Isolation, Culture, and Characterization of Endocrine Cells from 6-Month-Old Porcine Pancreas

The Use of Pancreas Biopsy Scoring Provides Reliable Porcine Islet Yields While Encapsulation Permits the Determination of Microbiological Safety

For clinical xenogenic islet transplantation to be successful, several requirements must be met. Among them is a sizeable and reliable source of fully functional and microbiologically safe islets. The inherent variability among porcine pancreases, with respect to islet yield, prompted us to develop a Biopsy Score technique to determine the suitability of each pancreas for islet isolation processing. The Biopsy Score consists of an assessment of five variables: warm ischemia time, pancreas color, fat content, islet size, and islet demarcation, each of which is assigned a value of −1 or +1, depending on whether or not the established criteria is met. For determination of islet size and demarcation, fresh biopsies of porcine pancreases are stained with dithizone (DTZ) solution and examined under a dissecting microscope. Based on the scoring of such biopsies in pancreases from 26—56-month-old sows, we report here that the presence of large (>100 μm diameter), well-demarcated islets in the pancreas biopsy is a reliable predictor of isolation success. Encapsulation of the isolated porcine islets within the inner layer of a 1.5% agarose and an outer layer of 5.0% agarose macrobead, containing 500 equivalent islet number (EIN), provides for extended in vitro functional viability (>6 months of insulin production in response to glucose), as well as for comprehensive microbiological testing and at least partial isolation of the xenogeneic islets from the host immune system. All microbiological testing to date has been negative, except for the presence of porcine endogenous retrovirus (PERV). Taken together, we believe that the Biopsy Score enhancement of our islet isolation technique and our agarose-agarose macroencapsulation methodology bring us significantly closer to realizing clinical porcine islet xenotransplantation for the treatment of insulin-dependent diabetic patients.

High-fat diet induces early-onset diabetes in heterozygous Pax6 mutant mice

BACKGROUND

Type 2 diabetes is caused by interactions between genetic and environmental factors. Our previous studies reported that paired box 6 mutation heterozygosity (Pax6(m/+)) led to defective proinsulin processing and subsequent abnormal glucose metabolism in mice at 6  months of age. However, high-fat diet exposure could be an important incentive for diabetes development. In this study, we aimed to develop a novel diabetic model imitating human type 2 diabetes by exposing Pax6(m/+) mice to high-fat diet and to explore the underlying mechanism of diabetes in this model.

METHODS

Over 300 Pax6(m/+) and wild-type male weanling mice were randomly divided into two groups and were fed an high-fat diet or chow diet for 6-10  weeks. Blood glucose and glucose tolerance levels were monitored during this period. Body weights, visceral adipose weights, blood lipid profiles and insulin sensitivity (determined with an insulin tolerance test) were used to evaluate obesity and insulin resistance. Proinsulin processing and insulin secretion levels were used to evaluate pancreatic β cell function.

RESULTS

After 6  weeks of high-fat diet exposure, only the Pax6(m/+) mice showed dramatic postloading hyperglycaemia. These mice exhibited significant high-fat diet-induced visceral obesity and insulin resistance and displayed defective prohormone convertase 1/3 production, an increased proinsulin:total insulin ratio and impaired early-phase insulin secretion, because of the Pax6 mutation. Hyperglycaemia worsened progressively over time with the high-fat diet, and most Pax6(m/+) mice on high-fat diet developed diabetes or impaired glucose tolerance after 10  weeks. Furthermore, high-fat diet withdrawal partly improved blood glucose levels in the diabetic mice.

CONCLUSIONS

By combining the Pax6(m/+) genetic background with an high-fat diet environment, we developed a novel diabetic model to mimic human type 2 diabetes. This model is characterized by impaired insulin secretion, caused by the Pax6 mutation, and high-fat diet-induced insulin resistance and therefore provides an ideal tool for research on type 2 diabetes pathogenesis and therapies.

Mutant PAX6 downregulates prohormone convertase 2 expression in mouse islets

Transcriptional factor paired box 6 (PAX6) is very important for the development of the eyes, central nervous system, and pancreas. PAX6 mutations are associated with a diabetic phenotype and abnormal glucose metabolism. Our previous study showed that PAX6 directly bound to and activated the prohormone convertase 1/3 (Pc1/3) gene promoter and subsequently regulated proinsulin processing. Prohormone convertase 2 (PC2) is the essential enzyme for pancreatic proinsulin processing. To study the regulation of PAX6 in Pc2 expression, we did research on the pancreas of Pax6 R266Stop mutant mice, where truncated mutations happened in the C-terminal of the PAX6 protein. Our studies showed that the mutant PAX6 protein was stable and regulated the activity of Pc2 promoter as shown by luciferase activity assays. We found that the wild-type PAX6 protein imparts a transcriptional effect, and the mutant PAX6 can also regulate the downstream molecules. The results provide new insights into the mechanism of truncated PAX6 in regulating the functions of the pancreas and endocrine system.

Paired box 6 (PAX6) regulates glucose metabolism via proinsulin processing mediated by prohormone convertase 1/3 (PC1/3)

AIMS/HYPOTHESIS

Human patients with aniridia caused by heterozygous PAX6 mutations display abnormal glucose metabolism, but the underlying molecular mechanism is largely unknown. Disturbed islet architecture has been proposed as the reason why mice with complete inactivation of paired box 6 (PAX6) in the pancreas develop diabetes. This is not, however, the case in human aniridia patients with heterozygous PAX6 deficiency and no apparent defects in pancreatic development. We investigated the molecular mechanism underlying the development of abnormal glucose metabolism in these patients.

