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

摘要

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 mye linogenesis 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 pre viously 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 splic ing in the qk~v mutant. Surprisingly, we found that transgenic ex pression of the cytoplasmic isoform QKI-6 in the qk~v OLs completely rescues the dysregulation of MAG splicing without increasing ex pression or nuclear abundance of QKI-5. In addition, cytoplasmic QKI-6 selectively associates with the mRNA that encodes heteroge neous nuclear ribonucleoprotein A1 (hnRNPA1), a well-characterized splicing factor. Furthermore, QKI deficiency in the qkv mutant re sults in abnormally enhanced hnRNPA1 translation and overproduc tion of the hnRNPA1 protein but not hnRNPA1 mRNA, which can be successfully rescued by the QKI-6 transgene. Finally, we show that hnRNPAl 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 hnRNPAl to govern alternative splicing in CNS myelination.