Lysosomal dysfunction in proteinopathic neurodegenerative disorders: possible therapeutic roles of cAMP and zinc

Abstract

A number of neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis, share intra- and/or extracellular deposition of protein aggregates as a common core pathology. While the species of accumulating proteins are distinct in each disease, an increasing body of evidence indicates that defects in the protein clearance system play a crucial role in the gradual accumulation of protein aggregates. Among protein degradation systems, the endosome-autophagosome-lysosome pathway (EALP) is the main degradation machinery, especially for large protein aggregates. Lysosomal dysfunction or defects in fusion with vesicles containing cargo are commonly observed abnormalities in proteinopathic neurodegenerative diseases. In this review, we discuss the available evidence for a mechanistic connection between components of the EALP-especially lysosomes-and neurodegenerative diseases. We also focus on lysosomal pH regulation and its significance in maintaining flux through the EALP. Finally, we suggest that raising cAMP and free zinc levels in brain cells may be beneficial in normalizing lysosomal pH and EALP flux.

Keywords: EALP; Lysosome; MT3; Zinc; cAMP.

Fig. 1

Schematic depiction of cAMP- and Zn²⁺-mediated lysosomal acidification. Increases in cAMP levels through activation of adenylyl cyclase and/or inhibition of PDEs activate PKA. PKA activation leads to increases in lysosomal free Zn²⁺ levels by an as yet unidentified mechanism. Increases in lysosomal Zn²⁺ levels restore lysosomal acidity through an unknown mechanism, even in the presence of BA, a potent and selective inhibitor of the main proton pump, V-ATPase

Fig. 2

Cilostazol (PDE3 inhibitor) increases lysosomal free Zn²⁺ levels, re-acidifies lysosomes, and promotes autophagy flux. a. Fluorescence photomicrographs of FluoZin3-loaded, cultured astrocytes before (left) and after a 1-h treatment (right) with 10 μM cilostazol alone or cilostazol plus the PKA inhibitor H-89 (10 μM) or Zn²⁺ chelator TPEN (500 nM). Cilostazol treatment raised free Zn²⁺ levels in lysosomes, an effect that was blocked by H-89 or TPEN. b. Fluorescence photomicrographs of astrocytes loaded with DND189 (a pH-sensitive lysosomal dye) before (left) and after a 1-h treatment (right) with 100 nM bafilomycin A1 (BA) alone, BA plus 10 μM cilostazol, BA plus cilostazol and PKA inhibitor (H-89, 10 μM), or BA plus cilostazol and TPEN (500 nM). c. Fluorescence images of H4 cells transfected with both GFP-LC3 and RFP-LC3 obtained after a 6-h treatment with 100 nM BA alone, BA plus 10 μM cilostazol, cilostazol alone, or sham washed (CTL). With BA treatment, GFP fluorescence (left) did not disappear, resulting in many yellow spots in the merged image. Addition of cilostazol substantially reduced GFP signals, resulting in a reduction in yellow spots in the merged image. d. Western blots (upper) for p62, a marker of autophagy flux, and corresponding β-actin in samples obtained from astrocytes after a 6-h treatment with 100 nM BA alone, BA plus cilostazol, BA plus PKA inhibitor (H-89, 10 μM), or sham washed (CTL). Another set of Western blots (lower) for p62 and corresponding β-actin in samples obtained from astrocytes after a 6-h treatment with BA alone, BA plus cilostazol, BA plus TPEN, or sham washed (CTL)