Alpha-, Delta- and PP-cells: Are They the Architectural Cornerstones of Islet Structure and Co-ordination?

Abstract

Islet non-β-cells, the α- δ- and pancreatic polypeptide cells (PP-cells), are important components of islet architecture and intercellular communication. In α-cells, glucagon is found in electron-dense granules; granule exocytosis is calcium-dependent via P/Q-type Ca(2+)-channels, which may be clustered at designated cell membrane sites. Somatostatin-containing δ-cells are neuron-like, creating a network for intra-islet communication. Somatostatin 1-28 and 1-14 have a short bioactive half-life, suggesting inhibitory action via paracrine signaling. PP-cells are the most infrequent islet cell type. The embryologically separate ventral pancreas anlage contains PP-rich islets that are morphologically diffuse and α-cell deficient. Tissue samples taken from the head region are unlikely to be representative of the whole pancreas. PP has anorexic effects on gastro-intestinal function and alters insulin and glucagon secretion. Islet architecture is disrupted in rodent diabetic models, diabetic primates and human Type 1 and Type 2 diabetes, with an increased α-cell population and relocation of non-β-cells to central areas of the islet. In diabetes, the transdifferentiation of non-β-cells, with changes in hormone content, suggests plasticity of islet cells but cellular function may be compromised. Understanding how diabetes-related disordered islet structure influences intra-islet cellular communication could clarify how non-β-cells contribute to the control of islet function.

Keywords: PP; communication; exocytosis; glucagon; granule; insulin; intra-islet signaling; non-β-cell; paracrine; somatostatin.

Figure 1.

Mouse islet immunolabelled for insulin (red), glucagon (blue), and somatostatin (green). This confocal image reconstruction of the cells at the exterior of the islet demonstrates the network of δ-cells and their proximity to α- and β-cells. Scale, 20 µm.

Figure 2.

Granule morphologies and islet cell network in an islet from (A) a mouse and (B) a human islet. β-, α-, δ-, and PP-cells viewed by electron microscopy. Insulin secretory granules are similar in both species with an electron-dense core and clear halo. However, human insulin granules sometimes appear crystalline, with angular shaped cores compared to the smooth spherical cores of the mouse islet. Glucagon secretory granules are electron-dense without a clear halo; in human α-cells, some secretory granules have a grey halo surrounding the dense core, whereas others are without a halo, as in the mouse. PP-cells contain spherical smaller granules, which are very heterogeneous in size in both species; some PP granules are similar to those found in α-cells and others have a small halo. Somatostatin-containing granule morphology is very different in mouse and human: in rodents, the granules are small, lozenge-shaped structures; in humans, the granules are larger, slightly electron-opaque but spherical and of similar size to that of glucagon granules. l, lipofuscin body; n, nucleus. Scale, 1.0 µm.

Figure 3.

Pancreatic islets demonstrating the species-specific differences in cellular architecture. Immunofluorescent labelling of pancreatic sections for insulin (green), glucagon (pink), and somatostatin (yellow). In mouse islets (A), the non-β-cells are situated at the periphery of the islet whereas, in non-human primates (B) and humans (C), the α- and δ-cells are found both at the periphery of the islet cross-section and towards the islet center. This reflects the location of these cells at the perivascular border of both circumferential capillaries (rodents and humans) and those penetrating the islet interior (non-human primates and humans). Scale, 200 μm.

Figure 4.

Part of an islet from a non-diabetic patient viewed with electron microscopy demonstrating the presence of insulin-containing β-cells (β), α-cells (α), and δ-cells (δ) all close to the peripheral capillary (cap). A large proportion of β-cells are not situated adjacent or near to a capillary in this thin section. l, lipofuscin body; n, nucleus. Scale, 5 μm.

Figure 5.

Electron microscopy image of mouse α-cell to demonstrate the dense granulation. α-Cells rarely show degranulation, even in diabetic human or mouse models. m, mitochondrion; n, nucleus; β, β-cell. Scale, 1 μm.

Figure 6.

δ-Cells in a mouse islet. Immunofluorescent labelling for somatostatin in a mouse intact islet and reconstructed from confocal images. The cells have long neuronal-like processes that connect with each other to create an interconnecting network of δ-cells in the islet. Scale, 20 µm.

Figure 7.

