BACE1-cleavage of Sez6 and Sez6L is elevated in Niemann-Pick type C disease mouse brains

Mirsada Causevic,Jenny M Gunnersen,Stefan F Lichtenthaler,Birgit Hutter-Paier,Kristina Dominko,Stjepko Cermak,Martina Malnar,Stefanie Flunkert,Peer-Hendrik Kuhn,Martina Pigoni,Manfred Windisch,Jessica Mcdonald,Sabina Tahirovic,Lea Vidatic,Silva Hecimovic,Alessio Colombo,Daniel Havas,Dimitri Krainc

doi:10.1371/journal.pone.0200344

Abstract

It is intriguing that a rare, inherited lysosomal storage disorder Niemann-Pick type C (NPC) shares similarities with Alzheimer’s disease (AD). We have previously reported an enhanced processing of β-amyloid precursor protein (APP) by β-secretase (BACE1), a key enzyme in the pathogenesis of AD, in NPC1-null cells. In this work, we characterized regional and temporal expression and processing of the recently identified BACE1 substrates seizure protein 6 (Sez6) and seizure 6-like protein (Sez6L), and APP, in NPC1-/- (NPC1) and NPC1+/+ (wt) mouse brains. We analysed 4-weeks old brains to detect the earliest changes associated with NPC, and 10-weeks of age to identify changes at terminal disease stage. Sez6 and Sez6L were selected due to their predominant cleavage by BACE1, and their potential role in synaptic function that may contribute to presentation of seizures and/or motor impairments in NPC patients. While an enhanced BACE1-cleavage of all three substrates was detected in NPC1 vs. wt-mouse brains at 4-weeks of age, at 10-weeks increased proteolysis by BACE1 was observed for Sez6L in the cortex, hippocampus and cerebellum of NPC1-mice. Interestingly, both APP and Sez6L were found to be expressed in Purkinje neurons and their immunostaining was lost upon Purkinje cell neurodegeneration in 10-weeks old NPC1 mice. Furthermore, in NPC1- vs. wt-mouse primary cortical neurons, both Sez6 and Sez6L showed increased punctuate staining within the endolysosomal pathway as well as increased Sez6L and BACE1-positive puncta. This indicates that a trafficking defect within the endolysosomal pathway may play a key role in enhanced BACE1-proteolysis in NPC disease. Overall, our findings suggest that enhanced proteolysis by BACE1 could be a part of NPC disease pathogenesis. Understanding the basic biology of BACE1 and the functional impact of cleavage of its substrates is important to better evaluate the therapeutic potential of BACE1 against AD and, possibly, NPC disease.

Highlights

Alzheimer’s disease (AD) is the most common form of dementia and the most common neurodegenerative disorder [1, 2]
The levels of soluble Sez6 (sSez6) and total soluble fragment of amyloid-β precursor protein (sAPPt) seem to increase in cerebellum of NPC1 compared to wt mice, this did not reach statistical significance (Fig 1C, 1D and 1F)
In our study, we did not detect an increase in β-site amyloid precursor protein cleaving enzyme 1 (BACE1) protein levels in NPC1 vs. wt mice, our results demonstrate significantly increased processing of the exclusive BACE1 substrates seizure protein 6 (Sez6) and seizure 6-like protein (Sez6L), as well as amyloid precursor protein (APP), in NPC1 mice, suggesting that BACE1-dependent cleavage in this mouse model of Niemann-Pick type C (NPC) disease is enhanced

Summary

Introduction

Alzheimer’s disease (AD) is the most common form of dementia and the most common neurodegenerative disorder [1, 2]. No disease modifying therapies against AD are available. About 30 million patients are suffering from AD worldwide. Due to an ageing population, it is estimated that by the year 2050 numbers will increase to 106 million AD patients [1, 2]. These numbers raise the concern of AD becoming an epidemic problem. BACE1 has become a prime therapeutic target for lowering Aβ and treating AD, and clinical development of BACE1 inhibitors is being intensely pursued [4]

Methods

Results

Discussion

Conclusion