Babies are torn from their placental food supply when thrust into the world at birth, and new research shows that they break down parts of their own cells, presumably to sustain energy, until momma comes by with her milk.
Cells from many organisms recycle their parts by breaking down sugars, fats, and proteins in a process called autophagy ("self eating"). Yeast and worms have many genes--some of which get kick-started by starvation--that help them eat bits of their own cells. Yet researchers have found only one mammalian autophagy gene so far: Atg5. In starving adult mice, the gene's protein product shows up throughout the body, although to a lesser extent in the brain. Molecular biologist Noboru Mizushima of the Tokyo Metropolitan Institute of Medical Science in Japan and colleagues wondered whether newborns, who had just lost their food supply from their mothers' wombs, might use autophagy to keep up their energy until they started on mother's milk.
To investigate, they generated mice that lacked both copies of the Atg5 gene. Newborn pups appeared relatively healthy, but if they didn't start on milk, they died about twice as fast as normal starved newborns, suggesting that they lacked some nutrients that normal mice had. The team had also inserted a green glowing protein into the mice that lights up cells during autophagy. When the researchers examined various tissues in normal mice 3 hours to 2.5 days after birth, they found active autophagy in heart, diaphragm, and lung cells, but not in the brain. This activity did not occur in mice without Atg5, confirming that the gene played a key role in the newborns' autophagy, they report in the 4 November issue of Nature.
The study is the first to show in mammals that "autophagy is clearly connected with a normal physiological process, which is birth," says cell biologist Daniel Klionsky of the University of Michigan, Ann Arbor. In addition, the result shows that babies use protein reserves as well as fat right after birth, he says.
Noboru Mizushima's laboratory