PROJECT SUMMARY The young brain displays remarkable plasticity. This includes an ability to remodel neuronal synaptic connections to learn new tasks in health, as well as being able to repair connections and restore function after injury. In the adult brain there are active mechanisms in place that maintain neuronal circuit connectivity and synaptic stability, which is necessary for typical brain function, but acts as a barrier to targeted synaptic remodeling in situations where this is beneficial, for example to enhance learning, or to repair neurons after injury. Removal of factors from the adult brain that limit plasticity is sufficient to enable enhanced remodeling and repair. However, these manipulations lead to a permanent reopening of plasticity states, which is detrimental to long-term brain function by non-specifically destabilizing synaptic connections. Therefore, new approaches are needed to transiently enhance neuronal plasticity state to enable controlled synaptic remodeling and repair. There is compelling evidence that manipulating proteins in the extracellular space surrounding neurons is sufficient to enhance plasticity and synaptic remodeling. For example, proteins secreted by non-neuronal glial cells, specifically astrocytes, are sufficient to induce synapse maturation and stabilization, and permanent removal of these factors from adult astrocytes enables enhanced plasticity and repair. This demonstrates a role for specific secreted proteins in repressing plasticity in the adult brain, suggesting their targeted removal may be beneficial. Therefore, the first goal of this proposal is to ask if acute degradation of specific extracellular proteins is able to reopen brief, controlled, windows of plasticity to enable enhanced learning or to promote synaptic repair after injury. This will be achieved by developing a genetically encoded system for Targeted Degradation of Extracellular Proteins (TDEP). TDEP will use nanobodies that bind the protein of interest, coupled to a degradation-targeting sequence for uptake and removal by endogenous brain cells. As proof-of-concept TDEP will be developed to degrade known astrocyte-secreted proteins that stabilize synapses, and determine whether acute protein degradation is sufficient to reopen transient windows of synaptic plasticity, assayed using visual system plasticity, injury models and learning and memory paradigms. The second goal is to identify the complete repertoire of extracellular proteins that contribute to repressing plasticity in adulthood, and their cellular source, to enable their precise targeting for degradation and plasticity enhancement. This will be achieved by labeling newly-synthesized proteins secreted from specific brain cells under different plasticity conditions, using proximity labeling of proteins with biotin by the enzyme TurboID, targeted to different subcellular compartments from which extracellular proteins originate. This will generate an atlas of the cellular origin of extracellular proteins, and will be used to identify candidates for TDEP targeting for plasticity enhancement. The outcome will be a toolkit of genetic reagents that enable precise control of the neuronal environment to promote brain health and repair, with lasting impact on multiple areas of neuroscience where enhancing brain plasticity would improve function.

PROJECT NARRATIVE The adult brain shows limited plasticity, which poses a challenge for learning new tasks in health, and for repairing neuronal connections after injury such as stroke, or in neurodegenerative disorders such as Alzheimer’s disease. Most current approaches to increase plasticity cause permanent alteration of neuronal state, with potential for detrimental outcomes, therefore new approaches are needed to transiently alter neuronal plasticity state to enable controlled synaptic remodeling and repair. This is achieved by developing Targeted Degradation of Extracellular Proteins (TDEP), a toolkit of genetic reagents to enable precise control of the neuronal environment to promote brain health and repair that can be applied to multiple neurological disorders where enhancing synaptic plasticity would be beneficial.