Vascular endothelial growth factor coordinates islet innervation via vascular scaffolding

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.

Heterotypic control of basement membrane dynamics during branching morphogenesis

Many mammalian organs undergo branching morphogenesis to create highly arborized structures with maximized surface area for specialized organ function. Cooperative cell-cell and cell-matrix adhesions that sculpt the emerging tissue architecture are guided by dynamic basement membranes. Properties of the basement membrane are reciprocally controlled by the interacting epithelial and mesenchymal cell populations. Here we discuss how basement membrane remodeling is required for branching morphogenesis to regulate cell-matrix and cell-cell adhesions that are required for cell patterning during morphogenesis and how basement membrane impacts morphogenesis by stimulation of cell patterning, force generation, and mechanotransduction. We suggest that in addition to creating mature epithelial architecture, remodeling of the epithelial basement membrane during branching morphogenesis is also essential to promote maturation of the stromal mesenchyme to create mature organ structure. Recapitulation of developmental cell-matrix and cell-cell interactions are of critical importance in tissue engineering and regeneration strategies that seek to restore organ function.

Islet formation in mice and men: lessons for the generation of functional insulin-producing β-cells from human pluripotent stem cells

The Islets of Langerhans are crucial 'micro-organs' embedded in the glandular exocrine pancreas that regulate nutrient metabolism. They not only synthesize, but also secrete endocrine hormones in a modulated fashion in response to physiologic metabolic demand. These highly sophisticated structures with intricate organization of multiple cell types, namely endocrine, vascular, neuronal and mesenchymal cells, have evolved to perform this task to perfection over time. Not surprisingly, islet architecture and function are dissimilar between humans and typically studied model organisms, such as rodents and zebrafish. Further, recent findings also suggest noteworthy differences in human islet development from that in mouse, including delayed appearance and gradual resolution of key differentiation markers, a single-phase of endocrine differentiation, and prenatal association of developing islets with neurovascular milieu. In light of these findings, it is imperative that a systematic study is undertaken to compare islet development between human and mouse. Illuminating inter-species differences in islet development will likely be critical in furthering our pursuit to generate an unlimited supply of truly functional and fully mature β-cells from human pluripotent stem cell (hPSC) sources for therapeutic purposes.

Surface coating of pancreatic islets with neural crest stem cells improves engraftment and function after intraportal transplantation

The present study aimed to develop techniques for surface coating of islets with neural crest stem cells (NCSCs) in order to enable cotransplantation to the clinically used liver site and then investigate engraftment and function intraportally of such bioengineered islets. Mouse islets were coated during incubation with enhanced green fluorescent protein (EGFP)-expressing mouse NCSCs and transplanted into the portal vein to cure diabetic mice. An intravenous glucose tolerance test was performed at 1 month posttransplantation. Islet grafts were retrieved and evaluated for vascular density, nerves, and glial cells. NCSCs expressed a vast number of key angiogenic and neurotrophic factors. Mice transplanted with NCSC-bioengineered islets responded better to the glucose load than recipient mice with control islets. NCSCs remained present in the vicinity or had often migrated into the NCSC-coated islets, and an improved islet graft reinnervation and revascularization was observed. Transplanted NCSCs differentiated into both glial and neural cells in the islet grafts. We conclude that bioengineering of islets with NCSCs for intraportal transplantation provides a possibility to improve islet engraftment and function. Pending successful establishment of protocols for expansion of NCSCs from, for example, human skin or bone marrow, this strategy may be applied to clinical islet transplantation.

Serum fetuin-A concentrations are positively associated with serum VEGF levels in patients with newly diagnosed type 2 diabetes

Fetuin-A was considered to be involved in pathogenesis of type 2 diabetes. On the other hand, higher vascular endothelial growth factor (VEGF) expression is associated with diabetes and its vascular complications, but the mechanisms leading to higher VEGF levels are still not clear. To the best of our knowledge, there are no data to show the associations between fetuin-A and VEGF in patients with type 2 diabetes. Therefore, the aim of this study is to investigate the relationship between serum fetuin-A concentrations and serum VEGF levels in patients with type 2 diabetes. We recruited 345 patients with newly diagnosed type 2 diabetes. Serum fetuin-A concentrations and serum VEGF levels were measured using enzyme-linked immunosorbent assay (ELISA) method. In this study, there was a significant positive correlation between serum fetuin-A concentrations and serum VEGF levels (r=0.223, P<0.001), and the correlation remained significant even after adjustment for other confounding factors in the multivariate regression model (β=0.151, P=0.006). Mantel-Haenszel (M-H) stratified analysis showed that the degree of association of high concentrations of fetuin-A with high levels of VEGF is higher than that with low levels of VEGF (odds ratio of M-H [ORM-H], 2.938; 95% confidence interval [CI], 1.896-4.553). In addition, this study showed that both fetuin-A and VEGF were positively associated with fasting plasma glucose (FPG), glycated hemoglobin A1c (HbA1c) and C-reactive protein (CRP). These data suggested that serum fetuin-A concentrations were positively associated with serum VEGF levels in patients with newly diagnosed type 2 diabetes.

