Abstract
Formation of functional myelin sheaths within the central nervous system depends on expression of myelin basic protein (MBP). Following process extension and wrapping around axonal segments, this highly basic protein is required for compaction of the multi-layered membrane sheath produced by oligodendrocytes. MBP is hypothesized to be targeted to the membrane sheath by mRNA transport and local translation, which ensures that its expression is temporally and spatially restricted. The mechanistic details of how this might be regulated are still largely unknown, in particular because a model system that allows this process to be studied in vivo is lacking. We here show that the expression of the zebrafish MBP orthologs, mbpa and mbpb, is developmentally regulated, and that expression of specific mbpa isoforms is restricted to the peripheral nervous system. By analysis of transgenic zebrafish, which express a fluorescent reporter protein specifically in myelinating oligodendrocytes, we demonstrate that both mbpa and mbpb include a 3’UTR sequence, by which mRNA transport and translation is regulated in vivo. Further functional analysis suggests that: (1) the 3’UTRs delay the onset of protein expression; and that (2) several regulatory elements contribute to targeting of the mbp mRNA to the myelin sheath. Finally, we show that a pharmacological compound known to enhance neuronal activity stimulates the translation of Mbp in zebrafish in a 3’UTR-dependent manner. A similar effect was obtained following stimulation with a TrkB receptor agonist, and cell-based assays further confirmed that the receptor ligand, BDNF, in combination with other signals reversed the inhibitory effect of the 3’UTR on translation.
Highlights
Myelin is a specialized multi-layered membrane structure, which ensheaths neuronal axons
The expression of different mbpa isoforms was assessed by RT-PCR, with RNA extracted from zebrafish embryos and larvae at different developmental stages as template (Figure 1B)
The onset of mbpa expression is in agreement with previously published data (Brösamle and Halpern, 2002; Buckley et al, 2010; Nawaz et al, 2013), and matches the timing of myelination in the zebrafish central nervous system (CNS), which is detectable by electron microscopy at 3 days post fertilization (Buckley et al, 2010)
Summary
Myelin is a specialized multi-layered membrane structure, which ensheaths neuronal axons. Increasing evidence of other important functions of MBP in myelinogenesis has emerged more recently. These include a role in regulating the composition and organization of the myelin membrane (Fitzner et al, 2006; Aggarwal et al, 2011; Steshenko et al, 2016), reorganization of the actin cytoskeleton during the myelination process (Zuchero et al, 2015), and a suggested role in cell signaling pathways through interactions with SH3 domaincontaining proteins (Harauz et al, 2009)
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