The Role of Synaptic Signalling Activity in Brain Development
by Frederieke Sophie Moschref née Scheiwe
Date of Examination:2025-06-20
Date of issue:2025-12-02
Advisor:Dr. Benjamin Cooper
Referee:Prof. Dr. Nils Brose
Referee:Prof. Dr. Dr. Oliver Schlüter
Referee:Prof. Dr. Joachim Lübke
Persistent Address:
http://dx.doi.org/10.53846/goediss-11629
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Abstract
English
Dynamic changes in synapse structure are essential for memory formation and require newly synthesized synaptic proteins to either replace aged or damaged proteins, or to alter synaptic signalling efficacy (Costa-Mattioli et al., 2007; Fornasiero et al., 2018; Ostroff et al., 2002). Whereas local protein translation has been observed to be crucial for the expression of postsynaptic plasticity (Greenough et al., 1985; Ostroff et al., 2002; Steward & Levy, 1982; Steward & Schuman, 2001; Steward, 1983), its contribution to forms of presynaptic plasticity is still under debate. Multiple transcriptomic, light and electron microscopic studies reported the presence of presynaptically localized ribosomal and ribosome-associated proteins in the mature mammalian brain (Hafner et al., 2019; Monday et al., 2022; Ostroff et al., 2019; Scarnati et al., 2018; Shigeoka et al., 2016; Younts et al., 2016). However, a quantitative ultratsructural analysis of presynaptic ribosomes embedded within an intact central nervous system circuit is currently lacking. Such information is ultimately necessary to fully understand local presynaptic translation capacity and the extent to which it might contribute to synaptic plasticity and synaptic heterogeneity in the brain. In this thesis I established a scanning transmission electron microscopy (STEM) tomographic approach capable of revealing presynaptically localized monosomes in hippocampal Schaffer collateral, mossy fiber-CA3, and perisomatic synapses. This three-dimensional ultrastructural imaging approach revealed a substantially lower abundance of presynaptic ribosomes compared to estimates derived from previously published light microscopic studies. My data reveals that the presence of presynaptic ribosomes only partially correlates with synapse-specific functional properties (e.g. fast-spiking parvalbumin positive interneurons) and structural indicators of synaptic strength (e.g. relative occupancy of presynaptic boutons by mitochondria). Given that most analyses of synaptic strength-related structural indicators were statistically insignificant, the relative sparsity of putative presynaptic ribosomes and their limited contribution to the required synthesis of presynaptic proteins, my data demonstrates that local protein synthesis within the presynapse is limited by the availability of translationally functional ribosomes. A second project of this thesis addresses the importance of neuron-microglia communication in brain homeostasis. In addition to their unique function as brain-resident immune cells, microglia also sense and respond to neuronal activity states (Li & Barres, 2018; Lowery et al., 2017; Sominsky et al., 2018; Wolf et al., 2017). However, pathways underlying activity-dependent neuron-microglia communication and their role in physiological processes, including synaptic plasticity, remain poorly understood (Lowery et al., 2017). In this thesis, I used genetic approaches to modulate neurotransmission (Munc13-1/2 DKO) and neuromodulation (Rab3ABCD QKO) followed by microscopic and proteomic techniques designed to quantify functionally relevant changes in microglial morphology and proteomic expression profiles, respectively. I developed the light microscopic workflow and have accumulated fixed organotypic hippocampal slice cultures from all the required genotypes, however, the analysis of this data set remains to be completed. I observed that selective silencing of both evoked and spontaneous neurotransmission in targeted neuronal subpopulations in cortical and hippocampal subregions induces subtle changes in microglia and astrocyte density without corresponding indications of neurodegeneration. Further, immunohistochemical experiments validated microglial-specific protein biotinylation in hippocampal organotypic slice cultures using the TurboID proximity-labelling approach. Analyses of the here established mouse lines remain to be completed to reveal the extent to which the function and survival of microglia and astrocytes are selectively influenced by neurotransmission and neuromodulation.
Keywords: Ribosomes; Presynaptic plasticity; Neuron-microglia communication
