Molecular mechanisms regulating mRNA transport and local translation in neurons.

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The developing human brain must create short- and long-distance connections between dendrites and axons in a highly regulated fashion. The creation of new dendrites and the development and differentiation of synapses are an essential part of this process and require, in turn, the local translation of a large set of mRNAs. There is growing evidence that up to thousands of mRNAs are differentially translated in the dendritic compartment, thus making local translation a general and key process in neuronal development. Mistakes in the regulation of local mRNA translation lead to a range of developmental diseases and neuro-pathologies.Local mRNA translation requires the selective transport and locally regulated translation of the mRNAs and is mediated by multi-functional, multi-domain RNA-binding proteins that recognise sequences or structures in the mRNAs and act as adaptors for the molecular motors and the cellular degradation and translation machineries. Despite several of these proteins having been identified, we know very little of how they target the cognate mRNAs and interact with the cellular machineries at the molecular level. Also we know very little of how these proteins are regulated by specific signals.We work on IGF2 mRNA Binding Protein 1 (IMP1, also called Zipcode Binding Protein 1, ZBP1) as a paradigm for the RNA-binding proteins regulating local mRNA translation. IMP1 is a multi-functional, multi-domain RNA-binding protein that plays a key role in defining synaptic morphology in neurons and has a general function in regulating cell motility and differentiation. Functional information in fly, worm, chicken and mammals has shown that IMP1 regulates the transport and local translation of a number of different mRNAs, and has linked the protein to the transport of specific mRNAs (e.g. beta-actin) and to a well-defined regulatory mechanism that promotes local mRNA translation in response to signalling in neurons.The questions we are asking are how IMP1 and other protein regulators recognise a diverse set of RNA targets, and how RNA recognition is linked to mRNA transport. We want to know how the RNA-binding proteins interact with the cellular mRNA transport machineries and how their functions are regulated by signalling at the molecular level. In the longer term, we want to obtain a broader understanding of the function of these proteins in local mRNA translation, that include their capability to localizing multiple, functionally related, targets, as a prelude to study the synergies between the locally translated proteins. We will use structural and biophysical techniques to answer these questions in the IMP1 system and determine the molecular rules of IMP1 target recognition and of its regulation by signalling. Further, we will characterise IMP1 protein and RNA partners in neurons and use in cell transcriptome-wide assays to look at how IMP1 achieves the selection of the RNA targets in the cell, and to understand the RNA binding and re-modelling of the RNA is linked to the functional output. This work will provide a unique structural and molecular analysis of the functional interactions mediating local mRNA translation in mammals. The output will be used to inform the investigation of the function of IMP1 in processes linked to neuronal development and function, for example in dendritic arborisation and in the changes in synaptic morphology. Importantly, our understanding of IMP1 will provide tools and concepts for investigating other RNA-binding proteins with a vital role in neuronal functioning (e.g. Syncrip, FMRP and TDP43) that are linked to widespread and severe neuro-pathologies.