Multi-molecular gradients of permissive and inhibitory cues direct neurite outgrowth

Correct development of neuronal tracts requires the coordination of multiple permissive and inhibitory signals. By generating an in vitro microenvironment using soft lithography and microfluidic techniques, multiple guidance cues can be presented in a spatially defined way. Here we evaluated how neurites of dorsal root ganglia neurons responded to permissive and inhibitory cues presented by substrate-bound molecular gradients. Linear gradients containing inhibitory chondroitin sulfate proteoglycan (CSPG) and/or permissive laminin-1 (LN) were generated as single-cue gradients, parallel double-cue gradients, and opposing double-cue gradients with varying slopes. Neurite growth was analyzed using circular statistical methods, and for all gradients examined, neurons extended neurites toward regions of lower CSPG and higher LN concentrations. Single-cue gradients elicited similarly directed neurite growth responses at the higher concentrations tested for both LN and CSPG, and both gradient slope and fractional concentration change affected neurite growth. When the two contrasting molecular cues were presented together, neurites responded differently depending on the directions of the gradients. Neurite growth on LN-CSPG double gradients of opposite direction was strongly directed, while neurite growth on LN-CSPG double gradients of parallel direction was uniform. These results represent an important step toward understanding how neurite growth is guided by complex microenvironments containing multiple molecular cues.