Reinnervation of pancreatic islets and regulation of insulin secretion by hepatic sympathetic nerves after intraportal transplantation of islets into livers of diabetic rats

Recent advances in the design of implantable insulin secreting heterocellular islet organoids

Islet transplantation has proved one of the most remarkable transmissions from an experimental curiosity into a routine clinical application for the treatment of type I diabetes (T1D). Current efforts for taking this technology one-step further are now focusing on overcoming islet donor shortage, engraftment, prolonged islet availability, post-transplant vascularization, and coming up with new strategies to eliminate lifelong immunosuppression. To this end, insulin secreting 3D cell clusters composed of different types of cells, also referred as heterocellular islet organoids, spheroids, or pseudoislets, have been engineered to overcome the challenges encountered by the current islet transplantation protocols. β-cells or native islets are accompanied by helper cells, also referred to as accessory cells, to generate a cell cluster that is not only able to accurately secrete insulin in response to glucose, but also superior in terms of other key features (e.g. maintaining a vasculature, longer durability in vivo and not necessitating immunosuppression after transplantation). Over the past decade, numerous 3D cell culture techniques have been integrated to create an engineered heterocellular islet organoid that addresses current obstacles. Here, we first discuss the different cell types used to prepare heterocellular organoids for islet transplantation and their contribution to the organoids design. We then introduce various cell culture techniques that are incorporated to prepare a fully functional and insulin secreting organoids with select features. Finally, we discuss the challenges and present a future outlook for improving clinical outcomes of islet transplantation.

Simulated Cholinergic Reinnervation of β (INS-1) Cells: Antidiabetic Utility of Heterotypic Pseudoislets Containing β Cell and Cholinergic Cell

Cholinergic neurons can functionally support pancreatic islets in controlling blood sugar levels. However, in islet transplantation, the level of cholinergic reinnervation is significantly lower compared to orthotopic pancreatic islets. This abnormal reinnervation affects the survival and function of islet grafts. In this study, the cholinergic reinnervation of beta cells was simulated by 2D and 3D coculture of INS-1 and NG108-15 cells. In 2D culture conditions, 20 mM glucose induced a 1.24-fold increase (p < 0.0001) in insulin secretion from the coculture group, while in the 3D culture condition, a 1.78-fold increase (p < 0.0001) in insulin secretion from heterotypic pseudoislet group was observed. Glucose-stimulated insulin secretion (GSIS) from 2D INS-1 cells showed minimal changes when compared to 3D structures. E-cadherin expressed in INS-1 and NG108-15 cells was the key adhesion molecule for the formation of heterotypic pseudoislets. NG108-15 cells hardly affected the proliferation of INS-1 cells in vitro. Heterotypic pseudoislet transplantation recipient mice reverted to normoglycemic levels faster and had a greater blood glucose clearance compared to INS-1 pseudoislet recipient mice. In conclusion, cholinergic cells can promote insulin-secreting cells to function better in vitro and in vivo and E-cadherin plays an important role in the formation of heterotypic pseudoislets.

Functional stimulation of graft nerves has minor effects on insulin release from transplanted rat pancreatic islets

INTRODUCTION

Morphological evidence for reinnervation of pancreatic islet grafts is plentiful. However, to what extent intra-graft nerves influence the endocrine functions of the islet transplant is largely unknown. We therefore aimed to directly stimulate nerves leading to islet grafts with electrodes and measure insulin secretion in response to this.

METHODS

We implanted syngeneic islets under the renal capsule of rats, and examined them 1 or 7-9 months later. In anesthetized rats blood samples were collected from the renal vein and femoral artery, respectively, during electrode stimulation of the nerves leading to the islet grafts.

RESULTS

As expected, nerve stimulation decreased renal blood flow. However, serum insulin concentrations in samples derived from the renal vein or femoral artery changed in concert with one another, both during normoglycemia and acute hyperglycemia.

CONCLUSION

Reinnervation which occurs after islet transplantation under the renal capsule has minor effects on graft endocrine function.

Human islet cell transplantation--future prospects

BACKGROUND

Islet transplantation has the potential to cure diabetes mellitus. Nevertheless despite successful reversal of diabetes in many small animal models, the clinical situation has been far more challenging. The aim of this review is to discuss why insulin-independence after islet allotransplantation has been so difficult to achieve.

METHODS

A literature review was undertaken using Medline from 1975 to July 2000. Results reported to the International Islet Transplant Registry (ITR) up to December 1998 were also analysed.

RESULTS

Up to December 1998, 405 islet allotransplants have been reported the ITR. Of those accurately documented between 1990 and 1998 (n = 267) only 12% have achieved insulin-independence (greater than 7 days). However with refined peri-transplant protocols insulin independence at 1 year can reach 20%.

CONCLUSIONS

There are many factors which can explain the failure of achieving insulin-independence after islet allotransplantation. These include the use of diabetogenic immunosuppressive agents to abrogate both islet allo-immunity and auto-immunity, the critical islet mass to achieve insulin-independence and the detrimental effects of transplanting islets in an ectopic site. However recent evidence most notably from the Edmonton group demonstrates that islet allotransplantation still has great potential to become an established treatment option for diabetic patients.

Islet graft-induced changes of beta-hydroxybutyrate dehydrogenase and glucose-6-phosphatase activity in liver cells of diabetic recipient rats

It is known that the liver is a favourable site for implantation of pancreatic islets since the grafted islets remain metabolically intact and provide long-term normoglycemia in diabetic animals. However, the long-term effects exerted by the grafted tissue on the host organ are not well defined. We therefore investigated by light and electron microscopy the effects of syngeneic islets on the host organ after intraportal transplantation into the liver of streptozotocin (STZ)-induced diabetic LEW.1W rats. In addition, tissue sections of graft-bearing liver were stained by enzyme histochemical methods for beta-hydroxybutyrate dehydrogenase (HBDH) and glucose-6-phosphatase (G6Pase). At 12 weeks after transplantation, the changes seen in the hepatocytes surrounding the grafted islets were hyperproliferation and accumulation of glycogen. Hepatocytes adjacent to the implanted islets displayed increased HBDH activity, whereas G6Pase activity was variable, either decreased or increased. Increased HBDH activity was also observed in the periportal region and in liver cells extending to the central veins. The results demonstrate that intraportal islet grafts, in addition to normalizing glucose homeostasis, exert remarkable effects on the liver parenchyma of experimentally diabetic recipient rats.