A cytoplasmic quaking I isoform regulates the hnRNP F/H-dependent alternative splicing pathway in myelinating glia

Abstract

The selective RNA-binding protein quaking I (QKI) plays important roles in controlling alternative splicing (AS). Three QKI isoforms are broadly expressed, which display distinct nuclear-cytoplasmic distribution. However, molecular mechanisms by which QKI isoforms control AS, especially in distinct cell types, still remain elusive. The quakingviable (qkv) mutant mice carry deficiencies of all QKI isoforms in oligodendrocytes (OLs) and Schwann cells (SWCs), the myelinating glia of central and peripheral nervous system (CNS and PNS), respectively, resulting in severe dysregulation of AS. We found that the cytoplasmic isoform QKI-6 regulates AS of polyguanine (G-run)-containing transcripts in OLs and rescues aberrant AS in the qkv mutant by repressing expression of two canonical splicing factors, heterologous nuclear ribonucleoproteins (hnRNPs) F and H. Moreover, we identified a broad spectrum of in vivo functional hnRNP F/H targets in OLs that contain conserved exons flanked by G-runs, many of which are dysregulated in the qkv mutant. Interestingly, AS targets of the QKI-6-hnRNP F/H pathway in OLs are differentially affected in SWCs, suggesting that additional cell-type-specific factors modulate AS during CNS and PNS myelination. Together, our studies provide the first evidence that cytoplasmic QKI-6 acts upstream of hnRNP F/H, which forms a novel pathway to control AS in myelinating glia.

The long and the short of TRF2 in neurogenesis

“…The significance of HNRNPH in controlling alternative splicing of neural genes was previously shown in oligodendrocytes. 55,56 Of note, HNRNPH1 regulates alternative splicing of the neuron-specific exon N of REST. 57 The role of HNRNPH on TRF2 splicing in neuronal differentiation was tested in two model systems.…”

Alternative Splicing of Neuronal Differentiation Factor TRF2 Regulated by HNRNPH1/H2

“…They included several RBPs [e.g., ELAVL1 (HuR), ELAVL3 (HuC), RBM14, and RNA helicases DDX17 and DDX5] and hnRNPs HNRNPH1 and HNRNPH2, which were shown to modulate alternative splicing (Huelga et al, 2012; Katz et al, 2010; Stark et al, 2011; Turunen et al, 2013; Uren et al, 2016), HNRNPF, HNRNPA3 and HNRNPD0. HNRNPF is functionally related to the HNRNPH family and was shown to regulate splicing (Mandler et al, 2014; Wang et al, 2012). Other RBPs identified in kidney extracts included Staufen 1 (STAU1) and the splicing regulator polypyrimidine tract-binding protein 1 (PTB) (Boutz et al, 2007; Li et al, 2014).…”

“…The most recent of these studies identified a CLIP tag on the human homolog of Trf2 (Uren et al, 2016) and identified potential binding sites as G tracts containing As (Huelga et al, 2012; Uren et al, 2016). HNRNPH1 was previously shown to regulate alternative splicing in oligodendrocytes (Mandler et al, 2014; Wang et al, 2012) and exon inclusion of several transcripts encoding brain-specific proteins C1 cassette exon of the glutamate NMDA R1 receptor (GRIN1) mRNA (Han et al, 2005), and was proposed to promote c-Src N1 exon inclusion in neuronal cells (Chou et al, 1999) and exon 2 inclusion on the OPRM1 (µ opioid receptor) (Xu et al, 2014). REST is also regulated via alternative splicing (Chen and Miller, 2013); interestingly, HNRNPH1 regulates the neuronal-specific exon N of REST in H69 small-cell lung cancer cells and is a pre-requisite for binding of the splicing factor U2AF65 and subsequent exon inclusion (Ortuno-Pineda et al, 2012).…”

STARs in the CNS

“…Minigene experiments suggest that hnRNP F/H also regulate MAG exon 12 splicing. This group also identified several new splicing targets in the qk v mouse that are part of the HnRNPF/H pathway [45]. Hence, these experiments show that Quaking may regulate splicing in the brain directly and indirectly.…”

“…Earlier experiments had already demonstrated that hypo-myelination and tremor in the qk v mice can be rescued by expressing a transgene encoding QKI-6 in oligodendrocytes [44]. More evidence for QKI-6 effects on splicing come from Mandler et al [45] who demonstrated that QKI-6 represses two splicing factors called hnRNP F/H in vivo. Minigene experiments suggest that hnRNP F/H also regulate MAG exon 12 splicing.…”