<p>The blood vessels of the pancreatic islets are of crucial importance for oxygen and metabolite supply as well as dispersal of secreted hormones. In addition to this, endothelial cells have an important role in the revascularization process after islet transplantation. Previous studies have reported signs of poor engraftment of transplanted islets, presumably due to impaired revascularization. The aims of this thesis were to investigate the revascularization process of transplanted islets and to examine the role of islet endothelial cells. In this context, the lectin Bandeiraea simplicifolia was found to stain endothelium of both endogenous and transplanted pancreatic islets. By using this lectin we investigated the vascular density of both endogenous and islets transplanted syngeneically beneath the renal capsule, into the spleen or intraportally into the liver of normoglycemic C57BL/6 mice. One month post-transplantation, a time point when the grafts are assumed to be completely revascularized, the vascular density was decreased at all three implantation sites when compared to endogenous islets. Furthermore, most of the blood vessels were located in the graft connective tissue stroma. Similar results were obtained when islet transplant vascular density was determined six months post-transplantation and in cured diabetic animals after one month. In order to evaluate the function of intraportally transplanted islets, we developed a method to retrieve such islets. We treated the implantation organ (liver) first enzymatically (collagenase) and then mechanically, thereafter we could re-isolate the transplanted islets for further in vitro studies. The retrieved islets had a decreased insulin relase, insulin content and glucose oxidation rate when compared to non-transplanted control islets. To understand the role of islet endothelium in the revascularization of transplanted islets we performed angiogenesis GEArray studies on islet endothelial cells, from non-cultured, cultured and transplanted islets. We found that the islet endothelium expressed mRNA for both inhibitors and inducers of angiogenesis, and that this expression differed with time. The functional consequences of this remain to be determined. In summary, the results presented above provide a useful platform for future studies of the morphology and function of islet endothelial cells, especially with a view for elucidating changes induced by islet transplantation.</p>

<p>In the field of transplantation, the increasing deficit of human donors have lead to an interest in animals as an alternative source of organs and tissues. </p><p>Different <i>in vitro </i>systems and rodent models of xenotransplantation were used to examine the most significant barriers that have to be overcome, before isolated islets of Langerhans from pigs can be used as a cure for insulin-dependent diabetes mellitus in humans.</p><p>In clinical transplantation, islets are infused into the liver through the portal vein. During this procedure the islets are susceptible to harmful innate reactions triggered in blood. Adenoviral vectors generating transgenic expression of human complement regulatory proteins were evaluated in pig islets and shown to confer protection against acute complement-mediated damage. </p><p>Transplanted islets escaping this immediate destruction will be targets of a cellular immune response. Using a new mouse model of islet xenograft rejection, it was demonstrated that macrophages, effector cells in the rejection, were part of an MHC-restricted xenospecific immune response mediated by T cells. In a strain of knockout mice it was further shown that this process can proceed in the absence of an important signalling system, mediated by Toll-like receptors, between cells in innate and adaptive immunity. These findings illustrate some of the mechanistic differences compared to cellular islet allograft rejection which partly explain why immunosuppressive drugs used in clinical allotransplantation is not sufficient for preventing xenograft rejection. </p><p>Porcine endogenous retroviruses (PERV) remain a safety concern in xenotransplantation. Characterization of PERV in pig islets indicated that virus expression is low <i>in vitro </i>but increases during the immediate time period following transplantation. This suggests that antiviral therapies administered at the time of transplantation could be used for preventing the risk of PERV transmission after xenotransplantation.</p>

