Growth-limiting role of endothelial cells in endoderm development

Innervation of the pancreas in development and disease

The autonomic nervous system innervates the pancreas by sympathetic, parasympathetic and sensory branches during early organogenesis, starting with neural crest cell invasion and formation of an intrinsic neuronal network. Several studies have demonstrated that signals from pancreatic neural crest cells direct pancreatic endocrinogenesis. Likewise, autonomic neurons have been shown to regulate pancreatic islet formation, and have also been implicated in type I diabetes. Here, we provide an overview of recent progress in mapping pancreatic innervation and understanding the interactions between pancreatic neurons, epithelial morphogenesis and cell differentiation. Finally, we discuss pancreas innervation as a factor in the development of diabetes.

Endothelial-derived angiocrine factors as instructors of embryonic development

Blood vessels are well-known to play roles in organ development and repair, primarily owing to their fundamental function in delivering oxygen and nutrients to tissues to promote their growth and homeostasis. Endothelial cells however are not merely passive conduits for carrying blood. There is now evidence that endothelial cells of the vasculature actively regulate tissue-specific development, morphogenesis and organ function, as well as playing roles in disease and cancer. Angiocrine factors are growth factors, cytokines, signaling molecules or other regulators produced directly from endothelial cells to instruct a diverse range of signaling outcomes in the cellular microenvironment, and are critical mediators of the vascular control of organ function. The roles of angiocrine signaling are only beginning to be uncovered in diverse fields such as homeostasis, regeneration, organogenesis, stem-cell maintenance, cell differentiation and tumour growth. While in some cases the specific angiocrine factor involved in these processes has been identified, in many cases the molecular identity of the angiocrine factor(s) remain to be discovered, even though the importance of angiocrine signaling has been implicated. In this review, we will specifically focus on roles for endothelial-derived angiocrine signaling in instructing tissue morphogenesis and organogenesis during embryonic and perinatal development.

Deep into the niche: Deciphering local endoderm-microenvironment interactions in development, homeostasis, and disease of pancreas and intestine

Unraveling molecular and functional heterogeneity of niche cells within the developing endoderm could resolve mechanisms of tissue formation and maturation. Here, we discuss current unknowns in molecular mechanisms underlying key developmental events in pancreatic islet and intestinal epithelial formation. Recent breakthroughs in single-cell and spatial transcriptomics, paralleled with functional studies in vitro, reveal that specialized mesenchymal subtypes drive the formation and maturation of pancreatic endocrine cells and islets via local interactions with epithelium, neurons, and microvessels. Analogous to this, distinct intestinal niche cells regulate both epithelial development and homeostasis throughout life. We propose how this knowledge can be used to progress research in the human context using pluripotent stem cell-derived multilineage organoids. Overall, understanding the interactions between the multitude of microenvironmental cells and how they drive tissue development and function could help us make more therapeutically relevant in vitro models.

Focal adhesion kinase-mediated signaling controls the onset of pancreatic cell differentiation

Signals from the endothelium play a pivotal role in pancreatic lineage commitment. As such, the fate of the epithelial cells relies heavily on the spatiotemporal recruitment of the endothelial cells to the embryonic pancreas. Although it is known that VEGFA secreted by the epithelium recruits the endothelial cells to the specific domains within the developing pancreas, the mechanism that controls the timing of such recruitment is poorly understood. Here, we have assessed the role of focal adhesion kinase (FAK) in mouse pancreatic development based on our observation that the presence of the enzymatically active form of FAK (pFAK) in the epithelial cells is inversely correlated with vessel recruitment. To study the role of FAK in the pancreas, we conditionally deleted the gene encoding focal adhesion kinase in the developing mouse pancreas. We found that homozygous deletion of Fak (Ptk2) during embryogenesis resulted in ectopic epithelial expression of VEGFA, abnormal endothelial recruitment and a delay in endocrine and acinar cell differentiation. The heterozygous mutants were born with no pancreatic phenotype but displayed gradual acinar atrophy due to cell polarity defects in exocrine cells. Together, our findings imply a role for FAK in controlling the timing of pancreatic lineage commitment and/or differentiation in the embryonic pancreas by preventing endothelial recruitment to the embryonic pancreatic epithelium.

Summary: Conditional inactivation of focal adhesion kinase in the developing pancreas reveals its role in controlling the timing of pancreatic lineage commitment and/or differentiation by preventing endothelial recruitment to the embryonic epithelium.

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.

