The Intracellular Threonine of Amyloid Precursor Protein That Is Essential for Docking of Pin1 Is Dispensable for Developmental Function

Background: Processing of Ab-precursor protein (APP) plays an important role in Alzheimer’s Disease (AD) pathogenesis. Thr residue at amino acid 668 of the APP intracellular domain (AID) is highly conserved. When phosphorylated, this residue generates a binding site for Pin1. The interaction of APP with Pin1 has been involved in AD pathogenesis. Methodology/Principal Findings: To dissect the functions of this sequence in vivo, we created an APP knock-in allele, in which Thr 668 is replaced by an Ala (T 668 A). Doubly deficient APP/APP-like protein 2 (APLP2) mice present postnatal lethality and neuromuscular synapse defects. Previous work has shown that the APP intracellular domain is necessary for preventing early lethality and neuromuscular junctions (NMJ) defects. Crossing the T 668 A allele into the APLP2 knockout background showed that mutation of Thr 668 does not cause a defective phenotype. Notably, the T 668 A mutant APP is able to bind Mint1. Conclusions/Significance: Our results argue against an important role of the Thr 668 residue in the essential function of APP in developmental regulation. Furthermore, they indicate that phosphorylation at this residue is not functionally involved i

Mice with combined gene knock-outs reveal essential and partially redundant functions of amyloid precursor protein family members.

The amyloid precursor protein (APP) involved in Alzheimer's disease is a member of a larger gene family including amyloid precursor-like proteins APLP1 and APLP2. We generated and examined the phenotypes of mice lacking individual or all possible combinations of APP family members to assess potential functional redundancies within the gene family. Mice deficient for the nervous system-specific APLP1 protein showed a postnatal growth deficit as the only obvious abnormality. In contrast to this minor phenotype, APLP2(-/-)/APLP1(-/-) and APLP2(-/-)/APP(-/-) mice proved lethal early postnatally. Surprisingly, APLP1(-/-)/APP(-/-) mice were viable, apparently normal, and showed no compensatory upregulation of APLP2 expression. These data indicate redundancy between APLP2 and both other family members and corroborate a key physiological role for APLP2. This view gains further support by the observation that APLP1(-/-)/APP(-/-)/APLP2(+/-) mice display postnatal lethality. In addition, they provide genetic evidence for at least some distinct physiological roles of APP and APLP2 by demonstrating that combinations of single knock-outs with the APLP1 mutation resulted in double mutants of clearly different phenotypes, being either lethal, or viable. None of the lethal double mutants displayed, however, obvious histopathological abnormalities in the brain or any other organ examined. Moreover, cortical neurons from single or combined mutant mice showed unaltered survival rates under basal culture conditions and unaltered susceptibility to glutamate excitotoxicity in vitro