Post-transcriptional Regulation of Cofilin and FMRP in Neurons

DoctorIn eukaryotes, mRNA translation begins with ribosome recruitment to the 5’ capping complex. However, some viral mRNAs adopt alternative initiation mechanism using internal ribosome entry sites (IRES) located in the 5’-untranslated region (UTR) of mRNA that can directly recruit ribosomes to initiate translation. Furthermore, few studies have discovered that cellular mRNAs are also translated by cap-independent translation: IRES mediated translation in the neuron. However, physiological significance of IRES-mediated translation is poorly understood. While screening for neuronal genes that have non-canonical translation activity, we discovered that the 5’UTR of cofilin and FMRP mRNA contains strong IRES activity. In the first chapter, I will discuss post-transcriptional regulation of cofilin. In neuronal development, dynamic rearrangement of actin promotes axonal growth cone extension, and spatiotemporal translation of local mRNAs in response to guidance cues directs axonal growth cone steering, where cofilin plays a critical role. While regulation of cofilin activity is well studied, regulatory mechanism for cofilin mRNA translation in neurons is unknown. Here, we report that 5’ UTR of cofilin mRNA contains an IRES element, and cofilin is predominantly translated by IRES-mediated mechanism in neurons. Furthermore, we show that IRES-mediated translation of cofilin is required for both axon extension and axonal growth cone steering. Our results provide new insights into the function of IRES-mediated translation in neuronal development. In the second chapter, I will discuss the regulatory mechanism of FMRP expression. Fragile X syndrome (FXS) caused by loss of fragile X mental retardation protein (FMRP) is the most common cause of inherited intellectual disability. Numerous studies show that FMRP is a RNA binding protein and regulates translation of its binding targets, which plays key roles in neuronal functions. However, the regulatory mechanism for FMRP expression is incompletely understood. Conflicting results regarding internal ribosome entry site (IRES)-mediated fmr1 translation were reported. Here, we unambiguously demonstrate that the fmr1 gene encoding FMRP exploits both IRES-mediated translation and canonical cap-dependent translation. Furthermore, we find that heterogeneous nuclear ribonucleoprotein Q (hnRNP-Q) acts as IRES transacting factor (ITAF) for IRES-mediated fmr1 translation in neurons. We also show that semaphorin 3A (Sema3A) induced axonal growth cone collapse is due to upregulation of hnRNP-Q and subsequent IRES-mediated expression of FMRP. These data elucidate the regulatory mechanism of FMRP expression and its role in axonal growth cone collapse