Oligodendroglial defects during quakingviable cerebellar development

Qki regulates myelinogenesis through Srebp2-dependent cholesterol biosynthesis

Myelination depends on timely, precise control of oligodendrocyte differentiation and myelinogenesis. Cholesterol is the most abundant component of myelin and essential for myelin membrane assembly in the central nervous system. However, the underlying mechanisms of precise control of cholesterol biosynthesis in oligodendrocytes remain elusive. In the present study, we found that Qki depletion in neural stem cells or oligodendrocyte precursor cells in neonatal mice resulted in impaired cholesterol biosynthesis and defective myelinogenesis without compromising their differentiation into Aspa⁺Gstpi⁺ myelinating oligodendrocytes. Mechanistically, Qki-5 functions as a co-activator of Srebp2 to control transcription of the genes involved in cholesterol biosynthesis in oligodendrocytes. Consequently, Qki depletion led to substantially reduced concentration of cholesterol in mouse brain, impairing proper myelin assembly. Our study demonstrated that Qki-Srebp2-controlled cholesterol biosynthesis is indispensable for myelinogenesis and highlights a novel function of Qki as a transcriptional co-activator beyond its canonical function as an RNA-binding protein.

Expression of Quaking RNA-Binding Protein in the Adult and Developing Mouse Retina

Quaking (QKI), which belongs to the STAR family of KH domain-containing RNA-binding proteins, functions in pre-mRNA splicing, microRNA regulation, and formation of circular RNA. QKI plays critical roles in myelinogenesis in the central and peripheral nervous systems and has been implicated neuron-glia fate decision in the brain; however, neither the expression nor function of QKI in the neural retina is known. Here we report the expression of QKI RNA-binding protein in the developing and mature mouse retina. QKI was strongly expressed by Müller glial cells in both the developing and adult retina. Intriguingly, during development, QKI was expressed in early differentiating neurons, such as the horizontal and amacrine cells, and subsequently in later differentiating bipolar cells, but not in photoreceptors. Neuronal expression was uniformly weak in the adult. Among QKI isoforms (5, 6, and 7), QKI-5 was the predominantly expressed isoform in the adult retina. To study the function of QKI in the mouse retina, we examined quaking^viable(qk^v) mice, which have a dysmyelination phenotype that results from deficiency of QKI expression and reduced numbers of mature oligodendrocytes. In homozygous qk^v mutant mice (qk^v/qk^v), the optic nerve expression levels of QKI-6 and 7, but not QKI-5 were reduced. In the retina of the mutant homozygote, QKI-5 levels were unchanged, and QKI-6 and 7 levels, already low, were also unaffected. We conclude that QKI is expressed in developing and adult Müller glia. QKI is additionally expressed in progenitors and in differentiating neurons during retinal development, but expression weakened or diminished during maturation. Among QKI isoforms, we found that QKI-5 predominated in the adult mouse retina. Since Müller glial cells are thought to share properties with retinal progenitor cells, our data suggest that QKI may contribute to maintaining retinal progenitors prior to differentiation into neurons. On the other hand, the expression of QKI in different retinal neurons may suggest a role in neuronal cell type specific fate determination and maturation. The data raises the possibility that QKI may function in retinal cell fate determination and maturation in both glia and neurons.