Tim Kieffer

Professor

Relevant Degree Programs

 

Great Supervisor Week Mentions

Each year graduate students are encouraged to give kudos to their supervisors through social media and our website as part of #GreatSupervisorWeek. Below are students who mentioned this supervisor since the initiative was started in 2017.

 

Tim Kieffer is exactly the #GreatSupervisor you hope to have. Dr. Kieffer never fails to challenge you to do better and never ceases to support you through adversity. Thank you for making the Kieffer Lab a place of creativity, productivity, and positivity. It is a privilege to recognize Tim as my mentor and I will proudly do so for my entire career ahead.

Adam Ramzy (2019)

 

Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - Mar 2019)
The effects of leptin knockout and insulin suppression on glucose metabolism in rodents (2017)

Background: The adipose-derived hormone leptin is well known for its effects on metabolic homeostasis, and in particular its role in suppressing appetite and regulating fat mass. Loss of leptin results in a phenotype characterized by obesity, insulin resistance, hyperinsulinemia and glucose intolerance. Treating leptin deficient mice with leptin is able to lower blood glucose and insulin levels without reducing body weight, indicating that leptin has independent effects on glucose homeostasis. While there have been many studies exploring the role of leptin in metabolism, the glucoregulatory actions of leptin remain to be fully elucidated. Scope of Thesis: The overall goal of this thesis was to understand the role of leptin in glucose regulation. To do so, we utilized a number of in vivo models in which leptin signalling was disrupted. First, we investigated the effects of reducing insulin gene dosage on glucose homeostasis and body mass in leptin deficient ob/ob mice. Next we explored the significance of leptin signalling in pancreatic β cells. To determine whether the glucoregulatory actions of leptin are conserved across rodent species, we explored the effects of leptin deficiency in rats, and used this model to investigate the hierarchy of metabolic defects that arise as a result of impaired leptin action. Finally, we describe a novel mouse model to selectively knock out leptin in a temporal and tissue-specific manner. Conclusions: We show that reduction of circulating insulin levels prevents obesity, but results in hyperglycemia in ob/ob mice. Furthermore, we show that direct leptin signalling in β cells is insufficient to restore euglycemia. In addition, we demonstrate that leptin deficiency produces a similar metabolic phenotype in rats as in mice. Finally, we suggest that loss of insulin sensitivity may be a primary metabolic defect that results from leptin deficiency in rats. Together, these results provide further insight into the glucoregulatory mechanisms of leptin that may be used to develop therapeutic strategies to treat obesity and diabetes.

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The metabolic effects of leptin therapy and glucagon suppression therapy in mouse models of diabetes (2017)

It has long been thought that the only hormone capable of reversing the catabolic consequences of diabetes is insulin. However, various studies have demonstrated that the adipocyte-derived hormone leptin can potently lower blood glucose levels in rodent models of insulin-deficient diabetes. In addition, the hormone glucagon is elevated in type 1 and type 2 diabetes, and glucagon suppression therapy has shown promise as an agent to treat diabetes in mice. Given the interest in both of these therapies, leptin treatment and glucagon antagonism were propelled into clinical trials for patients with type 1 and type 2 diabetes respectively. The overarching goal of this thesis was to perform preclinical studies to investigate the mechanism of the glucose lowering actions of leptin and the effects of glucagon suppression therapy in mouse models of diabetes. To achieve this, we probed the role of increased leptin action as a result of insulin therapy, determined the function of insulin-like growth factor binding protein-2 (IGFBP2) in the glucose lowering actions of leptin, investigated the necessity of insulin for leptin treatment and glucagon suppression therapy, and explored the potential of glucagon suppression therapy via glucagon receptor (Gcgr) small interfering ribonucleic acid (siRNA) delivered by lipid nanoparticle (LNP) technology. This thesis reveals that elevated leptin levels may contribute to the glucose lowering effect of insulin therapy in insulin-deficient diabetes. In addition, we demonstrate that physiological levels of IGFBP2 are neither sufficient nor required for the action of leptin on glucose homeostasis. Moreover, leptin can normalize many metabolic parameters in the complete absence of insulin, but blood glucose levels are volatile and the length of survival is finite. Furthermore, Gcgr siRNA can improve many diabetic symptoms in mouse models of type 1 and type 2 diabetes. Finally, we report that the metabolic manifestations associated with a complete lack of insulin cannot be overcome by Gcgr gene inactivation. Collectively the findings in this thesis contribute insight into the mechanism of action, and the therapeutic potential of leptin administration and glucagon suppression therapy as a treatment for diabetes.

