NOVA2-mediated RNA regulation is required for axonal pathfinding during development.

The first step of producing a protein involves the DNA of a gene being copied to form a molecule of RNA. This RNA molecule can often be processed to create several different “messenger” RNAs (mRNAs), each of which are used to produce a different protein by a process known as alternative splicing. A class of proteins that bind to RNA molecules controls alternative splicing. These “splicing factors” ensure that the right protein variant is produced at the right time and in the right place to carry out the appropriate activity. Many genes that play important roles in the nervous system have been reported to undergo alternative splicing to generate different protein variants. However, it is unclear whether alternative splicing is important for controlling how the nervous system develops, during which time the neurons connect to the cells that they will communicate with. Forming these connections involves part of the neuron, called the axon, growing along a precise path through the nervous system to reach its destination. Two RNA-binding proteins called NOVA1 and NOVA2 are produced exclusively in the central nervous system, where they regulate a number of actions including alternative splicing. So far, no differences in the roles of NOVA1 and NOVA2 have been identified, and relatively little is known about their actions in the brain. Saito et al. have addressed these missing puzzle pieces by combining RNA analysis methods with an analysis of the structure of the nervous system of mice that lack either NOVA1 or NOVA2. This approach identified where NOVA1 and NOVA2 bind on mRNAs, and showed that the mRNAs are processed in different ways in the developing mouse brain depending on which form of the NOVA protein is bound to it. Further analysis of the data revealed that NOVA2, and not NOVA1, regulates splicing in a series of RNA molecules that help to guide axons to the correct locations in the developing mouse brain. A related study by Leggere et al. also reported on the role that NOVA proteins play in the alternative splicing of one of these genes, called Dcc. Saito et al. also found defects in the nervous systems of the mice that lacked NOVA2 that only occurred in these mice and resulted from certain axons being unable to follow the correct path to their target cells. These led to major defects, such as agenesis of the corpus callosum (a complete lack of connection between the right and left sides of the brain). Further defects affected how specific subsets of motor neurons connect to muscles and how cochlear neurons in the brainstem connect to the inner ear. The next steps are to explore how the processing of RNA molecules by NOVA2 causes these defects, and to assess whether these actions relate to developmental brain disorders in humans.