Synergistic integration of Netrin and ephrin axon guidance signals by spinal motor neurons

TLDR

During neural circuit assembly, axonal growth cones encounter multiple guidance signals at trajectory choice points, yet the responses to combined cues and their underlying mechanisms remain poorly understood. We studied the convergence of signals directing trajectory selection of spinal motor axons entering the limb. Our investigations show that the Netrin receptor Unc5c and ephrin receptor EphB2 form a ligand‑dependent complex, and that Netrin–ephrin synergistic growth‑cone responses involve potentiation of Src family kinase signaling. We demonstrate that Netrin‑1 attracts or repels distinct motor axon populations according to receptor expression, and that motor axons synergistically integrate attractive or repulsive Netrin‑1 signals with repulsive ephrin signals in quantitative in vitro assays.

Abstract

During neural circuit assembly, axonal growth cones are exposed to multiple guidance signals at trajectory choice points. While axonal responses to individual guidance cues have been extensively studied, less is known about responses to combination of signals and underlying molecular mechanisms. Here, we studied the convergence of signals directing trajectory selection of spinal motor axons entering the limb. We first demonstrate that Netrin-1 attracts and repels distinct motor axon populations, according to their expression of Netrin receptors. Quantitative in vitro assays demonstrate that motor axons synergistically integrate both attractive or repulsive Netrin-1 signals together with repulsive ephrin signals. Our investigations of the mechanism of ephrin-B2 and Netrin-1 integration demonstrate that the Netrin receptor Unc5c and the ephrin receptor EphB2 can form a complex in a ligand-dependent manner and that Netrin–ephrin synergistic growth cones responses involve the potentiation of Src family kinase signaling, a common effector of both pathways.