The postnatal pancreatic microenvironment guides β cell maturation through BMP4 production

Engineered vasculature induces functional maturation of pluripotent stem cell-derived islet organoids

Blood vessels play a critical role in pancreatic islet function, yet current methods for deriving islet organoids from human pluripotent stem cells (SC-islets) lack vasculature. We engineered three-dimensional (3D) vascularized SC-islet organoids by assembling SC-islet cells, human primary endothelial cells (ECs), and fibroblasts in a non-perfused model and a microfluidic device with perfused vessels. Vasculature improved stimulus-dependent Ca2+ influx into SC-β cells, a hallmark of β cell function that is blunted in non-vascularized SC-islets. Moreover, vascularization accelerated diabetes reversal post engraftment of a subtherapeutic SC-islet dose into mice. We show that vasculature leads to the formation of an islet-like basement membrane that contributes to the functional improvement of SC-β cells. Furthermore, single-cell RNA sequencing (scRNA-seq) data predicted BMP2/4-BMPR2 signaling from ECs to SC-β cells, and correspondingly, BMP4 enhanced the SC-β cell Ca2+ response and insulin secretion. Vascularized SC-islets will enable further studies of crosstalk between β cells and ECs and will serve as an in vitro platform for disease modeling and therapeutic testing.

Bioprinting and synthetic biology approaches to engineer functional endocrine pancreatic constructs

Diabetes is a complex disease affecting over 500 million people worldwide. Traditional approaches, such as insulin delivery, are mainstay treatments, but do not cure the disease. Recent advances in biofabrication and synthetic biology offer new hope for the development of tissue constructs recapitulating salient organ functions. Here, we discuss recent progress in bioprinting a functional endocrine pancreas, ranging from cell sources to main advances in biomaterials. We review innovative areas for the development of this field, with a particular focus on the convergence of synthetic biology and cell engineering with bioprinting, which opens new avenues for developing advanced in vitro models and regenerative, transplantable grafts, with the potential to provide independence from exogenous insulin administration.

Enhanced BMP Signaling Alters Human β-Cell Identity and Function

Inflammation contributes to the pathophysiology of diabetes. Identifying signaling pathways involved in pancreatic β-cell failure and identity loss can give insight into novel potential treatment strategies to prevent the loss of functional β-cell mass in diabetes. It is reported earlier that the immunosuppressive drug tacrolimus has a detrimental effect on human β-cell identity and function by activating bone morphogenetic protein (BMP) signaling. Here it is hypothesized that enhanced BMP signaling plays a role in inflammation-induced β-cell failure. Single-cell transcriptomics analyses of primary human islets reveal that IL-1β+IFNγ and IFNα treatment activated BMP signaling in β-cells. These findings are validated by qPCR. Furthermore, enhanced BMP signaling with recombinant BMP2 or 4 triggers a reduced expression of key β-cell maturity genes, associated with increased ER stress, and impaired β-cell function. Altogether, these results indicate that inflammation-activated BMP signaling is detrimental to pancreatic β-cells and that BMP-signaling can be a target to preserve β-cell identity and function in a pro-inflammatory environment.

Embryonic macrophages support endocrine commitment during human pancreatic differentiation

Organogenesis is a complex process that relies on a dynamic interplay between extrinsic factors originating from the microenvironment and tissue-specific intrinsic factors. For pancreatic endocrine cells, the local niche consists of acinar and ductal cells as well as neuronal, immune, endothelial, and stromal cells. Hematopoietic cells have been detected in human pancreas as early as 6 post-conception weeks, but whether they play a role during human endocrinogenesis remains unknown. To investigate this, we performed single-nucleus RNA sequencing (snRNA-seq) of the second-trimester human pancreas and identified a wide range of hematopoietic cells, including two distinct subsets of tissue-resident macrophages. Leveraging this discovery, we developed a co-culture system of human embryonic stem cell-derived endocrine-macrophage organoids to model their interaction in vitro. Here, we show that macrophages support the differentiation and viability of endocrine cells in vitro and enhance tissue engraftment, highlighting their potential role in tissue engineering strategies for diabetes.

