Can I teach a depressed dog how to play with her toys?

[michelleandherguitar6391]

In November of 2007 I adopted a three year old chihuahua/miniature pinscher mix who had been previously abused and set out on the side of the road. When I first got the dog, she was terrified of everything and wouldn't go anywhere in the house without me. She wouldn't even eat unless I sat beside her. Now, she's getting a little better about getting along with and trusting the other members of our household, and our three other dogs, but I can't seem to get her to play or run or anything. She's very timid and easily frightened and she's always depressed, it seems. We've bought her toys, and the other three dogs in our house are very playful, but she hasn't seemed to pick up that it's OK to play. Every once in a while, she gets a burst of spunk and will run around chasing me outside, but it never lasts long, and shortly afterward she seems afraid again. What can I do to help her? I want her to be able to run and play like the rest of our dogs but it seems like she's afraid to.