PROJECT SUMMARY/ABSTRACT Perisynaptic astrocyte processes (PAPs) are fine, leaflet-like structures that are located on the distal processes of astrocytes. These structures contain numerous neurotransmitter receptors, ion channels, cell-adhesion molecules, and the ability to release synaptogenic molecules, enabling astrocytes to contribute to synapse homeostasis, development, and stabilization. Research indicates that PAPs enwrap or contact synaptic elements with increased coverage at mature, glutamatergic synapses. In addition, recent literature has indicated that once present at a synapse, these PAPs are highly motile structures. However, to date, the signaling mechanisms that initially recruit a PAP to a synapse remain to be elucidated. Brain derived neurotrophic factor (BDNF) is a critical factor that contributes to neuronal maturation, survival, and synapse development. RNA sequencing generated in our lab has recently revealed the novel and surprising finding that astrocytes express high levels of the BDNF receptor, TrkB, relative to other cell populations. Further isoform specific evaluation of this data indicated that astrocytes predominantly express a truncated isoform of TrkB, TrkB.T1. Our published work indicates that BDNF/astrocytic TrkB.T1 contributes to astrocyte morphogenesis. Global and astrocyte specific deletion of TrkB.T1 results in a decrease in astrocytic volumes, dysregulation of perisynaptic genes associated with mature astrocyte function, as well as inability for astrocytes to promote neuronal synapse formation and function in vitro. Preliminary data generated for this grant reveal that global deletion of TrkB.T1 results in a decrease in excitatory synapses in layer II/III cortex, assessed by immunohistochemical evaluation of pre- and post-synaptic excitatory elements. In addition, global deletion of TrkB.T1 disables astrocytes from responding to experience dependent plasticity in the whisker barrel cortex by inhibiting their ability to increase in cell volume and their ability to increase the number of synaptic elements contained within their territories. Finally, preliminary in vitro data indicate that relative to wildtype (WT) astrocytes, TrkB.T1 knockout (KO) astrocytes fail to form PAPs or fail to enwrap synaptic elements. Altogether, our findings and preliminary data have led to the hypothesis that BDNF signaling on to the astrocytic TrkB.T1 receptor is required for the recruitment of PAPs to glutamatergic synapses. We propose to test this hypothesis utilizing a combination of in vitro and in vivo pharmacological, genetic, functional, and imaging techniques. Successful completion of the outlined experiments could shed light on the pathophysiology of both neurodevelopmental and neuropsychiatric disorders, both of which have been implicated by aberrant BDNF/TrkB and synapse function but have never been studied in the context of BDNF/astrocytic TrkB.T1. The approaches employed in this proposal offer a breadth of innovativ...