Vascular and immune interactions in islets transplantation and 3D islet models

The aim of regenerative medicine is to restore specific functions to damaged cells or tissues. A crucial aspect of success lies in effectively reintegrating these cells or tissues within the recipient organism. This is particularly pertinent for diabetes, where islet function relies on the close connection of beta cells to the bloodstream for glucose sensing and insulin release. Central to this approach is the need to establish a fast connection with the host's vascular system. In this review, we explore the intricate relationships between endocrine, vascular, and immune cell interactions in transplantation outcomes. We also delve into recent strategies aimed at enhancing engraftment, along with the utilization of in vitro platforms to model cellular interactions.

Generation and application of novel hES cell reporter lines for the differentiation and maturation of hPS cell-derived islet-like clusters

The significant advances in the differentiation of human pluripotent stem (hPS) cells into pancreatic endocrine cells, including functional β-cells, have been based on a detailed understanding of the underlying developmental mechanisms. However, the final differentiation steps, leading from endocrine progenitors to mono-hormonal and mature pancreatic endocrine cells, remain to be fully understood and this is reflected in the remaining shortcomings of the hPS cell-derived islet cells (SC-islet cells), which include a lack of β-cell maturation and variability among different cell lines. Additional signals and modifications of the final differentiation steps will have to be assessed in a combinatorial manner to address the remaining issues and appropriate reporter lines would be useful in this undertaking. Here we report the generation and functional validation of hPS cell reporter lines that can monitor the generation of INS+ and GCG+ cells and their resolution into mono-hormonal cells (INSeGFP, INSeGFP/GCGmCHERRY) as well as β-cell maturation (INSeGFP/MAFAmCHERRY) and function (INSGCaMP6). The reporter hPS cell lines maintained strong and widespread expression of pluripotency markers and differentiated efficiently into definitive endoderm and pancreatic progenitor (PP) cells. PP cells from all lines differentiated efficiently into islet cell clusters that robustly expressed the corresponding reporters and contained glucose-responsive, insulin-producing cells. To demonstrate the applicability of these hPS cell reporter lines in a high-content live imaging approach for the identification of optimal differentiation conditions, we adapted our differentiation procedure to generate SC-islet clusters in microwells. This allowed the live confocal imaging of multiple SC-islets for a single condition and, using this approach, we found that the use of the N21 supplement in the last stage of the differentiation increased the number of monohormonal β-cells without affecting the number of α-cells in the SC-islets. The hPS cell reporter lines and the high-content live imaging approach described here will enable the efficient assessment of multiple conditions for the optimal differentiation and maturation of SC-islets.

Beneficial islet inflammation in health depends on pericytic TLR/MyD88 signaling

While inflammation is beneficial for insulin secretion during homeostasis, its transformation adversely affects β cells and contributes to diabetes. However, the regulation of islet inflammation for maintaining glucose homeostasis remains largely unknown. Here, we identified pericytes as pivotal regulators of islet immune and β cell function in health. Islets and pancreatic pericytes express various cytokines in healthy humans and mice. To interfere with the pericytic inflammatory response, we selectively inhibited the TLR/MyD88 pathway in these cells in transgenic mice. The loss of MyD88 impaired pericytic cytokine production. Furthermore, MyD88-deficient mice exhibited skewed islet inflammation with fewer cells, an impaired macrophage phenotype, and reduced IL-1β production. This aberrant pericyte-orchestrated islet inflammation was associated with β cell dedifferentiation and impaired glucose response. Additionally, we found that Cxcl1, a pericytic MyD88-dependent cytokine, promoted immune IL-1β production. Treatment with either Cxcl1 or IL-1β restored the mature β cell phenotype and glucose response in transgenic mice, suggesting a potential mechanism through which pericytes and immune cells regulate glucose homeostasis. Our study revealed pericyte-orchestrated islet inflammation as a crucial element in glucose regulation, implicating this process as a potential therapeutic target for diabetes.

