Location: Children's Nutrition Research Center
2021 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 started to use mouse models for the experiments as originally planned. Specifically, in Objective 1, we used a mouse model in which a protein, namely the small conductance potassium channel-3 (SK3), is selectively deleted from a group of brain neurons, called 5-HT neurons. Initial characterization of these animals revealed that they have normal chow refeeding response after an overnight food deprivation. However, when presented with a high palatable diet, they consume less food compared to normal mice. In Objective 2, we used a neuroscience technology, called optogenetics, to selectively activate the neural projections of 5-HT neurons to a brain region called the ventral tegmental area. We found that this stimulation does not affect chow refeeding response after an overnight food deprivation. However, this same stimulation can reduce high palatable diet intake in animals. These results support our original hypotheses, and we will pursue these findings with the experiments as we originally planned.
For Objective 3, researchers have completed the studies of the chemicals targeting a brain signaling pathway called the GSK3b — beta-Catenin pathway. To determine if this pathway suppresses the actions of the most potent anorectic hormone leptin in the brain, we inhibited the pathway in the brain explants to see if this manipulation blocked leptin's cellular action. Chemical inhibitors of the pathway resulted in attenuated leptin action in the brain explants.
For Objective 4, to determine if a brain signaling pathway (the GSK3b — beta-Catenin pathway) has an effect on leptin in the whole animals we removed one of the components of the pathway, beta-catenin, specifically from the hypothalamus of the brain. Then, we examined leptin action in mice lacking beta-Catenin in the hypothalamus. Due to the COVID-19 restrictions, we were only able to perform a pilot study with a small number of mice, we will re-examine the effect of Beta-catenin knockdown on leptin in the next year.
For Objective 5, we have completed a series of feeding tests to determine if a specific ground of neurons located within the hindbrain region (the parabrachial nucleus, PBN) is important to regulate the intake of a high-fat diet. Using various genetic approaches, stimulation of these hindbrain neurons reduced calorie intake of the high-fat diet. We also observed a long-term effect on a significant reduction in body weight, suggesting an important role of these PBN neurons in the control of obesity.
For Objective 6, we used genetic approaches to suppress or enhance the NR2B signaling (the key functional subunit of the N-methyl-D-aspartate NMDA glutamate receptor) within the PBN neurons. We found that enhancing NR2B signaling reduced consumption of a high-fat diet and body weight, whereas deletion of NR2B signaling promoted consumption of a high-fat diet. These results support our original hypotheses by suggesting a role of the NR2B signaling in regulation of high-fat diet consumption and diet-induced obesity.
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 highly palatable food for the enjoyment of eating, which can lead to the consumption of unnecessary calories. Researchers in Houston, Texas, have discovered that 5-hydroxytryptamine (5-HT) neurons 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. Additional studies are needed to reveal how the activity of brain 5-HT cells is regulated by nutrient intake and how these cells send signals to the downstream cells to regulate feeding behaviors.
2. Maintaining blood glucose within the normal range is vital for organisms. Elevated blood glucose levels that are above normal can lead to serious health complications such as heart attack, stroke, kidney failure, leg amputation, and vision loss. Scientists in Houston, Texas, have discovered a molecular switch in the brain that determines blood glucose levels. A signaling molecule called Rap1 in the hypothalamus of the brain acts as a determinant of blood glucose. When Rap1 is turned on, blood glucose is elevated. More importantly, when it is off, blood glucose is decreased and corrects diabetic conditions in mice. Researchers will next attempt to identify chemical(s) that can manipulate Rap1 to control our blood glucose, and novel therapeutic opportunities to improve type 2 diabetes from these research efforts.
3. Obesity is not only peripheral disease but also a group of brain disorders. When energy levels in the body are sensed to be high, fat cells send signals to the brain to inhibit hunger. The failure of this inhibition could cause the occurrence of obesity. Scientists in Houston, Texas, recently found a new neural circuit from the hypothalamus to hindbrain that could inhibit the activities of neurons in the hindbrain. The effect of activating this neural circuit was to enhance appetite toward a calorie-enriched high-fat diet, and the inhibition of this neural pathway could reduce feeding behavior accordingly. The long-term treatment on this pathway could suppress body weight gain. This work will help researchers gain new knowledge about the mechanisms of eating disorders and provide insights into the functional connection and key signaling components of a novel neural circuit in the control of feeding behaviors.
