Ian Sweet PhD
Dr. Sweet received his PhD from the University of Washington in Bioengineering in 1993. He received post-doctoral training in Biochemistry at the University of Pennsylvania in the lab of Dr. Franz Matschinsky until 1996.
Dr. Sweet’s research focuses on regulation and impairment of insulin secretion in Type 2 Diabetes; pancreatic beta cell death; metabolic basis of inflammation and T cell calcium metabolism in Type 1 Diabetes.
Regulation and impairment of insulin secretion in Type 2 diabetes
It is well established that inadequate insulin secretion by the pancreatic beta cell is a major cause for Type 2 Diabetes. Accordingly we focus on elucidating the factors regulating glucose stimulation of insulin secretion, and identifying those that mediate the loss of islet function in the progression of the disease. Specifically, calcium is the major intracellular signal whose transport across the plasma membrane is essential for insulin secretion to occur. We are trying to identify the critical process stimulated by calcium that enables exocytosis of secretory granules to occur, and facilitates the further control of glucose-stimulated insulin release by potentiators such as acetylcholine, GLP-1 and other incretins, and fatty and amino acids. Clues to its identity include a very high usage of energy that correlates with rates of insulin secretion, and dual control by both calcium and a metabolic factor downstream of electron transport. Importantly, we have shown that its activity, assessed by the rate of calcium-sensitive oxygen consumption, is highly correlated with control of blood sugar in vivo in a rat model of Type 2 diabetes. Therefore, this process, which we have termed the Ca2+/metabolic coupling process (CMCP), is operational in vivo, and its impairment may contribute to the progression of hyperglycemia in Type 2 diabetes. Identification of the proteins involved in the CMCP will lead to a greater understanding of the etiology of Type 2 Diabetes, and may be candidates for treatments aimed at increasing insulin secretion in patients with this disease. Recently we have obtained evidence supporting the dual control of the CMCP by calcium and cytochrome c and ultimately insulin secretionThis exciting line of research could have major implicatinos for the understanding and treatment of diabetes.
Pancreatic beta cell death
High rates of pancreatic beta cell death are a central problem in all aspects of diabetes including Type 1, Type 2 and in the performance of islet transplantation. The development of therapeutic strategies to prevent or slow the rate of cell death would be greatly facilitated by a fundamental characterization of beta cell population dynamics in vivo, and a precise understanding of the intracellular mechanisms mediating apoptosis, a primary cause leading to loss of functional beta cell mass. Major obstacles in the pursuit of these goals are a lack of methods to quantify and assess the progression of factors that mediate cell death. Thus, our laboratory has endeavored to develop both in vivo and in vitro methods to quantify the amount of functional beta cells. In developing an method to non-invasively assess beta cell mass In vivo, the method that appears to be the most promising is based on the use of Positron Emission Tomography, which boasts an exquisitely high sensitivity. Unfortunately, its spatial resolution is poor, and is unable to resolve an individual pancreatic islet, without the signal being contaminated by greater than 98% contribution from non-beta cells. Therefore we must rely on chemical specificity to obtain the necessary signal to noise for accurate quantification. Because the identification of a ligand-receptor pair with such high cellular specificity is unprecedented, a systematic approach to the search is warranted. We have thus developed in vitro screening criteria with which to evaluate candidate beta cell imaging agents that has correctly predicted in vivo response. We continue to evaluate molecules in vitro and in vivo to define the optimal properties of a successful beta cell mass marker.
In vitro assessment of beta cell death is also proven difficult. There has been particular need for such a method in the assessment of islet quality in the transplantation of human islets. There is a wide range of viability and yield from donor organ to organ and being able to rule out islet quality as a factor in graft failure would have great utility. To do this, we have focused on measures of electron transport, as an optimal “vital sign” of isolated islets. This is based on the fact that electron transport is the site of generation of reactive oxygen species, energy for the cell to meet cell requirements, and critical pro- and anti-apoptotic signals. We use glucose-stimulated oxygen consumption and cytochrome c reduction as parameters that 1. are specific for beta cells and 2. more importantly these parameters correlate with the ability of transplanted islets to lower glucose in a diabetic model of a mouse. Thus, it seems feasible that this method could become a Gold-standard in the assessment of islet quality.
Metabolic basis of inflammation
Chronically high levels of blood glucose and free fatty acids as seen in type 2 diabetes and obesity are associated with increased risk of cardiovascular disease. In endothelial cells, excess free fatty acids activate the pro-inflammatory IKK beta-NF-kB pathway via a mechanism that involves Toll-like receptor 4 signaling, and this effect in turn causes cellular insulin resistance and impaired nitric oxide production. Exposure of endothelial cells to excess glucose also induces inflammation and insulin resistance, but the underlying mechanisms remain to be established. Most current hypotheses are based on increased glucose utilization by various metabolic pathways leading to accumulation of pro-inflammatory intermediates or byproducts (such as reactive oxygen species). Candidate metabolic pathways include glycolysis, the pentose shunt, hexosamine biosynthesis, the tricarboxylic acid cycle, and the coupled mitochondrial processes of electron transport and oxidative phosphorylation. Work is currently being undertaken to determine how metabolic fluxes in primary endothelial cells respond to glucose concentrations above the physiologic range and relate metabolic flux to glucose-induced IKK beta activity.
Differentiation of Beta Cells From Stem Cells
Producing large numbers of functional beta cells would allow transplantation of the cells into diabetic patients. Our role in this endeavor is to develop methods to assess and improve protocols being used to induce stem cells to differentiate into insulin-producing cells. We have developed a method to measure oxygen consumption by cells during periods of hours and days, a time period when growth of cells makes a bigger contribution to changes in total metabolic rate than alterations in bioenergetics. Continuous profiles of oxygen consumption are an integration of the metabolic rate per cell times the number of cells. In order to separate these two effects, we are working on an approach to simultaneously measuring oxygen consumption and cell growth. Surprisingly, despite the central importance to cell growth and death to many biomedical fields, accurate determination of cell number is difficult, and growth rates are typically estimated with surrogate biomarkers such incorporation of thymidine and Ki-67 staining. Our results to date show a strong correlation between the rates of change in oxygen consumption and growth characteristics. Using a lifetime imaging camera, that operates by measuring the extremely reproducible phosphorescence decay following a short excitatory pulse, large and intermediate changes in oxygen consumption were seen in a fast and slow growing cell line (melanoma and INS-1 cells), whereas no significant change was observed by islets (Fig. 1). This method is unique in its ability to accurately quantify cell growth over such relatively short periods of time. Nonetheless, because in general physiologically significant changes in growth rate over the course of a day, which may be only 10-20%, require very stable detection schemes. It is necessary to keep in mind that even a 20% change in one day results in nearly a 4-fold increase in cell number in the course of a week. To this setup, we have incorporated automated control of the positioning of the imaging window to alternately assess oxygen sensors placed by each of the inflow and outflow ports. This system can be run in two ways. The first is where the plated cells are identically treated, but in one chamber they are exposed to a test agent, and the other chamber serves as the control. In this way, unwanted changes in temperature or media composition effects each chamber equally, and cannot contribute to any differences observed as a response to the test compound. Another setup would involve the perifusion of different cell populations with the same composition. The 2-chamber system will eventually be expanded to 4 or more chambers.