<p>Insulin release from single islets of Langerhans is pulsatile. The secretory activities of the islets in the pancreas are coordinated resulting in plasma insulin oscillations. Nutrients amplitude-regulate the insulin pulses without influencing their frequency. Diabetic patients show an abnormal plasma insulin pattern, but the cause of the disturbance remains to be elucidated. Ithe present thesis the influence of the cytoplasmic calcium concentratio([Ca²⁺]_i) and cell metabolism on pulsatile insulin release was examined in single islets of Langerhans from <i>ob/ob</i>-mice. Glucose stimulation of insulin release involves closure of ATP-sensitive K⁺ channels (K_ATP channels), depolarization, and Ca²⁺ influx in β-cells. In the presence of 11 mM glucose, pulsatile insulin secretion occurs in synchrony with oscillations i[Ca²⁺]_i. When [Ca²⁺]_i is low and stable, e.g. under basal conditions, low amplitude insulin pulses are still observed. When [Ca²⁺]_i is elevated and non-oscillating, e.g. when the β-cells are depolarized by potassium, high amplitude insulin pulses are observed. The frequency of the insulin pulses under these conditions is similar to that observed when [Ca²⁺]_i oscillations are present. By permanently opening or closing the K_ATP channels with diazoxide or tolbutamide, respectively, it was investigated if glucose can modulate pulsatile insulin secretion when it does not influence the channel activity. Under these conditions, [Ca²⁺]_i remained stable whereas the amplitude of the insulin pulses increased with sugar stimulation without change in the frequency. Metabolic inhibition blunted but did not prevent the insulin pulses. The results indicate that oscillations in metabolism can generate pulsatile insulin release when [Ca²⁺]_i is stable. However, under physiological conditions, pulsatile secretion is driven by oscillations in metabolism and [Ca²⁺]_i, acting in synergy.</p>

Insulin release from single islets of Langerhans is pulsatile. The secretory activities of the islets in the pancreas are coordinated resulting in plasma insulin oscillations. Nutrients amplitude-regulate the insulin pulses without influencing their frequency. Diabetic patients show an abnormal plasma insulin pattern, but the cause of the disturbance remains to be elucidated. Ithe present thesis the influence of the cytoplasmic calcium concentratio([Ca2+]i) and cell metabolism on pulsatile insulin release was examined in single islets of Langerhans from ob/ob-mice. Glucose stimulation of insulin release involves closure of ATP-sensitive K+ channels (KATP channels), depolarization, and Ca2+ influx in β-cells. In the presence of 11 mM glucose, pulsatile insulin secretion occurs in synchrony with oscillations i[Ca2+]i. When [Ca2+]i is low and stable, e.g. under basal conditions, low amplitude insulin pulses are still observed. When [Ca2+]i is elevated and non-oscillating, e.g. when the β-cells are depolarized by potassium, high amplitude insulin pulses are observed. The frequency of the insulin pulses under these conditions is similar to that observed when [Ca2+]i oscillations are present. By permanently opening or closing the KATP channels with diazoxide or tolbutamide, respectively, it was investigated if glucose can modulate pulsatile insulin secretion when it does not influence the channel activity. Under these conditions, [Ca2+]i remained stable whereas the amplitude of the insulin pulses increased with sugar stimulation without change in the frequency. Metabolic inhibition blunted but did not prevent the insulin pulses. The results indicate that oscillations in metabolism can generate pulsatile insulin release when [Ca2+]i is stable. However, under physiological conditions, pulsatile secretion is driven by oscillations in metabolism and [Ca2+]i, acting in synergy.

The blood vessels of the pancreatic islets are of crucial importance for oxygen and metabolite supply as well as dispersal of secreted hormones. In addition to this, endothelial cells have an important role in the revascularization process after islet transplantation. Previous studies have reported signs of poor engraftment of transplanted islets, presumably due to impaired revascularization. The aims of this thesis were to investigate the revascularization process of transplanted islets and to examine the role of islet endothelial cells. In this context, the lectin Bandeiraea simplicifolia was found to stain endothelium of both endogenous and transplanted pancreatic islets. By using this lectin we investigated the vascular density of both endogenous and islets transplanted syngeneically beneath the renal capsule, into the spleen or intraportally into the liver of normoglycemic C57BL/6 mice. One month post-transplantation, a time point when the grafts are assumed to be completely revascularized, the vascular density was decreased at all three implantation sites when compared to endogenous islets. Furthermore, most of the blood vessels were located in the graft connective tissue stroma. Similar results were obtained when islet transplant vascular density was determined six months post-transplantation and in cured diabetic animals after one month. In order to evaluate the function of intraportally transplanted islets, we developed a method to retrieve such islets. We treated the implantation organ (liver) first enzymatically (collagenase) and then mechanically, thereafter we could re-isolate the transplanted islets for further in vitro studies. The retrieved islets had a decreased insulin relase, insulin content and glucose oxidation rate when compared to non-transplanted control islets. To understand the role of islet endothelium in the revascularization of transplanted islets we performed angiogenesis GEArray studies on islet endothelial cells, from non-cultured, cultured and transplanted islets. We found that the islet endothelium expressed mRNA for both inhibitors and inducers of angiogenesis, and that this expression differed with time. The functional consequences of this remain to be determined. In summary, the results presented above provide a useful platform for future studies of the morphology and function of islet endothelial cells, especially with a view for elucidating changes induced by islet transplantation.

