Roles of zinc and metallothionein-3 in oxidative stress-induced lysosomal dysfunction, cell death, and autophagy in neurons and astrocytes

Abstract

Zinc dyshomeostasis has been recognized as an important mechanism for cell death in acute brain injury. An increase in the level of free or histochemically reactive zinc in astrocytes and neurons is considered one of the major causes of death of these cells in ischemia and trauma. Although zinc dyshomeostasis can lead to cell death via diverse routes, the major pathway appears to involve oxidative stress.Recently, we found that a rise of zinc in autophagic vacuoles, including autolysosomes, is a prerequisite for lysosomal membrane permeabilization and cell death in cultured brain cells exposed to oxidative stress conditions. The source of zinc in this process is likely redox-sensitive zinc-binding proteins such as metallothioneins, which release zinc under oxidative conditions. Of the metallothioneins, metallothionein-3 is especially enriched in the central nervous system, but its physiologic role in this tissue is not well established. Like other metallothioneins, metallothionein-3 may function as metal detoxicant, but is also known to inhibit neurite outgrowth and, sometimes, promote neuronal death, likely by serving as a source of toxic zinc release. In addition, metallothionein-3 regulates lysosomal functions. In the absence of metallothionein-3, there are changes in lysosome-associated membrane protein-1 and -2, and reductions in certain lysosomal enzymes that result in decreased autophagic flux. This may have dual effects on cell survival. In acute oxidative injury, zinc dyshomeostasis and lysosomal membrane permeabilization are diminished in metallothionein-3 null cells, resulting in less cell death. But over the longer term, diminished lysosomal function may lead to the accumulation of abnormal proteins and cause cytotoxicity.The roles of zinc and metallothionein-3 in autophagy and/or lysosomal function have just begun to be investigated. In light of evidence that autophagy and lysosomes may play significant roles in the pathogenesis of various neurological diseases, further insight into the contribution of zinc dynamics and metallothionein-3 function may help provide ways to effectively regulate these processes in brain cells.

Figure 1

Summary diagram depicting zinc and MT3 effects on autophagy and lysosomes. A. Under physiological conditions, normal signaling involving zinc release from MT3 contributes to the normal progression of autophagy, resulting in the degradation of abnormal protein aggregates and waste organelles. This pathway may be beneficial for cell survival. B. Under injurious oxidative stress conditions (e.g., H₂O₂ or HNE treatment), the events described in (A) are exaggerated. Hence, much more intense zinc release from MT3 is induced, and excess autophagy is activated. Excess autophagy and excess zinc accumulation in lysosomes ultimately leads to LMP and cell death. C. Downregulation of MT3 decreases zinc release from MT3, inhibiting lysosomal functions and reducing fusion between autophagosomes and lysosomes, resulting in a reduced autophagy flux. Under conditions of acute injury, this results in a reduction in both LMP and cell death, but it can be detrimental to cell survival under conditions of chronic stress by reducing autophagic degradation of abnormal proteins.