Location: Children's Nutrition Research Center
2021 Annual Report
Objectives
Objective 1: Determine if vitamin D receptors in the VMH of the brain are critical for glucose regulation.
Objective 2: In wild type (WT) and transgenic animals lacking functional leptin signaling (leptin knockout (KO), leptin receptor KO and leptin receptor agonist/antagonist treated WT mice) determine whether:
Objective 2A: Leptin is involved in the regulation of gluconeogenesis
Objective 2B: Regulation of gluconeogenesis through the leptin dependent mechanism operates via the leptin receptor
Objective 2C: Leptin agonist and small doses of hypoglycin-A or B reduces the rates of gluconeogenesis
Objective 3: Investigate the role of the mitochondrial deacetylase SIRT3 in regulation of pyruvate carboxylase and the gluconeogenesis pathway.
Subobjective 3A: To investigate SIRT3 regulation of pyruvate carboxylase
Subobjective 3B: To investigate SIRT3 regulation of gluconeogenesis in mouse models
Approach
One of the most significant abnormalities underlying type 2 diabetes is continued production of glucose by the liver (gluconeogenesis). Thus, understanding the mechanisms by which gluconeogenesis is regulated is paramount to effectively treating type 2 diabetes. In this project, researchers are investigating three mechanisms that are likely involved in glucose regulation by the liver. Scientists will determine if vitamin D receptors in a specific area of the brain, the ventromedial hypothalamus (VMH), are important for glucose control. We will create a preponderance of data to support the role of the vitamin D receptor in the VMH irrespective of the limitations of each model. We will determine the role of leptin and the leptin receptor in hepatic gluconeogenesis and investigate the nutritional significance of certain small molecules in reducing glucose production via gluconeogeneic pathway. Researchers will also determine the mechanisms of SIRT3, a mitochondrial protein, to regulate gluconeogenesis. Together, these projects will advance our understanding of how the liver regulates glucose production using neural, hormonal, and intracellular mechanisms, and increase the overall body of knowledge.
Progress Report
For Objective 1, research continued to determine how vitamin D receptors in the brain control blood sugar. Our goal for this year was to create the necessary mouse cohorts to perform the studies and we have succeeded in this goal. We used an existing database to determine if leptin was a useful biomarker for obesity in individuals with type 1 diabetes.
In Objective 2, we continued to study the effect of the hormone leptin and its receptor on glucose production from the liver. We completed tracer experiments which measured how leptin and its receptor changes how the liver produces glucose. Additionally, we collected liver tissues for further data collection using mass spectrometry-based techniques.
In Objective 3, we investigated the interaction between sirtuin 3 and an enzyme which helps produce a type of glucose named pyruvate carboxylase. We also induced sirtuin 3 expression in the mouse liver and measured how this affected the amount of glucose the liver produced.
Accomplishments
1. Creation of a new mouse model to identify vitamin D receptors in the brain. Vitamin D acts in the brain to control blood sugar, but the exact mechanisms are not known. Typically, the first step in understanding how vitamin D behaves would be to determine what types of neurons contain the vitamin D receptor. However, the available antibodies to identify vitamin D receptors do not work in the brain. Researchers in Houston, Texas, created a new mouse model which identifies vitamin D receptors in the mouse brain through a special fluorescent reporter. Researchers successfully validated that only those neurons containing the fluorescent reporter were activated by vitamin D. This new mouse model is important as it allows researchers to identify and manipulate neurons containing the vitamin D receptor. This is an important new tool which will aid researchers investigating the role of vitamin D in the brain on diabetes, Alzheimer's disease, obesity, and other brain-related disorders.
2. Leptin decreases fat synthesis in individuals with lipodystrophy. Lipodystrophy is a medical condition in which the body has abnormal distribution of fat in the body and results in nonalcoholic fatty liver disease, the most common cause of chronic liver disease in the U.S., and is likely to become the leading cause of liver transplantation. It is not known if a leptin analog treatment in individuals with lipodystrophy would reduce fat synthesis in the liver. Scientists in Houston, Texas, demonstrated that 6 months of the leptin analog (metreleptin) treatment in very insulin-resistant people with lipodystrophy led to near normalization of fatty acid synthesis. Improvements in fat synthesis in the liver were associated with reductions in blood glucose and improved insulin sensitivity. These findings indicate that metreleptin treatment lowered liver fat accumulation (hepatic steatosis) and blood lipid levels, and suggests that treatments targeting multiorgan insulin resistance may improve nonalcoholic fatty liver disease.
3. Insulin regulates glucose and triglyceride production differently in humans. Insulin resistance is a condition when cells in muscles, fat, and the liver do not respond efficiently to insulin and are unable to easily take up glucose from circulation. In the liver, insulin suppresses glucose production and increases lipid synthesis in the presence of glucose. It is not clear as to what mechanisms insulin regulates glucose and triglyceride (a type of lipid) production in humans with various types of insulin resistance conditions. Scientists in Houston, Texas, demonstrated that glycerol and free fatty acid availability (two precursors to triglyceride synthesis) are increased in two different models of insulin resistance, one involving the insulin receptor (receptor-level) and one caused by defects in the pathway after the insulin receptor (postreceptor). In receptor-level insulin resistance, free fatty acid oxidation increased glucose production rather than triglyceride production. In contrast, free fatty acids contributed to both glucose and triglyceride production in postreceptor insulin resistance conditions. The findings from this research provides details about glucose and fat metabolism in these insulin resistance conditions and insights for future research to find effective treatments.
4. Using blood markers to determine weight status in individuals with type 1 diabetes. Researchers often struggle to get accurate height and weight measurements when conducting studies outside of medical facilities. These measurements are critical for determining a person's body mass index which is used to determine if an individual is overweight or obese. Most studies obtain blood draws and there are many hormones which circulate in the blood that relate to a person's weight status; whether these hormones would be an accurate reflection of weight status in an individual with type 1 diabetes was not known. Researchers at Houston, Texas, used existing samples from a multi-site trial comprised of individuals with type 1 diabetes to determine if there were any markers identified in blood that would reflect weight status. They found that leptin was a good marker for weight status in boys but not in girls, however, waist circumference was a good marker in girls, which may be easier to collect than height and weight. Overall, this data shows promise and further studies should be performed to identify possible biomarkers since if successful, a blood biomarker would allow for many questions related to obesity to be asked in existing datasets of banked samples.
