For greater context, note that this is a Review article (that is to say, not an article describing new research, but rather an article synthesizing many recently published results and hypothesizing a theory that could unite them). It's part of a special issue that Science is doing this week that focuses on depression. See http://www.sciencemag.org/content/338/6103/67.full (not pay-walled) for a discussion of the various depression-centric papers that are in this issue.