The gain-of-function transgenic approach can provide valuable insights into gene function, but controlling when and where a transgene is expressed remains a considerable challenge, because promoters that are truly tissue specific are hard to find. An alternative tactic is to target DNA to specific regions using viral vectors, but it is difficult to restrict the spread of infection. To address these problems, Saito and Nakatsuji have combined in utero and exo utero surgical techniques with an electroporation-based gene-transfer system, and have managed to achieve stable, targeted transgene expression in the embryonic mouse brain.

Electroporation — the application of a pulse of electric current to make cells transiently permeable to large molecules — is widely used to introduce DNA into cells in vitro, and has also been used successfully in vivo in chick and cultured mouse embryos. However, because the mouse embryo can be maintained in culture for only a few days, it has not been possible to examine the long-term effects of transgene expression in this model.

In this new study, reported in Developmental Biology, Saito and Nakatsuji electroporated DNA constructs into the brains of mouse embryos without removing them from the uterus. The DNA was injected into the appropriate region using a micropipette, then an electric pulse was applied using forceps-like electrodes. Beyond 13.5 days post coitum, the brain was clearly visible through the uterine wall, but for younger embryos, it was accessed by exo utero surgery. The DNA was injected into one side of the brain only, so that the other side could act as a control. More than 90% of the embryos survived, and in many cases, transgene expression was maintained for at least six weeks after electroporation. In addition, the authors were able to introduce several constructs into the same cell simultaneously.

Another team has used a similar in utero electroporation protocol to label neurons and track their migration in the developing mouse brain, but Saito and Nakatsuji went one stage further by showing that the technique can be used to reveal gene function. They injected constructs that expressed genes for either Hes1 or a constitutively active form of Notch1, both of which are inhibitors of neurogenesis. In both cases, neuronal differentiation was suppressed around the site of injection, indicating that the transgenes were functioning normally.

Saito and Nakatsuji showed that their system can accurately target and restrict the expression of transgenes in the developing mouse brain. In these preliminary experiments, the genes were driven by ubiquitous promoters, but the authors speculate that by using region- or cell-type-specific promoters, they might be able to target expression even more precisely in future.