Diabetes as a Pancreatic Microvascular Disease-A Pericytic Perspective

Diabetes is not only an endocrine but also a vascular disease. Vascular defects are usually seen as consequence of diabetes. However, at the level of the pancreatic islet, vascular alterations have been described before symptom onset. Importantly, the cellular and molecular mechanisms underlying these early vascular defects have not been identified, neither how these could impact the function of islet endocrine cells. In this review, we will discuss the possibility that dysfunction of the mural cells of the microvasculature-known as pericytes-underlies vascular defects observed in islets in pre-symptomatic stages. Pericytes are crucial for vascular homeostasis throughout the body, but their physiological and pathophysiological functions in islets have only recently started to be explored. A previous study had already raised interest in the "microvascular" approach to this disease. With our increased understanding of the crucial role of the islet microvasculature for glucose homeostasis, here we will revisit the vascular aspects of islet function and how their deregulation could contribute to diabetes pathogenesis, focusing in particular on type 1 diabetes (T1D).

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.

A Supportive Role of Mesenchymal Stem Cells on Insulin-Producing Langerhans Islets with a Specific Emphasis on The Secretome

Type 1 Diabetes (T1D) is a chronic autoimmune disease characterized by a gradual destruction of insulin-producing β-cells in the endocrine pancreas due to innate and specific immune responses, leading to impaired glucose homeostasis. T1D patients usually require regular insulin injections after meals to maintain normal serum glucose levels. In severe cases, pancreas or Langerhans islet transplantation can assist in reaching a sufficient β-mass to normalize glucose homeostasis. The latter procedure is limited because of low donor availability, high islet loss, and immune rejection. There is still a need to develop new technologies to improve islet survival and implantation and to keep the islets functional. Mesenchymal stem cells (MSCs) are multipotent non-hematopoietic progenitor cells with high plasticity that can support human pancreatic islet function both in vitro and in vivo and islet co-transplantation with MSCs is more effective than islet transplantation alone in attenuating diabetes progression. The beneficial effect of MSCs on islet function is due to a combined effect on angiogenesis, suppression of immune responses, and secretion of growth factors essential for islet survival and function. In this review, various aspects of MSCs related to islet function and diabetes are described.

Pericytes modulate islet immune cells and insulin secretion through Interleukin-33 production in mice

Introduction

Immune cells were recently shown to support β-cells and insulin secretion. However, little is known about how islet immune cells are regulated to maintain glucose homeostasis. Administration of various cytokines, including Interleukin-33 (IL-33), was shown to influence β-cell function. However, the role of endogenous, locally produced IL-33 in pancreatic function remains unknown. Here, we show that IL-33, produced by pancreatic pericytes, is required for glucose homeostasis.

Methods

To characterize pancreatic IL-33 production, we employed gene expression, flow cytometry, and immunofluorescence analyses. To define the role of this cytokine, we employed transgenic mouse systems to delete the Il33 gene selectively in pancreatic pericytes, in combination with the administration of recombinant IL-33. Glucose response was measured in vivo and in vitro, and morphometric and molecular analyses were used to measure β-cell mass and gene expression. Immune cells were analyzed by flow cytometry.

Resuts

Our results show that pericytes are the primary source of IL-33 in the pancreas. Mice lacking pericytic IL-33 were glucose intolerant due to impaired insulin secretion. Selective loss of pericytic IL-33 was further associated with reduced T and dendritic cell numbers in the islets and lower retinoic acid production by islet macrophages.

Discussion

Our study demonstrates the importance of local, pericytic IL-33 production for glucose regulation. Additionally, it proposes that pericytes regulate islet immune cells to support β-cell function in an IL-33-dependent manner. Our study reveals an intricate cellular network within the islet niche.

