PROJECT SUMMARY Synaptic plasticity is well accepted as the basis of behavioral adjustability in the face of a constantly changing environment. Our lived experience is transmitted to our brain as electrical impulses along axons. Oligodendrocytes (OLs) increase the rate at which these electrical impulses are transmitted by insulating axons with myelin sheaths. Surprisingly, motor learning, sensory stimulation, and social enrichment induce the differentiation of precursor cells into myelinating OLs resulting in quantifiable structural changes in white matter. These findings point to myelin plasticity as a concurrent, and equally important contributor to the adaptability of neural circuits. However, the molecular and cellular mechanisms underlying myelin plasticity are not well understood. Single OLs can give rise to sheaths of different lengths and thicknesses to accommodate the needs of diverse axons. These observations suggest a local and independent regulation of myelination at the level of individual sheaths. How do sheaths assess the needs of specific axons? Action potentials cause axons to, not only release vesicles at their terminal ends, but also along their shafts. Our lab and others have shown that axons signal to myelin sheaths via these alternative release sites and that myelin sheaths express the canonical post-synaptic factors required to interpret these signals. These data suggest that the use of a shared transmission machinery enables synaptic and myelin plasticities to occur in parallel as a response to the same stimulus. While some components of axo-myelin communication have been elucidated, the intracellular mechanisms bridging signal receipt to myelin production remain unknown. In dendrites, the localization of mRNA transcripts and ribosomes to individual spines support their rapid, tailored adaptive responses. Similarly, diverse groups of mRNAs, along with ribosomes localize to myelin sheaths raising the possibility that local RNA translation underlies the ability of individual OL sheaths to fine-tune their responses to signals from various axons. Due to the dynamic nature of RNA translation, it would be best understood if studied in vivo. However, limitations in technological approaches stood in the way for decades. Using diverse transgene expression systems, protein photoconversion technology, and my expertise with 2-photon laser severing, I will determine if local translation of myelin-resident transcripts occurs in zebrafish. Additionally, I will investigate whether the myelin localization of an enriched group of transcripts we identified contributes to myelin plasticity. To accomplish this, I will create a loss-of-function mutation of Khdrbs1, an RNA binding protein predicted to bind to members of this enriched group. Finally, I will test if manipulating neuronal activity alters the translation of targeted myelin resident mRNAs. This work will add to our understanding of how axo-myelin exchanges impact the efficiency of...