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 innovative in vitro and in vivo techniques to the PI. Training for the
expertise of these techniques will take place in the sponsor’s laboratory. Throughout the fellowship, career
professional development training will occur through journal clubs, seminars, and associations at Virginia Tech,
as well as at national and regional conferences to prepare the PI for a career as an independent researcher.