The pathophysiology of type 2 diabetes (T2D) requires islet ?-cell dysfunction in the presence of insulin resistance. Islet amyloid is a pathological feature which occurs in the vast majority of individuals with type 2 diabetes (T2D) and contributes to the ?-cell dysfunction and loss that characterizes the disease. Islet amyloid arises due to aggregation of the ?-cell peptide islet amyloid polypeptide (IAPP), which is deposited in the islet extracellular matrix (ECM), between ? cells and endothelial cells; the latter being the major cell type of the islet capillary network. Despite decades of work in the field of islet amyloid, no studies have examined whether (i) islet amyloid is toxic to islet endothelial cells or (ii) whether islet endothelial cells contribute to the toxic effects of islet amyloid. Our preliminary data suggest that both occur, and that the ECM molecule hyaluronan (HA) produced by the islet endothelial cell is a major mediator of amyloid-induced toxicity. Specifically, we have found that islet amyloid deposition in vivo is detrimental to islet endothelial cells, while treatment of isolated primary islet endothelial cells in vitro with amyloidogenic human IAPP (hIAPP) peptide is cytotoxic and results in HA production. Our preliminary data also show that HA enhances hIAPP fibril formation in vitro, which is in line with our observation that HA deposition is also present in in vivo amyloid deposits in islets from hIAPP transgenic mice and human T2D. We and others have shown that IAPP aggregation is proinflammatory, activating macrophages, and resulting in IL-1? production. However, the mechanism by which recruitment and priming of these macrophages occurs has not been established. Based on the literature, we propose that islet endothelial- derived HA provides this missing link and mediates macrophage chemotaxis, adhesion and priming. Based on these data, we hypothesize that hIAPP aggregation induces HA production from islet endothelial cells, which exacerbates hIAPP fibril formation and cytotoxicity, and acts to recruit and prime macrophages. We will address this hypothesis in the following specific aims:
Specific Aim 1 : To determine the mechanism by which HA exacerbates hIAPP aggregation, and thereby ?-cell toxicity, in vitro. In this aim, we will determine how HA alters hIAPP peptide conformation, increases hIAPP fibril formation and whether hIAPP fibrils formed in the presence of HA show increased ?-cell cytotoxicity.
Specific Aim 2 : To determine whether islet endothelial cell-derived HA is necessary for migration, adhesion and priming of monocytes/macrophages under conditions of islet amyloid formation. We will determine whether HA produced by hIAPP-treated islet endothelial cells results in monocyte chemotaxis, macrophage adhesion, priming of the NLRP3 inflammasome and fragmentation of HA. We will also determine if cytokines produced by macrophages or endothelial cells in response to hIAPP treatment, are necessary for HA production, and which hyaluronan synthase isoforms contribute to this process.
Specific Aim 3 : To determine whether blockade of HA production or fragmentation inhibits islet amyloid deposition, and thereby its detrimental effects on ? cell mass and function in vivo. hIAPP transgenic and non- transgenic mice will be treated with the HA synthesis inhibitor 4-methylumbelliferone or bred to introduce a macrophage-specific deletion of hyaluronidase 1 (to inhibit HA fragmentation). These mice will then be followed for one year. We expect that reducing HA synthesis/fragmentation will reduce islet amyloid deposition and/or its toxic and proinflammatory effects, thus preserving ?-cell function and mass in vivo. These studies will provide novel data regarding the role of HA mediating ?-cell cytotoxicity. These are clinically relevant studies; islet amyloid is a feature of ~90% of subjects with T2D, a disease state that disproportionately affects the veteran population, and HA represents a truly ?druggable? target.!
Type 2 diabetes disproportionately affects individuals that receive care at the VA (20% versus 9% of the general population). Islet amyloid is a morphological feature of type 2 diabetes, which is known to be toxic to pancreatic islet ? cells. Islet amyloid deposition occurs in close proximity to islet capillaries, and the endothelial cells that line them. Our preliminary data show, for the first time, that islet endothelial cells are sensitive to islet amyloid?s toxic effects and that these cells also contribute to islet amyloid cytotoxicity, via the extracellular matrix (ECM) molecule hyaluronan. In this proposal, we will use human islets and animal models to determine whether hyaluronan produced by islet endothelial cells under conditions of islet amyloid deposition exacerbates amyloid deposition and its toxic effects, contributes to proinflammatory cytokine production from macrophages and increases islet amyloid formation in vivo. These studies will provide novel targets for interventions aimed at preventing ?-cell death, which can ultimately be applied to treat patients with type 2 diabetes.! !