Immunoisolation of islets in high guluronic acid barium-alginate microcapsules does not improve graft outcome at the subcutaneous site.

The survival and function of alginate microencapsulated islets is suboptimal when transplanted to the intraperitoneal site of diabetic animals. The large capacity and convenience of the subcutaneous site make it an appropriate and attractive alternative for microencapsulated grafts. Nonencapsulated and high guluronic acid barium-alginate microencapsulated islets were transplanted to the intraperitoneal and subcutaneous sites of diabetic mice. Microencapsulation improved graft success up to 28 days at the intraperitoneal site but not at the subcutaneous site. Samples of microencapsulated islets transplanted into normoglycemic mice confirmed that insulin secretion, insulin content, and adenosine triphosphate content were reduced more significantly in subcutaneous graft islets than intraperitoneal graft islets after 7 days. In addition, a greater proportion of dead cells were observed in the subcutaneous graft islets than in intraperitoneal graft islets after 28 days. We conclude that using alginate microencapsulated islets transplanted to the unmodified subcutaneous site is insufficient to reverse the diabetic state. This finding is likely to be related to the inability of the site to support islet function and viability.

Islet alpha cell number is maintained in microencapsulated islet transplantation.

Islet transplantation can reverse hyperglycaemia in Type 1 diabetes patients. One problem in islet transplantation is a loss of beta cell mass as well as blunted glucagon responses from the grafted islets. It has been suggested that alpha cell loss is associated with close contact of the alpha cells with the implantation organ. In the present study we made use of microencapsulation, where transplanted islets are not in direct contact with the host implantation site. After transplantation, the number of glucagon cells stained per microencapsulated islet section was increased whereas the number of insulin cells stained was decreased. DNA content of the islets was reduced, as was insulin content, whereas glucagon content was unchanged. This indicates that cell number in transplanted microencapsulated islets diminishes, which can be accounted for by loss of beta cells. However, in contrast to previous studies using non-encapsulated islets, alpha cell number seems to be maintained.

Transient beneficial effects of exendin-4 treatment on the function of microencapsulated mouse pancreatic islets.

Transplantation of microencapsulated islets may reduce hyperglycemia in the absence of immunosuppression. However, the efficiency of microencapsulated islet transplantation is low, requiring more islets to achieve normoglycemia than in vascularized islet transplantation. Exendin-4 (a glucagon-like receptor agonist) has been previously shown to improve islet transplantation outcome in rodents. We investigated whether this treatment would enhance the function of microencapsulated islets in vitro and in vivo. Encapsulated or naked islets were cultured with or without exendin-4 for 72 h. To test in vitro function, insulin release and glucose oxidation rates were measured in the absence or presence of exendin-4. In addition, in vivo function of a minimal mass of 350 microencapsulated islets was assessed by syngeneic transplantation into the peritoneal cavity of alloxan-diabetic mice. Glucose oxidation rates of microencapsulated islets were improved by 72-h pretreatment with exendin-4. Insulin release was increased both acutely after glucose stimulation and over a 40-h culture period by the presence of exendin-4. Transplantation outcome of microencapsulated islets cultured with exendin-4 was initially improved, but by day 7 there were no differences compared with control cultured microencapsulated islets. Culture of microencapsulated islets with exendin-4 increases glucose oxidation and insulin release rates, but the increased function seen in vitro was not enough to improve the long term outcome in a transplantation model.

Islet amyloid deposits preferentially in the highly functional and most blood-perfused islets.

Islet amyloid and beta cell death in type 2 diabetes are heterogeneous events, where some islets are affected early in the disease process, whereas others remain visibly unaffected. This study investigated the possibility that inter-islet functional and vascular differences may explain the propensity for amyloid accumulation in certain islets. Highly blood-perfused islets were identified by microspheres in human islet amyloid polypeptide expressing mice fed a high-fat diet for three or 10 months. These highly blood-perfused islets had better glucose-stimulated insulin secretion capacity than other islets and developed more amyloid deposits after 10 months of high-fat diet. Similarly, human islets with a superior release capacity formed more amyloid in high glucose culture than islets with a lower release capacity. The amyloid formation in mouse islets was associated with a higher amount of prohormone convertase 1/3 and with a decreased expression of its inhibitor proSAAS when compared to islets with less amyloid. In contrast, levels of prohormone convertase 2 and expression of its inhibitor neuroendocrine protein 7B2 were unaltered. A misbalance in prohormone convertase levels may interrupt the normal processing of islet amyloid polypeptide and induce amyloid formation. Preferential amyloid load in the most blood-perfused and functional islets may accelerate the progression of type 2 diabetes.

