Regulation of axonal growth and neuromuscular junction formation by neuronal PTEN signaling

During the development of the vertebrate neuromuscular junction (NMJ), motor axon tips stop growing after contacting muscle and transform into presynaptic terminals that secrete the neurotransmitter acetylcholine (ACh) and activate postsynaptic ACh receptors (AChRs) to trigger muscle contraction. The neuron-intrinsic signaling that retards axonal growth to facilitate stable nerve-muscle interaction and synaptogenesis is poorly understood. Here we report a novel function of presynaptic signaling by PTEN (phosphatase and tensin homolog) in mediating a growth-to-synaptogenesis transition in neurons. In Xenopus nerve-muscle cocultures, axonal growth speed was halved after contact with muscle compared with before contact, but when cultures were exposed to the PTEN-blocker bpV (bisperoxo (1,10-phenanthroline) oxovanadate; phen), axons touching muscle grew ∼50% faster than their counterparts in control cultures. Suppression of neuronal PTEN expression using morpholinos or the forced expression of catalytically inactive PTEN in neurons also resulted in faster than normal axonal advance after contact with muscle cells. Significantly, interference with PTEN by each of these methods also led to reduced AChR clustering at innervation sites in muscle, indicating that disruption of neuronal PTEN signaling inhibited NMJ assembly. We thus propose that PTEN-dependent slowing of axonal growth enables the establishment of stable nerve-muscle contacts that develop into NMJs.