Bildung, Aufrechterhaltung und strukturelle Veränderung von Synapsen
Zusammenfassung der Projektergebnisse
Synapses generally relay neuronal signals to their postsynaptic target cells. To establish and maintain these synapses during development organisms have evolved mechanisms that support and remodel these structures in order to balance presynaptic release with postsynaptic needs. To identify genes that control structural aspects of synapses we choose the Drosophila neuromuscular junction (NMJ) as a model system. NMJs are well suited for forward genetic screens as they form relatively large syptic terminals that can be identified through the translucent cuticle. In addition, NMJs are stereotypical ly arranged, i.e. their form and location is conserved from segment to segment, making it relatively easy to compare them within the same animal. In a large-scale mutagenesis screen, we identified several classes of mutations causing strong and permanent alterations of NMJ morphologies. Two of these genes, ankyrin 2 (ank2) and neuroligin 1 (dnlg1), were further characterized in this proposal. Ankyrins normally function in the anchorage of transmembrane proteins to the underlying Spectrin-based membrane skeleton, being an excellent candidate molecule for mediating important structural functions. Indeed, mutations in ank2 lead to irreversible retractions of presynaptic membranes, recognizable by the disturbed apposition of pre- and postsynaptic release sites. In an early step, presynaptic microtubules and membrane proteins fail to be maintained. Eventually entire presynaptic nerve endings, called boutons, are disassembled, causing synaptic terminals to degenerate over time, which eventually leads to denervation of a subset of muscles. Ank2 is an extremely large molecule that is alternatively spliced to form different isoforms the largest of which is 1,2 MDa in size. To analyze the function of ank2 we generated isoform-specific mutants and isoform-specific antibodies. While the large isoform Ank2-L is required for the stabilization of NMJs, the extra large isoform, Ank2-XL, mediates the connection to the microtubule cytoskeleton. In the absence of Ank2-XL, the microtubules are unable to extend to distal-most boutons, forming retraction bulb-like accumulations. In collaboration with Jan Pielage at the FMI in Basel, we found that Ank2-XL and the microtubule-associated protein Futsch synergistically function to control the organization and spacing of the microtubule network. Neuroligins are postsynaptic adhesion molecules that interact with presynaptic Neurexins, forming transsynaptic adhesion complexes that are conserved from invertebrates to vertebrates. In this project, we provide the first characterization of Drosophila Neuroligin (DNlg1). Drosophila larvae undergo a dramatic increase in body size during development, causing NMJs to steadily increase the output of neurotransmitters in order to supply rapidly growing muscle fibers. Larvae lacking DNlg1 display synaptic growth defects, with NMJs being too small compared to muscle size. In stark contrast to wild-type animals, a subset of presynaptic boutons is not apposed by corresponding postsynaptic domains, indicating that DNlg1 controls the alignment of synaptic membranes. Detailed molecular, ultrastructural and electrophysiological analysis in collaboration with Stephan Sigrist at the FU Berlin showed that DNlg1 is localized next to postsynaptic glutamate receptor fields, recruiting scaffolding proteins that initiate the assembly of postsynaptic densities. In addition, transsynaptic protein-protein interactions lead to accumulation of presynaptic neurexins and their associated proteins, such as Liprin-a and Syd-1. Thus, the Neurexin-Neuroligin complex promotes the differentiation of postsynaptic domains essential for synaptic maturation.
Projektbezogene Publikationen (Auswahl)
- (2009). No sidesteps on a beaten track: Motor axons follow a labeled substrate pathway. Cell Adh. Migr. 3 , 358-360
Aberle, H.
- (2009). Searching for guidance cues: Follow the Sidestep trail. Fly 3,270- 273
Aberle, H.
