Proteolytic Processing of Familial British Dementia-associated BRI Variants

Abstract

Different mutations in the BRI 2 gene cause rare neurodegenerative conditions, termed familial British dementia (FBD) and familial Danish dementia (FDD). The mutant genes encode BRI-L and BRI-D, the precursors of fibrillogenic ABri and ADan peptides, respectively. We previously reported that furin processes both BRI-L and its wild type counterpart, BRI, resulting in the secretion of C-terminal peptides; elevated levels of peptides were generated from BRI-L. In the present study, we show that inducible expression of ␣1-antitrypsin Portland, a furin inhibitor, inhibits the endoproteolysis of BRI and BRI-L in a dose-dependent manner. Moreover, comparison of the activities of several proprotein convertases reveals that furin is most efficient in endoproteolysis of BRI and BRI-L; PACE4, PC6A, PC6B, and LPC show much lower activities. Interestingly, LPC also exhibits enhanced cleavage of BRI-L compared with BRI. Finally, we demonstrate that BRI-D is also processed by furin and, like BRI-L, the cleavage of BRI-D is more efficient than that of BRI. Interestingly, while the ABri peptide is detected both intracellularly and in the medium, the ADan peptide accumulates predominantly in intracellular compartments. We propose that intracellular accumulation of amyloidogenic ADan or ABri peptides results in the neuronal damage leading to FDD and FBD, respectively. Familial British dementia (FBD),1 an autosomal dominant neurodegenerative disorder, is characterized by progressive spastic tetraparesis, cerebellar ataxia, and dementia (1). The principal pathological hallmarks of FBD include severe amyloid angiopathy, non-neuritic amyloid plaques affecting cerebellum, hippocampus, and cerebral cortex, hippocampal neurofibrillary tangles, and perivascular white matter changes. The genetic defect underlying FBD was recently identified (2); affected patients have a T to A transversion (TGA3 AGA) at the stop codon of the BRI 2 gene (3-5) that results in the generation of an arginine codon and a longer open reading frame that encodes a 277-amino acid protein, termed BRI-L (2). A 34-amino acid peptide, termed ABri, generated by endoproteolytic processing of this mutant precursor protein is deposited in the brains of FBD patients (2, 6). Interestingly, a different genetic defect of the BRI 2 gene was found in another rare, autosomal dominant, neurodegenerative disease, familial Danish dementia (FDD) (7). Clinical symptoms of FDD include progressive development of cataracts and hearing impairments as well as other neurological symptoms and dementia. The BRI 2 mutation in FDD is the presence of a 10-nucleotide (TTTAATTTGT) duplication between codons 265 and 266, just prior to the normal termination codon. This decamer duplication leads to the loss of serine 266 and a change in the reading frame to generate an 11-amino acid longer protein (BRI-D) that has a unique C-terminal sequence. The resulting C-terminal 34-amino acid peptide, termed ADan, is the major component of insoluble amyloid fibrils in brains of FDD patients (7). Thus, ABri and ...

Regulated Intramembrane Proteolysis of Bri2 (Itm2b) by ADAM10 and SPPL2a/SPPL2b

“…3A). Consistent with previous findings, the prodomain of Bri2 is proteolytically removed (32,33). Immunoblotting of cell lysates with an antibody to the N-terminal FLAG tag revealed both Bri2, lacking the C-terminal propeptide, and pro-Bri2 still containing the propeptide (Fig.…”

“…Interestingly, proteolytic processing of Notch and Bri2 share some remarkable similarities. Both contain a propeptide that is removed by furin or related proteases (32,33,46). Furthermore, in both cases, the ectodomain undergoes shedding via a protease of the ADAM family (46).…”

“…1A, proBri2) (32,33,40). Consequently, a membrane-bound fragment, containing the so-called BRICHOS domain, remains after this initial processing (Fig.…”

“…For clarity, we will use the term Bri2 throughout the manuscript. Bri2 undergoes processing by furin and related proteases within its ectodomain (32,33), which leads to the release of its 4-kDa C-terminal propeptide. Mutations of the stop codon in the bri2 (itm2b) gene are responsible for the generation of a longer open reading frame, therefore causing the release of a longer propeptide by furin-mediated proteolysis (31)(32)(33).…”

“…Bri2 undergoes processing by furin and related proteases within its ectodomain (32,33), which leads to the release of its 4-kDa C-terminal propeptide. Mutations of the stop codon in the bri2 (itm2b) gene are responsible for the generation of a longer open reading frame, therefore causing the release of a longer propeptide by furin-mediated proteolysis (31)(32)(33). The mutant Bri2 amyloid propeptides (ABri) tend to aggregate into oligomers, protofibrils, and further into amyloid deposits and cause familial British (31) and Danish dementia (34), which are autosomal dominant neurodegenerative diseases characterized by amyloid angiopathy, plaques, and neurofibrillar tangles (33,35).…”