Nerve Growth Cones as Sensing, Amplifying and Filtering Modules: A Single-Molecule and Microfluidic Approach

Abstract

Nerve growth cones (GCs) are chemical sensors that convert graded extracellular cues into oriented axonal motion. Ensuring a sensitive and robust GC response to directional signals requires the ability to amplify and filter external gradients. However, our knowledge of how these signal processing tasks are performed at the single cell level remains sparse. This is largely due to the limitations of conventional guidance assays that have precluded systematic measurements of the GC output response to variable input gradients. We developed a novel shear-free gradient-generating microfluidic device with a simple architecture that greatly facilitates the interface of cultured neurons with microcircuits. With this device, we probed the information-processing capabilities of single GCs during GABA directional sensing. By measuring at the single molecule level the polarization of GABAA chemoreceptors at the GC membrane as a function of the external GABA gradient, we found that GCs act as: (i) signal amplifiers over a narrow range of concentrations, (ii) low-pass temporal filters with a cut-off frequency independent of stimuli conditions. Furthermore, thanks to quantitative computational modeling, we related these systems-level properties to the saturable occupancy response and the lateral dynamics of GABAA receptors, and, thereby, provided an integrative view of individual GCs as sensing devices.