Ever taken antihistamines? Or heartburn medication? Along with others used for a variety of conditions, from diabetes to high blood pressure to depression, these drugs work by targeting the same class of protein molecules on our cells. They’re the most common type of drug on the market – and in medicine cabinets at home. Yet the available medications are only the tip of the iceberg as a University of Toronto study reveals a large swath of new therapeutic opportunities, including one that could lead to a better treatment for Parkinson’s disease.

Despite representing about a half of prescribed medications worldwide, these compounds target only a sliver of one of the largest – and most elusive – classes of human proteins, called G protein coupled receptors (GPCRs). Tapping into this vast unexplored therapeutic potential has been difficult because available tools weren’t up to the task of surveying the GPCRs on large scale. Enter Professor Igor Stagljar of U of T’s Donnelly Centre.

Stagljar’s new study, which earned the cover of the March issue of the journal Molecular Systems Biology, is based on a technology called MYTH. Previously developed in the lab, it allows detection of membrane protein interactions as they occur in their natural setting – on the surface of cells. Using MYTH, Stagljar’s team was able to capture almost 1,000 interactions between more than 600 proteins for almost 50 clinically important GPCRs. The largest survey of GPCRs to date, it revealed new associations among proteins involved in neurological disorders, such as motor neuron disease, schizophrenia, and neurodegenerative disorders, as potential targets for new drugs.

One association that stood out involved ADORA2A, a GPCR targeted by Parkinson’s disease drugs. By binding to ADORA2A, these drugs stimulate the release of dopamine, which helps communication between nerve cells to ultimately reduce tremor in patients with Parkinson’s. Stagljar’s team found that ADORA2A associates with another GPCR, called GPR37 or Parkinson’s disease associated receptor, in a way that affects movement in a mouse model of disease. This suggests that a combination of drugs targeting both receptors, may work better in patients.

The work on Parkinson’s was done in collaboration with Professor Francisco Ciruela’s team at the University of Barcelona in Spain, which will continue to investigate the clinical potential of the enhanced combination therapy involving ADORA2A and GPR37.

To appreciate just how pervasive the 800 or so human GPCRs are, you only need to take a deep breath and look around you. Nestled inside the eye, these proteins detect light and help us see; those in the nose detect scents, while the ones in taste buds let us taste chocolate and other sweet and bitter foods. But these proteins also detect glucose and hormones in the blood, neurotransmitters, or chemicals that help our brain cells communicate, as well as hold cells together ensuring that tissues don’t fall apart. It’s no surprise then, that when GPCRs go awry, this can lead to brain disorders, diabetes, cancer and a host of other diseases.

In the past, scientists would either focus on the GPCR parts that are easily accessible, such as those sticking out on either side of the cell. Or, to study the GPCRs in entirety, they would remove the surrounding membrane, which changes the proteins’ properties. Either way, researchers weren’t getting the full picture of how these proteins work. MYTH and MaMTH, another related technology developed in the lab, have revolutionized the study of membrane proteins, attracting interest from the pharmaceutical industry.