Abstract

Several microRNAs (miRNAs), including miR-23 and miR-27a have been reportedly involved in regulating myelination in the central nervous system. Although miR-23 and miR-27a form clusters in vivo and the clustered miRNAs are known to perform complementary functions, the role of these miRNA clusters in myelination has not been studied. To investigate the role of miR-23-27-24 clusters in myelination, we generated miR-23-27-24 cluster knockout mice and evaluated myelination in the brain and spinal cord. Our results showed that 10-week-old knockout mice had reduced motor function in the hanging wire test compared to the wild-type mice. At 4 weeks, 10 weeks, and 12 months of age, knockout mice showed reduced myelination compared to wild-type mice. The expression levels of myelin basic protein and myelin proteolipid protein were also significantly lower in the knockout mice compared to the wild-type mice. Although differentiation of oligodendrocyte progenitor cells to oligodendrocytes was not inhibited in the knockout mice, the percentage of oligodendrocytes expressing myelin basic protein was significantly lower in 4-week-old knockout mice than that in wild-type mice. Proteome analysis and western blotting showed increased expression of leucine-zipper-like transcription regulator 1 (LZTR1) and decreased expression of R-RAS and phosphorylated extracellular signal-regulated kinase 1/2 (pERK1/2) in the knockout mice. In summary, loss of miR-23-27-24 clusters reduces myelination and compromises motor functions in mice. Further, LZTR1, which regulates R-RAS upstream of the ERK1/2 pathway, a signal that promotes myelination, has been identified as a novel target of the miR-23-27-24 cluster in this study.

Introduction

In the central nervous system (CNS), myelin, produced by oligodendrocytes, supports axonal metabolism and allows for rapid transmission of action potentials along axons [1–۳]. In demyelinating conditions such as multiple sclerosis or spinal cord injury, myelin is lost and axons degenerate, resulting in permanent loss of function. Remyelination in these demyelinating conditions remains a challenge. Developmental myelination and remyelination have the common goal of attaching myelin sheaths to axons [4]. In addition, there is a common mechanism underlying developmental myelination and remyelination, and remyelination is considered as “rerunning” of myelination [4]. Therefore, understanding the mechanism underlying myelination will help in developing novel strategies for remyelination in various disease conditions. Normal myelination is a multistep process in which oligodendrocyte differentiation and initiation of myelination are tightly regulated [5]. Previous studies have shown that several genes, transcription factors, and pathways are involved in oligodendrocyte progenitor cell (OPC) proliferation and oligodendrocyte differentiation, maturation, and myelination [6–۸]. Furthermore, recent studies have shown that microribonucleic acids (miRNAs) play important roles in regulating myelination [9–۱۵].

Results

۳٫۱٫ Expression of the miR-23-27-24 Cluster in the CNS
KO mice were slightly smaller than wild-type mice but were born and grew without morphological abnormalities (Figure 1(a)). Compared to 10-week-old wild-type mice, the expression of all miRNAs in the miR-23-27-24 cluster was significantly lower in the KO mice, both in the spinal cord and brain () (Figures 1(b) and 1(c) and Table 1).

Next, we compared the expression of the miR-23-27-24 cluster in the spinal cord of mice at different ages (4 weeks, 10 weeks, and 12 months of age). For all miRNAs in the miR-23-27-24 cluster, the expression levels in 10- and 12- week-old wild-type mice were significantly higher than those in 4-week-old wild-type mice (p < 0:001) (Figure 2). The expression of miR-23a in 12-month-old wild-type mice was significantly lower than that in 10-week-old wild-type mice (p < 0:001) (Figure 2(a)), but there were no significant differences in the expression of other miRNAs (Figures 2(b)–۲(e)). The expression of all miRNAs in KO mice was significantly lower than that in wild-type mice at all ages (Table 2).