Positive Signal: Lone protons carry messages between cells

Roundworms need protons to poop.

MINIMESSENGER. Protons diffuse quickly from a roundworm’s gut and bind to receptors on surrounding muscles. The receptors then cause the muscles to contract, which makes the animal defecate. G. Ernstrom, Univ. of Utah

New research shows that protons released by roundworms’ intestines trigger surrounding muscles to contract, causing the worm to defecate. The discovery marks the first time that scientists have found protons transmitting signals between cells.

The research also adds new support to the idea that protons might relay electrical signals in the human brain.

“A proton acting as a transmitter between cells was pretty unexpected,” says Erik Jorgensen, lead scientist on the study at the University of Utah in Salt Lake City. “This is a fundamental subatomic particle, so it was a little hard to imagine it performing that role.”

Normally, signals between cells are carried by multi-atom molecules with specific shapes that fit into corresponding receptor molecules. A proton is what’s left when a hydrogen atom is stripped of its electron; it’s about 1/100,000th the size of the original atom.

In spite of their tiny size and lack of a characteristic shape, protons already were known to activate receptors on the surfaces of some nerve cells. These receptors, called acid-sensing ion channels (ASICs), allow pain nerves to sense the ambient concentration of protons—in other words, the pH. That’s why touching strong acid burns.

Jorgensen’s team found an ASIC-like receptor on the muscle cells surrounding the gut of the roundworm Caenorhabditis elegans. Previous studies by other researchers had shown that the worms’ intestines pump protons into the space next to the muscles, and Jorgensen and his colleagues wondered whether these protons could be causing muscle contraction by binding to the receptors.

Blocking the new receptor in one experiment caused the worms to become constipated, the team reports in the Jan. 11 Cell. In another experiment with working receptors, manually adding protons into the space adjacent to the muscles caused them to contract.

The study “supports the idea that protons can function in intercellular signaling, which is exciting,” comments John A. Wemmie of the University of Iowa in Iowa City.

Wemmie and his colleagues previously showed that removing proton receptors in the brains of mice impaired the animals’ learning and memory. Human brains also have proton-sensing ASIC receptors as well as proton pumps, so Jorgensen and others suspect that protons might carry signals between nerve cells in mammalian brains.

If so, protons would join the short list of fast-acting neurotransmitters, which work by opening pores in the surfaces of nerves and letting electrically charged particles flow through, resulting in an electrical impulse. Slow-acting neurotransmitters, which are more numerous, cause a metabolic change in the recipient nerves instead of an electrical one.

“Those fast neurotransmitters are an elite group,” Jorgensen says. “There’s not a lot of them.”