Person:

Fisher, Yvette

Building arborisations of the right size and shape is fundamental for neural network function. Live imaging in vertebrate brains strongly suggests that nascent synapses are critical for branch growth during development. The molecular mechanisms underlying this are largely unknown. Here we present a novel system in Drosophila for studying the development of complex arborisations live, in vivo during metamorphosis. In growing arborisations we see branch dynamics and localisations of presynaptic proteins very similar to the ‘synaptotropic growth’ described in fish/frogs. These accumulations of presynaptic proteins do not appear to be presynaptic release sites and are not paired with neurotransmitter receptors. Knockdowns of either evoked or spontaneous neurotransmission do not impact arbor growth. Instead, we find that axonal branch growth is regulated by dynamic, focal localisations of Neurexin and Neuroligin. These adhesion complexes provide stability for filopodia by a ‘stick-and-grow’ based mechanism wholly independent of synaptic activity.

We can maintain some sense of direction in the dark by keeping track of our own movements, but when visual landmarks are available, our sense of direction is more accurate and stable. Moreover, we can learn new landmarks in new environments. What mechanisms reconcile self-movement information with ever-changing landmarks to generate a coherent sense of direction? Using whole-cell recordings and calcium imaging from Drosophila heading neurons, we show that each heading neuron is inhibited by visual cues in specific horizontal positions, with different visual maps in different individuals. Inhibition arises from presynaptic axons that form an all-to-all matrix of potential connections onto heading neurons. Visual input to the heading network can reorganize over minutes when visuo-motor correlations change, causing persistent changes in the brain’s heading map. Plasticity of sensory inputs, when combined with network attractor dynamics, should allow the brain’s spatial maps to incorporate sensory cues in new environments.