Roach hormone: clue to human ancestry?

Roach hormone: Clue to human ancestry?

The strongest hormonal evidence yet of a common ancestry for insects and mammals has been provided by a pair of newly identified neuropeptides, isolated from cockroaches. These chemicals, called leucosulfakinins (LSKs), bear a strong similarity both in structure and function to hormones present in mammals, including humans. The strong similarity between the insect and mammalian neuropeptides, says one of the researchers, Ronald Nachman, is "evidence that our [human] hormones have very ancient roots.'

Biochemical similarities between primitive species, like the cockroach, and more recent branchings on the evolutionary tree, like mammals, serve as "molecular clocks'--a means for identifying and tentatively dating the evolutionary divergence of what were once closely related organisms. The high degree of similarity between the LSKs and two hormones present in humans--human gastrin II and cholecystokinin (CCK)-- indicates that these neuropeptides represent one of the slowest ticking of the molecular clocks, according to Nachman, a chemist at the U.S. Department of Agriculture (USDA) Western Regional Research Center in Berkeley, Calif.

Both LSK and LSK-II--amino-acid-chain molecules released by the Madeira cockroach's brain--appear to be hormones. Fifty-five percent of LSK's amino-acid sequence is identical to human gastrin II's, half of LSK-II's matches that in the frog neuropeptide caerulin and greater than 40 percent of LSK-II's amino-acid chain matches that of CCK. These are the highest percentages of structural similarity reported between insect and vertebrate neuropeptides, according to the researchers. Even more convincing, Nachman says, is that both LSK and LSK-II contain sulfate groups--a rare occurrence in hormones of any species. This same rare sulfation is present in the vertebrate neuropeptides they resemble.

Moreover, gastrin, CCK and both LSKs stimulate muscle contraction in the digestive tract; gastrin and both LSKs stimulate blood circulation. This homology of function further establishes the neuropeptide link between insects and mammals, Nachman believes. And because the activity of the LSKs has not yet been fully characterized, it's possible they may share even more attributes in common with the mammalian hormones--such as the ability to regulate digestive tract water content, to make organisms feel sated by a meal and to secrete enzymes. Nachman, William F. Haddon and colleagues from two other research laboratories described LSK's structure and function for the first time in the Oct. 3 SCIENCE; LSK-II's have just been published in the Oct. 15 BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS.

"This [work] is interesting in that it shows there's something common to us and insects that goes back 500 million years and has changed so little,' says Jerold M. Lowenstein, a molecular-evolution researcher at the University of California at San Francisco. "It gives you insight into how evolution works; it conserves those things that are important and work.' David Schooley, a biochemist at the Palo Alto, Calif.-based Zoecon Research Institute, agrees.

Two years ago Schooley coauthored one of the first papers identifying a homology between an insect hormone and a functionally similar mammalian hormone. But such investigations are still rare, notes G. Mark Holman, a collaborator on the LSK work at a USDA lab in College Station, Tex. "Up until about a year ago there were only four insect neuropeptides of known structure,' he says. "Now there are approximately 20.'

As more are found, Nachman says, one may expect to see more of these bio-chemical links between distant twigs on the evolutionary tree.