Strain rate-dependent induction of reactive astrogliosis and cell death in three-dimensional neuronal-astrocytic co-cultures.

A mechanical insult to the brain drastically alters the microenvironment as specific cell types become reactive in an effort to sequester severely damaged tissue. Although injury-induced astrogliosis has been investigated, the relationship between well-defined biomechanical inputs and acute astrogliotic alterations is not well understood. We evaluated the effects of strain rate on cell death and astrogliosis using a three-dimensional (3-D) in vitro model of neurons and astrocytes within a bioactive matrix. At 21 days post-plating, co-cultures were deformed to 0.50 shear strain at strain rates of 1, 10, or 30 s(-1). We found that cell death and astrogliotic profiles varied differentially based on strain rate at 2 days post-insult. Significant cell death was observed after moderate (10 s(-1)) and high (30 s(-1)) rate deformation, but not after quasi-static (1 s(-1)) loading. The vast majority of cell death occurred in neurons, suggesting that these cells are more susceptible to high rate shear strains than astrocytes for the insult parameters used here. Injury-induced astrogliosis was compared to co-cultures treated with transforming growth factor beta, which induced robust astrocyte hypertrophy and increased glial fibrillary acidic protein (GFAP) and chondroitin sulfate proteoglycans (CSPGs). Quasi-static loading resulted in increased cell density and CSPG secretion. Moderate rate deformation increased cell density, GFAP reactivity, and hypertrophic process density. High rate deformation resulted in increased GFAP reactivity; however, other astrogliotic alterations were not observed at this time-point. These results demonstrate that the mode and degree of astrogliosis depend on rate of deformation, demonstrating astrogliotic augmentation at sub-lethal injury levels as well as levels inducing cell death.