Recapitulating pancreatic cell-cell interactions through bioengineering approaches: the momentous role of non-epithelial cells for diabetes cell therapy

Over the past few years, extensive efforts have been made to generate in-vitro pancreatic micro-tissue, for disease modeling or cell replacement approaches in pancreatic related diseases such as diabetes mellitus. To obtain these goals, a closer look at the diverse cells participating in pancreatic development is necessary. Five major non-epithelial pancreatic (pN-Epi) cell populations namely, pancreatic endothelium, mesothelium, neural crests, pericytes, and stellate cells exist in pancreas throughout its development, and they are hypothesized to be endogenous inducers of the development. In this review, we discuss different pN-Epi cells migrating to and existing within the pancreas and their diverse effects on pancreatic epithelium during organ development mediated via associated signaling pathways, soluble factors or mechanical cell-cell interactions. In-vivo and in-vitro experiments, with a focus on N-Epi cells' impact on pancreas endocrine development, have also been considered. Pluripotent stem cell technology and multicellular three-dimensional organoids as new approaches to generate pancreatic micro-tissues have also been discussed. Main challenges for reaching a detailed understanding of the role of pN-Epi cells in pancreas development in utilizing for in-vitro recapitulation have been summarized. Finally, various novel and innovative large-scale bioengineering approaches which may help to recapitulate cell-cell interactions and are crucial for generation of large-scale in-vitro multicellular pancreatic micro-tissues, are discussed.

Shaping the thyroid: From peninsula to de novo lumen formation

A challenging and stimulating question in biology deals with the formation of organs from groups of undifferentiated progenitor cells. Most epithelial organs indeed derive from the endodermal monolayer and evolve into various shape and tridimensional organization adapted to their specialized adult function. Thyroid organogenesis is no exception. In most mammals, it follows a complex and sequential process initiated from the endoderm and leading to the development of a multitude of independent closed spheres equipped and optimized for the synthesis, storage and production of thyroid hormones. The first sign of thyroid organogenesis is visible as a thickening of the anterior foregut endoderm. This group of thyroid progenitors then buds and detaches from the foregut to migrate caudally and then laterally. Upon reaching their final destination in the upper neck region on both sides of the trachea, thyroid progenitors mix with C cell progenitors and finally organize into hormone-producing thyroid follicles. Intrinsic and extrinsic factors controlling thyroid organogenesis have been identified in several species, but the fundamental cellular processes are not sufficiently considered. This review focuses on the cellular aspects of the key morphogenetic steps during thyroid organogenesis and highlights similarities and common mechanisms with developmental steps elucidated in other endoderm-derived organs, despite different final architecture and functions.

Coordinated interactions between endothelial cells and macrophages in the islet microenvironment promote β cell regeneration

Endogenous β cell regeneration could alleviate diabetes, but proliferative stimuli within the islet microenvironment are incompletely understood. We previously found that β cell recovery following hypervascularization-induced β cell loss involves interactions with endothelial cells (ECs) and macrophages (MΦs). Here we show that proliferative ECs modulate MΦ infiltration and phenotype during β cell loss, and recruited MΦs are essential for β cell recovery. Furthermore, VEGFR2 inactivation in quiescent ECs accelerates islet vascular regression during β cell recovery and leads to increased β cell proliferation without changes in MΦ phenotype or number. Transcriptome analysis of β cells, ECs, and MΦs reveals that β cell proliferation coincides with elevated expression of extracellular matrix remodeling molecules and growth factors likely driving activation of proliferative signaling pathways in β cells. Collectively, these findings suggest a new β cell regeneration paradigm whereby coordinated interactions between intra-islet MΦs, ECs, and extracellular matrix mediate β cell self-renewal.

The Role of the Islet Niche on Beta Cell Structure and Function

The islets of Langerhans or pancreatic islets are pivotal in the control of blood glucose and are complex microorgans embedded within the larger volume of the exocrine pancreas. Humans can have ~3.2 million islets [1] which, to our current knowledge, function in a similar manner to sense circulating blood glucose levels and respond with the secretion of a mix of different hormones that act to maintain glucose concentrations around a specific set point [2]. At a cellular level, the control of hormone secretion by glucose and other secretagogues is well-understood [3]. The key signal cascades have been identified and many details of the secretory process are known. However, if we shift focus from single cells and consider cells within intact islets, we do not have a comprehensive model as to how the islet environment influences cell function and how the islets work as a whole. This is important because there is overwhelming evidence that the structure and function of the individual endocrine cells are dramatically affected by the islet environment [4,5]. Uncovering the influence of this islet niche might drive future progress in treatments for Type 2 diabetes [6] and cell replacement therapies for Type 1 diabetes [7]. In this review, we focus on the insulin secreting beta cells and their interactions with the immediate environment that surrounds them including endocrine-endocrine interactions and contacts with capillaries.

