Project 1: Heike Muenzberg-Gruening
Assistant Professor | View Bio
Co-Mentors: Jaqueline M. Stephens, Ph.D. and Leslie P. Kozak, Ph.D.

Role of galanin-expressing leptin receptor neurons in leptin action

We will determine the control of energy homeostasis, by a newly identified set of leptin-regulated neurons located in the dorsomedial hypothalamus (DMH) and adjacent perifornical area (PFA), collectively referred to as the extended perifornical area (exPFA).

The importance of leptin action in energy balance has been established by the severe hyperphagia and obesity in humans or rodents null for leptin or leptin receptor (LepRb). While leptin is known to act via LepRb located on neurons of the arcuate nucleus (ARC) melanocortin system (consisting of orexigenic AgRP and anorexigenic POMC neurons), other studies suggest that the action of leptin in the ARC alone only accounts for a fraction of leptin action on energy homeostasis.

The majority of LepRb expressing neurons have not been studied and their function and contribution to anorexigenic leptin actions is unknown. For example, the exPFA, which is known to regulate feeding circuits, contains a large population of LepRb expressing neurons, suggesting that leptin action in this site also contributes to the regulation of energy homeostasis. Since neurons in the exPFA interconnect with the paraventricular nucleus (PVN) - an output nucleus involved in the regulation of satiety as well as autonomic and endocrine outputs – leptin may act in the exPFA to modulate these functions. And indeed, we found that leptin injection into the exPFA significantly decrease food intake and body weight. Furthermore, LepRb neurons in the exPFA project densely into the PVN, consistent with an important role of the exPFA to mediate anorexigenic leptin action via PVN outputs.

The neuropeptide galanin (Gal) acts in the PVN to regulate energy expenditure and food intake. We have discovered that a large number of LepRb neurons in the exPFA co-express galanin. Thus, theco-localization of galanin in LepRb neurons represents a unique subset of leptin responsive neurons that we call LepRb(Gal) neurons.

We hypothesize that LepRb neurons in the exPFA, specifically LepRb(Gal) neurons, play a crucial role in mediating anorexigenic leptin actions via PVN outputs and thus control feeding behavior. Our overall goal is to understand the physiologic function and underlying neuronal network of LepRb(Gal) neurons as well as to investigate the overall population of LepRb neurons in the exPFA. We will thus:

1. Determine the role of LepRb(Gal) neurons in physiologic leptin action.
   We will examine the phenotype of mice with targeted deletion of LepRb in galanin neurons (LepRbKO(Gal) mice) to determine the contribution of LepRb(Gal) neurons in leptin dependent regulation of energy homeostasis. We will specifically investigate any perturbation in body weight, food intake or energy expenditure in these mice with disrupted LepRb in Galanin neurons. Thus, we expect LepRbKO(Gal) mice to be hyperphagic and obese.
2. Investigate the regulation of LepRb(Gal) neurons.
   We will use reporter mice to visualize LepRb(Gal) neurons immunohistochemically and define regulatory mechanisms of LepRb(Gal) neurons. We will determine if LepRb or LepRb(Gal) neurons in the exPFA are stimulated by leptin or physiological stimuli like fasting and refeeding (via cFos induction and/or regulation of galanin) and if they co-express the inhibitory transmitter GABA. We will further study the contribution of ARC inputs (e.g. melanocortin system) to regulate LepRb(Gal) neurons by investigating LepRb(Gal) regulation after blockade or ablation of ARC inputs.
3. Investigate the neuroanatomic circuits of LepRb(Gal) neurons.
   We will define axonal projection sites of LepRb(Gal) neurons (e.g. the PVN) by using traditional and novel tracing methods (LepRb or galanin neuron-specific tracer expression in the exPFA). Furthermore, we will identify the neurons that are innervated by LepRb(Gal) neurons (e.g. in the PVN) or that innervate LepRb(Gal) neurons (e.g. from the ARC), by using LepRb or galanin neuron-specific expression of anterograde or retrograde transsynaptic tracers in the exPFA.