Physiologically generated presenilin 1 lacking exon 8 fails to rescue brain PS1-/- phenotype and forms complexes with wildtype PS1 and nicastrin

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 (PS1δexon8). We previously reported that PS1 L271V increased amyloid beta (Aβ) 42/40 ratios, while PS1δexon8 reduced Aβ42/40 ratios, indicating that the former and not the exon 8 deletion transcript is amyloidogenic. Also, PS1δexon8 did not rescue Aβ generation in PS1/2 double knockout cells indicating its identity as a severe loss-of-function splice form. PS1δexon8 is generated physiologically raising the possibility that we had identified the first physiological inactive PS1 isoform. We studied PS1δexon8 in vivo by crossing PS1δexon8 transgenics with either PS1-null or Dutch APP E693Q mice. As a control, we crossed APPE693Q with mice expressing a deletion in an adjacent exon (PS1δexon9). PS1δexon8 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 PS1δexon8 interacts with nicastrin, participating in the δ-secretase complex formation. These data support that catalytically inactive PS1δexon8 is generated physiologically and participates in protein-protein interactions.