Autoimmune diseases ought to sound scary- That's what AIDS is.

What we know about Mitochondria:
We've only just begun to understand mitochondria. We know that they produce the ATP responsible for keeping us alive. We know that they don't produce it completely efficiently, because they also create a by-product of free radicals- oxygen ions that attach to molecules they shouldn't, thus causing cell deterioration, which causes aging. (The greatest irony of life- that which we need most to survive is also that which is slowly killing us.)

We know that mitochondria have their own DNA, separate from ours, and while you can identify a person via mitochondrial DNA, it's no where near as accurate. It has been theorized that mitochondria may in fact be a separate organism that evolved with us into a symbiotic relationship, however this theory is as yet unsupported.

As a side note, mitochondria are the inspiration for George Lucas's midichlorines (sp?), the tiny organisms that connect all living things to the Force. Cool, huh?

Mitochondrial related illnesses are extremely difficult to treat because the nature of mitochondria is still somewhat of a mystery.

It's possible that less efficient mitochondria may contribute to or even cause depression. It's also possible that the reverse is true. All such research needs to be repeated and tweaked on a wide scale with the same result before either can be confirmed.

Also, metabolism and mitochondria are only mildly related. The rate of metabolism is the net result of all chemical reactions occurring in your body at a given moment. The reactions in your mitochondria are only one part.

And here I thought my studies in genetic engineering would never come into use. *shrug*