Location: Children's Nutrition Research Center
2022 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, named the small conductance potassium channel-3 (SK3), is selectively deleted from a group of brain neurons, called 5-HT neurons. We found that deletion of SK3 significantly enhanced the electric activity of 5-HT neurons, which accounts for the reduction in animals' food intake. 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 confirmed that this stimulation significantly reduced animals’ consumption of a highly palatable diet. These results support our original hypotheses.
For Objective 3, we started to produce and use a reporter mouse model in which we can monitor a signaling pathway mediating insulin action in the brain. Using the reporter mouse model, we confirmed that insulin activates the signaling pathway in the brain explants. We performed a pilot study with a small number of explants, and we will use this model to examine the effect of inhibitors of insulin signaling on leptin next year. Objective 4 sought to determine if a brain signaling pathway, called the b-Catenin pathway, influences whole-body glucose balance. The experiments in Objective 4 were unable to be conducted as planned due to a critical vacancy. We anticipate this vacancy will be filled this year and that the proposed studies of Objective 4 are likely to be completed in the following year.
For Objective 5, we have used genetic approaches to test feeding in response to activation or suppression of the MC4R neurons located in the hindbrain. Notably, this group of neurons are previously known to regulate taste and other physiological functions related to metabolism. However, the direct role and mechanism in regulation of food intake were never explored. Here, we found that activation of this specific neuronal group can potently suppress feeding of both low and high-fat diets, while suppression of the same group of neurons led to opposite effects that promote overeating of high-fat diets. For Objective 6, we applied a series of cutting-edge genetic techniques to rapidly inactivate the inhibitory GABAA receptor signaling or the excitatory NMDA glutamate receptor signaling within these neurons. Our goal is to determine whether this genetic manipulation in the hindbrain would affect the loss of appetite as a result of the inaction of hypothalamic AGRP neurons. Our results indicated that deletion of hindbrain GABAA or glutamate receptor signaling caused mild to moderate deficiency in normal feeding when AGRP neurons were intact. However, such genetic manipulation failed to fully compensate for the acute ablation of AGRP neuron-induced starvation and body weight loss. This research suggests that the compensatory mechanism relevant to the ablation of AGRP neurons is perhaps situated in one or more downstream regions other than the established AgRP neural circuit.
Accomplishments
1. A new way to reduce eating in the absence of hunger. Hunger can drive humans and animals to eat, but in the absence of hunger, eating can also be triggered by hedonic (pleasant sensations) values of foods. This "pleasure-driven" eating is a contributing factor to obesity. Researchers at the Children's Nutrition Research Center in Houston, Texas, have discovered that a certain type of brain cells, called 5-hydroxytryptamine (5-HT) neurons, can suppress hedonic feeding. We revealed how 5-HT cells are regulated by nutrient intake and how these cells send signals to the downstream cells to regulate feeding behaviors. These findings are significant and provide a framework to potentially target these specific cells for the prevention and/or treatment of obesity.
2. A novel neural pathway can be regulated for the treatment of comorbidity of obesity and mental diseases. Obesity and depression are among the leading causes of disease worldwide and together they jointly form a vicious cycle disrupting the mental and metabolic health for >70% obese patients (i.e. >100 million in the United States and ~500 million worldwide). There has been no effective treatment for this global healthcare issue, however researchers at the Children's Nutrition Research Center in Houston, Texas, discovered how the brain exerts a reciprocal control of feeding of high-fat foods and psychological states. Similar to humans, mice that consumed a high-fat diet not only became obese, but also were anxious and depressed, a condition mediated by a defective brain circuit. When the disruptions were genetically or pharmacologically corrected within this neural circuit, the mice became less anxious and depressed and later lost excess body weight. More importantly, we successfully established a therapy using Food and Drug Administration approved medication for effectively treating this comorbidity. This new regimen displayed striking results that not only eradicate anxiety/depression but also reverse most of the obesity symptoms via a surprising effect to make the subjects voluntarily switch their dieting choice from high-fat foods to a low-fat, carb/protein-enriched healthy diet.
