Characterization of distribution, proteomic content, and dynamic behavior of inhibitory synapses onto L2/3 pyramidal neurons in visual cortex.

• Applicant: Dr. Bettina Schmerl
• Subject Area: Molecular Biology and Physiology of Neurons and Glial Cells ; Experimental and Theoretical Network Neuroscience ; Molecular and Cellular Neurology and Neuropathology
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 524025372

Synapse formation and stability are crucial for neural circuit development, function, and plasticity. How these processes are regulated in the brain is explored incompletely, especially in regards to inhibitory synapses that are indispensable for proper circuit formation and function. Three main classes of interneurons, defined by the expression of Somatostatin (SST), Parvalbumin (PV) or Serotonin receptors (5HT3aR), account for the majority of inhibitory innervation in the brain. These interneuron types differ not only in expression of the eponymous proteins and differential targeting of postsynaptic cells, but also exhibit very different firing properties, implying further molecular heterogeneity. The majority of inhibitory synapses on dendrites of pyramidal neurons, located on both the dendritic shaft and dendritic spines, are thought to be innervated by SST-expressing interneurons. PV-expressing cells are thought to mostly innervate the soma and proximal dendrites of pyramidal neurons, but this view is challenged by more systematic approaches that indicate a broad distribution of PV synapses on dendrites. The relative distribution of SST vs PV inputs onto individual neurons and their targeting of inhibitory shaft vs spine synapses is unknown. There is also no information regarding the molecular composition of SST and PV inhibitory synapses depending on their positioning on the dendritic arbor, and whether that correlates with differences in their dynamic potential. These questions remain unaddressed despite the implications of inhibitory synapse heterogeneity for signal transmission, neuronal computation, and circuit function. To address these open questions, we will take advantage of a genetic labeling strategy together with an exceptional multi-channel in vivo imaging approach to track daily dynamics of inhibitory synapses on the full dendritic arbors of individual neurons in the mouse visual cortex and analyze their molecular composition at single synapse level. Using different Cre driver lines, we will label subtype specific inhibitory presynaptic terminals and determine how synapse placement across the dendritic arbor and molecular composition relates to subtype identity. Moreover, chronic imaging, which allows dynamic tracking of synapse formation and elimination, will be paired with post hoc analysis of synaptic molecular composition using a tissue expansion and clearing protocol known as enhanced magnified analysis of proteome (eMAP). These studies will provide information on subtype-specific interneuron targeting, synaptic composition (receptor subunits, cell adhesion proteins, scaffold proteins), and remodeling dynamics, affording an unprecedented view of inhibitory circuit connectivity, function and plasticity.

• DFG Programme: WBP Fellowship
• International Connection: USA
• Host: Professorin Dr. Elly Nedivi, Ph.D.