Effects of UK-14,304, noradrenaline, and propranolol on insulin release from transplanted mouse islets

Local growth factors are beneficial for the autonomic reinnervation of transplanted islets in rats

INTRODUCTION

Transplanted islets, being avascular and denervated, receive blood vessels and nerves from the recipient. Reinnervation may account in part for the normalization of islet function in islet transplants. Whether reinnervation is possible to augment is not known.

AIMS AND METHODOLOGY

To explore whether reinnervation of transplanted islets is augmented by local addition of growth factors to the graft, syngeneic islets were transplanted to the pancreas of streptozotocin-diabetic Lewis rats with or without pellets locally releasing nerve growth factor (NGF) and vascular endothelial growth factor (VEGF), alone or in combination. The pellets released growth factors for 14 days at a rate of 20 ng/day. After 7 weeks, pancreatic tissue was processed for immunofluorescence of insulin and the neural markers neuropeptide Y (NPY) and tyrosine hydroxylase (TH).

RESULTS

Islets were larger and more numerous after treatment with NGF (p = 0.024) and with NGF in combination with VEGF (p = 0.044). Similarly, immunostaining for TH and the C-terminal flanking peptide of NPY (C-PON) was more pronounced after treatment with NGF in combination with VEGF than in controls (both p < 0.05).

CONCLUSION

Local growth factor treatment has a beneficial effect on autonomic reinnervation as well as islet integrity and survival of the graft after islet transplantation in rats.

Desensitization of alpha 2A-adrenoceptor signalling by modest levels of adrenaline is facilitated by beta 2-adrenoceptor-dependent GRK3 up-regulation

(1) Adrenaline (ADR) and noradrenaline (NA) can simultaneously activate inhibitory alpha(2)- and stimulatory beta-adrenoceptors (AR). However, ADR and NA differ significantly in that ADR is a potent beta(2)-AR agonist while NA is not. Only recently has the interaction resulting from the simultaneous activation of alpha(2)- and beta(2)-AR been examined at the cellular level to determine the mechanisms of alpha(2)-AR regulation following concomitant activation of both alpha(2)- and beta(2)-ARs by chronic ADR. (2) This study evaluates beta(2)-AR regulation of alpha(2A)-AR signalling following chronic ADR (300 nM) and NA (1 and 30 micro M) treatments of BE(2)-C human neuroblastoma cells that natively express both beta(2)- and alpha(2A)-ARs. (3) Chronic (24 h) treatment with ADR (300 nM) desensitized the response to the alpha(2A)-AR agonist, brimonidine, in BE(2)-C cells. Addition of the beta-AR antagonist, propranolol, blocked the ADR-induced alpha(2A)-AR desensitization. Unlike ADR, chronic NA (1 micro M) treatment had no effect on the alpha(2A)-AR response. However if NA was increased to 30 micro M for 24 h, alpha(2A)-AR desensitization was observed; this desensitization was partially reversed by propranolol. (4) Chronic ADR (300 nM) treatment reduced alpha(2A)-AR binding levels, contributing to the alpha(2A)-AR desensitization. This decrease was prevented by addition of propranolol during ADR treatment. Chronic NA (30 micro M), like ADR, treatment lowered specific binding, whereas 1 micro M NA treatment was without effect. (5) Chronic ADR treatment produced a significant increase in GRK3 levels and this was blocked by propranolol or GRK2/3 antisense DNA treatment. This antisense DNA, common to both GRK2 and GRK3, also blocked chronic ADR-induced alpha(2A)-AR desensitization and down-regulation. (6) Acute (1 h) ADR (300 nM) or NA treatment (1 micro M) produced alpha(2A)-AR desensitization. The desensitization produced by acute treatment was beta-AR independent, as it was not blocked by propranolol. (7) We conclude that chronic treatment with modest levels of ADR produces alpha(2A)-AR desensitization by mechanisms that involve up-regulation of GRK3 and down-regulation of alpha(2A)-AR levels through interactions with the beta(2)-AR.

