XSTIM1 is required for attractive turning responses of neuronal growth cones to a netrin-1 gradient. (A) Sample images of growth cone turning responses in a gradient of netrin-1 of a control Xenopus spinal neuron, and neurons derived from embryos injected with mRNA encoding wild-type (YFP-XSTIM1-WT), or dominant negative mutant (YFP-XSTIM1-DN). The left two columns of images show neuronal growth cones at the start (0 min) and the end of exposure (30 min) to a netrin-1 gradient (5 μg/ml in the pipette). The right column shows superimposed trajectories of neurite extension during the 30' period for a sample population of 12 neurons under the each condition. The origin is the center of the growth cone and the original direction of growth is vertical. Arrows indicate the direction of the gradient. Scale bars: 10 μm. (B) Summary of mean turning angles of growth cones in responses to a gradient of netrin-1 under different conditions. The number associated with the bar graph indicates the number of growth cones analyzed. Values represent mean ± s.e.m. (* indicates P < 0.01; Bootstrap-test). (C) Summary of net neurite growth during the 30 minutes turning assay under different conditions. Values represent mean ± s.e.m. (* indicates P < 0.05; Bootstrap-test).