Substrate Dependence of Mechanical Response of Neurons and Astrocytes

The response of neural cells to mechanical cues is a critical component of the innate neuroprotective cascade aimed at minimizing the consequences of traumatic brain injury (TBI). Reactive gliosis and the formation of glial scars around the lesion site are among the processes triggered by TBI where mechanical stimuli play a central role. It is well established that the mechanical properties of the microenvironment influence phenotype and morphology in most cell types. It has been shown that astrocytes change morphology [1] and cytoskeletal content [2] when grown on substrates of varying stiffness, and that mechanically injured astrocyte cultures show alterations in cell stiffness [3]. Accurate estimates of the mechanical properties of central nervous system (CNS) cells in their in-vivo conditions are needed to develop multiscale models of TBI. Lu et al found astrocytes to be softer than neurons under small deformations [4]. In recent studies, we investigated the response of neurons to large strains and at different loading rates in order to develop single cell models capable of simulating cell deformations in regimes relevant for TBI conditions [5]. However, these studies have been conducted on cells cultured on hard substrates, and the measured cell properties might differ from their in-vivo counterparts due to the aforementioned effects. Here, in order to investigate the effects of substrate stiffness on the cell mechanical properties, we used atomic force microscopy (AFM) and confocal imaging techniques to characterize the response of primary neurons and astrocytes cultured on polyacrylamide (PAA) gels of varying composition. The use of artificial gels minimizes confounding effects associated with biopolymer gels (both protein-based and polysaccharide-based) where specific receptor bindings may trigger additional biochemical responses [1].