A NATURAL BODY WINDOW TO STUDY HUMAN PANCREATIC ISLET CELL FUNCTION AND SURVIVAL

Abstract

The World Health Organization projects diabetes prevalence worldwide to be at 4.4% in 2030 compared to 2.8% in the year 2000. These alarming predictions come amid vigorous efforts in diabetes research which have failed so far to deliver effective therapies. Our incomplete understanding of the pathogenesis of diabetes is likely to contribute to the "disconnect" between our research efforts and their translation into successful therapies. Technically, studying the pathophysiology of the pancreatic islets is hindered by the anatomical location of the pancreas, which is deeply embedded in the body, and by lack of experimental tools that enable comprehensive interrogation of the pancreatic islets with sufficient resolution in the context of the natural in vivo environment non-invasively and longitudinally. Emerging evidence also indicates that challenges in successful translation of findings in animal models to the human setting are complicated by some inherent structural and functional differences between the mouse and human islets. In this review, we briefly describe the advantages and shortcomings of existing intravital imaging approaches used to study the pancreatic islet biology in vivo, and we contrast such techniques with a recently established intravital approach using pancreatic islet transplantation into the anterior chamber of the eye. We also provide a summary of recent structure-function studies in the human pancreas to reveal distinctive features of human islets compared with mouse islets. We finally touch on a recently renewed discussion of the validity of animal models in studying human health and disease, and we highlight the potential utility of "humanized" animal models in studying different aspects of human islet biology and improving our understanding of diabetes.

Keywords: Diabetes; Human pancreatic islet cell; Islets of Langerhans; in vivo imaging.

Figure 1

Transplantation of pancreatic islets into the anterior chamber of the eye is minimally invasive, and enables high-resolution in vivo imaging of islets non-invasively and longitudinally. This figure shows an illustration of islet transplantation into the anterior chamber of the mouse eye highlighting a magnified view of the eye during the transplantation procedure. The image on the top left shows isolated islets being introduced into the anterior chamber with air bubbles still trapped immediately after injection. The islets are injected using a cannula that is inserted through a small incision in the cornea. Notice that there’s no bleeding during this procedure. Shortly after transplantation, the islets begin engraftment on top of the iris where they become revascularized and innervated. The islets can now be imaged non-invasively and directly studied repeatedly in anesthetized animals for extended periods of time as shown in the top right image.

Figure 2

The unique cytoarchitecture of the human pancreatic islet. Human (B) pancreatic islets contain fewer beta cells (red) and a higher number of alpha cells (green) compared to mouse (A) islets. C, Human alpha, beta, and delta (blue) cells are intermingled throughout the islet with no particular order of distribution along islet blood vessels (outlined and marked with asterisks).

Figure 3

Alpha cells, but not beta cells, express functional glutamate receptors. (Top row) Three sequential images showing changes in free intracellular calcium concentration ([Ca²⁺]_i) in response to 3 mM glucose (rest), 100 mM glutamate (glutamate), and 11 mM glucose (glucose) in dispersed human islet cells. Images are shown in intensity scale to highlight the [Ca²⁺]_i responses. (Bottom left) Glucagon (green) and insulin (red) immunostaining of the same cells shown in top row. Glucagon-immunoreactive cells 2 and 3 responded to glutamate but not high glucose concentration. Insulin-immunoreactive cells 1 and 4 responded to high glucose concentration but not to glutamate. (Bottom right) Traces of the [Ca²⁺]_i responses of these cells. Bars under traces indicate stimulus application.

Figure 4

Insulin release from human pancreatic islets in response to ATP. Perifusion assays of insulin secretion show that ATP stimulates insulin secretion from isolated human islets in a concentration-dependent manner (n = 4 human islet preparations). Bars under trace indicate stimulation time with 11 mM glucose (11G) or the indicated ATP concentration (mM).

Figure 5

Sympathetic and parasympathetic innervation patterns of mouse and human pancreatic islets. Z-stacks of confocal images of islets in mouse and human pancreatic sections immunostained for (top row) the sympathetic marker tyrosine hydroxylase (TH; green), and (bottom row) the parasympathetic cholinergic marker vesicular acetylcholine transporter (vAChT; red). In contrast to evident vAChT expression in nerve fibers within the mouse islet, vAChT expression was detected in the endocrine cells of the human islet. A general axonal marker, acetylated tubulin (a Tubulin, green), confirmed sparse overall innervation within the human islet. Nuclear stain (DAPI; blue) and glucagon immunostain (red) are also shown in the top row. All images are shown as maximum projections.