A third of all Americans have difficulty with sleeping, and many of them turn to melatonin supplements to catch some Zs. However, scientists don't fully understand melatonin's role in the biological clock, which has made it difficult to develop drugs for sleep disorders without several side effects.

Now, an international team of scientists has shed much-needed light on melatonin's effects, opening the door to the development of new drugs for sleep disorders -- and other health issues affected by melatonin. They developed 3D models of the tiny antennae -- called receptors -- on the surface of cells that synchronize the body's internal clock with the day and night cycle.

"Our goal is to provide the structural information to other researchers who can use it for designing new drug compounds or to study mutations of these receptors in patients," said corresponding author.

Creating the 3D maps of the two melatonin receptors, MT1 and MT2, is critical for understanding how the biological clock works. The scientists can use this information to design drug molecules that bind to the melatonin receptors and monitor the potential effects. The benefits could go beyond improving sleep.

The findings on the melatonin receptors were published in two letters in the journal Nature.

Melatonin is generated in the center of the brain by the pineal gland, once described by the philosopher Descartes as the "soul" of the brain and body.

Humans respond naturally to daylight changes through the pineal gland, near the hypothalamus. As night falls, the gland produces more melatonin, which then binds to the MT1 and MT2 receptors of the cells. Before dawn, the gland decreases melatonin levels, signaling that it's time to wake.

MT1 and MT2 are among an estimated 800 receptors in the human body. These receptors, known as "G protein-coupled receptors," (GPCRs) appear on the surface of a cell. The receptors act as a sort of email inbox, relaying information into the cell to set off a cascade of activity.

About a third of all drugs on the market are designed to bind with GPCRs. Each receptor has a different role in regulating functions in the body, many of which are critical for basic survival, such as hunger and reproductivity. The bulk of these receptors also have some role in the human olfactory system -- taste and smell.

Scientists around the world have obtained structures of less than one-tenth of these receptors so far. MT1 and MT2 are among the latest. The MT1 and MT2 receptors are important for multiple processes, including reproduction and even some cancers.

The structures of both receptors were obtained using a laser, called the Linac Coherent Light Source (LCLS), which uses X-rays to take stop-action pictures of the receptor atoms and molecules in motion.

Although the MT₁ and 5-HT receptors have similar endogenous ligands, and agomelatine acts on both receptors, the receptors differ markedly in the structure and composition of their ligand pockets; in MT₁, access to the ligand pocket is tightly sealed from solvent by extracellular loop 2, leaving only a narrow channel between transmembrane helices IV and V that connects it to the lipid bilayer. The binding site is extremely compact, and ligands interact with MT₁ mainly by strong aromatic stacking with Phe179 and auxiliary hydrogen bonds with Asn162 and Gln181.

The structures provide an unexpected example of atypical ligand entry for a non-lipid receptor, lay the molecular foundation of ligand recognition by melatonin receptors, and will facilitate the design of future tool compounds and therapeutic agents, while their comparison to 5-HT receptors yields insights into the evolution and polypharmacology of G-protein-coupled receptors.

The structures of the human MT₂ receptor in complex with the agonists 2-phenylmelatonin (2-PMT) and ramelteon⁶ at resolutions of 2.8 Å and 3.3 Å, respectively, along with two structures of function-related mutants: H208^5.46A  and N86^2.50D, obtained in complex with 2-PMT.

Comparison of the structures of MT₂ with a published structure⁸ of MT₁ reveals that, despite conservation of the orthosteric ligand-binding site residues, there are notable conformational variations as well as differences in [³H]melatonin dissociation kinetics that provide insights into the selectivity between melatonin receptor subtypes. A membrane-buried lateral ligand entry channel is observed in both MT₁ and MT₂, but in addition the MT₂ structures reveal a narrow opening towards the solvent in the extracellular part of the receptor.

https://www.nature.com/articles/s41586-019-1141-3

https://www.nature.com/articles/s41586-019-1144-0