The importance of myelin integrity in reference to vision is most clearly represented in patients with multiple sclerosis (MS) or neuromyelitis optica (NMO) where inflammatory demyelination of the optic nerve results in impaired vision and eventual blindness. Newly generated oligodendrocyte progenitor cells (OPCs) proliferate in adult optic nerves during pathological conditions, but fail to reform myelin as they would during normal development. The goal of this proposal is to understand how the glutamate receptor AMPAR on OPCs responds to activity-dependent glutamate release from axons to initiate transcriptional regulation of myelination. Elucidating this glutamatergic signaling mechanism will have important implications in reprogramming newly proliferated OPCs in the adult diseased optic nerve to remyelinate. It is known that vesicular release of glutamate from en passant axonal synapses induces AMPAR currents in postsynaptic OPCs but, to date, the physiological relevance of this axon-glial signaling remains ill-defined. Our preliminary data suggest that retinal activity-dependent mechanisms generate action potentials causing axonal release of glutamate which activates AMPARs on OPCs. This AMPAR activation elicits a mitogen-activated protein kinase (MAPK) signaling cascade necessary for myelination. Consistent with this idea, inducible deletion of AMPARs in OPCs blocked activity-dependent evoked synaptic currents and myelin formation. We hypothesize that neuronal en passant synaptic activity modulates myelination of optic nerve axons via AMPAR-dependent MAPK signaling in OPCs. It is established that activity-dependence before eye opening (P1-P13), around eye opening (P11-P15), and after eye opening (P20-P27) modulates visual plasticity; however, its effect on myelination has not been studied. We developed a novel in vivo approach to mechanistically link retinal activity with glutamatergic axon-OPC signaling in the optic nerve by either (a) regulating retinal ganglion cell action potentials or (b) inducibly deleting AMPARs on OPCs to evaluate: 1) alterations in phosphorylation of the AMPAR-MAPK pathway in OPCs using fluorescence activated cell sorting (FACS) analysis, 2) changes in transcription and translation of myelin proteins, and 3) perturbations in myelin thickness per axon diameter in the optic nerve using electron microscopy. This grant proposal will ascertain the critical periods necessary for activity-dependent myelination establishing a fundamental mechanism in the development of the visual system. Knowledge gained from these studies will impact optic nerve inflammatory demyelinating diseases providing an important signaling mechanism necessary to rebuild the insulation around nerve fibers.
In adult demyelinating diseases of the optic nerve, new early stage myelin-producing cells proliferate but fail to rebuild the insulation around nerve fibers as they would during normal development. This proposal focuses on glutamate signaling between nerve fibers and early stage myelin-producing cells as a developmental signal that instructs the myelin sheath to form. Our work suggests that glutamate signaling must be properly activated in demyelinating diseases in order for new early stage myelin-producing cells to initiate remyelination.