Mind your Immune System
Another significant piece in the mind-body puzzle comes from this new study where obsessive-compulsive behavior in mice was cured by a bone marrow transplant.
A rare form of a genetic disorder in mice causes a “hair pulling” disorder, very similar to its human counterpart trichotillomania. In their new findings published in Cell, Mario Capecchi and his team at Salt Lake City, Utah found that the basis of this psychological aberration was a reduced population of microglia, which are the immune system cells in the brain. These cells have been long known to be the brain’s scavenger system, playing a vital role in clearing breakdown products and microbes, but its surprising that its depletion leads to a specific form of behavioral disorder.
The genetic mutation responsible was pinpointed to the Hox8 gene, which belongs to a family of genes that determine body plan and architecture in all vertebrates, apart from regulating development and growth of organs. The brain microglia cells are thought to originate from the bone marrow, and are the only brain cells that express this gene — thus they are thought to play a key role in the brain’s development.
The study found that if bone marrow containing Hox8 expressed stem cells (early forms which give rise to microglia) were transplanted to affected mice, their hair pulling disorder was cured within four months. By contrast when bone marrow from affected mice was transplanted into normal ones, the disorder appeared in normal ones.
Apart from the fact that this is the world’s first reported behavior transplant, this finding is an important landmark in our understanding of the genetic basis of behavior. To what extent are our behaviors pre-determined by our immune system and our mine interconnected? To what extent does a malfunction of one lead to problems with the other?
To quote Capecchi,
We know a lot more about the immune system than we know about our brain. We know almost nothing about how the brain works and less about how drugs work. If we say the immune system is important, this opens up a whole new vista of things we can do simply because we know more about the immune system.
Chen SK, Tvrdik P, Peden E, Cho S, Wu S, Spangrude G, & Capecchi MR (2010). Hematopoietic origin of pathological grooming in Hoxb8 mutant mice. Cell, 141 (5), 775-85 PMID: 20510925
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