Location: Children's Nutrition Research Center
2023 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 accomplished 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 increased electric activity of 5-HT neurons in animals, and these animals also displayed reduced food intake.
In Objective 2, we selectively activated the neural projections of 5-HT neurons to a brain region called the ventral tegmental area and confirmed that this stimulation reduced animals’ consumption of a high palatable diet. We then inhibited the activity of neurons in the ventral tegmental area and found that this inhibition attenuated effects of 5-HT to suppress food intake. These results support our original hypotheses.
For Objective 3, we completed the planned pharmacological experiments. Using pharmacological reagents in combination with the reporter mouse model we generated last year, we found that inhibition of the beta-catenin pathway attenuated cellular signaling downstream of the anorexic hormone leptin in hypothalamic explants.
In Objective 4, we carried out a chromatin immunoprecipitation assay and a functional leptin assay. We showed that a beta-catenin-related transcription factor binds to multiple sites within regions controlling the expression of a signaling molecule, known as the suppressor of cytokine signaling 3. In addition, we observed that selective deletion of beta-catenin in the hypothalamus resulted in decreased leptin action, suggesting a critical role for beta-catenin in this process.
For Objectives 5 and 6, we found that genetic disruption of GABAA5 or enhancement of NMDA NR2B signaling in the parabrachial nucleus Mc4r neurons reduced hyperphagic response to a palatable high fat diet, thereby rendering resistance from diet induced obesity. In contrast, gain-of-function of GABAA5 or disruption of NMDA NR2B signaling enhances the palatability of high fat diet, thereby promoting food intake and weight gain. These outcomes coincide with our original hypotheses.
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 the hedonic (pleasant sensations) value 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 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.
