Indiana University-Purdue University Indianapolis (IUPUI) / Glucose homeostasis is maintained through the coordinated actions of insulin secretion from pancreatic beta cells and insulin action in peripheral tissues. Dysfunction of insulin action yields insulin resistance, and when coupled with altered insulin secretion, results in type 2 diabetes (T2D). Exocytosis of intracellular vesicles, such as insulin granules and glucose transporter (GLUT4) vesicles is carried out by similar SNARE (soluble NSF attachment receptor) protein isoforms and Munc18 proteins. An additional regulatory protein, Doc2b, was implicated in the regulation of these particular exocytosis events in clonal cell lines, but relevance of Doc2b in the maintenance of whole body glucose homeostasis in vivo remained unknown. The objective of my doctoral work was to delineate the mechanisms underlying regulation of insulin secretion and glucose uptake by Doc2b in effort to identify new therapeutic targets within these processes for the prevention and/or treatment of T2D. Towards this, mice deficient in Doc2b (Doc2b-/- knockout mice) were assessed for in vivo alterations in glucose homeostasis. Doc2b knockout mice were highly susceptible to preclinical T2D, exhibiting significant whole-body glucose intolerance related to insulin secretion insufficiency as well as peripheral insulin resistance. These phenotypic defects were accounted for by defects in assembly of SNARE complexes. Having determined that Doc2b was required in the control over whole body glycemia in vivo, whether Doc2b is also limiting for these mechanisms in vivo was examined. To study this, novel Doc2b transgenic (Tg) mice were engineered to express ~3 fold more Doc2b exclusively in pancreas, skeletal muscle and fat tissues. Compared to normal littermate mice, Doc2b Tg mice had improved glucose tolerance, related to concurrent enhancements in insulin mumsecretion from beta cells and insulin-stimulated glucose uptake in the skeletal muscle. At the molecular level, Doc2b overexpression promoted SNARE complex assembly, increasing exocytotic capacities in both cellular processes. These results unveiled the concept that intentional elevation of Doc2b could provide a means of mitigating two primary aberrations underlying T2D development.

Indiana University-Purdue University Indianapolis (IUPUI) / FGF23 is the central regulator for phosphate homeostasis. Both FGF23 and phosphate dysregulation are highly related with the progression of chronic kidney disease (CKD), which is a global health problem. In previous studies, FGF23 was found to be produced in bone and targeting the kidneys to regulate phosphate reabsorption and excretion. In the FGF23 signaling axis, it binds a receptor complex (αKlotho and FGFRs) in the distal convoluted tubules (DCT) and causes its biological effects in the proximal tubules (PT). The mechanism of how the signals passing on from DCT to PT is not clear. In my research, experiments were focused on the FGF23 signaling pathway within the kidney to study the communication steps between tubular cells. HBEGF treatment was given to FGF23 signaling impaired mouse models resulting in significant change of genes regulated by FGF23, indicating that HBEGF was important in the FGF23 signaling axis. Then high quality rabbit anti-mouse HBEGF antibodies were made to better study HBEGF activity in vivo and in vitro. A new cell model was characterized to test FGF23 effects on HBEGF signaling using Western blots and immunofluorescence. Lastly, the location of HBEGF activity was examined in the kidney in vivo. Immunostaining suggested that HBEGF activated the mitogen activated protein kinase (MAPK) pathway. This mapping may provide important information for the molecular relationships between FGF23 and HBEGF.

Indiana University-Purdue University Indianapolis (IUPUI) / Solid organ transplantation is limited by available donor allografts. Pig to human transplantation, xenotransplantation, could potentially solve this problem if physiologic and immunologic incompatibilities are overcome. Genetic modifications of pigs have proven valuable in the study of xenotransplantation by improving pig to human compatibility. More genetic targets must be identified for clinical success. First, this study examines platelet homeostasis incompatibilities leading to acute thrombocytopenia in liver xenotransplantation. Mechanisms for xenogeneic thrombocytopenia were evaluated using liver macrophages, Kupffer cells, leading to identification of CD18, beta-2 integrin, as a potential target for modification. When disruption of CD18 was accomplished, human platelet binding and clearance by pig Kupffer cells was inhibited. Further, human and pig platelet surface carbohydrates were examined demonstrating significant differences in carbohydrates known to be involved with platelet homeostasis. Carbohydrate recognition domains of receptors responsible for platelet clearance Macrophage antigen complex-1 (CD11b/CD18) and Asialoglycoprotein receptor 1 in pigs were found to be different from those in humans, further supporting the involvement of platelet surface carbohydrate differences in xenogeneic thrombocytopenia. Second, immunologic incompatibilities due to antibody recognition of antigens resulting in antibody-mediated rejection were studied. Identification of relevant targets was systematically approached through evaluation of a known xenoantigenic protein fibronectin from genetically modified pigs. N-Glycolylneuraminic acid, a sialic acid not found in humans, was expressed on pig fibronectin and was identified as an antigenic epitope recognized by human IgG. These studies have provided further insight into xenogeneic thrombocytopenia and antibody-mediated rejection, and have identified potential targets to improve pig to human transplant compatibility.