Location: Children's Nutrition Research Center
2020 Annual Report
Objectives
Objective 1: Determine if potassium channels (SK3) expressed by serotonin neurons are required to regulate feeding behavior and body weight balance using a Cre-loxP strategy to generate mouse models that either lack SK3 selectively in serotonin neurons. Test whether these manipulations in mice alter animals’ food intake and body weight.
Objective 2: Identify downstream neural circuits that mediate serotonin neuron actions to regulate feeding behavior and body weight balance. Selectively stimulate specific downstream neural circuits that originate from brain serotonin neurons in mice, and measure effects on animals’ feeding behavior and body weight.
Objective 3: Identify upstream and downstream signaling molecules of glycogen synthase kinase 3 beta that controls suppressor of cytokine signaling 3 levels and cellular insulin and leptin actions in the hypothalamus by using an ex vivo brain slice model.
Objective 4: Determine if each component of the glycogen synthase kinase 3 beta -related pathway determines hypothalamic levels of suppressor of cytokine signaling 3 and hypothalamic leptin and insulin actions in vivo by using genetically engineered mouse models.
Objective 5: Determine the physiological roles of genetically defined Agouti-related protein/proopiomelanocortin-parabrachial nucleus circuit in differential control of feeding behavior and energy metabolism.
Objective 6: Determine the physiological roles of key gamma amino butyric acid and N-methyl-D-aspartic acid glutamate receptor subunits expressed in the Agouti-related protein/proopiomelanocortin-parabrachial nucleus circuit for the regulation of appetite, energy balance, and development of obesity.
Approach
Obesity and its associated metabolic disorders (e.g., diabetes) represent a serious health problem to our society. The central nervous system (CNS) senses multiple hormonal/nutritional cues and coordinates homeostatic controls of body weight and glucose balance. However, the mechanisms for CNS control of metabolism remain to be fully understood. Primarily using genetic mouse models, supplemented by optogenetic and chemogenetic approaches, research scientists will tackle this concern from multiple angles. Based on the previous observations that brain serotonin (5-HT) neurons regulate feeding, body weight and glucose balance, we will continue to identify the ionic mechanisms that regulate 5-HT neuron activity and the downstream neural circuits that mediate the metabolic effects of 5-HT. Additionally we will identify a previously unrecognized neural signaling pathway that controls leptin and insulin actions in the hypothalamus and mediates whole-body energy balance. Scientists will also identify a novel neural circuit with converged GABAergic and glutamatergic projections from hypothalamus to the brainstem in control of feeding, metabolism and body weight. Collectively, these studies will demonstrate the potential roles of metabolic cues (hormones/nutrients), CNS circuits, and the intra-neuronal signals in the control of energy and glucose homeostasis. Our research results should identify rational targets for the treatment or prevention of obesity and diabetes. Findings will provide evidence to support new guidelines in hormonal/chemical diet supplementation to prevent these diseases. Finally, numerous novel genetic mouse lines will be generated, which will benefit a broader research community.
Progress Report
For both Objectives 1 and 2, we have generated mouse models needed for the research as originally planned. We bred mice that carry the small conductance potassium channel-3 (SK3) flox mutation and those carrying tryptophan hydroxylase-2 (TPH2)-CreER mutation. These breedings produced offspring mice that carry both mutation which will lead to selective deletion of the SK3 (an ion channel protein) only in TPH2-expressing neurons. Similarly, we bred to produce mice that carry both TPH2-CreER and dopamine transporter (DAT)-CreER mutations. These mice are now housed in the Children's Nutrition Research Center vivarium and are ready for the experiments as we initially proposed. For Objective 3, researchers have completed the dosing studies of the chemicals targeting the GSK3b-Catenin-b pathway. To determine the pharmacologically effective dose ranges of GSk3b inhibitors, organotypic brain slices were treated with GSK3b inhibitors and we found that the appropriate dosing of the inhibitors to induce hypothalamic SOCS3 and elicit cellular leptin resistance. For Objective 4, the team also generated mouse models proposed in the project as originally planned. To delete B-Catenin gene from the mediobasal hypothalamic areas of the brain, we bilaterally injected AAV-Cre virus (AAV) into the brains of B-catenin floxed mice. Two weeks after AAV injections, we examined if this manipulation expressed Cre deleted hypothalamic B-Catenin gene, and confirmed that AAV was appropriately injected into the mediobasal hypothalamus by demonstrating the presence of a marker in the mediobasal hypothalamus of the mice receiving AAV injection. For Objective 5, to determine the functional role of a unique subset of neurons located in the brainstem that potentially mediated feeding and energy expenditure, we have completed the test of baseline feeding response and energy metabolism by chronic suppression of Mc4r neurons in the parabrachial nucleus. For Objective 6, to understand the role of GABA receptors expressed within this specific subgroup of neurons, we have completed the genetic disruption or enhancement of GABAA 5 signaling. Appetite, energy balance, and development of obesity were evaluated.
Accomplishments
1. Discovery of a brain cell type that controls hedonic eating. Hedonic eating behavior is a pleasure-driven type of overeating that occurs when an individual consumes food for the enjoyment of eating and this can lead to the consumption of unnecessary calories. Investigators at Houston, Texas, have discovered that a certain type of brain cell, called 5-hydroxytryptamine (5-HT), can suppress hedonic feeding. Since overeating highly palatable foods (often processed foods with high amounts of sugar, fats, and salt) is associated with obesity, a better understanding of the mechanisms by which brain 5-HT cells repress hedonic feeding will provide a framework to target these cells as a potential therapeutic strategy for the prevention or treatment of obesity.
2. Understanding the brain's role in the control of appetite and metabolism. Little is known about how the brain is functionally organized towards the control of hunger and energy metabolism. Researchers in Houston, Texas, recently identified a specific group of nerve cells that regulate feeding and nutrient sensing. Our research suggests that these cells precisely regulate body weight via a balanced control of various peripheral metabolic signals. Genetic mice with deficiency in these cells show uncontrolled overeating and eventually became obese. Conversely, stimulation of the neural signaling within the same group of cells reduces food intake whereas energy expenditure is enhanced, which together lead to a profound weight loss. Overall, the discovery of a group of nerve cells acting as a switch gating the caloric intake would accelerate the development of novel anti-obesity medication.
