Final Report Summary - GUIDANCE-MIR (microRNAs: ROLES IN AXON GUIDANCE DURING BRAIN WIRING) State-of-the art:The precise elaboration and specification of neuronal circuits is at the basis of any properly functioning nervous system. For circuits to be established, neurons form remarkably accurate connections with their target cells during development. How are these connections established? Neurons send out cell protrusions called axons, which navigate in a complex environment to reach their exact target, a process known as axon guidance or pathfinding. Understanding the key molecules that induce the formation of such precise circuits is crucial, because any failure in this process, either during development or following injury or disease, impairs the proper function of the nervous system. Such precision is achieved by tight regulation, in space and time, of guidance molecules and their concomitant signalling mechanisms. However the key molecules involved in this highly accurate regulation are to date largely unknown. Very recent findings by the fellow and others have suggested that specific microRNAs (miRNAs) could be involved in this process. However the possibility that miRNAs play a crucial role in axon guidance is to date largely unknown. Project Objectives:The fellow and her group have thus investigated whether miRNAs are important in the establishment of neuronal circuits using Xenopus retinotectal projection (composed of RGC axons) as a model, using a broad arrays of techniques and multidisciplinary approach pertaining to the fields of Developmental Biology, Neuroscience, RNA regulation and Bioinformatics. Results and main conclusions:The fellow, her group and collaborators have profiled which miRNAs are present in Xenopus RGCs during the period of axon guidance and found that numerous miRNAs are expressed in this projection neuron. They, next, investigated whether the most abundant miRNAs are important in axon guidance. They reveal that their loss-of-function approach lead to aberrant projections and behavioural impairments, indicating that miRNAs play a key role for neuronal circuit formation. They, finally, dissected out the mechanisms of action that these miRNAs employ to regulate this crucial developmental process. They revealed that miRNAs act locally to ensure that proteins are produced at the right time and place specifically within the axon. Collectively, their data reveal that miRNAs are key players in axon guidance and ensure the high degree of accuracy required in this process by fine-tuning the local production of proteins. Impact: The accomplishment of these objectives has helped understand the differential roles that miRNA-regulated pathways have in the elaboration of neuronal circuits in particular, and in postmitotic neurons in general. It also provided crucial background knowledge for the study of miRNAs in brain wiring of higher vertebrates including mammals where such functional screen-based study could not be performed nearly as easily. In addition, rewiring of central nervous system neurons is to date impossible to induce, following neuronal degeneration or injury. Finding new innovative approaches, possibly based on newly discovered molecular players is therefore of paramount important. The key findings of this project suggest that miRNAs, which are also emerging as new tools to treat difficult disease, could constitute novel therapeutic targets to promote rewiring. final1-guidancemir-graphicalabtract.png
