Possible therapeutic roles of metallothionein-3 and zinc in endosome-autophagosome-lysosome pathway (EALP) dysfunction in astrocytes

The accumulation of abnormal protein aggregates contributes to the pathological progression of diverse neurodegenerative diseases. An increasing body of evidence indicates that defects in the protein clearance system play a crucial role in this process. Cargoes delivered via endosomes, phagosomes, and autophagosomes converge on lysosomes for degradation, which process is collectively called “the endosome-autophagosome-lysosome pathway” or EALP. As such, dysfunction of lysosomes may result in the accumulation of all these upstream vesicles/cargoes and may play a key role in diverse neurodegenerative conditions. Over the years, we found that Zn-metallothionein-3 (MT3), the brain-enriched form of metallothionein, regulates lysosomal functions in cortical astrocytes. Zn-MT3 appears to interact with β-actin and activate c-Abl kinase. As a result, Zn-MT3 plays a role in maintaining lysosomal acidity, a prerequisite for vesicle fusion as well as cargo degradation. The reported downregulation of MT3 in Alzheimer’s disease (AD), hence, may contribute to lysosomal dysfunction in AD. Of interest, raising intracellular free zinc levels also caused lysosomal acidification and normalization of degradation, even in the context of arrested autophagy. Pending further research on the mechanisms of these effects, we propose that mea sures increasing Zn-MT3 and/or intracellular/lysosomal free zinc may be useful in normalizing lysosomal functions in neurodegenerative conditions.