A Role for Presenilins in Autophagy Revisited: Normal Acidification of Lysosomes in Cells Lacking PSEN1 and PSEN2

Zhang et al. report that autophagy is normal in presenilin 1 (PS1) KO or presenilin 1/ 2 (PS1/2) DKO cells, a conclusion differing from findings in five laboratories that have demonstrated autophagy pathology and accumulation of autophagy substrates consistent with impaired autophagy in PS1-deficient cells (Esselens et al., 2004; Wilson et al., 2004; Lee et al., 2010; Neely et al., 2011; Dobrowolski et al., 2012). The "vesicle" pH of ~ 6.6 they report for WT and PS1KO cells has questionable relevance to lysosomal pH, which is widely accepted to be ~ 5.0. Unlike the lysosome-targeted pH probe we used, the unconjugated Lysosensor DND-160 probe used in the Zhang et al. study labeled undefined intracellular compartments that the authors acknowledged in the Results were not specifically lysosomes, despite their paper's title. This may partly explain the surprising finding in their Figure 2 that more than 85% of the pH values measured in wild-type cells were higher than pH 7.0. Of additional concern, the authors eliminated, for unexplained reasons, >75% of their readings, which exceeded the upper limit of their standard curve (pH 8.0) in calculating the "vesicle" pH of wild-type cells. They also eliminated different percentages of the pH values measured in each of the PS1-deficient cell lines that were compared with wild-type cells.

Using an siRNA approach, Zhang et al. also report that, "We failed to detect any effects on the maturation of V0a1-6His by reducing the levels of either STT3A or STT3B." However, later in Results and consistent with our findings, they conclude the opposite, stating "... taken together our results indicate that the glycosylation site in V0a1 can be glycosylated by both the STT3A and STT3B isoforms of the OST." In this experiment, pro-Cathepsin C glycosylation, used as a positive control to establish the effectiveness of STT3B siRNA, remained unchanged after STT3B siRNA treatment, which would make any negative finding on V0a1 glycosylation after STT3B knockdown difficult to interpret.

References

Dobrowolski R, Vick P, Ploper D, Gumper I, Snitkin H, Sabatini DD, De Robertis EM (2012) Presenilin Deficiency or Lysosomal Inhibition Enhances Wnt Signaling through Relocalization of GSK3 to the Late-Endosomal Compartment. Cell Rep 2:1316-28.

Esselens C, Oorschot V, Baert V, Raemaekers T, Spittaels K, Serneels L, Zheng H, Saftig P, De Strooper B, Klumperman J, Annaert W (2004) Presenilin 1 mediates the turnover of telencephalin in hippocampal neurons via an autophagic degradative pathway. J Cell Biol 166:1041-1054.

Lee J-H, Yu WH, Kumar A, Lee S, Mohan PS, Peterhoff CM, Wolfe DM, Martinez-Vicente M, Massey AC, Sovak G, Uchiyama Y, Westaway D, Cuervo AM, Nixon RA (2010) Lysosomal Proteolysis and Autophagy Require Presenilin 1 and Are Disrupted by Alzheimer-Related PS1 Mutations. Cell 141:1146-1158.

Neely KM, Green KN, LaFerla FM (2011) Presenilin Is Necessary for Efficient Proteolysis through the Autophagy-Lysosome System in a g- Secretase-Independent Manner. J Neurosci 31:2781-2791.

Wilson CA, Murphy DD, Giasson BI, Zhang B, Trojanowski JQ, Lee VM (2004) Degradative organelles containing mislocalized a- and b-synuclein proliferate in presenilin-1 null neurons. J Cell Biol 165:335-346.

Conflict of Interest:

None declared