Lysosomal Dysfunction in Down Syndrome Is APP-Dependent and Mediated by APP-βCTF (C99)

Abstract

Lysosomal failure underlies pathogenesis of numerous congenital neurodegenerative disorders and is an early and progressive feature of Alzheimer's disease (AD) pathogenesis. Here, we report that lysosomal dysfunction in Down ayndrome (trisomy 21), a neurodevelopmental disorder and form of early onset AD, requires the extra gene copy of amyloid precursor protein (APP) and is specifically mediated by the ␤ cleaved carboxy terminal fragment of APP (APP-␤CTF, C99). In primary fibroblasts from individuals with DS, lysosomal degradation of autophagic and endocytic substrates is selectively impaired, causing them to accumulate in enlarged autolysosomes/ lysosomes. Direct measurements of lysosomal pH uncovered a significant elevation (0.6 units) as a basis for slowed LC3 turnover and the inactivation of cathepsin D and other lysosomal hydrolases known to be unstable or less active when lysosomal pH is persistently elevated. Normalizing lysosome pH by delivering acidic nanoparticles to lysosomes ameliorated lysosomal deficits, whereas RNA sequencing analysis excluded a transcriptional contribution to hydrolase declines. Cortical neurons cultured from the Ts2 mouse model of DS exhibited lysosomal deficits similar to those in DS cells. Lowering APP expression with siRNA or BACE1 inhibition reversed cathepsin deficits in both fibroblasts and neurons. Deleting one Bace1 allele from adult Ts2 mice had similar rescue effects in vivo. The modest elevation of endogenous APP-␤CTF needed to disrupt lysosomal function in DS is relevant to sporadic AD where APP-␤CTF, but not APP, is also elevated. Our results extend evidence that impaired lysosomal acidification drives progressive lysosomal failure in multiple forms of AD.

Iron Responsive Element (IRE)-mediated responses to iron dyshomeostasis in Alzheimer’s disease

“…The deleterious effects of this on the brains of mice (defective mitochondrial biogenesis and function and stimulation of inflammatory responses) could be alleviated by increasing the levels of iron in their diet. The observations of Nixon and colleagues that acidification of the endolysomal pathway is affected both by fAD mutations in PSEN1 [46] and excessive dosage of the APP gene [54], together with our observations from our fAD-like psen1 Q96_K97del /+ mutant zebrafish, support the possibility that fAD brains may suffer a ferrous iron deficiency in a background of ferric iron overload. Intriguingly, the greatest genetic risk factor for late onset AD, the e4 allele of the gene APOE, appears to increase lysosomal pH [55] but e4's increased risk of AD is alleviated in individuals who possess the HFE 282Y allele that predisposes to the iron overload disease hemochromatosis [56].…”

Transcriptome analysis indicates dominant effects on ribosome and mitochondrial function of a premature termination codon mutation in the zebrafish genepsen2

“…(We suggested that the involvement of PRESENILIN holoproteins in multimerization may explain the “reading frame preservation” rule that states that all EOfAD mutations in the PSEN genes must preserve an open reading frame that uses the original stop codon (12). ) Curiously, both of the PRESENILIN activities mentioned above, the regulation of MAM formation and of endolysosomal acidification, appear to be mediated by the C99 fragment (β-CTF) of APP (57, 60) and this implies that these activities of the PRESENILINs may share a common molecular mechanism.…”

Targeting Aggrephagy for the Treatment of Alzheimer’s Disease

“…Lysosomal hydrolase (e.g., cathepsin) activity and proteolysis indeed require optimal acidic pH [133]. In AD, presenilin-1 mutations and elevated CTFβ peptide levels impair lysosomal acidification and cathepsin activity, causing a massive pileup of autophagic vesicles with undigested autophagy substrates in neurons [134,135]. Presenilin-1 functional loss due to its mutation impairs the V-ATPase function, which leads to altered lysosomal Ca 2+ homeostasis and results in defective lysosomal acidification and cathepsin D activity [136].…”

“…Presenilin-1 functional loss due to its mutation impairs the V-ATPase function, which leads to altered lysosomal Ca 2+ homeostasis and results in defective lysosomal acidification and cathepsin D activity [136]. Also, amyloidogenic APP cleavage product CTFβ shifts lysosomal pH towards alkalinity and reduce cathepsin D activity [135]. These lysosome-related AD pathogenic events can be prevented with acidic nanoparticles.…”