Factors influencing Islet of Langerhans graft function and monitoring

Transplantation of islet of Langerhans represents a viable therapeutic option for insulin-dependent diabetes mellitus. Dramatic progress has been recently reported with the introduction of a glucocorticoid-free immunosuppressive regimen that improved success rate, namely, insulin independence for 1 year or more, from 8% to 100%. The fate of islet grafts is determined by many concurrent phenomena, some of which are common to organ grafts (i.e. rejection), while others are unique to nonvascularized cell transplants, including transplant cell mass and viability, as well as nonspecific inflammation at the site of implant. Moreover, islet grafts lack clinical markers of early rejection, making it difficult to recognize imminent rejection and to implement intervention with graft-saving immunosuppressive regimens. In the present review, we will address the problems influencing islet graft success and the monitoring of islet cell graft function.

Peptides and other neuronal markers in transplanted pancreatic islets

Functional alterations are developed in transplanted islets over time. Because islets in situ are densely innervated and isolation disconnects the endocrine organ from extrinsic nerves and from ganglia in the exocrine pancreas, it is important to examine the reinnervation of islet grafts. This review describes the patterns of appearances of intrinsic perikarya and reinnervating fibers demonstrating markers for parasympathetic, sympathetic or sensory nerve substances, most notably neuropeptides, in islet transplants. An altered innervation pattern, as compared to normal islets, develops. Presumably the expression of neuronal markers in the grafts is related to factors both in the islets and in the ectopic environment offered by the implantation organ.

Engraftment and growth of transplanted pancreatic islets

Transplantation of pancreatic islets may provide a cure for type 1 diabetes. However, this treatment can currently be offered only to very few patients. To improve transplantation success we need to understand better the mechanisms of how the implanted islets survive, grow and/or maintain adequate function. We herein report on our studies to evaluate the factors responsible for the engraftment, i.e. revascularization, reinnervation etc., of transplanted islets and relate these factors to the metabolism and growth of the islets. Graft metabolism can be monitored by microdialysis probes that allow for the measurement of minute amounts of islet metabolites and hormonal products. Growth of the endocrine cells can be stimulated both in vitro before implantation and in vivo post-transplantation. Another problem is rejection of transplanted islets, which may be overcome by the microencapsulation of islets. The knowledge gained by the present studies will enable us to elucidate the optimal treatment of islets to ensure a maximal survival of the transplanted islets, and may be applied also to clinical islet transplantation.

Expression of tyrosine hydroxylase, calcitonin gene-related peptide, substance P and protein gene product 9.5 in mouse islets transplanted under the kidney capsule

Pancreatic islets transplanted to the kidney of syngeneic mice were stained for calcitonin gene-related peptide (CGRP), substance P (SP), tyrosine hydroxylase (TH), acetylcholinesterase and the pan-neuronal marker, protein gene product 9.5 (PGP). Nerve fibers expressing TH-like immunoreactivity (TH-LI) and CGRP-LI were rare for 4 days but increased 2 (CGRP) or 6 (TH) weeks after transplantation. In 1-year-old grafts the CGRP-LI innervation resembled that in situ, while TH-LI and PGP-LI innervations were increased. SP-LI fibers remained rare throughout. Perikarya intrinsic to the islets did not show CGRP-LI or SP-LI. The results indicate a progressive ingrowth of sensory fibers into the grafts and that the TH-LI innervation becomes even more pronounced than in the pancreas. The post-transplantation reaction of islet intrinsic neurons does not involve CGRP and SP, contrasting with previous observations for vasoactive intestinal polypeptide.

Intrinsic and extrinsic NPY nerves in transplanted neuroinsular complexes

In mouse pancreatic islets, whether in situ or transplanted to kidney, nerve fibers and a few perikarya expressed NPY-like immunoreactivity (NPY-LI). In 4-5 day old grafts, NPY-LI coexisted with VIP-LI in randomly distributed nerve fibers. By 2-52 weeks, NPY mainly co-existed with tyrosine hydroxylase in fibers emanating from the kidney parenchyma. Radioimmunoassays indicated that the NPY levels increased with time, while those of VIP decreased. The study shows that NPY is primarily present in the intrinsic VIP-ergic innervation of islet grafts but later is mainly a constituent of the ingrowing sympathetic innervation.