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

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.