In the field of transplantation, the increasing deficit of human donors have lead to an interest in animals as an alternative source of organs and tissues. Different in vitro systems and rodent models of xenotransplantation were used to examine the most significant barriers that have to be overcome, before isolated islets of Langerhans from pigs can be used as a cure for insulin-dependent diabetes mellitus in humans. In clinical transplantation, islets are infused into the liver through the portal vein. During this procedure the islets are susceptible to harmful innate reactions triggered in blood. Adenoviral vectors generating transgenic expression of human complement regulatory proteins were evaluated in pig islets and shown to confer protection against acute complement-mediated damage. Transplanted islets escaping this immediate destruction will be targets of a cellular immune response. Using a new mouse model of islet xenograft rejection, it was demonstrated that macrophages, effector cells in the rejection, were part of an MHC-restricted xenospecific immune response mediated by T cells. In a strain of knockout mice it was further shown that this process can proceed in the absence of an important signalling system, mediated by Toll-like receptors, between cells in innate and adaptive immunity. These findings illustrate some of the mechanistic differences compared to cellular islet allograft rejection which partly explain why immunosuppressive drugs used in clinical allotransplantation is not sufficient for preventing xenograft rejection. Porcine endogenous retroviruses (PERV) remain a safety concern in xenotransplantation. Characterization of PERV in pig islets indicated that virus expression is low in vitro but increases during the immediate time period following transplantation. This suggests that antiviral therapies administered at the time of transplantation could be used for preventing the risk of PERV transmission after xenotransplantation.

Pancreatic ß-cells are heterogeneous in their secretory responsiveness, glucose sensitivity and metabolic rate. A diminished and delayed first-phase insulin release is an early sign of failing ß-cells in diabetes. Mechanisms controlling functional characteristics, such as lag time for insulin release or magnitude of the response in each individual cell are unknown. To find out whether the heterogeneity represents a random phenomenon in ß-cell or is a manifestation of reproducible characteristics, we compared parameters of Ca2+ response in Fura-2 labelled ob/ob mouse ß-cells during two consecutive stimulations with glucose. Lag times, as well as peak heights and nadirs of initial lowering showed a strong correlation between the first and second stimulation. Thus, timing and magnitude of the early Ca2+ response were specific for each cell. ß-Cells from lean mice, diabetic db/db mice and rats also showed cell-specific responses characteristics. This indicates that a cell-specific Ca2+ response to glucose is common in rodent ß-cells, both normal and diabetic. Another question was whether aggregated ß-cells show cell-specific responses. Using the same protocol as for dispersed ß-cells, we analysed Ca2+ responses in clusters of different size and in intact islets from ob/ob and lean mice. Correlations were found between the first and second stimulation for timing and magnitude of [Ca2+]i rise, and for the initial lowering. Next, we tested if the ß-cell response is cell-specific, when induced at different steps of the stimulus-secretion coupling. The glycolytic intermediate glyceraldehyde, the mitochondrial substrate KIC, the KATP-channel blocker tolbutamide and arginine were used as tools. [Ca2+]i changes were studied in dispersed ß-cells from lean, ob/ob and db/db mice. NADH responses to glucose and KIC were analyzed as a measure of metabolic flux. The correlation between Ca2+ and insulin response from individual ß-cells was tested using Fluo-3 and Fluozin-3. Both timing and magnitude of calcium responses were cell-specific in lean mouse ß-cells with all tested secretagogues. ß-Cells from ob/ob and db/db mice showed cell-specific timing of Ca2+ responses to glyceraldehyde but not to KIC, tolbutamide or arginine. However, ob/ob mouse ß-cells within intact islets showed cell-specific timing of tolbutamide-induced response. NADH responses to glucose were cell-specific in all three mouse models, but the timing of NADH responses to KIC was cell-specific only in lean mice. Thus, a cell-specific response can be induced in normal ß-cells at several steps of stimulus-secretion coupling for nutrient-stimulated insulin release. Cell-specific properties of ß-cell ion channels and the mitochondrial metabolism are affected in db/db and ob/ob mice. The relation between mitochondrial mass and parameters of Ca2+ responses were investigated in Mitotracker Red and Fluo-3 labelled ß-cells using confocal microscopy. Data show that ß-cell mitochondrial state may play an important role in determining the timing of [Ca2+]i changes. In summary, the early Ca2+ response pattern in ß-cells, including the lag time, the nadir of initial lowering and the height of the first peak response is cell-specific. Isolated and functionally coupled ß-cells show cell-specific timing of Ca2+ responses when stimulated with metabolic and non-metabolic agents. This may be a robust mechanism of importance for the adequate function of ß-cells and a basis for the pacemaker function of some cells. A disturbed cell specificity of the mitochondrial metabolism and ion channel function appears to be a marker of ß-cell dysfunction in hyperglycemia and diabetes and may explain the delayed insulin release in ß-cells from diabetic subjects.

