Dynamic axonal mRNA regulation by a major spliceosome protein

Neurons are specialised cells in our body that have long extensions (called axons) allowing them to form connections (called synapses) with other neurons, leading to the establishment of neuronal circuits. The main function of these circuits is to conduct signals that coordinate our bodily functions, thoughts, sensations, and perceptions of the world. Hence, changes in the way neurons are wired during development can lead to neurodevelopmental disorders such as autism spectrum disorder, intellectual disability, and Schizophrenia. Moreover, failure to properly maintain synapses throughout life often results in neurodegenerative conditions such as Alzheimer's and motor neuron disease. An essential requirement for any cell, including neurons, is to produce new proteins that are required for virtually all cell functions. Proteins are made up of long chains of amino acids linked together, and they are responsible for carrying out many crucial biological processes. Proteins come from the genetic information stored in the DNA. This information is transcribed into messenger RNA (mRNA), which is then translated into a chain of amino acids, forming a protein. Traditionally, it was believed that mRNAs were produced and processed in the cell nucleus and that fully processed mRNA molecules were transported from the nucleus to the cytoplasm for translation into proteins. However, recent research has revealed that mRNA processing can also take place in axons. This phenomenon is known as "local mRNA processing" or "axon mRNA processing". For instance, some mRNA molecules are transported from the cell body (where the nucleus is located) to the axon that can be millimetres or even centimetres away from the RNA-producing nucleus. Once mRNA molecules reach the axon, they can be locally translated into proteins. Local mRNA processing in axons plays a crucial role in neuronal connectivity and homeostasis by regulating axon growth and maintenance, respectively. Dysfunctions of local mRNA processing are known to play a fundamental role in the pathogenesis of many neurological conditions, however, how mRNA processing is regulated locally in axons remains a fundamental open question. Spliceosome proteins form a complex molecular machine responsible for processing mRNAs in the nucleus. These proteins work together in a highly coordinated manner to accurately identify splice sites, catalyse the removal of introns (non-coding regions) from pre-messenger RNA (pre-mRNA), and join exons (coding regions) to form mature mRNA. This process is essential to produce mature mRNAs that can be translated into functional proteins. Surprisingly, many spliceosome proteins can also be detected in axons and synapses, often far away from the mRNA producing nucleus. One such protein is SNRNP70, unexpectedly found to localise in motor neuron axons. Intriguingly, this extra-nuclear pool of SNRNP70 was shown to be functionally active, regulating motor connectivity. However, we know only very little about which axonal mRNA targets SNRNP70 binds to and what cellular processes these mRNAs control. Importantly, we don't know how local mRNA processing is regulated by SNRNP70 to control neuronal connectivity. In this research proposal, we will use a unique set of genetic tools to identify the mRNAs regulated by the axonal activities of SNRNP70. By using computational tools, we will also obtain a global picture of the precise SNRNP70 binding sites within the mRNA targets and determine the effects SNRNP70 depletion from axons has on mRNA metabolism. We will assess the physiological roles of SNRNP70 by performing imaging in the translucent zebrafish larval system or using neurons that grow outside the body in a culture dish. This project will provide new and significant insights into the regulation of axonal mRNA processing and enable us to better understand how axonal homeostasis and neuronal connectivity are influenced by local mRNA processing events.