Pancreatic acinar differentiation is guided by differential laminin deposition

Endothelial cells play multiple roles during pancreas organogenesis. First, they are required to instruct endoderm-derived pancreatic progenitor cells to initiate branching morphogenesis. Later, blood vessels promote β-cell differentiation but also limit acinar development. In this work, we show how endothelial cells might signal to pancreatic progenitors and spatially regulate acinar differentiation. Using an ex vivo culture system of undifferentiated E12.5 pancreata, we demonstrate that embryonic endothelial progenitor cells and their conditioned medium prevent the expression of two members of the pro-acinar transcriptional PTF1L-complex. This effect is not mediated by SPARC, a protein abundantly released in the medium conditioned by endothelial progenitors. On the contrary, heterotrimeric laminin-α1β1γ1, also produced by endothelial progenitor cells, can repress acinar differentiation when used on its own on pancreatic explants. Lastly, we found that laminin-α1 is predominantly found in vivo around the pancreatic trunk cells, as compared to the tip cells, at E14.5. In conclusion, we propose that expression or deposition of laminin-α1β1γ1 around the trunk cells, where blood vessels are predominantly localized, prevent acinar differentiation of these cells. On the contrary, transient decreased expression or deposition of laminin-α1β1γ1 around the tip cells would allow PTF1L-complex formation and acinar differentiation.

Vascularizing organogenesis: Lessons from developmental biology and implications for regenerative medicine

Organogenesis requires tightly coordinated and patterned growth of numerous cell types to form a fully mature and vascularized organ. Endothelial cells (ECs) that line blood vessels develop alongside the growing organ, but only recently has their role in directing epithelial and stromal growth been appreciated. Endothelial, epithelial, and stromal cells in embryonic organs actively communicate with one another throughout development to ensure that the organ forms appropriately. What signals tell blood vessel progenitors where to go? How are tissues influenced by the vasculature that pervades it? In this chapter, we review the ways in which crosstalk between ECs and epithelial or stromal cells during development leads to a fully patterned pancreas, lung, or kidney. ECs in all of these organs are necessary for proper epithelial and stromal growth, but how they direct this process is organ- and time-specific, highlighting the concept of dynamic EC heterogeneity. We end with a discussion on how understanding cell-cell crosstalk during development can be applied therapeutically through the generation of transplantable miniature organ-like tissues called "organoids." We will discuss the current state of organoid technology and highlight the major challenges in forming a properly patterned vascular network that will be critical in transforming them into a viable therapeutic option.

Signals in the pancreatic islet microenvironment influence β-cell proliferation

The progressive loss of pancreatic β-cell mass that occurs in both type 1 and type 2 diabetes is a primary factor driving efforts to identify strategies for effectively increasing, enhancing or restoring β-cell mass. While factors that seem to influence β-cell proliferation in specific contexts have been described, reliable stimulation of human β-cell proliferation has remained a challenge. Importantly, β-cells exist in the context of a complex, integrated pancreatic islet microenvironment where they interact with other endocrine cells, vascular endothelial cells, extracellular matrix, neuronal projections and islet macrophages. This review highlights different components of the pancreatic microenvironment, and reviews what is known about how signaling that occurs between β-cells and these other components influences β-cell proliferation. Future efforts to further define the role of the pancreatic islet microenvironment on β-cell proliferation may lead to the development of successful approaches to increase or restore β-cell mass in diabetes.

Pancreatic Mesenchyme Regulates Islet Cellular Composition in a Patched/Hedgehog-Dependent Manner

Pancreas development requires restrained Hedgehog (Hh) signaling activation. While deregulated Hh signaling in the pancreatic mesenchyme has been long suggested to be detrimental for proper organogenesis, this association was not directly shown. Here, we analyzed the contribution of mesenchymal Hh signaling to pancreas development. To increase Hh signaling in the pancreatic mesenchyme of mouse embryos, we deleted Patched1 (Ptch1) in these cells. Our findings indicate that deregulated Hh signaling in mesenchymal cells was sufficient to impair pancreas development, affecting both endocrine and exocrine cells. Notably, transgenic embryos displayed disrupted islet cellular composition and morphology, with a reduced β-cell portion. Our results indicate that the cell-specific growth rates of α- and β-cell populations, found during normal development, require regulated mesenchymal Hh signaling. In addition, we detected hyperplasia of mesenchymal cells upon elevated Hh signaling, accompanied by them acquiring smooth-muscle like phenotype. By specifically manipulating mesenchymal cells, our findings provide direct evidence for the non-autonomous roles of the Hh pathway in pancreatic epithelium development. To conclude, we directly show that regulated mesenchymal Hh signaling is required for pancreas organogenesis and establishment of its proper cellular composition.