METHODS

A human aniridia pedigree with a PAX6 R240Stop mutation was examined for abnormal glucose metabolism using an OGTT. The underlying mechanism was further investigated using Pax6 R266Stop mutant small-eye mice, which also have abnormal glucose metabolism similar to that in PAX6 R240Stop mutation human aniridia patients.

RESULTS

Paired box 6 (PAX6) deficiency, both in aniridia patients with a heterozygous PAX6 R240Stop mutation and in mice with a heterozygous Pax6 R266Stop mutation, causes defective proinsulin processing and abnormal glucose metabolism. PAX6 can bind to the promoter and directly upregulate production of prohormone convertase (PC)1/3, an enzyme essential for conversion of proinsulin to insulin. Pax6 mutations lead to PC1/3 deficiency, resulting in defective proinsulin processing and abnormal glucose metabolism.

CONCLUSIONS/INTERPRETATION

This study indicates a novel function for PAX6 in the regulation of proinsulin processing and glucose metabolism via modulation of PC1/3 production. It also provides an insight into the abnormal glucose metabolism caused by heterozygous PAX6 mutations in humans and mice.

A novel human stem cell coculture system that maintains the survival and function of culture islet-like cell clusters

Islet-like cell clusters (ICCs) have been suggested to be a source of insulin-producing tissue for xenotransplantation in type 1 diabetes. We designed an approach to maintain the cultured rat pancreatic ICC survival and function, when cocultured with human umbilical cord mesenchymal stem cells (HUMSCs). HUMSCs in coculture have the ability to maintain ICC survival and function, for which number and insulin secretion of ICCs are increasing and lasting for 3 months, while ICCs gradually crash, which results in cell death after a period of 12 days of culture without HUMSCs. Cytokine protein array showed it has more than a twofold increase in levels of several cytokines (interleukin-6, tissue inhibitor of metalloproteinases-1, tissue inhibitor of metalloproteinases-2, monocyte chemoattractant protein-1, growth related oncogene, hepatocyte growth factor, insulin-like growth factor binding proteins 4, and interleukin-8) on coculture medium, implying an important role of these cytokines in this coculture system. These findings suggest that coculture with HUMSCs may have a significant potential to protect ICCs from damage during culture, and may be employed in a novel culture approach to maintain islet cell survival and function before transplantation.

Impact of porcine islet size on cellular structure and engraftment after transplantation: adult versus young pigs

OBJECTIVES

To study the impact of porcine islet size on structural properties and cellular engraftment.

METHODS

The endocrine structure and collagen/vascular localization in pig islets were studied before and after enzymatic isolation on the pancreas from 6 young and 6 adult Landrace pigs. Isolated islets from both pig types were transplanted under the kidney capsula of diabetic nude rats to assess cellular engraftment.

RESULTS

In comparison with adult pig pancreata, a significantly greater number of small beta cells (<100 microm) were observed before and after isolation (82% vs. 32%, respectively, P < 0.005) from young pig pancreata. Small islets (<100 microm) showed a peripheral vascular structure, whereas large islets showed a more centralized vascular organization, thereby providing protection during the enzymatic digestion procedure. The islet endocrine structure was not affected by the islet size, but a loss of glucagon cells (-7.9%, P < 0.005) was observed in large isolated islets. The purity of islet preparation was better with pancreata from adult than young donors (86% vs. 64%, respectively, P < 0.05). A lack of engraftment was observed for small islets from young pig donors as compared with large islets from adult donors.

CONCLUSIONS

Large and well-structured islets, mainly found in adult pig pancreata, probably possess a better potential for cellular engraftment due to centralized vascularization and collagen distribution.

Subcutaneous transplantation of macroencapsulated porcine pancreatic endocrine cells normalizes hyperglycemia in diabetic mice

BACKGROUND

The ultimate goal of islet transplantation is the unlimited availability of insulin-secreting cells to be transplanted in a simple procedure that requires no use of immunosuppressive drugs. Immunoisolation of xenogeneic pig islets for transplantation has great potential therapeutic benefits for treatment of diabetes.

METHODS

Approximately 4 x 10(6) porcine pancreatic endocrine cells (PEC) isolated from 6-month-old pigs were macroencapsulated in agarose-poly(styrene sulfonic acid) mixed gel and implanted into a prevascularized subcutaneous site in streptozotocin-induced C57BL/6 diabetic mice. Animals receiving an equal number of free porcine PEC were used as controls. After transplantation, nonfasting blood glucose, body weight, intraperitoneal glucose tolerance test, and immunohistologic evaluations were processed.

RESULTS

All 10 animals receiving the subcutaneous xenografts of the macroencapsulated porcine PEC normalized hyperglycemia within 5 days after transplantation, maintained the duration of normoglycemia for 24 to 76 days, and gradually gained weight. The subcutaneous xenografts of free porcine PEC could not reverse hyperglycemia. The recipient became hyperglycemic again when the implanted graft was retrieved at day 45 after transplantation. The glucose clearances were significantly ameliorated at day 21 and day 45 after transplantation when compared with those in diabetic mice. The immunohistochemical results revealed an inherent intact structure of the macroencapsulated porcine PEC and positive double-immunofluorescence staining for insulin and glucagon.

CONCLUSIONS

Subcutaneous transplantation of macroencapsulated porcine PEC normalized hyperglycemia in diabetic mice. Our results identified a potential for a favorable development of subcutaneous transplantation of porcine PEC as a cure for diabetes.