Axonal localization of neuritin/CPG15 mRNA is limited by competition for HuD binding

Abstract

HuD protein (also known as ELAVL4) has been shown to stabilize mRNAs with AU-rich elements (ARE) in their 3' untranslated regions (UTRs), including <i>Gap43</i>, which has been linked to axon growth. HuD also binds to neuritin (<i>Nrn1</i>) mRNA, whose 3'UTR contains ARE sequences. Although the <i>Nrn1</i> 3'UTR has been shown to mediate its axonal localization in embryonic hippocampal neurons, it is not active in adult dorsal root ganglion (DRG) neurons. Here, we asked why the 3'UTR is not sufficient to mediate the axonal localization of <i>Nrn1</i> mRNA in DRG neurons. HuD overexpression increases the ability of the <i>Nrn1</i> 3'UTR to mediate axonal localizing in DRG neurons. HuD binds directly to the <i>Nrn1</i> ARE with about a two-fold higher affinity than to the <i>Gap43</i> ARE. Although the <i>Nrn1</i> ARE can displace the <i>Gap43 ARE</i> from HuD binding, HuD binds to the full 3'UTR of <i>Gap43</i> with higher affinity, such that higher levels of <i>Nrn1</i> are needed to displace the <i>Gap43</i> 3'UTR. The <i>Nrn1</i> 3'UTR can mediate a higher level of axonal localization when endogenous <i>Gap43</i> is depleted from DRG neurons. Taken together, our data indicate that endogenous <i>Nrn1</i> and <i>Gap43</i> mRNAs compete for binding to HuD for their axonal localization and activity of the <i>Nrn1</i> 3'UTR.