Vascular development in the vertebrate pancreas

The vertebrate pancreas is comprised of a highly branched tubular epithelium, which is intimately associated with an extensive and specialized vasculature. While we know a great deal about basic vascular anatomy of the adult pancreas, as well as islet capillaries, surprisingly little is known about the ontogeny of its blood vessels. Here, we analyze development of the pancreatic vasculature in the mouse embryo. We show that pancreatic epithelial branches intercalate with the fine capillary plexus of the surrounding pancreatic mesenchyme. Endothelial cells (ECs) within this mesenchyme are heterogeneous from the onset of organogenesis. Pancreatic arteries take shape before veins, in a manner analogous to early embryonic vessels. The main central artery forms during mid-gestation, as a result of vessel coalescence and remodeling of a vascular plexus. In addition, we show that vessels in the forming pancreas display a predictable architecture that is dependent on VEGF signaling. Over-expression of VEGF disrupts vascular patterning and arteriovenous differentiation within the developing pancreas. This study constitutes a first-time in-depth cellular and molecular characterization of pancreatic blood vessels, as they coordinately grow along with the pancreatic epithelium.

Human Islets Have Fewer Blood Vessels than Mouse Islets and the Density of Islet Vascular Structures Is Increased in Type 2 Diabetes

Human and rodent islets differ substantially in several features, including architecture, cell composition, gene expression and some aspects of insulin secretion. Mouse pancreatic islets are highly vascularized with interactions between islet endothelial and endocrine cells being important for islet cell differentiation and function. To determine whether human islets have a similar high degree of vascularization and whether this is altered with diabetes, we examined the vascularization of islets from normal human subjects, subjects with type 2 diabetes (T2D), and normal mice. Using an integrated morphometry approach to quantify intra-islet capillary density in human and mouse pancreatic sections, we found that human islets have five-fold fewer vessels per islet area than mouse islets. Islets in pancreatic sections from T2D subjects showed capillary thickening, some capillary fragmentation and had increased vessel density as compared with non-diabetic controls. These changes in islet vasculature in T2D islets appeared to be associated with amyloid deposition, which was noted in islets from 8/9 T2D subjects (and occupied 14% ± 4% of islet area), especially around the intra-islet capillaries. The physiological implications of the differences in the angioarchitecture of mouse and human islets are not known. Islet vascular changes in T2D may exacerbate β cell/islet dysfunction and β cell loss.

Endothelial cells control pancreatic cell fate at defined stages through EGFL7 signaling

Although endothelial cells have been shown to affect mouse pancreatic development, their precise function in human development remains unclear. Using a coculture system containing human embryonic stem cell (hESC)-derived progenitors and endothelial cells, we found that endothelial cells play a stage-dependent role in pancreatic development, in which they maintain pancreatic progenitor (PP) self-renewal and impair further differentiation into hormone-expressing cells. The mechanistic studies suggest that the endothelial cells act through the secretion of EGFL7. Consistently, endothelial overexpression of EGFL7 in vivo using a transgenic mouse model resulted in an increase of PP proliferation rate and a decrease of differentiation toward endocrine cells. These studies not only identified the role of EGFL7 as the molecular handle involved in the crosstalk between endothelium and pancreatic epithelium, but also provide a paradigm for using hESC stepwise differentiation to dissect the stage-dependent roles of signals controlling organogenesis.

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Endothelial cells play a stage-dependent role in embryonic pancreatic development

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Endothelial cells maintain pancreatic progenitor self-renewal by secreting EGFL7

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Overexpression of EGFL7 in vivo increases pancreatic progenitor proliferation

Regulation of tissue morphogenesis by endothelial cell-derived signals

Endothelial cells (ECs) form an extensive network of blood vessels that has numerous essential functions in the vertebrate body. In addition to their well-established role as a versatile transport network, blood vessels can induce organ formation or direct growth and differentiation processes by providing signals in a paracrine (angiocrine) fashion. Tissue repair also requires the local restoration of vasculature. ECs are emerging as important signaling centers that coordinate regeneration and help to prevent deregulated, disease-promoting processes. Vascular cells are also part of stem cell niches and have key roles in hematopoiesis, bone formation, and neurogenesis. Here, we review these newly identified roles of ECs in the regulation of organ morphogenesis, maintenance, and regeneration.

von Hippel-Lindau gene disruption in mouse pancreatic progenitors and its consequences on endocrine differentiation in vivo: importance of HIF1-α and VEGF-A upregulation

AIM/HYPOTHESIS

Different studies have linked hypoxia to embryonic development. Specifically, when embryonic pancreases are cultured ex vivo under hypoxic conditions (3% O2), beta cell development is impaired. Different cellular signalling pathways are involved in adaptation to hypoxia, including the ubiquitous hypoxia-inducible-factor 1-α (HIF1-α) pathway. We aimed to analyse the effects of HIF1-α stabilisation on fetal pancreas development in vivo.

