In frigid temperatures, mice ramp up the production of heat-generating brown fat by decreasing the levels of a small RNA molecule.

Because brown fat burns energy — unlike its unpopular cousin, white fat — its production is an attractive target for obesity and diabetes therapies. Markus Stoffel at the Swiss Federal Institute of Technology in Zurich and his colleagues found that exposure to cold reduced the expression of microRNA-133 in brown and subcutaneous white fat. Inhibiting miRNA-133 promoted brown-fat formation, whereas forcing miRNA-133 expression switched off brown-fat production.

The small RNA acted by directly inhibiting PRDM16, a protein that is central to the production of brown fat from white-fat-cell precursors.

Nature Cell Biol. http://dx.doi.org/10.1038/ncb2612 (2012)