Turnover of C99 is Controlled by a Crosstalk between ERAD and Ubiquitin-Independent Lysosomal Degradation in Human Neuroglioma Cells

Alzheimer's disease AD characterized by the buildup of amyloid-beta peptide (A beta) aggregates derived from proteolytic processing of the beta-amyloid precursor protein (APP). Amyloidogenic cleavage of APP by beta-secretase/BACE1 generates the C-terminal fragment C99/CTF beta that can be subsequently cleaved by gamma-secretase to produce A beta Growing evidence indicates high levels of C99/CTF beta are determinant for AD. Although it has been postulated that gamma-secretase-independent pathways must control C99/CTF beta levels, the contribution of organelles with degradative functions, such as the endoplasmic reticulum (ER) or lysosomes, is unclear. In this report, we investigated the turnover and amyloidogenic processing of C99/CTF beta in human H4 neurogliama cells, and found that C99/CTF beta is localized at the Golgi apparatus in contrast to APP, which is mostly found in endosomes. Conditions that localized C99/CTF beta to the ER resulted in its degradation in a proteasome. dependent manner that first required polyubiquitination, consistent with an active role of the ER associated degradation ERAD) in this process Furthermore, when proteasomal activity was inhibited C99/CTF beta was degraded in a chloroquine (CQ)-sensitive compartment, implicating lysosomes as alternative sites for its degradation. Our results highlight a crosstalk between degradation pathways within the ER and lysosomes to avoid protein accumulation and toxicity.