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Project

“Building a synapse: Investigation of the role of LRRTM1 and FLRT2 in synapse development in vivo”

Neural circuits form with remarkable specificity during development to permit the flow and computation of information in the brain. This extraordinary synaptic specificity that allows for connectivity between different neuronal subtypes with both cellular and subcellular precision gives rise to synapses of diverse shapes and properties. The functional significance of this synaptic diversity for the neural networks these synapses participate in is little understood. Furthermore, how this vast synaptic diversity is determined at the genetic and molecular level is largely unknown.

Recent studies indicate that cell-surface molecules are key determinants of cell type identity and suggest that these proteins are crucial players in the specification of synaptic connectivity. Synaptic adhesion molecules are a particular class of cell-surface proteins that form select trans-synaptic complexes and recruit diverse synaptic proteins, thus organizing synaptic development. One family of synaptic adhesion molecules, the leucine-rich repeat (LRR)-containing proteins, has emerged in recent years as important organizers of both excitatory and inhibitory synapses.

LRR-containing synaptic adhesion molecules possess various attributes that make them candidates for regulating synaptic specificity and diversity in neural circuits. Namely, this protein family is large and molecularly diverse, and displays distinctive expression patterns in specific neuronal cell types in the brain. Many LRR-containing adhesion proteins are postsynaptically localized and interact with diverse presynaptic partners. They furthermore exhibit a synapse formation-promoting, or synaptogenic, capacity in cultured neurons.

The overall aim of this thesis is to investigate the relevance of LRR adhesion complexes in intact neural circuits to better understand how this molecular diversity contributes to synaptic diversity in the brain. The hypothesis that drives this work is that cell type-specific repertoires of LRR adhesion proteins determine the specific properties of a neuron’s connections.

In order to investigate the significance of LRR protein diversity in hippocampal CA1 circuits, we chose to focus on a small selection of synaptogenic, postsynaptic LRR adhesion molecules belonging to different LRR subfamilies that each binds a different presynaptic partner – namely, FLRT2, LRRTM1 and Slitrk1. This work revealed that FLRT2, LRRTM1 and Slitrk1 are expressed in the same CA1 hippocampal neurons, localize to unique but partially overlapping hippocampal synapse types, and organize distinct aspects of pre- and postsynaptic structure, ultrastructure and function. These effects are context-dependent, as the role of individual LRR proteins can diverge at different CA1 synapse types. Our findings from this work in hippocampus indicate that a modular action of LRR proteins defines synaptic identity, because they operate in a non-redundant fashion to specify synaptic properties at designated synapse types.

We then aimed to further our investigation of how LRR protein diversity contributes to the specification of synaptic identity by studying how the cell type-specific expression of LRRTM1 controls the specification of thalamocortical synapses in primary somatosensory neocortex. Neocortical circuits are highly complex, harboring a wide array of neuronal cell types linked by specific connectivity patterns. This results in a magnificent synaptic diversity, but the molecular mechanisms that achieve this specificity and diversity are largely unknown. Our work revealed that Lrrtm1 exhibits a layer-specific expression pattern in neocortex, with a particularly prominent expression in superficial neurons in layer 5 (L5). Our preliminary data demonstrate that we now have the tools required to manipulate the expression of Lrrtm1 in a L5-specific manner and probe the function of specific thalamocortical circuits. In the near future, using these tools, we will be able to determine whether LRRTM1 determines specific properties of thalamocortical connections formed on L5 neurons.

Deciphering how cell type-specific repertoires of LRR proteins regulate the specificity of synaptic connectivity, as well as the diversity of a neuron’s synapses, is a major challenge. This thesis has yielded new insights into how these molecules differentially regulate synapse type-specific features, such as synaptic morphology and ultrastructural organization, and how they thereby differentially impact synaptic function. These results support our hypothesis that LRR adhesion molecules contribute to the diversity of synapses, in that they localize to select synaptic connections and act to specify synaptic properties. While we were able to demonstrate this for hippocampal circuits, future work built on the ideas and tools presented in this thesis will show whether this is also true in complex neocortical circuits.

The precise connectivity and function of neural circuits ultimately allows our brains to control our thoughts, feelings and actions. Because accurate neuronal wiring is necessary for cognitive function, efforts to better understand the molecular correlates that pattern synaptic specificity and diversity are important. LRR adhesion proteins are among these molecular correlates that help to define diverse synaptic connections both in health and disease, as dysfunction of these molecules has been shown to play a role in cognitive disorders. Going forward, these molecules therefore deserve to continue to be studied and characterized in diverse cell and synapse types, and across multiple brain regions.

Date:2 Sep 2013 →  26 Nov 2018
Keywords:Synapse Development, Slitrk1, Synaptic Function, Synapse Morphology, FLRT2, LRRTM1
Disciplines:Laboratory medicine
Project type:PhD project