate. Epithelial cells express the liver-enriched transcription factors HNF4 and HNF1alpha, and proteins associated with epithelial cell differentiation. Treatments that modulate their differentiation state, including acidic FGF, induce hepatic functions. Palmate cells show none of these properties. However, they can differentiate along the hepatic cell lineage, giving rise to: (a) epithelial cells that express hepatic transcription factors and are competent to express hepatic functions; (b) bile duct-like structures in three-dimensional Matrigel cultures. Derivation of epithelial from palmate cells is confirmed by characterization of the progeny of individually fished cells. Furthermore, karyotype analysis confirms the direction of the phenotypic transition: palmate cells are diploid and the epithelial cells are hypotetraploid. The clonal isolation of the palmate cell, an immortalized nontransformed bipotential cell that does not yet express the liver-enriched transcription factors and is a precursor of the epithelial-hepatocyte in MMH lines, provides a new tool for the study of mechanisms controlling liver development.

Transgenic expression in the liver of truncated Met blocks apoptosis and permits immortalization of hepatocytes

Hepatocyte growth factor induces proliferation, motility and differentiation of epithelial cells through the tyrosine kinase receptor encoded by the MET protooncogene. The cytoplasmic portion of Met (referred to as cyto-Met) is activated but only weakly transforming. In order to determine the effect of activated Met on hepatocytes, we have targeted truncated Met expression to the liver by incorporating the cDNA into a vector carrying the entire human alpha-1-antitrypsin transcriptional unit. Transgenic expression in the liver of truncated human Met, containing the regulatory and the catalytic cytoplasmic domains, renders hepatocytes constitutively resistant to apoptosis and reproducibly permits immortalization. The emerging stable cell lines are not transformed and maintain a highly differentiated phenotype judged by the retention of epithelial cell polarity and the expression of hepatocyte-enriched transcription factors as well as hepatic products.

Temporal and tissue-specific expression of the MET ORF driven by the complete transcriptional unit of human A1AT gene in transgenic mice

We inserted the sequence coding for the cytoplasmic portion of the human MET receptor into an 18-kb genomic fragment containing the entire human A1AT gene (encoding alpha-1-antitrypsin). Stringent control of gene expression, at the transcriptional, post-transcriptional and translational levels, was ensured by insertion of the MET open reading frame into A1AT, thus maintaining: (i) all the elements that confer tissue-specific transcription initiation, (ii) all the sequences involved in transcript processing and (iii) all the sequences which influence messenger stability and translational efficiency. The expression pattern of this vector in transgenic mice was identical to that of the human A1AT transgene, as well as to that of A1AT in humans with regard to both temporal and tissue-specific regulation.