From a Cone Snail Toxin to a Competitive MC4R Antagonist

The melanocortin 4 receptor (MC4R) plays a role in energy
homeostasis
and represents a target for treating energy balance disorders. For
decades, synthetic ligands have been derived from MC4R endogenous
agonists and antagonists, such as setmelanotide used to treat rare
forms of genetic obesity. Recently, animal venoms have demonstrated
their capacity to provide melanocortin ligands with toxins from a
scorpion and a spider. Here, we described a cone snail toxin, N-CTX-Ltg1a,
with a nanomolar affinity for hMC4R but unrelated to any known toxins
or melanocortin ligands. We then derived from the conotoxin the linear
peptide HT1-0, a competitive antagonist of Gs, G15, and β-arrestin2 pathways
with a low nanomolar affinity for hMC4R. Similar to endogenous ligands,
HT1-0 needs hydrophobic and basic residues to bind hMC4R. Altogether,
it represents the first venom-derived peptide of high affinity on
MC4R and paves the way for the development of new MC4R antagonists.