The first breath switches off a growth signal in mouse retinal neurons: exhaling CO₂ suppresses the signal, while inhaling CO₂ helps injured nerve fibers regrow in adult mice, according to a preprint
The first breath switches off a growth signal in mouse retinal neurons: exhaling CO₂ suppresses the signal, while inhaling CO₂ helps injured nerve fibers regrow in adult mice, according to a preprint
On October 11, Zhigang He’s laboratory at Harvard Medical School posted a preprint, a manuscript that has yet to undergo peer review, on bioRxiv. In mice, mTOR, a signal that promotes cell growth, switches off within minutes of birth. In retinal neurons, this is driven primarily by exhaling CO₂: in the authors’ model, the cell becomes more alkaline, activating SNRK, an enzyme that inhibits mTOR. In adult mice, inhaling CO₂ or deleting the Snrk gene restored mTOR activity and helped injured axons, the long extensions of neurons, regrow.
Injured axons in the adult brain and optic nerve usually fail to regrow. In 2008, He’s laboratory showed in Science that disabling PTEN, a gene that inhibits mTOR, in retinal ganglion cells of adult mice allows their axons to regrow after optic nerve injury. These cells’ axons form the optic nerve. mTOR activity in these cells falls sharply after birth, and the preprint examined whether birth itself switches it off.
The researchers measured mTOR activity using a phosphate tag on the S6 protein, which is present while mTOR is active. Before birth, most of these cells carried the tag; five minutes after birth, it had almost disappeared. The tag also declined within five minutes in the brain, heart, and limb muscles. Cesarean delivery allows researchers to separate the loss of maternal nutrient supply from the onset of breathing: a pup receives nutrients through an intact umbilical cord but cannot breathe air while enclosed in the amniotic sac. Pups that breathed lost the tag within five minutes, whether their umbilical cord was intact or severed, whereas mTOR remained active for up to 30 minutes in pups still enclosed in the sac. These results indicate that the onset of breathing switches off mTOR.
The researchers looked for a link between breathing and mTOR among kinases in the AMPK family, enzymes that attach phosphate tags to proteins and inhibit mTOR. A screen from the same laboratory pointed in the same direction: among more than a thousand genes individually disabled in adult mice with optic nerve injury, SNRK, a kinase in this family, emerged as the main inhibitor of regeneration. In newborn mice whose retinal neurons lacked the Snrk gene, the tag remained at its prenatal level five minutes after birth.
The researchers tested the sequence of events in live newborn mice by observing, through the pupil, a sensor in retinal neurons that responds to active AMPK-family kinases. Its signal began to rise 2–3 minutes after breathing started, before the mTOR tag disappeared. Without SNRK, most of this rise was absent.
In air containing 10% CO₂, kinase activity did not rise in newborn mice, and the mTOR tag persisted. Air containing 10% oxygen had neither effect. The usual triggers for activating these kinases seemed unlikely to explain the findings: the cellular energy supply (ATP) remained unchanged for ten minutes, and calcium rose only after kinase activity had increased. Acidity remained a possible explanation: CO₂ acidifies the cell, and exhaling it reduces that acidification. A pH sensor showed that neurons became more alkaline within a minute of the onset of breathing, whereas pH remained low in 10% CO₂.
In adult mice, the changes ran in the opposite direction: in air containing 10% CO₂, retinal neurons became more acidic within two minutes, kinase activity fell after an hour, and mTOR switched on after four hours. After 48 hours, about 70% of cells carried the tag, compared with a minority in ordinary air. Ten mice lived in 5% CO₂ for a week before optic nerve crush and for two weeks afterward. More axons regrew in these mice than in seven mice kept in ordinary air. Deleting Snrk also increased axon regrowth.
The authors suggest that rapidly switching off mTOR helps cells adapt to life outside the uterus, “but at the cost of reduced regenerative potential.” Because this switch can be reversed, they propose the possibility of a simple therapy that adjusts CO₂ levels and acidity without incisions or injections.