Asynchronous synapse elimination in neonatal motor units: Studies using GFP transgenic mice

In developing muscle, synapse elimination reduces the number of motor axons that innervate each postsynaptic cell. This loss of connections is thought to be a consequence of axon branch trimming. However, branch retraction ha s not been observed directly, and many questions remain, such as: do all mo tor axons retract branches, are eliminated branches withdrawn synchronously , and are withdrawing branches localized to particular regions? To address these questions, we used transgenic mice that express fluorescent proteins in small subsets of motor axons, providing a unique opportunity to reconstr uct complete axonal arbors and identify all the postsynaptic targets. We fo und that, during early postnatal development, each motor axon loses termina l branches, but retracting branches withdraw asynchronously and without obv ious spatial bias, suggesting that local interactions at each neuromuscular junction regulate synapse elimination.