No differences in efficacy between noncultured and cultured islets in reducing hyperglycemia in a nonvascularized islet graft model.

BACKGROUND: Although islet transplantation is a promising method to restore normoglycemia in recipients with diabetes, large numbers of pancreatic islets are still needed. It has been suggested that the use of freshly isolated islets could improve transplantation outcome through better vascular engraftment. Using a technique of microencapsulation, a model where revascularization is not possible, we investigated the importance of revascularization for transplantation outcome. METHODS: Either 700 or 350 3-day-cultured or noncultured encapsulated islets were transplanted intraperitoneally into syngeneic mice with alloxan-induced diabetes. In addition, 700 nonencapsulated islets were transplanted to mice with diabetes. Blood glucose concentrations were monitored, and glucose tolerance tests were carried out. After 42 days, the encapsulated islets were retrieved and assayed for glucose oxidation and insulin release rates. RESULTS: There were no differences between capsules containing fresh or cultured islets in their capacity to lower the blood glucose concentration of the recipients or in the in vitro function after capsule retrieval. Interestingly, mice that were intraperitoneally transplanted with 700 encapsulated islets had average blood glucose levels well below 11 mM for most of the study, whereas the same number of nonencapsulated islets had no beneficial effects on the blood glucose homeostasis. CONCLUSIONS: Encapsulated islets can reverse hyperglycemia after transplantation to the intraperitoneal site. This effect was not seen when nonencapsulated islets were grafted. Since a 3- day culture period did not influence the outcome of transplantation of encapsulated islets there is evidence to suggest that a more appropriate revascularization may explain why freshly isolated islets are more efficient than cultured islets.

Extensive amyloid formation in transplanted microencapsulated mouse and human islets.

OBJECTIVE: Deposition of cell toxic islet amyloid is a frequent finding in type 2 diabetes and also in transplanted human islets, where it is a possible explanation for their long-term failure. One suggested reason for amyloid in transplanted islets is that their low vascular density results in a disturbed local clearance of islet amyloid polypeptide (IAPP). To test this hypothesis we analysed accumulation of amyloid in microencapsulated islets, which exemplify a non-vascularised islet graft. METHODS: Isolated islets from human or transgenic mice expressing human IAPP were microencapsulated in alginate and cultured in vitro or transplanted under the kidney capsule of normoglycemic nude mice. The degree of amyloid was determined after Congo red staining and subcellular alterations were analysed with electron microscopy. RESULTS: Insulin and IAPP secretion from transgenic mouse islets were markedly increased during stimulation with glucose after one week of culture, but encapsulated islets in general released less insulin. Amyloid was detected after both one and three weeks of culture in the transgenic mouse islets and the encapsulated islets were most affected. After transplantation, electron microscopy displayed both intra- and extracellular amyloid in microencapsulated as well as in non-encapsulated human and transgenic mouse islet grafts. However, amyloid was more frequent in the encapsulated grafts. CONCLUSION: Micro-encapsulation of pancreatic islets might serve as an important tool for studies of amyloid formation under enhanced circumstances.

Amyloid deposition in transplanted human pancreatic islets: a conceivable cause of their long-term failure.

Following the encouraging report of the Edmonton group, there was a rejuvenation of the islet transplantation field. After that, more pessimistic views spread when long-term results of the clinical outcome were published. A progressive loss of the beta-cell function meant that almost all patients were back on insulin therapy after 5 years. More than 10 years ago, we demonstrated that amyloid deposits rapidly formed in human islets and in mouse islets transgenic for human IAPP when grafted into nude mice. It is, therefore, conceivable to consider amyloid formation as one potential candidate for the long-term failure. The present paper reviews attempts in our laboratories to elucidate the dynamics of and mechanisms behind the formation of amyloid in transplanted islets with special emphasis on the impact of long-term hyperglycemia.