Abstract
The presenilin 1 (PSEN1) L271V mutation causes early-onset familial Alzheimer’s disease by disrupting the alternative splicing of the PSEN1 gene, producing some transcripts harboring the L271V point mutation and other transcripts lacking exon 8 (PS1∆exon8). We previously reported that PS1 L271V increased amyloid beta (Aβ) 42/40 ratios, while PS1∆exon8 reduced Aβ42/40 ratios, indicating that the former and not the exon 8 deletion transcript is amyloidogenic. Also, PS1∆exon8 did not rescue Aβ generation in PS1/2 double knockout cells indicating its identity as a severe loss-of-function splice form. PS1∆exon8 is generated physiologically raising the possibility that we had identified the first physiological inactive PS1 isoform. We studied PS1∆exon8 in vivo by crossing PS1∆exon8 transgenics with either PS1-null or Dutch APPE693Q mice. As a control, we crossed APPE693Q with mice expressing a deletion in an adjacent exon (PS1∆exon9). PS1∆exon8 did not rescue embryonic lethality or Notch-deficient phenotypes of PS1-null mice displaying severe loss of function in vivo. We also demonstrate that this splice form can interact with wildtype PS1 using cultured cells and co-immunoprecipitation (co-IP)/bimolecular fluorescence complementation. Further co-IP demonstrates that PS1∆exon8 interacts with nicastrin, participating in the γ–secretase complex formation. These data support that catalytically inactive PS1∆exon8 is generated physiologically and participates in protein-protein interactions.
Highlights
Over the past decade, we have investigated the molecular mechanisms associated with a mutant PS1 lacking exon 8 (PS1∆exon8) that is generated by the L271V splice site mutation[9]
We confirmed the presence of hemorrhage in the intermediate zone near the lateral ventricle in the mice PS1(+ /− ) (mPS1)(−/−) KO embryo (Fig. 2E,I) and hPS1∆exon[8] (+ )/mPS1(−/−) embryo (Fig. 2F,J), while hPS1∆exon8(+ ) with a single endogenous mouse PS1 allele mPS1(+ /− ) (Fig. 2G,K) and the mPS1(+ /+ ) (Ntg) embryos (Fig. 2H,L) showed no hemorrhage
As an additional in vivo test in brain of the potential amyloidogenicity and pathogenicity of PSEN1∆exon[8 ], we employed bigenic mouse models to assess whether PS1∆exon[8] could promote cerebral amyloidosis as we previously reported when we crossed familial Alzheimer’s disease (FAD) Dutch APP mice with another line of mice harboring a deletion mutation in the adjacent exon[16]
Summary
We have investigated the molecular mechanisms associated with a mutant PS1 lacking exon 8 (PS1∆exon8) that is generated by the L271V splice site mutation[9] We identified this point mutation in a Tasmanian family (Tas-1) in 20039. Understanding the mechanism of action of these unusual PS1 molecules is highly relevant because their effects in the presence of wt PS1/2 suggests an intermolecular communication in trans between PS1∆exon[8] and wt PS1 that is not explicitly predicted by the current 1:1:1:1 stoichiometry model for the γ -secretase complex[12] This in trans model has been recently proposed by Kelleher and colleagues in his characterization of a severe loss-of-function FAD mutant PS113. This report constitutes evidence that PS1∆exon[8] is the first physiologically generated severely hypomorphic alternate splice form to be studied in this rescue paradigm
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