UC Santa Cruz
Mechanisms and Therapeutic Potential of the Agouti-Related Peptide's Long-Term Orexigenic Effects
- Author(s): Palomino, Rafael Ildefonso
- Advisor(s): Millhauser, Glenn
- et al.
The agouti-related peptide (AgRP) is a central nervous system-derived peptide that holds a critical role in appetite and energy balance. Through its interactions with the melanocortin-4 receptor (MC4R), AgRP promotes an anabolic physiological state that leads to increased feeding, reflex hyperphagia, and storage of energy, all to ensure a state of positive energy balance. While the magnitude of this effect has been long observed through exogenous administration of the peptide, more recent neurobiological studies have revealed that AgRP-secreting neurons respond to and manipulate feeding with significant temporal variance. AgRP appears to be unique in a physiological context with regards to its time course of action. Furthermore, due to the extraordinary long-term response driven by AgRP administration, it is considered a lead for cancer cachexia and other wasting disorders. Its impressive chemical stability also makes it an ideal scaffold for drug and imaging purposes. This dissertation aims to understand the mechanisms of AgRP’s extended orexigenic properties by addressing a long-standing incongruity in AgRP biology using biophysical tools. We use isothermal titration calorimetry and NMR, along with a panel of designed AgRP variants to characterize peptide interactions with glycosaminoglycan components of syndecans, which are hypothesized to potentiate AgRP action. We found compelling evidence that the mature AgRP(83-132) is a heparan sulfate binding protein and uses electrostatic complementarity primarily driven by nonreceptor-binding segments. To leverage these unique properties for AgRP’s therapeutic potential, we also utilize peptide chemistry to increase its utility as a drug or scaffold. We successfully engineered a head-to-tail cyclized versions of AgRP and its analog, agouti-signaling peptide (ASIP). Cyclization did not lead to enhanced stability for AgRP, though ASIP did display increases in serum resistance to proteolytic digestion. This study sets the stage for cyclization optimization of AgRP. This dissertation represents an advancement in our understanding of AgRP action and its potential as a therapeutic. In both cases, it is AgRP’s unique, long-term activity that motivated this work. As the neurobiology of feeding circuits continues to be driven forward, studying AgRP and its role within these circuits contributes the molecular pieces to the large and complex model of energy balance and its related diseases.