Electrostatic Similarity Analysis of Human β-Defensin Binding in the Melanocortin System

Abstract

The β-defensins are a class of small cationic proteins that serve as components of numerous systems in vertebrate biology, including the immune and melanocortin systems. Human β-defensin 3 (HBD3), which is produced in the skin, has been found to bind to melanocortin receptors 1 and 4 through complementary electrostatics, a unique mechanism of ligand-receptor interaction. This finding indicates that electrostatics alone, and not specific amino acid contact points, could be sufficient for function in this ligand-receptor system, and further suggests that other small peptide ligands could interact with these receptors in a similar fashion. Here, we conducted molecular-similarity analyses and functional studies of additional members of the human β-defensin family, examining their potential as ligands of melanocortin-1 receptor, through selection based on their electrostatic similarity to HBD3. Using Poisson-Boltzmann electrostatic calculations and molecular-similarity analysis, we identified members of the human β-defensin family that are both similar and dissimilar to HBD3 in terms of electrostatic potential. Synthesis and functional testing of a subset of these β-defensins showed that peptides with an HBD3-like electrostatic character bound to melanocortin receptors with high affinity, whereas those that were anticorrelated to HBD3 showed no binding affinity. These findings expand on the central role of electrostatics in the control of this ligand-receptor system and further demonstrate the utility of employing molecular-similarity analysis. Additionally, we identified several new potential ligands of melanocortin-1 receptor, which may have implications for our understanding of the role defensins play in melanocortin physiology.

The Dichotomous Responses Driven by β-Defensins

“…This may be due to complementary electrostatic interaction between the cationic peptide and receptors with anionic regions. HBD3, the most highly charged β-defensin (charge of +11) has been demonstrated to be a neutral antagonist, through charge based interaction with melanocortin receptor 1 and 4 (82). In dogs, a three base pair deletion in the canine orthologue of HBD3 results in an increase in the level of expression, which then allows this peptide to promiscuously bind the melanocortin receptor 1 (MC1R)-resulting in dogs with black, rather than agouti, fur (83).…”

60 YEARS OF POMC: Regulation of feeding and energy homeostasis by α-MSH

“…Cachexia is invariably linked to immunosuppression. As strongly charged molecules, defensins might interact with MCR solely through electrostatic interactions explaining their neutral antagonist effect (Nix et al 2013, 2015). There is little doubt that these interactions play a role in determining melanocortin response on MC1R, as defensins seem to block agouti in live animals (Candille et al 2007, Kerns et al 2007).…”

“…However, data are not as consistent for MC4R, and MC3R is yet to be tested as a potential defensin target. Electrostatic-based binding modalities open the possibility for additional undefined peptidic ligands to emerge as possible ‘modulators’ of AgRP and α-MSH signaling in energy balance (Nix et al 2015). …”

A Life without Hunger: The Ups (and Downs) to Modulating Melanocortin-3 Receptor Signaling

“…Melanocortin receptor pharmacology is complex, with two antagonists/inverse agonists (AgRP and agouti signaling peptide) and MSH ligands that exhibit varying degrees of receptor specificity (Figure 1B ; Cone et al, 1996 ). Other ligands and cell-surface proteins have been identified that regulate melanocortin signaling (e.g., melanocortin receptor accessory proteins 1 and 2, mahogany, mahoganoid, attractin-like protein, syndecans, ion channels and defensins) (Kaelin et al, 2008 ; Nix et al, 2013 , 2015 ; Anderson et al, 2016 ).…”