King’s College London: Scientists identify novel essential non-nuclear roles of spliceosome protein during neuronal connectivity

A new paper published today in Current Biology presents evidence of a novel and location-specific role of the spliceosome protein SNRNP70. The Houart Group in the Centre for Developmental Neurobiology found that non-nuclear SNRNP70 regulates biological events that control motor function by modifying the diversity of axonal mRNAs.

RNA processing, transport and translation are central processes in normal development and functioning of neurons. RNA processing involves a critical step called splicing to remove unwanted parts of the RNA. This step occurs in the cellular machines termed spliceosomes. Afterwards, the spliced RNA is transported out of the nucleus and translated into a functional protein.

These processes are regulated by RNA-binding proteins (RBPs), proteins that interact with RNAs inside and outside the nucleus. Defects in RBPs cause abnormal RNA metabolism and transport. Many RBPs participate in the regulation of splicing within the nucleus, however, some of these splicing regulators are also present outside the nucleus.

SNRNP70 is one of the RBPs that interact with the major spliceosome in the nucleus. Its non-nuclear role, however, was not understood until now. It has been associated with various neurodegenerative diseases such as Alzheimer’s Disease and Amyotrophic Lateral Sclerosis.

I was always fascinated by the stereotypical manner neurons establish connections during development, which to a large part depends on the correct RNA localisation and local protein synthesis. These processes occur far from the RNA-producing nucleus. The initial observation that a major RNA splicing protein can also be found outside the nucleus was the trigger I needed to investigate its non-nuclear roles’
– Dr Nikolas Nikolaou, lead author of this study.
Using zebrafish model, the scientists in Houart Group first sought to establish the distribution of SNRNP70 outside the nucleus. They found that SNRNP70 is present not only in the cell body but also in the neuron’s axon, a long extension responsible for transporting information to other cells, where it associates and moves along with RNAs.

The group generated a mutant line to eliminate SNRNP70 protein from all cells and found that young embryos and larvae have motor neuronal connectivity defects and motor function loss. They then restored the expression of SNRNP70 only within non-nuclear areas. They found that non-nuclear SNRNP70 is sufficient for the clustering of the chemical receptors AChR, which is a crucial process in establishing neuron-muscle connectivity.

Since SNRNP70 is both an RNA-binding and a splicing protein, the group examined whether non-nuclear SNRNP70 is involved in RNA processing. Upon loss of SNRNP70, they observed changes of expression in thousands of genes, many of them with roles in motor function. Surprisingly, the restoration of non-nuclear SNRNP70 was enough to rescue the expression of a significant number of these genes. Focusing on one of the rescued genes, rab1bb, the researchers learned that non-nuclear SNRNP70 regulates not only the abundance of transcripts, potentially through increased stability as well as decreased degradation but also their trafficking across axons.

In the nucleus, SNRNP70 regulates normal and alternative splicing – the latter allowing an RNA to be spliced in non-traditional ways to create different versions of a protein. It is widely believed that splicing can only happen in the nucleus, however, there have been increasing evidence to challenge this notion. The researchers investigated whether SNRNP70 has the same regulatory role when located outside the nucleus. They discovered that non-nuclear SNRNP70 modulates the alternative splicing of genes associated to neuronal development and connectivity.

These findings open many exciting questions regarding the complex roles spliceosome proteins play in axons and how these are affected in developmental and age-related neurological disorders. Local regulation of mRNAs in axons and dendrites is more complex than we ever imagined, and the next few years will reveal many amazing molecular mechanisms allowing neurons to take fast local decisions robustly.
– Professor Corrine Houart, last author of this study.