Neurexin and Neuroligins Jointly Regulate Synaptic Degeneration at the Drosophila Neuromuscular Junction based on TEM Studies

Abstract Background: The Drosophila larval neuromuscular junction (NMJ) is a well-known model system and is often used to study synapse development and signal transmission in neurology. Here, we show the synaptic degeneration at NMJ boutons, primarily based on transmission electron microscopy (TEM) studies.Methods: To this aim, we extensively acquire the ultrastructure of diffuse NMJ boutons using continuous sectioning and transmission electron microscope in the wide type and the dneurexin (dnrx) and dneuroligins (dngs) mutant.Results: When degeneration starts, the subsynaptic reticulum (SSR) swells, retracts and folds inward, and then, the residual SSR degenerates into a disordered, thin or linear membrane with a shrinking postsynaptic area (PSA) and axon terminal. The axon terminal begins to degenerate from the central region, and the T-bar structure detaches from the presynaptic membrane with clustered synaptic vesicles to accelerate large-scale degeneration, accompanied by retraction of synaptic vesicles and mitochondria. There are two degeneration modes for clear synaptic vesicles. In the first mode, synaptic vesicles without actin filaments degenerate on the membrane with ultrafine spots and collapse and disperse to form an irregular profile with dark ultrafine particles. In the second mode, clear synaptic vesicles with actin filaments degenerate into dense synaptic vesicles, form irregular dark clumps without a membrane, and collapse and disperse to form an irregular profile with dark ultrafine particles. Last, all residual membranes in NMJ boutons, including presynaptic and postsynaptic membranes, become very thin, most of the SSR degenerates into a linear shape, and all the residual elements in axon terminals, such as synaptic vesicles, mitochondria, the cytoskeleton, and the T-bar, degenerate in situ and eventually form a cluster of dark ultrafine particles. Axon terminals and elements within axon terminals degenerate with the postsynaptic SSR, and swelling and retraction of the SSR occurs prior to axon terminal degradation, which degenerates faster and with more intensity than the SSR. NMJ bouton degeneration occurs under normal physiological conditions, but degeneration in dnrx and dnlgs mutant is accelerated. Furthermore, there is a synergistic effect in dnrx;dnlgs double mutants and dnlg2;dnlg3 double mutants that promotes degeneration of NMJ boutons.Conclusions: NMJ bouton degeneration occurs under normal physiological conditions but is accelerated in dnrx and dnlgs mutants. Furthermore, there is a synergistic effect in dnrx;dnlgs double mutants and dnlg2;dnlg3 double mutants that promotes degeneration of NMJ boutons, which suggests that both neurexin and neuroligins play a vital role in preventing synaptic degeneration. This study proposes the model of NMJ bouton degeneration patterns, which is very conducive to in-depth study of neurodegeneration.