In the brain, nerve cells exchange information relevant for sensory perception, movement control, and higher brain functions at highly specific contact sites, the synapses. During sensory perception and sensorimotor integration, complex networks of neurons are activated by the incoming sensory information, interact with each other at multiple layers of synapses, and eventually initiate motor action. However, the action of specific neurons in these circuits is only incompletely understood. In this project co-funded by the Swiss National Science Foundation, two-photon (2P) fluorescence excitation microscopes will be acquired, and further optimized for use in the Brain Mind Institute of the École Polytechnique Fédérale in Lausanne.
In a first project, the signalling pathways that are active during the growth of nerve terminals and during activity-dependent plasticity will be imaged. Nerve terminals of the axon of a sending neuron connect with the cell body or the dendrite of a receiving neuron, and establish synapses. However, our knowledge of the signalling pathways which specify the size and therefore the signalling strength of a nerve terminal is limited. Here, proteins of a candidate signalling pathway will be labelled with a donor and an acceptor fluorophore based on green fluorescent protein (GFP) variants, and genetically engineered into nerve terminals. Fluorescence resonance energy transfer between the pair of the donor and acceptor flourophores will then be imaged in a 2P-fluorescence lifetime imaging approach. This will allow us to visualize the activation of specific signalling pathways during the growth of nerve terminals, and during activity-dependent plasticity. The focus of these studies will be on the so-called Rho-GTPases, which are known to signal to the cytoskeleton and regulate cell shape changes in cells. Specifically, we will investigate whether Rho-GTPases are activated during presynaptic plasticity and downstream of extracellular signals that act on the growing nerve terminal.
In a second project, neuronal activity in the cortical and subcortical areas of living mice performing tasks of sensory-motor integration will be imaged in a 2P-microscope. The activity of nerve cells will be read-out in the form of Ca2+ signals, detected by Ca2+ sensing proteins genetically engineered into specific nerve cell populations. In order to maximize the observation depth into the light-scattering brain tissue, the 2P excitation and the detection of fluorescence emission will be optimized. Together, this new equipment acquired with the help of the Swiss National Science Foundation is expected to significantly enhance our knowledge about signalling pathways activated during synaptic plasticity and nerve terminal growth, and about the role of specific nerve cell populations during sensory processing and motor integration in the living mouse brain.