Mouse models are essential tools in the quest to understand human diseases, but fine spatial and temporal control of gene expression are difficult to achieve with traditional transgenic and knockout techniques. The potential of post-transcriptional gene silencing, or RNA interference (RNAi), to overcome these problems in the mammalian brain has been explored by Ralph DiLeone and his team from The University of Texas Southwestern Medical Center at Dallas. Their data, published in the December issue of Nature Medicine, show that RNAi-mediated knockdown of the dopamine-synthesis enzyme tyrosine hydroxylase in the midbrain generates mice with a Parkinson's disease-like phenotype.

RNAi is based on a highly conserved defense mechanism in which short, double-stranded RNA fragments are enzymatically snipped from foreign genetic material and subsequently disrupt the translation of their messenger RNA of origin. DiLeone et al. designed hairpin-shaped RNA fragments to specifically target two unique 24-nucleotide stretches of the tyrosine hydroxylase mRNA. These fragments were co-expressed with enhanced green fluorescent protein (EGFP) in an adeno-associated viral vector, which was then injected into the midbrain of adult mice.

Twelve days after injections to the substantia nigra pars compacta, immunostaining and quantitative real-time reverse transcription-polymerase chain reaction showed that expression of tyrosine hydroxylase was markedly reduced in infected dopamine neurons (that is, those neurons positive for both EGFP and dopa-decarboxylase, a marker of dopamine neurons). This attenuation of expression persisted for at least 50 days. Levels of tyrosine hydroxylase in regions of the nucleus accumbens innervated by infected midbrain neurons were also reduced. By contrast, high levels of tyrosine hydroxylase expression were maintained in control neurons infected with a vector containing 'scrambled', non-tyrosine hydroxylase-specific short RNA fragments.

Importantly, interfering with tyrosine hydroxylase expression correlated with impaired motor function, as assessed by the rotarod test and a reduced response to amphetamine. This phenotype mimics that of neurotoxin-induced models of Parkinson's disease that show neurodegeneration of the substantia nigra, indicating that loss of dopamine in this brain region is sufficient to induce aberrant motor behaviour. So, DiLeone and colleagues succeeded in developing a potentially useful model for studying Parkinson's disease, while demonstrating the general utility of RNAi as a fast and effective means of probing the molecular mechanisms of disease.