Visualizing BDNF cell-to-cell transfer and its effects on synapse and circuit function

• Applicant: Dr. Camin Dean
• Subject Area: Molecular Biology and Physiology of Neurons and Glial Cells
• Term: from 2019 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 422153908

Brain-derived neurotrophic factor (BDNF) is essential for neuronal growth, differentiation, and synaptic plasticity. The release of BDNF from neurons, and the effects of BDNF on neuron and circuit function, have both been well-studied separately, where transfer of BDNF from neurons to other neurons has been assumed. But to date, the transfer of BDNF between cells has not been directly visualized and investigated. Using a four-fluorophore imaging approach to identify both the cell of origin and target cells, we quantified the transfer of BDNF between cells. Surprisingly, in preliminary data, we found that astrocytes appear to be the main recipient of neuronally expressed BDNF. We further found that astrocytes specifically take up mature (not pro) BDNF that is released by neurons. Over-expression of the TrkB receptor in neurons redirected released BDNF from astrocytes to neurons, suggesting that TrkB levels determine neuronal versus astrocytic BDNF uptake. In addition, increased neuronal activity further increased astrocytic (but not neuronal) uptake of neuronally expressed BDNF. Finally, we also found in preliminary experiments, that astrocytes do not simply act as a sink or buffer for excess BDNF, as previously proposed, but rather that BDNF taken up by astrocytes mediates physiological effects by increasing astrocytic territory. The goal of this project is to extend these initial observations to, 1) examine and quantify the transfer of endogenous BDNF from neurons to astrocytes in vivo - using CRISPR/Cas9 to label endogenous BDNF in the brain, 2) determine if astrocytic uptake of neuronal BDNF, and subsequent increases in astrocytic territory, are mediated by the TrkB.T1 receptor (the main BDNF receptor expressed in astrocytes) - using astrocyte-specific TrkB.T1 knockout mice, and 3) determine if the transfer of BDNF from neurons to astrocytes is necessary for aspects of synapse and circuit function - using optical readouts of synaptic strength and electrophysiological recordings. Together the proposed experiments will confirm and substantiate an unexpected function of BDNF, whereby it can affect synapse and circuit function in the brain via astrocytes, by changing astrocyte morphology.

• DFG Programme: Research Grants