Zinc ion homeostasis in cellular physiology and experimental models of traumatic brain injury
A major yet unsolved quest in treating traumatic brain injury (TBI) is the understanding of the secondary cellular injury that contributes to cell death. Whether zinc ions are toxic or protective in TBI is controversial. As an essential human micronutrient, zinc is needed for the structure and function of at least 3,000 proteins, and thus affects almost any aspect of cellular function. Although extremely low, intracellular zinc ion concentrations, [Zn2+]i, are tightly controlled to ensure optimal physiology and to avoid toxicity. Furthermore, zinc ions are now believed to be signaling ions, especially in neuronal systems. This dissertation addresses the dynamics of [Zn2+]i and quantitatively defines its safe range in particular cell types. [Zn2+]i was measured to be pico- to nanomolar in undifferentiated and differentiated rat pheochromocytoma (PC12) cells and in rat glioma (C6) cells. When PC12 cells proliferate, [Zn2+]i undergoes precisely controlled fluctuations with two peaks within one cell cycle. These results demonstrate that the already established requirement for zinc in the cell cycle and in differentiation relates to the availability of zinc ions. In a mechanical model of cellular injury, namely rapid stretch injury (RSI), nitric oxide induces an increase in [Zn2+]i that subsequently may protect cells by repressing the generation of ROS. A peak at one hour was followed by decreased [Zn2+]i. In PC12 cells, [Zn2+]i dropped below its normal level, indicating that these cells were in a state of ¡°zinc ion deficiency¡± hours after RSI. In an in vivo model of neural injury, namely fluid percussion TBI of rats, changes of [Zn2+]i were indirectly demonstrated by measuring the levels and states of the zinc-binding protein, metallothionein/thionein, in the hippocampus and the cortex. These results demonstrate that [Zn2+]i as well as zinc buffering dynamically fluctuate to adapt to the requirements of cellular functions, even when [Zn2+]i is extremely low inside the cell. They suggest that toxicity occurs when [Zn2+]i falls outside the safety thresholds. Therefore, when, where, how much and in which form zinc is present determine whether chelation or supplementation is an option for treatment. These new concepts provide new leads for developing strategies to treat TBI.