Two studies published on 26 October show peptides doing jobs their names would not suggest. In fruit flies, a germ-fighting peptide appears to help the gut clear out its own worn-out cells. In mice with Alzheimer's-like disease, a peptide known for keeping animals awake improved their sleep and was linked to less amyloid in the brain.
Both studies used animals, and neither says what would happen in people. They are worth reading closely all the same, because each points to a possible new role for a small, well-defined molecule.
Pileup: a germ-fighting peptide with a second job
Antimicrobial peptides are best known for killing bacteria. A team led from Seoul National University has found a new one in fruit flies, called pileup, that seems to have a second job as the gut ages. The work was reported in Current Biology by Lee, Kim and colleagues.
Older cells in the gut lining build up stress as their mitochondria, the structures that supply a cell's energy, begin to fail. According to the team, that stress switches on pileup inside those cells. The peptide damages their membranes and sets off a signal that the researchers think helps the cells to be shed, which makes room for new ones.
What happened without it
Flies that lacked pileup and its three related genes developed a tumour-like build-up of cells, a leaky gut and an unbalanced population of gut microbes, and they died early. That suggests the peptide's housekeeping role matters for keeping the gut lining healthy as flies age.
The result comes from fruit flies only, so it is not yet known how much of it carries over to people.
Neuropeptide S: a wake-promoting peptide used the other way round
Neuropeptide S is a peptide of 20 amino acids best known for keeping rodents awake after a single dose. A team at the Wuxi School of Medicine, Jiangnan University, in China, led by Peng Zhao, tested what happens when it is given every day instead. They published the results in Frontiers in Neurology.
The researchers used male APP/PS1 mice, a widely used model of Alzheimer's disease, at 8 months old. Each mouse received 1 nanomole of neuropeptide S a day for 14 days, injected directly into the fluid-filled spaces of the brain. Mice in the comparison group received saline.
Less time awake and fewer plaques
Given this way, the peptide had the opposite effect to a single dose. Treated mice spent less time awake and more time in non-REM sleep, and their episodes of non-REM sleep were longer, in both the light and dark phases. Time in REM sleep did not change significantly.
In the hippocampus, a brain region central to memory, the treated mice had lower levels of amyloid precursor protein (APP) and its phosphorylated form, a smaller area stained for amyloid-beta, and fewer plaques per field than the saline group. Untreated APP/PS1 mice slept badly, with more fragmented non-REM sleep.
In a separate experiment on cultured microglia, the brain's immune cells, the peptide was associated with signs of autophagy, the cell's recycling process, and with ERK signalling. That hints at a possible mechanism, but the study does not show whether the peptide helped clear amyloid or reduced how much was made.
Why the results need care
The authors list several limits. Only male mice of one age were studied, with six animals per group for the sleep and tissue work. There was no group of healthy mice given the peptide, so it is unclear whether the sleep effect is specific to the disease model. The peptide was injected straight into the brain, which is a long way from any treatment that could be used in people. The cell experiments used an immortalised cell line, and the study has not been tested in patients.
Even so, the finding that the same peptide can promote or reduce wakefulness depending on how it is given is a useful reminder of how much the dose pattern matters in peptide research.
What the two studies share
In both, a small peptide turns out to do something other than its best known job: an antibacterial molecule that also helps remove old cells, and a wake-promoting molecule that, given daily, seems to help sleep. In both, the evidence so far comes from animals. The next steps will be replication by other groups and, if the effects hold, work in more complex models.