mRNA compartmentalisation spatially orients tissue morphogenesis

Polarised targeting of diverse mRNAs to motile cellular protrusions is a hallmark of cell migration1-3. Although a widespread phenomenon, definitive functions for endogenous targeted mRNAs and their relevance to modulation of in-vivo tissue dynamics remain elusive. Here, using single-molecule analysis, endogenous gene-edited mRNAs and zebrafish in-vivo live-cell imaging, we report that mRNA polarisation acts as a molecular compass that orients motile cell polarity and spatially directs tissue movement. Clustering of protrusion-derived RNAseq datasets defined a core 192 bp localisation element underpinning precise mRNA targeting to incipient sites of filopodia formation at cell protrusions. Such targeting of the small GTPase, RAB13, generated tight spatial coupling of mRNA localisation, translation and protein activity, achieving precise subcellular compartmentalisation of RAB13 protein function to create a polarised domain of filopodia extension. Consequently, genomic excision of this localisation element and specific perturbation of endogenous RAB13 targeting - but not translation - depolarised filopodial dynamics in motile endothelial cells and induced miss-patterning of nascent blood vessels in-vivo. Hence, mRNA polarisation, not expression, is the primary spatial determinant of the site of RAB13 action, preventing ectopic functionality at inappropriate subcellular loci and orienting tissue morphogenesis. Considering the unexpected spatial diversity of other polarised mRNA clusters we identified, mRNA-mediated compartmentalisation of protein function at distinct subcellular sites likely coordinates broad aspects of in-vivo tissue behaviour.