Neurexin and Neuroligin-based adhesion complexes drive axonal arborisation growth independent of synaptic activity

Abstract

Building arborisations of the right size and shape is fundamental for neural network function. Live imaging studies in vertebrate brains strongly suggest that nascent synapses are critical for branch growth during the development of axonal and dendritic arborisations. The molecular mechanisms underlying such synaptotropic events are largely unknown.\n\nHere we present a novel system in Drosophila for studying the development of complex axonal arborisations live, in vivo during metamorphosis. In these growing axonal arborisations we see a relationship between the punctate localisations of presynaptic components and branch dynamics that is very similar to synaptotropic growth described in fish and frogs. These presynaptic components however do not appear to represent functional presynaptic release sites and are not paired with clusters of neurotransmitter receptors. Pharmacological and genetic knockdowns of evoked and spontaneous neurotransmission do not impact the outgrowth of these neurons. Instead, we find that axonal branch growth is regulated by the dynamic focal localisations of synaptic adhesion proteins Neurexin and Neuroligin. These adhesion complexes provide selective stability for filopodia by a stick and grow-based mechanism wholly independent of synaptic activity.