A new study in Cell shows that multipotent stem cells are committed to a neuronal lineage through the novel regulatory action of small, double-stranded RNAs (dsRNAs).

Small RNAs have received a lot of attention recently owing to their involvement in RNA interference, the process by which snippets of RNA bind specifically to target mRNAs and prevent their translation into protein. Now, Fred Gage and colleagues have discovered a new mode of action of dsRNAs — activating gene expression by interacting directly with proteins and DNA.

The discovery was made while investigating the molecular mechanisms that regulate neuron-specific gene expression. In a screen for small, non-coding RNAs that might participate in the differentiation of neurons, Gage's team isolated a short dsRNA whose sequence matched that of neuron restrictive silencer element/RE1 (NRSE/RE1) from adult hippocampal neural stem cells. NRSE/RE1 is a conserved DNA response element that is present in genes that code for neuronal proteins, including ion channels and neurotransmitter receptors. In non-neuronal cells, expression of neuronal genes is prevented when the NRSE/RE1 element in their promoters binds to neuronal restricted silencing factor/RE1 silencing transcription factor (NRSF/REST). This zinc finger protein mediates its repressive effect on gene expression by recruiting negative transcriptional regulators such as deacetylases.

The authors used viral vectors to introduce NRSE/RE1 dsRNAs into adult hippocampal stem cells. This transfection caused morphological changes consistent with the differentiation of neurons, such as the extension of processes. The expression of neuron-specific genes that contain NRSE/RE1 in their promoters (including synapsin 1 and mGluR2) increased in these cells. So NRSE/RE1 dsRNAs seem to have a crucial role in the acquistion of neuronal cell fate by counteracting the repressive action of NRSF/REST.

Do NRSE/RE1 dsRNAs exert their effect by silencing the NRSF/REST gene through RNA interference? As expression of NRSF/REST was unaffected in cells transfected with NRSE/RE1 dsRNAs, the authors concluded that this was not the case. Support for an alternative mechanism came from experiments in which stem cell extracts were incubated with biotin-labelled NRSE/RE1 dsRNAs. Immunoblots of biotin-positive conjugates and titration analysis revealed that NRSE/RE1 dsRNA binds NRSF/REST in a highly-specific manner.

So, a model is emerging whereby cells that are to become neurons activate the transcription of genes that contain the NRSE/RE1 sequence. These cells might then generate non-coding NRSE/RE1 dsRNAs that interact with the NRSE/RE1 DNA response element and NRSF/REST, switching this transcription factor from repressor to activator by disrupting its association with negative transcriptional regulators. No doubt many more examples of transcriptional regulation by other small modulatory RNAs of this type will come to light in the near future.