Quaking I controls a unique cytoplasmic pathway that regulates alternative splicing of myelin-associated glycoprotein

Abstract

Precise control of alternative splicing governs oligodendrocyte (OL) differentiation and myelination in the central nervous system (CNS). A well-known example is the developmentally regulated expression of splice variants encoding myelin-associated glycoprotein (MAG), which generates two protein isoforms that associate with distinct cellular components crucial for axon-glial recognition during myelinogenesis and axon-myelin stability. In the quakingviable (qk v ) hypomyelination mutant mouse, diminished expression of isoforms of the selective RNA-binding protein quaking I (QKI) leads to severe dysregulation of MAG splicing. The nuclear isoform QKI-5 was previously shown to bind an intronic element of MAG and modulate alternative exon inclusion from a MAG minigene reporter. Thus, QKI-5 deficiency was thought to underlie the defects of MAG splicing in the qk v mutant. Surprisingly, we found that transgenic expression of the cytoplasmic isoform QKI-6 in the qk v OLs completely rescues the dysregulation of MAG splicing without increasing expression or nuclear abundance of QKI-5. In addition, cytoplasmic QKI-6 selectively associates with the mRNA that encodes heterogeneous nuclear ribonucleoprotein A1 (hnRNPA1), a well-characterized splicing factor. Furthermore, QKI deficiency in the qk v mutant results in abnormally enhanced hnRNPA1 translation and overproduction of the hnRNPA1 protein but not hnRNPA1 mRNA, which can be successfully rescued by the QKI-6 transgene. Finally, we show that hnRNPA1 binds MAG pre-mRNA and modulates alternative inclusion of MAG exons. Together, these results reveal a unique cytoplasmic pathway in which QKI-6 controls translation of the splicing factor hnRNPA1 to govern alternative splicing in CNS myelination.selective RNA-binding protein QKI | heterogeneous nuclear ribonucleoprotein A1 | myelin development | 3′-UTR | translation regulation M yelination enables rapid conduction of nerve impulses and protects neuronal axons from extracellular insults, which is essential for the development and function of the central nervous system (CNS) (1). Oligodendrocytes (OLs) are responsible for CNS myelination, in which precise expression of OL-specific genes governs the formation and maintenance of CNS myelin (2, 3). Nearly all transcripts encoding myelin-specific structural proteins undergo extensive alternative splicing that is vigorously regulated during myelin development (4). As a result, the abundance of functionally distinct splice variants is markedly altered. A well-known example is the developmentally regulated alternative splicing of the pre-mRNA encoding myelin-associated glycoprotein (MAG), which generates two isoforms that are localized at the periaxonal membrane and mediate reciprocal signals toward axons and OLs (5). In rodents, exclusion of exon 12 generates the large MAG protein (L-MAG) predominantly expressed in the juvenile CNS, whereas inclusion of exon 12 introduces an in-frame stop codon, giving rise to the small MAG protein (S-MAG) mainly in the peripheral nervous system (PNS)...

hnRNP A1 and secondary structure coordinate alternative splicing of Mag

“…Together, the data reveal a specific interaction between hnRNP A1 and the Mag exon 12 5 ′ splice site, and suggest that binding of hnRNP A1 may interfere with U1 snRNP association. Our observations of an interaction between hnRNP A1 and Mag RNA is consistent with the observation by Zhao et al (2010) that the Mag pre-mRNA coimmunoprecipitates with hnRNP A1.…”

“…NMD generally requires a spacing of 50-55 nucleotides between the stop codon and the exon junction for efficient degradation (Nagy and Maquat 1998); in Mag exon 12, only 12 nucleotides are present. Recently, Feng and coworkers reported that hnRNP A1 enhances Mag exon 12 inclusion in HEK293T cells through an interaction with an element within exon 13 (Zhao et al 2010). The splicing reporter used in the current study does not include this exon.…”

“…Recently, we and others showed that hnRNP A1 contributes to the alternative splicing of Mag exon 12 (Zhao et al 2010;Zearfoss et al 2011). Moreover, we showed that the sequence UAGGU is enriched within and adjacent to exons that show alternative splicing changes upon hnRNP A1 knockdown in oligodendrocyte precursor cells (Zearfoss et al 2011).…”

Networking in a global world: Establishing functional connections between neural splicing regulators and their target transcripts

“…Additionally, QKI proteins can indirectly regulate AS through the modulation of other splicing regulators. Specifically, QKI-6, a cytoplasmic variant encoded by the qk locus, represses the translation of hnRNPA1, which, in turn, regulates MAG exon 12 AS (Zhao et al 2010). Several other transcripts encoded by genes relevant to myelin formation and maintenance display altered splicing patterns in Quaking mice (Wu et al 2002).…”

PIKEis essential for oligodendroglia development and CNS myelination

“…QKI proteins are members of signal transduction and activation of the RNA (STAR) family that play an essential role in OL development (36). These proteins, such as MBP, myelin-associated glycoprotein, and actin-interacting protein 1, control the splicing and stability of mRNA that are crucial to OL differentiation (37). The RNA-binding activity of QKI is governed by Fyn kinase (38,39), which is linked with PIKE function (9,40).…”