Picking Pathways: Small molecule boosts morphine effect

Some small molecules affect specific pathways in one of the body’s most common cell-regulating systems, according to a new report. The work could aid investigations of the pathways and lead to new drug therapies, the report’s authors say.

When certain cell-surface receptors bind a chemical stimulus, such as an opiate or a hormone, they interact with so-called G proteins. These three-subunit proteins then separate into two parts, known as alpha and beta-gamma. Alan V. Smrcka of the University of Rochester in New York and his colleagues focused on the beta-gamma part.

That double subunit binds to and activates many enzymes that carry out a cellular response, such as mediating pain relief, explains Smrcka.

In past work, Smrcka and his colleagues investigated how beta-gamma can recognize such a diverse group of enzymes. Smrcka’s team and other researchers found that the beta surface has a hot spot, an area that offers binding opportunities to many different target enzymes. Hydrophobic interactions bind some proteins there, while hydrogen bonding or other forces attach yet other proteins to the hot spot.

In the current study, the team showed that manipulating the hot spot “selectively interferes with what beta-gamma does,” says Smrcka.

The researchers screened nearly 2,000 compounds from a library of small organic molecules, identifying 85 that had a high affinity for the hot spot. The team reports on two of those compounds that affect beta-gamma in cultured cells.

“Both compounds are binding to the hot spot, but they interrupt different sets of interactions,” says Smrcka. For example, one of the compounds, called M119, blocked the subunit’s interaction with an enzyme called phospholipase C, but the other compound didn’t. Yet both compounds blocked the subunit’s binding to an enzyme called G-protein–coupled receptor kinase.

Because M119 blocked phospholipase C and previous studies had suggested that phospholipase C blunts morphine’s pain relief, the researchers tested M119’s effect on the morphine-triggered pathway. Smrcka’s team reports in the April 21 Science that morphine administered with M119 to mice is 11 times as potent as morphine alone is.

If M119 had shut off the beta-gamma subunits’ action, says Smrcka, it would have blocked morphine’s pain relief. Instead, the compound intensified pain relief by stopping the activity of phospholipase C while “leaving the rest of what beta-gamma is doing intact,” he says.

“It’s a very interesting mode of action for a small molecule to bind to one protein and augment its ability to act with another,” says pharmacologist Elliott Ross of the University of Texas Southwestern Medical Center in Dallas.

Smrcka’s team is now further investigating the morphine-enhancing compound. While the specific compounds that the team has so far identified may not become drugs, the study indicates the potential for small molecules to modify the many existing drugs that work by influencing G-protein–coupled receptors, Smrcka says.