Signal amplification in growth cone gradient sensing by a double negative feedback loop among PTEN, PI(3,4,5)P₃ and...

Axon guidance during neural wiring involves a series of precisely controlled chemotactic events by the motile axonal tip, the growth cone. A fundamental question is how neuronal growth cones make directional decisions in response to extremely shallow gradients of guidance cues with exquisite sensitivity. Here we report that nerve growth cones possess a signal amplification mechanism during gradient sensing process. In neuronal growth cones of Xenopus spinal neurons, phosphatidylinositol-3,4,5-trisphosphate (PIP₃), an important signaling molecule in chemotaxis, was actively recruited to the up-gradient side in response to an external gradient of brain-derived neurotrophic factor (BDNF), resulting in an intracellular gradient with approximate 30-fold amplification of the input. Furthermore, a reverse gradient of phosphatase and tensin homolog (PTEN) was induced by BDNF within the growth cone and the increased PTEN activity at the down-gradient side is required for the amplification of PIP₃ signals. Mechanistically, the establishment of both positive PIP₃ and reverse PTEN gradients depends on the filamentous actin network. Together with computational modeling, our results revealed a double negative feedback loop among PTEN, PIP₃ and actomyosin for signal amplification, which is essential for gradient sensing of neuronal growth cones in response to diffusible cues.