METHODS

We deleted the Vhl gene, which encodes von Hippel-Lindau protein (pVHL), a factor necessary for HIF1-α degradation, by crossing Vhl-floxed mice with Sox9-Cre mice.

RESULTS

HIF1-α was stabilised in pancreatic progenitor cells in which the HIF pathway was induced. The number of neurogenin-3 (NGN3)-expressing cells was reduced and consequently endocrine development was altered in Vhl knockout pancreases. HIF1-α stabilisation induced Vegfa upregulation, leading to increased vascularisation. To investigate the impact of increased vascularisation on NGN3 expression, we used a bioassay in which Vhl mutant pancreases were cultured with or without vascular endothelial growth factor (VEGF) receptor 2 (VEGF-R2) inhibitors (e.g. Ki8751). Ex vivo analysis showed that Vhl knockout pancreases developed fewer NGN3-positive cells compared with controls. Interestingly, this effect was blocked when vascularisation was inhibited in the presence of VEGF-R2 inhibitors.

CONCLUSIONS/INTERPRETATION

Our data demonstrate that HIF1-α negatively controls beta cell differentiation in vivo by regulating NGN3 expression, and that this effect is mediated by signals from blood vessels.

Islet microenvironment, modulated by vascular endothelial growth factor-A signaling, promotes β cell regeneration

Pancreatic islet endocrine cell and endothelial cell (EC) interactions mediated by vascular endothelial growth factor-A (VEGF-A) signaling are important for islet differentiation and the formation of highly vascularized islets. To dissect how VEGF-A signaling modulates intra-islet vasculature, islet microenvironment, and β cell mass, we transiently increased VEGF-A production by β cells. VEGF-A induction dramatically increased the number of intra-islet ECs but led to β cell loss. After withdrawal of the VEGF-A stimulus, β cell mass, function, and islet structure normalized as a result of a robust, but transient, burst in proliferation of pre-existing β cells. Bone marrow-derived macrophages (MΦs) recruited to the site of β cell injury were crucial for the β cell proliferation, which was independent of pancreatic location and circulating factors such as glucose. Identification of the signals responsible for the proliferation of adult, terminally differentiated β cells will improve strategies aimed at β cell regeneration and expansion.

In-silico models of stem cell and developmental systems

Understanding how developmental systems evolve over time is a key question in stem cell and developmental biology research. However, due to hurdles of existing experimental techniques, our understanding of these systems as a whole remains partial and coarse. In recent years, we have been constructing in-silico models that synthesize experimental knowledge using software engineering tools. Our approach integrates known isolated mechanisms with simplified assumptions where the knowledge is limited. This has proven to be a powerful, yet underutilized, tool to analyze the developmental process. The models provide a means to study development in-silico by altering the model’s specifications, and thereby predict unforeseen phenomena to guide future experimental trials. To date, three organs from diverse evolutionary organisms have been modeled: the mouse pancreas, the C. elegans gonad, and partial rodent brain development. Analysis and execution of the models recapitulated the development of the organs, anticipated known experimental results and gave rise to novel testable predictions. Some of these results had already been validated experimentally. In this paper, I review our efforts in realistic in-silico modeling of stem cell research and developmental biology and discuss achievements and challenges. I envision that in the future, in-silico models as presented in this paper would become a common and useful technique for research in developmental biology and related research fields, particularly regenerative medicine, tissue engineering and cancer therapeutics.

Pancreatic islet vasculature adapts to insulin resistance through dilation and not angiogenesis

Pancreatic islets adapt to insulin resistance through a complex set of changes, including β-cell hyperplasia and hypertrophy. To determine if islet vascularization changes in response to insulin resistance, we investigated three independent models of insulin resistance: ob/ob, GLUT4(+/-), and mice with high-fat diet-induced obesity. Intravital blood vessel labeling and immunocytochemistry revealed a vascular plasticity in which islet vessel area was significantly increased, but intraislet vessel density was decreased as the result of insulin resistance. These vascular changes were independent of islet size and were only observed within the β-cell core but not in the islet periphery. Intraislet endothelial cell fenestration, proliferation, and islet angiogenic factor/receptor expression were unchanged in insulin-resistant compared with control mice, indicating that islet capillary expansion is mediated by dilation of preexisting vessels and not by angiogenesis. We propose that the islet capillary dilation is modulated by endothelial nitric oxide synthase via complementary signals derived from β-cells, parasympathetic nerves, and increased islet blood flow. These compensatory changes in islet vascularization may influence whether β-cells can adequately respond to insulin resistance and prevent the development of diabetes.

