The QKI-6 and QKI-7 RNA Binding Proteins Block Proliferation and Promote Schwann Cell Myelination

Abstract

BackgroundThe quaking viable (qkv) mice have uncompacted myelin in their central and peripheral nervous system (CNS, PNS). The qk gene encodes 3 major alternatively spliced isoforms that contain unique sequence at their C-terminus dictating their cellular localization. QKI-5 is a nuclear isoform, whereas QKI-6 and QKI-7 are cytoplasmic isoforms. The qkv mice harbor an enhancer/promoter deletion that prevents the expression of isoforms QKI-6 and QKI-7 in myelinating cells resulting in a dysmyelination phenotype. It was shown that QKI regulates the differentiation of oligodendrocytes, the myelinating cells of the CNS, however, little is known about the role of the QKI proteins, or RNA binding proteins in PNS myelination.Methodology/Principal FindingsTo define the role of the QKI proteins in PNS myelination, we ectopically expressed QKI-6 and QKI-7 in primary rat Schwann cell/neuron from dorsal root ganglia cocultures. We show that the QKI isoforms blocked proliferation and promoted Schwann cell differentiation and myelination. In addition, these events were coordinated with elevated proteins levels of p27KIP1 and myelin basic protein (MBP), markers of Schwann cell differentiation. QKI-6 and QKI-7 expressing co-cultures contained myelinated fibers that had directionality and contained significantly thicker myelin, as assessed by electron microscopy. Moreover, QKI-deficient Schwann cells had reduced levels of MBP, p27KIP1 and Krox-20 mRNAs, as assessed by quantitative RT-PCR.Conclusions/SignificanceOur findings suggest that the QKI-6 and QKI-7 RNA binding proteins are positive regulators of PNS myelination and show that the QKI RNA binding proteins play a key role in Schwann cell differentiation and myelination.

MLCK regulates Schwann cell cytoskeletal organization, differentiation and myelination

“…Kip (Larocque et al, 2009) as well as that of pro-myelinating transcription factors and myelin proteins (MAG and P0) (Monuki et al, 1989;Parkinson et al, 2004), while reducing the expression of c-Jun, a negative regulator of myelination (Parkinson et al, 2008), and other markers of non-myelinating SCs such as p75 and Ncadherin (Monje et al, 2009;Morgan et al, 1991). Surprisingly, treatment with cAMP dramatically downregulated the expression of MLCK and the level of phosphorylated MLC (MLC-P), indicating an overall reduction of myosin II activity in promyelinating SCs.…”

The Quaking family of RNA-binding proteins: Coordinators of the cell cycle and differentiation

“…During differentiation of glial cells, QKI proteins regulate expression of the tumor suppressor p27, Krox20 (a transcription factor critical for myelination in the peripheral nervous system), Myelin Basic Protein (MBP) and Myelin-Associated Glycoprotein (MAG). [21][22][23][24][25] C. elegans contains two QR proteins, ASD-2 (alternative splicing defect) and GLD-1 (defective in germ line development). While relatively little is known about ASD-2, 26 GLD-1 functions to control several decisions during germ line development, such as the choice between mitosis and meiosis or spermatogenesis and oogenesis.…”

“…In mammalian myelin-producing cells, QKIs promote expression of factors required for terminal differentiation and myelin production (MBP, MAG and Krox 20). 21,[23][24][25] In contrast, in the worm germ line GLD-1 represses factors (such as GLP-1/Notch, TRA-2, and possibly OMA-1/2, or LIN-45/Raf) whose ectopic expression would interfere with the progression through meiosis and gametogenesis. 30,32,34 Similarly, in the early fly mesoderm, How represses expression of factors that can otherwise activate abnormal cell signaling, interfere with normal mesoderm formation, and perhaps induce an alternative cell fate (Fig.…”

Comparing peripheral glial cell differentiation in Drosophila and vertebrates

“…Drosophila proteins marked with a * were already identified as being expressed in glial cells by Altenhein et al [127] and proteins marked with ** were identified by Freeman et al [126] septate junction formation by splicing and possibly post-translational control of the nuclear access of splicing factors appears also evolutionary conserved. The HOW homolog Quaking is needed for proper development of myelinated nerves and has also been linked to the control of splicing [128][129][130][131]. This suggests that elucidation of the mechanism underlying the action of the Crn/HOW complex will shed some light on the biology of myelination.…”