Final Report Summary - SYNAPSE STABILITY (Molecular Analysis of Synapse Formation, Maintenance and Disassembly at the Drosophila neuromuscular junction)
Neuronal circuits are formed through synaptic connections between defined populations of neurons. While many synapses remain stable over prolonged periods of time, the regulated disassembly of functional synaptic connections is required to ensure precise connectivity during development and to enable plasticity of the mature circuit. In contrast, the inappropriate loss of synaptic connections in response to genetic mutations will lead to a disruption of neuronal circuits and to progressive neurodegenerative disorders. Therefore, the identification of the molecular mechanisms controlling synapse stability versus disassembly may help our understanding of neuronal circuit formation and plasticity and may advance our understanding of progressive neurodegenerative disease. We are using the Drosophila neuromuscular junction (NMJ) as a model system to unravel the molecular mechanisms underlying synapse formation, function and stability. We previously identified a presynaptic Ank2-Spectrin network that regulates synapse formation and stability at the Drosophila NMJ. This network has the potential to link cell adhesion molecules to the actin and microtubule cytoskeleton to control and regulate synapse formation and maintenance. The aim of this project was the identification of cell adhesion molecules (CAMs) functioning upstream of the Ank2-Spectrin network. We performed a large-scale RNAi screen targeting different classes of potential synaptic CAMs and identified the L1-type CAM Neuroglian as essential for synapse maintenance. By combining biochemical, biophysical and genetic assays at the Drosophila neuromuscular junction and the central Giant Fiber Synapse, we demonstrate that the association of Nrg with pre- and postsynaptic Ankyrin serves as a critical element controlling synapse growth, maturation and stability via trans-synaptic signaling. Modulation of this interaction allows local and well-defined alterations of synaptic connectivity while maintaining general neuronal circuit architecture. Several important findings arise from our work: (1) Control of synapse stability requires Nrg mediated cell adhesion and direct coupling to the presynaptic Ankyrin-based cytoskeleton. (2) Synapse elimination and axonal retraction display striking phenotypic similarities to developmentally controlled synapse elimination at the vertebrate NMJ suggesting common cellular mechanisms between developmental and disease processes. (3) Gradual alteration of Ankyrin binding capacities of Nrg provides a mechanism to control the delicate balance between synapse growth and stability. (4) Regulated association with the Ankyrin cytoskeleton enables trans-synaptic Nrg signaling to coordinate pre- and postsynaptic development. Our findings provide new insights into how synaptic connectivity can be controlled in a precise, local manner without disrupting general architecture of neuronal circuits. In addition, this work provides new insights into how mutations in L1-type CAMs might cause neuropathological diseases that include synaptic diseases like mental retardation and autism.