RNA binding proteins (RBPs) are involved in all aspects of the RNA life cycle and constitute a critical component of maintaining proper transcription and translation. RBPs act as mediators of the critical intermediate between DNA and proteins, messenger RNA (mRNA), which is required for cell survival and growth. RBPs form ribonucleoprotein (RNP) complexes with their target mRNAs. Over 500 genes encode RBPs in human DNA, and although RBPs have a crucial role in post-translational regulation of gene expression, few have been studied systematically. In the nervous system, RBPs and their associated mRNAs, play a key role in normal neuronal development and function and in neurological disease. Fragile X syndrome, a cognitive impairment disorder, results from the loss of expression of the Fragile X Mental Retardation Protein (FMRP). FMRP associates with the RNA helicase Moloney Leukemia Virus 10 (Mov10) in brain and modulates its translational activity through the microRNA (miRNA) pathway. We previously showed that MOV10 is important in developing and maintaining normal brain activity using both murine and Xenopus models. The deletion of Mov10 in Neuro2a cells caused abnormally decreased neurite outgrowth on differentiation which was restored upon exogenous expression of MOV10. Furthermore, culturing and staining of hippocampal neurons from MOV10 Heterozygotes (Het) confirmed these results showing markedly short dendrites as seen in the Mov10 knockout Neuro2a cells suggesting impaired neuronal function (Skariah et al., 2017). We were thus interested in investigating the consequences of Mov10 and Fmr1 reduction on dendritic development. Mov10 Het and Fmr1 KO neurons possess an abnormal morphology compared to WT neurons at day in vitro 14 (DIV14).
Additionally, Mov10 Het mice have reduced density of immature dendritic spines compared to WT and a smaller soma. The impaired neurite phenotype, spine maturation and reduced soma size have previously all been found to be associated with impaired miRNA biogenesis, and since MOV10 and FMRP are involved in regulation of the miRNA pathway, we sought to determine whether misregulation of the pathway was contributing to the abnormal neuronal phenotypes we observed in culture. We found a global reduction of Argonaute-2 (AGO2) – associated miRNAs in the absence of FMRP. This is important because AGO2 is the main effector of miRNA- mediated regulation. Furthermore, another component of the miRNA pathway, DICER, a ribonuclease which regulates biogenesis of miRNA and small interfering RNA (siRNA), was significantly decreased in the absence of MOV10 and FMRP. Through a series of knockdown and luciferase reporter experiments, we determined that MOV10 and FMRP modulate expression of the Dicer1 mRNA via the 3’ untranslated region (UTR). Overexpression (OE) of MYC-Dicer1 rescues the impaired neuronal phenotype in Mov10 Het neurons suggesting a mechanism for regulating local DICER expression when MOV10 and FMRP are present. Our work represents a new understanding of how FMRP and MOV10 regulate cobound mRNAs and neuronal development.LimitedAuthor requested closed access (OA after 2yrs) in Vireo ETD syste