Thesis (Ph. D.)--University of Missouri--Columbia, 2003. / Typescript. Vita. Includes bibliographical references (leaves 146-172). Also available on the Internet.

Thesis (Ph.D. in Epidemiology, Dept. of Preventive Medicine and Biometrics) -- University of Colorado Denver, 2008. / Typescript. Includes bibliographical references (leaves 81-92). Free to UCD Anschutz Medical Campus. Online version available via ProQuest Digital Dissertations;

NOD mice model human type 1 diabetes and have been used to investigate tolerance induction protocols for islet transplantation in a setting of autoimmunity. Costimulation blockade-based tolerance protocols that induce prolonged skin and permanent islet allograft survival in non-autoimmune mice have failed in NOD mice. To investigate the underlying mechanisms, we generated NOD hematopoietic chimeras. We were able to show that dendritic cell maturation defects seen in NOD mice are partially corrected in mixed hematopoietic chimeras. Furthermore, skin allograft survival was dependent upon the phenotype of the bone marrow donor, demonstrating that in the NOD the resistance to tolerance induction resides in the hematopoietic compartment. In addition, we studied congenic NOD mice bearing insulin dependent diabetes (Idd) loci that reduce diabetes incidence. The incidence of diabetes is reduced in NOD.B6 Idd3 mice, and virtually absent in NOD.B6 Idd3Idd5 mice. Islet allograft survival in NOD.B6 Idd3 mice is prolonged as compared to NOD mice, and in NOD.B6 Idd3Idd5 mice islet allograft survival is similar to that achieved in C57BL/6 mice. Alloreactive CD8 T cell depletion in NOD mice treated with costimulation blockade is impaired, but is partially restored in NOD.B6 Idd3 mice, and completely restored in NOD.B6 Idd3Idd5 mice. Idd3 results from variations in Il2 gene transcription. We hypothesized insufficient levels of IL-2 in NOD mice contributes to impaired deletion of alloreactive CD8 T cells and shortened islet allograft survival. We observed using synchimeric mice that co-administration of exogenous IL-2 to NOD mice treated with costimulation blockade led to deletion of alloreactive CD8 T cells comparable to that in C57BL/6 mice and prolonged islet allograft survival. However, some Idd loci impaired the induction of transplantation tolerance. These data suggest that Idd loci can facilitate or impair induction of transplantation tolerance by costimulation blockade, and that Idd3 (IL-2) is critical component in this process.