by Monya Baker
Transient expression of a single gene has lasting effects on others
Despite the bad publicity provided by One Flew Over the Cuckoo's Nest, electroconvulsive treatments can be effective for treating depression, and the procedure is known to cause adult mice to grow new neurons in their hippocampus. Now, researchers led by Hongjun Song at Johns Hopkins Medical School have found a mechanism through which this happens. A stimulus, in this case electrical activity, temporarily activates a gene whose protein product modifies DNA in such a way to affect the long-term expression of other genes.
"It is surprising to us that neuronal activity can induce changes of epigenetic DNA methylation that has been previously thought to be quite stable," says Song. The team began by looking for genes known to regulate gene expression that were also upregulated after electroconvulsive treatments. They identified Gadd45b, which is a member of a family of proteins believed to repair DNA as well as to perform a type of demethylation, in this case excising bases containing the chemical tags that designate silenced genes. Further work showed that Gadd45b was one of the so-called immediate early genes that prompt neurogenesis in the hippocampus.
Next, the team examined mice which lacked the gene for Gadd45b. Three days after ECT, they examined cells in the dentate gyrus, where neurogenesis is expected to originate. They first labeled DNA to get a sense of how rapidly cells were dividing. The density of labeled cells increased in both 'knockout' mice that did not express the Gadd45b and 'wildtype' mice that did. However, the wildtype mice showed a 140% increase in labeling, while the knockouts showed only 40%. A variety of more sophisticated experiments confirmed that Gadd45b was important for prompting neurogenesis after ECT, so the researchers began looking for a mechanism. Surprisingly, they found little difference in overall demethylation across the genome. However, demethylation was strongly observed at genes encoding two growth factors that are important in the brain, brain-derived neurotrophic factor and fibroblast growth factor-1. Chromatin immunoprecipitation to detect where in the genome GAdd45b binds DNA confirmed that the protein bound regulatory regions of the genes for these two growth factors.
Proteins that can demethylate specific regions of the genome have already been shown to be important regulatory factors for plants as well as mammalian cell cultures. This work now shows demethylation has a role in the adult brain. Though the researchers did not report behavioral tests on the animals, they note that altered expression of Gadd45b has been observed in some autistic persons. Even though GAdd45b expression itself is changed only for a short period, its effects on other genes that activate neurogenesis are much more lasting.
Tens of thousands of ECT procedures are conducted in the US every year, but no one knows why the technique works, says Karl Deisseroth, a Stanford professor, who studies how electrical activity helps new brain circuits form. The electrical excitation of cells in the hippocampus may drive the production of new neurons, he says, and other papers indicate that this involves calcium channels and depolarization in stem cell and progenitor populations. "While we still do not know precisely why ECT is so effective," says Deisseroth, "this elegant paper goes a long way toward elucidating the biochemical mechanisms that lead to the expression of excitation-neurogenesis coupling."
Song suspects that this mechanism will extend beyond the brain as well. "This is the first example of site-specific demethylation in changing stem cell behaviors through long-term modification of niche signals," he says. "It is very likely that other somatic stem cells in the adult tissue use similar epigenetic mechanisms to response to external stimuli."
Source: Nature