Gradient steepness influences the pathfinding decisions of neuronal growth cones in vivo

Abstract

Gradients of chemotropic molecules are generally thought to be fundamental for the guidance of neuronal growth cones in the developing embryo. Here we show that the grasshopper-secreted semaphorin Sema 2a is expressed in a gradient during the period of tibial Til pioneer axon pathfinding into the CNS. At two critical Til growth cone choice points, the Sema 2a gradient differs in steepness, whereas the absolute concentration is the same. Although Til growth cones enter and extend up both steep and shallow gradients of Sema 2a, fewer projection errors occur along the steep gradient, suggesting that the steepness of the gradient encodes the critical guidance information into the pathfinding growth cone. In contrast, an increase in the absolute concentration of Sema 2a appears to constrain the Til growth cone size. Using these in vivo gradients, we provide evidence that the Til growth cone detects the Sema 2a gradient by measuring the fractional change in Sema 2a concentration, thereby demonstrating one mechanism that neuronal growth cones may use to detect and read gradients in vivo.