Regulation of local mRNA translation by FMRP and m6A modification in axonal outgrowth and synaptogenesis

Neurons are one of the most highly polarized cells in our body, and they require sophisticated cellular mechanisms to maintain protein homeostasis in different subcellular compartments inside the cell. It has been widely acknowledged that the synthesis of new proteins occurs not only within neuronal cell bodies but also at distal synaptic sites along dendrites and axons. While local protein synthesis has been studied extensively in axonal guidance and growth cone turning responses in cultured spinal neurons, its involvement in the assembly of presynaptic and postsynaptic specializations at the vertebrate neuromuscular junction (NMJ) remains largely unclear. Our recent studies demonstrated that the temporally delayed expression of surface membrane-type 1 matrix metalloproteinase (MT1-MMP) in mature spinal neurons serves as an intrinsic molecular switch from axonal outgrowth to presynaptic differentiation, which requires local capturing of ribonucleoprotein-containing granules by agrin stimulation to regulate MT1-MMP mRNA localization and translation. However, the exact mechanisms underlying the axonal transport and synaptic targeting of MT1-MMP and other synapse-specific mRNAs in this process warrant further investigation. Given our preliminary data showing that fragile X mental retardation protein (FMRP), a major RNA-binding protein serving multifunctional roles in protein synthesis and neuronal development, is spatially localized in axonal growth cones and in agrin-induced synaptic specializations, this study will examine how FMRP regulates axonal transport, localization, and local translation of MT1-MMP and other synapse-specific mRNAs during neuronal outgrowth and synaptogenesis. On the other hand, the N6- methyladenosine (m6A) modification is a dynamic reversible modification that has been reported to have a wide range of effects on RNA metabolism. This project will also investigate the molecular regulation of dynamic m6A modification by different m6A regulators in axonal growth cones and agrin-induced presynaptic and postsynaptic specializations at developing NMJs. Together, the impact of this project will provide a better understanding of the roles of FMRP and m6A modification in mRNA transport, localization, and local translation underlying the assembly and maintenance of neuromuscular synapses in health and disease.