Sequential electron microscopy images (A–H) to demonstrate the pathway and adjacent cells of a δ-cell in a rat islet. Images were taken using serial block-face scanning electron microscopy of a rat islet. Sections were taken 50 µm apart throughout the islet. This series of sequential images (distance between images in µm listed on the panels) shows a δ-cell (δ) process penetrating the islet from the surface (A) between five adjacent β-cells (β1–5), an α-cell (α1), and an unknown cell type (x1) to a central capillary (c) in (H). Scale, 5 µm.

Figure 8.

Electron microscopy image of mouse δ-cells (δ) to demonstrate the lozenge-shaped granules and potential intercellular communication between cells. Omega-shaped thickened membrane invaginations (arrows) at the intercellular margin are characteristic of membrane recovery following granule exocytosis. m, mitochondrion; er, endoplasmic reticulum; cap, capillary. Scale, 1 μm.

Figure 9.

Tissue from the head region of a human pancreas to demonstrate differences in islet structure in the PP-rich lobule of the ventral pancreas. Immunoperoxidase labelling for glucagon (A) and PP (B). Adjacent sections through the head region show an exocrine lobule (outlined), which has large, diffuse islets containing a high proportion of PP-positive cells. An islet in an adjacent lobule (arrow) has no PP-cells and the usual population of glucagon-positive cells. The lower panel shows immunofluorescent labelling for (C) PP (green), (D) glucagon (red), (E) insulin (blue), and merged images (F). The diffusely structured islet contains mostly PP-positive cells, a single glucagon-positive cell (arrow), and insulin-positive cells distributed randomly throughout. Scale (A, B) 500 μm; (C, D, E, F) 50 μm.

Figure 10.

Immunofluorescent labelling of islet cells rodent and primate diabetes to demonstrate changes in islet architecture in hyperglycemia. Panels show cells immunofluorescently labelled for insulin, glucagon, and somatostatin and the merged signals. Hyperglycemic models: (A) Diabetic Goto-Kakizaki rat; (B) mouse model of diabetes exhibiting beta-cell specific deletion of the Krebs cycle enzyme fumarate hydratase (FH^-/-); (C) mouse with impaired insulin secretion due to the expression of an activating K_ATP channel mutation (βV59M); (D) non-human primate with diabetes; (E) patient with recent-onset T1D; and (F) patient with T2D. In all of these hyperglycemic models, there are marked changes in islet morphology (for comparison see Figure 2 for structure in the absence of diabetes). There was reduced insulin-positive areas and increased proportion of glucagon-positive cells. A typical feature of animal models of diabetes is an increased infiltration of α-cells into the core of the islet. Little change in the expression pattern of somatostatin is apparent in most of the diabetic models. However, somatostatin-expressing cells are increased in βV59M mice and in the patient sample of T1D. Scale, 200 μm.

Figure 11.

Pancreatic islet structure during the acute, ketotic phase of childhood-onset Type 1 Diabetes (T1D): Electron micrograph shows a few degranulated β-cells (β) but many α- (α) and δ- (δ) cells, most of which are located adjacent to a capillary (cap). Infiltration of lymphocytes (L) was present throughout the islet. Scale, 5 μm.

Figure 12.

Part of an islet from a patient with Type 2 Diabetes; electron microscopy montage demonstrating the presence of many insulin-containing β-cells (β) and a high proportion of α-cells (α). PP-containing cells (PP) and δ-cells (δ) are close to capillaries (cap). A large proportion of cells are not situated adjacent or near to a capillary in this thin section. Although other islets in this patient contained islet amyloid deposits, there were none in this islet cross section and no cells showing signs of apoptosis. Exo; exocrine tissue. Scale, 5 μm.

Figure 13.

Diagrammatic representation of intra-islet connections and regulatory signals in health and diabetes. In normoglycemia, cells are closely packed, with non-β-cells situated more peripherally and adjacent to capillaries. Secretion of insulin (green) and glucagon (purple) exert paracrine effects on α-, δ-(yellow), and β-cells. Somatostatin has inhibitory paracrine effects on all cell types (dotted lines). β-Cell products, Zn²⁺ and GABA, may affect α-cell function, and acetylcholine from α-cells may influence somatostatin secretion. In the diabetic islet, architecture is disrupted. There are less β-cells, and α-cells are increased and distributed throughout the islet. The influence of insulin on islet function is reduced but the paracrine effects of glucagon and somatostatin are likely to be increased (heavy solid and dotted lines, respectively).