Trafficking and translation of mRNA in osteocyte dendrites

Abstract

Osteocyte dendrites extend great distances through canaliculi in bone supported by an extensive cytoskeletal network. The formation and maintenance of osteocyte dendrites is closely linked to bone health, yet the mechanisms regulating these processes are poorly understood. Here we tested the hypothesis that osteocytes transport mRNA to their dendrites for local translation as a mechanism of regulating the formation and maintenance of these subcellular structures. Using molecular and imaging approaches, we demonstrated that a subset of mRNAs are enriched in Ocy454 osteocyte-like cell dendrites. To understand how certain mRNAs are directed to dendrites, we performed a massively parallel reporter assay with 7115 oligonucleotide sequences designed from the 3 untranslated regions of eight dendrite-enriched transcripts. We identified localization sequences within some 3UTRs that were both sufficient and necessary for trafficking to osteocyte dendrites, indicating that localization directions are specifically encoded in enriched transcripts. Furthermore, we found that dendrites contain the elements necessary for local translation, and trafficked transcripts were engaged in active translation within dendrites. Together this work describes a molecular regulatory mechanism novel to osteocyte biology with many potential consequences for the spatiotemporal control of the cytoskeleton and dendrites.