Conditional hypovascularization and hypoxia in islets do not overtly influence adult β-cell mass or function

It is generally accepted that vascularization and oxygenation of pancreatic islets are essential for the maintenance of an optimal β-cell mass and function and that signaling by vascular endothelial growth factor (VEGF) is crucial for pancreas development, insulin gene expression/secretion, and (compensatory) β-cell proliferation. A novel mouse model was designed to allow conditional production of human sFlt1 by β-cells in order to trap VEGF and study the effect of time-dependent inhibition of VEGF signaling on adult β-cell fate and metabolism. Secretion of sFlt1 by adult β-cells resulted in a rapid regression of blood vessels and hypoxia within the islets. Besides blunted insulin release, β-cells displayed a remarkable capacity for coping with these presumed unfavorable conditions: even after prolonged periods of blood vessel ablation, basal and stimulated blood glucose levels were only slightly increased, while β-cell proliferation and mass remained unaffected. Moreover, ablation of blood vessels did not prevent β-cell generation after severe pancreas injury by partial pancreatic duct ligation or partial pancreatectomy. Our data thus argue against a major role of blood vessels to preserve adult β-cell generation and function, restricting their importance to facilitating rapid and adequate insulin delivery.

The inactivation of Arx in pancreatic α-cells triggers their neogenesis and conversion into functional β-like cells

Recently, it was demonstrated that pancreatic new-born glucagon-producing cells can regenerate and convert into insulin-producing β-like cells through the ectopic expression of a single gene, Pax4. Here, combining conditional loss-of-function and lineage tracing approaches, we show that the selective inhibition of the Arx gene in α-cells is sufficient to promote the conversion of adult α-cells into β-like cells at any age. Interestingly, this conversion induces the continuous mobilization of duct-lining precursor cells to adopt an endocrine cell fate, the glucagon⁺ cells thereby generated being subsequently converted into β-like cells upon Arx inhibition. Of interest, through the generation and analysis of Arx and Pax4 conditional double-mutants, we provide evidence that Pax4 is dispensable for these regeneration processes, indicating that Arx represents the main trigger of α-cell-mediated β-like cell neogenesis. Importantly, the loss of Arx in α-cells is sufficient to regenerate a functional β-cell mass and thereby reverse diabetes following toxin-induced β-cell depletion. Our data therefore suggest that strategies aiming at inhibiting the expression of Arx, or its molecular targets/co-factors, may pave new avenues for the treatment of diabetes.

Type 1 diabetes is a condition that results from the loss of insulin-producing β-cells. Despite current therapies, diabetic patients are prone to vascular complications. Using the mouse as a model, we previously found that pancreatic glucagon-expressing cells can be regenerated and converted into β-like cells by the forced expression of a single gene, Pax4. Here, we generated transgenic mice allowing both the permanent labeling of α-cells and the inactivation of Arx solely in this cell subtype. Our results indicate that, upon Arx inactivation, α-cells can be continuously regenerated from duct-lining precursors and converted into β-like cells. Importantly, the additional loss of Pax4 does not impact these processes, suggesting that Arx is the main trigger of α-cell-mediated β-like cell neogenesis. Most interestingly, upon chemical induction of diabetes/β-cell loss, while control animals die or remain severely hyperglycemic, a normalization of the glycemia, a clear regeneration of the β-like cell mass, and an extended lifespan are noted in animals with the conditional inactivation of Arx. Our data therefore suggest that strategies aiming at inhibiting the expression of Arx, or its molecular targets/co-factors, may pave new avenues for the treatment of diabetes.

Relationship between vascular endothelial cells and pancreatic islet development and stem cell differentiation

As the main components of the pancreatic islet niche, endothelial cells participate in many processes of pancreatic development, including pancreatic cell fate decision, endocrine pancreatic cell differentiation and proliferation, and spatial distribution of the pancreas. On different occasions, endothelial cells play disparate roles by cross-talking with islet cells to influence endocrine pancreatic cell differentiation and islet morphology and function. Cytokines such as hepatocyte growth factor and sphingosine-1-phosphate as well as the extracellular matrixes such as laminin and collagen Ⅳ, which are produced and/or secreted by endothelial cells, play important roles in the regulation of islet development and function. Furthermore, endothelial cells are involved in the balance between self-renewal and differentiation of stem cells. Application of endothelial cells to induce the differentiation of stem cells into functional islet cells may be one of the most promising approaches to cell replacement therapy for diabetes.

