P4‐251: ALTERED METABOLISM OF BACE1 SUBSTRATES SEIZURE PROTEIN 6 AND SEIZURE 6‐LIKE PROTEIN 1 IN NIEMANN‐PICK TYPE C DISEASE

Nieman-Pick type C (NPC) disease is a lysosomal storage disorder caused by mutations in NPC1 or NPC2 genes. It shares a number of similarities with Alzheimer's disease, including disturbed cholesterol homeostasis and APP processing. Previously, in one of the models of NPC disease (CHO NPC1-null cells), we have shown a clear co-localization of APP and BACE1 within enlarged early/recycling endosomes leading to increased β-secretase processing of APP. In this study, in an effort to better characterize BACE1 (dys)function upon NPC1 loss, we investigated protein levels of BACE1 and several BACE1 substrates in two different experimental models of NPC disease. The levels of BACE1 and BACE1 substrates for which BACE1 is a major sheddase [seizure protein 6 (Sez6) and seizure 6-like protein 1 (Sez6L1)] were analyzed in the following models of NPC disease: 1) a cellular model of NPC disease, in which mouse, primary neurons were treated with the U18666A compound, that mimics the major phenotype of NPC disease - cholesterol accumulation and 2) in an animal model of NPC disease. We observed a marked increase in BACE1 protein levels in both NPC disease models examined in this study. BACE1 increase was more pronounced in hippocampi vs. cerebella of 4-week-old NPC1- mutant mice compared with wild-type mice. This increase paralleled the increase in Sez6 membrane-tethered precursor and its soluble ectodomain, suggesting full-length Sez6 accumulation and its enhanced shedding by BACE1 upon NPC1 dysfunction, respectively. Similar findings were observed for Sez6L1 and APP proteins. Currently, we are investigating other BACE1 substrates, including APP homologues APLP1 and APLP2. This study suggests the following: a) the NPC1 dysfunction has a strong effect on BACE1 as BACE1 protein levels are increased in all experimental models of NPC disease examined in this study; b) BACE1 alteration upon NPC1 dysfunction causes altered metabolism of the major BACE1 substrates Sez6 and Sez6L1; c) Sez6/Sez6L1 induction in hippocampi of NPC mice implicates their potential role in the NPC phenotype. Overall, altered BACE1 levels and processing of its substrates may contribute to the NPC phenotype and BACE1 may represent a novel target for an early intervention in NPC disease.