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A transcription factor guided approach to the generation of pancreatic endocrine-like cells from stem cells (2014)

Diabetes mellitus results in elevated blood glucose levels due to an insufficiency of the glucose lowering hormone insulin. In type 1 diabetes, insulin loss is due to an autoimmune destruction of the insulin producing pancreatic β-cells. One treatment for type 1 diabetes is the transplantation of cadaveric islets, although this process is limited by a lack of donor tissue. We and others are examining factors that influence the generation of new β-cells from stem cells. Specifically, the aims of this thesis were to examine the role of transcription factors in the formation of endocrine cells from stem cells. To do this, we developed a high content screening approach in human amniotic fluid stem cells to assess the effect of six pancreatic transcription factors on insulin expression. From this screen and subsequent studies, we observed that while transcription factor overexpression was capable of driving insulin expression, the resulting cells were unable to reverse diabetic hyperglycemia upon transplantation. Given the well-established developmental capacity of human embryonic stem cells (hESCs), we next characterized a novel hESC line (CA1S), which is amenable to high throughput screening and pancreatic differentiation. Using these cells, we found that the number of cells seeded into a culture system had a significant effect on the formation of endodermal, pancreatic progenitor and pancreatic endocrine cells. This effect correlated with hESC cell cycle status and resulted in the formation cells co-expressing insulin, glucagon and somatostatin. We next examined the effects of the transcription factors PAX4 and ARX on pancreatic endocrine specification. We revealed that increased PAX4 expression reduced ARX and glucagon expression leaving insulin positive cells. Reduced ARX expression by genomic editing resulted in fewer glucagon positive cells and increased PAX4 levels. Loss of ARX was also associated with an abundance of somatostatin positive cells and a partial reduction in insulin, which was rescued with re-expression of ARX adenoviral gene delivery methods. Collectively, the data presented in this thesis emphasise the role of transcription factor expression as a primary control point for the possible generation of β-cells from stem cells, which represents a potential cellular therapy for type 1 diabetes.

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The metabolic effects of leptin therapy in a mouse model of type 1 diabetes. (2014)

Type 1 diabetes is a life-long disease, with devastating consequences and no cure. While the hormone insulin has been the only effective treatment to date for type 1 diabetes, emerging evidence has revealed that the fat-derived hormone leptin can also exert profound glucose lowering effects, and reduce mortality in type 1 diabetes. This has generated interest in the therapeutic potential of leptin as an anti-diabetic treatment, and propelled leptin into clinical trials for type 1 diabetes. The fact that leptin monotherapy (without insulin administration) can so potently lower blood glucose in insulin-deficient diabetes is surprising, given that for almost a century, insulin has been assumed to be the only hormone that can lower blood glucose in type 1 diabetes. The overarching goal of this thesis was to perform pre-clinical studies to elucidate the mechanism of the anti-diabetic effects of leptin in type 1 diabetes. To this end, we thoroughly assessed the changes in metabolic and energy homeostasis that occur in a mouse model of type 1 diabetes receiving leptin therapy. The roles of hepatic and neuronal leptin receptor signalling in the anti-diabetic action of leptin were also investigated, through tissue specific disruption of leptin signalling using the Cre-lox method. In addition, we assessed whether leptin therapy can serve as an adjuvant to islet transplantation therapy in type 1 diabetes. This thesis revealed that leptin therapy lowers blood glucose in a mouse model of type 1 diabetes, correlating with decreased hepatic glucose production and enhanced insulin sensitivity. The anti-diabetic action of leptin is not blunted in mice with disrupted neuronal leptin signalling outside of the arcuate and ventromedial hypothalamus, or in mice with disrupted hepatic leptin signalling, suggesting that leptin acts through alternate or redundant pathways to lower blood glucose. Finally, low-dose leptin administration dramatically enhanced the ability of islet transplants to restore euglycemia, suggesting that leptin and islet co-therapy could be a successful therapeutic strategy for type 1 diabetes. Collectively the findings in this thesis contribute insight into the mechanism of action, and the therapeutic potential of leptin as a treatment for type 1 diabetes.

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Regulation of glucose and lipid metabolism by hepatic leptin signalling (2012)

Incidences of obesity and type 2 diabetes have risen worldwide at alarming rates. While there is an undeniable correlation between obesity and type 2 diabetes, a clear mechanistic link between these two conditions has not been fully elucidated. The adipocyte-derived hormone leptin may play a role linking obesity and type 2 diabetes. Leptin can regulate body weight through its effects on the brain to decrease food intake and increase energy expenditure, but these effects are disrupted in obesity. Interestingly, leptin also has effects on glucose and lipid metabolism independent of its effects on body weight, so it is possible that disrupted leptin signalling during obesity can also perturb glucose and lipid metabolism, leading to symptoms associated with type 2 diabetes. Since the liver plays a critical role in integrating and controlling glucose and lipid metabolism, it was hypothesized that leptin resistance in the liver could play a role in the development of diabetic symptoms. To investigate this hypothesis, three complementary mouse models were used to help identify the role of leptin signalling specifically in the liver. It was found that in lean mice, hepatic leptin resistance results in increased insulin sensitivity in the liver, leading to reduced hepatic glucose output but also increased lipid accumulation and secretion of larger, more triglyceride-rich very low density lipoprotein (VLDL) particles without an increase in total plasma triglycerides. In obese, hyperinsulinemic mice lacking hepatic leptin signalling, the effects of lost leptin signalling on triglyceride metabolism were exacerbated, resulting in decreased triglyceride clearance and elevated plasma triglycerides compared to controls. These effects on plasma triglycerides were reversed when hepatic leptin signalling was restored in a mouse model of total leptin resistance. Collectively, these data reveal a possible role for hepatic leptin resistance in the development of diabetic symptoms during obesity.