Vascular instruction of pancreas development

Blood vessels course through organs, providing them with essential nutrient and gaseous exchange. However, the vasculature has also been shown to provide non-nutritional signals that play key roles in the control of organ growth, morphogenesis and homeostasis. Here, we examine a decade of work on the contribution of vascular paracrine signals to developing tissues, with a focus on pancreatic β-cells. During the early stages of embryonic development, blood vessels are required for pancreas specification. Later, the vasculature constrains pancreas branching, differentiation and growth. During adult life, capillaries provide a vascular niche for the maintenance of β-cell function and survival. We explore the possibility that the vasculature constitutes a dynamic and regionalized signaling system that carries out multiple and changing functions as it coordinately grows with the pancreatic epithelial tree.

Enhanced expression of VEGF-A in β cells increases endothelial cell number but impairs islet morphogenesis and β cell proliferation

There is a reciprocal interaction between pancreatic islet cells and vascular endothelial cells (EC) in which EC-derived signals promote islet cell differentiation and islet development while islet cell-derived angiogenic factors promote EC recruitment and extensive islet vascularization. To examine the role of angiogenic factors in the coordinated development of islets and their associated vessels, we used a "tet-on" inducible system (mice expressing rat insulin promoter-reverse tetracycline activator transgene and a tet-operon-angiogenic factor transgene) to increase the β cell production of vascular endothelial growth factor-A (VEGF-A), angiopoietin-1 (Ang1), or angiopoietin-2 (Ang2) during islet cell differentiation and islet development. In VEGF-A overexpressing embryos, ECs began to accumulate around epithelial tubes residing in the central region of the developing pancreas (associated with endocrine cells) as early as embryonic day 12.5 (E12.5) and increased dramatically by E16.5. While α and β cells formed islet cell clusters in control embryos at E16.5, the increased EC population perturbed endocrine cell differentiation and islet cell clustering in VEGF-A overexpressing embryos. With continued overexpression of VEGF-A, α and β cells became scattered, remained adjacent to ductal structures, and never coalesced into islets, resulting in a reduction in β cell proliferation and β cell mass at postnatal day 1. A similar impact on islet morphology was observed when VEGF-A was overexpressed in β cells during the postnatal period. In contrast, increased expression of Ang1 or Ang2 in β cells in developing or adult islets did not alter islet differentiation, development, or morphology, but altered islet EC ultrastructure. These data indicate that (1) increased EC number does not promote, but actually impairs β cell proliferation and islet formation; (2) the level of VEGF-A production by islet endocrine cells is critical for islet vascularization during development and postnatally; (3) angiopoietin-Tie2 signaling in endothelial cells does not have a crucial role in the development or maintenance of islet vascularization.

Crosstalk between the developing pancreas and its blood vessels: an evolving dialog

Growth and development of embryonic organs goes hand in hand with development of the vascular system. Blood vessels have been known for centuries to supply nutrients and oxygen to all cell types in an organism, however, they have more recently been shown to provide specific cues required for the formation and functionality of a number of tissues. Here, we review the role of blood vessels during pancreas formation, from early specification of the initial pancreatic bud, to its growth and maturation. The overarching theme that emerges from the many studies carried out in the past decade is that the vasculature likely plays diverse and changing roles during pancreas organogenesis. Blood vessels are required for endocrine specification at the onset of pancreatic budding, while only a few days later, blood vessels suppress pancreatic branching and exocrine differentiation. In this review, we summarize our understanding to date about the crosstalk between the pancreas and its vasculature, and we provide a perspective on the promises and challenges of the field.

Tissue engineering 2.0: guiding self-organization during pluripotent stem cell differentiation

Human pluripotent stem cell (hPSC) differentiation aims to mimic development using growth factors or small molecules in a time-dependent and dose-dependent manner. However, the cell types produced using this approach are predominantly fetal-like in phenotype and function, limiting their use in regenerative medicine. This is particularly true in current efforts to produce pancreatic beta cells, wherein robust pancreatic progenitor maturation can only be accomplished upon transplantation into mice. Recent studies have suggested that hPSC-derived cells are capable of self-organizing in vitro, revealing a new paradigm for creating mature cells and tissues. Tissue engineering strategies that provide subtle and dynamic signals to developmentally naïve cells may be applied to mimic in vitro the self-organization aspects of pancreatic development.

