Ian Sweet, PhD

Background:

Dr. Sweet received his PhD from the University of Washington in Bioengineering in 1993. His post-doctoral training in Biochemistry, undertaken in the lab of Dr. Franz Matschinsky, at the University of Pennsylvania, was completed in 1996.  Dr. Sweet’s laboratory focuses on metabolic basis for disease with particular emphasis on diabetes, cancer and cardiovascular disease.  The research has utilized sophisticated methods that have been engineered to non-invasively assess energetics and function in a diverse variety of cells and tissue important to these diseases. The wide range and applicability of the experimental approaches and research have resulted in funding by multiple sources including NIH (National Institute of Diabetes, Digestive and Kidney, National Eye Institute, and National Cancer Institute), National Science Foundation, Juvenile Diabetes Research Foundation, Merck Inc, and the Seattle Foundation.  As Director of the Islet Cell and Functional Analysis Core of the Diabetes and Endocrinology Research Center, the methods developed by Dr. Sweet's group are available to other researchers as services carried out by two full time technicians in the laboratory. 
 
For more information on the DERC Islet Cell Core, visit their website at http://depts.washington.edu/diabetes/biomedcore/ifac.index.html.

Focus:

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.
 
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.
 
T cell calcium metabolism in Type 1 Diabetes
Calcium is the major intracellular signal mediating the activation of T cells.  It was previously established that T cells from patients with a variant of the PTPN22 gene (and resultant predisposition toward Type 1 diabetes) have a decreased calcium response to activation by the T cell receptor (TCR). This is thought to contribute to the progression of the disease state due to low levels of protective cytokines that normally keep the autoreactive lymphocytes in check.  The goal of the present studies is to elucidate the mechanism mediating the decreased calcium response. The original observations regarding calcium were made using FACS analysis, which only yields the average responses of a large population of T cells, so does not resolve the complex and oscillatory nature of the calcium dynamics, nor the heterogeneity of cellular responses.  In order to further characterize the calcium defect, we will take advantage of high-resolution calcium imaging of single cells. 
 
Therefore the kinetics of calcium dynamics of single T cells will be assessed to address the following questions:
1. Is the decrease in calcium response in T cells with a PTPN22 variant characterized by a decreased frequency of oscillations or lowered peaks?
2. Is the decrease in calcium response in T cells with a PTPN22 variant evenly distributed in all cells, or are some cells unaffected?
 
These questions are being determined for both memory and naïve cells, since it appears that the effect is bigger in memory cells.
 

Representative Publications:

Sweet IR, Cook DL, Lernmark A, Greenbaum CJ, Wallen AR, Marcum ES, Stekhova SA, Krohn KA.  Systematic screening of potential beta-cell imaging agents.  Biochem Biophys Res Commun. 314: 976-83, 2004.
 
Sweet IR, Gilbert M, Scott S, Todorov I, Jensen R, Nair I, Al-Abdullah I, Rawson J, Mullen Y, Kandeel F, Ferreri K.  Glucose-stimulated increment in oxygen consumption rate as a standardized test of human islet quality.  Amer. J. Trans. 8:183-192, 2008.
 
Gilbert M, Jung S-R, Sweet IR. Increased potency of calcium derived from L-type calcium channels compared to calcium from the endoplasmic reticulum on oxygen consumption and insulin secretion. J. Biol. Chem. 283:24334-42, 2008.
 
Sweet IR, Gilbert M, Maloney E, Hockenbery DM, Schwartz MW, Kim F. Endothelial inflammation induced by excess glucose is linked to intracellular accumulation of glucose-6-phosphate. Diabetalogia 52:921-3, 2009.
 
Jung S-R, Reed BJ, Sweet IR. A highly energetic process couples calcium influx through L-type calcium channels to insulin secretion in pancreatic beta cells. Am J Physiol. (Epub) 2009.
 
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Current Collaborations:

Within the Diabetes and Obesity Center of Excellence, The Diabetes and Endocrinology Research Center and its Affiliated Members

Karin Bornfeldt, PhD
Steven Chessler, MD, PhD (University of California at San Diego)
David Cummings, MD
Peter Havel, DVM, PhD  (University of California at Davis)
David Hockenbery, MD (Fred Hutchinson Cancer Research Center)
James Hurley, PhD
Francis Kim, MD
Martin Kushmerick, MD, PhD
Jerry Nepom, MD, PhD (Benaroya Research Institute)
William Osborne, PhD
Leslie Satin, PhD (University of Michigan)
Andrew Scharenberg, MD (Children’s Research Center)

Lab Members:

Seung-Ryoung Jung, PhD
Benjamin Reed, BS
Iok Teng “Denise” Kuok, BS
Drew Couron, BS
Kelli Geiger

Thesis Committees
Ken Lindsay  (Department of Biochemistry)