The exploration of polypharmacology of drug molecules has long been a focus and a challenge in the field of G protein-coupled receptor (GPCR) pharmacology due to its far-reaching implications on drug efficiency and safety. Drugs targeting dopamine receptors for neurodegenerative diseases such as Parkinson's disease and schizophrenia often exhibit polypharmacological characteristics. However, research on their working mechanisms is limited. 

Dopamine receptors belong to GPCR superfamily, which includes five receptor members (D1R to D5R). Over the past decade, progress has been made in the field of dopamine receptor structural biology, which improves the understanding of the system's mechanisms and promotes the development of drugs targeting the dopamine system. However, the structure of D5R and the activated state of D4R remain elusive, limiting the understanding of ligand recognition and activation mechanisms of the dopamine receptor family, thereby hindering the development of structure-based drugs targeting dopamine receptors. 

In a study published in Cell Research, teams of researchers led by XU Huaqiang (H. Eric XU) from Shanghai Institute of Materia Medica (SIMM) of the Chinese Academy of Sciences, Bryan Roth from the University of North Carolina at Chapel Hill, and ZHANG Yan from Zhejiang University revealed the structure of dopamine receptor D5R and activated dopamine receptor D4R, and systematically analyzed the polypharmacology and signaling mechanisms of the entire dopamine receptor system.  

The researchers employed single-particle cryo-electron microscopy to resolve the structures of all five dopamine receptors bound to the same agonist molecule, Rotigotine, a drug used to treat Parkinson's disease and restless leg syndrome, as well as downstream G protein complexes. 

Although all five dopamine receptors exert their physiological functions by binding to the same endogenous ligand dopamine, subtle structural differences between the receptors can affect their affinity with drug molecules and activate entirely different signaling effects. The researchers compared the structures of the five receptors and found obvious differences between D1-like and D2-like receptors. Rotigotine can activate all five dopamine receptors and is a typical polypharmacological molecule. The researchers systematically compared the binding modes of Rotigotine with different dopamine receptors and validated the interactions between ligands and receptors through pharmacological experiments.  

To understand the correlation between different receptors' ligand-binding pockets and the polypharmacological traits of small molecules, the researchers undertook a screening of Rotigotine's binding activity against more than 300 GPCRs. They discovered that Rotigotine's affinity extends beyond dopamine receptors to a variety of other GPCRs, including serotonin receptors, adrenergic receptors, somatostatin receptors, adenosine receptors, and opioid receptors. By comparing structures and sequences, the researchers identified a direct link between a ligand's ability to broadly bind and the conservativeness of orthosteric binding pocket.  

Besides, the researchers revealed the effect of cholesterol on the function of dopamine receptors. They found a cholesterol molecule near the second intracellular loop of D5R stabilizing the conformation of a residue Tryptophan, which contributes to the selectivity of positive allosteric modulators bind to D1R. A cholesterol molecule was also found between the first and seventh transmembrane helices of D4R, indirectly affecting ligand-receptor binding.  

This extensive structural study of dopamine receptors underscores the researchers' commitment to refining the understanding of polypharmacology, paving the way for more efficient and safer drug development.