Blood vessels restrain pancreas branching, differentiation and growth

How organ size and form are controlled during development is a major question in biology. Blood vessels have been shown to be essential for early development of the liver and pancreas, and are fundamental to normal and pathological tissue growth. Here, we report that, surprisingly, non-nutritional signals from blood vessels act to restrain pancreas growth. Elimination of endothelial cells increases the size of embryonic pancreatic buds. Conversely, VEGF-induced hypervascularization decreases pancreas size. The growth phenotype results from vascular restriction of pancreatic tip cell formation, lateral branching and differentiation of the pancreatic epithelium into endocrine and acinar cells. The effects are seen both in vivo and ex vivo, indicating a perfusion-independent mechanism. Thus, the vasculature controls pancreas morphogenesis and growth by reducing branching and differentiation of primitive epithelial cells.

G protein-coupled receptor signaling and sphingosine-1-phosphate play a phylogenetically conserved role in endocrine pancreas morphogenesis

During development pancreatic endocrine cells migrate in a coordinated fashion. This migration is necessary to form fully functional islets, but the mechanisms involved remain unknown. Therapeutic strategies to restore β-cell mass and islet functionality by reprogramming endogenous exocrine cells would be strengthened from simultaneous treatments that enhance endocrine cell clustering. We found that endocrine progenitors respond to and regulate G protein-coupled receptor (GPCR) signaling in order to cluster in islets. Rgs4, a dedicated regulator of GPCR signaling, was specifically expressed in early epithelial endocrine progenitors of both zebrafish and mouse, and its expression in the mouse endocrine progenitors was strictly dependent upon Ngn3, the key specification gene of the endocrine lineage. Rgs4 loss of function resulted in defects in islet cell aggregation. By genetically inactivating Gα(i)-mediated GPCR signaling in endocrine progenitors, we established its role in islet cell aggregation in both mouse and zebrafish. Finally, we identified sphingosine-1-phosphate (S1P) as a ligand mediating islet cell aggregation in both species acting through distinct but closely related receptors.

Connective tissue growth factor acts within both endothelial cells and beta cells to promote proliferation of developing beta cells

Type 1 and type 2 diabetes result from an absolute or relative reduction in functional β-cell mass. One approach to replacing lost β-cell mass is transplantation of cadaveric islets; however, this approach is limited by lack of adequate donor tissue. Therefore, there is much interest in identifying factors that enhance β-cell differentiation and proliferation in vivo or in vitro. Connective tissue growth factor (CTGF) is a secreted molecule expressed in endothelial cells, pancreatic ducts, and embryonic β cells that we previously showed is required for β-cell proliferation, differentiation, and islet morphogenesis during development. The current study investigated the tissue interactions by which CTGF promotes normal pancreatic islet development. We found that loss of CTGF from either endothelial cells or β cells results in decreased embryonic β-cell proliferation, making CTGF unique as an identified β cell-derived factor that regulates embryonic β-cell proliferation. Endothelial CTGF inactivation was associated with decreased islet vascularity, highlighting the proposed role of endothelial cells in β-cell proliferation. Furthermore, CTGF overexpression in β cells during embryogenesis using an inducible transgenic system increased islet mass at birth by promoting proliferation of immature β cells, in the absence of changes in islet vascularity. Together, these findings demonstrate that CTGF acts in an autocrine manner during pancreas development and suggest that CTGF has the potential to enhance expansion of immature β cells in directed differentiation or regeneration protocols.

Impaired spleen formation perturbs morphogenesis of the gastric lobe of the pancreas

Despite the extensive use of the mouse as a model for studies of pancreas development and disease, the development of the gastric pancreatic lobe has been largely overlooked. In this study we use optical projection tomography to provide a detailed three-dimensional and quantitative description of pancreatic growth dynamics in the mouse. Hereby, we describe the epithelial and mesenchymal events leading to the formation of the gastric lobe of the pancreas. We show that this structure forms by perpendicular growth from the dorsal pancreatic epithelium into a distinct lateral domain of the dorsal pancreatic mesenchyme. Our data support a role for spleen organogenesis in the establishment of this mesenchymal domain and in mice displaying perturbed spleen development, including Dh +/−, Bapx1−/− and Sox11−/−, gastric lobe development is disturbed. We further show that the expression profile of markers for multipotent progenitors is delayed in the gastric lobe as compared to the splenic and duodenal pancreatic lobes. Altogether, this study provides new information regarding the developmental dynamics underlying the formation of the gastric lobe of the pancreas and recognizes lobular heterogeneities regarding the time course of pancreatic cellular differentiation. Collectively, these data are likely to constitute important elements in future interpretations of the developing and/or diseased pancreas.