Deformation response and cytosolic calcium increases in NTera2 neurons subjected to traumatic levels of hydrodynamic shear stress

A biomechanical method was developed to model traumatic brain injury in vitro by hydrodynamically deforming NTera2 neurons while monitoring intracellular free calcium concentration ( (Ca$\sp{2+}\rbrack\sb{\rm i}$). Fluid shear stress caused membrane strain rates greater than 1 s$\sp{-1}$, loading conditions which mimic inertial traumatic loading. Cellular strain, measured by tracking membrane-bound microspheres, ranged from 0 to 52%. $\rm\lbrack Ca\sp{2+}\rbrack\sb{i}$, assesses by Fura-2, increased with strain and loading rate. Furthermore, $\rm\lbrack Ca\sp{2+}\rbrack\sb{i}$ increases correlated with strain (R$\sp2$ = 0.75), suggesting a deformation induced injury. Leakage of cytosolic lactate dehydrogenase (LDH) was used to assess cell viability in the first 24 hr following injury. Acute ($\u3c$5 min) LDH release correlated with loading rate (R$\sp2$ = 0.81) and, because no known transport mechanisms exist, this leakage was attributed to direct membrane damage. In addition, $\rm\lbrack Ca\sp{2+}\rbrack\sb{i}$ and LDH were significantly higher for loading rates above 100 $\rm dyne\cdot cm\sp{-2}\cdot s\sp{-1}$, whereas quasistatic controls (below 20 $\rm dyne\cdot cm\sp{-2}\cdot s\sp{-1}$) exhibited no significant increases in either parameter. Since LDH release at 24 hr was elevated for high loading rates, different mechanisms of calcium flux were examined to assess possible pathways which may lead to cell death. Most of the free calcium was found to enter the cytosol from the extracellular space. The presence of extracellular calcium was necessary for cell death. In addition, extracellular glutamate was found to increase significantly 24 hr following high loading rate conditions. The NMDA receptor antagonist MK-801 attenuated injury induced $\rm\lbrack Ca\sp{2+}\rbrack\sb{i}$ increases by 45% and reduced LDH release by 50%. Pretreatment with the calcium channel blocker nifedipine, the glutamate release inhibitor riluzole, and the ganglioside GM1 also significantly reduced $\rm\lbrack Ca\sp{2+}\rbrack\sb{i}$, but did not affect cell viability. The rate dependent behavior of NTera2 neurons suggests that mechanical loading directly causes changes in membrane permeability which result in calcium influx and membrane depolarization, which trigger cell damaging events. Hindered diffusion of LDH through a porous membrane was analyzed using hydrodynamic theory and then the behavior of calcium peaks and transients was predicted. Assuming short-lived pores formed from high rate deformation, this analysis agreed with the rate dependent behavior seen experimentally and may explain graded responses to injury as a function of mechanical input