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

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

Tau deletion reduces plaque‐associated BACE 1 accumulation and decelerates plaque formation in a mouse model of Alzheimer's disease

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The Metalloprotease ‘A Disintegrin and Metalloprotease 10’ (ADAM10) undergoes rapid, post-lysis autocatalytic degradation

The transmembrane protein ‘a disintegrin and metalloprotease 10’ (ADAM10) has key physiological functions, e.g. during embryonic development and in the brain. During transit through the secretory pathway, immature ADAM10 (proADAM10) is converted to its proteolytically active, mature form (mADAM10). Increasing or decreasing the abundance/activity of mADAM10 is considered a therapeutic approach for diseases, such as Alzheimer’s disease and cancer. Yet, biochemical detection and characterization of mADAM10 has been difficult. In contrast, proADAM10 is readily detected, e.g. in immunoblots, suggesting that mADAM10 is only a minor fraction of total cellular ADAM10. Here, we demonstrate that mADAM10, but not proADAM10 unexpectedly undergoes rapid, time-dependent degradation upon biochemical cell lysis in different cell lines and in primary neurons, preventing the detection of the majority of mADAM10 in immunoblots. This degradation required the catalytic activity of ADAM10, was efficiently prevented by adding active-site inhibitors to the lysis buffer and did not affect proADAM10, suggesting that ADAM10 degradation occurred in an intramolecular and autoproteolytic manner. Inhibition of post-lysis autoproteolysis demonstrated efficient cellular ADAM10 maturation with higher levels of mADAM10 than of proADAM10. Moreover, a cycloheximide chase experiment revealed that mADAM10 is a long-lived protein with a half-life of around 12 hours. In summary, this study demonstrates that mADAM10 autoproteolysis must be blocked to allow proper detection of the mature mADAM10, which is essential for the correct interpretation of biochemical and cellular studies of ADAM10

The β-Secretase Substrate Seizure 6–Like Protein (SEZ6L) Controls Motor Functions in Mice

The membrane protein seizure 6-like (SEZ6L) is a neuronal substrate of the Alzheimer's disease protease BACE1, and little is known about its physiological function in the nervous system. Here, we show that SEZ6L constitutive knockout mice display motor phenotypes in adulthood, including changes in gait and decreased motor coordination. Additionally, SEZ6L knockout mice displayed increased anxiety-like behaviour, although spatial learning and memory in the Morris water maze were normal. Analysis of the gross anatomy and proteome of the adult SEZ6L knockout cerebellum did not reveal any major differences compared to wild type, indicating that lack of SEZ6L in other regions of the nervous system may contribute to the phenotypes observed. In summary, our study establishes physiological functions for SEZ6L in regulating motor coordination and curbing anxiety-related behaviour, indicating that aberrant SEZ6L function in the human nervous system may contribute to movement disorders and neuropsychiatric diseases

Seizure protein 6 and its homolog seizure 6-like protein are physiological substrates of BACE1 in neurons

The protease BACE1 (beta-site APP cleaving enzyme) is a major drug target in Alzheimer's disease. However, BACE1 therapeutic inhibition may cause unwanted adverse effects due to its additional functions in the nervous system, such as in myelination and neuronal connectivity. Additionally, recent proteomic studies investigating BACE1 inhibition in cell lines and cultured murine neurons identified a wider range of neuronal membrane proteins as potential BACE1 substrates, including seizure protein 6 (SEZ6) and its homolog SEZ6L.status: publishe

Additional file 1: Figure S1. of Seizure protein 6 and its homolog seizure 6-like protein are physiological substrates of BACE1 in neurons

Comparison of BACE1 and BACE2 expression in MIN6 and WT primary neurons; Figure S2. UniProt subcellular location of proteins quantified in 3 out of 3 replicates of urea and SDC supported digestion; Figure S3. Sub-classification of membrane proteins quantified in three out of three replicates of urea and SDC supported digestion. (PDF 243 kb