Investigating the role of serotonergic signaling in glial development using human iPSC-derived organoids

PROJECT SUMMARY Astrocyte morphological maturation is a critical step for healthy central nervous system development. Immature astrocytes elaborate their processes and infiltrate the neuropil with fine, terminal, leaflet processes. These processes allow for cell-cell communication with neighboring astrocytes, ensheathment of the vasculature and enwrapping synapses, and where astrocytes participate in neurotransmitter uptake, synapse development and stabilization. Astrocyte morphogenesis coincides with neuronal maturation and synaptogenesis, implicating a common mechanism between these two events. Brain-derived neurotrophic factor (BDNF) is one developmental molecule crucial for development and activity-dependent plasticity in neurons. Our lab has shown that BDNF interacts with astrocytes through a TrkB receptor isoform (TrkB.T1) to promote astrocyte morphological maturation. Our published work demonstrates astrocyte deletion of TrkB.T1 results in a 25% reduction in astrocyte volume and branching complexity, a change that persists through adulthood. Our discovery is relevant to health, as neurodevelopmental disorders (NDDs) are associated with BDNF dysregulation, including the X-linked NDD, Rett Syndrome (RTT). Preliminary evidence demonstrates that astrocytes in RTT mouse models have volume deficits similar to TrKB.T1 knockout accompanied by dysregulated gene expression. Reports demonstrate that the TrkB.T1 isoform uniquely interacts with a small RhoGTPase inhibitor, though its functional significance is relatively unknown. We propose to evaluate astrocyte RhoGTPase signaling and cytoskeletal dynamics in response to BDNF and potentially therapeutic TrkB activators utilizing in vitro and in situ genetic, molecular, and imaging techniques. Furthermore, we will assess if TrkB therapeutics rescue RTT astrocyte morphology deficits and gene dysregulation. My specific aims seek to (1) determine signaling consequences of TrkB.T1 activation and (2) the role of TrkB.T1 activation on morphological change in astrocytes as well as to expand upon our discovery by (3) evaluating astrocyte TrkB.T1 in a relevant NDD model. This proposal meets the priorities of the BRAIN Initiate 2.0 of (1) demonstrating causality of relationships and (2) identifying fundamental principles of observable brain properties.