Using single-cell multi-omics screening of human fetal pancreas to identify novel players in human beta cell development

Islet transplantation from organ donors can considerably improve glucose homeostasis and well-being in individuals with type 1 diabetes, where the beta cells are destroyed by the autoimmune attack, but there are insufficient donor islets to make this a widespread therapy. Strategies are therefore being developed to generate unlimited amounts of insulin-producing beta cells from pluripotent stem cells, with the aim that they will be transplanted to treat diabetes. Whilst much progress has been made in recent years in the directed differentiation of pluripotent stem cells to beta-like cells, essential gaps still exist in generating stem cell-derived beta cells that are fully functional in vitro. This short review provides details of recent multi-'omics' studies of the human fetal pancreas, which are revealing granular information on the various cell types in the developing pancreas. It is anticipated that this fine mapping of the pancreatic cells at single-cell resolution will provide additional insights that can be utilised to reproducibly produce human beta cells in vitro that have the functional characteristics of beta cells within native human islets.

Maturation of beta cells: lessons from in vivo and in vitro models

The ability to maintain normoglycaemia, through glucose-sensitive insulin release, is a key aspect of postnatal beta cell function. However, terminally differentiated beta cell identity does not necessarily imply functional maturity. Beta cell maturation is therefore a continuation of beta cell development, albeit a process that occurs postnatally in mammals. Although many important features have been identified in the study of beta cell maturation, as of yet no unified mechanistic model of beta cell functional maturity exists. Here, we review recent findings about the underlying mechanisms of beta cell functional maturation. These findings include systemic hormonal and nutritional triggers that operate through energy-sensing machinery shifts within beta cells, resulting in primed metabolic states that allow for appropriate glucose trafficking and, ultimately, insulin release. We also draw attention to the expansive synergistic nature of these pathways and emphasise that beta cell maturation is dependent on overlapping regulatory and metabolic networks.

Mechanisms Underlying the Expansion and Functional Maturation of β-Cells in Newborns: Impact of the Nutritional Environment

The functional maturation of insulin-secreting β-cells is initiated before birth and is completed in early postnatal life. This process has a critical impact on the acquisition of an adequate functional β-cell mass and on the capacity to meet and adapt to insulin needs later in life. Many cellular pathways playing a role in postnatal β-cell development have already been identified. However, single-cell transcriptomic and proteomic analyses continue to reveal new players contributing to the acquisition of β-cell identity. In this review, we provide an updated picture of the mechanisms governing postnatal β-cell mass expansion and the transition of insulin-secreting cells from an immature to a mature state. We then highlight the contribution of the environment to β-cell maturation and discuss the adverse impact of an in utero and neonatal environment characterized by calorie and fat overload or by protein deficiency and undernutrition. Inappropriate nutrition early in life constitutes a risk factor for developing diabetes in adulthood and can affect the β-cells of the offspring over two generations. A better understanding of these events occurring in the neonatal period will help developing better strategies to produce functional β-cells and to design novel therapeutic approaches for the prevention and treatment of diabetes.

Metabolic Stress Impairs Pericyte Response to Optogenetic Stimulation in Pancreatic Islets

Pancreatic islets are highly vascularized micro-organs ensuring whole body glucose homeostasis. Islet vascular cells play an integral part in sustaining adequate insulin release by beta cells. In particular, recent studies have demonstrated that islet pericytes regulate local blood flow velocity and are required for maintenance of beta cell maturity and function. In addition, increased metabolic demand accompanying obesity alters islet pericyte morphology. Here, we sought to explore the effects of metabolic stress on islet pericyte functional response to stimulation in a mouse model of type 2 diabetes, directly in the pancreas in vivo . We found that high fat diet induced islet pericyte hypertrophy without alterations in basal local blood flow. However, optogenetic stimulation of pericyte activity revealed impaired islet vascular responses, despite increased expression of genes encoding proteins directly or indirectly involved in cell contraction. These findings suggest that metabolic stress impinges upon islet pericyte function, which may contribute to beta cell failure during T2D.