Axonal Transport of mRNA for Mitochondrial Proteins

SUMMARY Neurons use local translation to preserve the health of axons, dendrites, and synapses that are far from the soma. Local translation allows them to respond to local needs. While studying the local synthesis of PINK1 in axons and dendrites, we discovered that the PINK1 mRNA was transported into axons and dendrites by “hitchhiking” on mitochondria. The association of the PINK1 mRNA with mitochondria was mediated by synaptojanin2 and synaptojanin2 binding protein (SYNJ2BP). Synaptojanin2, like synaptojanin1, is an inositol lipid phosphatase, but also contains a predicted RNA-Recognition Motif (RRM); mutation of the RRM in synaptojanin2 prevents its ability to localize the PINK1 transcript to mitochondria. We know that synaptojanin2 can bind additional mRNA species, not just PINK1, including many for mitochondrial proteins. Thus synaptojanin2 represents an important addition to the roster of mechanisms that can transport neuronal mRNA. We therefore propose to study the broad significance of the synaptojanin2/SYNJ2BP mechanism for mRNA transport in neurons and for the function of neuronal mitochondria. To this end, we have made both mouse and human iPSC lines with mutations in the RRM of synaptojanin2 (SYNJAAA). In AIM 1 we will catalog the neuronal mRNA species bound to the RRM of synaptojanin2 and determine the extent to which it is required for their transport into axons and dendrites. In AIM 2 we will catalog the mRNA present on neuronal mitochondria. We will determine to what extent synaptojanin2 and SYNJ2BP are needed for this localization. We will also catalog the mRNA species present in synaptosomes from control and SYNJAAA mice to determine which presynaptic mRNAs require binding to synaptojanin2 to reach the synapse. In AIM3 we examine the functional consequences of mutations in the RRM of synaptojanin2 for the protein content of synapses. the morphology and functioning of neuronal mitochondria, and the health and survival of neurons.