Oxidative stress and autophagy in the regulation of lysosome-dependent neuron death

Abstract

Lysosomes critically regulate the pH-dependent catabolism of extracellular and intracellular macromolecules delivered from the endocytic/heterophagy and autophagy pathways, respectively. The importance of lysosomes to cell survival is underscored not only by their unique ability effectively to degrade metalloproteins and oxidatively damaged macromolecules, but also by the distinct potential for induction of both caspase-dependent and -independent cell death with a compromise in the integrity of lysosome function. Oxidative stress and free radical damage play a principal role in cell death induced by lysosome dysfunction and may be linked to several upstream and downstream stimuli, including alterations in the autophagy degradation pathway, inhibition of lysosome enzyme function, and lysosome membrane damage. Neurons are sensitive to lysosome dysfunction, and the contribution of oxidative stress and free radical damage to lysosome dysfunction may contribute to the etiology of neurodegenerative disease. This review provides a broad overview of lysosome function and explores the contribution of oxidative stress and autophagy to lysosome dysfunction-induced neuron death. Putative signaling pathways that either induce lysosome dysfunction or result from lysosome dysfunction or both, and the role of oxidative stress, free radical damage, and lysosome dysfunction in pediatric lysosomal storage disorders (neuronal ceroid lipofuscinoses or NCL/Batten disease) and in Alzheimer's disease are emphasized.

FIG. 1.

Convergence of the endosomal–lysosomal and autophagy–lysosomal degradation pathways. Lysosomal hydrolases are produced in the endoplasmic reticulum (ER) and, on delivery to the trans-Golgi network (TGN), are transported in vesicles by recognition of mannose-6-phosphate receptors (M6PRs) to the late endosome (or to the early endosome, which then matures to form the late endosome). The late endosome is then thought to deliver lysosomal hydrolases via a type of fusion event to their terminal location, the lysosome, which is M6PR negative. Damaged organelles and macromolecules are surrounded by a limiting membrane from the ER to form a preautophagosomal structure (PAS), which matures to form the double membraned autophagosome. The pH of autophagosomes is not sufficient to degrade their intraluminal contents, and fusion with lysosomes (forming the autophagolysosome) or with endosomes (forming an amphisome), which both contain pH-dependent acid hydrolases, must take place for autophagosomal contents to be effectively degraded. Please refer to text for further details.

FIG. 2.

Macroautophagy induction versus inhibition in oxidative stress–induced lysosome damage. The induction of lysosomal membrane damage, LMP, and cell death may be directly influenced by both the aberrant induction and inhibition of macroautophagy, which can lead to the induction of intralysosomal oxidative stress. It has also been proposed that an initial overinduction of macroautophagy induction may lead to an eventual inhibition of macroautophagy, which also may be related in part to the induction of oxidative stress. Please see text for further details.

FIG. 3.

Chemical structure of chloroquine. Chloroquine [7-chloro-4-(4-dimethylamino-1-methylbutylamino)quino-line] represents the class of fluoroquinolones.

FIG. 4.

Chloroquine-induced death of human SH-SY5Y cells follows alterations in the processing of CD. (A) Treatment of human SH-SY5Y cells with chloroquine (50 μM) significantly attenuates cell viability at 48 h vs. vehicle control but not at 24 h. ^*p < 0.05 vs. vehicle control (Student's unpaired t test). (B) By 24 h, chloroquine induces a modest decrease in the mature “active” form of CD, migrating at ~30 kDa, along with a marked increase in the inactive, “pre-pro” fragment migrating at ~50 kDa, in comparison to vehicle control. After 48 h of chloroquine treatment, levels of the mature active form of CD appear to be further reduced in comparison to 24 h. Levels of β-tubulin (migrating at ~50 kDa) serve as the loading control.

FIG. 5.

Oxidative stress, lysosomal membrane permeabilization, and the induction of necrotic versus apoptotic death. Agents that promote the direct or indirect production of oxidative stress may lead to lysosome membrane permeabilization (LMP) and cell death. It is thought that the induction of total LMP favors the onset of necrosis, whereas partial LMP favors the onset of apoptosis. LMP is associated with the release of lysosomal cathepsins into the cytosol and the interaction with pro-apoptotic Bcl-2 family members, which leads to the induction of mitochondrial apoptosis. Proapoptotic Bcl-2 family members may also act directly at the lysosomal membrane as a stimulus for LMP. For further details, please see the text.