From the article:

Levels of brain-derived neurotrophic factor (BDNF) stop declining when Huntington’s disease transgenic mice are housed in an enriched environment, the scientists say. BDNF promotes neuron growth and survival and can also regulate communication between neurons.

“The finding that environmental enrichment increases BDNF, and that this slows disease progression, provides a potential mechanism for the effects of environmental enrichment on Huntington’s disease,” says M. Flint Beal, chair of neurology at Cornell University Medical College in New York.