RNAi, or RNA interference, is a rapidly developing and powerful tool to achieve gene silencing (turning a gene “off”). Gene silencing shows what happened in a system or organism when that gene is no longer functional. In a recent study, described in a story in Technology Review, female mice lose all interest in sex when a specific gene in the brain is silenced.
(More, including a video of the un-horny mice, below the fold!)
The mating behavior of female mice is heavily influenced by the hormone estrogen–up-regulation of estrogen provokes “lordosis,” where the females arch their spines in preparation for mating. The estrogen receptor alpha (ER-alpha) binds estrogen in the blood, triggering lordosis. Transgenic mice who lack ER-alpha do not exhibit lordosis and reject the overtures of male mice attempting to mate. However, these knock-out mice lack ALL ER-alpha receptors, therefore its difficult to pinpoint the place in the brain where the ER-alphas really matter. that’s where RNAi comes in handy.
The study was published online in this week’s PNAS by Ogawa et al, and attempted to use targeted RNAi to just known down ER-alpha gene expression in the hypothalamus. This region is thought to play a large role in female sexual behavior in mice. Following delivery of the ER-alpha RNAi, the researchers injected the mice with estrogen to cause them to go into heat. A fascinating movie of these mice demonstrates that the treated mice (bottom cage in the movie) have no sexual response–no lordosis, and they rejected the male’s attempts to mount by running and kicking. On the other hand, untreated normal females (top cage) allowed males to mate.
One of the reasons that RNAi has not been widely used in the brain is due to the technical challenges of getting the RNA into the proper cells, and then getting the RNA to be reliably expressed by those cells. The study used a virus which can easily infect cells but does not cause an immune response to deliver the DNA sequences. This method is essentially the exact same way my lab delivers genes into the inner ear. A virus is modified to make a cell replicate a genetic sequence of interest. In the “wild” the virus would make the cell express more viral DNA, but the modified virus has been “tricked” into expressing what we WANT it to express. In this study, the authors engineered the virus to express a small hairpin of RNA whose action is to silence the gene that express ER-alphas. Since they delivered the virus directly to the hypothalamus, it stayed localized in that region.
(Shout-out to Edny for the tip!)