Differential, dominant activation and inhibition of Notch signalling and APP cleavage by truncations of PSEN1 in human disease

Abstract

PRESENILIN1 (PSEN1) is the major locus for mutations causing familial Alzheimer's disease (FAD) and is also mutated in Pick disease of brain, familial acne inversa and dilated cardiomyopathy. It is a critical facilitator of Notch signalling and many other signalling pathways and protein cleavage events including production of the Amyloidβ (Aβ) peptide from the AMYLOID BETA A4 PRECURSOR PROTEIN (APP). We previously reported that interference with splicing of transcripts of the zebrafish orthologue of PSEN1 creates dominant negative effects on Notch signalling. Here, we extend this work to show that various truncations of human PSEN1 (or zebrafish Psen1) protein have starkly differential effects on Notch signalling and cleavage of zebrafish Appa (a paralogue of human APP). Different truncations can suppress or stimulate Notch signalling but not Appa cleavage and vice versa. The G183V mutation possibly causing Pick disease causes production of aberrant transcripts truncating the open reading frame after exon 5 sequence. We show that the truncated protein potentially translated from these transcripts avidly incorporates into very stable Psen1-dependent higher molecular weight complexes and suppresses cleavage of Appa but not Notch signalling. In contrast, the truncated protein potentially produced by the P242LfsX11 acne inversa mutation has no effect on Appa cleavage but, unexpectedly, enhances Notch signalling. Our results suggest novel hypotheses for the pathological mechanisms underlying these diseases and illustrate the importance of investigating the function of dominant mutations at physiologically relevant expression levels and in the normally heterozygous state in which they cause human disease rather than in isolation from healthy alleles.

Increased localization of APP‐C99 in mitochondria‐associated ER membranes causes mitochondrial dysfunction in Alzheimer disease

“…Consistent with the data of others (Das et al , 2016), FL‐APP and BACE1 co‐migrated partially with a marker for endosomes (Rab7), but not with lysosomal, ER‐intermediate, or MAM markers (Fig 2C). Similarly, the APP‐CTFs C83 and C99 co‐migrated with endosomal and lysosomal markers (Rab5, Rab7, and LAMP‐2) (Haass et al , 2012; Das et al , 2016), whereas PS1 co‐migrated with MAM markers, such as FACL4 (Area‐Gomez et al , 2009; Newman et al , 2014; Schreiner et al , 2015). We reasoned that the difficulty in seeing APP‐CTFs and PS1 together was probably due to the rapid cleavage of the CTFs by γ‐secretase once both are in the same compartment.…”

“…We and others have shown that presenilins and γ‐secretase activity localize to MAM (Area‐Gomez et al , 2009; Newman et al , 2014; Schreiner et al , 2015). Moreover, MAM functionality (Area‐Gomez et al , 2012) and ER–mitochondrial apposition (Area‐Gomez et al , 2012; Hedskog et al , 2013) are increased in AD.…”

Evidence For and Against a Pathogenic Role of Reduced γ-Secretase Activity in Familial Alzheimer’s Disease

“…The Watanabe et al paper shows that these aberrant transcripts are not completely degraded in brain tissue although nonsense-medicated mRNA decay appears to remove them completely in other tissues. We have analyzed the putative protein products of these aberrant transcripts using assays in zebrafish embryos and have seen dominant negative effects specifically on Notch signaling but not on ␥-secretase cleavage of an A␤PP orthologue [87]. Zebrafish Psen1 peptides equivalent to those putatively translated from the aberrant transcripts also incorporate specifically and avidly into detergent-resistant higher molecular weight complexes and bind to both Psen1 and Psen2 proteins [87].…”

“…We have analyzed the putative protein products of these aberrant transcripts using assays in zebrafish embryos and have seen dominant negative effects specifically on Notch signaling but not on ␥-secretase cleavage of an A␤PP orthologue [87]. Zebrafish Psen1 peptides equivalent to those putatively translated from the aberrant transcripts also incorporate specifically and avidly into detergent-resistant higher molecular weight complexes and bind to both Psen1 and Psen2 proteins [87]. Therefore, it is possible that the action of truncated forms of PSEN1 protein may underlie the frontotemporal dementia pathology of the G183V mutation of human PSEN1.…”

“…Also, a truncated isoform of PSEN2, "PS2V", is upregulated in sAD brains [105]. PS2V is induced by hypoxia/oxidative stress, is associated with increased production of A␤ [106,107], can suppress the unfolded protein response (UPR) and can boost ␥-secretase activity [107,108] through an unknown mechanism that may involve interaction with fulllength PSEN protein [87].…”