mTOR

mTOR is a protein that senses whether nutrients are plentiful and switches the cell between building new material and recycling what it already has. When food is abundant it favours growth; when food is scarce it steps back and lets the cell break down and reuse its own components.

The protein is named after the drug that revealed it. mTOR stands for the mechanistic target of rapamycin, because rapamycin was found first and the protein was identified as the thing rapamycin acts on.

Its place in aging research comes from a consistent finding across species. Suppressing this signalling pathway, by genetic means or with a drug, extends lifespan in yeast, nematodes and fruit flies. Whether the same held in a mammal was an open question until 2009, when rapamycin was shown to extend both median and maximal lifespan in mice.

That consistency is the strongest argument for the pathway mattering, and it is also where the argument stops. A mechanism shared from yeast to mice is a good reason to look in humans. It is not evidence of what happens there, and mTOR is involved in immune function, wound healing and metabolism as well as in growth, so suppressing it is not a single-purpose act.