Identification of 3′ UTR motifs required for mRNA localization to myelin sheaths in vivo

Abstract

Myelin is a specialized membrane produced by oligodendrocytes that insulates and supports axons. Oligodendrocytes extend numerous cellular processes, as projections of the plasma membrane, and simultaneously wrap multiple layers of myelin membrane around target axons. Notably, myelin sheaths originating from the same oligodendrocyte are variable in size, suggesting local mechanisms regulate myelin sheath growth. Purified myelin contains ribosomes and hundreds of mRNAs, supporting a model that mRNA localization and local protein synthesis regulate sheath growth and maturation. However, the mechanisms by which mRNAs are selectively enriched in myelin sheaths are unclear. To investigate how mRNAs are targeted to myelin sheaths, we tested the hypothesis that transcripts are selected for myelin enrichment through consensus sequences in the 3′ untranslated region (3′ UTR). Using methods to visualize mRNA in living zebrafish larvae, we identified candidate 3′ UTRs that were sufficient to localize mRNA to sheaths and enriched near growth zones of nascent membrane. We bioinformatically identified motifs common in 3′ UTRs from 3 myelin-enriched transcripts and determined that these motifs are required and sufficient in a context-dependent manner for mRNA transport to myelin sheaths. Finally, we show that 1 motif is highly enriched in the myelin transcriptome, suggesting that this sequence is a global regulator of mRNA localization during developmental myelination.

Translation-dependent mRNA localization toCaenorhabditiselegans adherens junctions

“…Substitution of both the 5' and 3' UTRs with UTRs of unlocalized mRNAs did not disrupt association of dlg-1 mRNA with the CeAJ. This result mirrors orthologues of dlg-1 in other species, which also do not require their UTRs for subcellular localization (Yergert et al, 2021). Thus, dlg1 mRNAs are frequently localized within cells, but the mechanism of UTR-independent targeting is unknown in any species.…”

“…In zebrafish, instead, neuronal dlg1 mRNA localizes stably in myelin sheets of fully differentiated oligodendrocytes (Yergert et al, 2021). These data suggest that dlg1 mRNA localization may have pivotal roles in development.…”

Translation-dependent mRNA localization to Caenorhabditis elegans adherens junctions

“…For example, Drosophila dlg1 mRNA associates transiently to membranes during embryogenesis (mitotic cycle 14): laterally at stage 5 (cellularization) when membranes start to form around nuclei, all around the cell membrane at stages 6 and 7 (cellularization and gastrulation), and it becomes unlocalized at later stages (Lecuyer et al, 2007). In zebrafish, neuronal dlg1 mRNA localizes stably in myelin sheets of fully differentiated oligodendrocytes (Yergert et al, 2021). These data suggest that dlg1 mRNA localization may have pivotal roles in development.…”

Glial Cells Promote Myelin Formation and Elimination

“…One mechanism that occurs during OPC differentiation is the translocation of hundreds of mRNAs into premyelinating processes, presumably to enable local translation of proteins relevant for sheath maturation ( Thakurela et al, 2016 ). The archetypal mRNA studied in this process is Mbp mRNA, which translocates into myelinating processes both in culture and in vivo ( Ainger et al, 1993 ; Herbert et al, 2017 ; Yergert et al, 2021 ). MBP is classically known for driving myelin compaction ( Readhead et al, 1987 ) and has recently been implicated in actin disassembly during myelin wrapping ( Zuchero et al, 2015 ), raising the possibility that locally translated MBP is integral to wrapping.…”