Got nerve: making and breaking neuromuscular connections

In order to execute behaviors, the animal's nervous system needs to be accurately wired. One important step during this process is the formation of precise alignment of pre‐ and postsynaptic structures during embryonic development as well as during nerve regeneration. We use the zebrafish neuromuscular junction to study this processes during development and during regeneration. In genetic screens for motor axon guidance and synapse formation, we have identified a series of genes that have allowed us to dissect at the molecular and cellular level how motor axons navigate from the spinal cord to their muscle targets, and how they precisely align with postsynaptic elements on the muscle to form the first neuromuscular synapses [1–3]. One important question is whether the principles learned from these analyses also apply to axonal regrowth after injury. As a first step towards answering this question, we have established a zebrafish model in which to define the interplay between injured peripheral nerves and neighboring cells, i.e. muscle, glia cells and macrophages in vivo . Using a laser mounted on a spinning disc confocal microscope we axotomize identified spinal motor nerves and image the entire process of axonal fragmentation and debris removal. Minute‐by‐minute imaging reveals for the first time the dynamic changes preceding and during nerve fragmentation, with single axon resolutions. Moreover, simultaneous live cell‐imaging of injured nerves and neighboring cell types reveals extensive cell shape changes and cell‐cell contacts prior and during nerve fragmentation. We will discuss our ongoing efforts to integrate the process of axonal guidance and synapse formation during development with the reestablishment of a functional motor system after nerve injury.