Studies on neuron responses to simultaneous and competing extracellular cues
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The understanding of cell responses to extracellular signals is a field of major importance. In particular, cell interactions with designed surfaces are fundamental for creating successful interfaces between biomaterials and biological entities. Bioactive materials that promote specific behaviors such as cell proliferation, migration or differentiation are highly relevant for tissue engineering strategies. In particular, neural engineering applications including nerve conduits for nerve regeneration, neural prosthetics and artificial neural networks, would be highly benefited from studies on artificial substrates that modulate neuronal behavior. Extracellular stimuli influence critical phases in neuronal development such as neuron polarization (i.e., axon formation) and axon growth. There are numerous stimuli that have been studied for influencing neuron responses. Some of these stimuli are substrate topography, growth factors, extracellular matrix components, electrical activity and the presence of support cells. Although all these stimuli induce responses in neurons independently, in this investigation we focused on cell behavior when simultaneous cues were presented to the cell. This included both combinatorial and competitive cues. We studied the novel combination of chemical and physical stimuli by immobilizing nerve growth factor (NGF) on topographical features. We found that topography (i.e., physical stimulus) highly influences axon formation, whereas axon extension is controlled by a synergy of immobilized NGF and topography. We also investigated the combination of electrical and chemical stimulation. In that case, NGF was immobilized on polypyrrole, an electrically- conducting polymer, finding an enhanced effect on neurite extension. Finally, simultaneous but spatially independent stimuli (i.e., competitive stimuli) were investigated for influencing direction of polarization. We found that physical cues were preferred over chemical cues for axon formation. Our results have contributed to further knowledge regarding the modification of artificial substrates that better control neuronal responses. This knowledge can be applied in the design of materials for nerve regeneration strategies, the modification of electrodes that stimulate neurons in prosthetic devices, and for the control of neuronal mapping and connectivity in neural nets.