Extracellular matrix components in the pathogenesis of type 1 diabetes

Type 1 diabetes (T1D) results from progressive immune cell-mediated destruction of pancreatic β cells. As immune cells migrate into the islets, they pass through the extracellular matrix (ECM). This ECM is composed of different macromolecules localized to different compartments within and surrounding islets; however, the involvement of this ECM in the development of human T1D is not well understood. Here, we summarize our recent findings from human and mouse studies illustrating how specific components of the islet ECM that constitute basement membranes and interstitial matrix of the islets, and surprisingly, the intracellular composition of islet β cells themselves, are significantly altered during the pathogenesis of T1D. Our focus is on the ECM molecules laminins, collagens, heparan sulfate/heparan sulfate proteoglycans, and hyaluronan, as well as on the enzymes that degrade these ECM components. We propose that islet and lymphoid tissue ECM composition and organization are critical to promoting immune cell activation, islet invasion, and destruction of islet β cells in T1D.

An integrated cell purification and genomics strategy reveals multiple regulators of pancreas development

The regulatory logic underlying global transcriptional programs controlling development of visceral organs like the pancreas remains undiscovered. Here, we profiled gene expression in 12 purified populations of fetal and adult pancreatic epithelial cells representing crucial progenitor cell subsets, and their endocrine or exocrine progeny. Using probabilistic models to decode the general programs organizing gene expression, we identified co-expressed gene sets in cell subsets that revealed patterns and processes governing progenitor cell development, lineage specification, and endocrine cell maturation. Purification of Neurog3 mutant cells and module network analysis linked established regulators such as Neurog3 to unrecognized gene targets and roles in pancreas development. Iterative module network analysis nominated and prioritized transcriptional regulators, including diabetes risk genes. Functional validation of a subset of candidate regulators with corresponding mutant mice revealed that the transcription factors Etv1, Prdm16, Runx1t1 and Bcl11a are essential for pancreas development. Our integrated approach provides a unique framework for identifying regulatory genes and functional gene sets underlying pancreas development and associated diseases such as diabetes mellitus.

Discovery of specific pancreas developmental regulators has accelerated in recent years. In contrast, the global regulatory programs controlling pancreas development are poorly understood compared to other organs or tissues like heart or blood. Decoding this regulatory logic may accelerate development of replacement organs from renewable sources like stem cells, but this goal requires identification of regulators and assessment of their functions on a global scale. To address this important challenge for pancreas biology, we combined purification of normal and mutant cells with genome-scale methods to generate and analyze expression profiles from developing pancreas cells. Our work revealed regulatory gene sets governing development of pancreas progenitor cells and their progeny. Our integrative approach nominated multiple pancreas developmental regulators, including suspected risk genes for human diabetes, which we validated by phenotyping mutant mice on a scale not previously reported. Selection of these candidate regulators was unbiased; thus it is remarkable that all were essential for pancreatic islet development. Thus, our studies provide a new heuristic resource for identifying genetic functions underlying pancreas development and diseases like diabetes mellitus.

Neural control of the endocrine pancreas

The autonomic nervous system affects glucose metabolism partly through its connection to the pancreatic islet. Since its discovery by Paul Langerhans, the precise innervation patterns of the islet has remained elusive, mainly because of technical limitations. Using 3-dimensional reconstructions of axonal terminal fields, recent studies have determined the innervation patterns of mouse and human islets. In contrast to the mouse islet, endocrine cells within the human islet are sparsely contacted by autonomic axons. Instead, the invading sympathetic axons preferentially innervate smooth muscle cells of blood vessels. This innervation pattern suggests that, rather than acting directly on endocrine cells, sympathetic nerves may control hormone secretion by modulating blood flow in human islets. In addition to autonomic efferent axons, islets also receive sensory innervation. These axons transmit sensory information to the brain but also have the ability to locally release neuroactive substances that have been suggested to promote diabetes pathogenesis. We discuss recent findings on islet innervation, the connections of the islet with the brain, and the role islet innervation plays during the progression of diabetes.

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.