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Involvement of adenosine signalling in the release of gastric and pancreatic peptides (2011)

Adenosine is precursor and a metabolic intermediate of adenosine triphosphate in energy transfer, and cyclic adenosine monophosphate in signal transduction. Recent studies have demonstrated that the role of adenosine in the body is much more than just structural as it can also behave as an important regulator of homeostatic functions. Adenosine signalling relies on the activation of the A₁, A₂A, A₂B and A₃ adenosine receptors. Through the development of pharmacological tools and genetic knockout mouse models of specific receptor subtypes, the involvement of these receptors in various physiological systems is quickly being established. This thesis investigates the function of adenosine in the digestive system and specifically how adenosine regulates the release of gastric and pancreatic peptides. With the use of a novel vascularly perfused isolated mouse stomach model and specific A₁ and A₂A receptor knockout animals, the role of adenosine on the release of somatostatin and ghrelin was determined. Lower concentrations of adenosine can inhibit the release of somatostatin and ghrelin via the activation of A1 receptors, while higher concentrations can stimulate their release via activation of A₂A receptors. Given the importance of somatostatin in regulating gastric acid secretion and motility, and ghrelin in regulating systemic energy balance, better understanding of how the release of these two peptides is regulated may reveal potential therapies for eating disorders, gastrointestinal dysfunctions and metabolic diseases.In the pancreas, adenosine was shown to regulate both insulin and glucagon secretion from the pancreatic islets. Studies presented in this thesis demonstrate that adenosine signalling interacts with the effects of the incretin hormone GLP-1 in the pancreas such that concomitant administration of adenosine and GLP-1 in the perfused pancreas induced greater insulin release than GLP-1 administration alone. Furthermore, A₁ receptor knockout mice exhibited more frequent pulses of insulin secretion, which may have contributed to their superior glucose tolerance compared to wild type control mice. These findings on the role of adenosine signalling in the pancreas may have implications in the etiology of diabetes mellitus. The involvement of adenosine signalling in the digestive tract further illustrates the importance of adenosine as a metabolic regulator in homeostasis.

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Master's Student Supervision (2010-2017)
The role of leptin in the regulation of glucose homeostasis (2010)

The fat derived hormone leptin plays a crucial role in the normal maintenance of body weight and energy expenditure, as well as glucose homeostasis. Low doses of exogenous leptin administered to leptin deficient ob/ob mice are able to reverse the hyperinsulinemia and hyperglycemia without altering body composition. As leptin has the ability to directly suppress insulin secretion from ß-cells, we hypothesise that in the absence of leptin signalling, unregulated insulin secretion leads to hyperinsulinemia which in turn leads to increased adipogenesis and insulin resistance, ultimately culminating in the development of type 2 diabetes in this mouse model. To test this hypothesis we induced an acute state of leptin deficiency with a PEGylated mouse leptin antagonist (PEG-MLA) to determine the hierarchy of leptin action. Metabolic analysis by indirect calorimetry showed that PEG-MLA treatment resulted in increased food intake and respiratory quotient without altering body composition or energy expenditure. These changes in energy balance were accompanied with increased fasting, and glucose stimulated insulin levels. PEG-MLA treated mice also displayed decreased whole-body insulin sensitivity, elevated endogenous hepatic glucose production (HPG), and impaired insulin mediated suppressed of HPG as determined by euglycemic-hyperinsulinemic clamps. Overall, these findings demonstrate that leptin signalling is important in regulating insulin secretion, and that changes in insulin sensitivity occur prior to changes in body composition and energy expenditure in a state of acute leptin deficiency. It has been recently shown that the liver derived, leptin regulated insulin-like growth factor binding protein-2 (IGFBP-2) is responsible for the anti-diabetic effect of leptin in ob/ob mice. We investigated the mechanism by which leptin regulates IGFBP-2 levels. ob/ob mice with attenuated hepatic leptin signalling or a subdiaphragmatic vagotomy were utilized to determine if leptin acts directly on the liver or centrally to increase plasma IGFBP-2. Our results show that while leptin is able to increase plasma IGFBP-2 levels in ob/ob mice in a dose dependent manner, the mechanism does not involve heptic leptin signalling or vegal efferents and remains to be elucidated.

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